Note: Descriptions are shown in the official language in which they were submitted.
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MET~OD OF MARI~G PAP~R
The invention is directed to a method of making
adjustments at the headbox of a web-forming machine,
such as a paper machine, to provide a web of relatively
uniform density and layer height.
A headbox for a paper machine should adjust the
pulp density and fiber orientation profile of the paper
pulp suspension, at the latest, before the suspension
passes through the di~charge ~lit of the headbox, so
that the pulp den~ity and fiber orientation profiles of
the paper web correspond to the desired requirements ;~-
over the entire width of the web, meaning, as a rule,
that they are con~tant.
When operating a paper machine, there are many
perturbing factor~ which hinder the achievement of the
two above requirements. These perturbing factors
include, for example, temperature and pressure
fluctuations, manufacturing tolerances, and defects in
the design or adju~tment of the paper machine for the
production process after the paper pulp i9 discharged
from the headbox.
The following state of the art has become known
for influencing the transverse profile of a paper web.
German publication DE 35 14 554 proposes to change the
pulp density locally; that i~, to adjust the pulp
density at certain points, depending on demand.
However, it is not described how this should be
accomplished.
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German publication DE 40 19 593 Al recommends
that, upon devia~ion of the pulp den~ity profile of the
paper web at a certain point of the web width, the
concentration CM Of the respective section flow, and
thus that of the flow leaving the respective mixer,
should be changed correspondingly. In order to achieve
this, the ratio of the amounts of control flows QN/QL,
introduced to the mixture are changed. However, in the
case of valves of the usual construction, it is
difficult to avoid deviation of the section flow QM
leaving the mixer from the required value in an
uncontrolled and unwanted manner.
In addition, it i9 known from German publication
DE-OS 35 38 466 that a change of the volume flow of a
section leads to influencing the fiber orientation
angle in the discharge section of the headbox. If a
section flow deviates from the required value in an
uncontrolled manner, the fiber orientation will also
change in an uncontrolled manner.
Furthermore, it is known from German publications
DE 29 42 966 and DE-OS 35 35 ~49 that one can change
the width of the discharge slit, for example, with
threaded ~pindles for horizontally swinging or bending
the upper lip. As a result, the throughput of the
suspension can be altered locally. However, at the
same time, the flow direction is also influenced
locally, and thus the fiber orientation i3 affected.
Namely, at the narrow parts of the discharge slit, the
fibers will be disposed in a different flow direction
than at the other part~ of the discharge slit. This
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means that, although the consistency can be made
uniform over the width of the headbox by thi method of
control, called displacement control, the originally
good fiber orientation i9 de troyed.
It can be seen from the state of the art de3cribed
above that there are essentially two parameters that
are adjusted at the headbox, namely the flow rate of
the paper stock suspension at a given point of the
headbox and the stock consistency, and that those two
parameters have a different and conflicting influence
on the stock density profile and fiber orientation.
The present invention is directed to a method of
making adjuqtments at the headbox of a web-forming
lS machine to provide a web of relatively uniform density
and layer height. The headbox has a plurality of
transver~e sections, each of which i9 provided with a
web material at a variable flow rate and a variable
consistency. The method includes the steps of
measuring the layer height transver~e profile of the
web at a point along the web, measuring the density
transverse profile of the web at a point along the web,
and comparing a portion of the layer height profile
with a corresponding portion of the density profile to
determine whether there are corresponding deviations in
the profiles.
If there is a deviation in the density profile
without a corresponding deviation in the layer height
profile, then the magnitude of the consistency of the
web material provided to the transverse section of the
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headbox associated with the deviation i~ changed in a
direction opposite the sign of the deviation.
If there is a deviation in the layer height
profile without a corresponding deviation in the
density profile, then the magnitude of the flow rate of
web material provided to the transverse section of the
headbox associated with the deviation i9 changed in a
direction opposite the sign of the deviation.
If there is a deviation in the density pr~file
with a corresponding deviation with the same sign in
the layer height profile, then the magnitude of the
flow rate of the web material provided to the
transverse section of the headbox a~sociated with the
deviations is changed in a direction corresponding to
the sign of the deviations.
The web i~ preferably dewatered by providing it to
a dewatering apparatus, pressed and dried to provide a
finished web, and rolled into a roll. The method may
be used in connection with a paper machine to fonm a
paper web.
Instead of measuring the layer height profile, the
fiber orientation profile of the web may be measured,
and adjustments in the consistency and flow rate of the
~ web material may be made based on the presence or
absence of corre~ponding deviations in the measured
fiber orientation and density profiles.
The invention i9 based on the inventors'
xecognition that there are two parameters that
fundamentally influence the density profile, namely the
consistency and the flow rate of the web material at a
given point of the headbox, and that the fiber
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orientation of the web material is generally influenced
only by the flow rate of the web material at a given
transverse point or ~ection of the headbox.
These and other feature~ and advantages of the
presen~ invention will be apparent to those of ordinary
skill in the art in view of the detailed description of
the preferred embodiment, which is made with reference
to the drawings, a brief description of which i9
provided below.
