Note: Descriptions are shown in the official language in which they were submitted.
CA 02526192 2001-O1-19
- 1 -
SPECIFICATION
PULP SCREENING DEVICE
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
CA 02526192 2001-O1-19
- 2 -
in an annular flow passage 4, formed by an inner casing
3 and the inner wall of the container 17. When the pulp
suspensionis circulatingthroughthe annularflow passage
4 , heavy foreign obj ects 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 f lows inside the inner casing 3 through the f lowpassage
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
CA 02526192 2001-O1-19
- 3 -
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 obj ects
in the pulp are separated, and tangled f 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 cylfinder 1 at high speeds (10
to 30 m/s), holding a constant space from the cylinder
surface. When thevalve20isrevolving, negativepressure
is developed between the vane 20 and the screen cylinder
1, as shown in Fig. 30B. The suction force, developed
CA 02526192 2001-O1-19
- 4 -
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 l, 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
CA 02526192 2001-O1-19
- 5 -
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 axiallyintheinclinedportion121ofthewaveform.
In any of the cross sections shown in Figs. 32 and 33,
revolution flow caused by the vane 20 develops a turbulence
S at 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
suspensioniscirculatingthroughthe annularflow passage
4, heavy foreign obj ects 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
CA 02526192 2001-O1-19
- 6 -
suspension flows from the flow passage 4 to inside an inner
casing 3.
Cylindrical screen cylinders la and lb are
disposed inside the inner casing 3 . These screen cylinders
la and lb partition the inside of the inner casing 3 into
an agitation chamber 7 and exit chambers 14a, 14b. The
pulp suspension flowing in the flow passage 4 first flows
in the annular agitation chamber 7, formed between the
screen cylinders la and lb. When the pulp suspension is
flowing downward along the agitation chamber 7, part of
the pulp passes through the inner screen cylinder lb and
is filtered and sorted in the inner exit chamber 14a. The
remaining pulp passes through the outer screen cylinder
h 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 la, lb, as they
are, flow downward along the agitation chamber 7 and are
discharged from a reject exit 10.
In addition, within the agitation chamber 7,
a plurality of outer vanes 20a are disposed in opposition
to the outer screen cylinder la and a plurality of inner
vanes 20b are disposed in opposition to the inner screen
cylinder lb. The vanes 20a, 20b are fixedly attached to
a rotor 6 hung from the upper portion of a main shaft 11.
The outer vanes 20a are disposed at equal spaces in the
circumferential direction, holding a constant space (2.5
to 8 mm) from the outer screen cylinder la. Similarly,
CA 02526192 2001-O1-19
_ 7 _
the inner vanes 20b are disposed at equal spaces in the
circumferentialdirection, holingthe constantspace (2.5
to 8 mm) from the inner screen cyl finder lb . The main shaf t
11 is freely rotatably supported by bearings and is driven
to rotate by an electric motor (not shown) throughaV-pulley
18 mounted on the lower end portion thereof . If the rotor
13 rotates and therefore the vanes 20a, 20b 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
cylinders la, lb is prevented.
The aforementioned pulp screening devices,
however, have the following problems:
First, the conventional pulp screening device
shown in Figs. 28 and 29 has a limit to its processing
ability since it has only a single screen cylinder 1. In
addition,because ofthe configuration ofthe conventional
vane 20, the revolution flow caused by the vane 20 becomes
faster as it is near the surface of the vane 20 and slower
as it is away from the vane surface. Therefore, the
efficiency of cleaning the surface of the screen cylinder
1 is low, and there is a problem that the passage amount
of the pulp will be reduced. Furthermore, the surface
of the vane 20 remote from the surface of the screen cylinder
1 wastefully consumes the power required for friction,
because it makes no contribution to the cleaning of the
CA 02526192 2001-O1-19
_ g _
surface of the screen cylinder 1.
In the conventional pulp screening device shown
in Figs. 34 and 35, the speed of the revolution flow,
developed by revolution of the vanes 20a and 20b, is slower
at the inner screen cylinder lb than at the outer screen
cylinder la because of the difference in diameter between
the inner and outer screen cyl finders la and lb . In addi tion,
the pressure acting on the inner screen cylinder lb is
lower than that acting on the outer screen cylinder la
because of a difference in centrifugal force. Therefore,
the outer screen cylinder la tends to pass the pulp to
more than the effective area of the screen cylinder 1a,
whereas the inner screen cylinder lb tends to pass the
pulp to less than the effective area of the screen cylinder
lb.
Because of this, when the quantity of pulp to
be processed is excessively reduced, the outer screen
cylinder la will pass the pulp therethrough, but there
is a problem that the inner screen cylinder lb will be
liable to be clogged due to pulp flowing backward.
Conversely, when the quantity of pulp to be processed is
increased, the inner screen cylinder lb will properly pass
pulp therethrough, but there is a problem that the outer
screen cylinder la will increase in passage resistance
and will be likely to be clogged.
In addition, because revolution flow passes
through between the inner and outer vanes 20b, 20a, the
CA 02526192 2001-O1-19
_ g _
speed of the revolution flow within the agitation chamber
7 becomes faster only in the vicinities of the inner and
outer vanes 20b, 20a and slower at positions away from
the inner and outer vanes 20b, 20a. Because of this, the
efficiency of cleaning the surfaces of the screen cylinders
la, lb is low and there is a problem that the quantity
of pulp to be passed will be reduced. Furthermore, because
of underagitation of pulp, a good quality of pulp will
be discharged from the reject exit 10 without being
processed by the screen cylinders la, lb, and there is
also a problem that the screening efficiency will be
reduced.
In addition, as described above, the
conventional pulp screening device has the problem that
the quantity of pulp to be passed wil l be 1 invited by clogging
of the holes in the screen cylinder 1. The clogging of
the holes in the screen cylinder 1 results from the
configuration of the holes formed in the screen cylinder
1.
More specifically, the turbulence S (see Figs.
31 to 33), developed at the inlet of the hole by the
revolution flow resulting from revolution of the vane 20,
has the effect of preventing the hole from being clogged.
However, the strength of the turbulence S is affected by
the configuration of the front edge of the hole (located
on the upstream side of the revolution flow) . In addition,
the difficulty for tangled fibers to be caught, and the
CA 02526192 2001-O1-19
- 10 -
ease of removing foreign objects, are affected by the
configuration of the rear edge of the hole (located on
the downstream side of the revolution flow).
In the case of configuration such as that shown
in Fig . 31, the turbulence S develops at the incl fined surf ace,
on the upstream side, of the hole 100 formed by the
dish-shaped chambered surface 101, but the developed
vertex S is weak because the inclined surface is gentle.
Therefore, the turbulence S is less liable to reach the
front edge 102 or rear edge 103 of the hole 100. Because
of this, the effect of preventing clogging by the turbulence
S is low. In addition, because the dish-shaped chambered
surface 101 is formed coaxially with the hole 100, room
for forming the dish-shaped chambered surface is required
and the number of holes per unit area is thus Limited.
Because of this, there is a limit to increasing the quantity
of pulp to be passed, by increasing the number of holes
100.
In addition, in the case of configuration such
as the one shown in Fig. 32, the turbulence S which develops
is strong, because the vertical portion of the trapezoidal
groove 111 is located on the upstream side of f low . However,
since the front edge 112 of the hole 110 is positioned
at the groove bottom portion near the vertical portion
of the trapezoidal groove 111, the vortex S developed is
less likely to reach the front edge 112 and therefore the
effect of preventing clogging of the hole 110 is low.
CA 02526192 2001-O1-19
- 11 -
Similarly, as the rear edge 113 is positioned at the groove
bottom portion and is away from the inclined portion 114,
separation of tangled fibers, etc, caught in the hole 100,
is not easy. Besides, because the hole 110 can be disposed
only in the bottom portion of the trapezoidal groove 111,
the number of holes per unit area is also limited.
