Drying Diffusion Coefficients in Wood as Affected By Temperature

The purpose of this research was to determine the effective temperature at which wood dries when exposed to different drying temperatures. Rates of drying measurements were made on small, flat-grain specimens of eastern hemlock sapwood dried from the fiber saturation point to the dry condition at temperatures ranging from 50? to 120?C. One set of specimens was at room temperature as drying began, the other set was preheated to oven temperature under nondrying conditions before specimens were subjected to drying. In both cases, the temperature change with time was measured on matched controls at the surface and in the center of the specimens. With the room-temperature specimens, the temperature built up rapidly at first, followed by a slow rise as drying progressed. With the preheated specimens, the temperature dropped rapidly for the first few minutes and then recovered. The data were analyzed from 1) conventional plots of the loss of evaporable water versus the square root of time and from plots of the square of the loss of evaporable water versus the time. Diffusion coefficients calculated by the first method were higher for the specimens starting at room temperature than for the preheated specimens. Diffusion coefficients calculated by the second method were virtually the same, irrespective of whether the specimens were preheated or not. The second method of calculation is thus considered preferable for eliminating the effect of initial irregularities in the drying process. Effective drying temperatures were calculated from the temperature-time plots. The experimental drying diffusion coefficients of this research compare much more favorably with theoretically calculated diffusion coefficients when the comparison is made on effective temperature rather than on oven temperature. When the effective drying temperatures in previous drying studies are estimated from the temperature-time plots of this research, the use of effective temperatures in place of oven temperatures tends to over-correct the discrepancy between the experimental and the theoretical values. This is not surprising, because all of the previous measurements were started somewhat above the fiber saturation point. This should steepen the initial moisture gradients and tend to make the diffusion coefficients high.

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