Pressure Steam Drying of Lumber

A faster and more efficient process that could dry lumber with acceptable degrade would have a major impact on the forest products industry. A prototype pressure steam dryer of 100 board foot capacity that could dry full-size lumber in steam at temperatures to 320?F and pressures to 35 psi was designed and constructed. Charges of green 4/4 yellow-poplar, river birch, silver maple, white ash, and air-dried black walnut and red oak were dried to approximately 6 percent moisture content in the pressure steam dryer. Different schedules, as well as techniques for stress relief at the end of drying, were explained. Drying times ranged from 20 to 43 hours, depending on the operating conditions and physical properties of the wood. Quality of the dried lumber was best for yellow-poplar but still very good for the other species. Although pressure steam drying produced darkening throughout the lumber, the overall degrade was less than 3 percent of the total lumber value. Checking near knots and in mineral-streaked areas, end checks and some splitting were the primary defects. The cost of pressure steam drying, including capital investment, was slightly less than conventional kiln-drying. Pressure steam drying is a viable process which needs further investigation to expand its potential.

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