PROCEDE POUR COMPENSER DES DEFAUTS DANS UNE BANDE DE PAPIER

METHOD FOR COMPENSATING FOR FAULTS IN A PAPER WEB

Abrégé français

L'invention concerne un procédé pour compenser un rétrécissement inégal dans une bande de papier dans les sections de pressage et de séchage d'une machine à papier, dans laquelle il existe au moins un tirage ouvert. Dans le procédé, un ou des guides d'écoulement sont utilisés dans la caisse d'arrivée, au moyen desquels la tendance des fibres à s'orienter dans le sens machine est réduite dans les zones de bordure de la bande et l'angle de séparation de la bande par rapport au rouleau central est ajusté, en dirigeant sur la bande un jet d'air qui se trouve dans le sens travers et dont la pression peut être ajustée dans ladite direction.

Abrégé anglais

Method for compensating for uneven shrinkage in a paper web in the press and
drying sections of a paper machine,
in which there is at least one open draw. In the method, a flow guide/guides
are used in the head box, by means of which the machine
direction orientation tendency of the fibres is reduced in the edge areas of
the web and the angle of detaching of the web from the
centre roll is adjusted, by directing onto the web an air jet that is located
in the cross direction and the pressure of which can be
adjusted in the said direction.

Revendications

Claims
1. Method for compensating for faults, like uneven shrinkage, in a paper web
in
the press and drying sections of a paper machine, in which there is at least
one
open draw,
characterized in that
- the paper web, which has a property naturally or induced by a differing
release
angle from the centre roll of the press section over its width, is fed to the
press
and drying sections of the paper machine and
- the web's (4) release angle is adjusted by directing an air jet at the web
(6),
which is located in cross-direction of the web and the pressure of which can
be
adjusted in the said direction.
2. Method according to claim 1, characterized in that, in order to compensate
for
the great shrinking tendency of the edge areas of the web, a flow guide/guides
are
used, the length of which in the edge areas of the web is greater that in the
centre
area, in order to reduce the machine direction orientation of the fibres.
3. Method according to claim 1, characterized in that the pressure of the air
jet, in
the cross direction of the web, on the web (4) detaching from the centre roll
(3) of
the press section, is adjusted in such a way that it is greater in the edge
areas of
the web than in the centre area of the web.
4. Method according to any of the above claims, characterized in that the
release
angle of the web from the roll is monitored in real time and the result of the
monitoring is used directly and immediately to correct the pressure profile,
in order
to equalize the release angle of the web over the width of the web.
5. Method according to claim 4, characterized in that a suitable measuring
device, for example a laser camera/cameras is/are used for the monitoring.

Description

CA 02697054 2010-02-19
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Method for compensating for faults in a paper web
The present invention relates to a method for compensating for faults, such
non
uniform shrinkage, in a paper web. In particular, the matter concerns
compensating for greater shrinkage that occurs in the edge areas of the web
than
in both the centre, in the press and drying sections of a paper machine.
It is known that shrinkage in a paper web over its production width on a
papermachine is uneven, in such a way that the shrinkage in the edge parts can
be in the order of 7 %, whereas in the centre the shrinkage of the web can
remain
in the order of two or three percent. Refer to attached figure illustrating a
typical
shrinkage profile.
It is obvious that in the situation described above, it is extremely likely
that
unsaleable goods and other detrimental quality defects will be created, unless
the
shrinkage differences can be reduced in some manner.
Problems in the edge areas of the web include but are not limited to, an
increase
in the porosity of the edges, an increase in roughness at the edges, creasing
or
cockling, variation in the percentage of mineral filler content and of course
poorer
dimensional stability. The exact nature of the problem will depend on the type
of
the press and drying sections of the paper machine in question, and on the
paper
grade in general.
It is obvious that, in an analogous manner, other faults corresponding to
shrinkage
are also possible and that compensating for them belongs automatically to the
scope of the present invention.
The present invention is intended to create an improvement in the adverse
effects
of shrinkage profile variation. The intention is to achieve the improvement in
a
manner that is very reasonably priced and simple.

