Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE WET SCREENING OF STOCK SLTSPFNSInNS IN
PRESSURE GRADERS AND PRESSLIRF G1L~DER SC FFN
1. Field of the Invention
The invention relates to a process for wet screening of stock suspensions
in a pressure grader, and to a pressure grader screen for wet screening of
stock
suspensions.
2. Discussion o~Background Information
An important use of such processes and devices is the grading of stock
suspensions. In this process, the fibers contai~Yed in the s~asperaion are to
pass
through the screen, while the undesired solid components are rejected at the
slit
and conducted out of the screen again. Because the apertures have an
essentially
elongated shape, i.e., are slots or slits, fibrous particles pass through more
readily
than flat particles, even if both types are present in a similar order of
magnitude.
With such a grading technology, it is therefore possible to achieve a very
good
separation effect of non-fibrous disruptive substances from stock suspensions.
However, it is a prerequisite that the shape of the slot be extremely precise
over
the entire screen surface. Another important use is the separation of
different
fibers, in particular fractionation, e.g., based on fiber length or
flexibility.
Screens or revolving screens are known, e.g., from published Patent
Application DE 33 27 422 AI, in which the grading slots are formed by
essentially
parallel identical profiles connected to transverse holding ribs. The profiles
are
shaped and arranged so that, when the screen is in operation, the approach
surfaces
of the profiles have an oblique contour and a break-away edge behind. This
causes the suspension flow sweeping past
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to undergo a deflection that leads the suspension flow away from the grading
slot so that
eddies are formed that assist in keeping the grading slots clear and promoting
the passage
of the fibers through the screen.
In processes of this type, in almost alI casts a clearer that is moved past
class to
the screen and products pressure and suction impulses is used on one side of
the screen,
The brief pressure and suction impulses cause the flow direction to reverse in
the grading
aperture, The grading aperture is kept clear in that fiber clumps or particles
that cannot
pass through are conveyed back again against the normal flow-through
direction.
Pressure and suction impulses can also be introduced into the suspension
directly, e.g.,
via membranes.
The above-mentioned devices have proven to be effective in many applications.
However, it is still not always possible to reliably keep the grading slots
clear and, at the
same time, to conduct the desired high undexflow quantity through the screen
slots.
'!n U.S. Patent No. 4,898,665, a screen device is described in which step-
shaped
flora barriers ("obstacles' are installed in the inflow axes to each screen
apexture, which
barriers stem the approaching suspension. They also cover the screen apes
partially
or completely and produce turbulence in the flow. The eddies thus produced at
the end
of the flow barrier are intended to guide the flow directly into the screen
apcrlures.
However, the flow barriers can also lead to stagnation points on the screen
surface at
which disruptive deposits settle.
The present invention provides a process and a scxecn device of the type
generally
discussed above, in which the scrxn is more effectively kept char, and in
which a good
cleaning erect and a high screen throughput occur.
The present invention provides a process in which the inflow side of the
screen is
formed so that a flow direction of a break-away eddy is reversed by a back
flow, which
rinses a part of the screen lying between the screen apertr~,re in question
and a subsequent
downstream aperture.
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The process of the instant invention also provides that, as it exits from the
screen
apemlre, the back flow is turned so that its flow dixection runs in a
direction apposite to
a circumferential flow of the break-away eddy, and the back flow completely
rinses the
part of the screen lying between the screen aperture in question and the
subsequent
downstream screen aperture.
The pressure grader screen of the instant invention provides faces at a back
end
with extensions, each of which covers an adjacent gradsng slat and a part of a
front end
of an adjacent face of an adjacent grading slot. The extensions can extend
over a length
of at least approximately 0.5 mm of the front fact of the adj scent face of
the adj scent
grading slot, and ail underside of the extension facing and the adjacent face
can be
oriented an angle to each other of between approxitnatcly 3 ° - 45
°, and preferably
between approximately 5 ° - z5 ° .
In accordance with the features of the present invention, the flow situated in
the
inflow area o~the screen apertures can be advantageously developed. With the
aid of the
deflection produced, the return flow is not "caught" by the break-away eddy,
but flows
against its direction directly past the following downstream face of the
screen. Here, this
area of the screen is formed so that this face is rinsed up to the next
following
downstream screen aperture. It can also be the case that, during the change in
the ffow-
through direction, microeddies are formed in the inflow area whose rotation
direction is
opposite to that of the break-away eddy or that the rotation direction of the
break-away
eddy is even reversed. Due to this feature of the invention, fibers that might
adhere to the
screen surface are approached by a flow from another side, promoting their
detachment
from the screen surface, Such residues originate from the pre-flow phase. The
same is
also true for disruptive substances to be sepaxated, in particular when they
tend to stick
(e.g., "stickies")
Slot-shaped screen apertures may be particulaxly suitable for the process,
which
enable a possible advantage for the process to be achieved particularly
effectively. In
fact, it has been found that the content of long fibers in the screen
underflow (accepted
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stock) is higher than with conventional, screerns. Thus, a specific pxoblem of
slot grading,
namely the loss of long fibers, is more readily solved, However, other shapes
that can
cause a reverse eddy, e.g., round holes with deflection elements, are also
conceivable.
