Classifying cyclone

A classifying cyclone efficient at removing very fine particles. In preferred embodiments, the cyclone employs an annular bevel ring positioned along the interior wall to redirect separated particles from the wall back into the path of classifying air. A control air deflector is placed in the path of the classifying airstream and adjusted vertically to provide adjustment of the annular orifice through which the classifying air passes. The air deflector is positioned at approximately the height of the bevel ring.

BACKGROUND OF THE INVENTION 
This invention relates to classification, i.e., removal of fine particles 
from coarser particles in gas streams. Frequently it is desirable to 
separate very fine particles (e.g., less than 5 microns) from coarser 
particles. In one typical application, epoxy coating, heated parts are 
passed through a fluidized bed of ground epoxy particles. This bed of 
particles should contain no very fine particles, hence the need for 
classification. 
Two approaches are known in the field, rotating classifiers and cyclone 
classifiers. Rotating classifiers are subject to high power consumption, 
high noise generation, and unbalance susceptibility. Cyclone classifiers 
avoid these drawbacks, yet have been ineffective in removing very fine 
particles. 
In a typical cyclone classifier, one stream of air, containing particles, 
circulates and swirls around inside a cylindrical housing. Inertial, or 
centrifugal, forces tend to move the particles toward the outer wall of 
the housing. A second stream of air, flowing perpendicularly to the first, 
is intended to entrain and remove the fine particles while leaving the 
coarse particles behind. In general, these cyclones achieve adequate 
removal of some fine particles (e.g., 5-10 microns in length) but are 
inefficient at removing very fine particles (e.g. less than 5 microns). 
SUMMARY OF THE INVENTION 
I have discovered that classifying cyclones can be made much more efficient 
at removing very fine particles by redirecting separated particles from 
the outer wall of the cyclone back into the path of the classifying air. 
In preferred embodiments, particles are redirected by an annular ring 
located on the wall of the cyclone; the top of the ring is beveled at an 
angle of 30.degree. to 45.degree. from horizontal; the ring is from 0.5% 
to 33%, and preferably 3% to 5%, of the internal diameter of the cyclone; 
an air deflector is placed in the path of the classifying airstream, 
forming an annular orifice through which the classifying air flows; the 
air deflector is tapered (e.g., conical) and vertically movable, to allow 
adjustment of the size of the annular orifice, thus controlling the 
velocity of the classifying air and the overall efficiency of the cyclone. 
It is believed that improved classifying efficiency results because very 
fine particles travelling downwardly along the wall beneath coarser 
particles, and thus shielded from the classifying air, are forced back 
into the classifying air. The very fine particles tend to end up beneath 
coarser particles because of the inertial forces pushing the particles 
outwardly; the smallest particles are able to pass through interstices 
between coarser particles, and thus end up closest to the wall. In prior 
art classifying cyclones, those very fine particles that reached the wall 
in the upper portion of the cyclone were allowed to fall downwardly along 
the wall, shielded from further effects of the classifying air. 
In a second aspect, the invention features using the bevel ring in 
nonclassifying cyclones, for its ability to prevent the conical lower 
housing of the cyclone from becoming coated with very fine particles. With 
the bevel ring installed, the interior of the lower housing, which would 
have required cleaning in a conventional cyclone, is left glistening 
clean. 
Other features and advantages of the invention will be apparent from the 
following description of the preferred embodiment and from the claims.

