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 ID fan sits
- Furnace and grate
- Boiler
- Economiser
- Cyclone or multiclone
- Bag filter (if fitted)
- ID fan
- Chimney
The fan sits after the cyclone or bag filter, with suction typically 100 to 200 mmWC. Sawdust burns fast, and feed screws or pneumatic lines can bridge and surge, so the furnace sees short periods of low fuel followed by a burst. Each burst raises gas volume and lifts fines, which the cyclone partly catches. On plants without a bag filter, the fan handles fine ash and char directly.
Gas at the ID fan
| Gas temperature, normal | 160 to 220°C depending on heat recovery |
| Maximum or upset | Up to 260°C with fuel surges or afterburning |
| Dust at the fan | Fine ash and light char; 0.3 to 1 g/Nm³ after a cyclone |
| Moisture and dew point | Varies with sawdust: dry on shavings, 20 to 25% water vapour on wet green dust |
| Fire risk | Glowing fines reach collectors and occasionally the fan |
| Density at the design temperature | About 0.77 kg/m³ at 190°C, 64% 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 fired boilers
| Blade form | Backward curved or radial tipped, chosen on the collector and dust load |
| Material of construction | Mild steel with hard-faced leading edges |
| Drive and arrangement | Direct coupled or belt driven by size |
| Bearings and cooling | Pedestal or plummer block bearings with cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Casing drain, access door, inlet temperature alarm |
What this ID fan has to cope with
Sparks and burning fines
Sawdust is so fine that part of it leaves the furnace still burning. Sparks can ignite deposits in the cyclone, the bag filter and the fan casing. A spark arrestor before a bag filter, a drain and access door in the fan casing, and a temperature alarm at the fan inlet reduce the risk.
Gas volume swings with moisture
Green sawdust at 50% moisture produces far more gas per tonne of steam than dry shavings. A fan sized on dry fuel runs short when wet sawdust arrives, and the furnace goes positive. We size on the wettest expected fuel with drive control to cover the dry case.
Feed surges
Sawdust bridging in hoppers and then flooding the furnace gives sudden bursts of gas, flame and fines, followed by periods of low fuel. A variable speed drive on furnace draft responds faster than a damper and keeps the furnace negative. Hopper and screw design that avoid bridging remove the cause.
Energy and control
Sawdust boilers often run with variable load and fuel quality. A variable speed drive on furnace draft saves energy and handles feed surges better than a damper. Keeping sawdust under cover reduces moisture swings, gas volume and fan power together. Steady feeding, through well-designed hoppers and screws, also reduces excess air and fan load.
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 Forced Draft (FD) Fan for sawdust fired boilers.
Questions buyers ask
Which ID fan suits a sawdust fired boiler?
A backward curved ID fan with hard-faced leading edges after a bag filter, or a radial tipped fan after a cyclone alone, designed for about 160 to 220°C. It needs capacity for the wettest sawdust expected, a variable speed drive on furnace draft, and a drain and access door against spark deposits.
Why do we get fires in the cyclone or bag filter on a sawdust boiler?
Fine sawdust burns partly in suspension and some leaves the furnace still glowing. It collects in cyclone hoppers and on filter bags and ignites there. Spark arrestors ahead of the bag filter, steady feeding and combustion, and regular emptying of hoppers reduce the risk; a temperature alarm gives early warning.
Does wet sawdust need a bigger ID fan?
Yes. Water in wet sawdust becomes vapour in the flue gas, so wet green sawdust produces much more gas per tonne of steam than dry planer shavings, and operators usually raise air too. The ID fan must be sized for the wettest fuel the plant uses, with a drive to cover the dry case efficiently.
Information needed to quote an ID fan for sawdust 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.
- Sawdust source. Type of sawdust, moisture range and any shavings or chips burnt with it.
- Collector and spark control. Cyclone, multiclone or bag filter, and whether a spark arrestor is fitted.
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.