I US- 7 
 
 
 RJREST PRODUCTS LABORATORY t FOREST SERVICE 
 U. S. DEPARTMENT OF ACRICULTURE 
 
 SPECIAL METHODS OF SEASONING WOOD 
 
 HIGH-FREQUENCY DIELECTRIC HEATING 
 
 Large amounts of heat are needed to dry wood rapidly. Heat is needed to evaporate the 
 water and to keep the temperature of the wood high enough for rapid movement of 
 moisture from inside the wood to the surface. In modern dry kilns, heat is carried 
 from steam coils or other heating units to the wood by air. Much of the heat is lost by 
 radiation and leakage. To avoid drying defects in most hardwoods and some softwoods, 
 temperature and relative humidity conditions must be kept mild or moderate, except 
 during the final stages of drying. Because these woods must be dried more slowly than 
 other woods, the heat losses are larger. 
 
 When it was found that a high-frequency dielectric field could change electrical energy 
 into heat inside a solid, it was logical to attempt to dry wood by this method. Abramenko 
 apparently first used it to dry wood in 1934, but others began tests about the same time. 
 First work in this country apparently was done by Stephen and Holmquest in 1936. 
 Because the method appears to have limited uses, little work has been done on high- 
 frequency drying at the Forest Products Laboratory. This report on the work of others, 
 therefore, has been written to answer general questions on the method. A number of 
 references are given at the end of this report. A considerably larger number are given 
 in Kozlik's bibliography.— 
 
 Dielectric heating requires a generating apparatus capable of setting up an electric field 
 that will oscillate at a high frequency between condenser plates or electrodes. Wood 
 placed in a powerful electric field oscillating at more than 1 million cycles per second 
 is heated quickly throughout to a temperature above the boiling point of water. If wood 
 were very permeable, with no resistance to the movement of free water or vapor, it 
 could be dried rapidly. In general, however, wood is not very permeable. It varies 
 considerably in its structural characteristics. Some species have fairly permeable 
 sapwood, but others have a tight structure. Moisture does not move freely, either as 
 a liquid or as a vapor, through these "tight" woods. Moisture movement may be so 
 impeded that high internal pressures and temperatures well above the boiling point of 
 water are built up. 
 
 In permeable woods, the temperature levels off slightly above the boiling point as long 
 as free water exists. When only bound water (the water held within the walls of the 
 wood cells) remains, the temperature rises. If these high temperatures are prolonged, 
 they weaken the wood and lower its resistance to pressure. This may cause local 
 explosions or split the wood wide open. 
 
 - 
 
 - 'Kozlik, C. J. Bibliography of Special Seasoning Methods. Oregon Forest Research 
 Center, Corvallis, Oreg. 
 
 Report No. 1665-7 (Revised) 
 
 t Maintained at Madison 5, Wisconsin in cooperation with the University of Wisconsin 
 
Pratt and Dean proposed two methods of using high-frequency heat to dry wood. In the 
 first, the temperature is kept above the boiling point of water. This converts the 
 moisture within the wood to steam that can be rapidly driven off. In the second, only 
 enough heat is supplied to maintain a temperature gradient within the wood when the 
 surface is kept cool by air circulation. By the "boiling" method, European beech, 
 which is easier to dry than American beech, was dried in an hour or so without much 
 degrade. Green oak, however, was badly degraded by bursting, and air-dried oak 
 surface checked severely. Other tests showed the method might be satisfactory for 
 European poplar, sycamore, and a few exotic woods. Elm, Douglas-fir, Scotch pine, 
 Sitka spruce, western redcedar, and a number of exotic hardwoods, however, were not 
 tolerant to the process. The "temperature gradient" method was developed with beech, 
 then tried for oak. Although a very low temperature and a small temperature gradient 
 were used, there was some fine checking. The tests showed that drying time might be 
 cut to one-third or one-fifth of the time required for kiln drying. 
 
 Apart from technical difficulties, high-frequency heating is generally too expensive to 
 use for drying wood in this country. In Russia, however, the prospect of low-cost 
 electric power led to further investigation. One report says many high-frequency drying 
 plants are in use there. A technician's handbook by Birjukov describes cost factors 
 and "economic efficiency" of the method. Netushil has stated, however, that even in 
 Russia costs limit the use of this method to special cases, such as the drying of 
 large, high-quality structural parts or partial drying of ties, piles, and building 
 materials before they are treated with preservatives. 
 
 The main costs are for high-frequency generators, power tube replacement, and 
 electricity. A minimum estimate of these costs follows. It is based on drying green 
 beech, birch, or maple sapwood from a moisture content of 70 percent to one of 8 
 percent. The wood is assumed to have an ovendry weight of 35 pounds per cubic foot. 
 About 10, 000 board feet are to be dried per 24 hours. 
 
 To bring the lumber to drying temperature and to remove the calculated 764 pounds of 
 water per hour requires 263 kilowatts. Thus, a generator with a minimum output of 
 about 270 kilowatts would be needed. Because a generator is only about 50 percent 
 efficient and because a certain amount of heat would be lost, the power line would have 
 to deliver at least 600 kilowatts to the generator. At 1 cent per kilowatt hour, power 
 would cost $144 per 24-hour day or $14. 40 per 1, 000 board feet of lumber. 
 
 The generator would cost about $100,000. Three power tubes, such as type 862A with 
 a service life of about 5, 000 hours, would cost $1, 500 each. Total cost of investment 
 and upkeep spread over 10 years would amount to $55 per working day or $5. 50 per 
 1, 000 board feet of lumber. This is based on three hundred 24-hour working days a 
 year. Total cost for power, equipment, and maintenance is then about $20 per 1,000 
 board feet. This figure does not include cost of housing, lumber-handling equipment, 
 labor, or the salary of a skilled operator. 
 
