GIFT OF LIBRARY Works of Prof. Robt. H. Tlmrstori. MATERIALS OF ENGINEERING. A work designed for Engineers, Students, and Artisans in wood, metal, and stone. Also as a TEXT-BOOK in Scientific Schools, showing the properties of the subjects treated. By Prof. R. H. Thurston. Well illustrated. In three parts. Part I. THE NON-METALLIC MATERIALS OF ENGINEERING AND METALLURGY, With Measures in British and Metric Units, and Metric and Reduction Tables. 8vo, cloth, $3 00 Part II. IRON AND STEEL. The Ores of Iron ; Methods of Reduction ; Manufacturing Processes ; Chemical and Physical Properties of Iron and Steel ; Strength, Ductility, Elasticity and Resistance; Effects of Time, Temperature, and repeated Strain ; Methods of Test ; Specifications. 8vo, cloth, 4 00 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"; Strength of the Alloys as Commonly Made, and as Affected by Special Conditions ; The Mechanical Treatment of Metals 8vo, cloth, 3 00 "As intimated above, this work, which is soon to be completed, will form one of the most complete as well as modern treatises upon the Materials used in all sorts of Building Construc- tions. AS a whole it forms a very comprehensive and practical 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 OF CONSTRUCTION. A Text-book for Technical Schools, condensed from Thurston's " Materials of Engi- neering." Treating of Iron and Steel, their ores, manufacture, properties and uses ; the useful metals and their alloys, especially brasses and bronzes, and their "kal- chords " ; strength, ductility, resistance, and elasticity, effects of prolonged and oft- repeated loading, crystallization and granulation ; peculiar metals ; Thurston's "maxi-' mum alloys " ; stone ; timber ; preservative processes, etc., etc. By Prof. Robt. H. Thurston, of Cornell University. Many illustrations Thick 8vo, cloth, 5 00 " Prof. Thurston has rendered a great service to the profession by the publication of this throrough, yet comprehensive, text-book. . . . The book meets a long-felt want, and the well-known reputation of its author is a sufficient guarantee for its accuracy and thorough- ness. "Buildinj. TREATISE ON FRICTION AND LOST WORK IN MACHINERY AND MILL WORK. Containing an explanation of th3 Theory of Friction, and an account of the various Lubricants in general use, with a record of various experiments to deduce the laws of Friction and Lubricated Surfaces, etc. By Prof. Robt. H. Thurston. Copiously illustrated 8vo, cloth, 3 00 " It 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 and High Speed Engines. By Prof. R. H. Thurston. 12mo, cloth, 1 50 "This work must prove to be of great interest to both manufacturers and users of steam- engines." Builder and Wood-worker. CONVERSION TABLES Of the Metric and British or United States WECGHTS AND MEASURES. With an Introduction by Robt. H. Thurston, A.M., C.E 8vo, cloth, 1 00 A MANUAL OF STEAM BOILERS, THEIR DESIGN, CONSTRUC- TION, AND OPERATION. For Technical Schools and Engineers. By Prof. R. H. Thurston. Numerous illus- trations. 686 pages 8vo, cloth, 6 00 STEAM BOILER EXPLOSIONS IN THEORY AND IN PRACTICE. Containing Causes of Preventives Emergencies Low Water Consequences Management Safety Incrustation Experimental Investigations, etc., etc., etc. By R. II. Thurston, M.A., Doc. Eng., Director of Sibley College, Cornell University. With many illustrations 12mo, cloth, 1 50 " 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. *#* Will be Mailed and Prepaid on the receipt of the price. A MANUAL OF STEAM-BOILERS THEIR DESIGN, CONSTRUCTION, AND OPERATION. FOR TECHNICAL SCHOOLS AND ENGINEERS. BY R. H. ^HUsTOtf, M.A:, c Doc. ENG.; Director of Sibley College, Ojrnejl Univtrsity^ ?a$t\ grtsident American Society of Mechanical Engineers ; Author of a " History of the Steam-engine," ' ' Materials of Engineering" etc. , etc. , etc. NEW YORK: JOHN WILEY & SONS, 15 ASTOR PLACE. 1888. / 6^ Copyright, 1888, By R. H. THURSTON. GIFT OF ENGINEERING LIBRARY DRUMMOND & NEU, Electrotypers, 1 to 7 Hague Street. 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 869480 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 u 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 PREFACE. 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. SIBLEY COLLEGE, CORNELL UNIVERSITY, January, 1888. CONTENTS. CHAPTER I. HISTORY OF THE STEAM-BOILER; STRUCTURE; DESIGN. SEC. PAGE 1. Office of the Steam Boiler, r 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, 18 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. The Locomotive Boiler. ......... 26 17. Marine Boilers; older Forms, ........ 29 18. Marine Water-tube Boilers, . 30 19. The Scotch Boiler, .......... 32 20. Sectional Boilers, 35 21. Marine sectional Boilers, ......... 38- 22. Periods of Introduction, ......... 39 23. Special Forms of Boiler. ......... 42 24. Problems in Design and Construction, 43 25. Problems in the Use of Boilers, ........ 43 26. General Methods of Solution 43 CHAPTER II. MATERIALS OF STEAM-BOILERS; STRENGTH AND OTHER CHARACTERISTICS. 27. Quality of Materials required, 45 28. Principles relating to Strength, 45 29. Tenacity, Elasticity. Ductility, Resilience, ...... 56 Vlll CONTENTS. 30. Characteristics of Iron, physical and chemical, 57 31. " " " Steel, . 63 32. Effect of Variation of Form, ........ 64 33. " " Method of Treatment, . ...... .70 34. " " Time and Margin of Stress, ....... 74 35. Method of detecting- Overstrain, ........ 81 36. Effect of Temperature, 83 37. Crystallization and Granulation, ........ go 38. Iron and Steel compared, 92 39. Grades and Qualities of Iron Boiler-plate, ...... 94 40. Manufacture of Iron and Steel plate, . ...... 96 41. Methods of Test of Iron and Steel, ....... 98 42. Results of Tests, ........... 104 43. Specifications of Quality, 108 44. Choice for Various Parts, ......... 112 45. Methods of Working, ...... .... 113 46. Special Precautions in using Steel, ....... 113 47. Rivets and Rivet Iron and Steel, . . . . . . .114 48. Sizes, Forms, and Strength of Rivets, ...... 115 49. Strength of riveted Seams; Helical Seams, . . . . . .117 50. Punched and Drilled Plates, . ... . . . . . 123 51. Steam-riveting and Hand-riveting, ....... 125 52. Welded Seams, ........... 127 53. " Struck-up" or Pressed Shapes, ........ 127 54. Cast and Malleableized Iron, Brass, and Copper, . . . .127 55. Shells of Boilers, 129 56. Flues, Flanged and Corrugated, ........ 140 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 defined ; Perfect Combustion, ..... 152 63. Fuels; Coal defined, .......... 153 64. Anthracite Coals, 155 65. Bituminous Coals, .......... 156 66. Lignites, 158 67. Peat or Turf, 159 68. Wood, 159 69. Coke, ............ 160 70. Charcoal, ............ 162 71. Pulverized Fuel, 164 CONTENTS. ix SEC. PAGE 72. Liquid Fuels, .165 73. Gaseous Fuels, . 167 74. Artificial Fuels, . . 168 75. Heating Power of Fuels, 169 76. Temperature of the Fire, . 172 77. Minimum Air required, . . . . . - . . . . 178 78. Temperature of Products of Combustion, . . . . . .179 79. Rate of Combustion, 184 80. Efficiency of Furnace, ......... 185 81. Economy of Fuel, .......... 187 82. Weather Wastes, .......... 191 83. Composition of Fuels, 192 84. Heating Effects of Fuels, 194 85. Composition of Ash, 200 86. Commercial Value of Fuels, ........ 201 87. Furnace Management, ......... 204 88. Adaptation of Boiler, Furnace, and Fuel, ...... 206 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, 239 108. 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 W T ater ; 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, ......... 278 140. Regnault's Researches and Methods, ...... 280 141. Regnault's Tables, .......... 281 142. Stored Energy in Steam; Tables, 285 143. Curves of Energy, 289 144. Power of Steam ; of Boilers, . . . . . . . . 291 145. Horse-power of Boilers, ......... 292. CONTENTS. XI CHAPTER VII. CONDITIONS CONTROLLING BOILER DESIGN. SEC. PAGE 146. The Problem stated, 3 147. 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 3 1 ? 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, 340 165. Standard Sizes of Tubes; Spacing, 341 166. Details of the Problem, 345 CHAPTER VIII. DESIGNING STEAM BOILERS. 167. General Considerations 34 168. Parts defined ; Common Matters of Detail, 346 169. Designing the Plain Cylinder Boiler, 350 170. Stationary Flue Boilers, . 354 171. Cylinder Tubular Boilers, . . . . . . . . . 358 172. Marine Flue Boilers, .......... 3 01 173. Marine Tubular Boilers, 362 174. Sectional and Water-tube Boilers, 364 175. Upright and Portable Boilers, 369 176. Locomotive Boilers, ........* 37 1 CHAPTER IX. ACCESSORIES ; SETTING ; DESIGN OF CHIMNEYS. 177. Setting Steam Boilers; Suspension, 3?6 178. Covering 380 179. Form and Location of Bridge- wall, .381 Xll CONTENTS. SEC. PAOH 180. Disposition of Flues, . . . . . . . . .381 181. Location and Form of Dampers, ....... 381 182. Steam and Water pipes, 383 183. Safety Valves 385 184. Feed Apparatus ; Heaters, ........ 392 185. Steam Gauges, Fusible Plugs, and minor accessories, . . . 393 CHAPTER X. CONSTRUCTION OF BOILERS. 186. 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, . . . . . . . 4 2 7 202. Specification for Steam Boilers, ........ 427 203. Sample Specifications, .......... 4 2 7 204. Specification of Quality and Tests of Metal, ..... 436 205. Duties of the Inspector, 43 CHAPTER XII. OPERATION AND CARE OF BOILERS. 206. General Management, 44 207. 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. 44^ CONTENTS. Xlll SEC. P AGE 212. Operation of the Boiler, 445 213. Forced Draught 44$ 214. Closed and Open Fire-rooms, ........ 44$ 215. Control of Steam Pressures, 449 216. Regulation of Water-supply, 449 217. Emergencies, ........... 45 218. Low Water, 45<> 219. Priming; Sudden Stopping, ........ 451 220. 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, .......... 45 227. 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, 476 242. Theory of Commercial Efficiency, . . . . . . . 477 243. Efficiency of a Given Plant, . 481 CHAPTER XIV. STEAM-BOILER TRIALS. 244. Purposes of Boiler Trials, ........ 484 245. Test of Value of Fuel, 485 246. Determination of Value of Boiler, ....... 485 247. Evaporative Power of Fuels, ........ 485 248. Analysis of Fuels, .......... 486 XIV CONTENTS. SEC. PAGE 249. Efficiency and Economy of Fuel, . . 487 250. Relative Values of Boilers, . 489 251. Variation of Efficiency with Consumption of Fuel and Size of Grate, 489 252. Relation of Area of Heating Surface to Economy, .... 490 253. Combined Power and Efficiency, ....... 490 254. Apparatus and Methods of Test, ....... 490 255. Standard Test trials, ......... 492 256. Instructions and Rules for Standard Method, ..... 493 257. Precautions; Blanks and Record, ....... 502 258. Results of Test-trials, 504 259. Quality of Steam, . . ... . . . . . .517 260. Form of Barrel Calorimeter and use, ...... 519 261. Theory of Calorimeters, . . . . . . . . .521 262. Records ; Errors, .......... 523 263. The Coil Calorimeter, ......... 524 264. The Continuous Calorimeter, ........ 527 265. Analysis of Gases ; Form of Apparatus, ...... 531 266. Efficiency as indicated by Gas-analysis, . . . . . 535 267. Draught Gauges, .......... 535 CHAPTER XV. STEAM-BOILER EXPLOSIONS. 268. Steam-boiler Explosions, ......... 538 269. Energy stored in Boilers, . . . . . . . . . 541 270. Energy of Steam alone, ......... 548 271. Explosions denned and described ; Fulminating Explosions ; Col- lapsed Flues ; Bursting, . 549 272. Causes of Explosion : Probable ; Possible, and unusual ; improba- ble and absurd, .......... 550 273. Statistics of Explosions and Causes, 553 274. Theories and Methods of Explosion, ...... 558 275. 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, . . . . . . . . 502 285. Defects of Design, .......... 593 286. Defective Construction, 59 6 287. Developed Weakness ; Corrosion, . . . 601 CONTENTS. XV SEC. PAGE 288. General and Local Decay, 604 289. Methods of Corrosion and Decay ; Grooving or Furrowing, , . 606 290. Differences of Temperatures, 609 291. 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, ...,.,.,.... 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 steairu 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 cohesive forces, which measures the "latent heats" of evaporation and of 2 THE STEAM-BOILER. expansion; while ^th& rera^ink%rj ts fthe sensible heat of the steam. Thus tlie^fluid sit or ed in the^steam-boiler is a reservoir of energy which \i *itJa,vvT^iipGFi by^the^ ^team-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 FIG. i. THE GRECIA.N IDEA OF THE STEAM-ENGINE. o f operation included the appli- cation of the condensation of steam to the production of a History of the Steam-engine. R. H. Thurston. 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. FIG. 4. WORCES- TER'S ENGINE, A.D. 1650. 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'l after the steam has condenseol^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- 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 - 5- SAVERY'S ENGINE, A.D. 1699. it is alternately let into the vessels P P. 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. 6. NEWCOMERS 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, Fig. 7, the "wagon-boiler," as he called it, the FIG. 7. WATT'S FIRST 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. 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 1/93, Barlow invented, and FIG. 9. WATER-TUBE BOILER OF FULTON AND BARLOW, 1793. FIG. 10. STEVENS'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 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. STEPHENSON'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. 1 1) 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. 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, FlG - H--THE ROCKET, 1829. 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 Seguin 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. 2 or rather more than one fourth of the 320 -\- 461 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- =0.14 4; 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 i^ 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. I/ 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 = 0.15, 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 they 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 power. 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. Stationary { Mu l tin 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 are met with in all departments. Some of these are considerably employed, and in many cases possess special features of advantage. The FIG. 41. THE GALLOWAY BOILER. Galloway boiler (Figs. 41, 42) is one of the best known and suc- cessful modifications of the cylindrical flue-boiler. Its special feature is the conical stay-tube, which is used to increase the heating-surface and to strengthen the flue, without making the heating-surface difficult of access. Large numbers of these boilers have been built and used since about 1860 in Great Britain, and some have been constructed in the United States. * Trans. British Institution of Naval Architects, 1877. 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 boiler. In this flue are the conical water-tubes, each 10^ inches di- ameter at the top and 5^ 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 FIG. 43. UPRIGHT FLUE-BOILER. 10 % 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 BOTTOM BLOW 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 the gases pass up through the boiler in a set of fire-tubes seen con- necting the crown-sheet with the top of the boiler. This makes 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 if- to 2f inches in diameter outside, and T 3 ^ 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. 43 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 b'e 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 tjoiler 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 exposed. 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: j Tensile : resisting pulling force. ina ' ' ' ( Compression : resisting crushing force. r 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. 4/ "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 5 4 8 7 10 + 10 to 15 Ratio of ultimate strength to working load. The brittle metals and alloys The Proof Strength 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. 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, (i) Compression is similarly measured, and if C 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, Pl=M. ........ (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- 5, //'. The new diagram shows an elastic limit at e v \ and very much higher than the original limit e lv . 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 introduction 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: WV* (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, therefore, nearly two thirds the maximum, and />* - I pdx = p m s = \et = *Y nearly, . . (8) " 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 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 r 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 2,E 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 e^f'f", until it settles at such a point on that line as- suming the shock to have extended the piece to the point e 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 56 THE STEAM-BOILER. 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*. Taking the origin at the foot of /"/" since tne 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/t=Ws', and W=P\. . . (11) when the resisting force is /, the elongations x, while h and s are maximum fall and elongation, and P is the maximum resistance to the load at rest. Then r* f s a 2 2 ^ f x / pdx a I xdx = -s? = Ws; :. s = - . . (12) J * Jo 2 a For a static load, if s' is the elongation, W W = P = as'-, .'. s' = . Hence, 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. $? 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 securjed, 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 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, Lbs.~peiSq.lnch. 120.000 Kil's;perS.q.Cnn. 843ft Elongation per Cent FIG -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' /, 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 t g, and //, 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 60 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. 6 1 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, 7^, 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-place 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," 41 soft," or " mild " steel, and is, properly speaking, " ingot iron ;" all erf 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 duc- 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 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 secon.d 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 FIG. 50. MATERIALS STRENGTH OF THE STRUCTURE. 6/ 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 -J 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 ' = 18 + 7, ....... (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 n,457 Lowest ............. 136,490 9,595 Average ........... I53>6?7 10,803 Long cylinder ---- Highest ............ 123,165 8,658 Lowest ............. 103,255 7-259 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. rt , STRESS WHEN DIAME- g PIECE BEGAN BREAKING- "* o TER. C TO STRETCH STRESS. 5 uS OBSERVABLY. "o < rt < *1 Remarks. ji s Orig- inal. Final. 'er cent tion. g c deduced. 'ercent < tion of Ob- served Stress. Stress per square inch. Ob- served Stress. Stress per square inch. ^ PM o H M 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 3-500 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 57 1 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 59 1 45- 14,000 28,000 26,500 53.169 8 i .500 2.026 35-0 797 590 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 57,3^8 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 STKUCTUKE. 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 .16 T0201 FIG. 52. SHAPES FOR TEST-PIECES. tool steels (0.798 inch ; 2 centimetres diameter when round), and one-eighth or one-quarter square inch (0.8 1 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, ^ inch diameter (1.75 centimetres), or f square inch (2.44 square centimetres) area ; in other metals either inch (1.9 centimetres) 7O 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 \ 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 ta 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 i, 406 log d m . } Where T and T m measure the tenacity in British and metric MATERIALS STRENGTH OF THE STRUCTURE. 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 : _ y : 60,000 80,000 The Edgemoor Iron Company adopt, for wrought-iron in tension, the formula 7,000^4 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 f 56,000 3.947 2.54 I 55,500 3,902 3-18 Ii 54,500 3,838 3.8i Ii 53,500 3,761 4-45 If 52.000 3,656 5-o8 2 50,000 3.515 5-72 2i 49,OOO 3-445 6.35 2i 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. 7O 5 44,000 3.093 Kirkaldyf found that pieces of 1 1-inch (3.2 centimetres) * Ohio Railway Report, 1881, p. 379. f Experiments on Wrought Iron and Steel. 72 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 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 Seen pre- viously removed may be expressed approximately by the formula E' = $ 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 inch (o.oi 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. QO 35 ^ays. 5 minutes. 85 Unbroken at end of 16 mos. i day. 80 91 days. 266 days. 75 70 ! Unbroken. j 17 days. 455 days. 65 ( 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-BOJLER. 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 15 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. ?? 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 P r f* 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, Spaiigenberg, was of four kinds : I. To produce rupture by repeated load. * Transactions Institute Mining Engineers, 1880. MATERIALS STRENGTH OF THE STRUCTURE. ?$ 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 tcr 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 -f- 18,700 to -f- 30; -j- 1,309 to -j- 2.2 Wrought-iron, tension and compres. -|- 8.320 to 8,320; -(- 582 to 582 Cast-steel, tension only -j- 34.307 to -j- 11,440; -f- 2,401 to -f- 801 Cast-steel, tension and compression -j- 12,480 to 12,480; -f- 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 " 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., 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. /-. P 012 / *\ / j <*Q A , 9 r i 100 .^t 4^b -s^r 14 / *"-. . \ < - --*. ~~Z^ iC \ V <\ \ \ v \\ ! \ \\ \ \ \v \ S3 ^ v ""< \V V ^- 1 , . "^""4^- J 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. 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 MA TERIA L S S TRENG TH frF Tli'E J S TRUCTVRE: J I O I 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-BOILED. 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 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. 1 03 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 1 04 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 per cent ; 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 2,000 4.OOO 6,OOO 8,000 10,000 150 11.000 12,000 150 13,000 13,500 14,000 150 15,000 ~ 150 17,000 150 19,000 150 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 Q.< 9- 79!3 .7910 .7922 7930 .7946 .7948 .7914 7951 7953 .7915 .7967 7959 .8424 8351 .8712 .8632 .9618 .9518 .0856 .0732 .2811 .2663 .4381 .4212 5441 .6670 .8693 \1 54 4.000 8,000 12,000 16.000 20,000 22.000 24,000 26,000 27,000 28,000 0013 .0023 0035 .0050 .0058 .0064 .0070 .0080 .0087 0577 [0867 .1790 .3032 .5004 '.6586 .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 .OOOI .0003 .0486 .0763 !i666 .2391 4337 .6401 .001 .004 .608 .960 2.083 3-613 6.043 8.001 30 ooo 34.000 38,000 42,000 44,000 45,000 46,000 47,000 47.500 43,600 7752 .8884 1.0913 1.4700 1.5400 Lbs. 13,500 ORIGINAL Lbs. per sq. in. 47,500 ELASTIC LIMIT. ACTUAL. K Lbs. per Kgs. per sq. in. sq. cm. 6,140 27,000 1,898 BREAKING LOAD. SECT. FRACTURED SECT. Kgs. per Lbs. per Kgs. per sq. cm. sq. in. sq. cm. 3,340 69,840 4,910 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, 3 INCHES LONG (BETWEEN SHOULDERS) AND 0.622 INCH DIAMETER, TAKEN FROM BREECH-RECEIVER FOR n-INCH BREECH- LOADING RIFLE. Weight per s quare inch of Section. Extension per inch in Length. Successive Extension per inch in Length. Permanent Set per inch in Length. Successive Permanent Set per inch in Length. Pounds. Inches. Inches. Inches. Inches. Inches. Inches. 1,000 0.00000 o . ooooo o . ooooo o . ooooo 0.00000 o. ooooo 2,000 .00000 . ooooo . ooooo . ooooo .ooooo .ooooo 3,000 .00000 . ooooo . ooooo . ooooo .00000 .ooooo 4,000 .00033 .00033 .00033 .00033 .ooooo .ooooo 5.000 .00033 .ooooo .00033 .ooooo .00000 .ooooo 6,000 .00033 .00000 .00033 .ooooo .ooooo .00000- 7,000 .00033 .ooooo .00033 , .ooooo .00000 .00000 8,000 .00033 .00000 .00033 .ooooo .ooooo .00000 9,000 .00033 .ooooo .00033 .ooooo .00000 .ooooa 10,000 .00033 .ooooo .00033 .ooooo .ooooo .ooooo 11,000 .00033 .ooooo .00033 .ooooo .00000 .00000 12,000 .00033 .00000 .00033 .ooooo .ooooo .00000 13,000 .00033 .ooooo .00033 .ooooo .00000 .ooooo 14,000 .00033 .00000 . 00033 ooooo .00000 .ooooo 15,000 .00033 .ooooo .00033 1 .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 .00000 .ooooo 19,000 .00133 .00066 .ooioo -00033 .00033 .00033 20,000 .00233 .00100 .00100 .ooooo .00133 .OOIOO- 2I.OOO .00300 .00067 .00100 .00000 .00200 .00067 22.000 .00400 .00100 .OOIOO .OOOOO .00300 .00100 23,OOO .00467 .00067 .00100 .00000 .00365 .00067 24,000 00533 .00066 .OOIOO .ooooo 00433 .00066 25,000 .00633 .00100 .00133 .00033 .00500 .00067 26,000 .00700 .00067 .00133 .ooooo .00567 .00067- 27.000 .00767 .00067 .00133 .00000 .00633 .00066 28,000 .00900 .00133 .OOIOO .00033 .00800 .00167 29,OOO .00967 .00067 .00100 .00000 .00867 .00067 30,000 .01067 .00100 .00133 00033 .00933 .00066 31,000 .01200 .00133 00133 .00000 .01067 .00134 32,000 .01300 .00100 .00167 .00034 .01133 .00066 33,000 01433 .00133 .00167 .00000 .01267 .00134 34.000 .01567 00134 .00133 .00034 01433 .00166 35,000 .01700 .00133 .00133 .ooooo .01567 .00134 36,010 .OlSoO .00100 00133 .00000 .01667 .00100 37,000 .01967 .00167 .00133 .00000 01833 .00166 38,000 02133 .00166 .00167 .00034 .01967 .00134 39,000 02433 .00300 .00167 .ooooo .02267 .00300 40,000 02567 .00134 .00167 .ooooo .02400 .00133 41,000 02733 .00166 .00167 .ooooo 02567 .00167 42,000 .02867 .00134 .00167 .00000 .02700 .00133. 43,000 44.000 03033 .03300 .00166 .00267 .00200 .00233 .00033 .00033 02833 .03067 .00133. .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 .00200 49,000 .04700 .00333 .00267 .00034 04433 .00300 50,000 .05100 .00400 .00200 .00067 .04900 .00467 51,000 05533 00433 .00300 .00100 05233 .00333. 52,000 . .06067 00534 .00233 .00067 05833 . 00600 53,000 .06667 .00600 .00300 .00067 .06367 00534 54.000 .06897 .00200 .00233 .00067 .06633 .00266 55,ooo .07867 .01000 .00300 .00067 07567 .00934 56.000 08333 .00466 .001500 .oocoo .08033 .00466 57.000 .09500 .01167 .00300 .ooooo .09200 .01167 58,000 .10233 00733 .00333 .00033 .09900 .00700 59,000 .IISOO .01567 .00333 .00000 .11467 .01567 60,000 .13700 .01900 .00367 .00034 13333 .01866 61,000 62.000 .16900 0.30367 .03200 0.13467 0.00400 1 (*) 0.00033 (*) 0.16500 (*) 0.03.67 Tensile Strength per sq. in Ibs. 62,000 * Specimen broke. GENERAL SUMMARY. Elastic limit Ibs. 19,000 Extension per in. at elastic limit in. 0.00133 Extension per in. at rupture in. 0.30367 Original area of cross-section., .sq. in. 0.3038 Area after rupture sq. in. 0.1611 Position of rupture ^ from shoulder. Character of fracture Fibrous. MATERIALS STRENGTH OF THE STRUCTURE. IO/ times secure iron, if thin, capable of sustaining 60,000 pounds per square inch (4218 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 itseU, 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 flat. 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 55,000 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 0.8 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. 1 09 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. ii 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 110 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, MA TERIAL S 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. EXTENSION. Lbs. per sq. in. Kilos, per sq. cm. Per cent. 6o,OOO 4218 25 7O,OOO 4921 23 8o,OOO 5624 19 9O,OOO 6327 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. 11$ 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 U4 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 f-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 headed up, should not cause weakening by the stress incident to the strain so produced. A good iron rivet f inch (1.6 cm.) diameter can be doubled up and hammered together, cold, without exhibiting * Trans. Am. Soc. M. E., 1887, No. ccxlvi. 1889] THE LOCOMOTIVE. PRINTING HOUSE (68). A boiler exploded in a printing and publishing house in Cincinnati, on May 23d. Further particulars of this explosion are given in our Cincinnati letter in this issue. STEAM SHOVEL (69). On May 25th, -while three men were engaged at work in the engine room of the steam shovel which is being used for loading ballast on the cars at Wales, Out., for grading purposes on the Grand Trunk Railway double track, the boiler exploded, seriously injuring two of the men, one of whom, Mclntosh, is expected to die. PORTABLE SAW-MILL (70). The boiler of Gilfilen & Gibson's portable saw-mill, situ- ated on Poindexter's creek, near Winfield, W. Va., blew up on the morning of May 31st, with terrific force. William Doss was instantly killed ; George Gilfilen died two days later from a crushed skull, broken collar bone, and other injuries ; Burns Wooten was badly bruised and scalded, and cannot recover ; David Chambers, the sawyer, received several injuries about the body and head, none of which are serious. Several of John Gibson's bones were broken, and he was otherwise seriously but not dangerously hurt. One of John Niberl's eyes was blown out, and he was badly burned about the body. The lumber inspector of the McLaughlin Timber Company, of Columbus, O , happened to be on the grounds, and was seriously hurt. Engineer Wolford, who was slightly injured, fled to the woods after the accident, and Has not been seen since. About five feet of that portion of the boiler to which the fire-box is attached was blown about one hundred feet in the air, coming down about one hundred yards from the mill, imbedding itself into the ground three feet. The mill, which is estimated to have been worth two thousand dollars, is a total wreck. On the Longitudinal Riveted Joints of Steam-Boiler Shells.* BY JOHN H. COOPER, PHILADELPHIA, PA. The initial statement to the English Lloyd's rules for steam-boilers is embodied in the following words : " The strength of circular shells to be calculated from the strength of longitudinal joints," which assures us that this part of the boiler should be properly pro- portioned. To these rules a memorandum is added : " In any case where the strength of the longitudinal joint is satisfactorily shown by experiment to be greater than given by this formula (Lloyd's), the actual strength may be taken in the calculation." Later on, Lloyd's rules (under the head of " Periodical Surveys," regarding the examination of boilers after they have been several years in service) say : "The safe working pressure is to be deter- mined by their actual condition." These statements lie in the line of practical efficiency, and point to the necessity of providing material in accordance with the requirement of the load to be carried. Any one who takes the trouble to collect and compare data on this subject cannot fail to notice the great disparity of rules for determining the working pressure permissible for boilers. The case is clear by simple reasoning on the data collected, that boilers are held together, it would seem, more by conformity to rule than by the materials of which they are made. But of course the true course to pursue is to give to each member its proper allowance of section, in order that the components of the joint shall have an equal chance under strain according to its resisting power. The diminished strength of the shell of a boiler by a longitudinal joint is well known, and it becomes good engineering so to propor- tion its parts as to obtain the greatest strength possible within the limits of practical economy. When it became necessary to assure themselves confidently of the permanent safety of a structure composed of plates held together by rivets, engineers were not long in finding out that a certain allotment of rivet section to plate section at the joints was necessary, and that these sections were found to be nearly equal in the strongest joints. The experiments * Communicated to the LOCOMOTIVE by the author. 102 THE LOCOMOTIVE. [JULY, of Fairbairn, conducted in the year 1838, proved that " the sectional area of the rivets in a joint was nearly equal to the sectional area of the plate through the rivet holes." Subse- quent experiments by Clark on riveted plates for the Britannia and Conway Tubular Bridge fully corroborate the above statement; his conclusion was: "The collective area of the rivets is equal to the sectional area of the plate through the rivet-holes." This relation of the components of the joint in course of time became embodied in the English Board of Trade rules and in Lloyd's rules now in force, regulating the construction of steam-boilers. It also forms the basis of the Philadelphia steam-boiler inspection ordinance, first formu- lated in 1882. Referring now to those rules only which relate to the proportions of the longitudinal joints of the cylindrical shells of boilers, we are prepared to say they may be most con- veniently presented b}' the following notation and formulae : A = Percentage 'of punched plate to the solid plate. B = Percentage of driven rivet section to the solid plate. C ' The pressure in Ibs. per square inch which the boiler is allowed to carry. a = Area of driven rivet, or rivet -hole. d = Diameter of rivet-hole. n = Number of rows of rivets. p Pitch of rivets. t = Thickness of plates. R- Radius of boiler shell. 8 Ultimate shearing strength of rivets in Ibs. per square inch of section. T Ultimate tensile strength of plates in Ibs. per square inch of section. / = Factor of safety. E Limit of elasticity in the plates in Ibs. per square inch. % = Percentage of joint strength. The least of A or B should be inserted in the formula for finding C. All dimensions are in inches. The notation and the formulae mutually explain each other. p d A = - ..... (1), P a n B = - ..... (2), pt t (A or B) T These formulae are intended exclusively for the guidance of the inspector in ascertain- ing the exact strength of the joints in the boilers which come under his care, and which enable him to determine the working pressure of steam allowable under the rules. They do not, however, enable the boiler-maker to determine directly that proportion of pitch which he should use with any given plate thickness and rivet diameter, in order to secure the strongest joint and which will also pass the highest inspection. To secure these results, the following simple formulae were devised by the writer (early in 1882), in which the notation given above is similarly employed, and which may be thus expressed. For single riveted joints, when iron plates are secured by iron rivets and when the plate thick- ness and rivet diameter are given, it is desired to find a pitch that will secure equality of plate and rivet section the formulae will be : p = + d. . . . . (4). This plainly means that the pitch is equal to the area of the rivet hole, divided by the 1889.] THE LOCOMOTIVE. 