Distribution and Variation of Pressure and Temperature in Wood Cross Section during Radio-Frequency Vacuum (RF/V) Drying

The pressure and temperature at the same location in the middle cross section of Sugi wood were measured simultaneously during radio-frequency/vacuum (RF/V) drying. The distribution and variation of pressure and temperature in the wood cross section were investigated in different drying stages. The pressure behavior during the drying process was due to the pressure reduction rate and water vapor generation rate in wood. The temperature was higher in the center and was low from the center to surface layer. Pressure and temperature did not present symmetrical distributions along the vertical direction in the cross section. The pressure was irregular during the timber heating stage and became higher in the central zones than in the intermediate and surface layer zones during the drying process. Pressure curves exhibited three stages (irregular, rapid decreasing, and slow decreasing), in combination with an initial heating stage and a constant temperature stage. Above the fiber saturation point (FSP), the pressure (P) was greater than or equal to the saturated vapor pressure (Ps), corresponding to the temperature at the same location; below the FSP the pressure was maintained by superheated vapor and was smaller than Ps.