Centrifugal particle classifier having uniform influx distributor

A classifier rotor mounted for rotation about a rotor axis inside a classification chamber for separating a fines fraction includes a plurality of disks mounted in parallel, spaced-apart relation along the rotor axis and a plurality of classifier blades axially mounted between the disks at the disk periphery. A uniform influx fluid distribution tube disposed between the disks of the classifier rotor is connected to a vacuum source to remove the fines fraction. The uniform influx fluid distribution tube includes a plurality of perforations of varying sizes in the region between the disks to compensate for velocity and pressure variations, thus creating a uniform air flow along the entire length of the classifier rotor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to centrifugal particle classifiers in general, and 
in particular to a forced vortex particle classifier having a uniform 
influx fluid distributor to enhance particle classification and 
throughput. 
2. Background of the Invention 
In the field of powder technology, classification is generally defined as 
the process of separating a powder into a coarse fraction and a fine 
fraction. The coarse comprises coarse particles having sizes equal to and 
larger than a cut size, whereas the fine fraction comprises fine particles 
having sizes equal to and less than the cut size. The cut size is 
equivalent to the separation point, i.e., that particular size of 
particles about which the powder is separated. 
Numerous types of classification systems exist and have been used with 
varying degrees of success to separate or classify the powder into the 
coarse and fine fractions. Centrifugal air classifiers are among the more 
common types of classification systems and divide airborne powders 
(aerosols) by subjecting them to high centrifugal forces and opposing 
airflows. As is well-known, the centrifugal forces dominate the dynamics 
of large particles, whereas aerodynamic drag forces (Stokes forces) 
dominate the dynamics of the smaller particles. In a centrifugal 
classifier, the large particles above the design cut size are thrown 
outwardly by centrifugal forces and are therefore effectively separated 
from the small particles, which remain entrained in the classifying air. 
Centrifugal classification systems can take many forms, but usually can be 
grouped into one of two classes: "free" vortex systems or "forced" vortex 
systems, depending on the particular means used to create and maintain the 
vortices in the classification chambers. Most free vortex classifiers use 
curved vanes or stators to generate the vortices, whereas most forced 
vortex classifiers use spinning rotors to establish and maintain the 
vortices. 
Most forced vortex centrifugal classifiers have a rotor or classifier cage 
mounted for rotation within a hollow stator, so that the rotor and stator 
are separated by a narrow annular air gap. The rotor defines a hollow 
coaxial chamber that is in communication with the air gap along the 
periphery of the rotor and also in communication with a central opening 
for the egress of the classifying fluid, usually air, along with the 
entrained fine fraction of the powder. Typically, the classifying air is 
supplied to the chamber through the gap and a vortex is produced generally 
within the rotor by the rotation of the rotor itself. The powder to be 
classified is supplied to the vortex and the coarse fraction of the 
powder, which is forced by centrifugal action towards the stator, is 
removed through a coarse fraction passageway, while the fine fraction is 
removed with the fluid through the central axial opening in the rotor. 
Depending on the particular design of the forced vortex centrifugal 
classifier, the powder may enter the air gap through the stator along a 
radius of the chamber, so that the classifying air and the powder 
initially enter the air gap traveling perpendicular to each other. 
However, other forced vortex classifiers do not require that the powder 
and classifying air be separate, and instead mix the powder with the 
classifying air before it is introduced into the classifier. Examples of 
forced vortex centrifugal classifiers can be found the patents issued to 
Nomar, U.S. Pat. No. 2,991,844; Bouru, U.S. Pat. No. 3,561,195; Lapple 
U.S. Pat. No. 3,720,313; Voelskow, U.S. Pat. No. 3,767,045; Erickson, U.S. 
Pat. No. 4,268,281; Barthelmess, U.S. Pat. Nos. 4,409,097 and 4,390,419. 
The material handling capacity or throughput of a forced vortex centrifugal 
classifier is principally dependent on the axial length and diameter of 
the rotor or classifier cage, i.e., its circumferential surface or the 
cylindrically annular chamber in which the classification is performed. 
