FI92941B - Method and apparatus in the press portion of a paper machine for detaching the web from the surface of a press roll - Google Patents

Abstract

Method and device in the press section of a paper machine for detaching the web (W) from a smooth mantle face (18') of a press roll (18). In the method and in the device, a transfer zone (40a; 50a) is employed, over which a press fabric (23) or a particular transfer fabric (44) is passed so as to accomplish a closed draw of the web (W). The transfer zone (40a, 50a) as well as said fabric (23; 44) form a transfer nip (NS) or a transfer zone with the roll face (18'). At the transfer point, the temperature level (T) of the roll face (18') and/or of the web (W) is set or regulated so high that the pressure (pS) of saturated vapour of the water present in connection with the web (W) and with the roll face (18') which corresponds to said temperature level (T) is substantially equally high as, or just little higher than, the pressure (pO) that has been set to prevail in said transfer zone (40a, 50a), said pressure (pO) being preferably lower than the atmospheric pressure.

Description

92941

Method and apparatus for removing a web from the surface of a press roll with a press section of a paper machine Förfarande och anordning i presspartiet av en pappersmaskin för att lösgöra banan 5 frän ytan av en pressvals

The invention relates to a method for detaching a web from the smooth jacket surface of a press roll 10 by a press section of a paper machine, the method using a transfer zone through which a press fabric or a special transfer fabric is passed to effect closed export of the web. setting or adjusting the thermal space level of said roll surface and / or web.

15

The invention further relates to a press section of a paper machine comprising a press roll assembly forming one or more press nips, the web of which is detached from the smooth surface roll of the press section by a roll or similar transfer member and further transferred to a dryer section or a press roll the suction transfer box is arranged to form, in connection with a smooth roll surface, a transfer nip or transfer zone for carrying out the closed export of the web, by means of which a vacuum is applied to the web, which tends to release it from the roll surface.

Various compact and closed press parts are known in the art, which have a press roll combination, the press rolls of which generally form at least three press nips with one another. Of these known press parts, mention should be made of the applicant's "Sym-Press Π" press part and the "Sym-Press 0" press part ("" = trademarks).

In the "Sym-Press II" press section, a first two-felt nip is formed between a hollow-surface lower press roll and an upper press suction roll. Said suction roll forms a second nip with a smooth-surfaced central roll of the press, in connection with which a third press nip is formed 92941 2. This press section may further comprise a separate nip, usually a fourth nip of the press section, after which the web is transferred to the drying section.

In the "Sym-Press 0" press section, a first two-felt horizontal nip is formed between the lower hollow surface roll S and the upper press nip roll, after which the web follows the upper fabric in a vertical run to a second nip formed between the first nip rolls and the smooth surface center roll. A third felt press nip with a felt-faced press roll is formed in connection with said central roll.

10

As is already known, the web is detached using its traction force in the direction of its flow as a free unsupported pull from the smooth surface of the central roll of the press. This free pull is quite critical to the operation of the paper machine. Ko. in free traction, a speed difference is used to provide the release force, which stretches the web, resulting in certain disadvantages. In addition, said unsupported pull forms a problem area in the paper machine prone to breakage.

The prior art has not provided effective means of controlling the detachment of the web from the smooth-surfaced center roll and the subsequent free tension. The unfavorable properties of the central roll material-20 Iina commonly used granite have also contributed to limiting the control of web detachment and free tension.

As the speeds of paper machines continue to rise, the free tension of the web has become an even bigger problem. The speeds of paper machines are planned to be as high as about 2000 m / min in the near future. The open export of the web from the smooth-faced roll of the press forms or will form a bottleneck, which in turn prevents the speeds of the paper machine from being increased. Another problem is that the paper machine often produces different grades of paper with different adhesion to the smooth surface of the center roll, resulting in variations in the required web release tension.

3 92941 In the "Sym-Press ΙΓ press section, the surface properties of the center roll in the second and third press nips should be such that the moist web adheres as well as possible to the roll surface. On the other hand, the web should be easily detached from the roll surface for export to the drying section. means.