 The drying of green sapwood of softwoods, such as white fir, ponderosa pine, and 
 western white pine would cost more, at least $26 per 1 , 000 board feet. The reason 
 for the higher cost is the greater amount of water present in the softwoods. In this 
 estimate the weight of the ovendry wood was assumed to be 23 pounds per cubic foot 
 and its moisture content when green 160 percent. 
 
 Report No. 1665-7 -2- 
 
In view of the above, high-frequency dielectric heating does not appear practical for the 
 general drying of wood. Under some conditions, however, it could be used to dry 
 special items that would absorb the extra cost. A company in New England, for 
 example, used the method to dry turning squares of birch and maple. The drying unit, 
 which cost about $23, 500, held a charge of about 1, 500 board feet. The 1-1/2-inch 
 squares were given a drying period of 7 hours in each of three chambers. In only the 
 third chamber was dielectric heating used. The wood was removed from the unit after 
 the 21 -hour cycle and then dried in the air under a porous canvas cover. 
 
 References 
 
 (1) Abramenko, S. N. 
 
 1934. The drying of wood by electric currents of high-frequency. Woodworking, 
 U. S. S. R. 10:65-68. (See also Holztechnic 1 5: 165- 1 68, 221, 1935.) 
 
 (2) Bandi, C. 
 
 1959. High frequency drying. Timber of Canada 20(10), 46 et ref. (Also 
 
 reprinted in Northeastern Logger 8(10): 8-9, 35. ) 
 
 (3) Billig, K. 
 
 1950. High-frequency currents for timber processing. Timber News 58(21 29): 1 14, 
 117-8, March. (Reprinted from Civ. Engin. & Pub. Works Rev.) 
 
 (4) Birjukov, U. A. 
 
 1950. Kiln seasoning of timber in an R. F. electrical field. Goslesbumizdat. 
 Leningrad-Moscow, 1950, 102 pp. , 1 dgm. (Forestry Abs. 
 14(1): 110, 1953. ) 
 
 (5) Fessel, F. 
 
 1956. Practice of wood drying in the high-frequency electric condenser field in 
 different countries. Holz als Roh und Werkstoff 14(2): 41-47. (Also 
 Translation No. 95 by Forest Products Laboratory, Canada, 1956. ) 
 
 (6) Hunger, J. 
 
 1960. High-frequency drying of wood. Holzforschung und Holzverwertung 12 
 
 (4), 68-70. (Translation No. 423, U.S. Forest Products Laboratory, 
 1961. ) 
 
 (7) Ishaq, S. M. 
 
 1947. High-frequency drying of wood. Cosgrove's Mag., Oct. 1947; South. 
 
 Lbrmn. , Dec. 15, 1947. 
 
 (8) Miller, D. G. 
 
 1948. Application of dielectric heating to the seasoning of wood. Forest Products 
 
 Research Society Proc. 2:235-41. 
 
 (9) Netushil, A. V. 
 
 1947. Drying of wood with the aid of high-frequency current. Vestnik Ishenerov 
 i Tehnikov 4: 141-48, April; also Engin. Digest 9(10): 343 -46, 1948. 
 Abs. Wood (Chicago) 4(3): 40, March 1949. 
 
 Report No. 1665-7 -3- 
 
UNIVERSITY OF FLORIDA 
 
 (10) Pratt, G. H. , and Dean, A. R. 3 1262 08927 3733 
 
 1951. Report on an investigation into the drying of timber by the application of 
 
 radio-frequency heating. Forest Products Research Lab. (Great Britain) 
 report, Oct. 1951, 36 pp., illus. Abs. Jour. FPRS3(4):78; see also 
 Wood (London), Feb. 1949. 
 
 (11) Shinohara, U. , Otori, S. , and Iwata, K. 
 
 1952. On the drying mechanism of radio-frequency heating. Mt-m. Faculty 
 
 Engin. , Nagoya (Japan) Univ. 4(1). 
 
 (12) Siemens -Schucker twerke Akt. -Ges. 
 
 1936. Method for drying bodies containing moisture, particularly for drying 
 wood by means of alternating electric fields. Ger. patent 663,455, 
 appl. July 28, 1936, issued Aug. 8, 1938. 
 
 (13) Stephen, J. L. , and Holmquest, H. J. 
 
 1936. Drying lumber with high-frequency electric fields. Wood Products 
 41(10): 10-12 and 41(11): 15, 19. 
 
 (14) Villiere, A. 
 
 1956. Seasoning of lumber by high-frequency currents. Rev. Bois Appl. 
 
 11(1): 26-31. (Translation No. 103 by Forest Products Laboratory, 
 Canada, 1956.) 
 
 (15) Vodoz, J. 
 
 1957. The behavior of wood during drying by radio-frequency heating. 
 
 (In German) Holz als Roh-u. Werkstoff 1 5(8): 327 -340. 
 
 (16) 
 
 1958. Seasoning by high-frequency (R. F. ) current. Timber Tech. 
 66(2233): 535-538. 
 
 (17) Wood, H. P. 
 
 1951. Apparatus for drying wood. U.S. patent 2, 543, 6 1 8, Feb. 27, 1951. 
 
 (See also U. S. patent 2, 567, 983, and Northeastern Wood Util. Counc. 
 Wood Note 13, Nov. 1950. ) 
 
 J. M. McMillen 
 W. L. James 
 
 Forest Products Laboratory 
 Madison 5, Wisconsin 
 Revised February 1961 
 
 Report No. 1 665-7 -4