Method in a dryer section provided with single-wire draw and wire group in said dryer section

Abstract: The invention concerns a method and a wire group in a drying section provided with single-wire draw in a paper machine or paper finishing machine. The paper web (W) to be dried is passed on support of the drying wire (11) alternatingly over the heated cylinder faces (10') of the drying cylinders (10) and over the leading rolls (20) or equivalent. Then, on the drying cylinders (10), the paper web (W) is in direct contact against the heated cylinder face (10') and, on the leading rolls (20), the paper web (W) is placed on the outside face of the drying-wire loop (11) at the side of the outside curve. In the invention, the difference in speed (V₁ - V₂) of the paper web (W) which the paper web (W) would have during operation of the drying section on the turning sectors of the drying cylinders (10) and of the leading rolls (20) is reduced by means of dimensioning of the structure of the drying wire (11) in the direction of thickness (t) and by means of choice of materials.

Description: The invention concerns a method in a dryer section provided with single-wire draw in a paper machine or a paper finishing machine, wherein the paper web to be dried is passed on support of the drying wire alternatingly over the heated cylinder faces of the drying cylinders and over leading rolls or equivalent so that, on said drying cylinders, the paper web is in direct contact against the heated cylinder face and, on the leading rolls, the paper web is placed on the outside face of the drying-wire loop at the side of the outside curve (FI-A-83 680). Further, the invention concerns a wire group in a multi-cylinder dryer in a paper machine, comprising one or several smooth-faced and solid-mantle heated drying cylinders and leading rolls or equivalent drying cylinders placed at their proximity, said wire group comprising a drying-wire loop guided by guide rolls, which is arranged to be curved over said drying cylinders and leading rolls so that the drying cylinders are placed outside the drying-wire loop and the leading rolls are placed inside the drying-wire loop, and which drying wire is fitted to press the paper web to be dried against the heated cylinder faces of the drying cylinders, while the paper web is fitted to curve on the outside face of said drying wire over the leading rolls when the paper web runs from the preceding drying cylinder onto the following drying cylinder. In multi-cylinder dryers of a paper machine, for the passing of the paper web, either so-called twin-wire draw and/or single-wire draw is/are used. In twin-wire draw, the heated drying cylinders are arranged in two horizontal rows placed one above the other, the successive cylinders in said rows being placed as interlocked in the upper and lower row. In such a case, in each cylinder group, there are two drying wires, a so-called upper wire and a lower wire, by whose means the paper web is pressed against heated drying-cylinder faces, being guided by guide rolls placed in the gaps between the cylinders. In twin-wire draw, the web has usually free, unsupported draws as its runs between the rows of cylinders. Recently, in dryers, single-wire draw has become common, wherein one drying wire only is employed in a cylinder group, the paper web running through the whole group on support of said wire. Earlier, in single-wire draw, two rows of drying cylinders were employed commonly, one row placed above the other, but at present, only one row of drying cylinders is employed, in which case the other row consists of unheated leading rolls. Said cylinders, leading rolls, and the drying wire are arranged so that the drying wire presses the web to be dried against the cylinder face and, on the leading rolls, the web is placed at the side of the outside curve. The leading rolls are placed inside the drying-wire loop. In dryer sections provided with single-wire draw, the leading rolls are commonly suction rolls, e.g. suction rolls provided with a grooved outer mantle, marketed by the applicant under the trade mark "VA CROLL", by means of whose suction effect the staying of the paper web on the outer face of the drying wire on the reversing sectors of the suction-leading rolls is promoted. Compared with twin-wire draw, it is a substantial advantage of single-wire draw that free, unsupported draws of the paper web can be avoided. This is why single-wire draw is employed commonly in particular near the forward end of the dryer section, where the web has a higher moisture content and is, thus, of lower strength and more susceptible of breaks and stretching. Recently, such dryer sections have also become more common in which exclusively groups with single-wire