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This chapter will discuss what centrifuges are, their construction, and how they function.
A centrifuge is a scientific device that is used to separate fluids, gasses, or liquids based on the density of the subject. The separation is acquired by spinning a container with the material at a very high speed. The resulting forces created will then cause the heavier materials to travel to the bottom of the container.
A laboratory centrifuge is a device that makes use of a centrifugal force created by a rotating mechanism of rotors to separate the materials of different particle sizes in a solution or suspension. It is commonly used in pharmaceutical, food, environmental protection, and chemical tests.
The whole machine has a beautiful appearance, ergonomic design, speed control, temperature control, and other user-friendly functionalities that make the separation technique easier. The structure of a basic laboratory centrifuge has the following features in its construction:
This is a frame composed of a body box with a door cover and a container on the inside. The whole box is made up of steel and the inside tank is made of stainless steel.
The rotors are divided into several angular rotors and some horizontal rotors. Separation is done at high speeds and not at large capacity. The resistance in the horizontal rotors is larger than that in the angular rotor. Generally, it is not suitable for the high-speed separation but rather the large capacity separation is recommended.
The drive system is run by a motor, a drive shaft, and a mechanism for damping. The motor runs on an AC frequency and turns a sealed and lubricated drive shaft and the resultant forces and vibrations and noises are absorbed by the damping system.
A control system is achieved through a microcomputer with a touch panel and a digital display. This control unit can be used to set the rotating speed and the centrifugal force. The settings can be stored in memory for future or any experiments that may follow.
The cooling system adopts a wholly sealed air tight cooled Copeland Compressor unit which has circuits for both refrigeration and heating control.
This is a safety protection system that has a main current protection, high temperature protection, high speed protection, balance protection, and the door cover protection.
In any given solution, the particles whose density is greater than that of the solvent sink (sediment), the lighter particles will float up to the top. If the difference in density is notably greater the particles will move faster and if there is no difference, they will remain steady. A much more powerful centrifugal force can be applied on top of the usual gravitational force to take advantage of even the tiny differences in densities so as to separate the various particles in a solution.
A centrifuge is a mechanism that puts an object to be rotated in a fixed rotation axis and applies a strong force perpendicular to the axis of spin. It also functions using the sedimentation principle and the centripetal acceleration will cause the denser substances and any particles to shift and move outwards of the vessel. Almost simultaneously, the particles that weigh less are displaced and move to the center of the vessel. In a laboratory set up, the sample tubes are used and the denser particles settle at the bottom of the tube and those that are less dense rise up to the top of the vessel.
A centrifuge works through the principle of sedimentation which is under the influence of gravitational force and centrifugal force and thus separating the substances based on their densities. The various types of separation techniques are known as isopycnic technique, pelleting technique, density gradient technique, phase separation and ultrafiltration techniques. Pelleting is the most common application of most centrifuges. At the base of the tube of the centrifuge, the particles are concentrated as a pellet and they are separated from the remaining solution and it is known as supernatant. Chemicals are converted during the separation phase from a matrix or aqueous solution into a solvent.
When it is done through ultrafiltration, the molecules are cleaned/purified, separated and then concentrated through the use of a membrane. In Isopycnic centrifugation a self-generating gradient is used and it is established through sedimentation equilibrium. This method will concentrate the analysis matches with those in the surrounding solution. The protocols for centrifugation classically specify the relative force (centrifugal) and the acceleration degree in multiples of gravitational force. The use of rotational speed (like the use of revolutions per minute (rpm)) is often not precise.
A laboratory centrifuge is usually quite a huge investment if undertaken. The following are some of the main characteristics to be considered when choosing a centrifuge.
This entails how fast the centrifuge will spin and how much resultant force will be exerted. Relative Centrifugal force is much more important to consider as it will be the resultant force that will be exerted on the substances in the vessels. Most centrifuges have an RPM of up to 14 000 and an RCF of 18 624.
Each centrifuge model has a limited number of rotors and connected accessories. The rotor has other characteristics to follow and they are as follows:
The selected centrifuge should have rotor options that are compatible with the vessel of choice and appropriate sizes.
The styles available are usually fixed angle and swing out rotors and the centrifuges may have alternatives for one or the other counterpart and at times both. Some adopt vertical rotors, PCR strip rotors, and microlite rotors.
