The present invention concerns a system for keeping calender rolls of a paper ma- chine clean, including in a running direction, that is in the MD direction, of the web after a dryer section, a coating station, an additional dryer section and an On- Line calender prior to a reeling unit, in producing high-gloss paper or coated pa- per.

It is known in the art to combine the Off-Line coating either with the Off-Line or On-Line calender, preferably with a multiple-nip calender for producing high- gloss paper or coated paper, such as LWC or MWC. Hereby, the problem is, in addition to intermediate reeling of the web, that the wet spots and streaks of the coating become caught on the calender rolls, resulting in local hot spots in the smooth surface of the calender rolls, leading to a greater deformation and uneven heating of the smooth surface of the calender roll, and hence, to a self-induced local overheating of the smooth surface of the calender. The end result of the overheating is fatal because the ordinary material of a roll coating is poorly heat conductive.

The objective of the present invention is to provide a novel method and a system for producing high-gloss or coated paper, such as LWC or MWC for the ON-Line calender, wherewith problems related to the prior art technology caused by soiling of the smooth surface and local overheating of the calender rolls could be elimi- nated or at least reduced.

This aim is achieved by the method and the system described above, wherewith the characteristic unique special features are mentioned in the accompanying claim assembly.

Therefore, the present invention is based on the novel and inventive basic insight that in order to prevent the coating agent, such as a coating dye and impurities of additives, from addhering onto the smooth surface of the calender roll and, as a result, the smooth surface of the calender roll from being soiled, and to prevent the overheating phenomenon and an extra heat exchange resistance of the ther- moroll, the system includes another measurement station provided with measuring and indicator means for producing an output indicating the soiledness or the hu- midity profile of the web. When an output of the second measurement station in- dicates excessive soiledness or an uneven humidity profile, the system performs, as a response to the output of the second measurement station, at least one of the functions: cleaning of the surface of the smooth-surfaced calender roll; change of nip load in the calender; opening of nips; correction adjustment of the CD- direction humidity profile of the web; web cutting and conducting it into a pulper, and/or a change in the running direction of the web.

About the advantages of the invention, it is worthwhile to mention that the pro- duction loss is minimized by the invention being caused by cleaning and replace- ment operations of calender rolls and the intermediate reeling operations of the coated web.

The invention is described below more in detail with the aid of an embodiment considered advantageous, reference being made to the accompanying drawing in which Fig. 1 presents schematically part of a paper machine including the system of the invention, and Fig. 2 presents schematically a cleaning unit of the invention.

The part of the machine demonstrated in the drawing includes, in the running di- rection of the web W, that is in MD direction, arranged after the dryer section 1 a coating station 2, an additional dryer section 3, a measurement station 4 and a

calender 5, which is advantageously a multiple-nip On-Line calender, positioned before the reeling unit 6.

The aim of the present in vention is to prevent the coating dye from addhering onto the smooth surface of the calender roll and, as a result, to prevent the smooth surface of the calender roll from becoming soiled and to prevent the overheating phenomenon. To achieve said aim, the system includes a second measurement station 10, provided with measuring and indicator means to produce an output indicating the soiledness or the transverse direction, that is CD direction, humidity profile of the web W. When an output of the second measurement station 10 indi- cates excessive soiledness or an uneven humidity profile in the web W, the system performs, on an impulse of the output, one or more of the following functions to prevent the smooth-surfaced calender roll from becoming soiled: cleaning of the surface of the smooth-surfaced calender roll 51, change in nip load in the calender 5, correction adjustment of the CD direction humidity profile of the web W; cut- ting of the web W and guiding it into the pulper 7, and/or a change of the running speed of the web W.

In the CCC (Coated Calender Control) system of the present invention the essen- tial aspect is that after the last coating station the web is continously measured in the above-mentioned second measurement station 10, being in the running or MD direction to the web W positioned after the coating station 2. Hereby, on the basis of an output of the second measurement station 10, wet streaks and spots in the web can be detected, which will cause e. g. holes in the web W. Wet streaks and spots may also be caused by e. g. disturbances in the coating station 2, this in turn being caused by e. g a fixed particle entrapped between the measuring unit (not shown in the drawing) before the coating station and the web.

