METHOD AND APPARATUS FOR EXTRACTING DUST THAT IS RELEASED WHEN CREPING OFF A PAPER WEB

Abstract: In a tissue paper machine, the dust that is released when the web is creped off the Yankee dryer (3) constitutes a major work environment problem. To provide an improved extraction of the dust and thereby improve the work environment for the operating personnel concerned, a dust extractor (25″) having an internal space (39″) and a web stabilizing, imperforate plane (37″) is mounted immediately adjacent to an intended path of travel for the creped web (1), so that the web during its transportation will place itself in a fixed position close to the imperforate plane (37), and, at least an essential part of an entrained boundary layer of dust containing air is eliminated solely through suction to the internal space (39″). The suction is carried out at the rear and/or front edge of the extractor (25), and suitably both on the overside and the underside of the web (1). Dust that does not become entrained in the boundary layers will be sucked away by at least one dust suction box (55) having a sloping suction box cover (57), and mounted under the web (1) and spaced therefrom at doctor beam (11). Preferably, an air stream, which is of sufficient force to carry the dust and caused by the suction, is made rise towards the web (1) between the dust suction box (55) and a dust extractor (27) mounted downstream of the latter.

Description: The present invention relates to a method of extracting dust that is separated on creping a soft crepe paper web off a Yankee dryer by means of a creping doctor, part of the dust being carried along by the web in a boundary layer of air which, because of the transport of the web from the Yankee dryer towards a reel, becomes entrained and follows the web. The invention also relates to an apparatus for carrying out the method. When creping a soft crepe paper web, for example a tissue web, off a Yankee dryer, dust in the form of fibres and other particles is separated from the web. In order to lead that dust away, it has been proposed in Swedish Patent Publication SE-B-381 899 (= US-A-4 019 953) that a collecting receptacle should be mounted under the dust generating zone. To this receptacle, a compressed-air line and a suction line are connected for carrying away dust laden air entrained by compressed-air jets across the path of movement of the web. Directing a flow of compressed-air against the web is needed, since to just suck away the dust through ventilation has a negligible effect because of the poor action at this distance. Such an apparatus is unwieldy, space requiring and relatively inefficient, since it only takes care of that part of the dust that falls down under the dust generating zone. The high velocities of modern tissue machines contribute to the unsatisfactory result. Velocities in the range near to 25 m/s will cause dust laden air to be entrained on both sides of the travelling tissue web, which is fragile because of its low grammage. The apparatus proposed in SE-B-381 899 (= US-A-4 019 953) is not used in actual operation. The separation of dust from the surface of a paper web is a problem also at slitting, slitting-winding and rewinding of the paper web. Swedish Patent Publication SE-B-319 969 (= US-A-3 775 806) proposes an arrangement for vacuum-cleaning the surface of a paper web. In order to avoid that the paper web during the vacuum-cleaning is sucked against the suction device and thereby may be damaged, a flow of air is directed towards the web simultaneously with the vacuum suction. The proposed vacuum-cleaning device thus has e.g. an upward open, horizontal sheet steel channel surrounding a square blowing-air pipe and extending across the web. The side of the blowing-air pipe facing the web has eyelid type openings turned from each other, to direct compressed-air jets principally parallel to the web both against and along the path of transportation of the web and thereby attaining a fixed distance of the web from the vacuum-cleaning device. Intermediate openings of the same type can be provided for directing compressed-air jets towards the surface of the web, to improve the effect of the vacuum cleaning by blowing dust off the surface. The blown air and released dust are sucked away in a direction perpendicular to the web through the two gaps that are formed upstream and downstream of the blowing-air pipe between the same and the surrounding sheet steel channel, to which an air sucking means is connected. The wall parts of the sheet steel channel, which are situated nearest the web, may be vertically adjustable in such a way that the blown-in, dust conveying air does not flow past the suction gaps and that the paper web