CA1277166C - Press section with separate press nips in a paper machine, and methodfor pressing a web therein - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A press section of a paper machine, and method for pressing a web therein, with separate press nips being formed between respective pairs of press rolls, each roll disposed opposite another. An elastic resilient belt substantially thicker than a fabric passing through a press nip, is disposed to run through the same nip, whereby the nip is lengthened and compression impulse usable therein is increased. The paper web is also passed as a closed draw through the successive press nips in the press section, from a pick-up point to a sub-sequent transfer point thereof. Additionally, a transfer belt passes through at least one of the press nips, whereby a closed draw is obtained between two subsequent press nips and/or between a final press nip and a drying section of the paper machine.

Description

~;~7t7~6 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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~.2771~i6 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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lX~7~6 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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~ ~ ~ 7 1~6 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 . - 6 -. ' ' .

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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 - 6a -'~' .

~L;277~66 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 : 25 . 30 "

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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 - 7 ~ ~

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~L,f~77~L6~i 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 7b _ ,':

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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 : .. ,. ' ' : ' ' ~.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 _g_ 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 --10 - .

"' ' ' '. ' ' ~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 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 ~L2~7~66 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 ~ 7 7 ~ ~

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.

';

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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Claims (30)

1. In a press section of a paper machine, comprising at least two consecutive 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 comprising the combination of an elastic nip-extend-ing 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 receiving 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 wherein the web is sandwiched between a water-receiving press fabric con-tacting one side of the web and said substantially non-water receiving transfer belt contacting the other 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.

2. The combination of claim 1, wherein at least one roll in each pair of rolls forming a press nip, is provided with a hollow face.

3. The combination of claim 1, wherein said closed draw between said two press nips is formed by a lower or upper fabric passing through said first press nip, a transfer nip, and a suction transfer device in said transfer nip, said transfer belt or a press fabric passing through said second nip, whereby the web is transferred by said suction transfer device from said lower or upper fabric to said transfer belt or press fabric in said suction transfer nip, and said transfer belt passing through said second press nip and carrying the web in said closed draw from said second nip and onto a drying wire of the drying sec-tion, the web being transferred at the transfer point onto the drying wire by the effect of a suction zone of a suction device disposed inside a loop of the drying wire.

4. The combination of claim 1, additionally compress-ing a transfer nip disposed between said two pres nips, a lower fabric passing through said first nip and passing the web in said closed draw from said first press nip to said transfer nip, an upper fabric passing through said transfer nip and second press nip, and passing the web in said closed draw from said transfer nip to said second press nip, a suction device disposed in said transfer nip and disposed to transfer the web therein from said lower to said upper fabric, and said transfer belt passing through said second press nip and carrying the web on an upper surface thereof from said second nip to said transfer point in said closed draw.

5. The combination of claim 1, additionally comprising a transfer nip disposed between said two press nips, an upper fabric passing through said first nip and passing the web in said closed draw from said first press nip to said transfer nip, said transfer belt passing through said transfer nip and second press nip, and passing the web in said closed draw from said transfer nip, through said second press nip, and to said transfer point, a suction device disposed in said transfer nip and disposed to transfer the web therein from said upper fabric to an upper sur-face of said transfer belt, and wherein said transfer belt trans-fers the web to the drying section without substantial rewetting thereof.

6. The combination of claim 1, additionally comprising a third consecutive press nip formed between its own separate pair of press rolls, an upper pick up fabric passing through said first two slips, and transferring the web therethrough in said closed draw, and said transfer belt passing as a lower fabric through said second nip, and passing the web in said closed draw between said second and third press nips, wherein said resilient belt passes through said third press nip to extend the same and allow said higher compression impulse therein.

7. The combination of claim 6, wherein said transfer belt transfers the web in said closed draw from said third press nip to said transfer point.

8. The combination of claim 1, wherein the length of said nip having said resilient belt passing therethrough is from about 30-200 mm.

9. The combination of claim 8, wherein the length of said nip having said resilient belt passing therethrough is from about 40-150 mm.

10. The combination of claim 1, wherein said nip having said resilient belt passing therethrough being disposed to apply maximum compression pressure of about 40-60 bars.

