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 ID fan sits
- Hot air generator
- Rotary drum dryer
- Lint cyclone
- Alkali wet scrubber
- Mist eliminator
- ID fan
- Stack
The fan sits after the scrubber and mist eliminator, where the gas is cool, saturated and mostly free of lint. Placed before the scrubber it would see hot, dusty acid vapour that attacks every common steel. Even after the scrubber, a fine acid mist reaches the fan. It holds the drum outlet at a slight suction against the drum, cyclone, scrubber and eliminator, typically 120 to 250 mmWC in total.
Gas at the ID fan
| Air temperature, normal | 40 to 60°C after the wet scrubber |
| Maximum or upset | Up to about 100°C if scrubber water fails |
| Dust at the fan | Fine charred lint that passes the cyclone and scrubber |
| Moisture and dew point | Saturated, with acid mist droplets |
| Corrosive species | Sulphuric acid mist; SO₂ and SO₃ ahead of the scrubber; dilute acid in the condensate |
| Density at the design temperature | About 1.08 kg/m³ at 55°C, 90% 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 cotton seed delinting dryers
| Blade form | Backward curved or radial, moulded in FRP or fabricated in SS 316 |
| Material of construction | FRP with a chemical-resistant resin, or SS 316; rubber-lined mild steel for larger fans |
| Drive and arrangement | Belt driven arrangement 9, with the motor outside the gas path |
| Bearings and cooling | Bearings away from the casing, with a shaft seal against acid mist |
| Flow control | Variable speed drive holding drum outlet suction |
| Also specified | Casing drain to the scrubber sump, acid-resistant fasteners, and an inlet temperature alarm |
What this ID fan has to cope with
Acid corrosion of the fan
Sulphuric acid mist after the scrubber settles on the impeller and casing and concentrates as the film dries. Mild steel, even painted, is eaten through within months, starting at welds and the casing floor. FRP or SS 316 wetted parts, acid-resistant fasteners and a sealed shaft entry make the fan last.
Hot gas reaching an FRP fan
If the scrubber pump stops, hot acid gas from the drum goes straight to the fan and can soften FRP or blister a rubber lining. A temperature alarm before the fan that cuts the hot air generator, and a resin chosen for the upset temperature, protect it.
Lint loading the scrubber and fan
Lint that escapes a poorly maintained cyclone loads the scrubber packing and forms a wet felt on the fan blades, which unbalances the wheel. A correctly sized cyclone, regular emptying of its hopper and an inspection door on the fan casing prevent build-up between cleaning stops.
Acid fumes escaping into the plant
When the fan loses suction, acid vapour leaks from the drum seals and harms workers, motors and structures. A fan with margin for a fouled scrubber, a suction gauge at the drum outlet and an interlock that stops acid dosing on fan failure prevent it.
Energy and control
Delinting plants run in a season after the cotton harvest, often in long shifts. The exhaust flow sets how much hot air the generator must make, so excess exhaust wastes fuel as well as fan power. A variable speed drive on drum suction lets the fan follow seed throughput and hold only the suction needed, which also reduces the acid mist carried to the stack.
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 Forced Draft (FD) Fan for cotton seed delinting dryers.
Questions buyers ask
What material should a cotton seed delinting exhaust fan be made of?
The exhaust carries sulphuric acid mist and condensate, so mild steel corrodes quickly. FRP with a chemical-resistant resin and SS 316 are the usual choices, with rubber-lined mild steel for large fans. The fan is placed after the scrubber to reduce both temperature and acid load at the impeller.
Why is there an acid smell around the delinting dryer?
Acid vapour escapes when the drum is not under suction. Typical causes are a fouled scrubber or mist eliminator, an undersized or worn exhaust fan, or leaking drum seals. Measuring suction at the drum outlet and checking the fan against its curve usually shows where the problem lies.
Can an FRP fan handle the delinting dryer exhaust temperature?
After the scrubber the exhaust is typically 40 to 60°C, which suits FRP. The risk is an upset when scrubber water fails and hot gas reaches the fan. A temperature alarm that shuts the hot air generator, and a resin rated for the upset temperature, keep an FRP induced draft fan safe.
Information needed to quote an ID fan for cotton seed delinting dryers
- 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.
- Acid process. Delinting method, acid concentration and dosing rate, and the neutralising agent.
- Scrubber. Scrubber type, liquor used, and outlet 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.