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The aforementioned and other advantages and benefits of the present invention
are achieved in the manner described in the accompanying claims.
Stated simply, the basic idea of the invention is that the fault, no matter
whether it
is natural or the result of some operation, is affected in such a way that the
variation in properties can be minimized. Thus, for example, the shrinking of
a
paper web more at the edges than in the centre is a natural change, which when
attempting to minimise also promotes an improvement of the quality of the
paper
reels at the edges.
In the following, the invention is described in greater detail also with
reference to
the accompanying drawings, in which
Figure 1 shows one embodiment of a flow guide used according to the invention
in
the discharge nozzle of a head box of a paper machine in the direction of flow
of
the stock; and
Figure 2 shows a second sub-area of the invention, in connection with the
centre
roll of the press section.
In the discharge nozzle of a head box of a paper machine, seen in the
direction of
flow of the stock, flow guides are used, which are intended to affect the
properties
of the paper, by affecting the turbulence of the jet. The length of the flow
guides in
the direction of travel of the stock has a significant effect on the
properties of the
paper. This is because the machine direction orientation of the fibres changes
significantly as result of the turbulence generated by the flow guides. A
general
rule is that the machine direction orientation of the fibres decreases as the
length
of the flow guides increases. In the present invention, this aforementioned
property is exploited by making the flow guide or guides longer at the edges
than
in the centre. One example is shown in Figure 1, though only by way of example
and not restrictively. In the figure is a flow guide 1, which is generally a
plate-like
piece. In this embodiment, the edges 2 of the guides are clearly extended.

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Because the change in the machine direction magnitude of the fibre orientation
in
the edge areas of the web can also be a risk in terms of the operation of the
paper
machine, according to the invention other means are also used to achieve the
desired effect.
The angle of release of the paper web from the centre roll of the press
section in
an arrangement, which uses an open transfer from one roll to another, varies
according to the draw and the magnitude of machine direction fibre
orientation. If
the magnitude of machine direction fibre orientation has been altered in the
manner described above, in such a way that the orientation is less at the
edges of
the web, this means that the angle of release from the centre roll of the
press
section will change, in such a way that the web tends to remain attached to
the roll
at the edges of the roll more than the centre. The result of this is that the
edge
areas become permanently stretched more than the centre which can be
extremely detrimental to the machine's subsequent operation in the dryer
section.
Thus, it would be good to have available a device, with the aid of which it
would be
possible to counteract the release angle problems caused by the shape of the
flow
guide. This is solved according to the invention as follows. Here reference is
made
to Figure 2.
The angle, at which the paper web 4 running around the centre roll 3 of the
press
section detaches from the roll, is adjusted in a controlled manner, by using
an
adjustable air jet over the width of the web. In practice, this means that,
either
experimentally or by theoretical calculation, the pressure profile in the
cross
direction of the web is determined, by means of which it is possible to
achieve a
uniform release angle profile.
One device, by means of which it is possible to create and use the afore
mentioned profile, is a device developed by Runtech Systems Oy and marketed
under the trademark Air Curtain. By means of the said device, which is marked
with the reference number 5 in Figure 2, it is possible to create an air jet
extending
over the entire width of the web, the pressure profile of which can be varied
over
the entire width. The air jet being created is marked schematically with the

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reference number 6. The pressure of the air jet affects the angle of sheet
release,
thus by varying the cross machine pressure profile the release angle can be
influenced in such a manner as to compensate for the variation in release
angle
created by the length profile of the flow guides in the headbox. By this means
the
adverse effects on dryer section runnability can be controlled while retaining
the
positive effects of minimising variation in paper properties dependent on the
turbulence created by the length profile of the flow guides in the headbox.
One further development of the system described above is that, by using
suitable
means to measure the release angle, the information used by the said means can
be used directly to adjust the operation of the air jet device 5, so that the
desired
release angle will be obtained. In practice, a laser camera, for example, is a
suitable means for measuring the release angle.
The procedure described above is not the only one to which the principle of
the
present invention can be applied. With the aid of an airjet, it is possible to
correct,
more or less in real time, non-uniformity in the paper web arising from other
reasons. Thus it is possible to quickly adjust the air-jet pressure and the
release
angle at any point whatever in the width of the web. The result will be paper
of
improved quality.
Variations are possible while remaining with the scope of protection of the
inventive idea and the accompanying claims.

Dessin représentatif