Note: Descriptions are shown in the official language in which they were submitted.
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. Method for control of the curl of paper in the dryer section
of a paper machine and paper or board machine
The invention concerns a method in the dryer section of a paper or board
machine
for control of the curl of paper, in which method steam treatment and/or
moistening
of the paper web is/are employed.
The invention also concerns a paper or board machine, which comprises at least
a
headbox, a former, a press, and a dryer section, in which steam boxes or
moistening
devices are employed for control of the curl.
As is known from the prior art, in mufti-cylinder dryers of 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. 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
particu-
lar so when the web is still relatively moist and, therefore, of low strength.
This is
why, in recent years, ever increasing use has been made of said single-wire
draw,
in which each group of drying cylinders includes 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 the heated cylinder faces, whereas on the
reversing cylinders or rolls 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 said loop.
From experience it is known that, if paper is dried one-sidedly, the result is
'a
tendency of curling of the sheet. When paper is dried by means of normal
groups
with single-wire draw from the side of its bottom face and if such asymmetric
drying
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2
is extended over the entire length of the forward dryer section, the drying
takes
place so that first the bottom-face side of the paper web is dried and, when
the
drying makes progress, the drying effect is also extended to the side of the
top face
of the paper web. Under these circumstances, the dried paper is usually curled
and
becomes concave, seen from above.
As is known from the prior art, the tendency of curling of paper is already
affected
in connection with the web formation, in particular at the sheet fonmation
stage (for
example, the applicant's Sym-Former''' by means of selection of the difference
in
speed between the slice jet and the wire, by means of the choice of the former
and
its mode of running, and by means of other running parameters. As is known
from
the prior art, for example, in the case of copying paper, by means of
unequalsided-
ness of drying in the after-dryer a suitable initial curl foam is regulated
for the sheet
in order that the curling of the paper after one-sided or double-sided copying
could
be optimized. In the case of copying paper, the reactivity of curling, i.e.
the extent
to which curling occurs per unit of change in moisture content, is affected to
a
greater extent by means of a mufti-layer structure of the paper, which is
produced
in connection with the web formation in the wet end.
The most recent prior-art technology related to the present invention in high-
speed
paper machines has been based on dryer sections in which there is single-wire
draw
over the major part of the length of the dryer section, and in view of
controlling the
tendency of curling, an inverted group has also been used in order that the
drying
could be made sufficiently symmetric in the z-direction. However, it has come
out
that an inverted group produces obvious drawbacks in view of the runnability
and
the overall efficiency of the machine and in view of the profitability of the
paper
machine investment. Thus, from the point of view of the runnability of the
paper
machine, a dryer section fully supported over its entire length and based on
normal
groups with single-wire draw with no inverted groups would be a particularly
justified solution. People skilled in the art have, however, not had the
courage to
introduce this solution in operation, because it has been considered that it
would
result in solutions uncontrollable and unfavourable from the point of view of
the
CA 02284053 2001-03-09
3
tendency of curling of paper. One problem involved in the prior-art solutions
that
include inverted dryer groups is the removal of broke in the event of web
breaks, for
inverted groups are not self cleaning by the effect of gravity.
The present invention is directed towards the provision of a dry end of a
paper
machine in which no inverted groups are needed at all, but which, however,
meets all
other requirements that are imposed.
The present invention also is directed towards the provision of an approach to
these
prior art problems from a new point of view and of novel solutions for said
problems,
which solutions are contrary to conventional modes of thinking.
With respect to the prior art related to the present invention, reference is
made to the
applicant's FI Patent No. 91,900 (equivalent to US Pat. No. 5,416,980), in
which a
method is described in the dryer section of a paper machine in particular for
reducing the tendency of curling of paper, in which method the paper web is
dried by
means of drying cylinders, against whose heated faces the paper web is dried
by
means of drying cylinders, against whose heated faces the paper web is pressed
by
means of a drying wire, and in which dryer section groups of drying cylinders
are
used in which twin-wire draw and/or single-wire draw is/are applied. In this
method
it has been considered novel that in the dryer section, substantially across
the entire
width of the paper web, hot water steam is fed, by whose means the strains
that arise
or tend to arise in the fibre mesh in the paper web are relaxed by means of
heat and
moisture in, or substantially directly after, the area of formation of said
strains.
