Note: Descriptions are shown in the official language in which they were submitted.
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TWIN WIRE FORMER
FIELD OF THE INVENTION
The present invention relates to a twin wire
former particularly adapted to improve formation under
high speed forming conditions by forming two discrete
layers one on each of the wires by draining
substantially solely in one direction and then
substantially solely in the opposite direction each
drainage occurring over a discrete forming shoe.
BACKGROUND OF THE PRESENT INVENTION
Many different concepts for twin wire
formers have evolved over the years. For example, a
twin wire former sold by Dominion Engineering Works
under the trade mark Papriformer ~ is a roll type
former wherein the jet issuing from the headbox is
trapped between a pair of wires at the on-coming nip
between the wires formed on a roll and drainage is
substantially completed on this roll. Drainage is
relatively fast and formation is completed in a very
2U short period of time or distance along the roll
surface thereby freezing the fibres in their relative
positions as they will eventually appear in the
finished web very quickly.
The formation achieved using this equipment
is adequate for some purposes.
Other twin wire formers incorporate a
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forming shoe or shoes which reduced the drainage rate
since the curvature of the shoe surface has a much
longer radius than the radius that could practically
be used for a forming roll. Such use of the longer
radius reduced the unit pressure on the slurry between
the wires, which thereby reduced the rate of drainage
and had a tendency to improve formation. Such a
machine was marketed by Beloit under their trade mark
~el Baie ~ and was followed by the Bel Baie II and
Bel Baie III formers all of which used forming shoes
of one form or another with drainage either mainly in
one direction or simultaneous drainage in two
directions. All apply pulses to the stock by passing
over spaced elements forming the surface of at least
one shoe.
seloit also sell selbond ~ formers which
are basically a modified fourdrinier machine having a
forming shoe mounted above the fourdrinier wire at a
distance spaced from the headbox. The forming shoe
forms a twin wire section on the machine and is
provided with suction boxes etc., so that drainage
occurs in the area of the shoe in both directions,
i.e., through the top wire and bottom wire
simultaneously, the main sequence of drainage being
drainage downwardly only, followed by a first shoe
which normally permits drainage in both directions and
a second or main shoe positioned within the upper wire
to force water downwardly through the main fourdrinier
wire while at the same time drawing water up through
the top wire via an auto slice and suction boxes.
This particular device is primarly used to form
multi-layer board grades a low speeds.
Dominion Engineering Works Limited, within
the last several years, has sold a ~ynaformer
which is a roll type former primarily used as
fourdrinier re-build wherein no shoes were necessary.
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Such a machine is disclosed in Canadian Patents
1,144,795, issued April 19, 1983 and 1,157,302 issued
November 22, 1983 to Malashenko. A number of these
machines have been sold and are currently operating to
permit the formation of paper at high speed. It is
energy efficient in that only rolls are used and forms
a sheet having relatively good formation but the
machine has limited adjustments to ensure optimum
formation when the stock to the machine changes.
Similar type formers to the Dynaformer have
been sold under the trade mark Duo-Former H ~ by
Voith and Symformer ~ by Valmet oy.
The Black Clawson Company has recently sold
a twin wire former under the trade mark Top Flyte R
which is similar in some respects to the former shown
in the above referred to Canadian Patents of
Malashenko, however, instead of formation occurring on
the roll, formation takes place as the twin wires pass
between blades that alternately engage opposite wires
and apply pulses to the stock and force drainage in
two directions at each of the blades. This part~cular
former apparently produces paper web having reasonably
good formation and provides some adjustability by
changing the location of the blades, i.e., the
relative position of the edges of the blades to the
normal path of travel of the wire that they contact
provides an adjustment that may be made if required
when the stock is changed.
U.S. Patent 4,433,812, issued July 5, 1977,
to Riihinen discloses a machine especially adapted for
the manufacture of tissue paper wherein formation is
completed very quickly since tissue stock is
relatively free draining and the thickness of the
sheet being formed is relatively thin. In this
particular device drainage occurs downwards from the
bottom wire then upwards through the top wire and
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continues to drain upward through the top wire until
formation is completed. Obviously since the machine
relates to a tissue machine, formation is not a prime
objective and the considerations for drainage etc.,
and formation are significantly different than for a
newsprint sheet or a fine paper sheet.
Attention is also directed to U.S. Patent
4,154,645, issued May 5, 1979 to Kankaapaa. The
device disclosed in this patent is a machine for
making a multi-layered paper boards. The formation of
each layer is accomplished by a drainage sequence
wherein drainage initially takes place through a first
or lower wire, is followed by two way drainage which
in turn is then followed by one wa~ drainage through
the top wire to complete formation.
Generally the above described devices in one
manner or another apply sharp pulses to the stock
during formation for example as the stock passes the
doctor like element of the Top Flyte ~ former or
over spaced element forming the surface of the shoes
of most of the other formers.
