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Kiln dying consists of using one or more chambers that are designed to provide and control temperature, relative humidity and air flow for the proper drying of wood. During kiln drying, schedules are used to control the temperature, relative humidity, and air flow, allowing the lumber to dry at a safe rate without producing degrade. Drying schedules vary by species, thickness, and grade. The designs of the kilns have changed through years. There has been modification in the mechanism of heat, the arrangements of the fans, and the control of relative humidity or wet bulb temperature. Also, different materials have been used for the chamber construction. Section 2.2 and 2.3 will describe two types of kiln drying: conventional and vacuum. 2.2 CONVENTIONAL DRYING Conventional drying refers to air drying + kiln drying. Kiln drying is performed in a closed chamber or building in which air is rapidly circulated over the surface of the wood being dried. The initial drying temperatures that are used go from 100° to 170° F and final temperatures from 150° to 200° F. Control of relative humidity or EMC is necessary to avoid shrinkage- associated defects and to equalize and condition the wood to the degree of precision needed (MacMillen et. al 1978). Air velocities through the load in drying hardwoods generally are between 200 and 450 feet per minute and are required for uniform evaporation of water from the surface of lumber and to bring heat to the lumber. Moreover, the temperature and the relative humidity are managed by semiautomatic dry- and wet-bulb temperature recorder-controllers (MacMillen et al 1978). In general, for drying hardwoods, two types of conventional kilns are used: 1) package-loaded compartment kilns and 2) track-loaded compartment kilns. There are two basic heating systems: steam and hot air (or direct-fired). The most common are steam-heated. However, the direct-fired kiln, with supplemental steam or water spray for humidification, has been used occasionally for hardwoods (Simpson 1991). A conventional dry kiln is usually composed of the following parts (Rubberwood Processing Manual 2007): 7PDF Image | Impact of Vacuum-Drying on Efficiency of Hardwood Products
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