Classification of dryers

FIG. 12-45 Classification of dryers, based on method of beat transfer. [Re-oised from Marshall, Heat, Piping Air Cond., 18, 71 (1946).]... 

Classification of dryers based on method of operation Dryer... 

Classification of dryers by suitability for special features Process... 

Commercial dryers differ fundamentally by the methods of heat transfer employed (see classification of dryers. Fig. 12-45). These industrial-dryer operations may utilize heat transfer by convection, conduction, radiation, or a combination of these. In each case, however, heat must flow to the outer surface and then into the interior of the solid. The single exception is dielectric and microwave drying, in which high-frequency electricity generates heat internally and produces a high temperature within the material and on its surface. 

Elaborate classification of dryers has been presented by Kroll (1978) and Keey (1972) but less comprehensive (but perhaps more practical) classifications are shown in Table 9.1. In this table, the method of operation, physical form of the stock, scale of production, special features, and drying time are presented. Two other classifications of dryers may be found in Perry (1999). One classifies the equipment on the basis of heat transfer (Figure 12-45) and the other on the basis of the material handled (Table 12-9) in Perry. 

Classification of dryers on the basis of the mode of thermal energy input is perhaps the most useful since it allows one to identify some key features of each class of dryers. 

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