EP0723523B1 - Process and device for stabilising a travelling web - Google Patents

Abstract

The invention relates to a process and a suitable device for implementing it to stabilise a travelling web (B) in a free-running stretch between two supports, in which air is blown on the web (B) on one side from a nozzle arrangement (16) which is surrounded by a substantially flat wall surface (9). The wall surface (9) is parallel to the web (B) and comprises an air duct (14) extending transversely to the direction of travel of the web (B) and having a V-shaped cross section, in which the nozzle arrangement is fitted. The nozzle arrangement consists of mutually staggered hole-like outlets (16, 16') machined in the opposite walls of the air duct. The underpressure region generated by the crossing air flows draws the web towards the flat wall surface (9) on going below a critical distance so that it assumes a position of equilibrium at a certain distance which is determined by the diameter of the outlets and hence the thickness of the air cushion generated by the crossing air flows.

Description

The invention relates to a method of the type corresponding to the preamble of claim 1 and also includes a device suitable for the application of the method.

When gluing or coating a web made of a sensitive material, such as paper or the like, it has proven to be advantageous to avoid damage to the web or its surface to keep the web floating between two support points for the duration of the drying process of its surface coating . Since the trend in the coating of paper webs is towards ever higher operating speeds of the devices of known design used for this purpose, the drying rate of a surface piece of the web after coating, however, has an upper limit value, for example due to the maximum thermal load capacity of the web material and the coating, which leads to this that the surface piece must be in a floating state for a minimal period of time, the webs must be guided to float over ever greater lengths. Naturally, the tendency of the web sections running freely between adjacent floating end guides to flutter increases. whereby the uniformity of the drying process can be significantly impaired and which - due to the considerable mechanical load on the web - can possibly lead to a web break.

To suppress this harmful flutter movement, it is known to blow air into the web, seen in the direction of travel, between two adjacent floating end guides on opposite sides on both sides. DE-OS 36 26 016 discloses a device for carrying out this method, which has air channels with nozzle areas, in each of which individual outlets for air are provided on both sides of a transverse plane running in the longitudinal direction of the air channel opposite a guide surface for the air flow. Due to the special design of the air ducts, particularly stable working conditions, i.e. uniform "air cushions" to support the web are made possible.

With this method, transverse deflections of the web can be reliably suppressed, but it is for webs that, e.g. after gluing or pigmentation, have a sensitive surface, not applicable, since a strong air flow directed at the coated surface would lead to a reduction in the coating quality or even to a destruction of the coating. Therefore, this method cannot be used for webs coated in this way, and the only option is to counteract the flutter by reducing the web speed and the resulting possibility of reducing the distance between two adjacent floating end guides.

For floating guidance, in particular of webs which have a sensitive surface coating, the use of a method is known in which the web is blown with air only on one side from a nozzle arrangement which is at least partially surrounded by an essentially flat wall. With this method, the initially unexpected effect occurs that the web does not is deflected, for example, in the direction of the air flowing towards it, but is drawn towards falling below a critical distance from the flat wall, whereby it assumes an equilibrium position at a certain distance from it.

The reason for this effect is the fact that in a closed system in areas where flowable media flow faster, there is less pressure than in those areas where they flow more slowly flow. This physical law is formulated in the equation by Daniel Bernoulli, which states that the sum of the static pressure and the dynamic pressure comprising the flow component of a moving fluid medium exactly corresponds to the pressure that would prevail in the static fluid medium. In other words: the static pressure of a fluid medium moving in a system is always lower than that of the static medium. This connection, which strictly speaking only applies to the flow of ideal liquids, is responsible for the fact that there is a negative pressure in the air flowing along the flat wall, which pulls the web towards the flat wall so far that the blown-out air can still flow off unhindered.

By utilizing this effect, a running web can be stabilized without the surfaces being exposed to any significant mechanical stress.

A device for using this method is known from DE 36 30 571 A1.

