WO2000071813A1

WO2000071813A1 - Press section with web stabilizing device

Info

Publication number: WO2000071813A1
Application number: PCT/SE2000/000918
Authority: WO
Inventors: Leif Erik Helgesson; Per Anders Artur Leandersson
Original assignee: Valmet-Karlstad Ab
Priority date: 1999-05-21
Filing date: 2000-05-10
Publication date: 2000-11-30

Abstract

A press section in a machine for manufacturing a paper or paperboard web has a double-felted press (2) with first and second press elements (3, 4) and first and second press felts (5, 7). In accordance with the invention, the press section is provided with a web-stabilizing device to eliminate axial air flows that are detrimental to the stability of the web and which are caused by a partial vacuum arising in the press (2) after the press nip, which device has two air-deflection members (14) on the tender and drive sides outside the side openings of a wedge-shaped space (9) defined by the press felts. The air-deflection members (14) are arranged to deflect said air flows away from the wedge-shaped space defined by the press felts.

Description

Press section with web stabilizing device

The present invention relates to a press section in a paper or paperboard machine for manufacturing a continuous web, which press section comprises at least a one double-felted press with first and second press elements, forming a press nip, and first and second press felts, running through the press nip.

Because of the movements of the press felts and the web, partial vacuum is produced in the wedge-shaped space defined by the two press felts downstream of the press nip, which partial vacuum results in the adherence of the web to the web-carrying press felt being reduced and air being drawn into the wedge-shaped space defined by the press felts in an axial direction from the tender and drive sides via both open sides of the press. The axial air- flows thus formed pass across the free edge portions of the web-carrying press felt and across the edge portions of the web. The combination of said reduced adherence of the web to the press felt and said axial air flows results in a risk, of the edge portions of the web disengaging from the web-carrying press felt and becoming unstable in an undesirable way. The partial vacuum is a function of the machine speed raised to the second power, which means that said air flows into the wedge-shaped space become stronger with increasing machine speeds and that the problem involving unstable edge portions becomes greater to a corresponding, or substantially corresponding, degree. Unstable web-edge portions can entail creases being formed in the edge portions of the web when the web runs through the nip by the pick-up roll, which creases can create problems in a second subsequent press, in the drying section and/or in the reel-up. The creases can entail increased scrapping of the web, but they can also form indications of rupture, which sometimes can increase the risk of web rupture. Partial vacuum and consequent axial air flows also arise in every wedge-shaped space formed between a press felt and a press roll downstream of the press nip. The partial vacuum in the outer, wedge-shaped space defined by the press felt not carrying the web and the appurtenant press element can adversely affect the pressure condition in the inner, wedge-shaped space defined by the two press felts, when the partial vacuum in the latter is less, in that air will be drawn through the air-pervious press felt not carrying the web, from said inner space to said outer space. On the other hand, the partial vacuum in the outer, wedge-shaped space defined by the web-carrying press felt and the appurtenant press element can have a beneficial effect on the web in that the adherence of the web to the press felt is favourably affected due to the suction force acting from the last-mentioned outer, wedge-shaped space through the air-pervious press felt.

DE-19626418 describes single-felted presses in paper machines where the paper web after the press nip adheres to the upper press element to part from the same at a position a distance from the press nip. A blower with the width of the web is arranged in proximity to and upstream of said position to assist in separating the paper web from the press element. The blower is placed near the paper web to blow air in a direction perpendicular to the paper web so that air penetrates into the paper web and compresses the air already present in the paper web. Downstream of the blower, the compressed air will expand again to leave the paper web through its free side, which faces away from the upper press element . By way of this expansion, a spring back is created in the paper web that reduces the adherence of the paper web to the smooth envelope surface of the press element so that the paper web is disengaged more easily from the smooth envelope surface. Thus, this patent specification does not touch upon the problem associated with the partial vacuums that are created in the press section and, consequently, does not suggest any solution to the problem.

The object of the present invention is to alleviate the problems discussed in the introduction and to provide a press section with a web-stabilizing device that makes it possible, in an efficient and simple manner, to substantially reduce or even to completely eliminate the detrimental effect of partial vacuums on the stability of the web and, particularly, the edge portions of the web.

