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 FD fan sits
- Inlet screen
- FD fan
- Air duct and dampers
- Undergrate plenum
- Overfire air nozzles
The fan sits beside the furnace and supplies undergrate air to the shell bed, with a branch to overfire nozzles above it. On larger units an air heater sits between the fan and the grate. Raw shell releases most of its heat as volatile oil, so the overfire share is high and its nozzles need enough pressure to penetrate the flame. The fan works against the grate, dampers and nozzles, typically 100 to 180 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 40°C |
| Air after the air heater | 120 to 170°C where fitted, downstream of the fan |
| Pressure at the fan outlet | Typically 100 to 180 mmWC including overfire nozzles |
| Air split | A high share of overfire air when raw shell is fired |
| Dust | Clean air; shell grit from cutting sections can be drawn in |
| 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 cashew nut shell fired boilers
| Blade form | Backward curved, with pressure enough for the overfire nozzles |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive on steam pressure, with separate undergrate and overfire dampers |
| Also specified | Inlet screen, and an intake located away from the shell cutting and oil handling sections |
What this FD fan has to cope with
Smoke from too little overfire air
Raw shell gives off its liquid as vapour within seconds of landing on the bed. Without strong overfire air the vapour leaves unburnt as black smoke and tar. An FD fan with pressure for high-velocity overfire nozzles, and a separate damper to set that share, is the first line of defence against fouling downstream.
Puffs of volatile release
Shell fed in batches releases oil vapour in bursts. If air supply is steady, each burst gives a puff of smoke and a draft swing. More even feeding, and a variable speed drive that responds quickly to oxygen or furnace pressure, cope with this better than a fixed damper setting.
Oily film on the impeller
Air near shell cutting and oil handling areas can carry fine droplets of shell liquid. Drawn into the FD fan, they leave a sticky film that collects dust and unbalances the wheel. A clean intake location and an inlet screen are simple preventive measures. Cleaning the wheel at each scheduled stop keeps any film from building up.
Energy and control
Cashew processing runs on batches of roasting and steaming, so steam demand is uneven through the day. A variable speed drive on the FD fan saves power during low-demand periods and lets the air follow the fuel more closely. Lower excess air also reduces the gas the ID fan must move, but only as far as clean combustion allows: on raw shell, a little more air is cheaper than a tarred fan.
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 Induced Draft (ID) Fan for cashew nut shell fired boilers.
Questions buyers ask
How much overfire air does a cashew shell fired boiler need?
With raw shell, often 30 to 50% of the total air, because most of the energy is released as volatile oil above the bed. With de-oiled cake the share can be lower. The nozzles should give high velocity and good coverage across the furnace so the vapour burns before it cools.
Why does my cashew shell boiler produce black smoke?
Usually because the shell liquid vaporises faster than the air supply can burn it. Too little overfire air, poor mixing or batch feeding are the common causes. Increasing overfire air, feeding more evenly and keeping the furnace hot normally cure it. A flue gas oxygen check helps confirm whether air is short overall or simply poorly mixed.
Can the same FD fan serve raw shell and de-oiled cake?
Yes, if it has the pressure for the overfire nozzles and speed control. Raw shell needs more air above the bed and de-oiled cake more through the grate. Separate dampers for each branch let the operator shift the split when the fuel changes. Speed control then sets the total air for the load.
Information needed to quote an FD fan for cashew nut shell 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.
- Air split. Proportion of undergrate and overfire air, and the nozzle pressure required.
- Furnace type. Grate type, and whether the unit is a steam boiler or thermic fluid heater.
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.