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 ID fan sits
- Tap hole and runner hoods
- Stock house pick-up points
- Spark arrestor
- Bag filter
- Exhaust (ID) fan
- Stack
The fan sits after the bag filter. It draws fume from hoods at the tap hole, runners and tilting runner during casting, and dust from conveyor transfers and screens in the stock house. The suction at the fan inlet covers hoods, ducts, spark arrestor and filter, typically 250 to 400 mmWC. Cast house fume arrives at 80 to 150°C after dilution with air, so the fan handles hot dusty air, not combustion gas.
Gas at the ID fan
| Gas temperature, normal | 70 to 120°C at the fan, air with fume |
| Maximum or upset | Up to 180°C during casting with hoods close to the runner |
| Dust at the fan | Below 50 mg/Nm³ of fine iron oxide and graphite after the filter; coke and ore dust from the stock house |
| Moisture and dew point | Air; dew point not limiting |
| Corrosive species | Minor; some SO₂ from the slag |
| Density at the design temperature | About 0.95 kg/m³ at 100°C, 79% 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 mini blast furnaces
| Blade form | Backward curved with wear liners, or radial where the filter is unreliable |
| Material of construction | Mild steel with wear-resistant plate on blades and scroll |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive with higher speed during casting |
| Also specified | Inlet damper, inspection door, and dampers to switch flow between cast house and stock house |
What this ID fan has to cope with
Fume escapes during casting
If the fan cannot draw enough at the tap hole, fume escapes to the cast house floor. Capture velocity at the hoods sets the flow, and the fan is selected for casting duty. Separate dampers let the fan concentrate flow on the cast house during casting.
Abrasive dust at the fan
Coke, ore and sinter dust from the stock house is highly abrasive, and broken bags quickly erode the impeller. Wear liners and broken bag detection limit damage. Stock house dust is the hardest, and that branch may need wear-lined duct bends of its own.
Sparks and filter fires
Hot metal sparks, burning graphite and hot slag particles can be drawn into the ducts and reach the bag filter, starting a fire that destroys bags and lets dust through to the fan. A spark arrestor or cooling chamber ahead of the filter protects the bags, and with them the fan, from the dust surge that follows bag failure.
Energy and control
Casting takes place several times a shift, and between casts the cast house hoods need much less flow. A variable speed drive that raises speed during casting and drops it between casts saves considerable energy. Switching dampers between cast house and stock house duty also allows one fan to serve both.
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 Forced Draft (FD) Fan for mini blast furnaces.
Questions buyers ask
What fans does a mini blast furnace need?
A blast blower or compressor for cold blast, a combustion air fan for the stoves, and exhaust fans for cast house and stock house dedusting. Some plants also have ID fans for gas-fired boilers using BF gas. The duties are quite different. Each needs its own selection.
Is the blast blower of a mini blast furnace an FD fan?
No. Cold blast at 1 to 2 bar needs a centrifugal blower or compressor. The FD fan in a mini blast furnace is the stove combustion air fan. It is a separate machine with its own cooling and surge protection. It is specified separately from the fans.
What is the cast house dedusting fan duty in a mini blast furnace?
It draws fume from tap hole and runner hoods through a bag filter. The flow is set by capture velocity at the hoods, and the fan handles hot air with fine iron oxide and graphite. Stock house dust may be handled by the same fan through switching dampers.
Information needed to quote an ID fan for mini blast furnaces
- 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.
- Capture points. Number and type of hoods at the cast house and stock house, and casting schedule.
- Filter. Bag filter type and outlet dust level.
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.