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 FD fan sits
- Zone return air
- Heater coils or hot gas inlet
- FD (circulation) fan
- Supply plenum
- Jet nozzle boxes
- Veneer on rollers
Each zone has one or more circulation fans, mounted on the side or top of the dryer and often with the motor outside the hot casing. The fan draws air from the zone through the heating coils, or mixes in hot gas from a burner, and blows it into the plenum that feeds the jet boxes above and below each deck. The pressure covers the coils and the nozzles, typically 80 to 200 mmWC.
Air at the FD fan
| Air temperature at the fan | 150 to 200°C, recirculated zone air |
| Air after the coils | Same or slightly higher; coils may be before or after the fan |
| Dust | Fine wood dust and fibre in the recirculated air |
| Humidity | High at the wet end, lower towards the dry end |
| Pressure at the fan outlet | Typically 80 to 200 mmWC |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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 wood veneer dryers
| Blade form | Backward curved or plug fan, to give flat, even flow |
| Material of construction | Mild steel suitable for 200°C, or heat-resisting steel for direct gas fired zones |
| Drive and arrangement | Motor outside the casing, overhung or belt driven |
| Bearings and cooling | Cooling disc, heat shield and high temperature grease |
| Flow control | Variable speed drive per zone |
| Also specified | Shaft seal at the casing wall and access for cleaning |
What this FD fan has to cope with
Bearing heat
The fan shaft passes through a casing wall at 200°C, and bearings close to the wall overheat. A cooling disc, a heat shield and enough distance between the casing and the bearings keep the grease within its limit.
Uneven drying across decks
If the circulation fan does not deliver even pressure to the plenum, decks at one end dry more than the others and veneer comes out wet or brittle. Correct fan selection and plenum design give uniform jet velocity.
Fouling of coils and nozzles
Dust and pitch foul the heater coils and the jet nozzles, raising resistance and reducing flow. A speed-controlled fan can compensate between cleanings, but a cleaning schedule is still needed.
Energy and control
Circulation fans run continuously while the dryer runs and make up most of the dryer's electrical load. Variable speed drives let the operator set the air velocity to suit veneer thickness and species, instead of running all fans at full speed. High efficiency wheels and keeping the coils clean also cut fan power.
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 Induced Draft (ID) Fan for wood veneer dryers.
Questions buyers ask
Is the circulation fan in a veneer dryer an FD fan?
It does the work of a forced draft fan for each zone, pushing air through the heaters and the jet nozzles. It handles recirculated hot air at 150 to 200°C, not ambient air, which is why it needs high temperature construction.
How are veneer dryer circulation fans cooled?
The motor is placed outside the hot casing, and the shaft has a cooling disc and heat shield between the casing and the bearings. High temperature grease is used, and the bearing distance from the casing is chosen to keep the bearings cool.
Can veneer dryer fans be retrofitted with VFDs?
Yes. Variable speed drives on the circulation fans allow the air velocity to be matched to the veneer thickness and species, and they usually cut power substantially, especially when the dryer runs thin veneer or below capacity.
Information needed to quote an FD fan for wood veneer dryers
- 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.
- Zone data. Air flow per zone, temperature, and whether the fan is before or after the heater.
- Heating. Thermic fluid, steam, direct gas or wood waste hot gas.
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.