Furthermore, in the case of configuration such
as that shown in Fig. 33, the turbulence S develops at
the vertex of the waveform formed on the surface of the
screen cylinder 1. However, the front edge 122 of the
hole 120 is far from the vertex of the waveform and the
front and rear edges 122, 123 are at the inclined portion
121 of the waveform. Therefore, the turbulence S is less
likely to reach the edges 122, 123, and the effect of
preventing clogging of holes by the turbulence S is thus
low. In addition, since the rear edge 123 has an acute
angle, separation of a lump of pulp, etc. , caught on the
edge, is not easy. Moreover, the number of holes per unit
area is limited, because the hole 120 can be disposed only
in the inclined portion 121 of the waveform.
As described above, in any of the hole
configurations shown in Figs. 31 to 33, the effect of
preventing clogging by the turbulence S is not satisfactory.
Therefore, it is necessary to make the turbulence S stronger
by revolving the vanes 20 at high speeds in order to prevent
clogging of holes . The power required for revolving the
vanes 20, however, becomes greater in proportion to the
CA 02526192 2001-O1-19
- 12 -
square to cube of the revolution speed, so the quantity
of passage per consumption power is inversely reduced.
DISCLOSURE OF THE INVENTION
The present invention has been made in view of
the problems found in the prior art . Accordingly, it is
the primary object of the present invention to provide
a pulp screening device that is capable of screening a
large quantity of pulp with low power, by preventing
clogging of a screen cylinder.
To achieve this end and in accordance with one
important aspect of the present invention, there is
provided a pulp screening device, comprising:
a pait of inner and outer screen cylinders; and
one or a plurality of vanes which revolve within
an agitation chamber formed between the inner and outer
screen cylinders, holding a predeterminedsmallspacefrom
each of the inner and outer screen cylinders.
The agitation chamber can be practically
partitionedinthe circumferentialdirection,by providing
the vanes which revolve within the agitation chamber formed
between the inner and outer screen cylinders, holding a
predetermined small space from each of the inner and outer
screen cylinders. With this arrangement, the internal
pressure within the agitation chamber becomes higher, as
the revolution speed of pulp is increased. Therefore,
the separation and agitation of foreign objects and lumps
CA 02526192 2001-O1-19
- 13 -
of pulp are accelerated, and clogging of the screen
cylinders is prevented and the quantity of pulp to be passed
is increased. In addition, the distance between the inner
and outer screen cylinders can be shortened by sharing
a single vane with the inner and outer screen cylinders.
Because of this, the speed difference of the pulp between
the inner and outer screen cylinders caused by the
difference in diameter therebetween, and the pressure
difference caused by centrifugal force, become smaller
compared with prior art. Particularly, a reduction in
the quantity of pulp to be passed due to clogging of the
inner screen cylinder is prevented. Therefore, there is
no possibility that the screen cylinders will be clogged
even when the revolution speed of the vanes is relatively
slow, and there is obtained an effect that a large quantity
of pulp can be screened with low power.
In a first preferred form of the present
invention, the revolution-direction front portion of the
vane has a wall face extending radially toward the
2 0 peripheral surfaces of the inner and outer screen cyl finders .
With this arrangement, the direction of the revolution
flow of the pulp is changed from the circumferential
direction to the radial direction by the wall face. The
radial flow of the pulp renders it possible to partition
the agitation chamber efficiently.
In a second preferred form of the present
invention, the wall face is formed at a right or acute
CA 02526192 2001-O1-19
- 14 -
angle to the direction of revolution. With this
arrangement, the revolution flow of the pulp can
perpendicularly approach the peripheral surfaces of the
inner and outer screen cylinders, and it becomes possible
to partition the agitation chamber more efficiently.
In a third preferred form of the present
invention, the cross section of the vane is formed so that
the spacing between the cross section and each of the inner
and outer screen cylinders widens gradually from the wall
face in the direction of revolution. With this
configuration, the pressure within the agitation chamber
becomes negative on the rear portion side of the vane.
Therefore, the pulp suspensionflowsbackwardfrom outside
the inner and outer screen cylinders into the agitation
chamber. As a result, lumps of pulp, etc. , caught in the
screen cylinders, are removed. In addition, the pulp
density within the agitation chamber is diluted, and there
is obtained an effect that repassage of the high-density
pulp, which is not passed through the screen cylinders,
becomes easy.
In a fourth preferred form of the present
invention, the cross section of the vane is formed into
the shape of a wedge extending at an acute angle from a
revolution-direction tip end to both proximity portions
closest to the inner and outer screen cylinders. With
this shape, the position of the tip end of the vane can
be adjusted by adjusting the incidence angle of the vane,
CA 02526192 2001-O1-19
- 15 -
and it becomes possible to supply pulp to the inner and
outer screen cylinder equally.
In a fifth preferred form of the present
invention, a distance from the tip end to both proximity
portions is set to two to five times a distance between
both proximity port ions. With this, there is no reduction
in the screening efficiency of the screen cylinder and
no rise in the operating power per unit processing ability
of the screen cylinder. Therefore, clogging of the inner
and outer screen cyl finders is prevented, whereby it becomes
possible to assure a large quantity of pulp to be passed
with low power.
In a sixth preferred form of the present
invention, the aforementioned tip end is disposed at a
center between the inner and outer screen cylinders, or
at a position of f set from the center toward the outer screen
cylinder. With this arrangement, theloadfor processing
pulp can be balanced between the inner and outer screen
cylinders.
In a seventh preferred form of the present
invention, the cross section of the vane is formed so that
the spacing between the cross section and each of the inner
and outer screen cylinders widens gradually from both
proximity portions in the direction of revolution. With
this configuration, the pressure within the agitation
chamber becomes negative on the rear portion side of the
vane. Therefore, the pulp suspensionflows backwardfrom
CA 02526192 2001-O1-19
- 16 -
outside the inner and outer screen cylinders into the
agitation chamber. As a result, lumps of pulp, etc.,
caught in the screen cylinders, are removed. In addition,
the pulp density within the agitation chamber is diluted,
and there is obtained an effect that repassage of the
high-density pulp, which is not passed through the screen
cylinders, becomes easy.
In an eighth preferred form of the present
invention, adjacentvanesof the aforementioned plurality
of vanes are connected by a partition wall . This further
divides the agitation chamberinto two parts. Therefore,
flow from inside the agitation chamber to outside the
agitation chamber, which is caused by centrifugal force,
can be blocked, and it becomes possible to increase the
quantity of pulp to be passed at the inner screen cylinder.
In a ninth preferred form of the present
invention, the cross section of an inner discharge tube
at a point where the inner discharge tube j oins an outer
discharge tube is set greater than the cross section of
the outer discharge tube, pulp being passed through the
inner screen cylinder and flowing in the inner discharge
tube and also being passed through the outer screen cyl finder
and flowing in the outer discharge tube . With this setting,
an effect is obtainable that the flow of the pulp from
the inner discharge tube becomes satisfactory and that
the quantity of pulp to be processed is thus increased.
To achieve the aforementioned object and in
CA 02526192 2001-O1-19
- 17 -
accordance with another important aspect of the present
invention, there is provided a pulp screening device,
comprising:
a screen cylinder; and
one or a plurality of vanes which revolve within
an agitation chamber formed outside or inside the screen
cylinder, holding a predetermined small space from the
screen cylinder;
wherein a revolution-direction front portion
of the vane has a wall face extending radially toward the
peripheral surface of the screen cylinder, and the vane
is formed so that the spacing between the vane and the
screen cylinder widens gradually from the wall face toward
a revolution-direction rear end.