103 thickness of the plate, and to the result of which the diameter of the rivet hole must be added. For multiple riveted joints, when iron plates are secured by iron rivets, the same formula is used with the addition only of n, representing the number of rows of rivets, thus: n a p = +d . . . . (5). The different resisting power of equal areas of section, as many times found by tests of the shearing stress of the rivets and the tensile stress of the plates, is not taken into account in the make-up of these rules. They are treated in all cases as equals under the strains of continued use. That is to say The Philadelphia Boiler Ordinance and the English rules alike impliedly declare : The shearing strength of the rivets is just equal to the tensional strength of the plates per square inch of area in boilers made of iron plates and iron rivets. If any one takes exception to this treatment of the two Strains, the formulae permit him to introduce his own figures of difference into their make-up, by which he can get a result in accordance with his own belief ; but of the mathematical base embodied in the formulae, we are sure. For single and multiple riveted joints when steel plates are secured by iron or steel rivets, the relative resistance of the plates to tension and of the rivets to shear must be in- serted in the formula. First, let us assume, as the rules for inspection have done and do in all cases, that, area for area subjected to stress and acting together, iron plates and iron rivets are equal in resistance. The best Staffordshire iron boiler plates will stand 48,000 Ibs. tension per square inch of section ; but the Board of Trade and Lloyd's limit all best iron plates and rivets alike to 47,000 Ibs. The Philadelphia Ordinance will pass iron plates which have shown on test a tension of 50,000 Ibs. per square inch, but will allow no more whatever the plates may show, and will give full credit to a joint in which the driven rivets have equal section to the punched plates. And yet we well know it to be a matter of fact that the shearing strength is less than the tensile strength of the same material. Mr. William H. Shock's experiments on American iron gave as a mean for single shear 41.033 Ibs. per square inch, and 78,030 Ibs. for double shear, these experiments being made upon iron bolts in a shearing device which did not include the uncertain element of friction by the rough surfaces of the plates when bound closely by the rivets of a riveted joint made in the usual way. When iron rivets are used w T ith steel plates they are accepted under the rules for just what they are worth under shear and no more. The English rules say: " Iron rivets in steel boilers should have a section of -\ 3 - of the plate section." Steel rivets must be calcu- lated from their actual strength to resist shearing ; and for these the fraction |f will express the larger area they must have to the plates with which they are used to make joints, sim- ply because steel plates show an ultimate tensile strength of 28 tons, and steel rivets an ultimate shearing strength of 23 tons per square inch of section. The old rules published by Fairbairn, and used by him and by many boiler-makers since, are obsolete now, in the liirht of the later method of proportioning joints and the laws which sanction their use, although lie furnished the first material for the base upon which this law has been built. From an extended list of all iron single joints, proportioned on the principle of equality of sectional areas, the percentage of joint strength to the solid plate will reach to .64 and in double joints to .78 and be practically tight under pressures up to say 100 Ibs. of steam per square inch a material increase over the oft-quoted figures of .56 and .70 originated and recommended by Fairbairn. If we accept the inspection laws referred to, assuming even results of the two strains, then rules 4 and 5 will find the proper pitches for boiler joints made of iron plates and iron rivets ; but in composite boiler shells, the introduction of symbols representing the actual powers of resistance of the components, will be necessary : we will then have for double or multiple joints ; 104 THE LOCOMOTIVE. [JULY, n a S p = ._. + d . . . . (6), t T which can be applied also to an all-iron joint or to joints made of other materials than the usual iron and steel. If we desire to tind the pitch of the rivets, when the rivet diameter and a certain percentage of joint strength are given, we may use the following formula : . . . . (100-*) This do^s not include the thickness of the plates; it relates only to the proportion existing between the distance from center to center of the rivet holes and the space between the holes. Other convenient formulae are readily obtained from A, B, and C, by transposition ; as, for instance, if it is desired to know the shear to which the rivets are exposed in any par- ticular case after all the elements have been obtained the formula will take this shape : Cx Rxf Shear = - --, . . . (8), tX B and will give the Ibs. per square inch of cross-section to which the rivets are subjected in the seam by the steam pressure C, which has been obtained by the Ordinance formula. The rivet hole determines the size and measure of the rivet after it is driven, because it is then filled by it ; and in making calculations with the aid of these formulae, the trade sizes of the rivets must not be taken. In punching holes for rivets in boiler plates, it is the usual practice to use punches y 1 ^ of an inch greater in diameter than the trade diameter of the rivets, and it is also usual to make the dies which are used with the punches ^ of an inch larger in diameter than the punches to be used with them. The result of this method is to make conical holes in the plates, corresponding to the sizes of punch and die. If the punched holes are net to the dimensions of the 'punch and die here given, and if the material of the plate immediately around the hole has not suffered in the act of punching, then the proper size of holes to be used in the formula would be the mean diameter of the conical holes so made, instead of T y larger than the punch, as they are usually assumed to be. It is well known, however, that the material of the plates bordering the holes is weakened by the detrusion of the punch ; to what distance this reaches from the surface of visible separation of the metal may not be definitely known, and must necessarily be differ- ent with different materials and punches but it is certain to be a small measurable dis- tance into the plate around the hole. If we take the diameter of the punched holes to be equal to that of the die, we will not be far from the actual state of the case, especially as some of this disturbed metal is removed by the reamer or crushed by the drift-pin. We are safe in this assumption in so far as the ultimate strength of the joint is concerned, 'because, as usually happens in rupture, the plates give way, while the rivets rarely fail ; and again, the plates suffer loss of substance by wear and waste, while the rivets are preserved against deterioration, and therefore the initial strength of the plates ought to be favored. In view of these facts, the suggestion is here made that when we wish to determine pitches from given plates and rivets, that we use the greater diameter of the punched hole, whatever that may be, for the quantity expressed by a in all of these formulas, and that we assume the rivet diameter to be that of the lesser diameter, or reamed-out diameter of the rivet-hole. The result of this apportionment of the material will be effectively to strengthen the plates, which all experience has proven to be necessary ; so that while this. decision appears to be against reason and the isolated facts of experiment the resistance to shearing always proving, less than that to direct tension in the same material it must be constantly borne in mind that the strain on the plates and rivets are not direct in the ordinary lap-joint as they are used in a boiler, the plates being subjected to some transverse 1889.] THE LOCOMOTIVE. 105 strain while under tension, and the rivets to some tensile strain while under shear. Strictly speaking, the plate loses what is punched out of it, together with the metal de- stroyed around the punched hole, and the rivet gains by whatever increased diameter it gets in the process of riveting. They should be estimated upon what they actually are when the joint is made up. 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 2.25 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 d 1.2 V~t, 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. 1 Diameter, .......... . 2.0 When n = o the joint is parallel with the axis of the cylin- der; it becomes a longitudinal seam. When n =. oo , it becomes a girth-seam of twice the relative strength. When the angle of "rake" is 30, the gain is 10 per cent; when 45, the gain be- comes 0.4. It is obvious that this form of seam is very waste- ful of metal, if so much inclined as to secure any considerable gain of strength, if the boiler or the flue is built of a succession of ring courses laid side by side ; in such constructions as Root's " spiral pipe," in which the courses are helical, this objection does not hold. The " factor of safety," as stated where reference is made to the strength of steam-boilers, is usually misleading, as, for ex- ample, in the U. S. regulations. Pressures one sixth those computed from the reports of tests of strength of the plate are permitted ; but the real factor of safety is obtained by multi- plying this nominal factor by the coefficient of strength of seam. Thus, where the law r allows six the real factor is 0.56 X 6 3.36 MATERIALS STRENGTH OF THE STRUCTURE. 123 for the single-riveted seam, or 0.7 X 6 = 4.2 for double-riveting, Fairbairn's coefficients being accepted. The real factor should not be less than six, and some authorities, following Rankine, would make it eight, and others even ten. 51. Punched and Drilled Plates usually differ in strength, but each may be either stronger or weaker than unperforated metal of equal area of fractured section. When the metal is very soft and ductile, the operation of punching does no appre- ciable injury, and the Author has sometimes found it actually productive of increased strength, the flow of particles from the hole into the surrounding parts causing stiffening and strength- ening. With most steel and with hard iron the effect of punching is often to produce serious weakening and a tendency to crack, which has in some cases resulted seriously. With metal of the first class, punching is perfectly allowable ; with iron or steel of the second class, drilling should always be prac- tised. It is customary, in the practice of the most reputable engineers and builders, to drill all steel plates, but usually to punch iron. Sometimes the steel plate is punched with a punch of smaller diameter than the proposed rivet, and is sub- sequently reamed out or counterbored to size. It is generally assumed that this method is perfectly safe. Messrs. Greig and Eyth, after a long and carefully con- ducted investigation, say:* " The experiments show that the plates invariably lose part of their tensile strength in the section of solid material left between the rivets of a seam, this loss being greatest in lap- joints. It is also greater in punched than in drilled plates (iron as well as steel), and greater in plates riveted together by steam, than in those riveted by hydraulic pressure. On the other hand, the strength of rivets against shearing is greater than its normal figure, especially in lap-joints. " The usefulness of double-riveting appears to be mainly due to the fact that it more effectually prevents lap-jointed plates from bending under stress. At the same time the zig- zag riveting generally adopted, in double-riveting, increases * Lond. Engineering, June 29, 1879. 124 THE STEAM-BOILER. the tensile resistance of the material between the rivets con- siderably beyond its normal figure. " Butt-joints, with a cover on one side of the plate only, gave no advantage at all, the cover behaving simply as an intermediate plate attached to the two main pieces by an ordi- nary lap-joint. A marked improvement could, no doubt, be obtained by giving the cover greater thickness, so as to prevent its bending. "The most effective seams, as to tensile strength, were butt-joints with two covers, as not only do they nearly double the shearing strength of each rivet, but they entirely prevent the bending of the main plates. The main fact resulting from the tests of parts of boilers and complete boilers under hy- draulic pressure was the impossibility of bursting an ordinary rivet-seam in this way, the compression of the rivet and the elongation of the rivet-hole resulting invariably in leakage, which prevented the necessary pressure from being obtained. Each rivet becomes its own safety-valve, and the strain put on the weakest part of the structure never reached more than 70 per cent of the breaking strain. This is the point where addi- tional hardness of the material would be most useful, as it would prevent the opening of the rivet-holes, which now makes a boiler useless long before the breaking strain is reached." * Good steel is much more enduring than any iron, both against ordinary wear and extraordinary strain. The results of experiments on the best British steel for ship-building and for boilers, as reported to Lloyds, show that the injury done by punching is less as the plates are thinner, amounting, in the cases reported, to less than 10 per cent in sheets J inch (0.6 cm.) thick, and rapidly increasing, becoming 20 per cent at f inch (i cm.), and is still more serious with the heavy plate used for large ships and for boilers. But the injury was discovered to be local, and confined to a shell lining the punched hole, and but about one eighth of an inch (0.3 cm.) in thickness. This can be readily cut out, and the punched and counterbored, or reamed, holes produce no observable weak- * Lond. Engineering, 1879. MATERIALS STRENGTH OF THE STRUCTURE. 125 ness. In many instances no special precautions are taken in this direction where the metal is less than one half inch (1.25 cm.) in thickness. 52. Hand-riveting and Steam-riveting are both prac- tised by good makers, and authorities are somewhat divided in opinion as to their relative merit. With either system, good work may be done by a good workman ; by either method, dangerously defective boilers may be produced. With a prop- erly designed riveting machine of the right size for its work, and carefully manipulated, very perfect work may be done. Careless handling produces distorted rivets, eccentrically placed heads, and sometimes causes the formation of a " fin" on the rivet, which, entering between the sheets to be riveted to- gether, holds them apart and causes leakage along the seam. When the plates are well adjusted, in metallic contact, and per- fectly secured, before the rivet is " headed up," this last defect is not likely to appear. The careful adjustment of the rivet- head to the die which supports it against the blow of the ma- chine, and the exact alignment of rivet and striking die, will prevent distortion of the rivet by the blow. Sometimes the machine is too light for its work ; in such cases two blows may be necessary to completely form the head and to expand the body of the rivet sufficiently to fill the rivet-hole. In hand-riveting the action of the hammer often hardens the metal in the head, and gives it such rigidity and brittleness that it may even fly off at the last stroke of the riveting ham- mer. The cone-shaped head is a comparatively weak form, and it is better to use a cup-shaped die, or former, and a larger hammer striking fewer and heavier blows, to form a hemi- spherical head, which latter is much stronger, and neater in appearance. Work of this kind may be quite as good as the best machine-riveting, but it is usually not invariably more costly. Riveting machines constructed with two dies moved inde- pendently the one a hollow die, having for its office the closing up of the lap simply; the other a solid die, which immediately follows up the first and sets up the rivet are probably much better than the more common form of riveter having one die 126 THE STEAM-BOILER, only. Messrs. Greig and Eyth found the following to be the pressures attained on the heads of f-inch steel rivets :* Lbs. Steam- riveter 82,380 Hydraulic stationary 86,360 Hydraulic portable 44,018 Power light blow 69,384 Power heavy blow 115,640 The best work was done by the steam-riveter. They conclude that " The well-known fact of the superiority of riveting by machinery over hand-riveting has been again demonstrated most conclusively, while the experiments have shown that the effects of steam-riveting is, to say the least of it, not inferior to hydraulic riveting as far as the quality of the rivet is con- cerned, but that the hydraulic riveting is distinctly superior as to its effects on the plate, which is less injured by the slow pressure of the hydraulic ram. " Steel showed in this respect a decided superiority over iron beyond the proportion due to its greater tensile and shear- ing strength." The conclusions of Mr. J. M. Allen are that machine-rivet- ing probably results in a greater proportion of defective rivets than any other one cause. Machine-riveting to make good work must be very carefully done. The rivet-hole must be truly in line with the machine dies. The holes in the two plates must also be in line with each other. If there is an offset between them, the rivet is sure to be a very bad one. The most satisfactory riveting of boiler-plates is done by a prop- erly constructed and used button-set. By this means better and more rapid work can be done than by hand-riveting. A well-constructed machine will work quicker than the set, but we have rarely seen a complete job of machine-riveting which left nothing to be desired. It was not the fault of the machine, however. In hand-riveting the excellence of the joint depends * Lond. Engineering, June 29, 1879. MATERIALS STRENGTH OF THE STRUCTURE. upon the form of the set. With an improper set it is impos- sible to do good work, no matter how skilful the workmen may be. 53. Welded Seams are considered better than riveted, where facilities for welding are provided such that the weld may be made with certainty and invariably perfect. Unless special and very complete arrangements are made for securing absolute metallic contact, a good welding heat without oxida- tion, and thorough union by pressure or impact, welds are very apt to prove exceedingly unreliable. A gas-furnace, with a de- oxidizing flame of large volume and covering a considerable length of seam, has done good work, and some makers are adopting this system to the exclusion of riveting. Large boilers are sometimes made without the use of a single rivet in any important line of junction. It seems possible, and even prob- able, that welding may in time displace riveting in all good boiler construction. 54. " Struck-up" or Pressed Shapes are adopted, in pref- erence to riveted or even welded parts, wherever the form and size of the piece will admit. Dome-tops, manhole and hand- hole plates, and sometimes large tube or flue sheets, are thus made. The piece is made by compressing the sheet of which it is to be constructed between a pair of dies, and thus compel- ling it to take the shape of the intermediate space, which is that of the finished piece. The pressure is commonly applied by means of the hydraulic press. Small pieces are shaped in the drop-press, or drop-hammer, in which the dies are forced to- gether by the blow of a heavy " tup," or hammer, falling from a height of from two to six feet or more, according to the size and the intricacy of form of the part to be produced. 55. Cast and Malleableized Iron, Brass, and Copper all have limited application in steam-boilers. Cast-iron is used in the construction of manhole plates, of some of the fittings, and even, in many instances, in the heads of plain cylindrical and flue boilers. Its use is, however, always to be deprecated where wrought-iron can be substituted. When it is adopted, in places in which it may be subjected to heavy loading, and where its failure may prove a serious matter, 128 THE STEAM-BOILER. great care should be taken to secure the best possible quality. It would be advisable, probably, in such cases, to use " gun- iron," as it is called, which is cast-iron of the best grades, melted in an " air-furnace" a reverberatory furnace and refined by " poling," or stirring with a pole, usually a birch sapling, until its quality and composition are satisfactory. No contact being allowed with the fuel or any flux or other source of contami- nation by phosphorus or other objectionable element, greater strength and toughness can be obtained than when the melting is done in a " cupola" furnace, in which the iron, fuel, and any flux that may be used are mixed together. The process is ex pensive ; but the product is correspondingly valuable, the tena- city of good gun-iron exceeding, often, 30,000 pounds per square inch (2109 kgs. per sq. cm.), and its elasticity and elastic resili- ence approximating similar properties in wrought-iron. Malleableized cast-iron is usually given application in small castings forming parts of the various attachments to boilers. It is made by selecting a free-flowing cast-iron, as light in grade as possible, making the castings in the usual way and then sub- jecting them to a process of prolonged annealing at a red heat in the presence of substances capable of abstracting the carbon, such as iron-ore, blacksmith's scale, or other materials rich in oxygen. The abstraction of the carbon thus leaves the casting stronger, somewhat ductile and malleable, and, as a rule, a much safer material than when in its original state ; it has become a crude wrought-iron. Only small pieces can be successfully made in this manner, except by annealing for days, or even several weeks ; the larger the casting the longer the time de- manded. Some so-called " steel-castings" are thus made. Brass and bronze are used mainly in the encasing of pres- sure-gauges, water-gauges, and similar appurtenances, in the construction of gauge and other cocks, and in valves and their seats ; it is less liable to be cut away by steam, or by water, and hence brass valves keep tight longer than do iron valves or cocks. Bronze is better than brass, but its higher cost precludes its general use. Muntz metal, which consists of copper 60, zinc 40, and gun-bronze, 90 copper and 10 tin, are the most generally useful compositions ; but the brasses in common use generally MATERIALS STRENGTH OF THE STRUCTURE. 1 29 contain more or less of lead, and the bronzes are often also similarly adulterated. For surfaces exposed to friction the addition of lead is thought by many to be an advantage. The strongest of all such alloys is that consisting of copper 43, zinc 55, and tin 2, or one having a somewhat less proportion of tin ; this has been called by the Author, its discoverer,* " maximum bronze." The presence of zinc or other foreign element in the real bronzes is found to be particularly objectionable in those alloys intended for use in salt water, as it renders the latter especially liable to injury by local and rapid corrosion. 56. The Strength of the Shells and Flues of boilers may be readily calculated when the data can be safely relied upon. The two forms are subject to quite different laws, however ; and even the strength of cylinders subjected to internal pressure, as are the cylindrical shells of steam-boilers, when thick, is calcu- lated by different methods from those applicable when of thin plate; but it is not asserted that the heavy shells of large marine boilers, in which the metal is from three quarters to r sometimes, above an inch thick, may not be properly calculated by the rule applying to thick cylinders of cast-iron or other non-ductile material. Cylindrical Boiler-Shells, and other thin cylinders, have a thickness which is determined by the tenacity of the metal and the character of the riveted or other seam. If/ be the internal pressure, T the mean tenacity to be calculated upon along the weakest seam, r the semidiameter, and t the thickness, we have for axial stresses for equilibrium, pTtr* = 27trtT, and But for transverse stresses tending to rupture longitudinal seams, pr = tT, * See " Materials of Engineering:" The Alloys- I3O THE STEAM-BOILER. and tT pr / ; *=^. ...... (2) T J. With seams of equal strength in both directions, therefore, the cylinder is at the point of rupture along the longitudinal seams, while capable of bearing twice the pressure on girth seams. It is evident that spheres have twice the strength of cylinders of equal diameter. Tldck cylinders are considered later, as they are usually made in cast-iron. Flat Boiler-heads are made both in wrought and cast iron. For these Clark's rules may be used.* For elastic deflection, _ ^t For maximum pressure, tT p = 0.2 1 5--, (4) or, for iron, For steel, For cast-iron, /= 10,0007 u (5) p= 11,500-} (6) p = 4,000, ........ (7) when t is the thickness, d l 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, 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 * = -> ' - - ;- (9) 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) : b = iocA - 1 , for the plate; b = 100 - 1 , for rivets in punched holes ; b = 90 -, for rivets in drilled holes. The least of these values is taken. Here/, is the pitch of rivets, d l is their diameter, a l is the area of the rivet-section. When in double-shear, 1. 75#j is taken for a t . 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-pife 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, for vertical cylinders, where d is the internal diameter, and 4 () for horizontal cylinders of considerable size. In metric measures, kilogrammes and centimetres, these formulas become If r l 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, pr* = T(r, r,) = TV, nearly, . . . . (14) MATERIALS STRENGTH OF THE STRUCTURE. 133 and '- ........... OS (16) For the M/f 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 p> ; - - v . , . . - (18) and, taking the total resistance as/V,, when p f 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 other annulus ^ dx ; while the total resistance will be, on Ji- either side the cylinder, * = 7--f-L = T-- (.9) * 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 and the thickness ;j5 ' t = -T=j t > (2I > while the ratio of the radii ;:; : ;.: \= T i^^^ = T^p; - <> Lame's 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 2 by and the tension by which the ring resists that pressure greater, When rrvp = v, when r r v p = p l ; then /, = 2 b, and o = : , b ; " a = A MATERIALS STRENG7^H OF THE STRUCTURE. 135 and the maximum possible stress on the inner ring is (23) . (2 4 ) and the ratio of inner and outer radii is 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 2'f-A' <* (27) 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 be coincident with the ultimate strength, and T the tenacity of the metal, R = the ratio, external diameter divided by internal, p = the bursting pressure, (28) t R e T ........... (29) 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 fpr single-riveted boilers of various sizes : MATERIALS STRENGTH OF THE STRUCTURE. 137 TABLE OF PRESSURES ALLOWABLE ON BOILERS MADE SINCE FEB- RUARY 28, 1872. 45,000 TEN- 50,000 TEN- 55,000 TEN- 6o,000 T EN- 65,000 TEN- 70,000 TEN- SILE SILE SILE SI LZ SILE SILE y jo STRENGTH. STRENGTH. STRENGTH. STRENGTH. STRENGTH. STRENGTH. '3 CTl i 7,5oo 4,8,333.3 |,9,i66.6 i, 10,000 4, 10,833.3 i, 11,666.6 ] "o i | 3 -a 8| i 11 si 0*3 8.1 g go y O jj c"3 . | 8 1 1 _ 1 O *O ii 1 ii 3 1/-3 Q.T-I i i k'-S s ^s r g* 1 p I* 1 8* -i8 75 78.12 93-74 86.8 104. 16 95-48 "4-57 104. 16 124.99 ii2.84 i i 35 .4 121.52 145.82 .21 87-5 105. 97.21 116.65 106.94 128.3 116.66 139-99 126.38 151.65 136.11 J 63.33 23 95-83 114.99 106.47 127.76 117. 12 140.54 127.77 153-32 138.41 166.05 149.07 178.88 25 104. 16 124.99 ii5-74 138.88 127.31 152.77 138.88 166.65 150.46 180.55 162.03 193-43 36 .26 108.33 129.99 120.37 144.44 132.4 158.88 144.44 173.32 156.48 187.77 168.51 2O2 . 2 I Inches. .29 120.83 !44-99 i34- 2 5 161 . ii 147.68 177.21 161 . ii 193-33 174.53 209.43 187.90 225.48 -3125 130.2 156.24 144.67 173-6 159.I4 190.96 173-6 208.32 188.07225.68 202 5 243.04 33 137-5 165- 152-77 183.3; 168.05 201.66 183-33 219.99 198.61 238.35 213.88 256.65 35 145-83 174.99 162.03 194-43 178.23 213.87 194.44 233-32 210.64,252.76 226.84 272.20 375 156.2, 187.5 173.61 208.33 190.97 229. 16 208.33 249-99 225 69 271.82 243-05 291.66 1875 74.01 88.89 82.23 98.67 90.46 108.54 98.68 118.41 106.9 128.28 "5-13 138.16 .21 82.89 99.46 92 i 110.52 101 .31 121.57 110.521132.62 "9-73 !43- 6 7 128^93 I54-7 1 23 90.78 108.03 100.87 121.04 H0.96 i33-!5 121. 05(145. 2t 131.- 13 157-35 141.22 169.46 25 98.68 118.41 109 64 131.56 120.61 144-73 142.54 171 .04 153-5 184.20 38 Inches. .26 .29 IO2 . 63 114.47 123.15 I37-36 114.03 127.19 136.83 125.43 152.62 139.91 136-84 I52-63 164.2 183.15 148.24 165-35 177 88 198.42 159-64 178.06 191 .56 213.67 3125 123.35 148.02 164.46 150.76 180.91 I64-47 197.36 178.17 213-8 191.88 230.25 33 130.26 156.31 144-73 173-67 159.2 191.04 173-68 208.41 188.15 225.78 202 . 62 243-14 35 138.15 165.78 153-5 184.21 168.85 202.62 184.21 221 .05 199-56 239-47 214.91 257-89 375 I 4 8. 177.60 l6 4 73 197.67 180.81 217.09 '97-36 236.83 213.81 256-57 230.26 276.31 1875 70-3 1 84-37 78.12 93-74 85.93 103 . i i 93-75 II2.5 101 .56 121.87 109-37 131.24 .21 78.75 94-50 87.49 104.98 96.24 115-48 105. 126. '13-74 136.48 122.49 146.98 23 86.25 95-83 114.99 105.41 126.49 138. 124.58 149.49 160.99 25 93-75 112.5 104.16 124.99 114.58 137-49 125- I 5 0. 135.41 162.49 M5-83 174-99 40 .26 97-5 117. 108.33 129.99 119. 16 142.99 130. 156. 140.83 168.99 151 .66 181.99 Inches. 29 108.75 130-5 120.83 144.99 132.91 159-49 *4f-' 174. 157-08 188.49 169.16 202.99 3"5 117.18 140.61 130.2 156.24 143.22 171.86 156-25 187.45 169.27 203.12 182.29 218.74 33 I2 3-75 148.5 137-49 164.98 151.24 181.48 165- I 9 8. 178.74 214.48 192.49 230 98 35 131-25 157-5 145-83 174.99 160.41 192.49 210. 189.58227.49 204. 16 244.99 375 140.62 168.74 156.24 187.48 171.87 206 . 24 187-5 225. 2O3 . I 2 243-74 218.74 262.48 1875 66.96 80.35 74.40 89.28 81.84 98.20 89.28 107.13 96.72 116.06 104. 16 124.99 .21 75- 90. 83-32 99-99 91.66 109.99 IOO. 1 2O. 108.33,129.99 116.66 139-99 23 .82.14 98-56 91.23 109.51 100.39 120.46 109.52 131.42 118.65:142.38 127.77 25 89.28 107.13 99-2 119.04 IO9 . I 2 130-94 119.04 142.84 128.96 154-75 138.88 166.65 42 .26 92.85 111.42 103.17 123.8 113-49 136.18 123.8 148.56 134. 12 160.94 144-44 173-32 Inches. .29 103.57 124.28 115.07 138.08 126.57 151-85 138-09 165.7 149.6 179-52 i6i.n 193-33 3125 in .6 133-9 2 124. 148.8 136.4 16^.68 148.741178.56 161.2 193-44 173.61 208 . 23 33 117.85 141.42 130.94 157.12 114.04 172.84 157.14188.56 170.23 204.27 183-33 219.99 35 125 150. 138.88 166.65 152.77 183-32 166.66 199.99 i8o.55'2i6.66 194.44 233.32 375 133-92 160.7 148.8 178-56 163.68 196.40 I78-57 214.28 193-45232.14 208.33 249 99 1875 63.92 76.7 71 .01 85.22 78.12 93-74 85.22 IO2 . 26 92.32 110.78 99.42 "9-3 .2T 71-59 85-9 79-54 95-44 87.49 104.98 95.451114.54 103.4 124.08 in .36 133-63 23 78.4 94.08 87.12 104-54 95.83 114.99 104.54 125.44 113-25 '35-9 121.96 146.35 25 85.22 102.26 94.69 113.62 104. 16 124.99 113.63 136.35 123.1 147.72 132.56 159-07 44 .26 88.63 106.35 98.48 118.17 108.33 129.99 118.18141.81 128.02 153-62 137-87 165.44 Inches. .29 98.86 118.63 109.84 131.80 120.83 144-99 131.81 158.17 142-79 I7I-33 I53-78 184-53 3*25 106.53 127.83 118.36 142.03 130.2 156.24 142.04 170.44 153.88 184.65 165.71 198-85 33 112.5 135- 124.99 149.98 137-49 164,98 150. Ii8o. 162.49 194.98 174.99 209.98 35 375 119.31 127.81 143.17 132.57 153.37 142.04 159.08 170.44 145-83 '74 99 156.24 187.48 159.09 190.9 170.45^04.54 8$ 206.8 221.58 185.6 222.72 198.861238.63 138 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 tj 8 STRENGTH. STRENGTH. STRENGTH. STRENGTH. STRENGTH. STRENGTH. 1 rt K 4, 7,5oo 4,8,333-3 i 9.166.6 g, 10,000 i 10,833.3 &, 11,666.6 "o gc S G c c"rt 6 72,ooo- 9 (2) In metric measures, kilogrammes and centimetres diameter, and metres of length, / = 68,000, nearly (3) rT~ 68,000 k) 2. t = For elliptical flues take d = ~- ; where a is the greater and b the lesser semi-axis. These equations probably give too small values of t for heavy flues under high pressure. Belpaire's rule, deduced from Fairbairn's experiments, is ^2.081 p= i ( o57,i8 ~ (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, in which a is 90,000, provided, always, < 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 f]ues 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 P = -; *==--; r c in which the pressure, p, is in pounds per square inch ; the thickness, /, and the radius, r, of the flue in inches. The value of the constant c is 44. 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 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 1 6 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 >= (5) where t, T, and d are the thickness of sheet and its tenacity, and the " 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 f----\^dl' (6) in which / is the thickness of plate, and d the pitch of the stay- bolts. In design, we would make (7) * Journal Franklin Institute, 1872. 143 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, cd?t in which w = width of the fire-box ; p' the pitch of the supporting bolts ; d' = the distance from centre to centre of girders ; / = their length ; d = their depth ; t = their thickness ; all dimensions in inches except /, which is taken in feet. This is the formula approved by the British Board of Trade. The value of the coefficient c is from 500, when but one supporting bolt is used, to 750 and 800 when two or three and when four bolts are employed. The accompanying figure exhibits a common form of stay for water-legs and other narrow water spaces. The stay is cut from a long screwed rod, and is frequently fitted with a nut and washer at each end. They are sometimes drilled longi- tudinally in order that they may give warning by leakage if fractured. 58. The Relative Strength of Shell and FIG. 68. " Sectional " Boilers, and consequently, in large degree, their relative safety, "is measured by the relative magnitude of their largest parts. As remarked by John Stevens, MATERIALS STRENGTH OF THE STRUCTURE. 149 the inventor, the sectional boiler, with its smaller members and subdivided steam and water chambers, is safe in proportion as the sizes of the latter are diminished ; while the large shells of the common forms of boiler are liable to dangerous rupture in proportion as their diameters are increased. The strengths of cylindrical reservoirs subjected to internal pressure, as are the shells, steam-drums, and mud-drums of shell boilers, and the tubes and steam-reservoirs of sectional boilers, are subject to laws so simple, and are computed by methods of such easy ap- plication, that there never need be any doubt in regard to the margin of safety existing in either case when new. Flues and old boiler-shells are less amenable to calculation, and are thus more unsafe. Water-tubular boilers are comparatively safe under all conditions of ordinary operation, and, when compared with the other type of steam-generator, are vastly safer. 59. A Loss of Strength and of Ductility is very often ob- served in the iron of which boilers are composed, as they ad- vance in age, due to the progress of oxidation, probably, within and between the laminae of which the sheets may be composed. The plate may be thus very nearly destroyed, at times, before this action may be detected. In some cases the iron may be nearly all destroyed, and only a sheet of oxide may remain ; while the boiler, if not working under high pressure, may still appear sound. Such deterioration is often a source of great danger. Excessively high temperature not infrequently gives rise to a loss of tenacity of serious amount with, fortunately, in most cases, increase of ductility. This is not invariably the case, however, as, at a " black heat " just below redness, a critical temperature is reached at which the iron may exhibit great brittleness. The physical conditions thus modifying strength have been already described at considerable length. These changes occur in steam-boilers through the action of a variety of special causes. Ordinary oxidation, general and local, especially when accelerated by voltaic action, produces in many cases rapid de- terioration ; the constant and often great changes of tempera- ture due to not only the ordinary working of the boiler, but also THE STEAM-BOILER. at times to overheating of parts exposed to flame, may produce still more formidable effects ; and even the continual changing of form caused by variations both of pressure and temperature, after the lapse of considerable periods of time, may give rise to important losses of ductility, and sometimes of strength. Steel is especially liable, if too hard, to loss of quality and danger- ous injury by cracking, in consequence of such action. 60. The Deterioration of Boilers with age and with use is in nearly all cases due to modification of quality of metal,, and to reduction of section of parts exposed to stress and strain. This deterioration is certain to occur to a greater or less extent ; but its rate is usually indeterminate, and it conse- quently happens that, except by actual inspection and test, it is impossible to know, at any time after a boiler is built and set in operation, just what is its strength and whether it is safe. This deterioration may be to a certain extent controlled and retarded by care and by the adoption of proper precau- tions. The principal requisite is the keeping of every part dry, and at a temperature below that of " burning" or rapid oxi- dation. Loss of strength, elasticity, ductility, and resilience will, however, always take place ; and the boiler, whether in use or not, should always be very carefully examined at such inter- vals as shall insure its condition being known at all times, and such as shall secure a safe adjustment of the pressure main- tained within it to its reduced strength. Every element and member of the structure will inevitably depreciate, and the most insignificant part must be kept under proper supervision to insure safe operation. Experiment has shown that steel boiler-plate, exposed to repeated heating to high temperatures, and cooling down again, loses less by oxidation than does iron,* and retains its quality better. Steel loses rather more than iron when exposed to the action of sea-water, f and should never, if it can be conveniently avoided, be placed under such circumstances in contact with iron. Its own scale also produces an acceleration of galvanic * Engineering, April 20, 1883. ) Trans. Inst. Naval Architects, vol. xxiii. p. 143. MATERIALS STRENGTH OF THE STRUCTURE. l$l action, and it is best, where practicable, to remove all the scale by " pickling" in dilute hydrochloric acid or in sal ammoniac. 61. Inspection and Test of boilers, at regular intervals and by methods that are thoroughly reliable, is now universally rec- ognized as not only essential to permanent safe use of steam- generators, but also as necessary to secure maximum efficiency in their operation. Such examinations and tests are usually made by expert in- spectors who make a business of that work, and who have thus acquired exceptional, sometimes most extraordinary, skill in the detection of injury and its cause. The methods pursued and the rules adopted will be given later, in chapters devoted to the description of the methods of construction and to the pre- scription of forms of specification and contracts, and of the re- quisites of full conformance with the latter. CHAPTER III. THE FUELS AND THEIR COMBUSTION. 