In the past, some transfer suction rollers have been disclosed for forming a transfer nip against a smooth surface of the press portion. A common feature of these known transfer rollers is that they do not work as intended in practice. This is due to the fact that the paper web has not been able to be detached by known transfer suction rolls by suction from the center roll, because the web is tightly and usually still adhered to the smooth surface of the center roll at the transfer point by means of a water film. Thus, in the suction zone of the transfer suction roll, the vacuum applied to the web causes a corresponding vacuum between the web and the smooth surface of the center roll, with the result that the web does not come off but follows the smooth surface of the center roll. In view of the above, it is sufficient to consider it almost natural that the web cannot be removed from a smooth roll surface by vacuuming.

With regard to the state of the art related to the invention, reference is also made to the applicant's FI patent applications 870308 and 870309.

20

Em. FI application 870308 discloses a method and apparatus for detaching a web with innumerable tension from a smooth press roll, in which method the temperature of said press roll is controlled and this adjustment affects the adhesion between the roll surface and the detachable paper web and thereby sets the detachment angle and / or detachment tension. 1

Em. FI application 870309 discloses a method and apparatus in which a web is detached from a smooth roll surface in innumerable tension and an instantaneous and local induction heating effect is applied to the web outside the smooth surface press roll. roll face.

92941 4

In the following, the typical stresses occurring in the free tension of the web are considered in the light of one example.

The free tension of any web-like material creates a stress of 5 F = ρ · ν2 where F = stress [N / m] p = basis weight [kg / m2] 10 v = velocity [m / s]

Since the track has a certain limited breaking strength, the limit speed formula is given as 15% v = -

P

Typically, the newspaper has a wet breaking strength of FM «200 N / m and a basis weight of 20 42 g / m2, respectively, and a moisture in the first free space of the press of <42%.

-1 p = 0.1 kg / m 200 m m 25 - flow 4 - = 44.7 - 0.1 s s

On the other hand, typical strength values are average, but the track breaks at the weakest: 30 points. If the track has a random, local strength of 50 N / m, I '- \ 56 m m is obtained

Vraja = 1 - = W -

1 0.1 S S

35 5 92941

The speed limit shown above is currently the typical speed used in paper machines, which is why the track occasionally breaks in that open traction.

If the speed of the paper machine is to be increased, the web must be made generally stronger or S more homogeneous (no weak points) or free gaps must be eliminated. The present invention therefore solves these problems by eliminating free gaps.

It is a general object of the present invention to provide novel means for detaching and transferring a web from a center roll or the like of a press section to a separate nip-silicon or drying section.

The object of the present invention is to provide such a method and device for removing a web from a smooth-surfaced press roll, in which it is not necessary to stretch the web, i.e. it is not necessary to use the so-called traction and speed difference. It is therefore an object of the invention to provide such a press part in which, if necessary, a completely closed export can be used for transferring the web from the central roll of the press or a similar other roll to the drying section in general on its drying wire or separate nip press fabric.

An important object of said purposes is to make it possible to increase the speed of the paper machine 20 so that the degree of operation of the machine can be kept high.

In order to achieve the above and later objects, the method of the invention is mainly characterized in that the transfer zone is depressurized and the temperature level of said roll surface and / or web is set or adjusted so that the corresponding saturated vapor pressure of the water in connection with the web and roll surface is almost equal. high or higher than the pressure set in the said transfer zone below the ambient pressure.

The device according to the invention, in turn, is mainly characterized in that the device comprises a heating device or devices by means of which the temperature level of the web entering the transfer point and / or said roll surface can be raised so that no harmful vacuum is prevented at the transfer point.

92941 6

In the following, the invention will be described in detail with reference to some application examples of the invention shown in the figures of the accompanying drawing, to the details of which the invention is in no way narrowly limited.

Fig. 1 is a schematic side view of an application environment of the invention and a first embodiment thereof in which a press fabric of a third nip is used as a transfer fabric in the transfer of a center roll.

Fig. 2 shows, in a manner similar to Fig. 1, an embodiment of the invention in which a separate transfer fabric running around the transfer suction roll is used.

Figure 3 shows a transfer nip according to the invention implemented with a transfer coil in vertical section in the machine direction.

Figure 4 shows an embodiment of the invention in which the suction transfer takes place by means of a static, two-part transfer suction box.

Figures 1 and 2 schematically show the press section of a paper machine in which the paper web W coming from the former forming wire 10 is dried by pressing. With the pick-up felt 12, the web W is transferred to a first dewatering nip Nj formed between the press suction roll 16 and the lower press roll 14 provided with a hollow surface 14 '. The first nip Nj is double-felt and has a lower felt 13 guided by guide rollers 15.

