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 ID fan sits
- Burner
- Roasting drum
- Chaff cyclone
- Exhaust (ID) fan
- Afterburner or catalytic oxidiser
- Stack
The fan sits after the chaff cyclone and before the afterburner, so the drum runs under a small suction and smoke does not escape at the bean door or the sampling port. On recirculating roasters the fan also drives the return of hot gas to the burner chamber, and then handles much hotter gas. Suction at the fan is typically 50 to 150 mmWC on shop and industrial batch roasters.
Gas at the ID fan
| Gas temperature, normal | 150 to 250°C after the chaff cyclone |
| Maximum or upset | 300 to 450°C on recirculating roasters, or during a chaff fire |
| Dust at the fan | Fine chaff and smoke particles that pass the cyclone |
| Moisture and dew point | Moisture from the beans early in the roast; tars condense below about 120°C |
| Corrosive species | Organic acids and coffee oils in the smoke |
| Density at the design temperature | About 0.72 kg/m³ at 220°C, 60% 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 coffee roasters
| Blade form | Radial or backward inclined flat blade, plain so tar can be scraped off |
| Material of construction | Mild steel to about 300°C; SS 304 or heat-resisting steel on recirculating roasters |
| Drive and arrangement | Direct coupled with a cooling disc, motor clear of the hot casing |
| Bearings and cooling | Cooling disc and heat shield; bearings away from the casing |
| Flow control | Variable speed drive following the roast profile |
| Also specified | Large cleaning door, temperature sensor at the fan inlet and a water quench connection |
What this ID fan has to cope with
Tar and oil on the impeller
Smoke from dark roasts carries oils and tars that condense on the blades and the casing whenever the gas cools, building a hard, brittle layer that flakes off unevenly and causes vibration. A plain blade, keeping the fan hot, and scraping the wheel on a fixed schedule based on roast volume keep it running smoothly.
Chaff fires
Chaff that collects in the cyclone, ducts and fan can ignite from a hot roast, and a fire in the exhaust path can reach the fan. Regular emptying of the chaff collector, a temperature alarm at the fan inlet and a quench connection reduce the risk.
Profile repeatability
The air flow through the drum controls heat transfer to the beans, and a fan whose flow drifts as deposits build gives roasts that vary from batch to batch. A variable speed drive with a drum pressure signal, and clean ducts, keep the profile repeatable.
Energy and control
A coffee roaster runs batch after batch, with the exhaust fan speed changed within each roast. Variable speed control saves fan power, but the larger saving is in the afterburner: drawing only the air the roast needs keeps the volume the afterburner must heat to 650 to 750°C as small as possible. Recirculating roasters and catalytic oxidisers cut the gas used for smoke treatment further.
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 Forced Draft (FD) Fan for coffee roasters.
Questions buyers ask
Why does a coffee roaster exhaust fan need cleaning?
Coffee smoke carries oils and tars that condense on the fan blades and casing, especially with dark roasts. The layer grows unevenly and causes vibration, and it is a fire risk. Most roasteries clean the fan and ducts on a schedule based on the volume roasted.
How does the exhaust fan affect the coffee roast profile?
The fan sets the air flow through the drum, which controls how much heat reaches the beans by convection and how quickly smoke is removed. Roasters change the fan speed during the roast, so a variable speed drive is the normal control.
What temperature does a coffee roaster exhaust fan handle?
Typically 150 to 250°C after the chaff cyclone on a conventional roaster. Recirculating roasters, where the fan returns hot gas to the burner, can see 300 to 450°C and need heat-resisting construction.
Information needed to quote an ID fan for coffee 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 size, roasts per hour, and whether the roaster is single pass or recirculating.
- Smoke treatment. Afterburner or catalytic oxidiser 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.