The other essential parameter of the classification process, namely the 
particle size limit differentiating the fine material from the coarse 
material (the cut size), is on the one hand determined by the diameter and 
rotational speed of the classifier cage and on the other hand by the 
external diameter of the classifying chamber and on the inflow rate of the 
classifying air into the classifying chamber. In both cases, the cut size 
is dependent on the centrifugal forces acting on the particles being 
classified. 
While it is theoretically possible to increase the throughput of the 
classifier by increasing the diameter of the centrifuge cage, practical 
restrictions on the cage diameter exist since the centrifugal forces 
increase on a square law basis, thus rapidly increasing the forces acting 
on the centrifuge cage. Another factor limiting the size of the centrifuge 
cage diameter is the decrease in curvature of the classifying air path 
(i.e., the vortex) as the diameter of the cage increases. This curvature 
increase can be compensated by higher outflow speed or higher classifying 
air quantities, but these and the increasing resistance and frictional 
losses impose unacceptably high power requirements. 
The axial length of the centrifuge cage is also limited due to the 
increasing torsional loading (wind-up) and axial deflection of the cage 
shaft that typically accompany an increase in cage length. However, these 
structural problems usually can be reduced by proper cage design and by 
supporting the cage shaft at both ends. However, even if these structural 
problems are solved, the axial length of the centrifuge cage is 
particularly limited by the varying airflow rates along the cage edges 
where the classifying air and fines pass between the blades. 
Essentially, the flow rates along the jacket-like circumferential surface 
of the centrifuge cage are directly related to the suction within the 
cage. This suction is at a maximum level at the central coaxial opening 
(fines outlet) from the cage and decreases towards the closed end of the 
cage. These varying flow rates along the length of the cage lead to 
differences in the separation quality or selectivity of the classifier. As 
a result, oversize material passes into the fines in the vicinity of the 
fines outlet where maximum suction action occurs, while at the greatest 
distance from the fines outlet, undersize material will remain with the 
coarse material and be rejected with the coarse fraction. These 
selectivity disadvantages increase with the magnitude of the axial length 
of the centrifuge cage, and heretofore have limited the throughput of such 
classifiers if an acceptably narrow cut size is to be maintained. 
One solution to overcome the aforementioned disadvantages resulting from 
the different flow rates along the axial direction of the cage has been to 
add an additional fines outlet to the other end of the cage, as disclosed 
in European Patent 67 895B1. However, this is not a complete solution, as 
there will still be diminished air flow at the midpoint of the cage. 
Further, providing such an additional fines outlet can lead to 
difficulties in adequately supporting both ends of the cage, increasing 
the chances for cage vibration and flutter. 
The patent issued to Hanke, U.S. Pat. No. 4,869,786 recognizes this problem 
and instead solves the throughput limitation imposed by the flow 
variations along the cage by utilizing a multi-stage design with a 
plurality of centrifugal cages and chambers. More specifically, Hanke 
provides two stages, a pre-classification and a re-classification stage, 
to achieve improved separation efficiency and throughput. Unfortunately, 
however, Hanke's multi-stage design is large and cumbersome and requires 
relatively complicated apparatus. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of this invention to provide an 
improved forced vortex classifier having increased throughput. 
It is another general object of this invention to provide an improved 
forced vortex classifier having a more precise cut size. 
It is a further object of this invention is to provide a forced vortex 
classifier having a constant airflow and pressure drop ratio over a wide 
range of rotor speeds. 
It is yet another object of this invention to provide an improved forced 
vortex classifier that reduces the amount of suction required in the fines 
collection system. 
It is a more specific object of this invention to provide a forced vortex 
classifier with a longer classifying cage. 
To achieve the foregoing and other objects and in accordance with the 
purposes of the present invention, as embodied and broadly described 
herein, the improved centrifugal particle classifier according to this 
invention may comprise a classifier rotor or centrifugal cage mounted for 
rotation about a rotor axis inside a classification chamber. In the 
preferred embodiment, the classifier rotor includes four disks mounted in 
parallel, spaced-apart relation along the rotor axis and a plurality of 
classifier blades axially mounted between the disks at the disk periphery. 
