Suction roll for a paper machine

Abstract: The invention concerns a suction roll (10) for a paper machine, on which the paper web is pressed towards the outer face of the roll mantle. The suction roll (10) is divided, in the axial direction of the roll, into at least three vacuum spaces (A₁,A₂,A₃). In the interior of the suction roll (10), there are at least two partition walls (17a,17b), by means of which the suction space (13) is divided into separate zones of negative pressure. The lateral vacuum spaces (A₁,A₃) in the suction space can be subjected to a higher negative pressure than the vacuum space (A₂) in the middle area of the roll, whereby the profile of negative pressure is formed such that the negative pressure increases across the width of the roll towards the lateral areas. The invention also concerns a method for producing a desired pressure profile for a suction roll.

Description: The invention concerns a suction roll for a paper machine comprising the features of the pre-characterizing clauses of claim 1 and 5. A suction roll comprising these features is known from document US-A-4 483 083. One principal objective in paper manufacture is to achieve properties of paper as uniform as possible. In drying, the paper web shrinks more in the lateral areas than in the middle area. This may even have the consequence that the paper in the lateral areas can be unusable because of its properties. In view of the quality of paper, it would be preferable that the shrinkage of paper is little and uniform. In prior-art solutions, attempts have been made to avoid said problem by increasing the negative pressure in the suction space which extends along the entire length of the suction rolls. When the negative pressure is increased, the web shrinks less in the middle area, but the difference between the lateral areas and the middle area may even become larger. Thus, the result is not desirable. It is an object of the invention to improve the known suction rolls such that the percentage of shrinkage of the paper web in the lateral areas is reduced and such that a favourable effect is applied to the uniformity of said shrinkage. This object is achieved by means of the suction rolls according to claim 1 and according to claim 5. Document WO-A-9 002 840 is state of the art according to Art. 54(3) EPC. This document discloses a suction roll comprising all the features of claim 1 except the features that the stationary bottom wall extends in the direction of the axis of the roll towards end flanges of the roll, that said bottom wall is one of the elements defining the suction space, and that the partition walls are fixed to said bottom wall. The suction roll disclosed by document WO-A-9 002 840 comprises all the features of claim 5 except the feature that the partition walls are fixed to the inside face of the roll mantle. The suction roll in accordance with the invention is divided in the direction of width of the roll into at least three vacuum spaces and comprises at least two partition walls in the interior of the roll, by means of which the suction space is divided into separate zones of negative pressure, whereby the lateral vacuum spaces in the suction space can be subjected to a higher negative pressure than the vacuum space in the middle area of the roll, whereby the profile of negative pressure is formed such that the negative pressure increases across the width of the roll towards the lateral areas of the roll and whereby, when a roll in accordance with the invention is used, the percentage of shrinkage of the paper web in the lateral areas is reduced and a favourable effect is applied to the uniformity of said shrinkage. In the following, the invention will be described with reference to some preferred embodiments of the invention illustrated in the figures in the accompanying drawings, the invention being, however, not supposed to be confined to said embodiments alone. Figure 1 is an axonometric illustration in part of a first embodiment of a suction roll in accordance with the invention for a drying section. Figure 2A is an illustration of principle of the suction roll in accordance with the first embodiment, and what is shown is in particular a suction box arrangement. The illustration is axonometric. Figure 2B is a sectional view of the suction roll shown in Fig. 2A. Figure 2C is a sectional view taken along the line I-I in Fig. 2B. Figure 2D shows the roll as viewed from above. Figure 3A shows a second preferred embodiment of a suction roll in accordance with the invention. The roll is shown as a sectional view. Figure 3B is a sectional view taken along the line II-II in Fig. 2A. The wire and the paper web are also shown. Figure 3C illustrates