What is centrifugation Where is it used



A centrifuge is a technical device that uses centrifugal force to separate the components of suspensions, emulsions and gas mixtures (see also separation processes). In studies in space medicine, centrifuges are used to simulate the acceleration forces that act on the occupants of a spacecraft during the launch phase.

Efficiency

An important parameter for the performance of centrifuges is the maximum centrifugal acceleration that can be achieved in the centrifugation material space. The centrifugal acceleration a of a body in a centrifuge as the effect of the centrifugal force (an apparent force) is dependent on the distance of the body from the axis of rotation r and on the speed of the body on the circular path (e.g. angular velocity ω); it is independent of the mass of the body: a = ω2 r. The maximum centrifugal acceleration is thus achieved at the point of greatest distance in the centrifugation material space from the axis of rotation and at the maximum rotational frequency. It is therefore only dependent on the properties of the centrifuge, namely on the geometric shape of the centrifugal material space and its highest possible rotational frequency. It is therefore a suitable parameter. If SI units are used, the centrifugal acceleration is given in the unit of measurement Newton (N).

It is also common to specify the centrifugal acceleration as a multiple of the mean acceleration due to gravity (more precisely: normal acceleration due to gravity) g, which means that the unit of measurement is the mean acceleration due to gravity. Example: 1000 g means that the maximum centrifugal acceleration in a centrifuge is 1000 times the gravitational acceleration. In engineering, this quantity is often referred to as the "sling number", although it is not a number at all, but a number with a unit of measurement.

Occasionally, instead of the centrifugal acceleration, its ratio to the mean gravitational acceleration g is given. This parameter is then a pure number (dimensionless, i.e. without a unit of measurement).

It should be noted that the maximum centrifugal acceleration is reached at the point in the centrifugation chamber where the material to be separated has already reached the destination (at the point furthest away from the axis of rotation), and that at the point of the lowest centrifugal acceleration (closest to the axis of rotation ) the material to be separated still has the longest way to go. The required centrifugation time is therefore not only determined by the parameter centrifugal acceleration at the point furthest from the axis of rotation, but - in addition to other variables - also by the smallest distance between the material being centrifuged and the axis of rotation.

Applications

Examples of centrifuges in the home are the salad spinner and the spin dryer: The wet laundry is placed in the drum, which is rotated so fast that it is pressed against the wall of the drum. The water can drain through holes in the wall.

Centrifuges are used in a wide variety of applications in technology and in laboratories, for example they are used to separate cow's milk into cream and reduced-fat milk (invented by Wilhelm Lefeldt in 1877). Olive oil is also produced with the help of a centrifuge, as is mechanically separated meat. Even more often than for such a separation of liquids of different densities, centrifuges are used to separate solids from a liquid. In the laboratory, the mixture is added to centrifuge tubes, which are much thicker-walled and therefore more stable than test tubes. The laboratory centrifuge used to be driven by a crank, today it is driven by an electric motor. Centrifugation is often preferred to filtration, for example when strongly acidic solutions would attack paper filters or when the sludge is to be disposed of by type, i.e. without additional filter aid.

In the metal processing industry, centrifuges are used to de-oil metal chips, whereby it is possible to achieve a throughput of up to ten tons per hour in fully automatic operation. In this case, the previously shredded chips are fed into the drum at full speed (700 to approx. 1500 rpm) and ejected again.

Manual and fully automatic centrifuges are also used in electroplating companies to dry bulk goods such as screws, rivets, etc.

Extremely large centrifuges are used in the sugar industry. In it, sugar crystals and syrup adhering to them are separated from one another.

The ultracentrifuge was developed by Theodor Svedberg, who used it to determine the sedimentation speeds of macromolecules and thus their approximate molar mass. He received the Nobel Prize in 1926. Ultracentrifuges rotate their contents very quickly - up to 500,000 times a minute. This is why the rotor is usually in a vacuum so that no air friction occurs.

Centrifuges can also be used to separate isotopes. Huge centrifuges are used in acceleration tests for pilots and astronauts in order to simulate the forces acting mainly during take-off (several times the force of gravity).

Dangers, security

Due to the large kinetic energy of rapidly rotating rotors, safety precautions, especially a stable housing, are very important. Closed centrifuges often cannot be opened while the rotor is turning, and a safety switch prevents rotation when the housing is open.

See also

Categories: Centrifuge | Centrifugation | Laboratory device