draw are employed. In such a case, it is possible to use either so-called normal groups, in which the drying cylinders are placed in the upper row and the leading rolls in the lower row, or it is additionally possible to employ some so-called inverted groups, in which the cylinders and said rolls are placed one above the other in the reversed sequence. In a typical dryer section provided with single-wire draw alone, there are, e.g., seven successive wire groups. In dryer sections provided with single-wire draw, above all in their wire groups towards the rear end, glazing and wear stronger than average has been noticed in the face of the drying wire placed at the side of the paper. It has been noticed that the reason for this is the difference in speed between the wire and the paper web arising in single-wire draw, said difference in speed always arising in situations in which the paper web, as it runs on the face of the same drying wire or felt, is alternately placed now outside and now between the wire and the roll. This mode of formation of the difference in speed will be described in more detail later with reference to the figures in the accompanying drawing. The difference in speed between the paper and the wire produces a sort of "grinding" between them, which grinding abrades the surface portion of the wire placed at the side of the paper, and may also produce detrimental changes in the quality properties of the paper face. In the initial end of the dryer section, where the paper web has a higher moisture content and is therefore more elastic, the deformations of the paper web are usually mostly capable of compensating for said difference in speed between the wire and the paper. When the paper runs further in the dryer section and becomes dry, its elasticity becomes lower at the same time as the paper web shrinks to some extent. For example, in the dryer section mentioned above, provided with seven single-wire groups, the dry solids content at the beginning of group 6 is of an order of 65...70 % and at the end of the dryer section 90...98 %. In said range of dry solids content of the paper, its elasticity is no longer sufficient to compensate for the above drawbacks arising from the differences in speed between the drying wire and the paper web. Said problems, which are related to the wear of the wire considerably higher than average, usually increase steeply and become a significant drawback when the dry solids content of the paper web is higher than about 60...65 %. The differences in speed between the wire and the paper web may also cause problems of other sorts, such as dust formation in the paper and, with some grades, also other detrimental factors, mainly related to the quality of the paper. Also, the tightening of a relatively dry web inside a wire group causes problems of runnability, e.g., in the form of web breaks. As will come out from the formulae of calculation given below and from the exemplifying illustrations, it is possible to reduce the difference in speed between the paper to be dried and the supporting drying wire by making the drying wire thinner, which has also been experimented with in some places to reduce the problems. In the manufacture of a sufficiently thin drying wire, considerable difficulties are, however, encountered, for the manufacture and operation of a wire of a thickness of, for example, 1.1 mm is already "fine art", whereas the manufacture and operation of a drying wire of a thickness of 1.5 mm is fully conventional. The object of the present invention is to provide novel solutions for the problems discussed above. The object of the present invention is to provide said solutions both by means of process-technical operations, by means of a solution related to the construction of the dryer section, and by means of solutions related to the construction of drying wires intended for single-wire draw. For the sake of simplicity, above and in the following, exclusively drying wires have been and will be spoken of, but this term is to be understood generally as referring to all drying fabrics, including such drying fabrics whose structure is similar to a felt. In view of achieving the objectives stated above and those that will come out later, the method of the invention is mainly characterized in that the difference in speed of the paper web which the paper web would have during operation of the dryer section on the turning sectors of said drying cylinders and of said leading rolls and which difference in speed arises from the variations in the speed of the paper-side face of the drying wire between said drying cylinders and leading rolls is reduced by means of dimensioning of the structure of the drying wire in the direction of thickness and by means of choice