r REESE LIBRARY UNIVERSITY 'OF CALIFORNIA. ^n__n_n_ n MAR 16 1893 , 1*0 , No. *^Q U?>| . C/i7S5 Afo. Works of Prof. Robt. H. Thurston, Published by JOHN WILEY & SONS, 53 E. Tenth Street, New York. The Publishers and the Author will be grateful to tiny of the readers of this volume who will kindly call their attention to any errors of omission or of commission that they may find therein. It is intended to make our publications, so far as they may be demanded by the public, standard works of study and reference, and, to that end, the greatest accuracy is sought. It rarely happens that the early editions of works of any size are free from errors; but it is the endeavor of the Publishers to see them removed immediately upon being discovered, and it is therefore desired that the Author may be aided in his task of revision, from time to time, by the kindly criticism of his readers. JOHN WILEY & SONS. 53 EAST TENTH STREET, Second door west of Broadway. users of sTcain-ens-'ines ." Jiuifilfr K in Scientific Schools, show- ing the properties of the subjects treated. By I'rol. \i. II. Thurston. Well illustrated. In three parts. Part I. THE NON-METALLIC MATERIALS OF ENGINEER ING AND METALLURGY. With Measures in British and Metric Units, and Metric and Iledueti< >n Tables 8vo, cloth, $2 DO Part II- IRON AND STEEL. The Ores of Iron ; Methods of Reduction ; Manufacturing Processes; Chemical and Physical Properties of Iron and Steel; strength. Duc- tility, Elasticity and Resistance; Effects of Time, Teraperatuiv, ami repeated Strain ; Methods of Test ; Specifications Hvo, cloth, :>"<> Part III. THE ALLOYS AND THEIR CONSTITUENTS. Copper, Tin, Zinc, Lead, Antimony, Bismuth, Nickel, Aluminum, etc.; The Brasses, Bronzes; Copper-Tin-Zinc Alloys; Other Valuable Alloys; Their Qualities, Peculiar Characteristics; Uses and Special Adaptations; Thurston's "Maximum Alloys 1 ': strength of tin- Alloys as Commonly Made, and as Affected by Special Conditions; The Mechanical Treatment of Metals v< >, cl< >t h, :.' :M "As intimated above, this work will form one of the most complete as well as modern treatises upon the Materials used in all ports of Building Constructions. As a whole it forms a very comprehensive and pniciical book for Engineers, both Civil and Mechanical." American Machinist. " We regard this as a most useful book for reference in its departments ; it should be in every Engineer's library." Mechanical Engineer. MATERIALS OP CONSTRUCTION. A Text-book for Technical Schools, condensed from Thurston's "Materials of Engineering:." Treating of Iron and Steel, their oi manufacture, properties and uses; the useful metals and their alloys. especially brasses and bronzes, and their "kalchoids": strength, ductility, resistance, and elasticity, effects of prolonged and oft- repeated loading, crystallization and granulation; peculiar metals: Thurston's "maximum alloys"; stone; timber; preservative pro- cesses, etc., etc. By Prof. Robt. H. Thurston, of Cornell University. Many illustrations Thick HVO, cloth. :, m "Prof. Thurston has rendered a great service to the profession by tht publication of this thorough, yet comprehensive, text-book. . book meets a long-felt want, and the well-known reputation of its author is a sufficient guarantee for its accuracy and thoroughness."- liuili/imj. TREATISE ON FRICTION AND LOST WORK IN MACHIN- ERY AND MILL WORK. Containing an explanation of the Theory of Friction, and an account of the various Lubricants in general use, with a record oi \ar - experiments to deduce the laws of Friction and Lubricated Surfa etc. By Prof. Robt. H. Thurston. Copiously Illustrated.. 8va doth. . "II is not too high praise to say that the present treatise is exhaustive and a complete review of the whole subject." American Engineer. STATIONARY STEAM-ENGINES. Especially adapted to Electric Lighting Purposes. Treating of the Development of Steam-engines-the principles of Construction and Economy, with description of Moderate Speed ami Him Speed KM- yines By Prof. R. H. Thurston I2mo, cJoth. 1 " " This work must prove to he of great intrnM t.. both muimlartiux-r- and users of stcain-eiiL'iiH-s "-Bidtiler at.-d Wontl '-worker. DEVELOPMENT OF THE PHILOSOPHY OF THE STEAM- ENGINE. My Prof. K. H. Thurston 12mo, cloth, $0 7.1 "This email book of forty-eight pages, prepared with the care and pre- cision one would expect from the scholarly Director of the Sibley College of Kii'Miieering, contains all the popular information that the general student would want, and at the same time a succinct account covering so much ground as to be of great value to the specialist." Public Opinion. A MANUAL OF STEAM BOILERS, THEIR DESIGNS, CON- STRUCTION, AND OPERATION. For Ttvhnii-al Schools and Engineers. By Prof . R. H. Thurston. (183 engravings i n text.) Second edition . 8vo, cloth, 5 00 "We know of no other treatise on this subject that covers the ground so thoroughly as this, and it has the further obvious advantage ol being a new and fresh work, based on the most recent data and cognizant of the latest di.-covrrics and devices in steam boiler construction." Mechanical News. STEAM-BOILER EXPLOSIONS IN THEORY AND IN PRAC- TICE. Containing Causes of Preventives Emergencies Low Water Con- sequences Management Safety Incrustation Experimental In- vrstigations, etc., etc., etc. By R. H. Thurston, LL.D., Dr. Eng., Director of Sibley College, Cornell University. With many illus- trations 12mo, cloth, 150 "Prof. Thurston has had exceptional facilities for investigating the Causes of Boiler Explosions, and throughout this work there will be found matter of peculiar interest to practical men." American Machinist. " It is a work that might well be in the hands of every one having to do with steam boilers, either in design or use." Engineering News. A HAND BOOK OF ENGINE AND BOILER TRIALS, AND THE USE OF THE INDICATOR AND THE BRAKE. By R. H. Thurston, Director of Sibley College, Cornell University. Second edition revised 5 00 "Taken altogether, this book is one which every Engineer will find of value, containing, as it does, much information in regard to Engine and Boiler Trials which has heretofore been available only in the form of scat- tered papers in the transactions of engineering societies, pamphlet reports, note-books, etc." Railroad Gazette. CONVERSION TABLES. Of the Metric and British, or United States WEIGHTS AND MEAS- URES. With an Introduction by Robt. H. Thurston, A.M., C.E. 8vo, cloth, 1 00 " Mr. Thiirston's book is an admirably useful one, and the very difficulty and unfamiliarity of the Metric System renders such a volume as this almost indispensable to Mechanics, Engineers, Students, and in fact all classes of people." Mechanical News. REFLECTIONS ON THE MOTIVE POWER OF HEAT. And on Machines fitted to develop that Power. From the original French of N. L. S. Carnot. By Prof. R. H. Thurston. . . . 12mo, cloth, 2 00 From Mons. Haton de la Goupilliere, Director of the Ecole Nationale Superieitre des Mines de France, and President of La Societe a" 1 Encourage- ment pour r Industrie Nfitionale: " I nave received the volume so kindly sent me, which contains the trans- lation of the work of Carnot. You have rendered tribute to the founder of the science of thermodynamics in a manner that will be appreciated by the whole French people." A MANUAL OF THE STEAM ENGINE. A companion to the Manual of Steam Boilers. By Prof. Robt. H. Thurston. 2 vols Hvo, cloth, 12 00 Part I. HISTORY, STRUCTURE AND THEORY. For Engineers and Technical Schools. (Advanced courses.) Nearly 900 pages Hvo, cloth, 750 Part II. DESIGN, CONSTRUCTION AND OPERATION. For Engineers and Technical Schools. (Special courses in Steam Engineering.) 8vo, cloth, 750 TEXT BOOK OF THE PRIME MOTORS. For the Senior Year in Schools of Engineering. By Prof. R. H. Thurston. Ready, Fatt of '92. A MANUAL OF STEAM-BOILERS: THEIR DESIGN, CONSTRUCTION, AND OPERATION. FOR TECHNICAL SCHOOLS AND ENGINEERS. BY R. H. THURSTON, M.A., LL.D., DR. ENG'G; > t Director of Sibley College t Cornell University; Past President American Society of Mechanical Engineers; Author of a "History of the Steam-engine," "Materials of Engineering" etc., etc., etc. FOURTH EDITION. NEW YORK: JOHN WILEY AND SONS, 53 EAST TENTH STREET. 1892. Copyright, 1888, By R. H. THURSTON. Copyright, 1890, By R. H. THURSTON. Electrotyped by Printed by DRUMMOND & NEC, ^ KRIS BROS - 444 & 446 Pfearl Street, 326 Pearl Street, New York. New York. PREFACE. THE following treatise on the steam boiler, its design, con- struction, and operation, is the outcome of an attempt to meet a demand which has been repeatedly made for a fairly com- plete, systematic, and scientific, yet " practical," manual. It has been intended to work to a plan that should be sufficiently comprehensive to meet the wants of the engineer in his office, and yet so rigidly systematic as to be suitable for use as a text-book in schools of engineering. It has been the endeavor to incorporate the elements of the subject just so far as they are needed in preparing the way for the work of the designer, the builder, and the manager of steam-boilers ; while also amply complete and logical to permit the use of the book in the instruction of the student in applied science. It was not expected that it would be found practicable to make a manual of this kind absolutely complete as a workshop treatise to be used by the boiler-maker a trade manual; but it was hoped that it might, within these limits, be made fairly satisfactory to the engineer engaged in designing. The plan of the work is as follows : Beginning with an his- torical and descriptive introduction, in which are traced the various developments of the apparatus used by the engineers of the time of Watt and earlier, and by his successors, and the progress made since his time to date, the existing standard forms of boiler are described and classified, and their special adaptations indicated. A chapter is devoted to the study of the characteristics of the materials used by the engineer in the construction of steam-generators, and another to the strength of these metals in their several forms and compositions, the methods of adaptation to the purposes of construction, and to iv PREFACE. the statement of the precautions to be observed in their intro- duction into so important a structure. Another chapter is appropriated to the examination of the composition and rela- tive values of the various available fuels, and their economical use in the production of steam. These chapters on the mate- rials and their characteristics are adapted mainly from the notes of lectures from which the larger work of the Author- " Materials of Engineering" was compiled. It has been the endeavor of the Author to make this introductory portion of the book exceptionally complete, as it is the foundation of all that follows, and is a branch of the subject to which much at- tention is rarely given in treatises of this character. Follow- ing this part of the work are chapters upon the laws of ther- modynamics, so far as they find application in the subsequent portion of the work, as, for example, in the determination of the magnitude of the stock of heat-energy 'stored in steam, and in the calculation of the constants required in tabulation of its properties; and this part of the scheme is introductory to a study of the properties of water in its several character- istic forms, solid, liquid, gaseous, and especially of the essen- tial attributes of steam at the pressures and temperatures which are customarily met with in every-day practice. The tables, however, which are here given are carried up to a range of pressure and of temperature far exceeding those in common use, and it is thought are sufficiently complete to serve their purpose for many years, notwithstanding the unintermitted progress in the direction of higher pressures which is now ob- served, and which is not likely soon to completely cease. In these tables the constants of Rankine are adopted, not so much because it is considered by the Author, if we may judge from what is to-day known on this subject, that they are quite as likely to be correct as any others ; but for the reason that they have become so generally accepted among engineers, and differ so little from the best values taken by earlier authorities, that it is probably wisest and safest to retain them at least until the exact quantities are better settled than to-day. It is certain that the differences in the magnitudes now taken for the heat-equivalent, for example, and between those values PREFACE. V and the exact figures, are too small to be of moment to the en- gineer in the daily operations of professional work. Rankine's reconstruction of Regnault's results are here accepted, also ; and Buel's tables, the only tables known to the Author in which this correction has been applied, are, with the consent of their author and his publishers, here given. The tables of Porter, published in his treatise on the Richards Steam-en- gine Indicator, may be used where separate tables in con- venient and compact form are desired. The differences to be noted between the latter, which are compiled, with careful re- vision, directly from Regnault, and those of Rankine are not great; but the engineer should use either the one or the other exclusively in any one piece of work. In the study of the methods and principles of designing steam-boilers, an attempt is made to collate the most essential, and to apply them to the proportioning of the best forms of boilers now familiar to the engineer. This part of the work is of great importance to the designing engineer, and it has been the endeavor to give this treatment of it a shape that will prove at once sufficient for its purpose, and yet fairly con- cise and very definite. It includes chapters on the design of the chimney and other accessories, and on specifications and contracts subjects rarely touched upon in earlier manuals. The chapters on the operation and care of boilers, and their management generally, is largely based upon a somewhat ex- tensive personal experience during earlier life, on the part of the Author, when he was engaged, first in the business of con- stuction, and later in actual practice, during the civil war, as a member of the corps of U. S. Naval Engineers, as well as during two decades of desultory practice as a consulting en- gineer since that time. It is hoped that it may prove well suited to meet the needs of the class of young men to whom it is addressed. In the chapter on trials of steam-boilers, the methods re- ported favorably to the American Society of Mechanical En- gineers are adopted as standard, and the report of the com- mittee is taken almost bodily into the text. As this report, in part, was prepared by the Author from his lecture-notes VI PREFA CE. largely, and in consultation with the several distinguished en- gineers associated with him on that committee, it may, very probably, be admitted that this wholesale quotation is fully justified. The report will be found published in full in the Transactions of that Society, together with the discussion brought out by its presentation. The chapter on explosions is already in print, with a few additions, as a treatise on the subject, published by Messrs. J. Wiley & Son. It was considered that such publication would very possibly prove of some service in preventing this proba- bly absolutely preventable class of disasters, and that it would secure a wider circulation, and do so much the more good, if printed as a separate monograph. The work, as a whole, is a larger treatise than could be used profitably in the average technical school ; but it is thought that it may find its place in the special schools of mechanical engineering, in those which are properly entitled to be called professional schools, giving a training which really fits the student who may succeed in passing through them for en- trance into the ranks of a profession which demands of its cadets a more complete preparation and a higher standing than any other, even among the distinctively so-called learned professions. The Author is fully conscious of the vast discrep- ancy between his aim and his accomplishment ; but he hopes that the book may be of some service, nevertheless, to many engineers, old and young. CONTENTS. CHAPTER I. HISTORY OF THE STEAM-BOILER; STRUCTURE; DESIGN. SEC. PACK 1. Office of the Steam Boiler, j 2. Development of Standard Forms 2 3. The older Types of Boiler 4 4. Special purposes and modern Types, 7 5. Method and Limit of Improvement, 10 6. Principles involved in designing, n 7. Production, transfer, and storage of Heat, 12 8. Utilization of Heat, . . 15 9. Safety in operation, . iS 10. Appurtenances of Steam Boilers, 18 11. Classification of Boilers, 19 12. Modern Standard Forms, 20 13. Mixed Types, . 20 14. Mixed Application, 20 15. Common " Shell " Stationary Boilers, 21 16. Battery of Boilers, 27 17. The Locomotive Boiler. 27 18. Marine Boilers; older Forms, 28 19. Marine Water-tube Boilers, 30 20. The Scotch Boiler 31 21. Sectional Boilers, 32 22. Marine sectional Boilers, 38 23. Periods of Introduction 38 24. Special Forms of Boiler 39 25. Problems in Design and Construction, 42 26. Problems in the Use of Boilers, 43 27. General Methods of Solution, 43 CHAPTER II. MATERIALS OF STEAM-BOILERS; STRENGTH AND OTHER CHARACTERISTICS. 28. Quality of Materials required, ... .... 45 29. Principles relating to Strength, . . 4& 30. Tenacity, Elasticity, Ductility, Resilience, ... 56 viii CONTENTS, PAGE SEC. 31. Characteristics of Iron, physical and chemical, 32. ' " " Steel 6 3 3.3. Effect of Variation of Form, . 4 34. " " Method of Treatment 7 35. " " Time and Margin of Stress 74 36. Method of detecting Overstrain 37. Effect of Temperature, 38. Crystallization and Granulation, 9 39. Iron and Steel compared, 9 2 40. Grades and Qualities of Iron Boiler-plate, 94 41. Manufacture of Iron and Steel plate, 9 6 42. Methods of Test of Iron and Steel 9 8 43. Results of Tests , ' *4 44. Specifications of Quality, lo8 45. Choice for Various Parts, . . . II2 46. Methods of Working JI 3 47. Special Precautions in using Steel U3 48. Rivets and Rivet Iron and Steel, 114 49. Sizes, Forms, and Strength of Rivets, H5 50. Strength of riveted Seams; Helical Seams, . . . . . . n? 51. Punched and Drilled Plates, . . . . 123 52. Steam-riveting and Hand-riveting 125 53. Welded Seams 127 54. " Struck-up" or Pressed Shapes, ........ 127 55. Cast and Malleableized Iron, Brass, and Copper, . . . .127 56. Shells of Boilers; Flues, 129 57. Stayed Surfaces, Stays and Braces, 144 58. Relative Strength of Shell and Sectional Boilers, . . . .148 59. Loss of Strength and Ductility of Metal, 149 60. Deterioration of Boilers 150 61. Inspection and Test of Boilers 151 CHAPTER III. THE FUELS AND THEIR COMBUSTION. 62. Combustion denned ; Perfect Combustion, 152 63. Fuels; Coal defined, I53 64. Anthracite Coals, . . ice 65. Bituminous Coals, I5 6 66. Lignites I5 8 67. Peat or Turf, I59 68. Wood I59 69- Coke .-...;'.'..' 160 70. Charcoal, ....... !6 2 71. Pulverized Fuel, 164 CONTENTS. i x SEC. 72. Liquid Fuels 73. Gaseous Fuels, ..... 74. Artificial Fuels, ..... 75. Heating Power of Fuels, 76. Temperature of the Fire, 77. Minimum Air required, 78. Temperature of Products of Combustion, . . I7 79. Rate of Combustion, 80. Efficiency of Furnace, 81. Economy of Fuel, .... 82. Weather Wastes, .. IQI 83. Composition of Fuels 84. Heating Effects of Fuels, 85. Composition of Ash, 86. Commercial Value of Fuels, 87. Furnace Management, 88. Adaptation of Boiler, Furnace, and Fuel 2 o6 CHAPTER IV. HEAT; ITS NATURE, PRODUCTION, MEASUREMENT AND TRANSFER; EFFICIENCY OF HEATING SURFACE. 89. Nature of Heat, 207 90. Methods of Production ; Combustion, 208 91. Temperatures ; Quantities of Heat ; Specific Heat 210 92. Thermometry ; Calorimetry, 214 93. Transfer of Heat, 215 94. Radiation of Heat, 216 95. Conduction, 217 96. Convection, 219 97. Transfer of Heat in the Steam Boiler, 220 98. Formulas for Efficiency of Heating Surfaces, and Area of Cooling Surfaces, 221 99. Effect of Incrustation and Deposits, 228 CHAPTER V. HEAT AS ENERGY; THERMODYNAMICS. ico. Heat as a form of Energy, 229 101. Energetics ; Heat-energy and Molecular Velocity 233 102. Heat-energy as related to Temperature, 235 103. Quantitative measure of Heat-energy, 236 104. Heat transformations 237 105. Heat and Mechanical Energy, 237 106. Thermodynamics defined, 238 X CONTENTS. SEC. PAGE 107. First Law of Thermodynamics, 2 39 108. Second Law of Thermodynamics, ....... 240 109. Molecular Constitution of Bodies, ....... 241 no. Solids, Liquids and Gases defined ; the perfect gas, . . . .241 in. Heat and Matter ; Specific Heats, 242 112. Sensible and Latent Heats, 243 113. Latent Heat of Expansion, 243 114. Latent Heats of Fusion and Vaporization, . ..... 244 115. Distribution of Heat-energy, 244 116. Application of First Law ; Equations, ...... 245 117. Application of Second Law, ....... . 247 118. Computation of Internal and External Forces and Work, . . . 248 CHAPTER VI. STEAM ; VAPORIZATION ; SUPERHEATING ; CONDENSATION ; PRESSURE AND TEMPERATURE. 119. Steam Generation and Application, . 252 120. Properties of Water; Water as a Solvent, ..... 253 121. Composition and Chemistry of Water, ...... 254 122. Sources and Purity of " fresh" Water, ...... 255 123. Sea Water ; Deposits and Remedies, ...... 256 124. Technical Uses of Water- Filtration, 260 125. Water-analysis, ..... . 261 126. Purification of Water, . 262 127. Physical Characteristics of Water, 263 128. Changes of Physical State, 265 129. The " Critical Point," 265 130. The "Spheroidal State;" Superheated Water, 268 131. Vaporization; Superheating Steam, ...... 269 132. Thermal and Thermodynamic Relations, ...... 270 133. Internal Pressures and Work; Total and Latent Heats, . . . 271 134. Computation of Internal Work and Pressure, . . . . ,271 135. Specific Volumes of Steam and Water, 272 136. Relations of Temperatures, Pressures and Volumes, .... 273 137. Specific Heats of Water and Steam, 275 138. Computation of Latent and Total Heats, 276 139. Factors of Evaporation 27 g 140. Regnault's Researches and Methods, ... 280 141. Regnault's Tables, 2gl 142. Stored Energy in Steam; Tables, 285 143. Curves of Energy, 2 g 9 144. Power of Steam; of Boilers, \ 2QI 145- Horse-power of Boilers, ' 2 2 CONTENTS. xi CHAPTER VII. CONDITIONS CONTROLLING BOILER DESIGN. SEC. PAGE 146. The Problem stated, 300 147. Selection of Type and Location, 300 148. Choice of Fuel; Method of Combustion, ...... 302 149. Conditions of Efficiency ; Pressure chosen, ..... 303 150. Principles of Design, 304 151. Controlling Ideas in Construction 307 152. Factors of Safety; Efficiency and Cost, . . . . . . 311 153. Water-tubes and Fire-tubes, . 312 154. Shell and Sectional Boilers, 314 155. Natural and Forced Draught, 314 156. Special conditions affecting Design, 317 157. Chimney Draught, 317 158. Size and Form of Chimney, 322 159. Furnace and Grate, 329 160. Relative areas of Chimney, Flues and Grate 334 161. Common Proportions and Work of Boiler 335 162. Usual rates of Evaporation, ........ 338 163. Quality of Steam and Efficiency, 338 164. Boiler Power; Number and Size, 34 165. Standard Sizes of Tubes; Spacing, . ....... 341 166. Details of the Problem, 345 CHAPTER VIII. DESIGNING STEAM BOILERS. 167. General Considerations 34& 168. Parts denned ; Common Matters of Detail 34" 169. Designing the Plain Cylinder Boiler, 170. Stationary Flue Boilers 171. Cylinder Tubular Boilers 172. Marine Flue Boilers, . 173. Marine Tubular Boilers 174. Sectional and Water-tube Boilers, 175. Upright and Portable Boilers, 176. Locomotive Boilers, CHAPTER IX. ACCESSORIES; SETTING; DESIGN OF CHIMNEYS. 177. Setting Steam Boilers; Suspension, . 178. Covering 179. Form and Location of Bridge- wall, . XII CONTENTS. SEC. PA( -* 180. Disposition of Flues, . 3& 1 181. Location and Form of Dampers, 3i 182. Steam and Water pipes 383 183. Safety Valves, 385 184. Feed Apparatus ; Heaters, 39 2 185. Steam Gauges, Fusible Plugs, and minor accessories, . . . 393 CHAPTER X. CONSTRUCTION OF BOILERS. 1 86. Methods and Processes; Drawings, 400 187. Apparatus and Machinery, 401 188. Shearing; Planing; Fitting 402 189. Flanging and Pressing; Drilling and Punching, .... 402 190. Forming bent parts, .......... 403 191. Riveting and Riveting Machines; Welding, . . . . . 404 192. Setting Tubes and Flues; Staying, ....... 413 193. Chipping and Calking 417 194. Assembling, ........... 420 195. Inspection 420 196. Testing Steam Boilers, ......... 422 197. Sectional Boilers, .......... 423 198. Transportation and Delivery, 424 CHAPTER XI. SPECIFICATION; CONTRACTS; INSPECTION. 199. Purpose of Specification and Contract, ...... 425 200. The Contract 426 201. Form of Specifications, generally, 427 202. Specification for Steam Boilers, 427 203. Sample Specifications, 427 204. Specification of Quality and Tests of Metal, 436 205. Duties of the Inspector, 438 CHAPTER XII. OPERATION AND CARE OF BOILERS. 206. General Management, ......... 440 207. Starting Fires and getting up Steam .441 208. Managing Fires, ........... 442 209. Use of various kinds of Fuel, ........ 444 210. Liquid and Gaseous Fuels, ........... 444 211. Solid Fuels, 445 CONTENTS. xiii SEC. PACE 212. Operation of the Boiler, 44- 213. Forced Draught, 4^3 214. Closed and Open Fire-rooms, 448 215. Control of Steam Pressures, 44^ 216. Regulation of Water-supply, 44^ 217. Emergencies, 450 218. Low Water, 450 219. Priming; Sudden Stopping 45I 220. Fractured Seams; Leaky tubes, . 453 221. Deranged Safety Valves; Excessive Pressure 454 222. General Care of Boilers, 454 223. Chemistry of Corrosion, ......... 454 224. Method of Corrosion, 455 225. Durability of Iron and Steel, 457 226. Preservation of Iron, 458 227. Paints and Preservatives; Coverings, 458 228. Leakage; Contact with Setting 461 229. Galvanic Action, . 462 230. Incrustation; Sediment, . . 462 231. Repairs, 465 232. Inspection and Test, . 466 233. General Instructions 469 CHAPTER XIII. EFFICIENCIES OF STEAM BOILERS. 234. Efficiencies of the Steam Boiler, 472 235. Measures of Efficiency 473 236. Efficiency of Combustion 473 237. Efficiency of Transfer of Heat, 473 238. Net Efficiency, 473 239. Finance of Efficiency, 474 240. Commercial Efficiency, ......... 474 241. Algebraic Theory of Efficiencies 47 6 242. Theory of Commercial Efficiency 477 243. Efficiency of a Given Plant, 481 CHAPTER XIV. STEAM-BOILER TRIALS. 244. Purposes of Boiler Trials, 245. Test of Value of Fuel, 485 246. Determination of Value of Boiler, 485 247. Evaporative Power of Fuels, 4^5 248. Analysis of Fuels 486 xl - v CONTENTS. PAGE 249. Efficiency and Economy of Fuel, 250. Relative Values of Boilers, . . . ' ' ' 251 Variation of Efficiency with Consumption of Fuel and Size of Grate, 489 252.' Relation of Area of Heating Surface to Economy 49 253. Combined Power and Efficiency 254. Apparatus and Methods of Test 255. Standard Test-trials, 49 ' 256. Instructions and Rules for Standard Method, 257. Precautions; Blanks and Record, ' 258. Results of Test-trials 504 259. Quality of Steam, 5I ' 260. Form of Barrel Calorimeter and use, 261. Theory of Calorimeters, $21 262. Records ; Errors, 263. The Coil Calorimeter, 524 264. The Continuous Calorimeter, 527 265. Analysis of Gases ; Form of Apparatus, 53* 266. Efficiency as indicated by Gas-analysis 535 ^67. Draught Gauges, 535 CHAPTER XV. STEAM-BOILER EXPLOSIONS. 268. Steam-boiler Explosions, 53$ 269. Energy stored in Boilers, ......... 54 1 270. Energy of Steam alone, ......... 54 271. Explosions denned and described; Fulminating Explosions; Col- lapsed Flues ; Bursting, 549 272. Causes of Explosion : Probable ; Possible, and unusual ; improba- ble and absurd, 55O 273. Statistics of Explosions and Causes, 553 274. Theories and Methods of Explosion, 55 275. Colburn's Theory of Explosions, 559 276. Lawson's and other Experiments ....... 561 277. Energy stored in heated metal, 567 278. Strength of heated metal, ........ 568 279. Low-water ; Causes and Consequences, ...... 568 280. Sediment and Incrustation, ........ 574 281. Energy stored in superheated water; Experiments of Donny and Dufour ; De-aeration of water, . . . . . . 578 282. The Spheroidal State; Leidenfrost's and Boutigny's Experiments, 583 283. Steady rise of Pressure, ......... 589 284. Relative Security of Boilers, ........ 592 283. Defects of Design, .......... 593 286, Defective Construction, ........ 596 287. Developed Weakness ; Corrosion, ....... 601 CONTENTS. xv SEC - PACE 288. General and Local Decay, ....... 604 289. Methods of Corrosion and Decay ; Grooving or Furrowing, . 606 290. Differences of Temperatures, ........ 600 291. Management of Boilers, ........ 612 292. Emergencies ; Precautions 614 293. Results of Explosions ; Causes; Examples 616 294. Experimental Explosions and Investigations, 633 295. Conclusions, 642 APPENDIX. TABLE I. Properties of Steam, .... ... 646 la. Regnault's Table, . 653 II. Energy in Water and Steam, 656 INDEX, r . .'" ....... 659 THE STEAM-BOILER. CHAPTER I. HISTORY OF THE STEAM-BOILER ITS STRUCTURE. I. The Office of a Steam-boiler is to transfer the heat- energy produced by the combustion of fuel to the mass of en- closed water, and, by the conversion of the latter into steam, to store that energy in available form for use, as in the steam- engine. The source of this energy was, originally, that existing in the rays of the sun, and, by the action of chemical affinity as exhibited in the growth of vegetation, it has been transformed from its kinetic form, in heat and light rays, to the potential form, as now found in the recent or fossil fuels of forest and coal-bed. The process of absorption and storage of heat-energy in vegetable matter is reversed, in the furnace, in the combustion of the fuel ; and the combination of the carbon and hydrogen, constituting the familiar hydrocarbons, with the oxygen of the air entering the " firebox," retransforms their stored, poten- tial, energy into the available, kinetic, form of heat-motion, and it is then applied to the elevation of the temperature of the gaseous products of combustion and of the nitrogen passing through the boiler. By conduction and convection, and by radiation, in part, this heat is next transferred to the water in the boiler, raising its temperature, evaporating it, and " making steam" at a temperature fixed by the pressure under which the operation is carried on. By the formation of steam, a part of the heat is converted once more into the potential form by that method of performance of " internal work" in the separation of molecule from molecule, against the resistances due to coru forces, which measures the "latent heats" of evaporation and of THE STEAM-BOILER. expansion ; while the remainder is the sensible heat of the steam. Thus the fluid stored in the steam-boiler is a reservoir of energy which is drawn upon by the steam-engine when the latter is set in operation to transform that heat-energy into me- chanical energy ; and the steam sent from the boiler to the en- gine conveys to the latter this energy in the two forms of sensible and of latent heat, or of actual and potential energy. The steam-boiler should be capable of thus producing, stor- ing, and delivering heat-energy, in maximum quantity, and with maximum economy and safety. In other words, the steam-boiler should produce steam in the largest practicable quantity, with the least possible expenditure of fuel and of money, and with perfect safety. 2. The Development of the Standard Forms of Steam- boiler has been a process of trial and error, in some sense one of evolution of numerous types, and of the survival of the fit- test, extending over many years. In the earlier days of the steam-engine the shapes assum- ed were invariably simple, and comparatively easy of construc- tion. Thus the boiler shown by Hero (Fig. i), in his " Pneu- matica," two thousand years ago, was spherical ; as were those of many later engines, all being evi- dently expected to be capable of sustaining considerable pres- sures.* Thus, in 1601, Giovanni Bat- tista della Porta, in his work " Spiritali," described an appara- tus by which the pressure of steam might be made to raise a column of water, and the method of operation included the appli- cation of the condensation of steam to the production of a * History of the Steam-engine. R. H. Thurston. i. THE GRECIAN IDEA OF THE STEAM-ENGINE. HISTORY OF THE STEAM-BOILER-ITS STRUCTURE. 3 vacuum into which the water would flow. He used a separate boiler. Fig. 2 is copied from an illustration in a later edition of his work.* FIG. 2. PORTA'S APPARATUS, A.D. 1601. FIG. 3. DE CAUS'S APPARATUS, A.D. 1615. Again, in 1615, Salmon de Caus, who had been an engineer and architect under Louis XIII. of France, and later in the employ of the British Prince of Wales, published a work at Frankfort, entitled " Les Raisons des Forces Mouvantes avec diverses machines tant utile que plaisantes," in which he illus- trated his proposition, " Water will, by the aid of fire, mount higher than its level," by describing a machine designed to raise water by the expanding power of steam. {See Fig. 3.) This consisted of a metal vessel partly filled with water, and in which a pipe was fitted leading nearly to the bottom and open at the top. Fire being applied, the steam, formed by its * I Tre Libri Spiritali. Napoli, 1606. THE STEAM-BOILER, elastic force, drove the water out through the vertical pipe, raising it to a height depending upon either the wish of the builder or the strength of the vessel. In Worcester's apparatus, also (Fig. 4), we have a hardly less simple form of boiler, the operation of which is such as to render it subject to high pressure. Steam is generated in the boiler D, and thence is led into the vessel A, already nearly filled with water. It drives the water in a jet out through a pipe, F or F '. The vessel A is then shut off from the boiler and again filled " by suction" after the steam has condensed through the pipe G, and the operation is repeated, the vessel B being used alternately with A. The separate boiler, as here used, constitutes a very important improvement upon the pre- ' E7^NE, CK A*D. ceding forms of apparatus, although the idea was original with Porta. Denys Papin, contemporary with the Marquis of Worcester, and a distinguished man of science of that time, invented the common lever safety-valve, and applied it to his " digester," as his closed vessel for cooking under pressure was called ; he used it later (1690) on the steam-boil- ers connected with his own steam- engine. It has been continuously in use ever since. 3. Forms familiar in the Last Century approximate modern types. Thomas Savery, A.D. 1699, used ellipsoidal forms in his then "newly invented fire-engine," of which Fig. 5 is a good representa- tion, as first given by the inventor himself, in the " Miner's Friend." L L is the boiler, in which steam is raised, and through the pipes O O FlG - S.-SAVKRY'S ENGINB, A.D. 1699. it is alternately let into the vessels PP. Suppose it to pass into the left-hand vessel first. The HISTORY OF THE STEAM-BOILER ITS STRUCTURE. 5 valve M being closed and r being opened, the water contained in P is driven out and up the pipe 5 to the desired height, where it is discharged. The valve r is then closed, and also the valve in the pipe O. The valve M is next opened, and condensing water is turned upon the exterior of P by the cock F, leading water from the cistern X. As the steam contained in P is condensed, forming a vacuum, a fresh charge of water is driven by atmospheric pressure up the pipe T. Meantime, steam from the boiler has been let into the right- hand vessel P, the cock W having been first closed and R opened. The charge of water is driven out through the lower pipe and the cock R, and up the pipe 5 as before, while the other vessel is refilling preparatory to acting in its turn. The two vessels thus are alternately charged and discharged as long as is necessary. Savery's method of supplying his boiler with water was at once simple and ingenious. The small boiler D is filled with water from any convenient source, as from the stand-pipe 5. A fire is then built under it, and, when the pressure of steam in D becomes greater than in the main boiler Z, a communication is opened between their lower ends and the water passes under pressure from the smaller to the larger boiler, which is thus " fed " without inter- rupting the work. G and N are gauge-cocks by which the height of water in the boilers is determined, and these attachments were first adopted by Savery. It will be noticed that Savery, like the Marquis of Worces- ter, and like Porta, used a boiler separate from the water-reser- voir. A working model was submitted to the Royal Society of London in 1699,* and successful experiments were made with it. Newcomen's engine, of 1705 and later, superseded the Savery apparatus in consequence of his adaptation of his ma- chine to the use of low (atmospheric) pressure steam, quite as much as because of its greater economy. By introducing the * Transactions of the Royal Society, 1699. THE STEAM-BOILER. beam-engine, and pumps separate from the steam-vessel, he was able to avoid all danger of explo- sion, using his steam at a pressure but little exceeding that of the atmos- phere, and applying it simply to the displacement of the air, preliminary to the production of a vacuum. It thus became safe to use any convenient form of steam-vessel, and in Fig. 6 it is seen that he at once departed most signally from those shapes which had necessarily been earlier used, and took FIG. e.-NEwcoMEN's ENGINE AND advantage of this freedom in design to BOILER, A.D. 1705. secure a type of boiler of greater pro- portional area of heating-surface, as shown at d, and conse- quently of greater economy in use of fuel. It is seen that he used gauge-cocks, c c, and safety-valves, N. James Watt's first boiler illustrates another step in this latter direction. In this, A f Fig. 7, the " wagon-boiler," as he called it, the FIG. 7. WATT'S FIKST MODEL, 1765. FIG. 8. OLIVER EVANS'S ENGINE, 1800. vessel is so shaped as to permit flues to be formed on either side, as well as below, for the circulation of the products of combustion backward and forward from end to end of the boiler. A still further advance is illustrated in the now well-known Cornish Boiler," Fig. 8, as used by Oliver Evans in the United States, and by British engineers of his time (1800), of which HISTORY OF THE STEAM-BOILER ITS STRUCTURE. 7 the " shell" is cylindrical, and through which a single flue, of about one half the diameter of the boiler, passes from one end to the other. The gases traverse this flue and also partly en- velop the exterior of the shell, thus coming in contact with a comparatively large extent of heating-surface. This form was followed by the "two-flued" Evans or Lancashire boiler, which was a cylinder containing two flues, each about one third its diameter, and by others in which the number of flues was in- creased with continually decreasing diameter, and with con- stant gain in total heating-surface until the modern types of tubular boiler were developed. 4. Special Purposes produce the Modern Types of boilers. Thus a desire to secure maximum efficiency produced the tubular boilers, and the desire to secure safety the so-called " sectional boilers." As early as 1793, Barlow invented, and FIG. 9. WATER-TUBE BOILER BARLOW, 1793. FIG. 10. Si EVENS' s " SECTIONAL BOILER, 1804. with Fulton used, the " water-tube" boiler (Fig. 9), in which the water circulates through the tubes, instead of around them, as in " fire-tube" boilers. This was the pioneer of a great variety of boilers of this class. John Stevens, a distinguished statesman as well as engineer, of the early part of the nineteenth century, devised another ex- ample of this class, shown in Fig. 10, as early as the year 1804. The inventor says in his specifications : " The principle of this invention consists of forming a boiler by means of a system or combination of small vessels, instead of using, as is the com- mon mode, one large one ; the relative strength of the materials of which these vessels are composed increasing in proportion to the diminution of capacity." The steamboat boiler of 1804 was 8 THE STEAM-BOILER. built to bear a working pressure of over fifty pounds to the square inch, at a time when the usual pressures were from four to seven pounds. It consists of two sets of tubes, closed at one end by solid plugs, and at their opposite extremities screwed into a stayed water and steam reservoir, which was strengthened by hoops. The whole of the lower portion was inclosed in a jacket of iron lined with non-conducting material. The fire FIG. ii. GURNEY'S STEAM CARRIAGE, 1833. was built at one end, in a furnace inclosed in this jacket. The furnace-gases passed among the tubes, down under the body of the boiler, up among the opposite set of tubes, and thence to the smoke-pipe. In another form, as applied to a locomotive in 1825, the tubes were set vertically in a double circle sur- FIG. 12. STEPHBNSON'S LOCOMOTIVE, 1815. rounding the fire. These boilers are carefully preserved among the collections of the Stevens Institute of Technology. Still another modification of this type is illustrated in the boiler used by Gurney in steam-carriages (Fig. 11) built about the years 1830-5, in which the steam-generator consisted of bent steam-pipe of small diameter so connected with steam and mud HISTORY OF THE STEAM-BOILER-ITS STRUCTURE. 9 drums as to make a very efficient as well as safe and powerful boiler for use where lightness, strength, and safety were essen- tial characteristics. Similarly, the special demands of locomotive construction were not fully met by the single-flue boiler first used by George Stephenson (Fig. 12) and by his colleagues in 1815, and up to FIG. 13. STOCKTON AND DARLINGTON ENGINE No. i, 1825. the time of construction of the Stockton and Darlington Rail- way in 1825 (Fig. 13), an example of which is still preserved in the first engine built for that road. At the opening of the Liv- erpool and Manchester Railway (1829), Stephenson's Rocket was given the multitubular boiler, a form which had grown into shape in the hands of several inven- tors.* This boiler was three feet in diameter, six feet long, and had twenty-five three-inch tubes, extend- ing from end to end of the boiler. The steam-blast was carefully adjusted by experiment, to give the best effect. Steam-pressure was carried at fifty pounds per square inch. The average speed of the Rocket on its trial was fifteen miles per hour, F " : - M.-TH. ROCKET, , 8a9 . and its maximum was nearly double that twenty-nine miles an hour; and afterward, running alone, it reached a speed of thirty-five miles. * Barlow and Fulton, 1795 : Nathan Read, Salem, United States, 1796; Booth of England, and S6guin of France, about 1827 or 1828. 10 THE STEAM-BOILER. The shares of the company immediately rose ten per cent in value. The combination of the non-condensing engine with a steam-blast and the multitubular boiler, designed by the clear head and constructed under the eye of an accomplished engi- neer and mechanic, made steam locomotion so evident and decided a success, that thenceforward its progress has been un- interrupted and wonderfully rapid.* The special requirements of stationary steam-engine con- struction and operation, and of steam navigation, have, from these primitive types and forms, developed in the course of years the several now common and standard boilers which will be later described. 5. The Method and Extent of Improvement is now easily traced. Looking back over the history of the steam-engine, we may rapidly note the prominent points of improvement and the most striking changes of form ; and we may thus obtain some idea of the general direction in which we are to look for further advance.f Beginning with the machine of De Caus, at which point we may first take up an unbroken thread, it will be remembered that we there found a single vessel performing the functions of all the parts of a modern pumping-engine ; it was at once boiler, steam-cylinder, and condenser, as well as both a lifting and a forcing pump. The Marquis of Worcester, and, still earlier, Da Porta, divided the engine into two parts ; using one part as a steam-boiler, and the other as a separate water-vessel. Savery duplicated those parts of the earlier engine which acted the several parts of pump, steam-cylinder, and condenser, and added the use of the jet of water to effect rapid condensation. Newcomen and Cawley next introduced the modern type of engine, and separated the pump from the steam-engine proper ; in their engine, as in Savery's, we notice the use of surface- condensation first, and, subsequently, that of a jet of water thrown into the midst of the steam to be condensed. Watt finally effected the crowning improvement of the single-cylinder * History of the Steam-engine. R. H. Thurston. N. Y.: D. Appleton & Co., 1878. f Ibid. HISTORY OF THE STEAM-BOILER ITS STRUCTURE. II engine, and completed this movement of differentiation by separating the condenser from the steam-cylinder, thus perfect- ing the general structure of the engine. Here this movement ceased, the several important processes of the steam-engine now being conducted each in a separate vessel. The boiler furnished the steam ; the cylinder derived from it mechanical power ; the vapor was finally condensed in a separate vessel ; while the power, which had been obtained from it in the steam-cylinder, was transmitted through still other parts to the pumps, or wherever work was to be done. Watt and his contemporaries also commenced that move- ment toward higher pressures of steam, used with greater ex- pansion, which has been the most striking feature noticed in the progress made since his time. Newcomen used steam of barely more than atmospheric pressure, and raised 105,000 pounds of water one foot high, with a pound of coal consumed. Smeaton raised the steam-pressure to eight pounds, and in- creased the duty to 120,000. Watt started with a duty of double that of Newcomen, and raised it 320,000 foot-pounds per pound of coal, with steam at ten pounds. To-day, Cornish engines of the same general plan as those of Watt, but worked with forty to sixty pounds pressure, expanding three to six times, bring up the duty to 600,000 foot-pounds; while more modern compound engines have boilers carrying 150 pounds (ten atmospheres) above the normal air-pressure, and the duty has been since raised to above 1,200,000 foot-pounds per pound of fuel used. 6. The Requisites of Good Design are readily prescribed and defined : they are very simple, and although attempts are almost daily made to obtain improved results by varying the design and arrangement of heating-surface, the best boilers of nearly all makers of acknowledged standing are practically equal in merit, although of diverse forms. In making boilers the effort of the engineer should evidently be ist. To secure complete combustion of the fuel without permitting dilution of the products of combustion by excess of air. 12 THE STEAM-BOILER. 2d. To secure as high temperature of furnace as possible. 3d. To so arrange heating-surfaces that, without checking draught, the available heat shall be most completely taken up and utilized. 4th. To make the form of boiler such that it shall be con- structed without mechanical difficulty or excessive expense. 5th. To give it such form that it shall be durable, under the action of the hot gases and of the corroding elements of the atmosphere. 6th. To make every part accessible for cleaning and repairs. /th. To make every part as nearly as possible uniform in strength, and in liability to loss of strength by wear and tear, so that the boiler when old shall not be rendered useless by local defects. 8th. To adopt a reasonably high " factor of safety" in pro- portioning. 9th. To provide efficient safety-valves, steam-gauges, and other appurtenances. loth. To secure intelligent and very careful management. 7. Effective Development, Transfer, and Storage of Heat, in the best possible combination, is evidently what is demanded in the operation of the steam-boiler. In securing complete combustion an ample supply of air and its thorough intermixture with the combustible elements of the fuel are essential ; for the second, high temperature of furnace, it is necessary that the air-supply shall not be in excess of that absolutely needed to give complete combustion. The efficiency of a furnace burning fuel completely is measured by in which E represents the ratio of heat utilized to the whole calorific value of the fuel ; T is the furnace-temperature ; T the temperature of the chimney, and t that of the external air. Hence the higher the furnace-temperature and the lower that of the chimney, the greater the proportion of available heat. It is further evident that, however perfect the combustion, HISTORY OF THE STEAM-BOILER ITS STRUCTURE. 13 no heat can be utilized if either the temperature of chimney ap- proximates to that of the furnace, or if the temperature of the furnace is reduced by dilution approximately to that of the chimney. Concentration of heat in the furnace is secured, in some cases, by special expedients, as by heating the entering air, or, as in the Siemens gas-furnace, heating both the combus- tible gases and the supporter of combustion. Detached fire- brick furnaces have an advantage over the "fireboxes" of steam-boilers in their higher temperature ; surrounding the fire with non-conducting and highly heated surfaces is an effective method of securing more perfect combustion and high furnace- temperature. In arranging heating-surface the effort should be to impede the draught as little as possible, and so to place them that the circulation of water within the boiler should be free and rapid at every part reached by the hot gases. The directions of circulation of water on the one side and of gas on the other side the sheet should, whenever possible, be opposite. The cold water should enter where the cooled gases leave, and the steam should be taken off farthest from that point. The temperature of chimney-gases has thus been re- duced by actual experiment to less than 300 Fahr., and an efficiency equal to 0.75 to 0.80 the theoretical is attainable. The extent of heating-surface simply, in all of the best forms of boiler, determines the efficiency, and the disposition of that surface in such boilers seldom affects it to any great extent. The area of heating-surface may also be varied within wide limits without greatly modifying efficiency. A ratio of 25 to I in flue and 30 to I in tubular boilers represents the relative area of heating and grate surfaces in the practice of the best-known builders. This proportion may be often settled by exact calculation. The material of the boiler, as will be shown later, should be tough and ductile iron, or, better, a soft steel containing only suffi- cient carbon to insure melting in the crucible or on the hearth of the melting-furnace, and so little that no danger may exist of hardening and cracking under the action of sudden and great changes of temperature. I4 THE STEAM-BOILER. Where iron is used it is necessary to select a somewhat hard but homogeneous and tough quality for the firebox sheets or any part exposed to flames. The factor of safety is very often too low. The boiler should be built strong enough to bear a pressure at least six times the proposed working-pressure ; as the boiler grows weak with age, it should be occasionally tested to a pressure far above the working-pressure, which latter should be reduced gradually to keep within the bounds of safety. The factor of safety is seldom more than four in new boilers ; and even this is- reduced practically by the operation of the inspection laws. Effective development of heat is secured primarily by the selection of good fuel, by which is usually meant fuel which consists, to the greatest possible extent, of available combusti- ble material ; but for the purposes of the engineer who designs the boiler, or of the owner for whom it is to be constructed, the real criterion of quality is the quantity of heat which the com- bustible, as burned in the furnace, will yield for any given sum of money expended in obtaining that heat. The cost of a fuel to the consumer consists, not simply of money paid for it to the dealer who supplies it, but also of cost of transportation and of placing in the., grate, of removal of ash, of incidental ex- penses inseparable from its use, such as injury to boilers and other property, increased risks, and other such expenses, many if not most of which are very difficult of determination with any satisfactory decree of accuracy. Other things being equal, that fuel which gives the greatest quantity of available heat for the total money expenditure is that which permits most effec- tive development in the sense here taken. Effective heat-de- velopment from any selected fuel is secured, as already stated, by its complete combustion in such manner as to give the highest possible temperature. Effective transfer of heat is secured by such a form of steam-generator, and such extent and disposition of " heating- surfaces," as will most completely utilize the heat developed in the furnace and flues by causing it to flow, with the least pos- sible loss, into the water and steam contained within the boiler ; and this is effected by proper arrangement of surfaces absorb- HISTORY OF THE STEAM-BOILER ITS STRUCTURE. 15 ing heat from the gases and yielding it to the liquid as already generally described. Effective storage of heat can be secured by providing large volumes of water and of steam, within which the heat transferred from the furnace and flues can be stored, and by carefully pro- tecting the whole heated system from waste by conduction or radiation to adjacent bodies. Where the demand is steady, and the supply from the fuel fairly steady also, the amount stored need not be great, as the use of the reservoir is simply that of a regulator between furnace and engine or other apparatus re- ceiving it ; but where either supply or demand is variable, con- siderable storage capacity may be needed. 8. Efficient Utilization of Heat is as essential to the satis- factory working of any system of generation and application of heat as is efficient production, transfer, and storage. The mode of attaining maximum efficiency depends upon the nature of the demand and the method of expenditure ; and the considera- tion of this subject in detail would be here out of place. In general it may be said that where the heat and steam are re- quired for the impulsion of an engine, the higher the safe pres- sure and the practically attainable temperature at which the supply is effected, the more efficient the utilization of the heat. These limits of temperature and pressure are the higher as the actual working conditions are made the more closely to approxi- mate to the ideal conditions prescribed by pure science. Where heating simply, without transformation into work, is intended, the principal and only very important requisite, usually, is to provide such thorough protection for the system of transfer and use, that no wastes of importance can take place by radiation or conduction. The character of the steam made, as to humidity, is in this case comparatively unimportant ; but in the preceding case it will be found essential that it should be always dry, and it is often much the better for being super- heated considerably above the boiling-point due to its pressure. The actual standing of the best steam-engine of the present time, as an efficient heat-engine, is really very high, sources of loss are principally quite apart from the principles of design and construction, and even from the operation of the 1 6 THE STEAM-BOILER. machine ; and it may be readily shown that, to secure any really important advance toward theoretical efficiency, a radical change of our methods must be adopted, and probably that we must throw aside the heat-engine in all its forms, and substitute for it some other apparatus by which we may utilize some mode of motion and of natural energy other than heat. The very best classes of modern steam-engines very seldom consume less than two pounds (0.9 kilog.) of coal per horse- power per hour, and it is a good engine that works regularly on three pounds (1.37 kilog.). The first-class steam-engine, therefore, yields less than 10 per cent of the work stored up in good fuel, and the average engine probably utilizes less than 5 per cent. A part of this loss is unavoidable, being due to natural conditions beyond the control of human power, while another portion is, to a consid- erable extent, controllable by the engineer or by the engine- driver. Scientific research has shown that the proportion of heat stored up in any fluid, which may be utilized by perfect mechanism, must be represented by a fraction, the numerator of which is the range of temperature of the fluid while doing useful work, and the denominator of which is the temperature of the fluid when entering the machine, measured from the " absolute zero." Thus, steam, at a temperature of 320 Fahr., being taken into a perfect steam-engine, and doing work there until it is thrown into the condenser at 100 Fahr., would yield j- = 0.28 +, or rather more than one fourth of the 320 -(- 401 work which it should have received from each pound of fuel. The proportion of work that a non-condensing but other- wise perfect engine, using steam of 75 pounds (5 atmos.) pres- sure, could utilize would be ^ 2Q ~ 2I2 p \A i and while 320 + 461 the perfect condensing engine would consume two thirds of a pound (0.3 kilog.) of good coal per hour, the perfect non-con- densing engine would use \\ pounds (0.6 kilog.) per hour for each horse-power developed, the steam being taken into the engine and exhausted at the temperatures assumed above. HISTORY OF THE STEAM-BOILER ITS STRUCTURE. 1 7 Also, were it possible to work steam down to the absolute zero of temperature, the perfect engine would require but 0.19 pound (0.09 kilog.) of similar fuel. It may therefore be stated, with a close approximation to exactness, that of all the heat derived from the fuel about seven tenths is lost through the existence of natural conditions over which man can probably never expect to obtain control, two tenths are lost through imperfections in our apparatus, and only one tenth is utilized in even good engines. Boiler and engine are intended to be included when writing of the steam- engine above. In this combination a waste of probably two tenths at least of the heat derived from the fuel takes place in the boiler and steam-pipes, on the average, in the best of prac- tice, and we are therefore only able to anticipate a possible saving of 0.2 X 0.75 = 0.1$, about one sixth of the fuel now expended in our best class of engines, by improvements in the machine itself. The best steam-engine, apart from its boiler, therefore, has 0.85, about five sixths, of the efficiency of a perfect engine, and the remaining sixth is lost through waste of heat by radiation and conduction externally, by condensation within the cylinder, and by friction and other useless work done within itself. It is to improvement in these points that inventors must turn their attention if they would improve upon the best modern practice by changes in construction. To attain further economy, after having perfected the machine in these particulars, they must contrive to use a fluid which thoy may work through a wider range of temperature, as has been attempted in air-engines by raising the upper limit of temperature, and in. binary vapor engines by reaching toward a lower limit, or by working a fluid from a higher temperature than is now done down to the lowest possible temperature. The upper limit is fixed by the heat-resisting power of our materials of construction, and the lower by the mean tempera- ture of objects on the surface of earth, being much lower at some seasons than at others. In the boiler the endeavor must be made to take up all the heat of combustion, sending the gases into the chimney at as low a temperature as possible, and securing in the furnace perfect combustion without excess of 1 8 THE STEAM-BOILER. air-supply. The best engines still lack 1 5 per cent of perfec- tion, and the best boilers, as an average, over 30 per cent. 9. Safety in Operation is one of the most essential require- ments which the designer, constructor, and user of steam-boilers, must be prepared to fulfil. As will be seen later, the quantity of stored heat-energy in the steam-boiler is usually enormous, and this energy is stored under such conditions that, if set free by the rupture of the containing vessel, wide-spread disaster may ensue. This stored energy is at all times ready to instantly assume the kinetic form when permitted, and by doing mechani- cal work on all adjacent objects, to produce most extraordinary effects ; it is stored energy of the most perfectly elastic kind, as well as of high tension. The most absolutely reliable means known to the engineer must be adopted for the safe and per- manent control of such magazines of latent pow r er. Those methods of securing safety which have been found most satisfactory have been (1) The division of the confined energy among compara- tively small masses of steam and water contained in correspond- ingly small communicating chambers, so constructed that the rupture of one will be unlikely to produce fracture of any other. (2) The adoption of the very best material and of the best possible construction, and so proportioning all parts exposed to stress and strain that they may withstand pressures several times as great as the maximum intended to be carried. (3) Careful and intelligent operation and preservation. 10. The Appurtenances or Accessories of Steam- boilers are those attached parts and apparatus which, while not, strictly speaking, actually essential elements of the struc- ture specially designated as the boiler, are nevertheless essen- tial to its safe and economical operation : such as, for example, safety and other valves, gauge-cocks, feed-pumps, dampers, grates, and " settings." Safety-valves are automatically self-operating apparatus which open and permit the steam to issue from the boiler whenever the pressure reaches a limit at which they are ar- ranged to act. Steam-valves are the valves, usually operated by screws, which, when open, permit the steam to leave the boiler and pass away through the steam-pipes. Stop-valves are a HISTORY OF THE STEAM-BOILER-ITS STRUCTURE. 19 variety of valve which may be used to stop the passage of steam from the boiler: they may be "screw stop-valves," or simple valves moved directly by hand. Check-valves, commonly in- troduced at the junction of the feed-water supply-pipe with the boiler, are so arranged as to open automatically when the stream enters, but to close against a return current : they are sometimes pinned to their seats, when desirable, by a screw, in which case they are called "screw-checks." Gauge-cocks are set at, and above or below, the intended working water-level of the boiler, and, when opened, by discharging steam or water, indicate the actual position of the water-line. Glass water- gauges are glass tubes set in such manner that the water stands in a vertical tube at the same height as the water in the boiler, the top of the glass communicating with the steam- space, and the lower end with the water-space of the boiler. ii. The Classification of Steam-boilers may be based upon either a comparison of their forms or of their purpose. Under the former we have the plain cylindrical, the flue, the tubular, or the sectional boiler; under the latter, stationary, locomotive, or marine boilers. For the purposes of this work, the following may be taken as a satisfactory scheme : Plain cylindrical boilers. Cornish or single-flue. Lancashire or two-flue. , , . i Multiflue and return-flue boilers. Stationary . . . -\ , Cylindrical fire-tube boilers. Firebox boilers. Sectional boilers. ^ Peculiar forms. ( Common type. Locomotive >' Wooton boilers. ( Special devices. ( Flue. Older types < Flue and tube. ( Tubular. *i Scotch or drum boilers. Water-tube and sectional. Miscellaneous forms. 20 THE STEAM-BOILER. 12. The Modern Standard Types of Boiler are becom- ing rapidly settled in a few well-defined forms which have been found to be most satisfactory, all things considered, each in its own special province. These are specified in the list just presented. But many boilers have become so thoroughly well adapted to the special work to which they are customarily applied as to have almost or quite entirely displaced other forms, which in turn are as generally adopted for other uses. Thus, where the feed-water supplied to land boilers, in locali- ties where fuel is cheap, or water bad, and certain to produce serious incrustation, the plain cylindrical boiler is almost univer- sally employed ; where the fuel is costly and the feed-water pure, the tubular boiler is as universally adopted ; while inter- mediate conditions lead to the use of intermediate forms. The locomotive boiler is standard for its place and purpose, and no other form has ever yet competed with it in thorough adaptation to that peculiar case. The high pressures carried and the necessity of great economy at sea have made the so- called "Scotch" or "drum" boiler standard in trans-oceanic steam navigation. Where small area of floor-space and ample " head-room" are found, the upright cylindrical tubular boiler is the standard form ; if the head-room is less and the floor- space larger, a modification of the locomotive type finds appli- cation for stationary purposes. 13. Mixed Types of boiler are often constructed for special purposes or experimentally. In the shallow-water navigation of the United States of America, as on the Hudson River, the flue and tube boiler is much used ; the locomotive type of boiler, with fewer and larger tubes than are adopted in locomo- tive practice, has often found use in stationary practice. New designs are continually coming forward which illustrate such forms of boiler. As a rule, however, they are not found pref- erable to the simpler and standard types. 14. Mixed Applications are sometimes required, as where the same boiler supplies steam for power and for heating pur- poses. In this case the pressure carried on the boiler is fixed at the proposed maximum for the engine, and the lower pres- sures required for the other purpose are secured by the use HISTORY OF THE STEAM-BOILER ITS STRUCTURE. 21 of a "reducing" or "pressure-reducing" valve. The steam- heating systems of cities often illustrate this case, furnishing steam, as they do, for heating buildings, for cooking, and to steam-engines at all parts of the area covered by them. 15. Common Forms of " Shell " Boilers, as those boilers are called in which the structure consists of an external case enclosing steam and water, flues and tubes, are the following : (i) The Plain Cylindrical Boiler consists, as shown in section (Fig. 15), and in front elevation (Fig. 16), of a simple cylin- FIG. 15. SECTION OF CYLINDRICAL BOILER. drical vessel, A, made of boiler-plate, fitted with heads at each end, B, B; which heads are sometimes of sheet-iron and some- times of cast-iron. A steam-dome, C, on the upper side, usually serves as a collector and reservoir for the steam, as i rises from the water into the steam-space, and serves also as the point of attachment for the steam-pipe, D D, and safety- valve, E E, both of which thus take steam from the highes and driest part of the interior of the boiler. The fire is built in the detached furnace, F F, the product of combustion passing under the boiler to the rear, at G, where 22 THE STEAM-BOILER. a flue leads off to the chimney. The -setting" consists of side-walls and ends, H H, of brick, and a covering, 77, which is often merely a filling of ashes or other non-conductor, or an arch of brickwork carried over from the side-walls. " Binders," KK, and rods, L L, tie the whole together, and resist any change of form due to variations of temperature. The grates, FIG. 16. FRONT OK CYLINDRICAL BOILER AND SETTING. M M, are supported at the rear by the bridge-wall, N N, of which the upper part is usually built of fire-brick. The rear end of the boiler is often carried on rollers, to prevent danger of injury with the changes of form due to variations of tempera- ture such as are produced by the introduction of cold feed- water. (2) The Cylindrical Flue Boiler (Fig. 17) is a plain cylinder, like the preceding form, but with one or more flues passing through it from end to end. The setting is usually quite similar to that of the plain cylinder, except as necessarily modified to meet the requirements of the flue. The shell is generally shorter than that of the first-described boiler, the heating-surface considerably greater. HISTORY OF THE STEAM-BOILER ITS STRUCTURE. 23 (3) The Cylindrical Tubular Boiler is shown in one of the best forms in Fig. 18. It consists of a cylindrical shell con- FIG. 17. CYLINDRICAL FLUE BOILER. structed much as in Fig. 15, with a set of tubes carried from end to end, and set as closely as is practicable without inter- fering too seriously with the circulation of the water within it. FIG. 18. CYLINDRICAL TUBULAR BOILER. The peculiar feature of the illustration is the introduction of the very large single sheet which is seen to make the whole lower two thirds or more of the shell ; this construction pre- UNIVEBSITI 2 4 THE STEAM-BOILER. venting the fire reaching seams and riveting, as occurs in the usual construction. FIG. 19. CYLINDRICAL TI/BULAR BOILER AND SETTING. The setting of this kind of boiler is shown in Figs. 1} and 20. The weight of the boiler is here taken by " lugs" on each side and by them transferred to the brickwork of the setting. In other cases the boiler is suspended from girders crossing the structure later- ally ; and the suspension-rods carry- ing the boiler are sometimes allowed vertical play, under the action of expansion and contraction of the whole system, by the introduction of springs of rubber or steel, thus permitting very uniform distribu- tion of the weight at all times. In many -cases the gases, instead of F,0. 2 0.-SECTIO N 0,TUBU,.AK BoiLEK ^"8 ^'^ ^^ ^ b ilCr tO thC chimney, as shown in Fig. 19, are taken directly to the chimney from the front of the boiler, as HISTORY OF THE STEAM-BOILER ITS STRUCTURE. 2$ in Fig. 1 6. It is not always thought safe to expose the top and steam spaces of the boiler to the heat of the escaping gases ; but the practice is not an uncommon one, even with reputable builders. The air-spaces in Fig. 20, at either side in the walls of the setting, give an additional protection from loss of heat, and a certain amount of elasticity of setting. This is the most common of all forms of steam-boiler. FIG. 21. FIREBOX Tt BULAR BOILER. (4) The Firebox Flue Boiler is so made in order that the whole may become " self-contained," and brickwork dis- pensed with. Adding the firebox to the tubular (Fig. 21), forms the locomotive type of boiler. In stationary boilers, however, the tubes are, as a rule, larger and less numerous than in the locomotive boiler. These boilers require no set- ting or connections other than the parts needed to connect n FJG. 22. KIKEBOX BOILER SETTING. them with the chimney-flue. This arrangement is seen in Fig. 22. The advantages of this type are the low cost of installation, the more complete ac- 26 THE STEAM-BOILER. cessibility of the exterior for inspection and repair, the reduc- tion of floor-space occupied, and the portability of the boiler. FIG. 23. THE UPRIGHT BOILER. FIG. 24. UPRIGHT TUBULAR BOILER. (5) The Upright Boiler is usually a firebox tubular boiler, designed to stand vertically, as in Fig. 23, and to occupy mini- FIG. 25. BATTERY OF BOILERS. The above cut represents a pair of Cornish boilers set in brick-work, connected so as to be worked either together or separately. mum floor-space. Its construction at the upper end is often such as to permit the upper extremities of the tubes to be kept be- HISTORY OF THE STEAM-BOILER ITS STRUCTURE. low the water-line. In many cases, however, the tubes are car- ried directly through to the upper head, as is seen in Fig. 24. This figure also exhibits the method of attaching gauges and safety-valves. This boiler is much used where it is important to save floor-space, and where head-room can be obtained. It is the usual form in steam fire-engines. 16. A " Battery" of Boilers (Fig. 25) consists of two or more, placed side by side, the total power demanded being greater than it is considered advisable to construct a single boiler to supply. In such cases it is usually important that they should be so set and con- nected that either or any of them may be operated separately. To secure this result, the connections with the feed and steam-pipes must be so made that it may be perfectly practicable to put the feed on either or any FIG. 26.-UpR.GHT BOILER f tne boilers in the battery, and to take steam WITH F.ELD TUBES. from either Qr any> Each should have its own separate safety-valve, check-valve, and steam-gauge. An upright boiler fitted with " Field tubes " is shown in Fig. 26. The internal, cir- culating, tubes project slightly above the crown-sheet, and are carried down inside the main tube, nearly to the closed lower end. The water enters the centre tube, flows out at its lower end, and rises in the OUter tube FIG. 27.- LOCOMOTIVE BOILER. on all sides the smaller one issuing above the crown-sheet into the general body of water, and there discharging the accompany- ing steam which had been made during the period of circulation. 17. The Locomotive Boiler is always given a form sub- stantially as represented in Fig. 27, and consists of a firebox of rectangular form, attached to a cylindrical shell closely filled 28 THE STEAM-BOILER. with fire-tubes, through which the gases pass directly to the smoke-stack. Strength, compactness, great steaming capacity, fair economy, moderate cost, and convenience of combination with the running parts, are secured by the adoption of this form. It is frequently used also for portable and stationary engines. It was invented in France by M. Seguin, and in England by Booth, and used by George Stephenson at about the same time 1828 or 1829. FIG. 28. THE LOCOMOTIVE. Section. This form of steam-boiler has been found to lend itself with peculiar handiness to the special requirements of locomotive construction, and its use is universal for this purpose. 18. The "Marine" Boilers are often of very different form from those used on land. They have assumed their present forms after many years of experience and slow adapta- tion to the special conditions by which they are controlled. When steam-pressures were customarily low, the controlling condition was the form of the vessel, and boilers were given such shapes as would permit of their being compactly stowed on board ship ; in the later days of very high pressure which have followed the introduction of the surface-condenser, and of high expansion, the form of the steam-generator is deter- mined mainly by the demand for their safe operation. Fig. 29 shows one of the types of boiler in most common use on the steamers generally seen on the Eastern American rivers, and on the coast, before the period of high steam and great economy had opened. HISTORY OF THE STEAM-BOILER-ITS STRUCTURE. 29 It is known as the Return-flue" boiler, the flame and gases from the furnace passing back to the " back-connection- through one set of flues, usually of 10 to 20 or even 24 inches in diameter, and thence to the "front-connection" over the furnace, and to the " uptake," and chimney or "smokestack," by a set of flues of, as a rule, smaller size and larger number. This is seen to be a " firebox boiler, no brickwork setting being admissible on shipboard. FIG. 29. FLUES AND RETURN-TUBES. Surrounding the chimney uptake is a reservoir, called the " steam-chimney," which answers the double purpose of a steam-dome and a drier, or " superheater," in which the steam may part with its suspended water, and often become heated above the temperature of saturation by the heat from the chimney gases. An elaborate system of bracing and staying is required for a boiler of this type. The sketch (Fig. 29) shows one of a pair of boilers arranged to discharge their flue gases into a common chimney. An effort IT* secure increased steaming capacity and economy in this boiler resulted in the production of the boiler THE STEAM-BOILER. with direct flues and return-tubes, the latter being usually from three to five inches in diameter. This represents the later type, and one which is still very often used on paddle-steamers on Long Island Sound and on the rivers connected with that system of water communication. Fig. 30 illustrates a still more advanced type, the marine tubular boiler, extensively used in naval and other sea-going steamers, car- rying from twenty-five to forty pounds steam-pressure. The fur- nace discharges its gases directly into the back-connection, whence they pass forward into the front connection and stack through a set of tubes, which are commonly 2 \ to 3J inches in diameter. This arrangement gives a very compact, well-proportioned boiler, comparatively easy of calculation and construction, and especially convenient in bracing and staying. Several furnaces can in this boiler be conveniently placed side by side and connected to a common uptake. 19. The Marine Water-tube Boiler (Fig. 31) represents a type which has been often proposed for use at sea, but jvhich has never succeeded in finding its way into common use. FIG. 30. MARINE TUBULAR BOILER. FIG. 31. MARINE WATER-TUBE BOILER. Lord Dundonald in Great Britain and James Montgomery in the United States introduced boilers of the water-tube HISTORY OF THE STEAM-BOILER ITS STRUCTURE. 31 type before the middle of the century, and the form here il- lustrated, as originally designed by Mr. Martin of the U. S. Navy, was very extensively employed on the vessels of the navy during the Civil War. In these boilers the gases pass from the back-connection to the front through a " tube-box" placed in the water-space of the boiler, which tube-box con- tains a large number of vertical tubes within which the water circulates from the lower to the upper side, while the gases pass among and around the tubes. These boilers were found by Isherwood to give a somewhat larger steaming capacity and greater economy also than the corresponding boiler of the fire-tube type; but the difficulty of repairing leaky tubes and incidental disadvantages, as well as their greater cost, prevented their permanent adoption in either the navy or the merchant service. 20. The Scotch or Drum Boiler (Fig. 32) is the outcome of the attempt to secure a safe form of boiler for high pres- FIG. 32. SCOTCH OR DguM BOILER. sures, and it has, very naturally, assumed the cylindrical form of shell, while retaining the general disposition of furnace and tubes illustrated in the last-described fire-tube boiler. The furnaces are large, set in very thick flues; the grates are set in them at very nearly the horizontal diametrical line, and, in the case illustrated, the boiler is "double-ended." Heavy stay- rods, connecting the two ends, make the heads capable of safely carrying their enormous loads. These boilers often carry 100 and 150 pounds pressure, and sometimes even more, 32 THE STEAM-BOILER. and are built of between 10 and 20 feet diameter, and of iron or steel from to ij inches in thickness. Fig. 33 exhibits the method of setting and of connection of these boilers, as customarily practised where " single-ended," i.e., with the furnaces at one end only, as here seen. Either or any of these boilers, as so set, may be used or repaired sep- arately if necessary. For small powers these boilers are often given the form and structure shown in Fig. 31, which represents a boiler designed for a small yacht or a torpedo-boat ; it is three or four feet in FIG. 33. SETTING AND CONNECTION OF SCOTCH BOILERS. diameter and four to six feet long, and is calculated for from five to ten or twelve horse-power. ^ 21. Sectional Boilers are all constructed to meet the con- ditions and requirements so well stated by Col. Stevens in his specification for his British patent of 1805, in which he says that, to derive advantage from his principle, " it is absolutely necessary that the vessel or vessels for generating steam should have strength sufficient to withstand the great pressure from an increase of elasticity in the steam ; but this [total] pressure is increased or diminished in proportion to the capacity of the HISTORY OF THE STEAM-BOILER-ITS STRUCTURE. 33 containing vessel. The principle, then, of this invention con- sists in forming a boiler by means of a system or combination of a number of small vessels, instead of using, as in the usual mode, one large one ; the relative strength of the materials of which these vessels are composed increasing in proportion to the diminution in capacity." FIG. 34. 'MARINE BOILER OF SMALL POWER. Stevens' boilers were of two kinds : the one that shown in Fig. 10 ; the other, and that specifically shown in the patent, consisting of systems of small tubes grouped in circular con- centric rows, and connected at each end by annular heads and chambers of sufficiently small capacity to be safe, while still large enough to permit good circulation. The boiler adopted in Gurney's steam-carriage (Fig. 1 1) is a later type, which has been more than once since reproduced; and nearly all recent, familiar, forms of the sectional boiler are 3 34 THE STEAM-BOILER. constructed of systems of tubes united at the ends, and with the feed-apparatus, steam-drum, and mud-drum, by what are known as " headers," through which the general circulation is secured. In some cases the boiler has been made wholly or partly of cast-iron, as the early Babcock & Wilcox (Fig. 35), which consisted of a system of horizontal cast-iron tubes serv- ing both as water connections and as steam-chambers, and a second system of tubes set at a considera- .^ ble inclination from the horizontal, the two sets united by headers. The Babcock & Wilcox Boiler, in the -latest and best form, how- ever (Fig. 36), is wholly of wrought- iron or steel. The same general arrangement of tubes is preserved ; but the upper part of the construc- tion consists of one or more steam and water drums of com- paratively large diameter. These are away from the fire, and cannot be reached by the gases until they are cooled down to a safe temperature by passing through the lower system of FIG. 35. CAST-IRON SECTIONAL BOILEK. FIG. 36. BABCOCK & WILCOX BOILER. heating surfaces, the inclined tubes. The water-line in the drum is carried at about its middle, and a dry-pipe, seen at the top, carries off the steam made. The joints are all " milled," and so nicely fitted that no practicable pressure can cause leak- HISTORY OF THE STEAM-BOILER-ITS STRUCTURE. 35 age. The course of the furnace gases and the water-circulation can be readily traced in the drawing. The Root Boiler is shown in Fig. 37, differing from the pre- ceding in the arrangement of tubes and their connection. The form of header is peculiar, and cannot be seen ; but the general construction is well shown in the engraving. In various designs, as made at different times and for various purposes, the construction has been somewhat modified, and the location' FIG. 37. THE ROOT BOILER. size, and number of steam-drums has been varied. The tubes are four or five inches in diameter, and usually eight or ten feet long. The Harrison Boiler (Fig. 38) consists of an aggregation of spheres, of cast-iron, or steel as now made, connected by " necks" of somewhat smaller diameter. These spheres are 8 inches in diameter, f inch thick, capable of sustaining a pres- sure exceeding 100 atmospheres, and are set in clusters, as shown in the sketch ; they are fitted together with faced joints, 36 THE STEAM-BOILER. and secured by long bolts passing from end to end of each row. These boilers are intended to be so proportioned that a pres- sure far less than that which would produce rupture will stretch the bolts, thus allowing each joint to act as a safety- FIG. 38. THE HARRISON BOILER. valve. The three types of boiler which have just been de- scribed, and their various modifications are the most common and familiar forms of sectional boiler in use. The Allen Boiler (Fig. 39) has only been constructed ex- perimentally, and has never come into the general market ; but experiments made upon it, under the direction of a committee of the American Institute, in 1871, and under the immediate direction of the Author, its chairman, gave excellent results, both in steaming capacity and economy. In this boiler the tubes are suspended by one end, the lower end being closed, as in what is known as the Field system. The inclination of the tubes 30 from the vertical was found by experiment to be best. The horizontal cylinders above, to one of which each line of tubes is connected, serve as circulating tubes and passages by HISTORY OF THE STEAM-BOILER-ITS STRUCTURE. 37 which the steam made is conducted to the steam-drum. It will be noticed that the whole structure, steam-drum and all, is encased in the brick-work setting and exposed to contact with the heated gases. The circulation within the pendent tubes was excellent, and, with pure water and no sediment or in- Eng.by Ainer.Uk.Note Co. FIG. 39. THE ALLEN BOILER. crustation choking them at their lower ends, the boiler was considered capable of doing its work in a very satisfactory manner. Fairbairn has remarked that "danger in the use of high- pressure steam does not consist in the intensity of the pressure to which the steam is raised, but in the character and construe- tion of the vessel which contains the dangerous element ;" and this remark may be taken, like the propositions of Col. John THE STEAM-BOILER. Stevens, as part of the basis of the philosophy of construction of " sectional " boilers. 22. Marine Sectional Boilers have not as yet come into general use, although many attempts have been made to in- troduce them. The first boiler built by John Stevens was intended for use in a small steam-vessel; and in 1825 or 1826 Robert L. Thurston and John Babcock, then of Portsmouth, and later of Providence, R. L, built boilers of this class, con- sisting of coils of pipe within which the water and steam were contained, the fire and furnace gases passing around outside them. Modifications of the Root boiler, known as the Belle- ville, and others, have been used with success by French build- ers of marine machinery ; and the Babcock & Wilcox Co. have produced a marine boiler like that shown in Fig. 40, a com- bination of water-tubes below with fire-tubes and steam-space above, which is considered a good form for use at sea. The necessity of using a brick-work setting has prevented the introduction of the common forms at sea. Many designs are appearing constantly, and it is probably only a question of time, when, with continually rising steam-pressures, the older forms will be displaced by these modern and safer types. 23. The Dates of Introduc- tion of the principal devices no- ticed in modern boilers have been ^^ ^ HaSWell > who describes the various familiar forms as in- cluding the dry-bridge and combustion-chamber of Wright (1756), Dorrancc (1845) and Baker (1846); the dead-plate of Watt (1785); the water-bridge of Crampton (1842) and Mills (1851); the air-bridge of Slater (1831) ; the horizontal fire-tube of Bolton (1780), of Ericsson (1828), Seguin and Booth (1829), and of Hawthorne (1839) and Glasson (1852); the vertical fire- tube of Rumsey (1788); the water-bottom of Allen (1730) and Fraser (1827) ; the vertical water-leg of Stephens and Hardley FIG. 40. BABCOCK & WILCOX MARINE BOILER. HISTORY OF THE STEAM-BOILER ITS STRUCTURE. 39 (1748), Napier (1842) and Dundonald (1843); the steam-drum of John Stevens (1803); the superheaters of Hately (1768), of English (1809) and Allaire (who used a tall steam-chimney in 1827). The hanging bridge of Johnson (1818), the cylindrical return-flue boiler of Napier (1831), the cold-air supply above the fire, as by Thompson (1796), by Robertson (1800), Arnott (1821), and by Williams (1839), are a l so > he states, features of the modern boiler. The introduction of the water-tube boiler by Montgomery has not led to a change of type.* These various details will be described more at length in later chapters. 24. Peculiar and Special Forms of Boiler arc met with in all departments. Some of these are considerably employed, and in many cases possess special features of advantage. The _JL_J FIG. 41. THE GALLOWAY BOILER. Galloway boiler (Figs. 41, 42) is one of the best known and sue- cessful modifications of the cylindrical flue-boiler. Its special feature is the conical stay-tube, which is used to increase t heating-surface and to strengthen the flue, without making i heating-surface difficult of access. Large numbers of the. boilers have been built and used since about 1860 in ( Britain, and some have been constructed in States. * Trans. British Institution of Naval Architects, 1877. 4 o THE STEAM-BOILER. The exterior is a plain cylindrical shell, within which are two cylindrical furnaces which unite in one flue, having parallel FIG. 42. GALLOWAY BOILER. curved top and bottom, struck from a centre below the In this flue are the conical water-tubes, each ioj inches di- ameter at the top and 5-J inches diameter at the bottom, fixed in a radial position and perpen- dicular to the top and bottom so as to support and brace the flue and to intercept and break up the heated gases in their pas- sage from the furnaces. Along the sides of the flue there are boiler. FIG. 43. UPRIGHT FLUE-BOILER. FIG. 44. FIRE-ENGINE BOILER. HISTORY OF THE STEAM-BOILER ITS STRUCTURE. 41 several wrought-iron pockets, or " bafflers," which deflect the currents and cause them to impinge against the tubes the end pocket providing for necessary expansion and contraction. After leaving this flue the gases pass along the sides of the shell to the front end, thence back again under the centre of the boiler to the chimney. A simple form of upright flue-boiler, for heating purposes and where small power is required, is seen in Fig. 43. It is of simple design, and easy of access for repair. A steam fire-engine boiler (Fig. 44), as built by the Silsby FIG. 45. HERRESHOFF'S BOILER. Co. illustrates the use of the Field tubes, pendent from the crown-sheet of the furnace: these are water-tubes, but gases pass up through the boiler in a set of fire-tubes seen c necting the crown-sheet with the top of the boiler. This mak< an exceedingly compact, powerful, and light steam-boiler. 42 THE STEAM-BOILER. The Herreshoff boiler (Fig. 45)> as constructed for fast yachts and torpedo-boats, consists of a cone-shaped double coil of continuous wrought-iron pipe, five feet to five and a half feet in diameter, covered by a disk made up of a coil of smaller pipe. The feed-water passes through the latter, and downward through the boiler, inside, and then upward again, through the outside coil, finally passing to the separator, whence the steam, passes off to the engine, after circulating through the three top-coils of pipe which forms a super- heater, drying and superheating the steam en route. The water separated from the steam is driven back into the boiler, with the feed-water, by the feed and circulating pumps. The steam-pipe used in making up the boiler is lap-welded, and from i to 2f inches in diameter outside, and T \ inch in thick- ness. This boiler, as built for the yacht Leila, contained 22 cubic feet of steam and water space, of which about one third was steam-space; it had 485 square feet of heating-surface, 44 feet of superheating area, or 18.7 feet of heating-surface, and 1=7 feet of superheating surface, per square foot of grate, these areas being measured on the exterior of the tubes. The boiler developed 75 to 80 horse-power. The separator is ob- viously an essential feature of the system. 25. Problems in Steam-boiler Design and Construction are among the most interesting, as well as important, which arise in the practice of the engineer. These problems may, and usually do, take many distinct forms. It is almost invari- ably the fact that the quantity of steam to be obtained is specified either as a certain weight of water to be evaporated and an equal weight of steam to be furnished; or a stated amount of power is to be given through a specified form and cize of engine, the probable efficiency of which is known or as- certainable; or a stated volume of building, having a known exposure, is to be heated. In such cases the problem presented is to supply the steam so demanded at a minimum total cost, using a type of boiler to be selected with reference to the special conditions of location and use. It is often necessary, when dealing with a large " plant," to determine how many boilers should be employed, or to what HISTORY OF THE STEAM-BOILER ITS STRUCTURE. 4$ extent the steam made should be divided up among them: whether a larger number of small boilers should be built or fewer large boilers. The selection of the best type for a speci- fied location is an exceedingly common duty of the engineer. To secure the supply of a given quantity of steam with abso- lute safety, or with reasonable minimum risk, is another such problem. The usual case demands the production, with cer- tainty and with safety to life and property, of a stated weight of steam, day by day, for long periods of time, at minimum average total expense for the whole period of life of the boilers. Problems in construction, arising in connection with the design and application of steam-generators, are mainly related to the best methods of putting together the parts of a boiler of which the design has been made, and involve the continual application of a good knowledge of the nature and uses of the materials used, and especially of the facts and principles gov- erning the strength of materials, of parts, and of the structure as a whole. The selection of the best form of joint is a problem in the design of the boiler; but the determination of the best method of making that joint is a problem in construction. Such are all questions relating to the actual performance of work in the shop, the use of tools in the work of building the boiler, and the comparison of methods. 26. Problems in the Use of Steam-boilers are not less important and difficult of solution, often, than those which arise in the production of the design or in its construction. How to obtain a maximum quantity of steam ; how to secure dryness and uniformity of quality ; how to prolong the life of the structure ; and how to effect its preservation most effec- tively, at least cost in time, money, or loss of use are only a few examples of the many problems that continually present themselves for immediate solution while the boiler is in ser- vice. 27. The General Method of Solution of Problems in Design is to study the case very carefully in the light of all information that can be gained relating to the special conditions affecting it, and then, by comparison of the results of experi- 44 THE STEAM-BOILER. ence with various boilers under as nearly -as may be similar conditions, determining the best form for the case in hand. The designing engineer next endeavors to effect such improve- ment as his own talent and experience may enable him to originate, with a view to the most perfect possible adaptation of the design to its purposes. He next settles the general pro- portions, the forms of details, and finally the absolute dimen- sions and exact proportions. So much being done, he is pre- pared to make a preliminary study, which deliberately made alterations may convert into a finally complete design. CHAPTER II. MATERIALS STRENGTH OF MATERIALS AND OF THE STRUC- TURE. 28. The Quality of the Material used in the construc- tion of steam-boilers must obviously be very carefully consid- ered. Not only is the steam-boiler expected to bear great strains and high pressures, but the terrible consequences which are liable to follow its rupture make it important that it should sustain its load and do its work with the most absolute safety attainable. The structure is exposed to greater variety of con- ditions tending to weaken it and to shorten its life than any other apparatus familiar to the engineer ; and the results of its failure are more certain to be disastrous to human life, as well as to property. All parts of the boiler are, while under heavy stress, exposed to continually changing temperatures, with, usually, occasional variations extending over two hundred or more degrees Fahrenheit. Nearly every part is liable to cor- rosion, often of a kind which is the more dangerous because very difficult to detect or to gauge. The boiler is very liable to be subjected to peculiarly severe stresses due to accidental circumstances and to excessive steam-pressure or to deficiency of water. The material needed for the purposes of the boiler-maker should for all these reasons be as strong, tough, and ductile as it can possibly be made. Of these qualities it is evident that ductility, capability of bearing violent alteration of form without fracture, is even more vitally essential than strength. A lack of tenacity can be met by using more metal, but noth- ing can make amends for brittleness. Good boiler-plate must possess great strength, and must combine with it great ductil- itymust have high elastic and total " resilience," as such a combination is termed. 46 THE STEAM-BOILER. The various parts of the boiler require their material to exhibit somewhat different special qualities: tubes must be tough enough to bear the " upsetting" action of the " ex- pander" by which they are secured in the tube-sheets, and yet must be hard enough to sustain reasonably well the abrading effect of cinder-laden currents of gas ; flue-sheets and especially furnace-sheets must be hard, and capable of resisting both the mechanical wear and the corrosive action of the furnace-gases and their burden of coal, ash, and cinder, and must at the same time sustain safely the continual variation of temperature to which they are subjected by the alternate impact of flame and of cold air as the fires are worked. The "shell" of the boiler is less affected by such stresses; but it nevertheless must meet with a greater variety of loading, in a greater number of direc- tions, than perhaps any other known iron structure ; every change of pressure within it, every alteration of temperature, every rise or fall of the water-line, produces a variation of the amount and direction of the stresses to which its metal and joints are expos-ed. Great tenacity combined with ductility is the essential characteristic of all material used in the construc- tion of steam-boilers. 29. The Principles Relating to the Strength of Mate- rials of construction,* and other qualities useful in resisting the strains to which steam-boilers are subject, are very simple and, in the main, well established. The Resistance of Metal to rupture may be brought into play by either of several methods of stress, which have been thus divided by the Author : ( Tensile : resisting pulli'ng force. Longitudinal . \ ( Compression : resisting crushing force. i Shearing : resisting cutting across. Transverse . . . . j Bending : resisting cross breaking. ( Torsional : resisting twisting stress. When a load is applied to any part of a structure or of a machine it causes a change of form, which may be very slight, * Abridged and adapted from Part II., Chapter IX., " Materials of Engineer- ing," by the Author. MATERIALS STRENGTH OF THE STRUCTURE. 47 but which always takes place, however small the load. This change of form is resisted by the internal molecular forces of the piece, i.e., by its cohesion. The change of form thus pro- duced is called strain, and the acting force is a stress. The Ultimate Strength of a piece is the maximum resist- ance under load the greatest stress that can exist before rup- ture. The Proof Strength is the load applied to determine the value of the material tested when it is not intended that ob- servable deformation shall take place. It is usually equal, or nearly so, to the maximum elastic resistance of the piece. It is sometimes said that this load, long continued, will produce fracture; but, as will be seen hereafter, this is not necessarily, even if ever, true. The Working Load is that which the piece is proportioned to bear. It is the load carried in ordinary working, and is usually less than the proof load, and is always some fraction, determined by circumstances, of the ultimate strength. A Dead Load is applied without shock, and once applied remains unchanged, as, e.g., the weight of a bridge ; it produces a uniform stress. A Live Load is applied suddenly, and may produce a variable stress, as, e.g., by the passage of a railway train over a bridge. The Distortion of the strained piece is related to the load in a manner best indicated by strain-diagrams. Its value as a factor of the measure of shock-resisting power, or of resilience, is exhibited in a later article. It also has importance as indi- cating the ductile qualities of the metal. The Reduction of Area of Section under a breaking load is similarly indicative of the ductility of the material, and is to be noted in conjunction with the distortion. E.g., a considerable reduction of section with a smaller pro- portional extension would indicate a lack of homogeneousness, and that the piece had broken at the soft part of the bar. The greater the extension in proportion to the reduction of area in tension, the more uniform the character of the metal. Factors of Safety. The ultimate strength, or maximum capacity for resisting stress, has a ratio to the maximum stress 4 8 THE STEAM-BOILER. due to the working load, which, although less in metal than in wooden or stone structures, is nevertheless made of consider- able magnitude in many cases. It is much greater under mov- ing than under steady " dead " loads, and varies with the char- acter of the material used. For machinery it is usually 6 or 8 ; for structures erected by the civil engineer, from 4 to 6. The following may be taken as minimum values of this " factor of safety" for the metals : MATERIAL. LOAD. SHOCK. Dead. Live. Iron and steel, copper and other soft metals \ S 7 10 + 10 to 15 Ratio of ultimate strength to working load. The brittle metals and alloys The Proof StrengtJi usually exceeds the working load from 50 per cent with tough metals, to 200 or 300 per cent where brittle materials are used. It should usually be below the elas- tic limit of the material. As this limit, with brittle materials, is often nearly equal to their ultimate strength, a set of factors of safety, based on the elastic limit, would differ much from those above given for ductile metals, but would be about the same for all brittle ma- terials, thus: LOAD. Ayr ^ Dead. Live. Ratio of elastic Ferrous and soft metals. . . . 2 4 6 Resistance to Brittle metals and alloys. . . 3 6 8 to 12 working load. The figure given for shock is to be taken as approximate, but used only when it is not practicable to calculate the energy of impact and the resilience of the piece meeting it, and thus to make an exact calculation of proportions. The Measure of Resistance to Strain is determined in form MATERIALS STRENGTH OF THE STRUCTURE. 49 by the character of the stress. By stress is here understood the force exerted, and by strain the change of form produced by it. Tenacity is resistance to a pulling stress, and is measured by the resistance of a section, one unit in area, as in pounds or tons on the square inch, or in kilogrammes per square cen- timetre or square millimetre. Then if T represents the te- nacity and K is the section resisting rupture, the total load that can be sustained is, as a maximum, Compression is similarly measured, and if 7 be the maxi- mum resistance to crushing per unit of area, and K the section, the maximum load will be P=CK. . ....... (2) Shearing is resisted by forces expressed in the same way, and the maximum shearing stress borne by any section is P=SK. ....... (3) Bending Stresses are measured by moments expressed by the product of the bending effort into its lever-arm about the section strained, and if P is the resultant load, / the lever-arm, and M the moment of resistance of the section considered, (4) Torsional Stresses are also measured by the moment of the stress exerted, and the quantity of attacking and resisting mo- ments is expressed as in the last case. Elasticity is measured by the longitudinal force, which, act- ing on a unit of area of the resisting section, if elasticity were to remain unimpaired, would extend the piece to double its original length. Within the limit at which elasticity is unim- paired, the variation of length is proportional to the force act- ing, and if E is the " Modulus of Elasticity" or " Young's Mod- 50 THE STEAM-BOILER. ulus," / the length, and e the extension, P being the total load, and K the section, * = > (5) The Coefficients entering into these several expressions for resistance of materials are often called Moduli, and the forms of the expressions in which they appear are deduced by the Theory of the Resistance of Materials, and the processes are given in detail in works on that subject. These moduli or coefficients, as will be seen, have values which are rarely the same in any two cases ; but vary not only with the kind of material, but with every variation, in the same substance, of structure, size, form, age, chemical composition or physical character, with every change of temperature, and even with the rate of distortion and method of action of the distorting force. Values for each familiar material, for a wide range of conditions, will be given in the following pages. When a piece of metal is subjected to stress exceeding its power of resistance for the moment, and gradually increasing up to the limit at which rupture takes place, it yields and be- comes distorted at a rate which has a definitely variable rela- tion to the magnitude of the distorting force ; this relation, al- though very similar for all metals of any one kind, differs greatly for different metals, and is subject to observable altera- tion by every measurable difference in chemical composition or in physical structure. Thus in Fig. 46 let this operation be represented by the several -curves a, b, c, d, etc., the elevation of any point on the curve above the axis of abscissas, OX, being made proportional to the resistance to distortion of the piece, and to the equiva- lent distorting stress, at the instant when its distance from the left side of the diagram, or the axis of ordinates, OY, measures the coincident distortion. As drawn, the strain-diagram, a a' , is such as would be made by a soft metal like tin or lead ; b b' MATERIALS STRENGTH OF THE STRUCTURE. 5 I represents a harder, and c c' a still harder and stronger metal, as zinc and rolled copper. If the smallest divisions measure the per cent of extension horizontally, and 10,000 pounds per square inch (703 kilogrammes per square centimetre) vertically, d d' would fairly represent a hard iron, or a puddled or a "mild" steel; while//' and g g' would be strain-diagrams of hard and of very hard tool steels, respectively. The points marked e, e' , e" , etc., are the so-called " elastic limits," at which the rate of distortions more or less suddenly changes, and the elevation becomes more nearly equal to the permanent change of form, and at these points the resistance to further change increases much more slowly than before. FIG. 46. STRAIN-DIAGRAMS. This change of rate in increase in resistance continues until a maximum is reached, and, passing that point, the piece either breaks, as at/ and g ', or yields more and more easily until dis- tortion ceases, or until fracture takes place, and it becomes zero at the base-line, as at X. Such curves have been called by the Author " Strain-dia- grams." If at any moment the stress producing distortion is relaxed, the piece recoils and continues this reversed distortion until, all load being taken off, the recoil ceases and the piece takes its " permanent set." This change is shown in the figure at f" f", the gradual reduction of load and coincident partial res- toration of shape being represented by a succession of points 52 THE STEAM-BOILER. forming the line /'/" each of which P oints has a P osition which is determined by the elastic resistance of the piece as now altered by the strain to which it has been subjected. The distance Of measures the permanent set, and the distance f" f" measures the recoil. The piece now has qualities which are quite different from those which distinguished it originally, and it may be regarded as a new specimen and as quite a different metal. Its strain- diagram now has its origin at f", and the piece being once more strained, its behavior will be represented by the curve f f e vl f, a curve which often bears little resemblance to the original diagram 0, / /. The new diagram shows an elastic limit at e v , and very much higher than the original limit * >IV . Had this experiment been performed at any other point along the line//', the same result would have followed. It thus be- comes evident that the strain-diagram is a curve of elastic limits, each point being at once representative of the resistance of the piece in a certain condition of distortion, and of its elastic limit as then strained. The ductile, non-ferrous metals, and iron and steel and the truly elastic substances, have this in common that the effect of strain is to produce a change in the mode of resistance to stress, which results in the latter in the production of a new and elevated elastic limit, and in the former in the introductioa of such a limit where none was observable before. It becomes necessary to distinguish these elastic limits in describing the behavior of strained metals, and, as will be seen subsequently, the elastic limits here described are under some conditions altered by strain, and we thus have another form of elastic limit to be defined by a special term. In this work the original elastic limit of the piece in its or- dinary state, as at e, e' , e", etc., will be called either the Origi- nal or the Primitive, Elastic Limit, and the elastic limit cor- responding to any point in the strain-diagram produced by gradual, unintermitted strain will be called the Normal Elastic Limit for the given strain. It is seen that the diagram repre- senting this kind of strain is a Curve of Normal Elastic Limits. The elastic limit is often said to be that point at which a MATERIALS STRENGTH OF THE STRUCTURE. 53 permanent set takes place. As will be seen on studying actual strain-diagrams to be hereafter given, and which exhibit accu- rately the behavior of the metal under stress, there is no such point. The elastic limit referred to ordinarily, when the term is used, is that point within which recoil on removal of load is approximately equal to the elongation attained, and beyond which set becomes nearly equal to total elongation. It is seen that, within the elastic limit, sets and elongations are similarly proportional to the loads, that the same is true on any elastic line, and that loads and elongations are nearly proportional everywhere beyond the elastic limit, within a moderate range, although the total distortion then bears a far higher ratio to the load, while the sets become nearly equal to the total elongations. The behavior of metals under moving or "live" load and under shock is not the same as when gradually and steadily strained by a slowly applied or static stress. In the latter case the metal undergoes the changes illustrated by the strain- diagrams, until a point is reached at which equilibrium occurs between the applied load and resisting forces, and the body rests indefinitely, as under a permanent load, without other change occurring than such settlement of parts as will bring the whole structural resistance into play. When a freely moving body strikes upon the resisting piece, on the other hand, it only comes to rest when all its kinetic energy is taken up by the resisting piece; there is then an equality of vis viva expended and work done, which is ex- pressed thus: (7) in which expression W is the weight of the striking body, V its velocity, / the resisting force at any instant, p m the mean resistance up to the point at which equilibrium occurs, and s is the distance through which resistance is met. As has been seen, the resistance may usually be taken as varying approximately with the ordinates of a parabola, the 54 THE STEAM-BOILER. abscissas representing extensions. The mean resistance is r therefore, nearly two thirds the maximum, and WV * = f s pdx = p m s = \et = ae\ nearly, . . (8) 2g t/o where e is the extension, and t the maximum resistance at that extension, and a a constant. Brittle materials, like hard bronzes and brasses, have a straight line for their strain-dia- grams, and the coefficient becomes J instead of f , and WV* ,f = ae* = %et = %-- (9) *"o Resilience, or Spring, is the work of resistance up to the elastic limit. This will be called Elastic Resilience. The mod- ulus of elasticity being known, the Modulus of Elastic Resili- ence is obtained by dividing half the square of the maximum elastic resistance by the modulus of elasticity, E, as above, and the work done to the " primitive elastic limit" is obtained by multiplying this modulus of resilience by the volume of the bar.* The total area of the diagram, measuring the total work done up to rupture, will be called a measure of Total or Ulti- mate Resilience. Mallett's Coefficient of Total Resilience is the half product of maximum resistance into total extension. It is correct for brittle substances and all cases in which the primitive elastic limit is found at the point of rupture. With tough materials, the coefficient is more nearly two thirds and may be even greater where the metal is very ductile, as, e.g., pure copper, tin, or lead. Unity of length and of section being taken, this coefficient is here called the Modulus of Resilience. When the energy of a striking body exceeds the total re- silience of the material, the piece will be broken. When the" * Rankine and some other writers take this modulus as instead of - . E 2E MATERIALS STRENGTH OF THE STRUCTURE 55 energy expended is less, the piece will be strained until the work done in resistance equals that energy, when the striking body will be brought to rest. As the resistance is partly due to the inertia of the particles of the piece attacked, the strain-diagram area is always less than the real work of resistance, and at high ve- locities may be very considerably less, the difference being expended in the local deformation of that part of the piece at which the blow is received. In predicting the effect of a shock it is, therefore, necessary to know not only the energy stored in the moving mass and the method of variation of the resistance, but also the striking velocity. To meet a shock- successfully, it is seen that resilience must be secured sufficient to take up the shock without rupture, or, if possible, without serious deformation. It is in most cases necessary to make the elastic resilience greater than the maximum energy of any attacking body. Moving Loads produce an effect intermediate between that due to static stress and that due to the shock of a freely mov- ing body acting by its inertia wholly ; these cases are, there- fore, met in design by the use of a high factor of safety, as above. As is seen by a glance at the strain-diagram, ff (Fig. 46), the piece once strained has a higher elastic resilience than at first, and it is therefore safer against permanent distortion by mod- erate shocks, while the approach of permanent extension to a limit renders it less secure against shocks of such great inten- sity as to endanger the piece. When the shock is completely taken up, the piece recoils, as at * vl /"/" until Jt settles at such a P oint on that line as- suming the shock to have extended the piece to the point r vi that the static resistance just equilibrates the static load. This point is usually reached after a series of vibrations on either side of it has occurred. With perfect elasticity, this point is at one half the maximum resistance, or elongation, attained. Thus we have Of TTVT VFPRTTT $6 7 'HE STEAM-BOILED, but/ varies as A x within the elastic limit, which limit has now risen to some new point along the line of normal elastic limits, as e vl . Taking the origin at the foot of f"f ", since the varia- tions of length along the line Ox are equal to the elongations and to the distances traversed as the load falls, and as stresses are now proportional to elongations, p = ax; W/i=Ws] and W=P\. . . (11) when the resisting force is /, the elongations ;r, while h and s are maximum fall and elongation, and P is the maximum resistance to the load at rest. Then Fpdx = a Fxdx = -s* = Ws ; .-. s = . (12) iA> t/o 2 a For a static load, if s 1 is the elongation, W W = P = as'; .'. s' = . a Hence, 7 = *' ' 03) and the extension and the corresponding stress due to the sudden application of a load are double those produced by a static load. Where the applied load is a pressure and not a weight, i.e., where considerable energy in a moving body is not to be absorbed, as in the action of steam in a steam-engine, the only increase of strain produced by a suddenly applied load is that produced by the inertia of such of those parts of the mass attacked as may have taken up motion and energy. 30. Tenacity, Elasticity, Ductility, and Resilience are the four essential qualities of a good material for use in steam- boiler construction. In some cases, the relative values of MATERIALS STRENGTH OF THE STRUCTURE. 57 these several properties are very different from that relation in others. For example : while boiler-iron or steel must have ductility, even if tenacity is sacrificed to some extent to secure it, machinery irons and steels should have a certain amount of rigidity, and tool-steel a minimum allowable hardness, as their leading characteristics ; and in all, the essential property being secuned, as good a combination of all the other valuable prop- erties is sought as can possibly be obtained. The problem of proportioning parts to resist shock is seen to involve a determination of the energy, or " living force," of the load at impact, and an adjustment of proportion of sec- tion and shape of piece attacked such that its work of elastic or of ultimate resilience, whichever is taken as the limit, shall exceed that energy in a proportion measured by the factor of safety adopted. For ordinary live loads and moderate impact, requiring no specially detailed consideration, the factors of safety already given, as based upon ultimate strength simply, are considered sufficient ; in all cases of doubt, or when heavy shock is anticipated, calculations of energy and resilience are necessary, and these demand a complete knowledge of the character, chemical, physical, and structural, of every piece involved, of its resilience and method of yielding under stress, and of every condition influencing the application of the at- tacking force in other words, a complete knowledge of the material used, of the members constructed of it, and of the circumstances likely to bring about its failure. The form of such parts should usually be determined on the assumption that deformation may some time occur; and such expedients as that of Hodgkinson in enlarging the sec- tion on the weaker side, as well as the adoption of a larger factor of safety based on ultimate strength, are advisable. 31. The Chemical and Physical Characteristics of Iron determines the value of the metal for the purpose of the engi- neer in construction. The following set of strain-diagrams (Fig. 47) may be taken as representative of the behavior of good samples of the various grades of wrought-iron and of steel above described. The diagrams a a, b b, c c, are those of commercial irons of 58 THE STEAM-BOILER. good quality, soft, medium, and hard respectively, and all of high ductility. The elastic limits of a and b differ greatly in position, and the irons themselves are characteristically differ- ent. The one is in a condition of initial internal strain which has weakened it against external stresses ; but that strain being relieved by flow- under strain, the iron is finally found to be stronger than the -second piece. It is evident that the first is less valuable than the second, Kil's.per S.q.Ctn. Elongation per Cent FIG. 47. STRAIN-DIAGRAMS OF IRON AND STEEL. however, under any stresses that occur within the usual limits of distortion ; the engineer would choose b as having a higher elastic limit and much greater elastic resilience. The "elasticity line," e' e' , shows the amount of spring and of set at the point at which it is taken, and gives a measure of the modulus of elasticity. The harder iron, d d, is probably actually a puddled steel, and has been made by balling up the sponge in the puddling furnace too early to permit complete MATERIALS STRENGTH OF THE STRUCTURE. 59 reduction of carbon. The gradual increase in strength, with in- crease of carbon, and rise of the elastic limit, are shown, as well as the coincident loss of ductility, in the diagrams, e,f,g, and h, which are those of steels containing from 0.35 to I per cent carbon ; e and / are the diagrams from excellent samples of the product of the open-hearth and pneumatic processes, and the stronger specimens are representatives of the average crucible steel. The increase of resilience within the elastic range is seen to be very great as the percentage of carbon is increased. The chemical composition of iron and steel determines the real character of any sample, although differences of physical character and of molecular structure often seriously modify the value of pieces into the composition of which they enter. With cast metal, where sound castings have been secured, the chemical constitution of the metal being known from analyses, the value of the metal for purposes of construction may be usually well judged ; and a comparison of the data given by the chemist with the specific gravity of the metal, will gener- ally be sufficient to determine its character with great exact- ness. Specifications for cast-iron or cast-steel may usually be safely so drawn as to make the acceptance of the material de- pendent upon accordance with specified formulas of composi- tion and density. Thus: A good, gray foundry iron, free from phosphorus and low in silicon, and having a density of 7.25 to 7.28, is, un- less containing some peculiar and unusual constituent in excess, a safe iron to use for all purposes demanding strength Wrought- iron and " mild " steels are, on the other hand, so greatly mod- ified by the processes of preparation in the mill, that actual test can only be safely depended upon to determine their value in construction. Statements of the strength of iron or steel are not of great value in any case, when the metal of which the strength or ductility is given is specified by its trade or generic name sim- ply without a statement of its precise chemical composition and physical character. Wrought-iron varies in composition and in structure to such an extent that, while the softest and purest 6O THE STEAM-BOILER. varieties often have a tenacity of but about 40,000 pounds per square inch (2812 kilogrammes per square centimetre), some so-called wrought-irons (properly puddled steels) have been met with by the Author in the market having a tenacity of double that figure ; some samples extend 25 per cent before breaking, while others, with similar shape and size of test-piece are found nearly as brittle as cast-iron. Cast-iron varies in tenacity from as low as 10,000 pounds per square inch (703 kilogrammes per square centimetre) to more than 50,000 pounds (3515 kilogrammes per square centi- metre) ; while metals are sold under the name of " steel " hav- ing tenacities varying from that of wrought-iron up to over 100 tons per square inch (15,746 kilogrammes per square centi- metre). In the examples of results of tests of iron and steel which will be hereafter given, therefore, the character of the metal tested will usually be exactly defined by its chemical composi- tion. In comparing the results of test with the chemical constitu- tion of the material, it will be found that, in general, elements which increase tenacity also decrease ductility and resilience. Thus : carbon increases strength up to a limit beyond which an excess begins to weaken it, as at the limit which separates steel from cast-iron ; but every addition of strength takes place at the sacrifice of that ductility which is an essential property of good iron. Phosphorus adds strength, as do manganese and other less common constituents ; but in each case a limit to increasing strength is reached, and in each case the increase of strength noted is accompanied by an equally or more noticeable loss of ductility. It sometimes happens, however, that the elastic re- silience increases, with addition of such elements, up to a limit ; which limit is, however, reached long before the increase of strength ceases. The influence of the most common hardening elements upon the valuable qualities of " rail-steel " and similar metals has not been studied sufficiently to determine their precise effect and their modifying action as mutually reacting upon each other. MATERIALS STRENGTH OF THE STRUCTURE. 