The Mechanisms of Batch Drying

We now consider the various portions of the rate-of-diying curve in more detail. Our present knowledge permits us to describe the drying process in the constant-rate period reasonably well, but our understanding of the falling-rate periods is very limited. 

The constant-rate period In this period, where surface evaporation of unbound moisture occurs, it has been shown that the rate of drying is established by a balance of the heat requirements for evaporation and the rate at which heat reaches the surface. Consider the section of a material drying in a stream of gas as shown in Fig. 12,11. The solid of thickness is placed on a tray of thickness Zuf. The whole is immersed in a stream of drying gas at temperature Tq and humidity Y mass moisture/mass dry gas, flowing at a mass velocity G mass/ (time) (area). The evaporation of moisture takes place from the upper surface, area which is at a temperature T, The drying surface receives heat from several sources (1) by convection from the gas stream (2) by conduction through the solid (3) by direct radiation from a hot surface at temperature T/j, as shown, all expressed as a flux, energy/(area of solid for heat transfer) 

The rate of evaporation and the surface temperature can then be obtained by a heat balance [46]. If q represents the total heat arriving at the surface, then 

If we neglect the heat required to superheat the evaporated moisture to the gas temperature and consider only the latent heat of vaporization the flux of evaporation and the flux of heat flow are related, 

The heat received at the surface by convection is controlled by the appropriate convection heat-transfer coefficient 

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