WO2000071812A1

WO2000071812A1 - Press section with web stabilizing device

Info

Publication number: WO2000071812A1
Application number: PCT/SE2000/000896
Authority: WO
Inventors: Carl HÅKANSON; Per Anders Artur Leandersson; Börje SANDGREN
Original assignee: Valmet Karlstad AB
Current assignee: Valmet AB
Priority date: 1999-05-21
Filing date: 2000-05-05
Publication date: 2000-11-30
Anticipated expiration: 2001-11-21

Abstract

A press section in a machine for manufacturing a continuous web of paper or paperboard has a double-felted press (2) with first and second press elements (3, 4) and first and second press felts (5, 7), in which press partial vacuums that have a detrimental effect on the stability of the web are produced after the press nip. In accordance with the invention, the press section is provided with a web-stabilizing device to reduce said partial vacuums at least to a level at which the stability of the web is ensured, which device has an air-supply member (14) arranged for controlled, continuous blowing of air into the wedge-shaped space (9), defined by the press felts, in which said partial vacuums are produced.

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 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, in which press partial vacuums that have a detrimental effect on the stability of the web are produced after 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, thereby presenting a risk of the web disengaging from the press felt at its edge portions and across its entire width as well, which results in the web becoming unstable in relation to the press felt. The partial vacuum also causes air to be 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 caused by the partial vacuum pass across the free edge portions of the web-carrying press felt and across the edge portions of the web. The problem with the web becoming unstable is aggravated because of the axial air flows, which, due to said reduced adherence, can increase the risk of the edge portions of the web disengaging from the web-carrying press felt and becoming unstable. The partial vacuum is a function of the machine speed raised to the second power, which means that said adherence diminishes and that said air flows into the wedge-shaped space become stronger with increasing machine speeds and that the problem involving an unstable web or unstable web-edge portions is aggravated 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.

A partial vacuum and consequent axial air flows are also produced 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 then 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 surface, which faces away from the upper press element . By way of this expansion, a spring-back force is created in the paper web that reduces the adherence force 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 either.

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 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 a web-stabilizing device to reduce said partial vacuums at least to a level at which the stability of the web is ensured, which device comprises an air-supply member arranged for controlled, continuous blowing of air into at least one wedge-shaped space, defined by the two press felts, in which said partial vacuum is produced.

The web-stabilizing device, in accordance with the invention, eliminates the partial vacuums that can result in the adherence of the web to the web-carrying felt diminishing or the web disengaging and the device, furthermore, eliminates axial air flows that are detrimental to the stability of the web. 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 with two air-supply members in accordance with a first embodiment of the invention.

Figure 2 shows schematically a blowing box of the lower air-supply member in Figure 1.

Figure 3 is a view from above of parts of a press section in accordance with Figure 1 and illustrates the stream of air from the blowing nozzles of the blowing box towards the press nip.

Figure 4 shows schematically a blowing box in accordance with another embodiment.

Figure 5 is a view from above of parts of the press section in accordance with Figure 1, but with the modified blowing box in accordance with Figure 4, and illustrates the stream of air from the blowing nozzles of the blowing box towards the press nip.

Figure 6 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 an air-supply member in accordance with a second embodiment of the invention.

Figure 7 is a view from above of parts of the press section in accordance with Figure 6 and illustrates the streams of air from the blowing nozzles of the two blowing boxes of the air-supply member. 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 the 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, created when the press felts are separated, entails that the adherence of the web to the web-carrying press felt is reduced, thereby presenting a risk of the web being loosed from the surface of the felt. The axial air flows, caused by the partial vacuum, aggravate the problem with 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. 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, 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 sections of the web is further increased, 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 reducing the adherence of the web to the press felt and generating said uncontrolled, axial air flows, the partial vacuums in said two wedge-shaped spaces 9, 10 have a detrimental effect on the edge sections 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 one ore more air-supply members 14 for blowing air, in a controlled and continuous manner, into at least the wedge-shaped space 9 defined by the press felts 5, 7. In the embodiment shown in Figure 1, such air-supply members 14 are arranged in the wedge-shaped space 9 defined by the press felts 5, 7 as well as in the upper wedge-shaped space 10 between the upper press roll 3 and the upper press felt 5.

