Continuous dryers remove moisture from a constant flow of substances or items that enter the dryer, are dried and then exit, all without stopping. The dryer must provide enough heat to cause evaporation but not scorch the inside, so there is a limited window of opportunity because of the constant intake and withdrawal of the substances or items.
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Continuous dryers are necessary for high-paced applications where a steady stream of product needs to be dried quickly and in large quantities. The agricultural, food processing, publishing, apparel, and lumber industries use continuous dryers for drying ceramics, grain, cereal, paper, screen-printed graphics and treated wood among many other uses. Depending on the surface used to feed the products into the dryer, continuous dryers can be used to remove moisture from granules, gels, pastes, flakes, slurries, solids and large items. Dryers that are deemed continuous do not have one specific design because it is a term given to any dryer that doesn't work in batches. Dryers are frequently made from a metal such as steel or aluminum and use gas, electricity, oil or waste to generate heat. The time it takes for a substance or item to be fully dried varies based on the individual dryer; to dry platisol inks used on clothing, for example, only a few minutes are required. More complex machines are hundreds of feet long, and it can take days for the contents to dry. They range from 18 to over 100 inches wide, depending on the application.
Though some dryers use a series of hooks or mounts to carry products through a dryer, the most common dryer that is capable of running continuously is a rectangular apparatus made of metal with a conveyor belt running through the center. The surface of the belt can be a solid sheet of rubber or plastic, screen-like or perforated with holes. Heat sources mounted on the inside of the housing introduce hot air or infrared heat to the substances or items that are carried through the dryer on the conveyor belt. By the time they reach the other end of the machine, the moisture has evaporated and the substances or items are dry. The dried materials are removed quickly because the belt is running constantly, bringing more material. Continuous dryers that use a conveyor belt are occasionally confused with certain kinds of ovens; the drying process, however, should not be associated with cooking. Continuous dryers rely on the process of evaporation, which occurs when moisture inside the substance or item reaches a point where it changes from being in a liquid state to a gas. The vapor diffuses into the air and dissipates, leaving the remaining product with less moisture and therefore more dry. Cooking on the other hand changes the chemical makeup of the food and while the food may dry out in the process, it is not a simple matter of evaporation.