In the advantageous embodiment of the invention, the humidity of the web is measured by continuous point measurement, whereby disturbances in the dryer units and/or the coating station are detected, causing either inadequate drying of

the web in the dryer apparatus or excessive spreading of the coating mixture from the spreading unit.

In addition and in parallel to measuring the humidity profile or soiledness of the web W, it is advantageous to provide a correction adjustment of the CD direction humidity profile of the web in the last drying group 31 of the additional dryer section 3, said operation being, according to the invention, preferably imple- mented using infrared heaters. Said correction adjustment is preferably imple- mented by means of zone-controlled dryer units in the last drying group for cor- recting the CD-direction humidity profile of the web without any risk of overdry- ing the sufficiently dried web.

In the system of the invention, soiling of calender rolls can be reduced further and therethrough the calender rolls can be protected by reducing the nip load of the calender 5 when the humidity of the web W exceeds a permitted limit while being still sufficiently low for drawing the web W through a substantially unloaded cal- ender nip/nips. In addition, it is particularly of importance, according to the in- vention, to prevent the calender rolls from becoming soiled that the calender nip is opened when, on the basis of an output of the second measurement station 10, the local humidity content of the humidity profile of the web W exceeds a predeter- mined limit and when to a limited extent, wet spots of the coating agent, being apt to catch on, are detected on the web W.

It is furthermore essential, considering the soiling of calender rolls, particularly when, on the basis of an output of the second measurement station 10, the local humidity content of the humidity profile or the soiledness of the web W, resulting in a disturbance in the coating process, exceeds a predetermined limit value that the web W is cut before the calender 5 and conducted into the pulper 7.

When the web W is conducted into the pulper 7, it is important, considering the minimization of broke, that the running speed of the web W in the coating station 2 is decelerated.

After the humidity profile or the humidity of the web W is on an acceptable level after the coating station adjustments, the tail of the web W is again threaded through the calender 5, whereafter the running speed of the web W in the coating station 2 and in the calender 5 is accelerated back to the production speed.

Figure 2 presents as a schematic diagram a cleaning unit 11 of the invention, in- tended for cleaning a smooth-surfaced calender roll, which is positioned, as being an essential feature in the invention, to be moved away, said position being dem- onstrated by an intact line in Figure 2, and towards the calender roll to be cleaned, which position is demonstrated by a broken line in Figure 2. In this case, it is ad- vantageous, according to the invention that the cleaning unit is disposed to be ei- ther fixedly in the calender 5, e. g. with the aid of a fixed support arm in the sup- port frame of the calender rolls to be in connection with each calender roll with smooth surface or to be movable/transferrable in connection with the calender roll with a smooth surface, When the cleaning unit is a movable/transferrable cleaning unit 11, it is advantageously positioned to be supported by vertical guides in a portal support 14, so that the cleaning unit 11 can be moved to be in the adja- cency of a calender roll requiring cleaning.

As taught by the invention, the cleaning unit 11 is provided with a means 12 for generating a pressurized jet. Advantageously, said means is a pressure washer 12, the jet/jets of which and focussing of the jet/jets of which in CD direction towards the surface of the calender roll is controlled by an output of the second measure- ment station. It is advantageous, in addition to the pressurized jet, that the clean- ing unit is also provided with a doctor blade 15 which blade moves most advanta- geously together with the cleaning unit 11 on and off the surface of the calender roll, which in this manner aids in cleaning the smooth-surfaced calender roll.

The invention is described in the preceding merely in exemplary fashion with the aid of an embodiment considered to be advantageous. It goes without saying that the invention is not desired to be confined therewith; instead, a plurality of other alternative solutions and modifications, especially concerning the composition of the production line of paper, etc. and embodying of the present invention therein, are conceivable within the protective scope of the inventive idea as determined in the accompanying claims.