In the applicant's FI Patent Application No. 963734 (equivalent to provisional
US
Patent Application No. 60/030, 693) a method is described for drying a surface-
treated paper web or.equivalent in an after-dryer of a paper machine, as well
as an
after-dryer for a paper machine for applying the method, wherein, in order to
compensate for a tendency of curling of the paper web, in the after-dryer the
paper
web is dried in a dryer group/groups making use of a normal single-wire draw,
and
that, in connection with or after the drying, the paper web is treated by
means of a
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device/devices in order to compensate for a tendency of curling of the paper
web,
which devices are, for example, a steam box, a blower unit, a moistening
device,
and/or a soft calender.
On the other hand, in the applicant's FI Laid Open Publication No. 98,387, a
method
is described for the manufacture of paper, in particular fme paper, to be
surface-
treated as well as a dry end of a paper machine that makes use of the method.
The
paper web, which has been dewatered by pressing, is dried in a forward dryer
section,
in which drying energy is applied to the paper web over the entire length of
the
forward dryer section asymmetrically in the z-direction from the side of the
lower
face of the web. Said stage is carried out by means of a number of successive
groups
with single-wire dxaw open towards the bottom on the support of the drying
wire. In
this way, shrinkage of the web, which tends to take place both in the machine
direction and in the cross direction with an increase in the dry solids
content, is
substantially prevented. In connection with a web break, the paper broke is
removed
downwards from the dryer groups open towards the bottom substantially by the
foxce
of gravity onto the broke conveyor placed underneath. Owing to the asymmetric
forward drying, the paper web with a tendency of curling is passed into a
finishing
section, in which it is after-treated while it is, at the same time, moistened
or worked
plastically so that the tendency of curling that arose in it in the forward
drying stage is
eliminated. As examples can be mentioned, in the after-dryer section, groups
with
twin-wire draw and regulation of steam as well as steam boxes fitted to
control the
curl as well as the infra and airborne web drying.
In the applicant's published FI Patent Application No. 964830, a method is
described
for drying of paper as well as a dry end of a paper machine. The method for
drying of
paper comprises the following steps: the paper web to be dried is passed from
the
press section into a forward dryer section, in which the paper web is dried
from the
side of its bottom face in dryer groups that apply a normal single-wire draw,
said
forward dryer section comprising exclusively single-wire groups with normal
single-
wire draw, and
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from the forward dryer section the paper web is passed into a finishing
section, in
which the paper web is coated/surface-sized by means of a coating/surface-
sizing
equipment, dried in an after-dryer section, in which the paper web is dried in
at least
one dryer group that applies a normal single-wire draw, after which the paper
web
5 is calendered in a calender and passed to a reeling station, in which the
paper web
is reeled into a machine reel, and in the method the curling of the paper web
is
controlled by means of elements and/or by means of assemblies and combinations
formed out of said elements in the area of the forward dryer section and/or of
the
finishing section. On the other hand, the dry end of the paper machine
comprises a
forward dryer section and a finishing section, which finishing section
comprises a
coating/surface-sizing equipment, an after-dryer, a calender, and a reeling
station.
The dry end of the paper machine comprises elements and/or assemblies and
combinations formed out of said elements in view of controlling the curling of
the
paper web in the area of the forward dryer section and/or of the finishing
section.
The elements for the control of the curl include, among other things, blowing
of hot
moist air through the wire in the forward dryer section, steam boxes employed
in the
after-dryer, a combination in which steam treatment by means of a steam box is
combined with a cooling cylinder, a lower support belt or support wire in the
after-
dryer, twin-wire groups employed in the after-dryer, blowing through the wire
in
connection with at least one cylinder in the after-dryer, the use of a
suitable cylin-
der-diameter ratio, atomizing of water against the web in the after-dryer,
infrared
boxes for treatment of the web before the calender, transfer of moist air from
the
forward dryer to the after-dryer to be blown against the web, and mechanical
working of the web by means of a spreader bar.
With respect to the prior art, reference is also made to the US Patent 5, 557,
860, in
which a dryer section is described which is composed of dryer groups with a
normal
single-wire draw and of a moistening device fitte$ after the dryer groups, by
means
of which moistening device the curl is controlled.
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6
The present invention is directed towards the further development of the prior-
art
solutions described above in order that it should be possible to control the
curl of the
paper web in the dry end of the paper machine more efficiently.
The present invention additionally is directed towards the provision of a dry
end of a
paper machine with finishing devices in which the runnability can be brought
to a
particularly high level.
