Note: Descriptions are shown in the official language in which they were submitted.
2123820
GAP FORMER IN ~ PAPER MACHINE
BACKGRO~ND OF THE INVENTION
The present invention relates to a gap former in a paper
machine in which a twin~wire zone is formed by a loop of a carrying
wire and a loop of a covering wire. The twin-wire zone starts from
the forming gap into which the slice part of a headbox feeds a pulp
suspension jet. After the twin-wire zone, the web is separated
from the covering wire and is transferred on the carrying wire onto
a pick-up fabric to be carried into the press section of the paper
machine. The present invention also relates to a method for
dewatering a web in a gap former.
With increased running speeds of paper machines, several
problems in the web formation have been manifested with increased
emphasis. In the former section of a paper machine, the phenomena
that act upon the fiber mesh and upon the water, which is still -
relatively free in connection with the fiber mesh, such as
centrifugal forces, are increased in proportion to the second power
of the web speed. The highest web speeds of existing newsprint
machines are of an order of about 1500 meters per minute. However,
newsprint machines are being planned in which a web speed of up to
about 2000 m/min is aimed at.
The present invention is also related to further development
of the formers described in the assignee's Finnish Patent
Application No. 751774 (corresponding to the assignee's U.S. Patent
No. 3,996,068, the specification of which is hereby incorporated by
reference herein) and Finnish Patent Application Nos. 851650,
852662, and 902283, which formers are marketed by the assignee
under the trademark "Speed-Former". The "Speed-Former" formers are
characterized as gap-roll-shoe formers. In the "Speed-Former" web
former, there has arisen a need for further development in
particular in the case of papers made of slowly draining pulp
.
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2123820
qualities, such as SC-paper, and with particularly high machine
speeds. In view of satisfying this need of development so that a
sufficiently high dry solids content after the wire part can be
guaranteed with all web grades and web speeds in use, a former has
been developed that is described in the assignee's Finnish Patent
Application No. 902283, in whose gap-roll-shoe-roll former a
considerable proportion of dewatering is carried out on the last
(second) forming roll. Also, in connection with this second
forminq roll, the necessary dewatering equipment is arranged so
that, on the last forming roll, the dry solids content of the web
is still raised by about 3 per cent to about 5 per cent.
With respect to the prior art related to the present
invention, reference is also made to U.S. Patent No. 4,769,11 of
Messrs. A. Ahlstrom Corporation, to the assignee's Finnish Patent
Application No. 885609, as well as to the Finnish Patent
Application Nos. 885606 and 885607 of Messrs. Valmet-Ahlstrom Inc.,
in which formers marketed under the trademark "MB-former" are
described.
In the roll-gap formers mentioned above, the pulp suspension
jet is passed from the headbox into a gap formed by a forming roll
and by two wires. After the gap, the two wires and the pulp layer
placed between them follow the curve form of the forming roll,
whereby most of the water being drained is removed through the
wires. The necessary dewatering pressure is produced by means of
the tension of the outer wire, which tension produces a pressure in
the pulp layer which is proportional to the wire tension and
inversely proportional to the curve radius of the forming roll. In
the gap, part of the kinetic energy of the pulp jet is converted to
pressure energy, and the jet speed is lowered accordingly. The
dewatering through the outer wire away from the forming roll is
intensified by the centrifugal force. Dewatering towards the
forming roll can be intensified by means of negative pressure in a
- 2123820
suction zone of the forming roll.
With respect to additional prior art related to the present
invention, reference is further made to the assignee's Finnish
Patent Application Nos. 904489 and 920863. In FI '~89, a roll-gap
former is described and the application is primarily directed to
the novelty that the dewatering unit or units comprise(s) a
stationary press-support unit which guides the wire that enters
into contact with the unit as a substantially straight run, and
that the dewatering unit or units comprise(s) a dewatering
equipment placed facing press and support unit and provided with a
suction and foil equipment. The dewatering equipment removes a
substantial amount of water out of the web.
