Page 376 - Mechanical Engineers' Handbook (Volume 4)

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3 WARNINGS
Changing temperatures from end to end of high-temperature heat exchangers and from time
to time during high-temperature furnace cycles cause great thermal stress, often resulting in
leaks and shortened heat-exchanger life. Heat-transfer surfaces fixed at both ends (welded
or rolled in) can force something to be overstressed. Recent developments in the form of
high-temperature slip seal methods, combined with sensible location of such seals in cool
air entrance sections, are opening a whole new era in recuperator reliability.
Corrosion, fouling, and condensation problems continue to limit the applications of heat-
recovery equipment of all kinds. Heat-transfer surfaces in air heaters are never as well cooled
as those in water heaters and waste heat boilers; therefore, they must exist in a more hostile
environment. However, they may experience fewer problems from acid-dew-point conden-
sation. If corrosives, particulates, or condensables are emitted by the heating process at
limited times, perhaps some temporary bypassing arrangement can be instituted. High waste
gas design velocities may be used to keep particulates and condensed droplets in suspension
until they reach an area where they can be safely dropped out.
Figure 1 shows recommended minimum temperatures to avoid ‘‘acid rain’’ in the heat
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exchanger. Although a low final waste gas temperature is desirable from an efficiency stand-
point, the shortened equipment life seldom warrants it. Acid forms from combination of
water vapor with SO ,SO ,orCO in the flue gases.
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4 BENEFITS
Despite all the costs and warnings listed above, combustion air preheating systems do pay.
As fuel costs rise, the payback is more rewarding, even for small installations. Figure 2
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shows percent available heat (best possible efficiency) with various amounts of air preheat
and a variety of furnace exit (flue) temperatures. All curves for hot air are based on 10%

Figure 1 Recommended minimum temperatures to avoid ‘‘acid rain’’ in heat exchangers.