Note: Descriptions are shown in the official language in which they were submitted.
WO90/15187 2 0 ~ 7 9 3 ~ PCT/CA90/0018S
~ 1 .
CONTINUOUS CONTROLLED DRAINAGE
This invention is concerned with paper making machines of the
type having a "flat wire" or "open wire" forming section, which
includes means to remove water from the stock by the use of suction.
In this type of machine, as opposed to "twin wire" machines, or
"gap formers", an aqueous slurry known as the stock, which contains
both fibers and other substances in an amount of from about 0.1% to
1.5% by weight, is fed from a head box slice onto a single moving
forming fabric. Water is progressively removed from the stock
through the forming fabric in what is known as the "forming section"
of the paper making machine. In this forming section, a variety of
drainage devices are used, until the stock contains from about 2% to
about 4% by weight of solid material. At that point, the
distribution and orientation of the fibers and other solids in the
still very wet stock is largely determined, and will not change very
much in the remaining paper forming steps unless other devices such
as a dandy roll, or "top wire", is brought into contact with the
stock. Thus at this point the formation of the paper is largely
completed.
In outline, a conventional open wire forming section
includes a forming fabric which is supported at the head box slice
end by a breast roll, which is followed in sequence by a "forming
board" and a series of drainage devices, which may be drainage foils
or table rolls, and suction boxes. More recently, forming sections
have included a forming board followed by suction boxes of the type
described by Johnson, in U.S. Patent 4,140,573. These suction boxes
heretofore have been distributed along the length of the forming
section with gaps, or undrained spaces, in between them.
The one reported attempt to use vacuum assisted drainage for the
full length of an open wire forming section appears to have been a
failure. Such a paper making machine is described by E.J. Justus in
U.S. 3,052,296 (issued in 1962, assigned to Beloit Iron Works). As
described by Justus, the forming fabric is to be supported on a
"continuous or substantially uninterrupted" series of suction boxes,
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W O 90/15187 ~ 0 ~ 7 9 3 2 P ~ /CA90/00185
starting as near to the head! b~x sl ice as is practicable. These
suction boxes are provided with a foraminous surface to support the
forming fabric, for which several designs are proposed. Justus
proffers several advantages for such a machine: an increase in fiber
retention on the forming fabric of up to 70~, as compared to the
usual figure of less than about 50%, reduced wire marking on the
paper, and "better" paper. A further point made by Justus is that
his essentially flat surfaced suction boxes do not cause the
phenomenon known as "kick-up" in the stock associated with the table
rolls then used as the primary dewatering devices. Kick-up results
from the vertical deflection of the forming fabric caused by the
suction produced by the roll as described in U.S. 2,928,465. When
kick-up occurs, what is observed is an essentially vertical movement
of both the forming fabric and the stock carried on it in the
vicinity of a table roll: this movement can become so violent that
it will literally lift the stock off the forming fabric. Such an
occurrence is not conducive to the making of good paper. In a later
communication originating from Beloit Iron Works (~reported by P.
Wrist in "The Formation and Structure of Paper", British Paper and
Board Makers Association, London, England, 1962, at pages 863, 864)
it is noted that although many of the benefits proffered by the
all-vacuum assisted drainage technique proposed by Justus indeed are
obtained, nevertheless "the formation of the tpaper] sheet
deteriorated to an unacceptable level." (Communication to P. Wrist,
from Beloit Iron Works). In other words it proved to be impossible
to make acceptable quality paper using the modified paper making
machine proposed by Justus. Perhaps as a consequence of this
failure, this approach to stock dewatering was not pursued further.
Even Justus turned his attention to other methods (e.g. as in U.S.
3,l02,066).
It has now been realized that the failure of the Justus attempts
may be directly attributed to at least two seemingly unrelated
causes. First, Justus in setting out to avoid the then known
problems of heavy suction and kick-up becoming prevalent with table
rolls (and which were becoming a handicap serving to limit paper
WO 90/15187 PCr/CA90100185
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making speed, since as the linear speed of the forming fabric
increases the suction and kick-up effects become more violent)
endeavoured to eliminate all stock agitation in the forming section.
It has now been known for some time that improved paper making
operations can result if some deliberate and controlled agitation is
introduced into the stock on the forming fabric whilst it is still in
a highly fluid state.
