How rubber wood fired boilers work
Rubber wood comes from old rubber trees felled when plantations are replanted, mainly in Kerala, Tamil Nadu and Karnataka, and from offcuts of rubber wood sawmills and furniture units. It fires boilers in tyre plants, latex and rubber processing units and food factories in those regions. Fresh logs carry 35 to 50% moisture and seasoned wood 20 to 30%; ash is low, about 1 to 2%, and seasoned wood gives about 3,000 to 3,500 kcal/kg. Logs and split billets burn on fixed or step grates in smoke tube and water tube boilers, and chips and offcuts on reciprocating grates. The flue gas passes the boiler, an economiser and often an air heater, then a multicyclone or bag filter, before the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater (if fitted)
- Undergrate plenum
- Fixed or reciprocating grate
The FD fan sits beside the boiler and supplies the grate plenum, through an air heater on larger water tube boilers. On log-fired fixed grates the bed is deep and open, so resistance is modest, typically 80 to 150 mmWC. On reciprocating grates burning chips and offcuts the bed is denser and the air heater adds resistance, so pressure rises toward 150 to 200 mmWC. The fan is interlocked with the ID fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 38°C |
| Air after the air heater | 150 to 200°C where an air heater is fitted |
| Dust | Clean air; sawdust near the fuel store |
| Pressure at the fan outlet | Typically 80 to 200 mmWC |
| Humidity | High in coastal Kerala; negligible effect on the duty |
| Density at the design temperature | About 1.14 kg/m³ at 38°C, 95% 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 rubber wood fired boilers
| Blade form | Backward curved, for quick, stable response |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or overhung |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on steam pressure |
| Also specified | Inlet screen and interlock with the ID fan |
What this FD fan has to cope with
Slow response to steam demand
Wood on a grate responds to air faster than to fuel. If the FD fan reacts slowly, steam pressure sags when presses start and overshoots when they stop, and curing temperature drifts. A variable speed drive with a well-tuned steam pressure loop smooths this, and an air flow measurement lets the loop act on air rather than damper position.
Air leakage at firing doors
Log-fired boilers have large firing and ash doors that leak air. The leak bypasses the FD fan control, so excess air rises and the ID fan handles more gas than combustion needs. Good door seals, and control on measured oxygen rather than a fixed FD fan setting, recover most of this loss.
Clinker from treated wood
Boron-treated offcuts leave glassy clinker on the grate that blocks air openings. The FD fan pressure then rises while flow falls. Steady primary air, mixing treated offcuts with plain wood, and a fan with pressure margin to hold flow through a partly blocked grate keep the boiler at output.
Energy and control
Variable speed drives on the FD and ID fans, controlled together, save power and steady the steam pressure. In tyre plants steadier pressure also means steadier curing temperature, which is often the stronger reason for the change. Oxygen measurement at the boiler outlet shows quickly whether air leakage or fan setting is costing fuel.
ID fan and FD fan in rubber wood fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multicyclone or bag filter | Beside the boiler, feeding the grate |
| What it handles | Wet flue gas with low wood ash | Ambient air |
| Temperature at the fan | 160 to 190°C, up to 240°C | Ambient, 25 to 38°C |
| Main design concern | Draft response to press pulses | Fast firing rate control |
| Control | Furnace draft | Steam pressure |
See also the Induced Draft (ID) Fan for rubber wood fired boilers.
Questions buyers ask
How can I steady steam pressure on a rubber wood boiler?
Use a variable speed FD fan controlled from steam pressure, with the ID fan on furnace draft. Wood on the grate responds to air quickly, so good air control is the main lever. Tuning both loops together prevents the furnace going positive when the FD fan speeds up.
Is an air heater useful on a rubber wood boiler?
On larger water tube boilers, yes. It raises efficiency and helps dry fresh logs on the grate. The forced draft fan pressure must include the air heater resistance, which is often missed when an air heater is added to an existing boiler. Without the extra pressure, airflow falls and the efficiency gain is lost.
What FD fan pressure does a log fired rubber wood boiler need?
Typically 80 to 150 mmWC on a fixed grate burning logs, and more on a reciprocating grate with chips or with an air heater. We size it from the boiler maker's data or measured readings, with margin for partial grate blockage from treated wood clinker.
Information needed to quote an FD fan for rubber wood 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.
- Grate type. Fixed grate with logs, or reciprocating grate with chips and offcuts.
- Control. Present steam pressure control and whether an oxygen analyser 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.