This device comprises a so-called blow box, which essentially extends over the web width and into which the blown air can be introduced. The upper side of the blow box facing the web is designed as an airfoil to which air jets are blown out of the blow box, with the aid of which the low-pressure, contactless support of the web is achieved. The air flows from two parallel slot nozzles which run transversely to the web and are designed such that the outflowing air flows along a convexly curved flow surface which extends to the adjacent edge of the blow box.

The air streams blown out towards the edge produce between the slot nozzles above a support plane provided here, which is lower relative to the path plane than the flow surfaces running to the side of the slot nozzles is a negative pressure field, which exerts a force directed onto the supporting plane on the web.

In order to avoid that the web is pulled down from the vacuum field so far that it touches the supporting plane, an air pipe opens into it, through which air from the environment is fed to the vacuum field. A throttle valve is provided in the air duct, by means of which the air supplied can be controlled in terms of quantity and thus the force exerted on the web can be adjusted.

In principle, the floating guidance of a web is possible with this device without at least the surface of the web pointing away from the device being exposed to any significant mechanical stress, but the device is through the slot nozzles and the air duct opening into the supporting plane, which is used to adapt the forces on the web require an additional throttle valve, quite complex to manufacture. Also, through the continuous slot nozzles, the wing surface of the blow box facing the web is greatly weakened, so that stiffening ribs in the blow box, which are technically complex to implement, are particularly difficult to implement, particularly in the case of blow boxes. must be integrated in order to maintain sufficient stability of the wing. Furthermore, the energy requirement to be expected in this device is relatively high, since on the one hand the large amount of air required for the air flow to build up the air flow is quite large due to the large opening area of the slot nozzles and on the other hand additional air has to be entrained through the air duct in order to make contact between the web and the air box to avoid.

The invention is therefore based on the object of further developing the method for stabilizing the running web, in which it is only exposed to air on one side, in such a way that the energy requirement required for using the method is reduced. It is a further object of the invention, in particular for the application of the invention To develop a suitable device that is optimized in terms of its manufacturing costs. Of course, the method and the device according to the invention should also be suitable for stabilizing uncoated webs.

These objects are achieved by the method given in claim 1 and by the device given in claim 4.

Because the air flow necessary for the method according to the invention is generated by a blower, which is only supplied with electrical power when the web is actually fluttering, the method according to the invention can be used particularly economically. This is done by measuring their spatial position in situ while the web is running and only then applying air to the nozzle arrangement when a deflection of the web from its normal position is detected. The pressure is controlled by regulating the power supplied to the blower.

So that when fluttering begins, the web or its surface is not damaged by contact with the device provided for using the method according to the invention, it is advantageous according to claim 2 to move the nozzle arrangement while the web is running in its normal position at a distance from the web surface. so that such contact is excluded.

If a path deflection is then measured, it is advantageous if air is first applied to the nozzle arrangement and then shifted in the direction of the path until the path interacts with the air flow emerging from the nozzle arrangement (claim 3).

A device suitable for using a method for suspending a running web in which it interacts with the dynamic vacuum field generated by a wall around which a fluid medium flows, in particular a device suitable for using the method according to the invention, is the subject of claim 4 Bernoullic effect is usually achieved when the nozzle arrangement is completely surrounded on the side by the wall.

To generate a relatively low swirling air flow, the nozzle arrangement is arranged in an air duct extending transversely to the running direction of the web. The air duct comprises two duct walls arranged on both sides of a transverse plane standing in its longitudinal direction and perpendicular to the path plane, each of which forms the same acute angle with the transverse plane. The nozzle arrangement consists of a series of outflow openings provided in the channel walls, which are arranged on opposite walls of the air channel in such a way that the air streams emerging from the outflow openings of the one channel wall cross those which exit from the outflow openings of the other channel wall. The Bernoulli effect of the outflowing air creates an area above the air duct in which - compared to the environment - there is a negative pressure through which the material web is pulled in the direction of the device after the critical distance has been undershot In the area of the air ducts crossing air flows just above the air duct, an air cushion supporting the web, so that contact between the web and the device is effectively avoided without special measures being required.

In addition to this considerable advantage over previously known devices, tests have shown that by designing the nozzle arrangement as individual outflow openings, the efficiency of the device - i.e. the ability to stabilize the web at the desired distance using the device is increased.

