CA2074998A1

CA2074998A1 - Shoe press for the dewatering of a fiber web

Info

Publication number: CA2074998A1
Application number: CA 2074998
Authority: CA
Inventors: Christian Schiel
Original assignee: Individual
Current assignee: JM Voith GmbH
Priority date: 1991-08-01
Filing date: 1992-07-30
Publication date: 1993-02-02
Also published as: DE4125470C2; FI922976A0; JPH05195476A; DE4125470C1; FI922976A

Abstract

SHOE PRESS FOR THE DEWATERING OF A FIBER WEB

ABSTRACT OF THE DISCLOSURE
A shoe press having a press nip which is lengthened in direction of travel of the web passing through the nip. The nip is forms by a press shoe having a concave slide surface over which there slides an endless press belt which can be pressed against a backing roll by means of the press shoe. The press shoe is guided in the pressing direction within a pressure space which is provided in a stationary support member. The center axes of the backing roll and the annular, stationary support member lie in a press plane E. The slide surface of the press shoe is divided by the press plane into a web nip entrance zone and a departure zone.
The length of the entrance zone is greater than the length of the departure zone by at least a minimum ratio.
A porous belt press wire passes through the press nip between the web and the press belt. The porous belt has a denser cloth layer toward the web and less dense cloth layer away from the web. The surface of the press toward the porous belt is smooth and there are reinforcements in that press belt away from that smooth surface.

SPEC\19871

Description

2~7~8 SHOE_PRESS FOR THE DEWATERING OF A FIBER WEB

BACKGROUND OF THE INVENTION
The present invention relates to an extended nip shoe press îor dewatering a fiber web, particularly ~or use in a paper manuf acturing machine.
Prior art re.ferences applicable to this invention include (1~ German published application 37 08 18~ which corresponds to U.S. Patent 4,931,142; t2) German published application 37 05 241, which corresponds to U.S. Patent 5,071,513; (3) German published application 34 26 264, which corresponds to EP
application 0 168 718 and to Canadian patent 1,272,631;
(4) published International application WO 87/02080, which corresponds to U.S. Patent 4,908,103; (5) German published application 40 18 07~; and (6) German published application 40 22 800, which corresponds to U.S. patent 5,118,391.
The invention proceeds from above noted Reference 1, which shows a shoe press having a press nip which is lengthened or extended in the direction of travel of the web. The extended nip is formed at one side by a stationary press shoe havin~ a concave slide surface and at the other side by a rotating backing roll which cooperates with the press shoe. An endless, flexible, liquid-tight press belt, strengthened by a reinforcement within the belt, moves through the nip and slides over the slide surface of the shoe. The inner surface of the press belt, which is the surface that comes into contact with the press shoe, is smooth, in order to keep the sliding friction between them as slight SPEC~19871 2 ~

as possible. The sliding surface of the press shoe can be smooth when hydrodynamic lubrication of the press belt to press shoe interface is used ex~lusively. Instead, according to above Refersnce 2, the slide surface of the pr~ss shoe can alæo have pressure pockets to which lubricant is ~ed under pre~sure providing at least predominantly hydrostatic lubrication.
The press belt is prefer~bly deveLoped as a tubular, inflatable, endless loop press cover. Its path of travel outside the press nip is predominantly in a circular loop. Alternately, it is also known for the endless press belt to ~ravel over guide rolls located outside the press nip, which could then define a non-circular path for the press belt. The invention can be employed also with that embodiment of a pre~s belt.
The following features are also known from Reference 1. ~n endless loop, porous belt al50 passes through the press nip between the web to be dewatered, which is pressed by the rotatin~ backing roll, and the press belt. In Reference 1, this porous belt is developed as a paper making machine dewatering felt or felt belt, which has the ability to ab~orb water from the web when they are within the press nip. A second such felt can additionally be provided between the web and the backin~ roll.
The press shoe is guided in the pressing direction radially toward the backing roll, by the shoe . being within a pressure space which is provided in a stationary support body for the shoe. A press plane ~ is defined by the center axes of the backing roll and the support body. The press plane at the same time forms the central plane of the pressure space and the plane passes through the press shoe. The slide surface of the press SPf~C\19871 2~7~

