TM 5-810-15
CHAPTER 7
MECHANICAL AUXILIARY EQUIPMENT
Minimum recommended feedwater temperatures to
7-1. General.
the economizer are shown in figure 7-1. Minimum
This chapter addresses the criteria for the major
feedwater velocities are shown in figure 7-2.
steam plant auxiliary equipment.
7-3. Steam deaerators.
7-2. Closed
feedwater
heat
exchangers
a. General. The steam deaerator (DA) heats
(CFHE).
boiler feedwater to improve plant efficiency and
a. Applications. CFHE may be used to raise
lowers dissolved oxygen and gasses that are corro-
feedwater temperature to the plant economizer and
sive to internal metal surfaces of the boiler. The
thus maintain the exit flue gas temperature above
standards of the Heat Exchange Institute (HEI),
the acid dew point during low load operation. This
1992, Fifth Edition, state that a DA should be
application is possible for plant design in all size
guaranteed to remove all dissolved oxygen in
ranges. Other methods for keeping the exit flue gas
excess of 0.005 cc/i.
temperature above the acid dew point are
b. Deaerator types. There are several types of
bypassing flue gas around the economizer or
steam DA with three acceptable types being:
bypassing a portion of feedwater around the econo-
spray/tray type, atomizer or scrubber spray type
mizer which needs to be avoided. Bypassing the
and recycle type. DA heater should be counterflow
feedwater around the economizer at low load oper-
design. Although some tray and recycle type DA*s
ation creates a potential for steam formation in the
have a higher first cost, they will operate properly
economizer. The CFHE is the most positive
under rapid load changes and only require a 10 to
approach to maintaining the exist flue gas temper-
30 degrees F rise across the DA (inlet water
ature. An evaluation will be made to determine the
temperature 10 to 30 degrees F lower than the DA
economic practicality of each method.
outlet water temperature). Spray or atomizing type
b. CFHE design. Each CFHE will be a U-tube
DA*s can be used when steam loads are steady and
type heater to minimize stresses caused by thermal
the temperature rise across the DA is 30 to 50
expansion. Tube material selection is dependent on
degrees F or greater. Because of this performance
the quality of the water. Tubes of stainless steel
limitation, tray or recycle type DA*s will be used
construction will minimize the possibility of
unless there is a steady steam load and the
corrosion and erosion. High quality water will
temperature rise in the DA is 50 degrees F or
allow the use of 90/10 and 70/30 copper nickel
greater. If the latter conditions exist, the DA
(CuNi) material tubing.
selection will be decided by a LCCA.
c. CFHE design criteria. Data listed in table 7-1
c. Deaerator design criteria. Deaerating heaters
are necessary to size CFHE.
and storage tanks will comply with the ASME
Unfired Pressure Vessel Code, ASME Power Test
Table 7-1. Closed Feedwater Heat Exchanger Design
Code for Deaerators, Heat Exchange Institute,
Parameters.
American National Standards Institute, and Na-
tional Association of Corrosion Engineers Recom-
Parameter
Engineering Units
mendations. One steam plant DA can be sized for
multiple boiler units. At full load conditions, the
Feedwater flow
pph
water from the DA will have a temperature
Feedwater inlet temperature
degrees F
sufficiently high to prevent acid dew point corro-
Feedwater outlet temperature
degrees F
sion of the economizer. In no case will the temper-
Maximum feedwater velocity
fps
ature rise in the DA be less than 20 degrees F or
Maximum allowable tube side pressure
psi
the minimum storage capacity at normal operating
drop
level be less than 10 minutes at the DA*s maximum
Maximum tube side operating pressure
psig
continuous load rating or less than 12 minutes full.
Maximum shell side operating pressure
psig
Information contained in table 7-2 will be specified
after a heat balance around the DA has been
determined at full load conditions.
7-1
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