One basic type of a twin-wire former is the so-called ribbed
gap former in which the discharge jet of the headbox is passed into
the gap formed by two wires. The gap becomes narrower either as a
substantially straight dewatering area formed by means of
dewatering ribs placed at both sides of the wires transversely to
their running direction or as one or two successive curved
dewatering areas in which the ribs are placed at the side of the
curve center of the wires. In respect of these formers, reference
is made to U.S. Patent Nos. 3,578,558, 3,944,464, 4,125,428, to
German Published Patent Application No. DE 21 13 014, and to
Finnish Published Patent No. 50,647.
In addition, Finni~h Patent Application No. 913480, which was
originally applied for by Messrs. Tampella Papertech Oy, describes
a gap former wherein it has been considered novel that the breast
roll that guides the first wire is open, and the narrowed discharge
jet meets the first wire in the area of contact with the open roll
and meets the second wire after the breast roll that guides the
second wire. The distance between the wire placed on the open
breast roll, at the point of separation of the first wire from the
breast roll, and the second wire, which is placed between its own
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2123~20
breast roll and a guide member, is smaller than the thickness of
the narrowed discharge jet, preferably only from about 1 mm to
about 4 mm smaller.
03JECTS AND SUMM~RY OF THE INVENTION
It is an object of the present invention to further develop
the prior art web formers described above so that a novel gap
former is provided by whose means both good formation of the web
and good retention are achieved.
It is a further object of the present invention to provide a
former whose dewatering capacity is sufficient at high web speeds
and with thicker paper grades.
It is a further object of the present invention to provide a
former in which the advantages of the gap part of the former
described in the above-mentioned FI Pat. Appl. 913480 are
accomplished and in which a dewatering unit of the MD type can be
made to operate in an optimal range of dry solids content of the
web.
Further, it is an object of the present invention to provide
a former in which the ratio of tensile strength machine direction
/ cross direction MD/CD of the paper produced can be made lower
while, nevertheless, maintaining good formation and retention. In
this regard, an object of the invention is to provide a former in
which, if necessary, it is possible to achieve a ratio of tensile
strength of the paper MD/CD less than or equal to 2.5.
In view of achieving the objects stated above, those that will
come out later, and others, the web former of the present invention
includes a twin-wire zone comprising a combination of the following
web-forming and dewatering units, which are placed in the given
sequence in the running direction of the web that is formed and
which are known in themselves:
(a) a forming gap defined at one side by one of the wires
. . . ~
2123~20
which runs over the open face of a first forming roll, and at an
opposite side by the substantially straight run of the opposite
wire starting from its guide roll to the next forming member;
(b) a forming shoe provided with a curved ribbed deck whose
curve radius is Rl > 2 m to about 8 m, the tangential direction of
the inlet side of the ribbed deck of the forming shoe substantially
coincides with the principal direction of the forming gap;
(c) at least one MB unit comprising a dewatering unit and a
press and support unit placed one opposite to the other, the sets
of ribs in the dewatering and press and support units press the web
to be formed between the wires; and
(d) a large-diameter second forming roll which includes at
least one suction zone in or after the area of which the covering
wire is separated by means of the guide roll from the web, which is
passed further on the carrying wire to the press section of the
paper machine.
After the MB unit or units and before the large-diameter
second forming roll, preferably a suction box is placed either
inside the carrying wire or inside the covering wire, preferably
inside the loop of the carrying wire.
This specific combination of press elements and dewatering
elements enables the press section to run at higher speeds when
processing different pulp qualities and also facilitates a more
symmetric dewatering of the web. That is, substantially equal
amounts of water are removed from both sides of the web through the
covering wire and the carrying wire. This more symmetrical
dewatering is achieved as a result of the important feature that,
in the invention, water is removed on the forming roll in
conjunction with the forming gap to a considerably lower extent
than in the "Speed-Former" web formers. This lower amount of water
removal via the forming roll contributes to an improved retention.
