Typically, calenders comprise superimposed rolls, between which a nip is formed. A calender can comprise one or more roll pairs forming a nip. The surface of the rolls may be hard or soft. The rolls may be unheated or heated thermorolls. The medium used for heating the rolls may be fluid or steam, it can for example be water, aqueous vapour or oil. The selection of the medium depends, for instance, on the target surface temperature of the thermoroll. The web travels via the nips formed by the roll pairs, and by means of this process for instance the variations in the thickness of the web are reduced, the surface becomes smoother and the web becomes thinner.

The heating of known thermorolls is typically arranged by circulating the heated medium inside the roll. The earliest solutions comprised a channel extending via the central point of the roll in the direction of the longitudinal axis of the roll. Hot oil or the like was conveyed to the channel via the other end of the roll, and it was conveyed away from the roll via the opposite end of the roll.

One known heating of the thermoroll is arranged in such a manner that heating medium is conveyed on the ring-like area formed between the shell and the axle of the roll from the other end of the roll. The medium can be conveyed out of the roll from the opposite end of the roll, or the flow of the medium may be arranged in such a manner that the medium returns via the central channel of the axle to the same end of the axle from which it was conveyed inside the roll.

The heating of the roll may be arranged by means of channels which are arranged at fixed intervals on the perimeter of the shell of the roll, said channels being parallel to the axial direction of the roll. The channels are usually located within a distance of 20 to 60 mm from the surface of the roll, and their diameter is normally 25 to 50 mm. The

number of the channels varies between 15 and 50, depending on the purpose of use. In addition to the channels in the edge areas the roll also has a channel extending via the central point of the roll in the direction of the longitudinal axis of the roll.

There is also a known solution in which a conical displacing member is placed inside the channel, the purpose of which is to increase the flow rate of the heating medium in the channel, thus balancing the surface temperature of the roll.

The travel of the heating medium can be arranged in different ways in the channels. In mono-pass type rolls, the medium is passed inside from the other end of the roll and out from the opposite end. In duo- pass type rolls the medium, usually a fluid, is conveyed in the channel inside the roll from the other end of the roll, and it returns to the same end via a second channel connected to the first channel. In tri-pass type rolls the medium is first passed to one end of the roll, and therefrom via a second channel to the other end and via a third channel to the end opposite to the end from which it was passed inside the roll.

In tri-pass II type rolls, the liquid medium is connected to a third channel, wherein the flow rate of the fluid is increased. The fluid returns to the same end from which it was conveyed inside the roll.

Thermorolls are disclosed for example in the publication EP 333688, US 4,658,486 and US 4,920,623.

The drawbacks of known thermorolls include the uneven surface temperature as well as the uneven quality of the surface of the roll, due to the uneven heat distribution. The temperature of the surface of the roll is uneven both in the machine direction and in the transverse direction. The unevenness of the surface temperature in the transverse direction is due to the falling of the temperature of the heating medium when it circulates in the channels. Because in all known solutions the channels are parallel to the axial direction of the thermoroll, they are also parallel to the contact point of two rolls, i. e. the nip line formed between the thermoroll and its backing roll. The surface of the thermoroll is warmer in that point in which the heating medium travels in

a channel directly underneath the surface, and therefore the heat expansion on the surface of the roll varies locally. Thus, the shape of the roll becomes angular, which, when occurring at intervals, causes vibration and noise. Especially when machine speeds become higher and the use of multinip calenders more common, nip vibration has become a significant problem.

For example in Tri-pass II type rolls a variation of 2 ßm has been observed in the machine direction and in the transverse direction, which variation, however, is sufficient for triggering the nip vibration. Due to the angularity of the roll, deformations occur in the polymer coatings and the paper is marked. The medium travelling inside the roll also cools off the further it flows from the source of the warm medium. In known solutions, the flow rate of the medium should be increased in view of the surface temperature, but then for instance the size of the medium switch poses a problem. Thus, in known solutions the aim is to find a compromise between the flow rate of the medium and the evenness of the surface temperature.

