Centrifuge

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A laboratory centrifuge

Tabletop centrifuge

A centrifuge is a piece of equipment that puts a substance in rotation around a fixed axis in order for the centrifugal force to separate lighter and heavier substances. Generally, a motor drives the rotary motion of the sample. There are many different kinds of centrifuges, often for very specialised purposes.

Contents

1 Theory
2 History and predecessors
3 Different types and uses
4 Use and safety
5 Calculating relative centrifugal force (RCF)
6 See also
7 External links

Theory

Protocols for centrifugation typically specify the amount of acceleration to be applied to the sample, rather than specifying a rotational speed such as revolutions per minute. The acceleration is often quoted in multiples of g, the acceleration due to gravity at the Earth's surface. This distinction is important because two rotors with different diameters running at the same rotational speed will subject samples to different accelerations.

The acceleration can be calculated as the product of the radius and the square of the angular velocity.

History and predecessors

English military engineer Benjamin Robins (1707-1751) invented a whirling arm apparatus to determine drag, and Antonin Prandl invented the first centrifuge in order to separate cream from milk to make churning butter much easier.

Different types and uses

Simple centrifuges are used in biology and biochemistry for isolating and separating biocompounds on the basis of molecular weight. These will tend to rotate at a slower rate than an ultracentrifuge, and have larger rotors, and be optimized for holding large quantities of material at intermediate acceleration.
Standalone centrifuges for drying (hand-washed) clothes - usually with a water outlet.
Other centrifuges, the first being the Zippe-type, are used to separate isotopes, and these kinds of centrifuges are in use in nuclear power and nuclear weapon programs.
The ultracentrifuge is a device invented in 1925 by Theodor Svedberg, which by use of very high acceleration, and allowing the observation of sedimentation rates for macromolecules, allowed for the determination of their approximate molecular weights. Svedberg won the 1926 Nobel Prize in Chemistry for his invention.
Exceptionally large centrifuges are used to test the reactions of pilots and astronauts to acceleration above those experienced in the Earth's gravity.
In soil mechanics, centrifuges utilize centrifugal acceleration to match soil stresses in a scale model to those found in reality.
Large industrial centrifuges are commonly used in water and wastewater treatment to dry sludges. The resulting dry product is often termed cake and the water leaving a centrifugal after most of the solids have been removed is called centrate.
Centrifuges are used in the attraction , located at Epcot in Walt Disney World, which propels riders using a combination of a centrifuge and a motion simulator to simulate the feeling of going into space.
Gas centrifuges are used to enrich uranium. The heavier isotopes of uranium (uranium-238) in the uranium hexafluoride gas tend to concentrate at the walls of the centrifuge as it spins, while the desired uranium-235 isotopes are extracted and concentrated with a scoop selectively placed inside the centrifuge. It takes many thousands of centrifuges to enrich enough uranium for use in a nuclear reactor (around 3.5% enrichment), and many thousands more to enrich it to bomb-grade (around 90% enrichment).
Centrifuges can be used to generate artificial gravity, for example, in space craft.

Use and safety

The load in a laboratory centrifuge must be carefully balanced. Small differences in mass of the load can result in a large force imbalance when the rotor is at high speed. This force imbalance strains the spindle and may result in damage to centrifuge or personal injury.

Centrifuge rotors should never be touched while moving, because a spinning rotor can cause serious injury. Modern centrifuges generally have features that prevent accidental contact with a moving rotor.

Because of the kinetic energy stored in the rotor head during high speed rotation, those who have experienced the loss of a rotor inside of a ultracentrifuge compare the experience to having a bomb explode nearby.

Calculating relative centrifugal force (RCF)

Relative centrifugal force is the measurement of the force applied to a sample within a centrifuge. This can be calculated from the speed (RPM) and the rotational radius (cm) using the following calculation.

RCF = 0.00001118 × r × N²

where

r = rotational radius (centimetre, cm)

N = rotating speed (revolutions per minute, RPM)

External links

[RCF Calculator and Nomograph]
[Selection of historical centrifuges] in the Virtual Laboratory of the Max Planck Institute for the History of Science

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