METHOD AND DEVICE IN THE PRESS SECTION OF A PAPER MACHINE FOR DETACHING THE WEB FROM THE FACE OF THE PRESS ROLL

Abstract: Method and device in the press section of a paper machine for detaching the web (W) from the smooth face (10′) of a press roll (10). Within the area of the detaching point (R), a momentary and local heating effect is directed at the web (W) from outside the roll (10). In the method and in connection with the device, as the mantle or outer coating of the smooth-faced (10′) roll (10), a material is used which is magnetically at least to some extent conductive. To the roll mantle, an induction heating effect of such a high frequency (f,f s ) is applied free of contact that the depth of penetration (δ) of the heating effect remains sufficiently low in view of its local and momentary nature. By means of the heating effect, the water present between the web (W) and the roll face (10′) is heated, as a rule vaporized, locally within the area of the detaching point (R) so as to detach the web (W) from the said roll face (10′). The electric frequency of the induction heating is within the range of f = 0.5 to 2 MHz, preferably within the range of f = 0.8 to 1.5 MHz.

Description: The invention concerns a method in the press section of a paper machine for detaching the web from the smooth face of a press roll. The invention further concerns a device making use of the method of the invention in the press section of a paper machine, said press section including a smooth-faced press roll, preferably a central roll, from whose smooth face the web can be detached and thereinafter be passed to the drying section of the paper machine. In a paper machine, so-called closed press sections are commonly used, wherein one press nip or, as a rule, several press nips are formed in connection with the central roll (see US-A-4673462). An example of such a prior-art press section is the press section marketed by the applicant under the trade mark "Sym-Press II", whose smooth-faced central roll, whose diameter is larger than the diameters of the other press rolls, is usually made of rock, as a rule of granite. Being an unhomogeneous natural material of low tensile strength, granite is quite problematic in machine construction. If it is desirable to heat a granite roll, its deformations dependent on temperature are non-linear and difficult to predict. As a press roll material, granite has relatively good properties of detaching of the web, which is at least one of the reasons for its popularity. The detaching properties could, however, be better, in particular with unbleached paper qualities. In the way known in prior art, the web is detached as an open unsupported draw from the face of the said central roll in the press. This open draw is quite critical in view of the operation of the paper machine. In the said open draw, a difference in speed is used which extends the web, which results in certain drawbacks. Moreover, the said open draw forms a problematic point susceptible to breaks in a paper machine. The prior-art technology has not provided efficient means for controlling the detaching of the web from the smooth-faced central roll and the subsequent open draw. The said unfavourable properties of granite have, for their part, also made the control of the detaching and the open draw more difficult. The open draw of the web has become an ever more important problematic point with ever increasing running speeds of paper machines and because, by means of a paper machine, different paper qualities are often manufactured, whose adhesion to the face of the rock roll is different, which results in variations in the detaching tension required for the web. In a Sym-Press II press section, in the second and third press nip, the properties of the face of the central roll must be such that the moist web adheres to the roll face as well as possible. On the other hand, the web should be readily detachable from the roll face for the transfer to the drying section. The meeting of these paradoxical requirements has not been completely successful in all respects by the means known in prior art. It is a general object of the present invention to provide novel means in the detaching of the web from the central roll or equivalent in a press section and in its transfer to the drying section. The object of the present invention is to provide such a method and device for detaching the web from the smooth-faced press roll wherein the web does not have to be extended, i.e. it is not at all necessary to use a so-called draw and difference in speed. In relation to the above, the object of the invention is to provide such a press section in which, if necessary, it is possible to use a fully closed draw when the web is transferred from the central roll or from a corresponding other roll in the press to the drying section, as a rule onto its drying wire. In view of achieving the objectives given above and those that will come out later, the method of the invention is mainly characterized in that in the method, within the area of the detaching point, a momentary and local heating effect is directed at the web from outside the roll, that in the method, as the mantle or outer coating of the said smooth-faced