Note: Descriptions are shown in the official language in which they were submitted.
20~239
TWIN-WIRE FORMER
The present invention relates to a twin-wire former
for the production of a fiber web, in particular a paper
web, from a fiber suspension. The invention proceeds
from the basis of the twin-wire former known from British
Patent 1 125 906. They state, in other words, that the
forming of the fiber web from the pulp suspension fed
from the headbox takes place exclusively between two wire
belts. Thus, there is no so-called single-wire
predrainage path. In a first section of the twin-wire
zone, the two wire belts together form a wedge-shaped
inlet slot; a jet of pulp slurry coming from the headbox
discharges into it. The jet strikes the two wire belts
at a place they pass over a curved drainage element; in
the case of the aforementioned British patent, this is a
stationary, curved forming shoe. Its curved wire guide
surface is formed of a plurality of strips with drainage
slots between them. This forming shoe is followed (in a
second section of the twin-wire zone) by a drainage strip
arranged in the other wire loop and, behind the latter,
by a drainage strip arranged in the first-mentioned wire
loop (and formed by a first suction box). Finally, in a
third section of the twin-wire zone there are a plurality
of stationary drainage elements developed as flat suction
boxes.
It has been attempted for decades with twin-wire
formers of the known type to produce fiber webs (in
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- 2
particular, paper webs) of the highest possible quality
with relatively high operating speeds. Due to the
forming of the web between two wires, the result, in
particular, is obtained that the final fiber web has
substantially the same properties on both sides (little
"two-sidedness"). However, it is difficult to obtain as
uniform as possible a distribution of the fibers in the
final fiber web. In other words, it is difficult to
obtain a good "formation", since, while the web i8
formed, there is always the danger that fibers will
agglomerate and form flocculations. Therefore, it is
attempted to form a jet of pulp slurry which pulp slurry
is as free as possible of flocculations in the headbox
(for instance, by means of a turbulence producer). It
is, furthermore, endeavored so to influence the drainage
of the fiber suspension during the web-forming that
"reflocculation" is avoided as far as possible or that,
after possible flocculation, a ~deflocculation" (i.e. a
breaking up of the flocculations) takes place.
It is known that a curved drainage element
arranged in the first section of the twin-wire zone and,
in particular, a stationary curved forming shoe developed
in accordance with the aforementioned British Patent 1
125 906 counteracts the danger of reflocculation. This
is true also of the drainage strips arranged in the
British Patent in the second section of the twin-wire
zone. Nevertheless, the danger of reflocculation is not
completely eliminated in the arrangement according to
said British Patent. Since the number of drainage strips
there is very small, a large part of the web-forming
takes place in the region of the following flat-suction
boxes. They, to be sure, are of high drainage capacity
2~332?9
- 3
so that the web-forming can be completed in the region of
the last flat suction boxes (i.e. the so-called main
drainage zone, in which a part of the fiber material is
still in the form of a suspension, t~rm;n~tes in the
region of the flat suction box). The flat suction boxes,
however, are not able to avoid reflocculation or to break
up flocculations which have already occurred.
In order to control these last-mentioned
difficulties, a web-forming device known under the name
of "Duoformer D" has been developed (TAPPI Proceedings
1988 ~nml~l meeting, pages 75 to 80). This known web-
forming device is part of a twin-wire former which has a
single-wire pre-drainage zone. In the twin-wire zone
there are provided, in the one wire loop, a plurality of
strips which are fixed in position but adjustably
supported, namely, on the bottom of a suction box which
drains in upward direction. Furthermore, a plurality of
resiliently supported strips are provided in the other
wire loop. By this resilience of the last-mentioned
strips, the following result can be obtained: For
example, upon an increase of the amount of suspension
entering between the two wire belts, the flexibly
supported strips can move away somewhat. In this way,
the danger (which is present when only firmly supported
strips are used) is eliminated of a backing up taking
place in the fiber suspension in front of the strips.