25

In Figures 1 and 2, thanks to the suction zone 16a of the suction roll 16, the web W follows the nip Nj. then a felt 12 to a second nip N2 formed between said suction roll 16 and a smooth surface 18 'center roll 18. Between the nips N1 and N2 there is a steam box 17 directly heating the web W. A third nip N3 of the press is formed between said central roll 18 and a press roll 21 provided with a hollow surface 2Γ. A press felt 23 guided by the guide rollers 22 passes through the third nip N3. The sector 7 92941 below the central roll 18 has a formula 19 which keeps the smooth surface 18 'of the central roll 18 clean and removes the wrecked web from the surface 18'.

In Figures 1 and 2, the press rolls 14, 16, 18 and 21 form a compact roll combination, S with three successive nips Nj, N2, N3, through which the web W passes in a closed export all the time supported by the felt or roll surface. A fourth separate nip N4 is formed between the smooth-faced press roll 30 ', the large-diameter press roll 30 and the lower press roll 31 with the hollow surface 31'. After the fourth nip N4, the web W follows a smooth surface 30 'from which it is separated by a guide roll 39 and transferred to a drying section 10 36 of a drying section guided by a guide roll 35. The web W is attached to the drying wire 36 by a suction box 37 and applied by a wire 36 to a multi-cylinder dryer (not shown). The sector above the top press roll 30 of the separate nip N4 has fountains 29 and then a scraper 28 which keeps the roll surface 30 'clean.

As shown in Figure 1, after the fourth nip N4, the press fabric 26 is separated from the web W by a guide roll 38 and returned to the web W in a transfer nip 30/39 over the transfer suction roll 39. In the first suction zone 39a of the transfer suction roll 39, the web W is detached from the roll surface 30 ', and the web W passes over the roll 39 by the press fabric 26 secured by the vacuum of the suction zone 39b. The web W is detached from the fabric 26 by the suction box 37 and transferred to the drying wire 36. Contrary to that shown in Fig. 2, the press fabric 26 is separated from the web W under the guide roll 32b by the roll surface 30 'to the suction zone 39a of the transfer roll 30. A special transfer fabric 46 is passed over the transfer suction roll 39, by means of which the web W is transferred to the drying wire 36 secured by the second suction zone 39b of the roll 39.

25

The suction sector 16a between the first and second nips has a first steam supply box 17. Between the second and third nips N2 and N3, respectively, there is a second steam supply box 20 acting against the web W. The hot steam supplied from the boxes 17 and 20 was applied to the elastic properties of the web W fibers and the surface tension can also be influenced so that the dewatering in the nips N2 and N3 is more efficient. According to the invention, the temperature T of the surface 18 'of the web W and the central roll 18 is raised to such a level that in the transfer nip Ns the removal and transfer of the web W from the roll surface 18' is successful. The temperature level T of the web W is raised by internal heating of the central roll 18, as exemplified by the axial bores 18b made in the roll shell 18a in Fig. 4, to which a heating medium such as hot water or steam is introduced.

S Instead of or in addition to the internal heating of the roll 18, the roll jacket 18b can be heated by steam boxes 17 and 20, in particular by a steam box 20, and also by various electric heating devices, an induction heating device 60 shown in Fig. 2.

The press structure described above is essentially known in the art and has been described above only as a background and application environment of the present invention.

After the third nip N3, the web W follows the smooth surface 18 'of the central roll 18, from which it is removed as a fully closed export by a transfer roller 40 or a transfer suction box 50 provided in accordance with the invention (Fig. 4). In Fig. 1, in the suction zones 40a, 40b of the transfer roll 40, the web W is transferred in the suction zone 25a of the second lower transfer roll 25 to the press felt 26, which takes the web W on its upper surface to the fourth separate nip N4 of the press section.

According to Fig. 1, the press felt 23 of the third nip N3 is guided after the nip N3 to the guide roll 22a and from there to the transfer nip Ns so that said press felt 23 acts as a transfer fabric running around the transfer roll 40.

