A continuous-flow separator takes milk in at a steady rate through the top center and delivers cream and skim milk through two outlets, and the flow rate, outlet setting and drive power are linked. Per 100 kg of 4% fat milk, recovering 95-98% of the fat as 35% cream gives about 11 kg of cream, and delivering 100 liters an hour to the rim speed needs between about 32 and 113 W depending on where the skim milk leaves the bowl, so a hand crank can sustain it only toward the low end.
Part of Centrifuge Building
The continuous-flow system of a cream separator is the supply tank, inlet, bowl outlets and drive that let milk pass through the spinning bowl at a steady rate and leave as separate cream and skim streams.
Three Quantities Must Agree: Flow, Outlet Setting And Power
The carrier article names the parts: an inlet at the center top, a cream outlet at the center and a skim milk outlet at the periphery. A fourth part, not named there, makes it work: a way to hold the inlet flow steady. The outlets are connected by the fat balance, and the power is connected by the flow.
Worked Example 1: The Cream Ratio
Take 100 kg of milk at 4% fat, the midpoint of the 3.5-4.5% in Dairy Fermentation. The carrier says the separator extracts 95-98% of the fat.
- Fat in: 100 × 0.04 = 4.0 kg.
- Fat recovered in the cream: 3.80 to 3.92 kg.
- Cream at 35% fat: 3.80 ÷ 0.35 = 10.9 kg and 3.92 ÷ 0.35 = 11.2 kg.
- Skim milk: 100 − 10.9 = 89.1 kg and 100 − 11.2 = 88.8 kg.
So about 11 parts in 100 leave as cream, and the cream outlet must pass about one-eighth as much as the skim outlet. A thinner cream, say 20% fat, would need 3.8 ÷ 0.20 = 19 kg, about 1.7 times as much cream. The cream-to-skim ratio is set by how far the cream outlet is throttled; a more throttled cream outlet gives thicker cream. The 35% target is an example; choose the cream thickness the churn or the table needs.
Worked Example 2: Power For 100 Liters An Hour
The milk enters at the axis with no rotation and must leave rotating with the bowl at the radius of the outlet, so the drive supplies the angular momentum it carries off. The power is P = ṁ × (ω × r_o)², with ṁ the mass flow and r_o the outlet radius. This is a first-principles estimate, not a wiki figure, and it ignores bearing and air drag.
- 100 L/h × 1.03 kg/L = 103 kg/h = 0.0286 kg/s. ω = 837.8 rad/s (8,000 RPM).
- If the skim leaves at the rim, r_o = 75 mm: rim speed 62.8 m/s, P = 0.0286 × 62.8² = 113 W.
- If it leaves at r_o = 40 mm: speed 33.5 m/s, P = 0.0286 × 33.5² = 32 W.
- At 50 L/h, the values halve: 16 to 56 W.
Add the losses in the gear train from Speed Multiplication, 6-14% on the figures given there. A person at the crank can sustain only some tens of watts (an assumption in Hand-Cranked Centrifuge). So 50 L/h is within a hand’s reach if the skim leaves close to the axis, and 100 L/h usually is not. For a larger herd, put the separator on a water wheel or other source; see Water Wheel Types.
What You Need
- A supply tank with a tap or a float valve, so the inlet head, and so the flow, stays constant as the tank empties
- A bowl with two outlet spouts and a regulating screw or ring on the cream outlet
- Two catch vessels and a drive; see Cream Separator
- Warm, pasteurized milk (see Pasteurization Methods)
Method: Running A Batch
Step 1: Bring the bowl to full speed before any milk enters. A bowl that fills while slow spills milk and runs wet.
Step 2: Open the supply tap to the rated flow and keep the tank filled to a constant level.
Step 3: Check the streams after a minute. The cream should be thick and the skim thin and bluish-white.
Step 4: Adjust the cream outlet if the cream is too thin or too thick; wait a minute to see the change.
Step 5: Close the milk, flush the bowl with warm water and let it run until the water leaves clear.
Step 6: Let the bowl stop on its own or with the brake, then wash and dry all parts.
Common Mistakes
- Overfeeding. Above the rated flow the fat does not have time to separate and leaves in the skim.
- Cold milk. It is thicker and slower to separate; use the warm range in Cream Separation Techniques.
- Letting the speed drop mid-run. The separation worsens and the cream thins.
- Leaving the bowl dirty. Milk spoils in the bowl within hours.
Safety
Moving parts, coast-down and milk hygiene
Keep your hands out of the bowl area while it is turning. The drive can only be stopped by a brake or by waiting, and a bowl of this size coasts for minutes; write the measured coast-down time on the frame and see Safe Operating Procedures. A separator that is not scrubbed after each run spreads disease; raw milk through it remains raw milk.
What’s Next
- Learn next: Safe Operating Procedures — start-up, run and shutdown checks
- Combine with: Butter Production — what the cream becomes