750-211 (revised 2009)
Promethean Boilers, Model 4WI Manual
2-19
2.14 — Gas Fuel Flow
Oil flow to the burner is controlled by the movement of the metering stem of the oil metering valve, which varies
the flow to meet load demands. The metering valve and the air damper are controlled simultaneously at all times by
the modulating motor to proportion combustion air and fuel for changes in load demand.
Oil is purged from the burner gun upon each burner shutdown. The air purge solenoid valve opens as the fuel
valve closes, diverting atomizing air through the oil line. The air assures a clean nozzle and line for subsequent
restart.
2.14 — Gas Fuel Flow
Metered gas from the utility flows through the pressure regulator at a reduced pressure suitable to burner require-
ments, through the main gas shutoff cock, main gas valve(s), and modulating butterfly gas valve to the nonpremix-
orifice-type burner.
FIGURE 2-18. Gas Train
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 controlled
simultaneously by the modulating motor to proportion combustion air and fuel for changes in 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 regulator
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.
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