ABSTRACT
The most commonly encountered gaseous heat transfer media are combustion products, referred to as flue gases, and pure air. The former are generated at temperatures in the order of 1000°C when hydrocarbon fuels are burnt, mostly in furnaces used to heat process streams or in steam generators. Heat transfer from combustion products occurs in three steps, first by radiation just after the generation of flue gases and second, by convection, both of these steps occurring in the same basic enclosure but at different locations. The final step in heat removal can vary considerably depending on the application, varying from no heat recovery, where the gases may be rejected to the atmosphere at temperatures up to and above 500°C, to situations where the flue gases are ducted to process areas (such as reboilers of distillation columns) for heating purposes. More commonly, these gases are used in economizers, where boiler feedwater is heated, and recuperators and regenerators, where air is heated prior to entering the boiler or furnace. Some high-temperature flue gases, such as the gaseous effluent from gas turbines, can be used to raise steam in waste heat boilers.
