702 KNOWLEDGE CENTRIFUGATION As a rule, a mixture has to be separated in order to obtain single components. A simple and widely used separation method is centrifugation. Centrifuges use the centrifugal forces which have an impact on the mixture components during a rotation to accelerate the separation process. Centrifuges whose separation principle is based on the different behaviours of the mixture components in the gravity field are called sedimentation centrifuges. During sedimentation, the different components of a mixtures organise in certain layers. The sinking or sedimentation speed of the components mainly depends on their density. Particles of highest density settle at the bottom. The remaining substances following in the order of decreasing density. Under the influence of natural gravity, the sedimentation can take hours or days. Because the gravitation forces in the centrifuge are multiples of gravity, the mixture components separate much quicker. A significant number of different rotor types has been developed for centrifuges. In laboratory centrifuges, fixed-angle rotors and swing-bucket rotors are used most frequently. Both rotor types have specific benefits. The fixed-angle rotor Fixed-angle rotors are almost always solid bodies with smooth and closed surfaces. They have holes where the centrifuging vessels are inserted. The sample tubes are in a fixed angle to the rotor axes and they keep this angle during centrifugation. Due to their design, fixed-angle rotors provide low aerodynamic resistance and can reach higher revolutions than swing-bucket rotors at the same drive capacity. The centrifuged particles first move radially outward, then hit the tilted tube wall after a relatively short distance, and then slide down to the bottom of the tube, driven by gravity. Convection streams in the tube additionally contribute to a quicker sedimentation. Therefore, fixed-angle rotors make relatively short centrifuging times possible. The swingbucket rotor In this type of rotor, the pendants swing to up to 90° from the vertical position with increasing rotor revolution. Therefore, the centrifuging tubes orientate towards the effective field of gravity. The sedimentation process happens towards the tube axis, so that the sediment is formed at the bottom of the tube in evenly thick layers. Due to strong aerodynamic resistance, which is caused by the ragged form of the rotors with the swinging pendants, the swing-bucket rotors reach lower revolutions than comparable fixed-angle rotors. This and the longer way which the particles in the swing-bucket rotor have to cover to the tube bottom results in longer centrifuging times. Calculation of the centrifugal force The relative centrifugal force (RCF) is a major evaluation criterion of the separation performance of a centrifuge or a rotor. The relative centrifugal force, also called g-number, indicates the factor by which the centrifugal force exceeds the gravitational acceleration. The g-number is only a number and does not have a unit. To determine the RCF, the rotor diameter and the revolution are needed. The following formula is used: RCF = (n/1000)2 x r x 1,118 n = Revolution in min-1 (RPM) r = Radius in mm (= distance between rotor centre and bottom of the centrifuge tube) Important note for practical application: doubling the rotor radius of a centrifuge results in doubling the relative centrifugal force, doubling the revolution results in quadruplication.
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