roll, a material is used which is magnetically at least to some extent conductive, that to the said roll mantle, an induction heating effect of such a high frequency is applied free of contact that the depth of penetration of the heating effect remains sufficiently low in view of its local and momentary nature, and that, by means of the said heating effect, the water present between the web and the roll face is heated, preferably vaporized, locally within the area of the detaching point so as to detach the web from the said roll face. On the other hand, the device in accordance with the invention is mainly characterized in that in connection with the said smooth-faced press roll, at the proximity of the transfer point, an electroinductive heating device is provided, whereat the front face of the core of the said heating device forms an air gap at the detaching point with the roll face, which is made of a magnetically conductive, preferably ferromagnetic material, and that the said induction heating device comprises one or several magnetically conductive coil cores as well as one or several electric coils, and that the device further includes high-frequency apparatuses by means of which an electric current of such a high frequency can be passed to the said coil or coils that, in view of the momentary and local nature of the heating effect, a sufficiently low depth of penetration is obtained for the induction heating effect at the roll face. In the present invention, the central roll of the press or any other corresponding smooth-faced roll from which the paper web is supposed to be detached is a substantially metal-mantle roll coated with a metal or metal alloys, a cast-iron roll, or an uncoated metal roll, preferably a roll made of a ferromagnetic material. The invention can be applied advantageously to a press section in which the basic temperature of the smooth-faced central roll is about 70°C and the properties of the roll face are hydrophilic, i.e. such that the web adheres to the face well. In such a case, by means of the induction heating in accordance with the invention, at the detaching point the surface temperature of the roll is raised momentarily and locally to about 110...130°C, preferably to about 120°C. Thereby the water layer present between the web and the roll face is at least partly vaporized, and a thin vapour film is formed, which cannot keep the web in contact with the roll face, i.e. the web is detached from the roll face. Since, according to the invention, the web detaching work takes place by means of vaporization, in order to detach the web it is not at all necessary to extend the web, which again permits a closed draw from the central roll of the press section to the drying section, for example onto its drying wire. The invention is by no means restricted to be used for the detaching of the web from the central roll of closed press sections of paper machines alone, but the invention is suited and intended for the detaching of the web from a smooth-faced roll in a press in general. In the invention the temperature profile of the central roll or of a corresponding smooth face of a roll can be arranged adjustable in the axial direction of the roll. By means of this procedure, it is possible to set the distribution of the detaching tension in the trans verse direction of the web optimally and to prevent a curve formation of the detaching line in the lateral areas of the web, and thereby to prevent breaks of web, which usually start in the lateral areas of the web. The advantages of the invention are manifested with particular emphasis with thin paper qualities, with which, by means of the invention, it is possible to reduce the number of web breaks taking place in open draw to a substantial extent, as compared with prior art. In the following, the physical background of the invention and some of its exemplifying embodiments will be described in detail with reference to the illustrations in the figures in the attached drawings. Figure 1 is a schematical view of a prior-art closed press section provided with an induction device that makes use of the method of the invention. Figure 2 shows the rear end of a press section in which the invention is applied and in which, owing to the invention, the transfer of the web from the press section to the drying section is fully closed. Figure 3 is a schematical illustration of the principle of an induction heating device intended for the application of the invention, seen in the machine direction. Figure 4 shows a second solution of the principle of an induction heating device in a way corresponding to Fig. 3. Figure 5 shows a block diagram illustrating a first exemplifying embodiment of an induction heating device in accordance with the invention. Figure 6 is a graphic illustration of the current of the induction heating coil or coils in resonance as a function of frequency. Figure 7 shows a block diagram illustrating a second exemplifying embodiment of the invention in a way corresponding to Fig. 5. Fig. 1 is a schematical side view of the applicant's "Sym-Press" (TM) press section, wherein a web W detaching system in accordance with the invention is applied. To begin with, as a background of the invention, the