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 ID fan sits
- Cupola shaft
- Afterburner
- Recuperator
- Gas cooler
- Bag filter with lime injection
- ID fan
- Stack
The fan sits after the bag filter, so it handles cooled gas with only residual fume. Its suction covers the cupola top, the afterburner chamber, the recuperator tubes, the cooler, the filter at its dirty condition and the ducts, typically 250 to 450 mmWC. The suction at the charging door is the controlled variable: too little and CO and H₂S leak to the charging floor; too much and cold air enters, cools the afterburner and raises fuel use and gas volume.
Gas at the ID fan
| Gas temperature, normal | 130 to 180°C after the cooler and bag filter |
| Maximum or upset | Up to 250°C or more if the cooler or quench fails; filter protection usually bypasses or dilutes first |
| Dust at the fan | Low after the bag filter; fine oxide fume, coke fines and lime dust if bags leak |
| Moisture and dew point | Acid dew point raised by SO₂ and SO₃ from coke sulphur, typically 110 to 140°C |
| Corrosive species | SO₂, traces of H₂S if afterburning is incomplete, and fluorides from some slags |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 mineral wool cupolas
| Blade form | Backward curved, with a plain plate profile that tolerates light fume deposits |
| Material of construction | Mild steel or corrosion-resistant low-alloy steel, with stainless steel at the inlet where the gas runs close to its acid dew point |
| Drive and arrangement | Arrangement 3 or 8, direct driven through a coupling, with a variable speed drive |
| Bearings and cooling | Pedestal bearings with a cooling disc; no water cooling at normal temperatures |
| Flow control | Variable speed drive on charging door suction or afterburner pressure |
| Also specified | Insulated casing, casing drain, gas-tight shaft seal and a temperature trip linked to the cooler |
What this ID fan has to cope with
Acid corrosion at start-up and low load
When the cupola is banked or restarted the gas runs cooler than its acid dew point, and sulphuric acid condenses on the casing and impeller. Insulating the casing, keeping the filter outlet above about 130°C in normal running and using stainless steel at the inlet cone and backplate extend life between overhauls.
Temperature surges after a cooler trip
If the water quench or air cooler fails, gas at well above 250°C can reach the filter and fan. The fan is selected to survive a short excursion at its maximum temperature rating, with the dilution damper or bypass protecting the bags and the fan together.
CO at the charging floor
If the fan cannot hold suction as the filter loads, carbon monoxide escapes at the charging door, which is a serious hazard for operators. The fan duty is set at the dirty-filter condition with margin, and the suction is measured at the top of the shaft rather than at the fan.
Fume deposits on the blades
Fine fume with lime and sulphates can build up on the blades if bags leak or the gas cools. Regular inspection, a clean-out door and on-site balancing keep vibration within limits.
Energy and control
A cupola runs in long campaigns at a fairly steady melt rate, but the gas volume changes with the blast rate, the coke ratio and how much air leaks in at the charging door. A variable speed drive holding top suction avoids throttling losses and keeps tramp air low, which in turn cuts afterburner fuel. Closing up leaks at doors and expansion joints reduces the volume the fan must move.
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 Forced Draft (FD) Fan for mineral wool cupolas.
Questions buyers ask
Why does a rock wool cupola need an afterburner before the ID fan?
The top gas of a mineral wool cupola carries a high share of carbon monoxide and some hydrogen sulphide. The afterburner burns these to CO₂, water and SO₂ so they are not emitted, and the heat released is recovered in the recuperator to preheat the blast air.
What material should a cupola ID fan be made of?
Mild steel is usually adequate at normal temperatures after the filter, but the gas carries SO₂ and can reach its acid dew point at start-up. Stainless steel at the inlet cone and backplate, an insulated casing and a drain are common additions for a long life.
How much suction does a mineral wool cupola need at the charging door?
Enough to keep the top of the shaft just below atmospheric so CO cannot escape, usually a few mmWC. The ID fan must achieve this with the filter at its dirty pressure drop, so its duty includes a margin above clean conditions.
Information needed to quote an ID fan for mineral wool cupolas
- 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 sequence. Afterburner, recuperator, cooler type and bag filter, with the pressure drop of each and any lime or sorbent injection.
- Upset temperature. The highest gas temperature at the fan if the cooler or quench fails, and how long it lasts.
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.