6l The hardening elements most usually met with in iron and steel are carbon, silicon, manganese, and phosphorus. Dr. Dudley* takes the effect of manganese, carbon, silicon, and phosphorus to be as the numbers 3, 5, ;|, and 15, and reckons the sum of their effects in " phosphorus units" on this basis, allowing 0.05, 0.03, 0.02, and o.oi per cent respectively of these elements, taken in the order just given, as each equivalent to one unit. He concludes that the sum should not exceed 31 or 32 in rails and other soft ingot-metals, this figure being obtained, as above, by adding together the phosphorus percentage, one half the silicon, one third the carbon, and one fifth the manga- nese. Taken singly, the limit for phosphorus is placed at a maximum of o.io per cent, silicon at 0.04, manganese at 0.30 or 0.40, and for such metals, carbon at 0.25 to 0.30 per cent. Higher proportions make the material too brittle for rails and similar uses. For boiler-plate these elements should be re- duced nearly one half. Steels containing more carbon are still more carefully chosen with a view to the avoidance of the loss of ductility due to the action of other elements in presence of carbon. Manganese steels, i.e., steels containing a high percentage of manganese, having but little carbon or other of the harden- ing elements, are found to have peculiar value for many purpo- ses of construction ; but their use must be carefully avoided in steam-boilers, or elsewhere, when exposed to great and rapid changes of temperature. The chemical composition of cast-iron will usually, and es- pecially if checked by a determination of density, serve well as a guide to the selection of iron of any specified character for use in construction ; yet it is always advisable to supplement the analysis by the determination of its physical characteristics as revealed by inspection and by test. The openness or closeness of grain, the shade of color, the depth of chill, and other prop- erties capable ot detection by the senses, are valuable guides to the experienced engineer. The same is true of all forms of ingot metal, whether worked * Trans. Am. Inst. Mining Engineers, vol. vii. "62 THE STEAM-BOILER. or unworked. Steels are selected by visual inspection with great accuracy and certainty; but the engineer usually desires to compare the chemist's analysis with the results of mechanical tests, as well as to obtain the judgment of the steel-maker who inspects the topped ingots. The products of the pneumatic and of the open-hearth pro- cesses are now customarily tested both by the chemist's and by physical tests. The influence of mechanical treatment during the process of manufacturing wrought-iron and puddled steel the " weld " metals is very great in the modification of their valuable properties. This is the case to such an extent that the quality of these materials can but rarely be safely judged from chemi- cal analysis. The presence or absence of cinder, the amount of reduction in the rolls or under the hammer, and the tempera- ture and other conditions of working are circumstances that modify quality to such an extent as usually, with the better kinds of metal, to entirely obscure variations due to accidental differences in chemical constitution ; with other irons and steels both sets of conditions concur to determine quality. It is never safe, therefore, to base specifications for these materials upon chemical composition alone ; actual test is usually demanded as a basis for their acceptance or rejection. Cast-iron has some advantages as a material for steam-boil- ers, such as its durability in presence of corroding elements, its freedom from liability to rapid solution by acids, its compact structure and the impossibility of becoming laminated ; and it is found to have practically equal conducting power. Its cost is also low ; but it is exposed to danger of cracking, either from shrinkage strains or local variations of temperature ; it gives no warning when such danger arises, but is always treacherous and unreliable. Its composition is a matter of uncertainty, and is never absolutely known. The cast-iron boilers are usually so constructed that it is easy to substitute a new piece for a broken part, and the boiler is then as good as when new, instead of being weakened by the operation, as is apt to be the case with wrought-iron boilers. On the other hand, they are considered to be commonly somewhat defective in circulation, as a rule, MATERIALS STRENGTH OF THE STRUCTURE. 63 and deficient in steam-space. Cast-steel is now often substi- tuted for cast-iron in such boilers, and is at once stronger and more trustworthy ; it is subject to the same objection as cast- iron in the difficulty met with in securing sound castings. Could good castings be relied upon and shrinkage cracks and strain cracks be prevented, the material would undoubtedly be much more generally employed, especially in small boilers. 32. Steel for Boilers is always of the class known as " low," " soft," or " mild " steel, and is, properly speaking, " ingot iron ;" all of its characteristics being those of a homogeneous, tena- cious, and ductile iron, and quite distinct from those of the true steels. As compared with iron, its greater tenacity, per- mitting the use of thinner sheets for a given pressure, or giving a greater margin of safety; its greater homogeneousness, in- suring more certainty and security in attaining the conditions prescribed in designing; and its greater ductility, which adds enormously to the safety of the structure against dangerous strains and alterations of form : all make it, when of good qual- ity, much the more desirable material. It is rapidly supersed- ing iron in boiler-construction. The difficulties which have retarded its introduction have been mainly those of getting perfect uniformity of composition, not only in successive lots, but also in different parts of the same lot, and even in the same sheet. Many manufacturers have now become able to secure all the uniformity desirable, and to guarantee the quality of their product ; from them good boiler-plate can always be ob- tained. Steel boiler-plate is usually made by the Siemens-Martin or "open-hearth" process; although considerable quantities are produced from the Bessemer converter, and some by the more costly crucible process. The former possesses peculiar advan- tages in the making of " mild " steels and boiler-plate in conse- quence of the facility which it offers for testing the quality of the metal from time to time, while still molten on the furnace- hearth, and then, if it proves not to be of the desired character, modifying it, by addition of such material as may serve to im- prove it, until the required quality is obtained. While the Bessemer process in skilled hands has produced most excellent 64 THE STEAM-BOILER. steel, very uniform in grade, neither it nor the crucible process offers such facilities for test and adjustment of quality as characterize the Siemens-Martin system. The composition of good steel boiler-plate should always be such as will give great ductility and perfect freedom from liability to harden and " take a temper" in consequence of variations of temperature occurring while in use. The carbon should be less in amount than one fourth of one per cent, and it is often less than one tenth. Manganese, which usually con- stitutes an important element, should be as low as is possible consistent with soundness and homogeneousness. Any boiler- plate that, on being heated to a red-heat and suddenly cooled, is found to harden perceptibly, should be rejected. It should weld readily, and should be capable of sustaining all the tests customarily demanded of boiler-iron even more satisfactorily than the latter. Its ductility should be greater than that of iron. As ordinarily made, steel is rarely as easily manipulated, and, when subjected to the ordinary operations of boiler-making, seldom exhibits as little loss of quality as the best irons; it must often be very carefully treated, and even in many cases must be annealed after each operation to restore lost ductility. Shearing and punching steels too high in carbon, or containing too much manganese or phosphorus, is very certain to produce injury. 33. The Effect of Variation of Form of a piece of metal, a member, or a structure, is often extremely important. This generally so considerably modifies the apparent tenacity of iron and steel that it is necessary to note the size and shape of the specimen tested before an intelligent understanding of the value of the material can be arrived at by examination of data secured by test. When a piece of metal is subjected to stress and slowly pulled asunder, it will yield at the weakest section first ; and if that section is of considerably less area than adjacent parts (Fig. 48), or if the metal is not ductile, it will often break sharply, and without stretching appreciably, as seen in Fig. 50 ; the fractured surface will have a granular appearance, and the behavior of the piece, as a whole, may be like that of a MATERIALS-STRENGTH OF THE STRUCTURE. 65 brittle casting, even although actually made of tough and due- tile metal, when the piece is deeply scored. When a bar of very ductile metal, of perfectly uniform cross- section (Fig. 49) is broken, on the other hand, it will, at first, if of uniform quality, gradually stretch with a nearly uniform reduction of section from end to end. Toward the ends, where held by the machine, this reduction of area is less perceivable, and on the extreme ends, where no strain can occur, except from the compressing action of the grips, the original area of section \ > k FIG. 48. Incorrect. FIG. 49. Correct. FORMS OF TEST-PIECES FOR TENSION. is retained, diminution taking place from that point to the most strained part by a gradual taper or by a sudden reduction of section, according to the method adopted of holding the rod. When the stress has attained so great an intensity that the weakest section is strained beyond its elastic limit, "flow'" begins there, and, while the extension of other parts continues slowly, the portions immediately adjacent to the overstrained section stretch more and more rapidly as this local reduction of section continues, and finally fracture takes place. This locally reduced portion of the rod has a length which is depend- ent upon the character of the metal and the size of the piece. 5 66 THE STEAM-BOILER. Hard and brittle materials exhibit very little reduction, and the reduced portion is short, as in Fig. 50; ductile and tough metals exhibit a marked re- duction over a length of several diameters, and great reduction at the fractured section, as seen in Fig. 51. Of the samples shown in the figures, the first is of a good, but a badly worked, iron, and the second from the same metal after it had been more thoroughly worked. When the breaking section is determined by deeply grooving the test-piece, the results of test are higher by 5 or 10 per cent than when the cylinders are not so cut, if the metal is hard and brittle, and by 20 to 25 per cent with tough and ductile irons or steels. In ordinary work this difference will average at least 20 per cent with the ductile metals. A good bridge or cable iron in pieces of i inch (2.54 centimetres) diame- ter cut from 2-inch (5.08 centi- metres) bar, exhibited a tenacity of 50,000 pounds per square inch in long test-pieces, and 60,000 in short grooved specimens (3515 to 4218 kilogrammes per square centimetre). Cast-irons will give practically equal results by both tests, as will hard steels and very coarse- grained hard wrought irons. Since these differences are so great that it is necessary to ascertain the form of samples tested before the results of test can be properly interpreted, it becomes advisable to use a test- piece of standard shape and size for all tests the results of which are to be compared. The fig- ures given hereafter, when not otherwise stated, may be assumed to apply to pieces of one half square inch area (3.23 square centimetres) of FIG. 5 i. section, and at least 5 diameters in length. This length is MATERIALS STRENGTH OF THE STRUCTURE. 67 usually quite sufficient, and is taken by the Author as a mini- mum. For other lengths, the extension is measured by a con- stant function of the total length plus a function of the diame- ter, which varies with the quality of the metal and the shape of the test-piece. It may be expressed by the formula e = al+f(d) ........ (i) The elongation often increases from 20 up to 40 per cent as the test-piece is shortened from 5 inches (12.7 centimetres) to inch (1.27 centimetres) in length, while the contraction of section is, on the other hand, decreased from 50 down to 25 per cent, nearly. Fairbairn,* testing good round bar-iron, found that the extension for lengths varying from 10 inches (25.4 centimetres) to 10 feet (3.28 metres) could be expressed, for such iron, by the formula (2) where / is the length of bar in inches. In metric measures this becomes /= length in centimetres; e = elongation per unit of length. This influence of form is as important in testing soft steels as in working on iron. Col. Wilmot, testing Bessemer "steel" at the Woolwich Arsenal, G. B., obtained the following figures: TENACITY. FORM. TEST-PIECE. Lbs. per sq. in. Kilogs. per sq. cm. Grooved, Fig. 48, Highest ............ 162,974 Lowest ............. 136,490 Average ........... I53,6?7 IO ' 8 3 Long cylinder. . . .Highest ............ 123,165 Lowest ............. 103,255 Average ............ 114,460 _ 8,047 * Useful Information, second series, p. 301. 68 THE STEAM-BOILER. The difference amounts to between 30 and 35 per cent, the groove giving an abnormally high figure. It is evident from the above that the elongation must be proportionably much greater in short specimens than in long pieces. This is well shown below in tests made by Capt. Beardslee for the United States Board.* TESTS OF TEST-PIECES OF VARYING PROPORTIONS TENSION. 1 rt , STRESS WHEN & DIAME- rt PIECE BEGAN BREAKING- LENGTH. TER. C TO STRETCH STRESS. s <3 OBSERVABLY. "s *< Remarks. Number. OriK- mal. Final. Percent < tion. Original. Reduced. Percent c tion of Ob- served Stress. Stress per square inch. Ob- served Stress. Stress per square inch. In. In. In. In. Lbs. Lbs. Lbs. Lbs. i 5.000 6.522 30.0 .798 .568 49-3 13,400 26,800 26,000 51,989 Elastic limit, 26,795 Ibs. per sq. in. 2 3-938 5.204 32.0 .798 -564 50.0 14,000 28,000 26,200 52,389 Elastic limit, 28,194 Ibs. per sq. in. 3 4.500 5-853 30.0 -797 -584 46.3 14,000 28,290 26,190 52,495 Elastic limit, 28,062 Ibs. per sq. in. 4 3-5oo 4-605 31.6 .791 57 48.0 13,000 26,450 26,070 53,052 Elastic limit, 27,268 Ibs. per sq. in. 5 3.000 3-977 33-o .792 571 48.0 14.000 28,420 26,100 52,984 6 2.472 3.266 32.1 799 -589 45-6 14,000 27,920 26,500 52,852 7 1.989 2.644 32.9 .798 591 45-o 14,000 28,000 26,500 53,169 8 1.500 2.026 35-o 797 590 45-2 15,500 31,320 26,275 52,666 9 I.OOO 1-354 35-4 .798 .600 43-5 16,675 33,350 26,590 53,169 10 o. 500 0.708 41.6 .798 -635 36.6 18,760 37,520 28,665 57-318 With such brittle materials as the cast-irons, the difference becomes unimportant. Beardslee found a difference of but i per cent in certain cases. The more brittle the material the less this variation of the observed tenacity. As will be seen later, even more important variations follow changes of proportion of pieces in compression. No test-piece should be of very small diameter, as inaccuracy is more probable with a small than with a large piece, and the errors are more likely to be increased in reduction to the stress per square inch. The length should not be less than four times the diameter in any case, and with soft ductile metal five or six diameters would be preferable, for tension. Report, p. 104. MATERIALS STRENGTH Of THE STRUCTURE. 69 Where much work is to be done, it is quite important that a set of standard shapes of test-pieces should be selected, and that all the tests should be made upon samples worked to standard size and form. Thus, tension-pieces are often made of the shapes seen in the figure, when testing square, cylindrical, or flat samples, or samples cut from the solid. The last is a shape called for under the U. S. inspection laws when testing boiler-plate ; but it should never be used if choice is permitted, as it gives no chance of stretching, and is therefore nearly use- less as a gauge of the quality of the metal ; it will undoubtedly be abandoned in course of time, as it invariably gives too high a figure, and does not distinguish the hard and brittle from the better and tougher materials which are desired in construction. The dimensions adopted by the Author are one-half square inch (3. 23 square centimetres) section for all metals except the ._8'TOI2'__ FIG. 52. SHAPES FOR TEST-PIECES. tool steels (0.798 inch ; 2 centimetres diameter when round), and one-eighth or one-quarter square inch(o.8i to 1.61 square centimetres area; 0.398 or 0.565 inch, I or 1.4 centimetres diame- ter) for the latter, at the smallest cross-section. Kent, who sketches the above, takes these shapes, making them, if of tool steel, H inch diameter (1.75 centimetres), or f square inch (2.44 square centimetres) area ; in other metals either f inch (1.9 centimetres) 70 THE STEAM-BOILER. diameter or 0.44 square inch (2.84 square centimetres), or as above. The edges should be true and smooth, and the fillets inch radius. For compression tests of metal, I inch (2.54 centimetres) long and J inch (1.27 centimetres) diameter, ends perfectly square, is recommended ; for stone and brick, a 2-inch (5.08 centimetres) cube. Transverse test-pieces should not be less than i foot nor more than 4 feet in length, when to be handled in ordinary machines. The standard specimen will be taken as above, and good wrought-iron of such shape and size should exhibit a tenacity of at least 50,000 pounds (3515 kilograms per square centimetre) if from bars not exceeding 2 inches (5.08 centimetres) diameter, and should stretch 25 per cent with 40 per cent reduction of area. Such test-pieces have the advantage of giving uniform comparable and minimum figures for tenacity, and of permitting accurate determinations of elongation. Test-pieces are only satisfactory in form when turned in the lathe, as the coincidence of the central line of figure with the line of pull is thus most perfectly insured. When, as with sheet-metal, this cannot be done readily, care must be taken to secure proportions of length and cross-section as nearly like those of the standard test-piece as possible, and to secure sym- metry and exactness of form and dimension ; such pieces are liable to yield by tearing when not well made and properly adjusted in the machine. 34. The Method of Treatment of metal, either previous to its use in any structure or while under load, often seriously modifies its strength, its ductility, and its endurance. Bar-irons exhibit a wide difference of strength, due to difference of section alone. This variation may be expressed approximately with good irons, such as the Author has studied in this relation, by the formulas T = 56,000 20,000 log d\ \ T m = 4,500 - 1,406 log d m . \ Where T and T m measure the tenacity in British and metric MATERIALS STRENGTH OF THE STRUCTURE. J\ measures respectively, and d and d m the diameter of the piece, or its least dimension. Where it is desired to use an expression which is not loga- rithmic, it will usually be safe to adopt in specifications the following : 60,000 80,000 (2) The Edgemoor Iron Company adopt, for wrought-iron in tension, the formula ~ 7,000 A T= 52,000 - ^-, in which A is the area, and B the periphery of the section.* The figures in the following table have been taken by the Author as fair values of the tenacity of good average merchant- iron. TENACITY OF GOOD IRON. DIAMETER. TENACITY, T. Centimetres. Inches. Lbs. per square inch. Kilogrammes per square inch. .64 i 6o,OOO 4.218 1.27 * 58,000 4,077 I.QO i 56,000 3947 2-54 i 55.500 3,902 3-18 i* 54,500 3,838 3-8l ii 53,500 3.76i 4-45 if 52.000 3,656 5-08 2 50,000 3,515 5-72 2 49,000 3-445 6-35 2* 48,900 3-374 7.62 3 47-500 3.320 8.90 3i 47,ooo 3,304 10. 16 4 46,000 3.234 12.70 5 44,000 3.093 KirkaJdyf found that pieces of ij-inch (3.2 centimetres) * Ohio Railway Report, 1881, p. 379- f Experiments on Wrought Iron and Steel. J2 THE STEAM-BOILER. bar rolled down to I inch (2.54 centimetres), f inch (1.9 centi metres), and J inch (1.27 centimetres) diameter increased in tenacity 20 per cent while decreasing in ductility 5 per cent. Forging has the same effect as rolling. The elastic limit is also usually lower in large than in small masses. Turning iron down has no important effect on the tenacity. The considerable variations always observable in .the gen- eral rate of increase of tenacity, which, other things being equal, accompanies reduction of size of wire, are due to the hardening of the wire in the draw-plate, and occasional restora- tion to its softest condition by 'annealing. Beardslee has found the change of tenacity in forged and rolled bars to be due to differences in amount of work done in the mill upon the iron. The extent of reduction of the pile sent to the rolls from the heating-furnace is variable, its cross- sectional area being originally from 20 to 60 times that of the bar, the higher figure being that for the smallest bars. On making this reduction uniform, it is found that the tenac- ity of bars varies much less in different sizes, and that the change becomes nearly uniform from end to end of the series of sizes, and becomes also very small in amount. By properly shaping the piles at the heating-furnace, and by putting as much work on large as on small bars, it was found that a 2-inch (5.08 centimetres) bar could be given a strength superior by over 10 per cent, and a 4-inch (10.17 centimetres) could be made stronger by above 20 per cent than iron of those sizes as usually made for the market. The surface of a bar is usually somewhat stronger than the interior. The Limit of Elasticity will be found at from two fifths the ultimate strength in soft, pure irons to three fifths in harder irons, and from three fifths in the steels to nearly the ultimate strength with harder steels and cast-irons. Barlow found good wrought-iron to elongate one ten-thousandth its length per ton per square inch up to the limit at about 10 tons. The relation between the series of elastic limits and the maximum resistance of the iron or the steel is well shown in strain-diagrams, which exhibit graphically the varying relation MATERIALS STRENGTH OF THE STRUCTURE. 73 of the stress applied to the strain produced by it throughout the process of breaking. Repeatedly Piling and Reworking improves the quality of wrought-iron up to a limit at which injury is done by over- working and burning it. The iron thus treated exhibits increasing strength until it has been reheated five or six times, and then gradually loses tenacity at a rate which seems to be an accelerating one. Forging iron is similar in effect, and improves the metal up to a limit seldom reached in small masses. The forging of large masses usually includes too often re- peated piling and welding of smaller pieces, and it is thence found difficult to secure soundness and strength. This is par- ticularly the case where the forging is done with hammers of insufficient weight. The iron suffers, not only from reheating, but from the gradual loosening and weakening of the cohesion of the metal within the mass at depths at which the beneficial effect of the hammer is not felt. The Effect of Prolonged Heating is sometimes seen in a granular, or even crystalline, structure of the iron, which indi- cates serious loss of tenacity. Large masses must always be made with great care, and used with caution and with a high factor of safety. Ingot iron is always to be preferred to welded masses of forged material for shafts of steamers and similar o uses. The Tenacity of Ingot Irons and Steels is less subject to variation by accidental modifications of structure and compo- sition than is that of wrought-iron. The steels are usually homogeneous and well worked, and are comparatively free from objectionable elements, their variation in quality being determined principally by the amount of carbon present, which element occurs in a proportion fixed by the maker, and varying within a very narrow range. The softest grades of ingot iron and steel approach the character of wrought-irons ; but their comparative freedom from slag, and their purity, usually make them superior to all ordinary irons in combined strength and ductility. The products of the Bessemer and of the open- hearth processes vary in tenacity from 60,000 pounds per square 74 THE STEAM-BOILER. inch (4218 kilogrammes per square centimetre) to more than double that figure ; while the crucible steels often, and occa- sionally the preceding, are sometimes four times as strong, a tenacity of 200,000 pounds per square inch (14,060 kilogrammes per square centimetre) being sometimes exceeded. 35. The Time and the Margin of Stress, or loading, both affect greatly the life of the piece and the degree of safety with which it may be used. It has been shown by the Author, and by Commander Beardslee, U. S. N., by direct experiment in the Mechanical Laboratory of the Stevens Institute of Technology, and at the Washington Navy Yard, that the normal elastic limit, as ex- hibited on strain-diagrams of tests conducted without inter- mission of stress, is exalted or depressed when intermission of distortion occurs, according as the metal belongs to the iron or to the tin class. This elevation of the normal elastic limit by intermitting strain is, as has been shown, variable in amount with different materials of the iron class, and the rate at which this exaltation progresses is also variable. With the same material and under the same conditions of manufacture and of subsequent treatment the rate of exaltation is quite definite, and may be expressed by a very simple formula. The Author has experimented with bridge material, and Commander Beardslee has examined metal specially adapted for use in chain cables, for which latter purpose an iron is required, as in bridge-building, to be tough as well as strong and uniform in structure and composition. The experiments of the latter in- vestigator have extended to a wider range than have those of the Author, and the effect of the intermission of strains con- siderably exceeding the primitive elastic limit has been deter- mined by him for periods of from one minute to one year. From a study of the results of such researches and from a com- parison with the latter investigation, which was found to be confirmatory of the deduction, the Author has found that, with such iron as is here described, the process of exaltation of the normal elastic limit due to any given degree of strain usually nearly reaches a maximum in the course of a few days of rest after strain, its progress being rapid at first and the rate of in- MATERIALS STRENGTH OF THE STRUCTURE. 75 crease quickly diminishing with time. For good boiler irons, the amount of the excess of the exalted limit, as shown by sub- sequent test, above the stress at which the load had been pre- viously removed may be expressed approximately by the formula E' = 5 log T-\- 1.50 per cent ; in which the time, T, is given in hours of rest after removal of the tensile stress which produced the noted stretch. The Author has investigated the action of prolonged stress, using wire of Swedish iron : but one set of samples was an- nealed ; the other, of two sets, was left hard, as drawn from the wire-blocks. The size selected was No. 36, 0.004 mcn (- 01 millimetre) diameter, and was loaded with 95, 90, 85, 80, 75, 70, 65, and 60 per cent of the breaking load as obtained by the usual method of test. The result was: ENDURANCE OF IRON WIRE UNDER STATIC LOAD. TIME UNDER LOAD BEFORE FRACTURE. PER CENT MAXIMUM STATIC LOAD Hard wire (unannealed). Soft wire (annealed). 95 8 days. 3 minutes. 9 35 days. 5 minutes. 85 Unbroken at end of 16 mos. i dav. 80 91 days. 266 days. 75 70 65 Unbroken. j 17 days. 455 days. 455 days. 60 Unbroken. Several years. Soft irons and the " tin class" of metals and the woods are found to demand a higher factor of safety than hard iron. The elegant and valuable researches, also, of Mons. H. Tresca on the flow of solids,* and the illustrations of this action almost daily noticed by every engineer, seem to lend confirmation to the supposition of Vicat. The experimental researches of Prof. Joseph Henry, on the viscosity of materials, and which * Sur 1'Ecoulement des corps solides. Paris, 1869-72. 76 THE STEAM-BOTLER. proved the possibility of the coexistence of strong cohesive forces with great fluidity,* long ago proved also the possibility of a behavior in solids, under the action of great force, analo- gous to that noted in more fluid substances. On the other hand, the researches of the Author, indicating by strain-diagrams that the progress of this flow is often ac- companied by increasing resistance, and the corroboratory evi- dence furnished by all such carefully made experiments on tensile resistance as those of King and Rodman, Kirkaldy and Styffe, have made it appear extremely doubtful whether hard iron is ever weakened by a continuance of any stress not origi- nally capable of producing incipient rupture. Kirkaldy concludes that the additional time occupied in testing certain specimens of which he determined the elonga- tion "had no injurious effect in lessening the amount of break- ing strain." f An examination of his tables shows those bars which were longest under strain to have had highest average resistance. Wertheim supposed that greater resistance was offered to rapidly than to slowly produced rupture. The experiments of the Author prove that, as had already been indicated by Kirkaldy, a lower resistance is offered by ordinary irons as the stress is more rapidly applied. This effect conspires with vis viva to produce rupture. We conclude that the rapidity of action in cases of shock, and where materials sustain live loads, is a very important ele- ment in the determination of their resisting power, not only for the reason given already, but because the more rapidly common iron is ruptured the less is its resistance to fracture. This loss of resistance is about 1 5 per cent \ in some cases, noted by the Author, of moderately rapid distortion. The cause of this action bears a close relation to that operating to produce the opposite phenomenon of the ele- vation of the elastic limit by prolonged stress, to be de- * Proc. Am. Phil. Society, 1844. f Experiments on Wrought Iron and Steel, pp. 62, 83. | Compare Kirkaldy, p. 83, where experiments which are possibly affected by the action of vis viva indicate a very similar effect. MATERIALS STRENGTH OF THE STRUCTURE. 77 scribed, and it may probably be simply another illustration of the effect of internal strain. Metals of the " tin class" ex, hibit, as has been shown by the Author,* an opposite effect. Rapidly broken, they offer greater resistance than to a static or slowly applied load. It has also been seen that annealed iron has, in some respects, similar qualities. With a very slow distortion the " flow" already described occurs, and but a small amount of internal strain is produced, since, by the action noticed when left at rest, this strain re' lieves itself as rapidly as produced. A more rapid distortion produces internal stress more rapidly than relief can take place, and the more quickly it occurs the less thoroughly can it be relieved, and the more is the total resistance of the piece reduced. Evidence confirmatory of this explanation is found in the fact that bodies most homogeneous as to strain exhibit these effects least. At extremely high velocities the most ductile substances exhibit similar behavior when fractured by shock or by a sud- denly applied force, to substances which are really compara- tively brittle. f In the production of this effect, which has been frequently observed in the fracture of iron, although the cause has not been recognized, the inertia of the mass attacked and the actual depreciation of resisting power just observed, conspire to produce results which would seem quite inexpli- cable, except for the evidently great concentration of energy here referred to, which, in consequence of this conspiring of inertia and resistance, brings the total effort upon a compara- tively limited portion of the material, producing the short fracture, with its granular surfaces, which is the well-known characteristic of sudden rupture. Any cause acting to produce increased density, as reduction of temperature, evidently must intensify this action of suddenly applied stress. The liability of machinery and structures to injury by shock is thus greatly increased, and it is quite uncertain what * Trans. Am. Soc. C. E., 1874 et seq. \ Specimens from wrought-iron targets shattered by shock of heavy ord- nance exhibit this change in a very unmistakable manner. 78 THE STEAM-BOILER. is the proper factor of safety to adopt in cases in which the shocks are very suddenly produced. Meantime the precautions to be taken by the engineer are : To prevent the occurrence of shock as far as possible, and to use in endangered parts light and elastic members, composed of the most ductile materials available, giving them such forms and combinations as shall distribute the distortion as uniformly and as widely as possible. The behavior of materials subjected to sudden strain is thus seen to be so considerably modified by both internal and external conditions which are themselves variable in character, that it may still prove quite difficult to obtain mathematical expressions for the laws governing them. An approximation, of sufficient accuracy for some cases which frequently arise in practice, may be obtained for the safety factor by a study and comparison of experimental results. Egleston, studying the behavior of metal under long-con- tinued and repeated stresses, finds evidence of the existence of a " law of fatigue and refreshment of metals," occurring as in- dicated by the Author. He also concludes* that metal once fatigued may sometimes be restored by rest or by heating that " the change produced is a chemical one," accompanied by " a change in the size, color, and surface of the grains of the iron or the steel." Surface injuries by blows were found to affect the metal, in some cases, to a depth of 15 millimetres (0.6 inch). He informs the Author that he finds evidence of the formation of crystals in the cold metal during the process of becoming fatigued, and a decided change in the proportion of combined and uncombined carbon. The Effect of Repeated Variation of Load is most important. In the year 1859 Prof. Wohler, in the employ of the German Government, undertook a series of experiments to determine the effect of prolonged varying stress on iron and steel. These experiments were continued until 1870. The apparatus used by Wohler and his successor, Spangenberg, was of four kinds : i. To produce rupture by repeated load. * Transactions Institute Mining Engineers, 1880. MATERIALS-STRENGTH OF THE STRUCTURE. 79 2. For repeated bending, in one direction, of prismatic rods. 3. For experiments on loaded rods under constant bend- ing stress. 4. For torsion by repeated stress. The amount of the imposed stress was determined by breaking several rods of like material, ascertaining the break- ing load, and taking some fraction of this for the intermittent load. From the results of these experiments of Wohler, extend- ing over eleven years, the observations here appended were deduced : " WOHLER'S LAW : Rupture of material may be caused by repeated vibrations, none of which attain the absolute breaking limit. The differences of the limiting strains are sufficient for the rupture of the material." The number jf strains required for rupture increases much more rapidly than the weight of load diminishes. The work of Wohler and Spangenberg has proven what was long before supposed to be the fact that the permanence and safety of any iron or steel structure depends not simply on the greatest magnitude of the load to be sustained, but on the frequency of its application and the range of variation of its amount. The structure or the machine must usually be designed to carry indefinitely whatever load it is intended to sustain and to be permanently safe, however much the stress may vary, or however frequent its application. The stress permitted and calculated upon must therefore be less as the variation is greater, and as the frequency of its application is greater. Although it is customary to make the working load one fifth or one sixth the maximum load that could be sus- tained without fracture, it has now become well known that this is not the correct method except for an unvarying load ; although, as will be seen, these factors of safety are sufficient to cover the case studied by Wohler. Wohler found that good wrought-iron and steel would bear loads indefinitely as follows : 80 THE STEAM-BOILER. Lbs. per sq. in. Kilogs. per sq. cm. Wrought-iron, tension only + 18,70010+ 301+1,30910+ 2.2 Wrought-iron, tension and compres. + 8,320 to - 8,320; + 582 to 582 Cast-steel, tension only + 34-3O7 to + 11,440; + 2,401 to + 801 Cast-steel, tension and compression + 12,480 to 12,480; + 874 to 874 Thus rupture is produced either by a certain load, called usually the " breaking load," once applied, or by a repeatedly applied smaller load. The differences of stresses applied, as well as their actual amount, determine the number of appli- cations which may be made before fracture occurs, and the length of life of the member or the structure. This weakening of metal by repeated stresses is known as fatigue. It is not known that it may always be relieved, like internal stresses, by rest ; but it is apparently capable of relief frequently by either simple rest for a considerable period, or by heating, working, and annealing. The experiments described seem to indicate some relation between the action of variable loads and of prolonged stress where metals are soft enough to " flow." Wohler concluded that the allowable loads for the cases of stationary loading, loading in tension alternating with entire relief, and equal and alternate tensions and compressions, will be in the ratio 3:2: I. The method above described is still in the experimental stage ; but it may be provisionally accepted as safer than the usual method of covering cases of varying stresses by a factor of safety determined solely by custom or individual judgment. It has been the custom with some American bridge-builders to give members in alternate tension and compression a section equal to that calculated for a tension under static load equal to the sum of the two stresses a rough method of meeting the most usual and serious case. A number of engineers, commenting upon the work of Wohler, Spangenberg, Weyrauch, and Launhardt, consider that the result is simply to base upon the ultimate strength a deduced limit of working stress which corresponds closely to the elastic limit, and generally urge that reasonable factors of MATERIALS-STRENGTH OF THE STRUCTURE. 8l safety related to the limit of elasticity are preferable to the still uncertain method above described. It is admitted, however, that the results accord with those already indicated by experi- ence where a definite practice has become settled upon. There are many phenomena which cannot be conveniently exhibited by strain-diagrams ; such are the molecular changes which occupy long periods of time. These phenomena, which consist in alterations of chemical constitution and molecular changes of structure, are not less important to the mechanic and the engineer than those already described. Requiring usually a considerable period of time for their production, they rarely attract attention, and it is only when the metal is finally inspected, after accidental or intentionally produced fracture, that these effects become observable. The first change to be referred to is that gradual and imperceptible one which, occu- pying months and years, and under the ordinary influence of the weather going on slowly but surely, results finally in im- portant modification of the proportions of the chemical ele- ments present, and in a consequent equally considerable change of the mechanical properties of the metal. Exposure to the weather, while producing oxidation, has another important effect : It sometimes produces an actual im- provement in the character of the metal. Old tools, which have been laid aside or lost for a long time, acquire exceptional excellence of quality. Razors which have lost their keenness and their temper recover when given time and opportunity to recuperate. A spring regains its tension when allowed to rest. Farmers leave their scythes exposed to the weather, sometimes from one season to another, and find their quality improved by it. Boiler-makers frequently search old boilers carefully, when reopened for repairs after a long period of service, to find any tools that have been lost and so improved. 36. A Method of Detecting any Overstrain to which a structure or either of its parts may have been subjected, which was devised, or more properly discovered, by the Author, is sometimes of service in revealing danger of accident, or the cause of disasters already arrived. It has been shown by the 6 82 THE STEAM-BOILER. Author* and by other investigators, that when a metal is sub- jected to stress exceeding that required to strain it beyond its original apparent, or 4< primitive," elastic limit, this primitive elastic limit becomes elevated, and that strain-diagrams obtained autographically, or by carefully plotting the results of well-con- ducted tests of such metal, are " the loci of the successive limits of elasticity of the metal at the successive positions of set."f It has been shown by the Author also that, at the successive positions of set, strain being intermitted, a new elastic limit is, on renewing the application of the distorting force, found to exist at a point which approximately measures the magnitude of the load at the moment of intermission.^ Thus it is seen that a metal, once overstrained, carries per- manently unmistakable evidence of the fact, and can be made to reveal the amount of such overstrain at any later time with a fair degree of accuracy. This evidence cannot be entirely destroyed, even by a moderate degree of annealing. Often, only annealing from a high heat, or reheating and reworking, can remove it absolutely. Thus, too, a boiler, or any structure, broken down by causes producing overstrain in its tension members, or in its transversely loaded beams (and, probably, in compression members although the writer is not yet fully as- sured of the latter), retains in every piece a register of the maximum load to which that piece has ever been subjected ; and the strain sheet of the structure, as strained at the instant of breaking down, can be thus laid down with a fair degree of certainty. The Author has found by subsequent tests that transverse strain produces the same effect upon the elastic limit for tension. Here may be found a means of tracing the overstrains which have resulted in the destruction or the injury of any iron or steel structure, and of ascertaining the cause and the method of its failure, in cases frequently happening in which * See Trans. Am. Soc. C. E., 1874 et seq. t Journal Franklin Institute, 1874 ; Van Nostrand's Eclectic Engineering Magazine, 1874, etc., etc. f On the Strength, etc.. of Materials of Construction, 1874, Sec. 20. | On the Mechanical Treatment of Metals; Metallurgical Review, 1877; Engineering and Mining Journal, 1877. MATERIALS-STRENGTH OF THE STRUCTURE. 83 they are indeterminable by any of the usual methods of inves- tigation. This method may thus sometimes be used to ascertain the probable cause of a boiler explosion, by determining whether the metal has been subjected to overstrain in consequence of overpressure. The causes of accidents to machinery may also be thus detected, and many other applications might be sug- gested. 37. The Effect of Temperature and its Variation on iron and steel is probably the most important of all those phenom- ena which modify the behavior of iron or steel under load. 30O 4-OO BOO 000 872. 732 939 1112 1292 FIG. 53. HEAT vs. TENACITY. 80O 1472 eoo 1032 1000 1832 noo'Ci The diagram above* graphically represents the results of several series of experiments. Curves Nos. I and 2 represent Kollmann's experiments on iron, and 3 on Bessemer "steel." No. I is ordinary, and 2 steely puddled iron. Curve No. 4 represents the work of the Franklin Institute on wrought-iron. * Eisen und Stahl, A. Martens ; Zeitschrift des Vereins Deutscher Inge- nieure ; Feb. 1883, p. 127. 84 THE STEAM-BOILER. Curve No. 5 gives Fairbairn's results, working on English wrought-irons. Nos. 6 to II are Styffe's, and represent the experiments made by him on Swedish iron. The numbers do not appear, as these results do not fall into curves ; these results are indi- cated by circles, each group being identified by the peculiar filling of the circles, as one set by a line crossing the centre, another by one across, a third by a full circle, etc. The broken lines, 12 and 13, are British Admiralty experi- ments on blacksmiths' irons, and No. 14 on Siemens steel. The first five series only are of value as indicating any law ; and they exhibit plainly the general tendency already referred to, to a decrease of tenacity with increase of temperature. Fairbairn's experiments, No. 5, best exhibit the maximum, first noted by the Committee of the Franklin Institute, at a temperature between that of boiling water and the red heat. It will be observed that the measure of tenacity, at the left, is obtained by making the maximum of Kollmann unity. It will also be noted that Kollmann does not find a maximum as in curves 4 and 5, but, on the contrary, a more rapid reduction in strength at that temperature than beyond. It would seem, therefore, that that peculiar phenomenon must be due to some accidental quality of the iron.* The Author has attributed it to the existence in the iron, before test, of internal stresses which were relieved by flow as the metal was heated, disappearing at a temperature of 300 or 400 Fahr. (149 to 204 Cent.). The experiments of Mr. Oliver Williams f in determining" the change produced in the character of the fracture of iron by transverse strain, at extreme temperatures, indicate loss of duc- tility at low temperatures. Two specimens of nut-iron, from different bars, made at Catasauqua, Pennsylvania, were first nicked with a cleft on one side only, and then broken under a hammer, at a temperature * Isherwood suggests that this is simply due to repeatedly breaking the same piece. f The Iron Age, New York, March 13, 1873, P- J 6. MATERIALS STRENGTH OF THE STRUCTURE. 85 of about 20 Fahr. ( 7 Cent.). At this temperature both specimens broke off short, showing a clearly defined granular or steely iron fracture. The pieces were then gradually heated to about 75 Fahr. (24 Cent.), and then broken as before, de- veloping a fine, clear, fibrous grain. The two fractures were FIG. 54. FRACTURE AT ORDINARY TEMPERATURE. but four inches (10.16 centimetres) apart, and are entirely dif- ferent. The accompanying illustrations, from the Author's collection, exhibit this case. It has been long known that a granular fracture may be produced by a shock, in iron which appears fibrous when grad- ually torn apart. This was fully proven by Kirkaldy.* Mr. Williams was, probably, the first to make the experiment just described, and thus to make a direct comparison of the char- acteristics of fracture in the same iron at different temperatures. Valton has found f that some iron be- comes brittle at temperatures of 572 or 752 Fahr. (300 to 400 Cent.), and re- gains ductility and toughness at higher , , , if Fie;. 55. FRACTURR AT Low temperatures. On the whole, the frac- TEMPERATURE. * Experiments on Iron and Steel. f Bulletin Iron and Steel Assoc., Feb. 1877. 86 THE STEAM-BOILER. ture of iron at low temperatures has been found to be charac- teristic of a brittle material, while at higher temperatures it exhibits the appearance peculiar to ductile and somewhat vis- cous substances. The metal breaks, in the first case, with slight permanent set and a short granular fracture, and in the latter with, frequently, a considerable set, and the form of fracture in- dicating great ductility. The variation in the behavior of iron, as it approaches the welding heat, ifiustrates the latter condi- tion in the most complete manner. Valton found that a steel rod bent very well at a tempera- ture a little below dull red, but broke at a temperature which may be called blue, the fracture showing that color. Portions of the rod which were below this temperature manifested much toughness, and bent without fracture. Charcoal pig-iron from Tagilsk, made in 1770, irons obtained from the Ural in rods and sheets, soft Bessemer and Martin steels from Terrenoire, soft English steel and good English merchant-bars, all gave the same results, whether the metal tested had been hammered or rolled. Valton found that the phenomenon had been long known to the workmen under his direction. In working sheet- iron with the hammer they wait until the metal has cooled further when approaching the temperature which would give the blue fracture when broken. He concludes that wrought- irons, as well as some kinds of soft steel, even when of excel- lent quality, are very brittle at a temperature a little below dull red heat 577 to 752 Fahr. (between 300 and 400 Cent.). The variation of strength follows quite closely the change of density, which latter is illustrated in the preceding diagram, which exhibits increase of volume from the freezing-point. The sudden fall of the line before reaching the melting- point indicates the sudden increase of volume which castings exhibit while cooling, and which enables " sharp" castings to be secured. It is at the crest noted near this point that vis- cosity is observed. From this point back to the freezing-point the variation follows a regular law. It would thus seem that the general effect of increase or decrease of temperature is, with solid bodies, to decrease or increase their power of resistance to rupture, or to change of MATERIALS-STRENGTH OF THE STRUCTURE. 87 form, and their capability of sustaining "dead" loads; and we may conclude: (1) That the general effect of change of temperature is to produce change of ductility, and consequently change of resili- ence, or power of resisting shocks and of carrying " live loads." This change is usually opposite in direction and greater in de- gree at ordinary temperatures than the variation simultane- ously occurring in tenacity. (2) That marked exceptions to this general law have been noted, but that it seems invariably the fact that, wherever an exception is observed in the influence upon tenacity, an excep- tion may also be detected in the effect upon resilience. Causes which produce increase of strength seem also to produce a sim- ultaneous decrease of ductility, and vice versa. (3) That experiments upon copper, so far as they have been carried, indicate that (as to tenacity) the general law holds good with that metal. (4) That iron exhibits marked deviations from the law be- tween ordinary temperatures and a point somewhere between 500 and 600 Fahr. (260 and 316 Cent.), the strength increas- ing between these limits to the extent of about 15 per cent with good iron. The variation becomes more marked and the results more irregular as the metal is more impure. (5) That above 600 Fahr. (316 Cent.) and below 70 (21 Cent.) the general law holds good for iron, its tenacity increas- ing with diminishing temperature below the latter point at the rate of from 0.02 to 0.03 per cent for each degree Fahrenheit, while its resilience decreases in an undetermined ratio for good iron, and to the extent of reduction to one third its ordinary value or less, at 10 Fahr. ( 12 Cent.) when cold-short, and in the latter case the set may be less than one fourth that noted at a temperature of 84 Fahr. (29 Cent.). (6) That the viscosity, ductility, and resilience of metals are determined by identical conditions, and that the fracture of iron at low temperatures has accordingly been found to be characteristic of a brittle material, while at the higher tempera- tures it exhibits the appearance peculiar to ductile and some- what viscous substances. The metal breaks in the first case 88 THE STEAM-BOILER. with slight permanent set and a short granular fracture, and in the latter with frequently a considerable set and a form of frac- ture indicating great ductility. The variation in the behavior of iron, as it approaches a welding heat, illustrates the latter condition in the most complete manner. (7) That the precise action of the elements with which iron is liable to be contaminated, and the extent to which they modify its behavior under varying temperature, remain to be fully investigated, but that the presence of phosphorus and of other substances producing " cold-shortness," exaggerates to a great degree the effects of low temperature in producing loss of toughness and resilience. (8) That the modifications of the general law with other metals than iron and copper, and in the case of alloys, have not been studied, and are entirely unknown. The practical result of the whole investigation is that iron and steel, and probably other metals, do not lose their power of sustaining absolutely " dead " loads at low temperatures, but that they do lose, to a very serious extent, their power of sus- taining shocks or of resisting sharp blows, and that the factors of safety in structures need not be increased in the former case, where exposure to severe cold is apprehended ; but that machinery, rails, and other constructions which are to resist shocks should have larger factors of safety, and should be most carefully protected, if possible, from extreme temperatures. The Stress Produced by Change of Temperature is easily cal- culated when the modulus of elasticity and the coefficient of expansion are known, thus : Let E = the modulus of elasticity; A = the change of length per degree and per unit of length ; At* = the difference of initial and final temperatures; / = the stress produced. Then / : E : : \Af : I, (i) MATERIALS STRENGTH OF THE STRUCTURE. 89 For good wrought-iron and steel, taking E as 28,000,000 pounds on the square inch, or 2,000,000 kilogrammes on the square centimetre, and A as 0.0000068 for Fahrenheit, and as 0.0000120 for Centigrade degrees: p = \yzAf Fahr., nearly, ) \ ..... (2) = 25 J/ Cent., nearly. ) For cast-iron, taking E = 16,000,000; A = 0.0000062: / = ioo//* Fahr., nearly, ) (3) = \2Af Cent., nearly. ) This force must be allowed for as if a part of the tension, 7*, or compression, C, produced by the working load when the parts are not free to expand. Sudden Variation of Temperature has an effect upon steel which is very great when the proportion of carbon is not far from one per cent. With less carbon the effect is less observ- able, and with the wrought-irons and with ingot metals con- taining less than one third per cent carbon and other hardening elements, it becomes quite unimportant. Soft irons are still further softened by sudden reduction of temperature from the red heat. Cast-irons, unless of the class known as " chilling irons," are much less affected than steel, and when very rich in graphitic carbon are not perceptibly hardened. When either iron or steel is repeatedly heated and cooled, a permanent change of form takes place. Colonel Clarke has shown* that cylinders repeatedly heated to a high temperature and suddenly cooled, become enlarged in diameter perma- nently. Pieces of tempered steel are larger than when untem- pered. Cast-iron ordnance, after having been discharged many times, becomes unsafe in consequence of weakening, which is * Philosophical Magazine, 1863. Cp THE STEAM-BOILER. probably principally due to strains caused by sudden and irreg- ular changes of temperature in service. Such grades of steel as take a temper are greatly strength- ened unless too highly hardened, in which case they become brittle from internal stresses. The Author has found temper- ing in mercury to increase greatly both the strength and the toughness of small pieces of good tool steel. Kirkaldy has found, by an extended series of experiments, that tempering tool steels in oil greatly increases both strength and elasticity, while hardening in water reduces both. The higher the tem- perature at which, without risk, the steel can be cooled, the greater is this increase of strength. Hard steels exhibit the fact better than soft steels. Dividing steels into series in the order of their contents in carbon, beginning with the softest grades, the following were the percentages of increase of strength from weakest to strongest: 11.8, 24.2, 40.7, 53.2, 57.0, 64.1, 70.9, 77.6. The harder steels were highly heated; the soft steels only moderately. A singular change is observed in iron and in the soft steels, and may perhaps be found to occur with other metals, when the temperature approaches what is known as the black heat a temperature not far from 600 or 700 F. (316 to 370 C), and below a red-heat visible in the dark. At this temperature, metal which bends readily either cold or at the full red heat is found to be exceedingly brittle and to break easily, especially under percussion, without bending. This heat with its peculiar effect may be reached in a bath of boiling tallow at a little above the lower temperature above specified. The steels show less of this effect, usually, than the irons. The presence of more than a trace of sulphur, or phosphorus, or of other hardening elements, exaggerates this action. 38. Crystallization and Granulation are the two methods of alteration of molecular structure which are consequent upon the action of any cause which continually separates the par- ticles of the metal beyond the range marked and limited by the elastic limit. No evidence is to be found that a single suddenly applied force, producing fracture, may cause such a systematic and complete rearrangement of molecules. The MATERIALS STRENGTH OF THE STRUCTURE. 9 1 granular fracture produced by sudden breaking, and the crys- talline structure produced as above during long periods of time, are apparently as distinct in nature as they are in their causes. But simple tremor, where no sets of particles are separated so far as to exceed the elastic range, and to pass beyond the limit of elasticity, does not seem to produce such changes. In fact, some of the most striking illustrations of the improvement in the quality of wrought-iron with time have occurred where severe jarring and tremor were common. Metal has been sub- jected for many years to the strains and tremor accompanying the passage of trains without apparent tendency to crystalliza- tion, and with evident improvement in its quality. Wohler found cubic crystals in cast-iron plates which had been for some time kept at nearly the temperature of fusion in a furnace, and Augustine found similar crystals in gun-barrels; Percy found octahedra of considerable size in a bar which had been used in the melting-pot of a glass-furnace. Fairbairn as- serts the occasional occurrence of such change due to shock, jar, and long-continued vibration. Miller found cubic crystal- lization plainly exhibited in Bessemer iron, which may, how- ever, have been due to the presence of manganese. Hill shows * that heat may produce such crystallization. In a discussion which took place many years ago before the British Institution of Civil Engineers, Mr. J. E. McConnell produced a specimen of an axle which he thought furnished nearly incontestable evidence of crystallization. One portion of this axle was clearly of fibrous iron, but the other end broke off as short as glass. The axle was hammered under a steam hammer, then heated again and allowed to cool, after which it was found necessary to cut it almost half through and hammer it for a long time before it could be broken. The great testing- machine at the Washington Navy Yard has a capacity of about 300 tons, and has been in use 40 years. Commander Beardslee subjected it to a stress of 288,000 Ibs. (130,000 kilogrammes), which stress had frequently been approached before ; but it subsequently broke down under about 100 tons. The connect- * Iron Age, 1882 . Mechanics, 1882. 9 2 THE STEAM-BOILER. ing-bar which gave way had a diameter of five inches, and should have originally had a strength of about 400 tons (406,400 kilogrammes). Examining it after rupture, the fractured sec- tion was found to exhibit strata of varying thickness, each having a characteristic form of break. Some were quite granu- lar in appearance, but the larger proportion were distinctly crystalline. Some of these crystals are large and well defined. The laminae, or strata, preserve their characteristic peculiarities, whether of granulation or of crystallization, lying parallel to their axis and extending from the point of original fracture to a section about a foot distant, where the bar was broken a second time (and purposely) under a steam hammer. It thus differs from the granular structure which distinguishes the sur- faces of a fracture suddenly produced by a single shock, and which is so generally confounded with real crystallizion. 39. Irons and Steels Compared with reference to their composition and qualities, even when the latter are given as much of the character of the -best iron as is possible, will ex- hibit some marked differences. In composition the following may be considered good repre- sentative examples: IRONS. STEELS. Swedish. Dartmoor. Pennsylva- nia. "Mild." Very " Mild." Carbon 0.087 0.056 0.005 O.Ol6 O.O22 0.104 o. 1 06 0.280 99-372 0.067 0.020 O.OOI 0.075 0.009 99.828 0.238 o. 105 O.OI2 0.034 0.184 99.427 0.009 0.163 0.009 0.084 0.620 99-ns Silicon Sulphur Phosphorus Manganese Iron by diff 99.220 IOO.OOO IOO.OOO IOO.OOO IOO . OOO IOO.OOO All the hardening elements usually appear in larger propor- tion in the steels than in the irons; but this is not invariably the fact, especially with those very mild steels which can be made by the crucible process. Comparing the analyses of the two classes of metal, it will be found that the best irons are more irregular and uncertain MATERIALS STRENGTH OF THE STRUCTURE. 93 in composition than the best steels; that they contain con- siderable amounts of cinder, or slag, derived from the puddle- ball and the crude cast-iron from which it is made ; that the carbon and silicon are usually less in quantity, though very variable ; sulphur and phosphorus are commonly " higher" than in steels; and the whole list of elements, aggregating, slag aside, less in the irons than in the steels, varies greatly in pro- portions, and by no law. The steels are capable of more exact prescription of constitution than irons, and are especially dis- tinguished by their richness in manganese, silicon, and carbon, and their freedom from slag and from sulphur and phosphorus. The crucible steels contain, as a rule, much less manganese and silicon than do the others. For boiler-plate, the carbon should be kept below one fourth of one per cent, and all other ele- ments as low as possible ; but the effect of manganese and other hardening constituents is not sufficiently well settled, especially where the metal is exposed to the action of the fire, and to varying temperatures generally, to admit of the pre- scription of a formula for the best possible composition. Comparing the structure of iron and steel, it will be found that the latter is comparatively, often almost absolutely, homo- geneous ; while the former is very irregularly laminated, and exhibits the most remarkable fibrous texture when broken slowly, the slag separating threads of metal by encasing them in sheaths of mineral, and layers of cinder and oxide causing stratification by preventing the welding of the sheets of thinner iron of which the plate is made. The whole structure of the "pile" from which it is rolled is reproduced in a distorted fashion in the finished plates. The steel breaks with the same fracture, and offers the same resistance in both directions; while iron, especially the cheaper grades, usually resists longi- tudinal forces much better than transverse. In tenacity the best steel boiler-plate is but little, if any, stronger than the best boiler-iron : it excels the latter, however, in ductility as well as in homogeneousness, and resists the cor- roding action of the fluids with which it is brought in contact much better than iron. If too rich in manganese, too high in carbon or silicon, or if it contains an appreciable amount of 94 THE STEAM-BOILER. phosphorus, steel becomes unreliable, and more dangerous than ordinary irons. "Mild Steel" will take a temper, often, when containing over 0.30 per cent of carbon. Its uniformity and reliability decrease as its strength and hardness increase, and also with increase of thickness and size of the mass produced. This fact has caused the British Lloyds regulations to make the following allowances : Plates and stays o to I inch thick, maximum tensile strength 67,200 pounds (4/24 kgs. per sq. cm.). Plates and stays I to if inches thick, maximum tensile strength 64,960 pounds (4567 kgs. per sq. cm.). Plates and stays over if inches thick, maximum tensile strength 62,720 pounds (4410 kgs. per sq. cm.). The same proportions carried further would reduce the al- lowable tenacity of steel in heavy and thick masses to that of good iron, leaving its homogeneousness the only advantage. If used at all, the harder steels should be tempered in oil ; but they have no place in boiler-construction. The conclusions to be to-day reached after comparing steel and iron as materials for boiler-construction, and in view of ex- perience to date in their use, are fully confirmatory of the as- sertion of the late Mr. A. L. Holley, written a generation ago :* 41 It appears extremely probable that this material " (steel) " will gradually come into exclusive service, not only increasing the safety and decreasing the repair expense of boilers, but pro- moting the economy of steam generation and of railway work- ing generally." 40. The Characteristics of Iron Plate used in boiler- making must all be in accordance with the requirements already stated. A number of different qualities of both iron and steel are sent into the market for use in boiler-construction. Of these the makes and qualities of iron have been long well settled ; but the best qualities and compositions of steel are not as well established. No hard steels, however, are classed as boiler-steels. Good Boiler-plate is commonly assumed to be made of * American and European R ilway Practice, p. 29. MATERIALS STRENGTH OF THE STRUCTURE. 95 " charcoal iron," i.e., of iron made from pig-iron produced in the charcoal blast-furnace, no other fuel than wood-charcoal being used. The scarcity of charcoal and the cost of such irons is gradually making it more and more difficult to secure them. American boiler-plate is classed by the following-named brands : " Charcoal No. I iron" (C. No. i) is made entirely of char- coal iron ; it has a tenacity exceeding 40,000 pounds per square inch (2812 kgs. per sq. cm.), is hard, but not very ductile, and is never used when flanging or considerable change of form is required, as it is apt to break at the bend. When reheated and reworked to form what is called " charcoal No. I reheated iron" (C. No. I, R. H.) it becomes still harder, and is found to wear well in fireboxes, but is still less well fitted than before for flanging and working, on account of its increased brittleness. " Charcoal Hammered No. i Shell-iron" (C. H. No. I, S.) is a better worked iron than C. No. i ; but it is not always ham- mered. It is stronger, having a tenacity of 50,000 to 55,000 pounds per square inch (3515 kgs. per sq. cm. to 3838) in the direction of the fibre, and seventy-five or eighty per cent of this amount across the grain. This grade is not usually flanged, but may be bent if handled with care, and if the radius of curvature is made sufficiently great ; it is sold princi- pally for use in the shells of boilers. A better quality known as " flange-iron" (C. H. No. I, F.) is much more ductile, and may be worked into flanged sheets ; it is nearly equally strong in both directions, and has about the tenacity of the preceding. A still harder grade of hammered iron is intended for fireboxes mainly (C. H. No. i,F. B.), and especially for flue-sheets, which are flanged to receive the flues; and a still better grade (C. H. No. i, F. F. B.) called "charcoal hammered No. i, flange fire- box" iron, extra firebox, or, sometimes, best firebox, is made, which is more generally considered best for this use. All the grades of charcoal-irons have been made principally in Pennsylvania. " Shell" boiler-plate has often, if not generally, an outer skin of charcoal-iron, the " pile" from which it is rolled being composed of other irons, and covered top and bottom with g6 THE STEAM-BOILER. pieces of charcoal-iron. Although distinctively made for the shell of the boiler, the best makers usually prefer to use better grades for that purpose. "Refined" Iron is used for miscellaneous purposes when strength and toughness are not specially demanded, and where no risks are involved. It is not intended for boiler-making; it is made directly from the pig-iron. " Tank" iron is a still cheaper grade, used only for the most unimportant purposes. Neither of these grades should be used in boilers, or in any structure of great magnitude or value. The best British boiler and smith's irons are made in York- shire, the best known in the United States being those from the Low Moor, the Bowling, and the Farnley works, and sold in the trade as "best Yorkshire" irons. 41. The Manufacture of Boiler-plate, iron or steel, is not essentially different in method from the making of other iron and steel " uses." Iron boiler-plate is made from puddled or scrap iron, the process of puddling being always that which is resorted to in the reduction of the carbon and the production of the wrought-iron from the cast. In the rolling of plates, the wrought-iron, in bars, slabs, or miscellaneous scrap, is formed into " piles" of the proper size and form, which, after being heated to a full welding temperature, are passed through a heavy roll-train of sufficient size and pow r er to weld the con- stituent pieces into a comparatively solid mass, and to reduce that mass to the desired thickness. The pile is made of such size and shape as may be found to give the proper form and dimensions of sheet. Steel plate is oftenest produced by the Siemens-Martin process of reduction of cast with wrought iron of selected qual- ities, in the " open-hearth" or Siemens regenerative furnace, securing freedom from cinder by stirring, and from oxide by the addition of manganese in the form either of spiegeleisen for hard or of ferro-manganese for soft steels, and then, while still very fluid, tapping into the ingot-mould, whence the ingot, when sufficiently cooled, is taken to be rolled into plate. An intermediate reheating of the ingot, or a period of " soaking" in hot " soaking-pits," is very generally found advisable to secure MATERIALS-STRENGTH OF THE STRUCTURE. 97 a comparative uniformity of temperature throughout the ingot, in order that it may be successfully rolled. The Bessemer process produces "steel," or more correctly " ingot-iron," boiler-plate by a very similar scries of chemical operations; but it usually deals with larger masses, and fur- nishes, as a rule, harder steels. The rolling of steel demands the use of more powerful roll-trains than are needed in rolling iron. Comparing the two processes, it is seen that the wrought- iron plate must necessarily retain some of the slag which came into it from the puddle-ball, and that it must be liable to de- fects in welding where the several pieces of which the pile is composed come together, especially should those surfaces be covered, as is often the case, with a heavy coating of oxide. Iron plate must thus always exhibit some defect of homo- geneousness, and may be seriously defective in consequence of " lamination" produced as just described. On the other hand, steel, whether made by the crucible, the Bessemer, or the Siemens-Martin process, is always very uniform in texture, and is usually so in composition. The molten mass allows all slag and oxide to rise to its surface, and thus the fibrous and lami- nar character of iron is avoided, while the subsequent processes- do not involve necessity of welding part to part. It thus hap- pens that while iron boiler-plate is a mass of heterogeneous constituent elements, and liable to a thousand defects, steel is equally remarkable for its unity, homogeneousness, and re- liability. When an iron surface, parallel to the line of direction of rolling of plates, or of drawing down of pieces made or shaped under the hammer, is etched, it exhibits plainly the lines of " fibre" produced by the drawing out of the cinder originally present in the puddle-ball, and reveals any defective weld or the presence of any mass of foreign material. When a cross- section is made, as in the cases exhibited in the preceding figures, the character of the piling is shown, and also that of the workmanship. In these examples, which are reduced to one half the size of the originals, Fig. 56 is a section so etched of an iron locomotive axle, and Fig. 57 of a steel axle of similar 7 9 8 THE STEAM-BOILER. size and design. The beautiful homogeneousness of good steel is exhibited by the almost perfect uniformity of the color and texture of the surface; while the irregularity both of color and structure of the other illustration reveals plainly the reasons for the variable wearing quality and the inevitable uncertainty of strength which must always attend the use of forged iron, and especially when made of " scrap." It is evidently hope- FIG. 56. LOCOMOTIVE AXLE " SPECIAL" IRON. FIG. 57. LOCOMOTIVE AXLE STEEL. less to secure perfect uniformity of structure, texture, and strength, or even to obtain soundness, where such great num- bers of welds are to be made, and where so much impure and foreign material is distributed, hap-hazard, through the mass. 42. The Methods of Test of iron and steel, relied upon to reveal the properties and quality of the metal, are becoming well understood and standardized, and are universally practised in all important work by experienced and skilful engineers. Testing Machines are used for testing small sections and pieces of moderate length. They are usually built by manu- facturers who make a business of supplying them to engineers and other purchasers, and are generally made of several stand- ard sizes. The machine is frequently fitted up to test both longitudinally and transversely ; although the tests generally made are in but one direction. The Author has been accus- tomed to keep in use a machine specially intended to test in MATERIALS-STRENGTH OF THE STRUCTURE. 99 tension and compression, and also separate machines for trans- verse and torsional tests. Tension-machine is shown in Fig. 58: it consists of two strong cast-iron columns, secured to a massive bed-frame of the same material; above these columns is fastened a heavy cross-piece, also of cast-iron, containing two sockets, in which rest the knife-edges of a large scale-beam. The upper chuck is suspended by eye-rods from knife-edges. All the knife-edges are tempered steel, and the sockets and FIG. 58. TENSION TESTING-MACHINE. eyes are lined with the same material, thus reducing friction to a minimum. The load is applied by means of a screw, or by the hydraulic press, with a fixed plunger and movable cylinder. The stress to which the test-piece is subjected is measured by means of suspended weights and a sliding poise. The speci- men is secured in the chucks either by wedge-jaws or bored chucks. The extensions are measured by means of an instrument (Fig. 59) in which contact is indicated by an " electric contact apparatus." This instrument consists of two accurately made 100 THE STEAM-BOILER. micrometer screws, working snugly in nuts secured in a frame which is fastened to the head of the specimen by a screw clamp. It is so shaped that the mi- crometer screws run parallel to and equidistant from the neck of the spec- imen on opposite sides. A similar frame is clamped to the lower head of 'the specimen, and from it project two insulated metallic points, each opposite one of the micrometer screws. Elec- tric connection is made between the c two insulated points and one pole of a voltaic cell, and also between the mi- crometer screws and the other pole. As soon as one of the micrometer screws is brought in contact with the opposite insulated point a current is FIG. 59. MEASURING INSTRUMENT. 1 . , r i 1 established, which fact is immediately revealed by the stroke of an electric bell placed in the circuit. The pitch of the screws is 0.02 of an inch (0.508 mm.), and their heads are divided into 200 equal parts ; hence a rotary advance of one division on the screw-head produces a linear advance of one ten-thousandth (o.oooi) of an inch (0.00254 mm.). A vertical scale, divided into fiftieths of an inch (0.508 mm.), is fastened to the frame of the instrument, set very close to each screw-head and parallel to the axis of the screw; these serve to mark the starting of the former, and also to indicate the number of revolutions made. By means of this double in- strument the extensions can be measured with great certainty and precision, and irregularities in the structure of the material, causing one side of the specimen" to stretch more rapidly than the other, do not diminish the accuracy of the measurements, since half the sum of the extensions indicated by the two screws is always the true extension caused by the respective loads. The use of the hydraulic press is occasionally found to bring with it some disadvantages. The leakage of the press or of the pump is itself objectionable, and, where leakage occurs, it is difficult to retain the stress at a fixed amount during the time MATERIALS STRENGTH OF THE STRUCTURE. IOI required in the measurement of extensions. In such cases ab- solute rigidity in the machine is important, and the stress should be applied by mechanism, which usually consists of a train of gearing operated by hand or by power transmitted from some prime mover, and itself operating a pulling or compressing screw, as in Fig. 56. The "Autographic" Testing-Machine devised by the Author is used where it is desired to obtain a knowledge of the general character of the metal, including its elasticity and resilience, and the method of variation of its normal series of elastic limits, and where a permanent graphical record is found useful. It is shown in the accompanying figure. Fig. 61 is a perspective view of this machine. It consists of two A-shaped frames firmly mounted on a heavy bed-plate. The frames are secured to each other by cross-bolts. Near the top of each of these frames are spindles, each of which has a head with a slot or jaw to receive and hold the square heads of the specimens. The two spindles are not connected to each other in any way, excepting by the specimen which is placed in the jaws to be tested. To one spindle a long arm is attached, which carries a heavy weight at the lower end. The other has a worm-gear wheel attached to its outer end. This wheel is driven by a worm on the shaft which is turned by a hand crank. When a specimen is placed in the two jaws, and the spindle is turned by the worm-gear, the effect is to twist the specimen which would turn the spindle ; but in order to do this the weight on the end of the arm must be swung in the direction in which the specimen is twisted. But the farther the arm is moved from a vertical position, the greater will be the resistance of the weight to the turning of the shaft, while the movement of the arm and weight is effected by the force exerted through the specimen so that the position of the arm and weight will at all times give a measure of the torsional stress, which is exerted on the specimen by the one spindle, and transmitted by the former to the other spindle. But as this torsional stress which is exerted on the specimen is increased, it will at once commence to "give way," or be twisted more or less by the stress according to the quality of 102 THE STEAM-BOILER. the material. In making such torsional tests, it is essential that we should know how much the specimen was twisted, as the strains to which it was subjected were increased. If we could procure a record of this, it would be an indication of the capac- ity of the material to resist such stresses, or, in other words, of its quality. The testing-machine which has been described was designed by the Author for this purpose. The record is made in the following way : To one spindle a cylindrical drum is attached, which is covered with a suitable sheet of paper. To the pendulum, is attached a pencil, the point of which bears on the paper on the drum. Now supposing that the specimen in the machine should offer no resistance, but should merely twist, the pencil would then remain stationary, and as the drum is revolved the pencil would trace a straight line on i ._..;* **" ^ >s / *, ( V_ FIG. 60. TEST-PIECE. the paper, the length of which line would measure the amount by which the specimen was twisted. If, on the other hand, a specimen be supposed to resist and to twist simultaneously, as is always the case, then it will presently be seen that the spindle would be turned, and the arm with the weight would be moved from a vertical position a distance proportional to the strain resisted by the specimen. The pencil-holder, being attached to the arm, would move with it. As explained be- fore, the distance which the arm and its weight are moved from a vertical position indicates the stress on the specimen. Next, in order to make a record of this distance, a " guide- curve" is attached to the frame of the machine, so that when the pencil-holder is moved out of the vertical position the pen- cil is moved toward the left by the guide-curve, which is of such a form that the lateral movement which it gives to the pencil is proportional to the moment of the weight on the end MATERIALS-STRENGTH OF THE STRUCTURE. 103 of the arm. Now suppose, if such a thing were possible, that a specimen were tested which would not " give" or twist at all : in that case the spindles, the drum, and the pencil would turn together, or their movements would be simultaneous, so that the pencil would draw a vertical line along the paper. But FIG. 61. AUTOGRAPHIC MACHINE. there is no material known which would not yield or twist more or less, so that the pencil will always draw some form of curved line, which indicates the quality of the material tested. The test-pieces are held in a central position in the jaws by lathe " centres," which are placed in suitable holes drilled in the IO4 THE STEAM-BOILER. spindles for that purpose. The specimen is then held securely by wedges. In the diagrams each inch of ordinate denotes 100 foot-pounds of moment transmitted through the test-piece, and each inch of abscissa indicates 10 degrees of torsion. The fric- tion of the machine is not recorded, but is determined when the machine is standardized, and is added in calculating the results. By the use of this machine the metal tested is compelled to tell its own story, and to give a permanent record and graphical representation of its strength, elasticity, and every other quality which is brought into play during its test, and thus to exhibit all its characteristic peculiarities. The figures on page 105 are derived from a test by tension, as made for the Author. On page 106 is given the record of a test of steel made by the Ordnance Department, U. S. A. 43. Tests of Strength and Ductility of irons and steels have now been made in such numbers, and with such a variety of composition, that the engineer designing or constructing boilers need have no doubt in regard to the character of the metal to be incorporated in the structure. The mean of a considerable number of experiments on ex- cellent American iron boiler-plate, made under the eye of the Author, gave a tenacity of 54,000 pounds per square inch (3795.2 kilogs. per sq. cm.) with a variation of 9 percent; flange-iron averaged but 42,000 pounds (2952.6 kgs. per sq. cm.) with a variation of nearly 40 per cent ; the highest-priced, and presumably best, plate in the market averaged very nearly 60,000 pounds (4218 kgs.), varying 14 per cent ; and com- mon tank-iron showed practically the same tenacity and varia- tion as the flange-iron, and less ductility. Thoroughly good Pennsylvania plate, in other experiments, gave, for all good grades, tenacities not ranging much from 55,000 pounds per square inch (3866.5 kilogs. per sq. cm.), and an elastic limit at 60 per cent of the ultimate strength. Such tenacity is not usually to be expected when buying in the market, and it is very common, when designing boilers the material of which is not prescribed, for the designer to assume that its tenacity may not exceed 40,000 pounds (2812 kgs.). On the other hand, a contract and specification prescribing careful test may some- MATERIALS STRENGTH OF THE STRUCTURE. 10$ TEST OF WROUGHT-IRON; LENGTH 8" (19.32 cm.), DIAM. 0.798" (2.03 cm.). LOADS. MICROMETER READINGS. EXTENSIONS. SETS. Actual. Per sq. in. Actual. Per cent. Actual. Per cent. 150 2,OOO 4,000 6,000 8,000 10,000 150 11,000 12,000 150 I3,OOO 13,500 14,000 150 I5,OOO ~ 150 17,000 150 I9,OOO 150 21,000 150 22.000 150 22,500 23,000 23,500 23,750 21,800 .6600 .6628 .6637 .6646 . 6606 .6630 .6600 .6639 .6700 .6603 6715 .6728 .7242 .7133 7535 .7417 .8474 .8326 .9720 .9562 . 1710 .1524 .3303 .3102 4575 .5610 .7646 9- 9- 7913 .7910 .7922 7930 .7946 .7948 .7914 7951 7953 .7915 .7967 7959 .8424 8351 .8712 .8632 .9618 .9518 1.0856 1.0732 .2811 .2663 .4381 .4212 5441 1.6670 1.8693 \1 54 4,000 8,000 12,000 16,000 20,000 22.000 24,000 .0013 .0023 0035 .0050 .0058 .0064 .0070 .0080 .0087 0577 !o867 .1790 .3032 .5004 '.6586 7752 .8884 1.0913 1.4700 1.5400 .016 .029 .044 .063 073 .030 037 .100 .109 .721 1.084 2.238 3.790 6.255 8.233 9.690 11.105 13.841 18.375 19.250 .0001 .0003 [0486 .0763 !i666 .2391 4337 .6401 .001 .00 4 '!6o8 ". 9 60 2.083 3.613 6.043 8.001 26,000 27,000 28,000 30,000 34.000 38,000 42,000 44,000 45,000 46,000 47,000 47,500 43,600 Lbs. 13,500 ELASTIC LIMIT. ACTUAL. Lbs. per Kgs. per sq. in. sq. cm. 6,140 27,000 1,898 Kgs. BREAKING LOAD. ORIGINAL SECT. FRACTURED SECT. Lbs. per Kgs. per Lbs. per Kgs. per sq. in. sq. cm. sq. in. sq. cm. 47,500 3,340 69,840 4,9 10 Ultimate Elongation, per cent, of length = 19^. Reduction of Area, per cent, = 31-99 Modulus of Elasticity = 24,365,000 Ibs. on sq. in. Modulus of Elasticity = 1,712,860 kilogrammes on sq. cm. FINAL DIMENSIONS. Length = 9". 54 Diameter = o".6s8 io6 THE STEAM-BOILER. EXTENSION. RESTORATION, AND PERMANENT SET OF A SOLID CYLINDER OF STEEL * INCHES LONG (BETWEEN SHOULDERS) AND 0.622 INCH DIAMETER. ? FAKEN FROM BREECH-RECEIVER FOR n-INCH BREECH- LOADING RIFLE. Weight per tquare inch of Section. Extension per inch in Length. Successive R M , nrafi , , Successive Extension *g r ch Restoration per inch in P L e S per mch u in Length. n Len S lh - in Length. Permanent Set per inch in Length. Successive Permanent Set per inch in Length. Pounds. Inches. Inches. Inches. Inches. Inches, Inches. 1,000 O.OOOOO O.OOOOO O.OOOOO O.OOOOO 0.00000 0.00000 2,000 .00000 . ooooo . ooooo . ooooo .ooooo .ooooo 3,000 .00000 . ooooo . ooooo . ooooo .ooooo .00000 4,000 .00033 .00033 .00033 -00033 .ooooo .ooooo 5,000 .00033 .00000 .00033 .ooooo .00000 .00000 6,000 .00033 .ooooo . 00033 ooooo .ooooo .ooooo 7,000 .00033 .00000 .00033 .ooooo .00000 .00000 8,000 .00033 .ooooo .00033 .ooooo .ooooo .ooooo 9,000 .00033 .00000 .00033 .ooooo .00000 .00000 10,000 .00033 .ooooo .00033 .ooooo .ooooo .ooooo 11,000 .00033 .00000 .00033 .ooooo .00000 .00000 12,000 .00033 .00000 . 00033 . ooooo .ooooo .ooooo 13,000 .00033 .00000 .00033 .ooooo .00000 .00000 14,000 .00033 .00000 .00033 .ooooo .00000 .ooooo 15,000 .00033 .ooooo .00033 .ooooo .00000 .00000 16,000 .00067 .00034 .00067 .00034 .ooooo .ooooo 17,000 .00067 .ooooo .00067 .ooooo .00000 .00000 18,000 .00067 .ooooo .00067 .ooooo .ooooo .ooooo 19,000 .00133 .00066 .00100 .00033 .00033 .00033 20,000 .00233 .00100 .00100 .00000 .00133 .00100 21,000 .00300 .00067 .00100 .ooooo .00200 .00067 22.000 .00400 .00100 .00100 .ooooo .00300 .00100 23,000 .00467 .00067 .00100 .ooooo .00365 .00067 24,000 00533 .00066 .00100 .ooooo 00433 .00066 25,000 .00633 .00100 .00:133 .00033 .00500 .00067 26,000 .00700 .00067 .00133 .ooooo .00567 .00067 27.000 .00767 .00067 .00133 .ooooo .00633 .00066 28,000 .00900 .00133 .00100 .00033 .00800 .00167 29,000 .00967 .00067 .00100 .00000 .00867 . 00067 30,000 .01067 .00100 .00133 .00033 .00933 .00066 31,000 .01200 .00133 -00133 .ooooo .01067 .00134 32,000 .01300 .00100 .00167 .00054 .01133 .00066 33>ooo 01433 .00133 .00167 .ooooo .01267 .00134 34,000 .01567 .00134 .00133 .00034 01433 .00166 35iOOo .01700 .00133 .00133 .00000 .01567 .00134 36,010 .01800 .00100 .00133 .00000 .01667 .00100 37,000 .01967 .00167 .00133 .00000 .0.833 .00166 38,000 .02133 .00166 .00167 .00034 .01967 .00134 39,000 02433 .00300 .00167 .00000 .02267 .00300 40,000 .02567 .00134 .00167 .ooooo .02400 .00133 41,000 02733 .00166 .00167 .00000 .02567 .00167 42,000 .02867 .00134 .00167 .00000 .02700 .00133 43,000 03033 .00166 .00200 .00033 .02833 .00133 44,000 .03300 .00267 .00233 .00033 .03067 .00234 45,000 03433 .00133 .00200 .00033 .03233 .00166 46,000 .03900 .00467 .00233 .00033 .03667 .00434 47,000 .04167 .00267 .02223 .ooooo 03933 .00266 48,000 .04367 .00200 .00233 .00000 04133 . OO2OO 49,000 .04700 .00333 .00267 .00034 04433 .00300 50,000 .05100 .00400 .00200 .00067 .04900 .00467 51,000 5533 .00433 .00300 .00100 05233 .00333 52,000 .06067 00534 .00233 .00067 05833 . 00600 53,000 .06667 .00600 . 00300 .00067 .06367 534 54,000 .06897 .00200 .00233 .00067 .06633 .00266 55,ooo .07867 .01000 .00300 .00067 07567 .00934 56,000 08333 .00466 .00300 .00000 .08033 . 00466 57^000 .09500 .01167 .00300 .ooooo .OQ2OO .01167 $8,000 .10233 00733 00333 .00033 .09900 .00700 59,000 .11800 .01567 00333 .ooooo .11467 .01567 60,000 .13700 .01900 .00367 .00034 .13333 .01866 61,000 .!6 9 oo .03200 0.00400 0.00033 0.16500 0.03167 62.000 0.30367 0.13467 (*) (*) (*) (*) Tensile Strength per sq. in Ibs. 62,000 Elastic limit Ibs. 19.000 Extension per in. at elastic limit in. 0.00133 Extension per in. at rupture in. 0.30367 * Specimen broke. GENERAL SUMMARY. Original area of cross-section. . . sq. in. 0.3038 Area after rupture sq. in. 0.1611 Position of rupture % from shoulder. Character of fracture Fi brous. MATERIALS STRENGTH OF THE STRUCTURE. IO/ times secure iron, if thin, capable of sustaining 60,000 pounds per square inch (42 1 8 kilogs. per sq. cm.). A fair contract figure, and one that may be assumed in designing when the iron is to be thus selected and tested, would be considered to be 55,000 pounds (3867 kilogs.). Steel boiler-plate of high tenacity is so certain to involve in its use risk of cracking, either in the process of construction, or later, after exposure to variations of temperature, and to alter so seriously and so uncertainly in all its physical properties, that specifications usually prescribe that it shall not exceed 60,000 pounds (4218 kilogs.) tenacity, and in some cases the figure is put even lower. When first introduced, tenacities much greater were allowed for steels, and great risks, and often serious accidents and losses of life and property, were the conse- quence. All good boiler-irons should be expected to stretch at least 20 per cent of the length of the test-piece, the latter being made at least four or five, and better eight or ten, diameters, or breadths in length. The best irons stretch 25 per cent, and the best steels even more. Thick plates have less tenacity and less ductility than thin. The " bending test " is one which only the best of irons and the softer steels will bear. The strip cut from the sheet for test, the " coupon" as it is called, if of less than f inch thick- ness, should bend completely over and be hammered flat upon itseM, as in the figure. FIG. 62. BENDING TEST. Steels subjected to the " temper test," by heating the sam- ple red-hot and quenching in cold water, should then, if of good quality for boilers, be capable of successfully passing the bending test ; but it is not usually demanded that it shall close down ffat. If it bends to a circle of a diameter less than three times its own thickness, it is accepted. Steels subjected to the " drifting test " are commonly drilled with a f-inch drill, and the hole drifted out as large as possible. If it is enlarged to 108 THE STEAM-BOILER. double its original diameter, the metal is usually accepted, but it is sometimes demanded that it shall bear extension to two inches in diameter, as for example at Crewe, on the Lon- don and Northwestern Railway of Great Britain. 44. Specifications of Quality, as well as of kind and form, of materials proposed to be used in steam-boiler construction are so drawn as to secure not only an understanding on the part of the maker or vender of the exact nature of the intended provisions, but also a means of certainly determining whether those specifications and the contract are fully complied with. Wrought-iron and steel, as has been seen, are very variable in strength and other qualities. For small iron parts, a tenacity of 55,OOO to 60,000 pounds per square inch (3867 to 4218 kilo- grammes per square centimetre) is usually called for ; but the strength of plate or of large masses is rarely three fourths as great. The specification usually calls for " iron of the best quality," tough, of a definite tenacity, fibrous, free from cinder- streaks, flaws, lamination or cracks, uniform in quality, and with a prescribed elastic limit, and often a stated modulus of elasticity. Even the method of piling, heating, and rolling or hammering is specified. As has been shown fully in the preceding chapters, the di- mensions must be determined after a careful consideration of the character and the method of application of the load, as well as of its magnitude, and allowance must be made by the engineer for the effect of heat or cold, of repeated heating in the process of manufacture, for the rate of set under load, for the rapidity of its application, or for the effect of repeated or reversed strains. The differences in the behavior of the several kinds of iron or steel under the given directions must be considered in pro- portioning parts. Thus unannealed iron or " low" steel will be chosen for parts exposed to steady and heavy loads ; the use of annealed metal will be restricted to cases in which the primary requisite is softness or malleability ; steel containing about O.8 per cent carbon will be given the preference for parts exposed to moderate blows and shocks which are not expected to ex- ceed the elastic resilience of the piece ; tough, ductile metal, MATERIALS STRENGTH OF THE STRUCTURE. 109 preferably " ingot iron," will be chosen for parts exposed to shocks capable of producing great local or general distortion. " Wohler's Law" dictates the adoption of increased factors of safety, or of some equivalent device, as Launhardt's formula, when variable loads are carried. Thus the engineer is com- pelled to make a specification, in very important work, which shall prescribe all the qualities of materials and exactly the proportions of parts needed to make his work safe for an in- definite period. Steel has such a wide range of quality that few difficulties are met with in its introduction into any department of con- struction. In boiler-work, however, it must be kept low in car- bon, and therefore in tenacity ; and in machinery and bridge work, also, its composition must be carefully determined upon, and as exactly specified. The following are good specifications for boiler-work: Steel Sheets. Grain To be uniform throughout, of a fine close texture. Workmanship Sheets to be of uniform thick- ness, smooth finish, and sheared closely to size ordered. Tensile Strength To be 60,000 pounds to square inch for firebox sheets, and 55,000 pounds for shell sheets. Working Test A piece from each sheet to be heated to a dark cherry red, plung- ed into water at 60, and bent double, cold, under the hammer; such piece to show no flaw after doubling. Iron Sheets. Grain To be uniform throughout, showing a homogeneous metal with no layers or seams. Workmanship Sheets to be of uniform thickness, smooth finish, and sheared closely to size ordered. Tensile Strength To be 60,000 pounds to the square inch for firebox sheets, and 55,OOO pounds for shell sheets. Working Test A piece from each sheet to be bent cold to a right angle, showing no fracture. A piece bent double, hot, to show no flaking or fracture. Specifications for Boiler Tubes. Size Locomotive tubes to be 12 feet long and 2 inches diameter; to be of iron, No. 1 1 gauge. Quality of Metal When flattened under the ham- mer to show tough fibrous grain ; when polished and etched with acid to show uniform metal and a close weld. Working Tests When expanded and beaded into the flue-sheet to show HO THE STEAM-BOILER. no flaws; to stand " swaging down" hot without flakes or seams. The following are specifications for Boiler and Firebox Steel: (1) A careful examination will be made of every sheet, and none will be received that show mechanical defects. (2) A test strip from each sheet, tested lengthwise. (3) Plate will not be passed for acceptance when of strength of less than 50,000 or greater than 65,000 pounds per square inch, nor if the elongation falls below twenty-five per cent. (4) Should any sheets develop defects in working they will be rejected. (5) Manufacturers must send one test strip for each sheet (this strip must accompany the sheet in every case), both sheet and strip being properly stamped with the marks designated by the company, and also lettered with white lead, to facilitate marking. The U. S. Board of Supervising Inspectors of Steam-vessels restrict the stress on boiler stays and braces to 6000 pounds per square inch (4218 kilogrammes per square centimetre). For shells of boilers, a factor of safety of 6 is permitted in design- ing. The hydrostatic pressure applied in testing is one half greater than the steam-pressure allowed. All plates must be stamped by the maker with the tenacity, as determined by test, at the four corners and in the middle. The elongation is not noted, as the form of United States standard test-piece is unfitted to determine it. The contraction of area of section at fracture must be 0.15 when the tenacity is 45,000 pounds and one per cent more for each additional 1000 pounds. Hot-short, or red-short, and cold-short irons are detected by the forge tests ; the former is often found to be an excellent quality of iron if it can be worked into shape, as it is, when cold, tough and strong. Specially high qualities are rarely economi- cal, as they usually cost too much to make the difference worth what is paid for it. Shapes difficult to make or roll are usually weaker than others. Mills will usually supply " pattern iron," charging a little extra for it ; but it will often be found economi- cal to order them, if such shapes are necessary. In designing, MATERIALS STRENGTH OF THE STRUCTURE. Ill however, it is well to avoid the introduction of peculiar shapes, if possible. All wrought-iron, if cut into testing strips one and a half inches in width, should be capable of resisting without signs of fracture, bending cold by blows of a hammer, until the ends of the strip form a right angle with each other, the inner radius of the curve of bending being not more than twice the thickness of the piece tested. The hammering should be only on the ex- tremities of the specimens, and never where the flexion is tak- ing place. The bending should stop when the first crack ap- pears. All tension tests should be made on a standard test-piece of one and a half inches in width, and from one quarter to three quarters of an inch in thickness, planed down on both edges equally so as to reduce the width to one inch for a length of eight inches. Whenever practicable, the two flat sides of the piece should be left as they come from the rolls. In all other cases both sides of the test-piece are planed off. In making tests the stresses should be applied regularly, at the rate of about one ton per square inch in fifteen seconds of time. All plates, angles, etc., which are to be bent in the manu- facture should, in addition to the above requirements, be capable of bending sharply to a right angle at a working test, without showing any signs of fracture. All rivet-iron should be tough and soft, and pieces of the full diameter of the rivet should be capable of bending until the sides are in close contact, without showing fracture. All workmanship should be first-class; all abutting surfaces planed or turned, so as to insure even bearing, taking light cuts so as not to injure the end fibres of the piece, and protected by white lead and tallow. Pieces where abutting should be brought into close and forcible contact by the use of clamps or other approved means before being riveted together. Rivet-holes should be carefully spaced and punched, and in all cases reamed to fit, where they do not come truly and accurately opposite, without the aid of drift-pins. Rivets should completely fill the holes, and have full heads, and be countersunk when so required. The following are specifications originally issued by the 112 THE STEAM-BOILER. United States Navy Department, which indicate the relation of variation of tenacity to the corresponding change in ductil- ity where the quantity of carbon in steel is altered : TENACITY. Lbs. per sq. in. Kilos, per sq. cm. 6o,OOO 4218 70,000 4921 80,000 5624 90,000 6327 EXTENSION. Per cent. 25 23 12 A cold-bending test is demanded thus: Bend the strip over a mandrel of a diameter i times the thickness of the plate, through an arc of 90, and no cracks must appear with the softer grades, and any cracks seen in the case of the harder steels must be insignificant. Every reputable maker stamps his iron, not only with the figures indicating the tenacity, as required by law, but also, in the case of thoroughly good qualities, with their names. Where the brand is not found, it is assumed by the experienced en- gineer that the metal is not of such high quality as to do credit to the maker. All good plate is expected to have fair tenacity and high ductility, and good flange-iron should not deteriorate appreciably in working. 45. Choice of Quality of Metal for the Various Parts of a boiler or other structure is made with the greatest care by the designer and by the constructor. The furnace, exposed as it is to variations of temperature, to the corrosive effect of hot gases, and to the mechanical wearing action of the cinder and coal carried by their rapidly moving currents, is made of the harder qualities of iron or steel already described. The tubes, flues, and the flue-sheets are composed of comparatively ductile material, such as may be safely shaped in accordance with the plans of the designer ; the shell may be of cheaper material ; while all stays and braces must be made of the strongest and toughest metal available. Each grade should be carefully pre- scribed, and the iron or steel proposed for use as carefully in- spected and tested before it is introduced into the structure. It is sometimes advisable to substitute copper for iron, espe- MATERIALS STRENGTH OF THE STRUCTURE. 1 13 cially in the firebox ; and in such cases sheet-copper of a tena- cious and somewhat hard quality should be adopted. This ma- terial usually has about two thirds the strength of good iron, with greater ductility and flexibility, and resists the action of the furnace gases better than iron boiler-plate. 46. The Methods of Working the materials introduced into steam-boilers are adapted very carefully, in every case, to the known requirements of each quality so used. The frequent injury of steel and of hard iron plates by punching and by too abrupt change of form have led engineers to prescribe in many cases that all steel plate shall be drilled for the insertion of rivets, and not punched, and to direct the bending of the plate over rounded edges having comparatively large radii of curvature. All wrought-iron work in boilers, when subjected to any considerable change of form, should be worked at a bright-red heat, approaching the welding temperature; steel should be handled, in such cases, at a " cherry-red " heat. Great alteration of shape, if effected at ordinary tempera- tures, should be made slowly and carefully, and it may even be well in some instances to allow intermissions in such opera- tions sufficient to permit the particles some opportunity of self-adjustment. It may be taken as a general rule in the work- ing of all materials for steam-boilers, that the methods and pro- cesses chosen should always be such as will be least likely to strain or to injure, either generally or locally, the iron or steel so used. 47. Special Precautions in Using Steel are found to be necessary to secure safe construction. Construction in steel demands more care than the making of iron boilers, and a good boiler-maker for the latter class of work is not necessarily a good worker of steel. In handling steel for boilers there should be no unnecessary local heating. If so heated, steel should always be subsequently annealed. The plates for the cylindri- cal shells of boilers should be carefully bent to shape when cold. The rivet-holes should usually be drilled, not punched, and the drilling should be done after the plates are bent to shape, and bolted together in position. The longitudinal joints in the shell are best made with double butt-strips, one 8 114 THE STEAM-BOILER. being placed inside, and the other outside, to form a " butt joint." The tests of the plate supplied on specifications, and under contracts, should be even more carefully and minutely made than with iron ; every operation must be more carefully con- ducted and supervised, and the completed boiler should be inspected and tested with the greatest possible care. If it is well made and of good material, it will be a more satisfactory construction than any iron boiler can possibly be ; a mistake in accepting and using steel ill adapted to the purpose may produce an exceedingly dangerous and unsatisfactory boiler. Steel of good quality, and well adapted for other construction, is not necessarily safe for use in steam-boilers. Many engineers would anneal every plate of steel used, whatever its apparent quality, to insure its safety in the struc- ture, and it has even been suggested that it would be well, were it practicable,, to anneal the whole boiler after completion.* Too great care cannot be taken in selecting the metal. 48. Rivets and Rivet-Iron and Steel are necessarily of especially good quality. The rivet must be strong, tough, and ductile, and capable of bearing the severest deformation at all temperatures without injury. It is customary to " head-up" rivets hot ; but medium-sized and small rivets, in some locali- ties, are worked cold, and this is the most trying test of quality possible. Rivets of less than |-inch (0.95 cm.) diameter are very commonly driven cold. Rivet-iron should, in the bar, have a tenacity approaching 60,000 pounds per square inch (4218 kgs. per sq. cm.), and should be as ductile as the very best boiler-plate when cold. The rivet should be capable of bearing the change of form incidental to its use without ex- hibiting a tendency to split; the head should not seriously harden or become brittle under the blows of the hammer ; and the contraction on cooling, after it has been htaded up, should not cause weakening by the stress incident to the strain so produced. A good iron rivet | inch (1.6 cm.) diameter can be doubled up and hammered together, cold, without exhibiting * Trans. Am. Soc. M. E., 1887, No. ccxlvi. MATERIALS STRENGTH OF THE STRUCTURE. 11$ a trace of fracture. Such a rivet, split and " etched " on the cut surfaces, shows a smoothly curved grain, uniform texture and color, and no visible sign of the presence of slag. Such a rivet, made of good rivet-steel, will show absolute uniformity of surface, and no trace even of " grain." The chemical composition of these rivet-steels should be as nearly as possible that of the best rivet-irons ; they should con- tain the least possible proportion of the hardening elements, including carbon and manganese, as well as phosphorus, and should be so pure as to readily take a surface like that of a mirror, when polished. 49. The Sizes of Rivets, their form and strength, are quite well settled by experience and by test. The rivet con- sists, as supplied by the market, of a straight or slightly tapered body, circular in section, and having a head 1.5 or 1.6 the di- ameter of the shank ; the latter is 2 to 3 or 4 per cent smaller than the hole which it is to fill, and tapers toward the end to a diameter about 0.95 that of the hole. The head is cylindri- cal, and has a thickness 0.7 or 0.75 the diameter of the body of the rivet. The length of the shank or body is 2.2$ or 2.50 times the diameter of the hole, and the latter is often equal to the double thickness of plates held together by it. When in place, the small end is driven down by hand-hammers or by the riveting machines to form a cone-shaped or hemispherical head, the sheets riveted together being thus confined by the two heads and sustained by the strength of the shank against any force tending to separate them. The principal stresses exerted on the rivet are usually shearing. The rivets, when heated, should be brought up to a full, clear red heat. A simple rule sometimes used to determine the diameter of a rivets is that of Unwin, who makes this diameter = 1.2 in which / is the thickness of the single plate or sheet. The following table is thus obtained, taking the nearest THE STEAM-BOILER. Thickness of Plate. Diameter, d, of Rivet. i ............ i = 0.50 T 6 ff ............ A = 0.56 1 ............ It = 0.68 Thickness of Plate. Diameter, rf, of Rivet. -1 0.86 i TTF O.Q4 f .... lyg- -^ i. 06 i i . . il 1.25 i ........... If = 0.80 The driven rivet is something like four or five per cent larger than the undriven. The following table gives the proportions of rivets adopted in some of the best establishments in the United States,* and the relative strength of joint secured : TABLE OF THE PROPORTIONS OF RIVETS. Thickness of plate i" iV " 7" 4" Diameter of rivet if i 1| I Diameter o rivet-hole 1| 15 Pitch single-riveting 2 aJL T ? at 2A- 3 Pitch double-riveting 4 3 3* ** Strength of single-riveted joint . . . Strength of double-riveted joint... .66 77 .64 76 . .62 75 .00 74 58 73 Plates more than y thick should never be joined with lap- joints. When it is necessary to use them a butt-joint with a double fish-plate should always be used. In recommending the above proportions we assume that the workmanship is always fair. The common proportions of rivets, as given by Unwin,f are seen in the accompanying figure ; that illustrated is of such form as will permit the formation of the conical head, the total length being about 1\ times the diameter when a double thickness of plates is to be secured together. The next figures exhibit the differ- ence in proportions of rivets for hand- riveting and for steam-riveting, as given FIG. 6 3 . by the same authority; the first figure showing two forms of head for hand-work, the second two for * Locomotive, July, 1882. f Machine Design, MATERIALS STRENGTH OF THE STRUCTURE. II? steam-riveted work : one of each pair is set in a straight hole, -1*5 N 4* H J* j FIG. 64. FIG. 65. the other in a chamfered hole. The next figure gives the pro- portions for a countersunk rivet, used in ship-building. 50. The Strength of Seams, when riveting is used, varies with the character of the metal, the method of riveting, and the quality of workmanship. A single-riveted joint has usually not far from 60 per cent of the strength of the solid sheet, a double-riveted seam 70 per cent ; and the strength may be still further in- creased by adding to the number of rows of rivets, with proper distribu- tion. The joint is so proportioned that the fracture will occur by shear- ing the rivets rather than by breaking out the edge of the sheet or tearing away the lap bodily. The lap usually extends beyond the rivet-hole about 1.5 times the diameter of the rivet. To secure maximum " efficiency" of seam, i.e., equal and maximum resistance in all directions of possible stress, it is evident that the joint must be equally liable to tear along the line of rivets, to shear the rivets, and to tear them out by pull- ing them through the lap. For a single-riveted joint there- fore, if F represent the tearing force, T the tenacity of the sheet, SS' the shearing resistance of the rivet and sheet, C its resistance to crushing,/ the "pitch," and d the diameter of the*. FIG. 66. 118 THE STEAM-BOILER. rivets, / the width of lap, and / the thickness of the sheet, we must have F= nd*S' = Cdt = (p- or, if the lap is made over strong, as above, and if crushing is not anticipated, both of which are usual conditions, and \7td*S -\-dtT I 7td*S . *~- ~rr -~~TT Where, as sometimes is the case, the joint is a butt-joint and the rivets are thus " in double shear," and the same expression serves for the case of double-riveted seams made, as with single-riveting, with a lap, but having a second line of rivets behind and reinforcing the first. Where the rivet and the plate are of the same material, or wherever the resistance to shearing and the tenacity may be taken as substantially equal, the formula 0.7854***' may be adopted, in which />, d, n, and / are, respectively, the pitch of rivets, centre to centre, the diameter of rivet, the number of parallel rows, and the thickness of sheet. The following tables represent proportions for adoption in designing, the ratio of T to C being taken for iron and steel of various qualities, as assumed by Unwin :* * See Machine Design, by W. C. Unwin. London : Longmans, Green & Co. MATERIALS STRENGTH OF THE STRUCTURE. SINGLE-RIVETING. IRON RIVETS AND PLATES. STEEL RIVETS AND PLATES. d IN INCHES. Punched Plates. Plates Drilled. Plates Punched. Plates Drilled or Punched and Annealed. Nomi- nal. Actual. Pitch/ for values of T _ C ~ 0-75 0.85 0-95 I.O 1.05 I.I5 1.25 1-35 & H 0.72 2-45 2.25 2.1 2.0 2.0 1.8 5 1.8 I.? 1 i 0.78 2-5 2-3 2.1 2.1 2.0 1.9 1.8 T -7 1 i 0.85 0.92 2.6 2-7 2.4 2.5 2.2 2-3 2.15 2.2 2.1 2.1 2.0 2.1 1-9 2.0 1.8 I. 9 -1 0.98 2.6 2.4 2-3 2.2 2.1 2.0 2.0 1.9 iS 1. 10 2.8 2.6 2.4 2.4 2-3 2.2 2.1 2.0 i it I.I? 2.9 2.7 2.5 2.5 2.4 2.25 2.2 2.15 i i* 1.30 3-1 2.9 2.7 2.6 2.6 2.45 2.4 2-3 DOUBLE-RIVETING. T f= 0.215^7-, ..,.-... (4) = 10,000-7. . , ..... (5) a \ or, for iron, For steel, /=ii,5oo| ....... - (6) For cast-iron, p = 4,000^, ........ (7) when / is the thickness, d^ the diameter, both in inches, / the pressure and T the tenacity, both in pounds per square inch. * Inst. C. E., vol. liii., Abstracts. London, 1877-78. MATERIALS STRENGTH OF THE STRUCTURE. For spherical ends, (8) where a is 108,000 for wrought-iron, 125,000 for steel, 45,000 for cast-iron, and v is the versed sine or rise of the head. Lloyd's Rule for cylindrical shells of boilers is abt in which a is a constant, 155 to 200 for iron and 200 to 260 for steel, b the percentage of strength of solid sheet retained at the joint, / is the thickness of the plate, and d the diameter of the shell. The value of b is thus reckoned (n = number of rows of rivets) : = IOCP- A na -, for the plate ; b = 100 *, for rivets in punched holes ; nu b = 9OTTi f r rivets in drilled holes. The least of these values is taken. Here/, is the pitch of rivets, d l is their diameter, #, is the area of the rivet-section. When in double-shear, i. 75#, is taken for #,. The factor of safety is taken at 6, and boilers are tested by water-pressure up to 2p. The iron is expected to have a tenacity of at least 21 tons per square inch ; steel must bear 26 tons (3307 to 4095 kilogs. per sq. cm.). Welds are found, when well made, to carry 75 to 85 per cent of the strength of the sheet. Steam-pipe is usually made with an enormous excess of 132 THE STEAM-BOILER. strength to meet accidental stresses, such as those due to motion of water within them. The Author has tested pipes broken by " water-hammer," as the engineer calls it, to 1000 pounds per square inch (70 kilogrammes per sq. cm.) after it had been thus cracked in regular work in a long line, while the steam-pressure was less than 100 pounds (7 kilogs. per sq. cm.). They had all been previously tested to about one third this pressure. Cylinders of cast-iron, for steam^generators or for steam- engines, are usually given a thickness greatly in excess of that demanded to safely resist the steam-pressure ; often, according to Haswell, dp i for vertical cylinders, where d is the internal diameter, and for horizontal cylinders of considerable size. In metric measures, kilogrammes and centimetres, these formulas become dp i * ^ -f > nearl y ...... (13) IfV, is the external and r, the internal radius, T the tena- city of the metal, t its thickness, and/ the intensity of the in- ternal pressure, we have, for the thin cylinder, as an equation for equilibrium, (14) MATERIALS STRENGTH OF THE STRUCTURE. 133 and Tt 05) t=r l -r t ='-f; (,6) For the thick cylinder, however, the resistance at any inter- nal annulus of the cylinder is less than T. Thick Cylinders, technically so called, are those which are of such thickness that the mean resistance falls considerably below the full tenacity of the metal, as exhibited in thin cylin- ders, in low-pressure steam-boiler shells, for example. Such cylinders are seen in the " hydraulic" press, and in ordnance. Barlow* assumes the area of section unchanged by stress, although the annulus is thinned somewhat by linear extension. If this is the fact, as the tension on any elementary ring must vary as the extension of the ring within the elastic limit, the stress in such element will be proportional to the reciprocal of the square of its radius, i.e., it will be 08) and, taking the total resistance as/'r,, when p' is the internal fluid pressure, since the maximum stress at the inner radius is T, that on the inner elementary annulus is Tdx, and on any 7> 2 other annulus rdx\ while the total resistance will be, on either side the cylinder, Strength of Materials, 1867, p. 118. 134 THE STEAM-BOILER. The maximum stress is at the interior, and may be equal, as taken above, to the tenacity, T, of the metal ; then (20) I* If and the thickness while the ratio of the radii r 1 _T L . *(r-A)_ _7L_ r.-A A T=A' Lamfs Formula, which is more generally accepted, and which is adopted by Rankine, gives smaller and more exact values than that of Barlow. In the above, no allowance is made for the compressive action of the internal expanding force upon the metal of the ring. The effect of the latter action is to make the intensity of pressure at any ring less than before by a constant quantity, a P oo p - b, and the tension by which the ring resists that pressure greater, When r r^p Q', when r r v p /, ; then /, = b, and o -- -, b ; " " MATERIALS STRENGTH OF THE STRUCTURE. 135 and the maximum possible stress on the inner ring is ~^y| a _ r ] ' (23) and the ratio of inner and outer radii is 05. Of these two formulas, the first gives the larger and conse- quently safer results, and, in the absence of certain knowledge of the distribution of pressure within the walls of the cylinder, is perhaps best. For thick spheres, Lame's formula becomes Clark's formula* is more recent than the preceding. It is assumed that the expansion of concentric rings into which the cylinder may be conceived to be divided is inversely as their radii, and that the curve of stress will become parabolic if so laid down that the radii shall be taken as abscissas and the stresses as ordinates, the total resistance thus varying as the * Rules and Tables, p. 687. 136 THE STEAM-BOILER. logarithm of the ratio of the radii. Then if the elastic limit he coincident with the ultimate strength, and T the tenacity of the metal, R = the ratio, external diameter divided by internal, / = the bursting pressure, /= TX hyp log ^; ..'... (28) & = <* ( 2 9) In other cases, instead of T take the value of the resistance at the elastic limit, and base the calculation of proportions upon the elastic limit and its appropriate factor of safety. The for- mulas as given are considered applicable to cast-iron. The strength of thick cast cylinders with heads cast in may, however, sometimes be far in excess even of the calculated re- sistance of thin cylinders. The formulas for thick cylinders appear to be in error on the safe side ; and very greatly so when, as is usually the case, the cylinder is short, and strength- ened by having a head cast in. Such cylinders are generally also strengthened by very heavy flanges at the open end. The Pressure allowed by Law or by government regulations on any cylindrical shell is found by the following rule : " Multiply one sixth (-J-) of the lowest tensile strength found stamped on any plate in the cylindrical shell by the thickness expressed in inches or parts of an inch of the thinnest plate in the same cylindrical shell, and divide by the radius or half diameter also expressed in inches and the sum will be the pressure allowable per square inch of surface for single-riveting, to which add 20 per centum for double-riveting." The hydrostatic pressure applied under the above table and rule must be in the proportion of 150 pounds to the square inch to 100 pounds to the square inch of the working pressure allowed. The following table gives the pressures thus calculated for single-riveted boilers of various sizes : MATERIALS STRENGTH OF THE STRUCTURE. 137 TABLE OP PRESSURES ALLOWABLE ON BOILERS MADE SINCE FEB- RUARY 28, 1872. 45,000 TEN 50,000 TEN- 55,000 TEN- 60,000 TKN- 65,000 TBN- 70,000 TEN- . SILE SILE SILB SILZ ML I SIL E I u STRENGTH. STRENGTH. STRENGTH. STRENGTH. STRENGTH. STRENGTH. 1 i 7.500 i, 8,333.3 i, 9,166.6 i, 10,000 i, 10,833.3 J, 11,666.6 ^0 "o S73 3*3 J _. c < | gj r-5 5 1 i u .2 U O V U O v u.o 1 j 3 1 i'| 1 If u - ~ i S i 8.1 3 S3 5 \ E o a 8* Cu g rt 0U 8* .i8 7 S 78.12 93-74 86.8 104.16 95-48 "4-57 104.16 124.99 112.84 135.4 121.52 145.82 .21 87-5 105. 97.21 116.65 .06.94,128.3 116.66 139.99 126.38 151.65 .36..1 163-33 2 3 95-83 114.99 106.47 127.76 117.12 140.54 127-77 .53.32:138.41 166.09 149.07 .78.88 25 104. 16 124.99 115-74 138.88 127.31 I52.77 138.88 166.65 150.46 180.55 162.03 193-43 36 .26 108.33 129.99 120.37 144-44 132-4 .58.88 144.44 173-32 156-48 187.77 168.51 2O3.2I Inches. .29 120.83 144.99 i34- 2 5 161 .11 147.68 177.21 161 . ii 174.53209.43 187.90 225-48 3125 33 130-2 137-5 156.24 165. 144.67 173.6 159 14 152.77 183. 32 168.05 190.96 173.6 201.66,183.33 208.32:188.07 225.68 219. 99,, 98.61 238.33 202 5 E.88 243-04 256.65 35 162.03 194.43 178.231215.871194-44:233.32 210.64 252.76 >>- 272.2O 375 156.25 187.5 173.61 208.33 190.97 229.16 208.33 249.99 225 69271.82 .^05 291.66 1875 74.01 88.89 82.23 98.67 90.46 108.54 98.68 1,8.4, 106.9 128.28 115-13 138.16 .21 82.89 99.46 92 i 110.52 101.31 121-57 110.521132.62 "9-73 143-67 128.93 154-7 1 2 3 90 . 78 108 . 03 100.87 121.04 i 10.96 133.15 121.05 145.26 131.13 157-35 141.22 169.46 2 5 98.68 118.41 109.64 131.56 120.61 !44-73 131-57,157-88 .43.54 171-04 153-5 184.20 38 .26 102.63 12 3- r 5 114.03 136.83 125.43 I r o . 5 1 136.84 164.2 148.24 177. 8S 159-64 191.56 Inches. .29 114.47 J37-36 127.19 .52.62 139.91 .67.89 152.63 183.15 165-35 .98.42 .78.061213.67 .3125 33 123.35 148.02 130.26 156.31 137- 144-73 164.46 150.70 173-67 159-2 180.9. .91.04 '64-47 197-36 173.68 208.41 178.17 213.8 188.15 225.78 191.88 230.25 202.62 243.14 35 375 138.15 148. 165.78 177.60 153-5 164-73 184.21 168.85 197.67 180.81 2O2 . 62 217.09 184.21 I97-36 221.05 236.83 199.56 213.81 239-47 256-57 2,4.9, 257.89 230.26^76.31 1875 70. 3 1 84.37 78.12 93-74 85-93 103. 11 93-75 .12.5 101.56 1*1.87 109.37 131.24 .21 78.75 94-50 87.49 104.98 96.24 115.48 105. 126. "3-74 136.48 122.49 140.98 23 25 86.25 103.5 93-75 "2-5 95-83 104.16 114.99 124.99 .05.41 114.58 126.49 137-49 US- 125- 138- 150. 124-58 135-4I 149.49 134- 16 162.49 145-83 |.6o.o9 174-99 40 Inches. .26 29 97-5 108.75 117. 130-5 108.33 120.83 129.99 119.16 144.99 132-91 142.99 159-49 130- 145. 156. '74. 140.83 157-08 168.99 151.66 mi. 99 188.49 169.16:202.99 3125 33 35 117.18 140.61 123.75 148.5 131.25 157.5 130.2 137-49 145-83 164.98 174-99 143.22 151.24 160.41 171.86 181.48 192.49 156.25 165. 187-45 198. 210. 169.27 203. 12 ,78.74 214.48 ,89.58227.49 102.29 210.74 .92.49:230 98 204.16244.99 375 140.62 168.74 156.24 187-48 171.87 206.24 187-5 225. 203.12 243.74 218.74 262.48 1875 .21 66.96 75- 80.35 90. 74.40 83 32 89.28 99-99 81.84 98.20 91.66 109.99 89.28 107.13 100. 120. 96.72 ,08.33 .16.06 129.99 104.16 .24.99 .16.66 .39.99 42 23 .25 .26 82.14 98.56 89.28 107.13 92.85 111.42 91.23 99-2 103-17 109.51 119.04 123.8 100.39 120.46 109. 12 | 130. 94 113.49 136.18 109.52 131.42 119.04 142.84 12^.8 ,48.56 118.65 128.96 134-12 142.38 154-75 160.94 127.77 ,38.88 144-44 153 32 166.65 173.32 Inches 29 3125 33 35 375 I03.57 in. 6 117-85 125- I33-92 124.28 133-92 141.42 150. 160.7 115.07 124. 130.94 138.88 148.8 138.08 148.8 157-12 166.65 178-56 126.57 136-4 114.04 152-77 163.68 151.85 .63.68 .72.84 183.32 196.40 ,38.09165.7 148.74 178.56 157.14,188.56 166.66 199.99 178.57 2.4.28 149.6 179.5 2 l6l.2 ,193-44 ,70.23 204.27 .80.55 216.66 193-45 232.14 161.11,193.33 173.61 [208. 23 194 44 233.32 208.33249 99 .1873 .21 63-92 76.7 8s- 9 71.01 79-54 85.22 95-44 78.12 87.49 93-74 ,04.98 85.22 .02.26 95-45 "4-54 92.32 103.4 110.78 124.08 99-42 1 1 1 . 36 119-3 133-6? 23 78.4 94.08 87.12 85.22 102.26 94.69 104-54 113.62 95.83 114.99 104.16 124.99 104.54 .25.44 113.63 136.35 113.25 123.1 135-9 147.72 132.56 159 07 44 .26 88.63 106.35 08.481118.17 108.33 129-99 118.18 141.81 128.02 153.62 I37-87 | i5-44 Inches. .2 9 312;, 33 35 375 98.86 118.63 106.53 127.83 112.5 i35- 119.31 I43- X 7 127.81 153-37 109. 