Cleaver-Brooks CBH 40 - Gas Fuel Flow

Chapter !-General Description and Principles of Operation
G. GAS fUEl flOW
The gas flow system is shown with the direction of
flow indicated by arrows. (Figure 1-10)
Metered gas from the utility flows into the burner's
gas piping through a main gas shut-off cock;
through a pressure regulator where the pressure is
reduced to the pressure suitable to the burner's re-
quirements; through an electrically operated main
gas valve and through a butterfly gas valve to the
burner. Gas required for pilot operation is taken
from this line prior to the main gas shut-off cock.
Gas for the pilot operation flows through a shut-off
cock into a pressure regulator where the pilot oper-
ating pressure is established. A solenoid valve con-
trols the flow of the gas and when energized
(opened) allows the gas to flow to the pilot where it
is mixed with combustion air. This mixture is
ignited by a controlled electric spark to establish
the pilot flame. The pilot burns only during the
time required for main flame ignition.
At the beginning of the ignition cycle, the pilot
solenoid valve is energized through circuitry in the
programmer control and the pilot ignited. When
pilot flame is proven, the programming control
energizes the electrically operated main gas valve
allowing flow through the butterfly valve to the
burner. The rate of flow to the burner depends
upon the position of the vane in the butterfly valve.
This is mechanically controlled by the damper
motor and varied in the same maimer as the air
damper, thus properly proportioning gas and com-
bustion air.
1-12
High-Low Burner Operation
The standard burner is equipped for two position
firing; either low or high. A pressure control on a
steam boiler, or a temperature control on a hot
water boiler, makes or breaks the circuit to the
damper motor causing the burner to operate at
either the high fire or the low fire rate in accord-
ance with load requirements.
Modulating Burner Operation
An optionally equipped gas fired boiler has a
modulating damper motor. This type of motor is
reversible and will move in either direction or stop
at any position within its range.
Through a linkage arrangement it controls the air
damper and the butterfly gas valve to maintain a
constant air-fuel ratio throughout the firing range.
Gas flows through the burner orifice ring to enter
the combustion zone where it is intimately mixed
with combustion air to produce main flame.
The main gas valve cannot be energized (opened)
unless the combustion air proving switch is closed
to indicate a sufficient supply of combustion air.
Some insurance requirements specify two main gas
valves. Additional requirements call for a nor-
mally open vent valve to be placed between them
for venting gas should any be present in main gas
line when main gas valves are de-energized. The
vent valve closes when the main gas valves open.