The additional expense required ~or the shaping of the profile bars can be
held
within narrow limits. As already mentioned and also lrnown per se, it is
possible to
produce such devices by fixing profile bars arx~aaged next to one another by
fastening
elements, e.g., rings. In this way, it can then be sufficient to select a
different profile to
carry out the invention. While this may necessitate dispensing with standard
profiles, this
dispensing can be taken into account with the large number of the screens to
be produced.
The aevv profiles can generally be used without alte~adon for screens of
extremely
different sizes. If drawn profiles, for instance, of steel or a steel alloy,
are used as a blank
~or the bars, it is also possible to produce specific cross-section shapes
precisely and
economically.
Of course, the profile required for the invention can also be produced by
subsequent machining of the surface on the akeady produced screens.
The flow through the cylindrical screens of the type under consideration here
is
cithor centrifugal or centripetal. The meaning of the terms that era familiar
per se is as
follows; centrifugal relates to radially outwardly directed flows (i.e.,
inward to outward);
and centripetal related to radially inwardly directed flows (i.e., outward to
inward). The
28 position of the screen cleaners is also not always determined thereby. As
is known, there
is screen clearing on the accepted stock side and screen clearing on the
inflow side. The
invention is used advantageously when a clearer is on the inflow side.
The present invention is directed to a process for the wet screening of stock
suspensirn~s in a pressure grader that includes a screen with screen apertures
and at least
one clearer which is alovable relative to the screen. The process includes
conducting the
stock suspension along the screen, so that break-away eddies are formed on an
inflow side
of the screen, separating portions of the stock suspensions, so that a part of
the stock
suspension passes through the screen apertures as underflow and an other part
is rejected
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as overflow, and moving the at least one clearer relative to the screen, so
that one of
pressure- and suction impulses arc produced at the screen apertures to foam
pre- and back
flows, Xn this vcray, a flow direction of the break-away eddy is reversed by
the back flow
to rinse a part of the screen lying between the screen aperture with the back
flow and an
adj scent downstream aperture.
According to a feature of the invention, the screen apertures can be fonncd as
grading slots having longitudinal extensions that eatead essentially cross-
wise to a
movement direction of the at least one clearer, and an axis of the break-away
eddies can
be approximately para11c1 to the grading slots.
In accordance with another feature of the present invention, the rinsing of
the
screen can occur on a side of the screen facing the at least one clearer.
The present invention is directed to a process For the wet screening of stock
suspensions in a pressure grader that includes a screen with screen apertures
and at feast
one clearer that is movable relative to the screen. The process includes
conducting the
stock suspension along the screen provided with screen apertures, such that
break-away
eddies are formed on an inflow side of the screen, separating portions of the
stock
suspension so that a part of the stock suspension passes thmugh the screen
apertures as
underElow and an other part is rejected as overflow, and moving the at least
one cleaners
relative to the screen, so that one of pressure- or suction impulses are
produced at the
screen apertures to form pre- and back flows. In this way, as it exits the
screen apcctwe,
the baclc flow is turned so that its flow direction runs in a direction
opposite a
circumferential flow of the break-away eddy, such that the back flow
completely rinses
a part of the screen lying between the screen apcrtm~c with the back flow and
an adj scent
downstream screen aperture.
According to a feature of the invention, the screen apertures can be formed as
grading slots having longitudinal extension that extend es$entially crass-wise
to a
movement direction of the at least one clearer, and an axis of the break-away
eddies is
approximately parallel to the grading slots.
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According to another feature of the present invention, the rinsing of the
screen can
occur on a side of the screen facing the at least one clearer.
The present invention is directed to a pressure grader screen for the wet
screening
of stock suspensions that includes a plurality of bars arranged to form a
plurality of
essentially parallel grading slots through which au accepted part of the
stoclc suspension
passes during the wet screening. The bars have faces located between the
grading slots,
and the faces each include a front end and a back end. Each back end includes
an
extension arranged to cover an adjacent grading slot and at least a part of
the front end of
the face adjacent fine adjacent grading slot. Each extension has an underside,
such that
the underside and the front end are oriented obliquely to each other.
