Patent Grant
9181655
This application is a National Stage Application, filed under 35 U.S.C. §371, of International Application No. PCT/SE2013/050227, filed Mar. 12, 2013, which claims priority to Swedish Application No. 1250393-4, filed Apr. 19, 2012 and Swedish Application No. 1251000-4, filed Sep. 6, 2012, the contents of all of which as are hereby incorporated by reference in their entirety.
1. Related Field
The invention relates to an extended nip roll, to an extended nip press that comprises the inventive extended nip roll and to a paper making machine comprising such an extended nip press. The invention also relates to a method of operating the inventive extended nip wherein a wet paper web is passed through the extended nip.
2. Description of Related Art
The invention relates to the field of extended nip presses used in paper making machines. In a paper making machine, an extended nip press is normally used for pressing water out of a newly formed wet fibrous web but an extended nip may also be used for other purposes in a paper making machine, e.g. calendering. Although extended nip presses were first introduced for heavy grades such as paperboard, they later come to be used also for lighter grades such as printing paper. In recent years, such presses have also been used in machines for making tissue paper. In such machines, the extended nip press is often formed by an extended nip roll and a Yankee drying cylinder that acts as a counter roll for the extended nip roll. An extended nip press is typically formed by an extended nip roll that comprises a rigid shoe with a concave surface. The rigid shoe of such an extended nip roll can be made of a material such as, for example, steel or aluminum. U.S. Pat. No. 7,527,708 discloses how an extended nip can be formed by means of a device that does not use a steel shoe but instead a support body which is elastically deformable. The support body disclosed in that document has internal pressure chambers that can be connected to a pressure medium source.
Generally, it is desirable that the pressure in the nip of an extended nip press rises from the beginning of the nip to reach a peak at the end of the nip. Such a pressure profile is advantageous since it reduces re-wetting of the paper web when the web exits from the press nip. In U.S. Pat. No. 7,527,708, it is suggested that different pressure chambers are set under different pressures such that a pressure curve describing a stepped course is obtained.
It is an object of the present invention to provide an extended nip press that is designed in such a way that a suitable pressure profile is obtained.
Another object of the invention is to provide an extended nip press in which the lubrication functions in a reliable way.
The invention relates to an extended nip roll capable of forming a press nip with a counter roll which press nip has an extension in a machine direction when the extended nip roll cooperates with the counter roll. The inventive extended nip roll comprises; a flexible jacket with an interior surface and an exterior surface and a press body arranged inside the flexible jacket. The press body is elastically deformable and it has a top side that faces the interior surface of the flexible jacket. The top side of the press body is beveled in such a way that, in the machine direction, a working surface of the top side has a downstream end which, in the machine direction, is followed by an exit side surface that diverges from the interior surface of the flexible jacket. The press body further has at least a first and a second internal pressure chamber which internal pressure chambers can be pressurized such that the press body expands. The first internal pressure chamber has an extension in the machine direction that does not extend beyond the working surface and the second internal pressure chamber is located downstream of the first internal pressure chamber and has an extension in the machine direction beyond the downstream end of the working surface. The extended nip roll also comprises a support for the press body which support provides support to sides of the press body but allows the press body to expand in a direction towards the interior surface of the flexible jacket. The second internal pressure chamber has such an extension in the machine direction that more than 30% of the extension (the machine direction length of the second internal pressure chamber) extends beyond the downstream end of the working surface and in that the working surface and the exit side surface are made of a material that has a higher shore A hardness than the material in the rest of the press body.
The press body may be designed such that, in an unloaded state of the extended nip roll, the exit side surface downstream of the working surface forms an angle of 30°-65° with a tangent to the working surface, preferably an angle in the range of 35°-60°.
The working surface of the top side may be a flat surface in the unloaded state of the extended nip roll when the extended nip roll does not form a nip with a counter roll. Moreover, in the unloaded state of the extended nip roll, the exit side surface downstream of the working surface may form an angle of 40°-50° with the working surface. Instead of being flat in the unloaded state of the roll, the working surface may be, for example, a convex or concave surface.
In embodiments of the invention, a part of press body may comprise a sole which forms the working surface and the exit side surface. In such embodiments, the sole may preferably have a thickness in the range of 1 mm-30 mm, preferably 5 mm-25 mm, even more preferred 10 mm-25 mm.
