TM 5-810-15
conjunction with coal fired boilers. Another desir-
(2) In order to provide the required primary
able feature of the bare tube economizer is that the
air temperature, economizers may not be used on
tubes should be "in line" so as to have clear spaces
some pulverized coal fired units. They are generally
between each tube. This arrangement enables the
used on all gas, oil, and stoker fired boilers and as
sootblowers to keep the gas lines clear. Extended
indicated in b above, and may be the only type of
surface economizers for gas and/or oil fired units
heat recovery used on some stoker fired boilers and
offer economic advantages when compared to bare
the only type used on gas/oil fired boilers.
tube economizers. Extended surface economizers
Economizer sections on ACFB boilers are
are lower first cost and have smaller installation
sometimes called "heat recovery areas (HRA)" and
space requirements. For oil fired boilers fin spacing
are an integral part of the boiler and are typically
should take into account the particular grade of oil
not furnished and manufactured by an economizer
to avoid fouling problems.
company. As a rule of thumb, with the common
(1) The purpose of the economizer is to
fuels (coal, oil, gas) steam generator efficiency
raise the temperature of the boiler feedwater while
increases about 2.5 percent for each 100 degrees F
lowering the flue gas temperature. Economizer
drop in exit gas temperatures. By putting flue gas
surface is usually less expensive than heating
to work, air heaters and economizers can improve
surface in either the furnace area or the boiler
boiler unit efficiency by 6 to 10 percent and thereby
convection tube bank.
improve fuel economy.
3-22
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