Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to methods and apparatus for
multicylinder dryers of paper machines.
During the last few years running speeds of paper machines
have been increasing, and are now approaching the limit of
1500 metres per minute. At these speeds the flutter of the
web becomes a problem hampering the running of the paper
machine. Drawing the web from the press section to the
drying section and supporting the web at the single-wire-draw
area can be controlled with methods and equipment illustrated
in some of the Applicant's earlier Finnish patents and
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patent appl ications, but i~ the twin-wire draw area,
particularly in the third and fourth drive groups, there are
; difficulties at high running speeds.
By single-wire draw in this application is meant such a way
to draw a wire over heated drying cylinders in which the web
runs from one cylinder row to another supported by a drying
wire so that on one cylinder row the web is between the
drying wire and the surface of the cylinder, and that on the
other cylinder row the web is outside and the drying wire is
between the surface of the cylinder and the dryiny wire and
the we~ runs the draws between the cylinder rows supported by
the drying wire. An advantage of this single-wire draw is
that the wire is all the time supported by the drying wire,
and it has free draws not at all or at least no free draws of
any substantial length, which reduces the danger of creases
and breaks of the web.
By twin-wire draw in this application is meant the well-known
method of supporting and drawing a web in association with
heated drying cylinders in which a top wire is used in
association with the top cylinders and a bottom wire is used
in association with the bottom cylinders, which bottom wire
is guided by surfaces of the drying cylinders and by guide
rolls positioned between them so that, at the top cylinder
row, the web being pressed by the top wire has a direct
drying contact with the surface of the top cylinders, and,
and the bottom cylinder row, the web being pressed by the
bottom wire has a direct drying contact with the surface of
the lower cylinders.
In the twin-wire draw system there has usually been
essentially long free draws when the web has run from one row
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of cylinders to another. These free draws have been subject
to flutter and consequent breaks and creases, which drawback
has been particularly prevalent in the beginning of the
drying section, where the web is still relatively wet and
therefore weak and elastically sensitive to flutter. Efforts
have been made to eliminate this drawback by shortening said
free draws in the beginning of the drying section by
positioning tne planes imagined to pass via the axes of the
top and bottom cylinder rows at a shorter distance than
usually or what would be optimal from th- point of view of
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the efficiency of the drying procedure.
Another effort has been to convert the third and fourth
drying groups to single-wire draw arrangements, but this i5
an emergency solution, as it leads to lowered evaporating
efficiency and complicates the ventilation arrangements.
Efforts have been made to solve the drying section runability
problems brought about by higher speeds by changing over to
the single-wire draw system. As this lowers the drying
power, this solution is worth using only when quite
necessary, i.e. in the first and second drying groups~ In
other groups of the drying section one has to manage with a
twin-wire draw, which is more feasible from the point of view
of drying power.
The Prior Art has many such suggestions for solving said
problems in which the geometry of a machine equipped with a
twin-wire draw has been so changed that the web all the time
runs supported by a wire, without free runs. This kind of
drying sections are described in for instance U.S. Patents
Nos. 3,751,822, 3,753,298 and 4,510,698.
When the speed of a paper machine is higher than 1000 m/min,
the air currents caused by the wires become decisive from the
point of view of runability. Leaving these air currents
uncontrolled will result in web flutter, creases, uneven
drying and even web breaks and consequent expensive
interruptions of operation.
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The Applican~s earlier Finnish Patent Applications Nos.
