Method and device in the press treatment of a paper web

Abstract: Method and device in a machine for the manufacture of paper or board for heating the outer face (10′) of such a cylinder or roll (10) as is in direct contact with the web (W) to be pressed against said roll face. The cylinder or roll face (10′) is heated from outside inductively by using a magnetic field, by means of which a heating effect based on eddy currents is produced in the outer layer (3) of the roll. As the cylinder or roll face (10′), a relatively thin outer layer (3) of an electrically conductive ceramic material is used, in which the resistive heating effect is concentrated. The depth of penetration of the heating effect in the radial direction of the roll to be heated is restricted to a sufficiently low depth by means of the choice of the thickness of the ceramic outer layer (3) and/or of the electric frequency (f) of the induction heating. Besides on the basis of its electric properties, the ceramic material of the outer layer (3) is chosen so that the cylinder or roll face (10′) is given the necessary properties of strength, also in view of the thermal shock of the heating effect.

Description: The invention concerns a method in a machine for the manufacture of paper or board for heating the outer face of such a cylinder or roll as is in direct contact with the web to be pressed against said roll face, which said web is treated, such as dewatered or calendered, by means of the method. The invention further concerns a device for the press treatment of a paper web intended for carrying out the method in accordance with the invention, said device comprising a cylinder or roll whose outer face can be heated and in connection with which one or several roll nips and/or so-called extended nips are formed. As is known in prior art, water can be removed from a paper web by means of pressing so that the dry solids content of the web is ka ≈ 40...45 %. The rest of the water has had to be removed by evaporation, which consumes essentially more energy per unit of mass than is consumed by dewatering by pressing. In a way known in prior art, attempts have been made to intensify the dewatering of the paper web by raising the temperature of the web passing to the pressing and of the water contained in said web, e.g., by means of steam boxes and by thereby lowering the viscosity of the water and changing the elastic properties of the web so that the dewatering in normal roll nips or in so-called extended nips is intensified. By these means, it has not always been possible to obtain a sufficient increase in the dry solids content, but a significant proportion even of the so-called free water in the web, which is not immobilized on the fibre material by hydrogen bonds, has had to be removed out of the web by evaporation. 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. Being an inhomogeneous 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 said central roll in the press. This open draw is quite critical in view of the operation of the paper machine. In said open draw, a difference in speed is used which extends the web, which results in certain drawbacks. Moreover, said open draw forms a problematic point susceptible to breaks in a paper machine. With increasing production rates of paper machines, the dewatering performed as nip pressing has become a bottle-neck that limits the increasing of the running speeds. This comes from the fact that the press nips formed by a pair of rolls have a short area, so that with high speeds the time of stay of the web in these press nips remains short. Especially owing to the flow resistance of the fibre structure of the web, the water, however, requires a certain time in order to be removed from the web into the hollow face of a roll or into the press fabric. If attempts are made to increase the dewatering capacity in nip presses by increasing the nip pressure, with a certain linear load the limit is reached at which an increased nip pressure is no longer helpful, for the structure of the web no longer endures the compression. In prior art, so-called hot-pressing methods are also known, in whose respect, by way of example, reference is made to the US Patent No. 4,324,613, according to which the paper web is pressed in a roll nip in which one of the rolls or cylinders has been heated by means of surface heating to a temperature higher than 100°C. In said nip, the surface water in the paper web can be vaporized, and the pressurized vapour blows water, which has been pressed into the intermediate spaces in the fibre structure in the paper, into the press felt. The dry solids content achieved by means of this prior-art hot-pressing method is quite good, but a problem consists of the short nip time in a high-speed machine, because the compression time in a roll nip is only about 1...3 ms, whereby the vaporization has not time to be started properly, unless the roll temperature is very high (of an order of 500°C). The high temperature of the roll results in problems in particular in respect of the strength of the press fabric and of the roll. The press treatment and the press device in accordance with the method of the present invention can be applied both to dewatering-pressing of a paper or board web and to calendering of a web, and in particular to so-called gradient calendering. Reference is made, by way of example, to the applicant's US Patents Nos. 4,614,565, 4,631,794 and 4,653,395. US-A-4,631,794 forms the basis of the preambles of the independent claims. In respect of the applicant's recent inventions connected with the press treatment of web and closely related to the present invention, reference is made, by way of example, to the FI Patent Applications Nos. 871870, 870309 and 874136. The present invention is also closely related to inductive heating of a paper web and of a press roll, reference being made, in respect of earlier inventions of the applicant connected with said applications, to the FI Patent Applications Nos. 870308 and 870309. As comes out from the above, it is known in prior art to heat the face of a press roll or cylinder so