84! 131- 80 118.36)142.03 124.99 149-9* 132.57 159.08 142.04 170.44 120.83 130.2 137-49 145-83 156-24 144.99 1.56-24 164.98 '74 99 187.48 .31.8. 158.17 ,42.04 ,70-44 .50. 180. 159.09 190.9 .70.45 204.54 142.70 153-88 ,62.49 172.34 184.65 171-33 184.65 194.98 206.8 221.58 153.78 184.53 .65-71 198-85 174-99 209.98 185.6 222.72 .98.86238.63 138 THE STEAM-BOILER. TABLE OF PRESSURES ALLOWABLE ON BOILERS MADE SINCE FEB- RUARY 28, 1872. Continued. 45,000 TEN- 50,000 TEN- 55,000 TEN- 60,000 TEN- 65,000 TEN- 70,000 TEN- SILE SILE SILE SILE SILE SILE ^ s U -2 jj u .2 1 & 1 jil I a'l 1 V 'O a-o !3 flj *O 1 al al O I a* a 0* | o rt I ." 1 o re | 8* .1875 61.14 73-36 67-93 81.51 74-72 89.66 81.51 97.81 88.31 105-97 95-i 114.12 .21 68.47 82.16 76.08 91.29 83 69 100.42 91-3 109.56 98.91 118.69 106.52 127.82 23 2 5 75- - 81.51 90. 97.81 83-33 90-57 100. 108.68 01.66 99-63 109.99 119-55 100. 108.69 120. 130.42 108.33 ii7-75 129.99 141-3 116.66 126.8 139-99 152.16 46 .26 81.78 101.73 94.2 113.04 103.62 124.34 113.44 135-64 122.46 146-95 131.88 158.25 Inches .29 3 I2 5 101 .9 113.47 105 071126. 122.28 113.21 135.86 "5-57 138.68 126.09 151.3 124.54 149.44 135-86 163 03 136-59 147.19 163.92 176.62 147.1 158-51 176-52 190.21 33 107.6 129.12 119.56 143.47 131.52 157.82 143 57 172-16 155-43 186.51 167-39 200.86 35 114.13 136.95 126.8 1152.16 139.49 167.381152.17 182.6 164.85 197.82 177-53 213.03 375 122.28 146.73 135-86 163.03 149-45 179-34 163.04 195.641176.62 211.94 190.21 228.25 i875 58-59 70.30 65.1 78.12 71.61 85-93 78.12 93-74 84.63 ici.55 9 I - I 3 109-35 .21 65.62 78.74 72.91 87.49 80.2 96.24 87.49104.98 94.79 "3-74 102.08 122 .49 23 71.87 86.24 79.85 95.82 87.84 105.4 95.83 114.99 io3;8< 124-57 in. 8 I33-16 25 78.12 93-74 86.8 104 16 95-48 114-57 104. i6j 124.991112. 84 135-4 121 .52 145.82 48 .26 81.25 97.50! 90.27 108.32 99-3 119.16 108.33 129.99 117.36 140.83 126.38 15I-65 Inches. .29 90.62 i 08. 74 j 100.69 120.82 110.76 132.911120.83 144.99 x 3-9 157.08 140.97 169. 16 3125 97-65 117.18 108.5 130.2 119.35 143.22 130.21 156.25 141.05 169.26 15I-9 182.28 33 103. 12 123.74 114-58 137-49 126.04 I 5 I - 2 4 137-5 165. 148.95 178.74 160.41 192.49 35 109-37 131.24 121.52 145.82 133.67 160.4 145.83 174-99 I57-98 189.57 170.13 204.15 375 II7.I8 140.61 130.2 156.24 143.22 171.86 156.25 187.50 169.27 203. 12 182.29 218.74 1875 52.08 62.49 57.87! 69.44 63-65 76.38 69.44 82.44 75-23 Q0.27 8T.OI 97.21 .21 58.33 69-09 64.81; 77.77 71.29 85-54 77-77 93-32 84- 2 5 IOI . I 90.74 108.88 2 3 63.88 76-65 70.98; 85.17 78.08 93-69 85.18 102.21 92.28 110.73 99-38 119.25 25 69.44 83-32 77.16; 9 2 -59 84.87 101.84 92.59 in. 10 00.3 120.36 IO8.O2 129.62 54 .26 72.22 86.66 80.241 96-28 88.27 105.92 96.29 115.54 04-31 125.17 112-44 134-8, Inches. .20 80-55 96.66 89.5 07.40 98.45 118.14 107.41 128.88 16-35 139.62 125-3 150-36 3125 86 8 104. 16 96.44 15.72 106.09 127.30 115-55 138.66 25-38 '5-45 135-03 162.03 33 91.66 109.99 101.84 22.22 112.03 134-43 122. 22 146.66 3 2 -4 158.88 142-59 171 . 10 35 97.22 116.66 108.02 29.62 118.82 142-58 129.62:155.54 140.43 168.51 151-23 181.47 375 104. 161124.90 115.74 38.88 127.31 152-77 138.88 166.65 150.46 180.55 162.03 J94-43 1875 46.87 56.24 52.08 62.49 57-29 68.74 62.5 75- 67.7 8l.24 72 QI 87.49 .21 52-5 63- 58.33 69.99 64.16 76.99 69-99 84- 75.83 90.99 81.66 97-99 23 57-5 69. 6 3 .88j 76-65 70.27 84-32 76.66 91.99 83.05 99.66 89.44 107.32 25 62.5 75- 69.44 83.32 76.38 91.65 83.33 99-99 90.27 08.32 97.22 116.66 60 .26 65- 78. 72.22 86.66 79-44 95 -3 2 86.66!io3.qq 03.88 12.65 IOI . II I2I -33 Inches. .29 72.5 87- 80.55' 96.66 8.6l 106.33 96.66115.99:104.72 25.66 112.77 I35-32 3125 78. 12 93-74 86.8 104.16 95-48 114. 57 ! 104. 18 124.99 H2-95 35-54 121-52 145-82 33 82.5 99- 91.66 109.99 100.83 120.99 iog-99 132. 5119.16 42.99 128.33 153-99 87.5 105. 97.22 116.66 106.94 128.32 116.66 139.99 126.38 151-65 136.11 163-33 375 93-75 112.5 104.16 124.99 114.58 137-49 125. 150. I35-4 1 162.49 I45-83 175-99 1875 42.61 51.13 47-34 56-8 52.07 62.49 56.81 68.17 6i-55 73-86 66.28 79-53 .21 47-72 57-26 53- 63.63 58-33 69.99 63-63 76.35 68.93 82.71 74-24 89.08 2.3 52.27 62.72 69.69 63.88 76-65 69.69 83.62 75-5 90.6 81.31 97-57 .25 56.81 68.17 63-13 75-75 69.44 83-32 75.75 90.90 82.07 98.48 88.37 106.04 66 Inches. .29 59-09 65.90 70.9 79.08 65-65 73-23 78-78 87.87 72.22 80.55 86 66 78.78 94.53 85.35 96.66) 87.87 105.44 95-2 IO2'. 42 114.24 91 .91 I IIO.29 02.52 123.02 3125 7 1 - 85.2 78.91 94.69 86.89 104. 16 94 69 113.62 102.58 123.09 10.47 132-56 33 75- 90. 8 3-33 99-99 91.66 109.99 99-99 "o. 108.33 129.90 16.66 139.99 35 79-56 95.47 88.38 106.05 97.22 ii6.66jio6. 127.27 114.80 137.861 2H.73 1^8.47 375 85.22 I O2 . 26 94.69 113.62 104.16 124.99 113-62 136.341123.1 147.72 32.57 159.08 MATERIALS STRENGTH OF THE STRUCTURE. '39 TABLE OF PRESSURES ALLOWABLE OX BOILERS MADE SINCE FEB- RUARY 28, 1872. Continued. 1 1 1 E 45,000 TEN- SILE STRENGTH. 4, 7,5oo 50,000 TEN- SILE STRENGTH. 4,8,333-3 55,000 TEN- SILE STRENGTH. 60,000 TEN- SILE STRENGTH. 4, 10,000 65,000 TEN- SILE STRENGTH. 4, 10,833.3 70,000 TEN- SILE STRKNGTH. 4, 11,666.6 "o *J ' _ ' * r -_ a- 1 I > | tj 5 c o_o d s.I 1 v c O g Sj 8.1 5 M 3 "1 1 I 1 ! OJ *O I ? 1 1 8 1 -1875 .21 23 -25 39.06 43-75 47.91 52-08 46.87 52-5 57-49 62.49 43-4 48.6 53-24 57-87 52.08 53.33 63.88 69-44 47-74 53-47 58-56 63.65 57.28 64.16 70.27 76.38 52.08 & 69.44 62.49 69.99 76.65 83.32 56-42 63.19 62.21 75-22 67.70 75.82 83 05 90.26 60.76 68 05 74-53 81.01 72.01 81.66 89.43 97.21 72 .26 54.16 64.99 60.18 72.21 66.2 79.44 72.22 86.66 78 24 93 88 84-25 105.10 Inches. .29 60.41 72.49 67.12 80.54 73-84 88.60 80.55 96.66 87.26 04.71 9V 98 112.77 3 I2 5 65.10 78.12 72.33 86.8 79-57 95-48 86.8 104.16 94 03 112 83 01.27 121.52 33 68 75 82. S 76.38 9'-6s 84.02 100.82 91.66 109.99 (,9.3 119. l6 06.94 128.32 35 72.91 87-49 81.01 97-2' 89.11 106.93 97.22 116.66 105.32 26.38 13-42 136.1 375 78.12 93-74 86.8 104.16 95.48 i'4-57 104. 16 124.99 112.84 135-43 21 52 145.82 .1875 .21 2 3 36.05 40.38 44-23 43-21 48.45 53-07 40.06 44-87 49-14 48.07 53-84 58.96 44 07 49-35 54-05 52-87 59-22 64.86 48.07 5V 84 58.95 5:2 70.76 52.08 58.33 63.88 62.49 S:S 56.08 62.82 68.80 67-29 75 -3& 82.56 78 Inches. .29 48.07! 57.68 50. 60. 55 76 66.91 53-41 55-55 01 .96 64.09 66.66 74-35 58.76 66.11 68.16 70.5 73-33 81.79 64.4 66.66 74-35 76.92 79-99 89.22 69.44 72.22 80 55 ftg 96.66 74-78 77 77 86.75 89 73 93 32 104.1 .3125 60 09 : 72.1 66.77 80.12 73-45 88.14 80.12 96.14 86.8 104.16 93.48 112.17 33 35 63.46 67.3 76.15 80.76 70.51 84.61 74.78 89.73 77o6 82.26 93-07 98.71 84.61 101.53 8q 74 107.68 91.66 97.22 109.99 116.66 98.71 104 70 118.45 125.64 375 72.11 86.53 80.12 96.14 88.14 105.76 96.15 115.38 104.16 124.99 112.17 134 o 1875 33-48 40.17 37.2 44.68 40.92 49 i 44-64 53-56 48.36 58.03 52.08 62.49 .21 37-5 45- 41.66 49.99 45-83 54-99 50- 60. 54.16 64 99 58.33 69.99 .23 41 .02 49-22 45-63 54-75 50.19 60.22 54-75 65 71 59-32 71.18 63.65 76.38 84 Inches .29 44.64 46.42 5I-78 53-56 55-7 62.13 49-6 51-58 57-53 59-52 61.89 69.03 54 56 56.74 63.29 ! : H 59-52 61.9 69.04 71.42 74.28 82.84 64.48 67-05 Z 4 '! 77-37 80.46 89-76 69.44 72.22 2?'2 5 86:66 96.66 55-8 66.96 62. 74.4 68.2 8 1 . &4 74-4 89.28 80.6 96.72 86.8 104.16 33 58.92 70.7 65.47 78.56 72 02 86.42 78-57 94.28 85.11 IO2 . I " 91.661109.99 35 375 6?: 96 75- 80.35 69.44 83.32 74.4 ! 89.28 76.38 81.84 91.65 98.2 83.33 89.28 99-99 107.13 90.27 96.72 108.32 116.06 97.22 104 16 124-99 .187 .21 35- 37-5 42. 34-72 38.88 41.66 46.65 38-19 42.77 45.82 51- 3'-' 41.66 46.66 49-99 55-99 45-13 50.55 54 -15 60.66 48.68 54-44 58.33 65.32 90 Inches 23 25 .26 2 9 33 35 375 38.33 41.66 43-33 48.33 52.08 55- 62.5 45-99 49-99 51-99 57-99 62.49 66. 69 99 75- 42-59 46.29 48.14 53-7 57-86 6t.n 64.81 69.44 51.10 55-54 57-76 64-44 69.43 73-33 77-77 83-32 46.85 50.92 ; 5296 59-07 63.65 67.22 71.29 76.38 56.22 61.1 6355 70.8 76.38 80. 66 85.54 91.65 51." 55-55 57-77 64-44 69-44 73-33 77-77 83.33 6i-33 6666 69.32 77-32 83.32 87.99 93-32 99-99 62.59 69.81 75 -*3 84.2* 90.27 66.44 72.21 lln 90.27 95-32 IOI.I 108.32 6 7 V 75-iS 81.01 85.55 90.72 97-22 71-54 77-77 80.88 90.21 97-2. 102.60 108.88 116.66 .187 .SI 23 29.29 32.81 35-93 35-14 39-37 43- 46.87 32-55 36.45 39-93 43-4 39.06 43-74 47-91 52.08 358 40.1 43-92 47-74 42.96! 39.06 48.12 43.75 52-7 47-91 57.28 52.08 46.87 52.5 57-49 62.49 42-31 47-39 51-9 56-42 50-77 56. 8( 62.28 67-67 45-57 51.04 55-9 60.76 54-68 61.24 67-08 72.91 96 fetches .312 33 35 375 40.62 45-31 48 82 51-56 54-68 58.58 48.74 54-37 58-58 61.87 65.61 70.29 45-14 50-34 54-25 57-29 60.76 65.1 54 l6 60.4 65.1 68.74 72-91 78.12 49-65 55-38 .67 .02 71.6l 59- 68 66.4f 75-6s 80.15 85-9: 54-16 60.41 65-1 68.75 72.91 78.12 64.99 72.49 78.12 82.5 87-49 93 74 58.78 70.53 65-45 78.54 70.52! 84.62 74.47 89.36 78.99 94.78 84.63 101.55 63.19 70.48 75-95 80. a 85-06 91.14 75-82 84.57 91.14 06.24 102.07 109.6 I4Q THE STEAM-BOILER. Externally-fired boilers are not permitted by United States regulations to be made thicker than 0.51 inch (1.2 cm.). The 20 per cent higher pressure of the table is allowed on steam- vessels which carry no passengers. It will be observed that the rule above given allows an apparent " factor of safety" of six; while the loss of strength at a single-riveted seam reduces it to the actual value of four, nearly. It would probably be on the whole wiser to use as the actual value the higher figure, and thus to reduce the pressures carried to one third below those now permitted, except where inspection and test during construction, and constant supervision with frequent inspection during the life of the boiler, may give a safe margin with the lower figure. The operation of the law which allows old boilers to carry two thirds the test pressure is to reduce the real factor often to less than one and a half ; for it is well known that iron will not carry permanently a load which it will sustain for a short time without observable yielding. French regulations make the thickness of wrought-iron cylindrical shells of boilers not less than / = i.8A/ + 3 in millimetres, when the pressure, /, is in atmospheres and the diameter, d, is in metres. In no case, however, is a greater thickness allowed than 1 5 mm. (0.6 in.). German regulations give / = o.oo i d o.i inches. Flues and Cylinders subjected to external pressure resist that pressure in proportion to their stiffness and their com- pressive strength if thin, and if thick sustain a pressure pro- portional to their thickness and maximum resistance to crush- ing. Fairbairn,* experimenting on flues of thin iron, 0.04 inch * Useful Information. Second Series. MATERIALS STRENGTH OF TffE STRUCTURE. 14! (0.102 centimetre), of small diameter, 4 inches (10.2 centi- metres) to 12 (31 centimetres), and from 20 inches (50.8 centi- metres) to 5 feet (1.52 metres) long, found that their resistance to collapse varied inversely as the product of their lengths and their diameters, and directly as the 2.19 power of their thick- ness. The following equation fairly expressed his results when / is the external pressure in pounds per square inch, / their thick- ness in inches, d their diameter, and L the length in feet : or, for the length in inches, = 9,672,000-^- (2) In metric measures, kilogrammes and centimetres diameter, and metres of length, / 2 - 19 / = 68,000 -_-_-, nearly ...... (3) (4) 68,000 For elliptical flues take tfter-j^; where a is the greater and b the lesser semi-axis. These equations probably give too small values of / for heavy flues under high pressure. Belpaire's rule, deduced from Fairbairn's experiments, is (5) 142 THE STEAM-BOILER. Lloyd's rule for flues is in which a is made 89,600 pounds per square inch. The British Board of Trade Rule is, for cylindrical furnaces with butted joints, af P ~~ ' in which a is 90,000, provided, always, p < 8,000^; and for large joints a = 70,000, unless bevelled to a true circle, when a = 80,000. If the work is not of the best quality, these values of a are reduced to 80,000, 60,000, and 70,000. Flanged and Corrugated Flues are much stronger than plain, lapped, or butt-jointed flues. Experiment indicates that it is allowable to consider the length L in the formulae for strength of flues as the distance from flange to flange, and to assume that the flanges support the flue as effectively as the flue sheets. Where the several courses of a flue are flanged to- gether instead of being connected by the usual lap-jointed girth-seams, the strength of the flue is thus enormously in- creased. Another method of strengthening the flue is by sur- rounding it, at intervals, with a strongly made ring of angle or T-iron, which answers the purpose of a flange, while being less costly in construction. To prevent injury by overheating at those parts where the total thickness of metal traversed by the heat from the furnace-gases would be objectionably great, the ring is often supported clear of the flue by a set of thimbles through which the rivets holding it in place are driven. The corrugated flue is now very extensively used, the cor- rugations extending around the flue and having a pitch of ten MATERIALS^STRENGTH OF THE STRUCTURE. 143 or twelve times the thickness of the sheet. These flues pos- sess the double advantage of having more than twice the strength of equally heavy plain flues, and of being so much thinner for a given strength as to be vastly safer against over- heating and burning. These flues are less liable to distortion in the processes of^ working than are plain flues. By the United States regulations, lap-welded flues less than 1 8 feet long and 7 inches or more in diameter are allowed to carry pressures determined by the formula ct pr A=-; / = --, in which the pressure, /, is in pounds per square inch ; the thickness, /, and the radius, r y of the flue in inches. The value of the constant c is 4400. This gives, for example, an allowable pressure of 200 pounds per square inch on a flue 14 inches in diameter, less than 18 feet long and 0.32 thick. A minimum thickness is set at / = diam. X 0.022. For lap-welded flues exceeding 18 feet in length, 3 pounds is deducted from the pressure calculated as above, for each added foot, or o.oi inch is added to its thickness. When between 7 and 16 inches diameter and 5 to 10 feet long, one strengthen- ing ring is required ; and where 10 to 15 feet long, two such rings, each of a thickness of metal at least equal to that of the flue, and 2\ inches or more in width. Flues 16 to 40 inches diameter are allowed by the United States regulations a pressure in which/ = -, c = 0.31, or /= /44 THE STEAM-BOILER. which allows 100 pounds per square inch on a Jue 20 inches in diameter and 0.37 inch thick. Corrugated furnace-flues are allowed to " carry" a pressure, p =14,000-; a equivalent to 175 pounds on a flue 40 inches in diameter and 0.5 inch thick. Other flues are allowed pressures determined by Fairbairn's formula, / - 89,600^, in which, however, L is in feet. Rings are fitted in such man- ner as to reduce the maximum tension on the rivets to 6000 pounds per square inch of section. 57. Stayed Surfaces and Stays and Braces are parts and members which, in steam-boiler design and construction, should be studied with special care. Where it is possible to make the strength of the structure ample by correctly forming parts ex- posed to stress, as by making them cylindrical, it is usually con- sidered best to do so ; but in many types of boiler this is im- practicable, and staying must be resorted to. Properly designed stayed surfaces should be made the strongest parts of the boiler. The fireboxes of locomotives and of other firebox boilers, in which stay-bolts are well distributed, the water-legs of many marine boilers, and other parts composed of flat surfaces sus- tained by stays and braces, are common illustrations of the method of resisting pressure. Where flat surfaces are secured against lateral pressure by stay-bolts, as is done in steam-boilers, these bolts may yield either by breaking across, or by shearing the threads of the screw in the bolt or in the sheet. Such bolts should not be so proportioned that they are equally liable to break by either method, but should be given a large factor of safety (15 to 20) to allow for reduction of size by corrosion, from which kind of deterioration they are liable to suffer seriously. Wrought-iron MATERIALS STRENGTH OF THE STRUCTURE. 145 and soft steels are used for these bolts. They are screwed through the plate, and the projecting ends are usually headed like rivets. Nuts are sometimes screwed on them instead of riveting them when they are not liable to injury by flame. "Button-set" heads are from .25 to .35 stronger than the conical hammered head, and nuts give still greater strength. Experiments made by Chief Engineers Sprague and Tower, for the U. S. Navy Department, lead to the following formula* and values of the coefficient a, p being the safe working pres- sure, t the thickness of plate, and d the distance from bolt to bolt: VALUES OF a IN BRITISH AND METRIC MEASURES. A. A m . For iron plates and bolts ..................... 24,000 1,693 For steel plates and iron bolts ................ 25,000 1,758 For steel plates and steel bolts ............... 28,000 1,968 For iron plates and iron bolts with nuts ....... 40,000 2,812 For copper plates and iron bolts .............. 14, 500 1 ,020 The working load is given in pounds on the square inch and kilogrammes per square centimetre, the measurements being taken in inches and centimetres. The heads, where riveted, are assumed to be made of the button shape. The diameter of stay is made about 2 Vt, the number of threads per inch 12, or 14 (5 or 6 per centimetre). A very high factor of safety, as above, is recommended for stays, to afford ample margin for loss by corrosion. Lloyd's Rule for stayed plates is <> in which / is the working pressure in pounds on the square inch,,/! the. thickness of plate in sixteenths of an inch, and/, is the distance apart of the stays in inches. * Report on Boiler Bracing. Washington, 1879. 10 146 THE STEAM-BOILER. The coefficient a has the following value : a ~ go for plate T 7 ^ inch thick or less ; with screw stays and riveted heads ; a = 100 for plate $ inch thick or more; screw stays and riveted heads ; a = 1 10 for plate -$ inch thick or less ; screw stays and nuts; a = 1 20 for plate T 7 inch thick or more ; screw stays and nuts ; a = 140 for plate T 7 inch thick or more ; screw stays with double nuts ; a = 1 60 for plate -^ inch thick ; with screw stays double nuts and washers. The Board of Trade of Great Britain prescribes c. + 0' ... in which /, is the thickness of plate as above, and s is the area of surface supported, in square inches. a = 100 for plates not exposed to heat, and fitted with nuts and washers of 3" diameter and of the thickness of the plates ; a = 90 for same case, but with nuts only ; a = 60 in steam and having nuts and washers ; a = 54 if with nuts only ; a = 80 in water spaces, with screw stays and nuts ; a 60 if with screw stays riveted ; a = 36 in steam, screw stays, riveted. For girder stays, where the symbols are defined as on page 148. When one, MATERIALS STRENGTH OF THE STRUCTURE. \^J two or three, or four bolts carry the girder, a = 500, 750, and 800, respectively. Stay-bolts should have diameters considerably exceeding double the thickness of the plate. D. K. Clark allows, as a maximum, the pressure , (5) where t, 7", and d are the thickness of sheet and its tenacity, in tons per square inch, and the u pitch" of the stays in inches. In computing the strength of stayed surfaces, it is to be un- derstood that each stay sustains the pressure on an area bounded by lines drawn midway between it and its neighbors, and mea- sured by the product of the distances between stays in the two directions of the lines of their attachments to the sheet. Thus marine boiler stays spaced 8 inches apart sustain the pressure on 64 square inches ; while locomotive firebox stay-bolts spaced 4^ inches each way carry the pressure on 2oJ square inches. A common minimum factor of safety for stays, stay-bolts, and braces is 8, and when liable to serious corrosion the load applied is often reduced to 3000 or 4000 pounds per square inch of section of stay or brace, thus giving a factor of ten or more. The actual rupture of stay-bolted surfaces was found by the Author, by the study of the results of experimental steam- boiler explosions in 1871,* to be about the pressure in which / is the thickness of plate, and d the pitch of the stay- bolts. In design, we would make * Journal Franklin Institute, 1872. 148 THE STEAM-BOILER. a being the factor of safety, which, as has been seen, should al- ways be large, and/' the working pressure. Fairbairn showed that the diameter of a stay-bolt should exceed double the thickness of the sheet by the amount to be allowed for corrosion. He found that riveting over the ends of screwed stays increased the strength of the construction 14 per cent. Where the crown-sheet of the furnace of a boiler is supported by girders, the load to be permitted may be adjusted by the formula, already given, c13 Carbon 49 . 95 Ash !.6 5 Total loo.oo It will evaporate 7.98 times its own weight of water, develop- ing 7713 British heat-units per pound, demanding 6.08 times its own weight of air for complete combustion, the products of combustion containing 1089 heat-units per pound and attaining a temperature of 4287 F. (2382 C.). Pennsylvania petroleum, having the composition, according to Deering, of Carbon 85 | Hydrogen 15 requires 15 times its own weight of air for complete combus- tion, liberates 20,360 British thermal units per pound of the liquid, or 1267 per pound of products of combustion, and de- velops an increase of temperature of 4900 F. (2722 C.). Illuminating gas, according to Mr. Deering, having the composition, Carbon 61.26 Hydrogen 25.55 Nitrogen *. 8.72 Oxygen 4-47 Total loo.oo develops 20,801 British thermal units per pound, equivalent to * Howard Lecture. W.Anderson. London, 1885. 1 82 THE STEAM-BOILER. the evaporation of 21.53 times its own weight of water, the best mixture for complete combustion being 15.66 parts of air, by weight, to one of the gas. The rise in temperature with perfect combustion is 4567 F. (2537 C), the total heat liber- ated being 1250 British thermal units per pound of the mix- ture. The same gas, per 1000 cubic feet, weighs as follows : Carbon 18.19 Ibs. Hydrogen 7-58 ' Nitrogen '. 2.59 ' Oxygen 1-33 ' Total 29.69 Ibs. It produces 617,485 units of heat, and can evaporate 639 pounds of water, demanding 465 pounds of air for complete combus- tion. By using the data of Rankine, results are obtained for the two extreme cases of pure carbon and defiant gas, burned re- spectively in air ; British units are used thus : Carbon. Olefiant Gas. Total heat of combustion per pound 14,500 21,300 Weight of products of combustion in air, undiluted 13 Ibs. 16.43 Ibs. Their mean specific heat 0.237 0.257 Specific heat X weight 3-o8 4.22 Elevation of temperature, if undiluted 4>5So 5,050 If diluted with air = J air for combustion. Weight per Ib. of fuel. , 19. 24.2 Mean specific heat 0.237 0.25 Specific heat X weight 4.51 6.06 Elevation of temperature 3,215 3,515 If diluted with air = air for combustion. Weight per lb, fuel 25. 31.86 Mean specific heat o. 238 o. 248 Specific heat X weight 5-94 7-9 Elevation of temperature 2,440 2,710 For wet fuel, like sawdust, or spent tan from the leach, the Author has made the following estimation in one actual case THE FUELS AND THEIR COMBUSTION. 183 where the fuel consists of 45 per cent of woody fibre, and 55 per cent of water. Taking the available heat per pound of the dry portion at 6480 British thermal units, each pound of wet fuel yields 2916 units of heat. Of this, 531.6 are absorbed in the evaporation of the 55 per cent of water, leaving 2384.4 units to raise the temperature of the products of combustion. Of these there are, as a minimum, 3.7 pounds, having a mean specific heat of about 0.287. The elevation of temperature is therefore 2245.3 Fahr., and adding the mean temperature of the atmosphere, 74, the mean temperature of furnace, assuming no dilution with un- used air, and no losses, would have been about 2320 Fahr. (1271 Cent.). Losing 2 per cent by radiation and conduc- tion, etc., the actual temperature was 2260 Fahr. (1238 Cent.). The temperature of chimney flue was found by experiment to have been 544. The furnace gases were therefore cooled 2260 - 544 1716 Fahr. (937 Cent.) by the loss of the heat given up to the boiler. This is equivalent to 1716X0.287 = 492.5 British heat-units per pound of gas, and to 4049.4 units per pound of ligneous material in the fuel. The " equivalent evaporation," from and at 212, is 4049.4 -r- 966.6 = 4.18 pounds of water. The actual evaporation was equivalent to 4.24 pounds, and the difference less than one per cent of the total represents losses and errors of calculation. The actual existing temperature of furnace can be also thus estimated. The available heat per pound of fuel, including water, has been given at 2916 British thermal units. Of this '-^ = 0.182 passed off with vapor, and was not useful in rais- ing the temperature of either the furnace or the chimney. Hence, of all heat liberated, 1.00 0.182 = 0.8 1 8 was efficient in elevating the temperature of furnace, and 0.37 0.182 = o.i 88 was effective in producing the observed temperature, 5/1/1 Fahr., of chimney. Then, since the same quantity of gas passes at both places, the temperature of furnace was I x 470) + 74 = 2119 Fahr. To this is to be added \o. 1 88 / 1 84 THE STEAM-BOILER. the slight loss of temperature en route between furnace and chimney by conduction and radiation, which may make the figure very nearly 2260 Fahr., as above. The actual temperature of the furnace may be judged, in any case, by observing the brilliancy of the light radiated from any solid in its midst, and presumably at its own temperature, as by the following table given by Pouillet : Appearance. Temp. Fahr. Red, just visible 977 " dull 1290 " cherry, dull 1470 " " full 1650 " " clear 1830 Orange, deep 2010 " clear 2190 White heat 2370 " bright 2550 " dazzling 2730 To determine temperature by fusion of solids, we have also from the same authority Substance. Temp. Fahr. Tallow , 92 Spermaceti 120 Wax, white 154 Sulphur 239 Tin 455 Metal. Bismuth 518 Lead 630 Zinc 793 Antimony 810 Brass ^50 Silver, pure ^o Gold coin 2156 Iron, cast, medium 2010 Steel 255 o Wrought-iron 2910 79. The Rate of Combustion is determined principally by the quantity of air supplied. The amount of coal burned per square foot of grate with chimney draught varies very THE FUELS AND THEIR COMBUSTION. 185 nearly with the square root of the height of the chimney, and has been found by the Author, ordinarily, to be very nearly, as a maximum, -i, or =i where W and W are weights of fuel burned per hour per square foot of grate, and on the square metre, in pounds and kilogrammes, and H and H' are the heights of chimney in feet and metres. A chimney 64 feet or lo/J metres high, will, for example, under favorable conditions, usually support combustion of 15 pounds of coal per square foot of grate, or of 73 kilogrammes per square metre. The weight of combustible which may be burned in any unit of time may be calculated approximately by dividing the weight of air which can be supplied in that time, by its proportion to weight of fuel, as determined in the preceding paragraphs. In exceptional cases there is sometimes a large excess of air, and sometimes a considerable deficiency. In such instances, direct experiment only can determine the amount of fuel burned. 80. The Efficiency of the Furnace, considered as a heat- utilizing apparatus, is determined by the temperature of fur- nace gases, by the thoroughness with which complete combus- tion is secured, and with which losses of fuel and of heat are prevented. It is measured by the ratio of the amount of the total available heat of the fuel to that of the heat actually util- ized. This efficiency is rarely so high as 80 per cent, and fre- quently falls to 50 per cent. In all cases, efficiency is to be studied, in applications of heat, in two parts : (i) the efficiency of the heat-generating and absorbing apparatus, i.e., the furnace; (2) the efficiency of the heat-utilizing apparatus and methods, as the steam-boiler, the heating-chamber of the reverberatory furnace, or such other heat-absorbing arrangement as may be adopted. (i) The efficiency of the furnace is represented by 186 THE STEAM-BOILER. in which E is the ratio of the heat rendered available to heat developed ; T lt 7" 2 , T s , are the temperatures of furnace, of chimney, and of external air. For examples, in two actual cases, T lf T v T v were, 2118 F., 544 F., and 74 F., or 1176, 302, or 510, 251, and 48 C. for the second case. The values of the efficiencies of the two kinds of apparatus were and or for Centigrade degrees, 1176 - 302 510 - 251 ___ =0 .77; and __-_ = . 5 6; the first being nearly 40 per cent higher than the second. A certain change of fuel would have given the first a maximum temperature of 2644 F., 1451 C., and would have raised its efficiency to 2644 - 544 26^ o - 0.81, or 1451 - 279" - 23 -o = o.8i. (2) The efficiency of the heat-absorbing apparatus is de- pendent upon the character and proportion, and is not treated here. The highest efficiency in heat-production is secured by perfect combustion with the least practicable air-supply, thus obtaining the highest possible resulting temperature. A large part of the heat produced by combustion of fuel is expended in procuring chimney draught. This is not avail- able for producing any other useful effects. The amount of heat thus expended varies with the nature of the products of combustion, and the use to which the heat THE FUELS AND THEIR COMBUSTION. 187 is to be applied. In all cases the heat thus discharged is wasted. The temperature of the products of combustion cannot usually be reduced much below about 600 F., or 315 C. 81. Economy in Combustion of Fuels, where they arc- used simply in the production of high temperature, is so im- portant a matter, except in those favored localities where the proximity of coal, or of peat-beds, or of forests, renders its waste less objectionable, that the engineer should omit no precaution in the endeavor to secure their perfect utiliza- tion. To secure the greatest economy, it is necessary to adopt a form of grate which, while allowing a sufficient supply of air to pass through it to insure complete combustion, has such narrow air-spaces as to prevent waste of small fragments, by falling through them. The narrower the grate-bars and the air-spaces, the more readily can losses from this cause and from obstruction of draught be avoided. With a hot fire, however, the difficul- ties arising from the warping of the bars become so great, that it is only by peculiar devices for interlocking and bracing them that their thickness can be reduced below about of an inch at the top. Many such devices are now in use. In fur- naces burning wet fuel, with an ash-pit fire, fire-brick grate-bars are used. A certain amount of air must usually be allowed to enter the furnace above the grate, to consume those combustible gases which do not obtain the requisite supply of oxygen from below. The carbon, probably, in such cases usually obtains its oxygen from below the grate, while the gaseous constituents of the fuel are consumed by the oxygen coming in above. Chas. Wye Williams, who made most extended and care- ful experiments on combustion of fuel, recommended, for ordinary cases, where bituminous coal was burned, a cross area of passage, admitting air above the grate, of one square inch for each 900 pounds of coal burned per hour, or about one square centimetre for each 63 kilogrammes of fuel. This area should be made larger, proportionally, as the thickness of the 1 88 THE STEAM-BOILER. bed of the fuel is increased, and as the proportion of hydrocar- bons becomes greater. Chilling the gases, before combustion is complete, should be carefully prevented ; and comparatively cold surfaces, as those of a steam-boiler, should not be placed too near the burning fuel. A large combustion-chamber should, where possible, be provided, and more complete combustion may be expected in furnaces of large size, lined with fire-brick, and with arches of the same material, than in a furnace of small size where the fire is surrounded by chilling surfaces, as in a " fire- box steam-boiler." Finally, the greatest possible amount of heat being devel- oped in combustion, careful provision should be made for com- pletely utilizing that heat. In a steam-boiler this is accomplished by having large heat- ing-surfaces, and by so arranging the distribution of the adjacent currents of water and of hot gases that their differ- ence of temperature shall be the greatest possible. The gases should enter the flues at that part of the boiler where the tem- perature is highest, and leave them at the point of lowest tern perature. The feed-water should enter as near as possible to the point where the gases pass off to the chimney, and should gradually circulate until evaporation is completed at, as nearly as possible, that part of the boiler nearest to the point of entrance of the heated gases. Where a small combustion-chamber is unavoidably employ- ed, as in locomotives, various expedients have been devised with the object of producing complete intermixture of gases before entering the tubes. The most common and most suc- cessful is a bridge-wall, sometimes depending from the crown sheet, but sometimes rising from the grate, and which, by the production of eddies in the passing current, causes a more thorough commingling of the combustible gases with the accompanying air. None of these devices seem yet to have given such good results as to induce their general adoption. In the furnaces of steam-boilers it is usually considered advisable to allow the gaseous products of combustion to enter the chimney at a temperature of about 600 Fahr. (315 Cent.), THE FUELS AND THEIR COMBUSTION. 189 or about 2.08 times the absolute temperature of the external air, where natural draught is employed. Rankine has stated that the best temperature of chimney for natural draught is that at which the gases have a density equal to about one half that of the external air. Thus, the temperature of the external air being 60 Fahr. (15. 5 Cent.), its absolute temperature is 521. 2 (261. 75 Cent.), and the required absolute temperature of the gases in the chimney will be this temperature multiplied by 2 T V, i.e., 521. 2 X 2^ = io85.8, and the corresponding temperature on the ordinary scale is 624 .6 Fahr. (339. 2 Cent.). With forced draught, a considerable economy may be effected by the reduction of the temperature of escaping gases approximately to that of the boiler itself at the point of dis- charge of the gases. The fuel should be usually burned at a fair rate of combus- tion, and in such manner as to give that degree of efficiency which has been found financially desirable. The air-supply should be provided for, partly above as well as below the grates, bituminous coal demanding more above the bed of fuel than anthracite, partly because it is needed to burn the gaseous hydrocarbons driven off from the former, and partly because the bituminous fuel is burned in a thicker and less permeable bed of fuel. Ten or fifteen per cent of the total air-supply should usually be furnished above the flame-bed. The grate-area should always be so proportioned that it shall be possible to keep it, in ordinary working, at all times well and uniformly covered with incandescent fuel. The space above the grate, between it and the heating-surfaces, should always be so large that ample space and time are given for thorough intermixture of gases and complete combustion, and it should have such form that the air introduced above the fuel may become well mingled with the gases distilled from the coal. The effect of this air-supply, where bituminous coal is used, is well shown in an experiment by Mr. Houlds- worth,* made in 1842 for the British Association, at its Man- * Fuel Combustion and Economy; C. W. Williams. " On the Consumption of Fuel, etc.;" Wm. Fairbairn, Trans. Brit. Assoc. 1842. 1 9 o THE STEAM-BOILER. Air excluded : State of the Flues. Very Hack, ) Much smoke / Ditto Ditto Ditto " 1 I H - Air admitted : State of the Flues. / Clear flame, \ u \Hfeetlong. / en , o Ditto, 15 ft. longf. ' / o Ditto, 16 feet. & Ditto, 15 feet. Ditto, U feet. g; Ditto, 13 feet. Oj o en c g Ditto, 13 feet, o Ditto, 15 feet, a, ( Purple flame, \ < from carbonic > oo ( oxide. ) o en > i 1- \ ~*fr~. ~~-^ ^^ \ \ \ \ \ 1 Ditto Dark Bed ... Dingv Bed . . . Ditto, no flame . Ditto . . ', .. * \ Dark Bed . . . Dark . . . 1 5 Ditto . . . . v ; Ditto . . . . . - ; Ditto FUEL I l/ti i 1 Ditto ..... t L L L\ Dark Bed ... Dark . . . , . \ o fk 1 O i > i ; j Ditto .... / J /, / Ditto. ,, . , . / # * # * S ? g , % W2. MS ta =S 3333 H-* 1 i H-* CO O O tO O O U> CO O O o o o o papn ioxa .1 } -pa ui i p j I UBld p[0 UQ 'UClU M3U UQ FIG. 69. TEMPERATURE OF FURNACE. Chester meeting. As seen by reference to Fig. 69, the tern- perature in the flue fell to 750 F. (400 C.) on the introduc- tion of a fresh charge of fuel, rose at the end of a half-hour THE FUELS AND THEIR COMBUSTION. IQI to above 1200 F. (650 C.), then fell, until at the end of an hour and a quarter it had dropped to 1040 F. (560 C.), the fire meantime not having been disturbed. On then levelling off the surface of the bed of fuel, and thus filling all holes in the fire, the temperature at once rose nearly to the maximum, and then gradually fell again to 850 F. (454 C.). During this period, the air was admitted above the fire ; the lower line of the diagram shows the result of the usual method of handling the fires without air-supply above the fuel. The general method of variation of temperature is the same during the period between successive charges, but the temperature averages ten per cent lower. The transformation of a mass of black smoke into a flame many feet in length is the best possible evidence of the advantage of this operation. The gain in economy of fuel was estimated at about one third when the supply of air was properly adjusted and managed. The dotted line in the figure indicates the probable temperatures when the bed of fuel is kept level and free from holes. 82. Weather Waste. When coal is exposed to atmos- pheric influences, a " weather waste " occurs. Oxygen is absorbed, and a slow combustion injures the fuel. Berthelot found also that at temperatures not exceeding 530 Fahr. (277 Cent.) hydrogen may be absorbed, and succeeded in converting two thirds of the bituminous coal experimented with into liquid hydrocarbons. Coals freshly mined give out gaseous hydrocarbons, and even anthracite mines, where deep, are not free from danger by the explosion of such gases. The absorption of oxygen, and this loss of hydrogen and carbon, is injurious to the fuel. According to Mursiller, coals containing " fire-damp" give it up at or below 626 Fahr. (330 Cent.), and lose their coking property. Coals usually absorb carbonic acid freely. Poech concludes :* " Freshly-mined coal deposited on the rubbish piles is capable of condensing several times its volume of oxygen in its pores. The oxygen absorbed enters into chemical combination with the easily-oxidized constituents. * Van Nostrand's Magazine, 1884. 1 92 THE STEAM-BOILER. According as the absorption is rapid or slow, a greater or less elevation of temperature is produced. In the former it may lead to spontaneous combustion. The crumbling of coal is, among other causes, a consequence of the absorption and con- densation of oxygen in its pores, and the chemical changes tak- ing place. The escape of the hygroscopic moisture favors the absorption of oxygen. The pyrites can only produce a further- some effect on the increase of temperature w r hen present in considerable quantities, and then only in presence of moisture and air ; in the dry state they must be regarded as perfectly passive, and may even be detrimental to the warming. Freshly- mined coal placed in an atmosphere of steam can suffer no change. Even with incomplete exclusion of the air the steam will, in general, oppose oxidation and warming, principally by uniform moistening of the pieces of coal." 83. The Composition of the Common Fuels may be ob- tained from the following tables : COMPOSITION OF VARIOUS FUELS OF THE UNITED STATES. C. H. 0. if. S. Mois- ture. Ash. Spec. Grav. Pennsylvania Anthracite 78.6 2 K 17 o 8 O J. I 2 IA 8 Rhode Island " 85 8 IO ^ 37 1 45 Sc Massachusetts " Q2 O 6 o 2 O 5 T North Carolina " 83.1 7.8 Q. I . 70 Welsh 8J. 2 37 2 ^ o o f\ 7 Maryland Semi-Bituminous Penna. " " .... 80.5 75.8 4-5 2.7 2O. 2 u.y I.I u.y 1.2 1 ' J i-7 u.y 8-3 A O .40 33 < 59-4 38.8 i 8 <1Q Indiana " " ... 70. o 28 o 2 o 2J. M 52.0 30.0 9O 27 Illinois Bituminous 62 6 qc e. " (Block) Bituminous. ... 111. and Ind. (Cannel) Bituminous 58.2 CQ. C 37-i q6 6 .... i .y 4-7 3Q J<~ 2"* Kentucky 48.4 48.8 2 8 oc Tennessee Bituminous 71 O 17 O 41 ^ tti e, 45 Alabama " e A o J.2 6 .... I 2 5 . . . . . Virginia 55.0 41 .0 40 ..... n 7/1 o 18 6 Cal. and Oregon Lignite CQ I 3Q T-7 7 o o T A 7 4 w.y 1 D 1U. j i j. z 3* THE FUELS AND THEIR COMBUSTION. 193 MONONGAHELA GAS COAL. (CRESSON.) "Weight of sample, 60 Ibs. (27.27 kilogrammes). Volatile matter, per tent 35-74 Coke, per cent 64 . 26 Ash, per cent 6.66 Yield of gas, cubic feet per pound maximum 5.2 cubic metres per kilogramme maximum. . . . 0-324 Cubic feet per pound average 5.0 " cubic-metres per kilogramme average 0.312 Ton maximum 11,648.0 " average 11,200.0 Illuminating power, 5 feet per hour = candles 15.0 i ton coal = Ibs. sperm 576-Q COMPOSITION OF FOREIGN COALS. C H. if. o. s. Ash. 1.6 4.0 3.5 4.6 2.7 I.O 4.0 2.0 7-0 2.1 I.O 24.4 I 4 .6 1.6 12.5 5-5 IO.O 14.2 7.4 7.0 Specific Gravity. Authority. Welsh (Anthracite) 90.4 73.5 82.1 77-9 79-7 78.6 94-0 84.0 53-0 57-9 80.0 56.7 50.0 57-9 60.7 67.6 64-3 70.3 70.6 91-5 3-3 5.6 5-3 5-3 4.9 5-3 1.4 5-0 0.8 .0 4 3 4 .8 0.6 I.O 3.0 9-7 5-7 9-5 10.3 12.9 - 8.'o 0.9 I.I 1 .2 1.4 I.O 0.4 1.32 1.26 1.26 1.27 1.29 .... i-33 i! 2 6 1.29 1.47 1-37 1.29 1-33 1-34 1.27 i-37 1.29 Vaux. Muspratt. ii Vaux. Jacqueline. Ledieu. Johnson. Isherwood. Muspratt. n Scotch English (Newcastle) . . (Lancashire) " (Derbyshire) " (Staffordshire) French Anthracite " Bituminous 4-2 5-4 5-8 40 42 19 18 35 40 26 26 j I.O 0.7 I.O .0 1 .0 9 -4 .5 .8 9 . IO.O 19.2 13-2 oTe !.2 2.O 1-5 German (Silesia) . . Saxony Hindostan ... . Brazil Nova Scotia Cape Breton ... . Australia (Lignite) Borneo Chili Coke 13 194 THE STEAM-BOILER. COMPOSITION OF SUNDRY FUELS. C. H. N. 0. S. Ash. Specific Gravity. Authority. Wood (kiln-dried) " (air-dried) Peat (kiln-dried) " (air-dried) 50-5 40.4 60.0 46.1 O.I 4-9 6.8 4.6 O.Q 1-3 1.0 40.7 32.7 30.0 23.6 ;:;; 1.6 1.2 I. 9 i-5 0.510 1.2 0-5 Watts. Paul. Bitumen, United States " England 24.8 52.2 50.3 71.8 24.4 14.0 86.0 86.5 75-0 85-7 Volatile Matter. 2.8 0.3 O.I 1-5 7.6 13-6 Johnson. Watts. 7.0 25.0 14-3 72.4 47-5 41.6 26.7 68.0 72.6 14.0 France ... " South America. . . Asphaltum, Syria Petroleum, pure U. S 0.8 " Dead Oil" Refuse. 1-5 Gas Marsh ... . ... " Olefiant Carb. Acid. Carb. Oxide. H. N. Hydro- carbon. Authority. Gas from Wood ii 6 34 e O 7 co o Kbelmen ' Charcoal o 8 VA I O 2 64 Q ' Peat 14 O 22 4 O ^ 6q i ' Coke I 3 n.8 O I 64 8 M 2.O 4O.O 42.4 ^.2 12.4 Bituminous Coal*.. 4 .I 23-7 8.0 61.5 2.2 Siemens. * Burned in Siemens' gas-producers. 84. The Heating Effect, or calorific power of good specimens of the various kinds of fuel, is given in the follow- ing table, expressed in British thermal units : THE FUELS AND THEIR COMBUSTION. 195 CALORIFIC VALUE OF FUELS. FUEL. CALORIFI c POWER. Water vaporized at Boiling- Cubic Feet required to stow Weight. Pounds Relative. Absolute. point, Parts by one Part. one Ton of Furnace Coal. Foot as stowed. I OOO Hydrogen 4 280 62 5OO 62 7C .... i 816 u ^ /D oA AQ ****** .... defiant gas i 466 iu >4 1 :) 2 1 ^28 .... Coal, Anthracite i 020 IJ ST? *A O4 I J 08 ' * * * " Bituminous i .017 IJ 7o6 I J O5 J.2 tO A& Lignite, dry O 7 IO I ?O 47 to 53 Peat, kiln-dried O 7 jo 150 * u ' jb IO 2< V* Qr 53 " air-dried. . . . o 526 2 5 Wood, kiln-dried o <^m , _ TEMPERATURE OF THEORETICAL ATTAINABLE KE- COMBUSTION. VALUE. VALUE UNDER QUIRED. BOILER. "o a a i'S fe.g a.a o'oS 1 KIND OF " V "rt ** a en 3 -"rt u aJ >< JS t^3 COMBUSTIBLE. 33 w.h ^ & 'w ^" g.? >> ^o" O Zj a 1 o | S 5 ^Si^ s 8^ "c s s|"i| 1 ' I - &a gjj ?1^= <^L gll o-gVI sl j ^ 3* j-^" 5 ^ u=| So * *- w c j-. ~ U Q ~"o ^^ ^Sc^ " 2^*5 rn ^ ^ 5 S ^ ^ Hydrogen 36.00 5,750 3,86o 2,860 1,940 62,032 64.20 Petroleum Carbon 15-43 5,5 3,515 2,710 1,850 21,000 21-74 18.55 19.90 Charcoal ) Coke V 12.13 4,580 3,215 2,440 1,650 14,500 15.00 13-30 14.14 AnthraciteC'l ) Coal- Cumberland .. . Coking bitumi- 12. 06 n-73 4,000 3,36o 3,520 2,55 2,680 1,730 1,810 15.370 15.90 16.00 14.28 M-45 15.06- 15.19 nous Cannel Lignite 11.80 9. 3O 4,850 4,6OO 3,330 32IO 2,540 2 d.QO 1,720 i 670 35,080 1 1 7J.5 15.60 12 I ^ 14.01 10. 78 14.76 1 I . 46 Peat o v tAW )^yj A,(J/<_I 1 -M/TO *l . A^ Kiln-dried 7.68 4,47 3, I 4 2.420 I, 660 9,660 10.00 8.92 9.42 Air-dried, 25 p.c. water 5.76 4-OOO 2,820 2,240 I)55 o 7.OOO 7 25 6 41 6.78 Wood- Kiln-dried 6.00 4,080 2,910 2,260 I ,530 7,245 7-5 6.64 7 02 Air-dried, 20 p.c. water 4.80 3,700 2,670 2,100 1,490 5,600 5.8o 4.08 4-39 The above table gives the air required for complete com- bustion, the temperature attained with different proportions of air, the theoretical value, and the highest practically attainable value under a steam-boiler, assuming that the gases pass off at 320, the temperature of steam at 75 Ibs. pressure, and the in- coming air at 60 ; also, that with chimney draught twice, and with forced blast only, the theoretical amount of air is required for combustion. The effective value of all kinds of wood per pound, when * M. L. Gruner, Engineering and Mining Journal, xviii. THE FUELS AND THEIR COMBUSTION. 199 dry, is substantially the same. The following are the weights on other authorities of different woods by the cord : KIND OF WOOD. Weight. Hickory, shell-bark 4,469 red heart 3,705 White oak 3,821 Red oak 3, 254 Beech 3, 126 Hard maple 2,878 Southern pine 3,375 Virginia pin^. ...... - 2,680 ruce. . Sp 2,325 New Jersey pine t 2,137 Yellow pine 1,904 White pine : 868 The following table of American coals has been compiled from various sources : STATE. COAL. KIND OF COAL. Per Cent of Ash. THEORETICAL VALUE In Heat Units. In Pounds of Water Evaporated. Pennsylvania . Anthracite 3-49 6.13 2.90 15.02 6.50 10.77 5-00 5-60 9-50 2-75 2.OO 14.80 7.00 5-20 5.60 5.50 2.50 5-66 6.00 13.98 5.00 9-25 4-50 4-50 3-40 14,199 13,535 14.221 I3,M3 13.368 13,155 14,021 14-265 12,324 14.391 15,193 13,360 9.326 13.025 13,123 12,659 13.588 14.146 13.097 12,226 9.215 13.562 13,866 12,^962 11,551 20,746 14.70 14.01 14.72 13.60 13.84 I3-62 14.51 14.76 12-75 14.89 16.76 13.84 9-65 13.48 I3-58 13.10 14.38 14.64 13.56 12.65 9-54 14.04 14-35 13.41 11.96 21-47 M Cannel . .Connellsville . . .Semi-bituminous. . Stone's Gas . Youghiogheny. . . . . . .Brown . .Cannel ,, ,, . . . Bureau County . . . . . .Mercer County. . . Montauk Indiana ...Block t( . . Cumberland it Texas ii Washington Ter. Pennsylvania. .. ,4 200 THE STEAM-BOILER. Mr. D. K. Clark thus assigns the several portions of the heat of combustion of good coke, as burned in the locomotive :* Making steam 10,920 B. T. U. Loss at smoke-stack 2,316 Ash and waste 7 6 4 14,000 B. T. U. loo per cent. and concludes that combustion in the furnace of the locomotive may be, and often is, practically perfect, and anticipates that economy in the formation of steam will only be improved by utilizing heat now wasted at the chimney. The usual maxi- mum evaporation is about 8 times the weight of coke used a low figure, which is mainly due to the comparatively small proportion of heating-surface adopted. The nearer the compo- sition of the fuel approaches that of coke, the better, as a rule, the economical effect. Coal gives, as an average, about two thirds the effect of coke, as customarily burned ; and its value may be fairly approximated, the composition being known, by assuming the carbon to be the only useful constituent. ORDINARY CALORIFIC VALUES AS COMPARED WITH GOOD BITUMINOUS COAL. Lbs. Coal. i cord (3 .62 cubic metres) of seasoned hickory or hard maple 2,000 i " " " White oak i,75O i " " " beech, red or black oak 1,500 i " " " poplar, chestnut, or elm. . ., 1,000 i " " " " soft pine 960 85. Analyses of Ash. The following analyses represent the character of ashes of anthracite and bituminous coals. They may be taken as examples simply, since the ash of coal intended for metallurgical purposes should invariably be examined before taking the fuel for any important work. ANALYSES OF ASH. Specific Gravity. Color of Ash. Silica. Alum- ina. Oxide Iron. Lime. Mag- nesia. Loss. Acids S.&P. Pennsylvania Anthracite Bituminous Welch Anthracite sto 372 Reddish Buff. Gray. 45-6 76.0 42.75 21.00 44 8 9-43 2.60 1.41 o-33 0.48 0.40 Scotch Bituminous '26 37 6 5.O2 Lignite. S ^.u c 8 2 6 OQ 8 * Railway Machinery, p. 122. THE FUELS AND THEIR COMBUSTION. 2OI Where the difference between two coals lies principally in their relative percentages of ash, the comparison is made in the manner about to be described. The anthracites contain so little other combustible matter, that, as shown by Professor Johnson,* their calorific value is proportional very nearly to the percentage of contained carbon. 86. The Commercial Value of Fuels is somewhat modi- fied by the depreciation produced by presence of non-combus- tible matter ; this modification occurs in the following ways : (1) A certain amount of carbon is required to heat the whole mass to the temperature of the furnace. Of this a large part is lost. It follows, therefore, that a coal containing a cer- tain small quantity of combustible would have no calorific value, and consequently would be worthless in the market. (2) The presence of a high percentage of ash in a fuel checks combustion by its mechanical mixture with the com- bustible portion of the coal. A coal will, hence, have no com- mercial value when the proportion of refuse reaches a limit at which combustion becomes impossible in consequence of this action. (3) The cost of transportation of ash being as great as that of transporting the combustible, the consumer paying for ash at the same rate as for the carbon, and also being compelled to go to additional expense for the removal of ash ; these facts also determine a limit beyond which an increased proportion of ash renders the fuel valueless. (4) The determination of the financial losses due to in- creased wear and tear of furnaces and boilers, of incidental losses due to inequality or insufficiency of heat-supply, and to the many other direct and indirect charges to be made against a poor fuel, also indicate a limit which has a different value for each case, but which, in most cases, is difficult of even approxi- mate determination. The determination of the minimum pro- portion of combustible, under the first case, is made as follows, assuming this heat to be entirely wasted : (a) The specific heat of ash is usually nearly 0.20. Let X * Report to the Navy Departmenton American Coals. 2O2 7 'HE STEAM-BOILER. represent the percentage of ash which is sufficient to render the coal valueless. Then, since each pound of carbon has a heat- ing-power of 14,500 British thermal units (3625 calories), 14,500 (100 X) = A, represents the available heat of a unit in weight of the fuel ; 100 X 0.20 X 3000 = B, represents the heat required to raise this same amount of coal to a temperature equal to that of the furnace, which is here assumed at 3000 Fahr. (1633 Cent.) above the surrounding atmosphere. Since these quantities A and B are equal : 14,500 (ico X } = 100 X 0.2 X 3000, and X= 96 per cent. The minimum quantity of fuel permissible is, therefore, four per cent, where the first consideration only is taken into the account. () The influence of the second condition is at present not determinable in the absence of experiment. (c) The cost of transportation of ash to the consumer, as a part of the fuel, is not taken in the determination of its value to him. The removal of ash is a tax upon the consumer which may be considered as the equivalent of the loss of a certain weight of combustible received. Since this cost fluctuates with the market value of coal, and since its amount is determined by the same causes, it is easy to make the statement in that form. This cost is about ten per cent of the value of coal, weight for weight, and is therefore assumed at ten per cent of the propor- tion of ash found in the coal. (d) The losses, direct and indirect, coming under the fourth head, vary greatly, and are sometimes very serious. An ap- proximate estimate for an average example is taken, and is considered to be equal, at least, to a percentage of the total value of coal, in utilizable carbon, which equals one half the percentage of ash. Comparing two anthracites, which we will suppose to contain, respectively, fifteen and twenty-five per cent ash, eighty-five and seventy-five per cent carbon, the first being a well-known standard coal, selling in the market at six dollars per ton (1016 kilogrammes), we may, using this system of charging losses against equivalent values in combustible car- bon, determine the proper commercial value of the second kind. THE FUELS AND THEIlt COMBUSTION. 2OJ First Example. From the 85 per cent carbon : Deduct for heating to furnace temperature 0.040 " " transportation of refuse 10 per cent of 15 0.015 " " other losses 50 per cent of 15 0-075 Total 0.130 leaving valuable and available carbon 85 13 = 72 per cent. Second Example. From the 75 per cent carbon : Deduct for heating to furnace temperature 0.040 " " removal of ash 10 per cent of 25 0.025 ' " sundry losses 50 per cent of 25 0.125 Total 0.190 leaving valuable available carbon 75 19 = 56 per cent. Finally, if $6.00 is paid for 72 per cent available combustible, for 56 per cent we should pay = $4.66f. Third Example. Taking a third example, in which the fuel contains the exceptionally large proportion of 30 per cent ash, we should, by similar method, proceed as follows, deducting from the seventy per cent carbon as before the estimated charges against it : Deduct for heating 0.040 " " removal of ash 10 per cent of 30 0.030 " " sundry expenses 50 per cent of 30 0.150 Total 0.220 leaving available carbon, 70 22 = 48 per cent, which would be worth ^- = $4.00. Had the first coal had a market value of seven dollars per ton, the second and third would have been worth, respectively, $5-44i and $4.66$. Expressing this operation by symbols, if V represents the value of the fuel in percentage of pure carbon, and A equal the percentage of ash, V = 0.96 i.6oA. This method is evidently largely empirical, and its results 2O4 THE STEAM-BOILER. are but approximate. It is, however, simple and easily applied, and will often be found of use in the absence of more precise means of determination. The kind and quality of fuel employed in the production of steam for commercial purposes is often determined by condi- tions quite independent of the special quality of the fuel. In most cases the element of cost is the controlling one. Johnson, in his report to the Navy Department (1844) on American coals, proposes to grade coals according to (1) Their relative weights. (2) Rapidity of ignition. (3) Completeness of combustion. (4) Evaporative power under equal weights. (5) Evaporative power under equal bulks. (6) Evaporative power of combustible matter. (7) Freedom from waste in burning. (8) Freedom from tendency to form clinker. (9) Maximum evaporative power under equal bulks. (10) Maximum rapidity of combustion. He found it impossible to select any one coal which could be placed first in all these qualities or to attach equal impor- tance to all. For steam navigation he attaches most impor- tance to the fifth, " the evaporative power for equal bulks," as stowage-space is supremely important in steam navigation. With the fifth he combines the eighth and tenth, viz., " free- dom from clinker" and " maximum rapidity of action." Amerir- can coals are usually superior to foreign coals. 87. Good Furnace Management, to secure maximum heat-supply from the unit weight of fuel, is evidently as essen- tial to economy and efficiency of steam production as choice of proper fuels. In the management of the furnace the effort should be made to secure the best conditions for economy, and as nearly as possible perfect uniformity of those conditions. The fuel should be spread over the grate very evenly, and the tendency to burn irregularly, and especially into holes or thin spots, should be met by skilful " firing," or " stoking" as it is also termed, at such intervals as may by experience be found best. THE FUELS AND THEIR COMBUSJ'ION. The smaller the coal, where anthracite is used, the thinner should be the fire ; the stronger the draught the thicker the bed of fuel, of whatever kind. With too thin a fire, the dan- ger arises of excess of air-supply ; with too heavy a fire, carbon monoxide (carbonic oxide) may be produced. In the former case combustion will be complete, but the heat generated will be distributed throughout the diluting excess of air, and thus rendered less available, and the efficiency of the furnace will be correspondingly reduced ; while in the latter case a loss arises from incomplete combustion, and waste takes place by the passage of combustible gas up the chimney. The second is the less common cause of loss of the two, but both are liable to arise in almost any boiler, and we may even have both losses exhibited in the same boiler and at the same time. Successful working demands a very perfect mixture of the combustible with the supporter of combustion, and should this not be secured, serious waste will take place. The appearance of smoke at the chimney-top is not always indicative of serious loss, nor is its non-appearance always proof of complete combustion. With soft coals and other fuels con- taining the hydrocarbons some smoke usually accompanies the best practically attainable conditions; anthracites, charcoal, and coke never produce true smoke. Attempts to improve the efficiency of a heat-generating apparatus by " burning the smoke" usually fail by introducing such an excess of air as to cause a loss exceeding that before experienced from the forma- tion of smoke. Thorough intermixture of a minimum air-supply with the gases distilled from the fuel is the only means of at- taining high efficiency. In firing, or stoking, especial care should be taken to see that the sides and corners of the grate are properly attended to. Regulation of the fire is best secured by the careful ad- justment of the damper. The manipulation of the furnace doors for this purpose is likely to cause waste. Liquid fuels are especially liable to waste by excessive air-supply, and gas- eous fuel exhibits a peculiar liability to the opposite method of loss ; both should be, if possible, even more carefully handled than any solid fuels. 206 THE STEAM-BOILER. 88. The Fuels, Boiler, and Furnace must be adapted each to the others very carefully, if the best results are to be attained. Soft, free-burning fuels demand a different form of grate, as well as different air-distribution and furnace manage- ment, from the hard and slow-burning combustibles. The form and size of furnace, the extent and kind of heating-sur- face, and the type of boiler even, all influence the total effi- ciency of steam generation. Tubular boilers have small flues or tubes, and are better fitted for use with anthracite coal and with coke or other fuels burning with little flame ; while larger tubes or flues are better adapted for use with the bituminous and other soft, long-flaming combustibles. It thus happens, for example, that a locomotive using anthracite coal, another en- gine burning bituminous coal, and a coke-burning engine, all have different proportions of boiler. CHAPTER IV. HEAT PRODUCTION ; MEASUREMENT ; TRANSFER ; EFFICIENCY OF HEATING-SURFACE. 89. The Nature of Heat, long debated among men of science, has in the course of the last century become well determined. Heat consists in the vibrations of the molecules of which bodies are composed, and is a form of energy. This energy, although actually kinetic, being molecular is often taken to be potential or latent. The two forms in which energy is stored, when heat is communicated to any substance, are " sensible heat," of which the intensity is exhibited by the thermometer, and which is measured in quantity by the various methods of calorimetry ; and " latent heat," which is not detected or measurable as heat, and which in fact does not exist as heat, but has been transformed into the true potential energy of changed physical state and altered molecular rela- tions : it is manifested by a change of volume in the body affected. Thus all masses, of whatever kind, composition, or form, when heated increase in temperature and are altered in vol- ume, and the sum of the heat-energy producing the change in temperature and the potential energy measured by the prod- uct of the change of volume and the total intensity of the forces, internal and external, resisting that change measures the total heat transferred to effect the physical changes noted. The sensible heat retains its original form ; the latent heat, so- called, is no longer heat at all, but may be retransformed and may again appear as heat on reversing the first operation of transfer. In solids, by far the greater part of the heat received remains sensible, and takes effect in producing change of tem- perature ; in the transformation of the solid into liquid by fusion all heat absorbed becomes latent, and produces ex- 2O8 THE STEAM-BOILER. pansion of volume ; in heating the liquid the heat is employed mainly in elevation of temperature, but in part in doing work with the result of transformation into latent heat. During vaporization at any fixed temperature all heat is disposed of in causing change of volume, and this is known as the " latent heat of evaporation," or of vaporization ; while in the expan- sion of vapors and gases the increase of volume continues to be comparatively large in amount, and the " latent heat of ex- pansion" is a correspondingly large proportion of the total, and is especially large in vapors, such as steam, which have great internal potential energy due to the action of powerful molecular attractive forces. The heat-energy demanded to make steam in the boiler is thus, at ordinary temperatures, ten times greater than that required to overcome the external pressure measured by the steam-gauge. 90. Production of Heat by Combustion and other meth- ods involves, in all cases, the expenditure of an equivalent amount of energy in some transformable shape. The original source of all heat-energy is found far back of its first appearance in the steam-boiler. It had its origin at the beginning, when all Nature came into existence. After the solar system had been formed from the nebulous chaos of creation, the glowing mass which is now called the sun was the depository of a vast store of heat-energy, which was thence radiated into space and showered upon the attendant worlds in inconceivable quantity and with unmeasured intensity. During the past life of the globe the heat-energy received from the sun upon the earth's surface was partly expended in the production of great forests, and the storage, in the trunks, branches, and leaves of the trees of which they were composed, of an immense quantity of carbon, which had previously ex- isted in the atmosphere, combined with oxygen, as carbonic acid. The great geological changes which buried these forests under superincumbent strata of rock and earth resulted in the formation of coal-beds, and the storage, during many succeed- ing ages, of a vast amount of carbon, of which the affinity for oxygen remained unsatisfied until finally uncovered by the hand of man Thus we owe to the heat and light of the sun, HEAT PRODUCTION; MEASTREMEXT; TRANSFER. 2OO, as was pointed out by George Stephenson, the incalculable store of potential energy upon which the human race is so dependent for life and all its necessaries, comforts, and lux- uries. This coal, thrown upon the grate in the steam-boiler, takes fire, and, uniting again with the oxygen, sets free heat in pre- cisely the same quantity that it was received from the sun and appropriated during the growth of the tree. The actual energy thus rendered available is transferred, by conduction and radia- tion, to the water in the steam-boiler, converts it into steam, and its mechanical effect is seen in the expansion of the liquid into vapor against the superincumbent pressure. Transferred from the boiler to the engine, the steam is there permitted to ex- pand, doing work, and the heat-energy with which it is charged becomes partly converted into mechanical energy, and is ap- plied to useful work in the mill or to driving the locomotive or the steamboat. Thus we trace the store of energy received from the sun and contained in the fuel through its several changes until it is finally set at work ; and we might go still further and observe how, in each case, it is again usually retransformed and again set free as heat-energy. The transformation which takes place in the furnace is a chemical change; the transfer of heat to the water and the subsequent phenomena accompanying its passage through the engine are physical changes, some of which require for their investigation abstruse mathematical operations. A thorough comprehension of the principles governing the operation of the steam-boiler can only be attained after studying the phenom- ena of physical science with sufficient minuteness and ac- curacy to be able to express with precision the laws of which those sciences are constituted. The study of the philosophy of the generation and application of steam involves the study of chemistry and physics, and of the new science of energetics, of which the now well-grown science of thermo-dynamics is a branch. These sciences, like the steam-engine itself, have an origin which antedates the commencement of the Christian era ; but 14 210 THE STEAM-BOILER. they grew with an almost imperceptible growth for many cen- turies, and finally, only a century ago, started onward suddenly and rapidly, and their progress has never since been checked. They are now fully-developed and well-established systems of natural philosophy. Their consideration is the special province of works on the physical sciences and on applied mechanics. Combustion is simply the union of some combustible with oxygen ; but this phenomenon involves both chemical and physical operations. The first operation is a physical phenom- enon : it consists in the elevation of the temperature of one or both constituents of the compound to be formed, until, by some as yet not clearly understood modification of their mo- lecular relations, their chemical affinities come into play and combination takes place. But this combination consists in the enforced approximation of molecule to molecule, a relative motion taking place of great rapidity, and work is thus done of considerable amount. The resulting collision converts this energy of molecular motion into that energy of molecular vibration familiar to us as heat, and the quantity of heat so produced is the measure of the potential energy of chemical affinity in which it has its origin. With its development in this form this energy assumes an available and manageable form, and becomes at once capable of application to the pur- poses of the engineer. It may now be measured, stored, trans- ferred wherever wanted, and finally, as required, transformed into mechanical energy, and in that form applied to all kinds of useful work. 91. Temperatures and Quantities of Heat are related to each other as are pressures and work in dynamics. The one is a factor of the other, but the first is not a measure of the second. Temperature measures the intensity of molecular heat-vibrations and the tendency of heat-energy to transfer it- self to another body, very much as the pressure or tension of a confined gas or of steam measures the tendency to expand. In fact, the pressure of a confined gas and the total internal and external pressure of a vapor or other substance are directly and precisely proportional to the temperature, measured from the absolute zero of heat-motion. HEAT PRODUCTION; MEASUREMENT; TRANSFER. 211 Quantity of heat is the measure of the energy, whether in heat-units or in equivalent mechanical units, thermal units, calories, or foot-pounds, of the heat transferred in any change. It is equal to the product of the weight of the mass affected, its specific heat and the range of temperature marking the change. Temperatures are measured in either Fahrenheit or centi- grade degrees, and on either the common or the absolute scale. On the Fahrenheit thermometric scale the range of tempera- ture between the two standards, the melting-point of ice or the freezing-point of water, under normal atmosphere and pressure, and the boiling-point of pure water under one atmosphere, is divided into 1 80 equal parts or degrees, and the zero is con- ventionally placed thirty-two degrees below the former point, the freezing and boiling points thus being found at 32 Fahr. and 212 Fahr., respectively. On the centigrade thermometer the range between the standard temperatures is made 100, and the zero is taken conventionally at the lower of these two tem- peratures, the freezing and boiling points being thus at o Cent, and 100 Cent., respectively. The "absolute scale' of temperatures is one on which it is sought to place the zero-point at the absolute zero of heat- motion at that point at which all heat-energy becomes zero and temperature ceases to have existence. This is found to be at very nearly 461. 2 Fahr., or 274 Cent. ; so that, on the ab- solute scale, the standard temperatures are -f- 393. 2 Fahr. and + 573.2 Fahr., or + 274 Cent, and + 374 Cent. It is found that the scale of the air-thermometer is sensibly coincident with the absolute scale, provided its readings are made propor- tional to the volumes of the enclosed gas at the several tern peratures. Calling T the temperature on this scale the charac teristic equation y = constant is found correct for all true gases, / and v being the pressure and volume of unity of weight at any assumed temperature, T; hence for the air-ther- mometer, in which/ is constant, v oc T. The Thermal Unit, the unit by which quantity of heat is measured as heat, is that amount of heat-energy which is de- 212 THE STEAM-BOILER. manded to raise the temperature of unity of weight of water from the temperature of maximum density to one degree above that point. The British thermal unit is measured, cus- tomarily, by the engineer, by the " pound-degrees," and quanti- ties of heat are measured by the number of such thermal units transferred. The metric thermal unit or " calorie," as it was called by the French philosophers who first adopted the metric system, is that quantity of heat which is required to raise the temperature of one kilogramme of water one degree centi- grade, the " kilogramme-degree." Specific Heat is the quantity of heat in thermal units de- manded by unity of weight of any given material, as of water to raise its temperature one degree. When this heat is all sensible, it is simply called specific heat, but when it is in any observable amount latent, as in expansion of gases, a distinction must be made between the " Specific Heat at Constant Vol- ume," which is the real specific heat, and the " Specific Heat at Constant Pressure," and other specific heats involving more or less transformation of heat in the performance of the work of expansion. The specific heats of the gases are given in 78 for constant pressure. Those of the solids are given in the following table : SPECIFIC HEATS OF METALS AND MINERALS. Iron 0.11379 ace. to Regnault, o.noo ace. to Dulong and Petit. Zinc 0.09555 " " 0.0927 " " " Copper 0.09515 " " 0.0949 Brass 0.09391 " " " " '* Silver 0.05701 " '* 0.0557 " " " Lead 0.03140 " " 0.0293 " " " Bismuth 0.03084 " " 0.0288 " " Antimony 0.05077 " ** 0.0507 " " " Tin 0.05623 " " 0.0514 " " " Platinum 0.03243 " " 0.0314 " " " Gold 0.03244 " " 0.0298 " " " Sulphur 0.20259 " " 0.1880 " " " Coal 0.24111 Coke 0.20307 " " Graphite 0.20187 " " Marble 0.20989 HEAT PRODUCTION; MEASUREMENT; TRANSFER. 213 Unslaked Lime. 0.2169 according to Lavoisier and Laplace. Oak-wood 0.570 " Mayer. Glass 0.19768 " " Regnault. Mercury 0.03332 " Laplace and Lavoisier employed the method by melting ; Dulong and Petit, the cooling method ; Pouillet, and recently also Regnault, the method by mixture, which seems to be the most accurate method. Coke, coal, masonry, and the stones and earths may be taken as averaging very closely c 0.20. The woods range from c 0.50 to c 0.65. The specific heat of the same material, as has been seen, is not perfectly constant, but increases as the temperature in- creases. Thus, according to Dulong and Petit, the mean spe- cific heat is as follows : Iron between o and 100, 0.1098; between o and 300, 0.1218 Mercury " " " 0.0330; " " " 0.0350 Zinc " " " 0.0927; " " " 0.1015 Copper " " " 0.0947; " " " 0.1013 Platinum " " " 0-0335; " " " 0.0355 Glass ' " " 0.1770; " " " 0.190 Regnault found the ratio between the freezing and boiling points of the gases to be : ( i ronstant Volume. C Constant 'ressure. Air .q66<; . "^670 .3667 .3661 Nitrogen .^668 .3688 0660 .3667 37IQ Nitrous Oxide . .1676 2710 .3820 .3877 .3843 .3903 A relation between the specific heat and the atomic weight originally established by Dulong and Petit, and confirmed by Hegnault, is very interesting. The product of the specific 214 THE STEAM-BOILER. heats and the atomic weights is nearly constant, and varies only from 38 to 42 ; thus : C. At. Wts. Products. For Iron 0.11379 339- 21 38-597 " Silver 0.05701 675.80 38.527 " Platinum 0.03243 1233-5 39-993 " Sulphur 0.20259 201.17 40-754 92. Thermometry and Calorimetry are the processes em- ployed by physicists and engineers in the quantitative deter- mination of temperatures, and of quantities of heat and their variations. The instruments employed consist of the various kinds of thermometers and pyrometers for measuring tempera- tures, and of several sorts of calorimeter, the form being deter- mined by the character and accuracy demanded by the work to be done. Thermometers usually consist of a bulb, commonly of glass,, and a capillary stem which the fluid inclosed traverses as its volume changes, the position of the head of the column at any moment indicating the temperature attained by the instru- ment at the instant, the reading being taken from a scale established by the maker and standardized by reference to the standard temperatures or by comparison with another instru- ment of known accuracy. Mercury is generally used in thermometers ranging from below the freezing-point up to about 500 Fahr. (260 Cent.).. For the extremely low temperatures at which mercury might freeze, alcohol is used, and it may be employed also for familiar atmospheric temperatures. For temperatures approaching or exceeding the boiling-point of mercury, the various metallic thermometers or " pyrometers" are used, which depend for their operation upon differences in the rates of expansion of two metals. Siemens' electric pyrometer depends for its action on the variation of the resistance of a conductor of electricity with variation of temperature. The finer kinds of thermometer used in the thermometry of the engineer are mainly employed in the determination of temperatures of air and water, in the measurements connected HEAT PRODUCTION; MEASUREMENT; TRANSFER. 21 5 with steam-boiler trials. They are always mercurial thermom- eters, and are made and standardized with the utmost possible accuracy ; those used in the calorimeters employed in deter- mining the character of the steam furnished by boilers are often graduated to tenths, or even to twentieths, of degrees. The pyrometers used by the engineer are commonly constructed of a tube inclosing a rod of a different metal, the two secured together at one end, while at the other end the tube carries a case and dial, and the rod actuates a pointer, through some system of multiplying gear. The tube is usually of iron, and the rod of brass or copper. A more sensitive form is that in which the disposition of the two metals is reversed. The special forms of calorimeter used in connection with boiler tests will be described later. Regnault's and Wiedemann's experiments, made on simple gases, and on carbonic oxide which is formed without con- densation, proved that in these cases the specific heat between O and 200 C. is constant ; whilst their experiments on gases formed with condensation show that the specific heat varies, the mean being given in the following empirical formulae : For CO 2 = 44 gr. C. = 8.41 -f- 0.0053/1 Mean f Regnault " NO = 44 " = 8.96 -j- O.OOa&J and Wiedemann. " C 2 S 4 = 76 " = 10.62 -j- 0.007/, Regnault. " NH 3 =17 " = 8.51 -)- 0.0026s/, Wiedemann. " C 4 H 4 = 28 " = 9.42 -|- o.oiis/, Wiedemann. 93. The Transfer of Heat from the furnace to the boiler involves the application of chemical and physical principles which will be briefly stated in a succeeding part of this chapter. The production of heat by the chemical processes involved in construction has been seen to be governed by the nature of the fuel, by the relative proportion of combustible and of sup- porter of combustion, and by the quantity of diluting gases present. The heat, once produced, is the more completely available as the temperature of the products of combustion is higher ; it is the more completely utilized, also, as the arrange- ments for its transfer are the more complete and effective. The utilization and the waste of heat are dependent upon 2l6 THE STEAM-BOILER. the method and extent of its transfer to the absorbing appara- tus, or to other bodies. The heat generated in the furnace of a steam-boiler is usually mainly transferred to the boiler by radiation, conduction, and convection, partly, often in some- what large proportion, to the chimney and the outer air by convection, and to some extent to adjacent objects by conduc- tion or radiation through the furnace-walls and the occasionally opened furnace-doors. The laws and the extent of these utili- zations or wastes are fairly well understood, and can be some- times calculated with a satisfactory degree of accuracy and certainty. The tendency to transfer heat by either of the three meth- ods, radiation, conduction, or convection, and the quantity so transferred, depend upon (1) The difference of temperature between the source and the receiver of that heat. (2) The extent and character of the surfaces between which such transfer takes place. (3) The extent and nature of the intervening body or bodies. It is usually assumed that it is sensibly correct to take the quantity transferred, in any case, as measured by the product of the difference of temperature by a coefficient obtained for each substance by experiment. 94. Radiation of Heat is the direct transfer of that form of energy from one body to another across intervening space, the only medium of transfer being the " luminiferous ether/' the waves in which act as the vehicles of transportation, travel- ling at the rate of 186,860 miles (300,574,000 m.) per second. The vibrations of dark, pure heat-waves occur at the rate of 400,000,000,000,000 per second or less ; those of greater fre- quency, up to about double this rate, are light-waves ; and still more rapid vibration constitutes the actinic or chemical ray. The slowest heat-rays have about one fourth the rate of the fastest ; and the most rapid of known actinic rays vibrate one hundred times as rapidly as these last. Visibly hot bodies emit all kinds of rays. All bodies are continually receiving HE A TPROD UC TlON; ME A SURE ME X T; TRANSFER, 2 1 7 and emitting heat-rays, and, according to Prevost's theory of exchanges, gain or lose in total heat and in temperature accord, ingly as they gain by absorption from surrounding bodies more than they yield to the latter, or the reverse. A good radiator is always a good absorbent. Any body which absorbs a particular kind of ray will, when emitting energy, radiate the same form. Diathermous substances per- mit the heat-rays to pass through, as transparent substances admit light-rays : but diathermous bodies are not necessarily equally, even if at all, transparent ; and all substances are more diathermous to some rays than to others, while good absorbents are not diathermous. Radiation plays an important part in the operation of the steam-boiler, in the furnace of which, when the fire is bright, it is estimated that usually about one half of all the heat taken up by the generator is received direct from the fuel by radi- ation. 95. Conduction is the method of transfer of heat by flow from part to part in the same body, or from one to another of bodies in contact. These two phenomena are not precisely the same. The flow of heat from a hot to a cold body in contact depends not only upon the conducting power of the two sub- stances, but also, and often mainly, on the condition of the touching surfaces and the perfection of their contact. The rate of transfer within any given material depends solely on the variation of temperature along the line of flow, and on the character of the substance. Conductivity measures the rate of flow, or of transfer of heat, under any assumed and defined conditions; it is the power of transmission of heat. The rate of conduction, or the conductivity, may be expressed by the number of thermal units passing across a surface, or through an internal section, in the unit of time ; it is proportional to the rate of variation of temperature along the line of flow and to the constant co- efficient denominated the conductivity, or the coefficient of con- ductivity. Thus the quantity, Q, of heat passing in any given time, /, is measured by the product of that time into the con- 218 THE STEAM-BOILER. ductivity, k, and into , , the rate of variation of temperature with distance traversed, and area of section, A, The value of k varies greatly with different substances, be- ing comparatively high with the metals and very low with all organic materials and the minerals. Where k is constant, the equation above given becomes T - ....... (2) Where, as is often the case, the thermal resistance instead of the conductivity is taken, we shall have, when r is the co- efficient of resistance, r = -r, and (a) and the following values of r are found by experiment, accord- ing to Peclet, for x in inches and Q in British thermal units per hour:* Gold and silver .................................... 0.0016 Copper ................. . ......................... 0.0018 Iron ................................... , .......... 0.0043 Zinc ........ . ............................. , ...... 0.0045 Lead .......................................... _____ o . 0090 Stone ......... .. ................................. o. 0716 Brick ............................................. 0.1500 Where the plate consists of laminae, each may be considered by itself, and the total resistance obtained by adding together the resistances of the several parts. * Vide Rankine's Steam-engine, p. 259. HEAT PRODUCTION; MEASUREMENT; TRANSFER. 219 The surface resistance forms so large a part of the total in steam-boiler practice, that the formula Q = - - (4) a may be conveniently used to compute the amount of heat transferred, a being taken as from 1 50 to 200 in British meas- ures (15 to 20 in metric measures), accordingly as the surfaces are clean or not, the plate being of iron, with water on one side and hot gases on the other . i sq. ft. ; / = hrs. 96. Convection of Heat occurs by its communication to the particles of a fluid, and then by the flow of those particles into new positions, and by their contact with the receiver of heat by the transfer of that heat to such receiver. Convection is the only method of transfer in liquids, since conductivity is not appreciable, and it is only by its transportation by means of currents that it can be transferred at all. A good circulation is therefore essential to rapid transfer, and the rate of transfer is thus in a sense proportional to the efficiency of circulation. Thus the efficiency of a steam-boiler is dependent upon the effectiveness of its circulation, as well as upon the extent and conductivity of its heating-surfaces. A quiescent mass of water or of gas is incapable of transferring heat, and that element can only pass such a mass by penetrating it as radiated energy, its vehicle being the ether, which pervades all diathermic sub- stances. Heat applied to the surface of still water does not pass downward at all or in any direction by real conduction ; applied at one side or at the bottom of the mass, currents are at once set up, by means of which a rapid upward transfer of heat may take place. Thus convection invariably produces transportation of heated particles, and transfer of heat, from the source of heat to a receiver of heat, or a refrigerator, at a higher level. For best effect the heat must in all cases be applied at the lowest part of the fluid mass. These facts and deductions are equally true of liquids and gases, the latter being even more perfect non-conductors than the former. 220 THE STEAM-BQILEK. Condensation of steam and other vapors by contact with cooling surfaces at temperatures below those of vaporization always occur by a peculiar convection, the circulating or mov- ing currents of vapor streaming toward the refrigerating sur- faces, these streams having their origin in the condensation of the vapor in contact with the latter, and the formation thus of a vacuous space into which they are driven by the elasticity of the fluid. A continuous condensation and steady flow is pro- duced, and is sustained as long as these conditions persist. This operation is the most rapid of all known methods of con- vection or of transfer of heat, the mobility of the vapor per- mitting the most rapid movement of its currents, and its instan- taneous condensation preserving a constant head which forces the fluid in the direction of the condensing surface on which it is converted into a liquid of comparatively small volume and capable of prompt and complete removal. 97. The Transfer of Heat in Boilers is due to convec- tion largely. It is obvious that where transfer of heat takes place from one fluid to another through the sides of a contain- ing vessel, as in the steam-boiler, or the surface-condenser of the marine steam-engine, the two fluids should be so circum- stanced that their currents should flow in opposite directions, the heating or the cooled fluid entering on the heating-surface of the boiler or other vessel at its point of maximum tempera- ture, and passing off at the coolest part; while the coojng or heated fluid, the receiver of heat, should come into contact with the separating sheet of metal at its coldest part and pass off at the hottest. In the steam-boiler the feed-water should enter at that part at which the furnace-gases are entering the chimney-flue, and should circulate toward the furnace. In the surface-condenser the condensing water should enter near where the water of condensation is taken away by the pumps, and should issue near the point at which the steam enters. It is further evident that in the latter case, other things being equal, that disposition of apparatus which permits most rapid and complete removal of the drops and streams of water of condensation from the cooling surfaces, so as to give at all times the maximum possible area of effective surface, will pro- EFFICIENCY OF HEATING-SURFACE. 221 duce the highest efficiency. This has been found practically of essential importance in the design and construction of such condensing apparatus. Feed-water heaters for the above-stated reasons are placed in the chimney-flue, while superheaters are sometimes placed in the furnace. Considerations of convenience and economy, however, oftener compel the designing engineer to place the latter at the exit of the furnace gases from the boiler and between the latter and the feed-water heater. As a rule, how- ever, the rapidity and completeness of the circulation of the waters in a well-designed boiler are such that the point of introduction of feed-water is a matter of minor importance, so far as the boiler itself is concerned ; and the engineer usually seeks to enter the feed in such a manner as shall evade risk of injury by irregular strains due to excessive differences of tem- perature in its different parts. The mass of water in a good boiler, freely steaming, may be assumed to have substantially uniform temperature, and only the furnace gases need be con- sidered as flowing in definite paths with varying temperature. The use of the " counter current, as it is called, is better illus- trated practically in the case of the condenser. Experience shows that the thickness of the intervening plate has practically no important influence, as a rule, on the efficiency of transfer. Thick furnace-flues and thin tubes in the steam-boiler seem about equally effective ; and the Author has known cast-iron condenser-tubes to work practically with the same efficiency as the thin brass tubes, of one quarter their thickness, customarily employed. It should be stated, how- ever, that sheets of iron or steel in the furnaces of boilers, or in flues where exposed to nearly furnace temperatures, are liable to injury by " burning," if very thick, and especially if the laps of their seams are so exposed. In some cases the law forbids the use of heavy plates in furnace-flues or parts exposed to flame. 98. Efficiency of Heating or Cooling Surface measures the ratio of actual amount of heat transmitted across such sur- face to the total quantity available for such application ; in steam-boilers it is the ratio of the quantity of heat utilized in 222 THE STEAM-BOILER. heating and vaporizing the fluid to the total which is produced by the furnace, the unutilized heat being wasted by conduction and radiation to other bodies, or sent up the chimney. An expression was found by Rankine, based upon equation (4) of article 95, which has been found to give very satisfactory re- sults when properly used in application to the ordinary work of steam-boilers. This expression may be derived as below. Let w be the weight of furnace-gases discharged per hour, T t the difference between the temperatures of gas and water on opposite sides of any part of the plate on the elementary area dS, C the specific heat of the gas, and let q be the quantity of heat passing across unity of area in unity of time for a difference in temperature T t, in other words, the " rate of conduction" per unit of area per hour. The quantity of heat transferred across the area dS is then equal to qdS, and the fall of temperature of gas must be this quantity divided by the product of the weight, w, and specific heat, C, of the gas from which the heat is derived, and the gas flows on to the next elementary area and beyond, surrendering its heat as it goes, until it finally leaves the ab- sorbing surface and enters the chimney-flue. If 7^ and T^ are the initial and final temperatures of the gas, and t the temperature of the water entering the boiler, the heat produced, Q lt and that wasted, Q v per hour, are respec- tively measured by & = Cw(T, - /) : a = Cw(T, - t), nearly; . . (2) while the efficiency of the heating-surface is measured by the ratio of total heat to absorbed heat ; or, if the feed enters at atmospheric temperature, or nearly so, by a- g. T,- EFFICIENCY OF HEATING-SURFACE. 22$ The heat utilized, Cw(T l 7^), is also equal to that ab- sorbed and transmitted, qdS\ -T.) and ~ = ~. . . (4) The value of q has been found to be well represented by equation (4) of article 95, in which q = -j-, and hence Ai ( T O 3 q = - : -^ ; and thus _S- C T ^dT_ f T * dT CW-JT ' V T-t ri Assume (T /) = JT, then aCw * and the efficiency becomes Then, since T l -t_S(T l -f) , " T 9 t aCw aCw 224 THE STEAM-BOILER. and (T, - t) - (T, - t) T, - T, T- 1 T,-t r,-/ - s(r t -i)-i If the total heat absorbed per hour be taken as ff, H Cw> ' ' ' (I > and a simplified expression, is obtained, in which Civ may be taken as proportional to the weight of air supplied or of fuel burned, and H as proportional to the same quantity. Thus if F is the weight of fuel burned in the given time, on unity of grate-area, the efficiency may be expressed as BS B ~ ~ = ' which is the formula sought. A and B are constants to be ob- tained by experiment for the special type of boiler to be con- sidered. When 5 and F represent respectively the number of square feet of heating-surface per square foot of grate in any boiler, and the number of pounds of fuel burned as the square foot of p grate per hour, and R = -~, the values of A and B, as given by Rankine,* are as follows: * Steam-engine, p. 294. EFFICIENCY OF HEATING-SURFACE. BOILER TYPE. ^_ Class I. Best convection, chimney draught 0.5 " 2. Ordinary " " " 0.5 3. Best forced " 0.3 " 4. Ordinary " " 0.3 22 5 B. 1. 00 O.go 1. 00 0.95 These constants are derived from experience with good fast-burning bituminous coals; for anthracites of good quality the Author has usually found the following values more in ac- cordance with good practice : BOILER TYPE. A, Class i o 0-5 0-3 0-3 B. 0.90 0.80 0.90 0.85 When feed-water heaters are used, or superheaters are em- ployed, their surface should be included in the area 5. The formula assumes no loss by excess of air-supply. Where such excess is noted or anticipated, it may be allowed for by increas- ing the value of A in proportion to the square of the total quantity of air supplied. The following table presents values of efficiency for a wide range of practice : EFFICIENCY OF BOILERS. BITUMINOUS COAL. ANTHRACITE COAL. Class of Boiler. Class of Boiler. R. I. II. III. IV. I. II. III. IV. 10 o. 16 0.15 0.25 0.22 0.14 0.14 0.23 0.20 4 0-33 0.31 0-45 0.43 0.30 0.28 0.40 0-39 2 0.50 0.46 0.62 0.59 0-45 0.50 0.56 0-53 I 0.66 0.61 0.77 0.73 0.6o 0-55 0.70 0.66 0.80 0.71 0.65 0.81 0.77 0.64 0-59 o.73 0.69 0.67 0.75 0.69 0.83 0.79 0.67 0.63 o.75 0.72 0.50 0.80 0-73 0.87 0.83 0.72 0.65 0.78 0-75 0.40 0.83 0.76 0.89 0,85 0-75 0.68 0.80 0.77 0-333 0.86 0.80 0.90 0.86 0.77 0.72 0.81 0.78 0.167 0.93 0.85 0.95 0.90 0.84 0.77 0.86 0.81 O.III 0-95 0.87 0-97 0.92 0.86 0.78 0.88 0.83 These values have been found to agree well with practice up to rates of combustion exceeding 50 or 60 pounds per 226 THE STEAM-BOILER. square foot of grate-surface per hour, beyond which point the efficiency falls off. But agreement can only be expected where the combustion and air-supply are in accordance with the assumptions on which the formula is based. The problem of the designer of steam-boilers often takes the form : Required to determine the area of heating-surface needed to secure a stated efficiency. In this case the formula above given must be transformed thus : AF ~-' 03) 04) from which expressions, the efficiency aimed at being given, the ratio of heating to grate-surface and the extent of heating- surface may be computed. As will be seen later, the question to what extent efficiency may be economically carried by ex- tending heating-surface is one of the problems arising in de- signing boilers. The Area of Cooling-surface demanded to refrigerate liquids, or to condense steam or other vapor, is capable of somewhat similar calculation. Returning to the primary equations of the preceding article, we have fqdS=Cw(T,'-TJ\ (I) in which we may take 7^ as the measure of the total heat, per unit of weight of the steam entering the condenser or refriger- EFFICIENCY OF HEATING-SURFACE. 22/ -ator, and T t f the temperature of the water of condensation at its exit. As before, in which t becomes the temperature of the circulating or cool- ing water, while for such small differences of temperature we may take q= C(T /), whence 5 = MCw log, ' ~" *i * in which expression the value of N may be taken, for ordinary steam-engine condensers, at about 0.04, rising in exceptional case of inefficient apparatus to o.io, and falling in exception- ally good examples to o.oi, British units being used. M. Havez has found a similar expression to be practically correct for heating-surfaces, and asserts that we may take the quantity of heat transmitted in either case as decreasing in geometrical progression ; while the length of path swept over, measured from the origin, increases in arithmetical progres- sion.* Mr. Williams and M. Petiet both found, in experi- ments on locomotives, that the evaporation diminished about one half at each step, metre by metre, or yard by yard, from the furnace to the smoke-box end of the tubes. The efficiency of the heating-surfaces of boilers has been sometimes considerably increased by the expedient of setting pins in the plates in such manner that, projecting into the flue or furnace on the one side and the water-space on the other, they take up heat from the passing gases and conduct it into the midst of the water. A pin may be thus made to absorb and * Revue Industrielle, Mch., 1874. 228 THE STEAM-BOILER. utilize several times as much heat as could be taken up by the section of the sheet occupied by it. Such " conductor-pins" have often been introduced into marine and other boilers, with very evident improvement in results. Even corrugating a sheet will produce marked advantage in this manner, especially where the direction of the currents is across the lines of corru- gation. 99. The Effect of Incrustation, and of deposits of various kinds, is to enormously reduce the conducting power of heat- ing-surfaces; so much so, that the power, as well as the eco- nomic efficiency of a boiler, may become very greatly reduced below that for which it is rated, and the supply of steam fur- nished by it may become wholly inadequate to the require- ments of the case. It is estimated that a sixteenth of an inch (0.16 cm.) thick- ness of hard " scale" on the heating-surface of a boiler will cause a waste of nearly one eighth its efficiency, and the waste increases as the square of its thickness. The boilers of steam- vessels are peculiarly liable to injury from this cause where using salt water, and the introduction of the surface-condenser has been thus brought about as a remedy. Land boilers are subject to incrustation by the carbonate and other salts of lime, and by the deposit of sand or mud mechanically suspended in the feed-water. It has been estimated that the annual cost of operation of locomotives in limestone districts is increased $750 by deposits of scale. Professor T. B. Stillman finds that the carbonates are precipitated as such ; but that the temperature of the hotter portions of the heating surfaces may drive off the CO a and the water of hydration (J. Anal. Chem., Jan. 1890). CHAPTER V. HEAT AS ENERGY ENERGETICS AND THERMODYNAMICS. 100. Heat as a Form of Energy is subject to the general laws which govern every form of energy and control all matter in motion, whether that motion be molecular or the movement of masses. Under the title " Energetics" are comprehended all laws affecting bodies, molecules, or atoms in relative motion. That heat is the motion of the molecules of bodies was first shown by experiment by Benjamin Thompson, Count Rumford, then in the service of the Bavarian Government, who in 1798 presented a paper to the Royal Society of Great Britain, describing his work, and reciting the results and his conclusion that heat is not substance, but a form of energy. This paper is of very great historical interest, as the now accepted doctrine of the persistence of energy is a generaliza- tion which arose out of a series of investigations, the most im- portant of which are those which resulted in the determination of the existence of a definite quantivalent relation between these two forms of energy and a measurement of its value, now known as the " mechanical equivalent of heat." The experi- ment consisted in the determination of the quantity of heat produced by the boring of a cannon at the arsenal at Munich. Rumford, after showing that this heat could not have been derived from any of the surrounding objects, or by compression of the materials employed or acted upon, says : " It appears to me extremely difficult, if not impossible, to form any distinct idea of anything capable of being excited and communicated in the manner that heat was excited and communicated in these experiments, except it be motion." * He estimates the heat * This idea was not by any means original with Rumford. Bacon seems to have had the same idea; and Locke says, explicitly enough: " Heat is a very brisk agitation of the insensible parts of the object, ... so that what in our sen- sation is heat, in the object is nothing but motion." 230 THE STEAM-BOILER. produced by a power which he states could easily be exerted by one horse, and makes it equal to the " combustion of nine wax candles, each three quarters of an inch in diameter," and equivalent to the elevation of " 25.68 pounds of ice-cold water" to the boiling-point, or 4784.4 heat-units.* The time was stated at " 150 minutes." Taking the actual power of Rum- ford's Bavarian " one horse" at the most probable figure, 25,000 pounds raised one foot high per minute, f this gives the "mechanical equivalent " of the foot-pound as 783.8 heat-units, differing but 1.5 per cent from the now accepted value. Had Rumford been able to measure his power and to eliminate all losses of heat by evaporation, radiation, and con- duction, to which losses he refers, and to measure the power exerted with accuracy, the result would have been exact. Rumford thus made the experimental discovery of the real nature of heat, proving it to be a form of energy, and, publish- ing the fact a half-century before the now standard determina- tions were made, gave us a very close approximation to the value of the heat-equivalent. He also observed that the heat generated was " exactly proportional to the force with which the two surfaces are pressed together, and to the rapidity of the friction," which is a simple statement of equivalence be- tween the quantity of work done, or energy expended, and the quantity of heat produced. This was the first great step toward the formation of a Science of Thermodynamics. Sir Humphry Davy, a little later (1799), published the details of an experiment which conclusively confirmed these deductions from Rumford's work. He rubbed two pieces of ice together, and found that they were melted by the friction so produced. He thereupon concluded : " It is evident that ice by friction is converted into water. . . . Friction, conse- quently, does not diminish the capacity of bodies for heat." * The British heat-unit is the quantity of heat required to heat one pound of water i Fahr. from the temperature of maximum density. f Rankine gives 25,920 foot-pounds per minute or 432 per second for the average draught-horse in Great Britain, which is probably too high for Bavaria. The engineer's " horse-power" 33,000 foot-pounds per minute is far in excess of the average power of even a good draught -horse, which latter is sometimes taken as two thirds the former. HEAT AS ENERGY. 231 Bacon and Newton, and Hook and Boyle, seem to have an- ticipated long before Rumford's time all later philosophers, in admitting the probable correctness of that modern dynami- cal, or vibratory, theory of heat which considers it a mode of motion; but Davy, in 1812, for the first time, stated plainly and precisely the real nature of heat, saying: "The immediate cause of the phenomenon of heat, then, is motion, and the laws of its communication are precisely the same as the laws of the communication of motion." The basis of this opinion was the same that had previously been noted by Rumford. So much having been determined, it became at once evident that the determination of the exact value of the mechanical equivalent of heat was simply a matter of experiment ; and during the succeeding generation this determination was made, with greater or less exactness, by several distinguished men. It was also equally evident that the laws governing the new science of thermodynamics could be mathematically ex- pressed. Fourier had, before the date last given, applied mathemati- cal analysis in the solution of problems relating to the transfer of heat without transformation, and his " Theorie de la Cha- leur" contained an exceedingly beautiful treatment of the sub- ject. Sadi Carnot, twelve years later (1824), published his " Reflexions sur la Puissance Motrice du Feu," in which he made a first attempt to express the principles involved in the application of heat to the production of mechanical effect. Starting with the axiom that a body which, having passed through a series of conditions modifying its temperature, is returned to " its primitive physical state as to density, tem- perature, and molecular constitution," must contain the same quantity of heat which it had contained originally, he shows that the efficiency of heat-engines is to be determined by carry- ing the working fluid through a complete cycle, beginning and ending with the same set of conditions. Carnot was not a believer in the vibratory theory of heat,* and consequently was led into some errors ; but, as will be seen hereafter, the idea * Documents recently discovered (Comptes Rendus, 1878, p. 967) either show this to be an error cr prove his later conversion. 232 777^ STEAM-BOILER. just expressed is one of the most important details of a theory of the steam-engine. Seguin, who has already been mentioned as one of the first to use the fire-tubular boiler for locomotive engines, published in 1839 a W01 "k, " Sur I'lnfluence des Chemins de Per," in which he gave the requisite data for a rough determination of the value of the mechanical equivalent of heat, although he does not himself deduce that value. Dr. Mayer of Heilbronn, three years later (1842), published the results of a very ingenious and quite closely approximate, calculation of the heat-equivalent, basing his estimate upon the work necessary to compress air, and on the specific heats of the gas, the idea being that the work of compression is the equiva- lent of the heat generated. Seguin had taken the converse operation, taking the loss of heat of expanding steam as the equivalent of the work done by the steam while expanding. The latter also was the first to point out the fact, afterward experimentally proved by Hirn, that the fluid exhausted from an engine should heat the water of condensation less than would the same fluid when originally taken into the engine. A Danish engineer, Colding, at about the same time (1843), published the results of experiments made to determine the same quantity ; but the best and most extended work, and that which is now almost universally accepted as standard, was done by a British investigator. Joule commenced the experimental investigations, seeking a measure of the relations of heat and work, which have made him famous, at some time previous to 1843, at which date he published, in the Philosophical Magazine, his earliest method. His first determination gave 770 foot-pounds. During the succeeding five or six years Joule repeated his work, adopting a considerable variety of methods, and obtaining very variable results. One method was to determine the heat produced by forcing air through tubes ; another, and his usual plan, was to turn a paddle-wheel by a definite power in a known weight of water. He, in 1849, concluded these researches, and announced finally the value 772 foot-pounds as that of the mechanical equivalent of the British heat-unit. HEAT AS ENERGY. 233 101. Energetics treats of modifications of energy under the action of forces, and of its transformation from one mode of manifestation to another, and from one body to another, and within this broader science is comprehended that latest of the minor sciences, of which the heat-engines and especially the steam-engine illustrate the most important applications Thermodynamics. The science of energetics is simply a wider generalization of principles which have been established one at a time, and by philosophers widely separated both geographi- cally and historically, by both space and time, and which have been slowly aggregated to form one after another of the physi- cal sciences, and out of which, as we now are beginning to see, we are slowly evolving wider generalizations, and thus tending toward a condition of scientific knowledge which renders more and more probable the truth of Cicero's declaration : " One eternal and immutable law embraces all things and all times." At the basis of the whole science of energetics lies a principle which was enunciated before Science had a birthplace or a name: All that exists, whether matter or force, and in whatever form, is indestructible, except by t]ie Infinite Power which lias created it. That matter is indestructible by finite power became ad- mitted as soon as the chemists, led by their great teacher La- voisier, began to apply the balance, and were thus able to show that in all chemical change there occurs only a modification of form or of combination of elements, and no loss of matter ever takes place. The " persistence" of energy was a later dis- covery, consequent largely upon the experimental determina- tion of the convertibility of heat-energy into other forms and into mechanical work, for which we are indebted to Rumford and Davy, and to the determination of the quantivalence anticipated by Newton, shown and calculated approximately by Colding and Mayer, and measured with great probable accuracy by Joule. It is now generally understood that all forms of energy are mutually convertible with a definite quantivalence; and it is not certain that even vital and mental energy do not fall within 234 THE STEAM-BOILER. the same great generalization. This quantivalence is the basis of the science of energetics. Experimental investigation and analytical research have together thus created a new science, and the philosophy of the steam-engine has at last been given a complete and well-defined form, enabling the intelligent engineer to comprehend the opera- tion of the machine, to perceive the conditions of efficiency, and to look forward in a well-settled direction for further ad- vances in its improvement and in the increase of its efficiency. Energy is the capacity of a moving body to overcome resist- ance offered to its motion ;* it is measured either by the prod- uct of the mean resistance into the space through which it is overcome, or by the half product of the mass of a free body into the square of its velocity. Kinetic energy is the actual energy of a moving body ; potential energy is the measure of the work which a body is capable of doing under certain conditions which, without expending energy, may be made to affect it, as by the breaking of a cord by which a weight is suspended, or by firing a mass of explosive material. The British measure of energy is the foot-pound ; the metric measure is the kilo- grammetre. Energy, whether kinetic or potential, may be observable and due to mass-motion ; or it may be invisible and due to molecular movements. The energy of a heavenly body or of a coiled spring, and that of heat or of electrical action, are illus- trations of the two classes. In Nature we find utilizable poten- tial energy in fuel, in food, in any available head of water, and in available chemical affinities. We find kinetic energy in the motion of the winds and the flow of running water, in the heat- motion of the sun's rays, in heat-currents on the earth, and in many intermittent movements of bodies acted on by applied forces, natural or artificial. The potential energy of fuel and of food has already been seen to have been derived, at an earlier period, from the kinetic energy of the sun's rays, the fuel or the food being thus made a storehouse or reservoir of * The term " energy" was first used by Dr. Young as the equivalent of the work of a moving body, in his " Lectures on Natural Philosophy " (1807). HEAT AS ENERGY. 235 energy. It is also seen that the animal system is simply a " mechanism of transmission" for energy, and does not create but simply diverts it to any desired direction of application. All the available forms of energy can be readily traced back to a common origin in the potential energy of a universe of nebulous substance (chaos), consisting of infinitely diffused matter of immeasurably slight density, whose " energy of posi- tion" had been, since the creation, gradually going through a process of transformation into the several forms of kinetic and potential energy above specified, through intermediate methods of action which are usually still in operation, such as the poten- tial energy of chemical affinity, and the kinetic forms of energy seen in solar radiation, the rotation of the earth, and the heat of its interior. The measure of any given quantity of energy, whatever may be its form, is the product of the resistance which it is capable of overcoming into the space through which it can move against that resistance, i.e., by the product RS, or the equiva- wv* lent expressions and \MV*, in which W is the weight, M <~> the "mass" of matter affected by the motion, Fthe velocity, and g the dynamic measure of gravity. The tlirec great laivs of energetics are : (1) The sum total of the energy, active and potential, of the universe is invariable. (2) The several forms of energy are all interconvertible, and possess a definite quantivalence. (3) All forms of kinetic energy are tending toward reduc- tion to forms of molecular motion and final dissipation through- out space. 102. Heat-energy and Temperature are' closely related and directly proportional, the one to the other. The investigations of physicists have shown that when p and v are the pressure and volume of unit weight of any gas, and c is the velocity of molecules having the mass ;// and in number n, pv ^mnc*', (i) 236 THE STEAM-BOILER. it is also known that (2) when R = ^-, the subscripts denoting that these quantities * o are taken at the freezing-point of water, and T is the tempera- ture measured from the absolute zero, as hereafter defined (- 4 6i.2 F., or - 274 C); hence (3) and the temperature of any substance, measured on the abso- lute scale, is proportional to the kinetic energy of the molecules constituting the gas. In other words, as elsewhere stated, temperature is a measure of the intensity of molecular vibra- tion, while quantity of heat, as has been seen, is quantity of molecular energy of vibration. Thus temperature, as measured on the absolute scale and on the air-thermometer, is directly proportional to the molec- ular energy of any given mass, and thus, in the case of any confined gas, measures the intensity of pressure on the enclos- ing walls due to the heat-energy so imprisoned, which quan- tity is also proportional to the product of this pressure into the volume of the space throughout which it is exerted. 103. Quantitative Measures of Heat-energy, obtained by the various systems of calorimetry, always involve determina- tions of the magnitudes of factors the product of which give the quantity of molecular energy present. These factors have been seen to be either measures of the mass affected and of molecular velocity, or thermal equivalents. The quantity of heat-energy to be measured is obtained either by multiplying the mass by the square of velocity of vibration, or by the product of the weight into the range of temperature considered and the mean specific heat : these two measures are equivalent. It is by either method made evident that temperature is one factor of a product which is the measure of heat-energy, the HEAT AS ENERGY. 237 other factor being a measure of the mass of matter acting as the vehicle of that energy. 104. Heat Transformations may take place, through the action of physical and chemical forces, into any other known form of energy, and another form of energy may be transmuted into heat. Nearly all physical phenomena, in fact, involve heat-transformation in one form or another, and in a greater or a less degree, under the laws of energetics. According to the first of those laws, such changes must always occur by a defi- nite quantivalence, and when heat disappears in known quan- tity it is always certain that energy of calculable amount will appear as its equivalent ; the reverse is as invariably the case when heat is produced ; it always represents and measures an equivalent amount of mechanical, electrical, chemical, or other energy. 105. Heat and Mechanical Energy are thus evidently subject to the general laws of transformation of energy, and the transmutation of the one into the other must always be capa- ble of treatment mathematically. The relations of these two forms of energy are thought by the physicist and the engineer as of sufficient importance, and the phenomena involving these relations alone are so often found to demand and to permit in- dependent consideration, that they are taken as the subject of a division of energetics known as the science of thermodynamics, and a vast amount of study and research has been given by the ablest mathematical physicists of modern times to the investi- gation of its laws and their applications, and to the building up of that science. The conversion of water into steam in the steam-boiler and the utilization of the heat-energy thus made available, or in heated air and other gases, in steam- or other heat-engines, con- stitute at once the most familiar and the most important of known illustrations of thermodynamic phenomena and their useful application. The process of making steam is one of pro- duction of heat by transformation from the potential form of energy through the action of chemical forces, and its storage in sensible form for later use in the steam-engine, where it is changed into equivalent mechanical energy. The pure science 238 THE STEAM-BOILER. of the steam-engine is thus the science of thermodynamics, the first applications of which are made in the operations carried on in the steam-boiler. 106. Thermodynamics is that science which treats solely of the relations of heat and the mechanical form of energy, of the establishment of the laws governing their interconversion, and of the applications of those laws. The science of thermodynamics is, as has been stated, a branch of the science of energetics, and is the only branch of that science in the domain of the physicist which has been very much studied. This branch of science, which is restricted to the consideration of the relations of heat-energy to mechanical energy, is based upon the great fact determined by Rumford and Joule, and considers the behavior of those fluids which are used in heat-engines as the media through which energy is transferred from the one form to the other. As now accepted, it assumes the correctness of the hypothesis of the dynamic theory of fluids, which supposes their expansive force to be due to the motion of their molecules. This idea is as old as Lucretius, and was distinctly ex- pressed by Bernouilli, Le Sage and Prevost, and Herapath. Joule recalled attention to this idea in 1848, as explaining the pressure of gases by the impact of their molecules upon the sides of the containing vessels. Helmholtz, ten years later, beautifully developed the mathematics of media composed of moving, frictionless particles ; and Clausius has carried on the work still further. The general conception of a gas, as held to-day, including the vortex-atom theory of Thomson and Rankine, supposes all bodies to consist of small particles called molecules, each of which is a chemical aggregation of its ultimate parts or atoms. These molecules are in a state of continual agitation, which is known as heat-motion. The higher the temperature, the more violent this agitation ; the total quantity of motion is measured as vis viva by the half-product of the mass into the square of the velocity of molecular movement, or in heat-units by the same product divided by Joule's equivalent. In solids, the range of motion is circumscribed, and change of form cannot HEAT AS ENERGY. 239 take place. In fluids, the motion of the molecules has become sufficiently violent to enable them to break out of this range, and their motion is then no longer definitely restricted. The science of thermodynamics finds application in every phenome- non in which these various manifestations of heat-energy are accompanied by the performance of work or result from such work. 107. The First Law of Thermodynamics is a simple corollary of the first law of energetics ; it is enunciated as fol- lows : Heat-energy and mechanical energy are mutually convertible and have a definite equivalence. The British thermal unit being equivalent to 772 foot- pounds of work, nearly, and the metric calorie to 423.55, or, as usually taken, 424 kilogrammetres.* The first precise and direct determinations of the mechani- cal equivalent of the thermal unit were made by Joule, by sev- eral methods. He stated the results of his researches relating to the mechanical equivalent of heat as follows : (1) The heat produced by the friction of bodies, whether solid or liquid, is always proportional to the quantity of work expended. (2) The quantity required to increase the temperature of a pound of water (weighed in vacua at 55 to 60 Fahr.) by one degree requires for its production the expenditure of a force measured by the fall of 772 pounds from a height of one foot. This quantity is now generally called " Joule's equivalent." During this series of experiments Joule also deduced the position of the " absolute zero," the point at which heat-motion ceases, and stated it to be about 480 Fahr. below the freezing- point of water, which is not very far from the probably true value, 493. 2 Fahr. ( 273 C.), as deduced afterward from more precise data. This first law is that by the application of which we deduce a measure of the quantity of work done whenever a known * A committee of the British Association reported its value (1878)10 be 772.58 foot-pounds, and a later figure is 774, with a limit of error of about two per cent. 240 THE STEAM-BOILER. amount of heat is transformed ; it does not determine how much in any case will be transformed. For example, for any heat- engine we may calculate precisely how much is demanded for the performance of work when it is known how much work is done ; but this law affords no means of determining, in any such case, what proportion of the heat-energy sent into the system will be converted into work, or what part will pass through untrans- formed ; and it hence gives no clue to the total quantity of heat called for, or of steam to be made at the boiler, even though all wastes by conduction and radiation be discovered and measured. This clue is given by the second law, which will also enable us to determine the amount of thermodynam- ically unavoidable loss. 108. The Second Law of Thermodynamics is stated in a great variety of ways by various writers, and is not always clearly enunciated by the best authorities. The following method of statement is adapted especially to present purposes : In the transfer or the transformation of heat-energy, the total effect produced is directly proportional to the total quantity of heat present and acting. Thus, if the effect of heat be to produce change of pressure, change of volume, or variation of temperature, the magnitude of that alteration of pressure, of temperature, or of volume will be directly proportional to the quantity of heat concerned in its production. This law is based upon the almost axiomatic proposition, that heat-energy is homogeneous, and equal quan- tities must invariably be capable of causing equal effects. Since, in any mass of matter acting as a reservoir or vehicle of heat-energy, the quantity of heat present is proportional to its absolute temperature, it follows, from what has preceded, that the effect produced by any thermal variation in a heated mass is proportional to the absolute temperature at which the action takes place. These propositions and the second law of thermodynamics are expressed algebraically by the equations - - dT' f~ dT' HEAT AS ENERGY. 24! in which Q and T are the quantity of heat contained in the body and its absolute temperature. In other words, the prod- uct of the absolute temperature by the ratio of variation of any quantity with temperature is equal to the product of the heat acting into the rate of variation of that quantity with the variation of heat. The quantity of work performed by transformation of heat is measured by dW = QdU = TdU ; . . . . , (2) which will become known when the law of variation of work, dU, with heat, Q, can be given. 109. The Molecular Constitution of Matter and its physical structure and state determine precisely how heat will affect it, and just how it will behave in the storage, transfer, and transfor- mation of that energy into other forms. All matter consists of particles or molecules, sometimes simple, but usually complex, affected by the forces which become observable under the action of one body upon another. These forces are either attractive, repulsive, or directive. Thus, heat produces a mutual repul- sion of molecules, and, if permitted by surrounding masses, the body expands with its reception. Cohesion is an attractive force, as is gravitation, while magnetic and electric forces may be either attractive or repellent ; and the polarity seen in the formation of crystals and magnetism gives directive power the first determining the method of aggregation of approximating molecules, the last the positions assumed by the molecules affected by it. The property of inertia is common to all forms of matter, and is essential to the production of all the phe- nomena observed in the motion and mutual actions of free bodies. 