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A Sand 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 dryer are the simplest and the most economical class. They are used when a direct contact with the hot gas or air is not detrimental to the fed. When high temperature is required for 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. 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 or rotary dryers include;
Direct rotary dryer
Direct rotary kiln
Indirect steam-tube dryer
Indirect rotary calciner
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.
Whichever method you are using, there is one thing that all these instruments have in common: they all use methods of heat transfer to carry out their jobs. Heat transfer is how heat moves from one source to another. Understanding the types of heat transfer, and how they differ, is an important part in understanding how a rotary dryer, cooler, or kiln works. Knowing how heat transfer works can help in sizing a machine, and can also help to anticipate where heat will be lost in a system.
Even though we might not notice it, we think about heat transfer all the time – From cooking, to global warming – heat transfer is constantly at work around us. There are a couple basic rules to remember when thinking about heat transfer. First, heat ALWAYS transfers from a hotter object to a cooler one; the opposite of this is just not possible. Second, some materials can transfer heat better than others.
Materials are categorized into two basic groups, depending on their ability to transfer heat. Conductors are materials that transfer heat very well. Some of the best conductors are the non-ferous metals, such as copper and aluminum. Ferrous metals such as steel and stainless steel are also good conductors. The opposite of a conductor, is an insulator. Wood, rubber, ceramic, and brick, are all examples of insulators, or items that do not transfer heat well.
Advantages of Paddle-Harrow Rotary Dryer
Rotary dryers are less sensitive to particle size.
It can accept the highest flue gasses of any type of dryer.
Low maintenance cost; thus economical.
It has the greatest capacity than any type of dryer.
Disadvantages of Rotary dryer
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.