Constant rate period drying shrinkage

Experimentally, drying curves (Fig. 1) show two distinct regions a constant rate period (CRP), which is followed by a falling rate period (FRP). In the CRP, the evaporation rate is constant and shrinkage of the article occurs. At the end of the CRP, shrinkage stops and the FRP begins. 

These ideas can now be applied to the drying of lumber boards, which is assumed to be stress-free at the beginning. At the beginning of drying (constant drying-rate period), sap throughout the entire board remains free. No shrinkage occurs hence, stress buildup is absent. 

As opposed to Scherer, recent models are non-isothermal and not restricted to essentially one-dimensional problems (such as an infinite plate, cylinder or sphere). Some of them consider the drying gel as a biphasic medium, that is, a solid matrix that remains saturated, so that the second drying period cannot be described. Others can also simulate shrinkage and stress after the liquid/gas phase boundary has receded into the gel, as already explored by Scherer (1987b) for a flat plate by assuming a constant evaporation rate until the gel is completely dry. 

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