Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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.
Further, the invention concerns an after-dryer for a paper machine for
applying the
method in accordance with the 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
drying
cylinders are, as a rule, placed in the lower row and the reversing cylinders
in the
upper 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
mufti-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
mufti-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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group has also been used in order that the drying could be made sufficiently
symmet-
ric in the z-direction.
From the prior art, solutions are known for an after-dryer for paper to be
coated, in
S 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. This is why the heating of the web on the
cylinders
either in single-wire draw or in twin-wire draw has taken an unduly high
proportion
of the length of the machine in relation to the capacity that is provided.
It is an object of the present invention to provide a solution in which the
curling of
paper is controlled so that the after-dryer does, however, not become
substantially
longer.
Groups of the sort mentioned above for finishing 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 object of the present invention is further
develop-
ment of the solutions suggested in said application so that the tendency of
curling of
paper can be controlled more efficiently in the after-dryer.
With respect to the prior art, reference can also be made to the applicant's
FI Patent
Application No. 93l263 (equivalent to EP Application No. 0, 620, 3l3 and
equivalent
to L7S Patent No. 5, 495, 678), in which a favourable arrangement is described
for
combining impingement drying with a prior-art cylinder dryer group that
applies
single-wire draw.
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It is a further object of the invention to provide 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.
It is a further object of the present invention to provide such an after-dryer
for a
paper machine in which the runnability can be brought to a particularly high
level.
It is a further object of the invention to provide an after-dryer of a paper
machine
which has an efficient drying capacity.
la
From the prior art, it is known to use an electric or gas infra after a
coating station
or surface-sizing unit. Also, it is known from the prior art to use a what is
called
combination dryer, in which the hot air of a infra is passed into a what is
called
airborne web dryer.
it is a further object of the invention to provide an after-dryer for a paper
machine
which is advantageous in view of the consumption of energy.
A specific object of the invention is to create novel concepts for an after-
dryer of a
paper machine in which runnability, efficiency and control of curling are
optimized.
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
after-dryer the paper web is dried in at least one dryer group that makes use
of
single-wire draw, and that, at the same time, the paper web is dried by means
of an
impingement-drying equipment fitted in connection with at least one cylinder
or roll
in said dryer group.
Further, the after-dryer in accordance with the invention that makes use of
the
method is mainly characterized in that the after-dryer comprises at least one
dryer
group that applies single-wire draw, and an impingement-drying equipment is
fitted
in connection with at least one cylinder or roll in said dryer group.
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In accordance with one exemplifying embodiment of the invention, in the
beginning
of the after-dryer, an inverted group with single-wire draw is fitted, above
whose
suction rolls, suction cylinders or equivalent reversing rolls/cylinders
impingement
blawing hoods are placed. This solution is advantageous, because single-wire
draw
has a good runnability and does not require threading by means of a rope, and,
moreover, impingement drying dries the coated side of the paper web
efficiently. By
means of this arrangement, among other things, the advantage is obtained that
the
web is heated quickly, whereby efficient evaporation of the water present in
the web
surface is achieved. The impingement blowing does not affect the curling, and
the
curling is regulated in the following group with twin-wire draw by adjusting
the
steam pressures in the upper and lower cylinders to suitable levels. The
arrangement
in accordance with the invention permits the construction of a shorter after-
dryer and
has a better energy economy than infrared dryers etc. devices based on
electric
energy, because in impingement drying it is possible to use, for example,
combus-
tion gases from liquefied petroleum gas, which are more advantageous than
electric-
ity. Compared with an airborne web dryer, an important advantage of the system
in
accordance with the present invention is good runnability. Moreover, in the
arrange-
ment in accordance with the invention, the efficiency can be regulated readily
and,
also, the impingement drying provides a possibility for profiling.