With such a construction, clogging of the screen
cylinder can be prevented by making the difference in
pressure within the agitation chamber greater before and
after the wall face, and there is obtained an effect that
a great quantity of pulp can be screened with low power.
To achieve the aforementioned object and in
accordance with still another important aspect of the
present invention, there is provided a pulp screening
device, comprising:
a screen cylinder having a plurality of filter
holes; and
one or a plurality of vanes which revolve within
an agitation chamber formed outside or inside the screen
CA 02526192 2001-O1-19
- 18 -
cylinder, holding a predetermined small space from the
screen cylinder;
wherein a plurality of conical hollows are
provided in the peripheral surface of the screen cylinder
which faces the agitation chamber, and the filter hole
is formed to be offset from the center of the conical hollow
in the direction opposite to the direction in which the
vane revolves.
With construction like this, a strong,
turbulence is developed at the inlet of the filter hole
by the revolution flow of the pulp, and the pulp is
satisfactorily agitated. In addition, a lump of pulp and
foreign objects are prevented from being caught in the
filter holes, and clogging of the filter holes is prevented.
Therefore, there is obtainable an effect that a large
quantity of pulp can be screened with low power.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a part-sectional plan view showing
a pulp screening device constructed according to a first
embodiment of the present invention;
FIG. 2 is a part-sectional side view taken in
the direction of arrow A of FIG. 1;
FIG. 3 is a perspective view showing the
construction of the rotor of the pulp screening device
of the first embodiment of the present invention;
FIG. 4 is a sectional view showing the
CA 02526192 2001-O1-19
- 19 -
configuration of the vane of the pulp screening device
of the first embodiment of the present invention;
FIG. 5A is a diagram for explaining the
operational effect of the pulp screening device of the
firstembodiment, the positional relationship betweenthe
inner and outer screen cylinders and the vane being shown;
FIG. 5B is a diagram showing a pressure
distribution that acts on the screen cylinders in the
positional relationship shown in FIG. 5A;
FIG. 6 is a sectional view showing a first
variation of the vane of the pulp screening device of the
first embodiment of the present invention;
FIG. 7 is a sectional view showing a second
variation of the vane of the pulp screening device of the
first embodiment of the present invention;
FIG. 8 is a sectional view showing a third
variation of the vane of the pulp screening device of the
first embodiment of the present invention;
FIG. 9 is a perspective view showing a variation
of the rotor of the pulp screening device of the first
embodiment of the present invention;
FIG. 10 is a sectional view showing a fourth
variation of the vane of the pulp screening device of the
first embodiment of the present invention, the positional
relationship betweenthe inner and outer screen cylinders
and the vane being shown;
FIG. 11 is a perspective view showing the
CA 02526192 2001-O1-19
- 20 -
construction of a rotor that corresponds to the
configuration of the common vane shown in FIG. 10;
FIG. 12 is a sectional plan view showing a pulp
screening device constructed according to a second
embodiment of the present invention;
FIG. 13 is a sectional view taken substantially
taken along line B-B in FIG. 12;
FIG. 14 is a perspective view showing the
construction of the rotor of the pulp screening device
of the second embodiment of the present invention;
FIG. 15 is a sectional view showing the
configuration of the vane of the pulp screening device
of the second embodiment of the present invention;
FIG. 16A is a diagram for explaining the
operational effect of the pulp screening device of the
second embodimentofthe presentinvention,the positional
relationship between the inner and outer screen cylinders
and the vane being shown;
FIG. 16B is a diagram showing a pressure
distribution that acts on the outer screen cylinder in
the positional relationship shown in FIG. 16A;
FIG. 16C is a diagram showing a pressure
distribution that acts on the inner screen cylinder in
the positional relationship shown in FIG. 16A;
FIG. 17 is a diagram for explaining the
operational effect of the pulp screening device of the
second embodiment of the present invention, the
CA 02526192 2001-O1-19
- 21 -
configuration of a conventional vane which becomes an
object of comparison having been shown;
FIG. 18 is a sectional view showing a first
variation of the vane of the pulp screening device of the
second embodiment of the present invention;
FIG. 19 is a sectional view showing a second
variation of the vane of the pulp screening device of the
second embodiment of the present invention;
FIG. 20 is a sectional view showing a third
variation of the vane of the pulp screening device of the
second embodiment of the present invention;
FIG. 21 is a sectional view showing a fourth
variation of the vane of the pulp screening device of the
second embodiment of the present invention;
FIG. 22 is a sectional view showing a fifth
variation of the vane of the pulp screening device of the
second embodiment of the present invention;
FIG. 23 is a plan view showing the construction
of the screen cylinder of a pulp screening device
constructed according to a third embodiment of the present
invention;
FIG. 24 is a sectional view taken substantially
taken along line C-C in FIG. 23;
FIG. 25 is a diagram showing a first variation
of the positional relationship between the conical hollow
and round hole of the pulp screening device constructed
of the third embodiment of the present invention;
CA 02526192 2001-O1-19
- 22 -
FIG. 26 is a diagram showing a second variation
of the positional relationship between the conical hollow
and round hole of the pulp screening device of the third
embodiment of the present invention;
FIG. 27 is a diagram showing a third variation
of the positional relationship between the conical hollow
and round hole of the pulp screening device of the third
embodiment of the present invention;
FIG. 28 is a part-sectional plan view showing
a conventional pulp screening device;
FIG. 29 is a part-sectional side view taken in
the direction of arrow D of FIG. 28;
FIG. 30A is a diagram for explaining the
operational effect of the conventional pulp screening
device, the positional relationship between the screen
cylinder and the vane being shown;
FIG. 30B is a diagram showing a pressure
distribution that acts on the screen cylinder in the
positional relationship shown in FIG. 30A;
FIG. 31 is a sectional view showing the
configuration of the hole in the screen cylinder of the
conventional pulp screening device;
FIG. 32 is a sectional view showing a first
variation of the hole in the screen cylinder of the
conventional pulp screening device;
FIG. 33 is a sectional view showing a second
variation of the hole in the screen cylinder of the
CA 02526192 2001-O1-19
- 23 -
conventional pulp screening device;
FIG. 34 is a sectional view showing another
conventional pulp screening device; and
FIG. 35 is a sectional view taken substantially
along line E-E in FIG. 34.
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiments of the present invention will
hereinafter be described with reference to the drawings.
Figs . 1 through 5 show a pulp screening device
constructed according to a f first embodiment of the present
invention. The pulp screening device will hereinafter
be described with reference to Figs . 1 to 5 . Fig . 1 shows
a part-sectional plan view of the construction of the pulp
screening device. Fig. 2 shows a part-sectional side view
taken in the direction of arrow A of Fig. 1. Fig. 3 shows
a perspective view of the construction of the rotor of
the pulp screening device. Fig. 4 shows a sectional view
of the conf igurationof the common vane of the pulp screening
device of the first embodiment. Fig. 5 shows a diagram
for explaining the operational effect of the pulp screening
device. Note that the same reference numerals will be
applied to the same parts as the aforementioned
conventional pulp screening device.
The pulp screening device has two screen
cylinders la, lb differing in diameter, as illustrated
in Figs . 1 and 2 . An agitation chamber 7 is formed between
CA 02526192 2001-O1-19
- 24 -
the screen cylinders la and lb. An outer exit chamber
14a is formed outside the outer screen cylinder la, and
an inner exit chamber 14b is formed inside the inner screen
cylinder lb.
A pulp suspension, fed from a pump (not shown) ,
first flows in a tangential direction through the entrance
2 of a cylindrical container 17 and circulates through
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 flow passage 4, heavy
foreign objects such as sand, etc. , are discharged outside
the device froma trap 5 provided in the tangential direction
opposite from the entrance 2. The remaining pulp flows
from the flow passage 4 into the agitation chamber 7.
