Centrifuge
Centrifuge
Centrifuge studies in the space sciences
A centrifuge is a device for separating two or more substances by using centrifugal force—the tendency of an object traveling around a central point to continue in a linear motion away from that central point.
Centrifugation can be used to separate substances because materials with different masses experience different centrifugal forces when traveling at the same velocity and distance from a common center. For example, if two balls of different mass are attached to strings and swung around a common point at the same velocity, the ball with the greater mass will experience a greater centrifugal force. If the two strings are cut simultaneously, the heavier ball will fly farther from the common center than the lighter one.
Centrifuges increase the effects of Earth’s gravitational pull. For example, if a spoonful of clay is mixed vigorously with a cup of water and then allowed to sit for a period of time, the clay will eventually settle out because it experiences a greater gravitational pull than does the water. If the same mixture is centrifuged, however, the separation will take place much more quickly.
Types of centrifuges
Centrifuges can be subdivided into two major categories, stationary and rotating. Both types work by setting a collection of particles of different mass into motion around a common center. The faster these particles move, the greater the difference with
which they tend to escape from their common center, and the more easily they will be separated from each other.
In a stationary centrifuge, a fluid (a gas or liquid) consisting of two or more components is sprayed into a cylindrical or conical chamber at a high rate of speed. As the fluid travels around the inside of the chamber, it separates into its components, with the heavier substance(s) traveling to the outside of the container, and the lighter substance(s) remaining closer to the center of the cylinder.
Stationary centrifuges are used to separate uranium isotopes. Naturally occurring uranium consists of a mixture of uranium-235, which will undergo fission, and uranium-238, which will not. A uranium sample is first converted into the gaseous compound uranium hexafluoride and then injected into a stationary centrifuge. As the rapidly moving stream of uranium hexafluoride travels around inside the centrifuge, it begins to separate. The heavier uranium uranium-235-hexafluoride concentrates along the outer wall of the centrifuge, while the lighter uranium-238-hexafluoride is left toward the center of the stream. The heavier isotope can then be drawn out of the centrifuge, leaving behind a sample of uranium hexafluoride slightly richer in the desired uranium-235 isotope. This sample can then be recentrifuged and made still richer in the lighter isotope.
Rotating centrifuges
Rotating centrifuges are familiar to most beginning chemistry students. The fluid to be separated is introduced into a container, which is then set into rapid rotational motion. It is commonly used as a substitute for filtration in the separation of a solid precipitate from the liquid in which it is suspended.
In this kind of machine, hollow tubes about 2 in (5 cm) long are attached to arms radiating from the center of the machine. When the machine is turned on, the arms are spun around the center at a speed of about 30,000 revolutions per minute (rpm). The gravitational force experienced by materials inside the tubes—about 25,000 times that of gravity—separates materials much more efficiently than a conventional filtration system.
Laboratory centrifuges are invaluable tools in many kinds of scientific research. For example, a widely used method of studying cells is to break apart a tissue sample and centrifuge the resulting fluid. In this way, the discrete components of the cell can be separated and identified.
Applications of the rotating centrifuge
The basic centrifuge design described above can be adapted for use in many different settings. Industrial centrifuges, for example, tend to be quite large, ranging in size from 4 in to 4 ft (10 cm to 1.2 m) in diameter, with rotational velocities from 1,000 to 15,000 rpm. They can be designed to remove separated portions continuously, all at once after the machine has been stopped, or intermittently.
Large-scale centrifugation has found a great variety of commercial and industrial uses. Cream has been separated from milk by this process for well over a hundred years. Centrifuges are also used to remove water from oil and jet fuel, and to purify water by removing solid materials from waste water.
A centrifuge for use with very small particles of similar weight was first developed by the Swedish
KEY TERMS
Centrifugal force— The tendency of an object traveling in a circle around a central point to escape from the center in a straight line.
Gravitation— The pull of Earth’s mass on an object.
Revolutions per minute (rpm)— The number of times per minute an object travels around a central point.
Rotation— The spinning of an object on its axis.
chemist Theodor Svedberg in 1923. The ultracentrifuge uses containers no more than about 0.2 in (0.6 cm) in diameter that rotate at speeds of about 230,000 rpm to separate colloidal particles not much larger than the size of molecules.
Centrifuge studies in the space sciences
Centrifuge studies have been very important in the development of manned space flight. Human volunteers are placed into very large centrifuges and then spun at high velocities, creating the high gravitational velocities that correspond to high gravitational forces (g forces) that occur during the launch of space vehicles. Such experiments help scientists understand the limits of acceleration that humans can endure.
See also Gravity and gravitation.
Resources
BOOKS
Dufour, John W., and W. Ed Nelson. Centrifugal Pump Sourcebook. New York: McGraw-Hill, 1992.
Lobanoff, Val S., and Robert R. Ross. Centrifugal Pumps. 2nd edition. Houston: Gulf Publications, 1992.
OTHER
Gustavus Adolphus University. “Cell Biology Laboratory Manual, Appendix F: Centrifugation.” <http://homepages.gac.edu/~cellab/appds/appd-f.html> (accessed October 7, 2006).
