U.S. patent application number 16/463736 was filed with the patent office on 2019-11-07 for a forming section for forming a fibrous web, a papermaking machine comprising a forming section and a method of forming a fibrou.
The applicant listed for this patent is VALMET AKTIEBOLAG. Invention is credited to Antti Poikolainen, Karl-Johan Tolfsson.
Application Number | 20190338465 16/463736 |
Document ID | / |
Family ID | 61157387 |
Filed Date | 2019-11-07 |
United States Patent
Application |
20190338465 |
Kind Code |
A1 |
Poikolainen; Antti ; et
al. |
November 7, 2019 |
A FORMING SECTION FOR FORMING A FIBROUS WEB, A PAPERMAKING MACHINE
COMPRISING A FORMING SECTION AND A METHOD OF FORMING A FIBROUS
WEB
Abstract
The invention relates to a forming section (2) for forming a
fibrous web (W). The forming section (2) comprises a first forming
fabric (3) arranged to run in a loop supported by guide elements
(4) and a second forming fabric (5) arranged to nm in a loop
supported by guide elements (4). The second forming fabric (5) is
arranged so in 5 relation to the first forming fabric (3) that the
two forming fabrics (3, 5) converge towards each other to form an
inlet gap (6) into which stock can be injected. A forming roll (7)
is arranged within the loop of the second forming fabric (5) to
guide the second forming fabric (5) into the inlet gap (6) and to
guide the first and the second forming fabric (3, 5) along a part
of their path which is common to both the first and the second 10
forming fabric (3, 5) and which begins at the inlet gap. The
forming roll (7) comprises a flexible tubular jacket (8) which is
arranged to run in a loop around an axis of rotation (A) that
extends in a direction perpendicular to the direction in which the
first and second forming fabric (3, 5) are arranged to run and the
forming roll further (7) comprises a support ledge (9) located
inside the loop of the flexible tubular jacket (8) 15 and extending
in a direction parallel to the axis of rotation (A) of the flexible
tubular jacket (8). The support ledge (9) can press the flexible
tubular jacket (8) in a direction outwards away from the axis of
rotation (A) such that, in the area in which the flexible tubular
jacket (8) is pressed outwards by the support ledge (9), the
flexible tubular jacket (8) is caused to follow a path with a
radius of curvature which is smaller than the 20 radius of
curvature of the flexible tubular jacket (8) outside this area. The
invention also relates to a method of forming a fibrous web.
Inventors: |
Poikolainen; Antti;
(Jyvaeskylae, FI) ; Tolfsson; Karl-Johan;
(Forshaga, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
VALMET AKTIEBOLAG |
Sundsvall |
|
SE |
|
|
Family ID: |
61157387 |
Appl. No.: |
16/463736 |
Filed: |
October 10, 2017 |
PCT Filed: |
October 10, 2017 |
PCT NO: |
PCT/SE2017/050992 |
371 Date: |
May 23, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D21F 11/14 20130101;
D21F 9/003 20130101; D21F 5/181 20130101 |
International
Class: |
D21F 9/00 20060101
D21F009/00; D21F 5/18 20060101 D21F005/18 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 28, 2016 |
SE |
1651555-3 |
Claims
1-17. (canceled)
18. A forming section (2) for forming a fibrous web (W), the
forming section (2) comprising: a first forming fabric (3)
configured to run in a loop supported by guide elements (4); a
second forming fabric (5) configured to run in a loop supported by
guide elements (4), the second forming fabric (5) being arranged in
relation to the first forming fabric (3) that the two forming
fabrics (3, 5) converge towards each other to form an inlet gap (6)
into which stock can be injected; and a forming roll (7) arranged
within the loop of the second forming fabric (5), the forming roll
(7) being configured to guide the second forming fabric (5) into
the inlet gap (6) and to guide the first and the second forming
fabric (3, 5) along a part of their path which is common to both
the first and the second forming fabric (3, 5) and which begins at
the inlet gap, wherein: the forming roll (7) comprises a flexible
tubular jacket (8) which is configured to run in a loop around an
axis of rotation (A) that extends in a direction perpendicular to
the direction in which the first and second forming fabric (3, 5)
are configured to run; and the forming roll further (7) comprises a
support ledge (9) located inside the loop of the flexible tubular
jacket (8) and extending in a direction parallel to the axis of
rotation (A) of the flexible tubular jacket (8) and which support
ledge (9) is configured to be capable of pressing the flexible
tubular jacket (8) in a direction outwards away from the axis of
rotation (A) of the flexible tubular jacket (8) in an area along
the loop in which the flexible tubular jacket (8) is configured to
run such that, in the area in which the flexible tubular jacket (8)
is pressed outwards by the support ledge (9), the flexible tubular
jacket (8) is caused to follow a path with a radius of curvature
which is smaller than the radius of curvature of the flexible
tubular jacket (8) outside the area in which the support ledge (9)
contacts the flexible tubular jacket (8).
19. A forming section (2) according to claim 18, wherein the
support ledge (9) is arranged in a fixed position such that the
amount to which the flexible tubular jacket (8) is pressed outwards
by the support ledge (9) is constant.
20. A forming section (2) according to claim 19, wherein the
support ledge (9) is directly supported by or integral with a
support beam (10) located inside the loop of the flexible tubular
jacket (8) and remains fixed in position in relation to the support
beam (10).
21. A forming section (2) according to claim 18, wherein at least a
part of the support ledge (9) is configured to be movable towards
or away from the axis of rotation (A) of the flexible tubular
jacket (8) such that the amount to which the flexible tubular
jacket (8) is pressed outwards by the support ledge can (9) be
varied.
22. A forming section (2) according to claim 21, wherein: the
support ledge (9) is supported by a support beam (10) located
inside the loop of the flexible tubular jacket (8); and at least
one actuator (11) is mounted on the support beam (10) and
configured to be capable of moving the support ledge (9) outwards
away from the axis of rotation (A) of the flexible tubular jacket
(8).
23. A forming section (2) according to claim 21, wherein: the
support ledge (9) is supported by a support beam (10); and the
support ledge (9) is flexible and/or elastic and comprises an inner
cavity (12) that can be supplied with a pressurized fluid such that
the support ledge (9) expands and at least a part of the support
ledge (9) is caused to move in a direction outwards away from the
axis of rotation (A) of the flexible tubular jacket (8).
24. A forming section (2) according to claim 18, wherein the
forming section (2) further comprises a headbox (14) arranged to
inject stock into the inlet gap (6) between the first and the
second forming fabric (3, 5).
25. A forming section (2) according to claim 18, wherein the
support ledge (9) has a top surface (15) facing the inner surface
(16) of the flexible tubular jacket (8) and which top surface (15)
is convex.
