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 ID fan sits
- Infrared and flotation dryers
- Exhaust hoods
- Exhaust header
- Exhaust (ID) fan
- Heat recovery
- Stack
Exhaust from each flotation section and the infrared hood is collected into a header and drawn by one or more exhaust fans before heat recovery. The fan handles air at 120 to 180°C with water vapour, coating dust and volatile organics from latex. It works against the dryer hoods, header and recovery units, typically 100 to 250 mmWC. The exhaust rate keeps the dryer slightly negative so warm, dusty air does not spill into the coating room.
Gas at the ID fan
| Gas temperature, normal | 120 to 180°C |
| Maximum or upset | Up to 250°C on a web break before burner turndown |
| Dust at the fan | Fine pigment flakes from dried coating edges and web breaks |
| Moisture and dew point | Moderate humidity; condensation on cold ducts during shutdown |
| Other | Latex and binder volatiles that condense as sticky deposits on cooler surfaces |
| Density at the design temperature | About 0.83 kg/m³ at 150°C, 70% 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 paper coating line dryers
| Blade form | Backward curved, smooth and accessible for cleaning |
| Material of construction | Mild steel or stainless steel; stainless where condensate and latex deposits are heavy |
| Drive and arrangement | Direct coupled or belt drive, arrangement 8 or 9 |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | Variable speed drive on dryer balance or humidity |
| Also specified | Inspection doors, casing drain, and insulated casing to limit condensation |
What this ID fan has to cope with
Latex and binder deposits
Volatiles from latex and other binders condense on the impeller and duct when they cool, forming sticky deposits that trap pigment dust. Keeping the exhaust warm, insulating the casing and cleaning on a fixed schedule control build-up and unbalance. A clean fan also holds balance.
Pigment dust from web breaks
Web breaks and edge trimming release dried coating flakes that reach the exhaust. The dust is abrasive and builds in the scroll. Smooth blade surfaces and access doors keep cleaning practical. After a major break the exhaust fan is checked for packed dust before the line is restarted at full speed.
Dryer balance and room conditions
If exhaust falls, hot humid air spills into the room around the dryer, raising temperature and condensation on cold surfaces. Control of exhaust speed on hood balance keeps the dryer slightly negative. Balance is measured with a differential pressure sensor between the dryer enclosure and the coating room.
Energy and control
Coating lines change grade, coat weight and speed often, and each change alters the moisture to remove. Controlling the exhaust fan by humidity or balance with a variable speed drive avoids exhausting more heated air than needed. Every unit of excess exhaust must be replaced by make-up air heated by the burners, so fan control saves much more gas than fan power.
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 Forced Draft (FD) Fan for paper coating line dryers.
Questions buyers ask
What does the exhaust fan do on a paper coating dryer?
It removes humid air and binder volatiles from the flotation and infrared dryers, holding the dryer humidity and keeping it slightly negative to the room. On a coating line it is the dryer ID fan. It handles warm air with coating dust and binder volatiles.
How do I stop deposits in a coating dryer exhaust fan?
Keep the exhaust air warm so latex volatiles do not condense, insulate the casing, and clean on a fixed schedule. Smooth blades without pockets make cleaning quicker and reduce unbalance. After web breaks, check the fan for packed coating dust before restarting the line at full speed.
Should the coating dryer exhaust be controlled by humidity?
Yes, where possible. Controlling the exhaust fan speed on measured humidity or hood balance avoids exhausting more heated air than needed, which saves burner gas as grades and speeds change. It also keeps the coating room cooler and drier, because the dryer stays slightly negative.
Information needed to quote an ID fan for paper coating line dryers
- 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.
- Dryer sections. Number of infrared and flotation sections and the exhaust rate from each.
- Coating. Pigment and binder types, and coat weight range.
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.