The maximum RCF of the centrifuge should be considered and noted if the required is achievable.
Several types of centrifuges operate at room temperature without any cooling or heating options of any kind. Cooling types usually drop the temperature to down to -10O C or -20O C
These devices also come with numerous safety risks but they are usually equipped with safety features to protect users and their surrounding environment. Some features are as follows:
This feature prevents the lid from accidentally opening when the centrifuge is running.
This feature detects when the centrifuge is not balanced properly and also puts the centrifuge on hold if the level of vibration is increased.
These prevent spillages or leakages of biohazard chemicals or substances during the process
This feature detects which rotor is installed and makes sure that the rotor does not achieve speeds greater than the maximum operating speed.
One other notable factor to keep in mind is the location where it will be placed because the device is a vibrating mechanism so its placement is very important to note down. The machine also lets out quite some noise so the location should be in such a way that the sounds will be muffled.
There are two basic components in all centrifuges which are the rotor assembly and the electrical motor. The rotor assembly is the part that is responsible for generating the spin and transferring the rotational motion. The motor converts electrical energy into mechanical energy and kinetic energy.
This chapter will discuss the different classes and types of centrifuges.
There are two main categories of centrifuges and that is low speed centrifuges and high speed centrifuges.
Most of the laboratories make use of standard low speed centrifuges for the routine sedimentation of those heavy particles. The centrifuges have a maximum speed of up to 4000 to 5000 rpm. Basically, they are operated at room temperature and there are limited cases of temperature control. There are two types of rotors incorporated in these types of centrifuges and these are fixed angle rotors and swinging bucket rotors. A common use for these low speed centrifuges is the sedimentation of red blood cells to the extent they are tightly packed into pellets and the supernatant is divided by decanting.
Higher speed centrifuges are used in a lot of more sophisticated biochemical uses where higher temperatures and greater speeds are required for the sedimentation process.
The high speed centrifuge can go as fast as 15,000 to 20,000 rpm. The operator of the device cautiously controls the temperature and the speed of the instrument to suit the requirements desired by the sensitive biological samples. Three types of rotors are used in these types of centrifuges: fixed angle rotor, swinging bucket rotor, and vertical rotors.
The different types of centrifuges include:
A solid wall basket centrifuge makes use of centrifugal force to promote the separation of liquids or solids. The slurry feed is brought into the rotating basket and it is made to accelerate to the basket speed. Any rather large solids or contaminants are pulled away radically from the liquid due to the centrifugal force and they collect along the inner wall of the basket.
The clarified liquid then builds up along the inside of the walls and flows up and over the wall where it then collects and is discharged by gravity through an outlet opening. The process continues until the basket does not contain any solids and must be emptied.
In this type of centrifugation, large volumes of materials are centrifuged at high centrifugal forces without the filling tedium and decanting a lot of centrifuge tubes or repeatedly starting and stopping the rotors. When using this type of centrifuge, there will be a supernatant that needs to be collected or there is a pellet that needs scrapping.
Continuous centrifuges have short path lengths to minimize overall pelleting time. Therefore, they efficiently pellet the solids and facilitate a rapid material flow throughout the rotor. Also, they have large capacities and therefore they do not need to be started and stopped repeatedly as other conventional rotors. This saves time by minimizing the sample handling and minimizing the time lost in having to wait for rotor acceleration or deceleration.
Major applications for the continuous flow rotors include the following:
It is best to make use of the continuous flow rotor when the sedimentation coefficient of the particles that need to be collected is greater than 50 s. Since the rotor has a high pelleting efficiency, the solid material is separated from the liquid medium faster than a swinging bucket or a fixed angle rotor. The sample's solid to liquid ratio is low at 5 – 10 %. Above a solid or liquid proportion of 15%, the rotors tend to be very efficient.
When it comes to separating solid or liquid mixtures with a very high solid content, the decanter centrifuge is the best choice. The finely distributed solid particles are separated out of the suspension by very strong centrifugal forces. Decanters thereby play a pivotal role in a wide variety of industries worldwide. They are optimally adapted for a specific application and the applications range from dewatering of sludge to the classification of or wet classification of solids.
