CleaverBrooks CBLE, Operation And Maintenance Manual

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750-91 (revised 2009)
Model CB-LE Packaged Boiler Manual 2-15
2.8 — Combustion Air
2.8 — Combustion Air
Air for combustion of fuel (referred to as “secondary” air) is furnished
by the forced draft fan mounted in the boiler head. In operation, air
pressure is built up in the entire head and is forced through a diffuser
plate for a thorough mixture with the fuel for proper combustion. The
supply of secondary air to the burner is governed by automatically throt-
tling the output of the fan by regulating the rotary air damper. The
damper provides the proper amount of air for correct ratio of air to fuel
for efficient combustion at all firing rates.
FIGURE 2-9.
Secondary Air Flow Diagram
2.9 — Automatic Ignition
Oil or gas burners are ignited by an interrupted type pilot. The pilot flame is ignited automatically by an electric
spark.
In the case of a combination burner, the gas pilot is used to ignite either the main gas flame or the oil flame.
At the beginning of the ignition cycle, and governed by the program relay, the pilot solenoid valve and ignition
transformer are simultaneously energized.
The ignition transformer supplies high voltage current for the igniting spark. A gas pilot has a single electrode and
a spark arcs between the tip of the electrode and the wall of the tube surrounding it. The pilot solenoid valve and
the transformer are de-energized after main flame is ignited and established.
Fuel for the gas pilot is supplied from the utility’s main, or from a tank (bottle) supply. Secondary air flows into and
mixes with the pilot gas stream to provide an adequate flame.
Insurance regulations may require two gas pilot solenoid valves with a normally open vent valve between them. The
vent valve closes when the gas pilot valves open, and opens when the gas pilot valves shut to vent gas, should any
be present in the pilot line during the de-energized period of the gas pilot valves.