841137 (published print 68279), 850611 and 853670 describe
methods and e~uipment ~or improving the running o~ a machine
equipped with a twin-wire draw~
In accordance with tha method aspect of the invention there
is provided a method of operating a multi-cylinder dryer of a
paper machine, said dryer comprising two lines of drying
cylinders, at the connection of which a twin-wire draw is
applied so that, by means of a first and second drying wire,
a closed draw is obtained in the transfer of a paper web from
one cylinder line onto the other, and in which method, a
closed draw is provided, guide rolls of the respective first
and second drying wires are placed at the proximity of the
lS run of the other wire on which the drying wire runs from its
` drying cylinder to its guide roll whereat air is blown
through said drying wires so as to ventilate pockets defined
by said wires and free faces of the drying cyl~nders, wherein
an air jet or air jets are used as ventilation blows, said
jets being directed at the run of the first and second drying
wires at which the respective drying wire runs from its
drying cylinder onto its guide roll, and wherein said air
jets are applied within said run of the drying wire in the
area in which the respective drying wire contacts a portion
of the web supported by the other respective drying wire
which runs over a segment of the circumference of the guide
roll of the other respective drying wire and in which blow
area there is a widening wedge space de~ined by the
respective drying wire and its guide roll, through which
30 wedge space the pocket ventilation blowing takes place.
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Preferred embodiments of the method aspect of the invention
include:
The above method, wherein said air jet or jets are directed
substantially perpendicularly to a face ~f said drying wire
that is opposite a location facing the guide roll of the
drying wire.
~he above method, wherein said air jet or jets are applied
outside the drying wire to a location placed a certain
distance after a normal plane of the first mentioned drying
wire, said normal plane passing through the centre axis of
the guide roll of the other drying wire, said guide roll
baing placed facing said first-mentioned drying wire.
The above method, wherein besides providing ventilation of
said pockets, said air jet or jets also promote detaching of
the web from the wire to which the air jet or jets are
applied and transfer of the web onto the other drying wire.
The above method, wherein before the air jet or jets are
applied, a second air jet or air jets are applied by means of
which the second air jet or air jets are ejected out of an
area placed before the first-mentioned air jet or jets and
defined by a wall of a blow box adjacent to the drying wire
in which a negative pressure is formed in said area, by means
of which negative pressure, the keeping of the web on the
face of the drying wire is promoted.
The above method, wherein permeability of the drying wires
used in the method is within the range of 2000 to 7000
m3/hxm2, more preferably 3000 to 5000 m3/hxm2.
The above method, wherein said blowings are applied to the
face of the drying wire through a nozzle slot or slots in the
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blow box such that the velocity of the air jet or jets in the
blowing is within a range of v=10 to 50 m/s, more preferably
v=30 m/s.
The above method, wherein the air quantity, velocity and
alignment of the air jet or jets are arranged in such a way
that air flow applied through said drying wires into the
respective pockets is within a range of 100 to 400 m3/hxm2,
more preferably 100 to 2~0 m3/hxm2~
The above method, wherein in connection with the respective
drying wire or wires, blow devices are provided, by means of
air jets applied substantially parallel to the plane of the
drying wire at the opposite side of the drying wire relative
the web facing it so that, by means of said air jets, air is
ejected so that an area of negative pressure is produced by
means of which the keeping of the web in conformity with the
face of the drying wire facing the web is promoted.
In accordance with the apparatus aspect of the invention
there is provided a multi-cylinder dryer for a paper machine,
said dryer comprising two lines of drying cylinders placed
one above the other in which a twin-wire draw is applied so
that a drying section comprises an upper wire, which is
guided by guide rolls, and a corresponding lower wire, which
is guided by guide rolls, said guide rolls being placed so
that a web runs from upper cylinders onto lower cylinders and
vice versa at all times supported by the drying wires and in
said dryer, in gaps formed between the cylinders, the web is
transferred from one drying wire on to the other at the
respective guide roll of the latter drying wire, wherein
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facing the guide rolls of the drying wires, against
substantially straight runs of the drying wires running from
their respective drying cylinders as a substantially straight
run to their respective guide rolls, a blow box is fitted,
which is connected ~o an air source, a side of said blow box
is placed facing the drying wires and is provided with a
nozzle slot or slots which are directed in such a way that an
air jPt or jets are directed substantially perpendicularly to
the respective facing drying wires.
Preferred embodiments of the apparatus aspect of the
invention include:
The above dryer, wherein said blow box and its nozzle slot or
corresponding slots are placed in such a way that the air jet
or jets applied meet the drying wires after a tangent point
with the guide roll of the other drying wire.
