Centrifuges

View A Video on Centrifuges - A Quick Introduction

Capable of creating forces up to twenty thousand times that of normal gravity, centrifugal separators are essential to both isolating suspensions and isotope separating. Basket centrifuges accomplish this by containing materials in an enclosed space that is fixed around a constant axis. Spinning the suspensions and solutions at high speeds around this central point increases the pull of gravity causing denser particulates to separate out first creating layers of materials based on specific gravities. Beyond separation industrial centrifuges are used for product washing, extraction and purification. Laboratory centrifuges perform the same functions on a much smaller scale and are often implicated in both preparative and analytical contexts. Both applications often utilize high speed centrifuges or ultracentrifuges which allow for faster sorting and accumulation of elements. While batch operations are more common to laboratory centrifuge use, industrial mechanisms such as gas centrifuges and oil centrifuges are often continuously run to allow for increased productivity. Decanter centrifuges utilized in water treatment applications are likewise used in continuous operation to accommodate large facility and city water supply. For both batch and continuous centrifuges balance is an important consideration as even slight imbalances will throw off the equilibrium of the spinning cycle inflicting unnecessary and potentially dangerous stresses on the device. This should be considered when purchasing new or used centrifuges. Manufacturers and retail associates can help consumers determine the necessary centrifuge requirements for a given application.

Centrifuges can be used in industrial, commercial and residential settings. They are used to separate particles from solutions based on their size, shape, density and viscosity. Aerospace, medical, chemical, seismology, biotechnical and other research or explorative industries and applications utilize the unique properties of centrifuge operation to study everything from astronaut and pilot physiology under extreme conditions to virus and serum analysis in controlled environments. More accessible applications include the use of these mechanisms in washers and dryers to extract not only water, but dirt and debris as well. Many food and beverage processing industries also utilize the purification and degumming capabilities of centrifuges in the production of wine, beverages, edible oil products such as olive oil and dairy products such as cheese, milk and cream. Gas centrifuges in particular are used to enrich uranium for the production of nuclear energy. These devices are carefully monitored by several national and international agencies as they provide the possibility for nuclear weaponry. Pharmaceutical, agriculture and textile industries also utilize centrifuges to varying degrees in both daily and specialized operations. Centrifuges do not follow the idea that one size fits all because they are not mass produced and come in many different shapes, sizes and uses and are selected for a single and known application. With uses abound, the size and degree of force that a specific device can exert will help to determine its applicability and usage.

In any of these applications the basic function of the centrifuge remains relatively the same. An enclosure or arm-like apparatus is attached to a fixed central axis about which it can move in rotary motion. Often made of metal or plastic, this is what holds the items or materials to be isolated, analyzed or tested. Industrial centrifuges have a large cavity for materials while laboratory capacities are much smaller. Many small or micro-centrifuges hold several vials in order to process multiple samples at once. In some applications the solutions and suspensions are secured and in place prior to rotation while others require they be introduced during acceleration. An attached motor facilitates spinning. Force is applied to the axis perpendicular to the central axis in order to promote circular motion. This movement creates what is known as centrifugal force. The acceleration rather than speed is the integral consideration as centrifuges with different diameters will have variable acceleration even at the same speeds. At high enough acceleration the rotational pull causes more dense substances to separate out first in a radial direction. Heavier particles collect around the walls or the bottom of the device while lighter particles move up. This accelerated force can be used to augment normal gravitational pull to cause layered sedimentation based on mass or in the case of screening centrifuges drive the materials through a filtration sieve in order to facilitate particulate separation.

Centrifuges may be categorized by style of design as well as purpose. By purpose, there are two types of centrifuges: preparative and analytical. The former encompasses all those devices designed to sort or isolate specific particles or materials from slurries or liquids. Differential models separate particles from slurries or other particulates while density centrifuges separate types of fluid. Analytical centrifuges are more commonly used for inquisitive or research based applications as they measure the physical properties of a suspension or solution. Common data procured include molecular weight, sedimentation coefficients and macromolecular behavior. Designs are far more variable than purpose. Basket models are popular as are bowl models which may be classified as tubular or chamber types. Tubular bowl centrifuges use a tube-like enclosure that is much longer than it is wide. Chamber centrifuges are similar but use a series of bowls to progressively sort sediment. In addition to design and purpose, the acceleration or speeds a given centrifuge is capable of achieving are also important to consider. Acceleration is measured in G or gravitational force while speed is measured in RPMs or revolutions per minute. Additional application specific considerations include particulate size, sorting needs, slurry density, material viscosity and overall size. Centrifuge operation is relatively simple to understand on a basic level, though the complex physics of the devices should be carefully understood before use.

