Method and device in the drying section of a paper machine in the threading of the web

Description

[0001] The invention concerns a method of threading of a paper web in a multi-cylinder dryer of a paper machine as defined in the preamble of claim 1.

[0002] Further, the invention concerns a multi-cylinder dryer of a paper machine as defined in the preamble of claim 6.

[0003] As is known in prior art, in the drying section of a paper machine, so-called single-wire draw or twin-wire draw is employed. In such a case, for the threading of the web, as a rule, rope guide means are used, by whose means the leader, which has been cut out of a full-width web, is passed over the drying cylinders, whereupon the leader is widened to a web of full width. The system of threading ropes is quite a costly and complicated device, and disturbance occurs in its operation. This is why attempts have been made to arrange the threading without a set of threading ropes. Said modes of threading without ropes have been applied in particular with single-wire draw. In this respect, reference is made to the paper by Sam Palazzolo, "No-draw drying" in Tappi Journal, September 1990, pp. 225-228.

[0004] As is stated in said paper, with single-wire draw, the threading involves the drawback that the leader tends to follow the smooth face of the drying cylinder in stead of the drying wire as the wire runs from the drying cylinder to the following leading suction cylinder or roll. To make sure that the leader follows expressly the drying wire, according to said paper, arrangements of air nozzles have been employed, of which nozzle arrangements one has been fitted before each leading suction cylinder and through which nozzle arrangements air jets are blown to guide the run of the leader. Hereby, attempts are made to make sure that the leader remains on the drying wire. After the leader has reached the suction zone on the leading cylinder, the leader is transferred on the wire onto the next drying cylinder.

[0005] It is a drawback of the blow arrangement described above that, if the leader has been carried into connection with the blade of the doctor operating against the drying cylinder, the leader cannot be passed forwards from the blade without manually operated auxiliary blowings. In threading, there is, however, a need to drop the leader from the lower faces of several or all of the drying cylinders by means of the doctor blade down into the broke processing system placed underneath. This has not been readily possible in the prior-art rope-free web threading arrangements.

[0006] On the other hand, in a drying section, it is known in prior art to employ air-blow devices fitted in connection with doctor beams, but these blow devices are usually used for air-conditioning of the drying section, and their nozzles blow across the entire transverse width of the web. Also, in threading, blow devices operating in connection with a doctor have been used as an aid, but these have involved a number of drawbacks and they are not as such suitable for use in rope-free threading. In respect of said prior-art devices, reference is made, by way of example, to the FI Patents and Published Patent Applications Nos. 50,263, 54,954, and 62,571 and to the FI Patent Applications Nos. 833590 and 894609. In some of said publications, blow devices placed after the drying section are described, by whose means the leader is guided.

[0007] The principal object of the present invention is to provide novel solutions for the problems discussed above so that said drawbacks are largely avoided.

[0008] In view of achieving the objectives stated above and those appearing from below the method of the invention as defined in the preamble of claim 1 has got the features defined in the characterising clause of claim 1.

[0009] Further the multi-cylinder dryer as defined in the preamble of claim 6 has got the features defined in the characterising clause of claim 6.

[0010] The invention can be applied both in drying cylinder groups provided with single-wire draw and in cylinder groups provided with twin-wire draw, if necessary, over the entire length of the drying section. In connection with single-wire draw, the invention is suitable for use both in normal groups and in inverted groups. According to the present-day knowledge, the invention is most advantageously applicable in connection with single-wire draw.

[0011] According to the invention, when the blow pipes with their nozzles are placed expressly in connection with the doctor beams, an advantageous and solid "base" is obtained for the blow devices without having to make use of separate support constructions. Fitting of such constructions in the narrow gaps between the cylinders is difficult. Thus, the construction of the auxiliary devices for threading is simplified and the maintenance and operation of said devices become easier.

[0012] According to the invention, when the air jets are directed expressly at the area of the tip of the doctor blade at a suitable small angle in relation to the plane of the blade, the blowings can be deflected along the cylinder face that precedes the blade so that the effect of the blowing extends into the opening nip. In this way the blowings obtain a sufficient support and transfer effect on the leader. Of course, in the blowings, a sufficiently high energy must be used to provide a reliable transfer and support effect.

[0013] In the following, the invention will be described in detail with reference to some preferred exemplifying embodiments of the invention illustrated in the figures in the accompanying drawing, the invention being not support to be confined to the details of said embodiments.

