How oil fired thermic fluid heaters work
An oil fired thermic fluid heater burns light diesel oil, furnace oil or LSHS in a pressure jet or rotary cup burner at one end of a helical coil. The flame radiates to the inner coil, and the gas turns and passes over the outer coils before leaving at about 300 to 360°C. Many smaller heaters run with a pressurised furnace: the burner fan alone pushes the gas through the coils and up the chimney, and there is no ID fan. Larger heaters, and those with an air preheater, economiser or a long stack run, add an ID fan after the heat recovery equipment. Furnace oil usually contains 2 to 4% sulphur, so acid dew point governs how far the gas can be cooled.
Where the FD fan sits
- Inlet silencer
- FD (combustion air) fan
- Air preheater (if fitted)
- Air register
- Burner head
- Furnace
On a dual-block burner the FD fan stands beside the heater and pushes ambient air through the air preheater, where fitted, to the burner wind box. It sees the air register and burner head, which need a set pressure for good mixing, plus, on a pressurised furnace, the coils, any heat recovery equipment and the chimney. Fan pressures of 150 to 350 mmWC are common.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 150 to 250°C where an air preheater is fitted |
| Pressure at the fan outlet | Typically 150 to 350 mmWC |
| Dust | Clean air; an inlet filter keeps lint and dust off the burner |
| Duty | Pre-purge at full air, then modulating with the burner |
| 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 oil fired thermic fluid heaters
| Blade form | Backward curved, for a stable curve and good efficiency |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft, often at 2,900 rpm |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Air damper linked to the oil valve, or variable speed drive with oxygen trim |
| Also specified | Inlet silencer and an air flow switch for the burner safety system |
What this FD fan has to cope with
Air-fuel ratio drift
Mechanical linkages between the air damper and the oil valve wear and drift over time, and the heater either runs with too much air or starts to smoke. A variable speed drive with electronic ratio control and oxygen trim holds the ratio across the firing range and does not drift with wear, which also keeps the coils free of soot.
Purge air shortfall
Burner safety requires a pre-purge of several furnace volumes before ignition. A fan selected only for the combustion air at full fire may not purge a heater with a long gas path quickly enough. The purge flow and time are checked when the fan is selected.
Hot air at the burner
Where an air preheater is added to an existing heater, the burner receives air at 150 to 250°C. The air volume at the burner rises with temperature, and the fan pressure must cover the preheater resistance as well as the burner head. The burner itself must be suitable for preheated air, and the fan stays on the cold side.
Energy and control
The combustion air fan runs whenever the burner fires, through every modulation cycle. A variable speed drive with oxygen trim reduces fan power and holds excess air at the 10 to 20% typical of good oil firing, which also saves fuel. On older heaters, replacing a fixed speed fan and damper with a speed controlled fan often pays back through the fuel saving more than the electrical saving.
ID fan and FD fan in oil fired thermic fluid heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the air preheater or economiser | Ahead of the burner |
| What it handles | Flue gas with SO₃ and soot | Clean ambient air |
| Temperature at the fan | 180 to 220°C | Ambient |
| Main design concern | Acid dew point corrosion | Air-fuel ratio and purge |
| Control | Furnace pressure | Burner firing rate with oxygen trim |
See also the Induced Draft (ID) Fan for oil fired thermic fluid heaters.
Questions buyers ask
What is the FD fan on an oil fired thermic fluid heater?
It is the combustion air fan of the burner. On small heaters it is part of a monoblock burner. On larger heaters it is a separate fan supplying the burner wind box, sometimes through an air preheater, and it may also have to push the gas through the coils on a pressurised furnace.
Can a separate FD fan replace the fan in a monoblock burner?
Yes, by converting to a dual-block arrangement, which is often done when an air preheater is added or when the original fan cannot meet the pressure of a longer gas path. The burner control and safety interlocks, including the air pressure switch and purge timing, must be adapted and checked with the new fan.
What pressure is needed for the combustion air fan on an oil fired heater?
Usually 150 to 350 mmWC, depending on the burner head pressure, any air preheater and whether the furnace is pressurised so that the fan also pushes the gas through the coils and chimney. The burner maker's data gives the head pressure; the rest is calculated from the heater and duct layout.
Information needed to quote an FD fan for oil fired thermic fluid heaters
- 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.
- Burner. Burner type and make, rated firing range and the required wind box pressure.
- Preheat. Whether combustion air is preheated, and to what temperature.
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.