How cocoa bean roasters work
Fermented, dried cocoa beans are cleaned and roasted to develop flavour and loosen the shell, either as whole beans or as nibs after winnowing. Batch drum roasters and continuous roasters use hot air at about 120 to 250°C, heated by a gas burner directly or through a heat exchanger, or by thermic fluid, so that the beans reach about 110 to 150°C over 15 to 45 minutes. Some lines debacterise the beans or nibs with steam first. The roaster exhaust carries shell fragments, dust, moisture, acetic acid from fermentation, and in darker roasts cocoa butter aerosol and smoke. A cyclone removes shell and dust, and odour is treated with a thermal or catalytic oxidiser where needed. Winnowing aspiration fans separate shell from nib.
Where the ID fan sits
- Roasting drum or chamber
- Exhaust duct
- Cyclone
- Exhaust (ID) fan
- Thermal or catalytic oxidiser
- Stack
The fan sits after the shell cyclone and before the oxidiser or stack, so the roaster runs under a slight suction and smoke does not escape into the plant. The suction at the fan is typically 80 to 200 mmWC, covering the roaster, the cyclone and ducting. In recirculating roasters part of the exhaust returns to the heater and only the bleed passes this fan.
Gas at the ID fan
| Air temperature, normal | 120 to 200°C at the fan |
| Maximum or upset | About 250°C at the end of a dark roast or if the roaster runs empty |
| Dust at the fan | Fine shell and cocoa dust after the cyclone |
| Moisture and dew point | Moderate; moisture released in the early roast |
| Corrosive species | Acetic acid from fermentation; cocoa butter aerosol |
| Density at the design temperature | About 0.80 kg/m³ at 170°C, 66% 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 cocoa bean roasters
| Blade form | Radial or backward inclined flat blade, easy to clean |
| Material of construction | SS 304 for the impeller and casing |
| Drive and arrangement | Belt driven or direct coupled with a cooling disc |
| Bearings and cooling | Bearings away from the casing with a cooling disc |
| Flow control | Variable speed drive on roaster pressure or roast profile |
| Also specified | Cleaning door, drain and temperature monitoring for fire detection |
What this ID fan has to cope with
Cocoa butter deposits
In darker roasts and with nibs, cocoa butter leaves the beans as a fine aerosol. It condenses on the cooler fan casing and blades, picks up shell dust and forms a sticky, flammable layer that unbalances the wheel. Keeping the fan and duct insulated and warm, a plain impeller and cleaning on a schedule keep it under control.
Duct and fan fires
Shell dust soaked in cocoa butter in the exhaust duct can ignite from an over-roasted batch or a hot spot. A temperature alarm at the fan inlet, a steam or water quench connection, earthing and regular cleaning of the cyclone and duct reduce the risk.
Acetic acid condensate
Fermented beans release acetic acid and moisture early in the roast, and at start-up and shutdown these condense in a cold fan. The acid condensate corrodes mild steel and loosens deposits unevenly. Stainless construction and a casing drain give long life.
Energy and control
Cocoa roasters run in batches or continuously, and the exhaust fan's flow sets the heat loss from the roaster. Variable speed control matched to the roast stage, and recirculating part of the exhaust, save fuel. The oxidiser, where fitted, is a large energy user, so keeping the exhaust flow to a minimum saves fuel there too.
ID fan and FD fan in cocoa bean roasters
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone | Before the heater |
| What it handles | Hot air with shell dust and cocoa butter | Ambient air |
| Temperature at the fan | 120 to 200°C | Ambient |
| Main design concern | Deposits and fire | Steady flow |
| Control | Roaster pressure | Roast profile |
See also the Forced Draft (FD) Fan for cocoa bean roasters.
Questions buyers ask
What material is used for a cocoa roaster exhaust fan?
SS 304 is the usual choice, because the exhaust carries acetic acid and moisture that condense at start-up and shutdown, and stainless steel is easier to clean of cocoa butter deposits. Mild steel can be used on very hot, dry exhausts but corrodes when cycled.
Why does a cocoa roaster exhaust fan need regular cleaning?
Cocoa butter aerosol and shell dust form a sticky layer on the impeller and duct, which unbalances the fan and is a fire risk. Regular cleaning through an inspection door, with the frequency set by roast colour and throughput, keeps the fan safe and smooth.
Does a cocoa roaster need an afterburner after the exhaust fan?
Where the roaster is close to housing or odour limits apply, a thermal or catalytic oxidiser is fitted after the exhaust fan to burn the smoke and odour. The fan must then have enough pressure for the oxidiser, and its flow should be kept to what the roast needs.
Information needed to quote an ID fan for cocoa bean 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. Batch or continuous, whole bean or nib, and throughput.
- Odour control. Oxidiser type and its pressure drop.
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.