How low nOx gas boilers with flue gas recirculation work
Low NOx gas boilers limit flame temperature by recirculating 10 to 25% of the flue gas back into the combustion air. On package boilers this is usually induced FGR: a duct from the stack base or economiser outlet to the burner fan inlet, where the fan draws gas and air together. Larger water tube boilers use a separate FGR fan feeding the windbox, or take the recirculated gas from the ID fan discharge. Natural gas is the usual fuel, with NOx limits that burner staging alone cannot meet. The recirculated gas adds mass that the fans must move, and it carries the moisture of gas combustion back into the air side.
Where the ID fan sits
- Low NOx burner and furnace
- Boiler passes
- Economiser
- ID fan
- FGR take-off
- Stack
The ID fan sits after the economiser. On many balanced draft FGR units the recirculation duct branches from the fan outlet duct, so the ID fan pressure helps drive gas back to the burner or FGR fan, and the fan handles the net flue gas plus the recirculated gas. Suction covers furnace draft, the boiler passes and the economiser, typically 100 to 200 mmWC. Where FGR is taken upstream of the fan, the ID fan sees only net gas, but its suction interacts with the FGR flow.
Gas at the ID fan
| Gas temperature, normal | 120 to 160°C after the economiser |
| Maximum or upset | 200 to 240°C with economiser bypass |
| Dust at the fan | None |
| Moisture and dew point | Water vapour 17 to 19%; water dew point 55 to 60°C |
| Flow basis | Net flue gas plus 10 to 25% recirculated gas where FGR is drawn from the fan discharge |
| Density at the design temperature | About 0.85 kg/m³ at 145°C, 71% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
ID Fan selection for low nOx gas boilers with flue gas recirculation
| Blade form | Backward curved or aerofoil, for clean gas |
| Material of construction | Mild steel with insulated casing; stainless drain and outlet duct section |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | VFD on furnace draft, coordinated with the FGR damper |
| Also specified | Casing drain, outlet damper, tight shut-off on the FGR branch for start-up |
What this ID fan has to cope with
Draft interaction with the FGR damper
Opening the FGR damper draws gas from the ID fan outlet and changes the system the fan works against. Without coordination, furnace pressure swings each time the NOx control moves the FGR rate, and in the worst case the furnace trips. Feedforward from FGR damper position to ID fan speed keeps draft steady.
Condensation in the recirculation loop
Flue gas from a gas boiler is near its water dew point by the time it reaches cooler duct runs. During idle periods and cold starts, condensate forms in the FGR duct and drains back toward the fan casing. Sloped ducts, drains at the low points and an insulated casing protect the ID fan from wet corrosion.
Fans sized without FGR
When FGR is added to an existing boiler to meet a new NOx limit, the original ID fan may lack capacity for the extra flow and the changed system. The retrofit study must check the ID fan at the new flow and pressure, not only the burner and FD fan, or the boiler will be derated after the upgrade.
Energy and control
FGR raises fan power because part of the gas is moved twice. At low load many burners use higher FGR rates to hold NOx, so the extra flow does not fall in proportion to load. An ID fan on VFD with good efficiency across this wide range keeps the penalty small. Running only the FGR rate the permit actually requires, rather than a fixed high rate, is the main energy lever.
ID fan and FD fan in low nOx gas boilers with flue gas recirculation
| ID fan | FD fan | |
|---|---|---|
| Position | After the economiser, FGR branch at its outlet | At the burner, with FGR mixing at its inlet |
| What it handles | Net flue gas plus recirculated gas | Air and recirculated flue gas mixture |
| Temperature at the fan | 120 to 160°C | 50 to 80°C |
| Main design concern | Draft stability with FGR | Cold air overload and condensation |
| Control | Furnace draft with FGR feedforward | Air and FGR ratio for NOx |
See also the Forced Draft (FD) Fan for low nOx gas boilers with flue gas recirculation.
Questions buyers ask
Does flue gas recirculation increase ID fan load?
Yes, when FGR is taken from the ID fan outlet, the fan carries the recirculated gas as well as the net flue gas, typically 10 to 25% more flow. When FGR is taken upstream of the fan, ID fan flow is unchanged, but the FD fan or a separate FGR fan carries the extra.
Where should the FGR take-off be on a low NOx gas boiler?
It is usually after the economiser, where the gas is cooler and the duct can be smaller. Taking it from the ID fan outlet uses the fan's pressure to drive recirculation; taking it upstream needs a separate FGR fan or an induced arrangement at the burner fan inlet.
Why does furnace pressure swing when the FGR damper moves?
The FGR duct is a parallel path in the fan system. When the damper opens or closes, the resistance the ID fan sees changes and draft shifts. Coordinating ID fan speed with FGR damper position in the control removes the swing on a low NOx boiler with FGR.
Information needed to quote an ID fan for low nOx gas boilers with flue gas recirculation
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- FGR arrangement. Take-off point, FGR fan or induced FGR, and recirculation rate at full and low load.
- NOx limit. The NOx limit to be met, which sets the FGR rate.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.