The paper or paperboard web formed in the wire section is pressed in two points in the line of making paper or paperboard: in the press sec- tion, in which water is removed from the wet web by pressing, and in the calender, in which the surface is finished by applying pressure on the already relatively dry web. Even though the pressing has a different purpose in the press section and in the calender, the web has quite a different dry matter content when it enters these sections, and the tech- nical development of said sections is guided by the phenomena af- fecting the web therein, a common feature for both sections is that they both have a nip in which a given pressure affects the web, the pressure being dependent on the force with which the two moving surfaces forming the nip, normally the shell surfaces of two rotating rolls, are pressed against each other from both sides of the web.

For example in a shoe calender, a nip is formed by combining a roll with a soft surface and a hard roll, which nip extends in the longitudinal direction of the machine, and thus the paper web to be calendered has a long retention time in the calender nip. The shell of the roll with a soft surface is made of a flexible belt, and the roll with a hard surface is a metal roll, which is a heated roll functioning as a thermoroll that brings heat to the web. The shell of the soft roll is loaded from inside the roll against the hard roll by a loading shoe with a concave surface, and as a

result of this, the paper web travelling along the surface of the hard roll is pressed at a given pressure within a long distance in the nip between the surface of the soft shell and the hard roll surface. At the same time, the elastic shell can be compressed in its thickness direction in the area of the nip. The belt forming the shell of the soft roll can be made of a suitable flexible polymer, such as polyurethane, and inside the belt there is a weave that reinforces the same. As a result of the nip struc- ture, it is possible to attain a good bulk and stiffness for the paper or paperboard, as well as a uniform smoothness of the surface. To sum up, the calender is especially well suited for on-line calendering of printable paper or paperboard grades. Said calender type is called a shoe calender, and it is known by the trademark"OptiDwell". One fur- ther embodiment of the shoe calender is described in the international publication WO 99/28551.

The surface of the shoe guiding the shell of the soft roll through the nip is typically concave to form a long nip, and it corresponds approxi- mately to the curvature of the surface of the hard roll.

Long nip presses comprising a shoe element with a concave surface in the press section are disclosed in the Finnish patent 98843 and e. g. in the corresponding US patent 5908536, in the US patents 5084137, 5262011,5639351, and in the international publication WO 99/19562.

In these publications there is a shoe element inside a so-called hose roll, which shoe element is loaded against the inner surface of a flexible roll shell with a given force, and the web is guided together with one or two press felts through a nip formed between the hose roll and a counter roll.

The counter roll of the shoe calender, i. e. the thermoroll is typically heated by means of a heating medium circulation arranged inside the roll, for example oil circulation. In different calender types the heating medium can be replaced with so-called inductive heating of the roll that is based on producing a changing magnetic field in the electrically con- ductive material of the roll. Examples of induction heating of calender

rolls are disclosed for example in the Finnish patent 71375 and in the corresponding US patent 4,614, 565, in the Finnish publication 74825 and in the corresponding US patent 4,384, 514, as well as in the Euro- pean patent 196 264. These publications disclose induction heating by means of induction coils arranged outside the shell of the roll.

A known method in calenders is the placement of induction coils inside the thermoroll as well. Finnish patent application 904081 and the corre- sponding US patent 5074019 disclose induction coils attached inside the shell of an adjustable-crown roll to the side flanges of hydrostatic loading elements, which induction coils are utilized for induction heating of the inner surface of the metal shell in the same roll. Furthermore, the Finnish patent application 20000612 and the corresponding interna- tional publication WO 01/68979 disclose a so called shoe thermoroll equipped with a loading element and a convex glide shoe, in which shoe thermoroll the additional heating of the thin, hose-like metal shell, moving over the glide shoe, in addition to the heat obtained from fric- tion, is implemented by means of an induction heater located close to the inner surface of the shell. In this case the thermoroll is used in a calender in which there is a polymer-coated roll as a counter roll on the other side of the nip.

Besides calenders, induction heating is a known method in the press section. US patent 4948466 discloses the heating of a centre roll in the press by means of an induction heater outside the roll to carry out im- pulse drying of the web.

