WO2002033168A1

WO2002033168A1 - Suction roll for a paper or paperboard machine

Info

Publication number: WO2002033168A1
Application number: PCT/SE2001/002139
Authority: WO
Inventors: Philip HÅKANSSON
Original assignee: Stora Kopparbergs Bergslags Aktiebolag (Publ)
Priority date: 2000-10-17
Filing date: 2001-10-03
Publication date: 2002-04-25
Also published as: SE517426C2; SE0003743L; AU2001292511A1; SE0003743D0

Abstract

Suction roll (1) for a paper machine, which suction roll has an axis of rotation (11) and a perforated jacket (2) disposed to rotate around said axis of rotation, at least two stationary zones (S1, S2, O1) extending axially being arranged inside the jacket, of which a first (S1, S2) of these zones is connected to a negative-pressure-producing organ (14) and a second (O1) of said zones is connected to a positive-pressure-producing organ (13). The invention also relates to a paper or paperboard produced using the suction roll.

Description

Suc t i on ro l l fo r a pa pe r o r pa pe r boa rd mach i ne .

TECHNICAL FIELD

The present invention relates to a suction roll for a paper or paperboard machine, which suction roll has an axis of rotation and a perforated jacket disposed to rotate around said axis of rotation, at least two stationary zones extending axially being disposed inside the jacket, of which a first of these zones is connected to a negative-pressure-producing organ. The suction roll is intended primarily to be used in a press section in the paper or paperboard machine, it being provided to increase the dry content of a paper or paperboard web that runs through the press section. The invention also relates to a paper or a paperboard produced using the suction roll.

PRIOR ART AND PROBLEMS

When producing paper or paperboard in a paper or paperboard machine, it is essential that the paper or paperboard web running in the paper or paperboard machine is dewatered as effectively as possible in the press section of the paper or paperboard machine. The aim thereby is to keep the energy consumption in a subsequent drying part as low as possible. It is therefore of great importance that the suction roll or rolls arranged in the press section dewater the paper or paperboard web as effectively as possible without undesirable side effects arising.

Conventional suction rolls have a perforated, rotating jacket, inside which at least one stationary negative pressure zone extending axially is arranged. This negative pressure zone is formed by a suction box/vacuum box, which is arranged along an arc of the inner circumference of the suction roll and along the whole of its axial length, a negative pressure in the suction box resulting in the liquid forcedly being sucked out of the paper or paperboard web, via a press felt, when the paper or paperboard web passes over the area of the suction box. The suction effect also holds the press felt and the paper or paperboard web firmly against the suction roll in a desirable manner. Furthermore, a press roll is normally provided to press liquid out of the paper or paperboard web in a press nip with contact between the suction roll and the press roll. Dewatering is carried out in the press nip typically before, during and possibly also after the paper or paperboard web has been caused to pass over the area of the suction box. The suction box, which is used in all types of suction rolls, is intended to seal against the inside of the jacket surface of the suction roll by means of two or more sealing strips, which are arranged parallel to one another at an angular distance corresponding to the extension of the suction box. In the case of more than two sealing strips, the suction box can be divided into several vacuum zones. The sealing strips slide against the inside of the jacket surface when the jacket rotates. A negative-pressure-producing organ, i.e. one or more vacuum pumps, is connected to the suction box via the hollow axis of rotation of the suction roll. The zone or zones in which a vacuum is not created are under atmospheric pressure or under a slight negative pressure, caused by leakage from the vacuum zone/zones.

One evident disadvantage of this construction is that the holes in the jacket remain more or less filled with liquid even after the jacket and the paper web have passed the vacuum zone or vacuum zones. This liquid is returned to the press felt and paper web and thereby obviously reduces the dewatering function of the suction roll. This effect can be lessened to a certain extent by increasing the vacuum in the vacuum zone/zones, but the negative consequence of this is that the energy consumption for operating the suction roll is increased considerably. A further negative consequence of such an increased vacuum is that markings are produced on the surface of the paper. These markings, which are termed suction hole markings, appear most plainly in an unacceptable manner when the paper or paperboard is printed using the offset technique, amongst other things.

To reduce the formation of suction hole markings, development has gone in the direction of a larger number of holes of a smaller diameter. Due to this, however, the problem of liquid-filled holes has increased, since the capillary force is greater in the case of small holes.

Another less effective method of reducing the quantity of liquid that is returned to the press felt and the paper or paperboard web is to provide a scraper blade close to the jacket surface of the suction roll, immediately before an initial press nip. The disadvantages of this solution are that the blade must be at a very small distance from the roll to have any effect at all, meaning that it will often lie against the roll, with very expensive damage as a result. Yet another negative consequence of this solution is that a negative pressure is created on the underside of the blade, further liquid being drawn out of the suction holes just before the press nip, which liquid is returned to the upper side of the press felt, due to which the positive effect of the blade is thus reduced.