does not scrape against the wall parts and become damaged. The vacuum cleaning effect that is achieved by the arrangement according to SE-B-319 969 (= US-A-3 775 806) does not, though, meet today's demands when it comes to also taking care of the dust that follows in the air around a travelling paper web. Furthermore, this arrangement becomes both space demanding and expensive to buy and operate because of the compressed-air flow needed by the blowing-in technology. The object of the present invention is to achieve an improved catching and extraction of the dust that results from the creping of a tissue web or any other soft crepe paper web and to thereby achieve an improved work environment for the operating personnel concerned. According to the invention, this object is achieved in the above stated method by mounting a dust extractor having an internal space and a web stabilizing imperforate plane in the immediate vicinity of an intended path of travel of the creped web, so that the web during the travelling will take a fixed position close to the imperforate plane, and eliminating at least an essential part of the dust-containing boundary layer of air solely by vacuum suction to the internal space. In an apparatus for carrying out the method, the afore mentioned object is achieved in a corresponding way, according to the invention, in that the apparatus comprises a hood of paper web width, having a front side edge and a rear side edge in relation to the transport direction of the web, an imperforate plane cover plate located to substantially close the hood while defining an internal space and forming a gap between at least one of said side edges and the cover plate, means for producing a subatmospheric pressure in the internal space, so that air is sucked through the gap, said apparatus being adapted to be mounted with its cover plate in the immediate vicinity of an intended path of transportation of the creped web, whereby the plane imperforate cover plate has a web stabilizing effect, so that the web at transport will take a fixed position close to the cover plate and the gap will be situated in the dust-containing boundary layer. The invention is based on the realization that in order to solve the dust problems in tissue machines and the like it is necessary to suck away the dust laden boundary layers of air, which on creping build up around the fast travelling web, and to manage to suck sufficiently close to the creped web, the web must be stabilized and prevented from fluttering and the position of the web must be fixed. By the use of an imperforate plane, the web will be stabilized and fixed at a comparatively very short distance from the plane, without simultaneously getting a detrimental slowing down of the web against the plane, and hereby it is possible to work with vacuum suction only, instead of combined blowing and sucking to eliminate the dust. Advantageously, the distance is so short that the web very lightly touches the plane. When vacuum sucking at a front or upstream edge of the apparatus in relation to the direction of web travel, it is suitable that the air flow just before and in the suction gap is mainly parallel to the web plane, so as to minimize such detrimental whirling that could convey part of the dust laden boundary layer away from the dust extracting apparatus. At the rear or downstream edge, it is, however, suitable that the air just before getting sucked into the suction gap also has a component of movement directed towards the web, so as to entrain dust that is very close to or on the web surface. Preferably, the suction velocity is of the same order of magnitude as the travelling velocity of the web. Thereby, at said rear edge, the relative velocity between the air stream at the suction gap and the web will be in the range of twice the web velocity, which will give a really good dust extraction effect. At creping, most of the dust arises on the side of the web that comes into contact with the creping doctor, but since the boundary layers on both sides of the web will contain dust, it is of course suitable to extract an essential portion of each of the two boundary layers solely by suction. Part of the dust that is separated from the web at the doctor blade, which usually is located under the web and under the point where the web is doctored off the Yankee dryer, is not caught and conveyed off by the boundary layer on the underside of the web but falls downwards. To catch and extract even that falling dust, it is advisable to provide a dust suction box adjacent to the doctor beam. The suction box has a cover, which is sloping downwards and outwards from the Yankee dryer to prevent the web from getting stuck to the cover in the event