Note: Descriptions are shown in the official language in which they were submitted.
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BACKGRO~lND OF THE INVENTION
_
The present invention is directed to a press section
with separa~e nips in a paper machine,each nip being formed
between two press rolls placed one opposite the other, and
at least one roll in a pair being preferably provided with
a hollow ~ace. A method ~or pressing a web in such a press
section is also provided.
Removal of water out of a paper web by evaporation,
requires an abundance of energy and is consequently expensive
and uneconomical. Therefore, attempts have been made to de-
water the web as extensively as possible, by mechanical means
and methods, before the drying section. The last stage in
this process is the pressing of a web in a press section,
where water is removed out of the web by pressing the web
between rolls. In this respect, it has been well known that
water can be considerably removed more sasily when temperature
is raised, because viscosity of water as well as a springback
coefficient of the web, are thereby reduced, along with re-
duction in surface tension. In this way, considerable in-
crease in the dry solids content of the web after the press
section, and, consequently economies in the consumption of
dryi.ny energy, have been achieved.
The most common prior art mode of dewatering fibre
webs, in particular paper and board webs, has been to pass
the web through a press nip formed by two rolls acting one
against the other. In the prior art, one or two press fabrics
are used in the dewater nips, the press fabrics further
carrying the water removed out of the web, as we~l as acting
as means for carrying the web forwardly.
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With the increase in the rates of production of paper
machines, dewatering by means of nip pressing has become a
bottleneck, limiting the increase of speed in the paper
machines. This is due to the fact that the extension of
the press nips formed by a pair of rolls is short in the
direction of the machine, so that at high speeds, the residence
time of the web in these press nips remains short. In parti-
cular, owing to the flow resistance of the fibre structure
structure of the web, the water however requires a certain
minimum time ta be able to drain out of the web into the hollow
face o~ the roll and/or into the press fabric.
In the prior art, several successive separate, so-
called straight presses consisting of two rolls or so-called
compact presses have been used in press sections. The two-
roll presses, however r require quite a large space, especially
if there are several nips one after the other. A compact
construction of press sections, on the other ha~d, causes
difficulties in the optimal placing of the various components
as well as operational difficulties, e.g. in the removal of
paper broke. In two-roll presses, suction rolls are commonly
used, which are relatively expensive components and which
consume an abundance of suction energy. In suction rolls,
a perforated mantle must be used, the mechanical strength
thereof being problematic.
If, in two-roll presses, attempts are made to increase
the dewatering capacity by increasing the nip load, the limit
is reached at a certain linear load where the nip pressure
can no longer be increased, because o the limited st ength
of the paper web and/or of the press fabrics.
Due to the problems described above and other reasons,
in recent years so-called extended nip presses have been
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developed, in which two stationary shoes placed one opposite
the other, or a corresponding roll-shoe combination are used.
In the prior art, such presses formed by a pair of
rolls are also known in which attempts have been made to
increase the length of the nip zone by means of resilient
coatings, e.g. o~ rubber, on the rolls. The construction of
such a press is simple, but the nip cannot be made very long,
even with large rolls and with reasonable compression rates of
the coatings. Drawbacks include heat ormation in the coating,
rapid wear or detaching of the coating, as well as difficulties
involved in the renewal of the coating.
As is well known, various fabrics and roll coatings
soften and extend a nip formed by a pair of rolls in a wet
press and thereby affect the pressure acting upon the web in
the nip, as well as the duration of the pressure. Due to the
relative hardness and the scarce thickness of the most common
fabrics, the aff ct is, however, quite little, and above all
the nip length, and consequently, the compression pressure and
time, cannot be set in a controlled manner within sufficiently
wide limits.
By means of the prior-art so-called shoe solutions,
in which one o~ the rolls forming a nip is substituted by a
stationary device pressed against the face of the roll, it is
possible in principle, by selecting press shoes having
different lengths in the direction o running of the web,
to adjust the pressure prevailing in the gap between the shoe
and the roll and acting upon the web, as well as adjusting
the duration of this pressure. In practice however, it is
quite difficult to accomplish such a solution. The constructions
tend to become expensive, and the operating powers required
become high. Moreover, the strength of the mat to be passed
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into the nip be-tween the shoe and the roll, on the one hand, and
the reduction of the friction between the mat and the shoe and
the lubricants used on the other hand, constitute problems that
are difficult t~ resolve.
