What is Centrifuge?
A centrifuge is a device which applies centrifugal force to continuously subject a specimen to a constant force for separation purposes, like dividing the various components of a fluid.
This is achieved by spinning the fluid at high speed in a container, which separates fluids of different densities such as cream from milk, or liquids from solids.
It causes denser substances and particles to radially move outward. At the same time, diffrencially less dense objects are displaced and move to the center.
In a laboratory centrifuge that uses sample tubes, radial acceleration causes denser particles to settle to the bottom of the tube, while lower density substances rise to the top.
A centrifuge can be a very effective filter that separates contaminants from the main body of fluid.
Centrifuges are especially rampant in the industry of manufacturing and waste processing because of their ability to sediment suspended solids or seperate liquids which are immiscible.
The same can be said for cream separators that are used in dairies. Ultracentrifuges are able provide very high accelerations and separate fine particles down to molecules of different masses on a nano-scale.
Moreover, large centrifuges are used in the simulation of high gravity environments such as in high G training for test pilots.
Other medium-sized centrifuges are used in microwaves and washing machines or even in some swimming pools for the extraction of water from fabrics. Such is the case with gas centrifuges, which seperate isotopes for the enrichment of nuclear fuel of fissile isotopes.
What is the Principle of Centrifuges?
With centrifuges, particles that are of a relatively higher capacity than the solvent will settle at the bottom, while the lighter particles will rest on top. When there is a notable density difference, the particles will segregate at a faster rate.
Making use of excelerating forces alongside gravity enables the complete suspension of particles. If the densities are much alike, the particles are time bound to stay in one place.
Further accelerating the centrifugal force will aid in enhancing the degree of particle separation as it is now possible to exploit smaller differences in density.
A centrifuge works by rotating an object around a specific point that is fixed while applying a strong force perpendicular to the axis of rotation.
Employing the sedimentation principle, the centrifuge generates centripetal acceleration, which pushes the denser particles present in the medium outward towards the periphery of the vessel.
Conversely, the lighter particles are displaced towards the center along the axis of rotation. In a laboratory setup, sample tubes are used to obtain a clearer sample where particles like scrubbed cells are denser and settle at the bottom, while less dense particles like cell debris are suspended above them.
How does centrifuge work?
A centrifuge works on the sedimentation principle which utilizes centrifugal force in conjunction with gravity to separate substances based on their density.
There are different methods to achieve the separation such as: isopycnic separation, pelleting, density gradient, phase separation, and ultrafiltration. Among those methods, pelleting is the most common one.
With this process, particles will move toward the bottom of the centrifuge tube and will form a pellet that will be separated from the supernatant, which is the remaining solution. Separation of chemicals takes place from the matrix, or aqueous solution, into solvent.
Ultrafiltration utilizes membranes for the purification, separation, and concentration of molecules. In isopycnic centrifugation, self-generating gradients are formed through sedimentation equilibrium, where particles are aligned to the surrounding solution.
Unlike more precise measures of a centrifuge’s speed like revolutions per minute, traditional centrifugation protocols dictate the relative centrifugal force and acceleration in multiples of gravitational force, which can make determining speed less accurate.
Parts of Centrifuge
Understanding the crucial parts of a centrifuge enables one to select the most appropriate type for their laboratory needs and to operate it efficiently. Each of the components plays an essential role in the functionality of the machine.
Centrifuge Rotor
The sample tubes of a centrifuge are placed in the rotor, which spins and generates the centrifugal force necessary for content separation based on density.
The rotor type, whether swinging bucket or fixed angle, affects the direction and distribution of force on the samples, which affects how separation occurs. Particularly for high-speed and ultracentrifuges, the rotor speed is vital for the separation to be successful.
Centrifuge Motor
The motor is a critical component of centrifuges as it rotates the rotor to the required speeds for efficient centrifugation.
There are different types of motors for different types of centrifuges, ranging from a few thousand RPMs in bench models to over 100,000 RPMs in ultracentrifuges.
It is evident that the reliability and accuracy of the motor affects the performance of the centrifuge because it is a crucial part in its operation, particularly in its high speed and temperature sensitive applications.
Centrifuge Tubes
Centrifuge tubes are used to hold the samples during centrifugation and are therefore specialized containers. In most cases these tubes are made of either glass or plastic; glass is better due to its resistance to chemicals and durability while plastic is cheaper.
The tube size and its material must fit within the rotor and match the type of sample being processed. Large samples may be contained in centrifuge bottles while smaller samples are placed in traditional tubes.
Control Panel
The control panel enables users to predefine parameters such as RPM, time, and even the temperature for refrigerated ones, which the centrifuge will follow. More advanced models may come with further digital displays, safety programs, and alarms.