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Types of Centrifuges

• Basket centrifuges are the basic types of filtering centrifuges and consist of a basket rotating around the vertical axis.
  
• Centrifugal separators utilize centrifugal force to isolate particles and materials in heterogeneous slurries, suspensions and solutions. These devices not only separate elements from liquid, solid and gaseous mixtures; but may also be used to wash, extract and purify materials.
• Continuous centrifuges operate non-stop during a separation process, and generally contain intake and output channels to prevent buildup and ensure proper separation. Continuous centrifuges are often used in water treatment plants.
  
• Decanter centrifuges utilize centrifugal force to speed the dewatering or thickening of slurries and suspensions in many industrial and commercial applications.
• Disk centrifuges are used primarily for separating two liquid phases, but are also suitable for clarifying if the solid material content in liquids is small.
  
• Gas centrifuges are highly specialized devices used for the separation of isotopes in gaseous materials and mixtures. Uranium enrichment is one of the most notable and common application for this specific type of centrifuge.
• High speed centrifuges accelerate quickly and can be used for fast extraction, washing and separation purposes.
  
• Industrial centrifuges are wide ranging in styles and are used for the separation of larger volumes of liquid/solid from one another.
  
• Laboratory centrifuges are an apparatus used in the laboratory for separating substances of different density or particle size, when suspended in a fluid, by spinning them about an axis in a suitable container. Used in biotechnology and other chemical industry labs.
  
• Mini centrifuges are used for the separation of small samples in a laboratory setting, which includes the separation of DNA, RNA and proteins that need isolation. Mini centrifuges are sometimes referred to as a bench top centrifuge.
  
• Oil centrifuges are devices designed predominantly for the filtration and separation of contaminants suspended in oils. Some such centrifuges may be used in instances where oil is the contaminant, but more often industries rely on these devices to clarify industrial, edible and hydraulic oils which may carry undesirable solid, gaseous or liquid materials as a result of extraction or continued processing.
• Pharmaceutical centrifuges are used for the sterile processing of drugs and the separation of other fine chemicals in the pharmaceutical industry. This type of lab centrifuge deals with drugs only.
  
• Ultracentrifuges are a high speed centrifuge used to determine the relative molecular masses of large molecules in high polymers and proteins. These can be very dangerous if maintenance is not performed regularly because of the high speeds it reaches.
• Used centrifuges are refurbished devices designed for the separation of particles and isotopes, sedimentation and suspension isolation of materials and objects based on mass, specific gravity, size, shape or density.

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Centrifuges Terms Beach - In a bowl centrifuge, the dewatering part of the bowl. It is conical in shape. Bowl - The area of a bowl centrifuge which rotates and separated solids collect. Bowl Strips - Small, thin strips of metal or other material attached to the inside diameter of the bowl used to help the settled solids convey out of the machine and to protect the inside diameter of the bowl from wear by locking in a layer of settled solids. Cake Slides - The dryness or wetness of the discharged solids (solids phase). Usually measured as percent (%) by weight. Centrate - The liquid (light phase) which is separated from the heavy solids (heavy phase) during the classification/clarification process, then exits through the effluent head. Centrate is also referred to as "mother liquor," "effluent" and "overflow." Centrifugal Force - An outward force on a body rotating about an axis, assumed equal and opposite to the centripetal force and postulated to account for the phenomena seen by an observer in the rotating body. Chatter Torque - This type of torque is generated by settled solids, resisting conveyance. Clarification - Clearing action that occurs during waste water treatment when solids settle out. This is often aided by centrifugal action and chemically induced coagulation in waste water. Conveyance Speed - Speed at which the settled solids are moved along the bowl walls and removed from the bowl. Density - Measured in grams per cubic centimeter (or kilograms per liter); for example, the density of water is 1.0, iron is 7.9 and lead is 11.3. Dry Beach Area - This area dewaters the solids and allows liquid to drain back into the pool and not be discharged with the cake solids. Gradient Centrifugation - Separates particles on the basis of their buoyant densities. A gradient range is selected to encompass the densities of all particles to be banded. Heel - Layer of compacted solids on the inside diameter of the bowl. This layer is not conveyed out and settled solids removed by the conveyor are conveyed over this heel. Main Bearing - The frictionless central bearing on which the bowl is supported and rotates upon. Pillow Block - Term used to describe the central housing holding the main bearing. Sedimentation - The process where solids suspended in water settle to the bottom of a basin. Separation - This process divides or separates a mixture of particles or liquids into separate components. Slurry - Pourable mixture of solid and liquid. Supernatant - The liquid in a centrifuge tube above a pellet or sediment, after centrifugation. Tube Size - The length of a centrifuge tube, often given in both inches and millimeters.

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