shoe is divided in the direction of web travel by the press plane, as the slide surface is seen in cross section, into a web and press belt entrance zone and a web and press belt departure zone. Th length, in the direction of travel o* the web, of the departure zone is shorter than the length of the entrance zone. Thi~
causes the pressing pressure acting in the press nip to increase more gradually in the direction of travel of the web into the press nip up to the pressure plane and to decrease more rapidly upon emergence from the press nip, so that as little rewetting of the web as possible from the porous belt takes place.
Reference 3 discloses using a so called press wire instead of a water absorbing felt in pure roll presses of paper manufacturing machines. Such a roll press has a nip defined by two rotatable rolls, not by a slide shoe. The press wire can have the6Xadvantageover a traditional felt that the pre~s wire can more easily be kept clean by means of a wire cleaning device. This avoids the danger, which is sometimes experienced with a felt, that the web will be compressed as a result of dirtying or compacting of the felt. In other words, a press wire has a constant dewatering behavior over its entire life. One disadvantage of the known press wire is that it usually must be developed as a multi-layer cloth.
The outer side of the press wire that is contacted by the web is formed by a denser cloth material than the inner side that is away from the web. As a result, the cost of manufacture of the press wire is relatively high. This high price has not been compensated for up to now in many cases by a correspondingly higher solids content, i.e. a lower moisture con~ent, being obtained in the web. In addition, many uses ~uffer ~he following disadvantages SPBC~19871 2 ~

which also oppose generalized use of press wires. There is frequently a tendency toward a stronger rewetting of the web at the outlet from th press. Furthermore, in pure roll presses, there is a danger that the wires which rome into contact with the relatively hard roll wall are mechanically squashed so that the life of the press wire is reduced.

SUMMARY OF THE INVENTION
The object of the present invention is to develop a dewatering press for a paper manufacturing machine such that economical operation is possihle despite the use of a press wire instead of a customary felt. Another object is to compensate for the relatively high cost of manufacture of a press wire by having the least possible wear of the press wire and by reducing the rewetting of the web at the outlet from the press as much as possible, so as to obtain as high as possible a solids content of the web.
The invention is based on the discovery that shoe presses known from References 1 and 2 can be modified by a number of measures to enable the economical use of a known press wire.
A particularly important measure is an enlargement of the length ratio between the entrance zone and the departure zone of the slide surface of the press shoe. ~he necessary length ratios differ for different types of press shoes. For a shoe press with a smooth press shoe slide surface, the ratio is at least 1.5, while for a shoe press with a press shoe slide surface which has hydrostatic pressure pockets, the ratio is at least 1.2. In both cases, as seen in cross section, the discharge end of the slide surface i5 placed closer to SPEC\19871 .

' the press plane and thus also closer to the central press plane of the pressure chamber. This arrangement produces a pressing pressure curve which drops even more steeply than previously. This considerably reduces the period of time during which rewetting of the web from the press wire can take place. This in turn contributes essentially toward obtaining a relatively high solids content of the web.
Further measures which are necessary in order to economically operate a shoe press with a press wire are now explained. Press belts which are customarily used in shoe presses generally have a rough outer surface. This results from the fact that the reinforcement, which is generally a wire cloth, in the press belt extends up to the outer surface of the press belt. Frequently, outer bends of the reinforcement cloth even extend above the outer surface of the press belt.
If such a press belt were to come into contact with a press wire in the press nip, there would be a great danger of wear of both the press belt and the press wire.
In order to achieve the object of the invention, it is necessary to use a press belt in which the reinforcement is located inside the belt at a distance from the outer surface of the press belt, as is known from Reference 4.
Therefore, a press belt is used which has an outer sur~ace that is either entirely smooth or at least predcminantly smooth. The latter situation occurs when known storage depressions, for instance, grooves or blind holes, are provided in the outer surface of the press belt~ These storage depressions should take up at most ~0% of the outer surface. In other words, the smooth supporting part of the outer surface of the press belt should occupy at least 60% of the surface.
SP~19871 2~7~3~

Another measure which is necessary to avoid wear of the press wire is for the outer surface of the press belt to be relatively soft. It ha~ been found that the hardness of the out~r surface of the press belt should be at most 97 Shore D.
Other objects, features and advantages of the present inventisn will become apparent from the following descriptio~ of the invention which refers to the accompanying drawings.

