How cashew drum roasting plants work
Raw cashew nuts, conditioned to about 8 to 10% moisture, are fed into a slowly rotating perforated drum heated from below by a furnace fired on cashew shells, deoiled shell cake or firewood. As the shell heats to roughly 180 to 200°C, the cashew nut shell liquid (CNSL) it contains exudes, partly burns and partly vaporises, giving dense, oily, acrid smoke. Roasted nuts are discharged and quenched with water, which releases a burst of steam. The furnace gas and drum smoke are gathered in a hood, passed through a settling chamber or cyclone and usually a water scrubber, and discharged by the ID fan to a stack. Many units now steam-cook instead; drum roasting survives where its flavour and low capital cost are valued, and where the pollution board requires fume control on it.
Where the FD fan sits
- Inlet screen
- FD fan
- Air duct and damper
- Under-grate plenum
- Shell-fired furnace
- Roasting drum
The fan stands beside the furnace, away from the hot drum, and blows ambient air through a short duct and damper into the plenum under the grate, with a smaller branch for overfire air where fitted. Shells burn fast and fiercely because of their oil content, and the fan works against a grate that clogs with sticky clinker. Its outlet pressure is modest, typically 50 to 120 mmWC, but it has to stay steady as the bed resistance rises through the shift.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Shell fragments and husk dust in the shed; an inlet screen is needed |
| Radiant heat | Fan sits near a hot furnace; motor and bearings need shielding |
| Pressure at the fan outlet | Typically 50 to 120 mmWC including grate and shell bed |
| 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 drum roasting plants
| Blade form | Backward curved, for a stable curve on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft for small furnaces; belt driven for larger ones |
| Bearings and cooling | Standard motor or pedestal bearings; a heat shield on the furnace side |
| Flow control | Inlet damper, or a variable speed drive linked to drum temperature |
| Also specified | Inlet screen, tight-closing damper in the duct, and a separate overfire air branch |
What this FD fan has to cope with
Grate choking with tarry clinker
CNSL in the shells leaves a sticky residue that binds ash into clinker on the grate. Bed resistance rises through the shift and air flow falls unless the fan has a rising pressure curve with margin. A backward curved fan selected on the clinkered bed, with regular de-ashing, keeps the air flow up.
Smoke from air starvation
When the air supply drops, the shell oil cracks into dense black smoke that loads the scrubber and the ID fan downstream. Matching the combustion air to the shell feed rate, with an overfire air branch above the bed, burns more of the oil in the furnace and reduces the fume treatment load.
Flame travelling back into the duct
If the fan stops while the furnace is hot, flame and gas can travel back into the air duct and scorch the fan. A non-return flap or tight damper in the duct, and an interlock that stops shell feed when the fan trips, prevent this.
Energy and control
The combustion air fan is a small machine, so its own power saving is modest. What matters more is the fuel: the right air supply burns shells more completely, which lowers smoke, scrubber load and the power drawn by the ID fan. Variable speed drives on both fans, linked to drum temperature, let the furnace follow the roasting cycle instead of running at full fire through the shift.
ID fan and FD fan in cashew drum roasting plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the water scrubber, before the stack | Beside the furnace, feeding the grate |
| What it handles | Saturated flue gas and smoke with CNSL tar mist | Clean ambient air |
| Temperature at the fan | 60 to 80°C, up to 250°C if the scrubber fails | Ambient, 25 to 45°C |
| Main design concern | Tar build-up and corrosion | Steady air through a clinkering grate |
| Control | Hood suction | Drum temperature |
See also the Induced Draft (ID) Fan for cashew drum roasting plants.
Questions buyers ask
Does a cashew roasting furnace need an FD fan?
Many traditional drum roasters run on natural draft, but the fire then varies with the shell bed and the weather, and burns smoky. A small forced draft fan gives a steady combustion air supply, better shell burnout and a more even drum temperature. It also lets the furnace work against a scrubber and ID fan without starving the fire.
What pressure should a cashew shell furnace combustion air blower give?
Usually a modest pressure, typically 50 to 120 mmWC at the fan outlet, enough to push air through the grate, the clinkering shell bed and the duct dampers. The blower needs margin above the clean-grate figure, because bed resistance rises through the shift as tarry clinker builds up.
Can deoiled cashew shell cake be fired with the same combustion air fan?
Usually yes. Shell cake has less oil, burns more slowly and gives less smoke than raw shells, but forms a denser bed. The same fan normally serves both, provided it has pressure for the denser bed and flow for the higher feed rate the cake needs. Check both against the fan curve before switching fuel.
Information needed to quote an FD fan for cashew drum roasting plants
- 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.
- Furnace and grate. Grate area, fuel fired, and whether overfire air is required.
- Draft balance. Whether an ID fan and scrubber are fitted downstream, so the two fans can be matched to keep the hood under suction.
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.