It has also been recognized in the art that
relatively slow drainage is important to formation
thus drainage over the initial section of a
fourdrinier wire wherein the thin layer is formed, is
taught for example in Canadian Patent 983,750, issued
February 17, 1975, to Kankaapaa. In this particular
patent, drainage initially is downward through the
wire at a relatively gentle rate until formation of
the total thickness of the web is completed and then
dewatering takes place over a pair of consecutive
shoes which drive the water first in one direction and
then the other from the web, i.e., formation is
completed by slow drainage in one direction only,
namely through the bottom wire on a fourdrinier
section.
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It is also known from Canadian
Patent 930,580 issued July 24, 1973 to McCormick to
form a web by forcing water through the top wire of a
twin wire former over an extended length of soild shoe
having progressively decreasing radius of curvature
and scooping the water forced from the web from the
top wire without applying pulses. In this case
formation takes place against one wire only and the
pressure is gradually increased as required to drive
the water through the layer of formed pulp as this
layer increases in thickness.
BRIEF DESCRIPTION OF THE PRESENT INVENTION
It is an object of the present invention to
provide an improved twin wire former particularly
suited for the manufacture of fine paper, and
newsprint or the like.
Broadly the present invention relates to a
twin paper forming machine comprising a first forming
wire, forming a first continuous loop and a second
forming wire forming a second continuous loop, means
to feed low consistency paper making stock containing
paper making fibres and water between said forming
wires, a first forming shoe having a solid forming
surface within the loop of said first forminy wire and
engaging the inner surface of said first forming wire,
said second forming wire pressing said stock between
said wires against said first shoe to cause water to
drain from said stock through said second forming wire
and form first formed layer of stock thereon, a
second forming shoe having a solid forming surface
mounted within the loop of said second wire and
engaging the inner face of said second wire said
second shoe being spaced in front of said first shoe
in the direction of travel of said wires, said solid
surface of said first shoe having a curvature of at
least 50 inches radius and having a length sufficient
4697
so that between 25 and 70% of the water in the stock
is drained over said first shoe without completing
formation, said shoes being arranged so that said
wires curve smoothly over each said shoe and between
said shoes with no major abrupt changes in direction
thereby to gently drain said stock without subjecting
said stock to significant pulses.
Preferably the lenyths of the working wraps
of said shoes measured in the machine direction will
be between about 8 and 15 inches and the spacing
between adjacent shoes will be between about 1/2 and 3
inches so that formation can be completed on the
second shoe in a forming zone less than about 30
inches in length measured from the beginning of the
first shoe to the end of the second shoe.
BRIEF DESCRIPTION OF THE DRAWINGS
Further features, objects and advantages
will be evident from the following detailed
description of the preferred embodiments of the
present invention taken in conjunction with the
accompanying drawings in which:
Figure 1 is a schematic side elevation view
of one form of the invention;
Figure 2 is a view similar to Figure 1
showing a modified forming section;
Figure 3 is a view similar to Figure 1
showing a modified dewatering section;
Figure 4 is an enlarged schematic
illustration showing how formation is believed to take
30 place when using the present invention.
Figure 5 is a schematic arrangement of
another form of the invention.
DESCRIPTION OF THE PXEFERRED EMBODIMENTS
Before describing the present invention it
should be noted that terms such as formation, formed
and forming as used in the disclosure and claims
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unless otherwise indicated are intended to be read in
their paper making sense, that is designate that the
fibres are fixed in their relative positions
substantially as they will be in the finished or dried
sheet, i.e. the random distribution of fibres in the
finished sheet is fixed when "formation~ is
completed. After formation has been completed the
sheet is simply dewatered with only minimal changes in
fibre location, for example by stretching and the like
as the sheet traverses the driers, calendars etc in
the paper making process. Generally formation is
completed when the stock reaches about 4% consistency.
As shown in Figure 1, former 10 of the
present invention comprises an upper forming wire 12
and a lower forming wire 14 each of which forms a
continuous loop. These upper and lower wires pass
over a pair of breast rolls 16 and 18 respectively
which may be either solid or open rolls and stock is
ejected between these wires from the headbox generally
indicated at 20.
In the arrangement shown in Figure 1 the
wires 12 and 14 pass from the breast rolls 16 and 18
and converge over a shoe 22 having a solid forming
surface 24 which deflects the wires 12 and 14 so that
the tension in the wire 14 applies pressure against
the stock between the wires 12 and 14 and enforces
drainage into the saveall 26 appropriately positioned
to receive this draining water. If desired a forming
board shown in dotted lines at 28 maybe provided
adjacent the lower breast roll 18 however this is not
essential and is dependent to a degree on the length
of the incoming run from the breast roll 18 to the
shoe 22. The majority of drainage through the bottom
wire will occur as the wires traverse the shoe 22.