It has now been discovered that the precise spacing of the
devices used to generate stock agitation has a very important effect
on paper sheet quality. When the devices are spaced apart in a
uniform manner, they act in a periodic or harmonic relationship to
each other, so that later devices (that is, ones further from the
head box slice) can either reinforce and add to the stock agitation
produced by earlier devices, or diminish and dampen that agitation.
This provides a controlled and uniform stock agitation that is both
easily generated and easily controlled, to benefit the paper sheet
formation.
Second, Justus recommends to use a vacuum level ranging from a
low level of effectively zero in a suction box adjacent the head box
slice rising to a figure of 2 inches of mercury at the 3% point, that
is a value of about 70 cms of water. It has been discovered that
this is also a mistake, and that with dewatering devices somewhat
similar to those advocated by Justus a far lower level of vacuum is
often sufficient, rising from a very low level adjacent the head box
slice to a value of no more than 50 cms of water at the end of the
forming section. It has been discovered that much lower levels of
vacuum than those suggested by Justus can be used with great benefit
in retention and wire mark provided the above mentioned agitation or
kick-up can be achieved. This can be achieved by the use of the
static drainage unit known as the Isoflo (Trade Mark) which is
described by Johnson in U.S. 4,140,573.
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Thus it has now been discovered that if first, the correct level
of vacuum is used in the suction boxes and second, the path through
which the forming fabric moves whilst the stock is still highly fluid
is properly controlled to provide some agitation in the stock, then a
forming section can be successfully operated with continuous vacuum
assisted drainage. By this means, it becomes possible to increase
the area of the forming fabric that is subjected to vacuum assisted
drainage by an amount of often more than 30%, the actual figure
varying from machine to machine. Additionally, fiber retention is
also significantly improved.
Retention is fundamental in paper making. The commonly used
definition in paper making for first pass retention (FPR) is
Head Box Consistency - White Water Consistency x 100
Head Box Consistency
Values for FPR can range from 30 % in the case of papers with a
high filler content to over 90' for some long fibered grades.
Several factors affect the FPR including the type of stock, the kind
of forming fabric, the use of chemical retention aids, the amount of
stock agitation, the amount of suction used in forming the paper, and
particularly the velocity induced in the stock by that suction while
forming. Improving retention from 45% to 70% reduces the consistency
of the recirculating white water considerably if the amount of slice
opening is left unchanged. (By "consistency" in this context is
meant the total suspended solids content in percent by weight in the
stock or in the white water, as appropriate). This has beneficial
effects on the entire paper mill and reduces the amount of fiber and
filler loss. Alternatively, the paper maker may cut down on the
slice opening and use less water to form the paper. Thus one benefit
of this invention, which allows using low vacuums while still
achieving good formation, is to reduce the velocity of drainage
thereby improving retention and wire mark.
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WO90/15187 a-o5~932 PCl/CA90/0018
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Thus, in a first broad aspect this invention provides a process
for improving paper formation in a paper making machine having an
open surface forming section, including at least a travelling
continuous forming fabric which passes over a breast-roll adjacent a
head box having a head box slice through which aqueous stock is
deposited onto the forming fabric, and a plurality of stock
dewatering devices beneath the forming fabric which are provided with
white water drainage means, in which forming section the solids
content of the stock deposited from the head box through the head box
slice onto the forming fabric rises from an initial low value to a
value of from about 2% to about 4%, comprising the steps of:
(i) discharging onto the moving forming fabric an aqueous
stock;
(ii) causing the forming fabric to move over a dewatering
device comprising a continuous vacuum assisted suction box including
a plurality of chambers to each which a controlled level of vacuum is
applied, which includes a foraminous top support surface for the
forming fabric, and which provides a vacuum tight seal between the
forming fabric and the evacuated chambers of the suction box;
(iii) controlling the vacuum level applied to the chambers of
the suction box so that the vacuum level rises from an initial low
value of less than 5 cms water gauge adjacent the head box slice, to
a value of no more than 50 cms water gauge at the end of the forming
section; and
(iv) causing a desired level of uniformly spaced periodic
harmonic agitation in the stock on the forming fabric by providing
differences in level between different parts of the forming fabric
along the length of the forming section.
In a preferred embodiment, the vacuum in the suction box is
controlled in such a way that it rises in a stepwise fashion in the
separate sections of the box along the length of the forming section,
from the initial low value of below 5 cms water gauge to a maximum
value of no more than 50 cms water gauge. Desirably there are as
many vacuum levels as possible, preferably more than three, and most
preferably at least five.