In relation to the present invention, reference is made to
Canadian Patent Application No. 492,043 filed October 2, 1985.
This patent application describes a method in the press treatment
of a fibre web, in which a press nip formed between two press
rolls is used, through which the web to be dewatered by pressing
is passed while supported on a fabric or on a belt that receives
water, or while placed between a fabric andJor belt that receives
water. Moreover, an elastic press belt or several belts are
passed through this press nip.
In the method of this Canadian patent application, the maximum
compression pressure applied in the method is chosen between an
upper limit and a lower limit. The upper limit is lower than 130
bars or a certain function P2maX(L), whichever is lower. The
value of P2max according to this function, decreases with an
increase in the length (L) of the nip, the particular function
being determined principally by the maximum available compression
impulse. The lower limit ls higher than 50 bars.
The diameters of the press rolls are selected and the physical
characteristics of the press belt, e.g. thickness, structure, and
viscoelasticity are selected so that within the range of maximum
pressing load defined above, a range of the length of the
extended nip defined between the rolls is achieved, the upper
limit being less than about Z00 mm and the lower limit being
greater than about 30 mm.
Accordingly, the present inventlon provides for further
improvement and development of the features described in Canadian
Patent Application No. 492,043.
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The present invention also provides a press section in which
there are preferably at least two press nips, having rolls ~hich
are interchangeable with one another.
The present invention again provides a totall~ closed draw for
passing a web from a forming wire to a drying fabric, i.e. in the
first stage from the plck-up fabric to the first nip, in the
second stage from the first nip to the second nip, and thirdly
from the second or subsequent nips to tha drying ~abric of a
paper machine. By means of the closed draw, breaks in the web,
in particular webs of thin quality, and stoppages resulting from
such breakage, are reduced.
The present invention further provides a press sectlon in which
quality properties of paper can be controlled efficiently by
means of the choice of bands and mats utilized ln the press
section, such bands and mats moreover being disposable and easily
replaceable.
The present invention again provides a press section where
construction costs are low and in which there is sufficient space
between press nips so that adequate spacing is provided for
various apparatus, additionally in view of operation and
servicing, as well as of removal of paper broke. In this regard,
the structure and method in accordance with the present
invention, is also well-suited, e.g., for renewal or regeneration
of press sections in a paper machine.
The present invention further provides for increased dry solids
contents in a running web passing through a press section.
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The present invention also provides the possibility of
effectively preparing a web such as a paper web from pulps of
hlferior quality as compared with prior art pulps that have been
utilized.
The present invention further provides a press section
in whlch it is not necessary to utilize press-suction rolls, such
rolls having well known drawbacks such as discussed above.
The present invention again accomplishes a relatively
symmetric dewatering o~ a web passing through a press sectlon,
and thereby obtain a symmetric structure of the running web
itself.
According to the present invention there is provided in
a press section of a paper machine, comprising at least two con-
secutive separate press nips through which water-receiving press
fabric is passed, said consecutive press nips including a last
press nip, each press nip being formed between its own separate
pair of press rolls opposite one another, the improvement com-
prising the combination of an elastic nip-extending resilient
belt disposed to pass through at least one of said nips and being
thicker than the press fabric passing through the same nip,
whereby said nip is lengthened and compression impulse usable
therein is increased, a closed draw provided entirely through
said press section for passing a web running therethrough in a
closed draw from a pick-up point and to a transfer point at a
drying section, a substantially non-water recsiving transfer belt
passing through at least one of said nips, which forms the part
of said closed draw between two successive press nips or between
said last press nip and said transfer point, and wherein at least
the last press nip comprises an extended nip whereln the web is
sandwiched between a water-receiving press fabric contacting one
side of the web and said substantially non-water receiving
transfer belt contacting the other side o~ the web, and said nip
extending resilient belt contacting said transfer belt and
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passing through the nip b~tween said transfer bel~ and a press
roll formlng the nip to e~tend the nip, the web following the
transfer belt after passing through the nip.
