9.6B Method Using Predicted Transfer Coefficients for Constant-Rate Period 1. Introduction. In the constant rate period of drying, the surfaces of the grains of solid in contact with the drying air flow remain completely wetted. As stated previously, the rate of evaporation of moisture under a given set of air conditions is independent of the type of solid and is essentially the same as the rate of evaporation from a free liquid surface under the same conditions. However, surface roughness may increase the rate of evaporation.
During this constant-rate period, the solid is so wet that the water acts as if the solid were not there. The water evaporated from the surface is supplied from the interior of the solid