The sole may have a Shore A hardness that is higher than 90 Shore A while the part of the press body that surrounds the internal pressure chambers has a hardness which is less than or equal to 90 Shore A (i.e. at most 90 Shore A). Preferably the sole has a hardness of 93 Shore A-100 Shore A while the part of the press body that surrounds the internal pressure chambers has a hardness of 70 Shore A-90 Shore A.
The exit side surface is preferably separated from the working surface by a rounded edge which has a first radius in an area adjacent the working surface and a second radius in an area adjacent the exit side surface and wherein the second radius is smaller than the first radius. In embodiments of the invention, the first radius is in the range of 20 mm-40 mm and the second radius is in the range of 6 mm-15 mm.
The rounded edge that separates the working surface from the exit side surface may have an extension in the machine direction which is in the range of 6 mm-16 mm.
In embodiments of the invention, the press body comprises also a third internal pressure chamber which, in the machine direction, is located upstream of the first internal pressure chamber.
Preferably, the internal pressure chambers have a rectangular shape and a larger extension in the radial direction of the extended nip roll than in the machine direction.
In embodiments of the invention, the press body comprises a lip located upstream of the working surface and which lip protrudes in an upstream direction. The lip has an entry surface that faces the interior surface of the flexible jacket and forms an angle of 2°-50° with a tangent to the working surface.
A lubrication channel may advantageously be arranged to feed a lubricant to the working surface. However, it should be understood that such a lubrication channel is optional.
Preferably, the press body may be covered by an exchangeable wear protection layer. However, it should be understood that embodiments without such a wear protection layer are conceivable.
The invention also relates to an extended nip press comprising an extended nip roll as described above and additionally comprising a counter roll cooperating with the extended nip roll. The counter roll can preferably be a roll that is arranged to be heated, e.g. a Yankee drying cylinder.
The invention also relates to a paper making machine comprising an extended nip roll as described above, a counter roll cooperating with the extended nip roll to form an extended nip press and a forming section arranged upstream of the extended nip press.
The invention also relates to a method of operating an extended nip press as described above wherein a wet paper web is passed through the extended nip and subjected to pressure as the web passes through the nip. The extended nip is operated such that the pressure rises as the web passes through the nip and reaches a peak as the web passes over the downstream end of the working surface of the press body.
With reference to
With reference to
The extended nip roll 5 comprises a flexible jacket 10 with an interior surface 11 and an exterior surface 12. The flexible jacket 10 is typically made of polyurethane or comprises polyurethane. The flexible jacket 10 is shaped as a tube that extends in a cross machine direction. At its axial ends, it is normally connected to end walls that can rotate about an axis. Such arrangements are well known in the art of paper machinery and examples of fastenings for the axial ends of the flexible jacket are disclosed in, for example, U.S. Pat. No. 5,904,813 and European patent No. 1273701. The flexible jacket 10 may thus define an enclosed space. Advantageously, the extended nip roll 5 may be connected to a source of pressurized air such that the enclosed space within the flexible jacket 10 may be filled with pressurized air. Such an arrangement helps keeping the shape of the flexible jacket.
A press body 13 is arranged inside the flexible jacket 10. The press body 13 is elastically deformable and has a top side 14 that faces the interior surface 11 of the flexible jacket 10 (see also
It should be understood that the support 21 is in a fixed position and does not move during operation of the extended nip press. It may be supported by a support beam (not shown) that may carry rotatable end walls to which the axial ends of the flexible jacket 10 are fixed as is known in the art and as explained above. When the internal pressure chambers 18, 19, 20 are pressurized, the press body 13 will expand. The bottom wall 37 and the upstream and downstream walls 38, 39 of the support 21 prevent or limit expansion in the direction of those walls and the support 21 likewise has walls at its axial ends (not shown) that prevent or limit expansion in the axial direction. However, the top side 14 of the press body 13 is not limited by any wall of the support 21. At its top side 14, the press body 13 is therefore free to expand in a direction towards the internal surface 11 of the flexible jacket 10 when the internal pressure chambers are pressurized. When the internal pressure chambers 18, 19, 20 are pressurized, the press body 13 will therefore expand. It should be understood that, when the extended nip roll 5 is used, it is typically so that the press nip N is closed when the press body 13 is caused to expand due to pressure in the internal pressure chambers 18, 19, 20.
