How coconut shell fired boilers work
Coconut shell from desiccated coconut units, copra driers and shell charcoal yards is a dense, hard fuel. Crushed to 10 to 30 mm, it carries 8 to 15% moisture, a calorific value of about 4,200 to 4,600 kcal/kg and well under 2% ash. It burns hot on fixed, travelling or reciprocating grates, and most units in the coastal belt are smoke tube or packaged water tube boilers of 1 to 10 TPH, many without an economiser. Flue gas leaves the boiler at 200°C or more on the simpler units, passes a cyclone or multiclone and sometimes a bag filter, then the ID fan and chimney. The FD fan supplies undergrate air, which also keeps the grate bars cool under the hot shell bed.
Where the ID fan sits
- Grate furnace
- Smoke tubes or boiler bank
- Economiser, where fitted
- Cyclone or multiclone
- Bag filter, where fitted
- ID fan
- Chimney
The fan is mounted after the cyclone or bag filter, close to the chimney base. Coconut shell leaves little ash, so residual dust at the fan is light even after a single cyclone, and the main load on the impeller is thermal. On a smoke tube boiler without heat recovery the gas reaches the fan at 220 to 260°C, and inlet suction is usually modest, 80 to 150 mmWC, because there is no air heater. Units that have added an economiser run cooler but at higher suction.
Gas at the ID fan
| Gas temperature, normal | 200 to 250°C without an economiser |
| With an economiser | 160 to 180°C at the fan |
| Maximum or upset | Up to 300°C with sooted tubes or overfiring |
| Dust at the fan | Low; fine alkali-rich ash and soot, typically 0.2 to 1 g/Nm³ after a cyclone |
| Moisture and dew point | Water dew point about 45 to 50°C; very little sulphur |
| Density at the design temperature | About 0.71 kg/m³ at 230°C, 59% 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 coconut shell fired boilers
| Blade form | Backward curved, since the low ash load allows an efficient wheel even after a cyclone |
| Material of construction | Mild steel casing and impeller, checked for strength at the upset temperature |
| Drive and arrangement | Direct coupled, arrangement 8; belt driven on the smallest units |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc and heat shield; oil bath bearings at the higher temperatures |
| Flow control | Variable speed drive on furnace draft, with an inlet damper for isolation |
| Also specified | Casing insulation for personnel protection, an access door and a casing drain |
What this ID fan has to cope with
High fan inlet temperature
Without an economiser, gas at 230°C and above lowers density, raises the volume the fan must move and heats the shaft. A fan picked at 150°C from a standard curve runs short of draft and overheats its drive end bearing. Sizing at the real temperature, a cooling disc and a longer bearing span keep it reliable. If an economiser is added later, the fan duty is checked again.
Soot from banked fires
When steam demand drops, operators often bank the fire, and coconut shell smoulders with a sooty, tarry smoke. Soot sticks to the impeller and casing in uneven layers that cause unbalance and can occasionally ignite. Holding a minimum draft at low load rather than stopping the fan, and an access door for cleaning, limit the problem.
Draft lost to clinker
Shell ash is rich in potassium and fuses at a fairly low temperature. Clinker on the grate blocks air, the furnace turns smoky, and operators open the ID fan fully to clear it. A fan with some pressure reserve helps, but the lasting fix lies on the air side: regular grate cleaning and a steady bed temperature.
Energy and control
Coconut processing units often run to the day shift, with the boiler banked overnight. An ID fan on a variable speed drive can drop to a low holding speed while banked instead of stopping and restarting. On units without an economiser, the bigger gain is usually in the boiler: an economiser cuts fuel use, lowers the gas temperature at the fan and shrinks the gas volume, so fan power falls too.
ID fan and FD fan in coconut shell fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone or bag filter | Under the grate, beside the furnace |
| What it handles | Hot flue gas with light alkali ash and soot | Ambient air, often humid and salty |
| Temperature at the fan | 200 to 250°C without an economiser | Ambient, 25 to 40°C |
| Main design concern | Gas temperature and bearing heat | Air through a dense, hot bed |
| Control | Furnace draft | Steam pressure |
See also the Forced Draft (FD) Fan for coconut shell fired boilers.
Questions buyers ask
Why is the ID fan on my coconut shell boiler getting hot at the bearings?
Most small coconut shell boilers send gas to the fan at 220°C or more because there is no economiser. Heat conducts along the shaft into the drive end bearing. A cooling disc, a heat shield between casing and pedestal, and the right grease or oil for the temperature normally solve it. Also check that the gas has not become hotter because of sooted tubes.
Does a coconut shell fired boiler need a bag filter before the ID fan?
It depends on the emission limit. Shell ash is low and a good cyclone or multiclone meets some limits, but tighter ones need a bag filter. The ID fan must then be checked for the extra filter resistance, often 100 to 150 mmWC, and for a gas temperature low enough for the bags, which may mean adding an economiser.
Can I fit a VFD to an existing ID fan on a coconut shell boiler?
Usually yes. The motor should be checked for inverter duty and the fan for any resonance in the new speed range. Most fans on these boilers spend long hours throttled, so a drive tied to furnace draft gives worthwhile savings and smoother operation, especially where the boiler is banked overnight.
Information needed to quote an ID fan for coconut shell 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.
- Heat recovery. Whether the boiler has an economiser or air heater, and the measured gas temperature at the fan inlet.
- Fuel blend. Share of coconut shell and any other fuel such as husk, wood or briquettes.
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.