U.S. patent application number 09/867925 was filed with the patent office on 2002-01-24 for method and equipment for regulation of the initial part of the dryer section in a paper machine.
Invention is credited to Huhtelin, Taisto, Kaihovirta, Juha, Lipponen, Juha, Vahatalo, Harri.
Application Number | 20020007927 09/867925 |
Document ID | / |
Family ID | 8553040 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020007927 |
Kind Code |
A1 |
Vahatalo, Harri ; et
al. |
January 24, 2002 |
Method and equipment for regulation of the initial part of the
dryer section in a paper machine
Abstract
A method and an equipment for regulation of the initial part of
the dryer section in a paper machine. Moisture is removed from the
paper web in the press section (10) and the paper web is dried in
the dryer section (R). The paper web is dried against heated faces
of drying cylinders (20). The surface temperatures of the first
drying cylinders (20) in the dryer section (R) are measured. The
dry solids content/moisture content of the paper web (W) and the
temperature of the paper web (W) before said first drying cylinders
(20) are measured. In view of producing the desired running
situation, based on the measurement results obtained, the surface
temperatures of said first drying cylinders (20) and/or the dry
solids content/moisture content and/or the temperature of the paper
web (W) before said first drying cylinders (20) is/are
regulated.
Inventors: |
Vahatalo, Harri; (Jyvaskyla,
FI) ; Kaihovirta, Juha; (Jyvaskyla, FI) ;
Lipponen, Juha; (Kerava, FI) ; Huhtelin, Taisto;
(Tampere, FI) |
Correspondence
Address: |
LATHROP & CLARK LLP
740 REGENT STREET SUITE 400
P.O. BOX 1507
MADISON
WI
537011507
|
Family ID: |
8553040 |
Appl. No.: |
09/867925 |
Filed: |
May 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09867925 |
May 30, 2001 |
|
|
|
PCT/FI99/00990 |
Nov 29, 1999 |
|
|
|
Current U.S.
Class: |
162/198 ;
162/253; 162/262; 162/263 |
Current CPC
Class: |
D21F 5/06 20130101; Y10S
162/10 20130101; D21F 5/04 20130101 |
Class at
Publication: |
162/198 ;
162/262; 162/253; 162/263 |
International
Class: |
D21F 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 4, 1998 |
FI |
982622 |
Claims
We claim:
1. A method for regulation of the initial part of the dryer section
in a paper machine having a press section in which moisture is
removed from the paper web, and a dryer section in which the paper
web is dried, and in which the paper web is passed from the press
section into a first group of first drying cylinders forming a part
of the dryer section, in which the paper web is dried against
heated faces of the drying cylinders, the method comprising the
steps of: measuring the surface temperatures of the first drying
cylinders of the first group of drying cylinders in the dryer
section; measuring the dry solids content / moisture content of the
paper web and the temperature of the paper web upstream of said
first drying cylinders, and in view of producing the desired
running situation, based on the measured surface temperatures and
dry solids content/moisture contents, regulating the surface
temperatures of the first drying cylinders, the dry solids content
/ moisture content, and the temperature of the paper web before
said first drying cylinders.
2. The method of claim 1 wherein, based on said measurements, the
difference between the surface temperatures of the first drying
cylinders of the first group of drying cylinders and the web
temperature is regulated to a level lower than a preset value or a
value determined by means of computing on the basis of the
measurement results.
3. The method of claim 1 wherein, based on said measurements, the
surface temperatures of the first cylinders are regulated to a
level lower than the sum of the measured web temperature and a
predetermined temperature or a temperature determined by means of
computing.
4. The method of claim 1 wherein, based on said measurements, the
temperature of the web is regulated to a level higher than the
difference between the measured surface temperatures of the first
cylinders and a predetermined temperature or a temperature
determined by means of computing.
5. The method of claims 1 wherein the paper web to be dried is
passed from the press section to the dryer section over an
impingement drying unit into the first group of drying cylinders,
and based on said measurement results, blow parameters of the
impingement drying unit are regulated.
6. The method of claim 1 wherein, based on said measurement
results, regulating at least one of a press load of a press in the
press section, and at least one steam box in the press section.
7. The method of claim 1 wherein an additional group of drying
cylinders follows the first group of drying cylinders, and wherein,
based on said measurement results, regulating the difference of
draw between the press section and the dryer section as well as the
differences in draw between the first cylinders and the additional
group of drying cylinders.
