How cement clinker cooler (AQC) waste heat boilers work
Hot clinker from the kiln falls onto a grate cooler, where cooling fans blow ambient air up through the clinker bed. The hottest air returns to the kiln and calciner as secondary and tertiary air. The rest leaves the cooler as vent air, traditionally at 250 to 350°C, passing an ESP or bag filter, often after an air-to-air heat exchanger, and the cooler vent fan. An AQC (air quenching cooler) waste heat boiler takes air from a mid-cooler tap at about 350 to 450°C, removes coarse clinker dust in a settling chamber, and cools the air to about 100 to 130°C before the cooler dedusting unit and vent fan. This is hot air carrying clinker dust, not combustion gas.
Where the ID fan sits
- Grate cooler mid-tap
- Settling chamber
- AQC waste heat boiler
- Cooler ESP or bag filter
- Cooler vent fan (ID fan)
- Stack
The cooler vent fan sits after the cooler dust collector and before the stack. It holds the kiln hood at a slight suction, typically a few mmWC, which sets how much secondary and tertiary air goes to the kiln. With an AQC boiler it must overcome the settling chamber and boiler as well as the collector. The air reaching it is now cooler, about 100 to 130°C instead of 250°C or more, so its volume falls even as its resistance rises.
Gas at the ID fan
| Air temperature, normal | 100 to 130°C after the AQC boiler and collector |
| Maximum or upset | Up to 350 to 450°C cooler air if the boiler is bypassed, often limited by water spray |
| Dust at the fan | Fine clinker dust, very abrasive; low after a bag filter or ESP |
| Moisture and dew point | Ambient humidity only; no condensation concern |
| Hood pressure | Slight suction at the kiln hood, held by the vent fan |
| 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 clinker cooler (AQC) waste heat boilers
| Blade form | Backward curved with wear protection on the leading edges |
| Material of construction | Mild steel casing; impeller with wear-resistant plate or hard-faced edges against clinker dust |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings; shaft cooling disc for bypass temperatures |
| Flow control | Variable speed drive on kiln hood pressure |
| Also specified | Review of motor rating for the new duty and of impeller stress if the bypass temperature remains possible |
What this ID fan has to cope with
Hood pressure decides kiln operation
If the vent fan cannot hold hood suction steady, hot air puffs out at the hood or too much air is drawn off, starving the kiln of secondary air. The fan must hold the hood pressure across clinker flow changes with the boiler in and out of service. Speed control from hood pressure does this.
Clinker dust is very abrasive
Clinker fines are harder than raw meal. The settling chamber removes coarse particles before the boiler, and the dust collector cleans the air before the fan, but any bypass or bag failure sends abrasive dust to the impeller. Wear protection on the leading edges and periodic thickness checks are worth the cost.
Oversized fan after the retrofit
The cooler air is much denser after the boiler, so the existing vent fan, sized for hot air, often moves more volume than needed at much higher pressure capacity. It then runs throttled. Re-rating with a new impeller or speed control matches it to the new duty and saves power.
Energy and control
The cooler vent fan runs whenever the kiln runs, and after an AQC retrofit its duty often changes considerably. Cooler, denser air means lower volume, and a fan that was right for 250°C may now be oversized. Speed control on hood pressure, or a new impeller matched to the actual duty, recovers the power that a throttling damper would waste.
Questions buyers ask
Does an AQC boiler need a separate ID fan?
Usually not. The cooler vent fan already draws the excess air from the cooler and can serve as the ID fan for the AQC boiler. Its duty changes, though, so its impeller, speed and motor should be checked. Some plants with long ducts add a booster fan between the boiler and the collector.
Why does the cooler vent fan use less power after an AQC boiler?
The boiler cools the air from about 250 to 350°C to about 100 to 130°C, so its volume falls sharply. Even with the boiler's added resistance, the fan power can fall, provided the fan is controlled by speed rather than a throttling damper. Measuring before and after confirms it.
Is the AQC boiler ID fan handling flue gas?
No. It handles cooling air that has passed through the clinker bed, carrying clinker dust but no combustion products. That is why there is no acid dew point concern, and why abrasion from clinker dust is the main mechanical issue. The fan is selected at air density for the actual temperature.
Information needed to quote an ID fan for cement clinker cooler (AQC) 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 vent fan. Fan nameplate, speed, curve and the measured flow, temperature and pressure before the retrofit.
- AQC circuit. Tap air temperature and flow, settling chamber and boiler pressure drop, and the dust collector type.
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.