62. The Chemical and Physical Principles involved in the combustion of fuel, the development of heat and its trans- fer, are all well known and capable of very definite expression. Combustion may be defined as the rapid combination of any oxidizable substance with oxygen. The result of such combi- nation is the production of new compounds of definite charac- ter, and in quantities readily calculable when the amount of each of the combustible constituents is given. It is also known, very precisely, how much heat is produced by the combustion of any given weight of any one of the more familiar combusti- bles, and how much of that heat is available for transfer to a steam-boiler or other apparatus of utilization, when the com- bustion is complete and perfect. Perfect combustion occurs when all of the combustible is burned, and with the result of producing the highest stage of oxidation. Carbon is perfectly burned when it is wholly con- verted into carbon dioxide and carbonic acid. Wood, or other fuel containing hydrogen, is perfectly consumed when all its carbon is oxidized to carbonic acid, and all its hydrogen is united with oxygen to form steam. Chemical combination invariably produces heat, and de- composition as inevitably results in the absorption of heat in precisely the amount due to the opposite process. If both combination and decomposition take place in complex chemi- cal changes, the heat produced is the net result of both actions. Several interesting and important principles are recognized by writers on this general subject, as controlling the develop- ment of heat by combustion. Berthelot first called attention to the fact that the total heat evolved in any case of chemical THE FUELS AND THEIR COMBUSTION. 153 combustion is a measure of the energy expended in the separa- tion of the resulting compound into its elements. The same chemist announced a second law, also known by his name : The quantity of heat-energy evolved or absorbed in any chemi- cal change of this kind, where no mechanical work is done, is dependent purely on the initial and final states, and not at all on the intermediate process of change. Thus the heat pro- duced in a furnace depends on the final product of combustion, and not at all on whether the carbon, for example, has been, at intermediate stages, wholly or partly burned, and has existed in greater or less proportion in the state of carbon monoxide or of carbon dioxide. Berthelot's third law asserts that in any chemical action the tendency is toward that method of change which will yield the greatest amount of heat. In other words, the tendency always exists to produce complete transformation of potential into actual energy. 63. The Fuels used in Engineering* are anthracite and bituminous coals, coke, wood, charcoal, peat, and combustible gases obtained by the distillation of the solid kinds of fuel. The oils animal, vegetable, and mineral and the solid hydrocar- bons, of which bitumen is a type, are occasionally used also. All consist of either pure carbon or of combinations of carbon, hydrogen, and non-combustible substances. The mineral oils and liquid fuels generally promise excellent results when satis- factory methods shall have been found to secure the conditions of perfect combustion. In making a selection of a fuel the engineer is aided greatly by a knowledge of the origin and general characteristics of those combustibles from which he may be called upon to select the one best adapted to any given case. Each form of fuel, solid, liquid, and gaseous, is specially adapted to particular purposes ; and in selection the engineer and metallurgist should carefully examine all of the circum- stances of the case under consideration, in order to determine from which of these classes the fuel required should be selected ; * Adapted largely from the Author's "The Materials of Engineering," vol. i. N. Y. : J. Wiley & Sons, 1885. 154 THE STEAM-BOILER. and, this choice having been made, he will next select that quality which best fulfils the requirements of the case. COMPOSITION OF COMBUSTIBLES, CARBON TAKEN AS 100. Carbon. Hydrogen. Oxygen. Wood . ... IOO 12 4.8 8q O7 Peat IOO Q 8q ec 67 Lignite . IOO 8 17 42 42 Bituminous Coal . . . IOO 6 12 21 23 Anthracite Coal IOO 2 84. I 74. Coal, whether anthracite or bituminous, is a fossil of vege- table origin. It is always associated with some earthy matter, and the latter is sometimes present in such quantities as to destroy the value of the coal as a fuel. Coal is sometimes found so slightly altered as to differ but little in chemical composition and in physical structure from recent vegetable substances ; and in other cases it is so thoroughly changed as to have become, in all but its chemical constitution, a mineral. Some of the more completely fossilized bituminous coal breaks into cubic and rhomboidal fragments,, but the anthracite exhibits little or no traces of crystallization. Chemical examination shows coal, as already indicated, to be composed of both organic and inorganic matter. The for- mer is purely vegetable, and the latter consists of earthy mat- ter above which the ligneous portions once grew. Destructive distillation resolves the organic matter into its invariable ultimate constituents, carbon, hydrogen, and oxygen,, which come from the retort as solid carbon, or coke, liquid tar,, gaseous ammonia, benzole, naphtha, paraffine, illuminating gas, sulphurous acid, and other substances, in various proportions. The inorganic portion is left as an ash when the fuel is burned. It consists usually of silicates in varying proportions. The various fossil fuels having had a common origin, and being all more or less decomposed and mechanically altered vegetable matter, are found to exist in all states intermediate between that of recent vegetation and that of completely mineralized graphitic anthracite. THE FUELS AND THEIR COMBUSTION. 155 Their classification is therefore an arbitrary one, and it fre- quently happens that a particular species of coal lies so exactly between two classes as to make it difficult to determine to which it should be assigned. The anthracites are found among the older carboniferous strata; the bituminous coals come from the secondary, and the softest and least altered varieties from the tertiary, formations. The following, representing approximately the gradual change of composition as fossilization affects the alteration of woody fibre, is given by Dr. Wagner: CHANGE OF COMPOSITION OF FOSSIL FUELS. Carbon. Hydrogen. Oxygen. Cellulose . . ... C2 6* e 2^ 42 10 Peat 60 44 * 06 qo 60 66.06 5 - 27 27. 76 " (earthy brown coal). . . . Coal (secondary) 74.20 76 18 5-89 * 64 I9.QO 18 07 QO. ZO c (X 4 .40 Anthracite . . . . . Q2 8^ * 06 3IQ In the above analyses earthy matter is excluded. 64. Anthracite Coal, called sometimes^/^;?^, and sometimes blind or stone coal, consists of carbon and inorganic substances, and is usually free from hydrocarbons. Some varieties are thoroughly mineralized and have become graphitic. The or- dinary varieties of good anthracite are hard, compact, lustrous, and sometimes iridescent. The color is intermediate between jet black and that of plumbago. It is amorphous and somewhat vitreous in structure, the hardest varieties falling to pieces when suddenly heated, and sometimes breaking up into very small fragments, thus caus- ing considerable loss even when carefully "fired." It some- times gives out a ringing sound when struck. It is a strong, dense coal, its specific gravity ranging from 1.4 to 1.6. It has a high colorific value. It burns without smoke and without flame unless containing moisture, the vapor of which produces a yellow flame of com- paratively low temperature. It kindles slowly and with dif- THE STEAM-BOILER. ficulty; and, once kindled, requires to be carefully and skilfully managed to secure economic efficiency. A representative variety has a specific gravity 1.55, and con- tains, exclusive of ash, carbon, 94 per cent, hydrogen and oxygen (moisture) 6 per cent. Of the latter, 2-J per cent is hygroscopic, but is held with great tenacity. The percentage of ash varies greatly, even in the same variety, and in specimens from the same bed. It may be estimated, as an average, at above ten per cent, while the total loss in ash, fine coal, and clinker will be likely sometimes to reach double that proportion in ordinary furnaces. When selecting anthracite it is necessary to keep this fact carefully in mind. Twenty-four samples of anthracite from Pennsylvania, analyzed by Britton, gave as a mean Carbon 9 J -O5 Volatile matter 3.45 Moisture 1.34 Ash c 4 16 100.00 There was included in the above, sulphur 0.240, phosphorus 0.013. A variety of this class of coals, similar in composition, but differing from the typical anthracite above described in struc- ture, has been sometimes called semi-anthracite. It does not exhibit the conchoidal fracture of the latter, but is somewhat lamellar, and is marked by fine joints or planes of cleavage. It crumbles readily, and has less density than the preceding. One method of distinguishing good examples of the two varieties is found in the fact that the latter, when just fractured, soils the hand, while the former does not. The latter variety kindles quite readily and burns freely. An example of this coal contained, in one hundred parts, carbon, 90; hydrogen and oxygen, 1.5 ; ash, 8.5. 65. The Bituminous Coals are sometimes divided into three classes. Dry bituminous coal contains about 75 per cent of carbon, THE FUELS AND THEIR COMBUSTION. !$/ 5 per cent hydrogen, and 4 per cent oxygen. That part of the hydrogen which is combined with carbon is capable of adding to the heat-giving power of the coal. This coal is lighter than anthracite, its specific gravity being about 1.3. Its color is black or nearly black, and its lustre resinous ; it is moderately hard, and burns freely. Its structure is weak, brittle and splin- tery, fine-grained, and of uneven surface. It kindles with less difficulty than any variety of anthracite, but less readily than the bituminous coal to be described. It burns with a moderate flame, and gives off little or no smoke. Bituminous caking coal contains sometimes as little as 60 per cent of free carbon, and the maximum proportion is, perhaps, 70 per cent. It contains 5 or 6 per cent each of oxygen and hydrogen, and the remaining portion, amounting sometimes to 30 per cent, is incombustible. Its specific gravity is about 1.25. It is moderately compact ; its fracture is uneven, but not splin- tery; its color is a less decided black than the preceding, and its lustre is more resinous. When heated it breaks into small fragments if the proportion of bitumen is insufficient to cause it to cohere before becoming thoroughly softened, but afterward, as it becomes more highly heated, the pieces become pasty and adherent, and the whole mass becomes compact and hard as the gaseous constituents are expelled by heat. This coal, ignited in air, burns with a yellowish flame and very irregularly unless kept continually stirred to prevent ag- glomeration and consequent checking of the draught. It can- not be successfully used, therefore, when great heat is required. It is valuable for the manufacturer of gas and of coke, and can be used in small grates where but moderate heat is obtained. Long flaming bituminous coal is quite similar to the pre- ceding, differing chemically in composition and containing a larger proportion of oxygen. It burns with a long flame, and has a strong tendency to produce smoke. Some varieties cake like the preceding, others do not ; but all ignite readily and burn freely, consuming rapidly. There are many varieties of coal in each of the above- named classes, the gradation being sometimes marked and sometimes barely distinguishable. 153 THE STEAM-BOILER. American anthracites have been found, by experiments made under the direction of the United States Navy Depart- ment, to have a mean evaporative efficiency, in marine boilers, of 8.9 pounds of water evaporated from 212 Fahr. (100 Cent.) per pound of coal. The bituminous coals of the United States were found to evaporate an average of 9.9 pounds of water per pound of fuel, under similar conditions. The average efficiency of British coals is given by Bourne at about 8.7. American anthracites evaporated 10.69 pounds of water per pound of combustible matter contained in the fuels, and the bituminous coals 10.84, from 212 Fahr.* These results are practically identical for the two kinds of coal ; but the average of the best known varieties gives a dif- ference which is, with such good varieties, in favor of anthra- cite. 66. Lignite, or Brown Coal, is of more recent and of more in- complete formation than the bituminous coals, and occupies a position intermediate between the true coals and peat. It con- tains from 30 to 60 per cent of carbon, 5 to 8 per cent of hydrogen, and 20 to 25 per cent of oxygen. It is very light when pure, having, according to Regnault, a specific gravity of from i.io to 1.25. The heavier varieties contain much compact earthy matter. Lignite is found in tertiary geological formations. It is brown in color, has the woody structure well denned, and is usually lustreless. Where it approaches the bituminous coals in age, it also approximates to them in structure and other characteristics. It frequently contains considerable moisture, which can only be removed by high temperature or by long seasoning, and the lignite, once dried, must be carefully pre- served in dry situations if not used at once, as it reabsorbs moisture with great avidity. When thoroughly dry it kindles readily, burns freely, and is consumed rapidly. It is not usually considered a valuable kind of fuel. It occupies considerably more space weight for weight than the true coals, burns as an average a third more *See American Institute Reports: Tests of Steam Boilers, 1874. FUELS AND THEIR COMBUSTION. 159 rapidly, and its evaporation of water per pound of fuel is about 25 per cent less. To obtain maximum evaporative efficiency a slow rate of combustion is found most effective. 67. Peat, sometimes called Turf, is obtained from bogs and swampy places. It consists of the interlaced and slightly decayed roots of vegetation, which, although buried under a superincumbent mass of similar material and mingled with some earthy matter, retains its ligneous structure and nearly all the chemical characteristics of unaltered vegetable matter. Submitted to the great pressure and the warmth which have for ages acted upon the coal-beds, it would also probably become coal. Dried in the air, it, like the lignites, retains moisture per- sistently, and is usually found to contain 30 per cent after drying. After completely removing all water, an average specimen would contain about 60 per cent of carbon, 5 to 10 per cent hydrogen, and 30 or 40 per cent of oxygen. The ash varies very greatly, sometimes being as little as 5, and in other cases as high as 25 per cent. A pound of wood charcoal has nearly the same value as a fuel as 1.66 pounds of peat of average quality. Peat is frequently used in large quantities for heating pur- poses, and attempts have been made, with encouraging results, to use it in metallurgical operations. When to be thus used, it is cut from the bog with sharp spades, ground up in a machine specially designed for the purpose, and dried by spreading it where it can have full exposure to the sun and air. It is frequently compressed by machinery until its density approaches that of the lighter coals, and it is used in blocks of such size as are found best suited to the particular purpose for which it is prepared. Its charcoal makes excellent fuel for use in working steel and welding iron. It is frequently found to be a very excel- lent fuel for other purposes, and is extensively used in some localities. Its specific gravity is usually about 0.5. 68. ^A/ood, thoroughly seasoned, still contains about 20 per cent of moisture. 160 THE STEAM-BOILER. The moisture being completely driven off by high tem- perature, there is left about 50 per cent carbon, and combined oxygen and hydrogen compose the remainder, in very nearly the proportions which form water. The pines and firs contain turpentine, and other woods contain frequently a minute pro- portion of hydrocarbons peculiar to themselves. The proportion of ash varies* from about 0.5 per cent to 5 per cent. The woods all evaporate very nearly the same weight of water per pound of fuel. The lighter woods take fire most readily and burn most rapidly; the denser varieties give the most steady heat and burn longest. Where radiated heat is desired the hard woods are much the most efficient. The seasoning of wood has been described in that part of this work which treats of timber. Thorough seasoning in the open air requires from six months to a year, and is the only method generally adopted for wood intended to be used as fuel. One cord of hard wood, such as is used on the Northern lakes of the United States, is said to be equal in calorific power to one ton of anthracite coal of medium quality. One cord of soft wood, such as is used by steamers on Western rivers, is equal in heating power to 960 pounds (436 kilogrammes) or 12 bushels (423 cubic decimetres) of Pittsburg coal. One cord of well-seasoned yellow pine is equivalent to J ton (500 kilogrammes) of good coal. (See 84.) 69. Coke is made from bituminous coal by subjecting it to such high temperature as to deprive it of its volatile con- stituents. The presence of moisture in some of the coals largely reduces their heating power. The bituminous matter causes them to fuse and to form a coherent mass, and, by thus pre- venting the passage of air, destroys their efficiency for many purposes. The presence of sulphur and of deleterious volatile substances in many coals also precludes their application to the reduction of iron ores, and destroys their value for other metal- lurgical purposes. All of these volatile materials being driven off by heat, a mass of fixed carbon containing only earthy impurities remains, which " coke " constitutes the fuel with THE FUELS AND THEIR COMBUSTION. l6l which some of the most extensive and important metallurgical industries are conducted. These volatile matters are sometimes utilized, but are generally wasted, except where the coke is considered a secondary product, as in the manufacture of illu- minating gas. Coking is carried on by either of three methods in open heaps, in coke ovens, or in retorts. The first method is extremely wasteful, and is rarely prac- tised ; the second is more economical ; and the third is the best where gas is manufactured, and is the only one practised in that case. The second method is that generally adopted where the coke is the primary product, as, although not as economical as the last, it produces a strong coke which is much better adapted for use in furnaces than that afforded by the last method, which, although allowing of the complete separation and collection of the liquid and gaseous products of distillation r yields a coke which has too little density and strength to make it a valuable fuel. Coak made in ovens is usually of a dark gray color, porous, hard, and brittle. The best gives out a slight ringing sound when struck, and has something of the metallic lustre. It makes an intense, clear fire, and it should riot be forced so as to injure either the boiler or the grate by burning the iron. Where the coals contain sulphur but are free from moisture f provision should be made for the passage of a supply of steam through the oven. This will give up its oxygen to the metal with which the sulphur is combined, and the hydrogen, uniting with the latter, forms sulphuretted hydrogen. The coke is thus left comparatively free from the noxious ingredient, and as this is usually the only constituent of bituminous coal which injuriously affects iron, the coke is a better fuel than the coal from which it is made. Various coals yield from 33 per cent to 90 per cent of their weight in coke. The latter containing all the ash, the percent- age of ash in coke will be higher than in the coal from which it is prepared. Coke has a strong tendency to absorb moisture, and may, when unprotected from dampness, condense 15 or 20 per cent of its own weight within its pores, ii 1 62 THE STEAM-BOILER. Many cokes contain 15 percent ash and I or even 2 per cent sulphur; while others contain but 3 to 5 per cent ash and T L per cent sulphur. 70. Charcoal has the same relation to wood that coke has to bituminous coals. It is made from all kinds of wood, hard-wood charcoal being the best for fuel. Wood of about twenty years of age is preferred, and should be charred before decay has commenced. The methods of preparation are substantially the same, and the chemical constitution of the product is very similar, although its physical characteristics are quite different. Charcoal prepared by charring in heaps seldom amounts to more than 20 per cent of the total weight of wood used ; care- lessness in conducting the process may reduce the weight of product far below even that figure. A considerable loss is unavoidable, since the charring of one portion must be effected by the heat obtained from the combustion of another part of the wood. Sound wood is selected, cut in billets four or five feet in length, and, when large, split into sticks of from three to six inches in thickness. It is best to assort the w r ood, placing each kind in piles by itself. In making up the heap the ground is cleared, a stake is set at the centre of the cleared space, and a layer of wood is put down with all the sticks laid radially, and the interstices filled with smaller sticks. On this layer the rest of the wood is piled on end, beginning by leaning sticks against the centre stake. The whole is finally covered with another closely packed layer, which in turn is completely covered with sods. A central hole is left, and also an uncovered ring around the base five or six inches high, for the air-supply. One or two horizontal passages left in the pile conduct the gases to the centre, where they rise, passing out at the hole made by pulling out the centre stake before firing the pile. The fire being started and actively burning, all openings are closed, and combustion is perfectly controlled by altering their number and position. The condition of the fire is indi- cated by the color of the smoke, which should be black and thick ; when it is light and bluish the draft should be more THE FUELS AND THEIR COMBUSTION. 163 completely checked. The work is finished when the wood at the exterior of the pile is found charred. All openings are then closed, and the fire is thus extinguished. The pile can be usually opened on the following day, and the removal of charcoal begun. So crude a process is very liable to excessive losses from the difficulty experienced in adjusting the supply of air, and in conducting the heated products of combustion to precisely the right points, and in precisely the right proportions to secure maximum efficiency. The presence of moisture in wood is productive of loss by giving rise to the formation of carbonic oxide and of new hydrocarbons. They carry off carbon which would otherwise have been left in the solid state as so much charcoal. Dry wood, charred in a retort, yields as a maximum about 30 per cent of its weight in charcoal. Of the carbon originally contained in wood, therefore, by the first method of charring not above one half may be expected to be obtained as charcoal, while by the last method three quarters may be obtained by skilful management. The latter process requires the expen- diture of about one eighth of the weight of wood charred for the production of the heat demanded by that process. It therefore yields a net amount in charcoal of about 30 per cent of the total weight of wood used. The wood which is used for fuel, however, may be of less value than that charged into the retort. Peat charcoal is sometimes made by similar methods, but is little used. Wood heated to 300 Fahr. (150 Cent.) for a considerable length of time loses 60 per cent or more of its weight. If heated only to slightly above 212 Fahr. (100 Cent.), the loss is but from 50 to 55 per cent. The residue resembles charcoal, but in each case it retains some volatile matter which may be driven off by higher temperatures. Karsten found that, by rapid charring at high temperatures, he obtained as an average about 15 per cent charcoal in one series of experiments; while by slowly charring the same woods at a low temperature the percentage obtained averaged about 25 per cent. The combustibility of cJiarcoal is greater when prepared at a low than when prepared at a high temperature. 164 THE STEAM-BOILER. Good charcoal is black, with a high lustre, and has a con- choidal fracture. It is quite strong, and the best qualities ring when struck, although less than good coke. It burns without flame or smoke, and radiates heat strongly. It should not soil the hands. Charcoal and coke both make an intense, clear fire, and with a forced draught, giving a small air-supply, afford an extremely high temperature, which is liable to injure the grates or anything metallic which may be subjected to its action. 71. Pulverized Fuel, or Dust-fuel, is sometimes used in special processes. In the use of this form of fuel special ar- rangements become necessary to secure thorough intermixture of the fuel with the supporter of combustion, in order to effect complete oxidation. The fuel itself is sometimes prepared by pulverizing coal or other combustibles; and sometimes it is obtained from the large deposits of "slack," "breeze," or coal dust which are found wherever coal in large quantities is sub- jected to attrition. It is sometimes burned on a very fine grate, the requisite supply of air being secured by the use of a blast beneath the grate. One of the most successful methods is that pursued by Whelpley and Storer, and by Crampton. In this process a stream of mingled dust-fuel and air is driven into the furnace where combustion takes place, the quantity of fuel and of air being capable of adjustment in such a manner as to secure the most perfect combustion. This method has been applied suc- cessfully, not only in the production of heat simply, but also in the reduction of metals from their ores. The facility with which an oxidizing or a reducing flame may be produced at will is the great merit of the process in the latter application. Its advantage for heating purposes lies in the power which it gives of utilizing a fuel which would have otherwise no value. In making " muck-bar," an economy over that attained with coal of above 20 per cent has been reported to have been effected by the use of this process and fuel. The saving occurred in reduction of waste of metal, as well as in simple economy of fuel. At the United States Armory at Springfield, Massachusetts, 6.6 pounds or kilogrammes of fuel were con- 7 HE FUELS AND THEIR COMBUSTION 165 sumed per pound or kilogramme of iron heated to the welding heat, where 16 had been required by the old process.* 72. Liquid Fuels have been used to a limited extent. The liquids best adapted for use as fuel are the mineral oils. They yield an intense heat ; the products of combustion, as well as the fuels themselves, are comparatively free from deleterious -elements, and the temperatures obtained by their use are generally easily regulated, when they are burned in manageable quantities. Their tendency is to give off combustible gases, which may cause serious explosions ; and this fact, but especially the difficulty met with in uniformly distributing the oil, and in properly supplying it with air for its combustion, have hitherto prevented the general use of these fuels, even where their com- paratively high cost would not be a serious objection to their application. Crude petroleum, on distillation, breaks up into a large number of hydrocarbon compounds, having boiling-points varying from 32 Fahr. (o Cent.) to 700 Fahr. (371 Cent.), as. given by Van der Weyde. Its density is variable, but usually about 45 Beaume, corresponding to a specific gravity of about O.8, the gallon weighing 6.67 pounds, and the litre weighing 0.8 kilogramme. It contains by analysis: carbon, 84; hydrogen, 14; oxygen, 2. The latent heat of its vapor is about one fifth that of steam, and its volume 25 cubic feet to the gallon of oil, or 0.2 cubic metre per litre. The "creosote" or "dead oil" produced in gas-making is sometimes used as fuel. In experiments on board the British steamer Retriever, in 1868, where creosote was used for the generation of steam by what is called the Dorsett system, the evaporation was about 14 pounds or kilogrammes of water from a boiling-point per pound or kilogramme of liquid fuel used, or nearly double the average obtained where coal was used in the same boiler. Dr. Paul, reporting these results, gives the theoretic evapo- rative power of the constituents of this fuel, in units in weight of water per unit of fuel, as follows: phenol, 12.25; cressol, * Report, Lieut. H. Metcalf to Major Burton, Oct. 3ist, 1873. 1 66 THE STEAM-BOILER. 13.01; napthaline, 15.46; xylol, 16.59; cumol, 16.78; cymol, 16.94. Capt. Selwyn, R. N., reported an evaporative power from boiling-point of 16.77 parts water per part by weight of a liquid fuel which had a theoretical efficiency of 17.52 parts. In another instance he gives the evaporation of 14.98 from the boiling-point, by a fuel having a theoretical evaporative power of 17.5. Deville found oil from Oil Creek, Pennsylva- nia, to have a calorific value of 10,000 "calories," equivalent to the evaporation of 16.17 parts of water for one part by weight of oil. Of this 13^ per cent was lost by the chimney, and by conduction and radiation. Some other oils give slightly higher figures. Liquid fuels have probably had most genera! and success- ful application in Russia, where Mr. Urquehart and others have adopted it for locomotives, and many steamers in South- ern Russia have been fitted with petroleum furnaces. In these cases crude petroleum and refuse is injected into the furnace by means of a steam-jet in which highly-superheated steam is employed. The furnace is lined with fire-brick and the com- bustion-chamber as well, the burning jets passing first through the latter, then onward to the furnace, where combustion is completed. The brickwork serves as a reservoir of heat, regu- lating the supply, and also at times re-igniting the jets of oil- spray when they have been for a short time extinguished. The use of oil on the steamers of the Central Pacific Rail- way Co. gave in 1884 an economy of from 5 to 12 per cent in total running expense as compared with coal, with great saving of boilers also. Experiments made by Engineer-in-Chief B. F. Isherwood, U. S. N., under the direction of the U. S. Navy Department, upon various systems of utilization of petroleum as a fuel, gave a maximum economy over the use of anthracite of 68 per cent by Fisher's method of burning oil, and 38 per cent by Foote's process of burning liquid and solid fuel together ; he reports the failure of another method, in consequence of the obstruction of the tubes by deposition of solid carbon. Isherwood states the advantages attending the use of the THE FUELS AND THEIR COMBUSTION. l6/ mineral oils, which were the subject of his experiments, as fol- lows : 1. A reduction of weight of fuel amounting to 40^- per cent, 2. A reduction in bulk of 36^ per cent. 3. A reduction in the number of firemen (" stokers") in the proportion of 4 to I. 4. Prompt kindling of fires, and consequently the early attainment of the maximum temperature of furnaces. 5. The fire can at any moment be instantaneously extin- guished. Other advantages, unmentioned by him, are the uniformity of combustion and heating attainable, and the small propor- tion of ash. The disadvantages are given as follows : 1. Danger of explosions occurring by the taking fire of the vapors which are liable to arise from the fuel, and to escape from the tanks. 2. Loss of fuel by evaporation. 3. The unpleasant odors which distinguish these vapors. 4. The comparatively high price, which price would be rapidly augmented by any general introduction of the pro- posed application of the oils.* 73. Gaseous Fuels are used with marked success in some branches of metallurgical work, as well as in the generation of heat for ordinary purposes. The advantages possessed by gaseous fuels are : 1. Convenience of management of temperature. 2. Freedom from liability to injure material with which the products of combustion may come in contact, and conse- quently, also, allowing the use of fuel of inferior quality as a source of the gas. 3. The facility with which thorough combustion may be secured. 4. The readiness with which the flame may be given either an oxidizing or a deoxidizing character. * This may be questioned, since recent explorations of oil deposits, especially of the United States, indicate an immense supply as immeasurable and probably nearly as inexhaustible as the coal-fields. 1 68 THE STEAM-BOILER. 5. In many cases economy in expense of operation. The disadvantages are : 1. Danger of explosions when carelessly or unskilfully handled. 2. Expense of plant. 74. Artificial Fuels, other than charcoal, coke, and gases, are occasionally used in the production of high temperatures. They are prepared principally from refuse of natural fuels, which has but little value in its usual condition, but which, by special processes, is simply mixed with a small proportion of fuel of better quality or of more manageable form, and is compressed by machinery into conveniently shaped blocks, called briquettes. This refuse is found in large quantities in the neighborhood of coal-mines, and wherever coal is handled in considerable quantities. The total loss in this form in mining and transportation amounts to from one third to one half. It is called, as has been before stated, slack-coal. In the manufacture and transportation of coke and of charcoal, large quantities of refuse, called " breeze/' accumu- late ; which, although very rich in combustible matter, can- not be utilized in. the condition in which it is found, except by special contrivances. The sawdust which accumulates about saw-mills is another variety of combustible belonging to the same class ; as is also spent tan-bark, from tanneries, and " bagasse," or refuse crushed sugar-cane. They are most frequently mixed with some cohesive and at the same time combustible substance, as coal-tar. In districts abounding in mineral hydrocarbons, as in the neighborhood of the Caspian Sea, it has long been customary to mix them with clay, and thus to form a coherent and manageable fuel. The Norwegians have also long practised their method of utilizing sawdust by mixing it with clay and vegetable tar, and moulding it into bricks of such size and shape as to be conveniently handled, and at the same time to burn freely and without waste. It has been often urged, and with some reason apparently, that for many purposes a fuel made by careful mixture of dust-fuel with pitch or other combustible THE FUELS AND THEIR COMBUSTION. 169 cementing material is preferable to ordinary coal, in conse- quence of the greater convenience with which it can be stowed and handled. Another method of utilizing waste fuels consists in thor- oughly mixing, by grinding, charcoal-dust from the kilns with charred peat, spent tan-bark, and the proper proportion of tar or pitch to make a pasty, adhesive mass. This is moulded by machinery and dried in the open air, and then finally baked in closed retorts at a low heat. Dust-coal and pitch have been made into a good fuel in quite a similar manner to that just described. 75. The Heating Power of any Fuel is determined by calculating its total heat of combustion. This quantity is the sum of the amounts of heat generated by the combustion of the unoxidized carbon and hydrogen contained in the fuel, less the heat required in the evaporation and volatilization of con- stituents which become gaseous at the temperature resulting from the combustion of the first-named elements. It is meas- ured in " thermal units." A thermal unit is the quantity of heat necessary to raise a unit weight of water, at temperature of maximum density, one degree of temperature. The British thermal .unit is the quan- tity of heat required to raise a pound of water from the tem- perature 39. i to 40. i Fahr. The metric unit or calorie is the quantity of heat required to raise one kilogramme of water (2.2046215 pounds) from 3. 94 to 4.94 Centigrade. One metric or centigrade unit is equal to 3.96832 British units, and a British unit is equal to 0.251996 metric unit. An approximate estimate of the number of thermal units developed by the combustion of a pound or kilogramme of any dry fuel, of which the chemical composition is known, may be obtained by the use of the following formula : h = i4,5oo(7 + 62,000(11 --], . . . (i) THE STEAM-BOILER. where h is the number of British thermal units representing the total heat of combustion of one pound of the fuel ; h' is the number of metric units per kilogramme of fuel ; C represents the percentage of carbon, H that of hydrogen, and O that of oxygen. Thus an anthracite coal has been found to have the follow- ing composition : COMPOSITION OF ANTHRACITE COAL. Per cent. Carbon 8 1 . 34 Hydrogen, uncombined 3-45 Hydrogen, in combination o. 74 Oxygen and Nitrogen 5 . 89 Sulphur 0.64 Water . 2 . oo Ash 5 .94 Total 100.00 One pound or kilogramme of coal, of which the above is an analysis, can evaporate theoretically 14.4 pounds or kilogrammes of water from and at 100 Centigrade, or 212 Fahr. M. M. Scheurer, Kestner, and Meunier have adopted the common formula as first proposed by Dulong, but would omit all account of oxygen, thus reducing, as is claimed, the average error of the formula from about 12 per cent or more to 8 or 10. M. Cornut would separate the fixed from the volatile carbon, and would give the latter about one third more credit for heating power than the former ; closer approximations are thus made than by the other methods. Various methods of approximate determination of the heating power of fuels have been proposed. The use of the calorimeter is probably the most satisfactory ; another method is that of computation from the known chemical composition of the fuel, and the law of Walter, who found the quantity of heat produced in combustion very closely proportional to the weight of oxygen absorbed. Berthier's method is often em- ployed : this consists in heating the fuel sample to a red heat, in a closed vessel, with litharge or other source of oxygen. When lead oxide is thus used, the weight of lead reduced to THE FUELS AND THEIR COMBUSTION. I/I the metallic state is a measure of the oxygen absorbed. The method is simple and easy of practice, but is not sufficiently accurate to be generally approved. The value of h or of h' ranges between 5500 British or 1386 metric units for dry wood, and 16,000 or 4032 for the best known coals. The equation given is deduced from the experi- ments of MM. Fayre and Silbermann, who determined the total heat of combustion of one pound of pure carbon to be 14,500 British or 3654 metric thermal units, and of one pound of hydrogen to be 62,000 British units, or 15,624 calories. The combustion of one kilogramme of each would develop 31,967 British or 8080 metric units, and 136,686 British or 34,462 metric units, respectively. The combustion of the several kinds of carbon produces the development per unit of weight of : British Units. Metric Units. Material. 