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 FD fan sits
- Fresh air inlet
- Heater coils
- FD (circulation) fan
- Supply plenum
- Yarn packages, hanks or fibre bed
- Return duct
The circulation fan sits in the recirculating loop after the heater coils, so it handles hot air. On package pressure dryers it forces air through the carriers and packages, often reversing direction during the cycle, and develops a high pressure, typically several hundred mmWC. On hank and loose fibre conveyor dryers it blows hot air through the material at much lower pressure but larger volume. Fresh air enters the loop to replace what the exhaust fan removes.
Air at the FD fan
| Air temperature at the fan | 90 to 140°C after the heater coils |
| Moisture | Humid recirculated air with fresh air make-up |
| Dust | Lint carried round the loop in the recirculated air |
| Pressure at the fan outlet | High on package dryers, several hundred mmWC; lower on conveyor dryers |
| Density at the design temperature | About 0.90 kg/m³ at 120°C, 75% 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 yarn and fibre dryers
| Blade form | Backward curved for high pressure on package dryers; backward inclined for volume on conveyor dryers |
| Material of construction | Coated mild steel, or stainless steel for wetted parts |
| Drive and arrangement | Direct coupled with a variable speed drive |
| Bearings and cooling | Shaft cooling disc and heat shield for 120 to 140°C air |
| Flow control | Variable speed drive with cycle profiles |
| Also specified | Lint screen upstream, inspection door and insulated casing |
What this FD fan has to cope with
Uneven drying of packages
If air pressure across the carrier varies, outer packages dry before inner ones and the whole batch must run longer to finish the wettest. A fan with enough pressure margin, and stable delivery as packages dry and become more permeable, keeps drying even across the carrier.
Hot bearings
The fan handles air up to about 140°C, and heat travels along the shaft into the bearings. Without protection the grease breaks down and bearings fail early. A cooling disc, a heat shield, high-temperature grease and bearings mounted clear of the hot casing prevent it.
Lint in the circulation loop
Lint goes round with the recirculated air and builds up on fan blades and on the face of the heater coils, cutting both flow and heat transfer. A lint screen ahead of the heater and fan, cleaned every batch, keeps air flow and drying time steady.
Energy and control
The circulation fan is the largest motor on most yarn dryers. As packages dry they let air through more easily, so the fan can slow down without losing flow; fixed speed wastes power. A variable speed drive with a cycle profile, or controlled on differential pressure across the packages, saves energy without lengthening the drying cycle.
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 Induced Draft (ID) Fan for yarn and fibre dryers.
Questions buyers ask
What pressure does a package yarn dryer fan need?
Package pressure dryers force air through densely wound packages, so the circulation fan develops a high pressure, typically several hundred mmWC, depending on package density, the number of packages per spindle and the carrier design. Conveyor dryers for hanks or loose fibre need far less pressure but more volume.
Why does the yarn dryer fan bearing overheat?
The fan handles air at 90 to 140°C, and heat conducts along the shaft to the bearings. Without a cooling disc or heat shield the grease breaks down. Fitting a cooling disc, using high-temperature grease and mounting the bearings clear of the casing solve most cases of hot bearings on dryer fans.
Can the dryer fan be controlled during the drying cycle?
Yes. A variable speed drive can run the circulation fan at high speed while the yarn is wet and slow it as the packages dry and become more permeable. This saves power and avoids over-drying the outer layers. The forced draft fan profile is set per yarn count and package type.
Information needed to quote an FD fan for yarn and fibre 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.
- Packages. Package type, density and number per carrier, or fibre bed depth on conveyor dryers.
- Heater. Steam or thermic fluid coils, the air temperature required and the coil resistance.
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.