Effect of stacking position on drying rate during radio-frequency vacuum combined with mechanical press drying of Douglas-fir and Radiata pine

In this study, the effect of stacking position in the chamber on the drying rate during radio-frequency vacuum combined with mechanical press (RF/VP) drying was investigated for Douglas-fir and Radiata pine. Drying test was performed at a temperature of 40°C using RF/VP dryer with approximately 3m³. The results showed that the stacking position has a significant effect on the drying rate, with a more pronounced decrease in drying rate as the number of layers increases. Moisture content (MC) on day n-1 was important factor in determining the drying rate in both species. The MC distribution of specimens in the chamber before drying was very irregular, but over time, specimens stacked on the RF charge plate side in the chamber tended to have lower MC distributions than those stacked on the ground plate side, which was serious at the layer-10. In addition, as the MC decreases below the fiber saturation point during RF/VP drying, the difference in MC according to the stacking position became more prominent. These findings provide new insights into the effect of wood dielectric properties on drying rate and are expected to improve RF/VP drying technology.