26. A forming section (2) according to claim 23 wherein, when the
inner cavity (12) is filled with pressurized fluid such that when
the support ledge (9) is in an expanded state, the support ledge
(9) has a top surface (15) facing the inner surface (16) of the
flexible tubular jacket (8) and which top surface (15) is
convex.
27. A forming section (2) according to claim 25 wherein the support
ledge (9) has a varying radius such that, as the flexible tubular
jacket (8) moves over the support ledge (9) from an end adjacent
the inlet gap (6) to a point further away from the inlet gap (6),
the radius of the support ledge (9) will decrease from a greater
radius to a smaller radius.
28. A forming section (2) according to claim 27, wherein the radius
of the forming roll (7) in areas not in contact with the support
ledge (9) is in the range of 500 mm-1600 mm and the smallest radius
of the support ledge (9) is in the range of 40 mm-100 mm.
29. A forming section (2) according to claim 28, wherein the
smallest radius of the support ledge (9) is in the range of 45-80
mm.
30. A forming section (2) according to claim 28, wherein the
smallest radius of the support ledge (9) is in the range of 50
mm-75 mm.
31. A forming section (2) according to claim 25, wherein: the
support ledge (9) has a top surface (15) that contacts the flexible
tubular jacket (8) and a height (H) defined by the distance from
the axis of rotation (A) of the flexible tubular jacket (8) to the
top surface (15) of the support ledge (9); the support ledge (9)
has, in the direction of rotation of the flexible tubular jacket
(8) away from the inlet gap (6), an upstream end (18) and a
downstream end (19) and the support ledge (9) is shaped such that,
in the direction from the upstream end (18) to the downstream end
(19), the height (H) of the support ledge (9) increases to a peak
point (20) where the height (H) of the support ledge (9) reaches
its highest value; and the peak point (20) of the support ledge (9)
is located closer to the downstream end (19) of the support ledge
(9) than to the upstream end (18).
32. A forming section (2) according to claim 18, wherein: the
flexible tubular jacket (8) is closed at its ends such that the
interior of the forming roll (7) is an enclosed space (24); and the
forming roll (7) is connected to a source of pressurized air or gas
(25) such that the flexible tubular jacket (8) can be inflated.
33. A forming section (2) according to claim 18, wherein: the part
of their respective loops that is common to both the first and the
second forming fabric (3, 5) extends from a the inlet gap (6) to an
end point (27) where the first forming fabric (3) is separated from
the second forming fabric (5); and the smallest radius of the
support ledge (9) is located at a point where the first and second
forming fabric (3, 5) follow a common path but which is closer to
the end point (27) than to the inlet gap (6).
34. A papermaking machine (1) comprising a forming section (2)
according to claim 18, wherein: the second forming fabric (5) is a
felt; the machine (1) comprises a Yankee drying cylinder (28); and
the second forming fabric (5) is configured to carry a newly formed
fibrous web (W) to the Yankee drying cylinder (28) and transfer the
fibrous web (W) to the Yankee drying cylinder (28) in a nip formed
between the Yankee drying cylinder (28) and a roll (29) placed
within the loop of the second forming fabric (5).
35. A method of forming a fibrous web, the method comprising the
steps of: injecting stock in an inlet gap (6) formed between a
first forming fabric (3) and a second forming fabric (5), each of
the first and second forming fabric (3, 5) being configured to run
in a loop supported by guide elements (4), and wherein a forming
roll (7) is located in the loop of the second forming fabric (5)
and the forming roll (7) is configured to guide the second forming
fabric (5) into the inlet gap (6) and to guide the first and the
second forming fabric (3, 5) along a part of their path which is
common to both the first and the second forming fabric (3, 5) and
which begins at the inlet gap (6); causing the forming fabrics (3,
5) to run in their loops such that the stock that is injected into
the inlet gap (6) passes between the first and the second forming
fabric (3, 5) as the forming fabrics (3, 5) are guided by the
forming roll (7) such that water is removed from the injected
stock, wherein the forming roll (7) comprises a flexible tubular
jacket (8) configured to run in a loop around an axis of rotation
(A) that extends in a direction perpendicular to the direction in
which the first and second forming fabric (3, 5) are configured to
run and in that the forming roll further (7) comprises a support
ledge (9) located inside the loop of the flexible tubular jacket
(8) and extending in a direction parallel to the axis of rotation
(A) of the flexible tubular jacket (8) and which support ledge (9)
is configured to be capable of pressing the flexible tubular jacket
(8) in a direction outwards away from the axis of rotation (A) of
the flexible tubular jacket (8) in an area along the loop in which
the flexible tubular jacket (8) is configured to run such that, in
the area in which the flexible tubular jacket (8) is pressed
outwards by the support ledge (9), the flexible tubular jacket (8)
is caused to follow a path with a radius of curvature which is
smaller than the radius of curvature of the flexible tubular jacket
(8) outside the area in which the support ledge (9) contacts the
flexible tubular jacket (8).
36. A method according to claim 35, wherein the method further
comprises the step of applying such a tension in the first forming
fabric (3) that the pressure applied to the stock reaches a highest
value in the range of 8 kPa-20 kPa as the first and second forming
fabric (3, 5) pass over the support ledge (9).
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a forming section for
forming a fibrous web, to a papermaking machine comprising a
forming section and to a method of forming a fibrous web.
BACKGROUND OF THE INVENTION
[0002] In a papermaking machine, the web is first formed in a
forming section. In machines for making tissue paper, the forming
section typically includes two forming fabrics running in loops
around guide rolls. The forming fabrics converge to a gap in which
stock is injected by a headbox. Inside one of the forming fabrics,
a forming roll is located. The two forming fabrics will run
together over a part of the circumference of the forming roll as
water is squeezed out of the injected stock that is beginning to
form into a fibrous web. The dewatering achieved in the forming
section is normally not so high that the web is ready for pressing.
To increase the dry solids content of the web, it has been
suggested that suction rolls can be used. For example, the forming
roll itself may be a suction roll. However, suction rolls require
much energy and it would be an advantage if a higher dry solids
content could be achieved without using a suction roll. Therefore,
it is an object of the present invention to provide a forming
section that can achieve a high degree of dewatering without using
a suction roll.
DISCLOSURE OF THE INVENTION
[0003] The present invention relates to a forming section for
forming a fibrous web. The forming section comprises a first
forming fabric arranged to run in a loop supported by guide
elements and a second forming fabric which is also arranged to run
in a loop supported by guide elements. The second forming fabric is
so arranged in relation to the first forming fabric that the two
forming fabrics converge towards each other to form an inlet gap
into which stock can be injected. A forming roll is arranged within
the loop of the second forming fabric. The forming roll is arranged
to guide the second forming fabric into the inlet gap and to guide
the first and the second forming fabric along a part of their path
which is common to both the first and the second forming fabric and
which begins at the inlet gap. According to the invention, the
forming roll comprises a flexible tubular jacket arranged to run in
a loop around an axis of rotation that extends in a direction
perpendicular to the direction in which the first and second
forming fabric are arranged to run. The forming roll further
comprises a support ledge that is located inside the loop of the
flexible tubular jacket. The support ledge extends in a direction
parallel to the axis of rotation of the flexible tubular jacket.