Such a backing up could destroy the fiber layers which
have been formed up to then on the two wire belts. In
other words, with this known web-forming device, a
drainage pressure, once established, r~m~; nC constant due
to the resiliently supported strips even upon a change in
the amount of suspension fed or upon a change in the
_ 4 _ 2053~ 9
drainage behavior of the fiber suspension. Therefore,
automatic adaptation of the web-forming device to said
changed conditions occurs.
With this known web-forming device, fiber webs of
relatively good formation can also be formed. With
respect to this, however, the demands have increased
considerably recently, so that further improvements are
desirable.
The present invention is directed towards the
development of a twin-wire of the aforementioned kind
wherein the quality of the fiber web produced is further
improved, particularly with respect to its formation
(cloudiness), and wherein the twin-wire former can easily
be adapted to different operating conditions (for
instance, with regard to quantity and drainage behavior
of the fiber suspension).
The inventors have found that a combination of known
features, namely:
A. Twin-wire former without a single-wire
predrainage zone
B. Start of the drainage in the twin-wire zone at
a curved drainage element, for instance on a
rotating forming cylinder or, even better, on a
curved stationary forming shoe
C. Further drainage in the twin-wire zone between
strips which are arranged along a "zig-zag"
line, the strips which rest against the one
wire belt being resiliently supported,
leads to an extremely high increase in the quality of the
finished fiber web, so that it satisfies even the highest
requirements. At the same time, the twin-wire former of
the invention is insensitive to changes in the amount of
suspension fed and to changes in the drainage behavior of
the fiber suspension. Experiments have shown that it is
20~3239
possible by the invention to obtain both a high
increase in quality with respect to the formation and
also good values with regard to the retention of fillers
and fines. In contradistinction to this, in the known
double-wire formers it is constantly found that there is
a strong reduction in the retention upon an improvement
in the formation.
It was, furthermore, found in experiments that in
the second section of the twin-wire zone the number of
strips can be considerably reduced as compared with the
"Duoformer D". However, this number is substantially
greater than in the case of the twin-wire former known
from British Patent 1 125 906. It is advantageous to
increase the distance between adjacent strips as compared
with the "Duoformer D" (Claim 2).
From German OS 31 38 133, Fig. 3, a twin-wire former
is known the twin-wire zone of which is provided in a
first section with a curved stationary drainage element
and in a second section with strips arranged along a
"zig-zag" line, which strips may also be resiliently
supported and there being a relatively large distance
between them. However, in that case, in front of the
twin-wire zone there is a single-wire predrainage zone in
which the forming of the web starts initially only in a
lower layer of the fiber suspension fed while the upper
layer remains liquid and tends very strongly to
flocculation. It has been found that these flakes cannot
be broken up again to the desired extent in the following
twin-wire zone. Another disadvantage is that the twin-
wire zone is diverted by a guide roll (14b) behind thesecond section. This results (due to the so-called table
roll effect) in a further drainage which is uneven over
the width of the web and thus in undesired variations in
-
- 6 - 2053239
the quality of the web (recognizable, for instance, by
disturbing longitudinal stripes).