of a possible web break. At a certain distance downstream of the dust suction box, there suitably is a dust extractor working solely on suction basis to suck off the boundary layer on the web underside. Preferably, the distance will be such that under normal operating conditions, i.e. at normal web velocity and with normal suction forces in the dust suction box and in the dust extractor, between these two suction devices, an upwards directed air stream is created, which essentially extends over the whole mentioned distance and is strong enough to carry substantially all of the dust particles separated from the web. Since the creping doctor blade is not in close contact with the Yankee dryer, part of the dust particles will pass inbetween the blade and the roll. To take care of them, it is suitable to provide a second dust suction box on the opposite side of the doctor beam in relation to the first dust suction box. The second dust suction box can be designed as the first one but reversed. Preferably, the dust extractor is mounted in the immediate vicinity of the intended path of travel of the creped web in a position such that the web will touch the imperforate plane lightly. On a macroscopic scale, this touch is defined as at least a line of contact extending across substantially the whole width of the web. Such a line of contact is suitably located at least at a downstream end of the imperforate plane but can, in addition, be located also at an upstream end of the plane. The touch of the web against the plane gives a certain working of the creped paper web, which effectively contributes to the extraction of web carried dust by the suction at the downstream end of the extractor. There, the web will be subjected to a pulse of air that passes through the creped porous web from its under side to its upper side when the extractor is placed on the upper side of the web. On passing through the web, the air pulse will dislodge dust particles, so that they may be carried away by the air flow sucked into the downstream end of the extractor. A certain dust particle dislodging effect may also result from microturbulence caused by shear in the thin boundary layer of air between the web and the dust extractor. The invention gives a very efficient catching and extraction of the dust that is separated from the web at creping and, thus, it brings about a decided and very marked improvement of the work environment for the operating personnel concerned. Further features of the invention and what is achieved thereby will be apparent from the following description. The invention will hereafter be described more in detail with reference to the enclosed drawings. Figure 1 is a schematic side elevational view of a section situated between a Yankee dryer and a reel in a soft crepe paper machine, said section being provided with a plurality of preferred, varying embodiments of a dust extractor according to the invention. Figure 2 is a schematic side elevational view, which on a larger scale shows an assembly included in Figure 1 and comprising two dust extractors assembled together. In the soft crepe paper machine section shown schematically in Figure 1, a paper web adhering to the envelope surface of a clockwise rotating Yankee dryer 3 is lead downwards to a creping doctor 5. There, web 1 is creped off Yankee dryer 3 by a doctor blade 7, which is mounted on a holder 9 extending mainly vertically upwards from a doctor beam 11. From doctor blade 7, the creped paper web 1 runs obliquely downwards and through a grammage scanner 13 and a spreader formed as a curved bar 15 before it reaches reel 17. In the arrangement shown, the latter consists of a drum winder with a drive cylinder 19. An empty reel spool 21 has just been lowered down to contact with drive cylinder 19, next to which a ready made reel 23 of soft crepe paper is shown. At creping, dust separates from web 1, and part of this dust will remain on web 1 while another part will be entrained in a boundary layer on each side of the creped web, that can run forward at a velocity of close to 25 m/s. Most of the remaining dust is bound to fall down on both sides of doctor blade 7 and holder 9 towards doctor beam 11. To catch and extract at least an essential part of the dust contents in the boundary layers, a plurality of dust extractors are provided between creping doctor 5 and grammage scanner 13. In the embodiment shown in Figure 1, a first and a second such extractor 25′ and 25″, respectively, have been fitted together to form an assembly 25, which is mounted above web 1 to catch and extract the dust contents in the boundary layer on the upper side of web 1, whereas a third dust extractor 27 is mounted under web 1 upstream of assembly 