Device for curtain coating of a fiber web

The invention relates to a device for the curtain coating of a fiber web, said device including a supply means for producing a curtain formed by a coating liquid, said curtain being adaptable to drain during a production run in a free fall from a tip of the supply means onto the fiber web, and said device being provided with a doctor means for reducing the amount of boundary-layer establishing air from a surface of the fiber web upstream of a dynamic wetting line between the curtain and the fiber web, at least said supply means, tip of the supply means, and curtain being housed inside an enclosure defining an internal space.

Known from the prior art is EP 1 428 582 B1 , which describes the effect of various air flow components on a curtain coating process performed with a curtain coater, and appropriate elements for controlling the same. The cited document is principally concerned with air flows occurring in the immediate proximity of a contact point between the moving web and the falling curtain, i.e. a dynamic wetting line. These air flows, occurring or produced/self-inflicted in the immediate proximity of a dynamic wetting line, are critically vital for the success of a curtain coating process, but in view of the operation and runnability, as well as the energy consumption of a curtain coating station, the rest of the assembly has also a meaningful role.

In the context of this application, the term curtain coating is used in reference to a technique or technology, in which a curtain of coating color drains freely in response to earth gravity onto a moving fiber web or substrate. Because of friction, there is air traveling at the surface of a moving uncoated substrate, causing a displacement or deformations of the free-falling curtain. This air travels along with the moving substrate to a coating point, namely to the location at which the curtain of coating liquid makes a first contact with the substrate. At this point, the curtain is in the form of a cross-substrate line, and it is referred to as a dynamic wetting line. The near-substrate area, in which the air is in motion as a result of friction, is referred to as a boundary layer.

Commonly, in a paper machine or generally in a fiber web machine environment, the curtain coating stations used for fiber web coating are located in an air- conditioned "room" or in a closed space or enclosure, which is constructed around a curtain coating station performing the actual coating process. This way, the general ventilation of a machine hall cannot affect the highly sensitive curtain

coating process, in which all air flows, temperature, temperature changes and temperature differences, etc. have an impact on the obtained end result. In the context of this application, the air-conditioned room or closed space, defined by the walls of an enclosure, shall be referred to as an internal space.

The disposition of a curtain coating station in an internal space requires that the operator be able to monitor and, if necessary, to perform adjustment and maintenance procedures right in the immediate proximity of a contact point between the web and the falling curtain, even in the middle of ongoing production. The behavior of a curtain of coating color enables a knowledgeable operator to conclude numerous factors with an impact on the curtain coating process, which makes it desirable that the curtain of coating color be readily observable during a production run. Thus, the operator must be able to work and move inside the internal space. As well-known, the coating curtain is highly sensitive to all external interferences. The coating curtain is "alive" in response to a pressure difference of as little as one or a few Pascals, acting on the various sides of a falling curtain of coating color. Such a Pascal-rate pressure difference is created even by opening a door to the internal space surrounding a curtain coating station.

The disposition of a curtain coating station within the internal space is a bulky solution and thereby not very easily applied specifically to coating machine upgrades of modernizations intended for progressing from traditional blade coating technology to curtain coating technology. Oftentimes in these cases, the only available space is that claimed by the existing blade coater. In numerous coating machines currently in operation, the feeding geometry of a fiber web called for by the internal space constitutes a challenging task.

One notable aspect regarding such an internal space is the amount of energy required for the air-conditioning of a relative large internal volume. Since the fiber web to be coated is typically warmer than the temperature of a machine hall and the fiber web travels over a relatively long distance inside said internal space, the fiber web induces a thermal stress within the internal space, said thermal stress having in turn an effect on the actual coating operation. This thermal stress can be quite substantial in case the fiber web has a temperature which is considerably higher than that of the machine hall. Such a condition is particularly encountered when the preceding process step applied to the fiber web is drying. The extra heat must be removed from a closed internal space by supplying the space with large amounts of refrigerated air or by other appropriate means.

The breaking of a fiber web during a production run results naturally in an immediately required cleaning operation in order to return the coating machine to production run as quickly as possible. The recovery from such a production stoppage caused by a fiber web break requires that the structures be as simple as possible and easy to keep clean of fiber web shreds and coating color splashes. The traditional internal space of a curtain coating station is not necessarily one of the speediest solutions in terms of recovering from a fiber web break.

