Note: Descriptions are shown in the official language in which they were submitted.
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1
Method for drying a surface-treated paper web or equivalent
in an after-dryer of a paper machine and after-dryer
carrying out the method in a paper machine
The invention concerns a method for drying a surface-treated paper web or
equival-
ent in an after-dryer of a paper machine, in which method the paper web is
first
finished in a finishing section, in which finishing section the paper web is
surface-
sized or coated by means of a finishing device, after which the paper web is
dried.
The invention also concerns an after-dryer for a paper machine for applying
the
method in accordance with the present invention to the drying of a surface-
treated
paper web or equivalent, which after-dryer is placed after the finishing
device, by
means of which finishing device the paper web is surface-sized or coated.
As is known from the prior art, in multi-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.
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In the what is called normal groups with single-wire draw, known from the
prior art,
the heated drying cylinders are placed in the upper row and the reversing
cylinders
are placed in the lower row, which rows are, as a rule, horizontal and
parallel to
one another. On the other hand, in inverted groups with single-wire draw, the
reversing cylinders are placed in the upper row and the drying cylinders in
the lower
row. In the following, when the terms "normal (dryer) group" and "inverted
(dryer)
group" are used, what is meant is expressly groups with single-wire draw in
multi-
cylinder dryers, of the sort mentioned above.
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 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 circum-
stances, 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 formation
stage by
means of selection of the difference in speed between the slice jet and the
wire, 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 unequalsidedness of drying in the
after-
dryer a suitable initial curl form 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
multi-layer structure of the paper, which is produced in connection with the
web
formation in the wet end.
The most recent technology related to the present invention in high-speed
paper
machines, in particular in fine-paper machines, has been based on dryer
sections in
which there is single-wire draw over the major part of the length of the
machine,
and, in view of controlling the tendency of curling of paper, in practice, an
inverted
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3
group has also been used in order that the drying could be made sufficiently
symmetric in the z-direction.
In conventional after-dryers in paper machines, in which the web is dried
after coating
or surface-sizing, a problem has been the swelling of the web arising from the
moisture introduced by the coating into the web, which swelling produces
"wrinkles"
and equivalent problems that deteriorate the runnability in the web. In twin-
wire
draw, which is used traditionally in an after-dryer, the web flutters between
the upper
and lower rows, in which connection web breaks tend to occur and, moreover, on
the
drying cylinders in the lower row, when the web is wrinkled, the drying wire
of the
lower row has been readily damaged as a result of the wrinkles, the problem
consisting of frequent replacements of the lower fabric.
Further, as is known from the prior art, problems have also been caused by the
lowering of the web strength resulting from the coating, in which connection
long free
draws of the web have been quite problematic.
From the prior art, solutions are known for an after-dryer for paper to be
coated, in
particular for fine paper or equivalent, in which dryer there is first an
upper cylinder
and a lower cylinder and after this one group that employs normal single-wire
draw,
and after that dryer groups that make use of twin-wire draw. In these
applications, it
is a problem that, in view of the tendency of curling of paper, the ratio of
the upper
and lower cylinders is wrong if the curling is supposed to be regulated
efficiently. As
is known from the prior art, in the after-dryer it has been necessary to keep
the
temperature of the first cylinders low because of adhering of the web and the
size/paste to the cylinder.
Groups of the sort mentioned above for fmishing of paper to be coated, in
particular
of fine paper, have been described, among other things, in the applicant's FI
Patent
Application No. 950434. Thus, the present invention is directed towards
further
development of the solutions suggested in said application so that the
tendency of
curling of paper can be controlled more efficiently in the after-dryer.
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The present invention further is directed towards the provision of such a
solution for
an after-dryer as is suitable for use in particular in dryer sections in which
it has not
been possible or desirable to control the curling of the paper web in the
forward dryer.
The present invention also is directed towards creation of an arrangement in
which
there are no long free draws while the web is still moist after the coating.
The present invention is additionally directed towards the provision of such
an after-
dryer for a paper machine in which the runnability can be brought to a
particularly
high level.
Further, the present invention additionally is directed towards the provision
of such an
after-dryer for a paper machine in which a what is called ropeless threading
can be
applied favourably within the entire length of the after-dryer in the machine
direction,
which for its part contributes to making the constructions simpler and
standstills
shorter.
In accordance with one aspect of the present invention, there is provided a
method for
drying a surface-treated paper web or equivalent, in which method the paper
web is
first finished in a finishing section, in which finishing section the paper
web is
surface-sized or coated on both sides by means of a finishing device, after
which the
paper web is dried, wherein, in the after-dryer, the paper web is dried only
in a dryer
group/groups making use of a normal single-wire draw, and wherein, in
connection
with or after the drying, the paper web is treated by means of a
device/devices in
order to compensate for a tendency of curling of the paper web.
In accordance with a further aspect of the present invention, there is
provided an after-
dryer for a paper machine for applying the method defined above to the drying
of a
surface-treated web or equivalent, which after-dryer is placed after the
finishing
device, by means of which finishing device the paper web is surface-sized or
coated
or both sides, wherein the after-dryer comprises only at least one dryer group
that
apply/applies a normal single-wire draw so as to dry the paper web and a
device/devices for compensating for a tendency of curling of the paper web.
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In the arrangement in accordance with the invention, in which the whole after-
dryer is
formed as a group with single-wire draw or is composed of a group with single-
wire
draw, the web is supported over the entire length of the dryer section, in
which case
the runnability is very good. Possible curling of the web is compensated for,
for
example, by means of steam boxes and/or by means of soft-calendering. The web
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can also be passed to an upper cylinder directly after the coating, in which
case the
draw is closed right from the beginning of the after-dryer over the entire
length of
the after-dryer.
