Everything about Centrifuge totally explained
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 separate substances of greater and lesser density. There are many different kinds of centrifuges, including those for very specialised purposes. It can be used for viable counts, when shaking the culture for example
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.
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; in 1864,
Antonin Prandl invented the first dairy centrifuge in order to separate cream from milk, and in 1879,
Gustaf de Laval demonstrated the first continuous centrifugal separator, making its commercial application feasible.
Types and uses
There are basically four types of centrifuge:
- Tabletop/clinical/desktop centrifuge or microcentrifuge
- High-speed centrifuge
- Cooling centrifuge
- Ultracentrifuge
Laboratory centrifuge
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 atomic bomb-grade (around 90% enrichment).
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's 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 .
Commercial applications
Standalone centrifuges for drying (hand-washed) clothes - usually with a water outlet.
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.
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.
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.
»
where
» g = Relative centrifuge force
r = rotational radius (centimetres, cm)
» N = rotating speed (revolutions per minute, r/min)
Further Information
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