36 Drying of Wood

Low moisture contentHigh moisture contentFIGURE 36.16 Moisture diffusion in the hygroscopicrange: any combination in series or parallel of vapor diffusion(in lumen and pits) and bound-water diffusion (in thecell walls) is able to drive water from high to low moisturecontent regions.(e.g., impingement drying, radio-frequency, or microwaveheating). The problem of mass migration due to athermal gradient is a matter of scientific debate. Siau(1984, 1995) gives a good review of possible formulationsthat can be used in nonisothermal conditions.36.2.2.5 Physical FormulationSeveral sets of macroscopic equations are proposed inthe literature for the simulation of the drying process.The first fundamental difference between them lies inthe number of state variables used to describe theprocess:. One: moisture content (or an equivalent variable:saturation, water potential, etc.). Two: moisture content (or equivalent) and temperatureT (or an equivalent variable: enthalpyetc.). Three: moisture content (or equivalent), T (orequivalent), and gaseous pressure P g (or anequivalent variable: air density, intrinsic airdensity, etc.)Perré (1999) gives a critical review of the possibilitiesand limitations given by these different sets ofequations. The use of moisture content alone is thebasis of many correlations of lumber-drying rates(see Keey et al., 2000). Doe et al. (1996a) have foundthat, at relatively low temperatures (