Thus, the present invention is directed to an improve-
ment in a press section of a paper machine comprising at least
two separate press nips through which press fabric is passed,
with each nip being formed between ~ pair of press rolls opposite
one another. More speciically, the improvement ls directed to
the combination of an elastic resilient belt being disposed to
pass through at least one of the nips and being substantially
thicker than the press nip fabric passing through the same nip,
whereby the nip is lengthened and compression impulse usable
therein is increased; a closed draw situated entirely through the
press section for passing a web runnlng therethrough from a pick-
up point and to a transfer point at a drying section; and a
transfer belt passing through at least one nip, to form a closed
draw between two successive nips or between a final press nip and
the transfer point to the drying section.
Preferably, at least one roll in each palr of rolls
forming a press nip is provided with a hollow face. Addition-
ally, the transfer belt i5 substantially non-water-receptive.
The present invention is also directed to a method for
pressing a web in a press section of a paper machine.
According to the present invention there is also pro-
vided a method for pressing a web in a press section of a paper
machine, comprising the steps of forming a plurality of consecu-
tive press nips including a last press nip by disposing a pair of
rolls opposite one another to ~orm each nip, each nip being
~ormed by its own separate pair of press rolls, passing at least
one water-receiving press fabric through each press nip, passing
an elastic, resilient belt through at least one of said nips, the
belt being thicker than the fabric passing through the same press
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nip, thereby lengthening the same press nip and increaslng usable
compression impulse therein, passing a web in a closed draw
entirely through said press section, from a pick-up point to a
transfer point at a drying section, passing a substantially non-
water receiving transfer belt through at least one of said pressnips, thereby forming the part of said closed draw between two
successive press nips or between a final press nip and the trans-
fer point, and arranging said press section so that at least the
last press nip comprises an extended nip wherein the web is sand-
wlched between a water-receiving press fabric contacting on0 side
of the web and said substantially non-water receiving transfer
belt contacting the okher side of the web, and said nip-extending
resilient belt contacting said transfer belt and passing through
the nip between said transfer belt and a press roll forming the
nip to extend the nip, the web following the transfer belt after
passing through the nip.
A starting point for the present invention has been the
realization disclosed in Canadian Application No. 492,043, accor-
ding to which, by utilizing an elastic press mat wlth a view todewatering for each web to be dewatered, it is possible to pro-
vide an optimum form of a compression pressure function p , p(t)
of each
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~277~66
nlp or nlps, so that, In addltlon to utlllzlng fabrlcs belonglng
to the nlp and the web, an extra~, decldedly selected and dlmen-
sloned belt or beits are passed Into the nlp, the hardness,
~h I ckness, Inertla, rl~l d I ty, and other physlcal characterlstlcs
of such belts belng varled In a deslred manner, whlle sultable
roll dlameters are also chosen.
In thls manner, It Is posslble to vary the length of
the nlp In the runnlng dlrectlon of the machlne, and, by means of
the reslllence of the belt, to compensate for varlatlons In the
dry sollds content of the web.
In the present Inventlon, the concepts presented above
have been comblned In a novel manner wlth the advantageous use of
several nlps, as well as wlth a closed draw of the web from the
formlng sectlon to the drylng sectlon, along wlth control of sym-
metry of dewaterlng.
In one embodIment of the present Inventlon there are
provlded two press nlps, at least one of whlch belng extended by
sald reslllent belt, and sald closed draw between sald two press
nlps belng formed by a lower or upper fabrlc passlng through sald
fIrst press nlp, a transfer nlp, and a suctlon t ansfer devlce In
sald transfer nlp, sald transfer belt or a press fabrlc passlng
through sald second nlp, whereby the web Is transferred by sald
suctlon transfer devlce from sald lower or upper fabrlc to sald
transfer belt or press fabrlc In sald suctlon transfer nlp, and
sald transfer belt or a transfer fabrlc passlng through sald sec-
ond press nlp and carrylng the web In sald closed draw from sald
second nlp and onto a drylng wlre of the drylng sectlon, the web
beln~ transferred at the transFer polnt onto the drylng wlre by
the effect of a suctlon zone of a suctlon devlce dlsposed Inslde
a loop of the drylng wlre.
35In another embodlment of the present Inventlon th0re Is
provlded a transfer nlp dlsposed between sald two press nlps, a
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Iower fabrlc passlng through sald flrst nlp and passlng the web
In sald closed draw from sald flrst press nlp to sald transfer
nlp, an upper fabrlc passlng through sald transfer nlp and second
press nlp, and passlng the web In sald closed draw ~rom sal d
transfer nlp to sald second press nlp, a suctlon devlce dlsposed
In sald transfer nlp and dlsposed to transfer the web thereln
from sald lower to sald upper fabrlc, and sald transfer belt
belng substantlally water Impermeable and passlng through sald
second press nlp and carrylng along an upper surface thereof, the
web from sald second nlp to sald transfer polnt In sald closed
draw.