Individual outflow openings can also be incorporated into the duct walls of the air duct in a simple manner, for example by punching, and an air duct provided with such is mechanically weakened only to an insignificant extent. The special arrangement of the channel walls means that Air channel of the device according to the invention stabilizing on its surface facing away from the web, so that reinforcing ribs, which would significantly increase the production outlay, can also be dispensed with in devices provided for wider webs.

In an advantageous embodiment of the device, the outflow openings are hole-shaped. This measure optimizes the efficiency of the device on the one hand, and on the other hand, by choosing the diameter of the hole-shaped outlet openings, the thickness of the air cushion supporting the web and thus the distance between the web and the top of the device can be adapted to the respective requirements (claim 5).

The diameter of the outflow openings is preferably 5 mm to 20 mm.

In a particularly streamlined embodiment of the device, the mutually opposite outflow openings are arranged laterally offset and surrounded by concentric flow guide edges (claims 7 and 8). This configuration results in a particularly low-swirling air flow, which has a positive effect on the efficiency of the device.

In order to prevent the particularly endangered longitudinal ends of the device - seen in the web direction - from being touched by the running web, it is advantageous according to claim 9 to provide support nozzles pressurized by compressed air and spaced from the nozzle arrangement. The support nozzles can be designed in a simple manner in that, viewed in the direction of travel of the web, rows of holes arranged with compressed air and transversely to the web are worked into the wall at the beginning and at the end of the wall (claim 10).

The device according to the invention can be used economically if it is not in continuous operation, but is only used when needed, ie when the web is actually fluttering. To it is advantageous to connect a device for measuring the deflection of the web from its normal position to the nozzle arrangement. Advantageously, this device is connected to an arrangement for controlling the pressure at which the nozzle arrangement is pressurized with air (claim 12), so that the nozzle arrangement is pressurized with compressed air only when necessary and accordingly energy consumption takes place only when required.

In order to prevent the web from touching the not yet activated device according to the invention when fluttering begins, an arrangement for displacing the nozzle arrangement in the direction perpendicular to the web surface is provided, which is controlled by the device for measuring the deflection of the web. The control is advantageously carried out in such a way that when a detected fluttering of the web, air is first applied to the nozzle arrangement, in order to then be displaced in the direction of the web just enough that the web interacts with the air flow emerging from the device in the desired interaction.

Depending on the properties of the web, such as its nature or width, to stabilize the web, it may be sufficient, according to claim 14, to extend the extent of the housing transversely to the web only over a part of the web width lying in the edge regions of the web. Such a configuration allows the energy requirement of the device according to the invention to be considerably reduced.

• Fig. 1 eine Seitenansicht des Ausführungsbeispiels;
• Fig. 2 die Druckkammer desselben Ausführungsbeispiels in einer Ansicht von vorn (Ansicht A in Fig. 1);
• Fig. 3 einen Schnitt durch einen Luftkanal gemäß der Linie III-III in Fig. 2 in einer perspektivischen Darstellung sowie
• Fig. 4 eine schematische Darstellung einer an sich bekannten, jedoch mit einer erfindungsgemäßen Vorrichtung zur Stabilisierung der Bahn ausgerüsteten Beleimungspresse zur einseitigen Beleimung einer Papierbahn.

An exemplary embodiment of a device according to the invention is illustrated schematically in the drawing. Show it:

• Fig. 1 is a side view of the embodiment;
• Figure 2 shows the pressure chamber of the same embodiment in a view from the front (view A in Fig. 1).
• Fig. 3 shows a section through an air duct along the line III-III in Fig. 2 in a perspective view and
• Fig. 4 is a schematic representation of a gluing press known per se but equipped with a device for stabilizing the web according to the invention for one-sided gluing of a paper web.

When "top" or "bottom" is mentioned below, the upright operating position of the device shown in FIG. 1 is meant. Information such as 'front', 'rear', 'left' and 'right' refer to a view of the side of the device facing the web, corresponding to view A in FIG. 1.

