专业英语课文

专业英语课文

Equipment for Drying

Many types of commercial dryers are described in standard references. Here only a small number of important types are considered. They may be classified according to whether the material is a rigid or granular solid, a semisolid paste, or a liquid solution or slurry; whether or not the material is agitated during drying; whether the operation is batch or continuous. Still another division is between direct-contact dryers, in which the solids are dried by exposure to hot air or flue gas, and in direct dryers, in which heat is transferred to the material from a heating medium through a metal wall.

Tray Dryer In tray dryers, which are also called shelf ,cabinet, or compartment dryers, the material, which may be a lumpy solid or a pasty solid, is spread uniformly on a metal tray to a depth of 10 to 100 mm. Such a typical tray dryer contains removable trays loaded in a cabinet. Steam-heat air is recirculated by a fan over and parallel to the surface of the trays. Electrical heat is also used, especially for low heating loads. About 10% to 20% of the air passing over the trays is fresh air, the remainder being recirculated air. After drying, the cabinet is opened and the trays are replaced with a new batch of trays. A modification of this type is the tray-truck type, where trays are loaded on truck which are pushed into the dryer. This saves considerable time, since the trucks can be loaded and unloaded outside the dryer.