How cement kiln preheater waste heat boilers work
In a dry process cement plant, raw meal is preheated in a four to six stage cyclone preheater by kiln gas rising from the rotary kiln and calciner. The gas leaves the top cyclone at about 280 to 380°C, depending on the number of stages, carrying 50 to 100 g/Nm³ of fine raw meal. The preheater fan draws it through the preheater and sends it to the raw mill for drying or to a gas conditioning tower and the kiln bag filter. A preheater (PH) waste heat boiler is installed between the top cyclone and the preheater fan, cooling the gas to about 200 to 240°C. The existing preheater fan becomes the ID fan of the boiler, and the boiler adds resistance it must absorb.
Where the ID fan sits
- Kiln and calciner
- Cyclone preheater
- PH waste heat boiler
- Preheater fan (ID fan)
- Raw mill or conditioning tower
- Kiln bag filter
The preheater fan sits after the PH boiler and before the raw mill or conditioning tower, and it sees the full raw meal load not captured by the boiler hoppers. Its suction is already high, typically 600 to 900 mmWC across the preheater, and the boiler adds about 80 to 150 mmWC. The gas is cooler and denser after the boiler, which helps the fan, but the extra resistance usually moves its operating point beyond the original impeller's capability.
Gas at the ID fan
| Gas temperature, normal | 200 to 240°C after the PH boiler |
| Maximum or upset | Top cyclone temperature, 280 to 380°C, when the boiler is bypassed |
| Dust at the fan | Fine raw meal, typically 50 to 80 g/Nm³ after the boiler hoppers; abrasive |
| Moisture and dew point | Low to moderate water vapour; no acid dew point concern at these temperatures |
| Suction at the fan | Preheater draft plus boiler resistance, often 700 to 1,000 mmWC |
| Density at the design temperature | About 0.72 kg/m³ at 220°C, 60% 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 preheater waste heat boilers
| Blade form | Backward curved with wear protection, or radial tipped on heavily loaded older lines |
| Material of construction | High strength steel impeller with wear-resistant plate or hard-faced leading edges; liners at the scroll |
| Drive and arrangement | Direct coupled, arrangement 8, with the existing foundation reused where possible |
| Bearings and cooling | Pedestal bearings with shaft cooling, and continuous vibration monitoring |
| Flow control | Variable speed drive on preheater outlet draft |
| Also specified | Check of motor rating for the new duty, inspection doors for coating removal |
What this ID fan has to cope with
Added resistance with the same production
The boiler adds pressure drop, and the preheater draft must stay the same or the kiln output falls. With the cooler, denser gas the fan sees less volume but must deliver more pressure. Re-rating may need a larger impeller, higher speed or a new rotor on the existing casing, confirmed by a site measurement of flow and pressure.
Raw meal erosion
Fine raw meal at high concentration erodes the blade leading edges and the casing near the cut-off. Hard-facing, wear-resistant plate and liners reduce the rate. Boiler hoppers capture part of the dust, which helps the fan, but the remaining load is still heavy. Thickness checks at each kiln stop show the trend.
Coating build-up and unbalance
Raw meal can form coatings on the blades, especially where alkalis or moisture make it sticky. Uneven coating or its sudden loss throws the rotor out of balance. Vibration monitoring, inspection doors for cleaning and in-situ balancing keep the fan within limits. Water washing should be avoided on a hot rotor.
Bypass operation
When the boiler is bypassed, the fan sees the hotter top cyclone gas and lower resistance. The impeller and speed must suit both modes, so the fan does not overload the motor in bypass or lack pressure in boiler mode. Both cases are checked against the motor rating and impeller stress before the retrofit.
Energy and control
Cement kilns run continuously at high load, and the preheater fan is one of the largest drives on the line. A variable speed drive and an efficient impeller matched to the new boiler duty can partly offset the added resistance. The net power change from fitting the boiler should be calculated, because the denser gas reduces fan power while the extra pressure drop raises it.
Questions buyers ask
Is a new ID fan needed when adding a WHR boiler to a cement preheater?
Not always. The existing preheater fan usually serves as the ID fan. Whether it needs a new impeller, higher speed or a larger motor depends on the boiler's pressure drop and the cooler gas. A site measurement and a new duty calculation decide. A new rotor in the existing casing is common.
How much pressure drop does a PH boiler add?
Typically about 80 to 150 mmWC, depending on the boiler design and gas velocity. The preheater fan must absorb this while still holding the preheater draft that the kiln needs for its output. The figure should be confirmed with the boiler supplier, together with the gas temperature after the boiler, because both are needed to calculate the new preheater fan duty point.
Why does the preheater fan wear faster after a WHR boiler?
It may not, since the boiler hoppers capture part of the dust. But if the fan has been re-rated to a higher tip speed to cover the boiler resistance, erosion increases. Wear protection should be reviewed whenever the speed or impeller changes. Thickness checks give early warning.
Information needed to quote an ID fan for cement kiln preheater waste heat boilers
- 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.
- Present preheater fan. Fan nameplate, curve, present speed, flow, suction and power measured at site.
- Boiler data. PH boiler pressure drop, outlet gas temperature and dust capture in the boiler hoppers.
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.