Method of adhering a web to a drying wire

Abstract: The invention concerns a method to adhere a web (W) to a drying wire (H) against the centrifugal forces and various blow phenomena, which attempt to detach the web (W) from said wire (H) in the drying groups provided with single-wire draw in a multi-cylinder dryer of paper machine. The drying groups comprise as upper cylinders drying cylinders (K) heated by means of steam or equivalent and the web (W) is pressed by the drying wire (H) against outer cylinder faces of said drying cylinders (K). The drying groups further comprise as lower cylinders leading cylinders (10) on which the web (W) runs outside the drying wire (H), said leading cylinders (10) being provided with holes (15) passing through a mantle (11) of the leading cylinders (10). The holes (15) open into an interior space (14) of negative pressure (P 1 ) in the leading cylinders (10) and into the outer face of said cylinders (10), said negative pressure (P 1 ) being transmitted to said outer face through said holes (15). The leading cylinders (10) comprise shafts (18) on which the respective leading cylinder (10) is journalled revolving, and at least one of said shafts (18) has a duct (18a) through which said negative pressure (P 1 ) is applied to the space (14) inside said cylinder (10). The shafts (18) communicate with a suction duct (19) from a source (20) of said negative pressure (P 1 ). A part of the holes (15) in each leading cylinder (10) opens into the open air in the upper part of the leading cylinder (10) so that flow of air is directed into the interior (14) space of said cylinder. Said negative pressure (P 1 ) in said interior space (14) is selected within the range of 1000 Pa to 3000 Pa.

Description: The invention concerns a method of adhering a web to a drying wire. In the case of the leading rolls of the single-wire groups in the drying section of a paper machine, a problem is how to make the paper web adhere to the wire on the lower face of the cylinder. On the lower cylinders or leading rolls, the web runs as the topmost layer while the wire remains between the web and the outer face of the cylinder. This problem does not occur in the case of the other rolls in a single-wire group as the paper web runs between the wire and the mantle face of the cylinder. Attempts have been made to solve the problem concerned by using suction rolls. The suction roll solution is expensive. A suction box requires inside sealing ribs, which tend to be worn. When the sealing ribs operate as dry, the wear is even more extensive. Through the suction box of a suction roll, negative pressure is applied to the vacuum block inside the cylinder, which said vacuum block is, in the case of the lower cylinders in a single-wire group, placed in the lower half of the cylinder. By the intermediate of the negative pressure applied to said block, suction is applied expressly to the part of the cylinder face in which the paper web tends to be detached. Through bores in the mantle, negative pressure is applied through the felt to the paper web, which thereby adheres to the felt face. In prior art, a solution for said problem of the paper web tending to be detached is also known wherein an arrangement in accordance with documents F-B-71371 and GB-A-2 173 832 is used. In the Uno-Vac system concerned, a sort of a suction box is used in which no wearing seals placed against the inner face of the roll mantle are used. In said solution, a high-velocity air flow is applied to the proximity of the edge of the suction box and the mobile mantle face. Said air flow produces a suction flow from the interior of the box, which said suction flow thereby prevents flow of air through the edge area in the opposite direction into the space of negative pressure. A negative pressure is applied to the interior of the suction box and transferred through the bores in the mantle to the web face. It can be considered that a drawback of said solution of equipment is its high-cost construction. An abundance of holes must be made into the mantle. The inner axle requires blow ducts of its own and suction ducts of its own. In connection with single-wire groups, the use of a suction box outside the mantle is also known. The outer face of the drying cylinder is provided with grooves, and the negative pressure is applied to said grooves through an outside suction box placed on the cylinder. Thus, the negative pressure is transmitted through the grooves to the lower face of the cylinder, where a web W holding force is thereby produced. The solution requires space and the cost of the construction is high. Document GB-A-2125461 discloses a cylinder having an internal suction box. The above mentioned drawback with non-uniform vacuum distribution applies to this known cylinder. FR-A-2 104 562 discloses a paper web drying cylinder with perforated mantle. This known cylinder thus has a dewatering function and is intended to prevent clogging-up by fibres. This known cylinder too has a suction box. A method comprising the features according to the pre-characterizing clause of claim 1 is known from document GB-A-2 