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 FOREST PRODUCTS LABORATORY t FOREST SERVICE 
 U. S. DEPARTMENT OF AGRICULTURE 
 
 SPECIAL METHODS OF SEASONING WOOD 
 
 VACUUM DRYIi:C 01 W 00D 
 
 )RY 
 
 Many different processes have been proposed for using a vacuum to dry wood. 
 Charles Howard was granted a patent for a vacuum process in 1893 and numerous 
 variations have been patented since. No vacuum process for drying lumber has 
 come into general use either here or abroad, except the steaming and vacuum 
 process used to condition poles and other forms of pine timber prior to 
 preservative treatment. Although amazing results have been obtained by 
 drying lumber in small experimental vacuum-drying equipment, there are 
 several limitations to the use of the process in large-size commercial 
 equipment. Eecause of there limitations, the Forest Products Laboratory has 
 never thoroughly investigated this process. 
 
 A vacuum is merely a reduced air pressure, and the principles of drying are 
 no different at a very low pressure than at any other pressure. The use of 
 a vacuum alone is not effective for rapidly drying wood. Drying of wood in- 
 volves two processes, movement of water to the surface of the wood and 
 removal of that water from the surface. A vacuum maintained by continuously 
 withdrawing the vapors from a drying chamber materially aids removal of the 
 water from the surface for a short time, but considerable quantities of heat 
 are required to evaporate water. The evaporation of water from wood in a 
 vacuum quickly cools the wood so that evaporation becomes very slow i'n spite 
 of the vacuum. Furthermore, except at the very start, the rate of drying of 
 wood is governed by the rate of moisture, movement from the interior to the 
 surface. A vacuum has very little effect en the rate of moisture diffusion 
 through wood. The vacuum processes proposed for drying wood, therefore, have 
 incorporated some means of heating the wood before the vacuum is applied. 
 Hot air, hot water or other liquids, and steam have been used for heating. 
 
 A large load of lumber cannot be heated uniformly by ordinary means during 
 the vacuum period because there is no air or other substance in the vacuum 
 to carry heat from the heating elements to the lumber by conduction or 
 convection. There is no practical way of heating the lumber in the middle 
 of a large load by contact or radiant heat. Although radiant heat may 
 produce amazing results on a few boards in small apparatus, the only heat 
 available in the center of a large load is heat stored in the wood during 
 the Seating period. The maximum amount of heat possible to store in wood by 
 heating with any means is limited, depending on the maximum temperature used, 
 the density of the wood, and the amount of water in the wood. The amount of 
 evaporation that can be accomplished by one heatiag-and- vacuum period, 
 therefore, also is limited. 
 
 When the heating medium is withdrawn and the vacuum applied, some of the 
 liquid water present in the cell cavities can be forced out of pervious wood 
 by mechanical action, because of the differential in pressure between the 
 wood and the vacuum. This difference can be increased by steaming or other- 
 wise heating at pressures above atmospheric. The amount of water removed by 
 this mechanical action is limited and is negligible for refractory woods 
 
 Rept. 1TO.H1665-5 
 
 t Maintained at Madison 5, Wisconsin in cty 
 
UNIVERSITY OF FLORIDA 
 
 ■■■" mi 
 
 3 1262 08924 3405 
 
 trhieh water does not diffuse readily, such as sweetgum heartwood. 
 tory woods are also very resistant to moisture diffusion eyen 
 • temperatures above the boiling point of water. 
 
 Most of the water evaporation also takes place during the first part of the 
 vacuum, resulting in a rapid cooling of the wood. There is no advantage 
 prolonging the vacuum more than 2 hours, because, at the low temperatures 
 reached, evaporation is very slow. 
 
 Because of the limited amount of drying possible with one heat ing-and- vacuum 
 CJ "le, a number of cycles of alternating heating and vacuum must be used to 
 dry wood to low fuoisture content values. When the liquid water originally 
 present In the cavities of the wood cells has been taken out of the wood at 
 approximately 30 percent moisture content, no further drying by mechanical 
 action occurs. The rate of heating of the v/ood and the diffusion of water 
 through the wood become slower and slower as the moisture content is 
 lowered so that the vacuum process becomes increasingly inefficient. If 
 steam is used as a heating medium, a point of equilibrium is reached where 
 the moisture absorbed by the wood during the heating is equal to the amount 
 removed during the vacuum, and no further drying occurs. It would be ideal 
 to use humidified air as a heating medium, controlling the conditions so 
 that little or no moisture is taken from or given to the wood during the 
 heating, but such conditions are difficult to control in the vacuum process. 
 
 In addition to the limitations of the process, the fact that the equ' 
 is expensive works against commercial utilization. Also, the woods - 
 dry rapidly in a vacuum can be dried ranidly in an ordinary kiln. 
 
 FOREST PRODUCTS LABORATORY 
 : 'A oi son . c , "': SCO] 
 MARCH 194~ 
 
 Rept. No. R1661