Time, Costs, and Energy Consumption for Drying Red Oak Lumber as Affected By Thickness and Thickness Variation

The time required to kiln-dry lumber is influenced by its thickness. Thicker material dries more slowly, and an increase in drying time increases energy consumption and drying costs. The purpose of the study was to analyze the effect of thickness variation–of the size that would be encountered by sawing variation–on drying time, costs, and energy consumption in kiln-drying 4/4 to 8/4 red oak lumber. The basis of the study is empirical drying rate data of red oak as a function of temperature, RH, and thickness. This data was generalized into an analytical function to allow estimates of drying rate to be calculated at any combination of the three variables. The analysis showed that even the small difference in thickness between recognized values for minimum stipulated, target, and maximum recommended thicknesses, which is only 3/32 inch for 4/4 to 6/4 and 4/32 inch for 8/4 hardwoods, can have a significant effect on drying time. A 3/32-inch difference results in differences in drying time of 10 to 15 percent for 4/4 red oak lumber. The differences range down to 6 percent for thicker lumber. Differences in energy consumption range from 3 to 6 percent for the 3/32-to 4/32-inch thickness differences between minimum, target, and maximum thicknesses. Total drying costs are dependent upon specific operations, but if, as a first approximation, drying costs are assumed to be proportional to drying time, then differences in total drying costs due to thickness differences are the same, on a percent basis, as differences in drying time.

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