In an arrangement in accordance with a preferred embodiment of the present
invention, impingement drying is applied to the drying of a coating and, in
the same
way as in a combined infrared/airborne-web dryer, the air heated by the infra
is
utilized in the impingement drying unit. Warm air can be used as replacement
air for
the impingement drying unit, and also directly as impingement blow air, to
replace
air that has to be heated otherwise, or it can also be used as pre-heated air
for a
burner by whose means said blow air is heated. In view of the efficiency, a
situation
is particularly favourable in which gas infra dryers are available. In such an
arrange-
ment, in connection with the after-dryer, after the coating or surface-sizing
device,
before cylinder drying and impingement drying units, there is a gas dryer or
gas-
infra dryer in pre-drying. Advantages that are obtained include economies of
energy
and good draw of the web.
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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 confined to the details of said illustrations.
Figure 1 is a schematic illustration of an exemplifying embodiment in which,
in the
beginning of the after-dryer, there is a dryer group that makes use of single-
wire
draw, in which group the reversing rolls/cylinders are provided with an
impingement
drying device and which group is followed by a dryer group with twin-wire
draw.
Figure 2 is a schematic illustration of an after-dryer substantially similar
to that
shown in Fig. 1, in which dryer the first drying equipment is an airborne web
dryer.
The exemplifying embodiment that is illustrated schematically in Figure 3 is
substan-
dally similar to those shown in Figs. 1 and 2, but in this embodiment, after
coating,
the web is turned by means of a reversing blow box.
Figure 4 is a schematic illustration of an exemplifying embodiment in which,
after
coating/surface-sizing, the web is turned by means of a reversing blow box,
passed
into an inverted dryer group that applies single-wire draw and in which
impingement
drying devices are fitted in connection with the reversing rolls/cylinders,
after which
there follows a dryer group that applies twin-wire draw.
Figure 5 is a schematic illustration of an exemplifying embodiment which is
substan-
dally similar to that shown in Fig. 4, but the drying cylinder in the inverted
dryer
group with single-wire draw has been substituted for by a reversing blow box.
Figure 6 is a schematic illustration of an exemplifying embodiment in which
the first
dryer group is a group with normal single-wire draw, in which group, in
connection
with the reversing rolls, impingement drying units are fitted, which are
followed by
a dryer group with twin-wire draw.
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Figure 7 is a schematic illustration of an exemplifying embodiment in which
the first
group in the after-dryer is a normal dryer group with single-wire draw, which
group
is followed by a second normal group with single-wire draw, in which group
impingement drying devices are fitted in connection with some of the reversing
rolls.
Figure 8 is a schematic illustration of an exemplifying embodiment which is
substan-
tially similar to that shown in Fig. 7, but the first dryer device in the
after-dryer,
before the groups with single-wire draw, is an airborne web dryer.
Figure 9 is a schematic illustration of an exemplifying embodiment in which an
airborne web dryer equipment is followed by a normal dryer group with single-
wire
draw, after which group a cylinder of very large diameter is placed, in
connection
with which cylinder impingement drying devices are fitted and which cylinder
is
followed by a normal dryer group that applies single-wire draw.
Figure 10 is a schematic illustration of an exemplifying embodiment in which
the
first dryer group in the after-dryer is provided with a reversing
cylinder/roll of large
diameter, in connection with which impingement drying devices are fitted and
which
is followed by a dryer group that applies twin-wire draw.
Figure 11 is a schematic illustration of an exemplifying embodiment in which
the
first group in the after-dryer is a group with twin-wire draw, which group is
followed by an inverted dryer group with single-wire draw, followed by a dryer
group that applies twin-wire draw.
Figures 12A and 12B are schematic illustrations of an exemplifying embodiment
of
the nozzle face of an impingement drying device.
Figure 13 is a schematic illustration of an exemplifying embodiment in which,
in the
beginning of the after-dryer, connected with the first cylinder, an infra
dryer is
fitted, whose heat is used in connection with impingement drying, and in
which,
between the infra and the impingement dryer, a reversing blow equipment is
fitted.
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Figure 14 is a schematic illustration of an exemplifying embodiment in which,
after
the coating, the web is dried by means of an infra dryer or a gas dryer, whose
heat
is utilized in impingement drying, and in which, after the infra dryer, a
reversing
blow box is fitted, which is followed by an impingement dryer group.
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-SIZER'"', 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.