In the following, the invention will be described in detail
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with reference to some exemplifying embodiments of the invention
illustrated in the figures in the accompanying drawing. The
invention is by no means strictly confined to the details of these
embodiments.
BRIEF DE5CRIPTION OF ~ 13 DRAWING~;
The following drawings are illustrative of embodiments of the
invention and are not meant to limit the scope of the invention as
encompassed by the claims.
Figure 1 is a schema~ic side view of a former in accordance
with the invention in which there is a vertically rising twin-wire
zone and in which the open-faced forming roll in the gap area is
placed inside the loop of the covering wire.
Figure 2 shows a twin-wire zone in the other respects similar
to Fig. l except that the MB unit is in a position inverse in
relation to Fig. 1.
Figure 3 is an illustration corresponding to Fig. 2 of a twin-
wire zone in which the open-faced forming roll in the gap area is
placed inside the loop of the carrying wire.
Figure 4 shows a twin-wire zone in the other respects similar
except that the MB unit is placed in a position inverse in relation
to Fig. 3.
Figure 5 shows a former in accordance with the invention in
which the principal direction of the twin-wire zone is horizontal
and the forming roll in the gap area is placed inside the loop of
the upper wire which operates as the covering wire.
Figure 6 shows the initial part of the twin-wire zone and the
gap area in a former which is in the other respects similar to that
shown in Fig. 5 except that the open forming roll is placed inside
the loop of the lower wire, which operates as the carrying wire.
Figura 7 shows an exemplifying embodiment of a MB zone applied
in the invention, which zone is in most respects similar to that
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- shown in Fig. 3 except that the forming shoe is placed inside the
loop of the carrying wire.
Figure 7A shows the detail A indicated in Fig. 7 on an
enlarged scale.
DETAI~ED DESCRIPTION OF THE INVENTION
Referring to the drawings wherein like reference numerals
refer to the same elements, the common features of the twin-wire
formers illustrated in Figs. 1-6 will be described. The formers
generally comprise a twin-wire zone formed by a loop of a covering
wire 10 and a loop of a carrying wire 20. Between the lines B-C,
the wires 10,20 begin to form the twin-wire zone in which water is
removed from a paper web W through both of the wires 10,20. The
covering wire 10 is guided by guide rolls 14,14' and by the first
forming roll 11 or a corresponding breast roll llA directly. A
forming gap G is defined between the covering wire 10, which is
guided by the forming roll 11 or the corresponding breast roll llA,
and the carrying wire 20, which is guided by a forming roll 21 or
a corresponding breast roll 21A. The forming gap G becomes
narrower in a wedge-shape form in the running direction of the web.
It is an important feature of the invention that the wires
10,20 have no common curve sector on the first forming roll 11,21,
and that the principal direction of the forming gap G and the joint
run of the wires 10,20 following after the gap is straight until
the inlet edge of the forming shoe 12,22. The forming gap G is
determined by the first forming roll 11,21 having an open-face
11',21' and by the breast roll llA,21A having a smooth-face
11",21". The breast roll llA,21A is placed before the forming roll
in the feed direction of the pulp jet J. In such a case, the
forming gap G is defined, at one side, by the substantially
straight run of the wire 10,20 from the breast roll llA,21A to the
inlet edge of the forming shoe 22 and, at the other side, by the
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2123820
opposite wire 20,10 curved on the open face 21',11' of the first
forming roll 21,11, which opposite wire departs from the open face
21',11' of the forming roll 21,11 in the area of the bottom of the
gap G. The tangential direction of the inlet edge or side of the
ribbed deck subs~antially coincides with the principal direction of
the forming gap. In the area of the gap G, when the wire 10,20
runs on the open face 11',21' of the first forming roll 11,21,
dewatering takes place into the open face 11',21' and through the
substantially straight run of the opposite wire 20,10. Thus,
forming roll 11,21 constitutes a so-called "kissing forming roll"
(11,21).