13,986. . . 7,770 .Diamond. 13,968 7,760 Iron Graphite. 14,040 7,800 Natural Graphite. 14,490 8,050 Gas Carbon. 14,500 8,080 Wood Charcoal. Where the chemical composition of the fuel is unknown and cannot be readily ascertained, its heating effect may be determined experimentally by burning a known weight and passing the products of combustion through a calorimeter of such area of heating-surface as to reduce their temperature very nearly to that of the atmosphere before discharging them. The table given hereafter exhibits the total heating effect of various fuels as estimated from analyses of good specimens. Where the heat produced is not so thoroughly utilized as to cause the condensation of vapors which may pass off with the permanent gases resulting from combustion, there is necessarily a greater loss of the heat of combustion of hydrogen than of that of carbon, and the relative heating efficiency of carbon is considerably increased by the facts that it must be raised to red heat as a solid before combustion can occur, and that the specific heat of carbonic acid (0.216) is only about one half that of aqueous vapor (0.475). I7 2 THE STEAM-BOILER. The general formulas, as given by Watts, for ascertaining the thermal effect of any fuel of a known composition are as follows : For combustion in oxygen : ~ s.3.6 7 C+ 9 H+s'W For combustion in air : T=- Here T= increase of temperature produced by combus- tion ; 7 and H == quantities of carbon and hydrogen available in I part by weight of the fuel ; W= total quantity of water yielded by I part by weight of the fuel ; / = latent heat of water ; s, s f , s", s'" specific heat of carbonic acid, water-vapor, nitrogen, and air ; c and c' = calorific power of carbon and hydrogen ; N '= quantity of nitrogen in air necessary for con- verting combustible constituents of I part by weight of fuel into carbonic acid and water ; A = extra quantity of air supplied for combustion. 76. The Temperature of the Fire depends, not solely on the amount of heat generated by combustion, but also on the quantity and nature of the resulting products of combustion. The total heat generated by the combustion of fuel is all communicated to the products of combustion, which are usu- ally gaseous, giving them a temperature which is determined, partly by the calorific power of the fuel, and partly by their nature. Thus, carbon requires for its combustion to carbonic THE FUELS AND THEIR COMBUSTION. 173 acid 2.67 times its weight of oxygen, producing 3.67 times its weight of carbonic acid. The heat generated by combustion of carbon is capable of raising 8080 times its weight of water from 4 to 5 C., and would raise the temperature of water equal in weight to the carbonic acid produced, about 2202 C.* i.e., 8080 X i = 2201. 63 X 3.67. But the specific heat, or capacity for heat, of water is greater than that of carbonic acid ; the increase of temperature in the carbonic acid produced is correspondingly greater than the rise in temperature that would be produced in a quantity of water equal to 3.67 times the weight of carbon burnt. The quantities of heat necessary to produce equal increase of tem- perature in equal weights of carbonic acid and of water being in the proportion of 0.2164 : i.oooo, the amount of heat needed to raise the temperature of 3.67 parts water and 3.67 parts car- bonic acid one degree, are as 3-67 3.67 X 0.2164 0.794* Hence the rise in temperature of the 3.67 parts of carbonic acid, to which the heat of combustion of i part carbon is trans- ferred, maybe calculated by dividing the given number of heat- units by the amount of heat required to raise the temperature of the 3.67" parts carbonic acid one degree, or . = ,8,345 F. The heat of combustion of hydrogen is sufficient to raise the temperature of 34,462 times its weight of water 4 to 5 Cent., but it requires for its combustion 8 times its weight of oxygen, and produces 9 times its weight of vapor. The prod- * Watts' Dictionary of Chemistry. 174 THE STEAM-BOILER. ucts of combustion weigh nearly 2-J- times as much as those of the combustion of an equal weight of carbon. Some of the heat produced by the combustion of hydrogen becomes latent and does not increase the temperature of the gases. The latent heat of water, or that needed to convert I part of water at 100 C. into steam, is 537 times as much as is needed to raise the temperature of an equal weight of water from 4 to 5 C., and 966.1 times the quantity which will raise the temperature of one part from 39. I to 40. I Fahrenheit. The quantity of heat latent in the 9 parts vapor produced by the combustion of hydrogen will therefore be 4833 metric heat- units ; this must be taken from the total amount of heat gen- erated in calculating the quantity of heat producing rise in temperature. Parts by Metric British weight of Heat- Heat- water vapor, units. units. Total heat of combustion of I part hydrogen 34, 462 62,000.0 Latent heat of water in heat-units. . gX 537= 4,833 9X966.1= 8,694.9 Available heat , 29,629 = 53,305.1 The specific heat of water vapor is 0.475 J the heat raising the temperature of 9 parts water and 9 parts water vapor have the proportion 9Xi 9 9 X 0.475 4-275' and the rise in temperature will be 29629 _ 4 2 75 Thus the heating and the calorific power are not necessarily the same. The heating effect depends only partly upon the calorific power of the fuel burnt. THE FUELS AND THEIR COMBUSTION. 175 RECAPITULATION. (WATTS.) Weight. Weight of Oxygen. Ratio. Weight of Products. Ratio. Heat- units. Ratio. Thermal Effect. Ratio Carbon. . . . I 2.67 I 3.67 I 8080 I .OOO 10176 1. 000 Hydrogen.. I 8 3 9.OO 2.4 34,462 4.265 6930.7 0.681 In these examples combustion takes place in oxygen, and with no more than is theoretically needed. In all actual cases of combustion, atmospheric air supplies the oxygen supporting the combustion. Nitrogen, of which it contains 77 per cent, dilutes the products of combustion and reduces the tempera- ture. In the case of combustion of carbon in air, the nitro- gen in air containing 2.67 parts of oxygen amounts to 8.94 by weight. The specific heat of nitrogen is 0.244, and the quantity of heat needed to raise the temperature of the nitrogen from 4 to 5 C. is: 8.94 X 0.244 = 2.181 units. Adding to this the heat needed to raise the temperature of the carbonic acid produced, the amount of heat needed to raise the temperature of all the products of combustion in air from 4 to 5 C. will be 2.181 +0.794 = 2.975 units. And the elevation of temperature will be ~ = 2715 C. = 4887 F. Burning hydrogen in air, the nitrogen in air containing 8 parts of oxygen is, by weight, 26.78 parts, and the amount of heat needed to raise its temperature from 4 to 5 C. is: 26.78 X 0.244 6.534 units, THE STEAM-BOILER. and the consequent rise in temperature will be S i = 2 74 J 4.275 + 6.534 '* C. - 4934 F. The difference between the temperatures attainable by the combustion of carbon and hydrogen in oxygen and in air is much the greatest with carbon, as the quantity of heat pro- duced by its combustion is much less than that generated by burning hydrogen, thus : RECAPITULATION. (WATTS.) Calorific Power. Ratio. TEMPERATURE PRODUCED. Dif- ference. Ratio. In Oxygen. Ratio. In Air. Ratio. Carbon 8.080 34-400 1. 000 4-265 10,174 6,930 1. 000 0.68] 2,715 2,741 1.002 I.OOQ 7,459 4,189 1. 000 0.561 Hydrogen . Thus in all cases where high temperatures are demanded, it is of advantage to increase the amount of oxygen in the air supporting combustion, and to restrict the influx of nitrogen and of superfluous air. Thus also the reason of the attainment of high temperatures by combustion in pure oxygen with the oxyhydrogen blow-pipe is readily seen. The quantity of air supplied is usually much greater than that simply required to furnish the oxygen to consume the combustible. In practice it often amounts to twice as much, and is rarely less than one and a quarter times the quantity theoretically needed, and there consequently follows a propor- tionate reduction of the temperature attainable. When carbon is burnt with twice as much air as is theoretically needed, the products of combustion have 24.22 times the weight of the car- bon, and with hydrogen 80.56 times the weight of the hydro- gen. THE FUELS AND THEIR COMBUSTION. AIR REQUIRED TO SUPPLY A DOUBLE AMOUNT OF OXYGEN. Parts by Weight ot Air. Volume of Air at 60 F. per Lb. of Fuel, Cubic Feet. Parts by Weight of Gaseous Products. Carbon I . . 23.22 303 . 39 -25.22 Hydrogen I 70 ^6 908 . 62 80.56 The specific heat of air is 0.2377, and the quantities of heat needed to raise the temperature of the air demanded from 4 to 5, and the temperature resulting from combustion are : Combustion of carbon : 2.7597= ii.6rx 0.2377, 8080 and -.= 1408 C. and 2.759 + 2.975 Combustion of hydrogen : 34.78 X 0.2377 = 8.2672, 29,629 8.2672 + io.8o93 It is evidently always desirable to secure perfect combustion, and with the least possible air-supply. With the forced draught produced by a fan or blast-pipe, fuel may be burnt with less air than with a chimney draught, and can be utilized with greater economy of heat. This economy is greater with fuel contain- ing but little volatilizable matter. Dissociation is a phenomenon which probably rarely if ever occurs in familiar practice. Oxygen and hydrogen, combined to form water, or steam, at ordinary furnace temperatures, are separated again by heat-energy when the temperature is some- where below 6000 Cent. (10,832 Fahr.). St. Claire Deville, the first to observe and study this phenomenon, concluded that dis- 12 1 78 THE STEAM-BOILER. sociation may commence at 1000 Cent. (1832 Fahr.) or below that heat.* Deville and Debray reported the temperature of the common oxyhydrogen flame to be not above 2500 Cent. (4532 Fahr.), and Bunsen found that under increasing pres- sures the temperature limit as fixed by dissociation was raised until, at ten atmospheres, it had increased ten per cent or more. 77. The Minimum Quantity of Air required for the per- fect combustion of any kind of fuel may be readily calculated from its known chemical constitution. . Calling the weight of air W, and denoting the weights of carbon, hydrogen, and oxygen, C, H, and O, (4) The value of Granges from 6 for dry wood, to 12 for an- thracite and good bituminous coal. Charcoal and the softer bituminous coals require about 1 1 parts by weight of air per I part of fuel. These values can only be approximated, in practice, with extremely slow and carefully managed combustion. A perfect intermixture of the combustible with the supporter of combus- tion can only be secured by the admission of some excess of air to the furnace. Probably about double the estimated amount of air is usually provided, although in some cases, where a forced draught produces exceptionally complete intermixture of the gases, the quantity may be brought as low as 18 pounds of air per pound of coal. In one instance, in which a furnace burning wet fuel was tested by the Author, to determine its economic efficiency, the quantity of air supplied was very little in excess of that dictated by theory. This was, however, an exceptional case. As the excess of air must be heated to the temperature of the chimney, and then thrown away, it causes a notable waste of heat. The weight of a cubic foot of air at mean atmospheric tem- perature being 0.076361 pound, the volume of air required for Archives des Sciences Physiques, 1860, t. ix., p. 51. THE FUELS AND THEIR COMBUSTION. 1 79 perfect combustion, in any case, may be determined by the equation : (5) Eighteen and twenty-four pounds of air, required, as stated above for combustion, in the case mentioned, of one pound of coal, would measure, respectively, 236 and 314 cubic feet. The weight of a cubic metre of air is 1.224 kilogrammes. The volume, in metric measures, required in any case is there- fore (6) When eighteen and twenty-four times the weight of fuel are required respectively, the volumes in the case taken would be 15 and 19 cubic metres. 78. The Temperature of the Products of Combustion may be calculated, as has been shown, with approximation to accuracy, from the known weight of the fuel and of the prod- ucts of combustion, the heat-generating power of the former, and the specific heat of the latter. The specific heat of the products of combustion are, at con- stant pressure, and for equal weights : SPECIFIC HEATS OF PRODUCTS OF COMBUSTION. (REGNAULT.) (Water = I. Pressure constant.') Air 0.2374 Oxygen 0.2175 Nitrogen . 0.2438 Steam o. 4805 Carbonic acid 0.2164 The proportions in which these substances occur in the prod- ucts of combustion being known, the mean specific heat of all may be determined ; and the total heat of combustion of one pound of fuel being divided by the product of this weight by 180 THE STEAM-BOILER. this mean specific heat, the quotient is the probable tempera- ture of the furnace gases. Rankine gives the result of this calculation, in cases where carbon alone is burned with undiluted air, and diluted with one half and with equal weight of additional air, respectively, 4580, 3215, and 2440 Fahr., equal to 2627, 1824, and 1338 Cent. Olefiant gas, similarly treated, should give temperatures of 5050, 35 15, and 2710 Fahr.; or 2788, 1953, and 1488 Cent The mean specific heat of the products of combustion is practically equal to the specific heat of air. The following are the specific heats given by Rankine : SPECIFIC HEAT UNDER CONSTANT PRESSURE. Carbonic-acid gas 0.217 Steam 0.475 Nitrogen, probably 0-245 Air 0.238 Ashes o . 200 Durham (British) coke, having the composition (Deering) of Carbon 93-78 Sulphur 0.82 Ash 5.40 Total 100.00 liberates 13,640 British thermal units per pound and requires 10.91 pounds of air per pound of fuel, for complete combustion, the heat produced being 1 145 units per pound, the resultant rise in temperature being 4877 F. (2709 C), and the amount of water evaporated, as a maximum, being 14.12 times the weight of the coke. The best bituminous coalcontains, as an example, Carbon 81 . 47 Hydrogen 4-97 Nitrogen i . 63 Oxygen 5.32 Sulphur 1 . 10 Ash 5.51 Total ........... 100 oo THE FUELS AND THEIR COMBUSTION. l8l Its complete combustion requires 10.99 times its weight of air, giving a rise of temperature of 4830 F. (2683 C.) and an evaporation of 14.64 times its weight of water from and at the boiling-point. The heat produced is 14,143 units per pound of fuel, or 1 1 8 per pound of furnace gases. Oak wood, according to Deering,* has the composition, when kiln-dried, Oxygen. . . . . 41.27 Hydrogen. 6.00 Nitrogen ...... 1.13 Carbon. 49-95 Ash 1.65 Total 100.00 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 j 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 100.00 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 olefiant 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 ot products of combustion in air, undiluted 13 Ibs. 16.43 Ibs. Their mean specific heat 0.237 0.257 Specific heat X weight 3.08 4.22 Elevation of temperature, if undiluted 4,580 5,050 If diluted with air = -J- air for combustion. Weight per lb. of fuel. .. 19. 24.2 Mean specific heat 0.237 0.25 Specific heat X weight 4.51 6.06 Elevation of temperature 3, 2I 5 3,5 I 5 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 -=- 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 531.6 = 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, i.oo 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, 544 Fahr., of chimney. Then, since the same quantity of gas passes at both places, the temperature of furnace was oT8S" X 47 / ~^~ 74 2II 9 Fahn To this is to be 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 r. . . 2550 " dazzling 2730 To determine temperature by fusion of metals, 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 1650 Silver, pure 1830 Gold coin 2156 Iron, cast, medium 2010 Steel 2550 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, W=2V7T-i, or 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 19^- 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 oT 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 r, - r; 186 THE STEAM-BOILER. in which E is the ratio of the heat rendered available to heat developed ; 7\, T^ 7" 3 , are the temperatures of furnace, of chimney, and of external air. For examples, in two actual cases, T lt T v T 3 , 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 2118 - 544 . 919 ~ 452 or for Centigrade degrees, 1176 - 302 510 - 251 -_ -0 = 0.77; and ^- - - = 0.56; 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 _ 2644- 74 or 1451 - 279 - 23 (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. l8/ 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. 8l. Economy in Combustion of Fuels, where they are 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 -J 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 2^3-, i.e., 521. 2 X 2 T ^ = 1085. 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. 190 THE STEAM-BOILER. Air excluded s i- -- H- ^ State of the o o o o S o c Flues. Air admitted J State of the Flues. ( Clear flame, \ \ 14 feet long. J 1 ' Crt g Ditto, 15 ft. long. N: ^ Ditto, 16 feet. & Ditto, 15 feet. Ditto, 14 feet, g; Ditto, 13 feet. Ct OS o ^ Ditto, 13 feet. o ^ Ditto, 15 feet. 03 ( Purple flame, | from carbonic co ( oxide. on Very black, ) \ ~~~*-~ "> ^ Much smoke ) \ \ Ditto \ \\ \ } Ditt Dark Eed . . . Dingv Eed . . . Ditto, no flame . Ditto \ \ Dark Eed . . . Dark / ; / P Ditto p Ditto ..... Ditto FUEL EVCLLf i Ditto \ ZL ELLED Dark Eed . . . Dark \ B Ditto / / ' S Ditto '.*. J / o * -i' r ^C- * S as g . s B" sl 8 1 S3 it S" 21! 21! ? CO O O fcO O O fcO CO J13 5v-3 pspnioxa .IIB -panunpB are uoid pio no '"Cl 11 Aau "O FIG. 69. TEMPERATURE OF FURNACE. Chester meeting. As seen by reference to Fig. 69, the tem- 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 41 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 9 2 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 when 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. N. S. Mois- ture. Ash. Spec. Grav. Pennsylvania Anthracite . . . 78 6 25 17 0.8 O 4 I 2 14 8 I .15. Rhode Island " 85.8 IO 5 3-7 85 Q2.O 6.0 2.O .78 North Carolina " 83 i 7 8 Q I Welsh " 84.2 a 7 2.^ O.Q O Q 1 . 3 6 7 4O Maryland Semi-Bituminous Penna " " 80.5 75 8 4-5 2.7^ 2O 2 I. I 1.2 1-7 8. 3 A O 33 32 /5-o CQ A 38 8 i 8 2Q Indiana " 70 o JU .V 28.0 2 o 24 < i * ( c 2 o TQ o 9O 27 Illinois Bituminous . . 62 6 -ic 5 .j I Q OQ " (Block) Bituminous Ill and Ind (Cannel) Bituminous 5&-2 CQ C 37-1 36 6 .... .... 4-7 3 Q 27 Kentucky DV 48.4 48.8 2.8 25 Tennessee Bituminous . . 71 O 17 O 12 O 45 41 5 56 5 2 C Alabama K I.O 42 6 I.O 1.2 1 .2 55 -O 41 .0 4.O 74 O 18 6 7 4 Cal. and Oregon Lignite 5O. I 3. 13. 7 0.9 1 .5 Tfi 7 13.2 1.32 THE FUELS AND THEIR COMBUSTION. 193 MONONGAHELA GAS COAL. (CRESSON.) Weight of sample, 60 Ibs. (27.27 kilogrammes). Volatile matter, per cent 35-74 Coke, per cent 64 . 26 Ash, per cent 6.66 Yield of gas, cubic feet per pound maximum 5.2 44 44 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 .o< " " i ton coal = Ibs. sperm 576.0 COMPOSITION OF FOREIGN COALS. C. H. N. O. S. Ash. Specific Gravity. Authority. Welsh (Anthracite) 00.4 3-3 0.8 3.0 O.g 1.6 1.32 Vaux. Scotch " English (Newcastle) 78.5 82.1 5-6 5.3 .0 4 9-7 5.7 I.I I .2 4.0 3-5 1.26 1.26 Muspratt. .1 (Lancashire) 77 Q CO . a Q. C I .4 1 ft 1 .27 u 14 (Derbyshire) . . 7Q 7 4.Q 4 10. 3 I .O 2.7 I .20 il " (Staffordshire) 78.6 K .1 8 12. Q 0.4 I.O Vaux. French Anthracite " Bituminous 94.0 84.0 1.4 5-0 0.6 i .0 "s'.o 4.0 2.0 1-33 Jacqueline. Ledieu. C.3.O 4 C .0 7-O Johnson. German (Silesia) . . . C7 Q 42 .0 2. I 1.26 Saxony 80.0 IG .0 I.O I. 20 Prussia *6 7 11 Q 24 4 I 47 Hindostan ... . CQ O qe 4 14.6 1 . 37 Brazil C7.O 4 C .5 r 6 1 .2Q D/'V 60. 7 2t ft 12.5 1.33 Cape Breton 67 6 26 Q c . e I 34 Australia (Lignite) 64 3 4.2 I.O A IO.O o 6 IO.O 1.27 Isherwood. Borneo . . . ... 7O 3 c .4 O. 7 IQ 2 I . 2 14 2 I 37 Muspratt Chili , 70 6 S.8 I .O 13 .2 2 .O 7.4 1 ,2Q Coke 9 r -5 i-5 7.0 13 I 9 4 THE STEAM-BOILER. COMPOSITION OF SUNDRY FUELS. C. H. N. O. S. Ash. Specific Gravity. Authority. Wood (kiln-dried) " (air-dried) Peat (kiln-dried) 50-5 40.4 60.0 46.1 24.8 52.2 50.3 71.8 24.4 14.0 86.0 86.5 75-0 85.7 O.I 4.9 6.8 4.6 0.9 1-3 I.O 40.7 32.7 30.0 23.6 1.6 1.2 I Q 0-5tO 1.2 Watts. Paul. (air-dried) i-5 2.8 0.3 O. I 1.5 7-6 13.6 0-5 Bitumen, United States. . . . " England . . Volatile Matter. . 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. . . 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. c O. 7 51.2 Ebelmen. Charcoal 0.8 14. I O.2 64.0 4 Peat 14.0 22-4 O.5 63.1 ' Coke i "\ n.8 O I 64 8 ii ' Lignite ... . 2 O 40 o 42.4 3-2 12.4 ' Bituminous Coal*.. 4.1 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. CALORIFIC : POWER. Water vaporized at Boiling- Cubic Feet required to stow Weight. Pounds per Cubic Relative. Absolute. Parts by one Part. of Furnace Coal. Foot as stowed. Carbon pure I .OOO 14 5OO 15 .OO Hydrogen 4.280 62 500 62.75 1.816 26,415 26.68 Olefia.nl gas i 466 21 ^28 21 . 54 Coal Anthracite i .020 14 833 14.08 40 to 45 40 to 56 " Bituminous I OI7 M7d6 14. QC 42 to 4.8 47 to ^^ " Lignite dry . . . O 7 10 150 IO. ^5 42 57 Peat kiln-dried o. 7 10 150 10.25 81 25 o. 526 7 650 7.73 75 3 \Vood kiln-dried O 5^1 8 O2Q $ IO " air-dried o 4^0 6 185 6.45 56 to 100 22 tO 40 Charcoal 0.030 i'? =;oo 14.00 Coke 0.040 13,620 14.00 56 to 75 30 to 40 Petroleum, heavy. W. Va. . . . light, W. Va ' '* Penna 1.250 1.260 .240 18,200 18,350 18,050 18.75 18.90 18.60 45 50 ' heavy Ohio 27O 18 4.50 IQ O5 ' Asia . .240 1 8 ooo 18.60 ' Europe .240 1 8 ooo 18.60 Shale Oil France (crude) 24O 1 8 ooo 18.60 Animal fat o 650 Q OOO Q. ^O The difference between theoretical and effective heating power for various kinds of fuel is exhibited in the following table, which gives the number of pounds of water evaporated by one pound of fuel, according to European authorities : HEATING POWER. FUEL. Theoretical. Under Steam Boilers. Under Open Boilers. Petroleum 16 ^o 10 o to 14 o Anthracite 12 45 7.0 to n.o Bituminous Coal II 51 52 to 80 5 2 Charcoal IO 77 6.0 to 6.75 a 7 Coke 9.0 to 10 8 5.0 to 8 .0 7- 7 2.5 to 5.5 1.5 to 2.3 Peat 55 to i.A. 25 to 50 1.7 tO 2.3 Wood 4.3 to 5.6 2.5 to 3.75 1.85 tO 2.1 Straw . q O I 86 to I 92 196 THE STEAM-BOILER. RELATIVE VALUE OF VARIOUS WOODS. (OVERMAN.)* WOOD. Specific Gravity. Pounds in one Cord. Per- centage Charcoal. Specific Gravity of Charcoal. Pounds of Charcoal in a Bush. Relative Value of Wood. Hickory, shell bark.. . . Oak chestnut 1. 000 o 8Sq 4.469 3QC C 26.22 22 7^ 0.625 o 481 32.89 2C 2 Poplar, yellow... 0.563 2,5l6 2I.8I 0.383 20.15 0.52 * Metallurgy. N. Y. : D. Appleton & Co., 1864. Wood cut in January contains from 15 to 25 per cent less water than after the sap is in motion in April. As wood seasons naturally in the air, it loses from one sixth to one third its weight of water, but still contains from one seventh to one fourth its weight of moisture. A considerable part of the latter may be expelled by kiln-drying, and most of it if the kiln heat be raised to 212. A cord of wood contains 128 cubic feet as it lies piled up. But allowing for the interstices in fairly piled wood, we may reckon a cord to actually contain about seventy-two cubic feet. Thoroughly dry wood weighs nearly as follows : One cubic foot. 62 53 49 45* 45 Hickory, pounds, White oak White ash Red oak White beech. . One cord. 4,464 3,816 3,528 3,276 2,240 THE FUELS AND THEIR COMBUSTION. 197 One cubic foot. One cord. Apple tree 43 3,096 Black birch 43 3,096 Black walnut 42$ 3,060 Hard maple 40 2,880 Soft maple 37 2,664 Wild cherry 37 2,664 White elm 36^ 2,628 Butternut 35i 2,556 Red cedar 35 2,520 Yellow pine 34 2,447 White birch 33 2,376 Chestnut 32 2. 304 White pine 26 1,872 With hickory at $5 a cord, other woods are worth about as below : Hickory $5 oo White oak 4 05 White ash , 3 85 Apple , 3 50 Red oak 4 45 White beech 3 25 Black walnut 3 25 Black birch 3 15 Hard maple 3 oo White elm 2 90 Red cedar 2 08 Wild cherry 2 75 Soft maple 2 70 Yellow pine 2 70 Chestnut 260 Butternut 2 55 White birch 2 40 White pine 2 10 Experiments on combustion, conducted by MM. Scheurer- Kestner and Meunier-Dollfus,* indicate that the method em- ployed for determining the heating power of fuel, from its analysis, is not correct. A satisfactory explanation of this difference has not been given. The heating effect may depend * Bulletin de la Socitte Industrielle de Mulhouse, 1868, 1869, 108 THE STEAM-BOILER. on the state in which the carbon exists in the coal ; and that although the calorific effect of the combustion of charcoal has been determined, it may be higher in the case of other forms of carbon.* Mr. G. H. Babcock gives the following tables as representa- tive of familiar practice : A T r HIGHEST AIR TEMPERATURE OF THEORETICAL ATTAINABLE K.E COMBUSTION. VALUE. VALUE UNDER QUIRED. BOILER. "o .c'E, i'S Cfl fcd k'g.o 6"o rt KIND OF COMBUSTIBLE. 11 O ^ |h f SI S^H l|| filL V ^V i o 1 .C ji Pg i=l 5^ 21 ii! HHU c-J pi! > *m ttzS c |u If 1^ 8 "I o S3 5fw .C.C ^= v.h r l^ 5H:i "" O rt o ^2 ,gfi .-s|^8 ^o<3 S" 5 ^ c ^ 5 c J Hydrogen 36.00 5,750 3,860 2,860 1 ,940 62,032 64.20 Petroleum 5,050 2,710 1,850 21,000 21.74 18.55 19.90 Carbon Charcoal ) Coke V AnthraciteC'l ) 12.13 4,580 3,215 2,440 1,650 14,500 15.00 13-30 14.14 Coal- Cumberland .. . Coking bitumi- 12.06 "73 4,900 3,360 3,520 2,550 2,680 1,730 1, 810 15.370 15,837 15.90 16.00 14.28 14-45 15.06 15-19 nous. Cannel 11.80 4,850 3,330 2,540 1,720 15,080 15.60 14.01 14.76 Lignite Q- 3^ 4.600 1 670 II 7^1^ 10 78 Peat ' , 90 tfWyv 1 1 ,/4j ' 1U. /O 4 Kiln-dried 7.68 4-47 3,140 2.420 1, 660 9,660 10.00 8.92 9.42 Air-dried, 25 p. c. water Wood 5-76 4,000 2,820 2,240 1,550 7,000 7-25 6 41 6.78 Kiln-dried 6.00 4,080 2,910 2,260 1,530 7,245 7-50! 6.64 7 02 Air-dried, 20 p.c. water ... 4.80 3,700 2,670 2,100 1,490 5,6oo 5.80 4.08 4-32 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 pia^. ..... . t 2,680 Spruce * 2,325 New Jersey pine 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. Anthracite 3-49 6.13 2.90 15.02 6.50 10.77 5-00 5-6o 9-50 2-75 2.OO 14.80 7.OO 5-20 5.60 5-50 2.50 . 5-66 6.00 13.98 5-oo 9-25 4-50 4-50 3-40 14,199 13,535 14,221 13,143 13,368 13,155 14,021 14-265 12.324 I4-39 1 15,198 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 n,55i 20, 746 14.70 14.01 14.72 13.60 13.84 13.62 I4-5I 14.76 12-75 14.89 16.76 13.84 9-65 13.48 13.58 13.10 14.38 14.64 I3-56 12.65 9-54 14.04 14-35 I3-4I 11.96 21.47 M Cannel Semi -bituminous Stone's Gas Kentucky .... . . .Brown . Caking M ,, Lignite Mercer County ,, . Montauk Indiana . .Block Caking 4< Cannel Maryland . . .Cumberland \rkansas Lififnite Colorado . . . . Texas < < Washington Ter. Pennsylvania M Petroleum 2OO 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 764 ' ' 73 per cent. 16.5 " 5-5 " 14,000 B. T. U. 100 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 1,750 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. Tolor of Ash. Silica. Alum- ina. Oxide Iron. Lime. Mag- nesia. Loss. Acids S.&P. Pennsylvania Anthracite Bituminous Welch Anthracite 5*9 372 Reddish Buff. Gray. 45 6 76.0 42.75 21 .OO 44-8 9-43 2.60 1.41 12. o-33 trace 0.48 0.40 2.97 Scotch Bituminous .26 37.6 52.O 3-7 i . i 5.02 ~TT 6 5.8 23.7 2.6 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 Department on American Coals. 2O2 THE 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 (100 X) = loo 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. (b) 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 o 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 THEIR COMBUSTION. 2O$ 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 o. 125 Total 0.190 leaving valuable available carbon 75 19 = 56 per cent. Finally, if $6.00 is paid for 72 percent 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 o. 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, $5.44^ 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. arc 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." Ameri- 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 COMBUSTION. 20$ 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 smokei 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; MEASUREMENT; TRANSFER. 2OQ 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, andi 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 180 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 pv teristic equation -= = 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 i 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 o II37Q acc. to Re i i i i i i i i jnault, o.i 100 ' 0.0927 0.0949 Zinc CooDcr . Brass . O OQ3QI 0.0557 ' 0.0293 0.0288 0.0507 ' 0.0514 ' 0.0314 ' 0.0298 ' 0.1880 t it t t Lead . . . . o 03140 Bismuth . . . . . . o 03084 Antimony . O O5O77 Tin . O.O5623 Platinum . . Gold O O324.4. Sulphur O 2O25Q Coal . . . . O.24III Coke . . . O.2O3O7 Graphite . . . ... o. 20187 Marble.. . 0.20080 HEAT PRODUCTION; MEASUREMENT; TRANSFER. 21$ 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; " " Zinc " " " 0.0927; " " Copper " " " 0.0947; Platinum " " " 0.0335; " " Glass ' " " 0.1770; " " 0.0350 0.1015 o. 1013 o 0355 0.190 Regnault found the ratio of the specific heats of the gases to be: ( i Constant /olume. ( 1 Constant 'ressure. Air . . . .^66^ 2670 Hydrogen .1667 }66l Nitrogen .3668 Carbonic Acid . . . 3688 1660 Carbonic Oxide. q667 37] Q Nitrous Oxide 1676 ?7IQ Cyanogen . . q82Q -3877 Sulphurous Acid . . . . .384-} 7QO7 A relation between the specific heat and the atomic weight originally established by Dulong and Petit, and confirmed by Regnault, 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$ 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^ ) Mean of Regnault " NO =44 " = 8.96 -j- 0.0028^) and Wiedemann. " C 2 S 4 =76 " = 10.62 -f 0.007/, Regnault. " NH 3 = 17 " = 8.51 -f- 0.00265^, 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 HEAT PRODUCTION; MEASUREMENT; TRANSFER. 2IJ 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. //7" 1 ductivity, k, and into , , the rate of variation of temperature with distance traversed, and area of section, A, dT 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 (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 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 ---- . ....... . ................................. 0.0090 Stone ........................................... 0.0716 Brick ............................................ o. 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. The surface resistance forms so large a part of the total in steam-boiler practice, that the formula 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. 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. 22O THE STEAM-BOILER. 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 coo'ing 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-\vater 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 /, 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 T v and T t are the initial and final temperatures of the gas, and / 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 ft = Cw(T, - t) : Q, = Cw(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 EFFICIENCY OF HEATING-SURFACE. 22$ The heat utilized, Cw( T, T^), is also equal to that ab- sorbed and transmitted, qdS: t -T^ and - = ~. (4) The value of q has been found to be well represented by equation (4) of article 95, in which q -=-, and hence sit (T TV q = - ~ ; and thus dT - Assume (T t) = x, then _ -t f> . aCw JT (T-tf ~ J ' T ' aCw TI t TI and the efficiency becomes T t -T, Then, since ; - / 5(7; - /) + i ^ _ aCw aCw 224 THE STEAM-BOILER. and ( 7; -o- (?;-/) 7;- 7; 7>__7; _ 5(7; -/) "" - If the total heat absorbed per hour be taken as //", _*\ T" 1 * / \ , /); T l t = - 7 ^-\ . . . (io> / 10t \ / and a simplified expression, H is obtained, in which Cw 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 E- ~ 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 F 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. 225 BOILER TYPE. A, B. Class I. Best convection, chimney draught 0.5 i.oo " 2. Ordinary " " " 0.5 0.90 3. Best " forced " 0.3 i.oo " 4. Ordinary " " " 0.3 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. B. Class i 0.5 0.90 2 0-5 0.80 3 0.3 090 4 0.3 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 0.16 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 o-53 I 0.66 0.61 0.77 0-73 0.60 0-55 0.70 0.66 0.80 o 71 0.65 o 81 0-77 0.64 0-59 0.73 o.6q 0.67 0-75 0.69 0.83 0.79 0.67 0.63 0-75 0.72 0.50 0.80 0-73 0.87 0.83 0.72 0.65 0.78 o-75 0.40 0.83 0.76 0.89 0.85 0-75 0.68 0.80 -77 0-333 0.86 0.80 0.90 0.86 0.77 0.72 0.81 0.78 o. 167 0-93 0.85 o-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 15 226 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 : E ~-~~ i +AR A 1 + S F~ B -^ i 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-surf ace 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=Cl*K-T t r ), (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 TJ the temperature of the water of condensation at its exit. As before, TidT Ti dT in which / becomes the temperature of the circulating or cool- ing water, while for such small differences of temperature we may take q = C( T t\ whence = MCw \og e -=! - : (3) 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 Industrielk , 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. 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 or prove his later conversion. 232 THE 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 work, " Sur 1'Influence des Chemins de Fer," 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 year's 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 the Infinite Power which has 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 o the "mass" of matter affected by the motion, Fthe velocity, and g the dynamic measure of gravity. The three great taws 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 m and in number n, pv = \rnnc* ; (i) 236 THE STEAM-BOILER. it is also known that ...... ,. (2) when ^? = -^, 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 ( 46i.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 o^ " 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 vacuo 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 d_ d_ Q _. ,, d~ dT T~' 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 =QlU = TdU\ . . . . , (2) which will become known when the law of variation of work, dUj 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. 1 10. 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. *-7=r = R, a constant, * * i the product of pressure and volume always varying with the absolute temperature. ill. 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- 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 rrse 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. 245 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 -j. the thermal equivalent of the mechanical unit, we may write at once, as the expression of the first law of thermodynamics, (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, by Joule's equivalent ; T the absolute temperature ; 5 the sensible and W^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, (2) and making the " internal energy," as it is sometimes called,, , the sum of the sensible heat and internal work, Or, otherwise exhibited, dS 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 : -KdT^-pdv; 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, (5) and, from the preceding, we thus find dH _ dH ~Tn^ A; j P \ ...... (6) dT dv * 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 Q 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 dT' dH= TdQ = KdT+ TdT; . . , . (2) and the form and value of the thermodynamic function be- comes at once determinable : 248 THE STEAM-BOILER. ~^r d= T Tf ......... CO 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 completed 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 : 2/2 THE STEAM-BOILER. TOTAL PRESSURES OF STEAM. CENTIGRADE. EXTERNAL PRESSURE. Ratio Total Pressure Ratio dp P-T^- t. T. A- At. dT' * dT' *' 100 374 760 I 2/.2OO 10146 13.3 120 394 1520 2 4S-595 19150 12.6 134 408 2280 3 67.020 27277 11-9 144 418 3-|0 4 34.345 35172 n.$ 152 426 3800 5 100.375 42659 I I . 2 159 433 4560 6 116.085 50149 II.O- 166 440 5320 7 133-445 58502 10.8. 171 445 6080 8 146.910 65228 10.7 .176 450 6840 9 ioi .27 72410 10.6. i So 454 7600 10 173.425 78561 10.4 199 473 II4OO 15 239-57 II3077 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 H dp Av = ^f + dT'' a- r 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 m easures are employed. t. T. A v Calculated. By Experiment. 117.17 124.17 128.41 137.46 144.74 398.17 402.41 411.46 418.74 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. 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 : TEMPERATURE. VOLUMES AS PER Cent. Fahr. Kopp. Poiter. 4 39. I .00000 . OOOOO 5 41 .0 .00001 .OOOO I 10 50 .0 .OOO25 .00025 20 68 .0 .00169 .00171 30 86 .0 .00423 .00425 40 104 .0 .00768 .00767 50 122 .O .01190 .01186 60 140 .0 .01672 .01678 70 158 .0 .02238 .02241 80 176 .O .02871 .02872 90 194 .0 03553 03570 IOO 212 .O .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 : ' /? C* A- - ; (I) - . * [(= * Steam-engine, p. 237, 206. Ibid., pp. 559-5^4- 18 2/4 THE STEAM-BOILER. in which, for steam, B A = 8.2591 ; ,= 0.003441 ; *, log B = 343 6 42 ; 7? 