The process
Utility and large industrial boilers fire pulverised coal through mills and burners. The PA fan dries and conveys the coal from mill to burner, the FD fan supplies secondary air through the air heater, the ID fan draws the flue gas through the ESP to the stack, and seal air fans keep coal dust out of the mill and fan bearings. These are the largest fans in the plant.
The gas at the fan
After the ESP or bag filter the flue gas carries a light residual ash load. The ID fan handles large volume at moderate temperature, selected for efficiency with modest wear allowance.
Selection for this duty
| Blade form | Backward inclined or backward curved; the residual dust does not justify a radial wheel |
| Material of construction | Mild steel with replaceable wear liners on the blades and the scroll at the cut-off |
| Drive and bearings | Direct coupled or belt driven; belt drive where the duty point may need tuning at site |
| Density at the fan | At about 140°C the gas density is close to 0.85 kg/m³, 71% of standard air. Pressure developed and power drawn fall in the same ratio, so the selection is made at this density and the motor is checked at cold start. |
Feeds a burner with air at the pressure the burner register needs. Sized for the burner turndown range, and often fitted with an inlet silencer. Client-stated blade types for the Combustion Air Blower in general: Backward curved and radial tipped.
Industries
Other fans in pulverised coal fired boilers
Combustion Air Blower in other processes
Information needed to select or quote
- Duty point. Volume flow and static pressure at the fan inlet, with the gas density and temperature or the conditions to derive it.
- Operating conditions. Site altitude, normal operating temperature and continuous or intermittent duty.
- Gas stream. Dust loading and particle size, corrosive species, moisture, upset temperature and any tendency of the material to deposit on the impeller.
- Mechanical. Rotation viewed from the drive end, discharge orientation and motor power.