CN1738941B - Press section in a paper or board machine - Google Patents

Abstract

The invention relates to a press section in a paper or board machine, in which press section the transfer of a web (W) is accomplished as a closed draw through the press section and which press section is provided with at least three separate press nips (N1, N2, N3), dewatering in the first nip of said nips taking place on two sides. The closed draw of the web (W) through the pres s section is accomplished as a substantially straight run and that the press nips each have their own fabric loops (20, 21; 30; 40, 41), that the nips (N2, N3) following after the first nip (N1) have one water-receiving fabric (30, 40), which is placed in the nip (N2, N3) on the opposite side of the web (W) with respect to the other nip in the running direction of the web in the machine.

Description

Press section in a paper or board machine

technical field

The invention relates to a press section of a paper or board machine in which the transfer of a paper web is accomplished as a close draw through the press section, the press section being provided with at least three separate nips in which the dewatering takes place on both sides in the first nip.

The invention relates to a paper or board machine having at least one forming section, one press section and one dryer section, wherein the press section is provided with a separate press nip. In this application, a split nip means a nip in which opposing extrusions define only one nip. The separate nip can be a roll nip or an extended nip. The relative position of the separate nips is a press with a central roll forming a nip with at least two other rolls.

Background technique

Finnish patent application 990557 discloses a method and a device for treating paper and board webs. Figures 4-6 show some examples of chemical substances used to control the surface properties of the paper web and suitable for smoothing nips. In these illustrated embodiments, a tandem conveyor belt is utilized below the second and third nips. Because dewatering takes place in both directions on the first nip, upwards on the second nip, and does not occur on the third nip, dewatering is not symmetrical, but since the belts are on both sides of the web side, the paper web will be stable. Thus, even though the third nip in these embodiments improves the surface properties of the paper web, it does not increase the dry solids, thereby increasing the overall length of the paper machine.

Finnish patent 95610 discloses a press section of a paper machine provided with a smoothing press. The web is drawn under tight and supportive tension as it passes through the press section, which comprises at least two consecutive separate nips, dewatering the web at least in the first of said nips, preferably in both recovery nips. Water between press fabrics. The latter nip in the press section is a smooth nip, separate from the preceding nip, in which no actual dewatering takes place. The web is conveyed from the preceding dewatering nip through the smooth nip by a substantially non-absorbent conveyor belt, which travels substantially in a straight line and continues to travel in the same direction after passing the smooth nip, during which travel the paper Some elongation in the machine direction of the web, which elongation occurs in the smooth nip, can be compensated by the difference in speed of the conveyor belt.

US Patent 6,210,530 discloses a press section and a method of pressing with shoepresses of different lengths. Figure 2 shows an embodiment comprising three shoe press units, wherein the web first passes through first and second shoe press units arranged below a counter roll (i.e. below the web) , and the third shoe press is placed above a reaction roll, ie above the web.

Finnish patent application 981089 discloses a press section of a paper machine employing one or more nips through which the paper web to be pressed passes as a substantially tight draw, wherein the press section has at least one A nip of two water-absorbent press felts, through which the web travels between the press felts, and after the aforementioned nip the web is separated from one of the press felts and further Essentially as a tight stretch that is transferred to other press felt supports. The last nip in the press section is an extended nip zone where one roll is a shoe roll with a hose shell and press shoe configuration and the other roll is a suction roll with a suction area of the press suction roll. The web passes through the last extended nip region between the two absorbent press fabrics, and after this last extended nip region the web is separated directly from one of the press fabrics and passed between the press section rolls Aided by the vacuum present in the suction zone, the web is conveyed, without rehumidification, following a press fabric arranged on the side of said press suction roll, and the web is on this latter press fabric, as compact The draw is transferred to a dryer wire or equivalent located after the press section on which the web is transferred as the suction roll transfers, or by an equivalent arrangement send.

