All drying systems can be classifed primarily according to their operating temperature
ranges into two main groups of high temperature dryers and low temperature dryers.
However, dryers are more commonly classifed broadly according to their heating sources
into fossil fuel dryers (more commonly known as conventional dryers) and solar-energy
dryers. Strictly, all practically-realised designs of high temperature dryers are fossil fuel
powered, while the low temperature dryers are either fossil fuel or solar-energy based
systems (Ekechukwu and Norton, 1999).