the relationship between the cross-sectional flow areas of the grooves and of the perforations in the roll shown in Fig. 3A. Figure 4A shows the bearing means of the suction roll shown in Fig. 3. The roll is shown as a sectional view. Figure 4B shows a third embodiment of a paper machine suction roll in accordance with the invention wherein the partition wall is defined by a separate bottom part. Figure 4C is a sectional view of the suction roll shown in Fig. 4B as an illustration of principle. Figure 4D is a schematic illustration of a fourth embodiment of the invention wherein negative pressure is introduced into the central vacuum space in the roll through openings in both of the partition walls. Figure 4E shows a fifth embodiment of the invention wherein negative pressure is introduced into the central vacuum space in the roll through an opening formed in one of the partition walls only. Figure 5 illustrates a parabolic distribution of negative pressure across the roll width, produced by means of the use of partition walls. The vertical coordinate represents the negative pressure produced in the interior of the roll, and the horizontal coordinate represents the position of width of the roll and the locations of the partition walls. Figure 6 illustrates a profile of negative pressure across the width of the roll, produced by means of a suction roll in accordance with the invention. The vertical coordinate represents the percentage of shrinkage of the paper web from the original web width, and the horizontal coordinate represents the position of width of the roll. Fig. 1 is an axonometric view of a suction roll 10 in accordance with the invention for a drying section. The suction roll 10 comprises a roll mantle 11 and, inside the roll mantle, a suction box 12. Inside the roll mantle 11, the suction box 12 defines a suction space 13, and said suction space 13 can be subjected to negative pressure by means of separate means of negative pressure not shown, such as a pump device or equivalent. An important field of application of a suction roll in accordance with the invention is in the drying section of a paper machine in locations at which the paper web runs outermost on the wire, whereby the paper web is made to adhere to the wire face by means of negative pressure. Thus, by means of the suction roll, the running paper web in the drying section is supported. The longitudinal edges 14 of the suction box 12 are provided with sealing means 15, i.e. with edge seals. The sealing means are fitted in sealing seats 16 provided for the sealing means. According to the invention, the suction space 13 defined by the suction box 12 in the interior of the roll mantle is divided into at least three vacuum spaces A 1 ,A 2 and A 3 across the width of the roll, i.e. in the axial direction of the roll. In the embodiment of the invention shown in Fig. 1, the suction box is provided with partition walls 17a and 17b. In the figure, one partition wall 17a only is shown. By means of the partition wall 17a and 17b, the desired negative pressures can be produced at both sides of the partition walls. According to the invention, the highest negative pressure is produced in the lateral vacuum spaces A 1 and A 3 defined by the partition walls in the suction roll 10. In the vacuum spaces A 1 and A 3 a negative pressure of substantially equal magnitude is produced, and said negative pressure is higher than the negative pressure in the middle area of the roll in the vacuum space A 2 . In the embodiment of the invention shown in Fig. 1, the suction roll is provided with perforations 23 passing through the roll mantle 11. The passage of the air flow is illustrated by means of an arrow. Fig. 2A is an illustration of principle of a suction roll in accordance with one aspect of the invention, in particular of the inside construction. As is shown in Fig. 2A, the suction roll 10 comprises a central axle 18 of rotation, on which the roll mantle 11 of the suction roll 10 is rotated while the bearings at the ends of the roll mantle 11 are coupled with the outer face of the axle 18. A second function of the axle 18 is to act as a pipe arrangement by means of which the negative pressure is generated in the suction space 13 inside the roll mantle 11. As is shown in Fig. 2A, the interior duct of the axle 18 comprises a first duct 18a and a second duct 18b. A corresponding arrangement is provided at the other end of the axle. Through the first duct 18a, air is sucked from the vacuum space A 1 . Between the partition wall 17a and the end of the roll mantle, the axle 18 is provided with an opening 19 at the proximity of the partition wall 17a. Through the other duct 18b in the axle 18, air is