Note: Descriptions are shown in the official language in which they were submitted.
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APPARA.TUS AND METHOD OF GENERATING STOCK
TURBULENCE IN A FOURDRINIER FORMING SECTION
Field of the Invention
The present invention relates to an apparatus and method
for generating stock turbulence in the forming section of an
open surface paper making machine. More specifically, the
present invention relates to an apparatus and method for
generating sufficient turbulence in the stock layer of an open
surface forming section of a papermaking machine to assist in
deflocculating a relatively thick stock layer carried on a
relatively slowly movirig forming fabric. This invention thus
finds application in the manufacture of relatively heavy paper,
pulp and board products. Further, the apparatus can be
adjustable, so that the amount of turbulence imparted into the
stock layer may be controlled and optimized to suit the grade
of product being made.
Background of the Invention
In a conventional open surface forming section, an aqueous
stock, containing both paper making fibers and other paper
making solids in amounts of from about 0.1% to about 1.5% by
weight, is fed from a headbox slice onto a horizontal moving
forming fabric. In such a forming section, after receiving the
stock from the headbox slice, the moving forming fabric is
supported by a forming board, followed by a series of drainage
boxes. The drainage boxes commonly include dewatering devices
such as blades and foils mounted on the drainage box in contact
with the machine side of the forming fabric. In some modern
slow speed macY:iines table rolls are also still used as
dewatering and turbulence generating devices. The forming
section can also include other devices intended to generate at
least some turbulence within the stock, such as formation
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showers. As the stock on the open surface forming fabric moves
through the forming section, water is removed from the stock
until an incipient paper web is formed which contains from
about 75% to abciut 85% water. The remainder of the water is
removed in subse(luent parts of the papermaking machine.
The thickness of the stock layer deposited from the head
box slice onto the forming fabric is determined by the machine
speed, the water content of the stock delivered from the head
box, and the basis weight of the paper or board product being
manufactured. Fieavier grade products, such as linerboard,
corrugating medium, market pulp grades, and paperboard
products, require a greater initial stock thickness than
lighter grades, such as newsprint.
To provide an acceptable paper product, it is important
that the paper making solids, including the paper making
fibers, be thoroughly mixed and dispersed as randomly as
possible in the stock leaving the headbox slice. In practice
this is almost impossible to achieve: a proportion of the paper
making fibers tend to flocculate in the stock and are deposited
as flocs onto the: forming fabric. Flocculation will continue
in the stock on the forming fabric unless steps are taken to
generate turbulence within the stock. Once an incipient paper
web is formed i1t is essentially impossible to disperse any
remaining flocs. Thus, what occurs in the stock on the forming
fabric to convert. it from a dilute solution of fibers and other
solids into an incipient paper web is of vital importance to
the papermaker.
Numerous methods have been proposed to randomize fiber
distribution in the stock in the forming section. Most of
these methods involve creating a level of turbulence within the
stock to disperse flocs. For example, it is known to impart
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a rapid transverse vibrating motion to the forming fabric
adjacent to the headbox, so as to apply a destructive shear
force into the f].ocs and thereby redistribute the paper making
fibers. Formation showers, table rolls, and various air and
water jets, located either above or below the forming fabric,
have also been used to create turbulence in the stock layer.
The amount of energy required to impart a desirable level of
turbulence into the stock is generally a function of stock
layer thickness, machine speed, and the type of furnish present
in the stock.
A common means of creating turbulence within the stock on
an open surface moving forming fabric is to locate dewatering
elements (such as foils, agitator blades and the like) in
supporting contact with the machine side of the moving forming
fabric. Devices of this type are described by Wrist, US
2, 928, 456; Sepall, US 3, 573, 159; Johnson, US 3, 874, 998; Saad,
US 4, 420, 370; Kallmes, US 4,687,549 and US 4, 838, 996; and
Fuchs, US 4,789,433. Foils have a leading edge that skims
liquid from the forming fabric; the trailing portion is
declined downwards at an angle of from about 1 to about 8 ,
and serves to provide a suction effect which withdraws liquid
from the stock and causes the fabric to deflect sufficiently
to induce at least some turbulence within the stock.
Agitator blades are profiled so that some water is
withdrawn and ttien redirected back through the forming fabric
into the fluid stock layer. A carefully profiled cross-machine
direction channel is located in the blade surface to achieve
this; the water thereby redirected back through the forming
fabric creates turbulence in the stock on the fabric, which
provides a deflocculating effect and serves to randomize the
solids distribution.
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Another means of inducing agitation is disclosed by
Johnson, US 4,140,573. In this device, at least one of the
dewatering elemerits on a low vacuum dewatering box are lowered
a small amount relative to those on either side so that, as the
fabric passes over the sequence of elements, it is pulled down
a small amount by the dewatering box vacuum and then released,
causing some turbulence within the stock.
An alternative means of inducing stock turbulence is
described by Cabrera y Lopez Caram, US 5,830,322. In this
device a pair of fabric supporting elements are used, a primary
element with a declining surface together with a trailing
element with a horizontal surface. Drainage of water from the
stock is controlled by restricting the size of a cross machine
direction drainage gap between the two elements. The primary
element declining surface is configured to impart turbulence
into the stock above the drainage gap, without downwardly
deflecting the forming fabric into the drainage gap, utilizing
blade profiles substantially as described by Fuchs, US
4,789,433 and by Kallmes, US 4,838,996. The apparatus relies
on fluid flow into and out of the drainage gap and on the shape
of the declininci surface of the primary element within the
drainage gap, to cause turbulence within the stock after the
fluid has been returned through both the forming fabric and any
incipient paper mat formed thereon to the stock.
