Comparison Between Measured and Theoretical Drying Diffusion Coefficients for Southern Pine

Experimental drying diffusion coefficients were determined for loblolly pine sapwood in both the tangential and radial directions from rate of drying data from the fiber saturation point to the oven-dry condition at temperatures ranging from 50?C. to 120?C. The diffusion coefficients increase exponentially with an increase in temperature. The radial values are about 1.55 times the tangential values. This is probably due to a combination of reasons, such as the contribution of ray cells to radial diffusion, the misalignment of fibers in the tangential direction, the concentration of pits on the radial faces, and the effect of the annual rings. A theoretical analysis of diffusion of water through softwoods was made on the basis of considering diffusion through the different structural components of the wood combined in parallel and in series as in electrical conduction. The diffusion can be considered to occur through three different paths. Type 1, cavity-wall, is a combination of bound-water diffusion through the portion of the cell wall separating the fiber cavities in the diffusion direction in series combination with water-vapor diffusion across the fiber cavities. Type 2, cavity-pit, is the diffusion through the pit system in series with the fiber cavities. Type 3, continuous-wall, is the continuous diffusion of bound water through the portion of the cell wall in parallel with the fiber cavities. Under all of the experimental conditions, cavity-wall diffusion was chiefly responsible for determining the combined diffusion. It varied from 95 percent, for wood with a swollen-volume specific gravity of 0.2, to 55 percent, for wood with a swollen-volume specific gravity of 0.8.

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