How plywood and wood processing work
Plywood and block board plants need heat for veneer drying, glue curing in hot presses and sometimes log conditioning. Most use a thermic fluid heater, often 2 to 6 million kcal/h, fired on the plant's own wood waste: veneer clippings, peeler cores, bark, trimmings and sander dust. Some also run a wood-fired hot air generator feeding the veneer dryers directly. Waste moisture varies from about 10% for trimmings to 50% for green bark, and glued offcuts carry urea or phenol formaldehyde resin. Fine, dry sander dust is often burnt in a separate dust burner or blown into the furnace. Flue gas leaves the coil bank at 250 to 320°C, passes an air preheater and multicyclone, and reaches the ID fan at 180 to 250°C.
Where the FD fan sits
- Inlet screen
- FD fan
- Air preheater
- Under-grate plenum
- Grate and sander dust burner
- Furnace
The FD fan sits beside the heater and pushes ambient air through the air preheater into the grate plenum, with branches to the sander dust burner or overfire air nozzles. It handles ambient air; the preheated air is downstream of it. The resistance is the air preheater, ducts, grate and fuel bed, typically 100 to 200 mmWC. Sander dust itself is usually conveyed into the furnace by its own blower, separate from the FD fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air preheater | 120 to 180°C, downstream of the fan |
| Dust | Fine wood dust in the plant air; screened inlet |
| Pressure at the fan outlet | Typically 100 to 200 mmWC |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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 plywood and wood processing
| Blade form | Backward curved, for efficiency on clean ambient air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or direct mounted on the motor shaft |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on thermic fluid outlet temperature |
| Also specified | Inlet screen against wood dust, and branch dampers for the dust burner and overfire air |
What this FD fan has to cope with
Wood dust on the inlet and impeller
Sanding and trimming lines fill the plant air with fine wood dust, which is drawn onto the inlet screen and settles on the impeller. Flow falls slowly and the heater runs short of air. Taking the intake from a clean area, a screen with easy access and periodic cleaning keep the air supply steady.
Changing bed resistance
Wet bark forms a dense, slow-burning bed, while dry clippings burn fast and leave little bed. Undergrate air demand and resistance change with each mix fed. A fan with pressure margin, and speed control linked to thermic fluid temperature, handles these changes without smoke or loss of output.
Balancing air to the sander dust burner
Sander dust burns in suspension and needs its own share of air. If the FD fan feeds both grate and dust burner, the branch dampers must be set so each gets enough, or the dust burns poorly and sends unburnt fines and sparks downstream to the cyclone and ID fan.
Energy and control
Pressing and drying cycles make heat demand vary through each shift. A variable speed drive on the FD fan following thermic fluid temperature saves power and keeps excess air low, which saves fuel. Burning drier waste first and keeping the air preheater clean also reduce the air flow the fan must deliver for the same heat output.
ID fan and FD fan in plywood and wood processing
| ID fan | FD fan | |
|---|---|---|
| Position | After the multicyclone | Beside the heater, before the air preheater |
| What it handles | Flue gas with sandy ash and sparks | Clean ambient air |
| Temperature at the fan | 180 to 250°C | Ambient, 25 to 45°C |
| Main design concern | Erosion and sparks | Varying bed resistance |
| Control | Furnace draft | Thermic fluid temperature |
See also the Induced Draft (ID) Fan for plywood and wood processing.
Questions buyers ask
What does the FD fan do on a wood-waste thermic fluid heater?
It is the combustion air fan. It supplies air under the grate and sometimes to a sander dust burner or overfire nozzles. The heater then heats thermic fluid for the hot presses and veneer dryers. It handles clean ambient air, so a mild steel backward curved forced draft fan is usual.
Why does my wood-fired heater smoke when firing bark?
Wet bark forms a dense bed that needs more air pressure to penetrate, and it releases its volatiles slowly. If the FD fan has no pressure margin, air flow drops and combustion is incomplete. A fan with higher pressure capability, speed control and adequate overfire air usually solves it.
Should sander dust have its own combustion air fan?
Sander dust is usually conveyed pneumatically by a separate blower and burns with that conveying air plus secondary air. The secondary air can come from the FD fan through a damper, or from a separate fan where precise control is needed on a large dust burner.
Information needed to quote an FD fan for plywood and wood processing
- 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.
- Grate and burners. Grate type and area, and whether a sander dust burner or overfire air is fitted.
- Fuel mix. Typical proportions of bark, clippings and sander dust, with moisture.
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.