Drying 1-Inch Ponderosa Pine Lumber with a Combination of Microwave Power and Hot Air

This study was a preliminary investigation of the feasibility of drying 1-inch thick ponderosa pine lumber with a combination of 915 MHz microwave energy and convective hot air. Microwave energy inputs ranged from 0.13 to 0.49 kw.hr./cu.ft., and were applied intermittently at regular intervals of from 0.75 to 10.0 minutes. Air temperatures of 70?F. to 270?F. were employed without humidity controls. It was shown that it is, possible to rapidly dry 1 inch by 12 inch ponderosa pine boards 4 or 8 feet long without causing drying defects. Drying times of approximately 2 hours were required to dry sapwood boards from initial moisture contents of about 150 percent to final moisture contents of 12 percent. Some impermeable heartwood boards with initial moisture contents slightly above 30 percent also required nearly 2 hours drying time. Excessive microwave energy input resulted in internal checking. Casehardening appeared when microwave energy input was too low and air temperature too high. Lengthening the time between microwave applications increased drying time but reduced the amount of microwave energy required. Use of 220?F. convective air in combination with microwave energy produced twice as fast a drying rate as drying with microwave energy and ambient air, and allowed a 40 percent reduction in microwave energy input. Boards from two different geographical areas varied widely in their drying characteristics. This was attributed to differences in permeability. Moisture distribution of microwave-hot air dried boards varied ? 3 percent in moisture content along the length of the board, with the surface typically 2 percent higher than the core. Internal board temperatures during drying were non-uniform. This was ascribed primarily to unequal microwave energy distribution by the microwave applicator. Periodic temperature fluctuations were caused by intermittent microwave applications and evaporative cooling. Temperature runaway occurred in three of the 24 boards dried in this study and resulted in internal charring.

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