110. Solids are bodies in which the attractive and directive forces are sufficiently powerful to give stability both of form and of volume. Liquids have stability of volume but not of form ; while gases and vapors have stability neither of form nor of volume, and in them the repellent forces have more intensity 16 242 THE STEAM-BOILER. than the attractive. In gases the latter become insensible, and in the hypothetical " perfect " gas cease to exist. All interme- diate degrees of stability exist among the substances known in nature, and no known form of matter can be assigned to either class as a perfect representative of the combination of properties defining it. In passing from one state to another, substances traverse these intermediate conditions. Ice, water, and steam illustrate the three typical classes of matter. In the first the attractive and directive forces give stability of form and strength ; in the second, no stability of form exists, but some tenacity or cohesive power remains, which cannot be easily detected in con- sequence of the freedom of relative motion permitted among its particles when polarity disappears ; in the third form of the same substance the fluid must be confined within walls capable of sustaining its outward pressure to keep it from indefinite ex- pansion. The thermodynamic definition of a perfect gas is found in the equation pv p.v. -~r --yr- = R, a constant, * * i the product of pressure and volume always varying with the absolute temperature. in. Heat and Matter have this peculiar relation, that while all other forces which commonly, with that due to the presence of heat, determine to which of the three physical states the latter shall be assigned, are definitely related to the sub- stance, having magnitudes which are functions of volume and of molecular distances, the force introduced with heat, and which is always repellent, is variable, independently of all other conditions, and is, in fact, constantly so varying. It is the introduction or the removal of heat energy from mat- o/ ter which produces all familiar physical changes of states. When a solid is heated it is expanded against the resisting efforts of all other internal and external forces, and after a time the quan- tity of heat and the temperature attaining a limit which is per- fectly definite for each substance, the directive force becomes HEAT AS ENERGY. 243 insensible, and the mass becomes liquid. The introduction of heat continuing, the separation of molecules continues until the cohesive force becomes insensible, or at least less than the ex- pansive force of the heat, and the fluid is converted into a vapor ; and finally, when the attractive forces disappear en- tirely, into gas. In this process, internal forces being overcome, internal work is performed, and external forces being overcome, external work is done ; while a certain amount of heat, not so expended, is added to the mass as sensible heat, and thus raises its temperature. Specific Heats measure the quantity of heat absorbed by unit weight of any substance in a change of temperature of one degree, the heat being either all or partly unchanged. It has been already defined and values given in 91. Thermodynam- ically considered, it is seen specific heats may measure either heat or work, or both. 112. Sensible and Latent Heats must be carefully distin, guished in studying the action of heat on matter. The term " sensible heat " scarcely requires definition ; but it may be said that sensible and latent heats represent latent and sensible work ; that the former is actual, kinetic, heat-energy, capable of transformation into mechanical energy, or vis viva of masses, and into mechanical work; while the latter form is not heat, but is the equivalent of heat transformed to produce a visible effect in the performance of molecular, or internal as well as external, work, and visible alteration of volume and other phys- ical conditions. It is seen that heat may become "latent" through any transformation which results in a definite and defined physical change, produced by expansion of any substance in consequence of such transmutation into internal and external work ; whether it be simple increase of volume or such increase with change of physical state. 113. The Latent Heat of Expansion is a name for that heat which is demanded to produce an increase of volume, as distinguished from that untransformed heat which is absorbed by the substance to produce elevation of temperature. The latent heat of expansion may, by its absorption and transforma- 244 THE STEAM-BOILER. tion, and the resulting performance of internal and external work, cause no other effect than change of volume, as, e.g., when air is heated ; or it may at the same time produce an alteration of the solid to the fluid, or of the liquid to the vaporous state, as in the melting of ice or the boiling of water, in which latter cases, as it happens, no elevation of temperature occurs, all heat received being at once transformed. In the expansion of air, and in other cases in which no such change of state occurs, a part of the heat absorbed remains unchanged, producing elevation of temperature ; while another part is transformed into latent heat of expansion. The specific heat of constant volume, no molecular or other work being done, measures the heat untransformed, and, as sensible heat, producing rise in temperature. The specific heat of constant pressure measures the sum of the sensible and latent heats, when a gas is heated, and no alteration of physical state can occur. It usually is assumed to include both internal and external work, as well as sensible heat, but where used in an unaccustomed sense the conditions of the case are always stated. 114. The Latent Heats of Fusion and of Vaporization measure the quantities of heat transformed in these changes of physical state. In the first of these two cases the work done is mainly internal ; in the second the internal work performed is much greater, but is not so enormously in excess of the amount of external work done; and the higher the pressure under which vaporization takes place, the larger proportionally the measure of external work and of the heat demanded for its performance. In the case of steam, as will be seen later, at ordinary pressures, the ratio of internal to external work in this change of state is about as ten to one. All this work is performed in the expansion of the mass against resisting molec- ular attractive forces, unperceivable and incapable of measure- ment by any ordinary pressure-gauge or physical instrument. 115. The Distribution of Heat Energy in thermodynamic operations, and in physical changes produced by it, must be carefully studied, and must be represented in every algebraic expression in the mathematical theory of the subject. As has HEAT AS ENERGY. 24$ been fully shown, the absorption of heat by any substance often involves, and may in any given case involve, three different applications; it may be appropriated to the elevation of tem- perature ; to the expansion of the mass against internal forces, doing internal work ; or to the increase of its volume, overcom- ing external pressures and performing external work. On the other hand, if heat is received from any substance, it may be sensible heat simply transferred without change ; or it may be heat produced by transformation out of work through the action of cohesive forces ; or it may be heat similarly resulting from the work done by external pressure on the mass during its com- pression. Whatever the manner in which heat-energy is transferred or transformed, such phenomena as are observed during the pro- cess are subject to the principles which have been stated, and the theory of the process is constructed by the application of the two laws which have been enunciated, and in that manner only. Every algebraic expression representing such a process will be a statement of equality between the total amount of heat- energy entering or leaving the substance, and the sum of the variations of sensible and latent heats in the mass affected. 116. The Application of the First Law leads at once to the construction of the fundamental equations of thermody- namics, and permits the determination of their constants. The first equation to be established is simply a statement, in alge- braic language, of the fact that the total quantity of heat ab- sorbed or rejected by any substance during any elementary change must be the sum of the variation of the sensible heat of the mass and of the latent heats. The convertibility of the thermal unit into the mechanical unit of work or energy renders it a matter of indifference which unit is adopted. If Q repre- sent heat measured in thermal and H the same quantity in mechanical units, and if /be taken as the symbol of the me- chanical equivalent of heat, and A -, the thermal equivalent of the mechanical unit, we may write at once, as the expression of the first law of thermodynamics, dH = JdQ = KdT + dW, (i) 246 THE STEAM-BOILER. in which equation K is the dynamical specific heat, or in sym- bols CJ, the product of the thermally measured specific heat, C y by Joule's equivalent ; T the absolute temperature ; 5 the sensible and J^the total latent heat, measured in mechanical units. Hereafter all measurements will be given in mechanical units, unless otherwise stated. Separating the heat doing the work, W, as distinguished from other heat, into two parts, the one, Z, the internal latent heat, the other, U, the latent heat of external work, dH ' = JdQ = dS + dL + dU, .... (2) and making the " internal energy," as it is sometimes called, E, the sum of the sensible heat and internal work, ...... (3) Or, otherwise exhibited, dS 4- dW dL + dU dE And these expressions are true for all substances and for all possible cases. The sensible heat being the product of the specific heat into the range of temperature, and work being always the product of the alteration of volume into the intensities of the mean re- sistance, the preceding equations may be written : dH=KdT+( Pi +p f )dv HEAT AS ENERGY. 247 when//,/,, and / represent respectively the internal, the ex- ternal, and the sum of internal and external forces, and v is the volume of the mass, which is assumed to have unity of weight. When, as here, the two independent variables are tempera- ture and volume, and, from the preceding, we thus find dH dH and the values correspond with the definitions already given. 117. The Application of the Second Law of Thermo- dynamics establishes some important modifications of the equations just derived. Since every effect is proportional to the quantity of heat acting to produce it, and hence to the ab- solute temperature of the mass, (I) in which expression is that" thermodynamic function" which, being multiplied by the absolute temperature, will give a prod- uct measuring the quantity of heat demanded or rejected in the production of the change. Again, since dW ' = pdv, and since, according to the second law, the total pressure,/, must be equal to the product of the absolute temperature at which the change occurs by the rate of variation of pressure with temperature, dp (2) and the form and value of the thermodynamic function be- comes at once determinable : 248 THE STEAM-BOILER. dp (3) By a process which need not be here described, and which can be seen in every treatise on thermodynamics, an equation of somewhat similar form, but in which the variables taken are is obtained, thus: dv . ... (4) The fundamental equations of thermodynamics are thus completely established. As here given they are general, and applicable to all substances. In the present work, however, we are only concerned with their application to the operation of thermodynamic changes occurring in water and steam. 118. The Computation of Internal Forces and Work, and of external work, are now easily effected. Notwithstand- ing the fact, as already stated, that the molecular forces, and the work performed by or against them, are beyond the reach of any physical apparatus and are incapable of direct measure- ment, it becomes easy to calculate both force and work from measurable data by application of the second law of thermo- dynamics. The rate of variation of external pressure and work with temperature, at constant volume, may be determined easily by experiment ; this rate, according to the second law, is constant for all temperatures, and hence, being multiplied by the abso- lute temperature at which the total pressure or the work is to be determined, the product measures that total pressure or work. In symbols, let/, w, and T represent the total pressure and work, and the absolute temperature ; then the rates of va- . . dp d^v nation -j~, -T-,, with temperature may be ascertained by, for HEAT AS ENERGY. 249 example, noting the change of external pressure, as measured by the steam-gauge, for a change of one degree or other small but exactly measurable range, and taking this ratio of differences, Ap dp ^y, as sensibly equal to -r~. The work-ratio is obtained by multiplying the Ap by the volume and taking this product, Aw dw Ap-v Aw, as the numerator in -j-^= ~j^. Then the total A 1 a 1 pressure, internal and external, must be measured by (0 and the total work of expansion from zero ^dw dp = T Tt It thus becomes possible readily to determine the inter- nal and external pressures, the internal and external work, and the latent heats of the vapors, or of any other imperfectly gaseous or non-gaseous substance. Since the heat rendered latent, in any case, is the equivalent of the work performed by it, the latent heat of vaporization must be exactly equal, dynamically, to the work just measured, and if it be called H for unity of weight, (3) when Av is the increase of volume taking place during the change of physical state. If the value is made known, as is usual, by experiment, and Av is observed, it becomes easy to obtain dp 7\v, v t ) dT~ H (4) dp . The value of -^ is sometimes found to be negative, e.g., in 250 THE STEAM-BOILEK. the case of ice. Professor James Thomson found _ --= o.oi33 Fahr. = O .oo;4 Cent. as the amount by which the melting-point of ice is lowered by every increase of one atmosphere of pressure. The latent heat of fusion is similarly measured. The total heat of vaporiza- tion, as it is called, from a temperature 7^ and at a tempera- ture TV is the sum of the latent heat converted into work, as- just measured, and the sensible heat demanded to raise the temperature from 7i to T v The latent heat of vaporization per unit of volume is ob- viously measured by L H - T dp L = ^r " T Tr ...... (5 > and this permits the ready calculation of the heat demanded in supplying any steam, or other vapor, engine with the quantity of fluid required to do any given amount of work, or to drive its piston through any given space, and this without knowing the density of the fluid. The rate of variation of the pressure of the vapor at the boiling-point, with temperature, may be obtained from the tables, or from formulas such as have been given for steam by Regnault, and for that and other vapors by Rankine.* The latter are the most general and usually the most exact ; they have the form 7? C log/=^--y-y5; ..... (6) whence geio. . . . (7) The density of vapor may thus be readily computed from the known value of its latent heat, and much more satisfactorily * Steam-engine, 206, Div. III. HE A 7' A S EN ERG Y. 2 5 I and exactly than it can be derived by any known method of experimental determination. The increase of volume of unity of weight must always be H *, -*, = ; ....... (8) in which, practically, the values of z\ may usually be neglected. Then the density* is * Tables thus calculated for steam and for ether and other fluids are given by Rankine in his Miscellaneous Papers and in his treatise on the Steam-engine. CHAPTER VI. STEAM AND ITS PROPERTIES. 119. The Production and Use of Steam involves so im- portant and interesting a series of physical phenomena that they are deserving of special study. The generation of steam, and its supply to the steam-engine or other apparatus in which it finds application, is a process: first, of heating the " feed- water" from the temperature at which it is supplied up to that at which it is vaporized ; secondly, the change of its physical state at the latter temperature; thirdly, its expansion into a vapor; and finally, in some cases, the drying and even the superheating of the steam so formed until it assumes the truly gaseous state. The water supplied to the steam-boiler often comes from rivers or smaller streams, sometimes from springs, occasionally from rain-water cisterns, and, at sea, either from condensers or stills, or from the ocean. Each one of these sources of supply provides water having properties characteristic of its origin, and fitting it, or unfitting it, as the case may be, more or less per- fectly for its use in the boiler. The study of the properties of pure water, of its composition and chemical and physical char- acter, and of the nature and effects of the impurities dissolved or mechanically suspended in it, is thus made essential to an intelligent understanding of the problems presented to the engineer who designs, builds, or operates the steam-boiler. The chemistry and physics of water and steam, and of their changes of state and properties, must be studied in connection with the thermodynamics of steam in its application as a vehicle and a reservoir of heat-energy; and with this study must be combined also, and especially, that of the relations of heat and steam to mechanical power as developed by transformation of heat in the steam-engine. This latter division of the subject is commonly reserved for treatises on the steam-engine. STEAM AND ITS PROPERTIES. 253 120. The Properties of Water, as noted by the senses, are familiar to all. It occurs universally distributed throughout the world, in earth, air, and sea, in its three forms, ice, water, and vapor, and in its most common and familiar form covers three fourths of the surface of the globe. As ice and snow it per- manently covers the arctic regions and the tops of lofty moun- tains, and as vapor it forms an important constituent of the atmosphere. When pure, it is absolutely free from either taste or smell, and is colorless, except that in very large masses it assumes a beautiful blue tint. In the form of ice it weighs about 55 pounds per cubic foot (0.9 kilog. to the litre, nearly), and has considerable tenacity, while yet capable of flow, with breaking up and " regelation" under pressure. In the liquid state it still retains considerable cohesive force, but the lack of polarity among its molecules, and its con- sequent instability of form, so modify its properties that this tenacity cannot be perceived except by the adoption of special expedients directed to that end. Converted into vapor or steam, it assumes all the characteristics of the gases, except that, like other vapors, at temperatures and pressures near the boil- ing-point it gives evidence of the imperfection of its gaseous state by more rapid variation of pressure with temperature than the laws of the gases would indicate. Heated above the tem- perature of its boiling-point it rapidly takes on the properties of a true gas, and conforms to the laws of Boyle and Marriotte, PV and of Charles and Gay-Lussac, and the expression - = constant then becomes sensibly correct. Water is the most efficient of all known solvents, and under certain conditions dissolves nearly all kinds of matter, even at- tacking glass and other mineral substances at high temperatures and pressures. Its action on metals is often marked, and is sometimes very serious. It dissolves lead rather freely, so much so that lead-poisoning not infrequently occurs from the presence of that metal in drinking-water held in contact with it. The presence of carbonic acid in observable amount, how- ever, seems essential to the rapid solution of lead, as it invaria- bly is in oxidation. The lead in solder is dissolved more freely 254 THE STEAM-BOILER. than pure lead alone. Water, especially when containing car- bonic acid, dissolves iron and copper rather freely ; its presence, either as liquid or as vapor, is absolutely essential to the cor- rosion of iron ; both moisture and carbon dioxide are invariably present when iron " rusts" rapidly. Bunsen gives the following as the coefficient of absorption by water at the given temperatures, for familiar substances :* TEMPERATURES. Cent. o. Fahr. 32. 10 50 20 68 o.oiq^o O.OIQ3O O OIQ^O 0.04114 0.032^0 0.02838 Nitrogen o 020^ o 01607 o 00403 Atmospheric air ... o. 024.71 O OKK7 o 01704 Carbon dioxide . . . I . 7067 I 1847 O QOI4 0.03287 O.O26^ o 02312 Carb hydrogen CH4 o 0^440 O OJ.^72 O 0^400 C 2 H 4 O 2^6^ o 18^7 o 1488 Sulph " A, -3706 1 ^8^8 2 QO^^ IO4Q.6 812 8 6^4 O 121. The Composition of Water and its chemistry are well understood in all its technical relations. Cavendish showed its constitution by synthesis in 1781 ; Humboldt and Gay-Lus- sac, in 1805, found it to consist of one volume hydrogen and two volumes oxygen ; while Berzelius and Dulong determined its proportions by weight, hydrogen one and oxygen eight, i.e., H 2 Molecular Weight. 2 Calculated. II . Ill Berzelius and Dulong. II. I O 16 88.888 88.9 100.000 100. Dumas. II. II IOO.OO H 2 18 Lavoisier made the composition of water one of the bases of his new system. Water is a neutral compound, exhibiting, when pure, neither acid nor alkaline reaction ; but so freely does it dissolve sub- stances with which it is brought in contact, that it is rarely found in nature absolutely free from either acidity or alkalinity. Its presence is essential to nearly all the chemical operations of nature, as well as in the laboratory. Methods of Gasometry. STEAM A ND ITS PROPER TIES. 255 The fluid may be decomposed in either of several ways, as by heat alone, a process of " dissociation" of its elements tak- ing place at between 2000 and 4000 Fahr. (1100 to 22ooC.), or by the voltaic current, and by the action of various metals or metalloids at high temperatures, when the substance employed has a strong affinity for the oxygen, as have carbon, iron, etc. Water is found wherever hydrogen is burned, in air or oxy- gen, either alone or in combination with other elements. It enters into combination with many other substances, and as water of crystallization, for example, often influences the char- acter of the compound to a very important degree. 122. The Sources and Purity of Water demand careful at- tention from the engineer proposing to use it in the production of steam ; since the presence of any foreign matter is always productive of some and sometimes of serious difficulties, and even of dangers. Rain-water is the purest of all natural waters ; but even rain-\vater contains all such gaseous substances in solution as may have been dissolved in its fall through the at- mosphere, and such minute quantities of organic and other solid matter as are found floating in the air. The volume of dissolved gas is usually about 25 parts in 1000 of water. As oxygen dissolves more freely than nitrogen, their proportions in solution differ from those of the atmosphere, averaging not far from one third oxygen and two thirds nitrogen. Spring-waters hold in solution every soluble element or com- pound found in the rocks and soils through which they flow. The purest of them are those " soft" waters rising from gra- nitic formations ; those of limestone districts contain, often, con- siderable quantities of lime, and are very " hard." Spring-waters are often so heavily charged with dissolved substances as to be useless for domestic or manufacturing purposes. Good spring- water is, however, often found " fresh" and pure, and such water should always be sought for use as " feed-water" for steam-boil- ers. River-water is usually purer than spring-waters, even although largely consisting of such waters. The dilution of the stream by surface-water, the precipitation of lime and other salts held in solution only by carbonic acid, which is set free on exposure to 256 THE STEAM-BOILER. the atmosphere, and the purifying influences of the atmosphere, all together may very greatly reduce the proportion of impurity. River-water is apt to contain more organic matter than does spring-water; this is sometimes, though rarely, dangerous in boilers. It is liable to contain large quantities of sand, clay, or other kind of soil, mechanically suspended ; but this can usually be removed sufficiently well by filtering. A water carrying a considerable amount of the carbonate of lime and other alkalies in solution, and used in the boilers of locomotives in the Mississippi valley, deposited a scale having the following analysis : Iron peroxide 5 . 700 per cent. Silica 2.960 " Potassa 6-131 " Alumina 320 " Soda 2.137 " Sulphuric acid 006 " Lime 24.760 " Magnesia , 8.294 " Carbonic acid 41.060 " The effect was to produce some leakage and marked loss of economy. This may be taken as a fair sample of the incrusta- tion to be expected in limestone districts. 123. Sea-water is a " mineral water," strongly saline, con- siderably chlorinated, and slightly alkaline. The composition of the water of the ocean differs very slightly in different local- ities. It contains about % of its own weight of salts, mainly common salt, with various other chlorides and bromides, and some gases. The following analysis was made by Von Bibra : Sodium chloride 1671 . 34 Magnesium chloride 199.66 Sodium bromide 31.16 Potass, sulphate 108 . 46 Magnes. " 34.99 Calcium " 93.30 Total in i U. S. gallon 2138.91 grs., or 3.569 per cent, by weight. STEAM* AND ITS PROPERTIES. Forschammer finds for each 100 parts chlorine: SO 4 Mg Ca Total. Maximum 14-51 6.768 2.257 181.40 Mean 14.26 6.642 2.114 181.10 Minimum 13-98 6.570 2.050 180.60 In some inland seas, as the Great Salt Lake and the Dead Sea, the proportion of saline matter is enormously greater. Herapath found the latter to contain 19.73 per cent solid mat- ter, of which one half was common salt, and one third magne- sium chloride ; the next largest constituents were the calcium and potassium chlorides and sodium bromide. Deposits from sea-water, and from any other water contain- ing solid matter either in solution or suspended, will always occur on evaporating the water; and these deposits form the in- crustation and sediment which endanger the steam-boiler and reduce the efficiency of its heating-surfaces. They are pre- vented at sea, usually, by the adoption of the surface-condenser, or by the process of " pumping and blowing" where the jet-con- denser is employed, and when the sea-water is thus unavoid- ably used in the boiler. This will be referred to in describing the operation and management of the marine steam-boiler. The salts in sea-water are not precipitated at the boiling- point ; but, in a concentrated solution at 217 Fahr. (102 Cent.), sulphate of lime begins to come down, and at the tem- peratures customarily met with in marine steam-boilers it is all deposited. A saturated solution of common salt is obtained at a temperature of about 230 Fahr. (i 10 Cent.) and at one tenth the volume of the sea-water, the salt having increased in its per- centage from 3 to 30. A cubic foot of sea-water weighs about 64 pounds, or |-| that of fresh water, the one measuring 35 and the other 36 cubic feet to the ton, nearly. The boiling-point of salt water rises about i.2 Fahr. (o.7 Cent.) for every 3 per cent of salt added up to the point of saturation. (See 126.) The character of the water in a marine steam-boiler, after long working, and with the usual moderate concentration, is shown by analyses made for the Author by Dr. Albert R. Leeds, the report on which was as follows . 17 258 THE STEAM-BOILER. EXAMINATION or WATERS FROM A MARINE BOILER, WITH REFERENCE TO CAUSES OF RAPID CORROSION OF HEATING- SURFACES. Samples. I. Forward end of boiler ; 2, after end of boiler ; 3, hot well. Preliminary Instructions and A nalyses. The instructions were to examine for organic acids and copper. All the organic acids that could possibly occur under the circumstances were looked for in Nos. I, 2, and 3. None pres- ent. Of copper, none was found except in No. I, and then only a trace when the examination was repeated on a larger quan- tity of liquid. If the quantity of water at my disposal had not been so limited, a similar examination of No. 2 might have revealed a trace of copper in it also. These results being inadequate to explain the causes of cor- rosion, the following analyses were required : ANALYSES OF THREE SAMPLES. Amount of Solid Matter in Waters. 1. 100 cubic cent 4-562 grammes. (Corresponding to about 6.1 oz. in i U. S. gallon.) 2. 100 cubic cent, contained 4.8386 " 3. 100 " " " 0.2680 " Loss by Ignition. These residues were obtained by drying at 110 C. On ig- nition, water was given off, and a partial decomposition, attended with loss of chlorine, ensued. But unlike many samples of water, the loss by ignition in these cases is not to be attributed to organic matter present. No. i lost 9 . 43 per cent. " 2 " 6.01 " " 3 " 15-11 STEAM AND ITS PROPERTIES. 2$$ Results of Qualitative Analysis. I. 2. 3. Organic acids None. None. None. Chlorine Present. Present. Present. Ammonia None. None. None. Lime None. Trace. None. Magnesia Abundant. Abundant. Abundant. Oxide of iron None. None. None. Copper Trace. None. None. Sulphuric acid Present. Present. Present. Sodium Large. Large. Large. Bromine, . not tested for. Iodine, The most striking feature is the large amounts of the chlo- rides and sulphates of the alkalies and magnesium more espe- cially the magnesium salts. Specific Gravities and other Properties. Specific gravity of No. 1 1 .0300 15 C. " " 2 1.0309 " " " " 3 1.0030 " I was slightly turbid from suspended matter, but colorless ; 2, turbid, and of a slightly pinkish color; 3, colorless and clear. It will be noticed that the specific gravities of Nos. I and 2 are somewhat greater than the average specific gravity of sea- water, which is 1.027. Corrosive Properties of the Water. Ex. i. A galvanic pair was made of a plate of copper and one of iron, separated below but in contact above the liquid. On immersion into water No. I hydrated sesquioxide of iron was rapidly formed. No notable deposit of copper could be detected on the iron plate, and no trace of copper in the liquid. If the minute trace of copper was precipitated out, the coating was too slight to be visible. Ex. 2. A sheet of iron alone was immersed in the water No. i at the boiling-point. Oxide of iron was formed, but in much less quantity than in Ex. I. 26O THE STEAM-BOILER. Ex. 3. A galvanic pair, as in Ex. i, was put in the circuit of a galvanometer. On making contact a large deflection took place, showing high tension, the needle coming to rest with a permanent deflection of 3. At the same time oxidation of the iron went on rapidly. Conclusions. That water having a composition as above given, and with- out organic acids, is capable of producing corrosion of the iron ; that such water, when it is the exciting fluid in a galvanic com- bination, one element of which is iron, the other copper, pro- duces a galvanic current of notable quantity and intensity. Under such circumstances corrosion of the iron takes place more rapidly than when iron alone is in contact with the liquid. 124. Technical Uses and manufacturing operations com- monly require the purest possible water. In the steam-boiler, especially, where all the water evaporated necessarily leaves behind every particle of solid matter held in solution at its in- troduction, purity of the fluid is of great importance. Half a ton of lime " scale" has been taken from the boiler of a locomo- tive, and the Author has seen several tons of salt and scale in a large marine boiler which had been ruined by its presence, and the consequent destruction of its furnace and furnace-flues by overheating and oxidation. Boiler explosions have often been caused by such incrustations. The prevention and removal of scale is a matter of serious importance in steam-boiler manage- ment, and will be considered later. It may be stated here that various chemical reagents are relied upon to produce a remov- able and comparatively safe form of salt-deposit, and heating and filtration of the water before it enters the boiler are usually the best preventives. Filtration by means of filter-beds for large volumes of water, and by filtering apparatus of various kinds, may always be relied upon to remove the undissolved solid matter. Filtra- tion is often combined with heating and sometimes with chem- ical treatment in the purification of water. The temperatures at which calcareous matters are precipi- tated in ordinary boiler waters are as follows : STEAM AND ITS PROPERTIES. 26 1 Carbonates of lime, between 176 and 248 Fahr. (80 to 120 C.) Sulphates of lime, between 284 and 424 Fahr. (140 to 218 C.) Chlorides of magnesium, between 212 and 257 Fahr. (100 to 124 C.) Chlorides of sodium, between 324 and 364 Fahr. (160 to 184 C.) The presence of the chlorides causes retardation of the deposition of the sulphate to a very considerable degree. 125. Water-analysis is often resorted to by the engineer to determine the proportion of scale-forming constituents in water to be used in steam-boilers. The determination of the specific gravity is sometimes a first step ; but the variations from that of pure water are usually too slight to be observable. Where it is taken it is best done by weighing on the chemist's balance. Color is observed by filling a long glass tube, cap- ping the ends with plate glass, and looking through it at a white background, beside a tube similarly prepared containing pure water. The smell and taste are noted, both cold and warm, and the water is tested with litmus-paper to detect any acidity or alkalinity ; should the paper turn blue, and again lose the color on exposure to the air, ammonia is indicated. The total dissolved solid matter contained is ascertained by evaporating to dryness, after filtration, and weighing the de- posit. The final drying is usually completed in a steam-bath at the boiling-point, 212 F. (100 C.). The weight of fixed mineral contents is then estimated by igniting until all organic matter is decomposed and its carbon burned away, and the loss of weight noted. The suspended matter may be weighed from the filters, or may be obtained by allowing it to settle in a still tank or large bottle until the water is perfectly clear, decanting and weighing after drying. The " hardness 1 of water is gauged by several methods, of which Clark's is one of the best. It depends upon the fact that when water is pure it froths when shaken up with an alcoholic solution of soap ; while if mineral salts are present it remains free from " suds" until a considerably increased amount of soap is introduced. The quantity of soap required to produce ob- servable frothing is a fairly good gauge of the hardness of the water. This hardness is measured in "degrees," each of which is equal to o.oi gramme of calcic carbonate, or its equivalent, 262 THE STEAM-BOILER. to the litre, i.e., one part in 100,000. The standard solution is made by dissolving white curd-soap in alcohol of 0.92 s. g., until loo cubic centimetres will make a froth with an equal quantity of water of 20 hardness. This is preserved in glass- stoppered bottles, and sometimes diluted to make other stan- dard solutions. The presence of a considerable proportion of magnesian salts causes the indication of this test to be de- fective, giving too low a figure for the hardness. Carbonates precipitated by boiling are dried and weighed. Organic matter is calcined and so determined roughly, or may be measured by reaction with potassic permanganate. The amounts of the several solid constituents are customarily ex- pressed as parts in 1,000,000, by weight, of the water ; some- times as grammes in the litre, and also as grains to the gallon : the last may be reduced from the next preceding by multiply- ing grammes per litre by 0.07 ; they can be converted into degrees on Clark's scale by multiplying by 0.7. Cubic centi- metres per litre may be converted into grains per gallon by dividing by 3.738. 126. The Purification of Water is often essential both for sanitary and commercial purposes. The first and simplest process of purification of water containing dissolved substances is distillation. The liquid is boiled in closed vessels, and the steam conducted into a condenser, and there restored to the liquid state by cooling. All salts and solid matters are left be- hind in the evaporating vessel, and the distilled fluid is abso- lutely pure if the process is conducted in vessels of insoluble metal or of glass. In many cases the lime salts are precipitated by simple heating without vaporization, the solid precipitate coating the surfaces of the heater or the stone or other masses with which it is sometimes partly filled. The addition of com- mon washing soda is better than the use of the nostrums sold as " scale preventives." The safest course is always to have the water analyzed, and thus to ascertain the best method of treatment. Saccharine and amylaceous matters and extractive substances are useful in preventing the deposition of lime and magnesian carbonates in a hard scale, and barium chloride is effective in a similar manner, where the water contains calcic STEAM AND ITS PROPERTIES. 263 sulphate. Water may be purified of its lime salts, the lime being held in solution as a bicarbonate, by the addition of lime- water, which takes a part of the carbonic acid and causes com- plete precipitation. This process has been used in the purifi- cation of feed-water for use in steam-boilers ; but the great quantity of water used generally makes it a somewhat expen- sive system. M. E. Asselin recommends the use of glycerine to prevent incrustation in steam-boilers. Glycerine soluble in water in every proportion increases the solubility of combinations of lime, and especially of the sul- phate ; it appears besides to form with these combinations soluble compounds. When the quantity of lime becomes so great that it can no longer be dissolved, nor form with the glycerine soluble combinations, it is deposited in a gelatinous substance, which never adheres to the surface of the iron plates. Moreover, the gelatinous substances thus formed are not carried with the steam into the cylinder of the engine. M. Asselin advises the employment of one pound of gly- cerine for every 300 or 400 pounds of coal burnt, fifteen days' supply being introduced at once. Glycerine combines with all the salts, and leaves the plates perfectly clean. Filtration, as has been already stated, is the process by which all mechanically-suspended matter is removed from water ( 124). 127. The Physical Characteristics of Water, when pure, are the following: Its density is about 770 times that of air, and is a maximum at about 39. 2 Fahr. (4 Cent.), with exceedingly slight variation through ordinary ranges of temperature. This is taken as unity, and as a standard for all densimetric determi- nations with solids or liquids. Water is perfectly elastic with a very great modulus ; at low temperatures the compressibility increasing with temperature, and decreasing with its solution of salts. Grassi, Amaury and Descamps, and Cailletet, all find the coefficient of compressibility at mean atmospheric temperature to be 0.000045 to 0.000046. At the freezing- point it becomes 0.00005. On the application of heat, water expands from unity to 1.043, in passing from the freezing to the boiling point. It has a very high heat-capacity, which is taken as unity in comparing 264 THE STEAM-BOILER. specific heats of other substances. It is an almost perfect non~ conductor of heat, and only transfers heat readily by convec- tion. Its conductivity in absolute measure is about 0.002. On reducing its temperature to 32 F. (o C.) water freezes, and the ice produced has a specific gravity, when solid and pure, of 0.92, and floats on the surface of water at the same tempera- ture. The expansion observed at freezing takes place with immense force, and often bursts water-pipes when they are frozen up. The boiling-point of water, under atmospheric pres- sure, is at 212 Fahr. (100 Cent.), and very variable, as shown later, with change of pressure. The boiling-point also rises with the increase of density by the solution of other substances. One cubic foot of water weighs 62.425 pounds at maximum density, or nearly 1000 ounces (62.5 pounds). The cubic metre weighs 1000 kilogrammes. One atmosphere counterbalances a column of water 33.95 feet (10.35 m -) high. The following table gives the volume and weight of dis- tilled water at various temperatures : u Ratio of IU Ratio of li Ratio of g rt 1 volume to that of equal weight at maximum Weight of a cubic foot. imperatur volume to that of equal weight at maximum Weight of a cubic foot. jmperatur volume to that of equal weight at maximum Weight of a cubic foot. H density. P density. H density. Fahr. Lbs. Fahr. Lbs. Fahr. Lbs. 32- 1.000129 62.417 210. . 04226 59.894 390- 15538 54-030 39-i I. 000000 62.425 212. .04312 59-707 400. .16366 53-635 40. I .000004 62.423 22O. .04668 59.641 410. .17218 53-255 50. 1.000253 62.409 2 3 0. .05142 59-372 420. .18090 52.862 60. I .000929 62.367 2 4 0. 05633 59.096 430- .18982 52-466 70. I .001981 62 . 302 250. .06144 58.812 440. .19898 52.065 80. 1.00332 62.218 260. .06679 58-517 450- .20833 51.662 90. I . 00492 62.119 270. .07233 58.214 460. .21790 51.256 100. 1.00686 62.000 280. .07809 57-903 470. .22767 50.848 no. 1.00902 61.867 290. .08405 57-585 480. .23766 50.438 I2O. 1.01143 61.720 3 00. .09023 27-259 490. 24785 50.026 I 3 0. 1.01411 61.556 3 IO. .09661 56-925 500. .25828 49.611 140. I .01690 61.388 320. .10323 56.584 510. .26892 49-195 I 5 0. 1.01995 61 .204 330- . 11005 56.236 520. 27975 48.778 160. 1.02324 61 .007 340. . i i 706 55-883 530. . 29080 48.360 170. 1.02671 60.801 350. .12431 55-523 540- .30204 47-941 180. 1.03033 60.587 3 60. 13175 55-'58 550. 3^354 47-521 190. 1.03411 60.366 370. -3942 54.787 200. 1.03807 60.136 3 80. .14729 54-4" STEAM AND ITS PROPERTIES. 265 128. Changes of Physical State from ice to water, or from water to steam, or the reverse, are brought about by change of temperature and pressure. The heating of ice from any tem- perature below freezing up to its melting-point causes an ex- pansion of the mass and the conversion of a part of the heat supplied in the performance of the work of separation of mole- cules, and in less degree that of expansion of the mass against atmospheric pressure. At the melting-point rise in tempera- ture ceases, and all heat received is transformed into the work of " disgregation," as Clausius has called it, until such a separa- tion of molecules has been effected that stability of form is lost with the vanishing of the visible effect of the polarizing forces, and the mass becomes liquid. This change of physical condition having been effected, the addition of heat again causes rise in temperature, until a second halt takes place at the boiling-point and a second change of state produces vapori- zation. Above this latter point, the boiling-point, the absorp- tion of heat once more causes increase of temperature. There are thus two marked phenomena to be noted in applying heat to this substance, and at every stage the heat-supply is to be observed and compared with the amount of heating and of work done internally and externally ; the two quantities, that received and the sum of these expenditures, will always be found to balance. 129. The " Critical Point" is that at which the fluid is in- differently liquid or vapor at the same temperature and the same pressure. As the pressure increases and temperature rises, the quantity of heat rendered " latent" by conversion into the work of vaporization decreases, and with probably every fluid a point is finally reached at which a critical set of conditions is attained, the latent heat of expansion becoming zero, and the body exhibiting the properties of the liquid or of the vapor ac- cordingly as it is above or below this point on the thermometric and pressure scales. M. Cagniard de la Tour in 1822 first ob- served that, on raising the temperature of a confined fluid, part- ly liquid and partly gaseous, a point might be reached at which the whole mass suddenly became homogeneous in appearance ; and he supposed the action that of sudden gasification. Fara- 266 THE STEAM-BOILER. day found, a year later, as he stated it, that, above a certain temperature, definite for each case, no amount of pressure would cause liquefaction of a vapor ; and Dr. Andrews, who studied the phenomenon very carefully, finally concluded that at this " critical " point the properties of the two forms of matter blended that the one passes into the other without interrup- tion of continuity ; these physical states being thus found to be separate forms of the same condition of matter. M. de la Tour reported several critical temperatures and pressures, thus : Temperature. Pressure Atmos. Ether 160. *, F. 187.'; C 27. c 497. 5 F. 258. 5 C. IIQ.O 504. 5 F. 262. 5 C. 66.5 Water 773 .O F. 411 7 C ? At a temperature and pressure near that above given water dissolved glass. Steam in this condition, or of higher temperature and pressure, being worked in the steam-engine, would superheat while expanding ; at ordinary temperatures and pressures, and below this critical state, it partially condenses while expanding behind a piston, and thus performing work a fact predicted by Rankine and Clausius in 1849, before its experimental discovery. Isothermal lines of temperatures considerably above those of the critical points for the various pressures are sensibly hy- perbolic; but as these critical pressures and temperatures are approached the curve becomes distorted, and gives a combina- tion of nearly straight lines up to the boiling-point, a perfectly straight line of constant pressure and variable volume during vaporization, and finally it is hyperbolic when the gaseous state is attained, as in the vaporization of water. Fig. 65 exhibits a set of isothermals for carbon dioxide, as drawn from Dr. Andrews' data. The dotted lines indicating the probable form, as suggested by Professor James Thomson,* of portions not obtainable from those data, are by him given the Author, as shown. Each curve relates to one temperature, and * Rept. Brit. Assoc. 1871. STEAM AND ITS PROPERTIES. 267 pressures are represented by the horizontal ordinates, and cor- responding volumes of mass of carbonic acid constant through- out all the curves are represented by the vertical ordinates. A'->" tft S lVr?folVVl.-|;MI l*a[.7lV1s|>l I I i7|ol I I FIG. 70. ISOTHERMAL CURVES. CO 2 . Thomson points out that, by experiments of Donny, Dufour, and others, we have already proof that a continuation of the curve for the liquid state past the boiling stage for some dis- tance, as shown dotted in Fig. 70, from a to some point b towards /, would correspond to states already attained. The overhanging part of the curve from c to /may represent a state in which there would be unstable equiiibrium ; and thus, al- though the curve there appears to have some important theo- retical significance, yet the states represented by its various points would be unattainable throughout any ordinary mass of the fluid. It seems to represent conditions of coexistent tem- perature, pressure, and volume, in which, if all parts of a mass of fluid were placed, it would be in equilibrium, but out of which it would be led to rush, partly into the rarer state of gas, and partly into the denser state of liquid, by the slightest in- equality of temperature or of density in any part relatively to other parts. Above this point the fluid, as shown by the hy- 268 THE STEAM-BOILER. perbolic form of curve, is thoroughly gaseous ; below that point it may be called vapor. 130. The Spheroidal State of water is that condition ob- served when water lies in contact with highly heated metal sur- faces. When so situated, a liquid does not wet the metal, but is supported quite out of contact with it by a cushion of rapidly forming vapor. A very small mass assumes the form of a drop, a larger quantity that of a sheet of liquid of continually chang- ing outline. The supporting " Crookes's layer," as it is some- times called, consists of particles constantly bounding and re- bounding between the adjacent surfaces of fluid and metal, and gradually finding their way out of that capillary space as their places are taken by newly formed particles of vapor. Ether and bromine can be similarly supported on the surface of heated water, and ice can be produced in a red-hot crucible without contact. On cooling the metal, a temperature is finally reached (356 F., 1 80 C.) at which contact occurs, and an explosion often follows from the sudden and considerably increased evolution of steam. This, which is named, from its discoverer, Leidenfrost's phe- nomenon, or, otherwise, the " caloric paradox," has been very carefully studied, especially by Boutigny. The temperature of the spheroid of liquid is found always to be lower than its boil- ing-point ; contact never exists, during the continuance of the phenomenon, between metal and liquid. This action is pro- moted by any conditions which tend to prevent actual contact and wetting of the metal by the liquid, a fact having impor- tant bearing on the special danger of certain forms of oily or of pulverulent scale in steam-boilers. This interesting and im- portant action is illustrated in the impunity with which the hand may sometimes be dipped in molten metal, the moisture on its surfaces protecting it from contact and injury. Superheated water or other liquid may be sometimes ob- tained by careful management, as in the experiments of Donny, Dufour, and others. When water is deprived of air and of all impurities it may be raised to a temperature considerably ex- ceeding the boiling-point. The smaller the mass, the higher the temperature attainable. M. Donny raised water in a closed STEAM AND ITS PROPERTIES. 269 glass tube to 248 F. (i 38 C.), when explosive ebullition occurred, and the thermometer dropped to 212 F. (100 C.). Minute drops (i to 3 mm. or 0.04 to 0.12 in. in diameter) attained 346 F. (175 C.), when suspended in a mixture of oils of its own den- sity, a temperature at which the tension of steam in contact with its water is, under normal conditions, between 8J and 9 atmospheres. Water in glass vessels always boils, if pure, at a temperature slightly exceeding the ordinary boiling-point. Larger masses or impure water are not easily superheated. M. Dufour found that water retains the liquid state more per- sistently when the temperature is constant and pressure is made the variable than when the contrary conditions are arranged. MM. Donny, Dufour, and others have suggested that this phe- nomenon may be a frequent cause of a class of boiler-explosions known as " fulminating," in consequence of their violence ; and Mr. Radley, an English engineer, reported * having actually su- perheated the water in small steam-boilers 27 F. (15 C.) above the normal boiling-point for the pressure at which they were working. On the other hand, Mons. Hirsch, the well-known French engineer and author, reports to the Commission Centrale des Machines a Vapeur the results of experiments of a committee on the production of the superheated condition in the water of steam-boilers, in which, studying the history of such phenomena so far as they are recorded, and conducting a somewhat ex- tended series of experiments, the conclusion was finally reached that there is no evidence, up to the present time, that boiler explosions may be caused by the conditions studied, or that such conditions ever arise in practice. If they occur at all, it is only in extremely rare instances, and as a consequence of a coincidence of circumstances seldom to be observed, and which are neither well understood nor well defined. The use of the thermometer is advised to determine the facts bearing upon this question. The commission to which the report is made approve and adopt these conclusions. 131. Vaporization of water or other liquid has been seen to be a process of conversion of sensible heat-energy into the so- * Land. Mining Journal, June 28, 1856; Scientific American, Aug. 2, 1856. 270 THE STEAM-BOILER. called latent form by transformation into work in the separation of molecules, and consequent change of state of the fluid. This change has been found to be invariably produced at a tempera- ture fixed for every pressure under normal conditions, and to demand a certain exact and determinable quantity of heat. The vapor thus formed, so long as it is in contact with the liquid from which it issued, retains the precise temperature of ebullition, and in this condition the steam is said to be satu- rated. If it contains no unevaporated moisture, it is said to be dry and saturated. It is capable of being superheated by isolation and further addition of heat, and then rapidly assumes more or less perfectly the gaseous state. M. Hirn found that this state is sensibly reached when the superheating amounts to anything above 16 F. (9 C.). The fluid is known in this condition as " steam-gas." The specific heat of superheated steam is 0.48, equivalent to 371 foot-pounds, at constant pressure, and is 0.37 (286 foot- pounds) at constant volume. The quantity of heat doing in- ternal work here becomes insensible. The processes of conver- sion of the liquid into vapor and of the vapor into gas are seen to be, physically, very similar. 132. The Thermal and Thermodynamic Phenomena attending the production, storage, and transfer of heat-energy through the vaporization of steam are evidently in some sense identical phenomena. The communication of heat to a mass of water enclosed in a steam-boiler results in the raising of its temperature, the expansion of the mass, the performance of work, and the conversion of heat-energy in the doing of that work. The boiling-point is simply a point in the process at which the proportion in which the heat received is distributed to its several purposes is altered, and the superheating of steam is the result of passing another critical period in the process. The principles involved and these phenomena have been already fully explained, and it is only necessary here to apply those principles and the data obtained by experiment to the special case in hand the production and use of steam. It is perfectly easy to determine just how much sensible heat is em- ployed, untransformed, in raising the temperature of water or STEAM AND ITS PROPERTIES. 2Jl steam ; how much is transformed in producing expansion, and how much as the latent heat of change of state. 133. Internal Pressure and Work, in the case of steam, will illustrate the general case of thermodynamic change as already presented. The magnitude of the molecular resistance to expansion is well ascertained, and the quantity of work done in overcoming them in the process of making steam is as easily determinable. As has been shown, the quantity of heat be- coming latent is the equivalent of this internal work, and the sum of the latent and the sensible heat absorbed is the total heat demanded to produce the change. Both may be deter- mined by the processes which have been described in the earlier chapters. 134. The Computation of Internal Work or of internal pressure has been seen to be based on the principle expressed in the statement of the second law of thermodynamics. The total pressure, internal and external, at any tempera- ture, T, is always The rate of variation, -j~, of pressure as a function of tem- perature is determined experimentally, and the value of this expression may be obtained from the expressions already given, or from the tables of Regnault. The work done is the product of their total pressure, /, into the alteration of volume, Av, or (2) Internal pressure and work are computed by deducting exter. nal pressure and work from these totals. Clausius thus obtained the following values of/ for steam of the pressures given, all in millimetres of mercury, of which 760 measure one atmosphere of pressure : 272 THE STEAM-BOILER. TOTAL PRESSURES OF STEAM. CENTIGRADE. EXTERNAL PRESSURE. Ratio Total Pressure Ratio T. A- At. ~df ' ^ = 7 57" 7e' 100 374 7 6o I 27.2OO 10146 13-3 120 394 1520 2 48.595 19150 12.6 134 408 2280 3 67.020 27277 11.9 144 418 3040 4 84.345 35172 11.5 152 426 3800 5 100.375 42659 II. 2 159 433 4560 6 116.085 50149 II. 166 440 5320 7 133-445 58502 10.8 171 445 6080 8 146.910 65228 10.7 176 450 6840 9 161.27 72410 10.6 454 7600 10 173.425 78561 10.4 199 473 11400 15 239-57 113077 9-9 It is seen that the rate of variation of pressure with the temperature of steam continually increases as pressures and temperatures rise, and that the proportion of internal to ex- ternal work and pressure continually diminishes ; but that the latter ratio is large, about ten to one, for the whole range of pressures familiar in standard practice. 135. The Specific Volume of steam, or the volume of unity of weight, and its reciprocal, the density, have been seen to be capable of easy computation when the latent heat of vaporization at the given temperature is known ; since this latent heat measures the work done while the force resisting it is calculable as above. From the expressions already given we thus obtain very exact values. Clausius thus obtains the following values, and compares them with the somewhat uncertain figures of Fairbairn and Tate, derived experimentally. Metric measures are employed. ' T. A?/ Calculated. By Experiment. 117.17 124.17 128.41 137.46 144.74 39I.I7 398.17 402.41 411.46 418.74 0.947 0.769 0.681 0.530 0-437 0.941 0.758 0.648 0.514 0.432 STEAM AND ITS PROPERTIES. 273 Quite accurate results can also be obtained by taking the density of steam as 0.622 ; that of air at the same values of t and/ being unity. (See p. 282.) The volume of water increases with temperature, from the temperature of maximum density, more and more rapidly as the heat is increased. The following are the values as given by M. Kopp, who experimentally determined them, and as cor- rected by Mr. Porter to make the curve exhibiting the data perfectly smooth : TBM PERATURE. VOLUME 5 AS PER Cent. Fahr. Kopp. Potter. 4' 39- 1 .00000 .OOOOO 5 41 .0 .00001 .00001 10 50 .0 .00025 .00025 20 68 .0 .00169 .00171 30 86 .0 .00423 .00425 40 104 .0 .00768 .00767 50 122 .0 .01190 .01186 60 I4O .O .01672 .01678 70 158 .0 .02238 .02241 80 176 .0 .02871 .02872 90 194 .0 .03553 03570 IOO 212 .0 .04312 04332 136. Temperature, Pressures, and Volumes of Steam are related by natural law quite as definitely as those governing these relations for the gases ; but algebraic expressions of those laws are not yet obtained, except empirically. There have been numerous formulas proposed of the latter class, some of which are remarkably exact within a moderate range. The most ac- curate are probably those of Rankine,* already given for vapors generally : B C -~ A --f-j*' (0 T = (2) * Steam-engine, p. 237, 206. Ibid., pp. 559~564- iS 274 THE STEAM-BOILER. in which, for steam, B A = 8.2591; ^=0.003441; log B = 343 6 42 ; B* log C = 5-59873 ; -i = o.ooooi 184 ; pressures being taken in pounds on the square foot and tem- perature in degrees Fahrenheit on the absolute scale. The ex- periments of Regnault and of Fairbairn and Tate have furnished the generally accepted values. Unwin has proposed* a simpler formula than Rankine's, which, while not quite as exact, gives more manageable ex- dp pressions for -T= and its functions ; thus, for vapors generally : (3) I dp nb (a log/) n~ = 2.3025^ 1^ ;. ... (5) bn t dp nb (6) * Phil. Mag., April, 1886. STEAM AND ITS PROPERTIES. For steam, these formulas become : log / = 7-5030 -yrr s ; (7) / 7579 Y\ ~ \ 7 . 5030 - log// '() I dp 21815 (7-5030- p., ~W^~ -' (9) = 2.8782(7.5030- log /); . . . (10) which expressions give remarkably exact results. Metric meas- ures are used throughout. 137. The Specific Heats of Water and Steam vary somewhat with temperature ; this variation is noted with all solids, and occurs with the vapors, although in vastly less de- gree ; and this is one point in which they are distinguished from the gases. For all the purposes of the engineer the specific heat of either saturated steam or of steam-gas may be taken at the value obtained by Regnault, 0.305, the quantity of heat, in thermal units, demanded to raise the temperature of unity of weight of saturated steam one degree, while still keeping fi saturated by the evaporation of additional water ; which latter process demands the transformation of 0.695 unit of heat. The specific heat of isolated steam-gas, or superheated steam, is given by Regnault as 0.48051, and constant. The specific heat of water was determined by Regnault* very carefully and exactly, and the figures so obtained have been *Mem. of the Academy of Sciences, 1847. 2/6 THE STEAM-BOILER. found capable of being very accurately represented by the fol- lowing empirical formula of Rankine :* C= i + 0.000000309(7 39 -0 2 > (0 in which / is the temperature on the common Fahrenheit scale. The total heat demanded from /, to / 2 would thus be h = f*Cdt = t,-t 1 + o.oooooo 1 03 [(/ 2 - 39. i) 3 and the mean specific heat for such a range of temperature is + (',-39 -0*]-- - (3) On the Centigrade scale these expressions become C= i +0.00000 1 (/-4 )", .... ....... (itf) A=/,-* 1 +o.oooooo33[(*,-4)'-(' 1 -4 )'], ( 2a ) j-^j- = i + 00000033K*. - 4) ! + (t. - 4)C, - 4) '' The specific heat of ice is given by M. Person as 0.504. 138. The Computation of Latent and Total Heat of Steam is readily made by means of formulas given by Reg- nault or based upon his work, which covered a wide range of temperature from a little below the freezing-point to about 375 F. (190 C). The following is the formula of Regnault for latent heat as slightly modified and corrected by Rankine for the British and metric systems, respectively : /= 1091.7 -0.695(^-32) - 0.0000001030? - 39 - 1 ) 3 ; (0 l m = 606.5 0.695 * o.oooooo333(/ 4) 3 ;...'.. (10) * Trans. Royal Soc. of Edinburgh, 1851; Steam Engine, p. 246. STEAM AND ITS PROPERTIES. 277 or, approximately, as given by the investigator, I = 1092 - o.;(/ - 32) ^ = 966 0.7(/ 212) = 1147-0.7* ..... >J . ... (2) / TO = 606 0.7/ ...... . . .(20) The /0ta/ //ra/ #/ evaporation is the sum of the latent and sensible heats, and may be taken as h = ^ - = 1091. 7 + 0.3050 -32); .... (3) m = 606.5+0.305/1 ........ (30) in which the " total heat " measured is that from / a at / t , the original temperature of the water and that of evaporation, and the formulas given being based on the assumption that /, is taken at the melting-point of ice. For any other temperature the following will give satisfactorily exact measures : h = 1092 + o.3(/, - 32) - (/, - 32) ; = 1 146 + 0.3ft - 2 12) -(/,- 32); ... (4) h m = 606.5+ 0.3/, -/,; ........ (4*) h being obtained in British measures and h m in metric. For steam-gas, h = 1092 + o.48(/ f - O ........ (5) Professor Unwin proposes the following for the latent heat of vaporization of steam : 4 = 799 ~ (7.5030 - log/) - 8 ' ( 6 ) which is found to be extremely exact. He also obtains for the expansion during change of state, p being expressed, as above, in millimetres of mercury. 2 7 8 THE STEAM-BOILER. 139. Factors of Evaporation measure the relative amount of heat demanded to effect the heating of water from a given temperature, t 9J and its vaporization at a higher temperature, /!, and to simply produce vaporization at the boiling-point un- der atmospheric pressure, which latter is now usually taken as a standard. The value of this factor of evaporation is evidently /= 0.3(t l -212)+(212-t t ) 966.1 , nearly. . (i) The following are values of such factors, calculated as above : TABLE OF FACTORS OF EVAPORATION. BOILING-POINT, 7^. INITIAL TEMPERATURE OF FEED-WATER, 7" a . FAHR. 32 50 68 86 I0 4 122 140 158 I 7 6 I 94 212* 212 .19 :3 I5 r 13 .11 .10 .08 08 06 06 .04 .02 .00 230 248 20 .18 16 .14 13 .11 .09 07 03 .01 284 21 .20: 18 .16 . 12 .09 . 10 08 .06 .04 .04 .02 302 22 .20 18 .16 .14 .12 .11 o 9 .07 05 03 320 22 .21 19 J 7 15 13 . ii 09 .07 05 3 338 ' * 23 .21 in 17 15 .14 .12 10 .08 .06 .04 356 23 .22 20 .18 .l6 .14 . 12 IO .08 .06 .04 374 24 .22 20 .18 1 7 15 13 II .09 .07 5 392 2 4 23 21 iq 17 IS * *3 11 .09 .07 06 410 2.S 23 22 .20 .18 1.16 .14 12 .10 .08 .06 428 2 5 .24 22 .20 .18 .16 .14 12 . II .09 .07 A vastly more convenient form of table is that in which the pressures at which evaporation takes place are given ; thus : STEAM AND ITS PROPERTIES. M OO m l^ N t^ N * r ^ C S 2/9 ?$! \ 3- i^Ot- Os--CT> -* 0^ 000 t>. r^vo vO ? I" 8 - 00 O O O M o -*oo rooo rnco N t^ci lO'<- *(^imNN MM 8. ^ vg - m 5- ^ m - M w o o O> ooo oo rxvc *o n n ^- ** mmww t^oo^O^oooor^ I^NO \o o to ^- ^ m MNMNW M _M MMMM MMHM 55 OOOOOOOOOOO t^ * o> moo m oo moo mt^ Nt^Nrvw \OM\OMUI O'OO'oo *-o--*o\moo moo moo m M MOOOO>OOOO^ t^o voioiO'f^- m ro w IHOO &S oo f^ ^^o ^ >A irT* * m m M NN&MM HMMMM M^MWM M M - M MMMgC) OOOOO OOOOOO o f*>oo N NNwM B" S"^ 6 o" O 1 o" Q" O* moo moo N t* t o MVOMVOO looirio^- o>-*o^ -^f-oo moo moo m oo oorx^vovo ioio-f*m mNw-S o O o> aoo r^ txv5 vo m m + + m m t^wrs. N^W^M \OMVOMIO oioou^o* ^-ON^-o^moo moo mt^ wi^cir^ci ?!! =2^^^ ffy??? M 22H2 S8 4 8 >< 8'8 S'^'gff o 1 ???? - ^oo moo t; u 111 ill .'* c O MA >. 8 . 2 80 THE STEAM-BOILER. It is seen that the relative cost of using feed-water at any one temperature as compared with the use of water at any other temperature is as the reciprocal of their factors of evapo- rization. Thus if feed-water can be supplied, by means of a heater, at 210 F., where previously drawn from the mains at 50, the relative cost of making steam will be, at 100 pounds pressure, by gauge, -J-4f O.86, and a gain of fourteen per cent will be effected. As will be seen later, these tables are very useful in reducing the data obtained in trials of steam- boilers to the standard. 140. Regnault's Researches and Methods have furnished all the essential data relating to the production of steam in the boiler and the supply of stored heat-energy to the engine. The memoir of M. Henri Victor Regnault on " The Elastic Forces of Aqueous Vapors," * in which he described his re- searches, is a most magnificent exposition of a still more re- markable series of investigations. He repeated the methods and experiments of earlier physicists, invented new ways, and finally obtained a set of data of unexampled extent and accu- racy.f Regnault found that the density of aqueous vapor in vacua and under feeble pressure may be calculated according to the law of Boyle and Marriotte when the fraction of saturation is less than 0.8, while the density becomes sensibly greater when approaching saturation. He further found that the den- sity of vapor in air, in a state of saturation, may be similarly calculated, and the ratio of weight of equal volumes of vapor and air is a trifle less than that obtained theoretically. The data obtained by Regnault were carefully tabulated, and curves were constructed exhibiting the variation of pres- sure with temperature for saturated steam for the whole range covered by his experiments. Three formulas of interpolation were used for three different parts of the scale of temperatures ; for that part below the freezing-point he adopted the formula F=a + &cr, . ' (i) * Ann. de Chimie et de Physique, July, 1844 ; Mem. de I'lnstitut, tome xxi., p. 465 (1847) ; Mem. de 1' Academic des Sciences, xxi. xxvi. f Vide Dixon on Heat, vol. i., 724. STEAM AND ITS PROPERTIES. 28 1 in which F is the pressure, a and b constants, and a T a function of T t -\- 32, / being the temperature corresponding to F. Between the freezing and boiling points Regnault used Biot's formula, log F = a + bet - c& ; (2) and above the boiling-point, log F = a bee cft T ; (3) in which r t -f- 20. This last answers well, also, for the whole range. In it # = 6.2640348; log = 0.1397743; log c = 0.6924351 ; log a 1.994049292; log fi= 1. 998343862, as given by Regnault ; or, according to Dixon, a 6.263 509 686 5 log a = 1.998 343 377 8 log/? = 1.994 048 173 7 log b 0.692 450 419 2 logc = 0.139 553 958 4 For British measures, a = 4.859 984 524 7 log a = "1.999 079 751 3 log/5 = 1.996 693 778 3 log b = 0.659 317 975 2 log c = 0.020 517 432 4 A break was observed by Regnault, and is exhibited by the curves and the formulas, at the freezing-point, which had been attributed to error, the two curves cutting each other at a very small but appreciable angle ; but Professor James Thomson has supposed such a break to have a real existence, and to be produced by the physical change marking the freezing-point. 141. Regnault's Tables have been reproduced in many forms, usually with additions. The Appendix, among other tables, contains the data obtained by Regnault (Table I.), and 282 THE STEAM-BOILER. these values are accepted as standard universally. The table here given exhibits the temperatures and corresponding pres- sures of saturated steam throughout the full range now used in steam-boilers and far beyond ; the quantity of heat, sensible and latent, in unity of weight ; the total heat of evaporation and the density of the steam. Reference to these tables is vastly more convenient than calculation. Should it be neces- sary, or desirable, however, to make such calculations, the for- mulas already given will furnish the means. They also permit the calculation of data beyond the limits of Regnault's experi- ment, and are probably practically correct far beyond any pres- sure likely to become familiar in the operation of steam-boilers. Regnault's limit was at 230 C. (446 F.). Rankine's formula has been used beyond it. Fairbairn and Tate's formula for volume of steam is = 25.62 + -42513 r in which V is the volume of the steam formed at a pressure />, measured in inches of mercury, from one cubic foot of water, taken as unity and at the temperature of maximum density (see p. 272, 135). Nystrom has used this formula in the computation of very complete tables of specific volumes of steam (see Nystrom's Pocket Book of Mechanics). The formulas used in these calculations are elsewhere given, but are here grouped for convenience of reference. British measures are used throughout. STEAM AND ITS PROPERTIES. 28 3 J i % Jn col ^ 8 d 1 n I 1 vS ^ ( f I 1 ^ fr ^* c* t - M | 1 U1 S ej o" 1 o" fe fi. | ro 1 ^_ JP ^ fr M c m O : O *M ^ ^ i & *** |\o *3 8 IO ri pJ O ^J" ^ *. X i d r& 1 n x X X 1 i 8 X i n II ft,' ft, ft, 5s J d n 1 Ik i t ii II n II II 1 -c -j- N 1 s ^ he n k, -- * 1 ~ r - M fO C) ) fi -2 00 1 d II "^ ? 1 "^ N 1 * X ^~ ~- . II ft, + ? ft, II II II SYMBOL. - \ * 1 ^ - '- * N , " ' e B "rt be c ft,' o* ^ o c N "O m i a . JS 1 water f V J2 surroun I? t pressu o" 1 ^r o g (f. 4) B V c V g' 4 \t 3 ST u ! bo C! i *s i rt S B 1 S QUANTITY. Pounds per square ir Pounds per square fc Inches of mercury, at 32 of distilled water, at tempera density. Atmospheres. the atmosphere, in pounds pe Fahrenheit's scales. Absolute scale. Fahrenheit de ired to raise the temperature i 32 to t. ired to change the water into latent heat.) ed to overcome the pressure c medium. (External latei heat of evaporation, under co Total heat of evaporation a . of evaporation per pound of s in units of evaporation. 1 V 1 I I 1 ^ i 'otal heat A qy sjian jBuuaqi qsuug UI UJB31S JO punod J3J t* e i | HI 3* 1 fl a 284 THE STEAM-BOILER. ^ JSk I f X*N 'OO s 1!^ fO *O O w ,. O "+ Ts&a tn *n i ' 1 S o ^ K^ . p ~i *S j_ i ^ ^ X *L M . .gf^gg? D S PS zjf & 41 u X s-ifir y X X II g g II p||! a o D cubic foot of steam, in pou distilled water, in pounds, pound of steam, in cubic f steam to volume of equal 1 temperature of maximum i distilled water, at tempera ht at temperature of maxin "^ rt "o rt 08 "o ^ c o u as l] rt D rt c "o ^ 'o D 1 > cr Q u o o *j^ o 9 iS c o rt i o J cJ i t 8 j3 i $ j STEAM AND ITS PROPERTIES. 28$ 142. The Stored Energy in Steam at any pressure and temperature is now easily ascertained by calculation, in accord- ance with the first law of thermodynamics. The first attempt to calculate the amount of energy latent in the water contained in steam-boilers, and capable of greater or less utilization in expansion by explosion, was made by Mr. George Biddle Airy,* the Astronomer Royal of Great Britain, in the year 1863, and by the late Professor Rankinef at about the same time. Approximate empirical expressions are given by the latter for the calculation of the energy and of the ultimate volumes assumed by unit weight of water during expansion, as follows, in British and in metric measures : _ 772(^-212)' _ 423.55(^-100)' ~ _ 3676(^-212) 2.29(7- IOQ) " r+6 4 s These formulas give the energy in foot-pounds and kilo- grammetres, and the volumes in cubic feet and cubic metres. They may be used for temperatures not found in the tables to be given, but, in view of the completeness of the latter, it will probably be seldom necessary for the engineer to resort to them. The quantity of work and of energy which may be liberated by the explosion, or utilized by the expansion, of a mass of mingled steam and water has been shown by Rankine and by Clausius, who determined this quantity almost simultaneously, to be easily expressed in terms of the two temperatures be- tween which the expansion takes place. When a mass of steam, originally dry, but saturated, so expands from an initial absolute temperature, T t , to a final absolute temperature, 7^, if /is the mechanical equivalent of the unit of heat, and H is the measure, in the same units, of *" Numerical Expression of the Destructive Energy in the Explosions of Steam Boilers." f " On the Expansive Energy of Heated Water." 286 THE STEAM-BOILER. the latent heat per unit of weight of steam, the total quantity of energy exerted against the piston of a non-condensing en- gine, by unity of weight of the expanding mass, is, as a maxi- mum, U = ST.- i -hyp log - + - H. . . (A) This equation was published by Rankine a generation ago.* When a mingled mass of steam and water similarly ex- pands, if M represents the weight of the total mass and m is the weight of steam alone, the work done by such expansion will be measured by the expression, - i - hyp log + m > * H. (B) This equation was published by Clausius in substantially this form.f It is evident that the latent heat of the quantity m, which is represented by mH, becomes zero when the mass consists solely of water, and that the first term of the second member of the equation measures the amount of energy of heated wa- ter which may be set free, or converted into mechanical energy by explosion. The available energy of heated water, when explosion occurs, is thus easily measurable. The computers of the tables given in the Appendix were Messrs. Ernest H. Foster, and Kenneth Torrance. The tables range from 20 pounds per square inch (1.4 kgs. per sq. cm.) up to 100,000 pounds per square inch (7030.83 kgs. per sq. cm.), the maximum probably falling far beyond the range of possible application, its temperature exceeding that at which the metals retain their tenacity, and in some cases exceeding their melting-points. These high figures are not to be taken * Steam-engine and Prime Movers, p. 387. f Mechanical Theory of Heat, Browne's translation, p. 283. STEAM AND ITS PROPERTIES. 28? as exact. The relation of temperature to pressure is obtained by the use of Rankine's equation, of which it can only be said that it is wonderfully exact throughout the range of pressures within which experiment has extended, and within which it can be verified. The values estimated and tabulated are prob- ably quite exact enough for the present purposes of even the military engineer and ordnance officer. The form of the equa- tion, and of the curve representing the law of variation of pressure with temperature, indicates that, if exact at the familiar pressures and temperatures, it is not likely to be in- exact at higher pressures. The curve at its upper extremity becomes nearly rectilinear. The table presents the values of the pressures in pounds per square inch above a vacuum, the corresponding reading of the steam-gauge (allowing a barometric pressure of 14.7 pounds per square inch), the same pressures reckoned in atmospheres, the corresponding temperatures as given by the Centigrade and the Fahrenheit thermometers, and as reckoned both from the usual and the absolute zeros. The amount of the available stored energy of a unit weight of water, of the latent heat in a unit weight of steam, and the total available heat-energy of the steam, are given for each of the stated temperatures and pressures throughout the whole range in British measures, atmospheric pressures being assumed to limit expansion. The values of the latent heats are taken from Regnault, for mode- rate pressures, and are calculated for the higher pressures, be- yond the range of experiment, by the use of Rankine's modifi- cation of Regnault's formula. Studying the table, the most remarkable fact noted at the lower pressures is the enormous difference in the amounts of energy, in available form, contained in the water and in the steam, and between the energy of sensible heat and that of latent heat, the sum of which constitutes the total energy of the steam. At 20 pounds per square inch above zero (1.36 atmos.), the water contains but 145.9 foot-pounds per pound ; while the latent heat is equivalent to 16,872.9 foot-pounds, or more than 115 times as much; i.e., the steam contains 116 times as much energy in the form of laterj^Jie^tper pound, as 288 THE STEAM-BOILER. does the water, from which it is formed, at the same tempera- ture. The temperature is low ; but the amount of energy ex- pended in the production of the molecular change resulting in the conversion of the water into steam is very great, in conse- quence of the enormous expansion then taking place. At 50 pounds the ratio is 20 to I ; at 100 pounds per square inch it is 14 to i, at 500 it is 5 to I ; while at 5000 pounds the energy of latent heat is but 1.4 that of the sensible heat. The two quantities become equal at about 7500 pounds. At the high- est temperature and pressure tabled, the same law would make the latent heat negative ; it is of course uncertain what is the fact at that point. At 50 pounds per square inch the energy of heated water is 2550.4 foot-pounds, while that of the steam is 68,184, or enough to raise its own weight to a height, respectively, of a half-mile and of 12 miles. At 75 pounds the figures are 4816 and 90,739, or equivalent to the work demanded to raise the unit weight to a height of four fifths, and of about 17 miles re- spectively. At loo pounds the heights are over one mile for the water and above 20 miles for the steam. Comparing the energy of water and of steam in the steam- boiler with that of gunpowder, as used in ordnance, it will be found that at high pressures the former become possible rivals of the latter. The energy of gunpowder is somewhat variable with composition and perfection of manufacture, and is very variable in actual use, in consequence of the losses in ordnance due to leakage, failure of combustion, or retarded combustion in the gun. Taking its value at what the Author would con- sider a fair figure, 250,000 foot-pounds per pound, it is seen that, as found by Airy, a cubic foot of heated water, under a pressure of 60 or 70 pounds per square inch, has about the same energy as one pound of gunpowder. The gunpowder ex- ploded has energy sufficient to raise its own weight to a height of nearly 50 miles, while the water has enough to raise its weight about one sixtieth that height. At a low red heat wa- ter has about 40 times this latter amount of energy in a form to be so expended. One pound of steam, at 60 pounds pres- sure, has about one third the energy of a pound of gunpowder. STEAM AND ITS PROPERTIES. 28 9 At 100 pounds it has as much energy as two fifths of a pound of powder, and at higher pressures its energy increases very slowly. 143. The Curves of Stored Energy are most instructive. Plotting the tabulated figures and determining the form of the 5900 600* 500* WO* 800' ;oo ltd 00$ JOO 8(Xf 100 6000 7000 8000 9000 FIG. 71. CURVE OK HEAT IN STEAM. curve representing the law of variation of each set, we obtain the peculiar set of diagrams exhibited in the accompanying en- graving. In Fig. 71 are seen the curves of absolute tempera- 290 THE STEAM-BOILER. ture and of latent heat as varying with variation of pressure. They are smooth and beautifully formed lines, having no rela- tion to any of the familiar curves of the text-books on co-ordi- cr I. t -- t ::::::: ::: rt^i-rtrtrt H 85)OOOI I . . _j_ 1 1 373XDIJ- , ;d < \\ '* *' sooooott 1 LJ^ |::: :::::::::: z.i: : :::: _::: swootf __ z ^__ 275000 > - - ! _ " " / ~~ uJ r- :::: Si IIT / rt ^ t -, - - - i . ___ ff . 2 1 " H-M-- :::::::--::::::::::::: ^jJ'-rU- 200000 s:' ! " l!!liiiij::j|j:::jji:::: .| nr fiamffiC::::::::::::::::::::::::::^ !?::;::::::;!= =|iE:^!H" ffi:==: = :=^ |ig \& ' I26000HH trrorT -- 123000 (K^: ^L ^- TSOOoWh _- r !_- -- 100000 75000 EO)Oo|J|4- - - * - 50000 -- 25000 1000 SOOO 8000 4000 5000 6000 7000 8000 9000 ABSOLUTE PRESSURE IN FOOT POUNDS PER. SQ. IN. FIG. 72 CURVE OF HEAT-ENERGY IN STEAM. nate geometry. In Fig. 72 are given the curves of \available energy of the water of latent heat and of steam. The first and third have evident kinship with the two curves given in STEAM AND ITS PROPERTIES. 29 1 the preceding illustration ; but the curve of energy of latent heat is of an entirely different kind, and is not only peculiar in its variation in radius of curvature, but also in the fact of pre- senting a maximum ordinate at an early point in its course. This maximum is found at a pressure of about one ton per square inch a pressure easily attainable by the engineer. Examining the equations of those curves, it is seen that they have no relation to the conic sections, and that the curve, the peculiarities of which are here noted, is symmetrical about one of its abscissas, and that it must have, if the expression holds for such pressures, another point of contrary flexure at some enormously high pressure and temperature. The for- mula is not, however, a " rational " one, and it is by no means certain that the curve is of the character indicated ; although it is exceedingly probable that it may be. The presence of this characteristic point, should experiment finally confirm the deduction here made, will be likely to prove interesting, and it may be important ; its discovery may possibly prove to be useful. The curve of energy of steam is simply the curve obtained by the superposition of one of the two preceding curves upon the other. It rises rapidly at first, with increase of tempera- ture, then gradually rises more slowly, turning gracefully to the right, and finally becoming nearly rectilinear. The curve of available energy, of heated water, exhibits similar character- istics ; but its curvature is more gradual and more uniform. 144. The Actual Power of Steam and of Boilers evi- dently depends upon the efficiency of the method of applica- tion, and on the apparatus employed. The quantity of heat- energy supplied to the engine and yielded by the generator has been seen to be easily calculable by simply multiplying the quantity of heat given to the steam by the fuel, by the me- chanical equivalent of heat. The amount available as energy may be the total quantity so supplied, as when the steam is condensed in heating buildings or otherwise, and is returned as feed-water to the boilers ; or it may be any less amount, ac- cordingly as the method of utilization is more or less effective. The tables given in the Appendix furnish the data for calcu- THE STEAM-BOILER. lation in any case in which the efficiency of transfer and of transformation is known. Where no constant value can be assumed for the efficiency of the system employed, it is some- times, nevertheless, found to be important to establish a stand- ard conventionally. Thus, in the calculation of available stored energy, as given in the Appendix, Table II., it was as- sumed that the steam would be expanded to atmospheric pres- sure. Similarly, convention has established the unit horse- power of steam-boilers, in order to afford a standard of comparison in test-trials, and to give a means of rating boilers by the designer, the builder, or the purchaser and user. The operation of boilers occurs under a wide range of actual conditions the steam-pressure, the temperature of feed- water, the rate of combustion and of evaporation, and, in fact, every other variable condition, differing in any two trials to such an extent that direct comparison of the totals obtained, as a matter of information regarding the relative value of the boilers, or of the fuel used, becomes out of the question. It has hence gradually come to be the custom to reduce all results to the common standard of weight of water evaporated by the unit-weight of fuel, the evaporation being considered to have taken place at mean atmospheric pressure, and at the tempera- ture due that pressure, the feed-water being also assumed to have been supplied at the same temperature. This, in techni- cal language, is said to be the " equivalent evaporation from and at the boiling-point" (212 Fahr., 100 C.). This standard has now become generally incorporated into the science and the practice of steam-engineering. The " Unit of Evaporation " is one pound of water at the boiling-point, evaporated into steam of the same temperature. This is equivalent to the utilization of 965.7 British thermal units per pound of water so evaporated. The economy of the boiler may thus be expressed by the number of units of evaporation obtained per pound of combustible. 145. The Horse-power of Steam-Boilers must always be reckoned on an assumed basis involving the amount of heat supplied from the furnace, the conditions determining the availability of that heat as stored, and the circumstances con- STEAM AND ITS PROPERTIES. 293 trolling its expenditure and transformation. The term must evidently be purely conventional and technical, and its defini- tion must be strictly limited. The character and magnitude of the unit to be chosen to express the " power " of the steam-boiler is not fully settled, though the subject has attracted much attention among engi- neers. It is evident that since the boiler is merely an appara- tus for the generation of steam, and since the province of the steam-engine is to develop power from that steam, and with a degree of efficiency which may vary enormously, it is certain that we have no natural unit of power for steam-boilers. It may even be asserted that no natural unit can exist. The most scientific system of power-rating yet proposed considers the power of a boiler to be that expended by it in driving out all the steam which it makes against the pressure of the atmos- phere, a system suggested by Nystrom.* The weight of water to be evaporated per hour at any given pressure to produce one horse-power as the equivalent of its natural effect without expansion, by impelling a piston against its load, is calculable with sufficient accuracy by the formula of Nystrom : in which V is the volume of steam in cubic feet, p the absolute pressure in pounds per square inch, and v the volume of steam relatively to that of water at the freezing-point. By this method we obtain the following values : * Mechanics, i8th Ed., p. 562. 294 THE STEAM-BOILER. p H. P. per Cu. Ft. Lbs. per H. P. 5 .6600 29.852 10 .7253 28.723 14.7 7540 28.252 25 .7879 27.717 40 .8238 27.170 60 .8649 26.573 80 Q033 26.038 100 .9406 25-537 125 .9865 24-945 150 2.0321 24-387 What is sometimes called the "boiler-heat horse-power"* is the power corresponding to the energy imparted to the steam by its evaporation within the boiler. This power is measured by dividing the weight of steam made by that re- quired to produce unity of power, and the latter quantity is obtained by dividing the energy in foot-pounds of one horse- power per hour by the mechanical equivalent of the latent heat of steam ; i.e., w = 1,980,000 966 x 772 = 2.65 Ibs. Taking as a standard the quantity of steam demanded by a perfect engine, having no clearance, receiving steam at boiler- pressure, and expanding it down to a perfect vacuum, or to the atmospheric pressure, we may readily obtain figures for the weights demanded by which to rate steam-boilers, should it be found necessary to resort to such an ideal system. For such cases, Zeuner'sf figures are as below: * " Boiler-power and Boiler-heating Surface," by Professor R. H. Smith, In- dustrie -s, July i, 1887. fWarme Theorie. STEAM AND ITS PROPERTIES. WATER PER HORSE-POWER PER HOUR. PRESSURE ATMOSPHERES. Non-condensing Engine. Condensing Engine. Lbs. Kilogs. Lbs. Kilogs. 3 33 i5i 13 6 4 26 12 12 5* 5 23 10* Hi 5i 6 21 9* II 5 8 18 8 lOj 4i 10 i6| 7i 10 4i In this case the rated power of the boiler would be obtained by dividing the weight of steam made per hour by the proper figure from the above table. Assuming the actual kinetic energy of the issuing steam to measure the actual available power of the boiler, we find that if the size of the orifice is just sufficient to discharge the steam as rapidly as it is generated, the work done by the boiler will be u= (0 and the power = - +550, or H.P.= (2) when w is the weight of steam made, and v its velocity of out- flow per second, the one expression being in British, the other in metric measures. Again taking Zeuner's figures, we have PRESSURE ATMOSPHERES. 3- 4- 5- 6. 8. 10. VELOCITIES PER SECOND. Metres. Feet. 185 607 208 68 I 22 7 230 255 260 734 775 835 879 296 THE STEAM-BOILER. and the horse-power actually delivered on this basis would be obtained by inverting these values in the expression above. So using them, we obtain for the power of the boiler, PRESSURE H. P. = + 550 = =Z~ + 75 . ATMOSPHERES. *g ^S nt in Ibs. 5iw;w in kilos. 4 1407^ 5 165717 75w; 6 l84T/ 847'; The work done by the boiler is thus obtained by multiply- ing the weight of steam made per second by the figures here given. This system may be called the natural system of rating power. Where a similar system is adopted, but the total re- sistance of the atmosphere is allowed for, as proposed by Nystrom for the " legal " horse-power, the quantity of heat and of steam demanded is increased, at usual pressures, about one half. Nystrom proposed to assume a fixed rate of combustion and proportions of parts. His method may be illustrated as follows : A cubic foot of water, when evaporated, forms a definite volume of steam ; and if we take the product of the volume of water evaporated per hour, the increase of volume by its con- version into steam, the pressure of the steam, and divide this product by 1,980,000, the quotient, which is the power this steam can develop in a non-condensing engine, without expan- sion, is the horse-power of the boiler. Suppose, for example, that a boiler evaporates 25 cubic feet of water per hour, and that the pressure of the steam above the atmosphere is 130 Ibs. per square inch, or 18,720 Ibs. per square foot. The relative volume of steam of this pressure is 192.83, so that the increase of volume for each cubic foot of water, on its conversion into steam, is 191.83 cubic feet, and the horse-power of the boiler is the product of 25,191.83, and 18,720 divided by 1,980,000, or 45-3 + STEAM AND ITS PROPERTIES. He would take the power of a boiler to be 297 . FSVp. 10 (2) in which formula F and 5 are the areas of grate and heating surface in square feet. Thus a boiler having 100 square feet of grate and 3000 feet of heating surface, at 75 pounds pressure above vacuum, would rate at which is far above the usual power of steam-boilers with natural draught. Small engines, according to Buel, demand steam, ordinarily, as below : FEED-WATER REQUIRED BY SMALL ENGINES. Pressure of Steam in Boiler, by Gauge. 10 Pounds of Water per effective Horse- power per Hour. Il8 Pressure of Steam in Boiler, by Gauge. 60 15 III 70 20 105 80 25 TOO 90 30 40 93 84 100 120 50 79 150 Pounds of Water per effective Horse- power per Hour. 75 71 68 65 63 61 58 Pressures lower than 60 pounds are not usually adopted for small engines. Good examples of such engines have been found by the Author to demand from 25 to 33 per cent less steam, or- feed-water, than is above given. The following are considered by the Author as fair estimates of water and steam consumption for the best classes of engines in common use, when of moderate size and in good order : 298 THE STEAM-BOILER. WEIGHTS OF FEED-WATER AND OF STEAM. NON-CONDENSING ENGINES. STEAM PRESSURE. POUNDS PER H. P. PER HOUR. RATIO OF EXPANSION. Atmospheres. Lbs. per sq. in. 2 3 4 5 7 10 3 45 40 39 40 40 42 45 4 60 35 34 36 36 38 40 5 75 30 28 27 26 30 32 6 90 28 27 26 25 27 29 7 105 26 25 24 23 25 27 8 1 20 25 24 23 22 22 21 10 150 24 23 22 21 2O 2O CONDENSING ENGINES. 2 30 30 28 28 30 35 40 3 45 28 27 27 26 28 32 4 60 27 26 25 24 25 27 5 75 26 25 25 23 22 24 6 90 26 24 24 22 21 20 8 120 25 23 23 22 21 20 10 150 25 23 22 21 20 19 It is considered usually advisable to assume a set of practi- cally attainable conditions in average good practice, and to take the power so obtainable as the measure of the power of the boiler in commercial and engineering transactions. The unit generally assumed has been usually the weight of steam de- manded per horse-power per hour by a fairly good steam-en- gine. This magnitude has been gradually decreasing from the earliest period of the history of the steam-engine. In the time of Watt, one cubic foot of water per hour was thought fair; at the middle of the present century, ten pounds of coal was a usual figure, and five pounds, commonly equivalent to about forty pounds of feed-water evaporated, was allowed the best engines. After the introduction of the modern forms of en- gine this last figure was reduced twenty-five per cent, and the most recent improvements have still further lessened the con- sumption of fuel and of steam. By general consent, the unit has now become thirty pounds of dry steam per horse-power per hour, which represents the performance of good non-con- densing mill-engines. Large engines, with condensers and STEAM AND ITS PROPERTIES. 299 compounded cylinders, will do still better. A committee of the American Society of Mechanical Engineers* recommended thirty pounds as the unit of boiler-power, and this is now gene- rally accepted. They advised that the commercial horse-power be taken as an evaporation of 30 pounds of water per hour from a feed-water temperature of 100 Fahr. into steam at 70 pounds gauge pressure, which may be considered to be equal to 34^ units of evaporation, that is, to 34^ pounds of water evapo- rated from a feed-water temperature of 212 Fahr. into steam at the same temperature. This standard is equal to 33,305 British thermal units per hour.f It was the opinion of this committee that a boiler rated at any stated power should be capable of developing that power with easy firing, moderate draught, and ordinary fuel, while exhibiting good economy, and at least one third more than its rated power to meet emergencies. Any increase of temperature derived from a heater should not be credited to the efficiency of the boiler except by agree- ment ; and in the latter case tests should be made only with feed-water of the temperature observed during the regular operation of the boiler. * Trans., vol. vi., Nov. 1881. f According to the tables in Porter's Treatise on the Richards Steam-engine Indicator, which tables the committee adopt, an evaporation of 30 pounds of water from 100 F. , into steam at 70 pounds pressure, is equal to an evaporation of 34.488 pounds from and at 212 ; and an evaporation of 34^ pounds from and at 212 F. is equal to 30.010 pounds from 100 F., into steam at 70 pounds pressure. The " unit of evaporation" being equal to 965.7 thermal units, the commercial horse-power = 34.488 X 965.7 = 33.305 thermal units. CHAPTER VII. THE DESIGN OF THE STEAM-BOILER. 146. The Design of the Steam-Boiler is a problem in construction which involves vastly more than the mere applica- tion of chemical and physical principles, and the calculation of areas of grate and heating surfaces. The first step in its solu- tion is the study of the conditions under which the steam is to be produced and utilized ; the location and space available ; the kind and cost of fuel ; the nature and availability of the supply of feed-water ; the pressure to be adopted ; the facilities to be obtained for repairs ; and many other conditions, of which the financial and commercial are as important as any others, must all be taken into careful consideration. The problem, stated in the most general and comprehensive way, may be said to be the following : Required : To determine what type, proportions, size, and construction of boiler may be made, in the location chosen, and under all the natural and artificial conditions found there to exist, to supply a given amount of steam at least total risk and cost. The business aspects of the case must be as conscien- tiously studied by the designing engineer as those of pure en- gineering. The design of the steam-boiler is thus a problem in en- gineering which demands careful consideration, accurate knowl- edge of the principles controlling proportions and performance, and perfect familiarity with the conditions to be met in the case in hand. 147. The Choice of Type of Boiler and its Location is the first step to be taken preparatory to commencing the de- sign. The type best adapted for the special case is determined by the conditions of location and purpose, as whether station- THE DESIGN OF THE STEAM-BOILER. 30 1 ary, portable, locomotive, or marine ; by the pressure and quan- tity of steam demanded ; by the character of the feed-water and fuel, and the cost of obtaining it ; by the facilities to be had for repairs, etc. Where the boiler is to be used on land, the standard loco- motive and stationary boilers may be used, if found otherwise advisable ; but on shipboard it is essential that the boiler should be " self-contained," and the common stationary boilers cannot be employed. Each application is best made, as a rule, by the employment of some one of those forms which have been classed above, and certain types are thus standard for each location. Among stationary boilers the plain cylindrical is chosen when the cost of fuel is low, when the feed-water is bad, or when the facilities for repairing are not good. As the necessity for economy in fuel-consumption becomes greater, and when the character of the feed-water is good, the more complicated flue or tubular boilers are selected ; or the dictates of prudence may lead to the selection of some one of the so-called " safety" or " sectional " boilers, even where cost and other considera- tions would weigh against them. The most common form of stationary boiler in the United States, in ordinary good locations, is the cylindrical tubular boiler ; in Great Britain the Cornish and the Galloway boilers are much used ; while on the continent of Europe the " ele- phant" boiler is more common. In all directions, however, the safer forms of boiler are gaining ground. The " portable" boiler is usually an upright tubular, with firebox beneath, for very small powers, and a horizontal boiler of the locomotive type for larger sizes. It must always be " self- contained " in the sense of having no " setting," and is com- monly made the foundation or bed for its attached engine, somewhat as in locomotives. The locomotive boiler has become fixed in type, and nearly fixed in proportions. All builders adopt the horizontal, cylin- drical tubular shell with firebox. Here, as in all cases in which high pressures are employed, cylindrical or strongly stayed sur- faces are found essential to safety and durability. Many other 302 THE STEAM-BOILER. designs of boiler have been proposed and experimentally em- ployed for locomotives, but none has survived. The marine steam-boiler is the product of a long process of evolution which has led to the gradual reduction of a variety of forms to a few standards. Thus, at sea, the "drum" or Scotch boiler, described in article 19, has become almost uni- versally adopted where high pressures are employed, as it is stronger, more compact, and more economical than its rivals, and is self-contained. The location of a boiler is sometimes a matter of choice with the engineer preparing the plans, and may be one of serious importance. Where possible it should always be so chosen that the boiler may be easy of access for inspection and repair ; it should be free from special danger to lives or sur- rounding property in case of accident, and the site selected should be dry and well protected against the weather. The nearer the engine or other point at which its steam is delivered the better. Only sectional boilers should be placed under buildings. Shell-boilers should have boiler-houses constructed for them apart from the larger and more important structures to which they are auxiliary, and this precaution is especially advisable for cases, as mills, in which many lives may be en- dangered. The risk involved is not great where these boilers are well designed and constructed ; but the prudent engineer avoids even moderate risk where a life is involved. When the space is restricted in floor-area, but of good height, the upright tubular boiler is selected ; if the floor-area is unrestricted, but head-room is small, the horizontal forms of boiler are chosen. Good forms of " safety " boilers may be placed wherever they can be given room, provided they are accessible for inspection, cleaning, and repairs. 148. The Choice of Fuel and of Method of Combustion is commonly necessarily made before the design can be pro- ceeded with. The fuel is, as a rule, selected mainly \vith a view to commercial efficiency ; but the presence of any observable quantity of sulphur in coal justifies its rejection at even con- siderable pecuniary sacrifice. That fuel is best which produces the required quantity of steam with certainty and regularity THE DESIGN OF THE STEAM-BOILER. 303 under the given conditions, and at minimum total cost for purchase, transportation and handling, storage, interest and insurance, and wear and tear of apparatus. As a rule, the least costly fuels are most economical, if the furnace is properly adapted to them ; but it is not always so, and the user will generally solve the problem by experiment and experience. The conditions of the market are very apt to control, and anthracite fuel in the Eastern United States, bituminous coals throughout the West, and wood in forested countries are naturally the staple fuels. On the border lines, or even within either territory, prices may be so adjusted that the question may be difficult to decide until after prolonged trial of two or more kinds which may be available. In the case of the "soft" coals the decision of the question whether the fuel shall be used in its natural state, or coked, may often demand con- sideration. For metallurgical purposes coke is commonly used, but for steam-boilers the raw coal is most generally adopted. The combustion may be produced by either a natural chimney draught or a forced draught, created by a fan, a steam- jet, or other artificial means. With very fine coal, or where the grate-area or the boiler itself is so small as to make the rate of combustion due to natural draught insufficient, the blast is employed. The locomotive and the torpedo-boat illustrate this case. A closed fire-room, made air-tight, and into which the blast is driven and allowed to enter the furnace precisely as with a chimney draught, is regarded by many engineers as the best method of securing rapid combustion. Where the area of heating-surface is the same in proportion to the amount of coal burned, this system is fully as economical as the others. The proportion of heating to grate surface being fixed, or nearly constant, as is common, the slower combustion, down to certain limits is naturally the more efficient. Natural draught is to be preferred where the desired amount of steam may be made by that system. 149. The Conditions of Efficiency in steam-boilers are those affecting the production, the transfer, and the storage of the heat-energy derivable from the fuel. These have already 304 THE STEAM-BOILER. been considered. En resumt : the efficient production of heat requires the concentrated combustion of the fuel, with the minimum air-supply consistent with the complete combination of its oxidizable elements with oxygen, and the attainment of maximum temperature. The efficient transfer and storage in the steam of this heat demands that it be liberated at maxi- mum temperature, that the heating-surfaces be of great extent in proportion to the weight of fuel burned and to the quantity of heat liberated, and that these surfaces be effective in absorp- tion of heat. The formula deduced in Chapter IV. for effi- ciency of heating-surface gives a measure of the efficiency of the boiler when the value of the fuel is known, and includes efficiency of transfer and of storage. 150. The Principles of Design, in the case of the steam- boiler, involve those of strength of materials and of structures, the determination of the size, form, and proportions of parts ; the relation of area of heating and of grate surface to fuel burned ; the character and proportions of accessory parts ; in fact, the application of all the data and the laws which have been studied in the preceding portions of this work. The de- signing engineer must determine the form and proportions of a vessel in which is to be generated a given quantity of steam with satisfactory efficiency and safety, and with as nearly per- manent commercial success as possible. The settlement of the general proportions of the structure is made with reference to the above considerations ; but gen- eral experience has brought these proportions into a fairly definite relation, and, as an illustration, the better classes of boiler rarely have a less ratio of heating to grate surface, where natural draught is adopted, than about 25 to I, or a higher ratio than 40 to I. With more intense combustion and forced draught this proportion is considerably increased. The best proportion is probably usually capable of fairly exact calcula- tion by a method to be considered at some length in a later chapter. Boiler-power is very often calculated, in cases of ordinary practice, by allowing a certain number of square feet of heating-surface to the horse-power. Thus, the following may be taken as a fair average set of figures : THE DESIGN OF THE STEAM-BOILER. 305 Plain cylinder-boiler 8 Flue-boiler ' .... 10 Water-tube or sectional boiler 12 Locomotive boiler 13 Return tubular boiler 15 Upright tubular boiler 1 8 Careful calculation should be resorted to in every impor- tant case. In designing boilers the effort of the engineer should be (1) To secure complete combustion of the fuel without permitting dilution of the products of combustion by excess of air. A combustion-chamber is usually desirable. (2) To secure as high temperature of furnace as possible. (3) To so arrange heating-surfaces that, without checking draught, the available heat shall be most completely taken up and utilized and the most complete and rapid circulation se- cured, both for the water and for the furnace-gases. (4) To make the form of boiler so simple that it may be constructed without mechanical difficulty or excessive expense, and to arrange for ample water-surface, as well as large steam and water capacity, so as to insure against serious fluc- tuation of steam-supply. (5) To give it such form that it shall be durable, under the action of hot gases, and of corroding elements of the atmos- phere. (6) To make every part accessible for cleaning and repairs. (7) To make all parts as nearly as possible uniform in strength, and in liability to loss of strength with age, so that the boiler, when old, shall not be rendered useless or dangerous by local defects. (8) To adopt a reasonably high * factor of safety" in pro- portioning parts, and to provide against irregular strains of all kinds. (9) To provide efficient safety-valves, steam-gauges, mud- drums, and other appurtenances. (10) To secure intelligent and very careful management. In securing complete combustion the first of these desiderata an ample supply of air and its thorough intermixture with the 20 306 THE STEAM-BOILER. combustible elements of the fuel is essential ; for the second high temperature of furnace it is necessary that the air-supply shall not be in excess of that absolutely needed to give com- plete combustion. The efficiency of a furnace is measured by T T 1 F - ~ T- i' in which E represents the ratio of heat utilized to the whole calorific value of the fuel ; T is the furnace temperature ; T 1 the temperature of the chimney, and / that of the external air. Hence the higher the furnace-temperature and the lower that of chimney, the greater the proportion of available heat. It is further evident that, however perfect the combustion, no heat can be utilized if either the temperature of chimney approximates to that of the furnace, or if the temperature of the furnace is reduced by dilution to that of the chimney. Concentration of heat in the furnace is secured, in some cases, by special expedients, as by heating the entering air, or, as in the Siemens gas-furnace, heating both the combustible gases and the supporter of combustion. Detached fire-brick fur- naces have an advantage over the " fireboxes" of steam-boilers in their higher temperature ; surrounding the fire with non- conducting and highly heated surfaces is an effective method of securing high furnace-temperature. In arranging heating-surface, the effort should be to impede the draught as little as possible, and so to place them that the circulation of water within the boiler should be free and rapid at every part reached by the hot gases. The direction of circulation of water on the one side and of gas on the other side the sheet should, whenever possible, be opposite. The cold water should enter where the cooled gases leave, and the steam should be taken off farthest from that point. The temperature of chimney-gases has thus been re- duced by actual experiment to less than 300 Fahr., and an efficiency equal to 0.75 to 0.80 the theoretical is attainable. The extent of heating-surface simply, in all of the best forms of boiler, determines the efficiency, and the disposition THE DESIGN OF THE STEAM-BOILER. 30? of that surface seldom affects it to any great extent. The area of heating-surface may also be varied within very wide limits without greatly modifying efficiency. A ratio of 25 to I in flue and 30 to I in tubular boilers represents the relative area of heating and grate surfaces in the practice of many of the best-known builders. The factor of safety is usually too low. The boiler should be built strong enough to bear a pressure at least six times the proposed working-pressure. As it grows weak with age, it should be occasionally tested to a pressure at least double the working-pressure, which latter should be reduced gradually to keep within the bounds of safety. 151. The Controlling Ideas in designing dictate the follow- ing procedure. The engineer determines (1) The height of chimney, and rate of combustion desira- ble or practicable. (2) The type of boiler, having regard to the character of water to be used as " feed," and the costs of construction, opera- tion, and maintenance. (3) The quantity of steam that will be demanded. (4) The efficiency of boiler that it will be economical to se- cure, according to the principles to be given, and thus the ratio of heating to grate surfaces. (5) The kind and the quantity of fuel required, with the given or proposed efficiency, to produce the demanded quan- tity of steam. (6) The total areas of grate and of heating surface required to burn that fuel and to make that steam. (7) The forms, sizes, and proportions of details. The dimensions and proportions of the boiler plant being thus determined, the engineer decides what amount of power shall be obtained from a single boiler, and thus how many boil- ers are to be constructed, the area of heating and grate surface to be given each ; and he finally decides upon the form of set- ting, and method of making steam and water connections. It then remains only to make a drawing of the boiler, which shall show its form and dimensions, the arrangement of 3O8 THE STEAM-BOILER. stays, pipes, safety, and other attachments, and the setting. The first plan constructed will usually require some modifica- tion to adapt it exactly and satisfactorily to the wants of the user; which changes being made, the boiler may be constructed from the drawing. The thickness of shell, size of tubes or flues, sizes, methods, and distribution of stays, and similar matters of detail, are settled by well-known rules of practice, or by the consideration of the peculiar conditions met with in the case in hand. Especial care should be taken to give all parts ample strength, with a fair and safe allowance for corrosion ; to see that every part is easily accessible for inspection and repair; that all de- tails are of good form and proportions ; and that all accessories and attachments are the best and safest of their kind. The Steam-pressure to be adopted will necessarily be one of the first matters to be considered and settled ; both because it has an important bearing upon the efficiency of the engine and because it must be kept in view in the selection of the type and size of boiler. The tendency is constantly in the direction of higher steam-pressure, and the consequent adoption of the simpler, stronger, and safer kinds of boiler. This directly con- flicts with the commercial considerations affecting boiler-con- struction, especially of the common forms of shell-boiler. The larger the boiler, as a rule, the cheaper, comparatively, its con- struction, the less the cost of setting and of installation, and the higher its economy in operation. A large shell, however, must be made of thicker iron, and is always somewhat less ab- solutely safe than a similar smaller structure. A limit is thus being continually approached because of the fact that the net gain is less and less as the increase occurs at higher pressures. An increase from 100 to 200 pounds may give a calculated gain of 12 or 15 percent; but the net gain will be actually much less, and may not be enough to compen- sate the increased costs and risks. At the present day, pres- sures of 125 to 150 pounds are not unusual ; but many engineers consider it inadvisable to go much farther in the direction of increasing pressure, and the tendency of modern practice is THE DESIGN OF THE STEAM-BOILER. 309 to restrict the adoption of such higher pressures to the cases in which the sectional types of boiler are used. As illustrating the general effect of increasing pressures, and the progressive diminution of the rate of gain, Mr. H. F. Smith has given the following tables of weight of steam and coal demanded per hour and per horse-power, by a perfect steam-engine, calculated on the assumption that 1 100 thermal units per pound of coal are utilized by the boiler, which corre- sponds to an evaporation of about H T 4 F parts by weight of water from and at the boiling-point, per one part of coal a re- sult attainable with good coal : STEAM AND FUEL CONSUMPTION IN A PERFECT STEAM-ENGINE. BOILER PRKSSURE. Per Gauge. TEMPERATURE. Fahr. Cent. STEAM. Per I. H. P. per hour. COAL. Per I. H. P. per hour. Non-con- densing. Lbs. Kil. Con- densing. Lbs. Kil. Non-con- densing. Con- densing. Lbs. A tin os. Lbs. Kil. Lbs. Kil. 300 20 421.7 216.5 10.48 4.8 6.16 2.7 .98 .44 .64 .29 250 i65f 405.9 207.7 ".19 5-i 6.39 2.9 04 -45 .66 .30 200 I 3 J 387.6 197.5 12. 16 5.5 6.68 3.0 13 -Si .69 .31 175 " 377-1 191.7 12.81 5.7 6.87 3 i -18 .54 7i -32 150 10 365.6 185.3 13.63 6.2 7.09 3.2 25 -57 73 -33 "5 8* 352 6 167.0 14.71 6.7 7-37 3-8 -35 -60 75 -34 100 6 337- 6 J 59- 8 16.24 7-4 7-7' 3-5 .48 .67 78 -35 90 6 330.9 166.1 I7-05 7-7 7-89 3-6 55 -70 .80 .36 So 5 323.6 162.0 18.03 8- 2 8.09 3.7 64 -75 .82 .37 75 5 319.8 159.9 18.60 8.5 8-19 3-7 .69 .77 83 .38 7 4 315.7 157.6 19.25 8.7 8.32 3-8 -75 -80 84 -39 60 4 307.1 152.8 20.83 9-5 8-59 3-9 .88 .85 87 -39 50 3 297.5 147.5 22.95 10.4 8.92 4.1 .07 .90 .90 .40 45 3 292.2 144.5 24.53 ii. i 9.11 4.1 .19 i.oo .91 .40 The table shows that at high pressures the gain of economy is very slow, and that the very best modern engines waste a large part of the steam passing through the cylinder. At 125 pounds, if there were no losses, three fourths of a pound of coal per hour would furnish one indicated horse-power , but very few engine- builders can be found who are willing to guarantee an indicated 310 THE STEAM-BOILER. horse-power with less than one and three fourths of a pound of coal per hour under the best of conditions. A pound of coal, if all the heat were utilized, would evapo- rate 15 pounds of water from and at the boiling-point. Many boilers actually evaporate 11^ pounds of water with an effi- ciency of 75 per cent. An engine working perfectly would develop one indicated horse-power with /f pounds of steam (of 125 pounds initial pressure) per hour ; the best actual engines consume more than double this quantity. Mr. G. H. Barrus gives the following as the probable actual steam-consumption of good engines :* FEED-WATER CONSUMPTION FOR NON-CONDENSING ENGINES. Initial Feed-water Initial Feed-water pressure above atmosphere. Lbs. Mean effective pressure. Lbs. consumed per I. H. P. per hour. Lbs. pressure above atmosphere. Lbs. Mean effective pressure. Lbs. consumed per I. H. P. per hour. Lbs. AT 10 PER CENT CUT-OFF. AT 30 PER CENT CUT-OFF. 40 1.32 *53- 2 4 40 16.95 33-52 5 5-oi 52-52 50 23-71 29-35 60 8.70 37-26 60 3-47 27.24 70 12.39 3 -99 7 37-2i 25.76 80 16.07 27.61 80 43-97 24.71 90 19.76 25-43 9 5-73 23.91 IOO 23-45 23.90 IOO 57-49 23-27 AT 20 PER CENT CUT-OFF. AT 40 PER CENT CUT-OFF. 40 IO.22 38-13 . 40 22.24 32-79 50 I5- 6 7 30.98 50 29.99 29.72 60 21 . 12 27-55 60 37-75 27.92 70 26.57 2 5-44 70 45-50 26.26 80 32.O2 24.04 80 53-25 25.76 90 37-47 23.00 90 61.01 25.03 IOO 42.92 22.25 IOO 68.76 24-47 AT PER CENT CUT-OFF. 40 26.40 33 - l6 80 60.44 26.99 50 60 34-91 43-42 30-53 28.94 90 IOO 68.96 77.48 26.32 25.78 70 51-94 27-79 * The Tabor Indicator. THE DESIGN OF THE STEAM-BOILER. 311 FEED-WATER CONSUMPTION FOR CONDENSING ENGINES. Initial Feed-water Initial Feed-water pressure above atmosphere. Lbs. Mean effective pressure. Lbs. consumed per I. H. P. per hour. Lbs. pressure above atmosphere. Lbs. Mean effective pressure. Lbs. consumed per I. H. P. per hour. Lbs. AT 5 PER CENT CUT-OFF. AT 20 PER CENT CUT-OFE. 40 9-34 18.99 40 23-83 19.00 50 11.88 18.51 50 29.28 18.74 60 14.42 18.22 60 34-73 18.98 7 16.96 17.96 70 40.18 18.40 80 19-50 17.76 80 45.63 18.27 90 22.04 17-57 90 51.08 18.14 IOO 24.58 17.41 IOO 56.53 18.02 AT 10 PER CENT CUT-OFF. AT 30 PER CENT CUT-OFF. 40 14.96 18.25 40 30-54 20.57 18.65 22.34 17.91 17.68 50 60 ' 37-30 44.06 20. 35 20.19 70 26.03 17-47 ?o 50.81 20.04 80 90 29 72 33-41 17-30 17-15 80 90 57-57 64.32 19.91 19.78 IOO 37-JO 17.02 IOO 71.08 19.67 AT 15 PER CENT CUT-OFF. AT 40 PER CENT CUT-OFF. 40 50 19.72 24.36 ,8.41 i8.ii 40 5 35.84 43-59 21.94 21 .76 60 29.00 17-93 60 51-35 21.63 70 80 33-65 38.28 17.60 70 80 CQ TO Hiss 21-49 21.36 90 42.92 17-45 90 74.60 21.24 IOO 47.56 17-32 IOO 82.36 21.13 152. Safety and Efficiency vs. Cost may be taken as the most serious part of the problem to the designer and user of steam-boilers. The safety of the boiler being a first considera- tion, it becomes at onc*e a question how far the engineer is justi- fied in sacrificing money and special advantages to secure safety, and how closely he may be practically able to approximate ab- solute security. To increase strength of structure or of parts means to enlarge the dimensions, and to thus increase expense ; to select a specially safe type, or peculiarly safe construction, is usually to meet the same objection ; and it is soon found that there is a certain golden mean between maximum safety and impracticable expense which gives most satisfactory re- sults. For ordinary cases, this is probably found not far from those proportions which give a " factor of safety" of about six for the important parts of the boiler, although good authori- 312 THE STEAM-BOILER. ties advise eight, and even ten, and general practice often falls to less than four. The same difficulty arises when it is attempted to attain high efficiency. This must be done by extension of heating- surface and correspondingly increased first cost ; and it is readily shown, as in Chapter XIII., that business considera- tions fix the limit of efficiency to be sought. This efficiency being given, the size and proportions of boiler become at once determinable. Thus accepting Rankine's formula for effici- ency, already given in article 98, and taking the desired efficiency as given by calculation as , the ratio of heating-sur- face divided by fuel burned, = R, will be obtained thus : 5 B i -\-AR- B-E (i) AE Taking as common values E = 0.70, A = 0.5, B = i, and the ratio of heating to grate-surface would be S = -; if 0.80 F= 15, S = 17.5. Taking a rather high efficiency, E = 0.80, R 0.5, and 5 = 30. 153. Water-tubes and Fire-tubes have, respectively, their own special advantages and disadvantages, and these differ in their importance in different types of boiler. It was shown by experiments directed by Engineer-in-chief B. F. Isherwood of the U. S. Navy,* that the water-tube boiler as constructed for marine purposes with vertical tubes is some- what more economical than the horizontal fire-tube boiler of otherwise similar type, and the former excels in the perfection of its circulation and the readiness with which it can be freed from incrustation ; it, however, makes a heavier boiler, and the * Experimental Researches in Steam Engineering. THE DESIGN OF THE STEAM-BOILER. 313 water-tube is less easily plugged if leaking. This latter diffi- culty, and the inconveniences and dangers arising from the accumulation of salt in marine boilers when water from in- jured tubes evaporates in the tube-box, have caused the disuse of this class of boilers. The " sectional " class of water- tube boilers is less subject to such objections. Water-tubes are always set either vertical or steeply inclined, as horizontal or nearly horizontal water-tubes are liable to rapid destruction, and are comparatively inefficient because of the defective circulation invariably distinguishing them. The fire-tube may be used in any position, but is usually placed horizontally. The general experience of engineers has been such as to lead them to adopt the water-tube in the so-called " safety" class of boilers and the fire-tube in others. The water-tube is usually placed at an angle, in these boilers, of about thirty de- grees with the horizontal. In the " Field tube" the position is vertical, or nearly so ; the lower end is closed, and an in- ternal " circulating tube" permits the descent of a solid column of water while the mingled steam and water currents gene- rated by the heat applied to the exterior of the main tube rise unobstructed to the surface. Messrs. Porter and Allen found that water-tubes, closed at the bottom and set at an angle of about thirty degrees with the vertical, were capable of doing good work, and had a sufficiently good circulation to give extraordinarily high evapo- rative power. In all standard forms of " shell-boilers" the water-tubes are placed vertically, and are grouped in a low, long, and usually narrow tube-box, several of which tube-boxes are placed side by side in large boilers. The fire-tube stands vertically in the common " upright" boiler, and is set horizontally, as has been seen in Chapter I., in all the other common forms. As constructed by the best-known builders, the water-tube is expected to do about twenty per cent more work than the fire-tube of equal area. The water-tube shell-boiler is in some respects safer than the fire-tube boiler; since the water level can be carried below, and often a considerable distance below, 314 THE STEAM-BOILER. the top of the tube without endangering it. Low water with the horizontal fire-tube is always dangerous. 154. Shell and Sectional Boilers, compared in other re- spects than in reference to safety, in which attributes the latter are specially constructed to excel, are found, when equally well designed and constructed, and equally well managed, to stand on substantially the same level. The two types of boiler in most common use are the water- tube sectional and the cylindrical fire-tube (shell) boiler. The latter is in the more extensive use, its cost, as a rule, being less, its regularity of steam-supply and uniformity of water- level greater, while its unity of structure, its convenience of access for inspection and repair, and perhaps more than all, the fact of its having a longer history, and being the product of a kind of survival of the fittest of the older types, giving it a hold upon the market that later forms of boiler have not secured. The former of these two classes has the grand ad- vantage of safety against disruptive disastrous explosions, has. equally good or better circulation and general efficiency, less weight and volume for equal powers, and greater reliability in its details of structure. Its joints are an objection, and its usually less steady operation is a disadvantage ; but it is rapidly coming into favor among engineers, and into use as well. The Author would often use the shell-boiler where commer- cial reasons would dictate such use, and, wherever practicable, would select the externally fired cylindrical fire-tube boiler, but would never place a shell-boiler under a building in which its explosion would endanger life or much property : the " safety" class of boiler would be the only form to be wisely adopted in such locations. Shell-boilers should usually be placed in detached boiler-houses, and so set, a^s to position, that danger shall be made a minimum, i.e., never pointing toward other buildings. 