Centrifuges (Rotors)

The operating principle of a centrifuge is based on the exploitation of inertia and centripetal acceleration. The centripetal acceleration originates in the uniform circular movement of the sample in the centrifuge and is used for separating substances. A centrifuge is able therefore to separate components from suspensions, emulsions and gas mixtures.

Function principle of the centrifuge

Substances of higher density move outward on account of higher inertia. Consequently they displace the lower density components. The acceleration is independent of mass or absolute density of the substances, the substances are separated only by differences in density. The centripetal acceleration, however, depends on the distance from the axis of rotation and on angular velocity. The acceleration linearly increases with distance and quadratically with the angular velocity and the revolutions per minute. Therefore the centipetal acceleration can be boosted by increasing the radius and the rotary frequency. The centripetal acceleration is further dependent on the tangential velocity of the sample in the centrifuge. The tangential velocity is vital for the air resistance, meaning that many centrifuges can operate only in a vacuum. In general, centripetal acceleration is highest at the outside.

Application examples of centrifuges

Typical household centrifuges are the salad spinner, the spin-drier and the juice centrifuge. The wet product is spun in a drum so fast that it is pressed against the drum wall. Water is thus squeezed through the holes in the drum. By this procedure, the adhesion of the liquid to the solids is overcome. Another application area of the centrifuges is the metallurgic industry. Centrifuges are used there for de-oiling metal splinters. In industry, technology and laboratory, centrifuges are used for separating substances by their relative differences in density. A typical example is the processing of cow milk. Milk is separated into cream and low-fat milk. Beside the separation of liquids, laboratory centrifuges can be also applied to the separation of solids from a liquid. The mixture is then put in centrifuge tubes which feature much thicker walls than test tubes and are therefore more stable.

• http://de.wikipedia.org/w/index.php?title=Zentrifuge&oldid=87090230 (called: 2011-03-31).