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 FD fan sits
- Inlet silencer
- FD (combustion air) fan
- FGR mixing (if fitted)
- Air preheater (if fitted)
- Burner
- Furnace
The fan supplies the burner wind box, either from a monoblock burner or as a separate fan on dual-block burners. Where flue gas recirculation is used for low NOx, a duct from the stack joins the fan inlet, so the fan handles a mixture of air and flue gas at an elevated temperature. On a pressurised furnace the fan pressure also covers the coils, economiser and chimney.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C; higher with flue gas recirculation |
| With recirculated gas | Mixed inlet typically 60 to 100°C |
| Pressure at the fan outlet | Typically 150 to 300 mmWC |
| Dust | Clean air |
| Turndown | Burners modulate typically over 4:1 to 8:1 |
| 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 gas fired thermic fluid heaters
| Blade form | Backward curved, with a curve that stays stable at low flow |
| Material of construction | Mild steel; stainless inlet where recirculated gas can condense |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Variable speed drive with electronic air-fuel ratio control and oxygen trim |
| Also specified | Air pressure switch, inlet silencer and FGR damper where fitted |
What this FD fan has to cope with
Air-fuel ratio at low fire
Gas burners need tight ratio control across their turndown. A fan with a damper alone becomes coarse at low fire, where a small movement changes the air a lot, and excess oxygen rises. A speed controlled fan with oxygen trim holds the ratio and keeps the burner clean and efficient at every load.
Flue gas recirculation duty
Low NOx designs return 10 to 20% of the flue gas to the burner. The fan then handles a warmer, wetter mixture of lower density, and condensate can form at the mixing point in cold weather. The fan is sized at the mixed condition and the inlet is drained.
Noise
High speed combustion air fans are a main noise source in a heater house, and the burner front is where operators spend their time. An inlet silencer sized for the full air flow, lagging on the wind box duct and anti-vibration mounts are part of the selection, not an afterthought once the heater is running.
Energy and control
Natural gas is expensive, so excess air control saves more money than fan power. A variable speed fan with oxygen trim holding 2 to 3% oxygen in the flue gas typically saves both fuel and electricity compared with a damper controlled fan. Where flue gas recirculation is used, the extra gas volume adds to fan power and is set at the minimum that meets the NOx limit.
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 Induced Draft (ID) Fan for gas fired thermic fluid heaters.
Questions buyers ask
What is the FD fan on a gas fired thermic fluid heater?
It is the burner combustion air fan. It supplies air to the burner and, on most gas heaters, also pushes the flue gas through the heater and up the chimney because the furnace is pressurised. On low NOx burners it may also carry recirculated flue gas.
Why use a variable speed drive on the burner fan of a gas heater?
Gas burners modulate over a wide range, often 4:1 or more. A speed drive lets the fan follow the burner closely with an electronic air-fuel ratio and oxygen trim, which saves gas and power and keeps emissions stable at low fire, where a damper controlled fan wastes most energy.
Does flue gas recirculation change the combustion air fan size?
Yes. The fan handles air plus 10 to 20% recirculated gas at a higher temperature, so its volume and power rise and the density falls. The selection is made at the mixed temperature and density, the inlet is drained, and condensation at the mixing point in cold weather is allowed for.
Information needed to quote an FD fan for gas 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, firing range, turndown and required wind box pressure.
- Emissions. Whether flue gas recirculation is used and the NOx limit to be met.
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.