EP0055206B1

EP0055206B1 - Press section for substantially equally pressing both sides of paper web

Info

Publication number: EP0055206B1
Application number: EP81630083A
Authority: EP
Inventors: Donald Alexander Ely
Original assignee: Beloit Corp
Current assignee: Beloit Corp
Priority date: 1980-12-18
Filing date: 1981-12-18
Publication date: 1985-08-07
Also published as: DE3171746D1; ES8302824A1; BR8108279A; FI79731B; JPS57121695A; JPS5922838B2; KR850001609B1; EP0055206A1; KR830007957A; IN154483B; FI813825L; FI79731C; ES508018A0

Description

This invention relates to a papermaking machine dewatering press section for substantially equally pressing and dewatering both faces of a freshly felted paper web, comprising:

a plurality of porous dewatering felts;
a plurality of press rolls arranged to form:
a double felted first dewatering press nip through which the web passes in engagement between two of said porous dewatering felts so that the web is dewatered through both of its faces in said nip,
a single felted second dewatering nip through which the web passes with one of its faces in engagement with one of said porous dewatering felts and with its opposite face in direct compacting engagement with one of said press rolls, the web being dewatered through one of its faces in said second nip,
an unfelted third press nip through which the web passes with both faces in direct engagement with two of said press rolls,
a single felted fourth dewatering nip through which the web passes with its opposite face in engagement with one of said porous dewatering felts and with its one face in direct compacting engagement with one of said press rolls, the web being dewatered through said opposite face in said fourth nip,
three of said press nips being provided by a four roll stack, and one of said nips being formed between press rolls separate from said stack.

A dewatering press section for a papermaking machine of this type is known from US-A-4 188 262. Although the art of pressing and dewatering a paper web is already rather highly developed, as exemplified by US-A-4 188 262, there is still substantial room for improvement. The Fig. 1 embodiment of US-A-4 188 262 has disadvantages as regards equal pressing and felting of the opposite faces of the web, the web being dewatered after passage through a double felted nip, in the two successive nips through the same face and being thereafter compacted. The Fig. 2 embodiment of US-A-4 188 262 is more advantageous as regards equal pressing and felting of the opposite web faces but has other essential drawbacks. In that second embodiment a four roll stack defining three press nips is followed by a further pair of rolls defining an additional press nip. In the four roll stack one of the felts passes through successive nips on the same roll and the web must be transferred after the first nip from one felt to another. This may cause damaging of the web after passage through the first nip. Also, in the known press section the bottom side of the web is exposed to only one felted nip before the fourth nip. This is disadvantageous because it does not provide for appropriate compacting of the bottom side of a web which has different characteristics when formed in a single wire machine. Furthermore, in the known press section the web is unsupported by a felt or roll between the third and fourth nip. This "open draw" is a further disadvantage because it may cause web flutter and, ultimately, a break in the web.
Other prior press sections also suffer from several deficiencies among which may be enumerated undue complexity, excessively long press sections, unequal pressing and felting of the opposite faces of the web, open draws in the press section and requirement for large operating energy expenditure.
Representative of the general present state of the art are the following US patents:

2 672 078 - has an open draw for the web and does not attain balanced sheet compaction on both faces.
3 023 805 - does not have a sufficient number of pressing nips for high speed pressing and dewatering for at least some paper webs.
3 286 360 - has fewer nips than desirable for high speed operation and in the multi-nip arrangement requires excessive machine direction space.
4 075 056 - does not attain equal pressing and felting of the paper web.
4 086 131 - does not attain equal pressing and felting of the faces of the web because one face is subjected to extra felt nips.

