Steam continued dryer

Screw conveyor and indirect rotary. Indirect type, continuous operation. Includes paddle, horizontal agitated and steam-tube dryers, rotary kilns. Applicable with dry-product recirculation Applicable with dry-product recirculation Generally requires recirculation of dry product. Little dusting occurs. Chief advantage is low dust loss. Well suited to most materials and capacities, particularly those requiring drying at steam temperature Low dust loss. Material must not stick or be temperature-sensitive Not applicable Not applicable Not applicable... 

Finally, a novel high-pressure steam flash dryer (25 bar) has been operated successfully in Finland for drying of milled peat, which is then fed continuously to a high-pressure gasifier. Clearly, such a sophisticated dryer is not warranted for a drying application. 

Another problem that must be considered when designing the steam-heated dryers is the removal of the condensate. If the condensate is not removed from the cylinders continuously, the heat-transfer rate drops and the power load on the drive increases because of the buildup of a heavy load. 

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. 

In a continuous dryer, the wet solids travel horizontally in contact with a moving stream of gas. If tlie gas travels counlercuiTcntly to the solids, the hottest gas will encounter tlie driest solids, which tends to distribute the rate of evaporation somewhat, evenly through the dryer. But if the product is sensitive to high temperatures, coeurrent gas flow is safer, in that the wet solids arc loss likely to be damaged by hot gas. Food products or fine chemicals are ordinarily dried by air which is steam heated. Heavy chemicals are more often dried by direct exposure to a burning fuel gas. 

Cylinder Heat-Transfer Units Sometimes called can dryers or drying rolls, these devices are differentiated from drum diyers in that they are used for solids in flexible continuous-sheet form, whereas drum dryers are used for liquid or paste forms. The construction of the individual cyhnders, or drums, is similar in most respec ts to that of drum diyers. Special designs are used to obtain uniform distribution of steam within large drums when uniform heating across the drum surface is critical. 

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