The support ledge is arranged to be capable of pressing the
flexible tubular jacket in a direction outwards away from the axis
of rotation of the flexible tubular jacket in an area along the
loop in which the flexible tubular jacket is arranged to run such
that, in the area in which the flexible tubular jacket is pressed
outwards by the support ledge, the flexible tubular jacket is
caused to follow a path with a radius of curvature which is smaller
than the radius of curvature of the flexible tubular jacket outside
the area in which the support ledge contacts the flexible tubular
jacket.
[0004] In embodiments of the invention, the support ledge is
arranged in a fixed position such that the amount to which the
flexible tubular jacket is pressed outwards by the support ledge is
constant. For example, the support ledge may be directly supported
by or integral with a support beam located inside the loop of the
flexible tubular jacket and remain fixed in position in relation to
the support beam.
[0005] In other embodiments of the invention, at least a part of
the support ledge may be arranged to be movable towards or away
from the axis of rotation of the flexible tubular jacket such that
the amount to which the flexible tubular jacket is pressed outwards
by the support ledge can be varied.
[0006] In embodiments of the invention, the support ledge is
supported by a support beam located inside the loop of the flexible
tubular jacket and the at least one actuator may be mounted on the
support beam and arranged to be capable of moving the support ledge
outwards away from the axis of rotation of the flexible tubular
jacket.
[0007] In embodiments of the invention, the support ledge has a top
surface facing the inner surface of the flexible tubular jacket
which top surface is convex.
[0008] In some embodiments of the invention, the support ledge can
be supported by a support beam and wherein the support ledge can be
flexible and/or elastic and comprise an inner cavity that can be
supplied with a pressurized fluid such that the support ledge
expands and at least a part of the support ledge is caused to move
in a direction outwards away from the axis of rotation of the
flexible tubular jacket. In such embodiments, the support ledge may
advantageously, but not necessarily, be designed such that, when
the inner cavity is filled with pressurized fluid such that when
the support ledge is in an expanded state, the support ledge has a
top surface facing the inner surface of the flexible tubular jacket
which top surface is convex.
[0009] It should be understood that the support ledge may also be
made of a substantially massive block (without an inner cavity)
that is made of an elastic material such as rubber or a material
with properties comparable to rubber.
[0010] The forming section may advantageously comprise a headbox
arranged to inject stock into the inlet gap between the first and
the second forming fabric. However, the inventive forming section
can be sold to a paper mill without a headbox. This may be the case
when, for example, the inventive forming section is sold as a part
of a rebuild project to a paper mill that already has a head
box.
[0011] Instead of a support ledge of a flexible material, the
support ledge may be a rigid body of a material such as steel,
bronze, aluminum or some other metallic material. The support ledge
may conceivably also be formed of some other material such as glass
or a ceramic material. It could also be made of a rigid or
substantially rigid polymeric material. Both when the support ledge
is made of a flexible and/or elastic material and when the support
ledge is made of a rigid material, the support ledge can be
designed such that it has a varying radius such that, as the
flexible tubular jacket moves over the support ledge from an end
adjacent the inlet gap to a point further away from the inlet gap,
the radius of the support ledge will decrease from a greater radius
to a smaller radius.
[0012] In advantageous embodiments of the invention, the radius of
the forming roll in areas not in contact with the support ledge is
in the range of 500 mm-1600 mm and the smallest radius of the
support ledge is in the range of 40 mm-100 mm, preferably in the
range of 45-80 mm and even more preferred in the range of 50 mm-75
mm.
[0013] As should be clear from the above description, the support
ledge has a top surface that contacts the flexible tubular jacket.
The support ledge has a height that can be defined by the distance
from the axis of rotation of the flexible tubular jacket to the top
surface of the support ledge. It should be understood that the
support ledge has, in the direction of rotation of the flexible
tubular jacket away from the inlet gap, an upstream end and a
downstream end. Preferably, the support ledge is shaped such that,
in the direction from the upstream end to the downstream end, the
height of the support ledge increases to a peak point where the
height of the support ledge reaches its highest value and wherein
the peak point of the support ledge is located closer to the
downstream end of the support ledge than to the upstream end.
[0014] In preferred embodiments of the invention, the flexible
tubular jacket is closed at its ends such that the interior of the
forming roll is an enclosed space. The forming roll may then be
connected to a source of pressurized air or gas such that the
flexible tubular jacket can be inflated. During operation, the
forming roll can then be inflated by pressurized air such that the
flexible tubular jacket can retain its shape.
[0015] In all embodiments of the invention, the part of their
respective loops that is common to both the first and the second
forming fabric extends from a the inlet gap to an end point where
the first forming fabric is separated from the second forming
fabric. In advantageous embodiments of the invention, the support
ledge is located at a point where the first and second fabric
follow a common path. Preferably, the support ledge is in its
entirety located closer to the end point than to the inlet gap.
Preferably, at least the smallest radius of the ledge is located at
a point where the first and second forming fabrics follow a common
path but which is closer to the end point than to the inlet
gap.
[0016] The invention also relates to a papermaking machine that
comprises the inventive forming section. In embodiments of the
inventive machine, the second forming fabric is a felt and the
machine may comprise a Yankee drying cylinder. In such embodiments,
the second forming fabric, i.e. the felt, is arranged to carry a
newly formed fibrous web to the Yankee drying cylinder and transfer
the fibrous web to the Yankee drying cylinder in a nip formed
between the Yankee drying cylinder and a roll placed within the
loop of the second forming fabric.
[0017] The invention also relates to a method of forming a fibrous
web. The inventive method comprises the step of injecting stock in
an inlet gap formed between a first forming fabric and a second
forming fabric. Each of the first and second forming fabric is
arranged to run in a loop supported by guide elements and wherein a
forming roll is located in the loop of the second forming fabric.
The forming roll is arranged to guide the second forming fabric
into the inlet gap and to guide the first and the second forming
fabric along a part of their path which is common to both the first
and the second forming fabric and which begins at the inlet gap.
The inventive method further comprises the step of causing the
forming fabrics to run in their loops such that the stock that is
injected into the inlet gap passes between the first and the second
forming fabric as the forming fabrics are guided by the forming
roll such that water is removed from the injected stock. According
to the invention, the forming roll comprises a flexible tubular
jacket arranged to run in a loop around an axis of rotation that
extends in a direction perpendicular to the direction in which the
first and second forming fabric are arranged to run. The forming
roll further comprises a support ledge that is located inside the
loop of the flexible tubular jacket and extends in a direction
parallel to the axis of rotation of the flexible tubular jacket.