173 832. The object of the present invention is to elimate the drawbacks of the prior art solutions mentioned above. The method in accordance with the invention is defined by claim 1. The method according to the invention is well suited for supporting the web in connection with the lower rolls in a single-wire group in the initial part of the drying section of a paper machine. Centrifugal force and various blow phenomena attempt to detach the paper web from the wire off the face of the leading cylinder. In the invention, the starting point has been the basic fact that a relatively little force is capable of keeping the paper web on the wire face. According to the invention suction holes have been drilled that pass through the mantle of the roll. One end or both ends of the roll is/are provided with a shaft which comprises a suction duct, which said duct is further connected to a source of suction, most appropriately to a centrifugal blower. When the holes are dimensioned appropriately and when the roll mantle is provided with a certain limited number of bores that transmit the negative pressure, a permanent negative pressure can be produced in the interior of the roll. Said negative pressure can be maintained in spite of the fact that a part of the holes in the roll open into the open air in the upper part of the roll. The effect of negative pressure is applied to the web. In this way, a force pattern that attracts the web is obtained. By means of the negative pressure, the web is pulled towards the roll. The suction is applied to the web through the wire. The invention will be described in the following 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 restricted to said embodiments alone. Figure 1 is a schematical illustration of a Sym-Press II press and of the initial part of a drying section. Figure 2A shows a cylinder for executing the invention partly as an illustration of principle. Figure 2B shows a section I-I in Fig. 2A. Figure 3 is a graphic presentation of the relationship between the negative pressure prevailing in the space of negative pressure in the cylinder and the flow taking place through the holes as a function of the chosen total cross-sectional flow area of the holes. Figure 4 is a side view of a leading cylinder for executing the invention partly in section. Figure 5 illustrates the cross-sectional area of a hole and the cross-sectional flow area of a groove. Fig. 1 shows an equipment operating according to the invention. What is shown is the area of single-wire draw in a multi-cylinder dryer of a paper machine. The dryer comprises a line of heated drying cylinders, preferably upper cylinders, as well as a line of leading cylinders or leading rolls 10. The paper web W runs between said lines as supported by a drying wire H, e.g. a felt. The web W runs on the heated drying cylinders K as pressed by the drying wire H. The drying wire H presses the web W into direct contact with the heated face of the drying cylinder. At the leading cylinders or lower rolls, the web W runs on the outer face of the drying wire, e.g. a felt H. In such a case, in the prior art solutions of equipment, there is a major risk that the web W is detached from the face of the leading cylinders 10. As is shown in Fig. 1, the cylinders 10 are placed as leading cylinders in the single-wire group of the paper machine. The web W is passed through the nips N 1 to N 3 to the first single-felt or single-wire group. In said group, the wire H is passed over the leading rolls 10a, 10b and 10c. Said single-felt draw, i.e. Uno-Run, runs the paper web W alternatingly between the wire H and the outer face of the drying cylinder K and, in the case of leading cylinders 10 of a single-wire group, on the wire while the wire H runs between the paper web W and the outer face of the cylinder 10. As is shown in Fig. 1, the paper machine leading cylinders 10a, 10b and 10c in accordance with the invention are fitted as lower cylinders of the single-wire group. From a source 20 of negative pressure, advantageously from a centrifugal blower, negative pressure P 1 is applied through the ducts 19 into the interior spaces in each of the cylinders 10a, 10b and 10c. The source 20 of negative pressure is fitted to be placed underneath the floor level of the paper machine. Figures 2A and 2B show the principle of the web draw and support arrangement in accordance with the invention. In Fig. 2A, the cylinder 10 is shown in a longitudinal sectional view. The cylinder 10 comprises a mantle 11, which is attached to the end flanges 22 of the cylinder. The outer face 11a of the cylinder 10 mantle 11 comprises grooves 13. The grooves 13 are closed annular grooves, which are placed side by side in the outer face 11a of the mantle and in the area of the whole mantle. The groove formation may also consist of one single groove that runs as spiral-shaped. In such a case the groove runs from one end of the mantle to the other. The grooves are preferably turned into the mantle face, but such an embodiment of the invention is also possible in which a band that forms the grooves has been wound as spiral-shaped onto