11. The combination of claim 1, wherein said nip having said resilient belt passing therethrough being disposed to apply maximum compression pressure of about 60-120 bars.

12. The combination of claim 1, being arranged to pro-vide a dry solids content of about 47-52% in a web passing therethrough.

13. The combination of claim 12, being arranged to pro-vide a dry solids content of about 49-51% in the web passing therethrough.

14. The combination of claim 1, additionally comprising said resilient belt having a surface facing the web of about 10 to 80 P&J hardness.

15. The combination of claim 14, wherein said surface of said resilient belt has a hardness of about 20-40 P&J.

16. The combination of claim 14, wherein said resilient belt additionally comprises an underlying framework of about 3 to 50 P&J hardness.

17. The combination of claim 16, wherein the underlying framework has a hardness of about 5-30 P&J.

18. The combination of claim 1, wherein thickness of said resilient belt is about 5 to 25 mm.

19. The combination of claim 18, wherein the thickness of said resilient belt is about 8-15 mm.

20. A method for pressing a web in a press section of a paper machine, comprising the steps of forming a plurality of consecutive press nips including a last press nip by disposing a pair of rolls opposite one another to form each nip, each nip being formed 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 nip, thereby lengthening the same press nip and increasing 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 substan-tially non-water receiving transfer belt through at least one of said press nips, thereby forming the part of said closed draw between two successive press nips or between a final press nip and the transfer point, and arranging said press section so that at least the last press nip comprises an extended nip wherein the web is sandwiched between a water-receiving press fabric contact-ing one side of the web and said substantially non-water receiv-ing transfer belt contacting the other 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.

21. The method of claim 20, additionally comprising disposing a transfer nip between said two press nips, passing a lower fabric through a first one of said nips, whereby the web is passed in said closed draw from said first press nip to said transfer nip, passing an upper fabric through said transfer nip and second press nip, whereby the web is passed in said closed draw by said upper fabric from said transfer nip to said second press nip, transferring the web from the lower to upper fabrics in said transfer nip by a suction transfer device therein, and forming said transfer belt to be substantially water-impermeable and passing the same through said second press nip, whereby the web is carried in said closed draw from said second press nip to said transfer point by said transfer belt.

22. The method of claim 20, additionally comprising disposing a transfer nip between said two press nips, passing an upper fabric through a first one of said press nips, whereby the web is passed in said closed draw from said said first nip to said transfer nip, formins said transfer belt to be relatively water-permeable and passing the same through the transfer nip and second press nips, whereby the web is passed in closed draw from said transfer nip through said second press nip and to said transfer point by said transfer belt, without substantial rewetting of the web, and transferring the web from the upper fabric to an upper surface of the transfer belt in the transfer nip by a suction device.

23. The method of claim 20, additionally comprising providing a third consecutive press nip formed between its own separate pair of press rolls, passing an upper pick-up fabric through a first two of said three nips, whereby the web is passed through said first two nips in said closed draw by said pick-up fabric, passing said transfer belt as a lower fabric through said through said second press nip, whereby the web is passed in said closed draw between said second and third press nips by said transfer belt, and passing said elastic, resilient belt through said third press nip, thereby extending the same and providing the higher compression impulse therein.

24. The method of claim 20, additionally comprising lengthening the nip with the resilient belt passing therethrough to a total length from about 30-200 mm.

25. The method of claim 20, additionally comprising applying a maximum compression pressure in said nip having the resilient belt passing therethrough, of about 40-60 bar.

26. The method of claim 20, additionally comprising passing the web in the press section to have a total dry solids content of about 47-52%.

27. The method of claim 20, additionally comprising providing the resilient belt with a surface facing the web of about 10 to 80 P&J hardness.

28. The method of claim 27, additionally comprising providing the resilient belt with an underlying framework of about 3-50 P&J hardness.

29. The method of claim 24, additionally comprising providing said resilient belt with an overall thickness of about 5-25 mm.

30. The method of claim 20, additionally comprising applying a maximum compression pressure in said nip having the resilient belt passing therethrough, of about 60-120 bar.