The support ledge is arranged to be capable of pressing the
flexible tubular jacket in a direction outwards away from the axis
of rotation of the flexible tubular jacket in an area along the
loop in which the flexible tubular jacket is arranged to run such
that, in the area in which the flexible tubular jacket is pressed
outwards by the support ledge, the flexible tubular jacket is
caused to follow a path with a radius of curvature which is smaller
than the radius of curvature of the flexible tubular jacket outside
the area in which the support ledge contacts the flexible tubular
jacket.
[0018] In advantageous embodiments of the invention, the method
further comprises applying such a tension in the first forming
fabric that the pressure applied to the stock reaches a highest
value in the range of 8 kPa-20 kPa as the first and second forming
fabric pass over the support ledge.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic representation of a papermaking
machine that can make use of the inventive forming section.
[0020] FIG. 2 is a schematic representation of the inventive
forming section.
[0021] FIG. 3 is a Figure that shows, in greater detail, a possible
embodiment of some components of the forming section of FIG. 2.
[0022] FIG. 4 shows a possible embodiment of a support ledge
mounted on a support beam.
[0023] FIG. 5 is a figure similar to FIG. 4 but showing a possible
other embodiment of a support ledge.
[0024] FIG. 6 shows the support ledge of FIG. 5 mounted inside a
flexible tubular jacket.
[0025] FIG. 7 is a schematic representation of the varying pressure
level along the common path of the two forming fabrics.
[0026] FIG. 8 is a schematic representation in cross section of a
forming roll for the inventive forming section.
[0027] FIG. 9 is a figure similar to FIG. 6 but intended to
illustrate another aspect of the invention.
[0028] FIG. 10 is a schematic representation of another embodiment
in a state which is inactive.
[0029] FIG. 11 is a representation of the same embodiment as in
FIG. 10 but in an active state.
DETAILED DESCRIPTION OF THE INVENTION
[0030] With reference to FIG. 1, a machine 1 for making a fibrous
web W is shown. The machine of FIG. 1 is in particular suited for
making a tissue paper web W that may have a dry basis weight
(grammage) in the range from 10 g/m.sup.2 up to 50 g/m.sup.2 or 12
g/m.sup.2-40 g/m.sup.2. In many cases, the basis weight may be in
the range of 15 g/m.sup.2-25 g/m.sup.2. Tissue webs produced by
such a machine may be used for such purposes as, for example,
kitchen towel, bathroom tissue, facial tissue or table napkins. The
machine 1 shown in FIG. 1 has a Yankee drying cylinder 28 which is
preferably (but not necessarily) provided with a Yankee drying hood
30. The Yankee drying cylinder 28 may be connected to a source of
hot steam (not shown) that is arranged to supply hot steam to the
interior of the Yankee drying cylinder 28 such that the Yankee
drying cylinder 28 is heated. Thereby, a fibrous web W that travels
over the outer surface of the Yankee drying cylinder 28 can be
heated to such an extent that water in the fibrous web W is
evaporated. The Yankee drying cylinder is arranged to be rotatable
and in operation, it will rotate in the direction indicated by
arrow R in FIG. 1. A doctor 31 is arranged to crepe the dried
fibrous web from the outer surface of the Yankee drying cylinder
28. The Yankee drying cylinder 1 may be, for example, a Yankee
drying cylinder of cast iron but it could also be a welded Yankee
drying cylinder. For example, it may be a Yankee drying cylinder as
disclosed in U.S. Pat. No. 9,206,549 or U.S. Pat. No. 8,438,752.
The Yankee drying hood 30 may be of any known type and it may be,
for example, such a Yankee drying hood as disclosed in EP 2963176
A1.
[0031] Before the fibrous web W can be dried on the outer surface
of the Yankee drying cylinder 28, it must be formed. The machine 1
of FIG. 1 is provided with a forming section 2 that comprises a
first forming fabric 3 that is arranged to run in a loop around
guide elements 4. The guide elements 4 are suitably guide rolls
that are rotatably journalled. When the forming section 2 is
operating, the first fabric 3 will run in the direction indicated
by the arrows C such that, in FIG. 1, the first fabric 3 is
circulating in its loop in the "clockwise" direction. The forming
section 2 also comprises a second forming fabric 5 which is also
arranged to run in a loop supported by guide elements 4 that may
suitably be guide rolls 4 that are rotatably journalled. When the
forming section 2 is operating, the second forming fabric 5 is
running in the direction indicated by the arrows B such that, in
FIG. 1, the second forming fabric 5 is circulating in its loop in
the "counter-clockwise" direction. The second forming fabric 5 is
so arranged in relation to the first forming fabric 3 that the two
forming fabrics 3, 5 converge towards each other to form an inlet
gap 6 into which stock can be injected. The stock can be injected
by a headbox 14. The headbox 14 may be of any type suitable for
tissue making. For example, it may be a head box as disclosed in
U.S. Pat. No. 7,588,663, in U.S. Pat. No. 6,030,500 or in U.S. Pat.
No. 5,560,807. However, the skilled person is aware of many
commercially available headboxes that could all be suitable for the
present invention.
[0032] The forming section 2 further comprises a forming roll 7.
The forming roll 7 is arranged within the loop of the second
forming fabric 5 and the forming roll 7 is arranged to guide the
second forming fabric 5 into the inlet gap 6. The forming roll 7 is
also arranged to guide the first and the second forming fabric 3, 5
along a part of their path which is common to both the first and
the second forming fabric 3, 5 and which begins at the inlet
gap.
[0033] It should be understood that the inventive forming section 2
may be used in a machine as shown in FIG. 1 and that the inventive
forming section fits the general description given above. However,
it should be understood that the inventive forming section could
also be used in machine layouts that differ from the layout shown
in FIG. 1. For example, the inventive forming section 2 may be used
in a machine using through air drying (TAD) in which case a Yankee
drying cylinder 28 may not be present (although a TAD drying
arrangement can also be used in combination with a Yankee drying
cylinder). The inventive forming section may also be delivered
without a headbox as a part of a rebuild of a machine that already
has a headbox.
[0034] As the forming fabrics 3, 5 pass over the forming roll in
that part of their loops that is common to both fabrics, water will
be squeezed out of the stock that has been injected between the
forming fabrics 3, 5 such that a fibrous web begins to form. The
stock is squeezed or pressed between the two forming fabrics 3, 5
and water will leave the stock through the first forming fabric 3.
