Page 247 - Boiler plant and distribution system optimization manual
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232 Boiler Plant and Distribution System Optimization Manual
Figure 12.5—Economizer used to reduce stack temperature and raise feedwater temperature.
a. Average stack gas temperature of 450°F savings. The flue gas temperature can not
b. More than 2,500 operating hours a year. be reduced below the temperature of the
c. Stack flow rate more than 15,000 lb/hr incoming feed water or the acid formation
temperature.
The maximum benefit that can be safely
achieved is governed by a number of technical d. It may be possible that a controlling limita-
and physical limitations. tion may be imposed by the space available
for the installation of an economizer.
a. Economic considerations. Many economiz-
ers have paid for themselves in two years or e. When a boiler operates with a stack for in-
less, the economizer then continues to return duced natural draft on a negative pressure
dividends from reduced energy costs long furnace, a limitation is imposed on the fur-
after it has been paid for. nace draft due to the cooler exhaust gasses
caused by the economizer. In this case an
b. When limited fuel availability or steam pro- induced draft fan may be required. In other
duction threatens plant production an econ- cases of balanced draft or forced draft sys-
omizer can increase boiler capacity from tems, the additional boiler efficiency can off-
4% to 10%. If there is a need for more steam set the additional draft requirement caused
capacity, this may be the most cost effective by the lower stack temperatures and pres-
way to do it rather than invest in a new boil- sure drop across the economizer.
er.
f. Outlet water temperature is a limitation to
c. By the law of diminishing returns, an in- prevent steam formation and water hammer
crease in heating surface does not provide, in the economizer. An approach temperature
in equal proportion, for an increase in fuel of 40°F is customary for variable boiler load
