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 ID fan sits
- Shell-fired furnace
- Roasting drum and hood
- Settling chamber or cyclone
- Water scrubber
- ID fan
- Stack
The fan is placed after the water scrubber so that most of the tar droplets, soot and shell fragments are removed before the gas reaches the impeller. It then handles cooled, saturated gas carrying a fine mist of CNSL and phenolic compounds. Upstream it works against the hood suction, the duct, the cyclone and the scrubber, typically 100 to 200 mmWC in total. Older plants with the fan ahead of the scrubber see the gas at 150 to 250°C and collect a hard tar crust on the blades within weeks.
Gas at the ID fan
| Gas temperature, normal | 60 to 80°C after a water scrubber |
| Maximum or upset | 150 to 250°C if the scrubber water fails or the fan sits ahead of the scrubber |
| Dust at the fan | Soot, ash and shell char, mostly removed by the scrubber; the residual tar mist sticks to every surface |
| Moisture and dew point | Saturated after the scrubber, with free droplets; the quench at each drum discharge adds steam peaks |
| Corrosive species | Phenolic CNSL compounds and organic acids in the condensate; mildly acidic scrubber water |
| Density at the design temperature | About 1.02 kg/m³ at 75°C, 85% 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 drum roasting plants
| Blade form | Radial or radial tipped, with open blades that shed tar and can be scraped clean |
| Material of construction | SS 304 or SS 316 impeller and casing for scrubbed gas; mild steel with an acid-resistant lining as a lower-cost alternative |
| Drive and arrangement | Belt driven arrangement 9 or direct coupled arrangement 8, at moderate speed |
| Bearings and cooling | Bearings outside the gas stream, with a shaft seal against tarry condensate tracking along the shaft |
| Flow control | Variable speed drive or inlet damper holding hood suction |
| Also specified | Large access door, casing drain with a water seal, and wash nozzles for periodic cleaning |
What this ID fan has to cope with
CNSL tar build-up
Uncondensed CNSL and soot form a sticky film that hardens into a brittle crust on the blades and scroll. The crust grows unevenly and breaks away in patches, so vibration rises within weeks. Radial blades, a smooth stainless surface, wash nozzles in the casing and an access door for scraping at the weekly stop keep the impeller in balance.
Mist carry-over from the scrubber
A scrubber without a proper droplet eliminator throws slugs of tarry water into the fan, which corrode the blades, build deposits and load the motor. A mist eliminator before the fan, a casing drain with a water seal and an inlet duct that falls back towards the scrubber stop water collecting in the casing.
Smoke escaping at the hood
When tar fouls the duct or scrubber, the system resistance rises and the fan loses suction at the hood. Smoke then spills into the shed where people work. The fan is sized with pressure margin for a fouled system, and a suction gauge at the hood tells operators when the duct and scrubber need cleaning.
Fire in the duct
Tar lining the duct between hood and scrubber can be ignited by sparks from the shell furnace. A spark trap or settling chamber, duct sections that can be opened for cleaning, and an interlock that closes the hood damper and stops the fan on high duct temperature limit the damage.
Energy and control
Roasting runs in day shifts with drums loaded in batches, so the gas flow changes with how many drums are firing. A variable speed drive holding hood suction, in place of a fixed-speed fan with the damper half closed, saves power during light loading and breaks. Keeping the scrubber and duct clean matters as much: tar fouling can double the system resistance, and the fan then runs flat out just to keep smoke out of the shed.
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 Forced Draft (FD) Fan for cashew drum roasting plants.
Questions buyers ask
Why does the ID fan on a cashew roasting plant vibrate after a few weeks?
CNSL tar and soot build up unevenly on the blades and change the balance of the impeller. Regular cleaning through an access door or wash nozzles, a radial blade shape that sheds deposits, and placing the fan after the scrubber so most of the tar is removed first are the cures. Rebalancing on a clean wheel restores smooth running.
Which material is best for a cashew roasting fume exhaust fan?
After a water scrubber the gas is saturated and the condensate is mildly acidic with phenolic compounds, so SS 304 or SS 316 lasts longest. Mild steel with an acid-resistant lining costs less but needs regular inspection. Plain mild steel corrodes quickly in this service, especially at the bottom of the casing where condensate collects.
Is an ID fan needed for cashew drum roasting smoke control?
Yes, in practice. A chimney alone cannot pull the smoke through a cyclone and a water scrubber, and without suction at the hood the smoke escapes into the shed. An induced draft fan placed after the scrubber provides the draft for the whole system and holds the hood under suction, which is what fume control on drum roasting depends on.
Information needed to quote an ID fan for cashew drum roasting plants
- 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.
- Fume treatment. Whether the plant has a settling chamber, cyclone, water scrubber or mist eliminator, and where the fan is to sit among them.
- Drums and fuel. Number of roasting drums on the system, nut throughput per hour, and whether shells, shell cake or firewood is fired.
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.