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 ID fan sits
- Nozzle tower
- Cooled cone
- Cyclones
- Bag filter
- Exhaust (ID) fan
- Odour scrubber or stack
The fan sits after the bag filter and before any odour control. It holds the tower at a slight suction against the supply fan and the cone cooling air. Its suction covers the tower outlet, cyclones and filter, typically 250 to 400 mmWC. Coffee towers run hotter exhausts than dairy dryers because the higher outlet temperature keeps the moisture content of the powder low, so the fan handles more volume per kg of product at a higher temperature.
Gas at the ID fan
| Air temperature, normal | 95 to 115°C at the fan inlet |
| Maximum or upset | Up to about 150°C when feed is lost |
| Dust at the fan | Fine, hygroscopic coffee dust; low after the bag filter |
| Moisture and dew point | Dew point about 45 to 55°C; brown condensate at shutdown |
| Odour and volatiles | Aroma compounds and organic acids carried in the exhaust |
| Density at the design temperature | About 0.92 kg/m³ at 110°C, 77% 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 instant coffee spray dryers
| Blade form | Backward curved with smooth, flat blades that do not trap scale |
| Material of construction | SS 304 or SS 316 for wetted parts; coffee condensate is mildly acidic |
| Drive and arrangement | Direct or belt driven with a variable speed drive |
| Bearings and cooling | Bearings outside the air stream, cooling disc at higher temperatures |
| Flow control | Variable speed drive on tower pressure |
| Also specified | Wash nozzles, casing drain, insulated casing, earthing and inspection door |
What this ID fan has to cope with
Hard brown scale
Coffee fines absorb moisture, soften and then bake onto warm blades as a hard, glassy layer that does not shed by itself. Uneven scale causes vibration within weeks on poorly cleaned fans. Smooth blades, wash nozzles and a planned wash at each product change keep it under control.
Acidic condensate
Coffee extract contains organic acids, and the condensate that forms in the casing at shutdown is mildly acidic and stains. Stainless wetted parts and a drain at the lowest point avoid pitting and pooled residue.
Odour complaints
Exhaust from coffee towers carries a strong roasted odour. Where neighbours complain, a scrubber or thermal treatment is added after the fan, and the fan pressure must include it from the start.
Energy and control
Coffee dryers run long campaigns between washes. The exhaust fan is usually on a variable speed drive. Raising feed concentration, and recovering heat from the hot exhaust to the inlet air, are the main energy levers, because coffee towers exhaust at a high temperature and the air volume per kg of powder is large.
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 Forced Draft (FD) Fan for instant coffee spray dryers.
Questions buyers ask
Why does an instant coffee spray dryer run a higher outlet temperature than a milk dryer?
Coffee is dried in a single pass in a tall nozzle tower without a large integrated fluid bed, so the powder must leave the tower dry. A higher outlet temperature gives the low moisture needed, which means a hotter exhaust for the fan.
How is coffee scale removed from the exhaust fan?
By washing with hot water through nozzles in the fan inlet and casing at each product change or planned stop, with the drain open. Hard scale left too long may need manual cleaning through the inspection door, followed by a balance check.
Does the coffee dryer exhaust fan need to be stainless steel?
Yes for most plants. The condensate is mildly acidic, the fines are food product, and the fan is washed regularly with hot water. Stainless steel resists these and is easy to keep clean, while coated mild steel loses its coating under repeated washing.
Information needed to quote an ID fan for instant coffee spray dryers
- 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.
- Tower outlet temperature. Normal and maximum outlet temperatures, and how the tower behaves when feed is lost.
- Odour control. Whether a scrubber or other treatment follows the fan, with 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.