Note: Descriptions are shown in the official language in which they were submitted.
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OVERPRESSURE APPARATUS
The invention relates to an apparatus for removing a liquid, dusty or solid
substance from a moving fabric or web. The invention relates to an apparatus
for applying overpressure on a moving web path.
The conventional function of the wet end of a paper machine is based on, for
example, the so-called Fourdrinier principle, by which an endless rotating
wire is covered with pulp which consists of a liquid and the fibres required
for
paper manufacture and which is herein also called a web. Water is removed
from the web at several different steps by means of devices placed under the
wire and including, for example various suction boxes.
One suction box is presented in EP 0 639 667 Al. Said suction box com-
prises a roll rotating with the wire and having a jacket that is, for example,
meshed. The roll' is placed in the box under a negative pressure. Air flows
through the wire and the jacket of the roll. The document also discloses the
use of overpressure to make the removal of liquid more efficient. A device for
producing suction in the suction box is presented in WO 99/64667 Al. Appa-
ratuses equipped with a sealing are disclosed in publications WO-00/09914,
US-6497789-B1, WO-01/34904-A1 and WO-99/23296. WO 95/07387 Al
discloses an apparatus used for controlling a felt and a web supported on it,
applying a rotating roll whose perforated jacket is permeable to air. Other
rolls are also known which are provided with a perforated jacket and
equipped with suction by means of a separate apparatus. One apparatus is
disclosed in US 5,347,728.
A suction device according to Fig. 1 is also known, by which it is possible to
support a moving wire so that the friction is reduced when compared with, for
example, foils. In an opening in the upper surface of the device, against the
wire, a perforated rotating roll is provided, through which a negative
pressure
is effective on the wire. The device is used to provide a sufficient suction
effect and high-capacity dewatering, wherein it can also be applied on wires
with a high speed. In particular, the device can be used to replace suction
boxes of prior art. The device is suitable for the removal of liquid as well
as
CONFIRMATION COPY
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solids, for example dust, from a wire, a felt or various other fabrics which
are
used, for example, in a paper machine.
The device of Fig. I is also applicable for the removal of dust developed
during the manufacturing process of various fabrics or the like, or for the re-
moval of liquid, dust and solids accumulated during their cleaning. Con-
sequently, the device can be used as a cleaning device or a dewatering de-
vice for removing liquid or solids from moving pulp, a web or a fabric, for
example from a wire and a felt, which are sufficiently permeable to air or
liquid. As a suction device and an apparatus according to Fig. 1, an appa-
ratus according to publication WO-2006/090012-A1 is used.
The aim of the invention is to eliminate the above-presented drawbacks of
prior art. Another aim of the invention is to make the operation of the
suction
device more efficient, particularly by affecting the removal of liquid.
The apparatus according to the invention is presented in claim 1. The appa-
ratus according to the invention is also presented in claim 16. The other
claims present some embodiments of the invention.
The arrangement according to the invention for making the operation of the
suction device more efficient comprises an apparatus, by means of which
overpressure is effective on the opposite side of the wire when compared
with the effect of negative pressure. The overpressure makes, for example,
the removal of liquid from the web path and the operation of the suction de-
vice more efficient. The overpressure moves liquid from the web path to an
area subjected to negative pressure, and the negative pressure removes the
liquid further from the web path. The solution can be applied so that the web
path contains two wires and the web therebetween. The solution can be
applied especially in situations where the web path contains one wire and the
web on top of the same. The overpressure apparatus does not comprise
sealings positioned against the wire, but in one embodiment of the invention,
the sealing is an air gap, wherein the overpressure apparatus can be po-
sitioned above the web. On one side of the web path there is a suction de-
vice and on the other side an overpressure apparatus implemented by
means of a pressure box.
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In the arrangement according to the invention, overpressure is effective on
an area that is also subjected to the negative pressure of the suction device.
In one embodiment of the invention, the sealing between the overpressure
apparatus and the suction device by means of air gaps is provided by de-
fining the area which is subjected to the overpressure. When a suction roll is
used, the overpressure is restricted within the effective area of negative
pressure. Preferably, the overpressure is not effective in the point where the
web path is to be detached from the top of the roll, wherein the loosening
takes place more easily and friction is not unnecessarily increased.
