Band and Tunnel Dryers

Performance and Cost Data for Continuous Band and Tunnel Dryers Experimental performance data are given in Table 12-21 for numerous common materials. Performance data from several commercial through-circulation conveyor dryers are given in Table 12-22. Labor requirements vary d ending on the time required for feed adjustments, inspection, etc. These dryers may consume as little as 1.1 kg of steam/kg of water evaporated, but 1.4 to 2 is a more common range. Thermal efficiency is a function of final moisture required and percent air recirculation. 

Continuous Band and Tunnel Dryers This group of dryers is variously known as band, belt, conveyor, or tunnel dryers. 

Mechanical conveying (usually horizontal), e.g., band, tunnel, and paddle dryers... 

Band dryers are also of continuous operation type. Materials are conveyed on bands rotating in the drying chamber as shown in Figure 35.9. Band consists of a net or a perforated plate. Hot air flows crossing the band upward or downward because heat- and mass-transfer rates are greater than that from parallel flow. It is possible to control the temperature, humidity, and flow rate of air at each optimum state in some zones separated in the dryer. Although the operation temperatures are almost the same as the tunnel dryers, ha is 45-95 W/(K m -dryer volume) for the parallel flow. 

A number of factors influence the selection of a dryer from the many different types available. These factors are dominated by the nature of the feed, whether it be granular solids, a paste, a slab, a film, a slurry, or a liquid. Other factors include the need for agitation, the type of heat source (convection, radiation, conduction, or microwave heating), and the degree to which the material must be dried. The most commonly employed continuous dryers include tunnel, belt, band, turbo-tray, rotary, steam-tube rotary, screw-conveyor, fluidized-bed, spouted-bed, pneumatic-conveyor, spray, and drum dryers. 

Several plants based on the vapor recompression cycle have been constructed and tested in Europe. One system uses a horizontal conveyor band in a tunnel drying setup (Bertin, France). Another manufacturer has developed a fluidized bed dryer using high pressure steam (NIRO, Denmark). The results of both systems have been encouraging. 

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