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 ID fan sits
- Grate and furnace
- Thermic fluid coils
- Air preheater
- Spark arrestor
- Cyclone or multicyclone
- ID fan
- Chimney
The fan sits after the cyclone or multicyclone, which is the usual collector on plywood units, and before the chimney. Wood ash is light, and a large share of what reaches the cyclone is unburnt char, some of it still glowing. Suction at the fan covers the furnace, coils, air preheater and cyclone, typically 80 to 150 mmWC. Where a bag filter is added for emission norms, a spark arrestor ahead of it is essential.
Gas at the ID fan
| Gas temperature, normal | 170 to 220°C after the air preheater |
| Maximum or upset | Up to 320°C with the air preheater bypassed |
| Dust at the fan | Light wood ash and char after a cyclone, typically 0.5 to 1.5 g/Nm³ |
| Moisture and dew point | Wet bark and green veneer waste raise moisture; dew point about 55 to 65°C |
| Fire risk | Glowing char and sparks from sander dust firing |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 thermic fluid heaters for plywood hot presses
| Blade form | Radial tipped, or backward curved behind an efficient multicyclone |
| Material of construction | Mild steel; light wear protection on the blade tips |
| Drive and arrangement | Belt driven on smaller heaters, direct coupled arrangement 8 on larger units |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Inspection door, casing drain and a spark detection point upstream where a bag filter is fitted |
What this ID fan has to cope with
Sparks and burning char
Sander dust and fine sawdust burn fast and lift glowing particles out of the furnace. In a cyclone these mostly drop out, but behind a bag filter they burn holes in the bags and can start a fire in the fan casing. A spark arrestor or settling chamber, a cyclone first, and a fan casing with a drain and access door limit the risk.
Fuel moisture swings
Dry core waste and sander dust one hour, wet bark the next: the gas volume and moisture at the fan change by a large margin. The fan is sized for the wettest realistic fuel mix, when the gas volume per unit of heat is highest, and the speed drive trims it back on dry fuel.
Deposits from resin-bearing waste
Glued waste and sander dust contain cured urea or phenolic resin. Incomplete burning at low fire leaves tarry, sticky deposits in the coils and on the fan blades. Keeping the furnace hot, avoiding long low-fire periods, and cleaning the wheel at each shutdown stop the deposits causing unbalance.
Energy and control
The hot press draws heat in pulses as it opens, loads and closes, and the heater runs at moderate average load with short peaks. A variable speed drive holding furnace draft saves fan power across this cycle. Using sander dust and dry waste as fuel cuts both fuel cost and gas volume, so better fuel preparation reduces the ID fan duty as well as the grate maintenance.
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 Forced Draft (FD) Fan for thermic fluid heaters for plywood hot presses.
Questions buyers ask
What collector should be used before the ID fan on a plywood thermic fluid heater?
A cyclone or multicyclone is the usual first stage because wood ash is light and much of it is char. Where a bag filter is needed for emission limits, it must follow a spark arrestor and the cyclone, because glowing particles from sander dust firing burn filter bags. The ID fan follows the last stage.
Can sander dust be fired in the thermic fluid heater?
Yes, and many plywood units do, through a dust burner or blown over the grate. It burns quickly and raises the fire fast. It also carries more sparks and fine char to the collector and fan, so the gas path needs a spark arrestor and the fan needs access for cleaning.
Why does the ID fan on a plywood press heater collect tar?
Glued waste and sander dust contain cured urea or phenolic resin. If the heater runs long at low fire between press cycles, combustion is incomplete and tarry deposits form on cooler surfaces, including the fan. Keeping the furnace hot, avoiding long smouldering periods and cleaning the impeller at each shutdown prevents unbalance.
Information needed to quote an ID fan for thermic fluid heaters for plywood hot presses
- 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.
- Fuel mix. Share of bark, veneer waste, sawdust and sander dust, and how sander dust is fired.
- Press duty. Platen temperature, number of presses and whether a veneer dryer is on the same heater.
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.