High-Temperature Initial Drying of Wood: Potential for Energy Recovery

The reduction in energy consumption when drying wood is an important consideration of the forest products industry. A method whereby lumber is initially dried at high temperatures prior to final drying in a conventional kiln is explored, and energy saving possibilities are discussed. Green 4/4 boards of cottonwood and sugar maple were initially dried in a prototype jet dryer for 1 hour to remove approximately half the moisture in the wood, prior to transfer to a conventional kiln for completion of the drying by standard schedules. Cottonwood was initially dried at 300?F dry-bulb temperature (DBT) and 190?F wet-bulb temperature (WBT), whereas sugar maple was dried at 250?F DBT and 180″‘F WBT. The quality of the sugar maple boards was excellent (90% recoverable as clear and without defects) and that of cottonwood good (73% recoverable). Based on the experimental work with cottonwood and sugar maple, a drying method is proposed whereby a jet dryer and a standard kiln are used in tandem to dry lumber. The hot and humid vent air from the jet dryer is directed through a recovery heat exchanger located in the conventional kiln. Energy balances done for several examples utilizing this tandem process predict overall energy savings over conventional drying of from 12 to 15 percent.

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