8. The method of claim 1 wherein an additional group of drying
cylinders follows the first group of drying cylinders, and wherein,
based on said measurement results, regulating the difference of
draw between the first cylinders and the additional group of drying
cylinders.
9. The method of claim 1 wherein reversing suction rolls are
positioned between adjacent pairs of first drying cylinders, each
reversing suction roll having a level of vacuum, and wherein a blow
or suction box is positioned above each reversing suction roll,
and, based on at least one of said measurements, controlling the
level of vacuum in the reversing suction rolls, the capacity of the
blow or suction box, or both.
10. The method of claim 1, further comprising the step of employing
a multi-variable regulation and optimizing method, wherein the at
least one input variable is selected from the group consisting of:
the capacity of the steam box; the press load; the capacity of the
impingement drying; and the heating capacity of the first cylinder;
to thereby maintain the moisture content of the web and the
temperature of the web and the temperature of the first cylinder at
the desired level.
11. The method of claim 1 further comprising the step of optimizing
at least one of the web moisture content and the press load, and
differences in temperature between drying cylinders and the web
temperature are optimized by adjusting at least one of the heating
capacity of an impingement drying unit fitted before the first
drying cylinders and the heating capacity of the first cylinder
itself.
12. The method of claim 1 wherein, for the purpose of threading
after a web break in the paper web, the surface temperatures of
said first drying cylinders not covered by the web are measured,
and, based on the measurement results obtained, regulating at least
one of the surface temperatures of said first drying cylinders, the
press load in at least one press in the press section, the levels
of the steam boxes in the press section, and the blow parameters of
the impingement drying unit, in order to provide a desired
threading situation.
13. The method of claim 1 wherein the temperatures of said first
drying cylinders are regulated after a web break by means of a
separate steam valve and by means of a supply of water produced out
of a cooling cylinder.
14. The method of claim 1 wherein the temperatures of said first
drying cylinders are regulated after a web break by means of a
supply of water produced out of a cooling cylinder.
15. The method of claim 13 wherein condensate obtained from a
condenser of a steam system is used as cooling water.
16. The method of claim 14 wherein condensate obtained from a
condenser of a steam system is used as cooling water.
17. An apparatus for regulation of the initial part of the dryer
section in a paper machine having a press section and a dryer with
a plurality of first drying cylinders, the apparatus comprising:
measurement devices fitted in connection with the first drying
cylinders in the dryer section for measurement of the surface
temperatures of said first drying cylinders; a measurement device
for measurement of the dry solids content / moisture content of the
paper web and of the temperature of the paper web before said first
drying cylinders; and a control device for regulation of at least
one of the surface temperatures of said first drying cylinders, the
dry solids content / moisture content, and the temperature of the
paper web before said first drying cylinders, based on the
measurement results obtained by means of the measurement devices,
so as to produce the desired running situation.
18. The apparatus of claim 17 wherein the control device of the
equipment comprises means for regulation on the basis of said
measurement results obtained from the measurement devices of at
least one of the press load in at least one press in the press
section; the steam box/boxes; the impingement drying unit fitted
before said first drying cylinders; and the differences of draw
between the first groups of drying cylinders in the dryer section
on the basis of said measurement results obtained from the
measurement devices.
19. The apparatus of claim 17 wherein paper machine has reversing
suction rolls between pairs of the first drying cylinders, each
suction roll having a level of vacuum applied, and a blow or
suction box is positioned to act on at least one of the first
drying cylinders and the apparatus further comprises a controller
for regulation of the vacuums in the reversing suction rolls, and
the capacity of the at least one blow or suction box.
20. The apparatus of claim 17 wherein the equipment has been fitted
to be used during threading taking place after a web break in the
paper web so as to provide the desired threading situation.
21. The apparatus of claim 17 wherein the equipment comprises a
control unit for keeping the difference between the surface
temperatures of the first cylinders and the web temperature below a
preset value or a value determined on the basis of the measurement
results.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of PCT application No.
PCT/F199/00990, filed Nov. 29, 1999, and claims priority on Finnish
Application No. 982622, filed Dec. 4, 1998, the disclosures of both
of which applications are incorporated by reference herein.
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED
RESEARCH AND DEVELOPMENT
[0002] Not applicable
BACKGROUND OF THE INVENTION
[0003] The invention concerns a method for regulation of the
initial part of the dryer section in a paper machine, in which
method moisture is removed from the paper web in the dryer section
and in which method the paper web is dried in the dryer section, in
which method the paper web to be dried is passed from the press
section into the dryer section into the first group of drying
cylinders, in which the paper web is dried against heated faces of
drying cylinders.
