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 ID fan sits
- Drying chamber
- Integrated fluid bed
- Cyclones
- CIP bag filter
- Exhaust (ID) fan
- Wet scrubber or stack
The exhaust fan sits after the final powder separator, usually a CIP-able bag filter, and before the stack or a wet scrubber for heat and powder recovery. Together with the process air fan it holds the drying chamber at a small suction, typically −5 to −20 mmWC, so that powder does not escape through seals and doors. The fan must overcome the chamber outlet duct, the cyclones and the bag filter, and any scrubber downstream.
Gas at the ID fan
| Air temperature, normal | 70 to 90°C at the fan inlet |
| Maximum or upset | Up to 120°C if the feed is lost while the heaters are on |
| Dust at the fan | Fine, sticky milk powder; low after a bag filter, higher if the fan follows cyclones only |
| Moisture and dew point | Humid exhaust with a dew point of about 40 to 50°C; condensation forms on cold surfaces at shutdown |
| Hygiene and fire risk | Milk powder deposits can char and self-heat; the exhaust is part of a food-contact system |
| Density at the design temperature | About 0.99 kg/m³ at 85°C, 82% 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 milk powder spray dryers
| Blade form | Backward curved or aerofoil, with smooth blades that shed powder |
| Material of construction | Stainless steel AISI 304 or 316 for all wetted parts, with continuous welds and ground finish |
| Drive and arrangement | Direct coupled or belt driven; variable speed drive |
| Bearings and cooling | Bearings outside the air stream; shaft seal to stop powder and moisture tracking along the shaft |
| Flow control | Variable speed drive holding chamber pressure |
| Also specified | CIP spray nozzles in the casing, a drain at the lowest point, a quick-opening inspection door and earthing |
What this ID fan has to cope with
Powder build-up and unbalance
Milk fines that pass the separator stick to the blades in the humid air and build up unevenly. Smooth stainless blades, CIP nozzles in the casing and a regular wash schedule stop deposits growing to the point where they cause vibration.
Condensation at shutdown
When the dryer stops, the humid air in the casing cools below its dew point and wets the powder deposits, which then harden or go sour. A casing drain, continued running until the air is dry, and insulation on the casing reduce this.
Fire and explosion risk
Dried milk deposits can self-heat and smoulder, and milk powder is a combustible dust. The fan casing is earthed, rubbing contact is avoided, and the fan is included in the plant's fire detection and explosion protection philosophy.
Hygiene audits
The fan is part of the product contact path for returned fines. Crevice-free construction, accessible internals and documented materials make cleaning and inspection straightforward.
Energy and control
Exhaust fans on spray dryers run long campaigns, and a variable speed drive holding chamber pressure is the norm. Energy savings come mostly from the dryer itself: higher feed solids, lower outlet temperature and heat recovery from the exhaust air to the inlet air reduce the air volume the exhaust fan must handle.
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 Forced Draft (FD) Fan for milk powder spray dryers.
Questions buyers ask
Why must the spray dryer exhaust fan be stainless steel?
The exhaust air is humid and carries milk fines, and in many plants the fines are returned to the product. Stainless steel resists corrosion from condensate and cleaning chemicals and can be cleaned to food standards.
Can the exhaust fan be cleaned in place?
Yes. CIP spray nozzles can be fitted in the inlet and the casing, with a drain at the lowest point, so the fan is washed along with the bag filter during the CIP cycle.
What pressure is the spray dryer chamber held at?
Usually a small suction of a few to about 20 mmWC, set by the balance between the process air fan and the exhaust fan, so powder does not leak out of the chamber.
Information needed to quote an ID fan for milk powder 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.
- Separator and scrubber. Cyclones, CIP bag filter, or both, and any wet scrubber or heat recovery after the fan.
- Hygiene requirements. Material grade, surface finish, CIP and any food safety standard the plant follows.
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.