Page 381 - Mechanical Engineers' Handbook (Volume 4)

370 Air Heating

Preheating combustion air has some lesser benefits. Flame stability is enhanced by the
faster flame velocity and broader flammability limits. If downstream pollution control equip-
ment is required (scrubber, baghouse), such equipment can be smaller and of less costly
materials because the heat exchanger will have cooled the waste gas stream before it reaches
such equipment.
Somewhat related to air heating (other than heating air for a subsequent process) is
preheating combustion air for burners to make any heating process more efficient by recir-
culating waste heat through a heat exchange device (often built into a burner). This can be
either a regenerator (steady-state heat exchanger, Fig. 3) or a recuperator (alternating flow
heat exchanger, Fig. 4). Both use hot waste (flue) gases as the heat source to preheat com-
bustion air being fed to the burners on almost any kind of furnace. Preheating air not only
improves the thermal efficiency by recycling waste flue gas energy, but also raises the flame
temperature (Table 3), thereby increasing the heat transfer rate to the furnace loads.

REFERENCES
1. Heat Requirements for Direct-Fired Air Heating, North American Mfg. Co., Cleveland, OH 44105,
1981.
2. Steam—Its Generation and Use, Babcock & Wilcox, New York, 1978.
3. R. J. Reed, Combustion Handbook, 3rd ed., Vol. 1, North American Manufacturing Co., Cleveland,
OH 44105, 1986.
4. R. J. Reed, Combustion Handbook, 4th ed., Vol. 2, North American Manufacturing. Co., Cleveland,
OH 44105, 1997.
5. W. Trinks, M. H. Mawhinney, R. A. Shannon, R. J. Reed, and J. R. Garvey, Industrial Furnaces, 6th
ed., Wiley, Hoboken, NJ, 2003.