Timber Moisture Class Assessment in Kiln Drying

This study was carried out to evaluate the final moisture classes produced, the shape distortions, and the drying costs/profits for 115mm square Pacific coast hemlock timbers to the five different target moisture content levels of 12%, 15%, 20%, 25%, and 30% dried in an experimental heat-and-vent kiln. Kiln dried timbers were divided into four moisture classes, D15, D20, D25, and above, then were fully evaluated according to the Japanese standards for grade and worth. The drying times were inversely proportional to the target moisture levels, and the percentage of wets in the population was opposite and exponentially increased above the moisture content level of 18%. Developments in drying shape distortions such as twist and diamonding were cumulatively augmented from 30% to 12% target moisture content, but bow somehow wiggly increased. The target moisture content level of 12% favored to D15 moisture class and the levels of 15% and 20% favored to D20 class. The level of 25% favored to D25 and D30 classes. The on-grade yield of the Zairai was above 85%, independent of the target moisture content levels. The grading results for five test runs show that degradation in this study was not from drying losses, but resulted from the green wood quality. The most profitable target moisture content identified in this study was 15%, followed by the 12% and 20%. The 25% and 30% target moisture content levels produced negative profit due to higher wet percentages.