How mini blast furnaces work
Mini blast furnaces, typically 100 to 450 m³ working volume, produce hot metal or foundry pig iron from lump ore or sinter, coke and fluxes. Cold blast at about 1 to 2 bar from a centrifugal blower or compressor is heated to 900 to 1,100°C in three stoves fired with the furnace's own lean BF gas, often without enrichment. The top gas is cleaned in a dust catcher and wet scrubber or dry bag filter and used in the stoves, boilers and reheating furnaces. The blast blower is a compressor duty outside the normal fan range. The fan duties are the stove combustion air fan and the exhaust fans of the cast house and stock house dust extraction systems.
Where the FD fan sits
- Inlet filter
- FD fan
- Combustion air main
- Air valve
- Stove burner
- Combustion chamber
One fan with a standby serves the three stoves through a common air main. On most mini blast furnaces there is no waste heat recovery, so the air goes cold to the burner. The fan works against the main, air valves and burner, typically 300 to 600 mmWC. Where waste heat recovery is added later, the fan must overcome the extra exchanger resistance, so pressure margin is sensible.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air at the burner | Ambient, unless heat recovery is fitted |
| Dust | Ore and coke dust near the stock house; filtered inlet advisable |
| Fuel | Lean BF gas at 700 to 850 kcal/Nm³ |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| 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 mini blast furnaces
| Blade form | Backward curved, for a stable pressure curve |
| Material of construction | Mild steel casing and impeller |
| Drive and arrangement | Direct coupled, arrangement 4 or 8, with standby fan |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive or inlet damper on air-gas ratio |
| Also specified | Inlet filter, non-return dampers, and space for adding heat recovery later |
What this FD fan has to cope with
Low dome temperature on lean gas
Unenriched BF gas gives a limited flame temperature, and excess air lowers it further. Accurate air flow from a stable fan, matched to gas flow, achieves the highest dome temperature the gas allows. Oxygen in the waste gas confirms it.
Dust at the inlet
Ore and coke dust drawn into the fan erodes it and changes the air-gas ratio. A filtered inlet away from the stock house protects it. A duct from the clean side of the plant is the simple answer, and it keeps the air-gas ratio honest.
Single fan dependence
If the only fan trips, the stove on gas cannot fire, the dome cools and the blast temperature falls, which disturbs the furnace thermal state and raises coke rate. A standby fan with automatic changeover and a non-return damper on each fan keeps air on the burner when one unit fails.
Energy and control
Air demand follows stove gas flow, which varies with the stove cycle. A variable speed drive matches air to gas without throttling. Adding waste heat recovery to preheat air and gas raises dome temperature and saves gas, but adds resistance, and the fan should be selected with that in mind.
ID fan and FD fan in mini blast furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the dedusting bag filter | Ahead of the stove air main |
| What it handles | Hot air with fume and dust | Ambient combustion air |
| Temperature at the fan | 70 to 120°C | Ambient, 25 to 45°C |
| Main design concern | Capture flow and abrasion | Air-gas ratio on lean gas |
| Control | Casting schedule | Air-gas ratio |
See also the Induced Draft (ID) Fan for mini blast furnaces.
Questions buyers ask
What is the combustion air pressure for mini blast furnace stoves?
Typically 300 to 600 mmWC at the fan, depending on the burner type, the length of the air main and whether air preheating is fitted. The fan is selected for the stove furthest from it, with all losses included, so that every stove gets the air its burner needs.
Can mini blast furnace stoves run on BF gas alone?
Yes, many mini blast furnace stoves fire unenriched BF gas. The dome temperature is then limited, which limits blast temperature. Precise combustion air control from a stable FD fan gets the most from the gas, and preheating the air or gas with waste gas heat raises dome temperature further.
How many combustion air fans are needed for mini blast furnace stoves?
Usually one running fan and one standby serving all three stoves through a common air main. Since only one stove is on gas at a time on most mini blast furnaces, a single fan carries the duty, and the standby protects blast temperature if it trips.
Information needed to quote an FD fan for mini blast 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.
- Gas quality. BF gas calorific value and flow per stove.
- Future heat recovery. Whether air or gas preheating is planned.
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.