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Publication Date: 09/2004
Pages: 2
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Drying Desk: When to use an indirect-contact dryer

In this column, we’ll consider how an indirect-contact dryer’s characteristics make it particularly wellsuited for certain bulk solids drying applications. This dryer is one of two common types — direct contact and indirect contact — as classified by heat transfer method.

In a direct-contact dryer, heat is transferred by the direct contact of a hot gas with the material. Examples of direct-contact dryers that can be operated in either batch or continuous mode are fluid-bed dryers and hopper dryers. Examples that operate solely in continuous mode are spray dryers and flash dryers.

In an indirect-contact dryer (also called a contact dryer), heat is transferred by conduction across a metal wall that separates the heat source — such as steam, hot water, hot oil, or hot gas — from the material. Thus the dryer requires a substantially lower gas flowrate than the direct-contact dryer, reducing the size and cost of the indirect-contact dryer’s related gashandling equipment and increasing the dryer’s energy efficiency. Common continuous indirect-contact dryers are steam-tube rotary dryers; thin-film scraper drum dryers; thinlayer, high-speed paddle dryers; lowspeed paddle and disc dryers; and hollow-screw and -plate dryers. Batch indirect-contact dryers, which are well-suited for vacuum drying, include double-cone tumble dryers, vertical conical dryers, and horizontal heat-jacketed ribbon blenders.

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