Heated rolls are disclosed for example in the publications EP 333688, US 4,658,486 and US 4,920,623. The heating of the roll is usually con- ducted by circulating hot steam or liquid into the roll and out of the same. The publication EP 0967324 and the corresponding publication DE 19828156 disclose an arrangement where a nip is formed between a deflection compensated roll and a heated belt loop.

Rolls which can be heated electromagnetically are disclosed for exam- ple in the publication FI 73260 and in the corresponding publication US 4,675,487 as well as in US 5,895,598 and in the corresponding publi- cation DE 197 43 724. The publication FI 73260 discloses a roll which is heated by means of electromagnetic induction heating in such a manner that a magnetic flux is applied on the shell of the roll by means of a magnetic shoe device installed outside the shell, and such a high frequency is used as the frequency of the magnetization current that a sufficiently small penetration depth of the heating effect is attained.

The publication US 5,895,598 discloses a roll which is heated by electromagnetic induction heating in such a manner that excitation coils are installed inside the roll shell. The roll shell comprises channels containing a heated medium, for example water.

The drawback of the known solutions is the fact that the equipment necessary for circulating the heated medium is expensive. The proc- esses often require an arrangement in which the thermoroll is heated while the deflection compensated roll functioning as a counter roll for the same is cooled down, wherein energy is consumed both in heating and in cooling. In an open belt loop it is extremely difficult to keep the oil inside the belt loop, and it easily fumes outside the belt loop.

Furthermore, due to the thickness of the shell in the rolls, the rotating mass is large, which causes vibration problems. Known heating solu- tions that apply induction heaters are expensive, and the induction heaters which are placed outside the roll have a poor operating effi- ciency. When induction heaters placed inside the roll are used, the di- ameter of the roll has to be relatively small.

The thermoroll according to the invention provides an improvement in view of the state of art. The thermoroll according to the invention is characterized in that it comprises a glide shoe inside a thin, flexible shell, which shoe is arranged to touch said shell. The nip according to the invention is characterized in that it is formed between the flexible shell of the thermoroll pressed by the front surface of the glide shoe and a soft covered roll. In this application the term front surface of the glide shoe refers to the surface of the glide shoe which is in contact with the shell of the thermoroll.

The thermoroll according to the invention has several advantages: -basic heat of the roll is attained from friction work, -power dissipation is utilized extremely well, -an expensive machine unit and pipework for circulating the liquid are not necessary in the solution, -the service life of the shell of the thermoroll is long, because no fa- tigue stress is exerted thereon, -heat is conducted easily through the shell to the paper, and no sig- nificant thermal stresses are produced in the shell, -high operating efficiency of the induction heater can be attained, and the liquid, such as lubricant oil inside the roll can be used for cooling down the heater, -the deflection compensation of the roll pair forming the nip can be conducted by means of the thermoroll, and thus the second roll can be a cheap roll which is not deflection compensated.

-a spare roll is not required, but only a changeable shell part is nec- essary, and -the vibration problems are considerably reduced, because the rotating mass is reduced significantly.

The thermoroll according to the invention is used in the treatment of paper, especially in the calender. The invention can be used both in on- line and off-line processes. Typically, a calender comprises super- imposed rolls, between which nips are formed, and paper travels through the nips. The calender may also contain only one pair of rolls, or two or more pairs of rolls, wherein the nip/nips are formed between the pair of rolls/pairs of rolls. The purpose of the calendering is for in- stance to affect the thickness of paper and reduce variations in the thickness and to improve the surface properties, such as smoothness and gloss.

The invention comprises a shoe roll provided with heating. The thermo- roll according to the invention has a thin, hose-like metal shell. The thickness of the shell is 15 mm at the most, generally 10 mm at the most, advantageously between 1.0 to 1.5 mm. The shell is substantially non-compressible. The shell is arranged to rotate around a stationary shaft. Due to the fact that the shell is thin, the mass rotating around the shaft is small.

A glide shoe is arranged on the shaft of the roll, said glide shoe keeping the metal shell in the correct form in the nip. The front surface of the glide shoe is convex, wherein fatigue stress is not exerted on the shell of the roll, which is a problem in rolls with a concave glide shoe.