After this, the web W is passed into the after-dryer onto its first drying
cylinder 2 i
placed in the lower row RA, over which the web W is passed into its inverted
dryer
group with single-wire draw, onto its first reversing roll/cylinder 23 in the
upper
row RY, from which the web W is passed by means of the drying wire or an
equivalent support fabric 26 onto the drying cylinder 22 in the lower row RA
and
further onto the reversing roll or cylinder 24 in the upper row RY. In
connection
with the reversing rolls or cylinders 23,24 in the upper row RY, and
impingement
blowing equipment 25 is fitted, by whose means drying gas/air jets are blown
towards the web W. The guide rolls of the drying wire 26 are denoted with the
reference numeral 27, and by means of the guide roll 27A the run of the drying
wire
26 is shifted so that it does not reach contact with the web W to be dried
that runs
over the drying cylinder 21. From the reversing roll/cylinder 24, supported by
the
drying wire 26, the web W is passed as a closed draw into a dryer group with
twin-
wire draw onto the first drying cylinder 31 in its lower row RA. In the figure
the
drying cylinders in the lower row RA are denoted with the reference numerals
3l,32
and 33, over which cylinders the web W runs alternating and meandering onto
the
drying cylinders 41,42 and 43 in the upper row RY. The drying wire 34 of the
lower-row RA cylinders 31,32,33 runs guided by the guide rolls 35, and the
drying
wire 44 of the upper row RY runs guided by the guide rolls 45. Over the guide
roll
/ alignment roll 46 the web W is passed to further processing. In connection
with the
drying cylinders, a doctor 28 can be provided.
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As the reversing rolls/cylinders, particularly favourably are used the suction
cylin-
ders 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. S, 022) 163 and 5, l72, 49l ). In stead of the Vac-Roll suction
cylinders,
it is also possible to use rolls marketed by the applicant with the trade mark
UNO or
cold or hot cylinders in themselves known.
The exemplifying embodiment shown in Fig. 2 is substantially similar to that
shown
in Fig. 1, and corresponding parts are denoted with the same reference
numerals.
Unlike the exemplifying embodiment shown in Fig. 1, in Fig. 2 the web W coated
by means of the coating device 10 is first passed through an airborne web
dryer 15,
which operates as a pre-dryer, before the first drying cylinder 21.
The exemplifying embodiment shown in Fig. 3 is substantially similar to those
shown in Figs. 1 and 2, and corresponding parts are denoted with the same
reference
numerals. It is a difference in this exemplifying embodiment that after the
coating
device 10 the paper web W is passed over a reversing blow device 14 into an
airborne web dryer 15 operating as a pre-dryer, and from the airborne web
dryer 15
directly onto the first reversing roll/cylinder 23 in the upper row RY in an
inverted
group with single-wire draw.
The exemplifying embodiment shown in Fig. 4 is substantially similar to the
exemplifying embodiments shown in Figs. 1 to 3, but in this exemplifying
embodi-
ment, after the coating device 10, the web W is passed over a reversing blow
device
14 directly onto the first reversing roll/cylinder 23 in the upper row RY in
an
inverted group with single-wire draw.
The exemplifying embodiment shown in Fig. 5 is substantially similar to that
shown
in Fig. 4, but here the drying cylinder 22 in the lower row RA in the first
inverted
dryer group with single-wire draw has been substituted for by a reversing blow
device 28.
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In the exemplifying embodiment shown in Fig. 6, the coated web W is passed
from
the coating device 10 over the drying cylinder 21 onto the first drying
cylinder 51
in the upper row RY in a normal dryer group with single-wire draw. This dryer
group comprises two drying cylinders 51 and 52 in the upper row RY and two
5 reversing rolls or cylinders 53,54 in the lower row RA. In connection with
the
reversing rolls/cylinders, an impingement drying equipment 55 is fitted. The
drying
wire is denoted with the reference numeral 56 and the guide rolls with the
reference
numeral 57. After this dryer group the web W is passed as a closed draw by
means
of the drying wire 56 onto the first drying cylinder 41A in the upper row RY
in a
10 dryer group with twin-wire draw. In the other respects the dryer group with
twin
wire draw is similar to the twin-wire draw illustrated in the preceding
figures.
