Air current classifying separator

Details Air current classifying separator Air current classifying separator

1999-04-29

2001-08-07

Walsh, Donald P. (Department: 3653)

Classifying, separating, and assorting solids

Fluid suspension

Gaseous

C209S139100, C209S142000, C209S143000, C209S722000

Reexamination Certificate

active

06269955

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a classifier for centrifugally separating powder such as toner powder used in a copier, or inorganic compounds such as metal oxides, glass and ceramics into fine and coarse particles.
FIG. 6
shows a conventional classifier. It includes a plurality of pivotable louvers
33
provided around a classifying chamber
32
defined between a classifying cover
30
and a classifying plate
31
. Defined between the adjacent louvers
33
are passages through which secondary air is introduced into the classifying chamber
32
in a whirling flow. A powder supply tube
34
is provided over the classifying cover
30
to define a powder supply port
35
between the bottom inner edge of the tube
34
and the outer edge of the classifying cover
30
.
A fine powder discharge tube
36
is connected to the center of the classifying plate
31
, while a coarse powder discharge port
37
is provided around the classifying plate
31
.
In operation, while a suction force from a blower is applied to the fine powder discharge tube
36
, a fluid mixture of powder and compressed air is supplied into the top of the powder supply tube
34
so that the mixture will be supplied into the classifying chamber
32
through the powder supply port
35
in a whirling flow. At the same time, secondary air is blown into the classifying chamber
32
through the passages between the louvers
33
to increase the whirling speed of the mixture, thereby centrifugally classifying the powder so that finer particles will move toward the center of the chamber
32
to be discharge through the fine powder discharge tube
36
, while coarser particles will whirl along the outer periphery of the chamber
32
to be discharge through the coarse powder discharge port
37
.
In the powder classification in which a classifier of the above-described type is used, especially in the manufacture of fine powder such as ceramic powder used as material for electronic parts, the lower the classification point and the nearer the maximum particle diameter is to the classification point, the more uniform the particle diameter of the fine powder obtained.
The term “classification point” herein used refers to the particle diameter at the intersection of the particle diameter distribution curves for collected fine and coarse powder particles.
In this type of conventional classifier, micron-order classification is already possible. But there is a demand to further reduce the powder classification point in fields where the product is fine powder. Also it is required to reduce the maximum particle size of fine powder.
It is known that the classification point of powder is affected by the whirling speed of fluid in the classifying chamber
32
, and the classification point can be decreased by increasing the whirling speed of fluid.
The whirling speed of fluid in the classifying chamber
32
can be increased by increasing the supply pressure of the fluid mixture introduced into the classifying chamber
32
in a whirling flow. But the supply pressure cannot be increased without a limit.
Thus, in this type of classifier, the angles of the louvers
33
are adjusted with the supply pressure of fluid mixture kept constant to change the flow rate of secondary air introduced into the classifying chamber
32
, thereby adjusting the whirling speed of fluid in the chamber.
Based on the knowledge that there is a correlation between the whirling speed of powder in the classifying chamber and the classification point, the inventors thought that the whirling speed may have some influence on the maximum particle diameter, and measured the tangential flow speed at various positions in the classifying chamber
32
. The results are shown in FIG.
7
. For the test, the classifier shown in
FIG. 6
was used.
The dimensions of the classifier used for measurement are shown in Table 1.
TABLE 1
Symbol
D
3
D
4
D
5
d
2
H
2
&agr;, &bgr;
Size(mm)
280
195
270
60
20
24°
In the table,
D
3
=inner diameter of the classifying chamber
32
D
4
=outer diameter of the classifying cover
30
D
5
=outer diameter of the classifying plate
31
d
2
=inner diameter of the fine powder discharge tube
36
H
2
=height of the louvers
33
&agr;
2
, &bgr;
2
=inclination angles of the conical bottom surface of the classifying cover
30
and the conical top surface of the classifying plate
31
Measurement was made with high-pressure (2 kg/cm
2
) air being blown into the powder supply tube
34
and a suction force of −0.3 kg/cm
2
applied to the fine powder discharge tube
36
.
The velocity curves (I) to (IV) in
FIG. 7
represent whirling speeds when the gap between louvers
33
was set at 1 mm, 3 mm, 5 mm and 7 mm, respectively.
As will be apparent from
FIG. 7
, in this type of classifier, the fluid whirling speed is extremely high at a point slightly spaced from the center of the classifying chamber
32
(that is, a point slightly offset inwardly from the inner surface of the fine powder discharge tube
36
), and decreases gradually toward the inner surface of the classifying chamber
32
.
In the conventional classifier, because the powder supply port
35
formed along the outer edge of the classifying cover
30
is an area where the whirling speed is relatively low, the whirling speed of powder supplied into the classifying chamber
32
through the powder supply port
35
is low. It is thus impossible to impart a sufficient dispersing and whirling force to the powder.
Coarse particles are thus likely to mix into fine particles, increasing the maximum particle diameter of fine powders.
Also, in the conventional classifier, the classifying chamber
32
for classifying powder into fine and coarse particles by centrifugal force has a cylindrical inner surface
38
above the louvers
33
. Therfore, so that powder whirling in the outer circumference of the classifying chamber
32
tends to dwell, adhere to the cylindrical inner surface
38
, and build up without being sufficiently acted on by secondary air introduced into the classifying chamber
32
through the gaps between the louvers
33
. This tendency is especially remarkable if the powder particle diameter is small because such small-diameter powder particles tend to be more strongly pushed against the cylindrical inner surface
38
by centrifugal force. This reduces the recovery rate of classified powder. Also, due to the adhesion of powder, the shape of the classifying chamber tends to change, making it impossible to operate the classifier stably with a constant classification point.
An object of the present invention is to reduce the classification point of powder in a classifier of the above-described type.
Another object of this invention is to achieve a stable operation with a constant classification point, and to increase the recovery rate of classified powder.
A further object of this invention is to reduce the maximum particle diameter of powder as a product.
SUMMARY OF THE INVENTION
According to this invention, there is provided a classifier comprising a casing, a cover provided over the casing and having a conical bottom surface, a classifying plate provided under the cover and having a conical top surface which defines a classifying chamber between the conical bottom surface of the cover and the conical top surface of the classifying plate and a plurality of louvers arranged around the classifying chamber, passages defined between the louvers, a fine powder discharge tube connected to the central portion of the classifying chamber, a coarse powder discharge port defined around the outer edge of the classifying plate, where by powder supplied into and whirling in the classifying chamber is accelerated by secondary air introduced into the classifying chamber through the passages, whereby discharging fine powder through the fine powder discharge tube and discharging coarse powder through the coarse powder discharge port, characterized in that the conical bottom surface of the classifying cover has a larger inclination angle than the conical top sur

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