In accordance with one aspect of the present invention, there is provided a
method in
the dryer machine of a paper or board machine for control of the curl of
paper, in
which method steam treatment and/or moistening of the paper web is/are
employed.
The method in accordance with the invention is mainly characterized in that,
in the
method, the operations carried out in view of controlling the curl of the
paper web are
carried out in a number of stages before the finishing device that works the
web
mechanically in a dry solids content range Kl ...K2 of the web, wherein K1 =
ultimate
dry solids content - 7 % and K2 = ultimate dry solids content + 3 %, and when
the
web temperature is, during at least one stage, lower than 85 °C,
preferably lower than
75 °C.
In accordance with another aspect of the present invention, there is provided
a paper
or board machine, which comprises at least a headbox, a former, a press, and a
dryer
section, in which steam boxes or moistening devices are employed for control
of the
curl. The paper board machine in accordance with the invention is
characterized in
that there are at least two of said curl regulation devices in view of
achieving a
regulation of the curl in a number of stages, and that the operations carned
out in view
of controlling the curl of the paper web are carried out in a number of stages
before
the finishing device that works the web mechanically in a dry solids content
range
K1...K2 of the web, wherein K1 = ultimate dry solids content - 7 % and K2 =
ultimate dry solids content + 3 %, and when the web temperature is, during at
least
one stage, lower than 85 °C, preferably lower than 75 °C.
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6a
According to the invention, the curl is regulated when the web temperature is
lower
than 85 °C, preferably lower than 75 °C. Steam condenses into
the web more
efficiently when the web temperature has been lowered to a level below 85
°C,
preferably below 75 °C. In relation to this the applicant of the patent
has carried out
experiments with a production machine. When the last upper cylinder in the
dryer
section was hot, a steam box placed directly after said cylinder had no major
effect
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on the curl; when the supply of steam into said cylinder was closed, the
temperature
of the web became lower, and a significant effect was produced by the steam
box.
According to the invention, the curl of the paper web is regulated in the
final end of
the drying process. The applicant has also noticed that an excessive drying
promotes
the control of the curl when moistening/steam treatment is employed. According
to
the invention, optimal controllability is obtained in a range of dry solids
content that
extends from about 7 % lower than the desired ultimate dry solids content to
about
3 % above said content. In the case of surface-sized fine paper, said range is
typically 88 % to 98 % .
Further, according to the invention, the curl regulation operations, such as
steam
treatment/moistening and cooling of the web, are carried out in a number of
stages,
in which case a highly efficient curl regulation effect is obtained. This
comes out
from the accompanying Fig. 3, from whose test results it comes out that a web
that
has been dried from one side only is curled intensively (20 units). A mere
steam
treatment in one stage reduces the curl substantially, but the web is still
curled in the
original direction by about 5 units. When the web has been moistened slightly
before
the steam treatment, the web curl direction can be even reversed.
A moistening device is highly efficient in lowering the web temperature. In
some
applications, e.g., a cooling cylinder can be substituted for by a moistening
device.
In an arrangement in accordance with the present invention, the dry end of the
paper
machine is preferably exclusively based on dryer groups with single-wire draw,
in
which case the removal of broke takes place constantly by the force of gravity
and
does, thus, not cause problems. Likewise, in single-wire draw, the paper web
is
constantly supported by a wire, in which case the runnability is improved and
it is
possible to increase the speed. In view of controlling the unequalsidedness of
paper
and in particular the curl arising from unequalsided drying, in the after-
dryer in the
dry end of the paper machine elements have been provided for the control of
the
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tendency of curling in accordance with the principles described above so that
the
desired curl is obtained for the paper.
According to an exemplifying embodiment of the invention, in the after-dryer,
fQr
the control of the curl, cylinders which can be cooled and/or whose
temperature can
be regulated as well as moistening or steam boxes are used, which have been
placed
alternatingly so as to provide a multi-stage cooling/moistening-curl cycle,
together
with an optimal roll/cylinder diameter as a combination formed in a favourable
way.
In prior-art arrangements, it is affected by means of optimization of the
cylinder-roll
diameter ratio that when the diameter of a suction roll is made larger, the
evapor-
ation taking place through the lower face is increased, and the other way
round. A
favourable feature of the invention is the choice of a suitable cylinder-roll
diameter-
ratio so that, when the web runs around a suction roll or equivalent of larger
diameter, the web temperature is lowered. After this, for example, a steam box
is
fitted so as to provide moistening in view of regulation of the curl.