the outer face of the mantle frame. Negative pressure P 1 is applied to the space 14 of negative pressure in the cylinder 10 from a source 20 of negative pressure, e.g. from a centrifugal blower. The negative pressure is transmitted from the space 14 of negative pressure in the cylinder 10 through holes, preferably bores 15, into the grooves 13. The holes 15 pass perpendicularly to the central axis X of the cylinder. Each hole opens from one of its ends into the space of negative pressure in the cylinder and from the other end to the bottom 16 of the groove 12. The negative pressure is transmitted through the bores 15 substantially across the entire width L of the cylinder. The grooves 13 comprise a groove bottom 16 and side walls 17a and 17b. Each hole 15 opens into the groove bottom 16. The holes 15 are uniformly spaced in the groove 13. The cylindrical space 14 of negative pressure inside the cylinder mantle is substantially free from any constructions, and the negative pressure P 1 is applied to the entire inside mantle face 11b of the cylinder 10. The cylinder 10 comprises shafts 18, on which the cylinder 10 is journalled revolving. At least one of the shafts 18, the service-side shaft as shown in Fig. 2A, includes a duct 18a, through which negative pressure is applied to the space 14 of negative pressure placed inside the cylinder 10. The service-side shaft 18 is a tubular hollow shaft, and it communicates with a suction duct 19 from the source 20 of negative pressure, preferably from a centrifugal blower. In Fig. 2B, a section I-I is shown out of Fig. 2A. The figure also shows the runs of the paper web W and of the wire H at a leading cylinder of a single-wire group, e.g. single-felt group. The cylinder 10 comprises several holes ending in the groove 13, preferably bores 15. The bores 15 are fitted as uniformly spaced in the cylinder mantle. Into the space 14 inside the cylinder 10, negative pressure P 1 is introduced from the source 20 of negative pressure, and the negative pressure is fitted to prevail under all circumstances of operation in said interior space in the drying cylinder 10. The negative pressure P 1 is applied to the entire inner face 11b of the mantle 11 of the leading cylinder 10. As is shown in Fig. 2B, a holding force F is applied to the web W, which said force makes the web adhere to the face of the wire of good permeability to air, e.g. a fabric, and thereby to the outer face of the leading cylinder. Thereby detaching of the web from the cylinder 10 is prevented. As is shown in Fig. 2B, the upper face of the cylinder 10 remains free from the wire H and from the web W. Through said free face S, an unhindered flow of air is directed into the interior space 14 in the leading cylinder 10. Thereat the negative pressure P 1 tends to be reduced. However, in accordance with the invention, it has been realized to dimension the cross-sectional flow areas of the grooves and the holes so that negative pressure can be maintained in the interior space 14 in the cylinder 10 in spite of said free flow of air L 1 . According to the invention, the bores 15 are dimensioned so that the desired flow of air Q into the cylinder and the desired negative pressure P 1 in the interior 14 of the cylinder are achieved. A relatively low negative pressure P 1 is capable of keeping the web W on the face of the wire. Said effect of the negative pressure is spread in the groove 13, and thereby a band-shaped force pattern is obtained that holds the web. According to the invention, by dimensioning each hole 15 so that its diameter has a suitable dimension and providing only a certain limited number of holes in each groove, the air flow Q into the interior space 14 in the cylinder is kept limited. A certain holding force is kept as the starting point, which said force is further achieved by means of a certain negative pressure P 1 . Said negative pressure P 1 is transferred through the holes 15 into the grooves 13 placed on the face of the cylinder mantle. The transfer of negative pressure P 1 is most efficient when the number of holes is maximized. However, an upper limited is imposed on the number of holes by the air flow Q into the interior space 14 in the cylinder. The desired optimum value is obtained by choosing the required negative pressure P 1 and by choosing the total cross-sectional flow area of the holes such that the flow into 14 the cylinder is limited and remains within certain low limits in spite of minor variations in the negative pressure P 1 . Fig. 3 is a graphic presentation, wherein the horizontal coordinates represent the negative pressure inside the cylinder 10 and the vertical coordinates represent the flow of air into the cylinder 10 through the bores 15 or equivalent. The curves are shown for two chosen total cross-sectional flow areas A 2 and A 1 ' of the holes 15. In the total cross-sectional flow area is large, i.e. if the number of holes 15 is high and/or if their area is large, the flow passing through said holes 15 also increases steeply when the negative pressure 14 in the space is increased. Thereat, from the face S at the upper side of