The above dryer, wherein at an opposite edge of the blow box,
relative to said nozzle slot or slots, there is a further
no~zle slot from which an air jet is applied, said air jet
air being ejected out of a space defined by a wall of said
blow box placed facing the drying wires.
The invention and the State of Art associated with the
invention will now be described in detail with reference to
the figures in the accompanying drawing, in which:
Fig. A illustrates a part of a paper machine drying group
equipped with a previously known conventional twin-wire draw.
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Fig. B illustrates a part of a previously known paper machine
drying group equipped with a modified-geometry twin-wire
draw.
Fig. 1 shows the same as Fig. B but equipped with pocket
ventilation arrangements and runability-improving equipment
implementing the method in accordance with this invention.
Fig. 2 shows an apparatus implementing the method in
accordance with the invention in a larger scale.
In the known ~-wire draw of the web W being dried illustrated
in Fig. A the web W is on the bottom drying cylinders 11
between the bottom wire F2 and the heated cylinder surface
11'. Therefore in the twin-wire draw
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1 the contact between the cylinders 10, 11 and the web W ig good, which
result~ in good evaporation ~rom and efficient drying o the wire W. In
the known twin-wire draw of Fig. A, the web W i~ left with any wire
support on ~he long run W , becsuse the top wire Fl runs over the top
wire guide rolls 12 and the bottom wire F2 run3 over the bottom wire
guide rolls 13. The long unsupported runs W of the wire W cause
difficulties as the speed of the machine grows high, particularly
because of flutter at the run Wp.
In the previously known modified-geometry twin-wire draw shown in Fig. B
the wire guide rolls lZ and 13 have been moved as compared with the Fig.
A so that the guide roll 13 of the bottom wire F2 is located higher than
the guide roll 12 of the top wire Fl. In Fig. B, the web W is all the
way supported by wires Fl and F2. Air pumping induced by wires Fl and F2
will however ae high machine speeds cause so strong air currents and so
high pressure differences at the opposite sides of the web W that the
web W will not be able to adhere to the wires Fl and F2. The moisture
contents of the air within the pockets Tl and T2 will also increase,
which hampers evaporation and makes the web W dry unevenly in the cross
direction~
An effort has been made to improve the runability of the modified
geometry illustrated in Fig. B by changing the felt guide rolls-to
suction-blow rolls. This expensive solution may improve the detachment
of the web W from the guide roll at a certain point, but it does not
reduce air currents caused by pu~ping of wires Fl and F2. Therefore the
runability problem~ will not be ~olved in this way.
The invention will now be described in detail with reference to Fig. 1
and 2. The base of the inventional idea illustrated in these figures is
a cylinder dryer equipped with a closed twin-wire draw as shown in Fig.
B. Same reference are used for various items both in Fig~. 1 and 2 and
in Fig. A and B; thereiore Figs. A and B will not be re-explained in
these respects in the following.
In Fig. 1 such equipment arrangements have been added to the
modified-geometry ~win-wire draw which are particularly required when
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1 the speed of the paper machines grows high (usually the speed is > 1000
m/min). Devices 14 and 15 in accordance with the Applicants earlier
Finnish patent applications Nos. 850611 and 853670 stabilize the run of
the web W by maintaining the support contact between the web W and the
wire Fl and F2. A pocket ventilation arrangement 16 in accordance with
the present invention provides ventilation of pockets Tl and T2, which
lowers the air moisture level within the pocket~ Tl and T2, improves the
evaporation from the web W and provides for controlled drying.
0 A pocket arrangement 16 in accordance with the invention is positioned
close to the point where the top wire Fl and and the guide roll 13 of
the bottom wire and correspondingly the bottom wire F2 and the guide
roll 12 of the top wire are close to each o~her and touch (the web W and
the other wire Fl/F2 being in between).
