Pulp screening device

Details Pulp screening device Pulp screening device

2001-01-19

2003-04-22

Walsh, Donald P. (Department: 3653)

Classifying, separating, and assorting solids

Sifting

Drum sifters

C209S379000, C209S397000

Reexamination Certificate

active

06550620

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a pulp screening device for separating good-quality fibers and foreign objects in paper pulp.
BACKGROUND ART
On the upstream side of a paper machine, there is provided a pulp screening device (pulp screen). The pulp screening device is a device for screening and separating good-quality fibers and foreign objects in paper pulp (i.e., a pulp suspension with a pulp density of 0.2 to 5%) with a screen cylinder thereof. Typically, the pulp screening device is equipped with one or two screen cylinders. First, the construction of a pulp screening device with a single screen cylinder will be described with reference to
FIGS. 28 and 29
.
FIG. 28
shows a part-sectional plan view of a conventional pulp screening device.
FIG. 29
shows a part-sectional side view taken in the direction of arrow D of FIG.
28
.
A pulp suspension is fed to the pulp screening device by a pump. As illustrated in
FIGS. 28 and 29
, the pulp suspension flows in a tangential direction through the entrance
2
of a cylindrical container
17
, and advances in an annular flow passage
4
, formed by an inner casing
3
and the inner wall of the container
17
. When the pulp suspension is circulating through the annular flow passage
4
, heavy foreign objects such as sand, etc., are discharged outside the device from a trap
5
provided in the tangential direction opposite to the entrance
2
, and the remaining pulp flows inside the inner casing
3
through the flow passage
4
. Note that a cover
19
is provided on the upper surface of the container
17
so that the device can be operated under pressure.
A cylindrical screen cylinder
1
is disposed inside the inner casing
3
. The upper portion of the screen cylinder
1
is fixedly attached to the inner casing
3
, and this screen cylinder
1
partitions the inner side of the inner casing
3
into an agitation chamber
7
and an exit chamber
14
. The pulp flowing in the flow passage
4
first flows in the annular agitation chamber
7
formed inside the screen cylinder
1
.
A large number of slits of width 0.15 to 0.5 mm or holes of diameter 0.2 to 4.8 mm are provided in the peripheral surface of the screen cylinder
1
, and the pulp is filtered and sorted by these slits or holes when flowing downward along the agitation chamber
7
. That is, the good-quality fibers that can pass through the slits or holes in the peripheral surface of the screen cylinder
1
are discharged from an exit
9
via the exit chamber
14
, while the foreign objects of sizes that cannot pass through the slits or holes in the screen cylinder, as they are, flow downward along the agitation chamber
7
and are discharged from a reject exit
10
.
In addition, a rotor
6
is disposed within the agitation chamber
7
. The rotor
6
is hung from the upper portion of a main shaft
11
and is equipped with a plurality of vanes
20
at equal spaces in the circumferential direction. The vane
20
is positioned, holing a predetermined space (2.5 to 8 mm) from the inner peripheral surface of the screen cylinder
1
. The main shaft
11
is supported by bearings so that it is free to rotate, and is driven to rotate by an electric motor
13
through a V-pulley (not shown) mounted on the lower end portion thereof. If the rotor
13
rotates and therefore the vanes
20
revolve within the annular agitation chamber
7
, the pulp suspension within the agitation chamber
7
is agitated. The foreign objects in the pulp are separated, and tangled fibers are untangled. As a result, clogging of the slits or holes in the screen cylinder
1
is prevented.
FIG. 30
shows how clogging of the slits or holes in the screen cylinder
1
is prevented by the vanes
20
. As illustrated in
FIG. 30A
, the vane
20
revolves along the surface of the screen cylinder
1
at high speeds (10 to 30 m/s), holding a constant space from the cylinder surface. When the valve
20
is revolving, negative pressure is developed between the vane
20
and the screen cylinder
1
, as shown in FIG.
30
B. The suction force, developed by this negative pressure, causes the solution to flow backward into the agitation chamber
7
and therefore the tangled fibers or foreign objects, blocking holes
100
in the surface of the screen cylinder
1
, are removed. After passage of the vane
20
, the pulp suspension will flow from the agitation chamber
7
into the exit chamber
14
again, and the holes
100
in the screen cylinder
1
will be clogged with tangled fibers and foreign objects. However, the tangled fibers, etc., newly blocking the holes
100
, are removed by the negative pressure produced by passage of the next vane
20
. In the conventional pulp screening device, clogging of the holes in the screen cylinder
1
is prevented by repeating the aforementioned operation.
FIG. 31
shows a sectional view of the configuration of the hole
100
in the screen cylinder
1
. The hole
100
is circular in shape, and a chamfered face
101
in the form of a dish is formed coaxially at the inlet of the hole
100
(on the side of the agitation chamber
7
). When the vane
20
passes over the chamfered surface
101
in the surface of the screen cylinder
1
, a turbulence (separating vortex) develops at the inlet of the hole
100
, as shown by an arrow S in
FIG. 31
, and clogging of the hole
100
is suppressed by the turbulence S.
Furthermore, there are screen plates
1
of cross sections such as those shown in
FIGS. 32 and 33
. In the case of
FIG. 32
, trapezoidal grooves
111
are formed in the axial direction of the screen plate
1
(perpendicular to the paper surface) and forms a plurality of holes
110
at the bottoms of the grooves
33
. In the case of
FIG. 33
, an axial waveform is formed on the peripheral surface of the screen cylinder
1
, and a plurality of holes
120
are bored axially in the inclined portion
121
of the waveform. In any of the cross sections shown in
FIGS. 32 and 33
, revolution flow caused by the vane
20
develops a turbulence Sat the inlet of the hole, thereby preventing clogging of the hole.
Now, the construction of a pulp screening device with a double screen cylinder (inner and outer screen cylinders) will be described with reference to
FIGS. 34 and 35
.
FIG. 34
shows a sectional view of the conventional pulp screening device with two inner and outer screen cylinders, and
FIG. 35
shows a sectional view taken substantially along line E—E in FIG.
34
. Note that the same reference numerals will be applied to the same parts as the aforementioned conventional pulp screening device having a single screen cylinder.
As illustrated in
FIGS. 34 and 35
, a pulp suspension flows in a tangential direction through the entrance
2
of a cylindrical container
17
and circulates through an annular flow passage
4
. When the pulp suspension is circulating through the annular flow passage
4
, heavy foreign objects such as sand, etc., are discharged outside the device from a trap
5
provided in the tangential direction of the flow passage
4
, and the remaining pulp suspension flows from the flow passage
4
to inside an inner casing
3
.
Cylindrical screen cylinders
1
a
and
1
b
are disposed inside the inner casing
3
. These screen cylinders
1
a
and
1
b
partition the inside of the inner casing
3
into an agitation chamber
7
and exit chambers
14
a
,
14
b
. The pulp suspension flowing in the flow passage
4
first flows in the annular agitation chamber
7
, formed between the screen cylinders
1
a
and
1
b
. When the pulp suspension is flowing downward along the agitation chamber
7
, part of the pulp passes through the inner screen cylinder
1
b
and is filtered and sorted in the inner exit chamber
14
b
. The remaining pulp passes through the outer screen cylinder
1
a
, and is filtered and sorted in the outer exit chamber
14
. On the other hand, the foreign objects of sizes that cannot pass through the screen cylinders
1
a
,
1
b
, as they are, flow downward along the agitation chamber
7
and are discharged from a reject exit
10
.
In addition, within the ag

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