[0014] Figure 1 shows a drying section of a paper machine which is provided with single-wire draw and in which the method and the device of the invention are applied.

[0015] Figure 2 illustrates parameters that are of significance in the location and dimensioning of the method and the device in accordance with the invention.

[0016] Figure 3 is a side view of a first embodiment of equipment of the invention

[0017] Figure 4 shows the same device as Fig. 3, seen in the direction IV-IV.

[0018] Figure 5 shows a device in accordance with the invention as applied in an inverted cylinder group provided with single-wire draw.

[0019] Figure 6 shows an application of the invention in a gap between cylinder groups in which the operation is transferred from single-wire draw to twin-wire draw as well as in a cylinder group with twin-wire draw.

[0020] Figure 7 shows an application of the invention in cylinder groups with twin-wire draw and in the gaps between such cylinder groups.

[0021] Fig. 1 is a schematic illustration of a cylinder group with single-wire draw, in which, in the upper row, there are heated drying cylinders 10 and, in the lower row, leading suction cylinders 20 with perforated and grooved outer mantle. Fig. 5 shows a corresponding inverted group, wherein the drying cylinders 10′ are placed in the lower row and, correspondingly, the leading suction cylinders 20′ in the upper row.

[0022] As is shown in Fig. 1, the drying wire 11 of the single-wire draw runs meandering in contact with the drying cylinders 10 and the leading cylinders 20 so that, on the drying cylinders 10, the web to be dried enters into direct contact with the heated smooth faces of the cylinders 10 and, on the leading cylinder 20, the web is placed at the side of the outside curve on the outer face of the wire 11, where it is held by negative pressure acting from the interior of the cylinder 20 through the perforations and grooves (not shown) in its mantle. On the runs of the wire 11 and the web on which they run together from the drying cylinders 10 to the leading cylinders 20, at the side of the wire 11, a blow box 15 is provided, for example a device marketed by the applicant with the trade mark "UNO RUN BLOW BOX", by means of which formation of a detrimentally high negative pressure in the pressurized nip N+ is prevented and a suitable negative pressure is maintained at the side of the wire 11, which negative pressure guarantees that the web remains on the face of the wire 11 on said straight runs.

[0023] As is shown in Fig. 1, doctors operate against the free lower sector of the cylinders 10, which doctors comprise a doctor beam 12 and a doctor blade 14, which is attached to said beam by means of a holder 13 and doctors and keeps the smooth faces of the cylinders 10 clean. In an inverted cylinder group as shown in Fig. 5, corresponding doctors are fitted above the cylinders 10′ against their free sectors.

[0024] In accordance with the present invention, the doctors 12,13,14 are provided with blow devices, by whose means blowings F₁ are directed against the tip portions of the doctor blades 14, from which the blowings bounce and are "reflected" in the direction F₂ towards the opening nip N-. By means of the blowings F₁ and F₂, such a situation is prevented that the leader S were detached from the drying wire 11 and followed the cylinder 10 face.

[0025] In Fig. 1, the end of the leader S is denoted with the reference S_o, which represents the situation in which the end S_o is transferred on the suction sector of the leading suction cylinder 20 to the next cylinder 10. Figs. 2, 3 and 4 show an embodiment of the device in more detail. According to Figs. 3 and 4, an air distribution pipe 22 has been attached to the side of the doctor beam 12, one end of said pipe 22 being a closed end 22a, whereas the other end communicates with the blow-air supply duct 24. The blow pipe 22 is provided with a number of nozzle holes 23 across the width L₂. The blow pipe is arched to its place by means of flanges 25 and 26.

[0026] Out of the nozzle holes 23, air jeu F₁ are blown, which are directed precisely at the point K (Fig. 3) at the proximity of the tip of the doctor blade 14. Thus, in the invention, the blowings F₁ are not directed in the direction of the plane of the blade 14, but at a small angle B towards the blade.

[0027] By means of suitable choice of the angle B, the air jets F₁ can be directed at the area of the tip of the doctor blade 14 so that the air jets bounce and are "reflected" from the plane of the doctor blade and are deflected in a direction parallel to the outer face of the cylinder 10, being directed in the direction F₂ at the opening nip N-. In this way, a sufficiently high transfer and support effect is produced for the blowings F₁, said effect being applied to the leader. Of course, the energy of the blowings F₁ must be high enough to achieve said objectives.

[0028] In a way in itself known, the doctor beams 12 are attached by their ends, by means of axle journals 16, in connection with their pivoting and loading means (not shown).