The operation of a pocket ventilation method in accordance with the
invention is clarified by Fig. 2. An equipment arrangement implementing
the invention compri~es blo~ boxes 17 reaching over the whole wires Fl
and F2 in the croQs direction with re3pect to the running direction of
the web W, connected to elements supplyin~ air for air jets,
schematically illustrated by block 20. At the Qide of the wire Fl, in
its running direction at the trailing edge of the box 17 there i9 a
nozzle slot 18, through which an air jet Sl is blown perpendicu}arly or
almost perpendicularly against the wire Fl. When hitting the wire Fl
running at a high speed the air jet Sl turns parallel S2 with the wire
Fl, and part of air goes through the pervious wire Fl to ventilate the
pocket T2, which is shown with arrows S3.
T-T in Fig. 2 indicates the plane that includes the line P of the wire
Fl tangential to the guide roll 13 and the center axis of the guide roll
13. After this line start~ the nip N (the wedge-~haped ~pace) expanding
in the running direction of the wires Fl and F2 and the wire. In the
range of this nip, by means of a blow in accordance with the invention,
air is moved through the nozzle slot 18 to ventilate the pocket T2
(arrows S3). As shown in the figure, the nozzle slot 18 is positioned
close to the wire ~l after a small di~tance k after said touching line
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The air quantity (S3) going through the wire Fl being approx. 25-30% of
the total air quancity coming from the box 17 ii well sufficient to
ventilate the pocket T2. From ~he pocket T2 the air escapes to the sides
of the machine. The air flow S3 going through the wire Fl al~o helps co
detach the wire W from the wire Fl at the right point and to move to be
supported by the other wire F2, which improves the runability.
When required, a blow nozzle 19 deseribed in the Applicant's earlier
0 Finnish patent application No. 850611 may be positioned at the entry
adge of the blow box 17. The air jet S4 being di~oharged from this blow
nozzle 19 with i~s ejection effect (arrow ~) improves the 3upport
contact between the web W and the wire Fl by creating a vacuum in the
~pace A between the wall 21 and the wire Fl. However, the u3e of the
nozzle 19 does noe in any way - at least not in any detrimental way -
effect upon the operation of a pocket ventilation arrangeme~t 16
implementing the method of this invention.
At the sides of the wire Fl, the blow box 17 reaches close to the edges
of the wire Fl, and at the ends of the blow box 17 one may use edge
nozzles re~embling the nozzles 19 in order to prevent leak flow from the
sides of the machine into vacuum area~.
In association with the Fig. 2 there is a detailed description of the
phase in accordance with the invention in which the web W is moved as a
closed draw from the top wire Fl of a multicylinder dryer to its boteom
wire F2. It is ea3y to under3tand that corresponding method step~ shown
in Fig. 2 are carried out and the same equipment arrangements are used ,
when the web W is moved as a clo~ed draw from the bottom wire F2 to the
top wire Fl in association with the guide rolls 12 of the top wire Fl.
The explanation of Fi~A 2 i3 valid as such for these method steps and
equipment arangement~, when the tup wire Fl i5 changed to the bottom
wire, the guide roll 13 to the roll 12, and the pocket to another pocket
Tl.
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When u8ing the method of the invention, the permeability of the
transport wires Fl and F2 is usually in the range 2000 to 7000 m3/hxm2>
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preferably in the range 3003 to 5000 m3/hxm .
The speed of ~he air jet Sl being blo~n from the jet nozzle 18 or
several correspondint nozzle slots i3 usually in the range v = 10 to 50
m/s, preferably v ~ approx. 30 m/s.
With these parameters and with appropriate aiming of the air jet Sl or
of ehe air jet~, such an air jet (S3) can be applied into pocket Tl and
T2 through the drying wires, whose flow quantities are generally in the
0 range lO0 to 400 m3/hxm, preferably 150 to 250 m3/hxm.
The invention is by no means restricted to aforementioned details which
are described only as examples; they may vary within the framework of
the inventional idea as defined in the follo~ing claims.
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