Rotary dryer

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A rotary dryer is a type of industrial dryer used to reduce or minimize the moisture content of the material it is handling by bringing it into contact with a heated gas.[1] The dryer is made up of a rotating cylinder ("drum" or "shell"), a drive mechanism, and a support structure (commonly concrete posts or a steel frame). The cylinder is inclined slightly with the discharge end is lower than the material feed end so that material moves through the dryer under the influence of gravity. Material to be dried enters the dryer and, as the dryer rotates, the material is lifted up by a series of fins (known as flights) lining the inner wall of the dryer. When the material gets high enough, it falls back down to the bottom of the dryer, passing through the hot gas stream as it falls.

Applications

Rotary Dryers have many applications but are most commonly seen in the mineral industry for drying sand, stone, soil, and ore.[2] They are also used in the food industry for granular material such as grains, cereals, pulses, and coffee beans.

Design

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".[3] 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

References

The rotary dryer has a high capacity, low fuel consumption and low drying costs. The drum dryer is high temperature resistant and can use high temperature hot air to dry the material quickly. Scalable and rotary dryer design with production margins in mind, there is no need to replace equipment, even if production increases marginally. 

The equipment adopts the spherical tractor structure, the tractor and the rollers cooperate well, which greatly reduces the wear and power consumption. The specially rotary dryer designed retaining wheel structure significantly reduces the horizontal thrust caused by the tilting of the equipment. Strong overload resistance, smooth operation and high reliability of the cylinder.

High thermal efficiency

Since the dryer is heat transferring, the direction of heat transfer remains basically the same throughout the heat transfer cycle, so most of the heat supplied in the drum is used for the wet vaporization of the material with a thermal efficiency of 80% to 90%.

High drying rate

The heat and mass transfer process of the wet material film on the wall of the barrel, from inside to outside, the direction is the same, the temperature gradient is large, so that the surface of the material film maintains a high evaporation strength, generally up to 30 ~ 70 kg/(m.h).

Stable dry quality

Because the heating method is easy to control, the temperature inside the barrel and the heat transfer rate of the wall can remain relatively stable, so that the material film in the heat transfer state drying, product quality can be guaranteed. However, drum dryers also have their drawbacks, mainly the high surface humidity of the drum, which can damage the flavor or abnormal color of some products due to overheating. In addition, if a vacuum dryer is used, it is more costly and is only suitable for handling materials with very high thermal sensitivity.