sucked from the vacuum space A 2 placed at the opposite side of the partition wall 17a. The second duct 18b in the axle 18 opens through the opening 20 into the vacuum space A 2 . The ducts 18a and 18b are separated by a central transverse wall 21a. At its end, the first duct 18a is defined by the wall 21b. In the vacuum spaces A 1 and A 3 , a higher negative pressure is generated through the first duct 18a than in the vacuum space A 2 through the second duct 18b. Moreover, the negative pressures in the vacuum spaces A 1 and A 3 are substantially equally high. Moreover, the partition walls 17a,17b are connected with a bottom part 21c at both sides of the axle 18. Likewise, in the area between the partition walls 17a and 17b, at both sides of the axle 18 there is a bottom part 21d. The bottom parts 21c and 21d form a bottom wall. In this way a suction sector of 180° is formed. Fig. 2B is a sectional side view of the roll shown in Fig. 2A. As is shown in Fig. 2B, equal negative pressures are produced in the vacuum spaces A 1 and A 3 . A suction flow is directed towards the interior of the roll through perforations 23 fitted at the bottom of circumferential grooves 22 placed in the surface of the roll mantle 11. The partition walls 17a and 17b are placed substantially equally far from the roll ends. The arrows L 1 illustrate the flow through the perforations 23 in the roll mantle 11 into the central vacuum space A 2 defined by the suction box and further through the opening 20 into the second duct 18b in the interior of the axle 18. The partition walls 17a,17b, the bottom parts 21c,21d, and the axle 18 are fitted as stationary. The roll mantle 11 revolves on the bearings 11b,11c, while the bearings are supported on the axle 18. The arrows L 2 illustrate the suction flow through the perforations 23 in the roll mantle 11 into the vacuum spaces A 1 and A 3 and out of said spaces further into the first duct 18a in the interior of the axle 18. Fig. 2C is a sectional view taken along the line I-I in Fig. 2B. The groove 22 shown in the figure comprises a number of perforations 23 fitted with the same circumferential spacing. Through the perforations 23, air is made to flow, by means of the negative pressure inside the roll, from outside the roll into its interior and, at the same time, the paper web (not shown) running along with the roll is kept in contact with the wire (not shown) or equivalent, and in this way the running of the paper web is controlled. Fig. 2D hows the roll of Fig. 2B seen from above in the direction of the arrow K 1 (Fig. 2A). The roll comprises the circumferentially fitted grooves, but such an embodiment is also possible in which there is one groove which runs in spiral shape across the entire width of the roll. The perforations are fitted in the bottoms of the grooves. Fig. 3A shows a second preferred embodiment of the suction roll in accordance with the invention. In the embodiment shown in Fig. 3A, the suction roll is shown as a sectional view. The suction roll 10 comprises perforations 23 in the bottoms of the grooves 22. The roll shown in Fig. 3A does not comprise a suction box inside the roll. Instead, the roll shown in Fig. 3A comprises, in its interior, at least two partition walls 17a,17b at equal distances from the end flanges 27a and 27b of the roll 10. The partition walls 17a and 17b are fixed to the inside face 11' of the roll mantle 11. The axle 18 is a hollow axle, whose interior duct 18a opens into the vacuum spaces A 1 and A 3 between the partition walls 17a and 17b and the roll ends 27a and 27b. On the contrary, a lower negative pressure is introduced into the middle vacuum space A 2 in the roll in the space between the partition walls 17a and 17b. Into the middle vacuum space A 3 , a separate pipe passage 24 opens. Through the pipe passage 24 or equivalent air is sucked from the middle vaccum space A 2 . The negative pressure generated in the middle vacuum space A 2 is lower than the negative pressure in the lateral vacuum spaces A 1 and A 3 , in which substantially equally high negative pressures are provided. The pipe passage 24 is supported by means of ribs 25 or equivalent on the walls of the interior duct 18a. It is an essential feature of the embodiment of Fig. 3A that the negative pressure is applied at the same time to the interior of the whole roll mantle 11. Thus, the roll does not include a separate suction box inside the roll mantle 11. Fig. 3B is a sectional view taken along the line II-II in Fig. 3A. The figure shows the run of the paper web W and the wire H at a leading cylinder formed by the suction roll 10, in a single-wire group, for example a single-felt group. The suction roll 10 comprises a number