How MDF and particle board fibre dryers work
In an MDF line, wood chips are steamed, defibrated in a pressurised refiner, and blown with resin through the blowline into a flash tube dryer, where hot gas at about 140 to 200°C dries the fibre from about 100% to 8 to 12% moisture on a dry basis in a few seconds. The gas leaves at 60 to 75°C and large cyclones separate the fibre. Particle board lines instead dry particles in a direct-fired rotary drum, with inlet gas at 300 to 450°C and outlet gas at 100 to 130°C, down to 2 to 3% moisture. In both, the hot gas comes from an energy plant burning bark, sanding dust and rejects, often mixed with recirculated dryer exhaust. The exhaust goes to a wet ESP, scrubber or stack.
Where the FD fan sits
- Air intake
- FD fan
- Undergrate air and dust burner
- Energy plant furnace
- Hot gas mixing chamber
- Dryer
The fan sits beside the energy plant furnace and delivers primary air under the grate where bark and rejects burn, with branches or separate fans for secondary air and for the sanding dust burner. The hot gas from the furnace is mixed with recirculated exhaust or fresh air in a mixing chamber before the dryer. The fan pressure covers the grate, the bed and the air distribution, typically 150 to 300 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Dust | Clean air from outside the mill |
| Pressure at the fan outlet | Typically 150 to 300 mmWC |
| Fuel | Bark, sanding dust, trim and rejects |
| Turndown | Follows the dryer load and board thickness |
| 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 MDF and particle board fibre dryers
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled with a variable speed drive |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on energy plant demand |
| Also specified | Inlet filter and silencer |
What this FD fan has to cope with
Fuel mix changes
Bark, sanding dust and rejects have very different moisture and burn rates, so combustion air demand changes as the fuel mix changes. A speed-controlled fan with oxygen trim keeps combustion steady and limits sparks carried to the dryer.
Spark carry-over
Too much air under the grate lifts burning particles into the gas stream and carries sparks to the dryer, which can start fires. Correct primary and secondary air split from the fan system reduces carry-over.
Load swings
Board lines change thickness and speed, and the dryer load changes quickly. A combustion air fan that can follow these changes avoids over- or under-heated gas at the dryer inlet.
Energy and control
The energy plant burns the mill's own wood waste, so fuel cost is low, but fan power is not. A variable speed drive on the combustion air fan, linked to dryer demand and oxygen trim, saves power and keeps combustion clean. Burning fuel with the right excess air also reduces the gas volume through the dryer and the main dryer fan.
ID fan and FD fan in MDF and particle board fibre dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the product cyclones | Beside the energy plant furnace |
| What it handles | Humid gas with resin-coated fibre | Ambient air |
| Temperature at the fan | 60 to 75°C (MDF), 100 to 130°C (particle board) | Ambient |
| Main design concern | Deposits, fire and very large volume | Steady combustion with varying fuel |
| Control | Dryer suction | Energy plant demand |
See also the Induced Draft (ID) Fan for MDF and particle board fibre dryers.
Questions buyers ask
What is the FD fan on an MDF dryer?
It is the forced draft fan of the energy plant, which supplies combustion air to the grate where bark and rejects burn, and often to the sanding dust burner. The hot gas from this furnace dries the fibre in the flash tube dryer.
Does the MDF energy plant need separate fans for the dust burner?
Often yes. Sanding dust burners need their own combustion and transport air at a higher pressure than the grate. Some plants use one fan with branches and dampers, others use separate fans for each duty.
How do you control sparks from an MDF energy plant?
By keeping the undergrate air no higher than combustion needs, using secondary air for burnout, and fitting spark detection and extinguishing in the hot gas duct before the dryer.
Information needed to quote an FD fan for MDF and particle board fibre 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.
- Energy plant. Thermal output, grate type, and whether sanding dust burners are fitted.
- Air split. Primary, secondary and dust burner air quantities and pressures.
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.