WO 90/15187 PCI`/CA90/0018~
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In a second broad aspect this invention provides in a paper
making machine having an open surface forming section, including at
least a travelling continuous forming fabric which passes over a
breast-roll adjacent a head box having a head box slice through which
aqueous stock is deposited onto the forming fabric, and a plurality
of stock dewatering devices beneath the forming fabric which are
provided with white water drainage means, in which forming section
the solids content of the stock deposited from the head box through
the head box slice onto the forming fabric rises from an initial low
value to a value of from about 2~ to about 4k, an apparatus for stock
dewatering consisting essentially of in combination:-
(a) a suction box comprising a plurality of evacuated chambers,each of which is provided with a vacuum tight drainage means, located
in the forming section adjacent the head box slice and extending
continuously for the length of the forming section;
(b) a foraminous surface on the suction box adapted to support
the forming fabric, to provide apertures through which the forming
fabric drains under the influence of the vacuum in the suction box,
to provide a path through which the forming fabric moves which will
cause a controlled level of uniformly spaced periodic harmonic
agitation within the stock on the forming fabric; and to provide a
vacuum tight seal between the forming fabric and the evacuated
chambers of the suction box; and
(c) a vacuum control means whereby the vacuum in the evacuated
chambers of the suction box is controlled to a value that rises
progressively along the suction box for the length of the forming
section from an initial low value of less than 5 cms water gauge
adjacent the head box slice, to a maximum value of no more than 50
cms water gauge at the end of the forming section.
In one embodiment, the suction box comprises a plurality of
contiguously adjacent suction boxes, each of which is the full width
of the forming section. Alternatively, in a second embodiment, a
series of vacuum-tight transverse divisions can be provided in a
single large suction box. Thus the suction box comprises a sequence
of separated drainage chambers, to each of which a controlled level
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WO 90/15187 PCI`/CA90/00185
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of vacuum is applied, rising stepwise from a level of no more than 5
cms water gauge adjacent the head box slice to no more than 50 cms
water gauge at the other end of the suction box, that is at the end
of the forming section. Each drainage chamber is also provided with
a separate vacuum-tight drainage means.
In a preferred embodiment, the foraminous surface comprises a
slotted type fabric-supporting cover comprising a series of spaced
apart forming fabric-supporting blades having generally planar top
surfaces transverse to the direction of travel of the fabric in a
common essentially horizontal plane providing therebetween
suction-accessible gaps in which the forming fabric is substantially
unsupported and is drawn downward to form stock-agitating undulations
in said gaps, the cover including water seal-forming blades disposed
intermediately in the gaps between the fabric-supporting blades and
having top surfaces transverse to the direction of travel of the
fabric at a lower level than the top surfaces of the
fabric-supporting blades, and at least forming water seals at the
downward undulations of the forming fabric thereby interrupting the
suction temporarily to limit drainage while causing vertical
agitation of fibers on the fabric passing through the forming
section. In a most preferred embodiment, the fabric supporting
blades are spaced apart equally from each other for the length of the
forming section. This provides the desired and required agitation in
the stock, since the forming fabric and the paper stock thereon
undulate in a periodic or harmonic manner for the length of the
forming section.
In certain circumstances it is contemplated that it may not
prove to be either practicable or desirable to utilize a full length
open surface forming section using vacuum assisted drainage, for
example when modifying an existing paper making machine.
Furthermore, it has to be noted that obtaining and controlling the
very small vacuum levels needed adjacent a head box slice itself is
quite difficult. As noted above, a pressure difference of less than
5 cms water gauge, or approximately only 0.5% of an atmosphere, is
WO 90/15187 PCI/CA90/00185
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being used. The basic concepts of this invention can still be used,
nevertheless, in paper making machines having an open surface forming ..
section wherein a vacuum assisted section is preceded by a short
dewatering section using other static dewatering devices.
