The objective of this project was to provide the information required by Canadian lumber producers to evaluate the superheated-steam/vacuum (SS/V) drying technology as applied to 8/4-inch white oak with respect to productivity, wood quality, and energy consumption.
A drying test was conducted on a load consisting mostly of lumber in the Select&Better grade. Initial board moisture contents averaged 53.1%. The tests involved measurements of 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. A schedule that had been used successfully in SS/V drying tests with 8/4-inch red oak was used for this test.
A total of 21.6 days was required to reduce the lumber moisture content from 53% to 16%. In fact, this result underestimated actual drying time due to the cycle being interrupted prematurely and to relative humidity in the kiln at the beginning of the cycle being lower than set. As a result, a significant increase in surface and internal drying checks was observed. Relative humidity lower than the set point at the beginning of the cycle may account for such checking. The average final moisture content gradient of 12.3% was attributed to the cycle being interrupted when overage moisture content was still higher than target. Prong tests showed no indication of significant residual stresses, even though no conditioning had been performed. Total energy consumption was 18,602 kJ/kg of evaporated water with 48% as thermal energy and 52% as electrical energy.