METHOD AND DEVICE IN THE PRESS SECTION OF A PAPER MACHINE

Abstract: Method and device in the press section of a paper machine, in particular in a so-called closed press section provided with a smooth-faced press roll (10), for the control of the detaching of the paper web from the said press roll (10). In the method and by means of the device, the temperature of the face (10′) of the said smooth-faced press roll (10) is adjusted. By means of this adjusting, the adhesion between the roll face (10′) and the paper web (W) to be detached is affected, and thereby the detaching angle (ϑ) and/or the detaching tension (T) of the paper web is set within an optimal range. The temperature profile of the smooth-faced press roll (10) in the axial direction of the roll (10) can also be adjusted in view of controlling the detaching process.

Description: The present invention concerns a method in the press section of a paper machine, in particular in a so-called closed press section provided with a smooth-faced press roll, for the control of the detaching of the paper web from the said press roll. The invention further concerns a device in the press section of a paper machine, which said press section includes a smooth-faced press roll, preferably a central roll, from whose smooth face the web is detached and passed preferably as an open draw 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. 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 (See US-A-4673462). 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 adhesion, transfer and 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 open draw of a web taking place from a smooth-faced central roll. The said unfavourable properties of granite have, for their part, made the control of the said 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. It is a general object of the present invention to provide novel means in the detaching of the web from the central roll in a press section and in its transfer to the drying section. It is a particular object of the invention to provide a novel regulating system in which the detaching of the paper web from the smooth face of the central roll in a press section can be controlled better than in prior art. It is a particular object of the invention to provide such a system of regulating the detaching of the web, of the sort mentioned above, in which the tension of detaching of the web can be set optimally irrespective of the dry solids content of the paper web, of the surface energy of the substance, and of the running speed of the paper machine. 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 the temperature of the face of the said smooth-faced press roll is adjusted, and that by means of the said adjusting, the adhesion between the said roll face and the paper web to be detached is affected and, thereby, the detaching angle and/or detaching tension of the paper web is set within an optimal range. 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 heating devices are provided, by means of which the temperature of the smooth face of the said press roll and thereby the detaching of the web from the said roll face are affected. The present invention is based on the observation that the temperature at the interface between the paper web and the smooth roll face from which the web is to be detached affects the dry solids content of the web, the surface energies of the materials in contact, and the viscosity of water, which parameters again affect the adhesion between the paper web and the water contained therein, on one hand, and the smooth roll face, on the other hand. By establishing the interdependences of the said paramters, by controlling them, and by, on the basis of this information, adjusting the temperature of the roll face by means of the regulating system of the invention, it is possible to set the detaching tension of the paper web to a suitable level even within highly varying operating conditions. Thus, in the invention, when running different paper qualities and with different running speeds of the paper machine, it is possible to adjust the temperature of the smooth face of the roll to a certain set value, which provides an optimal detaching of the web and running quality with the web quality and machine speed that are used at each particular time. 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, with a ceramic material, or with mixtures of same, a cast-iron roll, or an uncoated metal roll, which is arranged to be heated by means of adjustable heating devices. The heating may take place from inside and/or outside the roll at least partly by means of techniques known in prior art. 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 controlling the detaching of the web from a smooth-faced roll in a press in general, i.e. also from a roll other than a central roll. The regulating system of the invention may be provided with a feedback, wherein the conduct of the web in the detaching draw is monitored either visually, by means of optical detectors, or by means of detectors that sense the location. In the said feedback, it is possible to use,as the measurement signal or adjustment signal, the difference in speed of the web between the drying group and the press, or a separate measurement roll by means of which the web tension can be measured. In a preferred embodiment of the invention, the temperature profile of the smooth face of the central roll or equivalent is 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 transverse 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 present 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 the said open draw to a substantial extent. In some cases, owing to the invention, it is possible to shorten the said open draw, or even to introduce a practically closed draw from the press section to the drying section. 