Note: Descriptions are shown in the official language in which they were submitted.
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P/563-106
METHOD AND DEVICE FOR CONTROLLING DEWATERING
OF A FIBER WEB
BACKGROUND OF THE INVENTION
The invention relates to the forming section of
a paper making machine and relates to a method and a
device for producing a fiber web, in particular a paper
web, from an aqueous fiber suspension. The invention
particularly concerns control over the dewatering
capacity of a part of the forming section.
The invention is based on EP 0 489 094 which
corresponds to U.S. Patent 5,389,206. This publication
describes the formation of a fiber web from stock
suspension which is supplied by a flowbox or headbox.
The web is formed in a twin-wire former, i.e.,
exclusively between two wire belts or wires of the
former. There is thus no so-called, single-wire, primary
dewatering section.
In a first dewatering unit or first part of the
forming section, the two wire belts form a wedge shaped
inlet nip. A stock suspension jet from the flowbox
enters the nip. The jet impinges on the two wire belts
preferably at a point where the wire belts run over a
curved dewatering element. This dewatering element may
be a stationary and curved forming shoe or a rotatable
forming roll. Web formation begins in this first
dewatering unit, and a first portion of the water escapes
through the meshes of the wire belts. The objective is
to produce fiber webs, and particularly paper webs, of as
high quality as possible and at relatively high operating
speeds. A web formed between two wires produces a
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finished fiber web that to a large extent has identical
properties on both sides (low "two sidedness"). At the
same time, as uniform fiber distribution as possible is
sought for the finished fiber web, that is, good
"formation" or "look-through" of the web.
During web formation, there is a constant risk
that fibers will agglomerate to form flocs. Efforts are
made to cause the flowbox or headbox to form a stock
suspension jet which is as free of flocs as possible, for
example, using a turbulence generator.
In addition, efforts are made to influence
dewatering of the fiber suspension during web formation
such that "re-flocking" is avoided as far as possible or
so that, following any possible formation of flocs, a
"deflocking", that is, disintegration of the flocs, takes
place once more. This is generally achieved quite well
using the method of U.S. Patent 5,389,206. In that
publication, pressure pulses are introduced several times
into the remaining part of the fiber suspension in a
second dewatering unit of the forming section. This is
carried out during the further dewatering and web
formation. The aforesaid patent is here incorporated by
reference particularly for its teachings as to
introducing pressure pulses into the fiber suspension.
This known method and the device used for
performing it have generally proven worthwhile. However,
difficulties occur if the stock composition changes
occasionally. These changes can result from inadvertent
disturbance in the stock preparation or from a deliberate
change in the paper grade or specific paper properties.
It is then observed that the entire web formation takes
place either too quickly or too slowly. Consequently,
the quality, and in particular, the "formation" of the
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finished web leaves much to be desired. This problem may
also occur in a former with a single-wire primary
dewatering section.
SUMMARY OF THE INVENTION
The object of the invention is therefore to
ensure good "formation" or "look-through" of the finished
web, even when there is a change in the stock
composition.
The invention is for producing a fiber web,
particularly a paper web, from fiber suspension supplied
to a twin wire former. The former includes a first
dewatering unit upstream in the path of the wire belts of
the former and a subsec~uent downstream second dewatering
unit. At least one discharge line communicates with the
second dewatering unit.
The first dewatering unit includes an open loop
control device to vary its dewatering capacity. The
second dewatering unit has a measuring device
communicating with its water discharge line to measure
the outgoing volume of water per unit time from the
second unit.
An additional closed loop control device
includes an actual value input, to which the measuring
device is connected, and a desired value input. The
closed loop control device has an output for an actuating
variable which triggers adjustment of the dewatering
capacity of the first dewatering unit in such a way that
the actual value approaches the desired value.
The essential idea of the invention is that at
the point where the two wire belts enter the second
dewatering unit, the water content of the suspension
should have a value which is as constant as possible and
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as optimum as possible. This can be achieved by keeping
the amount of water emerging from the web in the second
dewatering unit at least approximately constant,
specifically by influencing the dewatering capacity of
the first dewatering unit, for example by influencing the
vacuum that is applied to the first dewatering unit.
This ensures that the web dryness, which is established
at the boundary between the first and second dewatering
units, assumes a value which is as constant as possible
and is neither too high nor too low.
Other features and advantages of the present
invention will become apparent from the following
description of the invention which refers to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Each of Figures 1 to 5 shows in a simplified
schematic illustration a respective embodiment of a
forming section of a paper machine in which the invention
is embodied.