A uniform influx fluid distribution tube disposed between the disks of the 
classifier rotor is connected to a vacuum source to remove the fines 
fraction and includes a plurality of perforations of varying sizes in the 
region between the disks to compensate for velocity and pressure 
variations within the distribution tube. The perforations are sized to 
establish and maintain a uniform influx of air along the entire length of 
the classifier rotor. 
Additional objects, advantages, and novel features of this invention shall 
be set forth in part in the description that follows, and in part will 
become apparent to those skilled in the art upon examination of the 
following or may be learned by the practice of the invention. The objects 
and the advantages of the invention may be realized and attained by means 
of the instrumentalities and in combinations particularly pointed out in 
the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The improved forced vortex centrifugal classifier 10 according to the 
present invention is best seen in FIG. 1 and comprises a classification 
housing 12 and a fan housing 14 fluidically connected by a combination 
uniform influx distributor/draft tube assembly 16 and mounted to a support 
frame 18. The classification housing 12 defines a classification chamber 
48 (also shown in FIG. 2) and encloses a classifier rotor or centrifugal 
cage 36. A motor driven rotary air-lock or star valve assembly 26 attached 
to the coarse fraction collection chamber 28 of classification housing 12 
removes the rejected coarse fraction C from housing 12, as will be 
described in more detail below. Fan housing 14 encloses the centrifugal 
fan 52 and is connected to a suitable accept system or "baghouse" (not 
shown) via discharge chute 32, as is well-known. 
Both the classifier rotor or centrifugal cage 36 and the fan 52 are mounted 
to the combination uniform influx fluid distributor/draft tube assembly 
16, which is in turn mounted to a drive shaft 58. A single motor 20 
connected to drive shaft 58 via drive belt assembly 24 rotates the entire 
assembly, so that both the centrifugal cage 36 and fan 52 turn together. 
The combination uniform influx fluid distributor/draft tube assembly 16 
fluidically connects the classifier rotor 36 and fan 52, as best seen in 
FIG. 3. The uniform influx fluid distributor end 17 of combination 
distributor/draft tube assembly 16 includes a plurality of perforations 50 
having varying sizes through which pass the fines fraction F. The 
individual perforations 50 are sized and spaced to compensate for velocity 
and pressure variations within the distributor/draft tube assembly 16, 
resulting in a uniform airflow along the entire length of the classifier 
cage 36, as will be described in great detail below. The uniform airflow 
thus created enhances the classification process and allows a much longer 
classifier cage to be used for increased throughput while still 
maintaining an acceptably narrow cut size. 
During operation, the powder material to be classified, such as talc, 
barite, or any other powder-like material, is mixed with clean intake air 
and the mixture is fed into the classifier inlet 30 by suitable air/powder 
mixing and feed apparatus (not shown), as is well-known. The rotating fan 
52 in fan housing 14 creates a suction that draws the particle laden air 
from the classification chamber 48 and through the rotating centrifugal 
cage 36, via the combination distributor/draft tube assembly 16. See FIGS. 
2 and 3. In accordance with well-known principles, the centrifugal forces 
and opposing airflows within classification chamber 48 cause those 
particles larger than the predetermined cut size to be flung radially 
outward from the cage 36, whereas the smaller particles are drawn along 
with the airflow towards the center of the cage as the fines fraction F. 
The fines fraction F is drawn through the perforations 50 in the uniform 
influx end 17 of combination distributor/draft tube assembly 16 and on 
through the draft tube interior 46 by the suction created by fan 52. 
Finally, the fines fraction F passes through the centrifugal fan 52 and is 
exhausted out the discharge chute 32 where it is collected by the baghouse 
system (not shown). The rejected coarse fraction C accumulates in coarse 
fraction collection chamber 28, passes through rotary air-lock assembly 
26, and is carried away by a pneumatic conveyor system 34 to the grinding 
mill where it can be re-ground into smaller particles and re-classified. 