as to provide so-called hot pressing or impulse drying or to provide detaching of the web. An advantageous prior-art mode of heating a press roll or a calender roll is external inductive heating free of contact, in connection with which, as a rule, ferromagnetic roll coatings have been used, even though ferromagnetism is not a necessity in inductive heating, but what is necessary is expressly electric conductivity of the roll face in view of the heating eddy currents. It can be ascertained that a number of different properties are required from a roll or cylinder face to be heated, and the providing of said properties in one and the same roll face has not been solved in a satisfactory way in prior art. A problem has been how to provide a press-roll coating and equipment for heating of same by means of which it is possible to heat the roll face instantaneously, e. g., to about 350°C and/or to provide a sufficiently high thermal flow from the roll to the web to be heated and, at the same time, to obtain satisfactory properties of wear, thermal shock, and web detaching for the roll face. A further problem is how to permit crown-variation of a press roll in the same connection. Induction heaters operating at a high frequency (e.g. 25 kHz) are highly expensive to construct because of the power-transistor technology required by them. Impulse drying requires a power transfer capacity of about 0.5 MW/m as well as regulation of the temperature profile from a heat roll. Faces hotter than what is necessary in view of the process ought to be avoided, even in view of the fire risk alone. A common prior-art mode of solving the above problems is to pass hot gas into the interior of a press roll or calender roll. In such a case, the temperature of the inner face of the roll must be about 700°C, whereby the rigidity of the roll material is no longer sufficient, and crown variation cannot be effected by means of the present-day technology. If a press roll is heated from outside by means of infrared equipment or by means of combustion gases, that would require very high temperatures, which cause high heat losses and an obvious risk of fire. In a way known in prior art, induction heating can also be applied directly to the face of an ordinary press roll made of a ferromagnetic material, but in such a case a higher frequency, i.e. technology of higher cost, is required. Above, when a press roll has been spoken of, what is meant is both a roll of a wet press (including a roll used in impulse drying) and a calender roll and any other, corresponding roll in a paper finishing device. A general object of the present invention is to create new solutions for the problems discussed above. A particular object of the invention is to provide a method and a roll device which are applicable either to impulse drying or to dewatering-pressing taking place in conventional roll nips and/or extended nips, to detaching of the web from the central roll, and to calendering of a paper web. In view of achieving the objects stated above and those that will come out later, the method of the invention is characterized by the features appearing in the characterizing clauses of claims 1 and 9. When the invention is applied to impulse drying, owing to the invention the thermal energy can be advantageously applied to the relatively thin outer mantle face, made of an electrically conductive ceramic material, of the heating cylinder or roll. By means of a setting of the penetration depth, based on the choice of the thickness of the ceramic face and of an underlying insulation layer, if any, and/or of the frequency of the inductive heating, the heating depth can be made such that the thermal energy passed to the induction apparatus has time to be transferred into the web in the nips. Owing to the invention, heat losses and risks of fire can also be made considerably lower than, e.g., by means of infrared heating. The electrically conductive ceramic material used as the roll coating in the invention can be chosen so that it endures the thermal shocks and the mechanical wear and that by its means a suitable detaching of the web from the roll face can be obtained, which are particularly important properties in particular when the invention is applied in dewatering presses from whose central roll the web is detached as an open draw. The advantages of the method and the device of the invention are likely to be utilized best in connection with impulse drying, whereby, in induction heating, owing to the invention it is possible to use a relatively low frequency, e.g. about 300...1000 Hz. An advantageous device in accordance with the invention heats the face of a press roll, by means of electromagnetic induction, to about 350°C, the heat being transferred by said face to the web. The heating depth is determined as equal to the thickness of the layer of the electrically conductive ceramic material applied in the invention, in which case the heating frequency can be chosen more freely. The frequency is, however, preferably higher than 500 Hz in order that a uniform heating result should be obtained in the direction of rotation of the roll. In such a case, in the coating structure of the press roll, there is an electrically insulating layer below the electrically conductive ceramic outer mantle. Said electrically insulating layer is also thermally insulating, which permits a sufficiently low flow of heat into the roll, whereby it is possible to use variable-crown roll construction. The regulation of the transverse property profile of the web can be accomplished by regulating the distance of the ferrite cores that control the magnetic field from the roll (air-gap regulation). In the invention, as the outer coating of a press roll or cylinder, such an electrically conductive ceramic material is used whose specific resistance is lower than 10⁻² Ωm, preferably within the range of 2...8 Ωcm x 10⁻⁵. The wear properties of the ceramic material used in the invention are considerably better than those of structural metals, which is true in the case of ceramics in general. The resistance of a ceramic face to thermal shocks is