155. Natural and Forced Draught both have their advan- tages and their disadvantages. Chimney draught, unaided, gives a good supply of air to the fire, such as answers the pur- pose well for all ordinary work ; is free from the objections THE DESIGN OF THE STEAM-BOILER. 315 introduced with all machinery, and especially those arising from uncertainty of absolutely reliable continuous operation, and an equally certain expense for wear and tear. For the intense draught and large air-supply needed when a large amount of fuel is to be burned on a small area of grate, the size and especially the height of chimney required, and its cost, become serious matters, and for such cases a forced draught is the only suitable system. There are two principal systems of forced draught, as al- ready noted : that in which the air is forced directly into the ashpits through conduits leading from the fan or other source of the blast ; and that in which the current is driven into the fire-room, or " stoke-hole," which is made air-tight for this pur- pose, and thence finds its way to the furnaces precisely as when a natural draught is adopted. Of these the first is the older and more common method ; while the second is coming into use, particularly on torpedo-boats and elsewhere where enor- mously high rates of combustion are to be attained and kept up. By the older system the change from the forced to the natural draught is very conveniently made; but there is more difficulty in handling the fires, and the blowing of dust out into the room, and the danger of melting down the grate-bars, are two decided disadvantages, which are not inherent with the system involving the adoption of the air-tight fire-room. In the latter case the fires are as conveniently and nearly as comfortably managed as with natural draught ; and as all air passes to the furnaces through the fire-room, if it is well directed, the ventila- tion and cooling of the room and the comfort of the men are comparatively well insured. A later and in some respects most satisfactory system is that in which the air is drawn into the boiler-room by a fan placed as near the furnace as possible, r.nd then forced through ducts into the ashpit, and into the interior of hollow furnace- doors in 'such manner as to intercept any gas that would other- wise be liable to find its way outward at the furnace mouth. The Power required for Forced Draught is easily calculated thus: 316 THE STEAM-BOILER, Let/ = pressure of blast per square foot ; w = weight of fuel burned per minute ; F = volume of air per pound of fuel, at melting-point of ice ; T = temperature, absolute, at o Fahr.; T = " " of entering air; C = coefficient of efficiency of blast apparatus. Then the horse-power demanded will be H.P.= Thus for 100 square feet of grate, at 60 pounds burned per hour or one pound per minute, per square foot, 200 cubic feet of air at 32 F. per pound of fuel, when T =493.2, T= 532.2, C = ,/ = 3 inches of water = 16 pounds per square foot. 16 X 200 X i X 53 2 - 2 H. P. = -^- - = 20 nearly. 33,000 X 493-2 X i But good engines with such boilers should develop 2000 horse-power. The cost of blast would thus be about one per cent of the total power ; while with natural draught the cost would probably be in vastly greater proportion in the form of waste heat. An efficient water-circulation is very important, and the best boiler, as already stated, the most efficient as well as the safest, is that in which, other things being equal, the circulation is most complete, general, rapid, and steady. In nearly all boilers the circulation is a " natural " one ; but occasionally, as in Pierce's rotary boiler,* as tested by the Author, and later at the U. S. Centennial Exhibition of 1876, and in the boiler of Professor Trowbridge, the circulation is a " forced " one. The last-named engineer made experiments^ assisted by Messrs. T. W. Mather and J. F. Klein, graduate students of the Shef- * Reports on Steam-boilers at the U. S. Centennial Exhibition, 1876. f Heat and Steam-engines, p. 146. THE DESIGN OF THE STEAM-BOILER. 317 field Scientific School, to determine the efficiency of forced circulation. The difficulty of constructing very small steam- generators having sufficient strength to resist great pressure, and at the same time a high rate of evaporation with reason- able economy, has long been recognized. On account of this difficulty the use of very small engines is limited. The boiler in such engines must have such large proportions relatively to the engine that it ceases to be an economical apparatus. The object of these experiments was to reduce the heating- surface, and at the same time make it more efficient by a forced and continuous circulation of the water in the boiler, through the means of a circulating pump. Various combina- tions and modes of circulation were tried, with results which appear conclusive. A steam-generator of very small volume and weight, made of coils of gas-pipe, and consequently having a resistance of several thousand pounds per square inch, was made to evaporate quantities of steam per hour which by ordi- nary processes would require a boiler of very much greater volume. The principle of forced circulation has not often been employed for this purpose, but there is reason to believe thai it may become practically useful. 156. Special Conditions affecting Design thus arise in many cases, and may absolutely dictate the form of the boiler chosen and the place and method of its location and setting. Financial considerations often control ; the matter of safety should always be kept in view, and may often be the deciding element in the problem. Peculiarities of location may, and often do, determine the size and form of the boiler to be chosen, and even the character of the feed-water will frequently decide such choice. No design is satisfactory except it meets in the most satisfactory manner piacticable every element going to make up the whole problem, and is at the same time suitable for the location, the specific work to be done, and properly meets the pecuniary interests of those concerned, as well as gives the safest and most efficient arrangement possible under the circumstances. 157. The Chimney Draught, and the size, height, and general construction of chimney and flues, are among the first 318 THE STEAM-BOILER. of the details to be settled when preparing to design a steam- boiler. The chimney draught is the first condition to be studied, since upon it primarily depends the power and performance of the boiler. The intensity of the draught in a well-propor- tioned chimney will vary nearly as the square root of its height. The quantity of fuel burned on the unit-area of grate is thus determined, assuming the chimney section properly propor- tioned to the work. The sectional area of the chimney-flue should be carefully proportioned to the maximum weight of fuel to be burned in the unit of time. Chimneys are required to carry off obnoxious gases, and to produce a draught. Each pound of coal burned commonly yields from 1 5 to 50 pounds of gas, the volume of which varies directly as the absolute temperature. The weight of gas carried off by a chimney in a given time depends upon size of chimney, velocity of flow, and density of gas. But as the density decreases directly as the absolute temperature, while the velocity increases, with a given height, nearly as the square root of the temperature, there is a tem- perature at which the weight thus delivered is a maximum, perhaps at twice the absolute temperature, or 550 above, the surrounding air. At 550 the quantity is only four per cent greater than at 300 above the ordinary temperature. Height and area are practically the only elements necessary to consider in an ordinary chimney. The intensity of draught is independent of size, and varies directly with the product of the height into the difference of temperature. The intensity of draught needed varies with the kind of fuel and the rate of combustion desired, being least for wood and other free-burning fuels, and greatest for the finer coals and " slack" or "brees," the latter requiring a chimney one hundred and fifty to two hundred feet high, and a difference of pressure measured by an inch or more of water. The volume and weight of gas discharged from any furnace may be calculated as if it were of the density of air at the same temperature, the volume being \2\ cubic feet per pound, nearly, THE DESIGN OF THE STEAM-BOILER. 319 at o F., or three fourths of a kilogram to the cubic metre. Adopting British measures, if Fbe the volume per pound at T, absolute, Fahrenheit degrees, VV^\ (i) and we obtain, allowing, respectively, 12, 18, or 24 pounds to be equal to 150, 225, and 300 cubic feet, the following volumes of gases as originally calculated by Rankine: VOLUMES OF GAS PER POUND OF FUEL IN CUBIC FEET. (RANKINE.) 7* AIR-SUPPLY IN POUNDS PER POUND OF FUEL. JL 12 18 24 4640 1551 3275 1136 1704 2500 906 1359 1812 1832 6 97 1046 1395 1472 588 882 1176 1112 479 718 957 752 369 553 738 572 314 471 628 392; 259 389 519 212 205 307 409 104 172 258 344 68 161 241 322 32 150 225 300 If w denotes the weight of fuel burned in a given furnace per second ; V Q , the volume at 32 of the air supplied per pound of fuel ; T lt the absolute temperature of the gas discharged by the chimney ; A, the sectional area of the chimney; then the velocity of the current in the chimney in feet per second is u = AT. ' (2) and the density of that current, in pounds to the cubic foot, is very nearly as in (3). 32O THE STEAM-BOILER. Since one cubic foot of air at the temperature T weighs about 0.0807 pound, and the weight, on the assumption of uniform mean density of air and gases, is, at 7" , 0.0807 F +i> and its mean density is =^(0.0807 + -^). ....... (3) Multiplying D by the height of chimney, //, the weight of the column per unit section of its area, or, as here taken, in pounds on the square foot, becomes r; .... (4) or, expressed in inches of water, / = O.IQ/ = 0.19/^(0.0807 + -^r). . . . (5) The loss of head, as found by Peclet,* may be expressed by the equation in which / is the total length of flue from grate to chimney- top, m its hydraulic mean depth, or area divided by perimeter, and v the velocity of flow in feet per second. When this head, h r , is given we obtain 2gh' 0.012! ' J 3 H ~ * Traite de la Chaleur, vol. i. THE DESIGN OF THE STEAM-BOILER. 321 and the weight of gas discharged must be (8) 7, being the temperature of flue. The head, h, producing flow is obviously the difference be- tween the weight of chimney gases and that of the column of air of equal height outside ; or, if 7 a is the temperature of the latter, *'-'' - 8 7 - "'"^-iV.fe) 0.0807 + -^ #=//-=- (0.96^- - i) (10) *i The velocity of flow is measured by a Vh, a being a con- stant to be found by experiment, or by .96-p- - i), ..... (ii) varying as the quantity \ (0.96 J -- i j; while the density varies as I -r- 7 1 ,, and the weight flowing per second varies as the product of velocity and density, or as -^|/ (0.96 7", T 9 ). * i This becomes a maximum, 7", varying, as first indicated by Peclet,* when du T 2 T. T X = 7T, == and * Peclet, vol. i. p. 166. 21 322 THE s TEA M- BOILER. or, as Rankine states it,* T l -f- T 2 = ff, nearly ; and the most effective draught, but not the most economical, is obtained when the absolute temperature of the* flue-gases is 2.08 times that of the atmosphere, or about 550 Fahr. (288 Cent.), pro- vided the conditions of grate-resistance are as here assumed. For maximum efficiency of apparatus and economy of fuel the temperature must be made as low as possible. In constructing grates for boilers the air-spaces should be made as narrow as is practicable, the bituminous coals requir- ing more air-space than anthracite. A half-inch is usually con- sidered a minimum and three fourths a maximum. The area of grate should be somewhat more for wood than for coal, the same power being demanded. 158. The Size and Design of the Chimney, its height and area of flue, are modified somewhat by its form and pro- portions, and by the character of its interior surfaces. The greater the friction-head the less its effectiveness. A chimney of circular section and with a straight uniform flue is better than with any other section or wi,th less direct flue. The flue- area is either uniform or tapering toward the top, in which latter case the area for calculations is measured at the top. Mr. Kent assumes that the friction may be taken as equivalent to a reduction of section of two inches all around, and a square flue section as equivalent to a circular one of diameter equal to its side.f He thus obtains the following : Assuming a commer- cial horse-power to demand the consumption of 5 pounds of coal per hour, we have the following formulae : HP (i) increases. See On Chimney Draught, by. the Author, Trans. Am. Soc. M. E., 18.90- \ Trans. Am. Soc. M. E., 1884. THE DESIGN OF THE STEAM-BOILER. 323 in which HP= horse-power; H = height of chimney in feet; E = effective area, and A = actual area in square feet ; 5 = side of square chimney, and d=dia. of round chimney in inches. The following table* is calculated by means of these formulae : SIZES OF CHIMNEYS AND HORSE-POWER OF BOILERS. c Ji HEIGHT OF CHIMNEYS, AND COMMERCIAL HORSE-POWER. Side of square inches. ill S sr Is! < ( !S joojogmojog joomgu ^m^ I8S?8R8!?S," 1 R i/ >s R HHHISI vg ; : . : ; : : ::|fs n ^o; w ^vo 1 & - ^ 3 t> t - jj t^. tCoo'oo'oo'oo' o o CT< $ ''.','.'. .' .' M A M M ^ if -*f TJ- \r t in 10 lOVO VO VO vo vO I s * lx - CM M * mvo r-.oo m rfttl ii-i^H 1^ I 3 L^, HI, S3 ?* 3S 9 O -fl-00 MOO MOO M OO MOO MOO M f^ CM l-N CM ? 10 .f ro SCS^^^'S : : : : : mmMm ^^*^^^^^^^<^^>oocooooooo j. I ! I ; I ',&&&&<*><*>*** m m m mvo'vcfvo'vo'" I t^. 1 t^. g 00 lA tn'vO M fx CM 00 MOO -tf" M MVO 00^ -__ MVO CO "_ oooo'oo'~oooocf M ' '. ' vo* c^oo TJ-'O m 2" C rooo -* o c? CO ^ if pi O oo t>. m M M ooo XJOOOO OOOOO O & . -^-QvO MOmCMOO -^-0 t^ M vo coo m 2" f^ ^- o" CM m t-- o M moo VO r f M o m vo oo - HI OO ^- O t^ M OVO CM ^vo o CM -^- t^ o CM m s mr^-mm? ~. '6 co^ q m in invo'"vo"'vo' vo' r^ ^ ooo t^oo'oo o * m ooo vo * M xTocT o o o o o' o o" o M t^ o -^-oo CM vo o ^- tx M m VO <* - OvO M H 00 VO M O 00 m MVO o M * t^ O CM moo i- MVO 08 i^ M ^t-oo CM vo o ** $ p? moo" 2 M X z VO 1 CM O MMCO inMOOO in t^ o M moo TVC o M M M CM P) M' MMCOM-*Tt-Tj- * ^ m m\o vo vo vo t^ o tx t^OO 00 OOOOOOO O 9 85 oo cr H < SaB|S| ? |'&'|g ?r. moo 1-1 ^- r^ o CM >s ao 2 1 1 &;?: i^ol^ M m mvo vo vo vo ^ t> oo 00 oo 3OOOOOOOO" O *8 mo? 33 ?s 8 CM moo ^- P. o CM moo H" ' S.IIHS'SS ? VC oo ? H i H ^ ? co 00 00 OOOOOOOwM O w moo M w moo -^ MVO OPJ ^-t^O MMOO H< ^}-t^OCM vom- rxoo oOO^WfO^-^- N looo M ^ r-* o rovo O r^vo c> w moo mvo r^ r^oo O O w OD VO 0- '? s ::::::<:: oooooo ^ '. ' . a ^ & MvO If^clc? S^ISS SS^I^I MvS-Spr^a?! : < m o M CM m t^ < Q C N m t-^ o M moo O M vo OCMVO OCM inOCM inoo CM m H t- N, ^ I ; jj moo M eg m M^ mvo rovO 8^ c- ><- n rv -*oo moo CM vo o> MVO o -*oo O co ^O ^O a %& M tx ? ? m -i- ?? CM w ooo t-^ m * CM H ooo t^ OmOvO C100 ^J-QvO M f^M t^ M moo CM m o MVO O M t^. CM TJ-VO oo H m m t^ o * * CO PI vS M t-* S^C? CM % - ; * :::::::". O^O THE DESIGN OF THE STEAM-BOILER. 327 c c c c a c G ~j *j c c G CJ o ,j; c $* d : cc .S.S.S.S .s.s a .s ^'.s .s .s ^^^5^: t- r^ O-M w oo ^l^S M*2 Ji? 00 -- G c c c c O O< ro PO O f)vO ro PI ( M *o rod ro co i pr> OVO o ei oo ^ M ^M -^ s.s .s.s .s .s dd dd d.s aR:4f s s N ^C^C C G C_C _G _C C C ^C^C MOM OOO in M 10 vt 1 ^ O ""> ti c ' -d c ' .S.S .5 5 .S.S .S .S d.S .S.S 00 VO O> -^ 6 PI "1 00 w 10 N IOOO \OioromO-iriM f. ro * - ui"o^"c> d O "> ro MQP OOO INNOO^M PI \O CO * * *j' C -J OM O>> ' .s.s. s.s' .s s .s .s ^^sf^T^"" ^ ^ 0^00 * d je to se wall proje ng t pro all rst pn\ ll .............. cond projection w fi of om Mill s -! i-^^^: ^ jija 5^ = ^= Sj= eJS-c ^ atUubiootjCg-uPrt b y Jf 5 tt ^ 'I -Ji,S 'S '- 15 'w S 15 5 S '5 ' '5 15 '5 2 S I HE HE H SHS HE H se 328 THE STEAM-BOILER. assumed, as in common practice, that the plain cylindrical boiler on an average will, when supplying a good engine with detachable valve-gear, require about 4.7 square feet of heating- surface for actual indicated horse-power, and the tubular boiler 1 1. 8, the two boilers giving 2.1 and 3 horse-power, respectively, per square foot of grate. The following figure is the graphic representation of the 160 150 DIAGRAM FOR HEIGHT OF CHIMNEYS, * S GRATE SURFACE CONNECTED, IN 3O FT. 8 o 8 o 8 ! o i o CO i c* 8 8 W HORSE POWER : - GRATE SURFACE X 3 H. P. FIG. 73. DIMENSIONS OF CHIMNEY. law of variation of height with power required or size of boiler, as algebraically given in the formula above. In building the chimney for ordinary use in connection THE DESIGN OF THE STEAM-BOILER. 329 with steam-boilers, the fire-brick lining needed, at its lower part, when receiving gases from metallurgical or mill furnaces, is not required. The centre-line is fixed on the ground and preserved vertical, while under construction, by the use of a " plumb-line," preferably of fine brass wire, with a very heavy 41 bob" steadied by immersion in a pail of water, molasses, or other liquid. The shell should rarely be less in thickness than the length of a brick at the top, and the lining not less than one half that thickness ; this thickness, outside, should be increased by the width of a brick at every interval of 50 or 60 feet 'from the top, the lining being kept approximately, as near as may be, at one half the thickness of the main wall. The great chimney at St. Rollox. Glasgow, of the height of 45 5i f ee t, has the following dimensions : Division of the Chimney. Height above Ground. Outer Diameter in Feet. Thickness of the Wall in Feet and Inches. ( 435i 13* ) v - \ 35<>i I6f 1 2 IV. \ I I 6 210* 24 j III. I 10* 1 "4* 30* ) I II. r 2 3 I 544 35 i i L ' 7* \ 40 j The foundation of this chimney has a depth of 20 feet and a diameter of 50 feet. It has stood safely and has worked sat- isfactorily for near'y a half-century, and may be looked upon as a good example of successful construction. 159. Forms and Proportions of Furnace and Grate are settled upon so soon as the character of the fuel and the proportions of chimney are fixed. The rate of combustion is fixed, as a maximum, as already seen, by the height of chimney ; minimum rates are anything 330 THE STEAM-BOILER. less, and the customary rates may be taken as not far from the following. The rate of combustion of coal in a furnace is usually stated in pounds per hour, burned on each square foot of grate. Pounds per WITH CHIMNEY-DRAUGHT. square foot per hour. 1. The slowest rate of combustion in Cornish boilers. ... 4 to 6 2. Ordinary rate in these boilers 10 to 15 3. Ordinary rates in factory boilers 12 to 18 4. Ordinary rates in marine boilers 15 to 25 5. Quickest rates of complete combustion of anthracite coal, the supply of air coming through the grate only 15 to 20 6. Quickest rates of complete combustion of bituminous coal, with air-holes above the fuel -fa the area of grate... 20 to 25 FORCED DRAUGHT. 7. Locomotives 4010100 8. Torpedo-boats , 60 to 125 Fuels of the several classes should evaporate, respectively,. from feed-water at the boiling-point and at atmospheric pres- sure, under the most favorable possible conditions, about as fol- lows : Weight water Relatively. per unit weight of fuel. Best anthracite 100 13.5 Best semi-anthracite and bituminous. . no 15 Ordinary coals, soft 80 n Ordinary coals, anthracite 75 10 Examples of these several classes are seen in the best Pennsylvania anthracites, the Welsh and Maryland semi-anthra- cites, or semi-bituminous coals, the ordinary good bituminous fuels of Nova Scotia and of Western Pennsylvania, and the earthy coals of the West. The quantity of steam actually made will depend upon the temperature of the feed-water, and will be less as the water is colder. It is customary, as elsewhere stated, to reduce the results of experiments determining efficiency of boilers to- THE DESIGN OF THE STEAM-BOILER. 33! " equivalent evaporation from and at the boiling-point," under atmospheric pressure. When the maximum possible evaporation is given for feed at 212 F. (100 C), and at atmospheric pressure, i.e., under the standard conditions, multiplying that figure by" the reciprocal of the factor of evaporation for the proposed temperatures of feed and of steam will give the maximum possible evaporation under the latter conditions. Thus we get the following : RELATIVE EVAPORATION AT VARYING TEMPERATURES OF FEED. Temperature of) 212 F. 200 180 160 140 120 100 80 60 40 feed-water ) 100 C. 93.3 82.2 71.1 60.0 48.8 37.7 26.6 15.5 4.4. Relative steam) g 6 88 g6 evaporation.... ) The coals in common use in the United States are : The semi-bituminous coals from Maryland. The anthracites from Pennsylvania. The bituminous coals from Pittsburg and Western Pennsylvania. The bituminous coals from Ohio and the West. When burned in ordinary furnaces, these coals will make steam, per pound of coal, in nearly the following proportions, as given by Mr. T. Skeel :* Semi-bituminous no Anthracite 100 Pittsburg 90 Ohio 75 The weights that may be burned on the same grate, with the same chimney, will vary nearly as follows : Anthracite 100 Semi-bituminous 120 Pittsburg 120 Ohio 200 Relative areas of grate-surface that will be necessary to burn coal enough to furnish the same quantity of steam are nearly as follows : *Weisbach, Vol. II. 332 THE STEAM-BOILER. Anthracite 100 Pittsburg 90 Semi-bituminous 75 Ohio 67 This refers to the average coal of each kind in practice. The loss as refuse falling through well-proportioned grate- bars may be taken as 5 to 10 per cent for good bituminous coals, or 10 to 20 per cent for the lower grades, and about the same for anthracites. Wood may be taken by weight as hav- ing one half the value of coal. A cord of best hard wood should equal a ton of good coal. From the results of chemical analyses, the evaporative power of various kinds of fuel, expressed in pounds of water per pound of fuel evaporated from and at 212 F., which we will call E, has average values given by Prof. C. A. Smith* in the following table, which may be found useful, as supplement- ary to the several other sets of data already given in this con- nection. Kinds of Fuel. E Pure carbon completely burned to CO 2 15 Pure carbon incompletely burned to CO 4.5 CO completely burned to CO 2 10.5 Charcoal from wood, dry 14 Charcoal from peat, dry 12 Coke good, dry 14 Coke average, dry : . . 13.2 Coke poor, dry 12.3 Coal, anthracite 15.3 Coal, dry bituminous, best 15.9 Coal, bituminous ... 14 Coal, caking, bituminous, best 16 Coal, Illinois (from four mines near St. Louis) 12 Lignite 12.1 Peat, dry 10 Peat with one fourth water 7.5 Wood, dry 7.25 Wood with one fifth water 5.8 Wood, best dry pitch-pine 10 Mineral oils, about 22.6 * Am. Engineer, 1883. THE DESIGN OF THE STEAM-BOILER. 333 The anthracite coals burn completely with a thin fire and excess of air, but should have a thickness pretty nearly propor- tional to the rate of combustion, a good proportion being about one foot thickness on a rate of combustion of 20 pounds on the square foot of grate per hour (i decimetre per 65 kilogs. on the square metre). The bituminous coals will not burn well ex- cept in a thick bed and at high temperature, and when re- moved from the chilling influence of adjacent cold iron. A hot fire and large space for combustion are here essential. The furnace may therefore be of less capacity with hard than with soft coals ; but a good height over the grate and a large combustion-chamber are very desirable with the latter, and are of advantage in all cases. A fire-brick furnace, or an arch of brick-work over the grate, gives some gain usually. The rate of combustion to be anticipated and the intended efficiency of boiler, and evaporation per unit weight of fuel being ascertained, the area of grate is at once calculable by dividing the total weight of steam to be supplied by the evapo- ration to obtain the weight of fuel to be needed, and then di- viding this total weight per unit of time by the quantity to be burned on a unit area of grate. Thus, 1000 horse-power being called for, at 30 pounds (13.6 kilogs.) per H. P. per hour, 30,000 pounds (1361 kilogs.) of steam are demanded per hour. At 10 pounds' evaporation and 10 pounds burned on the square foot (48.8 kilogs. on the square metre) of grate-surface, 300 square feet (27.9 square metres) of grate must be provided, which would usually be divided, for convenience in construc- tion and operation, between several furnaces, as furnaces of greater depth than about 6 feet (1.8 m.) cannot be easily handled, that being about as far as coal can be well thrown ; while a greater width than 3 or 4 feet (0.9 to 1.2 m.) introduces difficulties of construction. The " combustion-chamber," which usually forms a part of a well-designed furnace, may be either simply an enlargement of the height of the furnace itself to obtain the space and time needed by the gas-currents for complete intermixture and thor- ough combustion, or it may be any separate chamber beyond the grate. The latter is often the best method of securing the 334 THE STEAM-BOILER. desired results ; but the more usual plan is that of giving con- siderable height of furnace-crown. Grate-bars are spaced differently for different kinds of fuel. Thus, for fine " pea" anthracite coal, the spaces between the bars are usually made about a quarter of an inch (0.6 cm.) ; for "chestnut," f inch (0.9 cm.); for " stove" coal, inch (1.27 cm.) ; and for large anthracite and for bituminous coals, f to f inch (0.95 to i. 9 cm.); while wood-burning calls for an inch (2.56 cm.). 160. The Relative Areas of Chimney, Flues, and Grate are seen to be variable with the circumstances under which the boiler is to be operated, but with natural draught and usual working conditions certain proportions have become almost universally accepted as standard in common practice. Thus it may be taken as well settled by experience, that in chimneys of circular section, smooth internal surfaces, and in the open, where draught is unobstructed by air-currents produced by surrounding objects, as, for example, with marine steam-boilers, the minimum ratio of chimney-flue section, section through the tuSes and that over the bridge-wall to grate-surface should be, at least, respectively, \, -J, ^-, while a maximum to be adopted with forced draught is not far from \, ^, and -J-, for anthracite coal. The latter ratios will also work well for bituminous, free- burning, coals and natural draught ; and the sections may often Be made still greater, with advantage when a blast is also used with such fuel. With restricted draught-area the amount of fuel that may be burned becomes reduced ; thus, assuming a chimney 50 feet (16 m.) high : Area of least flue-section (grate = i) ... 0.14 o.io 0.07 0.05 0.04 Relative coal burned I. 0.8 0.7 0.6 0.4 Average fuel, Ibs. per sq. ft. grate 15 12 10 9 6 " kilogs. per sq. m 7.5 6 5 4-53 For square sections of chimney-flue and with rough interior surfaces the size of chimney is increased both in weight and area of section. As a general rule, the height of factory chim- neys is increased with the size and number of boilers, irrespec- THE DESIGN OF THE STEAM-BOILER. 335 tive of the above-stated ratios, and a not uncommon proportion of " stack" is that which makes the height about twenty times the diameter of the flue. Ordinary mill-chimneys, for moder- ate powers, range between 50 and 75 feet (16 and 23 m.) in height. 161. Common Proportions of Boiler are found in ordinary practice to be not far from those given below. The interior space of the boiler is commonly divided into about two thirds or three fourths water-space, the remainder being steam-room. In marine boilers more steam-space should be given. RATIO OF HEATING TO GRATE SURFACE. Plain cylinder boilers 12 to 15 Cornish ... 15 to 30 Cylindrical flue 20 to 25 tubular 25 to 35 Marine tubular (fire) 30 to 35 (water) 3510 40 Locomotive tubular 50 to 100 The ratio of heating to grate surfaces should, where possible, be always carefully determined with reference to maximum com- mercial efficiency in the manner described in a later chapter. The above proportions produce ratios of weights of fuel burned per unit area of heating-surface, in general practice, about as follows : RATIO OF FUEL BURNED TO HEATING-SURFACE. Pounds per sq. ft. H. S. Kilogs. per sq. in. o 5 to i.o O.I to O.2 Marine (natural draught) ... o 5 'to o 6 o I to o 3 o 8 to i.o 04 to o. 5 Similarly, the power of such boilers may be reckoned roughly as below, and their relative standing in efficiency and capacity taken as follows : 336 THE STEAM-BOILER. HORSE-POWER AND ECONOMY. PER H. P. RELATIVE STANDING. Sq. ft. Sq. m. Capacity. Economy. 10 to 12 14 to 18 8 to 12 6 to 10 I tO 2 i.o to i.i 1.3 to 1.6 0.7 to i.i 05 to 0.9 O.I to 0.2 I. 0-75 0.50 O.2O 0.6 I. 0. 9 0.8 0.7 0.8 Flue Plain cylindrical . . The above, as with every proportion and detail of the steam- boiler, should always be made the subject of careful calculation whenever the case is in the least degree peculiar. The following are proportions frequently accepted by the trade in one of the most common varieties of stationary boiler sold in the market: PROPORTIONS OF CYLINDRICAL TUBULAR BOILERS. FLUES. DOME. STACK. o 1 1 i rt-u . 1 o V .c o 1 u 1 I 1 3 . 1 is ff-l c/5 B S v. .~ QJ .n o *X cw fc^ .5 W -^ m i- *** E 1 $ \ V i c V5 _C U 1 V **^ i *H o3 E Number c Horse-po Diameter y be i Number. Diameter Length Diameter I SB II 15 h II s IE H O **"* ^ ! Diameter tL 'S X i-9 B qj C C g >< f i I5 36 8' ii 3 3 8 20 20 H % 3 18 26 2,950 5,35 2 20 36 10' ii 30 3 10 20 20 /4 % 3/^5 r: 3 5.900 3 25 42 ii' 38 3 10 24 24 3% % 3/^ 20 30 4.400 7.100 4 30 42 13' 3 12 24 24 3* % 4 20 5.000 7,800 5 35 44 13' 4 6 3 12 24 26 A % 4 22 36 5,5OO 8,700 6 40 48 13 2 52 3 12 24 28 T5 % 4 24 36 6.400 9,900 7 45 5 14' 2 52 3 13 30 30 T^B % 4 24 36 6,800 10,400 8 50 54 13' 2 58 3 12 30 I 5 (i % 4 26 7,600 11.500 9 60 54 16' 2 3 15 30 30 & % 4^2 26 45 8.550 12.750 10 ii /o 75 60 60 'I/ 4 16' 4 76 76 3 3 15 30 30 30 II $8 4/S 28 28 45 5 IO.OOO 10.500 14,500 12 80 60 i?' 4 76 3 16 30 36 ]?* /B 5 28 55 II,2OO i6,joo 13 90 66 16' 5 100 15 36 36 7^ I 7 B 5 32 55 J3.50C 19.100 JOO 66 17' 5 100 3 16 36 36 % /B 5 32 55 14,200 19,800 15 125 72 17' 6 132 3 16 36 * 5 36 60 17,200 24,000 The upright tubular boiler is given less heating-surface than the above, is much lighter, and is less economical. The locomotive type of stationary boiler has about the same weight as the above, but rather less heating-surface. THE DESIGN OF THE STEAM-BOILER. 337 According to Professor Rankine,* a very useful mode of com- paring the capacities of different boilers is to divide the boiler- space by the area of heating-surface, and thus is obtained a mean depth. Of the following examples, the first three are given on the authority of Mr. Fairbairn's " Useful Information for Engineers:" 11 Mean depth." Feet. Plain cylindrical egg-ended boiler, with external flues below and at each side, but no internal flues 3. 50 Cylindrical boiler with external flues, and one cylindrical internal flue 1.65 Cylindrical boiler with external flues, and two cylindrical internal flue i.oo Stationary boilers according to Mr. Robert Armstrong's rules . . 3.00 Multitubular marine boilers, about 0.50 Locomotive boilers, and boilers composed of water-tubes, aver- age about o. 10 Boilers of large size and capacity exhibit steadiness in the pressure of the steam, ready deposition of impurities, space for the collection of sediment, and freedom from priming. Those of small capacity excel in rapid raising of the steam to any required pressure, small surface for waste of heat, economy of space and weight, of special importance on board ship, greater strength with a given quantity of material, and smaller damage in the event of an explosion. Mr. D. K. Clark considers that we may, in ordinary loco- motive practice, take the economical consumption of fuel as proportional to the square of the area of heating-surface, and make the grate-area vary in the same proportion. He adopts nine to one as the standard and desirable evaporation of water as compared with weight of fuel, makes the maximum and minimum allowable rates of combustion 150 and 14 pounds per square foot of grate, and the maximum evaporation in loco- motive boilers about 22 cubic feet per hour.f A rate of com- bustion of 112 pounds is considered a practical maximum, the ratio of heating to grate surface being 85 to I. * Steam-engine and Prime Movers, f Railway Machinery, p. 165. 338 THE STEAM-BOILER. 162. The Usual Rates of Evaporation and the effect of varying the proportions of tubes has been well determined by the experiments of Isherwood and others. The proportions of flues and tubes vary somewhat in prac- tice ; but it will be found seldom advisable to make tubes more than 50 or 60 diameters in length. Where the heating-surface consists principally of tubes, the efficiency will be found to vary with their length nearly as follows : Length of tube (diameters) 60 50 40 30 20 Water per unit weight of fuel 12 n 10 9 8 When the ratio of heating to grate area was 25 to i, Isher- wood found the evaporation to vary thus : Fuel per hour 8 10 12 16 20 24 Evaporation 105 10.1 9.5 8.2 7.3 6.8 which series is represented by W= r -= t nearly. \ r Clark obtained with locomotives an equal evaporation with Fuel (coke) 15 25 38 56 76 98 125 153 Ratio of H. S. to G. S.... .. 30 40 50 60 70 80 90 100 the evaporation being constant at 9 of water to I of fuel, which may be expressed by S = SV^, nearly, 5 being the ratio of the two areas and F the weight of coke burned on the unit of area of grate. In estimating area of heating-surfaces the whole surface exposed to the hot-furnace gases is reckoned. The formula for efficiency already given illustrates the progressive variation of the evaporative power with change of proportions of boiler. 163. The Relation of Size of Boiler to Quality of Steam demanded is one that occasionally becomes worthy of consideration. Where the steam is required for driving steam- engines it is very important that it should be thoroughly dry, arid it is an advantage to moderately superheat it. Maximum economy cannot be attained where wet steam is used. A boiler THE DESIGN OF THE STEAM-BOILER. 339 attached to a steam-engine, and especially where fuel is costly and efficiency important, should have ample heating-surface, some superheating-surface if practicable, ample extent of water- surface area to permit free separation of steam and water, and large steam-space. Steam employed for heating purposes is not necessarily dry ; it may carry a large amount of water with it into the system of heating-coils or radiators, and yet give good results, if the latter are of large section. Where the pipes are of restricted area of section, however, wet steam flowing less freely than when dry or superheated, there may result such a retarda- tion of flow and of circulation as may cause considerable increase of cost. This has been found sufficiently great, in some, cases, to justify drying, and perhaps superheating, the exhaust-steam from engines where used for heating purposes. As a general rule, the boiler must be made a trifle larger to supply perfectly dry steam and do good work. In the use of steam for heating purposes, one square foot of boiler-surface will supply from 7 to 10 square feet of radiating surface. Small boilers should be larger proportionately than large boilers. Each horse-power of boiler will supply from 250 to 350 feet of i-in. steam-pipe, or 80 to 120 square feet of radiat- ing surface. Under ordinary conditions one horse-power will heat about Brick dwellings, in blocks, as in cities 15,000 to 20,000 cub. ft. " stores " " 10,000 " 15,000 " dwellings, exposed all around 10,000 " 15,000 " mills, shops, factories, etc 7,000 " 10,000 Wooden dwellings, exposed 7,000 " 10,000 Foundries and wooden shops 6,000 " 10,000 Exhibition buildings, largely glass, etc 4,000 " 10,000 The system of heating mills and manufactories by means of pipes placed overhead is recommended. The air required for ventilation is usually warmed by the " indirect" system of radiation, the current passing through boxes or chambers in which a sufficient amount of pipe is coiled to heat it well. From 5 to 15 cubic feet per individual per 34 THE STEAM-BOILER. minute are allowed, the former in crowded halls, the latter in- dwellings, and about one tenth as much for each gas-burner or lamp. 164. The Number and Size of Boilers to be used in any case in which considerable power is demanded is determined mainly by practical considerations related to their construction. As a rule, the larger boiler is more economical in first cost and in operation, within certain limits, than several smaller boilers of equal aggregate power. But passing a limit which cannot be usually very exactly defined, expense is increased, trans- portation becomes difficult, location and setting involve prob- lems difficult of solution, and management becomes less easy. Mr. Leavitt has, however, constructed stationary boilers, of a peculiar modification of the locomotive type, of as high as one thousand horse-power ; and marine boilers of equal or greater power have been built not infrequently for steamers plying on the larger rivers of the United States. Stationary boilers of 100 horse-power and marine boilers of 500 are more usual and more commonly suitable sizes. Locomotive boilers are neces- sarily always sufficiently large to supply all the power de- manded of the engine. The type of boiler has much influence on the limit of size. Plain " cylinder boilers" are rarely made.more than from 3 to 4 feet (0.9 to 1.2 m.) in diameter, and this restricts the grate- area so that the power derivable from a single such boiler is seldom more than 15 or 20 horse-power, and is usually much less. The more complex structures often include several fur- naces, and yield from 100 to 200 horse-power each on land, and more at sea. Makers in the United States usually allow 1 5 square feet of heating-surface and one of grate to the horse-power, in plain cylindrical boilers, and the same area of heating-surface, but a fourth and a half less grate-area, respectively, with flue-boilers and tubular boilers, where estimating for the market. M. de Pambour found the priming of French locomotive boilers in 1834 to amount to about 30 per cent ; M. de Chatel- lier, in 1843-4, found it to be 30 to 50 per cent; but a large proportion of the moisture measured was undoubtedly the THE DESIGN OF THE STEAM-BOILER. 341 product of cylinder condensation, for which loss Clarke al- lowed as follows : * CONDENSATION. RATIO OF EXPANSION. Per cent, of Steam Per cent, of indicated. Total Steam. 1.25 12 II 1.6 7 12 II 2 OO 12 II 2.50 21 17 3.67 32 24 5.00 46 32 8-33 73 42 which figures indicate the proportion of steam by weight to be added to that calculated for the ideal engine, to obtain the probable requirement of the real engine. Builders of the more economical classes of engines supply them with .boilers often of less size than the accepted standard rating would dictate, as they demand less steam per horse- power than the average engine. A good engine of moderate size, with an automatically governing and adjusting valve-gear, if condensing, should give good results on as low as seven or eight square feet of heating-surface per actual horse-power, and if non-condensing, with ten or twelve square feet. Large en- gines are given a smaller allowance of heating-surface, propor- tionally, than are small engines. 165. The Standard Sizes of Tubes have become well set- tled by custom. So large an element of boiler-construction necessarily assumes, with time, a somewhat rigid set of propor- tions. The sizes employed range from I or ij inch (25.4 to 3 1 mm.) diameter in the smallest boilers, to 2 or 2 \ inches (5 1 to 63.5 mm.) in the locomotive and other boilers of moderate size; and to 3 or 4 inches (76 or 102 mm.), or even 5 or 6 inches (1.27 or 1.52 mm.), in large boilers, or where a very free draught or greater convenience of access are required. Water-tube boilers * Railway Machinery, p. 144. 342 THE STEAM-BOILER. are commonly given tubes 4 or 5 inches (102 or 127 m.) in diameter. The length of the tube is customarily not above 50 or 60 diameters in stationary boilers, and two thirds this length in marine work. The spaces between the tubes should be about one half their diameter ; they are, however, usually placed much closer. All tubes in our market are gauged to British measures, as below : When the dimensions of a tubular boiler are given, the out- side diameter of the tubes is usually stated, so that twice the thickness must be subtracted to obtain the diameter to be used in the calculation of heating-surface. The thickness of tubes by different makers varies somewhat, but those given below are average values, and can be used without serious error. The table gives dimensions of standard sizes of tubes. STANDARD TUBES. Outside diameter in inches. Thickness in inches. Internal diameter in inches. Internal diameter in feet. Heating-surface in square feet, per foot of length. 1.25 0.072 1. 106 0.0922 0.3273 1-5 0.083 1-334 O.III2 0.3926 1-75 0.095 1.560 J O.I3OO 0.4589 2. 0.095 I.8IO 0.1508 0.5236 2.25 0.095 2.060 0.1717 0.5890 2-5 0.109 2.282 o. 1902 0-6545 2-75 O.IOg 2.532 O.2IIO 0.7200 3- 0.109 2.782 0.2318 0.7853 3-25 0. 1 2O 3.010 0.2508 0.8508 3-5 0.120 3.260 0.2717 0.9163 3-75 O.T20 3-510 0.2925 0.9817 4- 0.134 3-732 O.3IIO 1.0472 4-5 0.134 4.232 0.3527 1.1790 5- 0.148 4.704 0.3920 1.3680 . 6. 0.165 5-770 0.4808 1.5708 7- 0.165 6.770 0.5642 1.8326 8. 0.165 7.770 0-6475 2.0944 9- 0.180 8.640 0.7200 2.3562 10. O.2O3 9-594 0.7995 2-5347 The following are the dimensions of standard tubes as made by some of the best makers in the United States: THE DESIGN OF THE STEAM-BOILER. 343 LAP-WELDED CHARCOAL-IRON BOILER-TUBES. Standard Dimensions. U 4 I J> 1 | y O * g ii si V 1 be 1 O Ii ll rt "rt _ cs a E &3 *i 11 1 II x Q u II 5 u H s$ 11 1- i- II II U U i M fS Ii 1 U u u H H H J OT 4; 3 In. In. In. No. In. In. Sq. in. Sq. in. Sq. in. Feet. Feet. Lbs. .86 .072 J5 3.14 2.69 .78 57 .21 3.82 4.46 7 1 125 .98 .072 15 3-53 3-08 99 76 24 3-39 3-89 .8 25 .11 .072 15 3-93 3-47 1-23 .96 27 3-o6 3-45 .89 32 15 083 14 4.12 3-6 1.03 32 .91 3-33 i. 08 375 .21 -083 14 3-8 5:Ji 34 .78 3.16 1-13 -5 33 .083 14 4-71 4-19 i-77 1-4 37 55 .86 1.24 .625 43 095 13 4-5 1 2.07 1.62 .46 35 .66 1 53 75 56 95 13 5-5 4.9 2-4 1.91 49 .18 45 1.66 875 .68 095 13 5-89 5-29 2.76 2.23 53 .04 27 1.78 .81 095 13 6.28 5.69 3-M 2 57 57 .11 1.91 125 25 93 .06 95 095 13 13 6.68 7.07 6.08 6.47 3-55 3-98 2-94 3-33 .61 .64 -7 97 -85 2.04 2.16 375 .16 .109 12 7.46 6.78 4-43 3-65 .78 .61 77 2.61 5 .28 .109 12 7-85 7-i7 4.91 4.09 .82 53 .67 2 75 :fe :I1 .109 .109 12 12 8.64 9-03 7-95 8-35 5-94 6-49 5-03 5-54 9 95 39- 33 44 3-04 3-i8 3- 78 .109 12 9.42 8.74 7 07 6.08 99 2 7 37 3-25 .01 .12 II IO.2I 9.46 7.12 1.18 .26 3 96 3-5 .26 .12 II II. 10.24 9.62 8-35 1.27 .09 17 4.28 3-75 : -51 . 12 II 11.78 11-03 11.04 9.68 '37 .02 .09 4.6 4 73 134 10 12.57 ii 72 12-57 10.94 1.63 95 .02 5-47 4-25 .98 134 IO 13-35 12.51 14.19 12-45 i-73 9 .96 5-82 4-5 .28 134 10 M-H 13.20 15-9 14.07 1-84 85 9 6.17 4-75 .48 134 IO I 4 .92 14.08 17.72 15-78 .8 85 5- 5-25 5-5 7 4-95 S- 2 .I 4 8 .I 4 8 .148 9 9 9 16.49 17.28 14.78 15-56 16.35 19.63 21.65 23.76 17-38 19.27 21.27 2-37 2-49 .76 73 7 .81 77 73 7-97 8.36 6. 5-67 -l6 5 8 18.85 17.81 28.27 25-25 3-02 .64 67 10.16 7- 6.67 .165 8 21.99 20.95 38-48 34-94 3-54 55 11.9 8. 9- 7.67 8.64 8 7 25-13 28.27 24.1 27.14 50.27 63.62 46.2 58.63 4.06 4-99 .48 .42 50 44 16.76 10. 9-59 -203 6 31-42 30-14 78.54 72.29 6.25 .38 4 20.99 ii. 12. 10.56 11-54 .22 .229 5 4-5 37-7 33-17 36.26 95-03 113-1 87.58 104.63 7-45 8-47 35 3 2 -36 33 25-03 28.46 13. 12.52 .238 4 40.84 39-34 132-73 123.19 9-54 29 3 32.06 14. .248 3-5 43.98 42.42 153-94 143.22 10 71 27 .28 36. 15. 14^48 2.S9 3 47.12 45-5 176.71 164.72 "99 25 .26 40-3 16. 15-43 .284 2 50.26 48-48 201.06 187.04 14.02 24 25 47.11 17- 16.4 3 I 53-41 5I-52 226.98 211.24 15-74 .22 23 52.89 18. 17-32 34 O 54-41 254-47 235-6i 18.86 .21 .22 63.32 The following table* gives the draught-areas of boiler-tubes and flues, which have been computed on the basis of the thick- ness of such tubes taken from the price-lists of American manu- facturers : * American Engineer, 1885. 344 THE STEAM-BOILER. DRAUGHT-AREAS OF TUBES AND FLUES. External diam- eter in inches. Draught-areain square inches. Draught-areain square feet. Number of tubes or flues = i square foot ofdraught-area. I 575 .0040 250.0 I .968 .0067 149-3 I* 1.389 .00964 103.7 If 1.911 .0133 75-2 2 2-575 0179 55.9 2i 3-333 .0231 43-3 1 4.083 .0284 35.2 2f 5.027 0349 28.7 3 6.070 .0422 23.7 g 7.116 .0494 2O. 2 3* 8-347 .0580 17.2 3f 9.676 .0672 14.9 4 10.93 0759 13.2 4* 14.05 .0976 IO.2 5 17-35 .1205 8-3 6 25-25 1753 5-7 7 34-94 .2426 4.1 8 46.20 .3208 3.1 9 58.63 .4072 2-5 10 72.23 .5016 2.0 In a flue-return tubular boiler the area of flues should be about 20 per cent, and the draught-area of uptake about 25 per cent greater than the draught-area of tubes. Good conditions for combustion and steaming are realized when the grate-sur- face is 8 times and the heating-surface about 200 to 240 times the draught-area of tubes. The location and arrangement of fire-tubes has an impor- tant bearing on the distance by which they may be safely separated. In locomotive boilers, where they only check the rise of currents laden with steam produced by their own action, they may be set closer than in those boilers, as many marine boilers, in which they lie above a crown-sheet from which enormous quantities of steam are liberated, which steam, as well as that made by the tubes themselves, must traverse the intermediate spaces. Where the circulation is forced and rapid the tubes may also be crowded more than where natural and sluggish. In locomotive boilers, the tubes, which are or- dinarily from if to 2 inches in diameter, are set apart from THE DESIGN OF THE STEAM-BOILER. 345 one third to one fifth their diameters ; but the larger space is probably none too great. 166. The Details of the Problem, as coming to the de- signer and the constructor of the steam-boiler, are so largely matters determined by experience, rather than by any scientific system or calculation, that much thought must be given to their consideration from the point of view of the practitioner in engineering and of the artisan engaged in building such structures from the boiler-maker's side rather than from that of the man of science. The selection of the iron or steel for shell, for stays, or of the rivets; the choice of style of riveting; the determination of the character of seam and lap ; the decision of the question whether the use of reinforced seams or of heavier plates is likely to prove best in the end ; the choice of type of boiler even, in view of known peculiarities of location or other conditions : these must all be settled in conference with the boiler-maker, even if not directed absolutely by him. It sel- dom happens that the engineer making the designs feels com- petent to act throughout without consultation with his lieu- tenants in the workshop. The method of designing in its details, as practised in the case of familiar forms of boiler, will be given in the next chapter. CHAPTER VIII. DESIGNING STEAM-BOILERS PROBLEMS IN DESIGN. 167. The General Considerations determining the design of a steam-boiler are, mainly, the following : (1) It must supply a defined quantity of steam in a speci- fied unit of time, or it must have a certain power. (2) It must be as absolutely safe as it is practicable to make it. (3) It must have reasonably high efficiency, and must be capable of working at the lowest total expense for fuel, attend- ance, interest on first cost, taxes^ insurance, and all other run- ning expenses, in proportion to work done, that may be attain- able. (4) It must be well suited to the location, and to all the special conditions affecting it when in operation. Marine steam-boilers must, for example, be given the mini- mum practicable weight and volume, since it costs as much to carry a ton of boiler as a ton of cargo, and every cubic foot occupied by boilers, fuel, or machinery displaces a cubic foot of paying load. Naval boilers, also, must usually be kept as low in the ship as possible to reduce risk of injury by shot. So important are these elements in naval construction, that the practical limits of space and power on shipboard are com- monly fixed by the space occupied by boilers ; and the reduc- tion of grate-area is the first problem attacked by the naval architect and engineer seeking high speed, whether for yachts, torpedo-boats, or larger craft. 168. The Parts and Details of the steam-boiler may be defined as follows :* * See Rankina, Steam-engine, p. 449. DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 347 The usual arrangements of furnace and boiler may be divided into three principal classes : (I.) In the external furnace, or " outside-fired boiler," the furnace is wholly outside of the boiler ; so that the boiler forms part of the superficies of the furnace; the other sides of the furnace being usually of fire-brick. Examples of this are the wagon boiler, the plain cylindrical boiler without internal flues, and all boilers in which the water and steam are con- tained in tubes surrounded by the flame. (II.) In the internal-furnace or " inside-fired boiler" the fire-chamber is enclosed within the boiler, as in boilers with furnaces contained in horizontal cylindrical internal flues, in most marine boilers, and in all locomotive boilers. (III.) The detached furnace, which is a fire-chamber built of fire-brick, in which the combustion is completed before the gas comes in contact with the boiler. The principal parts and appendages of a furnace are (1) The furnace proper, or firebox, being the chamber in which the solid constituents of the fuel, and the whole or a part of its gaseous constituents, are consumed. (2) The grate, which is composed of alternate bars and spaces, to support the fuel and to admit air. (3) The hearth is a floor of fire-brick, on which, instead of on a grate, the fuel is burned in some furnaces. (4) The dead-plate or dumb-plate, that part of the bottom of the furnace which consists of an iron plate simply. (5) The mouth-piece, through which fuel is introduced, and often some air. The lower side of the mouth-piece is the dead- plate. In many furnaces there is no mouth-piece. (6) The fire-door closes the doorway, and may or may not have openings and valves in it to admit air. Sometimes the duty of a fire-door is performed by a heap of fuel closing up the mouth of the furnace. (7) The furnace-front is above and on either side of the fire-door. (8) The ash-pit is the space into which the ashes fall, and through which, in most cases, the supply of air enters. (9) The ash-pit door is used to regulate the admission of air. 348 THE STEAM-BOILER. (10) The bridge is a low wall at the end of the furnace over which the flame passes to the chimney. This is meant when " the bridge" is spoken of ordinarily; but the word bridge, or bridge-wall, is also applied to any partition having a passage for flame or hot gas over it. Bridges are of fire-brick, or of plate iron and hollow, so as to form part of the water-space of the boiler, and are then called water-bridges. The top of a water-bridge should slope upwards at the ends to allow of the rapid escape of the steam on its internal surface. A water-bridge may project downwards from the boiler above the furnace ; it is then called a hanging bridge. (11) The combustion or flame-chamber is the space behind the bridge in which the combustion of the furnace-gases is completed. It may be lined with brick or tile to prevent ex- tinction of the flame. (12) Bafflers or diffusers are partitions so placed as to pro- mote the circulation of the gas over the heating surface of the boiler or of the currents of water within. Bridges fall under this head. (13) Dampers are valves placed in the chimney, flues, or passages to regulate the draught. The principal parts and appendages of a boiler are : (1) The shell of the boiler. The figures usually employed for the shells of boilers are the cylindrical and the plane, and combinations of those two figures. In locomotive boilers, part of the shell is a rectangular box, containing within it the firebox. The shells of marine boilers are often of irregular shapes, adapted to the space in the ship which they are to occupy, and approximating more or less to rectangular forms. For heavy pressures, however, they are usually cylindrical, with plane ends. (2) The steam-chest, steam-drum, or dome is a part which rises above the rest of the boiler, and provides a space in which the steam may deposit any spray carried by it ; it is usually cylindrical. (3) The furnace or firebox is usually within the boiler, so placed as to be covered with water. In cylindrical boilers it is often in one end of a horizontal cylindrical flue, as in Cornish DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 349 boilers ; in locomotive boilers it is a rectangular box. In marine boilers it is usually rectangular in the older kinds of boiler, and cylindrical in the high-pressure cylindrical tubular boiler. (4) A tube-plate forms part of the shell of the boiler, or one side of an internal firebox, or flue, and is perforated with holes, into which the ends of the tubes are fixed. Each set requires a pair, one for each end of the tubes. (5) The man-hole is an opening in the top or end of the boiler, large enough to admit a man. The bolts holding the man-hole cover must be capable of safely bearing their load. Commonly the cover opens inwards, and is kept closed by the pressure of the steam, and is held by bolts and nuts to a cross- bar outside the man-hole. (6) Hand-holes are openings usually placed at or near the lowest part of a boiler, and large enough to admit the hand, which are opened occasionally for the discharge of sediment. (7) The blow-off apparatus consists of a cock at the bottom of the boiler, which is opened to cleanse the boiler by empty- ing it or to discharge brine, and prevent salt from collecting. The surface blow-cock discharges the scum which collects on the surface of the water. (8) The pressure-gauge shows the pressure within the boiler. (9) The water-gauge shows the level of the water in the boiler. Gauge-cocks are set at different levels: one at the proper water-level, another a few inches above, and a third a few inches below. Opening these the engineer ascertains the level of the water. The glass water-gauge consists of a strong glass tube, communicating with the boiler above and below the water-level. The level of the water is thus rendered visi- ble. Every boiler ought to be provided with both forms of gauge. (10) Clothing and lagging prevent waste of heat. The former is made sometimes of hair felt, the latter covers it with a layer of thin wooden boards. Asbestus, ashes, and other ma- terials are similarly used. Hair-felt has sometimes been found to singularly accelerate internal corrosion. 350 THE STEAM-BOILER. 169. The Design of the Plain Cylindrical Boiler is the simplest problem of its class. This boiler, consisting of only a cylindrical shell and plane or domed heads, is not likely to afford opportunity for the display of either great knowledge in design and construction or of ingenuity in its details. This type is selected when cheap fuel or bad water make it unwise to adopt more economical forms. The shell is usually about twelve diameters in length, but is sometimes made fifteen or even twenty, and double the last fig- ure has been known. In some cases this boiler has been built as a cylindrical ring an annulus of large diameter and of circular section. Common sizes for this class of boiler range from 24 to 36 inches (63 to 91 cm.) diameter of shell, and 24 to 36 feet (7.3 to 1 1 m.) long. As the diameter of the boiler usually fixes the width of grate, and as the length of grate is rarely found to be profitably extended beyond about 6 feet (1.8 m.), the power of the boiler has a very simple relation to its size. The ratio of heating to grate surface is always thus made small, and the boiler is necessarily uneconomical of fuel. This boiler is usually designed with single-riveted seams throughout, although safety and even ultimate economy of cost and operation during its lifetime may be sometimes gained by double-riveting the longitudinal seams ; which would thus be strengthened in the proportion of about 70 to 55 or 60, or not far from 20 per cent, and the whole structure would be made correspondingly safer. The thickness of shell is determined by the pressure of steam to be carried and the factor of safety adopted. Assum- ing the iron to have a tenacity of 50*000 pounds per square inch (3515 kilogs. per sq. cm.), the joints will have, as may be assumed, 0.60 this resisting power, and the boiler-shell is to be calculated with this loss in mind, and will be made as if the sheets had a tenacity of 30,000 pounds per square inch (2109 kgs. per sq. in.), and were of uniform strength through the seams. In illustration, assume it to be demanded that a " 36- inch cylindrical boiler" shall be designed to sustain a pressure of 100 pounds per square inch (7 kilogs. per sq. cm.). The thickness of shell should be. DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 35 1 t fP d ' = 6 X IPO X 3^ g ~ 2kT 2 X 0.55 X 50,000 "" *' when /, d, and T are the pressure and the diameter of the shell and the tenacity of the metal, and k is the" efficiency" of the seam, which we may here assume to have k = 0.55, or 55 per cent of the strength of the solid sheet ; the factor of safety is taken as/ 6. The thickness of shell should be three- eighths of an inch (i cm. nearly). Such thickness is not usual, and a factor of safety of four and a thickness of one quarter of an inch (0.635 cm.) is more common for this case in general practice, and is allowed by the law as may be seen in article 55, to which reference may be made for tabulated legal dimen- sions of this class of boilers. The heads of the cylindrical boiler are sometimes made of cast-iron, the thickness made empirically from i to 2-J inches (3.8 to 6.4 cm.) for diameters of from 24 to 36 inches (63 to 91 cm.) respectively ; they are often of sheet-iron of the same thickness as the shell, and domed to give them resisting power, an excellent construction, especially when pressed into exact shape in the forming die of the hydraulic press. When the heads are plane, they are stayed either by stays running to the sides of the boiler at angles of from 10 to 30, or by triangu- lar "gusset-plates" riveted to the heads and sides. This last construction is subject to the objection that the gusset-plates are necessarily irregularly strained and liable to tear. Stay- rods are of sufficient size to safely carry the whole pressure re- ceived on the heads, and securing both heads, pass from the one to the other, the whole length of the boiler, with adjust- able nuts at each end, outside the head, and inside as well. A dished head is probably the best form to give, whether of boiler, of dome, or of steam and mud drums. As shown by Mr. Robert Briggs,* equal strength with the shell or with a stayed head can be obtained by giving the proper form to the head-sheet without any staying. Thus it is known that the strength of a spherical shell is twice as great as that of the * Journal Franklin Institute, 1878. 352 THE STEAM-BOILER. cylinder of the same diameter, when both shell and cylinder have the same thickness ; or that a spherical shell possesses the same strength as a cylindrical shell of the same thickness, when the radius of the spherical surface is equal to the diame- ter of the cylinder. When the rule stated is applied to the head of the dome or of the boiler, which is formed to a part of a spherical surface whose radius is the diameter of the dome or boiler, the head is " dished " out 0.134 the diameter of the head, in order to give the same strength to resist internal pres- sure, for both head and shell, of the same thickness of iron. A small allowance is needed for the thinning of the sheet-iron, in dishing. This allowance is easily computed thus : The sur- face of the flat circular plaj:e is to that of the dished plate as I to 1.072, and the thickness of the circle, before dishing, should be about 7 per cent (one fourteenth) greater than that of the shell. The flangeing of the head will inevitably upset the flange itself to a thickness much above the original ; and a dished head of ordinary thickness will be much stronger than the shell sheets at the joints, where they are weakened by rivet-holes, even if put together with the double-riveted longitudinal seams. Heads of sheet-iron are usually made ten or, better, twenty per cent heavier than the shell. A man-hole is commonly located in the most accessible end of the boiler, and, often, a hand-hole through which the boiler may be completely drained, and all mud and scale removed. The feed-pipe usually enters through the front head, but some- times at the rear. It should always be at a part readily reached for inspection and repairs. If on the shell, the opening should always be reinforced by a heavy wrought-iron ring and the strength of the boiler thus increased rather than diminished by its introduction. The ring should be riveted inside the open- ing. The steam-pipe is sometimes led directly out of the top of the boiler, but is better placed in connection with a steam- dome or steam-drum, in order to obtain as dry steam as is pos- sible. The safety-valve should here, as in all other cases, be so placed that no accident or carelessness can close its communi- cation with the steam-space ; a stop-valve placed between it DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 353 and the boiler has been known to produce a disastrous explo- sion, when shut by an ignorant or thoughtless attendant. Gauge-cocks should always be attached even if the glass water-gauge is in use. The experienced manager of boilers never feels perfect confidence in any other water-level indicator, however convenient and generally accurate. In setting the gauge-cocks it is usual to allow about one third the volume of the boiler for steam-space. The following table, calculated by Mr. W. F. Worthington, gives the volume of this space in unity of length of the shell, British measures : TABLE FOR CALCULATING THE CAPACITY OF THE STEAM - SPACE IN CYLINDRICAL BOILERS. DlAM 3 o. 32. 34' 36. 38- 40, 42. 48. 54' 60. 66. 72. In. Multipliers (cubic feet). In. i u I 05 05 05 -05 .05 .06 .06 .06 .06 .07 .07 .08 i 2 .14 1 5 '5 .16 .16 .16 17 .'9 .20 .21 .21 2 1 3 25 .26 27 .28 29 30 3 32 34 37 38 39 3 4 39 .40 .42 43 44 45 .46 50 53 55 58 .61 4 5 53 56 57 59 .61 6 3 .64 69 73 78 .82 85 5 6 .70 .72 75 77 .80 .82 83 .91 .96 1.02 1. 08 I. 12 6 7 .87 .90 93 .96 99 .02 05 1.14 i .20 1.27 1.35 1.41 7 S 8 1.05 1.09 I.tf i .20 .24 27 t .37 1-47 1-55 1.63 I.7I 8 u 9 1.24 1.29 1 1 -33 1.38 1.42 47 5 * i .62 i-73 1.85 1-94 2.04 9 := .E 10 1.43 1.49 1 -55 '59 1.65 -7 75 1.89 2.02 2.14 2.26 2. 3 8 "2 ii 1.63 1.69 1.76 1.82 1.89 95 .00 2.18 2-33 2.46 2-59 2-74 ii "^ i 12 1-83 1.91 1.98 2.06 2.13 2.20 2.26 2.46 2.63 2.79 2-95 3.08 2 > 13 2.04 2.13 2.21 2.30 2-38 2. 4 6 2-53 2.75 2-93 3-12 3.46 3 r^ *4 2.24 2 35 2-44 2-53 2.63 2.72 2.80 3.04 3-25 3-47 3-67 3-8 5 4 1 IS 2-57 2.68 2.79 2.89 2. 9 8 3-08 3-35 3-84 4-05 4 . 2 6 s a c l6 2.92 3-3 3- J 5 3.26 3-37 3.66 3-94 4.19 4-43 4.67 6 4, "aj X 7 3-28 3-53 3- 6 5 3 98 4.29 4-57 5-9 7 .5 g 18 1% 3-8 3-93 4-30 4-63 4-95 5-23 5-53 8 i rt 19 4.08 4.22 4.63 5-00 5-32 5-66 5-97 9 ii f> 20 4.52 4.96 5-35 5-72 6 08 6.41 i 5 21 ' 5.28 6.12 6.50 6.84 i " 5 61 6.10 6.51 6.92 7-3 2 e 5 95 6.46 6.92 7-35 7-76 23 6.82 7-33 7-79 8.2 4 2 4 y 7.20 7-75 8.22 8-71 25 o 7-57 8.15 8.70 9.20 26 cC RULE. Multiply the number in the table by the length of the boiler in feet, and the product will be the capacity of the steam-space in cnbic-feet. 8.57 8.97 9-39 9.14 9-59 10.04 10.49 9.68 10.17 10.67 ii. 16 27 1 28 Q 29 10.94 11.62 3 "39 12.12 32 12.62 33 13-12 34 13-63 35 In designing this, as any other boiler having a cylindrical shell and fired externally, it is advisable to secure as large 354 THE STEAM-BOILER. sheets, and as few seams on the under side and where exposed to the action of the fire and the furnace gases, as possible. Boilers are now often made with but a single sheet extending from end to end, and of such width that all longitudinal seams are above the reach of flame. The steam-space should be of such volume that the varia- tion of pressure produced by each stroke of the engine should be unimportant. An old rule given by Bourne made the space not less than twelve times the volume of steam taken out by the engine at each stroke ; it may, however, be less for a given power as the speed of ^rotation of the engine is higher and as the ratio of expansion is increased. Tredgold would restrict the variation of steam-pressure at each stroke to about three per cent of the normal amount, which would, if V be the volume of steam-space of the boiler, 5 that of the single cylin- der, up to the point of " cut-off," and r the ratio of expansion, adopting Tredgold's coefficient, 0.033, For coupled engines, a much smaller space may be al- lowed. According to Shock,* marine boilers of the older types work dry when they contain in their steam-space a supply suf- ficient for the engine during 14 seconds and give wet steam if the steam-space is sufficient for but 12 seconds ; while the more modern forms of high-pressure boilers will only furnish dry steam when containing a volume equal to 20 seconds' supply. Steam-space of considerable altitude is most effective. 170. Stationary Flue-boilers are designed, as to dimen- sions of shell, very much as are plain cylindrical boilers. They are commonly of somewhat larger diameter and of com- paratively less length. The Cornish boiler, in which the single great flue serves also as furnace, is rarely made of less than 6 feet (1.8 m.) in diame- ter, as a smaller flue than that so obtained gives too contracted * Steam-boilers, p. 306. DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 355 a furnace. The length of this boiler is usually from 25 to 40 feet (7.6 to 12 m.). The thickness of shell is made about -J inch (1.27 cm.) and of flue f inch (0.95 cm.) for the shorter and -J- inch (1.27 cm.) for the greater length, the steam-pressure adopted being usually about 40 pounds per square inch (3 at.). Both should, however, be carefully computed by the methods already given (g 55, 56), and a good factor of safety not less than 6 is advised to be adopted and permanently maintained. The flue is nearly always one half the diameter of the shell. Where the boiler is long, and the flue thus becomes structurally weak, strengthening rings, or flanged girth-seams, should be adopted to insure greater strength and safety in the flue, which should, because of its special liability to injury and general, as distinguished from local, failure, be even safer against collapse than the shell against bursting. Collapse of the flue, however, is less likely to be disastrous to life and surrounding property than explosion of the shell. The heads are so well stayed by the flue that they require no other bracing below the water- line ; above that level, however, they should be stayed by ei- ther stay-rods or gusset-pieces, like the plain cylindrical boiler. The same remarks also here apply, relative to appurtenances of the boiler, as in the preceding case. Multifluc Boilers are constructed either with or without fireboxes. The latter will be considered more at length in later articles. Flue-boilers without fireboxes are simply com- posed of a cylindrical shell with plane heads, and having flues running from end to end, below the water-line, and secured in the heads, at each end, by means of flanges turned in those 41 flue-sheets" and riveted to the ends of the flues. These flanges are usually turned inwards, but are sometimes on the exterior, the projecting end of the flue, extending beyond the plane of the head. The number and size of these flues is de- termined mainly by the judgment of the designer, and no rule exists ; but the better the water used and the more valuable the fuel consumed, the more numerous the flues. Where two are put in, they are commonly about one third the diameter of the boiler, each, and are set side by side below the horizontal diametral line of the shell. When more are used, the number 356 THE STEAM-BOILER. is first increased to five, each of about one fourth the size of the boiler-shell. With still further subdivision the designer puts them in as he best can, ordinarily keeping their centres at the intersections of horizontal and vertical lines, set apar' dis- tances equal to the diameter of flue plus the desired space for circulation, which varies from one half to one fourth the diam- eter of the flue accordingly as the latter are more or less nu- merous. Ample room for circulating currents is no less essen- tial to efficiency than extent of heating-surface, and, as a matter of safety, more so. Small flues are commonly made of iron of the same, or somewhat less, thickness with the shell, and, when numerous, have an excess of strength over that indicated by calculation and a correspondingly increased margin for safety. FIG 74. FLUE WITH RINGS. Flues of sizes below 5 or 6 inches (1.5 or 1.8 cm.) diam- eter are not usually riveted up, as is the case with the larger flues, but are commonly drawn in the tube-rolling mill and are known in the market as tubes. The larger mills also often pro- duce drawn tubes, or flues, of much larger size ; some, handled by the Author, have been as large as 16 inches (4.9 cm.V In consequence, partly, of such changes in modern facilities for DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 357 construction, and for various other obvious reasons, the tubu- lar has very generally superseded the flue boiler. Where still used, it is customary to allow about 12 square feet (1.16 sq. m.) of heating surface per horse-power, and not far from 20 to i as the proportion to grate-surface. Fig. 74 illustrates a case in which the flues are strength- ened by rings, placed at the girth-seams joining each adjacent pair of ring-courses. The domestic make of corrugated flue now used in the " Scotch" marine boiler is illustrated in the following engraving. FIG. 75. THE CORRUGATED FLUR. In this class of boiler it proves particularly valuable, since the construction here met with, of high steam-pressure and FIG. 76. FORM OF CORRUGATED FLUE USED AS FURNACE IN MARINE BOILER. a forced fire, is one which demands strength of structure, and, at the same time, compels the use of thin iron. One objection 358 THE STEAM-BOILER. to this form of flue is found in its liability to become encrust- ed with sc'ale or with sediment in the corrugations. Fig. 76 shows the form given the corrugated flue when constructed for use as a furnace in a marine boiler, and as made by Mr. Fox, who first successfully manufactured them. The joints are welded in a gas-flame, and are usually but little, if at all, weaker than the solid sheet. For good stationary boilers, according to Cave,* about 4 pounds of steam may he allowed as the evaporation to be ex- pected per square foot of heating surface (19 kgs. per sq. m.) ; but this quantity is very variable with the form and proportions of the boiler, locomotive boilers producing several times this quantity, and the amount so evaporated increasing generally as the efficiency of the generator diminishes. The Cornish boiler, as formerly customarily operated, supplied but about one fourth the above-mentioned quantity of steam. 171. The Cylindrical Tubular or Multitubular boiler like the flue boiler, may be made either with or without firebox ; it is now most frequently made " plain," consisting of a cylin- drical shell, with plane heads and a " nest " of tubes fitting and nearly filling the water-space up to the water-level. The com- mercial and accepted rating and proportions of this class of steam-boiler have already been given in 161. In all impor- tant work, the designing engineer will carefully determine the size and economical proportions for the special case in hand. The following may be taken as illustrating the process for this case, as well as for boilers generally : It is required to design a tubular boiler or a set of boilers capable of supplying steam to a condensing engine of 500 horse- power, guaranteed to demand not more than 22 pounds (10 kgs.) of steam per H. P. per hour, the pressure to be 100 pounds per square inch (6-J atmospheres), and the feed-water to be taken from the condenser at 120. Fahr. (48. 8 C.). The first step is to determine the quantity of steam to be made. Calculation on the above basis would make it 11,000 pounds (4990 kgs.) per hour, evaporated from 120 Fahr. (48. 8 * Trait6 des Machines a. Vapeur; Bataille et Jullin. DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 359 C.) at 338 Fahr. (170 Cent.) as shown by the steam-tables (Appendix, Table I.). At the customary rating, however (30 pounds or 13.6 kgs. per horse-power), the weight to be evapo- rated would be 15,000 pounds (6804 kgs.) per hour, and this larger figure is taken as permitting a good margin. Were this evaporated by the best fuel and in a boiler having the efficiency unity, it would require the supply of 1000 pounds (4536 kilogs) of coal per hour. The financially desirable efficiency of the boiler should be next determined as indicated in the chapter devoted to that subject ; it may be here assumed to have been found to be 0.75 ; and 1333 pounds (6048 kgs.) of fuel would be demanded per hour. By the use of the expression already found ( 98) we have in which we may take A 0.5 and B = i ; then AE S 075 X ; 05 3' and S = IF- ........ (2) Thus the best ratio-of heating to grate surface is twice the number representing in British measures the quantity of fuel burned on the unit area of grate. It thus becomes necessary as a next step to ascertain this last quantity, and therefore to ascertain the height of chimney. This is, in the case of con- siderable power, as here, to be determined by the principles de- tailed in 157 and 158. A height of 125 feet may be taken as the result of this investigation. A well-designed chimney of this altitude should permit the combustion of 15 pounds per square foot (7.5 kgs. per sq. m.) of grate with a mar- gin of at least one third for contingencies. On this basis, the 360 THE STEAM-BOILER. area of grate must be 82 square feet (7.6 sq. m.) and the area of heating surface 2460 square feet (228 sq. m. nearly). This is to be distributed among two or more boilers ; since, although a thousand horse-power, even, may be, and sometimes is, ob- tained from a single boiler, it is usually found inexpedient to concentrate power to such an extent. A boiler of 5 feet (1.5 m.) diameter and 2\ or 3 diameters long has become a very common and very satisfactory size. This permits a grate t>f about 30 square feet (2.8 sq. m.), and three such boilers having grates 6 feet (1.8 m.) in length would give the required grate-area with an allowance of ten per cent FIG. 77. TUBULAR BOILER. for ineffective surface along the edges and in the corners. It may be taken as a good rule to throw in all such differences on the side of increased boiler-power. A boiler of this charac- ter with 3-inch (7.6 cm.) diameter of tube will be found to have 63 square feet (5.9 sq. m.) area of heating-surface per unit length, and a length of 15 feet (4.57 m.) gives very exactly the desired total area for a single boiler. The proportion of length of tube to diameter, 60 to I, is considered a good one, although rather high ; and such a boiler operated under the assumed con- ditions would supply the power demanded with the intended economy of fuel. DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 361 The tube-sheets would be made, if of steel, a half-inch (1.27 cm.), or a little less, in thickness, to give good holding power, and the shell, if of metal having a tenacity of 60,000 pounds per square inch (4218 kgs. per sq. cm.), would be, if double- riveted in the longitudinal seams, as in Fig. 77, f inch (0.95 cm.) in thickness. The tubes would be 66 or 68 in number, and the braces of sufficient number and strength to sustain the heads safely. The dome would be probably given about one half the diameter of the boiler, and be made of metal rather more than one half as thick, as it would usually be single-riv- eted. The tubes should always be placed in vertical and horizon- tal rows; to "stagger" them would insure a defective circula- tion and injury to those thus exposed to overheating. The tubes should never be nearer than 3 inches to the shell of the boiler, and should never be carried down near the bottom of the boiler ; but there should be ample water-space at the bottom of the shell. The fire from the furnace first strikes the bottom of the boiler, and there should be a good body of water there. Pressures have risen in stationary-boiler operation until the common cylindrical tubular boiler of 6 feet (1.8 metres) di- ameter is made \ inch (1.27 cm.) in thickness of shell, and is safe, with usual construction, at a pressure of nearly ten at- mospheres. 172. Marine Flue-boilers are rarely used at sea, but remain in use on the rivers of the United States. In their design, the same principles which have just been applied are also applica- ble in the determination of the dimensions of shell and flues. The firebox forms an essential feature of this class, however ; and its construction involves calculations of strength of stayed surfaces. In the locomotive, the stay-bolts are placed 4 or 5 inches (10 to 12.7 cm.) apart; but in marine boilers they are more widely distributed, as working pressures are lower. In any case, the area of the flat surface should be estimated, and also the pressure upon it, and a sufficient number of braces used to provide for that pressure. If the braces are of iron of known strength, say 60,000 pounds per square inch (3515 kgs. 32 THE STEAM-BOILER, per sq. cm.), a factor of safety of 10 would give 6,000 pounds- (or 422 kgs.) on each brace of unit section, and the number of braces should be sufficient to safely carry the load on the total surface. The heavier the plate, the greater its resistance to the distorting action of the steam-pressure, and the heavier the stay-bolts and the wider their spacing. In the older forms of marine flue-boiler, in which steam-pressures ranged from 25 to 40 pounds per square inch (if to 2f atmos.), the stay-bolts were usually spaced from 10 to 8 inches (25 to 20 cm.) apart, and were given a diameter of from one eighth to one tenth those figures. This form of boiler has so generally been superseded by the tubular boiler that it has now comparatively little importance, except on the large rivers. The following are the proportions adopted on board a number of Ohio and Mississippi river steamers, all of which use the lap-welded and drawn tube in place of the older form of riveted flue : A steamer on the Ohio has two boilers, 47 inches diameter and 24 feet long, ten lap-welded flues in each, of 8 inches diameter ; two boilers 41 inches diameter, 24 feet long, with six lap-welded flues in each, of 10 inches diameter; steamer Golden Rule, three boilers, 44 inches diameter, 26 feet long, with three 8- and three lO-inch lap-welded flues in each. Such flues are more cylindrical in form than the riveted flue, thereby lessening the chances of collapsing. There are no rivet-heads or laps to in- terfere with the draught, and consequently the flues are not li- able to choke up with soot, are much less apt to scale, and hav- ing smooth surface, are much more easily cleaned. The water-level should be at least 6 inches (2.4 cm.) above the highest flue, and is usually fixed by law or regulation at a minimum of 4 inches (1.6 cm.). The highest line of heating- surface is usually required to be below that level. Where ex- posed to flame these boilers are not allowed to have a thick- ness exceeding 0.51 inch (1.2 cm.), and a water-space of at least 3 inches (7.6 cm.) is left between the flues and between flue and shell. 173. The Marine Tubular Boiler has now almost univer- sally been brought to a very definite standard form and propor- tions. It has been already described as consisting of a cylin- DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 363 drical shell with plane heads, traversed by large flues and comparatively small tubes, the furnaces being in the flues. Those designed for sea-going steamers are often of very large diameter, the steam-pressures often exceeding ten atmospheres (150 pounds per square inch), they are also made of very heavy boiler-plate. These boilers naturally are oftener double- riveted than those of smaller diameter, and every expedient known to the engineer is adopted to insure safety. Diameters of 1 5 and even of nearly 20 feet (4.6 and nearly 6 m.) are com- mon, and plates as thick as I J inches (3.2 cm.) have been used. These heavy plates are usually butted at the seams, and the joint is covered with a " butt-strap" or " covering-strip," double- riveted on each side, thus presenting to the eye four parallel rows of large rivets. The calculation of the shell is made in the same manner as in the cases of the forms of boiler already considered. The size is determined partly by the conditions and the method described in 171, and partly by the necessity of getting the whole set of boilers into a space limited both as to volume and form by the construction of the vessel and by the necessity of economizing as much as possible that space which might be otherwise used for lading and passengers. The stays are usually long rods, extending from end to end in the steam-space, and screwed stay-bolts, reinforced with nuts, in the water-spaces. The dimensions of a steel and of an iron boiler of this class, as actually constructed, are, as given by the builders, the following: STEEL BOILER FOR 6 ATMOSPHERES (90 LBS. PRESSURE). Diameter, 16 ft.; length, n ft.; shell, | in. thick. 4.901.; " 3.35m.; " 2.2 cm. " 3 Furnaces, 48 in. diameter; | in. " i. 2m. " 1.3 cm. " 250 Tubes, 3i- in. " 6 ft. long 7.7 cm. " 1.98 m. " Area heating-surface 1800 sq. ft. (167 sq. m.). Weight of boiler 70,000 pounds (3^,750 kgs.) nearly. water 50,000 " (22,680 " ) T^tal weight 120,000 " (54-43 " ) " 364 THE STEAM-BOILER. IRON BOILER (SAME PRESSURE). Diameter, 14.751'!.; length, n ft.; shell, \\ in. thick. " 4.501.; " 3.35 m. ; " 2.86cm. " 3 Furnaces, 39 in. diameter; if in. " 0.99 m. " >?3 cm. " 258 tubes, 3^ in. " 7 ft. long. 8.9 cm. " 2.1 m. long. Area heating-surface 2000 sq. ft. (186 sq. m.). Weight of boiler 75oo pounds (34,088 kgs.) nearly. " water,.. 45,ooo " (20,412 kgs.) " Total weight 120,000 " (54,430 kgs.) " The drawings herewith given, Fig. 78, illustrate the details of this construction. Marine steam-boilers require peculiar care in their design and construction. They must be as light and as small as is possible consistent with the efficiency demanded, and, being exceptionally liable to rapid corrosion and general deterioration, much depends on their being so made as to per- mit every precaution to be taken to prevent such injury and to insure their preservation. In the construction of cylindrical shells the longitudinal seams are usually all double-riveted, and often even butt-jointed, with double covering strips: this is almost always done in cases in which very high pressures compel the use of heavy plates. In ordinary practice, the heating-surface ranges from 30 to 40 times the grate-area ; the evaporation ranges from 6 or 8 to 10 or 1 1 pounds per pound of good coal consumed ; the crown- sheets are carried as high above the grate as the form of boiler allows ; the grate bars are inclined about one in twelve, from front to rear, and are given a length as little more than 6 feet as is practicable. In the cylindrical marine boiler, in which the grates must be set in furnaces which form the lower and larger set of flues, it is not possible to secure either as good a propor- tion of grate, or as great height above it as is desirable ; and the inefficiency sometimes noticed in boilers of this class is commonly due to these faults of the furnaces. 174. Sectional and Water-tube Boilers differ as radically in their design and construction, as in type, from the shell- DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 365 Boiler with Corrugated Flue. Three-furnace Boiler. FIG. 78. MARINE STEAM-BOILERS 366 THE STEAM-BOILER. boilers which have been here considered. As a rule, their de- sign involves but little calculation of strength, as their tubes and connections are always vastly stronger than is absolutely necessary as a mere matter of supporting the steam-pressure. The " headers " or other connections of parts are commonly without rivets, and are fitted, piece to piece, with machine- made " faced " joints, and held in place by bolts. Some special precautions are demanded, in designing this type of boiler, to secure safety against injury, and to avoid serious difficulties arising in management from the comparatively small body of water and of steam carried by them, and the consequent ab- sence of the self-regulating power observed in shell-boilers. Mr. Robert Wilson states that the following appear to be the points that require special attention in designing these water-tube boilers, to insure their satisfactory working and durability : (1) To keep the joints out of the fire. (2) To protect the furnace-tubes from the sudden impinge- ment of cold air upon them on opening the fire-door. (3) To provide against the delivery of the cold feed-water directly into the furnace-tubes. (4) To provide for a good circulation to take away the steam from the heating-surfaces. (5) To provide passages of ample size for upward currents so that they may not interfere with downward currents. (6) To provide passages of ample size, for steam and water, between the various sections of the boiler, to equalize the pres- sure and water-level in all. (7) To provide ample surface of water-level to permit the steam to leave the water quietly. (8) To provide a sufficiently large reservoir for steam to prevent the water being thrown out by suddenly opening a steam or safety valve. (9) To provide against the flame taking a short cut to the chimney, and impinging against tubes containing steam only. The several forms of this type of boiler now becoming fa- miliar have illustrated great ingenuity in securing efficient and novel arrangement of parts, rather than special knowledge of DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 367 the character and strength of materials. Some of these forms have been already described, and need not be here further il- lustrated. This class of boiler is generally in use on land, but attempts have been made to introduce them for marine pur- poses. The Author has under his hand sets of drawings of marine tubular boilers for a naval vessel, and of a " sectional " water- tube boiler intended for similar power and the same duty, which afford a means of comparing standard designs of the two types. It does not follow, however, that this comparison would in all cases yield similar deductions. The tubular boiler has a shell 9 feet (2.74 m.) in diameter, while the other is only 5 feet (1.52 m.). The tubular has i inches (3.2 cm.) thickness of metal between fire and water where the rear tube-sheet sets into the shell : the greatest thickness in the sec- tional boiler, between fire and water, is only f of an inch (9.5 mm.). The shell-boiler has a ratio of grate-surface to heating-sur- face of I to 23, and the ratio of grate-surface to calorimeter is 7.2 to I ; the sectional has a ratio of grate to heating surface of I to 41.3, and a ratio of grate-surface to calorimeter of 4.82 to I, which means the ability to burn more coal per unit of grate- surface. The steam-space is practically identical in both, but the water in the tubulars weighs 12.6 pounds against 15.5 pounds in the sectional per sq. ft. of heating-surface. The total iron-work of the tubular boilers is 35 \ pounds per sq. ft. of heating-surface, whereas in the sectional it is 25.8. The total weight per unit of heating-surface is as 48 in the former to 41 in the latter. The tabular comparison on page 368 was presented at the same time. On the other hand, it is objected, by those who oppose the introduction of these boilers on shipboard, that the following considerations are too important to permit their safe employ- ment.* (i) That they usually occupy as much space as shell- boilers. * Shock's Steam-boilers, pp. 280-1. or THE UNIVERSITY /^,. ._ r _ 368 THE STEAM-BOILER. SHELL TUBULARS. SECTIONAL. Shell o ft diameter X 9 ft. 7 in. long, 5 in. diameter X 20 ft. long, % in. % in. thick. thick. Heads. % in. thick. j To carry 1 ^ in. thick. 5 To carry Seams in fire i}4 in. thick. 1 115 Ibs. f None I 150 Ibs. Heating-surface ... 1322.7 sq. ft. 3100 sq. ft. Grate surface 57.3 sq. it. M* W *5 75 sq. ft. | Ratio of grate-surf'e "?g be to heating-surface. Steam-space i sq. ft. to 23 sq. ft. 169 cubic ft. ~& g i sq. ft. to 41.3 sq. ft. 392.6 cubic ft. '*: I Ratio of steam-space ( .012 cubic ft. per i I "". v ^ j .012 cubic ft. to i | U i *"^ to heating-surface. ) sq. ft. j 5 8" - 2 / sq. It. f d* ^ 'r> t/3 Water, weight of 16,660 pounds " J 48,262 pounds (Y> Weight per sq. foot I yQ .'H -C heating-surface 12.6 " V '". f i5-5 ' oT ^ s 1 Iron-work, weight of 47,040 *** rt ^ ^ 80,221 ^o -^ 2 Iron-work per sq. ft. 0)^ J jg iJ 1 ^ ^"^ heating-surface. .. 35-5 rt ? ^ 25 8 ; rt 3 ** Total weight 63.700 128,423 ^ < i Total weight per sq. T3 S S j^ P a = 6 ffi ft. heat-surface 48.18 s Ji-l 41.44 " 4> '^ O rt <" O O. . Calorimeter 8.27 sq. ft. 18 sq. ft. o rti . i- O._g u i Ratio of grate-sur- *J rt >~ rt u face to calorimeter. 7 . 2 to i sq. ft. p^ *** *C {j *fl 4.82 to i sq. ft. ( vi tial aim of all good systems of support. Where two points of support are chosen on each side, they should be placed one fourth the length of the boiler from each end ; where three supports are introduced, the outer ones should be one sixth the length of the boiler from the ends, and the third should be 380 THE STEAM-BOILER. placed in the middle, thus giving a uniform load on all. Horizontal boilers are sometimes supported at the rear end on plates resting on rollers to reduce frictional resistance to change of dimensions. It is probably as well not to attempt to carry the weight of the boiler on the "walls of its setting, and this can be avoided by adopting the plan of inserting vertical posts, made of a pair of channel-bars secured back to back, and thus forming strong, simple, and inexpensive columns, on which the load can be safely and permanently carried. The air-space between the walls is an important safeguard against injury by the change of form of the inner wall with variation of temperature. Where desirable, the space between the boiler and this continually moving mass can be closed by carrying a flange of angle-iron along it, and supporting this flange from the iron posts in the walls. Angle and channel irons are also best for use in making the binders or " buckstaves" by which the whole setting is kept in shape. Where cast-iron is used at all, as in the fronts, it should be heavy enough to keep its shape. Where a boiler is supported by lugs riveted to its sides and bearing on the side-walls of the setting, the principal risk is usually, probably, that of the failure of the riveting. The boiler-shell has a large margin of strength, and no injury need ordinarily be feared from the stress coming of its own weight between the points of support. When the rivets are placed not more than four or five diameters apart, the boiler may be con- sidered as perfectly safe, the workmanship being good. It is advisable to place covering strips on the inside to take the heads of the rivets securing the lugs in place. 178. Forms of Covering to prevent the loss of heat from the boiler and flues by conduction and radiation are of consid- erable variety. The rudest, though an effective one, is a layer of ashes over the top of the boiler, filling in between the side- walls of the setting. This is often objectionable, as giving rise to annoyance from dust ; and various mineral and fibrous sub- stances are preferred, such as asbestos, hair-felt, and several kinds of plaster and cement. Where hair-felt is used, it is often covered with canvas to give a neater appearance, and to DESIGNING A CCESSORIESSE TTING CHIMNE VS. 3 8 1 protect the felt from dust and injury. Occasionally, a brick arch is turned over the whole structure, and the air-space so produced relied upon to intercept heat. This construction is probably not quite as efficient as the other coverings, but it has the advantage of permitting easy access to the boiler for inspection and repair. A loose blanket is as good. 179. The Form of the Bridge-wall is not always the same in the same general design. A bridge-wall is needed at the rear end of the grate, and it is now rather unusual to build others ; but two, or even more, are sometimes introduced for the alleged purpose of securing intermingling of the currents of furnace-gas and their contact with the boiler. In some cases the bridge-wall is carried up to the boiler-shell nearly, and fitted rather closely to its form; a more approved system, how- ever, gives its top a perfectly straight and level line. Ample space should always be allowed for the passage of the gases, as well as above the grates, for the completion of combustion. The semi-diameter of the boiler is none too great for the depth of this latter space. Two feet is a good minimum. 180. The Disposition of Flues is subject to the same re- mark as was made relative to the bridge-wall. No standard practice can be described ; but it is continually becoming more usual to leave the whole space beneath the boiler without subdivision from bridge-wall to chimney-flue, taking off the gases from the tubes as directly to the chimney as possible, and controlling the flow of the gas-current by the damper. Oc- casionally a special direct flue is provided with its own dam- per, when a drop flue is ordinarily used, or when the flame is carried over the shell, the former being opened when the fires are started to secure rapid kindling, and closed again when the fires are fairly burning. The shortest line of flue from the boiler-setting to the chimney is best in all cases. 181. The Location and Design of Chimney may often be the first step to be taken preliminarily to designing the boiler ; or, as is oftener the case, the user purchases his boiler and then erects such a chimney as the designer and vender may recom- mend, in such location as he may find practicable. In many cases the chimney consists of a simple pipe of sheet-iron, ris- 382 THE STEAM-BOILER. ing directly from the flue, which, forming part of the boiler set- ting, also serves as the base of the pipe. In this case the rules for proportioning are to be taken as those governing marine prac- tice, and the draught as calculable on that basis, with a consid- erable margin to allow for variations of temperature, humidity, and mobility of atmosphere. In the majority of cases, how- ever, a chimney-stack of brickwork is preferred, both on the score of permanence and on that of better draught ; the iron flue permitting a loss of heat and cooling of the air-column, which does not take place to any observable extent in the brick stack. No. 10 or 12 iron is ordinarily used. The essentials of a good design are : adaptation in draught power and capacity, in height and area of flue, to the precise conditions to be met, with ample surplus for emergencies ; a solid and perfectly safe foundation ; a well-formed, straight, well-proportioned shaft ; stability against the pressure of the most violent winds ; security against injury by its own heated gases ; and economy in construction and maintenance. The first two of these requirements are met by the methods already detailed in 160: a safe foundation is obtained by going down to the rock wherever possible, or to firm, compact, stable soil, and there starting the bed courses, giving them ample area to carry the superincumbent weight safely. Where difficulty is met with in the endeavor to accomplish this, a broad concrete base is often laid on the yielding substratum of soil, and on this the masonry is laid up after ample time for hardening and settling is allowed. The more slowly the construction is car- ried on, the better the result. The form and proportion of the shaft is partly a matter of taste, judgment, and architectural effect, and partly of calculation based on the elements pre- scribed by the conditions under which the boiler is to be oper- ated. Stability is assured by carefully proportioning weight of stack and breadth at the foundation to meet the overturn- ing force of the highest winds, and allowing, further, a fair fac- tor of safety. A pressure of 55 pounds per square foot (268 kgs. per sq. m.) on chimneys of square section, and one half this amount on chimneys of circular or octagonal section, is a common assumption as a measure of the maximum force of DESIGNING ACCESSORIES SETTING CHIMNEYS. 383 the wind in exposed situations. In sheltered localities, a cal- culation of stability is rarely made. Security against the cut- ting or overheating which may sometimes occur where the fur- nace gases reach the chimney at a very high temperature is obtained in large chimneys by the construction of an inner chimney of fire-brick, separated from the main structure by a narrow air-space. In small chimneys a lining of fire-brick built into the walls of the chimney for some distance upward from the base is the usual safeguard, and even this is often omitted. Economy is obtained by making the design as simple, the height and the dimensions generally as small, as may be con- sistent with a good design. Circular and octagonal sections are best as a rule, but the square section is usually the least costly to build. Where an outer and an inner shell are put up separately from the foun- dation, provision is often made to cover, in some way, the annular opening between the two at the top of the inner stack to prevent the settlement of dust between them : this is not, however, usual or essential ; but a cleaning door should be placed at the bottom, through which access can be had both to this space and to the main flue. All the talent of the archi- tect is often demanded in the design of the exterior of large chimneys. The following are the dimensions of a large chimney of good design :* Height above grade 192 ft. 58. 5 m. Total height (with foundation). . . 204 ft. 62.18 m. Batter. 2 in 100, nearly. Diameter at grade 17 ft. 5.18 m. of flue at top 8ft. 2.4301. Thickness, stack 2.67 to 1.33 ft. 0.8 to 0.4 m. " inner shell 1.33 to 0.67 ft. 0.4100.201. Weight 2,187 tons. 2,222 tonnes. Horse-power 2, 700. Cost per H.-P '. . . $5.53. ' ' total $14,000. 182. Steam and Water Pipes and their connections should be as carefully designed and located as the members of the * Sri. Am. Supp., Jan. 29, 1887. 384 THE STEAM-BOILER. structure itself. Steam should be taken off at the point at which it will pass out most perfectly dry, or, if provision is made for it, superheated. If a steam-dome is attached to the boiler it should usually be placed at a distance from that part of the steam-space into which steam is rising most rapidly, and the steam-pipe should be led from the highest point within it. If a dry pipe is used it is better to so place it that its most contracted openings are nearest the furnace. Such area should be given this pipe that the frictional resistance to flow should not sensibly reduce its pressure, and the same precaution should be taken in placing valves. A velocity of 6000 feet (1829 m.) per minute should usually be a maximum rate of flow. The steam-pipe should be as carefully protected by non- conducting covering as the boiler itself, and it should be so set and drained that no water can collect at low points or in an- gles, to be thrown forward by the steam into the engine, there to cause danger of accident. The Author has frequently known this to occur, and the steam-pipe itself is sometimes burst open by its impact, causing loss of both life and property. Experi- ments conducted by the Author* have shown that pressures produced by this so-called " water-hammer " may amount to probably above ten times that which the pipe was expected to sustain in regular work. Drain-cocks and steam-traps suitably placed may be used to take away water collecting in bends where they are unavoidably introduced. Care must be taken, in long straight lines of pipe, to avoid danger of injury by the expansion and contraction taking place with change of temper- ature as the pipe is heated and cooled when steam is sent through it or when emptied. Where precautions are not taken, as in the introduction of bends, angles, or slip-joints or their equivalents, pipes are sometimes broken, joints are set leaking, or connections are completely broken, and serious results fol- low. If extensive systems of pipe are properly guarded against water-hammer and excessive temperature-strains by correct lo- cation, thorough drainage, and good designing, no other dan- ger than that of corrosion is to be apprehended. * Trans. Am. Soc. Mec. Engs., vol. iv., 1882-83, P- 44- DESIGNING A CCESSORIESSE TTING CHIMNE VS. 385 Similar principles control the location and proportioning of feed-water pipes. They should be of ample size and strength, should be so located as to be free from liability to injury by expansion and contraction, and should be led into the boiler in such manner and should so discharge the feed-water that in- jury should not be done the boiler by the impinging of cold water on heating-surfaces, or by the collection of a mass of cold water at times in the lower part of the boiler, thus introducing serious strains, along the line separating the cold from the hot water, or elsewhere. The entering feed should be warmed by flowing out into the general mass of circulating liquid, and should not be so directed as to impinge on metal. No calcu- lations of strength of ordinary steam and water pipe are ordi- narily made, as the internal pressure is usually the least impor- tant stress affecting them. If strong enough to bear other stresses and thick enough to resist corrosion for a considerable time, they are amply strong. All cocks, valves, and connections should be strong enough and sufficiently well put together to bear safely such accidental stresses as have been referred to without risk. 183. Safety-valves are absolutely essential to every steam- boiler. Many explosions have been known to have been caused by the failure of a safety-valve to open at the intended pres- sure, and it is considered good practice to evade such a danger by introducing two safety-valves into the design of every boiler. The office of a safety-valve, as used on a steam-boiler, is to discharge steam so rapidly, when the pressure within the boiler reaches a fixed limit, that no important increase of pressure can then occur, however rapidly steam may be made. It has also another office : it should be so constructed and arranged that should any accident occur it may be opened by hand and the steam-pressure lowered very rapidly, even when the fires in the boilers are burning brightly and generating steam with maxi- mum rapidity. The size of a safety-valve is determined by the character of the valve itself, by the pressure at which the steam is to be discharged, by the difference permissible between the pressure at which the valve is to open automatically, and that 25 386 THE STEAM-BOILER. at which it is intended to be capable of discharging steam as fast as the boiler can make it. A valve of defective design or badly constructed must nec- essarily be larger, to do the same work, than one of similar type well designed and constructed. Steam is discharged at any given rate through an orifice of smaller dimensions as the pressure increases ; the lower the pressure, on the other hand, the larger must be the valve. A boiler in which steam is car- ried at ordinary pressure may require a safety-valve of large area, while the same quantity of steam would escape through a rivet-hole in a boiler containing steam at pressures sruch as were attained by Perkins and Albans a generation ago. Rules by which to calculate the proper area of safety-valves for every case arising in his practice are used by every engineer accustomed to designing steam-boilers. These rules vary con- siderably with differences in the experience or the judgment of their authors. But a safety-valve, as has been stated, should be capable of discharging very much more than the maximum quantity of steam that the boiler can make when doing its best. The valve must be raised, ordinarily, by the action of the steam it- self, and the force exerted by the steam-pressure upon its disk rapidly diminishes as it rises from its seat. The seat is bev- elled, too, in such a manner that the effective area for dis- charge of steam is but a fraction of that due the rise of a valve having an unbevelled seat. It is therefore advisable to give a very large area to the valves. It has been common in the United States to allow but one square inch of area of valve-opening for 25 square feet of heating-surface, or a ratio of 0.0003, nearly ; while another rule gives one square inch to three feet of grate-surface : an English rule allows an area equal to a half square inch to a square foot of grate, or 0.003 the grate-surface, nearly ; while still another authority nearly doubles this area of valve. But the area should always be based on the quantity of steam made. The Author has been led by experience to adopt the rule : Multiply the maximum weight of steam which the boiler is expected to generate per hour by five and divide by ten times the gauge- DESIGNING A CCESSORIESSE TTINGCHIMNE YS. 38? pressure, increased by ten, in British measures ; or, divide that weight by twice the latter quantity. Thus, where w is the maximum weight of steam made per hour in pounds, / the pressure in pounds on the square inch, and a the area of the valve-opening in square inches. For important work it is advisable, especially for large boilers, to calculate carefully the area of opening needed, by the principles controlling the discharge of steam from orifices. A very large excess over the area demanded to just discharge steam at the maximum rate at which it is made should be given, as it is often necessary to rapidly reduce pressure just when the fires are brightest and vaporization most active. The design of the valve is rarely a problem solved by the de- signer of the boiler. Valves in great variety are made and sold by manufacturers, and it is customary to purchase such as are needed. One of the simplest of the common form, of lever safety- valve is that seen in Fig. 83, in which the valve, A, is held down to its seat by a lever, BC, having a fulcrum at the pin, C, and resting on the valve at D. The weight, W, can be adjusted at any distance from D that may give the mo- FIG. 83. LEVER SAFETY-VALVE. ment required to resist the intended steam-pressure. A guide at E, secured, like the pivot standard F, to the valve-chamber, G y keeps the lever in the designed vertical plane. The size of the valve is usually reckoned as that of the opening, H 9 of pipe and valve-seat. A " feather" on the outer side of the valve guides it and ensures its return fairly upon its seat when it falls with reduction of pressure. Fig. 84 shows the exterior of a better and more recent type of lever safety-valve. In some cases weights are carried directly on the top of the valve- 388 THE STEAM-BOILER. stem, a spindle rising from the latter over which they are threaded ; the pressure is then determined by adding or re- moving weights. In other instances the weights are suspended below the valve and inside the boiler, the idea being to make FIG. 84. SAFETY-VALVE, thm inaccessible to any one, except at times when no steam is on and when the inspector may adjust them. Often valves are so constructed that, once adjusted, they may be locked up, and thus made safe against the tampering of irresponsible or mali- cious persons. -40* TO POINT OF SUSPENSION OF WEIGHT FIG. 85. RECENT TYPE OF LEVER SAFETY-VALVE WITH KNIFE-EDGES. A better form of lever safety-valve than that just described is that proposed by the U. S. inspectors, Fig. 85, in which the contacts of valve and fulcrum with the lever are made by knife-edges, a system found to have marked superiority over DESIGNING A CCESSORIESSE TTINGCHIMNE YS. 389 the usual pin-construction. The valve is commonly covered by a "bonnet," and the steam flowing past the valve into the chamber so made is conducted away by an attached steam- pipe. The proportions adopted by the Board submitting it* are as follows: AREA OF VALVES EXPRESSED JN SQUARE INCHES. 5". 10". 15". 30". 25". 3O". Diameter of opening.. . Diameter of valves Length of lever 2.525 2. 7 6 2tr 3-37 3 77 qo 4-371 4.58 ae . 5-047 5.23 4O 5.642 5-86 AC . 6.781 6-375 4.7 ^ Distance of fulcrum. . . Angle of valve's face. . Width of face 2-5 45 . 1C 3 -45* *5 'Is- .12 4 '4S . 17 4 ' 5 o 45 1 7 4-75 o 45 .15 Length of fulcrum link. 4-5 4-5 4-5 4-5 4.5 4-5 When well proportioned and well made, these valves may be expected to keep the steam under usual conditions within FIG. 86. LEVER SAFETY-VALVE (U. S. BOARD OF INSPECTORS). one or two per cent of its working pressure ; but the smaller valves are less exact than the larger sizes. * Report on Safety-valve Test. Washington, 1877. 390 THE STEAM-BOILER. The essential requirements are considered to be (1) Capability of discharging any excess of steam above a fixed working pressure. (2) A minimum limit of variation of pressure within which the valve will open and close. (3) Uniformity of action at different pressures. (4) Reliability of action under continued use. (5) Simplicity. The form of valve just described meets these demands in a very satisfactory manner. The working drawings are seen in Fig. 86. The effective area of opening, a, required to discharge a given weight of steam, w, per hour was found to be, at various, usual pressures, as follows : 2 atmos. , 30 pounds per square inch a = w X 0.0009 4 atmos., 60 pounds per square inch a = w X 0.0006 6 atmos., 90 pounds per square inch a w X 0.0003 7 atmos. , 100 pounds per square inch a = w X 0.0002 The proportion a = o.oo$w is taken as giving a safe area, the factor of safety for the usual pressures being 10, and greater as the pressures increase. In many cases the lever and weight are too cumber- some, or otherwise objectionable, and a spring is used, acting either directly on the valve or on a short lever a common practice with both locomotive and marine boilers. Nearly all the later forms of valve are of the former of these two classes. It is found very difficult to avoid a considerable variation of steam-pressure with the common form of valve, as it is not often practicable to secure the full lift of the valve. Owing to a peculiar action of the impinging currents of steam, it is rarely possible to obtain a rise of more than about 0.2 inch (0.5 cm.) without serious excess of pressure, especially with low steam. Many expedients have been proposed to meet this difficulty, as, for example, in the Rochow valve of Fig. 87, in which a DESIGNING A CCESSORIESSE TTING CHIMNE VS. 39 1 piston is attached below the valve, having a slight excess of area, and thus continually forcing the valve upward to the limit of its rise until the pressure is relieved. A system now becoming very common, and giving most satisfac- tory results, is that known as the " reactionary" valve, of which a good example is that of Ashcroft (Fig. 88), in which the current issu- ing from under the valve is de- flected by a curved lip or flange in such manner as to cause a pressure by its reaction that aids effectively in raising the valve. This system of construction is in very extended use. When well designed, they open FlG - SJ.-ROCHOW'S SAFETY-VALVE. promptly and widely, discharge the surplus steam quickly, and seat themselves at once, thus preventing any observable varia- tion of working pressure. In designing safety-valves care is to be taken to secure ample area of opening, freedom from liability to stick or failure to rise fully, and to see that if the spindle passes through a guide the bearing-surfaces are not liable to rust fast. It is usual to line the opening, and to cover the spindle with brass. Narrow valve-seats are advisable to secure tightness and free working, and straight steam- The mechanism of one of the FIG. 88. ASHCROFT'S (REACTIONARY) f , . ,, . SPRING-LOADED SAFETY-VALVE. most rCCCllt of the " reactionary Safe- ty-valves is seen in Fig. 89, in which B B is a nickel seat, C C, the valve of which, C'C' f is the adjustable ring introduced to secure the desired reaction. F F is the spring and D D the 392 THE STEAM-BOILER. spindle, the one bearing against the fixed cross-bar, G G, and the other attached to it firmly. The channel, a a y turns the issuing current back into the verti- cal direction, and thus makes the re- actionary effect a maximum. . Brass or nickel valves and seats are free from the liability to dangerous corrosion that characterizes iron. The maximum intensity of pressure under any lever safety-valve is l'w'+uff fa FIG. 89. RICHARDSON'S SAFETY- VALVE. when a is its effective area ; w, w', w" , the weight applied, that of the lever and that of valve ; / /', the lengths of lever-arm from weight to fulcrum, and of that from centre of gravity of the lever; and f the distance from fulcrum to centre of valve. The actual value of a may vary enormously in any one valve having a wide seat, accordingly as it is tight or leaking. If perfectly tight, the valve will rise when an equilibrium is reached, assuming a to be the area within the inner periphery of the seat ; it will drop when the pressure has fallen so far that an equilibrium may be established, a being measured to the exterior periphery. If leaking, these two areas may have almost any apparent relation. The narrower the seat, the less these differences. For large boilers, "multiplex" valves, consisting of a set of two or more in one casing, are often used in preference to a single large valve. 184. The Feed Apparatus for steam-boilers is not usually designed by the engineer furnishing the plans for boilers, but is purchased of makers of feed-pumps or of "injectors" as it may be needed. Where open heaters are used, in which the feed is heated before it is pumped, the injector cannot, as a rule, be used ; but a large slow-moving pump, placed sufficiently low to fill with certainty at every stroke, should be employed. A DESIGNING ACCESSORIES SETTING CHIMNEYS. 393 pump driven by belt and by the main engine is more economi- cal in operation than a steam-pump. The independence of the latter, and their convenience of operation, have caused their very general introduction ; and they are commonly kept at hand for emergencies, even where the " power-pump" is used. With a closed or coil heater water may be forced by the feed- pump through the heating-coils and on into the boiler. In this case, either pump or injector may be used. The latter is, in this case the more economical, as no loss occurs except of heat from the steam and water pipes, and this loss may be ren- dered insignificant by carefully covering them. Even the effect of friction is to give a fully compensating increase of temperature to the water. The steam-pumps are not usually economical of steam, and often use ten times as much per unit of work done as good engines. A " duty" of ten millions is unusually large. All feed apparatus should be of the best possible construc- tion ; should, when possible, be in duplicate, and of far greater capacity than is demanded in regular work ; and should be placed where it will always be promptly and readily accessible, and kept in perfect order. Failure to act promptly and effec- tively in an emergency may lead to incalculable disaster. In many cases injectors are used in ordinary work, and very large steam-pumps kept in readiness for emergencies. Heating the feed-water by means of the waste gases is al- ways advisable if at all practicable, as well as the utilization of the heat of all exhaust-steam from engines and pumps and re- turns from systems of heating-pipe. The table on page 394 gives the percentage of saving ef- fected by heating the feed-water of a steam-boiler by means of heat otherwise wasted. 185. Minor Accessories and details, such as the kind and location of steam and water gauges, dampers, automatic con- trolling devices, etc., should be as carefully considered by the designer of the steam-boiler as any other parts of his work. The Steam-gauge is selected from among the numerous styles and makes in the market, and is never designed by the engineer preparing plans of boilers. The most common form 394 THE STEAM-BOILED. is the Bourdon Spring Pressure-gauge (Fig. 90), of which a number of modifications are in use. The case, A A, encloses a coil of flattened tube, B B, closed at the free end and open to boiler-steam at the supported extremity. As the pressure rises and falls, a tendency to expand the tube into circular section produces greater or leg i -jry/ effect, and the tube, as a whole, assumes a greater or a smaller radius of carvature, throwing its free end one way or the other in such manner as to measure, by the trav- erse of the attached pointer, the pressure at FIG. 90.- BOURDON GAUGE, g^h moment, of the confined fluid. Some- times the tube is held at its middle point, both ends being free, and their relative motion affecting the pointer. The more stable the tube and the more reliable the mechanism connecting it with the hand at the dial, the better the gauge. SAVING BY HEATING FEED-WATER. (Steam at 60 Ibs.) V 3 INITIAL TEMPERATURE OF WATER (FAHR.). 1M i m B * o 32 40 50 60 7 80 90 100 I20 140 160 180 200 ' 60 2-39 1.71 0.86 80 4.09 3-43 2-59 1.74 o 88 IOO 5-79 5-14 4-32 3-49 2.64 1.77 0.90 120 7-5 6.85 6.05 5-23 4.40 3-55 2.68 1. 80 O 140 9.20 8-57 7-77 6.97 6.15 5-32 4-47 3.61 1 .84 160 10.90 0.28 9-5 8.72 7.91 7.09 6.26 5-42 3-67 1.87 o 180 12.60 2.00 11.23 10.46 9.68 8.87 8.06 7-23 5-52 3-75 i 91 200 14.30 3-71 13.00 12.20 "43 10.65 9-85 9-3 7-36 5-62 3-82 1.96 220 16.00 5-42 14.70 14.00 I3-I9 12.33 11.64 10.84 9.20 7-50 5-73 3-93 I. 9 8 240 J 7-79 7- J 3 16.42 15.69 14.96 14.20 13-43 12 .65 II .05 9-37 7.04 5-90 3-97 260 280 19.40 21.10 8.85 20.56 18.15 19.87 17.44 19.18 16.71 18.47 17-75 15.22 17.01 14-45 16.26 11.88 14.72 11.24 13.02 9-56 11.46 7.86 9-73 5-90 7-94 3 00 22.88 22.27 21 .6l 20.92 20.23 18.81 18.07 16.49 14.99 13-37 11.70 9-93 Fig. 91 represents a section of the Bourdon tube. The major axis is placed vertically to the plane of the coil. Were it placed parallel to that plane, internal pressure ^^^^^ would close up the coil instead of, as in the usual IG. 91. case, uncoiling it. This latter is the disposition adopted by the Author, as in Fig. 92, in a gauge devised by him for very DESIGNING A CCE SSORIES SE T TING CHIMNE VS. 39$ high pressures, and especially to work steadily where exposed to heavy jar, as on locomotives. A pair of corrugated disks, secured together at the edges, and receiving steam-pressure within, is a form of pressure-gauge spring which has been found useful, and many gauges are thus constructed. All spring gauges, unless constructed with ex- traordinary care, are very liable to give after a time misleading indications, and they should be occasionally tested to ascertain to what pressures the readings on the dial actually correspond. FIG. 92. THURSTON'S HIGH-PRESSURE GAUGE. Mercury-gauges, in which the pressure is measured by the height of a mercury column balancing it, are much safer than spring-gauges, but are too cumbersome for common use. All other steam-gauges are, however, referred to the mercury-gauge in standardizing them. 396 THE STEAM-BOILER. Steam-gauge connections should be so made that the in- strument may not be liable to injury by heat, either externally or internally, and so that the spring shall always have a body of comparatively cold water interposed between itself and the steam. A coil or siphon-shaped bend in the gauge-pipe is gen- erally introduced with this purpose in view : it fills up with a body of water condensed from the steam which protects the spring from injury by exposure to heat. The point of entrance of the gauge-pipe into the boiler is simply a matter of conven- ience, usually. Gauge-cocks and water-gauges should be set where they will not be affected by any foaming that may occur within the boiler; they should be as far from the furnace as is conven- ient, or their coanections should be led to a quiet part of the boiler. A foaming boiler, by deceiving the eye at the gauges, may discharge a dangerously large amount of water undetected. The Low-water Detector and Alarm is an apparatus which is in very common use to give warning should the water-level ever fall below that considered safe. It com- monly consists of a vertical tube closed at the top by a fusible plug, or by a valve actuated by a rod having a different coefficient of ex- pansion from the tube itself. The tube com- municates at the lower end with the water- space of the boiler. It ordinarily stands full of water; but should the water-level fall below its. lower end, steam displaces the water in the tube, the fusible plug melts, or the valve is FIG. 93. LOW-WATER * ...... . ALARM. opened by the difference in expansion of the tube and rod, and steam at once issues, giving warning of dan- ger. The upper end of the tube is commonly fitted with a steam-whistle, the blowing of which when the steam makes its exit insures attention. Many forms of grate-bars are used in steam-boiler furnaces, some of which are provided with interlocking devices so con- trived that all are so bound together that it is impossible for single bars to warp and twist out of shape to such an extent that they will be liable to burn. In other cases the bars are fitted so as to be all capable of vibration or rotation by the ac- DESIGNING A CCESSORIESSE TTINGCHIMNE YS. 397 tion of a single handle, and thus to permit convenient cleaning of the fires. Such grates are in very common use in anthracite- burning furnaces. Fusible plugs are inserted at convenient points in plates lia- ble to be the first to be left dry on the falling of the water- level. A leaden rivet in an upper seam or in a rivet-hole made for the purpose at the highest part of a crown-sheet is often relied upon ; but it is better to use an alloy of lower melting-point, and to make it quite large. Several small plugs are sometimes inserted in a larger plug of cast-iron properly located, the idea being to thus secure greater safety by avoid- ing the chance of a single one failing to serve its purpose. A large plug of fusible metal, projecting well above the crown- sheet or other plate in which it may be placed, and having a central rod of copper passing completely through it and pro- jecting at top and bottom, is a very excellent device. When its upper end becomes exposed the copper rod melts out of its casing and falls down out of the way, exposing clean surfaces of fusible metal, which in turn melt, and the purpose of the appa- ratus is accomplished with certainty. In some cases alloys are so altered by long exposure to heat f.hat they fail to melt when the emergency arises. It is advised by the best engineers that they be renewed frequently. An accumulation of sediment or scale sometimes prevents their working, or may permit their melting without causing egress of steam and water, as is usu- ally intended. A coating of thin scale will often sustain all the pressure coming upon it over such an opening as is left by the dropping out of the plug. The best fusible plugs consist, as a rule, of an outer shell, as in the figure, filled with a fusible metal, C, in the form of a plug extending through the shell from top to bottom. The shell should be of hard brass to insure strength, with a good thread where it screws into the plate, and a good hexagonal or square head, and durability suf- ficient to permit several fillings. The thread cut in the shell should correspond with the gas-fitters' standard. The use of such plugs FIG. 44 . -FUSIBLE PLUG. is often required by law. 398 THE STEAM-BOILER. Low temperatures can be determined by the melting-points of compositions of lead, tin, and bismuth ; and the following may be used for fusible plugs :* An alloy of i part lead, i part tin, 4 parts bismuth, melts at 94 C., 201 F. e's metal 5 3 " 8 * 100 202 Rose's metal 5 3 ' 4 8 2 3 5 I 4 5 I .. i I i .. . . 2 i ' I 3 .. | 1 ' 3 ' i 100 IOO 118.9 141.2 241 167.7 167.7 200 202 202 2 4 6 257 4 66 334 334 392 " fusi- " con- It is customary to use such compositions in making ble plugs" to be inserted in the crown-sheets or tops of nections" liable to be injured by low water, to give warning of danger, and to act as safety devices by melting when uncovered and permitting steam to issue into the furnace and flues. All marine boilers subject to the rules of the United States Treasury Department are required to have plugs of Banca tin inserted, of not less than 0.5 diameter in the smallest part.f Cylinder boilers with flues must have one in each flue, and one in the shell not less than four feet from the forward end. Fire- box boilers must have a plug in the crown-sheet. , Upright tu- bular boilers must have a plug in one of the tubes, two inches or more below the lower gauge-cock, or in the upper tube- sheet if so preferred by the inspector. Where manhole covers can be " struck up" in wrought-iron, as many of them are now often made, they are much safer, as well as lighter and more convenient of manipulation. The accom- panying figure illustrates such a construc- tion as introduced some years ago. The two guards and bolts give greatly increased security as compared with the ordinary ar- rangement of a single guard and bolt at the middle of the cover. The M'Neil manhole cover and guard represent good recent practice, as seen in Fig. 96. The opening through the shell FIG. , 95. WROUGHT-IRON MANHOLE PLATE. * Weisbach. f Regulations, 22. DESIGNING ACCESSORIES SETTING CHIMNEYS. 