Preferably, the overpressure is not effective in the point where the web path
is to move on top of the roll, wherein the loosening for example from the even
guide surface takes place more easily and friction is not unnecessarily in-
creased.
In one embodiment of the invention, the pressures and amounts of air in the
overpressure apparatus and the suction device are set so as to minimize the
spurting and splashing of liquid between the overpressure apparatus and the
web path. The settings are selected so as to direct the air flow in said !o-
cations towards the inside of the configuration formed by the suction device
and the overpressure apparatus, or to minimize said air flow. Correct settings
may also be determined by means of tests, wherein by means of increasing
the overpressure the changes in the amount of removed water are observed
and the behaviour of the web path is examined at the location of the air gap.
The overpressure prevails within a suitable area when the amount of re-
moved water is not substantially increased and for example spurting does not
occur. It is also possible to determine the correct settings by measuring the
pressures of the overpressure apparatus.
In an embodiment of the invention the overpressure apparatus comprises a
chamber where overpressure prevails and affects a predetermined area in
the web path. Significant changes occurring in the overpressure have been
equalized or eliminated by means of air gaps both between the apparatus
and the web path, as well as by conveying air out of the chamber, if neces-
sary. Thus, the pressure peaks occurring in the chamber may be eliminated,
wherein the effect of negative pressure and overpressure on the web path
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remains steady, which increases the uniform quality of the web path. Espe-
cially the spurting of the liquid as well as the resulting web breaks are
elimi-
nated, because the pressure is capable of discharging from the chamber
through a route other than the air gap between the apparatus and the web
path, wherein the route is a special channel or gap. Thus, the pressure or air
flow remains constant in the air gap between the web path and the chamber.
In an embodiment of the invention the overpressure apparatus contains at
least one overflow chamber alongside with said overpressure chamber, to
which the air from the overpressure chamber can be discharged, if neces-
sary. Furthermore, air can be sucked from the overflow chamber through the
web path to the suction device and the ambient pressure typically prevails in
the overflow chamber. The overflow chamber extends at least on top of the
point where the web path is to be detached for example from the top of the
roll, and where the underpressure area ends. Preferably, at least one over-
flow chamber extends at least on top of the point where the web path is in-
tended to move on top of the roll, and where the underpressure area begins.
There is a small air gap as a sealing between the overflow chamber and the
web path. The overflow chamber covers either entirely or partly the stationary
supporting surface along which the web path is positioned before it moves on
top of the roll or off the same.
The overpressure chamber extends transversely across the web path, and so
does the roll of the suction device, and the overflow chamber also runs next
to the overpressure chamber. Preferably, there are overflow chambers on
both sides of the overpressure chamber.
In one embodiment of the invention the overpressure chamber forms a
tapering gap or throat, in which the narrower lower part of the throat faces
the
web path and the wider upper part is directed away from the web path. In the
narrow throat the movement of the air is accelerated. The upper part of the
throat is sealed with a cover and air is conveyed thereto, but in the upper
part
of the throat, for example between the cover and the throat there is a channel
or a gap, through which excess pressure and air are capable of being dis-
charged, preferably to the overflow chamber. The channel or gap is larger
than the corresponding gap between the web path and the overpressure
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chamber, and therefore the pressure and air are more easily discharged from
the chamber through the aforementioned channei or gap, and not through the
gap functioning as a sealing between the web path and the overpressure
chamber. This results in that the pressures and air settle down by the web
5 path between the web path and the overpressure chamber.
Through the channel air is discharged to the overflow chamber in which the
movement of air rapidly settles down. The overflow chamber has preferably
the form of a widening chamber comprising a narrower upper part at the Io-
cation of said channel and a wider upper part opening towards the web path.
The overflow chamber restricts an area from the web path, using the air gap
for sealing.
The arrangement according to one embodiment of the invention comprises,
for the suction device, a sealing device that is capable of following the
trans-
verse movement of a fabric, for example a wire. The function can be
described as being active, because the sealing device follows the movement
of the fabric in different situations. The sealing device is capable of
keeping
the ends of the opening of the suction device closed and sealed even if the
fabric moved in the transverse direction. The sealing device is placed onto
the ends of the opening and glides above the roll. Using the sealing device,
the openings can be made so long that they cover the full width of the fabric,
irrespective of even large movements of the fabric.
