How cashew nut shell fired boilers work
Cashew nut shell is left after raw nuts are roasted or steamed and cut in processing units in Kerala, Karnataka, Goa, Tamil Nadu and Odisha. Raw shell contains about 20 to 30% cashew nut shell liquid, a phenolic oil, and has a calorific value of about 4,500 to 5,000 kcal/kg with only 1 to 3% ash. Many plants first pass the shell through an expeller or solvent plant to recover the liquid and burn the de-oiled shell cake, which burns more cleanly. Raw shell gives a smoky, tarry flame unless the furnace is designed for it. Boilers and thermic fluid heaters on fixed or travelling grates serve roasting, steaming and drying. Gas passes the boiler, economiser, cyclone and a bag filter or wet scrubber to the ID fan.
Where the ID fan sits
- Grate furnace
- Boiler bank
- Economiser
- Cyclone
- Bag filter or wet scrubber
- ID fan
- Chimney
The fan is after the cyclone and final collector. Where raw shell is fired, oily soot blinds filter bags, so many plants use a wet scrubber and the fan handles saturated gas. With de-oiled shell cake a bag filter is common and the fan runs dry and warm. Suction at the inlet is typically 150 to 250 mmWC. The fan is kept downstream of the collectors so that the heaviest tar drops out before it reaches the wheel.
Gas at the ID fan
| Gas temperature, normal | 170 to 200°C dry; 60 to 75°C after a scrubber |
| Maximum or upset | About 250°C with tube fouling or overfiring |
| Dust at the fan | Little ash; oily soot and tar droplets when raw shell is fired |
| Moisture and dew point | Water dew point 45 to 55°C in dry gas; saturated after a scrubber |
| Corrosive species | Organic acids and phenolics in condensate, aggressive to mild steel |
| Density at the design temperature | About 0.77 kg/m³ at 185°C, 65% 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 cashew nut shell fired boilers
| Blade form | Radial tipped and open for easy cleaning where tarry soot is expected; backward curved on clean de-oiled cake firing |
| Material of construction | Mild steel for dry gas; SS 304 or SS 316 wheel and casing behind a wet scrubber |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Large access doors, water wash connections and a casing drain |
What this ID fan has to cope with
Tarry deposits on the impeller
Raw shell releases shell liquid vapour that cracks into soot and tar when combustion is incomplete. The deposit coats the blades unevenly, vibration climbs and eventually the fan trips. An open radial wheel, wash nozzles, access doors and a cleaning routine keep it under control; full burnout in the furnace or de-oiling the shell removes the cause.
Duct and fan fires
Oily soot is combustible. A layer in the duct or fan casing can ignite from a carried spark or during a hot restart, and the fire can damage the impeller and bearings. Cleaning deposits at every shutdown, avoiding long low-temperature running and providing access along the duct reduce the risk.
Acidic water behind a scrubber
Scrubbing gas from shell firing gives water laden with phenolics and tar. Behind the scrubber the fan runs wet, and condensate attacks mild steel within months. Stainless construction, a mist eliminator ahead of the fan and drains at the low points make a scrubber arrangement reliable, with the duty taken at saturated gas conditions.
Energy and control
Cashew units run roasting, steaming and drying on a day pattern, and steam demand jumps when batches start. A variable speed drive on the ID fan follows these steps and saves power during the long idle spells. Clean combustion is also the best energy measure for the fan: a sooted boiler sends hotter gas to the fan, and a tar-coated wheel loses efficiency.
ID fan and FD fan in cashew nut shell fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or scrubber | Beside the furnace, feeding grate and overfire air |
| What it handles | Flue gas with oily soot or saturated scrubber gas | Ambient air |
| Temperature at the fan | 170 to 200°C dry, 60 to 75°C wet | Ambient, 25 to 40°C |
| Main design concern | Tar deposits, fire and cleanability | Overfire air for clean burnout |
| Control | Furnace draft | Steam pressure and smoke |
See also the Forced Draft (FD) Fan for cashew nut shell fired boilers.
Questions buyers ask
Why does my ID fan get coated with tar on a cashew shell boiler?
Raw cashew shell contains shell liquid that vaporises in the furnace. If combustion air or mixing is poor, the vapour cracks into soot and tar that stick to the fan. Better overfire air, steady feeding and good furnace temperature reduce it; burning de-oiled shell cake largely removes it.
Can a bag filter be used on a cashew shell fired boiler?
With de-oiled shell cake and good combustion, yes. With raw shell, oily soot tends to blind the bags and creates a fire risk, so wet scrubbers are often used instead. The choice changes the ID fan duty: a scrubber means saturated, corrosive gas and stainless construction.
How often should the ID fan be cleaned on a cashew shell boiler?
It depends on the fuel and combustion. On raw shell with marginal burnout, deposits can build within weeks; on de-oiled cake, a check at each scheduled shutdown may be enough. Rising vibration readings are the best guide, and access doors and wash connections make each cleaning quicker.
Information needed to quote an ID fan for cashew nut 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.
- Shell type. Whether raw shell or de-oiled shell cake is fired, and in what proportion.
- Collector. Whether the fan follows a bag filter or wet scrubber, and the gas temperature at the fan.
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.