The screen cylinders la, lb are provided in the
peripheral surfaces thereof with a large number of slits
of width 0.15 to 0.5 mm or holes of diameter 0.2 to 4.8
mm. Because of this, when the pulp suspension is flowing
downward along the agitation chamber 7, part of the pulp
passes through the inner screen cylinder lb and is filtered
and sorted in the inner exit chamber 14b, while the remaining
pulp passes through the outer screen cylinder la and is
filtered and sorted in the outer exit chamber 14a. On
the other hand, the foreign objects of sizes that cannot
pass through the screen cylinders la, lb, as they are,
flow downward along the agitation chamber 7 and are
discharged from a reject exit 10 via a reject receiver
CA 02526192 2001-O1-19
- 25 -
25.
In the pulp screening device, the inner exit
chamber 14b and the outer exit chamber 14a are completely
partitioned, and the pulp suspension, sorted in the outer
exited chamber 14a from the agitation chamber 7, passes
through an outer discharge tube 16 and is discharged from
the exit 9 . On the other hand, the pulp suspension sorted
in the inner exit chamber 14b passes through an inner
discharge tube 15 provided in the outer discharge tube
16, and is discharged from the exit 9, joining the pulp
suspension flowing from the outer exit chamber 14a into
the discharge 16. Note that the dimension of the cross
section of the exit of the inner discharge tube 15 is set
equal to or greater than the dimension of the cross section
of the outer discharge tube 16 at a point where the outer
discharge tube 16 joins the innerdischarge tube 15. Also,
the bottom surface of the inner exit chamber 14b, the bottom
surface of the outer exit chamberl4a, and the bottom surface
of the reject receiver 25 are inclined
CA 02526192 2001-O1-19
- 26 -
downward toward the exits 9 and 10 in order to prevent
deposition of the pulp.
A cylindrical rotor 6 is hung from the upper
portion of a main shaf t 11 and disposed within the agitation
chamber 7. The rotor 6 has a plurality of vanes 12
(hereinafter referred to as common vanes, because each
vane in the first embodiment acts in common on the inner
and outer screen cylinders) at its peripheral surface,
as illustrated in Fig. 3. The common vanes 12 are
interconnected at their lower ends by a connecting ring
30 and are disposed at equal spaces in the circumferential
direction of the rotor 6. As illustrated in Figs. 1 and
2, each common vane 12 is located within the agitation
chamber 7, holding a predetermined space (preferably 2
to 6 mm) from the inner peripheral surface of the outer
screen cylinder la and the predetermined space from the
outer peripheral surface of the inner screen cyl finder lb .
With this arrangement, the agitation chamber 7 in the pulp
screening device of the first embodiment is practically
partitioned in the circumferential direct ion by the common
vanes 12.
Now, the configuration of the common vane 12
wil l be described . The common vane 12 in the pulp screening
device of the first embodiment has a front wall 201 and
a deflection wall 202, as illustrated in Fig. 4. The front
wall 201 extends from a tip end 205 in the direction opposite
to the direction of revolution,
CA 02526192 2001-O1-19
- 27 -
and the deflection wall 202 is continuous to the front
wall 201 and extends in the radial direction of the rotor
6 (perpendicular to the direction of revolution). The
deflection wall 202 is jointed to a pair of rear curved
faces 204 extending from a rear end 206 in the direction
of revolution, and the joined portion forms an acute-angle
edge 203.
Withthe aforementioned vane configuration, the
spacing, within the agitation chamber 7, between the common
vane 12 and the screen cylinder la or 1b becomes gradually
narrower from the tip end 205 toward the rear direction
and then becomes even narrower suddenly at the deflection
wall 202 and narrowest at the edge 203. In the pulp
screening device of the first embodiment, the spacing
between the edge 203 and the screen cylinder la or lb is
setto the aforementioned predeterminedspace (preferably
2 to 6 mm). Furthermore, the spacing widens gradually
from the edge 203 to a rear end 206 (refer to Fig. 5A).
Note that it is preferable that the deflection wall 202
be concave and also preferable that the angle of deflection
at the deflection wall 202 (which is an angle, indicated
by 6 in Fig. 5A, which is formed by both the direction
of revolution and the direction in which the deflection
wall 202 extends) be 90 degrees or less.
Now, a description will be made of the operation
of the pulp screening device of the first embodiment
constructed as described above.
CA 02526192 2001-O1-19
- 28 -
The pulp suspension, fed from an upstream pump
(not shown) , first flows in a tangential direction through
the entrance 2 of the container 17 and circulates through
the flow passage 4. When the pulp suspension is
circulating the flow passage 4, the heavy foreign objects
in the pulp suspension, such as sand, etc . , are discharged
outside the device fromthe trap 5 provided in the tangential
direction opposite to the entrance 2, and the remaining
pulp flows into the agitation chamber 7, formed between
the screen cylinders la and lb inside the inner casing
3.
If the common vane 12 revolves within and along
the annular agitation chamber 7, as shown in Fig. 5A, the
pulp within the agitation chamber 7 flows in the direction
opposite to the direction of revolution of the common vane
12, relative to the common vane 12 . However, because the
common vane 12 is provided with the deflection wall 202
extending in the radial direction, the circumferential
flow of the pulp strikes on the deflection wall 202 and
is therefore changed to the radial flow. As a result,
the flow of the pulp into the space between the screen
cylinder la or lb and the common vane 12 is suppressed.
That is, the agitation chamber 7 is practicallypartitioned
at the space between the screen cylinder la or lb and the
common vane 12, by the radial flow near the deflection
wall 202.
Thus, the agitation chamber 7 is practically
CA 02526192 2001-O1-19
- 29 -
partitioned into a plurality of parts in the
circumferential direction by the radial flow of the pulp
near the deflect ionwal1s202. Therefore, the pulp, within
the agitation chamber 7 partitioned into a plurality of
parts, is pushed by the common vanes 12 and revolved in
the circumferential direction at approximately the same
speed as that of the common vane 12. Since the radial
flow of the pulp toward the surface of the screen cylinder
la or lb is developed by the deflection wall 202, the
internal pressure within the agitation chamber 7 rises
greatly from the tip end 205 to the edge 203, as illustrated
in Fig. 5B. The rise in the revolution speed of the pulp
and the rise in the internal pressure accelerate the
separation and agitation of foreign obj ects and lumps of
pulp at the chamfered portions (see Figs. 31 to 33) of
the holes 100 in the surfaces of the screen cylinders la,
lb.
Note that for the revolution speed of the pulp
within the agitation chamber 7, there is a difference in
speed between the surface of the outer screen cylinder
la and the surface of the inner screen cylinder lb, because
of the difference in diameter therebetween. However, in
the pulp screening device of the first embodiment, the
spacing between the screen cylinders la and lb is
approximately the same as the thickness of a single common
vane 12, and is narrower, compared with the conventional
pulp screening device provided with two screen cylinders
CA 02526192 2001-O1-19
- 30 -
(see Figs. 34 and 35). Therefore, the speed difference
of the pulp between the inner and outer screen cylinders
la and lb is smaller compared with conventional pulp
screening device, and the pressure difference developed
by centrifugal force is also smaller compared with
conventional pulp screening device.
On the other hand, on the rear portion side of
the common vane 12 (behind the edge 203), the pulp is
inhibited from flowing into the screen cylinder la or lb
through the space between the surface of the screen cyl finder
la or lb and the edge 203 . In addition, the spacing between
the surface of the screen cylinder la or lb and the rear
curved face 204 widens gradually. Therefore, as
illustrated in Fig. 5B, the internal pressure within the
agitation chamber 7 results in a great negative pressure,
which causes the pulp suspension to flow backward from
the exit chambers 14a, 14b into the agitation chamber 7.