Other stock agitating devices are described by Cowan, US
3, 922, 190; Marx, Jr., US 4, 999, 086; Hansen et al., US
5, 011, 577; Hansen, US 5, 089, 090; and Neun, US 5, 681, 430.
However, in situations where the paper product being made
requires that the forming fabric moves at a relatively slow
speed, and carries a relatively thick stock layer, for example
in the producticin of heavy basis weight products, it becomes
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more difficult to generate the desired levels of turbulence
within the stock. As machine speed decreases, and stock
thickness increases, in the manufacture of heavy basis weight
products, it becomes increasingly difficult to create an
effective amount of turbulence within the stock, and hence to
improve formation. It is thus found that for open surface
forming sections in which the forming fabric is moving at
speeds of less than about 400 m/min, carrying stock layers
whose initial thickness is greater than about 2.0cm at the head
box slice, and producing heavier grade paper products with
basis weights in excess of about 160 gsm, there is still a need
for a device that is capable of generating an effective level
of turbulence w:tthin the stock sufficient to cause at least
some deflocculation within the stock. It would also be a
considerable advantage if such a device could be readily
adjustable so tY:Lat the level of turbulence can be matched to
the paper maker's requirements.
An additional problem occurs with stock compositions using
a furnish having a hig:h content of relatively short fibers or
recycled materials. In these stocks, an almost impenetrable
mat can be formed on the paper side of the forming fabric
surface, effectively sealing the fabric and preventing adequate
drainage of the stock; a phenomenon commonly referred to as
"sheet sealing". A need therefore exists for a dewatering
device capable of at :least alleviating drainage restrictions
arising from this phenomenon
Summary of the Invention
The present: invention seeks to provide an apparatus and
a method for geiierating stock turbulence sufficient to cause
at least some stock deflocculation, and to improve formation
in an open surface paper making machine forming section in
which the stock layer is relatively thick, and in which the
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forming fabric moves at a relatively low speed. This invention
thus seeks to improve formation in open surface papermaking
machines which are used to make relatively heavier basis weight
products such as board stock and the like. This invention also
seeks at least to alleviate, if not eliminate, sheet sealing
by generating sufficient turbulence within the stock so as to
redistribute the fibre mat forming a more or less impenetrable
layer on the paper side of the forming fabric. This invention
consequently is of particular relevance to the use of stock
compositions containing a significant content of relatively
short fibers, or of recycled materials.
Further, ir.t one particular embodiment, this invention
seeks to provide an adjustable apparatus for generating a
controllable level of stock turbulence sufficient to cause at
least some stock deflocculation, and to improve formation in
an open surface paper making machine forming section in which
the stock layer is relatively thick, and in which the forming
fabric moves at a relatively low speed.
In the context of this invention, a "relatively low speed"
refers to an open. surface forming fabric that is moving through
the forming section at a linear speed of less than about
400m/min; a"relativel.y heavier basis weight product", and a
"relatively thick stock layer", each refer to an open surface
forming fabric machine that is being used to make a product
with a finished basis weight over about 160 gsm, which will
generally require a stock layer more than about 2.0cm thick
adjacent the headbox slice. It should also be noted that
although this invention is concerned with the manufacture of
products with a relatively high basis weight it is not so
limited, and under some circumstances is of benefit with
lighter products, and at higher machine speeds.
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According to a first aspect of the present invention,
there is provided an apparatus for generating turbulence in the
stock on a forming fabric in an open surface forming section of
a paper making machine, the forming section including a
relatively slowly moving forming fabric having a paper side and
a machine side, a relatively thick stock layer on the paper
side thereof, a dewatering box means located beneath the
forming fabric connected to a controlled vacuum supply means
operable to create a reduced pressure within the dewatering
box, and a plurality of forming fabric supporting dewatering
elements carried by the dewatering box comprising:
(i) a lead-in dewatering element having a fabric
supporting surface comprising in sequence:
a doctoring leading edge;
a substantially horizontal intermediate surface; and
a declining trailing surface;
(ii) a riser dewatering element having a fabric supporting
surface comprising in sequence
a doctoring leading edge;
an inclined surface;
an exit surface; and
a portion comprising the junction of the inclined and exit
surfaces; and
(iii) at least one intermediate dewatering element located
between the lead-in dewatering element and the riser element
and spaced from each other dewatering element by a gap, the or
each intermediate element having a fabric supporting surface
comprising in sequence:
a doctoring leading edge;
a declining surface; and
a trailing edge;
wherein:
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(a) the portion of the riser element located at the
junction of the inclined and exit surfaces is chosen from an
apex at the junction of the inclined surface and the exit
surface, a short substantially horizontal surface linking the
inclined surface and the exit surface, and a curved surface
linking the inclined surface and the exit surface;
(b) the intermediate surface of the lead-in dewatering
element, and the portion of the riser element comprising the
junction of the inclined and exit surfaces define a first
plane;
(c) the declining trailing surface of the lead-in
dewatering element and the declining surface of the or each
intermediate dewatering element(s) define a second plane
inclined at a pre-selected downward trailing angle with respect
to the first plane;
and
(d) the doctoring leading edge of the riser element is
located above the trailing edge of the adjacent intermediate
dewatering element, such that movement of the forming fabric
from the trailing edge of the adjacent intermediate dewatering
element to the doctoring lgading edge of the riser element
results in a vertical movement of the forming fabric, and of
the incipient paper web and the stock carried thereon.