In the exemplifying embodiment shown in Fig. 7, the coating device 10 is
followed
by a dryer group with normal single-wire draw, into which the coated paper web
W
is passed over the drying cylinders 61 and 62. Supported on the drying wire
66, the
paper web W to be dried runs meandering over the reversing rolls/cylinders 65
in
the lower row RA and the drying cylinders 63 in the upper row RY onto the
drying
cylinder 64, from which the web W is passed into the next dryer group with
normal
single-wire draw as a closed draw onto the drying wire 76 of said group, on
whose
support the paper web W runs meandering over the reversing rolls/cylinders
75,78
in the lower row RA and the drying cylinders 73 in the upper row. In
connection
with the middle reversing rolls/cylinders 78 in the lower row, impingement
drying
devices 79 are fitted for further drying of the web by means of drying
blowings.
Over the drying cylinder 74 the web W is passed to further processing. In
dryer
groups with single-wire draw blow boxes 68 or equivalent are used, for example
devices 68 which are marketed by the applicant with the trade mark UNO-RUN-
BLOW-BOX and which improve the runnability. Of course, these devices can also
be used in the exemplifying embodiments shown in the other figures.
The exemplifying embodiment shown in Fig. 8 is substantially similar to that
shown
in Fig. 7, but in this embodiment the web W is passed from the coating device
10
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through the airborne web dryer I S operating as a pre-dryer over the drying
cylinders
61,62 into dryer groups that apply a normal single-wire draw.
The exemplifying embodiment shown in Fig. 9 is substantially similar to that
shown
in Fig. 8, and in this embodiment, after the coating device 10, the coated
paper web
W is passed through the airborne web dryer 15 over the cylinders 61,62 into
the first
normal dryer group with single-wire draw, after which the web W is passed over
the
drying cylinder 81 around the large-diameter roll 80, in connection with which
roll
an impingement drying equipment 85 is provided. The diameter of the large-
diameter
roll is 20. . .150 % , preferably 25 . . . 90 % larger than the diameter of
the ordinary
drying cylinders. After this, there follows the drying cylinder 82 in the
lower row,
from which the web W is passed over the drying cylinder 91 into a normal dryer
group with single-wire draw, whose reversing rolls/cylinders are denoted with
the
reference numeral 95 and drying cylinders with the reference numeral 93. The
drying wire is denoted with the reference numeral 96, and its guide rolls with
the
reference numeral 97.
In the exemplifying embodiment shown in Fig. 10, after the coating device 10,
the
paper web W is passed over the drying cylinder l01 onto the face of the large-
diameter roll 100, in whose connection an impingement drying equipment l05 is
fitted. The large-diameter roll 100 is followed by a drying cylinder 102 and
by a
reversing roll/cylinder 103, after which the web W is passed into a dryer
group with
twin-wire draw, which is substantially similar to that described above, for
example,
in relation to Fig. 1. In connection with the large-diameter roll l00, a
drying wire
106 of its own is provided, whose guide rolls are denoted with the reference
numeral
l07. By means of the guide roll 107A the run of the drying wire is shifted so
that it
does not reach contact with the paper web W to be dried which runs over the
drying
cylinder 101.
In the exemplifying embodiment shown in Fig. 11, the paper web W, which was
coated in the coating device 10, runs first over the drying cylinder 111 into
a dryer
group with twin-wire draw, in which the upper wire 114 runs guided by the
guide
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rolls 15 over the drying cylinders 112 and 1l3 and the lower wire 122 runs
over the
guide rolls 123 and the drying cylinder 121. The web W runs meandering from
one
row RA to the other RY, and between the rows RA,RY the web W has free
unsupported draws. From the group with twin-wire draw the web W is passed into
the next dryer group, onto the drying cylinder 131 in the lower row, where the
web
runs on support of the drying wire 136. After this the web is passed onto the
reversing roll/cylinder 132 in the upper row, in connection with which an
impinge-
ment drying equipment l35 is fitted. After this the web is passed back to the
lower
row and further onto the drying cylinders 141, l42 in the upper row, and in
the final
part of the group an ordinary group with twin-wire draw is fitted, in which
the wire
that operates as the lower wire 136 is common with the wire running through
the
impingement dryer group, and the drying cylinders 141,142 in the upper row are
provided with a drying-wire cycle 146 of their own with guide rolls l47. Over
the
guide roll l43 the web W is passed to possible further processing.