[0029] In Fig. 4, the width of the leader S is denoted with the reference L₁. The extension L₂ of the blowings F₁ in the transverse direction is of the same order (width L₂) as the width L₁ of the leader S. Air from the blowings F₁ is guided along the face of the cylinder 10 in the direction of the arrow F₂ into the opening nip N-. In this way, it is achieved by means of the blowings F₁,F₂ that the leader S is pressed against the wire 11, in which position the suction effect of the blow box 15 keeps the leader S.

[0030] When the leader must be made to fall down onto the broke processing means (not shown) at a certain cylinder 10, the blowings F₁ of the doctor placed below said cylinder 10 are closed, whereby the leader S is carried by nature to the doctor blade 14, which detaches it from the cylinder 10, and the leader falls down onto the broke conveyor placed underneath, which conveyor carries the broke into the pulper. When the leader S has to be passed further from said cylinder 10, the blowings F₁ at this cylinder 10 are controlled to make them operative, whereby, owing to the direction of the blowings F₁, they detach the leader from the doctor blade 14 and guide the leader S onto the support of the wire 11 placed facing it, which wire carries the leader S over the suction zone of the leading suction cylinder 20 onto the next cylinder 10.

[0031] Fig. 2 illustrates parameters significant in the dimensioning and alignment of the device in accordance with the invention. In the following, some typical preferred ranges of various parameters will be given, the invention being not confined to said ranges. When the diameter of the upper cylinders is of an order of 1800 mm, the angle A is typically A = 35°-45°. The perpendicular distance of the doctor blade 14 from the adjacent wire 11 is P = 160-200 mm, and the distance of the nozzle holes from the tip of the doctor blade is L ≤ 200 mm, and the angle C > 0°. The angle B of the direction of the jets F₁ is, as a rule, in the range of B = 5°-45°, preferably in the range of B = 10°-30°. When the width L₁ of the leader is about 200 mm, the width L₂ of the blowings in the transverse direction is typically about 200-350 mm.

[0032] The energy of the blowings F₁ and F₂ must be sufficiently high to produce the desired effects. This is why the air pressure p used in the blow pipe 22 must be set at a sufficiently high level. As a rule, said pressure p is in the range of p = 50-350 kPa. Said pressure range is used advantageously in connection with the dimensioning of the nozzle holes 23 to be described later. In the invention, the blow width is, however, not equally critical as the angle B of the direction of the blowings F₁.

[0033] As the blow pipe 22, it is possible to use a perforated pipe, in which the diameter of the nozzle holes is typically 3-4 mm and the spacing 30-50 mm. The blow width L₂ may be narrower than the width L₁ of the leader, but, as a rule, the blow width L₂ should preferably not be very much larger than the width L₁ of the leader. For example, with blowings F₁ extending across a width of about 300 mm it is possible to guide a leader S even as wide as about 600 mm, but, as a rule, the width proportions cannot be reversed.

[0034] Fig. 5 shows the use of the blowings in an inverted cylinder group provided with single-wire draw, wherein the drying cylinders 10′ are placed in the lower row and the leading suction cylinders 20′ in the upper row. The operation of the blowings F₁ and F₂ is similar to that described above. Of course, from the upper face of the cylinder 10′, the leader cannot be doctored onto the broke conveyor.

[0035] According to Fig. 5, in connection with the doctor beams 12, additional blowings F₃ and F₄ are provided, by whose means the leader S is held on the leading cylinder 20′. Moreover, between the leading cylinders 20′, it is possible to employ additional blow pipes 17, from which blowings F₅ are directed against the leader S so as to guide the leader over the suction zones of the leading cylinders 20′ onto the following drying cylinder 10′.

[0036] Fig. 6 shows an application of the invention in a group gap formed between a group with single-wire draw and a group with twin-wire draw. The web is transferred from the drying wire 11 of the group with single-wire draw by means of a suction box 32 after a guide roll 33a as a free draw W_p onto the lower wire 11B of the group with twin-wire draw. In the group with twin-wire draw, in the way known in prior art, there are two rows of steam-heated drying cylinders 10A and 10B, one row above the other, as well as an upper wire 11A and a lower wire 11B, which are guided by guide rolls 31A; 31B. In connection with the drying wires 11A and 11B, at the proximity of their guide rolls 31A; 31B, blow boxes 32 are placed, by whose means the support contact between the web and the wire is promoted.

