622 



Yearbook of Agriculture 1949 



is to be done. The structural materials 

 in the building, whether wood, con- 

 crete, brick, or tile, also have a bearing 

 on the expected life and maintenance 

 costs of the kiln. Manufacturers of dry 

 kilns and engineers can provide such 

 engineering services. 



The early dry-kiln designs involving 

 forced-air circulation with internal 

 fans were developed at the Forest 

 Products Laboratory, and the patents 

 on them were dedicated to public use. 

 The dry-kiln companies and engineers 

 whose designs are based on those pat- 

 ents attest to the soundness of the de- 

 sign. The development by the Forest 

 Products Laboratory of the internal- 

 fan type of dry kiln resulted from a 

 need for drying freshly sawn lumber 

 quickly, cheaply, and with control of 

 seasoning defects. 



Perhaps the most important aspect 

 of kiln operation is the changing of 

 the conditions of temperature and rela- 

 tive humidity within the kiln so as to 

 control the drying of lumber in accord- 

 ance with a definite schedule. In most 

 schedules, the initial drying conditions 

 for lumber that is freshly sawn are mild 

 enough to prevent seasoning defects, 

 such as end and surface checking. For 

 some hardwoods, the initial tempera- 

 ture may be 105 F.; and some soft- 

 woods can be subjected to 180. The 

 initial relative humidities are quite 

 high (80 percent) , but they are rapidly 

 reduced as the stock becomes drier. As 

 the lumber dries, the temperature is 

 usually raised until rather high tem- 

 peratures and low humidities are 

 reached near the end of the process. 

 Final temperatures are often near 200% 

 and final relative humidities as low as 

 15 percent. 



A typical drying schedule is based on 

 the moisture condition of the lumber, 

 and changes in temperature and rela- 

 tive humidity are made when certain 

 stages of lumber dryness are obtained. 

 Samples in the kiln are periodically 

 weighed to determine the moisture 

 condition of the stock, and the rate at 

 which the samples dry determines 

 when the changes in temperature and 



relative humidity are made. Some of 

 the softwoods, however, are dried at 

 such high temperatures and in such 

 short periods of time that the changes 

 in drying conditions are placed on a 

 time basis. In that case, freshly cut 

 lumber is subjected to certain initial 

 drying conditions that are changed 

 after a certain number of hours of dry- 

 ing, the time of the changes having 

 been determined by previous studies 

 or experience. 



BEFORE ANY CHARGE of lumber is re- 

 moved from the dry kiln, it is desirable 

 to operate the kiln at conditions that 

 tend to bring all of the boards to the 

 same moisture content. Some boards 

 dry faster than others and the drying 

 conditions are changed so that the dry 

 boards will not overdry but the high 

 moisture-content boards will continue 

 to lose moisture. This is called the 

 equalizing period. The time required 

 to equalize a charge of lumber depends 

 on the species, its thickness, and the 

 degree of nonuniformity of its mois- 

 ture content at the time the stock is 

 ready for equalizing. After equalizing, 

 the lumber may be subjected to a con- 

 ditioning treatment to relieve stresses 

 that develop during the drying process. 

 If hardwood lumber, for example, is 

 not properly conditioned after kiln dry- 

 ing, boards, when resawn or cut into 

 two thinner pieces, will tend to cup 

 toward the newly sawn faces and may 

 not be suitable for the use intended. 



Wood that has been kiln-dried to 

 low moisture-content values that are 

 more nearly in equilibrium with those 

 of winter-heated homes will absorb 

 moisture from the atmosphere if it is 

 stored in warehouses where normal 

 outdoor air conditions prevail. 



One of two courses is recommended 

 for lumber stocks that have been kiln- 

 dried to low moisture-content values, 

 such as required in the furniture in- 

 dustry. Either the dry lumber should 

 be fabricated immediately and the 

 products protected from moisture 

 changes with finish coatings, or the 

 stock should be stored in warehouses 



