Twin-wire papermaking machine and method of dewatering a paper web in a twin-wire papermaking machine

In a particularly simple construction a twin-wire papermaking machine possesses a reduced number of rolls as well as a compact arrangement. The foregoing and the greatest possible velocity of a through-passed paper web can be obtained by employing non-suction rolls, instead of suction rolls, in a pressing section of such papermaking machine, by upwardly entraining the paper web conjointly with a top or upper wire of a twin wire arrangement and by transferring the paper web to a pressing roll having a smooth surface, and furthermore, by providing at the pressing roll an extended or wide-nip supporting shoe by means of which the paper web is dewatered in an extended pressing nip zone. Advantageously, the dewatered paper web is taken off from the surface of the pressing roll in a downward direction and fed to a drying section of the papermaking machine. The drying section may be arranged below the pressing section and directly adjacent the bottom or lower wire of the twin wire arrangement. There is thus obtained a particularly compact construction.

BACKGROUND OF THE INVENTION 
The present invention relates to a new and improved construction of a 
twin-wire papermaking machine and also relates to a new and improved 
method of dewatering a paper web in a twin-wire papermaking machine. 
In its more particular aspects, the present invention specifically relates 
to a new and improved construction of a twin-wire papermaking machine 
containing a top or upper wire or screen and a bottom or lower wire or 
screen, each of which is guided in an endless loop at related guide rolls. 
The endless loops have a common section in which the top and bottom wires 
or screens form a twin-wire arrangement which passes at least one sheet or 
web forming roll. A paper web is formed between the twin-wire arrangement 
from a fiber stock suspension delivered by a headbox and such formed paper 
web is guided to a pressing roll at which the paper web is dewatered. 
In a twin-wire or double-screen papermaking machine as described, for 
example, in U.S. Pat. No. 4,116,763, granted Sept. 26, 1978, and U.S. Pat. 
No. 4,176,005, granted Nov. 27, 1979, the sheet or web forming rolls are 
preferably constructed as open rolls. The paper web, after separation of 
the two wires or screens, is withdrawn in an approximately horizontal 
direction either from the top or upper wire or from the bottom or lower 
wire. Thereafter, the paper web, if desired, with the interposition of a 
take-off roll, is transferred to a felt web and fed to a pressing section 
which generally contains one or a number of suction rolls. In such 
arrangement a pressing roll cooperates with at least one, generally with a 
number of counter rolls. The paper web is thus dewatered under the action 
of a line-shaped pressing force and such dewatering operation is assisted 
by the felt web. As a result, the paper web can be subsequently fed to a 
drying section of the papermaking machine, and this drying section is 
provided consecutive to the pressing section. 
In the presently required high-speed operation of such papermaking 
machines, i.e. at high travel speeds of the paper web, the employment of 
suction rolls as well as the series-arrangement of a number of pressing 
nips hitherto has been deemed indispensable in order to obtain safe web 
guidance and a sufficiently efficient mechanical dewatering process in the 
pressing section. However, suction rolls are expensive and cost-intensive 
machine elements in terms of manufacture as well as in terms of 
maintenance. Furthermore, a wire or screen section, the pressing section 
and the drying sections had to be series-arranged in the known papermaking 
machines, with the undesirable result that there prevailed considerable 
total or overall dimensions of the papermaking machine. 
SUMMARY OF THE INVENTION 
Therefore, with the foregoing in mind, it is a primary object of the 
present invention to provide a new and improved construction of a 
twin-wire papermaking machine which is not afflicted with the 
aforementioned drawbacks and limitations of the prior art constructions 
heretofore discussed. 
It is a further important object of the present invention to provide a new 
and improved construction of a twin-wire papermaking machine which is 
further developed, particularly with respect to the wire or screen section 
and the pressing section thereof, and which does not require any 
complicated machine elements which are expensive, particularly with 
respect to their maintenance. 
Another significant object of the present invention aims at a new and 
improved construction of a twin-wire papermaking machine which possesses a 
reduced number of rolls and reduced total dimensions, so that a twin-wire 
papermaking machine of a compact structure is obtained. 
Now in order to implement these and still further objects of the invention, 
which will become more readily apparent as the description proceeds, the 
twin-wire papermaking machine of the present development is manifested by 
the features that, the top or upper wire, after separation from the bottom 
or lower wire, entrains the paper web substantially in an upward 
direction. The paper web is transferred from the top wire to the pressing 
roll at a transfer location. The pressing roll is structured as a 
non-suction roll with a smooth circumferential or outer surface. There is 
further provided a pressing or supporting shoe which extends over part of 
the smooth circumferential surface of the pressing roll and forms 
conjointly therewith an extended pressing nip zone through which the paper 
web is passed and thereby dewatered during the operation of the twin-wire 
papermaking machine. 