It is evident that the control panel serves the purpose of ensuring that the centrifuge operates within the desired limits which optimizes performance while preserving sample integrity.
Types of Centrifuges
#1. Low Speed Centrifuge.
Routine sedimentation of heavy particles is performed in almost all laboratories with the aid of a standard low-speed centrifuge. They are able to achieve speeds of 4,000 to 5,000 RPM and generally remain at room temperature with limited temperature control options.
These Centrifuges come with two options of rotors, fixed angle rotors and swinging bucket rotors. Charging of red blood cells is one of the common uses of low speed centrifuges.
This charging process culminates in the formation of pellets which are further decanted to separate the supernatant.
#2. High Speed Centrifuge.
At more advanced stages of biochemical work, high-speed centrifuges become necessary when the effectiveness of sedimentation requires higher speeds and temperatures.
These high-speed centrifuges are capable of achieving up to 15,000 to 20,000 RPM. Control over temperature and speed requires extreme care as they are to be tailored to sensitive biological specimens being used.
These centrifuges usually come with 3 types of rotors; fixed angle rotors, swinging bucket rotors and vertical rotors.
#3. Basket Centrifuge.
A solid wall basket centrifuge utilizes centrifugal force for the separating of liquids or solids. The slurry feed is brought into the rotating basket and is made to accelerate to the basket speed.
For any fairly large solids or impurities, they are radially pulled away from the liquid through centrifugal force and they accumulate at the inner wall of the basket.
The purified liquid that has separated from the remains accumulates along the inner surface of the basket until it overflows.
The overflowed liquid is captured and is allowed to exit through an outlet that utilizes gravity. The basket filled with liquid maintains this cycle until it reaches a point of solid liberation and requires waste removal.
#4. Continuous Centrifuge.
This method of centrifugation allows the treatment of high amounts of substances at a high centrifugal force without the need of filling multiple centrifuge tubes, frequently restarting the rotors, or stopping the centrifuge.
For this method, collecting the supernatant or removing the step pellet that is manually gathered needs to be done.
Due to short path lengths, continuous centrifuges are able to shorten the time needed for pelleting. This design permits the effective separation of solids with a high rate of material movement through the rotor and the smooth flow of material throughout the rotor.
In addition, these centrifuges are of high capacity, which means that there are not frequent stops and starts as in conventional rotors. This efficiency leads to reduced sample manipulation and more time savings during acceleration and deceleration of the rotor.
The continuous flow rotors have the following main applications:
- Large viruses
- Mitochondria
- Tissue homogenates
- Bacteria
- Cell harvest
- Algae harvest
The sedimentation coefficient of the particles to be collected should ideally be greater than 50 s for the continuous flow rotor to be the most functional.
Thanks to its high pelleting efficiency, solid material is separated from the liquid medium far more quickly than with swinging bucket or fixed angle rotors.
This rotor is best for materials suspected to have a solid-liquid ratio of 5 – 10%. For values greater than 15%, the efficiency of these rotors is even more significant.
#5. Decanter Centrifuge.
The fitting equipment that thrives best when separating solid/liquid mixtures with a high degree of solid content is the decanter centrifuge. Extremely high centrifugal forces will separate the finely distributed solid particles from suspension.
Decanters are important in many different industries around the world. These devices are well suited for a particular application and the applications include everything from dewatering of sludge to classification of or wet classification of solids.
A decanter is a special type of sedimentation vessel which is placed on a rotating axis. Due to the action of gravity, heavier solid particles move towards the bottom of the tank where they accumulate as sediment, which can also be referred to as a pourable cup.
On the other hand a centrifuge separates solid and liquid phases using centrifugal force. With respect to a sedimentation centrifuge, stronger dense solids are pushed towards the bottom of the centrifuge bowl.
Permitting a solid sediment to form along the inner walls of the bowl. The centrifugal force can reach up to 3000g compared to the gravitational force of 1g achieved in a standard Earth environment.
#6. Industrial Centrifuge.
In the broader global context of mineral processing, industrial centrifuges are critical for classification and dewatering.
They are used in the industrial separation of solids from liquids. These centrifuges are effective in coal and copper as well as many other valuable minerals and ores.
Industrial centrifuges can be broadly classified into two basic types. These include sedimentation and filtration centrifuges.
Sedimentation centrifuges rely on centrifugal force to separate solids from liquids, while filtration centrifuges force a liquid through a filter medium.
Examples of industrial processes that utilize centrifugation include:
- Pharmaceutical and Biotechnology: the production of drugs and medicines, vaccines, and enzymatic waste processing.
- Chemical Processing: the production of raw salts, acids, oils, and some water solids.
- Waste Water Processing: the separation of treated water from mud and dirt, environmental wastewater sludges, and some other minute particles found in water.