The object of the present invention is to overcome the disadvantages, drawbacks, inefficiencies, shortcomings and problems inherent in prior press sections and their operation, and to provide a new and improved press section which will attain substantially equal ink, size or coating receptivity or absorption, i.e. compaction, on both faces of a paper sheet resulting from a freshly felted paper sheet web, in a simplified manner, in compact space, free from open draws in the press section, conserves operating energy, and provides for equal pressing and felting of both faces of the web.
To achieve this the papermaking machine dewatering press section is characterized in that the double felted first nip is provided by press rolls arranged in the direction of web movement forwardly of said four roll stack, and the first nip in said stack is the single felted second nip which is formed between a grooved roll and a porous covered roll, the second nip in said stack is the unfelted third nip which is formed between the porous covered roll and a smooth hard surface roll and the third nip in said stack is the single felted fourth nip which is formed between the smooth hard surface roll and a further grooved roll.
By the provision of these features the above mentioned disadvantages are overcome. Opposite faces of the web are equally dewatered and compacted in a compact space machine free from open draws. By disposing the double felted press nip forwardly of the four roll stack and providing the four roll stack with a first single felted nip, the bottom face or wire face of the web may be dewatered in two successive nips. None of the felts passes through two successive nips on a single roll of the four roll stack. It is accordingly not necessary to transfer in the four roll stack the web from one felt to another, thus reducing the risk of damaging the web. The porous covered roll takes over the web after passage through the first nip in the four roll stack and releases the web to the smooth hard surface roll after passing through the unfelted nip formed therebetween.
Other optional features of the press section are claimed in the subclaims.
Other features and advantages of the press section will be readily apparent from the following description of a certain representative embodiment thereof, taken in conjunction with the accompanying drawing in which:

Fig. 1 is a schematic side elevational view of a paper making machine press section;
Fig. 2 is an enlarged fragmentary sectional detail view taken substantially along the line II-II of Fig. 1;
Fig. 3 is a fragmentary enlarged sectional detail view taken substantially along the line III-III of Fig. 1; and
Fig. 4 is an enlarged fragmentary sectional detail view taken substantially along the line IV-IV of Fig. 1.

