How thermic fluid heaters for plywood hot presses work
A plywood or particle board unit uses a thermic fluid heater to heat the platens of its multi-daylight hot press, typically to 110 to 120°C for urea formaldehyde glue and 140 to 160°C for phenolic resin, and often a veneer dryer as well. The heater is fired on the plant's own waste: veneer clippings, core waste, bark, sawdust and sander dust, usually on a fixed or inclined grate, with sander dust sometimes blown in through a burner. Fluid temperatures are lower than on textile duty, so the gas leaves the coils at about 250 to 320°C and an air preheater brings it to 170 to 220°C. A cyclone or multicyclone cleans it before the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air preheater
- Undergrate plenum and overfire nozzles
- Grate and dust burner
The fan sits beside the heater and pushes ambient air through the air preheater into the undergrate plenum, with a branch to overfire nozzles and, where fitted, the sander dust burner. Wet bark needs most of its air under the grate to dry and burn the bed; dry dust needs most above it. The fan works against the preheater, plenum and bed, typically 60 to 120 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 120 to 170°C, downstream of the fan |
| Dust | Clean air; wood dust in the press area can be drawn in |
| Pressure at the fan outlet | Typically 60 to 120 mmWC |
| Air split | Shifts between undergrate and overfire with the fuel mix |
| 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 thermic fluid heaters for plywood hot presses
| Blade form | Backward curved, on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft or direct coupled |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive from the fluid temperature controller, with dampers on the air branches |
| Also specified | Inlet screen, and a separate small fan for sander dust transport where the pressure differs |
What this FD fan has to cope with
Wrong air split for the fuel
A single fixed split between undergrate and overfire air suits one fuel only. On dry dust the bed is starved of overfire air and smokes; on wet bark the bed gets too little air to dry. Separate dampers on each branch, set by the operator or by a fuel selection, keep combustion clean as the fuel changes.
Wood dust drawn into the fan
Plywood plants have fine wood dust in the air around the presses, sanders and trimming saws. Drawn into the fan, it coats the blades and inlet screen, unbalances the wheel and slowly reduces air flow. Ducting the inlet from a clean location away from the sanding line and keeping the screen accessible for cleaning prevent it.
Dust burner pressure
Sander dust is conveyed to a burner pneumatically at a higher pressure than undergrate air needs. Taking this air from the main FD fan forces the whole fan to run at the higher pressure. A small separate transport fan is usually the more efficient choice.
Energy and control
Hot press demand is cyclic, and the heater's fluid temperature controller moves the firing rate up and down with each press cycle. A variable speed FD fan follows these changes smoothly and avoids running at high excess air between cycles. Burning dry sander dust as part of the fuel lowers the air needed per unit of heat compared with wet bark, so better fuel management also cuts FD fan power.
ID fan and FD fan in thermic fluid heaters for plywood hot presses
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone or multicyclone | Before the air preheater |
| What it handles | Flue gas with light ash, char and sparks | Clean ambient air |
| Temperature at the fan | 170 to 220°C | Ambient |
| Main design concern | Sparks, char and fuel moisture swings | Air split for mixed wood waste |
| Control | Furnace draft | Fluid temperature |
See also the Induced Draft (ID) Fan for thermic fluid heaters for plywood hot presses.
Questions buyers ask
What is the FD fan on a plywood thermic fluid heater?
It is the combustion air fan that blows air under the grate and through overfire nozzles above it, through the air preheater. On some heaters it also supplies the burner for sander dust. It handles clean air and its speed follows the fluid temperature controller.
Should sander dust transport air come from the FD fan?
Usually not. Pneumatic transport of sander dust needs a higher pressure than undergrate air. If it comes from the FD fan, the whole fan must run at that pressure and the surplus is throttled away at the grate dampers, wasting power. A small dedicated transport fan is more efficient and easier to control.
How much air does a plywood thermic fluid heater need for wood waste?
It depends on the fuel mix, its moisture and the heater rating. Wood waste on a grate usually burns with 40 to 70% excess air. Send us the heater rating and fuel details or the existing fan data, and we will work out the air quantity and pressure.
Information needed to quote an FD fan for thermic fluid heaters for plywood hot presses
- 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 burner. Grate type and whether sander dust is fired through a separate burner.
- Air distribution. Undergrate and overfire air split and the pressure at each branch.
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.