How food and dairy spray dryers work
Food spray dryers turn a pumpable liquid into powder by atomising it into hot air. The family covers dairy and whey, instant coffee and tea, egg, fruit and vegetable powders, starch derivatives such as maltodextrin, flavours and encapsulated oils. Plants differ in atomisation (rotary wheel in wide-body chambers, pressure nozzles in tall towers), in the number of drying stages (single-stage, two-stage with an external fluid bed, or multi-stage with an integrated bed), and in heating (steam coils, indirect oil or gas, or direct natural gas burners). Inlet air is typically 150 to 250°C and exhaust 65 to 100°C, depending on how heat-sensitive and sticky the product is. A supply fan pushes filtered air through the heaters and an exhaust fan draws it out through cyclones and bag filters; larger plants add fluid bed, cooling air and conveying fans.
Where the FD fan sits
- Intake louvre
- Pre-filter and fine filter
- FD (supply) fan
- Air heaters
- Air disperser
- Drying chamber
The main supply fan sits after the filters and before the heaters, so it handles cool, dense, filtered air. In direct gas-fired dryers it supplies the burner duct; in steam or indirect dryers it feeds the heater coils. Its pressure covers the dirty filters, the heaters, any heat recovery coil and the air disperser. Fluid bed supply fans need higher pressure because they must pass the distributor plate and the bed.
Air at the FD fan
| Air temperature at the fan | Ambient, 15 to 45°C, or after dehumidification for sticky products |
| Air after the heaters | 150 to 250°C, downstream of the fan |
| Dust | Filtered air; nothing must be added to the product air |
| Humidity | Ambient humidity sets drying capacity; sticky products need dehumidified air |
| Other supply fans | Fluid bed, wall cooling and powder conveying fans each with their own duty |
| 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 food and dairy 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 or belt driven, with a variable speed drive |
| Bearings and cooling | Standard bearings, sealed against oil mist into the air |
| Flow control | Variable speed drive on measured air flow |
| Also specified | Food-grade flexible connections and an inspection door |
What this FD fan has to cope with
Seasonal humidity
In humid seasons the inlet air carries more moisture, which lowers drying capacity and makes sticky products harder to dry. Plants that dehumidify the inlet add a cooling coil or desiccant wheel, and the extra pressure drop is part of the supply fan duty.
Air flow drift as filters load
Filter resistance can double between clean and change-out. Without flow control the drying air falls and the powder moisture rises. A flow measurement and speed control hold the air constant.
Balancing several supply fans
Main, fluid bed and cooling air fans all discharge into a chamber held at suction by one exhaust fan. Each supply fan must hold its flow independently while the exhaust fan controls pressure.
Energy and control
The supply fan moves all the drying air for the whole campaign. An efficient wheel and a variable speed drive pay back quickly. Heat recovered from the exhaust preheats the inlet air and cuts steam or fuel use; the recovery coil adds pressure drop, which is included in the fan duty from the start.
ID fan and FD fan in food and dairy spray dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or scrubber | After the filters, before the heaters |
| What it handles | Humid exhaust with product fines | Clean filtered ambient air |
| Temperature at the fan | 65 to 100°C | Ambient |
| Main design concern | Product deposits, hygiene and cleaning | Steady flow and air quality |
| Control | Chamber pressure | Drying air flow |
See also the Induced Draft (ID) Fan for food and dairy spray dryers.
Questions buyers ask
How many fans does a food spray dryer have?
A simple single-stage dryer has a supply fan and an exhaust fan. Multi-stage dryers add fluid bed supply and exhaust fans, wall-sweep or cooling air fans and pneumatic conveying fans. The main supply and exhaust fans carry most of the power.
Does the supply fan see hot air?
Not normally. It sits before the heaters and handles filtered ambient air. Only in plants that recirculate part of the exhaust, or place the fan after the heater for layout reasons, does the supply fan handle warm air.
Why do some food spray dryers dehumidify the inlet air?
Sugar-rich and hygroscopic products become sticky in humid air. Drying the inlet air lets the dryer run at a lower outlet temperature without the powder sticking, and keeps output steady through the monsoon.
Information needed to quote an FD fan for food and dairy 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.
- Air treatment. Filter grades, any dehumidification, and heater types with their pressure drops.
- Supply fans. Duties of the main, fluid bed and cooling air fans required.
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.