General Description Chapter 3
750-177 3-3
System Pressure
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SYSTEM PRESSURE - PSI
SUPPLY WATER TEMPERATURE - F
Figure 3-2: Minimum System Operating Pressure
The design of the system and the usage requirements
often will dictate the pressure exerted upon the boiler.
Some systems are pressurized with nitrogen. Caution
must be
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 3-2.
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 Water Temperature
The recommended minimum boiler water temperature (outlet or supply) should be no less than 160°F (71°C)
when the
boiler is operating. This is specifically relevant during times when there is little load or during
intermittent firing. The recommended minimum system return water temperature shall be 140°F (60°C)
when firing natural gas or 150°F (66°C) when firing oil. And for special burner applications firing digester gas,
minimum return water should always be above 160°F (71°C) and supply outlet temperature greater than
170°F (77°C).
These recommendations are provided to reduce or minimize the formation of condensation on the fireside of
the boiler. Failure to do so will lead to corrosion of the boiler tubes and will not be covered by warranty. When
supply temperature is lower than 160°F (71°C), the combustion gases are cooled to the point where the water
vapor in the gases condenses. When this occurs, fireside corrosion may result if the condensed moisture stays
on the steel fireside surfaces. This may occur more frequently during seasonal or light load conditions and
specifically if the boiler is oversized for the system load.
If the system water temperature requirement is to be less than 160°F (71°C), mixing valves or blend pump
should be employed to mix the supply water with return water. NOTE: When system 3-way valves are used,
they should be set so that the boiler is not by-passed, thus impairing circulation within the boiler when the
burner is firing. This could lead to over-temperature and nuisance shutdowns, as the high limit control will
trip, requiring a manual reset of the control. Repeated overheating could lead to other damage.
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.
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