To attain an optimal calendering and pressing result, the surface tem- perature of the thermoroll may be even 250°C to 300°C. Because the nip is long, even over 250 mm, the power consumed by the paper web is over 200kW/m. In the press section the power consumption with a wet web may even exceed 2000 kW/m.

At present, such power figures are attained only by means of external induction. It is a problem in external induction that the induction heater

must be placed before the nip and high powers must be transferred to the roll within a short distance. Thereafter the point that has been heated within a short period of time is transferred to the nip in which heat transfer to the paper is high. The roll experiences a thermal shock twice during one revolution.

Another problem is that in a long nip the surface temperature of the roll is reduced to a much lower level than is desired, when the surface moves forward in the nip. After the nip the surface temperature is re- stored slowly.

It is a purpose of the invention to present a treatment method by means of which a nip with a good heating efficiency is attained, said nip having extension in the machine direction (long or extended nip). To attain this purpose, the method according to the invention is primarily character- ized in that the induction effect is exerted from inside the shell or a belt loop through the moving element on the counter element, to the area of the counter element which is bounded by the nip and in which the heating of the counter element is effected.

The solution can be applied both in calender nips of a shoe calender, in which heat is conducted to the web, as well as in the nips of a long nip press as impulse drying.

It is a clear advantage of the invention that the thermal power trans- mitted to the paper or paperboard web is transferred directly to the tar- get of use, i. e. to the nip (the surface of the counter element that is bounded by the nip). The fatigue-promoting stress exerted on the roll as a result of temperature differences is significantly reduced. The fall in the temperature of the surface of the thermoroll or the corresponding counter element in the nip can be eliminated at the same time.

According to a preferred embodiment the induction effect is applied to the counter element from inside the shoe element. This can be imple- mented by placing the induction coils inside the shoe element.

By means of the induction effect it is also possible to conduct a profiled heating of the counter element, if several induction coils are arranged in the cross direction of the web.

If the device for treating the web is a calender, the counter element can be a calender roll that is arranged rotatable, or a belt passed over the calender roll that forms a closed loop. If the device for treating the web is the press of a press section, the counter element can be a press roll that is arranged rotatable.

As is well known, the shoe element may contain loading devices, which load the shoe against the counter element on the other side of the nip.

In the following, the invention will be described in more detail with ref- erence to the appended drawings, in which Fig. 1 shows a general side view of the treatment device, Fig. 2 shows the solution according to the invention in a shoe calender, and Fig. 3 shows a solution according to a second embodiment of the invention in a shoe calender.

Fig. 1 shows a treatment device, in which a paper or paperboard web W is guided through a calender nip N formed between two rolls. The first roll 1 is a calender roll equipped with an elastic flexible shell 1a.

The second roll, the counter roll 2, is a roll with a hard surface, against which the shell of the first roll 1 is supported by means of a shoe ele- ment 3 with a concave surface that is stationary with respect to the movement of the shell 1a. The shoe element 3 is loaded against the in- ner surface of the shell 1a by means of loading devices 4. For lubrica- tion, a static oil pocket is formed between the shoe element 3 and the inner surface of the shell 1a. The loading and the lubrication between

the inner surface of the shell 1 a can be arranged by means of known solutions, as is known for example from the OptiDwell shoe calender.

The length of the nip N in the machine direction is for example at least 50 mm. In this area the web W is guided by the nip and positioned against the hot surface of the counter roll 2 under the loading caused by the nip pressure. The invention is especially well suited for such cal- enders in which the nip length is at least 200 mm, advantageously at least 250 mm, because in relatively long nips the thermal power re- quirements that can be met by means of the invention, are higher.

Fig. 2 shows a calender according to the invention in a side-view similar to Fig. 1, but on a larger scale. In the calender the web W to be calen- dered is guided through a nip N formed between two superimposed rotating calender rolls. The lower first roll 1 comprises a shell 1 a made of an elastic material, which rotates around the rotating axis of the calender. The upper counter roll 2 is a heated calender roll with a hard surface, for example a roll equipped with a metal shell, the surface of which is smooth, for example a polished metal surface or a surface equipped with a smooth coating. Inside the rotating shell 1a induction coils 5 are arranged, by means of which an induction effect is exerted through the shell 1a on the electrically conductive material of the sur- face of the counter roll 2 in an area in which the surface of the counter roll 2 is in nip contact with the shell 1 a and in contact with the web W (areas marked with arrows). The induction coils 5, i. e. electromagnetic coils are used for increasing the temperature on the outer surface of the counter roll 2 to a predetermined value by producing eddy currents in the shell of the counter roll 2 by means of induction, said eddy cur- rents heating the shell of the roll. The induction principle is known as such, and it is not described in more detail herein.

