Vacuum shelf dryer

Vacuum-Shelf Dryers Vacuum-shelf diyers are indirec t-heated batch diyers consisting of avacuumtight 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 resihent gaskets of rubber or similar material (Fig. 12-49). 

Removal of 95% or more of the water from a material, usually a foodstuff, by exposure to high temperature by various means. Its primary purpose is to reduce the volume of the product, increase its shelf life, and lower transportation costs. Special equipment for dehydration includes tunnel dryers, vacuum (shelf) dryers, drum dryers, etc., in which the bulk product is exposed to a hot-air environment. Another method is spray drying, in which a liquid product is ejected from a nozzle into hot air dried milk and egg white are prepared in this way. The term dehydration is not applied to loss of water by evaporation or sun drying. See drying. 

Radiative Heat Transfer Heat-transfer equipment using the radiative mechanism for divided solids is constructed as a table which is stationary, as with trays, or moving, as with a belt, and/or agitated, as with a vibrated pan, to distribute and expose the burden in a plane parallel to (but not in contacl with) the plane of the radiant-heat sources. Presence of air is not necessary (see Sec. 12 for vacuum-shelf dryers and Sec. 22 for resubhmation). In fact, if air in the intervening space has a high humidity or CO9 content, it acts as an energy absorber, thereby depressing the performance. 

Vacuum-shelf dryers require auxiliary stream jets or other vacuum-producing devices, intercondensers for vapor removal, and occasionally wet scrubbers or (heated) bag-type dust collectors. 

Performance Data for Vacttum-Shelf Dryers The purchase price of a vacuum-shelf dryer depends upon the cabinet size and number of shelves per cabinet. For estimating purposes, typical prices (1985) and auxiliai y-emiipment requirements are given in Table 12-12. Installed cost of the equipment will be roughly 100 percent of the carbon steel purchase cost. 

The thermal efficiency of a vacuum-shelf dryer is usually on the order of 60 to 60 percent. Table 12-13 gives operating data for one... 

TABLE 12-13 Performance Data of Vacuum-Shelf Dryers... 

Description A tray or compartment dryer is an enclosed, insulated housing in which solids are placed upon tiers of trays in the case of particulate solids or stacked in piles or upon shelves in the case of large objects. Heat transfer may be direct from gas to sohds by circulation of large volumes of hot gas or indirect by use of heated shelves, radiator coils, or refractory wmls inside the housing. In indirect-heat units, excepting vacuum-shelf equipment, circulation of a small quantity of gas is usually necessary to sweep moisture vapor from the compartment and prevent gas saturation and condensation. Compartment units are employed for the heating and drying of lumber, ceramics, sheet materms (supported on poles), painted and metal objects, and all forms of particulate solids. 

Vacuum-shelf dryers may vary in size from 1 to 24 shelves, the largest chambers having overall dimensions of 6 m wide, 3 m long, and 2.5 m high. 

Rubber and electrical insulating material should be treated in standard chamber dryers or vacuum-shelf dryers, with or without the recovery of the so Ivents. Shel dryers are more expensive to install, but the saving in steam quickly pays for the extra expense. Explos ves are handled in vacuum-shelf dryers, especially built for this purpose, with and without recovery of the solvents. The drying housing is equipped with large automatic doors which will open in case of explosion. 

Design methods for vacuum-shelf dryers Heat is transferred to the wet material by conduction through the shelf and bottom of the tray and by radiation from the shelf above. The critical moisture content will not be necessarily the same as for atmospheric tray drying, as the heat-transfer mechanisms are different. 

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