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 ID fan sits
- Fixed or reciprocating grate
- Boiler
- Economiser
- Air heater (if fitted)
- Multicyclone or bag filter
- ID fan
- Chimney
The fan follows the multicyclone or bag filter. Rubber wood ash is low and soft, so dust at the fan is light. Suction at the fan is typically 120 to 220 mmWC. In a tyre plant the boiler often runs at high pressure for curing presses that draw steam in pulses as their cycles start, and the ID fan must follow a firing rate that changes within a minute or two, many times a shift.
Gas at the ID fan
| Gas temperature, normal | 160 to 190°C after the economiser |
| Maximum or upset | Up to 240°C during sustained peak steam demand |
| Dust at the fan | Low, soft wood ash |
| Moisture and dew point | Water dew point about 55 to 65°C with fresh logs |
| Corrosive species | Low; offcuts treated with boron preservative add glassy deposits on heat surfaces |
| Density at the design temperature | About 0.79 kg/m³ at 175°C, 66% 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 rubber wood fired boilers
| Blade form | Backward curved, for efficiency on low dust gas |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive with a fast furnace draft loop |
| Also specified | Standby fan or spare rotor for critical plants, casing insulation and drain |
What this ID fan has to cope with
Curing press load swings
Tyre curing presses draw steam in pulses as their cycles start together. The boiler fires harder, gas volume rises, and the furnace goes positive if the fan lags. A drive with a fast draft loop, tuned with the FD fan so air and draft change together, keeps the furnace at suction through each pulse and protects the firing doors.
Boron-treated offcuts
Rubber wood for furniture is usually treated with boron compounds against borers. Boron in the ash forms glassy deposits on the boiler tubes and economiser, raising resistance and gas temperature at the fan between cleanings. Where treated offcuts are a large share of the fuel, the fan needs pressure and temperature margin for the fouled boiler.
Fresh logs in the replanting season
When plantations are felled, fresh logs arrive in bulk at 40% moisture or more. They burn slowly and wetly, raising gas volume and dew point. Plants that buy fresh logs in quantity should size the fan on that wet case and season wood under cover wherever space allows.
Energy and control
Tyre plants run three shifts, and the ID fan runs continuously. A variable speed drive saves power in low demand periods and responds faster than a damper to press pulses, which also steadies steam pressure. Seasoning wood before firing reduces the gas volume and the fan power and raises boiler efficiency, often by more than any change to the fan itself.
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 Forced Draft (FD) Fan for rubber wood fired boilers.
Questions buyers ask
Why does my rubber wood boiler go positive when the curing presses start?
Curing presses draw steam in pulses, so the boiler suddenly fires harder. If the ID fan responds slowly, furnace pressure rises before the draft catches up. A variable speed drive with a fast draft control loop, tuned together with the FD fan, holds suction through each pulse.
Does treated rubber wood affect the boiler fans?
Rubber wood treated with boron preservative leaves glassy deposits on the boiler tubes. Resistance and gas temperature at the ID fan rise between cleanings. The induced draft fan should have pressure and temperature margin if treated offcuts are fired in quantity, and the tubes need regular cleaning.
Is a standby ID fan needed for a tyre plant boiler?
Many tyre plants keep one, or at least a spare balanced rotor, because curing stops without steam and partly cured tyres are scrapped. The cost of a standby is small compared with lost production. A spare rotor is the minimum where a full standby fan is not practical.
Information needed to quote an ID fan for rubber wood 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.
- Steam demand pattern. Curing press cycle times and the peak steam draw.
- Fuel form. Logs, billets, chips or offcuts, and whether treated offcuts are fired.
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.