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This chapter will cover the basics of centrifuges, including their design, construction, and operational principles.
A centrifuge is a scientific instrument designed to separate fluids, gases, or liquids according to their density. This separation is achieved by rapidly spinning a container filled with the material. The centrifugal forces generated during this process push the denser substances toward 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 centrifuge is designed with an aesthetically pleasing appearance and ergonomic features, including speed and temperature controls, to enhance ease of use. A typical laboratory centrifuge includes the following structural elements:
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 centrifuge rotors are categorized into angular and horizontal types. Separation occurs at high speeds but typically at lower capacities. Horizontal rotors have greater resistance compared to angular rotors. As a result, horizontal rotors are generally more suited for large capacity separation rather than high-speed separation.
The drive system consists of a motor, a drive shaft, and a damping mechanism. The motor operates on AC power, driving a sealed and lubricated drive shaft. The damping system absorbs the resulting forces, vibrations, and noise to ensure smooth operation.
The control system is managed by a microcomputer equipped with a touch panel and digital display. This unit allows users to adjust the rotation speed and centrifugal force, with the ability to save settings for future use or experiments.
The cooling system features a fully sealed, airtight Copeland Compressor unit, which includes separate circuits for both refrigeration and heating control.
The safety protection system includes features such as main current protection, high-temperature protection, high-speed protection, balance protection, and door cover protection.
In a solution, particles with a higher density than the solvent will settle at the bottom, while lighter particles will rise to the top. When there is a significant density difference, the particles will separate more quickly. If the densities are similar, the particles will remain in place. To enhance separation, a stronger centrifugal force can be applied in addition to the gravitational force, allowing even small density differences to be exploited for more effective particle separation.
A centrifuge operates by rotating an object around a fixed axis and applying a strong force perpendicular to this axis. Utilizing the sedimentation principle, the centrifuge generates centripetal acceleration, which pushes denser substances and particles outward toward the edges of the vessel. At the same time, lighter particles are displaced inward towards the center. In a laboratory setup, sample tubes are used where denser particles settle at the bottom, while less dense particles rise to the top of the tube.
A centrifuge operates on the principle of sedimentation, utilizing both gravitational and centrifugal forces to separate substances based on their densities. Various separation techniques include isopycnic separation, pelleting, density gradient separation, phase separation, and ultrafiltration. Among these, pelleting is the most commonly used method. In this process, particles concentrate at the bottom of the centrifuge tube, forming a pellet that is separated from the remaining solution, known as the supernatant. During this separation, chemicals transition from a matrix or aqueous solution into a solvent.
In ultrafiltration, molecules are purified, separated, and concentrated using a membrane. Isopycnic centrifugation involves creating a self-generating gradient through sedimentation equilibrium, which aligns the particles with the surrounding solution. Traditional centrifugation protocols specify the relative centrifugal force and acceleration in multiples of gravitational force, rather than relying on rotational speed (e.g., revolutions per minute), which can be less precise.
Purchasing a laboratory centrifuge represents a significant investment. When selecting a centrifuge, it is important to consider the following key characteristics:
This refers to the spinning speed of the centrifuge and the resulting force it generates. The relative centrifugal force (RCF) is crucial, as it indicates the actual force exerted on the substances in the containers. Many centrifuges can reach speeds of up to 14,000 RPM and produce an RCF of 18,624.
Each model of centrifuge comes with a specific set of rotors and accessories. The rotors have additional characteristics that should be considered, which include:
The chosen centrifuge should offer rotor options that are compatible with the selected vessels and are available in the appropriate sizes.
Centrifuges typically offer different rotor styles, including fixed-angle and swing-out rotors. Some models provide options for one type, both types, or additional styles such as vertical rotors, PCR strip rotors, and microlite rotors.
It is important to consider and verify whether the centrifuge can achieve the required maximum relative centrifugal force (RCF).
Some centrifuges operate at room temperature without cooling or heating features. In contrast, centrifuges with cooling options can lower the temperature to as low as -10°C or -20°C.
While centrifuges come with several safety risks, they are typically equipped with features designed to protect users and their environment. Some of these safety features include:
This feature ensures that the lid remains securely closed while the centrifuge is in operation, preventing accidental openings.
This feature monitors the balance of the centrifuge and halts operation if excessive vibration is detected, ensuring safe and stable performance.
These features help prevent spills or leaks of biohazardous chemicals or substances during operation.
This feature identifies the installed rotor and ensures that it does not exceed its maximum allowable operating speed.
Another important consideration is the placement of the centrifuge. As a vibrating mechanism, it requires careful positioning to ensure stability. Additionally, since the machine generates significant noise, it should be placed in a location where the sound can be adequately dampened.
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 cover the various classes and types of centrifuges.
