By means of calendering the gloss of a paper web is improved by smoothening its surface on the microlevel and its smoothness is im- proved by equalizing the thickness variations of the paper web on the macrolevel. The paper web can be calendered in a paper machine in connection with the papermaking process (so-called on-line calen- dering) or the calendering can be conducted as a separate processing stage for the already reeled paper web (so-called off-line calendering).

Furthermore, for example before the paper web is coated, it is possible to conduct so-called precalendering as an on-line calendering process to improve the coating result.

In its simplest form the calender is composed of two rolls between which one nip is formed. Such a one-nip calender is for example a machine calender comprising two hard-covered rolls, or a soft calender in which one of the rolls has an elastic coating. Various end products, however, have quality and property requirements of their own. Different calendering conditions can be attained by changing the coatings and diameters of the rolls, the length and number of the nips as well as the running speed. There are also machine calenders and soft calenders that comprise more than one nip. In an extended-nip calender, which is also a one-nip calender, the length of the nip can be increased with a separate extended-nip roll, wherein the nip distributes the compression pressure to a wider area. A calender suitable for almost all paper grades is a multinip calender containing typically 6 to 12 primary rolls between which the calender nips are formed. The multinip calender is advantageously also used in such situations where the aim is to pro- duce several paper grades in the same machine.

In the multinip calender the nips are formed between on top of each other stacked calender rolls. The paper web to be calendered travels along a winding path downward from above via the calender nips. The roll stack contains both hard-covered rolls and rolls equipped with an

elastic coating. Fig. 1 shows a typical multiroll calender containing calender rolls 103 placed on top of each other. Typically, the rolls of the multiroll calender are of different types. Typically, the rolls have hard or soft surfaces, some of them a coated, some heated, and some of them are variable crown rolls. Nips 105 are formed between the calender rolls 103 placed on top of each other. The paper web 101 travels along a winding path via the nips 105 in such a manner that it is first con- veyed to the uppermost nip 105 in the calender, and the paper web 101 is conveyed out of the calender when it has travelled through the lowermost nip 105 in the calender. In Fig. 1 arrows illustrate the travel direction of the web 101 and the paper web that is being run in the calender. After each nip 105 the web 101 is guided off the surface of the calender roll 103 following the nip 105 by means of fly rolls 104.

The multiroll calender shown as an example in Fig. 1 can be used both in connection with the paper machine line as an on-line calender and as an off-line calender separate from the paper machine line. Fig. 1 shows guide rolls 102 that guide the travel of the web both to the calen- der and to the stage following the calendering process. Typically in an on-line calender the paper web is guided from the dryer section to the uppermost nip 105 by means of the guide roll/rolls 102. In an off-line calender the guide rolls 102 guide the paper web from the paper reel to the uppermost nip 105 in the calender. From the calender the paper web is guided by means of the guide roll/rolls 102 typically to a reel-up.

Typically, the calenders are driven with their nips open in a threading situation. At first, the web is passed as a narrow tail through the open nips, whereafter the web is spread into its determined width. Generally the nips are not closed until the web has been spread. The nips of the calender following the coating machine are not closed in a threading situation until the desired measured quality values of coating have been reached.

When the paper web travels through the open nip, i. e. when two ro- tating rolls are not in contact with each other, the paper web flutters heavily. The machine calenders, soft calenders and extended-nip calenders can be driven in a threading situation with an open nip, and a

multiroll calender with all nips open. When the tail is spread into its full width, the flutter may begin already at a certain width before the full- width web.

The flutter of the paper web results from the air flows in the machine di- rection and in the cross-machine direction. When the paper web travels through an open nip, a negative pressure is generated between the paper web and the roll closest thereto. As a result of the negative pressure the paper web is drawn against the roll. When the paper web is attached to the roll, overpressure is generated between the roll and the paper web as a result of the gap between the inlet side of the web and the roll. The generated overpressure forces the paper web off the roll. Because of the mass and the slowness of the paper web, the position of the paper web is not stabilized in either position, but the pa- per web flutters in the open nip. The flutter can be eliminated only if the gap between the rolls is sufficiently large or the paper web travels along the surface of one of the rolls with a sufficient bending angle, i. e. the paper web is in contact with the roll within a longer distance.

In addition to the above-mentioned flutter in the machine direction, there also appears flutter at the edges of the paper web. This results from the air flows on the surface of the paper web in the cross-machine direction. On the inlet side of the nip the transverse flows are typically directed from the middle of the web towards the edges, and on the outlet side of the nip from the edges of the web towards the middle.

It is an aim of the invention to stabilize the travel of the paper web in such a manner that the flutter of the paper web in the calender nip both in the longitudinal direction of the machine and in the cross-machine di- rection is minimized.

