A cream separator is a continuous-flow centrifuge in which milk enters a spinning bowl of 150-200 mm diameter, turning at 6,000-10,000 RPM, and leaves as two streams: cream from near the axis and skim milk from the rim. A well-built one processes 50-100 liters of milk an hour and recovers 95-98% of the cream, which is more than a few hand-cranked hours a day can supply, so the larger herds need a powered drive.
Part of Centrifuge Building
A cream separator is a spinning bowl with a stack of conical discs that splits milk continuously into cream and skim milk, replacing a day of gravity creaming with seconds in the bowl.
The Separator Is The Dairy’s Most Valuable Centrifuge
The carrier article calls it the most commercially valuable centrifuge for a village. Gravity creaming takes 12-24 hours in a pan (see Cream Separation Techniques); a separator does the same sorting continuously, while milk flows through, so the cream is ready as soon as the milk is. The physics is the same law, and the numbers are in Separation Theory.
How It Works In Five Parts
- Bowl. A machined steel shell, 150-200 mm in diameter, spinning at 6,000-10,000 RPM. See Bowl Construction.
- Disc stack. 20-40 conical stainless discs with 0.5-1 mm spacing between them, so each liquid layer is thin and the fat has only a fraction of a millimeter to cross. See Disc-Stack Design.
- Inlet. Milk enters at the top center and is carried to the outer edge of the stack.
- Separation. Skim milk, the heavier part, moves outward; the lighter cream moves inward along the discs.
- Outlets. Two separate spouts: cream from near the axis, skim milk from the periphery. See Continuous Flow System.
The Numbers At A Glance
For the reference bowl, 150 mm diameter at 8,000 RPM, the relative centrifugal force at the rim is 5,366 g and the rim speed is 62.8 m/s; the derivation is in Centrifugal Force Calculations. A 1 g imbalance at that radius pulls on the bearings with about 53 N.
The carrier Centrifuge Building states that a well-built 150 mm separator recovers 95-98% of the cream (fat) in the milk. The fat balance that figure implies, for 100 kg of 4% milk, is worked in Continuous Flow System; the remaining 2-5% of the fat leaves in the skim milk.
Worked Example: Is The Herd Claim Right?
The carrier says 50-100 liters an hour handles the daily output of a 10-20 cow herd. Check against the parent dairy article’s cow yield of 5-8 gallons a day.
- One cow: 5-8 gal × 3.785 = 18.9-30.3 L a day.
- 10 to 20 cows: 189-606 L a day.
- Running time at 50-100 L/h: from 189 ÷ 100 = 1.9 hours up to 606 ÷ 50 = 12 hours a day.
That is hours of continuous running, at a power cost worked out in Continuous Flow System, well beyond what one person can crank. A small community herd of 2-4 cows gives about 38-121 L a day, which is under one to a few hours a day of cranking and is a plausible hand-powered job. For larger herds use a water wheel or other prime mover; see Water Wheel Types and Speed Multiplication.
What You Need
- A lathe-turned bowl and discs; see Lathe Fundamentals
- Stainless or tinned surfaces where milk touches; see Stainless Steel Production
- Two bearings and a drive to 6,000-10,000 RPM; see Hand-Cranked Centrifuge
- A frame, a shield and a brake; see Safety Considerations
- Pasteurized, warm milk; see Pasteurization Methods
Common Mistakes
- Treating the nominal throughput as a power-free number. The throughput needs a drive that can supply it.
- Skipping cleaning. Milk left on the discs spoils in hours and is the main way a separator makes people ill.
- Putting raw milk through and pasteurizing only the cream. Skim milk from raw milk is raw milk.
Safety
The bowl is a stored-energy rotor, and the crank keeps turning
A 150 mm bowl at 8,000 RPM carries thousands of joules and turns for minutes after you stop cranking. Never open, touch or brake it by hand until it has stopped, and never run it with a damaged or unbalanced bowl; the rules are in Safety Considerations.
What’s Next
- Learn next: Disc-Stack Design — why discs raise capacity
- Combine with: Butter Production — what the cream becomes