The stock is dewatered by the pressure that the stock is subjected
to as it travels between the forming fabrics 3, 5. The centrifugal
force also assists in throwing the water out through the first
forming fabric 3 since the forming fabrics 3, 4 travel over the
curved surface of the forming roll which has a shape that is
substantially circular cylindrical. The first forming fabric 3 may
advantageously be a fabric with a high permeability to water. In
particular, the first forming fabric 3 may be a foraminous wire
which does not absorb water. The second forming fabric 5 may also
be a wire but it can preferably be a water-absorbing felt which is
less permeable than the first forming fabric 3. In this way, it
will be easier for the water in the stock to pass through the first
forming fabric 3.
[0035] The amount of water that is squeezed or pressed out of the
stock as the stock travels between the forming fabrics 3, 5 in that
part of their respective paths that is common to both forming
fabrics depends to a large extent of the pressure to which the
stock is subjected. The pressure to which the stock is subjected
can be calculated as P=T/R where P is the pressure to which the
stock is subjected, T is the tension in the first forming fabric 3
and R is the radius of the forming roll 7. In theory, it would be
possible to increase the pressure simply by using a small forming
roll with a correspondingly small radius. However, experience has
showed that the draining zone, i.e. the part where the stock
travels between two forming fabrics 3, 5, needs to have a certain
length. Therefore, a forming section with a forming roll that is
too small would be insufficient. Likewise, the tension in the
forming fabrics 3, 5 can be increased but there are technical
problems also with such a solution, for example the amount of
tension to which the forming fabrics 3, 5 can be subjected.
Therefore, it is difficult to achieve a dry solids content during
forming that is much higher than about 12%. With such a low dry
solids content, it is normally not possible to subject the fibrous
web to pressing since the web would then risk crushing. Therefore,
in order to increase web dryness before pressing, it has been
suggested that a suction roll can be placed in the loop of the
second forming fabric which suction roll can act through the second
forming fabric 5 at a point after the first and second forming
fabrics have been separated from each other. An example of such a
solution is disclosed in WO 2010/033072 and FIG. 1 of that
publication shows a suction roll 25 placed inside the loop of the
forming fabric that carries a newly formed web to a press. It has
also been suggested that the forming roll itself may be a suction
roll and an example of such an arrangement is disclosed in U.S.
Pat. No. 6,821,391 in which FIG. 2 shows a forming section with a
forming roll 18 that is a suction roll with a suction zone 38.
However, suction rolls require much energy for their operation
which of course also costs money. In addition, suction rolls make
noise. Therefore, it is desirable to find a solution that can give
a higher dry solids content during forming even when a suction roll
is not used. The present invention offers a solution to this
technical problem.
[0036] The inventive forming section will now be explained in
greater detail with reference to FIG. 2 and FIG. 3.
[0037] In FIG. 2, it can be seen how the forming roll 7 has a shell
8. The shell 8 is a flexible tubular jacket that may also be termed
"a sleeve". The flexible tubular jacket 8 or sleeve may
advantageously be made of a polyurethane or a material that
partially comprises polyurethane or has material properties similar
to those of polyurethane. The flexible tubular jacket 8 is arranged
to run in a loop around an axis of rotation A. In other words, the
flexible tubular jacket 8 is arranged to rotate. It should be
understood that, in FIG. 2, the flexible tubular jacket (the
sleeve) will be rotating in the direction indicated by the arrow R.
It should likewise be understood that, just as in FIG. 1, the first
forming fabric 3 moves in the direction indicated by the arrows C
and the second forming fabric 5 moves in the direction indicated by
the arrows B. It should be understood that the axis of rotation A
for the flexible tubular jacket 8 extends in a direction that is
perpendicular to the direction in which the first and second
forming fabrics 3, 5 are arranged to run, i.e. it extends in the
cross machine direction, of the forming section. It should be
understood that, in FIG. 2, the flexible tubular jacket will rotate
in the direction of arrow R when the forming section is operating.
The actual thickness of the belt may be selected while taking the
choice of material into consideration and factors such as machine
speed, machine width and other factors. However, in many realistic
embodiments, the flexible tubular jacket may have a thickness in
the range of 2-7 mm. For example, it may have a thickness that is 3
mm, 4 mm or 5 mm. The flexible tubular jacket 8 may also comprise
several layers of different materials. As can be seen in FIG. 7,
the forming roll further comprises a support ledge 9 that is
located inside the loop of the flexible tubular jacket 8 and
extends in a direction parallel to the axis of rotation A of the
flexible tubular jacket 8. Of course, the flexible tubular jacket 8
itself extends in the same direction. The support ledge 9 is
arranged to be capable of pressing the flexible tubular jacket 8 in
a direction outwards away from the axis of rotation A of the
flexible tubular jacket 8 in an area along the loop in which the
flexible tubular jacket 8 is arranged to run. This has the result
that, in the area in which the flexible tubular jacket 8 is pressed
outwards by the support ledge 9, the flexible tubular jacket 8 is
caused to follow a path with a radius of curvature which is smaller
than the radius of curvature of the flexible tubular jacket 8
outside the area in which the support ledge 9 contacts the flexible
tubular jacket 8.
[0038] In the embodiment of FIG. 2, the support ledge 9 is
supported by a support beam 10 to which the support ledge is
directly or indirectly fastened. The support beam 10 may be a
welded box beam but other sorts of support beams could also be
used, for example a support beam of cast iron.
[0039] The flexible tubular jacket 8 is preferably impermeable to
water but embodiments are conceivable in which the flexible tubular
jacket is permeable to water. If the flexible tubular jacket 8 is
impermeable to water, which it preferably is, this assists in
making the water in the stock pass out through the first forming
fabric 3.
[0040] From the above description, those skilled in the art to
which the invention pertains will now understand that the forming
roll 7 with the flexible tubular jacket 8 is substantially similar
to a shoe press unit such as a shoe press roll. Such units are sold
commercially under such trade names as SymBelt.TM. shoe press or
NipcoFlex shoe press and have been described in many patent
publications, for example in U.S. Pat. No. 7,387,710 or U.S. Pat.
No. 5,662,777. The support ledge 9 may alternatively be called
"support body" or "elongate support body". The support ledge 9
could also equally well be termed "shoe" since it is placed in the
position where a shoe would be placed in a shoe press unit.
However, while the support ledge 9 of the present invention is used
in connection with dewatering while a certain pressure is applied
as the forming fabrics 3, 5 pass over the support ledge, the
purpose of the support ledge 9 differs in some ways from that of a
shoe in a shoe press as will be explained in the following.
[0041] Since the support ledge 9 is capable of pressing the
flexible tubular jacket 8 outwards, it can achieve the effect that,
over a part of the circumference of the flexible tubular jacket 8,
the radius becomes smaller. Over that part of the circumference of
the flexible tubular jacket 8, the pressure to which the stock is
subjected will rise and have a peak that it would otherwise not
have. The support ledge 9 is arranged to or capable of pressing the
flexible tubular jacket 8 out from the path it follows at those
parts of its circumference where it does not pass over the support
ledge 9. As the support ledge 9 does this, it forces the flexible
tubular jacket 8 and the forming fabrics 3, 5 to follow a path
where the radius over which the fabrics 3, 5 pass is actually
smaller than what is the case at other points along the
circumference of the flexible tubular jacket. As a result, the
pressure to which the stock is subjected increases as the forming
fabrics 3, 5 pass over that part of the forming roll 7 where the
support ledge 9 is acting.