prior-art overall construction of the press section shown in Fig. 1 will be described. The paper web W is drained on the forming wire 50 of the paper machine, from which said wire the web W is detached on the downwardly inclined run of the wire 50 between the wire guide rolls 51 and 52 at the detaching point P and transferred within the suction zone 53a of the pick-up roll 53 onto the pick-up felt 55, on whose lower face the web W is transferred into the first dewatering press nip N₁. The first nip N₁ is formed between a press-suction roll 54 and a hollow-faced 57 lower press roll 56. Two felts run through the nip N₁, viz. the lower felt 60 guided by guide rolls 58 and 59, and the pick-up felt 55, which acts as the upper felt in the first nip N₁. After the first nip N₁, the web W follows along with the upper roll 54 by the effect of the suction zone 54a of the press-suction roll 54, moving into the second dewatering press nip N₂, which is formed between the said press-suction roll 54 and the smooth-faced 10′ central roll 10. The diameter D₁ of the central roll is substantially larger than the diameters of the other press rolls 54, 56, 61. This is why there is space for various apparatuses to be fitted around the central roll 10, including the heating apparatus applied in the invention. Within the suction sector 54a of the suction roll 54 there is a steam box 81, which acts upon the outer face of the web W and raises the temperature of the web W and of the water contained therein, thereby lowering the viscosity of the water. Substantially at the opposite side of the central roll 10, relative the second nip N₂, there is a third dewatering press nip N₃, through which the press felt 65 runs, guided by the guide rolls 63 and 64. The rolls of the nip N₃ consist of the central roll 10 and the hollow-faced 62 press roll 61. The adhesion properties of the smooth face 10′ of the central roll 10 are such that, after the second nip N₂, the web follows along with the face 10′ of the central roll 10. On the lower free sector of the central roll 10, there is a doctor 69, which keeps the roll face 10′ clean and detaches the paper web supposed to become broke from the roll face 10′. From the face 10′ of the central roll 10, the web is detached at the detaching point R as an open draw W₀ and transferred onto the drying wire 70, whose loop has been brought to a distance as short as possible from the roll 10 face 10′, being guided by the guide roll 66. After the guide roll 66, there are suction boxes 67 inside the loop of the drying wire 70, which said suction boxes ensure that the web W adheres to the drying wire 70 and passes reliably to the drying section, whose first drying cylinder or a corresponding lead-in cylinder is denoted with the reference numeral 68. Generally speaking, it can be ascertained that the smaller the detaching angle of the web W, i.e. the angle between the tangential plane imagined at the detaching point and the plane of running of the web, the higher is the detaching tension that is required. On the other hand, the detaching tension is determined by the difference of the speeds of the drying wire 70 and of the face 10′ of the central roll 10, i.e. by the so-called draw difference. The steam box 80 shown in Fig. 1 is not necessarily needed, but it may be used either for intensification of the dewatering in the last nip N₃ and/or in order to set the basic level of the temperature of the face 10′ of the central roll 10 suitable, e.g. about 70°C, and optimal in view of the operation of the detaching device 20 in accordance with the invention. In accordance with the invention, within the area of the detaching point R, an induction heating device 20 is used, which is fed with high-frequency electricity, whose frequency f is, in a way coming out in the following, set so high that the depth of penetration at the face 10′ of the roll 10 is very little. In the invention, e.g., a frequency of f = 1 MHz is used, whereat with steel a penetration depth of 0.02 mm is obtained at a web speed of 20 m/s and with an electric capacity of 40 kW per metre of roll length. This means that the temperature of the roll face 10′ rises within the detaching area R only momentarily and locally by about 50°. If the basic temperature of the roll is about 70°C, in respect of the induction heating within the detaching area the temperature rises locally to about 120°C. Thereby the water layer present between the web W and the roll face 10′ is at least partly vaporized, and a thin vapour film is formed which cannot keep the web W in contact with the roll face 10′, but the web W is detached from the roll face and can be passed immediately to the drying section, e.g. onto its drying wire 70. According to Fig. 2, in the detaching of the web from the face 10′ of the central roll 10 a fully closed draw is used so that, guided by the guide roll 72, the drying wire 70 of the drying section is passed onto the face 10′ of the roll 10 before the detaching point R, in connection with which an induction heating device 20 in accordance with the invention is placed. Around the heating device 20, a blow box 90 placed, which is marketed by the applicant under the trade mark "Press Run" and into which air is passed