A decanter is a sedimentation tank that is wrapped around an axis. The solid particles that are heavier than the liquid move to the tank base by gravity and form a sediment at the bottom. The solid and liquid phases of a centrifuge are separated by centrifugal acceleration. The solid particles because they have a higher density and therefore drop down to the bottom and a sediment forms inside the centrifuges` bowl inner walls. The force of the centrifuge generated is larger and is up to 3000g instead of the usual 1g of gravity.
Another type of centrifuge is a large capacity high speed centrifuge. This large machine is grounded on the floor and is mostly designed for large volume daily working laboratories. It has a program memory of about 100 protocols and has automatic rotor recognition. The centrifuge is equipped with acceleration or deceleration ramps, a spin pulse function and castors that can be locked for safety reasons. The centrifuge works on the principle generation of a desired centrifugal force and also takes in gravitational force. The rate of sedimentation depends on applied centrifugal force G that is radically directed outwards and it depends on the following:
Industrial centrifuges are used to dewater and classify applications in the global mineral processing industry. They are used to achieve solid or liquid separation from a range of industrial substances. They are very suited for a very wide range of minerals and ores like coal, copper, and other precious metals.
Industrial centrifuges are classified into two main categories namely sedimentation or filtration type centrifuges. Sedimentation centrifuges use centrifugal force to split solids from liquids, while filtration centrifuges use force to pass a liquid substance through a filtration media.
Some industrial processes that make use of centrifugation are:
This is a laboratory build centrifuge meant for lab experiments that require separation of filtration through the use of centrifugal devices. The centrifuge is used to separate particles based on their density, size, shape, or viscosity of the substance by applying an artificial gravitational force.
Materials with higher densities or particle size move to the bottom (sedimentation) while those with lower particle size or density will move away from the axis of centrifugation. Usually the cells, cellular components, very small virus particles, and precipitant proteins are separated this way.
The oil centrifuge is also referred to as a centrifugal oil filter since it is used in areas where oils need to be filtered. The dirty lube oil circulates from the machine into the centrifuge oil filter. The oil is then pressure driven through the filter into a bowl. The bowl is then spun at very high speeds of up to 5 000 rpm to up to 8 000 rpm with respect to the oil inlet pump pressure. The resultant gravitational force draws any particles to the exteriors of the bowl walls while the clean purified oil passes back into the machine for usage. Soot and other contaminants are repeatedly removed during regular operation of the engine.
It is important to note that the oil filter is not a barrier-type filtration mechanism and therefore it does not rely on any filtration media to eradicate the hydraulic or lube oil contaminated particles.
Peeler centrifuges are batch centrifuges with perforated bowls that have cloth filters. Liquids are centrifuged at high speeds forming solids on the filter cloth in the shape of cakes. A spinning process is used to dry the cakes. Once the cakes are dried, they are scraped off by peeler knives. After the scraping process, residual heel remains and must be removed. The position of the scraper is changed, and high pressure nitrogen is used to remove the residual.
A horizontal peeler centrifuge is a continuous flow filtration system that can be switched between automatic control and manual control.
The design of an inverting filter centrifuge is the complete removal of the cake material without leaving any residual heel. Like a horizontal peeler centrifuge, an inverting filter centrifuge has a rotating drum. Instead of having the cake removed using peeler knives, the cake is pushed out of the bottom of the drum. The pushing process extends so far out that the filter cloth is inverted such that it empties any possible residual, which removes the need for any form of mechanical cleaning.
The process of inverting filter centrifuges ensures the highest possible product purity. They are used with difficult to filter products and where it is necessary to avoid contact between products and operators.
Solid bowl vertical centrifuges are clarification centrifuges that use centrifugal force to separate mixed materials. The slurry is loaded into the spinning bowl. As the speed of the bowl increases, the g forces remove the dense and solid materials from the liquid and collect on the interior of the wall of the bowl.
Since a solid bowl vertical centrifuge does not have any openings, it has to be periodically opened and scraped out. This process is completed during the scrape cycle. Solid bowl vertical centrifuges are not continuous flow centrifuges due to the fact that they have to be stopped and scrapped out periodically.
Pusher centrifuges, known as horizontal basket centrifuges, have metal screening sheets or slotted sieves with a hydraulic piston. They are able to filter liquids to form filter cakes using oscillating pushing motion from the hydraulic piston to move the cakes out of the bowl or have them washed away. Pusher centrifuges are continuous operation centrifuges designed for high volume solids where there are demands on the solids purity and residual moisture.
