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The rate at which wood in the form of rough-sawn sawmill products can be safely dried is determined by its structure and physical properties. Water moves out predominately by diffusion. Drying conditions must be controlled to prevent defects occurring because of shrinkage stresses. Two war periods stimulated drying research and equipment design. Prior to World War I, dry kilns were of the natural circulation type and often described as “hot boxes.” Modern dry kiln equipment features forced-air circulation with automatic reversal of air flow direction, automatic dry- and wet-bulb temperature control, and controlled venting. The application of impingement drying processes is gaining favor. Kiln drying schedules to accelerate the drying rate and to minimize defect formation have been greatly improved. Time-based program controllers are being given industrial trial. Moisture content quality control is being attained to assure users of fabricated wood items better performance, enhancing satisfaction.
Rietz, Raymond C.
Seasoning specialist, U.S. Department of Agriculture, Madison, Wis.