G~Tith the back flow of the pulp suspension, the lumps of
pulp, etc. , caught in the holes 100 of the screen cylinders
la, 1b, are removed and the pulp densitywithintheagitation
chamber 7 is diluted.
The pulp suspension, passed through the outer
screen cylinder la via the agitation chamber 7, and sorted
in the outer exit chamber 14a, is discharged from the outer
discharge tube 16. Also, the pulp suspension, passed
through the inner screen cylinder lb via the agitation
chamber 7, and sorted in the inner exit chamber 14b, is
CA 02526192 2001-O1-19
- 31 -
discharged from the exit 9 through the inner discharge
tube 15. When this occurs, a static pressure component
in the flow from the inner exit chamber 14b is increased
and a static pressure component in the flow from the outer
exit chamber 14a is conversely decreased, because the
dimension of the cross section of the exit of the inner
discharge tube 15 is set equal to or greater than the
dimension of the cross section of the outer discharge tube
16 at a point where the outer discharge tube 16 joins the
inner discharge tube 15.
From the foregoing description, the pulp
screening device of the first embodiment has the following
advantages:
First, in the pulp screening device, a single
common vane 12 is shared with the inner and outer screen
cylinders la, lb so that the distance between the screen
cylinderscan be reduced. Therefore,thespeed difference
of the pulp between the inner and outer screen cylinders
la, lb caused by the difference in diameter therebetween,
and the pressure difference caused by centrifugal force,
become smaller compared with conventional pulp screening
device . As a result, the holes in the inner screen cyl finder
lb are less likely to be clogged and a reduction in the
quantity of pulp to be passed is prevented.
Also, the common vane 12 is provided with the
deflection wall 202. Because of this, the agitation
chamber 7 is practically partitioned into a plurality of
CA 02526192 2001-O1-19
- 32 -
parts by the radial flow of the pulp near the deflection
walls 202. This causes the revolution speed of the pulp
to rise and the internal pressure within the agitation
chamber 7 to rise . Therefore, the separation and agitation
of the foreign objects and lumps of pulp at the chamfered
portions of the holes 100 in the screen cylinders la and
lb are accelerated, and clogging of the holes 100 is
prevented and the quant i ty of pulp to be pas sed i s increased .
In addition, the radial flow of the pulp near
the deflection wall 202 inhibits the pulp from flowing
through between the surface of the screen cylinder la or
lb and the edge 203. The formation of the rear curved
face 204 behind the edge 203 causes the internal pressure
within the agitation chamber 7 to be negative on the rear
portion side of the common vane 12. Therefore, the pulp
suspension flows backward from the exit chambers 14a and
14b into the agitation chamber 7. As a result, lumps of
pulp, etc . , caught in the holes 100 of the screen cylinders
1a, lb, are removed, and the pulp density within the
agitation chamber 7 is diluted and repassage of
high-density pulp not passed through the screen cylinders
1a, lb becomes easy.
That is, the pulp screening device of the first
embodiment is capable of effectively utilizing both the
operating surfaces of the common vane 12 and the surfaces
of the inner and outer screen cylinders la, lb and therefore
has the advantage that a large quantity of pulp can be
CA 02526192 2001-O1-19
- 33 -
screened and processed with low power at a relatively slow
revolution speed, while preventing clogging of the holes
in the screen cylinders la, lb.
Besides, the dimension of the cross section of
the exit of the inner discharge tube 15 is set equal to
or greater than the dimension of the cross section of the
outer discharge tube 16 at a point where the outer discharge
tube 16 joins the inner discharge tube 15. Because of
this, a static pressure component in the flow from the
inner exit chamber 14b is increased, whereas a static
pressure component in the flow from the outer exit chamber
14a is conversely reduced. Therefore, the flow of the
pulp from the inner exit chamber 14b, which is less liable
to flow compared with the outer exit chamber 14a, becomes
satisfactory. Because of this, there is also an advantage
that the quantity of pulp to be passed can be increased.
Furthermore,inthe conventionalpulpscreening
device, the tip end portion of the vane is round and the
spacing between the tip end portion and the screen cylinder
is gradually reduced, and consequently, foreign objects
are liable to be caught in the reduced spacing and are
dif f icult to remove . However, in the pulp screening device
of the first embodiment, the deflection wall 202 is formed
in the common vane 12, whereby there
CA 02526192 2001-O1-19
- 34 -
is also an advantage that foreign objects are not caught
in the space between the common vane 12 and the screen
cylinder la or lb, as is done in conventional pulp screening
device by wedge effect.
Note that the common vane 12 in the pulp screening
device of the first embodiment is not limited to that shown
in Fig. 4. The radial thickness, circumferential width,
axiallength,number ofaxialdivisions,axialinclination,
configuration of the front wall, deflection wall, and rear
curved face, etc. , can be varied according to pulp type,
pulp density, screen cylinder hole dimensions, rotor speed,
etc. For example, the configuration of the common vane
12 will be satisfied if it has at least a deflection wall
and a rear curved face extending from the edge of the
deflection wall to the rear end of the vane . Therefore,
a f rout wal l 2 O 1 may be formed into a f 1 at shape such as
that shown in Fig. 6. Also, as illustrated in Fig. 7,
the front wall 201 may be formed into a semicircular shape
with a tip end 205 as avertex. Furthermore, as illustrated
in Fig. 8, the front wall can be omitted and the vane can
be formed with both a concave (or flat) deflection wall
302 and a pair of rear curved faces 204 extending from
an edge 203 to a rear end 206.
Similarly, the configuration of the rotor 6 is
not limited to the one shown in Fig. 3. For instance,
as illustrated in Fig. 9, the rotor may be axially divided
CA 02526192 2001-O1-19
- 35 -
into two and the upper common vanes and the lower common
vanes may be connected by two connection rings 30, and
the upper and lower common vanes may be disposed so that
they are shifted in phase. According to the construction
illustrated in Fig. 9, as with the first embodiment, the
agitation chamber 7 can be practically partitioned into
a plurality of parts in the circumferential direction by
the common vanes 12, and the mechanical strength of the
common vanes 12 is enhanced, whereby deformation of the
common vanes 12 by centrifugal force can be prevented.
Moreover, as illustrated in Figs. 10 and 11,
the common vanes 12 may be interconnected by partition
walls 301 and the agitation chamber 7 may be separated
into an inner agitation chamber 7a and an outer agitation
chamber 7b. If constructed in this manner, the radial
flow of the pulp within the agitation chamber 7 (from the
inner screen cylinder toward the outer screen cylinder) ,
which results form centrifugal force, can be blocked by
the partition walls 301. Therefore, it becomes possible
2 0 to further increase the quantity of pulp to be passed through
the inner screen cylinder la.
Furthermore, the configuration of the common
vane 12 in the pulp screening device of the present invention
is not 1 invited to devices provided with two screen cylinders,
as in the first embodiment. For instance, it is also
applicable to devices having a single screen cylinder
outside or inside an agitation chamber, as illustrated
CA 02526192 2001-O1-19
- 36 -
in Fig. 28. In this case, the vane will be satisfied if
only the portion of the vane opposite to the screen cylinder
has at least a deflection wall and a rear curved face
extending from the edge of the deflection wall to the rear
end of the vane. Even in this case, clogging of holes
in the screen cylinder can be reduced, compared with the
conventional device having a single screen cylinder
outside or inside an agitation chamber (see Fig. 28) , and
there is an advantage that it becomes possible to screen
and process a large amount of pulp.
Now, a pulp screening device
according to a second embodiment of the present invention
will be described with reference to Figs. 12 to 17. Fig.
12 shows a sectional view of the construction of the pulp
screening device of the second embodiment . Fig . 13 shows
a sectional view taken along line B-B in Fig. 12. Fig.
14 shows a perspective view of the construction of the
rotor of the pulp screening device of the second embodiment .
