Centrifugal Equipment

Centrifuges and rotors are commercially available in literally hundreds of shapes, sizes and configurations. They range from small laboratory-scale units equipped with capillary tubes, operating at speeds in excess of 100 000 rpm or forces approaching 1 000 000gto large industrial decanters that may continuously process up to 300 000 L h"1. The primary rotor or centrifuge selection criteria must centre on the objective for conducting the separation. Parameters such as batch versus continuous; required centrifugal force and purity; throughput; the number of components to be recovered; sample toxicity/corrosiveness; time; cost; available space; noise tolerances, and so forth must be considered when selecting the appropriate centrifuge/rotor for a given application.

Early rotors were often manufactured of steel or brass, but are now more commonly constructed of aluminium and titanium. Newer carbon composites are also gaining acceptance, with plastics commonly used for small-scale applications and stainless steel for industrial-scale units. Though somewhat more expensive, titanium is particularly suitable as it has both a higher strength-to-density ratio and a high resistance to corrosion and erosion. Selected properties for steel, aluminium and titanium are shown in Table 2.

Centrifuge bottles and tubes are also constructed from a variety of materials. Early tubes were usually glass or stainless steel, but these have largely been replaced by plastics, e.g. polycarbonate, nylon,

Table 2 Strength data for commonly used rotor construction materials (from Sheeler, 1981)

Material

Density

Ultimate

Strength: density

(g cm~3)

strength

ratio

(g cm~z)

Aluminium

2.79

2159

774

Titanium

4.84

6088

1258

Steel

7.99

7915

991

cellulose nitrate and cellulose acetate, etc. Polycarbonate is one of the more popular materials owing to its transparency and strength. The choice of material is generally dictated by the properties of the particles to be fractionated and, in high speed separations, by the maximum rated gforce.

An exhaustive discussion of the many equipment options along with their advantages and disadvantages is beyond the scope of this article. Rather, a brief overview is offered of the more common centrifuge designs together with typical applications. Much of the discussion will assume batch operation, though in most cases rotors are available or may be adapted for batch, semi-batch or continuous-mode operation. However, since continuous-mode centrifuges are so widely used in industrial applications and their analogues are often unavailable in laboratory-scale units, a section describing the more common or innovative continuous-flow configurations is included.

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