A wide variety of rotary dryer designs are available for different applications. Gas flow, heat source, and drum design all affect the
efficiency and suitability of a dryer for different materials.
Gas flow The stream of hot gas can either be moving toward the discharge end from the feed end (known as co-current flow), or toward the feed end from the discharge end (known as
counter-current flow). The direction of gas flow combined with the
inclination of the drum determine how quickly material moves through the dryer.
Heat source The gas stream is most commonly heated with a burner using
gas, coal or oil. If the hot gas stream is made up of a mixture of air and combustion gases from a burner, the dryer is known as "directly heated". Alternatively, the gas stream may consist of air or another (sometimes inert) gas that is preheated. Where burner combustion gases do not enter the dryer, the dryer is known as "indirectly-heated". Often, indirectly heated dryers are used when product contamination is a concern. In some cases, a combination of direct-indirect heated rotary dryers are also used to improve the overall efficiency.
Drum design A rotary dryer can consist of a single shell or several
concentric shells, though any more than three shells is not usually necessary. Multiple drums can reduce the amount of space that the equipment requires to achieve the same
throughput. Multi-drum dryers are often heated directly by oil or gas burners. The addition of a combustion chamber at the feed end helps ensure efficient fuel usage, and homogenous drying air temperatures.
Combined processes Some rotary dryers have the ability to combine other processes with drying. Other processes that can be combined with drying include cooling, cleaning,
shredding and separating. == See also ==