Microwave drying of hardwood: simultaneous measurements of pressure, temperature, and weight reduction

This study investigated how certain factors affect different wood species during microwave drying. Microwave drying of ash, beech, and oak was carried out and continuous measurements of weight, internal vapor pressure, and temperature were made. Maximum dimension during the tests was 50 by 50 by 500 mm. The vessels in the wood species were measured with respect to size and quantity by using a digital image processor, and permeability calculations were made. The experiments showed that ash and beech are suitable for microwave drying, while oak is more difficult to dry. The increases in the internal temperature were similar in every test. Speed of heating varied between 0.2? and 3?C per second, depending on specimen dimension. Internal vapor pressure curves were similar in beech and ash but the maximum values were considerably higher in beech, where the pressure could reach up to 8 kPa. The increase in pressure in oak appeared later on in the process and continued until the process was interrupted. The pressure was independent of specimen dimension in ash and oak, but increased with increasing length in beech. Moisture flux varied between 0.05 and 0.32 gram per second but was independent of specimen dimension and lower in oak than in the other species. Two different methods for determining the permeability constants gave almost the same values. The permeability constants for the species were determined to [1-35]|10 to the -12th power m3/m and there was no significant difference between the wood species.

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