The process
A sugar mill boiler fires bagasse straight from the mill at around half moisture. The fuel bed needs a large volume of under-grate air from the FD fan and over-fire air from the SA fan, and the wet, fibrous fly ash leaving the furnace goes to a multiclone or wet scrubber before the ID fan. The season runs for months without a stop, so every fan on the boiler is sized for continuous hot running and for ash that builds up on the blades.
The gas at the fan
Flue gas from bagasse firing carries a heavy load of fibrous fly ash and unburnt char, with high moisture from the wet fuel. The ash is abrasive and tends to build up on blades, so the wheel is chosen to shed deposit and the blades carry wear protection.
Selection for this duty
| Blade form | Radial tipped with wear liners, or straight radial where the dust is coarse |
| Material of construction | Mild steel with replaceable wear liners on the blades and the scroll at the cut-off, with a casing drain for the moisture |
| 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; casing drain at the lowest point |
| Density at the fan | At about 180°C the gas density is close to 0.78 kg/m³, 65% 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. |
Injects ambient air into a hot flue gas stream to bring it down to a temperature the downstream bag filter or fan can accept. Client-stated blade types for the Dilution Air Blower in general: Backward curved.
Industries
Other fans in bagasse fired boilers
Dilution 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.