How alumina calciners work
An alumina calciner converts aluminium hydroxide from the Bayer process into smelter grade or specialty alumina by driving off free moisture and chemically bound water at about 950 to 1,100°C. Most plants use gas suspension or circulating fluidised bed calciners; older and specialty plants use rotary kilns. Wet hydrate enters a flash dryer or venturi drier, is preheated in cyclone stages by hot gas from the calciner, calcined in a furnace fired with natural gas or fuel oil, and cooled in cyclones and a fluid bed cooler that preheat the combustion air. The exhaust gas is very humid, with 35 to 50% water vapour, and leaves the drying stage at 140 to 200°C carrying fine alumina to the ESP and ID fan.
Where the FD fan sits
- Inlet filter
- FD (fluidising or combustion air) fan
- Fluid bed cooler air box or burner
- Cooling cyclones
- Calcining furnace
Fluidising air blowers sit near the fluid bed cooler and push air into the air box under the bed, where it fluidises the hot alumina and picks up heat. That warm air often goes forward as combustion air. A combustion air fan supplies the burner and any primary air. Pressure for fluidising is typically 500 to 1,000 mmWC, depending on bed depth.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the cooler | Heated by the product, downstream of the fan |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | Typically 500 to 1,000 mmWC for fluidising air |
| Turndown | Steady at production rate; limited turndown for fluidisation |
| 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 alumina calciners
| Blade form | Backward curved with a stable, rising pressure curve |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled at two-pole speed |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive or inlet vanes on air flow |
| Also specified | Inlet filter, inlet silencer, and often a standby blower for the fluid bed |
What this FD fan has to cope with
Loss of fluidisation
If the fluidising air pressure drops, the alumina bed slumps and stops flowing, and hot product backs up into the cooling cyclones. A fan with a steep, stable curve and a standby unit with automatic changeover protect the cooler. The fan curve should have no unstable region near the duty point, since the bed resistance changes as the product level varies.
Air distribution in the bed
Uneven air to the bed compartments leaves cold spots that do not cool or flow. Measured air flow to each compartment, from a fan with steady header pressure, keeps the bed uniform. Compartment dampers are set at commissioning and checked against bed temperatures. A blower without enough pressure margin cannot hold flow to the deeper compartments, which is often the first sign that the fan is undersized.
Noise from high pressure blowers
Fluidising air blowers at two-pole speed are loud at the inlet. An inlet silencer and acoustic lagging are usually needed to meet workplace limits. Blower noise also travels along the air ducts into the cooler area. Flexible connections, lagging on the ducts near the fan and a well-placed inlet, away from walkways, keep the noise down without affecting the duty.
Energy and control
Fluidising air blowers run continuously at a fairly steady flow, so efficiency at the duty point matters. A well-chosen backward curved fan at its best efficiency point, with speed control for production changes, uses the least power. The air heated in the cooler returns heat to the calciner, so excess fluidising air does not waste much heat but does waste blower power.
ID fan and FD fan in alumina calciners
| ID fan | FD fan | |
|---|---|---|
| Position | After the ESP, before the stack | At the fluid bed cooler and burners |
| What it handles | Humid exhaust with fine alumina | Clean ambient air |
| Temperature at the fan | 140 to 200°C | Ambient |
| Main design concern | Erosion and dew point | Steady fluidising pressure |
| Control | Calciner draft and production rate | Fluidising air flow |
See also the Induced Draft (ID) Fan for alumina calciners.
Questions buyers ask
Does an alumina calciner have an FD fan?
Yes, but not in the boiler sense. The ID fan draws most of the air through the product coolers. FD fans supply fluidising air to the fluid bed cooler and combustion air to the burners. They handle clean ambient air. The fluidising air is heated in the cooler and reused as combustion air, which is why the arrangement is so efficient.
What pressure does a fluid bed cooler blower need?
Typically 500 to 1,000 mmWC, set by the bed depth, the air distributor and the air box. We calculate the duty from the bed design and confirm it against the existing blower where one is being replaced. Distributor plate wear and fouling raise the pressure drop over time, so a margin is included.
Why is a standby blower used on a fluid bed cooler?
Loss of fluidising air causes the bed to slump and stops product flow, which can back up hot alumina into the cyclones and stop the calciner. A standby blower with automatic changeover prevents this. The standby unit is usually started automatically on a low air box pressure signal, without operator action.
Information needed to quote an FD fan for alumina calciners
- 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.
- Fluid bed cooler. Bed area, depth, number of compartments and air distributor pressure drop.
- Combustion air. Burner type and any primary air requirement.
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.