also considerably better than that of metals. According to the invention, it is possible to achieve an induction heating arrangement which has a good efficiency, and the embodiments of equipment can be made of relatively small size and even in other respects advantageous. Another advantageous form of application of the invention is the so-called gradient calendering, in whose respect reference is made to the applicant's FI Patent 71,374 (corresponding US Pat. 4,653,395). In the following, the invention will be described in detail with reference to some exemplifying embodiments of the invention illustrated in the figures in the drawing, the invention being by no means strictly confined to the details of said embodiments. Figure 1 is a schematical illustration of a prior-art closed press section, which includes an induction apparatus making use of the method of the present invention as well as a central roll of the press. Figure 1A is an enlarged cross-sectional view of a press roll provided with a coating in accordance with the invention and of a magnetic-shoe device operating in its connection. Figure 2 illustrates the induction heating and a hot cylinder in accordance with the invention as applied to an impulse drying device. Figure 2A illustrates the distribution of the press load in a device in accordance with Fig. 2. Figure 3 is a schematical illustration of the principle of an induction heating device intended for application of the invention as seen in the machine direction. Figure 4 illustrates a second solution of principle of an induction heating device in a way corresponding to Fig. 3. Figure 5 is a block diagram illustration of a first exemplifying embodiment of an induction heating device. Figure 6 is a graphic illustration of the current in an induction heating coil or coils in resonance as a function of frequency. Figure 7 is a block diagram illustration, corresponding to Fig. 5, of a second exemplifying embodiment of an induction heating device. Figure 8 illustrates the application of the method and of the device of the invention in a gradient calender. Figure 9 is a vertical sectional view of a magnetic shoe device in accordance with the invention and of a mechanical regulation device used in connection with same, by means of which said regulation device the air gaps of the magnetic shoes, and thereby the heating capacity, can be controlled in the axial direction of the roll. Fig. 1 is a schematical side view of a "Sym-Press II" (TM) press section of the applicant, wherein the method and a press roll 10 in accordance with the invention are applied. To begin with, as a background of the invention, the prior-art general 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 the web W is detached on the downwardly inclined run of the wire 50 between the guide rolls 51 and 52 at the detaching point P, being transferred on 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 the press-suction roll 54 and the hollow-faced 57 lower press roll 56. Two felts pass through the nip N₁, i.e. the lower felt 60 guided by the 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, by the effect of the suction zone 54a of the press-suction roll 54, the upper roll 54 and runs into the second dewatering press nip N₂, which is formed between said press-suction roll 54 and the smooth-faced 10′ central roll 10 in accordance with the present invention. 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 enough for various devices to be fitted around the central roll 10, including the inductive heating device 100 in accordance with the invention. On the suction sector 54a of the suction roll 54 there is a steam box 71, which acts upon the outer face of the web W and raises the temperature of the web W and of the water contained therein, while 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, being guided by guide rolls 63 and 64. The rolls of this nip N₃ are 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 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 any paper web passing to 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 W o , being transferred onto the drying wire 70, whose loop has been brought to a distance as short as possible from the face 10′ of the roll 10, being guided by the guide roll 66. After the guide roll 66, suction boxes 67 are provided inside the loop of the drying wire 70, which said suction boxes ensure that the web W adheres to the drying wire 70 and is transferred reliably to the drying section, whose first drying cylinder or corresponding lead-in cylinder is denoted with the reference numeral 68. According to Fig. 1, between the nips N₂ and N₃ an induction heating device 100 in accordance with the invention is fitted, which supplies heating power through the air gap V by means of a magnetic field into the particular outer layer 3 in accordance with the invention, provided on the roll 10. In Fig. 1, the regulating devices and the devices for the supply of electric power in the induction heating device 100 are illustrated schematically as the block 110. Moreover, in Fig. 1, a second induction heating device 100′ is shown to be used in the area of the web W detaching point R, the purpose and operation of said device 100′ being described in more detail below. In the following, with reference to Fig. 1A, the construction of the roll 10, which has a particular outer layer in accordance with the invention, will be described. The roll 10 comprises a frame mantle 1 of, e.g., cast iron, which gives the roll the necessary basic strength. If necessary, crown variation devices (not shown) may be provided in the interior of the roll mantle 1. Onto the outer face of the mantle 1, a ceramic inner layer 2 has been applied, which acts as a thermal insulation and as an electric insulation. On said insulation layer 2, there is an electrically conductive ceramic outer layer 3, whose outer face constitutes the outer face 10′ of the roll 10, which said outer face is sufficiently smooth and has adhesion properties suitable for the web W. The thickness s₁ of the frame mantle 1 is, as a rule, within the range of s₁ = 60...140 mm. The thickness