36 Drying of Wood

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FIGURE 36.25 Two examples of mechanical degrade during wood drying: board deformation and internal checking.GreenwoodSIPOven-driedLength0SIPMoisture contentFIGURE 36.26 Wood shrinkage: the shrinkage intersection point (SIP), often close to 30%, depends on species andtemperature.ß 2006 by Taylor & Francis Group, LLC.
ConstantmoisturecontentTime t = 0 +LoadTime tLoadAfter cycles ofmoisture contentLoadMoisturecontentLoadTimeTimeLengthvariationViscoelasticMechanosorptiveTimeFIGURE 36.27 Viscoelastic and mechanosorptive behavior of wood. (Adapted from Perré, P., The numerical modeling ofphysical 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 Wood Drying Conference, Québec, Canada, 1996, 9–38.)subjectremainsamatterofsomescientificdebate(Keeyet al., 2000).Nevertheless, in the case of drying, the moisturecontent only decreases and some simplificationsapply. Here, only the most common way to expresscreep and mechanosorptive effect will be presented.The general formulation of the time dependency ofthe creep property involves the whole stress history:« ij ¼ð t 1J ijkl (t t 0 ) d s kldt 0 dt0 (36 :18)where J ijkl (t) is the creep compliance tensor and t isthe actual time. The experimental creep function isoften analyzed as a number of Kelvin elements inseries (Figure 36.32), each having the property ofa spring and dashpot in parallel (Genevaux, 1989;Martensson, 1992; Mohager and Toratti, 1993; Hanhijärvi, 1999 Passard and Perre, 2005). In the case ofuniaxial load, this leads toJ(t) ¼ J 0 1 þ XNa n (1 et t!n ) (36:19)n ¼1The temperature and moisture dependency of thatfunction can be expressed using a material time orchanging the characteristic time t n . The thermal activation,for example, is often expressed with the aid ofan Arrhenius law: t n ¼t 1 n exp DW n(36:20)RTTime t = 0 + highmoisture contentTime t lowmoisture contentLoad1 2 3 Drying1 2 3LoadFIGURE 36.28 Dimension changes of a specimen loaded during drying. (Adapted from Perré, P., The numerical modeling ofphysical 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 Wood Drying Conference, Québec, Canada, 1996, 9–38.)ß 2006 by Taylor & Francis Group, LLC.
Page 1 and 2: 36Drying of Wood: Principlesand Pra
Page 3 and 4: Primary growthSecondannual ringFirs
Page 5 and 6: FIGURE 36.4 Wood is formed by cell
Page 7 and 8: TABLE 36.1Elementary Composition of
Page 9 and 10: FIGURE 36.8 Tension wood in Peduncu
Page 11 and 12: 36.2.1.3 Bound WaterBound moisture
Page 13 and 14: TABLE 36.6Order of Magnitude of Shr
Page 15 and 16: The transverse permeability and the
Page 17 and 18: FIGURE 36.14 Relative permeability
Page 19 and 20: Low moisture contentHigh moisture c
Page 21 and 22: Boundary layersExternal flowTP vHea
Page 23 and 24: HeatVaporVessel ortracheidLiquidPit
Page 25 and 26: Moisture content (%)15010050Sapwood
Page 27: to the history of that point (« me
Page 31 and 32: Element 1 Element 2 Element NElemen
Page 33 and 34: Averaged moisture content (%)907560
Page 35 and 36: Moisture content (%)12010080604020A
Page 37: Second drying periodTensionParamete
Page 42 and 43: eaction wood produced by environmen
Page 44 and 45: where the boards butt up due to shr
Page 46 and 47: 1.2Normalized moisture content (Φ)
Page 48 and 49: set at a constant value at constant
Page 50 and 51: oards by conduction whereas the vac
Page 52 and 53: KilnSolarcollectorCondenserControlv
Page 54 and 55: Doe, P.E., Oliver, A.R., and Booker
Page 56 and 57: Perré P., 1992. Transferts couplé

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