Softwood heating in radio frequency fields

The aim of this study was to investigate the heating rate of lodgepole pine (Pinus contorta) and western red cedar (Thuja plicata) during radio frequency (RF) heating. Wood specimens (40 × 150 × 1000 mm³) with various moisture content and power density were heated using a laboratory size RF dryer at a frequency of 40.7 MHz, until shell temperature reached 56 °C that is approved as a lethal temperature for phytosanitation. Heating rate was positively correlated with power density and negatively correlated with moisture content. The ratio of heating rate to power density had a negative correlation with moisture content and density in both pine and cedar. The regression lines for moisture content had good coefficient of determination (R ²) values of 0.63 and 0.61 °C m³/min kW (pine), 0.50 and 0.55 °C m³/min kW (red cedar) in both shell and core, respectively. The results demonstrate that the ratio of heating rate to power density is a useful parameter to estimate heating rate. The derived empirical equations made possible the calculation of the heating rate within test conditions applied to this study. The initial temperature rise in shell and core allowed a rather accurate determination of local power density. One dimensional mathematical model to describe the heating rate of wood during RF heating that was derived from the governing heat transfer equation combined with internal heat generation was developed and verified. The model using local power density had high R ² of 0.71 and 0.93 in both shell and core, respectively, indicating that the model was able to predict heating rate of wood with various moisture content under the known power density distribution.