Accordingly, in one aspect of the present invention,
there is provided a twin-wire former for the production
of a paper web from a fiber suspension, the twin wire
former comprising:
first and second web forming wire belts, means for
directing the wire belts to travel along a path together
for forming a twin wire zone of the twin wire former,
with the web between the wire belts as the wire belts
travel along the path through the twin wire zone, neither
wire belt defining a single wire predrainage zone;
each wire belt forming an endless loop;
the twin wire zone having a first section at the
start of the path through the twin wire zone, means for
supporting the belts for forming a wedged shaped entrance
slot into the first section, a fiber suspension supplying
headbox having an outlet placed and directed for
delivering fiber suspension from the headbox to the wedge
shaped entrance slot of the first section of the twin
wire zone; a curved drainage element in the first
section, the curved drainage element having an open
surface to enable drainage of water from the fiber
suspension and being curved along the path of the belts
through the twin wire zone, the curved drainage element
being engaged by one of the wire belts, for curving the
path of the belts around the curved drainage element
after the entrance of the suspension into the entrance
slot;
the twin wire zone having a second section following
the first section along the path of the belts through the
twin wire zone; in the second section, a plurality of
first drainage strips are positioned within the loop of
the first wire belt and are for contacting the first wire
belt; in the second section, a plurality of second
-- 2053239
- 6a -
drainage strips are positioned within the loop of the
second wire belt and are for contacting the second wire
belt; the first strips being shifted in position along
the path of the wire belts with respect to the second
strips so that the first and second strips are offset and
in a non-opposing relationship; first support means for .
resiliently supporting the first drainage strips against
the respective wire belt that the strip contacts;
second support means supporting the second drainage
strips rigidly against the second wire belt; and
means for supplying a vacuum in the area of the
second drainage strips;
the twin wire zone having a third section following
the second section along the path of the wire belts
through the wire zone; a stationary drainage element in
the third section, for being engaged by one of the wire
belts as the wire belts travel over the stationary
drainage element, the stationary drainage element having
an open surface to enable water to be drained through the
wire belt in contact therewith;
the twin wire zone being free of guide rolls which
deflect the twin wire zone and which cause a table roll
effect.
Other developments of the invention will be
explained below with reference to embodiments which are
shown in the drawing. As seen therein:
Figure 1 is a diagrammatic representation of a twin-
wire former provided in accordance with the embodiment of
the invention;
Figure 2 is a diagrammatic representation of a twin-
wire former provided in accordance with a second
embodiment of the invention;
Figure 3 is a diagrammatic representation of a twin-
wire former provided in accordance with a third
embodiment of the invention;
~J
2053239
- 6b -
Figure 4 is a diagrammatic representation of a twin-
wire former provided in accordance with a fourth
embodiment of the invention; and
Figure 5 is a diagrammatic representation of a twin-
wire former provided in accordance with a fifthembodiment of the invention.
The twin-wire former shown in Fig. 1 has a
substantially horizontally extending twin-wire zone; this
zone comprises three sections I, II and III arranged one
behind the other. The endless wire belts (lower wire 11
and upper wire 12) , shown only in part, travel in the
direct vicinity of a headbox 10 over, in each case, a
breast roll 13 and 14 respectively, so that the two wire
belts together form a wedge-shaped entry slot 15 at the
start of the twin-wire zone. The jet pulp discharged by
the headbox 10 comes into contact with the two wire belts
11 and 12 only at the place where the lower wire 11 in
the first section I of the twin-wire zone travels over a
stationary curved forming shoe 16. The curved travel
surface thereof is formed of several strips 16' with
drainage slits present between them. The distance
between the two breast rolls 13 and 14 is variable. The
forming shoe 16 can be operated with or without vacuum.
In the second section II of the twin-wire zone, the
two wire belts 11 and 12 (with the partially still liquid
fiber suspension present between them) travel between a
lower drainage box 17 and an upper drainage box
~3
~ 2~5~9
18. In the lower drainage box 17 there are a row of at
least two strips 27 (preferably of approximately
rectangular cross section) which are pressed from below
resiliently against the lower wire 11. For this purpose,
they are supported, for instance, on springs 24 (or
pneumatic pressure cushions) on a, preferably water-
perme~hle, plate. It is obvious that the force of the
springs (or of the pressure prevailing in the pressure
cushions) is individually adjustable.
