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Chapter 12
Waste Heat Recovery
The value of waste heat comes from the fact On the average the temperature of flue gas-
that it supplants additional input energy, reduc- ses leaving boilers is about 400°F ranging between
ing overall energy costs. 350°F and 650°F. Flue gas leaves the boiler at a
temperature higher than the steam temperature
for heat transfer to take place.
WASTE HEAT RECOVERY OPPORTUNITIES The boiler exhaust approach temperature,
in general, will rise from 400F to 150°F from low
Flue gasses from a boiler represent a 17% to load to maximum (Figure 12.1). Many tests have
30% (plus) opportunity for savings investments. shown these numbers to vary widely, so each
This chapter will cover the technology for waste boiler should be tested for its characteristic ex-
heat recovery including practical approaches to haust temperature, preferably after a cleaning
boiler efficiency improvement and other concepts and tuneup, to establish the ideal temperatures
for utilizing the recovered energy from flue gas- for that particular boiler.
ses.
Figure 12.1— Typical exit gas temperature above steam temperature.
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