Transfer of a lead strip of a paper web

Abstract: Method and apparatus for transferring a lead strip of a paper web, in particular the beginning of a still wet lead strip (or "tail" 9), from a press roll (12) of a paper making machine to a following section (21) of that machine. Provided is an air jet device (16) for peeling off the beginning of the tail (9) from said press roll (12) and for transferring it across a paper roll (20) to an infeed area of a felt (24) which guides the web (9') into said following section (21). On an infeed area of said felt (24) an air cushion is created by an air table (40) having a plurality of tiny holes (44) delivering air from a low pressure plenum (41) to the air cushion.

Description: FIELD OF INVENTION This invention relates to a method for transferring a lead strip of a paper web from a first travelling surface (e.g. of a press roll or of a press belt) of a paper making machine to a following section (e.g. further wet press or drying section) of that machine. In particular, the beginning of a still wet lead strip (or "tail") is transferred according to the invention. This is to facilitate the threading of the paper web into the machine. The present invention also relates to an apparatus for carrying out said method. DESCRIPTION OF PRIOR ART The present invention may preferably be used in a dewatering press section of a papermaking machine as disclosed in Fig. 1 of US patent 5,404,811. Therein a still wet paper web 9 is travelling from a wire belt 8 of a forming section through three press nips 16/17, 1/2 and 2/3 and thereafter via a paper roll 59 to a further dewatering press 60; then the still wet web is transferred to a drying section (not shown). When the machine is started up or after a web break, the web is threaded into the machine in a known manner: Initially the web (having the full web width) is running behind the third press nip downwardly and is guided by a doctor 14 into a broke pulper (not shown, positioned below the machine). The web comprises a small edge strip, namely the so-called lead strip or "tail", severed from the web by a water jet positioned in the forming section. This tail is now transferred across paper roll 59 and via a bottom press felt through press 60 as well as through the following drying section. Then the width of the tail is increased up to the full width of the web. During the threading operation the web is already running with the full machine speed which may be more than 1000 m/min, in modern high speed machines up to about 2000 m/min. Therefore, the transfer of the tail from press roll 2 to the press felt of press 60 is a very difficult step of the threading operation. Sometimes, this is done manually by means of an air jet being directed onto the surface of roll 2, thusly severing the tail and forming a new beginning of the tail guided across paper roll 59 to the further press 60. A modern high speed paper making machine normally comprises an apparatus for carrying out this difficult step. One known apparatus of this type is disclosed in Fig. 2 of US patent 5,635,030. Here again, a paper web 1 is travelling downwardly across a press roll 5 from a press nip toward a doctor 7. A blast nozzle 6 (or "separating blow pipe") is provided to peel off the tail from press roll 5 and to transfer the tail to paper roll 2. A further blast nozzle 3 is arranged between the two rolls 5 and 2, which blast nozzle creates an air stream, the velocity of which is greater than the velocity of paper roll 2. Due to the Coanda-Effect, the air stream adheres to the rotating shell of paper roll 2 and guides the tail up to a stationary guide plate 9 which deflects the air stream and the tail toward subsequent press unit 8. It is believed that the method and the apparatus disclosed in US '030 have some disadvantages. Among others, two blast nozzles are needed, one being positioned between the press roll 5 and the paper roll 2. Also, there is a large distance between the paper roll 2 and the infeed area of the press felt 17 of the following press 8. As a result, the tail transfer to the following press may not always be successful in a reliable manner. In another concept, only a short distance has been provided between the paper roll and the infeed area of the following press felt. In other words: A felt roll guiding the following press felt to the following press has been arranged relatively close to the paper roll (as shown in Fig. 1 of US '811). Also, an air cushion has been created onto said infeed area by means of a series of blast nozzles. However, this concept too, does not always operate satisfactorily. Also, a so-called pony roll has been suggested to be arranged on said infeed area instead of an air cushion; however this is mechanical complicated and therefore not desirable. SUMMARY OF THE INVENTION It is an object of the present invention to significantly improve the transfer of a tail from a first travelling surface to a following machine section by means of a novel method and by means of an improved apparatus which will operate more reliably than previous proposals. In particular, it is an object of the invention to provide a method which may be carried out without a lot of an operator's skill, so that new people, too, would be able to start the threading operation without a lot of practice. It is a further object of the invention to allow an easy start of the threading operation by a reliable transfer of the tail, even with different paper grades (e.g. different basis weight) and with different machine speed including