of perforations 23, preferably bores, ending in the groove 22. The perforations 23 are fitted as equally spaced in the roll mantle on the distance of its circumference. In the vaccum space A 1 of the suction roll (10), a negative pressure is produced, e.g., by means of a pump device, and the negative pressure is fitted to be present in all operating conditions in said vacuum space in the suction roll 10. The negative pressure is applied, at the same time, to the inner face 11' of the entire roll mantle 11 of the suction roll 10. In the way shown in Fig. 3B, a holding force F is applied to the web W, which holding force fixes the web to the face of a wire H of good permeability, for example a fabric, and thereby to the outer face of the suction roll 10. In this way, detaching of the web from the suction roll 10 is prevented. As is shown in Fig. 3B, the upper face of the suction roll 10 remains free from the wire H and the web W. Through said free face S, an air flow is directed without obstacle into the interior space in the suction roll 10. A lowering of the negative pressure in the interior vacuum space in the roll is prevented by dimensioning the cross-sectional flow areas of the grooves and the perforations so that the negative pressure P can be maintained in the interior of the suction roll 10 in spite of said free flow of air L 3 through the area S. The perforations through the roll mantle are carried out so that a controlled air flow Q into, and a desired negative pressure in, the interior of the suction roll are achieved. A relatively low negative pressure is capable of keeping the web W on the wire face. The effect of said negative pressure is spread in the groove 22, and thereby an area of the force F of the shape of a band, acting upon the web, is obtained. Fig. 3C is a schematic illustration of the ratio of the area of the perforations to the area of the grooves provided with perforations. The ratio of the total cross-sectional flow area A o of the perforations 23 on the suction roll to the total cross-sectional flow area A 4 of the perforated grooves 22 is in the range of 1:10...1:150, and preferably in the range of 1:50...1:110. Advantageously, the flow Q per metre of width of the cylinder into the interior space of negative pressure in the cylinder is in the range of 500...1500 m 3 /m/h, and preferably in the range of 800...1200 m 3 /m/h. The negative pressure in the interior of the suction roll is in the range of 1000...3000 Pa. Thus, when a roll in accordance with Fig. 3A is used, the paper web can be fixed and guided efficiently while the holding force is applied to the paper web in such a way that the web is pressed against the felt along with the roll surface also in such draws of the web in which the web is placed outermost and on the surface of the felt. The suction roll in accordance with the invention permits application of the pressure profile across the entire width of the roll and, moreover, in the suction roll in accordance with the invention, the quality of the paper that is formed is influenced favourably by applying a higher negative pressure to both of the lateral areas of the roll as compared with the middle area of the roll. In Fig. 4A, a suction roll similar to the preceding embodiment is shown, in particular the bearing means is shown. The roll mantle 11 is supported by its end flanges 27a and 27b on the bearing means 26 on the bearing block. The axle 18 is a hollow axle, and through its interior duct 18a and through the partition wall 17a a pipe passage 24 is passed. The roll is divided, by the partition walls 17a and 17b, into three vacuum spaces A 1 ,A 2 ,A 3 . In the vacuum spaces A 1 and A 3 in the lateral parts of the roll, there are substantially equal negative pressures, which are higher than the negative pressure in the middle vacuum space A 2 in the roll between the partition walls 17a and 17b. Fig. 4B is an axonometric illustration in part of a further embodiment of a suction roll in accordance with the invention for a drying section. In the embodiment shown in Fig. 4B, the middle vacuum space A 2 is formed by means of partition walls 17a and 17b, which partition walls are defined by the inner face of the roll mantle 11 of the suction roll and so also, at the other end, by the cylindrical bottom part 28 of circular section. The partition wall 17a comprises holes or openings 29. Through the hollow interior space in the axle 30, air is sucked from the vacuum space A 1 and thereupon, through the openings 29, from the vacuum space A 2 . The throttle of the holes 29 has the effect that the negative pressure in the vacuum space A 2 is lower than in the vacuum space A 1 . The perforations in the roll mantle 11 