2 log C = 5-59 8 73 ; T^- 8 = 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 - and its functions ; thus, for vapors generally : (3) i dp nb ^ (a ;. ... (5) t dp nb p df T n = 2.30257*0 log/) (6) * Phil. Mag., April, 1886. STEAM AND ITS PROPERTIES. For steam, these formulas become : log/ = 7-5030--^; ..... (7) / 7579 \'\ ~ V- 5030 -log// i dp 21815 (7.5030 -log/) 1 ' 8 . . . 441.3 = 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 it 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. 276 THE STEAM-BOILER. found capable of being very accurately represented by the fol- lowing empirical formula of Rankine :* C i -f o.oooooo309(/ 39. i) 2 , . . . . (i) in which t is the temperature on the common Fahrenheit scale. The total heat demanded from /, to / would thus be h = Cdt = / 3 /, + o.oooooo 1 03 [(V 2 - 39. i) 3 and the mean specific heat for such a range of temperature is + (' 1 -39.i) 2 ].. (3) On the Centigrade scale these expressions become C=^i+o.oooooi(t 4)", ......... . (10) *=/,-/H-o.oaxxx>33[(',-4T-(',-4) 1 ], (*) ~j = i + 00000033K/. - 4) + ('. - 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 o.695(/ 32) o.ooooooio3(/ 39. i) 3 ; . (i) 4 = 606.5 0.695 1 0.000000333^ 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, / = 1092 - o.7(/ 32) = 966 o.7(/ 212) = 1147-0.7*. ........ (2) l m 606 0.7* ...... ... -(20) The total heat of evaporation is the sum of the latent and sensible heats, and may be taken as k = Cfe-ig + 4 = 1091.7 + o.305(f- 32); .... (3) h m = 606.5 +0.305*; ....... .(30) in which the " total heat " measured is that from , 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 + 0.3ft - 32) - ft - 32) ; = ii 4 6 + o.3ft-2i2)-ft-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.48ft - *,) ........ (5) Professor Unwin proposes the following for the latent heat of vaporization of steam : 4 = 799 ~ (7.5030 - log/) - 8 ' 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 S TEA M-B OILER. 139. Factors of Evaporation measure the relative amount of heat demanded to effect the heating of water from a given temperature, t lt and its vaporization at a higher temperature, t 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 966.1 -, nearly. . (i) The following are values of such factors, calculated as above : TABLE OF FACTORS OF EVAPORATION. INITIAL TEMPERATURE OF FEED-WATER, 7" a . FAHR. 32 50 68 86 I 4 22 ] 40 ] 58 I7 6 94 I2 212 .19 17 15 13 11 .10 .08 .06 .04 .02 .OO 230 .20 .18 .16 .14 12 .10 .08 .06 .04 .02 .01 248 .20 .18 16 .14 13 .11 .09 .07 .05 .03 .01 266 .21 .19 17 15 13 .11 .09 .07 .06 .04 .02 284 .21 .20 18 .16 *4 . 12 . IO .08 .06 .04 .02 302 .22 .20 18 .16 .12 .11 .09 .07 05 03 320 .22 .21 19 . 17 15 *3 . ii .09 .07 05 O3 338 23 .21 .19 I 7 15 14 .12 .10 .08 .06 .04 356 23 .22 .20 .18 16 .14 . 12 . IO .08 .06 .04 374 39 2 .24 .24 .22 23 .20 .21 .18 .19 17 15 13 .11 . II .09 .09 .07 .07 :3 410 25 2 3 .22 .20 18 !i6 .14 .12 . IO .08 .06 428 25 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. 2JQ ^ ptpTprSSS S8g?2 >< S 2^^^ v ^^^JT' m 22r:s 22 ^o < S c S- o ? ^ ? * {? TO N S ^m^g ^ 2 8 So? o??.^^ ^S^^P; m?TS Jr^ ^ 5 ^ gxff g K 8.^ S o m t.j o inir)-^--*m mp)pio oo OTO TO ti. tv.\o o ^> K s M 2 Q mmn m-<--.pit^ti vonvonin o*nono ^o-^omTO mTO mTO m PI PI M M o O o ooo TO t^ txvo voinin -^-^mmpi PIMWOO ^ TOt^t^vovotnin^ ^ td 8^ 7 H ' 1 Tf- MVCO^OQ lOQiTJO^t- ON^C> ^-00 rOOO COOO N t^Nl^WO VOMVOO lOO^O"^ O N C4 M M O O O> O^OO OOt^vOvOiOiO-^- ^J-rnrONN HwOOO* OOO OO^t^VOvOiOiO-^- -*f M ^vd H X c I h So ^*-oo moo moo moo N f>.cNts.Nvo MVOMVOO lootoc^" o^-o -^-oo moo moo m oo M M o O ON ooo ootxt^vovomm^ ^mm J9 ^ 4 H td o o C 1 ^ 4 M S W i M HOOOOOOOOt^. t^-VO miOTt-^m mNNww OOO ^OO OO I s -* t*xVO 'O iri ir> ^ -^- m m H 1 2, U M < X jn o s f j'&ss SSRSS fia?^ Ss2ss ??H ??f^ ????? 1 H D s < s 1 ^^ cS s.\s 'ft^iK mj?g yy ? ; jff ????? 1? Is 5 ? s Id w H O 9 osvrjMinnto O>oOioo ^o^'Omoo rooo mt^ ctt^wi^M vo>-'VO )- io O^Ou^O 10 n o o o> ooo oo hs, t^.\o 10 m-^-^t-mm N N -< O OO ooo oo t^ t^.vo *O"^ to^-^hmm w O ^ N ^8 ^o^-S f^^vS^ S.^^8, J?8^2? |^o?^ glS^nR JlgU - 2 H 3- 5^o^ R&^S bias's S^ra 8 ;o^^ ^.S^Pn Jlgg.? J *O fl Si i o 'S:: g J rt g^ jjaa* swas MS &ciw wasi* II i j s, ffi^uas^^aic^ ^asss- s^ss-a sisas^^ais^ ^aiss? 2 B 55 280 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-fff = 0.86, 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 tlie 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 =*+&*', ....... (i) * Ann. de Chimie et de Physique., July, 1844 ; Mem. de 1'Institut, tome xxi., p. 465 (1847) ; Mem. de 1' Academic des Sciences, xxi. xxvi. \ Vide Dixon on Heat, vol. i.. 724. STEAM AND ITS PROPERTIES. 28 I in which F is the pressure, a arid b constants, and a r a function of T t -\- 32, t being the temperature corresponding to F. Between the freezing and boiling points Regnault used Biot's formula, logF=a + &a t c0 t ; (2) and above the boiling-point, log F = a bo? cft T ; (3) in which T = / + 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 ft = 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 log c = 0.139 553 958 4 For British measures, a = 4.859 984 524 7 log <* = 1.999 079 751 3 log/? = i". 99 6 693 778 3 log = 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. The formulas used in these calculations are elsewhere given, but are here grouped for convenience of reference. British measures are used throughout. 77-5 PROPERTIES. 28 3 -t- i fc 5 % *t- c c. S d 1 | Si 1 n I 5 $ 6 ! _ s 1 h i \ 1 ** s ?. cT r n "ro (t N 8 J R ^ ? R 1 -f | N ' o" 1 fc t H _3 j 1 fi cJ 0i d ^" ^- ^s i 1 d ^ 1 N* od X X X 1 I g g X S II i 1 i ft, ft, ft. ft, EN, O T- l O n S | 1 JJ- ^ ft. < >s II II II II || M ^ + s S j ifi i ^ ^ ^ '0 1. m H } V? M od 1 d si ^ ? t- -t- V II 1 ^ > < ^VM*^ M | ^ + 5 ft, i 1 II 11 ^ K, ^ ft, - tt h ^ s ^ 1 bx a; Q ctf c o " 3 5 i % X ja IN c: $ V 2 .S rt c u 1 QUANTITY. Pounds per square inch. 1 u 1 L, CO 1 Inches of mercury, at 32 Fahr. Feet of distilled water, at temperature of r density. Atmospheres. Above the atmosphere, in pounds per square Fahrenheit's scales. 1 o '5 a u 1 3 1 Required to raise the temperature of the w 32 to t. Required to change the water into steam, latent heat.) Required to overcome the pressure of the su medium. (External latent heat.) c a u 4> 1 g i a A > cu "o I J Total heat of evaporation above 32' 'otal heat of evaporation per pound of steam, a in units of evaporation. A IV snun inuuaq 1 qsnuq t- 1 ui iuB3;s jo punod jaj I 8 3 i >> I jl H a 284 THE STEAM-BOILER. t | ^ i o'% ^ gta ^ O o* H H 2 8^ i j f Jx Si S, g. vS llll s B CM M It ^ & II X 8 ' "8 5 ? i X II | II <*> li ll[-? 1 X ^ H -ll 1 1 9 o 1 ei O [ ^ + II 8 s *Xi 8 r O ^ 2 o w u u <* R & o s^ m s^ S i> {j J^ fe H S . P t/) w (X s cu i 1 I 1 CU to rt 5 > w' g 1 a w "o . || 3 o o 1 i ^o 3 3 tJ C FORMULAS RELATI: QUANTITY. t heat of evaporation, per cu a cubic foot of steam, in pou of distilled water, in pounds, f a pound of steam, in cubic I of steam to volume of equal at temperature of maximum of distilled water, at tempen :ight at temperature of maxii c i Q I ^ .2 e o o -I 11 3 > ** s* i 3 o *o^- M o .2 s i 1 o 1 3 r^ a M g "o o ^ > fe STEAM AND ITS PROPERTIES. 285 142. 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 : _ _ 423.55(^-100)' T+ 1 134.4 r+6 4 8 _ 3676(^-212) 2.29(^-100) T+ 11344 T+6 4 8 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 Jt to a final absolute temperature, T 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, log + -#. . . (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, U= MJT& - i - hyp log ^) + m ^^H. (B) X -L J- ' J- 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 mff, 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 1 contains 116 times as much energy in the form of latent heat per 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 100 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 800 1500* 1100* 300' ?0tf \ 1100 000 x? axf 500 00 1WW 20UO 8000 4000 6000 7000 8000 9000 10000 11000 12000 FIG. 71. CURVE OF HEAT IN 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- iq 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- 875000 300099 1 790001 raooo 128000 w 200000 O 75000 25000 cool 1000 2000 8000 4000 5000 6000 7000 8000 9000 ABSOLUTE PRESSURE IN FOOT POUNDS PER. SQ. IN. FIG. 7 2 CURVE OF HEAT-ENERGY IN STEAM. 10000 11000 UOQO 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! 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 st^am, 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 : r .37484 in which V is the volume of steam in cubic feet, / 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., ^ 2 - 65 lbs - 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,- Ze-uner's f figures are as below : * " Boiler-power and Boiler-heating Surface," by Professor R. H. Smith, In- dustries, July i, 1887. f War me Theorie. STEAM AND ITS PROPERTIES. 2 95 WATER PER HORSE-POWER PER HOUR. PRESSURE ATMOSPHERES. Non-condensing Engine. Condensing Engine. Lbs. Kilogs. Lbs. Kilogs. 3 33 1 51 13 6 4 26 12 12 51 5 23 io Hi 5i 6 21 9* II 5 8 18 8 10* 4f 10 idi 7i 10 4i 1 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 and the power H.P. = --,55* 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.. VELOCITIES PER SECOND. Metres. Feet. 185 607 208 68l 10. 227 230 255 260 734 775 835 879 296 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 ATMOSPHERES. H. P. = ^ 1 1 2W j 1407^ 4. c 1657*!' 6 I847C' 8 2OI7' 10. . 2MW m Ibs. 5i7c/w in kilos. 6 4 7C', 75WW 842001 9 1 7c'; 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 HLfc-.4/S3; 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 H. P. = X 3000 X V'7S = 5IQ . 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 Pounds of Water per effective Horse- power per Hour. 75 15 III 70 71 20 105 80 68 25 100 90 65 30 93 100 63 40 84 120 61 50 79 ISO 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 : 2Q8 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 8 120 25 24 23 22 22 21 10 150 24 23 22 21 20 20 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 1 20 25 23 23 22 21 2O 10 150 25 23 22 21 20 9 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 yd 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. 301 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 with 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 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- 7' 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 t 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. 307 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 308 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 iiy 4 ^ 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. STEAM. COAL. BOILER Per I. H. P. per hour. Per I. H. P. per hour. PRESSURE. TEMPERATURE. Per Gauge. Non-con- Con- Non-con- Con- densing. densing. densing. densing. Lbs. Atmos. Fahr. Cent. Lbs. Kil. Lbs. Kil. Lbs. Kil. Lbs. Kil. 300 20 421.7 216.5 10.48 4.8 6.16 2.7 0.98 .44 .64 .29 250 i6 405.9 207.7 11.19 5-i 6.39 2.9 04 -45 .66 .30 200 13^ 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 * .18 .54 .71 .32 150 10 365.6 185.3 13.63 6.2 7.09 3.2 -25 -57 73 -33 125 8 352 6 167.0 14.71 6.7 7-37 3-8 35 - 6o 75 -34 100 6 337- 6 J 59- 8 16.24 7-4 7-7 1 3-5 .48 .67 78 -35 90 6 330.9 166.1 I7-05 7-7 7.89 3.6 55 -7 .80 .36 80 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 3*5-7 W- 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 33 297-5 M7-5 22.95 10.4 8.92 4.1 .07 .90 .90 .40 45 3 292.2 144.5 24-53 "- 1 9.11 4 i .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 3io 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 1 1 \ pounds of water with an effi- ciency of 75 per cent. An engine working perfectly would develop one indicated horse-power with 7f 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 pressure above atmosphere. Lbs. Mean effective pressure. Lbs. Feed-water consumed per I. H. P. per hour. Lbs. Initial pressure above atmosphere. Lbs. Mean effective pressure. Lbs. Feed-water consumed per I. H. P. per hour. Lbs. AT 10 PER CENT CUT-OFF. AT 30 PER CENT CUT-OFF. 40 5 60 70 80 90 IOO 1.32 5.01 8.70 12.39 16.07 19.76 23-45 153-24 52-52 37.26 30-99 27.61 25-43 23.90 40 50 60 7 80 90 IOO 16.95 23-71 30-47 37 21 43-97 50-73 57-49 33-52 29-35 27.24 25-76 24.71 23.91 23.27 AT 20 PER CENT CUT-OFF. AT 40 PER CENT CUT-OFF. 40 50 60 70 80 90 IOO IO. 22 IS-G? 21 . 12 26.57 32.O2 37-47 42.92 38.13 30.98 27-55 25-44 24.04 23.00 22.25 40 5 60 70 80 90 IOO 22.24 29.99 *7-75 45-50 53-25 61.01 68.76 32.79 29.72 27.92 26.26 25.76 25.03 24.47 AT 50 PER CENT CUT-OFF. 40 26.40 33-i6 80 60.44 26.99 50 34.91 3-53 90 68.96 26.32 60 43-42 28.94 IOO 77.48 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 5 29.28 18.74 60 14.42 18.22 60 34-73 18.98 70 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 18.14 100 24.58 17.41 100 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 50 18.65 17.91 50 37-3 20.35 60 22.34 17.68 60 44.06 20.19 70 26.03 17-47 70 50.81 20.04 80 29 72 I 7-3 80 57-57 19.91 90 33 4i 17-15 90 64.32 19.78 100 37 -!0 17.02 IOO 71.08 19.67 AT 15 PER CENT CUT-OFF. AT 40 PER CENT CUT-OFF. 40 19.72 I8. 4 I 4 3^.84 21.94 50 24.36 i8.ii 50 43-59 21.76 60 29.00 17-93 60 51.35 21 .63 70 33-65 70 59 - I0 21.49 80 38.28 17.60 80 66.85 21.36 90 42.92 17.45 90 74.60 21 .24 100 17-32 100 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 once 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 E, the ratio of heating-sur- face divided by fuel burned, - = R, will be obtained thus : wj ........ <* Taking as common values E = 0.70, A = 0.5, B = i, *-%-'?-? -.- and the ratio of heating to grate-surface would be S = -- ; if o.So F 15, 5 = 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 L, 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, $14 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 \vould be the only form to be wisely adopted in such locations. Shell-boilers should usually be placed in detached boiler-houses, and so set, as 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 a = 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 i,/ = 3 inches of water = 16 pounds per square foot. 16 X 200 X i X 532.2 H. P. = ~ = 20 nearly. 33,ocx) 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,f 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, 18760 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 c-f very much greater volume. The principle of forced circulation has not often been employed for this purpose, but there is reason to believe that 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 15 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 of gas delivered is a maximum. This is about 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 12^ cubic feet per pound, nearly, THE DESIGN OF THE STEAM-BOILER. 319 or about three fourths of a kilogram to the cubic metre. Adopting British measures, if Fbe the volume per pound at T, absolute, Fahrenheit degrees, V Fl 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.) T AIR-SUPPLY IN POUNDS PER POUND OF FUEL. 12 18 24 4640 1551 3275 1136 1704 . . 2500 906 1359 1812 1832 697 1046 1395 1472 588 882 1176 1112 479 718 957 752 369 553 738 572 314 47i 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 ; 7 1 ,, 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 - ^- (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 0.080; pound, and the weight, on the assumption of uniform mean density of air and gases, is, at J" , 0.0807 F -(-i, and its mean density is .and at r. (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 or, expressed in inches of water, / = O .i 9 / == 0.19^(0.080; + Y)> (5) The loss of head, as found by Peclet,* may be expressed by the equation in which / is the total length of flue from boiler 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', is given we obtain ~y& . L 13 * Trait6 de la Chaleur, vol. i. THE DESIGN OF THE STEAM-BOILER. 321 and the weight of gas discharged must be vA T 7*! being the temperature of flue. The head, /z, 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 T t is the temperature of the latter, 0.0807+- * H = h -r- (0.96^- - l) ...... (10) The velocity of flow is measured by a Vh, a being a con- stant to be found by experiment, or by (11) varying as the quantity |/f 0.96 ^ -- ij; while the density varies as I -=- T lt and the weight flowing per second varies as the product of velocity and density, or as--y (0.96 7^ T 9 ). L \ This becomes a maximum, T l varying, as first indicated by Peclet,* when , Vo.967; - T, du T, 2T, and * Peclet, vol. i. p. 166. 21 322 7^HE STEAM-BOILED. or, as Rankine states it,* 7^ -f- T^ = f f, 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, as c.lready stated, ordinarily about 550 Fahr. (288 Cent.) above that of the latter, and their density is about one half that of the atmosphere, and the volume discharged about twenty-six feet per pound. 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 with 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 : 0. HP . .... . . (i) _ . . . ... . (2) *= 12*^+4; ....... (3) d= 13.541/^ + 4; ...... (4) * Steam-engine, p. 289. f 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. sj HEIGHT OF CHIMNEYS, AND COMMERCIAL HORSE-POWER. ? 4J 51 5 oft 60 ft 70 ft. Soft. 90 ft. 100 ft. 1 10 ft. 125 ft. 150 ft. 175 ft. 200 ft. Eg." C/3 .S ^"i-l^ 18 23 25 38 2 7 16 0.97 1.77 24 54 S8 62 22 2.08 - 3.14 27 6" 72 78 83 2 78 i n8 3 84 92 100 107 "3 27 3.58 4.91 P "5 141 125 ^63 141 I7 2 3 32 4.48 5-47 5-94 7.07 182 .. 39 i8i IQ^ 208 219 35 6-57 8.30 42 216 231 245 258 271 .. .. 38 7.76 9.62 48 3 11 348 3^5 39 43 10.44 12 57 427 536 449 472 593 503 632 692 74 8 54 16^98 15.90 19.64 66 694 728 776 849 918 98T 59 20.83 23.76 72 835 876 934 1023 1105 1181 64 25.08 28.27 78 1038 1107 1212 I3IO 1400 70 29-73 33-i8 84 1214 1294 I 4 l8 1531 1637 75 34.76 38.48 90 1496 1639 1770 '893 80 40.19 44-18 96 l8 7 6 2027 2167 86 46.01 50.27 The external diameter at the base should be one tenth the height, unless it be supported by some other structure. The " batter" or taper of a chimney should be from T 3 ^ to J inch to the foot on each side. The thickness of brick-work should be, usually, one brick (8 or 9 inches) for 25 feet from the top, increasing \ brick (4 or 4J inches) for each 25 feet from the top downwards. If the inside diameter exceed 5 feet the top length should be i bricks, and if under 3 feet it may be \ brick for ten feet. To find the maximum draught for any given chimney, the heated column being 612 F., and the external air 62 : Multiply the height above grate in feet by .0075, and the product is the draught-power in inches of water. For natural draught it is found that the weight in pounds of anthracite coal which can be burned on the square foot of grate per hour is, as a maximum, for example, under the best conditions in marine boilers, *" Steam," 1885. 324 THE STEAM-BOILER. F=2 V7f I, nearly; ..... (6) * and, under more ordinary conditions, P=!. S Vff-i ........ (7) From this we obtain the following : HEIGHTS OF CHIMNEY AND RATES OF COMBUSTION. Chimney-section = \ to grate-area. Fuel, Anthracite. Best Conditions. 4 H = height of chimney in feet ; W= weight of coal burned per square foot of grate per hour. Thus for H= 50, W=is; ff= 65, W=i$; H= 80, W= 17; H= 100, F= 19. These figures represent very exactly the results of Isher- wood's experiments* with anthracite coals. The best Welsh and Maryland semi-anthracites, or good bituminous and semi-bituminous coals, should give, as maxima, F= 2.25 and the less valuable soft coals, more nearly Thus, average coals of each quality stand, relatively, nearly as follows : Weight per Sq. Area Grate Foot Grate. per Pound. Good anthracite coals i.oo i.o " semi-anthracite and bitum. 1.05 0.9 Ordinary low-grade coals, soft 1.5 0.7 " " " anthracite 0.9 i.i * Trowbridge, Heat and Heat-engines, N. Y., 1874; Isherwood, Researches in Engineering. THE DESIGN OF THE STEAM-BOILER. 325 Some of the soft coals will burn still more freely, while some anthracites will burn even less rapidly than above stated. The figures given may be taken as fair averages. The height of chimney being known in advance or settled upon, the total quantity of fuel to be burned determines the area of grate. This total quantity is known from the chemical constitution of the fuel, or by experiment under denned conditions, and from the work demanded and the intended efficiency of the boiler, as estimated by the methods already described. Mr. Lowe, a builder of large experience, finds the following good proportions* for stationary boilers, presumably allowing about 30 pounds of water per hour, and 15 square feet of heating-surface per horse-power : STEAM-BOILER CHIMNEYS. Heights in feet ....................... 50 60 70 80 90 100 Sq. in. area per H.P .................... 9 8.67 8.34 8.01 7.68 7.35 Heights in feet .............. ......... no 120 130 140 150 Sq. in. area per H.P .................... 7.02 6.69 6.36 6.03 5.70 Heights in feet ....................... 160 170 180 190 200 Sq. in. area per H.P .................... 5.37 5.04 4.71 4.38 4.05 Professor C. A. Smith f gives the following formulas for the relation of height of chimney to fuel consumption : - 4/77' -77- where If is the height of chimney in feet, A its flue-section in square feet, and ^Pthe pounds of coal burned per minute. Mr. J. T. HenthornJ gives the following tables of dimen- sions of chimney as obtained by the empirical formula: I2 G - A- in which he takes the area, G, of grate in square feet, the height, H, in feet, and the area of flue, A, in inches. It is * Am. Machinist, March 27, 1886. \Am. Engineer, Sept. 21, 1883, p. 123. \ Journal of Commerce, July 5, 1884. 326 THE STEAM-BOILER. .91BJ3 JO nf) ?? oK8c?aK8c?^K8^^K8 ^5,^8^5,^80? 5,^8 J?S>K8,??,1C8 H 1 O ^ ^ - oo to o (^ ^ M o^o ro o tx 10 c^ oo ro o oo^ in M o^o roiotvOcNLOt^-OCN -^-O O i- -^-"O oo ro 1000 O ro 10 t^- w ^t- 9 w ro *noo n moo N mONvo orotNO TJ-INM oo H . rooo ^o-^-O u" vo HI INN IN rooo roo^oinG innivo 04 INNOO rooo *^- . . . IN Nrorororo^^-^^.o^ioinvovovovotNtNiN tNOO 000000 0000 O" 3 : ? Hlf SvSo? H- OOO vo -r ro - OOO vo <* ro M o-oo vo * ro HI cfw N*cTror? ; ro4-4''<}-4inioio invo'vcTvo" vo' tC t^. t^.oo' oo oc' oo' o o o o o" 6 o" s 1J Ovo rr^Htoovo roOoo inN O iNinN otN-^N ovo ^-MOOVO rOHOo inroooo in IO O OOO OO ! i o? M rose C m C ro H? o t^ En^ro^S ^^R ininininTj-Tj-^-^-rorororON N N 01 M M M M o O O O O N^ in IN q ^.vq_ ON M -^ tN O. ro inoo * ?l o N" Snco" H rovo" O c? In ^ o" rovo^ o nT oo "- $ r^ O ro*o O CJ 1000 O roo o N moo w -rt- r^ o ro*O O i- TJ- t-x o roo o - - N O HI N ro rr rovo O PI " 3 invO tN tNOO OOO^NrO 1 ^"^ invO tN t^oO OO'-'^NrO 1 ^-. H ro -^-O OC co-0 ON"; ? ro 0*0 Noo-^OtNroou'iNoo^-OtNrooinM HI N ^j-vo INO- N Ti-intNOO N ^-m tNoo ON N'NN rororo4'-oio invo'vcTvo tC . '. . '. '. '. '. '. '. '. '. '. '. '. '. '. '. . I eg _.rS5 H ^^44^^ :::::::::::::::::::: R -*j- o rooo r^ ro^O ro tN | cTt-xt-tvO ~ inQ ^-O ^-OO ofwr^cOfOTf^-^iOiniri III. II. . I I . ! . a oo IN in * N M ro tN(? S H? u"ioo N u"> O rovO Orot^* ... . >o ro ^ "__ ao * -1-co S ^^?^?t~8^ :::::::::::::::::;:: : : vS SSI 1 ^InliH : : :::::::::::::::::::::[ iH vieiir 8^8^8^8^J^8^8* ^8^ Sg M THE DESIGN OF THE STEAM-BOILER. 1 c c' c' c* c' G c' c' , G ^ * J O O ro'ro'l^ t^O to l^. "w CO 10 O tovo 10 (N 1000 vo N t- t-^vo aw ro N oo CON ro M ro 11 ro ; rovo vo 00 vo $ S c' d G' G' c' G' G' "-" *-J *J G' >.; c" ,*j G' .jj G' 4.5 G' ^ 1 3t3*3?3t*3CR 5t 3? ^COVOVO^COH H VO CO. M : *- Tl 8 c' c c G' c G" c' c itvi il S NOVO ON OOO. | ^00 io 10 ro H . c* c' c c c' c _G _a i^d OO"l>OOC>w>O IO r<1 HON 000 N O 00 00 * ^^^"^ ^^ 3t 3s HEIGHT IN FEET. M : I = 3 ': : :'^ -a 2 : i'o" : a : |l||!!|lll^ | '1=^=1;= ESil^lSMil I ^ :ig ;i [|lill|i|iH|! js|l i||gj | i 1 i 3 : ! | 1 8 :t ' 8 : '5 :< sS.i] ;a w e O ; i 1 i 2 : 111 3 o '* \t\l !a| : w 2x ^ffi f- S ''$' '-2 3 ^llll 5 -^ .-4 g g :8 IS. 2 y ' w I < 3 fc 1 CO 5 f Q 3 Q 'v K H H 3 Q 'C I !. ^ I 15 36 8' ii 30 3 8 20 20 H % 3 18 26 2,950 5,350 2 20 36 10' ii 30 3 10 20 20 /4 % 3^ 18 30 3,500 5.900 3 25 42 1 1' 38 3 10 24 24 ?% % 3/^ 20 30 4,400 7.100 4 5 3 35 42 44 $ 38 46 3 3 12 12 24 2 4 11 % 4 4 2O 22 36 36 5,000 5,500 7,800 8,700 6 40 48 13' 2 52 3 12 24 28 % 24 36 6,400 9,900 7 45 50 14 2 52 3 13 30 3 % 2 4 36 6.800 10,400 8 50 54 13' 2 58 3 12 3 30 % 26 36 7,600 11.500 9 60 54 16' 2 58 3 15 30 30 % < ^3 26 45 8,550 12.750 10 70 60 J 5' 4 76 3 30 30 T ? B ' /^ 28 45 10,000 14,500 ii 75 60 16' 4 76 3 15 30 30 T ? B ' ^2 28 5 10.500 15.100 12 80 60 *7' 4 76 3 16 30 36 TB 5 28 55 n,2oo 1 6, TOO 13 90 66 16' 5 100 3 15 36 36 J^ 5 32 55 13,500 19.100 100 66 J 7' 5 100 3 16 36 36 ^B 5 32 55 14,200 19,800 15 125 7 2 17' 6 132 3 16 3 6 3 6 ? 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:" " 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 freedo'm 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. 22 33 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 S 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 10 5 10.1 9.5 8.2 7.3 6.8 which series is represented by W ^ = , nearly. vr 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 So 90 100 the evaporation being constant at 9 of water to I of fuel, which may be expressed by S = SVF, 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 Quantity 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, and 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 340 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 15 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 4 6 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 I J inch (25.4 to 3 1 mm.) diameter in the smallest boilers, to 2 or 2-J- 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 I.I06 0.0922 0.3273 i-5 0.083 1-334 O.III2 0.3926 i-75 0.095 1.560 O.I3OO 0.4589 2. 0.095 1.810 o. 1508 0.5236 2.25 0.095 2.060 0.1717 0.5890 2-5 o. 109 2.282 0.1902 0-6545 2-75 0.109 2.532 0.2IIO 0.7200 3- 0.109 2.782 0.2318 0.7853 3-25 O. 1 2O 3.010 0.2508 0.8508 3-5 O. I2O 3.260 0.2717 0.9163 3-75 O. T2O 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- O.lSo 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. 1 i j. I i 1 a 21 C g |-a J^ . Si iS V ^ c . N K-3 n\ C rt "rt Sj g o-S CO s Ii X ." O S 6 & i if |i u c S.T S-I ff .2 * rt c ft Si H 1 3 K 3.S * c > " c bt-5 S " t o ' 1 H H 1 J^ ^ * In. In. In. No. In. In. Sq. in. Sq. in. Sq. in. Feet. Feet. Lbs. .86 .072 15 3-H 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 i. ii .072 15 3-93 3-47 1.23 .96 27 3-o6 3-45 .89 32 375 I. 21 -083 -083 14 4.12 4 32 3-6 3-8 1:3 1.03 i-i5 3 2 34 2.91 2.78 3-33 3.16 .08 '3 5 J -33 .083 14 4-71 4.19 1.77 1.4 37 2-55 .86 .24 625 1.43 095 13 4-5 1 2.07 1.62 .46 2-35 .66 53 75 1.56 95 13 5-5 4-9 2-4 1.91 49 2.18 45 .66 875 1.68 095 13 5-89 5- 2 9 2.76 2.23 53 2.04 27 .78 1.81 095 13 6.28 5-69 3 14 2 57 57 1.91 . ii .91 125 1.93 95 13 6.68 6.08 3-55 2-94 .61 1.8 97 .04 25 375 2.06 2.16 095 .109 12 7.07 7.46 6.47 6.78 3-98 4-43 iS .64 78 i!6i 85 77 .16 .61 5 2.28 .109 12 7-85 7.17 4.91 4.09 .82 i-53 .67 75 75 2-53 .109 12 8.64 7-95 5-94 5-03 9 1 39 51 3-04 875 2.66 .109 12 9-03 8-35 6.49 5-54 95 44 3- 2.78 .109 12 9.42 8.74 7 07 6.08 99 1.27 37 3-33 3-25 3.01 .12 II 10.21 9.46 8 3 7.12 1.18 1.17 .26 3-96 3-5 3.26 .12 II II. 10.24 9.62 8-35 1.27 1.09 17 4.28 3-75 3-5i . 12 II 11.78 ii .03 ii .04 9.68 '37 i .02 .09 4.6 4- 3-73 134 IO 12.57 ii 72 I2 -57 10.94 1.63 95 .02 5-47 4-25 3'98 134 10 13-35 12.51 14.19 12.45 t-73 9 .96 5.82 4-5 4-28 10 14.14 13.20 15-9 14.07 1.84 85 9 6.17 4-75 4.48 134 10 14.92 14.08 17.72 15.78 1.94 .8 85 6.53 5- 4-7 .148 9 1 5-7 I 14.78 19.63 17-38 2.26 .76 .81 7.58 5-25 4 95 .148 9 16.49 15-56 21.65 19.27 2,37 73 77 7 97 5-5 5-2 .148 9 17.28 16.35 23.76 2[.2 7 2-49 7 73 8.36 6. -l6 5 8 18.85 17.81 28.27 25.25 3-02 .64 67 10. 16 8.' 6'.6 ? 7 7.67 .I6 5 .165 8 8 21.99 20.95 24.1 38.48 50.27 34-94 46.2 1:3 55 ,48 S7 5 11.9 13-65 9- 8.64 .18 7 28.27 27.14 63.62 58.63 4-99 .42 44 16.76 0. 9-59 203 6 31.42 30.M 78.54 72.29 6.25 38 4 20.99 i. 2. 10.56 "54 .22 .229 5 4-5 34-56 37-7 33- T 7 36.26 95-03 "3-1 87-58 104.63 7-45 8-47 35 3 2 .36 33 25-03 28.46 3- 12.52 2 3 8 4 40.84 39 34 132-73 123.19 9-54 .29 3 32.06 4- .248 3-5 42.42 J 53-94 143.22 10 71 .27 .28 36. 5- 14^48 259 3 47.12 45-5 176.71 164.72 11.99 25 .26 40-3 6. *5-43 .284 2 50.26 48.48 201.06 187.04 14.02 24 25 47.11 7- 16.4 3 I 53-41 5I-52 226.98 211.24 iS-74 .22 23 52.89 18. 17-32 34 O 56-55 54-41 254-47 235.61 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 ii 1.389 .00964 103.7 if 1.911 -0133 75-2 2 2-575 0179 55-9 at 3-333 .0231 43-3 2* 4.083 .0284 35-2 2| 5-^27 .0349 28.7 3 6.070 .0422 23-7 3i 7.116 .0494 2O. 2 3* 8-347 .0580 17.2 3* 9.676 .0672 14-9 4 10.93 .0759 13-2 4i 14.05 .0976 1O.2 5 17-35 .1205 8-3 6 25-25 1753 5-7 7 34-94 .2426 4-i 8 46.20 .3208 3-i 9 58.6 3 .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 Rankine, 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. 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 body 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, 01 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 6 _o ~ 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|- 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. 35 2 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 plate 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 30 32" 34" 3 6" 38. 40 42 48, 54" 60 66 . 721 In. Multipliers (cubic feet). In. '& * 05 05 05 05 .05 .06 .06 .06 .06 .07 .07 .08 i 2 14 .14 *S 15 .16 .16 .l6 17 .'9 .20 .21 . 2 1 2 J 3 25 .26 27 .28 .29 3 3 32 34 37 .38 39 3- v- 4 39 .40 .42 43 44 45 .46 50 53 55 -58 .61 4 c 5 53 56 57 59 .61 63 .64 .69 73 .78 .82 85 5- c 6 .70 7 2 75 77 .80 .82 83 .91 .90 i .02 1. 08 . 12 6 w 7 .87 .90 93 .96 99 .02 1.05 1.14 i . 20 1.27 '35 41 7 w 8 05 .09 13 1.17 1.20 .24 1.27 i-37 1-47 i -55 1.63 71 8 v 9 .24 .29 33 1.38 1.42 47 i-5i 1.62 i-73 1.85 1.94 .04 ~y J3 I0 43 49 55 1.6 5 .70 i-75 1.89 2.02 2.14 2.26 .38 10 7, II .63 69 .76 1.82 1.89 95 2.OO 2.18 2-33 2.46 2-59 74 ii :l S 2 83 .91 .98 2.06 2.13 2.20 2.26 2.46 2 63 2-79 2-95 3.08 12 C > 3 2.04 2 13 2.21 2.30 2. "8 2.46 2-53 2-75 2-93 3-12 3-3i 3.46 i- a. I 4 2.24 2 35 2-44 2 -53 2.63 2 7 2 2.80 3-4 3 2 5 3-47 3-67 3-85 14 3- 1 5 2-57 2.68 2-79 2.89 2. 9 8 3.08 3-6< 3-84 4-05 4.26 X 5 6 2.92 3-3 3-<5 3.26 3-37 3.66 3-94 4.19 4-43 4-67 16 u 7 3.28 3.41 3-65 3 98 4 29 4-57 4-83 5-09 17 -5 e 8 3.67 3 .8l 3-93 4-30 4-6j 4-95 5-23 5-53 18 3 9 4.08 4.22 4-63 5.00 5^2 5-66 5-97 19 ii .2 20 4-52 4.96 5-35 5-72 6 08 6.41 20 *; C 21 5.28 5-72 6.12 6.50 6.84 21 * 5-6i 6.10 6.51 6.92 7-3 22 f 5 95 6.46 6.92 7-35 7.76 23 I' 6.82 7-33 7-79 8.24 ! 24 v 7.20 7-75 8.22 8.71 ! 25 g 7-57 8.15 8.70 9.20 26 ctf: 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 27 .ic ft. per i 1 "1 sq. ft. )" 16,660 pounds 12 6 47,040 35-5 " 63.700 48.18 8.27 sq. ft. 7 . 2 to i sq. ft. 6 in. Proposed battery, 14 boilers, 18.517 ~^*~ -j sq. ft. heating-surface, 802 sq.ft. grate- * 5- surface. U1 o : Fire-rooms, 9 ft. 6. in. wide. Coal passed through whole length ? ^ .= of fire-room. , : 5 in. diameter X 20 ft. thick. L in. thick. None 3100 sq. ft. 75 sq. ft. i sq. ft. to 41.3 sq. ft. 392.6 cubic ft. j .012 cubic ft. to i ) I sq. It. f 48,262 pounds 15-5 80,221 25 8 128,423 " 41.44 " 18 sq. ft. 4.82 to i sq. ft. 24 in. Proposed battery, 8 boilers, 24,800 ~^\ 5* sq. ft. heating-surface, 600 sq.ft. grate- o ^ surface. _o - Fire-rooms, 12 ft. wide. cr^ ^ Coal passed through 3 ft. passage- !*<3 _. 1 way. Seams in fire Heating-surface ... Grate surface Ratio of grate-surf e to heating-surface. Ratio of steam-space to heating-surface. Water, weight of Weight per sq. foot heating surface. . Iron-work, weight of Iron-work per sq. ft. heating-surface Total weight Total weight per sq. ft. heat-surface Calorimeter Ratio of grate-sur- face to calorimeter. Height of water-line above crown-sheet. (2) That they are subject to rapid and serious fluctuations of water-level and steam-pressure. (3) That the circulation is less free and steady. (4) That, for the above reason and because of their liability to accumulation of incrustation, overheating is sometimes pe- culiarly apt to take place. Notwithstanding these objections, which are undoubtedly to a certain extent valid, these boilers are thought likely, by many engineers, to find their way into use at sea. Every good " sectional " boiler consists of a system of water- tubes, or their equivalent, so arranged as to permit a rapid, steady, and certain circulation ; a system of " headers " or con- nections by which the steam and water find their way into the steam-space, where separation and settling may occur ; and of this steam-space, usually in the shape of a large drum or set of drums of small section from which the steam is discharged, dry, into the steam-pipe, and by it conveyed to the point at which it is to be utilized. In some cases, the steam-drum is also partly a water-reservoir, and thus assists in producing a regu- larity of operation very difficult to secure unless obtained by the presence of a considerable body of water, somewhere in the structure. In this last case, the greatest care must be taken to DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 369 secure this drum against the direct action of flame, the nest of tubes being ordinarily so disposed as to intercept the gases leaving the furnace. 175. Upright and Portable Boilers are chosen when the location or use is such as demands concentration of space or facility of transportation. The upright boiler, occupying little floor-space, having, for the small powers for which it is most commonly used, no great height, and being self-contained and thus requiring no setting, is a form that meets these special conditions most perfectly. Its design is precisely that, in method, of the cylindrical tubular boiler, except that it must have a firebox. The latter is made in the form of a short cy- lindrical, upright, flue, occupying so much of the lower part of the boiler as will give the needed height of furnace and ash- pit. The water-space between this flue and the shell is usually about one tenth the diameter of the latter. In the design of this flue or furnace, care should be taken to introduce stay-bolts to prevent collapse from overpressure or weakness produced by corrosion, a method of yielding which causes the greater proportion of explosions of boilers of this kind. The thickness of furnace sides is commonly the same as that of the shell ; the bottom ring and the tube-sheet, at its up- per end, giving additional security and making the furnace very much safer against accident so long as it is in good order. The calculations of this detail are the same as for any other cy- lindrical flue subjected to external pressure. The steam -space in the upright boiler, as often built, consists only of the volume of the upper part of the boiler above the water-level, and as the tubes occupy a considerable proportion of the total volume of the shell, the steam-space is correspondingly restricted. This extension of the tubes above the water-level to the upper tube-sheet also renders their upper ends liable, at times, to injury by overheating. A better plan is that shown in 15, in which the upper tube-sheet is sunk be- low the water-level, and all the steam-space needed is obtained by carrying the shell upward to any desired additional height, and connecting the two by a frustum of a cone having its upper end no larger than is needed for the chimney-flue ; the tubes. 24 370 THE STEAM-BOILER. are thus protected, and the steam-space made ample. The same remarks apply to the computations of this cone as to those of the furnace ; it is, however, of stronger form and less likely to require staying. Tlic Portable Boiler is sometimes upright, as when used by itself independently of the engine, or when it has to carry the frame of an upright engine ; or it is horizontal, if of large size, or if forming the bed-piece of a horizontal engine, as is a more common arrangement. In either case, no very important dif- ference arises in either the design or method of construction, except that somewhat greater care is taken to make it safe against injury either by transportation or by the stresses com- ing of the action of the attached machinery. It is always bet- ter that the boiler should carry an engine with its frame than that it should itself act the part of that member. In all cases, the connection of engine and boiler and of boiler with its car- riage, where locomotive, should be so arranged that the changes of form and dimension due to variations of temperature and the stresses caused by difference of temperatures of adjacent parts as well as changes of pressures may have no ill-effect. A good steam-drum or dome is of even greater advantage on the portable than on the stationary boiler. Their attached engines are usually wasteful, take steam in very variable quan- tity, and are peculiarly liable to cause " foaming." The following are the proportions adopted for portable engine-boilers by a well-known firm of British builders : * PORTABLE ENGINE-BOILERS. HEAT-SURFACE SQUARE FEET. GRATE-SURFACE. GRATE- SURFACE. TUBES. Horse- Horse- power. Fire- box. Tubes. Total. Per Horse- power. Total. Per Horse- power. Heat- surface. Draught- way Sq. ft. power. 