How milk powder spray dryers work
Concentrated milk from the evaporator, at 45 to 52% solids, is atomised by a rotary atomiser or pressure nozzles into a drying chamber where it meets filtered air heated to 180 to 220°C. The droplets dry in seconds and the powder falls to an integrated fluid bed for final drying and agglomeration, while the exhaust air, now at 70 to 90°C and humid, carries the fines to cyclones and a bag filter, or a CIP-able bag filter alone. The fines are returned to the atomising zone. A process air fan supplies the drying air through filters and heaters, and an exhaust fan draws air from the chamber so that the chamber runs at a slight suction.
Where the FD fan sits
- Weather louvre
- Pre-filter and fine filter
- FD (supply) fan
- Steam and indirect air heaters
- HEPA or final filter (if fitted)
- Air disperser
- Drying chamber
The supply fan sits between the filter house and the heaters, so it handles filtered ambient air and pushes it through the steam coil heaters and an indirect gas or oil-fired heater, or an electric heater, to the air disperser. Placing the fan before the heaters keeps it cool and small. Its pressure must cover the filters at their dirty condition, the heaters, the air disperser and the chamber pressure.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the heaters | 180 to 220°C, downstream of the fan |
| Dust | Filtered air; the fan must not add contamination to the product air |
| Humidity | Seasonal humidity changes the drying capacity; some plants dehumidify the inlet air |
| Pressure at the fan outlet | Set by the filters, heaters 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 milk powder spray dryers
| Blade form | Backward curved or aerofoil for efficiency on clean air |
| Material of construction | Stainless steel, or mild steel with a food-grade coating, for the air path |
| Drive and arrangement | Direct coupled or belt driven, with a variable speed drive |
| Bearings and cooling | Standard bearings outside the air stream; no cooling needed |
| Flow control | Variable speed drive holding the air flow constant as the filters load |
| Also specified | Flexible connections with food-grade material and an inspection door |
What this FD fan has to cope with
Filter loading changes the flow
As the filters load, resistance rises and a fixed-speed fan delivers less air, which changes the outlet temperature and powder moisture. Flow measurement and a speed-controlled fan hold the air flow constant.
Hygiene of the product air
Anything the fan adds to the air reaches the powder. Coated or stainless surfaces, no oil leaks at the bearings and filters placed correctly protect the product.
Matching the exhaust fan
The chamber pressure depends on the balance between the supply and exhaust fans. The supply fan must have enough margin to hold its flow while the exhaust fan controls the chamber pressure.
Energy and control
The supply fan moves all the drying air, so it runs at high load for the whole campaign. A high-efficiency backward curved or aerofoil wheel and a variable speed drive repay quickly. Preheating the inlet air with heat recovered from the exhaust reduces steam or fuel consumption, and the extra pressure drop of the recovery coil is added to the fan duty.
ID fan and FD fan in milk powder spray dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack or scrubber | After the filters, before the air heaters |
| What it handles | Warm humid air with milk fines | Clean filtered ambient air |
| Temperature at the fan | 70 to 90°C | Ambient |
| Main design concern | Hygiene, deposits and condensation | Steady flow as filters load |
| Control | Chamber pressure | Drying air flow |
See also the Induced Draft (ID) Fan for milk powder spray dryers.
Questions buyers ask
Should the supply fan be placed before or after the air heater?
Before. The fan then handles cool, dense air, which makes it smaller and avoids high temperature construction. The heaters are on the pressure side of the fan.
Does the supply fan need to be stainless steel?
Many dairy plants specify stainless steel for the whole product air path. A mild steel fan with a food-grade coating is also used where it meets the plant's hygiene standard.
How is the air flow kept constant as the filters get dirty?
A flow measurement in the duct and a variable speed drive on the supply fan raise the fan speed as the filter resistance rises, keeping the drying air flow and outlet temperature steady.
Information needed to quote an FD fan for milk powder 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.
- Filtration. Filter grades and their clean and dirty pressure drops.
- Heaters. Type of heaters, air side pressure drop and the inlet air temperature 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.