Finnish patent application 961518 discloses a press section geometry in which a tightly drawn press section of a paper web in a paper or board machine is provided comprising a first absorbent upper fabric, the web on the suction zone of a pick-up roll It is conveyed onto it from the pick-up web of the pick-up point, which is located after its suction roll, and which has a wire drive roll behind it, from which the return journey of the web starts. The press section consists of one or two consecutive nips which press water from the web, between which nips the web has a fully tight draw so that it is always supported by a fabric. The nip where this water is removed is the so-called extended nip, the press zone of which is substantially longer than the press zone of the tip roll nip, and all nips are additionally provided with two water-absorbing press fabrics, so that the moisture in them passes through the Both surfaces are removed substantially symmetrically. In addition, the embodiment shown in Figures 1, 2 and 3 utilizes a lighter load transfer zone defined by a press roller having a concave surface and positioned in the same position as a first drying cylinder or equivalent. Internet loop.

A known problem with press sections in the prior art is absorption asymmetry, which is caused by variations in the composition/structure of the web in the thickness direction, during each press operation due to some Fines and filler material are soaked with moisture that has not been removed in equal amounts in both directions. Fines accumulate on the parts of the web surface where dewatering is more intense. The problem can also be affected in one wire section, but the press is the last drain, whereby the problem is actually affected. Even in good paper, the interlayer has less fines, but on both sides of the web, an equal amount of fines moves and adheres to the surface.

In terms of surface topography, in modern press sections with tight draws, printing leaks, as well as the adsorption and asymmetry of the upper and lower surfaces are notable drawbacks when applied to printing paper. As speeds increase, taking them beyond 2000 m/min, compact stretching offers substantial flow benefits in terms of speed potential and efficiency. It is an object of the present invention to further develop a press section which has a tight draw and is suitable for speeds of more than 1800 m/min, preferably more than 2000 m/min, without the aforementioned aspects of surface topography and water absorption symmetry The problem.

Contents of the invention

It is an object of the present invention to provide a press section of a paper and board machine which optimizes surface topography and absorption at speeds exceeding 1800 m/min, preferably exceeding 2000 m/min.

In order to achieve the above-mentioned and subsequent objects, according to the present invention there is provided a press section in a paper or board machine, wherein in the press section, as a tight draw through the press section, the adjustment of a The transfer of the paper web, the press section is provided with at least three separate nips, in the first of which the dewatering takes place on both sides, by which the paper web is tightly stretched is done as a substantially straight run, and each nip has its own fabric loop, with the nip after the first nip having an absorbent fabric relative to the direction of travel of the web in the machine The other nip on the nip is placed on the opposite side of the web on the nip, and in the nip after the first nip, in the so-called smoothing or leveling nip, on the side opposite the absorbent fabric The web surface is arranged to be smoothed either against a smoothing roll or against a conveyor belt.

In the press section of a paper or board machine according to the invention, the closed draw through the press section runs substantially straight, and the press section comprises at least three separate press nips, The nips do not have any fabric loops in common with each other.

In the press section according to the invention, the first nip in the direction of travel of the web is a roll nip or a press suction roll nip, preferably running at a lower speed of more than 1800 m/min, but When the speed exceeds 2000 m/min and when the dewatering demand increases, it may be preferred to use an extended nip. It is conceivable that where all three nips are high speed, all nips are extended nips and, on the other hand, where more than three nips are low speed, only roll nips may be used. Each of the lower two nips has only one absorbent fabric, which is placed on the opposite side of the paper in each nip relative to the other nips in the machine direction. In said subsequent nip, both sides of the web are mechanically smoothed/levelled one at a time against a smoothing surface, which may be a smoothing roll, known as the center roller set, or conveyor belt. In this connection, in the nip, the action of removing water from the web takes place only in the direction away from the smooth surface, ie on the side of the absorbent fabric. In successive nips, equal amounts of fines affecting absorption are placed adjacent to both surfaces.

According to the invention, as is known, in an embodiment, from the press section is transferred from a conveyor belt to a dryer section, wherein the smooth surface is located in the upper position of the last nip of the press section, using the transfer from the lower felt.