The paper web is preferably also cooled primarily from the bouom side when the
forward dryer and the after-dryer in the dryer section are, as a rule,
provided with
a normal single-wire draw, i.e., in principle, attempts are made to produce a
temperature gradient in the paper web so that the side that is moistened for
control
of the curl is colder than the opposite side of the web.
In the following, the invention will be described in more detail with
reference to the
figures in the accompanying drawing, the invention being, however, by no means
supposed to be strictly confined to the details of said illustrations. In the
drawings,
Figure 1 is a schematic illustration of an exemplifying embodiment of the
present
invention in respect of the dry end of a paper machine, the dry end of the
paper
machine being illustrated from the forward dryer section up to the machine
reel ;
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Figure 2 is a schematic illustration in part of Fig. 1, mainly in respect of
the last
dryer group in Lhe after-dryer of the finishing section in the dry end of the
paper
machine ; and
Figure 3 is a schematic illustration of test results in relation to regulation
of the curl
in paper by means of water moistening and steam treatment.
As is shown in Fig. 1, the paper web W is brought to the forward dryer section
D 1
from the press section onto the drying wire 15 of the first group Rp with
single-wire
draw, to which wire the web is made to adhere by the effect of the vacuum in
the
suction boxes 13. The forward dryer section includes 7 groups Rp...R6 with
single-
wire draw, over whose group gaps the web W has a closed draw. In the figures,
the
machine direction, i.e. the running direction of the web W, is denoted with
the
arrow S. In the forward dryer section D1 in accordance with the invention, all
the
single-wire groups Rp...RN are so-called normal gmups, in which the, for
example,
steam-heated smooth-faced drying cylinders 10 arc placed in the upper
horizontal
row and the reversing suction cylinders 11 are placed in the lower horizontal
row.
The number of dryer groups RQ...N is, as a rule, N = 4...12, preferably 6...8.
Each normal group Rp...RN has a drying wire 15 of its own, which is guided by
guide rolls 18. The drying wires 15 prcss the web W to be dried on the drying
cylinders 10 against the smooth heated faces of the cylinders, and on the
reversing
cylinders 11 the web W remains at the side of the outside curve on the outer
face of
the wire 15. On the reversing cylinders 11 the web W is kept reliably on the
support
of the wire 15 against the effects of centrifugal forces by the effect of the
vacuum
present in the grooved faces of the reversing cylinders 11 or in the
perforated
mantles of equivalent suction rolls, by which means cross-direction shrinkage
of the
web W is also counteracted. As the revcrsing suction cylinders 11,
particularly
advantageously the suction cylinders marketed by the applicant with the trade
mark
VacRoll"' are used, which cylinders have no insidc suction boxes and in
respect of
the details of whose constructions reference is made to the applicant's FI
Patent No.
83. 680 (equivalent to US Patents Nos. S, 022,163 and 5,172.491).
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In a forward dryer D1 in accordance with a preferred embodiment of the
invention,
the support contact between the web W and the drying wire 15 is kept adequate
also
on the straight draws between the drying cylinders 10 and the reversing
cylinders 11
by, at least on the runs from the drying cylinders 10 to the reversing
cylinders 11,
5 making use of blow-suction boxes 17, by means of which boxes formation of
pressures induced by the wire 15 is also prevented in the closing wedge-shaped
nip
spaces between the wire 15 and the cylinder 11 mantles. Blow-suction boxes 17
are
understood as blow boxes whose air blowing produces a vacuum, and said boxes
17
do not communicate with sources of vacuum. With respect to the details of the
10 constructions of these blow-suction boxes 17, which are marketed by the
applicant
with the trade mark "UnoRun BlowBox"'~, reference is made to the applicant's
FI
Patents Nos. 59, 63 7, 65, 460 and 80, 491 (equivalent to US Patents Nos. 4,
441, 263,
4, 516, 330 and 4, 905, 380. Blow-box solutions of other types, in themselves
known,
are also included in the scope of the overall concept of the present
invention.
In the forward dryer section Dl, in the single-wire groups R~...RN, blow boxes
16
are also employed in the gaps between the reversing cylinders 11, by means of
which boxes 16 said gap spaces are air-conditioned and evaporation from the
web W
is promoted. The faces of the drying cylinders i0 are kept clean by doctors
14.
