A
centrifuge is a piece of equipment, generally driven by an electric motor (some older models were spun by hand), that puts an object in
rotation around a fixed axisRotation around a fixed axis is a special case of rotational motion. The fixed axis hypothesis exclude the possibility of a moving axis, and cannot describe such phenomena as wobbling or precession. According to Euler's rotation theorem, simultaneous rotation around more than one axis at the same...
, applying a force perpendicular to the axis. The centrifuge works using the
sedimentation principleSedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
, where the centripetal acceleration causes more dense substances to separate out along the radial direction (the bottom of the tube). By the same token, lighter objects will tend to move to the top (of the tube; in the rotating picture, move to the centre).
In the picture shown, the rotating unit, called the rotor, has fixed holes drilled at an angle (to the vertical). Test tubes are placed in these slots and the motor is spun. As the centripetal force is in the horizontal plane and the tubes are fixed at an angle, the particles have to travel only a little distance before they hit the wall and drop down to the bottom. These angle rotors are very popular in the lab for routine use.
Theory
Protocols for centrifugation typically specify the amount of
accelerationIn physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
to be applied to the sample, rather than specifying a
rotational speedRotational speed tells how many complete rotations there are per time unit. It is therefore a cyclic frequency, measured in hertz in the SI System...
such as
revolutions per minuteRevolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
. This distinction is important because two rotors with different diameters running at the same rotational speed will subject samples to different accelerations. During
circular motionIn physics, circular motion is rotation along a circular path or a circular orbit. It can be uniform, that is, with constant angular rate of rotation , or non-uniform, that is, with a changing rate of rotation. The rotation around a fixed axis of a three-dimensional body involves circular motion of...
the acceleration is the product of the
radiusIn classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...
and the square of the
angular velocityIn physics, the angular velocity is a vector quantity which specifies the angular speed of an object and the axis about which the object is rotating. The SI unit of angular velocity is radians per second, although it may be measured in other units such as degrees per second, revolutions per...
, and the acceleration relative to "
g" is traditionally named "relative centrifugal force" (RCF). The acceleration is measured in multiples of "
g" (or × "
g"), the standard acceleration due to gravity at the Earth's surface, a
dimensionless quantityIn dimensional analysis, a dimensionless quantity or quantity of dimension one is a quantity without an associated physical dimension. It is thus a "pure" number, and as such always has a dimension of 1. Dimensionless quantities are widely used in mathematics, physics, engineering, economics, and...
given by the expression:
-
where
is earth's
gravitational accelerationStandard gravity, or standard acceleration due to free fall, usually denoted by g0 or gn, is the nominal acceleration of an object in a vacuum near the surface of the Earth. It is defined as precisely , or about...
,
is the rotational radius,
is the
rotational speedRotational speed tells how many complete rotations there are per time unit. It is therefore a cyclic frequency, measured in hertz in the SI System...
, measured in revolutions per unit of time. In physics, this is usually denoted
and is called the frequency of revolution.
This relationship may be written as
-
where
is the rotational radius measured in centimetres (cm), that is,
,
is
rotational speedRotational speed tells how many complete rotations there are per time unit. It is therefore a cyclic frequency, measured in hertz in the SI System...
measured in
revolutions per minuteRevolutions per minute is a measure of the frequency of a rotation. It annotates the number of full rotations completed in one minute around a fixed axis...
(RPM), that is,
, and
.
History and predecessors
English military engineer
Benjamin RobinsBenjamin Robins was a pioneering English scientist, Newtonian mathematician, and military engineer.He wrote an influential treatise on gunnery, for the first time introducing Newtonian science to military men, was an early enthusiast for rifled gun barrels, and his work had substantive influence...
(1707–1751) invented a whirling arm apparatus to determine
dragIn fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
. In 1864, Antonin Prandtl invented the first dairy centrifuge in order to separate cream from milk. In 1879,
Gustaf de LavalKarl Gustaf Patrik de Laval was a Swedish engineer and inventor who made important contributions to the design of steam turbines and dairy machinery.-Life:De Laval was born at Orsa in Dalarna...
demonstrated the first continuous centrifugal separator, making its commercial application feasible.