Preferably, the at least one intermediate dewatering
element located between. the lead-in dewatering element and the
riser element and spaced from each other dewatering element by
a gap, is adjustably attached to the dewatering box permitting
location of the or each declining surface thereof in the
desired second plane, and permitting movement to a different
desired second plane. In this embodiment, as is set forth in
more detail below,
the included angle between the first and second planes instead
of being determined by the angle to which the intermediate
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element declininci surface is cut, is determined by the setting
of the adjustable attachment to the dewatering box. In this
embodiment, since the lead-in element is not adjustably
mounted, it is preferred that its declining trailing surface
is arcuate.
In an alternative preferred embodiment, the apparatus
further includes a drainage restricting element, which is
interposed between the riser element and the adjacent
intermediate element, having a fabric supporting surface
comprising in sequence:
a doctoring leading edge; and
an upwardly inclined surface;
wherein the attachment of the drainage restricting element to
the dewatering box is constructed and arranged to locate the
upwardly inclined surface at an angle to the second plane so
as to-provide a shallow "V" angle therebetween conforming to
the inclined surface of the riser element. In this embodiment,
the attachment of the drainage restricting element to the
dewatering box can be chosen from the group consisting of a
fixed attachment, an adjustable attachment, and a second
adjustable attachment incorporated into a first adjustable
attachment for the intermediate elements.
Preferably, all of the intermediate fabric supporting
elements are of the same width in the machine direction.
Alternatively, all of the intermediate fabric supporting
elements are not of the same width in the machine direction.
Preferably, the or each intermediate fabric supporting
element has a substantially flat declining surface.
Alternatively, at least one intermediate element has an
agitator blade profile.
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In an alternative aspect, this invention seeks to provide
a method for creating a desired level of turbulence in a stock
layer carried on a forming fabric in an open surface forming
section of a papermaking machine, consisting essentially of
moving the formir.-g fabric carrying the stock over at least one
dewatering box means carrying a plurality of fabric supporting
elements beneath, and in supportive contact with, the forming
fabric, and applying a controlled vacuum supply to create a
controlled reduced pressure in the dewatering box, the
dewatering fabric supporting elements consisting essentially
of:
(i) a lead-in dewatering element having a fabric
supporting surface comprising in sequence:
a doctoring leading edge;
a substantially horizontal intermediate surface; and
a declining trailing surface;
(ii) a risez" dewatering element having a fabric supporting
surface comprising in sequence
a doctoring leading edge;
an inclined surface;
a exit surface; and
a portion comprising the junction of the inclined and exit
surfaces; and
(iii) at least one intermediate dewatering element located
between the lead-in dewatering element and the riser element
and spaced froni each other dewatering element by a gap, the
or each intermediate element having a fabric supporting surface
comprising in sequence:
a doctoring leading edge;
a declining surface; and
a trailing edge;
wherein:
(a) the portion of the riser element located at the
junction of the inclined and exit surfaces is chosen from an
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apex at the junction of the inclined surface and the exit
surface, a short substantially horizontal surface linking the
inclined surface and the exit surface, and a curved surface
linking the inclined surface and the exit surface;
(b) the intermediate surface of the lead-in dewatering
element, and the portion of the riser element comprising the
junction of the inclined and exit surfaces define a first
plane;
(c) the declining trailing surface of the lead-in
dewatering element and the declining surface of the or each
intermediate dewatering element(s) define a second plane
inclined at a pre-selected downward trailing angle with respect
to the first plane;
and
(d) the doctoring leading edge of the riser element is
located above the trailing edge of the adjacent intermediate
dewatering element, such that movement of the forming fabric
from the trailing edge of the adjacent intermediate dewatering
element to the doctoring leading edge of the riser element
results in a vertical movement of the forming fabric, and of
the incipient pa;per web and stock carried thereon.
Preferably, the desired level of turbulence is created and
controlled by at least one adjustable intermediate dewatering
element located between the lead-in dewatering element and the
riser element which is adjustably attached to the dewatering
box permitting location of the or each declining surface
thereof in the :second plane; and the level of turbulence is
controlled by ad-Justing the adjustable intermediate supporting
element to a desired second plane location.
More preferably, the desired level of turbulence is
created by an apparatus further including a drainage
restricting element, which is interposed between the riser
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element and the adjacent intermediate element, having a fabric
supporting'surface comprising in sequence:
a doctoring leading edge; and
an upwardly inclined surface;
wherein the attachment of the drainage restricting element to
the dewatering box is constructed and arranged to locate the
upwardly inclined surface at an angle to the second plane so
as to provide a shallow "V" angle therebetween in conformance
with the inclined surface of the riser element.
Most preferably, the desired level of turbulence is
created and controlled by:
(i) at least one adjustable intermediate dewatering element
located between the lead-in dewatering element and the riser
element which is adjustably attached to the dewatering box
permitting location of' the or each declining surface thereof
in the second plane; and
(ii) a drainage restricting element, which is interposed
between the riser element and the adjacent intermediate
element, having a fabric supporting surface comprising in
sequence:
a doctoring leading edge; and
an adjustable upwardly inclined surface;
wherein the level of turbulence is controlled by:
(a) adjusting the adjustable intermediate supporting element
to a desired second plane location; or
(b) adjusting the drainage restricting element to a different
location; or
(c) adjusting both the adjustable intermediate supporting
element to a desired second plane location, and adjusting the
drainage restricting element to a different location.
Preferably, the angle between the first and second planes
is from greater than 0 to about 10 *
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One advantage that has been found with the apparatus of
this invention is that with relatively thick stock layers once
a desired level of turbulence has been induced in the stock,
it is less difficult to maintain a desired level of turbulence
further along the forming section. Hence, although the known
devices are not always capable generating an acceptable level
of turbulence, they are sufficient to maintain that level of
turbulence once it has been generated. The present invention
thus can be used to optimize the performance of these prior art
devices.
