Energy Conservation in Wood Dryers Using Vent Air Energy Recovery Devices

Three types of heat exchangers–the finned tube, the crossflow finned plate, and the finned heat pipe–were used to recover energy from the vent stream of a prototype wood jet dryer. The dryer was operated to simulate two kinds of drying operations–batch runs where humidity in the dryer changed as drying progressed and continuous runs where humidity in the dryer remained constant. Heat recovery was determined for dryer vent temperatures of 250?, 300?, and 350?F and vent air flow rates of 42 and 82 cfm. Heat exchanger effectiveness ranged from 35 to 62 percent depending on vent air temperature, flow rate, and type of heat exchanger. The effectiveness of the finned tube heat exchanger, where only sensible heat was recovered, was not significantly influenced by air temperature or flow rate. The effectiveness of the finned heat pipe and the crossflow finned plate exchangers increased as the vent air temperature increased. The finned heat pipe was more effective than the crossflow finned plate over the range of dryer operating conditions. An example of energy recovery from a veneer dryer showed that fuel savings could recover the cost of the heat exchange equipment in less than 1 year.

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