Mass transfer from wooden surfaces and internal moisture non-equilibrium

The transfer of moisture from a wooden surface and the corresponding mass transfer coefficient in relation to the heat transfer coefficient are considered. Experimental data are reviewed that show that the mass transfer coefficient often is one order of magnitude smaller than what could be expected from the analogy between heat and mass transfer. The nature of this deviation and its influence on wood drying models is discussed. It has been suggested by Wadso that moisture flow inside the wood exhibits non-Fickian behavior and that this may be due to a slow desorption/absorption by the wood fibre. A model is thus developed where the normal assumption of internal local equilibrium between the vapour and bound phases is replaced by a mass transfer resistance. The model is applied to one-dimensional moisture flow in a body of finite thickness and solved analytically. Numerical values calculated with the model are analysed in order to establish whether 'apparent' non-Fickian behavior of the kind observed could be explained by such a model. It is also analysed whether the model could explain an apparent decreased mass transfer coefficient at the surface.