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 ID fan sits
- Energy plant hot gas
- Flash tube or rotary drum
- Product cyclones
- ID (main dryer) fan
- Wet ESP or scrubber
- Stack
The main dryer fan sits after the product cyclones, so the dryer and cyclones run under suction and fibre is discharged cleanly. Upstream are the hot gas duct from the energy plant, the dryer tube or drum, and the cyclone battery. Suction at the fan is typically 300 to 500 mmWC on an MDF flash dryer. Some two-stage MDF dryers have a fan at each stage, and some lines place a fan before the cyclones to push the fibre through.
Gas at the ID fan
| Gas temperature, normal | 60 to 75°C on MDF; 100 to 130°C on particle board drums |
| Maximum or upset | 150 to 200°C when feed is lost or during a fire |
| Dust at the fan | Resin-coated fibre fines that pass the cyclones, typically below 1 g/m³ |
| Moisture and dew point | Very humid; dew point near 60°C on MDF |
| Corrosive species | Formaldehyde, organic acids and VOCs from wood and resin |
| Density at the design temperature | About 1.02 kg/m³ at 75°C, 85% 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 MDF and particle board fibre dryers
| Blade form | Backward inclined or radial tipped, sized for a very large volume |
| Material of construction | Mild steel with wear plate on blade tips; SS 304 for wet ESP-integrated exhausts |
| Drive and arrangement | Direct coupled, arrangement 8, double inlet on very large flows |
| Bearings and cooling | Oil-lubricated or grease bearings with temperature monitoring |
| Flow control | Variable speed drive on dryer suction or outlet temperature |
| Also specified | Spark detection upstream, water deluge or extinguishing, vibration monitoring and a wash connection |
What this ID fan has to cope with
Resin fibre build-up
Fibre fines carrying uncured resin stick to the blades and harden as the resin cures, building an uneven layer that causes vibration. Regular washing, a blade shape that sheds deposits and a high-efficiency cyclone in front of the fan reduce cleaning stops.
Dryer fires
MDF and particle board dryers are prone to fires from sparks from the energy plant, overheated fibre or hot spots. Spark detection and extinguishing in the duct, temperature trips, and a fan that is robust enough to survive a fire and the water deluge are needed.
Large flow and vibration
The main dryer fan handles a very large gas volume, and deposits or erosion quickly cause high vibration. Continuous vibration monitoring and balancing to ISO 21940-11 after cleaning keep the fan reliable.
Condensation at shutdown
The exhaust is near saturated; at shutdown, condensate wets the fibre deposits, which then harden or corrode the casing. Running the fan with the energy plant on idle until the system dries helps.
Energy and control
The main dryer fan is one of the largest power users in an MDF or particle board mill and runs continuously. Variable speed control on dryer suction, instead of damper throttling, saves a great deal at reduced output and during species changes. Recirculating part of the exhaust to the energy plant reduces both fuel use and the fresh gas the fan must handle.
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 Forced Draft (FD) Fan for MDF and particle board fibre dryers.
Questions buyers ask
Where is the ID fan placed on an MDF dryer?
Usually after the product cyclones, so the dryer and cyclones run under suction and the fibre discharges cleanly. The fan then handles humid exhaust with fine resin-coated fibre and sends it to the wet ESP, scrubber or stack.
Why do MDF dryer fans need spark detection?
Fibre dryers often have fires, started by sparks from the energy plant or hot spots in the dryer. Spark detection and extinguishing in the duct protect the dryer, the cyclones and the main dryer fan, which is one of the most costly items in the line.
What causes vibration on an MDF main dryer fan?
Mostly uneven build-up of resin-coated fibre on the blades, which hardens as the resin cures, and erosion of the blade tips by fines. Regular washing, cleaning at planned stops and rebalancing after cleaning keep the fan within its vibration limits.
Information needed to quote an ID fan for MDF and particle board fibre 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 type. MDF flash tube or particle board drum, single or two-stage, and the dryer capacity.
- Fire protection. Spark detection and extinguishing in place, and the fire water arrangement near the fan.
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.