How kaolin and clay calciners work
Kaolin is calcined to metakaolin at about 650 to 850°C, or to fully calcined kaolin at 1,000 to 1,100°C, to raise brightness and opacity for paper coating, paint, plastics and ceramics. The feed is washed, refined and spray-dried or pulverised clay of 1 to 2 µm, with about 13 to 14% combined water released during calcination. Calciners are direct or indirect-fired rotary kilns, multiple hearth furnaces or flash calciners, fired with natural gas, LPG or clean oil to avoid colouring the product. The exhaust carries water vapour, combustion gas and a large share of ultrafine clay dust. It passes cyclones and a bag filter, where the dust is recovered as product, before the ID fan and stack.
Where the FD fan sits
- Inlet filter
- Combustion air FD fan
- Burner
- Combustion chamber
- Calciner
The fan sits near the burner or combustion chamber. On rotary calciners it feeds the main burner at the discharge end; on hearth furnaces it feeds burners on several levels; on flash calciners it feeds the combustion chamber that produces the hot gas stream carrying the clay. Outlet pressure is typically 300 to 800 mmWC, depending on the burner and calciner design.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Clay dust in the plant; filtered intake essential |
| Pressure at the fan outlet | Typically 300 to 800 mmWC |
| Fuel | Natural gas, LPG or clean oil |
| Air-fuel ratio | Kept close to target to avoid soot or iron reduction that dulls the product |
| 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 kaolin and clay calciners
| Blade form | Backward curved, for stable flow and good efficiency |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive with air-fuel ratio control |
| Also specified | Inlet filter, flow measurement, air pressure interlock |
What this FD fan has to cope with
Clean flame for brightness
Soot or reducing pockets from a poorly controlled flame lower the brightness of calcined kaolin. Measured air flow and ratio control, not a fixed damper, keep the flame clean.
Multiple burners on hearth furnaces
Hearth furnaces have burners at several levels, each needing correct air. A fan with a steady pressure curve and individual burner valves gives even air distribution as burners turn up and down.
Clay dust intake
Fine clay drawn into the fan coats the impeller and burner passages, upsetting the air flow and the flame. A filtered intake from a clean area, with a pressure gauge across the filter, prevents it.
Energy and control
Kaolin calcining is fuel-intensive, and the combustion air fan affects fuel use through excess air control. Speed control linked to firing rate, with correct excess air, saves both fan power and fuel. Running with high excess air out of caution costs fuel and adds gas volume to the ID fan.
ID fan and FD fan in kaolin and clay calciners
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | At the burners or combustion chamber |
| What it handles | Exhaust with released water and clay fines | Ambient air |
| Temperature at the fan | 150 to 230°C | Ambient |
| Main design concern | Steady draft for consistent calcination | Clean flame for brightness |
| Control | Calciner outlet pressure | Air-fuel ratio |
See also the Induced Draft (ID) Fan for kaolin and clay calciners.
Questions buyers ask
What is the FD fan on a kaolin calciner?
On a kaolin calciner the forced draft fan is the combustion air fan for the burners. On flash calciners the combustion gas also carries the clay through the calciner.
Why is air-fuel ratio important in kaolin calcining?
The product is sold on brightness. A smoky or reducing flame can discolour it. Accurate combustion air control keeps the flame clean and the product white.
What pressure does a kaolin calciner combustion air fan need?
Typically 300 to 800 mmWC, depending on the burner and the calciner type. Flash calciner combustion chambers usually need the higher end of the range.
Information needed to quote an FD fan for kaolin and clay 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.
- Burners. Number of burners, fuel and firing rate.
- Calciner type. Rotary, hearth or flash, and how the hot gas is used.
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.