36 Drying of Wood

TABLE 36.3Typical Values of Moisture Content Found inGreenwoodWood Type Moisture Content Dry Basis (%)wall substance (r s ffi 1530 kg m 3 ), and r ‘ is the sapdensity (r ‘ ¼ 1000 kg m 3 ).To highlight that the development of wood intrees leads to a very anisotropic material, Table 36.4indicates some order of magnitude for dimensionlessanisotropy ratios found in wood for the most importantproperties involved in drying. The ease of fluidmigration in wood (i.e., the permeability) is by far theproperty that presents the highest anisotropy ratio.The reduction in wood permeability from sapwood toheartwood affects particularly the longitudinal directionin hardwoods (especially for ring-porous speciesdeveloping tyloses) and all directions in softwoods.36.2 BOARD SCALE36.2.1 WATER IN WOOD: SORBED AND CAPILLARYWATER, AND SHRINKAGE36.2.1.1 Moisture Content of WoodThe moisture content of wood (dry basis) is definedby the following mass ratio:X ¼Sapwoodmass of wateroven-drymassHeartwoodSoftwoods 150–200 40–80Hardwoods 80–120 60–100(36:2)Moisture in wood is distinguished between the liquidsap present unattached in the cell lumens (free water)and the water molecules held in the cell walls (boundwater), with an activity less than 1.36.2.1.2 Free WaterThe sap flows in wood through the vascular systemproduced by the cambium. This liquid, present in thecell cavities, is usually referred to as ‘‘free water’’ justbecause its properties are very close to those of liquidwater: density, viscosity, saturated vapor pressure, etc.However,onehastokeepinmindthatthiswateristied to the solid matrix through capillary forces. Due tothe surface energy of the interface between liquid andgas (the surface tension), together with the contact anglebetween this interface and the woody matrix, a pressuredifference exists between liquid and gaseous phases.This pressure difference, which obeys Laplace’s law,increases with decreasing pore diameter. Because liquidis the wetting phase in the case of water and wood, theliquid pressure is less than the gaseous one.In addition, due to the curvature of the interface,a deviation of the saturated vapor pressure exists.This deviation can be calculated from the definitionand properties of the Gibbs free energy and leads toKelvin equation (Dullien, 1992):ln w ¼ lnP v¼ s 1 þ 1 Mv(36:3)P vs r 1 r 2 r ‘ RTIn Equation 36.3, P vs is the saturated vapor pressure, P vis the equilibrium vapor pressure at the curved interface,s is the surface tension, r 1 and r 2 are the twoprincipal radii of the surface, and M v is the molarmass of water. The quantity w is known as the relativehumidity. However, a very small pore radius is requiredfor the deviation to become significant (Table 36.5).TABLE 36.5TABLE 36.4Order of Magnitude of the DimensionlessAnisotropy Ratios Encountered inWood Relative to Tangential ValuePropertyDirectionRadius of theCapillaryT R LStiffness 1 2 20Shrinkage 1 0.5 0Thermal conductivity 1 1.5 2Mass diffusivity 1 1–2 20Pressure Difference DP and Relative Humidity wat the Surface for Different Radii Values (ValuesCalculated at 208C, for a Perfectly Wetting Liquidand Cylindrical Tubes)DP (Equivalent WaterColumn)w ¼ P v /P vs1 mm 0.146 kPa (14.9 mm) 0.999999100 mm 1.46 kPa (0.149 m) 0.99998910 mm 14.6 kPa (1.49 m) 0.999891 mm 0.146 MPa (14.9 m) 0.99890.1 mm 1.46 MPa (149 m) 0.9890.01 mm 14.6 MPa (1.49 km) 0.898ß 2006 by Taylor & Francis Group, LLC.