Method for compensating for faults in a paper web

The present invention relates to a method for compensating for faults, such non uniform shrinkage, in a paper web. In particular, the matter concerns compensating for greater shrinkage that occurs in the edge areas of the web than in both the centre, in the press and drying sections of a paper machine.

It is known that shrinkage in a paper web over its production width on a papermachine is uneven, in such a way that the shrinkage in the edge parts can be in the order of 7 %, whereas in the centre the shrinkage of the web can remain in the order of two or three percent. Refer to attached figure illustrating a typical shrinkage profile.

It is obvious that in the situation described above, it is extremely likely that unsaleable goods and other detrimental quality defects will be created, unless the shrinkage differences can be reduced in some manner.

Problems in the edge areas of the web include but are not limited to, an increase in the porosity of the edges, an increase in roughness at the edges, creasing or cockling, variation in the percentage of mineral filler content and of course poorer dimensional stability. The exact nature of the problem will depend on the type of the press and drying sections of the paper machine in question, and on the paper grade in general.

It is obvious that, in an analogous manner, other faults corresponding to shrinkage are also possible and that compensating for them belongs automatically to the scope of the present invention.

The present invention is intended to create an improvement in the adverse effects of shrinkage profile variation. The intention is to achieve the improvement in a manner that is very reasonably priced and simple.

The aforementioned and other advantages and benefits of the present invention are achieved in the manner described in the accompanying claims.

Stated simply, the basic idea of the invention is that the fault, no matter whether it is natural or the result of some operation, is affected in such a way that the variation in properties can be minimized. Thus, for example, the shrinking of a paper web more at the edges than in the centre is a natural change, which when attempting to minimise also promotes an improvement of the quality of the paper reels at the edges.

In the following, the invention is described in greater detail also with reference to the accompanying drawings, in which

Figure 1 shows one embodiment of a flow guide used according to the invention in the discharge nozzle of a head box of a paper machine in the direction of flow of the stock; and

Figure 2 shows a second sub-area of the invention, in connection with the centre roll of the press section.

In the discharge nozzle of a head box of a paper machine, seen in the direction of flow of the stock, flow guides are used, which are intended to affect the properties of the paper, by affecting the turbulence of the jet. The length of the flow guides in the direction of travel of the stock has a significant effect on the properties of the paper. This is because the machine direction orientation of the fibres changes significantly as result of the turbulence generated by the flow guides. A general rule is that the machine direction orientation of the fibres decreases as the length of the flow guides increases. In the present invention, this aforementioned property is exploited by making the flow guide or guides longer at the edges than in the centre. One example is shown in Figure 1 , though only by way of example and not restrictively. In the figure is a flow guide 1 , which is generally a plate-like piece. In this embodiment, the edges 2 of the guides are clearly extended.

Because the change in the machine direction magnitude of the fibre orientation in the edge areas of the web can also be a risk in terms of the operation of the paper machine, according to the invention other means are also used to achieve the desired effect.

The angle of release of the paper web from the centre roll of the press section in an arrangement, which uses an open transfer from one roll to another, varies according to the draw and the magnitude of machine direction fibre orientation. If the magnitude of machine direction fibre orientation has been altered in the manner described above, in such a way that the orientation is less at the edges of the web, this means that the angle of release from the centre roll of the press section will change, in such a way that the web tends to remain attached to the roll at the edges of the roll more than the centre. The result of this is that the edge areas become permanently stretched more than the centre which can be extremely detrimental to the machine's subsequent operation in the dryer section.

Thus, it would be good to have available a device, with the aid of which it would be possible to counteract the release angle problems caused by the shape of the flow guide. This is solved according to the invention as follows. Here reference is made to Figure 2.

The angle, at which the paper web 4 running around the centre roll 3 of the press section detaches from the roll, is adjusted in a controlled manner, by using an adjustable air jet over the width of the web. In practice, this means that, either experimentally or by theoretical calculation, the pressure profile in the cross direction of the web is determined, by means of which it is possible to achieve a uniform release angle profile.

One device, by means of which it is possible to create and use the afore mentioned profile, is a device developed by Runtech Systems Oy and marketed under the trademark Air Curtain. By means of the said device, which is marked with the reference number 5 in Figure 2, it is possible to create an air jet extending over the entire width of the web, the pressure profile of which can be varied over the entire width. The air jet being created is marked schematically with the

reference number 6. The pressure of the air jet affects the angle of sheet release, thus by varying the cross machine pressure profile the release angle can be influenced in such a manner as to compensate for the variation in release angle created by the length profile of the flow guides in the headbox. By this means the adverse effects on dryer section runnability can be controlled while retaining the positive effects of minimising variation in paper properties dependent on the turbulence created by the length profile of the flow guides in the headbox.

One further development of the system described above is that, by using suitable means to measure the release angle, the information used by the said means can be used directly to adjust the operation of the air jet device 5, so that the desired release angle will be obtained. In practice, a laser camera, for example, is a suitable means for measuring the release angle.

The procedure described above is not the only one to which the principle of the present invention can be applied. With the aid of an airjet, it is possible to correct, more or less in real time, non-uniformity in the paper web arising from other reasons. Thus it is possible to quickly adjust the air-jet pressure and the release angle at any point whatever in the width of the web. The result will be paper of improved quality.

Variations are possible while remaining with the scope of protection of the inventive idea and the accompanying claims.