According to one example, the suction device is coupled to the sealing de-
vice, with which the overpressure apparatus moves. The sealing device
utilizes a movable carriage which can also be utilized for moving the over-
pressure apparatus as such, without properties relating to the sealing. The
overpressure apparatus is equipped with height adjustment, wherein it is
possible to adjust the size of said air gaps.
In the following, the invention will be described by means of an example and
with reference to the appended drawings, in which:
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Fig. 1 illustrates the principle of a suction device according to prior art,
the suction device comprising a rotating roll against a moving
fabric,
Figs. 2, 3, 4, and 5 illustrate suction devices equipped with overpressure
apparatuses according to the invention, and
Fig. 6 illustrates the pressure box according to Fig. 2, in a cross section
seen from its end.
As shown in Fig. 1, the dewatering suction device comprises a box-like and
sealed chamber 1 used as a basin, in which a rotating roll 2 has been placed.
The upper surface 1 a of the box, on and against which a fabric 3 is placed,
is
substantially flat and typically horizontal. The upper surface 1 a may also
form
a very gently sloping angle, the roll 2 being placed in its angular point. The
chamber 1 is elongated and extends preferably underneath the fabric 3, sub-
stantially across its entire length. The chamber 1 is placed crosswise, pri-
marily perpendicularly to the travel direction of the fabric 3, in the same
way
as the opening I b in the upper surface 1 a of the chamber 1. The opening 1 b
is uniform and continuous and extends over the full width of the fabric 3. The
chamber 1 is closed with end parts I c.
A roll 2 is placed inside the chamber 1 and is allowed to rotate with the
fabric
3. The opening 1 b exposes at least part of the outer jacket 2a of the roll 2
in
such a way that the moving fabric 3 or web that is tangential to the chamber
1 can be supported to the outer jacket 2a. Preferably, the roll 2 extends
slightly higher than the upper surface 1 a and causes a gently sloping ridge
or
a shape having a curved cross-section in the fabric 3, when the wire is
placed or aspirated against the outer jacket 2a. The roll 2 and the upper sur-
face 1 a form a surface, against which the fabric 3 is pressed by the effect
of,
the suction. This part of the outer jacket 2a covers a sector-like part
limited
for instance by beams 4 and 5 at the front and back edges of the opening 1 b.
The exposed sector-like part encompasses about a quarter or less of the
outer jacket 2a shown in Fig. 1. In the presented embodiment, said beams 4
and 5, at the same time, constitute the upper surface 1 a and are placed as
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close to the roll 2 as possible. The beams 4 and 5 are connected to the upper
part of the chamber 1 or to supporting structures above the chamber 1.
The fabric 3 travels along the upper surface 1 a, and the negative pressure
effective on the fabric 3 removes, for example, liquid from the fabric 3
further
into the chamber 1. The roll 2 is perforated, and the suction prevailing
inside
it pulls the fabric 3 against the outer jacket 2a of the roll and the liquid
to the
inside of the roll 2 and into the chamber 1. The roll 2 is arranged to rotate
around a rotation axis X, and the rotation axis X is parallel to the
longitudinal
axis of the roll 2 and also to the opening 1 b. The width of the opening 1 b
is a
fraction of the length of the roll 2 and the opening 1 b. The longitudinal
axis is
parallel to the chamber 1 and transverse to the travel direction of the fabric
3.
The roll 2 rotates or is rotated at a speed that corresponds to the speed of
the fabric 3.
The roll 2 comprises the structure of a shaft 6, for example a solid shaft,
appropriately mounted with bearings at its ends. In the presented embodi-
ment, the ends of the shaft 6 protrude from both ends of the chamber 1,
wherein the bearings can be easily arranged outside the chamber 1. The gap
between the end of the chamber 1 and the shaft 6 is sealed with appropriate
sealings. At least one end of the shaft 6 may be connected to a separate ro-
tating device, typically an electric motor, to rotate the roll 2 via the shaft
6.
