EP2072672A1

EP2072672A1 - Arrangement in connection with the press section of a web-forming machine

Info

Publication number: EP2072672A1
Application number: EP07150200A
Authority: EP
Inventors: Kari RÄISÄNEN; Petter Honkalampi
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2007-12-20
Filing date: 2007-12-20
Publication date: 2009-06-24
Anticipated expiration: 2027-12-20
Also published as: EP2072672B1; CN101463575A; ATE518989T1; CN101463575B

Abstract

The invention relates to an arrangement in connection with the press section of a web-forming machine. The arrangement includes at least one press nip (10) for dewatering the web (11) manufactured on a web-forming machine. The arrangement also includes a smoothing press (12) including a smoothing roll (13) arranged after the press nip (10). The smoothing press (12) includes smooth heat carrying means (14') arranged as a loop around at least two guide rolls (15). The smoothing roll (13) is arranged into contact with the means (14') between two guide rolls (15). The smoothing roll (13) and/or at least one of the guide rolls (15) is a deflection compensated roll.

Description

The present invention relates to an arrangement in connection with the press section of a web-forming machine, which arrangement includes

at least one press nip for dewatering the web manufactured on a web-forming machine, and
a smoothing press including a smoothing roll arranged after the press nip.

In the press section of a web-forming machine lots of water is rapidly removed from the web. The press section has usually one or two press nips and press felts are also used. The press nip is often formed of two rolls arranged to be loaded against each other.
The quick removing of water causes surface densification of the same side of the web where the water is removed. Simultaneously, an oil absorption reduction appears and the print quality degrades. To avoid these problems a smoothing press is arranged after the press section. Traditionally the smoothing press includes two rolls forming a nip. The web is thus pressed in the nip without any press felt. Due to this fact water is not removed from the web in the smoothing press. However, both surfaces of the wet web get smoother which improves drying of the web in the drying section.
One target in the web-forming is to maintain both sides of the web as symmetrical as possible. However, this has occasionally required very complicated structures in the press sections and smoothing presses. Also, when heating the paper or board some problems have occurred. In the conventional short heated smoothing nip the water may turn to steam with a sudden heat or press impulse. This leads to delamination of the web. Furthermore the brightness and opacity of the web may decrease and the web can stick to the surfaces, for example to the surface of the roll.
The invention is intended to create a new type of arrangement in connection with the press section of a web-forming machine by means of which arrangement the quality of the surfaces of the web can be improved together with better drying properties. The characteristic features of this invention are stated in the accompanying Claims. The arrangement according to the invention is a new kind of combination of a press nip and a smoothing press. In the smoothing press according to the invention the web is heated efficiently without any risk of delamination. Simultaneously, both surfaces of the web get smooth which intensifies drying of the web. Other properties and advantages of the arrangement are discussed later. The paper or board produced in the arrangement according to the invention has surprisingly improved properties, above all, the initial smoothness of the paper or board product is improved resulting in improved stiffness and more efficient drying of the paper or board. Dry content of 45% or more, preferably 55% or more before cylinder drying section enables smoothing with lower nip loads and without the need of moisturising.
In the following, the invention is described in detail with reference to the accompanying drawings showing some embodiments of the invention, in which

Figure 1a: shows the first embodiment of the arrangement according to the invention,
Figure 1b: shows the second embodiment of the arrangement according to the invention,
Figure 2a: shows the third embodiment of the arrangement according to the invention,
Figure 2b: shows the fourth embodiment of the arrangement according to the invention.