As a consequence of this, the shape of the exit surface
on the riser blade will be determined by what follows the
dewatering device of this invention in the forming section. For
example, if it i:; immediately followed by a second set of the
same elements so that the riser element is both the last
element on one set, and the first element in the next set, the
exit surface of the riser blade will be the same shape as that
of the corresponding part of a lead-in element, so that it will
have a substantially horizontal intermediate surface, and a
declining trailing surface in the same second plane as the
following elements. Alternatively, if it is followed by an
undrained gap, or by a drainage box equipped with foils, the
exit surface of the riser blade will be generally shaped as a
foil blade, with a foiling angle generally of from about 0.5
to about 5 .
Brief Description of the Drawings
The present invention will now be described with reference
to the attached Figures, wherein:
Figure 1 shows schematically a cross section in the
machine direction of a stock turbulence generating unit in
accordance with a first embodiment of the present invention;
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Figures 2, 3 and 4 show cross sections of the fabric
supporting elements used in Figure 1;
Figures 5 ar.kd 6 show alternative element arrangements to
that shown in Figure 1;
Figure 7 shows schematically a cross section in the
machine direction of a stock turbulence generating unit
incorporating two sets of fabric supporting elements;
Figure 8 shows schematically an intermediate element
including an agitator blade profile;
Figures 9 and 10 show schematically partially sectioned
a stock turbulence generating unit in accordance with a second
embodiment of the present invention;
Figures 11 and 12 show details of the pivot and adjustment
device used in F:lgures 10 and 11;
Figure 13 shows schematically a cross section of the unit
of Figures 9 - 12;
Figure 14 shows schematically a cross section in the
machine direction of a stock turbulence generating unit in
accordance with a third embodiment of the present invention;
Figure 15 shows a cross section of the drainage
restriction element shown in Figure 14; and
Figure 16 shows alternative intermediate element
arrangements applicable to Figures 1, 7 and 14 including
agitator blade profiles for the intermediate elements.
Detailed Description
In the context of this invention, the following
directional terms have the meanings given:
"machine di.rection" means a direction along the machine
substantially parallel with the direction of travel of the
forming fabric;
"cross machine direction" means a direction substantially
perpendicular to the machine direction generally parallel to
the plane of the forming fabric;
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"upstream" refers to a direction closer to the headbox
from a given point in the machine direction;
"downstream"' refers to a direction further from the
headbox in the machine direction;
"leading" refers to an upstream element edge;
"trailing" refers to a downstream element surface or edge;
"paper side" refers to the face of the forming fabric upon
which the stock is deposited, and the paper web is formed; and
"machine side" refers to the side of the forming fabric
in contact with the fabric supporting elements, and thus is the
other side from 'Che paper side.
In all of the schematic dewatering box cross sections
shown in the Figures, the fabric supporting elements all extend
in the cross machine direction for the full width of the
forming fabric. Additionally, all of the angles shown have
been enlarged for clarity.
In Figure 1 is shown a first embodiment of this invention.
In both this Figure and later Figures most of the other
conventional parts of a forming section, such as the headbox,
headbox slice, breast roll, a forming board (if present), a
forming shower or showers, and any other drainage or formation
devices are not shown. The stock turbulence generating device
1 includes a dewatering box 2, which is provided with a
hydraulically sealed drain 3 at the bottom, through which the
water 3A drained from the stock escapes. Dewatering box 2 is
attached by the pipe 4 to a vacuum source which provides a
controlled reduced pressure in the range of from ambient
pressure to about 7.5 kPa below ambient pressure.
A desired level of turbulence is generated in the stock
by the set of fabric supporting elements 5, 6, 7 and 8, which
are mounted onto the top rail of the dewatering box 2 using a
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conventional T-bar arrangement, as at 9A, 9B and 9C. In
combination, the spacing of the T-bars and the widths of the
elements determines the widths of the drainage gaps 10, 11, and
12. These gaps are sealed at their lateral edges with end
deckles (not shown). In the illustrated embodiment, gaps 10
and 11 are the same width and gap 12 is wider. The factors
influencing the choice of gap widths is discussed below. The
elements 5, 6, 7 and 8 can be formed from high density
polyethylene, with inserted ceramic wear surfaces, or any other
material appropriate for forming fabric support surfaces.
Within the set of' fabric supporting elements shown, element 5
is the lead-in blade, element 8 is the riser blade, and
elements 6 and 7 are the intermediate blades.
The forming fabric 13 moves in the direction of the arrow
A with its machine side in contact with the supporting elements
- 8. Over the gap 12, the forming fabric 13 rises from the
last intermediate element 7 onto the riser element 8. This
vertical movement; of the forming fabric, and of the incipient
paper web and stock carried by it, induces turbulence within
the stock adjacerit to, and downstream of, the exit surface of
the riser element 8.
The cross section. of the lead-in element 5 is shown in
Figure 2. This includes a doctoring leading edge 14, a flat
intermediate surface 15, and a declining trailing surface 16.
In this embodiment, the trailing surface is substantially flat,
and is at an angle of inclination a, relative to the surface
15. The element is mounted onto the T-bar 9A so that the
surface 15 is substantially horizontal. The doctoring leading
edge 14 removes at least some of water that has drained
through the forming fabric upstream of the lead-in element.
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The cross section of the riser element 8 is shown in
Figure 3. This includes a doctoring leading edge 17, an
inclined surface 18, an exit surface 19, and a portion 20
comprising the junction between the inclined and exit surfaces.