In accordance with the invention, the press section is characterized in that it is provided with web-stabilizing device to eliminate axial air flows that are detrimental to the stability of the web and which are caused by partial vacuums arising in the press after the press nip, which device comprises two air-deflection members, arranged on the tender and drive sides of the machine outside and in immediate proximity to the side openings of a wedge-shaped space defined by the two press felts, through which side openings air normally flows into said space, and which air-deflection members extend from the area of the press nip to a pre-determined point located downstream, which air-deflection members are arranged to deflect said air flows away from the wedge-shaped space defined by the press felts.

The detrimental effect of the partial vacuum and, more precisely, of the axial air flows is thus reduced or eliminated, in accordance with the invention, by the axial air flows being deflected so that they are prevented from passing across the web edges, where web instability may otherwise arise immediately after the press nip. The invention will be further described in the following with reference to the drawings.

Figure 1 shows schematically in perspective parts of a press section in a paper or paperboard machine, which press section is provided with a web-stabilizing device having two air-deflection members in accordance with a first embodiment of the invention, only one of the air-deflection members being shown.

Figure 2 is a view from above of parts of the press section in accordance with Figure 1, where both air-deflection members being shown.

Figure 3 shows schematically in perspective parts of a press section in a paper or paperboard machine, which press section is provided with a web-stabilizing device with two air-deflection members in accordance with a second embodiment of the invention, only one of the air-deflection members being shown.

Figure 4 is a view from above of parts of a press section in accordance with Figure 3, where both air-deflection members are shown.

Figure 1 shows schematically in perspective parts of a press section in a machine for manufacturing paper or paperboard in a continuous web 1, which is formed in a wet section (not shown) and dried in a drying section (not shown) . In the embodiment shown, the press section comprises a double- felted press 2, having a first press element 3 and a second press element 4 , which press elements 3, 4 define a press nip with each other. In the embodiment shown, the first and second press elements 3, 4 are arranged one above the other and may therefore also be designated upper and lower press elements 3, 4. The press shown is a roll press 2, the press rolls 3, 4 of which can be smooth, grooved or blind-drilled and alike or different in this respect. Alternatively, the press is a shoe press. The roll press 2 further comprises a first or upper press felt 5, running in an endless loop through the press nip and around a plurality of guide rolls 6, and a second or lower press felt 7, running in an endless loop through the press nip and around a plurality of guide rolls 8. After the press nip, the two press felts 5, 7 diverge from each other so that they define between them an intermediate wedge-shaped space 9. Furthermore, the upper press roll 3 and the upper press felt 5 define between them an upper, outer wedge-shaped space 10 and the lower press roll 4 and the lower press felt 7 between them a lower, outer wedge-shaped space 11. All three wedge-shaped spaces 9, 10, 11 are, by way of side openings, open on the tender and drive sides of the machine, which side openings are defined by diverging lower and upper felt edges 16, 17. After the press nip, the web 1 accompanies the lower press felt 7. In an alternative configuration, the web accompanies the upper press felt. Downstream of the roll press 2, a clothing 12 runs in an endless loop around a plurality of guide rolls (not shown) and a pick-up roll 13, which, in the embodiment shown, is arranged adjacent to the lower press felt 7 so that said clothing 12 and the lower press felt 7 run in contact with each other to transfer the web 1 from the lower press felt 7 to the clothing 12. The clothing 12 can be a felt or a wire, in which case the pick-up roll 13 is a suction roll. In another configuration (not shown) of the paper or paperboard machine, the clothing 12 is an impermeable belt, such as a transfer belt.