Thus in a third broad aspect this invention provides in a paper
making machine having an open surface forming section, including at
least a travelling continuous forming fabric which passes over a
breast-roll adjacent a head box having a head box slice through which
aqueous stock is deposited onto the forming fabric, and a plurality
of stock dewatering devices beneath the forming fabric which are
provided with white water drainage means, in which forming section
the solids content of the stock deposited from the head box through
the head box slice onto the forming fabric rises from an initial low
value to a value of from about 2% to about 4%7 an apparatus for stock
dewatering consisting essentially of in combination:-
(a) a relatively short foraminous dewatering device adjacentthe head box slice consisting essentially of a forming board section,
including at least one forming board, situated adjacent the slice,
and a foil section adjacent to the forming board section, which
includes a plurality of foils, in which foil section each foil
comprises a flat support surface, and a trailing portion (in the
direction of forming fabric travel) diverging from the plane of the
fabric at an angle greater than zero degrees and less than five
degrees;
(b) a suction box adjacent the dewatering device extending for
the remainder of the length of the forming section, which suction box
consists essentially of either a single box provided with transverse
vacuum tight divisions each provided with a vacuum supply means and a
water drainage means or a plurality of drainage boxes each extending
the full width of the forming fabric, which are placed contiguously
with each other thereby effectively having a common dividing wall,
and which are each provided with a vacuum supply means and a water
drainage means;
(c) a foraminous surface on the suction box adapted to support
the forming fabric consisting essentially of a slotted-type fabric
P ~ /CA90/00185
W O 90/1~187
2~57932
g
cover comprising a series of uniformly spaced forming
fabric-supporting blades having a generally planar top surface
transverse to the direction of travel of the fabric in a common
essentially horizontal plane providing therebetween
suction-accessible gaps in which the forming fabric is substantially
unsupported and is drawn downward to form stock-agitating undulations
in said gaps, said cover including water seal forming blades disposed
intermediately in said gaps between the fabric supporting blades and
having top surfaces transverse to the direction of travel of the
fabric at a lower level than the top surfaces of the fabric
supporting blades and at least forming water seals at the downward
undulations of the forming fabric, thereby interrupting the suction
temporarily to limit drainage while causing vertical agitation of
fibers on the fabric passing through the forming section; and
wherein both the first, the last, and any intermediate support-blades
placed over either an internal transverse vacuum tight division or a
pair of contiguous transverse walls of two adjacent suction boxes,
are all forming fabric-supporting blades, thereby inducing uniformly
spaced periodic harmonic agitation in the stock; and
(d) a vacuum control means whereby the vacuum in the suction
box is controlled to a value that rises progresively stepwise along
the length of the suction box from a low value of less than 5 cms
water gauge adjacent the foraminous dewatering device to a maximum
value of no more than 50 cms water gauge at the end of the forming
section.
Further, in a fourth broad aspect this invention provides a
process for improving paper formation in a paper making machine
having an open surface forming section, including at least a
travelling continuous forming fabric which passes over a breast-roll
adjacent a head box having a head box slice through which aqueous
stock is deposited onto the forming fabric, and a plurality of stock
dewatering devices beneath the forming fabric which are provided with
white water drainage means, in which forming section the solids
content of the stock deposited from the head box through the head box
WO 90/15187 PCr/CA90/00185
~G~93~ ~
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slice onto the forming fabric rises from an initial low value to a
value of from about 2% to about 4~, comprising the steps of:
(a) discharging onto the moving forming fabric an aqueous paper
making fiber stock across the width of the forming fabric;
(b) causing the forming fabric to move over a relatively short
foraminous dewatering device adjacent the head box slice, which
includes a plurality of stationary drainage elements disposed in a
supporting relationship transversely of the forming fabric,
(c) thereafter causing the forming fabric to move over a
continuous vacuum assisted suction box including a plurality of
chambers to each of which a controlled level of vacuum is applied and
which includes a foraminous top support for th~e forming fabric which
extends for the remainder of the length of the forming section and
which provides a vacuum tight seal between the forming fabric and the
evacuated chambers of the suction box;
(d) controlling the vacuum in evacuated chambers of the suction
box to a value that rises progressively along the length of the
suction box from an initial value of less than 5 cm~ water gauge
adjacent the short foraminous dewatering device to a maximum value of
no more than 50 cms water gauge at the end of the forming section;
and
(e) causing a desired level of uniformly spaced periodic
harmonic agitation in the stock on the forming fabric by providing
differences in level between different parts of the forming fabric
along the length of the suction box.
