How sugar dryers and sulphitation work
In a plantation white sugar factory, two parts of the process use fans. Wet sugar from the centrifugals, at about 0.5 to 1.5% moisture, is dried in a rotary drum or a fluid bed or louvre dryer with air heated to about 80 to 100°C by exhaust steam coils, and then cooled with ambient air before grading. The dryer exhaust carries sugar dust to a wet scrubber or cyclone and is drawn out by an exhaust fan. In the sulphitation section, sulphur is burnt with air in a sulphur burner to make SO₂ gas, which is cooled and absorbed in the juice and syrup sulphiters. Air is supplied by a blower or compressor, preferably dried, and the sulphiter vents release SO₂-laden moist air that must be extracted.
Where the FD fan sits
- Air intake and filter
- FD (supply) fan
- Steam air heater
- Sugar dryer
- Cooler (separate air)
The supply fan sits before the steam air heater, so it handles ambient air. It pushes the air through the heater into the dryer, and a separate fan supplies ambient air to the cooler section. In sulphitation, a blower supplies air to the sulphur burner, often through an air dryer with silica gel or a refrigeration unit, because moist air forms sulphuric acid and sublimed sulphur in the burner. Pressure for the dryer fan is typically 100 to 250 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 40°C |
| Air after the heater | 80 to 100°C, downstream of the fan |
| Dust | Clean filtered air |
| Humidity | Humid air reduces drying capacity and harms the sulphur burner |
| Pressure at the fan outlet | Typically 100 to 250 mmWC for the dryer |
| 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 sugar dryers and sulphitation
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel with coating |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Damper or variable speed drive |
| Also specified | Inlet filter on dryer air; air dryer before the sulphur burner blower |
What this FD fan has to cope with
Humid inlet air
In the humid months, air carries more moisture and dries the sugar less, and the sugar can cake in storage. A supply fan with margin and good control of air temperature hold the final moisture.
Moist air in the sulphur burner
Moisture in the burner air forms sulphuric acid and sublimed sulphur, which block pipes and corrode the burner and SO₂ cooler. Drying the air before the blower, and a steady blower flow, keep the burner clean.
Filter loading
Sugar dust in the sugar house loads the dryer air filter, and a fixed-speed fan delivers less air. Regular cleaning or a fan with speed margin keeps the flow steady.
Energy and control
Sugar dryer fans run steadily through the season and use exhaust steam for heating, so the energy cost is low. Correct fan sizing and keeping filters clean are the main savings. On the sulphur burner, matching the air flow to sulphur burning rate avoids excess air that dilutes the SO₂ and reduces absorption.
ID fan and FD fan in sugar dryers and sulphitation
| ID fan | FD fan | |
|---|---|---|
| Position | After the scrubber, or on the sulphiter vents | Before the steam heater, or at the sulphur burner |
| What it handles | Saturated air with sugar; SO₂ vent gas | Ambient air |
| Temperature at the fan | 40 to 55°C | Ambient |
| Main design concern | Sugar crust and acid corrosion | Dry, steady air |
| Control | Dryer suction | Air flow |
See also the Induced Draft (ID) Fan for sugar dryers and sulphitation.
Questions buyers ask
What is the FD fan on a sugar dryer?
It is the forced draft fan that supplies air through the steam heater into the sugar dryer. A separate fan supplies ambient air to the cooler. Both handle clean air and are placed before any heater.
Why does the sulphur burner need dry air?
Moisture in the air reacts with the SO₂ and SO₃ formed in the burner to make sulphuric acid, which corrodes the burner and pipes, and sublimed sulphur blocks the lines. Dry air keeps the burner running cleanly.
Is the sulphur burner air supplied by a fan or a compressor?
Both are used. Many factories use a blower or a small compressor, often with an air dryer. The choice depends on the pressure needed to push the SO₂ gas into the sulphiters.
Information needed to quote an FD fan for sugar dryers and sulphitation
- 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.
- Dryer air. Air flow and temperature for the dryer and cooler.
- Sulphur burner. Sulphur burning rate and the air pressure needed at the burner.
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.