How Arecanut husk fired boilers work
Arecanut husk is the fibrous outer layer of the areca fruit, removed when the nut is dehusked, mainly in Karnataka, Kerala and Assam. Fresh husk carries 40 to 60% moisture; sun-dried husk falls to 15 to 25%. Ash is low, typically 2 to 5%, and dried husk has a calorific value of about 3,500 to 4,000 kcal/kg. The husk is coarse, fibrous and springy, so it bridges in hoppers and packs unevenly on the grate. It is fired on fixed or inclined grates of smoke tube and small water tube boilers in areca processing and food plants, often blended with firewood or coconut shell. The flue gas passes the boiler and a cyclone or multicyclone before the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Damper
- Undergrate plenum
- Fixed or inclined grate
The FD fan stands beside the boiler front and supplies the plenum under the fixed or inclined grate, with a branch to overfire air ports where they are fitted. Few small areca husk boilers have an air heater, so the air enters the furnace cold. The fan pressure is typically 60 to 140 mmWC, set by the grate and a bed of husk whose resistance changes as it packs, dries and burns through.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 40°C |
| Air after the fan | Ambient; an air heater is uncommon |
| Dust | Husk fibre near the fuel store |
| Pressure at the fan outlet | Typically 60 to 140 mmWC |
| Bed resistance | Varies widely as fibrous husk packs and burns through |
| 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 Arecanut husk fired boilers
| Blade form | Backward curved, with a steep pressure curve |
| Material of construction | Mild steel |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Variable speed drive or damper linked to steam pressure |
| Also specified | Inlet screen and a damper on the overfire air branch |
What this FD fan has to cope with
Channelling through the bed
Fibrous husk packs into mats with gaps between them. Air rushes through the gaps and blows holes in the fire while packed areas smoulder. A fan with a steep curve holds flow steadier as bed resistance changes, and regular raking, or a moving grate on larger units, breaks up the mats so air reaches the whole bed.
Wet husk needs more pressure
Wet husk packs more densely and takes longer to dry, so bed resistance rises in the harvest season. A fan selected with no pressure margin delivers less air just when the fire needs more, and the boiler loses steam pressure. The selection is made at the wet-husk bed resistance, with the drive trimming it back for dry fuel.
Cold air on wet fuel
With no air heater, wet husk dries slowly on the grate and the fire burns back. Where a small air heater or a flue gas heat recovery coil can be added, it helps considerably, but the FD fan must then have the extra pressure for its air-side resistance, which is often overlooked on retrofits.
Energy and control
Seasonal operation and frequent load changes suit a variable speed FD fan matched to the ID fan. Where the fan now runs at full speed against a part-closed damper, a drive gives a quick saving. Adding a simple air heater to dry wet husk can raise boiler efficiency more than any fan change, provided the fan has the pressure to push air through it.
ID fan and FD fan in Arecanut husk fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone, before the chimney | At the boiler front, feeding the grate |
| What it handles | Flue gas with fibrous char flakes | Ambient air |
| Temperature at the fan | 170 to 220°C, up to 260°C | Ambient, 20 to 40°C |
| Main design concern | Seasonal gas volume range | Pressure through an uneven bed |
| Control | Furnace draft | Steam pressure |
See also the Induced Draft (ID) Fan for Arecanut husk fired boilers.
Questions buyers ask
Why does my arecanut husk fire burn unevenly?
Fibrous husk packs into mats, and air from the FD fan channels through the gaps between them. Some areas burn hard while others smoulder. A forced draft fan with a steep pressure curve, together with regular raking or a moving grate, gives more even burning and less unburnt husk in the ash.
Does an arecanut husk boiler need an air heater?
Not always, but it helps a great deal with wet husk. Hot undergrate air dries the fuel faster on the grate and steadies the fire. If one is added, the FD fan must have enough pressure for the extra air-side resistance, or the gain is lost to lower airflow.
What size FD fan does an arecanut husk fired boiler need?
It depends on boiler capacity, excess air and the resistance of the grate and husk bed. Send us the boiler data or the existing fan nameplate and we will calculate the duty, with pressure margin for wet harvest-season husk and for any air heater planned.
Information needed to quote an FD fan for Arecanut husk 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, inclined or moving grate, and the usual bed depth.
- Air heater. Whether an air heater is fitted or planned, and its air-side pressure drop.
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.