How ladle furnace secondary fume extraction work
A ladle furnace reheats and refines steel after tapping, using three graphite electrodes through a water-cooled roof while argon stirs the bath through a porous plug. Lime, fluorspar, alloys and carbon are added through the roof. Arcing, stirring and additions release fume and dust: lime and calcium compounds, iron and manganese oxides, fluorides and graphite. The fume is captured at the roof, through a gap or a dedicated suction port, and by a canopy over the station, and is ducted to a pulse jet bag filter. At the roof the gas can be 400 to 800°C, but so much ambient air is drawn in that the mixture reaches the filter at about 80 to 130°C. On many melt shops the LF shares the EAF fume system; on others it has its own filter and ID fan.
Where the ID fan sits
- LF roof suction
- Canopy hood
- Spark arrester or drop-out box
- Bag filter
- ID fan
- Stack
The fan is on the clean side of the bag filter, so it sees only residual dust, typically under 30 to 50 mg/Nm³. The suction it works against is the sum of the roof port and canopy capture, long ducts across the melt shop, the spark arrester and the filter, often 250 to 400 mmWC. Dampers split the flow between roof and canopy, and on shared systems between several stations.
Gas at the ID fan
| Gas temperature, normal | 80 to 130°C at the filter outlet |
| Maximum or upset | 150 to 180°C during heavy arcing with low dilution |
| Dust at the fan | Low after the bag filter; fine lime and oxide particles |
| Dust character upstream | Hygroscopic lime-based dust that cakes in humid conditions |
| Corrosive species | Fluorides from fluorspar additions; minor |
| Density at the design temperature | About 0.90 kg/m³ at 120°C, 75% 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 ladle furnace secondary fume extraction
| Blade form | Backward curved aerofoil or plate, for efficiency on clean high volume duty |
| Material of construction | Mild steel; wear liner on the leading edges if filter bags fail often |
| Drive and arrangement | Direct drive, arrangement 8, double inlet on large volumes |
| Bearings and cooling | Pedestal bearings, no special cooling at this temperature |
| Flow control | Variable speed drive linked to LF operating state |
| Also specified | Vibration monitoring, inlet and outlet dampers for isolation |
What this ID fan has to cope with
Wear after bag failures
A torn bag lets lime and oxide dust straight to the fan, and lime dust settles on the blades and abrades the leading edges. Broken bag detection and a fan with replaceable wear strips limit the damage.
Dust caking in the monsoon
Lime-rich dust picks up moisture when the filter runs cool or idle, cakes on bags and hoppers and raises the pressure drop. The fan then cannot hold capture at the roof. Keeping the filter warm and sizing the fan for a loaded filter are the design response.
Poor capture during additions
Alloy and flux additions, and arc start under a fresh slag, give sudden bursts of fume that spill from the roof gap into the crane bay. A fan with enough margin and a drive that ramps up as additions start, signalled from the LF controls, keeps the melt shop clear without running at full speed all the time.
Energy and control
The LF arcs for only part of each heat; the rest is waiting, sampling and stirring. A fan running at full speed throughout spends much of its power moving clean air. A variable speed drive linked to the LF signals, at full speed during arcing and additions and lower in between, typically cuts fan energy considerably on secondary fume systems.
Questions buyers ask
What does an ID fan on a ladle furnace fume system handle?
It handles the cleaned air and fume leaving the bag filter, at about 80 to 130°C with very little dust. The volume is large because much ambient air is drawn in at the roof and canopy to capture the fume and cool it.
Can the ladle furnace use the EAF fume extraction fan?
Many melt shops connect the LF to the EAF secondary or direct evacuation system with dampers, sharing the bag filter and ID fan. A dedicated LF system is used when the EAF system has no spare capacity or the LF is far from it.
What fan pressure is needed for LF fume extraction?
Typically 250 to 400 mmWC, made up of the roof and canopy losses, duct runs across the melt shop, spark arrester and bag filter. The exact figure depends on duct length and the filter design pressure drop, and the fan should be selected at the dirty filter condition, not the clean one.
Information needed to quote an ID fan for ladle furnace secondary fume extraction
- 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.
- Capture points. Roof suction design, canopy size and whether the system also serves the EAF or other stations.
- Operating cycle. Arcing time per heat, additions practice and the signal available for fan speed control.
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.