How wood veneer dryers work
Rotary-peeled or sliced veneer comes off the lathe at 60 to 120% moisture on a dry basis and is dried to about 6 to 10% for plywood glueing. The common machine is a multi-deck roller dryer, where sheets travel between driven rollers while hot air is blown onto both faces through jet nozzles, or across the decks in older longitudinal flow designs. Air in the hot zones is typically 150 to 200°C, heated by thermic fluid or steam coils, or by hot gas from a direct gas burner or a wood waste furnace. Circulation fans inside the dryer recirculate most of the air, and exhaust fans on the wet end remove the moist air together with wood volatiles, resin and fines. Cooling sections at the outlet use ambient air.
Where the ID fan sits
- Hot zones and jet boxes
- Wet end exhaust hood
- Exhaust duct
- Exhaust (ID) fan
- Stack or emission control
The exhaust fans sit on the dryer roof or beside the wet end, drawing from the zone where the veneer enters and releases most of its water. The exhaust rate sets the humidity inside the dryer: high humidity speeds heat transfer and reduces case hardening, but too high slows drying. Pressure is low, typically 30 to 100 mmWC, but the gas is hot and dirty.
Gas at the ID fan
| Air temperature, normal | 120 to 160°C at the fan |
| Maximum or upset | About 200°C if the dryer runs empty |
| Dust at the fan | Fine wood fibres and dust from the veneer |
| Moisture and dew point | Very humid; dew point typically 70 to 85°C |
| Corrosive species | Wood acids, resin and terpenes that condense as pitch |
| 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 wood veneer dryers
| Blade form | Radial or backward inclined flat blade, plain and easy to clean |
| Material of construction | Mild steel for most species; SS 304 where acidic condensate forms |
| Drive and arrangement | Belt driven or direct coupled with a cooling disc |
| Bearings and cooling | Bearings outside the hot air with a heat shield and cooling disc |
| Flow control | Variable speed drive or damper on dryer humidity |
| Also specified | Large cleaning door, water spray or steam snuffing connection, and earthing |
What this ID fan has to cope with
Pitch build-up
Resin and terpenes from the wood condense on the blades and casing as a sticky pitch that traps fibre and dust. It builds unevenly and causes vibration, and it can catch fire. A plain wheel, keeping the fan above the condensing temperature and regular cleaning keep deposits under control.
Exhaust fires
Dry fibre and pitch in the exhaust duct and fan can ignite from a hot spot in the dryer. A steam or water snuffing connection, temperature monitoring in the duct and a cleaning routine reduce the risk.
Humidity control
Exhausting too much air wastes heat and can case harden the veneer; exhausting too little slows drying. A fan with speed control, set on wet end humidity or exhaust temperature, keeps the dryer at its best.
Energy and control
Exhaust air is the largest heat loss on a veneer dryer. Controlling the exhaust fan on humidity rather than leaving a damper open saves thermic fluid or steam, and some plants recover heat from the exhaust. The fan power itself is small compared with the circulation fans, but correct control of the exhaust has the biggest effect on dryer heat use.
ID fan and FD fan in wood veneer dryers
| ID fan | FD fan | |
|---|---|---|
| Position | On the wet end exhaust | Inside each zone, feeding the jet boxes |
| What it handles | Hot humid air with pitch and fibre | Recirculated hot zone air |
| Temperature at the fan | 120 to 160°C | 150 to 200°C |
| Main design concern | Pitch deposits and fire | Bearing heat and even flow |
| Control | Dryer humidity | Zone air velocity |
See also the Forced Draft (FD) Fan for wood veneer dryers.
Questions buyers ask
What temperature is the exhaust from a veneer dryer?
Typically 120 to 160°C at the fan, with high humidity and wood volatiles. It can rise to about 200°C if the dryer runs without veneer, so the fan is designed for the higher figure.
Why does the veneer dryer exhaust fan get covered in pitch?
Wood releases resin and terpenes during drying, and these condense on cooler surfaces in the duct and the fan. The deposits trap fibre and build up, so regular cleaning and keeping the fan warm are needed.
Does the exhaust fan affect veneer quality?
Yes. The exhaust rate controls the humidity in the dryer. High humidity in the early zones gives faster heat transfer and less case hardening, while too little exhaust slows drying and leaves wet veneer.
Information needed to quote an ID fan for wood veneer 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. Dryer type, number of decks, zones and the veneer throughput in m³/day.
- Wood species. Main species dried, since resin content affects deposits.
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.