The roll 2 comprises an outer jacket 2a with a circular cross-section. The
outer jacket 2a can be made of a sheet or, for example, as a wire-cloth that
is
permeable to liquid and air. In a way known as such, the outer jacket 2a may
be perforated or equipped with holes or openings, through which the suction
is effective on the moving fabric 3. Through the outer jacket 2a, the liquid
or
air flow can enter the inside of the roil 2. The air flow effected by the
suction
and passing through the fabric 3 and the web carries liquid, dust and possibly
other solids that can penetrate the fabric 3 and the outer jacket 2a.
The ends of the upper surface 1 a of the chamber 1 are provided with inclined
sealing surfaces 8 which are positioned partly underneath the fabric 3. The
fabric 3 rises from the upper surface 1 a onto the sealing surface 8. The
sealing surfaces 8 are placed onto the roll 2, and simultaneously they cover
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the ends of the opening 1 b. The sealing surface 8 is connected to the cham-
ber I or to the beams 4 and 5. The sealing surface 8 is formed in a part,
whose lower side conforms to the shape of the roll 2 at the opening 1 b. (n
Fig. 1, said part is connected next to a cover 7 fixed on the end part 1c to
close the opening 1 b. The sealing surface 8 and the cover 7 may also consti-
tute a single integrated element.
The chamber 1 is coupled, for example by means of a connection 9, to a
separate suction system to provide a suction effect. The pressure difference
causes a liquid or air flow through the outer jacket 2a and simultaneously
through the fabric 3 or web into the chamber 1.
The beams 4 and 5 as well as the sealing surfaces 8 define a suction area,
i.e. the underpressure area, or the area subjected to negative pressure,
having a length Yl and a width Xl. The suction area is placed at the opening
1 b. Similarly, in the example of Fig. 3, the beams 4 and 5 and the carriages
11 define the suction area. The suction area may also be shorter than the
distance between the two sealing surfaces 8 or the carriage 11, because the
length Yl of the suction area is also confined to that area of the outer
jacket
2a of the roll 2 which is provided with holes, openings or gaps. More specifi-
cally, the underpressure area X1 can be defined as an area within which the
fabric 3 is in contact with the outer jacket 2a.
In the example of Fig. 2, the overpressure apparatus is installed as a pair to
the suction device of Fig. 1, on the opposite side of the web path. The over-
pressure apparatus comprises a box or a hood that is also called a pressure
box 19 in this description, wherein the web path is placed between the
pressure box and the roll 2. The height of the pressure box 19 in relation to
the web path can be adjusted by adjusting means, such as screw members
attached to the chamber 1. The pressure box 19 is placed at the location of
the underpressure area defined by the suction device, and the size of the
pressure box is selected so that the pressure is effective at least on the
underpressure area (Xl, Y1). The pressure box shown in Fig. 2 comprises
both an overpressure chamber and two overflow chambers, wherein the
pressure box extends within the entire shared width of the upper surface 1 a
and the outer jacket 2a, and beyond. The pressure box is coupled, for
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example by means of one or several connections 20, to a separate pressure
system by means of which a pressure effect and an air flow to the pressure
box is attained. The pressure box can be constructed in a variety of ways and
of various materials. The pressure box may be made for example of metal or
plastic.
In this example, the web path consists of a fabric 16, a web 17 being placed
on top of the same. In accordance with Fig. 6, the pressure box is sealed
against the fabric 16 by means of the air gap 14 located by the outer jacket
2a, and by means of an air gap 15 located at the outer edge of the pressure
box. The outer jacket 2a of the roll is sealed directly against the fabric 16.
The pressure box is installed at a distance from the suction device, and it is
supported by structures whose construction may vary. The pressure box is
also supported, for example, to structures on top of the web path, or struc-
tures placed on both sides of the web path.
In practice, it has been found that particularly the position of new wires in
relation to the suction device varies from several millimetres all the way to
several centimetres; that is, the wire "wanders" in the direction transverse
to
the travel direction of the web path. The variation of the web path may have a
negative effect on the quality of the web, particularly at the edge of the web
and the wire. For example, the suction device of Fig. 1 has an opening 1 b,
through which the suction is effective on the wire and the web which is
narrower than the wire. If the wire and the web move vigorously in the cross
direction, the ends of the opening 1 b are covered to a varying extent, which
will affect the negative pressure maintained inside the apparatus and makes
it more difficult to maintain a uniform suction. If the opening 1 b is not
sealed
under the wire, the suction becomes weaker.