In a further embodlment of the present Inventlon there
Is also provlded a transfer nlp dlsposed between sald two press
nlps, an upper fabrlc passlng through sald flrst nlp and passlng
the web In sald closed draw from sald flrst press nlp to sald
transfer nlp, sald transfer belt belng relatlvely water-permeable
and passlng through sald transfer nlp and second press nlp, and
passlng the web In sald closed draw from sald transfer nlp,
: 20 through sald second press nlp, and to sald transfer polnt, a suc-
tlon devlce dlsposed In sald transfer nlp and dlsposed to trans-
fer the web thereln from sald upper fabrlc to an upper surface of
sald transfer belt, and sald transfer belt dlsposed to transfer
the web to the drylng sectlon wlthout substantlally rewettlng
2~ thereof. Sultably the comblnatlon addltlonally comprlses three
consecutlve press nlps, an upper plck up fabrlc passlng through
sald fIrst two nlps, and transferrlng the web therethrough In
sald closed draw, and sald transfer belt passlng as a lower fab-
rlc through sald second nlp, and passlng the web In sald closed
draw between sald second and thlrd press nlps, whereln sald
reslllent belt Is dlsposed to pass through sald thlrd press nlp
to extend the same and allow sald hlgher compresslon Impulse
thereln.
35The present Inventlon wlll be descrIbed In greater
detall below, wlth reference to certaln exempllfylng embodImen~s
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~277~;6
illustrated in the accompanying drawingsl to which the pr~sent
invention is not intended to be exclusively confined. In the
drawings,
FIG. 1 is a schematic side view of one embodiment
of the present invention having two press nips;
FIG. 2 is a schematic illustration of a second embodi-
ment of t,he present invention with two press nlps;
FIG. 3 is a schematic side view of a third embodiment
of the present in~ention with three press nips;
FIG. 4 illustrates distribution of compresion pressures
in an extended resilient belt nip within the scope of the present
invention; and
FIG. 5 is a sectional view along line V-V in Fig. 3,
i.e. of a nip suitable for use in the present invention, in
which the resilient belt is used.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figs. 1, 2 and 3, a web W is formed on a
wire 10, which is either a fourdrinier wire or one of the wires
of a twin-wire former. On a downwardly slanting run between
the wire 10, the wire suction roll 11 (over suction zone lla)
and the wire drive roll 12, the web W is transferred at a
detaching line Pl within a suction zone l9a oE a pick-up roll
19, and onto a first upper Eabric 20;21;22, which is either a
transfer fabric or an ordinary pick-up felt.
The fabric 20;21;22 carrie,s the web W along the
bottom face thereof into the irst press nip Nl, which is
formed between two press rolls 24 and 25. The nip Nl is provided
with two press fabrics, mainly the upper press fabric 20;21;22
~ and a first lower fabric 40;41;42 which is a press fabric that
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~.X7~66
receives water, e.~. an ordinary prcss felt.
The upper fabric 20;21;22 passes about guide rolls
23 and reconditioning devices 2~, as illustrated in Figs. 1-3.
In accordance with Figs. 1 and 2, a particular resilient
belt 100 is passed through the first nip Nl, with the lengthen-
ing of the first nip Nl being accomplished with the belt 100,
e.g. in a manner presented in Fig. 4. Therefore, it is now
possible in this nip to use a considerably higher compression
impulse, as compared with sharp roll nips. The resilient belt
100, having a structure which will be described in greater
detail below, runs about a smooth-faced 24' upper press roll
24, and is guided by guide rolls 121 and a tensioning roll 122
so that the inner face of the resilient belt 100 is in direct
contact with the face 24' of the press roll 24.
As illustrated in Fig. 1, an ordinary press felt 40
is passed through the first nip Nl as a lower fabric. The
guide rolls of the felt 40 are denoted by reference numerals
43, and the reconditioning devices with numeral 46. After the
nip Nl, a suction device 28 is fitted to act against an inner
surface of the lower felt 40, so that the web W is drawn onto
the lower felt 40. This ensures that the web W will follow
along with the lower felt 40, on which the web W is passed as
a closed draw into a transfer nip No~
The transfer nip No is formed in connection with a
suction zone 37a of a transfer-suction roll 37 as illustrated.