In the press section according to the invention, in the smoothing/levelling nip provided with smoothing rolls or conveyor belts, there is a water-absorbing press fabric and on the other side a press roll. At lower loads, a conventional pressure suction roll can be used, at higher loads preferably a press suction roll produced by powder metallurgy, wherein the wall thickness of the suction roll does not need to be increased. For example, under the action of the last smoothing nip, it is ensured that the web follows the felt and does not require high loads. Similarly, powder metallurgy suction rolls are used to achieve enhanced dewatering with high loads in the initial roll/extended nip. For example, in Finnish patent 100422 a press suction roll manufactured by powder metallurgy is described. The suction roll ensures that the web remains on the surface of the porous fabric so that it does not follow the more viscous smooth surface of the roll/belt. The smooth surface of the nip also mechanically affects the surface of the web and compensates for the loss of smoothness caused by the rough press fabric. The present invention can also use a shoe press.

The press section according to the invention provides the web with better surface properties in relation to printability and can be processed at higher speeds of over 1800 m/min, and preferably over 2000 m/min, and has Higher efficiencies are combined with better moisture characteristics.

The press section according to the invention also facilitates the adjustment of the two sides, which needs to be done in-line in the paper machine, mainly by calendering and application of chemicals. The volume that may be lost during pressing can be retained later by lighter calendering, which can be further optimized according to other desired surface properties since no correction of absorption symmetry is required. On the other hand, when pressing dry solids increases in more nips than in the prior art, it can be handled with a shorter dryer section despite the longer press section.

In a preferred embodiment of the invention there are three nips, the tight drawing of the web passes through the press section and in the first nip dewatering takes place on both sides, whereby a symmetrical web can be provided . After the first zone, the web is transferred from the surface of an upper felt to a second nip where dewatering takes place below and the upper surface of the web is either against a smooth roll or against a A smooth conveyor belt. In the third nip, where dewatering takes place above, a conveyor belt is applied against the lower surface of the web and conveys the web from the lower fabric to a dryer section. According to a preferred additional feature, a press roll is arranged in the second nip after the first nip, whereby a sufficient material displacement can be achieved in the second nip, i.e. displacement. At the same time, the smoothness of the upper surface of the paper web is improved because the paper web is pressed against a smooth surface.

Hereinafter, embodiments of the present invention are described with reference to the accompanying drawings, but the present invention is not limited entirely to the contents of the accompanying drawings.

Description of drawings

Figure 1A is a schematic diagram of one embodiment of the present invention.

Figure IB is a schematic illustration of a variation of the first nip in the embodiment of the invention shown in Figure IA.

Fig. 2A is a schematic diagram of a second embodiment of the present invention. Figure 2B is a schematic diagram of an alternative application of the embodiment of the invention shown in Figure 2A.

Fig. 3 is a schematic diagram of a third embodiment of the present invention.

Detailed ways

In the following drawings, the same reference numerals are used for parts corresponding to each other.

In Fig. 1A, the web W formed in the forming section is conveyed to the pick-up point P position before reversing rolls 11, 12 on the forming wire, which are arranged on the forming wire. Net 10 interior. The pick-up point P is formed between the forming wire 10 and a roll 24 arranged on the circulation of the first water-absorbing press felt 20 and provided with a suction 24a so that the web W passes through the forming wire 10 and the pick-up point P between the first press felt 20 . At the pick-up point P, the web W is separated from the forming wire 10 and transferred to support by the first press felt 20 . The web W is then transferred, supported by the lower surface of the first press felt 20, to a first nip N1 delimited by two concave press rolls 22 and 23 so that Between are formed a roll nip N1 or a press suction roll nip ( FIG. 1B ), wherein the upper roll 22 is a press suction roll provided with a suction zone, and the other roll 23 is a concave roll. In this regard, the felt suction roll 25 can be omitted. The web W travels to the first nip N1 between a first absorbent press felt 20 as an upper press fabric and a first absorbent press felt 21 as a lower press fabric. The dewatering of the web W takes place in the first nip N1 with water entering the two press felts 20 , 21 . In embodiments where a press suction roll is used, moisture may also be removed through the suction zone and into the suction roll.