Figures 1a - 2b shows some arrangements according to the invention in connection with the press section of a web-forming machine. The arrangement includes at least one press nip 10 for dewatering the web 11 manufactured on a web-forming machine. In Figure 1a there is actually three press nips. The arrangement also includes a smoothing press 12 including a smoothing roll 13. The smoothing press 12 is arranged after the press nip 10. When arriving to the smoothing press, the web has a dry content of 45% or more, preferably 55% or more before cylinder drying section. According to the invention the smoothing press 12 also includes smooth heat carrying means 14', such as a belt 14. The belt is preferably made of partly or wholly metal. The metal belt is smooth and has no void volume for water removal. The belt 14 is arranged as a loop around at least two guide rolls 15. The smoothing roll 13 is then arranged into contact with the belt 14 between two guide rolls 15. Thus, a non-water removing long nip area is formed with the metal belt. During the processing of the web, the web is guided between the belt and the smoothing roll. Then both surfaces of the web will be smoothened in this long wrap metal belt smoothing nip. In addition, the smoothing roll 13 and/or at least one of the guide rolls 15 can be deflection compensated. A guide roll is deflection compensated at least when it forms a nip with the smoothing roll. A deflection compensated roll adapts to the bending forces of the smoothing nip and/or metal belt tension which also gives a better control of evenness of smoothing. The smoothing roll and/or guide rolls can be heated. Thus, the web is also heated in an efficient way. In practice the rolls are internally heated with heating medium, for example with oil or steam.
According to the invention, the press nip 10 is a roll nip or a long nip press. A long nip and low specific pressure in the press nip is very beneficial to the final bulk and bending stiffness of the paper or board. Preferably, the long nip press is a shoe press. With the shoe press high amount of water can be removed from the web. This increases both bulk and dry content of the paper or board but it can make the surfaces of the web porous. However, the web is then smoothened in the smoothing press according to the invention. This results in a paper or board with improved bulk or in a paper or board with improved surface properties with standard bulk. Simultaneously, drying efficiency can be improved since the contact between the web and the drying rolls is improved. This results in, for example, reduced drying shrinkage in the cross direction of the paper or board. The belt and the smoothing roll have very smooth surfaces which is important in view of smoothing. Due to the long nip area in the smoothing press heat has time to penetrate through the web and thus also to increasing the temperature of the whole web.
Heating of a web influences the smoothness of the web. The smoothing press according to the invention has a more efficient heating equipment compared to prior art. First, the smoothing roll 13 can be a heated roll. Secondly, at least one of the guide rolls 15 can be a heated roll. A combination of the first two embodiments is also possible. Also all guide rolls 15 can be heated rolls. Temperatures of smoothing press are around 50 - 300 °C, preferably 70 - 170 °C. If needed, there can be also some auxiliary heating means 23 for the rolls 13, 15. The auxiliary heating means can include, for example infrared, resistive, inductive or gas-burner heater. There can be also a combination of the above mentioned heaters and the belt can also be heated. Some special arrangements can also be equipped with above mentioned auxiliary heating means without any steam heated solutions. In practise the diameter of the smoothing roll 13 and/or the guide roll 15 is 1000 - 2500 mm, preferably 1300 - 2000 mm. Thus, the nip area becomes long and the rolls are stiff. In spite of large rolls the actual size of the smoothing press stays quite small. Preferably the smoothing press 12 also has an own hood 16. In this way it is possible to independently control the smoothing press and runaway heat flows can be avoided. Also excess or extra moisture can be handled with removal of moist air. The hood can also be common with the hood of the drying section.
In Figure 1a there is an open draw from the press nip 10 to the smoothing press 12. The tail threading is handled for example by ropes arranged to the side of the press section. However, vacuum belt conveyors and air blowings can be used. In Figures 1b, 2a and 2b between the press nip 10 and the smoothing press 12 there is a closed draw. This makes the tail threading more secure and the actual web-forming process is also stable.
The belt can be made of metal, for example of steel. The thickness of the metal belt is about 1 mm. However by using large rolls thicker belts can be used. The belt can be made also from other material. In this situation the belt has, for example, metallic coating for achieving as smooth surface of the belt as possible. This kind of belt can be thicker than the metal belt. In order to achieve a stable run of the belt quite high tensions are needed. According to the invention, the tension of the belt 14 is 3 - 15 kN/m, preferably 5 - 10 kN/m. The position and alignment of one or more guide rolls can be adjusted. Thus, the smoothing press can be controlled for every situation.
