How gas fired thermic fluid heaters work
A gas fired thermic fluid heater burns natural gas, LPG or, in some plants, biogas or process off-gas in a forced draft burner firing into a coil type heater. The fuel is clean and free of sulphur and ash, so the gas leaving the coils at about 280 to 340°C can be cooled further in an air preheater or economiser without acid dew point limits. The water vapour content is high, typically 15 to 18% by volume for natural gas, with a water dew point near 55 to 60°C. Most gas fired heaters run with a pressurised furnace and no ID fan; an ID fan is added where heat recovery, a condensing economiser or a long flue makes balanced draft necessary.
Where the ID fan sits
- Burner and furnace
- Coils
- Economiser or air preheater
- Condensing stage (if fitted)
- ID fan
- Chimney
The fan sits after the last heat recovery stage. With a dry economiser or air preheater the gas reaches it at 130 to 180°C and is dry. With a condensing economiser it arrives at 45 to 60°C, saturated, often carrying droplets. Suction is low, typically 30 to 80 mmWC, because there is no dust collector. Placing the fan at the cold end gives the smallest volume but sets the material choice.
Gas at the ID fan
| Gas temperature, normal | 130 to 180°C after a dry economiser |
| Condensing duty | 45 to 60°C, saturated, where a condensing economiser is fitted |
| Dust at the fan | None in normal operation |
| Moisture and dew point | Water dew point about 55 to 60°C for natural gas |
| Corrosive species | Weakly acidic condensate from CO₂ and NOx; chlorides or H₂S if biogas is fired |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 gas fired thermic fluid heaters
| Blade form | Backward curved aerofoil, for efficiency on clean gas |
| Material of construction | Mild steel for dry gas; SS 304 or SS 316 for condensing or biogas duty |
| Drive and arrangement | Direct coupled or direct mounted |
| Bearings and cooling | Standard bearings outside the gas stream; cooling disc for dry gas duty |
| Flow control | Variable speed drive on furnace pressure |
| Also specified | Casing drain and sloped inlet duct for condensing duty |
What this ID fan has to cope with
Condensate in the fan
Downstream of a condensing economiser the gas is saturated and carries droplets. Water collects in the casing, corrodes mild steel within months and can unbalance the wheel as it pools and drains. A stainless fan with a drain at the lowest point, a sloped inlet duct and a mist eliminator upstream handle this duty reliably.
Biogas and off-gas impurities
Biogas carries H₂S and siloxanes; some process off-gases carry chlorides. These form acids and silica-like deposits that a natural gas heater never sees, both in the coils and on the fan. The fan material and inspection access are chosen for the actual fuel mix and its worst impurity level, not for natural gas.
Low pressure draft control
With no collector the system resistance is low, so small changes in fan speed cause large changes in furnace pressure, and the draft loop can hunt. The loop is tuned carefully, and the fan is selected so its normal point is not at very low speed where control is coarse and the motor runs cool on a weak fan curve.
Energy and control
Gas fired heaters are often run as the efficient unit in a plant, with long operating hours and modulating load. A high efficiency aerofoil wheel and a variable speed drive keep the fan's power small. The bigger gain is heat recovery: a condensing economiser recovers latent heat that a dry economiser leaves in the gas, and the fan for that duty is a stainless fan at low temperature.
ID fan and FD fan in gas fired thermic fluid heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the economiser, before the chimney | Ahead of the burner |
| What it handles | Clean, moist flue gas | Ambient air, sometimes with recirculated gas |
| Temperature at the fan | 130 to 180°C, or 45 to 60°C condensing | Ambient, up to 100°C with FGR |
| Main design concern | Condensate and draft control | Air-fuel ratio across turndown |
| Control | Furnace pressure | Firing rate with oxygen trim |
See also the Forced Draft (FD) Fan for gas fired thermic fluid heaters.
Questions buyers ask
Does a gas fired thermic fluid heater need an ID fan?
Usually not. Most gas 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 used when an economiser or condensing heat recovery adds resistance the burner fan cannot overcome, when the chimney is long, or when the plant needs a balanced furnace.
Can a mild steel ID fan be used on a gas fired heater with a condensing economiser?
Not reliably. The gas after a condensing economiser is saturated, carries droplets and forms weakly acidic condensate from CO₂ and NOx. Condensate corrodes mild steel quickly and the rust flakes unbalance the wheel. Stainless steel, usually SS 304 or SS 316, with a casing drain is the normal choice for this duty.
What efficiency can an ID fan on a natural gas heater reach?
Because the gas is clean and free of ash, a backward curved aerofoil wheel can be used without wear concerns, with peak static efficiencies typically in the high seventies to low eighties per cent. Actual efficiency depends on where the operating point sits on the curve, so the selection matters as much as the wheel type.
Information needed to quote an ID fan for gas 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. Natural gas, LPG, biogas or off-gas, with impurities where known.
- Heat recovery. Dry economiser, air preheater or condensing economiser, and gas temperature at the fan.
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.