Fig. 15 shows a sectional view of the configuration of
the vane of the pulp screening device of the second
embodiment. Fig.l6isusedfor explainingthe operational
effect of the pulp screening device of the second embodiment .
Fig. 17 is used to explain the operational effect of the
configuration of the vane of the pulp screening device
of the second embodiment. Note that the same reference
numerals will be applied to the same parts as the
aforementioned conventional pulp screening device or the
CA 02526192 2001-O1-19
- 37 -
pulp screening device of the first embodiment.
The pulp screening device of the second
embodiment, as with the f first embodiment , has two screen
cylinders la, lb differing in diameter, as illustrated
in Figs . 12 and 13 . An agitat ion chamber 7 is formed between
the screen cylinders 1a and lb. An outer exit chamber
14a is formed outside the outer screen cylinder la, and
an inner exit chamber 14b is formed inside the inner screen
cylinder lb. The outer exit chamber 14a is in fluid
communication with the inner exit chamber 14b through the
bottom portion.
A pulp suspension, flowing in a tangential
direction through the entrance 2 of a cylindrical container
17, circulates through an annular flow passage 4. When
the pulp suspension is circulating through the flow passage
4 , heavy foreign obj ects such as sand, etc . , are discharged
outside the device from a trap 5, and the remaining pulp
flows from the flow passage 4 into the aforementioned
agitation chamber 7. The screen cylinders la, lb forming
the agitation chamber 7 are provided in the peripheries
thereof with a large number of slits of width 0.15 to 0.5
mm or holes of diameter 0.2 to 4.8 mm. Because of this,
when the pulp is flowing downward along the agitation
chamber 7, the pulp passes through the inner and outer
screen cylinders la, 1b and are filtered and sorted in
the exit chambers 14a, 14b and are discharged from an exit
9. On the other hand, the foreign objects of sizes that
CA 02526192 2001-O1-19
- 38 -
cannot pass through the screen cylinders la, lb, as they
are, flow downward along the agitation chamber 7 and are
discharged from a reject exit 10.
A cylindrical rotor 6 is hung from the upper
portion of a main shaft 11 and disposed within the agitation
chamber 7. The rotor 6 has a plurality of vanes 21
(hereinafter referred to as distribution vanes, because
the primary obj ect of the vanes in the second embodiment
is to properly distribute pulp to the inner and outer screen
cylinders) at its peripheral surface, as illustrated in
Fig. 14. The distribution vanes 21 are interconnected
at their lower ends by a connecting ring 30 and are disposed
at equal spaces in the circumferential direction of the
rotor 6. As illustrated in Figs. 12 and 13, each
distribution vane 21 is located within the agitation
chamber 7, holding a predetermined space (preferably 2
to 6 mm) from the inner peripheral surface of the outer
screen cylinder la and the predetermined space from the
outer peripheral surface of the inner screen cylinder lb.
V~Iith this arrangement, the agitation chamber 7 is
practically partitioned into a plurality of parts in the
circumferential direction by the distribution vanes 21.
The distribution vane 21 in the pulp screening
device of the second embodiment is in the shape of a wedge
and made up of four flat faces, namely an inner distribution
wall 402, an outer distribution wall 403, an inner suction
wall 406, and an outer suction wall 407, as illustrated
CA 02526192 2001-O1-19
- 39 -
in Fig. 15. An acute-angle front edge 401 is formed at
a point where the inner distribution wall 402 and the outer
distribution wall 403 join each other. Similarly, an
acute-angle rear edge 408 is formed at a point where the
inner suction wall 406 and the outer suction wall 407 join
each other. An obtuse-angle inner edge 404 is formed at
a point where the inner distribution wall 402 and the inner
suction wall 406 join each other. Likewise, an
obtuse-angle outer edge 405 is formed at a point where
the outer distribution wall 403 and the outer suction wall
407 join each other. When a distance from the inner edge
404 to the outer edge 405 (i.e., the thickness of the
distribution vane 21) is taken to be "d," a distance from
the f ront edge 4 O 1 to a 1 ine j oining both the inner edge
404 and the outer edge 405 ( i . a . , the height of the wedge
with the distribution vane thickness as its base and the
front edge 401 as its vertex) is set to 2 to 5d.
As illustrated in Fig. 12 or Fig. 16A, the
distribution vane 21 within the agitation chamber 7 is
disposed so that the spacing between the inner edge 404
and the inner screen cylinder lb, and the spacing between
the outer edge 405 and the outer screen cylinder la, become
narrowest. In the pulp screening device of the second
embodiment, the spacing between the inner edge 404 and
the inner screen cylinder lb, and the spacing between the
outer edge 405 and the outer screen cylinder 1a, are each
set to the aforementioned predetermined space (preferably
CA 02526192 2001-O1-19
- 40 -
2 to 6 mm) . In addition, the position of the front edge
401 is set so that it is at the center of the agitation
chamber 7 or at a position slightly offset from the center
toward the outer screen cylinder la.
Now, a description will be given of the operation
of the pulp screening device of the second embodiment
constructed as described above.
The pulp suspension, fed from an upstream pump
(not shown) , first flows in a tangential direction through
the entrance 2 of the container 17 and circulates through
the flow passage 4. When the pulp suspension is
circulating the flow passage 4, the heavy foreign objects
in the pulp suspension, such as sand, etc . , are discharged
outside the device from a trap 5, and the remaining pulp
flows into the agitation chamber 7, formed between the
screen cylinders 1a, lb inside the inner casing 3.
If the distribution vane 21 revolves within and
along the annular agitation chamber 7, as shown in Fig.
16A, the pulp within the agitation chamber 7 flows in the
direction opposite to the direction of revolution of the
distribution vane 21. The revolution flow of the pulp
is distributed at the front edge 401 of the distribution
vane 21 into a radially inner f low and a radial ly outer
flow. The inwardly distributed pulp flows along the inner
distribution wall 402 of the distribution vane 21 and is
supplied to the inner screen cylinder lb, while the
outwardly distributed pulp flows along the outer
CA 02526192 2001-O1-19
- 41 -
distribution wall 403 and is supplied to the outer screen
cylinder la.
The pulp being revolved tends to flow to the
side of the outer screen cylinder la by a difference in
pressure, developed by the centrifugal force exerted on
the pulp suspension. However, in the pulp screening device
of the second embodiment, it becomes possible to equally
supply the pulp to the inner and outer screen cyl finders
lb, la in accordance with a dimensional ratio of the holes
100 in the inner screen cylinder lb and the holes 100 in
the outer screen cylinder 1a by adjusting the position
of the front edge 401, because, as described above, the
revolution flow of the pulp can be distributed at the front
edge 401 into radially inner and outer flows.
The reason the position of the front edge 401
can be adjusted in this manner is that the distribution
vane 21 is formed into the shape of a wedge having an
acute-angle front edge. Assume that in the conventional
pulp screening device (see Fig. 34) , the maximum thickness
of the vane 20a or 2ob is "d", as illustrated in Fig. 17.
In the conventional pulp screening device, the distance
from the maximum thickness portion to the front end of
the vane 20a or 20b is about 0.5 to 1.5d, and the vane
front end portion is circular in shape and the radius of
curvature is about 0.5d (see Fig. 17). Because of such
a vane configuration, the position of the front end (the
foremost position with respect to the direction of flow)
CA 02526192 2001-O1-19
- 42 -
of the conventional vane 20a or 20b hardly changes even
when the incidence angle a of the vane is adjusted (see
the two-dotted line in Fig. 17). This is because the
conventional vane 20a or 20b is provided solely for the
purpose of the agitation of pulp within the agitation
chamber 7, and the blocking prevention of the screen
cylinders la, lb at the rear portion of the vane by negative
pressure, and also because the adjustment of the incidence
angle a is made for the purpose of varying the spacing
between the rear portion of the vane and the screen cylinder
la or 1b in order to adjust the magnitude of the negative
pressure.