After the first nip N1, the vacuum in the suction zone 25a of the upper suction roll 25 ensures that the web W travels with the upper press felt 20 and passes through the suction zone of the lower transfer suction roll 34 The vacuum in 34a separates the web W from the first press felt 20 so that it adheres to a second lower press fabric 30, which may also be a press felt, the web W is conveyed on its upper surface to a second nip N2 defined between a roll 32 and a press suction roll 33 having a suction zone 33a. The suction zone 33a of the suction roll 33 preferably has a large sector and a high vacuum to ensure that the web has adhered to the desired felt before the nip and after the nip Follow the felt so that the web does not follow the smooth roll. Thereafter, the web W is transferred with the support of the second press fabric 30 and transferred through the suction zone 44a of a transfer suction roll 44 to the support of the subsequent upper press fabric 40 and transferred to the third nip N3, the third nip N3 is defined between an elongated nip roll 42 and a counter roll 43 . Below is a conveyor belt 41, by means of which the paperboard W is conveyed to a conveying suction roll 54 having a suction zone 54a, whereby the paperboard is conveyed onto a drying wire 50 of a dryer section. Dewatering of the paperboard W takes place in the third nip N3, with moisture entering the upper press felt 40 .

The embodiment shown in FIG. 2A essentially corresponds to the embodiment shown in FIG. 1A and its variants. However, in the embodiment shown in FIG. 2A, the second nip N2 is formed such that in the second nip 32, there is an upper conveyor belt 31 and, as in the embodiment shown in FIG. 1A, there is a press A felt 30 is used as the lower fabric 30 . The web W is transferred from the second nip N2 on the lower surface of the transfer belt 31 onto a transfer fabric 60, on which the web W is transferred by a suction zone 64a of a transfer suction roll 64, And supported by the felt 60, the paper web W is conveyed by the suction zone 44a of the suction roll 44 to be supported by the upper fabric 40 of the third nip N3.

The embodiment of the invention shown in FIG. 2B substantially corresponds to the embodiment shown in FIG. 2A , except that each nip N1 , N2 , N3 is formed as an extended nip. One of the rollers is an extended press roller 22; 33; 42 and the other roller is a counter roller 23; 32; 43.

Figure 3 shows an embodiment of the invention in which the web W is transported from a forming wire 10 of the forming section in front of the reversing roll 11 to a pick-up point P where the web W is drawn in a drawer. The suction zone 24a of the suction roll 24 is conveyed by the action of the suction zone 24a, thereby being supported by the first water-absorbing press felt 20, and the paper web W is conveyed to a first nip N1, which is defined in two concave press nips. Between rolls 22 and 23, or defined between a press suction roll and a concave press roll (as in FIG. 1B ). The lower press felt 21 also absorbs water. Adhesion to the surface of the lower press felt 21 is ensured by a vacuum in a suction zone 26a of a transfer suction roll 26, and the web W is transported by a vacuum in a suction zone 35a of a suction roll 35, whereby Supported by an upper press felt 31 of the second nip N2. The second nip N2 is formed as an extended nip between an extended roll 32 and a counter roll 33 . The lower fabric of the nip N2 is a water-absorbent press felt 30 . Under its support, the web W is transferred to a transfer suction roll 44, and under the action of the vacuum 44a in the suction region of the transfer suction roll 44, the web W is transferred to be supported by an upper felt 40. , on which the web is conveyed to a third nip N3, which is a smooth nip formed between rolls 42 and 43. The roll 42 is a press suction roll provided with a suction zone 42a. Dewatering takes place in the suction zone 42a of the suction roll. The web is transferred from the felt 40 to the subsequent felt 70 under the action of the vacuum in the suction zone 74a of the suction roll 74 . Through the felt 70 , the web is conveyed to a subsequent smoothing nip N4 formed between a press suction roll 73 and a suction roll 72 . The suction zones 42a and 73a of the suction rolls 42 and 73 preferably have a large fan shape and a high degree of vacuum to ensure that the web has been adsorbed to the desired felt before the nip, and in the nip The felt is then followed so that the web does not follow the smoothing roll. Thereafter, the web W is conveyed to a dryer section and is supported by a dryer wire 50 as described above in connection with Figures 1A and 2A. The embodiment of Fig. 3 can also be realized so that only one nip that absorbs water in two directions is arranged before the smooth nips N3 and N4, in this case, the first nip N1 can be a roller formed by a shoe roll) and a concave roll, or a nip defined by a shoe roll and a press suction roll.