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 devices making use of the method of the invention. Figure 2 shows the rear end of the press section and the geometry of the open draw of the web as well as various parameters of same. Figure 3 illustrates the dependence of the viscosity and surface tension of water on temperature. Figure 4 is a graphic presentation of the test results with the invention, i.e. the detaching angle of the web from a smooth-faced roll as a function of the web temperature with newsprint. Figure 5 is a schematical illustration of the principle of the induction heating apparatus suitable for an application of the present invention, seen in the machine direction. Figure 6 shows an exemplifying embodiment of an induction heating apparatus in accordance with the invention as a block diagram. Fig. 1 is a schematical side view of the applicant's "Sym-Press II" (™) press section wherein a control 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 suction roll 51 and the wire drive roll 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 10 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 apparatuses 20, 80, 100 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. In the following, with reference to Fig. 2, the detaching of the web W from the smooth face 10′ of the central roll 10 and its transfer as an open draw W₁ or W₂ onto the drying wire 70 will be described. In Fig. 2, the detaching angle of the draw W₁ is denoted with ϑ₁ and the corresponding detaching point with R₁. The detaching angle of the second draw W₂ is denoted with ϑ₂ and the detaching point with R₂. The detaching orders of the open draws W₁ and W₂ are denoted with T₁ and T₂. Generally speaking, it can be ascertained that the smaller the detaching angle ϑ, the higher is the detaching tension T that is required. On the other hand, the detaching tension T is determined by the difference between the speed v₂ of the drying wire 70 and the speed v₁ of the face 10′ of the central roll 10, i.e. by the so-called difference in draw Δv = v₂ - v₁ (v₁ = web speed in the press section before detaching, v₂ = web speed at the beginning of the drying section). As a rule, Δv/v₁ is within the range of Δv/v₁ = 1%...3%. The web tension, i.e. the detaching tension T can be calculated as follows: wherein From the above formulae (1) and (2), the following circumstances essential in view of the invention come out. When the temperature at the contact point between the web W and the roll face 10′ rises, the detaching tension T becomes lower, because the viscosity µ is reduced, and the surface energy σ is also reduced, which, for its part, results therein that the detaching work W S (formula (2)) is reduced and the dry solids content is increased (owing to the last-mentioned fact, the term mv² becomes lower). Fig. 3 shows the dependence of the viscosity and surface tension of water on temperature. As is seen, the surface tension is lowered in a substantially linear way as the temperature is raised, whereas the viscosity is lowered within the temperature range of 0°C to 80°C very steeply, and thereinafter substantially in the same proportion as the surface tension is lowered with a rising temperature. From Fig. 3, and so also from the above equations (1) and (2), it can be concluded that with a rising temperature of the roll face 10′ the web tension T required to detach the web W from the roll face 10′ is lowered, i.e. the web W is detached from the roll face 10′ more readily at higher temperatures. As was stated above, a reduced web tension T results in an increased detaching angle ϑ. 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 preferable to rock material both constructively and in view of the operation. In the present invention, active use has been made of the, usually inverse, interdependence between the web tension T and the roll-face 10′ temperature, which was described above. For this purpose, as is shown in Fig. 1, a steam box 80 is fitted in connection with the face 10′ of the central roll 10 between the nips N₂ and N₃, the temperature of the web W and the temperature and viscosity of the water contained in the web as well as, indirectly, also the temperature of the face 10′ of the roll 10 being affected by means of steam S in passed into the said steam box 80. As is shown in Fig. 1, before the detaching point R, inductive heating apparatuses 20 are placed substantially in the horizontal plane placed through the centre of rotation of the central roll 10, which said heating apparatuses act, free of contact, through an air gap V, substantially upon the temperature of the thin surface layer of the web face 10′. As is shown in Fig. 1, a heating medium F in is fed into the roll 10 through a pipe 91 and a connection 90, the said medium being taken out of the roll (F out ) through the same connection 90, or through another connection (not shown) placed in connection with the opposite roll shaft, and a pipe 92. The apparatuses for the circulation and heating of the heating medium are denoted schematically with the block 100 in Fig. 1. Even though, in Fig. 1, three different sets of equipment 20; 80 and 100 are shown for the heating of the face 10′ of the central roll 10 and of the web W in view of controlling the draw tension T, T₁, T₂ of the open draw W₀, W₁, W₂, i.e. the steam box 80, the inductive heating apparatuses 20, and the devices 90, 91, 92 and 100 for the heating and circulation of the heating medium, in practice, as a rule, all of these means do not have to be used at the same time in one application. As came out preliminarily above, when the method of the invention is being applied, a granite roll or any other rock roll is not used as the central roll in the press or as any other, corresponding smooth-faced roll, but a metal-mantle roll coated with a metal, a ceramic substance, or with a mixture of same, a castiron roll, or an uncoated metal roll, such a roll being constructively preferable to a rock roll of natural material. The face 10′ of a metal roll 10 or equivalent can be heated to a temperature optimal in view of the invention without uncontrolled phenomena of alteration. In connection with the invention, as the central roll, it is favourably