How dual fuel oil and gas boilers work
Dual fuel boilers fire natural gas as the main fuel with furnace oil, LSHS or HSD as backup, or switch between them on price and availability. The burner carries a gas ring or spuds and an oil gun, with a common air register. Gas gives a clean, moist flue gas; oil adds soot, SO₃ and a higher acid dew point. Sizes range from package shell boilers to water tube units of 100 TPH and more in refineries, fertiliser and chemical plants. Larger units run balanced draft with ID and FD fans, and the fans must meet the duty on both fuels, often with the boiler changing fuel while on load.
Where the FD fan sits
- Air intake
- FD fan
- Steam coil air preheater
- Air heater
- Windbox
- Dual fuel burners
The FD fan stands at ground level and supplies air through the air heater, and a steam coil preheater where oil firing needs it, to the windbox. Each burner has one air register for both fuels. The fan works against the preheaters, windbox and registers, and on pressurised boilers against the whole gas path as well. Oil and gas call for different register settings and excess air, so the fan's operating point shifts with the fuel being fired.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | Typically 200 to 450 mmWC depending on firing mode |
| Excess air | About 10 to 15% on gas, 15 to 25% on oil |
| Dust | Clean air |
| Turndown | Wider on gas than on oil |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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 dual fuel oil and gas boilers
| Blade form | Backward curved or aerofoil, efficient at the gas duty |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Pedestal bearings |
| Flow control | VFD with fuel-specific air curves and O₂ trim |
| Also specified | Inlet silencer, air flow measurement, steam coil preheater for oil firing |
What this FD fan has to cope with
One air curve for two fuels
If the air control uses a single characteristic, excess air is right on one fuel and wrong on the other, usually leaving oil smoky or gas wasteful. Separate air curves for each fuel in the burner management system, each with its own O₂ trim set point, keep both fuels clean and efficient across the load range.
Air lag during changeover
During changeover both fuels may fire together, and total air must follow the combined input. A fan with slow response, or a damper with dead band, lets the air lag and creates smoke or CO spikes. VFD control with tight air-fuel cross-limits handles the transition and keeps the furnace safe.
Oil firing on cold air
Boilers that fire oil only for tests often run it with the steam coil preheater out of service. Cold air on oil gives poor burnout and drives the air heater cold end below the acid dew point. Keeping the preheater ready, even for short oil runs, protects both the combustion and the air heater.
Energy and control
Since most hours are on gas, the FD fan should run at its best efficiency on the gas duty, with the oil case covered by speed margin. VFD control with O₂ trim on each fuel keeps excess air low, which saves far more fuel than the fan saves in power. Regular checks of the air flow measurement, which drifts over time, keep the control honest on both fuels.
ID fan and FD fan in dual fuel oil and gas boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the air heater, before the stack | Ahead of the air heater and windbox |
| What it handles | Wet gas on gas, acidic sooty gas on oil | Ambient air |
| Temperature at the fan | 140 to 190°C, varying with fuel | Ambient, 25 to 45°C |
| Main design concern | Two duty points and acid deposits | Separate air curves for each fuel |
| Control | Furnace draft | Fuel-specific air flow and O₂ |
See also the Induced Draft (ID) Fan for dual fuel oil and gas boilers.
Questions buyers ask
Does a dual fuel boiler need different FD fan settings on oil and gas?
Yes. Air per unit of heat is similar, but excess air and register settings differ between oil and gas. The burner management system holds separate air curves and O₂ set points for each fuel, and the FD fan follows whichever is in use on the dual fuel boiler.
What happens to FD fan duty during oil to gas changeover?
During changeover both fuels may fire together for a short time, and total air must follow the combined heat input. The control uses cross-limits so that air always leads fuel on an increase and lags it on a decrease, avoiding smoke and CO while the burner changes over.
Why is a steam coil air preheater used on dual fuel boilers?
Mainly for oil firing. Warmer air helps heavy oil burn out and keeps the main air heater above the acid dew point. On gas it is usually not needed, but it should be kept ready so that it can be put in service as soon as the boiler switches to oil.
Information needed to quote an FD fan for dual fuel oil and gas boilers
- 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.
- Oil grade. Backup fuel type, sulphur content and atomising method.
- Burner management. Whether separate air curves and O₂ trim for each fuel are available in the control system.
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.