Effect of High-Temperature Drying on Quality and Strength of Western Hemlock

The study investigates the feasibility of reducing kiln-drying time by employing temperatures above 212?F. Three schedules were used: the first started with a dry bulb temperature of 220?F. and a wet bulb temperature of 180 or 200?F and maintained these temperatures until the wood was dry; the second schedule used conventional temperatures until the fiber saturation point was reached and then used temperatures of 218 to 230?F. dry bulb and 200 to 205?F. wet bulb; and the third was a conventional schedule. Two- by 10-inch western hemlock was used; 72 pieces, each 3 feet long, were-end-coated and dried. Drying times of the low-high schedule were 10 percent and the constant high-temperature schedule were 30 percent shorter than conventional schedules at an air circulation rate of 600 feet per minute. The variation in final moisture content between pieces was less in lumber dried at temperatures above 212?F. than in lumber dried conventionally. The moisture differential between core and shell was also lower, but all treatments produced stress-free lumber. The least degrade was found in lumber dried under the low-high temperature schedule and the greatest under the conventional schedule. Although more surface checks occurred in lumber dried at a constant high temperature, knot damage was comparable for all schedules. Most strength properties showed an increase, a trend opposite to that experienced with Douglas-fir. Analysis showed that 1) the average strength values of the lumber dried under a low-high temperature schedule ranged from not significantly different to significantly higher than those of conventionally dried specimens, and 2) the average strength values of lumber dried under a constant high-temperature schedule were not significantly different from lumber dried conventionally. Strength property changes and their causes as related to high-temperature drying is reserved for another publication.

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