The invention relates to an impingement dryer for drying a web of fibrous material, e.g. paper, board or the like.

In conventional impingement dryers, hot drying gas is jetted towards the web carried by a supporting surface by means of a nozzle plate which is arranged opposite the support surface. The nozzle plate has a surface comprising blow openings and exhaust openings. It was found that web breaks occur in the impingement dryer, which web breaks start from the edge of the web and which web breaks adversely affect the machine runability of the web.

The inventors have studied this problem which can be understood as lifting the web by side gas streams. In particular, referring to Fig. 7, there is a support surface 2 carrying a web 5 which passes underneath a blowing device 1 having a nozzle plate 11 facing the web 5.

When a drying gas 4 jetted onto the surface of the web 5 is loaded with evaporated water from the web 5, this used drying gas 43 is removed from between the nozzle plate 11 and the web 5. A stream of side gas 42 enters the space under the nozzle plate 11 to partly compensate for the used drying gases 43 which are exhausted.

This stream of side gas 42 may cause the web 5 to rise or lift off at its edge portion 51.

Therefore, it is the object of the invention to suggest an improved impingement dryer and an improved method for drying the web in which the machine runability of the web is improved.

This object is solved with an impingement dryer according to claim 1 and with a method according to claim 18.

According to the invention, an impingement dryer is provided for drying a fibrous web which is carried by a support surface and opposite a nozzle plate. The nozzle plate has openings for blowing a drying gas towards the web. The distance between the nozzle plate and said support surface is larger in a support surface edge portion supporting an edge portion of the web than in a support surface central portion supporting a central portion of the web.

The inventors have found that the web travelling through the impingement dryer tends to rise or flutter in the edge portion and that a contact of the web with the nozzle plate above the web may initiate web breaks. Therefore, increasing the distance between support surface and nozzle plate in the edge portion of the web avoids web contact to the nozzle plate and the increased sectional area of the flow path of the side gas reduces its speed. Therefore, lifting the web edge portion is less likely. It is noted that neither the nozzle plate nor the support surface have to be flat; in particular various shapes of curvature especially along the machine direction are commonly used in impingement dryers and may be used here as well. The curvature of the nozzle plate may be concave or convex and the corresponding support surface is suitably curved in a convex and concave shape, respectively.

The support surface may be selected from a group comprising fabrics, wires and rolls or similar materials suitable for supporting the web.

Preferably, the nozzle plate has a central plate portion, and has a lateral plate portion facing the support surface edge portion. This lateral plate portion may be parallel to the central plate portion.

Preferably, the lateral plate portion includes an inclined plate portion which faces the support surface edge portion, and which is inclined with respect to the central plate portion, so that the distance between the lateral plate portion and the support surface increases towards the support surface edge.

In an alternative preferred form, the lateral plate portion includes a curved plate portion which faces the support surface edge portion, and which is curved, so that the distance between the lateral plate portion and the support surface increases towards the support surface edge.

In a preferred form, the lateral plate portion is offset with respect to the central plate portion in a direction away from the support surface and may, in this form, be connected to the central plate portion by a step. Preferably an adjustable edge plate may be provided between the lateral plate portion of the nozzle plate and the support surface edge portion so as to be movable towards and away from the central portion of the nozzle plate. According to the shape of the lateral plate portion, the adjustable edge plate may be flat and movable in parallel to the lateral plate portion, or the adjustable edge plate may be curved and movable in parallel to a curved lateral plate portion. It is further noted that the extension direction of the support surface and the corresponding nozzle plate in the machine direction (MD) can be any direction with regard to gravity, i.e. substantially horizontal arrangements or substantially vertical arrangements of the mentioned plate/surface elements or combinations thereof are possible and can be used with the invention .

Preferably, the central portion of the nozzle plate comprises openings that are directed to blow the drying gas in a direction perpendicular to the support surface and/or comprises openings which blow the drying gas in the moving direction of the web. Additionally or alternatively, it may be arranged that a plurality of openings of the nozzle plate are directed to blow the drying gas in a direction perpendicular to the support surface including the support surface edge portion. Additionally or alternatively, the lateral plate portion may comprise openings which are directed to blow the drying gas obliquely towards the edge of the support surface.

In a preferred form, the nozzle plate is provided with at least one suction opening for the passage of used drying gas, wherein support blowing openings are provided on the nozzle plate which are directed to blow drying gas towards the support surface below said suction opening. Preferably, edge support blowing openings are provided, which are directed to blow drying gas to a support surface edge portion adjacent said suction opening. In an advantageous form, the suction opening is formed in the nozzle plate, and its shape is selected form a group of shapes including a round hole, slits, discrete rows of openings, an array of openings, and openings covered with a grid.