[0004] Further, the invention concerns an equipment for regulation
of the initial part of the dryer section in a paper machine, which
equipment has been fitted in connection with the press section and
with the dryer section in the paper machine.
[0005] As is known from the prior art, in multi-cylinder dryers in
paper machines, twin-wire draw and/or single-wire draw is/are
employed. In twin-wire draw the groups of drying cylinders comprise
two wires, which press the web, one from above and the other one
from below, against heated cylinder faces. In twin-wire draw,
between the rows of drying cylinders, which are usually horizontal
rows, the web has free and unsupported draws, which are susceptible
of fluttering, which may cause web breaks, in particular in the
stages of drying in which the web is still relatively moist and,
therefore, of low strength. This is why, in the course of the last
15 years, ever increasing use has been made of the single-wire
draw, in which each group of drying cylinders comprises just one
drying wire, on whose support the web runs through the whole group
so that the drying wire presses the web on the drying cylinders
against heated cylinder faces, and on the reversing cylinders or
rolls placed between the drying cylinders the web remains at the
side of the outside curve. Thus, in single-wire draw, the drying
cylinders are placed outside the wire loop and the reversing
cylinders or rolls inside the loop. In order to heat the faces of
the drying cylinders, steam is passed into their interior, and the
temperature of the surface of a heated drying cylinder is regulated
so that the pressure of the steam present in the interior of the
cylinder is regulated.
[0006] In view of the cylinder drying and in view of the
runnability of the paper web and of the quality properties of the
paper, the first drying cylinders placed in the beginning of the
dryer section are highly important. If the temperatures of the
first drying cylinders are too high, the paper web has a tendency
to follow the face of the cylinder, which causes problems of
runnability and thereby restricts the running speed. Likewise, the
brightness of the paper web becomes lower, the porosity increases,
the roughness increases, formation of dust increases, and the bulk
may become lower. On the other hand, if the temperatures of the
first drying cylinders are too low, the drying cylinders in the
initial part of the dryer section are inefficient.
[0007] During threading of the web, even though the steam pressure
has normally been switched off for this time or the steam pressure
has been lowered to a level lower than a normal running situation
in respect of the cylinders in the initial part of the dryer
section, most commonly by lowering the pressure in the principal
steam group, the lead-in strip has adhered to the drying cylinders,
because the faces of the first drying cylinders are hot since the
energy-consuming paper web is not present on them, in which case,
thus, the web does not consume the thermal energy of the drying
cylinders. Adhering of the lead-in strip has caused threading
problems and thereby lowered the efficiency of the paper
machine.
[0008] In the prior-art multi-cylinder dryers, in the initial part
of the dryer section, it has been a further problem that it has not
been possible to use a temperature sufficiently high in view of
optimal drying, because, when the paper web is in direct contact
with the hot faces of the drying cylinders, at temperatures higher
than a certain level the web tends to adhere to the hot faces of
the cylinders, which results in web breaks and standstills. It has
also been noticed that excessively hot contact drying cylinders
have detrimental effects on the quality properties of the paper. On
the other hand, excessively low temperatures result in lowered
drying efficiency.
[0009] At the first drying cylinders, problems have also arisen
from the fact that, when the paper web starts becoming warm, it
stretches, and when it stretches, it loses some of its tension, in
which connection the web tends to follow the drying cylinder, which
may cause problems of runnability. Traditionally, these problems
have been solved so that a difference in speed has been arranged
between the press section and the dryer section in order to
maintain the web tension. Further, differences in speed have been
employed between the first dryer groups in order to produce an
adequate tension. However, excessive differences of draw have
negative effects on the properties of the final product, because,
when the paper is pulled, in particular its strength properties
both in the machine direction and in the cross direction
change.
[0010] One prior-art solution of the type described above is known
from the publication EP 0,769,587. In said prior-art arrangement
the draw applied to the web is increased constantly so that a
number of short dryer groups are employed in the initial part of
the dryer section, which is unfavourable in view of the costs,
because, among other things, extra drives, wires and wire guides
are needed.
[0011] Important factors which affect the temperatures of the first
cylinders in a dryer section include, among other things, the paper
grade to be produced, the web moisture after the press section, and
the temperature of the paper web. Usually, information on these
factors in combination with the steam pressures in the first drying
cylinders employed in cylinder drying has been obtained empirically
only.
