The process
A circulating fluidised bed boiler recirculates solids through a hot cyclone and loop seal. The primary air fan runs at high pressure to fluidise the bed, the secondary air fan stages combustion, and the ID fan handles the flue gas after the cyclone and ESP. Loop seal and fluidising blowers add high pressure duties around the furnace.
The gas at the fan
Circulating fluidised bed boilers return most solids through the cyclone, so the flue gas reaching the fan carries fine ash at moderate loading. Erosion is slower than on an AFBC unit but still sets the wheel design.
Selection for this duty
| Blade form | Radial tipped, or backward inclined where the fan sits after the collector |
| 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 with a shaft cooling disc and bearings outside the casing |
| Also specified | Hard facing or liners on the blade pressure face |
| Density at the fan | At about 150°C the gas density is close to 0.83 kg/m³, 69% 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 cfbc 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.