Continuous Tunnel Kiln Direct-Fired with Bark to Dry 1.75-Inch Southern Pine in 12 Hours

A kiln design is described in which length-sorted lumber surfaced on one side to 1.75-inch thickness is mechanically stacked 5 feet wide and 10 feet high on 1-1/4-inch-thick sticks and continuously transported through a zone-controlled, 96-foot-long tunnel kiln at 8 feet per hour to yield 500,000 board feet of lumber dried to 9-percent average moisture content per 168-hour week. In the tunnel, the lumber is dried for 8 hours at 270?F, conditioned for 3 hours at 195?F, and cooled for 1 hour before discharge to cooling sheds and subsequent unstacking. During drying and conditioning, wet-bulb temperature is held constant at 185?F. Air velocities vary by zone–i.e., 1,600 fpm at inlet, 300 fpm while conditioning, and 1,000 fpm while cooling. To control warp, a top load (12 inches of concrete) is automatically placed on the moving lumber stacks as they enter the tunnel kiln and removed as they exit. Total equipment cost for the entire system–including fuel preparation and multiple direct-firing green bark suspension burners–is estimated at about $1,000,000. Weekly consumption of green bark to dry 500,000 board feet should be less than 200 tons; mechanical energy consumed to drive motors will total 34,267 horsepower-hours, and labor requirements will total 288 man-hours weekly. Strength tests indicate that mechanical properties (stiffness, bending strength, toughness, and hardness) of southern pine dried on the 12-hour schedule will be equal to those of matched lumber dried for 5 days at 180?F.

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