Types
There are at least six types of centrifuge:
- preparative centrifuge
- analytical centrifuge
- angle fixed centrifuge
- swing head centrifuge
- haematocrit centrifuge
- continuous tubular 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
A sieve, or sifter, separates wanted elements from unwanted material using a woven screen such as a mesh or net. However, in cooking, especially with flour, a sifter is used to aerate the substance, among other things. A strainer is a type of sieve typically used to separate a solid from a liquid...
of some sort, through which the solids cannot go (due to granulometryGranulometry is the measurement of the size distribution in a collection of grains.* Granulometry : Granulometry computation using the morphological opening operation....
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
Settling is the process by which particulates settle to the bottom of a liquid and form a sediment. Particles that experience a force, either due to gravity or due to centrifugal motion will tend to move in a uniform manner in the direction exerted by that force...
due to centrifugal acceleration.
- Continuous liquid; common types are:
- Solid bowl centrifuges
- Conical plate centrifuges
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 centrifugeThe Zippe-type centrifuge is a device designed to collect Uranium-235. It was developed in the Soviet Union by a team of 60 Austrian and German scientists captured after World War II, working in detention...
, separate
isotopeIsotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s, and these kinds of centrifuges are in use in
nuclear powerNuclear power is the use of sustained nuclear fission to generate heat and electricity. Nuclear power plants provide about 6% of the world's energy and 13–14% of the world's electricity, with the U.S., France, and Japan together accounting for about 50% of nuclear generated electricity...
and
nuclear weaponA nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
programs.
Gas centrifugeA gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centripetal force accelerating molecules so that particles of different masses are physically separated in a gradient along the radius of a rotating container.A prominent use of gas...
s are used in uranium enrichment. The heavier
isotopeIsotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
of uranium (
uranium-238Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
) in the
uranium hexafluorideUranium hexafluoride , referred to as "hex" in the nuclear industry, is a compound used in the uranium enrichment process that produces fuel for nuclear reactors and nuclear weapons. It forms solid grey crystals at standard temperature and pressure , is highly toxic, reacts violently with water...
gas tends to concentrate at the walls of the centrifuge as it spins, while the desired
uranium-235- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
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 reactorA nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
(around 3.5% enrichment), and many thousands more to enrich it to
weapons-gradeA weapons-grade substance is one that is pure enough to be used to make a weapon or has properties that make it suitable for weapons use. Weapons-grade plutonium and uranium are the most common examples, but it may also be used to refer to chemical and biological weapons...
(above 90% enrichment) for use in nuclear weapons.
Aeronautics and astronautics
Human centrifuges are exceptionally large centrifuges that test the reactions and tolerance of
pilotsAn aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...
and
astronautAn astronaut or cosmonaut is a person trained by a human spaceflight program to command, pilot, or serve as a crew member of a spacecraft....
s to acceleration above those experienced in the Earth's gravity.
The US Air Force at
Holloman Air Force BaseHolloman Air Force Base is a United States Air Force base located six miles southwest of the central business district of Alamogordo, a city in Otero County, New Mexico, United States. The base was named in honor of Col. George V. Holloman, a pioneer in guided missile research...
, New Mexico operates a human centrifuge. The centrifuge at Holloman AFB is operated by the
aerospace physiologyAerospace physiology refers to effects on the human body caused by characteristics of the aerospace environment.This subject includes, but is not limited to, the following topics:* Spatial disorientation and airsickness* Vestibular system...
department for the purpose of training and evaluating prospective fighter pilots for high-
g flight in Air Force fighter aircraft.
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 decalcificationBone decalcification is the removal of calcium ions from the bone through histological process thereby making the bone flexible and easy for pathological investigation....
and
muscle atrophyMuscle atrophy, or disuse atrophy, is defined as a decrease in the mass of the muscle; it can be a partial or complete wasting away of muscle. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass...
that affect individuals exposed to long periods of freefall. An example of this can be seen in the film
2001: A Space Odyssey2001: A Space Odyssey is a 1968 epic science fiction film produced and directed by Stanley Kubrick, and co-written by Kubrick and Arthur C. Clarke, partially inspired by Clarke's short story The Sentinel...
.