of materials, said dimensioning being such that the drying wire is conferred an asymmetric structure. On the other hand, the dryer section in accordance with the invention is mainly characterized in that said drying wire has an asymmetric structure in the direction of its thickness, in which structure the plane of constant speed, i.e. the neutral plane of deflection, is arranged to be placed, in the direction of thickness of the drying wire, between the face of the wire that is placed at the side of the paper and the centre plane of the wire. The problems that are eliminated by means of the invention and the advantages offered by the invention are manifested with particular emphasis in modern high-speed paper machines whose running speeds are or will be of an order of 900...1800 metres per minute, or even higher. The invention can be applied either to the manufacture of new paper machines or paper finishing machines or to improvement of the operation of existing dryer sections. Even though, in the following, the invention has been described mainly with reference to horizontal multi-cylinder dryers of paper machines, the invention can also be applied in various paper finishing machines, such as coating devices or size presses, for example in their intermediate dryers. The invention can also be applied to multi-cylinder dryers in which the drying cylinders and leading rolls in the cylinder groups are placed in vertical rows or in inclined rows. It should be emphasized that, even though, in the following, the invention is described expressly with reference to a construction in which unheated leading rolls, preferably suction-leading rolls, are used in addition to heated drying cylinders, the scope of the invention also includes such older dryer sections as have corresponding drying cylinders in stead of leading rolls. The scope of the invention also includes dryer sections in which cold cylinders are also used in the rows placed one above the other. Said cold cylinders are used above all in paper coating machines. In prior art, some such drying wires are known whose cross-section in the direction of thickness is asymmetric, but this asymmetry and the related constructional solutions of the drying wire have been dictated by factors other than the reduction and minimization of said differences in speed in single-wire draw in accordance with the present invention. With respect to asymmetric constructions of wire and felt, reference is made, by way of example, to the following patents: US-4,186,780, SE-227,396, and SE-429,769. It is expressly the maintenance of the speed of the paper-side face of the wire as constant as possible that is also the aim of the invention. In theory, a constant speed of the paper-side face is the ideal state, but, in practice, it is not fully possible to reach this state. In theory, the speeds of the web and of the neutral plane (= constant-speed plane) of the wire are equal - what goes in will also come out. In practice, shrinkage of the web inside the group, also in the machine direction, produces a minimal difference in speed between the web and the drying wire. The web speed may also become equal to the speed of the paper-side face of the wire, in which case, depending on the holding forces, the web speed is either the speed of the paper-side face of the wire on a leading-suction roll or on a drying cylinder - possibly even something in between. In the following, the invention and its theoretical background will be described in detail with reference to the figures in the accompanying drawing and to preferred exemplifying embodiments of the invention illustrated in them. Figure 1 is a schematic side view of a dryer section which is provided with single-wire draw over its entire length. Figure 2 is a side view of a part of a wire group with single-wire draw. Figure 3 is a schematic illustration of parameters essential in view of the invention regarding the thickness of the web and of the wire on a highly exaggerated scale. Figure 4 is a sectional illustration of principle of an asymmetric drying wire that is employed in the invention. Figure 5 is a sectional view in the machine direction of an asymmetric construction of a drying wire that is employed in the invention. Figure 6 is a column diagram illustration of paper speeds on a leading-suction roll and on a drying cylinder in single-wire draw and of differences of said speeds with wires of different thicknesses and different asymmetries. Figure 7 illustrates the speeds of the drying cylinders and of the adjacent leading-suction rolls in different wire groups in a dryer section provided with seven single-wire groups and similar to that illustrated in Fig. 1 as well as a simulated development of the percentage of dry solids