As shown, the portion 20 is the apex at the junction of the two
surfaces 18, 19 on either side; alternative shapes are a short
horizontal surface, and a curved surface. The underlying
requirement for the portion 20 of the riser element is that it
provide a continuum of support for the forming fabric moving
and bending over it, and that together with the substantially
horizontal surface 15 of the lead-in element it defines the
first plane, below which the fabric is deflected during its
passage over the intermediate elements. The exact shape of the
portion 20 is chosen based on the constructional materials
used, and the desired lengths of the inclined surface 18 and
the exit surface 19. The inclined surface 18 is at an angle
R, which is measured between the inclined surface and the first
plane defined by the surface 15 on the lead-in element, and the
portion 20 of the riser element. The shape of the exit surface
19 is discussed below.
In Figure 1, two intermediate elements 6 and 7 are shown,
which are generally the same. The cross sections of these are
generally the same, and that of intermediate element 6 is shown
in Figure 4. This includes a doctoring leading edge 21, a
declining surface 22, and a trailing edge 23. The set of three
elements comprising the lead-in element and the two
intermediate elements supported by the T-bars 9C are spaced
apart so that the surface 16 and the two surfaces 22 are in a
common second plane at the angle a relative to the first plane.
In the apparatus of this embodiment of this invention, as
shown in Figure ]., as fabric 13 moves over the dewatering box
2, the machine side of fabric 13 first engages leading edge 14
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of lead-in element 5 which skims liquid from the machine side
of fabric 13. Ttie forming fabric 13 continues downstream and
passes over, in succession, inclined surface 16, gap 10,
inclined surface 22 and trailing edge 23 of intermediate
element 6, gap 11., inclined surface 22 and trailing edge 23 of
intermediate ele:ment 7, gap 12, and finally leading edge 17,
and surfaces 18, 20, and 19(in that order) of riser element 8.
'The fabric is pulled down onto the surface 16 and the two
surfaces 22 in sequence by the controlled low vacuum in the
dewatering box so as to form a fluid seal on these surfaces.
Finally, the forming fabric rises upwardly over the gap 12 and
the surfaces of the riser element B. This upward movement
generates turbulence in the stock in the vicinity of the riser
element 8.
In this embodiment, the value chosen for the angle a is
determined by the machine characteristics, which includes the
overall separation of the lead-in and riser elements, the
number of intervening intermediate elements, the machine speed,
the thickness of the stock layer, and the level of turbulence
desired in the product being made. Consequently, the value of
a determines the vertical distance through which the forming
fabric must rise from the locus where it loses contact with the
last intermediate element, which is at or near to the trailing
edge 23 of this element, to the doctoring leading edge 17 of
the riser element. Generally, a is in the range of from about
0.25 to about 1-0 . For most purposes it has been found that
a is less than 6 ' and often is in the range of from about 2 to
about 4 . The gap widths between each of the elements making
up the set, in combination with the applied vacuum, and the
properties of the stock and of the furnish in the stock also
affect both the amount of drainage that occurs, and the amount
of turbulence that is generated. The level of applied vacuum
in combination with the gap widths must be sufficient to ensure
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that the forming fabric is in hydraulic contact with the fabric
supporting surfaces of all of the elements. The actual value
of the applied vacuum also influences the level of turbulence,
since it influences the transition of the forming fabric from
the last intermediate element onto the riser element. At this
point, the forming fabric has a shallow "V" shape, which is
sharper or flatter depending at least in part on the vacuum
applied. The actual values chosen for a, and of the other
identified variables, will be determined by the amount of
turbulence that is desired in the stock at that point in the
forming section; some experimentation may be required to
determine optimum values for a given set of paper making
conditions.
The shape of the exit surface 19 of the riser element 8
depends to a large extent on what follows this element
downstream in the forming section, for which there are several
choices. The riser element may be followed, for example, by
another identical stock turbulence generating unit, an
uncontrolled drainage gap, by a set of foils, or by an
Isoflo(trade mark) drainage unit. When the next drainage unit
is another more or less identical unit contiguous with, or even
mounted on the same drainage box as the preceding unit, the
riser element becomes common to both units. The exit surface
of the riser element is then profiled as if it is a lead-in
element, so that, it matches the chosen value of a for the
following unit, which may not be the same as that of the
preceding unit. When the riser element is followed by a gap,
or a foil unit, it appears to be sufficient to use an exit
surface that is either substantially horizontal, or is
downwardly inclin.ed at more or less the same angle as is used
for a conventional foil blade, that is up to about 5 , without
an intervening short horizontal surface.
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The inclined surface of a riser element, as at 18 in
Figure 3, is generally at a fairly steep rising angle, as it
defines the path of the rising forming fabric as shown in
Figure 1. The angle 0 shown in Figure 3 will generally be in
the range of from about. 00 to about 300. In practice, an angle
of from about 10 to about 200 is often sufficient. The value
of the angle R is determined by the vertical displacement of
the forming fabric as it rises from the declining surface 22
of the last intermediate element to the surface 20 of the riser
element. The value of R should be selected to minimize fabric
deflection with a. low vacuum level. If in operation the fabric
deflection is, or becomes greater than this, it is found that
the forming fabric still engages with and follows the shape of
this surface. However, some experimentation may be necessary
to determine the optimum value of R for a given set of machine
conditions.
