Dry Kiln Pressure Venting and Lumber Drying

Traditionally, fresh, dry air is taken in and hot, moist air exhausted through vents in the roof of the kiln. A new system is being advocated that creates a pressurized atmosphere within the kiln and necessitates forcing air into and out of the unit. Faster drying, less degrade, better control, lower maintenance, and reduced steam consumption are claimed. This study set out to evaluate these claims. A conventional single-track kiln and a pressurized single-track kiln of identical overall design were chosen. Each had a capacity of 120MBF. Matched 2 by 8 lumber was charged into each kiln and dried according to the same schedule. Three such trials were carried out with observations of total drying time, average moisture content, degrade loss, and air velocity. The results differed slightly in each trial. The first trial showed the pressure-vented kiln produced slightly lower moisture content and higher degrade (average moisture content 14.4 percent, degrade 2.73 percent) than the conventional unit (average moisture content 16.0 percent, degrade 1.97 percent). The second trial produced almost identical moisture contents between kilns but the degrade figures were not calculated. The third trial reversed the differences between the pressure-vented (average moisture content 15.1 percent, degrade 1.58 percent) and the conventionally-vented (average moisture content 14.1 percent, degrade 1.70 percent) kilns. The pressure-vented kiln consumed $0.09/MBF less steam than the conventional unit but used $0.01/MBF more power, which balances to a net saving of $0.08/MBF in favor of the pressure-vented unit. This savings will more than compensate for the added cost of equipping new kilns with pressure venting. It is concluded that no differences in drying results are obtained between the two kilns but that a net savings of $0.08/MBF justifies the cost of installing pressure venting in new kilns.

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