via the pipe 91 in the direction of the arrow L in . The web W is detached from the roll face 10′ by means of the vaporizing effect of the device 20 in accordance with the invention and, by means of the "Press Run" box 80, it is made to adhere to the outer face of the drying wire 70, being passed under the suction effect of the box 90 onto the leading roll 71, whose suction zone 71a keeps the web W on the outer face of the drying wire 70 when the web is at the side of the outside curve. Hereupon the web W moves on the drying wire 70 onto the first drying cylinder 68 and from there further as a closed draw onto the following drying cylinders, which are arranged in a way in itself known. As is known in prior art, a certain detaching tension has been necessary for the web W, which said tension has been produced by means of a difference in speed, i.e. so-called draw difference, between the roll face 10′ and the drying wire 70, which said difference has extended the web W. Owing to the vaporization detaching of the invention, the detaching tension is not needed necessarily, so that it is possible to use a closed draw of the sort shown in Fig. 2, or any other, corresponding closed draw, for example one in which, in the case shown in Fig. 1, the guide roll 66 has been displaced so as to reach contact with the roll face 10′ and to form a slightly loaded transfer nip (not shown) with the said roll face 10′. In the invention, the central roll 10 that is used is, as a rule, a roll with a metallic mantle, preferably a roll of a ferromagnetic material, i.e. of a roll material that is also preferable to rock material both constructively and in view of the operation. The press roll 10 shown in Figures 3 and 4 has a smooth and hard face 10′, and it has a cylindrical mantle, which is made of a suitable ferromagnetic material, which has been chosen in consideration of the strength properties of the roll and of the inductive and electromagnetic local heating in accordance with the invention. The roll 10 is mounted as revolving around its central axis K-K by the intermediate of its ends 11 and axle journals 12. On the axle journals 12 there are bearings, which are fitted in bearing housings. The bearing housings are attached to the supporting frame of the roll, which is placed on a base. In the interior of the roll 10, it is possible to fit crown-variation or crown-adjustment devices in themselves known, for which there is plenty of room owing to the invention, because, in the interior of the roll 10, it is not necessary to use heating apparatuses operating with a liquid medium or other, corresponding heating apparatuses, which said heating apparatuses are, however, not completely excluded as apparatuses that may be used in connection with the present invention. In accordance with the invention, the roll 10 is arranged as inductively and electromagnetically heatable by means of eddy currents so that the temperature of a very thin surface layer of the roll 10 is raised, owing to this heating, to a considerably high level, as a rule to about 110...130°C. In view of accomplishing the inductive local heating, at the proximity of the roll 10, in the same horizontal line with each other in the axial direction of the roll, component cores 20₁, 20₂...20 N of an iron core are arranged. These component cores 20 n form a magnetic-shoe apparatus 20, which further includes an excitation winding 30, or a winding of its own 30₁...30 N for each component core (Fig. 3). The inductive heating is carried out free of contact so that a small air gap V remains between the iron core and the roll 10 face 10′, the magnetic fluxes of the iron core being closed through the said air gap V via the roll 10 mantle, thereby causing a heating effect in it. In Fig. 3 an excitation winding 30₁...30₇ of its own is shown for each component core 20₁...20 N . A second, alternative embodiment of the invention is similar to that shown in Fig. 4, wherein all the component cores 20₁...20 N (N = 16) have a common excitation winding 30, which has two turns, according to Fig. 4. According to Fig. 7, the excitation winding 30 of the iron core 20 has only one turn. According to an alternative embodiment of the invention, each component core is arranged separately displaceable in the radial plane of the roll 10 so as to adjust the magnitude of the active air gap V and, at the same time, the heating capacity. For this purpose, each component core is attached to the frame by means of an articulated joint. The displacing of the component cores can be arranged by means of various mechanisms. As a rule, the said air gaps may vary, e.g., within the range of 1 to 100 mm. In respect of the mechanical devices for the adjustment of the air gaps, whose construction is not described in this connection, reference is made to the applicant's said Finnish Patent Application No. 833589. In respect of the electrotechnical background of the invention, the following is ascertained. When a variable magnetic field is provided in a material that conducts electricity, as is well known, in the material eddy-current and hysteresis losses are produced and the material is heated. The power (P) of the eddy currents depends on the intensity (B) of the magnetic field