How sawdust suspension fired boilers work
In a suspension fired boiler, sawdust from sawmills, plywood and furniture units is screened to below about 3 to 5 mm, dried where necessary to under 15 to 20% moisture, and blown into the furnace through burner nozzles by a stream of transport air. The particles ignite and burn in flight, often inside a refractory pre-furnace or cyclone combustor ahead of the boiler, with only the coarser fraction finishing on a small hearth or grate. Dry sawdust has a calorific value of about 3,800 to 4,300 kcal/kg and only 1 to 3% ash. Flue gas leaves through the boiler passes, economiser and air heater, then a cyclone and a bag filter or wet scrubber to the ID fan. Combustion air is split between the FD fan, the fuel transport blower and secondary air ports.
Where the ID fan sits
- Pre-furnace or cyclone combustor
- Boiler and economiser
- Air heater
- Cyclone
- Bag filter or wet scrubber
- ID fan
- Chimney
The fan is placed after the final collector, normally a bag filter with a cyclone or spark trap ahead of it, or a wet scrubber on smaller units. Sawdust ash is light and soft, so the residual dust at the fan is fine rather than abrasive. Suspension firing needs only a modest furnace suction, around 3 to 8 mmWC, but the fan must add the air heater, cyclone and filter losses, which typically puts the inlet suction at 180 to 280 mmWC. Sitting last keeps it clear of burning particles that a cyclone alone does not stop.
Gas at the ID fan
| Gas temperature, normal | 160 to 190°C after the air heater |
| Maximum or upset | Around 240°C during a fuel surge or with the air heater bypassed |
| Dust at the fan | Fine, low-density wood ash with unburnt carbon; usually below 50 mg/Nm³ after a bag filter |
| Moisture and dew point | Water dew point near 50 to 55°C with dried fuel; saturated gas at 60 to 75°C behind a wet scrubber |
| Pressure transients | Short furnace pulses of 20 to 50 mmWC when the feed surges or a burner trips |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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 sawdust suspension fired boilers
| Blade form | Backward curved, for efficiency and a steep pressure curve that steadies furnace draft; radial tipped where the fan follows a wet scrubber |
| Material of construction | Mild steel casing and impeller behind a bag filter; SS 304 or coated wheel with a casing drain behind a wet scrubber |
| Drive and arrangement | Direct coupled, arrangement 8, with motor margin for start-up on cold air |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on a fast draft loop, with the pressure tapping close to the burner, and an inlet damper for isolation |
| Also specified | Purge interlock with the FD fan and fuel blower, and a casing access door for clearing fines |
What this ID fan has to cope with
Furnace puffs and trips
A choked feeder followed by a slug of sawdust can raise furnace pressure sharply, and a slow fan lets flame and gas blow back through the sight ports. A wheel with a steep pressure curve, a drive tuned for quick response and a draft transmitter close to the furnace hold suction within a few mmWC through these swings.
Glowing fines reaching the bag filter
Fine particles that leave the pre-furnace partly burnt can stay alight through the boiler passes. In a bag filter they burn holes in the bags or start a hopper fire, and the fan then draws hot gas. A cyclone or spark trap ahead of the filter, a temperature trip at the filter inlet and an ID fan rated for the bypass temperature are the usual defence.
Light carbon deposits and unbalance
Unburnt carbon and soft wood ash settle on the back of backward curved blades when the fan stands idle in humid weather, then cake. The layer is light but uneven and appears as slowly rising vibration. Washing through an access door and a check balance at each shutdown keep it in hand.
Saturated gas behind a scrubber
On small units with a wet scrubber instead of a bag filter, the fan handles saturated gas with droplets. Water collects in the casing and attacks mild steel, and wet ash builds on the blades. A droplet eliminator, a drain at the lowest point and a stainless or coated wheel are needed, with the duty worked out at the real saturated gas density.
Energy and control
Suspension fired sawdust boilers often run on the shift pattern of the plywood press or sawmill, with steam demand stepping up and down as presses and dryers start. Because the furnace responds instantly, a variable speed drive on the ID fan pays twice: it cuts power at part load and gives tighter draft control than a damper actuator. Holding excess oxygen at about 5 to 7% instead of 9 to 10% also reduces the gas volume the fan must move.
ID fan and FD fan in sawdust suspension fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or scrubber | Ahead of the air heater and burner |
| What it handles | Flue gas with fine wood ash and sparks | Clean ambient air |
| Temperature at the fan | 160 to 190°C, up to 240°C upset | Ambient, 25 to 45°C |
| Main design concern | Fast draft response to feed swings | Burner air velocity and turndown |
| Control | Furnace draft, fast loop | Steam pressure, cross-limited with fuel |
See also the Forced Draft (FD) Fan for sawdust suspension fired boilers.
Questions buyers ask
Why does furnace pressure fluctuate on a sawdust fired boiler?
Sawdust burns within seconds of entering the furnace, so any change in feed rate turns directly into a change in gas volume. Bridging in the hopper, wet patches in the fuel or a stalled rotary valve cause the swings. The ID fan control has to respond as quickly, which is why a variable speed drive and a fast draft loop are preferred to a slow damper actuator.
Is a bag filter safe ahead of the ID fan on a sawdust suspension fired boiler?
Yes, with precautions. A cyclone or spark trap upstream removes most burning particles, the filter has an inlet temperature trip and a bypass, and the bags are chosen for the gas temperature. The ID fan should be rated for the bypass temperature so it can keep the furnace under draft while the filter is isolated.
What ID fan is needed for a sawdust burner boiler?
The size follows from the steam output, excess air, gas temperature and the total resistance of the boiler, air heater and collectors. For most plants it is a backward curved, direct coupled fan on variable speed. Share the boiler data or the present fan nameplate and we will work out the duty point and check it against site readings.
Information needed to quote an ID fan for sawdust suspension fired boilers
- 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 preparation. Sawdust particle size and moisture as fired, and whether a dryer or pre-furnace is used.
- Collector and bypass. Whether the fan follows a bag filter, cyclone or wet scrubber, and the gas temperature at the fan during filter bypass.
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.