To exert the induction effect accurately and efficiently to the desired point, the induction coils 5 are placed inside the shoe element 3 close to the outer surface of the shoe element that guides the shell 1 a travel- ling over the shoe element. Structurally it is possible to place the induc-

tion coils 5 in cavities formed inside the shoe element 3. The attach- ment to the shoe element can also be conducted in such a manner that at least in the direction of the counter element 2, a free space remains between the induction coil 5 and the material of the shoe element 3.

The placement is advantageously conducted in such a manner that the induction coil 5 is not rigidly connected to the material of the shoe ele- ment 3, but between the shoe element 3 and the induction coils 5 it is possible to use such locking by shape that the induction coil 5 is in pre- determined limits free to move in relation to the shoe element 3, espe- cially to eliminate stresses caused by thermal expansion.

Thus, the surface of the counter roll 2 is heated according to the known principle as a result of the eddy currents induced in the roll when it ro- tates and moves by the nip N past the induction coils 5 located inside the shell 1a. The induction coils 5 are directed in such a manner that the flux lines of the magnetic field produced by said induction coils pass through the material of the counter roll 2 in the area of the nip N. Thus, the material of the counter roll 2 is electrically conductive material at least at its surface. The material of the shoe element 3, in turn, is of non-conductive material at least between the induction coils 5 and the outer surface guiding the shell 1a, for example some firm electrically non-conductive polymer material, for example reinforced polymer com- posite. Thus, the shoe element 3 does not heat up under the influence of the induction coils and the induction effect of the magnetic field can be exerted on the counter roll. The belt that forms the shell 1a is made of non-conductive elastic polymer.

In view of the manufacture, the induction coils 5 can be arranged inside the shoe element 3 when the shoe element is molded in its shape. It is, however, possible to assemble the shoe element of several parts, for example parts premolded from polymer material, in such a manner that the induction coils remain inside the shoe element 3. Furthermore, the parts can be prefabricated of different materials in such a manner that in connection with the assembly the non-conductive material remains

between the induction coils and the guiding outer surface of the shoe element.

There are induction coils 5 for example at fixed intervals in different points in the axial direction of the counter roll 2. Thus, it is possible to implement the induction heating of the counter roll 2 over the entire width of the roll (in the cross-machine direction, i. e. in a direction per- pendicular to the travel direction of the web W). Thus, it is possible to use the induction coils 5 for profiling induction heating in which the surface of the counter roll 2 is heated in the nip N by means of each in- duction coil 5 with the desired efficiency that deviates from the effi- ciency of at least one other induction coil 5 that is located in a different point in the axial direction. The profiling is attained by adjusting the electric variable of the operation of each induction coil 5 that affects the heating response in the counter roll 2, such as the power of the alter- nating current supplied to the coil 5.

Fig. 2 shows that there may also be induction coils in different points in the longitudinal direction of the nip N, i. e. in the machine direction (travel direction of the web W). Thus, it is possible to adjust the surface temperature of the counter roll 2 and the heat transferred to the web W in the nip in the machine direction as well, in accordance with the same adjustment principles as in the cross-profiling. The temperature of the nip can be maintained on a sufficient level, irrespective of the heat transfer to the web W as it proceeds in the nip. The shoe element 3 may contain two or several transverse rows of induction coils 5 succes- sively in the machine direction, which makes cross-profiling and ad- justment of the heating efficiency in the machine direction possible.