content (k-a %) of the web. According to Fig. 1, the paper web to be dried is passed from the press section of the paper machine to the multi-cylinder dryer at the arrow W in . The dryer section comprises seven successive wire groups R 1 ...R 7 . From the last wire group R 7 , the dried paper web W is removed in the direction of the arrow W out to reeling or finishing. At this point, the dry solids content of the web W is 90...98 %, whereas it was of an order of 35...40 % on its arrival at the multi-cylinder dryer. The dryer section shown in Figs. 1 and 2 consists of heated drying cylinders 10, which have a solid and smooth outer mantle 10', against which the web W to be dried is pressed by means of the drying wire 11. As is shown in Figs. 1 and 2, the drying cylinders 10 are placed outside the loop of the drying wire 11, which is guided by guide rolls 24. In the gaps between adjacent cylinders 10, there are leading-suction rolls 20 fitted inside the loop of the drying wire 11, preferably the applicant's "VA CROLL" rolls, whose construction comes out, e.g., from the applicant's FI Patents Nos. 82,849 and 83,680. According to Fig. 1, the wires of the different wire groups R 1 ...R 7 are denoted with the reference numerals 11 1 ...11 7 . Between the wire groups 11 1 ...11 7 , the web has a closed draw. The wire groups R 1 ...R 5 and R 7 are so-called normal groups, in which the drying cylinders 10 are placed in the upper row and the leading rolls 20 in the lower row. The group R 6 is an inverted group, in which the drying cylinders 10 are placed in the lower row and the leading rolls in the upper row. In Fig. 1, the reference numerals 27 denote the suction ducts of the leading-suction rolls 20, which ducts communicate with a vacuum pump in itself known. The leading rolls 20 are provided with a perforated and grooved mantle 20', through which a negative pressure acts from the interior of the roll 20. This promotes the keeping of the web W on the turning sectors of the leading rolls 20 while the web is at the side of the outside curve with no support outside. Against the lower faces of the drying cylinders 10, doctors 22 are fitted to operate, whose blades 23 keep the cylinder faces 10' clean. On the joint straight runs of the drying wire 11 and the web W from the drying cylinder 10 to the leading roll, blow boxes 21 are arranged to act, by means of which boxes attempts are made to prevent formation of differences in pressure in the opening nip spaces N- and in the closing nip spaces N+, which differences attempt to separate the web W from the wire 11 detrimentally. The dryer section illustrated in Figs. 1 and 2 and described above is in itself known, and it is described in this connection just as a background for the invention and as a typical and preferred environment of application. In the following, mainly with reference to Fig. 3 and to its denotations, the theoretical background of the invention will be described. The denotations in Fig. 3 are as follows: D = diameter of drying cylinders 10 d = diameter of leading-suction rolls 20 A = thickness of drying wire e = thickness of paper web W N = percentage of asymmetry of drying wire t 1 = distance of the constant-speed plane, i.e. of the neutral plane T-T during bending, of the drying wire 11 from the paper-W-side face 12 of the drying wire 11 t 2 = distance of said wire plane T-T from the face 13 of the drying wire 11 placed at the side of the mantle of the leading-suction roll 20' V 1 = speed of paper web W on drying cylinder 10 V 2 = speed of paper web W on leading roll 20 V n = speed of neutral plane T-T of drying wire V s = speed of cylinder face 10' of drying cylinder 10 V v = speed of cylinder face 20' of leading roll 20 Z = distance of neutral plane T-T from inner face of wire loop Between the quantities listed above, the following equations can be derived: Z = (N %) * A t1 = (1 - N) * A t2 = N * A = Z Vn = Vv(d+2Na)/d Vn = Vs(D+2e+2A(1-N))/D V1 = Vn(D+e)/(D+2A(1-N)+2E) V2 = Vn(d+2A+e)/(d+2NA) V2 - V1 = Vn ( d +2 A + e d +2 NA - D + e D +2 A (1- N )+2 e ) V2 - Vs = Vn ( d +2 A + e d +2 NA - D D +2 A (1- N )+2 e ) Vs - Vv = V n ( D D +2 e +2 A (1- N ) - d d +2 NA ) In the following description, when the location of the neutral plane of the wire and its asymmetry in accordance with the invention are defined, a percentage of asymmetry N is used, which is defined as N = distance of neutral plane from the face of the bottom side (non-paper side) of the wire as a percentage of the thickness A of the wire. Thus, in a fully symmetric wire, N = 50 %, and in the state aimed at in the invention, N = 100 %. The thickness A is preferably dimensioned to be in a range of A=0,3... 5mm, more preferably A = 0,5... 