[0012] The current development in paper manufacture is about to
lead in a situation in which there is no open draw between the
press section and the dryer section, but a closed draw is
concerned, in which case no high differences in speed can be
employed, for which reason the problems described above in relation
to the stretch of the web are emphasized.
SUMMARY OF THE INVENTION
[0013] The object of the present invention is to provide a solution
for the problems described above.
[0014] It is a further object of the invention to provide a drying
method and a dryer section that makes use of said method in which
adhering of the web to the cylinders in the initial end of the
dryer section is prevented and in which, at the same time, improved
quality of the paper and improved runnability of the paper machine
are achieved.
[0015] In view of achieving the objectives stated above and those
that will come out later, the method in accordance with the
invention is mainly characterized in that, in the method, the
surface temperatures of the first drying cylinders in the dryer
section are measured, that, in the method, the dry solids
content/moisture content of the paper web and the temperature of
the paper web before said first drying cylinders are measured, and
that, in the method, in view of producing the desired running
situation, based on the measurement results obtained, the surface
temperatures of said first drying cylinders and/or the dry solids
content/moisture content and/or the temperature of the paper web
before said first drying cylinders is/are regulated.
[0016] On the other hand, the dryer section in accordance with the
invention is mainly characterized in that the equipment comprises
measurement devices fitted in connection with the first drying
cylinders in the dryer section for measurement of the surface
temperatures of said first drying cylinders, a measurement device
for measurement of the dry solids content/moisture content of the
paper web and of the temperature of the paper web before said first
drying cylinders, and a control device for regulation of the
surface temperatures of said first drying cylinders and/or of the
dry solids content/moisture content and/or of the temperature of
the paper web before said first drying cylinders, based on the
measurement results obtained by means of the measurement devices,
to produce the desired running situation.
[0017] In accordance with the invention, the surface temperatures
of the first drying cylinders in the dryer section and the dry
solids / moisture content of the paper web and the temperature of
the web before the first drying cylinders are measured, and on the
basis of the measurement results the surface temperatures of the
first drying cylinders are regulated to the desired level so that a
good running situation is achieved.
[0018] Besides by means of the steam pressure/flow, the surface
temperatures of the first drying cylinders can, of course, also be
regulated by means of other known methods, for example by means of
induction heating or by means of electrical resistors.
[0019] The highest possible difference in temperature that provides
good runnability depends on a number of different factors: paper
grade and basis weight, stock that is used, web temperature,
moisture content, fibre orientation, and machine speed. This is why
it is often not possible to present a precise model for a correct
maximal difference in temperature, but the regulation algorithms
must be based on empiric information. It is known that the strength
properties of different paper grades depend on the fibrous
raw-material that is used and on the various properties of said
material. The fibrous raw-material, its grinding degree, and
possible additives and fillers affect the tendency with which the
web, when moist, attempts to adhere to a hot cylinder face. In
practice, these are, however, factors which cannot be changed for
the sake of the runnability in the initial part of the dryer
section alone, but the decisive factor is the paper quality. The
same is true concerning the fibre orientation, even though it is
known that a highly oriented web would be stronger in the machine
direction.
[0020] The web temperature has two factors which act in opposite
directions. On one hand, it has been noticed that, when the web is
hotter, it does not adhere to the first hot faces equally well. On
the other hand, the web strength becomes lower when the temperature
becomes higher. The machine speed does not have a direct intensive
effect on the adhering of the web to the first cylinder, but, on
the other hand, it produces a vacuum in the opening web-cylinder
gap, as a result of which vacuum the web tends to follow the
cylinder face. This diverts the web from the face of the support
wire and, thus, subjects the web to centrifugal forces which are
increased in direct proportion to the second power of the
speed.
[0021] The web moisture has a great negative effect on the
runnability of the initial part of the dryer section. First,
experience has shown that a moist web has an intensive tendency to
adhere to a cylinder face. In practice, this has caused, e.g.,
formation of dust and a so-called Tinting effect. On the other
hand, the web strength becomes lower when the moisture content
becomes higher.
[0022] A solution is included in the scope of the present invention
in which a regulation model has been formed by using at least some
of the parameters mentioned above and by regulating the surface
temperatures of the first drying cylinders so that the difference
between the temperatures of the cylinders and the web is smaller
than the maximal temperature given by said model. In its simplest
form, this model can have, for example, the following form:
[0023] .DELTA.Y=T0-K * M, wherein
[0024] T0 is an empiric invariable or an invariable determined by
computing,
[0025] K is an empiric invariable or an invariable determined by
computing, and
[0026] M is the moisture content of the web before the first
cylinder.