As alluded to above, the invention is not only concerned with the 
aforementioned apparatus aspects, but also relates to an improved method 
of dewatering a paper web in a twin-wire papermaking machine. To achieve 
the aforementioned measures, the inventive method comprises the following 
steps: 
(a) upwardly entraining the paper web by the top or upper wire after the 
twin wires have been separated from each other, 
(b) transferring the paper web from the top or upper wire to the pressing 
roll, 
(c) providing as the pressing roll a non-suction roll possessing a smooth 
circumferential surface, and 
(d) dewatering the paper web in an extended pressing nip zone which is 
formed by the pressing roll and a pressing or supporting shoe extending 
over part of the circumferential surface of the pressing roll. 
It is of particular advantage when the paper web is directly transferred 
from the top or upper wire to the smooth circumferential surface of the 
pressing roll. 
Preferably, the dewatered paper web, after exiting from the pressing nip 
zone, is downwardly guided from the pressing roll to a drying section, 
preferably to a first drying cylinder thereof which is arranged below the 
pressing roll and adjacent the bottom or lower wire. 
As a result of the coaction of the aforementioned features of the inventive 
apparatus and as a result of the coaction of the aforementioned method 
steps of the invention, there is obtained in high-speed operating 
papermaking machines this type, i.e. at high paper web speeds, despite a 
smaller number of rolls and without the necessity of using suction rolls, 
a safe web guidance and an improved dewatering result. Additionally, and 
due to the specific arrangement of the components there is obtained a 
papermaking machine of a compact construction and which has comparatively 
smaller total overall dimensions and possesses a simplified roll stand or 
frame. The roll stand or frame may even constitute a common structure for 
the wire or screen section and for the pressing section of such 
papermaking machine. 
It is a further advantage of the inventive construction of the twin-wire 
papermaking machine that there are avoided open or unsupported runs of the 
paper web, so that the paper web is stretched or extended only to a 
minimum degree.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Describing now the drawings, it is to be understood that only enough of the 
construction of the twin-wire papermaking machine has been shown as needed 
for those skilled in the art to readily understand the underlying 
principles and concepts of the present development, while simplifying the 
showing of the drawings. Turning attention now specifically to FIG. 1 of 
the drawings, there is schematically illustrated therein a first exemplary 
embodiment of the inventive twin-wire papermaking machine comprising a 
wire or screen section or dewatering section A, a pressing section B and a 
consecutively arranged drying section C. 
The dewatering section or wire or screen section A contains a bottom or 
lower wire 1 and a top or upper wire 2, each of which comprises a wire or 
screen or any other suitable liquid pervious structure defining 
web-carrying means and each of which forms an endless loop of such wire or 
screen. The top wire 2 may be structured, for example, as a simple plastic 
screen or mesh or may constitute a felt web. The endless loops of the 
bottom or lower wire 1 and the top or upper wire 2 form a common twin-wire 
section or arrangement 2a which is passed over at least one, in the 
illustrated embodiment two dewatering rolls which constitute sheet or web 
forming rolls 3 and 4. The sheet or web forming rolls 3 and 4 are 
constructed as solid rolls but, if desired, they can also be provided with 
an open, i.e. interrupted or non-smooth surface. 
Additionally, the sheet or web forming roll 4 constitutes at least part of 
separating means and performs the function of separating, at a 
predetermined separating location 2b, the bottom wire 1 and the top wire 2 
which previously formed the twin wire section or arrangement 2a. Guide 
rolls 5, 6 and 7 are provided to guide the bottom wire 1 and the top wire 
2 in their related endless loops. Additionally, there is provided a 
displaceable directing or control roll 11 which also is part of the 
aforementioned separating means and which assists in or controls the 
separation of the bottom wire 1 from the top wire 2. A further 
displaceable directing or control roll 15 is operatively associated with 
the pressing roll 14 which consitutes a single pressing roll and its 
function will be described in more detail hereinbelow. 
The fiber stock suspension for forming the paper web P issues from an only 
schematically indicated headbox H and the formed paper web P is fed by 
means of the bottom wire 1 to a sheet or web forming zone which is mainly 
defined by the twin-wire section or arrangement 2a. Such sheet or web 
forming zone starts at a control or guide roll 8 which has an open, for 
example, a grooved or fluted surface. In the sheet or web forming zone the 
paper web P is fed to the two dewatering or web forming rolls 3 and 4. 