2,5mm. From the point of view of the invention, it would be an ideal situation that the neutral plane of bending of the wire, i.e. the constant-speed plane, were in the plane of the face of the wire that is placed against the paper, and the further away one comes from that plane, the worse does the situation become. In practice, a value of percentage of asymmetry, mentioned above, can be reached that is N = 90 % and even higher. The aims of the invention are always approached when the percentage of asymmetry is N > 50 %, and the advantage that is obtained depends on the thickness of the wire that is used. In practice, however, the advantages provided by the invention are already manifested significantly when N > 60 %. In the formulae given above, for the sake of simplicity, the percentage of asymmetry N is given as a decimal number, so that N = 50 % -> N = 0.5, and, for example, N = 80 % -> N = 0.8. In the following, an example of calculation will be given with the equations (1) to (10) given above while assuming the following typical starting values: D = 1.83 m d = 1.5 m A = 1.5 mm e = 0.1 mm t1 = t2 Thus, above, it has been assumed that the drying wire 11 has a symmetric structure (t 1 = t 2 ) and the paper web W is fully elastic in the machine direction. With the values given above, theoretically, it is possible to calculate the following differences in speed: V2 - V1 = 1.9 m/min. V2 - Vs = 2.0 m/min. Vs - Vv = 0.07 m/min The speeds V s and V v can be measured highly accurately. The theory given above is very well in agreement with practical measurements in the groups 1 to 6 in the dryer section described above in relation to Fig. 1, and also in group 7 when the paper web W is not on. Fig. 4 is a schematic sectional view of an asymmetric drying wire 11 applied in the invention, whose construction is asymmetric expressly so that its constant-speed plane, i.e. neutral plane T-T of bending, is, in the direction of thickness of the wire, between the paper-side face 12 of the wire 11 and the centre plane K-K of the wire. In Fig. 4, the wire 11 is shown schematically as composed of two layers 14 and 15, of which the outer face of the layer 14 forms the face 12 of the drying wire 11 that is placed against the paper W, whereas the outer face of the other layer 15 forms the face 13 that is placed against the cylinder faces 20' on the leading cylinders 20. The layer 14 is substantially thinner than the layer 15, and the modulus of elasticity of the layer 14 is substantially higher than the corresponding modulus of elasticity of the layer 15. Thus, the neutral plane T-T of bending can be defined as the plane in which, when bending in a non-prestressed state, a pressing strain is converted to tensile strain in the direction of thickness of the wire 11. In the invention, the percentage of asymmetry N defined above in the equation (1) is N > 50 %, as a rule N = 60...99 %, preferably N = 70...95 %. An asymmetric drying wire 11 in accordance with the invention has, of course, the other properties suitable for a drying wire 11, such as the properties of smoothness and adhesion of the face 12 placed against the web W, and permeability, which is typically in a range of 1000...2000 m 3 /hm 2 (cubic metres per hour per square metre). Normally, the rear face 13 of an asymmetric wire in accordance with the invention tends to become coarser, so that it carries an increased amount of air along with it into the closing nip spaces N+ between the drying wire 11 and the leading-suction roll 20. For this purpose, the above blow boxes 21 illustrated in Fig. 2 are needed, which boxes are marketed by the applicant under the trade mark "UNO RUN BLOW BOX". According to the present-day knowledge, as a rule, no other particular requirements have to be imposed on an asymmetric wire 11 in accordance with the invention. Fig. 5 is a sectional view of an asymmetric wire applied in the invention, in which wire the neutral plane T-T is, thus, at the side of the paper-side face 12 of the wire 11. The wire 11 consists of three integrated layers of wefts 14a, 18a and 18b. The wefts 14a in the face 12 at the side of the paper W are interconnected by means of warps 17 so that the modulus of elasticity of the layer 14a is substantially higher than the modulus of elasticity of the layers 18a and 18b. In the layers 18a and 18b, there are relatively "loose" warps 19. For the warps and wefts, it is possible to use suitable plastic and/or metal materials so that an asymmetric wire unit in accordance with the invention is obtained which has properties also in other respects suitable for a drying wire of single-wire draw. Fig. 6 is a column diagram that illustrates theoretical calculated speeds of paper in single-wire draw when the circumferential speed of the suction roll 20 is V v = 1000 