The invention is characterized in what will be presented in the charac- terizing part of the independent claims. Advantageous embodiments of the invention will be described in the dependent claims.

The objective is attained by means of keeping the pressure field stable in the nip.

The pressure variation in the open nip as well as the air flows ap- pearing in the opening nip cause reciprocating flutter of the paper web travelling through the nip. The air discharged from the nip with the pa- per web causes pressure variations in the nip. This air discharged from the nip can be replaced according to the invention by blowing air back to the opening or open nip. In this way a stable pressure field is ad- vantageously generated in the nip. When the generated pressure is low and remains stable, the paper web seeks its advantageous position and also keeps it while the pressure field remains constant.

The equalization or stabilization of the pressure field in the nip in accor- dance with the preferred embodiment of the invention is implemented in such a manner that air is blown towards the nip in a direction oppo- site to the travel direction of the paper web. The blowing is advan- tageously implemented on that side of the web that is located closer to the roll forming the nip. Replacement air is advantageously blown into the nip within the entire width of the paper web. The air nozzle is ad- vantageously positioned close to the surface of the paper web. Ac- cording to a preferred embodiment, a rapid air flow is discharged from the air nozzle, the static pressure of said air flow being low. Thus a pressure difference is formed between the air flowing from the air nozzle and the environment. In this situation the paper web seeks a position in which the static pressure is lower. Now the pressure at- tained by means of the air flow produced by the air nozzle is lower than the varying pressure of the nip. Thus the paper web remains in lower pressure, i. e. in the blown rapid air flow, and attached to the roll when the nip is open. The stable pressure produced in this way also sta- bilizes the flutter of the web.

The act of stabilizing the web in the opening or open nip improves the runnability and reduces web breaks. According to a preferred embodi- ment of the invention, the travel of the web through the open nip re- mains stable, i. e. the web does not flutter even at high running speeds.

Thus, the threading stage and especially the spreading of the web can be conducted in a more controlled manner. Furthermore, after the web has been spread, the closing of the nip becomes easier, and the risk of a web break is reduced in this situation as well.

The invention and its preferred embodiments will be described in more detail hereinbelow with reference to the appended drawings, in which Fig. 1 shows a multiroll calender of prior art, Fig. 2 shows an apparatus according to a preferred embodiment of the invention for stabilizing the web in a calender nip, and Fig. 3 shows an apparatus according to a preferred embodiment of the invention for stabilizing the web in a calender nip.

The multiroll calender according to Fig. 1 has already been discussed in connection with prior art. In the following, a solution according to a preferred embodiment of the invention will be described by means of Fig. 2. Fig. 2 shows two rolls 203 between which a nip 205 is formed.

Here, the nip is open and the paper web 201 travels through the nip in the direction indicated by the arrow on top of the paper web 201 in Fig.

2. The nip can be the lowermost nip in the multiroll calender of Fig. 1 or the nip of a two-roll calender. The lowermost nip in the multiroll calen- der is in a special position with respect to the stabilization, because the web extends over it with a relatively long clear space, i. e. an un- supported section from the last fly roll (roll 104 in Fig. 1) to the web guiding roll (roll 102 in Fig. 1) following the calender.

There is a varying pressure field between the upper roll 203 of the open or opening nip 205 and the paper web 201 travelling through the nip. Typically, air is discharged from the open nip to the direction away from the nip to the travel direction of the paper web 201. In a running situation like this, the paper web 201 typically flutters in such a manner that it is alternately attached to the upper roll 203 of the Figure and detached therefrom. Furthermore, the transverse air flows of the web cause lighter flutter at the edges of the paper web 201. According to a preferred embodiment of the invention, a blower 206 is installed at the outlet side of the nip and at the upper roll 203 side of the web, the nozzles of said blower blowing air close to the surface of the paper web 201 and towards the nip, against the travel direction of the paper web.

The direction of the replacement air blown to the nip is indicated in Fig.

2 by means of an arrow in the blower 206.

Advantageously, a rapid flow of air is produced from the nozzles of the blower 206, said air flow producing low static pressure. According to a preferred embodiment of the invention, the pressure attained by means of the air flow is lower than the varying pressure prevailing in the nip 205. Similarly to other kind of matter, the paper web 201 travelling via the nip 205 seeks a state and location where the static pressure is lower. As a result of the low, steady pressure field, the web also re- mains in the produced rapid air flow. The stable pressure and the rapid air flow efficiently prevent the fluttering of the web.

By adjusting the air flow and thus the static pressure of the nip, it is possible to determine a desired location for the paper web in the open nip. Typically, the paper web is guided to travel close to the other roll.

