How paper coating line dryers work
Paper and board coating lines apply a pigment coating of kaolin clay or calcium carbonate with starch or latex binder by blade, rod or film press, then dry it before the next coat or the calender. Drying usually starts with gas-fired or electric infrared emitters to set the coating, followed by air flotation dryers that float the web between nozzle rows blowing hot air at typically 150 to 300°C, and finally steam cans. Each flotation section is a recirculation loop with a supply fan, a gas burner and nozzle boxes. Part of the humid air is exhausted to hold humidity and remove binder volatiles, through an exhaust fan and often heat recovery. Infrared dryers have their own combustion air and exhaust fans.
Where the FD fan sits
- Return air
- Make-up air
- Supply (FD) fan
- Burner
- Nozzle boxes
- Web
Each flotation section has its own supply fan in a recirculation loop. The fan draws return air mixed with make-up air at about 150 to 220°C and pushes it through the burner and supply duct to the upper and lower nozzle boxes. Outlet pressure is typically 200 to 400 mmWC. The fan is upstream of the burner, so it handles air below the nozzle temperature. Infrared dryers have separate small combustion air blowers.
Air at the FD fan
| Air temperature at the fan | 150 to 220°C recirculated air |
| Make-up air | Ambient air mixed into the loop |
| Dust | Fine coating dust carried with the return air |
| Pressure at the fan outlet | Typically 200 to 400 mmWC at the nozzles |
| Upset | Higher temperature on a web break before burner turndown |
| Density at the design temperature | About 0.76 kg/m³ at 190°C, 64% 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 paper coating line dryers
| Blade form | Backward curved, with a stable, rising pressure curve |
| Material of construction | Mild steel or stainless steel suited to 200 to 250°C |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | Variable speed drive on nozzle pressure |
| Also specified | Insulated casing, inspection door, and filter on the make-up air |
What this FD fan has to cope with
Web flutter and contact
Flotation dryers hold the web on an air cushion. If nozzle pressure drifts or pulses, the web flutters, touches the nozzles and the wet coating is marked. A fan with a stable curve and pressure control keeps the cushion steady. Sudden speed changes are avoided.
Coating dust on the impeller
Dried coating flakes in the return air deposit on the blades and inlet. Build-up lowers pressure and causes unbalance. Make-up air filters, smooth blade surfaces and inspection doors limit the effect. Cleaning during scheduled grade changes keeps the wheel in balance and nozzle pressure at its design value.
Uneven drying across the web
If the fan cannot hold pressure at the far nozzles, moisture streaks and curl appear. The fan is sized for actual nozzle and duct losses with margin for nozzle fouling. Measured nozzle pressures at each box, compared across the width, show whether the fan or the ducting is the cause.
Energy and control
Coating lines run many grades and coat weights, and drying demand changes with each. A variable speed drive on nozzle pressure lets the supply fans run at the velocity each grade needs. Lower velocity on light coats saves fan power and reduces heat loss from the dryer, so the savings show up on the gas bill as well as the electricity bill.
ID fan and FD fan in paper coating line dryers
| ID fan | FD fan | |
|---|---|---|
| Position | On the exhaust header, before heat recovery | In each flotation loop, before the burner |
| What it handles | Humid air with coating dust and volatiles | Recirculated hot drying air |
| Temperature at the fan | 120 to 180°C | 150 to 220°C |
| Main design concern | Deposits and dryer balance | Steady nozzle pressure |
| Control | Humidity or balance | Nozzle pressure |
See also the Induced Draft (ID) Fan for paper coating line dryers.
Questions buyers ask
Is the supply fan on an air flotation dryer an FD fan?
Functionally yes, because it pushes air into the dryer. But it handles hot recirculated air at about 150 to 220°C, so it is built as a hot air fan with cooled bearings. Its pressure must be steady, because the web floats on the air cushion it creates.
What pressure does a flotation dryer supply fan need?
Typically 200 to 400 mmWC at the nozzles, depending on the nozzle design, web tension and drying rate. The pressure must be steady, because the web floats on it. Upper and lower nozzle boxes are balanced so the web runs centrally between them, without touching either side.
Why does my coating dryer mark the web?
Often because nozzle pressure is unstable or uneven, so the web flutters and touches the nozzles. Checking fan pressure, fouling of the impeller and nozzle condition usually finds the cause. Uneven pressure between upper and lower nozzle boxes is another common cause, and is corrected with the balancing dampers.
Information needed to quote an FD fan for paper coating line 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.
- Nozzle data. Nozzle type, open area and required nozzle pressure per section.
- Air temperature. Return and make-up air temperatures at the fan 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.