Batch trays and dryers

Batch Furnaces This type of furnace is employed mainly for the heat treatment of metals and for the drying and calcination or ceramic articles. In the chemical process industry, batch furnaces may be used for the same purposes as batch-tray and truck dryers when the drying or process temperature exceeds 600 K (620°F). They are employed also for small-batch calcinations, thermal decompositions, and other chemical reactions which, on a larger scale, are performed in rotary Idlns, hearth furnaces, and shaft furnaces. 

TABLE 9.6. Performance Data of Batch Tray and Tray-Truck Dryers... 

A tray and compartment dryer is a batch unit that uses a stationary tray or compartment to dry the waste, generaUy before transport for disposal or further treatment. Some units can be mounted on removable tmcks. 

Design Methods for Turbo-Tray Dryers The heat- and mass-transfer mechanisms are similar to those in batch tray diyers, except that constant turning over and mixing of the solids significantly improves diying rates. Design must usually be based on previous installations or pilot tests by the manufacturer apparent heat-transfer... 

Batch tray dryers are used for drying small quantities of solids, and are used for a wide range of materials. 

Contact Tray and Vacuum-Shelf Dryers Vacuum-shelf dryers are indirectly heated batch dryers consisting of a vacuum-tight chamber usually constructed of cast iron or steel plate, heated, supporting shelves within the chamber, a vacuum source, and usually a condenser. One or two doors are provided, depending on the size of the chamber. The doors are sealed with resilient gaskets of rubber or similar material. It is also possible, but much less common, to operate at atmospheric pressure without vacuum. 

The batch tray dryer can often be converted into a continuous unit. Figure 23.11 shows the so-called Turbo dryer, which consists of a stack of coaxial circular trays mounted on a single vertical shaft. The product layer fed into the first shelf is leveled by a set of stationary blades that scratch a series of grooves into the surface layer of the solid particles. The blades are staggered to ensure mixing of the material. After one rotation, the material is wiped off the shelf by the last blade and falls onto the next lower shelf. Up to 30 trays or more can be accommodated. 

TRAY DRYERS. A typical batch tray dryer is illustrated in Fig, 24,9. It consists of a rectangular chamber of sheet metal containing two trucks that support racks H. Each rack carries a number of shallow trays, perhaps 750 mm (30 in.) square and 50 to 150 mm (2 to 6 in.) deep, that are loaded with the material to be dried. Heated air is circulated at 2 to 5 ra/s (7 to 15 ft/s) between the trays by fan C and motor D and passes over heaters E. Baffles G distribute the air uniformly over the stack of trays. Some moist air is continuously vented through exhaust duct B makeup fresh air enters through inlet A. The racks are mounted on truck wheels /, so that at the end of the drying cycle the trucks can be pulled out of the chamber and taken to a tray-dumping station. 

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