How ferro alloy furnace waste heat boilers work
Ferro alloys such as ferrosilicon, silicomanganese, ferromanganese and ferrochrome are produced in submerged arc furnaces, where electrodes heat a charge of ore, coke or coal and fluxes. On open and semi-closed furnaces, CO rising from the charge burns with air drawn under the hood, and the off-gas leaves at about 500 to 800°C, carrying fine fume: amorphous silica on ferrosilicon and silicon metal, manganese oxide on manganese alloys. A waste heat boiler cools this gas to about 180 to 220°C before a bag filter, where the silica fume is collected as microsilica. On closed furnaces the CO-rich gas is cleaned wet and used as fuel, and a waste heat boiler is not usually fitted.
Where the ID fan sits
- Submerged arc furnace hood
- Off-gas duct
- Waste heat boiler
- Spark arrestor or cooler
- Bag filter
- ID fan
- Stack
On a suction bag house the fan sits after the filter and handles cleaned gas at about 160 to 200°C. On a pressure bag house, common in microsilica collection, the fan sits before the filter and handles the full fume load. Either way, it sets the suction at the furnace hood, which controls how much air enters and therefore the gas temperature and volume reaching the boiler.
Gas at the ID fan
| Gas temperature, normal | 160 to 200°C after the boiler |
| Maximum or upset | Up to 300°C or more during furnace blows or boiler bypass, often limited by dilution air |
| Dust at the fan | Very fine silica or manganese oxide fume; low after a suction bag house, several g/Nm³ before a pressure bag house |
| Moisture and dew point | Low; gas is mostly air with CO₂ from burned CO |
| Volume variation | Large swings with hood air ingress, furnace blows and charging |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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 ferro alloy furnace waste heat boilers
| Blade form | Backward curved on a suction bag house; radial or backward curved with deposit-shedding profile on a pressure bag house |
| Material of construction | Mild steel; wear protection where manganese oxide or ferrochrome dust is abrasive |
| Drive and arrangement | Direct coupled, arrangement 8, double inlet on large furnaces |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on hood suction or boiler inlet temperature |
| Also specified | Inspection doors for cleaning fume deposits, casing drain and vibration monitoring |
What this ID fan has to cope with
Fine fume builds up on the blades
Silica fume is sub-micron and spherical, so it does not erode much, but it packs onto blade surfaces and inside the casing, especially before a pressure bag house. Deposits break off unevenly and cause vibration. A blade profile that sheds fume, cleaning access and in-situ balancing keep the fan running.
Hood suction against boiler steam
More hood suction draws in more air, which cools the gas and lowers the boiler's steam output but captures fume better. The fan control must find the balance the plant wants, usually holding hood suction within a band set for emission capture while the boiler takes what heat it can.
Furnace blows and hot surges
Furnaces blow when gas breaks through the charge, sending flame and hot gas to the hood. The boiler inlet temperature rises suddenly, and the fan sees a short surge in temperature and volume. Dilution air dampers and impeller stress design at the upset temperature protect the fan.
Energy and control
Ferro alloy furnaces run continuously but with variable gas flow from charging, stoking and blows. A fan running at fixed speed on a damper wastes power in the long periods of moderate flow. Speed control from hood suction or boiler inlet temperature cuts power and improves capture. Reducing excessive hood air also raises gas temperature to the boiler and improves steam output.
Questions buyers ask
Where is the ID fan placed on a ferro alloy furnace bag house?
It depends on the bag house. With a suction bag house, the fan is after the filter and sees cleaned gas. With a pressure bag house, common for microsilica collection, the fan is before the filter and handles the full fume load, so it is designed for deposits.
Why does a ferro alloy waste heat boiler produce less steam when hood suction increases?
More suction draws more cold air into the hood, cooling the gas and increasing its volume. The boiler receives cooler gas, so steam output falls, and the fan uses more power. The plant must balance fume capture against heat recovery. Speed control from hood suction helps find that balance.
Does a closed ferro alloy furnace use a waste heat boiler?
Usually not. A closed furnace produces CO-rich gas that is cleaned in a wet scrubber and used as fuel. Waste heat boilers are fitted mostly on open and semi-closed furnaces, where the CO burns at the hood and the gas leaves hot. Gas fans on closed furnaces are a separate duty.
Information needed to quote an ID fan for ferro alloy furnace waste heat boilers
- 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 and alloy. Furnace rating, alloy produced, hood type and the measured off-gas flow and temperature.
- Bag house type. Whether the bag house is suction or pressure type, and its design pressure drop.
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.