By means of the thermoroll according to the invention it is possible to reduce the above-mentioned problems. The thermoroll according to the invention is characterized in that the thermoroll comprises channels which form an angle with a straight line extending via the central point of the roll in the longitudinal direction of the roll.

The thermoroll according to the invention has a more even surface temperature than solutions of prior art. The channels therein, intended for conveying a heating medium, are not parallel to the nip line, wherein the parallel deviations in the shape of the roll do not occur simultaneously on the nip line.

The evenness of the surface temperature requires that the channels conveying the heating medium in the thermoroll are distributed close to the surface of the roll in a suitable manner. As for the surface temperature, changes also occur in the axial direction of the roll, and thus, to make the temperature even on the entire area of the surface, it has to be taken into account that the medium emits heat into the roll,

and is thus cooled off when it travels forward in the channels.

Generally, channels are produced in the roll in such a manner that they are bored from both ends of the roll, and the borings meet in the middle of the roll. Thus, the channels have a constant diameter and they extend along a straight line at least in that section which is bored from the same end of the roll.

In the thermoroll according to the invention, the problem of the cooling of the roll has been solved by boring the channels diagonally in such a manner that the end of the channel from which the heating medium is supplied in, is further apart from the surface of the roll than the other end of the channel, in other words the distance between the channel and the surface becomes smaller in the direction of flow of the heating medium. The distance of single channels from the surface of the roll within the same longitudinal point of the roll can vary, so that an optimal temperature distribution is attained.

The channels conveying in the heating medium are located relatively close to the surface of the roll to attain a good heat transfer from the roll to the paper. The heat expansion at the location of the channels causes irregularities in the shape of the surface of the roll. If the channels are bored in such a manner that they form an angle with the nip line formed between two rolls, the irregularities in the shape are more evenly distributed in the nip. Thus, several channels constantly extend through the nip line, forming an angle with the nip line. Advantageously, the angle is 1° to 5°. The channels can also be bored so that they form an arrow pattern, wherein the subchannels bored from both ends of the roll meet each other in the angle. When the channels form an angle with respect to the nip line, the distance of single channels from the surface of the roll varies slightly due to the round shape of the surface of the roll.

In the thermoroll, it is also possible to use a solution whose aim is to solve the problems related to the even surface temperature and the heat expansion. The channels are bored on the shell of the roll in such a manner that they form an angle with the nip line, and the distance of a single channel from the surface of the roll varies depending on the point

of location of the channel in the longitudinal direction of the roll. Thus, the irregularities in shape caused by the channels enter the nip line in an angle, and at the same time the distance of the channels from the surface of the roll is reduced in the direction of flow of the heating medium (with the exception of the variation in the distance due to the round shape of the surface of the aforementioned roll).

The flow of the medium travelling in the channels can be arranged in a similar manner as disclosed in the section of the application describing the state of the art. For example channels whose distance from the surface of the roll is reduced in the direction of the flow of the heating medium, can be connected to channels of similar kind from their other end. When the heating medium is supplied into the channels which come closer to the surface of the roll in the direction of flow of the medium, they can be connected from their other end to channels which also come closer to the surface of the roll in the direction of flow of the medium. Thus, the channels that are connected to each other are positioned crosswise inside the roll and the cooled medium can be removed from the roll from the same end of the roll from which it was supplied in.

When an arrow pattern channel arrangement is used, it is also possible that both sides of the arrow form an angle with the nip line and the distance of single channel halves from the surface of the roll is reduced in the direction of flow of the heating medium.

In the following, the invention will be described by means of the appended drawings, in which, Figs. 1 a, 2a, 3a and 4a show a part of the roll in a side-view, and Figs. 1 b, 1 c, 2b, 2c, 3b, 3c, 4b, 4c, 4d show cross-sections of the roll.