Related catch containers 9 and 10 for collecting thrown-off liquid are 
provided for the dewatering or web forming rolls 3 and 4. 
Following the last dewatering or web forming roll in the sequence, namely 
the dewatering or web forming roll 4, the bottom wire 1 and the top wire 2 
which up to the predetermined separation location 2b formed the twin-wire 
section or arrangement 2a in the dewatering section or sheet or web 
forming section A of this embodiment of the twin-wire papermaking machine, 
are separated from each other by means of the displaceable wire separation 
control roll 11. This displaceable wire separation control roll 11 is 
arranged in such a manner that the paper web P is upwardly entrained by 
the top wire 2 i.e. by an upwardly directed top wire section 2c, after the 
bottom wire 1 and the top wire 2 have been separated from each other. A 
steam box 12 is arranged intermediate the wire separating means 
constituted by the displaceable wire separation control roll 11 and the 
pressing roll 14. The paper web P passes through the steam box 12 by means 
of which the paper web P is thus heated. The steam box 12 is arranged on 
the side of the top wire 2 or upwardly directed top wire section 2c on 
which the paper web P is entrained. On the opposite side of the top wire 2 
or upwardly directed top wire section 2c and at the level of the steam box 
12 there are arranged suction boxes 13 which constitute means for 
effecting additional dewatering of the paper web P. 
Subsequently the paper web P is directly transferred to the pressing roll 
14 at a predetermined transfer location 14b. This direct transfer of the 
paper web P is effected without the assistance of a felt web and is 
accomplished exclusively due to the action of the tension of the top or 
upper wire 2. The single pressing roll 14 constitutes a non-suction solid 
roll possessing a smooth circumferential or outer surface 14a. The direct 
transfer of the paper web P from the top wire 2 or upwardly directed top 
wire section 2c to the smooth surface 14a of the pressing roll 14 is made 
possible due to the fact that the paper web P still has a certain water 
content which is required for the transfer to this smooth surface 14a of 
the pressing roll 14. For this purpose the dry content of the paper web P 
should not substantially exceed a value of 25%. 
The separation or transfer of the paper web P from the top wire 2 or 
upwardly directed top wire section 2c is additionally assisted by the 
aforementioned further displaceable control roll 15. Particularly at high 
paper web speeds the web transfer from the top wire 2 or upwardly directed 
top wire section 2c to the pressing roll 14 can be facilitated by 
additionally providing a web transfer roll 16 at the web transfer location 
14b at which the paper web P is separated or taken off from the top wire 2 
or upwardly directed top wire section 2c and transferred to the pressing 
roll 14. This transfer roll 16 makes the paper web P adhere more readily 
to the smooth circumferential or outer surface 14a of the pressing roll 
14. 
The pressing section B of the first exemplary embodiment of the inventive 
twin-wire papermaking machine contains a single pressing nip which is 
formed by the pressing roll 14 possessing the aforementioned smooth 
circumferential surface 14a, and a pressing or supporting shoe 17 which 
extends along part of the circumference of the pressing roll 14 and which 
forms conjointly therewith an extended pressing nip zone Z. The paper web 
P is passed through the extended pressing nip zone Z along a predetermined 
passage direction. During such paper web passage the pressing or 
supporting shoe 17 presses the paper web P conjointly with a felt web 19, 
which is guided at guide rolls 18, along with a flexible band or cover 20 
against the surface of the pressing roll 14 in the region of the extended 
pressing nip zone Z and thereby dewaters the paper web P. 
The pressing or supporting shoe 17 may be designed in a manner known as 
such as a hydrostatic support element or as a series of a number of such 
hydrostatic support elements such as known, for example, from U.S. Pat. 
No. 3,783,097, granted Jan. 1, 1974, and U.S. Pat. No. 3,974,026, granted 
Aug. 10, 1976. The pressing or supporting shoe 17, however, can also be 
designed as a wide-nip or extended supporting shoe. Furthermore, the 
pressing or supporting shoe 17 may also be constructed in such a manner 
that the pressing force increases in the passage direction of the paper 
web P through the extended pressing nip zone Z. The prolonged dewatering 
time which the paper web P experiences in the extended pressing nip zone Z 
is sufficient, particularly for paper possessing low unit area mass in 
order to obtain a high dry content comparable with the dry content 
otherwise obtained by using a number of line-shaped pressing nips. 