The web may dust during smoothing. Also fibres, fines, chemical deposit or stickies can stick to the surface of the rolls or belt. According to one preferred embodiment of the invention, there are cleaning equipments 17 which include high-pressure water cleaners preferably in connection with both the smoothing roll 13 and the belt 14. The cleaners are used continuously and bigger particles are removed with doctors 18. For better cleaning results the temperature of the cleaning medium can be risen close to 100 °C which also reduces the cooling effect of the cleaners.
The smoothing effect of the smoothing press is achieved by long wrap metal belt nip area together with smooth surfaces of the belt 14 and the smoothing roll 13, and heating. The length of the nip is 500 - 2500 mm, preferably 1000 - 2000 mm. There is no need to press the web which would decrease the bulk. Thus, the smoothing press 12 has a linear load which is 1 - 150 kN/m, preferably 5 - 100 kN/m. This nip pressure can be adjusted for example by moving one guide roll. If additional pressing is needed the smoothing roll 13 can be equipped with a optional counter roll 20 which is arranged inside the belt 14 arranged as a loop. By loading the counter roll against the smoothing roll, the nip pressure is locally increased. The counter roll is preferably a deflection compensated or tube roll.
The smoothing roll 13 is arranged into contact with the belt 14 between two guide rolls 15. The rolls are situated in such a way that the smoothing roll 13 and the belt 14 form two gaps 19 which open downwards. The gaps are under the loop which keeps the smoothing press clean if threading tail breaks. Also, if something else than the web enters into the closing gap, it will come out and fall underneath the smoothing press by way of gravity.
An other embodiment of the present invention shown in Figure 2b includes an impingement drying section 21 arranged between the press section and the smoothing press 12. This drying section is for increasing the temperature of the web and decreasing the water content therein. Thus the smoothing process becomes more effective. This web-preheating arrangement according to Figure 2b also enables less or no heating in the actual smoothing press.
In Figure 1a, for example, the web is guided from the smoothing press to the drying section. Due to the smoothing press according to the invention the first drying cylinder can be warmer than previously. However, between the press nip 10 and the smoothing press 12 there can be at least one impingement drying section 21 as mentioned earlier. This increases the dry content and the smoothing press can be even warmer. In Figure 2b there is one horizontal impingement drying section while in Figure 2a there is also a vertical impingement drying section. Also the smoothing press can be followed by one or more impingement 22 drying sections, as shown in Figure 1b. With preheating, higher initial cylinder temperatures can be used in the drying section. This improves the drying capacity of the drying section as a whole. Also the improved smoothness of the web will increase the friction between drying cylinders and drying fabrics. This will improve the surface properties of the board especially in the edge areas.
The arrangement according to the invention is compact and it has many advantageous features. Due to the long wrap metal smoothing nip, the web has higher dry content than in the ordinary smoothing press. Also, the surfaces of both sides of the web are very similar. The components of this arrangement are simple so the founding costs are low. Also, there are only minor press felt markings on the web after leaving the smoothing press. The web has very good smoothness and bulk both after the press section and also for the end product. As mentioned earlier, this leads to improved contact between drying cylinders and drying fabrics. Also the drying shrinkage is reduced and the temperature transmission from drying cylinders to the web is improved. When the drying efficiency gets higher the production capacity can be improved or higher bulk can be achieved after the press section. When the smoothness is improved in the metal belt smoothing press there is less or no need for calendaring after the drying section. This leads to improved bending stiffness because the caliper of the web is improved.
The press section has few press nips. The structure of the press section is thus simple which improves the runnability of the press section and reduces the investment costs. This gives a short payback period for the smoothing press arrangement according to the invention. Also, the space needed for the press section is reduced. The metal belt smoothing press has also effect on the strength properties of the end product. The smoothing press also changes the surface properties of the web. Also, it is possible to adjust the temperatures of the sides of the paper or board in the smoothing press. Hence, it is easy to change the sidedness of the paper or board. When the web is already preheated in the smoothing press, higher initial cylinder temperature can be used in the drying section without fibre picking and dusting or linting problems.
The press section is intended for dewatering and/or processing paper of board webs. In spite of the end use, the paper or board has no warpages or curlings and it has a good stiffness. According to the invention, the heating means or heat carrying means or cleaning equipments includes one or more suitable device for heating, heat carrying or cleaning, respectively.

