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a) Kiln body and door: metal structure framework. b) Air circulation system: the air inside kiln is circulated by fans, which are directly connected to motors, and located in the upper side or end position inside the kiln. c) Heating and spraying system: the heat sources are steam or hot water. d) Temperature and humidity system: The temperature and humidity detection usually use dry and wet bulb temperature method, and the moisture content detection could use weighing or electric resistance methods. e) Stacking and stack transportation system: this system consists of folks, or stacking machine, stack trolley, and unstacking machine. f) Lab equipment: includes electric oven with fan inside, electric balance, moisture content meter, air speed meter, saws, ruler, digital vernier calipers, and record paper. 2.3 VACUUM DRYING The primary force in vacuum drying of wood is the total pressure difference between the wood and the chamber rather than diffusion as in conventional drying. The prevailing control in the moisture transfer mechanism is water vapor bulk flow from the ends and diffusion through the lateral faces (Chen 1998; Waananen et. al 1993). Moreover, when temperatures approach the boiling point of water, steep total pressure differences are caused by the fast generation of vapor (Waananen et. al1993), contributing to the acceleration of the moisture movement through wood. Free water, in both liquid and vapor phases, travels in the longitudinal direction, and bulk flow is accelerated by the much greater longitudinal permeability [longitudinal-to-transverse ratios ranging between 30,000 to 400,000,000 (Siau, 1984)], which causes water to migrate lengthwise, leaving the individual lumber pieces through the ends (Avramidis and Zwick, 1992; Zwick and Avramidis, 2000). Consequently, the movement of water through wood increases, leading to shorter drying times than can be achieved at atmospheric pressure, and with similar or better quality than conventional drying. 8PDF Image | Impact of Vacuum-Drying on Efficiency of Hardwood Products
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