The objective of this project was to provide the information required by Canadian lumber producers to evaluate the superheated steam/vacuum (SS/V) drying process as applied to 8/4-inch red oak with respect to productivity, final product quality, efficiency and energy consumption. Another objective was to compare SS/V drying with conventional drying in terms of productivity and wood quality.
Two homogeneous test loads were prepared from a single lumber lot for the SS/V and conventional tests. Initial lumber moisture content in the two loads averaged 28%. The tests involved measuring drying time as well as wood quality before and after the drying operation, including: distortion, drying checks and splits, final moisture content gradient, dimensional shrinkage, and residual stresses (prong tests). In addition, the tests included measurements of energy consumption, both thermal and electrical, with meters. The schedule used for conventional drying was drawn from Boone et al. (1993) and that for SS/V drying was supplied by Iwotech, the equipment manufacturer. In both cases, the drying cycle was terminated when the all boards used for control had reached the 7 ± 1.5% moisture content range.
Drying was achieved in 7.6 days with the SS/V process, representing a significant reduction (by a factor of 2.2) relative to the time required with the conventional method; and wood quality after drying was comparable. The results obtained with respect to distortion, checking, moisture content gradients, and residual stresses attest to the quality of the drying operation with both processes. Total energy consumption for the SS/V process was 11,307 kJ/kg of evaporated water with 58% as thermal energy and 42% as electrical energy.