The basket of a pusher centrifuge provides the centrifugal force to remove solids from the slurry. A hydraulically driven piston that rotates with the basket pushes the solids out of the basket with each of its forward strokes. Solids are deposited on the screen of the basket while liquids drain through the basket’s metal screen or slotted sieve.
This chapter will discuss the applications and benefits of centrifuges.
The process of centrifugation consists of a wide spectrum of laboratory and industrial applications. The process is used to separate the dissolved particles that will be in a solution based on their viscosity, shape, size, and density. Some common applications of the centrifuges are described in this segment.
It is used to separate a mixture containing two dissimilar miscible liquids. Centrifugation takes advantage of the different densities of the two liquids and thus achieving the separation. The technique is also used to aid in the analysis of macromolecules and their special hydrodynamic properties. Another centrifugation application is the fractionating of numerous subcellular organelles and the analysis of membranes. Mammalian microcells are purified with the aid of a special type of centrifuge.
Chalk manufacturing is completed by centrifugation as the water is separated from the chalk (dewatering). Skimmed milk is another form of milk that has a lower quantity of dissolved fats. Skimmed milk is obtained from regular milk and this is accomplished through centrifugation. The centrifuge separates the fat in the milk leaving it skimmed in the bowl.
Another practical application is in the stabilization and dewatering of wine. Aging wine is passed into a centrifuge to help stabilize it to make it more profitable. The field of forensic chemistry also makes use of centrifugation as this technique can be used to analyze blood, DNA, urine and other samples hard to notice.
The centrifugation process has vastly taken over the commercial dewatering market in recent times. The following are some advantages of centrifuges.
Despite being very useful mechanisms, the centrifuges do come up with their significant share of drawbacks.
There are numerous factors to consider when looking into acquiring a centrifuge and these must be considered so that you don’t purchase the wrong device.
The speed of the rotors should be considered so that the experimenter knows in the event that their line of work requires a very high centrifugal force and therefore they should consider looking for a centrifuge with high up revolutions per minute.
The centrifuge should also have an option for setting desired settings so that it can be left to function automatically without having to manually change configurations or need human supervision.
An ideal centrifuge should have sensors that detect any rotor imbalance to avoid spillage and disruptions of separating procedures.
The buyer should also look into the display type they require ranging from Liquid Crystal Display (LCD), AMOLED display or QLED display and also specific lines of display.
The overall dimensions of the whole device are worth considering depending on how or what it is to be used for. Some industrial centrifuges are up to 5m wide whilst some medical centrifuges are no wider than a 30cm ruler
The overall weight again is also important to take note. Some scientific centrifuges are travel size and are very light whilst others weigh up to 200 kg.
Depending on the location of where the centrifuge needs to be used, functional voltages need to be considered. Some areas have a plug voltage of 240V (residential) while some have a larger working voltage (industrial). A large industrial centrifuge cannot be used in a residential area as it requires a lot of voltage for it to barely function.
A high-power consuming mechanism requires a sturdy and reliable energy source so that it functions properly without failing.
It is important to take note of the work that needs to be done before getting into acquiring a centrifuge. There are many types of centrifuges and each is designed for its specific task. Some are light task centrifuges like those used in pharmaceuticals whilst others are robust like those used in the industries.
Floor standing centrifuges are those that are placed on the ground and are usually meant for high speed and high capacity functions. They can attain speeds of up to 150 000 rpm and create a resultant force of up to 300 000N.
These are centrifuges that are small and portable and following are typical examples.
They are meant for low volume tubes and provide a relatively low speed and force. Some are capable of both high and low temperature controls but most have just one functionality.
Multipurpose centrifuges offer a wider variety of options and are known to be versatile as they can perform multiple tasks.
A centrifuge is a scientific device that is used to separate fluids, gasses or liquids based on the density of the subject. The separation is acquired by spinning a container with the material at a very high speed. The resultant forces created will then push out the heavier materials to the extreme outside of the container.
In summary, centrifuges are very important devices that help us in our day to day lives as they can be used in various walks of life. Ranging from laboratories, pharmaceuticals, biology and chemical technology experiments, mining, and industrial processes. There are also many different types of centrifuges with different capabilities. These centrifuges, though important, do come in with a notable number of drawbacks that need to be considered. They have a wide range of specifications that need to be considered before acquiring one.
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