General Characteristics of Dryers

Removal of water from solids is most often accomplished by contacting them with air of low humidity and elevated temperature. Less common, although locally important, drying processes apply heat radiatively or dielectrically in these operations as in freeze drying, the role of any gas supply is that of entrainer of the humidity. 

The nature, size, and shape of the solids, the scale of the operation, the method of transporting the stock and contacting it with gas, the heating mode, etc. are some of the many factors 

Effects of Moist Air Recycle and Increase of Fresh Air Rate in Belt Conveyor Drying 

The conditions of Example 9.3 are taken except that recycle of moist air is employed and the equilibrium moisture content is assumed constant at We = 0.014. The material balance in terms of the recycle ratio R appears on the sketch  

Humidity of the air at any point is obtained from the water balance 

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Direct Dryers The general operating characteristics of direct diyers are these ... 

In a later section, the characteristics and performances of the most widely used equipment will be described in some detail. Many types are shown in Figure 9.4. Here some comparisons are made. Evaporation rates and thermal efficiencies are compared in Table 9.2, while similar and other data appear in Table 9.3. The wide spreads of these numbers reflect the diversity of individual designs of the same general kind of equipment, differences in moisture contents, and differences in drying properties of various materials. Fluidized bed dryers, for example, are operated as batch or... 

Generally the chamber design depends on the atomizer used and on the air-fluid contact system selected. The selection of atomizer and air-fluid layout is determined by the required characteristics of the dry product and production rate. The product specifications are almost always determined from small-scale tests in an experimental spray dryer. Sometimes they are available from the literature. If heat-sensitive material is involved, attention must be paid to the temperature... 

Methods of supplying heat and transporting the moisture and the drying product are the basic variations among different types of dryers. In general, the types of dryers and their operational design characteristics are reviewed. 

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