Fig. 1 is a schematic illustration of a preferred
embodiment of a paper machine in accordance with the
present invention; and
Fig. 2 illustrates a density profile and a layer
height profile of a paper web generated by the paper
machine of Fig. 1.
Fig. 1 is a schematic illustration of a paper
machine 10 with a headbox 12 on the left side, which i9
fed with a web material such a~ paper pulp, or stock
suspension, across a plurality of transverse sections.
The magnitude of the flow of stock ~uspen~ion provided
to each transverse section is regulated with a valve
~2. The consistency of the stock suspension provided
to each transverse section is de~ermined by the
combination of two ~tock flows at a mixer mi, one
having a maximum con3istency b~ and the other having a
minimum consistency b~. The actual consistency is
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determined by a valve Vl, which controls the magnitude
of the stock flow with the minimum consistency b~.
In the operation of the paper machine 10, the
headbox 12 displaces the stock suspension onto a screen
14 for dewatering the stock suspension. The screen 14
is translated past the headbox 12 from left to right in
Fig. 1 via a plurality of roller~ 16. The point at
which the velocity of the impinging stock suspension is -~
adjusted to the screen velocity i8 designated with S.
At thi~ point, the stock layer height is hi, where i
represents a particular transverse portion of the stock
suspension, i.e. a particular portion along the width
of the web. It should be understood that the layer
height of the stock suspension may be different at
different transverse points h; of the suspension.
The end of the dewatering section of the screen 14
is designated by Sl, which i9 then followed by a
conventional pressing and drying apparatus 1~. After
the stock suspen~ion is pressed and dried, at a point
S2, it i8 wound up on a roll 20 as paper.
The consistency of the stock suspension at a
transverse point i of the headbox 12 is designated
herein as b;, and the amount or flow rate of the stock
suspension introduced at a transverse point i of the
headbox 12 is designated as q;.
After the stock suspension is ejected onto the
moving scree~ 14, the velocity of the stock suspension
adjusts to the velocity of-the screen 14 at a point S,
at which point tho~e two velocities are substantially
the same. At that point S, the flow rate of the stock
2 ~ ~ 2 ~ 7 1
suspension is proportional to the layer height hj of
the stock suspension. If the velocity of the screen 14
is known, the flow magnitude of the stock suspension
can be determined by measuring the layer height hj and
multiplying it by the screen velocity.
The layer height hj can be determined in any
conventional manner, such aq by a plurality of sensors
(not shown), each of which is provided at a different
transverse section of the screen 14. The mean or
average layer height value haVe is determined by
dividing the sum of all the layer height measurements
h; by the number of such measurements.
The density of the stock suspension at a
transver~e point i i8 designated herein as fj. That
density fj, which may be represented for example, in
grams/square meter (GSM), can be measured in a
conventional manner by a plurality of sensors (not
shown), each o~ which is provided at a different
tran~verse section of the screen 14, at a position
between the points Sl and S2 along the length of the
stock suspension.
After the stock density f; at each such point is
mea~ured, the average stock density fDve may be
determined by dividing the sum of all the transverse
density measurements fj by the number of such
measurements.
As described below, the consistency bj of the
stock suspension and the stock flow rate qj are
controlled via the valves Vl and V2, respectively, based
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upon the measurements of the layer height hj and the
stock density fj.
Referring to Fig. 2, a stock density transverse
profile f'j is shown above a layer height transverse
profile h'j. Each point h'j on the layer height profile
is determined from the measurements described above in
accordance with the following equation: h'; = (hj -
h~Ve)/h~ve. Similarly, each point f'j on the stock
density profile is determined from the measurement~
made above in accordance with the following equation:
f i (fi fgve) /f~vc-
The stock density f'j is shown to increase above
the average stock density at a transverse point A of
the paper machine and to decrease below the average
stock density at a transverse point ~. At other points
on the density profile, the stock density f'j is shown
to be approximately equal to the average stock density.
The layer height hj is shown to increase above the
average layer height at the transver~e point A and to ~ ~-
decrease below the average layer height at a transverse
point C. At other points on the layer height profile,
the layer height h'; is shown to be about equal to the
averag~ layer height.
It should be noted that, since both the layer
height h'i and stock density f'j both increa~ed at the
same tran~verse point A of the paper machine, there is
a high correlation between the layer height and stock
density at that point A. Here, one would expect
defective fiber orientation and a high density
deviation. In order to correct such condition, the
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flow rate qj is decreased in the region i = A via the
valve V~ associated with the transverse point A of the
headbox 12.
It should be noted that, at transverse point B of
Fig. 2, there is only a deviation in the density f'j,
with no corresponding deviation in the layer height
h'j. In this case, the deviation in the density f'j is
corrected by adjusting the valve Vl associated with the
transverse point B of the headbox 12 so that less of
the stock solution having the consistency b~ is
provided to the headbox 12.
At transverse point C, there is a deviation in the
layer height h'j, but no deviation in the density
profile f';. In this case, in order to avoid
undesirable fiber orientation, the flow rate qj at the
transverse point C is increased by adjusting the valve
V2 associated with the transverse point C and, at the
same time, reducing the stock consistency at the
transverse point C by adjusting the valve V~ so that
more ~tock solution having the consistency b~ i9
provided to the mixer mi associated with the transverse
point C.