Earthquake and blast simulation
The
geotechnical centrifuge is used for simulating blasts and earthquake phenomena. For a discussion of their design, see
Geotechnical Centrifuges by Philip Turner.
Commercial applications
- Centrifuges with a batch weight of up to 2,200 kg per charge are used in the sugar industry to separate the sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
crystals from the mother liquorA mother liquor is the part of a solution that is left over after crystallization. It is encountered in chemical processes including sugar refining....
.
- Standalone centrifuges for drying (hand-washed) clothes – usually with a water outlet.
- Centrifuges are used in the attraction Mission: SPACE
Mission: SPACE is a motion simulator thrill ride at Epcot. It simulates what an astronaut might experience aboard a spacecraft on a mission to Mars, from the higher g-force of blastoff to the speculative hypersleep....
, located at EpcotEpcot is a theme park in the Walt Disney World Resort, located near Orlando, Florida. The park is dedicated to the celebration of human achievement, namely international culture and technological innovation. The second park built at the resort, it opened on October 1, 1982 and was initially named...
in Walt Disney World, which propels riders using a combination of a centrifuge and a motion simulatorA motion simulator or motion platform is a mechanism that encapsulates occupants and creates the effect/feelings of being in a moving vehicle...
to simulate the feeling of going into spaceSpace is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...
.
- In soil mechanics
Soil mechanics is a branch of engineering mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids and particles but soil may also contain organic solids, liquids, and gasses and other...
, 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
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
and wastewaterWastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations...
treatment to dry sludgeSludge refers to the residual, semi-solid material left from industrial wastewater, or sewage treatment processes. It can also refer to the settled suspension obtained from conventional drinking water treatment, and numerous other industrial processes...
s. 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
In geotechnical engineering, drilling fluid is a fluid used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also used for much simpler boreholes, such as water wells. Liquid drilling fluid...
.
- Disc-stack centrifuges used by some companies in Oil Sands industry to separate small amounts of water and solids from bitumen
- Centrifuges are used to separate cream (remove fat) from milk.
Separators in dairying
- Separators are centrifuges used on farms or in industrial dairies to separate milk
Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...
into creamCream is a dairy product that is composed of the higher-butterfat layer skimmed from the top of milk before homogenization. In un-homogenized milk, over time, the lighter fat rises to the top. In the industrial production of cream this process is accelerated by using centrifuges called "separators"...
and skimmed milkSkimmed milk , or skim milk is made when all the cream is removed from whole milk .Sometimes only half the cream is removed, this is called semi-skimmed milk....
.
Further reading
Naesgaard et al., Modeling flow liquefaction, its mitigation, and comparison with centrifuge tests
KEM-TRON Technologies KT-1448 Centrifuge
See also
- Lamm equation
The Lamm equation describes the sedimentation and diffusion of a solute under ultracentrifugation in traditional sector-shaped cells. It was named after Ole Lamm, later professor of physical chemistry at the Royal Institute of Technology, who derived it during his Ph.D...
- Sedimentation
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
- Centrifugal force
Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...
- Centrifugation
Centrifugation is a process that involves the use of the centrifugal force for the sedimentation of mixtures with a centrifuge, used in industry and in laboratory settings. More-dense components of the mixture migrate away from the axis of the centrifuge, while less-dense components of the mixture...
- Gas centrifuge
A gas centrifuge is a device that performs isotope separation of gases. A centrifuge relies on the principles of centripetal force accelerating molecules so that particles of different masses are physically separated in a gradient along the radius of a rotating container.A prominent use of gas...
- Sedimentation coefficient
The sedimentation coefficient s of a particle is used to characterize its behaviour in sedimentation processes, notably centrifugation. It is defined as the ratio of a particle's sedimentation velocity to the acceleration that is applied to it .The sedimentation speed v_t is also known as the...
- Clearing factor
In centrifugation the clearing factor or k factor represents the relative pelleting efficiency of a given centrifuge rotor at maximum rotation speed...
- Hydroextractor
Hydroextractors are machines which are used in textile processing industry. These are mainly centrifuges. The material wet material is placed in the extractor, which has a wall of perforated metal, generally stainless steel. the internal drum rotates at high speed thus throwing out the water...
External links