Cleaver-Brooks CBH 40 - Page 22

Chapter 2- The Pressure Vessel
The air vent tapping on the top center line of the
boiler should be piped into the expansion or com-
pression tank. Any air which is trapped at the top
of the boiler will find its way out of the boiler
through this tapping.
Minimum Boiler Water Temperature- The mini-
mum recommended boiler water temperature is
170 degrees F. When water temperatures lower
than 170 degrees F are used, the combustion gases
are reduced in temperature to a point where the
water vapor condenses. The net result is that
corrosion occurs in the boiler and breeching.
This condensation problem is more severe on a
unit which operates intermittently and which is
greatly oversized for the actual load. This is not a
matter which can be controlled by boiler design,
since an efficient boiler extracts all the possible
heat from the combustion gases. However, this
problem can be minimized by maintaining boiler
water temperatures above 170 degrees F.
Rapid Replacement of Boiler Water - The
system layout and controls should be arranged to
prevent the possibility of pumping large quantities
of cold water into a hot boiler, thus causing shock,
or thermal stresses. A formula, or "magic number"
cannot be given, but it should be borne in mind
that 200 degrees or 240 degrees water in a boiler
cannot be completely replaced with 80 degrees
water in a few minutes' time without causing ther-
mal stress. This applies to periods of "normal-
operation" as well as during initial start-up.
This problem can be avoided in some systems by
having the circulating pump interlocked with the
burner so that the burner cannot operate unless
the circulating pump is running.
When individual zone circulating pumps are used,
it is recommended that they be kept running - even
though the heat users do not require hot water.
The relief device or by-pass valve will thus allow
continuous circulation through the boiler and can
help prevent rapid replacement of boiler water
with "cold" zone water.
Contiru.1ous Flow Through the Boiler - The
system should be piped and the controls. so
arranged that there will be water circulation
through the boiler under all operating conditions.
The operation of three way valves and system con-
trols should be. checked to make sure that the
boiler will not be by-passed. Constant circulation
through the boiler eliminates the possibility of
stratification within the unit and results m more
even water temperatures to the system.
A rule of thumb of 1/2 to 1 gpm per boiler horse-
power can be used to determine the minimum con-
tinuous flow rate through the boiler under an
operating conditions.
Before initial firing or refiring after boiler has
2-2
been drained, the operator should determine that a
flow
of water exists through the boiler.
Water Circulation
The chart in Fig. 2-1 shows the maximum G.P.M.
circulation rate of boiler water in relation to full
boiler output and system temperature drop.
Multiple Boiler Installations - When multiple
boilers of equal or unequal size are used, care must
be taken to insure adequate or proportional flow
through the boilers. This 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 (e.g. 3-5
psi) must be taken across the balancing device to
accomplish this purpose.
If care is not taken to insure adequate or propor-
tional flow through the boilers, this can result in
wide variations in firing rates between the boilers.
In extreme cases, one boiler may be in the "high
fire'' position, and the other boiler or boilers may
be loafing. The net result would be that the com-
mon header water temperature to the system
would not be up to the desired point. This is an im-
portant consideration in multiple boiler in-
stallations.
Pressure Drop Through Boiler - There will be a
pressure drop of less than three feet head ( 1 psi-
2.31 ft.
hd.) through all standardly equipped
Cleaver-Brooks boilers operating in any system
which has more than a 10 degree F. temperature
drop.
Pump location - It is recommended that the
system circulating pumps take suction from the
outlet connection on the boiler and that they
discharge to the system load. This puts the boiler
and the expansion tank on the suction side of the
pump. This location is preferred because it
decreases air entry into the system and does not
impose the system head on the boiler.
It is common practice to install a standby system
circulating pump, and these main circulating
pumps are usually located adjacent to the boilers
in the boiler room.
Pump Operation - Pumps are normally started
and stopped by manual switches. It is also
desirable to interlock the pump with the burner so
that the burner cannot operate unless the cir-
culating pump is running.
Pressure
The design of the system and the usage
requirements will often dictate the pressure exer-
ted upon the boiler. Some systems are pressurized
with air or with an inert gas, such as nitrogen.
Caution must be exercised to make sure that the
proper relationship of pressure to temperature
exists within the boiler so that all of the boilers' in-
ternal surfaces are fully wetted at all times. It is