extremely high speed (e.g. more than 2000 m/min). The aforementioned object as well as further objects that will come out later are attained by the features defined in the appended claims. According to the method of the present invention (claim 1), a lead strip or "tail" of a paper web is transferred from a first travelling surface of an element of a papermaking machine to an infeed area of a second travelling surface which guides the tail into a following machine section. Preferably, said first travelling surface is the surface of a rotatable shell of a press roll which directly contacts the still wet paper web and which is part of a web dewatering press. It may also be the surface of a press belt travelling through a press nip of a web dewatering press. Preferably, said second travelling surface is the web carrying surface of a dewatering press felt which guides the web through a subsequent dewatering press. However, said second travelling surface may also be the surface of a subsequent press roll or of a subsequent press belt. In another embodiment of the invention, the second travelling surface is the surface of a dryer fabric which guides the web through a part of a dryer section following the press section of the papermaking machine. The method of the present invention comprises providing at least one air jet for peeling off of the tail from the first travelling surface and for transferring it across a rotating paper roll to the infeed area of the second travelling surface. More particularly, the air jet initially severes the tail running with the first travelling surface, thereby forming a new beginning of the tail which is now transferred to the second travelling surface. The most important step of the method of the present invention is to provide - on the infeed area of the second travelling surface - an air cushion, this being created by an air table having a plurality of tiny orifices delivering air from an air plenum to the air cushion. According to the invention, the air cushion is created by a large number of tiny orifices which connect said air plenum to the air cushion and which are distributed substantially equally on said air table. This results in a significant advantage, namely that the oncoming tail (including its new beginning) is forced to lay on the second travelling surface with only a small amount of air moving with the second travelling surface. Therefore, the tail which is running at the high machine speed together with the second travelling surface, is reliably pressed onto the second travelling surface. In other words: According to the invention, it is avoided that the air bounces off the second travelling surface and that the tail is lifted with the air. Such an undesirable behavior would result from creating an air cushion by means of a series of blast nozzles which deliver too much air at a too high pressure. Also, it is avoided that too much air is directed into the following machine section, e.g. into a further dewatering press. Therein the press nip (or a similar wedge-like gap) would cause the air to flow sideways and to take the tail with it. This could happen both before the beginning of the tail has arrived the press nip and after that. In summary, the method of the invention results in a very easy and reliable transfer of the tail so that the transfer does not need much operator skill. Also, the transfer will work well with different paper grades and with different machine speeds including high speed of modern paper machines. The method of the invention may be further improved by the features defined in the claims depending from claim 1. The present invention also includes an apparatus as claimed in claim 5 for transferring the tail as described above, the apparatus comprising a peeling jet device for providing the at least one air jet as mentioned above; said peeling jet device being arranged close to the first travelling surface (e.g. the surface of a press roll). Said apparatus further comprises a conventional paper roll which guides the web between the first and second travelling surfaces. The most important element of the apparatus of the invention is a so-called air table having a large number of tiny holes which connect an air plenum with the infeed area of the second travelling surface, thereby creating an air cushion which is almost stationary on the second travelling surface. This results in the advantages described above. Further improvements of the apparatus may be provided according to the claims depending from claim 5. BRIEF DESCRIPTION OF THE DRAWING The attached drawing illustrates the invention by way of example. Fig. 1 is a schematic side view of two dewatering presses of a papermaking machine including an air table. Fig. 2 is a view along arrow II of Fig. 1. Fig. 3 is a sideview of a segmented air table. DESCRIPTION OF THE EMBODIMENTS SHOWN IN THE DRAWING In Fig. 1, the normal path of a paper web to be dewatered in two presses 11 and 21 is shown as a broken line and designated 9'. Web 9' is travelling through the nip of a top-felted press 11 comprising a bottom press roll 12 (which directly contacts the web) and a top press roll 13 positioned in the loop of an endless dewatering felt 14. Bottom press roll 12 comprises a (downwardly) "first travelling surface" 15. Close to it, a peeling jet device 16 and a conventional doctor 17 are arranged. A subsequent dewatering press 21 is bottom felted. It comprises a top press roll 22 (which directly contacts the web) and a bottom felt 24 guided by felt rolls 28 and 29, one (28) of which