The evaporation of water from wood at high temperatures

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When predicting the performance of wood drying processes, film coefficients for heat and mass transfer are among the parameters that need to be estimated. Customary methods such as the use of the Lewis relation were originally derived for vapour pressures which were small compared with atmospheric pressure. For high temperature drying of wood it is necessary to extend the associated psychrometric equation to one of more general applicability. Such extension is the subject of this paper. Essential to the analysis is the determination of the rate of change of vapour pressure with temperature adjacent to the wood surface. It is found that when the vapour pressure is not assumed small compared with atmospheric pressure, the above gradient contains a factor P - po where P is the atmospheric pressure and po is the free stream vapour pressure. At high temperatures and humidities, this factor becomes important and the film coefficient for mass transfer increases without limit.