[0042] With reference to FIG. 3, the first forming fabric 3 and the
second forming fabric 5 are caused to run together around the
forming roll 7. Initially, they follow a curve defined by a first
radius R.sub.1 of the forming roll 7. The radius R.sub.1 may be
understood as the radius from the axis of rotation A of the
flexible tubular jacket 8. As the forming fabrics 3, 5 pass over
the support ledge 9, they will be forced to follow a curve with a
radius R.sub.2 which is defined by the shape of the support ledge
9. The radius R.sub.2 is smaller than the radius R.sub.1 and the
pressure will thus increase such that the dewatering is intensified
as the forming fabrics pass over the support ledge 9. It should be
understood that the radius of the support ledge 9 may vary in the
machine direction from the upstream end of the support ledge 9 to
the downstream end of the support ledge 9.
[0043] In embodiments of the invention, the support ledge 9 can be
arranged in a fixed position such that the amount to which the
flexible tubular jacket 8 is pressed outwards by the support ledge
9 is constant. For example, the support ledge 9 may be directly
supported by or integral with a support beam 10 located inside the
loop of the flexible tubular jacket 8 and remain fixed in position
in relation to the support beam 10.
[0044] Instead of a support ledge 9 that is held in a fixed
position, it could be so that at least a part of the support ledge
9 is arranged to be movable towards or away from the axis of
rotation A of the flexible tubular jacket 8 such that the amount to
which the flexible tubular jacket 8 is pressed outwards by the
support ledge can 9 be varied. Possible embodiments of such an
arrangement will now be explained with reference to FIG. 4-6. In
FIG. 4, a support ledge 9 is shown that is supported by a support
beam 10. In FIG. 4, two actuators 11 are shown and the actuators 11
may be hydraulic cylinders as is known from shoe press technology.
The actuators 11 are supported by and fixed/secured to the support
beam 10 and the actuators 11 are arranged to be capable of acting
on the support ledge 9 to press it outwards and thereby also press
the flexible tubular jacket 8 outwards. It should be understood
that the two actuators 11 that are shown in FIG. 4 may represent
two rows of actuators 11 that extend in the cross machine direction
(see also FIG. 8).
[0045] FIG. 5 and FIG. 6 show an arrangement in which only one
actuator 11 can be seen in the figures but it should be understood
that this single actuator 11 may represent a row of actuators that
extend in the cross-machine direction (see also FIG. 8).
[0046] However, it should be understood that the actuator 11 of
FIG. 5 and FIG. 6 may be formed as a single actuator extending in
the cross-machine direction (the CD direction) which may even be
integral with the support ledge 9. Such a design of an actuator is
known from, for example, U.S. Pat. No. 5,223,100 that relates to a
shoe press but a similar arrangement may be used also for the
forming roll according to the present invention. If several
actuators 11 are used, the arrangement and design of the actuators
could be similar to or identical to any known arrangement of
actuators for a shoe in a shoe press. For example, the actuator 11
or actuators 11 could be designed and arranged as disclosed in U.S.
Pat. Nos. 5,662,777, 6,083,352, 7,387,710, 4,917,768 or European
patent No. 2808442. However, other actuator arrangements for shoe
presses are also known from the patent literature and from what is
commercially available on the market and those skilled in the art
of paper making can select among known solutions for actuators.
[0047] It will now be understood that the at least one actuator 11
is arranged to be capable of moving the support ledge 9 outwards
away from the axis of rotation A of the flexible tubular jacket 8.
By having the support ledge 9 supported by/carried by the support
beam 10 which is located inside the loop of the flexible tubular
jacket 8 and at least one actuator 11 mounted on the support beam
10, the technical effect is achieved that the at least one actuator
11 can vary the amount to which the flexible tubular jacket (the
sleeve) is pressed outwards from its otherwise circular cylindrical
path.
[0048] With continued reference to FIGS. 4-6, it can be seen that
the support ledge 9 has a top surface 15 which is facing the inner
surface 16 of the flexible tubular jacket 8 (see FIG. 6) and
contacts the inner surface 16 of the flexible tubular jacket 8, at
least when the inventive forming section 2 is operating. In the
embodiment of FIG. 4, the top surface 15 is convex and the top
surface 15 of the support ledge 9 (i.e. the surface that faces the
inner surface 16 of the flexible tubular jacket 8) has a varying
radius such that, as the flexible tubular jacket 8 moves over the
support ledge 9 from an end adjacent the inlet gap 6 to a point
further away from the inlet gap 6, the radius of the support ledge
9 will decrease from a greater radius to a smaller radius. In FIG.
4, it can be seen that, at one end of the support ledge 9, the
support ledge 9 (or the top surface 15 of the support ledge 9) has
a radius R.sub.3. The top surface 15 has a peak point 20, i.e. the
highest point on the top surface 15 that is at the greatest
distance from the axis of rotation A of the flexible tubular jacket
8. At the peak point 20, the radius R.sub.4 of the support ledge 9
(i.e. the radius of its top surface 15) is smaller such that
R.sub.4<R.sub.3. The radius of the support ledge 9 will thus
decrease from a higher value to a smaller value which is reached
when the amount to which the flexible tubular jacket 8 is pressed
outwards from its otherwise circular path reaches its maximum. This
will lead to a peak in the pressure to which the stock between the
forming fabrics 3, 5 is subjected and the dewatering will
increase.
[0049] Reference will now be made only to FIG. 5 and FIG. 6. In the
embodiment of FIG. 5 and FIG. 6, the support ledge 9 is designed
such that, in the direction of rotation of the flexible tubular
jacket 8 (see FIG. 6 in which the arrow R indicates the direction
of rotation of the flexible tubular jacket 8), the top surface 15
of the support ledge 9 increases in height to a peak point 20 that
is closer to the downstream end 19 of the support ledge 9 than to
the upstream end 18. In this way, the pressure peak is not reached
until the end of the area of the support ledge 9 and the pressure
is built up gradually until it goes down after the peak point 20.
By this design of the support ledge 9, a sudden pressure pulse can
be avoided which might otherwise have damaged the fibrous web that
is forming.