In an extended nip press, the pressure profile should be unsymmetrical in such a way that the peak pressure is achieved shortly before the end of the press nip whereafter the pressure is quickly reduced. Such a pressure profile reduces re-wetting of the paper web. It is also desirable that the pressure gradient at the beginning of the nip is relatively small such that the pressure initially is increased gently. Thereafter, the pressure should increase progressively until it reaches a peak at the end of the nip.
With reference to
According to one aspect of the invention, the press body 13 is designed in such a way as to assist in producing a pressure profile where the peak pressure appears at the end of the nip. To achieve this purpose, the press body is designed in the following way. With reference to
As best seen in
The inventors have found that, when the press body 13 is made of an elastic material, this has many advantages. One advantage is that the elasticity of the press body makes it possible to expand the press body by means of pressurized fluid. Another advantage is that the press body can adapt very accurately to the profile of the counter roll such that a regular pressure distribution can be obtained. However, the inventors have found that the elasticity of the press body 13 can also result in a problem that is related precisely to the pressure distribution. With reference to
The inventors have found that the second peak can be prevented or reduced if the working surface 15 and the exit side surface 17 are made of a material that has a higher shore A hardness than the material in the rest of the press body 13.
With reference to
Preferably, the sole 22 has a hardness higher than 90 Shore A while the softer part 23 of the press body 13 that surrounds the internal pressure chambers 18, 19, 20 has a hardness which is less than or equal to 90 Shore A, preferably the sole has a hardness of 93 Shore A-100 Shore A while the softer part 23 of the press body that surrounds internal pressure chambers 18, 19, 20 has a hardness of 70 Shore A-90 Shore A. In one realistic embodiment considered by the inventors, the sole 22 may have a shore A hardness of 95 while the softer part 23 that surrounds the internal pressure chambers 18, 19, 20 may have a shore A hardness of 90. This means that the softer part 23 of the press body 13 that surrounds the internal pressure chambers 18, 19, 20 is sufficiently soft to deform and expand in response to increased pressure in the internal pressure chambers 18, 19, 20. At the same time, the sole 22 has such a hardness that it will not easily deform to cause a second peak.
In an unloaded state of the extended nip roll 5, the exit side surface 17 downstream of the working surface 15 preferably forms an angle β of 30°-65° with a tangent to the working surface 15, preferably an angle β in the range of 35°-60° (see
To further reduce the risk that a second peak occurs, the inventors have found that the area in which the working surface 15 goes over into the exit side surface 17 should preferably be shaped in a way that counteracts any tendency of the press body 13 to deform such that a second peak occurs.
With reference to
With reference to
The internal pressure chambers 18, 19, 20 preferably have a rectangular shape and a larger extension in the radial direction of the extended nip roll 5 than in the machine direction.
Another aspect of the invention will now be explained with reference to
Preferably, a lubrication channel 29 is arranged to feed a lubricant to the working surface 15. In the embodiment that is shown in
Although not shown in the figures, it should be understood that the inventive extended nip roll may also be provided with an evacuation system for removing such lubricant fluid that has already been used such that spent lubricant fluid may be continuously replaced by fresh lubricant fluid (e.g. oil).
With reference to
In operation, a web W is formed in the forming section and passed to the extended nip press formed between the extended nip roll 5 and the counter roll 7 and subjected to pressure as the web W passes through the nip N. The pressure will rise as the web W passes through the nip N and reach a peak as the web passes over the downstream end of the working surface 15 of the press body 13.
When the press body 13 has two internal pressure chambers 18, 19, the hydraulic pressure in the first internal pressure chamber 18 may be about 0.16 MPa during operation of the extended nip press while pressure in the second internal pressure chamber 19 may be about 3.2 MPa. The peak pressure may then be on the order of about 6.5 MPa. In another embodiment, pressure in the first chamber 18 may be about 0.9 MPa while pressure in the second internal pressure chamber 19 may be about 2.6 MPa and the peak pressure about 6 MPa. Depending on the shape of the press body 13 and the pressure in the second internal pressure chamber 19, peak pressure may be significantly higher than 6.5 MPa. Due to the design of the press body, the peak pressure is thus clearly higher than the actual pressure in the second internal pressure chamber 19. Generally speaking, a suitable pressure level in the first internal pressure chamber 18 during operation may, in many embodiments, be 0.1 MPa-0.6 MPa while the pressure in the second internal pressure chamber 19 may be in the range of 1.5 MPa-5 MPa in many practical embodiments.