On reference to Fig. 1, a paper making machine dewatering press section 5 includes a pickoff device 8 adapted to be located adjacent downstream from paper web forming means and operating to lift a freshly formed, i.e. felted, paper sheet web W from a forming belt, wire or fabric 8 for further processing in the press section. For this purpose, the pickoff device 7 comprises a pickoff roll 9 equipped with a suction gland 10 and running in engagement with the upper face of an endless belt dewatering felt 11 which is pressed against the web W on the forming fabric 8 at the nip of the roll 9 with the fabric. Suction from the gland 10 causes the web W to separate from the generally upwardly facing forming fabric 8 and adhere to the generally downwardly facing surface of the dewatering felt 11 which carries the web onward to a first press roll means 12.
In the first press roll means 12, the dewatering felt 11 runs together with an endless upwardly facing second porous dewatering felt with the web W engaged therebetween through a first pressing and dewatering nip N-1 provided by and between a suction roll 14 and a grooved roll 15. In this instance the suction roll 14 is on the lower side of the nip N-1 and the grooved roll 15 on the upper side of the nip N-1, so that although in the nip itself substantially equal dewatering results through both faces of the web W, the suction provided by a suction device 17 in the roll 14 effects separation of the web W from the upper felt 11 so that the web W can travel onwardly in machine direction carried on top of the lower felt 13. Desirably, the upper press roll 15 may embody a structure substantially according to U.S. Patent 3 198 697, that is the perimeter of the roll is provided with a set of axially spaced annular grooves 18.
Beyond the nip N-1, the web W is caused to be further pressed and dewatered in second press roll means 19 comprising a four roll stack providing a second nip N-2, a third nip N-3 and a fourth nip N-4, through which the web runs successively. The nip N-2 is provided by an upper porous covered press roll 20 and a lower grooved roll 21. In the nip N-2 the dewatering felt 13 engages the grooved roll 21 and the lower face of the web W, and the porous covered roll 20 engages the other or upper face of the web. By preference, the grooved roll 21 is similar to the grooved roll 15, having an axially spaced set of circumferential annular grooves 22 (Fig. 2). In addition to facilitating dewatering drainage through the felt 13, the grooves 22 avoid any tendency towards vacuum retention of the web W on the felt 13, so that the web easily releases from the web 13 at the exit from the nip N-2 for travel on the perimeter of the roll 20 to the nip N-3.
For optimum results, the press roll 20 has a porous elastomer cover 23. In a typical press roll configuration for this purpose, the cover 23 may be about 1" thick and formed from a suitable elastomer in a 5 to 30 P & J hardness range, which enables the pressing perimeter of the roll 20 to yield slightly under web pressing nipping pressure to minimize crushing the paper web fibers. The degree of hardness of the cover 23 can at least to some extent be determined by the degree of porosity of the cover, that is the size and number of porosity voids 24 in the material of the cover. As noted in Fig. 2, some of the pores 24 open outwardly at the nipping perimeter of the roll 20 and facilitate operation of the roll 20 by capturing water pressed from the web W and thus facilitating release of the web from the roll 20 after the web passes through the nip N-3. In order to avoid excessive accumulation of water on the perimeter of the roll 20, a doctor 25 acts on the free offrunning perimeter of the roll 22 beyond the nip N-3. If desired, a felt wick 27 may engage the perimeter of the roll 20 either before or after the doctor 25, but preferably after, for applying a web release agent supplied from supply means 28 with which the wick communicates. Instead of the wick 27, a suitable spray device may be used, if preferred. It will be appreciated, of course, that the pressing perimeter porosity of the cover 23 should be of a character to avoid marring the surface of the web W in contact with the roll at the nips N2 and N3.
In the nip N-3, the web W which has travelled from the nip N-2 on the perimeter of the roll 20, is pressed between the perimeter of the roll 20 and the nipping perimeter of a hard surfaced roll 29. Although the roll 29 may have a plastic cover, it should be of bone hardness and which according to paper machine standards is equivalent to about 0 P & J hardness. Since the nipping perimeter of the roll 29 is smooth, the softer porous perimeter of the roll 20 will easily release the web W from the perimeter of the roll 20 to follow the perimeter of the roll 29 on the offrunning side of the nip N-3. Such release may, of course, be further facilitated by treating the surface of the roll 20 with a release agent, as by means of the wick 27. In a preferred arrangement, the roll 29 may be driven rotatably by suitable drive means 30 and cause driving of the other rolls in the stack in the press means 19.
In the nip N-4, the web W is engaged on its lower face directly by the perimeter of the roll 29, and on its upper face by a third endless belt dewatering felt 31 which runs through the nip N-4 with the web W and in engagement with the perimeter of a grooved press roll 32 of substantially the same type as the press rolls 15 and 21, and having a set of axially spaced annular peripheral grooves 32 (Figs. 1 and 4). At the offrunning side of the nip N-4, the web now substantially dewatered and densified leaves the roll 29 and the felt 31 and travels on, as for example, past a guide roller 34 as a sheet W/S which is possessed of substantially equal ink, size or coating receptivity or absorption characteristics.
Since the leading end of the web W on leaving the nip N-4 during threading of the press roll means stack 19 may tend to cling to the perimeter of the roller 29, a doctor blade 35 on the perimeter of the roll 29 adjacent to the offrunning side of the nip N-4 will effect separation of the advancing leading end, which can then be directed about the guide roller 34 and onward to a calender, winder, or the like. During normal operation, of course, the doctor 35 is adapted to maintain the perimeter of the roll substantially dean.
As will now be apparent, the substantially equal surface characteristics of the paper sheet W/S leaving the press section 5 results from the substantially equal dewatering and pressing treatment applied to each of the web faces in progressing through the press section. In the nip N-1, dewatering is effected through both of the web faces, that is, water is caused to leave through both of the faces and thus fines in the web migrate substantially equally from the core of the web toward each face.
In the nip N-2 dewatering is effected through the lower face of the web W as indicated by the directional arrow in Fig. 2, while the upper face is compacted by the perimeter of the roll 20.
On traveling through the nip N-3, the web W is substantially equally compacted by the direct nipping engagement of the rolls 20 and 29 with the opposite faces, as exemplified in Fig. 3 by the double headed arrow.
Then, in the nip N-4, the upper face of the web W is finally dewatered as indicated by directional arrow in Fig. 4, while the lower face of the web is finally compacted by the roll 29.
Therefore, not only is the web W thoroughly compact to provide a strong sheet WS, but each face of the web has substantially equal concentration of fines so that the desirable equal receptivity or absorption characteristics are attained.