The partial vacuum is greater in the central area of each wedge-shaped space and in the two side areas of the space located closest to the edges of the press felts than in other, interjacent areas of the wedge-shaped space, as seen across the machine direction. As the axial air flows enter through the side openings of the wedge-shaped space, it is important that a controlled supply of air takes place primarily in the two said side areas. Accordingly, each air-supply member 14 comprises two blowing bodies 15, which, in the embodiment shown in Figure 1, are in the shape of elongate blowing boxes. Alternatively, they can be in the shape of elongate blowing pipes. Such blowing boxes 15 are arranged on the tender side as well as the drive side inside the edges 16, 17 of the felt, i.e. by said side areas of the wedge-shaped space, the blowing boxes 15 being horizontally lined up with each other, perpendicularly to the machine direction. Experiments have shown that the greatest partial vacuum is created within said side areas of the space 9 at a point located within a distance of 50 to 300 millimetres from the edge 16 of the felt, depending on the machine speed. In accordance with a preferred embodiment of the invention, the blowing box 15 therefore emits a stream of air with an effective width (across the machine direction) of about 400 millimetres and extends inwards from a point located immediately above the felt edge 16, 17 of the lower and upper press felts 7, 5, respectively, or located inside this felt edge 16, 17, within a distance of 0-50 millimetres, for instance. Each blowing box 15 has a plurality of blowing nozzles 18, arranged equidistantly in a horizontal row.

The discharge openings of the blowing nozzles 18 face the press nip to emit a stream of air, during operation, which is aimed against the feeding direction of the web 1 and fills that part of the space 9, 10 that is located between the blowing box 15 and the press nip so that the pressure is increased, i.e. the partial vacuum is reduced, in this part and a little outside of it. The blowing boxes 15 are located at a distance from the press felts 5, 7 such that the function of the press felts is not affected and they are not otherwise impeded. The four blowing boxes 15 are connected to a common air source 19 and are suitably mounted to the machine stand (not shown) . As the partial vacuum is reduced, the quantity of air flowing in axially from the outside on the tender side and drive side is also reduced and this flow of air may even cease altogether if the partial vacuum is reduced to a zero value . At a given adherence force between the web and the press felt, only a certain maximum quantity of air, having no detrimental effect on the web edges, can be permitted to flow in from the sides. If so desired, a blowing box is also arranged in the middle of the space 9, 10 to blow air into said central area.

The blowing box in accordance with Figures 4 and 5 is designed to emit a stream of air at a speed which is greatest at the felt edge 16 and a short distance inward from the same to diminish gradually thereafter, as illustrated in Figure 5, whereby more air is admitted to that part of the side area of the space 9 where the partial vacuum has been shown to be greatest, i.e. the distribution of the stream of air is controlled and adapted to the partial-vacuum profile across the web.

To cover a sufficient length of the wedge-shaped space, viewed in the machine direction, the blowing box 15 or another blowing body is located at a distance of about 0.5-2 metres, preferably about 1 metre, from the press nip.

The blowing bodies 15 are directly or indirectly mounted on the machine stand in such a way that they can easily be moved from their operative positions in the wedge-shaped spaces 9, 10 so as not to obstruct threading or other manoeuvres that must be performed. The suction boxes 15 can, for instance, be supported by foldable movement devices mounted on the machine stand.

In the embodiments shown, the blowing bodies 15 have a plurality of blowing nozzles 18. It is, of course, possible to use a blowing body with a single nozzle having a slot-shaped, horizontal discharge opening. This can have a constant width or variable width towards the felt edge to emit a stream of air of variable quantity to achieve the same effect as illustrated in Figures 4 and 5. In an alternative embodiment, not shown, the air-supply member is designed as an elongate blowing body extending across the machine direction and having a length corresponding to or substantially corresponding to the width of the press felts . The discharge openings can be distributed along the entire blowing body or solely along its end portions, which should operate within said side areas, and, potentially, also along its middle portion, which should operate within said central area of the wedge-shaped space.

Figures 6 and 7 show schematically a device in accordance with a second embodiment of the invention. The device comprises an air-supply member 14, arranged to blow air between the press felts 5, 7, in a controlled and continuous manner, into the space 9 defined by the press felt. The air-supply member 14 comprises two blowing bodies 20 which, in the embodiment shown, are in the shape of elongate, straight blowing pipes. Such blowing pipes 20 are arranged on the tender side as well as on the drive side immediately outside the wedge-shaped space. Each blowing pipe 20 has a plurality of equidistantly arranged blowing nozzles 18. The blowing pipe 20 is mounted on the machine stand and situated outside the wedge-shaped space 9 defined by the press felts and slightly below the level of the upper press felt 5, i.e. above the symmetry plane of the space 9, so as not to disturb the web edges. The discharge openings of the blowing nozzles 18 face inward towards the wedge-shaped space 9 defined by the press felts to emit a stream of air that is aimed at the same across the feeding direction of the web 1. The blowing nozzles can be aimed straight into the wedge-shaped space 9 defined by the press felts or obliquely into the same so that the stream of air is aimed at the web, although, for obvious reasons, without impinging or affecting the web edges. Said stream of air thus fills that part of the wedge-shaped space 9 defined by the press felts that is located between the press nip and a downstream point level with the downstream end of the blowing pipe, the extension of this part across the machine direction being dependent on the speed of the stream of air and the prevailing partial vacuum in the wedge-shaped space 9 that "absorbs" the air supplied. The blowing pipes 20 are connected to a common air source 19. To cover a sufficient length of the wedge-shaped space, viewed in the machine direction, the downstream end of the blowing pipe is located at a distance of about 0.5-2 metres, preferably 1 metre, from the press nip. If so desired, such blowing pipes 20 are arranged in connection with the upper wedge-shaped space 10 as well.