An objective of the invention is to provide a curtain coating station design which, in terms of the above-described aspects, is better than the prior known solutions intended for the paper machine environment. In the paper machine environment, the fiber webs are typically more than 3 meters in width, and even much wider than this, such as more than 9 meters in width. An objective is to improve the controllability of a curtain coating process, such that the operator is readily able during a production run to monitor a falling coating color curtain without such monitoring action causing any effect on the actual curtain and curtain coating process. Another additional objective is also to provide a design, which enables a speedy recovery from a web break back on production run, which in practice may amount, for example, to good accessibility and cleanability as far as the equipment configuration is concerned. Yet another objective of the invention is to reduce the energy consumed by a curtain coating process and to provide thereby a coating machine more efficient than before in terms of its energy balance.

A device according to the invention for the curtain coating of a fiber web is characterized in that the enclosure providing and defining the internal space is formed with an aperture, by way of which the curtain is allowed to fall onto the fiber web establishing a dynamic wetting line, said aperture being bounded, in the running direction of the fiber web, upstream relative to the position of the dynamic wetting line by the doctor means and downstream by a guide panel, as well as in lateral directions by side fences.

Thus, the curtain coating station in itself constitutes a closed enclosure, housing thereinside an actual curtain coater unit and its essential components. The proposed invention fulfils all the above objectives. By means of the invention, the curtain coater can be made very compact in size, and by virtue of its compact size, it can be substituted with very slight modifications for a blade coater present in the coating machine. Likewise, the incidence and departure angles of a fiber

web can be chosen over an extensive range, which further enhances a capability of accommodating the device in cramped spaces. In the context of this invention, the term upstream is used in reference to a location or orientation which lies in the incoming direction of a fiber web and the term downstream refers to a location or orientation which lies in the outgoing direction of a fiber web. The device of the invention can be of a so-called "slot" type, wherein the coating liquid is drained upon a fiber web from a supply means making up a V-shaped space, or the device can be of a so-called "slide" type, wherein the coating liquid is delivered onto a supply means including a draining surface. Furthermore, the fiber web can be supported or unsupported at a dynamic wetting line, whereby said support can be provided for example by means of a roll, a wire, or the like.

According to one preferred embodiment, the enclosure is provided with a transparent window or an array of windows, by way of which the tip of the supply means and the position of the wetting line are capable of being observed over the entire width of the fiber web during a production run. This enables the operator to monitor the curtain coating process during a production run without disturbing the coating process in any way. This also prevents the movements or other activities of an operator from affecting the curtain itself.

Preferably, the enclosure is formed with an openable service hatch. A properly designed and positioned service hatch enables a during-the-run washing of surfaces, soiled or color-splashed by the coating liquid e.g. at the start of a coating process, without a risk of interfering with the curtain or wash water splashes ending up on the fiber web. Likewise, by way of a service hatch the operator is able to carry out maintenance actions, such as to remove a possible foreign object or a piece of debris from impeding a flow of the curtain. According to a particularly preferred embodiment, said window or array of windows is provided on the enclosure as the openable service hatch.

It should be appreciated, however, that the enclosure, the service hatch, or the window must be located excessively close to or excessively far away from the curtain. The enclosure structures located excessively close to the curtain induce an excessively high speed in the air flowing inside the enclosure, as the fiber web advancing at a high rate of speed in the aperture re-establishes a moving film of air in the boundary layer very quickly after the dynamic wetting line. On the other hand, the structure located excessively far away from the curtain hampers maintenance, adjustment, surveillance, or another similar action possibly

performed by the operator. Preferably, the structure is such that the shortest distance between the service hatch and the curtain is more than 100 mm, preferably within the range of 600-900 mm, but less than 1000 mm.

The invention will now be described in more detail with reference to the accompanying drawing figure.

Fig. 1 shows a simplified device for curtain coating a fiber web.

In reference to fig. 1 , there is shown a curtain coating device 1 of the invention in one simplified cross-section. The figure depicts a device for curtain coating a fiber web W, said device including a supply means 3 for producing a curtain 21 formed by a coating liquid 2, said curtain 21 being adaptable to drain during a production run in a free fall from a tip 31 of the supply means onto the fiber web W, and said device being provided with a doctor means 4 for reducing the amount of boundary-layer establishing air from a surface of the fiber web W upstream of a dynamic wetting line between the curtain 21 and the fiber web W, at least said supply means 3, tip 31 of the supply means, and curtain 21 being housed inside an enclosure 7 defining an internal space 70, as opposed to the rest of the coating machine. The enclosure 7 providing and defining the internal space 70 is formed with an aperture 8, by way of which the curtain 21 is allowed to fall onto the fiber web W establishing a dynamic wetting line, said aperture 8 being bounded, in the running direction of the fiber web W, upstream relative to the position of the dynamic wetting line by the doctor means 4 and downstream by a guide panel 5, as well as in lateral directions by side fences 6.