399 is strengthened by a wrought-iron " struck-up" ring, the section of which is L-shaped. The inner edge is faced to re- ceive the faced bearing-surface of the cover, and thus makes a steam-tight joint without requiring packing. FIG. 96. M'NEIL MANHOLE COVER. The " blow-off cock," which controls the discharge of water through the " blow-off " pipe, should never have a valve substi- tuted for it, but only a good conical cock should be used. It should be of the best of brass or bronze, and of extra strength. A valve is liable to be caused to leak by the catching of dirt or of chips between it and its seat, and thus to endanger the boiler by undetected leakage. With the cock no uncertainty can exist in regard to its being open or closed, and foreign matter caught by the plug will be cut off, or the cock will be opened an instant to wash it away. A " T " placed outside the cock and so arranged that the plug can be taken out to see whether the blow-cock leaks, and if so how much, will be found an important element of security. The " feed-valve" which controls the introduction of the feed-water into the boiler should always be a strong, well-made brass valve, of the best of metal and heavier than the customary market valves. The ordinary steam-fitter's valve and other brass-work is usually much too light, and it is often thought wise to make special patterns for boiler connections. The valve should be placed close to the boiler and the check-valve outside, and as near it as possible. Often a single valve a " screw-check" serves both purposes. It should be so placed that in case of the valve getting loose it may not pre- vent the entrance of the water into the boiler. CHAPTER X. 4 CONSTRUCTION OF STEAM-BOILERS. 186. The Methods and Processes employed in the shop in the construction of steam-boilers are usually simple, and in- capable of very great refinement. The boiler-maker receives a set of drawings from the designing engineer, which exhibit the general form and proportions of the boiler, and complete representations of all details. These drawings should include front and side elevations, and plan, together with sections taken wherever necessary to exhibit the internal arrangement and structure. All dimen- sions should be carefully marked on each sheet, and the work- men instructed to "go by the figures," as attempts to measure by scale are apt to lead to mistakes. The thickness of each sheet should be indicated, and the location, form, and size of every opening to be made in the shop. General plans are commonly made on a scale of from T \ to -J full size, ac- cording to circumstances ; but detail drawings are often all made full size. The boiler-maker often reproduces the general drawings, as well as all details, full size, on a set of large boards provided for the purpose, and, measuring all parts anew, makes sure that the originally given dimensions are all right and consis- tent with each other. The location of each sheet and its seams being thus determinable, the dimensions of the rectangu- lar, or other simple form of sheet, as it is to come from the mill, are ascertained, and if not in stock, the iron or steel is ordered. Mills will usually be able to supply sheets cut very exactly to the ultimate size and shape, and thus save great expense in cutting and fitting in the shop. Every sheet should be ordered as exactly as to size as possible, and the grade and quality should be as precisely specified in the order-list thus made. CONSTRUCTION OF STEAM-BOILERS. 4 s~^. S~\ 6 ,% oo * P DOUBLE RIVETED, CHAIN. , 4 FIG. 105*5. Where, as should always be the case, steel plates are drilled, or are punched and the holes sufficiently enlarged by counter- boring or reaming and the plates finally well annealed, no al- 412 THE STEAM-BOILER. lowance need be made for loss of strength in the metal be- tween the plates. The best makers of boilers endeavor to reduce the number of seams to a minimum, as well as to make those retained of uniform and ample strength. Double-riveted longitudinal seams are becoming constantly more common, and in some cases welding is resorted to with great success. The latter plan permits the securing of perfectly cylindrical courses or rings of plates. It seems not improbable that welding may ultimately become the usual method of making all joints. The < TRIPLE RIVETED. ZIGZAG. Hfc" > FIG. lose. lap-joints are disappearing in good designs, and the butt-joints, single and double riveted or other, are taking their place. In these cases the cover-plates, or covering strips, or straps, as they are variously called, should be cut from plate, and in such manner that the grain shall run parallel with the direction of stress. Welding, if it can be safely relied upon, offers so many ad- vantages over riveting, that there can be no question that it will in time supplant entirely the older method of uniting the parts of boilers. It has been the practice of a few makers to employ this system for many years, and the use of welded seams is slowly but steadily increasing. A good weld gives more nearly the full CONSTRUCTION OF STEAM-BOILERS. 413 strength of the iron than can any arrangement of rivets, and en- ables all risks arising from defects in workmanship peculiar to riveting, such as drifting or careless chipping and calking, and such as cold-hammering, to be avoided. It permits dispens- ing with calking entirely, and consequently the avoidance of the grooving or furrowing which so often proves dangerous. It is also possible to reroll the course or ring of welded plates, and thus to secure greater accuracy of dimension and perfection of form than could be obtained with riveted work. Welding is less likely to prove unreliable in flues than in shells of boilers ; as the steam-pressure there tends to force the parts together, rather than to separate them, as in the latter case. Great experience is in any case demanded, as well as great care and skill, in making long lines of weld, such as are required in this work. It is stated by Mr. Adamson, who has been one of the most successful makers using the process, that the metal must be of the best possible quality, and that steel containing enough carbon and other elements, to perceptibly harden it cannot be safely employed. 192. Flues and Tubes are set after the parts of the boiler are assembled, or in the construction of the tube-boxes and " connections." The flue is commonly riveted into the flanged opening cut into the two flue-sheets to receive it ; the tube is "expanded" into the tube-sheet by means of a " tube- expander," of which there are many kinds ; which tool forces out the tube into metallic and firm contact with the hole bored to receive it, and at the same time expands it a trifle on each side the sheet, and thus tightens its hold and gives it the effect of a stay, while still further insuring against leakage. Care must be taken to avoid too great expansion, as the tube- sheet is sometimes strained and weakened by excessive stretch- ing, and the tube itself is sometimes split. Properly set and expanded, a tube makes an exceedingly effective stay. A locomotive tube should safely carry 3000 pounds (1360 kgs.) and a marine boiler tube, of double the diameter, 5000 pounds, (2268 kgs.), or the full boiler-pressure ordinarily carried. For very high pressures, as now often attained with three and four cylinder " compound " engines, stay-tubes are introduced at in- 414 THE STEAM-BOILER. FIG. 106. STAYING FLAT SURFACES. tervals which are made of heavier iron, and have nuts screwed = outside to sustain the excessive pressure. Many build- CONSTRUCTION OF STEAM-BOILERS. 41$ ers prefer not to bead over the ends of the tubes, fearing that the operation may loosen them and cause leakage. The ends of the tubes are annealed before expanding them. In laying out the flue or tube sheets, the centres are located by reference to the drawings, and the outline of the hole is laid out by the dividers. For flue-sheets, a row of small holes is drilled around the circle, marking the opening to be made; the centre part is cut out, the opening trimmed and flanged, and the sheet is then ready to receive the flue. Tube-sheets are similarly laid off, a small hole drilled at each centre, and the hole then " counterbored " to the required size and the edges of the enlarged holes smoothly rounded to prevent cutting the tubes when expanded in place. Ferrules are often driven into the tube-ends, partly to give greater tightness, partly often to reduce the draught-area when, as sometimes occurs, it is too great. Staying is variously practised, and marine-boilers especially exhibit a great variety of methods. Fig. 106 illustrates a somewhat peculiar method of staying adopted in the boilers of the U. S. S. Nipsic. A set of gusset-plates is riveted to the shell, and the connection is stayed to them by means of lugs riveted to both. The long stay-rods running lengthwise of the boiler are connected to these gusset-plates. Other gusset-plates stiffen the junction of the adjacent parts of the shell above the connection. The water-spaces are stayed by riveted stay- bolts in the usual manner. Fig. 107 illustrates the staying of the heads of the boilers of the U. S. S. Monadnock. Fig. 108 exhibits the method of staying adopted in the boilers of the U. S. S. Miantonomoh, which differs from the more common practice in the manner of fastening the heads of the stay-rods. The eyes to which the rods are attached at the end are made fast to the shell by means of bolts passing through the plates and held by nuts on the outside. The usual method of securing the stay-rods and "crow- feet" in marine-boiler construction is seen in Fig. 109, as prac- tised in the boilers of the U. S. S. Terror.* A set of l-irons is * Shock on Boilers. 416 THE STEAM-BOILER. o o c| o 6 c o o n FIG. 107. STAYING FLAT SURFACES. TOT FIG. 108. STAYING FLAT SURFACES. CONSTRUCTION OF STEAM-BOILERS, 417 riveted on the inside of the shell which gives an anchorage for the crow-feet to which the stay-rods lead, the connections being made by bolts in shear. Fig. 1 10 represents the method of staying adopted in the boilers of the S.S. Lord of the Isles to secure the heads. 193. Chipping and Calking seams after they are riveted up is a process which is relied upon to insure against leak- age. The workman, with hammer and chisel chips the edge of the lap smoothly from end to end sometimes only on the outside, but often, if accessible, on the inside, and thus ob- taining a smooth edge; then drives a blunt " calking-tool " against it, and thus expands the metal against the opposite plate, and securing metallic contact closes every leak. FIG. 109. STAYING FLAT SURFACES. The process of chipping is a dangerous one, and the score produced by the chisel as its corner marks the under sheet has been often known to lead to the formation of a groove or a crack, and later to explosion. Planing the edge before final assembling and riveting up is much to be preferred. The use of the calking-tool has sometimes resulted in similar injury; and split-calking, which consists in driving the edge of a chisel into the edge of the sheet and thus splitting it slightly and ex- panding the lower part against the adjacent sheet, is advised as a safer plan. The Connery system, regarded by many as very much better than either of the preceding, is similar to the last ; 27 4 i8 THE STEAM-BOILER. but a round-nosed tool is employed which makes a smooth, semi-cylindrical groove instead of a sharp crack. The expand- ing effect is also felt further back under the lap, the seam is thus tighter and more permanently so, and the use of the tool is not liable to injure the lower sheet. 9 - 9 a L D 6 v^ b o - 6 b ( f 9 cx X) o a o o O C bo o QL ,o" o 'o FIG. no. STAYING FLAT SURFACES. In European practice, even where the builders have not gone so far as to adopt the system of calking with a round- nosed tool, they have very generally substituted for the old and dangerous form of calking-tool a wider-edged tool called the fullering-tool, and the specifications usually prescribe ful- lered seams as well as planed edges. No calking-tool should ever be permitted to be used which has a sharp edge or corner that may by careless handling be made to cut, or even mark, the sheet at the edge of the lap. CONSTRUCTION OF STEAM-BOILERS. 419 Butts are calked with a tool which makes a depression on each side the line of junction, expanding the two sides into contact and making that line tight. It is customary with some makers to calk" around the heads of rivets, and when found leaking this process is resorted to as a remedy. Calking should not be done while the boiler is under pressure, and should be very carefully done at all times. The " concave" calking, so-called, is exhibited in the ac- companying figure, which shows the difference in the effect of the new and old methods upon the sheet. The plate is shown, as bent after the operation, to determine the extent to which injury of the plate may have been incurred. On the left is seen the action of the concave system, the effect of the tool being somewhat more marked than is customary, but perfectly representing samples in possession of the Author. On bending FIG. in. CONCAVE AND COMMON CALKING. down the sheet, as shown, it is seen to be quite sound, and en- tirely unaffected by the action of the tool. On the other hand, the ordinary tool, as, commonly used and as illustrated on the right in the same engraving, is almost invariably found to pro- duce a slight indentation of the sheet along the edge of the lap, and then to cause a tendency to crack when the sheet is flexed by the changing temperatures of the boiler and accompanying strains. By this method either the edge of the tool or the edge of the lap is liable to produce a dangerous groove, at once or after corrosion has progressed somewhat. The more rational system gives a broad band of metallic contact between the two sheets, and makes the joint tight without injury to the structure.* In using the " round-nosed " calking tool, the following directions should be observed : * Journal Franklin Institute, 1874. 42O THE STEAM-BOILER. Chip or plane the plates to an angle of about 1 10, seeing that the seams are perfectly close inside and outside. Apply the tool in the usual manner, forming a channel, and always keeping the burr between the tool and plate, and calk until found solid, smooth, and brought to a feathered edge, free from pin-holes. Do not cut off the burr, as it may injure the under plate. Upon testing, if pin-holes are found, apply the same tool as before, until made perfectly tight. The convex tool should taper about two inches from the point, which is about half an inch wide, otherwise perfectly straight, save when un- avoidable, and ground to a radius that will finish the concave channel to about one half the bevelled edge ; if too wide it will thicken the edge ; if too small it will wedge it off. 194. Assembling is the process of fitting, and finally rivet- ing permanently together, all the details and members, which, separately constructed, are finally brought together to make the complete structure. The shell, the tubes and their tube-sheets, with the front and back connections and the steam-drum, are the several principal parts thus dealt with. The shell is first set in position and riveted up ; the flues or tubes and connec- tions are next finished, placed in their proper position within the shell, and riveted into place ; the drum or dome is attached ; and, finally, all minor details are added, and the boiler is ready for examination, test, and finally for calking and " finishing." 195. Inspection of the work should take place, not only when the boiler is reported completed, but should be kept up constantly throughout the whole period of construction. Where extensive contracts are filled, it is usual for the pur- chaser to have a skilled inspector constantly employed to see that the material introduced is in accordance with the contract ; that the construction is precisely what is called for by the drawings and specification, the work well done, and the whole properly put together. A special inspection is usually provided for, to take place at completion and before acceptance. At this time the inspector very carefully and minutely examines the boiler inside and out, overhauling the braces and stays, their connections with the shell and other parts, and their welds and fitted parts ; he observes the character of the riveting, the method of attach- CONSTRUCTION OF STEAM-BOILERS. 421 ment of the various accessory members ; the valves, cocks, and gauges, if attached ; and every detail, great or small, comparing all with the specifications and drawings, and noting any defect, either in general construction or in workmanship. Any defec- tive material or bad work is condemned, and must be replaced by good material and with better workmanship. The final in- spection proving satisfactory, the boiler is tested. The work of inspection is often, perhaps in good practice almost invari- ably, provided for by specifications attached to the contract and forming part of it. Such specifications direct every step FIG. us. DEFECTIVE PINNING. from the preliminary visual examination of the material when received, or even sometimes the watching of its manufacture in the mill, through all intermediate tests of iron, steel, or fin- ished parts, to the final examination and pressure-tests of the completed structure, and the method of recording measure- ments and tabulating them and of making the reports for which they furnish the texts. The defects sometimes revealed by inspection are flaws in the iron in parts not readily seen ; inferior iron in concealed portions of the boiler ; cracked flanges or laps, either in lines 422 THE STEAM-BOILER. from rivet-hole to rivet-hole, or from the rivet to the edge of the plate; " unfair" or "half-blind " rivet-holes; weak and narrow FIG. 113. CORRECT CONSTRUCTION. laps ; injury by calking or by chipping ; laps not well closed ; narrow water-spaces ; injured tube-ends ; loose and badly set and fitted braces and pins ; omitted stays or braces ; and minor de- fects. To ascertain whether a sheet is of the right thickness, a small hole is sometimes drilled at the suspected point. The connecting of stays should be con- demned if not as in Fig. 113; they are FIG. 115. FIG. 116. sometimes found as dangerous as the case illustrated in Fig. 112. The junctions of plates meeting at the intersections of seams are given the shape seen in the accompanying figures, the first showing the junction ot three sheets as where the longitudinal CONSTRUCTION OF STEAM-BOILERS. 423 and transverse seams meet in overlapping courses, the middle plate being thinned to give proper bearing. 196. Testing Boilers, when under inspection, at the time of acceptance, usually consists simply in filling them with cold water, applying a pump, and subjecting them to a pressure ex- ceeding that at which they are to be used. It is better to warm the water to the boiling-point nearly, as the pressure then affects a boiler more nearly as under the conditions of actual use. The temperature should not exceed the boiling-point under atmospheric pressure, as an explosion or serious rupture might follow the revelation of a defect a result which has actually occurred in more than one instance. The pressure should be applied very carefully and steadily, and the steam-gauge watched to detect any sudden drop of pressure which would indicate yielding. The breaking of a brace is usually revealed by a sharp report. Gradual yielding is shown by a cessation of rise of pressure, or by its falling off. Leaks show themselves whenever seams have not been made tight, and are traced out by trickling drops or running streams, and are marked with chalk or a pencil for later calking. The connection of large steam-drums or domes with the shell are apt to show weakness, and should be carefully watched as pres- sure rises. The pressure is finally relieved, the boiler emptied, all leaks stopped, and the test repeated if the result is not satisfactory. In filling boilers, care should be taken to run them full of water to the very top of the safety-valve case ; as any con- fined air might make trouble. Testing a boiler by filling it to the safety-valve with cold water and then starting a fire is advised by some writers as a very safe method ; since the pressure can be run up, if the boiler is tight, to any desired point without exceeding the boil- ing-point under atmospheric pressure, and thus without danger in case of a weak spot revealing itself. The temperature should not be allowed to go higher than that limit, as a boiler filled with water at the temperature due a high steam-pressure is more dangerous than when under steam at the same pressure. 197. Sectional Boilers are constructed, so far as composed 424 THE Sl^EAM-BOILER. of riveted work, precisely as are other boilers; but they are usually constructed mainly of nests of tubes, connected by cast or forged " headers," which are fitted together with machined or " faced " joints, and held by bolts. Each header has its tube-end either screwed or expanded into it, or in some cases simply slipped into place and made tight by packing. In these boilers the special precautions to be observed are to see that the joints are well made and permanently tight. The facing off should be so perfectly done that a thin coat of red-lead paint, at most, should be all that is necessary to make the joints tight against any steam-pressure. The best makers do not even use this precaution. 198. Transportation and Delivery are effected usually by the maker. Small boilers are simply loaded on strong wagons and carted off to the place at which they are to be delivered. Heavier boilers often require specially constructed vehicles, and the very cumbersome structures often seen where marine flue-boilers are employed are sometimes transported on skids and rolls as houses are moved. In hoisting boilers to place them on the vehicles on which they are to be transported, or in setting them, great care is re- quired to see that they are so handled as to introduce no risk of straining them. The best method of slinging them should be carefully studied ; the tackles used should be of more than ample strength, and no risk of sudden fall or change of position should be taken. CHAPTER XL SPECIFICATIONS AND CONTRACTS. 199. The Purpose of Specification and Contract is to present a perfectly definite and exact statement of the charac- ter and extent of the work to be done : the forms and propor- tions of details, the time to be allowed in construction, and the amount and method of payments to be made by the purchaser. These documents are always prepared when any work of im- portance is to be done, and are signed by the two contracting parties, or by authorized representatives or agents. They con- sist of a formal contract, or statement of obligation, with spe- cifications describing all work to be done, and, where the case permits, of a set of drawings of everything to be made, in full and in detail ; which drawings form a part of the contract as well as of the specification. 200. The Contract is an agreement in writing by which the one party to the bargain agrees to do a certain exactly specified work, and the other to make compensation in a cer- tain stated manner, and often with provision of penalties for failure to fulfil the terms of the contract. This agreement rep- resents as exactly and clearly as possible the mutual under- standing between the contracting parties in regard to all busi- ness relations involved in the performance of the work to be done. Everything needed to make the understanding definite is embodied in the contract. Advertisements, proposals, and preliminary agreements are often taken as parts of the contract, as well as drawings and specifications. These papers are made out in duplicate, and are signed by both sides, each retaining a copy. Where many interests are involved, representatives of each should sign and each should retain a copy. The essential conditions of a legal contract are that it shall be definite as to the obligations of both sides ; that the com- 426 THE STEAM-BOILER. pensation be stated and valid ; that mutual consent be secured by voluntary act ; and that the parties in interest shall all sign of their own free will, and with a full understanding of the ob- ligation assumed. The mentally or legally incompetent cannot take part in any contract while such disability exists. The agreement is interpreted by its own reading, and the private intentions of the makers have no weight, nor have their mental reservations. The document is its own commentary and proof. Interpretations of terms are settled by the customary and habit- ual use of the term, and if technical, the word or phrase must be taken as having the meaning usual in the business. Obscur- ity of wording may vitiate the agreement. The duty of each party to the contract is to be separately defined and described. The contractor is bound to perform a specified work in a satisfactory manner, to complete it in a speci- fied time, and to accept a stated compensation, made in a man- ner and as to time clearly prescribed. The other party to the bargain is bound to make full compensation to the extent and in a manner stated, to aid in all proper ways in the carrying of the agreement into effect, and to at all times meet the con- tractor in a fair and helpful spirit. The work is the contractor's until paid for as prescribed by contract ; when so paid for, it becomes the property of the employer, who only then carries any risks on it, unless otherwise provided in the agreement. Penalties incurred by non-fulfilment of the terms of the con- tract are of the nature of a standing debt, and may be similarly held and collected. Non-fulfilment of an agreement by the one side does not necessarily give freedom from obligation to the other, except where such failure on the one side may interfere positively with the operations of the other. In statements of time, a day ends at midnight. No time being stated, the work must be done within what may be decided to be a " reasona- ble" period. Action at law must usually be entered against one guilty of breach of contract within six years ; but the Statute of Limita- tions varies in different states. A guaranty and bond is some- times exacted to insure the completion of the contract ; but this is usual only in public work. SPECIFICATIONS AND CONTRACTS. 427 201. The Form of Specification is such that every descrip- tive portion of the contract may be embodied in it, in a sys- tematic manner, in proper relative order, and in thoroughly definite shape. The character of materials to be employed ; the method of working them ; their final form ; the quality of the workmanship ; all instructions that may be needed in regard to the performance of the work are given in the specifications. Since this document is that on which the intending contractor makes his offer, it must be absolutely complete, and as concise as possible. No detail should be omitted, and nothing should be left to be assumed or disputed. 202. Specifications for Steam-boilers should not only com- ply with all the legal conditions of a sound contract, but should represent the best known practice of the time. They should be prepared by the designing engineer, and, with all drawings, advertisements, blank proposals, agreements, and intended form of contract, laid before the employer for careful discussion and final approval before any step is taken in the receiving of bids or the acceptance of proposals. They should include a full de- scription of the boiler to be built, with complete drawings, gen- eral and in detail ; statements of the kind, make, and quality of the iron or steel to be used, the character of the workmanship to be demanded, the kind of tests to be applied, and every con- dition having a bearing on the subject. 203. Sample Specifications are as follows, illustrating stan- dard practice in common forms of boiler-work. The first* is that of the tubular boiler already illustrated in 15. SPECIFICATION FOR A HORIZONTAL TUBULAR STEAM-BOILER. Type. Boiler to be of the horizontal tubular type, with overhanging front and doors complete. Dimensions. Boiler to be 16 feet 3 inches long outside, and 60 inches in diameter. Tube-heads to be 15 feet apart outside. Steam-dome to be 33 inches in diameter and 33 inches high. Tubes How Set and Fastened. Boiler to contain 66 best lap- welded tubes, 3 inches in diameter by 15 feet long, set in vertical and * See Am. Engineer, Nov. 1883: Specifications by the Hartford Inspection and Insurance Co. 428 THE STEAM-BOILER. horizontal rows, with a space between them, vertically and horizontally, of not less than one inch (i"), except the central vertical space, which is to be two inches (2"), as shown in drawing. Tubes to be set sufficiently high from bottom of boiler to give room for man-hole and access to boiler underneath tubes, as shown in drawing. No tube to be nearer than 3 inches to shell of boiler. Holes through heads to be neatly chamfered off. All tubes to be set by a Dudgeon expander, and slightly flared at the front end, but turned over or beaded down at back end. FOR IRON PLATES. Quality and Thickness of Iron Plates. Shell plates to be of an inch thick, of the best C. H. No. i iron, with brand, tensile strength, and name of maker plainly stamped on each plate. Tensile strength to be not less than 50,000 pounds per square inch of section, with a good per- centage of ductility. Heads to be of an inch thick, of the best C. H. No. i flange-iron. FOR STEEL. Steel />/#/.?. Shell-plates to be of an inch thick, of homogene- ous steel of uniform quality, having a tensile strength of not less than 60,000 pounds per square inch of section, nor more than 65,000 pounds with 45 per cent ductility, as indicated by the contraction of area at point of fracture under test. Name of maker, brand, and tensile strength to be plainly stamped on each plate. Heads to be of same quality as plates of shell in all particulars, of an inch thick. Flanges. All flanges to be turned in a neat manner to an internal ra- dius of not less than two inches (2"), and to be clear of cracks, checks, or flaws. Riveting. Boiler to be riveted with f-inch rivets throughout. All girth seams to be single-riveted. All horizontal seams and flange of dome at junction of shell of boiler to be double staggered riveted. Rivet- holes to be punched or drilled so as to come fair in construction. No drift -pin to be used in construction of boiler. A reamer to be used in all cases to bring the holes " fair." Braces. There are to be twenty-two (22) braces in boiler seven (7) on each head above the tubes, and six (6) on rear head and two (2) on front head below the tubes, as shown in drawing, none of which are to be less than three (3) feet long. Braces to be made of best round iron, of one (i) inch in diameter, and of single lengths. How Set and Fastened. There are to be five (5) lengths of T-iron, four (4) inches broad and one half (i) inch thick, Three (3) being eight (8) inches long, Two (2) being fourteen (14) inches long, placed radially, and riveted with f-inch rivets to each head above the tubes, as shown SPE CIFICA TIONS A .VD COX 7 7tA C 7\S, 429 in drawing. There are to be four (4) lengths of T-iron, four (4) inches broad and one half () inch thick, two (2) being six (6) inches long and two (2) being twelve (12) inches long, placed radially, and riv- eted on rear head below the tubes, also two (2) lengths, six (6) inches long, riveted on front head below the tubes, each side of man-hole, as shown in drawing. The holes for fastening the braces to these radial brace-bars are all to be drilled. The braces are to be fastened with suit- able jaws and turned pins or bolts, so as to realize strength equal to inch- round iron. Braces to be set as shown in drawing, and to bear uniform tension, and to be fastened on shell of boiler with two (2) f-inch rivets each. Dome. Dome to be constructed of same quality of iron or steel as heads of boiler, of an inch thick, and head to be of same quality of iron or steel as heads of boiler, of an inch thick. Dome-head to be braced with six (6) f-inch braces, reaching from head well down on shell, as shown in drawing, and fastened at each head with two (2) f- inch rivets. Opening from boiler into dome to be inches in diam- eter. There are to be two pieces of T-iron riveted on to outside of boiler shell, within the dome girthwise, one on each side of opening, as shown in drawing ; also suitable drip holes to be cut at junction of shell and dome. Man-holes. .Boiler to have two man-holes, each eleven (11) inches by fifteen (15) inches, with strong internal frames (as shown in drawing), and suitable plates, yokes, and bolts, the proportions of the whole such as will make them as strong as any other section of the shell of like area. One to be placed in front head underneath the tubes, and one to be placed on shell of boiler, as shown in drawing. Hand-holes. Boiler to have one hand-hole, with suitable plate. yoke, and bolt, located in rear head below the tubes, as shown in the drawing. Nozzles. Boiler to have two cast-iron nozzles, four (4) inches in- ternal diameter, one for steam and the other for safety-valve connections, securely riveted to head of dome, as shown in drawing. Wall-plates. Boiler to have four cast lugs, two on each side, securely riveted in place, each twelve (12) inches long, with a projection of nine (9) inches from the boiler, the rear lugs each to rest on three transverse rollers, one inch in diameter, which are to rest on suitable cast-iron wall-plates, as shown in drawing, front lugs to rest on suitable wall-plates, without rollers. Blow-out. For blow-out connection, one plate, one half inch thick, to be secured with rivets driven flush on inside of the shell, and tapped to receive a two (2) inch blow-pipe. Front. Boiler to be provided with cast-iron front and all the requisite doors and fastenings for facility of access to tubes, furnace, and 430 THE STEAM-BOILER. ash-pit. All to be of substantial construction, neat appearance, and close-fitting. Buckstaves Grate-bars. Boiler to be provided with buckstaves ; also all bolts, rods, nuts, and washers, anchor-bolts to ex- tend in setting beyond bridge-wall ; also bearer and grate bars (pattern to be selected); also cast-iron door, to be at least two (2) feet by three (3) feet and provided with liner plate, for back tube-door and door fifteen inches by fifteen inches for flue for side or rear end. Fittings. Boiler to be provided with one safety-valve, inches in diameter, one inch steam-gauge of standard make, three gauge-cocks properly located, also one glass water-gauge, a two-inch open-way blow-valve, and feed and check valves, each one and one quarter inch. Feed to be introduced into front head of boiler, above tubes. Feed-pipe to extend well back towards rear of boiler, across tubes, and turn down between tubes and shell, as shown in drawing. Fusible Plug. Boiler to be provided with a fusible plug so located that its centre shall be two inches above upper row of tubes at back end. Damper. Boiler to be provided with a damper with suitable hand attachments, easily accessible at the front of the boiler, damper to be fitted to the throat of the smoke-arch, as near as practicable to the tube-openings, and of area equal to the cross-section of all the tubes. The size and description of parts to conform substantially to the de- tails of the accompanying plan. All the above to be delivered at and all the material and workmanship to be subjected to inspection and approval. The following are specifications for a marine flue-boiler for SPECIFICATION FOR FLUE-BOILER. There is to be one boiler of the flue and return-tube type, of the fol- lowing general dimensions : Extreme length 13 feet. Diameter of shell 8 " Width of front 8 " Diameter of steam-chimne)' 1 5 " Diameter of steam-chimney lining 3 " Height of steam-chimney above shell 5 " *For very elaborate and complete naval specifications, see Shock's "Treat- ise on Steam Boilers." New York: D. Van Nostrand. PECIFICATIONS AND CONTRACTS. 431 There are to be two furnaces, each forty-two inches wide and six feet long. Bottom of furnace-legs to drop six inches below shell. Bridge-wall seven inches thick. Combustion-chamber of back furnaces in one twenty- four inches deep. Back connection twenty-eight inches deep. Front connection twenty-eight inches deep. Furnace-crowns to be a semi- circle. To have two 16 inch, two 12-inch, two n-inch, and four 9-inch direct flues, all fifteen inches long, and ninety return tubes, seven feet ten inches long. All the horizontal shell-seams to be double- riveted, also the bottom course of steam-chimney where riveted to shell and vertical seams. Back part of furnace, where connected to shell, to be double-riveted one third distance around, the remainder of riveting about the boiler to be single. Thickness of Plates. To be as follows : tube-sheets T \, shell of boiler (round part) f, bottom course of steam-chimney T 7 ^, inside liningof steam- chimney f , the balance of the iron in the boiler to be -&, except bottoms of furnace-legs, which are to be f. Material. Furnaces to be of steel, and the balance of the iron in the boiler to be of the best C. H. No. i. and flange iron, and all stamped 50,000 pounds T. S. All flat surfaces to be braced 6^-inch centres, with hot sockets wherever practicable. Boiler to be fitted with man-hole in top of shell, also in front in the spandrels over furnace-crowns. Openings to be surrounded by a wrought- iron ring 2| inches wide by f inch thick, riveted to shell. Hand-holes to be cut in legs and every part where necessary to facilitate cleaning. Man and hand holes to be furnished with plates and bolts complete. Front connection to be fitted with wrought-iron doors, fitted with wrought-iron linings, and fitted with two registers. Furnace-doors to be of wrought-iron with cast-iron perforated linings, to be fitted with wrought-iron hinges, latches, etc., complete. A suitable opening with door to be provided in back connection. Grates and Bearers. Boiler to set on three cast-iron legs under fur- naces running the whole length, about 12 inches high, and fitted with supports for grate-bar bearers. Grates to be 6 feet long in two lengths. Ash-pans of cast-iron to be laid in brick and cement. Back ends of legs to be closed in with No. 10 sheet-iron. Shell of boiler to rest on a cast-iron saddle in two halves firmly bolted. Test. The boiler before being hoisted into the vessel is to be sub- jected to a hydrostatic pressure of 100 pounds per square inch. Boiler Connections. Smoke-pipe 36 inches diameter, and to extend 1 6 feet above top of steam-chimney, to be made of No. 12 iron, to be finished with angle-iron top, bead-iron joints, six chain-stays and damper, arranged to be operated from the fire-room. Lower part of smoke-pipe 432 THE STEAM-BOILER. to be bolted to the steam -chimney, the inside lining being carried up for this purpose. Chain-stays to be provided with turn buckles to take up the slack. Steam-chimney to be encased with No. 16 sheet-iron and fitted with a stopping-cap in two halves. A chamber of cast-iron is to be bolted to the steam-chimney, containing the safety-valve and stop valve, each to be five inches in diameter, with top of trumpet shape. Surface and bottom blows to be provided with screw stop-valve for the former and cock for the latter, secured on the boiler. Blows to be led out of the vessel below the water-line through a suitable valve. There is to be a feed-valve on each side of boiler in front, in con- nection with check-valve, one to be for the donkey and the other for the main feed-pumps, both to be of composition and 2 inches diameter. Gauge-cocks and glass water-gauge to be placed on a stand-pipe, con- nected to the boiler. Boiler to be covered with i^-inch felt, canvased and painted, felt to be secured with necessary bands around steam-chim- ney. Steam Pump To be an approved steam-pump with 2^-inch water- plunger, and fitted with hand-gear. To be connected with necessary receiving-pipes from bilge and sea cock, and delivery-pipes to boiler, over- board and for fire hose, each branch to be fitted with a proper screw- valve. Exhaust-pipe to lead overboard, awash with water-line ; all the donkey pump-pipes to be of wrought-iron, galvanized. The following is a general proposal-specification for " sec- tional boilers," purposely left somewhat elastic to admit all bidders : * SECTIONAL STEAM-BOILERS. Boilers. Proposals will be received for two (2) sectional or water- tube steam-boilers of nine hundred (900) superficial feet of heating-sur- face each, or eighteen hundred (1800) superficial feet of heating in the aggregate for both boilers. Details. The proposals must be for the two (2) steam-boilers complete with cast-iron fronts, grate bars and bearers, ash-pit and side doors and frames, steam and water gauges, check and blow-off valves, safety-valves of the pop pattern, smoke connection for chimney, damper and rods, and a steam main connected with the steam drums of the two boilers, together with all bolts, beam-columns and materials necessary for the proper erection of said boilers upon the grounds of the gas company in the city of Cincinnati. * Issued by the Cincinnati Gas Co., as prepared by Mr. J. W. Hill, 1883. SPECIFICATIONS AND CONTRACTS. 433 Erection. The proposals must embrace the construction, erection, and trimming of said boilers complete, excepting connection of steam-main with company's steam-pipe. The contractors to turn over the plants to the company ready for use. Tubes. The tubes in the boilers shall be lap-welded, of three and one half (3.5) inches, or four (4) inches, external diameter (at the option of the contractor), of such length and arrangement in connection with steam and water drums as may seem proper to the contractor. Steam Drums. The steam-drums shall be twenty-eight (28) inches diameter, of Otis or equivalent soft steel plates, of a tensile strength of seventy thousand (70,000) pounds per square inch of section, of three- eighths (.375) inch thickness, with double-riveted longitudinal seams, and furnished with heads corresponding in quality and strength with the steel shell. Steam Mains. The steam-mains shall be eighteen (18) inches diam- eter, of Otis or equivalent steel plates, of a tensile strength of seventy thousand (70,000) pounds per square inch of section, of one quarter (.25) inch thickness, with double-riveted longitudinal seams, and with heads corresponding in quality and strength with the steel shell. Water Drums. The water-drums may be of cast-iron or wrought- iron, at the option of the contractor, of sixteen (16) inches diameter, and shall be of same relative strength as the steam-drums. Sample Joint. Each proposal shall be accompanied by a sample joint, such as will be used in connecting the tubes to the headers, or to the steam and water pumps ; and shall contain a detailed schedule (writ- ten or printed) of all the material dimensions of parts subject to strain, (pressure) and shall be accompanied by a scale-drawing [one and one half (1.5) inches to the foot] of front elevation, transverse and longi- tudinal sections, and plan of boilers set in brick-work ready for smoke connections with chimney. Chimney. The company will furnish a brick chimney, properly located, octagonal in form, of an internal cross-section of twelve (12) superficial feet, increasing gradually in internal diameter from bottom to top, and ninety (90) feet six (6) inches high from level of boiler-house floor. Heating Surface. The proposals must state exact heating-surface, measured upon inner diameter of tubes, and outer diameter of steam- drums (or steam and water drums). Grate Surface. Grate-surface and area of cross-section of smallest throat through which the hot gases must pass to chimney, and area of cross-section of smoke connection with chimney to be stated. Smoke Holes. (The openings one upon either side of stack to receive the smoke connections will have an area of six (6) superficial feet each, and will be two (2) feet wide horizontal diameter, and three and forty- 28 tf& 0 Min. " ) 1 18 in. STANDARD P. R. R. CLASS " P" PASSENGER ENGINE WITH TENDER. Boiler material, Steel. Thickness of boiler-sheets, dome T 5 ^- in. " " barrel, and outside fire-box, . . in. Thickness of boiler-sheets, slope, roof, waist, and smoke-box, ^ in. Max. internal diameter of boiler, j. w t $ 5^ * n ' Min. ...... f } 53 in> Height to centre of boiler from top of rail 86 in. No. of tubes, . ........ 240. Inside diameter of tubes, . . . . " . . . . if in. Outside " " 2 in. Tube material, Wrought-iron. Length of tubes between tube-sheets, .... I 3TV in > External heating surface of tubes, . . . 1,365.81 sq. ft. Fire-area through tubes, ....... 4 sq. ft. Length of fire-box at bottom (inside) 9 ft. n| in. Width " ' .... 3 ft. 5f in. Height of crown-sheet above top of grate, centre of fire-box, 3 ft. 10 in. Inside fire-box material Steel. Thickness of inside fire-box sheets, sides, . i in. " front, back, and crown, 1 . . T 5 ^ in. Thickness of tube-sheets, in. Tube-sheet material, Steel. 436 THE STEAM-BOILER. Heating-surface of fire-box, . . 164.39 sq. ft. Total heating-surface, i,53- 2 sq. ft. Fire-grate area, . , . 34.8 sq. ft. Diameter of smoke-stack (straight), . 18 in. Height of stack above top of rail, . ., , 15 ft. o in. 204. Quality of Material and methods of test are often specified very minutely, and are sometimes settled by legal provisions. Thus the British " Admiralty" issue the following requirements, other than the ordinary tensile tests, for test of irons: Samples of B. B. iron i inch (2.54 centimetres) thick are to bend cold, without fracture, to an angle of 15 with the grain and 5 across the grain; -J inch (1.27 centimetres) plates, 35 and 15 respectively; T 3 inch (0.48 centimetre) and under must bend 90 and 40. When hot, plates I inch (2.54 centimetres) and under must bend 125 with and 90 across the grain. For B. iron, the requirements are : THICKNESS. ANGLE. ANGLE. Inches. Centimetres. With the grain. Across the grain, i 2.54 10 5 i 1.27 30 10 T 8 ff 0.48 and under. 75 30 Test-pieces to be 4 feet (1.22 metres) long with the grain and full width of plate across the grain. The plate should be bent from 3 to 6 inches (7.62 to 15.24 centimetres) from the edge. The Admiralty tests for steel are the following when selecting mild-steel ship-plates : Tenacity from 26 to 30 tons per square inch (4100 to 4700 kilogrammes per square centimetre). Extension at least 20 per cent in a length of 8 inches (19.3 centimetres). Longitudinal strips planed down, i j- inches (3.8 centimetres) wide, heated to low cherry-red, cooled in water 82 Fahn (28 Cent.), must bend, in the press, to a curve of radius equal to one and a half times the thickness. Plates must be free from lamination and injurious surface defects. One plate in every fifty in any invoice is to be tested. SPECIFICATIONS AND CONTRACTS. 43? Test-pieces to be 8 inches (20.32 centimetres) long, or more, and parallel. Weight is estimated at forty pounds per square foot for one inch thick, with a variation allowable of 5 per cent (lighter weight only) on plates of half inch thick or thicker. The same specifications apply to bulb, bar, and angle steel. Lloyd's rules allow for one ton higher tenacity and one half the bend specified by the Admiralty. Masts and yards are to be made of iron having a tenacity of 20 tons per square inch (3150 kilogrammes per square centimetre). In working, all plates and bars are to be bent cold when possible, and heating only resorted to when unavoidable. All parts that have been heated must be annealed as a whole, if possible, and if not, a little at a time. When necessary, long pieces may be made up of shorter ones with butted joints shifted and strapped securely. No pieces failing in the working can be used, but samples must be cut from them and forwarded to the Admiralty for examination. Work must be finished above a black heat. Hammering is objected to, and the hydraulic press used for bending when practicable. An American railroad makes the following specifications for materials supplied to the repair-shops : Specifications for Common Bar Iron. Grain. To be uni- form and fibrous, rather than granular in texture. Workman- ship. All bars to be smoothly rolled and to be accurately gauged to size ordered. Tensile Strength. To average 55,000 pounds per square inch (3,867 kilogrammes per square centi- metre), and no iron to be received less than 50,000 pounds to square inch (3,515 kilogrammes per square centimetre). Work- ing Test. A three-quarter-inch bar bent double, cold, to show no fracture ; the same bar, heated, to be bent and also to be drawn to a point showing no tendency to " red-shortness." Specifications for Stay-bolt Iron. Grain. To be uniform and of a fibrous nature. Iron to be soft and easily worked. Tensile Strength. To be 60,000 pounds to the square inch (4218 kilogrammes per square centimetre). Working Test. A bar three-quarter inch diameter to be bent cold, showing no flaw ; a piece of same diameter, having thread cut on it, may 438 THE STEAM-BOILER. show opening when bent double, cold, but such opening should not extend more than one eighth of an inch in depth. When put into the boiler the metal should not become brittle when hammered down to form a head. 205. The Duties of the Inspector are such as demand the utmost care, considerable skill, and a large amount of experience, together with a good judgment and absolute conscientiousness. He must also be a man of sufficient strength of character to do his duty by his employers, whatever influences may be brought to bear upon him to induce him to pass work or material which does not fully comply with the specification. He is expected to examine all material with a view to the determination, both of its full compliance with the terms of the specification and contract, and of its general fitness for the work. The first step in inspection is a careful measurement of the piece offered for examination, and a comparison with the draw- ing, model, pattern, or template, to ascertain if it is made exactly to size. Exact workmanship is often secured by a system of standard gauges. This is especially the case where machines are made in large numbers. The modern method of manufacturing machinery for the market compels the adaptation of special tools to the making of special parts of the machines, and the appropriation of a certain portion of the establishment to the production of each of these pieces, while the assembling of the parts to make the complete machine takes place in a room set apart for that purpose. But this plan makes it necessary that every individual piece of any one kind shall fit every individual piece of a certain other kind without expenditure of time and labor in adapting each to the other. This requirement, in turn, makes it necessary that every piece, and every face and angle, and every hole and every pin in every piece, shall be made precisely of this standard size, without comparison with the part with which it is to be paired ; and this last condition compels the construction of gauges giving the exact size to which the workman or the machine must bring each dimension. Sizes being found right, the quality of the material is SPECIFICATIONS AND CONTRACTS. 439 determined by examination and test ; defective welds, lamina- tion, and cracks are found and condemned. A blow with a hammer often reveals unsoundness, and a laminated plate may be detected by suspending it and tapping it all over. If the defect appears on the surface, the sheet may be supported by the corners in the horizontal position, and water poured on it at the line indicating lamination, and then tapping it with a hammer. The liquid will work into the sheet, lifting the surface lamina and revealing the extent of the defect. CHAPTER XII. THE MANAGEMENT AND CARE OF BOILERS. 206. The Management of Steam Boilers, it may be stated generally, demands in the highest degree care, conscientious- ness, and uninterrupted vigilance. The value of the property entrusted to the attendants is so great and the consequences of ignorance or neglect in operation are so serious, and may be so disastrous, that no possible excuse can be given for negli- gence on the part of the proprietor or his responsible repre- sentative, in securing intelligent, experienced, and trustworthy attendants, or on the part of the attendants, whether engineer in charge, fireman (" stoker"), or water-tender, in the manage- ment of the boiler. The care demanded, in ordinary working, to keep a full sup- ply t>f water, to preserve the fires in their most effective condi- tion, to keep an even steam-pressure, an ample and unintermit- tent supply of steam, is such as tries the best of men ; but, added to this, it is imperative that the responsible man in charge of boilers have that presence of mind and readiness in action and promptness in expedients, in time of accident or of emergency, which is hardly less necessary than on the battlefield. In still further addition to these requirements, any person taking charge of boilers must understand so much of the trades of the boiler- maker and the machinist that he can if necessary make minor repairs, reconstruct his feed-apparatus, and refit the valves. He must know something of the nature and of the peculiar methods of combustion of all ordinary fuels, and enough of the principles of combustion to be able to realize the waste that may follow the introduction of an excess of air on the one hand or the produc- tion of incomplete combustion on the other, and enough of the nature and dangers of sediment and incrustation to understand the necessity of adopting the usual expedients for prevention. THE MANAGEMENT AND CARE OF BOILERS. 44! He should know how to adjust the safety-valve, and should un- derstand its office and the liability to accident coming of its maladjustment or neglect. Intelligence, experience, and conscientiousness are the best and only real insurance against accident. 207. Starting Fires is an art which is not always familiar to even experienced firemen. With the soft coals it is only neces- sary to have a supply of some kind of kindling material that can be lighted by a match or a lamp, and to begin by building with it a small fire and then adding a little coal, and thus grad- ually increasing the flame-bed until the grate is fully covered with the burning fuel. On a large grate the whole area is usually first covered with fuel, from end to end and side to side, so that no currents of air can enter the boiler through the ash-pit, and so as to insure that all air entering the furnace may pass over the wood used in kindling the fire. The wood is placed on the front of the bed of coals, with oily cotton-waste, shavings, small chips, or other easily ignited material under it. The ash-pit doors are kept closed until the fire is fairly burning, so that the draught may be concentrated on the point at which the flame is started. After a few minutes, the fire being well started, the upper part of the mass burning in front is pushed back over the grate, and the flame is rapidly communicated to the whole bed of fuel. When this is effected the ash-pit doors are opened and the fire managed in the customary way. The precaution must be taken to see that the air has free access to the boiler-room and to the furnace. The process just described will work well with anthracite coal ; but the operation is a slower one, and more wood is usu- ally required. Building a fire of wood and then gradually adding coal is a more expeditious method than the above, but it 'is less econom- ical. When it is known that steam will be needed the boiler should be at once closed up and filled, in order that, should a leak be discovered or a misfit occur in setting a man-hole or a hand-hole plate, time may be allowed to get it right without causing delay in getting up steam. A leak discovered after 442 THE STEAM-BOILER. steam has been raised may sometimes be checked by driving in pine wedges. The rubber " gaskets" used in making the joints under man-hole and hand-hole plates may be " blackleaded " on one side to prevent their adhering to the boiler. All valves should be carefully examined before starting fires, and especial care should be taken to see that the safety-valves and the feed- check valves are in good order. All flues should be clean, and every part of the boiler and all its accessories should be given a last and thorough inspection. Before starting the fires the precaution should be taken to see that the fuel is not allowed to be placed in the furnaces until the boilers have been filled with water ; even the kindling material should never be permitted in an empty boiler. The fires should not be forced at the first, as hot gases passing over heating-surfaces in contact with cold water, and the sudden ex- pansion due to too rapid increase of temperature, may cause strain and leakage. 208. The Management of Fires is an important l>ut often neglected branch of instruction in fitting firemen for their special duties. The economy of boiler management is very largely dependent upon the skilful handling of the fuel and the furnace. In general, the fires should be kept of even thickness, clear of ash and clinkers, and as clean at the sides and in the corners as elsewhere. The depth of the fuel is determined by its nature and size and by the intensity of the draught. Hard coals can be used in greater depth than soft, and large coal in deeper fuel-beds than 'small. A strong draught demands a thick fire, a mild draught a thin one. With a low chimney and natural draught small anthracite or fine bituminous coal may be most successfully burned in a layer but a hand's breadth in thickness ; while with large " steamboat" coal of the hardest varieties and with a heavy forced draught, fires have been actually worked successfully of five times that depth, or more. The secret of success in hand- ling fires is to find the best depth of fire for the conditions existing ; to keep that thickness at all times, allowing for the ash that may accumulate ; to throw the fuel on the grate at such frequent intervals as will prevent the fire burning into holes or in irregular thickness at different points ; to introduce THE MANAGEMENT AND CARE OF BOILERS. 443 the coal so quickly and with such exactness of direction that no serious loss may occur from the inrush of cold air, and so that every shovelful should go precisely where needed, the place for the next shovelful being at the same instant located. The removal of ash is best done by means of a rake or other tool used under the grate, rather than by stirring and breaking up the bed of fuel by working through the furnace-door. The various forms of shaking grate now in use are often very effi- cient. For best working, the fire should usually be kept bright beneath, and the ash-pit clear. With light draught, however, and thin fires, it is sometimes advisable, if sufficient steam can be so made, to allow the fire to be less frequently raked out, and some accumulation of ash may be thus produced when working with maximum economy. " Firing," or " stoking," as the replenishing of the fuel is called, must be done very quickly and skilfully to avoid serious annoyance by variation of steam-pressure and supply. Where several furnaces are in use this difficulty is less likely to be met with, as the fires may be cooled and cleaned in rotation. A skilful man will find it possible to keep steam very steadily with but two furnaces, even. Ash-pits should not be allowed to become filled with ashes, as the result would be the checking of the draught, the reduc- tion of the steaming capacity of the boiler, and loss of efficiency, even if not the melting down of the grates. It is customary at sea to clean out the ash-pits and send up ashes, throwing them overboard once in every watch of four hours, when in full steaming. If much unburned fuel is found in the ashes, it should be, if possible, cleaned out and returned to the fire, or used elsewhere. The gases should have 10 per cent CO 2 , usually. Cleaning fires consists in thoroughly breaking up the mass of fuel on the grate, shaking out all the ashes, quickly raking out all " clinker," as the semi-fused masses of ash and fuel are called, and, after getting a level, clean bed of good fuel, as promptly as possible covering the whole with a layer of fresh coal. This is done, usually, once in four hours at sea and twice a day on land ; but different fuels require somewhat different treatment. The work should be performed with the greatest 444 THE STEAM-BOILER. possible thoroughness and dispatch, to avoid serious loss of steam-pressure. Mr. C. W. Williams' instructions for handling the fires, where bituminous coal is used and an air-supply above the fuel is provided, are substantially as follows : Charge the furnace from the bridge-end, gradually adding fuel until the dead-plate is reached and the whole grate evenly covered. Never permit the fire to get lower than four or five inches in thickness, of clear and incandescent fuel, uniformly distributed, and laid with especial care along the sides and in the corners. Any tendency to burn into holes must be checked by filling the hollows and securing a level surface. All lumps should be broken until not larger than a man's fist. Clean out the ash-pit so often that there shall be no danger of overheating the grate-bars. An ash-pit, brightly and uniformly lighted by the fire above, indicates that it is in good order and working well. A dark or irregularly lighted ash-pit is indicative of an uncleaned and badly working fire. The cleaning of the fire is best done, in ordinary working, by a "rake" or other tool working on the under side of the grates, and not by a " slice-bar " driven into the mass of fuel and above the grate. 209. Different Fuels require different treatment. The principles just stated apply generally, but more, perhaps, to an- thracite coals. The soft coals are commonly so disposed on the fire that a charge may have time to coke and its gases to burn before it is spread over the grate ; liquid fuels must be so sup-- plied that they may burn completely, at a perfectly uniform rate, and especially in such manner as to be safe from explosive combustion ; the same precaution is demanded with the gaseous fuels. Special arrangements of grate and a special routine in working may be, and often are, demanded in such cases.* 210. The Liquid and Gaseous Fuels are often and suc- cessfully burned in conjunction with solid fuels. In such cases the same methods are to be adopted and precautions observed in handling the latter as when burned alone. * For the peculiarities of these fuels and their use, see Chap. III. THE MANAGEMENT AND CARE OF BOILERS. 445 The liquid fuels are almost invariably the crude petroleums. They are sometimes burned in a furnace in which they are allowed to drip from shelf to shelf in a series arranged verti- cally at the front of the furnace, the flame passing to the rear, with the entering current of air supporting their combustion. In many cases they are sprayed into the furnace by a jet of steam which should be superheated and at high pressure. The use of the steam is considered to have a peculiar and beneficial effect, possibly through chemical reactions facilitating the for- mation of hydrocarbons. The petroleums are all liable to cause accident if carelessly handled, and special precaution must be observed in their application to the production of steam. The gaseous fuels are seldom used under steam-boilers, except where " natural " gas from gas-wells is obtainable, or where a very large demand or the use of metallurgical processes justi- fies the construction of gas-generators. Even greater precau- tions against accidents by explosion are needed than with the liquid fuels. In burning gas, maximum economy is secured by careful apportionment of the air-supply to the gas-consump- tion, and especially in avoiding excess. The regenerator sys- tem is not generally economically applicable to boilers. 211. The Solid Fuels, coal and wood, are burned in fur- naces which are proportioned especially for the intended fuel. With soft coals, the grate-bars are set closer together than for hard coals ; the provision for the introduction of air above the grate is larger, and a " dead-plate" is usually provided on which to coke the coal. In the use of this device, the fresh fuel is piled on the dead-plate at the furnace-mouth, and then left until the next charge is to be thrown in ; the first is then pushed in and spread over the fire, and the second charge is coked. In some cases the fuel is replenished on one side of the fire at a time ; but oftener it is spread over the whole surface of the grate. A furnace for burning wood is deeper than one intended for coal. Wood burns so freely that the ingoing charges must be continually replaced by fresh fuel. 212. The Operation of the Boiler, aside from the man- agement of the fires, in such manner as to make steam regu- 446 THE STEAM-BOILER. larly and in ample quantity, mainly consists in adjusting the draught so as to make the production of steam keep exact pace with the demand, and in keeping the supply of feed-water as precisely proportional to the amount demanded, and thus pre- serving the water constantly at a safe level, and reducing to a minimum the danger, on the one hand, of uncovering heating surfaces, and on the other of causing heavy " priming" or foaming, or the production of wet steam. As the working conditions of a steam-boiler are always those of steady motion, constant vigilance and an undisturbed and unconquerable equi- librium of mind on the part of the attendants are essential to perfect safety and thorough efficiency. So long as the water is kept at the proper height in the boiler, the boiler itself being in good repair, safety is assured ; and if the steam-pressure can be held at the proper point, effi- ciency is equally well insured ; but to maintain a state of abso- lute safety and efficiency, it is essential that something more than careful feeding and skilful firing be practised. Every apparatus upon which the working of the boiler is in any de- gree dependent must be known to be in good order and abso- lutely reliable. Feed-pumps must be kept in good repair, well packed, and ready for service on the instant ; the safety-valve must, by at least daily trial, be seen to be in good working order; the pressure-gauges must be frequently compared with a standard test-gauge to make certain that its error it will usually have some error is known and unimportant ; and the gauge-cocks and water-gauge glass the latter, especially, is lia- ble to deceive must be tried often and their reliability made evident. Blow-off and feed valves often leak, must be often exam- ined, and should be repaired or reground whenever perceptibly affecting the water-supply. A grain of sand or a chip under a valve has sometimes given rise to unfortunate results. In salt water, when using sea-water in the boilers, frequently blowing off from the bottom or a continuous discharge from the " surface-blow" or " scum-pipes" is essential to keeping the water so fresh as not to produce deposits or incrustation. The higher the " saturation" permitted, however, provided that THE MANAGEMENT AND CARE OF BOILERS. 447 common salt is not actually deposited, the less the expense of operation and the less the amount of lime-scale formed. About twelve times the quantity of salt found in sea-water is thus the maximum ; and three or four is probably as high as is safe, two thirds the water entering the boiler being converted into steam, the remaining third blown out into the sea again. And generally, if n represent the ratio of saltness of boiler to that of the sea, and m the ratio of feed-water blown out to that made into steam, m = ; n = 1- i; n I m and if the ratio of total feed-water to total evaporation is/, m -f- i n " * i n \ If large boiler-power is demanded, and a battery consisting of a considerable number of boilers is in use, one man should be detailed especially to see that the water is properly sup- plied ; he is called the " water-tender." On a large steamer several are often employed, each caring for a set of boilers and supervising the firemen or " stokers" and coal-handlers employed at his section. All these workmen should be carefully chosen, and known to be skilful and trustworthy. A careless or unskil- ful man will waste vastly more in bad firing than can be saved in the difference of wages between a good and an inefficient man. One good man should handle a ton of coal an hour sev- eral times the value of his own wages the total charges for the boiler-room amounting usually to about one fourth or one fifth wages, three fourths or four fifths fuel, and wear and tear. The coal-handler should be able to supply two to four firemen, according to distance of coal-bunkers and convenience of trans- portation. Firing stoking should be done with promptness and pre- cision during a few seconds, while the nearest man holds the furnace-door open. Every moment of needless delay allows great volumes of cold air to rush into the furnace, reducing the 448 THE STEAM-BOILER. efficiency of the boiler and causing strain by cooling the sur- faces just before exposed to gases of high temperature. The damper should be partly closed while working the fire. With a number of furnaces the order of opening the furnace-doors may be systematically arranged, and a very noticeable saving thus effected. 213. A Forced Draught is produced by the use of a blower or fan, or by the steam-jet. The former is the best method where practicable. In using the forced draught, the fires should be managed precisely as with a natural draught ; but the rate of combustion is so greatly increased that they must be made heavier, and the process of replenishing the fuel even more carefully conducted. The draught should indeed must usually be checked while adding fuel ; but where the closed fire-room or stoke-hole is adopted, or with the steam-jet, this is not absolutely necessary, though best both on the ground of economy and of safety. When the blast is driven into the ash- pit, care should be taken to open the ash-pit doors the instant the fan is stopped, or danger is incurred of melting down the grate-bars by the intense heat concentrated beneath them, un- tempered by the entering current of cold air. 214. Closed and Open Boiler-rooms, with forced draught, have each their advantages and their special methods of man- agement. With the closed, air-tight, fire-room all air supplied to the fire passes through the room, ventilating it thoroughly and cooling it, while at the same time enabling the fires to be worked precisely as where a natural draught is employed. No peculiarities of management are introduced other than come of the rapidity of combustion. In providing for the opening and closing of the fire-room doors for entrance and exit of the at- tendants, a double system must be so arranged that one will always act as a valve to close communication with adjacent apartments. In putting on and taking off the blast the fan should be first " slowed down," the doors then opened, and finally the blower stopped. In putting on the blast these steps should be precisely reversed. With the open boiler-room and closed conducting passages leading from fan to ash-pit, the special precautions to be taken THE MANAGEMENT AND CARE OF BOILERS. 449 are simply to open the ash-pit the instant the blast is stopped, or to start the blower the instant the ash-pit doors are shut. 215. The Regulation of the Steam-pressure should be effected by varying the intensity of the draught by means of the damper at the chimney, or, where a forced draught is em- ployed, by properly adjusting the speed of the blower; it should never be attempted, except in a serious emergency, to regulate it by opening furnace, ash-pit, or " connection" doors. The latter method is certain to accelerate corrosion, strain the seams, and produce leakage of tubes, as well as to waste fuel. The rushing of currents of air, alternately cold and hot, through the flues and over the heating-surfaces has been found in some cases to have probably been the cause of injury leading to ex- plosion ; and the introduction of cold air over the fire is invari- ably a cause of serious loss of economy of fuel. Automatic dampers, if well made and reliable, are very use- ful. 216. The Control of Water-supply should always be en- trusted only to experienced and proven men ; this is the main precaution to be taken in every case. The more uniform the supply, and the more perfectly the proper water-level is main- tained, the safer and the more economical the operation of the boiler. It is better that the feed-water be supplied continu- ously than to feed intermittently. Steam is then made more regularly, and of better quality ; the heating of the feed is more steady and more thorough ; the boiler itself suffers less from varying temperatures, either local or general ; and every opera- tion goes on more easily and more satisfactorily. The feed-pump, if used, should be amply large for cases of emergency, but should be ordinarily worked continuously and slowly ; the injector, if employed, should be of such size that it may never cease working while the boiler is in normal opera- tion ; and a second instrument or, better, an independent feed- pump, should be always ready for use should occasion arise. The necessity for watchfulness is greater with boilers having small water-space for their power, as the modern tubular and sectional boilers, than in the older types, in which the regulating effect of a large body of water is felt. 29 45O THE STEAM-BOILER. The first duty of engineer or of fireman, on taking charge of a boiler, for the day or for a watch, is to see that the water is at the right height ; and his constant care throughout the whole period for which he is responsible is to keep it right, and to provide against any contingency that may introduce a liabil- ity of its rising or falling beyond the intended and safe range of fluctuation. 217. Emergencies are liable to arise unexpectedly in the operation of the steam-boiler and demand the highest qualities of mind and character on the part of him who may be called upon to meet them, Self-possession and coolness, with full control of every faculty, will usually enable the attendant to successfully meet any form in which they may appear, with the single exception of an explosion of the boiler ; for that case prevention is the only cure. Minor emergencies occur so fre- quently that the experienced engineer or fireman will generally meet them promptly and effectively, and greater events often find him equally ready and prompt of action. Every attend- ant, whether in engine or boiler-room, should have constantly in mind the best course to take in the event of any accident ; and every intelligent and conscientious man w r ill have often gone over, in his own mind, the methods and means by which he should attempt to prevent every probable accident, or to render its consequences as unimportant as possible. There is often no time to think, and whatever is to be attempted can only be done intuitively, on the instant, on the impulse of the moment, guided by earlier thought or earlier experience. This quality of readiness in emergencies is perhaps the most valua- ble of all those especially required in the management of engines, boilers, and machinery generally. 218. " Low-water" is the most serious and trying of the conditions liable to arise in steam-boiler management. Once the water-level has fallen below that of the crown-sheet or the upper row of tubes, but one thing can be done reduce the temperature of the furnace and flues as rapidly as possible to a safe point. To introduce a larger quantity of feed might cause a sudden and dangerous increase of pressure by flooding the overheated metal ; to attempt to haul out the fires might pro- THE MANAGEMENT AND CARE OF BOILERS. 45 1 duce a similar effect by the momentarily higher temperature often caused by breaking up the bed of fuel, and by the pro- longed exposure of the already endangered metal it might cause the hot sheets or flues to give way. The proper course to pursue is at once to dampen the fires, preferably by quickly covering them with wet ashes. Coolness, promptness, and rapidity of action are the only safeguards in this case. With high steam-pressure, the danger is that the overheated and softened and weakened sheets may be forced out ; the intro- duction of the feed-water is in itself a less serious source of danger. The Author has many times, in experimental work, pumped water into a red-hot boiler,* but has only once seen an explosion so produced. He has experimentally allowed the water to be completely evaporated from an outside-fired boiler, and has then succeeded in covering the fires with ashes and re- filling the boiler without injury.f When the boiler has cooled down and no steam is forming, it will be safe to blow off steam, then haul fires, blow out the water, and examine to see if any injury has occurred. Dangers of this kind rarely arise where the gauges are kept in order; but carelessness in regard to the water-gauges and gauge-cocks is said to be a more frequent cause of accident than all other causes combined. Equal care should be taken to see that the fusible plugs, if used, are clean and in good condi- tion. 219. Priming or Foaming takes places whenever the quan- tity of steam drawn from the boiler exceeds that which can be liberated, dry, from the mass of water which it at the time contains. This action may be due either to forcing the boiler beyond its real capacity, or to the presence of foreign matters in solution, which tend to cause the retention of the bubbles of steam in the mass, and, when leaving it, to carry spray into the steam-space. A boiler will foam badly if the design and con- struction are such that a rapid circulation is not insured, sufficient to carry all steam made below the upper level freely to the sur- * In the work of the U. S. Commission on Steam-boiler Explosions, 1875. \ This might not be as safe an operation with an inside fired boiler. 452 THE STEAM-BOILER. face, where it may be naturally discharged ; or where currents conflict; and where a mass of water, entangled among the tubes or flues, finds no natural way of egress, laden as it is with the steam bubbles which convert it into foam ; and priming may thus occur, even when the boiler is working well within its rated capacity. Any boiler will foam if overworked. Priming is also produced by the presence of mucilaginous, oily, or other foreign matter in the water ; or by changing from a salt-water feed to fresh-water, and sometimes by the reverse ; by sudden and heavy demand for steam at the engine, or by suddenly and widely opening the safety-valve; and by other causes less well understood. When foaming takes place, it often throws water from the boiler so rapidly and in such quantities that the engine may be liable to have a cylinder-head knocked out, and the height of the water-level in the boiler may be dangerously lowered. The instant such dangers arise the throt- tle-valve should be partly closed, when the water will usually immediately settle down in the boiler, making it possible to ascertain its height in the gauges. If dangerously low, a rare occurrence, however, proceed as already indicated ; if other- wise, the draught should be promptly lessened, the fires checked, and, by thus reducing the quantity of steam made, the pro- duction of foaming and its attendant dangers may be quickly stopped. If the cause is suspected to be dirty water, contin- uous feeding and blowing, and thus changing the water, should be resorted to to remove that cause of danger. With boilers heavily driven, as is usual at sea, and too common elsewhere, priming is always one of those contingencies which those in charge of the boilers must be prepared to meet. Where sur- face-condensers are used and the boiler is fed with water of un- changing and pure quality, foaming rarely occurs. The method of circulation of water in a plain cylindrical or other " outside-fired " boiler, and the course of the steam pro- duced, is well illustrated in the accompanying figure, the fire being assumed to be located at the left. The greater part of the steam made in the boiler is produced immediately over the fire, here assumed to be at the left, and rises at once, as seen, into the steam-space above, thus determining the circulation THE MANAGEMENT AND CARE OF BOILERS. 453 in currents rising at that end and falling at the rear end of the boiler. In all cases the rising currents are at the hottest part, the descending currents at the cooler portions of the boiler. Were a boiler so constructed as to be uniformly heated, an efficient cir- culation would not be obtainable. " False water" is a term applied to the apparent increase of volume of the water in a boiler when priming takes place. It may be imperceptible ; but it often causes an apparent rising cf the water-level to the extent of several inches. It is considered that a well-proportioned boiler should be capable of evaporating five times the volume of its own steam-space each minute FIG. 117. CIRCULATION OF WATER AND STEA without serious priming ; but it is not thought wise to attempt an evaporation exceeding one half this amount. 220. Fractures, whether of seams, sheets, or tubes, are liable to occur in all boilers; but the danger is diminished as the care .taken in selection of material is the greater, the construction better, and the management more intelligent. Such injuries rarely occur so suddenly or are so extensive as to be imme- diately dangerous, and ample time is commonly allowed for their detection and safe remedy. Cracks in sheets or seams are re- paired by patching and in tubes by plugging each end, or by the removal of the sheet or tube. The duty of the attendant, for the moment, is to reduce steam-pressure at once, and as soon as possible blow off steam, to empty the boiler and to see it 454 THE STEAM-BOILER. properly repaired temporarily if necessary, but preferably per- manently. A blistered sheet should be treated as if fractured. 221. A Deranged Safety-valve may sometimes cause dan- ger by making it difficult to reduce the steam-pressure or to keep it below a dangerous point. This is sometimes a conse- quence of the rusting of the stem or of the valve and its sticking to its seat, or in such a manner that an insufficient area for exit is obtainable. In such a case the steps to be taken are to check the fires, to reduce the production of steam, and to find other di- rections of egress, as through gauge-cocks, all available valves, by the engines taking steam from the boiler, and by means, even,, of their cylinder, water, and drip cocks, until the safety-valve can be made to work or until the steam can be disposed of in other ways. If the valve be daily or oftener raised to its full height, no such danger will be incurred. 222. The General Care of a steam-boiler demands much experience, some knowledge of the causes and the methods of prevention and of remedy of injury, and thorough reliability on the part of those to whom it is entrusted. Aside from the in- juries and the deterioration which occur in its daily operation,, there are others which are to be anticipated quite independently,, and which may become even more serious when the boiler is out of use : these are principally the various forms and conse- quences of corrosion. Such general care includes the preserva- tion of the boiler against decay or loss of efficiency, the reten- tion of its setting in good repair, and the keeping in order of all its accessories and connections. 223. The Chemistry of Corrosion has been studied by many distinguished modern chemists, and is now well under- stood. Corrosion of iron and steel and the changes which characterize that method of deterioration cannot go on in the air except when both moisture and carbonic acid are present, or unless the temperature is considerably higher than that of the atmosphere. When exposed to the action of free oxygen, however, under either of these conditions, the metal is cor- roded rusts rapidly or slowly, according to its purity. Wrought-iron rusts quickly in damp situations, and especially when near decaying wood or other source of carbonic acid ; THE MANAGEMENT AND CARE OF BOILERS. 455 while steels are corroded with less rapidity, and cast-iron is comparatively little acted upon. The presence of acids in the atmosphere accelerates corrosion, and the smoke of sulphur- charged coal, or smoke charged with pyroligneous acid, fre- quently causes the oxidation of out-of-door iron structures. The composition of the rust forming upon surfaces of iron is determined by the method of oxidation, but is principally per- oxide of iron. Calvert gives the following : Rust from Con way Bridge. Llangollen. Fe 2 O 3 93-094 92.900 FeO 5.810 6.177 Carbonate of iron 0.900 0.617 Silica 0.196 o.i 21 Ammonia traces traces Carbonate of lime 0.295 A series of experiments made to determine the effect of dif- ferent oxidizing media, after four months' exposure of clean iron and steel blades, gave results * indicating that oxidation is principally due to the presence of carbonic acid with oxygen. When distilled water was deprived of its gases by boiling, and a bright blade introduced, it became in the course of a few days here and there covered with rust. The spots where the oxidation had taken place were found to mark impurities in the iron, which had induced a galvanic action, precisely as a mere trace of zinc placed on one end of the blade would establish a voltaic current. 224. The Methods of Corrosion vary with circumstances. Kent has shown f that the rusting of iron railroad bridges is sometimes greatly accelerated by the action of the sulphurous gases and the acids contained in the smoke issuing from the lo- comotive, and that sulphurous acid rapidly changes to sulphuric acid in the presence of iron and moisture, thus greatly acceler- ating corrosion. Iron and steel absorb acids, both gaseous and liquid, and are therefore probably permanently injured when- ever exposed to them. Calvert experimented upon iron immersed in water contain- * Chemical News, 1870-71. \ Iron Age, 1875. 45 6 7^ HE STEAM-BOILER. ing carbonic acid, in sea-water, and in very dilute solutions of hydrochloric, sulphuric, and acetic acids. A piece of cast- iron placed in a dilute acetic-acid solution for two years was reduced in weight from 15.324 grammes to 3^ grammes, and in specific gravity from 7.858 to 2.631, while the bulk and outward shape remained the same. The iron had gradually been dis- solved or extracted from the mass, and in its place remained .a carbon compound of less specific weight and small cohesive force. The original cast-iron contained 95 per cent of iron and 3 per cent of carbon, the new compound only 80 per cent of iron and 1 1 per cent of carbon. Iron immersed in water containing carbonic acid was also found to oxidize rapidly. Iron exposed to the wash of the warm aerated water of the jet- condensers of steam-engines is often very rapidly oxidized, and the mass remaining after a few years often has the appearance, texture, and softness of plumbago, so completely is the iron re- moved and the carbon isolated. Mallett, experimenting for the British Association,* found the rate of corrosion of cast-iron greatly accelerated by irregu- lar and rapid cooling, and retarded by a slow and uniform re- duction of temperature while in the mould. The rate of corrosion is usually nearly constant for long periods of time, but it is retarded by removal of the coating formed by the rust, as if left it creates a voltaic couple, which accelerates corrosion. Hard iron, free from graphite, but rich in combined carbon, rusts with least rapidity, and with about equal rapidity in the sea as in the air, in an insular climate. Two metals of differ- ent character as to composition or texture being in contact, the one is protected at the expense of the other. Foul sea-water, as " bilge-water," corrodes iron very rapidly. The rate of corrosion of iron is too variable to permit any statement of general application. In several cases the plates of iron ships have been found to be reduced in thickness in the bilges and along the keel-strake, at the rate of 0.0025 inch (0.06 millimetres) per year, as ordinarily protected by paint ; * Proc. Inst. C. E. 1843. THE MANAGEMENT AND CARE OF BOILERS. 457 while it is stated that iron roofs, exposed to the smoke of loco- motives, have sometimes lasted but four years. The iron hulls of heavy iron-clads have sometimes been locally corroded through in a single cruise, where peculiarities of composition or of structure, or the proximity of copper or of masses of iron of different grade or quality, had caused local action. 225. Durability of Iron and Steel. Twaite* gives the fol- lowing as the measure of the probable years' life of iron and steel undergoing corrosion, assuming the metal to be uniform in thickness. Thin parts corrode most rapidly. T= W ~CL in which Wis the weight of the metal in pounds, of one foot in length of the surface exposed ; L is the length in feet, of its perimeter; and C a constant, of which the following are values : VALUES OF c MATERIAL IN SEA WATER. RIVER WATER. IMPURE AIR. AVERAGE SEA WATER. Foul. Clear. Foul. Clear, or in air. Cast-iron .0656 .1956 .1944 .2301 .0895 copper, o ass, copp .0635 1255 .0970 .0880 359 r gun-brc 2r, or gun .0381 .1440 "33 .0728 037 1 nze .0113 .0123 .0125 .0109 .0048 .0476 1254 .1252 .0854 .0199 Wrought-iron Steel . ... . .... Cast-iron, skin removed " galvanized in contact with brass, Wrought-iron in contact with br 0.19 to 0.35 0.30 to O.AI; -bronze. . When wear is added to the effect of oxidization, the " life" of a piece of iron or steel may be greatly shortened. If kept well painted, multiply the result by two. The mean duration of rails of Bessemer steel is, accord- ing to experiments in Germany, about sixteen years. Ten years of trial at Oberhausen, on an experimental section of the * Molesworth, p. 32, 2ist ed., 1882. 458 THE STEAM-BOILER. line between Cologne and Minden, has shown that the renewals during the period of trial were 76.7 per cent of the rails of iron of fine grain, 63.3 of those of cementation steel, 33.3 per cent of those of puddled steel, and 3.4 per cent Bessemer steel. 226. The Preservation of Iron and Steel is accomplished usually by painting, sometimes by plating it. As the more porous varieties will absorb gases freely and some liquids to a moderate extent, Sterling has proposed to sat- urate the metal with mineral oil ; heating the iron and forcing the liquid into the pores by external fluid pressure, after first freeing the pores from air by an air-pump, or other convenient means of securing a vacuum in the inclosing chamber. Temperatures of 300 to 350 Fahr. (150 to 177 Cent.) and pressures of ID to 20 atmospheres are said to be sufficient for all purposes. Voltaic action may be relied upon to protect iron against corrosion in some situations. Zinc is introduced into steam- boilers for the double purpose of preventing corrosion and of checking the deposition of scale. It is sometimes useful in the open air, where rusting is so seriously objectionable as to justify the use of so expensive a preventive. The zinc itself is often quickly destroyed. Zinc has been used as a plating, or sheathing, on iron ships, as by the plan proposed by Daft,* and in some cases with good results. Mallett has proposed the use of lime-water to check the internal corrosion of the bottoms of iron ships where exposed to the action of bilge-water, and uses a solution of the oxy- chloride of copper, or other poisonous metallic salts, in the paint applied externally, to check fouling and consequent oxidation ; the amalgam of zinc and mercury is also some- times used to protect iron plates. 227. The Paints and Preservation Compositions in use are very numerous : Coal-tar, asphaltum, and the mineral oils are all used, the latter having the advantage, in the crude state, of being free from oxygen and having no tendency to absorb it. The animal and vegetable fats and oils are used temporarily in many cases, and if free from acid, are useful. * Fouling and Corrosion of Iron Ships. London. 1867. THE MANAGEMENT AND CARE OF BOILERS. 459 Surfaces of iron are painted with red-lead and oil, with oxide of iron mixed with oil, or with oxide of zinc similarly prepared. Sterling prepares a varnish by dissolving gum copal in paraffine oil, placing the iron in it, and heating it under in- creased pressure. Iron vessels, tinned inside, which can be her- metically sealed, are used, heated by superheated steam. Scott uses the following mixture : Coal tar. . . 6 gallons. Black varnish 3 " Wood-tar oil 2 " Japanese glue o . . I " Red lead 28 Ibs. Portland cement 14 " Arsenic 14 " The Author has used fish-oil as a preservative of steam-boil- ers out of use for long periods of time, with success, and has found some vegetable paints of unknown composition far more durable, when exposed to the weather, than red-lead and oil. " Iron paints" bear heat well, and are often better than any other cheap paint. Iron to be painted should first be carefully cleaned by scraping and washing, and then coated once or twice with linseed-oil. One pound of good oxide of iron paint should cover 20 square yards (16.7 square metres) of iron. Where practicable the Barff method of protection may be adopted for small parts. It consists in heating the iron or steel to be treated to a temperature of 500 Fahr. (260 Cent.) in an atmosphere of steam, and thus securing an even and imperme- able coating of the black (ferric) oxide. Where more complete protection is demanded, the iron is heated to 1200 Fahr. (649 Cent.), and is said to be thus made impregnable against the attack of even the acrid vapors of the chemical laboratory. Steam-boilers are preserved, in mass, against corrosion by various special methods. They are sometimes dried thoroughly by means of stoves, if necessary, and then closed up with a quantity of caustic lime in their water-bottoms or lower water- 460 THE STEAM-BOILER. spaces. Occasional inspection prevents injury occurring unde. tected in any case. When new boilers are stored they are usually painted inside and out. Air should be excluded from them. by closing all man-holes, etc. Working boilers are best preserved by a thin coating of scale on their heating-surfaces. Mineral oils being used for lubrication of their engines, decay is far less likely to take place rapidly. Steel corrodes more rapidly than iron, and the common brands of iron corrode less than the finer. Zinc placed within boilers, and in amount one thirty-fifth the area of the heating-surface, was found, by the British Admiralty, to pro- tect them perfectly. A pound (0.45 kilogrammes) of carbon- ate of soda to every ton (or tonne] of coal burned is ordered to be pumped into boilers at sea, to give the water an alka- line reaction. Boilers of sea-going vessels average a life of nine or ten years. Boiler Coverings having for their object the protection of the external surfaces against loss of heat and from any inju- rious action liable to occur in consequence of their exposure, are of very various kinds, and are always considered the better the more perfect they are as non-conductors. Care should be taken, however, that they do not themselves cause injury more serious than that which they are designed to pre- vent. Hair-felt has been known to cause possibly by some peculiar galvanic or electric action observable acceleration of corrosion on the inner sides of the sheets to the exterior of which it has been applied, as, for example, where used to cover the steam-drums of marine boilers, mineral-wool, when con- taining sulphur-compounds, has been known to absorb moist- ure, and to thus cause rapid corrosion of parts with which it was in contact. When free from sulphur no such danger is incurred. They should be air-tight. The experiments of Mr. C. E. Emery give the following as the relative values of available covering materials:* * Trans. Am. Society Mech. Engrs., vol. ii., 1881. THE MANAGEMENT AND CARE OF BOILERS.* 461 Non-Conductor. Value. Non-Conductor. Value. Non-Conductor. Value. Wood felt Charcoal 672 Asbestos ,5, 8^2 Pine-wood across fibre. Coal-ashes r> ' h t 2 . . . Coke in lumps 680 Slacked lime .480 Air-space, undivided .136 Mineral-wool No. i... .676 Gas-house carbon .470 Hair or wool felt is injured by high temperature ; woods are liable to char, and all organic matters, in presence of grease and dampness, to take fire spontaneously. Asbestos is much used, as is also " rock-wool," which is less likely to absorb moisture than the " mineral-wool " from the blast-furnaces. Sand, ashes, and other earthy matters are often used to fill in over boilers. They are, however, liable to conceal and accelerate corrosion whenever leakage takes place beneath them. In all cases the values of successive layers of non-conductor decrease in a geometric ratio. Anything that will encage air in its pores is a good covering. Large boilers and their pipes, as designed by Mr. E. D. Leavitt, Jr., were covered with about two inches and a half of plaster and sawdust, and one inch of hair-felt outside that. The proportion of the mixture is about one part of plaster and two parts of sawdust. The plaster and the sawdust are mixed up like mortar. They are first put in together dry, and then wet and mixed up. For steam-pipes, the mixture is applied from one and a half to two and a half inches thick. For boilers, wooden battens f by 2\ inches wide are used. Between the edge of the batten and the boiler half an inch of the compound is put. These are fastened all around the boiler , then a band of hoop-iron is put around it, and filled between the battens with plaster. The practice of putting it on in little blocks about a foot square has been adopted. Outside of that, the specifications call for an inch of hair-felt and canvas.* 228. Leakage, and contact of damp portions of supports and setting, produce the most serious corrosion. A leak, once started, will keep everything near it damp, and thus cause acceleration of oxidation to a very marked degree. Where the leakage, or the dampness produced by it, finds its way between the iron of the boiler and the brickwork about it, there is no * Trans. Am. Soc. Mech. Engrs., 1882. 462 THE STEAM-BOILER. opportunity of evaporation and drying the moistened surfaces, and the dampness thus held in contact with the metal promotes decay. When inspecting the boiler, care should be taken to detect every such cause of deterioration, and to immediately repair the injured part. It is well to so design and construct the boiler that there will be as little liability as possible to this kind of injury. 229. Galvanic Action is liable to occur, and enormously to accelerate corrosion, either local or general, whenever a mass of brass, bronze, or copper, large or small, is in metallic contact with the boiler at any point, or with any of its connections. The brass tubes of a surface-condenser have been often known to thus cause the ruin of a boiler in a few months, and very serious general corrosion in few weeks. Copper boiler-tubes, brass valve-seats, and any other minor part made of such electro-negative metals, may similarly cause local deterioration and leakage or weakness. The remedy is either to remove the cause of the trouble ; to protect the metal attacked, as by allowing it to become coated with a thin layer of incrustation ; or to counteract the effect of the electro-negative metal by in- troducing a mass of another element, as zinc, which is electro- positive to both the iron of the boiler and the copper or other material producing the destructive action. In the latter case, the zinc will be corroded instead of the iron of the boiler, and must be occasionally renewed. 230. Incrustation and Sediment are deposited in boilers, the one by the precipitation of mineral or other salts previously held in solution in the feed-water, the other by the deposition of mineral insoluble matters, usually earths, carried into it in suspension or mechanical admixture. Occasionally also vege- table matter of a glutinous nature is held in solution in the feed-water, and, precipitated by heat or concentration, covers the heating-surfaces with a coating almost impermeable to heat and hence liable to cause an overheating that may be very dangerous to the structure. A powdery mineral deposit some- times met with is equally dangerous, and for the same reason. The animal and vegetable oils and greases carried over from the condenser or feed-water heater are also very likely to cause THE MANAGEMENT AND CARE OF BOILERS. 463 trouble. Only mineral oils should be permitted to be thus in- troduced, and that in minimum quantity. Both the efficiency and the safety of the boiler are endangered by any of these de- posits. The amount of the foreign matter brought into the steam- boiler is often enormously great. A boiler of 100 horse-power uses, as an average, probably a ton and a half of water per hour, or not far from 400 tons (406 tonnes) per month, steaming ten hours per day, and, even with water as pure as the Croton at New York, receives 90 pounds (41 kgs.) of mineral matter, and from many spring waters a ton (1.016 tonnes), which must be either blown out or deposited. These impurities are usu- ally either calcium carbonate or calcium sulphate, or a mixture; the first is most common on land, the second at sea. Organic matters often harden these mineral scales, and make them more difficult of removal. Mineral oils often soften them. The only positive and certain remedy for incrustation and sediment once deposited is periodical removal by mechanical means, at sufficiently frequent intervals to insure against injury by too great accumulation. Between times, some good may be done by special expedients suited to the individual case. No one process and no one antidote will suffice for all cases. Where carbonate of lime exists, sal-ammoniac may be used as a preventive of incrustation, a double decomposition occur- ring, resulting in the production of ammonium carbonate and calcium chloride both of which are soluble, and the first of which is volatile. The bicarbonate may be in part precipitated before use by heating to the boiling-point, and thus breaking up the salt and precipitating the insoluble carbonate. Solu- tions of caustic lime and metallic zinc act in the same manner. Waters containing tannic acid and the acid juices of oak, su- mach, logwood, hemlock, and other woods, are sometimes em- ployed, but are apt to injure the iron of the boiler, as may acetic or other acid contained in the various saccharine matters often introduced into the boiler to prevent scale, and which also make the lime-sulphate scale more troublesome than when clean. Organic matters should never be used. The sulphate scale is sometimes attacked by the carbonate 464 THE STEAM-BOILER. of soda, the products being a soluble sodium sulphate and a pulverulent insoluble calcium carbonate, which settles to the bottom like other sediments and is easily washed off the heat- ing-surfaces. Barium chloride acts similarly, producing barium sulphate and calcium chloride. All the alkalies are used at times to reduce incrustations of calcium sulphate, as is pure crude petroleum, the tannate of soda, and other chemicals. Marine boilers have been effectively treated for the preven- tion or the removal of scale, by introducing sheet-zinc, or zinc in balls or in blocks of any convenient size and form. The in- crustation met with in marine boilers, properly managed, being always nearly pure sulphate of lime, the zinc, probably by some voltaic action, causes the deposit to become pulverulent, in- stead of compact, and very hard and strong, as when formed in the unprotected boiler, and it also compels the precipitation of the mineral upon the zinc itself principally. The water in boil- ers of any kind is very liable at times to become acidified per- ceptibly by the decomposition of the lubricants entering with the feed-water from the engine cylinders and condensers, and corrosion is thus accelerated. In such cases the zinc suffers and the boiler is preserved, if metallic contact is secured be- tween the iron or steel and the zinc precisely as, when the boiler itself is constructed of different qualities of metal, one part is preserved while another part is corroded. Zinc, as, relatively, an electro-positive metal, protects iron ; which latter is electro-negative to the former, and takes the hydrogen of so much water as may be decomposed by the voltaic action occur- ring, the zinc being attacked by the oxygen set free on that element of the voltaic pile so formed. Marine boilers thus protected have shown no trace of decay after years of use. Whenever zinc is used, the precaution should be taken to secure a perfect metallic connection between it and the boiler ; otherwise it will be neither uniform in action nor reliable. The zinc is sometimes amalgamated to prevent wasteful oxidation by local action. A little soda, or sodium carbonate, introduced into the boiler may often insure the formation of a softer deposit where it is found to be hard, and to so incrust and embalm the zinc THE MANAGEMENT AND CARE OF BOILERS. 465 that it ceases to do its work. A surface of zinc of 25 to 50 square inches (2.5 to 4.5 square decimetres, nearly) per ton of water contained in the boiler, and per month, is usually found ample. After studying the use of zinc as an " anti-incrustator," and the reports of M. Lesueur, who first introduced it extensively in France,* M. Euvrard concludes that it should be used in the form of " pigs" or ingots, and in any type or in any part of a boiler, although it is better not to place it on the heating-sur- faces of the firebox. He advises from one pound to two pounds for every ten square feet of heating-surface at a time (-J to I kg. per sq. m.). It is found that zinc is valuable with calcareous feed-waters when not excessively hard, causing the deposit to become pulverulent, and thus altering an incrustation or scale into a sediment. Water-tube Boilers have been successfully treated by M. C. Quehaut,f where the incrustation was calcareous, and largely consisting of calcic sulphate, by using instead of the " tartri- fuges" commonly employed for such cases, none of which proved satisfactory, sheet-zinc of thickness No. 18. Sheet? about two metres (6.56 feet) long and 0.8 metre (3 feet) wide were cut into strips each about -fa metre (3.28 inches) wide, and wrapped helically on a mandril, forming coils of which about 45 kilograms (100 pounds) were introduced into a boiler rated at 40 horse-power at each charge. The making of the coils cost about one dollar. One of the strips so coiled was pushed into each tube after each cleaning, and withdrawn at the succeeding period of washing out. Heavier zinc did not answer as well, as the strips were liable to be displaced by the circulating current. Incrustation takes place on the zinc instead of upon the adjacent iron surfaces. It is pulverulent, and easily removed. The cost was $2.50 per annum per horse-power. 231. Repairs are the source of the great expense of main- tenance of steam-boilers, and sometimes of new dangers hardly less serious than those which they are expected to prevent. * Annales des Mines, 1877; Jour. Franklin Inst. 1878. f Ann. de 1'Association des Ingenieurs de Liege, 4016 serie, t. v., 1886. 30 466 . THE STEAM-BOILER. Frequent and systematic inspection and test will always reveal the approaching necessity of repairs long before serious risks are run, and, if promptly attended to and skilfully performed, the life of the boiler may often be very greatly prolonged. At sea it is customary to have on hand a good stock of extra boiler-plate, rivets, tubes, and other material for use in making repairs, and to have all minor and temporary repairs made by the engineer's crew. On land this is rarely necessary, as boiler- makers are usually close at hand, and the work can be done more perfectly, quickly, and cheaply by regularly employed workmen. Leaky tubes are often plugged until it becomes convenient to replace them by new ones. In such cases wooden or iron plugs are driven into the ends, and leakage thus checked. Sometimes special apparatus, devised with a view to con- venience of application while steam is still kept on, are em- ployed. Local defects, as oxidation or blisters, are remedied by bolting on " soft-patches" of boiler-plate fitted to the weak- ened surface and made tight by a cement of red-lead and oil, or a mixture of red and white lead and oil, with iron borings and some other constituent, as sal-ammoniac, the effect of which is to promote the oxidation of the borings and the production of a hard, stone-like cement. A permanent " job" is made by cutting out the defective metal, and riveting in a piece of new boiler-plate, thus making a " hard-patch." A patch secured by tap-bolts is also sometimes called a " hard-patch." Leaks in steam-pipes are stopped by placing sheet-rubber packing over the crack or joint, covering this with sheet copper or brass, and wrapping with tightly wound wire or cord. Feed- pipes may be similarly temporarily repaired, or by covering the leak with a " putty" of red and white lead and wrapping it with canvas and twine. Where a crack appears in any part of the heating-surface, if not more than two or three inches long, it should be stopped by drilling at each end and inserting a screw-plug. A long crack must be patched. Hard-patches are used when in con- tact with the fire ; soft-patches elsewhere : 232. Inspection and Tests of strength should be occa- THE MANAGEMENT AND CARE OF BOILERS. 467 sionally resorted to for the purpose of determining the precise condition of the boiler at the time, and its absolute safety under the conditions of its regular use. Custom and opinion differ somewhat, among the ablest and most experienced engineers, as to the precise method and the extent to which such exami- nations and tests should be carried. It may be safely assumed, however, that the following principles and processes will be considered as, at least, on the safe side. The complete visual inspection and examination of a boiler, inside and out, should be considered one of the primary duties of the person responsible for its safe operation at every avail- able opportunity, and during its operation a watchful eye should be kept upon it uninterruptedly. With marine boilers, a complete examination is expected to be made every time that steam is off usually at the end of every trip ; stationary and locomotive boilers are inspected at regular intervals by skilled inspectors or by the master-mechanic having charge of them. The former should be so examined at least once in each three months, and a complete inspection and thorough test should be made as often as once a year ; the latter still oftener de- mands attention. In a careful inspection, the inspector goes underneath and examines all the fire-sheets, and inside and with hammer and chisel and lamp examines every portion of the boiler. If a corroded or grooved place is found, or a blister, it receives care- ful attention. If for any reason the examination should be made more complete, the hydrostatic test is applied. In the course of the examination, the safety-valves, the gauge-cocks, water-gauges, feed and stop valves, pumps, dampers, every de- tail, should receive careful attention. The tap of the hammer will, to the experienced ear and inspection should only be intrusted to experienced men reveal the thickness of a sheet, the presence of a crack, groove, or any form of serious oxida- tion or injury, the soundness of stays and braces and their con- nections, and the nature and extent of any defect that may exist. After this inspection the defects, if any, are removed, and after the repairs are completed the inspection should be repeated to make sure that the work has all been done, and 468 THE STEAM-BOILER. properly done. Finally, the boiler is closed, filled to the safety- valve, all stop-valves being closed, and is subjected to a pres- sure exceeding its working pressure by at least one half, and preferably more. Many authorities advise a double pressure. While this operation is going on, the inspector carefully watches to see that no new weakness is revealed. That testing by hydraulic pressure is not alone sufficient to reveal dangerous defects or to insure against disaster is un- questionable. The Author has repeatedly met with evidence that explosions have occurred at pressures less than those at which tests had been made ; and it is well known to experienced engineers, and especially to inspectors, that a dangerously thin boiler may sustain high pressures for a time. A case is related * in which a water-pressure of 12 atmospheres was sustained by a boiler which in places was exceedingly thin, and, as reported, at several points not thicker than paper. It not infrequently occurs that the inspector's hammer is driven through sheets by which very considerable pressures had been sustained. It was at one time common to test boilers to three times their working pressure, or even more ; but it is less usual now. The United States regulations controlling steam-vessels pre- scribe a ratio of i to I ; French regulations direct that a ratio of 2 to i shall be adopted for new boilers, annually, on naval vessels, and the same on merchant vessels at first, but later re- duced the ratio to i^ to i, although even then this pressure must not be kept up more than five minutes.f The British regulations prescribe 2 to i, the tests to be made semi-annual- ly. If signs of weakness are observed the pressure may be re- duced. All boilers should be drilled occasionally wherever thinness of plate is suspected. All such tests and inspections should be made before painting, and inspection should be made while the boiler is still under the test-pressure. Leaks are often more easily detected under cold water than under steam- pressure , and the inspection rather than the test is the insur- ance against accident. This inspection and the hammer-test are especially relied upon where the boiler is one with the his- * Locomotive, Sept. 1873, P- 3- f Ledieu, Appareils a Vapeur, vol. ii. THE MANAGEMENT AND CARE OF BOILERS. 469 tory of which the inspector is unfamiliar, and when old and worn ; as it is only by this plan that cracks, leaks, blisters, dis- tortion of parts, and corrosion can be satisfactorily found and gauged. All boilers are usually very carefully inspected inside and out at least once a year, and thoroughly tested. It is custom- ary to make quarterly examinations also as complete as possi- ble, but not, as a rule, to make the extended inspection and test which is insisted upon at the annual inspection. Where the feed-water is impure, however, and where sediment and in- crustation are found to give occasion, these periodical examina- tions should be made so frequently that all possible danger may be avoided. Every boiler should be cleaned out and thor- oughly freed from incrustation at intervals whether a year, a month, or a week such as will secure immunity from danger of overheating and from serious loss of economy. 233. General Instructions for the management and care of boilers should always be written out and placed in the hands of attendants whenever they are not known to be in every re- spect familiar with their duties. Especially should they be cautioned against raising steam too rapidly, or emptying the boiler while the setting is hot, and against pumping cold water in large quantities into a hot boiler, and other errors of either omission or commission by which the boilers may be injured. All air-leaks about the setting should be found and stopped. The most perfect cleanliness should be enjoined. The most complete codes of instructions are those issued to naval officers, of one of which the following is an abstract :* The engineer officers are to make themselves acquainted with the general construction and with any special fitting of the boilers under their care. In order to protect the plates and stays from corrosion, it is essential that the interior surfaces should be coated with some impervious substance. A thin layer of hard scale, deposited by working the boilers with sea- water, has been found to be the most effectual preservative ; and therefore all boilers when new, or at any time when any of * London Engineering, 1884. 4/0 THE STEAM-BOILER. the plates or stays are bare, are to be worked for a short time with the water at a density of about three times that of sea- water, until a slight protective scale has been deposited ; but in this case care is to be taken not to allow a scale to be formed of such a thickness as would in an appreciable degree impair the efficiency and economy of the boilers. During the first six months' service the boilers should be frequently examined ; and afterwards, where possible, at least once a month, or after steaming twelve days. The boilers are to be examined care- fully after steaming ; and every judicious measure is to be used for the prevention and removal of scale, especially on the fur- nace crowns and sides. Whenever serious corrosive action has been discovered it is to be at once reported, together with full information as to the circumstances and the supposed cause. The tubes and tube-plates are to be cleaned as soon as possible after steaming. It is essential that at first the water should be kept for a short time at about three times the density of sea-water, until the thin protective scale has been formed, as before directed. After this, in the ordinary working of the boilers, the engineer officers in charge of machinery are to use their discretion as to the most suitable density at which the water in the boilers should be kept for the service on which the ship is employed. This density, which is in no case to exceed three times, nor be less than one and a half times that of sea-water, will probably vary to some extent, on different stations and under different conditions of working, of regular service, and the engineer offi- cers will be guided in their selection of the working density by their experience of the economy of fuel under steam, and of the state of the boilers after steaming. No tallow or oil of ani- mal or vegetable origin is to be put into the boilers to prevent priming, nor for any other purpose whatever. When the boilers are empty, the fires are not to be kept laid ; the boilers are to be kept dry and warm ; all accessible parts are to be frequently examined and cleaned ; and the lower parts are to be coated with red and white lead, or other protecting substance. Where the boilers cannot be kept thor- THE MANAGEMENT AND CAKE OF BOILERS. 471 ongJily dry and warm, they are ; at the discretion of the engineer officer in charge, to be kept quite full* The boilers should not be exposed to sudden changes of temperature ; the steam should not be raised rapidly ; the smokebox doors should not be opened suddenly, as a rush of cold air through the tubes affects the ends and the tubes leak ; and . the stop and safety valves should be opened grad- ually. The safety-valves should be partially raised each watch to test the fittings, and the smokebox doors should not be opened except when absolutely necessary. The blow-off cocks are to be kept in good condition. The spaces at the backs and sides of the boilers are at all times to be kept clear ; and on no account is anything combus- tible to be placed on the top of the boilers or in contact with them. Every care is to be taken to prevent any accumulation of soot or coal-dust between the uptake and casings of the boil- ers, and, when necessary, means should be provided for exam- ining the air-space between the uptake and the air-casing, and every possible precaution taken to prevent the clothing of the boilers being set on fire. It is well to keep a log in the boiler-room, where a large " plant" is operated, and the record so kept should exhibit all important data relating to its operation. The following is a good form of ruling for the blanks or log-book employed : BOILER RECORD. WEEK ENDING 188 No. of Boiler. Average Pressure. Hours Steaming. Coal, Tons. Ashes Removed Water Used. REMARKS. Totals.. MEMORANDA. * A small quantity of washing soda or other alkali may be introduced with ad- vantage. CHAPTER XIII. THE EFFICIENCIES OF STEAM-BOILERS. 234. Steam-boiler Efficiency is not difficult of definition when the nature of the quantity to be measured is itself first understood. There are, however, as will be presently seen, several different efficiencies of the steam-boiler, as of the steam- engine ; and it is important that each be distinctly defined be- fore a study of either, or of total efficiency, can be made. In general, it may be said that efficiency is measured by the ratio, in common or similar and definitely related terms, of a result produced to the cost of its production. As, in the study of the steam-engine, either efficiency is measured by the ratio of work done in the specified manner to the work or work-equiva- lent expended in doing it ; so, in the case of the steam-boiler, either efficiency is measured by the ratio of a heat-effect, or its equivalent, to the quantity of heat, actual or latent, paid for its accomplishment. In some cases it is not practicable to thus establish a nu- merical value of an efficiency ; and it can only be shown that efficiency, in the sense of quantity of result compared with magnitude of means used, is increased or decreased by the op- eration of defined phenomena, or by conditions which can be specified. A common measure cannot always be found, or an exact law of relation established. Increasing steam-pressure gives increasing economy up to a limit somewhere above customary pressures. The higher the pressure the greater the economic value of the steam in a steam-engine, but on the other hand the lower the efficiency of the boiler ; and it is perfectly possible to reach a point at which the gain on the first score is more than counterbalanced by the loss on the second. Where the object sought is simply heating-power, the advantage lies, on the whole, on the side of low pressures. THE EFFICIENCIES OF STEAM-BOILERS. 473 235. The Measure of Efficiency of boilers is commonly a ratio of heat applied to a defined purpose or obtained in store, in a stated form, to the total quantity of heat from which it has been saved, another part having been diverted to other purposes, and, for the use considered, wasted. Thus, a given quantity of heat being stored as potential energy of chemical action in fuel, a small proportion of that energy is received at the steam-engine when that fuel is burned under a steam-boiler ; the ratio of these two quantities always a fraction and often small is the total efficiency of the whole apparatus employed in the combustion of fuel, the transfer of heat-energy to the fluid in which it is stored, and its further transfer to the point at which it is usefully applied by transformation into mechani- cal energy and work. 236. The Efficiency of Combustion thus measures the ratio of the available heat-energy of the fuel to that set free by its union with oxygen, and is less than unity in the proportion in which the combustible portion of the fuel escapes such chemical change or is imperfectly burned, as when a part of the fuel falls into the ash-pit, is imbedded in clinker, or remains on the grate when the fire is extinguished ; or as when carbon is only oxidized to carbon monoxide instead of being com- pletely burned into dioxide. In well-managed furnaces the value of this efficiency approaches unity ; it ought not to fall below 0.90, probably, in any ordinary case. 237. The Efficiency of Transfer of Heat similarly meas- ures the ratio of heat received from the furnace by the boiler to that produced by combustion. That not transferred to the boiler is either sent up the chimney, where it is, in a certain degree, useful in producing draught, or it is lost by conduction and radiation to surrounding bodies. In good examples, the value of this ratio exceeds 0.75, and it should not usually fall under fifty or sixty per cent. Its best value depends on con- siderations, however, to be hereafter stated, and it is not al- ways desirable that it should have the highest value possible, or approximate unity. 238. The Net Efficiency of Boiler is the continued prod- uct of all efficiencies of the several operations constituting the 474 THE s TEA M ~ B OILER. process of production and supply of steam ; and it can only be exactly known by direct experimental determination, either as a whole, or in detail, by the ascertainment of the values of each of its factors. It is this quantity with which the engineer and the proprietor are principally concerned, and the study of the elementary efficiencies is mainly useful in revealing the causes and the extent of wastes in the several steps of the whole process. 