Because of the movements of the web 1, the press felts 5, 7 and the press rolls 3, 4 during operation, partial vacuums arise in said wedge-shaped spaces 9, 10, 11. The pressure differences thus produced cause air to flow axially into the wedge-shaped spaces. The partial vacuum in itself entails that the adherence of the web to the web-carrying press felt is reduced. Consequently, the reduced adherence of the web to the press felt in combination with the axial air flows caused by the partial vacuum result in a significant risk of the edge portions of the web becoming unstable. The axial air flows enter, in an uncontrolled manner, from the tender and drive sides into the wedge-shaped space 9 defined by the press felts 5, 7, by way of its side openings, said air flows passing across the free edge portions of the lower press felt 7 and the edge portions 21 of the web 1, which results in the web-edge portions 21 disengaging from the press felt 7 and being lifted from the same and thus becoming unstable. The partial vacuum in the wedge-shaped space 9 defined by the press felts 5, 7 may even increase and thus also the speed of the axial air flows so that the instability of the edge portions of the web is further increased, if the partial vacuum in the wedge-shaped space 10 defined by the upper press felt 5 and the upper press roll 3 increases to a level above the partial vacuum in the wedge-shaped space 9 defined by the press felts 5, 7, i.e. air will be drawn through the press felt 5 not carrying the web, in an upwards direction, according to the embodiments shown, for as long as such a difference in pressure prevails. Thus, in generating said uncontrolled, axial air flows, the partial vacuums in said two wedge-shaped spaces 9, 10 have a detrimental effect on the edge portions of the web. The partial vacuum is a function of the machine speed raised to the second power and increased machine speeds therefore increase the detrimental effect of the partial vacuums on the web edges to a corresponding or substantially corresponding degree. To eliminate such a detrimental effect of said partial vacuums, the press 2 is provided with a device comprising two air-deflection members 14, arranged to prevent air being drawn into, at least, the wedge-shaped space 9 defined by the press felts after the press nip. One of the air-deflection members 14 is arranged on the tender side and the other on the drive side of the press section. In the embodiment shown in Figures 1 and 2, each air-deflection member 14 comprises a suction box 15, extending in the machine direction immediately outside the wedge-shaped space 9 from the press nip to a certain point downstream of the press nip. The two suction boxes 15 are connected to a common suction source 19 and are mounted on the machine stand (not shown) . At a given adherence force between the web and the press felt, a certain maximum quantity of air can be permitted to flow in from the sides, which maximum quantity of air then has no detrimental effect on the web edges.

The suction box 15 is arranged in front of the side opening of the wedge-shaped space 9 defined by the press felts, which side opening is defined by the diverging edges 16, 17 of the press felts. The suction box 15 is so designed and its suction inlets so distributed around the suction box 15 that an effective suction area is obtained around the suction box so that air is prevented from flowing past the suction box 15 into the wedge-shaped space 9 defined by the press felts and, instead, drawn into the suction box 15. To achieve the drawing-in of the air flow into the suction box 15, a partial vacuum must consequently be created around the suction box 15 and between the suction box 15 and the wedge-shaped space 9 defined by the press felts 5, 7, i.e. within said suction area, that is greater than the partial vacuum prevailing in the wedge-shaped space 9 just downstream of the press nip and especially by or in the proximity of the felt edges 16, 17, where the partial vacuum is greatest. This partial vacuum is a function of the machine speed raised to the second power, which means that the suction capacity of the suction box 15 must be adapted to the machine speed and the partial vacuum that is created at the machine speed in question. The suction capacity of the suction box 15 is adjusted so that a partial vacuum is generated within its said suction area, that is at least about 5 and preferably at least about 10 Pa units greater than said maximum partial vacuum in the wedge-shaped space 9 defined by the press felts within its edge portions. At a web speed of 900 metres per minute, the maximum partial vacuum created within the edge portions inside the felt edges 16, 17 is in the magnitude of 13 Pa, and, at 1200 metres per minute, 28 Pa. The downstream point of the suction box 15 is located at a distance of about 0.5-2 metres, preferably about 1 metre, from the press nip, its upstream point being located as close to the exit of the press nip as possible.

The suction boxes 15 are directly or indirectly mounted on the machine stand in such a way that they can easily be removed from their operative positions so as not to obstruct threading or other operations that must be performed. The suction boxes 15 can, for instance, be supported by foldable movement devices mounted on the machine stand.

The suction boxes 15 can have a plurality of round suction inlets or a plurality of slot-shaped suction inlets.