5 19.6 81.8 101.4 2O. 2 3-6 0.72 28.2 0.66 5 10 3 2 -4 161 .9 194-3 19.4 6.2 0.62 3 1 - 1 i. 08 10 15 43.0 228.7 271.7 16.9 8.6 0.53 31-6 1.39 15 20 53- 279.2 332-2 16.6 10.5 0.52 31-7 i .60 20 25 59-3 34 -5 405.8 16.0 12.8 0.51 31.8 1.87 25 30 68.1 408.8 476.9 i5-9 14.9 0.49 3i-9 2-35 3 * Wansbrough, p. 8r. DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. $?l A source of danger to which the upright boiler is peculiarly liable is that of " burning" the firebox or tube-sheet in conse- quence of the collection of sediment in the water-legs about the furnace or on the lower tube-sheet. The water-leg is sometimes found filled with solid matter, and the tube-plate so heavily in- crusted that the metal is readily overheated and burned. All boilers of this kind should be provided with hand-holes at the level of the crowmsheet of the furnace, and so placed as to permit thorough inspection and complete removal of the sedi- ment at frequent intervals. Comparing the vertical with the horizontal tubular boiler, it will be observed that a large item of expense is avoided in the cost of setting ; and that an incidental advantage is secured for the former in the fact of its accessibility at all times, whether working or cold, for examination of the exterior. The upright boiler is also less liable, while in operation, to injury from a small depression of the water-level ; the fire never comes in contact with its shell, and this permits the safe use of plates as heavy as may be desired ; no strains from unequal expansion are to be apprehended, and experience shows this to be an ele- ment contributing to the exceptional durability of this class of boiler. Its only setting is a foundation with an ashpit, and its connection to the chimney-flue. In the vertical tubular boiler, loss of water, and the falling of the water-level even a consid- erable proportion of the whole depth of boiler, does not neces- sarily involve danger ; and the upper part of the tubes may be utilized as superheating surface, and the extent of the super- heating adjusted very conveniently by varying the water-level. Where the feed-water is not very pure, however, the great and often fatal objection to this form of boiler arises in the danger of sediment or scale being deposited on the lower tube- head, the furnace-crown, and introducing danger of overheating and of explosion. A considerable proportion of the explosions of this kind of boiler, which have been investigated, are known to have been due to this cause. 176. " Locomotive " Boilers whether stationary or actually forming a part of the locomotive, are of the same general design and construction. They consist of a horizontal, cylindrical, 37 2 THE STEAM-BOILER. tubular boiler, crowded, as far as is safe and practically eco- nomical, with tubes, and with a firebox added as an integral part of the structure. In such boilers, designed to be station- ary, the tubes are often larger than those adopted in the boiler of the locomotive, as the draught is commonly vastly less intense, and the power demanded also comparatively small. The boiler of the locomotive represents the highest art of the engineer in the combination of the essential desiderata for its purpose : great power in small weight and volume, combined with maximum economy of fuel consistent with such concen- tration of power. The locomotive must always use steam of maximum pressure, must use enormous quantities because of its neces- sarily great power, and must be at once safe and fairly econom- ical. In consequence of its exposure to the action of its own great inertia in its constant motion over, often, an irregular roadbed, and because it must sustain the stresses due to the action of its own machinery and to frequent collisions, of greater or less violence, while making up and transporting trains, the whole structure must be designed with especial re- gard to such extraordinary and unreckoned strains as may be thus caused. Since the power demanded is a maximum, the tubes must be as numerous, and therefore as small and as closely packed, as is possible without affecting sensibly the circulation of water and thus losing steaming capacity ; and since economy of fuel is hardly less important than steam- ing capacity, the tubes must have sufficient length to give a ratio of area of heating surface to weight of fuel burned such as will insure that efficiency found to be practically desirable. With all this, the designer must keep in mind the special ne- cessity of compactness of structure, and of a limit in weight fixed, in many cases, at least, by the magnitude of the friction on the rail and the tractive power demanded by the special kind of work for which the engine is intended. To reconcile so many and oftentimes conflicting conditions, and to secure a maximum total efficiency, is evidently a problem of immense importance and of corresponding difficulty, and one which can only be fully solved by the gradual evolution of the precise DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN. 373 form and proportions best fitted for each of a number of spe- cialized types and duties, such as is illustrated by the different passenger and " freight " or " goods " engines now becoming standard. The methods of computation of size and strength of parts are in no way peculiar, and no special consideration of them is here demanded. Custom guided by experience has led to the production of such proportions as are illustrated in standard practice. Common faults of design in this, as in other forms of hori- zontal tubular boilers, are the excessive crowding of tubes and serious contraction of the water-spaces about the furnace. It would probably be found advantageous not only to preserve good water-channels between adjacent tubes, but to leave out a vertical row of tubes along the diameter of the boiler, and to allow an equal space between the nest of tubes and the shell all around. This has often been done by good constructors, with evident advantage, when boilers are doing much work. It is a safe arrangement to adopt for all cases. Water-legs should be made to widen from the bottom upward. The crown-sheet is supported by girders, " crown-bars," rest- ing at each end on the upper edge of the side sheets of the furnace and carrying the load by stays set at frequent intervals in their length. They should be very carefully designed. Stays to the shell are unsafe. The material used .in this class of boiler is becoming univer- sally soft steel, containing so little carbon that it will not tem- per. Harder steels crack in the firebox-sheets, especially where deep and hard-worked. The thickness of the shell is often re- duced 15 or 20 percent, as compared with iron. Good steel neither cracks nor blisters. Asa rule, with steam at 120 pounds, the general practice is, in the United States, to use -f-inch iron or steel for outside sheets, T 5 ^ inch iron or steel for fireboxes, and from - to j- inch for tube-sheets. Water-spaces around firebox from 2\ to 3^ inches inside, and from 2f to 4 inches in front. At straight seams \\ inch rivets are used, spaced if inches between centres. Longitudinal seams double-riveted, centres of the two lines of rivets ij inches apart, centre to cen- 374 THE STEAM-BOILER. tre of rivets on same line 2f inches. Stay-bolts \ inch di- ameter, 4 inches centre to centre. It is thought that thin plates give the best result in fireboxes, sides and back of -inch steel, crown-sheet T 5 -inch steel, and tube-sheet f inch. Tube-sheets of T ^-inch iron, the other plates being steel, have also given good results. It is believed that ^-inch steel plates are strong enough for side sheets and less liable to crack than thicker plates. Crown-sheets are more easily straightened when sagged down from mud collecting, and will not crack so quickly from overheated crown-bar bolts. The life of a good boiler is usually from ten to twelve years. Tubes are removed to permit inspection every three or four years. Steel and iron are now used for wood-burning fireboxes, with a result usually declared to be in favor of steel, in conse- quence of the lighter sheets and the metal not blistering. With bituminous coal copper, steel, and iron are used. Copper will not crack, but wears away, and is soon reduced to a dan- gerous thinness. A copper firebox lasts from three to five years, The objection to iron fireboxes is that the iron blisters, becomes " burnt " and very brittle, and cracks. Three years is the average life of an iron firebox. The only objection to steel is that it sometimes cracks. The average life of the best is 9 years and 6 months ; of the worst, 4 years and 4 months ; of the total reported, 6 years and 4 months. The following is considered a good specification for a steel locomotive boiler: Boiler to be made of mild steel T \- inch thick, riveted with f-inch rivets placed not over 2J- inches from centre to centre ; all horizontal seams and junction of waist and firebox double riveted ; all longitudinal seams provided with lap welt, with rivets alternating on both sides of main seams, to protect calk- ing edges, and all parts well and thoroughly stayed ; top and sides of outside firebox all in one sheet ; back-head a perfect circle. All plates planed on edges and calked with round- pointed calking tools, insuring plates against injury by chipping and calking with sharp-edged tools. Boiler tested with 180 Ibs, to the square inch, steam-pressure. Waist 52 inches in diame- ter at smoke-box end, made wagon-top with extended arch with DESIGNING STEAM-BOILERSPROBLEMS IN DESIGN 3/5 one dome 30 inches diameter on the wagon-top ; tubes of char- coal-iron, No. 12 B, wire-gauge, 200 in number, 2 inches outside diameter and 1 1 feet 8f inches in length, with copper ferrules on firebox end; firebox made of mild steel, 78 inches long and 34 inches wide ; all plates thoroughly annealed after flanging ; side T 5 F and back-sheets f inches thick ; crown-sheet f inches thick ; flue-sheet ^ inch thick ; water-space 5 inches wide at sides, 3-J inches wide at back, and 3^ to 4-|- inches wide at front ; stay-bolts -J inch diameter, screwed and riveted to sheets, and not over 4^ inches from centre to centre ; fire-door opening formed by flanging and riveting together the inner and outer sheets ; 2 rows of hollow stay-bolts above fire ; 2 rows of telltale stay-bolts at top on sides ; crown supported by crown-bars, each made of two pieces of 5 X | inches wrought-iron ; placed not over 4 inches between centres, bars to extend across, with ends resting on castings on the side-sheets ; crown-bar bolts -J in. diameter, with flat heads under the crown-sheet, the fit in the crown-sheet to be tapered and drawn to its place by a nut above the crown-bar; the crown to be well and thoroughly stayed by braces to dome and outside shell of boiler ; clean- ing holes in corner of firebox, and blow-off-cock in side ; smoke- stack straight ; grates cast-iron, rocking with dump ; ash-pan wrought-iron, dampers front and back ; balanced poppet throttle-valve of cast-iron in vertical arm of dry-pipe. The firebox first introduced by Mr. Wooten on the Phila- delphia and Reading Railway is carried higher than ordinary, so as to obtain room for broadening the grate and thus enlarging it, so as to be capable of successfully burning the hitherto use- less anthracite culm. The dimensions of their common loco- motive firebox are 60 and 66 by 32 inches ; the first of new design is 8 feet 6 inches long by 7 feet 6 inches wide ; the heating-surface of the firebox is 106 square feet, and of the combustion-chamber 26 feet, making a total of 982 square feet. The grate-rest is between water-bars to prevent burning out, and the area is 64 feet. The consumption of coal is only 16 pounds per hour per square foot of grate-surface against 40 to 60 pounds in the ordinary locomotive. The fuel remains perfectly quiet in the firebox, the consump- THE STEAM-BOILER. tion is slow, the steam is more freely made than in the common style of locomotive boiler, and no smoke or sparks are ejected from the smoke-stack. FIG. 79. STATIONARY " LOCOMOTIVE" BOILER. The stationary boiler of the locomotive type is shown in the accompanying figure, as customarily mounted on skids for transportation, with gauge-cocks, water-gauge, steam-gauge, and safety-valve attached, and in working order. CHAPTER IX. DESIGNING ACCESSORIES SETTING CHIMNEYS. 177. The Setting of Boilers which are not self-contained involves the construction of a system of side-walls and bridge- walls, customarily of brickwork, and entails so great an expense as often to make the question of the adoption of the firebox or TUBULAR BOILER. the plain boiler one of serious importance. It is usually found to be economical to adopt the firebox boiler for small powers, and to employ the other type where large quantities of steam are to be made. The form of the setting, the arrangement of bridge-walls, and the number, size, and disposition of flues, are all matters of ready determination once the style of boiler is settled ; but while the best engineers have come to a nearly uniform and 378 THE STEAM-BOILER. standard design, a great variety of forms and proportions are actually in use for every one of the familiar boilers. General practice prescribes the use of a cast-iron front protected from the action of the fire by a fire-brick lining. Side-walls are of red or common brick, lined with fire-brick wherever exposed to the direct action of the flame. The bridge-wall adjacent to the furnace is of fire-brick, except in parts so located as to be pro- tected from the impinging flame ; and the flues, even, are some- times similarly lined. The brickwork is held in place and the whole structure kept together by tie-rods and binding-bars, of which the fastening bolts are so located as to be exposed only to moderate temperatures. The following figure illustrates such a setting for a horizon- tal tubular boiler of good proportions : FIG. 81. SETTING OF HORIZONTAL TUBULAR BOILER. Here a set of 12-inch-side walls are lined with an inner wall, and an air-space between intercepts the heat, and is itself partly or wholly of fire-brick. Vertical binders on each side, tied together by heavy transverse bolts at top and bottom, hold all in place ; and similar bolts tie the front to the rear wall. The bridge-wall is set inside, at the rear of the grates, and is raised just high enough to prevent fuel falling or being thrown back under the boiler. The practice of the Hartford Boiler Insurance Co. is illus- trated by the next figure, in which are given the dimensions of setting for a " 6o-inch" tubular boiler, as published in the speci- DESIGNING A CCESSORIP:SSE TTINGCHIMNE vs. 3 79 fication. In this sketch the fire-brick used in lining the walls is sharply distinguished from the remainder. Where no circulation is permitted there is no objection to allowing the spaces above and below the boiler to communicate. In some cases the space above the boiler, when closed in, is used as a flue, with the effect of drying, and sometimes of superheating, the steam. There is an unquestioned advantage in keeping the boiler as nearly of uniform temperature as pos- sible ; but many engineers consider this system to involve some risk. The suspension of the boiler is a matter demanding the greatest care. It was formerly the custom to pay little atten- tion to this matter ; but the occasional explosion of a boiler in consequence of irregular strains so induced, has led to more FIG. 82. SETTING OF TUBULAR BOILER. careful design. The most common system is probably that in which the boiler has a set of cast-iron lugs riveted on its sides and resting on plates built into the brickwork of the side-walls, thus distributing the weight. In some cases the boiler is sus- pended from transverse girders resting, at each end, on the side- walls of the setting ; and the heads of the supporting bolts have sometimes been carried on springs to insure an equalization of load and its uniform and safe distribution which is the essen- tial aim of all good systems of support. Where tw r o 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- CPIIMNE VS. 3 8 I 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. 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. 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- 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 SET 7^1 NG 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. 2.43m. Thickness, stack 2 67 to 1.33 ft. 0.8 to 0.4 m. inner shell 1.33 to 0.67 ft. 0.4 to 0.2 m. Weight 2, 187 tons. 2,222 tonnes. Horse-power 2.700. Cost per H.-P $5-53. " total $14 ooo. 182. Steam and Water Pipes and their connections should be as carefully designed and located as the members of the * Sci. 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. 404. DESIGNING A CCESSORIESSE TT1NG CI1IMNE YS. 38 $ 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 cam 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 CCESSORIES SETTING CHI MNE VS. 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, keeps the lever in the designed vertical plane. The size of the valve is usually reckoned as that of the opening, //, 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. them 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 ACCESSORIES SETTING CHIMNEYS. 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 IN SQUARE INCHES. 5". 10". 15". 20". 35". 3O". Diameter of opening.. . Diameter of valves . . . Length of lever. . . . 2.525 2.76 2C. 3-37 3 77 ^o. 4-371 4.58 ac . 5-047 5-23 4O 5.642 5-S6 1C 6.781 6-375 47 ^ Distance of fulcrum. . . Angle of valve's face. . Width of face Length of fulcrum link. 2 1s 15 4-5 % 5 - 15 45 3-5 45 .12 4-5 4- 45 .17 4-5 4-5 45 17 4-5 4-75 45 15 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 DE SIGNING A CCK SSOR IE S SE T TING- CHIMNE VS. 3 9 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 - S/.-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- ways. The mechanism of one 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, CC, is the adjustable ring introduced to secure the desired reaction. FF is the spring and D D the FIG. 88. ASHCROFT' s (REACTIONARY) SPRING-LOADED SAFETY-VALVE. IYlOSt rCCCllt OI the 39 2 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, 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 p _ wl -\- I'w' -\- w"f FIG. 89. RICHARDSON'S SAFETY- VALVE. when a is its effective area ; w, iv' , 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 /"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 A CCESSORIES SE TTINGCHIMNE VS. 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-BOILER. 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 less H^HBW effect, and the tube, as a whole, assumes a greater or a smaller radius of curvature, 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. QO.-BOURDON GAUGE. eac 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.) E nfi INITIAL TEMPERATURE OF WATER (FAHR.). EI* OJ 32 40 50 60 70 80 9 o 100 I20 140 1 60 180 200 H 60 2.39 I.7I 0.86 O 80 4.09 3-43 2 59 i-74 o 88 100 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. 80 140 9.20 8-57 7-77 6.97 6.15 S-S 2 4-47 3.61 1.84 o 160 10.90 o 28 9-5 8.72 7.91 7.09 6.26 5-42 3.67 1.87 1 80 I2.6o 1 2.OO 11.23 10 46 9.68 8.87 8.06 7-23 5-52 3-75 i 91 o 200 14-3 3-71 13.00 12.20 "43 10.65 9-85 9-03 7.36 5-62 3-82 1.96 220 16.00 i 5.42 14.70 14.00 I 3- I 9 I2 -33 ii .64 10.84 9.20 7-50 5-73 3-93 1.98 240 17-791 7-13 16.42 15.69 14.96 14.20 13-43 12.65 11.05 9-37 7.64 5-9 3 97 260 19.40 ! 8.85 18.15 17-44 16.71 T 5-97 15.22 14-45 11.88 11.24 9-56 7.86 5-96 280 21.10 20.56 19.87 I 9 .l8 18.47 17-75 17.01 16.26 14.72 13.02 11.46 9-73 7-94 3 00 22.88 22.27 21 .6l 20.92 20 23 19.52 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. 9 i. 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 ACCESSORIES SETTING CHIMNEYS. 39 5 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. I Fig. 84 THURSTON'S HIGH-PRESSURE GAUGE 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. 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 connections 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 ii.ii-rr r^u 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 ACCESSORIES SETTING CHIMNEYS. 3Q/ 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 that 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 .-FusiB L E PLUG. is often required by law. 39* 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. Rose's metal 5 3 ' " 8 2 4 " 3 1 ' 5 I ' " 4 ' ' 5 I ' " i * * i (i 2 ' i I ' " 3 ' ' ' . . ' " 3 1 ' i 100 IOO 118.9 141.2 241 167.7 167.7 200 202 2O2 246 257 4 66 334 334 392 It is customary to use such compositions in making " fusi- ble plugs" to be inserted in the crown sheets or tops of " con- 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 FIG. 5. WROUGHT-IRON . .. ., r^. . . , , in PLATE. in r ig. 90. I he opening througn the shell 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. 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. 4OI All special sheets should be exhibited by sketches as well as by figures, and in arranging their location and dimensions care is taken to bring just as few seams into the furnace and to expose riveting to the heated gases as little as possible ; heavy laps, two, three, or even more sheets coming together in the joint, as is sometimes the case, are very apt to make trouble. Laps should be so planned, also, as to be easily reached for chipping and calking when necessary. The larger the sheets, generally, the better. The order being filled, the work of construction is begun. 187. The Apparatus, Tools, and Machinery employed in boiler-making are of the simplest character ; although the tendency is constantly observed to introduce more machine- work to the exclusion of hand-work, and to make steam-boiler construction, like iron-bridge construction, approximate more and more to the art of the machinist. The boiler-maker is coming to work more and more to gauges and standards, and the boiler is getting to be more and more a machine-made product. The apparatus used in taking off the dimensions from the working drawings and laying them down on the sheet consists of a set of rules, scales, straight-edges, and templates. The latter are usually strips or frames, which may be laid down on the sheet, and which contain carefully spaced holes correspond- ing to the rivet-holes to be made, in number, size, and loca- tion ; they permit the location of the rivet-holes with accuracy and dispatch. , The tools employed in boiler-making consist of tongs with which to handle hot rivets ; riveting-hammers, especially de- signed for their work ; chipping chisels for use in trimming the edges of plates ; cape-chisels with narrow cutting edges for cutting off portions of the sheet, or making openings in it ; and hammers for driving these chisels. Drift-pins tapering iron pins which are inserted in the rivet-holes to draw them into line are also used, sometimes endangering the construc- tion ; calking tools are used for making seams tight, and " expanders" to " set" tubes. The machinery of the boiler-maker consists of heavy rolls 26 4O 2 THE STEAM-BOILER, for giving the sheets the cylindrical form ; shears for cutting them to correct outline ; punches for making rivet-holes ; boring-lathes or drill-presses for making the large holes in tube or flue sheets ; and riveting-machines. Where large boilers, to carry high pressure, and therefore made of heavy plates, are to be built, all these tools must be very heavy and powerful. Reamers, or " rimmers," are used to enlarge holes found to be too small for their purpose. In the best-equipped establish- ments a planer is used to give the edges of heavy plates their bevel, and that exactness of line that is essential to neatness of appearance along the lap, as well as to secure immunity from injury by the chisel when the edge of the lap is chipped in the older way, preparatory to calking. Various kinds of rivet-heating furnaces complete this list of apparatus of the boiler-shop. All such machinery should be very substantial and powerful, as it is always liable to be sub- jected to very heavy stresses. 188. Shearing, Planing, and Shaping the sheets to the prescribed size and form are operations preliminary to the fitting together and riveting up of the work. Shearing is performed by the " shears" or shearing-machine, which consists of a pair of strong jaws, of which the one is fixed, the other movable, and actuated by a powerful toggle- joint or by an eccentric. The cutting edges are usually straight, but set at a slight inclination the one with the other, in such manner that the cut begins at one end of the blade and runs across to the other, thus enormously reducing the force required to effect it. This operation is rapid and inex- pensive, but is liable to injure the metal near the cut if it is hard, and usually leaves so rough an edge that it is advisable to give a better finish by means of the planer. Sharply curved and irregular outlines cannot be given by the shears or the planer, and are formed by the chisel. Occasionally, the rough work is done by drilling a series of holes along the line to be cut, and dressing out to the line with the chisel. 189. Flanging sheets which are to receive the ends of flues, or are to be used as heads and riveted to the shell, is performed at open fires, by means of which an even heat is ob- CONSTRUCTION OF STEAM-BOILERS. 403 tained over the whole area to be worked, and the flange is then made by hammering the edge to be turned, over an anvil or properly shaped " former." In some cases, when considerable numbers of circular or other simple forms are to be flanged, flanging-machines are used, in which the whole flange is formed at one operation, the disk being forced by hydraulic pressure into a die which turns up the flange all around. In some cases dome-tops, manhole-rings, manhole-plates, and other parts are similarly " struck up." Punching and drilling are performed by machinery usually, and for the latter process the gang-drill is often found an economical machine : it consists of a collection of drills so set as to be driven together, and so to make a number of holes at once. Punching is generally practised with very soft steels, and with all iron ; but drilling is always preferable where steel is employed of appreciably greater hardness than good iron, and is probably safest with hard irons. A good rule in working steel plate is to punch the holes T 3 g- inch (0.476 cm.) smaller than the size of rivet, and then to enlarge the hole to full size by either counterboring or ream- ing. The sharp edge, or fin, if any, around the hole should finally be trimmed so as to make a slightly rounded fillet under the head of the rivet, and thus reduce the risk of fracture at that point. For these operations the holes have been previously marked off by the template, and the art of successfully doing the work is mainly that of securing exact location of the punch or drill at starting. A table, carrying the plate and moving automati- cally the correct distance to give the desired spacing at each operation, is often adopted, and with advantage. When punch- ing, the sheet should be so placed that the side at which the punch enters shall be that next the adjacent sheet when riveted up, thus producing a hole for the rivet largest on the outside, next the heads, and smallest at the middle. 190. Bending Plates to the required curvature is often the first process, though frequently performed after the opera- tions just described are completed. The bending rolls are so set as to produce a moderate degree of curvature at the first 404 THE STEAM-BOILER. passage of the plate, and repeatedly adjusting the rolls and successive passes of 'the sheet finally give the full curvature desired. Where the shape to be secured is the frustum of a cone, one end of the sheet is more closely pressed in the rolls than the other, and a sharper curvature given it. The use of a template determines when the plate has the right curvature. 191. Riveting is done partly on detached portions of the boiler, as in making flues and fireboxes, and partly in assem- bling such parts and building up the complete structure. As a rule, all parts which can be easily handled are completed separately, and later joined to adjacent parts in the final work of putting them all together. Each member being compara- tively light and small, the work can be done on it, detached, much more conveniently, rapidly, and cheaply than when it is attempted to construct it as an attached portion of the larger mass. Before riveting up, each scam is examined to see that the two halves of the lap are precisely matched, the edges parallel and well adjusted, and opposite rivet-holes all exactly located and fair with each other. Should any fault appear it is cor- rected before riveting is begun. While making this trial of parts and doing this fitting, and while making this examina- tion, the seam is temporarily held by bolts which should nearly fit the holes intended for the rivets. Should a pair of rivet- holes be unfair, the bolt will not enter, and one or both the holes must be dressed over with a reamer until the rivet can enter. The drift-pin is used to bring companion sets of holes opposite, when in doing this the plate requires to be slightly sprung ; but it ought never to be employed to enlarge the holes or to force them fair by visibly distorting the sheet or the metal about the hole. When this process seems necessary, and when enlargement by chipping produces a seriously mis- shapen hole, the faulty sheet, or pair of sheets, should both be in fault, should be condemned, and better prepared sheets substituted. When the seam is found to be right, the two edges are bolted firmly in position, with the laps in perfect contact, and riveting proceeded with. Every rivet should be of the length CONSTRUCTION OF STEAM-BOILERS. 405 and size required by specification, and of the best material. In heating, the shank is given a good forging temperature, the head left barely red, and the point safely inside the burning heat. A few blows on the laps, with the rivet in place, deter- mine whether metallic contact exists, and the rivet is then rapidly headed up and shaped. Quick work means easy and good work, as the riveting is then finished before the rivet is hardened by cooling. Riveting-hammers are comparatively light ; but the rivet is held up to its place by heavy hammers, FIG. 97. STEAM RIVETING-MACHINE. weighing from ten to sometimes thirty or forty pounds (4.5 to 14 or 1 8 kgs.), and capable by their inertia of resisting the heaviest blows struck during the operation. Two or three hundred blows are required for each rivet in ordinary boiler- work. Very heavy rivets are headed up with a " button-set," a forming tool which is cup-shaped at one end, where it rests on the point of the rivet, while blows of a sledge-hammer on its other end drive it down and so give the head a hemi- spherical shape. This form of head is stronger than the cone- shape usually given in hand-riveting. 406 THE STEAM-BOILER. Machine-riveting, either by steam, compressed air, or hy- draulic machines, is, if well done, preferred to any hand-rivet- ing ; although on work which is not too heavy the latter is thoroughly capable of giving satisfaction. In machine-riveting the machine should be amply powerful ; the die should be carefully brought in line with the rivet ; the laps should be very closely secured together, and the pressure fully up to the working standard. A machine which will clamp and hold the lap while the rivet is driven is to be preferred. FIG. 98. HYDRAULIC RIVETER. Well-constructed steam-riveters of angular size do their work by pressure, not by impact or blow. The boiler-pressure should be varied to suit the size of the rivets being driven, and main- tained at a uniform pressure during the entire work. A good steam-riveter should drive ordinary sizes of rivets ten times as rapidly as a single gang of riveters working by hand, notwith- standing the time and labor required in the handling and ad- justment of the boiler, the rivet, and the machine. The lighter machines are compelled to strike a blow : this gives less satis- factory and far less reliable work than when the machine has CONSTRUCTION OF STEAM-BOILERS. 407 sufficient power to head up the rivet by steady pressure. In working this machine the rivets are inserted from the outside of the boiler, instead of, as in hand-riveting, from the inside. The boiler, suspended in slings attached to a crane, is drawn up to the riveting-hammer, and the pressure heads up the rivet in a moment, and the steam-pressure is retained until the rivet is cool. The charge of steam used in riveting is sometimes utilized in its expansion to draw back the ram. \o 9 9 9 9\ 910 9 9 9j o|' FIG. 115. FIG. 116. demned if Fig. 113; sometimes not as in they are 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 STEAM-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 mast 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 Plates. 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 (-J-) 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 SPECIFICATIONS AND CONTRACTS, 429 in drawing. There are to be four (4) lengths of T-iron, four (4) inches broad and one half (J) 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 suitabLs 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-chimney 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 ii-inch, and four p-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 ^, shell of boiler (round part) f, bottom course of steam-chimney -fa, inside lining of steam- chimney -f, the balance of the iron in the boiler to be T 5 -^, 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 i\ 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 16 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 43 2 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. Boiiers. 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- 28 434 THE STEAM-BOILER. three hundredths (3.43) feet long vertical diameter, with semicircular ends struck upon a radius of one (i) foot.) Fuel. The fuel to be fired under the boilers will be " Breeze" or coke screenings, a smokeless fuel containing from twelve (12) to fifteen (15) per cent of non-combustible matter. Guarantees. Each proposal must contain a guarantee of capacity of not less than four (4) pounds of steam per hour per superficial foot of heating-surface, with moderate firing; and shall contain a guarantee of economy of not less than eight (8) pounds of steam exclusive of water (if any) entrained from and at 212 Fahr. per pound of " Breeze." Test Trial. When the boilers are erected and completed, a test-trial for capacity and economy shall be made by the contractor, under direc- tion of the company. Failure to Comply. Should the boilers fail to comply with the con- tractors' guarantees for economy or (and) capacity, or in any other respect, a reasonable time not in excess of sixty (60) days shall be given the con- tractor to remedy such defects; failing in which the boilers and all appurtenances belonging thereto and furnished by the contractor shall, at the option of the company, be removed within thirty (30) days from order of such removal. Time. The proposal must name the time after acceptance required for completion of boilers as per invitation. Terms of Payment. One half of the contract price for said boilers will be paid upon completion, and after the test-trial and acceptance as herein provided ; and the balance within thirty (30) days thereafter. The company reserves the right to reject any or all proposals submitted. The following are two dimension-specifications of boilers and locomotives as issued by the Pennsylvania Railway Motive Power Department : STANDARD P. R. R. CLASS " R" FREIGHT ENGINE WITH TENDER. Boiler material, Steel. Thickness of boiler-sheets, dome, and extended smoke-box, . T 5 ^ in. Thickness of boiler-sheets, barrel, T \ in. " " " outside fire-box, . . . . f in. " " " smoke-box, sheet under dome, waist, and throat, -^ in. Max. internal diameter of boiler, / Bdpaire fire . box? \ 6of in. Min. internal diameter of boiler, ) ( 59 in. Height to centre of boiler, from top of rail, . . . . 89 in. No. of tubes, 183. SPECIFICATIONS AND CONTRACTS. 435 nside diameter of tubes, ........ 2j in. Outside " " ........ 2% in. Tube material, ........ Wrought-iron. Length of tubes between tube- sheets, ..... 156^! in. External heating-surface of tubes, .... 1,564.24 sq. ft. Fire-area through tubes, ....... 5 sq. ft. Length of fire-box at bottom (inside), ..... 107 in. Width of " " ..... 42 in. Height of crown-sheet, above top of grate (centre of fire-box), 51^ in. Inside fire-box material, ........ Steel. Thickness of inside fire-box sheets, sides, ..... i in. front, back, and crown, . -^ in. Thickness of tube sheets, ........ -J- in. Tube-sheet material, ......... Steel. Heating-surface of fire-box, ...... 166.8 sq. ft. Total heating-surface, ..... . . 1,731.04 sq. ft. Fire-grate area, ........ 31.1 sq. ft. Max. diameter of smoke-stack, I r - \ j 2 6f in. Min. " f ( 18 in. STANDARD P. R. R. CLASS " P" PASSENGER ENGINE WITH TENDER. Boiler material, . . . ..... . Steel. Thickness of boiler-sheets, dome ...... . T 5 -g- in. " barrel, and outside fire-box, . . f in. Thickness of boiler-sheets, slope, roof, waist, and smoke-box, T 7 ^ in. Max. internal diameter of boiler, ) Wagon . top { 5^1 in. Min. " " ( 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, .... I3O T V 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. 1 1| 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, . . . J in. " front, back, and crown, . . T 5 ^- in. Thickness of tube-sheets, ....... ^ in. Tube-sheet material, Steel. 43 6 THE STEAM-BOILER. Heating-surface of fire-box, 164.39 sc l- ft. Total heating- surface, ....... 1,530.2 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 \ 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 Fahr. (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. 437 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 43$ 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 unintermitted 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 of 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. Star ing 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 44 2 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 but 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. 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- 44-6 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 ;/ 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, i i m ; n =. hi; n i m and if the ratio of total feed-water to total evaporation is/, m + i n P ' i n I* 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 u 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 45 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 will 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. 451 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. 45 2 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 c f 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 STEAM. 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 Conway 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 0.121 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. f Iron Age, 1875. THE 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 3-3- 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 11 percent 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 off 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. .0656 .1956 .1944 .2301 .0895 copper, o ass, copp 0635 1255 .0970 .0880 0359 r gun-brc ;r, or gur .0381 .1440 "33 .0728 0371 nze -bronze . 