According to one example, the web path, for example the fabric 3, moves the
sealing device which comprises parts placed at both edges of the fabric 3
and coupled to each other for a synchronized movement.
Figure 3 shows in more detail a suction device according to one embodiment
of the invention, equipped with a moving sealing device and an overpressure
apparatus. The principle of operation of the suction device of Fig. 3
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corresponds primarily to the operation of the suction device of Fig. 1.
Figure 3 shows one of two carriages 11 detached from a coupling 13 and
separated from a bearing part 10, for better illustration and to expose the
bearing part 10, and the pressure box 19 is also detached. As in Fig. 2, the
5 overpressure apparatus is installed as a pair to the suction device, com-
prising a pressure box 19 placed at the underpressure area defined by the
suction device. The pressure box 19 extends over the carriages 11 or the
end of the pressure box 19 is positioned at the carriages 11.
10 The bearing parts 10 close the ends of the openings 1 b, being placed at
the
edges of the beams 4 and 5, and they are connected e.g. on top of the end
parts 1 c. The bearing parts 10 are also placed as close as possible to the
ends of the roll 2. The shape of the upper surface of the bearing parts 10
corresponds to the joint shape of the upper surface I a and the outer jacket
2a of the roll 2. For the bearing, the upper surface of the bearing part 10 is
provided with a cut grooving 12, to which a liquid is supplied, for example
pressurized water, to carry the carriage 11 and to act as a bearing between
the bearing part 10 and the moving carriage 11, and also between the
carriage 11 and the beams 4, 5. The water also seals the gap between the
bearing part 10 and the carriage 11, so that no suction into the roll 2 takes
place via said gap, and variations of negative pressure are avoided. The
liquid is supplied, for example, via an inlet at the end of the bearing part
10.
From the inlet, a channel extends inside the bearing part to the grooving 12.
Figure 4 shows an example in which the suction device of Fig. 3 is used, but
the end of the pressure box 19 is located by the carriages 11. At the end of
the pressure box 19 there is a groove in which the carriage is positioned and
in which it is capable of moving back and forth in relation to the pressure
box.
The invention can also be implemented in such a manner that the pressure
box is supported on top of the carriages 11, wherein they move together. In
one example, two carriages are provided at the same end of the roll 2, the
first carriage moving with the fabric 3 and taking care of the sealing, and
the
second carriage supporting the pressure box. The second carriage is
stationary and is moved, if necessary, for example by actuators.
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The single carriage 11 can be utilized for sealing independently of the
pressure box, as shown in Fig. 3, or solely by moving the pressure box sup-
ported on top of the carriage 11, or jointly for both sealing and moving of
the
pressure box. The pressure box 19 can also be arranged to be movable in
another way, for example, for monitoring the transverse movement of the
carriage 11 or the web path. With reference to Fig. 3, the shape of the lower
surface of the moving carriage 11, in turn, corresponds to the joint shape of
the upper surface 1 a and the outer surface 2a of the roll 2 so that the
carriage 11 can be placed tightly on top of them. Because the shapes are the
same, the carriage 11 can glide on the bearing part 10 and the beams 4, 5. In
the example shown in Fig. 4, the carriage 11 has a substantially vertical sur-
face, against which the moving edge of the fabric 3 is sealed. When the fab-
ric 3 moves in the cross direction, it simultaneously also pushes aside the
carriage 11, wherein no gap can be formed between the fabric 3 and the
carriage 11. The end of the roll 2, as well as the gap between the roll 2 and
the bearing part 10, are placed underneath the carriage 11. The roll 2 is
wider than the fabric 3, so that the suction effect remains constant over the
entire width of the fabric 3, even if the fabric 3 moved in the lateral
direction.
The other carriage 11 on the other side of the fabric 3 is coupled to the
carriage 11 that the fabric 3 forces to move. Thanks to a coupling 13, the
carriages 11 follow each other's movements, wherein no gap is formed
between said second carriage 11 and the fabric 3 either. The coupling 13 is,
for example, mechanical, comprising two bars coupled to both sides of the
carriage 11 and to both carriages 11. The coupling 13 is placed so that it
will
not be hit by the fabric 3. For example, the coupling 13 may have a control
function, by means of which the distance between the carriages 11 can be
changed as desired. The coupling 13 can also be placed underneath the
beams 4 and 5 or inside channels formed therein. The pressure box coupled
on top of the two carriages 11 can function as a corresponding coupling.