the upper leading cylinder 10, an abundance of air (L 1 ) flows into the inside space, i.e. space 14 of negative pressure, in the cylinder. Said case is illustrated by curve A 1 ' in Fig. 3. The curve A 1 ' mainly corresponds to the characteristic curve of the suction roll. On the contrary, if the number of holes is lower and/or the cross-sectional area of said holes has been chosen correct, i.e. the total cross-sectional flow area A 2 is considerably smaller than A 1 ', the form of the curve is different. In such a case, the curve includes an almost horizontal part D 1 . From said curve portion it can be read that the negative pressure P 1 can be chosen within a wide range of variation while the flow Q, nevertheless, remains low and within controlled limits. Under these circumstances, the total number of the suction holes and their total area must be in a certain relationship to the desired level of negative pressure. For example, if the roll diameter  is 1500mm and the roll length 9300mm and the grooves are b = 5mm, a = 4mm, t = 20mm, the total number of grooves is 455; b = groove width, a = groove height and t = groove pitch. The perforation is carried out as follows. The number of holes per groove is chosen as 30, i.e. the spacing of the holes is 12°, or on the face of the roll mantle about 157mm. In the area of threading, the holes 15 are made into each groove 13. At the rear edge the holes are made into every second groove, and in the middle area of the cylinder into every fourth groove. The total number of holes is 3960, and the diameter  of the hole is chosen as 4.5mm. Thereat, the total cross-sectional area of the holes is 630cm 2 . The desired negative pressure is 1176 Pa (120mm H 2 O). From the formulae Q = µ x A 0 x v and v = 2 g h γ the velocity of the air flow is obtained as v = 45.5 m/s and Q as Q = 2m 3 /s. The friction resistance in the hole is about 392 Pa (40mm H 2 O). In other words, to maintain a negative pressure or 1176 Pa (120mm H 2 O), at the maximum a suction quantity of 800m 3 per hour and per metre of length of the roll is required, i.e. a total of 7200m 3 per hour. It should be noticed that the web and the wire seal the roll partly, whereby the air quantity is reduced and/or the suction becomes deeper. If it is supposed that, in the suction shaft of our example, the air velocity is v = 35 m/s, the inner diameter of the shaft is obtained as 270mm. When Δ P = 392 Pa (40mm H 2 O) is reserved for the exhaust losses, the pressure in the blower is P = 1960 Pa (200mm H 2 O), and the power requirement of the blower is Po = Q x P 102 (Kw) = 4 kW. The following values were used in the accompanying formulae. h = difference in pressure, mmH 2 O (1mm H 2 O = 9,8 Pa) v = velocity m/s γ = 1.128 kg/m 3 (+40°C) A 0 = air flow area m 2 µ = throttle factor 0.7 Friction resistance in the hole Δp = λ x 1 x v2 x γ d x 2 g and λ = 0.005 (according to Bradke). By means of the example given above, it has been possible to establish the suitability of the device in practice. A combination of a groove and a hole pattern of low density is advantageous to manufacture, because the total number of holes is only a little portion of the corresponding number of holes in a suction roll proper. Likewise, the internal structures required by a suction roll are omitted completely. Also, in absence of external box constructions, the initial end of the drying section of the paper machine can be monitored and serviced readily. The masses of air to be dealt with are linked in the overall air-conditioning of the paper machine. Fig. 4 shows a construction of a cylinder for executing the invention. The cylinder 10 comprises a mantle 11 and grooves 13 on said mantle. One groove 13 is illustrated in the figure for the sake of example. The grooves 13 are provided side by side across the whole mantle face of the cylinder. The other grooves 13 are represented in Fig. 4 by means of dashed-dotted lines. The negative pressure P 1 is introduced through the hollow interior space 18a in the shaft 18 at the service side K p into the space 14 of negative pressure in the cylinder 10. The shaft 18 is attached by means of screws 18' to the flange 22. The shaft 18 is fitted to revolve on bearings 21a and 21b. Fig. 5 is a schematical presentation of the ratio of the area of the perforations to the area of the perforated grooves. The ratio of the total cross-sectional flow area A 0 of the holes 15 in the cylinder to the total cross-sectional flow area A 1 of the perforated grooves 13 is within the range of 1:10 to 1:150 and most advantageously, the flow Q per metre of width of the cylinder into the space 14 of negative pressure in the cylinder is within the range of 500 m 3 /m.h to 1500 m 3 m.h, and most advantageously within the range of 800 m 3 /m.h to 1200 m 3 /m.h. The negative pressure P 1 in the interior space 14 in the cylinder 10 is within the range of 1000 Pa to 3000 Pa. The ratio of the total cross-sectional flow area A 0 of the holes 15 to the entire outside mantle area A of the cylinder 10 mantle 11 is within the range of 1 to 2 per mill, and preferably within the area of 1.5 per mill.

