How iron ore pellet plants work
An iron ore pellet plant grinds fine ore, often with bentonite and limestone, rolls it into green pellets of 9 to 16 mm in discs or drums, and hardens them by drying, preheating and firing at 1,250 to 1,350°C before cooling. Two induration systems dominate: the straight grate, where the pellets stay on one travelling grate through every zone, and the grate kiln, where a travelling grate dries and preheats, a rotary kiln fires and an annular cooler cools. Both reuse the hot cooling air as combustion and drying air, so the gas circuit is a chain of process fans. Smaller plants use shaft furnaces. Fuel is natural gas, oil, coal or coke oven gas, and the waste gas leaves through an ESP and the main waste gas fan.
Where the FD fan sits
- Inlet filter
- FD (cooling) fan
- Cooler air box
- Pellet bed
- Hood or kiln
- Firing and drying zones
On a straight grate the cooling fan blows air up through the pellets in the cooling zone of the same grate. On a grate kiln several fans feed the sections of an annular cooler, and the hottest air goes to the kiln as secondary air. On shaft furnaces cooling air is blown into the bottom of the shaft. The pressure needed is typically 400 to 800 mmWC, depending on bed depth and the system.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the pellet bed | 800 to 1,100°C in the first cooling section, downstream of the fan |
| Dust | Fine ore dust in plant air; filtered intake advised |
| Bed resistance | Depends on bed depth and pellet size, typically 400 to 800 mm of pellets |
| Pressure at the fan outlet | Typically 400 to 800 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 iron ore pellet plants
| Blade form | Backward curved, high efficiency on clean ambient air |
| Material of construction | Mild steel casing and impeller |
| Drive and arrangement | Arrangement 8, single or double inlet |
| Bearings and cooling | Oil-lubricated pedestal bearings with condition monitoring |
| Flow control | Variable speed drive or inlet vanes on cooling air flow |
| Also specified | Inlet filter and silencer, and dampers for each cooler section |
What this FD fan has to cope with
Loss of the heat recovery loop
The cooling air becomes the hot air for firing and drying. If the cooling fan loses capacity, firing zone temperature falls, pellets are underfired and fuel use rises. The fans have margin for bed resistance at full output. Fuel rate is a good indicator of a cooling air shortfall.
Dusty intake
Air around a pellet plant carries fine, hard ore and bentonite dust, especially near the discharge end and the screens. Drawn through a fan, it erodes the blades and works into the bearings. Filtered intakes placed away from the dustiest areas, and sealed bearing housings, keep cooling fans running for long periods without attention.
Different coolers, different duties
A straight grate cooling zone uses one or two large fans at high pressure through a bed with a hearth layer. A grate kiln annular cooler uses several smaller fans, one per section, each at its own pressure. A shaft furnace blows air into the bottom of the shaft. Each fan is selected for its own cooler, not copied from another.
Energy and control
Cooling air flow sets both the pellet discharge temperature and the hot air available for firing, so it cannot simply be minimised. Variable speed control matched to production keeps the loop balanced without throttling. Sealing leaks at cooler and hood seals helps both fuel and fan power. Each cooler fan is controlled for its own section.
ID fan and FD fan in iron ore pellet plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP, before the stack | Below the cooler or cooling zone |
| What it handles | Moist drying gas with ore dust | Ambient cooling air |
| Temperature at the fan | 100 to 160°C | Ambient, 25 to 45°C |
| Main design concern | Moisture, build-up and erosion | Stable heat recovery loop |
| Control | Hood or windbox pressure | Cooling air flow |
See also the Induced Draft (ID) Fan for iron ore pellet plants.
Questions buyers ask
What are the FD fans in a pellet plant?
They are the cooling air fans that blow ambient air through hot fired pellets. The heated air is then used as combustion and drying air, so they feed the whole heat recovery loop of the plant. Some plants also use them for drying air. Each is matched to its zone.
How is a straight grate cooling fan different from a grate kiln cooler fan?
A straight grate has one or two large cooling fans feeding the cooling zone of the grate. A grate kiln uses several smaller fans, one per annular cooler section, each at its own pressure. Their selection points therefore differ. The first annular section usually has the highest duty.
What pressure do pellet cooling fans need?
Typically 400 to 800 mmWC, depending on bed depth, pellet size and the cooler design. We confirm this from the cooler data before selecting the fan. The pressure varies with bed depth and with how the cooling air is reused, and fan curves are checked against measured bed pressure at commissioning.
Information needed to quote an FD fan for iron ore pellet plants
- 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.
- Cooler type. Straight grate cooling zone, annular cooler or shaft furnace, and the cooling air flow.
- Heat recovery. Where the hot cooling air is used, and the air temperatures required.
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.