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.