David E. Newton
Centrifuge
Centrifuge
A centrifuge is a device for separating two or more substances from each other by using centrifugal force . Centrifugal force is the tendency of an object traveling around a central point to continue in a linear motion and fly away from that central point.
Centrifugation can be used to separate substances from each other becausematerials with different masses experience different centrifugal forces when traveling at the same velocityand at the same distance from the common center. For example, if two balls of different mass are attached to strings and swung around a common point at the same velocity , the ball with the greater mass will experience a greater centrifugal force. If the two strings are cut simultaneously, the heavier ball will tend to fly farther from the common center than will the lighter ball.
Centrifuges can be considered devices for increasing the effects of the earth's gravitational pull. For example, if a spoonful of clay is mixed vigorously with a cup of water and then allowed to sit for a period of time, the clay will eventually settle out because it experiences a greater gravitational pull than does the water. If the same clay-water mixture is centrifuged, however, the separation will take place much more quickly.
Types of centrifuges
Centrifuges can be sub-divided into two major categories, stationary devices and rotating devices. Both types of centrifuge work on a common principle, however. A collection of particles of different mass is set into motion around a common center. The faster these particles move, the greater will be the difference with which they tend to escape from their common center, and the more easily they will be separated from each other.
In a stationary centrifuge, a fluid (a gas or liquid) consisting of two or more components is sprayed into a cylindrical or conical chamber at a high rate of speed. As the fluid travels around the inside of the chamber, it separates into its components, the heavier substance(s) traveling to the outside of the container, and the lighter substance(s) remaining closer to the center of the cylinder.
One application of the stationary centrifuge is in the separation of the isotopes of uranium isotope from each other. Naturally occurring uranium consists of a mixture of uranium-235, which will undergo fission, and uranium-238, which will not. A sample of uranium is first converted into the gaseous compound uranium hexafluoride and then injected into a stationary centrifuge. As the rapidly moving stream of uranium hexafluoride travels around inside the centrifuge, it begins to separate into two parts. The heavier uranium uranium-235-hexafluoride concentrates along the outer wall of the centrifuge, while the lighter uranium-238-hexafluoride is left toward the center of the stream. The heavier isotope can then be drawn out of the centrifuge, leaving behind a sample of uranium hexafluoride slightly richer in the desired uranium-235 isotope. This sample can then be re-centrifuged and made still richer in the lighter isotope.
Rotating centrifuges
Another type of centrifuge is one in which the fluid to be separated is introduced into a container, and the container is then set into rapid rotational motion. Most beginning chemistry students are familiar with this instrument. It is commonly used as a substitute for filtration in the separation of a solid precipitate from the liquid in which it is suspended.
In this kind of machine, hollow tubes about 2 in (5 cm) in length are attached to arms radiating from the center of the machine. When the machine is turned on, the arms are spun around the center at a speed of about 30,000 revolutions per minute. The gravitational force experienced by materials inside the tubes-about 25,000 times that of gravity-causes the separation of materials much more efficiently than would a conventional filtration system.
Laboratory centrifuges have become invaluable tools in many kinds of scientific research. For example, today a widely used method of studying cells is to break apart a tissue sample and then centrifuge the resulting fluid. In this way, the discrete components of the cell can be separated and identified.
Applications of the rotating centrifuge
The basic centrifuge design described above can be adapted for use in many different settings. Industrial centrifuges, for example, tend to be quite large, ranging in size from 4 in to 4 ft (10 cm to 1.2 m) in diameter, with rotational velocities from 1,000 to 15,000 revolutions per minute. They can be designed so as to remove separated portions continuously, all at once after the machine has been stopped, or intermittently.
Large-scale centrifugation has found a great variety of commercial and industrial uses. For example, the separation of cream from milk has been accomplished by this process for well over a hundred years. Today, centrifuges are used to remove water from oil and from jet fuel and in the removal of solid materials from waste water during the process of water purification.
A centrifuge for use with very small particles of similar weight—the ultracentrifuge—was first developed by the Swedish chemist Theodor Svedberg in about 1923. In the ultracentrifuge , containers no more than about 0.2 in (0.6 cm) in diameter are set into rotation at speeds of about 230,000 revolutions per minute. In this device, colloidal particles, not much larger than the size of molecules, can be separated from each other.
Centrifuge studies in the space sciences
Centrifuge studies have been very important in the development of manned space flight programs. Human volunteers are placed into very large centrifuges and then spun at high velocities. Inside the centrifuge, humans feel high gravitational velocities that correspond to high gravitational forces ("g forces") that occur during the launch of space vehicles. Such experiments help space scientists understand the limits of acceleration that humans can endure in such situations.
See also Gravity and gravitation.
Resources
books
"Centrifugation." McGraw-Hill Encyclopedia of Science &Technology. 6th edition. New York: McGraw-Hill Book Company, 1987, volume 3, pp. 392—398.
Dufour, John W., and W. Ed Nelson. Centrifugal Pump Source-book. New York: McGraw-Hill, 1992.
Lobanoff, Val S., and Robert R. Ross. Centrifugal Pumps. 2nd edition. Houston: Gulf Publications, 1992.
Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.
David E. Newton
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Centrifugal force
—The tendency of an object traveling in a circle around a central point to escape from the center in a straight line.
- Gravitation
—The pull of the earth's mass on an object.
- Revolutions per minute
—The number of times per minute an object travels around some central point.
- Rotation
—The spinning of an object on its axis.
Centrifuge
Centrifuge
Gravity can eventually separate a sediment (material that settles to the bottom of a liquid) from a liquid or separate two liquids which do not mix. The heavier element within a container sinks to the bottom, while the lighter element rises to the surface. This process is very slow if left up to nature alone. It can also be wasteful, as evidenced by the way farmers used to separate cream from milk. They would let whole milk stand for several hours until the lighter cream rose to the top. They then skimmed off the cream with a wooden spoon, but as much as 40 percent of the cream was left in the milk. Later, small strainer dishes were used to extract the cream, yet this too was a slow process.
In 1877 Swedish inventor Carl Gustaf Patrik de Laval introduced a high-speed centrifugal cream separator. Milk was placed in a chamber where it was heated. Once heated, it was sent through tubes to a container that was spun at 4,000 revolutions per minute by a steam engine. The centrifugal (moving away from the center) force separated the lighter cream, causing it to settle in the center of the container. The heavier milk was pushed to the outer part and forced up a discharge pipe. Thus, only the cream was left in the container. Several years later an improved cream separator was introduced with the capability for self-skimming and self-emptying. This type of separator can be used for other purposes and can extract impurities from lubricating oils, beer and wine, and other substances.
Spin Dryers
Other types of centrifuges were created in which spin dryers were used for filtering solids. In these dryers, a perforated (full of holes) drum is spun, driving any separated liquids to the outside where they were collected. Spin dryers can now develop accelerations of up to 2,000 times the force of gravity. They are used in the food, chemical, and mineral industries to separate water from all sorts of solids. Other centrifuges remove blood serum (plasma) from the heavier blood cells.
Scientists needed faster rotations for separating smaller particles. Particles, like DNA (deoxyribonucleic acid), proteins, and viruses are too small to settle out with normal gravity. The banging of water molecules is enough to keep the particles from separating. The key to separating smaller particles was to build an ultracentrifuge. A centrifuge that could spin fast enough to cause these small particles to settle out. In 1923 the Swedish chemist Theodor Svedberg developed a device that could spin fast enough to create gravity over 100,000 times normal. It could take small samples in glass containers, balance them on a cushion of air, and send jets of compressed air that touched the outer surface. By 1936 Svedberg had produced an ultracentrifuge that spun at 120,000 times per minute and created a centrifugal force equal to 525,000 times that of normal gravity. Newer models can accelerate samples to 2,000,000 times the force of gravity.
The ultracentrifuge enabled biologists, biochemists, physicians, and other life scientists to examine viruses, cell nuclei, small parts within cells, and individual protein and nucleic acid molecules. These new tools helped make the genetic engineering field ripe with possibility.
Centrifuge
Centrifuge
A centrifuge is a device that uses centrifugal force to separate two or more substances of different density or mass from each other. Centrifugal force is the tendency of an object traveling around a central point to fly away from that point in a straight line. A centrifuge is able to separate different substances from each other because materials with heavier masses move faster and farther away from the central point than materials with lighter masses. The first successful centrifuge was invented in 1883 by Swiss engineer Carl de Laval.
A centrifuge consists of a fixed base and center stem to which arms or holders containing hollow tubes are attached. When the device is turned on, the arms spin around the center stem at a high rate of speed. In the process, the heavier material is thrown outward within the tube while the lighter material stays near the center of the device.
Applications of the centrifuge
Large-scale centrifugation has found a great variety of commercial and industrial uses. For example, the separation of cream from whole milk has been accomplished by this process for more than a century. Today, the food, chemical, and mineral industries use centrifuges to separate water from all sorts of solids. Medical laboratories use centrifuges to separate plasma from heavier blood cells.
Modern centrifuges can even separate mixtures of different sized molecules or microscopic particles such as parts of cells. These instruments, called ultracentrifuges, spin so fast that the centrifugal force created can be more than one-half million times greater than the force of gravity.
Centrifuge studies have been very important in the development of manned space flight programs. Human volunteers are placed into very large centrifuges and then spun at high speeds. Inside the centrifuge, humans feel intense gravitational forces (g forces) similar to those that occur during the launch of space vehicles. Such experiments help space scientists understand the limits of acceleration that humans can endure in such situations.
[See also Gravity and gravitation ]
centrifuge
cen·tri·fuge / ˈsentrəˌfyoōj/ • n. a machine with a rapidly rotating container that applies centrifugal force to its contents, typically to separate fluids of different densities (e.g., cream from milk) or liquids from solids.• v. [tr.] (usu. be centrifuged) subject to the action of a centrifuge. ∎ separate by centrifuge: the black liquid is centrifuged into oil and water.DERIVATIVES: cen·trif·u·ga·tion / ˌsentrəˌfyoŏˈgāshən; senˌtrif(y)ə-/ n.