How cotton seed delinting dryers work
Planting seed from the gin carries short lint fuzz that must be removed before sowing. In acid delinting, the fuzzy seed is wetted with dilute sulphuric acid and passed through a rotary drum dryer, where hot air, typically 100 to 160°C at the inlet, evaporates the water and concentrates the acid, which weakens the lint until it is brittle. The seed itself must be kept cool enough to protect germination. The charred lint is then rubbed off in tumblers and the seed is neutralised with lime or ammonia. Hot air comes from an oil, gas or biomass hot air generator. The dryer exhaust, carrying acid vapour, water vapour and lint dust, passes a cyclone and usually an alkali wet scrubber before the exhaust fan.
Where the FD fan sits
- Inlet filter
- FD (supply) fan
- Hot air generator
- Hot air duct
- Rotary drum dryer
The supply fan is at the cold end of the hot air generator, taking ambient air and pushing it through the furnace or heat exchanger into the drum inlet. Keeping it ahead of the generator means it handles cool, clean air. It works against the generator, the hot duct and the drum, typically 80 to 200 mmWC, and together with the exhaust fan it sets the pressure balance in the drum. The burner has its own small combustion air fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the hot air generator | 100 to 160°C, downstream of the fan |
| Dust | Lint and seed dust in the plant; an inlet filter is needed |
| Pressure at the fan outlet | Typically 80 to 200 mmWC |
| 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 cotton seed delinting dryers
| Blade form | Backward curved, for stable flow on clean air |
| Material of construction | Mild steel with a coating suited to acid fumes in the plant |
| Drive and arrangement | Belt driven or direct coupled |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive or damper, with temperature held by the burner |
| Also specified | Inlet filter against lint, and a flow or pressure switch interlocked with the burner |
What this FD fan has to cope with
Lint clogging the inlet
Lint floats in the air of every delinting plant and collects on the fan inlet or filter, reducing air flow. Less air through the generator means hotter air at the drum for the same firing, and a risk to the seed. A filter sized for lint, regular cleaning and a low-flow alarm protect the process.
Acid fumes attacking the fan outside
Acid vapour in the plant attacks painted mild steel even on a clean-air fan. Acid-resistant coatings, stainless fasteners and an air intake placed away from the drum, tumblers and scrubber stop external corrosion of the casing, base and motor. Inspection at each season's start catches coating damage early.
Imbalance with the exhaust fan
If the supply fan delivers more than the exhaust fan removes, the drum goes positive and acid vapour leaks at the seals. Letting the exhaust fan control drum suction while the supply fan sets air flow, and sizing the exhaust fan with margin, keeps the balance.
Energy and control
The supply fan runs with the hot air generator, so its flow also sets fuel use. A variable speed drive lets the flow be set for seed throughput and moisture, instead of running full flow and wasting heat to the exhaust. Heat recovery from the acid exhaust is difficult, so cutting excess air flow is the main saving available on most delinting lines.
ID fan and FD fan in cotton seed delinting dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the scrubber, before the stack | At the cold end of the hot air generator |
| What it handles | Saturated air with acid mist and lint | Clean ambient air |
| Temperature at the fan | 40 to 60°C | Ambient, 25 to 45°C |
| Main design concern | Acid corrosion | Steady flow and air temperature |
| Control | Drum suction | Seed throughput |
See also the Induced Draft (ID) Fan for cotton seed delinting dryers.
Questions buyers ask
What does the FD fan do on a delinting dryer?
It is the hot air supply fan. It pushes ambient air through the hot air generator into the drum dryer, where the hot air dries the acid-treated seed and concentrates the acid that chars the lint. It handles clean ambient air, so a mild steel backward curved forced draft fan is the usual choice.
Why must drying air temperature be controlled in cotton seed delinting?
Too little heat leaves lint uncharred and the seed hard to clean; too much damages the embryo and lowers germination. Supply fan flow and generator firing together set the air temperature, so a fan with stable flow and a well-controlled burner are needed for consistent seed quality.
How are supply and exhaust fans balanced on a delinting dryer?
The exhaust fan holds a slight suction at the drum outlet so acid fumes cannot escape, while the supply fan delivers the hot air flow. The exhaust fan is sized with margin for scrubber fouling, so it can always remove more than the supply fan delivers, and the supply fan is interlocked to stop if exhaust fails.
Information needed to quote an FD fan for cotton seed delinting dryers
- 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.
- Hot air generator. Fuel, rating and air-side resistance.
- Drum. Seed throughput, drum size and the required air temperature at the drum inlet.
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.