How cement kiln ID fan systems work
In a dry process cement plant the kiln exhaust leaves the preheater at about 280 to 350°C and is drawn out by the preheater fan. It then passes a gas conditioning tower, where water sprays cool it to about 150 to 200°C, and splits between the raw mill and a bypass. When the raw mill runs, the gas dries the raw meal and leaves the mill circuit at 90 to 120°C. Both streams join at the main bag filter or ESP, where the dust is collected and returned to the kiln feed. The final fan after this collector, called the kiln ID fan, bag house fan or main stack fan, draws the combined gas through the filter and discharges it to the stack. This page covers that fan; the preheater fan is a separate duty.
Where the ID fan sits
- Preheater fan
- Gas conditioning tower
- Raw mill or bypass
- Main bag filter or ESP
- Kiln ID fan
- Stack
The fan sits at the outlet of the main bag filter or ESP, at the end of the kiln gas train. The preheater fan delivers gas at a small positive or neutral pressure into the conditioning tower, so the kiln ID fan works mainly against the tower, the raw mill or its bypass, the filter and the ducts, typically 200 to 350 mmWC on a bag filter. Its suction sets the pressure at the preheater fan outlet and at the raw mill inlet, so it is the fan that ties the kiln and mill circuits together.
Gas at the ID fan
| Gas temperature, normal | 90 to 120°C with the raw mill running; 150 to 200°C in direct mode |
| Maximum or upset | Up to 250°C if conditioning tower sprays fail, limited by bag ratings |
| Dust at the fan | Usually below 30 mg/Nm³ after a bag filter; higher after an old ESP |
| Moisture and dew point | Raw mill mode carries moisture from the meal, with a dew point of about 50 to 60°C |
| Gas volume | Very large; the actual volume and density change markedly between the two modes |
| Corrosive species | SO₂ is low after the raw mill, which absorbs much of it; condensation is the main risk |
| Density at the design temperature | About 0.90 kg/m³ at 120°C, 75% 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 cement kiln ID fan systems
| Blade form | Backward curved or aerofoil, for efficiency on clean gas at large volume |
| Material of construction | Mild steel or boiler quality plate; wear liners only where an ESP lets dust through |
| Drive and arrangement | Double inlet, double width on large kilns; direct coupled through flexible coupling, with variable speed drive |
| Bearings and cooling | Pedestal sleeve or roller bearings with temperature and vibration monitoring; no cooling needed |
| Flow control | Variable speed drive on preheater fan outlet or raw mill inlet pressure |
| Also specified | Rotor designed for the direct mode volume and the mill mode density; outlet damper for isolation; casing drains |
What this ID fan has to cope with
Two operating points
With the raw mill off the gas is hotter and lighter, so the fan moves a larger actual volume at lower pressure; with the mill on the gas is cooler and denser and the resistance changes. A fan sized for one mode can run in an inefficient or unstable region in the other. Both points are checked on the curve, and speed control covers the range.
Conditioning tower upsets
When the tower sprays fail or the water is cut at a kiln stop, hot gas reaches the filter and fan. The fan must be stress-checked at this upset temperature, and the filter protection, with dilution air or a bypass, must act before the bags are damaged.
Condensation in mill mode
At kiln stops the humid mill gas cools in the fan and ducts and condenses, wetting the meal dust that has settled on the blades. The result is hard deposits and corrosion. Drains, insulation and running the fan to purge before stopping reduce it.
Higher pressure after an ESP to bag filter change
Converting an ESP to a bag filter adds 100 to 150 mmWC of pressure drop. The existing fan often cannot meet it. A new impeller for the higher pressure, or a new fan, is assessed together with the motor and foundation.
Energy and control
The kiln ID fan runs continuously at large volume and is one of the main power consumers of the pyroprocess. Variable speed control saves power at reduced kiln output and in mill mode, and a correctly sized fan avoids the heavy losses of a throttled damper. False air entering through the raw mill, filter and duct joints is paid for directly in fan power, so sealing the system is often as valuable as improving the fan.
Questions buyers ask
What is the difference between a preheater fan and a kiln ID fan?
The preheater fan draws gas from the top cyclone of the preheater at about 300°C with heavy dust. The kiln ID fan, or bag house fan, comes later, after the conditioning tower, raw mill and main bag filter, and handles cooler, clean gas at a larger volume. On some plants the names are used the other way round, so the position matters more than the name.
Why does the cement kiln ID fan need variable speed?
It must run with the raw mill on and off, at different kiln outputs, and against a bag filter whose pressure drop varies between cleaning cycles. A variable speed drive covers all these points efficiently, where an inlet damper would waste much of the power.
Can the impeller be upgraded when the ESP is changed to a bag filter?
Usually yes. The extra pressure drop can often be met with a new impeller of larger diameter or different blade form in the existing casing, provided the motor, shaft and bearings are checked for the new duty.
Information needed to quote an ID fan for cement kiln ID fan systems
- 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.
- Operating modes. Gas flow, temperature and pressure at the fan inlet for both raw mill on and raw mill off.
- Collector. Type of main collector, ESP or bag filter, its pressure drop and any planned conversion.
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.