In the open nip the paper web can travel off contact with the closest roll as well or so that it barely touches the rotating roll. Typically the paper web is never guided to travel via the centre of the nip. When the nip is open, the aim is to keep the paper web against the other rotating roll.

When the nip is open, air can flow between the rolls and the pressure conditions change. Thus, the blowing of replacement air into the nip against the travel direction of the paper web in accordance with the preferred embodiment of the invention equalizes the pressure dif- ferences. The blowing is conducted on that side of the web which is meant to be positioned against the rotating roll when the nip is open.

The blowing of replacement air in accordance with the preferred em- bodiment of the invention to the nip that is opening or to an open nip can be implemented for example by means of a centrifugal blower shown in Fig. 3, in which a pump is marked with the number 307. The blower can also be any other kind of blowing device known as such.

The blowing is typically guided on the entire width of the web, wherein air is guided from the centrifugal pump 307 to the entire width of the web by means of the nozzles 308 of the blower 306. From the narrow nozzle slots 308 a rapid air flow is typically obtained.

According to a second embodiment of the invention, the blower is computer controlled and it is possible to take into account in its function other known and/or measurable parameters of the paper web and the

paper machine. The embodiment shown in Fig. 3 comprises a com- puter 309 that controls the pump 307 and the nozzles 308 in accor- dance with the values it has measured and/or commands programmed in and/or given to said computer. The blower can be programmed to always function in an open nip or at a particular stage of the threading or when the nip is about to be opened. The blower can also be started when pressure variation is detected in the nip. Furthermore, the blower can be started manually, if necessary, or the controlling can always take place as a response to a particular action, such as opening the nip, or the blower can be started when a measured variable so re- quires.

According to a preferred embodiment, it is in Fig. 3 also possible to adjust for example the blowing efficiency of the blower and the tem- perature of the air to be blown for example on the basis of the measured state or in accordance with the running stage. The devices 310, 311, by means of which the properties of the blower are adjusted, are controlled by means of a computer. This embodiment shows as an example a device 310 for adjusting the temperature of the air to be blown, by means of which device it is possible to adjust the tem- perature of the blast air, thus affecting the moisture of the paper web.

The moisture also affects other properties of the paper web, such as runnability. Furthermore, Fig. 3 shows an adjustment device 311 for the nozzles of the blower, by means of which the direction of the nozzles and/or the size of the openings can be adjusted. Thus, the air to be blown can be directed in an advantageous angle to the paper web, and the travel path of the paper web in the open nip can be adjusted into an advantageous location in the nip. The speed of the air flow can be ad- justed by changing the size of the nozzle openings. According to a preferred embodiment of the invention it is also possible to use several blowers in parallel.

The blower according to the invention can be advantageously moved in such a manner that it has a so-called operating position and a separate rest position. The blower can move in the transverse direction of the calender, wherein when the blowing apparatus is positioned aside in its so-called rest position, it is advantageously possible to perform

cleaning and maintenance measures. The blower can also be lifted from the operating position close to the paper web to the rest position upward with respect to the paper web for example for the duration of the calendering process. Before the intended opening of the nip it is thus possible to bring the blower into its so-called operating position in which the nozzle parts of the blower from which air is blown, are brought close to the paper web coming from the nip. Advantageously, the blowing can be directed in accordance with the paper grade that is being run or in accordance with the needs of the current situation in such a manner that at least the direction and intensity of the blowing can be adjusted. Advantageously, the settings of the blower are set automatically to comply with the other parameters of the paper machine, such a running speed, properties of the paper web or the running stage of the web. Thus, according to the invention the travel of the web is stabilized so that is becomes suitable for the situation, and it is not necessary to adjust other functions, such as running speed or spreading of the web in accordance with the behaviour of the web.

The invention is not limited solely to certain running situations. The blowing can be started when the web reaches a determined critical width when it is spread from a narrower tail and the nip is open. The blowing can be stopped after the nip is closed. Similarly, when the nip is opened, the blowing can be started already before the nip is opened, and it can continue for as long as the nip is open. The blowing can be arranged to start and stop automatically on the basis of the afore- mentioned conditions. An automatic apparatus can for example monitor whether the nip to be monitored is open, and guide the blowing appa- ratus accordingly. Similarly, the automatic apparatus can monitor whether the web is spread into a determined width and give a start command for the blowing when the width of the web has reached a given limit value in connection with the threading. The closing of the nip after the web has reached the full width again automatically interrupts the blowing to the corresponding nip.

Furthermore, the method can also be used when the nip is closed if the aim is to stabilize the travel of the web after the nip by means of a counter-blowing directed towards the nip.