BRIEF DESCRIPTIQN OF THE DRAWINGS
Fig. 1 diagrammatically shows a shoe press for dewatering a web, with a press wire passing through the press nip between the web and a press belt;
Fig. 2 is a greatly enlarged partial cross-section through the press wire;
Figs. 3 is a partial section through one embodiment of a press belt;
Fig. 4 is a partial section through another embodiment of a press belt;
~ig. 5 diagrammatically shows a portion of a shoe press embodiment which differs from that shown in Fig. 1.
Fig. 6 is a partial longitudinal section through one end of a shoe press roll.

DETAILED DESCRIPTION OF THE INVENTION
The shoe press 10 in Fig. 1 has a press shoe 13 with a smooth concave upper slide surface over which an endless loop, flexible, liquid tight press belt 14 slides. The concave slide surface of the press shoe 13 is adapted to the shape and diameter of a generally same radius of curvature complementary backing roll 11. Thus, SPEC~19~71 2~7~

the press shoe 13, the press belt 14, and the backing roll 11 form a press nip 15 which is lengthened or extended in the direction of tra~el of the web, which is from left to right in Fig. 1. There is an annular, stationary support body 12 around which the press belt 14 travels. At the top of the body 1~, there is a recess which defines a pressure space 2 0 . That space receives a piston like bottom or base part of the press shoe 13.
Pressure fluid is ~ed to the pressure space 20 through a pressure ~luid line 19. Lubricant is fed in known manner to the inner surface of the endless loop press belt which slides over the shoe 13. The lubricant serves in a known manner for hydrodynamic lubrication of the slide surface of the press shoe.
The center axis 12a of the support member 12 and the axiæ of rotation lla of the backing roll both lie in the so called press plane E. The plane E also extends through the center of the pressure space 20. It is the plane along which the greatest radial pressing force in the nip is applied, or where the resultant of pressing foroes is concentrated.
A web 18 to be dewatered, for instances a paper web~ travels through the press nip 15 in the direction of the arrow P, along with the so called press wire 17 which is between the web 18 and the press belt 14. If necessary, a further press wire 16 or a traditional press section web dewatering felt can also travel through the press nip between the web 18 and the backing roll 11.
The slide surface of the press shoe 13, which is one limit of the press nip 15, is arranged asymmetrically ~o the press plane E. The slide surface is divided by the press plane E into an entrance zone 21 and a departure zone 22. For the smooth slide surface SPE~C\19871 2 ~

press shoe in Fig. 1, the length of the entrance zone of the nip 21 should be at least 50% greater than the length of the departure zone 22, measursd in the direction of travel of the web. The path of travel o~ the press belt 14 is preferably predominantly circular. For that path to be adapted to the asymmetric arrangement of the slide surface of the press shoe 13, the axis 14a of the path of traYel of the press belt 14 is arranged in a plane that i8 parallel to and is displaced from the press plane E in the direction opposite th2 direction of travel of the web and back toward the entrance zone 21. The distance e between the axes 12a and 14a should be at least 40 mm.
The path of the press belt, its generally circular loop shape and the offset between axes 12a and 14a are set by the known manner of supporting the enclosed ends of the generally tubular press belt by supporting and guidance discs, as in U.S. Patent Re.33,034; 4,944,089 and 4,931,142, incorporated by reference.
Fig. 6 shows one manner of supporting the enclosed ends of the generally tubular press belt. The elements of Fig. 1 which are seen in Fig. 6 have the same numerals~
- The support body 12 has a journal 12b positioned in a bearing bracket 41.
On each end of the roll, a bearing ring 44 is displaceable axially on the journal 12b, but is not rotatable. A belt support disk 42 is supported for rotation around axis 14a on the ~earing ring 44 by means of an anti-friction bearing 43. on the outer end side of the belt support disk 42, the radially inwardly shaped zone of the press belt 14 is fastened by means of a clamping flange 45 and screws 46.

