When striving for higher running speeds, the temperatures of thermo rolls in on-line calenders, especially in multi-nip calenders, have risen. To improve the calendering result, fine water mist is sprayed onto the surface of the fibrous web going to the roll nip/roll nips of a calender. It has now been found that problems are often caused due to the improvement in the calendering technique disclosed above, because material adheres to the hot thermo roll from the fibrous web arriving from the drying section after the manufacture of pulp. A filler, such as kaoline, talcum, or calcium carbonate added to the base paper adheres especially easily to the surface of the thermo roll.

Although calenders have doctor blades, by means of which the surface of each thermo roll is continuously cleaned, the detaching material will still fast wear the surface of the thermo roll, thus graining it. This again deteriorates the calendering result, and the fall in quality has to be compensated by changing the process pa- rametres, such as, for example, by raising the temperature of the thermo roll and/or by increasing the moisture level. Nevertheless, changing the process parametres will increase costs, and the change may cause problems arising from the equipment, if this involves moving within the upper limits for the control of the equipment- specific process parametres.

The object of the invention is to eliminate the drawbacks in the state of the art.

Thus, the main object of the invention is to achieve an arrangement and a method for preventing the wear of thermo rolls.

Further, it is an object of the invention to achieve an arrangement and a method for preventing the wear of thermo rolls without changing the process parametres, such as the amount of heat to be brought to the roll nip, or the moisture of the fibrous web arriving at the roll nip.

A third object of the invention is to substantially reduce the adherence of the fibrous web to the surface of a hot thermo roll in a roll nip.

The said objects are achieved by means of the arrangement according to claim 1 and the method according to claim 6.

In the arrangement of the invention, the calender has one or several roll nips, be- tween which the fibrous web travels, while being simultaneously profiled, and of which roll nips at least one consists of a pair of rolls with a thermo roll and a counter roll located opposite to it. Before the said pair of rolls, the fibrous web is moistened by a wetting device, and before the wetting device, adjacent to the fibrous web arriving at the calender, there is a dust separator for cleaning the fibrous web.

In the method according to the invention, the fibrous web is cleaned by blowing gas onto its surface before the moistening of the fibrous web.

The invention is based on the basic idea that, after the drying of the fibrous web, and before the moistening of the fibrous web arriving at the calender, easily detaching material is removed from the fibrous web, the material often being a filler, which is removed from the fibrous web as dry dust. In the manufacture of pulp at the wet end of a paper machine, the fibrous web is manufactured onto a wire, in which connec- tion filler is added to it. Subsequent to the manufacture of pulp, the pulp manufac- tured onto the wire will be dried to form a fibrous web with a low moisture level.

Thus, in this connection, drying refers primarily to the drying of the fibrous web coming from the pulp drying section, manufactured onto the wire. Also the fibrous web coming from the coating of the fibrous web may dust on the drying drums or similar drying equipment after the drying, because of which the dust separating ar- rangement according to the invention may also be located after the drying section of the coating device, immediately before the wetting of the fibrous web arriving at the calender.

The removal of dust from the fibrous web after drying and subsequent to the coating is disclosed, for example, in the patent specifications US-A-4 019 953 and US- 5 878 462 and US-5 577 294, but these do not include any reference to how this could be used for improving the calendering result in a calender with at least one roll nip, and with one of the counter rolls being a eatable thermo roll.

By means of the arrangement of the invention and the method used therein, the wear of the thermo rolls is effectively reduced without changing the process parametres, because the amount of loose material, such as filler, detaching from the fibrous web to the surface of the thermo roll is essentially reduced by the blowing of the fibrous web performed before the calender. It has also been noted that although the fibrous web were a paper web, to which no fillers are normally added (for example, news-

print, packing papers), the adherence of the fibrous web to the thermo roll can be reduced by means of the arrangement according to the invention.

The invention is demonstrated in more detail referring to the enclosed drawings.

Figure 1 is a schematical view of the arrangement of the invention seen from the side, Figure 2 is a schematical view of the dust separator used in the arrangement of the invention, also seen from the side.

In the following, the main features of the arrangement according to Figure 1 will be gone through first, and then the main structures of the dust separator according to Figure 2.

In the arrangement according to Figure 1, the fibrous web W travels from left to right. The fibrous web W comes from the drying drum 6; 61 of the drying equip- ment at the wet end of the paper machine via the guide rolls to the dust separator 3.

From the dust separator, the fibrous web W continues its way via the guide rolls to the first water box 2; 2'and the second water box 2,2", both sides of the fibrous web W being moistened by means of the water boxes 2. From the water boxes, the fibrous web W continues to the multi-nip calender 5 with seven roll nips N, six of the roll nips including a pair of rolls, comprising a heatable thermo roll opposite to the metal or polymer coated roll.

The dust separator 3 of Figure 2 contains the blowing device 32 for blowing air to the fibrous web W, and the suction box 31 for recovering the dust detaching from the fibrous web.

The invention will next be described in more detail referring to the enclosed draw- ings and their general specification.

