How bell annealing furnaces work
Bell annealing furnaces anneal stacks of cold rolled steel coils under a protective atmosphere, usually hydrogen or a hydrogen-nitrogen mix. The coils sit on a base with a powerful convection fan in it, under a sealed inner cover filled with the atmosphere. A heating bell, fired by gas burners or heated electrically, is lowered by crane over the inner cover and heats it, bringing the coils to about 650 to 720°C over many hours. The burner flue gas is drawn off the heating bell, often through a recuperator mounted on the bell that preheats the combustion air. After soaking, the heating bell is moved to another base and a cooling bell with fans, or water spray, cools the inner cover.
Where the FD fan sits
- Inlet
- Combustion air (FD) fan on bell
- Bell recuperator
- Air ring main
- Burners
The fan is mounted on the heating bell and delivers air through the recuperator to a ring main feeding the burners around the bell. Air pressure needed at the fan is typically 300 to 600 mmWC, covering the recuperator, ring main and burners. The fan works in the warm air around the bells and is disconnected and reconnected each time the bell moves to a new base.
Air at the FD fan
| Air temperature at the fan | Ambient, often warm near the bells, 35 to 50°C |
| Air after the recuperator | 250 to 450°C, downstream of the fan |
| Dust | Shop dust |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Mounting | On a crane-moved bell |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% 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 bell annealing furnaces
| Blade form | Backward curved, compact |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, overhung, on a rigid frame |
| Bearings and cooling | Bearings rated for shock loads |
| Flow control | Variable speed or damper on air pressure |
| Also specified | Inlet filter and plug-in electrical connection |
What this FD fan has to cope with
Shock loads
Bell handling shocks are transmitted straight into the fan bearings at every landing. Bearings with suitable clearance, a rigid frame tied into the bell structure and a balance check after any heavy landing prevent early bearing failure on fans that move several times a week.
Hot ambient
Air around the bells is warm, often 35 to 50°C, so it is lighter than standard air and a fan selected at 20°C delivers less mass flow and pressure than expected. Selecting at the actual inlet temperature, or ducting cooler air to the inlet, avoids a shortfall at full firing.
Uneven burner air
Burners around the bell are fed from a ring main. If the fan has little pressure margin, burners furthest from the fan run short of air and the bell heats unevenly, which affects coil properties. A correctly sized ring main and enough fan pressure keep all burners supplied.
Energy and control
Bell annealing cycles are long, with full firing at heat-up and low firing through the soak. A variable speed combustion air fan saves power during soak compared with a throttling damper. Correct air to fuel ratio, held through the whole cycle, saves the most fuel and keeps the bell recuperator clean.
ID fan and FD fan in bell annealing furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | On the bell, after the recuperator | On the bell, before the recuperator |
| What it handles | Burner flue gas | Ambient air |
| Temperature at the fan | 250 to 350°C | Ambient, warm |
| Main design concern | Shock loads and hot gas | Shock loads and even burner air |
| Control | Bell suction | Air pressure |
See also the Induced Draft (ID) Fan for bell annealing furnaces.
Questions buyers ask
What is the combustion air fan on a bell annealing furnace?
It is the forced draft fan mounted on the heating bell of a bell annealing furnace. It takes ambient air, pushes it through the bell recuperator and supplies it to the burners around the bell. It is separate from the base convection fan and from the fans on the cooling bell.
Does the bell furnace FD fan move with the bell?
In most designs, yes. The combustion air fan is mounted on the heating bell and travels with it from base to base, with a plug-in electrical connection. This keeps the air path short, but it means the fan must be compact and withstand crane handling shocks without losing balance.
What pressure does a bell annealing combustion air fan need?
The combustion air fan on a bell annealing furnace typically delivers 300 to 600 mmWC, covering the bell recuperator, ring main and burner pressure. The exact figure comes from the burner data and the recuperator air side pressure drop at full firing. Fan selection should use the warm air temperature around the bells.
Information needed to quote an FD fan for bell 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.
- Burners. Number of burners on the bell, firing rate and air pressure needed at each burner.
- Mounting and handling. Space on the bell, air temperature near the bells and the crane handling arrangement.
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.