[0037] According to Fig. 6, the doctor beams 12 are provided with blow pipes 22, from which the blowings F₁ in accordance with the invention are directed at the area of the tip of the doctor blade 14. According to Fig. 6, in connection with the doctor beam 12 of the first lower drying cylinder 10B, a second blowing F₃ is also employed, which is directed towards the path of running of the leader S. The blowings F₁ are not necessarily always needed in an area of twin-wire draw, or the blowings F₁ may be very weak. On the contrary, the blowings F₃ are even more helpful. In a group with twin-wire draw, the web has free draws W_o, and the leader S also runs along the same route when it is passed through the group with twin-wire draw, being aided by means of the method and the device of the invention.

[0038] Fig. 7 shows two subsequent groups with twin-wire draw. The web is transferred as a free draw W_p from one group to the other and onto the first lower cylinder 10B in the latter group between the guide rolls 31A and 31B. Both of the groups with twin-wire draw are provided with doctor beams 12, at which blow pipes 22 in accordance with the invention are placed at suitable locations, from which blow pipes blowings F₁ and/or F₃ in accordance with the invention are applied to produce the effects described above.

Claims

1. Method of threading of a paper web in a multi-cylinder dryer of a paper machine by means of air jets (F₁), said threading taking place substantially without a system of threading ropes, wherein the web is passed along a meandering path in twin-wire draw over drying cylinders (10A, 10B) or in a single-wire draw on a drying wire (11) over drying cylinders (10, 10') and leading suction cylinders or rolls (20, 20'), and wherein the free sector of the drying cylinders (10, 10', 10A, 10B) is cleaned by the blade (14) of a doctor beam (12), characterized in that after passing over a drying cylinder (10, 10', 10A, 10B), the leader (S) of the web is guided away from the drying cylinder (10, 10', 10A, 10B) by means of air jets (F₁) extending across a substantial proportion or the whole of the width (L₁) of the leader (S), said air jets (F₁) being applied from the doctor beam (12) to the area (K) of the tip of its doctor blade (14) applied against the drying cylinder (10, 10', 10A, 10B) at an angle (B) in the range of 5° to 45° in relation to the plane of the doctor blade (14).

2. Method according to claim 1, characterized in that by means of said air jets (F₁), the leader (S) is both detached from the doctor blade (14) and transferred onto the drying wire (11, 11A, 11B) to be carried further, whereby it is ensured that the leader (S) follows on the drying wire (11, 11A, 11B).

3. Method according to claim 1 or 2, characterized in that said air jets (F₁) are applied to the area (K) of the tip of the doctor blade (14) by means of nozzles of a nozzle pipe (22) connected to said doctor beam (12) and are deflected into air jets (F₂) substantially parallel to the mantle of the drying cylinder (10) and guided into the opening nip (N-) defined by the drying cylinder (10, 10', 10A, 10B) and the drying wire (11, 11A, 11B).

4. Method according to any of the preceding claims, characterized in that said angle (B) is in the range of 10° to 30°.

5. Method according to any of the preceding claims, characterized in that the nozzles have holes (23) with a diameter in the order of 3 to 4 mm and the air jets (F₁) are applied to the area (K) of the tip of the doctor blade (14) from a distance (L) of less than or equal to 250 mm at a pressure of 50 to 350 kPa.

6. A multi-cylinder dryer of a paper machine for drying a paper web comprising a plurality of drying cylinders (10, 10', 10A, 10B), the web being passed along a meandering path in twin-wire draw over the drying cylinders (10A, 10B) or in a single-wire draw on a drying wire over the drying cylinders (10, 10') and leading suction cylinders or rolls (20, 20'), the multi-cylinder dryer further comprising doctor beams (12) with a blade (14) operating against the free sector of the drying cylinders (10, 10', 10A, 10B) and with a blow device connected thereto, characterized in that the blow device comprises a transverse nozzle pipe (22) or equivalent nozzle arrangement placed at the level of the leader (S) of the paper web at the proximity of one of the ends of the doctor beam (12) at the driving side of the paper machine, said nozzle pipe (22) or equivalent comprising a number of nozzle openings (23) placed side by side or an equivalent nozzle slot, whereby the blowing direction of said nozzles is directed towards the area (K) of the tip of the doctor blade (14) applied against the drying cylinder (10, 10', 10A, 10B) at an angle (B) of 5° to 45° in relation to the plane of the doctor blade (14), said nozzle pipe (22) and its nozzle openings (23) or equivalent nozzle slot extending substantially across the entire width (L₁) of the leader (S).

