# Centrifuge

A laboratory tabletop centrifuge.

A centrifuge is a piece of equipment, generally driven by a motor, that puts objects in rotation around a central, fixed axis, applying a force perpendicular to the axis. The equipment consists of a fixed base and a rotating component, called a rotor, that holds the objects or samples to be spun. The spinning action, called centrifugation, subjects the samples to forces that far exceed the force of gravity.

In general, a centrifuge is useful for separating materials in certain types of mixtures. If a liquid (or solution) contains materials of different densities or widely differing molecular weights, these materials may be separable by using a centrifuge. For example, a centrifuge may be used to separate cream from milk, or to separate biological cells or virus particles from suspension in a liquid. A household washing machine acts as a centrifuge during the spin cycle, when the liquid (water) is separated from the solids (clothing). Specialized gas centrifuges are employed for enrichment of the isotope uranium-235, for use in nuclear reactors or nuclear weapons. Some centrifuges are designed to accommodate humans or animals to test the effects of high gravitational forces on their bodies.

## Historical highlights

A nineteenth century hand cranked laboratory centrifuge.

English military engineer Benjamin Robins (1707-1751) invented a whirling arm apparatus to determine drag. In 1864, Antonin Prandtl invented the first dairy centrifuge to separate cream from milk. In 1879, Swedish engineer Gustaf de Laval demonstrated the first continuous centrifugal separator, making its commercial application feasible. In the 1920s, Swedish chemist Theodor Svedberg built the ultracentrifuge, using it to determine the molecular weights of viruses and proteins.

## Theoretical background

During the process of centrifugation, the centrifugal force acts to separate various components of a mixture. More-dense components of the mixture migrate away from the axis of the centrifuge, while less-dense components migrate toward the axis. The rate of centrifugation is specified by the acceleration applied to the sample, typically quoted in revolutions per minute (RPM) or in multiples of g, the acceleration due to gravity at the Earth's surface. The particles' settling velocity during centrifugation is a function of their size and shape, centrifugal acceleration, the volume fraction of solids present, the density difference between the particles and the liquid, and the viscosity of the liquid.

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. This distinction is important because two rotors with different diameters running at the same rotational speed will subject samples to different accelerations. The acceleration is often quoted in multiples of g, the standard acceleration due to gravity at the Earth's surface.

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

## Types

There are at least five types of centrifuge:

• Tabletop/clinical/desktop centrifuge or microcentrifuge
• High-speed centrifuge
• Cooling centrifuge
• Ultracentrifuge
• Geotechnical centrifuge

Industrial centrifuges may otherwise be classified according to the type of separation of the high density fraction from the low density one :

• Screen centrifuges, where the centrifugal acceleration allows the liquid to pass through a screen of some sort, through which the solids cannot go (due to granulometry larger than the screen gap or due to agglomeration). Common types are :
• Pusher centrifuges
• Peeler centrifuges
• Decanter centrifuges, in which there is no physical separation between the solid and liquid phase, rather an accelerated settling due to centrifugal acceleration. Common types are :
• Solid bowl centrifuges
• Conical plate centrifuges

## Uses

### Separations based on density or molecular weight

Centrifuges are often used in chemistry, biology, and biochemistry laboratories for isolating and separating materials of differing densities or molecular weights. These centrifuges vary widely in speed and capacity. They usually consist of a rotor containing two, four, six, or more numbered wells that carry centrifuge tubes containing the samples.

### Isotope separation

Cascade of gas centrifuges used to produce enriched uranium. U.S. gas centrifuge plant in Piketon, Ohio, 1984.

Other centrifuges, the first being the Zippe-type centrifuge, separate isotopes, and these kinds of centrifuges are in use in nuclear power and nuclear weapon programs.

Gas centrifuges are used in uranium enrichment. The heavier isotope of uranium (uranium-238) in uranium hexafluoride gas tends to concentrate near the walls of the centrifuge as it spins, while the desired uranium-235 isotope is extracted and concentrated with a scoop selectively placed inside the centrifuge. It takes many thousands of centrifuges to enrich uranium enough (around 3.5 percent enrichment) for use in a nuclear reactor, and many thousands more to enrich it to weapons-grade (around 90 percent enrichment) for use in nuclear weapons.

### Aeronautics and astronautics

The 20 G centrifuge at the NASA Ames Research Center.

Human centrifuges are exceptionally large, designed to test the reactions and tolerance of pilots and astronauts to accelerations much higher than those experienced in the Earth's gravitational field.

The U.S. Air Force at Holloman Air Force Base, New Mexico, operates a human centrifuge. The centrifuge at Holloman AFB is operated by the aerospace physiology department for the purpose of training and evaluating prospective fighter pilots for high-g flight in Air Force fighter aircraft. It is important to note that the centrifuge at Holloman AFB is far more difficult for a pilot to tolerate the high-g environment in the centrifuge than in a real fighter aircraft. This well-known fact is based on countless accounts from experienced operational fighter pilots.

The use of large centrifuges to simulate a feeling of gravity has been proposed for future long-duration space missions. Exposure to this simulated gravity would prevent or reduce the bone decalcification and muscle atrophy that affect individuals exposed to long periods of freefall. An example of this can be seen in the film 2001: A Space Odyssey.

### Earthquake and blast simulation

The geotechnical centrifuge is used for simulating blasts and earthquake phenomena.[1][2]

### Commercial applications

• Standalone centrifuges for drying (hand-washed) clothes - usually with a water outlet, known as an extractor.
• Centrifuges are used in the amusement park attraction Mission: SPACE, located at Epcot in Walt Disney World, in Florida, which propels riders using a combination of a centrifuge and a motion simulator to simulate the feeling of going into space.
• 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 centrifuge after most of the solids have been removed is called centrate.
• Large industrial centrifuges are also used in the oil industry to remove solids from the drilling fluid.
• Disc-stack centrifuges used by some companies in Oil Sands industry to separate small amounts of water and solids from bitumen before it's sent to Upgrading.

## 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.

$g = RCF = 0.00001118\,r \, N^2 \,$

where

g = Relative centrifuge force
r = rotational radius (centimeters, cm)
N = rotating speed (revolutions per minute, RPM)

## Notes

1. I.S. Ha, et al. 2006. Development of a large scale geotechnical centrifuge in KOWACO. In Physical Modelling in Geotechnics, edited by C.W.W. Ng, Y.H. Wang, and L.M. Zhang. Proceedings of the Sixth International Conference on Physical Modelling in Geotechnics. (6th ICPMG'06) (Hong Kong, 4-6 August 2006.) Balkema-proceedings and monographs in engineering, water and earth sciences. (London: Taylor & Francis. ISBN 0415415861), 135. Retrieved October 10, 2008.
2. Philip Turner, Geotechnical Centrifuges. (A discussion of their design.) Department of Engineering, Cambridge University. Retrieved October 10, 2008.

## References

• Graham, John. 2001. Biological Centrifugation: The Basics. Oxford, UK: Bios. ISBN 1859960375
• Leung, Wallace Woon-Fong. 1998. Industrial Centrifugation Technology. New York, NY: McGraw-Hill. ISBN 0070371911
• Lindley, J. 1987. User Guide for the Safe Operation of Centrifuges, 2nd ed. Rugby: Institution of Chemical Engineers. ISBN 085295218X
• Regel, Liya L., and William R. Wilcox. 2001. Processing by Centrifugation. New York, NY: Kluwer Academic/Plenum Publishers. ISBN 0306466546
• Taylor, R. N. 1995. Geotechnical Centrifuge Technology. London: Blackie Academic & Professional. ISBN 0751400327