Note: Descriptions are shown in the official language in which they were submitted.
CA 02332677 2001-O1-31
DEVICE AND METHOD FOR VENTILATING AN OFFSET
POCKET SPACE IN A PAPERMAKING MACHINE
BACKGROUND
Papermaking is a sophisticated operation involving massive and very expensive
machines. These machines are increasingly running at higher speeds, meaning
that their overall efficiency must be very high, and in particular, the
efficiency of
their sub-components must also be very high. The papermaking process requires
that water be removed from the initial pulp fiber solution as the paper is
formed.
The pulp fiber solution, once in the drying section of a papermaking machine,
is
110 referred to as the paper web. The paper web is supported as it travels
across the
machine following a path during which moisture is progressively removed
therefrom. The support is provided by endless sheets of porous fabric, felts,
wires
or other water and gas permeable support means, all of which are generically
referred to as the « felt or felts » in the description and appended claims.
'I 5
The paper web travels from what is referred to as the wet end of the machine
to
the dry end thereof. In its path, the paper web runs over numerous heated
drying
cylinders where moisture is evacuated therefrom either by direct evaporation
or
transfer of moisture to the felts or to the surface of the drying cylinders. A
network
?0 of ventilator devices are used throughout the drying section in order to
inject
heated dry air at numerous locations and promote the removal of moisture from
the papermaking machine. At the end, the machine outputs the resulting paper,
which is then generally reeled to be shipped elsewhere.
;25 Papermaking machines can be built according to numerous possible
configurations. One configuration in particular is the twin-wire draw, where
the
papermaking machine comprises two superposed rows of axially-parallel and
horizontally disposed heated drying cylinders. The paper web runs in a
serpentine
or zigzag path where it defines mops by alternating between the two rows of
drying
.30 cylinders as it advances along the drying section. The paper web is being
CA 02332677 2001-O1-31
2
supported in most of its path with the assistance of the felts. There is
generally
one felt for each row of drying cylinders. Each felt presses the paper web on
a
portion of the surface of the drying cylinders of the corresponding row. Each
felt
also runs over a felt roll between each pair of adjacent drying cylinders of a
same
corresponding row. The felt rolls are located deep in the space between the
two
adjacent drying cylinders. This configuration allows to maintain the paper web
in
supporting contact with the felt as long as possible. The felt rolls
essentially
redirect a felt to the next drying cylinder of the same row.
It should be noted at this point that the terms « roll or rolls » and «
cylinder or
cylinders » are synonyms since both are elongated member with a circular cross-
section, the only distinctions in the present context being that the drying
cylinders
are generally much larger in diameter than the felt rolls and are heated by
appropriate means that are well known in the art. The segregated use of the
'15 terms in the text is only for the purpose of clarity. The « cylinder or
cylinders » are
sometimes referred to as « drum or drums » in other documents.
Traditional drying cylinders and felt roll arrangements feature the felt roll
being
positioned intermediate the axis of rotation of adjacent drying cylinders,
thus
;20 halfway between two adjacent drying cylinders. Inherent in these
arrangements is
the fact that the paper web is repetitively unsupported wherever the felt
separates
from the paper web to pass over a felt roll.
In order to increase the speed of papermaking machines, it was desirable to
;25 reduce the length of unsupported paper web because of the risks of rupture
thereof, particularly near the wet end where the paper web is weaker. One
solution to this problem was to move the axis of rotation of the felt rolls
backwards,
more particularly towards the wet end of the machine, as illustrated in FIG.
1. This
offset configuration has resulted in the felts and the paper web being in
contact
.30 longer, thus minimizing the length of unsupported paper web as it travels
from ane
drying cylinder to another.
CA 02332677 2001-O1-31
3
The offset configuration of the felt rolls has also resulted in creating what
is known
as offset pockets. In FIG. 1, the pocket spaces (12) are identified as hatched
areas. Each pocket space (12) is situated between three successive drying
cylinders (20) over which consecutively run a paper web (14). There are thus
many offset pockets (12) since there are many groups of three successive
drying
cylinders (20. A pocket space (12) may be roughly defined as the space limited
by
a felt (16) between two successive drying cylinders (20) of a same row, a
first draw
of paper web (14) from a first drying cylinder (20) to the next drying
cylinder (20) of
the other row, a second draw of paper web (14) between that second drying
cylinder (20) and a third successive drying cylinder (20) on the same row as
the
first one, and the free surface the second drying cylinder (20). The pocket
spaces
(12) are only open at each side of the machine. It should be noted that the
pocket
ventilators have been omitted from FIG. 1 for to simplify the drawing.