[0027] One such equation of regulation might be
.DELTA.T(.degree.C)=48-5 * M, wherein
[0028] M is the moisture content of the web before the first
cylinder as a percentage of the total weight of the web.
[0029] The equation is valid in the range 35%<M<60%.
[0030] A form that is slightly more common is represented by
.DELTA.T=T0(Tr, BW)-k1* f(M)-k2* f(v), wherein
T0 is an invariable dependent on web temperature Tr and on basis
weight BW,
[0031] k1 and k2 are invariable coefficients, and
[0032] f(M) and f(v) are functions of web moisture M and on machine
speed v.
[0033] An example of such a regulation model is the equation:
.DELTA.T(.degree.C.)=60-6*M-0.01* v, wherein
[0034] v is the machine speed (metres per second).
[0035] Of course, in its commonest form, the regulation model is,
for each paper grade and paper quality to be produced, of the
form:
.DELTA.T=.DELTA.T (Tr, v, M, BW),
[0036] but, as was stated earlier, this is often quite difficult to
determine precisely.
[0037] The control algorithm solutions described above are also
affected by limitations arising from quality factors, so that the
initial part of the dryer section is quite significant from the
point of view of the paper quality. In the initial part of the
dryer section the web may end up in such a range of
temperature-moisture that the fibres are softened. When the initial
drying temperature becomes higher, among other things the following
changes are noticed in the properties of the web: the coefficient
of scattering of light becomes lower, the tensile strength and
density become higher, the permeability to air is lowered, and the
roughness of the surface is increased. At the same time the
calendering quality of the paper is deteriorated. The magnitude of
the effect depends on the yield of stock so that the effects are
highest with mechanical pulps, but are reduced rapidly when the
yield becomes lower. One consequence of the softening of the fibres
by the effect of the drying temperature is adhering of fibres to
the faces of the cylinders in the initial part of the dryer section
if the temperatures of the cylinders are excessively high in the
beginning, which effect occurs with printing papers that contain
mechanical pulp. When the fibres adhere to the cylinders, they rise
upright when the web is separated from the cylinder. This causes
dust formation in the paper during printing. A certain contribution
to such adhesion is given by various additives of paper, such as
stock sizes. In accordance with the present invention, these
factors present in the initial part of the dryer section are taken
into account when the surface temperatures of the drying cylinders
are regulated in accordance with the invention so that a good
running situation is achieved.
[0038] The surface temperatures of the drying cylinders can be
measured by means of IR measurements, for example by means of an IR
camera, or by using particular apparatuses developed for
measurement of the surface temperature of a cylinder. Such an
apparatus has been described, for example in the journal Paper
Technology, June 1998, page 17. From the prior art, various
temperature detectors that reach contact with the face of a drying
cylinder are also known.
[0039] After a paper web break, the steam pressures in the first
cylinders are lowered from the level of a normal running situation.
Based on the results obtained from measurements of the surface
temperatures of the first drying cylinders, the surface
temperatures of the first drying cylinders are regulated to a
suitable level for the time of threading, and after threading the
temperatures are raised, based on a pre-determined sequence or on
measurement of the ultimate dry solids content, to the level
corresponding to the normal running situation. If necessary, the
moisture content and the temperature of the lead-in strip before
the first drying cylinders can be measured.
[0040] Based on the moisture content of the lead-in strip, the
possibilities of success of the following threading can be
estimated, because the moisture value predicts the runnability. An
excessively moist lead-in strip is of low strength, and it adheres
readily to the first drying cylinders. It also stretches easily and
makes successful threading of the web more difficult.
[0041] Based on the measurement results, if necessary, it is also
possible to regulate the press loads in the press section in order
to obtain sufficient strength of the lead-in strip so as to provide
a successful threading. In addition to this, based on the
measurement results, it is possible to select suitable differences
of draw between the press section and the dryer section and between
the first groups in the dryer section as a function of the
speed.
[0042] In accordance with a favourable additional feature of the
invention, in the event of a web break, the steam pressures in the
first cylinders are lowered in the usual way, and after this the
evaporation load is equalized by moistening the wire. The web
moisture and the web temperature preceding the first drying
cylinders are measured after the web break in order that it should
be possible to make the necessary corrections either by means of
impingement drying, if there is an impingement drying unit in the
initial part of the dryer section before the first drying
cylinders, and/or by means of a steam box in the press section
and/or by means of press loads. If the measurement also provides
the cross-direction profiles of moisture and temperature, by means
of said devices it is also possible to correct possible profile
defects.