Centrifuges are generally categorized into two main types: low-speed centrifuges and high-speed centrifuges.
Standard low-speed centrifuges are commonly used in laboratories for routine sedimentation of heavy particles. They typically operate at speeds ranging from 4,000 to 5,000 RPM and usually function at room temperature, with limited temperature control options. These centrifuges are equipped with two types of rotors: fixed-angle rotors and swinging bucket rotors. A typical application for low-speed centrifuges is the sedimentation of red blood cells, where they are concentrated into pellets, and the supernatant is then separated by decanting.
High-speed centrifuges are employed in more advanced biochemical applications, where higher speeds and temperatures are necessary for effective sedimentation.
High-speed centrifuges can reach speeds of up to 15,000 to 20,000 RPM. Operators carefully manage both the temperature and speed to meet the specific needs of sensitive biological samples. These centrifuges typically use three types of rotors: fixed-angle rotors, swinging bucket rotors, and vertical rotors.
The various 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 purified liquid accumulates along the interior surfaces and eventually spills over the edge, where it is then collected and allowed to exit through a gravity-fed outlet. This cycle persists until the basket is free of solids and requires emptying.
This centrifugation method allows for the processing of large volumes at high centrifugal forces without the hassle of filling numerous centrifuge tubes or frequently starting and stopping the rotors. With this approach, you'll either need to collect the supernatant or remove the pellet that has formed.
Continuous centrifuges feature brief path lengths to reduce the time needed for pelleting. This design ensures efficient solid separation and allows for a swift flow of material through the rotor. Additionally, these centrifuges have large capacities, which eliminates the need for frequent starts and stops, unlike traditional rotors. This efficiency minimizes sample handling and reduces delays caused by rotor acceleration and deceleration.
Common uses for continuous flow rotors include the following:
The continuous flow rotor is most effective when the sedimentation coefficient of the particles to be collected exceeds 50 s. With its high pelleting efficiency, this rotor separates solid material from the liquid medium more rapidly compared to swinging bucket or fixed angle rotors. It is particularly suited for samples with a solid-to-liquid ratio of 5 – 10%. For ratios above 15%, the efficiency of these rotors is even greater.
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 type of sedimentation vessel mounted on a rotating axis. Heavier solid particles migrate to the base of the tank due to gravity, accumulating as sediment. In contrast, a centrifuge separates solid and liquid phases using centrifugal force. The higher density solids are forced to the bottom of the centrifuge bowl, forming a sediment along the inner walls. The centrifugal force can be up to 3000 times greater than the force of gravity, compared to the standard 1g of gravitational force.
Another type of centrifuge is the high-capacity, high-speed centrifuge. This substantial machine is floor-mounted and is typically used in laboratories that handle large volumes on a daily basis. It features a program memory capable of storing around 100 protocols and includes automatic rotor recognition. Additionally, it comes with acceleration and deceleration ramps, a spin pulse function, and lockable castors for safety. This centrifuge operates based on generating a specific centrifugal force while also considering gravitational force. The sedimentation rate is influenced by the applied centrifugal force, which acts radially outward, and depends on the following factors:
Industrial centrifuges play a crucial role in dewatering and classifying within the global mineral processing sector. They are employed to separate solids from liquids across various industrial materials. These centrifuges are highly effective for processing a diverse array of minerals and ores, including coal, copper, and other valuable metals.
Industrial centrifuges are generally divided into two primary categories: sedimentation and filtration centrifuges. Sedimentation centrifuges utilize centrifugal force to separate solids from liquids, whereas filtration centrifuges apply force to push a liquid through a filtration medium.
Examples of industrial processes that utilize centrifugation include:
This laboratory centrifuge is designed for experiments that require particle separation through centrifugal force. It separates particles based on factors such as density, size, shape, or viscosity by applying an artificial gravitational force.
Particles with higher densities or larger sizes migrate towards the bottom of the centrifuge (sedimentation), whereas those with lower densities or smaller sizes are pushed outward from the axis of rotation. This method is commonly used to separate cells, cellular components, tiny virus particles, and precipitated proteins.
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 understand that the oil filter does not use a barrier-type filtration method. Instead, it does not depend on filtration media to remove particles contaminating hydraulic or lubricant oil.
Peeler centrifuges are batch-type units featuring perforated bowls with cloth filters. During operation, liquids are spun at high speeds, causing solids to accumulate on the filter cloth and form cakes. The cakes are then dried through a spinning process. Once dried, peeler knives scrape off the cakes, and any remaining residual material, known as heel, is removed. The scraper's position is adjusted, and high-pressure nitrogen is employed to clear the residual material.
A horizontal peeler centrifuge operates as a continuous flow filtration system and can be set to either automatic or manual control modes.
