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Basket Centrifuge

5 min read

Parent
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Chemical/Industrial Centrifuge
Separating precipitates, crystals, and suspensions
Current
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Basket Centrifuge
Perforated basket for filtering solids from liquids

A basket centrifuge is a perforated cylinder, usually lined with cloth, spun at 1,000-3,000 RPM so that liquid passes out through the holes while crystals or precipitate build up as a cake on the wall. The cake loads the basket heavily: 10 kg of wet cake at 2,000 RPM on a 300 mm basket presses outward with about 61 kN, and the perforations leave the wall weaker than a solid one, so load it evenly, spin it no faster than needed, and never run it with the lid open.

Part of Centrifuge Building

A basket centrifuge separates solids from a slurry by spinning it in a perforated drum, so the liquid is thrown through the holes and the solids stay behind on a cloth lining as a drained cake.

The Cake Is Heavy, And It Builds Where The Slurry Lands

The carrier’s design is a perforated basket with holes of 0.5-2 mm depending on particle size, lined with cloth for fine particles, loaded with slurry and spun at 1,000-3,000 RPM; the liquid passes through, solids build up on the wall, and you stop and scrape out the solids. The build is a welded stainless cylinder on a vertical shaft in a catch basin, driven by belt from a water wheel or motor.

Worked Example: How Hard The Cake Pushes

Take a 300 mm basket (radius 0.15 m), 200 mm tall, a wall of 3 mm and a 10 kg wet cake (an assumption) at a mean radius of 0.14 m, running at 2,000 RPM. Steel density 7,800 kg/m³.

  • ω = 2π × 2,000 ÷ 60 = 209.4 rad/s, ω² = 43,858. Rim speed = 209.4 × 0.15 = 31.4 m/s. RCF at the wall = 1.118 × 10⁻⁵ × 15 × 2,000² = 671 g.
  • Force of the cake: F = m × ω² × r = 10 × 43,858 × 0.14 = 61,400 N, about 6.3 tonnes-force.
  • Wall area: π × 0.30 × 0.20 = 0.1885 m². Pressure on the wall = 61,400 ÷ 0.1885 = 0.326 MPa.
  • Wall stress from that pressure: σ = p × R ÷ t = 0.326 × 10⁶ × 0.15 ÷ 0.003 = 16.3 MPa. Add the basket’s own rotation, ρ × v² = 7,800 × 31.4² = 7.7 MPa. Solid-wall total: 24 MPa.
  • Holes weaken it. Holes of 2 mm on a 4.5 mm pitch leave a ligament efficiency (4.5 − 2) ÷ 4.5 = 0.56 along the weakest line, so the stress along that line is 24 ÷ 0.56 = 43 MPa. Each hole also concentrates stress locally by a factor typically between 2 and 3 (general knowledge). With that local concentration on top of 43 MPa the peak is about 86-130 MPa, a margin of about 1.9-2.9 against the 250 MPa yield. The perforated wall is clearly closer to its limit than the solid wall of a dairy bowl, before any weld or corrosion is counted.

Double the speed to 4,000 RPM and every one of these stresses goes up four times. Check the maximum against Speed Limits and Material Stress.

What You Need

  • Stainless or mild steel sheet, perforated, rolled and welded; holes 0.5-2 mm (carrier)
  • A cloth lining sized to the holes and particles
  • A vertical shaft, bearings and a catch basin around the basket; see Bearings and Bushings
  • A drive by belt; see Belt Drives
  • A lid, a brake and a heavy base; see Containment Shield

Method: A Run

Step 1: Line the basket and check the cloth lies flat without folds.

Step 2: Start slowly and feed the slurry through the lid while the basket turns at low speed, spreading it evenly around the wall by feeding steadily rather than pouring into one spot.

Step 3: Bring to working speed and hold until the liquid stops running out of the casing drain.

Step 4: Wash the cake with a little clean liquid sprayed over it if the crystals need it.

Step 5: Let the basket stop completely (brake if fitted), then lift the lid and scrape out the cake.

Common Mistakes

  • Pouring the load in one place. It spins out of balance within seconds; the cake mass at 2,000 RPM is a 61 kN swing if it is all on one side.
  • A cloth with folds. It breaks the cake and unbalances the basket.
  • Starting at full speed. Slurry sloshes and the drive overloads.
  • Opening the lid before the basket stops. The basket is open to the room.

Safety

An open-top rotor with a heavy, wet load

Never reach into a basket that is turning or coasting; the rim moves at over 30 m/s. An out-of-balance basket will walk the machine across the floor and can tear off the lid. Wear eye protection, stop at the first sign of shaking, and bolt the machine to a heavy base. The liquid thrown out can be corrosive; keep the casing drain clear.

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