One common structure of a curtain coater unit used in a paper/board machine and its operation are known, for example, from the published application FI 19991863, in which the curtain coater unit is comprised of an elongated nozzle beam fitted transversely with respect to the direction of travel of the material web to be coated, above the material web and on the support frame. The coating agent is applied through the wide nozzle part of the nozzle beam, or a corresponding feeder part, such as an inclined plane, as a curtain-like stream onto the surface of the material web moving below the nozzle, to an impact point (a point in the web onto which the curtain drips). In addition to this, in conjunction with the material web, in the area before the impact point, are fitted boundary-layer air exhaust means that are separate from the nozzle part. The means include a suction surface fitted so as to face the surface of the material web to be coated, through which suction surface the layer of air on the surface of the material web is sucked into the exhaust ducts, and an air doctor between the suction surface and the impact point.

The problem with this type of a curtain coater unit relates to controlling the support frame, and thus the straightness of the nozzle beam, in the changing loading environment. Even the slightest deflection of the nozzle unit will affect the size of the nozzle slot and thus the feed rates of the coating agent to be fed. Presently, these problems are solved by manufacturing sufficiently rigid nozzle beams. Such structures are large, difficult to manufacture and thus also expensive to manufacture. Although the external mechanical loads are generally essentially constant, the relatively instable thermal load in the surroundings of the nozzle part and the actual thermal expansion of the nozzle part and air currents bring additional problems to controlling straightness and thus also the amounts of the coating agent

to be fed. Temperature variations are caused by heat brought along by the material web to the curtain coater unit area. Nowadays, the problems caused by temperature changes have been eliminated, or at least reduced, by making the surroundings of the curtain coater unit isothermal by locating the unit in a so-called calibration room. The calibration room is a large space or room where the prevailing ambient conditions are maintained suitable for coating, especially as concerns the support frame and the nozzle beam. This type of solution takes up much space and is expensive.

The aim of the present invention is to provide a curtain coater unit by means of which some of the above-mentioned disadvantages are eliminated or can be substantially reduced.

To achieve the above-mentioned aim, the present invention is characterised in that the boundary-layer air exhaust means are connected with the frame element, and that at least a part of the frame element is fitted between the material web and the nozzle beam of the curtain coater.

By means of the curtain coater unit structure according to the invention, the effects (temperature changes and air currents) of external loads and other factors disadvantageous to the application of the coating agent, that otherwise directly affect the curtain coater or the coating agent curtain being applied, are partly or completely avoided. At the same time, the structure of the unit can be fitted into a smaller, more compact space, whereby also the desired height of the coating agent curtain is easier to achieve.

Preferred embodiments of the present invention are disclosed in the dependent claims.

The invention is described in greater detail in the following, with reference to the accompanying Figures, in which: Figure 1 shows a cross-section of a curtain coater unit according to the invention, and

Figure 2 shows another embodiment of the curtain coater unit according to the present invention.

Figure 1 thus shows a curtain coater unit according to the invention, which is marked by reference numeral 1. The curtain coater unit 1 is located essentially above a material web W travelling on its track. The curtain coater unit 1 comprises a curtain coater 4, or application beam. By means of the nozzle part of the application beam 4, which is of essentially equal width with the material web W, or a corresponding feeder part, such as an inclined plane, the coating agent is formed into a curtain 20 of equal width with the material web W, which is applied from a predetermined height onto the surface of the moving material web W at the impact point 21.

The application beam 4 is arranged in the frame element 2 of the curtain coater unit 1, for example, by means of a support belonging in conjunction with the frame element 2. The frame element 2 is elongated in the lateral direction of the material web W. The cross-section of the frame element 2 is preferably C-shaped, which means that it is open on one side and its curved walls limit the space 10. The shape of the cross-section, therefore, makes it possible to fit the application beam at least partly through the open side into the space 10. As can be seen in Figure 1, the frame element 2 remains partly between the lower material web W and the application beam 4. The frame element 2 itself thus forms an application beam 4 cover which prevents substantially the passage of harmful air currents and heat carried with the web W onto the application beam 4. The application beam 4 is located in the space 10 in such a way that its nozzle part is outside the space 10, which means that through it the coating agent 20 can be applied past the wall formed by the space 10 and further to the impact point 21 of the material web W moving below. The material used for the frame element 2 is usually steel. The frame element 2 may also be coated or covered by any thermally insulating material to improve thermal insulation.

In conjunction with the frame element 2 are further arranged boundary-layer air A exhaust means 5, 3, which are preferably located at a point before the impact point

21 with respect to the direction of travel of the material web W. The boundary-layer air A is a layer of air formed on the surface of the material web W, which moves with the travelling material web W. The boundary-layer air A exhaust means include a suction box 5 or the like, in which is formed a suction surface facing the material web W. On the suction surface are made, for example, openings, through which the boundary-layer air moves from the surface of the material web W to the suction box 5. The suction box is further in contact with the suction duct 3, through which the boundary-layer air A is removed from the material web W and from the impact point 21 area. The suction duct is arranged in connection with the outer surface of the frame element 2 wall, that is, on the opposite side of the wall with respect to the application beam 4. In this embodiment, the suction duct 3 is preferably formed of the curved wall of the frame element 2 and a curved duct element 30 arranged at a distance from the outer surface of the wall, the said element essentially surrounding the outer surface of the frame element 2. The suction duct is thus formed in the space remaining between the frame element 2 and the duct element. In addition to this, to enhance the removal of air, in connection with the suction box 5, in front of the impact point 21, is preferably arranged an elongated doctor blade means 5b, which is located transversely to the direction of travel of the material web W.