According to Fig. 2, a separate transfer fabric 41 is arranged to pass around the transfer suction roll 40, which is introduced into the transfer nip Ns and separated from the transfer suction roll 40 so that the web W: follows the lower fabric 26 of the fourth press nip.

by selecting the surface properties of the fabrics 26 and 41 and possibly by a suction box 33. In connection with the loop of the transfer fabric 41 there are guide rollers and rehabilitation devices known per se, which are not shown in Fig. 2.

92941 9

According to Figures 1 and 2, the transfer of the web W from the smooth surface of the central roll 18 takes place in a completely closed export by the transfer suction roll 40 with the aid of a vacuum acting on the suction zone 40a inside it. After the suction zone 40a, a second suction zone 40b of the transfer suction roll 40 follows, with a lower vacuum level than the zone 40a, which ensures that the web W remains in support of the transfer fabric 23; 41. An important feature of the closed export implementation of the invention is that the smooth surface 18 'and / or web W of the center roll 18 is heated to a temperature level T before the transfer point so that in the suction zone 40a of the suction roll 40 the web W is sucked between the surface 18' and the web W vacuum, but the temperature T corresponds to -10 va vapor pressure ps, which in the present invention is traced in absolute value higher than the absolute pressure p0 of the suction zone (Ps> Po) · The pressure difference p§ - Po provides a force field which presses the paper web W from the roll surface 18 'to the transfer fabric 23; 44.

In the following, the physical background of the IS method according to the invention will be described in more detail, mainly with reference to Figure 3.

Figure 3 shows the environment of the central transfer nip Ns of the central roll 18 and the transfer suction roll 40. The transfer suction roll 40 includes a roll jacket 41 with a perforation 42 and suction boxes inside. In the case of the suction boxes, only the sealing strips 43 of the first suction-zone 40a, which act against the smooth inner surface 41 'of the roll shell 41, are shown in Fig. 3.

The paper web W enters the transfer nip Ns on the smooth surface 18 'of the center roll 18. The paper web W is generally tightly adhered to the surface 18 'via a non-uniform water film. Prior to the transfer nip 25, the temperature T of the surface 18 'and the water film adhering the web W and its surface 18' is raised to such a level that the water between the roll surface 18 'and the web W, partially internal to the web W structure, evaporates as its volume increases in the transfer nip Ns. With the vacuum p0 of the suction zone 40a 40, the web W is sucked towards the transfer tissue 44. When there is water at temperature T between the web W and the surface 18 '30, the maximum vacuum that can be created for this is the saturated steam pressure ps at that temperature T.

»· • · 92941 10

Air begins to flow into the bottom of the opening wedge space A between the dough W after the nip Ns and the roll surface 18 'as soon as the gap formed between the roll surface 18' and the outer surface of the web W is wide enough compared to the speed of the surfaces. When the flow resistance of the paper web W is large enough, a vacuum can be sucked between the paper web W and the transfer fabric 44, the absolute value p0 <p§ · The length of the transfer nip Ns is dimensioned so that a full vacuum can develop between the paper web W and the transfer fabric 44. For this purpose, the outer surface of the sheath 41 of the transfer suction roll 40 may have a soft coating with which the length Ns of the transfer nip can be increased large enough. In Fig. 3, the point or line at which the paper web W detaches from the surface 18 'is indicated by the reference S, because a vacuum less than the saturated water vapor pressure ps corresponding to the temperature T cannot be generated in the opening 10.

The web temperature T of the surface 18 'of the central roll 18 as it enters the transfer nip Ns in the invention is generally in the range T <50-100 ° C, which corresponds to the range ps »0.13-1.0 bar of the saturated water vapor pressure 15 (absolute values). Particularly preferred and practical for the purposes of the invention is a temperature range T <65-75 ° C, which corresponds to a water-vapor pressure range ps <0.25-0.40 bar. Thus, for example, if a temperature T = 65 ° C is used, the corresponding vapor pressure ps <0.25 bar, in which case the absolute pressure p ^ of the suction zone 40a in closed suction must be p0 <0.25 bar, i.e. at least a negative pressure must prevail in the suction zone 20a ( 1.00-0.25) bar = 0.75 bar.