[0043] During normal running, based on the measurement results, it
is possible to optimize the surface temperatures of the drying
cylinders, the differences of draw in the initial part of the dryer
section, and, if necessary, to change the press loads or to control
the steam box of the press section so that an optimal running
situation is achieved. Also, in a preferred embodiment of the
present invention, based on the measurement results, it is possible
to regulate the blow parameters of an impingement drying unit,
which may have been fitted before the first drying cylinders, in
compliance with the dry solids content and the temperature of the
web.
[0044] In accordance with the invention, the surface temperatures
of the first cylinders are monitored. The temperature is used as an
indirect indicator of the conduct of the web on the cylinder
concerned. Of course, it would also be possible to use, for
example, apparatuses based on a video camera and picture
processing, or a laser rangefinder, by whose means the conduct of
the web in the opening wire-cylinder gap is monitored directly.
Such a technology is, however, clearly more expensive and more
complicated than measurement of temperature, and it is probable
that even in such a case measurement of the surface temperature of
the cylinder would be advantageous in view of regulation of the
system.
[0045] In an additional embodiment of the invention, based on said
measurements of the surface temperatures of the cylinders and/or on
the measurements of the web moisture and/or web temperature, it is
possible to regulate the vacuums at least in the first suction
rolls and/or the capacities of the blow/suction boxes. With respect
to the regulation of the vacuum, reference is made to U.S. Pat. No.
5,535,527. The applicant has noticed that, as a rule, when the web
moisture and/or temperature become(s) higher, the web must be
supported more efficiently in the initial part of the dryer
section. The same is true regarding the surface temperature of a
cylinder, but not to an equally great extent, in particular if the
difference in temperature between the web and the cylinder is not
excessively large.
[0046] In accordance with the present invention, the measurements
can also be carried out with a web of full width in order that
information could be obtained on the profile parameters of the web
and that necessary regulation could be performed in respect of the
profile properties of the web. In a preferred embodiment of the
invention, the running parameters of the initial part of the dryer
section are chosen on the basis of the most unfavourable point
measured from the cross-direction profile.
[0047] In the following, the invention will be described with
reference to the figures in the accompanying drawing, the invention
being, however, not supposed to be strictly confined to the details
of said illustrations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 is a schematic illustration of an initial part of a
dryer section in which the regulation in accordance with the
invention is applied, comprising an impingement drying unit between
the press section and the cylinder drying.
[0049] FIG. 2 is a schematic illustration of a press section and of
an initial part of a press section following after the press
section, wherein the regulation in accordance with the invention is
applied.
[0050] FIG. 3 is a schematic illustration of the arrangement in
accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0051] FIG. 1 shows a preferred exemplifying embodiment of the
invention, in which the paper web W is passed from the press
section 10 of the paper machine on the bottom face of a transfer
fabric 11, while supported by a PressRun.TM. box 11a, onto the top
face of the drying wire or belt 12 over its guide roll 13. The
planar drying unit R.sub.1, comprises a blow hood 15, under which
the web W to be dried runs on the horizontal run of the wire or
belt 12, which run is supported by rolls 14. By means of the unit
R.sub.1, an intensive impulse of drying energy is applied to the
web W. In the unit R.sub.1 the paper web runs on support of the
upper run of the drying wire 12 along a linear path in the
horizontal plane so that it has no great changes in the direction
and, thus, the web is not subjected to high dynamic forces, which
might produce a web break in the web, which is still relatively
moist and, therefore, of low strength. Inside the blow hood 15,
there is a nozzle arrangement, by whose means hot drying gases,
such as air or steam, are blown onto the top face of the web.
Additionally, or alternatively, it is possible to use infrared
heaters. Said blow devices and/or radiation devices in the unit
R.sub.1 can be arranged so that their capacity can be regulated in
the cross direction of the web W in order to achieve
cross-direction profiling of the web W.