The web is transerred in the transfer nip No onto an upper
press felt 30, which runs through a second nip N2.
Referring to Fig. 2, the web W is disposed after the
first nip Nl to follow along with the upper felt 21, which may
also be an ordinary pick-up felt. If a transfer elt 21 is
used, then it must be quite permeable, because the pick-up roll
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7~66
19 must suction the paper web W off the wire 10. In accordance
with Fig. 2, the web W is transferred along the upper felt 21
into the transfer nip No, which is formed in connection with
the suction zone 57a of a suction roll 57 fitted inside a lower
transfer ~elt 51 of the second nip N2. In the transfer nip No,
the web W is transferred from the upper felt 21 onto the lower
~elt 51, as a completely closed draw.
As illustrated in Figs. 1-3, there is a second press
nip N2 along the run of the web W after the first press nip Nl,
the second nip N2 being formed between two press rolls 34 and 35
provided with drives 39. In Figs. 1 and 2, the upper rolls 34
are provided with hollow faces 34',while in Fig. 3, both of
the rolls 34, 35 are hollow-faced 34' and 35' respectively.
However, the roll 35 may also be smooth-faced 35' as illustrated
in Figs. 1 and 2. As also illustrated in Figs. 1 and 2, a
second resilient belt 110 may be disposed to pass through the
second press nip N2 and about respective guide rolls 121 and
tensioning roll 122.
According to Fig. 3, the press section may include
two conventional short and sharp press nips as the first and
second nips Nl and N2 respe~tively. An ordinary press felt 22
runs through the nips Nl and N2 as the upper fabric. The press
rolls 24 and 25 in the first nip Nl are provided with hollow
faces 24';25' and drives 29. The lower fabric in the first
nip Nl is an ordinary press felt ~2. The lower fabric in the
second nip N2 is a transfer felt or trans~er belt 52, which may
also be completely impermeable.
The surface proper1~es of the outer surface of the
transfer belt 52 are chosen so that, after the nip N2, the
web W follows exactly along with the transfer belt 52, on
whose support the web W is passed as a closed draw into a
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~Z77~66
third press nip N3, which is a substantially dewatering, ex-
tended nip.
The nip N3 is formed between two press rolls 54 and 55
that are provided with drives 59, the upper roll being pro-
vided with a hollow face 54' and the lower roll 55 being
smooth-faced 55'. In accordance with Fig. 3, the third nip
N3 is an extended nip, for which reason a resilient belt 100
is fitted to pass through this nip N3, being guided by guide
rolls 121 and by an alignment roll 123.
As illustrated in Fig. 3, after passing through the
extended nip N3, extended by means of the resilient belt 100,
the web W is ~itted upon the transfer ~elt 52 so that it
continues to follow along with the trans~er belt 52, based
on the surface properties of the belt 52. The web W is trans-
ferred from the transfer belt 52 as a closed draw to the
drying section of the paper machine, in a manner to be des-
cribed in greater detail below. The upper fabric 32 of the
nip N3 is an ordinary press felt, which is guided by guide
rolls 33, and in connection therewith, prior art recondition-
ing devices 36 are disposed.
As illustrated in Figs. 1-3, the transfer fabric 50;
51;52 carries the web W after the nip N2;N3 onto the transfer-
suction roll 62, with a drying wire 60 o~ the drying section
passing over the suction zone 62a thereof. At the line Pz,
the web W is txans~erred, by the effect o~ the suction zone
62a, from the transfer ~abric 50;51;52 onto the drying wire
60, with the web W being causecl to adhere to the surface of
the wire 60. The wire 60 then passes over a first drying
cylinder 63 in the drying section, or over a so-called "baby"
cylinder, within a suitable sector. The web W continues its
passage on support of the drying wire 60 as a so-called single
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fabric draw, at least in the initial portion o~ -the drying
section, which also contains a second drying cylinder 64.
The transfer fabric 50;51;52 passes about guide rolls
53, with appropriate reconditioning devices 56 being optionally
disclosed, as illustrated in phantom in Figs. 1-3.
