Energy efficiency and drying rates during vacuum microwave drying of wood

The application of vacuum-microwave drying can dramatically decrease drying times in comparison to conventional kiln drying. Instead of thermal energy for the conventional drying, electric energy is used for heating the material in microwave drying. To investigate the energy efficiency specimens of beech, maple and spruce wood were dried in a semi-industrial and a laboratory vacuum-microwave kiln. Both allowed measurement of the energy consumption during the process. In addition, a continuous monitoring of the weight loss was possible in the laboratory kiln. In the first experiments energy input was controlled manually. To avoid material degradation the process was controlled by measurement of wood surface temperature in further experiments. Energy efficiencies of up to 80% were reached depending on the moisture content of the samples and wood species. Drying rates depended additionally on mass specific energy input. For beech drying rates between 1.0 and 4.5%/min were reached. The period of fairly constant drying rate extended to values below the fibre saturation point. The wood was in most cases free of checks and always free of colour changes because of the short process duration and the absence of oxygen.