Dryers are used to remove water or solvents from solid material by evaporation. The solid material usually has a rigid structure. However, pastes, slurries, and liquid solutions are also handled in dryers. In this case the formation of the rigid structure of the solid material occurs during the drying process itself. The effect of drying on the quality of the dried product depends on the drying conditions. The use of fast drying at high temperatures reduces the size of dryer required; however, it may also destroy the solid product by shrinking it too fast and by formation of fissures or by denaturing and other forms of deterioration of the constituents. The selection of drying conditions is usually a compromise between these two requirements-minimizing the size of the drying equipment and optimizing the product quality. Also, energy consumption has to be taken into account.

Dryers are designed in many different ways in order to serve various requirements, such as high- or low- temperature drying at normal pressure, or at vacuum with air or nitrogen or superheated vapor as the drying agent.

Other requirements result from whether the product is a liquid, a slurry, or a solid. Solid material may be in the form of large pieces, sheets, threads, granules, powder, or other.

Another aspect is the required residence time of the product in the dryer, which may vary from a fraction of a second up to several days. This leads to different concepts of dryer design.

The heat necessary to evaporate the moisture may be supplied by convection through the drying agent, by radiation, by direct contact conduction, or by micro- waves. This again leads to different concepts of dryer design. The majority of dryers, however, are furnished with convective heat supply. A classification of types of dryers must take into account all these aspects in order to provide a good tool for the selection of a proper type with respect to the nature of the product.