In the arrangement according to the invention, the fibrous web W is a paper web, which has been manufactured at the wet end of the paper machine onto a transport wire (not shown in the Figure) in a way known in itself, so that also talcum, kaoline, calcium carbonite, or other known fillers have been added to it in an amount de- pending on the paper grade. The fibrous web W arrives at the arrangement accord- ing to the invention from the drying 6 of the fibrous web supported on the transport wire. In the Figure, there is shown the last drying drum 61 of the drying equipment.

From the drying drum of the drying equipment, the fibrous web W arrives at the

dust separator 3, the fibrous web being further supported from below on the trans- port wire. A strong air blow is directed to the surface of the fibrous web opposite to the side of the fibrous web W supported on the transport wire by means of the blow- ing device 32 of the dust separator 3, which advantageously extends from one end to the other in the lateral direction of the fibrous web. The strong air blow is used for removing loose dust from the fibrous web W, said dust most often being a filler, such as calcium carbonite that has imperfectly adhered to the fibres of the fibrous web in the pulp manufacture. The dust blown from the fibrous web W is recovered by means of the suction box 31 of the dust separator 3 located beneath the blowing device 32, on the same side of the fibrous web as the blowing device, and at such a distance from the said blowing device that the dust detaching from the surface of the fibrous web travels into the suction box on the basis of the suction effect of the suc- tion box. In the Figure, the suction box 31 has been presented schematically without the vacuum pump or a similar arrangement achieving the suction function of the suction box. After the suction box 31, both sides of the fibrous web W are mois- tened by the water boxes 2; 2'and 2; 2". The water boxes 2 are located near the sur- face of the fibrous web, and they comprise nozzles extending from one end to the other transverse to the direction of travel of the fibrous web, and used for spraying water onto the surface of the fibrous web. By spraying the water, it is the intention to raise the moisture level of the fibrous web to be suitable for the calendering and to adjust the moisture profile of the line.

The fibrous web 2 can be supported or without support as it arrives at the water boxes 2. From the water boxes 2, the fibrous web W is transferred to the multi-nip calender 5 by means of guide rolls.

The multi-nip calender of Figure 1 comprises seven roll nips N between seven pairs of rolls. The line P in the roll system of the multi-nip calender, travelling through the centre line of the rolls, is located approximately in the vertical position. In the multi-nip calender, the uppermost roll 51 and the lowermost roll 52 are variable crown rolls with a metal or polymer coating, equipped, for example, with loading equipment inside the roll. The uppermost and the lowermost rolls can be moved in the direction of the line P of the roll system by suitable hydraulic equipment linked to the axle boxes of the said rolls. The intermediate rolls 501-506 are either metal or polymer coated intermediate rolls so that the intermediate rolls 501 and 504 and 506 are eatable thermo rolls, the other intermediate rolls 502, 503, and 505 being unheated polymer rolls. In addition, the multi-nip calender has loading arms linked to the axle box of each intermediate roll, by means of which the own gravity of the

intermediate rolls. and the weight of the instruments at the ends of the intermediate rolls, such as recovery rolls and doctor blades in the thermo rolls can be compen- sated in a desired way.

Thus, the roll nips N1, N2, N4, N5, N6 and N7 have a eatable metal coated thermo roll as the one roll in the pair of rolls, opposite to which there is a metal or polymer coated roll as the counter roll. The roll nip N3 is a so-called reversing nip with two unheated polymer coated intermediate rolls opposite to each other. The desired nip pressure is generated to the roll nips N, for example, by moving the upper and/or lower roll 51, 52 in relation to the line P of the roll system, and/or by using the in- ternal loading equipment of the upper and lower rolls. The reversing nip is needed for profiling both sides of the fibrous web W. The state-of-the-art technology is re- ferred to concerning the more detailed structure of the multi-nip calenders.

In the multi-nip calender, the fibrous web W is guided from one roll nip to another by take-out rolls linked to the ends of the intermediate rolls so that the fibrous web travels between the roll nips, its surface being simultaneously profiled by the com- binatory effect of heat brought to the fibrous web and the pressure prevailing in the roll nips. Generally, filler and other loose material would detach to the thermo rolls (here intermediate rolls 501 and 504 and 506) of the roll nips from the fibrous web in substantial amounts, and this detached material would have to be removed by means of doctor blades and, in spite of this, this material would roughen and wear the sur- face of the roll. Because of the wear of the surface, the control parametres for cal- endering, such as the amount of water and/or the temperature of the thermo rolls would have to be changed in order to reach the same profiling of the surface of the fibrous web.

In the arrangement of Figure 1, considerably less loose material adheres to the sur- face of the thermo rolls 501 and 504 and 506 from the fibrous web, because the sur- face of the fibrous web has been cleaned by blowing the loose material away before moistening the fibrous web so that even the wear of the rolls is smaller. In this case, the calendering result stays more easily constant without changing the control pa- rametres.

Above, only one embodiment of the arrangement 1 for preventing the wear of thermo rolls according to the invention and of the dust separator 3 used in it has been disclosed, and it is obvious for one skilled in the art that it is possible to realise the invention in many other ways within the inventive idea disclosed in the patent claims. Thus, the multi-nip calender may have more or fewer roll nips. The dust

separating arrangement of the invention can also be used in calenders with a simple roll nip, in which a thermo roll is used as the second roll. The wetting device used for moistening the fibrous web can also be a device of some other type used for moistening the fibrous web, such as a device based on the adding of steam, or a roll application device.