How instant coffee spray dryers work
Soluble coffee is made by extracting roasted ground coffee with hot water under pressure, stripping and recovering aroma, and concentrating the extract to about 40 to 55% solids. The concentrate is sprayed through high-pressure nozzles at the top of a tall co-current tower, where it meets air at typically 200 to 280°C. Coffee is hygroscopic and turns sticky when warm and moist, so towers have cooled walls, a cool air sweep at the cone and a short residence at the bottom. The powder is often agglomerated afterwards with steam to give the granular appearance. Exhaust air leaves at about 95 to 115°C with fine brown dust and aroma volatiles, passes cyclones and a bag filter, and is drawn out by the exhaust fan.
Where the FD fan sits
- Air intake
- Filters
- FD (supply) fan
- Air heaters
- Tower air disperser
- Nozzle tower
- Cone cooling air fan
The main supply fan sits after the filters and before the heaters, so it handles cool, filtered air. On many coffee towers the heaters are indirect gas or oil fired or steam coils, and the duct to the top of the tower is long, which adds pressure. The cone cooling air fan draws ambient air, sometimes through a chiller and dehumidifier, and blows it into the jacket or through slots at the cone.
Air at the FD fan
| Air temperature at the fan | Ambient, 15 to 45°C |
| Air after the heaters | Typically 200 to 280°C, downstream of the fan |
| Dust | Filtered air |
| Cooling air | Ambient or dehumidified and chilled to keep the cone cool and dry |
| Pressure at the fan outlet | Filters, heaters, long riser duct and air disperser |
| 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 instant coffee spray dryers
| Blade form | Backward curved or aerofoil for clean air efficiency |
| Material of construction | Stainless steel or food-grade coated mild steel |
| Drive and arrangement | Direct driven with a variable speed drive |
| Bearings and cooling | Standard bearings outside the air stream |
| Flow control | Variable speed drive on measured flow |
| Also specified | Separate cooling air fan sized for the cone and wall sweep |
What this FD fan has to cope with
Powder sticking at the cone
If the cone cooling air fails or is too warm and humid, coffee powder sticks to the cone and chars. The cooling air fan is treated as essential, with its own flow monitoring and alarm.
Riser duct pressure
Coffee towers are tall, and the hot air duct from the heater to the roof is long with several bends. Underestimating its loss leaves the supply fan short of flow. The fan duty uses the actual duct route.
Steady drying conditions
Small changes in drying air flow shift the powder moisture, bulk density and colour. A flow-controlled supply fan holds the air steady as the filters load, so the tower conditions stay where the product needs them.
Energy and control
The supply fan runs at near-constant duty through each campaign. A variable speed drive holding flow, rather than a damper, saves power as the filters load. Recovering heat from the hot coffee exhaust to the supply air is worthwhile because the exhaust temperature is high, and the recovery coil pressure drop is added to the fan duty.
ID fan and FD fan in instant coffee spray dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | After the filters, before the heaters |
| What it handles | Hot humid air with coffee fines | Clean filtered ambient air |
| Temperature at the fan | 95 to 115°C | Ambient |
| Main design concern | Hard scale and acidic condensate | Steady flow and cone cooling |
| Control | Tower pressure | Drying air flow |
See also the Induced Draft (ID) Fan for instant coffee spray dryers.
Questions buyers ask
Why does a coffee spray tower need separate cooling air?
Coffee powder is hygroscopic and turns sticky when warm and moist. Cool, dry air on the cone and walls keeps the powder below its sticky point so it slides out instead of building up and scorching.
Is the coffee dryer FD fan before or after the heater?
Before. The fan handles cool filtered air and pushes it through the heaters to the top of the tower, so it does not need high temperature construction. The heaters, riser duct and disperser are on the pressure side of the fan.
What pressure does the coffee tower supply fan need?
It must cover dirty filters, the heaters, a long riser duct to the top of the tower and the air disperser. Several hundred mmWC is common, but the actual figure comes from the dryer layout.
Information needed to quote an FD fan for instant coffee spray dryers
- 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.
- Heater and duct route. Heater type and pressure drop, and the length and bends of the hot air riser.
- Cooling air. Flow and temperature required for the cone and wall cooling, and any dehumidification.
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.