Forced circulation air heaters

GT V02 1961 005-010

The heating of air or other fluids by heat exchange from hot gases frequently sets severe problems in choice of equipment and materials of construction owing to the high temperature of the hot gases being handled. Such difficulties as scaling of metals and loss of strength are
aggravated by the possibility of local overheating caused by stratification in the hot gas or by uneven cooling on the air side of the heat exchanger.
The source of hot gas may be either furnace waste gases, or, with independently $red heaters, combustion
products from gas or oil burners. This paper describes
methods employed on independently .fired heaters to
reduce and control combustion product temperatures
without loss of efficiency.
Reduction of combustion product temperatures can be effected by introducing atmospheric air or, less wastefully, by recirculating combustion products through the equipment. Two types of forced circulation methods are available. A recirculating fan may be installed within the heater or in an external duct, or alternatively forced circulation may be achieved by means of high intensity combustors with converging exit nozzles to give maximum velocity and momentum to the exhaust gases.
The relative merits of fan-induced and jet-induced forced circulation are described, and typical pieces of
equipment are illustrated. Incandescent Heat Company designs for forced circulation air heaters have concentrated on the jet-induced system, and units with heat inputs of several million Btu per hour, both gas and oil fired, have been supplied for such applications as metallurgical blast heating, aircraft test rigs and many chemical processes. The extremely uniform temperature of the recirculated combustion products makes the system particularly suitable for heating temperature- Presented at the Meeting of the Society on 28 April 1960. sensitive liquids and gases where local overheating must be avoided. Overall thermal efficiencies of 85% can be obtained in equipment of this type, which lends itself equally well to
heating fluids under pressures ,from atmospheric up to 500 lb/in2 gauge.
Details of suitable oil 01. gas combustors for jet-induced recirculation and combustion products are
described and illustrated. These combustors comprise high intensity burner systems, designed for high combustion efficiency and for optimum pressure recovery.

C. J. V. Denning
The Incandescent Heat Co. Ltd., Smethwick