The invention may be easily realized by modifying commercially available 
cyclones, such as the XQ 120 Series Cyclone, Size 10, manufactured by 
Fisher-Klosterman, Inc. Referring to FIG. 1, a cyclone comprises an inlet 
10 which carries a mixture of air and particles of varying size, an upper 
cylindrical chamber 12 (26.5 inch I.D.), a lower conical chamber 13, a 
classifying air inlet 14, an air deflector 22, and an outlet 18. The inlet 
10 is oriented tangentially to the circumference of the cylindrical 
chamber 12 in such manner as to cause swirling in the chamber 12. A second 
stream of air, known as classifying air, which enters the chamber 12 
through classifying air inlet 14 and air deflector 22 (both of which are 
added as a modification), selectively removes lighter particles from the 
air, by carrying them in the exhaust airstream flowing out through outlet 
18. The heavier particles are pulled downwardly by gravity and collected. 
In order to prevent very fine particles from being trapped under larger 
particles on the wall of the cylinder and thus shielded from the 
classifying air, a bevel ring 20 is fixed to the inner wall of the 
cylindrical chamber 12 across from the air deflector 22, at about 10 
inches above the boundary between cylindrical chamber 12 and conical 
chamber 13. The bevel ring 20 forces particles which are falling along the 
side of the chamber 12 inward, into the path of the classifying air, as 
shown diagrammatically by the dotted line in P in FIG. 2. In this manner, 
all particles are exposed to the classifying air, and efficiency of 
classification is improved. 
The bevel ring may have a wide range of shapes and sizes. It is expected 
that the practical minimum and maximum thicknesses are 0.5% and 33% of the 
cyclone diameter. Good results have been achieved with a bevel ring 
thickness of 1/4 inch (or 1% of cyclone diameter), and even better results 
with thicknesses of 1/2 inch (3%) and 3/4 inch (7%). Accordingly, the 
range 3% to 7% is preferred. 
A wide range of bevel angles are also possible. An angle of about 
30.degree. to 45.degree. has proved most successful. If too large an angle 
is used (e.g., 90.degree.), particles will accumulate on the upper surface 
of the ring, to form in effect a smaller angle. Too small an angle will 
not achieve the purpose of redirecting particles into the classifying air. 
The vertical dimension, or height, of the bevel ring can also vary. Heights 
of 1 and 2 inches have worked successfully. 
The vertical position of the bevel ring within the cyclone can be varied. 
It is preferable that it be positioned below the level at which 
substantially all the particles have been driven to the wall by the 
centrifugal action of the cyclone. If positioned higher, the improvement 
in classifying efficiency can be expected to lessen. 
The invention's efficiency at removing very fine particles can be seen in 
examples of its use. In one application, the cyclone was operated with a 
particle loading of 0.15 lb. of powder per cubic meter of air, where the 
powder contained 2.6% fine particles, 5 microns or smaller. There were no 
particles detected at 5 microns or smaller after classification. When the 
particle loading was increased to 0.30 lb. per cubic meter, the classifier 
produced a powder with approximately 0.7% fines. 
A comparison of classifying efficiency with and without the invention was 
conducted. Using a mixture of particles in which 4% were very fine (less 
than 5 microns), it was necessary, without the bevel ring installed, to 
exhaust one third of the particles to remove all of the very fine 
particles. With the bevel ring, however, all of the very fine particles 
could be removed with a loss of only 3% of the larger particles (7% of the 
total particles). Efficiency--i.e., the amount of waste of coarser 
particles required to achieve the desired reduction in fine particles--is 
dramatically better with the invention. 
The invention is especially suited to efficiently removing very fine 
particles (less than 5 microns), but will improve the classifying 
efficiency of a cyclone for any size particle. For example, if it is 
desired to remove all 20 micron or smaller particles from a stream 
containing larger particles, the invention makes it possible to do so with 
less waste of particles larger than the 20 micron cutoff. 
A further refinement in classifying efficiency is provided by air deflector 
22 (FIG. 2), which is tapered so as to be narrower at the bottom end, and 
installed so as to vertically movable. The deflector has a 23.5 inch 
outside diameter at its top, an 18 inch outside diameter at its bottom, 
and an 18 inch overall height. Vertical adjustment of the deflector 22 has 
the benefit of allowing adjustment of the size of the annular orifice 
between the deflector 22 and the bevel ring 20, so that classification 
efficiency can be maximized for different operating conditions. Generally, 
narrowing the orifice will result in higher velocity of classifying air, 
leading to a more complete removal of fines but also the removal of more 
of the coarser particles. Conversely, widening the orifice tends to reduce 
the loss of coarse particles but also reduces the velocity of classifying 
air, and thus the degree of removal of fines. 
Efficient classification of very fine particles has been achieved over a 
wide range of positions of the deflector 22, anywhere from the top of the 
deflector even with the bevel ring to the bottom of the deflector even 
with the bottom of the ring. If the deflector is raised higher, an 
undesirable turbulence seems to occur in the annular region between the 
deflector and the ring, with a resulting drop in efficiency. The deflector 
could be lowered below the height at which its top is even with the bevel 
ring, so long as the particles deflected by the ring do not travel onto 
the top of the deflector. The best efficiency seems to result when the 
bottom of the deflector is about two inches below the bottom of the bevel 
ring. 
OTHER EMBODIMENTS 
Other embodiments are within the following claims. For example, the bevel 
ring can be used in a nonclassifying cyclone (i.e., one used to separate 
all particles). In such an application it has the unexpected benfit of 
keeping fine particles from accumulating on the conical wall of the lower 
half of the cyclone.