Dryers for heated air drying are complete units, incorporating both a heater unit where fuel is burned, and a fan to force the heated air thorough a canvas connecting duct and into the air distribution system of the drying bin. Safety features are also included such as automatic thermostatic high limit temperature control, which cuts off the burner flame if the air temperature exceeds a certain safe maximum; and a flame failure control which automatically cuts off fuel flow to the burner if the flame should go out. It is also highly desirable to have a thermostat which will automatically maintain the air temperature at a desired setting. Such thermostat are either included as a standard feature in many dryers, or can be purchased as an option.
Dryers come into two main types, according to manner in which heat is supplied to air:
1. Direct Fired
2. Indirect Fired
1. Direct Fired:
In a direct fired dryer, the fuel is burned and the hot combustion gases are thrown directly into the air stream which goes into the air distribution system. The use of heat is highly efficient, but there is possibility of blowing soot, unburned fuel and the objectionable flumes into the seed. This happen if the burner is allowed to gate out of adjustment so that fuel is not completely burned. With some fuels there is danger of blowing small sparks into the seed.
2. Indirect Fired:
In an indirect fired dryer, the hot combustion gases pass into a chamber. The drying air circulates around this chamber, thus heat exchanger and picks up heat just as in a hot air fumes. Thus, drying air can include combustion gases , sparks, soot or fumes. Indirect fired dryers are less efficient in the use of heat, than direct fired once, but are safer.
The dryers are also designed to burn various fules, E .g Liquid propane( LP) or butane , natural gas, fuel oil, and coal ( coal is now really used as a fuel).
The liquid propane and natural gas which are readily burned, with minimum possibility of soot, are best for direct fired dryers, and fuel oil (Kerosene oil) for indirect fired dryers.
The fan of the dryer may be driven by either an electric motor or gasoline engine and power take off (PTO). Electric motors are the most efficient, economical and most commonly used.
Several makes of crop dryers are available and could be chosen as per requirements of the operation.