How sawdust fired boilers work
Sawdust from sawmills, furniture plants and plywood units is fired in boilers of about 1 to 15 TPH. Most burn it on a grate, fed by screw or pneumatic conveyor onto a fixed, step or reciprocating grate; some blow it into the furnace for suspension firing. Moisture varies widely, from 10 to 15% for dry planer shavings to 40 to 55% for fresh green sawdust. Ash is low, typically 0.5 to 2%, but the fuel is very fine, so much of it lifts off the grate and burns in suspension, and some leaves the furnace as glowing particles. Gas passes the boiler, economiser where fitted, then a cyclone or multiclone and sometimes a bag filter before the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater (if fitted)
- Undergrate plenum
- Grate
- Furnace
The FD fan delivers ambient air, through an air heater where fitted, into the plenum under the grate. Because sawdust is fine, a large share of combustion air is supplied as overfire air above the bed, from the same fan through a separate duct or from a secondary air fan. Pressure is modest, typically 60 to 150 mmWC, depending on grate type and air heater.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Pressure at the fan outlet | About 60 to 150 mmWC |
| Air preheat | 120 to 200°C where an air heater is fitted |
| Dust | Sawdust in the air around the fuel store |
| Duty variation | Load and sawdust moisture |
| 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 sawdust fired boilers
| Blade form | Backward curved, compact |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or motor-mounted |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on steam pressure |
| Also specified | Inlet screen against sawdust |
What this FD fan has to cope with
Too much undergrate air
High air velocity through the grate lifts fine sawdust into suspension before it has dried, raising carry-over, sparks and unburnt carbon in the ash. A fan and control that deliver gentle, even undergrate air, with more of the total supplied above the bed, keep the fuel burning on the grate where it should.
Sawdust drawn into the fan
Sawdust-laden air around the fuel store and conveyors is drawn into the FD fan and builds up on the impeller, causing unbalance and blinding the inlet. A screened intake taken from a clean area, away from the fuel yard, prevents it and keeps the fan delivering its rated flow.
Wet sawdust needs preheat
Wet green sawdust needs warm air to dry and ignite. Where an air heater is added for this, the FD fan must push through its resistance, which is often forgotten when an air heater is retrofitted to an existing boiler. The fan is checked, and re-rated if needed, for the added pressure drop.
Energy and control
The FD fan on a sawdust boiler is a modest load, but its setting has a big effect on excess air and carry-over. A variable speed drive tied to steam pressure, with the undergrate/overfire split set correctly, saves fuel, reduces gas volume to the ID fan and cuts sparks to the collector.
ID fan and FD fan in sawdust fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After cyclone or bag filter | Under the grate |
| What it handles | Flue gas with fine ash and sparks | Ambient air |
| Temperature at the fan | 160 to 220°C | Ambient, 25 to 45°C |
| Main design concern | Sparks and moisture-driven volume swings | Gentle undergrate air for fine fuel |
| Control | Furnace draft | Steam pressure |
See also the Induced Draft (ID) Fan for sawdust fired boilers.
Questions buyers ask
How should air be split on a sawdust boiler?
Because sawdust is fine and burns partly in suspension, a smaller share of air goes under the grate and more above it, from a separate overfire duct or a secondary air fan. The exact split depends on the grate and the sawdust type; dry shavings need less undergrate air than wet green dust.
What FD fan pressure does a sawdust boiler need?
Typically about 60 to 150 mmWC at the fan outlet, depending on grate type and whether an air heater is fitted. Sawdust beds are thin, so grate resistance is modest. Boilers with air heaters and step grates sit at the upper end, while simple fixed grates with screw feeding sit lower.
Can the FD fan cause sparks on a sawdust boiler?
Indirectly, yes. Too much undergrate air lifts sawdust into suspension and carries burning fines out of the furnace toward the cyclone and bag filter. Reducing undergrate air and supplying more above the bed, with a drive or damper that can be set precisely, reduces sparks considerably.
Information needed to quote an FD fan for sawdust fired 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.
- Feeding method. Screw, pneumatic or spreader feeding, and grate type.
- Air split. Share of undergrate and overfire air.
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.