How groundnut and peanut roasters work
Groundnut kernels are roasted to develop flavour and loosen the red skins, either in batch rotary drums or in continuous hot air roasters with a belt or rotating drum. Hot air at about 160 to 220°C, from a direct gas or diesel burner or an indirect hot air generator, passes through the kernels for roughly 10 to 40 minutes until they reach about 150 to 170°C and lose most of their 5 to 7% moisture. In continuous roasters most of the air is recirculated through the burner box, and a part is bled off as exhaust carrying moisture, oil vapour, loose skins and dust through a cyclone to the exhaust fan. The roasted kernels then pass to a cooler, where ambient air is drawn through the bed, before blanching or packing.
Where the ID fan sits
- Burner box
- Roasting chamber
- Recirculation fan
- Exhaust bleed duct
- Skin cyclone
- ID fan
- Stack or afterburner
The exhaust fan takes air from the roasting loop, usually from the return side after the recirculation fan, and sends it through a cyclone that drops out skins, broken kernels and dust. The fan sits after the cyclone so it handles only the fines that escape, but these are light, dry and combustible. It works against the bleed damper, the cyclone and the stack, typically 80 to 180 mmWC. In larger plants the exhaust then goes to a thermal afterburner to destroy roasting odour and smoke.
Gas at the ID fan
| Air temperature, normal | 130 to 180°C in the exhaust bleed |
| Maximum or upset | Above 220°C if a batch scorches or the burner overshoots |
| Dust at the fan | Fine skin flakes and kernel dust that pass the cyclone; light and combustible |
| Moisture and dew point | Moderate humidity; oil vapour condenses on casing walls that run below about 100°C |
| Fire risk | Skins can ignite in the roaster and carry sparks into the duct |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 groundnut and peanut roasters
| Blade form | Backward curved, with smooth self-cleaning blades; radial where the skin load is heavy |
| Material of construction | Mild steel for most duties; SS 304 where food plant rules or wash-down apply |
| Drive and arrangement | Belt driven arrangement 9 or direct coupled, with a variable speed drive |
| Bearings and cooling | Shaft cooling disc and heat slinger; bearings outside the hot air stream |
| Flow control | Variable speed drive on roaster chamber pressure or exhaust humidity |
| Also specified | Insulated casing, cleaning door, drain point, and a spark detection and quench point in the duct |
What this ID fan has to cope with
Oily skin deposits on the blades
Peanut skins carry some oil, and oil vapour condenses on any part of the casing that runs cooler than the exhaust. The deposit grows on the blade pressure face and pulls the wheel out of balance. Insulating the casing, keeping the exhaust warm and cleaning through an access door at each sanitation stop prevent it.
Roaster fires carried into the duct
A scorched batch, or skins resting on the burner box, can ignite and send burning flakes down the exhaust. A spark detector with water quench ahead of the cyclone, a fan that can be stopped and isolated quickly, and a casing with no flat ledges where skins can settle reduce the risk of a duct fire.
Roast colour drifting with exhaust flow
If the exhaust flow varies as the cyclone fills or the damper wears, the moisture held in the roasting loop changes and roast colour drifts from batch to batch. A speed-controlled exhaust fan holding a constant bleed, independent of the cyclone condition, gives repeatable roasting.
Energy and control
Every cubic metre of hot air bled from the roaster must be replaced by ambient air that the burner heats again, so the exhaust fan governs fuel use as well as its own power. A variable speed drive holding the minimum bleed needed to remove moisture, in place of a damper set for the worst case, saves heat and power. On batch drums the fan can slow during loading, cooling and discharge steps.
ID fan and FD fan in groundnut and peanut roasters
| ID fan | FD fan | |
|---|---|---|
| Position | After the skin cyclone, on the exhaust bleed | On the burner or hot air generator |
| What it handles | Hot, moist air with skins and oil vapour | Clean ambient air |
| Temperature at the fan | 130 to 180°C | Ambient, 25 to 45°C |
| Main design concern | Oily deposits and fire | Steady air-fuel ratio |
| Control | Chamber pressure or bleed rate | Burner firing rate |
See also the Forced Draft (FD) Fan for groundnut and peanut roasters.
Questions buyers ask
What temperature does a peanut roaster exhaust fan handle?
In most groundnut roasters the exhaust bleed is at about 130 to 180°C, with short peaks above 200°C if a batch scorches or the burner overshoots. The fan is designed for the peak, with a shaft cooling disc and bearings kept outside the hot air stream, and its rating is corrected for the lower density of hot air.
Why does the groundnut roaster exhaust fan vibrate?
Peanut skins and condensed oil form an uneven, sticky layer on the blades. A small mass of deposit changes the balance enough to raise vibration, especially on a fan running at high speed. Regular cleaning, an insulated casing kept above the oil condensation point, and in-situ balancing after cleaning bring it back to smooth running.
Does the roaster exhaust fan need to be stainless steel?
Not usually. The exhaust is hot and fairly dry, and mild steel serves well. Stainless steel is chosen where the plant follows food safety rules for equipment near the product, where the fan is washed down, or where the exhaust is cooled in a heat recovery unit and condensate forms in the casing.
Information needed to quote an ID fan for groundnut and peanut roasters
- 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.
- Roaster type. Batch drum or continuous hot air roaster, its capacity in kg per hour, and whether the air is recirculated.
- Exhaust treatment. Skin cyclone, afterburner or scrubber after the fan, and the resistance each adds.
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.