Note: Descriptions are shown in the official language in which they were submitted.
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DOUBLJ~: WIRE FOl~ME]R WIT}~ LOCALLY
ADJUSTABLE: DEWATERING SLAT
BACK~:ROUND OF T~HE INVENTION
The invention concerns a doubl~ wire former for
producing a fiber web, specifically a paper web, from a
fiber suspension.
The invention is based on the double wire former
known from the British patent document 1,125,906. The
characteristics known from this publication are providing
two wires which together form a double wire, providing at
least one double wire dewatering zone, and providing at
least one dewatering slat in the double wire dewatering
zone. This publication illustrates a double wire former
which, in a first section of the double wire zone, ~o~ms
with the two wires a wedge-shaped entrance gap. A jet of
stock suspension coming from the headbox empties into it,
impinging on the two wires at a point where they run
across a curved dewatering element. In the case o~ the
said British pate~t document, this element is a
stationary, curved forming shoe~ Its curved wire support
surface is formed of numerous slats with interposed
dewatering slots. This forming shoe is followed (in a
~econd section of the double wire zone) by a dewatering
slat arranged in the other wire loop and, behind it, a
dewatering ~lat arranged in the former wire loop (and
formed by a first suction box). Lastly, ~ollowing in a
third section of the double wire zone are several
stationary dewatering elements fashioned as suction flat
boxes~
Using double wire formers of the prior type,
attempts have already been made for decades at producing
fiber webs (specifically paper webs) of maximally high
quality and at relatively high operating speeds. The web
formation between two wires has achieved, in particular~
a finished fiber web which possesses on both sides
extensively the same properties (slight "twosidedness").
However, difficulties are encolmtered in achieving a
maximally uniform fiber distribution in the finished
fiber web. In other words, it is difficult to achieve a
good "formation." During the web formation there
constantly exists the risk that ~ibers will coagulate to
flocks. Therefore, attempts are made to form at the
headbox a maximally flock-free jet of stock suspension
(for instance with the aid of a turbulence generator).
In addition, efforts are being made toward influencing
the dewatering of the fiber suspension during web
formation to the effect that a "reflocculation" is
maximally avoided or that after any flock formation a
"deflocculation" (i.e., dissolution of the flocks) occurs
again.
It is known that a curved d~watering element
arranged in the first section of the double wire zone,
specifically a stationary, curved forming shoe fashioned
according to the British patent document 1,125,906,
counteracts the risk of reflocculation. This is true
also for the dewatering slats which acc~rding to the
British patent document are arranged in the second
ection of the double wire zone. Nevertheless, the risk
of reflocculation is not completely eliminated with the
arrangement according to this British patent document.
Namely t with the number of dewa~ering slats being very
small there, the web formation largely takPs place ln the
area of the following suction flat boxes. While these
have a high dewatering capacity so that the web formation
can be terminated in the area of the last ~uction flat
box (i.e., the so-called major dewatering zone in which
part of the fiber material still on hand in the form of a
suspension ends in the area of the suction flat boxes),
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the suction flat boxes are unable to avoid a
reflocculation or to redissolv~e flocks which have been
created.
To master the latter difficultles, a web forming
system known as "Duoformer Dl~:has been developed (TAPPI
Proceedings 1988 Annual Meeting, pp. 75-80~. This prior
web forming system is part of a double wire former
featuring a sin~le wire primary dewatering zone.
Provided in the double wire zone, in the one wire loop,
is a number of fixed but adjustably supported slats on
the underside of a suction flat box dewatering upwardly.
In addition, a number of flexibly supported slats are
provided in the other wire loop. Due to the flexibility
of the latter slats, the following can be achieved. For
instance, at an increase of the suspension quantity fed
between the two wires, the flexibly supported slats can
somewhat yi~ld. This eliminates the risk (which exis~s
when fixedly supported slats are solely used) that a
backup will form in the fiber suspension in front of the
slats. Such backup could again destroy the fiber layers
formed up to here on the two wires. In other words, a
dewatering pressure once adjusted remains constant with
this web forming system, owing to the flexibly supported
slats, also at a change of the supplied suspension
quantity or of the dewatering performance of the fiber
suspension. Thus, an automatic adaptation of the web
forming system to the said changed conditions is taking
place.
Fiber w~bs with relatively good formation can be
formed with this prior web forming system. Requirements
have recently risen considerably, however, making further
improvements desirable. It would be a considerable
improvement of the sheet forming mechanism if the basis
weight cross profile and the fiber orientation cross
profile were more specific and influenceable at the
points where the defects occur.
First variations, or defects, in the basis weight
and fiber orientation cross profiles occur in the
headbox. From the German patent disclosure DE 31 09 227
A1, for example, it is known that variations o~ the basis
weight cross profile from the ideal condition are
corrected by adjustments on the headbox profile bar. In
the aforementioned patent disclosure, the basis weight
cross profile is influenced by bending the headbox
profile bar directly at the slice, in such a way that
both errors that have nccurred in setting the headbox and
errors which in the subsequent course of the paper
machine occur in the dewatering zone, for instance by
nonuniform contact pressure in the press section or
nonuniform contact pressure of dewatering slats, will
thereby be eliminated.
For onel however, it is technically unsatisfactory
to compensate for defects occurring at one point of the
machine by "maladjustments," seen in the ideal sense at
another point of the paper machine; for another, this
also entails the problem that adjustments made with
regard to the basis weight cross profile will undesirably
in~luence, as the case may be, the fiber orientation of
the paper web.