25 to catch and extract the dust contents in the boundary layer on the underside of web 1. According to the invention, each of the dust extractors 25′ and 25″, as is best shown in Figure 2, comprises a web-wide hood 29′ and 29″, respectively, with a front side edge and a rear side edge in relation to the travelling direction of web 1. The front edge of hood 29′ is denoted by 31 and the rear edge of hood 29″ is denoted by 33. The hoods 29′ and 29″ are built together tandemwise to form a unit and are separated by a common partition wall 35. An imperforate flat cover plate 37 has a front half 37′ and a rear half 37″, located to substantially close the corresponding hood 29′ and 29″, respectively, so as to define an internal space 39′ and 39″, respectively, and form a gap 41′ between hood 29′ and the front cover plate half 37′, and a gap 41″ between hood 29" and the rear cover plate half 37″. The two internal spaces 39′ and 39″ are not interconnected. Instead, there are separate means for the two spaces 39′ and 39″, shown as suction pipe stubs 43′ and 43″, each connected to an air exhauster, not shown, for separately regulating the subatmospheric pressure in spaces 39′ and 39″ and thereby the suction of air through gaps 41′ and 41″. Preferably, the plane cover plate 37 has a relatively high degree of flatness accuracy. A tolerance range in the order of 1.5 mm is recommended. To avoid that dust extractors 25′ and 25″ become deformed under operation to an extent that would be detrimental to their function, it is advisable that they are provided with stiffeners in a manner conventional for sheet-metal designs. In the embodiment shown in Figure 2, a stiffening, inclined sheet-metal member 45′ and 45″, respectively, extends in each of spaces 39′ and 39″ from a line somewhat inside each of gaps 41′ and 41″ to a horizontal line about half-way up the common partition wall 35. These inclined sheet-metal members also contribute to deflect the air stream from the gaps towards the suction pipes. Furthermore, at the gaps 41′ and 41″, the hoods 29′ and 29″ are internally stiffened with each a sheet-metal angle strip 47′ and 47″, respectively, extending along the gap, and from each of these angle strips, a plurality of legs 49′ and 49″ protrude down to the top surface of the plane cover plate 37. On the external side of the part of the hood 29′ and 29″ that is stiffened with angle strips 47′ and 47″, a flat bar 51′ and 51″, respectively, is extending along gap 41′ and 41″. In each one of these flat bars, there is a row of screws screwed-in, not shown, which extend through slots, not shown, running perpendicularily to the plane of the cover plate 37 and provided in a sheet-metal strip 53′ and 53″, respectively, that can slide to and from the plane of cover plate 37 to adjust the widths of the associated gaps 41′ and 41″, respectively. As can be seen from Figure 2, the gap defining edge of the sheet-metal strip 53″ is situated downstream of the downstream edge of cover plate 37, whereby the actual suction gap at operational conditions will be formed between sheet-metal strip 53″ and web 1. In the soft crepe paper machine the assembly 25 that consists of the two extractors 25′and 25″ is mounted with its cover plate 37 located immediately adjoining an intended travelling path for the creped web 1. Thereby, the plane imperforated cover plate 37 will have a web stabilizing effect, so that web 1 when travelling will have its position fixed close to and very lightly touch cover plate 37, said touch on a macroscopic scale being defined by at least a line of contact extending over substantially the entire width of the web, and gaps 41′ and 41″ will be situated in the dust-containing boundary layer. In the embodiment shown in Figure 2, the paper web is deflected a few degrees in counter clockwise direction when passing the upstream edge of cover plate 37. At the downstream end of cover plate 37, about 1.5 cm of plate 37 is somewhat tilted upward, so that paper web 1 is deflected some more than 10 degrees in counter clockwise direction, before web 1 runs further in straight direction to grammage scanner 13. Said very light touch is obtained both at the upstream and the downstream edge of cover plate 37. The touch gives a certain working of paper web 1, which at the downstream edge of cover plate 37 effectively contributes to the extraction of dust carried by web 1. As described above, at the downstream edge of cover plate 37 web 1 will be subjected to a pulse of air that passes through the creped porous web 1 from its under side to its upper side, when dust extractor 25 is located on the upper side of web 1. On passing through web 1, the air pulse will dislodge dust particles so that they may be