On the other hand, in the pulp screening device
of the second embodiment, the position of the tip end of
the distribution vane 21, i.e. , the position of the front
edge 401 can be adjusted by adjusting the incidence angle
a , since the tip end is formed into an acute-angle wedge
shape, not a circular shape. Therefore, it becomes
possible to equally supply pulp to the inner and outer
screen cylinders lb, la in accordance with a dimensional
ratio of the holes 100 in the inner screen cylinder lb
and the holes 100 in the outer screen cylinder la.
The internal pressure within the agitation
chamber 7 gradually rises between the front edge 401 and
the inner edge 404, when the revolution flow of the pulp
passes through the spacing, which is gradually reduced,
between the innerdistributionwall 402 and the inner screen
CA 02526192 2001-O1-19
- 43 -
cylinderla. Similarly, the internal pressure within the
agitation chamber 7 gradually rises between the front edge
401 and the outer edge 405, when the revolution flow of
the pulp passes through the spacing, which is gradually
reduced, between the outer distribution wall 403 and the
outer screen cylinder lb . When this occurs, the revolut ion
flow of the pulp is equally distributed at the front edge
401 to the side of the outer screen cylinder la and the
side of the inner screen cylinder lb in accordance with
the aforementioned dimensional ratio of the holes 100.
Therefore, the internal pressure within the agitation
chamber 7, regardless of a difference in pressure due to
centrifugal force, rises approximately the same, between
the side of the outer screen cylinder la and the side of
the inner screen cylinder lb, as illustrated in Figs . 16B
and 16C.
On the other hand, on the rear portion side of
the distribution vane 21 (behind the inner and outer edges
404, 405) , the spacing between the inner suction wall 406
and the inner screen cylinder lb, and the spacing between
the outer suction wall 407 and the outer screen cylinder
la, widen gradually from the inner edge 404 and the outer
edge 405, respectively. Therefore, as illustrated in Figs .
16B and 16C, the internal pressure within the agitation
chamber 7 results in a great negative pressure, which causes
the pulp suspension to f low backward from the exit chambers
14a, 14b into the agitation chamber 7 . With the back f low
CA 02526192 2001-O1-19
- 44 -
of the pulp suspension, the lump of pulp, etc., caught
in the holes 100 of the screen cylinders la, lb, are removed
and the pulp density within the agitation chamber 7 is
diluted.
From the foregoing description, the pulp
screening device of the second embodiment has the following
advantages:
First, in the pulp screening device, as with
the first embodiment, a single distribution vane 21 is
shared with the inner and outer screen cylinders la, lb
so that the distance between the screen cylinders can be
reduced. Therefore, the speed difference of the pulp
between the inner and outer screen cylinders la, 1b caused
by the difference in diameter therebetween, and the
pressure difference caused by centrifugal force, become
smaller compared with conventionalpulp screening device.
As a result, the holes in the inner screen cylinder lb
become less liable to be clogged and a reduction in the
quantity of pulp to be passed is prevented.
Also, the revolution flow of the pulp can be
distributed into a radially inner flow and a radially outer
flow by the front edge 401 of the distribution vane 21.
Therefore, the pulp can be supplied equally to the outer
screen cylinder la and the inner screen cylinder lb
independently of centrifugal force action. As a result,
when the quantity of pulp to be passed is excessively reduced,
clogging due to a back flow at the inner screen cylinder
CA 02526192 2001-O1-19
- 45 -
lb is prevented. Also, when the quantity of pulp to be
passed is increased, clogging due to an increase in passage
resistance at the outer screen cylinder la is prevented.
That is, the load required for processing the pulp can
be balanced between the inner screen cylinder lb and the
outer screen cylinder la, and consequently, a flow-rate
range for the pulp is not limited as is done in conventional
pulp screening device.
In addition, the agitation chamber 7 is
practically partitioned into a plurality of parts by a
plurality of distribution vanes 21, so the revolution speed
of the pulp becomes approximately the same as the revolut ion
speed of the distribution vane 21. Because of this,
agitation of the pulp within the agitation chamber 7 is
accelerated, and there is nopossibilitythatagoodquality
of pulp will flow downward without being processed and
will be discharged from the rej ect exit 10, and consequently,
the screening efficiency rises. In addition, a rise in
the revolution speed of the pulp accelerates the separation
and agitation of the foreign objects and lumps of pulp
at the chamfered portions of the holes 100 in the screen
cylinders la and lb. As a result, clogging of the holes
100 is prevented and the quantity of pulp to be passed
is increased.
Besides, the spacing between the inner suction
wall 406 and the inner screen cylinder lb, and the spacing
between the outer suction wall 407 and the outer screen
CA 02526192 2001-O1-19
- 46 -
cylinder la, widen gradually from the inner edge 404 and
the outer edge405,respectively. Therefore, the pressure
within the agitation chamber 7 becomes negative on the
rear portion side of the distribution vane 21, and the
pulp suspension flows backward from the exit chambers 14a,
14b into the agitation chamber 7. As a result, lumps of
pulp, etc . , caught in the holes 100 of the screen cylinders
la, lb, are removed. Furthermore, the pulp density within
the agitation chamber 7 is diluted, and repassage of
high-density pulp, which is not passed through the screen
cylinders la, lb, becomes easy.
Thus, the pulp screening device of the second
embodiment, as with the first embodiment, is capable of
obtaining the advantage that a large quantity of pulp to
be passed can be assured with low power, by preventing
clogging of the screen cylinders la, lb.
Furthermore, the pulp screening device of the
second embodiment has also the following advantages,
because the height of the wedge shape of the distribution
vane 21 is set to a range of 2 to 5 times the base of the
wedge (i.e., when a distance from the inner edge 404 to
the outer edge 405 is taken to be "d," a distance from
the front edge 404 to a line joining both the inner edge
404 and the outer edge 405 is set to 2 to 5d).
That is, in the case where the height of the
wedge shape of the distribution vane 21 is less than twice
the base of the wedge shape, the revolution flow within
CA 02526192 2001-O1-19
- 47 -
the agitation chamber 7 changes sharply and results in
a radial flow toward the surface of the screen cylinder
la or lb. Therefore, this radial flow can effectively
partition the agitation chamber 7, but there is a
possibility that foreign objects will pass through slits
or holes along with the radial f low and, by this amount ,
the screening efficiency will be reduced.
On the other hand, if the height of the wedge
shape of the distribution vane 21 exceeds five times the
base of the wedge shape, the friction resistance of the
distribution vane 21 will increase and therefore the
operating power per unit processing ability will rise.
In addition, a plurality of distribution vanes 21 are
disposed, but if the height of the wedge shape becomes
higher ( i . a . , if the vane width becomes wider) , adj acent
distribution vanes 21 will become too close . As a result,
there is also a possibility that proper distribution of
the pulp cannot be performed.
Therefore, it is suitable that the height of
the wedge shape of the distribution vane 21 be set to a
range of two to five times the base of the wedge shape.
Since the pulp screening device of the second embodiment
is correctly set to the aforementioned range, there is
no reduction in the screening efficiency and no rise in
the operating power per unit processing ability.
Therefore, it becomes possible to prevent clogging of the
screen cylinders 1a, lb and assure a large quantity of
CA 02526192 2001-O1-19
- 48 -
pulp to be passed with low power.
Moreover, for the vane of the conventional pulp
screening device, the cross section, taken in the direction
perpendicular to the axis, is not a curved surface formed
in a f fixed curvature and requires straightness in the axial
direction. Because of this, there is a problem that the
manufacturing cost will be increased. However, the
distribution vane 21 in the pulp screening device of the
second embodiment is formed with four flat faces, an inner
distribution wall 402, an outer distribution wall 403,
an inner suction wall 406, and an outer suction wall 407.
