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 ID fan sits
- Burners on heating bell
- Annular space around inner cover
- Bell recuperator
- Flue gas (ID) fan on bell
- Exhaust stack or duct
The fan is mounted on the heating bell, after the recuperator, and travels with it each time the bell is moved by crane. Gas reaches it at about 250 to 350°C. It holds the space between bell and inner cover at slight suction, so flue gas does not escape at the bell seal. Some plants instead use a fixed exhaust fan with a connection at each base. Suction is typically 100 to 250 mmWC.
Gas at the ID fan
| Gas temperature, normal | 250 to 350°C after the bell recuperator |
| Maximum or upset | 450 to 550°C if the recuperator is fouled or bypassed |
| Dust at the fan | Very low; clean gas firing |
| Moisture | High with natural gas or coke oven gas |
| Mounting | On a crane-moved bell, with shock loads at landing |
| Density at the design temperature | About 0.62 kg/m³ at 300°C, 52% 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 bell annealing furnaces
| Blade form | Backward curved, compact for bell mounting |
| Material of construction | Heat-resisting steel or boiler quality plate for the impeller; mild steel casing |
| Drive and arrangement | Direct coupled, overhung, with a rigid frame on the bell |
| Bearings and cooling | Cooling disc and heat shield; bearings rated for shock loads during crane handling |
| Flow control | Variable speed or damper control on bell suction |
| Also specified | Plug-in electrical connection, lifting points, vibration-resistant mounting |
What this ID fan has to cope with
Shock loads during crane handling
The fan travels with the bell and is bumped at every landing. Bearings and mountings must withstand shock without brinelling, and the rotor balance must stay stable after handling. Bearings with suitable internal clearance, a rigid frame tied into the bell structure, and a balance check after any heavy landing keep the fan running smoothly through many moves a week.
Hot exhaust when the recuperator fouls
Fouling or failure of the bell recuperator sends hotter gas to the fan. The impeller and bearings must handle the upset temperature. The impeller and bearings must handle the upset temperature, not only the normal one. An inlet temperature alarm warns the operator, and recuperator cleaning at planned intervals keeps the fan in its normal range.
Seal leakage at the bell
If suction is too low, flue gas escapes at the bell seal; too high draws air in and cools the bell. Stable suction control keeps the seal tight. A variable speed drive or damper controlled from the measured suction under the bell keeps the seal tight through heat-up and soak, when the flue gas flow changes by a large factor.
Energy and control
Bell annealing cycles last many hours, with high firing at heat-up and low firing through the long soak. A variable speed exhaust fan holding bell suction cuts power during soak and avoids drawing cold air in at the bell seal. Keeping the bell recuperator clean holds both fuel use and fan inlet temperature down.
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 Forced Draft (FD) Fan for bell annealing furnaces.
Questions buyers ask
Where is the ID fan on a bell annealing furnace?
On gas-fired bell annealing furnaces the flue gas exhaust fan is usually mounted on the heating bell, after the bell recuperator. It moves with the bell between bases. Some plants use a fixed central fan with connections at each base. Each design has its own handling and connection needs.
What temperature does the bell furnace exhaust fan handle?
Typically 250 to 350°C after the recuperator, rising if the recuperator is fouled. The fan must be designed for the upset temperature. Upset temperatures of 450 to 550°C are possible if the recuperator is fouled or bypassed, so the impeller and bearing protection should be designed for that case, with an inlet temperature alarm.
Is the base convection fan the same as the ID fan?
No. The base convection fan circulates protective atmosphere inside the inner cover. The ID fan handles burner flue gas outside the inner cover. They are separate duties. The base fan runs inside a sealed hydrogen-rich atmosphere and is a specialised design supplied with the furnace base.
Information needed to quote an ID fan for bell 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.
- Bell design. Firing rate per bell, recuperator details and space available on the bell for the fan.
- Handling. Crane handling arrangements and electrical connection method.
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.