In the embodiment shown in Figures 3 and 4, each air-deflection member 14 comprises a wall element 20, extending in the machine direction immediately outside the wedge-shaped space 9 defined by the press felts from the press nip to a certain point downstream of the press nip. The wall element 20 can consist of stable plates of suitable material, for instance metal sheets. The wall elements 20 are arranged in front of the side openings of the wedge-shaped space 9 defined by the press felts, which side openings are defined by the diverging edges 16, 17 of the press felts, and as close to the side openings as possible to prevent undesirable quantities of air penetrating the narrow gaps formed by the wall elements 20 and the felt edges 16, 17. The wall elements 20 are advantageously extended a short distance upwards and downwards beyond the felt edges 16, 17. In accordance with an embodiment not shown, the wall elements 20 are designed so that they also cover the side openings of the upper wedge-shaped space 10. The downstream point of the wall elements 20 is located at a distance of about 0.5-2 metres, preferably about 1 metre, from the press nip, its upstream point being located as close to the exit of the press nip as possible. Beyond the downstream end of the wall element 20, air is permitted to flow into the wedge-shaped space 9 defined by the press felts, which space is considerably expanded at this location so that the partial vacuum is not as great here as immediately following the press nip and the speed of the air flowing in from the sides beyond the wall element 20 therefore becomes significantly lower. This lower speed should therefore normally not affect the stability of the web edges. It should thus be understood that the length of the wall element 20 is sufficient, at a certain web speed, to obtain this lower, non-detrimental speed of the air flow into the wedge-shaped space beyond the wall elements. For a web speed of 900 metres per minute, the downstream point of the wall element 20 can be located at a distance of about 1.0 metres from the press nip. To further reduce the quantity of air that can penetrate past the wall element 20 into the wedge-shaped space 9, the wall element 20 can be extended rearwardly to also overlap a part of the press rolls 3, 4, the wall element 20 being designed or provided with deflected parts corresponding to the difference between the width of the felt and the length of the rolls.

Claims

C L I M S

1. A press section in a paper or paperboard machine for manufacturing a continuous web (1) , which press section comprises at least one double-felted press (2) with first and second press elements (3, 4), forming a press nip, and first and second press felts (5, 7) , running through the press nip, characterized in that the press section is provided with a web-stabilizing device to eliminate axial air flows that are detrimental to the stability of the web (1) and which are caused by partial vacuum arising in the press (2) after the press nip, which device comprises two air-deflection members (14) , arranged on the tender and drive sides of the machine outside and in immediate proximity to the side openings of a wedge-shaped space

(9) , defined by the two press felts (5, 7) , through which side openings air normally flows into said space (9) , and which air-deflection members (14) extend from the area of the press nip to a pre-determined point located downstream, which air-deflection members (14) are arranged to deflect said air flows away from the wedge-shaped space (9) defined by the press felts (5, 7) .

2. A press section as claimed in claim 1, characterized in that each air-deflection member (14) extends in the machine direction immediately outside the wedge-shaped space (9) defined by the press felts (5, 7) to a pre-determined point downstream of the press nip and is arranged in front of the side openings of the wedge-shaped space (9) defined by the press felts (5, 7) , which side openings are defined by the diverging edges (16, 17) of the press felts (5, 7) .

3. A press section as claimed in claim 1 or 2 , characterized in that each air-deflection member (14) comprises at least one suction member (15) , connected to a suction source (19) and having a plurality of suction inlets distributed around the suction member (15) to generate an effective suction area for receiving said air flows so that the same are prevented from being drawn into the wedge-shaped space (9) defined by the press felts (5, 7) .

4. A press section as claimed in claim 3, characterized in that the suction member consists of a suction box

(15) .

5. A press section as claimed in claim 3 or 4, characterized in that the suction member (15) is arranged to create a partial vacuum in its said suction area that is greater than the greatest partial vacuum in the wedge-shaped space (9) defined by the press felts (5, 7) .

6. A press section as claimed in claim 5, characterized in that said partial-vacuum difference is at least 5 Pa, preferably at least 10 Pa.

7. A press section as claimed in any one of claims 3-6, characterized in that said pre-determined point is located at a distance of about 0.5-2 metres, preferably about 1 metre, from the press nip.

8. A press section as claimed in claim 1 or 2, characterized in that each air-deflection member (14) comprises a wall element (20) that is impermeable to air and designed to cover the side opening of the wedge-shaped space (9) defined by the diverging edges (16, 17) of the press felts (5, 7) from the side, in which the wall element (20) and the felt edges (16, 17) define between them narrow gaps that permit the influx of small quantities of air that do not detrimentally affect the stability of the web edges (21) .

9. A press section as claimed in claim 8, characterized in that the wall element (20) extends a short distance beyond both felt edges (16, 17) .

10. A press section as claimed in claim 9, characterized in that the wall element (20) is designed to also cover the wedge-shaped space (10) after the press nip which is defined by the press felt (5) not carrying the web, and the press element (3) arranged in its loop.

11. A press section as claimed in any one of claims 8-10, characterized in that the wall element (20) is extended rearwardly in relation to the machine direction to overlap a part of the press elements (3, 4) before the exit from the press nip.

12. A press section as claimed in any one of claims 8-11, characterized in that said pre-determined point is located at a distance of about 0.5-2 metres, preferably about 1 metre, from the press nip.

990812 P1414SE.TP1