Centrifuge

The centrifuge is a technical device that works by taking advantage of inertia; The operation is based on the centrifugal force which comes into being due to a uniform circular motion of the material to be centrifuged. Which is used for separating materials, a similar procedure used in the centrifugal separator.

A centrifuge, the components of suspensions, emulsions and separate gas mixtures (see also separation process ). In investigations in the space medicine centrifuges are used to simulate the acceleration forces acting on the occupant of a spacecraft during the launch phase.

Principle of operation

Centrifuges use the inertia in Zentrifugiergutraum for material separation. Particles or media with higher density migrate due to the higher inertia to the outside. They displace the lower density components, which pass therethrough the center. The process is over sedimentation by gravity much faster or is at all possible - opposing forces such as adhesion, thermal molecular motion or the viscosity can be overcome.

The acceleration is independent of the density of the substances. However, the centripetal force is greater, the higher the density of the substance. The material separation takes place (for example, laundry and salad spinner or centrifugal juicer ( fruit juice centrifuge) ) due to the density differences (for example, laboratory centrifuge, gas centrifuge) or by the solids -retaining sieve.

The centripetal acceleration of a body in a centrifuge as the effect of the centripetal force on the distance of the body from the axis of rotation and on the angular speed dependent, it will increase linearly with the distance from the rotational axis in Zentrifugiergutraum. In addition, it increases with the square of the angular velocity or the rotational speed:

They can therefore be increased by increasing the radius of the Zentrifugiergutraums and increasing the rotation frequency. This, however, the forces increase on the rotor.

From the peripheral speed of the material to be centrifuged is as follows depending on:

This peripheral speed is important for the air resistance. This results in that many can only work in a vacuum centrifuge.

The centripetal acceleration is outside, that is highest at the point in Zentrifugierraum where the deposited goods has reached the destination. At the point of the lowest centripetal acceleration ( more inside ) the deposited goods nor the broadest way to go. The required spin time for material separation is thus not only by the speed, but - also determined by the smallest distance from the rotational axis of the Zentrifugierguts - along with other variables.

Common one is to specify the acceleration as a multiple of the average acceleration of gravity. means, for example, the maximum acceleration in a centrifuge is 1000 times the acceleration of gravity. This factor is in engineering often referred to as spin index, although it is not a number but a number.

In DIN 58970-2 is the relative centrifugal force ( RCF briefly ) mentioned in this context. This size is also specified as a multiple of the average value of and used today in most devices documentation. According to the DIN of the RCF value ( " spin index " ) can be calculated using the following formula, which can be easily calculated from the above formula ( here R is the radius in centimeters and N is the rotational speed in revolutions per minute):

If one uses SI base units for (m ) and (), we obtain for RZB:

For the simplified application of the conversion, there are two possibilities: the use of conversion tables, for which stating the centrifugal radius and either the RPM ( revolution per minute; international use: revolutions per minute or rpm) or relative centrifugal force (RCF, used internationally: relative centrifugal force or rcf) the other value can be determined. Modern laboratory centrifuges also have an automatic conversion of RZB in the corresponding speed, this function is usually marked with " rcf / rpm".

Applications

Examples of centrifuges in the household are the salad spinner, spin the laundry and fruit juice centrifuge. The wet material ( lettuce, wash, pomace ) is rotated rapidly in a perforated drum that it is pressed against the wall and drains water or juice through the holes to the outside. Characterized the adhesion of the liquid to overcome the solids. These and laboratory centrifuges have a direct drive.

Olive oil and MSM are also in this way from the pomace ( in Decanter ) or separately ( in the separator ) of liquids; on the same principle win beekeeper with a honey extractor honey. Another field of application for centrifuges in the household is the molecular gastronomy.

In the metalworking industry, centrifuges are used for de-oiling of metal shavings, and it is possible to achieve a throughput of up to ten tons per hour in fully automatic mode. Here, the pre- comminuted chips are full speed ( about 700 to 1500 rpm ) of the drum is supplied and ejected. Galvanic companies use manual and fully automatic centrifuges for drying bulk material, such as bolts, rivets, etc., a.

Extremely large centrifuge with a fill weight of up to 1,750 kg can be used in the sugar industry. This sugar crystals from the adhering syrup are separated. These centrifuges are today largely equipped with a PLC and frequency-controlled drives. In addition, large centrifuges are used for example for wastewater treatment in municipal wastewater treatment plants in the chemical industry and mining to separate minerals. In industry, technology and laboratory centrifuges are used for separating substances due to their different densities. For example, so is cow's milk in cream and reduced-fat milk isolated (invented in 1877 by Wilhelm Lefeldt ).

In addition to a separation of liquids of different densities Laboratory centrifuges are used to separate solids from a liquid. One gives to the mixture in centrifuge tubes, the clear thick-walled and thus are more stable than test tubes.

Laboratory centrifuges are previously driven by a crank, today by an electric motor. Centrifugation is often preferred to the filtration, for example when strongly acidic solutions would attack filter paper or if you want the disposal of the sludge unmixed, ie without additional filter aid done.

The ultracentrifuge was developed by Theodor Svedberg, the resulting measuring the sedimentation of macromolecules and thus their approximate molar mass. He received the 1926 Nobel Prize. Ultracentrifuge rotate their content very fast - up to 500.000/min. The rotor is in this vacuum, in order to avoid the otherwise extremely high air resistance.

For isotope separation, among other gas centrifuges are used.

Large-sized centrifuges are used in acceleration tests for pilots and astronauts in order to simulate during takeoff, flight maneuvers and during re-entry into the atmosphere forces which are a multiple of the Earth's gravity.

In the experimental soil mechanics, centrifuges are used for the simulation of earth pressure.

Hazards, safety

Due to the large kinetic energy of fast-rotating rotors safety precautions are necessary. The stronger the centrifugal forces in a centrifuge, the more stable the rotor chamber and cover shall be constructed. Boiler and equipment enclosures need to protect the environment of the centrifuge from injury or damage in the event of bursting rotor itself. Stainless steel bowls are here in the case of a rotor break far more stable and thus safer for the user as a plastic kettle cheap product. By weight of a symmetrical loading of the rotor ( balancing of the samples prior to centrifugation ), the risk of imbalance during the centrifuge run, can be significantly reduced. Especially with powerful laboratory centrifuges an automatic imbalance detection is often installed. In emerging imbalance of the centrifuge run is immediately terminated automatically.

Laboratory centrifuges or even spin dryers can usually not open as long as the rotor rotates. With the lid safety switch coupled and / or brakes prevent rotation of the housing is open.

The safety of centrifuges is regulated in the European standard EN 12547. In addition, the safety standards by professional associations such as the sugar trade association established.