7. Multi-cylinder dryer according to claim 6, characterized in that said nozzle pipe (22) is fitted in such a way that its nozzle openings (23) are placed at a distance of about 100 to 300 mm, preferably about 200 mmm, from the tip of the doctor blade (14).

8. Multi-cylinder dryer according to claim 6 or 7, characterized in that additional nozzles are connected to the doctor beam (12), from which additional air jets (F₃, F₄) are directed at the leader (S) passing by the doctor beam (12) on the drying wire (11) or as a free draw (W_o).

9. Multi-cylinder dryer according to any of the preceding claims, characterized in that the tip of the doctor blade (14) is placed at a perpendicular distance (P) in the range of 150 to 500 mm from the drying wire (11) or corresponding free draw (W_o) of the web passing by the doctor blade (14), while the distance (L) between the nozzle openings (23) and the tip of the doctor blade (14) is less than or equal to 250 mm.

Ansprüche

1. Verfahren zum Aufführen einer Papierbahn in einem Mehrzylindertrockner einer Papiermaschine mittels Luftstrahlen (F₁), wobei das Aufführen im wesentlichen ohne ein System von Aufführ-Seilen stattfindet, wobei die Bahn in Doppelsiebführung entlang einem meanderförmigen Pfad um Trockenzylinder (10A, 10B) oder in einer Einzelsiebführung auf einem Trockensieb (11) um Trockenzylinder (10, 10') und Leitsaugzylinder oder -walzen (20, 20') geleitet wird und wobei der freie Sektor der Trockenzylinder (10, 10', 10A, 10B) mittels der Klinge (14) eines Schaberbalkens (12) gereinigt wird, dadurch gekennzeichnet, daß der Aufführungsstreifen (S) der Bahn, nachdem dieser um einen Trockenzylinder (10, 10', 10A, 10B) geleitet worden ist, von dem Trockenzylinder (10, 10', 10A, 10B) weggeleitet wird, und zwar mittels Luftstrahlen (F₁), die sich über einen wesentlichen Anteil oder über das gesamte Ausmaß der Breite (L₁) des Aufführungsstreifens (S) erstrecken, wobei die Luftstrahlen (F₁) von dem Schaberbalken (12) auf den Bereich (K) der Spitze seiner Schaberklinge (14) aufgetragen werden, die bezogen auf die Ebene der Schaberklinge (14) in einem Winkel (B) in dem Bereich von 5° bis 45° auf den Trockenzylinder (10, 10', 10A, 10B) aufgetragen wird.

2. Verfahren gemäß Patentanspruch 1, dadurch gekennzeichnet, daß der Aufführungsstreifen (S) mittels der Luftstrahlen (F₁) sowohl von der Schaberklinge (14) abgelöst wird als auch auf das Trockensieb (11, 11A, 11B) transferiert wird, um weitergefördert zu werden, wodurch es sichergestellt ist, daß der Aufführungsstreifen (S) auf dem Trockensieb (11, 11A, 11B) nachfolgt.

3. Verfahren gemäß Patentanspruch 1 oder 2, dadurch gekennzeichnet, daß die Luftstrahlen (F₁) mittels Düsen einer mit dem Schaberbalken (12) verbundenen Düsenrohrleitung (22) auf den Bereich (K) der Spitze der Schaberklinge (14) aufgetragen werden und in Luftstrahlen (F₂) abgelenkt werden, die im wesentlichen parallel zu dem Mantel des Trockenzylinders (10) sind, und in den Öffnungsspalt (N-) geleitet werden, der durch den Trockenzylinder (10, 10', 10A, 10B) und das Trockensieb (11, 11A, 11B) definiert ist.

4. Verfahren gemäß einem der vorhergehenden Patentansprüche, dadurch gekennzeichnet, daß sich der Winkel (B) in dem Bereich von 10° bis 30° befindet.

5. Verfahren gemäß einem der vorhergehenden Patentansprüche, dadurch gekennzeichnet, daß die Düsen Löcher (23) mit einem Durchmesser in der Größenordnung von 3 bis 4 mm haben und die Luftstrahlen (F₁) in einem Abstand (L) von kleiner oder gleich 250 mm bei einem Druck von 50 bis 350 kPa auf den Bereich (K) der Spitze der Schaberklinge (14) aufgetragen werden.

