Unigas RG91 Manual page 70

10 Configurations for burner with FGR
10.1 Recommendations
Note!
Reduction of maximum burner output
Use of the flue gas recirculation (FGR) function or the flue gas mass introduced to the supply air duct might lower the burner's
maximum output.
This means that the maximum amount of combustion air that can be introduced will be reduced.
It is recommended to consider a proper air excess during the regulation of the burner in order to have to the right O2 content in
the smoke, after the flue gas recirculation.
Hence, the amount of fuel for high-fire operation must be reduced to ensure correct combustion values.
Caution!
Temperature-compensated flue gas recirculation (FGR) can be correctly set only when selecting with DriveLowfire in operation!
A change in the curve point without the corresponding flue gas recirculation temperature (e.g. without driving in operation or in
standby) results in an incorrect pairing of the values Flue gas recirculation position and Flue gas recirculation temperature.
This can lead to excessive amounts of recirculated flue gas, which might cause the flame to lift: Stability limit of flame.
Caution!
A subsequent change of the curve point without an associated flue gas recirculation (FGR) temperature (e.g. without
DriveLowfire in operation or standby) leads to an incorrect pairing of flue gas recirculation-position and flue gas recirculation-
temperature.
This can lead to excessive amounts of recirculated flue gas, which might cause the flame to lift: Stability limit of flame.
Note!
Flue gas recirculation (FGR) in combination with O2 trim control Recommendation: Do not use flue gas recirculation (FGR) in
combination with O2 trim control.
This has no impact on the use of the O2 alarm.
The physical effects are the following:
1. Pressures have reciprocal effects.
2. The reduction of O2 can lead to a significant increase of NOx levels.
As a result of these reciprocal effects, it is difficult, if not impossible, to adjust fuel-air ratio control, O2 trim control, and the flue
gas recirculation (FGR) function.
Even if an adjustment was possible, the flame may become instable during operation, or the required NOx levels might not be
reached.
Note!
The full scope of setting TCautoDeact is possible only when the flue gas temperature is acquired via the load controller input
(X60...).
When the temperature is acquired via the PLL52... input (X86...) and the O2 trim controller / alarm is active (not CtrlAutoDeac),
flue gas recirculation (FGR) mode temperature-compensated cannot be used (would lead to error C:F6 D:2).
When operating mode O2 Control is deactivated (man deact), operating mode TCautoDeact can be used if the flue gas
temperature is acquired via PLL52... (X86...).
Attention!
If at an dual-fuel burner the FGR function is used for only one fuel (e.g. gas operation with FGR and oil operation without FGR)
pay attention to the following:
When the fuel selection is switched over to the fuel without FGR it must be assured that the FGR actuator is closed and is
supervised kept in the closed position.
This is accomplished by making the following settings for the fuel without FGR:
- Activation of the AUX3 actuator
- Parameterization of the positions Home, Prepurge , Ignition and Postpurge to closed
- Parameterization of all AUX3 actuator positions at all curve points to closed
- Parameterization of the FGR operating mode to Aux3onCurve
Before to activate the FGR system, it is mandatory to complete the air/fuel ratio curve for each point, up to the maximum burned output.
Check the previous chapter for instructions.
WARNING: Activating or increasing the FGR butterfly valve opening, it is mandatory to check the combustion by means a
properly and calibrated smoke analyzer.
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