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 FD fan sits
- Air intake
- FD fan
- Air box under the grate
- Bagasse grate
- Furnace
- Flue under pans
The fan sits beside the furnace mouth and delivers air into an air box or ash pit under the grate, sometimes with a small branch to overfire nozzles above the fire. The operator feeds dried bagasse by hand or by a simple feeder, and the fan provides the air to burn it quickly. Pressure is low, typically 30 to 80 mmWC, because the bed of dried bagasse is thin and open.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 40°C |
| Dust | Dusty yard air with bagasse fines |
| Pressure at the fan outlet | Typically 30 to 80 mmWC |
| Fuel | Sun-dried bagasse, 25 to 35% moisture |
| Turndown | Wide; fired hard to boil, eased back at striking |
| 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 jaggery plant furnaces
| Blade form | Backward curved or radial, small and direct driven |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Damper or variable speed drive under the operator's control |
| Also specified | Inlet screen and a backflow flap |
What this FD fan has to cope with
Wet bagasse
When the bagasse has not dried fully in the sun, the fire dies down and the operator raises the fan air to revive it, which cools the furnace further and makes more smoke. Drying the bagasse properly and keeping the grate area right for the unit matter more than a bigger fan.
Bagasse fines in the intake
The fan stands in a yard full of loose bagasse fibre, which blocks the inlet guard and coats the blades. A screened intake facing away from the bagasse heap and a quick clean each morning keep the fan delivering its air.
Too much air
Excess air cools the gas under the pans, carries heat up the chimney and lifts burning fibre into the flue as sparks. A fan sized for the furnace, with a damper or speed control within reach of the operator, avoids this.
Energy and control
A jaggery unit is small and the fan is a few kilowatts, but excess air is the main heat loss. Controlling the fan to match the stage of boiling saves bagasse, which can then be sold or used for more cane. Units with more pans in series use the heat better and need less air per kg of jaggery.
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 Induced Draft (ID) Fan for jaggery plant furnaces.
Questions buyers ask
What size FD fan does a jaggery furnace need?
Small: typically a few hundred to a few thousand m³/h at 30 to 80 mmWC, depending on the bagasse burnt per hour. The fan should let the operator change the firing rate quickly as each pan approaches the striking point.
Does a forced draft fan improve jaggery quality?
Indirectly. Steadier, controllable firing lets the operator bring each batch to the striking point without scorching, which helps colour and consistency. It also reduces the bagasse burnt per kg of jaggery.
Can a jaggery furnace work with only an ID fan?
Yes. Many improved jaggery units use only an ID fan or a chimney. A forced draft fan under the grate gives faster response and better combustion, especially when the bagasse is not fully dry.
Information needed to quote an FD fan for jaggery 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.
- Firing. Bagasse burnt per hour and the grate area.
- Control. Manual damper or variable speed control.
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.