High Temperature Kiln-Drying of Lumber–A Summary of Canadian Progress

The Ottawa Laboratory conducted exploratory investigations with a German-built high temperature kiln of limited versatility. Results were promising enough to warrant construction of a new experimental kiln. This kiln was an overhead cross-shaft type with two fans mounted centrally powered by a single motor. The kiln had a capacity of 500 FBM, a temperature range of 60-300?F., and circulation rates of 350-1000 fpm. Heating was done electrically (12 KW) and humidification was accomplished with low pressure steam. A typical test run for Eastern white spruce dried from 80 percent MC to 5 percent MC was as follows: 900 fpm, dry-bulb temperature 240?C., wet bulb temperature 200?F., 18-1/4 hours total time including 2-1/2 hours warm-up and 1-1/2 hours conditioning. The range of final moisture content is a function of circulation and fan reversal. Very high circulation rates were highly beneficial on the final moisture content range and on reducing the total drying time. Schedules for drying softwoods have been published in the F.F.L. of Canada Tech Note No. 2. “High Temperature Kiln-Drying of Eastern Canadian Softwoods – Drying Guide and Tentative Schedules”. Drying hardwoods such as birch is more difficult than softwoods although the quality was good in comparison with tests in the German kiln. Collapse is the more prevalent defect. Steaming appeared to prevent honeycombing. From practical considerations softwoods can be dried commercially with complete success by high temperature methods. Hardwoods require more research. Economics are hard to determine from this kiln, but quick turnover of inventory and kiln size are extremely advantageous factors. Lumber dried by high temperature is less hygroscopic and more dimensionally stable.

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