Yet another problem in connection with suction rolls is that the suction holes tend to be stopped up by components used in paper production. Examples of such components are clay, styrene butadiene latex, calcium carbonate and dispersions of hydrophobizing agents.

Another problem in connection with suction rolls is that the sealing strips of the suction box wear out and are often subjected to mechanical damage. They are also arranged in such a way that it is difficult to diagnose them.

BRIEF ACCOUNT OF THE INVENTION

The object of the present invention is to eliminate or at least reduce the aforementioned problems without the positive effect of the suction roll being impaired. This is achieved by means of the suction roll as presented in claim 1.

It has surprisingly proved to be the case that the water that has collected in the suction holes is effectively transported away if a zone directly following the conventional negative pressure zone is connected to an organ producing a positive pressure. This water is best collected on the outside of the jacket, in a position corresponding to the positive pressure zone, by means of a collecting vessel, e.g. a trough or the like. The connection to the positive-pressure-producing organ is best executed in the same, known manner as the connection to the negative-pressure-producing organ which produces a negative pressure in the negative pressure zone/zones. The magnitude of the positive pressure can be optimized for each given suction roll, but is best between 0.1 and 10 kPa. A fan is preferably used as a positive-pressure-producing organ, but the use of a compressor or the like is naturally also conceivable.

It has also surprisingly turned out to be the case that the holes of the suction roll stay open without any problems of stopping up when the suction roll according to the invention is used.

According to one aspect of the invention, the suction roll also comprises a measuring device for measuring a differential pressure between the environment and the positive pressure zone according to the invention and/or a measuring device for measuring a differential pressure between the environment and the conventional negative pressure zone or zones "Environment" here means the environment on the outside of the jacket Due to this, a further advantage is obtained in that the condition of the sealing strips of the suction box can be diagnosed during operation This measurement of differential pressure can also be used to diagnose any possible but unlikely stopping up of the suction holes in the jacket of the suction roll How this can be done is illustrated below

If only the positive pressure fan is started and the pressure in the positive pressure zone is greater than its setpoint value, this shows that the suction holes are stopped up If on the other hand only the vacuum pump is started and the negative pressure in the negative pressure zone is not as great as its setpoint value, 1 e a poorer negative pressure is present, this shows that the sealing strips are worn or damaged If both the positive pressure fan and the vacuum pump are started, I e normal operating mode, and the pressure in the positive pressure zone is lower than its setpoint value at the same time as the negative pressure in the negative pressure zone is not as great as its setpoint value, this indicates that the suction holes are open but that the sealing strips are worn or damaged etc

According to another aspect of the invention, one or more measuring devices can be provided alternatively to or in combination with the measuring device just described, to measure a differential pressure between the positive pressure zone and the conventional negative pressure zone or zones

One essential advantage of the invention is that the paper or paperboard manufactured using the suction roll has a higher quality, primarily with regard to the lack of suction hole markings, compared with a paper or paperboard produced using a suction roll according to the prior art Alternatively, the invention can naturally be used for the manufacture of paper or paperboard with the quality maintained while increasing production

Yet another advantage is that the invention can also be implemented in existing suction rolls, as a so-called retrofit

DETAILED ACCOUNT OF THE INVENTION

The invention will be explained in greater detail with reference to Fig. 1 which shows a conceivable embodiment of the suction roll according to the invention, which is fitted in a press section with two press rolls adjacent to a press nip.

Fig. 1 shows a suction roll 1 according to the invention. The suction roll 1 has an outer rotating jacket 2, with a length of typically 2 - 10 metres and a diameter of typically 0.8 - 2 metres. This has evenly distributed suction holes over the whole of its jacket surface, e.g. 20-60 suction holes per dm², with a diameter of typically 2.8 - 3.2 mm. Arranged between the jacket 2 and an inner, concentrically arranged cylinder 4, are two stationary negative pressure zones, i.e. suction boxes, Si, S₂, which are delimited by sealing strips 10, which run against the inside 3 of the jacket 2, and a stationary positive pressure zone Oi.

