How aluminium billet homogenising furnaces work
Cast aluminium billets are homogenised before extrusion by soaking them at about 550 to 600°C for several hours, then cooling them at a controlled rate. Batch furnaces take a load of billets on a car or in baskets; continuous furnaces move billets through heating, soaking and cooling sections. Heat is supplied by gas burners firing into a recirculating air stream, by indirect radiant tubes or by electric heaters, and large high temperature circulation fans drive the hot air across the billets. Products of combustion and excess air are vented through an exhaust fan or stack. The cooling chamber then uses large air fans, sometimes with water mist, to cool the billets uniformly.
Where the ID fan sits
- Billet load
- Recirculation fans and heating chamber
- Exhaust outlet
- Recuperator or dilution (if fitted)
- Exhaust (ID) fan
- Stack
The exhaust fan takes gas from the heating chamber, usually through a damper that sets how much of the recirculating atmosphere is vented. On direct-fired furnaces it removes combustion products at the rate they are made, and on indirect radiant tube furnaces it serves the radiant tube flue system. Suction is low, typically 50 to 150 mmWC, and the gas is hot unless a recuperator or dilution air brings it down.
Gas at the ID fan
| Gas temperature, normal | 400 to 550°C at the fan |
| Maximum or upset | Up to 620°C at soak temperature without dilution |
| Dust at the fan | Very low; traces of oxide |
| Moisture and dew point | Combustion moisture; no condensation at fan temperature |
| Corrosive species | Minimal with natural gas firing |
| Density at the design temperature | About 0.47 kg/m³ at 480°C, 39% 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 aluminium billet homogenising furnaces
| Blade form | Backward curved or radial blade, for clean hot gas |
| Material of construction | Heat-resisting or stainless steel impeller and inner casing; insulated outer casing |
| Drive and arrangement | Direct coupled, arrangement 8, casing on sliding supports |
| Bearings and cooling | Shaft cooling disc and heat shield; water or oil-cooled bearings at the upper temperatures |
| Flow control | Variable speed drive or exhaust damper on furnace pressure |
| Also specified | Slow rotation after shutdown, insulated inlet box and flexible duct connections |
What this ID fan has to cope with
Shaft bow after a hot stop
If the fan stops while full of gas at 500°C, the shaft heats unevenly and bows. On restart the fan vibrates heavily. Running the fan at low speed until the gas cools, or turning gear on larger units, prevents this. The sequence is built into the furnace controls so it happens at every stop.
Creep at soak temperature
Steel loses strength rapidly above about 450°C. The impeller is designed for stress at the maximum soak temperature using heat-resisting steel, not mild steel. Overheating during an over-temperature fault shortens impeller life. The stress check covers both normal running and any over-temperature fault the furnace controls permit. Inspection for blade distortion at each major stop shows whether the impeller is creeping.
Over-venting wastes heat
If the exhaust fan vents more atmosphere than needed, fresh air enters and must be heated to soak temperature. Pressure-based control, rather than a fixed damper, keeps venting to the minimum needed for combustion products. Venting is set during commissioning with oxygen measurement in the chamber, and checked whenever burners or seals are changed, so that heat loss stays at the minimum.
Energy and control
Homogenising is energy intensive, and most of the energy goes into heating billets and the furnace. The exhaust carries heat out, so venting the minimum and recovering heat from the exhaust to preheat combustion air are where the savings lie. A variable speed drive on the exhaust fan, controlled by furnace pressure, avoids over-venting during the soak.
ID fan and FD fan in aluminium billet homogenising furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | On the furnace exhaust outlet, before the stack | Outside the casing, feeding the burner header |
| What it handles | Hot furnace atmosphere and combustion products | Ambient air |
| Temperature at the fan | 400 to 550°C | Ambient |
| Main design concern | Heat-resisting construction and shaft bow | Turndown between heat-up and soak |
| Control | Furnace pressure | Header pressure and burner ratio |
See also the Forced Draft (FD) Fan for aluminium billet homogenising furnaces.
Questions buyers ask
What is the exhaust fan on a homogenising furnace for?
It vents the products of combustion and a small share of the recirculating hot atmosphere so that the burners can keep firing and the furnace pressure stays steady. It is the induced draft fan of the furnace, separate from the large recirculation fans inside it.
What material is used for a homogenising furnace exhaust fan?
At 400 to 550°C mild steel is not strong enough for a high speed impeller. Heat-resisting or stainless steel is used for the impeller and inner casing, with shaft cooling and heat shields to protect the bearings. Bearing temperature monitoring is advisable at the upper end of this range.
Is the exhaust fan the same as the recirculation fan?
No. Recirculation fans move very large volumes of hot air across the billets inside the furnace. The exhaust fan handles a much smaller volume, removing combustion products from the system. Both are high temperature fans, but they are selected differently. Their materials and bearings differ accordingly.
Information needed to quote an ID fan for aluminium billet homogenising 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.
- Furnace type. Batch or continuous, direct-fired or radiant tube, and the soak temperature.
- Exhaust volume. Burner firing rate and the vented volume at soak and heat-up.
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.