Air Preheating, Lighting Burners, and Heat Balancing

A typical air preheater will reduce the fuel required to liberate a given amount of heat by 10 percent. The debit for this improved thermal efficiency is a higher flame temperature and the possibility of overheating the radiant section. There is a clear advantage to fit an air preheater 

The wheel type is subject to air leaks across the mechanical seals. This leakage is identified by increased oxygen content in the flue gas, low flue-gas outlet temperature, and a greater temperature loss in the flue than rise in the air temperature across the preheater, as shown in Fig. 29.9. 

In general, air preheater leaks are a problem because they 

All air preheaters are subject to corrosive attack caused by condensation of sulfur trioxide. At a 150-ppm sulfur in the fuel gas, operating experience shows that a minimum temperature of 350 to 400°F minimizes corrosive attack. Uneven coohng in the preheater results in the need to keep the flue-gas outlet temperature 50 to 100°F above the calculated SO dew point. The flue-gas outlet temperature can be 

Air preheaters or any type of waste-heat recovery device designed for horizontal flow across vertical tubes may be subject to vibration produced by the velocity of gas across the tube banks. The velocity produces a vortex-shedding wave pattern that could correspond to the natural harmonic frequency of the tube bank. If the natural harmonic frequency is reached, excessive vibration of the tubes will occur. Redesign of the internal baffle system by inserting dummy baffles can stop the vibration. 

All air preheaters are subject to corrosive attack caused by condensation of sulfur trioxide. At a 150-ppm sulfur in the fuel gas, operating 

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