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 ID fan sits
- Burner and furnace
- Inner coil
- Outer coil pass
- Air preheater or economiser
- ID fan
- Chimney
Where fitted, the fan sits after the air preheater or economiser and before the chimney. Upstream it sees the coil passes, the heat recovery equipment and the ducts, usually 40 to 100 mmWC, with the burner fan carrying the furnace and coil resistance. There is no dust collector on most oil fired heaters. Placing the fan on the cool side keeps it small, but it must stay above the acid dew point of the gas.
Gas at the ID fan
| Gas temperature, normal | 180 to 220°C after the air preheater |
| Maximum or upset | Up to 360°C with heat recovery bypassed |
| Acid dew point | About 130 to 160°C for furnace oil with 2 to 4% sulphur; far lower for LDO |
| Dust at the fan | Soot and unburnt carbon, low in quantity, higher at start-up and with poor atomisation |
| Corrosive species | SO₃ and sulphuric acid at cold surfaces; vanadium in some furnace oils |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 oil fired thermic fluid heaters
| Blade form | Backward curved, on clean gas with only soot present |
| Material of construction | Mild steel for gas above the acid dew point; Corten-type weathering steel or stainless for the casing where the fan runs near the dew point |
| Drive and arrangement | Direct coupled or direct mounted on smaller heaters |
| Bearings and cooling | Bearings outside the gas stream with a cooling disc |
| Flow control | Variable speed drive on furnace pressure, linked to burner firing rate |
| Also specified | Casing drain, insulation and an inspection door for soot removal |
What this ID fan has to cope with
Acid corrosion at low load and start-up
When the heater runs at low fire or is started cold, the gas at the fan and duct can fall below the acid dew point and sulphuric acid condenses on the casing and blades. Insulating the casing, keeping the air preheater exit above the dew point, and a corrosion-resistant casing material where low loads are common prevent rapid thinning.
Soot build-up and unbalance
Poor atomisation, a worn nozzle, oil that is not heated to the right viscosity or too little air leaves soot that collects on the impeller as acid-wet deposits. These build up unevenly and cause vibration. Burner maintenance is the real cure, and an inspection door allows the wheel to be cleaned before the deposit hardens and has to be chipped off.
Burner and ID fan fighting each other
With a pressurised burner and an ID fan in series, a poorly tuned draft loop can starve or flood the burner: too much suction pulls extra air through the burner, too little backs pressure onto the flame. The ID fan holds a fixed small suction at the furnace outlet so that the burner's own air-fuel ratio control is not disturbed.
Energy and control
Oil fired heaters often modulate between low and high fire through the day. A variable speed drive on the ID fan tracks the burner firing rate and avoids the throttling losses of a damper. The larger saving is in the heat recovery: each 20°C reduction in exit gas temperature, kept safely above the acid dew point, improves heater efficiency by roughly 1%.
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 Forced Draft (FD) Fan for oil fired thermic fluid heaters.
Questions buyers ask
Does an oil fired thermic fluid heater need an ID fan?
Not always. Many smaller oil fired heaters run with a pressurised furnace, where the burner fan pushes the gas through the coils and up the chimney. An ID fan is added when an air preheater or economiser, a long stack run or a balanced furnace makes the burner fan alone insufficient.
What material should the ID fan be on a furnace oil fired heater?
Mild steel works when the gas at the fan stays well above the acid dew point of about 130 to 160°C. Where the heater runs at low fire for long periods or starts cold often, a weathering steel or stainless casing and insulation reduce acid corrosion.
Why does soot collect on the ID fan of an oil fired heater?
Soot comes from incomplete combustion, usually poor atomisation, a worn nozzle, oil below its atomising temperature or too little air. It sticks to the fan as an acidic deposit and absorbs moisture when the heater stops. Fixing the burner is the real cure; regular cleaning through the inspection door keeps the fan balanced in the meantime.
Information needed to quote an ID fan for oil fired thermic fluid heaters
- 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 oil. Grade of oil and its sulphur content.
- Furnace arrangement. Whether the furnace is pressurised or balanced, and the burner fan pressure.
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.