How wood fired thermic fluid heaters work
Wood fired thermic fluid heaters burn logs, chips, offcuts or plywood waste on a fixed or step grate, fed manually through a firing door or by a screw or ram feeder. Wood typically has 20 to 45% moisture, a calorific value of about 2,500 to 3,500 kcal/kg as fired and only 1 to 3% ash. The volatiles burn above the grate with secondary air. The flue gas leaves the coil at around 330 to 380°C, passes an air preheater, and goes through a cyclone or multicyclone to the ID fan and chimney. Manual firing gives large swings in combustion as each charge of wood is added and the door is opened.
Where the FD fan sits
- Inlet screen
- FD fan
- Air preheater
- Undergrate plenum
- Grate
- Furnace
The fan pushes ambient air through the air preheater, where it heats to 150 to 200°C, and into the plenum under the grate. Pressure is low, typically 60 to 120 mmWC including the preheater and the wood bed. Overfire air enters above the grate from a small secondary fan or through natural draft ports. The FD fan is interlocked with the firing door so that it stops or slows when the door opens, preventing flame blowback onto the operator.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 150 to 200°C, downstream of the fan |
| Dust | Wood dust and bark fibre in the fuel yard |
| Pressure at the fan outlet | 60 to 120 mmWC |
| Control | Interlocked with firing door |
| 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 wood fired thermic fluid heaters
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on oil temperature |
| Also specified | Inlet screen and door interlock |
What this FD fan has to cope with
Blowback at the firing door
If the FD fan keeps blowing while the firing door is open, flame and hot gas can blow out towards the operator. A door limit switch interlocked with the fan, so that it stops or slows while the ID fan holds suction, prevents this. The interlock is part of the safety checks at every inspection.
Wood dust at the inlet
Sawdust, bark and fibre from the wood yard collect on the impeller of an unscreened fan and cause unbalance, and they also clog the air preheater. A screen on the inlet, an intake away from the fuel stack and a periodic clean keep the fan balanced and the air flow steady.
Overfiring after charging
Fresh dry wood burns fast right after a charge, and with full undergrate air it can overheat the oil film on the coil. FD fan control linked to oil outlet temperature cuts the air quickly to slow the fire, and the oil flow interlock stops the fan altogether if circulation is lost.
Energy and control
The burning rate of wood changes through each charge, and a variable speed FD fan linked to oil outlet temperature gives steadier firing than an on-off or damper control. It also saves power on a heater whose load varies with the process all day. Reducing air when the oil is near setpoint keeps excess air down and the stack loss low.
ID fan and FD fan in wood fired thermic fluid heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone, before the chimney | Before the air preheater, under the grate |
| What it handles | Flue gas with char, sparks and tar | Ambient air |
| Temperature at the fan | 190 to 230°C | Ambient |
| Main design concern | Draft at the open firing door, tar deposits | Door interlock and burning rate |
| Control | Furnace draft | Oil outlet temperature |
See also the Induced Draft (ID) Fan for wood fired thermic fluid heaters.
Questions buyers ask
Why is the FD fan interlocked with the firing door?
To stop flame blowing out when the door is opened for charging. The fan stops or slows while the door is open, and the ID fan keeps the furnace under suction. It is a basic safety interlock on a manually fired wood thermic fluid heater and should be tested regularly.
Does the FD fan on a wood heater handle hot air?
No. It pushes ambient air into the air preheater, and the air is heated downstream on its way to the grate. The fan is therefore a cold-air machine, selected at ambient density in mild steel, and needs no shaft cooling.
How much FD air pressure does a wood fired grate need?
Usually about 60 to 120 mmWC in total, covering the air preheater, ducts, plenum and the wood bed on the grate. A deeper bed or a step grate with small air openings needs the higher end. The exact figure follows from the grate area and design air flow.
Information needed to quote an FD fan for wood fired thermic fluid heaters
- 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. Grate type and area, and the design air flow under the grate.
- Air split. Share of undergrate and overfire air, and whether overfire air comes from a fan or natural draft.
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.