From the shoe 22 the wires pass onto a
second shoe 30 having a solid forming surface 32
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curved in the opposite direction to the surface 24
over which the wires 12 and 14 pass with the tension
in the wire 12 forcing water up through the wire 12 to
be scrapped from the wire 12 via scrappers or doctors
generally indicated at 34 that preferably just contact
the wire and into a saveall generally indicated at 36.
The wires pass from the forming shoe 30 to a
solid roll or as illustrated to an open roll 38 which
causes drainage in two directions namely out through
the wire 12 and into the saveall 40 and into the
roll 30 and down into the saveall 42. The open roll
will only be used where the amount of dewatering
demands d~ainage in two directions. The wrap on the
roll 38 is not critical but will generally be in the
order of 20 to 60 degrees.
From the roll 38 the wires pass over a
roll 44 which will normally be a solid roll although
in certain circumstances might be a suction roll or
open roll provided with suitable water clearing
equipment such as savealls and suction glands etc.
The use of such more expensive rolls will normally be
avoided since it is not believed to be necessary.
Generally the roll 44 will be a solid roll and
drainage will take place through the wire 14 into the
saveall 42.
After the roll 44 the wires 12 and 14 are
separated with the web following with the wire 14 to
be carried to the normal couch roll etc (not shown).
It will be apparent that suitable guide roils such
as those indicated at 46 will be provided for the
upper wire 12 and those indicated at 48 for the lower
wire 14 to complete the loops.
In the illustrated arrangement the shoe 22
is mounted on leading and trailing variable length
pillars 50 and 52 so that the angle of the shoe 22 may
be adjusted as well as its vertical position relative
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to the wire path i.e. it can be bodily raised or
lowered. Similarly the shoe 30 will be mounted on
leading and trailiny adjustable length pillars 54
and 55 which permits its angle of orientation to be
adjusted and operating both the pillers in unison its
height or position relative to the breast rolls may be
changed.
It is important that there be no major
abrupt change in direction of the wires as they
approach or leave the shoes 22 and 30 and thus the
leading end 58 of the shoe 30 must be positioned
relative to the trailing end 60 of the shoe 22 so that
there is no major abrupt change in direction of the
wires as they pass from the shoe 22 to the shoe 30.
It has been found that the leading end 58 of shoe 30
may deflect the wires but up to 1/16 of an inch i.e.
"dig into" the wire by distance of up to 1/16 of an
inch measured substantially perpendicular to the
direction of wire travel and that such a defection
will tend to increase drainage with little effect on
formation. Slightly greater deflection has been seen
to tend to crush the paper web being formed.
The curvatures of the surfaces 24 and 32
will be such that they will have radiuses of at
least 50 inches probably not over 250 inches and
generally closer to 100 inches say between 80 and 150
inches. Obviously, the precise curvature of the shoe
will depend on the rate of the drainage desired (the
greater the radius the slower the rate of drainage for
any given tension in the outside wire passing over the
shoe and this determines the unit pressure applied by
this outside wire to cause drainage therethrough).
If desired the surface 32 of the trailing
shoe 30 may be divided into a first section 62 that is
substantially planar and has a length of about 2 to 4
inches followed by a second section 64 that is curved
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as above described and provides the forming section of
this shoe. The working wrap of the shoe will be
about 8 to 15 inches in length measured in the path of
wire travel. The working wrap or working length of a
shoe is that portion of the wrap of the shoe by the
wires in which the tension in wire 14 drives water
from the stock through the outside wire.
The surface 24 of the shoe 22 may be
similarly formed with a straight or planar lead ln
section substantially equivalent to the section 62 at
its lead in end and a curved forming section
equivalent to the section 64 i.e. both of the
shoes 22 and 30 may be provided with similar formi~g
surfaces 24 and 32 respectively. The working wrap of
shoe 22 will also be between about 8 to 15 inches in
length measured in the path of wire travel. More
generally the length of working wrap of the wires on
the shoe 22 will be such that the total drainage over
the shoe 22 will drain 25 to 70~ of the water in the
stock leaving the headbox without completing formation.
A further embodiment of the present
invention in shown in Figure 2. In this embodiment
the first forming shoe 22' is substantially equivalent
to the shoe 22 but is located within the loop of the
bottom wire 14 so that drainage is upward through the
top wire 12 first, with the water pressed from the
slurry via the wire 12 passing up the ramp generally
indicated at 66 into the saveall 68 substantially
equivalent to the saveall 36, as described
hereinabove. Mounted below the saveall 66 is shoe 30'
that may be substantially equivalent to the shoe 30
described hereinabove but located within the top
wire 12 to force drainage through the bottom wire 14.