The upper drainage box la i8 suspended on both
the front and rear ends on vertically displaceable
support elements as indicated diayL~L~,Latically by double
arrows. On its lower side, there is a row of at least
three strips 28 of preferably parallelogram cross section
which rest against the upper side of the upper wire 12
and are rigidly attached to the box 18. Above the strips
28, a front vacuum chamber 21 and a rear vacuum chamber
22 are present in the drainage box 18. In the region of
the forming shoe 16, a part of the water of the fiber
suspension is led off downward; another part penetrates -
due to the tension of the upper wire 12 - upwards through
the upper wire and is deflected by the furthest in front
of the strips 28 into the front vacuum chamber 21. The
water passing upward between the upper strips 28 enters
into the rear vacuum chamber 22. The water penetrating
between the lower strips 27 through the lower wire 11 i9
led off downward. ~etween adjacent upper drainage strips
28 there is a m;niml~m distance X of about three times the
thic~ness Y of the strips. The same ig true of the lower
resiliently supported strips 27. It i9 important that
each of the strips 27 and 28 lies in the region of a
space between two opposite strips so that a "zig-zag"
-
2~32~9
- 8
arrangement is present. The two wires 11 and 12
preferably travel on a straight path through section II.
Gentle curvature of this section of the path is, however,
also possible; see Figs. 2 and 5. Differing from Fig. 1,
the resiliently supported strips could also be arranged
in the upper box 18 and the firmly supported strips in
the lower box 17.
In the third section III of the twin-wire zone,
both wire belts 12 and 13 travel over another curved
forming shoe 23 which (a~ shown) is arranged preferably
in the lower wire loop 11. ~ehind it, an additional
strip 29 with vacuum chamber 30 can be arranged in the
loop of the upper wire 12. Furthermore, flat suction
boxes 31 can be present in the loop of the lower wire.
There (as is shown by dash-dot lines) the upper wire 12
can be separated by means of a guide roll 19 from the
lower wire 11 and from the fiber web formed. Lower wire
and fiber web then travel over a wire suction roll 20.
The guide roll 19 can, however, also lie further bac~, 30
that the upper wire 12 is separated from the lower wire
11 only on the wire suction roll 20.
It is important that two drainage boxes 17 and
18 with the alternately resiliently and firmly supported
ledge strips 27 and 28 lie not in the front or the rear
sections but in the middle section II of the twin-wire
zone, since only here can they develop their full effect,
namely, intensive drainage of the fiber suspension fed
while retA ~ n lng the fine flocculation-free fiber
distribution. This is achieved in the m~nnPr that the
corresponding wire belt is imparted a slight (scarcely
visible~ deflection on each strip so that turbulence is
constantly produced in the still liquid part of the fiber
2~32~9
g
pulp. For success it is, however, aIso decisive that
previously, in section I, a known pre-drainage towards
both sides has already taken place and that this also
takes place with the greatest possible retention of the
flocculation-free condition of the fiber suspension.
For this two-sided pre-drainage, a stationary
curved forming shoe is provided in the first section I of
the twin-wire zone (in accordance with Figs. 1 and 3-5)
whenever it is a question of satisfying the highest
quality ~m~n~C with respect to the formation. This
effect of the forming shoe is due to the fact that at
least the one wire belt travels polygonally from strip to
strip, each strip not only leading water away but also
producing turbulence in the pulp which is still liquid.
With such a forming shoe, it is, however, difficult at
times to obtain a stable operating condition upon the
starting of the paper machine. Therefore, it may be
advantageous to provide a known forming roll 40 in
accordance with Fig. 2 in Section I instead of the
stationary forming shoe and the breast roll lying in
front of it. This possibility will be utilized when, in
particular, the highest productivity is demanded from the
paper manufacturing machine.
In the third section III, the aforementioned
strip 29 can serve either solely to lead away water
upwards or, in addition, for the further production of
turbulence (for further impL~v~ent in quality). The
latter is possible if a part of the ~iber pulp is still
in liquid condition at this place.