Unfortunately, ventilating offset pocket spaces is more difficult than in
symmetrical
pockets spaces found in traditional configurations. This results from the
reduction
of the length of felt which does not support the paper web on the side
upstream of
the felt roll. Since the felt is permeable to air and the paper web is not,
the
conventional dry heated air ventilators which were hitherto provided to
ventilate
the felt cannot be used the same way since air cannot be blown through the
felt
when it is supporting the paper web. Moreover, an offset pocket is often too
small
for receiving a ventilating device therein. All of this has resulted in
decreased air
flow into offset pocket spaces, thus a decrease in the efficiency of the
ventilation.
An example of a pocket ventilator previously known in the art is disclosed in
U.S.
Patent No. 5,074,278 to Turcotte and issued December 24, 1991. It illustrates
a
traditional symmetrical arrangement of pocket spaces. This patent is hereby
incorporated by reference.
CA 02332677 2001-O1-31
4
The geometry of a pocket and the natural air currents generated in the pocket
space by the moving paper web and felt, as well as the rotation of the drying
cylinders and the felt rolls, are key factors which increase the difficulty in
ventilating a pocket space. Air introduced in a pocket space has a natural
tendency to follow the movement of the felt and also tends to be trapped in a
cleft
defined by the felt and a downstream drying cylinder. This cleft, known as the
closing nip, is a zone of positive air pressure where air tends to flow
through the
felt to evacuate the pocket space. Furthermore, a zone of negative air
pressure is
created in a cleft defined where the paper web leaves the felt roll on its way
to the
'10 next drying cylinder. Some air evacuates the pocket space to satisfy the
negative
pressure created therein at the opening nip. However, these natural air
currents
do not provide an adequate ventilation of the pocket space as they do not
effectively sweep of the pocket space. As a result, air becomes entrapped
therein,
thereby increasing the humidity level within the pocket space and decreasing
the
'15 overall drying capacity of the papermaking machine.
SUMMARY
The main object of the present invention is to provide a solution to the above-
discussed ventilation problems so that offset pocket spaces of the drying
section
of a papermaking machine could be adequately ventilated.
;20
In summary, the invention may be defined as a device for ventilating an offset
pocket space located in a drying section of a papermaking machine by injecting
air
from at least one heated dry air supply. The offset pocket space is situated
between three axially parallel drying cylinders over which consecutively runs
a
.25 paper web, a first and a third of these cylinders being vertically spaced
from a
second of the cylinders. The paper web is pressed against the first and the
third
cylinder by a felt which further runs over a felt roll having a rotation axis
parallel to
that of the cylinders. The felt roll is disposed between the three cylinders
in an
offset position which is closer to the first cylinder than the third cylinder.
The offset
;30 pocket space being delimited by a first cleft defined where the felt and
the paper
CA 02332677 2001-O1-31
web separate, a first draw of the paper web from the first cleft to the second
cylinder, a free portion of the second cylinder, a second draw of the paper
web
from the second cylinder to a second cleft defined where the felt and the
paper
web rejoin, and the felt between the first cleft and the second cleft. The
device
5 comprises:
- a first air outlet where a first air stream is directed into a third cleft
defined where the felt and the felt roll join; and
- a second air outlet where a second air stream is directed in close
proximity of the felt at a location which is upstream the third cleft, the
second air stream being in a direction which is substantially opposite
the running direction of the felt.
In use, the first air stream flows through the felt and lifts the paper web
away from
the felt to reach the offset pocket space, and the second air stream creates
an air
curtain to lower the pressure in a zone between the first and the second
outlet so
as to maintain the paper web on the felt as long as possible.
The present invention may also be defined as a method for ventilating an
offset
pocket space located in a drying section of a papermaking machine. The method
comprises the steps of:
- directing a first air stream from a first outlet into a third cleft defined
where the felt and the felt roll join, the first air stream flowing through
the felt and lifting the paper web away from the felt to reach the offset
pocket space; and
- directing a second air stream from a second outlet in close proximity
of the felt, the second air stream being supplied at a location which is
upstream the third cleft and in a direction which is substantially
opposite the running direction of the felt so as to create an air curtain
which lowers the pressure in a zone between the first and the second
outlet.