Because the water does not form a puddle on the bottom
wire as it does on the top wire when drainage is
downward the length of shoe 30' in the machine
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direction may be extended beyond the above
described 15 inch maximum length.
Another embodiment of the present invention
is shown in Figure 3. This particular arrangement is
substantially the same as that shown in Figure 1
however instead of the roll 38 being located within
the loop of the bottom wire 14 a similar roll 70 is
located within the loop of the top wire 12 to cause
drainage downwardly into the saveall 72 and if an open
roll is used upwardly into the saveall 74 positioned
behind the second roll 76. This second roll 76 is
equivalent to the roll 44 but it is located within the
loop of the bottom wire 14 and forces water up through
the top wire 12 into the saveall 74.
Obviously, the arrangement in Figure 2 may
also be used with the roll arrangement shown in
Figure 3 i.e. with the rolls 70 and 76 replacing the
rolls 38 and 44 respectively and located within the
loops of the different wires 12 and 14.
In the above described embodiments the
rolls 38 and 44 or 70 and 76 have been used for
dewatering the stock after the forming shoes, i.e.
after formation has been completed. These rolls could
be replaced by a second pair of solid forming shoes
that further dewater the stock. The advantage of such
shoes is reduced cost and reduction in shear that
accompanies wrapping of the two rolls by the two wires.
An example of an arrangement using shoes in
place of the rolls is illustrated in Figure 5 in
combination with the Figure 2 arrangement but wherein
the forming shoes 22' and 30' are followed by
dewatering shoes 100 and 120 which dewater the formed
web while keeping the wires in a more or less straight
line path.
Obviously be repositioning the shoes 100
and 102 the shoes may be used to replace rolls in the
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Figure 1 embodiment.
Referring now to Figure 4 the operation of
the device will be described particularly for the
drainage over the two shoes 22 and 30 wherein
formation is substantially completed before the wires
leave the shoe 30.
As shown in Fiyure 4 stock issues from the
headbox 20 as indicated by the arrow 80 between the
two wires 12 and 14 as they converge on the shoe 22.
Water is driven through the wire 14 and drained into
the saveall 26 over the curved section of the
surface 24 of the shoe 22 to form the first layer 82
of formed fibres adjacent the wire 14. The two wires
then pass through an open stretch generally indicated
at 84 in Figure 4 which normally will be between
approximately half an inch to about 3 inches in length
(there being little point in extending the spacing 84
much beyond three inches) to flow onto the surface 32
of the shoe 30 without, as above defined, any major
change in direction of wire travel. As the wires pass
over the shoe 30 the tension in the wire 12 forces the
wire 12 towards the shoe 30 in the curved section 64
thereof to force water to drain through the wire 12.
This water is then scraped by a doctor blade 34 which
just contacts, or as above described may defect the
wire up to 1/16 of an inch, the inner surface of the
wire 12 to deflect the water into the saveall 36. The
water draining through the upper wire 12 forms a
second layer 86 adjacent the wire 12. As this layer
increases in thickness due to drainage through the
wire 12 the amount of water throughout substantially
the whole thickness of the slurry between the wires is
reduced and consistency increases so that formation is
substantially completed as the wires leave the shoe 30
as schematically illustrated at 88 in Figure 4 i.e.
the formed layers span the gaps between the wires 12
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and 14.
The total drainage over the first shoe 22
or 22' should not complete formation of the total
thickness of the web and normally should only result
in drainage of between 25 and 70% of the water in the
stock leaving the headbox.
It will noted that as the wires traverse the
respective shoes 22 and 30 initially there is no or
substantially no build up of fibres or formed layer on
the wire through which drainage is forced as the
respective shoe is traversed i.e. as the shoe 22 is
traversed initially there is substantially no layer 82
and this layer gradually builds up as the two wires
with the slurry therebetween pass over the shoe 22.
The shoe 22 has sufficient curvature that the tension
in the wire 14 gradually drives water through the
wire 14 into the saveall 26. Obviously this drainage
is also aided by the influence of the gravity, thus
the curvature of the forming section or curved
section 64 may be of a greater radius than the
equivalent curved section 64 of the shoe 30 since
drainage over the shoe 30 must overcome gravity and be
forced through the upper wire 12.
It will be evident since the working length
or working wrap in the machine direction of the curved
section 64 of shoes 22 and 30 will be in the range of
about 8 to 15 inches preferably 10 to 14 inches and
that the total forming length from the beginning of
the shoe 22 to the end of the shoe 30 may be as short
as about 2 feet i.e. ~formation" rnay be completed in
about 2 feet of running length of the wires yet
relatively gentle drainage obtained with the resultant
formation being equivalent or better than that
obtained in using much more extensive forming sections
or that obtained using rolls.
Having described the invention modifications
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-14-
will be evident to those skilled in the art without
departing from the spirit of the invention as defined
in the appended claims.
