Press section of a paper or board machine

The present invention relates to a press section in a paper or board machine, comprising in combination at least one dewatering press nip and a smoothing press positioned downstream in terms of the web running direction. The invention relates further to a smoothing press for the press section of a paper or board machine.

The press section of a paper or board machine has a duty of dewatering a wet fibrous web (dry content 15-23%) as effectively as possible, such that, when passing over to the dryer section, the dry content would be as high as possible (in practice about 40-55%). In modern wet presses, the web is delivered into a nip contact, wherein the web is pressed against an absorbent felt for displacing the water from web to felt in response to pressure forces. Commonly employed are press designs based on a conventional roll nip or a so-called shoe nip. The press nip can be either single-felted with the web having a felt on just one side or double- felted with water draining into felts on both sides.

In an effort to provide a dry content as high as possible downstream of the press section, use is made of felts as open-structured as possible. With open-structured felts, the web becomes extremely coarse and has to be subjected to abundant smoothing with a calender, resulting in the loss of bulk and thereby strength. In order to improve smoothness, it is possible to use smoother felts in the final nip or to use a separate smoothing press (finishing press). Especially in board machines, it is a common practice to use a smoothing press. The purpose of a smoothing press is to make the treated web smoother, not dewatering any more. Hg. 1 shows one press section application of the prior art, wherein a web W is passed from a wire section WS to a press section P and delivered thence further to a dryer section D. Viewed in the direction of travel, the press section P includes two successive wet press nips 1, 2 followed by a smoothing press 3.

A drawback in the use of smoother felts is poorer dewatering in the actual pressing process, the machine's production suffering. A drawback with the smoothing press is poor runnability as well as higher investment.

It is an objective of the present invention to provide an enhanced press section solution, which is capable of avoiding prior art drawbacks, further improving smoothness, reducing a length of the press section, and improving runnability.

In order to achieve this objective, a press section of the invention is characterized in that the smoothing press is positioned at a location in which the dry content is less than 75%, the smoothing press is provided with a metal belt circle comprising a metal belt adapted to extend around at least one guide element, on the outside of said belt being mounted at least one counter-element providing a contact surface with the belt so as to establish between the belt and the counter-element a web treatment zone, the web to be treated being carried therethrough.

The advantage is an improved smoothness of the web and thereby an enhanced heat transfer along the dryer section. The smoothing press provided with a metal belt circle is preferably outfitted with heating elements for heating the metal belt and/or the counter-element in view of warming up the treated web upstream of the dryer section, thus enabling the use of higher steam pressure along the dryer section right from the start.

On the other hand, a smoothing press of the invention for the press section of a paper or board machine is characterized in that the smoothing press is provided with a metal belt circle comprising a metal belt adapted to extend around at least one guide element, on the outside of said belt being mounted at least one counter- element providing a contact surface with the belt so as to establish between the belt and the counter-element a web treatment zone, the web to be treated being carried therethrough.

The invention will now be described more closely with reference to the accompanying drawings, in which:

Rg. 1 shows schematically one board machine press section of the prior art,

fig. 2 shows in a schematic view of principle one press section of the invention, and

figs. 3-6 show in schematic views of principle preferred further embodiments for a solution of the invention.

Fig. 2 is a schematic view, showing one embodiment for a press section of the invention, wherein a second wet press 2 and a smoothing press 3 of the press section are replaced with a smoothing press 4 based on a metal belt circle and comprising a metal belt 5 adapted to extend around at least one guide element 6, and a counter-element 7 mounted on the outside of the belt circle and comprising preferably a roll, the counter-element providing a contact surface with the belt so as to establish between the belt and the counter-element a web treatment zone PN, the web to be treated being carried therethrough. A wet press nip 1 preceding the smoothing press 4 is preferably implemented as a shoe press.