possible to use such a synthetic press roll as is described in the applicant's Finnish Patent Application No. 853544 or No. 854748, corresponding to the documents US-A-4748736 and US-A-4796342, according to which specifications the basic level of the surface energy of the roll can be chosen appropriate from the point of view of the invention, in consideration of the adhesion between the web W and the roll face 10′ and of the detaching process. In stead of, or in addition to, the steam box 80 described above, it is possible to use a radiation heater, e.g. an infrared heater, whose construction is in itself known and an exemplifying embodiment of whose construction comes out, e.g., from the applicant's Finnish Patent Application No. 861086, corresponding to US-A-4854052, true enough, applied in connection with an airborne web dryer. In addition to the heating taking place by means of a heating medium circulating (F in -F out ) in the roll 10, shown in Fig. 1, it is possible to use electric heating apparatuses, such as resistance heating or inductive heating. An example of heating apparatuses fitted inside the roll and suitable for use in connection with the present invention is described in the applicant's Finnish Patent No. 69,151, whereat the distribution of the temperature in the axial direction of the roll 10 can also be controlled by means of the apparatuses described in the said patent. As condensed it can be ascertained that for the heating of the central roll before the last nip N₃ it is possible to use either a steam box 80, infrared heating and/or inductive heating. For internal heating of the central roll 10, it is possible to use either a circulating medium, such as steam or water, and/or electric heating, such as inductive heating or resistive heating. Within the area of the detaching point R of the web W, as adjustable heating apparatuses of the roll 10, it is possible to use either infrared heating and/or an inductive heating apparatus 20. Fig. 4 shows the effect of the steam box 80 on the detaching process. In the graphic presentation in Fig. 4, the vertical axis represents the detaching angle ϑ of the open draw as degrees, and the horizontal axis represents the web temperature. The curve in Fig. 4 was obtained with the applicant's test paper machine and with newsprint when the web speed was 20 m/s. The measurement points in Fig. 4 were obtained with the web W speed of 20 m/s so that, e.g., the difference in speed Δv was kept constant and only the web temperature was varied. From Fig. 1 it can be seen that the detaching angle ϑ becomes larger when the web W temperature rises. In the following, with reference to Figures 5 and 6, the inductive heating apparatuses 20 will be described, which are, according to the present estimate, the most advantageous embodiments of the invention both in view of efficiency and in view of a possibility to adjust the transverse profile of the control and heating effect. The smooth-faced 10′ press roll 10 shown in Fig. 5 is the roll from which the web W is detached. The roll 10 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 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 excluded as apparatuses that may be used in connection with the present invention. The roll 10 is arranged as inductively and electromagnetically heatable by means of eddy currents so that the temperature of the face 10′ of the roll 10 is raised by means of this heating to a considerably high level, as a rule to about 70°C to 100°C. In view of accomplishing the inductive 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 for each component core (not shown). 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. According to Figures 5 and 6, all the component cores 20₁...20 N (N = 16) have a common excitation winding 30, wherein there are two turns in Fig. 5, and only one turn in Fig. 6. Each component core 20 n 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 20 n can be arranged by means of various mechanisms. As a rule, the said air gaps may vary, e.g., within the range of about 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 Finnish Patent Application No. 833589 corresponding to the documents US-A-4675487 and US-A-4775773. 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: The variable magnetic field produced on the roll 30 is closed between the front face of the iron core 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 currants induced in the mantle 10 in the direction x of the radius of the roll follows the law I x is the current density at the depth x counted from the mantle face 10′, I o 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 I o . For the depth of penetration, the following expression is obtained: wherein ρ = specific resistance of the material, f = frequency of the magnetizing current, and µ = 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 1 to 30 kW/m. As is well known, the smaller the air gap V, the larger is the proportion of the electric power that is passed to the apparatus through the winding 30 which is transferred into the roll mantle 10 to be heated. In accordance with Fig. 6, 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 system shown in Fig. 