Therefore, there is also an advantage that machining is
easy and manufacturing costs can be reduced.
Note that the distribution vane 21 in the pulp
screening device of the second embodiment is not limited
to the configuration shown in Fig. 15. The radial depth,
circumferential width, axial length, axial inclination,
number of vanes, configuration of the inner distribution
wall, outer distribution wall, inner suction wall, and
outer suction wall, etc. , can be varied according to pulp
type,pulp density,screen cylinder hole dimensions, rotor
speed, etc., without departing from the scope of the
invention hereinafter claimed.
That is, the configuration of the distribution
vane 21 will be satisfied, if it is formed from at least
four wall faces, an inner distribution wall, an outer
distribution wall, an inner suction wall, and an outer
CA 02526192 2001-O1-19
- 49 -
suction wall and is in the form of an acute-angle wedge
in the tip end direction, and if, when it is assumed that
a distance from the inner edge to the outer edge is ~~d, "
a distance from the front edge to a line joining both the
inner edge and the outer edge is set to 2 to 5d.
Therefore, for example, as illustrated in Fig.
18, an outer distribution wall 403 and an outer suction
wall 407 may be formed into convex faces, and an inner
distribution wall 402 and an inner suction wall 406 may
be formed into concave faces. In addition, as illustrated
in Fig. 19, an inner distribution wall 402 and an outer
distribution wall 403 may be formed into flat faces, and
an outer distribution wall 407 and an inner suction wall
406 may be formed into convex and concave faces,
respectively. Furthermore, as illustrated in Figs. 20
to 22, the front and rear edges 401, 408 in the distribution
vanes 21 of Figs. 15, 18, and 19 may be rounded.
Note that it is possible to make the thickness
d of the distribution vane 21 constant, since the spacing
between the inner screen cylinder lb and the outer screen
cylinder la can be made constant within the operational
range of the device independently of cylinder diameter.
In the case where a small-diameter screen cylinder with
a large curvature is employed, however, there are cases
where the height of the wedge shape of the distribution
vane 21 is limited to less than 5d (i.e., less than five
times vane thickness).
CA 02526192 2001-O1-19
- 50 -
Next, a pulp screening device according to a
third embodiment of the present invent ion will be described
with reference to Figs. 23 and 24. Fig. 23 shows a plan
view of the construction of the screen cylinder of the
third embodiment. Fig. 24 shows a sectional view taken
along line C-C in Fig. 23. Note that the same reference
numerals will be applied to the same parts as the
aforementioned embodiments.
While the first and second embodiments are
characterized in vane construction, the pulp screening
device of the third embodiment is characterized only in
screen cylinder construction, particularly hole
configuration, and the remaining construction is the same
as the conventional pulp screening device (refer to Figs .
28 and 29, or Figs. 34 and 35) . In the third embodiment,
therefore, only the screen cylinder construction will
preponderantly be described, and a description of the
remaining construction is omitted. Note that in the third
embodiment, a description will be made in the case where
the present invention is applied to the outer screen
cylinder la of a double screen cylinder.
In the pulp screening device of the third
embodiment, conical hollows 51 are bored zigzag in the
surface of the screen cylinder la, as illustrated in Figs .
23 and 24 . A hole (round hole) 50 is provided to be offset
on the upstream side of revolution flow (i.e., in the
direction opposite to the advancing direction of the vane)
CA 02526192 2001-O1-19
- 51 -
from the center of the corresponding conical hollow 51.
The front edge 52 (positioned on the upstream side of the
revolution flow) of the round hole 50 is positioned outside
the outer peripheral circle of the conical hollow 51, and
the rear edge 53 (positioned on the downstream side of
the revolution flow) is positioned inside the outer
peripheral circle of the conical hollow 51. With this
arrangement, the front edge 52 is formed substantially
perpendicular to the surface of the screen cylinder la,
while the rear edge 53 has an obtuse angle and constitutes
the inlet of the conical hollow 51 along with the inclined
face of the conical hollow 51 . The round hole 50 is bored
toward an exit chamber 14a (see Fig. 13) and forms an axial
wall 55, and is joined with an enlarged passage 56 widening
toward the exit chamber 14a.
Next, a descriptionwill be made of the operation
of the pulp screening device of the third embodiment
constructed as described above.
The front edge 52 of the round hole 50 is formed
substantially perpendicular to the surface of the screen
cylinder la. Therefore, when the revolution flow of pulp
takes place, a strong, turbulence S develops at the inlet
of the roundhole50, and the pulp is satisfactorily agitated.
Since the rear edge 53 is formed to have an obtuse angle,
a lump of pulp and foreign objects are prevented from being
caught in the rear edge 53 . Furthermore, the turbulence
S is near the front edge 52, so foreign objects are easily
CA 02526192 2001-O1-19
- 52 -
removed and clogging-of the round hole 50 is prevented.
Therefore, there is an advantage that clogging can be
prevented even when vanes are revolved at relatively low
speeds and that a large quantity of pulp can thus be screened
and processed with low power.
In addition, in the pulp screening device of
the third embodiment, the center of the round hole 50 is
offset from the center of the conical hollow 51 in the
direction opposite to the direction of the revolution flow,
whereby the front edge 52 for developing the turbulence
S is also used as the hole inlet and the dimension of the
inclined portion 54 is assured. Therefore, the zigzag
pitch can be reduced and there is also an advantage that
the number of round holes 50 per unit area can be increased
and that the quantity of pulp to be passed is thus increased .
Furthermore, the conical hol low 51 can be formed
into the required configuration with a minimum amount of
machining (e. g., mechanical machining such as drilling,
etc . , or electron beam machining such as laser machining,
etc. ) . Therefore, the conical hollow 51 is advantageous
in mechanical strength and there is also advantage that
a thin flat plate can be employed in the screen cylinder
la.
Note that the construction of the screen
cylinder la of the pulp screening device of the third
embodiment is not limited to the one illustrated in Figs.
23 and 24, but will be satisfied if at least the front
CA 02526192 2001-O1-19
- 53 -
edge 52 of the round hole 50 is formed substantially
perpendicular to the screen cylinder surface, and if the
rear edge 53 has an obtuse angle and constitutes the hole
inlet along with the inclined portion 54 of the conical
hollow 51 . Therefore, as illustrated in Fig. 25, the outer
peripheral circle of the conical hollow 51 may coincide
with the front edge 52 of the round hole 50 . As illustrated
in Fig. 26, the diameter of the outer peripheral circle
of the conical hollow 51 may coincide with the diameter
of the round hole 50, and the rear edge 53 of the round
hole 50 may be disposed at the center of the conical hollow
51. Furthermore, as illustrated in Fig. 27, the round
hole 50 is disposed within the outer peripheral circle
of the conical hollow 51 . In this case, however, the front
edge 52 of the round hole 50 is formed substantially
perpendicular to the screen cylinder surface, and the
center position of the round hole 50 is offset on the
upstream side of the revolution flow.
Furthermore, the construction of the
screen cylinder in the pulp screening device of the present
invention is not limited to devices provided with two screen
cylinders, as in the third embodiment. For instance, it
is also applicable to devices having a single screen
cylinder outside or inside an agitation chamber, as
illustrated in Fig. 28.
Although the present invention has been
described by way of the three embodiments thereof, the
CA 02526192 2001-O1-19
- 54 -
invention is not limited to the embodiments . For example,
the common vanes in the f first embodiment may be combined
with the screen cylinders of the third embodiment. The
distribution vanes in the second embodiment maybe combined
with the screen cylinders of the third embodiment. With
these combinations, clogging of the screen cylinder is
more effectively prevented, and furthermore, it becomes
possible to process agreat quantity of pulp with low power.