DETAILED DESCRIPTION OF THE INVENTION
The twin-wire former for a paper making
machine, which is illustrated in Figure 1, has a twin-
wire zone that run essentially horizontally. That zone
is comprised of three dewatering units or sections I, II
and III arranged one after another along the joint path
of the two wire belts. Two endless wire belts comprising
the bottom wire 11 and the top wire 12, which are each
illustrated only in part, run in the direct vicinity of a
flowbox or headbox 10. Each wire runs over a respective
breast roll 13 and 14 which is placed so that the two
wire belts together form a wedge shaped entry nip 15 at
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the beginning of the twin-wire zone. The stock jet
emitted by the flowbox or headbox 10 only comes into
contact with the two wire belts 11 and 12 at the location
along the path of the wires where the bottom wire 11 runs
over a stationary, curved forming shoe 16 located in the
first section I of the twin-wire zone. The curved
running surface of the forming shoe 16 is formed from
several strips, foils or ledges 16b that extend across
the wires and are spaced apart along the path of the
wires to define dewatering slots that extend across the
wires and are located between adjacent strips. As is
known, e.g. from U.S. Patent 5,389,206, those strips or
ledges generate pressure pulses in the suspension being
dewatered. A line 16a discharges the removed water into
a container 59. The distance between the two breast
rolls 13 and 14 can be varied. The forming shoe 16 is
preferably operated under vacuum supplied through the
suction line 60, with control valve 61 and suction blower
62 communicating into the shoe 16.
In the second section II of the twin-wire zone,
i.e. the second dewatering unit, the two wire belts 11
and 12 together, with the still partially liquid fiber
suspension located between them, run between a bottom
dewatering box 17 and a top dewatering box 18. A row
comprised of at least two strips, foils or ledges 27,
preferably having an approximately rectangular cross
section and extending across the wires, are located in
the bottom dewatering box 17. The strips are pressed
compliantly from below against the bottom wire 11. For
this purpose, the strips are supported, for example via
springs 24 or via pneumatic pressure cushions, on a water
permeable plate 26. The force of the springs or of the
pressure prevailing in the pressure cushions can be
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adjustable individually. Again this generates pressure
pulses in the suspension.
The top dewatering box 18 is suspended from
vertically displaceable supporting elements at both at
the upstream and downstream ends, as illustrated
schematically with double arrows. The bottom side of the
box carries a row of at least two strips foils or ledges
28 spaced apart along the web path and each having a
preferably parallelogram shaped cross section and
extending across the wires. These rest on the top side
of the top wire 12 and are firmly connected to the box
18. Above the strips 28 in the dewatering box 18, there
are a front or upstream vacuum chamber 21 and a rear or
downstream vacuum chamber 22. Suction lines 63 connect
the chambers 21, 22 to a vacuum source 64.
A portion of the water from the fiber
suspension is discharged downward through the line 16a in
the region of the forming shoe 16. Some of the water
penetrates upward through the top wire, due to the
tension of the top wire 12 and is then deflected by the
upstream strip, foil or ledge 28 into the upstream vacuum
chamber 21. This water then passes into the container 59
via line 21a.
Water that penetrates upward between the top
strips, foils or ledges 28 passes into the downstream
vacuum chamber 22 and from there, via line 22a, into a
container (for example 59 or 59').
The water that penetrates through the bottom
wire 11 then passes between the bottom strips, foils or
ledges 27 and is discharged downward via line 17a.
The volume of water per unit time flowing away
through the line 22a is kept constant in the following
way. A measuring device, for example, a flow meter 65 in
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or communicating with the line 22a, is connected via a
measuring line 67 to a closed loop control device 66. A
desired value transmitter 68 is also connected via
desired value line 69 to the control device 66. The
closed loop control device compares the measured value
with the desired value. Depending on the deviation
between the measured and the desired values, the closed
loop control device 66 sends an actuating variable via
line 70 to the control valve 61. For example, if the
volume of water flowing away via line 22a is too low,
which may impair the "look-through" of the finished
paper, then the closed loop control device 66 reduces the
vacuum in the forming shoe 16 and thus reduces the
dewatering capacity of the first dewatering unit I.
Consequently, more water passes into the second
dewatering unit II, so that the latter can in turn ensure
better "formation" (look-through). The closed loop
control device 66 operates in a similar way when the
volume of water flowing away via the line 22a is too
great.
In the third unit or section III of the twin-
wire zone, the two wire belts 11 and 12 run over a
further curved forming shoe 23, which is preferably
arranged within the bottom wire loop 11. An additional
strip 29 extending across the wires and with a vacuum
chamber 30 can be provided in the loop of the top wire
12, downstream of the forming shoe 23. In addition, flat
suction boxes 31 may be provided within the loop of the
bottom wire 11. As illustrated with broken lines, the
top wire 12 can be separated from the bottom wire 11 and
from the fiber web formed by a guide roll 19. The bottom
wire and the fiber web then run over a wire suction roll
20. However, the guide roll 19 may also lie further
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downstream, so that the top wire 12 is separated from the
bottom wire 11 only on the wire suction roll 20.
