What is a rotary dryer?

A rotary tumble dryer.

A rotary dryer is a device that uses rotation, gravity and hot gases to dry a material. These dryers range from small household clothes dryers to large industrial systems. Regardless of size, most rotary dryers work the same way. A drum rotates and moves the material as the hot gases quickly dry it. The main difference in these dryers is the type of gases 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, the inner workings of a rotary dryer are basically the same as any other. The main user component is the drum where the work material is placed. The interior of the drum contains many large fins that help lift and agitate the work material as the dryer rotates. This drum is connected to the inner workings of the dryer, but those are hidden on the outside. A door covers the opening of the drum so the compartment can be sealed.

As the rotary dryer rotates, the fins lift the work material from the bottom of the drum and lift it along the side. As the fin approaches the top of the drum, gravity causes the lifted material to fall back to the bottom. As the working material falls, it is sprayed with hot gas. This gas is very effective in drying the material, as it is completely open on all sides.

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There are several different gases used in rotary dryers. In domestic dryers, most of the gas is simply hot air. In commercial or industrial dryers, almost any gas can be used, but the most common are exhaust gases and various inert gases. In these dryers, the main concern is the possible contamination of the working material and the maximum heat of the gas used.

The only common difference in a rotary dryer is the method used to heat and distribute the air in the drum. A direct-heated dryer uses the combustion gases generated by the heater mixed with an artificially heated gas. This second gas is almost always ordinary 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 heats a second gas that enters the drum while the flue gases remain separate. This way of drying is the one most likely to be encountered by the average person, as it is the type commonly used to dry clothes.

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