To attain a suitable nip pressure, the glide shoe can be loaded by means of loading elements located between the shaft and the glide shoe. The liquid medium inside the roll, such as lubricant oil, reduces the friction between the glide shoe and the rotating metal shell. How- ever, between said surfaces there is always a certain amount of friction, and the friction work turns into heat which can be utilized in the solution according to the invention to maintain a basic heating in the roll.

The liquid medium inside the roll, such as lubricant oil, can be special heat resistant oil. The amount of liquid inside the roll constitutes at least 25 % of the volume inside the roll, but the space remaining inside

the roll may also be entirely filled with liquid. However, the amount of liquid has to be so large that the lubrication between the shell of the roll and the glide shoe is attained. The heat produced by the friction work between the roll shell and the glide shoe heats up the lubricating liquid, which is not circulated to be heated outside the roll, but the heat of the liquid is conducted through the thin metal shell to be utilized in the heating of the outer surface of the roll. The generated heat can be fully utilized, and by means of a suitable auxiliary heating method, e. g. by induction heating or electric resistors it is possible to cover the need for additional heat. Thus, the required efficiency in the heater, such as an induction heater or electric resistor, is small, because the basic heat is produced in another manner.

It is possible that liquid is circulated outside the roll for other purposes than heating, for example for cleaning, such as filtering.

When an induction heater is installed in the shaft of the roll as an aux- iliary heater, it heats up the ferromagnetic, thin metal shell. The induc- tion heater is placed close to the nip formed by the thermoroll and its counter roll, wherein heat losses are small. The heat is easily trans- ferred through the thin metal shell to the paper web travelling via the nip. The induction heater comprises an electromagnetic coil/coils, which are cooled down by means of liquid inside the roll, wherein the liquid is heated up and this heat can also be utilized.

By means of induction heaters it is also possible to conduct zonal ad- justment of the temperature in the longitudinal direction of the roll, wherein separate induction heaters may be located over the entire length of the roll or on a part of the length of the roll. The temperature can be adjusted so that it varies in different longitudinal locations in the roll by adjusting the heating efficiency of individual induction heaters.

The heating capability of the induction heater is based on electro- magnetic eddy-current losses and hysteresis losses. When a variable magnetic field is arranged in an electrically conductive material, eddy- current losses and hysteresis losses are generated in the material, and

the material heats up. The efficiency of the eddy currents depends on the strength of the magnetic field and on the frequency of the magnetic field according to the following formula P-B2 f2, in which P = efficiency of the eddy currents B = strength of the magnetic field f = frequency of the magnetic field The magnetic flux lines of the magnetic field form a closed curve. The ferromagnetic substance, the most well known examples of which are iron, cobalt, nickel and alloys of the same, attracts the flux lines. Thus, the flux lines travel via the thin metal shell of the thermoroll. The fre- quency of the magnetization current is selected in accordance with the thickness of the shell in such a manner that the thinner the shell, the higher the frequency of the magnetization current.

In the following, the invention will be described in more detail with ref- erence to the appended drawing, in which Fig. 1 shows a pair of rolls formed by a shoe roll provided with heating and a polymer roll.

According to Fig. 1, the thermoroll 1 comprises a shell 2, a shaft 3, a loading element 4, a glide shoe 5 and an induction heater 6. The sub- stantially cylindrical shell 2 of the thermoroll 1 is made of metal, for ex- ample steel, and the thickness of the wall of the shell is 1 to 15 mm, typically 1 to 10 mm, advantageously 1.0 to 1.5 mm. The outer surface of the shell 2 may be coated with an abrasion resistant coating, for ex- ample with allenite. The shell 2 is made of ferromagnetic material.

Inside the shell 2 there is a shaft 3, a loading element 4, a glide shoe and an induction heater 6. The shaft 3 has, for example, a cross-sec- tion with the shape of an I-beam and extends over the entire length of the roll shell 2. Against the inner surface 2a of the roll shell there is a glide shoe 5 which is convex on the side of the inner surface of the shell and extends continuously on the entire length of the roll shell 2.

The convex shape of the glide shoe 5 ensures that fatigue stress is not exerted on the shell 2, and therefore the shell 2 is usable for a long pe- riod of time. The glide shoe 5 is a substantially continuous, solid piece.

