How khandsari sugar plant furnaces work
A khandsari unit makes open-pan sugar on a larger scale than jaggery, typically from 100 to 500 tonnes of cane a day. Juice is clarified, often by sulphitation or with hydros and lime, and concentrated in a train of five to seven open pans set in series over a long furnace fired with bagasse straight from the mill. The concentrated syrup is crystallised in coolers and spun in centrifugals. The flue gas passes under each pan in turn, so it leaves cooler than in a jaggery unit, typically 180 to 280°C. The bagasse is burnt wet, about 45 to 50% moisture, on a step or inclined grate, and the units run around the clock through the crushing season.
Where the FD fan sits
- Air intake
- FD fan
- Air box under the step grate
- Wet bagasse bed
- Furnace
- Flue under pan train
The fan sits beside the furnace and delivers air into an air box under the step or inclined grate. Bagasse from the mill is fed continuously at the top of the grate, dries as it slides down, and burns. Pressure is typically 60 to 120 mmWC, higher than for a jaggery unit because the bed of wet bagasse is deeper.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 40°C |
| Dust | Mill yard air with bagasse fines |
| Pressure at the fan outlet | Typically 60 to 120 mmWC |
| Fuel | Wet bagasse, 45 to 50% moisture |
| Turndown | Follows the crushing rate |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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. |
FD Fan selection for khandsari sugar plant furnaces
| Blade form | Backward curved or radial |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Damper or variable speed drive |
| Also specified | Inlet screen and a standby fan for continuous running |
What this FD fan has to cope with
Wet bagasse needs more pressure
Bagasse straight from the mill at 45 to 50% moisture forms a dense, deep bed on the step grate. Too little air pressure gives a smouldering fire and smoke under the first pans; too much blows fines into the long flue. Sizing the fan for the actual bed depth and grate area gives a strong, clean fire.
Fines in the intake
The fan draws from a mill yard full of bagasse fines that block the inlet guard and coat the blades, cutting air flow during the night shift. A screened intake away from the bagasse carrier and a cleaning routine keep the fan delivering.
Continuous duty
The unit runs round the clock through the season, and losing the combustion air fan stops the boiling across the whole pan train and can spoil the syrup. A standby fan or a spare motor kept ready reduces the downtime.
Energy and control
The FD fan runs continuously through a season of 100 to 150 days. Speed control linked to the crushing rate saves power when cane supply is short, and correct air reduces the excess air that carries heat up the chimney. Drying bagasse with waste heat from the flue before firing improves combustion and reduces air demand.
ID fan and FD fan in khandsari sugar plant furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | End of the long flue, before the chimney | Beside the furnace, under the step grate |
| What it handles | Moist gas with bagasse fly ash | Ambient air |
| Temperature at the fan | 180 to 280°C | Ambient |
| Main design concern | Steady draft, fly ash and continuous duty | Air through a deep wet bed |
| Control | Furnace draft | Crushing rate |
See also the Induced Draft (ID) Fan for khandsari sugar plant furnaces.
Questions buyers ask
What does the FD fan do on a khandsari furnace?
It is the forced draft fan that supplies combustion air under the step grate where wet bagasse from the mill is burnt. It keeps the fire strong enough to boil the full train of open pans, round the clock through the season.
Why does a khandsari furnace need more air pressure than a jaggery furnace?
Khandsari units usually burn wet bagasse straight from the mill in a deeper bed on a step grate, which resists the air far more than the thin bed of sun-dried bagasse in a jaggery furnace.
Should a khandsari unit have a standby FD fan?
It is advisable, because the unit runs continuously and losing the fan stops the boiling across the pan train. A standby fan, or at least a spare motor and impeller kept on site, reduces downtime in the season.
Information needed to quote an FD fan for khandsari sugar plant furnaces
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Grate. Grate type and area, and bagasse burnt per hour.
- Duty. Whether a standby fan is required.
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.