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 FD fan sits
- Air intake and filter
- FD (process air) fan
- Burner chamber or heat exchanger
- Hot air duct
- Roasting drum or chamber
The process air fan sits before the burner chamber or heat exchanger, so it handles ambient air and pushes it through the heater into the roaster. On recirculating roasters a hot air circulation fan returns part of the exhaust to the heater and handles air at 150 to 250°C. The burner itself has a combustion air blower. Pressure is typically 100 to 250 mmWC, covering the heater, the hot air duct and the bean bed.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 40°C; 150 to 250°C on recirculation fans |
| Air after the heater | 120 to 250°C |
| Dust | Filtered room or outdoor air; cocoa dust near cleaning and winnowing |
| Humidity | Ambient; humid air slows the drying stage of the roast |
| Pressure at the fan outlet | Typically 100 to 250 mmWC |
| 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 cocoa bean roasters
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel with food-safe coating, or stainless where the plant requires it |
| Drive and arrangement | Direct coupled with a variable speed drive |
| Bearings and cooling | Standard bearings; cooling disc on recirculation fans |
| Flow control | Variable speed drive on air flow or roast profile |
| Also specified | Inlet filter and access for cleaning |
What this FD fan has to cope with
Uneven roast between batches
If the process air flow drifts as the filter loads or the heat exchanger fouls, beans in successive batches receive different heat and the flavour varies. A speed-controlled fan with flow measurement holds the air flow steady across the day.
Recirculation fan heat
Where part of the exhaust is recirculated, the circulation fan runs at 150 to 250°C with cocoa butter in the air. It needs a cooling disc, bearings away from the casing and a cleanable impeller, much like the exhaust fan.
Shell dust in the intake
Cocoa cleaning, breaking and winnowing release fine shell dust into the plant air. An intake filter, or an intake from outside the process area, keeps it out of the heater and the roaster.
Energy and control
The process air fan runs for every roast, and on continuous roasters for the whole shift. Variable speed control matched to the roast stage saves power, and recirculating part of the exhaust saves burner fuel. Correct air-fuel ratio at the burner and a clean heat exchanger give the largest fuel savings.
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 Induced Draft (ID) Fan for cocoa bean roasters.
Questions buyers ask
What is the FD fan on a cocoa roaster?
It is the process air fan that pushes air through the burner chamber or heat exchanger into the roaster. The burner has its own combustion air blower; together they do the work of a forced draft fan.
Are cocoa roasters heated directly or indirectly?
Both designs are used. Indirectly heated roasters pass the air through a heat exchanger or thermic fluid coil so that combustion products do not touch the beans; direct gas fired roasters mix the burner gas with the process air.
Why do cocoa roasters recirculate hot air?
The roaster exhaust is still hot after the cyclone, so returning part of it to the heater saves fuel. The recirculation fan then handles hot air with cocoa butter and fine dust, and needs high temperature construction.
Information needed to quote an FD fan for cocoa bean 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.
- Heating. Burner, heat exchanger or thermic fluid heating, and the process air flow.
- Recirculation. Whether a recirculation fan is needed and the air temperature it handles.
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.