The main purpose of calendering is to bring the surface properties and/or thickness of paper to a desired level. In the calendering treat- ment a continuous paper web is guided via nips formed by rolls. In cal- enders, polymer-coated rolls form a soft, long nip with their counter roll, in which the local surface pressure exerted on the paper web is smaller than in the corresponding hard nip. In addition to the rolls that form nips, multi-nip calenders also contain take-out rolls by means of which the paper web can be guided between the nips, and its travel path can be stabilized. Between the nips and take-out rolls it is possible to sub- ject the web to additional treatment to attain effects that are advanta- geous in view of the calendering.

A special multi-nip calender is a supercalender, which comprises alter- nating hard and soft rolls. Soft rolls enable great loading, which is nec- essary to attain smooth paper without damages. It is possible to use polymer-coated rolls as soft, resilient rolls.

In multi-nip calenders problems are caused by the uneven temperature distribution of the polymer rolls. As an example it can be mentioned that when the normal operating temperature of the roll is 50 to 70°, 80 to 100° at the most, the temperature difference between the central and edge areas in the metal shell of the polymer roll can be 10 to 20°.

There are several reasons for the uneven temperature distribution, for example strong cooling of the roll via the ends, heating caused by the bearing temperatures of the take-out rolls and irregularities in the tem- perature caused by variations in the moisture content of paper. Local tensions, for example stresses caused by irregularities in the thickness of the paper web, cause local heating in the roll, and thus, the polymer surface of the roll can be damaged. When the polymer-coated roll

forms a nip with the hot thermo roll and when the ends of the polymer- coated roll are not protected in the area remaining outside the paper web, radiation and direct contact with the thermo roll result in high tem- peratures and great loading. Loading is increased by thermal expansion of the polymer coating which is usually stronger than that of metal. In treatment of paper such an uneven temperature distribution impairs the profiles of paper.

Known solutions for adjusting the temperature of a polymer roll include for instance water circulation arranged inside the roll, filling the roll with water, sealing, heating or cooling of the end areas of the roll and placement of bearings that generate heat in the take-out rolls in differ- ent locations. The heating of the end areas of a polymer-coated roll is disclosed for instance in the publication US 5,932,069 and cooling of the same in the publication US 5,289,766.

There are several known solutions for circulating water in order to equalize the surface temperature of a polymer roll. Liquid, usually wa- ter, can be conducted to a channel that extends via the centre of the roll in parallel to the longitudinal axis of the roll, and it can be conducted away from the roll via the opposite end of the roll.

Another known solution is arranged in such a manner that liquid is con- ducted to the ring-like area formed between the shell and shaft of the roll from the other end of the roll. The liquid can be conveyed out of the roll from the opposite end of the roll, or the flow of the liquid may be ar- ranged in such a manner that it returns via the central channel of the shaft to the same end of the shaft from which it was conveyed inside the roll.

The equalization of the temperature of the roll may also be arranged by means of channels in which water is circulated, said channels being ar- ranged at fixed intervals on the perimeter of the shell of the roll, in par- allel to the axial direction of the roll. In addition to the channels in the edge areas the roll also has a channel extending via the centre of the roll in the direction of the longitudinal axis of the roll.

As for the state of art, it can be mentioned that the publication DE 19743724 discloses an induction-heated hard-faced roll, whose tem- perature is equalized by utilizing a two-phase liquid existing in the channels extending in the axial direction of the roll. The channels are closed in such a manner that a negative pressure prevails in therein.

Induction heaters are placed inside the shell of the roll to heat the shell of the roll.

The polymer-coated roll according to the invention provides an im- provement for the act of maintaining an even temperature in a polymer- coated roll. The polymer-coated roll according to the invention is char- acterized in that its temperature is arranged to remain constant by means of a phase change in the medium located inside the roll.

The calender roll according to the invention is unheated as such, and the purpose of equalizing its temperature is to equalize the effect of heat transmitted from outside primarily via the polymer surface, in the axial direction of the roll.

The calender roll according to the invention utilizes a so-called heat pipe phenomenon. The heat pipe phenomenon is known as such. The heat pipe phenomenon functions in such a manner that the liquid me- dium evaporates at a point in which the temperature is sufficient for causing its evaporation, i. e. the temperature of that point is at least slightly under the boiling point of liquid. The liquid can change into va- pour either by steaming off or evaporating from the surface. The evapo- ration of liquid causes a small increase in pressure. Due to the increase in pressure, the evaporated liquid moves to an area where evaporation does not occur, in other words, the temperature of the area is substan- tially lower than the evaporating point of the liquid. The liquid evapo- rated in this area condenses, thereby releasing the latent heat stored therein, i. e. when the vapour condenses into liquid, it releases the amount of heat stored therein.

By utilizing the heat pipe phenomenon it is possible to implement the system according to the invention for equalizing the temperature of a polymer-coated calender roll in such a manner that channels are bored in the metal shell of the polymer roll in parallel to the shaft of the roll, and liquid, for example water or suitable alcohol is placed in the chan- nels. Suitable liquid media are for example ethanol, the evaporating point of which in normal air pressure is 78°C and water, the evaporating point of which in normal air pressure is 100°C, or mixtures of these, the evaporating points of which vary between 78°C and 100°C.

The amount of liquid in the channel is approximately 10 to 20% of the volume of the channel. The pressure prevailing in the channels can be adjusted for example by means of a pressure controller to be placed in the pipe shaft of the roll, in such a manner that a suitable evaporating point is attained for the liquid in the channel. Negative pressure can be sucked in the channels until a vacuum is attained and the ends of the channels are closed in an air-tight manner. In a pressure lower than the normal air pressure the boiling point of liquid drops to a lower level, wherein the evaporating point is in a lower temperature. Due to the heat pipe phenomenon, heat is efficiently transferred from a warmer section of the roll shell to a cooler one.

In the following, the invention will be described with reference to the appended drawings, in which an embodiment of the invention is de- picted in such a manner that Fig. 1 a shows the profile of a polymer-coated calender roll, and Fig. 1 b shows the cross-section of a polymer-coated calender roll.

In the profile of a polymer-coated calender roll 1 shown in Fig. 1a, an end flange 2 can be seen which is fixed to the end of the roll by means of bolts or the like. The roll has a polymer coating 3 underneath of which there is a metal shell 4. The metal shell 4 is provided with chan- nels 5 located at fixed intervals on the periphery of the roll shell, in par- allel to the longitudinal direction of the shell, to which channels the liq-

uid equalizing the temperature of the roll is placed. The liquid evapo- rates at that point of the channel 5 in which the temperature is sufficient for evaporation. The evaporated liquid causes a slight pressure in- crease, due to which vapour can travel in the channel 5. When the va- pour reaches the point whose temperature is lower than the evaporat- ing point, it condenses into liquid, releasing the latent heat stored therein.

The polymer-coated metal shell 4 can be seen in cross-section A-A of the polymer-coated calender roll 1, shown in Fig. 1b. The channels 5 are located relatively close to the surface of the metal shell 4, so that they could efficiently influence the equalization of the temperature in the roll 1.

The above description does not restrict the invention, but the invention may vary within the scope of the claims. The number of the channels in the metal shell is not restricted, and it is possible that the orientation of the channels forms an angle with the axial direction of the roll. The polymer surface of the roll can be composed of several layers, and it is possible that the surface layer of its outer surface is made of a sub- stantially harder material than the lower layers of the polymer coating.

The main aspect in this invention is that the temperature of a polymer- coated roll can be equalized by utilizing latent heat set and released in phase change.