CA 02332677 2001-O1-31
6
Similarly, the present invention may be defined as a the method comprising the
steps of:
- producing an air stream directed into a third cleft defined where the
felt and the felt roll join, the air stream flowing through the felt and
then between the paper web and the felt in order to ventilate the
offset pocket space; and
- establishing a zone of reduced air pressure in close proximity of the
felt at a location which is immediately upstream the third cleft so as to
maintain the paper web on the felt as long as possible.
The objects, features and advantages will be apparent from the following
detailed
description of a preferred embodiment thereof, which proceeds with reference
to
the accompanying figures.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a schematic side view of a drying section of a papermaking machine
as
found in the prior art, illustrating an example of a twin-draw arrangement and
the
location of offset pockets.
FIG. 2 is an enlarged side view of a device in accordance with a preferred
embodiment of the present invention.
FIG. 3 is a schematic view of a portion of a cylinder and of the device,
showing an
example of how the device is connected to the air duct network.
DETAILED DESCRIPTION
The device (10) is used for ventilating an offset pocket space (12) located in
a
drying section of a papermaking machine in which travels a paper web (14) to
be
dried. The device (10) essentially provides heated dry air coming thus air
ducts
from a conventional heated dry air supply inlet (11 ), as schematically
represented
CA 02332677 2001-O1-31
7
in FIG. 3. Air is first treated to remove an important amount of moisture and
pushed through the air duct network by blower. Proper ventilation of the
offset
pocket space (12) is achieve by injecting heated dry air in sufficient
quantity so as
to lower the humidity level and maximize water evaporation from the humid
components, particularly the paper web (14). It should be noted that the exact
arrangements and constructions of the air ducts or the air supply inlet (11 )
are well
known to a person skilled in the art and do not need to be described in
detail.
An offset pocket space (12) is situated between each group of three axially-
parallel
drying cylinders (20) over which consecutively runs the paper web (14). Among
these cylinders (20), the first and third ones are vertically spaced from the
second
one, as shown in FIG. 1. This is due to the fact that there are usually two
superposed rows of cylinders (20) in a papermaking machine, more particularly
a
lower row and an upper row. Since the paper web (14) follows a serpentine or
zigzag path across the drying section, there is a plurality of successive
pocket
spaces (12) in a papermaking machine. The first and third cylinders (20) of a
given group will be either on the lower or upper row, while the second
cylinder (20)
belongs to the opposite row.
:20 As already explained in the background section, and as shown in FIG. 2,
the paper
web (14) is pressed against the corresponding first and third cylinders (20)
by a
felt (16) which further runs over a felt roll (26) having a rotation axis
which is
parallel to that of the cylinders (20). The felt roll (26) is disposed between
the set
of three cylinders (20) in an offset position, more particularly in a position
which is
closer to the first cylinder (20) than the third cylinder (20), as best shown
in FIG. 1.
The offset pocket space (12) is delimited by a first cleft (30) defined where
the felt
(16) and the paper web (14) separate, a first draw (14a) of the paper web (14)
from the first cleft (30) to the second cylinder (20), a free portion of the
secand
.30 cylinder (20) where there is no paper web or felt, a second draw (14b) of
the paper
web (14) from the second cylinder (20) to a second cleft (32) defined where
the felt
CA 02332677 2001-O1-31
8
(16) and the paper web (14) rejoin, and the felt (16) between the first cleft
(30) and
the second cleft (32).
The device (10) preferably comprises an elongated hollow body (40) extending
parallel to and between the first and third cylinders (20). It extends
substantially
along the entire length of the cylinders (20), more particularly from one side
of the
machine to another. The body (40) encloses at least one plenum chamber with
which a first air outlet (50) and the second air outlet (52) are in fluid
communication. In the preferred embodiment illustrated in FIG. 2, the body
(40)
'10 comprises the three plenum chambers (42,43,44). The first (42) and second
(43)
plenum chambers are physically separated by an air diffuser, preferably
consisting
of a perforated plate (45). The main purpose of the perforated plate (45) is
to
distribute air uniformly across the second plenum chamber (43). Preferably,
the
second (43) and third (44) plenum chambers are separated from each other by at
115 least one perforated plate. For instance, there could be a set of three
superposed
perforated plates (46a,46b,46c), the plate (46b) in the middle being movable
with
reference to the others. This allows to adjust the flow of air by shifting the
position
of the movable plate (46b), thus moving its holes out of alignment with the
corresponding holes of the other plates (46a,46c).
