How mineral wool cupolas work
A mineral wool cupola is a coke-fired shaft furnace charged with basalt, diabase, dolomite and slag or cement-bonded briquettes made from recycled wool and fines. Coke burns at the tuyeres in a blast of air, often preheated to 400 to 600°C, and the melt leaves at about 1,450 to 1,550°C for the spinning machine. The top gas leaves the charge at roughly 150 to 300°C and is rich in carbon monoxide with hydrogen sulphide from the coke and slag, so it is burnt in an afterburner at 800°C or more. The hot clean-burnt gas passes a recuperator that heats the blast air, then a gas cooler or quench, a bag filter, often with lime injection for SO₂, the ID fan and the stack. The blast air comes from a high-pressure FD fan.
Where the FD fan sits
- Air intake and filter
- FD (blast) fan
- Recuperator
- Hot blast main
- Bustle pipe
- Tuyeres
- Cupola shaft
The blast fan takes ambient air and pushes it through the recuperator tubes, the hot blast main and the bustle pipe to the tuyeres. Placing it ahead of the recuperator keeps it cool and compact. Its pressure covers the recuperator, the hot blast valves and the resistance of the charge, which rises as the coke and stone column fines up; blast pressures in the range of several hundred to over a thousand mmWC are typical.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the recuperator | Typically 400 to 600°C at the tuyeres, downstream of the fan |
| Dust | Clean ambient air, filtered at the intake |
| Pressure at the fan outlet | High; set by the charge resistance, recuperator and blast main |
| Oxygen enrichment | Some cupolas add oxygen to the blast, which changes the air volume needed for a given melt rate |
| 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 mineral wool cupolas
| Blade form | Backward curved or radial-tipped high-pressure wheel, chosen for a steep curve that holds flow as charge resistance changes |
| Material of construction | Mild steel; the fan handles cold air |
| Drive and arrangement | Direct driven, with a variable speed drive |
| Bearings and cooling | Standard pedestal bearings; no cooling required |
| Flow control | Variable speed drive on measured blast flow |
| Also specified | Inlet silencer, outlet non-return damper and a pressure relief to protect the recuperator |
What this FD fan has to cope with
Blast falls as the charge tightens
As fines build up in the shaft the resistance rises, and a fan with a flat curve loses flow, which drops the melt temperature. A fan selected with a steep pressure curve and a flow-controlled variable speed drive holds the blast steady.
Recuperator protection
If the blast stops while the recuperator is hot, the tubes overheat. The blast fan is interlocked with the afterburner and recuperator bypass, and a standby or emergency air supply is often provided for controlled shutdown.
Noise
A high-pressure blast fan running continuously is loud at the intake and the casing, and the melting floor is already a noisy area. An inlet silencer, acoustic lagging on the casing and a flexible connection to the blast main are normally part of the supply.
Energy and control
The blast fan runs for the whole campaign and is one of the larger motors in the melting area. Flow control by speed rather than by a throttle valve saves a large share of its power, because the charge resistance, not the fan, should set the pressure. Recovering more heat to the blast in the recuperator lowers coke consumption without changing the fan duty much.
ID fan and FD fan in mineral wool cupolas
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | Before the recuperator, feeding the tuyeres |
| What it handles | Afterburnt top gas with SO₂ and fine fume | Ambient blast air |
| Temperature at the fan | 130 to 180°C | Ambient |
| Main design concern | Acid dew point and CO-safe suction | High pressure and steady blast |
| Control | Charging door suction | Blast flow |
See also the Induced Draft (ID) Fan for mineral wool cupolas.
Questions buyers ask
What pressure does a cupola blast fan need?
It depends on the shaft height, the charge size and the recuperator, but mineral wool cupolas usually need several hundred to over a thousand mmWC at the fan outlet. The duty should be set at the highest charge resistance expected, with the variable speed drive trimming it back in normal running.
Is the cupola FD fan the same as a combustion air fan?
Yes. On a cupola the forced draft fan supplies the blast air that burns the coke at the tuyeres. It is the combustion air fan of the furnace, but at much higher pressure than a burner fan because the air passes through the charge.
Why is the blast air preheated after the fan, not before?
A fan handling cold air is smaller, cheaper and needs no high temperature construction. Pushing cold air through the recuperator puts all the hot parts on the pressure side of a cool fan.
Information needed to quote an FD fan for mineral wool cupolas
- 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 duty. Blast flow and pressure at the tuyeres across the melt rate range, with any oxygen enrichment.
- Recuperator. Air side pressure drop and the protection philosophy if the blast is lost.
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.