TM 5-810-15
usages of plant air occurs when air soot blowers
(4) Plant air compressor will be designed to
are used in the boiler. Steam soot blowers may be
be loaded 50 percent of the time at maximum load.
used eliminating the need for air soot blowers.
Instrument air compressor is to be sized for 40
Other usages which must be considered are air tool
percent loading at maximum load. Centrifugal
demand and cleaning coal handling dust bags.
compressors can be loaded 100 percent of the time.
(2) Outlet pressure. The compressor outlet
(5) Provisions will be made to allow
pressure will be sufficient to supply air at the
drainage of water from all coolers and receivers by
required pressure, after line losses, to the device
means of traps or manual valving.
requiring the highest pressure in the plant air
(6) Separate receivers will be placed near
system. Pressure regulators will limit the pressure
area of large air demands. It may be more
to devices operating at lower pressures.
economical to supply separate air systems for air
e. Compressor auxiliaries.
soot blowers and baghouse cleaning systems.
(1) Aftercoolers/intercoolers. Intercoolers
are used on any compressor having more than one
7-7. Boiler feedwater treatment.
stage, and all compressors will have aftercoolers.
a. General. Feedwater treatment is necessary to
Aftercoolers will be pipeline type units with air-in-
prevent corrosion of metals, formation of deposits
tube, water-in-shell construction and designed with
and to minimize boiler water solids carryover.
a 20 degrees F approach.
Boiler water treatment guidelines are discussed in
(2) Air dryers. All air compressors will have
AR 42049. For boilers operating at 400 psig,
air dryers installed immediately downstream of the
constituents in the feedwater must be controlled so
aftercoolers. The dryers will be designed to
that the maximum water limits for boiler feed-water
maintain a dew point at line pressure which is lower
and boiler water shown in tables 7-4 and 7-5 can be
than any ambient temperature to which pressurized
maintained with minimal boiler blow-down, since
air lines are exposed.
the higher the blowdown rate, the greater the
(3) Receivers. Receivers will be sized based
thermal loss. An evaluation will be made to
upon a timed usage of a volume of air. The
determine the costs of thermal losses due to
required tank volume will be determined using
blowdown versus the costs of high quality treated
equation 7-1:
water.
T ' V x (P1 & P2)
(eq 7-1)
Table 7-4. Boiler Feedwater Limits.
C x P0
T = time in minutes receiver will supply air from
Total hardness
Drum
Calcium
upper to lower pressure limits (use 15 sec-
Pressure
Iron
Copper
carbonate
onds)
(psig)
(ppm Fe)
(ppm Cu)
(ppm CaCo3)
V = volume of tank, in cubic feet
P0 = absolute atmospheric pressure, psia
0-300
0.100
0.050
0.300
P1 = maximum tank pressure, psia (compressor
3010-450
0.050
0.025
0.300
discharge pressure)
P2 = minimum tank pressure, psia (pressure re-
Table 7-5. Boiler Water Limits.
quired to operate tool)
C = amount of cubic feet of free air needed per
Total
minute, cfm (air at ambient temperature and
Alkalinity
pressure)
Drum
Silica
calcium
Specific
f. General design criteria.
pressure
silicon dioxide carbonate
(1) The total air capacity will be increased by
(psig)
(ppm SiG) (ppm CaCo3) (micromhos./cm)
a factor of 1.1 to 1.2 to account for leakage.
(2) Both the instrument air and the plant air
0-300
150
700
7000
301-450
90
600
6000
systems will consist of two compressors tied to a
common header. Backup capacity of 100 percent
b. Design requirements. Before a plant water
will be provided so maximum compressed air
treatment system is designed, a thorough raw water
demand can be satisfied with one compressor out
analysis will be obtained as shown in table 7-6.
of service.
The raw water condition can vary widely even
(3) The headers for instrument air and plant
within a small regional area and can greatly effect
air will have an emergency cross-connection
the options and economics available for water
equipped with oil removal equipment to protect the
treatment equipment. Also, the purity and quantity
instrument air system.
7-5
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