The device designated 100 in the drawing as a whole comprises a holder 1 which rests at its lower end in a receptacle 2 fixedly mounted, for example, on the floor. The receptacle 2 is designed in such a way that the holder 1 can be displaced in the direction normal to the path B, as indicated by the arrow V in FIG. 1.

The holder 1 carries at its upper end a completely closed housing 3, the inner volume of which forms a pressure chamber 4. The approximately cuboidal housing 3, which extends with a long side over a substantial part of the width of the web B, comprises a connection piece 6 arranged on its rear side facing away from the web 5, which connecting a - not shown in the drawing - pressurized with compressed air serves. The top 7 and the bottom 8 of the housing 3 are provided near the surface 9 facing the web with two step-like housing shoulders 10, 10 'parallel to one another and to the surface 9. Extending from the housing shoulders 10, 10 'toward the web is a housing extension 11 which widens in a funnel shape in the web direction, the end of which forms the surface 9 facing the web.

As can be seen from FIG. 2, the surface 9 of the housing 3 facing the web comprises two flat walls 12, 12 'which run transversely to the web and parallel to one another and which - seen in the running direction of the web - at the beginning and at the end comprise transverse rows of holes 13, 13 'which communicate with the pressure chamber 4 and form support nozzles for the web B.

Provided between the two walls 12, 12 'is an air duct 14 which runs transversely to the web and is open to it and which is let into the housing 3. The air duct comprises on both sides a channel wall 15, 15 'arranged in its longitudinal direction and perpendicular to the plane E of the web, which each form the same acute angle β with the transverse plane Q (cf. FIG. 3). Outflow openings 16, 16 'are worked into the channel walls 15, 15' in a regular row arrangement, which serve for the air outlet from the pressure chamber 4. The outflow openings 16, 16 'are surrounded by flow guide edges 17, 17' which are arched into the air duct 14 and which serve to reduce the formation of eddies when the air flows out. In the exemplary embodiment shown in the drawing, the mutually opposite outflow openings 16, 16 'are arranged laterally offset, as a result of which the escaping air streams do not meet directly, as a result of which a reduced eddy formation can also be achieved.

The mode of operation of the device will be explained below. To stabilize the web B, compressed air is supplied via the flange 6 to the housing 3 acting as a pressure chamber 4 according to the double arrow S in FIG. 1, for example by means of a blower (not shown in the drawing). On the side of the housing 3 facing the web, part of the supplied air flows through the rows of holes 13, 13 'acting as support nozzles, thereby reliably preventing the web B from touching the edges of the walls 12, 12' during a transient process. The remaining part of the supplied air flows through the outflow openings 16, 16 'into the air duct 14 and, caused by the so-called Coanda effect, essentially follows the duct walls 15, 15' and further out the walls 12, 12 ', as through the Double arrows S in FIG. 3 clarifies. As already explained, there is a lower pressure due to the increased flow velocity of the air compared to the air at rest in the surroundings according to Bernoulli's equation, as a result of which the web essentially aligns itself along surface 9.

4 schematically shows a glue press known per se for one-sided glue application, which is equipped with a device according to the invention. In such a size press, the web is guided around a drying cylinder 20 and passes through a downstream guide roller 21 before it is fed to the size press rollers 22, 22 ′ arranged on both sides of the web. Seen in the running direction of the web B, a glue application device 23 is provided in front of the size press rolls 22, 22 'on one side of the web. The layer of glue applied to the web B is evened out by the size pressing roller 22 ', excess glue being taken up by a glue collecting trough 24 surrounding the size pressing roller 22'. Seen in the running direction of the web B, a device 100 according to the invention for stabilizing the web is provided behind the size-pressing rollers 22, 22 ', which acts on the uncoated side of the web B and, if necessary, prevents the web from fluttering. The web is then passed through a spreading device 25, which prevents the web B from wrinkling, which would lead to a bonding of web parts and thus to their destruction. Via a downstream guide roller 26, web B is finally fed to a drying cylinder 27, on which the drying process of the coated web side takes place.