In the drawings, all details are not shown in order to maintain the illustrative nature of the drawings. For the same reason, the drawings only show eight channels, even though the number of them is typically larger. Figs 1 a to 1c show an embodiment of the invention in which the

distance of a single channel 1 from the surface of a roll 2 varies. Fig. 1 a shows a part of the roll 2 in a side view and illustrates the fact that the channels 1 are located conically inside the shell of the roll 2, i. e. the channels 1 form an angle with a straight line L extending via the central point of the roll in the axial direction of the roll. A single channel 1 extends in a straight line inside the roll 2 in such a manner that the section of the channel 1 via which the hot heating fluid, typically oil, enters the channel, is located further away from the surface of the roll than the section of the channel located further away from the fluid source, in which the heating fluid is colder. Thus, the surface temperature of the roll 2 remains constant quite well, because the colder fluid flows closer to the surface than the warmer fluid. Fig. 1b shows a cross-section A-A, which is located closer to the end from which the hot heating fluid is arranged to flow inside the channel 1.

Thus, the channels 1 are located further away from the surface of the roll than in the cross-section B-B shown in Fig. 1c, which is close to the opposite end of the roll, and thus the channels 1 are close to the surface of the roll.

Figs. 2a to 2c show an embodiment of the invention. When the thermoroll 2 and its backing roll (not shown in the drawing) form a nip through which the paper web or the like travels in the calendering process, a nip line extending over the rolls is formed in the contact point of the rolls, said nip line being linear when hard-surface rolls are used, and covering a wider area when at least one of the rolls forming the nip has a soft surface. The channel 1 shown in Figs. 2a to 2c is produced in such a manner that it forms an angle with the straight line L extending through the central point of the roll 2, and with the nip line formed between two rolls. In Fig. 2a, part of the roll 2 is shown in a side-view, the channels 1 diagonal with respect to the nip line appearing therein. Because the channel 1 is diagonally oriented with respect to the nip line, the irregularities due to the heat expansion are distributed evenly on the nip line, and thus no sequential variation of the pits and the peaks does not occur, which results in vibration causing deformations on the polymer coatings and markings on the paper web.

Fig. 2b shows a cross-section C-C, and Fig. 2c a cross-section D-D, in which it can be seen that the channels travel diagonally inside the roll

with respect to the nip line and the straight line L extending via the central point of the roll.

Figs 3a to 3c show an embodiment of the invention in which the channels 1 form an angle with the nip line and the distance of single channel 1 from the surface of the roll 2 is reduced in the direction of flow of the heating medium. Fig. 3a shows a part of the roll in a side view. Figs 3b and 3c show cross-sections E-E and F-F, in which it can be seen that the distance of single channels 1 from the surface of the roll varies, and they form an angle with the nip line. The cross-section E-E lies close to the point where the heating medium is supplied into the channels 1.

Figs. 4a to 4d show an embodiment of the invention, in which the channel 1 forms an arrow pattern. The advantage of the arrow pattern is that the vibration of the roll 2 and the noise resulting therefrom are reduced considerably, it is easier to make the channels meet in the middle of the roll, and the error in the distance of the channel due to the round shape of the roll is insignificant. Both halves of the arrow pattern form an angle with the nip line. Fig. 4a shows a part of the roll 2 in a side view. Fig. 4b shows a cross-section G-G, Fig 4c a cross-section H- H and Fig. 4d a cross-section I-I. The cross-sectional drawings illustrate what kind of an angle the arrow pattern forms with the nip line at a given time.

The above-described facts do not restrict the embodiments of the invention, but the embodiments may vary within the scope of the claims. New embodiments can be produced by combining the above- described embodiments. The heating medium and the travel directions of the same in the channels as well as the act of combining the channels together can vary according to the need. In addition to heating, the channels described in the application can also be used in conveying a medium intended for cooling for example in polymer coated rolls. In that case, however, the distance of the channels from the surface is arranged in a manner suitable for the cooling. Some of the channels in the roll may be parallel to the axial direction of the roll.

In addition to calenders, the thermoroll according to the invention can

also be used in the press section of a paper machine or the like. The main aspect of the invention is that the surface temperature of the thermoroll is even, and that the irregularities in the shape of the surface of the roll are distributed evenly on the nip line in such a manner that they cause as small a number of problems as possible in the treatment of a paper web or the like.