General Description
1-22
750-96 (revised 2010)
Model CB Packaged Boiler Manual
1.17 — Gas Fuel Flow
Metered gas from the utility flows through the pressure regulator at a reduced pressure suitable to the burner’s
requirements, through the main gas shutoff cock, main gas valve(s), and modulating butterfly gas valve to the
non-premix orifice type burner.
The main gas valve is of the normally closed type and is opened (energized) in proper sequence by the program-
ming relay.
The butterfly gas valve modulates the flow of gas from low through high fire settings. The position of the butterfly
valve disc is governed by the gas modulating cam. The butterfly gas valve and the air control damper are con-
trolled simultaneously by the modulating motor to proportion combustion air and fuel for changes to load
demand.
The gas flow rate required for rated burner input depends upon the heating valve (BTU/cubic foot) of the gas sup-
plied. The gas pressure regulator adjusts the gas pressure (flow rate) to the entrance of the gas train. The regula-
tor is not always supplied with the burner, but may be provided by others.
The main gas valves cannot be energized (opened) unless the combustion air proving switch is closed to indicate
a sufficient supply of combustion air. The low gas pressure and high gas pressure switches must be closed to
prove sufficient, but not excessive, gas fuel pressure.
When two main gas valves are used, a normally open vent valve is installed between them. This valve is shut
when the main gas valves are opened. When they are closed the vent valve is open to vent gas should any be
present.
1.18 — Modulating Firing
The modulating motor, through a linkage arrangement, controls the air damper and the butterfly gas valve or the
oil metering valve to maintain a constant air-fuel ratio throughout the firing range.
During burner operation, the motor is controlled by a modulating pressure control on a steam boiler, or by a mod-
ulating temperature control on a hot water boiler. A manually operated potentiometer is provided to permit the
positioning of the motor at a desired burner firing rate. This is used primarily for initial or subsequent checking
and setting of fuel input. Normal operation should be with the manual-automatic switch in the “automatic” posi-
tion and under the control of the modulating control.
The modulating motor (commonly called a damper motor) is reversible. It has an internal limit switch that
restricts shaft rotation to 90º. During normal operation the motor will move in either direction or stop at any
position within this range.
The motor’s potentiometer is electrically connected to a matching potentiometer in the modulating control.
Changing steam pressure or water temperature alters the electrical resistance of the modulating controller’s
potentiometer. This change in resistance compels an integral balancing relay to start, stop, or reverse the motor
rotation. Rotation in either direction continues until the resistance ratio of the two potentiometers is equal.
When this occurs, the motor stops in a position that allows the proper fuel and combustion air flow to meet oper-
ating demands.
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