are denoted with the reference numeral 23. In a corresponding way, suction is produced through the bottom part 28 into the vacuum space A 3 placed at the other side. In said vacuum space, a negative pressure is produced that is substantially equal to the negative pressure in the vacuum space A 1 . Thus, only one of the partition walls 17a, 17b is provided with holes 29. Fig. 4C is a sectional view illustrating the principle of the suction roll shown in Fig. 4B. By means of the partition walls 17a and 17b and the bottom part 28, the space in the interior of the roll is divided into pressure areas P 1 and P 2 . The suction flow is passed out through the axle 30 at one end only. The bottom part 28 and the partition walls 17a and 17b are fixed permanently to the inner face of the roll mantle 11 and, thus, revolve along with the roll mantle. Fig. 4D is a sectional view of a suction roll for a drying section, which comprises partition walls 17a and 17b and therein openings or holes 31. Negative pressure is produced through the axles 32 and 33 for the vacuum spaces A 1 ,A 2 and A 3 . Fig. 4E shows an embodiment of the invention wherein the suction roll 10 of the drying section comprises partition walls 17a and 17b and, in the partition wall 17a, at least one opening 34. Through the axle 35, air is sucked from the vacuum space A 1 and thereupon from the vacuum space A 2 in the middle area of the roll, and through the axle 36 air is sucked from the vacuum space A 3 at the other end of the roll. Fig. 5 is a schematic illustration of a parabolic pressure profile, accomplished by means of partition walls, on the face of a paper web. Between the positions 1 and 2 as well as 3 and 4, there is a higher negative pressure than between the positions 2 and 3 in the vacuum space A 2 . In the lateral vacuum spaces between the width points 1 and 2; 3 and 4, there is a negative pressure which is about 3000 Pa, and between the width positions 2 and 3 in the vacuum space A 2 , there is a negative pressure which is about 1500 Pa. In the figure, the curve is shown by dashed line that corresponds to the negative pressure at the face of a paper web. The curve is substantially parabolic. Said parabolic shape is effected by the porosity of the wire or equivalent; even though partition walls are used, the negative pressure is equalized efficiently, and there are no points of discontinuity in the curve of the profile of negative pressure measured on the face of the paper web W. Fig. 6 illustrates the percentage of shrinkage of the paper web across the width of the paper. In the figure, a conventional solution of suction roll is illustrated by the dashed line, in which solution the same pressure prevails in the interior of the suction roll across the entire width of the suction roll. In the figure, the full line represents the curve that is obtained when a suction roll in accordance with the invention, which is provided with partition walls, is used. In the vertical system of coordinates, the shrinkage is indicated as a percentage from the original width of the paper web. The horizontal coordinates represent the width position of the roll. It is seen from the figure that, when a suction roll in accordance with the invention is used, the percentage of shrinkage is, across the entire width of the paper web, lower than in the prior-art solution of suction roll. Also, when a suction roll in accordance with the invention is used, besides the fact that a lower percentage of shrinkage of the paper web W is obtained at all width positions, the use of said equipment also permits to produce a paper quality wherein the proportion of the lateral shrinkage is not emphasized in relation to the shrinkage in the middle area of the paper web. Within the scope of the invention, an embodiment is also possible in which the interior of the roll is divided by more than two partition walls into several vacuum spaces A 1 ,A 2 ,A 3 ,A 4 ...A n , in which case in both of the end vacuum spaces A 1 and A n in the roll, substantially the same negative pressure prevails, and in the spaces A 2 and A n-1 the same negative pressure prevails, etc., and in which case the negative pressure becomes lower when moving towards the middle area of the roll. It is an advantage of such an embodiment that the shape of the profile of negative pressure can be determined more accurately. In the suction roll of the invention, the desired pressure profile is obtained for the suction roll and further for the paper web W by dividing the suction roll in its axial direction into vacuum spaces of different pressures by means of partition walls, whose wall planes are substantially perpendicular to the axis X of rotation of the roll 10.