5 In the following the invention will be described in more detail with
reference to the
figures in the accompanying drawing, the invention being, however, in no way
supposed to be strictly confined to the details of said illustrations.
Figure 1 is a schematic illustration of an exemplifying embodiment of an after-
dryer,
which is composed of a dryer group that applies a normal single-wire draw, and
Figure 2 is a schematic illustration of an arrangement in which, after the
coating, the
web is passed directly onto the upper cylinder in the group with single-wire
draw
over a reversing blow-box or a reversing roll or equivalent, in which case a
closed
draw is obtained over the entire length.
In Fig. 1 the coating device is denoted with the reference numeral 10, and the
coating device 10 is, for example, a coating device marketed by the applicant
with
the name SYM-SIZERT"', which device comprises two opposite coating rolls 11
and
12, in connection with each of which there are size feed devices so that the
paper
web W is coated from both sides in the coating nip between the rolls 11 and
12. The
direction of progress of the paper web W is denoted with the reference arrow
S.
After the coating device 10 the web W is passed over the drying cylinder 171
into
the dryer group that applies a normal single-wire draw, in which group the
reversing
rolls/cylinders, fitted inside the drying-wire loop 176 guided by the guide
rolls 177,
are denoted with the reference numeral 175 and the drying cylinders are
denoted
with the reference numeral 174. In the dryer group the paper web W to be dried
runs from the drying cylinders 174 placed in the upper row RY onto the
reversing
rolls 175 placed in the lower row RA so that on the drying cylinders 174 the
web W
is placed against the heated face of the drying cylinder 174, and on the
reversing
rolls 175 placed in the lower row RA the web W remains at the side of the
outside
curve on the wire 176. After the dryer group the paper web W is passed over
the
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guide roll 195 to the soft calender 191, which comprises two calender nips Nl
and
N2. In connection with the soft calender, two steam boxes 192 are also placed,
and
curling of the paper web W is compensated for by means of the soft-calendering
and
by means of the steam boxes. Over the guide roll 197 the paper web is passed
through the nip device 198 to further processing. Between the calendering nips
N 1, N2 the web is guided over the guide roll 196.
In Fig. 1, after the coating device 10, a pre-drying unit 15 is fitted, which
does not
contact the web W and which can be an infrared dryer or an airborne web dryer
or
a combination of same, in itself known from the prior art. In the final
portion of the
after-dryer, a blower unit 201 is fitted, by whose means attempts are made to
promote evaporation through the wire, i.e. to equalize the distribution of
moisture
in the direction of thickness in the paper that departs from the after-dryer.
Further,
in the arrangement, there may be a steam box 202 in itself known, of the sort
described, for example, in the FI Patent Application No. 906216, or a
moistening
device 202, which is also in itself known, by means of which devices 202 the
distribution of moisture in the Z-direction of the web is equalized and/or the
curling
of the web is reduced. All of these additional devices are, of course, not
necessarily
in use in the same construction.
In relation to curling of the paper web, the applicant has carried out tests
with a
newsprint machine, whose dryer section was exclusively provided with single-
wire
draw and which dryer section was followed by a two-nip soft calender preceded
by
a steam box. In these reference test run conditions, it was noticed that the
web was
curled in the direction of the top face - the edges of a sample of a diameter
of 100
mm rose 3 centimetres in the direction of the top face. When the running
conditions
were kept in the other respects unchanged but the bottom face of the web was
steam-
treated before the latter soft-calender nip, the curling was reduced
significantly to the
value 1.2 cm. In a second test the dryer section was run normally, in which
case the
curling was measured as 1.5...2 cm, and when the bottom face of the web was
steam-treated in the way mentioned above, the web was no longer curled at all.
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As regards its principal features, the exemplifying embodiment shown in Fig. 2
is
similar to the exemplifying embodiment shown in Fig. 1, but in the embodiment
of
Fig. 2, after the coating device, a reversing blower device 14 is fitted, by
whose
means the web is turned so that it is passed directly onto the drying cylinder
174 in
the upper row. After this, there follow two dryer groups that apply a normal
single-
wire draw, and between said groups there is a closed draw. The parts in the
first
dryer group that applies a normal single-wire draw are denoted with the
reference
denotations corresponding to Fig. 1, and the drying cylinders in the second
dryer
group are denoted with the reference numeral 182 and the reversing
rolls/cylinders
with the reference numeral 181. The drying wire 186 in the second dryer group
runs
guided by the guide rolls 187. After the dryer groups, after the guide roll
184, the
dried paper web W is passed, in compliance with what is shown in Fig. 1, to
the
soft calender 191, in whose connection preferably also steam boxes 192 are
fitted,
in which case it is possible to control the curling of the web W by means of
the
steam boxes 193 and by regulating the calendering nips N1,N2 of the soft
calender
191 in a suitable way.
In the arrangement in accordance with the invention, as the reversing
rolls/cylinders,
particularly favourably are used the suction cylinders marketed by the
applicant with
the trade mark "VAC-ROLL"T" and provided with no inside suction box, reference
being made, with respect to the details of the constructions of said rolls, to
the
applicant's FI Patent No. 83,680 (equivalent US Patents Nos. 5,022,163 and
5,172, 491) .
Above, the invention has been described just with reference to some preferred
exemplifying embodiments of same only, the invention being, however, by no
means
supposed to be strictly confined to the details of said embodiments.
