36 Drying of Wood

TABLE 36.6Order of Magnitude of Shrinkage Values in theTransverse Plane for Some Species. FSP is Assumedto be Equal to 30% for All SpeciesSpecies Radial Shrinkage Tangential ShrinkageTotal(%)Coefficient(%/%)Total(%)Coefficient(%/%)Spruce, pine 5.0 0.17 9.0 0.30Oak 6.0 0.20 11.0 0.37Beech 6.5 0.22 12.0 0.40Balsa 3.0 0.10 6.0 0.20Teak 3.0 0.10 5.0 0.17Okoumé 4.0 0.13 6.5 0.22Azobé 8.0 0.27 11.0 0.37Source: Kollmann, F.P. and Côté, W.A., Principles of WoodScience and Technology, Solid Wood, Vol. 1, Springer, Berlin,1968; Aléon, D., Chanrion, P., Négrié, G., and Perré, P.,FormaXylos 4—Le séchage (Training in Wood Science: Drying,Vol. 4), CD-Rom français/English, CTBA, Paris, 2003.c. Quartersawn sections, which include the pith,present a nonuniform thickness once dried;near the pith, the thickness shrinks along theradial direction whereas the tangential shrinkageis involved elsewhered. A squared section cut along the grain directionbecomes rectangular after dryinge. A diamond shape is obtained if the grain directionis along the diagonal36.2.2 HEAT AND MASS T RANSFER IN WOOD36.2.2 .1 Flui d Migration in Wood:Singl e-Phase FlowThe understanding of the fluid migration in wood isof utmost importance to understand the drying process.The differences in drying behavior between speciesand within the log primarily come from thepermeability value and from the initial moisture saturation.When only one fluid phase is present, theclassical Darcy’s law applies:v ¼ K r(P) (36:5)mwhere v is the apparent velocity of the fluid throughthe specimen (m s 1 ), K is the permeability (m 2 ), andm is the dynamic viscosity of the fluid (Pa s 1 ).Fluid migration in wood uses the vascular systemdeveloped by trees for their physiological requirements.For this reason, wood has several specificfeatures concerning permeability among which themost important are the anisotropy ratios. Wood hasdramatic anisotropy ratios: the longitudinal permeabilitycan be 1000 times greater than the transversepermeability for softwoods and more than a millionfoldfor hardwoods (Banks, 1968).Table 36.7 summarizes some values of directionalpermeability available in the literature for differentspecies. Depending on the experimental apparatusand the protocol used by the authors, some data aremissing in the papers. For example, it is not alwaysFlatsawn(d)(b)(e)Quartersawn(a)(c)FIGURE 36.12 Section deformations depending on the sawing pattern. The shape after drying results from the anisotropyratio between radial and tangential shrinkage. These deformations exist even when the equilibrium is achieved and with auniform moisture content throughout the section. (Adapted from Aléon, D., Chanrion, P., Négrié, G., and Perré, P.,FormaXylos 4—Le séchage (Training in Wood Science: Drying, Vol. 4), CD-Rom français/English, CTBA, Paris, 2003.)ß 2006 by Taylor & Francis Group, LLC.