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Rotary dryer: how it works, what it is for, parts, applications

Rotary dryers are made up of a metal cylinder or drum that rotates and has a slight inclination to facilitate the discharge of the material from the dryer. The dryer is mounted on a concrete base or steel beams that support it.

Rotary dryer typical of industries. Source: De Boer / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

The material is dried thanks to a stream of hot air, produced by a combustion furnace. The stream of hot air passes through the wet material, flowing through the rotary dryer, thus increasing the temperature of the material and losing its moisture.

Rotary Dryer Operation

General process

The material to be treated enters the rotary dryer through its upper part and leaves, once it has been treated to reduce its humidity, through its lower part. This process is favored due to the slight inclination of its support. The wall is equipped with a kind of fins that form an angle with it.

Material entering the top of the dryer tends to fall toward the bottom of the dryer. However, the rotation in combination with the fins that it has on its wall, produces the elevation of the material to be dried until it reaches a certain height on the internal wall.

There the fins are not able to hold the material they carry and this falls towards the bottom of the dryer, passing through a stream of hot air that causes the loss of moisture from the material. This process is repeated several times until the desired drying of the wet material is achieved.

Drying of wet material

Direct heating

Hot air is produced by mixing an air stream and the gases produced in a combustion furnace. When the heating of the humid material is a consequence of the direct action on it of the combustion gases, then we speak of direct heating.

The hot air stream can be directed from the discharge site to the rotary dryer loading site; that is, the hot air moves in the opposite direction to that of the falling material. In this case we speak of a countercurrent flow of hot air.

Indirect heating

In some rotary heaters the air stream is preheated, the humid material being heated without the intervention of the combustion gases. In this case, we speak of indirect heating.

Evaporation and rotation

There are drum dryers, or rotary dryers, in which the water evaporates on the inner wall due to its heating. Meanwhile, a metal blade scrapes the solid material that is adhered to the wall.

The dry material then descends by gravity towards the outlet of the rotating drum, functioning as a hybrid of a dryer and an evaporator.

What is a rotary dryer for?

The usefulness of rotary dryers is very wide, although their most widespread use is the drying of various materials including sludge of different origins, cereal grains, feed for dairy cattle, pigs and poultry, etc.

Rotary dryers are also used in some chemical reactions, in the mixing of materials, in the recovery of solvents, agglomeration of solids. They can also be used in roasting nuts, sesame seeds and cocoa beans.

Parties

Drum

The drum, cylinder or casing of the rotary dryer is metallic and usually has a kind of fins that intervene in the mobilization of the material present inside. It is the place of drying of the humid material.

Loading site

Found in the upper part of the rotary dryer, since it has a slight inclination.

Download site

Found at the bottom of the rotary dryer, which favors the output of the processed material.

Medium

A concrete or steel beam support that gives the rotary dryer its inclination. There are some dryers in which the degree of inclination can be adjusted in order to favor the exit of the treated material.

Electric motor

An electric motor with a transmission system that acts by causing the rotary dryer drum to rotate.

Combustion furnace

A furnace or a combustion burner that produces gases, which when mixed with a stream of air, cause a flow of hot air. This constitutes the main source of the heating of the humid material, thus producing the evaporation of the water content.

Gas eliminator

A part that takes part in the elimination of the gases that are produced in the rotary dryer.

Applications

Raw material processing

The rotary dryer is used for drying clay in industrial cement, as well as for coal sludge from mines. It is also used in the drying of sandstone, powdered minerals, mineral slag and the treatment of mud of different origins and characteristics.

For example, rotary dryers are used to dehydrate textile sludge, papermaking sludge, electroplating sludge, oil sludge, sewage sludge, etc. They are also used to remove moisture from diatomaceous earth, kaolin, potassium chloride and sulfate.

In addition, rotary dryers are used in the drying of milk of lime, phosphate rock slag and red sludge from aluminum plants .

Treatment of biological and organic material

Rotary dryers are used in the treatment of animal manure, wood slag, citrus peels, vegetable trimmings, alfalfa feed, rice with a high moisture content, orange pulp used to complete the feeding with cereals of livestock animals.

Some dryers can use very hot air that can be used to roast seeds.

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