s₂ of the insulation layer is, as a rule, within the range of s₂ = 3...10 mm, preferably s₂ = 4...5 mm. The thickness s₃ of the outer layer 3 is, as a rule, within the range of s₃ = 0.1...3 mm, preferably s₃ = 0.1...0.5 mm. According to Fig. 1A, at the proximity of the electrically conductive ceramic outer layer 3 of the roll 10, a magnetic-shoe device is arranged as an induction heating device 100, which said device extends substantially across the entire length of the roll 10. In a way that comes out later, the magnetic-shoe device consists of several ferritic magnetic cores 20 placed side by side, which said cores apply a magnetic flux through the air gaps V to the electrically conductive outer layer 3. The magnetomotoric force is generated by means of the coils 30, to which an adjustable power is fed, which is obtained from control and electric devices 110. In the following Table 1, data are given on electrically conductive composite ceramics suitable for use as materials for the outer layer 3 of the roll 10. The qualities 1, 2 and 3 given in Table 1 are boride ceramics, which are based on diborides of titanium and zirconium. The specific resistance of the qualities 1 and 2 is particularly suitable for use in the invention, whereas the specific resistance of the quality 3 is so high that it is not heated sufficiently well in all applications of the invention. Quality 1 2 3 Density (g/cm³) 4.9 4.0 3.2 Bending strength 20°C (N/mm²) 340 400 830 K₁C (MN/m 3/2 ) 3.7 4.4 5.0 Coefficient of elasticity (GPa) 310 360 500 Thermal expansion coefficient (10⁻⁶/°C) 6.2 5.3 5.5 Resistance to thermal shock (°C) 550...600 250...300 300...350 Specific resistance ( cm x 10⁻⁵) 2.1 6.5 50 In Table 1 above, the quality 1 is the ceramic material marketed by Messrs. Asahi Glass under the product name "Ceraborex". The coefficients of thermal conductivity of the composite ceramics 1, 2 and 3 at different temperatures are as follows: RT 200°C 400°C 600°C quality 1 34.3 44.4 41.5 37.9 quality 2 21.4 14.2 13.2 13.5 quality 3 24.6 25.9 23.5 22.0 The numbers are given in the unit In a preferred embodiment of the invention, underneath the electrically conductive thin ceramic layer 3, a layer 2 is used, which acts as an electric and thermal insulation and which is made, e.g., of the ceramic XG manufactured by Yamaguchi. The insulation layer 2 confines the inductive heating effect to the electrically conductive outer layer 3 alone. Moreover, the insulation layer 2 restricts the flow of heat from the outer layer 3 to the metal mantle 1 so that the temperature of the metal mantle remains at a sufficiently low level, among other things, in view of other equipment placed inside the mantle, e.g. crown variation means. In Fig. 1, in addition to the device 100, a second induction heating device 100′ is also used in the area of the detaching point R of the web W, which said device 100′ is supplied with electricity whose frequency f is, e.g., f = 500 Hz. This means that the temperature of the roll face 10′ rises in the detaching area R, e.g., by about 50°C. If the basic temperature of the roll is maintained at about 70°C, by the effect of the additional heating by the device 100′ in the detaching area R the temperature of the face 10′ 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 forms a thin vapour film, 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 directly to the drying section, e.g. onto its drying wire 70. As is known in prior art, a certain detaching tension has been necessary in the web W, which has been produced by means of a difference in velocity, i.e. a 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 transfer carried out by means of the device 100′, a detaching tension is not necessarily needed, so that it is also possible to use a closed draw, for example a draw in which, in the case illustrated in Fig. 1, the guide roll 66 has been shifted so that it reaches contact with the roll face 10′ and forms a lightly loaded transfer nip with said face 10′. The embodiment of the invention illustrated in Fig. 1 may also be accomplished so that there is no heating device 100, but exclusively a detaching-heating device 100′ in accordance with the invention is used, which is fitted in the web W detaching area R. In such a case, a sufficiently high basic temperature of the outer face 10′ of the central roll 10 is maintained by other means, e.g. by means of a heating medium fed into the interior of the roll 10, or by corresponding other means known in prior art. The embodiment of the invention in accordance with Fig. 1 may also be carried into effect without a detaching-heating device 100′ by means of the heating device 100 and the related electric and control devices 110 alone. The hot-pressing or impulse-drying device illustrated in Fig. 2 comprises a hot cylinder 10 with a relatively large diameter and with a drive gear 10a, which said cylinder 10 has a smooth or porous outer face 10′, which consists of an electrically conductive ceramic layer 3 in the way described above. The cylinder 10 face is heated by means of induction heating devices 100 based on eddy currents through an air gap V. The temperature T₀ of the cylinder 10 face 10′ is arranged to be T₀ > 100°C when the face 10′ meets the web W that is being passed to the hot pressing on the face of the press felt 12, the dry solids content of the web W being denoted with KA in . Depending on the location of the hot-pressing device in accordance with the invention in the process, KA in varies within the range KA in = 25...75 %. The device further includes a press roll 81 placed before the press shoe device 30, which said roll 81 has a smooth or patterned mantle face 81′ and is provided with a drive 81a. The press roll 81 is placed inside the loop of the glide belt 85, and the roll 81 forms a nip N₁₀ with the hot cylinder 10. The web W is passed on the support of the press felt 12 directly into the nip N₁₀ so that the web W is placed in direct contact with the smooth face 10′ of the cylinder 10 which has been heated inductively by means of the device 100. In a corresponding way, the press felt 12 is detached from the