How coffee roasters work
Green coffee at about 10 to 12% moisture is roasted in batch drum roasters, continuous drum roasters or hot air fluid bed roasters. A gas, LPG or diesel burner heats air to about 200 to 450°C, and the beans reach 200 to 230°C over roughly 8 to 20 minutes in a drum, or a few minutes in a fluid bed. The burner and the drum air flow are changed through drying, browning and development to follow a roast profile. The exhaust carries chaff (silverskin), smoke, oils, CO and volatile organics; a chaff cyclone removes the chaff and an afterburner at about 650 to 750°C, or a catalytic oxidiser at 300 to 400°C, burns the smoke. Recirculating roasters return part of the exhaust to the burner. A separate cooling fan pulls ambient air through the beans in the cooling tray.
Where the FD fan sits
- Air intake
- FD (combustion air) fan
- Burner
- Combustion chamber
- Roasting drum
On small roasters the burner has its own built-in blower. On industrial roasters a separate combustion air fan supplies the main burner and often the afterburner burner, through control valves that follow the gas valve. The pressure needed is typically 100 to 300 mmWC at the burner. Hot air for roasting is drawn through the burner chamber by the exhaust fan, so the FD fan handles only combustion air.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C, often roastery room air |
| Dust | Room air with some chaff and coffee dust |
| Pressure at the fan outlet | Typically 100 to 300 mmWC at the burner |
| Fuel | Natural gas, LPG or diesel |
| Turndown | Wide and fast; the burner is modulated through each roast |
| 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 coffee roasters
| Blade form | Backward curved, for a stable pressure curve |
| Material of construction | Mild steel or aluminium |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive or air valve linked to the gas valve |
| Also specified | Inlet filter against chaff and an air pressure switch for burner safety |
What this FD fan has to cope with
Air-fuel ratio during profile changes
The burner is turned up and down several times in each roast. If the combustion air does not follow quickly, the burner runs rich and smoky or lean and unstable. A fan and air valve that track the gas valve closely keep the flame clean.
Chaff in the intake
Roastery air carries chaff and coffee dust that block the inlet filter and coat the impeller, reducing air flow. A filter with regular cleaning, or an intake from outside the roasting room, keeps the air supply steady.
Afterburner air supply
The afterburner needs enough combustion air to hold 650 to 750°C as the smoke load changes. Undersized air supply leaves smoke unburnt; oversized wastes gas. The fan duty is set for the peak smoke load at the end of a dark roast.
Energy and control
The combustion air fan is small, but it decides how efficiently the roaster and afterburner burn gas. Close air-fuel ratio control through every roast saves fuel and avoids smoke. Where one fan serves both the burner and the afterburner, speed control matched to the combined demand saves power compared with throttling.
ID fan and FD fan in coffee roasters
| ID fan | FD fan | |
|---|---|---|
| Position | After the chaff cyclone, before the afterburner | At the burner |
| What it handles | Hot smoky air with chaff and oils | Ambient air |
| Temperature at the fan | 150 to 250°C | Ambient |
| Main design concern | Tar deposits, chaff fires and profile air flow | Air-fuel ratio through each roast |
| Control | Roast profile | Burner firing rate |
See also the Induced Draft (ID) Fan for coffee roasters.
Questions buyers ask
Does a coffee roaster have an FD fan?
Yes. The forced draft fan is the combustion air fan of the burner. On small roasters it is built into the burner; on industrial roasters it is a separate fan that may also supply the afterburner.
Is the coffee roaster cooling fan an FD fan?
No. The cooling fan pulls ambient air through the beans in the cooling tray after the roast. It is a separate fan with its own cyclone or chaff filter, and it handles warm air with chaff.
What pressure does a coffee roaster burner fan need?
Typically 100 to 300 mmWC at the burner, depending on burner type and the roaster's combustion chamber. Afterburner burners can need a similar or higher pressure.
Information needed to quote an FD fan for coffee 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.
- Burners. Main burner and afterburner ratings, fuel and air pressure needed.
- Control. How the burner is modulated: gas valve, air valve or fan speed.
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.