In yet another refinement of these broad apparatus and process
aspects of this invention it is also contemplated that the short
foraminous dewatering device can also provide a path causing changes
in level in the forming fabric thereby initiating a controlled level
of agitation in the stock before it reaches the foraminous surface of
the suction box. Thus it is preferred that the short foraminous
dewatering device includes both a forming board adjacent to the head
box slice, and a foil section (between the forming board and the
suction box), wherein the foil section may comprise up to four fifths
of the area drained by the short foraminous dewatering device. In
WO90/15187 2~57932 PCI/CA90/00185
.
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the foil section, each foil comprises a flat support surface, and a
trailing portion (in the direction of forming fabric travel)
diverging from the plane of the fabric at an angle greater than 0
and less than 5. The foils generally are grouped above drainage
boxes of a suitable size.
Preferably the separate foils are also so placed as to
contribute toward controlled agitation of the stock on the forming
fabric. Preferably, the combination of forming boards and foil
section comprises no more than about 20% of the length of the forming
zone .
As was discussed above, it is preferred that the suction box
either is a plurality of contiguously adjacent suction boxes (each of
which is the full width of the forming section), or is provided with
a series of vacuum-tight-transverse divisions to provide a series of
separated drainage chambers to each of which a controlled level of
vacuum is applied. The applied vacuum will thus rise in a stepwise
fashion in the suction box from an initial low value of no more than
5 cms water gauge up to a maximum of no more than 50 cms water gauge
at the end of the forming section. Such a mode of construction
readily permits control of the applied level of vacuum.
Similarly, in a most preferred embodiment of these aspects of
this invention, the same foraminous surface, comprising a slotted
type fabric supporting cover, as described by Johnson, in U.S.
4,140,573, is used for this aspect of this invention. Thus in a more
limited broad aspect this invention provides a process for improving
stock formation on a paper making machine, including the steps of:
(a) discharging onto the moving forming fabric an aqueous paper
making fiber stock across the width of the forming fabric,
(b) causing the forming fabric to move over a relatively short
foraminous dewatering device comprising in combination a forming
board section, adjacent the head box slice, and a foil unit section
in which section each foil comprises a flat support surface, and a
trailing portion (in the direction of forming fabric travel)
WO 90/15187 ~ 7 9 3 2 PCl /CA90/00185
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diverging from the plane of the fabric at an angle greater than zero
degrees and less than 5 degrees;
(c) supporting the forming fabric over a continuous suction box
including a plurality of chambers to each of which a controlled level
of vacuum is applied at uniformly spaced apart zones transverse to
the direction of the travel of the fabric and permitting the fabric
to sag in gaps between the supported zones and forming vertical
fabric undulations in said gaps, thereby inducing uniformly spaced periodic harmonic agitation in the stock;
~ d) providing water seal means intermediately of the gaps in a
plane below where the fabric is supported to interrupt the suction;
(e) applying vacuum in said gaps to the underside of the
forming fabric to draw the fabric downwardly between the gaps, the
suction applied in each gap being interrupted by the water seal
forming means as the aqueous paper-making suspension of fibers is
dewatered; and
(f) controlling the vacuum applied in the gaps so that the
applied vacuum rises progressively from an initial value of less than
5 cms water gauge adjacent the foil unit to a maximum value of no
more than 50 cms water gauge at the end of the forming section.
The invention will now be described by way of reference to the
attached drawings in which:
Figure l shows diagrammatically the initial part of a
conventional paper making machine;
Figurc 2 shows a conventional foil blade;
Figure 3 shows a so-called Isoflo unit;
Figure 4 shows schematically harmonic stock agitation associated
with a series of foils;
Figure 5 shows diagrammatically a paper making machine modified
according to one aspect of this invention;
Figure 6 shows a modification to the machine of Figure 5; and
Figure 7 shows a detail of Figure 5.
In these Figures, relevant like parts have been given the same
numbers.
WO 90/15187 PCI/CA90/00185
2~7~3:2-
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In Figure 1, the forming section of a conventional open surface
paper making machine is shown, incorporating a forming fabric 1,
which moves in the direction of the arrows shown at lA and lB. The
forming fabric moves over a breast roll 2, and various tensioning and
idling rollers 3. The stock is deposited onto the forming fabric 1
from the head box shown diagrammatically at 4, through a slice 5,
which extends across the forming fabric 1. Beneath the forming
fabric in the dewatering zone are placed a sequence of drainage
devices 6, 7, 8, 9, 10, 11 and 12, provided with white water drains
15, 16, 17, 18 and 19. The first of these drainage devices, 6,
comprises a forming board, the second, 7, comprises an open foil
unit, and the remainder are so-called Isoflo units (Trade Mark).
