The Pressure Vessel
2-2
750-96 (revised 2010)
Model CB Packaged Boiler Manual
Those steam boilers designed for operation exceeding 15 psig are constructed in accordance with Section I,
Power Boilers, of the ASME Code. Hot water boilers for operation over 250º F are likewise built to this Code.
2.3 — Water Requirements
2.3.1 — Hot Water Boiler
2.3.1.1 — Air Removal
The hot water outlet includes a dip tube which extends 2 to 3 inches into the boiler. This dip tube reduces the pos-
sibility of any air which may be trapped at the top of the shell from entering into the system.
Any oxygen or air which is released in the boiler will collect or be trapped at the top of the boiler shell.
The air vent tapping on the top center line of the boiler should be piped into the expansion or compression tank.
Any air which is trapped at the top of the boiler will find its way out of the boiler through this tapping.
2.3.1.2 — Minimum Boiler Water Temperature
The minimum recommended boiler water temperature is 170º F. When water temperatures lower than 170º 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 170º F.
Another reason for maintaining boiler water temperature above 170º F is to provide a sufficient temperature
“head” when No. 6 fuel oil is to be heated to the proper atomizing temperature by the boiler water in a safety type
oil preheater. (The electric preheater on the boiler must provide additional heat to the oil if boiler water tempera-
ture is not maintained above 200º F.)
2.3.1.3 — 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 kept in mind that 200º F or 240º F water in a boiler cannot be completely replaced with 80º F
water in a few minutes time without causing thermal stress. This applies to periods of “normal operation,” as well
as during initial startup.
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 bypass valve will thus allow continuous circulation through
the boiler and can help prevent rapid replacement of boiler water with “cold” zone water.
NOTE: If the water temperature going to the system must be lower than 170º F, the boiler water temperature should
be a minimum of 170º F (200º F if used to preheat No. 6 oil) and mixing valves should be used.
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