m/min, the diameter of the drying cylinder is D = 1830 mm, and the diameter of the leading-suction roll 20 is d = 1500 mm. The vertical axis of Fig. 6 represents the running speed of the paper W as metres per minute (m/min). The first group of columns represents the speed V 2 of the paper on the leading-suction roll 20 with different thicknesses of the drying wire: A = 1.1 mm, A = 1.35 mm, A = 1.5 mm, and with three different percentages of asymmetry: N = 50 %, N = 75 %, and N = 85 %. A corresponding series of five columns is shown concerning the speeds V, on the dryer cylinder 10, and at the right side of the column diagram, the differences in speed V 2 - V, corresponding to said speeds V 2 and V, are given. From the latter data, it is noticed that, when the asymmetry of a 1.5 mm thick wire 11 is increased from the value N = 50 % to the value N = 85 %, the difference in speed V 2 - V 1 is lowered from 1.9 per mille to 0.65 per mille. In a corresponding way, when the asymmetry of a 1.1 mm thick wire 11 is increased from 50 % to 75 %, the difference in speed V 2 - V 1 is lowered from 1.45 per mille to 0.8 per mille. Fig. 7 illustrates the measured circumferential speeds V s of the dryer cylinders 10 and the corresponding circumferential speeds V v of the leading-suction cylinders 20 (black column = V s , and diagonally shaded column = V v ). In the groups R 1 ...R 6 , no essential difference in speed V s - V v can be noticed, because in those groups the paper web is sufficiently elastic. In Fig. 7, the speeds in the group R 7 behave in contradiction with the presented theory, because, in group R 7 , on the freely revolving drying cylinders 10, speeds occur that are higher than theory, which is an indication of the very problems arising from differences in speed that are eliminated by means of the invention. In Fig. 7, the simulated dry solids content K as a percentage is indicated by the dashed line (right vertical axis). The problems that are supposed to be eliminated by means of the invention begin after the dry solids content has reached the value K ≈ 60...65 %, and with dry solids contents higher than this, the advantages of the use of an asymmetric wire 11 in accordance with the invention are manifested with particular emphasis. TEST EXAMPLE: By means of speed measurements with the 1st dryer group of the applicant's test paper machine at Rautpohja, Finland, attempts were made to find out whether it is possible to reduce said detrimental difference in speed between the wire and the paper web by making use of an asymmetric structure of the drying wire. In the measurements, such a wire of twin-wire draw was employed as had an asymmetric structure and as could be assumed to operate in an asymmetric way. Herein, by means of asymmetry, a situation was aimed at in which the speed of the paper-side face of the wire remains as invariable as possible in the dryer group. The asymmetric test wire was run with the normal paper side of the wire in the wire loop both outwards and inwards in order to confirm the results. With a drying wire normally used in the test machine, reference measurements were carried out additionally. In the measurements, the percentages of asymmetry of the test wire were noticed to be N = 31...38 % with the wire run one way, and N = 71...76 % with the wire run the other way. With the reference wire, the values in normal operation were N = 43...50 %, i.e. the reference wire was substantially symmetric. Said value of asymmetry represents the distance of the constant-speed line of the wire, i.e. of the neutral plane of bending, from the face of the wire placed at the side of the suction-leading roll, so that the whole thickness of the wire is represented by the percentage number 100 %. The results described above indicate that it is possible to affect the differences in speed between the wire face and the paper web by means of asymmetry of the structure of the drying wire. With a normal wire (thickness A = 1.5 mm), the theoretical difference in speed between the wire face and the paper web in a normal "Sym-Run" draw is about 2 per mille. With a wire whose asymmetry is N = 72 %, said difference in speed would be 1.1 per mille, which means a noticeable reduction in the wear of the drying wire. An asymmetric wire in accordance with the invention that is best suitable for single-wire draw has preferably an asymmetry higher than that of the wire described and tested above, the value of asymmetry of N = 85 % of said wire permitting a speed difference between paper and wire as little as 0.6 per mille (A = 1.5 mm). In the following, the patent claims will be given, and the various details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from those stated above for the sake of example only.