The combination uniform influx fluid distributor/draft tube assembly 16 
forms the heart of the invention as it results in a uniform influx of air 
along the entire length of the centrifugal cage 36, thereby eliminating 
the problems associated with non-uniform airflow along the cage. For 
example, the uniform airflow made possible by the uniform influx fluid 
distributor/draft tube assembly 16 allows the classifier according to the 
present invention to make precise cuts at the desired cut size and at 
lower centrifugal forces and pressure drops than any existing classifier 
system known to the inventor. This uniform airflow also allows a longer 
classifier cage to be used, thus significantly increasing throughput. 
Furthermore, the combination of the classifier cage 36 and fan 52 on a 
single drive shaft 58 permits more stable and consistent operation of the 
classifier. While most existing classifier systems experience significant 
changes in the ratio between pressure drop and operating speed, which 
adversely affects the classification process and cut size, the combined 
classifier/fan design according to the present invention results in nearly 
constant pressure drop ratios at all rotor speeds. Finally, because the 
fines fraction is removed from the combination distributor/draft tube 16 
by the suction developed by the fan 52, the classifier according to the 
present invention reduces the amount of suction pressure required in the 
baghouse system, thus further reducing operating costs. 
Another significant feature of this invention is that it uses two separate 
housings, classification housing 12 and fan housing 14, for rejects and 
accepts, therefore dispensing with the need for labyrinth seals or other 
complex sealing mechanisms to prevent rejected particles from passing into 
the classifier cage by short circuiting. The separate housings 12, 14 
allow simple air-gap seals 70 (FIG. 3) to be used to seal out ambient air. 
The small amount of ambient air leaking through the seals 70 does not 
adversely effect the classification process. 
The details of the forced vortex centrifugal classifier 10 are best 
understood by referring to FIGS. 2, 3, and 4 simultaneously, with 
occasional reference to FIG. 1. As mentioned above, the classification 
housing 12 encloses the classifier cage 36 and, in combination with baffle 
plate 38 and intake chute 30, forms a conventional scroll or 
classification chamber 48. Gap 39 between the housing 12 and baffle plate 
38 allows the coarse particles C to drain from the bottom of the 
classification chamber during the shut-down cycle when the centrifugal 
cage 36 is spinning down from full speed. The coarse fraction collection 
chamber 28 attached to the bottom of the classification housing 12 
communicates with a pneumatic conveyor system 34 via rotary air lock valve 
assembly 26 for removal of the rejected coarse fraction. 
Fan housing 14 defines a scroll chamber 53 and encloses the fan 52, as best 
seen in FIG. 4. Essentially, fan 52 is a simple centrifugal fan and 
comprises two end plates 54, 55 separated by a plurality of radial fan 
blades 56. The center annulus, or intake 51, of fan 52 is open to the 
interior 46 defined by combination uniform influx fluid distributor/draft 
tube assembly 16, as best seen in FIG. 3. 
Both the classifier cage 36 and fan 52 are integral with the uniform influx 
fluid distributor/draft tube assembly 16, which is in turn mounted to 
drive shaft 58 by end plates 80, 82, as best seen in FIG. 3. As mentioned 
above, the entire assembly, including classifier cage 36, fan 52, and 
combination distributor/draft tube assembly 16, rotate together, 
permitting more stable and consistent operation of the classifier 10. The 
combined classifier/fan design also results in constant pressure drop 
ratios regardless of rotor speed, and allows for the reduction of the 
suction pressure in the baghouse system, as mentioned above. 
Classifier cage 36 comprises two end plates 40, 41 and two perforated 
flanges 43, 45 that are welded in parallel, spaced apart relation to the 
exterior surface of the uniform influx end 17 of combination 
distributor/draft tube assembly 16. A plurality of classifier blades 42 
attached to pivot rods 44 are axially mounted between the end plates 40, 
41 and perforated flanges 43, 45. The pivot rods 44 pass through the 
perforated flanges 43, 45 and are secured at either end to the end plates 
40, 41 by means well-known in the art. The two perforated flanges 43, 45 
divide the annulus defined by the combination distributor/draft tube and 
the classifier blades 42 into three separate classification zones 72, 74, 
and 76 and provide radial support to the classifier blades 42 and pivot 
rods 44. Each perforated flange is identical and includes a plurality of 
perforations 78, as shown in FIG. 5, to equalize the pressure between and 
among the classification zones 72, 74, and 76 to further enhance 
classification and ensure a narrow cut size. 