It should be understood that the actual peak pressure (the highest pressure acting on the web W in the nip N) achieved may vary depending on, for example, the pressure in the internal pressure chambers and the shape of the press body. In many embodiments, the actual peak pressure achieved in the nip N may be in the range of, for example, 1.6 MPa-6.5 MPa. However, both lower and higher peak pressures are conceivable.
In many realistic embodiments of the invention, the nip length may be in the range of, for example, 80 mm-150 mm although other dimensions are also conceivable. The nip length is dependent on the length of the working surface 15 of the press body 13. In one realistic embodiment that has been contemplated, nip length may be 130 mm.
The dimensions of the extended nip roll 5 may of course vary. However, in many realistic embodiments, it may have a diameter in the range of, for example, 800 mm-1500 mm. For example, it may have a diameter of 1100 mm.
In practice, the machine in which the inventive extended nip roll is used may be operated at a speed in the range of, for example, 800 m/minute-1800 m/minute.
Speeds higher than 1800 m/minute may also be contemplated. Generally speaking, higher speeds are usually desirable since higher speeds normally mean higher productivity. However, a higher machine speed may entail certain difficulties. For example, a higher machine speed means a shorter dwell time in the nip N. A shorter dwell time in the nip N may have the result that less water is pressed out of the web. It has been noted that, in an application where the counter roll 7 was a hot Yankee cylinder and where the linear load was 150 kN/m, the dryness level after the nip N decreased by 1% when the machine speed was increased from 1500 m/minute to 1800 m/minute.
It can be added that, when the pressing of tissue paper takes place at a low temperature, for example when the counter roll 7 is at room temperature (about 10° C.-30° C.), trials have indicated that the dewatering is less influenced by machine speed. However, when the temperature in the nip is high (as when the counter roll is a hot Yankee cylinder), dwell time in the nip makes a significant difference for the dewatering effect on a tissue web. Dewatering is also more effective when the nip N is formed against a hot counter roll such as a Yankee cylinder.
In principle, the extended nip roll 5 may be pressurized already when the papermaking machine is started and the nip N may be closed. However, the extended nip roll 5 is normally not pressurized when the papermaking machine is started—or at least not fully pressurized. Instead, the nip N may actually be open when the papermaking machine is started. When the machine has reached a certain speed, for example a speed in the range of 550 m/minute-650 m/minute, the nip N may be closed and the internal pressure chambers may be pressurized. Preferably, the entire extended nip roll 5 may be movable towards and away from the counter roll 7 (which may be a Yankee cylinder). The starting sequence may then be that the extended nip roll 5 is moved close to the counter roll 7 when the machine has reached a certain speed (for example 600 m/minute). When the extended nip roll 5 has reached a position close to the counter roll 7, the internal pressure chambers may be pressurized such that the press body 13 is caused to expand radially outwards. When the press body 13 expands, the flexible jacket 10 will be pressed towards the counter roll 7 and the nip N will be closed. When it has been established that the nip N is closed, the load can be increased by increased pressure in the internal pressure chambers. Alternatively, the internal pressure chambers may be lightly pressurized while the extended nip roll 5 is moved towards the counter roll 7 until the nip N has been closed. When it has been established that the nip N is closed, the load can be increased by increased pressure in the internal pressure chambers.
If it is desired to produce a product with high bulk, the pressure profile can be altered such that the pressure in the last internal pressure chamber is decreased (the last internal pressure chamber is the second internal pressure chamber 19 since this is the last internal pressure chamber in the machine direction). Such a profile may increase bulk but dewatering becomes less effective. If instead the energy consumption should be kept low, an effective dewatering in the nip N is desirable. If effective dewatering is desired, the pressure in the last internal pressure chamber (i.e. the second internal pressure chamber 19) should be higher. It is generally known that bulk decreases with increased nip load.
It should be understood that, in the context of this application, the machine direction is the direction in which the web moves from the forming section to the reel-up.
Although the invention has been described above in terms of an extended nip roll, an extended nip press, a paper making machine and a method of operating an extended nip press, it should be understood that these categories only reflect different aspects of one and the same invention. The inventive extended nip roll can thus be used in the inventive extended nip press and the inventive extended nip press is used in the inventive machine and the inventive method.
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|---|---|---|---|
| 1250393 | Apr 2012 | SE | national |
| 1251000 | Sep 2012 | SE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SE2013/050227 | 3/12/2013 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2013/158010 | 10/24/2013 | WO | A |
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