~~0
The first air outlet (50) provides a first air stream which is directed into a
third cleft
(34}. The third cleft (34) is defined where the felt (16) and the felt roll
(26) join.
The second air outlet (52) provides a second air stream which is directed in
close
proximity of the felt (16) and at a location which is upstream the third cleft
(34).
2.5 The second air stream is in a direction which is substantially opposite
the running
direction (R) of the felt (16). The first air stream is preferably ejected
from the third
plenum chamber (44) and the second air stream is preferably ejected directly
from
the second plenum chamber (43).
3.0 In the illustrated embodiment, the first air outlet (50} comprises a
nozzle (50a)
projecting towards the third cleft (34). The nozzle (50a) longitudinally
extends
CA 02332677 2001-O1-31
9
substantially along the entire length of the body (40), thus along the entire
length
of the cylinders (20). It should be noted that the term « nozzle » covers the
case
where the device (10) has a set of consecutive nozzles or similar structures
that
are closely spaced to one another. As for the second air outlet (52), it is
preferably
in the form of a slot which is longitudinally extending on a side of the
device (10)
facing the first cylinder of the group of three cylinders (20). The slot (52)
extends
substantially along the entire length of the body (40). It should be noted
that the
term « slot » also covers the case where the device (10) has a set of
consecutive
slots or similar structures that are closely spaced to one another. Various
other
'10 means may also be added to control or adjust the air flow through the
first air
outlet (50), the second air outlet (52), or both.
In use, the above-described arrangement allows the first air stream to flow
into the
third cleft (34) and then through the felt (16). The air lifts the paper web
(14) away
from the felt (16) to reach the offset pocket space (12), creating an air
cushion
(30a) curving the path of the paper web (14). It also allows air to be
introduced
using the natural pumping effect of the felt (16) in addition to air coming
out the
first outlet (50). This has been found to be a very effective way to ventilate
the
offset pocket space (12). As for the second air stream, it creates an air
curtain to
lower the pressure in a zone (Z) between the first air outlet (50) and the
second air
outlet (52). The second air stream thus establishes a reduced air pressure in
the
zone (Z) at a location which is immediately upstream the third cleft (34).
This
allows to maintain the paper web (14) on the felt (16) as long as possible and
to
stabilize the paper web (14) so as to reduce the risks of fluttering.
~!5
Since the parts are rotated or moved at very high speeds, the natural air
pumping
effect may be a concern, particularly in the center region of the papermaking
machine. This pumping effect is due mainly to two phenomenon. The first is
that
air tends to follow the surface of the third cylinder of a group of three
cylinders
;.~0 (20). The second is that air is extracted from the pocket area into a
negative-
pressure cleft formed at a fourth cleft (36) defined when the felt (16) and
the felt
CA 02332677 2001-O1-31
roll (26) separate. In each case, the air' flow through the felt (16) between
the
fourth cleft (36) and the second cleft (32). A strong air pumping reduces the
pressure in the pocket space (12), in spite of the first air stream.
5 Whenever air pumping is too important, it is possible to restrict air flow
through the
draw of the felt (16) through which air is pumped. This is preferably achieve
by
creating an air flow restriction above a location on the third of the
cylinders (20)
over which the felt (16) runs and which is immediately downstream the second
cleft (32). Accordingly, and as best shown in FIG. 2, the air flow restriction
can be
10 realized by an elongated mechanical seal (60) extending from an outer
portion of
the body (40) towards a location on the third cylinder (20) where the felt
runs
thereon. The seal (60) does not necessarily extend over the entire length of
the
third cylinder (20) but needs at least to be place at the center position
where the
pumping effect is the strongest. The seal (60) has a tip edge (60a) in close
115 proximity of the felt (16) and is only separated from the surface of the
felt (16) by a
small gap. The leading portion (60b) of the seal (60) is preferably made of a
resilient material and preferably comprises means for adjusting the distance
between the tip edge (60a) and the felt (16). This means may take many forms,
as apparent to a person skilled in the art. One of them is to firmly hold the
leading
~'.0 portion (60b) of the seal (60) between two rigid strips (62,64) which can
be loosen
to shift the position of the leading portion (60b). Of course, other
arrangements
are possible, as apparent to a person skilled in the art.
Although a preferred embodiment of the invention has been described in detail
?5 herein and illustrated in the accompanying figures, it is to be understood
that the
invention is not limited to this precise embodiment and that various changes
and
modifications may be effected therein without departing from the scope or
spirit of
the present invention.