[0050] In many realistic embodiments of the invention, the radius
of the forming roll 7 in areas not in contact with the support
ledge 9 is in the range of 500 mm-1600 mm. The smallest radius of
the support ledge 9 may then be in the range of 40 mm-100 mm,
preferably in the range of 45-80 mm and even more preferred in the
range of 50 mm-75 mm. The amount to which the support ledge 9 is
pressed outwards must then be sufficient for achieving the effect
that, as the forming fabrics 3, 5 pass over the area of the support
ledge 9, the forming fabrics must actually conform to and follow
the smaller radius of the top surface 15 of the support ledge 9
such that the fabrics 3, 5 are forced to follow a path with a
radius that is smaller than that of the forming roll 7 in areas
where the flexible tubular jacket 8 is not in contact with the
support ledge 9. In many realistic embodiments, this means that the
support ledge 9 will press the flexible tubular jacket out from its
otherwise circular cylindrical path by distance in the range of 2
mm-20 mm but other values are conceivable and the exact amount may
vary depending on the diameter of the forming roll and the length
of the support ledge in the circumferential direction of the
flexible tubular jacket 8.
[0051] If the radius of the top surface 15 of the support ledge
decreases gradually from a greater value to a smaller value, this
has the technical effect that the pressure to which the stock is
subjected rises gradually which can provide for a smoother
dewatering without a sudden pressure pulse that may harm the web
that is forming.
[0052] Reference will now be made to FIG. 9 to further explain how
the embodiment of FIG. 4 differs from that of FIG. 5 and FIG. 6. As
previously explained, the support ledge 9 has a top surface 15 that
contacts the flexible tubular jacket 8. The height H of the support
ledge 9 can be defined as the distance from the axis of rotation A
of the flexible tubular jacket 8 to the top surface 15 of the
support ledge 9. In the direction of rotation R of the flexible
tubular jacket 8, the support ledge 9 has an upstream end 18 and a
downstream end 19 and the support ledge 9 is shaped such that, in
the direction from the upstream end 18 to the downstream end 19,
the height H of the support ledge 9 increases to a peak point 20.
In the embodiment of FIG. 4 and as indicated in FIG. 9, the peak
point 20 is symmetrically placed such that it has the same distance
to the upstream end 18 as to the downstream end 19. In the
embodiment shown in FIG. 5 and FIG. 6, the peak point is
asymmetrically placed such that the peak point 20 of the support
ledge 9 is located closer to the downstream end 19 of the support
ledge 9 than to the upstream end 18, i.e. the height H of the
support ledge 9 reaches its highest value at a point closer to the
downstream end 19 than to the upstream end 18. When the forming
roll 7 according to the invention is placed in the forming section
2 and operating, the upstream end 18 will be that end of the
support ledge 9 that is closest to the inlet gap 6 of the forming
section.
[0053] Another embodiment of the inventive forming section will now
be explained with reference to FIG. 10 and to FIG. 11. In FIG. 10,
the support ledge 9 is seen supported by a support beam 10. In this
embodiment, the support ledge 9 is flexible and/or elastic, i.e. it
is made of a material that is flexible and/or elastic. The support
ledge 9 of this embodiment comprises an inner cavity 12 that can be
supplied with a pressurized fluid such that the support ledge 9
expands and at least a part of the support ledge 9 is caused to
move in a direction outwards away from the axis of rotation A of
the flexible tubular jacket 8. In FIG. 10, the support ledge 9 is
shown in a state in which the inner cavity is not filled with
pressurized fluid and the flexible tubular jacket 8 can pass over
the support ledge 9 without being forced very much away from its
circular path, possibly without being forced out to any extent at
all from its circular path. In FIG. 11, the inner cavity 12 has
been filled with pressurized fluid such that the support ledge 9
has expanded. As a result, the flexible tubular jacket 8 is forced
out from its otherwise circular path as it passes over the support
ledge 9. Such a support ledge solution is disclosed in for example
U.S. Pat. No. 7,527,708 where a "support body 7" is described and
the support ledge 9 of the present invention may have a similar
design. The support ledge 9 may then be designed such that, when
the inner cavity 12 is filled with pressurized fluid such that when
the support ledge 9 is in an expanded state, the support ledge 9
has a top surface 15 facing the inner surface 16 of the flexible
tubular jacket 8 and which top surface 15 is convex.
[0054] Reference will now again be made to FIG. 1 and to FIG. 2.
The part of their respective loops that is common to both the first
and the second forming fabric 3, 5 extends from a the inlet gap 6
to an end point 27 where the first forming fabric 3 is separated
from the second forming fabric 5. Preferably, the support ledge 9
is placed at a point along the common path of the first and the
second forming fabrics 3, 5 that is closer to the end point 27 than
to the inlet gap 6 such that the pressure peak is attained at the
end or close to the end of the common path of the forming fabrics
3, 5. The smallest radius of the support ledge 9 will then also be
located at a point where the first and second forming fabric 3, 5
follow a common path but which is closer to the end point 27 than
to the inlet gap 6. In embodiments of the invention, the pressure
peak is reached immediately before the end point 27 such that the
maximum pressure that the stock (or the forming web) is subjected
to is reached at or immediately before the end point 27. In this
way, where the dewatering ends with a pressure peak, an effective
dewatering can be achieved.
[0055] Reference will now be made to FIG. 7. In FIG. 7, it can be
seen how the pressure P acting on the stock (or the forming web)
between the first forming fabric 3 and the second forming fabric 5
lies at a constant value of P1 over a large part of the
circumference of the forming roll 7. However, as the forming
fabrics 3, 5 come close to the end point 27 where the forming
fabrics are separated from each other, the pressure rises to a
higher level, P2. In this way, the pressure peak lies at the end of
the zone where the forming web is sandwiched between the two
forming fabrics 3, 5.
[0056] A further feature will now be explained with reference to
FIG. 8. In preferred embodiments of the invention, the flexible
tubular jacket 8 is closed at its ends such that the interior of
the forming roll 7 is an enclosed space 24. In this embodiment, the
forming roll 7 can be connected to a source of pressurized air or
gas 25 such that the flexible tubular jacket 8 can be inflated. In
FIG. 8, it can be seen that the forming roll 7 has two end walls
21, 22 and bearings 23 allow the end walls 21, 22 to rotate. The
bearings 23 can be mounted on a fixed part of the support beam 10.
The flexible tubular jacket 8 is fastened at its ends to the end
walls 21, 22 and the flexible tubular jacket 8 can be fastened to
the end walls 21, 22 in the same way as is known from shoe presses.
Known solutions for fastening the flexible tubular jacket 8 to the
end walls 21, 22 are disclosed in, for example, U.S. Pat. Nos.
4,625,376, 5,700,357, 6,010,443, 5,098,523 and 5,904,813. By
inflating the flexible tubular jacket 8, the advantage is gained
that it will be easier for the tubular flexible jacket 8 to retain
its shape.
[0057] As an alternative to inflating the tubular flexible jacket
8, it can be provided with supports that do not press it outwards
but merely helps it retain its shape (not shown in the
figures).