239. The Finance of Efficiency is a more important mat- ter, if possible, than the theory of either or all the efficiencies already defined. It is obvious that, in any case in which steam is demanded at a given pressure and in stated quantity, it may be obtained either expensively by using ill-chosen types, con- struction, and proportion of boiler, and operating under un- fortunate conditions, or economically by an opposite method. In general, the larger the boiler the less the cost of steam in fuel and operating expenses ; the smaller the boiler the heavier the coal bills and related accounts. On the other hand, the larger boiler is of great first cost, expensive in its interest, in- surance, and perhaps maintenance, accounts ; while the oppo- site is true of the smaller boiler. It is equally evident that a boiler may be too large and costly for real and ultimate financial economy ; or it may be too small and too wasteful of fuel to give best results as read on the final balance-sheet, at the end of its period of service. There must in every case be some proportion of size and cost to quantity of steam demanded which shall, on the whole, prove in the end a financial success, and give the work required of it at the least total cost. 240. Commercial Efficiency must thus be added as the final and most important of all efficiencies, as judged from the standpoint of the proprietor, and as measuring also the success of the designer of the steam-generating apparatus ; and the fol- lowing definitions and principles may be admitted as a basis for the mathematical theory of the finance of steam-boiler operation : In the design and construction of a steam-boiler, and in its operation, problems arise which must be solved by the mechan- ical engineer in their natural order before he can say with. THE EFFICIENCIES OF STEAM-BOILERS. 475 confidence that the best interests of the purchaser or proprietor of the apparatus are fully met in its construction and manage- ment. Such are the following : (1) The " Efficiency of the Steam-boiler' is the ratio of the total quantity of heat utilized in the production of steam to that set free in the combustion of the fuel. It has as the maximum limit unity, and is a function of area of heating-sur- face, and of factors dependent upon the character of the fuel and its combustion, and upon the design of the boiler. (2) The " Commercial Efficiency" or the " Efficiency of Capi- tal" employed in the maintenance of steam-generating appa- ratus of a given power is measured by the ratio of quantity of steam produced to the total cost of its continuous production, Le., by the reciprocal of the total cost of steam per pound or per cubic foot at the required pressure. This efficiency is a maximum when that cost is a minimum. (3) The "Efficiency of a Given Boiler Plant" as the Author has called it, or the commercial efficiency of a steam-boiler already in place and in operation, is still another quantity. It is a maximum when the work done by the boiler can be in- creased beyond that for which it was proportioned if de- signed originally to give maximum efficiency of capital at a pre- arranged power, as above until the amount of steam made by that boiler per dollar of working expense is made a maximum. These three efficiencies differ essentially in their character, and are determined by different processes. In the first case, the engineer designing a boiler finds himself called upon to deter- mine what is the maximum efficiency that it will be economical, or otherwise advisable, to endeavor to secure, and then cal- culates the proportions necessary to secure that efficiency. Or, knowing the proportions of any boiler already designed and built, he may be required to calculate its probable efficiency and the quantity of fuel required to make a certain quantity of steam, i.e., to estimate the quantity of steam which will be generated per pound of coal burned. In the second case, the designing engineer calculates the proportions of heating-surface to grate-surface or to fuel burned, where the quantity of steam required is known, and the 4/6 THE STEAM-BOILER. conditions determining costs, which shall give that quantity of steam at least total running expense. The investigation de- termines how large a boiler or what extent of heating-surface will, all things considered, pay best. In the third case, the boiler is in place and in operation, and it is found that it is advisable to ascertain what quantity of steam is made when the cost of that steam, per unit of weight or of volume, becomes a minimum. In the first two cases, the variable element is usually the area of heating-surface per pound of fuel burned in the unit of time; in the last, the variable may be either the quantity of fuel burned or of steam made. (4) To what Capacity may any Given Boiler be forced with- out exceeding that Cost of Steam at which a Paying Profit is given? is another problem in steam-boiler efficiency, and one which is of more frequent occurrence and is usually more im- portant than the preceding. The economical maximum of steam-production is evidently determined by the money value, to the producer, of the steam made. 241. Efficiency of the Steam Boiler. This case has been studied by Rankine, who deduces a very simple and handy formula for the efficiency of a boiler of known proportions, using a fuel of known calorific value. (See 98, p. 221.) Taking the rate of conduction of heating-surfaces as varying as the square of the difference of temperatures of the gas and of the water on opposite sides of the sheet, the formula is readily deduced, in which E is the efficiency, a a constant, c' the specific heat of the furnace-gases, and W their weight ; while H is the total heat expended and 5 the heating-surface. This expression is further transformed into THE EFFICIENCIES OF STEAM-BOILERS. 477 in which E is the theoretical evaporative power of the fuel per pound, E, the probable actual evaporation in a boiler in which F is the weight of fuel burned on the unit of area of grate, and 5 is the area of heating-surface per unit of the same area. A and B are here coefficients, having values respectively of 0.3 to 0.5 and 0.9 to i for bituminous coals, according to Ran- kine, and from 0.3 to 0.5 and from 0.8 to 0.9 with anthracite coal, as determined by experiments made by the Author. The lowest and best values of A are obtained when using a minimum needed air-supply, and the value of that coefficient is seen, by comparing the two equations just given, to vary as the square of the quantity of air supplied to the fuel. The value of B is dependent upon the character of the boiler, being greater as the design and construction are improved. The following are illustrations of the results thus obtained : EFFICIENCY OF STEAM-BOILERS. I. II. III. IV. A = Q.$\B=i. A = o.3;l?=i. ^=0.5; B= 0.9. ^=0.3; Bo.q. 0.17 0.92 0.95 0.83 0.86 0.33 0.87 0.91 0.78 0.82 0.40 0.83 0.89 0.75 0.80 0.50 0.80 0.87 0.72 0.78 0.67 0.75 0.83 0.68 0.75 242. Commercial Efficiency of the Boiler. The expenses of operating a steam-boiler may be classed under three heads : (1) Those costs of boiler and its maintenance which are de- pendent upon the size and the character of the boiler itself and its attachments, such as interest on cost of boiler and setting, rent of building, and other items on construction account, such as taxes, insurance, repairs and depreciation, etc., etc. (2) Those costs of operation which are dependent upon the quantity of steam made and of fuel consumed, such as market price of fuel, cost of transportation, storage (an important item on shipboard especially), and of feeding into the furnace, cost of feed-water and its introduction into the boiler, and often a certain part of other costs of attendance and supply. (3) In addition to these variable expenses are often, perhaps usually, to be counted certain constant expenses which are un- 4/8 THE STEAM-BOILER. affected by any change of proportions of boiler likely to be made in the assumed case, such as nearly all, or frequently quite all, the costs of attendance. A given amount of steam being demanded, it may be ob- tained either from a boiler so small as to use fuel extravagantly, or from a large boiler using fuel economically. In each case arising in practice, there will be found a certain easily deter- mined proportion of heating-surface to grate-surface, and a definite size of boiler which will, on the whole, supply the de- sired quantity of steam most economically. Thus : Let the total cost of fuel per annum and per pound burned per hour on the square foot of grate or on the square metre be called C. Let the total cost per annum of boiler, per square foot or per square metre of heating-surface, be called D, and let = R. In the first item is included Class I, and in the second Class 2. Then the cost of boiler maintenance per annum is DSG, where 5 is the area of heating-surface per unit of area of grate and G is the area of grate. The cost of fuel, etc., per annum, as per Class 2, is CFG, if F is the weight of fuel burned per unit of area of grate. The total of costs variable with change of proportion of boiler is P= DSG+CFG. The profitable work of the boiler is measured by the quantity, by weight, of steam made, FGE 1 ^ W\ E l being the evapora- tion of water per unit of weight of fuel. The ratio of cost to work done is P_ _ DGS + CFG _ CF+DS ~ W'~ FGE, E,F This quantity being made a minimum by variation of the area 5, the most economical boiler is obtained. But E l is a function of S, and, taking the value of E l from the equation F BE '" AF' THE EFFICIENCIES OF STEAM-BOILERS. 479 we obtain BEFG which is a minimum when BEFG DS + ADF+CF + ACF* BEF In illustration : Let a boiler, set in place, complete with all its appurtenances and in running order, cost $3 per square foot of heating-surface, and the annual charges on all accounts en- tered in Class I, above, be 20 per cent on this cost, the annual charge becomes DS = $0.60 X S per square foot of grate, i.e., D $0.60. Let the cost of operation, as for Class 2, amount to $15 per annum per pound of fuel burned per hour on the square foot of grate; then CF= $15 X F\ C= $15; -^ = R = 2$. Assume F= 10 pounds of fuel per hour per square foot of grate, A =0.5. For this case, then, the boiler should have per square foot of grate, S, = F \TAR = 10 x (0.5 x 25)^ = 35; 35 square feet of heating-surface. Similarly we get the following values : COMMERCIAL EFFICIENCY OF BOILERS. Ratio of Areas of Heating and Grate Surfaces. Values of S. F 6 10 12 *5 20 3 40 SO R 25 16 9 4 21 17 12 8 35 28 21 14 42 34 24 16 52 42 32 21 70 56 42 28 105 8 4 63 42 140 112 8 4 56 175 140 105 70 480 THE STEAM-BOILER. These values are 20 or 25 per cent lower for forced draught. Where the boiler is worked almost continuously, as in flour- mills and some other establishments kept in operation night and day throughout the year, the higher values will be found correct; when the boiler is worked discontinuously or, as in steam fire-engines and some classes of steam-vessels, a com- paratively small proportion of the annual working time of the establishment or whole plant, the values of S, become very small. It is seen that the best area of heating-surface will vary nearly as the square root of the total working time per annum. Boilers worked continuously, worked twelve hours out of the twenty-four, and eight hours in the day, will require, respective- ly, values of 5 having the proportion I, 0.7, and 0.6 nearly. W The total required area of grate is -=-= G\ the total area , W(S,+AF) of heating-suriace is The following are examples, in greater detail, of the appli- cation of the above : EXPENSE ON BOILER ACCOUNT AND MAXIMUM COMMERCIAL EFFICIENCY. CASES. STATIONARY. MARINE. I. II. III. IV. Class i (Z>) Cornish. Tubular. Tubular. Tubular. Total annual cost of boiler per unit of S.. . . . $1.50 $2.00 $3.00 $2.00 Interest .................................. . .09 .12 .15 .12 Repairs and depreciation .................... 15 .20 .45 .30 Rent, insurance, and miscellaneous ..... ...... .10 .07 i.oo .20 Total value of D ................... 34 .38 1.60 .62 Class 2 (0. Fuel (@ $5 for I., II., IV.; $4 for III.) per unit of F .................... ............. 7.50 7.20 12.00 2.00 Transportation and storage ................. i.oo i.oo 10.00 i.oo Attendance (variable cost) .................. o.oo o. 50 o. 50 o.oo Total ............................. 8.50 9.00 22.50 3.00 Value of- =R .................. 25 23 14 5 Value of A ...................... 0.5 0.3 0.3 0.5 Value of \/AR .................. 3.5 2.7 2.0 1.6 Value of F ...................... 8 10 16 20 Value of t/Z/V = Si .............. 28 27 32 32 THE EFFICIENCIES OF STEAM-BOILERS. 481 R varies in magnitude very greatly in practice, falling as low as 4 and rising as high as 50 with varying cost of fuel and length of working time. The engineer thus solves this important problem in boiler- design which may be thus enunciated : To determine the com- mercial efficiency of a steam-boiler doing a fixed amount of work ; or, given all variable expenses of boiler installation, maintenance, and operation, to determine what proportion of heating-surface to grate-surface, or to fuel burned, will give the required amount of power at least total cost. 243. Commercial Efficiency of a Fixed Plant. A second commercial problem may sometimes be presented to the engineer : A steam-boiler is in place and in operation ; all con- stant expenses are known and all variable costs of mainten- ance and operation are determinable. The question arises, or may arise whenever additional steam may be usefully employed : How much work can be obtained from the ap- paratus when driven to such an extent as to yield the maximum amount of steam per dollar of total cost of operation ? The independent variable is now the quantity of fuel burned in the boiler, and this is, in the established equation, represented by P y the fuel burned per unit of area of grate. This problem is thus stated : Given : All expenses, constant and variable, the method of variation of the latter, and the proportions of the boiler being given, to determine that rate of combustion which will make the commercial efficiency of the given plant a maximum. For this case let K represent that total annual expense of working which is independent of Classes I and 2 and which falls. TS- into Class 3, and let k = -^-. Let all other symbols stand as before. Then the total cost of maintenance and operation will be P = kG+DGS+CFG, while the work done will be, as before, 31 482 THE STEAM-BOILER. The quantity to be made a minimum is, for the present case, the quotient of F by W, F k + DS+CF -W Ef F being taken as the independent variable. This becomes a minimum when we substitute for E^ its value T> E* E , = j-^, and make the first derivative equal zero. Then we find AC When, in this expression for the value of F, giving maxi- mum weight of steam for the dollar expended, we make k = o, the expression maybe reduced, as obviously should be possible, to the form shown already to be that giving the solution of the first problem : The following cases illustrate this problem : EXPENSES OF BOILER AND MAXIMUM ECONOMY OF PLANT. CASES. STATIONARY. MARINE. I. II. III. IV. Cost of maintenance : D ............... ... $0.34 $0.58 $0.88 $0.62 Cost of operation : C. ....... , ......... 8.20 9.00 14.50 3.00 Cost of operation : K. ................. 30.00 25.00 10.50 10.00 For maximum fuel and work : FI .......... 16 13 17 21 For maximum efficiency, as before : F ..... 8 10 16 20 Case No. I is that of a Cornish boiler, No. 2 that of a mul- titubular stationary boiler, No. 3 that of a sea-going steamer, and No. 4 that of a yacht. It is seen that in all cases the weight of steam delivered from the boiler and the quantity of fuel burned at maximum com- mercial efficiency, for the case assumed, are less than where the boiler once set and still capable of being forced to deliver THE EFFICIENCIES OF STEAM-BOILERS. 483 more steam than originally proposed and calculated upon is worked up to a maximum delivery per dollar of total expense. The figures above given should be found amply large. Water-tubular boilers have been known, frequently, to work, for years, steadily without repairs ; and if well handled, all boilers should give low figures for such expense. "Maximum commercial efficiency of boiler" and "Maxi- mum efficiency of a given plant " are therefore by no fneans identical conditions ; and it will usually be found that when this maximum work can be put on the boiler, it might be done still more economically by a boiler specially designed, as in the first problem, to do the increased quantity of work: the conclusion from this fact being simply that economy dictates that as much steam-power as possible should be grouped into a single plant in order to diminish the proportional cost of the constant part of running expenses, i.e., otherwise stated, there being given a certain necessary expenditure, invariable within certain limits with variation of size of boiler or of quantity of steam made, the larger the amount of work done without increasing this constant expense, the cheaper will the steam be made. The larger the plant supervised by the engineer the less the total cost per pound of steam made, other conditions of econ- omy being unchanged. CHAPTER XIV. STEAM-BOILER TRIALS. * 244. The Object of a Trial of a Steam-boiler is to de- termine what is the quantity of steam that a boiler can supply under definitely prescribed conditions ; what is the quality, as to moisture or dryness, of that steam ; what is the amount of fuel demanded to produce that steam ; what the character of the combustion, and the actual conditions of operation of the boiler when at work. The conditions prescribed for one trial may differ greatly from those of another trial, and such differ- ences are often the essential matters to be studied. In any case it is assumed that the conditions under which the boiler is to be worked are to be definitely stated, and the engineer con- ducting the experiments is expected to ascertain all the facts which go to determine the performance of the boiler, and to state them with accuracy, conciseness, and completeness. In the attempt to ascertain those facts the engineer meets with some difficulties, and finds it necessary to exercise the utmost care and skill. In conducting a steam-boiler trial the weight of the water supplied to the boiler must be determined ; the weight of the fuel consumed must be obtained ; the state of the steam made must be determined ; and these quantities must all be noted at frequent intervals. It is also necessary to know whether the combustion is perfect or imperfect, and to what extent the conditions and facts noted are due to the boiler, and what to external conditions. It has now come to be considered that the determination of power and economy of a steam-boiler demands all the care, skill, and perfection of method and of apparatus of any purely scien- tific investigation. It is essential that all work of this kind shall be done in substantially the same way, in order that compari- sons may be made. STEAM-BOILER TRIALS. 485 245. Tests of Value of Fuel are sometimes the sole object of a trial of a steam-boiler, the intent being to ascertain by actual experiment what quantity of water a fuel of unknown quality can evaporate in a boiler of which the general efficiency is fairly well established. In such cases the fuel is employed in the usual manner and the results compared with those ob- tained with fuels of known excellence. Thus, in a good type c/ boiler, having a good proportion of area of heating-surface to weight of fuel burned per hour, it may be found that a fuel of established reputation for uniform excellence will evaporate ten times its own weight of water " from and at " the boiling-point. The trial of a fuel of unknown quality may prove that this boiler will, under precisely similar conditions, evaporate an equal amount of water into steam, and yet the market price of the fuel may be considerably less than that of the other. The immediate result would be the substitution of the second for the first, should no counterbalancing disadvantages exist. In such cases the method of conducting the experiment is precisely the same as where the efficiency of the boiler is de- termined ; but the object sought is quite a different one. This also commonly compels at least two trials, the one of the old and standard, the other of the new and uncertain fuel, and a comparison of boiler-efficiency as found in the two trials. 246. The Determination of the Value of a steam-boiler involves the measurement of its efficiency, independently of the nature of the fuel, and it is thus important that a standard system of measuring the effectiveness of the fuel should be settled upon, or that all variations of such effectiveness should be eliminated. The latter is commonly the course taken ; and the determination of the efficiency of the boiler is based upon the measurement of the evaporation of water, under stated standard conditions, per unit weight of the combustible and burned portion of the fuel supplied during the trial. But the power of the boiler is as important an element of its value as its efficiency, and a complete trial includes, usually, measurements of efficiency at both the rated and the maximum working power of the boiler as operated for its special purpose. 247. The Evaporative Power of Fuels depends upon 486 THE STEAM-BOILER. not only their chemical composition as fuels, but also to an important extent upon their structure and their physical con- dition in every aspect ; on their greater or less purity, and the admixture of earths, moisture, or other foreign matters ; the fitness of the furnace for their utilization ; the air-supply ; its quantity, temperature, and humidity ; the proximity of chilling surfaces ; the extent of the combustion-chamber in which the gases rising from the bed of coal or other combustible may be more or less completely consumed ; and many other minor con- ditions, all of which tell, in a more or less important degree, upon their value and the efficiency of the system of heat- generation. 248. Analyses of Fuels are sometimes made, either as a check upon the results of the trial or in substitution for it. Should analysis show that a given fuel is rich in heat-producing elements, while trial fails to give the results that should have been obtained, and such as the use of other fuels in the same boilers indicates to be possible, it will at once appear that the fuel demands peculiar treatment, or some other arrangement of furnace. Should ^doubt exist which of a number of fuels of the same class is best, chemical analysis may give a quicker and cheaper answer to the question than a formal trial. It rarely happens, however, that any system is as satisfactory, in the end, as actual trial extending over so long a period as to eliminate uncertainties. Methods of analysis differ somewhat. The following is a standard method of general treatment as prescribed by the Union of Engineers of Germany :* In order to take a sample of the fuel, a shovelful from each barrow or wagon will be thrown into a box with a cover. The coal will be mixed up and spread in the form of a square upon a level floor, and then divided by two diagonals into four parts. Of these, two opposite parts will be taken away, the other two will be broken up small and mixed together. Another shovelful will then be thrown in, and the method continued until about 10 kilogrammes are in the box. This will then be * American Engineer, August, 1883. STEAM-BOILER TRIALS. 487 closed and reserved for chemical analysis. For accurate ex- periments the halves which have been taken away should also be analyzed. To determine the moisture in the coal, about 10 grammes from the above-named sample is to be heated for two hours to 105 or 110 C. The loss in weight shows the moisture in the coal. Coal which happens to have been wetted by rain or otherwise should not be used. The test should be applied to coal in the average state of moisture at which it is delivered from the pit mouth, and this state should, if necessary, be determined beforehand. The remainder of the sample, pow- dered and mixed thoroughly, serves to determine the ash, the carbon, the hydrogen, the nitrogen, and the sulphur. The heating-value of the coal is determined as follows : Suppose that it is found to contain c per cent of carbon, h per cent of hydrogen, s per cent of sulphur, o per cent of oxygen, and w per cent of water, then the theoretical heating-value is given by the formula of Dulong as follows : (a). Referred to Water at o Cent. 8 1 ex* + 34320 (//A) +2500?. ' O/ (b). Referred to Water at 100 Cent. 8 1 oc*+ 34200 (A - 1) + 2500.5: - 636.5 (g// + w.) To determine the quantity of air required for burning coal we have the following: One kilogramme of coal requires to burn it, 2.66?c + 8/1 + s o " cu ' metres of ox yg en ; or > x 1.43 2.667;: + S/i + s o cu. metres ot air containing 21 per ct. I<4 3 of oxygen. The analyses should be made with care, by a skilled and experienced chemist, if any important question is to be settled. 249. Economy of Fuel is nearly synonymous with effi- ciency of boiler, as a matter of engineering simply ; but when the finance of the case is studied, it is often found, from that 488 THE STEAM-BOILER. point of view, a very different mattter. It is perfectly possible to adopt so great a proportion of heating-surface, so large a boiler, that the gain in fuel saved, as compared with boilers of similar type and usual proportions, may be more than offset by the increased charges on account of enlargement of boiler. The efficiency of boiler, in the ordinary sense in which that term is used, is, however, a measure of economy. The varia- tion of efficiency and of economy in fuel consumption is a func- tion of the proportion of area and of heating-surface to fuel burned, and the object of a boiler-trial is to ascertain these rela- tions with precision. An understanding should be had before the trial in regard to the kind of fuel to be used ; where no reason of controlling importance exists to the contrary, the best obtain- able coal should be selected, for the reason that a boiler can be better judged, and the results of its trial may be more satisfac- torily compared with similar trials of other boilers, when the very best work of which it is capable is done by it. The differences between separate lots of the best coals are less than the differences between separate lots of inferior fuels, and the comparison is thus less difficult where the former are used. The results of a boiler-trial at Cassel are reported to have given the following distribution of heat :* B. T. U. per cent. Heat of i Ib. coal utilized .11,498.4 80.34 Carried off by gases 1.031.4 7.21 " " " brickwork 286.2 2.00 "ashes 2340 1.63 " " " radiation, etc 1,261.8 8.82 14.311.8 100.00 The coal contained : c 82.51 percent. H 4-73 " " 4.68 " " H 3 1>3 s " ' Ash and Waste 6.70 " " 100.00 * Abstracts of Papers, XC., 1887, p. 70, Inst. C. E. STEAM-BOILER TRIALS. 489 The data of the trial were : Steam pressure (atmos.) 6.36 Water e vap. per hr. , Ibs 4. 501 . 79 " " " sq. ft. H. S. per hr., Ibs 2.99 " " " Ib. coal, Ibs 10.50 Temp, feed-water in tank (Fahr.) 64. 4 " " " from heater 115. 52 " air in boiler-house 69. 8 " gas leaving flues 345-2 Ratio air to theoretical quantity 1.31 Coal per sq. ft. G. S. per hr., Ibs 14.67 " " ' H. S. " " " 0.297 250. The Relative Values of Boilers depend not only on their efficiencies, but also on their capacities for furnishing steam, and on various other qualities and attributes : as their greater or less complication in structure ; their safety and durability ; their volume, weight, and cost. The boiler-trial only settles questions relating to their efficiency and capacity, and their real relations of value, only just so far as those elements enter the problem. These are usually, however, the main factors, and their measurement by a test-trial gives the means of deciding, in nearly all cases, every question likely to present itself in the use of the apparatus. 251. Variations of Efficiency occur with variations in grate-area, in rate of combustion and in kind of fuel. In any given boiler, within a wide range of which the limits are usually far outside of practical conditions, the greater the quantity of fuel burned the less the amount of steam made per unit weight of that fuel; the -smaller the quantity of fuel, burned under proper conditions, in the boiler, the higher the efficiency ; and it has been seen in an earlier chapter, that the gain in efficiency, with increasing proportion of heating to grate surface or to fuel burned, is less and less as this increase goes on. By enlarging or reducing the grate, or by increasing or diminishing the draught and air-supply, and during a suc- cession of trials, noting the method of variation of efficiency and of capacity for making steam, the law of such variations 49O THE STEAM-BOILER. may be established, and the best arrangement, all things con- sidered, may be determined. 252. Variations of Proportions in different boilers, other- wise similar, have been seen to be capable of expression by a very simple algebraic expresssion on which all theories of effi- ciency are based. But in some cases this law is not found to be precisely applicable, and only test-trials of boilers so differing can be relied upon to give correct relations. The general relations already stated invariably hold ; but it often happens that a steam-boiler exhibits peculiarities which make that exact statement inapplicable. It is not uncommon not only to compare actual performance, as shown by trial, with the results indicated by the theory, but also to alter the ratio of heating to grate surface by bricking over more or less of the grate, and by this or other expedients so varying that ratio in successive trials as to obtain an empirical and approxi- mately exact expression for the law of variation of efficiency for the particular case in hand. 253. Combined Power and Efficiency distinguish the best types of boiler. That which, at a given cost, exhibits highest steam-producing power combined with greatest efficiency, is the best boiler. These qualities, however, are not usually com- patible, and increased steam-production from any boiler is com- monly attended with a decrease in efficiency; and as the one or the other of these qualities is the more important, the combi- nation which will give best total result will vary. In no two cases will the same combination be equally desirable. Every boiler must be tested for both before it can be said whether it is satisfactorily adapted to its place and work. 254. The Apparatus and Methods of test-trials should be prescribed in the preliminary arrangements for every trial, and if possible should be in exact accordance with some accepted standard rules. The apparatus consists of scales and tanks for measurement of weights of coal and of water ; gauges to give the pressure of steam ; thermometers of great accuracy to determine the temperatures of water, steam, and flue-gases ; and calorimeters to determine the quality of the steam and STEAM-BOILER TRIALS. 49 1 the extent of superheating, or the percentage of moisture en- trained by it. The establishment of the correctness of this apparatus is the first of the preliminaries to their use. The standardization of the instruments is a matter of supreme importance, since upon their accuracy the whole work of the engineer is depend- ent. It is also a work demanding, in most cases, unusual skill and care, and, to be satisfactory, must generally be performed either at the manufacturer's, or at the office of the engineer conducting the trial. The scales can usually be standardized by the official sealer of weights and measures, and sealed by him ; the water-meters, if used, can be readily tested by the use of the scales so sealed ; the thermometers are, as a rule, best tested by their makers, and should be sent to the maker for test immediately before and directly after the test. The engineer often has a carefully preserved standard with which they may be compared in his own office. The same remarks apply to the examination of the gauges used, which should be standardized both before and after their use. The apparatus used in connection with the calorimeter, in the determina- tion of the quality of the steam made, demand exceptional care in this process. Where it is unavoidable, the use of coarsely graduated thermometers and roughly constructed scales may be permitted, but only then when a very large number of observations are taken, and an average thus ob- tained which may befairly expected to fall within reasonable limits of error. The method of starting and of stopping the trial is a very important matter, and one upon which engineers of experience and acknowledged authority are not in complete accord. The principles to be adhered to in this matter, as in every other detail of the operation of testing a boiler, are easily specified, but they are not always as easy of practice. All conditions should be as exactly the same at the beginning and at the end of the test as they can possibly be made. The period of the trial and the times of stopping and of starting should be capa- ble of being exactly fixed, and the method of test should be 492 THE STEAM-BOILER. such as should permit of the commencement and the end occurring at these exactly defined times, or, as an alterna- tive, they should be such that the work done by the boiler during the less precisely determinable time of beginning and eiiding of the trial should be as nearly as possible nil, so that a slight error as to time may not appreciably affect the results. During the trial, provision should be made for the preserva- tion of the utmost possible uniformity of working conditions throughout the whole period of the trial. Every irregularity gives rise to more or less loss of efficiency, and to uncertainty in regard to the correctness of the reported figures. The nearer the working of the boiler is kept to the final average for the trial, the better. Uniformity of operation and maximum efficiency are best attainable during a trial when a system of record is adopted which allows of that regularity being shown at all times ; and records in proper form are the best possible security against error of observation. Graphical methods should be adopted wherever practicable. Such methods of record exhibit most satisfactorily the accordance with or the deviation from the uniformity of operation considered so desirable on the score of efficiency and accuracy. 255. Standard Test-trials are made under established sys- tems, and in accordance with codes of regulations which are accepted as representing a satisfactory system of procedure. In such cases the first step is to settle upon a standard of measurement and comparison that may be accepted by all who may be interested in the result. The standard nominal horse- power has already been described as now accepted by the best authorities. The Committee of Judges of the Centennial Exhibition, to whom the trials of competing boilers at that exhibition were intrusted, adopted the unit, 30 pounds of water evaporated into dry steam per hour from feed-water at 100 Fahrenheit, and un- der a pressure of seventy pounds per square inch above the atmos- phere, these conditions being considered to represent fairly STEAM-BOILER TRIALS. 493 average practice. The quantity of heat demanded to evaporate a pound of water under these conditions is 1 110.2 British ther- mal units, or 1.1496 " units of evaporation." The unit of power proposed is thus equivalent to the development of 33,305 heat- units per hour, or 34.488 units of evaporation. The " unit of evaporation" is taken as a certain weight preferably unity of water, evaporated " from and at " the boiling-point under atmos- pheric pressure. The now-accepted unit of boiler-power, in the code constructed for the American Society of Mechanical En- gineers,* is the equivalent of the Centennial Standard, and in all standard trials the commercial horse-power is taken as an evaporation of y> pounds of water per hour from a feed-water temperature of 100 Fahr. into steam at 70 pounds gauge-pres- sure, which is equal to 34^ units of evaporation, that is, to 34^- pounds of water evaporated from a feed-water temperature of 212 Fahr. into steam at the same temperature. This standard is equal to 33,305 thermal units per hour.f A boiler rated at any stated horse-power should be capable of developing that power with easy firing, moderate draught and ordinary fuel, while exhibiting good economy; and the boiler should be capable of developing one half or one third more than its rated power to meet emergencies at times when maximum economy is not the most important object to be at- tained. 256. Instructions and Rules governing the standard sys- tem of boiler-trial, prepared by a committee of the American Society of Mechanical Engineers, may be taken as a good illus- tration of such regulations as, in one form or another, have been customarily agreed upon by engineers conducting such work. They are (Thurston's Engine and Boiler Trials) : * Transactions, vol. vi., 1884. f An evaporation of 30 pounds of water from 100 F. into steam at 70 pounds pressure is equal to an evaporation of 34.488 pounds from and at 212; and an evaporation of 34! pounds from and at 212 F. is equal to 30.010 pounds from 100 F., into steam at 70 pounds pressure. The "unit of evaporation" being equal to 965.7 thermal units, the commercial horse-power is 34.488 X 965.7 = 33-3O5 thermal units. 494 THE STEAM-BOILER. x PRELIMINARIES TO A TEST. I. In preparing for and conducting trials of steam-boilers, the specific object of the proposed trial should be clearly defined and steadily kept in view. II. Measure and record the dimensions, position, etc., of grate and heating surfaces, flues and chimneys, proportion of air-space in the grate-surface, kind of draught, natural or forced. III. Put the Boiler in good condition. Have heating-surface clean inside and out, grate-bars and sides of furnace free from clinkers, dust and ashes removed from back connections, leaks in masonry stopped, and all obstructions to draught removed. See that the damper will open to full extent, and that it may be closed when desired. Test for leaks in masonry by firing a little smoky fuel and immediately closing damper. The smoke will then escape through the leaks. IV. Have an understanding with the parties in whose inter- est the test is to be made as to the character of the coal to be used. The coal must be dry, or, if wet, a sample must be dried carefully and a determination of the amount of moisture in the coal made, and the calculation of the results of the test corrected accordingly. Wherever possible, the test should be made with standard coal of a known quality. For that portion of the country east of the Alleghany Mountains good anthracite egg coal or Cumberland semi-bituminous coal may be taken as the standard for making tests. West of the Alleghany Mountains and east of the Missouri River, Pittsburg lump coal may be used.* V. In all important tests a sample of coal should be selected for chemical analysis. VI. Establish the correctness of all apparatus used in the test for weighing and measuring. These are : * These coals are selected because they are almost the only coals which con- tain the essentials of excellence of quality, adaptability to various kinds of fur- naces, grates, boilers, and methods of firing, and wide distribution and general accessibility in the markets. STEAM-BOILER TRIALS. 495 1. Scales for weighing coal, ashes, and water. 2. Tanks, or water-meters for measuring water. Water- meters, as a rule, should only be used as a check on other meas- urements. For accurate work, the water should be weighed or measured in a tank. 3. Thermometers and pyrometers for taking temperatures of air, steam, feed-water, waste gases, etc. 4. Pressure-gauges, draught-gauges, etc. VII. Before beginning a test, the boiler and chimney should be thoroughly heated to their usual working temperature. If the boiler is new, it should be in continuous use at least a week before testing, so as to dry the mortar thoroughly and heat the walls. VIII. Before beginning a test, the boiler and connections should be free from leaks, and all water-connections, including blow and extra-feed pipes, should be disconnected or stopped with blank flanges, except the particular pipe through which water is to be fed to the boiler during the trial. In locations where the reliability of the power is so important that an extra feed-pipe must be kept in position, and in general when for any other reason water-pipes other than the feed-pipes cannot be disconnected, such pipes may be drilled so as to leave openings in their lower sides, which should be kept open throughout the test as a means of detecting leaks, or accidental or unauthorized opening of valves. During the test the blow-off pipe should remain exposed. If an injector is used, it must receive steam directly from the boiler being tested, and not from a steam-pipe, or from any other boiler. See that the steam-pipe is so arranged that water of con- densation cannot run back into the boiler. If the steam-pipe has such an inclination that the water of condensation from any portion of the steam-pipe system may run back into the boiler, it must be trapped so as to prevent this water getting into the boiler without being measured. 496 THE STEAM-BOILER. STARTING AND STOPPING A TEST. A test should last at least ten hours of continuous running, and twenty-four hours whenever practicable. The conditions of the boiler and furnace in all respects should be, as nearly as possible, the same at the end as at the beginning of the test. The steam-pressure should be the same, the water-level the same, the fire upon the grates should be the same in quantity and condition, and the walls, flues, etc., should be of the same temperature. To secure as near an approximation to exact uniformity as possible in conditions of the fire and in tempera- tures of the walls and flues, the following method of starting and stopping a test should be adopted : X. Standard Method. Steam being raised to the working pressure, remove rapidly all the fire from the grate, close the damper, clean the ash-pit, and as quickly as possible start a new fire with weighed wood and coal, noting the time of starting the test and the height of the water-level while the water is in a quiescent state, just before lighting the fire. At the end of the test, remove the whole fire, clean the grates and ash-pit, and note the water-level when the water is in a quiescent state ; record the time of hauling the fire as the end of the test. The water-level should be as nearly as pos- sible the same as at the. beginnihg of the test. If it is not the same, a correction should be made by computation, and not by operating pump after test is completed. It will generally be necessary to regulate the discharge of steam from the boiler tested by means of the stop-valve for a time while fires are being hauled at the beginning and at the end of the test, in order to keep the steam-pressure in the boiler at those times up to the average during the test. XI. Alternate Method. Instead of the Standard Method above described, the following may be employed where local conditions render it necessary : At the regular time for slicing and cleaning fires have them burned rather low, as is usual before cleaning, and then thoroughly cleaned ; note the amount of coal left on the grate as nearly as it can be estimated ; note the pressure of STEAM-BOILER TRIALS. 497 steam and the height of the water-level which should be at the medium height to be carried throughout the test at the same time ; and note this time as the time of starting the test. Fresh coal, which has been weighed, should now be fired. The ash-pits should be thoroughly cleaned at once after starting. Before the end of the test the fires should be burned low, just as before the start, and the fires cleaned in such a manner as to leave the same amount of fire, and in the same condition, on the grates as at the start. The water-level and steam-pressure should be brought to the same point as at the start, and the time of the ending of the test should be noted just before fresh coal is fired. DURING THE TEST. XII. Keep tlic Conditions Uniform. The boiler should be run continuously, without stopping for meal-times or for rise or fall of pressure of steam due to change of demand for steam. The draught being adjusted to the rate of evaporation or com- bustion desired before the test is begun, it should be retained constant during the test by means of the damper. If the boiler is not connected to the same steam-pipe with other boilers, an extra outlet for steam with valve in same should be provided, so that in case the pressure should rise to that at which the safety-valve is set, it may be reduced to the desired point by opening the extra outlet, without checking the fires. If the boiler is connected to a main steam-pipe with other boilers, the safety-valve on the boiler being tested should be set a few pounds higher than those of the other boilers, so that in case of a rise in pressure the other boilers may blow off, and the pressure be reduced by closing their dampers, allowing the damper of the boiler being tested to remain open, and firing as usual. All the conditions should be kept as nearly uniform as pos- sible, such as force of draught, pressure of steam, and height of water. The time of cleaning the fires will depend upon the character of the fuel, the rapidity of combustion, and the kind of grates. When very good coal is used, and the combustion not too rapid, a ten-hour test may be run without any cleaning 498 THE STEAM-BOILER. of the grates, other than just before the beginning and just be- fore the end of the test. But in case the grates have to be cleaned during the test, the intervals between one cleaning and another should be uniform. XIII. Keeping tJie Records. The coal should be weighed and delivered to the firemen in equal portions, each sufficient for about one hour's run, and a fresh portion should not be de- livered until the previous one has all been fired. The time required to consume each portion should be noted, the time be- ing recorded at the instant of firing the first of each new por- tion. It is desirable that at the same time the amount of water fed into the boiler should be accurately noted and recorded, in- cluding the height of the water in the boiler, and the average pressure of steam and temperature of feed during the time. By thus recording the amount of water evaporated by successive portions of coal, the record of the test may be divided into sev- eral divisions, if desired, at the end of the test, to discover the degree of uniformity of combustion, evaporation, and economy at different stages of the test. XIV. Priming Tests. In all tests in which accuracy of re- sults is important, calorimeter tests should be made of the per- centage of moisture in the steam, or of the degree of super- heating. At least ten such tests should be made during the trial of the boiler, or so many as to reduce the probable average error to less than one per cent, and the final records of the boiler test corrected according to the average results of the calorimeter tests. On account of the difficulty of securing accuracy in these tests the greatest care should be taken in the measurements of weights and temperatures. The thermometers should be ac- curate to within a tenth of a degree, and the scales on which the water is weighed to within one hundredth of a pound. ANALYSES OF GASES. MEASUREMENT OF AIR-SUPPLY, ETC. XV. In tests for purposes of scientific research, in which the determination of all the variables entering into the test is de- sired, certain observations should be made which are in general not necessary in tests for commercial purposes. These are the measurement of the air-supply, the determination of its con- STEAM-BOILER TRIALS. 499 tained moisture, the measurement and analysis of the flue- gases, the determination of the amount of heat lost by radiation, of the amount of infiltration of air through the setting, the direct determination by calorimeter experiments of the absolute heating value of the fuel, and (by condensation of all the steam made by the boiler) of the total heat imparted to the water. The analysis of the flue-gases is an especially valuable method of determining the relative value of different methods of firing, or of different kinds of furnaces. In making these analyses great care should be taken to procure average samples, since the composition is apt to vary at different points of the flue, and the analyses should be intrusted only to a thoroughly competent chemist, who is provided with complete and accurate apparatus. As the determination of the other variables mentioned above are not likely to be undertaken except by engineers of high scientific attainments, and as apparatus for making them is likely to be improved in the course of scientific research, it is not deemed advisable to include in this code any specific direc- tions for making them. RECORD OF THE TEST. XVI. A " log" of the test should be kept on properly pre- pared blanks, containing headings as follows : PRESSURES. TEMPERATURES. FUEL. FEED- WATER. u . u d TIME. 1 Steam-gaug Q External Ai Boiler-room d 3 E Feed -water. 1 c7) 1 Pounds. i H Pounds ore. 5oo THE STEAM-BOILER. REPORTING THE TRIAL. XVII. The final results should be recorded upon a properly prepared blank, and should include as many of the following- items as are adapted for the specific object for which the trial is made. The items marked with a * may be omitted for or- dinary trials, but are desirable for comparison with similar data from other sources. Results of the trials of a. Boiler at To determine. . i Date of trial 2. Duration of trial hours DIMENSIONS AND PROPORTIONS. Leave space for complete description. See Ap pendix XXIII. 3. Grate surface. . . ,wide long Area 4. Water-heating surface sq. ft. sq. ft. 5. Superheating-surface sq. ft. 6. Ratio of water heating surface to grate-sur- face AVERAGE PRESSURES. 7. Steam-pressure in boiler, by gauge .... Ibs. *8. Absolute steam-pressure Ibs. *9. Atmospheric pressure, per barometer in. 10. Force of draught in inches of water in. AVERAGE TEMPERATURES. *u. Of external air *I2. Of fire-room *I3. Of steam . 14. Of escaping gases deg. 15. Of feed-water V FUEL. 16. Total amount of coal consumed f. ......... deg. Ibs 17. Moisture in coal 18. Dry coal consumed. ... IKt- 19. Total refuse, drv pounds 20. Total combustible (dry weight of coal, Item 18, less refuse, Item 19) . It* *2i. Dry coal consumed per hour. . Ibs *22. Combustible consumed per hour Ibs. * See reference in paragraph preceding table. t Including equivalent of wood uSed in lighting fire, i pound of wood equals 0.4 pound coal. Not including unburnt coal withdrawn from fire at end of test. STEAM-BOILER TRIALS. 501 RESULTS OF CALORIMETRIC TESTS. 23. Quality of steam, dry steam being taken as 2 i Percentage of moisture in steam . per cent 25 Number of degrees superheated deer WATER. 26. Total weight of water pumped into boiler and apparently evaporated * Ibs 27. Water actually evaporated, corrected for Quality of steam \ . IKc 28. Equivalent water evaporated into dry steam from and at 212 F.f Ibs ~*29. Equivalent total heat derived from fuel in British thermal units f . B T U 30. Equivalent water evaporated into dry steam Ibs ECONOMIC EVAPORATION. 31. Water actually evaporated per pound of dry coal, from actual pressure and tempera- ture f . . Ibs 32. Equivalent water evaporated per pound of dry coal from and at 212 F.f Ibs 33. Equivalent water evaporated per pound of combustible from and at 212 F.f Ibs. * Corrected for inequality of water-level and of steam-pressure at beginning and end of test. f The following shows how some of the items in the above table are de- rived from others: Item 27 = Item 26 X Item 23. Item 28 = Item 27 X Factor of evaporation. TT L Factor of evaporation = . Hand h being respectively the total heat- units in steam of the average observed pressure and in water of the average observed temperature of feed, as obtained from tables of the properties of steam and water. Item 29 = Item 27 X (ff h}. Item 31 = Item 27 -f- Item 18. Item 32 = Item 28 -f- Item 18 or = Item 31 X Factor of evaporation. Item 33 = Item 28 -*- Item 20 or = Item 32 *- (per cent 100 Item 19). Items 36 to 38. First term = Item 20 X - Items 40 to 42. First term = Item 39 X 0.8698. Item 43 = Item 29 X 0.00003 r = ~ Difference of Items 43 and 44 Item 45 = . Item 44. 502 THE STEAM-BOILER. COMMERCIAL EVAPORATION. 34. Equivalent water evaporated per pound of dry coal with one sixth refuse, at 70 pounds gauge-pressure, from temperature of 100 F. = Item 33 multiplied by 0.7249 *37- RATE OF COMBUSTION. 35. Dry coal actually burned per square foot of grate-surface per hour I Per sq. ft. of grate- surface. Per sq. ft. of water- heating surface one sixth refuse. f I Per sq. ft. of least j area for draught. . . RATE OF EVAPORATION. 39. Water evaporated from and at 212 F. per square foot of heating-surface per hour. . . f Water evaporated "| Per sq. ft. of grate- I per hour from tem- 4 j perature of 100 F. 4 ' ) into steam of 70 4 2 ' j pounds gauge-pres- j I sure. f surface Per sq. ft. of water- heating surface. . Per sq. ft. of least area for draught. COMMERCIAL HORSE-POWER. 43. On basis of thirty pounds of water per hour evaporated from temperature of 100 F. into steam of 70 pounds gauge pressure, ( = 34i Ibs. from and at 212) f 44. Horse-power, builders' rating, at square feet per horse power 45. Per cent developed above, or below, rat- ingf Ibs. Ibs. Ibs. Ibs. Ibs. Ibs. Ibs. Ibs. Ibs. H. P. H. P. Per cent. 257. Precautions are to be taken in every possible way to prevent and avoid irregularities in the conduct of the trial and errors of observation.* In preparing for and conducting trials of steam-boilers the specific object of the proposed trial should be clearly defined and steadily kept in view, and as suggested by Mr. Hoadley (i) If it be to determine the efficiency of a given style of boiler or of boiler-setting under normal conditions, the boiler brickwork, grates, dampers, flues, pipes, in short, the whole ap- paratus, should be carefully examined and accurately described, * The appendix to the report above quoted should be read in this connection. STEAM-BOILER TRIALS. 5O3 and any variation from a normal condition should be remedied, if possible, and if irremediable, clearly described and pointed out. (2) If it be to ascertain the condition of a given boiler or set of boilers with a view to the improvement of whatever may be faulty, the conditions actually existing should be accurately observed and clearly described. (3) If the object be to determine the relative value of two or more kinds of coal, or the actual value of any kind, exact equality of conditions should be maintained if possible, or, where that is not practicable, all variations should be duly al- lowed for. (4) Only one variable should be allowed to enter into the problem ; or, since the entire exclusion of disturbing variations cannot usually be effected, they should be kept as closely as possible within narrow limits, and allowed for with all possible accuracy. Blanks should be provided in advance, in which to enter all data observed during the test. The preceding instructions contain the form used in presenting the general results. Rec- ords should be, as far as possible, made in a standard form, in order that all may be comparable. The observations must be made by the engineer conduct- ing the trial, or by his assistants, with this object distinctly in mind ; and each should have a well-defined part of the work assigned him, and should assume responsibility for that part, having a distinct understanding in regard to the extent of his responsibility, and a good idea of the extent and nature of the work done by his colleagues, and the relations of each part to his own. No observations should be permitted to be made by unauthorized persons for entrance upon the log ; and no duties should be permitted to be delegated by one as- sistant to another, without consultation and distinct under- standing with the engineer in charge. The trial should, wher- ever possible, be so conducted that any error that may occur in the record may be detected, checked, or, if advisable, removed, by some process of mutual verification of related observations. It is in this direction that the use of graphical methods of rec- ord and automatic instruments have greatest value. UNIVERSITY 504 THE STEAM-BOILER. Several methods of weighing fuel have been found very satis- factory, but it should be an essential feature that the weights shall be made by one observer and checked by another, at as distant a point as is convenient. The weighing of the fuel by one observer at the point of storage, and the record at that point of times of delivery, as well as of weights of each lot, and the tallying of the number and record of the time of receipt at the furnace-door, will be usually found a safe system. The fail- ure to record any one weight leads to similar error, and can only be certainly prevented by an effective method of double observation and check. The same remarks apply, to a considerable extent, to the weighing of the water fed to the boiler. A careful arrangement of weighing apparatus, a double set of observations, where pos- sible, and thus safe checks on the figures obtained, are essential to certainty of results. With good observers at the tank, and with small demand for water, a single tank can be used ; but two are preferable in all cases, and three should be used if the work demands very large amounts of feed-water, as at trials of very large boilers, or ^f " batteries." The more uniform the water-supply, as well as the more steady the firing, the less the liability to mistake in making the record. The two blanks which follow were prepared by the Author for use in laboratory as well as professional work. 258. The Results of Trials actually conducted under ac^ ceptable conditions, and with all the precautions which have been advised, are illustrated by the following examples : The first case was a trial which was carried out in ac- cordance with the above programme. The measurements of the feed-water were made by passing the water through a Worthington metre into two wooden tanks located on Fair- banks Standard Platform Scales. The pipe connections were so arranged that one tank could be filled and weighed while the other tank was being emptied into the boiler. Each tank was filled once every half hour. As soon as the tank was full and the pumping into the boiler commenced, the temperature of the feed-water was taken by sensitive ther- mometers reading to one-tenth of a degree. STEAM-BOILER TRIALS. 505 W bib _G I | 1 C/5 i jj s ! 1 & Cq II * *^ * 1 1 oo i 11 m ^ 1 5 H 1 ' ^ ft, H ^|g X ^ u' o o M (V " 1 S &U 5 1 ^ * ^j s 3 1 E t/5 i" 13 *c/i H o jj u- ^ s B < 1 O M 1 S ^ H || Cu 1 1 . i r Q So 5 s i S^ s M c/5So tN ^3 ii rto 'S.6 r. w U 3 lil REMARKS. HORSE-POWER. pjtnov pajBH ff+^A v=d ^ NI ff- QNV y jo samvA ^ EFFICIENCY. pajcumsg 'tBjuauiuadxg psmaiisg J i IBjusuiuadxg per cent. EVAPORATION FROM AND AT 212 F., EQUIVALENT TO TOTAL HEAT-UNITS DERIVED FROM FUEL. jnoq 43d 3DBjjns-Sui -1B3H }0 )} 'bS J3J 1 aiqnsnqnuv-) jo panoj aaj 1 \3n jo punodjaj en gg SauBaqaadng jo junonjy aSujsAV i_* 1 508 THE STEAM-BOILER. The measurements of the coal were effected by weighing the coal previous to its being wheeled into a- pile in the coal- room. The second weighing was made when the coal was fed into the furnace. As far as it was possible, ithe furnace was supplied with coal at intervals of every half: hour, so as to correspond as nearly as could be to the feeding of the water. After the completion of the test, a careful analysis of the coal was made, to determine upon a sufficiently large scale its calorific power and the quantity of contained moisture. The steam from the boiler was condensed by means of a continu- ously acting calorimeter, formed by placing four tanks on Fairbanks Standard Platform Scales. The steam from the boiler was passed through a surface-condenser having a condensing surface of 631 sq. ft. As fast as the steam was con- densed from the boiler it was received in small tanks located on platform-scales. These tanks were similar in size to the feed-water tanks, and were so arranged as to be filled and emptied once every half hour, one tank receiving the condensed water from the boiler while the other was being emptied. The condenser was supplied with a large volume of cold water from a weir just outside of the works, and after flowing through the condenser and thereby cooling the steam and receiving therefrom the contained heat, this water was caught in two large tanks placed on platform-scales. These tanks were also arranged so that one tank could be emptied while the other was being filled, and were of sufficient capacity so as to insure catching all of the water required for half an hour's run in the condenser. The temperature of the inlet water of the condenser, of the outlet water, and of the condensed steam were carefully noted by means of thermometers reading to a tenth of a degree. Readings of the inlet water and of the condensed steam were taken once every half hour at the same time that the quantities of the water in the tanks were weighed. Inasmuch as the outlet to the condenser varied considerably in temperature, readings on this were taken every five minutes during the entire time of the test. It will thus be seen that a very correct average of the amount of heat given to the condenser was obtained. The quantity of air supplied STEAM-BOILER TRIALS. by the blowers to the furnace was measured by continuously acting anemometers placed in the supply-pipes. The readings of the anemometers were checked by means of the number of revolutions of the blowers and their cubic feet per revolution. The steam-pressure was kept by a recording pressure-gauge, which was checked by an exceedingly delicate and sensi- tive gauge, which previously, and subsequently to the test, was carefully verified by means of a mercury column. Constant records of the hygrometer, barometer, and thermometers, both in the boiler-room and of the external air, were kept during the entire period of the test. It will be seen from the above, that all of the processes and measurements were kept in duplicate in such a way as to afford a constant check on each other and preclude the possibility of any errors. Samples of steam were taken in a small calorimeter for the purpose of ascertaining whether the boiler supplied wet steam. The following is a brief condensed summary : EFFICIENCY AS PER TEST, 7.50 A.M. to 7. 50 A.M. Total heat of boiler 64,536,613 heat-units. Steam 42,933,141 " " 66.6 per cent. Heat escaping in flue -gases 9,669,036 " " 15 " " Radiated heat 5,162,939 " " 8 " " Heat to vaporize moisture in coal 141.372 " ' " 0.2 " " Heat to vaporize moisture in air supplied to furnace 345.978 " " 0.4 " " Leakage 3.531.645 " " 4-O " " " from pump 127,936 ' 0.2 ' Heat absorbed by fire-brick 2,581,645 " " 4.0 " " Unaccounted for 1,092,941 " 1.6 " " In the trial of an upright boiler reported on by Sir Frederick Bramwell, in 1876, coke being used as the fuel and wood in starting the fires, the following data* were obtained : Ash and moisture 43-79 Ibs. Combustible. 194.46 " Total fuel 238. 25 Air used per pound combustible 17-! * Conversion of Heat into Work. Anderson. 5 io THE STEAM-BOILED. Heat generated, net 2,798,312 B. T. u. per Ib. fuel n,745 available.net 2,101,700 "" Water evaporated 1,620 Ibs. The efficiency of the furnace was 0.643 " The balance-sheet stands thus : Dr. Available heat 2,101.700 B. T. u. Or. Per Cent. 88.29 Heat expended in evaporation 1,855,900 7.03 Displacing atmosphere 147, 720 3.35 Loss by conduction and radiation 70,430 .05 Heat in ashes 1,129 1.26 Unaccounted for 25,521 B. T. Ui 100.00 2,101,700 The following are data from a trial of a Galloway boiler, as reported to the Edgemoor Iron Co., in the year 1885, by Messrs. G. N. Comly and R. Dawes, and the efficiency too near the theoretical maximum to be often duplicated. The boiler tested was fitted with an " economizer," or feed- water heater, and the power developed was considerably under its rating. The fuel was a Pennsylvania bituminous. The draught was obtained by a high chimney, and was, as shown in the table, quite powerful. The tabular statement is mainly given as illustrating a very compact form of record of results. TABLE OF RESULTS OF THE TEST OF A GALLOWAY BOILER AT FRANKFORD JUNCTION, PHILADELPHIA, PA. Date of Trial . . ... . . .w April 8th, 1885. nJ4 hours. Height of Stack 200 feet. Boiler, seven feet in diameter, twenty-eight feet long. ' Grate-surface . . . ' . . 35.75 sq. ft. DIMENSIONS AND PROPORTIONS: Water-heating-surface .... ..- Superheating-surface . . . . . Ratio of Water-heating Surface to Grate-sur- 853 225 face 23.86 to i sq. ft. Economizer Heating-surface, per each boiler 609 sq. ft. Force of Draught, in inches, at stack base, after leaving economizer .... .75 ins. of water. Force of Draught, in inches, at back of boiler, AVERAGE PRESSURES: before entering economizer Force of Draught, in inches, at front of boiler, before entering economizer Absolute Steam-pressure .5625 .6063 Atmospheric Pressure, per barometer . Steam-pressure in boiler, by gauge 29.975 inches. 78.875 pounds. STEAM-BOILER TRIALS. AVERAGE TEMPERATURES:' FUEL: RESULTS OF CALORIMETRIC TESTS: WATER: ECONOMIC EVAPORATION; Of External Air Of Fire-room Of Steam Of Chimney-flue, escaping gases Of Side-flue, at back end of boiler, escaping gases Of Side-flue, at front end of boiler, escaping gases Of Feed-water . . . . . . Of Feed-water, after leaving economizer, and entering boiler Total amount of Coal consumed Total Refuse from coal . . . . Moisture in Coal Total Combustible Dry Coal consumed, per hour .... Combustible consumed, per hour Dry Coal consumed, per indicated horse-power, per hour ....... Combustible consumed, per indicated horse- power, per hour ...... f Quality of Steam, dry steam being taken as J unity ........ I Percentage of Moisture in steam (^ Number of Degrees superheated Height of Water in gauge-glasses Total weight of Water pumped into boiler Of this there was used as hot water . Converted into Steam Water actually evaporated, corrected for qual- ity of steam Equivalent Water evaporated into dry steam from and at 212 F Percentage of increase of Evaporative Capacity by using economizer Equivalent Water evaporated into dry steam from and at 212 F. per hour . Equivalent total Heat derived from fuel, in British thermal units ..... Equivalent total Heat derived from one pound of dry coal ....... Equivalent total Heat derived from one pound of combustible ...... Water actually evaporated, per pound of dry coal, from actual pressure and temperature Water actually evaporated, per pound of com- bustible Equivalent Water evaporated, per pound of dry coal, from and at 212 F Equivalent Water evaporated, per pound of combustible, from and at 212 F. Equivalent Water evaporated, per pound of combustible, from and at 212 F. Boiler and Economizer used together. By boiler exclusive of econo- RATE OF COMBUSTION: Dry Coal actually burned, per square foot of grate-surface, per hour .... Consumption of dry f Per square foot of grate- Coal per hour, coal I surface assumed with onel Per square foot of sixth refuse, (. water-heating surface . 58 degrees. 66 381 " 200 " 3 60 589 " 84 " 155 " 6925 pounds. 569 301 6055 1.87 " .72 1.019984. None. 58 degrees. 4.63 inches. 68, 138 pounds. 2,782 " 65,356 " 66,854 " 78,112 " 6& per cent. ( 6943 pounds. ( 1 1 6 cubic feet. 75.432885. .11389. .12459. 10.093 pounds. 11.041 " 11.79$ 12.907 12.153 " 16.46 18.07 '" 0-745 " 5*2 THE STEAM-BOILER. RATE OF EVAPORATION: Water evaporated from and at 212* F., per square foot of water-heating surface, per hour Water evaporated, per hour, f Per square foot from temperature of of grate-surface 100 F. into steam oH Per square foot seventy pounds' gauge- of water-heat- pressure, i ing surface Horse-power of engine, as per indicator-cards taken on day of boiler-test Kind of Coal used . . . . . .-' V Condition of Chimney-damper .... Cleaned fires, number of times on each fur- nace during the test 8. 139 pounds. 168.9 " 7.079 t: 311.45 horse-power. Ocean bituminous. 58 p.c. of full open'g. In trials conducted by the Author, for a committee of the American Institute, of which he was chairman, in testing a number of different types of boiler,* a surface-condenser was employed to condense all steam made, and results thus for the first time obtained which gave exact measures of net efficiency, the quality of all steam made being determined. In calculating the results from the record of the logs, the committee first determined the amount of heat carried away by the condensing water by deducting the temperature at which it entered from that at which it passed off. To this quantity is added the heat which was carried away by evaporation from the surface of the tank, as determined by placing a cup of water in the tank at the top of the condenser at such height that the level of the water inside and outside the cup were the same, noting the difference of temperatures of the water in the cup and at the overflow, and the loss by evaporation from the cup. The amount of evaporation from the surface of the water in the cup and in the condenser, which latter was ex- posed to the air, was considered as approximately proportional to the tension of vapor due their temperatures, and was so taken in the estimate. The excess of heat in the water of con- densation over that in the feed-water also evidently came from the fuel, and this quantity was also added to those already mentioned. * See Transactions, 1871; also, Report on Mechanical Engineering at Vienna International Exhibition, 1873, R. H. T. STEAM-BOILER TRIALS. 513 The total quantities were, in thermal units, as follows : A 34,072,058.09 B 48,241,833.60 C 24,004,601 . 14 D 38,737,217 57 E 11,951,002.10 These quantities, being divided by the weight of combus- tible used in each boiler during the test, will give a measure of their relative economical efficiency; and, divided by the num- ber of square feet of heating-surface, will indicate their relative capacity for making steam. But as it was the intention of the committee to endeavor to establish a practically correct meas- ure that should serve as a standard of comparison in subsequent trials, it was advisable to correct these amounts by ascertaining how and where errors have entered, and introducing the proper correction. There were two sources of error that are considered to have affected the result as above obtained. The tank being of wood, a considerable quantity of water entered it, leaked out again at the bottom, without increase of temperature, instead of passing through the tank and carrying away the heat, as it is assumed to have done in the above calculation. The meters also registered rather more water than actually passed through them, and this excess assists in making the above figures too high. The sum of these errors the committee estimated at 4 per cent of the total quantity of heat carried away by the condensing water. The other two quantities were considered very nearly correct. Making these deductions, we have the following as the total heat, in British thermal units, which was thrown into the con- denser by each boiler : A 32, 75 1, 835 . 34 B 46,387,827.10 C 23,066,685 . 39 D 37,228, 739-07 E 11,485, 777 . 35 That the figures thus obtained are very accurate, is shown by calculating the heat transferred to the condenser by the the boilers marked A and B (both of which superheated their 33 514 THE STEAM-BOILER. steam), by basing the calculation on the temperature of the steam in the boiler, as given by the thermometer, the results thus obtained being 32,723,681.76 and 46,483,322.5, respec^ tively. Dividing these totals by the pounds of combustible con-~ sumed by each boiler, we get as the quantity of heat per pound, and as a measure of the relative economic efficiency : A 10,281.53 B 10, 246 . 92 C 10,143.66 D 10,048.24 E 10,964.94 Determining the weight, in pounds, of water evaporated per square foot of heating-surface per hour, we get as a measure of the steaming capacity : A 2.65 B 3-59 C 2.83 D 3.10 E... 1.92 The quantity of heat per pound of combustible, as above determined, being divided by the latent heat of steam at 212 Fahrenheit (966. 6), gives as the equivalent evaporation of water at the pressure of the atmosphere, and with the feed at a temperature of 212 Fahrenheit: A 10.64 B 10. 60 C 10.49 D 10.40 E 10.34 For general purposes this is the most useful method of com- parison for economy. The above figures afford a means of comparison of the boilers, irrespective of the condition (wet or dry) of the steam furnished by them. All other things being equal, however, the committee consider that boiler to excel which furnishes the driest steam ; provided that the superheating, if any, does not exceed about 100. STEAM-BOILER TRIALS. 515 In this trial the superheating was as follows : A i6.o8 B 13. 23 C o. D o. E o. As the boilers C, D, E did not superheat, it became an inter- esting and important problem to determine the quantity of water carried over by each with the steam. This we are able, by the method adopted, to determine with great facility and accuracy. Each pound of saturated steam transferred to the condens- ing water the quantity of heat which had been required to raise it from the temperature of the water of condensation to that due to the pressure at which it left the boiler, phis the heat required to evaporate it at that temperature. Each pound of water gives up only the quantity of heat required to raise it from the temperature of the water of condensation to that of the steam with which it is mingled. The total amount of heat is made up of two quantities, therefore, and a very simple algebraic equation may be constructed which shall express the conditions of the problem: Let H = heat-units transferred per pound of steam. // heat-units transferred per pound of water. U = total quantity of heat transferred to condenser. W = total weight of steam and water, or of feed-water. x = total weight of steam. W x = total weight of water primed. Then U Hx + h(W - x) = U; or x = 77' Substituting the proper values in this equation, we deter- 5 i6 THE STEAM-BOILER. mine the absolute weights and percentages of steam and water delivered by the several boilers as follows : Weight of Steam. Weight of Water. Percentage of Water Primed to Water Evaporated. 27,896. 0. 0. B 39,670. O. O. r 19.782.94 645 . 06 3.26 j) 31,663.35 2,336.65 6.9 E 9,855.6 296.9 3. And the amount of water, in pounds, actually evaporated per pound of combustible : A 8.76 B 8.76 C 8.70 D.. 8-55 9.41 Comparing the above results, the committee were enabled to state the following order of capacity and of economy in the boilers exhibited, and their relative percentage of useful effect, as compared with the economical value of a steam-boiler that should utilize all of the heat contained in the fuel: Steaming Capacity. Economy of Fuel. Percentage of Economical Effect. A No d No 2 O 7OQ B , No I No 3 O 7O7 C No 3 No 4 o 600 D No 2 No 5 E No 5 No i O 7^6 The results obtained as above, and other very useful deter- minations derived from this extremely interesting trial, were given in the table, as a valuable standard set of data with which to compare the results of future trials, and as a useful aid in judging of the accuracy of statements made by boiler-venders in the endeavor to effect sales by presenting extravagant claims of economy in fuel. Mr. Drewitt Halpin found the following net results of test of a variety of English-built boilers : STEAM-BOILER TRIALS. 517 No. DESCRIPTION OF BOILER. POUNDS WATER EVAPORATED. THERMAL UNITS. Efficiency. TL 1| Per square foot of heating-surface per hour. Per pound of fuel from and at 212 degrees. In fuel. Transmitted per hour per sq. ft. heating-surface per hour. Per pound fuel. II u ^Idl? 7 1 5 14,296 14.004 14.600 I3-550 13,550 13,550 J 3,55o 14,727 M,7 2 7 14.727 14.727 4,4M 2,202 2,482 1,468 2,183 1,700 3.438 1.516 2,733 1,816 4>5Q5 2,482 1,381 . 9,495 4,462 12,142 13,263 6-530 7,138 12,113 J 4,354 17,291 20.034 d 8,529 10,461 10,558 9,882 J o,i33 11.408 9o92 I2 ,393 9,553 ",833 7.500 10,529 11,125 9,930 10,287 7,940 8,636 9,669 10,819 8,085 7,523 7,235 6,800 e 67 68 77 ii 75 % 78 ; 7 1 70 5 i 51 49 2 Field Field 98,356 148,444 130.900 118,248 108.248 185,844 136.200 229.750 166,294 107,718 664,650 312,340 704,236 835,569 463,630 549,626 654,102 732,054 847.259 921,564 Portable \ $J Portable (;= Portable f }j Portable ) (j Lancashire Lancashire Jacketed Lancashire . ... Loco. (Webb) Loco. (Marie) Loco. ) L co - I Coke Loco, f '- Loco. ) Torpedo ... Torpedo Torpedo Torpedo The " locomotive" boiler is found to be more efficient as a part of the engine and on the track than when mounted as a stationary boiler, an unexpected result. 259. The Quality of Steam made in any boiler, or as sup- plied to an engine, is hardly less important than the quantity. When the steam is required for heating purposes simply, or even when all the heat issuing as waste, necessary or other, from the exhaust-ports of a non-condensing engine cylinder can be utilized for useful and paying purposes, this is a matter of no importance; but when it is essential that loss in the engine shall be made a minimum, and that the engine shall have maximum efficiency, the quality of the steam becomes exceedingly important. Dry steam is very much more efficient as a working substance in the steam-engine than wet ; since, where the latter is supplied from the boiler, the waste by cylinder-condensation is greatly increased and so greatly that the more obvious direct loss by the passing of heat through the engine in unavailable form, hot water acting as its vehicle, becomes comparatively small. The determination of the quality 518 THE STEAM-BOILER. of steam by any boiler is thus as important as the measure of its apparent evaporation. The difference between the apparent and the actual evapo- ration is often very great. A good boiler properly managed will usually " prime" less than five per cent, even though having no superheating-surface, and less than two per cent may usually be hoped for. Steam is often made practically dry. But a hard-worked boiler, or one having defective circu- lation, will often prime ten or twenty per cent ; and cases have been found in the experience of the Author in which the quan tity of water carried out of the boiler by the current of steam ex- ceeded the weight of the steam itself. It has thus happened that, where no measure of this defect has been made, the apparent evaporation only being reported, the quantity of water said to have been evaporated has equalled, and sometimes has even greatly exceeded, the theoretically possible evaporation of an absolutely perfect boiler. It is thus essential that, when the apparent evaporation has been determined by trial, the quantity of water entrained with the steam be measured and deducted, and then real evaporation thus ascertained and reduced for the standard conditions. Under ordinarily good conditions, a real evaporation of ten or eleven times the weight of the fuel, cor- responding to an efficiency of 0.75 to 0.80, represents the best practice, and a real evaporation of twelve of water by one of combustible, from and at the boiling-point, or an efficiency of eighty per cent, is rarely observed under the usually best con- ditions of steam-boiler practice. Where more than the efficiency here given as probable is reported, the work should be very care- fully revised, and errors sought until absolute certainty is secured. Trials not including calorimetric measurement of the water entrained with the steam are comparatively valueless, and should be rejected in any important case. Reports of extra- ordinary economy are often based on this kind of error. The experiments of M. Him at Mulhouse showed an average of about 5 per cent priming ; Zeuner makes it approximately from J\ to 15 per cent; while the experiments of the Author at the American Institute in 1871 give from 3 to 6.9 per cent. STEAM-BOILER TRIALS. 519 A recently devised method of measuring the amount of moisture in the steam is to introduce into the boiler with the feed-water sulphate of soda, and at intervals to draw from the lower gauge-cock a small amount of water, and also from the steam, condensing either by a coil of pipe in water or a small pipe in' air. A chemical analysis gives the proportion of sul- phate of soda in each portion, and the quotient of the propor- tion of sulphate of soda in the portion from the steam by the proportion in that from the water gives the ratio of water entrained, as steam does not carry sulphate of soda, which is only brought over by the hot water entrained. This method was used by Professor Stahlschmidt at the Diisseldorf Exhibi- tion Trials. 260. The Calorimeters used in determining the quantity of moisture in steam have several forms, widely differing in construction, and to some extent in value. They nearly all embody the same principles, however. The objects sought to be attained in their construction are : The exact measure- ment of the weight of steam received by them from the boiler, and of its temperature and pressure at the boiler ; the determi- nation of the weight of water used in its condensation and the range of temperature through which it is raised in the operation ; the reduction of wastes of heat in the calorimeter to a minimum, and the exact measurement of that waste if it is sensibly or practically noticeable. The Barrel or Tank Calorimeter as employed by the Author, is the simplest form o this instrument which has been employed. It consists of a strong barrel or tank, of hardwood, absorbing little of either water or heat, and having a movable cover. This tank is mounted on platform-scales capable of accurate adjustment and having as fine readings as possible. It is filled with water to within about one fourth its height from the top, and the steam is led into it through a rubber tube or hose of sufficient capacity to supply the steam to the amount of one eighth or one tenth the weight of the water in three or five minutes. A steam-gauge of known accuracy gives the boiler-pressure, and the corresponding temperature and total heat of the steam are ascertained from the steam-tables. 520 THE STEAM-BOILER. In using this apparatus the steam is rapidly passed into the mass of water contained in the tank, until the scales show that the desired quantity has been added. The steam is so directed by varying the position of the end of the tube, and by inserting it so deeply in the water that the whole mass is very thoroughly stirred, and a very perfect mixture secured of condensing water with the water of condensation ; and so that the temperatures indicated by the inserted thermometer shall be the real mean temperature of the mass. The weights and FIG. 118. THE CALORIMETER. temperatures are then inserted in the log of the trial, as below, and the proportion of water brought over with the steam is thence easily calculable. The thermometers employed usually read to tenths of a degree Fahrenheit, or to twentieths of a centigrade degree, accordingly as the one or the other scale is employed. Readings must be made with the greatest pos- sible accuracy, and in sufficient number to insure a satis- factorily exact mean. With good thermometers and scales, a reliable gauge, and care in operation, good results can be obtained by averaging a series of trials.* The Hirn Calorimeter is substantially the same as the above, with the addition of an apparatus for stirring the water * Report on Boiler Trial, Trans. A. S. M. E. 1884, vol. vi. STEAM-BOILER TRIALS. 521 in the tank to insure thorough mixture and readings of tem- perature of condensing water exactly representative of the true mean temperature of the mass after the introduction of the steam. This is not an essential feature of the apparatus, if the Author may judge by his own experience, provided the jet of entering steam is so directed as to cause rapid circulation. No stirring apparatus could operate more efficiently than the force of the steam itself, properly directed. Hirn was probably the first (1868) to attempt the determination of the quality of steam as delivered from steam-boilers.* A similar apparatus was used at the trials of the Centennial International Exhibition, Philadelphia, 1876^ 261. The Theory of the Calorimeter is as follows :J Each pound of saturated steam transferred to the condens- ing water the quantity of heat which had been required to raise it from the temperature of the water of condensation to that due to the pressure at which it left the boiler, plus the heat required to evaporste it at that temperature. Each pound of water gives up only the quantity of heat required to raise it from the temperature of the water of condensation to that of the steam, with which it is mingled. The total amount of heat is made up of two quantities, therefore, and a very simple algebraic equation may be constructed, which shall express the conditions of the problem : Let, as in 258, H = heat-units transferred per pound of steam ; h = heat-units transferred per pound of water ; U = total quantity of heat transferred to condenser ; w = total weight of steam and water, or of feed-water ; x = total weight of steam ; W = condensing water; w x = total weight of water primed. * Bulletin de la Societe Industrielle de Mulhouse, 1868-9. f Reports of Judges, vol. vi. \ First published by the Author, who had not then become aware of the work done by M. Hirn, in Trans. Am. Inst. Report on Boiler Trial, 1871. See also Vienna Reports, vol. iii. p. 123. THE STEAM-BOILER. -X)=U', or * = U - wh ~ H -h ' Substituting the proper values in this equation, we deter- mine the absolute weights and percentages of steam and water delivered by the boiler. Or, let Q = quality of the steam, dry saturated steam being unity ; H' total heat of steam at observed pressure ; T = " " " water h' =. " " " condensing water, original ; /^ " " " " final. And we have the equivalent expression, as written by Mr. Kent, The value of the quantity [7 is obtained by multiplying the weight of water in the calorimeter originally by the range of temperature caused by the introduction of. the steam from the boiler. Mr. Emery employs another form, as below, in which Q is the quality of steam as before ; W the weight of con- densing water ; w the weight added from the boiler ; T the temperature due the steam-pressure in the boiler ; t the initial and /, the final temperature of the calorimeter ; / the latent heat of evaporation of the boiler-steam ; and x the weight of steam corresponding to /. Thus, , /) - w(T- /,) w x and n = - = , ~ ~ w lw STEAM-BOILER TRIALS. 523 If Q exceeds unity, the steam is superheated by the amount ( -^r-= 2 -833/(e-/);* and if less than unity, the priming is, in per cent, 100 (i Q). 262. Records of calorimetric tests should be even more carefully and more frequently made than in any other part of the work of a boiler-trial. The following, from work conducted by the Author, illustrates the method. The symbols relate to the first of the above formulas. PRIMING TESTS. w w T T' _ 3* 1 ^T3 "o 1" rt a rt . * ft H. w *O cfl * Date. Gauge- a 5 SH ^ UH 3 u g ^S G^ . c * fe pressure. ^ a ctf 3 O *" cd "* oJ Number f Number ( let stea heated. Number outlet superhe Number wet stea heated. i^is pit "55 tfl , N ^JO 2I-79-S 21 y N unity of weight of this coal will then give, at a temperature of o and a pressure of one atmosphere, C = carbonic acid : 10 KO =.- = oxygen: KN = nitrogen. The quantity of moisture in the escaping gases may be cal- culated from the moisture in the coal, from that formed by burning the hydrogen, and from that contained in the air ad- mitted to the furnace where the latter has been determined. Any serious break in the setting can be detected by filling the grate with smoky coal and then closing the damper. The apparatus designed by Professor Elliott, and employed in work carried on under the direction of the Author, consists, STEAM-BOILER TRIALS. 533 FIG. i2i. APPARATUS FOR GAS ANALYSIS. as shown in Fig. 121, of two vertical glass tubes, AB\ A' B' , joined by rubber-tubing, E, at their upper ends. The large tube, AB, is the treating, the smaller, A'B', the measuring tube; the latter is suitably graduated to cubic centi- metres. Water-bottles, K, L, are connected with the lower ends of the tubes by tubing, NO, N'O\ and are used in effecting transfer of the gas from tube to tube. M is a funnel through which the reagents used may be in- troduced. G, F, and / are cocks of suitable size and construction. In filling the apparatus it is set up conveniently near the flue, and the line of tubing from the collector, within the latter, is connected with the tube AB. The receiver L being de- tached the lower end of AB is connected with an aspirator or equivalent apparatus, such, for example, as might be improvised by the use of an air-tight tank or a barrel ; and the flow thus produced, when the aspirator is emptied of its water, fills the tube AB with gas drawn from the flue. It is retained by clos- ing the valves F and /, which had been open during the opera- tion of filling. The tube is then disconnected from the aspi- rator, and the receiver, or bottle, L, connected as shown, and in such manner that no air can reach the tube AB. Removing the apparatus to the laboratory or other con- venient location, the analysis is made as follows : Pass into A ' B' a convenient volume, as 100 c.c. of the gas, .and discharge the remainder through the valve and funnel F and M, filling the tube AB with water from L. Transfer the measured gas back to AB, through E, and add a solution from M, which will absorb some one constituent. Return the gas to A'B', and again read its volumes. The difference is the quan- tity of gas absorbed. Repeat this process, using next an ab- sorbent which will take up a second constituent of the gas, and thus obtain a second measure of volume ; and thus continue until all the desired determinations are made. All readings should be made at the same temperature, or practically so. The tube 534 THE STEAM-BOILER. AB should be well washed at each operation, in order that no> reagent should be affected by traces of that previously used. The absorbents employed are best taken in the following order: 1. Caustic potash to absorb carbonic acid. 2. Potassium pyrogallate to absorb free oxygen. 3. Cuprous chloride in concentrated hydrochloric-acid solu- tion to absorb carbonic oxide. After their use nitrogen will remain, and will be measured as a balance which, added to the sum of the measured volumes of gases absorbed, should give the original total. Where weights are to be determined, the volumetric measures ob- tained as above are to be reduced by the usual process. The atomic weights of the principal constituents being, oxygen, 16; nitrogen, 14; carbon monoxide, 28; carbon dioxide, 44, we shall have by percentages, where the symbols represent per cent in volumes, for each, when the total is M = iN i6O 2SCO i*N \6O 28CO 44CO, ~W' ~W' ~W' -^-.respectively. Since the total per cent of oxygen is measured by -- CO, + 44 16 12 -gCO + free oxygen, and the total per cent of carbon is CO t I r + ^' we S ^ a ^ ^ ave ^ or *ke P ercenta ge of each, __ 32 x 44 X CO, 16 X 28 X CO i6O ~ ~ M c = _ 12 X 44 X CO, 12 x 28 X CO ; STEAM-BOILER TRIALS. 535 or, + O) c - M The total oxygen is that which entered the furnace as the supporter of combustion, and is a measure of the air supplied. The ratio of free to combined oxygen is a measure of the ratio of the air acting as a diluent simply to that supporting com- bustion. Thus these measurements exhibit the efficiency of combus- tion, the quantity of air employed, and the magnitude of the wastes of heat at the chimney, occurring through imperfect combustion or excess of air-supply. It is evident, however, that where moisture or steam accompanies the gases, it escapes measurement ; this, however, introduces no important error in ordinary work. 266. Efficiency of Combustion is indicated by the analysis of the flue-gases with very great certainty. The appearance of carbon monoxide at the chimney proves the combustion to be imperfect in proportion as it is more or less abundant. The presence of unconsumed oxygen, on the other hand, in the ab- sence of carbon monoxide, proves an excess of air-supply. Both gases appearing is a proof of incomplete intermixture of air and combustible, or of so low a temperature of furnace as to check combustion. This analysis being compared with that of the fuel reveals the character and the perfection of combus- tion, and permits a very exact determination to be made of the specific heat of the gases, and is thus a check on calculations of wasted heat. 267. Draught-gauges are made for the purpose of deter- mining the head-producing draught and the intensity of the draught, which are of many forms, but which usually depend upon the measurement of the head of water which balances that head at the chimney. A very compact and accurate form 536 THE STEAM-BOILER. of draught-gauge, used by the Author with very satisfactory results, is that of Mr. J. M. Allen (Fig. 122). A and A' are glass tubes, mounted as shown, communicating with each other by a passage through the base, which may be closed by means of the stop-cock shown. Surrounding the glass tubes are two brass rings, B and B '. These rings are attached to blocks which slide in dovetailed grooves in the F F' FIG. 122. DRAUGHT-GAUGE. body of the instrument, and may be moved up and down by screws at F F f . The scales are divided into fortieths of an inch, and read to thousandths of an inch by the verniers e and e' y which are attached to the sliding rings B B '. If the two short rings are set at different heights, the difference in readings will give the difference of level. The thermometer is for the purpose of noting the temperature of the external air. The method of using the instrument is as follows :* At a con- * The Locomotive, May, 1884, p. 67. STEAM-BOILER TRIALS. 537 venient point near the base of the chimney a hole is made large enough to insert a thermometer. The height from this opening to the top of chimney, and also of grates, should be noted. The chimney-gauge is attached to some convenient wall. The tubes are filled about half full of water, when the verniers afford an easy means of setting it perpendicular. One end of a flexible rubber tube is then inserted into the upper end of one of the glass tubes, and the other end of the tube is in the chimney-flue. The tubes B B' are adjusted until their upper ends are just tangent to the surface of the water in the two tubes. The reading of the two scales is then taken, and their difference. At the same time the temperature of the flue is noted, as well as that of the external atmosphere. Com- parison may then be made with the following table, computed for use in this connection for a chimney 100 feet high, with various temperatures outside and inside of the flue, and on the supposition that the temperature of the chimney is uniform from top to bottom an inaccurate though usual assumption, however. For other heights than 100 feet, the theoretical height is found by simple proportion, thus : Suppose the exter- nal temperature is 60, temperature of flue 380, height of chimney 137 feet, then under 60 at the top of the table, and opposite to 380 interpolated in the left-hand margin, we find .52". Then 100 : 137 :: .52" : .71", which is the required height for a 137-foot chimney, and similarly for any other height. HEIGHT OF WATER COLUMN DUE TO UNBALANCED PRESSURE IN CHIMNEY 100 FEET HIGH. Temperature in the Chimney. Fahr. TEMPERATURE (FAHR.) OF THE EXTERNAL AIR BAROMETER, 14.7. M. 40 60 8O 100 220 25O 300 350 4OO 450 500 .419 .468 541 .607 .662 .714 .760 355 405 478 543 .598 .651 697 .298 347 .420 .486 541 593 .639- .244 .294 .367 432 .488 540 .586 .192 .242 315 .380 .436 .488 534 CHAPTER XV. STEAM-BOILER EXPLOSIONS.* 268. Steam-boiler Explosions are among the most terrible and disastrous of all the many kinds of accident, the introduc- tion of which has marked the advancement of civilization and its material progress. Introduced by Captain Savery at the beginning of the i8th century with the first attempts to apply steam-power to useful purposes, they have increased in fre- quency and in their destructiveness of life and property con- tinually, with increasing steam-pressures and the unintermitted growth of these magazines of stored energy, until to-day the amount of available energy so held in control, and. liable at times to break loose, is often as much as two or even three millions of foot-pounds (276,500 to 414,760 kilogrammetres), and sufficient to raise the enclosing vessel 10,000 or even 20,000 feet (3048 to 6096 m.) into the air, the fluid having a total energy, pound for pound, only comparable with that of gun- powder. In this and the following article it is proposed to present the results of a series of calculations relating to the magnitude of the available energy contained in masses of steam and of water in steam-boilers. This energy has been seen to be meas- ured by the amount of work which may be obtained by the gradual reduction of the temperature of the mass to that due atmospheric pressure by continuous expansion. The subject is one which has often attracted the attention of both the man of science and the engineer. Its importance, both from the standpoint of pure science and from that of science applied in engineering and the minor arts, is such as * This chapter has been separately printed with slight modifications as a monograph " On Steam-boiler Explosions," and published by the Messrs. Wiley. STEAM-BOILER EXPLOSIONS. 539 would justify the expenditure of vastly more time and atten- tion than has yet ever been given it. Mr. Airy * and Professor Rankine f published papers on this subject in the same number of the Pliilosophical Magazine (Nov. 1863), the one dated the 3d September and the other the 5th October of that year. The former had already presented an abstract of his work at the meeting of the British Association of that year. In the first of these papers it is remarked that " very little of the destructive effect of an explosion is due to the steam which is confined in the steam-chamber at the moment of the explosion. The rupture of the boiler is due to the expansive power common at the moment to the steam and the water, both at a temperature higher than the boiling-point ; but as soon as the steam escapes, and thereby diminishes the compressive force upon the water, a new issue of steam takes place from the water, reducing its temperature ; when this escapes, and further diminishes the compressive force, another issue of steam of lower elastic force from the water takes place, again reducing its temperature : and so on, till at length the temperature of the water is reduced to the atmospheric boiling-point, and the pressure of the steam (or rather the excess of steam-pressure over atmospheric pressure) is reduced to o." Thus it is shown that it is the enormous quantity of steam so produced from the water, during this continuous but exceed- ingly rapid operation, that produces the destructive effect of steam-boiler explosions. The action of the steam which may happen to be present in the steam-space at the instant of rupture is considered unimportant. Mr. Airy had, as early as 1849, endeavored to determine the magnitude of the effect thus capable of being produced, but had been unable to do so in consequence of deficiency of data. His determinations, as published finally, were made at his request by Professor W. H. Miller. The data used are the results of the ex- periments of Regnault and of Fairbairn and Tate on the relations of pressure, volume, and temperature of steam, and of an experi- * ' ' Numerical Expression of the Destructive Energy in the Explosions of Steam-boilers." j- " On the Expansive Energy of Heated Water." 540 THE STEAM-BOILER. ment by Mr. George Biddle, by which it was found that a locomo- tive boiler, at four atmospheres pressure, discharged one eighth of its liquid contents by the process of continuous vaporization above outlined, when, the fire being removed, the pressure was reduced to that of the atmosphere. The process of calculation assumes the steam so formed to be applied to do work expand- ing down to the boiling-point, in the operation. The work so done is compared with that of exploding gunpowder, and the conclusion finally reached is that " the destructive energy of one cubic foot of water, at a temperature which produces the pres- sure of 60 Ibs. to the square inch, is equal to that of one pound of gunpowder." The work of Rankine is more exact and more complete, as well as of greater practical utility. The method adopted is that to be described presently, and involves the application of the formulas for the transformation of heat into work which had been ten years earlier derived by Rankine and by Clausius, inde- pendently. This paper would seem to have been brought out by the suggestion made by Airy at the meeting of the British Association. Rankine shows that the energy developed during this, which is an adiabatic method of expansion, depends solely upon the specific heat and the temperatures at the beginning and the end of the expansion, and has no dependence, in any manner, upon any other physical properties of the liquid. He then shows how the quantity of energy latent in heated water may be calculated, and gives, in illustration, the amount so de- termined for eight temperatures exceeding the boiling-point. This subject attracted the attention of the engineer at a very early date. Familiarity with the destructive effects of steam- boiler explosions, the singular mystery that has been supposed to surround their causes, the frequent calls made upon him, in the course of professional practice and of his studies, to exam- ine the subject and to give advice in matters relating to the use of steam, and many other hardly less controlling circumstances, invest this matter with an extraordinary interest. A steam-boiler is a vessel in which is confined a mass of water, and of steam, at a high temperature, and at a pressure greatly in excess of that of the surrounding atmosphere. The STEAM-BOILER EXPLOSIONS. 541 sudden expansion of this mass from its initial pressure down to that of the external air, occurring against the resistance of its 44 shell " or other masses of matter, may develop a very great amount of work by the transformation of its heat into mechani- cal energy, and may cause, as daily occurring accidents remind us, an enormous destruction of life and property. The enclosed fluid consists, in most cases, of a small weight of steam and a great weight of water. In a boiler of a once common and still not uncommon marine type, the Author found the weight of steam to be less than 2 50 pounds, while the weight of water was nearly 40,000 pounds. As will be seen later, under such con- ditions, the quantity of energy stored in the water is vastly in excess of that contained in the steam, notwithstanding the fact that the amount of energy per unit of weight of fluid is enor- mously the greater in the steam. A pound of steam, at a pres- sure of six atmospheres (88.2 pounds per square inch), above zero of pressure, and at its normal temperature, 177 C. (319 F.), has stored in it about 75 British thermal units (32 calories), or nearly 70,000 foot-pounds of mechanical energy per unit of weight, in excess of that which it contains after expansion to atmospheric pressure. A pound of water accompanying that steam, and at the same pressure, has stored within it but about one tenth as much available energy. Nevertheless, the dispro- portion of weight of the two fluids is so much greater as to make the quantity of energy stored in the steam contained in the boiler quite insignificant in comparison with that contained in the water. These facts have been fully illustrated by the figures presented already. 269. The Energy Stored in steam-boilers is capable of very exact computation by the methods already described, and the application of the results there reached gives figures that are quite sufficient to account for the most violently destruc- tive of all recorded cases of explosion. A steam-boiler is not only an apparatus by means of which the potential energy of chemical affinity is rendered actual and available, but it is also a storage-reservoir, or a magazine, in which a quantity of such energy is temporarily held ; and this quantity, always enormous, is directly proportional to the 542 THE STEAM-BOILER. weight of water and of steam which the boiler at the time con- tains. Comparing the energy of water and of steam in the steam- boiler with that of gunpowder, as used in ordnance, it has been found that at high pressures the former become possible rivals of the latter. The energy of gunpowder is somewhat variable, but it has been seen that a cubic foot of heated water, under a pressure of 60 or 70 pounds per square inch, has about the same energy as one pound of gunpowder. The gunpowder exploded has energy sufficient to raise its own weight to a height of nearly 50 miles, while the water has enough to raise its weight about one sixtieth that height. At a low red heat water has about 40 times this latter amount of energy in a form to be so ex- pended. Steam, at 4 atmospheres pressure, yields about one third the energy of an equal weight of gunpowder. At 7 at- mospheres it has as much energy as two fifths of its own weight of powder, and at higher pressures its energy increases very slowly. Below are presented the weights of steam and of water con- tained in each of the more common forms of steam-boilers, the total and relative amounts of energy confined in each under the usual conditions of working in every-day practice, and their relative destructive power in case of explosion. In illustration of the results of application of the computa- tions which have been given in 142, and for the purpose of obtaining some idea of the amount of destructive energy stored in steam-boilers of familiar forms, such as the engineer is con- stantly called upon to deal with, and such as the public are continually endangered by, the following table has been calcu- lated. This table is made up by Mr. C. A. Carr, U. S. N., from notes of dimensions of boilers designed or managed at various times by the Author, or in other ways having special interest to him. They include nearly all of the forms in com- mon use, and are representative of familiar and ordinary prac- tice. No. I is the common, simple, plain cylindrical boiler. It is often adopted when the cheapness of fuel or the impurity of the water supply renders it unadvisable to use the more com- STEAM-BOILER EXPLOSIONS. vo MIOW $& P ^ fOOO t^ o> ~ < g cf ^ 5 A VO O ro CO C? in r^ vo" ?. o o o H *- <(- OOO % vo - t^ ? s JH tx ef w oo ro Om ior^ -jjirj-jr OOO jnjojn HD.MI anvnos saNnoj 888 MWfOCO invoOOOO>n N N OOO McoNcoMcowiHcoint^ txt^OO .1 iul S Sfli^i i 1 i i PL .eldom d - .erious injury to property OUtiidc t lie engine- itself. 'I he explosion of one ol i h< ,< marine boilers while .it Mi would he likely to be destructive of in.mv live*, if not ol the vessel ilself and all on hoaid. Nos. i i .MX! LI are boilers of tin- older types, such as an- still to he seen in steamboats plyini; upon the Hudson and other of our rivers, and in New Y<>ik h.nhoi and hay. No. n is a feturfl-tubular boiler having a shell n > feet in diametei by ; let long, 2 furnaces each /I h < t deep, 8 15-inch and 2 9-inch il,i , and 85 return-tubes, 4^ inches by 15 feet. The boiler weighs -5 tons, contains nearly 20 tons of water and 70 pounds of strain, and at 30 pounds prrs.m. tores 92,000,000 foot- pounds ol available ener;;y, o| \\liu h 2 per Cent H sides ill tllC ste.un. Thb b enough to hoist tin- hoilei oni third ol a mile with a velocity of projection of 330 feet per second. The second ol these two boilers is of t h ,.i in. w . i h! , also of about 2OO h<>rse power, but < irri< ,i little m<>iv watei and strain and stoics I04,OOO,OOO foot-pounds of energy, or enou;.;h \ I.H .. it I'eet. This was a return line boiler, ;;l .ind hav .In II :: , : I eel in d 1. 1 meter, flues 8 A to 15 indie:, in diameter, accoidin;; to loe.il ion. 'I'he ' sect ional " boilers are here seen to have, for 250 horse- power each, weights lair-.in^ from about 35,ooo to 55,000 pounds, to contain from 15,000 to 30,000 pounds of water and Ironi 25 to 58 pounds of steam, to store from 110,000,000 to 230,000,000 foot-pounds of en- i ;\ , <|ual to from 2000 to 5000 fool pounds per pound of boiler. The stored available energy is thus usually less than that of any of the other statinnaiv boilers, and not very far from the amount sioied, pound for pound, by the plain tubular boiler, the best of the older forms. hi evident that their admitted safety from destructive explo- sion does not conn- from this i elation, however, but from the division of the contents into small portions, am I < p< < ially from those details of construction which make it tolerably certain that any rupture shall be local. A violent explosion can only * ome of the ^eneral disruption of a boiler and the liberation at once of large masses of steam and water. 548 THE STEAM-BOILER. 270. The Energy of Steam alone, as stored in the boiler, is given by column 10 of tfie preceding table. It has been seen that it forms but a small and unimportant fraction of the total stored energy of the boiler. The next table exhibits the effect of this portion of the total energy, if considered as acting alone, STORED ENERGY IN THE STEAM-SPACE OF BOILERS. TYPE. Total Energy. Stored in Steam (ft.-lbs.) per Ib. of Boiler. Height of Projection. Initial Velocity per second. i Plain Cylinder 676,693 709,310 2,377.357 1,022,731 1,483.896 2,135,802 1,766,447 1.302,431 1.462.430 2,316,392 i-570,5i7 1,643-854 2,108.110 3-513,830 i, 3U.377 271 42 351 108 76 85 86 107 54 61 28 29 61 79 24 271 f 42 351 1 08 76 85 86 107 l\ 28 29 61 79 24 t. 132 f 32 150 83 69 74 74 83 59 62 42 43 59 7i 39 t. 2 Cornish 4 Plain Tubular ^ Locomotive . . 6 " 7' * 8 " o Scotch Marine 10 , ii Flue and Return-tube 12 " " 13 Water-tube . i; " The study of this table is exceedingly interesting, if made with comparison of the figures already given, and with the facts stated above. It is seen that the height of projection, by the action of steam alone, under the most favorable circumstances, is not only small, insignificant indeed, in comparison with the height due the total stored energy of the boiler, but is proba- bly entirely too small to account for the terrific results of ex- plosions frequently taking place. The figures are those for the stored energy of steam in the working boiler ; they may be doubled, or even trebled, for cases of low water; they still remain, however, comparatively insignificant. The enormous power of molecular forces, even when heat is not added to reinforce them, is illustrated by the often de- STEAM-BOILER EXPLOSIONS. 549 scribed experiments of an artillery officer at Quebec* and others in which a large bombshell is filled with water, safely plugged, and exposed to low temperature. In such cases the expansive force ex- erted, when freezing, by the for- mation of ice and the increase of volume accompanying the forma- tion of the crystals, either drives out the plug, sometimes project- ing it hundreds of yards (Fig. FlG - ^--EXPANSIVE FORCE OF ICE. 123), or actually bursts the thick iron case. In the more familiar cases of purposely produced explosion, the expansion is caused by the production of great quantities of gas previously in solid form. The violence of the familiar explosives as used in ordnance, in mining operations, is com- FIG. 124. AN EXPLOSION. monly due to this combined effect of heat and chemical action, occurring by the sudden action of powerful forces. In the steam-boiler explosion mighty forces previously long held in subjection finally overcome all resistance, and their sudden ap- plication to external bodies constitutes the disaster. 271. Explosion and Bursting are terms which, as often tech- nically used by the engineer, represent radically different phe- nomena. The explosion of a steam-boiler is a sudden and violent * Phenomena of Heat. Cazin. 550 THE STEAM-BOILER. disruption, permitting the stored heat-energy of the enclosed water and steam to be expended in the enormously rapid ex- pansion of its own mass, and, often, in the projection of parts of the boiler in various directions, with such tremendous power as to cause as great destruction of life and property as if the explosion were that of a powder-magazine. The bursting of a boiler is commonly taken to be the rupture, locally, of the structure, by the yielding of its weakest part to a pressure which at the moment may not be deemed excessive, but which is too great for the weakened spot. The collapse of a flue is a form of rupture which is ordinarily considered as of the second class. With high steam-pressure, bursting or the collapse of a flue may occur with a loud report, and may even cause some displacement of the boiler ; but it is not generally termed an explosion when the boiler is simply ruptured, and is not torn into separated pieces. There is, however, no real boundary, and the one grades into the other, with no defined line of de- marcation. It occasionally happens that an explosion takes place with such extraordinary violence and destructive effect that it has- been thought best, especially by French writers, to class it by itself, and it is denoted a detonant or fulminant explosion, " explosion fulminante" In such cases the report is like that of an enormous piece of ordnance ; the boiler is often rent into many parts, or even completely broken up, as if by dynamite ; and surrounding objects are destroyed as if by the discharge of a park of artillery. In any steam-boiler there may at any time exist a state of equilibrium between the resisting power of the boiler and the steam -pressure. In ordinary working, the latter is far within the former; but as time passes the limiting condition is gradually approached, and in every explosion the line is passed. The pressure may rise until the limit of strength is at- tained, or the resisting power of the boiler may decrease to the limit : in either case the passage of the line is marked by ex- plosion, or a less serious method of yielding. 272. The Causes of Boiler-explosions are numerous, but are usually perfectly well understood. Where uncertainty exists,, STEAM-BOILER EXPLOSIONS. 551 it is probably the fact that, were the cause ascertained, it would be found to be simple and well known. It is nevertheless true that some authorities, including a few experienced and distin- guished members of the engineering profession, believe that there are causes, at once obscure and of great potency and en- ergy, which are not yet satisfactorily understood. In this work the many causes to which explosions are, by various practi- tioners and writers, attributed may be divided into the known, the probable, the possible, the improbable, and the impossible and absurd. To the first class belong the general and fairly uniform weakness of boilers as compared with the steam-pressures car- ried ; the sticking of safety-valves, and the thousand and one other causes having their origin in the ignorance, the carelessness, or the utter recklessness of the designer, the builder, or the attend- ants intrusted with their management. To this class may be as- signed the causes of by far the greater proportion of all explo- sions ; and the Author has sometimes questioned whether this category may not cover absolutely all such catastrophes. To the second class may be assigned " low-water," a cause to which it was once customary to attribute nearly all explosions, but which is known to be seldom operative, and so seldom that some authorities now question the possibility of its action at all.* Among the possible causes, acting rarely and under pe- culiar conditions, the Author would place the overheating of water, and the storage of energy in excess of that in the liquid at the temperature due the existing pressure ; the too sudden opening of the throttle-valve or the safety-valve, producing priming and shock ; the spheroidal state of water ; and perhaps other phenomena. The improbable include the latter, however. The action of electricity a favorite idea with the uninformed may be taken as an example of the impossible and absurd. The actual causes of a vast majority of boiler-explosions are now determined by skilled engineers, inspectors, and insurance experts ; and it is by them generally supposed that no so-called mysterious" causes exist, in the sense that they are phenom- * See opinion of Mr. J. M. Allen, Sibley College Lecture, Sd. Am. Supple- ment, Feb. 19, 1887, p. 9272. 552 THE STEAM-BOILER. ena beyond the present range of human knowledge and scien- tific investigation. All recent authorities agree in attributing boiler-explosions, almost without exception, to one or another of the following general classes of causes, and the Author is inclined to make no exception : (1) Defective design: resulting in weakness of shell, of flues, or of bracing or staying ; in defective circulation ; faulty arrangement of parts ; inefficiency of provision for supplying water or taking off steam ; and defects in arrangement leading to strains by unequal expansions, and other matters over which the designer has control. (2) Malconstruction : including choice of defective or im- proper material ; faulty workmanship ; failure to follow instruc- tions and drawings ; omission of stays or braces. (3) Decay of the structure with time or in consequence of lack of care in its preservation ; local defects due to the same cause or to some unobserved or concealed leakage while in operation. (4) Mismanagement in operation, giving rise to excessive pressure; low water; or the sudden throwing of feed-water on overheated surfaces ; or the production of other dangerous conditions ; or failure to make sufficiently frequent inspection and test, and thus to keep watch of those defects which grow dangerous with time. Weakness of boiler or over-pressure of steam are the usual immediate causes of explosions. It has often been suggested that the most destructive boiler- explosions may be attributable to electricity, and may illustrate the effect of an unfamiliar form of lightning. Such hypotheses are, however, absurd. No storage and concentration of elec- tricity could be produced in a vessel composed of the best of conducting materials and enclosing a mass of fluid incapable of causing electrical currents, either great or small, under the con- ditions observed in the steam-boiler. The production of elec- tricity seen in Armstrong's experiments, a phenomenon some- times thought to support this theory, is simply the result of the friction of a moving jet of steam on the nozzle from which STEAM-BOILER EXPLOSIONS. 553 it issued, and presents not the slightest reason for supposing that the electrical hypothesis of the origin of boiler-explosions has any basis of fact. Professor Faraday, in a report to the British Board of Trade, May, 1859, states his belief in the absurdity of the idea that the water within a steam-boiler may become decomposed, and the explosion of a mixture of gases so produced may burst a boiler: " . . . . As respects the decomposition of .the steam by the heated iron, and the separation of hydrogen, no new danger is incurred. Under extreme circumstances, the hydro- gen which could be evolved would be very small in quantity, would not exert greater expansive force than the steam, and would not be able to burn with explosion, and probably not at all, if it, with the steam, escaped through an aperture into the air or even into the fire-place." Decomposition cannot occur in the steam-boiler, ordinarily ; and if it were to happen in consequence of low-water and overheated plates, no oxygen could remain free to explosively combine with it. A half-century ago, M. Arago, in writing of steam-boiler explosions,* asserted that " no cause of explosion exists which cannot be avoided by means at once simple and within reach of every one." A committee of the Franklin Institute, in 1830, asserted f of boiler-explosions that " they proceed, it is be- lieved, in most cases, from defective machinery, improper ar- rangement or distribution of parts, or, finally, from carelessness in management." These conclusions are fully justified by all later experience ; and it is now admitted by all accepted author- ities that a careful examination and study of the facts of the case will almost invariably enable the experienced engineer to determine the origin of the disaster. It follows that it is per- fectly practicable to so design, construct, and manage steam- boilers that there shall be absolutely no danger of explosion. 273. The Statistics of Explosions have been very care- fully collected for many years in some European countries, * Mem. Roy. Acad. Sci. Inst. France, xxi. f Journal Franklin Institute. 1830. 554 THE STEAM-BOILER. notably in France, and are now given for the United States in very reliable form by inspectors, governmental and private, who are thoroughly familiar with the subject. The following is a list reported for the year 1885 : CLASSIFIED LIST OF BOILER-EXPLOSIONS. 1 >, 1 V u - 3 js g i i E S * c a 1 * 1 rt s. s 3 i "5 1 ! s 1 S g Saw-mills and wood-working 5 2 A *2 o 2 2 o <2 ^ T o 35 A T T 2 T I TO 2 2 I I I j 2 Tft Portables, hoisters, and agri- I 2 4 2 3 2 T6 Mines, oil wells, collieries, _ * _, ty ft 3 o T T 3 I T I Orv Paper-mills, bleachers, digest- I Rolling-mills and iron-works I 2 I . . . I I I I 2 Distilleries, breweries, sugar- houses, dye houses, ren- dering establishments, etc. 3 I . . . 3 I I 3 4 2 18 Flour-mills and elevators 7 I 2 i 2 v 10- I i 3 3 ? ? T -3 ,, ^ Total per month 14 20 14 7 12 12 IO 9 ii 14 15 17 155 Persons killed total 220 per month 4. 22 2O 18 14 _ jj i:r IQ 34 <5T Persons injured total 288 per month . . 3e Of) O and its tota , ( "g men ) weight j gs- ( ^ Then the product of weight into range of temperature, into specific heat (o.iu), is the measure of the heat-energy stored. 568 THE STEAM-BOILER. i 375 X 1000 X o.i 1 1 =41,625 B. T. U., nearly; 170 X 556 X o.i 1 1 =10,492 calories, nearly; and in mechanical units, 41,625 X 7/2 = 32,134,500 foot-pounds nearly; 10,502 X 423.55 = 4,443,886 kilog.-metres nearly; which is fifteen or twenty times the energy stored in the steam in a locomotive-boiler in its normal condition, and about one half as much as ordinarily exists in water and steam together. It is evident that the limit to the destructiveness of explosions so caused is the rate of transfer of this energy to the water thrown over the hot plate, and the promptness with which the steam made can be liberated at the safety-valve. A sudden dash of water or spray over the whole of such a surface might be expected to even produce a " fulminating" explosion. For- tunately, as experience has shown, so sudden a transfer or so complete a development of energy rarely, perhaps never, takes place. 278. The Strength of Heated Metal is known usually to decrease gradually with rise in temperature, until, as the weld- ing or the melting-point, as the case may be, is approached, it becomes incapable of sustaining loads. Both iron and steel, however, lose much of their tenacity at a bright-red heat, at which point they have less than one fourth that at ordinary temperatures. A steam-boiler in which any part of the furnace is left unprotected by the falling of the water-level is very likely to yield to the pressure, and an explosion may result from simple weakness. At temperatures well below the red heat this will not happen. 279. " Low Water," in consequence of the obvious dangers which attend it, and the not infrequent narrow escapes which have been known, has often been by experienced engineers considered to be the most common, even the almost invariable, cause of explosions. This view is now refuted by statistics and a more extended observation and experience ; but it re- mains one of the undeniable sources of danger and causes of accident. Its origin is usually in some accidental interruption of the STEAM-BOILER EXPLOSIONS. 569 supply of feed-water ; less often an unobserved leak or ac- celerated production of steam. Whatever the cause, the result is the uncovering of those portions of the heating- surface which are highest, and their exposure, unprotected by any efficient cooling agency, to the heat of the gases passing through the flue at that point. Should it be the case of a locomotive or other boiler having the crown-sheet of its furnace so placed as to be first exposed when the water- level falls, the iron may become heated to a full red heat ; if the highest surfaces are those of tubes, through which gases approximating the chimney in temperature are passing, the heat and the danger are less. In either case danger is incurred only when the temperature becomes such as to soften the iron, or when the return of the water with considerable rapidity gives rise to the production of steam too rapidly to be relieved by the safety-valve or other outlet. Such explosions probably very seldom actually occur, even when all conditions seem fa~ vorable. Every boiler-making establishment is continually col- lecting illustrations of the fact that a sheet may be overheated, and may even alter its form seriously when overheated, without completely yielding to pressure; and the Author has taken part in many attempts to experimentally produce explosions by pumping feed-water into red-hot boilers, and has but once seen a successful experiment. The same operation, in the reg- ular workings of boilers, has been often performed by ignorant or reckless attendants without other disaster than injury to the boiler, but it has unquestionably on other occasions caused terrible loss of life and property. The raising of a safety-valve on a boiler in which the water is low, by producing a greater violence of ebullition in the water on all sides the overheated part, may throw a flood of solid water or of spray over it ; and it is probable that this has been a cause of many explosions. The Author has seen but a single explosion produced in this way, although he has often attempted to so produce such a re- sult. In three experiments on a plain cylindrical boiler, empty and heated to the red heat, the result of rapidly pumping in a large quantity of water was in the first the production of a vacuum, in the second an excess of pressure safely and easily 57O THE STEAM-BOILER. relieved by the safety-valve, and in the third case a violent ex- plosion of the boiler and the complete destruction of the brick masonry of its setting.