Figure 4 shows an embodiment of the invention in which a static suction box 50 is used instead of a rotating suction roller 40. Inside the suction box 50 are arranged two suction chambers 50a and 50b, the open ends of which are delimited by horizontal sealing strips 53, 25 over which a transfer fabric 44 there is such a negative pressure pQ that in the transfer nip Ns the web W differs as a closed export: from the roll surface 18 '. The second suction zone 50b has a lower vacuum level (usually in the range pj <0.1 bar) to ensure that the web W follows the transfer tissue 44. The suction chambers 50a and 50b are connected via pipes 54a and 54b to the suction connections 55a and 55b of the suction box 50. After the suction box 50, the web W is transferred by the transfer fabric 55 to the transfer nip formed by the rollers 32 and 45, after which the web W separates from the transfer fabric 92941 and moves to the support of the press felt 26 where it is attached by the vacuum of the suction box 33.

Said temperature level T of the web W and the roll surface 18 'entering the transfer nip Ns is adjusted to the invention by internal heating systems 18b, steam boxes 17 and / or 20 inside the roll 18. The internal heating systems of the roll 18 may also include heating media such as steam or water inside the roll shell 18a. In addition to the devices shown above, a heating device before the transfer nip Ns according to Fig. 2, preferably an induction heating device 60, can be used to raise the temperature T of the surface 18 'of the roll-10 to suit the suction transfer according to the invention and the closed export of the web W. An electric power supply unit 61 is connected to the induction heating device 60. The induction heating device 60 and other heating devices can also be monitored, if necessary, to control the transverse temperature profile of the roll surface 18 'and the web W before and in the region of the transfer nip Ns. optimal, usually as uniform as possible, e.g. taking into account the different adhesion properties of different pulp grades on the roll surface 18 '.

A prerequisite for the successful closed transfer according to the invention is that the vacuum in the suction zone of the transfer roll 40 does not act for too long so that the steam generated at the transfer point between the web W and the roll surface 18 'has time to cool the space between the web W and the roll surface 18'.

Although the invention has been described above with reference to an application environment in which a separate nip N4 is used, the invention can also be applied in a press section where there is no separate nip N4 but the web W is transferred as a closed export from the fabric passing over the transfer roll 40 to the drying section 1 and so on The press fabric 26 shown in Fig. 2 corresponds to the drying wire of the first wire group, by means of which the web W is passed as a single wire pass through the first drying group.

Figures 1 and 2 further show a "Sym-Press-ΙΓ press part in which the suction roll 16 forms two nips N1 and N2. The invention can also be applied to other press geometries 92941 12, e.g. in the applicant's" Sym-Press-O "press part, wherein after the press roll 16, the web W has a vertical outlet supported by the fabric 12 and a press press roll is replaced by a separate press roll in connection with the center roll 18.

S It is not always necessary that the vacuum in the suction zone 40a, 50a of the suction roll 40 or the suction box 50 be exactly equal to the vapor pressure corresponding to the temperature T of the roll surface 18 or web W, but in some special situations a lower vacuum level is used. detachment of the web W without its detrimental high tensile stress. This is the case with e.g. the thickest paper grades and / or stronger pulps. If, for example, the temperature T of the detachable web W and the roll surface T <70 ° C, it corresponds to a saturated steam pressure of 0.30 bar, in which case the required vacuum is preferably 1-0.30 bar = 0.7 bar. As stated above, in special cases the invention can operate at an estimated "vacuum level" of about 80%, i.e. the vacuum p0 used in the suction zone 40a; 50a in the example above could be at least in the order of 0.80 * 0.7 bar = 0.56 bar.

Also within the scope of the method of the invention are embodiments other than those described above, in which the temperature level T 'of the surface 18' of the central roll 18 is set to T> 100 ° C, whereby a vacuum zone inside the transfer fabric 20 23; 44 is not necessarily required for web transfer. However, according to the present assessment, the method and apparatus of the invention are most preferably implemented by using specifically a suction zone.

• I

40a, 50a, which is set to a pressure Pq lower than the outside air pressure.

Although the invention has been described above and in the embodiments of the figures with reference to 25 ns. for compact and closed press parts, the invention can also be applied in presses in which separate press nips are applied, between which the web W has, for example, an export supported by the transfer fabric.

The following claims set forth within the scope of the inventive idea defined by the invention, the various details of the invention may vary and differ from those set forth above by way of example only.

