How edible oil solvent extraction plants work
A solvent extraction plant recovers oil from oilseed cake, soybean flakes or rice bran by washing the material with hexane in a continuous extractor. The oil-rich miscella goes to distillation, and the solvent-wet meal goes to a desolventiser-toaster heated by steam, where hexane is stripped and recovered by condensers. The meal then passes a dryer-cooler, where heated air dries it and ambient air cools it to near ambient before storage. Steam comes from a separate boiler, often rice husk or coal fired, which has its own draft fans. The fans covered here are those of the meal dryer-cooler: an air supply fan pushing air through a steam air heater into the dryer decks, and an exhaust fan drawing moist air and meal fines through cyclones. The whole area is a hazardous zone because of hexane.
Where the FD fan sits
- Inlet filter
- FD fan
- Steam air heater
- Dryer decks
- Cooler decks (separate air)
The fan draws ambient air from a safe location through a filter and delivers it to a finned steam air heater, which raises it to about 100 to 130°C before it enters the dryer decks. Cooling air for the cooler decks comes from a separate fan or from the same fan before the heater. Outlet pressure is moderate, typically 100 to 300 mmWC. Taking the inlet air from outside the hazardous zone reduces the risk of drawing in vapour.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the heater | 100 to 130°C, downstream of the fan |
| Dust | Filtered air |
| Pressure at the fan outlet | Typically 100 to 300 mmWC |
| Hazardous area | Fan location usually Zone 1 or Zone 2 for hexane |
| 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 edible oil solvent extraction plants
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel, with non-sparking inlet cone or rub ring |
| Drive and arrangement | Direct coupled with a flameproof motor |
| Bearings and cooling | Standard bearings with temperature monitoring |
| Flow control | Variable speed drive on air flow or dryer temperature |
| Also specified | Inlet filter, earthing and bonding |
What this FD fan has to cope with
Drawing hexane into the air system
If the fan inlet is in an area where hexane vapour collects, vapour passes over the hot steam coil and into the dryer. Taking air from a safe area and duct sealing avoid this.
Air heater fouling
Meal dust from plant air fouls the fins of the steam heater, reducing heat and flow. An inlet filter keeps the heater clean and holds flow steady. A differential pressure gauge shows when to clean it.
Balance with the exhaust fan
If supply air exceeds exhaust, the dryer goes positive and dust and vapour escape. Supply and exhaust fans must be controlled together to keep a slight suction. A pressure transmitter in the dryer is the usual reference.
Energy and control
The dryer air fan runs continuously while the plant runs. Air flow should follow meal throughput and moisture. Speed control saves fan power and steam, because every cubic metre of air heated in the dryer and then exhausted carries steam energy with it. Interlocks with the exhaust fan keep the dryer under suction at every speed.
ID fan and FD fan in edible oil solvent extraction plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the dryer-cooler cyclones | Before the steam air heater |
| What it handles | Warm humid air with meal fines | Clean ambient air |
| Temperature at the fan | 50 to 80°C | Ambient |
| Main design concern | Hazardous area and fines build-up | Safe air intake and balance with exhaust |
| Control | Dryer-cooler suction | Air flow or dryer temperature |
See also the Induced Draft (ID) Fan for edible oil solvent extraction plants.
Questions buyers ask
What is the FD fan in a solvent extraction plant?
It is the air supply fan of the meal dryer-cooler, pushing ambient air through a steam air heater into the dryer decks. Cooling air for the cooler is often supplied by a second fan.
Does the dryer air fan need a flameproof motor?
Usually yes, because the dryer-cooler is in the hexane hazardous area. The motor, construction and earthing must suit the zone classification of the location. Non-sparking inlet parts and antistatic belts, if belt driven, complete the build.
Where should the dryer-cooler air fan take its air from?
From a safe area outside the solvent zone, through a filter. This avoids drawing hexane vapour into the air heater and dryer. The intake should be well away from vents, pumps and the extractor building.
Information needed to quote an FD fan for edible oil solvent extraction plants
- 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.
- Hazardous area. Zone classification and gas group at the fan location, and where inlet air can be taken from.
- Air heater. Steam pressure, air heater pressure drop and required air temperature.
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.