In Figs. 1 to 3, the distance between the two
wires 11 and 12 in the twin-wire zone has been shown
greatly exaggerated. By this, it is intended to make it
- 2`~3239
- 10
clear that the two wires 11 and 12 converge towards each
other over a relatively long path within the twin-wire
zone. This makes it clear that the process of web-
forming on the first forming shoe 16 (in Section I)
commences relatively slowly and is completed only in
Section III. In this connection, the end of the main
drainage zone in which the two wireg converge towards
each other (and thus, the end of the web-forming process)
can lie approximately in the center of the wrapping zone
of the second forming shoe 23, as is indicated, merely by
way of example, in Figs. 1 to 3. The end of the wire
convergence is symbolically indicated there by the point
E; the solids content of the paper web has reached there
approximately the value of 8%. This point can, however,
also lie, for instance, on one of the flat suction boxes
31. ~ehind this point, it is attempted further to
increase the solids content, if possible even before the
separation of the two wires. One goal is, namely, for
the separation of the wires to take place with the
highest possible solids content of the web so that as few
fibers as possible are torn out of the web upon the
separation. The nature and number of the drainage
elements necessary for this within the twin-wire zone
may, however, differ greatly and is dependent, among
other things, on the type of paper and the raw-material
components thereof, as well as on the operating speed.
The embodiments shown in Figs. 2 and 3 differ
from the others primarily by the fact that the twin-wire
zone rises substantially vertically upward in the
direction of travel of the wires. In this way, the
removal of the water withdrawn from the fiber suspension
is simplified since the water can be discharged
- 2Q~32~
relatively uniformly towards both sides. No vacuum
chambers are required in particular in the central
section II of the twin-wire zone. To be sure, the
forming roll 40 of Fig. 2 is, as a rule, developed as a
suction roll. The forming shoes 16, 23, particularly
those arranged in the third section III, can, if
necessary, be provided with a suction device.
Further elements of the twin-wire former shown
in Fig. 2 are water-collection containers 41, 42 and 43,
guide plates 44 associated with the fixed strips 28, and
a water removal strip 45. The other elements are
provided with the same reference numbers as the
corresponding elements in Fig. 1. The same is true with
regard to Fig. 3. One possible modification of Fig. 3
can consist therein that, instead of the wire suction
roll 20, a forming roll is provided, and instead of the
guide roll 19 the wire suction roll. A similar
arrangement is known from German Utility Model 88 06 036
(Voith File: P 4539). Aside from this exception and
aside from the embodiment according to Fig. 2 (with
forming roll 40), the invention will, however, be used -
whenever possible - so to design the twin-wire former
that the relatively expensive forming roll (as to
purchase and operation) can be dispensed with. Thus, as
a rule, the wire suction roll 20 ig present as the sole
suction roll. Furthermore, in all embodiments of the
invention it can be seen to it that no guide roll which
deflects the twin-wire zone (and has the above-mentioned
injurious table-roll effect) is present.
The embodiment of Fig. 4 differs from Fig. 1
among other things by the fact that, in the first section
I of the twin-wire zone, a second curved stationary
20532~
forming shoe 16a i8 arranged in the ioop of the lower
wire 11 behind and spaced from a first curved stationary
forming shoe 16. Furthermore, in the loop of the upper
wire 12 in the region between the two stationary forming
shoes 16 and 16a there is provided an individual strip 50
which in known m~nner iS part of a vacuum chamber 51.
This vacuum chamber 51, similar to the upper drainage box
18 of Fig. 1, is suspended on its front and rear ends in
vertically displaceable mounts. In this way, both the
depth of penetration of the strip 50 into the path of
travel of the upper wire 12 as well as the angle of
attack of the strip 50 can be varied. With slight depth
of penetration, the strip 50 serves ~olely for removal of
water, while with greater depth of penetration it serves,
in addition, for the production of turbulence in the
suspension and, thus, for improvement of the formation.