SPBC\~9871 2 ~ 7L~

In order to seal of the inside of the shoe press roll (defined by the tubular press belt 14 and the belt support disk 42) from the outside, the press belt 14 consists essentially of a liquid-tight ~aterial, for instance, polyurethane. It is pre~errably reinforced with a fabric or with threads wrapped therein. Details of the belt construction appear below. There i~ an o-sealing ring 47 betwaen the disk 42 and the belt 14. On the axial outside of the anti-friction bearing 43, there is a shaft sealing ring 49 which rests in a housing ring 50 fastened to the belt support disk 42.
Due to all these sealing means, the interior of the shoe press roll i5 infatable and retains the inflati~n pressure. For the inflation purpose, an air pressure feeding line 48 is provided.
As shown in Fig. 2, the press wire 17 is comprised of two, and in some other embodiments of more than two, layers of cloth 17a and 17b. The outer cloth layer 17a, which is contacted by the web 18, is woven from relatively fine threads. It forms a denser cloth layer than the inner layer of cloth 17b, which comes into contact with the press belt 14 and is woven vf thicker threads.
The press belt 1~ in Fig. 3 is preferably developed in accordance with Reference 6. It is comprised of an elastomeric material layer 23 in which there are reinforcement threads 24, 25. The reinforcing threads are in two layers which lie one above the other and whose paths intersect. All of the reinforcement threads are arranged in a distance from the two smooth opposite outer surfaces of the press belt.
In Fig. 3, the reinforcement threads 24, 25 are not woven together although they pass across each other, SPI~C\19871 2 ~

the threa~s 24 extending lengthwise while the threads 25 extend across the belt. Fig. 4, however, shows that a woven cloth can also be used. In accordance with Fig. 3, both the inner surface of the press belt which slides over the press shoe 13 and its outer surface are entirely ~mooth and free fro~ any depressions. Differlng ~rom what is shown in Fig. 3, however, the elastomeric layer 23, if n~cessary, can be made ssmewhat thicker. In that case, storage depressions are worked into the outer surface. The press belt shown in Fig. 4 is provided with such storage depressions 26.
In Fig. 1, in order to keep the press wire 17 clean, a wire cleaning device 27 having the following features is provided. A blast chamber 28 is provided on the outside of the press wire 17 and extends transversely over the entire width of the wire. It includes two ledges 29, 30 which are close enough together to form a blast slot between them. Between the upstream, feed side ledge 29 and the press wire 17, ~ wedge 31 is defined~
which tapers down in the direction of travel of the wire.
The jets of a water spray pipe 32 are directed into this wedge. In the simplified showing in the drawing, a water pump 33 pumps water into the spray pipe 32. Furthermore, compressed air from a source of compres~ed air 34 passes inside the blast cham~er 28. The parts o~ the wire cleaning device described above are arranged on the outer, relatively finely woven side of the press wire 17.
on the opposite side of the press wire, there is a collecting r~ceiver 35 having a water return line 36 leading away from it. An improved cleaning effect is obtained by this arrangement, as compared with the reverse arrangement previously used, since air and water SP~C119871 2~7~8 are fed first through the fine layer of cloth and then through the coarse layer.
Fig. 5 shows a shoe pres~ in which the press shoe 13' does not have a smooth sliding surface but has hydrostatic pressure pockets 13a defined in that surace.
In this type of press shoe, the length of the entrance zone 21' is at least 20% greater than the length of the departure zone 22'. The other elements of this arrangement are pro~ided with the same reference numbers as the corresponding parts in Fig. 1.
Although the present invention has been descr$bed in relation to particular embodiments thereo~, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