Claims: Method of adhering a web (W) to a drying wire (H) against the centrifugal forces and various blow phenomena, which attempt to detach the web (W) from said wire (H) in the drying groups provided with single-wire draw in a multi-cylinder dryer of a paper machine, said drying groups comprising as upper cylinders drying cylinders (K) heated by means of steam or equivalent, the web (W) being pressed by the drying wire (H) against outer cylinder faces of said drying cylinders (K), said drying groups further comprising as lower cylinders leading cylinders (10), on which the web (W) runs outside the drying wire (H), said leading cylinders (10) being provided with holes (15) passing through a mantle (11) of said leading cylinders (10), said holes (15) opening into a interior space (14) of negative pressure (P 1 ) in said leading cylinders (10) and into the outer face of the mantle (11) of said leading cylinders (10), said interior space (14) of negative pressure being communicated with a suction duct (19) from a source (20) of said negative pressure (P 1 ), said negative pressure (P 1 ) being transmitted to said outer face through said holes (15), each of said leading cylinders (10) comprising shafts (18) on which said leading cylinders (10) are journalled revolving, a part of the holes (15) in said leading cylinders (10) opening into the open air in the upper part of said leading cylinders (10), characterized in that in said outer face (11a) of the mantle (11) of said leading cylinders (10) grooves (13) are provided in which the holes (15) in the mantle (11) end, so that said negative pressure (P 1 ) transmitted to said outer face is spread by said grooves (13) on said outer face and thus holds the web (W) to the drying wire (H), in that at least one of said shafts (18) has a duct (18a) through which said negative pressure (P 1 ) is applied from said suction duct (19) to said interior space (14) inside said leading cylinders (10), in that the negative pressure (P 1 ) is applied to the entire inner face (11b) of the mantle (11) of said leading cylinders (10), so that a flow of air is directed into the interior space (14) of said leading cylinders (10) through said holes (15) opening into the open air in the upper part of said leading cylinders (10), in that said negative pressure (P 1 ) in said interior space (14) is selected to be within the range of 1000 Pa to 3000 Pa, and in that the dimensions and number of said holes (15) are selected such that the air flow rate (Q) per meter of width of said leading cylinders (10) into said interior space (14) of negative pressure (P 1 ) is within the range of 500 m 3 /m • h to 1500 m 3 /m • h. The method according to claim 1, characterized in that the air flow rate (Q) per metre of width of said leading cylinders (10) into said interior space (14) of negative pressure (P 1 ) is within the range of 800 m 3 /m • h to 1200 m 3 /m • h. The method according to claim 1 or 2, characterized in that said leading cylinders (10) are in absence of external box constructions. The method according to any of the claims 1 to 3 characterized in that the ratio of the total cross-sectional of area (A 0 ) of the holes (15) to the entire outside mantle area (A) of the mantle (11) of said leading cylinders (10) is selected within the range of 1 to 2 per mill and preferably within the area of 1.5 per mill. The method according to claim 4, characterized in that in the area of threading said holes (15) of said leading cylinders (10) are made into the each groove (13), that at the rear edge of said leading cylinders said holes (15) are made into every second groove (13) and that at the middle area of said leading cylinders (10) said holes (15) are made into every fourth groove (13). The method according to any of the claims 1 to 5, characterized in that said negative pressure (P 1 ) is of order 1176 Pa (120 mm H 2 O) when the diameter (⊘) of said leading cylinders (10) is of order 1500 mm. The method according to any of the claims 1 to 6, characterized in that all of said leading cylinders (10a, 10b, 10c) of a same single-wire group are connected by said shafts (18) and said suction ducts (19) to a same source (20) of said negative pressure (P 1 ).

Inventors: ILVESPAEAE HEIKKI, LIEDES ALLAN, VIITANEN OLAVI, ILVESPAEAE, HEIKKI, LIEDES, ALLAN, VIITANEN, OLAVI, Ilvespää, Heikki
Applicants: METSO PAPER INC, METSO PAPER, INC.
Application year: 1989
Main classification: D21F5
IPC codes: D21F3, D21F5