web W after the second nip N₂₀, said web W following the smooth face 10′ of the cylinder 10, from which it is detached as an open draw W p . The press-shoe device 90 in the hot-pressing device shown in Fig. 2 comprises an extended-nip press shoe 91, which has a hydrostatic pressure chamber 92 placed facing an impervious glide belt 85. The press-shoe device 90 comprises a frame beam 90a, which extends across the entire width of the paper web W. A cylinder block is arranged as supported on the frame beam 90a, into the pressure space of which said cylinder block the pressure or pressures of a pressure medium can be passed from a pressure source. The cylinder block is provided with a sealed piston, which has a glide face operating against the inner face of the glide belt 85 in the extended-nip zone. Pressure-fluid lubricant is passed out of the pressure space through bores into the hydrostatic pressure chamber 92. Around the loop of the glide belt 85, a splash-water collecting trough 87 is provided. The second press roll 80 is provided with a smooth face 80′ and with a drive 80a, and at its rear side there is a lubricant collecting trough 84, from which the lubricant is fed by means of a recirculation device. The second press roll 80 forms the nip N₂₀ with the hot cylinder 10, after which said nip the web W follows the smooth face 10′ of the cylinder 10, from which it is detached as a draw W p by means of a guide roll 13 provided with a drive 13a, being transferred onto the support of the drying fabric 15 guided by the guide roll 14. The fabric 15 carries the web W to the drying section, where the dewatering is continued by evaporation. The dry solids content of the web W after the hot-pressing device is denoted with KA out . As a rule, said dry solids content is KA out = 50...70 %. The paper web W is pressed by means of an extended-nip press shoe 31 of relatively low pressure (p₁) by the intermediate of the belt 25 and the press felt 12 against the hot (T₀ > 100°C) cylinder 10 face 10′, and heating of the face of the paper web W that is placed against the roll face 10′ is achieved to a temperature higher than 100°C. Said temperature of the face 10′ when it reaches contact with the web is within the range of T o = 150... 500°C. The corresponding temperature T o1 at the time when the web W is detached from the face 10′ is, as a rule, within the range of T o1 = 100...300°C. The pressure level of the extended-nip press shoe 31 is, e.g., p₁ = 0.1...5 MPa. The extended-nip shoe 31 is hydrostatic, hydrodynamic, or a combination thereof. After the extended-nip pressing stage C the pressure applied to the paper web W is lowered to the level p o determined by the tension of the belt 25 within the zone D, and the vaporization of the water in the paper web W is intensified by the effect of the lowering of the pressure p₁ -> p o . The pressure p o = T/R, wherein T = tensioning strain of the belt 25 and R = radius of the cylinder 10. The zone D is followed by the intensive pressing stage taking place in the nip N₂₀, wherein the paper web W is pressed with a high pressure between the cylinder 10 or a corresponding roll and the press roll 20. In Fig. 2A this stage is denoted with E, and the maximum of the compression pressure is thereat preferably p max2 = 7 MPa. In the pressing stage E the water vapour is blown through the paper web W and produces blowing-off of the water present in the intermediate spaces between its fibres and, consequently, an intensified pressing result and a higher dry solids content KA out . As the glide belt 25, it is also possible to use a so-called resilient belt, by means of which the zones A and E of the roll nips N₁₀ and N₂₀ and, at the same time, their compression times can be extended and the compression impulse be increased. If necessary, it is also possible to use a separate resilient band, which is guided to run between the glide belt 85 and the felt 12. Since water cannot be pressed out of the press felt 12 into hollow faces of the rolls, it is possible to form a follow face into the belt 85, which is referred to by the dashed line 85′ of the outside face of the belt 85. In the stage A illustrated in Fig. 2A, wherein a peak compression pressure p max1 is used in the nip N₁₀, the first hot-pressing stage is concerned. The stage B is a pressure-lowering stage, the stage C is the second preliminary hot-pressing stage and the stage D is the pressure-lowering and vapour-formation stage, and the stage E (peak pressure P max2 ) is the (intensive) compression and blowing-through stage proper. In Fig. 2A, on the middle line (L) below the zone denotations A...E, examples are given on advantageous lengths (mm) of said zones and on the lowest line (t) the corresponding times of dwell (ms) when a machine speed of v = 20 m/s is used. In accordance with the invention, the press roll 10 illustrated in Figures 3 and 4 is provided with an outer mantle 3 of an electrically conductive ceramic material. The roll 10 is journalled as revolving around its central axis K - K by the intermediate of its ends 95 and its axle journals 96. On the axle journals 96, bearings are provided, which are fitted in bearing housings. The bearing housings are fixed to the roll support frame, which is supported on a base. In the interior space of the roll 10 it is possible to fit crown variation or crown adjustment devices in themselves known, for which an abundant space is provided, because inside the roll 10 it is not necessary to use heating devices operating by means of a liquid medium or equivalent, said heating devices being, however, not completely excluded from use in connection with this invention. According to the invention, the roll 10 is arranged to be heated inductively and electromagnetically by means of eddy currents, so that, by the effect of this heating, the temperature of the thin surface layer 3 of the roll 10, made of an electrically conductive ceramic material, is raised to a remarkably high level, as a rule about 110...350°C. In view of effecting 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 a ferrite core 20 are arranged. These component cores 20 