Drying rate and temperature profile for superheated steam vacuum drying and moist air drying of softwood lumber

Two charges of green radiata pine sapwood lumber were dried, either using superheated steam under vacuum (90 degrees C, 0.2 bar abs.) or conventionally using hot moist air (90/60 degrees C). Due to low density of the drying medium under vacuum, the circulation velocity used was 10 m/s for superheated steam drying and 5.0 m/s for moist air drying, and in both cases, the flow was unidirectional. In drying, stack drying rate and wood temperatures were measured to examine the differences between the superheated steam drying and drying using hot moist air. The experimental results have shown that the stack edge board in superheated steam drying dried faster than in the hot moist air drying. Once again due to the low density of the steam under vacuum, a prolonged maximum temperature drop across load (TDAL) was observed in the superheated steam drying, however, the whole stack dried slower and the final moisture content distribution was more variable than for conventional hot moist air drying. Wood temperatures in superheated steam drying were lower than those in the hot moist air drying, and this could reduce some drying defects such as kiln brown stain and drying stresses. In order to reduce drying time, higher circulation velocity can be used for the vacuum superheated steam drying.