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FGR Adapter
The FGR adapter provides an interconnection between
the housing and air damper, placed in the air flow stream
to introduce the FGR. This location allows the FGR to
be "induced" into the air stream, because of the negative
pressure downstream of the air damper and created by
the burner blower wheel.
FGR Inlet Tube
This tube is inside the FGR adapter, and is positioned
to enhance the induction or negative pressure in the
FGR line. The tube can be adjusted to provide more or
less pressure by sliding it into or out of the air steam.
Setscrews are used to lock the tube into position. 60 ppm
systems do not use this tube.
Airflow over this tube creates a negative pressure at
the FGR duct. The more this tube is moved into the air
steam, the more negative pressure is created.
FGR Control Valve
The FGR control valve controls the flow of recirculated
flue gas. The valve is connected to the FGR adapter and
inlet tube, which creates the pressure differential for flow.
This valve is normally smaller then the FGR duct line to
provide better flow control.
The FGR control valve modulates in conjunction with the
fuel/air valves to provide different input rates. On a single
point positioning system (linkage), it is connected to the
jackshaft. With a parallel positioning system (linkageless),
an actuator is connected to the FGR control valve and
modulated by electronic control to the desired position.
FGR Shutoff Valve
Single point positioning systems (linkage) require a
separate FGR shut-off valve that prevents flow during
the purge cycle. The valve is driven by a motor to close
the FGR line during the purge cycle. Parallel positioning
systems will modulate the control valve shut during purge
and do not require a shut-off valve.
The shutoff valve should be installed in the FGR duct
close to the boiler connection. The valve stem should be
horizontal, to prevent condensate from building in the
shaft bore, causing it to seize.
When firing oil, this valve may be closed or it may be
partially open to provide some FGR. If the valve is
intended to be partly open, there will be a potentiometer
in the control panel to adjust the position of this valve.
FGR Duct
The FGR duct provides the connection between the
boiler outlet and the control or shut-off valve. The design
of this duct is very important for proper operation and to
prevent maintenance problems (see Section C).
7. Fuel-Air-Ratio Controls
The burner may be equipped with single point positioning
(linkage), multiple setting modulating motor or parallel
positioning system (linkageless). All of these systems
JBE(X) Manual
provide the basic fuel-air-ratio control required for good
combustion, however they can provide different features
and setup capabilities.
Modulating Control
The burner modulates to match the energy requirements
of the load. It does this by using a sensor that measures
the pressure or temperature of the system and a
matching sensor in the modulating motor that moves to
match the readings of the sensor.
In some optional systems, a similar process is used with
an external control that provides a signal to the motor to
go to a certain rate. These systems may include multiple
burner sequencing, outside temperature compensation
and numerous other control strategies.
Single Point Positioning (Linkage)
Single point positioning systems use a single modulating
motor to vary the fuel input, air flow and other flow
changes like FGR and atomizing air flow. Linkage is
used to connect these flow control elements together
to provide a unified fuel-air-ratio control system. Other
elements in this system would typically include a
jackshaft, fuel cam and modulating motor.
Jackshaft
The jackshaft is a shaft that is used to tie the fuel, air
and FGR valves together with linkage, to provide a
uniform change in the flow as the burner modulates. A
modulating motor is used to drive the jackshaft, driven
by the requirement for heat in the system and as allowed
to operate by the flame safeguard.
The jackshaft is a round shaft that rotates and is
mounted in bearing supports. This provides a common
means of modulating all of the valves from a single
drive mechanism. The length can vary to meet overall
dimensions and individual drive arms are used to
connect to each valve.
Fuel Cam
A fuel cam is a mechanical linkage that allows for small
fuel rate changes without changing the linkage setting.
It can simplify the fuel-air-ratio adjustments during the
burner setup (Figure B-4).
Modulating Motor
The jackshaft is driven by a modulating motor that
o
rotates 90
to modulate the burner input from minimum
rate to maximum rate. Linkage is used to connect the
modulating motor to the jackshaft and the fuel cams
along with connecting the fuel, air and FGR control
valves to the jackshaft.
The standard modulating motor has two internal proving
switches. One switch, the Low Fire switch, proves the
low fire position where the burner will light. This is also
the position the modulating motor will travel to when the
burner shuts down. The second switch, the High Fire
Purge switch, proves the high fire purge position during
pre-purge.
Page 14
Section B - Components
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