Moisture Movement in Beech Below the Fiber-Saturation Point

Radial and tangential moisture diffusion coefficients were calculated by analyzing the changes in moisture distribution during transverse drying of 1-inch quarter-sawn and flat-sawn boards from three trees of American beech (Fagus grandifolia Ehrh.) from the fiber-saturation point (f.s.p.) to about 16 percent moisture content at temperatures of 10?, 20?, 30?, 40?, 50?, and 60?C. Both vapor and boundwater movement are believed to operate in this range. The radial diffusion coefficient increases with temperature to nearly the same extent as the vapor pressure of water, indicating that vapor movement dominates in this case. Also the radial coefficient increases exponentially with increasing moisture content from 10 percent to f.s.p., further supporting the importance of vapor movement in this case. The tangential diffusion coefficient increases only about 60 percent as rapidly with temperature as does the radial coefficient, and appears to be independent of moisture content. This indicates that boundwater movement is more limiting in this case. The tangential coefficient is also consistently lower than the radial coefficient over most of the moisture and temperature range. It is proposed that the differences in radial and tangential moisture movement are caused by the rays which occupy up to 20 percent of the volume of beech. Presumably these rays provide radial paths for vapor movement in which relatively few cross walls must be penetrated compared with the case of tangential drying. Therefore radial drying is controlled by vapor movement through the rays and tangential drying is controlled by boundwater movement through the cell walls.

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