Claims

Arrangement in connection with the press section of a web-forming machine, which arrangement includes
- at least one press nip (10) for dewatering the web (11) manufactured on a web-forming machine, and

- a smoothing press (12) including a smoothing roll (13) arranged after the press nip (10),
characterized in that the smoothing press (12) includes smooth heat carrying means (14') arranged as a loop around at least two guide rolls (15), and the smoothing roll (13) is arranged into contact with the means (14') between two guide rolls (15), and the smoothing roll (13) and/or at least one of the guide rolls (15) is a deflection compensated roll.
Arrangement according to Claim 1, characterized in that the smoothing roll (13) and/or at least one of the guide rolls (15) is heatable.
Arrangement according to Claim 1 or 2, characterized in that the smoothing press (12) is arranged in the direction of motion of the web (11) when the dry content of the web (11) is 45 % or more, but before a cylinder drying section belonging to the web-forming machine.
Arrangement according to Claim 3, characterized in that the dry content of the web (11) is 55 % or more.
Arrangement according to any if Claims 1 - 4, characterized in that the smooth heat carrying means (14') consist of a belt (14) made partly or wholly of metal.
Arrangement according to any of Claims 1 - 5, characterized in that the smoothing roll (13) and/or at least one of the guide rolls (15) is arranged to be heated by heating medium, infrared, resistive, inductive or gas-burning heating means.
Arrangement according to any of Claims 1 - 6, characterized in that all guide rolls (15) are heatable.
Arrangement according to any of Claims 1 - 7, characterized in that the diameter of the smoothing roll (13) and/or the guide roll (15) is 1000 - 2500 mm, preferably 1300 - 2000 mm.
Arrangement according to any of Claims 1 - 8, characterized in that the smoothing press (12) has an own hood (16).
Arrangement according to any of Claims 5 - 9, characterized in that the tension of the belt (14) is 3 - 15 kN/m, preferably 5 - 10 kN/m.
Arrangement according to any of Claims 5 - 10, characterized in that in connection with both smoothing roll (13) and belt (14) there are cleaning equipments (17) which include high-pressure water cleaners.
Arrangement according to any of Claims 1 - 11, characterized in that the smoothing press (12) has a linear load which is 1 - 150 kN/m, preferably 5 - 100 kN/m.
Arrangement according to any of Claims 5 - 12, characterized in that the smoothing roll (13) has a counter roll (20) which is arranged inside the belt (14) arranged as a loop, and the counter roll (20) preferably is a deflection compensated roll.
Arrangement according to any of Claims 5 - 13, characterized in that the smoothing roll (13) and the belt (14) form two gaps (19) which open downwards.
Arrangement according to any of Claims 1 - 14, characterized in that in connection with the smoothing press (12) there are auxiliary heating means (23) for heating the web (11) and/or the smooth heat carrying means (14').
Arrangement according to any of Claims 1 - 15, characterized in that between the press nip (10) and the smoothing press (12) there is a closed draw.
Arrangement according to any of Claims 1 - 16, characterized in that between the press nip (10) and the smoothing press (12) there is at least one impingement drying section (21).
Arrangement according to any of Claims 1 - 17, characterized in that right after the smoothing press (12) there is at least one impingement drying section (22).
Arrangement according to any of Claims 1 - 18, characterized in that the arrangement is intended for dewatering and/or processing paper or board webs.