How closed submerged arc furnaces for ferro alloys work
In a closed submerged arc furnace the roof seals the furnace so that the CO-rich gas from the reduction reactions is collected without burning. The gas is typically 70 to 85% CO with some hydrogen and CO₂, and its volume is a small fraction of that from an open furnace. It leaves the cover at 400 to 800°C, carrying fine dust and fume, and is cleaned either wet, in venturi scrubbers or disintegrators that cool it to 30 to 60°C, or dry, in high temperature filters. A gas blower or exhaust (ID) fan after the cleaning section holds the furnace under the cover at a set pressure and sends the clean gas to a flare or to users such as preheating kilns, sinter plants and power generation.
Where the ID fan sits
- Furnace cover
- Raw gas duct
- Venturi scrubber or dry filter
- Mist eliminator
- Gas (ID) fan
- Flare or gas users
The fan sits after the gas cleaning plant, so it handles cleaned gas: wet and saturated at 30 to 60°C after a scrubber, or dry and warm after a hot filter. It works against the losses of the raw gas duct, scrubber and mist eliminator, and it holds the cover pressure close to zero or slightly positive, so that air is not drawn in to form an explosive mixture under the cover.
Gas at the ID fan
| Gas temperature, normal | 30 to 60°C, saturated, after wet cleaning |
| Maximum or upset | Higher if scrubber water fails; protected by temperature trip and bypass to flare |
| Dust at the fan | Low after cleaning, but wet sludge and carry-over droplets are present |
| Gas composition | 70 to 85% CO, some H₂, combustible and toxic |
| Moisture | Saturated with entrained droplets after the scrubber |
| Density at the design temperature | About 1.08 kg/m³ at 55°C, 90% 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 closed submerged arc furnaces for ferro alloys
| Blade form | Backward curved or radial, depending on pressure and deposit tendency |
| Material of construction | Stainless steel or coated mild steel for wet, slightly acidic gas |
| Drive and arrangement | Direct drive with a gas-tight casing and shaft seal suitable for CO |
| Bearings and cooling | Standard bearings; shaft seal with inert gas purge |
| Flow control | Variable speed drive on furnace cover pressure |
| Also specified | Water wash nozzles, casing drains with seal pots, and earthing for static charge |
What this ID fan has to cope with
CO leakage at the shaft seal
A leaking shaft seal releases carbon monoxide into the fan house, where it is lethal at low concentrations. Gas-tight casings, labyrinth or carbon ring seals with nitrogen purge, and CO detectors in the area are standard on this duty.
Sludge and mist deposits
Fine dust carried through the scrubber as droplets forms a sticky sludge on the impeller and unbalances it. A good mist eliminator ahead of the fan, water wash nozzles in the casing and drains keep the rotor clean.
Air ingress and explosion risk
If the fan pulls the cover negative, air enters the furnace and can form a combustible mixture with the CO. The fan must hold the cover pressure within a narrow band, with interlocks that open the flare and stop the fan on oxygen rise.
Energy and control
A closed furnace gives a small, steady volume of gas compared with an open furnace, so the gas fan is much smaller. The energy question is less about fan power and more about using the gas: its calorific value can heat charge preheating kilns, sinter or a boiler. A fan that holds steady pressure to the users, rather than flaring, is what makes recovery possible.
Questions buyers ask
What does the ID fan do on a closed submerged arc furnace?
It is the gas blower that draws the CO-rich furnace gas through the cleaning plant and sends it to the flare or gas users. It also controls the pressure under the furnace cover, keeping air out of the closed furnace.
What material is used for a furnace gas blower after a wet scrubber?
The gas after the scrubber is saturated, often slightly acidic, and carries fine sludge. Stainless steel impellers and casings, or mild steel with a protective coating, are used, with drains, seal pots and wash nozzles to deal with deposits. The casing and shaft seal must also be gas tight because the gas is mainly carbon monoxide.
Why is furnace gas from a closed furnace hazardous?
It is mostly carbon monoxide, which is toxic and flammable. Any leakage from the fan or ducting can harm people, and air drawn into the system can form an explosive mixture. Gas-tight design and interlocks are therefore essential.
Information needed to quote an ID fan for closed submerged arc furnaces for ferro alloys
- 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.
- Gas cleaning. Wet or dry cleaning, gas temperature and moisture at the fan inlet.
- Gas use. Whether gas is flared or used, and the pressure needed at the users.
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.