n constitute the heating device 100, which further includes a magnetizing coil 30 or a component coil 30₁...30 N of its own for each component core (Fig. 3). The inductive heating is carried out free of contact so that a little air gap V remains between the ferrite cores 20 n and the roll 10 face 10′, via which said air gap the magnetic fluxes of the ferrite cores 20 n are closed through the electrically conductive ceramic layer 3 of the roll 10, therein producing a heating effect in the eddy currents. In Fig. 3, each component core 20₁...20 N is illustrated as having a magnetizing coil 30₁...30₇ of its own. An 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 magnetizing coil 30, which is, according to Fig. 4, provided with two turns. According to Fig. 7, the magnetizing coil 30 of the iron core 20 has one turn only. According to an alternative embodiment of the invention, each component core 20 n is separately arranged 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 basic level and/or the distribution of the heating effect. For this purpose, each component core 20 n 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 different mechanisms. As a rule, said air gaps may vary, e.g., within a range of 1...100 mm. In respect of the mechanical means for the adjustment of the air gaps, whose construction is not described in this connection, reference is made to Fig. 9 as well as to the applicant's FI Patent Application No. 833589, which was mentioned above. In respect of the electrotechnical background of the invention, the following is ascertained. When the electrically conductive ceramic layer 3 of the roll 10 or cylinder is provided with a variable magnetic field, as is well known, losses of eddy currents and hysteresis are produced in the material, and the material is heated. The power (P) of the eddy currents depends on the strength (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 in the ceramic layer 3 of the roll 10 is closed through the front faces of the ferrite cores 20 of the device 100 and the air gaps V. This magnetic field induces eddy currents in the ceramic layer 3 of the roll mantle 10, which said currents produce heat owing to the relatively high resistance (see Table 1) of the layer. The distribution of the eddy currents, induced in the ceramic layer 3, in the direction x of the radius of the roll 10 follows the law I x = I o e -x/δ wherein I x is current density at the depth x from mantle face 10′, I o is current density at the mantle 10 face 10′, and δ is the 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 at the surface. For depth of penetration, the following equation is obtained: ρ is the specific resistance of the ceramic material in the layer 3 (see Table 1), f is the frequency of the magnetizing current, and µ is the relative permeability of the ceramic material in the outer layer 3. The equation (3) indicates that, when the frequency becomes higher, the depth of penetration becomes lower. In the invention, as a rule, heating powers are used that are of an order, in impulse drying of about 10 MW, in gradient calendering of about 500 kW, and in web detaching of about 100 kW. As is known, the smaller the air gap V is, the larger is the proportion of the electric power passed to the device through the coil 30 that is transferred to the ceramic layer 3 on the roll 10 to be heated. According to Fig. 7, the electric power that supplies the induction coil 30 is taken from a 50 Hz three-phase network (3 x 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 either to constant-frequency or variable-frequency (f s ) AC electricity. The regulation of the positions of the component cores 20₁...20 N of the ferrite core 20 can be arranged, e.g., by means of the automatic closed regulation systems shown in Figures 5 and 6. The regulation motors consist of stepping motors 29, which receive their control signals S 1...N from the regulation system 42. The regulation system is controlled by a detector device 41, which is, e.g., a temperature measurement device, by means of which the factual values of the surface temperatures T o1 ...T ok of the roll are measured at several points on the roll 10 in the axial direction K - K of the roll. The regulation system 42 includes a set-value unit, by means of which the axial K - K temperature profile of the roll can be set optimally. The power of the inverter 34 is fed through a matching transformer 35 to an LC-resonance circuit in accordance with the invention, the effect and operation of said circuit being illustrated by Fig. 6. In a way in itself known, the transformer 35 comprises 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 through a change-over switch 36 to the resonance circuit 37, by means of which the power is fed to the induction coil 30. Of course, as is well known, the resonance frequency of a connected RLC-circuit can be calculated from the formula In Fig. 6, the dependence of the current I in the circuit 37 from the frequency f s is illustrated. 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 power is at the optimum when the operation takes place at the resonance frequency f r . Out of a number of reasons, it is, however, not optimal to operate at the resonance frequency f r and/or at the same time at both sides of it, but the frequency of operation is chosen within the area f a1 ...f y1 above the resonance frequency f r or, in a corresponding way, within the area f a2 ...f y2 below the resonance frequency f r . Within the scope of the invention, said frequency ranges are chosen preferably as follows: f a1 ...f y1 = (1.01...1.15) x f r or f a2 ...f y2 = (0.85...0.99) x f r . According to Fig. 7, in the RLC-circuit a series capacitor C s is used. The basic tuning of the circuit 37 is carried out 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 placed at the correct position in accordance with the principles explained above. In Fig. 7, a parallel capacitor C r illustrated by dashed lines, which said capacitor can be used in stead of, or at the side of, the 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) has a coefficient dependent on resistance R. From