The structure thus formed by the combination distributor/draft tube 
assembly 16 and perforated plates 43, 45 together provide a large degree 
of torsional stiffness and radial support for the classifier blades 42, 
thereby allowing relatively long classifier cages to withstand the large 
torsional and centrifugal forces with a minimum of deflection. 
Having described the structure of the forced vortex centrifugal classifier 
10 in detail, the operating principles of the classifier can now be more 
easily described and understood. Referring now to FIGS. 2 and 3 
simultaneously with occasional reference to FIG. 4, the shaft 58 of rotor 
22 is driven at high speed by motor 20 in the direction of arrow 60. The 
rotating fan 52 creates a suction that draws air from the classification 
chamber 48, through the perforations in the uniform influx distributor end 
17 of combination distributor/draft tube assembly 16, through draft tube 
16 itself, and pumps it out through discharge chute 32 attached to fan 
housing 14. Once this air flow is established, the powder to be classified 
is mixed with intake air and fed into the classification chamber 48 via 
inlet 30, as indicated by arrows 61 in FIG. 2. The centrifugal action of 
the classifier blades 42 and the swirling vortex associated therewith 
tends to throw the larger particles or coarse fraction C outward and into 
coarse fraction collection chamber 28. However, because of the small sizes 
and lower terminal velocities of the particles in the fines fraction F, 
they remain entrained in the airflow 61 and are drawn into draft tube 16 
through the perforations 50. The fines fraction F is drawn through the 
draft tube assembly 16, through fan 52, and is blown out discharge chute 
32 and collected by the baghouse system (not shown). 
As was described above, the sizing and spacing of the perforations 50 in 
the combination uniform influx fluid distributor/draft tube 16 are 
critical in achieving the desired uniform fluid influx along the length of 
the cage 36 and in each classification zone 72, 74, and 76. Accordingly, a 
system of equations based on recognized fluid flow principles and methods 
has been derived for the purpose of selecting the proper hole size and 
spacing required to achieve such a uniform fluid influx along the 
classifier cage length. 
Referring now to FIG. 6, a generic uniform influx fluid distributor 116 can 
be "unwrapped" and considered to be a flat plate having a plurality of 
perforations 150 arranged along a number of rows 112. Van Winkle et al. 
developed the following relation for the weight rate flow of fluid through 
a perforated plate: 
##EQU1## 
where: w=weight rate of flow 
C=orifice coefficient; 
A.sub.f =total free area of holes; 
Y=expansion factor; 
g.sub.c =dimensional constant; 
.rho..sub.1 =fluid density at upstream pressure and temperature; and 
.DELTA.p=pressure drop across the plate. 
Equation (1) accurately predicts the flow for fully turbulent flows with 
hole Reynolds numbers ranging from 16,000 to 65,000. See Perry's Chemical 
Engineers' Handbook, Sixth Edition, page 5-37, for a full description of 
Equation (1) and a graph of the perforated plate orifice coefficient C 
versus hole Reynolds number and physical characteristics of the plate. 
The orifice coefficient C varies depending on the pitch-to-diameter ratio 
and the hole thickness-to-diameter ratio of the uniform influx fluid 
distributor tube 116. In the preferred embodiment, both of these ratios 
will vary with hole size since the distance between hole centers is kept 
constant in the perforation pattern, as seen in FIG. 5. For the range of 
hole thickness to diameter ratios associated with the preferred 
embodiment, the orifice coefficient C is consistently predicted with: 
##EQU2## 
where t is the wall thickness of distributor 116 and D is the hole 
diameter. This orifice coefficient Equation (2) was derived based on 
ranges of hole sizes from 3/8" to 1" and wall thicknesses of 0.12" to 
0.5". However, the applicability of Equation (2) beyond these ranges has 
not been confirmed, in such a case it would be necessary to consult the 
orifice coefficient tables in Perry's to determine the correct orifice 
coefficient. 
