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 FD fan sits
- Inlet filter
- FD fan
- Air heater
- Secondary air header
- Burner register or pre-furnace ports
The FD fan stands at ground level near the air heater and delivers ambient air through it into a secondary air header around the burner or pre-furnace. A separate fuel transport blower carries the sawdust into the furnace with about 10 to 20% of the total air at higher pressure, and a small share may go under a hearth to finish the coarser particles. The forced draft fan works against the air heater, ducts, register dampers and burner throat, a resistance that stays fairly constant because there is no thick fuel bed.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air heater | 150 to 220°C, downstream of the fan |
| Pressure at the fan outlet | Typically 120 to 220 mmWC including air heater and burner register |
| Dust | Clean air, though sawdust fines in the boiler house can be drawn in |
| Turndown required | About 3 to 1, set by the burner stability limit |
| 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 suspension fired boilers
| Blade form | Backward curved, aerofoil on larger units, for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, or impeller on the motor shaft for smaller duties |
| Bearings and cooling | Standard pedestal bearings; no cooling required |
| Flow control | Variable speed drive cross-limited with the fuel feed, so air leads fuel on a load rise and lags it on a load fall |
| Also specified | Inlet filter, inlet silencer and an air flow measuring element in the duct |
What this FD fan has to cope with
Flame instability at low load
A burner that loses air velocity at low load lets the flame lift or detach, and the boiler trips on flame failure. If the FD fan is oversized and throttled hard, it runs in the unstable part of its curve exactly when stability matters most. Sizing close to the real duty and controlling by speed keeps register velocities inside the burner's working range.
Air and fuel out of step
Sawdust feed is less steady than oil or gas. If the fuel jumps and the air does not follow, the flame smokes and unburnt carbon rises; if air is left high, efficiency falls. Air flow measurement in the duct and a cross-limited loop between fan speed and feeder speed manage this far better than a fixed damper position.
Fines in the hot air duct
Sawdust handling sheds fine dust into the boiler house air. Drawn into the FD fan, it builds on the blades and the air heater tubes and can smoulder in the hot air duct downstream. An inlet filter sited away from the fuel conveyors and a periodic check of the air heater air side prevent both problems.
Energy and control
Because suspension firing stores almost no fuel in the furnace, the FD fan does the real load following. A variable speed drive on steam pressure, with oxygen trim, cuts fan power at the part loads seen between press cycles and keeps excess air low. Measuring oxygen at the boiler outlet rather than after the air heater gives a truer picture, since air heater leakage inflates the reading downstream.
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 Induced Draft (ID) Fan for sawdust suspension fired boilers.
Questions buyers ask
What is the difference between the FD fan and the fuel transport blower on a sawdust burner?
The transport blower carries sawdust from the feeder to the burner at a velocity high enough to stop it settling in the pipe, and supplies only a small part of the combustion air. The FD fan supplies most of the air, usually preheated, around the burner. The two are interlocked so the burner never receives fuel without combustion air.
How much pressure does a forced draft fan need for a sawdust suspension fired boiler?
Typically 120 to 220 mmWC at the fan outlet, made up of the air heater, ducts, dampers and burner register. The figure depends on the burner design and air heater size, so the burner maker's register pressure and the air heater pressure drop at full load are the starting points for the fan duty.
Should the FD fan on a sawdust burner have a VFD?
In most cases, yes. A suspension flame needs air to follow fuel closely, and a drive responds faster and more precisely than a damper. It also saves power at part load. The motor should be inverter rated, and the control should keep a minimum air flow for flame stability and purge.
Information needed to quote an FD fan for sawdust suspension 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.
- Air split. Proportion of air through the fuel transport blower, secondary air ports and any hearth or grate.
- Burner data. Register pressure drop at full load and the turndown the burner needs at minimum load.
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.