carried away by the air flow sucked into the downstream end of dust extractor 25. A certain dust particle dislodging effect may also result from microturbulence caused by shear in the thin boundary layer of air between web 1 and dust extractor 25. Since not all dust that is separated from web 1 at the creping-off from Yankee dryer 3 will be entrained by the boundary layers on both sides of web 1, part of the separated dust will fall down from the area adjacent doctor blade 7. To catch a major portion of this dust, a dust suction box 55 is provided, as shown in Figure 1, immediately adjacent doctor beam 11 on the side facing the creped web 1. The dust suction box 55, which for example can be screwed onto beam 11 by means of screws, not shown, has a suction box cover 57. This is sloping downwards and outwards from Yankee dryer 3, and the lower longside edge of suction box cover 57 is situated at a considerable distance from the boundary layer on the under side of the creped web 1. Suction box cover 57 is suitably made of perforated sheet-metal and is sloping in such a manner that web 1, when possibly breaking, will not fall down onto and remain lying on suction box cover 57 but will instead slide further downwards to a collecting place. Preferably, a second dust suction box 59, which comprises a suction box cover 61 and is of a design that in all essentials may be reversed in comparison to dust suction box 55, is provided immediately adjacent the opposite side of doctor beam 11, onto which it may be screwed. As mentioned above, the third dust extractor 27 is mounted under the creped web 1 upstream of assembly 25 in order to eliminate at least an essential portion of the dust-containing boundary layer on the under side of web 1. Extractor 27 is of the same principle construction as the second, rearmost dust extractor 25″ and, consequently, is working solely by suction at the rear edge of an imperforate plane, web-stabilizing cover plate. The distance between the third dust extractor 27 and dust suction box 55, which is located on the one side of the creping doctor 5 that faces the creped web, and the suctions in these dust extractors are so balanced in relation to each other that the space between these extractors is in all essentials completely occupied by a rising air stream, strong enough to carry substantially all of the dust particles that are separated from web 1. Such an air stream in the dust densest space contributes effectively to the mastering of the problem of dust removal and results in an important improvement of the work environment for the personnel concerned. In order to facilitate the desired flow of air, the wall members, between which the distance in question is defined, can be designed as illustrated in Figure 1 so as not to unnecessarily prevent the flow. The invention is not restricted to what is shown on the drawings, but a plurality of modifications, not shown and not described, are possible within the scope of the appended claims. For example, dust extractor 27 under web 1 can be exchanged against an assembly like assembly 25 on the upper side of the web and having a suction gap also at the front edge, provided that there is space enough for such an assembly. Furthermore, a sheet-metal strip of the same design as sheet-metal strip 53″, which at the rear edge of the assembly 25 defines the suction gap against the web, may replace sheet-metal strip 53′, which, at the front edge of the assembly 25, defines the suction gap against the plane, imperforate cover plate 37.

Claims: 1. Method of extracting dust that is separated on creping a soft crepe paper web (1) off a Yankee dryer (3) by means of a creping doctor (5), part of the dust being carried along by the web in a boundary layer of air, which because of the transport of the web (1) from the Yankee dryer (3) towards a reel (17) becomes entrained and follows the web (1), characterised by providing a dust extractor (25′, 25″) having an internal space (39′, 39″) and a web stabilizing, imperforate plane (37) immediately adjacent an intended path of travel for the creped web (1), so that the web (1) when travelling will place itself in a fixed position close to the imperforate plane (37), and eliminating at least an essential part of the dust-containing boundary layer solely through suction to the internal space (39′, 39″). 2. Method according to claim 1, characterised by carrying out the suction at a rear edge (33) of the dust extractor (25′, 25″) in relation to the direction of travel of the creped web (1). 3. Method according to claim 2, characterised by carrying out the suction through an elongated and in relation to the creped web (1) transversal gap (41″) that is defined between the web (1) and a parallel, preferably adjustable portion (53″) of said edge (33). 