The flexible tubular shell can be fixed at its ends to the rotating end structures of the roll by means of known techniques. The shoe element 3 is supported by means of loading devices 4 to an axial static element arranged inside the roll shell, said static element being brought through the ends of the roll and supported by carriers outside the roll. The roll shell is clearly wider than the shoe element, so that it can have a circu-

lar shape in its fixing point to the ends and can be formed within the width of the nip line to a shape defined by the shoe element, which shape possibly deviates from the circular shape. The thickness of the shell 1a is generally between 1 to 20 mm, advantageously between 3 to 8 mm.

The shell 1 a of the first roll 1 can be a belt, which is compressible in its thickness direction, and the belt can thus be made of e. g. reinforced elastic polymer, e. g. fibre-reinforced polyurethane. Such a belt can have a lower dynamic coefficient of elasticity than conventional polymer coatings, of the order of below 0.5 GPa in the temperature range of 25 to 125 °C. It is also possible to use a belt disclosed in the international publication WO 99/28551, the material of which belt has a compressi- ble volume, for example as a result of polymer with a cellular structure.

Although hereinabove the endless element travelling over the shoe element 3 and bounding the nip N on the other side of the counter roll 2, is a tubular or hose-like shell 1 a of a roll, it can be a belt instead of a roll shell, said belt forming an endless belt loop whose shape depends on the positions of the elements that guide the belt from inside, such as guiding rolls.

Fig. 3 shows an alternative in which a counter element on which the nip N borders, is a belt 6 passed over the calender roll 2. The belt forms a closed loop. The belt travels through the nip N at web speed and the rotating calender roll 2 guides its travel in the nip. The belt 6 is made of electrically conductive material, in which eddy currents are produced by means of the induction by the induction coils 5, said eddy currents heating up the belt. Instead of the surface of the calender roll 2, the hot belt 6 is positioned against the web W to be calendered within the length of the nip N. The belt 6 is flexible at least in such a degree that it can be taken in a loop, under the guidance of guiding rolls or the like around the calender roll 2. The belt also has a smooth surface, it is in- compressible and advantageously made of metal, for example a steel belt. The surface material of the calender roll 2 does not have to be

material that heats up under the influence of induction, and thus the effect of induction can be restricted to the belt 6.

In addition to a calender, the invention can also be applied in the press section in impulse drying. The nip N of Fig. 2 is thus a nip of a so-called extended-nip press, and the shape and dimensions of the same can be similar to those described above, and thus the roll functioning as a counter element is a press roll.

Hereinabove, the invention is not restricted solely to the embodiment described in the drawings, but it can be modified within the scope of the inventive idea presented by the claims. Several separate induction coils 5 placed in the cross-machine direction can be replaced with such an induction heater that does not contain a profiling possibility. Such an in- duction heater is composed of a longitudinal conductor loop that ex- tends in the transverse direction, by means of which conductor loop it is also possible to produce a magnetic field. Such induction heaters may be positioned successively in the machine direction, or there may be at least one transverse row of separate induction coils 5 and a continuous crosswise extending induction heater successively in the machine di- rection in a suitable order. By means of this combination it is also pos- sible to implement profiling induction heating in the transverse direction and adjustment of the heating efficiency in the machine direction.

Inside the shoe element 3 there are conductors required by induction heaters for supply of current and a medium circulation, the purpose of which is to cool the induction coils 5 or the uniform induction heater.

The cooling medium can be for example oil.

The counter roll that is in contact with the web either as a press roll or a calender roll can be for example the solution disclosed in the US patent 4948466, in which the surface layer of the roll is made of electrically conductive material that can be heated up by means of induction, and underneath the same there is an insulating layer that is made of non- conductive thermally insulating material that restricts the effect of in-

duction to the surface layer. The surface layer may be for example made of electrically conductive ceramic material that is discussed in more detail in the patent in question.

The method according to the invention can be used for the treatment of all continuous paper or paperboard webs, in which the webs are pressed with the aim of producing paper or paperboard which fulfils, after possible aftertreatment steps, certain quality requirements and can be wound to a roll. Such paper in the form of wound rolls can later be printed or it can be used for other purposes, depending on the paper grade. By means of the invention it is possible to treat all paper and paperboard grades made of pulp and existing in the form of a continuous web, which undergo a production stage where it is possible to remove water from the web by pressing the web by means of the method according to the invention, or the surface of the web can be finished by calendering it by means of the method according to the in- vention.