Claims: Suction roll (10) for a paper machine, comprising a perforated roll mantle (11), and a suction space (13) inside the roll mantle (11), which suction space can be subjected to negative pressure, wherein an air flow passes through the perforations in the roll into the interior of the roll and wherein the paper web (W) is pressed towards the outer face of the roll mantle (11), said suction roll further comprising a stationary axle (18) through whose interior duct (18a, 18b) the negative pressure is introduced into the suction space (13) within the roll mantle (11), and a stationary bottom wall (21c, 21d) which extends in the direction of the axis (X) of the roll towards and flanges (27a, 27b) of the suction roll, the suction space (13) being defined by the roll mantle (11) and by the end flanges (27a) and by the bottom wall (21c, 21d), characterized in that the suction roll (10) comprises at least two partition walls (17a, 17b) in the interior of the roll, by means of which the suction space (13) is divided in the direction of width of the roll into at least three vacuum spaces (A 1 , A 2 , A 3 ) forming separate zones of negative pressure, wherein the lateral vacuum spaces (A 1 , A 3 ) in the suction space can be subjected to a higher negative pressure than the vacuum space (A 2 ) in the middle area of the roll, so that the profile of negative pressure is formed such that the negative pressure increases across the width of the roll towards the lateral areas of the roll, and in that the partition walls (17a, 17b) are fixed permanently to the roll axle (18) and the bottom wall (21c, 21d). Suction roll as claimed in claim 1, characterized in that the interior duct of the axle (18) of the suction roll comprises two ducts (18a,18b), of which the first duct (18a) is fitted to be opened through an opening (19) placed on the surface of the axle (18) into at least one of the two lateral vacuum spaces (A 1 , A 3 ) between one of the partition walls (17a) and one of the end flanges (27a) of the suction roll. Suction roll as claimed in claim 2, characterized in that the first duct (18a) is defined by a central transverse wall (21a) in the axle (18), by the tubular cover of the axle (18) and by an end wall (21b), whereby an air flow can be passed out of the lateral vacuum space (A 1 or A 3 ) placed between the partition wAll (17a or 17b) and the end flange (27a) through the opening (19) provided on the axle (18) into the duct (18a) and further out of the suction space (arrow L 2 ). Suction roll as claimed in claim 3, characterized in that a suction flow can be passed through the perforations (23) in the roll mantle (11) into the second duct (18b) placed in the roll axle (18) through a second opening (20) on the axle (18). Suction roll (10) for a paper machine, comprising a perforated roll mantle (11) and a suction space (13) inside the roll mantle (11), which suction space can be subjected to negative pressure, wherein an air flow passes through the perforations in the roll into the interior of the roll and wherein the paper web (W) is pressed towards the outer face of the roll mantle (11), characterized in that the suction roll (10) comprises at least two partition walls (17a, 17b) in the interior of the roll, by means of which the suction space (13) is divided in the direction of width of the roll into at least three vacuum spaces (A 1 , A 2 , A 3 ) forming separate zones of negative pressure, wherein the lateral vacuum spaces (A 1 , A 3 ) in the suction space can be subjected to a higher negative pressure than the vacuum space (A 2 ) in the middle area of the roll, so that the profile of negative pressure is formed such that the negative pressure increases across the width of the roll towards the lateral areas of the roll, and in that the partition walls (17a, 17b) are fixed to the inside face (11') of the roll mantle (11). Suction roll as claimed in claim 5, characterized in that the suction roll includes a separate pipe passage (24), which is passed through a hollow axle (18) of the roll and which is passed through one of the partition walls (17b) to open itself into the vacuum space (A 2 ) in the middle