Boxes 8 to 12 are also provided with a controlled vacuum, through the
vacuum pipes 20, 21, 22, 23 and 24 respectively. The vacuum applied
will typically range from zero to 5 cms water gauge in box 8, to no
more than 50 cms water gauge in box 12, the white water drains 15,
16, 17, 18 and 19 contain suitable vacuum legs. A key feature, from
the aspect of this invention, is that not all of the forming section
is being actively drained. The drainage and suction boxes are
separated by the spans marked a, b, c, d, e and f which represent
undrained areas, apart from any water which may happen to drain
through under gravity. In the machine shown, these spans represent
nearly 30% of the total area of the forming section.
In this machine, which is typical of existing prior art
machines, three different forms of drainage element are used, in
sequence away from the head box slice 5. The first of these is a set
of conventional flat forming board blades associated with box 6.
In box 7 the drainage elements 25 are conventional foil blades
broadly conforming to the design shown in section in Figure 2. These
foils comprise a supporting bar 28 with a tee-shaped head, onto which
is slid the foil blade proper, 29. This includes a flat face 30 onto
which the forming fabric 1 rests, and a divergent trailing face 31.
In the figure the divergent angle Z is shown exaggerated for clarity.
Generally it is far smaller than it is shown, ranging from about 1
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degree to about 5 degrees, with angles of 2 to 2.5 degrees being
commonly used. As the forming fabric moves over the foil in the
direction of the arrow 32, as a consequence of hydraulic phenomena
created in the nip provided by the trailing face 31, water is sucked
from the stock through the forming fabric.
In boxes 8 through 12 a so-called Isoflo unit is used, which is
described in detail in Johnson, U.S. 4,140,573. This is shown in
Figure 3 (which corresponds broadly to Johnson's Figure 4), and can
be seen to incorporate two groups of static devices 26 and 27.
Devices 26 and 27 are each supported on a tee-bar 28, these tee bars
28 are supported across the width of the box by suitably placed
supports 33. Although similar in appearance to the foil blades of
Figure 2, the static devices 26 and 27 differ in two separate ways.
The top faces of all of these devices which bear against the forming
fabric 1 are generally planar and either in the plane of the forming
fabric (devices 26) or a little below it (devices 27). As shown in
Figure 3 the vertical lowering of the devices 27 is indicated at A,
which is exaggerated for clarity. In practise, this distance
generally will range from about 0.5 mm to about 5.0 mm. The forming
fabric in moving over such a foraminous surface undulates between
successive devices 26, and the intervening devices 27 are so placed
vertically as to provide a water seal to the underside of the forming
fabric. Sealing elements, not shown, are also provided along the
sides of the boxes in between the drainage devices, parallel to the
sides of the forming fabric. Water is drawn from the stock through
the forming fabric by the application of vacuum to the box.
There is a further feature which is common to both of these
forms of static drainage devices. Figure 4 shows diagrammatically
the harmonic, or periodicj stock agitation that can be generated by a
regular and uniform spacing of the vertical pu1ses generated by foil
blades supporting a forming fabric. In Figure 4, a small section of
the forming fabric 1 is shown moving in the direction of arrow lA.
The forming fabric passes over a series of foil blades all uniformly
spaced apart by the distance Y, as indicated between foil blades 45
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and 46 mounted on the tee bars 41 and 42. Because the stock
agitation is generated by vertical movement of the forming fabric
caused by the foil blades, which are each spaced apart by the
constant distance Y, the area of vertical stock agitation shown by 48
is followed by another area 50. Similarly, the quiescent zone 49 is
followed by another quiescent zone 51, following the area 50. As
Figure 4 indicates, both the areas of vertical agitation 48 and 50
and the zones of quiescence 49 and 51 are each spaced apart at the
same distance Y. As shown in Figure 4, with no foil blade on the tee
bar 43, vertical agitation of the stock still occurs at the location
52 (which is differently shaded in Figure 4 to emphasize that there
is no foil blade on tee bar 43), although the amplitude of the
agitation at the location 52 is somewhat less than is obtained with a
foil blade in place on tee bar 43. The occurrence of this activity
in the vicinity of the tee bar 43 (which has no foil blade) is
referred to as occurring at a "ghost blade". It is also important to
note that these areas of agitation and quiescence in the stock do not
move with the forming fabric, but rather remain in essentially the
same place. A similar quiescent zone 53 follows the "ghost blade",
and foil induced agitation 54 occurs in the vicinity of foil 47.
