Drying equipment dryer classification

Drying equipment may be classified in several ways. The two most useful classifications are based on (1) the method of transferring heat to the wet solids or (2) the handling characteristics and physical properties of the wet material. The first method of classification reveals differences in dryer design and operation, while the second method is... 

A classification chart of drying equipment on the basis of heat transfer is shown in Fig. 12-45. This chart classifies dryers as direct or indirect, with subclasses of continuous or batchwise operation. 

Another useful classification is whether or not a dryer is a direct or indirect dryer. A direct contact dryer is one in which the material is dried by exposure to a hot gas, whereas in an indirect contact dryer, the heat required for evaporation is transferred from a heating medium through a metal wall to the material. Generally, direct heat dryers are more efficient. Dryer efficiency is defined by the fraction of energy supplied to the drying equipment which actually causes the evaporation of the liquid. As we shall see later in the chapter, heating is not always necessary to achieve drying. 

Drying equipment may be classified in several ways. Effective classification is vital in selection of the most appropriate dryer for the task and in understanding the key principles on which it operates. The main categories are as follows ... 

CLASSIFICATION OF DRYERS. There is no simple way of classifying drying equipment. Some dryers are continuous, and some operate batchwise some agitate the solids, and some are essentially unagitated. Operation under vacuum may be used to reduce the drying temperature. Some dryers can handle almost any kind of material, while others are severely limited in the type of feed they can accept. 

Mujumdar and Menon (1995) and Mujumdar (2000) provide detailed classification schemes for industrial dryers along with the numerous criteria that are important in making an appropriate selection. It is noted that one should select a drying system—including pre- and post-drying equipment—that can impact the choice of the dryer itself as well as its operating conditions. 

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. 

It is difficult to generate rules for both classification and selection of dryers because exceptions occur rather frequently. Often, minor changes in feed or product characteristics result in different dryer types as the appropriate choices. It is not uncommon to find that different dryer types are used to dry apparently the same material. The choice is dependent on production throughput, flexibility requirements, cost of fuel as well as on the subjective judgment of the individual who specified the equipment. 

Mujumdar AS. Classification and selection of industrial dryers. In S Devahastin, ed., Mujumdar s Practical Guide to Industrial Drying Principles, Equipment and New Developments. Brossard, Quebec, Canada Exergex Corporation, pp. 23-36, 2000. 

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