In accordance with a feature of the instant invention, the at least a part of
the front
end covered by the extension can be a length of at least approximately 0.5 mm,
and the
underside and the front end can be oriented at an angle of between
approximately 3 ° and
45 ° , Further, the underside and the front end can be oriented at an
angle of between
~ approximately S ° to 25 ° ,
According to still another feature of tho in ention, the at least a part of
the front
end cowered by the extension can be a length of at least approximately 1 nun.
The extensions can lie on inflow sides of the faces at ~uvhich the stock
suspension
flows into the grading slot during operation.
The back end is located on a downstream portion of the fact, relative to an
approach flow of the stock suspension during operation of the pxessure grader,
According to a further aspect of the prosent invention, the fads can be
oriented
at an angle to a screen surface of between approximately 5 ° and 50
° , Further, the angle
betweon the faces and the screen surface can be between approximately 10
° to 30°.
~25 Ia accordance with a still further aspect ofthe invention, the face is
free of edges
on which stagnation points could occur during operation.
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Moreover, the underside can be oriented at an angle of between approximately 2
°
and 40 ° to a screen surface. Further, the underside and the screen
surface may be
oriented at an angle between approximately 5 ° to 25 °.
Still filrrther, the invention includes reinforcing elements having
depressions
adapted to accept art last some of the plurality of bars. The bars can include
predetermined profiles and at least some of the plurality of bars can be fixed
in the
reinforcing elements. The at least some of the plurality of bars can be fixed
in the
reinforcing elements by stresses resulting from plastic deformation of the
reinforcing
elements. Further, several spacer noses can be provided, such that the several
spacer
noses are coupled to the undersides and distributed over a longitudinal
extension of the
bars, and the several spacer noses are supported rnz the faces covered by the
undersides.
Moreover, the reinforcing elements can include rings, and the screen is
adapted as a
revolving cylindrical screen, or the reinforcing elements cad the at least
some of the
plurality of bays are arranged in planes parallel to one another, and the
screen is a flat
screen.
According to another aspect of the invention, a screen width can be between
approximately 0.0$ and 2 mm.
In accordance with still another aspect of the invention, the extensions may
extend
essentially along an entire length of the grading slots.
According to a Riither feature of the invention, the extensions may be
repeatedly
partitioned in the longitudinal direction of the grading slots.
In accordance with a still further feature of the present invention, a shape
of the
extensions along the grading slots can be non-uniform.
In accordance with another feature of the instant invention, a length of the
grading
slots may not be greater than approximately IO mm,
The present invention is directed to a process for the wet screening of stock
suspensions in a pressure grader that includes a screen with screen apertures
and at least
one clearer which is movable relative to the screen. The process includes
creating brcak-
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away eddies on an inflow side of the screen, separating portions of the stock
suspensions,
so that a part of the stock suspension passes through the screen apertures as
undezdlow
and an other part is rej ected as overflow, producing one of pressure- and
suction impulses
at the screen apertures, thereby forming pre- and back flows, and rinsing a
part of the
screen lying between the screen aperture with the baclc flaw and an adjacent
downstream
aperture.
According to an aspect of the invention, the rinsing can include reversing a
flow
direction of the break-away eddy with the back flow,
In accordance with yet another feature of the present invention, the rinsing
can
include turning the back flow so that its flow direction runs in a direction
opposite a
circumferential flow of the break-away eddy.
Other exemplary embodiments and advantages of the present invention may be
ascertained by reviewing the present disclosure and the accompanying drawing.
The present invention is further desen'bed in the detailed descxiption which
follows, in reference to the noted plurality of ~awings by way of non-limiting
examples
of exemplary embodiments of the present invention, in which like reference
numerals
represent similar parts throughout the several views of the drawings, and
wherein:
Figure 1 illustrates flow conditions at an inflow of a pt'essure grader screen
24 according to the prior art;
Figure 2 illustrates flow conditions in accordance with the features of the
instant
invention;
Figure 3 illustrates a partial sectional view of a pressure grader screen of
the
present invention;
p'igure ~ illustrates a cylindrical revolvi,ag screen;
Figure 5 illustrates a perspective view of a variation of the exemplary
embodiment;
Figure 6 illustxates a sectional view of atxother variation of the exemplary
embodiment;
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Figure 7 illustrates a variation of the exemplary embodiment having.round
screen
apertures;
Figure 8 illustrates a detail of the inflow area; and
Figure 9 illustrates a variation of the flow conditions.
g p~TAI FD DFS ItTPTION OF T f PRE ENT TIN
The particulars shown herein arc by way of example and for purposes of
illustrative discussion of the embodiments of the present invention only and
are presented
in the cause of providing what is believed to be the most useful and readily
tutderstood
description of the principles and conceptual aspects of the present invention,
In this
14 regard, no attempt is made to show structural details of the present
invention in more
detail than is necessary' for the fiiiidamental understanding of the present
invention, the
description taken with the drawings making apparent to those skilled in the
art how the
se~reral forms of the present invention may be embodied in practice.
