36 Drying of Wood

HeatVaporVessel ortracheidLiquidPitsEndpiecefully saturatedOverpressureLiquid evacuationpossible inmicrowave heatingFIGURE 36.20 Drying at high temperature (second drying period): a high-temperature regime means that an overpressuredevelops inside the medium. Depending on the moisture content, this overpressure induces liquid or gaseous flow; inaddition, as wood is strongly anisotropic, the most part of the flow occurs in the longitudinal direction (see the magnifiedviews). (Adapted from Perré, P., The numerical modeling of physical and mechanical phenomena involved in wood drying:an excellent tool for assisting with the study of new processes, Tutorial, Proceedings of the Fifth International IUFRO WoodDrying Conference, Québec, Canada, 1996, 9–38.)the case of high-temperature drying, the increase ofinternal pressure gives rise to longitudinal migrationof liquid toward the end pieces, provided that thepermeability and the moisture content are highenough. This is a good way to spot sapwood after afew hours of drying (Figure 36.21b). This phenomenoncan be observed in industrial kilns (Figure 36.21c).To illustrate the effect of these differences on thedrying process, Figure 36.22 depicts drying experimentscarried out with superheated steam at 150 8Con both sapwood and heartwood of Norway spruce(Picea abies). They are representative of the trendsobserved by different authors (Salin, 1989; Pang et al.,1994; Perré and Martin, 1994).After the initial transient period, the constant drying-rateperiod takes place for the sapwood board.During this period, which lasts several hours, alltemperatures are equal to the wet-bulb temperatureand the overpressure remains very small. At the beginningof the second drying period (around 350min), an important overpressure develops due tothe temperature increase. It disappears only when the(a) A stack at the beginning of the dryingHeartwoodSapwoodSapwood Heartwood(b) The same stack after a few hours of dryingat high temperature(c)FIGURE 36.21 A stack of boards during high-temperature drying (shaded areas indicate wet zones).ß 2006 by Taylor & Francis Group, LLC.