The Potential of the Forced-Air Drying of Northern Hardwoods

The drying of red oak at low temperatures in a forced-air dry-kiln was investigated. A partial charge (10 percent of chamber capacity) of 4/4 lumber and an 8700 bd. ft. 9/4 charge (67 percent capacity) was successfully dried from green, approximately 75 percent moisture content, to 14 percent without producing abnormal degrade. The principal, significant results derived from the 9/4 lumber drying run. The kiln was operated at 85?F. with a 5?F. wet bulb depression for 40 days, bringing the moisture content from 75 percent to 31 percent. Following this a 27-day period at 100-112?F. brought the moisture content down to 14 percent. A conditioning treatment at 7.7 percent E.M.C. was carried out during the last 20 hours of the drying period. Average air velocity through the charge was 700 fpm, with measurements showing a variation of 450 to 900 fpm. A 3-hour air flow reversal schedule was used. Thermal efficiency during the first 40 days was 28 percent, 8 percent during the last 27 days. With steam at 72 per million B.T.U. and electricity at 2 per kilowatt hour, operating costs were $19.83 per Mbf of 9/4 red oak lumber. Most other hardwoods should dry at comparable or lower costs. For a full charge of 13,000 bd.ft. costs are estimated at $16 per Mbf. Fixed charges, 5 years, 6 percent interest, 300-day operating year are $12.80 per Mbf. These preliminary studies indicate that a low-cost unit is capable of controlling conditions to allow for uniform drying and minimum defect development. Predrying of other northern hardwood species will be the subject of future studies.

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