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1991). Above fiber saturation point (FSP) the limiting factor is energy transfer; below FSP, mass transfer becomes the controlling factor (Koumoutsakos et. al 2001). As drying progresses, less free water is available and most of the mass transfer occurs by diffusion, which is a much slower process than bulk flow (Rosen, 1980), thus temperature is increased significantly in the late stages of drying to maintain an optimum drying rate. Although, longitudinal diffusion is 10 to 15 times faster than transversal diffusion, this difference is more than offset by the relatively large ratio between length to width (or thickness) of lumber. Drying is affected by environmental factors such as temperature, relative humidity and air flow. Wood is often dried using a single method, such as kiln drying, or via combined methods, such as air drying and then kiln drying. A combination of methods is usually used to increase the throughput of dry kilns and reduce the drying cost for slow drying species or thicknesses. Air drying is where the lumber is placed outdoors and the natural flow of air and heat from the sun is used for drying the wood. The lumber is stacked in piles allowing the air to pass through the pile and dry the wood. It is often the most economical and energy saving method to remove water from wood (Dening et. al 2000). Usually, it is used to obtain a moisture content of 20- 25% and then the drying process is complemented by kiln drying to achieve a final average moisture content of 7-8% (Forest Product Laboratory1999). The drying rate completely depends on the weather conditions, which means that the temperature, relative humidity and air flow cannot be controlled. Degrade is frequent due to rainy periods or excessive sunny periods, which can increase checks, splits or warp, for example. Also, the drying times are longer which causes excessive inventories (Dening et. al 2000). One of the advantages of air drying is that is a method that reduce energy costs. Dening et. al (2000) established that each 1% of moisture content removed by the use of air drying can save around 50 to 85 BTUs per board feet; while for a kiln drying of 50 MBF, each 1% moisture content that is removed can save approximately 2.5 to 4.25 x 106 BTUs (Dening et. al 2000). 6PDF Image | Impact of Vacuum-Drying on Efficiency of Hardwood Products
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