Drying 4/4 Red Oak By Solar Heat

Three small charges of 1-inch red oak flooring stock were dried in an experimental solar dry kiln at. Madison, Wisconsin, and the results compared with those obtained by air-drying. Each charge was made up of 17 40-inch-wide tiers of 8-foot unseasoned lumber. The experimental dryer was constructed with a 2-by-4-inch wood frame lined with corrugated aluminum painted dull black to absorb solar energy and covered on the outside with transparent plastic film. Side walls and roof had a second plastic film outside the first. Air warmed by solar energy was picked up from both sides of the aluminum lining and moved crosswise through the lumber pile by a 24-inch fan. Refinements in the dryer included a light sensitive cell to increase fan operating time on bright days and vents operated by a wood element control in response to relative humidity changes. Temperature inside the solar dryer averaged 14 to 22?F above the outside temperatures from mid-May to late October and from 8 to 16?F. higher than outside temperatures from November through February. A charge begun in mid-May 1960 dried to 20 percent moisture content in 33 days; one begun in mid-August dried to 20 percent in 23 days; one begun in mid-November reached 20 percent in 105 days. The shorter drying time for the second charge was attributed to the higher temperatures prevailing inside the dryer–about 11?F. higher than the average during the first charge. The average relative humidity prevailing inside the dryer was actually somewhat higher during the second charge (64 vs. 58 percent). When the estimated costs of solar drying (over the year) were compared with the estimated costs of air-drying to a 20 percent moisture content there was little difference between the two. The comparison included costs of inventory, investment, maintenance, and power. The author concluded that in areas where more than a moderate amount of solar energy is received, the potential of solar drying would be improved.

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