Vibrating-tray dryers

Continuous-tray dryers such os continuous metol belts, vibrating troys utilizing hot goses, vertical turbodryers... 

Sublimation temperatures are in the range of —10 to —40°C and corresponding vapor pressures of water are 2.6-0.13 mbar. Cabinet tray dryers are the most commonly used type. The trays are lifted out of contact with hot surfaces so the heat transfer is entirely by radiation. Loading of 2.5 lb/sqft is usual for foodstuffs. Drying capacity of shelf-type freeze dryers is 0.1-1.0kg/(hr)(m2 exposed surface). Another estimate is 0.5-1.61b/(hr)(sqft). The ice surface has been found to recede at the rate of 1 mm/hr. Freeze drying also is carried out to a limited extent in vacuum pans, vibrating conveyors, and fluidized beds. Condensers operate as low as —70°C. 

Tray Dryers. Material is placed on vibrating trays over or under which hot gases flow. 

Closed-system dryers, spray dryers, fluidized bed dryers Batch or continuous tray, vibrated bed dryers Use dielectric heating (microwave or radio frequency) over part of drying... 

FIGURE 45.19 Continuous vibrogravitational freeze dryer for liquid and pastes (1) vacuum granulator (2) material feed (3) condenser (4) vacuum pump (5) feeding valve (6) drying chamber (7) radiators and (8) vibrating trays. (From Novikov, P.A. et al., Continuous freeze-dryer for liquid materials, Russian Patent No. 273,734, 1970.)... 

The two types of dryers are static trays and conveyors, which are often vibrated as they move along. Atmospheric tray dryers are not suitable for some materials, notably nylon molding powder that oxidizes in air and requires vacuum drying. A commonly used variant of the tray type is the truck dryer. Here the loaded trays are placed on trucks and rolled into the drying enclosure. Continuous conveyor drying systems are extensively used where the product must be dried from the wet solid state and where production requirements are high. 

The efficiency of a vibrating tray freeze-dryer is enormous and the drying times drop by more than an order of magnitude. We are speaking in terms of minutes instead of hours and throughputs of tens of tonnes per day are no longer unrealistic. Moreover, the process is intellectually clean and the end product constant in quality. 

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. 

Because of the presence of the horizontal component of vibration and the louvered tray ensuring a horizontal air velocity component, the material can be transported at desired rate even on a horizontal tray. Therefore, an alternative model of the dryer termed the ring-to-ring dryer [82] is designed as a series of annular trays equally spaced along the central column. 

An important feature of the Helix dryer is its compact size. The dryer with an evaporative capacity of about 8400 kg of water/h at air temperature 315 and 82° C at the dryer inlet and outlet, respectively, is about 3.5 m in diameter and 5 m high, and contains about 75 m of tray area. A conventional vibrated fluid bed dryer for a similar application would be about 1.2 m wide and 48 m long [84]. Cost and energy efficiency comparisons, however, are not available. 

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