6. Mehrzylindertrockner einer Papiermaschine zum Trocknen einer Papierbahn, mit einer Vielzahl von Trockenzylindern (10, 10', 10A, 10B), wobei die Bahn in Doppelsiebführung entlang einem meanderförmigen Pfad um die Trockenzylinder (10A, 10B) oder in einer Einzelsiebführung auf einem Trockensieb um die Trockenzylinder (10, 10') und Leitsaugzylinder oder -walzen (20, 20') geleitet wird, wobei der Mehrzylindertrockner ferner Schaberbalken (12) mit einer gegen den freien Sektor der Trockenzylinder (10, 10', 10A, 10B) wirkenden Klinge (14) und mit einer damit verbundenen Gebläsevorrichtung aufweist, dadurch gekennzeichnet, daß die Gebläsevorrichtung eine querverlaufende Düsenrohrleitung (22) oder eine gleichartige Düsenanordnung aufweist, die auf der Höhe des Aufführungsstreifen (S) der Papierbahn in der Nähe von einem der Enden der Schaberklinge (12) an der Antriebsseite der Papiermaschine angeordnet ist, wobei die Düsenrohrleitung (22) oder dergleichen eine Anzahl von Seite an Seite angeordneten Düsenöffnungen (23) oder einen gleichartigen Düsenschlitz aufweist, wodurch die Blasrichtung der Düsen auf den Bereich (K) der Spitze der Schaberklinge (14) gerichtet ist, die bezogen auf die Ebene der Schaberklinge (14) in einem Winkel (B) von 5° bis 45° auf den Trockenzylinder (10, 10', 10A, 10B) aufgebracht wird, wobei sich die Düsenrohrleitung (22) und ihre Düsenöffnungen (23) oder ihr gleichartiger Düsenschlitz im wesentlichen über die gesamte Breite (L₁) des Aufführungsstreifens (S) erstrecken.

7. Mehrzylindertrockner gemäß Patentanspruch 6, dadurch gekennzeichnet, daß die Düsenrohrleitung (22) auf derartige Weise angebracht ist, daß ihre Düsenöffnungen (23) in einem Abstand von etwa 100 bis 300 mm, vorzugsweise etwa 200 mm, von der Spitze der Schaberklinge (14) angeordnet sind.

8. Mehrzylindertrockner gemäß Patentanspruch 6 oder 7, dadurch gekennzeichnet, daß zusätzliche Düsen mit dem Schaberbalken (12) verbunden sind, von dem aus zusätzliche Luftstrahlen (F₃, F₄) auf den Aufführungsstreifen (S) gerichtet sind, der auf dem Trockensieb (11) oder als freier Zug (W₀) an dem Schaberbalken (12) vorbeigeleitet wird.

9. Mehrzylindertrockner gemäß einem der vorhergehenden Patentansprüche, dadurch gekennzeichnet, daß die Spitze der Schaberklinge (14) in einem senkrechten Abstand (P) in dem Bereich von 150 bis 500 mm von dem Trockensieb (11) oder dem entsprechenden freien Zug (W₀) der an der Schaberklinge (14) vorbeigeleiteten Bahn angeordnet ist, während der Abstand (L) zwischen den Düsenöffnungen (23) und der Spitze der Schaberklinge (14) kleiner oder gleich 250 mm ist.

Revendications

1. Procédé pour engager une bande de papier dans un séchoir multicylindre d'une machine à papier à l'aide de jets (F₁) d'air, ledit engagement s'effectuant essentiellement sans système de câbles d'engagement, dans lequel la bande est amenée à suivre un parcours sinueux sur des cylindres de séchage (10A, 10B) d'un tendeur de toile double ou sur des cylindres de séchage (10, 10') et des cylindres ou rouleaux d'entraînement par aspiration (20, 20') sur la toile de séchage (11) d'un tendeur de toile simple, et dans lequel le secteur libre des cylindres de séchage (10, 10', 10A, 10B) est nettoyé par la lame (14) d'une poutre docteur (12), caractérisé en ce que, après être passée sur un cylindre de séchage (10, 10', 10A, 10B), l'amorce (S) du papier est guidée à l'écart du cylindre de séchage (10, 10', 10A, 10B) à l'aide de jets d'air (F₁) s'étendant en travers d'une forte proportion ou de la totalité de la largeur (L₁) de l'amorce (S), lesdits jets (F₁) d'air étant appliqués depuis la poutre docteur (12) sur la surface (K) de l'extrémité de sa lame docteur (14) appliquée contre le cylindre de séchage (10, 10', 10A, 10B) suivant un angle (B) compris entre 5° et 45° par rapport au plan de la lame docteur (14).