After the extended pressing nip zone Z the paper web P follows the surface 
of the pressing roll 14 until it is taken off therefrom by means of a 
take-off or pick-up roll 21 and is further downwardly fed to the drying 
section C of the twin-wire papermaking machine. 
The aforementioned drying section C contains a number of heatable drying 
cylinders 22 to 25. The paper web P is passed over the surfaces of the 
drying cylinders 22 to 25 conjointly with a drying wire or screen 28 which 
is guided by guide rolls 26 and 27. The paper web P is thereby subjected 
to a final drying operation. A couch pit 29 is provided below the pressing 
section B, i.e. intermediate the dewatering section or screen or wire 
section A and the drying section C. 
The upward entrainment of the paper web P by the top wire 2, the 
arrangement of the pressing roll 14 at the upwardly directed section 2c of 
the endless loop formed by the top wire 2, the further downward direction 
of the paper web P after leaving the extended pressing nip zone Z, and the 
arrangement of the first drying cylinder 22 of the drying section C below 
the pressing section B permits a compact structure of the twin-wire 
papermaking machine. As a consequence, the overall length of the inventive 
twin-wire papermaking machine can be maintained shorter than that of 
hitherto known papermaking machines. As a further consequence, the 
dewatering section or wire or screen section A, particularly the 
dewatering or sheet or web forming rolls 3 and 4 as well as the pressing 
section B containing the pressing roll 14, can be accommodated in a common 
roll frame or stand, and thereby the installation is considerably 
simplified and more favorable in terms of costs. 
Guiding the paper web P along a winding or serpentine travel path at the 
dewatering or sheet or web forming rolls 3 and 4 increases the dewatering 
capacity in this dewatering section or wire or screen section A. The 
overall dewatering capacity is further increased by extending the pressing 
nip by employing the aforementioned pressing or supporting shoe 17 which 
forms the extended pressing nip zone Z in the pressing section B. Guiding 
the felt web 19 merely within the extended pressing nip zone Z laterally 
adjacent the pressing roll 14 furthermore permits outfeeding the paper web 
P from the pressing roll 14 in a downward direction. As a consequence, the 
drying section C can be arranged below the pressing section B and 
immediately adjacent the dewatering section or wire section A. 
It is a further advantage that in the arrangement of the dewatering section 
or wire section A, the pressing section B and the drying section C as 
described hereinbefore, there is only required a single couch pit 29 or 
equivalent structure. 
Due to the heretofore described measures, specifically due to the reduction 
in the number of rolls which are employed and due to the more simple roll 
construction, the entire installation is distinctly more favorable in 
terms of costs and less expensive with respect to maintenance. 
A second exemplary embodiment of the inventive twin-wire papermaking 
machine is schematically illustrated in FIG. 2 and constitutes a variant 
of the first embodiment described hereinbefore with reference to FIG. 1. 
In this second embodiment the paper web P is taken off from the pressing 
roll 14 by means of a take-off or pick-up roll 31 which is directly 
arranged at the pressing roll 14. The paper web take-off is thus effected 
without any free or unsupported run of the paper web P and in a closed 
guiding system by means of a drying mesh felt 30 which is guided by means 
of guide rolls 32. Also in this case there is obtained a simplified and 
compact arrangement and design of the twin-wire papermaking machine which 
is favorable in terms of costs. This second embodiment has the additional 
advantage that there are avoided open or unsupported runs of the paper web 
P, so that stretching or extension of the paper web P is kept at a 
minimum. 
During the use of certain types of paper it may be preferable to guide the 
paper web P conjointly with a further felt web at the pressing roll 14. 
Such guidance of the paper web P at the pressing roll 14 is realized in 
the third exemplary embodiment of the inventive twin-wire papermaking 
machine illustrated in FIG. 3. An additional felt web 34 is guided at 
guide rolls 33 and is further guided conjointly with the paper web P after 
its transfer from the top wire 2 to the smooth circumferential surface 14a 
of the pressing roll 14 as well as through the extended pressing nip zone 
Z formed between the pressing or supporting shoe 17 and the pressing roll 
14. Otherwise this third embodiment corresponds to the first embodiment 
described hereinbefore with reference to FIG. 1 and this third embodiment 
has analogous advantages. 
It should be noted that the invention is not limited to the exemplary 
embodiments described hereinbefore, but that further modifications and 
developments are possible within the spirit and scope of the inventive 
concepts. 
While there are shown and described present preferred embodiments of the 
invention, it is to be distinctly understood that the invention is not 
limited thereto, but may be otherwise variously embodied and practiced 
within the scope of the following claims. Accordingly,