Figs. 12A...12B are schematic illustrations of the construction of the nozzle
face of
the impingement drying device, in which the blow holes are denoted with the
reference N2 and the exhaust air pipes with the reference N 1. The diameter of
the
exhaust air pipes N 1 is about 50. . .100 mm, preferably 75 mm, and the
diameter of
the blow holes is about 3...8 mm, most commonly 5 mm. The paper web W runs at
a distance of about 10...150 mm, preferably 25 mm, from the face of the nozzle
plate, and the nozzle chamber of the hood is denoted with the reference letter
N.
The cylinder face is denoted with the reference C. The open area of the blow
holes
in the nozzle plate in the area of the web W is about 1...5 % and most
commonly
1. 5 % . The velocity of air in the blow holes is about 40. . .150 metres per
second,
preferably 100 m.p.s. The air quantity that is blown is about 0.5...2.5 cu.m
per
second per sq.m, which is calculated for the effective area of the hood. Most
commonly an air quantity of 1...1.5 cu.m per second per sq.m is used. The open
area of the exhaust air pipes is 5 . . .15 % , most commonly 10 % . In
addition to the
nozzle face illustrated in this figure, it is possible to use a commonly known
slot
nozzle construction, fluid nozzle construction, foil nozzle construction, or a
direct-
blow nozzle construction as well as, for example, infra dryers, as well as any
of
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those mentioned above alternatingly in the cross direction of the machine as
what is
called combination dryers.
The impingement drying equipment to be used in connection with the
exemplifying
embodiments illustrated above in Figs. 1 to 11 can be an arrangement of a
number
of different types and in itself known to a person skilled in the art, in
which
arrangement drying air flows are blown towards the web to be dried. The
impinge
ment drying equipment can be constructed in blocks in the running direction S
of the
web or in the direction transverse to the running direction, in which case
each block
can be regulated separately if necessary.
In the exemplifying embodiment shown in Fig. 13, after the coating device 10,
the
paper web W is passed onto the drying cylinder 151, in whose connection an
infra
drying equipment 1S2 is fitted. After this the paper web to be dried is passed
over
the reversing blow box 153 onto the drying cylinder 154 and further onto a
reversing
roll or a drying cylinder or equivalent 155. In connection with the
cylinder/roll 154,
an impingement drying equipment 156 is fitted, which can be opened in the way
indicated by the arrows S 156 for cleaning. The wire draw takes place as a
normal
single-wire draw as described above in relation to the preceding figures. By
means
of the coating device 10 the coating is applied to the wire side, i.e., as
shown in the
figure, to the bottom side of the web face W. The hot exhaust air of the infra
dryer
152 is passed along the duct 157 through a blower 15 $ for use in the
impingement
dryer 156. This infra-heated air can be used directly as impingement blow air,
for
example, to replace air that would have to be heated otherwise, or as pre-
heated air
in the burner in which said blow air for the impingement dryer is heated, or
as
replacement air for the impingement dryer. The impingement drying equipment
156
can be openable towards the bottom by means of pivot members 171.
The exemplifying embodiment shown in Fig. 14 represents an arrangement in
which
the coating of the web W by means of the coating device 10 is applied to the
top
side of the web, and the web W is dried first by means of an infra/gas dryer
161,
after which the run of the web W is turned by means of a reversing blow box
162
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and the web W is passed to run into a group with inverted single-wire draw, in
which, in connection with the reversing rolls/cyiinders 163,165, impingement
drying
devices 167 are provided. The exhaust air from the infra/gas dryer 161 is used
as
drying air for the impingement drying, and the exhaust gas is passed through
the
ducts 168 and through the blower 169 into the air system of the impingement
drying.
Above, the invention has been described with reference to some preferred
exemplify-
ing embodiments of same only, and the invention is, however, by no means
suppos-
ed to be confined to the details of said embodiments. Many variations and
modifica-
tions are possible within the scope of the inventive idea defined in the
following
claims.