In general terms, by determining the deviations in
the layer height profile h'j and the stock density
profile f'j and whether the deviations correspond with
each other, one can determine which of the parameters
qj or bj to adjust in order to achieve a uniform stock -~
density profile and layer height profile.
More specifically, the method of control includes
the steps of determining the layer height transverse
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profile of the web at a point along the web,
determining the density transverse profile of the web
at a point along the web, and comparing a portion of
the layer height transverse profile with a
corresponding portion of the density transverse profile
to determine whether there are corresponding deviations
in the profiles.
If there i9 a deviation in the density transverse
profile withou~ a corresponding deviation in the layer
height profile, then the magnitude of the consistency
of the web material provided to the transverse section
of the headbox associated with the deviation i9 changed
in a direction opposite the sign of the deviation.
Thus, at transverse point B of Fig. 2, where the sign
of the deviation is negative, i.e. f'j decreased, the
change in consistency i9 made positive, i.e. the
consistency is increased in response.
If there i9 a deviation in the layer height
profile without a corresponding deviation in the
density profile, then magnitude of the flow rate of web
material provided to the transverse section of the
headbox associated with the deviation is changed in a
direction opposite the sign of the deviation. Thus, at
transverse point C of Fig~ 2, where the sign of the
deviation is negative, i.e. h'j decreased, the change
in flow rate is made positive, i.e. the flow rate is
increased in response.
If ~here is a deviation in the density profile
with a corresponding deviation with the same sign in
the layer height profile, then the magnitude of the
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flow rate of the web material provided to the
transverse section of the headbox associated with the
deviations i9 changed in a direction opposite the sign
of the deviations. Thus, at transverse point A of Fig.
2, where the signs of the deviations are the same, i.e.
both f'j and h'j increased, the change in flow rate is
changed in the opposite direction, i.e. the flow rate
is decreased in response.
In the control method, if there is a deviation in
the density transver e profile with a corresponding
deviation in the layer height profile of opposite sign,
then both deviations may be treated separately as
uncorrelated deviations.
Also, counter-control may be carried out with the
other parameters of the change that was performed. As
used herein, the "counter-control" refers to a type of
control in which the mathematical product of the stock
flow rate and stock consistency i9 held substantially
constant. Thu~, if the stock flow rate is increased,
the stock consistency would be decreased by an amount
so that the product of the two remained constant. If
the stock flow rate is decreased, the stock consistency
would be increased by an amount to maintain the product
of the two constant.
In another preferred method in accordance with the
invention, instead of measuring the layer height
profile of the web, the fiber orientation profile is ~;~
measured at a point along the web. The fiber
orientation can be measured by any conventional manner,
such as by ultrasound or with the aid of a laser.
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The remaining steps are similar to the first
method described above. In particular, after the fiber
orientation profile and the density profile of the web
are measured, a portion of the fiber orientation
S profile is compared with a corresponding portion of the
denRity profile to determine whether there are
corresponding deviations in the two profiles.
If there is a deviation in the den~ity profile
without a corresponding deviation in the fiber
orientation profile, then the magnitude of the
consistency of the web material provided to the
transverse section of the headbox associated with the
deviation is changed in a direction opposite the sign
of the deviation. If there is a deviation in the fiber
15 orientation profile without a corresponding deviation ~ ~ ;
in the density profile, then the magnitude of the flow
rate of web material provided to the transverse section
of the headbox associated with the deviation is changed
in a direction opposite the sign of the deviation. If
there is a deviation in the density profile with a
corresponding deviation with the same sign in the fiber
orientation profile, then the magnitude of the flow
rate of the web material provided to the transverse
section of the headbox associated with the deviations
is changed in a direction opposite the sign of the
deviations.
The above methods can be applied to all types of
headboxes, for example, one layer headboxe~, multi-
layer headboxes, headboxes for slit formers, headboxes
for long sieves, etc.
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The above methods could be carried out via any
conventional control scheme. For example, a
conventional proportional-integral (PI) or
proportional-integral-derivative (PID) controller could
be provided for each transverse section of the stock
solution. Each controller would be connected to
receive both the inputs from the sensor~ which measure
the stock density and layer height at its transverse
point or section, and each controller would be
connectPd to control both the valves V~ and V~
associated with its transverse section.
Alternatively, a single controller could be
connected to the transversely located sensors for
sensing the density and layer height of the web and to
the transversely located valve~ for controlling the
stock consistency and flow rate. In the case of a
single controller, the control method could be
accomplished on a time-shared or round-xobin basi~,
with each transverse section of the paper machine being
controlled sequentially.
Modification~ and alternative embodiments of the
invention will be apparent to those skilled in the art
in view of the foregoing description. This description
is to be construed as illustrative only, and is for the
purpose of teaching tho~e skilled in the art the best
mode of carrying out the invention. The details of the
structure and method may be varied ~ubstantially
without departing from the spirit of the invention, and
the exclusive use of all modifications which come
within the scope of the appended claims is reserved.