As mentioned in the introduction, the partial vacuum in the lower wedge-shaped space 11 is beneficial in that the adherence of the web to the press felt 7 can increase due to prevailing pressure differences. A consequential effect of the invention is that the beneficial adherence of the web is further increased due to an increased difference in pressure.

990811 P1350SE.TP1

Claims

C L A 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, in which press partial vacuums that have a detrimental effect on the stability of the web are produced after the press nip, characterized in that the press section is provided with a web-stabilizing device to reduce said partial vacuums at least to a level at which the stability of the web is ensured, which device comprises an air-supply member (14) arranged for controlled, continuous blowing of air into at least one wedge-shaped space (9) , defined by the two press felts (5, 7), in which said partial vacuums are produced.

2. A press section as claimed in claim 1, characterized in that said air-supply member (14) is arranged inside the wedge-shaped space (9) defined by the press felts (5, 7) .

3. A press section as claimed in claim 1, characterized in that said air-supply member (14) is arranged outside the wedge-shaped space (9) defined by the press felts (5, 7), as viewed across the machine direction, on the tender and drive sides of the machine.

4. A press section as claimed in claim 2, characterized in that the air-supply member (14) comprises an elongate blowing body (15) , arranged across the machine direction and having a length corresponding to or substantially corresponding to the width of the press felts (5, 7) .

5. A press section as claimed in claim 4, characterized in that the blowing body (15) has a plurality of nozzles, distributed along the entire blowing body or at least along its end portions and having their discharge openings aimed at the press nip and/or at the press felt (7) not carrying the web, at a location between the press nip and the blowing body (15) .

6. A press section as claimed in claim 2, characterized in that each air-supply member (14) comprises two elongate, separate blowing bodies (15), arranged across the machine direction inside the wedge-shaped space (9) defined by the press felts (5, 7) and having a limited length relative to the felt width, which two blowing bodies (15) are arranged inside or immediately inside the parallel side openings, defined by the edges of the press felts (5, 7) , of the wedge-shaped space (9) defined by the press felts (5, 7) .

7. A press section as claimed in claim 6, characterized in that the blowing bodies (15) have a length of about 400 millimetres.

8. A press section as claimed in claim 6 or 7, characterized in that each blowing body (15) has a plurality of nozzles (18) , distributed along the entire blowing body (15) and having their discharge openings aimed at the press nip and/or at the press felt (7) not carrying the web, at a location between the press nip and the blowing body (15) .

9. A press section as claimed in claim 4 or 6, characterized in that the blowing body or blowing bodies (15) , respectively, are arranged at a distance of about 0.5-2 metres, preferably about 1 metre, from the press nip.

10. A press section as claimed in any one of claims 4-9, characterized in that each blowing body (15) is arranged to emit a stream of air, the speed profile of which is adjusted according to the variations in the partial-vacuum profile across the web (1) so that the greatest quantity of air is admitted to that part of the area where the partial vacuum is greatest.

11. A press section as claimed in claim 3, characterized in that said air-supply member (14) , which is arranged outside the wedge-shaped space (9) defined by the press felts (5, 7), has two blowing bodies (20) disposed on the tender and drive sides of the machine and exhibiting a plurality of blowing nozzles (18) , distributed along the entire blowing bodies (20) and having their discharge openings aimed into the wedge-shaped space (9) defined by the press felts (5, 7) via its parallel side openings, defined by the diverging edges of the press felts (5, 7) , to emit a stream of air straight into the wedge-shaped space defined by the press felts (5, 7) and/or obliquely at the press felt (7) not carrying the web.

12. A press section as claimed in claim 11, characterized in that the downstream ends of the blowing bodies (20) are located at a distance of about

0.5-2 metres, preferably 1 metre, from the press nip.

13. A press section as claimed in any one of claims 4-12, characterized in that the blowing bodies are in the shape of blowing boxes (15) or blowing pipes (20) .

14. A press section as claimed in any one of claims

1-13, characterized in that an air-supply device (14) is also arranged for controlled, continuous blowing of air into a wedge-shaped space (10) defined by the press felt (5) not carrying the web, and the press element (3) located in the loop of this press felt (5) .

990811 P1350SE.TP1