Further, it appears that once a desired level of
turbulence has been created in the stock by the apparatus of
this invention it is easier to induce turbulence in the stock
downstream in the forming section, thus facilitating the use
downstream of subsequent turbulence generating devices. This
enhances the operation of subsequent conventional
deflocculation and dewatering devices and improves the
formation in the product being produced. In a similar fashion,
it is observed that the apparatus of this invention will
enhance a lower level of turbulence created in the stock by an
upstream device, such as a formation shower.
While the embodiment described above is applicable when
the fabric speed is 400 m/min or less and the stock relatively
thick, for example 2.0cm or more adjacent the head box slice,
for manufacturing paper products whose basis weights is 160 gsm
or greater, it is contemplated that the present invention will
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also provide advantages in other circumstances, such as higher
fabric speeds and/or thinner stock layers.
Unexpectedly, it has been discovered that, once fabric 13
is running at machine speed over the dewatering box 1 under an
applied vacuum, it will often continue to follow the path
defined by the supporting elements 5, 6, 7 and 8 even if the
vacuum is reduceci. This permits a reduction in the amount of
drainage over 'the dewatering box 2. This provides an
additional benefit in reducing any tendency for sheet sealing.
In the embodiment, shown in Figure 1, the unit shown has
two intermediate dewatering elements 6 and 7. Depending on the
machine characteristics and the product being made, other
configurations can be used. Figure 5 shows one intermediate
dewatering element 6 between a lead-in element 5 and a riser
element 8 and Figure 6 shows a configuration using five
intermediate elements 24, 25, 26, 27 and 28, in which all five
intermediate elements are arranged to be in the second plane
at a common angle a to the first plane. It is also shown in
Figure 6 that the intermediate elements need not all be the
same width.
It is also possible to utilize this invention with two
dewatering units in sequence, with the riser element of the
first unit also serving as the lead-in element of the second
one. This arrangement is shown in Figure 7. The first set of
elements includes a lead-in element 5, and two intermediate
elements 29 and :30. The second set of elements includes again
two intermediate elements 32 and 33, and a riser element 8.
The central element 31 functions as riser for the first set,
and lead-in elempnt for the second set. Its upstream inclines
surface 18 is s:haped to conform to a riser element, and its
downstream declining trailing surface is shaped to conform to
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a lead-in element. This arrangement also can be set up in two
different ways:
(i) a single dewatering box 2 can be used, with a single vacuum
supply 4, as shown essentially in Figure 1; or
(ii) a dewatering box with two hydraulically separate
compartments 2A and 2B, separated by wall 34, each of which
have their own vacuum supplies 4A and 4B, as shown in Figure
7.
In this latter arrangement, the vacuum applied to the two
compartments need not be the same. It is also possible that
the angles al anci a2, both of which are measured relative to
the first plane as shown in Figure 7, need not be the same,
depending on the level of turbulence desired in each unit.
In the embodiments shown, the intermediate elements have
an essentially planar forming fabric supporting surface. In
certain circumstances, depending on both the machine
characteristics, the stock characteristics, and the product
being made, it has been found desirable to cause more
turbulence in the stock than is caused by utilizing planar
forming fabric supporting surfaces on the intermediate elements
in the second plane. As is shown in Figure 8 an intermediate
element with a so-called agitator blade profile with a single
channel 35 can be used to induce additional turbulence.
Agitator blades having this surface profile are described, for
example, by Johnson in US 3,874,998; other profiles are known
and used. It appears that an agitator blade profile can
enhance the turbulence effects provided by the turbulence
generating unit of this invention.
In a similar fashion, it is also contemplated within this
invention for the dewatering device to share a common
dewatering box with a different dewatering device, such as an
Isoflo(trade mark), agitator blades, or a set of foil blades.
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In an experimental trial a stock turbulence generating
unit according to the present invention was located downstream
of a formation shower in an open surface forming section of a
paper machine. Z'he unit used was that shown in Figure 7, but
without the internal dividing wall 34, and only a single vacuum
supply. Two suction boxes provided with covers substantially
as described by Johnson in U.S. 4,140,573 were located
immediately downstream of the unit. The machine speed of the
forming section was approximately 320 m/min, and the paper
board product had a basis weight of approximately 299 gsm. The
lead-in element was 38.1mm wide, with a declining surface
8.5mm wide. The two intermediate elements were the same in
each pair, and had a declining surface width of 150.9mm The
drainage gap between each of the elements was 9.5 mm, except
for the gap downstream of each of the last intermediate
elements, which was 12.7 mm. In both sets of elements, the
value of a was 2 . The dewatering element acting as a common
lead-in and riser element at the middle of the set had an
inclined surface 9.5 mm.wide, and the value of the angle Q was
. The downstream exit surface of this common element was
substantially f:lat, and inclined downwardly at 2 , thus
matching the valtie of a. The exit surface of the second riser
blade was horizontal. All of the element widths and the
element separation gaps are measured in the machine direction.
During the trial, the vacuum level applied by the suction
box was varied from ambient pressure to about 5 kPa below
ambient pressure. It was found that when the formation shower
located upstream was turned off, the visual appearance of the
stock as it passed over the turbulence generating unit did not
indicate any increased activity within the stock. however, it
was found that both the drainage of the incipient sheet and
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quality of the resulting paper product, as evidenced by its
formation and smoothness, improved as compared to its quality
before the unit was installed. This indicated that the unit
was effective in. generating turbulence within the stock, and
in preventing sheet sealing, despite the fact that the
formation shower had been turned off.
When the f'ormation shower was turned on, the visual
appearance of the stock as it passed over the stock turbulence
generating unit changed dramatically, indicating an increased
level of stock activity. This shows that the stock turbulence
generating unit of this invention is effective both in
imparting turbulence into the stock so as to improve formation
and prevent sheet sealing, and can enhance the performance of
other drainage and turbulence generating devices.
