How heat treatment furnaces work
Heat treatment furnaces harden, temper, carburise, anneal and normalise steel components, from fasteners and gears to automotive parts. Common types are sealed quench batch furnaces, mesh belt and roller hearth continuous furnaces, and pit furnaces. Many fuel-fired units heat through radiant tubes, so the flue gas never touches the work and a protective or carburising atmosphere can be maintained inside; others fire directly into the chamber for tempering or normalising. Process temperatures range from about 150°C for tempering to 950°C for carburising and hardening. Radiant tube exhaust leaves at 600 to 900°C, often through self-recuperative burners or a common recuperator. Electrically heated furnaces have no combustion fans at all, only atmosphere circulation and quench fans.
Where the FD fan sits
- Inlet filter
- Combustion air (FD) fan
- Air header
- Recuperator or self-recuperative burner
- Radiant tube burner
The fan feeds an air header serving all burners on the furnace. Radiant tube burners, especially self-recuperative types, need air pressure of about 200 to 500 mmWC at the burner. With header, valve and recuperator losses the fan typically delivers 300 to 700 mmWC. Each fan is small, but in a heat treatment shop several furnaces often share one central fan, so the header must keep pressure even at the furthest furnace.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after recuperation | 300 to 600°C, downstream of the fan |
| Dust | Clean shop air; inlet filter |
| Pressure at the fan outlet | Typically 300 to 700 mmWC |
| Turndown | Wide, with on-off or pulse firing on many furnaces |
| 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 heat treatment furnaces
| Blade form | Backward curved, steep curve for stable pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on header pressure |
| Also specified | Inlet filter, low air pressure interlock with burner management |
What this FD fan has to cope with
Pulse firing pressure swings
Many radiant tube furnaces fire burners in pulses, high or off. As burners switch, header pressure swings. A fan with a steep curve and fast speed control keeps pressure steady so each burner lights correctly. A non-return damper on each furnace branch prevents interaction between furnaces on a shared header, and the fan must be selected well clear of stall so pressure does not hunt.
Shared fan for several furnaces
A central fan serving several furnaces must cover all at full fire, but usually runs well below that. Speed control on header pressure keeps efficiency reasonable and pressure stable. A standby fan, or a cross-connection to another header, is worth considering because one fan failure stops every furnace it serves.
Low air causing sooting
Insufficient air in radiant tubes causes soot, which shortens tube life. The fan must provide enough pressure at the furthest burner. Soot also insulates the tube, so the furnace needs more fuel for the same temperature and the tube runs hotter at the burner end. Checking air pressure at each burner, not only at the fan, avoids this.
Energy and control
Heat treatment shops often run several furnaces from one combustion air fan, and the furnaces are rarely all at full fire. Variable speed control on header pressure saves power when furnaces are at low fire or idle at temperature. Correct air to fuel ratio reduces fuel use, limits soot inside the radiant tubes and extends their life, which matters more than the fan power.
ID fan and FD fan in heat treatment furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After radiant tube exhaust header | Before the burners |
| What it handles | Clean combustion products | Ambient air |
| Temperature at the fan | 250 to 400°C | Ambient |
| Main design concern | Holding suction on every radiant tube | Stable pressure at pulse or low fire |
| Control | Exhaust header suction | Air header pressure |
See also the Induced Draft (ID) Fan for heat treatment furnaces.
Questions buyers ask
What does the FD fan do on a heat treatment furnace?
It is the combustion air fan, supplying air to the radiant tube or direct-fired burners. The forced draft fan handles ambient air, often preheated afterwards in recuperators or self-recuperative burners. On radiant tube furnaces it must give stable pressure at low fire and during pulse firing, because uniformity and atmosphere quality depend on steady combustion in every tube.
Can one combustion air fan serve several heat treatment furnaces?
Yes, a central fan with a common header is common. It must be sized for the combined maximum firing, with speed control on header pressure to keep pressure stable as furnaces change load. A non-return damper on each branch prevents interaction between furnaces. A standby fan is worth considering.
What pressure is needed for radiant tube burners?
Typically 200 to 500 mmWC at the burner, depending on burner type, with header and valve losses added at the fan. Self-recuperative burners usually need more pressure than simple radiant tube burners. The burner data sheet gives the exact requirement at full fire, and the fan should have margin for filter loading and header losses.
Information needed to quote an FD fan for heat treatment furnaces
- 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.
- Burners. Number and type of burners, and the air pressure needed at each.
- Furnace count. Number of furnaces served by the fan and their maximum firing rates.
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.