the point of view of the objectives of the invention, as a rule, a series-resonance circuit is, however, preferable, in particular in 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 = 200 Hz to 30 kHz. The frequency range f r = 200 to 400 Hz is estimated to be particularly advantageous. To keep the efficiency of the power supply high and to eliminate phenomena of instability, i.e. the "risk of runaway", the operating frequency f s 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. in the areas f y1 ...f a1 or f y2 ...f a2 illustrated in Fig. 6. The measurement of the impedance of the resonance circuit 37 may be based, e.g., on measurement of the current I that flows in the circuit. This mode of measurement is illustrated in Fig. 7 by the block 46, from which the regulating 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 regulating signal b. Another mode of measurement of said impedance, which may be used as an alternative or in addition to the current measurement, is to pass a regulating signal c from the block 42, from which information can be obtained concerning the positions of the component cores 20n, i.e. the air gaps V, which primarily determine said impedance by acting upon the inductance L. An alternative mode of regulation is to pass a return signal from the stepping motors 29 to the block 47 and further so as to act upon the output frequency f s of the frequency converter 34. Fig. 5 illustrates 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 supply wires 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 separate resonance circuit is altered. The measurement of the impedance of each separate 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 series of signals e₁...e N received from said current meters, which said signals contain the data concerning, e.g., the air gaps V of the different component cores. Thereat each frequency f₁...f N can be optimized in view of the efficiency of the power supply of each component core and in view of the stability of the regulation. In view of obtaining a sufficiently low depth of penetration, the frequencies f₁...f N are, e.g., within the range of 0.3 to 1.0 kHz. The calender in accordance with Fig. 8 comprises a frame construction 150, which is fixed to a base 111. In connection with the frame construction 150 of the calender, a calender stack 120 is journalled by means of support and loading members (not shown), said calender stack consisting, from above, of an end roll 10, intermediate rolls 122 and 123, as well as of a lower end roll 10, in connection with which there is a doctor 129. Both of the end rolls 10 are provided with crown variation or crown adjustment means 125;128, which are placed inside the roll 10 in the way known in prior art and which operate either by means of a pressure medium and/or magnetically. The intermediate rolls 122 and 123 in the calender are most appropriately so-called double-mantle heated rolls, whose ends are provided with connector means 126 and 127, by means of which the interior spaces in the intermediate rolls 122,123 communicate with a heating/cooling aggregate. The cooling/heating medium may be, e.g., circulating water. In accordance with Fig. 8, in connection with the end rolls 10 there are external induction heating devices 100 in accordance with the invention, the details of the construction of said devices 100 coming out from the above or from Fig. 9. By means of the heating devices 100, by the intermediate of the magnetic shoes 20 provided in them, a magnetic flux is applied to the outer mantle 3, made of an electrically conductive ceramic material, of the rolls 10 through the air gap V, free of contact, within the sector e, which said magnetic flux induces eddy currents in the outer mantle 3. These eddy currents produce a heating effect owing to the resistance of the mantles 3 of the rolls 10. In Fig. 8, the entering of the web W into the calender, e.g., from the drying section of the paper machine is denoted with the reference W in , and its outlet from the calender with the reference W out . As is shown in Fig. 1, at the inlet side of the web W in there is a cooling roll 112, with which the web W in is in contact over a sufficiently large sector a. At the ends of the cooling roll 112 there are connecting means 12a, by means of which the space between the double mantle of the roll 12 communicates with a cooling-water aggregate. The run of the web W between the roll 112 and the first nip N₁ is guided by the guide roll 115. Before the guide roll 115, in connection with the run of the web W, there are moistening devices 113 and 114 at both sides of the web. By means of the devices 113,114, water jets S are sprayed onto one or both of the faces of the web W to produce a suitable moisture gradient in the direction of thickness of the web. Moistening is not always necessary. The elevated temperatures T₁,T₂ = 150...200°C of the end rolls 10 are produced by means of the induction heating devices 100 described above, by means of which it is also possible to control the temperature profile in the axial direction of the rolls 10. The temperature range T₂,T₃ = 40...50°C of the central rolls 122 and 123 is produced either without any particular operations or, if necessary, by cooling or heating the rolls 122,123. In the following, mainly with reference to Figs. 8 and 9, an exemplifying embodiment of the heating devices 100 used in the invention will be described, which said devices 100 are placed in connection with one or both of the end rolls 10 of a calender and, in some special applications, if necessary, also in connection with the other rolls, i.e. with the intermediate rolls 122,123. The devices to be described can also be used in the applications illustrated in Figs. 1 and 2. If necessary, in connection with one roll there may also be several heating devices. The outer mantle of the roll 10 is made of an electrically conductive ceramic layer 3, below which there is preferably an insulation layer 2. In the interior of the rolls 10, there are crown variation or crown adjustment devices in themselves known, for which a free space remains