Comparison of efficiency of radiofrequency/vacuum kiln and conventional kiln for drying Scots pine wood

In this study, the efficiency of radiofrequency/vacuum (RF/V) technology for drying of pine wood (Pinus sylvestris) was investigated compared to the hot air convective method. For this purpose, flat-sawn boards with dimensions of 4.8 × 21 × 120 cm, heartwood percentage of 72.9% and average green moisture content of 60% were cut. The boards were dried using three methods (1) an industrial conventional kiln at low temperature (maximum temperature of 50 °C) and (2) normal temperature (maximum temperature of 71.3 °C) according to a 20 step-modified drying schedule and (3) RF/V kiln. RF/V drying was performed in an industrial kiln with a power of 100 kW and a frequency of 50 MHz according to a 6 step-drying schedule with a maximum temperature of 65 °C. Then, final moisture content, moisture gradient, discoloration, wood drying defects and stresses were measured. The results showed that in addition to a significant reduction in the drying time of the boards in the RF/V kiln, the intensity of surface and end-checks in RF/V drying was significantly lower than that in conventional drying. All dried boards were free from internal checks. The maximum twist and cup were 4.8 and 5.4 mm, respectively, and no difference was observed in the efficiency of the three wood drying methods. Unlike conventional dried boards, the boards dried in RF/V kiln showed no casehardening. Moisture gradient in all dried boards was negligible (less than 2%). The discoloration of the boards dried by RF/V was lower compared to other dried boards.