Between one side 3a of the shaft 3 and the glide shoe 5 there is a loading element 4 by means of which the glide shoe 5 is loaded against the inner surface 2a of the shell 2. The loading element 4, several of which have been placed on the width of the roll, can be a known force- producing device, for example a hydraulic cylinder. The cross-section of the convex surface 5a of the glide shoe 5 has substantially the same shape as the part of the periphery of the circle that has the same di- ameter as the shell 2.

Inside the shell 2 of the thermoroll 1 there is lubricant oil (not shown in the drawing), which lubricates the contact surface between the convex front surface 5a of the glide shoe 5 and the rotating shell 2. The con- tact between the convex front surface 5a and the rotating shell 2 gen- erates friction work, which turns into heat, thereby heating the lubricant oil. The heat of the lubricant oil is easily conducted through the thin metal shell 2, thereby heating the outer surface of the thermoroll 1, wherein heat can be utilized in the calendering.

The heat produced by the friction work is usually not sufficient as such for heating the thermoroll, but additional heating is required, for exam- ple induction heaters or electric resistors. In Fig. 1 the additional heat- ing is arranged by means of an induction heater 6 which is located close to the inner surface 2a of the shell 2, advantageously slightly be- fore the glide shoe 5 in the direction of rotation of the shell, wherein the section of the shell 2 that presses the paper web W only produces small heat losses before it enters in contact with the paper web W.

The induction heater 6 comprises cores arranged inside the shell 2 on the other side of the shoe roll 1 provided with heating 1 and in the same horizontal line, said cores being enclosed by an excitation winding. The core is usually made of iron. Each core may have a winding of its own, or several cores may share one winding. However, the core inside the excitation winding is not necessary, but the induction heater 6 can be

formed of mere excitation windings. The core affects the operating effi- ciency of the induction heater, but it also increases costs.

The section containing one winding can be adjusted in a desirable manner and thus it is possible to implement for example the zonal ad- justment of the surface temperature of the thermoroll e. g. to profile the gloss of paper.

The core and the winding form an electromagnetic coil, which is ar- ranged to generate a magnetic field. The flux lines of the magnetic field form a closed curve, and they travel via the section of the ferromagnetic metal shell 2, heating the same by heating up of the material due to the eddy-current losses and hysteresis losses produced in the shell 2. To control the axial temperature profile of the roll, a magnetization current whose intensity and/or frequency can be adjusted is conveyed to the core of the coil. A small gap remains between the induction heater 6 and the inner surface 2a of the shell 2, and the flux lines of the mag- netic field that form a closed curve travel via said gap. The lubricant oil inside the roll 1 cools down the electromagnetic coil, wherein the lubri- cant oil heats up, and heat is conducted via the thin shell 2 of the roll 1 to be utilized in the processing of paper.

The shell 2 is arranged to rotate around a stationary shaft 3. Thus, there is a bearing assembly or a corresponding arrangement between the shell 2 and the shaft 3, and the ends of the shaft 3 are connected with a suitable stationary fastening to the supporting structure support- ing the shoe roll 1 provided with heating. Said shoe roll 1 has actuators in one or both of its ends, and force is transmitted via the end flanges of the roll 1. The actuators can also be located inside the roll.

According to Fig. 1, a preferred embodiment of the invention is such that a roll 7 which is not deflection compensated forms a nip N with the shoe roll 1 provided with heating, said roll 7 containing for example a cast iron shell 7a and a polymer cover 7b outside the same. Thus, the deflection compensation of the pair of rolls forming the nip N is ar- ranged in the shoe roll 1 provided with heating, wherein the roll pair

forming the nip N is a cheap solution. The deflection compensation can be implemented in such a manner that the loading elements 4 are used to load the glide shoe 5 with forces that can be adjusted separately.

The above-described facts do not restrict the invention, but the inven- tion may vary within the scope of the claims. It is possible that the shell of the thermoroll has another shape than substantially the shape of a cylinder. It can be for example a belt rotating around two rolls, the edges of which belt are closed by means of a suitable arrangement.

The induction heater may also be located outside the shell of the thermoroll. The glide shoe may contain points of discontinuity. In addi- tion to the induction heater, there may be other heaters inside the roll, for example electric resistors. The liquid medium inside the roll may be any liquid that is suitable for the purpose. The counter roll of the thermoroll may also be another shoe roll, the shell of which is made of polymer material, for example polyurethane, for example a shoe roll which is known from the OptiDwelIT" calender. In addition to paper, the invention can also be utilized for treatment of different paperboard grades.