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 ID fan sits
- Dual fuel burners
- Furnace
- Superheater and bank
- Economiser
- Air heater
- ID fan
- Stack
The ID fan sits after the economiser or air heater, before the stack, with no collector in normal practice. Suction differs by fuel: oil firing fouls the surfaces, raising resistance and gas temperature over time, while gas firing keeps them clean. The fan is placed at the cold end to handle the smallest volume, but this is also where oil-fired gas is closest to its acid dew point, so the minimum gas temperature has to be set for the oil case even if the boiler fires gas most of the year.
Gas at the ID fan
| Gas temperature, normal | 140 to 160°C on gas, 160 to 190°C on oil |
| Maximum or upset | Up to 250°C with surfaces fouled after long oil firing |
| Dust at the fan | None on gas; soot and cenospheres on oil |
| Moisture and dew point | Water dew point about 55 to 60°C on gas; acid dew point 120 to 150°C on high-sulphur oil |
| Gas volume | Slightly higher per unit of heat on gas; higher resistance on oil as surfaces foul |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 67% 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 dual fuel oil and gas boilers
| Blade form | Backward curved, balancing gas-case efficiency against oil-case deposits |
| Material of construction | Mild steel with insulated casing; SS 316 impeller where oil campaigns are long or sulphur is high |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | VFD on furnace draft, with fuel-specific tuning |
| Also specified | Casing drain, washing access, outlet damper |
What this ID fan has to cope with
One curve, two duty points
The gas and oil cases differ in volume, temperature and system resistance. A fan selected only for the gas duty can lack pressure once oil firing has fouled the boiler. The selection is checked at both points, with the fouled oil case as the maximum, and the best efficiency placed on the fuel the plant fires most.
Corrosion after returning to gas
Oil firing leaves acidic soot on the impeller and casing. When the boiler goes back to gas, the very moist flue gas wets this soot and forms acid on the metal underneath. Washing the fan after long oil campaigns, or sootblowing before changeover, prevents corrosion that neither fuel would cause on its own.
Draft upset at fuel changeover
Switching fuel on load changes flame shape, heat absorption and gas volume within seconds. The ID fan must hold furnace pressure through the transition or the boiler trips. Feedforward from the combustion control and separate tuning of the draft loop for each fuel avoid trips during changeover.
Energy and control
Most dual fuel boilers spend most of their hours on gas, with oil firing only during supply interruptions or periodic tests. The ID fan should therefore be most efficient at the gas duty, with the oil case covered by speed margin on the VFD rather than by a larger fan running throttled on gas. Regular oil firing tests keep the system proven without changing the energy picture much.
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 Forced Draft (FD) Fan for dual fuel oil and gas boilers.
Questions buyers ask
How is an ID fan sized for a dual fuel oil and gas boiler?
The fan is checked at both the gas and oil duty points, with flow, temperature and resistance for each, including fouled surfaces after oil firing. It is then selected so that the fuel fired most is near peak efficiency and the other lies within the speed and power range of the drive.
Why does my ID fan corrode after switching from oil to gas?
Oil firing leaves acidic soot on the fan. When gas firing resumes, the high water content of natural gas flue gas wets the deposits and forms acid on the metal. Cleaning after oil campaigns, keeping the casing insulated and warm, and draining the casing prevent this.
Do dual fuel boilers need separate fans for oil and gas firing?
No. One set of ID and FD fans serves both fuels, but the selection and control must account for both. The air register settings and fan control curves are tuned separately for each fuel within the burner management system, and the fan is checked at both duties.
Information needed to quote an ID fan for dual fuel oil and gas boilers
- 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.
- Fuel hours. Expected annual hours on gas and on oil, and the oil grade.
- Changeover. Whether fuel change is made on load or with a shutdown.
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.