area of the roll, whereby, through said pipe passage, a suction flow (L 1 ) can be passed from the middle vacuum space (A 2 ) out of the roll construction. Suction roll as claimed in claim 5, characterized in that said partition walls (17a and 17b) are, at one end, defined by a bottom part (28), preferably a circular cylinder, in the interior of the roll, and in that at least one partition wall (17a and/or 17b) comprises holes (29), whereby the negative pressure is produced in said vacuum space (A 2 ) defined by the partition walls (17a,17b), by the bottom part (28) and by the inner face (11') of the roll mantle through the holes (29), the air flowing out of said vacuum space (A 2 ) in the middle area of the roll into one of the adjoining lateral vacuum spaces (A 1 ) in the lateral area and further out of connection with the roll. Suction roll as claimed in claim 7, characterized in that the negative pressure in the vacuum space (A 3 ) placed at the other end of the roll is introduced centrally through the roll through the tubular structure formed by the bottom part (28) of the circumferential zone (A 2 ) of negative pressure. Suction roll as claimed in claim 5, characterized in that at least one of the partition walls (17a) includes at least one hole or opening (29,31,34) passing through the partition wall, through which hole or opening the negative pressure is introduced into the space placed at the opposite Side of the partition wall (17a) while the hole operates as a throttle member, whereby in the vacuum space placed at one side of the partition wall a lower negative pressure can be produced than in the space placed at the other side of the partition wall. Suction roll as claimed in one of claims 1 to 9, characterized in that the partition-wall planes of the partition walls (17a,17b) are placed substantially perpendicularly to the axis (X) of rotation of the roll (10). Suction roll as claimed in one of claims 1 to 10, characterized in that in the interior of the roll (10), in the suction space (13), the vacuum spaces (A 1 ,A 2 ,A 3 ) are arranged symmetrically to the axial center point of the central axis of the roll, whereby, at the same distances from said center point (X), similar vacuum spaces (A 1 ,A 3 ) are placed, into which substantially the same negative pressure is introduced, said negative pressure being higher than the negative pressure in any vacuum space between said vacuum spaces (A 1 and A 3 ). Suction roll as claimed in any of the preceding claims, characterized in that the suction roll includes grooves (22) fitted in the mantle face of the roll mantle (11) and running in the direction of the circumference or running spirally, said perforations (23) in the roll mantle (11) being passed through the bottom parts of said grooves so as to direct a suction flow from outside the roll into the interior space in the roll. Suction roll as claimed in claim 12, characterized in that the ratio of the total cross-sectional flow area (A o ) of the perforations (23) in the roll mantle to the total cross-sectional flow area (A 4 ) of the perforated grooves (22) is in the range of 1:10...1:150, and preferably in the range of 1:50...1:110. Suction roll as claimed in claim 13, characterized in that the suction roll comprises such perforations (23) that the flow (Q) through the holes into the interior of the roll is in the range of 500...1500 m 3 /m/h, in which case the flow into the roll also through the area (S) free from web and felt remains within controlled limits, whereby the negative pressure in the vacuum spaces in the interior of the roll is maintained and whereby an adequate holding force (F) is achieved by means of the negative pressure to keep the paper web (W) in contact with the wire (H) or an equivalent thereof also in such roll positions in which the paper web runs outermost and, in particular, in the drying section at the beginning of the drying section.

Inventors: AULA JOUKO, ILVESPAEAE HEIKKI, LIEDES ALLAN, AULA, JOUKO, ILVESPAEAE, HEIKKI, LIEDES, ALLAN, Ilvespää, Heikki
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1990
Main classification: D21F3
IPC codes: D21F3, D21F5