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 FD fan sits
- Inlet filter and silencer
- FD fan
- Combustion air main
- Zone air valves
- Radiant tube burners
The fan usually stands at floor level beside the furnace and feeds an air main running its full length, with branches to each zone and each burner. Where the radiant tubes have their own recuperators, the fan delivers cold air and each burner heats its own air; with a central recuperator the fan pushes air through it first. Typical fan pressures are 300 to 700 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperators | 350 to 550°C at the burner, downstream of the fan |
| Dust | Clean filtered air |
| Pressure at the fan outlet | Typically 300 to 700 mmWC |
| Load pattern | Pulse firing switches burners every few seconds; average load follows the mill schedule |
| 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 tube and pipe annealing furnaces
| Blade form | Backward curved, with a steep pressure curve near the duty point |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled at two-pole speed on high pressure duties |
| Bearings and cooling | Standard bearings, no cooling |
| Flow control | Variable speed drive on air main pressure |
| Also specified | Inlet silencer, acoustic lagging, and a standby fan for continuous lines |
What this FD fan has to cope with
Main pressure ripple
When many burners open together in pulse firing, main pressure dips and the last burner on the main can fail to light. A fan with a steep curve, a generously sized main and a drive that holds pressure without hunting solve this.
Noise in the finishing bay
High pressure combustion air fans running at two-pole speed are loud at the inlet, with a strong blade-pass tone. Tube finishing, ultrasonic testing and inspection bays are often next to the furnace, so an inlet silencer, casing lagging and a flexible connection to the air main are part of the duty, not extras.
Air main condensation
Where the plant uses a central recuperator, the long hot air main cools during holiday shutdowns, and moist air trapped in it can condense and collect at low points with scale from the duct. Drains at low points, and a purge with the fan running before relighting, keep water and debris away from the burners.
Energy and control
With burners cycling, the average air demand on a radiant tube furnace is often far below the fan rating. A drive holding constant main pressure follows that average and saves power during holding periods. Keeping air ratio correct at each burner matters more than fan power: excess air through a radiant tube cools it and cuts heat transfer to the tubes.
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 Induced Draft (ID) Fan for tube and pipe annealing furnaces.
Questions buyers ask
How much pressure does a radiant tube burner need?
It depends on the burner design. Typical recuperative radiant tube burners need about 200 to 500 mmWC of air at the burner. The combustion air fan adds the losses of the main and valves, so fans of 300 to 700 mmWC are common.
Is the FD fan on an annealing furnace the same as the combustion blower?
Yes. On tube annealing furnaces the forced draft fan is the combustion air blower. It supplies air to the burners and nothing else; the protective atmosphere comes from its own generator or gas supply, and the cooling sections and roller hearth fans are separate units with their own duty.
Should the combustion air fan have a standby?
On a continuous tube annealing line, losing combustion air stops production, leaves tubes part-annealed inside the furnace and can overheat recuperators. A standby fan with automatic changeover is common. On batch or car bottom furnaces a single fan is usually accepted, with a spare impeller or motor in stores.
Information needed to quote an FD fan for tube and pipe annealing 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.
- Burner list. Number of burners per zone, burner type and air pressure required at the burner.
- Firing mode. Whether burners are modulated or pulse fired, and the switching frequency.
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.