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 ID fan sits
- Step grate
- Under the pan train
- Last pan flue
- Settling chamber or cyclone
- ID fan
- Chimney
The fan sits at the end of the long flue under the pan train, after a settling chamber or cyclone. The flue is often 20 to 40 metres long, with several bends, so the resistance is higher than in a jaggery unit and the suction at the fan is typically 50 to 120 mmWC. The fan replaces or supplements a tall brick chimney.
Gas at the ID fan
| Gas temperature, normal | 180 to 280°C after the last pan |
| Maximum or upset | About 350°C if a pan is out of service |
| Dust at the fan | Bagasse fly ash and unburnt fibre; higher with wet bagasse |
| Moisture and dew point | High from wet bagasse; water vapour 15 to 25% by volume |
| Corrosive species | Low; condensation possible at start-up |
| Density at the design temperature | About 0.69 kg/m³ at 240°C, 58% 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 khandsari sugar plant furnaces
| Blade form | Radial or backward inclined, robust for fly ash |
| Material of construction | Mild steel with wear plate at blade tips |
| Drive and arrangement | Belt driven or direct coupled with a cooling disc |
| Bearings and cooling | Bearings away from the casing with a cooling disc |
| Flow control | Variable speed drive or damper on furnace draft |
| Also specified | Settling chamber or cyclone before the fan, and an inspection door |
What this ID fan has to cope with
Wet bagasse and fly ash
Burning wet bagasse produces more fly ash and unburnt fibre, carried a long way through the flue. Without a settling chamber or cyclone, the fan erodes and the chimney emits heavy smoke. A collector before the fan and wear plate on the blades extend fan life.
Draft in a long flue
The long flue under the pan train has many bends and leaks, and natural draft varies with weather. A fan with enough suction margin, and good sealing of the flue, keep every pan boiling steadily.
Continuous running
Khandsari units run round the clock through the season, and a fan failure stops the pans and spoils the batch. Robust bearings, monitoring and a spare rotor or standby fan reduce the risk.
Energy and control
Khandsari units run for 100 to 150 days a year, continuously. A variable speed drive on the ID fan, set on furnace draft, saves power when the crushing rate is lower. Reducing air leaks into the long flue reduces the gas volume the fan handles and improves the heat available to the pans.
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 Forced Draft (FD) Fan for khandsari sugar plant furnaces.
Questions buyers ask
How is a khandsari furnace different from a jaggery furnace?
A khandsari unit is larger, runs continuously, and has a longer train of pans, so the flue is longer and the gas leaves cooler. It often burns wet bagasse straight from the mill, which gives more fly ash and moisture.
Does a khandsari unit need an ID fan?
Many use a tall chimney, but an ID fan gives steady draft through the long flue regardless of weather, and allows better control of the firing. It is common in larger and modernised units.
What temperature is the flue gas in a khandsari furnace?
Typically 180 to 280°C after the last pan. It can reach about 350°C if a pan is out of service or the flue is short-circuited.
Information needed to quote an ID fan for khandsari sugar plant furnaces
- 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.
- Unit size. Cane crushed per day, number of pans and the flue length.
- Bagasse. Bagasse moisture and quantity burnt per hour.
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.