As also shown in Figs. 1-3, steam boxes 70 and 71 are
disposed between the irst and second nips Wl, N2 to act
against the web W. Steam i.s supplied into the r~spective
steam boxes 70 and 71 in the direction of arrow S. In a
corresponding manner, a steam box 71 is disposed between the
second and third nip N2, N3 in the embodiment of Fig. 3, to
act against the web W. The steam boxes 70 and 71 provide a
web W having a higher dry solids content. In other words,
this is achieved in that, in the nipsN2, N3 following the
steam boxes 70,71 more water can be removed. This is accom-
plished through a lower viscosity o~ water on the one hand,
and through a reduced springback coefficient on the other hand,
both of which are based upon a higher compression temperature.
In accordance with the above, a fully closed draw is
achieved between the lines P1 and P2, which mean that the
web W is supported at all times by a face either of a press
fabric or of a particular transfer fabric 50;52, there being
no unsupported runs o the web W at all. This promotes the
running reliability o the press section, by substantially
reducing the risk of breaks.
In the press section according to Figs. 1-3, dewatering
takes place in the first nip Nl in both directions (please see
arrows V), i.e. through both surfaces of the web W. This pro-
motes symmetry o the web W itself. In accordance with Fig. l,
the dewat0ring take.s place in the second nip N2 principally
upwardly (please see arrow V), because the transfer fabric 50
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1277~L~i6
is not substantially water-receiving. According to Fig. 2,
dewatering in the second nip N2 takes places principally up-
wardly, but also to some extent downwardly, because the upper
fabric 31 is an ordinary press felt that receives water, and
the lower fabric 51 is a transfer felt or belt having a re-
latively high penetrability, because suction-roll trans~er
is utilized in the transfer nip No. The web W is also trans-
ferred with support of the upward surface of the fabric 51 to
the drying Yection of the paper machine as a closed draw,
substantially without rewetting of the web W.
In Fig. 3, dewatering in the second and third nips
N2, N3 takes place upwardly (arrow V), because the lower fabric
is a transfer belt 52 in these nips, which is substantially non-
water-reaeiving. The choice of dewatering direc~ions described
above has the practical advantage that a web W of a quite
symmetric distribution of fines and fillers, is obtained.
An important feature of the present invention is the
use of the transfer belts 50, 51, 52 described above, which
either do not receive water, or receive only relatively little
water, and which moreover transfer the web W as a closed draw
to the drying section. In accordance with Figs. 1-3, the
transfer belt 50;51;52 is the second lower Eabric, and runs
through the extended nip N2 carrying the web up to the point P2,
where the web W i9 transferred as a closed draw onto the drying
wire 60~ ~he feature of not receiving water, which is peculiar
to the transfer belts 50;51;52, as a rule but not necessarily
means that the transfer belt 50, 52 is relatively impervious.
As stated above, due to the suction-roll transfer 57, 57a, the
transfer belt 51 in Fig. 2 must be relatively permeable. The
transfer belt 50;52 is, for example, a fabric produced by im-
pregnating an ordinary press felt with an appropriate plastic
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material.
Moreover, the following may be noted with respect to
the application of the various embodiments of the present
invention illustrated in Figs. 1-3, for different products to
be prepared. According to Fiys. 1 and 2, two press nips Nl
and N2 extended by means of a resilient belt 100, 110 are used.
As a rule, it is sufficient to use two press nips to obtain a
suficiently high dry solids content for the web W. Alterna-
tively, according to Fig. 3,two ordinary, sharp, and short
press nips Nl and N2, as well as a third, extended nip N3,
which provides the web W with an adequate dry solids content,
are used.
By using the several nips in the manner described above,
sufficient variation in dewatering direction is also obtained.
Regarding the embodiments illustrated in Figs. 1 and 2
it can be ascer~ained that thege are suitable for almost all
paper qualities, but especially well for rather thin paper
qualities for which the closed draw of the web W i5 important.
As an example, the kraft sack paper is noted, in which exten-
sions should be avoided. By using nips Nl and 1~2 provided with
a resilient belt 100, 110 it is possible to achieve long nips
with high compression impulses.