and on the frequency (f) of variation of the magnetic field as follows: P ≙ B² · f² The variable magnetic field produced on the roll 30 is closed between the front face of the iron core in the apparatus 20 and the air gaps V through the mantle of the roll 10. This magnetic field induces eddy currents in the surface layer of the roll mantle 10, which said eddy currents generate heat owing to the high resistance in the roll mantle 10. The distribution of the eddy currents induced in the mantle 10 in the direction x of the radius of the roll follows the law I x = I₀e -x/δ I x is the current density at the depth x counted from the mantle face 10′, I₀ is current density on the face 10′ of the mantle 10, and δ is depth of penetration. The depth of penetration has been defined as the depth at which the current density has been lowered to 1/e of the current density I₀. For the depth of penetration, the following expression is obtained: wherein ρ is the specific resistance of the material, f is the frequency of the magnetizing current, and µ is the relative permeability of the material. The expression shows that with a higher frequency the depth of penetration is reduced. When steel is heated, both the electric conductivity and the permeability are reduced with a rising temperature. In the invention, as a rule, heating capacities are used which are of the order of 400 kW/m². As is well known, the smaller the air gap V, the larger is the proportion of the electric power passed to the apparatus through the winding 30 which is transferred into the roll mantle 10 to be heated. In accordance with Fig. 7, the electric power feeding the induction coil 30 is taken from a 50 Hz three-phase network (3 × 380 V). By means of a rectifier 33, the AC current is converted to DC current, which is, by means of an inverter in itself known, based on power electronics, converted to either constant-frequency or variable-frequency (f s ) AC current. The adjustment of the positions of the component cores 20₁...20 N in the iron core 20 can be carried out, e.g., by means of the automatic closed regulating systems shown in Figs. 5 and 6. The adjusting motors are stepping motors 29, which receive their control signals S 1-N from the regulating system 42. The regulating system is controlled by a detector device 41, which is, e.g., an apparatus for the measurement of temperature, by means of which the factual values of the surface temperature T₀₁...T 0k of the roll are measured within the detaching area R at several different points in the axial direction K-K of the roll 10. The regulating system 42 includes a set-value unit, by whose means it is possible to set the temperature profile so as to optimize the web W detaching process. The output of the inverter 34 is fed through a matching transformer 35 into an LC resonance circuit in accordance with the invention, whose effect and operation are illustrated by Fig. 6. In a way in itself known, the transformer 35 has a primary circuit 35a, a core 35b, and a secondary circuit 35c. The secondary circuit has n pcs. of taps 45₁...45 n , which can be connected via a change-over switch 36 to the resonance circuit 37, by means of which the power is fed into the induction coil 30. As is well known, the resonance frequency of a RLC circuit connected in series can be calculated from the formula: Fig. 6 shows the dependence of the current I in the circuit 37 on the frequency f s . In resonance, the current I r = U R , wherein R is the resistance of the circuit 37. In Fig. 5 it is assumed that the voltage U is invariable. The efficiency of the transfer of heating capacity is at the optimum when the operation takes place at the resonance frequency f r . Out of several reasons, it is, however, not optimal to operate at the resonance frequency f r and/or simultaneously at both sides of it, but the frequency of operation is chosen within the areas f a1 to f y1 above the resonance frequency f r or correspondingly within the area f a2 to f y2 below the resonance frequency f r . Within the scope of the invention, the said frequency ranges are preferably chosen as follows: f a1 ...f y1 = (1.01...1.15) × f r or f a2 ...f y2 = (0.85...0.99) × f r . According to Fig. 7, in the RLC circuit a series capacitor C s is used. The circuit 37 is tuned with basic tuning so that the transmission ratio of the transformer 35 is chosen by means of the switch 36 so that the resonance frequency f r calculated from the formula (4) becomes positioned correctly in accordance with the principles given above. Fig. 7 shows a parallel capacitor C r by means of broken lines, which said parallel capacitor can be used in stead of, or along with, a series capacitor C s . As is well known, the resonance frequency f r in a parallel resonance circuit, whose induction coil (L) has a resistance R, is calculated as follows: The above equation (5) includes a factor dependent on the resistance R. In view of the objectives of the invention, as a rule, however, a series resonance circuit is preferable, in particular from the point of view of adjustment and control. Within the scope of the invention, the resonance frequency is, as a rule, chosen within the range of f r = 0.5 to 2 MHz. The frequency range f r = 0.8 to 