Types of Centrifuges and their Applications
On one level, all centrifugation is essentially the same. Gravity will always naturally separate materials of different densities. Using a centrifuge you can speed up and fine tune this natural separation and sedimentation process.
Broadly speaking, in terms of applications, centrifuges break into two major categories:
- laboratory centrifuges
- industrial centrifuges
Different types of centrifuges have different applications. While most lab centrifuges are designed and built to meet fairly narrow applications, industrial centrifuges are different. They tend to be built as more general-purpose devices and are then programmed or customized to excel in certain applications. Many high-speed industrial centrifuges can actually serve several entirely different purposes in the same production process (for example, a brewery might use the same disc stack centrifuge first to pull the yeast bodies from their beer when it comes out of the fermenters and then again during a final polishing stage).
Laboratory Benchtop Centrifuges
Laboratory centrifuges are usually bench-top devices. Common benchtop centrifuges seen in research labs include hematocrit centrifuges, microcentrifuges, vacuum concentrators, slide-spinners, ultracentrifuge, and more. (This isn’t to say that labs use nothing but benchtop centrifuges. For example, floor-mounted analytical ultracentrifuges aren’t uncommon in some fields.)
In most labs, centrifugation is largely about separating solids from liquids. Samples-each held in their own centrifuge tube or bottle—are loaded into a rotor. (There are two common centrifuge rotor types in labs: fixed angle rotors, which hold all the sample tubes in the same position for the entire operation, and swing-bucket rotors, which permit the samples to be loaded vertically, and then to swing up into an entirely horizontal position as the rotor gains speed.) As the rotor spins, the density of the particles (relative to the fluid they’re suspended in) forces them to the bottom of the tube. There, they form a sediment called the “pellet.” Left behind is the “supernatant,” an isolated specimen solution ready for further processing or analysis.
This arrangement-with fixed-angle or swinging bucket rotors holding individual samples-makes it possible to readily calculate the RCF (relative centrifugal field) each sample experiences. This is a measure of how much “g-force” was applied to the sample. It’s expressed in multiples of the earth’s gravitational field and usually a vital piece of data labs need to track during their work.
But being able to reliably and accurately track RCF comes at the cost of throughput: only so many samples can be processed over a given period, and performing any large-scale fluid separations is simply not possible.
High speed, high throughput centrifugation, sedimentation, and separation are the domain of industrial centrifuges.
Types of Centrifugation: Industrial High-Speed Centrifuges
Compared to lab centrifugation, industrial centrifugation is an inherently high-throughput domain. No one is especially interested in the RCF experienced by CNC cutting fluid or raw milk during sedimentation. So, in these centrifuges, there are no individual sample vessels. Instead, the rotor is a solid bowl (or in some cases a perforated basket). As such, materials can be continuously fed into the bowl while it spins, allowing for (essentially) in-line centrifugation.
There are two different types of high-speed industrial centrifugal separation: two-phase centrifugation and three-phase centrifugation. “Two-phase” centrifugation separates the incoming mixed materials into a “solid phase” and a “liquid phase.” Contrast this to “three-phase” centrifugation, which separates your fluid into two liquid phases (one “heavy”—usually water or water-based—and the other “light”—usually oil—as well as a solid phase.
“Disc Stack” Industrial Centrifuges
For a broad range of industries (from breweries and dairies to aerospace manufacturers and ethanol), the most useful industrial centrifuges are high-speed disc stack centrifuges. The “discs” are a series of cone-shaped plates arranged in a vertical stack inside of the centrifuge bowl. This increases the amount of available settling surface as well as controlling how mixed fluids mingle and move throughout the bowl. This both speeds the separation/sedimentation and gives you a great deal of fine control over what’s happening inside your centrifuge. Enhanced control makes it possible to separate several phases in a single pass. By changing the stack arrangement, angle, spacing, and shape, you can accurately control how materials are separated and adjust for changes in liquid flow and composition. For example, in the field, disc stack centrifuges like Trucent ‘s DX series regularly handle challenging three-phase separations with 70 percent efficiency (or better) on a single pass. As needed, the DX can easily switch to two-phase liquid-solid separation and hit 90 percent efficiency on a single pass.
“Scraper Bowl” Industrial Centrifuges
In many industrial centrifuges (such as those with a tubular bowl or a horizontal “screw” decanter centrifuge), it’s the overall shape of the rotor vessel that sorts the materials as they separate. Denser solid particles collect on the walls of the rotor and are forced out of one end of the rotor, while less dense liquid flows out the other. One might even argue that this is essentially how a disc stack functions: the geometry of the bowl internals and arrangement of the discs determines how materials separate and sediments collect.
A “scraper bowl” centrifuge is different. In this high-performance, high-speed two-phase centrifuge, the vertical rotor retains the solid material during the process cycle. Mixed liquid is introduced into the spinning rotor by a feed pump. It accelerates to match the vessel’s speed, at which time centrifugal force drives the solid particles out of suspension in the liquid and holds them against the rotor walls. The clean liquid then drains away out of the bottom of the vertical rotor. Periodically, the feed pump is shut off and the “scrape cycle” begins: A blade is brought up against the bowl’s interior, collecting all of the captured solids.
In the past, many users have shied away from scraper bowl centrifuges because they’ve proven hard to maintain. That’s because older scraper bowl designs relied on redundant arrangements of motors and drive-trains that required a complex clutch interlock and frequent servicing. Trucent’s S series of liquid-solid industrial centrifuges eliminated this complexity. The S series is built from the ground up for hassle-free and reliable operation with minimal operator involvement.
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