Rotary dryers are designed using a rotating cylinder. With two basic designs, directly and indirectly heated, they are readily adaptable to a wide variety of processing applications, as stand-alone units or as part of complete thermal processing systems.
Directly Heated Rotary Dryers
The direct heat rotary dryer design consists principally of an inclined rotating cylinder with internal flighting. This type of design brings process material into direct contact with the drying medium, either in a concurrent or countercurrent flow configuration. The drying medium may be products of combustion tempered with air, heated clean air or special gases. Direct heat rotary dryers are most appropriate when product characteristics reflecting particle size range, specific gravity, and shape permit reasonable drying air velocities through the cylinder, as well as for applications involving product temperatures upwards to 930°F / 500°C.
Indirectly Heated Rotary Dryers
The indirect heat rotary dryer design provides for heating under oxidizing, reducing, or inert atmospheric conditions. The rotary dryer design consists principally of a rotating cylinder that is enclosed in a furnace along its active length. With heat transfer indirect and through the cylinder wall, the gas velocities within an indirect dryer are low as compared to a direct heat dryer, and correspondingly, the off-gas system is smaller and more economical.