In the press sectlon illustrated in Figs. 1 and 2,
the resilient belt 100 in the first nip Nl is preferably subs-
tantially softer than the resilient belk 110 in the second
nip N2 so that in the first nip Nl, which is a limited-flow
nip, a smaller load, i.e. a lower maximum compression pressure
Pm~X, can be utilized than in the second nip N2. A higher
elongation of the nip is obtained by way of the softer first
resilient belt 110.
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By using a softer first resilient belt 110, it is
possible to use a higher overall impulse right in the first
nip Nl, without exceeding the crushing limit of the web W.
In the second nip N2, typically, e.g., for newsprint, a
harder resilient belt 110 and a higher maximum compreæsion
pressure are utilized. The second nip N2 may, however, be
shorter than the first nip Nl.
In the embodiment illustrated in Fig. 3, the ~irst
resilient-belt nip Nl, is replaced with two conventional short
and sharp roll nips. The embodiment illustrated in Fig. 3
is suitable for all paper qualities, in particular for high-
speed newsprint and kraft sack paper machines, due to the
closed draw provided therein.
Several different variations of the embodiments in
accordance with Figs. 1-3 may be utilized within the scope of
the present invention. For example, variations of the embodiments
in Figs. 1 and 2 are possible in which a resilient belt 100 is
not used in the first nip Nl. Such a variation is also possible,
but as a rule not advantageous, in which a resilient belt 100
is used in the first nip Nl but not in the second nip N2. In
Figs. 1 and 2, it is also possible to carry out a type of in-
version, in which the transfer felt or belt 50, 51 of the second
nip N2 is disposed at the upper side, with the resilient belt 110
o~ the second nip N2 being disposed about the upper roll 3~, i.e.
at the top side thereof.
In the embodiment according to Fig. 3, such variations
may also be made in which resilient belts 100 are also used in
the second nip N2, or e~en in both the first and second nips Nl,
N2, whereby resilient belts 100 are provided in all nips. An
inversion may also be made in the press section illustrated in
Fig. 3, so that the transfer belt 52 runs through the upper
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side of both the second and third nips N2 and N3, whereby the
resilient belt 100 is also placed at the top side of the nip N3,
i.e. about the upper press roll 54.
Fig. 4 illustrates some compression pressure curves
P = P (L) of an extended nip as used in a paper machine in
accordance with the present invention. The pressure curves
illustrated in Fig. 4 have been obtained by means o~ a press
section illustrated in accordance with Fig. 3, by measuring
the pressure curves of the extended nip N3 with pressure detec-
tors itted in connection with the press rolls, and with an
oscilloscope synchronized with the revolutions of the press
rolls 54 and 55.
More particularly, Fig. 4 illustrates the compression
pressure curves A,B,C,D, and E obtained with diferent linear
loads (kN/m). Fig. 4 includes a table which, in addition to
the linear load, also gives the percent dry solids contents
obtained in a web by means of a press in accordance with
Fig. 3. In Fig. 4, the measurement result C (150 kN/m, dry
solids = 45.4%) is a reference value obtained when the press
of Fig. 3 had no resilient belt 100. In such a case, the
nip N3 was sharp, with its length denoted by I,c.
The pressure distribution curves given in Fig. 4 were
obtained wi.th a resilient belt 100 having an uncompressed
thickness of 10 mm, with the hardness of the surface facing
the web being about 18 P ~ ~. This resilient belt 100 was
made of polyurethane, and provided with a reinforcement fabric.
The test results were obtained by means of a press of Fig. 3,
in which the diameter of the roll 54 and 55 were 1300 mm. The
upper roll 54 had a grooved steel face 54'. The lower roll
55 was a smooth steel-faced 55' roll. The test results o~
Fig. 4 were obtainec1 with a web speed of 15 m/s when the web
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to be run was newsprint of a grammage of 48 g/m2.
When the measurement results c and B in Fig. 4 are compared,
which have substantially equal maximum compression pressure Pmax~
it is noted that in accordance with the invention, a substantial
improvement has been obtained in the dry solids content by using
a resllient belt 100, because the dry solids content has
increased from 45.4% to 48.9~. On the basis of the measurement
result A, in which a fully useable maxlmum compression pressure
Pmax = 102 b~rs has been used, a dry solids content of the web W
as high as 50.0% has been accomplished with a nip wldth of LA =
100 mm.