[0052] In FIG. 1, the unit R.sub.1 is followed by a first,
so-called normal (not inverted) single-wire unit R.sub.2, onto
whose drying wire 22 the web W is transferred as a closed draw in
the area of the first reversing suction roll 21. The single-wire
unit R.sub.2, and so also the following single-wire unit R.sub.4
which is open towards the bottom, comprise steam-heated
contact-drying cylinders 20 fitted in the upper row and reversing
suction rolls 21 fitted in the lower row, for example Metso Paper,
Inc.'s said VAC roll.TM.. Below the cylinders 20, there are doctors
and ventilation blow devices 25. The paper web W to be dried enters
into direct contact with the faces of the steam-heated drying
cylinders 20, and on the reversing suction rolls 21 the web W
remains at the side of the outside curve on the drying wire 22. In
order to improve the runnability, blow boxes 27, for example the
Metso Paper, Inc.'s UnoRunBlowBox blow boxes, have been fitted
above the suction rolls 21. In the place of said boxes, it is, of
course, possible to use suction boxes.
[0053] In FIG. 1, the group R.sub.2 with single-wire draw is
followed by a drying unit R.sub.3, which comprises two
contact-drying cylinders 30 and a large-diameter D.sub.1
impingement-drying/through-drying cylinder 31 with a perforated
mantle, which cylinder will be called large cylinder in the
following. A drying wire 32 has been fitted to run around the
contact-drying cylinders 30 and around the large cylinder 31, which
drying wire 32 is guided by guide rolls 33. The
impingement-drying/throug- h-drying hood module M.sub.1 of the
drying unit R.sub.1 has been fitted in the basement space KT below
the floor level K.sub.1 of the paper machine hall on the floor
plane K.sub.2 of said space KT. The central axes of the
contact-drying cylinders 30 in the unit R.sub.3 are placed
substantially in, or in the vicinity of, the floor level K.sub.1 of
the paper machine hall, most appropriately slightly above said
level. The paper web W to be dried is passed from the single-wire
unit R.sub.2 as a closed draw onto the first drying cylinder 30 in
the drying unit R.sub.3, after which the web W is passed on the
wire 32 of the unit R.sub.3 over the large cylinder 31 of the first
module M.sub.1 within a remarkably large sector on support of the
drying wire 32, and further onto the second drying cylinder 30 in
the unit R.sub.3. From this drying cylinder 30 the web W is
transferred as a closed draw into the following normal unit R.sub.4
with single-wire draw, which unit is, for example, substantially
similar to the unit R.sub.2 described above. After this, there
follow groups of drying cylinders and/or impingement-drying units.
The drying units R.sub.3,R.sub.4 can also be, for example,
substantially similar to the drying unit R.sub.2 described
above.
[0054] The press section shown in FIG. 1 has a drive of its own, so
also the transfer belt, the planar dryer and all of the dryer
groups following after it. A fabric is most commonly driven by
means of a driven roll or rolls. In FIG. 1, and likewise in FIG. 2,
driven rolls have been denoted with a mark {circle over (x)}
applied to the shaft.
[0055] FIG. 2 shows a second preferred exemplifying embodiment of
the invention. The web W is passed between the felts H.sub.1 and
H.sub.2 when it enters into the press section 10. The felt H.sub.1
has been passed over felt guide rolls 61.sub.1, and similarly the
felt H.sub.2 has been passed over felt guide rolls 61.sub.2. The
web W is passed into the first press 71 in the press 10. From the
nip N.sub.1 of the first press 71 the web is transferred further
into the nip N.sub.2 of the second press 72, and further into the
nip N.sub.3 of the third press 73 in the press section 10. After
this the web W is transferred from the nip N.sub.3 of the third
press 73 through a free draw F.sub.1 onto a transfer roll 65. The
web W is guided further from the roll 65 into connection with a
press felt H.sub.4, and along with said felt into the nip N.sub.4
of the fourth press 74. The felt H.sub.4 has been passed over the
felt guide rolls 61.sub.4. Similarly, the felt H.sub.3 has been
passed over the felt guide rolls 61.sub.3.
[0056] From the fourth press 74 the web W is guided further through
a free draw F.sub.2 onto a transfer roll 68 and further into the
dryer section.
[0057] The web is passed further from said second transfer roll 68
into the dryer section R into its first dryer group R.sub.2 onto
the first drying cylinder 20 in said group along with the bottom
face of the wire 22 of the single-wire draw while the web is
supported by a vacuum produced by a blow box 69.
