Most high-value agricultural, food and wood products need drying to minimize spoilage, preserve quality and reduce transportation costs [1]. Drying is a complex and energy intensive process where the water is evaporated from a product by supplying heat by convection, conduction, radiation, microwave, etc. About 85% of industrial driers are the convective type with hot air as drying medium, and 99% of them involve removal of water [2]. They consume up to 25% of the national industrial energy in the developed countries. Driers incorporating dehumidification cycles are called drying heat pumps. They recover energy by condensing moisture from the drying air. The recovered waste heat is recycled back to the dryer at higher temperature for reuse in the drying process. This is a method of improving the energy efficiency and reducing primary energy consumption of drying processes.
Drying processes fall into three categories: