Centrifuge

History

English military engineer Benjamin Robins (1707–1751) invented a whirling arm apparatus to determine drag. In 1864, Antonin Prandtl proposed the idea of a dairy centrifuge to separate cream from milk. The idea was subsequently put into practice by his brother, Alexander Prandtl, who made improvements to his brother's design, and exhibited a working butterfat extraction machine in 1875.

Types

A centrifuge machine can be described as a machine with a rapidly rotating container that applies centrifugal force to its contents. There are multiple types of centrifuge, which can be classified by intended use or by rotor design:

Types by rotor design:

Fixed-angle centrifuges are designed to hold the sample containers at a constant angle relative to the central axis.

Swinging head (or swinging bucket) centrifuges, in contrast to fixed-angle centrifuges, have a hinge where the sample containers are attached to the central rotor. This allows all of the samples to swing outwards as the centrifuge is spun.

Continuous tubular centrifuges do not have individual sample vessels and are used for high volume applications.

Types by intended use:

Laboratory centrifuges, are general-purpose instruments of several types with distinct, but overlapping, capabilities. These include clinical centrifuges, superspeed centrifuges and preparative ultracentrifuges.

Analytical ultracentrifuges are designed to perform sedimentation analysis of macromolecules using the principles devised by Theodor Svedberg.

Haematocrit centrifuges are used to measure the volume percentage of red blood cells in whole blood.

Gas centrifuges, including Zippe-type centrifuges, for isotopic separations in the gas phase.

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

Generally, there are two types of centrifuges: the filtration and sedimentation centrifuges. For the filtration or the so-called screen centrifuge the drum is perforated and is inserted with a filter, for example a filter cloth, wire mesh or lot screen. The suspension flows through the filter and the drum with the perforated wall from the inside to the outside. In this way the solid material is restrained and can be removed. The kind of removing depends on the type of centrifuge, for example manually or periodically. Common types are:

Screen/scroll centrifuges (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))

Pusher centrifuges

Peeler centrifuges

Inverting filter centrifuges

Sliding discharge centrifuges

Pendulum centrifuges

In the sedimentation centrifuges the drum is a solid wall (not perforated). This type of centrifuge is used for the purification of suspension. For the acceleration of the natural deposition process of suspension the centrifuges use centrifugal force. With so-called overflow centrifuges the suspension is drained off and the liquid is added constantly.Common types are:

Pendulum centrifuges

Separator centrifuges (Continuous liquid); common types are:

Solid bowl centrifuges

Conical plate centrifuges

Tubular centrifuges

Decanter centrifuges, in which there is no physical separation between the solid and liquid phase, rather an accelerated settling due to centrifugal acceleration.

Though most modern centrifuges are electrically powered, a hand-powered variant inspired by the whirligig has been developed for medical applications in developing countries.

Laboratory separations

A wide variety of laboratory-scale centrifuges are used in chemistry, biology, biochemistry and clinical medicine for isolating and separating suspensions and immiscible liquids. They vary widely in speed, capacity, temperature control, and other characteristics. Laboratory centrifuges often can accept a range of different fixed-angle and swinging bucket rotors able to carry different numbers of centrifuge tubes and rated for specific maximum speeds. Controls vary from simple electrical timers to programmable models able to control acceleration and deceleration rates, running speeds, and temperature regimes. Ultracentrifuges spin the rotors under vacuum, eliminating air resistance and enabling exact temperature control. Zonal rotors and continuous flow systems are capable of handing bulk and larger sample volumes, respectively, in a laboratory-scale instrument.

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 tends 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 centrifugations to enrich uranium enough for use in a nuclear reactor (around 3.5% enrichment), and many thousands more to enrich it to weapons-grade (above 90% enrichment) for use in nuclear weapons.

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 first centrifuges used for human research were used by Erasmus Darwin, the grandfather of Charles Darwin. The first largescale human centrifuge designed for Aeronautical training was created in Germany in 1933.

The US 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.

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.

Geotechnical centrifuge modeling

Geotechnical centrifuge modeling is used for physical testing of models involving soils. Centrifuge acceleration is applied to scale models to scale the gravitational acceleration and enable prototype scale stresses to be obtained in scale models. Problems such as building and bridge foundations, earth dams, tunnels, and slope stability, including effects such as blast loading and earthquake shaking.

Synthesis of materials

High gravity conditions generated by centrifuge is applied in the chemical industry, casting, and material synthesis. The convection and mass transfer are greatly affected by the gravitational condition. Researchers reported that the high-gravity level can effectively affect the phase composition and morphology of the products.

Commercial applications

Standalone centrifuges for drying (hand-washed) clothes – usually with a water outlet.

Washing machines

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 the oil sands industry to separate small amounts of water and solids from bitumen

Centrifuges are used to separate cream (remove fat) from milk; see Separator (milk).

Mathematical description

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. During circular motion the acceleration is the product of the radius and the square of the angular velocity ω , 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 quantity given by the expression:

RCF = r ω 2 g

where

g is earth's gravitational acceleration, r is the rotational radius, ω is the angular velocity in radians per unit time

This relationship may be written as

RCF = 10 − 3 r mm 2 π N RPM 60 2 g

or

RCF = 1.11824396 × 10 − 6 r mm N RPM 2

where

r mm is the rotational radius measured in millimeters (mm), and N RPM is rotational speed measured in revolutions per minute (RPM).

To avoid having to perform a mathematical calculation every time, one can find nomograms for converting RCF to rpm for a rotor of a given radius. A ruler or other straight edge lined up with the radius on one scale, and the desired RCF on another scale, will point at the correct rpm on the third scale. Based on automatic rotor recognition, modern centrifuges have a button for automatic conversion from RCF to rpm and vice versa.