How wood log fired fixed grate boilers work
Wood log fired fixed grate boilers are small units, typically 0.5 to 6 TPH, used by textile dyeing and processing units, food plants and laundries. Split logs or firewood are loaded by hand through a firing door onto a fixed grate in a refractory furnace or a large furnace tube. Combustion is cyclic: each charge flares, burns down and smoulders until the next. Gas passes through the smoke tubes of a shell boiler, or a small water tube bank, and leaves at about 220 to 300°C, as most units have no economiser or air heater. Wood ash is low, but soot, tar and fine ash collect in the tubes. A cyclone, and sometimes a wet scrubber, precedes the ID fan.
Where the FD fan sits
- Inlet
- FD fan
- Damper
- Ash pit
- Fixed grate and log bed
- Furnace
The fan is mounted at the front or side of the boiler and blows into the closed ash pit. Air passes up through the grate into the log bed. Many designs also have air ports above the grate, fed from the same fan or by natural draft, to burn the volatiles released by fresh logs. Pressure needed is low, typically 30 to 80 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | About 30 to 80 mmWC |
| Preheat | Usually none |
| Duty variation | Cyclic with each log charge |
| Dust | Clean air; sawdust or bark near the wood store |
| 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 log fired fixed grate boilers
| Blade form | Backward curved or radial, compact motor-mounted |
| Material of construction | Mild steel |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Damper or small variable speed drive |
| Also specified | Interlock with the ID fan and firing door |
What this FD fan has to cope with
Smoke from the door during charging
If the FD fan keeps blowing when the firing door is opened, the furnace pressurises and smoke and flame come out at the fireman. A door limit switch that stops or slows the FD fan while the door is open, and restores it once closed, keeps smoke and flame inside during every charge.
Too much air on a burnt-down bed
As a log charge burns down, the same air flow gives very high excess air, cooling the furnace and sending heat up the chimney. Reducing FD air as the bed thins, by a damper or drive linked to steam pressure, improves efficiency and reduces the gas volume the ID fan has to handle.
FD fan running without ID fan
Ash pit doors that are left ajar for de-ashing, or that warp with heat, let uncontrolled air into the grate and take combustion away from the FD fan. Air then bypasses the damper settings and the boiler runs with high excess air. Well-sealed ash pit doors are as important as the fan selection.
Energy and control
The FD fan on a log-fired boiler is small. Its main energy effect is on excess air: too much air on a burnt-down charge wastes far more fuel than the fan's own power consumption. A damper or small drive linked to steam pressure, smaller and more frequent charges, and good firing practice give the main savings.
ID fan and FD fan in wood log fired fixed grate boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After cyclone, before chimney | Under the grate |
| What it handles | Hot gas with soot and fine ash | Ambient air |
| Temperature at the fan | 220 to 280°C | Ambient, 25 to 45°C |
| Main design concern | Heat, soot and draft recovery at charging | Interlocks and excess air |
| Usual drive | Belt driven | Motor mounted |
See also the Induced Draft (ID) Fan for wood log fired fixed grate boilers.
Questions buyers ask
Does a wood log boiler need an FD fan?
Many do, to burn a deep log charge at the boiler rating. Some smaller units work on ID fan draft alone, drawing air naturally through the grate. Where fitted, the FD fan is a small blower, typically 30 to 80 mmWC, blowing into the closed ash pit under the fixed grate.
Why stop the FD fan when the firing door is opened?
With the door open, the furnace already has a large air inlet. If the FD fan keeps blowing, the furnace turns positive and smoke and flame come out of the door toward the fireman. Stopping or slowing the FD fan during charging, through a door switch, prevents this.
Is overfire air needed on a wood log boiler?
It helps. Fresh logs release a burst of volatiles that need air above the bed to burn cleanly. Overfire ports, fed from the FD fan or by natural draft, reduce smoke and soot, which also keeps the smoke tubes and the ID fan cleaner between weekly cleanings.
Information needed to quote an FD fan for wood log fired fixed grate boilers
- 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.
- Door interlock. Whether the firing door has a switch for fan interlock.
- Overfire air. Whether over-bed air is supplied by the FD fan or by 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.