A rotary dryer is a device that uses rotation, gravity and warm gasses to dry a material. These driers range from small household clothes dryers to large industrial systems. Regardless of their size, the majority of rotary dryers work the same way. A drum rotates and moves material while hot gasses quickly dry it out. The main distinction in these dryers is the type of gasses used and the method used to generate heat. As a result of the tremendous kinetic energy they create, rotary dryers of all but the smallest sizes require fixed support systems such as concrete columns or floor bolts.
Regardless of its size and purpose, one rotary dryer’s inner workings are basically the same as any other. The main user component is the drum where the work material is placed. The inside of the drum contains many large fins that help to lift up and agitate the work material as the dryer spins. This drum is connected to the inner workings of the dryer, but those are hidden from the outside. A door covers the drum opening so the compartment may be sealed.
As the rotary dryer spins, the fins lift the work material from the bottom of the drum and raise it up along the side. As the fin nears the top of the drum, gravity makes the lifted material fall back to the bottom. As the work material falls, it is sprayed with warm gas. This gas is very effective at drying the material as it is completely open on all sides.
There are several different gasses used in rotary dryers. In household dryers, most of the gas is simply warm air. In commercial or industrial dryers, nearly any gas can be used, but the more common ones are exhaust fumes and various inert gasses. In these dryers, the main concern is possible contamination of the work material and the maximum heat of the used gas.
The only common difference in a rotary dryer is the method used to heat and distribute the air into the drum. A direct heated dryer uses the combustion gases generated by the heater mixed with an artificially heated gas. This second gas is nearly always common air, but it can be any inert gas. These systems are only used when the heater exhaust cannot contaminate the work material.
The second system is called an indirect heat rotary dryer. In this case, the heater warms a second gas which enters the drum while any combustion gases remain separate. This form of dryer is the most likely for an average person to encounter, as it is the kind commonly used to dry clothing.
By M. McGee