How paper machine dryer sections work
In the dryer section of a paper machine, the wet web from the press section, at about 40 to 50% dryness, is dried to 90 to 95% by passing it over steam-heated cast iron cylinders. The water evaporated from the sheet is carried away by air under a hood. Modern machines use a closed hood, with supply air at about 90 to 110°C blown into the pockets between cylinders, and exhaust air at about 80 to 90°C with a high moisture content. Older machines have open hoods or a canopy with much more air at lower humidity. Exhaust heat is recovered in air-to-air and air-to-water heat exchangers, which preheat the supply air and process or building water. Tissue machines use a Yankee cylinder with a gas-fired hot air hood instead.
Where the FD fan sits
- Fresh air intake
- Heat recovery unit
- FD (supply) fan
- Steam coils
- Pocket ventilation ducts
- Dryer pockets
The supply fans are in the machine room basement, after the air-to-air heat recovery units, which preheat the fresh air with heat from the exhaust. The fan pushes the air through steam coils to about 90 to 110°C and into the pocket ventilation ducts. The pressure covers the heat recovery, the coils and the pocket blow boxes, typically 150 to 300 mmWC.
Air at the FD fan
| Air temperature at the fan | 30 to 60°C after heat recovery |
| Air after the coils | 90 to 110°C, downstream of the fan |
| Dust | Clean outdoor or machine room air |
| Humidity | Fresh air, preheated |
| Pressure at the fan outlet | Typically 150 to 300 mmWC |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% 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 machine dryer sections
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel with coating |
| Drive and arrangement | Direct coupled with a variable speed drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on supply air flow |
| Also specified | Inlet filter and silencer |
What this FD fan has to cope with
Uneven pocket ventilation
If the supply air does not reach every pocket evenly, the sheet dries unevenly across and along the machine, giving moisture streaks and curl. Correct fan duty and well-designed ducts give even flow.
Heat recovery fouling
Fibre and deposits foul the heat recovery units, raising resistance and reducing the supply air flow. A speed-controlled fan can hold the flow, but the units still need cleaning.
Balance with the exhaust
If the supply air is too low compared with the exhaust, cold air leaks into the hood; if too high, humid air escapes into the hall. The supply and exhaust fans are controlled together.
Energy and control
Supply air fans run continuously on a paper machine. Heat recovery from the exhaust to the supply air saves a large share of the steam needed to heat the air. Variable speed drives on the supply fans allow the air flow to be matched to machine speed and grade, saving power and steam.
ID fan and FD fan in paper machine dryer sections
| ID fan | FD fan | |
|---|---|---|
| Position | After the hood and heat recovery | After heat recovery, before the steam coils |
| What it handles | Humid hood air with fibre | Preheated fresh air |
| Temperature at the fan | 80 to 90°C | 30 to 60°C |
| Main design concern | Condensation and corrosion | Even pocket ventilation |
| Control | Hood zero level and humidity | Supply air flow |
See also the Induced Draft (ID) Fan for paper machine dryer sections.
Questions buyers ask
What is the supply air fan on a paper machine hood?
It is the forced draft fan for the dryer section, pushing preheated air through steam coils into the pockets between the dryer cylinders. It replaces the humid air removed by the exhaust fans.
Is the supply air preheated before the fan?
Usually yes. Fresh air is preheated in an air-to-air heat recovery unit using exhaust heat, so the fan handles air at about 30 to 60°C. The steam coils after the fan heat it to the final temperature.
Why is pocket ventilation important?
Without fresh, dry air in the pockets between the cylinders, humidity builds up and the sheet dries unevenly. Pocket ventilation gives even drying and allows higher machine speed.
Information needed to quote an FD fan for paper machine dryer sections
- 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.
- Supply air. Air flow, temperature after the coils, and the number of pocket ventilation boxes.
- Heat recovery. Air side pressure drop of the heat recovery unit.
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.