Water Requirements (Steam Boilers); Multiple Boiler Installations - CleaverBrooks FLX Operation, Service And Parts Manual

Pressure Vessel Care
exercised to make sure that the proper relationship of pressure
to temperature exists within the boiler so that all of its internal
surfaces are fully wetted at all times. It is for this reason that
the internal boiler pressure, as indicated on the water pressure
gauge, must be held to the level shown in Figure 3-3.
It is advisable to install a thermometer in the return line to
indicate return water temperature. With the return water
temperature and the supply water temperature to the system
known, the temperature differential will be established.
Knowing the flow rate, the operator easily can detect any
excessive load condition and take appropriate corrective
action.
Pressure Drop
There will be a pressure drop of less than 4 psi through all
standardly equipped Cleaver-Brooks boilers operating in any
system that has more than the 20°F temperature drop. This
drop will vary with boiler size and circulation rate. For
specific information, refer to table 3-1, Minimum Flow Rates
for Hot Water Boilers.
Minimum Boiler Outlet Water Temperature
The minimum recommended operating boiler outlet water
temperature is 150°F. When water temperatures lower than
150°F are used, the combustion gases are reduced in
temperature to a point where the water vapor in the gases
condenses. A result of condensation is that fireside corrosion
occurs in the boiler and breeching. The condensation problem
is more severe on a unit that operates intermittently or is
oversized for the actual load. This is not a matter that can be
controlled by boiler design, since an efficient boiler extracts
all the possible heat from the combustion gases.
Note: In order to maintain a minimum outlet
water temperature of 150°F the low limit of
the Operating Temperature Control should
be set at least 10° higher.
If the operating water temperature going to the system must
be lower than 150°F, the operating boiler outlet water
temperature should still be held to a minimum of 150°F.
Mixing valves are used to reduce the supply temperature
going to the system.
N o t e : T h e m i n i m u m r e t u r n w a t e r
temperature through the boiler is 140°F
(150°F on #2 Oil).
CAUTION
! DANGER
Three-way valves and system controls
should be installed or set so that the boiler
cannot be bypassed. A reduced circulation
of water or no water circulation through the
boiler when the burner is operating may re-
sult in damage to the equipment.
750-177

Multiple Boiler Installations

When multiple boilers of equal or unequal size are installed,
care must be taken to ensure proportional flow through the
boilers. Proportional flow can best be accomplished by use of
balancing cocks and gauges in the supply line from each
boiler. If balancing cocks or orifice plates are used, a
significant pressure drop (for example, 3-5 psi) must be taken
across the balancing device to accomplish proportional flow.
Variations in water temperature and firing rates will result if
care is not taken to ensure proportional flow through the
boilers. In extreme cases, differences in firing rates could
result in a net header water temperature below the desired
temperature.
C. WATER REQUIREMENTS
(STEAM BOILERS)
Deaeration
The most important factor in the life of a steam pressure
vessel is the proper conditioning of the boiler feed water.
Corrosive gasses, such as oxygen and carbon dioxide, must
be removed from the feed water in order to prevent
degradation of the pressure vessel. For this reason Cleaver-
Brooks recommends the use of a deaeration system as an
integral part of a comple te bo il er i nst al la ti on . If
circumstances do not allow the implementation of a
deaeration system, then serious consideration should be given
to effective alternatives such as a feed water preheater
combined with a chemical oxygen scavenger. Complete
boiler water chemistry parameters are given in Table 3-3.
Feed Water Supply
The internal dynamics of the Model FLX steam boilers
require the capability to deliver large quantities of feed water
to the boiler on demand. (Feed water inlet Figure 3-3.)
Sudden changes in firing rate or operating pressure of the
Figure 3-3: Feed Water Inlet
Steam Boiler
Chapter 3
FEED WATER
SUPPLY PORT
3-3