Above, the invention has been described in terms of some preferred embodiments, but the invention is not limited to the above details.

Claims (18)

1. A press section in a paper or board machine in which the transfer of a paper web is accomplished as a close draw through the press section, the press section being provided with at least three Separate nips, the first of which dewatering takes place on both sides, characterized in that the tight drawing of the web through the press section occurs as a substantially straight run completed, and each nip has its own fabric cycle, the nip after the first nip has an absorbent fabric placed at the nip relative to the other nips in the direction of travel of the web in the machine In the nip after the first nip, in a so-called smoothing or leveling nip, the surface of the web on the side opposite to the absorbent fabric is placed on or against A smoothing roll or is smoothed against a conveyor belt. 2. A press section as claimed in claim 1, characterized in that the press section is arranged for use at operating speeds in excess of 1800 m/min. 3. A press section as claimed in claim 1, characterized in that the press section is arranged for use at operating speeds exceeding 2000 m/min. 4. A press section as claimed in claim 1 or 2, characterized in that the first nip is a roll nip or a press suction roll nip. 5. A press section as claimed in claim 1 or 2, characterized in that the first nip is an extension nip or an extension/suction roll nip. 6. A press section as claimed in claim 1 or 2, characterized in that all nips are extended nips. 7. A press section as claimed in any one of claims 1 to 3, characterized in that, after the first nip, the press section comprises an extended nip and two smoothing nips. 8. The press section of claim 1, wherein at least one of the rolls in the preceding two nips is an extended nip roll. 9. A press section as claimed in claim 7, characterized in that at least one of the rolls in the preceding two nips is an extended nip roll. 10. The press section according to claim 7, characterized in that, the nips of the press section are all roll nips. 11. A press section according to any one of claims 1-3, characterized in that one press roll in the smoothing/levelling nip is a press suction roll manufactured by powder metallurgy. 12. The press section according to any one of claims 1-3, characterized in that one press roll in one nip is a press suction roll manufactured by powder metallurgy. 13. A press section as claimed in any one of claims 1-3, characterized in that at least one of the rolls in the nip is a press suction roll. 14. A press section according to any one of claims 1-3, characterized in that at least one nip after the first nip is an extended nip, ie a shoe press nip. 15. A press section according to any one of claims 1-3, characterized in that after the first nip the web is arranged to be transferred from the surface of the upper fabric of the first nip to the subsequent Press down on the fabric/felt. 16. A press section as claimed in any one of claims 1-3, characterized in that, in the second nip, dewatering is downwards and the upper surface of the web rests either against a smooth roll or against a smooth conveyor belt; in the third nip, dewatering is upwards, and the lower surface of the web is attached to a conveyor belt. 17. A press section according to any one of claims 1-3, characterized in that the web is arranged to be transferred as a tight draw from the last lower fabric cycle of the press section to the drying wire of a dryer section . 18. Press section according to any one of claims 1-3, characterized in that, in a smooth nip without fabric provided with a counter-roll, there is a large fan-shaped press suction roll, the press suction roll The fan-shape begins before the nip and continues after the nip, on which the web is arranged to follow a press felt before, while passing through the nip and after passing through the nip.

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