By means of the gap geometry described above, the
controllability and the trans~erse stability of the wires can be
improved to some extent in comparison with a case in which the
forming gap G would be more open at the forming roll 11,21 than the
"kissing forming roll" 11,21 described above.
In the twin-wire zone, after the forming gap G, there is a
curved forming shoe 12,22 which has a ribbed deck 12a,22a with a
large curve radius R1. The forming shoe 12,22 is followed by an MB
unit 50 in the twin-wire zone. The MB-unit comprises a dewatering
unit 40 and a press and support unit 60 operating one opposite to
the other so that the wires 10,20 are pressed against one another
by means of the press and support unit 60 so as to remove water out
of the web W placed between the wires.
There may a variable number of MB units 50, for example two,
in the twin-wire zone placed one after the other and preferably
arranged so that in the successive units 50, the units 40 and 60
are placed inside different wire loops 10,20.
The MB unit 50 or units is/are followed by a suction box 29
which is provided with a ribbed deck and is placed inside the loop
of the carrying wire 20. The suction box 29 is followed by a
large-diameter D2 second forming roll 23 which is placed inside the
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2123820
carrying wire 20. Forming roll 2~ constitutes a suction roll in
which there are two successive suction zones 23a and 23b, as shown
in Figs. 1-4, or only one suction zone 23a, as shown in Fig. 5.
The diameter D2 of the second forming roll 23 is generally selected
in the range of about 1.0 m to about 2.0 m. The covering wire 10
is separated at the line C from the covering wire 20 by means of
the guide roll 14' so that the web W follows the carrying wire 20.
As shown in Figs. 1 and 5, the web W is passed further on a suction
zone 24a of suction/guide roll 24 and from there further in a
downward inclined run to the pick-up point where the web W is
transferred onto a pick-up fabric 31 on a suction zone 30a of the
pick-up roll 30. After this, the carrying wire 20 continues its
run, guided by the guide rolls 24, onto the breast roll 21A in the
gap area (Figs. 1, 2 and 5) or onto the forming roll 21 placed in
the gap area, which forming roll has an open face 21' which
receives water (Figs. 3 and 4).
As shown in Figs. 1-4, the main direction of the run of the
twin-wire zone B-C is vertical. The twin-wire zone starts in the
forming gap G into which a slice part 32 of the headbox feeds a
pulp suspension jet J. The jet J is directed towards the bottom of
the straight gap G which becomes narrower in a wedge-shaped form.
As shown in Figs. 1 and 2, inside the loop of the covering
wire 10, there is a forming roll that is provided with an open face
11' and that receives water. Breast roll 21A is arranged at a
level lower than the level of the forming roll 11 at the distance
of the height difference h" inside the loop of the carrying wire.
The breast roll 21A is provided with a smooth solid face 21". The
diameters D1 of the rolls 11 and 21A are preferably substantially
equal in comparison with one another, being of an order of about
0.5 m to about 1.5 m, preferably from about 0.7 m to about 1.0 m.
With the above roll diameters D1, the difference in height h1 is
preferably in the range of about 50 mm to about 300 mm. The
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2123820
forming roll 11 may also be a suction roll provided with an inside
or interior suction chamber lla, the negative pressure employed in
this type of roll being preferably in the range of about 10 kPa to
about 20 kPa.
In Fig. 1, the outlet end of the forming gap G is in the
horizontal plane T-T determined by the central axis of the first
forming roll 11 or near this plane. In the horizontal formers
shown in Figs. 5 and 6, the outlet end of the forming gap is
arranged in proximity to the vertical plane passing through the
center of the forming roll 11.
As shown in Figs. 3 and 4, a water-receiving forming roll 21
provided with an open face 21' is placed inside the loop of the
carrying wire 20, and the breast roll llA provided with a smooth
face 11" is placed inside the loop of the covering wire 10. For
fine adjustment of the gap geometry, the forming roll 11,21 is
arranged so that its position can be adjusted substantially in the
horizontal plane (arrow E), via displacement means (not shown).