[0058] In FIGS. 1 and 2, in accordance with the invention, in
connection with the first drying cylinders 20 in the initial part
of the dryer section, devices 91 for measurement of the surface
temperatures of said cylinders have been fitted. Besides this,
before the first drying cylinders, measurement devices 92 have been
fitted for measurement of the temperature and of the moisture/dry
solids of the paper web W. From the prior art, for example, various
infrared-based measurement apparatuses not contacting the web are
known for this purpose. From these measurements, there is a
connection through a control unit 93 to the press section 10 for
regulation of the press loads and/or of the steam box/boxes 81, 82
in the press section and/or for regulation of the impingement
drying unit R.sub.1. In addition to this, based on the measurement
results, the steam pressures in the first cylinders 20 are
regulated so as to reach the desired surface temperatures by means
of regulation 95 of the steam pressures.
[0059] As is shown in FIGS. 1 and 2, the surface temperatures of
the first drying cylinders 20 in the dryer section R are measured
by means of the measurement apparatuses 91, and the temperature and
the dry solids/moisture of the paper web are measured by means of
the measurement apparatus 92 before the first drying cylinders 20,
and based on the measurement results, by means of the control unit
93, the surface temperatures of the first drying cylinders 20 are
regulated to the desired level by means of regulation 95 of steam
pressure. Further, it is possible to regulate the blow parameters
of the impingement drying unit (FIG. 1) and/or the press loads in
the presses 71,72,73, 74 and/or the steam boxes 81,82 and the
differences in speed 94 between the dryer groups. The measurement
apparatuses can be, for example, apparatuses in themselves known
based on IR measurements or corresponding suitable measurement
apparatuses.
[0060] In a situation in which there has been a web break in the
paper web and threading is carried out, the steam pressures in the
first cylinders 20 are lowered in the first dryer group R.sub.2,
and the temperature of the impingement drying hood R.sub.1 (FIG. 1)
is also lowered. After this, the surface temperatures of the first
drying cylinders 20 are regulated to a suitable level by means of
regulation 95 of steam pressures by means of the control unit 93,
and likewise, in the exemplifying embodiment shown in FIG. 1, the
blow parameters of the impingement drying hood R.sub.1 are
regulated based on the results obtained from the measurements of
the surface temperatures of the drying cylinders 20 by means of the
measurement apparatus 91. In the regulation, it is possible to use
the measurement results obtained from the apparatus 92 for
measurement of the dry solids/moisture of the web. Based on the
measurement results, if desired, it is also possible to regulate
the press loads in the presses 71,72,73,74 in the press section 10
in order to obtain a correct moisture for the lead-in strip, and,
further, in the regulation operations, it is possible to employ
regulation of the steam boxes 81,82 in the press section 10.
Further, the measurement results can be used as an aid when the
differences in speed 94 between the dryer groups in the dryer
section are regulated.
[0061] FIG. 3 is a schematic illustration of the arrangement in
accordance with the invention. Said illustration shows the steam
box 81;82 in the press section 10, the press nip
N.sub.1;N.sub.2;N.sub.3;N.sub.4, of which there can, of course, be
several, apparatuses 92 for measurement of the moisture and the
temperature of the paper before the first cylinder 20, as well as a
possible impingement drying unit 15 and the first drying cylinder
20. Of course, the steam box 81;82 of the press 10 can be
controlled directly on the basis of the measured moisture and
temperature data (dashed line), but in addition to this they can be
regulated by using a control based on a computing model as an aid,
in which control the parameters 96 include, among other things,
machine speed, stock data, basis weight, etc. This program attempts
to optimize the runnability of the dryer section R, while taking
into account possible criteria of quality, if any, by means of said
differences in moisture and temperature and by means of the surface
temperature of the cylinder measured by means of the measurement
apparatus 91. In the case shown in the figure, the actuators that
can be regulated are the surface temperature of the cylinder 20,
capacity of the impingement drying 15, and the steam box 81;82 of
the press 10 and the press loads. As the optimizing algorithm it is
possible to use any prior-art method whatsoever provided that the
effects of said parameters are known, for example, based on an
empiric or computed model.
[0062] According to a preferred embodiment of the invention, a
model-predictive multi-variable regulation and/or optimization
is/are employed. The input parameters of the model are the capacity
of the steam box, the press load and/or any other control parameter
that affects the drying capacity, the heating capacity of the
impingement drying unit (flow speed and/or temperature), and/or the
heating capacity of the first cylinder (pressure or equivalent).
The output parameters of the model are moisture and/or temperature
of the web after the press and/or temperature of the first
cylinder.
[0063] Above, the invention has been described with reference to
some preferred exemplifying embodiments of same only, the invention
being, however, not supposed to be in any way strictly confined to
the details of said embodiments. Many variations and modifications
are possible within the scope of the inventive idea defined in the
following patent claims.
* * * * *