Notes on the Case Described in Section

1 This method of calculation is of great interest to describe the process of drying, but only when the diffusivity is constant and does not depend on the concentration of the diffusing substance in the sheet. 

2 In the same way, the surroundings can be considered as infinite provided that the sheet is not confined in a closed volume, because in this case, the concentration of the evaporated substance in the atmosphere, far from remaining constant, would increase regularly as the evaporation proceeds. 

3 It should be noted that the rate of convection h is related to the rate of evaporation of the diffusing substance per unit area by the relationship  

Of course, when the concentration of the liquid is expressed in terms of mass per volume, the rate of evaporation is also expressed in terms of mass of substance evaporated per unit time and per unit surface of the solid. 

The most common case encountered in food packages occurs when the volume of food located in the plastic package is finite, and the liquid is not so strongly stirred that the coefficient of convection h would be infinite. 

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