How tube and pipe annealing furnaces work
Tube and pipe annealing furnaces are long roller hearth, walking beam or car bottom furnaces that take tubes up to 12 m or more. Carbon steel tubes are normalised or stress relieved at about 600 to 950°C; stainless tubes are solution annealed at 1,050 to 1,100°C and quenched. Where the tubes must stay bright or lightly oxidised, the furnace is heated by gas-fired radiant tubes and filled with a protective atmosphere; where scale is acceptable, open-fired burners are used. Radiant tube burners, often with their own recuperators, discharge into an exhaust manifold along the furnace, and an exhaust (ID) fan pulls the products of combustion from all the tubes to the stack, usually with dilution air to bring the temperature down.
Where the ID fan sits
- Radiant tube burners
- Tube recuperators
- Exhaust manifold
- Dilution air inlet
- Exhaust (ID) fan
- Stack
The fan sits at the end of the exhaust manifold, which runs along the furnace roof or side. Each radiant tube discharges into the manifold through a damper, and ambient air is drawn in ahead of the fan through a dilution damper controlled on temperature. On open-fired furnaces the fan instead takes flue gas from the charge end, much like a reheating furnace. The suction needed at the manifold is small, about 20 to 60 mmWC, but the manifold is long and the fan must hold it across all zones.
Gas at the ID fan
| Gas temperature, normal | 250 to 400°C after dilution air |
| Maximum or upset | 500 to 650°C if the dilution damper sticks or a tube recuperator fails |
| Dust at the fan | Very low; occasional refractory and scale particles |
| Moisture and dew point | About 15 to 18% water vapour on natural gas, well above dew point |
| Leakage gas | Protective atmosphere (nitrogen with hydrogen) can enter if a radiant tube cracks |
| Density at the design temperature | About 0.57 kg/m³ at 350°C, 47% 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 tube and pipe annealing furnaces
| Blade form | Backward inclined, on a clean hot gas duty |
| Material of construction | Heat-resisting or stainless steel impeller for the upset temperature; insulated casing |
| Drive and arrangement | Arrangement 8 direct drive with shaft cooling disc |
| Bearings and cooling | Air-cooled pedestal bearings; water cooling where the fan may see over 450°C |
| Flow control | Variable speed drive on manifold suction, with temperature-controlled dilution damper |
| Also specified | Low suction alarm interlocked to the burners, and a standby fan on continuous lines |
What this ID fan has to cope with
Uneven draft along the manifold
Tubes at the far end of a long manifold see less suction than those near the fan. Their burners run rich, soot the tubes and lose heating, giving a temperature gradient along the charge. Correct manifold sizing, balancing dampers and holding suction at the far end rather than at the fan inlet solve this.
Pressure swings with pulse firing
Many radiant tube furnaces fire burners on and off in pulses. As tubes switch, the gas flow into the manifold changes every few seconds. A fan with a stable curve and a drive tuned for steady suction, not fast hunting, keeps the burners lighting reliably.
Cracked radiant tube
A failed tube lets protective atmosphere with hydrogen into the exhaust, or exhaust into the furnace. The fan should keep the manifold negative so that leakage goes outward, and the system needs a way to detect and isolate the failed tube.
Energy and control
Tube annealing furnaces run to the mill schedule, often with long holding periods between orders when firing drops to a fraction of full load. A radiant tube exhaust fan on a fixed speed with a throttled damper wastes most of its power at these times. Speed control on manifold suction, with dilution air limited to what temperature protection needs, cuts both fan power and the volume handled.
ID fan and FD fan in tube and pipe annealing furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | End of the radiant tube exhaust manifold | Feeding the combustion air main |
| What it handles | Radiant tube exhaust with dilution air | Ambient filtered air |
| Temperature at the fan | 250 to 400°C | Ambient |
| Main design concern | Even suction at every tube | Steady pressure during pulse firing |
| Control | Manifold suction | Air main pressure |
See also the Forced Draft (FD) Fan for tube and pipe annealing furnaces.
Questions buyers ask
What is a radiant tube exhaust fan?
It is the ID fan that draws the combustion products out of the radiant tubes of an annealing furnace and sends them to the stack. Each tube burner needs a set suction at its exhaust leg, so the fan holds a controlled negative pressure in the common manifold.
Why is dilution air added before the exhaust fan?
Radiant tube exhaust can leave the tube recuperators at 500°C or more. Adding ambient air brings it down to 250 to 400°C, which allows a smaller, cheaper fan construction and protects the bearings. The fan must be sized for the extra volume of dilution air.
Does a tube annealing furnace with open burners need an ID fan?
Open-fired tube furnaces often work on chimney draft. An ID fan becomes necessary when a central recuperator or a long flue is added, or when the furnace pressure must be held closely to control scale on the tubes.
Information needed to quote an ID fan for tube and pipe annealing furnaces
- 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.
- Radiant tubes. Number of radiant tubes, firing mode (modulating or pulse) and exhaust suction required at each tube.
- Dilution. Exhaust temperature before dilution and the target 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.