Claims (10)

92941 13 A method for detaching a web (W) from a smooth jacket surface (18 ') of a press roll (18) on a press section of a paper machine, the method using a transfer zone 5 (40a; 50a) over which a press fabric (23) or a special transfer fabric ( 44), and which transfer zone (40a, 50a) and said fabric (23; 44) are traced to form a transfer nip (Ns) or transfer zone with said roll surface (18 '), and in which method said roll surface (18') is set or adjusted at the transfer point. and / or the temperature level (T) of the web (W), characterized in that in the method the transfer zone (40a; 50a) is depressurized and the temperature level (T) of said roll surface (18 ') and / or web (W) is set or adjusted so high at the transfer point that the corresponding saturated water vapor pressure (ps) in connection with the web (W) and the roll surface (18 ') is almost equal to or greater than in said transfer zone (40a, 50a) to set a set pressure (Pq) below the outside air pressure. 15 A method according to claim 1, characterized in that the vacuum level (p1) of said transfer zone (40a; 50a) is selected to be at least about 80% of the temperature (T) corresponding to the temperature (T) of said roll surface (18 ') or web to be transferred (W). saturated water vapor pressure (ps). 20 A method according to claim 1 or 2, characterized in that the method uses a transfer roller (40) over which the press fabric (23) or the own transfer fabric (44) of the nip (N3) preceding the press section is passed and that said transfer roller (40) ) is traced to form a transfer nip (Ns) on the smooth roll surface (18 ') from which the web (W) 25 is transferred as a closed export without a substantial draw and speed difference. Method according to claim 1 or 2, characterized in that the method uses a static transfer suction box (50), the suction zone (50a) or zones (50a, 50b) of which are tracked to form a closed transfer area of the web (W) on a smooth roll surface (18 ') connection (Figure 4). 92941 14 Method according to any one of claims 1 to 4, characterized in that the temperature level of said roll surface (18 ') from which the web (W) is removed as a closed export without substantial tension is set or adjusted in the range T <65-75 ° C and that the pressure in said transfer zone Po is adjusted or set in the range Pq «0.25-0.4 5 bar. Method according to one of Claims 1 to 5, characterized in that the web (W) transferred from the smooth roll surface (18 ') by the press fabric (23) or the special transfer fabric (44) is introduced as a closed export to the next transfer nip 10 or the like from which the web (W) transferring the separate nip (N4) of the press section to the press fabric (23) or to the drying wire (36) of the drying section. A press section of a paper machine comprising a press roll assembly (14,16,18,21,30,31) which together form one or more press nips (NrN3), the press part of which is smooth or has a web (18 ') from the roll (18) web (W) is removed by a roller (40) or a similar transfer member (50) and transferred to a drying section or a separate nip (N4) of the press section, in which the rotating suction transfer roller (40) or other similar static suction transfer box (50) is traced to form a smooth roller surface (18 '). ) in connection with a transfer nip (Ns) or a transfer zone for the closed export of the web, by which a vacuum (Pq) tending to release the web (W) 20 from the roll surface (18 ') is applied, characterized in that the device comprises a heating device or devices (20,60) the temperature level (T) of the web entering the transfer point and / or of said roll surface (18 ') can be raised so that no harmful large closed transfer of the web is formed at the transfer point. vacuum. 25 Device according to claim 7, characterized in that the transfer suction device is • a transfer suction roller (40) having a suction zone (40a) at the transfer nip (Ns) and then a second, preferably lower vacuum suction zone (40b) and via said transfer nip (Ns) is tracked to pass the compression fabric (23) or the special transfer fabric (44) of the previous press nip (N3) (Fig. 2). 92941 15 Device according to claim 7 or 8, characterized in that said smooth-surfaced (18 ') roll (18), which is preferably the central roll (18) of a compact and closed press section, has an internal heating arrangement (18b) and / or (18) is provided with a steam box (20) and / or an induction heating device (60) or the like, with which the temperature level (T) of the roll surface (18) can be raised to a level high enough for the closed suction transfer of the web (W). Device according to any one of claims 7 to 9, characterized in that after said transfer roller (40) or static transfer box (40) the web (W) is guided by a fabric (23; 44) to a transfer nip or the like with which the web (W) is transferred to the lower fabric 26 of the separate nip (N4) of the press section or to the drying wire (36) of the drying section of the paper machine or other similar fabric. 15 «92941 16