OIL/GAS FIRED INDIRECT HAG: - It consists of multiple concentric cylinders through which suitable amount of ambient air and combustion gas circulate without mixing with each other. This ambient air absorbs the radiant heat from the surface of concentric cylinders thus achieving the required temperature.
SOLID FUEL FIRED INDIRECT HAG: - It consists of a fluidised bed chamber where combustion of fuel takes place to produce combustion gas. This combustion gas passes through multiple heat exchangers. Fresh air is blown through a blower over these heat exchangers without mixing with combustion gas which results in the achieving the required temperature of air by absorbing radiant heat from the surface of heat exchangers.