Equations (1) and (2) can be combined to yield the following equation for 
uniform fluid influx rates along the tube length: 
##EQU3## 
where: CFM=total influx of air in cubic feet per minute; 
M=number of holes per row; 
D=hole diameter, in inches; 
t=wall thickness, in inches; 
.DELTA.p=pressure drop across the perforations, in inches of water; and 
.rho.=air density in pounds per cubic foot. 
If the .DELTA.p's are kept below 20 inches of water, which is preferred for 
efficient operation, the expansion factor Y in Equation (1) is 
approximately equal to unity and can be ignored. 
The fluid influx through each successive row of perforations changes the 
flow conditions as the fluid flows down the interior cavity 46 of 
distributor 116, so it is necessary to determine the various static 
pressures within the distributor tube 116 at each successive row of 
perforations. The static pressures in the subsequent rows can be 
calculated from the well-known momentum balance equations given in Perry's 
for flow of a fluids in a duct with the addition of a secondary fluid at a 
plane 2 (in the nomenclature of Perry's): 
EQU (p.sub.2 -p.sub.1)g.sub.c A=w.sub.p (V.sub.p -V.sub.m)+w.sub.s (V.sub.s 
-V.sub.m) (4) 
where: 
p.sub.1 =pressure at plane 1; 
p.sub.2 =pressure at plane 2; 
g.sub.c =dimensional constant; 
A.sub.c =cross sectional area of the tube; 
w.sub.p =weight rate of flow of the primary fluid; 
V.sub.p =velocity of the primary fluid; 
w.sub.s =weight rate of flow of secondary fluid; 
V.sub.s =velocity of the secondary fluid; and 
V.sub.m =velocity of the combined fluids. 
Assuming that the velocity of the primary fluid V.sub.p is zero, since it 
enters the distributor 116 orthogonal to the axial flow in the tube, 
Equation (4) can be rewritten in the dimensional units and nomenclature 
adopted herein as: 
##EQU4## 
where: SP.sub.n =static pressure at row n; 
SP.sub.n-1 =static pressure at the row immediately before row n; 
DC=the total fluid influx for the tube (CFM) divided by the number of rows 
of perforations; 
CFM.sub.n =is the summation of the DCs up to a given row n of perforations. 
For example, CFM.sub.3 =DC.sub.1 +DC.sub.2 +DC.sub.3, or for the desired 
uniform fluid influx in the preferred embodiment, CFM.sub.3 =3DC. 
Note that the coefficient of A.sub.c in Equation (5), i.e., 542,500, also 
reflects a 5% reduction in the cross sectional area of the tube to 
compensate for flow irregularities within the tube due to the incoming 
perpendicular flow through the holes. Therefore, the true cross sectional 
area of the tube A.sub.c is always used in Equation (5), since the 5% 
reduction is already accounted for in the coefficient. 
The required hole sizes of the tube can now be calculated, stepwise, in the 
following manner. Equation (3) is used as a starting equation to calculate 
an initial static pressure .DELTA.p (SP.sub.1) for the first row of 
perforations at the closed end of the tube 114. A desired hole diameter is 
selected, which will then help to determine a reasonable number of 
perforations to be placed around the perimeter of the tube. This number of 
perforations per row 112 will then remain the same for all rows regardless 
of the calculated hole size. Since it is desired to calculate the .DELTA.p 
for row 1 only, the CFM value in Equation (3) should be the total CFM 
influx for the entire tube divided by the number of rows of holes. 
Equation (3) can then be solved to yield the static .DELTA.p at row 1 
(i.e., SP.sub.1). 
Having established .DELTA.p for row 1 (SP.sub.1), Equation (5) can be used 
to calculate the successive values for the static pressure SP.sub.n at 
each subsequent row n. Then, after having calculated the static pressures 
for each respective row n, the required hole diameter for that respective 
row n can be calculated by solving Equation (3) for hole diameter D, thus: 
##EQU5## 
By way of example, the method and equations described above should yield 
the following static pressures and hole diameters for a sample uniform 
influx distributor 116 having the following dimensions: 
Inside Diameter=3.068" 
t=0.185" 
Total CFM=600 cubic feet per minute 
Number of Rows=7 
M=8 holes per row 
.rho.=0.075 pounds per cubic foot 
A.sub.c =0.051 square feet 
DC=600/7 or 85.7 cfm/row 
The following values were calculated for each row of perforations: T1 -? 