The problem underlying the invention is to design a
double wire former of the initially described type such
that defects in the basis weight cross pro~ile which
occur through peculiarities of the dewatering zone also
will be in~luenced, or corrected, within the dewatering
zone.
SUMMARY OF THE INVENTION
This problem is solved by the inventive features of
the present invention. A number of adjustment devices
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are provided for a dewatering slat across the width of
the paper web, wherein each adjustment device, for a
limited width, allows for influencing the position of the
contact surface of the dewatering slat.
The inventors have recognized that it is possible to
influence the basis weight croRs profile o~ a fiber web
in the area of the dewatering zone through the use of one
or several flexible dewatering slats whose position is
individually adjustable, across the entire width of the
web, nearly or approximately perpendicularly to the
travel direction of the wire.
BRIEF DESCRIPTION OF THE_DRAWINGS
The invention will be more fully explained with the
aid of the drawings.
Eigs. 1 and 2 show two embodiments of the adjustable
dewatering slat.
Eig. 3 shows an embodiment of a flexural slat of two
materials.
Fig. 4 shows a flexibly arranged dewatering slat.
Figs. 5 through 7 show various positions for
arrangement of the adjustable dewatering slat.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 shows one embodiment of the dewatering slat
where the flexural slat B is forced on the wire by a
spindle type actuator in keeping with setting
requirements. Fig. 1 shows a cross section of this
device, where the device consists of a stable body having
approximately the shape of a flat U-profile through the
short topside of which holes are tapped matching the
number of actuators. A threaded spindle S is passed
- through each hole. Threaded spindle S connects via a
holder H in the form of a claw with a deflector slat D,
which in turn bears on the lower short side of the
U-profile. The part of the U-profile now remaining open
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is covered by a contact plate A across the machine width,
causing the deflector slat to be forced on the U-profile
at its underside. The spindles are each adjustable
through a device V. The de~lector slat slidably mounted
between the guide element F and the contact plate A can
be positionally adjusted with the aid of the pindle/
causing the slat to exert variable pressure on the wires
Sl and S2 passing beneath it.
Fig. 2 shows another embodiment of the dewatering
slat described by the invention. Illustrated in cross
section in Fig. 2 is a deflector slat D deflected at an
angle ~ to the vertical and bearing on a wire S1 situated
underneath. On its nonbearing side, the deflector slat
is retained by a holder H in ths fonn of a claw. Holder
H is in turn attached to a guide body having
approximately the shape of a flat iron, such that the
guide body and deflector slat assume the shape of a
lying, left-opening V whose topside i9 situated parallel
to the wire passing beneath. A number of tapped bores
are again provided in the guide body, through which the
spindles, adjusted by the device V, bear down on the
deflector slat, again enabling the position-dependent
adjustm~nt of the deflector slat.
Fig. 3 shows schematically the embodiment of the
deflector slat corresponding to the shape illustrated in
Fig. 1 where the deflector slat consists of two
materials. The slat includes an upper continuous,
flexurally elastic material B where the spindles S are
arranged on the topside with the aid of claws. On the
opposite sicle there are a number of individual elements
set side by side which consist of a more resistant, more
brittle material E and which bear dir ctly on the wire.
A relatively high flexibility of the overall slat, and a
good wear performance, is thereby achieved.
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Fig. 4 shows a dewatering slat similar to Fig. 1,
but here the guide element F connects with a carrier T by
way of a web S arranged on the underside. Two pressure
cushions are arranged bstween the guide element F and
carrier ~ The size of the pressure cushions can be
adjusted by application of different pressures, such that
the guide element F, in relation to the carrier, can move
in its entirety, thus exerting on the wire an adjustable,
flexible pressure. In the present form, th~ two pressure
cushions are prevented from slipping out of the
intermediate gap between the guide element and carrier by
slats arranged above them.
Fig. 5 shows in exemplary fashion the sheet
formation zone of a double wire former with a headbox
coming ~rom the left and injecting its fiber suspension
between two wires Sl and S2 passing over two reversing
cylinders Zl and Z2. The two wires S1 and S2 extend in
sandwich fashion across a dewatering unit E, behind which
a locally adjustable dewatering slat according to the
invention is provided. The locally adjustable dewatering
slat is bearing from above.
Fig. 6 shows a double wire former as well. In this
embodiment, the stock suspension is injected between two
wires from below. The two reversing cylinders Zl and 22
carrying the wires are situated with their axes on a
nearly horizontal plane, whereby the wire Sl coming from
above is deflected to the right, first across cylinder Z1
and thent bearing on wire S2, across cylinder Z2. In
this embodiment, there are two inven~ional dewatering
slats Ll and L2 provided at a mutual offset, above and
below the w:ire, bordering directly on the cylinder Z2.
Fig. 7 once again shows a headbox of a double wire
former, this time with a primary dewatering. Coming from
the left, the headbox applies the stock suspension on the
p ~
wire S2 which, coming from below, is de~lected via
cylinder Z2 and is passed to the dewatering zone El.
Following the dewatering zone E1, a wire S1 is moved
across cylinder Zl ~rom above and, with a steadily
diminishing gap, to the wire S2. Both wires run to a
double-sided dewatering zone 2, and wire Sl is then
deflected upward via cylinder Z3 while wire S2 is
deflected downward via cylinder Z4. This embodiment
proposes placing an inventional dewatering slat L between
reversing cylinder Zl and the double-sided dewatering
zone E2 on the topside o~ wire S1, and the slat~ on the
lower wire in the dewatering zone E2.