Figure 1 schematically illustrates the prior art, and, in particular, a screen
having
15 grading slots 30 that are formed between bars ZO arranged in parallel. Flow
lines in two
different flow situations are represented by free-hand arrows. The suspension
flows
through grading slot 30 (shown on the right] from an inflow side ofthe screen
aperture
to an outflow side in slot flow-through direction S. In flow direction S, the
screen is
utilized for it actual purpose, i.e., enabling a part of the suspension fine
enough to pass
20 grading slot 30 into underflow A, while another part of the suspension is
retained as
overffour. Dextmrotatory break-away eddies 16 can be fornied due to the
approach flovv~
direction and the profile shape of grading bars 20. In view of this flow, a
stagnation point
13, which is symbolically depicted as a small deposit, can be formed between
eddy 16 and
the flowing-away, v~rhich cannot be avoided. The suspension flows through
screen
25 aperture 34 at first based upon a constantly applied excess pressure,
Moreover, as is
ldxown in the aft, suction impulses can be produced by movement of a clearer
12. The
conditions at gxading slot 30 dozing the suction impulse are depicted on the
lefthand side
of Figure I, in which the flow in grading slot 30 is briefly directed
upwardly, i.c., against
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the actual production flow. Dextrorotatory break-away eddy 16 is maintained by
the
suction impulse.
The conditions are different in the exemplary embodiment of Figure 2, where
bars
2 are structured and arranged so that rinsing o~th~ screen occurs on a side of
the screen
facing clearer 12. As illustrated, it is apparent that the sttuctiub and
arrangement ofbars
2 substantially avoids dextrorotatory break-away eddy 16 at grading slot 3,
but instead
forms a levorotatory eddy 17. Thus, the danger of deposits forming in the
inflow area can
be considerably reduced, and the clearing effect its decisively improved.
Further, the bacit
flow can flow directly and unhindered along face 4 so as to rinse Exec
stagnation point 13
(formed in a pre-flow phase).
rn cases in which clearer 12 is on an undcrflow side of the screen (not shown
here),
instead of a suction impulse, a pressure impulse Ieads to the reversal of the
flow direction
in the screen aperture. In such an embodiment, the instant invention can also
be utilized.
A sectional view of a portion of a pressure grader screen in accordance with
the
features of the present invention is illustrated in Figure 3. T'he pressure
grader screen is
a bar screen in which grading slots 3 are formed between bars 2, which are
inserted into
a reinforcing element 1. A profile of bars 2 is depicted in a somewhat simplif
ed form,
such as without the customary curvatures. When the device is in operation, an
accepted
portion of the suspension flows through grading slot 3 as a screen underflow
in slot flow-
through direction S, i.e., downwardly in the illustrated cxatnple. Screen
surfaces 5 or 5'
depict imaginary connecting surfaces of bars 2 on the inflow or outflow side
of the screen,
respectively. Bars 2 have faces 4 on the inflow side and faces 4' on the
outflow side.
Faces 4 of the inflow side are delimited by or formed with a fiont end 7 and a
back end
8. After the suspension has passed inflow-side fact 4 in flow direction R,
i.c., at its back
end 8, it forms an eddy with an axis lying approximately in the longitudinal
direction of
grading slot 3, which improves the clearing effect when the pressure grader is
in
operation.
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According to the invention, faces 4 can be provided at their back end 8 with
extensions b, which are shaped so as to cover not only grading slot 3, but
also front and
7 of an adjacent bar 2 following grading slot 3. The portion of face 4 of the
following bar
2 which is covered, i.e., measured parallel to the face, has a length b, e.g.,
at least
approximately 0,5 mm. Extension 6 has an underside 9 facing front end 7 of
adjacent bar
2, and underside 9 and front end 7 can be oriented to form an angle a to one
another that
is, e.g., between approximately 3 ° and 45 °, and preferably
between approximately 8 ° and
25 °. The selection of angle a ensures that a flow cross-section,
viewed in the pre-flow
direction, broadens. In this way, the suspension can be accelerated in the
opposite
direction, i.e., in the direction of the back flow, which is favorable for the
dissolving and
rinsing off of clumps or stagnation poiunts 13.
