How hot blast cupolas work
A hot blast cupola melts iron with blast air preheated to about 450 to 600°C, which raises flame temperature, lowers coke consumption and allows more steel scrap in the charge. The top gas is drawn off below the charging door through an offtake ring and burnt with air in a combustion chamber. The hot combustion gas then heats the blast in a recuperator before passing a cooler, a bag filter or scrubber, and the ID fan. Large hot blast cupolas run long campaigns, often for weeks, with water-cooled shells and liningless shafts in some designs. Because the top gas is a fuel source for the recuperator, the gas offtake, combustion and blast preheat form a single linked system.
Where the ID fan sits
- Shaft and offtake ring
- Combustion chamber
- Recuperator
- Gas cooler
- Bag filter
- ID fan
- Stack
The fan sits at the end of the line after the bag filter, handling cleaned gas at 120 to 180°C. Upstream are the combustion chamber, where the top gas burns at 800 to 1,000°C, the recuperator, which takes the gas down to about 400 to 600°C, and a cooler. The fan must provide the suction for the whole chain, typically 300 to 600 mmWC, and hold a small negative pressure at the offtake ring.
Gas at the ID fan
| Gas temperature, normal | 120 to 180°C after the cooler and bag filter |
| Maximum or upset | 250 to 300°C if the cooler fails or the recuperator is bypassed |
| Dust at the fan | Below 50 mg/Nm³ after the bag filter |
| Dust character upstream | Coke breeze, silica, iron oxide and zinc oxide fume |
| Corrosive species | SO₂ from coke sulphur; matters at cold starts below acid dew point |
| 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 hot blast cupolas
| Blade form | Backward curved, for efficiency on long campaigns with clean gas |
| Material of construction | Mild steel; wear liners on leading edges as protection against bag failures |
| Drive and arrangement | Direct drive, arrangement 8 |
| Bearings and cooling | Pedestal bearings with heat slinger |
| Flow control | Variable speed drive on offtake pressure |
| Also specified | Vibration monitoring and online bearing temperature for continuous campaigns |
What this ID fan has to cope with
Offtake suction balance
Too much suction pulls air down through the charge to the offtake, burning coke and oxidising metal in the upper shaft and, in extreme cases, starting a fire at the top. Too little lets gas escape at the charging door. The fan must hold a narrow offtake pressure band as the blast rate changes.
Recuperator and filter protection
If combustion in the chamber falters, unburnt CO reaches the recuperator or filter, and if it burns there it overheats them. Burner support in the combustion chamber, temperature trips and a fan rated for the upset temperature protect the system.
Long campaigns without stops
A hot blast cupola may run for weeks without a stop, and an ID fan failure forces the cupola to bank or shut down. The fan must run for the whole campaign without maintenance, so bearing life, online vibration monitoring and balancing quality matter more here than on a cold blast cupola melting a few hours each day.
Energy and control
A hot blast cupola recovers heat from its own top gas, so the ID fan is part of the energy system: correct offtake suction gives enough fuel to the combustion chamber and a steady blast temperature. Holding suction with a speed-controlled fan rather than a damper saves power and prevents the air ingress that dilutes the top gas and lowers its calorific value.
ID fan and FD fan in hot blast cupolas
| ID fan | FD fan | |
|---|---|---|
| Position | After the cooler and bag filter | Before the recuperator |
| What it handles | Burnt top gas, cleaned | Ambient air for the blast |
| Temperature at the fan | 120 to 180°C | Ambient |
| Main design concern | Offtake suction balance | Steady blast for weeks |
| Control | Offtake pressure | Measured blast volume |
See also the Forced Draft (FD) Fan for hot blast cupolas.
Questions buyers ask
What does the ID fan do on a hot blast cupola?
It draws the top gas from the offtake below the charging door, through the combustion chamber, recuperator, cooler and bag filter, and sends it to the stack. It also controls the pressure at the offtake, which affects how much air leaks into the shaft and how well the top gas burns.
What is the difference between a hot blast and cold blast cupola fan system?
In a cold blast cupola the ID fan only cleans and exhausts the top gas. In a hot blast cupola the same gas fuels the recuperator that heats the blast, so the ID fan suction affects blast temperature and coke rate as well as emissions.
What temperature does a hot blast cupola ID fan handle?
After the recuperator and cooler, the gas reaches the bag filter and fan at about 120 to 180°C. The fan should be rated for 250 to 300°C to cover cooler or recuperator upsets during a long campaign.
Information needed to quote an ID fan for hot blast 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.
- Offtake and combustion. Offtake design, combustion chamber burner support and the target offtake pressure.
- Recuperator. Gas temperature before and after the recuperator and blast temperature.
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.