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 FD fan sits
- Air intake
- FGR duct and mixing box
- FD fan
- Low NOx burner
- Furnace
The fan sits on or beside the burner. An FGR duct from the stack base or economiser outlet joins the air inlet at a mixing box ahead of the fan, with a damper to set the recirculation rate. The fan draws both and delivers the mixture to the burner. With 15 to 20% FGR the mixture is typically 50 to 80°C, and fan pressure covers the burner, the furnace and, on pressurised boilers, the gas passes. Larger units use a separate FGR fan into the windbox instead.
Air at the FD fan
| Air temperature at the fan | Air and FGR mixture, typically 50 to 80°C |
| Air alone | Ambient, 25 to 45°C, at purge and light-off before FGR opens |
| Pressure at the fan outlet | Typically 200 to 400 mmWC with a low NOx head |
| Moisture | Flue gas water vapour in the mixture; condensation at the mixing box on cold starts |
| Flow | Combustion air plus 10 to 25% recirculated gas |
| Density at the design temperature | About 1.03 kg/m³ at 70°C, 86% 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. |
FD Fan selection for low nOx gas boilers with flue gas recirculation
| Blade form | Backward curved, sized for the mixture flow and density |
| Material of construction | Mild steel, with coated or stainless inlet and mixing box |
| Drive and arrangement | Direct coupled or direct mounted, motor sized for cold air at light-off |
| Bearings and cooling | Motor or pedestal bearings |
| Flow control | VFD on mixture flow, with a separate FGR damper |
| Also specified | Drains at the mixing box and fan casing, inlet temperature measurement |
What this FD fan has to cope with
Motor overload on cold air
The fan is sized for a warm, light mixture. During purge and light-off FGR is closed, and the fan handles denser cold air at the same speed. Without allowance the motor overloads. The motor is rated for the cold air case, or the VFD limits speed until the FGR damper opens and the mixture warms.
Condensation at the mixing box
Moist flue gas meeting cooler ambient air at the mixing box can fall below its dew point, especially at start-up and in cool weather. Water with traces of acid collects at the fan inlet and in the casing. Drains, coated or stainless parts and opening FGR only once the boiler is warm reduce the problem.
Flame stability at high FGR
High FGR rates lower the oxygen in the combustion mixture and can destabilise the flame at low fire. The fan and FGR control must keep the mixture oxygen within the burner's limits across the load range, which needs measurement and careful tuning at commissioning rather than a fixed damper setting.
Energy and control
Induced FGR adds the recirculated gas to the FD fan's flow and raises its power by roughly the same proportion. A VFD with an accurate FGR damper, set for the NOx limit at each load rather than a fixed high rate, keeps the penalty small. Low-fire operation with high FGR is where most of the extra power goes, so the FGR schedule at low load deserves attention.
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 Induced Draft (ID) Fan for low nOx gas boilers with flue gas recirculation.
Questions buyers ask
Does the FD fan handle flue gas on a low NOx boiler with FGR?
Yes. With induced FGR the FD fan draws recirculated flue gas into its inlet together with ambient air. It handles a warm, moist mixture, typically 50 to 80°C, and must be sized for the extra flow and lower density, with the motor rated for cold air at start-up.
What is the difference between induced FGR and an FGR fan?
Induced FGR uses the FD or burner fan suction to draw flue gas into the air stream, which is simple and common on package boilers. A separate FGR fan pushes flue gas into the windbox and is used on larger boilers, where it gives independent control and keeps the FD fan on clean air.
Why is there condensation in my low NOx boiler FD fan?
Moist flue gas from the FGR duct cools when it meets ambient air at the fan inlet, and at start-up or in cool weather it falls below its dew point. Drains, stainless parts at the mixing box and delaying FGR until the boiler is warm reduce condensation in the fan.
Information needed to quote an FD fan for low nOx gas boilers with flue gas recirculation
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- FGR rate. Recirculation rate at full and low load, and the gas temperature at the take-off.
- Burner data. Low NOx burner type, head pressure and allowed FGR range.
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.