For the Johnson Isoflo device shown in Figure 3, the area of the
stock vertical agitation is due to the downward deflection of the
fabric as it moves from fabric support surfaces 26 to surfaces 27,
and periodicity similar to that of Figure 4 is observed.
Thus it has been found that spacing of the various support
surfaces for the forming fabric can be used to generate, to optimize
and to control the agitation occurring in the stock on the forming
fabric. The dewatering support surfaces can be placed to control the
vertical movement which is initiated by earlier drainage devices.
This flexibility of control of the amount of vertical agitation
permitted to occur in the stock on the forming fabric allows for
better dewatering of the stock as it travels on the forming fabric
through the forming section. Two possible ways of utilizing this
WO 90/15187 pcr/cA9o/ool8~
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205~9~
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invention are shown in Figures 5 and 6. Each of these figures shows
essentially the same portion of a paper making machine as is shown in
Figure l, but with certain differences. In common with Figure l, the
forming fabric l passes over the rollers 3, around the breast roll 2,
and then past the head box slice 5, at which point the stock is
deposited onto it. Drainage is initiated by the forming board
section on box 6, and continued by the foils associated with box 7;
it is to be noted that boxes 6 and 7 are still separated by the gap
a.
The remainder of the forming section comprises a single extended
suction box lO0, which is separated into the sequence of separate
chambers 8, 9, lO, ll and 12, either by using a single continuous
suction box with dividers, or by using a plurality of smaller boxes,
butting up closely to each other. These suction units also differ
from the arrangement shown in Figure 3 in another way. In that
figure, the first support surface 26 is an upper one, and is followed
by a lower one, 27. As described by Johnson in U.S. 4,140,573, the
last support surface in the box is also an upper one. When a
sequence of boxes of this type are brought into the contiguous
relationship of this invention, one of these surfaces becomes
redundant, since the last support surface in any one box also becomes
the first support surface for the next one. This is shown more
clearly in Figure 7.
There are two different ways in which the continuous suction box
of this invention can be utilized to improve the sort of machine
shown in Figure l. In Figure 5, the gaps c through f have been
eliminated by lengthening the boxes 8 through 12, and therefore the
overall length of the forming section has not been changed.
Alternatively, the same effect could be achieved by adding another
box to match the span of gaps c through f, and moving boxes 9, lO and
ll to accommodate it. In either case, the length of the forming
section is retained unchanged. The other option is simply to move
the suction boxes 8 through l2 together to eliminate gaps c through
f. Whilst this is effective, it has two disadvantages. The first is
WO 90/15187 2 0~ 7 ~ 3 2 PCI /CA90/00185
- 17 -
that it will still leave an equivalent length of forming fabric
(corresponding substantially to the eliminated gap length)
effectively unused, unless the somewhat drastic step of reducing
overall machine length is also taken. The second is that although
the same amount of water is being removed from the stock, the use of
an overall shorter forming section means that water is being removed
more rapidly, than is the case if the length of the forming section
is retained unchanged. It appears to be advantageous to retain the
forming section length, since removing the same amount of water over
a longer length of forming fabric reduces the rate at which that
water is removed. Decreasing the drainage rate generally improves
the quality of the paper being made, since better paper mat formation
occurs and wire marking is lessened. Further, the FPR figures also
improve; it appears to be feasible to obtain an improvement of the
order of 20% with the method of this invention.
In Figure 5 two of the noted gaps which are not actively drained
are still present in the forming fabric: these are a and b. As is
shown in Figure 6, it is also possible to eliminate gap b, and hence
to lengthen the suction box 100 a little further. The same
considerations will also apply concerning whether the gap b is
removed by lengthening box 100, by adding another box, or by
laterally displacing box 100 as are mentioned above in respect of
Figure 5. It appears to be preferable to lengthen box 100, to remove
gap b. It should also be noted that if gap b is to be eliminated,
then the last foil in box 7, which will generally be over the wall of
box b adjacent the gap, will become redundant, as the arrangement
shown broadly in Figure 7 needs to be used, with a fabric support
blade over the contiguous walls of chamber 7 and the suction box 100.