The gap area is substantially immediately followed by the
forming shoe 12,22. The forming shoe 12 shown in Figs. 3 and 4 is
placed inside the loop of the covering wire 10, and the forming
shoe 22 shown in Figs. 1, 2, 5 and 6 is placed inside the loop of
the carrying wire 20. The forming shoes 12,22 are provided with a
ribbed deck having a large curve radius Rl. The curve radius R1 is
usually chosen within the range of about 2 m to about 8 m,
preferably in the range of about 3 m to about 5 m. If necessary,
! the box of the forming shoe 12,22 may be connected to a source of
negative pressure p3~ which is illustrated by the corresponding
arrow in Fig. 1. The negative pressure p3 promotes the draining
through the gaps in the ribbed deck 12a,22a. However, in the area
of the forming shoe 12,22, the dewatering takes place primarily in
a direction away from the shoe by the effect of the pressure PT =
T/R~ produced by the tightening tension T of the wires, which
2123820
dewatering is aided by centrifugal forces.
The forming shoe 12,22 is substantially immediately followed
by the MB unit 50, which, as shown in Fig. 1, comprises two
draining chambers 40a and 40b, which are arranged inside the loop
of the covering wire 10 and are separated from one another by a
partition wall 49. Both draining chambers 40a,40b communicate with
different negative pressures P1 and P2. At the side of the chambers
40a and 40b placed against the twin-wire zone, there is a set of
ribs 80. The press and support unit 60 is provided with a
corresponding set of ribs 70, and the construction and the
operation of these sets of ribs 70,80 will be described in more
detail later with reference to Figs. 7 and 7A. The box 61 in the
press and support unit 60 can also, if necessary, be connected to
negative pressure P~. The forming shoe 12a may also be connected
directly to the first draining chamber 40a as shown in Fig. 4.
As shown in Fig. 2, the units 40 and 60 operating one opposite
to the other in the MB unit 50 are placed in an inverse order,
compared with Fig. 1, so that in Fig. 2 the dewatering unit 40 is
placed inside the loop of the carrying wire 20 and the press and
support unit 60 is placed inside the loop of the covering wire 10.
The MB unit 50 or units is/are followed by a suction box 19
arranged inside the loop of the covering wire 10 to operate against
the inner face of the wire 10 and which is provided with a ribbed
deck having slots.
In the web former shown in Fig. 3, the dewatering unit 40 of
the MB unit 50 is placed inside the loop of the carrying wire 20,
and the press and support unit 60 is placed inside the loop of the
opposite wire 10, whereas in Fig. 4, compared with Fig. 3, the
units 40 and 60 are placed in an inverse order. The principal
direction of the twin-wire zone guided by the sets of ribs 80,70 in
the units 40 and 60 is straight, but alternatively it is possible
to use a ribbed zone between the sets of ribs 70,80 whose principal
- ~23~2~
direction is slightly curved, the curve radius of this ribbed zone
is denoted by R2 in Figs. 1 and 2. The curve radius R2 is generally
selected within the range of about 3 m to about 8 m. In Fig. 1,
the center of curvature of the curved ribbed zone is placed at the
side of the dewatering unit 40, and in Fig. 2 the center of
curvature of the curved ribbed zone is placed at the side of the
press and support unit 60.
In the web former shown in Fig. 4, a second MB-unit 40C (shown
in phantom lines) may be arranged after the MB unit 40. The second
MB unit has a dewatering unit 40A arranged inside a loop of the
carrying wire and a press and support unit 40B arranged inside a
loop of the covering wire, i.e., in opposite relation to the
arrangement of these units in the first MB unit 40.