1.2 MHz has been estimated particularly advantageous. In accordance with the main principles of the present invention, in order to keep the efficiency of the power supply high and to eliminate any phenomena of instability, i.e. "risk of runaway", the operating frequency f s is arranged automatically adjusted in accordance with the impedance of the resonance circuit 37 so that the operating frequency f s remains near the resonance frequency f r but, yet, at a safe distance from it, in view of the risk of runaway, i.e. within the areas f y1 ...f a1 or f y2 ...f a2 shown in Fig. 6. The measurement of the impedance of the resonance circuit 37 may be based, e.g., on measurement of the current I passing in the circuit. This mode of measurement is illustrated in Fig. 7 by the block 46, from which the control signal b is controlled to the regulating unit 47, which alters the frequency f s of the frequency converter 34 on the basis of the control signal b. A further mode of measurement of the said impedance, which may be an alternative mode or which may be used in addition to the current measurement, is to pass a control signal c from the block 42, from which information can be obtained on the positions of the component cores 20n, i.e. on the air gaps V, which substantially determine the said impedance by acting upon the inductance L. An alternative mode of adjustment is to pass feedback signals from the stepping motors 29 into the block 47 and further so as to act upon the output frequency f s of the frequency converter 34. Fig. 5 shows an alternative embodiment of the invention, wherein each component core 20 n is provided with an induction coil of its own in accordance with Fig. 3. Into each component core 20 n , a separately adjustable frequency f₁...f N of its own is passed from the frequency converter 34 by means of the feed cable 44₁-44 N . Now, when the air gap V of each component core 20 is adjusted by means of the stepping motors 29, the resonance frequency f r of each individual resonance circuit is changed. The measurement of the impedance of each individual resonance circuit is carried out by means of separate current meters 48₁...48 N , the frequency converter unit 34 or group being controlled by means of the signal series e₁...e N received from the said current meters, which said signal series contain the information, e.g., on the air gaps V of the different component cores. Thereat each frequency f₁...f N can be made optimal in view of the efficiency of the power supply and of the stability of the adjustment in each component core. In view of achieving a sufficiently low depth of penetration, the frequencies f₁...f N are within the said range of 0.5 MHz to 2 MHz. By means of a circuit similar to that shown in Fig. 5, within the scope of the invention, it is also possible to accomplish a different sort of adjustment of the heating capacity, even so that either the component cores 20₁...20 N may be made static, or the adjustment of their air gaps V may be arranged just as an adjustment similar to a basic-setting adjustment and not as an operating adjustment proper. Thereat, by varying each frequency f₁...f N individually, it is possible, on the basis of Fig. 6, to act upon the current I fed into the circuit and, thereby, on the heating capacities of the individual component cores 20 n , and thereby on the temperature profile of the roll 10. If the operation takes place within the areas mentioned above, below or above the resonance frequency f r , by varying the feed frequencies f₁...f N it is possible to affect the current I within the area I y ...I a . The intensity B of the magnetic field (formula (1)) is substantially proportionally dependent on the magnetization current. The mode of adjustment based on variation of frequency may be used either alone for controlling the temperature profile of the roll 10, or it may be used in addition to the air-gap adjustment. In the invention, the temperature of the roll face from which the web W is supposed to be detached is kept, as a rule, within the range of 60°C to 90°C, preferably about 70°C. In accordance with the invention, by means of momentary and local induction heating, the temperature of the roll face is raised from the basic level given above by about 30°C to 70°C, preferably by about 50°C, to a temperature of about 110°C to 130°C, preferably to about 120°C, whereby a vaporization phenomenon takes place momentarily and locally within the detaching area R, and a vapour film is formed between the web W and the roll face 10′ and detaches the web W efficiently. In particular cases, relief may already be obtained for the problems concerned even with roll face temperatures of an order of 95°C. The detaching powers required in the invention, e.g., at a web speed of about 20 m/s are of an order of 30 to 50 kW/m, most appropriately of an order of about 40 kW/m, which latter value means, in the case of steel, a momentary rise in the temperature of the roll face by about 50°C. In the following, the patent claims will be given, whereat the various details of the invention may show variation within the scope of the inventive idea defined in the said claims and differ from the details given above for the sake of example only.

