Basically, they are a big drum which is heated, then materials passed through to dry them quickly.

http://www.rotarydrumdryers.com/sitebuilder/images/set_drum_dryer-150x112.jpg

http://www.iqsdirectory.com/info/industrialdryers/images/rotary_dryersA.jpg

http://www.iqsdirectory.com/rotary-dryers/
"Rotary Dryers

Rotary dryers pass material through a heated rotating drum. Rotary dryers, also referred to as drum dryers, can be fired by gas, liquid or solid fuel. In some rotary dryers, the drums have multiple chambers that allow a longer pass to get through the drum.

Rotary drums can be made for direct or indirect heat processes with either co-current flow or counterflow operations. Direct heat dryers deliver heated air through the same compartment as the material being dried. In comparison, when using an indirect heat dryer, the heated air and the material being dried do not flow through the same compartment. In co-current flow, heat sensitive, wet materials are dried when exposed to hot air. In counterflow operations, dried materials are exposed to hot air, resulting in very little moisture.

Applications for rotary drums include metallic oxides, powdered detergents, insulation materials, metal powders, carbon black pigment, and industrial chemicals. Rotary dryers can often be found in the chemical, food and mineral industries. "

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