Figs. 5 and 6 show two horizontal versions of a gap former in
accordance with the invention. As shown in Fig. 5, the gap G is
determined by the lower breast roll 21A which has a smooth solid
face 21" and by the upper forming roll 11 which is provided with an
open face 11' that receives water. Between the rolls 21A and 11,
there is a horizonal distance v,. The upper forming r~ll 11 has
been arranged to be adjustable mainly in the vertical direction
(arrow E) in view of fine adjustment of the gap geometry (via
displacement means (not shown). In Fig. 6, in the area of the gap
G, in the horizontal direction, first there is a breast roll 11
provided with a smooth face 11' and a lower forming roll 21 which
is provided with an open face 21'. The horizontal distance between
these rolls is vl which is generally in the range of about 50 mm to
about 300 mm.
In the web formers shown in Figs. 5 and 6, the forming gap G
is followed by a forming shoe 22 placed inside the loop of the
lower wire 20, which forming shoe is provided with a ribbed deck
22a having a curve radius Rl. After the forming shoe 20, the MB
unit 50 is arranged and comprises a first water drain chamber 41
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~ 2123820
placed above the forming shoe 22 inside the loop of the upper wire
10 in the manner shown in Fig. 6. Drain chamber 41 has a water
drain duct 42 arranged preferably at the driving side of the
machine only. Underneath the first chamber 41, a gap space 43
remains through which the water is drained, while being aided by
negative pressure P1, through a suction-deflector duct 44 in the
direction of the arrow V1 into the chamber 41 and from it further
through the duct 42 to the side of the paper machine and into the
wire pit (not shown). In the direction of the arrow V~, through
the suction-deflector duct 44, mainly water that has been separated
from the web W through the upper wire 10 in the area of the shoe 22
is drained.
The first water drain chamber 41 is followed directly by a
second water drain chamber 45 having a rear wall against which
there is a water drain duct 47. Through the duct 47, water is
drained in the direction of the arrow V2 through the slot gaps in
the sets of ribs 80 in the dewatering unit 40. This amount of
water is removed out of the chamber 45 through a water drain duct
46 which is placed preferably at the operating side of the machine
only. Against the set of ribs 80, a set of ribs 70 operates which
is loaded by means of the pressure of the pressure medium of the
press and support unit 60. The construction and the operation of
said sets of ribs 70,80 will be described in more detail in the
following with reference to Figs. 7 and 7A.
The MB unit 50 or units is/are followed by a suction box 29
which is placed inside the loop of the carrying wire 20 and which
is provided with a ribbed deck that includes slots. After the
suction box 29, the twin-wire zone is curved in the suction zone
23a of the second forming roll 23 to have a downward inclined run,
and the web W is separated from the upper wire 10 and guided to
follow the lower wire 20. This separation is aided by the negative
pressure present in a ribbed deck 25a of a suction box 25 placed
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inside the loop of the lower wire 20.
In Figs. 7 and 7A, the dewatering unit 40 of the MB unit 50 is
arranged directly after the curved ribbed deck 22a of the forming
shoe 22 and inside the loop of the carrying wire 20. Inside the
S loop of the covering wire lo, there is the press and support unit
60 of the MB unit 50, whose construction comes out in more detail
from Fig. 7A. The unit 60 includes a set of ribs 70 which
comprises ceramic loading ribs 71,72, which are interconnected in
pairs by means of support structures 73. The ribs 71,72, and so
also back-up ribs 81 in the dewatering unit 40, extend in the
transverse direction across the entire width of the web W and the
wires 10,20. The set of ribs 70 is loaded by means of pressure Pk
passed into loading hoses 75 mounted against stationary frame
constructions 74. Into the hoses 75, pressure Ph is applied
through pipes 77 from a source 76 of negative pressure, which is
illustrated schematically. The units 40 and 60 may also alternate
locations with one another, i.e., be situated in the opposite wire
loops.
