Direct fired hot air generators are used to heat the air for processes which can allow mixing of products of combustion with the products which come in contact with hot air. The construction of this type of HAG consists of concentric double shells and cones. For air temperatures above 350oC up to 1200oC the inner shell is refractory brick lined in the firing zone. Either gas or liquid fuel is fired in the combustion chamber and the process air passes through the annular space between the two shells. The mounting of the HAG can be either horizontal or vertical. The construction of direct solid fuel fired is either grate type or fluidised bed combustion type depending upon the fuel to be used.
Indirect fired hot air generators are used to heat the air for processes where clean air is a necessity and the temperature of air is below 350oC. For oil or gas fired systems the construction of this type of HAG consists of multiple concentric shell in shell having separate passes for process air and combustion air. The fuel i.e. oil or gas is fired in the innermost combustion chamber and the process air enters the annular space between the outermost two shells. The flue gas has two passes while the process air has three passes due to which there is maximum recovery of heat and high efficiency. The flue gas is exhausted to the atmosphere through the flue gas duct. The mounting of the HAG can be either horizontal or vertical. For solid fuel fired indirect HAG the process air is passed through the tubes and the flue gases generated by solid fuel firing are passed over the tubes facilitating heat exchange and heating of process air inside the tubes. There are multiple passes to recover maximum possible heat from the flue gases. The flue gases are then passed through cyclone and bag filter to make it dust and ash free before exhausting it to the atmosphere. The firing zone can be either grate type or fluidised bed combustion type depending upon the fuel to be used.