﻿KTLN DRYING HANDBOOK. 49 



Irawing the air from under the lumber on the floor of the kiln, and 

 mother having the vents located in the roof. Almost as wide a range 

 s to be found in the location of the inlet openings in the kiln; al- 

 ;hough the air may be brought into the kiln in ducts running along 

 ;he floor, several kiln designers cany it up in risers at various points 

 tlong the length of the kiln and deliver it at convenient heights above 

 ;he rails. While it is usual to provide considerable outlet flue area, 

 ;here is a wide difference in the amount of inlet area. One maker 

 Drovides none at all, another allows about a square foot for a kiln 

 10 feet long, and a third insists upon at least 4 or 5 square feet for 

 i similar kiln only 40 feet long. 



The cross circulation in most ventilated compartment kilns depends 

 argely on the draft of the chimneys or vents. It may be assisted by 

 steam jets placed in air intakes or outlets, and even by the steam used 

 n the kiln for humidification. If the circulation caused by the cooi- 

 ng of the air as moisture is evaporated from the wood can be made 

 ;o augment the draft of the chimneys, the maximum circulation and 

 he most satisfactory drying will be secured. Figure 7 shows the 

 general construction of a ventilated compartment kiln. This figure 

 s a composite representing no particular make of dry kiln. While 

 t is not offered as a scale drawing for an ideal kiln, very good results 

 ■an be obtained from kilns built upon the principles illustrated. 



The principles of the kiln can best be understood by following the 

 arrows which indicate the air flow. The air enters through the inlet 

 Iiict, which has suitable openings along its length. The steam jet 

 located in the inlet duct where it enters the kiln increases the rate of* 

 low. The air from the duct passes over the heating coils and into 

 he chimney or flue in the center of the lumber pile, thence outward 

 md downward. Some is exhausted through the flue outlets and some 

 returns past the steam-spray line and the baffles to the heating coils 

 ind around again. The downward-pointing steam sprays are always 

 used for steaming and high-humidity treatments, and may be used 

 :o assist the steam jet or to act in its place during the drying period, 

 rhe baffles prevent the air from rising in any passages except the 

 jhimney, thus assisting materially in producing and maintaining the 

 lesired air flow. They also prevent the steam from spraying against 

 the lumber or the heating pipes. The floor boards under the lumber 

 pile protect the lower layers from direct radiation and prevent the 

 short-circuiting of the air through them. 



WATER SPRAY AND CONDENSER KILNS. 



The water-spraj 7 kiln was invented and developed at the Forest 

 Products Laboratory. As ordinarily designed it embodies the prin- 

 ciples of the condenser kiln, and the two may be described together. 

 Figure 8 is a cross section of a typical water-spray kiln. The cir- 

 culation is similar to that in Figure 7, although there are no intakes 

 or outlets. The baffles at the bottom of the spray chambers prevent 

 spray or mist from passing along with the air and thus increasing 

 the humidity beyond the desired point. The condensers and the 

 water sprays are located close together, and both serve to regulate 

 the humidity and increase the circulation. The sprays and con- 

 densers are usually used for high and low humidity, respectively. 

 When the sprays are in use the air is cooled to the dew point each 



23241°— 23 1 



