Drying Southern Pine At 240 Degrees F–Effects of Air Velocity and Humidity, Board Thickness and Density

In all, 108 kiln loads (24 boards per load) of southern pine lumber were dried at 240?F in an air-steam mixture. Boards were 8 feet long, 4 inches wide, and planed green to exact thicknesses. Prior to drying, the lumber was, stored in water; green moisture content averaged 122 percent with range from 35 to 201 percent, standard deviation was 28.8 percent. Factors in the experiment were 1) wet-bulb depressions of 40, 80, and 115?F; 2) velocity of cross-circulating air of 510 and 930 fpm; 3) board thicknesses of 1, 1.5, and 1.9 inches, 4) board specific gravity (ovendry weight and green volume) of 0.34-0.45, 0.45-0.48, and 0.49-0.75; and 5) two replications of kiln charges. Kiln time to reach 10 percent moisture content was shortened by circulating air at high velocity, but was little affected by, board specific gravity. A wet-bulb depression of 80?F provided faster drying than depressions of 40 or 115?F. At 80?F depression with air circulated at 930 fpm, kiln time to 10 percent moisture content was directly proportional to board thickness. Under these optimum conditions, 1 by 4’s required 10.4 hours to reach 10 percent moisture content; boards 1.5 and 1.9 inches thick required 15.8 and 20.7 hours respectively. Total energy expended in drying to 10 percent moisture content (including heat energy and energy for air humidification and circulation) was positively correlated with board thickness; it was minimum with wet-bulb depression of 80?F and maximum at 40? depression. Air circulation velocity (510 and 930 fpm) and lumber specific gravity did not significantly affect total energy required to dry to 10 percent moisture content. Boards kiln-dried for 24 hours at 240?F and then ovendried showed significant variation in shrinkage according to drying schedule. The greatest percentage of shrinkage from green to ovendry occurred in dense, thin boards dried in low-velocity air with a 40?F wet-bulb depression. Conversely, least percentage of shrinkage occurred in thick boards of low density dried in high-velocity air with a 115?F wet-bulb depression. No significant differences in modulus of elasticity, proportional limit, modulus of rupture in bending, or toughness were detected in clear-wood specimens cut from lumber dried by the six different schedules.

You must be logged in to download any documents. Please login (login accounts are free) or learn how to Become a Member