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 FD fan sits
- Inlet filter
- FD fan (blast fan)
- Recuperator
- Hot blast main
- Bustle pipe
- Tuyeres
The fan is placed before the recuperator, so it handles cold air and stays compact; putting it after the recuperator would mean a hot, large fan. Its pressure covers the recuperator air side, the insulated hot blast main and bustle pipe, the tuyeres and the burden, typically 800 to 1,500 mmWC on larger hot blast cupolas. A separate combustion air fan usually supplies the air for burning the top gas in the combustion chamber.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 450 to 600°C, downstream of the fan |
| Dust | Filtered air |
| Pressure at the fan outlet | Typically 800 to 1,500 mmWC |
| Campaign | Continuous for days or weeks |
| 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 hot blast cupolas
| Blade form | Backward curved, high pressure, with a steep curve near duty |
| Material of construction | Mild steel, high strength plate where tip speed is high |
| Drive and arrangement | Direct coupled at two-pole speed |
| Bearings and cooling | Standard bearings with temperature monitoring |
| Flow control | Variable speed drive on measured blast volume |
| Also specified | Standby blower or quick changeover, non-return damper, inlet silencer |
What this FD fan has to cope with
Loss of blast during a campaign
If the blast stops, the tuyeres can freeze and the recuperator overheats because no air cools it while hot gas keeps flowing. A standby blower, an emergency recuperator bypass and interlocks with the combustion chamber are usual on hot blast cupolas.
Recuperator leaks
A leak on the recuperator lets blast air into the flue gas side. The blower then delivers more than reaches the tuyeres, and the ID fan sees extra volume. Monitoring blast volume at both the fan and the tuyeres reveals leaks early.
Blast temperature swings
If the blast fan cannot hold constant volume as the burden resistance changes, the air velocity through the recuperator changes, its outlet temperature swings, and tapping temperature and carbon pick-up follow. Speed control on measured volume, with enough pressure margin over the highest burden resistance, keeps the blast and the metal steady.
Energy and control
Preheating the blast is what saves coke in a hot blast cupola, and that only works when blast volume and recuperator gas flow are both steady. A speed-controlled blast fan on measured volume avoids over-blowing and gives a stable blast temperature. Its power is significant at these pressures, so an efficient backward curved fan repays over a long campaign.
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 Induced Draft (ID) Fan for hot blast cupolas.
Questions buyers ask
Does the hot blast cupola FD fan handle hot air?
No. The blast fan pushes ambient air into the recuperator, and the air is heated downstream of the fan. This keeps the fan small and avoids high temperature construction. Only the hot blast main and tuyeres see the preheated air.
What blast pressure does a hot blast cupola need?
Higher than a cold blast cupola of the same size, because the recuperator and hot blast main add resistance. Pressures of about 800 to 1,500 mmWC at the fan are common. The exact figure depends on shaft size, burden height and recuperator design.
Why does a hot blast cupola need a standby blast fan?
Because the blast cools the recuperator tubes and keeps the tuyeres open. If it stops during a campaign, hot combustion gas keeps flowing over an empty recuperator, which overheats, and slag and metal can freeze in the tuyeres. A standby fan with quick changeover protects both the equipment and the campaign.
Information needed to quote an FD fan for hot blast 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.
- Recuperator air side. Air side pressure drop of the recuperator and hot blast temperature.
- Campaign length. Typical campaign duration and the standby philosophy.
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.