- Fuel and Biofuel Technology: This refers to the processes involved in manufacturing ethanol, biodiesel, lube oil, as well as undergoing petrol refinement and biomass dewatering.
- Food Processing: It encompasses the putting into use of vegetable oils, milk and cheese manufacturing, as well as the processing of poultry, pork, and beef and beer, wine, and liquor clarification.
- Mining and Mineral Processing: It involves the refining and purification of tar sands, coal, calcium carbonates, precious metals, and kaolin clay.
#7. Laboratory Centrifuge.
This type of laboratory centrifuge is intended for scientific work that involves the separation of particles using centrifugal force. It separates particles due to density, size, shape, viscosity, and other factors by means of an external gravitational field.
The particles which have the larger density or bigger size settle at the bottom of the centrifuge (sedimentation), whereas the particles of lower density or smaller size move towards the center or rotation axis.
This method is used widely in cell separations, for small virus particles, cellular components, and even proteins that have been precipitated.
#8. Oil Centrifuge.
The oil centrifuge is also known as a centrifugal oil filter because of its usage in areas that involve filtering oils. The lube oil which is dirty is circulated from the machine to the centrifuge oil filter.
The oil, in turn, is pressure fed to a filter that leads to a bowl. This bowl is spun to extremely high rotational speeds of 5 000 rpm to 8 000 rpm with respect to oil inlet pump pressure.
As a result, shear force is generated that separates the remaining particles to the sides of the bowl and the clean purified oil is fed back to the machine for use. During the normal engine operation soot and other contaminants are removed continuously.
It should be noted that the method of filtration for the oil filter is not barrier-type filtration. It does not rely on filtration media for the removal of particles contaminating hydraulic or lubricant oil.
#9. Horizontal Peeler Centrifuge.
The peeler centrifuges are batch-type machines with a perforated bowl and cloth filter. As liquids are spun at very high speeds, solids shoot form onto the filter cloth and accumulate into cakes.
These cakes are then dried through spinning. Once dried, the cakes are scraped off by peeler knives, and any leftover sui generis heel is left beneath. The scraping mechanism is altered, and the high-pressure nitrogen discharge shuts off the residual.
A horizontal peeler centrifuge is automated to function as a continuous flow filtration system, but can be switched on and off to allow for handpicking controls.
#10. Inverting Filter Centrifuge
This style of the inverting filter centrifuge features a rotor that completely expels all cake residual without them being flaked off.
Like horizontal peeler centrifuge, it has a rotary drum but a different way of dealing with the cake. Instead of pulling the cake out with peeler knives, the material is expelled from the lower rim of the drum.
The force behind it is sufficient to turn the filter cloth inside out, and in doing so, wipe any residual dirt that would require other means to clean up.
This approach guarantees maximum pureness of the product while filtering in circumstances that require low operator interaction with the device.
#11. Solid Bowl Vertical Centrifuge.
Solid bowl vertical centrifuges employ centrifugal force for the separation and clarification of materials. Slurries that undergo this process are put in a container that spins.
Auxiliary components within the container multiplies the spinning/slushing action within the vessel. As the bowl turns, the stronger, denser particles are pushed towards the walls of the bowl while the liquid is remaining at the center of the bowl.
Due to the lack of openings for continuous discharge in the solid bowl vertical, these machines require periodic ‘cleaning.’ This cleaning allows the solid particles that are scraped to accumulate.
Thus these machines are unsuitable for operations that require a continuous, unbroken flow of material, as the machines need to be stopped and emptied after a fixed interval.
#12. Pusher Centrifuge.
Sometimes referred to as horizontal basket pusher centrifuges, these form filter cakes by using a metal screening sheet or slotted sieves while simultaneously employing a hydraulic piston within the machine.
A hydraulic piston pushes in alternating directions, while simultaneously washing out the cakes left in the bowl.
These machines are fitted to work non-stop and are best suited to larger workpieces with high levels of solid particles while still maintaining clean output and low moisture levels.
The pusher centrifuge type has a basket that provides centrifugal force for separating solids from the slurry. With each advance stroke, the hydraulically driven piston that rotates about the basket solos, forces the solids out.
The liquid constitutes flows through the metal mesh or slotted sieve and the solid particles get retained on the basket screen.
Benefits of Using Centrifuge
Centrifugation, as a process, has gained more popularity recently for the purpose of commercial dewatering. Below are the most common benefits of utilizing centrifuges.
- Increases workflow efficiency by reducing unpleasant odors that would otherwise be generated by dewatering using other traditional methods. This helps reduce olfactory nuisances as well as spraying deodorants around the work premises.
- If the supplied sludge is constant, the system can be set to operate automatically. To add on, there are dependable mechanical safety features and sensors that are standard equipment for reliable operation.
