A sedimenting centrifuge is a solid-walled bowl that separates liquids of different density, clarifies a liquid of suspended solids, or thickens a slurry. When two liquids are separated, the heavy one collects at the wall and the light one near the axis, and the boundary between them sits where the two outlet weirs balance the pressures, so a small change in a weir radius moves the boundary a long way when the two densities are close.
Part of Centrifuge Building
A sedimenting centrifuge uses a solid bowl, with no perforations, to settle denser material to the wall so that immiscible liquids, suspended solids or slurries can be parted by density.
Three Jobs For A Solid Bowl
The carrier lists them: separate immiscible liquids of different densities, clarify liquids by spinning out suspended solids, and concentrate slurries. All three follow Separation Theory: the heavier phase moves out, at a speed set by particle size, density difference and viscosity.
Worked Example: Where The Interface Sits
Two liquids in a spinning bowl form concentric layers. Each leaves over its own weir, a ring at a set radius that fixes the free-surface radius of that outgoing stream: the light liquid over a weir at radius r_L and the heavy one over a weir at radius r_H. At the boundary between them, radius r_i, the pressure from each side must be equal. A spinning liquid has pressure ½ρω²(r² − r_weir²), so equal pressure at r_i gives:
ρ_L (r_i² − r_L²) = ρ_H (r_i² − r_H²), which solves to r_i² = (ρ_H × r_H² − ρ_L × r_L²) ÷ (ρ_H − ρ_L).
The speed drops out; the interface position depends only on the densities and the weirs. Example: a vegetable oil (ρ_L = 920 kg/m³, an assumed density) over water (ρ_H = 1,000 kg/m³), with r_L = 20 mm and r_H = 30 mm:
- ρ_H × r_H² = 1,000 × 0.0009 = 0.900; ρ_L × r_L² = 920 × 0.0004 = 0.368; difference = 0.532.
- r_i² = 0.532 ÷ (1,000 − 920) = 0.532 ÷ 80 = 6.65 × 10⁻³, so r_i = 0.0815 m = 81.5 mm.
- Move the heavy weir out by 2 mm to r_H = 32 mm: 1,000 × 0.001024 = 1.024; (1.024 − 0.368) ÷ 80 = 8.2 × 10⁻³, so r_i = 90.6 mm. A 2 mm change in the weir moves the interface 9.1 mm.
The bowl must be larger than r_i, and the interface must sit inside the bowl, not outside it or in the outlet. The closer the two densities, the more a small weir change moves the boundary: here the denominator is only 80. So the weirs are set to the liquids, and a changed feed (a different oil, a different temperature) needs a new weir setting.
What You Need
- A solid cylindrical bowl with no holes, spun about a vertical or horizontal axis, with outlets at two different radii
- Interchangeable weir rings, so the interface radius can be set
- A catch basin for each outlet
- A scraper or a means to flush out settled solids; solids accumulate on the wall and must be removed when the bowl is stopped
- A bearing set and drive as in Belt Drives
Method: Parting Two Liquids
Step 1: Compute the interface radius for your liquids and weirs, as above, and check it is inside the bowl.
Step 2: Bring to speed and feed the mixed liquid at a steady rate.
Step 3: Watch both outlets. Pure heavy liquid from one and pure light from the other means the interface is right. Light liquid in the heavy outlet means the interface is too far out; swap to a smaller heavy weir.
Step 4: Stop and clear the wall of solids before the layer reaches the interface.
Method: Clarifying A Liquid
Feed slowly, since time in the bowl is time to settle. Solids build against the wall, so the batch size is set by how much sludge the bowl holds before it unbalances and before the layer reaches the liquid surface.
Common Mistakes
- Treating the speed as the cause of a wrong interface. The interface depends on the weir radii and densities, not on the speed.
- Letting the sludge build. An unequal sludge layer unbalances the bowl and shortens the settling distance.
- Different weirs on the two outlets without computing. The boundary can end up in the wrong outlet.
- Feeding too fast. Fine droplets or particles pass straight through.
Safety
Stored energy and a heavy, uneven load
A solid bowl holds a large mass of liquid and sludge at speed. If sludge builds unevenly the vibration grows with each rotation; stop at the first shaking. Handle the liquids according to their hazard: flammable liquids must not be used near flames, and a flame is never to be used to test the vapor. The rules for the rotor are in Safety Considerations.
What’s Next
- Learn next: Balancing and Vibration Control — keeping the bowl steady
- Combine with: Industrial Centrifuge — choosing between the types