Hot air, thus generated is let out from Hot Air generator at controlled temperature to the process. Most of the times the combustion chamber is refractory lined. The thermal efficiency is almost 100% with proper insulation and air temperature can be up to 1100 deg.C.

Fresh Air from atmosphere is fed onto the surface of combustion chamber through a statically & dynamically balance blower. The shape of passage ensures to form an envelope on the combustion chamber to extract maximum heat, thus higher heat transfer efficiency. The air continues to pass through outer surface of heat exchanger, gaining heat & is collected at the other end of hot air generator. The generation of hot air is within minutes of its start.

Indirect fired hot air generator types are,

a) Fuel fired HAG

This consists of multiple shell in shell construction or shell and tube construction. The process air does not come in contact with the flame or the combustion gases. The combustion gas path and process air paths are different. The heat from flue gases is recovered for maximum thermal efficiency. Thermal efficiency up to 88% are easily achieved in these hot air generators. Air temperatures up to 350 degree C are possible in these types of heaters. The material of construction of combustion chamber is SS 310 and the outer shells are either made up of carbon steel and spray galvanised or are made up of stainless steel. For GMP plants the complete construction is in stainless steel.

b) Steam/ Thermic Fluid heated air heater

These consist of a bank of fin tube radiators. The process air passes over the fin tubes through which either steam or thermic fluid flows. Thus indirect heating of air takes place. Temperatures from 250 to 300 deg. C are achievable depending upon the steam and thermic fluid temperatures. The material of construction of tubes is either carbon steel or stainless steel depending upon process requirement. The mounting of these heaters can be either horizontal or vertical.