Static Pressure? Hole Diameter? -Row? (Inches H.sub.2 O)? (Inches)? -1 
-0.43 1.00 -2 -1.60 0.71 -3 -3.55 0.58 -4 -6.29 0.50 -5 -9.80 0.44 -6 
-14.09 0.40 -7 -19.17 0.37 - 
Note that Equations (3), (5), and (6) incorporate the dimensional constants 
required for the system of units used in the above example. However, it 
should be understood that these equations will also accurately predict the 
required hole sizes for any other system of units, provided that the 
corresponding dimensional constants are incorporated into the respective 
equations. Therefore, the present invention should not be considered as 
being limited to the particular system of units incorporated herein and 
the use of such other systems of units should be considered to be 
equivalents. 
Finally, it should be noted that the hole sizes calculated for the above 
example will provide a uniform velocity influx over a wide range of flow 
rates and are not limited to a flow rate of 600 CFM. Thus, the uniform 
influx draft tube according to the present invention will allow consistent 
classifier operation over a wide range of flow rates, further enhancing 
the utility of the invention. 
This completes the detailed description of the preferred embodiment of the 
improved classifier 10 according to the present invention. While a number 
of specific components were described above for the preferred embodiment 
of this invention, persons skilled in this art will readily recognize that 
other substitute components or combinations of components may be available 
now or in the future to accomplish comparable functions to make 
centrifugal air classification apparatus according to this invention. For 
example, numerous draft tube sizes may be used depending on the particular 
size of the classifier, and the above-described method for calculating the 
sizes of the various perforations can be used for any size draft tube. 
Likewise, myriad configurations for the perforation pattern are possible 
and could be used to produce the uniform fluid influx along the length of 
the classifier cage, and the present invention should not be considered as 
limited to the particular hole patterns shown and described herein. 
While the distributor/draft tube assembly 16 of the preferred embodiment 
rotates with drive shaft 58, classifier cage 36, and fan 52, it would also 
be possible to replace the rotating distributor/draft tube with a 
stationary distributor/draft tube assembly that does not rotate with the 
classifier cage. The necessary modifications required to do so are 
well-known and could be easily accomplished by persons having ordinary 
skill in the art. Moreover, the draft tube need not have a closed end 114 
as seen in FIG. 6. An equivalent alternative would be to use an open end 
tube placed in close proximity to the end plate of the classifier cage. 
The gap between the end of the tube and the end plate of the cage would 
then be treated as a plurality of holes of such large diameter and spaced 
so closely together, that they could be considered to have an area 
equivalent to the area created by the actual gap. 
Other possible substitutes have been mentioned throughout this description, 
and many more equivalents are possible. For example, the uniform 
influx/draft tube assembly 16 shown and described herein is not limited to 
use with a blade-type centrifugal cage, and could be easily adapted for 
use with centrifugal classifiers having bladeless rotors, such as the 
design disclosed by Lawless et al., in U.S. Pat. No. 4,923,491. Moreover, 
the uniform influx/draft tube assembly 16 shown and described herein is 
not limited to use with centrifugal classifiers and could be readily 
adapted to any use that requires a uniform fluid influx along an elongated 
cylindrical volume. Furthermore, the uniform influx draft tube could also 
be used in reverse to uniformly eject and distribute fluid along an 
elongated cylindrical volume. Therefore, it would be feasible to someone 
having ordinary skill in the art, in light of this disclosure, to assemble 
the necessary components to practice this invention, regardless of whether 
some of such components might not be the same as those described herein. 
The foregoing is considered illustrative only of the principles of the 
invention. Further, since numerous modifications and changes will readily 
occur to those skilled in the art, it is not desired to limit the 
invention to the exact construction and operation shown and described, and 
accordingly, all suitable modifications and equivalents may be resorted to 
as falling within the scope of the invention as defined by the claims 
which follow.