TM 5-810-15
stainless steel tubes, outer tubes 1 inch outside
Table 7-8. Blowdown Tank Pressure.
diameter and 0.049-inch minimum wall thickness
and inner tubes 5/8-inch outside diameter and
Maximum Allowable Boiler
Blowdown Tank Design
0.022-inch minimum wall thickness. The supply and
Pressure, psig
Pressure, psig
return connections are to be on the same end of the
coil. Tubes will be pitched to the drain. The coil
50
25
should be removable in a manner that does not
100
50
disturb connecting ends of breeching. The coil
200
70
outer casing is typically 10 gauge steel welded into
300
90
an airtight structure. The core header plate will be
500
125
gasket sealed to the casing.
750
165
1000
200
7-11.
Steam coil drain tank.
1500
275
2250
325
a. Application. The steam coil drain tank will
2500
400
collect the condensate from the steam coil air
heater for transfer to the DA.
b. Design. The steam coil drain system must be
b. Design. Heat is recovered in a blowdown heat
sized large enough to drain the maximum expected
recovery system by passing the blowdown water
steam flow rate to the air heater and to maintain a
from the blowdown tank through a heat exchanger
reasonable condensate level allowing proper
to recover the sensible heat of the water and
operation of the steam coil drip return pumps (if
transferring blowdown tank steam to the DA. The
included). Another consideration is the possibility
heat exchanger will be sized to reduce the temper-
of freezing. The steam coil tank should be located
ature of the blowdown main to 20 degrees F above
indoors, if possible, and sized small enough that
the inlet temperature of the fluid being heated,
outdoor drain piping is not allowed to fill with
typically feedwater heating, makeup water heating,
condensate. The steam coil drain tank is normally
building heating, oil heating or process steam
equipped with a level controller, gauge glass and
generation. The blowdown tank for a heat recovery
high level alarm. The steam coil drain tank will be
system will be smaller to allow the blowdown drain
designed and constructed in accordance with the
water to be hotter for an effective heat recovery
ASME Boiler and Pressure Vessel Code, Section
system. If flash steam is used in the DA, the
VIII.
blowdown tank will be designed to minimize
carryover. A normal blowdown system consisting
7-12.
Fans.
of a large blowdown tank venting to atmosphere
a. Applications. Boiler furnaces are either pres-
and draining directly to waste may have to be
surized or have balanced draft for combustion. Gas
available to allow maintenance on the heat recovery
and oil fired boilers are normally of the pressurized
equipment during operation. There are several
furnace design. Modern coal fired boilers have
package blowdown heat recovery systems available
balanced draft type furnaces. Balanced draft type
consisting of blowdown tanks, heat exchangers,
boilers use FD fans to supply combustion air to the
flow control valves, thermostatic control valves,
furnace and ID fans exhaust the products of
sample coolers and high level float switches.
combustion or flue gas. The furnace is kept at a
slightly negative pressure ranging from 0.1 to 0.25-
7-10.
Steam coil air heater.
inches w.g., by the ID fan which is located
a. Application. The steam coil air heater preheats
downstream of the particulate removal equipment.
the combustion air before it enters the main air
b. Forced draft fans. FD fans operate with rea-
heater. The heat dries the air and reduces corrosion
sonably clean, cool or warm air and will be
of the air heater tube metals.
designed for quietness and efficiency. This source
b. Design. The installation will be designed for
of combustion air is frequently taken from within
reasonable air velocities with pressure loss not to
the steam plant to promote ventilating and to take
exceed one inch of water. The heating coils will be
advantage of the higher ambient temperatures.
designed in multiple elements to maintain average
Inlets for the fans will have silencers with screens
cold and metal temperatures of the air heater
to attenuate entrance noises and to keep birds and
surfaces above 180 degrees F at all loads up to 15
other objects from entering the system. The static
percent above full rated load. The uncorrected air
pressure of the FD fan will be calculated for the
heater gas outlet temperature should be used to
pressure drop through the inlet air duct, steam coil
determine the average cold end metal temperature.
air heater, air heater (if used), air metering devices,
A typical steam coil would have seamless type 321
dampers or vanes, air ducts, static fuel bed or
7-11
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