The rotary dryer also known as tumbling dryer is an equipment employed to minimize the moisture content of feed materials by bringing it in direct contact with a heated gas. It consists of an inclined long drum or cylindrical shell often fitted with internal flights or lifters; rotated slowly upon bearings through which the material to be dried flow with a tumbling/cascading action in concurrent (for heat-sensitive materials) or counter-current flow with the heating air or gases.
The movement of the material is due to the combined effect of inclination of the shell to the horizontal and the internal tumbling action or mechanical turn over thus the name tumbling dryer. The nature of the feed determines the directions of gas flow through the cylinder and it is relative to the solid. This drying equipment can also perform batch or continuous processing of the wet feed.
The material held up time is determined by;
Based on the method of heat transfer, rotary dryers can be classified into;
1. Direct-heated rotary dryer.
2. Indirect- Direct rotary dryer.
3. Indirect rotary dryer.
4. Special type.
A rotary dryer is said to be of the direct type if, by virtue of its design, heat is added to or removed from the solids by direct exchange between the gas and solids. The direct heat dryers are the simplest and the most economical class. They are used when direct contact with the hot gas or air is not detrimental to the fed.
When high temperature is required for the drying process in a direct-heated rotary dryer, a combustion chamber is used and when low temperature is required on the other hand, for thermolabile materials, steam coil is used.
Rotary dryer can also be said to be indirect when the heating medium is separated from contact with the solid by a metal wall or tube.
Read Also: Spray Dryer
Although there is an infinite variation of rotary dryers, which present characteristics suitable for drying, chemical reactions, mixing, solvent recovery, thermal decompositions, sintering and agglomeration of solids, the main types of rotary dryers include;
a. Direct rotary dryer
b. Direct rotary kiln
c. Indirect steam-tube dryer
d. Indirect rotary calciner
e. Direct Roto-Louvre dryer
Of all the types stated above, the direct rotary dryer is the one that has been extensively studied while very little scientific work has been published for other types.
1. Rotary dryers are less sensitive to particle size.
2. It can accept the highest flue gasses of any type of dryer.
3. Low maintenance cost; thus economical.
4. It has the greatest capacity than any type of dryer.
1. Excessive entrainment losses in the exist gas stream is possible especially if the material contains extremely fine particles due to the large gas volumes and high gas velocities that are usually required.