It is a further substantial advantage of the forward dryer section D 1 used in
the
invention that in the groups Rp...RN with single-wire draw, which extend over
the
entire length of the dryer section, removal of broke by the effect of gravity
can be
applied, for the single-wire groups R~...RN are open towards the bottom, so
that the
paper web W that becomes broke can be removed without any special arrangements
onto the broke conveyor (not shown) placed in the basement spaces of the paper
machine and on said conveyor further into the pulper or pulpers.
In view of prevention of cross-direction shrinkage of the web W, it is of
particular
importance that, in the forward dryer section D1, the web W is kept in
reliable
contact with the drying wires 15 all the time. This holding effect is produced
on the
reversing cylinders 11 by means of a vacuum present in the grooved mantle 12
or
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equivalent on said cylinders and, on the straight runs between the cylinders
10 and
the reversing cylinders 11, by means of pressure levels arranged by means of
the
blow-suction boxes 17 and partly also by means of the tension T of the web W
in
the machine direction, which tension produces a contact pressure pk = T/R (R =
radius of the cylinders 11) between the web W and the wires 15.
As was stated above, as the reversing cylinders 11 in the forward dryer D 1,
favour-
ably the applicant's Vac' rolls are used. This vacuum effect is spread through
the
perforations on the reversing cylinders 11 onto the grooved mantle 12 so that
the
wedge-shaped nip spaces between the reversing cylinders 11 and the drying wire
can
also be evacuated efficiently, so that pressures cannot be induced into these
wedge
spaces, which pressures would attempt to separate the web W from the drying
wire
when the web W is placed outside. If suction rolls provided with inside
suction
boxes are used as the reversing cylinders 11 in the forward dryer section D1,
the
suction zone should preferably be extended over an area wider than the turning
sector of the drying wire 15 and the web, so that the suction effect and the
free flow
of air can be extended into said wedge spaces, for the purposes mentioned
above.
Besides the forward dryer section D 1 described above, the dry end of a paper
machine in accordance with the invention includes a finishing unit D2 placed
after
the forward dryer D1, which finishing unit includes a machine reel-up 50, for
example a Pope-type reel-up. A machine reel that is being produced on-line by
means of the reel-up 50 is denoted with the reference MRo, and one complete
machine reel is denoted with the reference MR. The web W is brought to the
machine reel-~p 50 through the calender 40 from an after-dryer 30, which is
placed
after the coating device 20 in the finishing section D2.
As is shown in the figures, after the forward dryer section D1 the paper web
W,
which has been dried to a dry solids content of k2 = 96. . .99 % , is passed
over
paper guide rolls 25 and over a measurement beam 26, which is placed between
said
guide rolls 25 and which measures the property profiles of the paper, into a
coating
device 20, which is, for example, a coating device marketed by the applicant
with
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12
the name SymSizerT"'. The coating device 20 includes two coating rolls 21 and
22
placed one opposite to the other, and size feed devices 23 and 24 are placed
in
connection with both of said rolls so that the paper web W is coated from both
sides
in the coating nip NS between the rolls 21 and 22. Owing to the water-
containing
coating agent, the web W is partly moistened in the coating nip NS from both
sides.
Then, the web W, which was dried in the forward dryer D1 asymmetrically from
the
side of its bottom face W and which has a tendency of curling, is treated into
such
a state that its internal strains are partly relaxed or at least substantially
reduced.
In the exemplifying embodiment shown in Fig. 1, the after-dryer 30 in the
finishing
section D2 is also exclusively composed of dryer groups R21,R22~R23 with
single-
wire draw. The last dryer group R23 in the after-dryer 30, in which group the
curl
control arrangements are fitted, will be described in more detail in relation
to Fig.
2. The first two groups R21,R22, which are provided with a possibility of
impinge-
IS ment blowing 19, are basically similar to the dryer groups Ro...R6 in the
forward
dryer D 1 , and the same reference numerals are used for corresponding parts.
As is shown in Fig. 2, the last dryer group R23 in the after-dryer first
includes a
drying cylinder 10 in the group gap and a following reversing roll or cylinder
11 and
further a second drying cylinder 10 and a following reversing cylinder i 1.