* A committee of the Franklin Institute, conducting similar experiments^ had the same experience, the pressure " rising from one to twelve atmospheres within two minutes" after starting the pump. The most rapid vapor- ization occurs, as is well known, at a comparatively low temper- ature of metal ; at high temperature the spheroidal condition is produced, and no contact exists between metal and liquid. Mr. C. A. Davis, President of the New York and Boston Steamboat Co., in a letter addressed, Dec. 7, 1831, to the Col- lector of the Port of New York, and answering inquiries of the United States Treasury Department, wrote \\ "I have noted that by far the greatest number of accidents by explosion and collapsing of boilers and flues I might say seven tenths have occurred either while the boat was at rest, or immediately on starting, particularly after temporary stoppages to take in or land passengers. These accidents may occur from directly opposite causes either by not letting off enough steam r or by letting off too much : the latter is by far the most de- structive." The idea of this writer was that the " letting off of too much " steam, producing low-water, was the most frequent cause of explosions an idea which has never since been lost sight of. The chief-engineer of the Manchester (G. B.) Steam-boiler Association, in 1866-67, repeatedly injected water into over- heated steam-boilers, but never succeeded in producing an explosion. Yet, as has been seen, such explosions may occur. A writer in the Journal of the Franklin Institute,! a half- century or more ago, asserted that " the most dreadful accidents from explosions which have taken place have occurred from low-pressure boilers." It was, as he states, " a fact that more persons had been killed by low than by high pressure boilers." * Set. Am., Sept. 1875. f Jour. Franklin Inst. 1837, vol. xvii. \ Report on Steam-boilers, H. R., 1832. Mechanics' Magazine, May, 1867. || Vol. iii. pp 335, 418, 420. STEAM-BOILER EXPLOSIONS. 571 Nearly all writers of that time attributed violent explosions to low-water, and some likened the phenomenon to that observed when the blacksmith strikes with a moist hammer on hot iron. Thus, if the boiler is strong, and built of good iron, and not too much overheated, or if the feed-water is introduced slowly enough, it is possible that it may not be exploded ; but with weaker iron, a higher temperature, or a more rapid development of steam, explosion may occur. Or, if the metal be seriously weakened by the heat, the boiler may give way at the ordinary or a lower pressure ; which result may also be precipitated by the strains due to irregular changes of dimensions accompanying rapid and great changes of temperature. Explosions due to low-water, when there is a considerable mass of water below the level of the overheated metal, are some- times fearfully violent ; a boiler completely emptied of water t and only exploded by the volume of steam contained within it, is far less dangerous. Low-water and red-hot metal in a loco- motive or other firebox boiler are for this reason far more dan- gerous than in a plain cylindrical boiler, since, as was indicated by the experiments conducted by the Author, the latter must be entirely deprived of water before this dangerous condition can arise. In the course of the numerous experiments already al- luded to, many attempts were made to overheat the latter class of boiler; but none were successful until the water was entirely expelled. Experiments with apparatus devised for the purpose of keeping the steam moist under all circumstances indicate that it is difficult if not impossible to overheat even an un- covered firebox crown-sheet if the steam be kept moist, and that such steam is very nearly as good a cooling medium, in such cases, as the water itself. Fig. 125* represents a boiler exploded by the introduction of water after it had been emptied by carelessly leaving open the blow-cock. This boiler was about five years old ; and the explosion, as is usual in such cases, was not violent, the small amount of water entering and the weakness of the sheet con- spiring to prevent the production of very high pressure or the * The Locomotive, Sept. 1886, p. 129. 572 THE STEAM-BOILER. storage of much energy. The whole of the lower part of the shell of the boiler was found, on subsequent examination, to have been greatly overheated. One man was killed by the fall- ing of the setting upon him ; no other damage was done. FIG. 125. BOILER EXPLODED. CAUSE, LOW-WATER. Fig. 126 shows the effect of a similar operation on a water- tube boiler. The feed-water was cut off, and not noticed until the water-level became so low that the boiler was nearly empty and the tubes were overheated. One FIG. I26.-TUBE BL-RST: LOW-WATER. o f the tubes burst, and the damage was speedily repaired at a cost of $15, and the works were running the next day.* That low-water and the consequent overheating of the boiler 'does not necessarily produce disaster, even when the water is again supplied before cooling off, was shown as early as 1811, by the experience 6% Captain E. S. Bunker of the Messrs. Stevens' steamboat Hope, then plying between New York and Albany. During one of the regular passages he dis- covered that the water had been allowed by an intoxicated fire- man to completely leave both the boilers. He at once started the pump, and, filling up the boilers, proceeded on his way, no other sign of danger presenting itself than " a crackling in the * G. H. Babcock. STEAM-BOILER EXPLOSIONS. 573 boiler as the water met the hot iron, the sound of which was like that often heard in a blacksmith's shop when water is thrown on a piece of hot iron." * A year later Captain Bunker repeated this experience at Philadelphia on the Phcenix, where the boilers were of the same number and size as those of the Hope.f Defective circulation may cause the formation of a volume of steam in contact with a submerged portion of the heating- surface. The Author, when in charge of naval boilers during the civil war, 1861-5, found it possible on frequent occasions to draw a considerable volume of practically dry steam from the water-space between the upper parts of two adjacent furnaces at a point two or three feet below the surface-water level. After drawing off steam for a few seconds, through a cock provided to supply hot water for the engine and fire- rooms, water would follow as in the normal condition of the boiler. This condition often occurs in some forms of boiler, and has been occasionally observed by every experienced en- gineer. It would not seem impossible, therefore, that steam might be sometimes thus encaged in contact with the furnace, and thus cause overheating of the adjacent metal. Many such instances have been related ; but they have been commonly regarded by the inexperienced as somewhat apocryphal. ;f In order that the danger of overheating the crown-sheet of the locomotive type of boiler may be lessened, it is very usual to set it lower at the firebox end, when employed as a station- ary boiler, so as to give a greater depth of water over the crown-sheet than over the tubes at the rear. The plan of giv- ing greatest depth of water, when possible, at that end of the boiler at which the heating-surfaces near the water-surface are hottest is always a good one. Mr. Fletcher concluded from his experiments that low-water is only a cause of danger by weaking the overheated plates. He says : *Doc. No. 21, H. R., 25th Congress, 3d Session, 1838, p. 103. f Ibid. \ See London Engineer \ Dec. 7, 1860, pp. 371, 403. London Engineer, Mar. 15, 1867, p. 228. '^\ CNIVEBSITl) X^UFORN)*^/ 574 THE STEAM-BOILER. " These experiments, it is thought, may be accepted as conclusive that the idea of an explosion arising from the in- stantaneous generation of a large amount of steam through the injection of water on hot plates is a fallacy." The conclusion of the Author, in view of the experiments of the committee of the Franklin Institute and of his own per- sonal experience in the actual production of explosions by this very process, as elsewhere described, does not accord with the above ; but it is sufficiently well established that low-water may frequently occur and feed-water may be thrown upon the overheated plates without necessarily causing explosion. Dan- ger does, however, unquestionably arise, and such explosions have most certainly occurred possibly many in the aggre- gate. Low-water is certainly very rarely, perhaps almost never, the cause of explosion of other than firebox boilers ; in these, however, the danger of overheating the crown-sheet of the furnace, if the supply of water fails, is very great, and in such cases explosion is always to be feared. The most disastrous explosions are usually those, however, in which the supply of water is most ample. 280. Sediment and Incrustation sometimes produce the effect of low-water in boilers, even where the surfaces affected are far below the surface of the water. Every increase of re- sistance to the passage of heat through the metal and the in- crusting layer of sediment or scale causes an increase of tem- perature in the metal adjacent to the flame or hot gases, until, finally, the incrustation attaining a certain thickness, the iron or steel of the boiler becomes very nearly as hot as the gases heating it. Should this action continue until a red heat, or a white heat even, as sometimes actually occurs, is reached, the resistance becomes so greatly reduced that the sheet yields, and either assumes the form of a " pocket" or depression, as often happens with good iron or with steel, or it cracks, or it even opens sufficiently to cause an explosion. " Pockets" often form gradually, increasing in extent and depth day by day, until they are discovered, cut out, and a patch or a new sheet put in, or until rupture takes place. In such cases the STEAM-BOILER EXPLOSIONS. 575 incrustation keeps the place covered while permitting just water enough to pass in to cause the extension of the defect. In some cases the process is a different and a more disas- trous one : The scale covers an extended area, permitting it to attain a high temperature. After a time a crack is pro- duced in the scale by the unequal expansion of the two sub- stances and the inextensibility of the incrustation ; and water entering through this crack is exploded into steam, ripping off a wide area of incrustation previously covering the overheated sheet, and giving rise instantly, probably, to an explosion which drives the sheet down into the fire, and may also rend the boiler into pieces, destroying life and property on every side. Such an explosion usually takes place with the boiler full of water and its stored energy a maximum, and the result is correspondingly disastrous. Certain greasy incrustations and some floury forms of min- eral or vegetable deposits have been found peculiarly danger- ous, as, in even exceedingly thin layers, they are such perfect non-conductors as to speedily cause overheating, strains, cracks, leakage, and often explosion. M. Arago mentions a case in which rupture occurred in consequence of the presence of a rag lying on the bottom of a boiler.* The effect of incrustation in causing the overheating of the fire-surfaces, the formation of a " pocket " and final rupture, is well shown in the illustrations which follow. When the water is fully up to the safe level, as at the right in the first of the two figures, the heat received from the fur- nace-gases is promptly carried away by the water, and the sheet is kept cool. When the water falls below that level, or is pre- vented by incrustation from touching the metal, as in the left- hand illustration, the sheet be- comes red-hot, SOft, and Weak, ^ FIG^T^-OVERHEATING THE SHEET. and yields as shown. When this goes on to a sufficient extent, * Report of the Committee of the Franklin Institute. 57 6 THE STEAM-BOILER. as on a horizontal surface (Fig. 128), a pocket is produced. The illustration represents a sheet removed from the shell of an ex- ternally fired boiler thus injured. FIG. 128. A " POCKET.' Finally, when the defect is not observed and the injured sheet removed, the metal may finally give way entirely, per- FIG. 129. RUPTURED POCKET. mitting the steam and water to issue, as in the last illustration of the series, in which this last step in the process is well represented. Where the area thus affected is considerable, FIG. 130. SHELL RUPTURED. the result may be a general breaking up of that portion of the shell, as in the next figure, and an explosion may prove to be the final step in the chain of phenomena described. In other cases, where, as in the next sketch, a line of weakness may be STEAM-BOILER EXPLOSIONS. 577 the result of other causes, a large section of the boiler may be broken out, as at AD, Fig. 131. FIG. 131. EXTENDED RUPTURE. The deposition of sediment and of scale takes place not only in the boiler, but also with some kinds of water, in the feed-pipe, as is illustrated in the accompanying engraving, which is made from an actual case in which the pipe was so nearly filled as to become quite incapable of perform- ing its office. A current has appar- ently no effect, in many such cases, f . FIG. 132. INCRUSTATION IN in preventing the deposition of scale. FEED-PIPE. The Author has known hard scale to form in the cones of a Giffard injector under his charge, where the stream was mov- ing with enormous velocity, and loudly whistling as it passed. Instances are well known of the explosion, with fatal effect, of open vessels, in consequence of the action above described. Mr. G. Gurney in 1831 gave an account of such an explosion of the water in an open caldron, at Meux's brewery, by which one person was killed and several others injured.* It was found that the bottom had become incrusted with sediment, and the sudden rupture of the film, permitting contact of the water above with the overheated metal below, caused such a sudden and violent production of steam that it actually ruptured the vessel. The process of which this is an illustration is precisely analogous to suddenly throwing feed-water into an overheated boiler. * Report on Steam Carriages. Doc. 101, 22d Congress, ist Session, p. 31. 37 5/8 THE STEAM-BOILER. 281. Energy stored in Superheated Water has been sometimes considered a source of danger to steam-boilers and a probable cause of. explosions. The magnitude of this stock of energy is not likely to differ greatly from that of water at the same temperature under the pressure due that tempera- ture, and for present purposes specific heat may be taken as unity. The quantity of heat so stored is therefore measured very nearly by the product of the weight of water so overheated, the mean range of superheating, and the specific heat here taken as unity. It is not known how large a part of the water in any boiler can be superheated, or the extent to which this action can occur. It is often doubted, however, whether it can take place at all in steam-boilers. This condition occurring, the experiments of MM. Donny, Dufour, and others show that the larger the mass of water the less the degree of superheating attainable ; the more im- pure the water, or the greater the departure from the condi- tion of distilled water, and the larger the proportion of air or sediment mechanically suspended, the more difficult is it to at- tain any considerable superheating. As early as 1812,* Gay-Lussac observed a retardation of ebullition in glass vessels; thirty years later, f M. Marcet found that water deprived of air can be raised seveial degrees above its normal boiling-point; while Donny, $ Dufour, Magnus, || and Grove^f all succeeded in developing this phenomenon more or less remarkably. Donny, sealing up water deprived of air in glass tubes, succeeded in raising the boiling point to 138 C. (280 F.), at which temperature vaporization finally occurred explosively. Dufour, by floating globules of pure water in a mixture of oils of density equal to that of the water, succeeded with very minute globules in raising the boiling-point to 175 C. (347 F.), at which temperature the normal tension of its * Ann. de Chimie et de Physique, Ixxxii. f Bibl. Univ. xxxviii. \ Ann. de Ch. et de Phys. ; 3tne serie, xvi. Bibl. Univ., Nov. 1861, t. xii. l| Poggendorff's Ann. t. cxiv. *([ Cosmos, '863. STEAM-BOILER EXPLOSIONS. 579 steam is 1 1 5 pounds per square inch (nearly eight atmospheres) by gauge. In such cases the touch of any solid or of bubbles of gas would produce explosive evaporation. Solutions always boil at temperatures somewhat exceeding the boiling-point of water, but usually quietly and steadily. In all these cases the rise in temperature seems to have been the greater the smaller the mass of water experimented with. In all ordinary cases of steam-boiler operation the mass of water is simply enormous as compared with the quantities em- ployed in the above-described laboratory experiments ; the water is almost never pure, and probably as invariably contains more or less air. It would seem very unlikely that such super- heating could ever occur in practice. There is, however, some evidence indicating that it may. Mr. Wm. Radley* reports experimenting with small labora- tory boilers of the plain cylindrical form, and continuing slowly heating them many hours, finally attaining temperatures ex- ceeding the normal by 15 F. (8. 3 C.). The investigator con- cludes: " Here we have conclusive data suggesting certain rules to be vigorously adopted by all connected with steam-boilers who would avoid mysterious explosions : First, never feed one or more boilers with surplus water that has been boiled a long time in another boiler, but feed each separately. Second, when boilers working singly or fed singly are accustomed, under high pressure, to be worked for a number of hours consecutively, day and night, they should be completely emptied of water at least once every week, and filled with fresh water. Third, in the winter season the feed-water of the boiler should be sup- plied from a running stream or well ; thaw water should never be used as feed for a boiler." " Locomotive, steamboat, and stationary engine boilers have their fires frequently banked up for hours, without feeding water, and the steam fluttering at the safety-valve, so as to have them all ready for starting at a moment. This is a dangerous prac tice, as the foregoing experiments demonstrate. While so * London Mining Journal, June 28, r8s6. 580 THE STEAM-BOILER. standing, all the atmospheric air may be expelled from the water, and it may thereby attain to a high heat, ready to gen- erate suddenly a great steam-pressure when the feed-pump is set in motion. This is, no doubt, the cause of the explosion of many steam-boilers immediately upon starting the engine, even when the gauge indicates plenty of water. The remedy for such explosions must be evident to every engineer keep the feed-pump going, however small may be the feed re- quired." On the other hand, the report of a committee appointed by the French Academy to inquire into the superheated-water theory of steam-boiler explosions indicates at least the difficulty of securing such conditions.* The committee constructed suit- able apparatus, experimented in the most exhaustive manner, and investigated several explosions claimed by the advocates of the theory to have been due to this cause. They failed to su- perheat water under any conditions which could probably occur in practice, and the explosions investigated were shown conclu- sively to have resulted from simple deterioration of the boilers, or from carelessness. It is unquestionably the fact that explo- sions due to this cause are at least exceedingly rare, although it is not at all certain that they may not now and then take place. The ocean is constantly being traversed by thousands of steamers having surface-condensers and boilers in which the water is used over and over again, and in which every condition is seemingly favorable to such superheating of the water ; but no one known instance has yet occurred of the production of this phenomenon, there or elsewhere, on a large scale, where boilers are in regular operation. M. Donny, who first suggested the possibility of this action as a cause of boiler-explosions, has had many followers. M. Dufour,f who doubts if such explosions are possible in the or- dinary working of the boiler, points out the fact, however, that boilers which are not in operation, but which are quietly cool- ing down after the working-hours are over, are peculiarly well * Annales de Mines, 1886. f Sur T^bullition de 1'Eau, et sur une cause probable d' Explosion des Chau- dieres a Vapeur, p. 29. STEAM-BOILER EXPLOSIONS. 581 situated for the development of this form of stored energy. He points out the known fact that many explosions have taken place under such conditions, the pressure having fallen below the working-pressure. M. Gaudry* makes the same observa- tion. Such cases are supposed to be instances of " retarded ebullition" with decrease of pressure and superheating of the water. Many circumstances unquestionably tend to strengthen this view. So tremendous are the effects of many explosions that M. Audrand has expressed the belief that a true explosion must be preceded by pressures approaching or exceeding 200 atmos- pheres ;f an intensity of pressure, however, which no boiler could approximate. Mr. Hall also thinks that the shattering effect sometimes witnessed, resulting in the shattering of a boiler into small pieces, must be the effect of a sudden and enormous force, partaking of the nature of a blow ;: and cites cases, such as are now known to be common, of an explosion taking place on starting an engine, after the boiler has been at rest and making no steam for a considerable time. M. Arago cites a number of similar instances, and Robinson a number in still greater detail. || Boilers after quietly " simmering" all night exploded at the opening of the throttle-valve or the safety-valve in the morning. The locomotive Wauregan, which exploded within sight and hearing of the Author at Prov- idence, R. I., in February, 1856, is mentioned by Colburn as such a case. The engine had been quietly standing in the en- gine-house two hours, the engineer and fireman engaged clean- ing and packing, preparatory to starting out. The explosion was without warning and very violent, stripping off the shell and throwing it up through the roof, and killing the engineer, who was standing beside his engine. Mr. Robinson^f thinks the usual cause of such explosions is * Traite des Machines a Vapeur. f Comptes Rendus, May, 1855, p. 1062. \ Civil Engineers' Journal, 1856, p. 133 ; Dingler's Journal, 1856, p. 12. Annuaire, 1830. | Steam-boiler Explosions, p. 62. 1" Ibid. p. 66. 582 THE STEAM-BOILER. the overheating of the water, the phenomenon being in its ef- fects very like the " water-hammer " in steam-pipes, producing shocks which the Author has shown to give rise to instantaneous pressures exceeding the working pressures ten or twenty times ; the action seems, however, rather to be that "boiling with bumping" familiar to chemists handling sulphuric acid in con- siderable quantities. Instances have been known in which this bumping has burst pipes or severely shaken boilers and setting without producing explosion. The deaeration of water, and the consequent superheat- ing of the liquid, to which some explosions have been attrib- uted, are phenomena which have been often investigated. Mr. A. Guthrie, formerly U. S. Supervising Inspector-General of Steam-vessels, states that he has made many such experiments, as follows :* "(i) In my experiments I first procured a sample of water from the boiler of an ordinary condensing-engine ; here, of course, in addition to being subjected to long-continued boil- ing, it had passed through the vacuum. " (2) I procured a sample from the ordinary high-pressure non-condensing engine-boiler, which before entering the boiler had passed the heater at 210. " (3) I procured some clean snow and dissolved it under oil, so that there was no contact with the air. " (4) I froze some water in a long, upright tube, using only the lower end of the ice when removed from the tube, and dis- solved under oil. " (5) I placed a bottle of water under a powerful vacuum- pump worked by steam, for two hours ; agitating the water from time to time to displace any air that might possibly be confined in it, then closed it by a stop-cock, so that no air could possibly return. " (6) I boiled water in an open boiler for several hours, and filled a bottle half-full, closed and sealed it up, so that when it became cool it would in effect be under a vacuum, agitating it as often as seemed necessary. * American Artisan; Locomotive, 1880. STEAM-BOILER EXPLOSIONS. 583 " (7) Another bottle was filled with the same, and sealed. " (8) I next took some clean, solid ice, dissolved it under oil, and brought it to a boil, which was continued for an hour or more, after which it was tightly corked. " (9) I procured a bottle of carefully-distilled water, after long boiling and having been perfectly excluded from air during the distillation. "(10) I obtained a large number of small fish, placed them in pure, clean water in an open-headed cask, on a moderately cold night, so that very soon it became frozen over, conse- quently excluding the air, the fish breathing up the air in the water, so that (if I am correct in this theory) a water freed from air would be the result ; but in some of these different processes, if not in all, I was likely to free the water from air, if it could ever possibly occur in the ordinary course of operating a steam- boiler. " Having procured a good supply of glass-boilers adapted to my purpose, and so made that the slightest changes could be noted, and using as delicate thermometers as I could obtain, I took these samples, one after another, and brought them to the boiling-point ; and every one, with no variation whatever, boiled effectually and positively at 212 Fahrenheit or under ; nor was there the slightest appearance of explosion to be observed." This evidence is, of course, purely negative. The superheating of water, on even the small scale of the laboratory experiments of Donny, Dufour, and others, has never been successfully performed, except with the most elaborate precautions. The vessel containing the liquid must be abso- lutely clean ; the washing of all surfaces with an alkaline solu- tion seems to be one of the customary preliminary operations. The vessel must usually be heated in a bath of absolutely uni- form temperature in order that currents may not be set up within the body of the liquid to be heated ; no solid can be per- mitted to enter or come in contact with it; no shock can be al- lowed to affect it ; even contact with a bubble of gas may stop the process of superheating. All these conditions are as far re- moved as possible from those existing in steam-boilers. 282. The Spheroidal State, or Leidenfrost's phenomenon, 584 THE STEAM-BOILER. as it is often called, is a condition of the water, as to tempera- ture, precisely the opposite of that last described, its tempera- ture being less, rather than greater, than that due the pressure ; while the adjacent metal is always greatly overheated, and thus becomes a reservoir of surplus heat-energy which can be trans- ferred at any instant to the water. This peculiar phenomenon was first noted by M. Leidenfrost about 1746. It was studied by Klaproth, Rumford, and Baudrimont,* and more thoroughly by Boutigny. When a small mass of liquid rests upon a surface of metal kept at a temperature greatly exceeding the boiling-point of the liquid under the existing pressure, the fluid takes the form of a globule if a very small mass, or of a flattened spheroid or round-edged disk if of considerable volume, and floats around above the metal, quite out of contact with the latter, and grad- ually, very slowly, evaporates. The higher the temperature of the plate, the more perfect this repulsion of the liquid. Should the temperature of the metal fall, on the other hand, the globule gradually sinks into contact with it, and, at a temperature which is definite for every liquid, and is the lower as it is the more volatile, finally suddenly absorbs heat with great rapidity and evaporates often almost explosively. If contact is forcibly pro- duced at the higher temperature of the supporting plate of metal, as under a blacksmith's hammer, a real explosion takes place, throwing drops of the liquid in every direction. M. Boutigny found the temperature of contact to be, for water, alcohol, and ether, respectively, 142 C, 134, and 61 (287 F., 273, and 142). In all cases the temperature of the liquid was independent of that of the metal, and somewhat be- low the boiling-point. It is found, also, that a real and power- ful repulsion is produced between metal and liquid ; this is sup- posed to be due, in part at least, to the cushion of vapor there interposing itself. Contact is accelerated by the introduction of soluble salts into the liquid. It is supposed by many writers that this phenomenon may play its part in the production of explosions of steam-boilers, * Ann. de Chemie et de Physique, 2d series, t. Ixi. STEAM-BOILER EXPLOSIONS. and especially in cases in which there seems some evidence that, immediately before the explosion, there was no apparent over- heating of the parts exposed to the action of the fire, and in those still more remarkable instances in which the shattered parts had been, to all appearance, much stronger than other por- tions which had not been ruptured ; no evidence existing of low- water or overheating at the furnace, and the pressure being, the instant before the accident, at or below its usual working figure. Bourne* has no doubt that this does sometimes take place. Colburn gives a number of instances of explosions taking place under, apparently, precisely such conditions; and Robinsonf also cites several, in some of which the plates of the shell were badly shattered, as by a concussive force. In some such in- stances evidences of overheating, but only far below the water- level, known to have existed immediately before the explosion, have been observed, indicating repulsion to have there occurred. This latter is simply still another instance of bringing about the same results as when pumping water into an overheated boiler in which the water is low. Mr. Robinson;): tells of a case in which a nearly new locomo- tive, standing in the house, with a pressure, as shown but a moment before by the steam-gauge, of but 40 pounds, one third its presumed safe working pressure, the fire low and every- thing perfectly quiet, exploded with terrible violence, shatter- ing the top of the boiler directly over the firebox into many parts. That such explosions might occur were the metal actu- ally overheated under water, is shown by experiences not at all uncommon. In the work of determining the temperatures of casting al- loys tested by the Author for the United States Board ap- pointed in 1875 to test iron, steel, and other metals, at the first casting of a bar composed of 94. 10 copper, 5.43 tin, while pouring the metal into the water for the test, an explosion took place which broke the wooden vessel which held the water, and threw * Treatise on the Steam-engine, 1868. f Steam-boiler Explosions, p. 33. \ Steam-boiler Explosions, p. 62. Report on Copper-tin Alloys. Washington, 1879. 586 THE STEAM-BOILER. water and metal about with great violence. It appears probable that the metal was heated to an unusually high temperature, as in pouring other metals when at a dazzling white heat explo- sions sometimes took place, but they were usually not violent enough to do more than make a slight report as the hot metal touched the water. Another bar was cast at an extremely high temperature, being at a dazzling white heat. On pouring a small portion into water in attempting to obtain the temperature, a severe explosion took place, and this was repeated every time that even a small drop of the molten metal touched the water. The cold ingot-mould was then filled with this very hot metaL After the metal remaining in the crucible had stood for several minutes and had cooled considerably, it could be poured into water without causing the slightest explosion. Thus it would seem that the temperature at which contact with the water is produced may have an important effect upon the violence w r ith which the steam is generated, and that of the explosion so pro- duced. The explosions sometimes taking place with fatal effect in foundries when molten metal is poured into damp or wet moulds are produced in the manner above illustrated. They are usually apparently of the "fulminating class." Another in- stance occurred within the cognizance of the Author, even more striking than either of the above.* Two workmen in a gold and silver refinery were engaged in "graining" metal, which process consists in pouring a small stream of melted metal into a barrel of water, while a stream of water is also run into the barrel to agitate the water already there. Suddenly an explosion occurred which literally shivered the barrel, and threw the workmen across the room. Every hoop of the barrel, stout hickory hoops, was broken. The staves, seven eighths of an inch thick, and of oak, were not only splintered, but broken across ; and the bottom, which was resting on a flat surface, and which was of solid oak an inch in thick- ness, was split and broken across the grain. A box on which stood the man who was pouring the metal was converted into kindling wood. The metal, though scattered somewhat, for the * Reported in the Providence (R. I.) Journal. Feb. 2, 1881. STEAM-BOILER EXPLOSIONS. $87 most part remained in place, but the water was thrown in all directions. This explosion of an open barrel, like the preceding cases, was evidently due to the deferred thermal reaction of the water with a mass of very highly heated metal, with which it was finally permitted to come in contact at a temperature which allowed an explosive formation of steam. This class of explo- sions, by which open vessels are shattered and the water con- tained in them atomized, are by many engineers believed to exemplify the terrible explosions fulminantes of French writers on this subject. The temperature of maximum vaporization, with iron plates, was reported by the committee of the Franklin Institute to be 346^ F. (175 C.) and that of repulsion 385 F. (196 C), and to be the same under all pressures. Any cause which may retard the passage of heat from the iron to the water, though but the thinnest film of sediment, grease, or scale, may permit such in- crease of temperature as may lead to repulsion of the water, the overheating of the metal, the production of the spheroidal condi- tion, and the accidents due to that phenomenon, provided that the fire be so driven as to supply more heat than can be dis- posed of in ordinary working by the circulation and vapori- zation then going on. Robinson's experiments with safety- plugs indicate that a good circulation is usually a sufficient insurance against this action ; and experience with the boilers of locomotives and of torpedo-boats, in which from 50 to 100 pounds of coal per square foot (244 to 488 kilogs. on the square metre) of grate are burned every hour, shows that the risk, with steam-boilers of good design, is not great. With impure water and defective circulation Robinson observed many instances of singular and dangerous phases of this action.* It is suggested that many explosions of locomotives on the road or at stations may be due to the impact, on the shells of their boilers, of water thus projected from overheated iron below the water-line. In many such cases the engines have not left the rails, the break taking place just back of the smoke-box or near the fire- * See his Steam boiler Explosions, pp. 40-46. 588 THE STEAM-BOILER. box, and from the impact of water thus thrown from the tube- sheets. M. Melsen* experimentally proved it possible to prevent the occurrence of the spheroidal condition by the distribution of spurs or points of iron over the endangered sheets. The conductivity of the metal has an important influence on the effect of contact, suddenly produced, between the red-hot solid and the liquid. Professor Walter R. Johnson observed, in his elaborate experiments,f that brass produced much greater agitation of the water when submerged at the red heat than did iron. He also noted the singular fact that water at the boiling- point, thrown upon red-hot iron, requires more time for evapo- ration than cold water, probably in consequence of the greater efficacy of the latter in bringing down the temperature of the metal to that of maximum rapidity of action. The contact with the iron of incrustation, oxide, or other foreign matter ac- celerated this process also. Johnson found that beyond the temperature of maximum repulsion vaporization was acceler- ated by further elevation of temperature. At the meeting of the British Association in 1872, Mr. Bar- rett read a paper upon the conditions affecting the spheroidal state of liquids and their possible relationship to steam-boiler explosions. The presence of alkalies or soaps in water percep- tibly aids in the production of the spheroidal state. A copper ball immersed in pure water produced a loud hissing sound and gave off a copious discharge of steam. On adding a little soap to the water the ball entered the liquid quietly. Albumen, glycerine, and organic substances generally produced the same result. The best method is to use a soap solution, and to plunge into this a white-hot copper ball of about two pounds weight. The ball enters the liquid quietly, and glows white hot at a depth of a foot or more beneath the surface. Even against such pressure the ball will be surrounded with a shell of vapor of an inch in thickness. The reflection of the light from the bounding surfaces of the vapor-bubble surrounding the glowing * Bull, de I'Academie Royale de Belgium, April, 1871. f Reports on Steam-boilers, H. R., 1832, p. in. STEAM-BOILER EXPLOSIONS. 589 ball gives to the envelope the appearance of burnished silver. As the ball gradually cools, the bounding envelope becomes thinner, and finally collapses with a loud report and the evolu- tion of large volumes of steam. Mr. Barrett makes the sugges- tion that the traces of oil, or other organic matters which find their way into a steam-boiler, may similarly produce a sudden generation of steam sufficient to account for certain problemati- cal explosions, and thus lends some strong confirmatory evidence to the idea often promulgated by others within and without the engineering profession. 283. Steady Rise in Pressure has been shown by the experiments of the committee of the Franklin Institute, and by numerous cases of explosion, both before and since their time, to be capable of producing very violent explosions. In such cases, the steam being formed more rapidly than it is given exit, the pressure steadily increases until a limit is found in the final rupture of the weakest part of the boiler. Should this break occur below the water-line, and be the result of long decay or injury, no explosion may ensue ; but should the rup- ture be extensive, or should it occur above or near the surface of the water, the succession of phenomena described by Clarke and Colburn may follow, and an explosion of greater or less violence may take place. The intensity of the effect will de- pend largely upon the quantity of stored energy liberated, and partly upon the suddenness with which it is set free. A slowly- ripping seam or gradually extending crack would permit a far less serious effect than the general shattering of the shell, or an instantaneously produced and extensive rent. The time required to produce a dangerous pressure is easily calculated when the weight of water present, W, the range of temperature above the working pressure and temperature, t l t^ and the quantity of heat, Q, supplied from the furnace are known, and is W(t-t t ) Q' Professor Trowbridge gives the following as fair illustrations of such cases :* * Heat as a Source of Power, p. 191. 590 THE STEAM-BOILER. (i) A marine tubular boiler is of the largest size, such that W = 79,000 Ibs. of water. Suppose the working pressure to be 2% and the dangerous pressure 4 atmospheres. The boiler contains 5000 square feet of heating-surface ; and supposing the evaporation to be 3 Ibs. of water per hour for each square foot, we shall have, taking 1000 units of heat as the thermal equivalent of the evaporation of i Ib. of water, = t t - t = 29 F. 5000 X 3 X looo 60 79,000 X 29 60 (2) A locomotive boiler, containing 5000 Ibs. of water, hav- ing 1 1 square feet of grate-surface, and burning 60 Ibs. of coal per hour on each square foot of grate, each pound of coal evaporates about 7 Ibs. of water per hour, making 77 Ibs. of water evaporated per minute. Suppose the working pressure to be 90 Ibs., and the danger- ous pressure to be 175, /,-/=5oF. 5000 X 50 * ~~ = 3 * minutes - (3) The Steam Fire-engine. The boiler contains 338 Ibs. of water and 157 square feet of heating-surface. Supposing each square foot of heating-surface to generate but i Ib. of steam in one hour, the pressure will rise from 100 to 200 Ibs. in T = 7 minutes. (4) To find, in the same boiler, how long a time will be re- quired to get up steam; that is, to carry the pressure to 100 Ibs. STEAM-BOILER EXPLOSIONS. 59 1 If we suppose but \\ cubic feet of water in the boiler, we shall have T= 93XI17 =4-1 minutes. I5/X iQQQ 60 Thus, if W\s diminished, the time T is diminished in the same proportion. The lowering of the water-level from failure of the feed-apparatus increases the danger, not only by expos- ing plates to overheating, but by causing a more rapid rise of pressure for a given rate of combustion. Gradual increase of pressure can never take place if the safety-valve is in good order, and if it have sufficient area. The sticking of the safety-valve, either of its stem or its seat, the bending of the stem or the jamming of the valve by a superincumbent object or lateral strains, and similar accidents, have produced, where boilers were strong and otherwise in good order, some of the most terrific explosions of which we have record. The parts of the boiler have been thrown enor- mous distances, and surrounding buildings and other objects levelled to the ground, while the report has been heard miles away from the scene of the disaster. The records of the Hartford company up to 1887 include accounts of 26 explosions of vessels detached from the generat- ing boiler, used at moderate pressures for various purposes in the arts, and there have been many others of less importance that were not considered worthy of public mention. It is con- cluded that the percentage of explosions among bleaching, digesting, rendering, and other similar apparatus is ten times greater than among steam-boilers at like average pressures, and the destructive work done is quite as astonishing as that by the explosion of ordinary steam-generators.* This is sufficiently decisive of the question whether it is possible to produce destructive explosions of boilers simply by excess of pressure above that which the vessel is strong enough to withstand. In these cases low-water and all the other special causes operating where fire and high temperatures exist, * The Locomotive, 1887. 592 THE STEAM-BOILER. and such absurd theories as the generation of gas or the action of electricity, are eliminated ; and it is seen that mere deteri- oration and loss of strength, or a rise of steam-pressure, even where there is an ample supply of water, may produce explo- sions of the utmost violence. 284. The Relative Safety of Boilers of the various types is determined mainly by their general design, and their greater or less liability to serious and extensive injury by the various accidents and methods of deterioration to which all are to a greater or less extent liable. The two essential principles by which to compare and to judge the safety of boilers are : (1) Steam-boilers should be so designed, constructed, oper- ated, inspected, and preserved as not to be liable to explosion. (2) Boilers should be so designed and constructed that, if explosive rupture occurs at all, it shall be with a minimum of danger to attendants and surrounding objects. The prevention of liability to explosion, and the provision against danger should explosion actually take place, are the two directions in which to look for safety. As Fairbairn has remarked, the danger does not consist in the intensity of the pressure, but in the character and construc- tion of the boiler.' 55 ' Other things being equal, the boiler, or that form of boiler in which the original surplus strength of form and of details is greatest, and which is at the same time best preserved, is the safest. That class in which original strength is most certainly and easily preserved has an impor- tant advantage ; those boilers in which facilities for constant oversight, inspection, and repairs are best given are superior in a very important respect to others deficient in those points. For example, the cylindrical tubular boiler, if properly set, is very accessible in all parts, and may be at all times examined : it offers peculiar facilities for inspection and the hammer-test, and can be readily kept in repair ; but it is liable, in case of its becoming weakened by corrosion over any considerable area or along any extended line of lap, to complete disruptive ex- plosion. * Engineering Facts and Figures, 1865. STEAM-BOILER EXPLOSIONS. 593 On the other hand, the various " sectional," or so-called u safety," boilers are rarely as convenient of access or of inspection, and cannot usually be as readily and completely cleaned ; but they are so designed and constructed as to be little, if at all, liable to dangerous explosive rupture, and if a tube or other part bursts it is not likely to endanger life or property. That boiler is, therefore, on the whole, best which is least liable to those kinds of injury which lead to explosion, and which is least likely to do serious harm should explosion actually take place.* Those who select the tubular boiler are commonly influenced mainly by considerations of cost and the first of the above considerations ; while the users of the water- tube sectional boiler are controlled by the second, in so far as either considers this form of risk at all. During the experiments of Jacob Perkins, about 1825 and later, the value of the " sectional " boilers, where high-pressures are adopted, was well shown. He frequently raised his steam- pressure to 100 atmospheres,! and in his earlier work rupture often took place, but no ill effects followed. The division of the boiler into numerous compartments saved the attendants from injury. In a letter to Dr. T. P. Jones, dated March 8, 182/4 Mr. Perkins states that he had worked at the above- mentioned pressure with a ratio of expansion of 12; his usual pressure was about two thirds that amount, and the ratio of expansion 8. Mr. Perkins was then building an engine to safely carry a pressure of 2000 pounds per square inch. 285. Defective Designs, causing explosion, are not as common as many other causes. They exist, however, more frequently than is probably usually supposed. The defects are generally to be observed in the staying of such boilers as re- quire bracing; in the insertion of the heads of plain cylindrical boilers; in the attachment of drums, and the arrangement of * Dr. E. Alban, following John Stevens, was probably the first to enunciate the principle, " so construct the boiler that its explosion may not be dangerous." The High-pressure Steam-engine, 1847, p. 70. f Jour. Franklin Tnst. , vol. Hi., p. 415. \ Ibid., p. 412. Reports on Steam-boilers, H. R., 1832, p. 188. 38 594 THE STEAM-BOILER. man-holes and hand-holes; and, less frequently, in the selection of the proper thickness and quality of iron for shells and flues. Such defects as these are the most serious possible ; they are not only serious in themselves and at the start, but are of a kind which is commonly very certain to be exaggerated, and rendered continually more dangerous with age. A thin shell grows constantly thinner, a weak stay or brace weaker, and an unstayed head more likely to yield every day ; while a flue originally too thin is all the time overstrained, not simply by the steam-pressure, but also by the action of the relatively stronger parts around it. The most minute study of every detail and the most careful calculation of the strength of every part, with an allowance of an ample factor of safety, are the essentials to safety in design. Faulty design in bracing is illustrated by an explosion which took place in New York City, January 15, 1881, by which, for- tunately, however, no loss of life was caused. A dome-head, proportioned and braced as shown in the next figure, was blown out and tore up a sidewalk, under which the boiler was set, FIG. 133. DOME AND HEAD. doing no other damage. The case was examined by Mr. Rose, who reported substantially as follows : The dome-crown tearing around the edge at A, also tore across at B, being thus completely severed. The iron at the fractures was of excellent quality. The plate showed lamina- tion in places, and the crack around A was rusty, and evidently STEAM-BOILER EXPLOSIONS. 595 not of recent formation. The six stays, three of which are shown in place at C, Fig. 133, were all in position in the dome, and their surfaces of contact with the dome were covered by a black polish, indicating movement and abrasion. FIG. 134. EXPLOSION OF DOME. Apparently, as the pressure and temperature increased and decreased the dome-head might lift and fall, bending on A as a centre ; thus, taking / as a centre, the movement of C would FIG. 135. DEFECTIVE FORM. be in the direction of F, while at D the direction would be toward/, and the direction of motion of the two would nearly 596 THE STEAM-BOILER. coincide. The exploded dome shows an indentation at /, due to the motion of the foot of the stay. Another error in the design of this boiler is that the diameter of the dome-shell is 34 inches, and a circle of iron about 18 inches in diameter is punched out of the shell at D. This opening is required only to admit an inspector or workman to the interior of the boiler; hence it is several inches wider than it should be. Defective design is illustrated in the case of the next boiler, the explosion of which left it in the form shown in Fig. 135.* This boiler consisted of two incompletely cylindrical shells, united as in the next figure, and ineffectively stayed at the \ / lines of contact. This is a form which, Mf Vv insufficiently braced, becomes peculiarly f \ dangerous. In the case illustrated, the FIG. i 3 6.-j UNCTION OF SHELLS, braces yielded, after having been weak- ened by continual alteration of form, and split the two shells apart as seen. It is probably possible to brace boilers of this type safely, but it is better to avoid their use. They have some- times been used for marine purposes, where lack of space com- pelled special expedients, the bracing consisting of strong bolts with nuts and washers on the outside of the shell a compara- tively strong and safe construction. Steam-domes are a source of some danger and of additional expense, however well designed and attached ; and it is proba- bly good economy, all things considered, to dispense with them altogether, using a dry pipe instead, and expending the amount of their extra cost on an increase in size of boiler over that which would have otherwise been selected. The large boiler will steam easier and more regularly, will give drier steam, and will be less liable to danger of deterioration or of explosion. A steam-drum above the boiler and connected by two separate nozzles, or a drum connecting the several boilers of a battery, is not subject to the objections which apply to the attached dome. 286. Defective Construction, material, and workmanship are responsible for many explosions of steam-boilers. Thin, * Locomotive, Feb. 1880. STEAM-BOILER EXPLOSIONS. 597 laminated, or blistered sheets, imperfect welds in bracing, the strain produced by the drift-pin, carelessness in the attachment of nozzles and drums, and in neglect of the precaution of strengthening man-holes and hand-holes, and bad riveting, are all common causes of weakness and accidents. Only the most careful and skilful, as well as conscientious, builders can be re- lied upon to avoid all such faults, and to turn out boilers as strong and safe as the designs may permit. In all cases, careful and unintermitted inspection by an ex- perienced, competent, and trustworthy inspector should be provided for by the proposing purchaser and user of the boiler. In the case of some of the more modern forms of boiler, con- structed under a system of manufacture which includes some machine fitting and working to gauge of interchangeable parts, with regular inspection before assemblage, this supervision be- comes less essential, and a careful test and trial, previous to acceptance, may be all that is necessary to insure a satisfactory and safe construction. Wherever defective material or bad workmanship is detected, the fault should always be corrected before the boiler is accepted, and previous to any trial or use under steam. Careless riveting and the use of the drift-pin are defects which cannot often be readily detected afterward, and they are such common causes of explosion that too much care cannot be taken to avoid any establishment of which the repu- tation in this regard is not the best. FIG. 137. DEFECTIVE WELDING. Defective welds, the cause of many unfortunate accidents following the yielding stays or braces, are among the most 598 THE STEAM-BOILER. common and least easily detected of all faults. They are due to the difficulty of producing metallic contact in abutting sur- faces between which particles of scale and superficial oxidation may interpose. The grain of the iron, as illustrated in the ac- companying engraving, is broken at such junctions, and. it is difficult to secure a good weld, and next to impossible to de- termine until it actually breaks whether it is seriously un- sound. Defective workmanship is often exhibited most strikingly by the distorted forms of rivets, revealed after explosion has caused a fracture along the seam, or when the yielding of the weakened seam has resulted in an explosion. The following illustrations of a variety of cases of such distortion, all taken from a single boiler,* show how very serious this kind of de- fect may be. It is not to be presumed that such carelessness or worse, as is here exemplified, is to be attributed to the builder himself, but rather to the fault of workmen carefully concealing their action from the eye of the foreman or inspec- tor. No law or rule can protect the purchaser from this kind of fault; his only reliance must be upon the reputation of the maker and his workmen, and the vigilance and skill of his in- spector. FIG. 138. Rivet "driven" in overset holes, the conical point broken off by the tearing apart of the plates, the head FIG. 138. FIG. 139. nearly severed from the body, and probably weakened in " driving." FIG. 139. Rivet "driven" in overset holes, head broken off by the tearing apart of the plates, conical point also nearly * Locomotive > Feb. 1880. STEAM-BOILER EXPLOSIONS. 599 broken off, bad sample of "driving," cone too flat to properly hold down the plate. The next figure illustrates a group of similar distorted riv- ets which played their part in the production of an explosion. FIG. 140. DEFECTIVE RIVETS. FlG. 141. Rivet " driven" in slightly overset holes, point excentric and not symmetrical, too flat to properly secure the edge of the plate. FlG. 142. Rivet " driven" in badly overset holes, very weak. FIG. 141. FIG. 142. See Figs. 143, 144, 145, which were " sheared " at the time of the explosion. The dark shading on lower end, Fig. 142, indi- cates an old crack. FlGS. 143, 144, 145. Samples selected from a number taken from a " sheared " seam, which was believed to be the initial break from which the explosion arose. They were no doubt similar to Fig. 142 before they gave way. 6OO THE STEAM-BOILER. The Author, on one occasion, picked out with his fingers FIG. 143. FIG. 144. FIG. 145. twelve consecutive rivets, deformed like those here illustrated, from a torn seam in an exploded boiler. FIG. 146. FlG. 146. Rivet " driven" in overset holes ; it was probably fractured under the head in driving. Taken from a seam that was broken through the rivet-holes. FIG. FIG. FlGS. 147 and 148. Long rivets taken from a broken casting which they were intended to secure to the wrought-iron head STEAM-BOILER EXPLOSION'S. 6oi of the boiler. The holes in the wrought-iron plate were " drifted " and chipped to allow the rivets to enter, as shown by the enlarged portion of the body. This irregular upsetting and the sharp little wave of iron on the body of Fig. 147 indi- cate the thickness of the wrought-iron plate. 287. Developed Weakness, usually a consequence of progressing decay by corrosion, is the most common of all causes of the explosion of steam-boilers. A boiler, designed and constructed of the best possible proportions and of the best of materials, having at the start a real factor of safety of six, may be assumed to be as safe against this kind of accident as possible ; but with the beginning of its life decay also begins, and the original margin of safety is continually lessened by a never-ceasing decay. The result is an early reduction of this margin to that represented by the difference between the work- ing pressure and that fixed as a maximum by the inspector's tests. Should this difference be sufficient to insure against ac- cident resulting from further depreciation in the interval be- tween inspector's or other tests, explosion will not occur ; should this margin not be sufficient, danger is always to be ap- prehended, and almost a certainty that rupture, and possibly explosive rupture, will at some time occur. This margin is, legally, usually fifty per cent ; it is too small to permit the pro- prietor to feel a real security. It is usually thought that the tests should show soundness under pressures at least double the regular working pressure at which the safety-valve is set.* Many cases have been known in which the boiler has yielded at the working pressure not very long after the regular official inspection and pressure-test had taken place. Such an example was that of the explosion of the boiler of the Westfield, in New York Harbor, in June, 1871. The steam ferry-boat Westfield is one of three boats which have formed one of the regular lines between New York and Staten Island. The Westfield made her noon trip up from * Experiments made by the Author, and later by other investigators, have in- dicated the possibility that an apparent factor of safety of two, under load momen- tarily sustained, may not actually mean a factor exceeding one for permanent loading. " Materials of Engineering," vol. i. , 133; vol. ii., 295. 6O2 THE STEAM-BOILER. the island to the city on Sunday, July 3Oth, and while lying in the New York slip her boiler exploded, causing the death of about one hundred persons and the wounding of as many more. The boiler is of a very usual form, as represented in Fig. 149, and is known as a " marine return-flue boiler." The diameter of its shell the cylindrical part was ruptured is ten feet ; its thickness, No. 2 iron, twenty-eight hundredths inch. FIG. 149. BOILER OF THE WESTFIELD. The evidence indicated that the explosion occurred in con- sequence of the existence of lines of channelling and long-exist- ing cracks, by which the boiler was gradually so weakened that, six weeks after its inspection and test, the pressure of steam being allowed by the engineer to rise slightly above the pres- sure allowed, the boiler was ruptured, giving way along a horizontal seam and tearing a course out of the boiler. The common lap-joint customarily adopted in the construc- tion of boilers is liable to such serious distortion under very heavy pressures as to produce leakage before actually yielding, and this leakage is sometimes so great as to act as a safety- valve. Thus, suppose a straight strip of plate riveted up in parts as in Fig. 1 50.* A heavy pull will cause distortion as shown, in all cases except where a butt-joint is made with a covering strip on each side. If the metal is brittle and the rivet-heads strong, preventing the bending of the plate on the * See Locomotive, Oct. 1880. STEAM-BOILER EXPLOSIONS. line of rivet-holes, the plate will probably break adjacent to G or F, Fig. 150; or in the middle, / and H. But should the plates be ductile or the rivet-heads weak, the break would occur at the line through the holes. Of A 7 FIG. 150. YIELDING JOINTS. If the plates, Fig. 150, A, etc., were straight at the joint, the extreme end, Z, must contract and the outer one expand at M, involving in the one a compression or upsetting, and in the other drawing the metal. If the joint be a butt, with "a single outer cover, C, a similar contraction must take place at both ends, and a contraction of the middle of the covering strip, while the opposite would take place in the case of the joint with the inner cover, B. These distortions are not likely to take place in a transverse seam of a cylindrical boiler-shell from internal pres- sure. The butt-joint, with two covering plates, , would re- tain its shape. Lapped longitudinal joints are shown at A' . Single-riv- eted and single-covered butts at B' and C '. D' shows a double- riveted, single-covered butt. The next figures (151, 152) show FIG. 151. Before Stretching. After Stretching. FIG. 152. the effect of strain on rivet-holes and on holes filled by the rivet. Multiple explosions are not infrequent. They usually occur in consequence of the explosion of one of a battery, with the result of injuring adjacent boilers in such manner that they also 604 '/'//A' STEAM-BOILER. explode, the phenomena following each other ::o quickly as to produce the appearance of simultaneous explosion. It is pos- sible also that in some cases an accession of pressure in a set of boilers may take place with such suddenness as to explode several, notwithstanding there may exist a difference in their resisting power, the weakest not being given time to act as a safety-valve to the rest. It is doubtful, however, whether such cases can often if ever arise. 288. General and Local Decay introduce vastly different degrees and elements of danger. As has been elsewhere stated, in effect, an explosion comes of extended rupture ; while local injuries or breaks, if they do not lead to wider injury, cannot cause widespread disaster. Hence, general corrosion, extend- ing over considerable areas of plate or along lines of considera- ble length, is a cause of danger of complete disruption and ex- plosion. A corroded spot in a firebox, a loosened rivet, or even a broken stay, if the boiler be otherwise well proportioned, well built, and in good order, may not be a serious matter; but a thinned sheet in the shell, a long groove under a lap, a line of loose rivets, or a cluster of weakened stays or braces, will certainly be most dangerous. General or widespread corrosion is very liable to lead to explosion ; local and well-guarded cor- rosion may cut quite through the metal, and simply cause a leak or an unimportant " burst." Old fireboxes are often seen covered with " patches" in places, and yet they very rarely ex- plode. Such a state of affairs may, nevertheless, by finally producing large areas of patched and fairly uniformly weak portions of the boiler, lead to precisely the conditions most favorable to explosion. A steam-boiler experimentally ex- ploded at Sandy Hook, N. J., September, 1871,* had previ- ously, by repeated rupture by hydraulic pressure and patching, been gradually brought into precisely this state, and exploded under steam at 53!- pounds, about four atmospheres pressure, a slightly lower pressure than it had sustained (59 pounds) at its last test. On this occasion, when a pressure was reached of 50 pounds per square inch, a report was heard which was prob- * Journal Franklin Institute, January, 1872. STEAM-BOILER EXPLOSIONS. 605 ably caused by the breaking of one or more braces, and at pounds the boiler was seen to explode with terrible force. The whole of the enclosure was obscured by the vast masses of steam liberated ; the air was dotted with the flying fragments, the largest of which the steam-drum rising first to a height variously estimated at from 200 to 400 feet, fell at a distance of about 450 feet from its original position. The sound of the explosion resembled the report of a heavy cannon. The boiler was torn into many pieces, and comparatively few fell back upon their original position. FIG. 153. CORROSION. Thus corrosion may affect a single spot in a boiler, in which case a " patch," if properly applied, should make the boiler nearly as strong as when whole. A. series of weak spots near each other may so weaken a boiler as to produce explosion, as may any considerable area of thin plate, although, when occur- ring in the stayed surfaces of a firebox, the metal may become astonishingly thin. A sketch of spots of corrosion is shown in Fig. 153, which represents the cause of an actual explosion. This cause of explosion may be either internal or external, and is induced internally by bad feed-water, and externally by dampness or by water leaking from the boiler, either unseen or neglected. It is always dangerous to have any portion of a boiler concealed from frequent observation. The effect of covering a part of a sheet subject to corrosion by solid iron, as by the lap of a seam, is shown in the next fig- ure, whicji also exhibits a common method of corrosion along a seam. The same effect is seen still more plainly in the sue- 6o6 THE STEAM-BOILER. ceeding figure, in which the pitting which so often attends the use of the surface-condenser is also well shown. 289. The Methods of Decay are as various as the forms and location of the parts subject to corrosion. As Colburn* has said : " As a malady, corrosion corresponds, in its comparative frequency and fatality, to that great destroyer of human life, consumption ;" and it has as innumerable phases and periods of action. The two most common methods of decay are the gen- eral, and here and there localized, corrosion that goes on in all boilers, and in fact on all iron exposed to air and carbonic acid, in presence of moisture ; and the concentrated and localized oxi- FIG. 154. CORROSION AT A SEAM. FIG. 155. " PITTING.' dation that is often seen along the line of a seam at the edge of the lap, where the continual changing of form of the boiler is as constantly producing an alternate flexing and reflex motion of the sheet, which throws off the oxide as fast as formed along that line, and exposes fresh, clean metal to the corroding influ- ence. A groove or furrow is thus in time produced, which may, as occurred in the case of the Westfield (Fig. 149), actually cut through the sheet before explosion takes place. The phenomenon known as " grooving" or " furrowing" is well illustrated by the case just mentioned, in which this action was originally started, probably, by the carelessness of the work- man, who, either in chipping the edge of the lap along a girth- * Trans. Brit. Assoc. 1884. STEAM-BOILER EXPLOSIONS. 6o/ seam, or in calking the seam, scored the under-sheet along the edge of the lap with the corner of his chisel or with the calk- ing-tool. This is a very common cause of such a defect. The boiler was broken into three parts. The first, and by far the largest part, consisted of the furnaces, steam-chimney and flues, with a single course of the shell ; the second con- sisted of two courses of the outside of the shell next the back- head, together with that head, to which they remained attached ; the third piece consisted of a single complete course from the middle of the cylindrical shell, which was separated at one of its longitudinal seams, partially straightened out and flung against the bottom and side of the boat. This last piece re- mained opposite its original position in the boiler before the explosion, while the first and second pieces went in opposite directions, the former finally lying several feet nearer the en- gine than when in situ, and against the timbers of the " gallows- frame," while the latter piece was thrown fifty feet forward into the bow of the boat, where it fell, torn and distorted. The longitudinal seam, along which piece number three separated, and the deep score or " channel " cutting nearly through in many places, and presenting every evidence of being an old flaw, were plainly seen. The mark made by a chisel in chipping, and that of the calking-tool, were seen, and indicated the probable initiative cause of the flaw. The Author examined this piece and found an old crack or " channel " cut along the edge of the horizontal lap referred to as being at the ends of the sheet, and in some places so nearly through that it was difficult to detect the mere scale of good iron left, while in other places there remained a six- teenth of an inch of sound metal. Fig. 156 FIG. 156. exhibits a section of the crack. Were this the weakest place in the boiler, and the least thick- ness here one sixteenth of an inch, the tensile strength being equal to the average determined by the tests made of the iron, the pressure required to rupture such a boiler, ten feet in diameter, 608 THE STEAM-BOILER. would be 44079 X iV X 2 -h 120 = 47 pounds per square inch, nearly. A pressure of 27 or 28 pounds would burst it open where the least thickness was slightly more than one thirty- second of an inch. One portion may be supported, to some extent, by a neighboring stronger part. Along this longitudi- nal seam the limit of strength would seem to have been about 30 pounds per square inch, which is about the pressure at , which the boiler exploded, this seam ripping for a distance of several feet. The original strength of the boiler was equal to about 1 20 pounds along the horizontal seams, its then weak- est parts, provided that the iron had, when new, the average strength of the specimens which we have tested. In the ver- tical seams may be seen, in some places, similarly weakened portions, the cracks running usually from rivet to rivet, and here and there exhibiting marks that show the wedging action of the " drift-pin," and many places, both in longitudinal and girth-seams, are cut by the chisel and marked by the " calking- tool." These lines of " furrowing" are sometimes continuous, and sometimes interrupted by portions of good iron. They are probably in most cases caused by changes in form of the boiler with variations of temperature and pressure, some line of local weakness determining the line along which the plate shall bend, and this bending taking place continually, though ever so slightly, along the same line precisely, finally produces rupture. This change of form of the shell of a boiler may be due to either the constantly occurring variation of pressures, as steam is made or is blown off during working hours ; or it may be pro- duced by changes of temperature. Large and thin boilers are especially liable to this form of injury. Bad methods of sup- port may permit or may cause variations of form and this defect, which is all the more dangerous that it is difficult in many cases to detect it. Water trickling from leaks sometimes causes a kind of grooving along its path, hardly less serious in its nature and extent. Sometimes this action produces a narrow crack, and at other times, as above stated, as the rust formed is thrown or scoured off the iron at the bend, leaving a comparatively clean surface. STEAM-BOILER EXPLOSIONS. 609 oxidation is probably accelerated, and the fault takes the form of a groove or furrow. If unperceived, this goes on until a rup- ture or an explosion occurs. Of forty explosions of locomotive boilers noted in British Board of Trade reports,* eighteen gave way at the firebox and twenty at the barrel. Of these twenty, every one was the re- sult of " grooving" or cracks along the lap of seams, all of which were lap-joints. The grooves were most common ; they always occurred along the edge of the inside overlap, just where the changes of form with varying pressure would concentrate their effects. Such results are sometimes also seen at butt- joints, especially where a strip has been used inside. The rack- ing action of the engines may produce precisely the same effect. Wherever change of form is felt, grooving or furrowing and cracking may be expected to be found in time. Where the boiler is already heavily strained along one of these lines of re- duced thickness, any slight added stress, as a jar, or the action of a calking-tool, as when leaks in boilers under pressure are being calked, may precipitate an explosion, the break follow- ing the groove or crack just as a stretched drum-head may yield to the scratch of a knife. 290. Differences in Temperature between parts of a boiler more or less closely connected in- the structure may pro- duce serious strains, and some instances of explosion have been attributed to this cause. Changes of temperature occur as steam is raised or blown off from a boiler, and its temperature becomes at one time that due the steam-pressure, and then it falls to that of the atmosphere each time steam is blown off. It will change its form more or less, and will usually be subjected to some strain by this pro- cess. Again, while actually at work, the steam-space and upper portion of the water-space are at the temperature of 'steam at the working pressure, while the lower part is continually vary- ing in temperature from that of the feed-water to the maximum which it attains after entrance. This difference of temperature * "Wear and Tear of Steam-boilers." F . A. Paget, Trans. Soc. of Arts, 1865 ; London, 1865, p. 8. 39 6lO THE STEAM-BOILER. between the upper and lower parts of the boiler, as well as be- tween other portions, causes a continual tendency to distortion ; and if this distortion be resisted, a stress is thrown upon the parts equal to that which would be required, acting externally, to remove the distortion, if produced. The stress is also equal to the mechanical force that would be necessary to produce similar distortion. Thus, had the temperature of the main and upper part of the Westfield's boiler been, after the entrance of the feed- water, 273, or that due to about twenty-seven or twenty-eight pounds steam, while the feed-water had a temperature of 73, the bottom of the boiler having a temperature, in consequence, 200 below that of the top, the difference in length would be about one eight-hundredth, and, if confined by rigid abutments, iron so situated would be subject to a stress of twelve and a half tons per square inch. But in this case one part would yield by compression and the other by extension, and if they were to yield equally it would reduce the stress to six and a quarter tons. Actually, in this case, the lower fourth and upper three fourths would be more likely to act against each other, and the stress, if the boiler had no elasticity of form, would be about nine tons. Any elasticity of form and boilers generally possess considerable would still further reduce the strain, and it very frequently makes it insignificant. It is thought, by some experienced engineers and other authorities, that many of the explosions known to have taken place, after inspection and test, at pressures lower than those of the test, are caused by the weakening action of unequal expan- sion, the stresses and strains produced in this manner being superadded to those due to simple pressure, against which latter the boiler might otherwise have been safe. Such effects may also be the final provocative to explosion when cold feed-water is pumped into a boiler, on getting up steam, or possibly, some- times, when cooling off. It has even been asserted that an empty boiler has been ruptured by such changes of form conse- quent on building a light fire of shavings in a flue to start the scale. The Author has known of instances in which the girth- seams of large marine flue-boilers were ruptured along the line STEAM-BOILER EXPLOSIONS. 6ll of rivet-holes a distance of several feet by the introduction of a large volume of cold feed-water, when steam was up, but the engine at rest. The differences of temperature on the two sides the sheet may be important. While it is true that the heat supplied by the furnace-gases is absorbed by the boiler to the same extent, practically, without much regard to the thickness of the plates of the boiler, it is a well-known fact that the resistance of iron to the flow of heat is so great that the effect of heat on the metal itself is seriously modified by the thickness of the sheet. Heavy plates " burn" away, projecting rivet-heads are destroyed, -and the laps of heavy plates are especially liable to be thinned seriously where they are employed. A variation of temperature of considerable range, and often recurring, frequently causes injury by hardening the metal of the boiler, making it brittle and liable to crack with change of form, and also produces the very change of form causing this cracking. The experiments of Lt.-Col. Clark, R.A.,* show that great distortion may be thus produced. It is probably thus that iron and especially steel fireboxes so often crack, in consequence of a continual swelling of the metal under varying temperatures and the stresses so caused. This action, combined with oxidation, external and internal, sometimes makes the sheets and oftener the stays of a boiler remarkably weak and brittle ; they some- times become more like cast than wrought iron. The thicker the sheet, the more readily is it overheated and overstrained. The extent to which alteration of form under pressure may go, with good material, before actual rupture, is illustrated by the following :f During the summer of 1868, a cylindrical boiler, made of -inch steel plates, built at the Fort Pitt Iron Works, Pittsburg, was tested under authority of the government, with a view to determining the relative advantages of steel and iron as a material for navy boilers. When the pressure of cold water had reached 780 pounds, the girth of the boiler was found to * Proc. Royal Society, 1863; Journal Franklin Institute, 1863. f Iron Age, Sept. 26, 1872. 6l2 THE STEAM-BOILER. have permanently increased 3} inches, and at 820 pounds rup. ture occurred. Cases have been known in which a steel crown-sheet has be- come overheated, and has sagged down until, the tube-sheet going with it, a basin-shaped form has been produced, convex toward the fire, and yet no fracture produced, even when the pump was put on and the boiler filled up again under pressure. 291. The Management of the Steam-boiler, or, more correctly, its mismanagement, while in operation, and a neglect of proper supervision and inspection, may be considered, on the whole, the usual reason of explosion, as the deterioration of the boiler is the immediate cause; and this deterioration is almost invariably so gradual and so readily detected by intel- ligent and painstaking examinations that there is rarely any excuse for its resulting disastrously. A well-made boiler under good management and proper supervision may be considered as practically free from danger. The person in direct charge of the boiler is usually a pre- sumably experienced and trustworthy man. He should be thoroughly familiar with his business, generally intelligent, of good judgment, ready and prompt in emergencies, and abso- lutely reliable at all times. His first duty is to see that the boiler is full to the water-line, trusting only the gauge-cocks ; he must keep constant watch of the furnaces, flues, and other surfaces subject to the action of the fire, and thus be certain that no injury is being done by overheating or sediment ; he must keep the feed-apparatus in perfect working order, keep up the supply of water continuously and regularly, and see that the safety-valve is in good order at all times. Such careful management, conscientious inspection and cleaning, and repair- ing at proper intervals will insure safety. To keep the safety-valve in good working order and to make certain that it is operative, provision should be made for opening it by hand, and it should be daily raised, before getting up steam, to the full height of its maximum lift. Explosions of Gas sometimes precipitate steam-boiler explo- sions. Should the gases leaving the fuel and the furnace not be completely burned, but become so mingled in the flues as to STEAM-BOILER EXPLOSIONS. 613 produce an explosive mixture, combustion finally occurring, the shock may be sufficient to cause rupture of the boiler, and, as has actually sometimes happened, its explosion. Sewer-gases have been known to find' their way into an empty boiler through an open blow-off pipe, and have been exploded by the first light brought to the man-hole, and with serious damage to adjacent property. Mineral oils used to detach scale have caused similar dangerous and sometimes fatal explosions by the ignition of the mixture of their vapors and the air within the boiler. It is important that care be taken in using lights about boilers in such cases of application of mineral oils. Explosions of gas within a boiler at work cannot occur ; but the suggestion of the possibility of such an occurrence is often made. No decomposition of water can take place except a portion of the boiler is overheated ; this happening, all the oxygen produced is absorbed by the iron, and no recombination can occur later, even \vere it possible for ignition to take place under the conditions producing decomposition. The flooding of a boiler with water until it is filled to the steam-pipe or safety-valve may cause so serious a retardation of the outflow of the mingled fluids as to result in overpressure and great danger. Mr. W. L. Gold * gives the following instances, and the experience of the Author justifies fully his statement. The steam-pipe or the safety-valve cannot relieve a full boiler rapidly and safely. First, a boiler 38 inches in diameter, two flues, shell \ inch Juniata iron, ruptured in the sheet a crack 9 inches long, steam- gauge indicating 60 pounds, safety-valve weighted at 80 pounds pressure. This rupture closed instantly ; and if he had not seen it made, he might possibly have been surprised by an explosion, with water and steam in their normal condition, very shortly after. Second, a steam-drum (spanning a battery of five boilers) 30 inches in diameter. The blank-head forced (bulged) out i inches, the stay-rods stretched, and the corner of the head-flange cracked one third around. Third, a vertical boiler, built especially to carry high pressure (safe running pressure * Am. Manufacturer, Feb. 1881. 6 14 THE STEAM-BOILER. 150 pounds), the hand-hole and man-hole joints forced out past the flanges, the steam-pipe joints and union forced out, the packing in the engine-piston destroyed, and the engine gener- ally racked, so as to be almost useless. Steam-pressure by gauge from 40 to 60 pounds; safety-valve weighted at 90 pounds. Mr, Gold suggests that, as this is a not infrequent occur- rence, many explosions may be simply the final act in the drama commenced by the feed-pump. 292. Emergencies must be met with a clear head and ready wit, with perfect coolness, and usually with both prompt- ness and quickness of action. Every man employed about steam-boilers, as well as every engineer and every proprietor,., should have carefully thought out the proper course to take-in any and every emergency that he can conceive of as likely or possible to arise, and should have constantly in mind the means, available for meeting it successfully. When the time comes to act, it is not always, or even often, possible to take time to study out the best thing to be done ; action must be taken, on the instant, based on earlier thought or on either the intuition or the impulse of the moment. " Low-water" presents perhaps the most common, as well as one of the most serious, of such emergencies. The instant it is detected, the effort must be made to check the fall to a lower level ; the fire must be dampened, preferably by throwing on wet ashes, and the boiler allowed to cool down. Care should be taken that the safety-valve is not raised so as to pro- duce a priming that might throw water over the overheated metal, and that no change is made in the working of either en- gine or boiler that shall produce foaming or an increased pres- sure. If, on examination, it is found that the water has not fallen below the level of either the crown-sheet or any other extended area of heating-surface, the pump may be put on with perfect safety; but if this certainty cannot be assured, the boiler should be cooled down completely, and carefully inspected and tested, and thoroughly repaired if injured. If no part of the exposed metal is heated to the red heat there is no danger, except from a rise in the water-level and flooding the hot iron. If any por- STEAM-BOILER EXPLOSIONS. tion should be red-hot, an additional danger is due to the steam-pressure, which should be reduced by continuing the en- gine in steady working while extinguishing the fire. If the safety-valve be touched at such a time, it should be handled very cautiously, allowing the steam to issue very steadily and in such quantities that the steam-gauge hand shows no fluctua- tion, while slowly falling. The damping of the fire with wet ashes will reduce the temperature and pressure very promptly and safely. The Author has experimentally performed this operation, standing by a large outside-fired tubular boiler while all the water was blown out, and then covering the fire. The pyrometer inserted in the boiler showed no elevation of tem- perature until all the water was gone, and the fire was then so promptly covered that the rise was but a few degrees, and the boiler was not injured. As it proved, there was not the slight- est danger in that case; but with less promptness of action some danger might have arisen of injury to the boiler, although probably not of explosion. Overheated plates, produced by sediment, or over-driving, resulting in the production of ' pockets" or of cracks, are, virtu- ally, cases of low-water, and the action taken should be the same. The boiler being safely cooled down, the injured plate should be replaced by a sound sheet, all sediment or scale carefully re- moved, and a recurrence of the causes of the accident effectively provided against. Cracks, suddenly appearing in sheets exposed to the fire, or elsewhere, sometimes introduce a serious danger. The steps to be taken in such a case are the immediate opening of the safety- valve and reduction of steam-pressure as promptly and rapidly as possible, meantime quenching the fire and then cooling off the boiler and ascertaining the extent of the injury, and repair- ing it. In such a case, unless the crack is near the safety-valve itself, no fear need be entertained of too rapid discharge of the steam. Blistered sheets should be treated precisely as in the cases preceding. It is not always possible to surmise the extent of the injury or the danger involved until steam is off and an ex- amination can be made. It is not, however, absolutely neces- 6l6 ' THE STEAM-BOILER. sary to act as promptly as in the preceding cases ; and where the blister is not large and is not extending, it is sometimes perfectly allowable to await a convenient time for blowing off steam and making repairs. An inoperative safety-valve, either stuck fast, or too small to discharge all the steam made, or to keep the pressure down to a safe point, produces one of the most trying of all known emer- gencies. In such a case steam should be worked off through the engine, if possible, and discharged through any valves available, through the gauge-cocks, or even through a few scat- tered rivet-holes, out of which the rivets may be knocked on the instant ; the fire being meantime checked by the damper or by free use of water. The throwing of water into a furnace is often a somewhat hazardous operation, however, and if neces- sary, should be performed with some caution, to avoid risk of in- jury of either the person attempting it, or of the boiler. The use of wet ashes is preferable. In all cases in which it is to be at- tempted to reduce the rate of generation of heat, closing the ash- pit doors as well as opening the fire-doors will be of service by checking the passage of hot air from below and accelerating the influx of cold air above the grate ; but the closing of the ash-pit involves, with a hot fire, some risk of melting down the grates. 293. The Results of Explosions of steam-boilers, in spreading destruction and death in all directions, are so famil- iar as scarcely to require illustration ; but a few instances may be described as examples in which the stored energy of various types of boiler has been set free with tremendous and impres- sive effect. Referring to the table in 269, and to case No. I : The explo- sion of a boiler of this form and of the proportions here given, in the year 1843, m the establishment of Messrs. R. L. Thurston & Co., at Providence, R. I., is well remembered by the Author. The boiler-house was entirely destroyed, the main building seriously damaged, and a large expense was incurred in the pur- chase of new tools to replace those destroyed. No lives were lost, as the explosion fortunately occurred after the workmen had left the building. A similar explosion of a boiler of this STEAM-BOILER EXPLOSIONS. size occurred some years later, within sight of the Author, which drove one end of the exploding boiler through a i6-inch wall, and several hundred feet through the air, cutting off an elm- tree high above the ground, where it measured 9 inches in di- ameter, partly destroying a house in its further flight, and fell in the street beyond, where it was found hot and dry immedi- ately after striking the earth. Long after the Author reached the spot, although a heavy rain was falling, it was too hot to be touched, and was finally, some time later, cooled off by a stream of water from a hose, in order that it might be moved and inspected. It had been overheated, in consequence of low- water, and cold feed-water had then been turned into it. The boiler was in good order, but four years old, and was considered safe for 1 10 pounds. The attendant was seriously injured, and a pedestrian passing at the instant of the explosion was buried in the ruins of the falling walls and killed. The energy of this explosion was very much less than that stored in the boiler when in regular work: A boiler of class No. 3, which the Author was called upon to inspect after explosion, had formed one of a " battery" of ten or twelve, and was set next the outside boiler of the lot. Its explosion threw the latter entirely out of the boiler-house into an adjoining yard, displaced the boiler on the opposite side, and demolished the boiler-house completely. The exploding boiler was torn into many pieces. The shell was torn into a helical ribbon, which was unwound from end to end. The fur- nace-end of the boiler flew across the space in front of its house, tore down the side of a " kier-house," and demolished the kiers, nearly killing the kier-house attendant, who was standing be- tween two kiers. The opposite end of the boiler was thrown through the air, describing a trajectory having an altitude of fifty feet and a range of several hundred, doing much damage to property en route, finally landing in a neighboring field. The furnace front was found by the Author on the top of a hill, a quarter of a mile, nearly, from the boiler-house. The attendant, who was on the top of the boiler at the instant of the explo- sion, opening a steam-connection to relieve the boiler, then con- 6l8 THE STEAM-BOILER. taining an excess of steam and a deficiency of water, was thrown over the roof of the mill, and his body was picked up in the field on the other side, and carried away in a packing- box measuring about two feet on each side. The cause was low-water and consequent overheating, and the introduction of water without first hauling fires and cooling down. Both this boiler and the plain cylinder are thus seen to have a projectile effect only to be compared to that of ordnance. The violence of the explosion of the locomotive boiler is naturally most terrible, exceeding, as it does, that of ordnance fired with a charge of 150 pounds of powder of best quality, or perhaps 250 pounds of ordinary quality fired in the usual way.* On the occasion of such an explosion which the Author was called upon to investigate, in the course of his professional prac- tice, the engine was hauling a train of coal cars weighing about 1000 tons. The steam had been shut off from the cylinders a few minutes before, as the train passed over the crest of an in- cline and started down the hill, and the throttle again opened a few moments before the accident. The explosion killed the engineer, the fireman, and a brakeman, tore the firebox to pieces, threw the engine from the track, turning it completely around, broke up the running parts of the machinery, and made very complete destruction of the whole engine. There was no indication that the Author could detect of low-water ; and he attributed the accident to weakening of the fire box sheets at the lower parts of the water-legs by corrosion. The bodies of the engineer and fireman were found several hundred feet from the wreck, the former among the branches of a tree by the side of the track. This violence of projection of smaller masses would seem to indicate the concentration of the energy of the heat stored in the boiler, when converted into mechanical en- ergy, upon the front of the boiler, and its application largely to the impulsion of adjacent bodies. The range of projection was, * The theoretical effect of good gunpowder is about 500 foot tons per pound (340 tonne-metres per kilogramme), according to Noble and Abel. STEAM-BOILER EXPLOSIONS. 619 in one case, fully equal to the calculated range. The energy- expended is here nearly the full amount calculated. Figs. 157, 158, 159, 1 60 illus- trate the explosion of two large boil- ers which produced very disastrous effects,* killing the attendant and de- stroying the boiler-house and other property. These boilers were hori- zontal, internally-fired, drop-flue boil- ers, seven feet diameter and twenty- one feet long, the shells single-riv- eted, originally five sixteenths of an inch thick. The two exploded boilers were made twenty-one years before the explosion, and worked, as their mak- ers intended, at about thirty pounds per square inch, till about twenty months before the explosion, at which k, , 57 .-Ex,.Los time additional power was required, and the pressure was increased to and limited at fifty pounds. A third boiler did not explode, but was thrown about fifty feet out of its bed. BOILERS A OOKLVN, N. Y. FIG. 158. POSITION OF THE THREE BOILERS AFTER THE EXPLOSION. A few minutes before noon, while the engine was running at the usual speed, the steam-gauge indicating forty-seven pounds * Scitnti fie American, May 20, 1882. 62O THE S TEA M-B OILER. pressure, and the water-gauges showing the usual amount of water, the middle one exploded : the shell burst open, and was nearly all stripped off. The remainder of the boiler was thrown high in the air. While this boiler was in the air, No. i, the left-hand boiler, having been forcibly struck by parts of No. 2, also gave way so FIG. 159. INITIAL RUPTCRE. that its main portion was projected horizontally to the front, arriving at the front wall of the building in time to fall under No. 2, as shown in Fig. 158. The most probable method of rup- ture is indicated in Fig. 159, as the line AB separates a ring of plates which was found folded together beneath the pile of -n * m^o tvoo Os s.s'.w ^ u^vo t^ f s B >v _SJ | > X 1 2 ^ Q s. VOLUME. 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CN! O rOOO ro O I NO M lOOO w fO ^ ^- CO co^ooio ^^o^^h 9 ^^^^^^? 10 to IONO NO NO NO NO t-. i< X ri. t^CO ON O M CN) CO -^ IONO ^CO C 10 10 10 IONO NO NO NNNCMMCMINN CN1NCN1CNI (NNINCN!CN!CN!?) O fONO ON -- N ON 10 O tx ON O NO>inN ONUIM t^- ico co'oc?co 1 co'co" O^OO VO ^ w O'CX'^' fONO ON CJ ^- t^ O f oooooooooo ooooooooco OO 00 000 00 5-^^ ^^S 2 SZS NO oo ON N m U-NO t-co OOOOOOOOOOOOOOCOOOOJ (N 100 NO NO fxONO^lOJNOlOMOt-l W M ONOO tx O- O ON CO M tx c^. Cx Cx NO ^" N 00 rooo O OO O 10 r^ r^ coiO| txOror^-ioON^O-'* - rX I^NO I VO NO NO" NO NO NO NO'NO^NO'NO'' MOO NOO - ON -- ON f'l ro ro co ro ro V - <2S ?| ^l^^sss 1 "S 2" CO ? I ioNo" KcO 0' < 0- V 2" CNICO I er, ON^OlOMNOi-i C< CJ I CO CO ^ 1O IONO 'O Ix txCO \O NO NO N NO NO N O NO CO '(NO CN) CN! NO N j ON ONNO loco NO o NO co NO f.ao M N1 tr> * IOVO t^OO O> TfTf^-^-^-^^^-^-lO 2 8 S, 5- 6 5 2 THE STEAM-BOILER. The column headed " U" in the table of the properties of saturated steam is useful for reducing the performance of differ- ent boilers to a common standard this standard being that most generally accepted by engineers : the equivalent evapora- tion at atmospheric pressure and the temperature of boiling water, or, as it is frequently called, the evaporation from and at 212. In the table it is assumed that the temperature of the feed-water is 32, and an auxiliary table is added, giving corrections for any temperature of feed from .32 to 212. CORRECTION FOR TOTAL HEAT IN UNITS OF EVAPORATION. Tempera- ture of feed, Fah- renheit degrees. U Tempera- ture of feed, Fah- renheit degrees. _o Q (J Tempera- ture of feed, Fah- renheit degrees. Correction. Tempera- ture of feed, Fah- renheit degrees. Correction. Tempera- ture of feed, Fah- renheit degrees. Correction. 33 .0010 69 383 I0 5 .0756 141 1129 177 .1504 34 .0021 70 0393 106 .0766 142 1140 178 1 5 I 4 35 .0031 71 .0404 107 .0777 i43 1150 179 1525 36 .0041 72 .0414 108 .0787 144 1160 180 1535 37 .0052 73 .0424 109 .0797 i45 1171 181 1545 38 .0062 74 0435 no .0808 146 .1181 18-2 1556 39 .0073 75 0445 III .0818 J 47 .1192 183 .1566 40 .0083 76 .0450 112 .0829 148 .1202 184 1577 4i .0093 77 .0466 "3 .0839 149 .1213 185 .1587 42 .OIO4 78 .0476 114 .0849 150 .1223 1 86 .1598 43 .0114 79 .048 7 "5 .0860 J5 1 I2 33 187 .1608 44 .0124 80 .0497 116 .0870 J 5 2 .1244 188 .1618 45 0'35 81 .0507 117 .0880 153 I2 54 189 .1629 46 .0145 82 .0518 118 .0891 J 54 1264 190 .1639 47 0155 83 .0528 119 .0901 155 I2 75 191 .1650 4 1 .0166 84 .0538 1 20 .0911 156 1285 192 .1660 4* .0176 85 0549 121 .0922 *57 1296 193 . 1670 5 .0186 86 0559 122 .0932 158 1306 194 .1681 5' .0197 ll .0569 123 0943 X 59 1316 195 . 1691 5 2 .0207 88 .0580 I2 4 0953 160 1327 196 .1702 53 .0217 89 .0590 j 125 .0963 161 I 337 197 .1712 54 .0228 90 .060I 126 .0974 162 .1348 198 I 7 2 3 55 .0238 91 .06ll I2 7 .0984 163 1358 199 r 733 56 .0248 92 .0621 128 .0994 164 .1368 200 1743 57 .0259 93 .O632 I2 9 .1005 165 1379 2OI I 754 58 .0269 94 .0642 I 3 .1015 1 66 .1389 202 .1764 .0279 .0290 95 96 .0652 .0663 131 I 3 2 .1025 .1036 167 168 . 1400 .1410 203 204 1775 1785 61 .0300 97 .06 7 3 133 . . 1046 ,69 . 1420 20 5 .1796 62 .0311 98 .0683 ^34 1057 170 I43 1 206 .1806 63 .0321 9Q .0694 135 .1067 171 .1441 207 .1817 64 33i loo .0704 I 3 6 .1077 172 MS 2 208 .1827 & .0342 101 .0714 137 .1088 173 .1462 20 9 1837 66 .0352 102 0725 138 .1098 174 I 473 2IO .1848 67 .0362 I0 3 0735 139 .1109 J 75 .1483 211 .1858 68 .0372 I0 4 .0746 I 4 .1119 176 1493 212 .1869 \PPENDIX. 653 TABLE la. TEMPERATURES AND PRESSURES, SATURATED STEAM. IN METRIC MEASURES AND FROM REGNAULT. o t 3 STEAM-PRESSURE. 3 STEAM-PRESSURE. fS 3 1 L S s In Centimetres. In Atmospheres S. e In Centimetres. In Atmospheres H H - 32 C 0.0320 0.0004 + 14 C. I . 1908 0.016 31 0.0352 0.0005 15 1.2699 0.017 30 0.0386 0.0005 16 1.3536 0.018 29 0.0424 0.0006 17 1.4421 0.019 28 o . 0464 0.0006 18 1-5357 0.020 27 0.0508 0.0007 19 1.6346 O.O22 26 0.0555 0.0007 20 I-739I 0.023 25 o . 0605 0.0008 21 1.8495 0.024 24 0.0660 0.0009 22 1.9659 0.026 23 0.0719 0.0009 23 2.0888 0.028 22 0.0783 O.OOIO 24 2.2184 0.029 21 0.0853 O.OOII 25 2.3550 0.031 20 0.0927 0.0012 26 2.4988 0.033 19 0.1008 0.0013 27 2.5505 0.034 18 0.1095 O.OOI4 28 2.8101 0.037 17 o. 1189 0.0015 29 2.9782 0-039 16 o . i 290 0.0017 30 3.1548 0.042 15 0.1400 O.OOlS 31 3.3406 0.044 14 0.1518 O.OO2O 32 3-5359 0.047 13 0.1646 0.0022 33 3-74H 0.049 12 0.1733 O.OO24 34 3-9565 O.O52 II 0.1933 O.OO25 35 4.1827 0-055 10 0.2093 0.0027 36 4.4201 0.058 9 0.2267 O.OO3O 37 4.6691 0.061 8 0.2455 O.OO32 38 4.9302 0.065 7 0.2658 0.0035 39 5 2039 0.068 6 0.2876 O.OO38 40 5.4906 0.072 5 0.3113 O.OO4I 4i 5-791 0.076 4 0.3368 0.0044 42 6.1055 0.080 3 0.3644 0.0048 43 6.4346 0.085 2 0.3941 0.0052 44 6.7790 0.089 I 0.4263 O.OO56 45 7-I39 1 0.094 0.4600 0.0061 46 7-5158 0.099 + I 0.4940 0.0065 47 7.9093 0.104 2 0.5302 0.0070 48 8.3204 0.109 3 0.5687 0.0073 49 8.7499 0.115 4 0.6097 0.0080 50 9.1982 O.I2I 5 0.6534 0.0086 51 9.6661 0.127 6 0.6998 0.0092 52 IO.I543 0.134 7 0.7492 0.0199 53 10.6636 O.I4O 8 0.8017 0.0107 54 11.1945 0.147 9 0.8574 O.OII 55 11.7478 0.155 10 0.9165 0.012 56 12.3244 0.163 ri 0.9792 0.013 57 12.9251 o. 170 12 1.0457 0.014 58 I3-5505 0.178 13 1.1162 0.015 59 14.2015 0.187 654 THE STEAM-BOILER. TABLE la. Continued. I STEAM-PRESSURE. 3 rt u STEAM-PRESSURE. i s In Centimetres. In Atmospheres Q. E u In Centimetres. In Atmospheres H H + 60 c. 14.8791 0.196 +noC. 107.537 .415 61 15.5839 0.205 in III. 209 463 62 16.3170 0.215 112 114.983 .513 63 17.0791 0.225 "3 118.861 .564 64 17.8714 0.235 114 122.847 .616 65 18.6945 0.246 115 126.941 .670 66 19.5496 0.257 116 I3LI47 .726 67 20.4376 0.267 117 135.466 .782 68 21.3596 0.281 118 139.902 .841 69 22.3165 0.294 119 144.455 .901 70 23.3093 0.306 120 149.128 .962 71 24-3393 0.320 121 I53.925 2.025 72 25.4073 0-334 122 158.847 2.091 73 26.5147 0-349 123 163.896 2.157 74 27.6624 0.364 124 169.076 2.225 75 28.8517 0.380 125 174.388 2.295 76 30.0838 0.396 126 179.835 2.366 77 31.3600 0.414 127 185.420 2.430 78 32.6811 0.430 128 191.147 2.515 79 34-0488 0.448 129 197.015 2.592 80 35-4643 0.466 130 203.028 2.671 81 36.9287 0.486 131 209. 194 2.753 82 38.4435 0.506 132 215.503 2.836 83 40.0101 0.526 133 221.969 2.921 84 41.6298 0.548 134 228. 592 3.008 85 43.3041 0.570 135 235.373 3.097 86 45 0344 0-593 I 3 6 242.316 3.188 87 46.8221 0.616 137 249.423 3.282 88 48.6687 0.640 138 256.700 3.378 89 50.5759 0.665 139 264.144 3.476 90 52.5450 0.691 140 271-763 3.576 91 54.5778 0.719 141 279-557 3.678 92 56.6757 0.746 142 287.530 3.783 93 58.8406 0-774 143 295.686 3.890 94 61.0740 0.804 144 304.026 4.000 95 63.3778 0.834 145 312.555 4.1^3 96 65-7535 0.865 I 4 6 321.274 4.227 97 68.2029 0.897 147 330.187 4-344 98 70.7280 0.931 148 4.464 99 73.3305 0.965 149 348.609 4.587 100 76.000 .000 150 358.123 4.712 101 76.7590 036 151 367.843 4.840 102 81.6010 .074 152 377-774 4.971 103 84.5280 . 112 153 387.918 5.104 104 87.5410 .152 154 398.277 5.240 105 90.6410 193 155 408.856 5.380 1 06 93-8310 235 156 419.659 5-522 107 97.1140 .278 157 430.688 5.66 7 1 08 100 4910 -322 I 5 8 441-945 109 103.965 -368 159 453-436 5-966 APPENDIX. TABLE la. Continued. 655 t II a 3 STEAM -PRESSURE. s STEAM-PRESSURE. a rt flj a a 1 In Centimetres. In Atmospheres 1 In Centimetres. In Atmospheres -f-i6oC. 465.162 6.120 +196 C. 1074-595 14.139 161 477.128 6.278 197 1097.500 14.441 162 489-336 6.439 198 1120.982 14.749 163 501.791 6.603 199 1144.746 15.062 164 514.497 6.770 200 1168.896 I5-380 165 527-454 6.940 2OI "93-437 15.703 1 66 540.669 7.114 202 1218.369 16.031 167 554-143 7.291 203 1243.700 16.364 168 567.882 7-472 204 1269.430 16.703 169 581.890 7.656 205 1295.566 17.047 170 596.166 7.844 206 1322. 112 17.396 171 610.719 8.036 207 1349.075 17.751 172 625.548 8.231 208 1376.453 18.111 173 640.660 8.430 209 I404.252 18.477 174 656.055 8.632 210 1432.480 18.848 175 67L743 8.839 211 I46I.I32 19.226 176 687.722 9.049 212 1490.222 19.608 177 703.997 9-263 213 1519.748 19.997 178 720.572 9.481 214 I549.7I7 20.391 179 737-452 9-703 215 I580.I33 20.791 180 754.639 9.929 216 1610.994 21.197 181 772.137 10.150 217 1642.315 2 r . 690 182 789.952 10.394 218 1674.090 22.027 183 808.084 10.633 219 1706.329 22.452 184 826.540 10.876 220 1739.036 22.882 185 845.323 11.123 221 T772.2I3 23-3I9 186 864.435 11-374 222 1805.864 23-761 187 883.882 11.630 223 I839-994 24.210 188 903.668 11.885 224 1874.607 24.666 189 923.795 12.155 225 1909.704 25.128 190 944.270 12.425 226 1945.292 25.596 191 965.093 12.699 227 1981.376 26.071 192 986.271 12.977 228 2017.961 26.552 193 1007.804 13.261 229 2055.048 27.040 194 1029.701 13-549 230 2O92 . 640 27-535 T 95 1051.963 13-842 6 5 6 THE STEAM-BOILER. H < X s i si O "">OO 04 IX N M ^-mtx M 00 cr>00 N V III! . " >,c.o "fiPll \o M -4- M oo moo ^- O tx moo m-^t^t^mM-n i/^o io Nin-ioMtsiooLot^Nmt^i- --oo *o m o w M OOO t^ <* O "^ O ^"OO N IOOM ^-^00* ^VC OO O M 04 CO ^- io\6 VO tx tx fxOO 00 00 O < . i t^oo ooo oo oo *o rn 04 mOO 04 VO O tOO 04 \O tOO N VO TO *O VO t- C^OO OO OO O O ( t^oo oo'oo o o ' . O 0) t\O oo o M m in\O oo i\o vo^o^ovo^o t^t^r-^cxtx -O O Ot^-*O T*- in m\o t>.oo e* N N 04 PI o tx moo OO 10 w moo O 1 O O lx 01 04 M N wmmmmmmmmmmmmmmm moo mvo \o 04 M M ot04oo ixtomM inir)" mn lAininoi 04 t>-o t^04 ovo O ^o 1000 t04 moj mi* uioi^o **^'00 Si m^ m^ o moo t 1? o K\> S> too o M * mvo OOOOOOOOOOOOOOOOOOOOOOi VO O tOO N \O O tOO 04 *O O too ft \O C tOO N \O tOO M mtxo mrvq ttxq tr^M rj-t^w too * -*oo w 1000 M ui H M 01 04 04 rommttt>oo io\d vd vd tx tx txoo oo oo o o J. 4> l" rt rt o 3 mmrommmmmmmmmmmmmmmmmmmmmm v> Q >n O M d in d n o v> p tno r> Q v> O w O ^* O CNI O ^*vo ^ -^- O OO vOOMVONroO^SNT^-loS mvO MOO r^^-oo ro O O t^ 10 M r^ MOO t^ oo * r-oo oo Ooo tv. o M (N -*-\o t^o>O romt^M mmr^iot^ rp in\O \O N . 1O tv "^ N ^ $y g ;r . Jill! ^t- O> rovo O> N -\O t^ OO O -< N rn u->\o t^OO WMNNNWNWN rt- --v N oo <> fOO^M O~ N lOQ OlOM ON CJv !>. M 5 irjOO N >0 S crjvo ro ro c? ro , SS 2 e 8*2 i-O = v 2 3 i* ro irjvo oo O O N ro .-^- 10 t^ oo O O N ro .-^- 10 ^o u-)\c vovovovo x ON H M (X, M 1000' M \O ON M --O O M T-\O 00 O NVO M lOO^M M M N N M N romrOrO-^-^-^-lOlO ul\O OOOOOOCOOOOOOOOOOOOOCOOOCOOOOOCO WS-SEiag 1 22?;r;rs ON M M ** IOVO OO ON O >-* CO -^-VO OO -i rOlO(N)\o t^NlOiOM W M O ^M ^ t^oo oooooooooooo O>ONONONONONO O O -*oo mtx-o mu->tx.o-O woo MMI-ll-IMHMMM(-IMHI-tMC404NC4MrOrO^* < ^ > ^^-^-lOlO*-l ' u"jvO t^ Cvoo OO ON O> O ^ a = _ N rOVO O * N O 00 VO if-OO V i- t^ * 10 * t-s o^oo -*ro W M |-"xVO ONNO \O O CO ON VO t-.00 O IN 1000 N 10 ON ^t-VO IO(^)O O t^r^- \O COCO t>OO VO txO O CO VO 10 ro M ONOO VO -^-OIOCO mCNl O O O fOO ^-101- ONO>N IT, i vovS ioioioio>o-<--<--4--.j-^-^rnrorr f^oo M *& Orinoco 10 i OOOOOOOOCOOOOOOOOOOOOOOOOOOOOOOOOO tx, tx,vO lOlOlO-*-<--rOCOi OO 1000 ^-WO W vo<^. 6o O 100 i/*O oo 100 100 lOWNioioiotoioioioioioioir ro ro * * 10 iov5 vo r^ tx-oo oo ON O - M <*ioo oo oo oo oo oo oo MMHIMMMI-tMl-.l-lwCNlO 1 NONONONO' 3 S en Ulis P<3 u a 3 4- i<-vOOOioOOO.>oO>oQO w 10 iov5 vo ti. t^oo ooONONOM INDEX. A SEC. PAGE Air, minimum, required in Fire, 77 178 Anthracite Coals, 64 155 Apparatus, forms o'f gas-analysis 265 531 Applications of Boilers 14 20 Appurtenances of Steam Boilers, 10 18 Area of Cooling Surfaces, formulas for 98 221 B Barrel Calorimeters, forms of, ....... 260 519 use of, 260 519 Bituminous Coals, 65 156 Bodies, molecular constitution of, 109 241 Boiler, common proportions and Work of, . . . . . 161 335 conditions of efficiency of, 149 303 design of Plain Cylinder 169 350 determination of Value of, ...... 246 485 form and Location of Bridge-wall of 179 381 forming bent parts of, 190 403 general decay of 288 604 management of, . . . . , . . 206 440 local decay of, 288 604 matters of detail of, 168 346 number of, 164 340 office of the Steam, i i operation of, 212 445 parts of, defined, 168 346 selection of Type and location of, 147 300 size of, 164 340 the Locomotive, ........ 16 26 transfer of Heat in the Steam, 97 220 The older Types of, 3 4 The Scotch, 19 32 Upright and portable 175 369' Boiler-construction, controlling ideas in, 151 307 660 INDEX. SEC. PAGE Boiler-Design, details of the problem of, . . . , . 166 345 general consideration of, 167 346- principles of, ....... 150 304 problem stated, 164 340 special conditions affecting, .... 156 317 Boiler-Power 164 340 Boiler-pressure, choice of . . . 149 303 Boiler-trials, errors of, ......... 262 521 precautions observed at, 257 514 purposes of, 244 484. record-blanks for 257 514 records of, . 262 521 Boilers, appurtenances of Steam, ...... 10 18 assembling of, . . . . . ... 194 420 classification of n 19, common " Shell " stationary, 15 21 corrosion of, ......... 287 601 cost of, 152 311 covering of, .178 380 coverings of, ......... 227 456 cylindrical Tubular, 171 358 defects of construction of 286 596 design of, 285 593 deterioration of, . .57 144 developed weakness of, . ...... 287 601 drawings of construction of, ... ... 186 400 efficiency of, .... , 152 311 efficiency of the Steam, ....... 234 472 energy stored in, ........ 269 541 factors of safety for, ....... 152 311 general care of, ... ..... 222 454 general instructions in management of, ... 233 469 Horse-power of, . . . . . . 145 292 inspection of, ......... 195 420 inspection and test of, ..... 56, 232 140. 466 management of, 291 612 Marine Flue, 172 361 Marine ; older-forms. ....... 17 29 Marine Sectional, ........ 21 38 Marine Tubular, . . . . . . .173 362 Marine Water-tube, ....... 18 30 Methods of construction of, ... ... 186 400 corrosion in, . . . . . . 289 606 decay in, 289 606 of locomotives ....... 177 377 periods of introduction of, .... 22 39 INDEX. 66 1 SEC. PAGE Boilers, power of 144 291 problems in the use of, ....... 25 43 processes of construction of, . . . . . .186 400 relative security of, ....... 284 592 relative strength of Shell and Sectional, ... 58 148 relative value of, ........ 250 488 repairs of, 231 465 sectional, .-......, 20 33 Sectional and Water-tube, 174 364 setting of I77 369 special forms of, . ....... 23 42 shells of, 55 I2 9 specification for Steam, ....... 202 427 stationary Flue, ........ 170 354 staying in, I 9 2 413 Steam, explosions of, ....... 268 538 suspension of, ........ 177 377 testing Steam, ........ 196 422 transportation and delivery, ...... 198 424 Braced and Stayed Surfaces, ....... 60 151 Brass. 54 I27 Bridge-wall of Boiler, Form and location of, .... 179 381 Bursting, . 271 549 C Calorimeters, Theory of, 261 521 Calorimetry, .......... 92 214 Calking and chipping, ......... 193 417 Charcoal 70 162 Chemical characteristics of Iron, ...... 30 57 Chimney Draught, ......... 157 317 Forms of ......... 158 322 Flues, and Grate, relative areas of, . . 160 334 size of - . . .158 322 Chipping and Calking, 193 417 Coal Calorimeter, The 263 524 defined, .......... 65 153 Coals, anthracite, ......... 64 155 Bituminous, ......... 65 156 <^oke, 69 160 Colburn's Theory of Explosions, 275 559 Combustion defined ; Perfect combustion, ..... 62 152 efficiency of, 236 473 method of, 148 302 rate of 79 184 temperature of products of, . . . . . 78 179 662 INDEX. Commercial efficiency, 2 4 474 theory of, 242 477 Conclusions relating to explosions 295 642 Construction of Boilers, defective, 286 596 Construction, problem in Design and, 24 43 Continuous Calorimeters, The, 263 52^ Contract, 2O 426 purpose of Specification and, 199 4 2 S Cooling Surfaces, Area of, Formulas for, 9 8 221 Copper, 54 127 Corrosion, chemistry of, 223 454 method of, 224 455 methods of, in Boilers, 289 606 of Boilers . . 287 601 Critical Point, 129 265 Crystallization and Granulation, ....... 37 9 Curves of Energy, 143 28 9 Cylindrical Tubular Boilers, .171 35& D Dampers, location and Form of . 181 381 Decay, general, of Boilers, . 288 604. local, of Boilers, 288 604 Methods of, in Boilers, 289 606 Delivery of Boilers, ... 198 424 Deposits, Incrustation and effect of, 99 218 Design and construction, problems in, ..... 24 43 of Boilers, special conditions affecting, .... 156 317 defects of 285 593 Designing Boilers, principles involved in, . . . . . 6 n Deterioration of Boilers, 57 144 Donny and Dufour, experiments of, . . . . . .281 578 Draught Gauges, .......... 267 535 natural and forced, 155 3 X 4 Drilling and punching, 189 402 Ductility, 29 56 of Metal, loss of, 59 149 E Economy, relation of Area of Heating Surface to, . . . 252 490 Efficiency and Quantity of Steam, 163 338 as indicated by Gas-analysis, . . . . . 216 449 combined power and ....... 253 489 INDEX. 663 Efficiency, commercial 240 474 finance of, 239 474 measures of, 235 473 of Boiler, conditions of 149 303 of Heating surfaces, Formulas for, .... 98 221 Theory of commercial, ...... 242 471 Efficiency, variations of, with consumption of Fuel and size of grate, 251 488 Efficiencies, algebraic Theory of, 241 476 Elasticity, ........... 29 56 Emergencies, 217, 229 450, 462 Energetics; Heat-energy and Molecular Velocity, . . . 101 233 Energy, curves of, . . 143 289 Heat and Mechanical, 105 237 Heat as a form of, . . . . . . . .98 221 of Steam alone, ........ 270 548 stored, in Steam, 142 285 stored, in Boilers, . . . . . . . . 269 541 heated Metal, 277 567 superheated Water, 281 578 Evaporation, factors of, 139 278 usual rate of, . . . . . . . . 162 338 Excess of Pressure, 221 454 Expansion, Latent Heat of, ....... 113 243 Experimental explosions and investigations, .... 294 633 Experiments of Donny and Dufour, 281 578 Leidenfrost and Boutigny, 282 583 Explosions, absurd, causes of 272 550 causes of 272, 293 550, 616 Colburn's Theory of, 275 559 definition of 271 549 description of, ........ 271 549 examples of, ........ 293 616 experimental, . . . . . . . . 294 633 fulminating, 271 549 improbable causes of, 272 550 Lavvson's and others' experiments of, ... 276 561 methods of, 274 558 of Steam-boilers, 268 538 possible causes of, ....... 272 550 probable causes of, ....... 272 550 results of, 293 616 statistics of causes of, 273 559 Theories of, 274 558 usual causes of, . ... 272 550 664 INDEX. F SEC. ' PAGE Feed apparatus, ........ l8 4 39 2 Filtration, ........... I2 4 260 Fitting ....... ..... 188 402 Fire, minimum air required in, ....... 77 178 temperature of, . . . . ..... 76 172 Fire-rooms, closed and open, ..... . . 214 448 Fire-tubes ............ 153 312 Fires, starting of ........... 207 441 the management of, ........ 208 442 Flanging and Pressing, ....... , 189 402 Flue-boilers, Marine, ......... 172 361 stationary, . . . . . . . .170 354 Flues, Chimney and Grate, relative areas of, . . . 160 334 collapsed, .......... 271 549 disposition of .......... 180 381 flanged and corrugated, ....... 61 151 setting of, .......... 192 413 Forced Draught, .......... 213 448 Forces and Work, computation of External, . . . .118 248 Internal, . . . .118 248 Form, effect of variation of, ........ 32 64 Forms of Boilers, modern standard, ...... 12 20 Fuel, adaptation of, ......... 88 206 choice of .......... 148 302 economy of, ........ 81, 249 187, 487 pulverized, .......... 71 164 test of Value of ... ..... 245 485 use of various kinds of, ........ 209 444 Fuels, ............ 63 153 analysis of, ......... 248 486 artificial, ..... ..... 74 168 commercial value of, ....... 86 201 composition of, ........ 83 192 efficiency of, ........ . 249 487 evaporative Power of ........ 247 485 Gaseous ........... 73 ^7 heating effects of, ........ 84 194 heating-power of, ........ 75 !6 , .......... 72 I65 and Gaseous, ........ 210 444 solid . ..... ..... 2ii 445 Furnace, adaptation of, ........ 88 206 and grate, ......... I59 32 g efficiency of ........ .80 185 management, . ....... 87 204 INDEX. 665 SEC. PAGE Fusible Plugs 185 393 Fusion and Vaporization, latent heats of, 114 214 G Galvanic Action, .......... 229 462 Gas-analysis, efficiency as indicated by, 266 535 Gases, analysis of, ......... 265 531 denned; the perfect gas, . no 241 Gaseous Fuels, .......... 73 167 Gauges, draught, 267 535 Granulation and Crystallization, ....... 37 90 Grate and Furnace, 159 227 Flues, Chimney, relative areas of, ..... 160 334 Grooving and furrowing, . 289 606 H Heat, as a form of energy, ........ 100 229 and matter; Specific heat, . . . . . .in 242 and mechanical Energy, 105 237 conduction- of, -95 217 convection of, ......... 96 219 efficiency of Transfer of, ....... 237 473 methods of Production of, 90 208 nature of, 89 207 production, transfer, and strength of, .... 7 12 quantities of, . . 91 210 radiation of, ......... 94 216 Sensible and Latent, . . . . . . . .112 243 Specific, 91 210 transfer of, 93 215 in Steam Boilers, 97 200 Transformations, 105 237 utilization of, ......... 88 15 Heat-energy, as related to Temperature, 102 235 distribution of, 115 244 quantitative measure of, 103 236 Heaters 184 382 Heating effects of Fuels, 84 194 power of Fuels, 75 169 Heating- surface to economy, relation of area of, ... 252 489 efficiency of, Formulas for, .... 98 221 Heats, computation of Latent and Total, 138 276 specific, of Steam and Water 137 275 Total and Latent; Internal Pressures and Work, . . 133 271 Helical Seams -49 "7 Horse-power of Boilers, 145 292 666 INDEX. I SEC. PACK Improvement in Boilers, method and limit of, ... 5 i Incrustation ........ ... 230 462 Incrustation and Deposits, effect of, ...... 99 228 Sediment, ....... 280 574 Inspections and Test of Boilers, ..... . . 56 140 Inspector, duties of the, ...... . . 205 438 Internal Pressures and Work; Total and Latent Heats, . . 133 271 computation of, . . . . 134 271 Investigations and Experimental explosions, .... 294 633 Iron, Cast and Malleableized, ....... 54 127 choice of, for various parts, ...... 44 112- preservation of, ......... 226 564 Physical and Chemical characteristics of, . . . .30 57 specification of quality of ........ 43 108 Iron and Steel compared ...... .. . .38- 9 2 durability of, ....... 225 457 method of Test of, ...... 41 98 Latent and sensible heat, 112 243 Heat of Expansion, 113 243 Heats, computation of, . . . . . . 138. 276 of Fusion and Vaporization, . . . . .114 244 Lawson's and others' Experiments, . . . . . . 276 561 Leakage, 228 461 Leidenfrost's and Boutigny's Experiments, .... 282 583 Lignites, . . . . . . . . . . .66 158 Liquid Fuels, . .72 165 Liquids defined, .......... no 241 Location and Type of Boiler, Selection- of, .... 147 300 Locomotive Boiler, The, ........ 16 26 Boilers, . . .176 371 Low-water, . . . . . . . . . .218 450 causes of, ......... 279 568 consequences of, ....... 279 568 M Marine Boilers, older Forms, ....... 17 29 Flue Boilers, 172 361 Tubular Boilers, 173 362 Water- tube Boilers, 18 30 Materials required, Quantity of, . . . . . .27 45 Metal, heated, energy stored in 277 567 loss of Strength and Ductility of, 59 149 INDEX. 667 SEC. PAGE Methods of Explosions, 274 558 Method of Treatment, effect of 33 7O Minor accessories, 185 393 Mixed applications of Boilers, 14 20 Types of Boilers, 13 20 Molecular constitution of Bodies, 109 241 N Net efficiency, 238 473 Number of Boilers, 164 340 O Operation of Boilers, safety in, . 9 18 Overstrain, method of detecting, 35 8r P Paints and Preservatives, 227 458 Peat or Turf, . . . . . ... . .67 150 Physical characteristics of Iron, 30 57 State of Water, changes of, 128 265 Pipes, Steam and Water, 182 383 Plain Cylinder Boiler, design of, 169 350 Planing 188 402 Plant, efficiency of a given, ....... 243 481 Plate, Grades and Quantities of Iron in Boilers, ... 39 94 manufacture of Iron and Steel, 40 96 Plates, drilled, 50 123 punched, . 5 I2 3 Portable Boilers, . J75 3&9 Power and efficiency, combined, 253 489 of Boilers, 175 3^9 Steam Boilers, 144 291 Precautions, ; 292 614 Preservatives and Paints, . 227 458 Pressing and Flanging, 189 402 Pressure, computation of Internal Work and, .... 134 271 excess of 221 454 in Boiler, choice of 149 33 steady rise of 283 589 Pressures, control of Steam, 215 449 relations of, I3 6 273 Priming 219 451 Principles of Boiler-Design 150 34 Problem of Boiler-Design, details of the, 166 345 Production of Heat, methods of, 9 2oS 668 INDEX. Products of Combustion, temperature of, 78 179 Pulverized Fuel, 71 164 Punching and Drilling, . . . . . . .189 402 Q Quality of Metal, specifications of, 204 436 Quantities of Heat, 91 210 R Rate of Combustion, 79 184 Records for Boiler-trials, 257 514 Regnault's researches and methods, 140 280 tables, 141 281 Resilience, 29 56 Riveting and riveting machines, . . . . . . .191 404 Rivet-iron and Steel, rivets and, ...... 47 114 Rivets and rivet-iron and Steel, -47 114 forms of, 48 115 Rivets, sizes of, 48 115 strength of, 48 115 Riveting, Steam and Hand, 51 125 S Safety Valves, 183 385 Sample specifications, ....*... 203 421 Scotch Boilers, . . 19 32 Sea- water; deposits and remedies, ...... 123 256 Seams: fractured, 22O 453 Helical, 49 117 strength of riveted, . 49 117 Welded, 52 127 Sectional Boilers sees. 20, 154, 197; pp. 35, 314, 423 and Water-tube Boilers, .174 364 Security of Boilers, relative, 284 592 Sediment, 230 462 and Incrustation, 280 574 Sensible and Latent Heat, I I2 243 Setting, contact with, 22 8 461 Shapes, " Struck-up" or Pressed, 53 127 Shearing l88 4O2 Shell and Sectional Boilers, relative strength of, ... 58 148 Shell Boilers, ^ 3I4 common stationary, !g 21 Shells of Boilers, 55 I29 INDEX. 669 SEC. PAGE Size of Boiler, 164 340 Sizes of Tube, standard, ........ 165 341 Solid Fuels, . . . . . . . . . .211 445 Solids defined * no 241 Solution of Problems, general methods of, .... 26 43 Spacing of Tubes, ........ 165 341 Specific Volumes of Steam and Water, 135 272 Specifications and contract, purpose of, ..... 199 425 generally, form of, ...... 201 427 Spheroidal State . 130 268 of Water, . . 282 583 Standard Boilers, Marine, 21 38 Forms of Boiler, development of, .... 2 2 modern 12 20 method, instructions and Rules for, .... 256 491 Stayed and Braced Surfaces, ....... 60 151 Staying in Boilers, ......... 192 413 Steam alone, energy of, ........ 270 548 gauges, . 185 393 generation and application. ...... 119 252 getting up of, . . , . . . . ; . 207 441 stored energy in; Tables 142 285 superheating, ... ..... 131 269 quantity of, 259 517 and efficiency, . . . . . .163 338 and Water pipes, 182 383 specific Heats of, 137 275 volumes of, ..... 135 272 Steam Boilers, Powers of, 144 291 Steam Pressures, control of, 215 449 Steel, characteristics of, 31 63 Steel, choice of, for various parts, 44 112 special precautions in using 46 113 specification of quality of, 43 108 Rivets and rivet-iron and, 47 114 and Iron compared, ....... 38 92 durability of, 225 457 method of Test of, ...... 41 98 Stopping suddenly, 219 9 Stored energy in Steam; Tables, 142 285 Strength of Metal, loss of, 59 149 principles relating to 28 45 Stress, margin of, ..... . . 34 74 Surfaces, Stayed and Braced, 60 151 6/0 INDEX. T SEC. PAGE Technical uses of Water, 124 260 Temperature, differences of 290 609 effects of 36 83 Heat energy as related to, 102 235 of Fire, 7 172 of products of combustion, 78 179 Temperatures 9* 210 relations of, 136 273 Tenacity 29 56 Test, apparatus and Method of, 254 489 of Boilers, inspection and, 56 140 of Iron and Steel, method of, 41 9 8 Tests of Metal, specification of, 204 436 results of, 4 2 104 Test-trials, results of, 258 504 Standard 255 491 Theory of Calorimeters, 261 521 explosions, Colburn's, ...... 275 559 Theories of explosions, ........ 274 558 Thermal and Thermodynamic relation, 132 270 Thermodynamic relation, Thermal and, ..... 132 270 Thermodynamics, 116 245 defined, 106 238 first law of, 107 239 second law of, 108 240 application of 117 247 Thermometry, . . . . . . . . . . .92 214 Time, effect of, .......... 34 74 Total Heats, computation of, 138 276 Transfer of Heat, efficiency of, . . , 237 473 Transportation of Boilers, 198 424 Tubes, leaky, .......... 220 453 setting of, 192 413 standard sizes of, 165 341 spacing of, 165 341 Tubular Marine Boilers, . 173 362 Type and Location of Boiler, selection of, .... 147 300 Types of Boilers, mixed, 13 20 special purposes and modern, 4 7 U Upright Boilers, 175 369 INDEX. 671 V SEC. PAGE Value of Boilers, determination of 246 485 Valves, deranged safety, ........ 221 454 Vaporization, 131 269 Latent Heats of Fusion and, 114 244 Variation of Form, effect of, ....... 32 64 Volumes, relations of, 136 273 W Water analysis, 125 261 and Steam pipes, 182 383 Specific Heats of, ..... 137 275 changes of Physical states of, 128 265 composition and chemistry of, . . . . . 121 254 de-aeration, 281 578 low, causes of, 279 568 consequences of, 279 568 Physical characteristics of, ...... 127 263 properties of; Water as a Solvent, ..... 120 253 purification of, 126 262 sources and purity of "fresh," 122 255 specific Volumes of Steam and, 135 272 Spheroidal State of, 282 583 superheated, ......... 130 268 energy stored in, ..... 281 578 Technical uses of, 124 260 Water-supply, regulation of, 216 449 Water-tubes, 153 314 and Sectional Boilers, ...... 174 364 Weather waste of fuel, . .82 191 Weakness, developed, of Boilers, ...... 287 601 Welded Seams, -52 127 Welding, 191 404 Wood, . 68 159 Work, internal pressure and, ....... 133 271 Working iron, method of, . . . . , . . 45 113