Seasoning of Wood 



621 



will overhang the front end 1 or 2 feet 

 and should be held about 6 inches 

 above the top layer of boards by a few 

 timbers laid crosswise on the pile. Piles 

 should be built so there will be no over- 

 hanging boards at the rear. 



When conditions favor too rapid 

 drying and excessive checking, the 

 checking can be reduced by making 

 wider piles, narrowing the space be- 

 tween piles, piling the boards edge to 

 edge, narrowing the vertical air chan- 

 nels, using thinner stickers, and, some- 

 times, by using shields around the pile 

 for protection against wind, rain, and 

 sun. When the season of the year re- 

 duces likelihood of checking, or the 

 species being dried is not likely to check 

 easily, the circulation of air through 

 the pile can be stimulated by opening 

 up the pile. Faster drying is thereby 

 obtained, and stain and decay are re- 

 tarded. Roof boards or pile covers pre- 

 vent exposure of the boards in the top 

 layer to the direct heat of the sun, 

 which will invariably cause checking. 

 At some plants, lumber of high value 

 is air-dried in open sheds to prevent 

 loss of quality by more direct exposure 

 to the elements. 



KILN DRYING is a process designed to 

 hasten drying by circulating large 

 volumes of warmed air through care- 

 fully piled lumber. Modern drying 

 kilns equipped with blowers or fans 

 can dry wood more thoroughly in a few 

 days than can be done by air seasoning 

 in months. For some kinds of lumber, 

 kiln drying is indispensable. 



In modern dry kilns, conditions can 

 be had that favor the fastest possible 

 drying with a minimum of drying de- 

 fects. In kiln drying, as in air drying, 

 the atmosphere is used as the medium 

 whereby heat is conducted to the wood 

 to evaporate the water it contains. In 

 kiln drying, however, the atmospheric 

 conditions of temperature and relative 

 humidity can be controlled with a rea- 

 sonable degree of accuracy. Thus, the 

 dry kiln is independent of weather 

 conditions. 



Most modern dry kilns are of the 



forced-air-circulation type. Steam coils 

 are generally used for heating air that 

 is circulated through the loads of lum- 

 ber. Recently, furnace-type dry kilns 

 have been developed for use where it 

 is not economical to install steam 

 boilers. In such dry kilns, the steam- 

 heated coils are replaced by large pipes 

 or manifolds in which the hot gases 

 resulting from the burning of sawdust, 

 gas, oil, or other fuel are circulated. 



Good results depend largely on good 

 piling practices. For kiln drying, lum- 

 ber is usually flat-piled on kiln trucks 

 with an adequate number of stickers. 

 Warping of boards is prevented by 

 good piling, stickers used in good verti- 

 cal alinement, and other mechanical 

 devices that make better loads. 



The way air circulates within the 

 kiln determines how the loads are to be 

 piled. Thus, in internal-fan kilns de- 

 signed to move air across the loads, the 

 lumber is piled edge to edge in each 

 layer. In external-fan kilns designed to 

 deliver air upward into the load from 

 a central delivery duct, the lumber is 

 usually stacked with an A-shaped flue 

 in the middle of the load to distribute 

 the delivered air. Many natural-draft 

 kilns are still in use, however, and in 

 such kilns the lumber is piled with 

 spaces between the boards and usually 

 with one or more flues. In contrast to 

 the forced-air-circulation kiln with its 

 edge-to-edge piled loads, the load as 

 piled for a natural-draft kiln contains 

 considerably less volume or footage. 



Designing a lumber dry kiln requires 

 a knowledge of mechanical heating 

 as well as ventilation engineering. Some 

 kilns seem to be of simple engineering 

 arrangement, but actually the relation 

 of the size of the room to the size of the 

 kiln charge and the placement of fans, 

 fan baffles, ducts, and heating coils are 

 not simple. 



The design of the heating system and 

 the method of coupling it to the tem- 

 perature-control apparatus (so as to 

 provide uniform temperatures along 

 the length and height of the entering- 

 air side of the kiln charge) are par- 

 ticularly important if precision drying 