After this
there follows a cylinder 61 of adjustable temperature, preferably a cylinder
that can
be cooled. After this the web W is passed onto the reversing roll or cylinder
11 in
the lower row and onto a second cylinder 63 of adjustable temperature,
preferably
a cooling cylinder. The drying wire is denoted with the reference numeral 15,
and
its guide rolls are denoted with the reference numeral 18, the runnability
components
with the reference numeral 17, and the doctors with the reference numeral 14,
as is
the case in Fig. 1. At the outlet side of the last two reversing rolls or
cylinders l I
in the last dryer group R23 in the after-dryer 30, before the cylinders 61, 63
of
adjustable temperature, preferably a moistening device 62 and preferably a
steam
box 64 are placed. Both devices can, of course, be both moistening or steam
treat-
ment devices. The paper web W is moistened/steam-treated before the web W is
passed onto the cylinders 61,63, which are, thus, preferably cooled cylinders.
From
r r
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13
the latter cooling cylinder 63 the web W is passed past the steam box 65 of
the guide
roll 71 to the calender 40 and further to the reel-up 50.
In accordance with the present invention, by means of alternation of moisten-
ing/steam-treatment devices 62,64 and cooling of the web W by means of cooling
cylinders 61,63, a regulation of the curl in several steps has been achieved,
in which
case it is highly efficient. At this stage the dry solids content of the web W
is in the
range of K1...K2, wherein K1 = ultimate dry solids content - 7 % and K2 =
ultimate dry solids content + 3 %, and the temperature of the web at the steam
boxes 64,65 is lower than 85 °C, preferably lower than 75 °C.
As comes out from the figure, the diameter D 11 of the reversing rolls or
cylinders
11 in the last dryer group R23 is larger than the diameter D of the reversing
rolls or
cylinders 11 in the preceding groups. In view of regulation of the tendency of
curling, in the last dryer group R23, the diameter ratio of the diameter D 11
of the
reversing cylinder 11 to the diameter D~ of the drying cylinders 10 / cooling
cylin-
ders 61,63 has been chosen so that the web W is also cooled in this way on the
face
of the reversing cylinder 11 of a diameter larger than usual so as to
intensify the
effect of regulation of the curl. The diameter ratio D~/D11 is 0.75...2.5,
preferably
1...1.5.
Since attempts are made to cool the web by means of cylinders of a lower
tempera-
ture placed in the end of the dryer section and possibly by using larger
reversing
cylinders, it is also advantageous to employ a wire circulation of their own
on said
cylinders. In this way the other, heatabie cylinders and the long wire
circulation
cannot heat the wire and, thereby, the web in the end of the dryer section. In
order
to provide additional capacity, the wire can be cooled further, for example,
by
blowing of cold air.
Even though it is not illustrated in the figure, the dryer section is placed
inside a
hood, in a conventional way, for recovery of the moist air, to improve the
energy
economy, and to intensify the air-conditioning of the dryer section. In
particular,
CA 02284053 1999-09-17
WO 98/44191 PCT/FI98/00253
14
when a wire circulation of its own is employed in the end, it is preferable to
isolate
said area from the rest of the hood by means of a partition wall. By means of
this
solution, the wire is kept colder and both the energy efficiency and the
operability
of the air-conditioning of the dryer section are improved.
S
The applicant of patent has also noticed that, from the point of view of the
ultimate
properties of the paper, it is advantageous that said curl regulation
operations are
carried out when the web is supported, e.g., against the drying wire, as is
shown in
Figs. 1 and 2. The final result is also better when the web is subjected to a
draw
either in the machine direction or in the cross direction. In Fig. 2, the web
is drawn
between the last drying cylinder and the calender, i.e. the web is subjected
to :a
tension in the machine direction when it is steam-treated/moistened by means
of the
device 6S.
1S Fig. 3 is a schematic illustration of test results related to regulation of
the curl of
paper by means of water moistening and steam treatment. The Y axis represents
the
curl, and the X axis represents the test points 1 to 3 . In the test point 1,
one-sided
drying has been used, in the test point 2 one-sided drying and steam-
treatment, and
in the test point 3 one-sided drying, water moistening and steam-treatment.
When the
sign of the curl is positive, the curling takes place towards the top side,
and when
the sign of the curl is negative, the curling takes place towards the bottom
side. As
is seen from the figure, a one-sidedly dried web curls intensively (20 units).
Steam
treatment alone at one stage reduces the curl substantially, but the web still
curls in
the original direction by about five units. When the web has been moistened
slightly
2S before steam treatment, the web curl direction can be even reversed.
Even though the invention is preferably applied in connection with the dryer
section
solution described in the exemplifying embodiment, the invention is by no
means
supposed to be strictly confined to said solution, but the invention can also
be
utilized in connection with conventional cylinder dryer concepts or while
using
drying other than cylinder drying, such as, for example, impingement drying.