[0058] The inventive forming section 2 may further comprise a
headbox 14 arranged to inject stock into the inlet gap 6 between
the first and the second forming fabric 3, 5. However, the forming
section may conceivably be delivered without a headbox, for example
as a part of a rebuild of a paper machine that already has a head
box.
[0059] It is to be understood that the invention may also come in
the shape of a papermaking machine 1 that comprises the inventive
forming section 2. Such a machine may take many forms but the
inventors have in particular contemplated a papermaking machine in
which the second forming fabric 5 is a felt and the machine 1
comprises a Yankee drying cylinder 28 as shown in FIG. 1. In such a
machine, the second forming fabric 5 may be arranged to carry a
newly formed fibrous web W to the Yankee drying cylinder 28 and
transfer the fibrous web W to the Yankee drying cylinder 28 in a
nip formed between the Yankee drying cylinder 28 and a roll 29
placed within the loop of the second forming fabric 5. The roll 29
may be, for example, an extended nip roll such as a shoe press
roll. For example, it may be such a roll as disclosed in U.S. Pat.
No. 7,527,708, European patent No. 2085513 or as disclosed in
European patent No. 2808442. In the nip between the roll 29 and the
Yankee drying cylinder 28, the web W is further dewatered by
pressing. The web is at the same time transferred to the smooth
outer surface of the Yankee drying cylinder 28. Due to the smooth
outer surface of the Yankee drying cylinder, the web will follow
the smooth outer surface of the Yankee drying cylinder instead of
the (relatively) rough surface of the felt since the web W has a
strong tendency to follow the smoothest surface.
[0060] Due to the higher dry solids content that has been achieved
with the inventive forming section, the newly formed fibrous web
can be taken direct to a press nip against the Yankee drying
cylinder 28 without having to pass a suction roll.
[0061] Embodiments of the machine are conceivable in which the
newly formed fibrous web is first brought by the second forming
fabric 5 which is a felt to a press nip between to press rolls and
then transferred to a following Yankee drying cylinder 28. One of
the press rolls may then be an extended nip roll, for example shoe
roll. Possible rolls are such rolls that are disclosed in, for
example, U.S. Pat. No. 7,527,708, European patent No. 2085513 or as
disclosed in European patent No. 2808442.
[0062] Embodiments are also conceivable in which both forming
fabrics 3, 5 are foraminous wires and in which the second forming
fabric transfers the newly formed fibrous web W to a felt that then
carries the web W to a nip against a Yankee drying cylinder 28.
Alternatively, both forming fabrics 3, 5 can be foraminous wires
and the second forming fabric 5 is arranged to carry the web W to a
felt. The felt can then be arranged to pass the web through a press
nip between two rolls and then to a Yankee drying cylinder.
[0063] Embodiments are also conceivable in which the forming
section is followed by a through-air drying unit (TAD) and in which
the second forming fabric 5 may be a felt or a foraminous wire that
carries the web to a TAD wire where the web W can be transferred to
the TAD wire, for example by a suction device arranged inside the
loop of the TAD wire. The TAD wire can then carry the web W to the
through-air drying unit. It is also conceivable that the second
forming fabric 5 can be a foraminous wire that is also used as a
TAD wire. Examples of through-air drying units are disclosed in,
for example, U.S. Pat. No. 6,398,916 and the inventive forming
section of the present invention may be used also in an arrangement
such as that disclosed in U.S. Pat. No. 6,398,916.
[0064] In all embodiments of the inventive machine, the width of
the machine may be in the range of 2.5 m-7 m in realistic
embodiments. For example, the machine width may be in the range of
3 m-5.5 m.
[0065] The invention can also be defined in terms of a method of
forming a fibrous web. The method comprises the steps: of injecting
stock in an inlet gap 6 formed between the first forming fabric 3
and the second forming fabric 5 while each of the first and second
forming fabric 3, 5 is arranged to run in a loop supported by guide
elements 4. The forming roll 7 is located in the loop of the second
forming fabric 5 as described previously and the forming roll 7 is
arranged to guide the second forming fabric 5 into the inlet gap 6
and to guide the first and the second forming fabric 3, 5 along a
part of their path which is common to both the first and the second
forming fabric 3, 5 and which begins at the inlet gap 6. The
forming fabrics 3, 5 are caused to run in their loops such that the
stock that is injected into the inlet gap 6 passes between the
first and the second forming fabric 3, 5 as the forming fabrics 3,
5 are guided by the forming roll 7 such that water is removed from
the injected stock. As explained previously with reference to the
inventive forming section, the support ledge 9 will force the
flexible tubular jacket 8 outwards from the circular path it
otherwise follows such that the flexible tubular jacket 8 is caused
to follow a path with a radius of curvature which is smaller than
the radius of curvature of the flexible tubular jacket 8 outside
the area in which the support ledge 9 contacts the flexible tubular
jacket 8. In this way, the forming web will be subjected to a
pressure peak.
[0066] Optionally, the method may further comprise the step of
applying such a tension in the first forming fabric 3 that the
pressure applied to the stock (or the forming web) reaches a
highest value in the range of 8 kPa-20 kPa as the first and second
forming fabric 3, 5 pass over the support ledge 9. This pressure
level at the peak of the pressure is suitable to achieve a good
dewatering.
[0067] In the inventive method, the forming fabrics may move at a
speed in the range of, for example, 1200 m/min-2200 m/min. In many
realistic embodiments, the forming fabrics 3, 5 may move at a speed
in the range of 1600 m/min-2000 m/min. However, the inventive
forming section, machine and method may also operate at speeds
above 2200 m/min. For example, the speed of operation could be from
2200 m/min up to 2500 m/min or even higher.
[0068] The stock used may advantageously be virgin pulp that
comprises softwood fibers.
[0069] The flexible tubular jacket 8 may be caused to rotate about
its axis of rotation A by the forming fabrics 3, 5. Alternatively,
if the forming roll is provided with end walls 21, 22, it may be
provided with a drive arrangement acting on the end walls 21, 22.
Such a drive arrangement is known from shoe calenders and is
disclosed in, for example, U.S. Pat. No. 6,158,335.
[0070] While the invention offers a possibility to achieve a high
dryness without a suction roll, it should be understood that the
inventive forming section may optionally include a suction roll
located between the separation point 27 and the nip against the
Yankee drying roll 28 (see FIG. 1) if even higher dryness is
wanted.
[0071] Although the invention has been described above in terms of
a forming section, a papermaking machine and a method of forming a
fibrous web, it should be understood that these categories only
reflect different aspects of one and the same invention. The
inventive method may thus comprise such steps that would be the
inevitable result of operating the inventive forming section and/or
the inventive machine, regardless of whether such steps have been
explicitly mentioned or not. In the same way, the inventive forming
section may comprise means for performing any method step that is
part of the inventive method, regardless of whether such steps have
been explicitly mentioned or not.
* * * * *