2. Procédé selon la revendication 1, caractérisé en ce que, à l'aide desdits jets d'air (F₁), l'amorce (S) se détache de la lame docteur (14) et est en outre transférée sur la toile de séchage (11, 11A, 11B) pour être transportée plus loin, grâce à quoi il est assuré que l'amorce (S) suit sur la toile de séchage (11, 11A, 11B).

3. Procédé selon la revendication 1 ou 2, caractérisé en ce que lesdits jets d'air (F₁) sont appliqués à la surface (K) de l'extrémité de la lame docteur (14) par l'intermédiaire d'injecteurs d'un tuyau (22) à injecteurs relié à ladite poutre docteur (12) et sont déviés en jets d'air (F₂) sensiblement parallèles à l'enveloppe du cylindre de séchage (10) et guidés jusqu'à l'ouverture de pincement (N-) définie par le cylindre de séchage (10, 10A, 10B) et la toile de séchage (11, 11A, 11B)).

4. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que ledit angle (B) est compris entre 10° et 30°.

5. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que les injecteurs ont des trous (23) d'un diamètre de l'ordre de 3 à 4 mm et les jets d'air (F₁) sont appliqués, d'une distance (L) inférieure ou égale à 250 mm et à une pression de 50 à 350 kPa, à la surface (K) de l'extrémité de la lame docteur (14).

6. Séchoir multicylindre d'une machine à papier pour sécher une bande de papier, comprenant plusieurs cylindres de séchage (10, 10', 10A, 10B), la bande étant amenée à suivre un parcours sinueux sur des cylindres de séchage (10A, 10B) d'un tendeur de toile double ou sur des cylindres de séchage (10, 10') et des cylindres ou rouleaux d'entraînement par aspiration (20, 20') sur la toile de séchage d'un tendeur de toile simple, le séchoir multicylindre comportant en outre des poutres docteurs (12) avec une lame (14) agissant contre le secteur libre des cylindres de séchage (10, 10', 10A, 10B) et avec un dispositif d'insufflation relié à ceux-ci, caractérisé en ce que le dispositif d'insufflation comporte un tuyau transversal (22) à injecteurs ou un système d'injection équivalent disposé au niveau de l'amorce (S) de la bande de papier à proximité d'une des extrémités de la poutre docteur (12) du côté entraînement de la machine à papier, ledit tuyau (22) à injecteurs ou équivalent comportant un certain nombre d'ouvertures d'injection (23) placées côte à côte ou une fente d'injection équivalente, grâce à quoi l'insufflation depuis lesdits injecteurs est dirigée vers la surface (K) de l'extrémité de la lame docteur (14) appliquée contre le cylindre de séchage (10, 10', 10A, 10B) suivant un angle (B) de 5° à 45° par rapport au plan de la lame (14) de racle, ledit tuyau (22) à injecteurs et ses ouvertures d'injection (23) ou la fente d'injection équivalente s'étendant essentiellement en travers de toute la largeur (L₁) de l'amorce (S).

7. Séchoir multicylindre selon la revendication 6, caractérisé en ce que ledit tuyau (22) à injecteurs est installé de telle manière que ses ouvertures d'injection (23) sont placées à une distance d'environ 100 à 300 mm, de préférence environ 200 mm, de l'extrémité de la lame docteur (14).

8. Séchoir multicylindre selon la revendication 6 ou 7, caractérisé en ce que des injecteurs supplémentaires sont reliés à la poutre docteur (12), depuis lesquels des jets d'air supplémentaires (F₃, F₄) sont orientés vers l'amorce (S) passant devant la poutre docteur (12) sur la toile de séchage (11) ou sur un tendeur libre (W_o).

9. Séchoir multicylindre selon l'une quelconque des revendications 6 à 8, caractérisé en ce que l'extrémité de la lame docteur (14) est placée, à une distance (P) de 150 à 500 mm, perpendiculairement à la toile de séchage (11) ou au tendeur libre correspondant (W_o) de la bande passant devant la lame docteur (14), tandis que la distance (L) entre les ouvertures d'injection (23) et l'extrémité de la lame docteur (14) est inférieure ou égale à 250 mm.

Drawing