The counter-element 7 can also be disposed inside a fabric loop (not shown), the web being transferred within the treatment zone into the smoothing press nip while supported by the fabric. In this case, the fabric not only facilitates feeding the web into the nip but can also take up water draining from the web, thereby further improving the dry content of the web. Thus, within the treatment zone PN the web is over one side thereof in contact with the metal belt 5 and over the other side thereof either directly with the surface of the counter-element 7 or with a fabric present on the surface of the counter-element 7. The latter case is pertinent especially with heavy-duty grades, wherein the dewatering in a single nip is not sufficient. Changing the treatment zone length and/or the belt tension is preferably performed by designing at least one guide element 6 to be displaceable and/or adjustable in terms of its size or position, e.g. its angular position.

As opposed to fig. 2, the number of wet press nips upstream of the smoothing press can be more than one. In terms of structural and cost aspects, the most favorable is nevertheless a solution in which the number of traditional press nips is just one.

Reference numeral 8 is used to represent metal belt heating elements, which may comprise e.g. an induction heater, an infrared radiator or a gas burner. The heating may also be based on resistive heating. The counter-element 7 can also be provided with separate external heating elements or the counter-element may comprise a

thermo roll. Heating the web prior to its passage to the dryer section enables higher steam pressure to be applied from the very start. In addition, a long treatment zone in the smoothing press enables improving the web smoothness, which enhances heat transfer along the dryer section.

Inside the metal belt circle can be further provided an extra press element (not shown), pressing the belt 5 against the roll 7 so as to establish a nip zone of higher pressure within the web treatment zone. This type of extra press element is beneficial, especially when using a dewatering fabric loop disposed around the counter-element 7. The extra press element is preferably a roll which, like a roll functioning as the counter-element 7, may or may not be a deflection-compensated roll and which is selected from a group, including: a flexible surface roll, such as a polymer-coated roll, a rubber-coated roll or an elastomer surface roll, a shoe roll, a thermo roll, a metal roll, a fibrous roll, and a composite roll. Instead of a roll, the counter-element may comprise some other variable profile or permanent profile press element, which may further consist of a number of cross-machine directed successive components. A counter-element implemented in the form of a roll may also consist of a number of cross-machine directed successive components. The press element may have its surface designed to be continuous or discontinuous. In addition, the press element can be adapted to be displaceable for changing the treatment zone length and/or the belt tension.

An open draw to a smoothing press and thence further, as depicted in fig. 2, is a good solution at lower web speeds of e.g. less than 1000 m/min and preferably less than 800 m/min. Higher speeds can be employed when the web has a higher dry content or more strength. In this case, the activation of a belt smoothing press can be performed by means of tail threading blows and conveyors or preferably by drawing the tail with ropes from the side of a smoothing press, followed by spreading the tail across the machine fill.

What is special about tail threading is a closed belt smoothing press nip that cannot be opened the way of an ordinary press/smoothing nip for the duration of a tail threading process. Thus, it is preferred that the belt 5 be in a high position and that the gaps, established thereby jointly with the counter-element 7, open downwards, whereby the gaps are cleaned gravitationally and a closing gap, in particular, does

not gather the web into a heap that could ruin the belt. The high position also makes it easier to replace a belt as well as to provide with an overhead extractor hood and thereby to control heat balance and dehumidification.

It is also possible to arrange a closed draw, especially in high-speed web forming machines of more than 800 m/min, preferably more than 1000 m/min, the lower limit being determined by a grade to be run. In a closed draw, the web is brought as early as in the very tail threading process in full width to a belt smoothing press and through its treatment zone and further to a drying process while being supported. In this arrangement, the draws are challenging as the upstream press nips are in relieved condition and the web is thereby inconsistent with running parameters.

Rg. 3 depicts an application, wherein a counter-element 7 is contacted not with an extra press element but with a guide element 6. The nip zone established with a guide element is beneficial as it develops a long preheating range upstream of a higher pressure nip zone PN between the rolls 6, 7. In addition, the application provides an easier tail threading process from the roll 7 further to drying, specifically the transition point being disposable at a higher level than in solutions with a longer treatment zone.