The beams 4 and 5, the bearing parts 10 and the carriages 11 are made of,
for example, a plastic material having good sliding properties to reduce fric-
tion.
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According to another embodiment of the invention, an inclined sealing sur-
face 8 according to Fig. 1 can be placed next to the carriage 11, underneath
the fabric 3, the fabric 3 rising above the sealing surface 8, which is con-
nected to the carriage 11. The fabric 3 pushes and moves the carriage 11,
being pressed against the sealing surface 8. The carriages 11 slide on the
suction device, crosswise to the fabric 1, and the web path is placed there-
between.
The above-presented principles can also be applied when the upper surface
1 a is substantially flat throughout and it does not comprise a roll. Even in
such a case, the shape of the upper surface of the bearing part conforms to
the upper surface 1 a, and the lower surface of the carriage 11 conforms to
the shape of the upper surface of the bearing part and the upper surface 1 a.
The carriage 11 covers those openings, gaps or holes in the upper surface
1 a, which are not placed underneath the fabric 3.
Figure 4 shows an example in which the suction device of Fig. 3 is used, but
the end of the pressure box 19 is located by the carriages 11. The under-
pressure area restricted by the carriages 11 moves along with the web path
and remains inside the pressure box, and due to the arrangement the over-
pressure is always effective by the underpressure area.
Figure 5 shows an example in which the suction device of Fig. 2 is used, but
the end of the pressure box is supported on top of the covers 7.
Alternatively,
the pressure box is supported on top of the bearing part 10 or the end 1 c, in
a movable or stationary manner.
Fig. 6 shows in more detail an embodiment of the invention in a cross-
section, seen from the side of the web path. The figure also shows the web
path to illustrate the mutual placement of the web path and the overpressure
apparatus. The web path is elevated when it is positioned on top of the outer
jacket 2a of the roll and the elevated section substantially corresponds to
the
width Xl of the underpressure area, which is also positioned between the
beams 4 and 5. Fig. 6 also shows the pivot of the roll 21, and in this example
the overpressure apparatus and the suction device are positioned symmet-
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rically in relation to the plane Z that extends via the pivot 21 and
perpendicu-
larly to the web path.
The pressure box 19 comprises at least one overpressure chamber 22 which
is positioned at the underpressure area and extends transversely across the
web path. The cross section of the overpressure chamber has the form of a
narrowing slit or throat 23. The overpressure chamber 22 is open towards the
web path and it narrows down towards the web path. The opening angle of
the walls 26 restricting the overpressure chamber, i.e. the angular difference
in their mutual position is substantially the same or preferably larger than
that
of the sector 24 which has the pivot 21 as its tip and whose sides travel via
the narrowest point of the throat 23. At the same time the sector 24 defines
the overpressure area. The sector 24, in turn is narrower or smaller than the
sector 25 whose tip is formed by the pivot 21 and whose sides extend via
those points of the throat 23 in which the web path moves on top of the outer
jacket 2a and leaves from the same, thus determining the width Xl of the
underpressure area more precisely. The pressure of the overpressure cham-
ber 22 affects the web path within the area of the sector 24, but not outside
the same. Between the sectors 24 and 25, in the edge of the underpressure
area Xl there is a narrow area in which only the suction affects the web path.
In the embodiment of Fig. 6, the narrowest point of the throat 23 is in its
lower
part, and in the narrowest point the distance of the wall 26 from the web path
is in its smallest, thus forming an air gap 28 of 3 to 8 mm, preferably 5 mm.
The throat 23 is positioned between the walls 26. In the wide upper end of
the throat 23, in turn, there is a channel 29, for example a gap of 6 to16 mm,
preferably 10 mm. The gap must be larger than the air gap 28, preferably
twice as large. The height of the wall 26 is 50 to 70 mm. The width Xl of the
underpressure area is approximately 130 to 150 mm, when the size of the roll
of the suction device is approximately 170 to 180 mm. The largest width of
the overflow chamber 30 is thus approximately 130 to 150 mm.