0113 0123 0125 0109 0048 .0476 1254 .1252 .0854 .0199 Wrouerht-iron Steel Cast-iron skin removed .... 0.19 to 0.35 0.30 to 0.45 " in contact with brass, Wrought-iron in contact with br 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. 45$ 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 i D 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. 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 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- * Fouling and Corrosion of Iron Ships. London, 1867. 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. 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 i .000 Charcoal .632 Asbestos 363 Mineral-wool No. 2... Do. with tar Sawdust Mineral-wool No. i... 83* 715 .680 .676 Pine-wood across fibre. Loam, dry and open... Slacked lime Gas-house carbon 553 550 .480 .470 Coal-ashes Coke in lumps Air-space, undivided 345 .277 .136 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. 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. 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 y 1 ^- 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, 4tne 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-wafer 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. 470 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 CARE OF BOILERS. 471 oiigJily 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. 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. 4/3 235. 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 STEAM-BOILER. 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, 4?$ 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) TJie "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, i.e., 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 ^vhat 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. 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 r 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 l 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. p ,4 = 0.5; .5=1. .4 = 0. 3; ,5=1. ^=0.5;^ = !. A= 0.3; B=i. 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- 47 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 j. 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, FG 1 = W\ E l being the evapora- tion of water per unit of weight of fuel. The ratio of cost to work done is /> _ DGS + CFG _ CF+DS " W~~ FGE, Ef This quantity being made a minimum by variation of the area S, the most economical boiler is obtained. But E l is a function of S, and, taking the value of E l from the equation '+-5- THE EFFICIENCIES OF STEAM-BOILERS. 479 we obtain ACF^G BEFG which is a minimum when BEFG BEF -v = VAR. 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 5 per square foot of grate, i.e., D $0.60. Let the cost of operation, as for Class 2, amount to $15 per annum per pound of fuel burned per hour on the C square foot of grate; then CF=$i$ X F\ ^=$15 ; -^ = R = 25. 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,=FV~AR= 10 X (0.5 X25) = 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 T 5 20 3 40 5 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 1 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 1 +AF) of heating-surface is -j=^- = S.G : 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 5.. ... $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 (C). Fuel (@ $5 for I., II., IV.; $4 for III.) per unit Of F ................................. 7.50 7.20 12.00 2.00 Transportation and storage ................. r.oo i.oo 10.00 i.oo Attendance (variable cost) .................. o.oo 0.50 0.50 o.oo Total ....... . ..................... 8.50 9.00 22.50 3.00- Value oi=R .................. 25 23 14 5 Value of A ...................... 0.5 0.3 0.3 0.5 Value of tf~AR .................. 3.5 2.7 2.0 1.6 Value of F ...................... 8 10 16 20 Value of t^AR 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 F, 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. 7f into Class 3, and let k = -^. Let all other symbols stand as before. Then the total cost of maintenance and operation will be while the work done will be, as before, 31 THE STEAM-BOILER. The quantity to be made a minimum is, for the present case, the quotient of P by W, _P ~ VV : F being taken as the independent variable. This becomes a minimum when we substitute for E l its value BE l = , and make the first derivative equal zero. i +-s- ; Then we find When, in this expression for the value of /% 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 : F\ .......... 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. " Maximum commercial efficiency of boiler" and "Maxi- mum efficiency of a given plant " are therefore by no means 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 w r ork. 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 of 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. 8100^ + 34320 (/i gj +250*. (b\ Referred to Water at 100 Cent. SIQOC+ 34200 \k ?j + 2$oos 636.5 (g/i + w.) To determine the quantity of air required for burning coal we have the following: One kilogramme of coal requires to burn it, 2.667^ + S/i + s o " cu> metres of ox yg en ; or > loo x 1.43 2.667*: -f 8^ + s o -- -- cu. metres of air containing 21 per ct. 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 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 2 1.38 " " 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 evap. 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 49 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. 491 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 49 2 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 ending 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 tJie 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 1 10.2 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 ail standard trials the commercial horse-power is 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-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 as follows : * 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. 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. 49^ 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 beginning 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. XL 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 the 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 2-2 49^ 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 the 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. TIME. L: 1 rt i u 1 1 a - M PQ Steam-g 1 Q Externa 1 i c T3 1 rt 9 C/5 1 Pounds. g H Pounds < BOO 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 hours. DIMENSIONS AND PROPORTIONS. Leave space for complete description. See Ap- pendix XXIII. 3. Grate- surf ace. . . .wide. . . .long. . . .Area. . . . sq. ft. sq. ft. ^ Superheating-surface sq. ft. 6. Ratio of water heating surface to grate-sur- AVERAGE PRESSURES. 7 Steam-pressure in boiler by gauge . Ibs. *8 Absolute steam-pressure Ibs. *q Atmospheric pressure per barometer in. in. AVERAGE TEMPERATURES. deer ucg. deer ucg. dee:. IA Of escaping gases . deer deer FUEL. 16 Total amount of coal consumed \ ..... Ibs 17 Moisture in coal per cent 18 Dry coal consumed Ibs 19. Total refuse, dry pounds = 20. Total combustible (dry weight of coal, Item 18 less refuse Item IQ) per cent. Ibs Ibs. *22. Combustible consumed oer hour. . Ibs. * See reference in paragraph preceding table. f 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 unity 24. Percentage of moisture in steam per cent. 25. Number of degrees superheated deg. WATER. 26. Total weight of water pumped into boiler and apparently evaporated * Ibs. 27. Water actually evaporated, corrected for quality of steam f Ibs. 28. Equivalent water evaporated into dry steam from and at 212 F.f Ibs. 2g. Equivalent total heat derived from fuel in British thermal units f B. T. U. 30. Equivalent water evaporated into dry steam from and at 212 F. per hour 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. J-T h Factor of evaporation = , ^Tand 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 -|- Item 18 or = Item 31 X Factor of evaporation. Item 33 = Item 28 -4- 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 = ~. 34i _ 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 Ibs. RATE OF COMBUSTION. 35. Dry coal actually burned per square foot of grate-surface per hour Ibs. i"j Per sq. ft. of grate- Consumption of | surface. . , Ibs. dry coal per hour. I Per sq. ft. of water- Coal assumed with | heating surface.. .. Ibs. one sixth refuse. f Per sq. ft. of least j area for draught. . . Ibs. RATE OF EVAPORATION. 39. Water evaporated from and at 212 F. per square foot of heating-surface per hour. . . Ibs. f Water evaporated J Per sq. ft. of grate- "Mo ! P er nour f rom tem- surface Ibs. *li \ P erature f I00 F - I Per S< 1- ft- f water- *A2 ' into steam f 7 f heating surface. . Ibs. J pounds gauge-pres- | Per sq. ft. of least [sure.f j area for draught. Ibs. 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 H. P. 44. Horse-power, builders' rating, at square feet per horse-power H. P. 45. Per cent developed above, or below, rat- ing f 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. 503 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. 5O4 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 cc bo H i 1 w tf 1 u t> be ^ * UN II 1! JJ 1 Ml i d Q ,_ u o | w ii K, H fe g I I 1 E rt O H fci^ *i ^ H "u O O g tu 1 H s i u 2 (X 1 CQ L | E 1 g fi 3 5 M |a S3 3 c/5 bo Is U i u H 1 c c 5 8 < u fi SUPER- HEATING. sjiun ' -1B9H saaaSaa f Suiuiuj JO 3SB1U3O43J X J319CUUOIB3 oiui una taBajs ,? - ; = v 43112 M UJO4J 1E3H. J - J. = H CDB91S OJ04J }3H /? =y XyM 43J3U1UOIB3 OJ P94J3JSUE4J JB3H HEAT-UNITS PER POUND FROM BOILER. X ureais ; J35B^V\. CALORIMETER. TEMPERATURE. ^7 - u c< rt " !| 5 'c WEIGHTS. lls C/5 Condensing Water. W saanssand -wvaxg u s H 1 506 STEAM-BOILER. on Composition W 9 M S (4 5 REAL EVAPORATION. Square feet of Heat- ing-surface required to Evaporate one Cubic Foot of Water. 34nSS34d-mB31S IBmoB IB PUB j { ^ ZIZ IB PUB UJO4J }U3[BAinb3 } IVXOJL 1 -UIB31S JBniDB IB pUB 43JBAV-p33J JO 34m -B43dUI31 (BniDB UJO4JJ 1 3j uoiuodo4 c j t) i Per Square Foot of Heating-surface, per Hour. 34nSS34d-UIB31S IBmOB }B pUB ' J 9 ziz ujo4j ^usiBAinbg 1 IBJOX i CONSUMPTION OF FUEL. | unoq 43d 30BJ4nS -3upB3J JO 'Ij'-bS 43J i PUB uio4j ;u3iBAinbg | *34nSS34d -UIB31S |BmOB IB pUB 431BA\-p33J JO 34m -B43dUI3^ [BniDB UIO4^J 1 unoq JO JOOJ 34BnbS 43J jQ Per Pound of Com- bustible. 34nSS34d-UIB31S lBn;oB JB PUB -a a ziz uio4j lusjBAinog 1 HUM, | | g O ziz 5B I AVERAGE PRESSURES. ffSSwa 1 -01B3JS |BmDB >B pUB 431BA\-p33J JO 34ttl -B43dCU3J JBmDB UIO4jJ 1 3J3nBJ3 1 e/j -UJB31S 1 = U3}3UIO4Bg M C Per Pound of Fuel. |BmDB 3B pUB ' J 1 AVERAGE TEMPERATURES. U3JEM. -P39J A"3uujiq3 OJ 30UB4^Ug 1 PUB OJO4J !U3[BAinbg I 34nSS34d -UIB3JS (BmOB IB pUB 431BM-D33J JO 34HJ -B43dai3; jBnioB uiojj t 4iy 1 Is" si? ^1 3jnss34d-mB3}S IBHJOB ITS PUB -j i U1004 -43 [I0g 1 g ziz 3 B PUB uiojj luaiBAjnbg l -DNIXV3J-J OX 3XVH) dO OIXV^ 34n&s3jd -UJB33S IBmOB }B pUB J3JBAV-P33J JO 34 m -B43dai31 |BmOB OT04J i AREAS. JO UOIJD3S -SS043 ;SB3^ sr Per Pound of Com- bustible. IBnjoB ve pne -jj I 3DBJ4nS -43dng 1 PUB rao4j 3U3iBAinbg I 9DBJ4nS -SUUBSJJ J 43JBM-p33J JO 34m -B43dlU31 |mDB UlOJjJ l '31B4Q 1 JO HXON3q e 3 ffi Per Pound of Fuel. '34nSS34d-UlB31S JBmOB IB pUB 'J O ziz rao4j lusjEAinbg l puB rao4j ;u3iHAinbg I "IV1HJ, JO 3J.VQ '34nSS34d -UIB31S ^BmOB IB pUB 431BM-P33J JO 34m -B43dlU3J [BmOB UIO4jJ i '1VMX jo aaawnj^ 1 STEAM-BOILER TRIALS. SO/ Equivalent from 212 F. and at actual steam- pressure. * > rt o 'i.8 s MO O Z I* S fc ' I.: c 3 E erg 1 HORSE-POWER. petuav pai*H '8+tffl v = d CQ NI ff dNV y do samvA ^ EFFICIENCY. paremnsg 'IBjuauiuadxa pajBunjsg per cent. pnaamuadxg per cent. EVAPORATION FROM AND AT 212 F., EQUIVALENT TO TOTAL HEAT-UNITS DERIVED FROM FUEL. jnoq jgd 3D^jjns-Sui -jean jo }; -bs jaj 1 aiqijsnqraof) jo'punoj J3745 " " " " available, net 2,101,700 " " Water evaporated 1,620 Ibs. The efficiency of the furnace was o. 643 ' ' The balance-sheet stands thus : Dr. Available heat , 2, 101 , 700 B. T. u. Cr. Per Cent. 88.29 Heat expended in evaporation 1,855,900 B. T. u. 7.03 Displacing atmosphere .147,720 " " 3.35 Loss by conduction and radiation 70,430 " " .05 Heatinashes 1,129 " " Unaccounted for 25,521 " " 1.26 100.00 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. DIMENSIONS AND PROPORTIONS: AVERAGE PRESSURES: Date of Trial . Duration of Trial Height of Stack Boiler, seven feet in diameter, twenty-eight feet long. Grate-surface . . . . . . . Water-heating-surface . Superheating-surface . Ratio of Water-heating Surface to Grate-sur- face Economizer Heating-surface, per each boiler 'Force of Draught, in inches, at stack base, after leaving economizer .... Force of Draught, in inches* at back of boiler, before entering economizer . . . Force of Draught, in inches, at front of boiler, before entering economizer Absolute Steam-pressure Atmospheric Pressure, per barometer [ Steam-pressure in boiler, by gauge April 8th, 1885. 1 1*4 hours. 200 feet. 35.75 sq. ft. 853 225 23.86 to i sq. ft. 609 sq. ft. .75 ins. of water. .5625 .6063 93-575 pounds. 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 ........ | Percentage of Moisture in steam L 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, and at 212 F. . . . \ mizer. Boiler and Economizer used together. By boiler, f rom J exclusive m 1 ofecono- IDry Coal actually burned, per square foot of grate-surface, per hour .... Consumption of dry f Per square foot of grate- Coal per hour, coal j surface assumed with one | Per square foot of sixth refuse, ( water-heating surface . 58 degrees. 66 381 " 200 " 3 60 589 " 8 4 155 " 6925 pounds. 569 301 6055 1.87 " 1.72 " 1.019984. None. 58 degrees. 4.63 inches. 68, 138 pounds. 2,782 " 65,356 " 66,854 " 78,112 " 6^5 per cent. \ 6943 pounds. | 116 cubic feet. 75.432885. .11389. .12459. 10.093 pounds. 11.041 "793 12.907 12.153 16.46 18.07 o-745 5 I2 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 of 4 Per square foot seventy pounds' gauge- of water-heat- l in] pressure, ing surface Horse-power of engine, as per indicator-cards taken on day of boiler-test Kind of Coal used ...... Condition of Chimney-damper .... Cleaned fires, number of times on each fur- nace during the test 8. 139 pounds. 168.9 " 7.079 311-45 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 32,751, 835 . 34 B 46,387,827.10 C 23,066,685 . 39 D 37,228,739.07 E i 1,485, 777 . 35 That the figures thus obtained are very accurate, is shown by calculating the heat transferred to the condenser by the Root and the Allen boilers (both of which superheated their 33 5H 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, plus 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. 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. Wx = total weight of water primed. Then + h(Wx)= U',or x = * H h Substituting the proper values in this equation, we deter- S 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. A 27,896. o. O. B 2Q 6?O o. O. c iq 782.04 645 . 06 3.26 D 1l,66l. 35 2,336.65 6.0 E . . .. a Sqt; 6 2o6 . Q q And the amount of water, in pounds, actually evaporated per pound of combustible : 8.76 8.76 8.70 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. 4 No. 2 O. 7OQ B No i No 3 O 7O7 C No 3 No 4 o 699 D No 2 No. 5 o 6cn E No. 5 No. i o 7 c ;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 J^RIALS. 517 POUNDS WATER EVAPORATED. THERMAL UNITS. il "o o "3 (N 5 . *! . II u No. DESCRIPTION OF BOILER. 11 u <" u "o - . *O c^ . 3 y>g^t ill ^ a w 3 rt tl j; b3 Bu-S^ T3 C I ogr'3 &S' a eA ct u " flj-C ^ II! "w J3 t 3 ti = o S fc 2^=J= a b - *o IE f|| C H W j Field . 4 * 57 8 83 4 414 8,529 Field 2.28 U . UJ jo 8^? 2,202 10.461 Field 2.57 1U.UJ | 10 93 i 2,482 10.558 4 5 6 7 Portable \ %* Portable ( ~ Portable f..}i Portable ) 1.52 2.26 1.76 3-56 10.23 10.49 11.81 9-93 14.718 14,718 14,718 1,468 2,183 1,700 3,438 9.882 10,133 11.408 9,592 67 68 77 98.356 148.444 130 900 118.248 g Lancashire 1 . 57 12 83 T C 71 C T.Cl6 TO OQO 77 108 248 9 Lancashire 2.83 i^.Oj 9.89 Ow * J 13,833 .^ AU 2,733 i "t^5?J 9-553 68 i8.s,8 44 10 Lancashire 1.88 12 2^ ' 15*715 1,816 ii 8^q 7^ i ^6.200 ii Jacketed 4.70 i^ . 4$ i 7-7 14.805 45Q5 a L ,oo 7.500 /O 50 229.750 12 Lancashire 2-57 10.9 2,482 10.529 6 7 166.294 -jg Compound 1-43 11.51 14,296 1,381 11.125 78 107.7:8 14 15 Loco. (Webb) Loco. (Marie) 9.83 4.62 10.28 10.65 14.004 14.600 9,495 4,462 9,930 10,287 7 70 664.650 312.340 16 Loco, j 8.22 13.550 12,142 7.940 58 704,236 18 L co - (-Coke f:P 8-94 IO.OI 13,550 13,550 13.263 8,636 9,669 i 63 7 1 8,5.569 463.630 Loco, f ^ *9 Loco. ) 7-39 II. 2 7,138 10,819 77 549.626 20 Torpedo 12.54 8-37 14,727 12,113 8,085 54 654,102 21 Torpedo ** O,wx=- - = H _ k 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 ; h, " " " " " final. And we have the equivalent expression, as written by Mr. Kent, The value of the quantity U 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 ; / 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, and * _ w Iw STEAM-BOILER TRIALS. $2$ If Q exceeds unity, the steam is superheated by the amount 0.48 = 2.o8 3 3/(<2-/);* and if less than unity, the priming is, in per cent, 100(1 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. CALORIMETER. HEAT-UNITS Weights. Tempera- ture. FROM BOILER. 111 Q || | M if . ul rt fc o rt iJ rt^ rt< " C 7.29 The boiler was a water-tubular boiler, which was not so handled as to give as dry steam as was desired; and one object of the trial, of which the above is a part of the record, was to ascertain how seriously was the quality of the steam affected. It is seen that the priming amounted to seven or eight per cent, with fairly uniform figures through the period of test. The steam should have entrained less than one half this proportion, had the boiler been all that was expected of it. Errors of small magnitude, absolutely, may greatly affect the results of calculation, as is well illustrated by the following example presented by Mr. Kent : * Centennial Report, pp. 138-9. 524 THE STEAM-BOILER. Assume the values of the quantities to be, as read, column I : OBSERVED READING. TRUE READING. AMOUNT OF ERROR. Weight of condensing-vvater, corrected for 200 5 Ibs. 200 Ibs. -i- pound. \Veight of condensed steam w 9Q ' ' IO O ' ' yW Pressure of steam by gauge P 78 80 " TIT 2 pounds. Original temperature of condensing water, /. . . . Final temperature of condensing water, t' 44. 5 " I OO. 5 " 45 " 100 " i degree. i Then let it be assumed that errors of instruments or of ob- servation have led to the recording of slightly different figures from the true quantities, as given in column 2 : Moisture Error Substituting in the formula the true per cent, per cent. readings," we have for the value of Q = 0.9874 = 1.26 = o. All readings true except W = 200.5, Q = .9906 = 0.94 = 0.32 " " " w = 9.9, (? = i.oooo = o.oo =1.26 " " " " P = 78.0, Q= .9880=1.20 =0.06 "*== 44.5, Q= .9989 = 0.11 =1.15 " " " ?."**- t' =100.5, Q= .9994 = 0.06 =1.20 " " incorrect Q = 1.0272 = (minus)= 3.98 The last case is equivalent to 50.2 degrees superheating. Errors of o.i or even 0.25 per cent in weights and of tem- perature of equal amount not infrequently occur, probably, where ordinary instruments are employed. The errors due to false weight in measurement of the condensed steam are liable to be very serious, and it is only by making a consider- able number of observations and obtaining the mean that re- sults can be secured, ordinarily, of real value. 263. The " Coil Calorimeter" has been devised to secure more exact results in the weighing of the water of condensation than can be obtained when it is weighed as part of the larger mass. In this instrument a coil of pipe is introduced into the tank and serves as a surface-condenser in which the boiler-steam is received and condensed, and from which it is transferred to another vessel in which it is weighed by itself with scales con- structed to weigh such small weights with accuracy ; or the coil is removed and weighed with the contained water. In the * Correction made only for coil calorimeter to be described. STEAM-BOILER TRIALS. 525 former case, drops of water may adhere to the internal surfaces of the coil and escape measurement ; in the latter, the weight to be determined is increased by the known weight of the coil, and less delicacy of weighing becomes possible. The following is Kent's description of his calorimeter, which is of this class, and has been found to give good results : * A surface-condenser is made of light-weight copper tubing f " in diameter and about 5O / in length, coiled into two coils, one inside of the other, the outer coil 14" and the inner 10" in diameter, both coils being 15" high. The lower ends of the coils are connected by means of a brazed T-coupling to a shorter coil, about $' long, of 2" copper tubing, which is placed at the bottom of the smaller coil and acts as a receiver to contain the condensed water. The larger coil is brazed to a f " pipe, which passes upward alongside of the outer coil to just above the level of the top of the coil and ends in a globe-valve, and a short elbow-pipe which points outward from the coil. The upper ends of the two f " coils are brazed together into a T, and con- nected thereby to a | /x vertical pipe provided with a globe-valve, immediately above which is placed a three-way cock, and above that a brass union ground steam-tight. The upper portion of the union is connected to the steam-hose, which latter is thoroughly felted down to the union. The three-way cock has a piece of pipe a few inches long attached to its middle outlet and pointing outward from the coil. A water-barrel, large enough to receive the coil and with some space to spare, is lined with a cylindrical vessel of galva- nized iron. The space between the iron and the wood of the barrel is filled with hair-felt. The iron lining is made to return over the edge of the barrel, and is nailed down to the outer edge so as to keep the felt always dry. The barrel is furnished also with a small propeller, the shaft of which runs inside of the inner coil when the latter is placed in the barrel. The barrel is hung on trunnions by a bail by which it may be raised for weighing on a steelyard supported on a tripod and lifting lever. The steelyard for weighing the barrel is graduated * Trans. Am. Soc. M. E. 1884. 526 THE STEAM-BOILER. to tenths of a pound, and a smaller steelyard is used for weigh- ing the coil, which is graduated to hundredths of a pound. In operation, the coil, thoroughly dry inside and out, is carefully weighed on the small steelyard. It is then placed in the barrel, which is filled with cold water up to the level of the top of the globe-valves of the coil and just below the level of the three-way cock, the propeller being inserted and its handle con- nected. The barrel and its contents are carefully weighed on the large steelyard ; the steam-hose is connected by means of its union to the coil, and the three-way cock turned so as to let the steam flow through it into the outer air, by which means the hose is thoroughly heated ; but no steam is allowed to go into the coil. The water in the barrel is now rapidly stirred in reverse directions by the propeller and its temperature taken. The three-way cock is then quickly turned, so as to stop the steam escaping into the air and to turn it into the coil ; the thermometer is held in the barrel, and the water stirred until the thermometer indicates from five to ten degrees less than the maximum temperature desired. The globe-valve leading to the coil is then rapidly and tightly closed, the three-way cock turned to let the steam in the hose escape into the air, and the steam entering the hose shut off. During this time the water is being stirred, and the observer carefully notes the thermometer until the maximum temperature is reached, which is recorded as the final temperature of the condensing water. The union is then disconnected and the barrel and coil weighed together on the large steelyard ; the coil is then withdrawn from the barrel and hung up to dry thoroughly on the outside. When dry it is weighed on the small scales. If the temperature of the water in the barrel is raised to 1 10 or 120 the coil will dry to con- stant weight in a few minutes. After the weight is taken, both globe-valves to the coil are opened, the steam-hose connected, and all of the condensed water blown out of the coil, and steam allowed to blow through the coil freely for a few seconds at full pressure. When the coil cools it may be weighed again, and is then ready for another test. If both steelyards were perfectly accurate, and there were no losses by leakage or evaporation, the difference between the STEAM-BOILER TRIALS. $2? original and final weights of the barrel and contents should be exactly the same as the difference between the original and final weights of the coil. In practice this is rarely found to be the case, since there is a slight possible error in each weighing, which is larger in the weighing on the large steelyard. In making calculations the weights of the coil on the small steel- yard should be used, the weight on the large steelyard being used merely as a check against large errors. The late Mr. J. C. Hoadley constructed exceedingly accu- rate apparatus of the " coil " type and obtained excellent re- sults. It is evident that this calorimeter may be used continuously, if desired, instead of intermittently. In this case a continuous flow of condensing water into and out of the barrel must be established, and the temperature of inflow and outflow and of the condensed steam read at short intervals of time. 264. The Continuous Calorimeter is an instrument in which the operations of transfer of steam to the instrument and its examination are not intermitted, as is necessarily the case in the more commonly employed forms of the apparatus. The instrument being thus kept in use continuously, every variation in the quality of steam can be observed and the num- ber of observations can be increased to any desired extent, and, the apparatus being accurate, any degree of exactness of mean results can be attained. One of the earliest forms of this instrument was devised by Mr. John D. Van Buren, of the U. S. N. Engineers, and In- structor in Engineering at the Naval Academy, about 1867. This instrument, as constructed by Mr. T. Skeel, and used by a committee of judges* at the exhibition of the American In- stitute, 1874-5, of which the Author was chairman, was made as follows : Steam was drawn from the steam-drum, near the safety- valve, through a felted pipe i% inches (3.8 cm.) diameter, into a rectangular spiral or coil consisting of 80 feet (24.4 m.) of pipe of similar size. Condensing water from the street-main was led into the tank surrounding the coil or " worm," and * Trans. Am. Inst. 1875; Van Nostrand's Mag. 1875. 5 28 THE STEAM-BOILER. issued at the bottom through a " standard orifice," the rate of discharge from which had been determined and the law of its variation with change of head ascertained. The quantity of condensing water thus became known by observing the head of water within the tank. The water of condensation from the coil was caught in a convenient vessel, and weighed on scales provided for that purpose. The temperature of the condensing water at entrance and exit was shown by fixed thermometers, and that of the water of condensation at its issue from the coil was similarly shown, while the steam-gauge placed on the boiler gave the other needed data. The calculations are evidently precisely the same as with the preceding type of calorimeter. The Barrus Calorimeter* (Fig. 119) is essentially of a small surface-condenser. The steam enters by the pipe/. The con- densing-surface, a, is a continua- tion and enlargement of the supply-pipe, a i-inch (2.54 cm.) iron pipe with a length of 12 inches (30.4 cm.) of exposed sur- face. This pipe is under the full pressure of steam. The con- densed water collects in the lower parts of the apparatus, where its level is shown in the glass, e, and is drawn off by means of the valve, d. The injection-water, cooled to a temperature of 40 Fahr., or less, enters the wooden vessel, o, through the valve, b, and circulates around the con- densing pipe, carried downward FIG. n 9 .-THE CONTINUOUS CALORIMETER. to the bottom by means of the tube k, and overflows at the pipe, c, after passing through the mixing chambers, m. The amount of water admitted is regu- lated so as to secure a temperature at the overflow of 75 or 80 Fahr., or the approximate temperature of the surrounding atmosphere. The thermometers, f and g, which are read to * Trans. Am. Soc. M. E. 1884. STEAM-BOILER TRIALS. S 2 9 tenths of a degree, show the temperature of injection and over- flow water, and the thermometer, //, shows that of the con- densed water. The overflow water and the condensed water are collected in a system of weighing tanks. The steam-pipe down to the surface of the water, and the pipes in the lower part of the apparatus, are covered with felt. There is no wire-drawing of the steam, and no allowance to be made for specific heat of the apparatus. The only correc-; tion to be made of material amount is for radiation from the pipes covered with felt, and this can be accurately determined by an independent radiation experiment, made when the con- denser vessel is empty. Another form of instrument devised by the same engineer is arranged in such manner as to permit the steam from the boiler to be dried and the quantity of heat so employed meas- ured as a gauge of the amount of water contained in the steam. This form of this apparatus is found very satisfactory.* The pipe conveying the steam to be tested is usually a half-inch (1.27 cm.) iron pipe. A long thread is cut on this pipe, and it is screwed into the main steam supply-pipe of the boiler in such a manner as to extend diametrically across to the opposite side. The inclosed part is perforated with from 40 to 50 small holes, and the open end of the pipe sealed. If the pipe is screwed into the under side the perforations begin at a distance of one inch (2.54 cm.) from the bottom. The connection is made as short as possible, and covered with felt. Where the calorimeter can be attached to the under side of the main, the distance to the top valve need not exceed six inches (15 cm.). In this position it is self-supporting. The steam for the superheater is also supplied by a half-inch iron pipe, but this may be at- tached to the main at any convenient point. Steam to be tested enters by the pipe, which has a jacket. On passing out the thermometer gives its tem- perature, and it is discharged through a small orifice -J- inch (0.32 cm.) in diameter. Steam to be superheated enters and is superheated by a gas-lamp, passes the thermometer, * Trans. Am. Soc. Mach. Engrs., vol. vii. p. 178. 34 530 THE STEAM-BOILER. and issues through an opening like that for the steam. The thermometers are immersed in oil-wells surrounded "by the current of steam to be tested, or of that used in drying the boiler-steam. In the operation of this calorimeter steam at full pressure enters the apparatus, and the jacket-steam is heated until a perceptible rise of temperature above that due the pres- sure indicates that its moisture has been evaporated. The working having become steady, the difference between the temperatures is noted and corrected by deducting the ex- cess" above that of moist steam at the observed pressure, and the number of degrees of superheating thus determined, as the rate of flow is the same from both orifices. Here the evaporation of one per cent of moisture from steam at 80 pounds pressure (5.6 kilogs. per sq. cm.) reduces the tempera- ture of superheated steam about iS ./ Fahr. (io.4 Cent.), and the percentage of moisture is obtained by dividing the range of superheat, as above, by this number, or generally by the quotient of the latent heat at the observed pressure by 47.5. The following are data and results obtained by the use of this apparatus : DATA AND RESULTS IN FULL OF CALORIMETER TESTS. rence. U bo U is. Sfg IP ^ Q. Amount of Moisture in the Wet Steam. * OB 9 " v 2 $ <* Date Gauge- T3 flj *o *,> *~ Z S v- c^e c* pressure. *o 9 rt rt o 2 58 I sit lit S5 * * 2; W Wo. I Apr. 13 89. 99. 54-5 8. 8. 9-5 19 I. O2 2 14 89. 75- 37- 5-5 8. 9-5 16. 0.86 3 15 86. 74- 37- 7- 10.5 9-5 10. o 54 4 " 16 86. 74- 39- 9-5 7- 9-5 9- 0.49 5 " 30 85. 72. 38. 10.5 8. 9-5 6. 0.32 6 May 4 80. 77-5 41-5 9-5 8. 9-5 9- 0.49 7 5 84. 68. 36.5 6-5 7-5 9-5 8. 0-43 NOTE. The duration of each of these tests was about one hour. * Obtained by dividing the preceding column by 18.6, the number of degrees corresponding to I per cent of moisture. STEAM-BOILER TRIALS. 531 Many other forms of calorimeter have been devised, but space will not permit their description. 265. The Analysis of Gases* issuing from the furnace and passing up the chimney is sometimes an important detail of the work of testing a steam-boiler. Such an investigation involves only an operation of great simplicity which can easily be performed by any engineer. If it is not found convenient to make the analysis in the office of the engineer, he can have the work done, at little expense, by a chemist of known skill and reliability. It is only by a knowledge of the proportions of constituents of the flue-gases that it can be determined whether the combustion is complete, whether the products of combustion are diluted with excess of air, and whether the fuel used has been so burned as to give its best effect. Such analyses also enable the engineer to ascertain the best method of burn- ing the fuel. In sampling the gases, a matter in regard to which some precaution is advisable, the method of Mr. Hoadley is found very satisfactory.! Very great diversities in composition often exist in the same flue at the same time. To obtain a sample, allow one orifice to draw off flue-gases for each 25 sq. inches (161 sq. cm.) of cross-section of flue. The pipes must be of equal diameter and of equal length. These should be secured in a box of galvanized sheet-iron, equal in thickness to one course of brick, so that the ends may be evenly distributed over the flue A (Fig. 120), and their other open ends inclosed in the IB; FIG. 120. FLUE-GAS SAMPLING. * Consult Handbook of Gas Analysis, by C. Winkler. London : J. Van Voorst. 1885. f Trans. Am. Soc. M. E., vol. vi. 53 2 THE STEAM-BOILER, receiver B. If the flue gases be drawn off from the receiver B by four tubes C C, into a mixing box Z>, beneath, a good mixture can be obtained. The sampling of the gas should be carried out at intervals of 10 to 15 minutes throughout the trial. The gas should be received in an air-tight pipe or jar. The composition of the gases should be determined as far as regards carbonic acid, car- bonic oxide, and oxygen. The tube should be of porcelain or glass for very hot flues, since iron tubes at such temperatures are oxidized. Supposing an analysis of the gas give K per cent of carbonic acid, O per cent of oxygen, and N per cent of nitrogen, then the proportion of air actually used to the theoretical quantity required is I to x. Where N 21 or unity of weight of this coal will then give, at a temperature of o and a pressure of one atmosphere, 1854 ~ C = carbonic acid : 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 1 atter 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 I, 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 = 14^+ i6O + 2SCO + 44CO,, i6O 2SCO 44CO, ~M> ~M~> -^-respectively. Since the total per cent of oxygen is measured by - CO, -f- 44 16 12 ~^CO -f- ^ee oxygen, and the total per cent of carbon is CO 9 28 44 12 -| sCO, we shall have for the percentage of each, 28 32 X 44 X CO, 16 X 28 X CO i6O 44M ~ 2SM ' + M ' _ 12 X 44 X CO, 12 X 28 X CO 44M 2SM STEAM-BOILER TRIALS. 535 or, CO, CO 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 FIG. 122. DRAUGHT-GAUGE. body of the instrument, and may be moved up and down by screws at F F'. The scales are divided into fortieths of an inch, and read to thousandths of an inch by the verniers e and e' , 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 tJie 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 TEMPERATURE (FAHR.) OF THE EXTERNAL AIR BAROMETER, 14.7. in the Chimney. Fahr. 2O 40 6O 80 1OO- 22O .419 355 .298 .244 .192 250 .468 .405 347 .294 .242 300 541 .478 .420 .367 315 350 .607 543 .486 432 .380 400 .662 .598 541 .488 436 450 .714 .651 593 540 .488 500 .760 .697 639 .586 534 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 Rankinef published papers on this subject in the same number of the Philosophical 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." f " On the Expansive Energy of Heated Water." 54 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 " 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 54 2 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. 