is positioned near the bottom press roll 12 of press 11. The part of felt 24 travelling from felt roll 28 to the nip of press 21 forms a so-called "second travelling surface" 25 comprising a so-called "infeed area" (positioned on felt roll 28) wherein the web comes into contact with felt 24. Web 9' is guided by a paper roll 20 from the first (15) to the second (25) travelling surface. There is a short distance between surface 15 and roll 20 as well as between roll 20 and surface 25. Behind press 21, web 9' is travelling across a further paper roll 30 to a dryer fabric 34 running across a fabric roll 33 transferring the web to a first drying cylinder 32 of a drying section 31. Normally, at least one further press nip (not shown) may be arranged upstream of press 11. Typically, press 11 forms a third press nip of a complete press section; then press 21 forms a fourth press nip. Before the web 9' is travelling along its normal path it runs across press roll 12 and doctor 17 downwardly into a broke pulper (not shown). For threading the web into the bottom felted press 21 (and further to dryer section 31) an edge strip or "tail 9" of the web is severed by peeling jet device 16, peeled off from the first travelling surface 15 and transferred to the second travelling surface 25. An air table 40 extends along the infeed area of the second travelling surface 25 of felt 24. The air table also extends from paper roll 20 across the gap between paper roll 20 and felt 24 at felt roll 28. The width of air table 40 is slightly larger than the width of the tail 9 (see Fig. 2). The air table 40 comprises a box 41 connected to an air pressure source 43 providing a relatively low air pressure, the box 41 thusly forming a so-called air plenum, wherein the air pressure is preferably between 0,35-1,38 bar (5 and 20 psi). Box 41 comprises a convex wall 42 facing to felt 24, the convex wall having a plurality of tiny holes 44 or orifices. The diameter of each hole may be about 0,5 to 2 mm; the percentage of open area will be about 0,03 % to 0,1 %. Thereby a stable air cushion is created between air table 40 and the travelling surface 25 of felt 24 pressing the tail onto felt 24 without moving much air toward the nip of press 21. As a further improvement, box 41 is arranged in such a way, that the air cushion formed between box 41 and surface 25 is converging with respect to the web travel direction. Close to paper roll 20, an air jet pipe 45 is mechanically connected to box 41, the pipe 45 being connected to a high pressure source 46. Peeling jet device 16, too, may be connected (not shown) to high pressure source 46. Air jet pipe 45 may provide an air jet approximately tangential to the shell of paper roll 20 contrary to the travel direction of the shell. At most one further air jet or air curtain may be directed from pipe 45 or from an additional pipe 67 (Fig. 3) toward the infeed area, i.e. toward felt roll 28. Peeling jet device 16 produces an air jet about perpendicular to the first travelling surface 15. If needed, jet device 16 may additionally create an air jet toward paper roll 20. In order to further improve the reliable transfer of the tail to the second travelling surface 25 or 34 it may be helpful to provide suction means (70 - 73) in the area of the tail within roll 28 and/or within roll 33 and/or beneath felt 24 and/or fabric 34. Fig. 2 shows the peeling jet device 16 and the air table 40 in their operating position held during a threading operation. During the normal operation of the papermaking machine, the peeling jet device 16 is removed to the outside of the machine. For this purpose, a pneumatic cylinder (not shown) or a similar equipment is provided which moves the device 16 automatically into or out of its operating position, as indicated by a double arrow. If needed, the air table 40 may also be connected to a pneumatic cylinder. In order to facilitate the transfer of tail 9 from press roll 22 via paper roll 30 to the drying fabric 34, a further peeling jet device 26 is arranged close to the surface of press roll 22. Also, a further air table 50 may be arranged below fabric roll 30. In this case, press roll 22 forms the said first travelling surface and the drying fabric 34 forms the said second travelling surface. Fig. 3 shows a further developed air table 60. It is divided into various (e.g. three) segments 61 to 63. Each segment is formed as a box similar to box 41 of Figs. 1 and 2. One (61) of the boxes is mechanically connected to a supporting device 65 which supports the complete air table 60 and which may be connected (if needed) to a pneumatic cylinder (not shown) of the type explained above. The segments 61 and 62 as well as the segments 62 and 63 are mechanically connected one to the other by a pivot 64. Also, the outer end of the third segment 63 is connected to support 65 by a spindle 66 (or threaded bolt). Therefore, the distance between the outer end of segment 63 and support 65 may be changed. In other words: The average radius R of the curvature of the complete air table 60 may be changed. This allows to change the form of the complete air table 60 in order to adapt the same to various operating conditions, e.g. to various paper grades or to various machine speeds. As an example: The form of the complete air table may be approximately flat. The segments 61-63 may be provided with equal air pressure or with different air pressures. Finally, it is possible to create air tables of different size by using a different number of segments (e.g. two or three or four etc.).

