﻿KILN DRYING HANDBOOK. 45 



always enough, however, since there may be a vast difference as to 

 seasoning between two lots of stock dried down to the same moisture. 

 The specification should include a clause concerning the presence 

 of drying stresses, based upon the use of stress sections. To be com- 

 plete and accurate such a clause would be quite lengthy and cumber- 

 some, and therefore more or less impractical. However, a simple 

 statement that the wood shall be free from injurious drying stresses, 

 while very broad, affords reasonable protection to the purchaser. 



STORAGE OF KILN-DRIED STOCK. 



Whenever possible, the stock should be cooled before it is removed 

 from the kiln, since exposure of the hot stock to the cool air is liable 

 to cause checking". All of the boards in the kiln are not of the same 

 moisture content at the end of the drying period. It is therefore 

 necessary that they be held in storage until both dry and moist 

 boards have the same moisture content. The required time for storage 

 varies with conditions. Where little accuracy is needed, as with soft- 

 wood, the stock need be stored but a short time. One week is con- 

 sidered long enough for furniture stock, and two weeks are specified 

 for aircraft stock. Careful conditioning in the kiln reduces the re- 

 quired time of storage. 



Dimension stock and finished wood products which have to be 

 stored should be held in the proper atmospheric conditions, or they 

 will absorb or lose too much moisture. Later, when the stock is 

 manufactured and put into actual use, this loss or gain may damage 

 its serviceability. Stock taken from damp, unheated storerooms into 

 heated shops is too moist for the best utility. The moisture is un- 

 evenly distributed not only in the individual pieces of stock but in 

 the entire pile. The boards on the sides and top have a different 

 moisture content from those in the pile. Products made from such 

 stock may be end-checked or distorted. Short stock with large end 

 surfaces warps when stored in a damp atmosphere. Such stock used 

 in chair seats of the common saddle style, if dried too rapidly, shows 

 end checks and open glue joints. 



KILN TYPES. 



Dry kilns for wood, may be grouped in two general classes, com- 

 monly known as progressive and compartment. Progressive kilns are 

 sometimes called " continuous " kilns, and compartment kilns are 

 known as " box " or " charge " kilns. The differences between the two 

 types depend on the method of handling the stock through the kiln. 

 In the progressive kiln (Fig. 6), the stock enters at one end and moves 

 progressively through to the other end, emerging, presumably dry, 

 at the proper time. The stock is fed in and removed periodically, 

 and the process is continuous. In the compartment kiln the entire 

 kiln is loaded at one time, and the charge remains in place throughout 

 the drying period. In the progressive kiln the temperature and 

 humidity at any point remain constant,, but the kiln is hotter and 

 drier at the discharge end than at the receiving end ; in the compart- 

 ment kiln the temperature and humidity are as nearly uniform as 

 possible throughout the kiln at any given time, and are changed from 

 time to time as the stock dries. 



