How foundry cupola furnaces work
A foundry cupola is a vertical shaft charged from the top with alternate layers of pig iron, steel scrap, returns, foundry coke and limestone. Cold air from a blower enters through tuyeres near the base, burns the coke and melts the metal, which collects in the well and is tapped continuously or in batches. Many Indian foundries use divided blast cupolas with two rows of tuyeres to improve coke economy. The top gas is rich in CO, typically 10 to 18%, and carries coke breeze, sand, metal oxides and fume. In a below-door or above-door offtake, it is burnt with air, cooled by dilution, water spray or a heat exchanger, and cleaned in a venturi scrubber or bag filter before the ID fan and stack. Melting runs in campaigns of a few hours per day.
Where the ID fan sits
- Cupola shaft
- Charging door offtake
- Afterburner (if fitted)
- Quench or cooler
- Venturi scrubber or bag filter
- ID fan
- Stack
The fan is placed after the collector. With a venturi scrubber and separator, it sees saturated gas at 60 to 80°C carrying droplets and fine sludge. With a bag filter the gas must first be cooled, by dilution air, spray or air cooler, to 120 to 180°C, and the fan sees clean, dry gas. The suction it provides covers the offtake, the cooler and the collector; a venturi scrubber alone can need 500 to 900 mmWC.
Gas at the ID fan
| Gas temperature, normal | 60 to 80°C after a wet scrubber; 120 to 180°C after a bag filter |
| Maximum or upset | 250°C or more if the spray or dilution fails, especially at burn-down |
| Dust at the fan | Wet sludge carry-over after a scrubber; under 50 mg/Nm³ after a bag filter |
| Dust character upstream | Coke breeze, silica sand, iron oxide and zinc oxide from galvanised scrap |
| Moisture and corrosion | Saturated and mildly acidic after a scrubber, from SO₂ in the coke |
| Density at the design temperature | About 0.86 kg/m³ at 140°C, 72% 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 foundry cupola furnaces
| Blade form | Radial or radial tipped after a scrubber, to shed sludge; backward curved after a bag filter |
| Material of construction | Stainless steel or coated mild steel for wet duty; mild steel with wear liners for dry duty |
| Drive and arrangement | Belt or direct drive, arrangement 8 or 9 |
| Bearings and cooling | Pedestal bearings, heat slinger on dry duty |
| Flow control | Variable speed drive or inlet damper, with reduced speed between melts |
| Also specified | Water wash nozzles and casing drain on wet duty, inspection doors on both |
What this ID fan has to cope with
Sludge build-up after a wet scrubber
Fine dust that passes the separator sticks to the blades as a wet cake, dries during idle hours and breaks off unevenly when the fan restarts. Vibration rises fast. Radial blades that shed deposits, a wash spray before shutdown and a good mist eliminator keep the fan running smoothly.
Hot gas at burn-down
At the end of a melt the cupola is emptied and the burden burns down, sending hotter gas with less dilution to the collector. A bag filter needs protection by a temperature-controlled dilution damper, and the fan must be rated for this peak, not only for normal melting.
Acid corrosion
Coke sulphur gives SO₂ that dissolves in scrubber water and lowers its pH. Mild steel fans after a scrubber corrode quickly in this condition. Stainless steel construction or effective coating, plus pH control of the scrub water, protect the fan.
Fumes at the charging door
If the fan cannot hold the door under suction while charging, CO-bearing gas and dust escape onto the platform where the chargers work. The fan is sized with the charging door open and the large volume of air it admits, not just the shaft gas, and the drive is set to speed up during charging.
Energy and control
Cupolas melt in campaigns of a few hours a day, with a long burn-down and idle period. The ID fan should run only as fast as needed: high during melting, lower during burn-down and stopped or at a minimum when the cupola is idle. A venturi scrubber spends a large share of its fan power on pressure drop, so a bag filter often uses less fan energy for the same cleaning.
ID fan and FD fan in foundry cupola furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the scrubber or bag filter | Feeding the wind box and tuyeres |
| What it handles | Burnt top gas with dust or sludge | Ambient air |
| Temperature at the fan | 60 to 80°C wet, 120 to 180°C dry | Ambient |
| Main design concern | Sludge, acid and charging door suction | Constant blast against changing burden |
| Control | Charging door suction | Measured blast volume |
See also the Forced Draft (FD) Fan for foundry cupola furnaces.
Questions buyers ask
What ID fan is needed for a cupola wet scrubber?
A fan that handles saturated, slightly acidic gas with sludge carry-over, usually radial or radial tipped blades in stainless steel or coated mild steel, at a pressure high enough for the venturi scrubber, often 500 to 900 mmWC in total. A wash system and drain are normally included.
Can a cupola use a bag filter instead of a scrubber?
Yes, if the top gas is first burnt and cooled to about 120 to 180°C. The bag filter gives cleaner gas and a smaller fan pressure, but it needs reliable cooling and spark protection because hot particles and unburnt CO reach it otherwise.
Why does the cupola ID fan need to be variable speed?
The gas volume changes between charging, melting and burn-down, and the charging door suction must be held throughout. A variable speed drive matches the fan to each stage and saves energy between melts, instead of throttling a damper and running at full speed.
Information needed to quote an ID fan for foundry cupola 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.
- Cupola size and melt rate. Shaft internal diameter, melt rate in tonnes per hour and campaign hours per day.
- Gas cleaning. Wet scrubber or bag filter, with type of cooling and any afterburner.
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.