The enclosure 7 is further provided with a transparent window 71 or an array 71 of windows, by way of which the tip 31 of the supply means and the position of the dynamic wetting line are capable of being observed over the entire width of the fiber web W during a production run. The enclosure 7 is also formed with an openable service hatch 72. According to a particularly preferred embodiment, these windows or the windows and the service hatch are combined for a single structural element, i.e. said window 71 or array 71 of windows is provided on the enclosure 7 as the openable service hatch 72.

From the standpoint of a curtain coating process, it is desirable that the air flows in the internal space 70 be under control as well as possible. Therefore, it is preferred for the structure that the shortest distance between the service hatch 72

and the curtain 21 be more than 100 mm, preferably within the range of 600-900 mm, but less than 1000 mm. The invention testing trials have revealed that, if the advancing rate of a fiber web is among the values typical of a paper machine environment, i.e. more than 1000 m/min, even more than 2500 m/min, the distance of the curtain from the closest structure of the enclosure 7, for example from the service hatch 72 or the window 7, is preferably more than 100 mm. At the shortest distances less than this, the air flowing between the curtain and the structure begins to flow at such a high speed that this air flow induces undesired movement in the curtain 21 , hindering control over the curtain coating process. It is further preferred that the enclosure 7 have its shape in a cross-section along the running direction of the fiber web W established principally by means of arcuate surfaces. Consequently, the air flows and vortices within the internal space 70 remain quiet in wide arches and the restless turbulence vortices produced by sharp corners are as insignificant as possible. On the other hand, in terms of the device's operation it is preferred that said shortest distance not be excessively long. When the shortest distance is within the range of 600-900 mm, but less than 1000 mm, the operator or any person of normal height is able to reach from outside the enclosure without auxiliary equipment to carry out adjustment, maintenance, washing, repair and the like procedures within the region of supply means, a tip of the supply means, and a dynamic wetting line. When the structure is designed correctly, the service hatch 72 is openable during a production run without substantially disturbing the air flows acting around the curtain 21 falling freely from the tip 31 of the supply means. One such recommended structural feature is a seal 721 of the service hatch 72, by means of which the air leaks between the service hatch and the enclosure are denied an access to disturb the curtain during a production run. Likewise, by means of the appropriate seal 721 , the pressure or flow variations inside the enclosure, caused by opening the service hatch, remain at an acceptable level.

Because, according to the invention, the encapsulated internal space in its cross- section is just a fraction of a room housing the entire station and its catwalks, the control of air flows present therein is considerably easier and the amount of possibly refrigerated air to be supplied thereinside is very small in comparison with a generally employed enclosure covering the entire station area. Conveniently, in some devices according to the invention, the ratio between a cross-sectional area (m ² ) of the internal space 70 in the fiber web's W running direction and a volume (m ³ ) thereof is dependent on the fiber web's W width, such that: - when the fiber web (W) has a width of 3-5 m, the ratio is less than 0,3 1/m,

- when the fiber web (W) has a width of 5-8 m, the ratio is less than 0,2 1/m,

- when the fiber web (W) has a width of more than 8 m, the ratio is less than 0,125 1/m.

The above calculated ratios are naturally just approximate values.

The control over temperatures is also considerably facilitated by the fact that the communication established by the aperture 8 from an encapsulated internal space to a warm fiber web covers only a length of about 100 mm in the running direction of the fiber web. Furthermore, in most cases there is an air flow through the aperture outward from the enclosure, i.e. away from the internal space 70. Through the same aperture 8, the curtain 21 consisting of the coating color 2 is falling onto the fiber web W. The easiest way of controlling the air flows existing in the internal space 70 of the enclosure 7 is by designing the device in such a way that the internal space (70) is provided with an air pressure and temperature other than that existing outside the enclosure, preferably the air pressure in the internal space 70 being higher than that outside the enclosure 7. Control over the temperature in the encapsulated space is assisted additionally or among other things by the fact that one segment of the enclosure is constituted by a doctor means 4, which functions for reducing the amount of air establishing a boundary layer. As an example, this may comprise the cell-structured body member of a boundary-layer air knife, inside which a blast nozzle(s) is supplied with air at ambient temperature.

The temperature of air supplied into the internal space can be used for adjusting the temperature of the internal space to as close as possible to the temperature of a supplied coating liquid. This enables minimizing deformations caused by the thermal expansion of equipment, e.g. a nozzle block, present in the internal space.