In the embodiment described above, the location of the
intermediate elements is determined by fixed structures, and
the cross sectional profile of the intermediate elements
determines the value of a. Since the value of a is never very
large, this construction requires precision machining and
installation of 'the intermediate elements in order to provide
a set of surfaces accurately located in the second plane.
In a second embodiment of this invention, instead of
mounting each intermediate element directly onto the structure
of the dewaterinq box, each intermediate element is adjustably
mounted onto the structure of the dewatering box. It is then
feasible to control the value of a by moving the whole
intermediate eleinent to provide an appropriate declining angle
for the declining surface by adjusting the adjustable mounting,
instead of constructing the element to provide the required
fixed declining angle. In this configuration, where more than
one intermediate element is used, it is preferred that all of
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the intermediate elements are mounted onto a single adjustable
mounting at the desired machine direction separation with their
forming fabric supporting surfaces in a common plane. The
desired value of ia is then obtained by adjusting the mounting,
or mountings, as required.
In addition to greatly simplifying the construction of the
turbulence generating unit, as all of the intermediate elements
can be fabricated to essentially the same dimensions, this
configuration has the added advantage that the value of a can
be readily chancied so as to alter the level of generated
turbulence. This can be required for several reasons, such as
a change in product, a change in furnish for the same product,
and less than perfect mixing in the headbox causing problems
on the forming fabric. Thus in addition to providing a means
to generate turbulence within the stock on the forming fabric,
this embodiment of this invention additionally provides a means
whereby the level of turbulence created can be controlled, and
either enhanced or diminished as paper making conditions
require.
This embodiment of this invention is shown in Figures 9 -
13. In the Figures 9 - 12 the forming fabric is omitted for
clarity.
Referring first to Figures 9 and 10, which show partially
cut away three quarter views of the unit, the unit includes a
single dewatering box 2 supporting a lead-in element 5, three
intermediate elements 35, 36 and 37 of which the middle one 36
is narrower than the other two, and a riser element 8. The
lead-in element 5 and the riser element 8 are supported by T-
bar structures 9A. and 9B, both of which are directly supported
by the frame 38 on the top of the dewatering box 2. The three
intermediate elements are supported by similar T-bar structures
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9C, each of which is mounted onto an adjustable supporting
frame 40. At its upstream end, adjacent the lead-in element
5, the adjustable frame 40 is supported by a pivot assembly 41.
At its downstream end, the adjustable frame 40 is provided with
a vertical adjustment assembly 42, which in its turn is
controlled by the adjustment bar 43 which is moved in the
directions shown by arrow B by means of the handle 44. The
adjustment bar 43 is supported by suitable bearing surfaces
(not shown) on the beam 45 carried by the supporting framework
of the dewatering box top shown generally as 46.
The upstreattL pivot is shown in more detail in Figure 11.
The frame 40 pivots through a small arc (which provides
sufficient angular movement to obtain any desirable value for
a) about rod 47 which is supported by the wall of the
dewatering box 2, as at 50. The frame 40 is attached to the
rod 47 by means of an adjustable bearer block 48 carried by a
bracket 49. The bearer block is held in place by the lockbolt
51 which passes through the slot 52. This form of attachment
allows fine control of the location of the surface of element
35 relative to the declining surface of the lead-in element 5.
Figure 12 shows only one pivot assembly; in practice there will
be at least two, and often more, so that the upstream end of
frame 40 is adequately supported for the full width of the
forming section.
The downstream vertical adjustment assembly is shown in
more detail in Figure 12. The vertical adjustment assembly 42
is attached to the downstream face of the frame 40 by the bolts
53 and 54 which are provided with enlarged holes 53A and 54A.
The assembly 42 also includes an angled slot 55, into which is
fitted a captive pin 56. The outer end of the pin 56 engages
into the aperture 57 in the adjustment bar 43. As a result,
horizontal movement of the bar 43 in the directions of arrow
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B causes vertica:L movement of the frame 40 in the directions
of arrow C. The enlarged holes 53A, 54A are provided to permit
fine adjustment of the assembly 42 relative to the frame 40 so
that the same value of a is obtained across the full width of
the forming section. If desired, the bar 43 can be locked in
a particular setting by using any appropriate locking
mechanism. Figure 12 shows only one adjustment assembly; in
practice there will be at least two, and often more, so that
the downstream end of frame 40 is adequately supported for the
full width of the forming section.
It is also contemplated that other vertical adjustment
means are useable: for example, the adjustment bar 43 can be
replaced by a screw thread system, which can be motorized, and
the whole adjustment means can be replaced by a hydraulic or
pneumatic system.. If the vertical adjustment means is to be
freely operable, the fact that it is placed in an environment
where it can be clogged with solids from the stock should be
borne in mind.
The cross section of the unit of Figures 9 - 12 is shown
schematically in Figure 13. The lead-in and riser elements 5
and 8 are supported by their T-bars 9A and 9B attached directly
to the dewatering box 38. The three intermediate elements 35,
36 and 37 are each supported by T-bars 9C carried on the
subframe 40. The subframe 40 is supported at its upstream end
by the rod 47, about which it rotates to provide the required
value for a. It is supported at its downstream end by the
adjustment assembly 42 controlled by the adjusting bar 43. The
actual value fo:r a is determined by the position of the
adjustment bar 43 relative to the vertical adjustment assembly
42.
