Hardwood Veneer Drying

Three hundred samples representing 22 species of hardwoods were dried in an experimental veneer drier at temperatures above 212?F. Most of the samples were red gum. Variables in the drying operation included drying temperature, time in the drier, and relative humidity. Variables in the material being dried included initial moisture content, species, thickness, and heart or sap condition. Drying curves demonstrating constant rate/falling rate phenomena were derived. A drying rate equation is given. Major causes for differences in drying rate include: initial moisture content, drying rate in constant rate period, critical moisture content, moisture content separating distinct zones in the falling rate period if more than one zone exists, and the values of the drying equation constants for each zone. Curves representing these differences are shown. A discussion of practical aspects of veneer drying points out that variables such as thickness, initial moisture content, species, and heart or sap condition must be considered at the green end of the drier. Changes made in drier feed rates and temperature based on observations at the dry end are apt to be misleading. Splits, buckling, wavy ends, casehardening, and collapse are reviewed as drying defects. These vary by species, thickness, initial moisture content, and drying conditions. Means by which these can be detected and minimized include using more moderate drying conditions and more careful green veneer sorting.

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