Claims: Method for transferring a lead strip (9) of a paper web (9'), in particular the beginning of a still wet lead strip (9), from a first travelling surface (15) of a paper making machine to a following section (21, 31) of that machine, the method comprising: providing at least one air jet (16) for peeling off the beginning of the lead strip (9) from said first travelling surface (15) and for transferring it across a paper roll (20) to an infeed area of a second travelling surface (25) which guides the web (9') into said following machine section, and providing onto the infeed area of said second travelling surface (25) an air cushion created by an air table (40) having a plurality of fine or tiny holes (44) delivering air from an air plenum (41) to the air cushion. Method of claim 1, further including: creating said air cushion by means of said plurality of fine or tiny holes (44) which are equally distributed on said air table (40). Method of claim 1, further including: providing in said air plenum (41) a low air pressure of between 0,35-1,38 bar (5 and 20 psi). Method fo claim 1, further including: delivering air from said air plenum (41) through said holes (44) substantially vertically onto said second travelling surface (25). Apparatus for transferring a lead strip (9) of a paper web (9'), in particular the beginning of a still wet lead strip (9), from a first travelling surface (15) of a paper making machine to a following section (21, 31) of that machine, comprising a peeling jet device (16) provided for peeling off the beginning of the lead strip (9) from said first travelling surface (15) and for transferring it across a paper roll (20) to an infeed area of a second travelling surface (25) which guides the web (9') into said following machine section, characterized by an air table (40) extending along the infeed area, said air table being perforated by having a plurality of tiny holes (44) being connected to an air pressure source (43) and opening onto said second travelling surface (25). Apparatus of claim 5, further comprising the following features: a) said paper roll (20) being arranged in a distance from the infeed area of said second travelling surface (25) thereby forming a gap therebetween; b) said air table (40) extending from the paper roll (20) across said gap and along the infeed area of the second travelling surface (25). Apparatus of claim 5 wherein the air table (40) is curved, with said holes (44) being arranged at the convex side of the air table. Apparatus of claim 5, wherein the air table (40) supports an air jet device (45) for directing an air jet onto the surface of the paper roll (20). Apparatus of claim 8, wherein the air jet is directed approximately tangential onto the surface of the paper roll (20) contrary to the travel direction of the roll shell. Apparatus of claim 5, wherein the air table (40) supports an air curtain device (67) for providing an air curtain which flows along the perforated outer surface of the air table (40) in the web travel direction. Apparatus of claim 5, wherein the air table (60) is divided into at least two segments (61-63) which are connected one to the other by a pivot (64), for providing the possibility to change the average radius R of curvature of the air table. Apparatus of claim 5, wherein the diameter of the holes (44) is about 0,5 to 2 mm. Apparatus of claim 5, wherein the percentage of open area formed in the air table by said plurality of holes is about 0,03 % to 0,1 %. Apparatus of claim 5, wherein the air table (40) is forming with the second travelling surface (25) a convergent air cushion, with respect to the web travelling direction. Apparatus of claim 5, wherein close to said second travelling surface (25) in the area of said lead strip (9) suction means (70 - 73) are provided.

Inventors: BROOM ALLAN, MOHRSEN LEIF, BROOM, ALLAN, MOHRSEN, LEIF
Applicants: VOITH PAPER PATENT GMBH
Application year: 2001
Main classification: D21G9
IPC codes: D21G9