- Ford some sludges, there is a considerable increase in the amount of dry solids, making the centrifuges the right candidate for cleansing or purifying the substance.
- The technique is also useful for oily sludges.
- Most centrifuges are simple to operate because they have microcomputers which save rotation speeds and centrifugal forces that can be accessed by different users.
- With regard to depreciation, the process of centrifugation is inexpensive because it is automated and only requires minimal supervision from human attendants, thereby reducing labor costs.
- Provided it is used for its intended purpose, a centrifuge will last a very long time as it has a rugged design, especially the decanter.
Limitations of Centrifuge
Centrifuges are extremely helpful tools, but are also accompanied by a list of notable disadvantages.
- The determination of the lightest particles is indeed very tedious because they are separated based on either size or weight. It is very difficult to perform this process for particles that are close to having no mass because they are very hard to segregate.
- They are not particularly efficient because, consuming power for a myriad of functions, such as: maintaining temperature control (where some substances need to be heated and others cooled), and generating high degrees of centrifugal force which requires rapid rotor rotation, leading to heavy strain on the power system.
- Due to high RPMs, centrifuges create a significant amount of noise and vibrations. Since the device makes use of rotations, noise pollution and vibration are unavoidable hindering use in some locations.
- Because of malfunctions, the maintenance of these devices is both complicated and expensive. Every type of centrifuge is a complicated electronic device, so when it comes to malfunctions, expert servicing is a requirement.
- Most often a standby unit is required to take over this role when the operating machine ceases to work. Occasionally, the device is placed in an environment where it is crucial to function, like in municipal or any water treating organizations. In case the device malfunctions, one more centrifuge is required to ensure uninterrupted supply of clean water.
- Excessive use of polymers
Application of Centrifuge Machines
- Centrifugation can be used to separate a mixture of two different miscible liquids.
- This method can be employed for studying and analyzing macromolecules and their hydrodynamic behavior.
- A certain type of centrifuge enables the purification of mammalian cells.
- Together with the separation of subcellular organelles, centrifugation is also known to be very important in the fractionation of membranes and membrane bound organelles.
- Centrifugation is useful in the separation of membrane vesicles too.
- A centrifuge can also separate chalk suspended in water.
- Skimmed milk is a type of milk that contains less dissolved fats. It can be derived from regular milk through the process of centrifugation, which separates fat from milk and leaves behind the desired skimmed milk.
- The process has important applications in particle separation from air flows by cyclonic separation.
- Wine stabilization and clarification is another important application of this technique.
- When used in conjunction with other purification techniques, this is quite useful during protein separation. Also used are salting out techniques like ammonium sulfate precipitation.
- Centrifuges have a large application in forensic chemistry. In this science, it is used to separate different blood constituents from blood specimens. In addition, this method is used in some laboratories for the separation of the different constituents of urine from urine specimens.
- This method of centrifugation is sometimes referred to as differential centrifugation and is useful in locating organelles.
How Much Does a Centrifuge Cost?
A centrifuge is going to vary in cost based on the machine type, its features and size. Whether you are indending to purchase an ultracentrifug for advanced research or basic benchtop centrifuges for general lab work, one should consider both initial and the ongoing costs.
Following is the classifications of the types and relevant centrifuge’s cost ranges provided:
Small Benchtop Centrifuges:
General purpose tasks and small sample volumes can be achieved using these types of centrifuges. They are priced bewteen $1,000 and $5,000 in relation to their brand, speed, and rotor options.
Usage of benchtop centrifuges is high within academic labs, clinics, and biotech companies where use is attributed to cell sedimentation and small volume separation.
Large Capacity and High-Speed Centrifuges:
Large or high speed centrifuges are required to perform advanced sample volume and speed centrifugation and are often used within industrial and biotech laboratories.
Their cost is speculated between $10,000 and $25,000, again depending on features and spec. Temperature control and different application rotors are almost always included.
Ultracentrifuges:
These machines are highly specialized and are used to separate proteins or viruses at very high speeds (up to 600,000 x g). They are among the most expensive machines periodically coming with a price tag of around $10,000 to $50,000 or more depending on functionality
and additional features. Ultracentrifuges are used in Molecular biology, virology, pharmacology etc.
It is also necessary to mention that the total cost of ownership for the ultracentrifuge is not only its purchase price. This cost includes installation, maintenance, and operational costs. Some centrifuges require routine servicing or rotor changes which adds to the overall cost.
For instance, ultracentrifuges with freon refrigerating units are more expensive to maintain because they require the samples to be maintained at specific temperatures for longer periods. Types of Centrifuge Rotors
The rotors for centrifuge machines come in various forms, each having a different relative cost to the performance of a particular centrifuge.