Drying curve for different materials

During constant-rate period, the free moisture leaves the saturated surface of the material by diffusion through a stationary air film into air stream. GiUiland[3] showed that the drying rate for this period for variety of material is essentially the same (around 2.0-2.8 kg/rrP/K). 

The mass rate of evaporation (W) is found to be dependent on partial pressure 

surface area A, velocity of air stream u, and ratio of length L to width B of the surface and is given below [3]. 

However, for most practical purposes drying at a constant-rate period can be assumed to be proportional to (p s — Pw) given by, 

The simple expression of time required for drying period can be expressed in terms of moisture content Wi and final moisture content equal to critical moisture content Wc as  

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