The inverting filter centrifuge is designed to fully expel cake material without leaving any residual heel. Like horizontal peeler centrifuges, it features a rotating drum. However, instead of using peeler knives to remove the cake, the material is pushed out through the bottom of the drum. This pushing action is so thorough that it inverts the filter cloth, effectively clearing any remaining residue and eliminating the need for mechanical cleaning.
This method ensures the highest level of product purity and is ideal for handling challenging filtration processes and situations where minimizing operator contact with the product is crucial.
Solid bowl vertical centrifuges are designed for clarification by using centrifugal force to separate mixed materials. The slurry is introduced into the spinning bowl, and as the bowl’s speed increases, the centrifugal forces drive the dense, solid particles against the interior walls of the bowl, leaving the liquid phase separated.
Because solid bowl vertical centrifuges lack openings for continuous discharge, they require periodic maintenance to remove accumulated solids. This is done during the scrape cycle. Consequently, these centrifuges are not suitable for continuous flow operations as they must be halted and emptied at intervals.
Pusher centrifuges, also known as horizontal basket centrifuges, are equipped with metal screening sheets or slotted sieves and utilize a hydraulic piston. These centrifuges filter liquids to form filter cakes, with the hydraulic piston providing an oscillating pushing motion to either move the cakes out of the bowl or wash them away. They are designed for continuous operation and are particularly effective for handling large volumes of solids where high purity and low residual moisture are required.
In a pusher centrifuge, the basket generates centrifugal force to separate solids from the slurry. A hydraulically driven piston, which rotates with the basket, pushes the solids out with each forward stroke. Solids accumulate on the basket’s screen, while liquids pass through the metal screen or slotted sieve.
This chapter will explore the various applications and advantages of centrifuges.
Centrifugation encompasses a broad range of laboratory and industrial applications. This process is used to separate particles in a solution based on their viscosity, shape, size, and density. Some of the common uses of centrifuges are outlined in this section.
Centrifugation can separate mixtures of two immiscible liquids by exploiting their density differences. It is also employed in analyzing macromolecules and their hydrodynamic properties. Additionally, it is used to fractionate various subcellular organelles and analyze membranes. Specialized centrifuges are used to purify mammalian microcells.
In chalk production, centrifugation is used to remove water from the chalk (dewatering). Similarly, the process is employed to produce skimmed milk by separating the fat from regular milk. The centrifuge effectively separates the fat, resulting in milk with reduced fat content.
Another practical application of centrifugation is in the stabilization and dewatering of wine. During the aging process, wine is processed in a centrifuge to enhance its stability and improve its quality. Additionally, centrifugation is utilized in forensic chemistry to analyze complex samples such as blood, DNA, urine, and other substances that are difficult to detect.
In recent times, centrifugation has increasingly dominated the commercial dewatering market. The following are some of the key advantages of using centrifuges.
Although centrifuges are highly useful, they do have their share of significant drawbacks.
When considering the purchase of a centrifuge, it is important to evaluate several factors to ensure you choose the right device for your needs.
The rotor speed is an important factor to consider, as it determines the level of centrifugal force. If your work demands very high centrifugal forces, you should look for a centrifuge that offers high revolutions per minute (RPM).
The centrifuge should offer programmable settings, allowing it to operate automatically without the need for manual adjustments or constant supervision.
An ideal centrifuge should be equipped with sensors to detect rotor imbalances, preventing spillage and disruptions during the separation process.
The buyer should also consider the type of display required, such as Liquid Crystal Display (LCD), AMOLED, or QLED, and ensure it meets their specific needs.
Considering the overall dimensions of the device is crucial based on its intended use. Industrial centrifuges can be as wide as 5 meters, while medical centrifuges may be as compact as 30 centimeters.
The weight of the centrifuge is another important factor. Some scientific centrifuges are compact and lightweight, whereas others can weigh up to 200 kg.
The required operating voltage depends on the centrifuge’s location. Some areas use a 240V residential plug, while others require higher voltages for industrial applications. Large industrial centrifuges necessitate more voltage and are unsuitable for residential settings.
Centrifuges with high power consumption need a reliable and robust energy source to operate effectively without interruption.
Before selecting a centrifuge, it’s essential to understand the specific tasks it needs to perform. Different centrifuges are designed for various purposes; some are suited for lighter tasks, such as those in pharmaceuticals, while others are built for more demanding industrial applications.
Floor standing centrifuges are designed for high-speed and high-capacity operations, often reaching speeds of up to 150,000 rpm and generating forces up to 300,000N.
Benchtop centrifuges are compact and portable, typically suited for smaller-scale operations. Examples of these are provided below.
Micro centrifuges are intended for small volume tubes, offering relatively low speeds and forces. Some can handle both high and low temperatures, though many only provide one type of temperature control.
Multipurpose centrifuges are versatile, providing a range of options and capabilities for various 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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