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 temperatures T o1 ...T ok of the roll are measured at several different points in the axial direction K-K of the roll 10. If the regulating system 42 includes a set-value unit, by its means it is possible to set the temperature profile in the axial direction K-K of the roll 10 so that an optimal detaching of the web W is obtained. The output of the inverter 34 is fed through a matching transformer 35 into a LC resonance circuit. In a way in itself known, the transformer 35 has a primary circuit 35a, an iron 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: In resonance, the current wherein R is the resistance of the circuit 37. The efficiency of the transfer of heating capacity, is at the optimum when the operation takes place at the resonance frequency f r . It has, however, been noticed that, out of several reasons, it is 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 yi = (1.01...1.15) × f r or f a2 ...f y2 = (0.85...0.99) × f r . According to Fig. 6, 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 (6) becomes positioned correctly in accordance with the principles given above. Fig. 6 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 (7) includes a factor dependent on the resistance R. As a rule, however, a series resonance circuit is preferable, in particular in view of adjustment and control. Within the scope of the invention, the resonace frequency is, as a rule, chosen within the range of f r = 2...35 kHz. Depending on the dimensioning of the coil cores 20 and on the air gap V between the roll 10 and the cores 20 n , the inductance of the resonance circuit is, e.g. with a roll 10 of a length of 8 m, of an order of 10...250 µH. For example, if L = 60 µH and f r = 20 kHz, the value of the capacitance of the capacitor becomes C s = 1.06 µF. 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 impendance 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. 6 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. The mode of adjustment based on change in frequency, described above, can be used either alone in adjusting the temperature profile of the roll 10, or in addition to and together with air-gap adjustment to improve the accuracy and/or rapidity of adjustment. In some cases, by using the above mode of adjustment based on change in frequency, it is possible to omit mechanical regulating devices acting upon the air gap V completely. In this way it is possible to increase the rapidity of the regulating system and, in some cases, to improve the accuracy of adjustment, even though, in such a case, it may be necessary to be content with a somewhat lower efficiency of the power supply. 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, in particular in a so-called closed press section provided with a smooth-faced press roll (10), for the control of the detaching of the paper web from the said press roll (10), characterized in that in the method the temperature of the face (10′) of the said smooth-faced press roll (10) is adjusted, and that by means of the said adjusting, the adhesion between the said roll face (10′) and the paper web (W) to be detached is affected and, thereby, the detaching angle (ϑ) and/or detaching tension (T) of the paper web is set within an optimal range. 2. Method as claimed in claim 1, characterized in that the temperature profile of the said smooth-faced press roll (10) in the axial direction of the roll (10) is adjusted for the purpose of controlling the detaching process. 3. Method as claimed in claim 1 or 2, characterized in that in the method an outside contact-free inductive heating of the smooth face (10′) of the press roll (10) is used (Figs. 5 and 6). 4. Method as claimed in claim 1, 2 or 3, characterized in that in the method the temperature of the roll face is adjusted within the range of 30°C...150°C, preferably within the range of 50°...100°C. 5. Method as claimed in any of the claims 1 to 4, characterized in that in the method a heating medium, such as water or steam, fed and circulated inside the roll mantle (F in -F out ) is used for heating the smooth-faced press roll (10). 6. Method as claimed in any of the claims 1 to 5, characterized in that in the method a heating outside the roll mantle is used, such as a steam box (80) and/or infrared-radiator heating. 7. Method as claimed in any of the claims 1 to 6, characterized in that the method is applied to the central roll (10) of such a closed press section of a paper machine as comprises a first two-felt nip (N₁) and a second nip (N₂), which is formed between a press roll, preferably a press-suction roll (54), and the said central roll (10), and in which said press section there is preferably a third press nip (N₃), which is formed in connection with the said central roll (10) before the web (W) detaching point (R). 8. Device in the press section of a paper machine, which said press section includes a smooth-faced press roll, preferably a central roll (10), from whose smooth face (10′) the web (W) is detached and passed preferably as an open draw (W₀, W₁, W₂) to the drying section of the paper machine, characterized in that in connection with the said smooth-faced press roll (10) heating devices (20, 80, 100) are provided, by means of which the temperature of the smooth face (10′) of the said press roll (10) and thereby the detaching of the web (W) from the said roll face (10′) are affected. 9. Device as claimed in claim 8, characterized in that the device includes inductive heating apparatuses (20), which are placed preferably slightly before the web (W) detaching point (R). 10. Device as claimed in claim 9, characterized in that the inductive heating apparatuses 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 their own or all of the said cores having a common excitation winding (30), and that 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, both the basic level of the heating effect and the distribution of the heating effect in the axial direction (K-K) of the roll are adjusted. 11. Device as claimed in any of the claims 8 to 10, characterized in that the device includes means (90, 91, 82, 100) for the heating and circulation of a heating medium, by means of which the heating medium is circulated inside the mantle of the smooth-faced (10′) press roll (10). 12. Device as claimed in any of the claims 8 to 11, characterized in that the device includes a steam box (80) and/or infrared radiation apparatuses fitted in connection with the smooth-faced press roll (10), by means of which said box and/or apparatuses the temperature of the press-roll face (10′) and possibly also the temperature of the web (W) are affected.

Inventors: ILMARINEN, ANTTI, LAAPOTTI, JORMA, MIIHKINEN, VEIJO
Applicants: VALMET PAPER MACHINERY INC.
Application year: 1988
Main classification: D21F3
IPC codes: D21F2, D21F3