How tissue machine Yankee hoods work
A tissue machine dries the sheet in one pass on a large steam-heated Yankee cylinder, typically 4 to 6 m in diameter. Steam alone cannot dry the sheet at modern speeds, so a high-velocity hood covers most of the Yankee and blows hot air at about 350 to 500°C onto the sheet at 100 to 150 m/s through nozzles. The air is heated by gas burners in each hood half. Most of the air is recirculated: a supply fan pulls return air from the hood and pushes it through the burner back to the nozzles. An exhaust fan removes part of the return air, holding humidity in the hood and keeping the hood in balance with the room. Exhaust heat is usually recovered to heat the combustion and make-up air and process water.
Where the FD fan sits
- Hood return
- Supply (FD) fan
- Burner chamber
- Supply duct
- Hood nozzles
- Yankee sheet
Each hood half, wet end and dry end, usually has its own supply fan. The fan draws return air at about 250 to 350°C, mixed with some make-up air, and pushes it through the burner chamber, which raises it to 350 to 500°C, then to the nozzle boxes. The fan sits before the burner so it handles the lower temperature. Outlet pressure is high, typically 500 to 900 mmWC, to drive the 100 to 150 m/s nozzle velocity. A small separate blower supplies combustion air to the burner.
Air at the FD fan
| Air temperature at the fan | 250 to 350°C recirculated hood air |
| Humidity | High, typically 0.3 to 0.6 kg water per kg dry air |
| Dust | Tissue fibre carried with the return air |
| Pressure at the fan outlet | Typically 500 to 900 mmWC to the nozzles |
| Upset | Higher temperature on a sheet break before burner turndown |
| Density at the design temperature | About 0.62 kg/m³ at 300°C, 51% 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 tissue machine Yankee hoods
| Blade form | Backward curved, high efficiency at high pressure |
| Material of construction | Stainless steel or heat-resisting steel impeller, stainless casing |
| Drive and arrangement | Direct coupled, arrangement 8, with heat shield and cooling disc |
| Bearings and cooling | Pedestal bearings outside the hot zone with shaft cooling disc |
| Flow control | Variable speed drive on nozzle pressure or impingement velocity |
| Also specified | Insulated casing, smooth fibre-free inlet, and inspection access |
What this FD fan has to cope with
High pressure at high temperature
The fan must deliver high pressure while handling low-density hot air, so tip speed is high and the impeller is stressed at temperature. Impeller design checks stress at the actual air temperature, and materials keep strength at the design temperature with margin for upsets. Overspeed checks follow.
Fibre on the impeller
Return air carries tissue fibre, which collects on the impeller and in the inlet. Build-up reduces pressure and unbalances the wheel, and it is a fire risk. Smooth, open blade passages and regular cleaning keep it in check. Inspection doors allow cleaning during short machine stops without removing ducting.
Uneven drying across the sheet
Nozzle velocity must be even across the machine width. A fan whose pressure sags as filters or nozzles foul leaves wet streaks. Speed control on nozzle pressure holds velocity as the system changes. Cross-direction moisture profile readings show when nozzles or the fan need attention.
Energy and control
Supply fans are among the largest motors on a tissue machine, and impingement velocity is a strong lever on drying rate. Grade changes, lighter basis weights and slower speeds need less velocity. A variable speed drive set on nozzle pressure lets operators reduce velocity when it is not needed instead of throttling, with significant savings over a year of grade changes.
ID fan and FD fan in tissue machine Yankee hoods
| ID fan | FD fan | |
|---|---|---|
| Position | On the return duct, before heat recovery | On the return loop, before the burner |
| What it handles | Humid exhaust air with fibre | Recirculated hot hood air |
| Temperature at the fan | 250 to 350°C | 250 to 350°C |
| Main design concern | Condensation and fire risk | High pressure at high temperature |
| Control | Hood humidity and balance | Nozzle pressure |
See also the Induced Draft (ID) Fan for tissue machine Yankee hoods.
Questions buyers ask
Is the supply fan on a Yankee hood a forced draft fan?
In function, yes: it pushes air into the hood. But it handles recirculated hood air at 250 to 350°C, not ambient air, so it is built as a hot air fan with heat-resisting materials and cooled bearings. It is usually one of the largest motors on the machine.
What pressure does a tissue hood supply fan need?
Typically 500 to 900 mmWC to drive the nozzle velocity of about 100 to 150 m/s, plus burner and duct losses. The exact figure depends on hood design and nozzle open area. The fan is selected with margin for nozzle fouling and the need to raise velocity on heavier grades.
Why should the supply fan sit before the burner?
Because the air there is cooler, about 250 to 350°C instead of 350 to 500°C after firing. The fan is denser, smaller and needs less exotic material, and the burner is on the pressure side. Combustion products then pass straight to the nozzles without passing through the fan.
Information needed to quote an FD fan for tissue machine Yankee hoods
- 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 velocity, open area and pressure required at the nozzle box.
- Air condition. Return air temperature and humidity 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.