A paper or paperboard web W, with a dry content of around 20%, runs together with a press feltF through a first press nip Ni between the suction roll and a first adjacent press roll 5. The paper web W and the feltF then run on the surface of the jacket 2 of the suction roll and onwards into a second press nip N₂, against a second press roll 6. The negative pressure zones Si and S₂are arranged here between the first and second nip, Ni and N₂ respectively, and the positive pressure zone Oi according to the invention is disposed directly following the second negative pressure zone S₂. The linear load in the press nip can be up to 100 kN/m, or in certain cases even higher. In the embodiment shown, the linear load in the first press nip Ni is around 40 - 50 kN/m and in the second press nip around 60 - 70 kN/m. The negative pressure in the first zone Si is around 40 kPa in the embodiment shown and around 50 kPa in the second zone S₂. Following the second press nip N₂, the dry content of the paper or paperboard web W has been increased by around 20 percentage units.

According to the invention, the positive pressure zone Oi is connected to a fan 13, to create a positive pressure of around 0.1 - 10 kPa. The actual connection is executed practically in a known manner, via the hollow axis of rotation 11 of the suction roll. The negative pressure zones Si, S₂ are connected to one or more negative-pressure-producing organs, i.e. one or more vacuum pumps 14. The connections and fan 13 and vacuum pump 14 are only shown symbolically in Fig. 1. In the embodiment shown, the positive pressure zone Oi extends all the way from the second end of the second negative pressure zone S₂ (seen in the direction of rotation of the jacket 2) to the first end of the first negative pressure zone Si It is naturally also conceivable for the positive pressure zone Oi to have an extension only within a small part of this area, it also being possible to provide a principally pressureless zone in this area In such a case, the positive pressure zone best lies directly after the second negative pressure zone S₂, and is followed by the pressureless zone However, it is also conceivable for the order to be reversed, the pressureless zone lying directly after the second negative pressure zone S₂, and being followed by the positive pressure zone Oi

Without restricting the invention, it is assumed that the effect according to the invention is greater the lower the speed of rotation of the jacket 2, since conventionally it is the centrifugal effect that shall remove the water remaining in the suction holes

Arranged by the positive pressure zone Oi, by the outside of the jacket 2, is a collecting vessel 12 for collecting the water or liquid that is pressed by the positive pressure out of the suction holes in the jacket 2

The invention is not restricted to the embodiments described above, but can be varied within the scope of the following claims It is perceived for example that the number of negative pressure zones can be both greater and smaller, and that it is also conceivable to provide more than one positive pressure zone Furthermore, it is also conceivable that the invention can be used in the drying part of the paper machine

Claims

1. Suction roll (1) for a paper or paperboard machine, which suction roll has an axis of rotation (11) and a perforated jacket (2) disposed to rotate around said axis of rotation, at least two stationary zones (Si, S₂, Oi) extending axially being arranged inside the jacket, of which a first (Si, S₂) of these zones is connected to a negative-pressure- producing organ (14), characterized in that a second (Oi) of said zones is connected to a positive-pressure-producing organ (13).

2. Suction roll according to claim 1, characterized in that said positive-pressure- producing organ (13) comprises a fan or a compressor, which is disposed to provide a positive pressure of 0.1 - 10 kPa.

3. Suction roll according to claim 1 or 2, characterized in that said second zone (Oi) is arranged directly after said first zone (S₂) seen in a direction of rotation of the jacket (2), said second zone (Oi) preferably extending all the way around the jacket, back to said first zone (S₂) or a third zone (Si), which third zone also consists of a negative pressure zone arranged adjacent to said first zone (S₂).

4. Suction roll according to claim 1 or 2, characterized in that said second zone (Oi) is arranged directly after said first zone (Si, S₂) seen in a direction of rotation of the jacket (2), a fourth zone being arranged directly after said second zone (Oi), which fourth zone consists of a principally pressureless zone.

5. Suction roll according to claim 1 or 2, characterized in that a fourth zone is arranged directly after said first zone (Si, S₂) seen in a direction of rotation of the jacket (2), which fourth zone consists of a principally pressureless zone, said second zone (Oi) being arranged directly after said fourth zone.

6. Suction roll according to any of the above claims, characterized in that it is arranged in a press section in said paper or paperboard machine, at least one press roll (5, 6) being arranged against the suction roll (1) to form a press nip (Ni, N₂).

7. Suction roll according to any of the above claims, characterized in that a liquid collecting vessel (12) is arranged by the outside of the jacket (2), in a position by said second zone (Oi).

8. Suction roll according to any of the above claims, characterized by a measuring device for measuring a differential pressure between the environment and said second zone (Oi) and/or a measuring device for measuring a differential pressure between the environment and said first zone (Si, S₂).

9. Suction roll according to any of the above claims, characterized by a measuring device for measuring a differential pressure between said second zone (Oi) and said first zone (S,, S₂).

10. Paper or paperboard, characterized in that it was produced using a suction roll according to any of claims 1-9.