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Mills producing spruce–pine–fir (SPF) dimension lumber are required to carry out heat treatment before shipping the lumber to markets. Kiln drying/heat treatment is the most common strategy for drying SPF lumber. Currently there is a question about whether to use air drying before or after heat treatment to improve lumber grade recovery and reduce energy consumption. We tested three drying strategies for spruce/pine and sub-alpine fir lumber: heat treatment followed by air drying (HT+AD), air drying followed by heat treatment (AD+HT), and kiln drying heat treatment alone (KD/HT). Results related to final moisture content uniformity, warp, and drying stress were better when air drying was incorporated in the strategy. The standard deviation of the final moisture content was reduced from 3.9 to 1.3 percent for spruce/pine and from 7.2 to 3.1 percent for sub-alpine fir. Warp was reduced by 27 to 42 percent for spruce/pine and 14 to 41 percent for sub-alpine fir. Using the prong tests, drying stress was reduced by 23 to 60 percent for spruce/pine and 35 to 71 percent for sub-alpine fir. Improved final moisture content uniformity, less warp, and lower drying stresses were obtained for the lumber tested under the HT+AD strategy compared with the AD+HT strategy.

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