36 Drying of Wood

ParametersLow valueHigh valueExternalVelocityDrying potential(dry bulb−wet bulb)ThicknessInternalMass diffusivityThermal diffusivityTemperatureFIGURE 36.38 Guidelines on how to obtain a fast drying operation (from good to excellent and from poor todisastrous). (Adapted from Perré, P., The drying of wood: the benefit of fundamental research to shift fromimprovement to innovation, Proceedings of the Seventh International IUFRO Wood Drying Conference, Tokyo, Japan, 2001,2–13.). The air velocity also plays an important role inuniform drying within the stack. However, itseffect becomes less important as the drying progresseswhen internal transfer mainly controlsthe moisture migration.In addition, we have to keep in mind some moresubtle effects:. The internal transfer (diffusion, liquid migration)becomes easier when the temperaturelevel increases.. Above the boiling point of water, an additionaldriving force, the gradient of total pressure, actswith a dramatic effect (Perré , 1995). Dryingby internal vaporization takes place in suchconditions.. The internal transfer rates depend on the localmoisturecontent(liquidmigrationisusuallymuchmore effective than bound or vapor diffusion).In addition, diffusion becomes very slow whenthe bound-water content decreases toward zero.In general, the drying time is reduced when thevelocity and the temperature of air are high and itsrelative humidity is low. However, an excessively lowrelative humidity may produce a surface zone with lowmoisture content, thus reducing moisture migrationclose to the surface. All high-temperature arrangements(convective drying at high temperature, vacuumdrying, contact drying, etc.) are processes that accelerateinternal moisture migration due to the overpressuresgenerated within the product.Finally, drying with electromagnetic heating(microwave or radio frequency) offers an entirelynew possibility: any internal temperature can beattained without resorting to a heating medium suchas a hot gas.36.2.5 .2 Fac tors Affecti ng the Dry ing Qual ityDrying stresses originate from shrinkage; as soon asthe shrinkage field within the board is not geometricallycompatible, a stress field develops in the material,which is responsible for mechanical degradation. Inorder to reduce the stress level throughout the process,and thereby the surface checking, the internalchecking, and the residual stress, several conditionsshould be fulfilled (Figure 36.39):. Low shrinkage coefficients, not under control. Small thickness, not under control. Low moisture content values between surfaceand core. Retaining important possibilities of viscoelasticcreep (mechanosorptive creep is always a sourceof stress reversal); such an effect is obtained athigh temperatures, provided the moisture contentis sufficiently high (Irvine, 1984)It may be noted that a low-temperature level is sometimesdesired (for example to avoid collapse), becausea high-temperature level may produce thermal degradationor discoloration.36.2.5 .3 Cr iteria for Obtaini ng a Fast and GoodDry ing Pro cessA fast and good drying process should incorporate thecriteria listed in Section 36.2.5.1 and Section 36.2.5.2,ß 2006 by Taylor & Francis Group, LLC.