4. Method according to claim 3, characterised by carrying out the suction with a volumetric flow rate such as to make the velocity of the air in the gap (41″) be of the same order of magnitude as the velocity of the web (1) and opposed to the latter. 5. Method according to any one of claims 2-4, characterised by carrying out the suction also at a front edge (31) of the dust extractor (25′, 25″). 6. Method according to any one of claims 1-5, characterised by eliminating, solely by suction, at least a substantial portion of the boundary layers on both sides of the creped web (1). 7. Method according to claim 6, characterised by leading the creped off web (1) away from the Yankee dryer (3) above the creping doctor (5), that comprises a doctor beam (11), providing immediately adjacent to the doctor beam (11) on the side facing the creped web (1) a dust suction box (55) having a suction box cover (57), which slopes downwards and outwards from the Yankee dryer(3) and has a lower longside edge situated at a considerable distance from the boundary layer on the under side of the creped web (1), providing at a distance from the creping doctor (5) a web stabilizing, solely through suction acting dust extractor (27) to remove at least an essential part of the dust-containing boundary layer from the under side of the web (1), and balancing said distance and the suction in the dust suction box (55) and in the dust extractor (27) so in relation to one another, that the space between the dust suction box (55) and the dust extractor (27) is in all essentials completely occupied by a rising air stream, strong enough to carry substantially all of the dust particles separated from the web (1). 8. Method according to claim 7, characterised by providing immediately adjacent the opposite side of the doctor beam (11), which side faces the Yankee dryer (3), a second dust suction box (59) having a suction box cover (61). 9. Method according to claim 7 or 8, characterised by carrying out the suction of the boundary layer on the upper side of the web (1) at a location that is downstream of the location of the suction of the boundary layer on the under side of the web (1). 10. Method according to any one of claims 1 to 9, characterised by mounting the dust extractor (25′, 25″) immediately adjacent the intended path of travel of the creped web (1) in a position such that on a macroscopic scale the web (1) will touch the imperforate plane (37) in at least a line of contact extending across substantially the whole width of the web (1). 11. Method according to claim 10, characterised by placing the line of contact at a downstream end of the plane (37). 12. Apparatus for extraction of dust that is separated at the creping of a soft crepe paper web (1) off a Yankee dryer (3) by means of a creping doctor (5), part of the separated dust being entrained in a boundary layer of air, which, because of the travelling of the web (1) from the Yankee dryer (3) towards a reel (17), becomes entrained and follows the web (1), characterised in that the apparatus comprises a hood (29′, 29″) of paper web width having a front side edge (31) and a rear side edge (33) in relation to the path of travel of the web (1), an imperforate plane cover plate (37) located to substantially close the hood (29′, 29″) while defining an internal space (39′, 39″) and forming a gap (41′, 41″) between at least one of said side edges (31, 33) and the cover plate (37), means (43′, 43″) for producing a subatmospheric pressure in said internal space (39′, 39″), so that air is sucked in through the gap (41′, 41″), said apparatus being adapted to be mounted with its cover plate (37) immediately adjacent an intended path of travel for the creped web (1), whereby the plane, imperforate cover plate (37) will get a web stabilizing effect, so that the web (1) when travelling will place itself in a fixed position close to the cover plate (37), and the gap (41′, 41″) will be situated in the dust containing boundary layer. 13. Apparatus according to claim 12, characterised in that said side edge (33, 31) is situated downstream of a rear edge and upstream of a front edge, respectively, of the cover plate (37), whereby an actual suction gap is defined against the web (1). 14. Apparatus according to claim 12 or 13, characterised in that said side edge (31, 33) is a strip (53′, 53″) adjustably mounted on a hood body, said strip being movable towards and from the web (1) to set the width of the gap (41′, 41″).

Inventors: MYREN, HARRY INGEMAR, Myrén, Harry Ingemar
Applicants: VALMET PAPER MACHINERY INC.
Application year: 1988
Main classification: D21G9
IPC codes: B31F1, D21G9, B65H23, D21G3