Gas and Heat Requirements in Convection Drying

Consider a continuous adiabatic single stage dryer as shown in Fig. 5-15. The wet product flow, the, is dried from an initial moisture content Xq to a final state Xf,, . 

When drying using flue gas, as the heating medium, the total mass flow rate of all noncondensable components are summed to rhi (e.g., nitrogen N2, nitric oxide NO, oxygen O2, carbon dioxide CO2, carbon monoxide CO, sulfur dioxide SO2, etc.). If fuel of known composition is burned completely with an appropriate excess air coefficient, 

The specific drying gas requirement ( specific gas need ), related to the exchanged moisture / is 

The moisture flow rate of the flue gas m p is the sum of the water content of the fuel and combustion air, and the water generated during the combustion reaction. If flue gas cleaning stages are installed between the combustion and drying stages, their effect on the mass flow rate and composition of the flue entering the dryer has to be considered. 

With discontinuous operation of the convection drying process, the moisture content Xq and moisture load X of the drying gas are time dependent. Considering the moisture rhp, (t) removed from the product and carried by the continuous flow drying gas. 

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