centrifuge

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A laboratory tabletop centrifuge

A centrifuge is a piece of equipment, generally driven by a motor, that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis. The centrifuge works using the sedimentation principle, where the centripetal acceleration is used to evenly distribute substances (usually present in a solution for small scale applications) of greater and lesser density. There are many different kinds of centrifuges, including those for very specialized purposes. It can be used for viable counts, when shaking the culture e.g. yeast, out of suspension.

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 standard 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.

Since the motion is circular the acceleration can be calculated as the product of the radius and the square of the angular velocity. Traditionally named "Relative centrifugal force" (RCF), it is the measurement of the acceleration applied to a sample within a centrifuge and it is measured in units of gravity (times gravity or × "g"). It is given by

where

is earth's gravitational acceleration,

is the rotational radius,

is the rotating speed, measured in revolutions per unit of time.

When the rotational speed is given in revolutions per minute (RPM) and the rotational radius is expressed in centimetres (cm) the above relationship becomes

where

is the rotational radius measured in centimetres (cm),

is rotating speed measured in revolutions per minute (RPM).

History and predecessors

A 19th 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 in order to separate cream from milk. In 1879, Gustaf de Laval demonstrated the first continuous centrifugal separator, making its commercial application feasible.

Types

There are at least five types of centrifuge:

• preparative centrifuge
• analytical centrifuge
• angle fixed centrifuge
• swing head centrifuge
• haemacrotic 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

Isolating suspensions

Simple centrifuges are used in chemistry, biology, and biochemistry for isolating and separating suspensions. They vary widely in speed and capacity. They usually comprise a rotor containing two, four, six, or many more numbered wells within which the samples containing centrifuge tips may be placed.

Isotope separation

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 the uranium hexafluoride gas tend to concentrate at 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 for use in a nuclear reactor (around 3.5% enrichment), and many thousands more to enrich it to weapons-grade (around 90% enrichment) for use in nuclear weapons.

The 20 G centrifuge at the NASA Ames Research Center

Aeronautics and astronautics

Human centrifuges are exceptionally large centrifuges that test the reactions and tolerance of pilots and astronauts to acceleration above those experienced in the Earth's gravity.

The US Air Force at Holloman Air Force Base, NM 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 unrealistic in that it 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.^{[citation needed]}

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, while the James Bond film Moonraker shows an astronaut-training centrifuge in action on earth (albeit sabotaged with murderous intent).

Earthquake and blast simulation

The geotechnical centrifuge is used for simulating blasts and earthquake phenomena.^[1] For a discussion of their design, see Geotechnical Centrifuges by Philip Turner.

Commercial applications

• Standalone centrifuges for drying (hand-washed) clothes - usually with a water outlet.
• Centrifuges are used in the attraction Mission: SPACE, 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.
• 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.

References and notes

1. ^ C. W. W. Ng, Y. H. Wang, L. M. Zhang (2006). Physical Modelling in Geotechnics: proceedings of the Sixth International Conference on Physical Modelling in Geotechnics. Taylor & Francis. p. 135. ISBN 0415415861. http://books.google.com/books?id=mzQlFBqJC1wC&pg=RA1-PA186&dq=centrifuge+earthquake&lr=&as_brr=0&sig=ACfU3U1SZImDanYOwrq15Jre19qtAqpyJw#PRA1-PA135,M1. 

Further reading

Naesgaard et al., Modeling flow liquefaction, its mitigation, and comparison with centrifuge tests

See also

• Lamm equation
• Sedimentation
• Centrifugal force
• Centrifugation
• Gas centrifuge
• Sedimentation coefficient
• Clearing factor

External links

Wikimedia Commons has media related to: Centrifuge

Look up centrifuge in Wiktionary, the free dictionary.

• RCF Calculator and Nomograph
• RPI centrifuge video
• Centrifugation Rotor Calculator
• Selection of historical centrifuges in the Virtual Laboratory of the Max Planck Institute for the History of Science