It is important that the two dewatering boxes
17 and 18, together with the alternately compliant and
fixedly supported strips foils, or ledges 27 and 28, not
be located in the upstream or in the downstream section I
or III but rather in the central section II of the twin-
wire zone. This is because only here can they completely
perform intensive dewatering of the fiber suspension
supplied, while maintaining the fine, floc-free fiber
distribution.
The device according to Figure 1 may be
modified, e.g. in a manner similar to Figure 2, by
providing a known forming roll 40 in section I instead of
the stationary forming shoe 16 and the breast roll 13
lying upstream of that shoe. This possibility is used
when the primary demand made on the papermaking machine
is to provide the highest productivity. In addition, it
may be advantageous to select an obliquely rising wire
running direction, as shown in Figure 4, instead of the
predominantly horizontal wire running direction in the
twin-wire zone.
Figures 1 to 3 illustrate the vertical spacing
between the two wires 11 and 12 in the twin-wire zone in
an exaggeratedly large manner. This is intended to show
that the two wires 11 and 12 converge toward each other
over a relatively long distance within the twin-wire
zone, so that the process of web formation at the first
forming shoe 16 or at the forming roll 14 in section I is
established relatively slowly and is completed only at
the end of section II or only in section III. In this
case, the end of the main dewatering zone, in which the
two wires converge toward each other, and thus the end of
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the web formation process, are located, for example,
approximately in the center of the wrap zone of the
second forming shoe 23, as is shown, for example, in
Figures 1 to 3. The end of the twin wire convergence is
illustrated symbolically there by the point E. At that
point, the paper web has reached approximately 8%
dryness. However, the point E can also be located, for
example, on one of the flat suction boxes 31.
The embodiments in Figures 2 and 3 primarily
differ from the others in that the twin-wire zone rises
essentially vertically from bottom to top along the wire
running direction. This orientation simplifies the
discharge of the water drawn out from the fiber
suspension because the water can to a large extent be
discharged uniformly to both sides. In the central
section II of the double-wire zone in particular, no
vacuum chambers are needed as the water merely falls.
However, the forming roll 40 in Figure 2 iS a suction
roll. The forming shoe 23 in the third section III can,
if required, also be provided with a suction device.
Figure 2 also shows water receiving containers
41, 42 and 43, sheet metal guides 44 assigned to each of
the stationary strips, foils or ledges 28 and a water
discharge strip or ledge that extends across the wires in
the third section 45. The remaining corresponding
elements have the same reference numbers as the
corresponding elements of Figure 1. The same numbering
scheme applies to Figure 3. In particular, control of
the vacuum in the forming roll 40 or, respectively, in
the forming shoe 16 by means of the closed loop control
device 66 is carried out in the same way as described
with reference to Figure 1. This also applies to the
further embodiments of Figures 4 and 5.
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- 10 -
In addition, in the embodiment of Figure 4, the
wire belts 11, 12 run obliquely upward from the forming
roll 40. The top dewatering box 18 is bellied out or
convexly curved slightly downward with a large radius of
curvature R. The closed loop control device 66 does not
control a valve arranged in the suction line 60, but
instead controls the rotational speed of the motor 61A,
which drives the suction blower 62, as also shown in
Figures 2 and 5. The effect will be the same as in
Figure 1.
In addition, in the embodiment of Figure 5, the
forming roll 40' is not located in the bottom wire 11 but
in the top wire 12. As illustrated, the forming roll 40'
may be followed on the wire path by a forming shoe 16'
located in the bottom wire. But the forming shoe 16' may
also be omitted. The top dewatering box I8 is also
bellied out or convexly curved downward. It is followed,
as in Figure 4, by a suction separator 23' which is
located in the bottom wire 11. The closed loop control
device 66 controls the vacuum both in the forming roll
40' and in the forming shoe 16', if a shoe is present.
The invention can also be used in a twin-wire
former having an essentially horizontal wire running
direction in the web forming region, similar to Figure 1,
but in which section I, the first dewatering unit, is a
single-wire primary dewatering section rather than a
twin-wire section. Then, the top wire 12 comes against
the bottom wire 11 only at the boundary between sections
I and II, and onto the fiber suspension that has already
been partially dewatered in section I.
In this control process, if an operating limit
is reached while adjusting the dewatering capacity of
section I, for example, when adjusting the vacuum, a
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- 11 -
signal is output. Based on the signal, the stock supply
may be changed, for example, by intervention at the
flowbox.
Although the present invention has been
described in relation to particular embodiments thereof,
many other variations and modifications and other uses
will become apparent to those skilled in the art. It is
preferred, therefore, that the present invention be
limited not by the specific disclosure herein, but only
by the appended claims.
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