High temperature dryers
High temperature dryers are necessary when very fast drying is desired. They are usually
employed when the products require a short exposure to the drying air. Their operating
temperatures are such that, if the drying air remains in contact with the product until
equilibrium moisture content is reached, serious over drying will occur. Thus, the products
are only dried to the required moisture contents and later cooled. High temperature dryers are usually classify into batch dryers and continuous-flow dryers. In batch dryers, the
products are dried in a bin and subsequently moved to storage. Thus, they are usually
known as batch-in-bin dryers. Continuous-flow dryers are heated columns through which
the product flows under gravity and is exposed to heated air while descending. Because of
the temperature ranges prevalent in high temperature dryers, most known designs are
electricity or fossil-fuel powered. Only a very few practically-realised designs of high
temperature drying systems are solar-energy heated (Ekechukwu and Norton, 1999).