How yarn and fibre dryers work
After package or hank dyeing, yarn and loose fibre are hydro-extracted to remove free water and then dried. Package yarn is dried in pressure dryers, where hot air is forced through the wound packages on their carriers, or in radio frequency dryers; hanks pass through conveyor or chamber dryers; loose fibre is spread on a perforated conveyor and dried by hot air blown through the layer. Air is heated by steam or thermic fluid coils, typically to 90 to 140°C depending on the fibre, with acrylic and some synthetics dried cooler than cotton. Most dryers recirculate the air and bleed off part of it as humid exhaust carrying lint and fibre fly, replacing it with fresh air.
Where the ID fan sits
- Heater coils
- Circulation fan
- Yarn packages, hanks or fibre bed
- Return duct
- Lint filter
- Exhaust (ID) fan
- Atmosphere
The exhaust fan draws from the return duct of the recirculating loop, after the air has passed through the yarn or fibre and picked up moisture, and after a lint filter or screen. It discharges to the roof. Its suction is low, typically 30 to 100 mmWC, but it decides how much fresh air is drawn into the loop. On pressure package dryers the exhaust is a bleed from the high-pressure loop, so the fan itself sees little resistance.
Gas at the ID fan
| Air temperature, normal | 60 to 90°C at the exhaust |
| Maximum or upset | Up to about 140°C at the end of a cycle when the material is nearly dry |
| Dust at the fan | Lint and fibre fly that pass the filter |
| Moisture and dew point | Close to saturation early in the cycle; condensation in cold ducts and casing |
| Corrosive species | Traces of dye auxiliaries and softeners; mildly corrosive condensate |
| Density at the design temperature | About 1.00 kg/m³ at 80°C, 83% 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 yarn and fibre dryers
| Blade form | Backward curved, with clean blade edges that shed lint |
| Material of construction | Coated mild steel, or SS 304 for wetted parts |
| Drive and arrangement | Direct drive or belt driven, motor outside the air stream |
| Bearings and cooling | Standard bearings with a heat slinger |
| Flow control | Variable speed drive on exhaust humidity or dryer cycle stage |
| Also specified | Lint filter upstream, casing drain and cleaning door |
What this ID fan has to cope with
Lint on the impeller
Lint and fibre fly that escape the filter catch on blade edges and build up on the back plate, especially when damp. The wheel goes out of balance and flow falls. An effective, easily cleaned lint filter, smooth blades and a cleaning routine tied to the batch count keep the fan clean.
Condensation in casing and ducts
Exhaust air early in the drying cycle is close to saturation. In unheated ducts it condenses, wetting the lint into a paste and corroding the casing floor. Insulated ducts, a drain at the lowest point and stainless or coated wetted parts prevent the damage to casing and impeller.
Over-drying and wasted heat
Exhaust at a fixed flow through the whole cycle removes too much warm air near the end of drying, when little water is left to carry away. Heat is wasted and outer layers of yarn can over-dry and turn harsh. Speed control that follows exhaust humidity or cycle stage reduces exhaust as the material dries.
Energy and control
Batch dryers go through a cycle: a short period of fast evaporation while the material is wet, then a long falling-rate period. The exhaust needed falls as drying proceeds. A variable speed drive that follows humidity or a set cycle profile cuts both fan power and heat loss. On continuous fibre dryers, separate exhaust control for each zone does the same job.
ID fan and FD fan in yarn and fibre dryers
| ID fan | FD fan | |
|---|---|---|
| Position | On the return duct, after the lint filter | In the loop after the heater coils |
| What it handles | Humid exhaust air with lint | Hot circulating air |
| Temperature at the fan | 60 to 90°C | 90 to 140°C |
| Main design concern | Lint and condensation | Pressure through the packages |
| Control | Exhaust humidity | Cycle profile or package pressure drop |
See also the Forced Draft (FD) Fan for yarn and fibre dryers.
Questions buyers ask
What does the exhaust fan do on a yarn package dryer?
It removes humid air from the drying loop so that the recirculated air can go on absorbing moisture from the yarn. The circulation fan moves the air through the packages; the exhaust fan bleeds off part of this air with the water it has picked up, and draws fresh air in to replace it.
Why is lint blocking my fibre dryer exhaust fan?
Lint and fibre fly in the exhaust stick to the fan blades, especially when the air is damp. A lint filter before the fan, cleaned on a schedule, is the first defence. Smooth blades and a casing door for cleaning help, and insulation that stops condensation in the duct slows the build-up.
What temperature does a yarn dryer exhaust fan handle?
Typically 60 to 90°C, rising towards 140°C at the end of the drying cycle when the yarn or fibre is nearly dry and the air leaving it is barely cooled. The fan is designed for the upper value, with a heat slinger to protect the bearings and a rating corrected for warm, humid air.
Information needed to quote an ID fan for yarn and fibre 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.
- Dryer type. Package pressure dryer, hank dryer or loose fibre conveyor dryer, and its capacity per batch or per hour.
- Material and cycle. Fibres dried, moisture after hydro-extraction and drying cycle time.
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.