Claims: 1. Method in the press section of a paper machine for detaching the web (W) from the smooth face (10′) of a press roll (10), characterized in that in the method, within the area of the detaching point (R), a momentary and local heating effect is directed at the web (W) from outside the roll (10), that in the method, as the mantle or outer coating of the said smooth-faced (10′) roll (10), a material is used which is magnetically at least to some extent conductive, that to the said roll mantle, an induction heating effect of such a high frequency (f, f s ) is applied free of contact that the depth of penetration (δ) of the heating effect remains sufficiently low in view of its local and momentary nature, and that, by means of the said heating effect, the water present between the web (W) and the roll face (10′) is heated, preferably vaporized, locally within the area of the detaching point (R) so as to detach the web (W) from the said roll face (10′). 2. Method as claimed in claim 1, characterized in that in the method, as the electric frequency of the induction heating, a frequency is used that is within the range of f = 0.5 to 2 MHz, preferably f = 0.8 to 1.5 MHz. 3. Method as claimed in claim 1 or 2, characterized in that in the method the temperature of the roll from which the detaching takes place, preferably the central roll in the press section, is within the range of 60°C to 90°C, preferably about 70°C. 4. Method as claimed in any of the claims 1 to 3, characterized in that in the method such a heating capacity is used that the temperature of the roll surface rises within the detaching area locally and momentarily maximally by about 40°C...60°C, preferably by about 50°C. 5. Method as claimed in claim 4, characterized in that the heating capacity per metre of length of the roll (10) is about P = 30 to 50 kW/m, preferably P = 40 kW/m. 6. Method as claimed in any of the claims 1 to 5, characterized in that the heating effect is arranged adjustable in the axial direction (K-K) of the roll (10). 7. Method as claimed in any of the claims 1 to 6, characterized in that the level or the axial distribution of the induction heating effect is adjusted by means of adjustment of the air gap, adjustment of the frequency, adjustment of the current, adjustment of the voltage, or by means of a combination of the adjustments mentioned above. 8. Method as claimed in any of the claims 1 to 7, characterized in that in the method a closed draw is used for the transfer of the web (W) from the roll (10) in the press from which the web (W) is detached by means of the induction heating in accordance with the invention. 9. Method as claimed in claim 8, characterized in that in the method the drying wire (70) or any other, corresponding transfer fabric is passed to the detaching point (R) and that the web (W) is, after the detaching, made to adhere to the drying wire (70) or to a corresponding transfer fabric by means of a suction effect or equivalent and passed either onto the first drying cylinder (68) or onto a corresponding lead-in cylinder or, over the suction sector (71a) of a suction roll (71), onto the first drying cylinder (68) (Fig. 2). 10. Method as claimed in claim 8, characterized in that in the method the guide roll (66) of the drying wire (70) or of any other, corresponding transfer fabric is arranged to form a transfer nip with the smooth-faced central roll (10) or equivalent of the press section. 11. Device intended for carrying out the method as claimed in any of the claims 1 to 10 for being applied in a press section which includes a smooth-faced press roll (10), preferably a central roll (10), from whose smooth face (10′) the web (W) can be detached and thereinafter be passed to the drying section of the paper machine, preferably onto its drying wire (70) or a corresponding transfer fabric, characterized in that in connection with the said smooth-faced (10′) press roll (10), at the proximity of the transfer point (R), an electroinductive heating device (20) is provided, whereat the front face of the core of the said heating device forms an air gap (V) at the detaching point with the roll face (10′), which is made of a magnetically conductive, preferably ferro-magnetic material, and that the said induction heating device (20) comprises one or several magnetically conductive coil cores as well as one or several electric coils (30), and that the device further includes high-frequency apparatuses by means of which an electric current of such a high frequency can be passed to the said coil or coils that, in view of the momentary and local nature of the heating effect, a sufficiently low depth of penetration is obtained for the induction heating effect at the roll face (10′). 12. Device as claimed in claim 11, characterized in that the inductive heating devices (20) comprise a series of magnetic cores (20₁...20 N ) fitted at the proximity of the roll face (10′) to be heated, each of the said cores having an excitation winding of its own or all of the said cores having a common excitation winding (30), that both the basic level of the heating effect and the distribution of the heating effect in the axial direction (K-K) of the roll (10) are controlled by adjusting the air gap (V) between each core and the roll face (10′) to be heated and/or by adjusting the magnetization current and/or by adjusting the frequency of the magnetization current. 13. Device as claimed in claim 11 or 12, characterized in that in the method, as the supply circuit of electric power, a LC resonance circuit is used, the frequency (f) of the electric power fed into the said circuit having been chosen at a certain safety distance above or below the resonance frequency (f r ) of the said LC resonance circuit.

Inventors: NISKANEN, JUHANI
Applicants: VALMET PAPER MACHINERY INC.
Application year: 1988
Main classification: D21F3
IPC codes: D21F2, D21F3