The air is introduced inside preferably from the top edge of the internal space, preferably over almost the entire width of the internal space, whereby the air flows migrate as smoothly as possible in the internal space from top to bottom. This serves to achieve as even a temperature distribution as possible in the internal space. The in-flowing air can be supplied into the internal space by way of a nozzle or opening extending across the entire width of the internal space. The amount of inlet air can possibly be regulable in cross-direction. Regulation for the amount and temperature of in-flowing air is naturally subject to automation. The flow rate of air can be regulated for example as a function of the fiber web's speed. According to one preferred embodiment, the internal space is capable of

being supplied, essentially across the breadth of the internal space, with air or a gas regulable in terms of its temperature and/or flow rate.

In the device for curtain coating a fiber web, the fiber web W having been fed in its path through the device and the doctor means 4 being in a run position, the distance between said doctor means 4 and the fiber web W is less than 10 mm, preferably less than 5 mm, most preferably less than 3,5 mm. Preferably, the doctor means is in the type of an air knife, blasting an air jet counter to the fiber web's running direction, which stops the air advancing in the boundary layer and prevents a significant amount of boundary-layer air from reaching the impact point between the fiber web and the curtain. Prior known are also appropriate doctor means of other types or operating on another principle. As regards the invention, such particular type of doctor means does not bear a particular significance as long as the amount of air traveling in the boundary layer is reduced considerably upstream of the dynamic wetting line.

Downstream of the curtain, i.e. with the coating liquid and the fiber web traveling jointly forward, it is preferred that the fiber web be provided with an appropriate and/or sufficient amount of space. The structural element closest to the fiber web in this direction is a guide panel 5, which is used for controlling, among other things, the air flows subsequent to the actual coating operation. Preferably, with the guide panel 5 in a run position, the distance between an apex of the guide panel 5 and the fiber web W is more than 10 mm, preferably more than 15 mm, most preferably within the range of 25-75 mm. The fiber web's W propensity to breaking increases remarkably as a result of wetting caused by the coating process, which is why there should be enough space for tails or pieces of the fiber web W, advancing in the aftermath of a possible web break, to escape from the effective range of the enclosure. Likewise, a high-speed air flow, applied to a wet coated fiber web immediately after a curtain coating process, may produce an undesired undulation on the surface of the coating color. When the coating machine is subjected to a start-up for beginning a production run, the flow of coating liquid 2 is switched on for producing a curtain 21. The curtain 21 is first directed into a coating liquid collector which, when the fiber web speed and other production parameters have been set up correctly, is displaced aside for bringing the curtain 21 to drain onto the fiber web W. Preferably, the structure is such that the guide panel 5 is provided structurally as part of a collector 51 for the coating liquid 2. Most conveniently, the structure is chosen to be such that a length of the fiber web W limited by the internal space 70, which length is more or less equal to

the width of the aperture 8 in the fiber web's W running direction, shall be less than 150 mm, preferably less than 100 mm. Moreover, the doctor means 4, the enclosure 7, and the guide panel 5 constitute a continuous, integrally coupled structure.

In lateral direction, the aperture 8 is limited by side fences. The construction of these can be chosen in such a way that the side fences define the width of either the aperture 8 or the curtain 21. In most cases, it is desirable that the width of the aperture 8 and/or the curtain 21 defined by side fences 6 be adjustable or modifiable. In this context, adjustability refers to a procedure performed during a production run and modifiability refers to a procedure that can be performed in downtime, i.e. when the production run is in a standstill. The widths of a fiber web to be coated fluctuate to a certain degree and the operator or a person in charge of production may also prefer to carry out the coating process exactly to his/her liking. Thus, preferably, the width to be coated can be selected such that the width of the aperture 8 or the curtain 21 in a lateral direction of the fiber web W is narrower than, wider than, or equal to that of the fiber web. The side fences can be designed in such a way that the width of either the curtain or the aperture or both is adjustable. Along the sides of the fiber web are several structural elements inflicting irregularity or possibly turbulences in the air flows. These include for example air vortices produced by the end faces of rolls, such as those of a coating backup roll, vortices caused by the end brackets of the supply means, the structures of web tail threading equipment, and so forth. Thus, the most preferred structural solution for most applications is probably such that both the curtain width and the aperture width are adjustable or modifiable independently of each other. Another factor in this may be the applied fiber web running speed, since the falling curtain reduces its area somewhat upon impact against the advancing fiber web and this reduction in area is partly proportional to running speed.

List of reference numerals:

W Fiber web or substrate

1 Curtain coating station

2 Coating liquid

21 Curtain

3 Supply means

31 Tip of supply means

4 Doctor means

5 Guide panel

51 Coating liquid collector

6 Side fence

7 Enclosure

70 Internal space

71 Window

72 Service hatch

721 Seal

8 Aperture