Air classifiers are preferred over fine screening in one or both of the following situations: 1) the rate is too high for fine screening; 2) the cut point is too fine for screening. Take for example an application in which calcium carbonate must be separated at 75 microns. A common round vibrating separator with a 200 mesh screen can make a very sharp separation in such an application at about 1 tn/hr. But what if the desired processing rate is 30 tn/hr? Sure, you could install 30 vibrating screens, but that’s not really practical. A single air classifier could handle the entire 30 tn/hr.
As another example, let’s say we have a toner powder that we want to separate at 5 microns. It is not possible to accomplish this with a screen for the simple reason that it is not physically possible to weave a screen with openings as small as 5 microns. But a high-performance turbo classifier can easily make such a separation.
There are several types of air classifiers available. Perhaps the simplest is the "falling bed aspirator". This is a static device in which the material to be classified falls by gravity through an upwardly moving air stream. The lighter, finer material is carried off by the air and the heavier, coarser particles fall out the bottom. This extremely simple device can be used when the difference in particle size is huge, such as de-dusting a granular or pelletized material.
For more accurate and finer separations, one can use a rotary air classifier. Sometimes these are referred to as "dynamic" or "whizzer" classifiers. In these devices the material to be classified is dropped onto a rotating circular disk that flings the material out to its edges. Below the disk is a fan generating an air stream such that particles flow upward all around the periphery of the disk. Coarser, heavier particles fall off the disk into a hopper. Finer, lighter particles are lifted by the air and flow up and over a wall into a separate hopper. The cut point can be changed by varying fan speed and adjusting directional vanes. While these types of classifiers are relatively inexpensive, they cannot make very sharp cuts and are generally limited to finenesses in the range of 300 microns down to 45 microns.
For applications that require very sharp cuts or very fine cut points, a turbo classifier is the way to go. The turbo classifier is not a standalone unit. It requires a separate air moving system to pull air through the classifier. The turbo classifier itself resembles a squirrel cage blower, but, in this case, the spinning rotor is not there to generate an air flow, but rather to make a separation based on particle size. The external air moving system is configured to suck air "backwards" through the rotor, that is, air is drawn through to the inside of the rotor. The material to be classified is introduced to the air flow upstream of the classifier. As the material enters the classifier, it encounters the spinning rotor where a "tug-of-war" is played out on each particle. Aerodynamic forces from the flowing air try to pull the particle into the rotor. Simultaneously centrifugal forces from the spinning rotor try to push the particle out away from the rotor. Which force wins depends on the particle’s size.
For coarse particles, centrifugal force wins. These particles are flung out away from the rotor and fall into a collection hopper. For smaller particles, aerodynamic force wins. These fine particles are drawn through the rotor and conveyed to a dust collector where they are dropped out of the air flow. Most turbo classifiers operate in the range of around 150 microns down to 5 or 10 microns. But highly advanced turbo classifiers can achieve cut points as fine as 1 micron, making them suitable for nanotechnology applications.
Several manufacturers offer various types of air classifiers for a wide range of applications. As with other process equipment, selecting the right manufacturer requires thorough research and often involves laboratory testing.
A.J. DeCenso is business development manager for Sweco (Florence, KY), a business unit of M-I LLC. He has a BSME from Tufts University and an MBA from Xavier University. He has 20 years of process industry experience and holds several patents.