36 Drying of Wood

Moisture content (%)15010050SapwoodHeartwood00 200 400 600Time (min)FIGURE 36.23 Moisture content loss obtained for sapwoodand for heartwood (same experiments as in Figure 36.22).is detectable at the rear end of the board (T 8 ).Consequently, the overpressure remains high (especiallyfor the center pressure P 3 ) up to the end of thedrying. The maximum pressure is higher for heartwoodthan for sapwood. The differences in dryingkinetics are of great interest. In spite of the high initialmoisture content of sapwood (170% against 60%), thepermeability of heartwood is so low that the curvescross each other at 450 min of drying (Figure 36.23).This is consistent with the observations on entirestacks (Salin, 1989).The strategy of simulating the differences betweenheartwoodandsapwoodliesinonlytwosetsofparameters:the permeability and the initial moisture content(fortheseexperiments,180%forsapwoodand70%forheartwood). The values of permeability used to differentiatesapwood from heartwood (Table 36.8) arebased on the considerations concerning pit aspiration.By using only these differences, Perré and Turner(1996) found that all the trends observed for sapwoodand heartwood were found in the simulated results.The most spectacular effect is the longitudinal flowdue to the overpressure (Figure 36.24). In the case ofhigh-temperature convective drying, the sapwoodboard delivers a large supply of water to the endpiece after 5 h, while the heartwood end piece isalready within the hygroscopic range. These carpetplots should be compared with Figure 36.21.36.2.4 MECHANICAL A SPECTS OF WOOD DRYING36.2.4 .1 Mech anical Behavior of W oodIndustrial wood drying consists of not only removingmoisture from greenwood but also ensuringthat its quality (fitness for purpose) is adequatein end use. Because wood shrinks during drying,deformations and stresses develop that can lead toTABLE 36.8Intrinsic Permeabilities Used in the Drying ModelingDirection Heartwood SapwoodLongitudinalGas 10 13 m 2 2.10 13 m 2Liquid 10 13 m 2 10 12 m 2TransverseGas 10 16 m 2 2.10 16 m 2Liquid 10 16 m 2 10 15 m 2unusable products (Figure 36.25). The understandingof these aspects must account for the complex mechanicalbehavior of wood, including its memory effect.Shrinkage is the ‘‘driving’’ force for drying stress,i.e., without shrinkage, no drying stresses would develop.Figure 36.26 exhibits the dimensional variationof an unladen sample with the moisture content(the latter is assumed to be uniform). Under normalconditions, the dimensions do not change until themoisture content attains a moisture content close tothe acknowledged FSP. This condition is sometimescalled SIP. Then, the dimension variations are almostproportional to the change in moisture content. Thisstrain field is called free shrinkage.A sample subjected to tensile or compressive stress(Figure 36.27) exhibits the instantaneous deformation(elastic part) at first, which then increases with time(viscoelastic creep). After cycling the moisture contentto and from a higher moisture content, the creephas been significantly greater due to mechanosorptiveaction.Thus, a sample subjected to a compressive stress(as shown in case 1, Figure 36.28) exhibits a smallerlength at the end of drying than an unloaded specimen(case 2), which itself has a smaller length thanthe sample subjected to a tensile stress (case 3). Inthis experiment, the viscoelastic behavior acts becauseof time and the mechanosorptive behavioracts because of the removal of water molecules dueto drying.These ideas can now be applied to the drying oflumber boards, which is assumed to be stress-free atthe beginning. At the beginning of drying (constantdrying-rate period), sap throughout the entire boardremains free. No shrinkage occurs; hence, stressbuildup is absent.At the beginning of the second drying period,shrinkage exists close to the exposed surfaces(Figure 36.29a). At this moment, if the section wascut into slices, the outer slices would have a shorterlength than the inner ones (Figure 36.29b). This displacementfield is not compatible and induces, in theß 2006 by Taylor & Francis Group, LLC.