Claims

1. Papermaking machine dewatering press section for substantially equally pressing and dewatering both faces of a freshly felted paper web, comprising:

a plurality of porous dewatering felts (11, 13, 31),

a plurality of press rolls (14, 15, 20, 21, 29, 32) arranged to form:

a double felted first dewatering press nip (N-1) through which the web (W) passes in engagement between two of said porous dewatering felts (11, 13) so that the web (W) is dewaterd through both of its faces in said nip (N-1),

a single felted second dewatering nip (N-2) through which the web (W) passes with one of its faces in engagement with one (13) of said porous dewatering felts (11, 13, 31) and with its opposite face in direct compacting engagement with one (20) of said press rolls (14, 15, 20, 21, 29, 32), the web (W) being dewatered through one of its faces in said second nip (N-2),

an unfelted third press nip (N-3) through which the web (W) passes with both faces in direct engagement with two (20, 29) of said press rolls (14, 15, 20, 21, 29, 32),

a single felted fourth dewatering nip (N-4) through which the web (W) passes with its opposite face in engagement with one (31) of said porous dewatering felts (11, 13, 31) and with its one face in direct compacting engagement with one (29) of said press rolls (14,15,20, 21, 29,32), the web (W) being dewatered through said opposite face in said fourth nip (N-4),

three of said press nips (N-1 to N-4) being provided by a four roll stack (19), and one of said nips being formed between press rolls separate from said stack (19),

characterized in that the double felted first nip (N-1) is provided by press rolls (14, 15) arranged in the direction of web movement forwardly of said four roll stack (19), and the first nip in said stack is the single felted second nip (N-2) which is formed between a grooved roll (21) and a porous covered roll (20), the second nip in said stack is the unfelted third nip (N-3) which is formed between the porous covered roll (20) and a smooth hard surface roll (29) and a third nip in said stack is the single felted fourth nip (N-4) which is formed between the smooth hard surface roll (29) and a further grooved roll (32).

2. Press section according to claim 1, characterized in that the double felted first press nip (N-1) is formed between a pair of press rolls (14, 15) comprising a suction roll (14) and a grooved roll (15) in nipping relation.

3. Press section according to claim 1 or 2, wherein a pickoff device (7) is located adjacently downstream from the paper web forming means and includes a suction roll (9) for picking-off the freshly formed paper web (W) from a forming belt (8), characterized in that the pickoff suction roll (9) is disposed in the loop formed by a first (11) of said felts (11, 13, 31) and is adapted to transfer the web (W) from the forming belt (8) onto a downwardly facing portion of said first felt (11) and passing with the web through the first nip (N-1), a second (13) of said felts (11, 13, 31) also passing through the first nip (N-1) and transporting the web on its upper side through the second nip, (N-2) the first felt (11) being separated from the web (W) after the first nip (N-1) and the second felt (13) being separated from said web (W) after the second nip (N-2).

4. Press section according to any one of claims 1 to 3, characterized in that means (28, 27) are provided for treating the surface of the porous covered roll (20) with a release agent.

5. Press section according to any one of claims 1 to 4, characterized in that the smooth hard surface roll (29) is a driven roll of the four roll stack.

6. Press section according to any one of claims 1 to 5, characterized in that the porous covered roll (20) has open pores at its perimeter.