In cases in which the screen is not constructed of bass, but, e.g., of slots
of a
previously closed screen blank, extensions 6 can be formed, e.g., by
subsequent plastic
deformation such that they likewise cover grading slot 3 and the adjacent area
of the
~ screen body.
Slot width w, which is decisive for the screen characteristics, is formed, as
is
shown for the second and third grading bars 2, between underside 9 of
extension 6 and
face 4 of the adj scent bar. Faces 4 are inclined in approach flow direction R
so that they
rise at an angle of inclination ~i to screen surface 5, which results in the
approaching
suspension being deflected from slot slow-through direction S. As a result of
this
inclination, a differCnce h from face 4 of the adjacent bar, measured
perpendicularly to
screen surface 5, is formed at back end 8 of an upstream bar. The volume
formed by
di~'erencc h promotes the desired eddy,
In general, extensions 6 cars run uniformly along an entire length of bars 2,
so that
same flow~conditions prcwail overall, ~Iowever, it is also conceivable to
install a number
of smaller extensions instead of a continuous extension, e.g., in order to
produce
advantageous cross-eddies. Anothtr possibility is to make the projecting
length of
extensions 6 different. Since different solid concentrations, c.g., disruptive
material
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concentrations, occur along bars 2 when the pressure grader is in operation, a
change in
the shape or size of extensions 6 can offer an additional opportunity' for
optimization,.
The pressure grader screen of the present invention can be formed in
accordance
with the exemplary illustration shown in Figure 4 as a revolving cylindrical
screen held
together by several ring-shaped reinforcing elements 1. While the figure shows
only a
portion of the bars 2 present and does not depict the extensions, it is noted
that the
structure and arrangement of bars 2 is in accordance with the features of the
instant
invention.
In Figure 5, a perspective view of a part of an another exemplary embodiment
of
the pressure grader screen of the invention is shown. A reinforcing element 1
is shown,
into which bars 2' are to be inserted. The profile of bars 2' on the side
inserted into
reinforcing elements 1 has thickenings, the specific shape of which is
connected with its
manufacture. 'GVhen bars 2' are being inserted, depressions 10 in reinforcing
elements 1
are opened far enough to enable the bars to be inserted easily. The
depressions are
narrowed by plastic deformation ofrcinforcing elements 1, and bars 2' are then
clamped
in reinforcing elements 1. The finished screen is then equipped with perfectly
fixed bars.
According to Figure 6, underside 9 of extensions 6 of bars 2 " is provided
with
spacer noses 1 I, which enable the gap ~ridih to be maintained with particular
precision,
As depicted in the longitudinal extension ofba~rs 2", spacer noses 11 can be
far apart from
one another, e.g., about 240 mm. In cases in which relatively short slofs,
e.g.,
approximately 0.5 to 2 mm, are utilized for reasons of semen technology, a
correspondingly large number of spacer noses can be utilized,
Figure 7 illustrates an clnbodimcnt in which pressure grader screen 14 is
provided
with round screen apertures 15. A reversal of the direction of eddy rotation
is brought
about by extensions d'.
In Figure 8, geometric relationships on the inflow side of the screen are
shown in
somewhat greater detail. Similar to the embodiment depicted in Figure 6,
underside 9 of
extensions 6 can be oriented at an angle of inclination y to screen surface 5,
i.e., not
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P19131.501
parallel to it, with the inclination being between approximately 2 °
atld ~0 ° . Face 4 can
also be inclined relative to screen surface 5, e,g., at an angle of
inclination ~i between
approximately 5° and 50°. 'Thus, a diagonal channel may be
formed at a diagonal face
4, which ensures the desired rinsing of faces 4 even under unfavorable
conditions,
Figure 9 shows a particularly strong effect of the measures of the present
invention, in which dextrorotatory break-away eddy 1b is converted into
levorotatory
eddy 17 under back flow so as to rinse face 4 of bars 2.
It is noted that the foregoing examples have been provided merely for the
purpose
of explanation and are in no way to be construed as limiting of the present
invention.
1Q While the present invention has been described with reference to an
exemplary
embodiment, it is understood that the words which have been used herein are
words of
description and illustration, rather than words of limitation. Changes may be
made,
within the purview of the appended claims, a,s presently stated and as
amended, without
departing from the scope and spirit of the present invention in its aspects,
Although the
present invention has been described herein with reference to particular
means, anaterials
and embodiments, the present invention is not intended to be limited to the
particulars
disclosed herein; rather, the present invention extends to all functionally
equivalent
structures, methods and uses, such as are within the scope of the appended
claims.
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