Equipment for Drying

In tray dryers, which are also called shelf, cabinet, or compartment dryers, the material, which may be a lumpy solid or a pasty solid, is spread uniformly on a metal tray to a depth of 10 to 100 mm. Such a typical tray dryer, shown in Fig. 9.2-1, contains removable trays loaded in a cabinet. 

Steam-heated air is recirculated by a fan over and parallel to the surface of the trays. Electrical heat is also used, especially for low heating loads. About 10 to 20% of the air passing over the trays is fresh air, the remainder being recirculated air. 

After drying, the cabinet is opened and the trays are replaced with a new batch of trays. A modification.of this type is the tray-truck type, where trays are loaded on trucks which are pushed into the dryer. This saves considerable time, since the trucks can be loaded and unloaded outside the dryer. 

In the case of granular materials, the material can be loaded on screens which are the bottom of each tray. Then in this through-circulation dryer, heated air passes through the permeable bed, giving shorter drying times because of the greater surface area exposed to the air. 

Vacuum-shelf dryers are indirectly heated batch dryers similar to tray dryers. Such a dryer consists of a cabinet made of cast-iron or steel plates fitted with tightly fitted doors 

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A comparative calculation for the sub-structure of the stalls housing a population of 25,000 laying hens in 3- and 4-tier batteries has been made for different drying systems. The calculation also takes account of the equipment for drying and removing the manure. 

The process is quite slow—even with the high airflow per bushel (0.83—1.1 m / (min-t) [0.75—1.0 cfm/bu] available on bins equipped for drying. It would be difficult to accomplish significant reconditioning using the low airflow aeration systems common on storage bins. 

More information can be obtained from drying kinetics and experimental measurements. These experiments provide the ultimate basis for selection of the appropriate dryer and drying condition. Often one may arrive at several alternate drying equipment for drying a given product. 

Perrut M, Francais E (1999) Process and equipment for drying polymeric aerogel in the presence of a supercritical fluid. US patent 5 962-539. 

A large variety of equipment for drying is available. The selection criteria are beyond the scope of this book. However, in general, agitated dryers and static dryers have to be differentiated, namely, due to their different potential effects on the crop. In the following, drying aspects related to the properties of the crop are discussed. 

Figure 5. Static frictional force (cantilever equipment) for dry PET fibres as a function of applied load W. The theoretical curve was calculated using Equation 13.

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