SPEC~19871

Claims

1. A shoe press for dewatering a fiber web in a paper manufacturing machine, the shoe press comprising:
a press shoe having a slide surface which is concave and is extended in the direction of web travel over the shoe;
a stationary support body having a pressure space defined in it, and the press shoe being supported in the pressure space; pressure means connected to the pressure space for urging the press shoe out of the pressure space;
a backing roll with an outer peripheral surface opposite the slide surface, the backing roll peripheral surface having a curvature generally corresponding to the curvature of the concave slide surface, the backing roll having a rotation axis about which the backing roll rotates past the slide surface, the backing roll peripheral surface and the press shoe slide surface defining a press nip between them, and the pressure means urging the press shoe out of the pressure space toward the surface of the backing roll;
the backing roll, the press shoe and the support body together defining a press plane in which the resultant of the shoe pressing forces extend, the press plane extending through the axis of the backing roll and through the concave slide surface of the press shoe;
the concave slide surface of the press shoe is divided along the web travel direction and along the concave slide surface into an entrance zone of the press nip up to the press plane and a departure zone of the press nip following the press plane, the length of the departure zone from the press plane is shorter than the length of the entrance zone to the press plane, and the SPEC\19871 ratio of length of the entrance zone to the length of the departure zone is at least 1.2 to 1;
a flexible, liquid tight, press belt passing between the concave slide surface and the peripheral surface of the backing roll, and with the press shoe pressed toward the backing roll, the press nip is lengthened in the direction of web travel;
the press belt having an outer side toward the backing roll, and the press belt outer side having a smooth surface toward the backing roll; a reinforcement in the press belt spaced a distance from the smooth surface thereof toward the backing roll so that the smooth surface is at least predominantly smooth;
a porous belt in the form of a press wire passing through the press nip between the smooth surface of the press belt and the backing roll, and the porous belt having a surface toward the backing roll for supporting the web to pass with the porous belt through the nip.

2. The shoe press of claim 1, wherein the surface of the press belt toward the porous belt and the backing roll has a hardness of at most 97 Shore D.

3. The shoe press of claim 1, wherein the press belt is an endless loop belt and the porous belt is also an endless loop porous belt.

4. The shoe press of claim 3, wherein the support body is so shaped and the press belt is an endless loop and is so shaped that the press belt passes around the support body.
SPEC\19871

5. The shoe press of claim 1, wherein the entire concave slide surface is a smooth slide surface without pockets and the ratio of length between the entrance zone and the departure zone is at least 1.5 to 1.

6. The shoe press of claim 1, wherein the concave slide surface of the press shoe is shaped to define a plurality of pressure pockets the slide surface, and the pressure means supplying hydrostatic pressure in the pressure pockets of the slide surface; and the ratio between the length of the entrance zone and the length of the departure zone of the press shoe is at least 1.2 to 1.

7. The shoe press of claim 1, wherein the support body is shaped to have and define a center axis thereof; and the press plane extends from the center axis of the support body through the press shoe to the axis of the backing roll.

8. The shoe press of claim 7, wherein the press belt is an endless press belt and is developed as a tubular, inflatable covering, which upon inflation, is shaped and sized to have a path of travel outside the press nip which is predominantly circular in shape.

9. The shoe press of claim 1, further comprising a second porous belt disposed between the first mentioned porous belt and the backing roll for sandwiching the web to be dewatered between the first and the second porous belts, and the second porous belt SPEC\19871 traveling between the web and the backing roll through the press nip.

10. The shoe press of claim 9, wherein the second porous belt is also a press wire.

11. The shoe press of claim 9, wherein the second porous belt is a dewatering felt.

12. The shoe press of claim 1, wherein the press belt is an endless press belt and is developed as a tubular, inflatable covering, which upon inflation, it is shaped and sized to have a path of travel outside the press nip which is predominantly circular in shape.

13. The shoe press of claim 12, wherein the press shoe slide surface is so shaped with its entrance and departure zones and the tubular press belt is so shaped and supported that the axis of the path of travel of the press belt is in a plane parallel to the press plane and displaced therefrom by at least 40 mm in the direction opposite the web travel direction and back toward the entrance zone of the press shoe.

14. The shoe press of claim 1, wherein the porous belt comprises a press wire having at least two different density cloth layers, the porous belt has a side contacted by the web which is formed of a denser cloth layer and the porous belt has a side which is contacted by the press belt which is of a less dense cloth layer.
SPEC\19871

15. The shoe press of claim 1, further comprising a porous belt cleaning device disposed at the porous belt and out of the press nip and at the side of the porous belt which would be toward the backing roll, the wire cleaning device including means for blasting air across the porous belt.

16. The shoe press of claim 15, wherein the porous belt cleaning device includes two ledges which are spaced apart to define an air blast slot which opens toward the porous belt, and the ledges and the slot extend across the porous belt; one of the ledges being on the upstream side of the slot with respect to the direction of web travel and the one ledge being shaped toward the porous belt for defining a wedge between the one ledge and the porous belt in the direction of travel of the porous belt; a water spraying device discharging into the wedge.

SPEC\19871