543 omO ^CNro m in O ro in O -^-io ovOM 10 Tt-i-irc ro in oo ro -> jj . u js S . c rt J3 M hi | B i 3 C rt jQ I 1 rt S ', rt 3 CUD s < a u c/) s V rt 1 Saw-mills and wood-working 5 2 4 3 3 o o 2 o <1 T ? />O . T T T T TO Steamboats tugs etc 2 2 i I I q 4 T6 Portables, hoisters, and agri- T a 4 2 q 2 2 Tlace, experience has shown. In the experiments above described they had many times opened the glass boiler without causing an explosion ; with the second boiler, too, they had done so without being able to bring about explosion, both with high and low pressure. In the former class of explosions the steam shatters, twists, and contorts everything in an instant. " Water-hammer' has, by the bursting of steam-pipes, by a process somewhat closely related to that described by Clark and Colburn, sometimes caused fatal injury to those near at the instant of the accident. This is a phenomenon which has long been familiar to engineers, and the author has been cog- nizant of many illustrations, in his own experience, of its remarkable effects, and has sometimes known of almost as seri- ous losses of life as from boiler-explosions. It is rarely the cause of serious loss of property. When a pipe contains steam under pressure, and has intro- duced into it a body of cold water, or when a cold pipe con- taining water is suddenly filled with steam, the contact of the two fluids, even when the water is in very small quantities, results in a sudden condensation which is accompanied by the impact of the liquid upon the pipe with such violence as often to cause observable or even very heavy shocks ; and often a succession of such blows is heard, the intensity of which is the greater as the pipe is heavier and larger, and which may be startling, and even very dangerous. It is not known precisely how this action takes place ; but the Author has suggested the following as a possible outline of this succession of phe nomena:* * " Water-hammer in Steam-pipes." Trans. Am. Soc. Mech. Engrs., vol. iv. p. 404. $66 THE STEAM-BOILER. The steam, at entrance, passes over or comes in contact with the surface of the cold water standing in the pipe. Con- densation occurs, at first very slowly, but presently more quickly, and then so rapidly that the surface is broken, and condensation is completed with such suddenness that a vacuum is produced. The water adjacent to this vacuum is next pro- jected violently into the vacuous space, and, filling it, strikes on the metal surfaces and with a blow like that of a solid body, the liquid being as incompressible as a solid. The intensity of the resulting pressure is the greater as the distance through which the surface attacked can yield is the less, and enormous pressures are thus attained, causing the leakage of joints, and even the straining, twisting, and bursting of pipes. In some cases the whole of an extensive line or system of pipes has been observed to writhe and jump to such extent as to cause well-grounded apprehension. The Author once had occasion to test the strength of pipes which had been thus already burst. They were 8 inches in diameter (20.32 cm.), and of a thickness of f inch (0.95 cm.), and had been, when new, subjected to a pressure of about 20 at- mospheres (300 Ibs. per sq. in.). When tested by the Author in their injured condition they bore from one third more to nearly four times as high pressures before the cracks which had been produced were extended. It is perhaps not absolutely certain that some of these pieces of pipe may not have been cracked at lower pressures than the above ; but it is hardly probable. It seems to the Author very certain that the pres- sures attained in his tests were approximately those due to the water-hammer, or were lower. The steam-pressure had never exceeded about four atmospheres (60 Ibs. per sq. in.). It is evident that it is not safe, in such cases, to calculate simply on a safe strength based on the proposed steam-pres- sures; but the engineer may find those actually met with enormously in excess of boiler-pressure, and a " factor-of-safety >v of 20 may prove too small, it being found, as above, that the water-hammer may produce local pressure approaching, if not exceeding, 70 atmospheres (1000 Ibs. per sq. in.). These facts, STEAM-BOILER EXPLOSIONS. $6? now well ascertained and admitted, lend some confirmation to the Clark and Colburn theory of explosions. 277. Energy Stored in Heated Metal is vastly less in amount, with the same range of temperature, than in water. The specific heat of iron is but about one ninth that of water, and the weight of metal liable to become overheated in any boiler is usually small. If the whole crown-sheet of a locomo- tive-boiler were to be heated to a full red heat, it would only store about as much heat per degree as forty pounds (18 kgs.) of water, or not far from 30,000 thermal units (7560 calories), or 23,016,000 foot-pounds (3,030,000 kilog.-m., nearly), or about three tenths of the total energy of the fluids concerned in the explosion. It would be sufficient, however, to considerably in- crease the quantity of steam present in the steam-space ; and this increase, if suddenly produced, and too quickly for the prompt action of the safety-valve, might evidently precipitate an explosion, which would be measured in its effects by the total energy present. It thus becomes at once obvious that the danger from the presence of this stock of excess energy is determined not only by the weight of metal heated and its temperature, but even more by the rate at which that surplus heat is communicated to the water that may be brought in contact with it, by pump- ing in feed-water, or by any cause producing violent ebullition. It is probable that this cause has sometimes operated to pro- duce explosions ; but oftener that the loss of strength pro- duced by overheating is the more serious source of danger. It is also evident that the first is the more dangerous as the pres- sures are lower, the second with high pressures. As illustrating a calculation in detail, assume range of temperature, into specific heat (o.m), is the measure of the heat-energy stored. of crown-sheet, or boiler-shell, overheated ! T/ ;;Lo T-' f > tne i i ooo Jr \ metal being { ^centimetres J in thicknesSj and its total weight I7 kll V . Then the product of weight into 568 THE STEAM-BOILER. 375 X 1000 X o.i 1 1 41,625 B. T. U., nearly; 1 7 X 556 X o.i 1 1 =10,492 calories, nearly; and in mechanical units, 41,625 X 772 = 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 57 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, f 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, 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 ? 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. 57 2 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 BURST: LOW-WATER. Q f the tubes burstj 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, evn when the water is again supplied before cooling off, was shown as early as 1811, by the experience of 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 Phoenix, 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.;): 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. 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, and yields as shown. When this goes on to a sufficient extent, * Report of the Committee of the Franklin Institute. FIG. 127. OVERHEATING THE SHEET 5/6 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, in preventing the deposition of scale. ' IG< 1 *'F5 A H 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. | Bibl. Univ. xxxviii. \ Ann. de Ch. et de Phys., 3me serie, xvi. Bibl. Univ., Nov, 1861, i. xii. !j Poggendorff's Ann. t. cxiv. *[[ Cosmos, '863. STEAM-BOILER EXPLOSIONS. 579 steam is 115 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, 1856. 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 1'Ebullition 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 Rendtis, May, 1855, p. 1062. \ Civil Engineers' Journal, 1856, p. 133; Dingler's Journal, 1856, p. 12. Annuaire, 1830. I 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. "(id) 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 EXPLOSION'S. 585 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 with 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.* TAVO 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. 587 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. $88 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 1'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 acco'unt 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 v and the quantity of heat, Q, supplied from the furnace 'are known, and is W(t-t Q~ Professor Trowbridge gives the following as fair illustrations of suoh cases :* * Heat as a Source of Power, p. 191. 59O 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 = 29 F. 5000 X 3 X 1000 79,000 X 29 T = 5000X3XIOOQ 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, 5000 X 50 1 = ----- = s* minutes. 77 looo (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 ii cubic feet of water in the boiler, we shall have rr 93 X 117 T= - . =4.1 minutes. I57X IOQO 60 Thus, if IV is 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. 59 2 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.* 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 00 O ^ N C4 t^ t- 1 (NCOCNOGroi-.o-tO^O ^ l-^ f IA inoo ro O>\O ro Soi?" f- 0-00 t^ -* o * VO '-O Tj M M 133J oiqno ui ureais jo punod B JQ w vo -^- O to t^ ro ^Oromnwoo^o t^oo ? t^ ro in rooo oo ro o COt.MOO^^^^-^CO roroo. d spunod ui 'uiB3}s jo }ooj oiqno B jo }u.Sp.A\ ^ t^OO d N M t^OO VO -<4- t^ M M -! vo -^J-OO O t^ rooo ro 6 irioo H ro in t^ 01 6 cTKH i^^^ 1 vc vo vo -* O OOO VO t^OO on.^ SfSM SefH- S||s i^s-n |ta ^ ^-O O OH iriion CJ :>oo t^. o * Ooo o m O <"O rooo ^vo -^*vo (N ^S ^ cT 8 SN 00? oo' S O O O O OOOOO O oo vo t^ O 5 o" roK VO ro O t^- r^ r^ t^vo O O O O ? $ 12 ^ ro oo in ro o ^O^^O S c *i Si! 8 >< rtJ3 w ^ rt-inroO rOH MVO O w in t^vO O ^1- O O *O t^ N ro ro vo rovo r-x O rovo O t*^ S?K N moo V VOvOVO\OVO>O\Ot^t^ O H M M H (N N IN "t3 4J a a w Bsg I" 5 s H>0 tx OOO T)- fO Tj- CM ca O co ro 1000 oo t^ t^ m N o vo oo mvo oovo ?5-roN"S^2 M 1 ? OOOOOOOOOO rovo ro **- 00 VO OO 00 O i^vo c^ in M oo 10 000?0? 4 cl *t-oo t^ t ofoll S *IS^ llilf 8 8 a l'2 S 5 s !b| OVO rot^lN in^-0) MVOVO 6 *" O H O 00 10 H VO' H t-^OO N >* N w tt" vo 6 -^-od O t^. t>. tx. ) oi O W IT) 00 00 00 saajSap iiaqaajqBjj 'ajniBaadujaj, *K4 OOMM-(MOrO01 * lovO tvOO O O M Pi ro TJ- IDVO t^oo O O M Pi rn * IDVO r^OO O O w Pi ro <) IDVO t^ U 1? VO VO O CO CO Pi CO r^CO *H r^ PI CO ro ovO Pi OvO * M M O O O O OCO CO CO ID M C>CO t^CO O CO t^ Pi *- OVO ^ Pi O O t^ ID <}- CO t^ tv t^ t^ t^>O VO VO O t^mor^-i-rOM 6 O O M M OOO t-.VO ID - ro Pi VOVO U-JIOIDIDIDIOIOID 8 o oa| Kv|vS to t^co PI 00 VO VO t^ O rnOO O \O t^-; O\O fO O t^ 10 PI ro pi PI' M M H M 6 VO O fO t^OO 00 1DOO OO ID Pi O O O '-' f)VO O CO Ooovo ^J-Pi Ocovo -*ro O O O O O OCO OO 00 CO u"> t>. ^vO -^-vO N ^O^tCv^ro ^ O Pi \O loo? O o" o" o 1 *vO N'-'4-OOir>iow rooc^O (N J -^-t^o^-^ - fcj III t^i^roO t^^Mco -^-O p> ro < ID IDVO r>. t^co c> OCT>OOOC3NOC>OO vo Pi oo rooo rrco roco ro 8-88ooooS ) o 1 '^~ co rooo W vo O -^--MO *O PJVO t^lOMVOOO OOO Pi O * "> ro O\ rj-co Pi VO ID M 1DCO O O O CO \O PI Pi OVO -^ ID O'O VO CO tD copivoOMr;roropiM ^vg^^gvS ? ^ O 6 M rj- 10 10 pi m * in u"> ID\O r^ t^-co IDlDlDlDlDiDlDlDlDl^l COOOQt-tMNpiPiro ID ID 1DO VO VO \O ^O VO CO Tj- -^- ID IDVO VO VO t^% t^ vovovovovovovovovovo ^SSvS^^R. i^ . t^OO t*- O O *O t^ ro ro OOO O P) loco Pi r^ pi (^ m O VO fOHOOVO IDCOPl M O CO ** P> M rrjvO Hi CO VO VO OOOOOOO O H Pi ssvo^-o^a ooo lDID^--^--^--^-^f^-^-fO OO-OO-OOOONOO cr. roror^rororoN Pi Pi 0-000. CJNOOOOO oooooooooo vO ID -^- ro Pi PI H OOOOOOO VO'OP? OPiVOrO^CO^MOOO VOCO^OO ^CO-DXDCDOO ^ O IT) ^ ? M . ? ? 9 -. ? vo t..* 8 M ei ro ^vo ^co o Pi ro t ^ VO Pi CO VO + N rO"O \O flvO "^ OOO w w N o^iot^O rof^-w t^. Pi txr^ovo roocovo * vO M CO oo O roco ir-, Tt- ID Pi i OCO t^. t^vo VO VO VO t^ M vo Pi O O O VO t-- t^OO O H ID -0 M ot^mo^M ooo*O u^m vS vS 5;%^ % 5 S> 5> 8 ?.^. ^ ^ ? 5. 0; 00 t^vo ID - ^- ro CO CO 00 oooooooocooooooooooo cooooooococooocococo CO CO OO OO 00 OO CO ^ npiooiDOvo M Pi I s * *O t^ -^-VO 00 ^ -^" VO Pi IDVO * ro * ro O vo w ID O ro t^ rovo O H^iisHp? P! Pi N O ro >D t- O N -^-\O CO O r*irororoM p ^^"*-* ( t'iO pi -^IDI^OO Pi -^10^- ID ID ID ID IDVO VO VO VO VO OO O w Pi * u~, t^OO O O vo i^t^t^r^t^t^t>* t^oo O CO -f IO*O OO O ft, SS^ H Pi CO <* >0\O 1^00 C> PiPiPiNMNPiMPiW rorororrirororororo-*' M N ro * IDVO t^CO O O -^-^^^^^"^-^ID H N m -^- iovo tv 10 m 10 10 10 10 o 648 THE STEAM-BOILER. qoui ajsnbs jad spunod ui 'umrioEA B SAoqB ajrissajj *, ^^ vS0'O VOLUME. .foisuap uiniuixBiu jo aaniEjadcuai IB jajEM pajinsip jo iqSiaM pmba jo aumioA oj tuEais jo auinioA jo OUB^ ^ (N 1000 M u-100 ro CS M M o OOO 00 r-. 1 -4- o^ ^- O n M t^vo VO vo LO 10 laaj oiqn ui ureais jo punod B JQ g w tx C^* t-N. M no2Nro^ SJ^ OOO t^^O lO -^ ro N M O vdOt^O (N -^-IT) t^OO 1 ON O M M o N u-100 O!NiOt^Oi-.mu-> t^O!N -^-VO OO Tf-^j C>M miot^O o t^vo 10 ro OO 1000 HI ^ t^0> Ififfl H OC^ Hi: IIP |r| O HI OOO Rfcl 2 g^'g o> o> o^ t"*.vO mo t^f^t^O w O> lOO-3-OvO -^(N (N M IN M rovo O -< ^ t^ ro*O OO l-^so 10 10 -^- ro ro N M aoo^^o^o^o ^vo^ O C rooo 1 ro 0,(N VO (N VO o o 9"9<9 ^ O OOO 00 OO OO h Z 1 III x JS w q ^E- oo oo oo ! g2_v| ^ss?:?:^ OvC roo r^-*J-M c-^Tj-0 M- ir.vo tv r>.oo O O O M III c -Q s 00 w 1^ N \o o M ro ID 01 M O rr ro ro 00 00 00 ro 10 w oo vo Tt-w ^ VO fO O O"00 O w T^-CO N vo oo O ^ ro ir>oo (N in OOO OO hs^O iA Tfr- Tt- CO CS 00 00 00 00 00 00 M OT tS J oo H N o rovo r^ -^- M OO t^vo t-x O ro looo M ; M M O OOO 00 2 Ji^^ ifilf 11 1^ rt iJo to g ^5- 8 i?JT i-ii OO rOVO vO N -^-VO -^ ON h*. ^- m O VO HI rj-\O t^.vO iO ro ^j- m LOVO vo vo *O vo *O ro *od ON O H N rfRWKftS H|alS S SSI* SI qoui aannbs jad spunod ui 'umnTCA -e aAoqn aanssajj ft, ^SvS vS^v^^^g^S^g. w W CO ^ XOVO tx tx tx tx e^ ix APPENDIX. 649 o r^oo ON S t>. tx txc M . O H O w m t^. O w ro looo If) CO O VO CM (x * O VO CM (^ COOO VO t^ ON | w CM -^* in t^OO I t^ O* M Tj-VO OO O CNI -3- -^- $ in i in m invo vo vo ' CM CM CM CM CM CM HMMMHCNlCNlCNlCNlCNl CMCNlCMCNlCMCNlCNlCMCMCNl . , a s s s s s s s 3 a ^- t^ M -^ 1^ ON CM -^-NO OO O "-i ro -^- invo t^OO < ON ON ON ON 00000000 I 00 00 00 00 OO 00 00 00 OO OO 00 03 CO OO CO 00 OO O 'O CM O O' C^ C^ t>.OO ON HI COVO ON c*^ I ON ro t^ M Tt-oo CMNOO-^-ONfOt^Hi |VO( vo vo' to in I 4- ^2^22-groS: CN) t^CM t^CNl t^CNl t^ ON ON O-O^OO rO * ONOO H t> * CN! t~- rooo ro ON * 00 00 00 OO 00 OO 00 lO>-I^Nt^ O ^ ON -^-oo ro ro ro ro ^- -*f in ON CN! T- in mvo vom^cM , Ot^^Mt^r t^ t^ fvOO OOONONONOO MMMCMCMCNlrO oooo oo co oo oo co oo co f^ O NJ vo H in ro CM M M M M HVO H\0 HVO M oooooocooooooooo I in O vo CM vo r--vo in CM H O ON ro O vo i^ ON CM invo t-s CNI CO ** invo t^ r^OO ON O OOOOOOOOOOOOOOOOOO ON vo O t^t^ oo ONTj-romM o ro i ti oo -^- O in ON H M N ro * in mvo t^oo ONONONONONONONONONON invo f^ CM r- M in t^ m -t CM O * rovo -<*-oo ON M N ro ro - nvo \ rorororororororo in invc r^ r^oo ON ro ro ro ro ro ro ro t>-00 ON O t* t^ t^CO OO CM ro ** "1VO 650 THE STEAM-BOILER. qoui ajBnbs jad spunod ui 'omrioBA B 3AoqB aanssajj Jjj S8?cr^s-s^& RfcR 33*55% VOLUME. A"}isu9p tunuiJXBUi jo ajmBjadulai ;B JSIBM paijnsip jo }qJo*i3M jBnba jo amhjoA o; uiBajs jo aamioA jo OUB'JJ - 1 M O_OO VO -4- CO w r>00 VO CO w 6 00 t-. 10 * PH O "OOOOOOO NNWWNNNN oiqna ui UIBSIS jo punod B JQ - 10 IOVO oog^oo.voo^O *SS2J 10 CO CO 2SSJiJ -------- spunod ui k uiB3}s jo }ooj oiqno B jo }qSi3jy\ ^ 1000 O ir> o ^ f> TJ-CO fo t^- * M M CO u^vo t^ O O W(N corororornr^j .&, prcTc7- c s' : c s < s^s N S N h < M E h 8 < D a uopBaodBA3 jo simn ui ' O z 3AoqB uoi;BaodBAa jo iBaq IBJOX ^ II! QO O P* M^vO OO O P* CO 10 f-00 00000000 ONOOO> NP)P)P) Ill O o t^ * o covo oo o o 10 m M M oco vo TJ- o o M CO 10 t^CO O IN - o*O oo o t^ VO 00 P) OOO H t- IOVO OO (N t^ COOO -1- H t^ Tj- M 00 m ._ ,,o oo oo'oo co co pi pi H w 6 6 o o d-ocj oo os t^, t-.vo vo VOVOVOVOVOVOVCVO oooooooooooooooo oooocooooooooooooooo P 1 , vo m o ^ocooo.voo^^ 10 C--00 O M N Pi M O (OVO COVO O P) COCOCO OOOOO T(- io 10 iovo vovot-.t-.rs. 000000 CO 00 00 00 00 00 00 00 00 00 cooooocooocoooco EgS 0) 4J U c JS -^ s M M CO vovo oo H covo O> co t^ 1-1 C S)o"^0 > S^'tQro co co pi pi H' M 6 <> ood oo' t>- O O O O> OOO 00 00 00 CO oo"oo"o? o? co 5 <5?oTco' "OX! 5 rt O II Pi ^ u ^l jH| *?*? 8, " ro ro ro cocococococococococo - O r*^ >O fltUUlftl lttfK JJ si* H CV1 p) CO Tj- Tj- IO IOVO t-. t-.oo oo o O O H M cocococococococococr) cocococococococo qoui sjBnbs jad spunod m 'rannDBA B 3AoqB ajnssajj - 00 N N P? N P?^ P^'S pT CO H N CO * IOVO t>.OO cocococococococo APPENDIX. 6 5 I ^ ft* ~<<--^^$ vgacgas OOOCOQOOOQ 5 S S? ct $' S"S S^ , S,8S>8 ^jass^sas^s M ^ < - "N rj cj SN SNOOPS ocTcS oo*" KvO 10 -t- -^ TJ-00 rOOO ON 10 IN ON 10 ,g^ K?^ig;^^Sri^ C, oo vO - M rf- w ON t^ to ro M OOOVO MWQOOOOOONON *&m ^-MVOOO lOONl^ONlOlO iovo t^. o> N 10 ON r^oo ro M O ONOO 00 t^VD VO 10 10 ro M O O* O^O M ovo t^roN ino in O- rooo fo O>vo ro o oo oo t>. t^.vo vo in in in in TJ- ^ 8ff^ rrvo' tx O rovo ON M IOOO w ** T)-VO 1-.00 ON w (S ro IOVO Hl a ,5 N W *-i OOOVO rOHCOlO -i- IOVO t^ t^OO ON O O w vo oo O ro 10 ^ TI- 10 10 10 10 IOVO VO VO rooo ro 8 Sfo-SvS r-oo ON IIII1III1I H MHMMHHMMMN k tovo 00 O w ro TJ-VO OO O vo w vo O * ^-VO t^ ON O M rO Tj- IOVO t^OO ON O H ^--^-^--*-|---oo O IN ro >o t^.oo O IN M "- M W ON ON ON ON ON ON t^ O O H ON rovo O vo OO ?)00 'ON - M" ro ro'w C? ON O *-" ro ^t- iovo t^CO O^ ONOOOOOOOOO 3 N 04 IN 8 04 0?C?0?oToToToToT 8 ft ??si*mt* d ON ro ro ro w IOVO O Cx ON ro ON t^ 10 vo cv> CO ro ONVO w ro ON 10 lOTt-IN OiOONO txO O IOVO OO l-i rj-OO 4- ONVO rO 00 * P4 (M * ON M O t** M W* ^ vO VO OO 00 cooooooooooooooooooo oowo^worT Zffffff &&&& O ONOO CO OO tx Cx 1^ _____ * o o H MMMMCSNCvirororo VO 00 O (N * O oo vo mvo oo O ro M m rj-Qvo M 1000 n W roro CO O * M 0) SO CO O 04 OO O ON -*00 O M O CO M M N M 1-1 O O ONOO VO 0000 00000000000000000000 OO OO OO OO CO 0000000000000000000,, 00 CO 00 CO cooooococococooocooo s II (N rooo VO vo t-x O VO * 1O Tt M 00 VO -. S. O>00 00 O- N IOOO rOOO * m rhoo O H Tj-^r-^-H OOO ro ^* *-t o roOO t^.vo vo - SI ro M \r> 10 rooo O> -o t^ VO 0) ON ONVO * O VO CO -*vo 04 0) VO O IOOO vo M O vo oo vo rooo in 10 ro ro <* 10 iovo vo f^ t^co rooo N t^ w vo vo r- t^oo a? < SS;8o"oS2'^ ro^^vo co c? o 2 ? "8 fo -" ^ roS- M W ro ^ IOVO t^OO ON O O O O v6 f-oo ON 2 S ro 3-ov8 S-co ON 8 o o o o in ^n O OOOOOOOOOO too 1OO lOO too ^Q H 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. Correction. Tempera- ture of feed, Fah- renheit degrees. Correction. 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 .0383 I0 5 .0756 141 .1129 177 .1504 34 .0021 70 0393 106 .0766 142 .1140 178 1514 35 .0031 7i .0404 107 .0777 M3 .1150 179 1525 36 .0041 72 .0414 108 .0787 144 .1160 180 1535 37 .0052 73 .0424 109 .0797 MS .1171 181 I 545 38 .0062 74 0435 no .0808 146 .1181 182 -1556 39 .0073 75 0445 III .0818 i47 . 1192 183 .1566 40 .0083 76 .0450 112 .0829 148 .1202 184 !577 4i .0093 77 .0466 "3 .0839 149 1213 185 1587 42 .0104 78 .0476 114 .0849 150 .1223 186 .1598 43 .0114 79 .0487 "5 .0860 151 1233 187 .1608 44 .0124 80 0497 116 .0870 152 .1244 1 88 .1618 45 0135 81 .0507 117 .0880 i53 1254 189 .1629 46 .0145 82 .0518 118 .0891 i54 .1264 190 .1639 47 0155 83 0528 119 .0901 155 1275 191 165 48 .0166 84 .0538 120 .0911 156 .1285 192 .1660 49 .0176 85 0549 121 .0922 i57 .1296 193 .1670 50 .0186 86 0559 122 .0932 158 . 1306 194 .1681 5 1 .0197 87 .0569 123 0943 159 .1316 *95 . 1691 52 .0207 88 .0580 124 0953 160 1327 196 .1702 53 .0217 89 .0590 I2 5 .0963 161 I 337 .1712 54 .0228 90 .0601 126 .0974 162 .1348 198 X 723 55 .0238 9 1 .0611 I2 7 .0984 163 .1358 199 r 733 56 .0248 92 .0621 128 .0994 164 .1368 200 I 743 % .0259 .0269 93 94 .0632 .0642 I2 9 I 3 .1005 .1015 3' 1379 .1389 201 2O2 1754 .1764 59 .0279 95 .0652 131 .1025 i6 7 .1400 203 I 775 60 .0290 96 .0663 I 3 2 .1036 168 . 1410 204 1785 61 .0300 97 .0673 133 . 1046 169 .1420 20 5 .1796 62 63 .0311 .0321 98 90 .0683 .0694 T 34 135 1057 . 1067 170 171 I43i .1441 206 207 .1806 .1817 64 OSS 1 100 .0704 136 .1077 172 1452 208 .1827 6| .0342 101 .0714 137 .1088 173 . 1462 209 1837 66 .0352 102 .0725 138 .1098 J 74 I 473 210 .1848 67 .0362 I0 3 735 139 .1109 175 .1483 211 .1858 68 .0372 I0 4 .0746 140 .1119 176 1493 212 .1869 \PPENDIX. 653 TABLE la. TEMPERATURES AND PRESSURES, SATURATED STEAM. IN METRIC MEASURES AND FROM REGNAULT. jj 1 QJ STEAM-PRESSURE. fll STEAM-PRESSURE. S e V In Centimetres. In Atmospheres a 6 In Centimetres. In Atmospheres t_l H - 32 c 0.0320 0.0004 '+ 14 C .1908 0.016 31 0.0352 0.0005 15 .2699 0.017 30 0.0386 0.0005 16 3536 0.018 29 0.0424 O.OOO6 17 .4421 o.org 28 o . 0464 0.0006 18 5357 O.O2O 27 0.0508 0.0007 19 .6346 0.022 26 0.0555 o . 0007 20 7391 0.023 25 0.0605 O.OOOS 21 .8495 0.024 24 o . 0660 O.OOOg 22 9 6 59 0.026 23 0.0719 O.OOOg 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 O.OOI3 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 0.1290 O.OOI7 30 3-1548 0.042 15 o . 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 0.1783 o . 0024 34 3-9565 0.052 II 0.1933 0.0025 35 4.1827 0.055 10 0.2093 0.0027 36 4.4201 0.058 9 0.2267 o . 0030 37 4.6691 0.061 8 0.2455 0.0032 38 4.9302 0.065 7 0.2658 O.OO35 39 5.2039 0.068 6 0.2876 0.0038 40 5.4906 0.072 5 0.3113 0.0041 4i 5-791 0.076 4 0.3368 0.0044 42 6.1055 0.080 3 0.3644 0.0048 43 6.434 6 0.085 2 0.3941 0.0052 44 6.7790 0.089 I 0.4263 O.OO56 45 7-I39 1 0.094 O 0.4600 0.0061 46 7.5158 0.099 + I 0.4940 0.0065 47 7.9093 o. 104 2 0.5302 0.0070 48 8.3204 0.109 3 0.5687 0.0073 49 8.7499 0.115 4 o . 6097 0.0080 50 9. 1982 O.I2I 5 0.6534 0.0086 5i 9.6661 O.I27 6 0.6998 0.0092 52 10.1543 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 O.OI2 56 12.3244 0.163 ii 0.9792 0.013 57 12.9251 0.170 12 1.0457 0.014 58 13.5505 0.178 13 1.1162 0.015 59 14.2015 0.187 654 THE STEAM-BOILER. TABLE la. Continued. ti u a STEAM-PRESSURE. 3 STEAM-PRESSURE. e 1 1 d. i In Centimetres. In Atmospheres B < j I , -w t>.N M t^ O OOO 10 VO vo ^-fo COOO w 4 M oo roo - O t. ro CJ ID'-' IOM C4OO lOt^.(N 88 H. M o oo 10 oo r-.oo moo m t~. M 1000 CM vo O^-CMOim-^-Tj-LO IOVO VO t^ t^OO 00 OO ON ON JHMwOOOVOmOVOW ) M 4-VO* OO ON M 1 CO lOVO OO >vovovovovo C^t^t^c^t^ ONO O M OOO ON O CN CM -^-M O C^.C lO!>.rOrOON Tt-VO , ON me oo CMVO MVO OOOOOOOOOOOOOOOO - ' .* " CM VO O -^-OO CM r-sOO 00 OO ON CJN inQino moino mo ino mo ino mo ino ino mo in HMwwcoro^-Tj-io mvo vo K txoo 00 & O O M M |l Cl APPENDIX. 6 57 . *? *?" r ? C0 . e *. e . "? "? "f ? 'T ^ , *f ^ *? * 1 lOlOfOfOM \O ^J- O ^J- t^ O <""> rn CO lOVO 00 O 6 M tf> T(- \0 1^.00 O P< >0 t^. ON^O C i rO O> ON ir>\O 1O -4- M in 10 10 10 IDNO NOOVO\OVONO t^t>.t-xrxr^MN6 M -*ti.o N *-*& oo --rrt-T-^.^-^-^-^.^.^.^.^.^^.^.^. lo mvo vo vo tx t^ t^ tx ti. -2 < a 8 '5 fitS VO ON M --1-VO ON w -4-VO OO O NOO M w CJ 0) CNI CN1 rOfOC r im i ^ - ^''t-iOlO IOVO O> oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo OO ON M O CNIOO M M M M M H M M K H M S S S W Si * W * if *T) Si M IO\O NO NO NO t^ 1-^ fx t^ f^O <*-^-O l O O ^-OO ^- t*vO ' lOVO ^ t^OO OO OO ON ON ON M O< ro-O O * N O 00 NO ^00 NOt^-*MONONO ONOO -OO VO t^OO ON ^-VO CN) CNI O OO VO IO fO H ONOO VO ^-fOOOO fOCS O C O COO -^-IOM ONONCN11OIO vovo ioioiooio-*Tj-Tj--f^-.^-rororr rooo & V ON ^ O tx m O oo So oooooooooooooooooooooooooooooooooo t^ rxvo toioiOTi-Tt-^-romm VO O ^OO CNI VO O -^-OO WVOOOOVO-^-CNlOMCNJ IOOO O COVO ON M -^- (^ CNI 00 (M IOOO CNI lOONCNl lOONCIvfi NVOvJmO O O O O_ M MM M IN cT N t-. N 222MMM2M2 f0r ' 1 >T Ti " "'^ ^ ^" ^ "* " VO * N O CN! M CS CN! CO -oop MMMMMMMHMMMWMNCNlCNlCNllOO INDEX. A SEC. PAGE Air, minimum, required in Fire, 77 178 Anthracite Coals, . 64 155 Apparatus, forms of 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, denned, . 168 346 selection of Type and location of, . . . . .14? 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, . ..'.-... . , *9 32 Upright and portable, . ._-.-. . .175 3^9 Boiler-construction, controlling ideas in, . . . . . 151 37 660 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, ........ u 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 Boilers, power of, ........ SEC. . 144 PAGE 2 9 I problems in the use of, . 25 43 processes of construction of , . . . . 1 86 400 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 . 174 364 setting of, ........ 177 369 special forms of, 23 42 shells of, 55 129 . 202 427 stationary Flue, ....... . 170 354 . I 9 2 413 Steam, explosions of, . . . ... . 268 538 suspension of, . 177 377 testing Steam, . . . . . . I 9 6 422 transportation and delivery . 198 424 Braced and Stayed Surfaces, . 60 151 54 127 . 179 38i 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, . . . . . . . . . .63 153 Coals, anthracite, , 64 155 Bituminous, 65 156 Coke, 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. SEC. PAGE Commercial efficiency, . . . . * . . . 240 474 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, ..... . . 263 524 Contract, . . . . 200 426 purpose of Specification and, . . '. . 199 425 Cooling Surfaces, Area of, Formulas for, . . . . .98 221 Copper, . ........... . 54 I2 7 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, ......... 143 289 Cylindrical Tubular Boilers, . . . . .171 358- 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, ....... 155 314 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. SEC. Efficiency, commercial, 240 finance of, 239 measures of, ........ 235 of Boiler, conditions of, . . . . . . 149 of Heating surfaces, Formulas for, .... 98 Theory of commercial, ...... 242 Efficiency, variations of, with consumption of Fuel and size of grate, 251 Efficiencies, algebraic Theory of, 241 Elasticity, ... ........ 29 Emergencies, ......... 217, 229 Energetics; Heat-energy and Molecular Velocity, . . . 101 Energy, curves of, ......... 143 Heat and Mechanical, ....... 105 Heat as a form of, . . 98 of Steam alone, . . . . . . . . 270 stored, in Steam, . . ... . . . 142 stored, in Boilers, ........ 269 heated Metal, 277 superheated Water, . . . . .281 Evaporation, factors of, 139 usual rate of, 162 Excess of Pressure, ......... 221 Expansion, Latent Heat of, . . . . . . .113 Experimental explosions and investigations, .... 294 Experiments of Donny and Dufour, 281 Leidenfrost and Boutigny, ..... 282 Explosions, absurd, causes of, 272 causes of, 272, 293 Colburn's Theory of, ...... 275 definition of 271 description of, 271 examples of, 293 experimental 294 fulminating, ........ 271 improbable causes of, ...... 272 Lavvson's and others' experiments of, ... 276 methods of, . . . . . . . . . 274 of Steam-boilers, 268 possible causes of, 272 probable causes of, 272 results of, . 293 statistics of causes of, 273 Theories of, 274 usual causes of, . ... 272 664 INDEX. F SEC. PAGE Feed apparatus, i4 39 2 Filtration, . 124 260 Fitting, . 188 402 Fire, minimum air required in, ....... 77 J 7S temperature of, ......... 76 I/ 2 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, .......... 73 167 heating effects of, ........ 84 194 heating-power of, ........ 75 169 liquid, 72 165 and Gaseous, . . . . . . . .210 444 solid, . . . . . . . . . .211 445 Furnace, adaptation of, 88 206 and grate, ......... 159 329 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 defined; 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 2I 7 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, . . . . 9 1 2I radiation of, 94 216 Sensible and Latent, . . . . . . . .112 243 Specific, . - 9 1 210 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 ^69 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, 13? 275 Total and Latent; Internal Pressures and Work, . . 133 271 Helical Seams, 49 "7 Horse-power of Boilers, . 145 292 666 INDEX. I SEC. PAGE Improvement in Boilers, method and limit of, .... 5 10 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- 92 durability of, . ... . . . . 225 457 method of Test of, . 41 98 L 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 *49 INDEX. 667 SEC. PAGE Methods of Explosions, . ... . . . . 274 558 Method of Treatment, effect of, 33 70 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 81 P Paints and Preservatives, 227 458 Peat or Turf 67 159 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, 175 39 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, *49 3O3 steady rise of, 283 589 Pressures, control of Steam, 2I 5 449 relations of, 136 273 Priming 2I 9 45* Principles of Boiler-Design .150 304 Problem of Boiler-Design, details of the, 166 345 Production of Heat, methods of, 9 2o8 668 INDEX. SEC. PAGE 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, 5* I2 5 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 JI 7 strength of riveted, 49 XI 7 Welded, 52 127 Sectional Boilers, .... sees. 20, 154, 197; pp. 35, 3*4, 423 and Water- tube Boilers 174 3 6 4 Security of Boilers, relative, ....... 284 592 Sediment, 230 462 and Incrustation, 280 574 Sensible and Latent Heat, 112 243 Setting, contact with, . 228 461 Shapes, " Struck-up" or Pressed, ...... 53 127 Shearing, 188 402 Shell and Sectional Boilers, relative strength of, . . . 58 148 Shell Boilers, , 154 314 common stationary, ...... 15 21 Shells of Boilers, . . 55 129 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/O 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, . ...... 76 172 of products of combustion, ..... 78 179 Temperatures, . . . . . ..... 91 210 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 98 Tests of Metal, specification of, . . . . . . . 204 436 results of, ......... 42 104 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 Upright Boilers, . ~. . . . . ^ . . 175 369 INDEX. 671 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, * I 9 i 4O4 Wood, . 68 159 Work, internal pressure and, 133 271 Working iron, method of 45 u 3 CAPITAL, $250,000.00. 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Johnson, C.E., Prof, of Civil Engin- eering, Washington University, etc., etc. Illustrated by up- wards of 150 engravings, with folding Maps, Tables, etc., etc. Third edition, thoroughly corrected, 1887. 8vo, cloth 3 50 "On the whole this is the best treatise on Surveying that we know off." - Railroad Gazette. ' Whatever branch, of work the Surveyor is in, he will find this book valuable and exhaustive." American Engineer. KIDDER. THE ARCHITECT'S AND BUILDER'S POCKET- BOOK of Mensuration, Geometry, Trigonometry, Rules, Tables, and Formulas relating to the strength and stability of Founda- tions, Walls, Buttresses, Piers, Arches, Posts, Ties, Beams, Girders, Trusses, Floors, Roofs, etc., etc. Statistics and Tafcles Relating to Carpentry, Masonry, Drainage, Painting and Glazing, Plumbing, Plastering, Roofing, Heating and Ventilation, Weights of Materials, Capacity and Dimensions of Churches, Theatres, Domes, Towers, Spires, etc., etc. By F. E. Kidder, C.E., Con- sulting Architect. Upwards of 600 p&ges and over 400 plates. Fourth edition, enlarged (1887) ... 3 50 " The book admirably fulfills its purpose of becoming an indispensable com- panion in the wori: of every Architect, young or old." American Architect. MERCUR & MAHAN. IV.ICHIE. PENCOYD. REED. RUSKIN. SEARLES. SINCLAIR. SMITH. TRAU TWINE. PERMANENT FORTIFICATIONS. By Prof. D. H. Mahan. Kevisedand brought up to date by Col. James Mercur, U. S. Mil- itary Academy, WestPoint. Numerous Plates, 1887. 8vo, 3^ mor. $7 50 "This Work is a Text-Book in the U. S. Military Academy, West Point." ELEMENTS OF ANALYTICAL MECHANICS. By Col. Peter S. Michie, ^? U. S. Military Academy. 8vo, cloth, 1887.. 3 " A revised edition, as taught to the Cadets of U. S. Military Academy, West 00 Point. IRON CO.'S POCKET-BOOK (THE). Containing many valuable Tables and Calculations of interest to Bridge Engineers and Users of Iron Beams in general, together with the Standard Shapes and Dimensions of this Company. Fourth edition, re- vised and enlarged, 1887. Containing 39 pages of Plates giving Pencoyd Shapes. 12mo, morocco flaps ....................... 2 50 "To'Ensineers ;md Builders in Iron and Steel this volume is presented with the hope that it may be of assistance to them in their daily labors." TOPOGRAPHICAL DRAWING AND SKETCHING. Including Applications of Photography. Illustrated with Plates, colored and plain. By Lt. Henry A. Reed, Ass't Professor of Drawing, U. S. Military Academy, West Point. 4to, cloth . . 3 50 " This is decidedly the best work of its class that we have ever met with." Engineering News. "An expert at our elbow says that this is one of the best, works on tie subject in the English or any other language." Engineering and Mining Journal. "We can commend without reservation Lt. Reed's work." Franklin Institute. THE WORKS OF JOHN RUSKIN. New issue of the 12- volume edition, 1887. NEW STYLES OF BINDING. 12 Volumes, containing several hundred woodcuts, some of them full page. Extra brown cloth. .......... . ... .............. 12 00 12 Volumes, with all the woodcuts and 206 full-page Plates, colored and plain. Extra blue cloth ................... ....... 18 00 12 Volumes, with all the woodcuts and Plates. Fine % calf. . 36 00 FIELD ENGINEERING. A HANDBOOK of the Theory and Practice of RAILWAY SURVEYING, LOCATION, and CONSTRUC- TION, designed for CLASS-ROOM, FIELD, and OFFICE USE, and containing a large number of Useful Tables, Original and Se- lected. By Wm. H. Searles, C.E., late Prof, of Geodesy at Rens- selaer Polytechnic Inst.,Troy. This volume contains many short and unique methods of Laying Out, Locating, and Constructing Com- pound Curves, Side Tracks, and Railroad Lines generally. It is also intended as a Text-Book for Scientific Schools. Pocket-book form. Eighth edition, with additions, 1887. 12mo, morocco ... 3 00 "I have made an examination of ' Field Engineering ' by W. H. Searles, C.E. The book i* admirable. The internal arrangements and appearance are excellent. It is an easy work to refer to and isplainand clear. There is no useless lumber in it. Every sentence belongs there." Prof. Davis, University of Michigan. LOCOMOTIVE-ENGINE RUNNING AND MANAGE- MENT. A Practical Treatise on the Locomotive Engines, showing their performance in running different kinds of trains with economy and dispatch. Also, directions regarding the care, management, and repairs of Locomotives and all their connec- tions. Illustrated by numerous engravings. 8th edition, 1887. 12mo, cloth ........ * ............. ............................ 2 00 "It is a very comprehensive and thoroughly practical work." American Machinist. "The more it is studied, the more thoroughly will it be appreciated." National Car Builder. ' The book only needs to be known to have a wide sale." Railway News. A TREATISE UPON CABLE OR ROPE TRACTION, as applied to the working of STREET and other RAILWAYS. (Revised and Enlarged from Engineering.) By J. Bucknall Smith, C.E. With illustrations and folding Plates. 4to, cloth ........ 3 50 " The publication of this book seems to us to be most timely. The subject is ably handled by an experienced Engineer." American Machinist CIVIL ENGINEER'S POCKET-BOOK of Mensuration, Trigonometry, Surveying, Hydraulics, Hydrostatics, Instruments and their Adjustments, Strength of Materials, Masonry, Princi- ples of Wooden and Iron Roof and Bridge Trusses, Stone Bridges and Culverts, Trestles, Pillars, Suspension Bridges, Dams, Rail- roads, Turnouts, Turning Platforms, Water Stations, Cost of Earthwork, Foundations, Retaining Walls, etc. In addition to which the elucidation of certain important Principles of Con- struction is made in a more simple manner than heretofore. By J. C. Trautwine, C.E. 12mo, morocco, flaps, gilt edges. 28th thousand, revised and enlarged, with new illustrations, by J. C. Trautwine, Jr., C.E. (1887.). . - .................... 5 00 "It is the best Civil Engineers' Pocket-Book in existence." American Engineer. TRAUTWINE. WADDELL. A METHOD OP CALCULATING THE CUBIC CON- TENTS OF EXCAVATIONS AND EMBANKMENTS BY THE AID OP DIAGRAMS, together with Directions for Estimating the Cost of Earthwork By John C. Trautwine, C.E. Ninth edition, revised and enlarged by John C. Traut- wine, Jr., 1887. 8vo, cloth $2 00 THE PIELD PRACTICE OP LAYING OUT CIRCU- LAR CURVES POR RAILROADS. By J. C. Trautwine, Civil Engineer. 12th edition, revised by J. C. Trautwine, Jr., 1887. 12mo, limp morocco , 2 50 " Probably the most complete and perfect treatise on the sinirle subject of Rail- road Curves that is published in the English language." J~ -Engineering News. THE DESIGNING OP ORDINARY IROiv HIGHWAY BRIDGES. A new Practical Work, with many Tables and Illustrations. Third editibn. With an additional Plate, etc. By J. A. L. Waddell, Member of the Society of Engineers, and Pro- fessor in the University of Tokio, Japan. 8vo, cloth 4 00 "His book is probably the most valuable contribution to the literature of Iron Bridge building that has yet appeared." American Engineer. " We strongly recommend the book to Students and Engineers desirous of learning the art of Bridge design." Engineering News. ' The book will be found of the greatest value to County Commissioners, and all others who have to make contracts with Builders of Iron Highway Bridges, etc., etc." Railway Age. WARREN. A PRIMARY GEOMETRY. With simple and practical ex- amples in Plane and Protective Drawing, and suited to all begin- ners. By S. Edward Warren, C.E., late Professor in the Rensselaer Polytechnic Institute, Troy, N. Y., etc., etc. 12mo, cloth 75 " The object of this volume is to contribute to a general earlier beginning of the study of Geometry; in the belief, moreover, that without such a beginning, elementary education is somewhat one sided. "It is expressly adapted to ' MANUAL TRAINING.' " GENERAL PROBLEMS IN THE LINEAR PERSPEC TIVE OP FORM, SHADOW, AND REFLECTION. OR THE SCENOGRAPHIC PROJECTIONS OP DESCRIPTIVE GEOM1ETRY. ' By Prof. S. Edward War- ren, C.E. New edition, revised, 1887. 17 folding Plates. 8vo, cloth 3 50 " In use as a Text-Book in the Rensselaer Polytechnic Institute, Troy, N. Y., etc., etc. WELLINGTON. THE ECONOMIC THEORY OP THE LOCATION OP RAILWAYS. An Analysis of the Conditions controlling the laying out of Railways to effect the most judicious expenditure of capital. By Arthur M. Wellington, Chief Engineer of the Vera Cruz and Mexico Railway, etc. New and improved edition. 8vo, 5 00 " Mr. Wellington has done great service to the Railroad profession ; more particularly to Engineers, Managers, and Superintendents, by bringing together in a single volume such a mass of valuable matter. It, should be in every Rail- way Library. 1 ' Railway Age. WEST. AMERICAN FOUNDRY PRACTICE. Treating of Loam, Dry Sand, and Green Sand Moulding, and Containing a Practical Treatise upon the Management of Cupolas, and the Melting of Iron. By Thomas D. West, Practical Iron Moulder and Foundry Foreman. Fully illustrated. Sixth edition, 1887, revised through- out. 12mo, cloth. 250 " We can commend the book as the best we know off in its special line." Sanitary Engineer. "Every possible disaster that can be suggested by a long training is set forth with its cause and the method by which it can be avoided. "Engineering (London}. UNIVERSITY OF CALIFORNIA LIBRARY BERKELEY Return to desk from which borrowed. This book is DUE on the last date stamped below. EN MAY 25 1953^ LD 21-100m-9,'48(B399sl6)476 YC 33301 THE UNIVERSITY OF CALIFORNIA LIBRARY