The method of the invention for drying a fibrous web using impingement drying, passing a web through an impingement dryer in which a drying gas is blown onto the passing web to be dried which is supported on an support surface. Here, blowing towards the edge portion of the web is made from further away from the web surface than blowing onto a central portion of the web. Preferably, the method comprises exhausting used drying gas including evaporated water from above the web in localized areas and blowing fresh drying gas onto the web in the vicinity of suction areas where the drying gas is exhausted. In the method, the drying gas may be blown perpendicularly onto the web in a central portion, and may be blown obliquely and outwardly onto the edge portion of the web.

Control of the drying process may be achieved by control of openings of the nozzle plate in lateral plate portions of the nozzle plate by moving an edge plate in machine cross direction (CD) closing selected ones of the openings. Preferably, drying of the fibrous web is carried out after the edges of the web have been trimmed.

The invention will now be explained in detail by way of possible embodiments of the invention, wherein reference is made to the attached drawings in which: Fig. 1 is a schematic view of a part of a first embodiment of an impingement dryer as seen in machine direction;

Fig. 2 is a schematic view of a part of a second embodiment of an impingement dryer as seen in machine direction;

Fig. 3 is a schematic side view of an impingement dryer according to a third embodiment;

Fig. 4 shows a schematic partial bottom view on nozzle plates of the apparatus of Fig. 3;

Fig. 5 shows an arrangement of openings in a side portion of a nozzle plate of an impingement dryer according to a fourth embodiment;

Fig. 6 is a schematic view of a part of a fifth embodiment of an impingement dryer as seen in machine direction; and

Fig. 7 is an explanatory drawing for explaining problems of impingement dryers of the prior art.

Fig. 1 shows a schematic view of a part of a first embodiment of an impingement dryer as seen in machine direction. On the bottom of Fig. 1, there is shown a support surface 2 on which a web 5 is supported. The support surface can be a wire, a belt, a felt or similar equipment, which travels in the direction into the drawing plane of Fig. 1. Above the support surface 2 there is arranged a box shaped blowing device 1 which blows drying gas 4 towards the web 5. On the surface of the blowing device 1 facing the support surface 2 and the web 5, respectively, there is provided a nozzle plate which has a central portion 11, a lateral portion 12 and a step portion 13 connecting the central portion 11 and the lateral portion 12. Further, an edge plate 3 is provided in front of the lateral portion 12 of the nozzle plate, which edge plate is movable along the drawing plane of Fig. 1, i.e. towards and away from the central portion 11 of the nozzle plate, so as to selectively close individual openings in the lateral portion 12 of the nozzle plate.

The left part of the web 5 in Fig. 1 is referenced with reference sign 51. There, it is indicated that the web does not lie flat but has the trend to lift off, which may cause contact to the lateral portion 12 of the nozzle plate and may initiate web breaks. The lateral portion 12 is arranged 50-400mm away from the edge of the web 5 towards the central portion 11, preferably 150-300mm.

Reference is now made to Fig. 2 which shows another embodiment in schematic drawing, wherein the same elements have the same reference signs as those elements of Fig. 1, so that a detailed description of these elements may be omitted here. The blowing device of Fig. 2 differs from that of Fig. 1 in that the central portion 11 of the nozzle plate is flat while the lateral portion 12 of the nozzle plate is inclined so as to lower from outside towards the center of the web. Here, the step portion 13 of the nozzle plate can be omitted. Also, similar to Fig. 1, there is provided an edge plate 3 slidably arranged opposite the lateral portion 12 of the nozzle plate, so as to selectively close openings in that lateral portion 12. As can be seen in Fig. 2, the inclined lateral portion 12 is arranged in a distance from the web 5, so that even an uplifted edge portion 51 of the web 5 will not contact the nozzle plate.

Here, reference is made to Fig. 5 which shows a modification of an inclined nozzle plate lateral portion 12 which can be used in the general layout of the blowing device as shown in Fig. 2. As can be seen in Fig. 5, an oblique nozzle plate lateral portion 12 has openings 15 provided in horizontally arranged portions of the inclined nozzle plate lateral portion 12, so that a drying gas 4 is vertically (i.e. perpendicularly with respect to the support surface (not shown) ) blown downwards towards the web 5. This is also indicated with the vertical arrows in Fig. 2 opposite the uplifted web portion 51.