By means of this arrangement, the boundary-layer air A can be removed from the surface of the material web W in a controlled manner. At the same time, the air A flowing in the suction duct 3 and the duct element for its part both insulate and carry away heat otherwise conveyed from the material web W to the frame element 2 and further to the application beam 4. In addition, the suction duct 3 is made so long that the air A flowing in it will even out possible transverse temperature differences in the material web W. Thus the heat possibly conveyed to the frame element 2 and application beam 4 is as uniform as possible in the transverse direction of the frame element 2 and the application beam 4. The application beam 4 and the boundary-layer air A exhaust means 5,3 are located, in accordance with the invention, in the same protective frame structure, whereby a compact structure is obtained where the equipment effecting the different functions can be placed in their optimal locations. For example, the desired height of the coating agent curtain 20 is easier to obtain.

Figure 2 shows diagrammatically another preferred embodiment of the invention.

Also in this case, the curtain coater unit 1 is comprised of a frame element 2 according to the first embodiment which is located partly between the material web W and the application beam 4, and in the space 10 formed by the walls of which the application beam 4 is at least partly located. In addition to the suction duct 3, also other means intended for handling air are arranged conjunction with the frame element 2, the structure and operation of the said means being described in greater detail below.

The behaviour of the coating agent curtain 20 in the impact point 21 area is controlled by means of stabilisation air C, which is supplied to the impact point 21 area through blasting means 7,7a, 7b and removed from the impact point 21 area through exhaust means 6,6a, 6b.

The stabilisation air C blasting means comprise a blasting duct 7a through which the stabilisation air C is conveyed to the curtain coater unit 1. From the blasting duct 7a the stabilisation air C is fed to a distributing duct 7. The distributing duct 7 is formed in the space remaining between the inner surface (the surface on the application beam 4 side) of the curved wall of the frame element 2 and the curved duct element 70 arranged at a distance from the inner surface. One end of the distributing duct 7 forms the nozzle part 7b of the stabilisation air C blowing means, which is fitted to guide the stabilisation air at an appropriate force in the impact point 21 area, over the entire width of the coating agent curtain 20. Owing to the elongated distributing duct 5 formed in connection with the frame element 2, it has been possible to fit the actual blasting duct 7a at such long distance from the material web W that the heat carried from the material web W to the surroundings will not essentially affect the temperature of the blasted stabilisation air C.

The stabilisation air C is thus removed by the exhaust means 6,6a, 6b, including suction duct 6b which is fitted in the immediate vicinity of the nozzle part 7b, in front of the impact point 21. As an extension of the suction duct 6b is arranged a transfer duct 6. The transfer duct 6 is made in compliance with the transfer duct 3 described in connection with the first embodiment. In other words, the transfer duct 6 is formed in the space between the outer surface of the frame element 2 and a

second duct element 60 arranged at a distance from the frame element 2. At the end of the transfer duct 6 is provided the actual exhaust duct 6a transverse to the web, through which the stabilisation air C is removed from the curtain coater unit 1.

In the same way as the blasting duct 7a, the exhaust duct 6a is also located as far as possible from the material web W, however as close to the frame element 2 as possible, whereby the structure of the curtain coater unit 1 will remain compact.

In another preferred embodiment of the invention, in addition to the above- mentioned ducts are also arranged boundary-layer air A exhaust means 5,3, 3a and support air B blasting means 8,8a, 8b in connection with the frame element 2.

Here, the boundary-layer air A exhaust means include a suction box 5 or the like, which is fitted in the immediate vicinity of the suction duct 6b, in front of the suction duct 6b in the direction of travel of the material web W. As an extension of the suction box 5 is arranged a boundary-layer air A transfer duct 3. In this case, the transfer duct 3 is formed in the space remaining between the second duct element 60 arranged at a distance from the frame element 2 and the third duct element 30. The third duct element 30'thus also forms the outermost surface or cover of the curtain coater unit 1, which for its part insulates the nozzle beam 4 from the heat released by the material web. At the end of the transfer duct 3, as far from the material web W as possible, is arranged the actual exhaust duct 3a transverse to the track, through which the boundary-layer air A is removed from the curtain coater unit 1.

In conjunction with the boundary-layer air A exhaust means 5,3, 3a are further fitted support air B blasting means 8,8a, 8b, by means of which the air B supporting material web W is blasted under low pressure onto the surface of the material web W. Typically, the blast pressure is 5 to 10 pascals, but it may deviate from this. The support air B blasting means 8,8a, 8b include the blasting duct 8a, through which the support air B is conveyed to the curtain coater unit 1. In this case, the blasting duct 8a is located adjacent to the boundary-layer air A exhaust duct 3a, in the suction duct 3 limited by the second duct element 60 and the third duct element 30'. As an extension of the blasting duct 8a is arranged a distributing duct 8, into which the support air B from the blasting duct 8a is fed. The duct

elements 60 and 30'are arranged at such a distance from one another that it has been possible to locate the distributing duct 8 in the transfer duct 3 limited by the duct elements 60 and 30'. In Figure 2, the distributing duct 8 is the duct 8 remaining between the elements 80 shown in broken line, whereby the suction duct 3 essentially surrounds it. The end of the distributing duct 8 forms the nozzle part 8b which is preferably fitted in connection with the suction box 5, in the vicinity of the material web W. Through the nozzle part 8b the support air B is guided over the total width of the material web W surface.

The present invention is not limited to the embodiments described but is applicable within the scope of the claims presented below.