How white cement kilns work
White cement clinker is made from very low-iron raw materials, such as white limestone, kaolin and silica sand, and burnt at about 1,450 to 1,550°C, hotter than grey clinker because there is little iron to act as a flux. Fuel must be low in ash and iron, so furnace oil, petcoke or natural gas is used rather than coal. Kilns are smaller than grey kilns, typically 300 to 1,500 tpd, with a cyclone preheater. The hot clinker is quenched in water or cooled under reducing conditions to keep it white, then finished in an air cooler. The kiln gas leaves the top cyclone at about 300 to 380°C, is cooled in a conditioning tower and cleaned in a bag filter. The kiln ID fan draws gas through the preheater; the burner primary air fan and the cooler fans supply air.
Where the FD fan sits
- Inlet filter and silencer
- Primary air FD fan
- Burner air ducts
- Multi-channel burner
- Kiln
The fan stands on the kiln firing floor near the burner platform and feeds the axial and swirl channels of a multi-channel burner. Primary air is a small share of the combustion air, typically 8 to 12%, but it leaves the burner tip at high velocity, so the fan pressure is high, usually 1,500 to 3,000 mmWC. Separate fans supply the cooler, and secondary air reaches the kiln hot from the cooler, drawn in by the kiln ID fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Plant air with clinker and meal dust; inlet filter needed |
| Pressure at the fan outlet | Typically 1,500 to 3,000 mmWC |
| Flow | 8 to 12% of the total combustion air |
| Variation | Adjusted by the burner operator to shape the flame |
| 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 white cement kilns
| Blade form | Backward curved or radial, at high tip speed for the pressure |
| Material of construction | High-tensile steel impeller suitable for the tip speed |
| Drive and arrangement | Direct coupled at two-pole speed, often with a standby fan |
| Bearings and cooling | Standard bearings with vibration monitoring |
| Flow control | Variable speed drive, with channel dampers at the burner for axial and swirl split |
| Also specified | Inlet silencer, emergency air supply for burner tip cooling on power failure |
What this FD fan has to cope with
Burner tip overheating on fan trip
If primary air stops with the kiln hot, the burner pipe overheats and distorts within minutes. A standby fan, a DC or diesel-backed emergency fan, or an automatic changeover protects the burner.
Flame shape and clinker whiteness
White clinker colour depends on burning conditions. A fan that cannot hold pressure as the filter loads, or that hunts on its control, lengthens and destabilises the flame. A steep fan curve and pressure control hold the flame steady.
Noise at the firing floor
High pressure primary air fans at two-pole speed are loud, and the burner platform is a place where operators spend long periods. Inlet silencers and casing insulation are usually needed to keep noise at the firing floor within limits.
Energy and control
Primary air fans are smaller than the kiln ID fan but run at high pressure around the clock. Speed control instead of a throttled damper saves power, and getting the primary air quantity right affects fuel use: too much cold primary air displaces hot secondary air from the cooler and raises the heat consumption of the kiln.
ID fan and FD fan in white cement kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the top cyclone of the preheater | On the firing floor, feeding the burner |
| What it handles | Hot kiln gas with fine meal dust | Ambient air |
| Temperature at the fan | 300 to 380°C, up to 450°C upset | Ambient |
| Main design concern | Sulphate build-up and hot strength | High pressure and steady flame shaping |
| Control | Kiln draft | Burner air pressure |
See also the Induced Draft (ID) Fan for white cement kilns.
Questions buyers ask
What is the FD fan on a cement kiln?
On a cement kiln the forced draft duty is split. The primary air fan supplies the burner, the cooler fans blow air through the clinker bed, and the hot air from the cooler becomes secondary combustion air. On a white cement kiln the term FD fan usually means the primary air fan.
What pressure does a kiln primary air fan need?
Typically 1,500 to 3,000 mmWC, depending on the burner design. Modern low primary air burners use less air at higher pressure, which raises the tip velocity and shortens the flame.
Why is a standby primary air fan used?
The burner pipe sits inside a kiln at over 1,000°C and relies on the primary air for cooling. Losing the fan without a backup can damage the burner within minutes, so a standby or emergency fan is common.
Information needed to quote an FD fan for white cement kilns
- 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.
- Burner. Burner type, number of air channels, and the air flow and pressure required.
- Fuel. Fuel or fuel mix fired and any coal or petcoke conveying air supplied by the same fan.
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.