In Fig. 4, the center line of the nlp is denoted with the
vertical line K-K. When the measuremenk results C,D and E are
examined, it can be ascertained that the resilient belt 100 acts
upon the shape of the pressure distribution above all, so that
the resilient belt 100 widens the sharp compression pressure
curve C at the top portion thereof, and lowers the maximum
compression pressure Pmax~ while the nip length is changed only
slightly when the linear load (kN/m) is maintalned substantially
constant.
The area remaining underneath the curves in Fig. 4, illustrates
the compression impulse effective in the nip.
Fig. 5 illustrates a cross-sectional view along line v-v in Fig.
3, i.e. a cross-section of an extended nip N3 in accordance with
Fig. 3. Fig. 5 is drawn substantlally to scale so that lt
discloses the relative thicknesses of the different belts and
fabrics, as well as of the web. The thickness H of the resilient
belt 100, is as a rule, wlthin the range of about 5 to 25 mm,
preferably about 8 to 15 mm. With respect to the detai.ls o~ the
structure of the resilient belt 100, reference is made to 499,107
filed January 7, 1986 and U.S. Patent No. 4,767,501.
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9L~77~66
The resilient belt loo may also be of unlform hardness ln the
direction of thickness. The hardness of the face of the
res~lient belt 100 facing the paper web W, iOe. of the outer face
101 disposed against the fabric 52, is preferably within a range
of about 10 to 80 P & J, most preferably within a range of about
20 to 40 P & J. By means of a layer 130 of the resilient
component having this hardness, small-scale variations in the
compression pressure are equalized, principally such variations
lo whose range of variatlon is about 6 mm and less.
In the resilient belt 100 of Fig. 5, there is a frame work layer
120 substantially harder than the above hardness o~ the outer
surface 101, and also thicker. Thls gives the resilient belt 100
the necessary mechanical strength, and, for its part, equalizes
variation in compression pressure taking place on a larger scale
as compared with that described above. The hardness of the
framework layer 100 of the resilient belt 100 ls, as a rule,
within a range of about 3 to 30 P & J, preferably within a range
of about 5 to 20 P & J. The thickness Hl of the harder framework
layer 120 in the resilient belt loO, is substantially more than
one-half of the entire thickness H of the belt 100, preferably
about 60 to 70% of the thickness of the entire belt. The
resilient belt 100 may have net structure or any other equivalent
framework layer acting as a reinforcement. The resilient belt
loo is made, e.g., of rubber and/or polyurethane, and lt may have
one or several net-like or equivalent reinforcement fabric or
layers.
Moreover, the transfer belt 50 51:52 used is preferably inelastic
and has a face which does not rewet the web after passing through
a respective nip, a conslderable advantage in addition to other
advantages of the use of a transfer belt.
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. ','"' ' ' ' ' , ' .
~27~6~i
In Figs. 1 and 2~ the diameters of the press roll 24,
25, 34, 35 are preferably within a range of about 800 to 1800 mm,
within which range the diameters of the press rolls 54 and 55
of the nip N3 illustrated in Fig. 3 also fit. The diameter
of the press rolls 24, 25 and 34, 35 forming the first and
second short and sharp nips Nl and N2 in Fig. 3, are, as a rule,
preferably within a range of about 700 to 1400 mm. As ascer-
tained above, the choice of diameters of the press rolls,
for its part, also affects the length of the nips. If necessary,
variable-crown devices are used in the press rolls, there being
no restrictions for their use, because the press rolls are not
suction rolls but solid-mantle press rolls.
In the nips used in accordance with the invention and
extended by means of the resilient belt 100, 110 or some other,
corresponding resilient component, a linear load is used which,
as a rule, is within a range of about 50 to 500 kN/m, preferably
within the range of about 200 to 400 kN/m.
In the present invention, the diameters of the press
rolls forming the nips Nl, N2, N3, the resilient-component
material or materials used, and the linear load in the nips,
are preferably chosen so that the nip length L ls within the
range of about 30 to 200 mm, preferably within the range of
about 40 to lS0 mm, and the maximum compression pressure PmaX
of the nip within a range o~ about 40 to 60 bars in the case
of limited-flow paper qualities, and about 60 to 120 bars in
the case of limited-compression paper quality.
The preceding description of the present invention is
merely exemplary and is not intended to limit the scope thereof
in any way. Various details within the scope of the present
invention may be var:ied and differ from the details described
above, which have been presented for the sake of examples only.
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