Centrifugal Separators: Working Principle and Design
If you’ve ever bought Italian salad dressing, you understand the fundamental principles of sedimentation and centrifugal separation.
Before you dig into your salad, you give that bottle a good hard shake, mixing together the vinegar, olive oil and herbs. As you munch, the herbs sink to the bottom (sedimentation) and the vinegar and oil form two distinct layers (separation). After a few minutes, the light oil rests on top of the somewhat denser vinegar, and all of the herbs and spices rest on the bottom of the bottle.
If the dinner conversation has been less than thrilling, you probably gave that dressing a moment’s consideration and realized that gravity was doing all the work:
- Oil and water don’t mix
- Herbs don’t dissolve
- They are all different densities
- Gravity is pulling on all of them
Given enough time, gravity is always going to sort out mixtures of solid particles and immiscible liquids—with the emphasis on that phrase “given enough time.”
The oregano in your Italian dressing settles out in a few minutes. The smallest fines in your CNC cutting fluid won’t settle out for days. You can’t make time go any faster—but you can make gravity pull harder, and thus do its job faster. That, in a nutshell, is the principle behind every centrifugal separator.
Centrifugation with High Speed Industrial Centrifuges
There are many different ways to approach liquid-liquid and solid-liquid separation. A centrifuge is one piece of separation technology that allows for high speed separation of immiscible (think “non-mixable”) liquids and particles.
Centrifuges are built around a rotating chamber (often called a “bowl” or “rotor”). The rotor’s acceleration generates a centrifugal force that is, essentially, a sort of artificial gravity that pushes everything inside the rotor bowl toward the walls. Mixed liquids and particles are fed into the spinning bowl via an inlet. By adjusting the acceleration of the rotor, the centrifuge can hold the solid particles (or “solid phase”) to the rotor wall while permitting the liquids (i.e., the “liquid phase”) to flow out. The higher the rotor’s rotational speed, the greater the artificial gravity. As the artificial gravity increases, the time it takes to separate your materials decreases.
This is centrifugation at its most basic. It’s often called “two-phase” or “solid-liquid” separation. Most high speed, high-efficiency two-phase centrifuges are scraper bowl centrifuges. Such separators periodically enter a “self-cleaning” cycle, during which the inlet is closed and a piston or blade pushes through the bowl, clearing off the collected particles.
Disc Stack Centrifuges: Greater Separation Control in Less Time
But speeding up separation is just the start. Once you’ve got your mixture of liquids and particles under acceleration, you can exert a great deal of control over how the separation proceeds.
For example, it is entirely possible to separate a single mixed liquid input into three separate outputs in a single pass using in-line centrifugation. This is called “three-phase” or “solid-liquid-liquid” separation. One popular separator design for accomplishing this sort of liquid/particle separation is a high speed industrial separator called a “disc stack centrifuge.”
The “discs” in a disc stack separator are actually a series of cone-shaped plates arranged in a vertical stack inside of the centrifuge’s rotor bowl. These increase the amount of available settling surface (or “relative surface area”) within the bowl. This serves several purposes. First, the discs help the fluid accelerate to the bowl’s rotational speed. As the fluid’s rotational speed increases, liquids of different densities begin to separate. These discs also create a shorter settling distance, which means shorter settling times, and a shorter particle sedimentation process overall. It also gives you a great deal of fine control over what’s happening inside your separator. By changing the flowrate, back pressures, bowl rotational speed, and other variables the operator can fine tune the separation. That can mean relatively fine adjustments to compensate for changes in liquid flow and composition. Or it can mean a gross alteration in how centrifugation is performed, allowing extremely high efficiency two-phase or three-phase operation from a single centrifuge.