Drying Equipment: Types and Operating Principle
Moisture removal from solid and pasty materials allows reducing cost of their transportation and giving them the necessary properties (e.g. reducing caking of fertilizers or improving solubility of dyes), as well as reducing corrosion of equipment and piping when storing or subsequently processing these materials.
Moisture can be removed from the materials mechanically (by squeezing, settling, filtration, and centrifugation). However, more complete dehydration is achieved by evaporation of moisture and removal of vapors, i.e. by heat drying.
This procedure is widely used in chemical processes. It is often the last operation before production of finished products. At that, preliminary moisture removal is usually carried out using more accessible mechanical methods (e.g. by filtration), and final moisture removal – using drying. Such a combined process of removing moisture can improve the process efficiency.
In chemical industry, as a rule, they use artificial drying of materials in special dryers, since natural drying in the open air is quite a long process.
Drying is a complicated diffusion process, the speed of which is determined by the speed of moisture diffusion from the bottom of the material being dried to the environment. Moisture removal at drying is actually moving heat and substance (moisture) within the material and transferring them from the material surface to the environment. Thus, drying process is a combination of related to each other processes of heat and mass transfer (moisture transfer).
As for the way of supplying heat to the material being dried, there are the following types of drying:
1) convective drying - by direct contact of the material dried with a drying agent, usually being heated air or flue gas (usually mixed with air);
2) contact drying - by transferring heat from the heat carrier to the material through the wall separating them;
3) radiation drying – by transferring heat by infrared rays;
4) dielectric drying - by heating in the field of high-frequency currents;
5) freeze-drying is drying in a frozen state under high vacuum. As for the method of heat transfer, this type of drying is similar to the contact one, but the uniqueness of the process refers freeze-drying to a special group. The last three types of drying are used relatively rarely and are usually called special types of drying.
The dried material at any method of drying is in contact with wet gas (air in most cases). At convective drying, wet gas (being a drying agent) plays the main role in the process. Therefore, studying wet gas properties is necessary when considering drying processes and their calculations.
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Strommashina Samara Plant has a huge experience in manufacturing drying equipment. Industrial drying machines manufactured by Strommashina – drying drums, dryers and rotary kilns – have gained recognition in Russia and abroad as reliable, cost-effective and high-performance equipment.