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In this arrangement, although the intermediate elements
are adjustable to any desired value for a, the lead-in element
is still fixed, and is unadjustable, so that its declining
trailing surface is at a constant angle. In some
circumstances, it has been found that this can result in the
forming fabric deflection over the trailing edge of the lead-in
element, which is not desirable for several reasons. It is
therefore preferred that in this arrangement, as indicated at
70(see also Figure 10), the lead-in element has an arcuate
trailing edge.
It can thus be seen that in this preferred embodiment,
rather than adjust each intermediate element individually to
obtain the desired value of a, which requires either precise
machining and installation, or precise individual vertical and
angular adjustment, the set of intermediate elements are made
all the same, and are mounted onto the subframe so that all of
their forming fabric engaging surfaces are in a common plane,
which is conveniently substantially parallel to the frame
itself. When the frame is installed, after making any required
adjustments by means of the bolts 51, 53 and 54, a desired
value for a is obtained by moving the bar 43 to the required
position, which inclines the surfaces of the intermediate
elements to the desired position determining the second plane.
In a further embodiment, a fourth drainage restricting
element is included in the dewatering device, located in the
gap between the riser element and the immediately preceding
upstream intermediate element. In certain configurations,
particularly where the inter-element spacing is chosen to be
relatively large, or the value of a combined with the machine
direction length of the unit provides relatively high vertical
distance between the last intermediate element and the
doctoring leading edge of the riser element, a significant
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length of the forming fabric can be exposed to vacuum assisted
drainage between the point where the machine side of the
forming fabric loses contact with the last intermediate element
adjacent its trailing edge, and the leading doctoring edge of
the riser element. This allows an excessive amount of water
to be withdrawn from the stock at this point. This can be
controlled by insertion of a fourth drainage restricting
element in this gap, with a fabric supporting surface that is
upwardly angled to be in supporting contact with the forming
fabric, so that the intermediate element supporting surfaces
and the drainage restricting element supporting surface form
a shallow "V" which supports the machine side of the forming
fabric, and which limits the area of the machine side of the
forming fabric exposed to vacuum assisted drainage at this
point.
There are several options for the construction of the
additional drainage restricting element; for example:
(a) it can be unadjustably mounted, more or less as
described above for the other elements; or
(b) it can :be adjustably mounted; or
(c) it can be adjustably mounted onto a subframe
supporting a set of intermediate elements.
For the same reasons as set out above for the intermediate
elements, it is preferred that the additional drainage
restricting element is adjustably mounted. More preferably,
more or less the same subframe assembly as that described for
the intermediate elements is used for the additional drainage
restricting element.
In Figure 14 is shown a schematic cross section embodying
a drainage restriction element. The lead-in and riser elements
and 8 are supported by their T-bars 9A and 9B. The three
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intermediate elements 35, 36 and 37 are each supported by T-
bars 9C carried on the first subframe 40. The first subframe
40 is supported at its upstream end by the rod 47, about which
it rotates to provide the required value for a. It is
supported at its downstream end by the adjustment assembly 42
and the adjusting bar 43. The actual value for a is determined
by the position of the adjustment bar 43 relative to the
vertical adjustment assembly 42. The drainage restriction
element 55 is supported by a T-bar 9D carried by a second
subframe 56, which is rotatably supported at its downstream end
(in much the same fashion as the first subframe 40) by the rod
57. The angular position of the drainage restriction element,
indicated by the angle y between the surface 61 and the first
plane, is controlled by the vertically adjustable upstream
mounting 58 for the second subframe 56. A similar arrangement
to that described.for the first subframe is conveniently used.
In most cases, the angles and y will be more or less the
same.
The cross section of the drainage restriction element is
shown in Figure 15. The upstream face 59 includes a doctoring
leading edge 60, which is followed by an upwardly inclined
surface 61, which terminates in a trailing edge 62. The
element is suitably supported by a T-bar as at 9D. The value
of the angle b is chosen to allow a value for the angle y which
provides a smooth transition of the moving forming fabric from
the locus at which it loses contact with the last intermediate
element 37 onto the inclined surface of the riser element 8.
Depending on the form of mounting used for the drainage
restriction element, the angle b can be quite small, and can
be zero, so that the upwardly inclined surface is substantially
perpendicular to the upstream face 59. As noted above, the
point at which the forming fabric loses contact with the
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element 37 depenc3s inter alia on the level of vacuum applied
to the dewatering box.
Figure 16 shows schematically alternative intermediate
element profiles to those shown in Figures 1, 7 and 14. Figure
16 shows a seven element set. The first set of elements
comprises a lead-in element 5, and two intermediate elements
63, 64 each of which have an agitator blade profile. The
central element 31 is both riser element for the first set, and
lead-in element for the second set. The second set comprises
two intermediate elements 65 and 66, followed by a riser
element 8. The elements 65 and 66 have a substantially planar
surface. As shown, the two sets are placed over a divided
dewatering box 2 with separate drainage spaces 2A and 2B to
which the same, or a different, level of vacuum can be applied.
It is also contemplated that the elements 63 and 64 can form
the second set, with elements 65 and 66 forming the first set.
From this it can be seen that combinations of element profiles
can be used to generate a desired level of turbulence within
the stock.
The present invention provides a number of advantages over
the prior art. The stock turbulence generating unit can be
used to advantage to dewater and deflocculate thick and/or
heavy grade stocks while applying low vacuum pressure, or, in
some circumstances, minimal vacuum once the section is
operating. The ability to diminish the applied level of
vacuum significar.-tly reduces drainage and sheet sealing during
passage of the stock over the unit. The turbulence generated
throughout the stock thickness can also be used to enhance the
even and efficient deflocculation of the stock by other
agitation devices located both upstream and downstream of the
unit.
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