Tray dryer

Manually loaded shelf dryers are not continuous so we would have to calculate a batch-surge arrangement to accommodate the continuous operation. 

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Fig. 11. Two-truck tray dryer. A, air inlet duct B, air-exhaust duct with damper C, axial flow fan D, fan motor, 2—15 kW E, air heaters F, air-distribution plenum G, distribution slots and H, wheeled tmcks and trays. The arrows iadicate air and vapor flow pattern.

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

VIbrotIng-tray dryers. Heoting occompijshed by steam or hot woter... 

Batch thi ough-cii culation. Direct type, hatch operation Not apphcahle Not apphcahle Suitable only if material can he preformed. Suited to hatch operation. Shorter drying time than tray dryers Not apphcahle... 

Direct-Heat Tray Dryers Satisfactory operation of tray-type dryers depends on maintaining a constant temperature and a uniform air velocify over all the materi being dried. 

Circulation of air at velocities of I to 10 m/s is desirable to improve the surface heat-transfer coefficient and to eliminate stagnant air pockets. Proper air flow in tray dryers depends on sufficient fan capacity, on the design of ductwork to modify sudden changes in direction, and on properly placed baffles. Nonuniform airflow Is one of the most serious problems in the operation of tray di yers. 

Tray dryers may be of the tray-truck or the stationary-tray type. In the former, the trays are loaded on trucks which are pushed into the dryer in the latter, the trays are loaded directly into stationary racks within the dryer. Trucks may be fitted with flanged wheels to run on tracks or with flat swivel wheels. They may also be suspended from and moved on monorails. Trucks usually contain two tiers of trays, with IS to 48 trays per tier, depending upon the tray dimensions. 

Determination of the Temperature of the Evaporating Surface in Direct-Heat Tray Dryers When radiation and conduction are negligible, the temperature of the evaporating surface approaches the wet-bulb temperature and is readily obtained from the humidity and diy-bulb temperature. Frequently, however, radiation and conduction cause the temperature of the evaporating surface to exceed the wet-bulb temperature. When this occurs, the true surface temperature must be estimated. 

Performance Data for Direct-Heat Tray Dryers A standard two-truck diyer is illustrated in Fig. 12-48. Adjustable baffles or a perforated distribution plate is normally employed to develop 0.3 to 1.3 cm of water-pressure drop at the wall through which air enters the truck enclosure. This will enhance the uniformity of air distribution, from top to bottom, among the trays. In three (or more) truck ovens, air-reheat coils may be placed between trucks if the evaporative load is high. Means for reversing air-flow direction may also be provided in multiple-truck units. 

Type of dryer 2-tnick 16-tray dryer 16-tray 3-truck 2-tnick... 

Turbo-Tray Dryers The turbo-tray diyer is a continuous diyer consisting of a stack of rotating annular shelves in the center of which turbo-type fans revolve to circulate the air over the shelves. Wet material enters through the roof, falling onto the top shelf as it rotates beneath the feed opening. After completing one revolution, the mate-... 

By viriue of its vertical construction, the turbo-type tray dryer has a stack effect, the resulting draft being frequently sufficient to operate the dryer with natural draft. Pressure at points within the dryer is maintained close to atmospheric, as low as 0.1, usually less than 0.5 mm of water. Most of the roof area is used as a breeching, lowering the exhaust velocity to settle dust back into the dryer. 

The turbo-tray dryer can handle materials from thick slurries [1 million (N s)/m (100,000 cP) and over] to fine powders. It is not suitable for fibrous materials which mat or for doughy or tacky materials. Thin slurries can often be handled by recycle of dry product. Filter-press cakes are granulated before feeding. Thixotropic materials are red directly from a rotary filter by scoring the cake as it leaves the drum. Pastes can be extruded onto the top shelf and subjected to a hot blast of air to make them firm and free-ffowing after one revolution. 

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... 

FIG. 12-77 Turbo-tray dryer in closed circuit for coutiuiioiis drying with solvent recovery. Wyssmont Company, Inc.)... 

Performance and Cost Data for Turbo-Tray Dryers Performance data for three apphcations of closed-circuit diying are included in Table 12-29. Operating, labor, and maintenance costs compare favorably with those of direct-heat rotating eqmpment. 

TABLE 12-29 Turbo-Tray Dryer Performance Data in Wyssmont Closed-Circuit Operations ... 

Use alternate drying method (ex. vacuum drying instead of atmospheric drying vacuum tray dryer, freeze drying, cryogenic CO2 drying, instead of vacuum rotary dryer) where material is subdivided in multiple locations... 

Wiped film stills in place of continuous still pots —Centrifugal extractors in place of extraction columns —Flash dryers in place of tray dryers —Continuous reactors in place of batch —Plug flow reactors in place of CFSTRs —Continuous in-line mixers in place of mixing vessels... 

Drying hygroscopic plastics should not be taken casually. The simple tray dryers or mechanical convection hot-air dryers that are... 

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

While drying in a tray dryer at 60°C, a batch decomposed fairly exothermically but without fire. As a blowing agent, it is designed to have limited thermal stability [1], One of the recently calculated values of 87° and 78°C for induction periods of 7 and 60 days, respectively, for critical ignition temperature coincides with the previous value of 8777 days. Autocatalytic combustion is exhibited. 

A 100 kg batch of granular solids containing 30 per cent of moisture is to be dried in a tray dryer to 15.5 per cent moisture by passing a current of air at 350 K tangentially across its surface at the velocity of 1.8 m/s. If the constant rate of drying under these conditions is 0.7 g/sm2 and the critical moisture content is 15 per cent, calculate the approximate drying time. It may be assumed that the drying surface is 0.03 m2/kg dry mass. 

A solid is to be dried from 1 kg water/kg dry solids to 0.01 kg water/kg dry solids in a tray dryer consisting of a single tier of 50 trays, each 0.02 m deep and 0.7 m square completely filled with wet material. The mean air temperature is 350 K and the relative humidity across the trays may be taken as constant at 10 per cent. The mean air velocity is 2.0 m/s and the convective coefficient of heat transfer is given by ... 

Two-truck tray dryer, 9 118—119 Two-way memory, of shape-memory alloys, 22 341, 346-350... 

A modem tray dryer consists of a well-insulated cabinet with integral fans and trays which are stacked on racks, or loaded on to trucks which are pushed into the dryer. Tray areas are 0.3-1 m2 with a depth of material of 10-100 mm, depending on the particle size of the product. Air velocities of 1-10 m/s are used and, in order to conserve heat,... 

A 100 kg batch of granular solids containing 30 per cent of moisture is to be dried in a tray dryer to... 

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