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 FD fan sits
- Inlet filter
- FD fan
- Air-fuel ratio damper
- Burner
- Burner box
- Roasting chamber
On direct-fired roasters the combustion air fan is mounted on or beside the burner, feeding air through a modulating damper linked to the gas or oil valve. On roasters heated by a separate hot air generator, it sits on the generator and pushes air into the combustion chamber, from which hot gas passes a heat exchanger. The large fan moving roasting air round the loop is a separate recirculation fan, and the cooler has its own fan pulling ambient air through the roasted kernels.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Peanut dust and skin flakes in the roasting hall; inlet filter needed |
| Pressure at the fan outlet | Typically 100 to 300 mmWC at the burner |
| Turndown | Burner modulates through each batch and with throughput changes |
| 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 groundnut and peanut roasters
| Blade form | Backward curved, for a stable curve and quiet running |
| Material of construction | Mild steel, painted |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Characterised damper or variable speed drive linked to the fuel valve |
| Also specified | Inlet filter, air pressure switch for burner safety, and a low-noise inlet |
What this FD fan has to cope with
Inlet filter clogging with skins
Skin flakes and peanut dust in the hall are drawn onto the inlet filter. As it loads, combustion air falls, and the flame turns rich and sooty, which can taint the roast. A filter sized for low face velocity, an air pressure switch and a fixed filter-changing routine keep the air flow where the burner needs it.
Air-fuel ratio drift at low fire
Roasters spend long periods at low fire between batches or at reduced throughput. A fan with a flat curve, or a damper with slack in its linkage, lets the ratio drift so the burner runs with too much or too little air. A variable speed drive or characterised damper tied to the fuel valve holds it.
Burner trips from low air pressure
A worn impeller, a dirty inlet or a slipping belt on older units reduces air pressure until the burner safety switch trips mid-batch and spoils the roast. A direct-drive fan with pressure margin over the burner requirement, and a filter gauge the operator can read, avoid nuisance trips.
Energy and control
The burner fan is small, but it runs every hour the roaster is hot. The larger energy lever is the excess air it delivers: surplus air is heated and then carried out with the exhaust bleed. Setting the fan, by speed or damper, to the burner maker's air-fuel curve and checking flue oxygen periodically cuts fuel use while keeping the flame clean.
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 Induced Draft (ID) Fan for groundnut and peanut roasters.
Questions buyers ask
What does the FD fan do on a peanut roaster?
It supplies combustion air to the burner, either on the roaster itself or on the hot air generator feeding it. The burner then heats the roasting air that passes through the kernels. The fan handles clean ambient air, so it is a small backward curved unit whose job is a steady air supply as the burner modulates.
Why is the groundnut roaster burner flame sooty?
Usually the burner is short of air. A clogged inlet filter, a slipping belt, a worn impeller or a damper linkage that has moved all reduce combustion air. Restoring fan pressure and resetting the air-fuel ratio with a flue gas analyser clears the soot, which otherwise taints the roasted groundnuts and fouls the burner box.
Can one fan supply both burner air and roasting air?
On small batch roasters a single fan sometimes does both, but larger roasters separate them. The burner needs a modest, controlled flow of combustion air, while roasting air is a much larger recirculated flow. Separate fans let burner firing and air flow be controlled independently, which gives a more even roast.
Information needed to quote an FD fan for groundnut and peanut roasters
- 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.
- Burner details. Fuel, burner rating and the air pressure the burner maker requires at full fire.
- Firing arrangement. Whether the burner fires direct into the roasting air or into a separate hot air generator.
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.