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 FD fan sits
- Inlet filter
- FD fan (cupola blower)
- Blast main
- Wind box
- Upper and lower tuyeres
- Coke bed
The blower sits near the cupola and delivers air through a blast main to the wind box surrounding the shaft. In a divided blast cupola a damper splits the blast between the upper and lower rows of tuyeres. The blower pressure covers the coke bed and burden resistance plus the tuyere and wind box losses, typically 400 to 1,200 mmWC depending on shaft diameter and charge height.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air at the tuyeres | Ambient on a cold blast cupola |
| Dust | Foundry dust and sand near the cupola; inlet filter recommended |
| Pressure at the fan outlet | Typically 400 to 1,200 mmWC |
| Load pattern | Steady blast during melting, varying back pressure as the burden settles |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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. |
FD Fan selection for foundry cupola furnaces
| Blade form | Backward curved or radial, chosen for a steep pressure-volume curve |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled at two-pole speed for higher pressures |
| Bearings and cooling | Standard bearings, no cooling |
| Flow control | Variable speed drive on measured blast volume |
| Also specified | Blast volume meter, non-return damper, and an outlet silencer if required |
What this FD fan has to cope with
Blast volume falls as the burden packs
A fan with a flat pressure curve gives less air as the burden resistance rises, and melt rate and metal temperature drop just when they are needed. A steep curve, or a speed-controlled fan holding measured volume, keeps the blast constant through the melt.
Back flow of gas at stop
When the blower stops with the cupola full, CO-rich gas can flow back through the tuyeres into the wind box and blast main, where it can ignite on restart. A non-return damper and purge or open tuyere peep holes before stopping prevent this.
Wrong split in a divided blast cupola
The upper and lower tuyere rows need a set share of the blast. A blower with too little pressure margin cannot hold that split as the burden resistance changes. Margin in the fan and separate measurement on each row solve it.
Energy and control
The cupola blower runs only during melting but at fairly high pressure, and the blast volume directly sets coke consumption. A blower controlled on measured volume rather than on a fixed damper avoids over-blowing, which burns extra coke and oxidises silicon and manganese. A divided blast arrangement with correct air split is one of the most effective ways to reduce coke rate in a cold blast cupola.
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 Induced Draft (ID) Fan for foundry cupola furnaces.
Questions buyers ask
What pressure does a cupola blower need?
It depends on shaft diameter, charge height and coke size. Small cupolas may need 400 to 600 mmWC, larger ones with tall burdens up to about 1,200 mmWC. The blower is selected for the maximum burden resistance with a margin, so volume does not fall during the melt.
Is a centrifugal fan or a roots blower better for a cupola?
Both are used. A roots blower gives nearly constant volume against changing pressure. A centrifugal blower with a steep curve and speed control on measured volume does the same with lower noise and maintenance, and is common on medium and large cupolas.
How much air does a cupola need?
Blast volume is usually set in proportion to shaft cross-sectional area and the desired melt rate, and must match the coke charge. The foundry metallurgist sets it; the blower is then sized for that volume at the maximum burden pressure.
Information needed to quote an FD fan for foundry cupola furnaces
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Blast requirement. Shaft diameter, blast volume, and pressure at the wind box.
- Tuyere arrangement. Single row or divided blast, and the required split between rows.
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.