Claims: Method in a dryer section provided with single-wire draw in a paper machine or paper finishing machine, wherein the paper web (W) to be dried is passed on support of the drying wire (11) alternatingly over the heated cylinder faces (10') of the drying cylinders (10) and over leading rolls (20) or equivalent so that, on said drying cylinders (10), the paper web (W) is in direct contact against the heated cylinder face (10') and, on the leading rolls (20), the paper web (W) is placed on the outside face of the drying-wire loop (11) at the side of the outside curve, characterized in that the difference in speed (v 1 - V 2 ) of the paper web (W) which the paper web (W) would have during operation of the dryer section on the turning sectors of said drying cylinders (10) and of said leading rolls (20) and which difference in speed (V 1 - V 2 ) arises from the variations in the speed of the paper-side face (12) of the drying wire (11) between said drying cylinders (10) and leading rolls (20) is reduced by means of dimensioning of the structure of the drying wire (11) in the direction of thickness (t) and by means of choice of materials, said dimensioning being such that the drying wire is conferred an asymmetric structure. Method as claimed in claim 1, characterized in that the structure of the drying wire (11) in the direction of thickness (t) is chosen as in such a way asymmetric that the constant-speed plane of the drying wire (11), i.e. the neutral plane (T-T) of bending, is placed in the area between the paper-side face (12) of the drying wire (11), i.e. the outer face of the wire loop, and the centre plane (K-K) of the drying wire (11). Method as claimed in claim 2, characterized in that the percentage of asymmetry (N) of the drying wire (11) in the direction of thickness (t), which percentage is defined as (N %) = Z/A, wherein Z is the distance of the neutral plane (T-T) of bending of the drying wire (11) from the inner face (13) of the wire loop (11), and A = thickness of the drying wire, is chosen in the range of N = 60 % ... 99 %, preferably N = 70 % ... 95 %. Method as claimed in any of the claims 1 to 3, characterized in that, in the method, said difference in speed (V 1 - V 2 ) is lowered to a range of about 0.1...1.5 per mille, preferably to a range of 0.5...1.0 per mille, of the circumferential speed (V v ) of the leading roll (20). Method as claimed in any of the claims 1 to 4, characterized in that the method is applied in a multi-cylinder dryer of a paper machine at least in the wire groups (R 6 ,R 7 ) in whose area the dry solids content of the paper web (W) has reached the value of K > 60...65 %. Wire group (R n ) in a multi-cylinder dryer in a paper machine, comprising one or several smooth-faced and solid-mantle heated drying cylinders (10) and leading rolls (20) or equivalent drying cylinders placed at their proximity, said wire group (R n ) comprising a drying-wire loop (11) guided by guide rolls (24), which is arranged to be curved over said drying cylinders (10) and leading rolls (20) so that the drying cylinders (10) are placed outside the drying-wire loop (11) and the leading rolls (20) are placed inside the drying-wire loop (11), and which drying wire (11) is fitted to press the paper web (W) to be dried against the heated cylinder faces (10') of the drying cylinders (10), while the paper web (W) is fitted to curve on the outside face of said drying wire (11) over the leading rolls (20) when the paper web (W) runs from the preceding drying cylinder (10) onto the following drying cylinder (10), characterized in that said drying wire (11) has an asymmetric structure in the direction of its thickness, in which structure the plane of constant speed, i.e. the neutral plane (T-T) of deflection, is arranged to be placed, in the direction of thickness (t) of the drying wire, between the face (12) of the wire (11) that is placed at the side of the paper and the centre plane (K-K) of the wire (11). Wire group as claimed in claim 6, characterized in that the percentage of asymmetry (N) of the drying wire (11) in the direction of thickness (T), which percentage is defined as (N %) = Z/A, wherein Z = distance of the neutral plane (T-T) of bending of the drying wire (11) from the inner face (13) of the wire loop, and A = thickness of the drying wire, is in a range of N = 60 % ... 99 %, preferably in a range of N = 70 % ... 95 %. Wire group as claimed in claim 6 or 7, characterized in that the thickness (A) of the drying wire (11) has been dimensioned to be in a range of A = 0.3...5 mm, preferably A = 0.5...2.5 mm. Wire group as claimed in any of the claims 6 to 8, characterized in that one or several of said wire groups (R n ) have been fitted one after the other towards the final end of the multi-cylinder dryer of the paper machine at least in the area in which the dry solids content of the paper web (W) to be dried is K > 60 % ... 65 %.

Inventors: KUHASALO ANTTI, KUHASALO, ANTTI
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1993
Main classification: D21F5
IPC codes: D21F5, D21F1