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 ID fan sits
- Yankee hood return
- Return air duct
- Exhaust (ID) fan
- Air-to-air heat recovery
- Air-to-water heat recovery
- Stack
The exhaust fan usually draws from the return duct before the heat recovery, so it handles air at 250 to 350°C with high water vapour content. Some lines place it after heat recovery to keep it cooler, at the cost of running the exchangers under suction. It works against the return duct, the exchangers and the stack, typically 150 to 300 mmWC. The exhaust rate controls hood humidity and the balance between hood and machine room.
Gas at the ID fan
| Gas temperature, normal | 250 to 350°C before heat recovery |
| Maximum or upset | Up to 450°C on a sheet break if burners are not turned down |
| Dust at the fan | Tissue fibre and fine dust, highly combustible |
| Moisture and dew point | Very humid, typically 0.3 to 0.6 kg water per kg dry air; dew point above 70°C |
| Other | Products of gas combustion; condensation on cold surfaces during shutdown |
| 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. |
ID Fan selection for tissue machine Yankee hoods
| Blade form | Backward curved, smooth and without pockets for fibre to collect |
| Material of construction | Stainless steel, typically SS 304 or SS 316, for hot humid service with condensation on shutdown |
| Drive and arrangement | Direct coupled, arrangement 8, with heat shield |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | Variable speed drive on hood humidity or balance |
| Also specified | Insulated casing, casing drain, inspection door, and water spray or fire detection connections |
What this ID fan has to cope with
Tissue dust and fire
Tissue fibre is fine and flammable, and in a hot exhaust it can smoulder in deposits. Hood fires are a recognised risk on tissue machines. Smooth surfaces, accessible inspection doors, regular cleaning and fire detection with spray connections in the duct and fan are used.
Condensation on cooling
The exhaust is so humid that any surface below about 70°C collects water. On shutdown, condensate mixes with fibre and corrodes mild steel. Stainless construction, insulation and a drain at the casing low point handle it. Running the fan briefly on dry air before shutdown helps.
Temperature spike on sheet break
When the sheet breaks, the Yankee stops removing moisture and the hood temperature rises quickly unless the burners turn down. The fan must accept the short excursion, and the control should ramp burners on a break signal. Bearings and seals are rated for the break temperature.
Energy and control
Hood exhaust rate is a direct lever on machine energy. Exhausting more than needed drags hot air and its gas firing out of the loop; exhausting too little raises hood humidity and slows drying. A variable speed drive on hood humidity, measured continuously, keeps the exhaust near its optimum, and the fan's power is small compared with the gas it saves. Heat recovery downstream amplifies the benefit.
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 Forced Draft (FD) Fan for tissue machine Yankee hoods.
Questions buyers ask
What does the exhaust fan do on a Yankee hood?
It removes part of the humid return air from the hood recirculation loop, holding hood humidity at the target and keeping the hood balanced with the room. On a tissue machine it is the ID fan of the hood system. It handles hot, humid air.
What material is used for a tissue hood exhaust fan?
Stainless steel, usually SS 304 or SS 316, because the air is hot and very humid and condenses on any cold surface during shutdown. Mild steel corrodes quickly in that service. The casing is insulated and drained, and the impeller is smooth so tissue fibre does not collect and become a fire risk.
How can I reduce gas use on a tissue machine hood?
Control the exhaust by hood humidity rather than a fixed damper, so you do not throw away more hot air than necessary. A variable speed drive on the exhaust fan and good heat recovery are the usual measures. A humidity sensor in the return duct is needed.
Information needed to quote an ID fan for tissue machine Yankee hoods
- 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.
- Hood data. Hood supply temperature, humidity target and exhaust rate at design speed.
- Heat recovery. Type and pressure drop of the heat recovery units, and fan position relative to them.
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.