How jaggery plant furnaces work
Jaggery is made by boiling sugarcane juice in open pans until it is concentrated enough to set. A typical unit has two to four pans in series over a long furnace, called a chulha, fired with dried bagasse from the same cane. The fire is at one end, and the flue gas passes under each pan in turn before leaving through a chimney. Traditional units use natural draft and lose much of the heat to the chimney at 300 to 500°C. Improved units have better grates, more pans or a longer flue, and fans for forced and induced draft. Bagasse used for firing is sun dried to about 25 to 35% moisture, with low ash but a lot of light fibre and fines.
Where the ID fan sits
- Bagasse grate
- Under first pan
- Under second and third pans
- Flue duct
- ID fan
- Chimney
The fan sits at the end of the flue, after the last pan and before the chimney. Upstream are the grate, the flue passages under each pan and often a simple settling chamber. Suction at the fan is low, typically 20 to 60 mmWC, but the gas reaching it is hot because open pans absorb heat less effectively than a boiler. The fan lets the unit use a shorter chimney and gives steady draft regardless of wind.
Gas at the ID fan
| Gas temperature, normal | 250 to 400°C after the last pan |
| Maximum or upset | About 500°C on a heavily fired unit or when a pan is removed |
| Dust at the fan | Light bagasse ash with unburnt fibre and sparks |
| Moisture and dew point | Moderate; bagasse moisture gives water vapour, but gas stays hot |
| Corrosive species | Low; bagasse is low in sulphur |
| Density at the design temperature | About 0.60 kg/m³ at 320°C, 50% 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 jaggery plant furnaces
| Blade form | Radial or open paddle, robust and simple |
| Material of construction | Mild steel for gas up to about 350°C; heat-resisting steel above |
| Drive and arrangement | Belt driven with a cooling disc, arrangement 1 or 3 |
| Bearings and cooling | Bearings away from the casing with a cooling disc and heat shield |
| Flow control | Damper or variable speed drive on furnace draft |
| Also specified | Settling chamber or spark arrestor before the fan |
What this ID fan has to cope with
High gas temperature
Open pans take less heat than a boiler, so the gas reaching the fan is hot, often 300 to 400°C. Mild steel impellers lose strength and warp at the upper end. Designing for the upset temperature, with heat-resisting steel where needed, avoids failure.
Sparks and burning fibre
Light bagasse fibre burns in suspension and is carried through the flue as sparks, which wear the fan and can set fire to the bagasse store near the chimney. A settling chamber or spark arrestor before the fan reduces the carry-over.
Hot stops
When the unit stops at the end of a day or a power cut, a fan stopped full of hot gas can bow its shaft. Running the fan until the gas cools, or keeping the bearings well clear of the casing, prevents trouble.
Energy and control
Jaggery units run seasonally, often in daytime only, and the fire is adjusted batch by batch as each pan reaches the striking point. A fan with a variable speed drive or a good damper lets the operator match draft to the stage of boiling, saving bagasse. Adding a fourth pan or a juice preheater to use the hot gas saves more than the fan itself.
ID fan and FD fan in jaggery plant furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the last pan, before the chimney | At the furnace mouth, under the grate |
| What it handles | Hot gas with bagasse ash and sparks | Ambient air |
| Temperature at the fan | 250 to 400°C | Ambient |
| Main design concern | Temperature and sparks | Quick fire control |
| Control | Furnace draft | Firing rate by stage |
See also the Forced Draft (FD) Fan for jaggery plant furnaces.
Questions buyers ask
Does a jaggery furnace need an ID fan?
Traditional units use a tall chimney and natural draft. An ID fan gives steady draft regardless of weather, allows a shorter chimney and a longer flue under more pans, and makes it easier to control the fire.
What temperature is the flue gas in a jaggery furnace?
After the last pan the gas is typically 250 to 400°C, higher on traditional units with few pans. The fan must be designed for the hottest gas it can see, which sets the material.
How do you stop sparks from a jaggery furnace chimney?
Fit a settling chamber or spark arrestor in the flue before the fan, avoid excess air under the grate that lifts burning fibre, and keep the bagasse store away from the chimney.
Information needed to quote an ID fan for jaggery 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. Number of pans, cane crushed per day and the bagasse burnt per hour.
- Furnace. Grate area and chimney height.
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.