| Several methods for measuring the moisture diffusion coefficient for wood were reviewed. The relationship of the diffusion coefficient to the rate of drying of wood was given, including three-dimensional drying. The moisture diffusion coefficient for beech (Fagus grandifolia Ehrh.) below the fiber-saturation point was determined experimentally by three different methods. One of these was a steady-state method, another was a three-dimensional drying method and the third was a technique in which the change in moisture distribution with drying time was measured. The longitudinal diffusion coefficient for beech was about 13 times the mean transverse coefficient at 40{dollar}spcirc{dollar}C and 10 percent moisture content. The tangential coefficient was essentially constant with moisture content change for temperatures from 10{dollar}spcirc{dollar} to 60{dollar}spcirc{dollar}C. The radial coefficient increased logarithmically with moisture content over the same range of conditions. The logarithms of both the radial and tangential coefficients decreased linearly with the reciprocal of absolute temperature but the radial decrease was more pronounced. The activation energy for moisture diffusion in the radial and tangential directions for beech was calculated from the temperature effect. For radial movement this energy was near the magnitude expected if vapor movement predominates, but for tangential movement it was considerably lower. |