Claims (14)

1. Process for the stabilisation of a travelling web (B), preferably a moist paper web, in web zone running freely between two support points, in which the web (B) is blown with air on one side from a nozzle arrangement, which is provided in an air duct (14) extending transversely to the direction of travel of the web (B) and is surrounded at least partially by an essentially smooth wall face (9), which runs approximately parallel to the web (B),
   wherein the nozzle arrangement is charged with air by a blower, and with which for the travelling web (B) its deflection from its normal position is measured in perpendicular direction to the direction of travel of the web, characterised in that the pressure under which the nozzle arrangement is charged with air is regulated in dependence upon the deflection of the web from its normal position by a regulatory system of the power fed to the blower in such a manner that with the web (B) travelling in the normal position no power is fed to the blower.
2. Process according to claim 1, characterised in that for a web (B) travelling in its normal position the nozzle arrangement is displaced at a distance to the web surface, that the web (B) does not touch the nozzle arrangement and these surrounding components even for the maximum deflections of the web to be anticipated.
3. Process according to claim 2, characterised in that after measuring a deflection of the web the nozzle arrangement (16, 16') is firstly charged with air and then displaced sufficiently far enough in the direction of the web (B) until the web (B) reacts with the air current escaping from the nozzle arrangement.
4. Device for the application of the process for the stabilisation of a travelling web, preferably a moist paper web, in web zone running freely between two support points, in which the web (B) reacts with the dynamic sub-pressure field generated over a wall immersed in a current of a fluidal medium, especially according to any of the claims 1 to 3, having a compressed air source designed as an electrical blower, having charged with compressed air a nozzle arrangement, which is provided in an air duct (14) extending transversely to the travelling direction of the web (B) and comprises facing the web an essentially smooth wall face (9), which is aligned approximately parallel to the web (B) and at least partially surrounds the nozzle arrangement, characterised in that the air duct (14) comprises, arranged on both sides of a transverse plane (Q) standing in longitudinal direction of the air duct and perpendicular to the plane of the web (E), two duct walls (15), which form the same acute angle (β) with the transverse plane (Q) in each instance, that the nozzle arrangement comprises a series of flow outlet orifices (16, 16') provided in the duct walls (15, 15'), that means are provided for measuring the deflection of the web in perpendicular direction to the direction of travel of the web from its normal position and means for regulatory control of the pressure with which the nozzle arrangement is charged with air, and that means for the regulatory control of the pressure are equipped such that with the web running in the normal position no power is fed to the blower.
5. Device according to claim 4, characterised in that the flow outlet orifices (16, 16') are designed hole-shaped.
6. Device according to claim 5, characterised in that the diameter of the hole-shaped flow outlet orifices (16, 16') amounts to between 5 mm and 20 mm.
7. Device according to any of the claims 4 to 6, characterised in that flow control edges (17) are provided, which surround concentrically the flow outlet orifices (16, 16').
8. Device according to any of the claims 4 to 7, characterised in that the flow outlet orifices (16, 16') opposite each other are arranged laterally offset.
9. Device according to any of the claims 4 to 8, characterised in that seen from the nozzle arrangement in the direction of travel of the web spaced support nozzles (13, 13') charged with compressed air are provided.
10. Device according to claim 9, characterised in that seen in the direction of travel of the web the support nozzles a series of holes (13, 13') are arranged transversely to the web (B) at the start and at the end of the walls (12, 12').
11. Device according to any of the claims 4 to 10, characterised in that a mechanism for measuring the deflection of the web (B) from its normal position to the casing (3) containing the air duct (14) is installed on the feed side of the system.
12. Device according to claim 11, characterised in that the mechanism for measuring the deflection of the web is connected to an arrangement for controlling of the electrical power of the blower with which the flow outlet orifices (16, 16') is charged with air.
13. Device according to claim 11 or 12, characterised in that for displacement of the casing (3) in the direction vertical to the surface of the web is provided an arrangement (2), which is controlled by the mechanism for measuring the deflection of the web.
14. Device according to any of the claims 4 to 13, characterised in that the extent of the casing (3) transverse to the web (B) amounts to just a part of the web width and the casing (3) is opposite the edge zone of the web (B).

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