The channel 29 is located for example between the upper edge of the wall 26
and the cover 27. The channel 29 may be implemented in other ways as well
in the upper part of the throat 23, for example by means of an opening or
perforation of corresponding size, via which the air and pressure of the pres-
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sure of the overpressure chamber 22 are capable of being discharged more
easily than through the gap 28. The discharging preferably takes place by the
side of the overpressure chamber 22, where, according to an embodiment,
there is an overflow chamber 30 evening out the air flow and the pressure.
The air gap 28 also leads to the overflow chamber 30.
In the example of Fig. 6 the throat 23 is constructed by means of two walls 26
extending crosswisely across the web path and inclining with respect to the
web path. The walls 26 are for example plate structures fastening for
example to the ends 32 and/or the cover 27 of the pressure box 19, which
cover is made for example of metal. The walls 26 forming the side walls of
the throat 23, are directed almost radially in relation to the outer jacket 2a
or
to the curved web path. In the presented example the walls 26 are
substantially perpendicular to the cover 27, which comprises inclinations 14
above the overflow chambers 30. In the presented embodiment the cover 27
is substantially parallel to the web path and horizontal above the over-
pressure chamber 22. Thanks to the inclinations 14 the pressure box 19 be-
comes lower in height towards the front and back edge of the pressure box,
wherein it can be more easily positioned closer to the other structures, such
as rolls. In the presented example the end walls 32 of the pressure box 19
are substantially vertical.
The overflow chamber 30 is positioned on top of the web path in an area lo-
cated outside the sector 24, extending in the example of Fig. 6 over the beam
4 and beam 5, in parallel to the web path. The overflow chamber 30 is re-
stricted by an air gap 15 of 3 to 8 mm, preferably 5 mm. The overflow cham-
ber 30 preferably has a widening shape, starting from the channel 29 or at its
vicinity, and it is at its largest between the air gaps 15 and 28 by the web
path. Thanks to the widening shape, the movement of the air coming through
the channel settles down rapidly and does not affect the web path. From the
overflow chamber 30 the air can also be sucked to the suction device through
the narrow edge area (the distance between the edges of sectors 24 and 25)
of the underpressure area Xl (distance between the outer edges of sectors
24 and 25), which is affected by the pressure of the overflow chamber 30,
which is preferably substantially 0 bar. The pressure of the overpressure
chamber 22, in turn, is approximately 0.3 bar, 0.7 bar at the highest. The
CA 02686721 2009-10-28
WO 2008/093000 PCT/F12008/050036
pressures can be monitored by means of meters 31. The width of the edge
area corresponds to the width of the air gap 28, or it is slightly larger, 5
to 10
mm. The aforementioned edge area belongs to the area X2 restricted by the
overflow chamber 30 and subjected to the.pressure of the overflow chamber
5 30. In this example the area X2 and the overpressure area are also located
in
parallel to each other and there is an air gap 28 in the transition point.
In the example shown in Fig. 6 the pressure box 19 is symmetrical in relation
to the underpressure area Xl (sector 25), but the placement of the walls 26
10 may also be asymmetrical and the size of the air gap 28 may vary, as well
as
the distance of the wall 26 from the edge of the underpressure area. The
reason may be, for example, that the air flows behave in different ways in the
opposite edges of the suction area, resulting from the fact that on one edge
the moving web path arrives in the suction area (inlet side) and on the other
it
15 leaves the suction area (outlet side). Preferably, the apparatus comprises
overflow chambers 30 on both sides of the overpressure chamber, as well as
on the inlet side and on the outlet side of the web path. The shapes of the
cover 27 and the inclinations 14 may vary, wherein the cross-sections of the
overflow chambers 30 may vary slightly from each other.
The shape of the walls 26 may also differ from the planar shape, but the
throat 23 substantially keeps its tapering or narrowing shape when moving
from the upper part to the lower part towards the web path. The air gaps 28
in the lower end of the throat 23 restrict the area subjected to the over-
pressure. The ends of the pressure box 19 may comprise a similar air gap,
but typically the overpressure area is restricted for example by a sealing
carriage 11 in accordance with Fig. 3, or a sealing surface 8 in accordance
with Fig. 1.
The invention is not limited solely to the above examples, but it may vary
within the scope of the appended claims.
