ALBERT

Notes: Pinus sylvestris ), Norway spruce (Picea abies) and Birch (Betula pubescens) was measured above fibre saturation point (FSP) using a computer tomography (CT) scanner and digital image processing. A test volume was selected in the interior part of the samples and the density changes were determined every 10 minutes. Two different drying schedules were used, which had two periods seperated by a climate step. Schedule I was conducted with constant dry bulb temperature and schedule II with constant wet bulb temperature. The climate in the first period, A, was equivalent to 16% equilibrium moisture content (EMC) and period B, 8% EMC. Tests with schedule I were run at 50 °C, 60 °C, 70 °C and 80 °C (dry bulb temperature) and with schedule II also at 50 °C, 60 °C, 70 °C and 80 °C (wet bulb temperature). The results showed that there was no difference between the moisture flux during period A from the test volume for the different species. Between the different temperatures no significant differences of the rate of density changes in the test volumes for the different species were found. Spruce reacted more slowly than pine and birch on the external climate step, but after a while the flux rate was equal to that for pine. Birch had a slightly lower flux, about 60% of the rate for pine and spruce in period B. The wood rays in softwood are probably the most important flow path. The different shape and size of the pits between the tracheids and the rays in pine and spruce may be one explanation why spruce reacted more slowly than pine. A receding front was also observed and, by image processing, the distance from the surface and the receding front were determined. The receding front was defined at a fixed density level, approx. 30 kg/m3 above FSP and it was assumed that the evaporation occurs at approximately the position of the receding front. The receding front took the position at 0.5–1 mm below the surface just after the beginning of the drying process. For pine and birch the distance from the surface to the front increased slowly, even after the climate step, but for spruce the distance to the front instantly increased to a new position. Spruce lost its liquid water at the edges first and after a few hours the distance to the front stabilized. At approximately 60% moisture content (MC) the liquid water continuity broke down and the drying entered the irreducible saturation phase. This transition phase is in between a heat transfer controlled phase of drying when liquid water moves with low resistance to an evaporation front and a diffusion-controlled phase below FSP.