By the presence of two separate forming shoes 16 and 16a,
the pre-drainage on both sides is temporarily
interrupted; it is only continued after the strip 50 has
removed from the upper wire 12 the water which has
penetrated upward on the first forming shoe 16. In this
way, higher operating speeds are possible.
Another difference from Fig. 1 is that, in the
second section II of the twin-wire zone, the lower,
flexibly supported strips 57 and the upper, firmly
supported strips 58 are developed as individual strips.
This means that each strip has its own supporting body
55/56. The lower strip-supporting bodies 55 are
swingably moùnted, the strip 57 being pressed resiliently
by the force of springs 54 against the bottom of the
lower wire 11. The supporting body 56 of each of the
upper strips 58 is developed as vacuum chamber in the
20~3239
- 13 -
same way as that of the strip 50. The suspension of
these vacuum chambers 56 corresponds to that of the
vacuum chamber 51. It is important that each of the
strips 57 and 58 rest with a given force of application
(corresponding to the suspension pressure) against its
wire belt 11 or 12. The strips 57 and 58 are adjusted in
~uch a m~nn~r that a slight deflection of the wire belts
takes place preferably on each strip. Due to the
resilient supporting of the lower strips 57, the
adjustment, once effected, is insensitive to changes in
the quantity or quality of pulp, so that no backing up
takes place in front of the strips and, nevertheless, an
effective introduction of turbulence forces into the
fiber suspension takes place. In contradistinction to
Figs. 1 to 3, there is the possibility of adjusting each
one of the strips 57/58 individually with respect to
position in height and inclination relative to the travel
path of the wire. In this way, one can even better
control the quality of the paper produced, with respect
to both the formation and the nature of its surface
(printability). Differing from Fig. 4, the upper strips
5~ could be supported resiliently and the lower strips 57
stationary. Another alternative could consist therein
that not only the upper strips 58 but also the lower
strips 57 are fastened in vertically displaceable mounts
(as shown on the vacuum chamber 51~. In such case, the
springs 54 might possibly be eliminated.
Another difference between Figs. 1 and 4
resides in the fact that in Fig. 4 the twin-wire zone
rises in the direction of travel of the wires upwards
with an inclination of, on the average, about 20 with
respect to the horizontal. In thig way, it is possible
--- 2~532~9
- 14 -
to keep the entire height of the twin-wire former
relatively slight. In the third section III of the twin-
wire zone, a flat forming shoe 23' i9 provided rather
than a curved one, differing from Fig. 1. The separation
of the upper wire 12 from the lower wire and the fiber
web formed can take place, as in Flg. 1, on one of the
flat suction boxes 31. Instead of this, however, the
upper wire 12 can also be conducted up to the wire
suction roll 20. There, as shown, it can wrap around a
small part (or, alternatively, a larger part) of the
circumference of the wire suction roll and then be
returned via the reversing roll 19.
In the embodiment shown in Fig. 5, the twin-
wire zone, as a whole, extends substantially in
horizontal direction. The individual elements are
substantially the same as in the embodiment of Fig. 4.
However, there is the difference that the drainage strips
57 and 58 lying in the second section II of the twin-wire
zone are arranged along a downwardly curved path of the
twin-wire zone. Accordingly, an upwardly curved forming
shoe 16, 23 is provided in the first section I and in the
third section III of the twin-wire zone. This embodiment
is advisable, in particular, for the modernizing of
existing Fourdrinier paper mach; n~C .
The embodiments shown have the feature in
common that, in the second section II of the twin-wire
zone, there are present preferably n flexibly supported
strips 27/57 and n~1 rigidly supported strips. However,
it is also possible to make the number of flexibly
supported strips equal to or greater by one than the
number of rigidly supported strips. Instead of a rigidly
supported strip, a feed or discharge edge of a drainage
~3532~9
box can also be provided. The m; n; mllm number B of
flexibly supported strips is two (see Fig. 4). However,
three or four flexibly ~upported strips are preferred.