Fig. 4 depicts a belt smoothing press 4, including just two guide rolls 6 for a belt circle 5. One of the rolls may function as a tension regulator. A higher web temperature produced by the belt smoothing press is utilized in an immediately downstream blast-on drying process 20, which further enhances an increase of the dry content and functions as a transition in the closed draw to a single-wire draw cylinder dryer section. The full-width web is brought to the belt smoothing press 4 by a transfer nip TN, in which the web engages with the surface of the more adhesive metal belt 5 from a final fabric 30 of the press section P. Respectively, the draw to a drying process D can be effected by using a suction-roll transfer 12 from a roll immediately downstream of the treatment zone. In addition to adhering the web, the transfer nips shown in figs. 3-6 have a moving effect on at least one side of the web.

The treatment by a belt smoothing press can also be enhanced by means of steam boxes 9 located upstream of the treatment zone for moisturizing the web as well as for raising the web temperature, which also reduces the adherence of web fibers/stickies to the treatment surfaces. In addition, it is a good idea to provide the metal belt smoothing press 4 with cleaning means for smooth treatment surfaces, e.g. with doctors 11 or high-pressure washers 10 or a combination thereof, in order to prevent the dirt sticking to said surfaces from removing more material from the web or from pressing a pattern to the web surface.

A preferred solution is to apply drying to the web upstream of the smoothing press for discouraging adherence to its hot surfaces.

Fig. 5 depicts an embodiment, in which a single-nip press Nl is followed by a pre- drying section PD, including e.g. 5-18, preferably 8-15 drying cylinders upstream of a belt smoothing press 4. Consequently, the web has an increased dry content prior to feeding into the belt smoothing press. In this case, there is further depicted, along an open web draw to the belt smoothing press, a special engagement transfer nip 13, which is furnished with a fabric loop and which contributes to a web-and-belt contact and thereby improves heat transfer to the web. The open draw to the dryer section is indicated to include a belt conveyor 14 for a tail threading path. A particularly apt solution is the use of blast-on as shown in fig. 6, which neither contacts/engages the web nor densities its surfaces. The belt smoothing press 4 present downstream of such a top blasting arrangement 20a-20c drying both sides of the web constitutes a preferred application of the invention. The discussed belt smoothing press can be operated at higher temperatures and there is nevertheless a lesser demand of cleaning the treatment surfaces by virtue of the incoming web having drier surfaces. The arrangement is beneficial in connection with a single-nip press, the dry content of a web produced thereby not being as high as that of multi-nip applications.

A higher dry content of the web also enables in the arrangement the use of open draws in connection with a belt smoothing press. The web draw from blast-on drying to the belt smoothing press is preferably implemented by means of a transfer fabric 21, wherein a suction roll 22 picks up the web from a drying wire 23 and a transfer nip 24 sets the web in engagement with the metal belt 5.

In such an application, the web upon entering the treatment zone has a dry content preferably within the range of 55-75%, even more preferably within the range of 60-70%, whereby especially the web surfaces are drier and thus particularly suitable for a long treatment-zone belt smoothing press at high temperatures.

The treatment surfaces provided in a treatment zone PN have a temperature which is most conveniently less than 150 ⁰ C, preferably 1-3 times the temperature of an incoming web, the highest multipliers being found in solutions in which the belt smoothing press is located immediately downstream of the actual press. In addition, a temperature difference between the treatment surfaces can be used to manage the advancement of a web after the treatment zone, e.g. by setting up a temperature difference of more than 15 ⁰ C between the treatment surfaces, the web following the warmer surface downstream of the treatment zone. In the event that a temperature difference cannot be set up in favor of the desired surface, it is also possible to use a release chemical on the discussed surface. Fibers and stickies from the web may adhere to the treatment surfaces of a belt smoothing press, which should therefore be provided with doctors and, in addition to or instead of the latter, it is even more advisable to effect cleaning of the surfaces by means of traversing high-pressure washers to eliminate possible dusting.

The belt smoothing press according to the invention has been surprisingly found applicable to a web having a distribution of mass density of 4-7%. This range, which represents the consistency of web formation (good formation, normalized formation less than about 0,9), has been surprisingly found beneficial for a belt smoothing press as the visual impression of a web with a poorer formation may even deteriorate with a belt smoothing press of the invention.