As shown in Fig. 7A, the set of ribs 80 of the dewatering unit
40 placed inside the loop of the carryinq wire 20 comprises forming
ribs 81 attached to frame constructions 84 by means of dovetail
joints. The ribs 81 are placed as alternating with, i.e., not
directly opposite to, the ribs 71,72 in the set of ribs 70 so that,
the twin-wire zone runs between the units 40,60 along a very gently
meandering path while being guided by the sets of ribs 70,80. The
whose principal direction of the twin-wire zone is straight or
curved and, if curved, the curve radius is, in the embodiments of
Figs. 1 and 2, denoted by R2.
If necessary and~or desired, in the area of the MB unit 50,
the dewatering can be intensified by means of the effect of
negative pressures P" P2, P4 present in gaps between the ribs
71,72,81. The symmetry of dewatering and of web formation can be
14
2123~20
controlled by means of regulation of the levels of negative
pressure P1,P2, P3, P~ in the units 40 and/or 60. In Fig. 7, the
dewatering unit 40 has been divided by a partition wall 49 into two
compartments 40a and 40b, each of which communicates separately
with a suction pump, so that the compartments 4Oa and 4Ob can be
provided with negative pressures P~ and P2 having different levels
of suction to promote the dewatering.
In Fig. 1, the dewatering proportions at the different
dewatering units are illustrated by the arrows F1,F2,F3,F~,F5,F6,F,
and F8. One particularly favorable distribution of the dewatering
proportions that is achieved by the particular combination of press
members and dewatering elements in a former in accordance with the
invention is the following:
F, K 15% F2 ~ 27% F3 ~ 20% F4 ~ 4%
F5 ~ 29% F6 ~ 1~ F7 ~ 3.5% F8 ~ 0-5%
Of the dewatering percentages given above, the proportion of
the total dewatering taking place through the covering wire 10 are
F3 + Fs + F6 which total about 50%, and the proportion of the total
dewatering taking place through the carrying wire 20 are F, + F2 +
F~ + F7 + F8 which totals about 50~. Thus, as comes out from the
above, in the invention an extremely symmetrical and equal
dewatering is achieved through both of the wires 10 and 20, whereby
the symmetry of the web structure is promoted. This more
symmetrical dewatering is achieved as a result of the essential
feature, and one of the differences of the present web former when
compared with the assignee's prior-art "Speed-Former" concept, that
in the invention, water is removed on the forming roll 11,21 to a
considerably lower extent than in the "Speed-Former". This lower
amount of water removal via the forming roll 11,21 contributes to
an improved retention.
21~3~20
The consistencies, i.e., dry solids contents, of the web that
is being formed, which are indicated for the web former illustrated
in Fig. 3, are preferably as follows:
- consistency in the headbox ~ from about 0.5% to about 1.7%;
- consistency before the forming shoe 12,22 (k1) z from about
0.6% to about 1.9%;
- consistency after the forming shoe 12,22 and before the MB
unit 50 (k2) ~ from about 0.90% to about 3.0%;
- consistency after the MB unit 50 (k3) ~ from about 7% to about
12%; and
- consistency after the second forming roll 23 (k4) ~ from about
10% to about 14%.
The various parameters in accordance with the invention that
affect the web formation, such as the gap G geometry, the diameters
of the roll 11,21, the curve radius R~ of the forming shoe 12,22,
the pressures Pk,P1,P4 and P4 that act upon the operation of the MB
unit, the diameter of the roll 23 and the negative pressures in its
suction zone 23a,23b, and the negative pressure in the suction zone
lla, if any, of the forming roll 11, are preferably selected so
that the dry solids contents k1,...,k4 defined above and given in
the following table will be carried into effect.
Table
Dry solids content Widest range of dry Preferabledry solids
of the web, (%) solids content, (%) content, (%)
kl 0.6% to 1.9% about 1.5%
k2 0.9% to 3.0% about 2.2%
k3 7% to 12% about 10%
k~ 10% to 14% about 13%
. .
16
,,
; . -. .,
. - ~
3~20
The examples provided above are not meant to be exclusive.
Many other variations of the present invention would be obvious to
those skilled in the art, and are contemplated to be within the
scope of the appended claims.
,: : ", ., : ~