Fig. 3 and Fig. 4 show a schematical side view and a bottom view in which blowing devices 1 of the type shown in Fig. 1, 2 or 5 are arranged in a row and under which the web 5 passes, while being supported on the support surface 2. In particular, the support surface 2 is a wire which runs over rolls 6 to move with the web carried thereon along the row or array of blowing devices 1. The blowing devices 1 each blow drying gases 4 towards the surface of the web 5. When the drying gas has taken up evaporated water, it is removed from the space between the nozzle plates of the blowing devices 1 and the web. The used drying gas is indicated with reference sign 43 in Fig. 3. In order to avoid that the web lifts off following the exhaust flow of the used drying gas 43 from between the individual blowing devices, drying gas 44 is jetted towards a point where the used drying gas 43 is removed from the web. The drying gas jets 43 serve to hold the web 5 down on the support surface 2.

Turning now to Fig. 4, the apparatus of Fig. 3 is shown as seen from the web that is looking from the web upwards to the nozzle plate of the individual blowing devices. As can be well seen in Fig. 4, there is a number of individual nozzle plates 11, 12 each belonging to one of the blowing devices 1 of Fig. 3. Each nozzle plate 11, 12 of each blowing device 1 is divided into a central portion 11 and two lateral portions 12, of which only one is shown here because the arrangement is substantially symmetric. In the nozzle plates, individual openings are indicated with reference sign 15 and are distributed over the nozzle plate in regular or irregular patterns in a number and shape adapted for the respective drying task. As can be seen in Fig. 4, the edge plate 3 is arranged to overlap a part of the lateral portion 12 of the nozzle plate, so as to close openings 15 at need. The edge plate 3 can extend further over suction openings than over blowing openings, which also contributes to avoid edge lifting of the web 5. Between the individual nozzle plates 11 of Fig. 4, there is shown a grid 14 indicating a grid in front of a channel or passage, so as to protect the exhaust opening for used drying gas 43 from debris. Of course, the grid 14 can be replaced by slits or the like, and the channels for removing used drying gas may also be tubes, slits or may have any other desired shape. In this connection, reference can be made to FI 20080300 filed on April 21, 2008 which shows various arrangements of openings and channels for fresh and used drying gas. Therefore, in this respect, especially the layouts of Fig. 2, 3a, 3b, 4a, 4b are included in this specification by way of reference.

Fig. 6 shows another embodiment in a similar view as in Fig. 1 and Fig. 2 in which the lateral portion 12 of the nozzle plate is curved and is offset or stepped from the central portion 11 of the nozzle plate. This central portion 11 of the nozzle plate is connected at its edge by a step portion 13 to the lateral portion 12. The lateral portion 12 is curved, so that its distance from the support surface 2 carrying the web 5 increases when approaching the edge of the web. The lateral portion 12 of the nozzle plate also has openings which may be arranged to jet the drying gas 4 vertically or perpendicularly to the web or the support surface; or they may be arranged such that they release the drying gas 4 substantially perpendicular to the curved surface of the lateral portion 12 of the nozzle plate. Similar to the foregoing embodiments, also an edge plate 3 is provided which can be used to close a selected number of openings of the lateral portion 12 of the nozzle plate if required. Here, the edge plate 3 is also curved in a manner to match the curvature of the lateral portion 12 of the nozzle plate. Instead of a linear movement as is indicated in Fig. 1 and Fig. 2 by double-headed arrows, the edge plate 3 in Fig. 6 is arranged to pivot around a fulcrum 31. In this way, it can be easier to control the movement of the edge plate, since a stiffer body of the edge plate can be generated when putting the construction shown in Fig. 6 into practice. It is noted that the other elements of the blowing device 1 of Fig. 6 which have the same reference signs as that of Fig. 1 and Fig. 2 are similar and for their description reference to the mentioned figures appears sufficient.

It is noted that Fig. 1, Fig. 2 and Fig. 6 only show one side of the blowing device which extends in the machine cross direction to the other edge portion of the web. Of course, at the other end of the blowing device, there is the same edge portion design as shown in Fig. 1, 2 and 6 in a symmetrical arrangement.

It is further noted that the support surface 2 has been shown to be a substantially flat surface extending horizontally, on which the web 5 is borne. Of course, the support surface 2 can be curved and may have any direction with regard to gravity, and suitable means can be provided to carry the web on the surface. For example, the surface may be arranged above the web, holding it attached thereto by negative pressure. A big wire covered vacuum roll can also be used as support surface 2. Also multiple dryers may be provided in subsequent order and also in a way that they act on different sides of the web. In all these arrangements, the invention may be applied in the edge portion of the web in order to avoid web breaks.

Finally, the impingement dryer may be surrounded by a suitable housing which allows maintaining the temperatures inside the dryer, promotes the intended gas flows and protects the environment around the machine from the considerably hot drying gases which are used in impingement dryers .