because of the external heating device 100, because in the interior of the roll 10 it is unnecessary to use heating devices operating with a liquid medium or equivalent. The device 30 comprises a number of component cores 131₁,131₂...131 N (N pieces) placed side by side, whose positions can be regulated independently from each other in the direction of the arrow B in Fig. 9 for adjustment of the magnitude d of the active air gap V between the front faces of the cores 31 and the roll 10. The magnitude d of the air gap V is adjustable, e.g., within the range of d = 10...60 mm. The component cores 131 have, e.g., a common magnetizing coil, which is supported on the box part 133 by means of projections 133b. An adjustable AC-current of adequate frequency is supplied into the coil 132. The position of each component core 131 can be adjusted independently from the other component cores 131 so as to regulate the magnitude d of the air gap V and the axial distribution of the heating effect. For this purpose, the component cores 131 are attached to arms 135 by means of a flange 135a, which said arms are placed in guide tubes 137 by means of slide fittings 138. Screws 142 are connected to the arms 135 by means of threadings 141, which said screws are operated by screw motors 136. Tile screw motors 136 are connected to the regulation system in a way in itself known. By setting the level of the air gaps d of the component cores 131 and/or by setting the level of the magnetizing current of the coil 132, it is possible to regulate the temperature level of the rolls 10. By means of individual regulation of the positions of the component cores 131, it is possible to regulate the axial temperature profile and, thereby, on the basis of changes in the radius of the roll 10 and in a way in itself known, the nip and the thickness profile of the web W to be calendered. In front of the front face of the component cores 131, there is a protective box 133, which is attached to the frame part 140 of the heating device 130 by means of a groove-projection fitting 134. The frame part 140 of the heating device or devices 130 is fixed permanently either to the frame part 150 of the calender or to support members by means of which the heating device 130 can be shifted further apart from the calender rolls, e.g., in connection with a web W break or with servicing.

Claims: Method in a machine for the manufacture of paper or board for heating the outer face (10') of such a cylinder or roll (10) as is in direct contact with the web (W) to be pressed against said roll face, which said web (W) is treated, such as dewatered or calendered, by means of the method, comprising the steps that said cylinder or roll face (10') is heated from outside its mantle inductively by using a magnetic field, by means of which a heating effect based on eddy currents is produced in an outer layer (3) of the roll, characterized in that, as said cylinder or roll face (10'), a relatively thin outer layer (3) of an electrically conductive ceramic material is provided, in which layer said resistive heating effect is concentrated, that the depth of penetration of said heating effect in the radial direction of the roll to be heated is restricted by means of the choice of the thickness of said ceramic outer layer (3) and/or of the electric frequency (f) of the induction heating, and that, besides its electric properties, the ceramic material of said outer layer (3) is chosen so that the cylinder or roll face (10') is given the necessary properties of strength, both in view of the wear resistance and in view of the thermal shock of the heating effect. Method as claimed in claim 1, characterized in that the method is applied in connection with hot pressing of a paper web to the heating of the outer mantle of such a hot cylinder (10) in connection with which one or several roll nips (N₁₀,N₂₀) and/or a so-called extended nip (91/10) are formed. Method as claimed in claim 2, characterized in that in the method the roll face is heated to a temperature that is within the range of 140...500°C. Method as claimed in claim 1, characterized in that the method is applied in dewatering-pressing in a paper machine to the heating of the outer mantle (3) of such a press roll, most appropriately the central roll (10) in the press, in connection with which said roll one or several press nips (N₂,N₃) provided with a press felt are formed, the temperature of the outer layer (3) of said roll (10) being raised by means of said method so as to intensify the pressing and/or to promote the detaching of the web (W). Method as claimed in claim 4, characterized in that the method is applied by producing a heating effect in accordance with the invention at the point at which the paper web is detached from said press roll, most appropriately from the central roll (10) of a compact press section, and is passed further. Method as claimed in claim 1, characterized in that the method is applied in the calendering of a paper web by providing one or several calender rolls, preferably one or both of the extreme rolls (10), with a heating device in accordance with the invention, which said device produces an eddy-current heating in the outer layer (3), made of an electrically conductive ceramic material, of a calender roll placed in connection with the device. Method as claimed in any of the claims 1 to 6, characterized in that in the method the inductive heating effect is confined to the outer layer (3) by excluding any conduction of heat by means of an electrically and preferably also thermally insulating inner layer (2) placed underneath said outer layer (3). Method as claimed in any of the claims 1 to 7, characterized in that the specific resistance of the electrically conductive ceramic material in the outer layer (3) is lower than 10⁻² Ωm, preferably within the range of 2...8 Ωcm x 10⁻⁵. Device for the press treatment of a paper web intended for carrying out the method as claimed in any of the claims 1 to 8, comprising a cylinder or roll (10) whose outer face can be heated and in connection with which one or several roll nips and/or so-called extended nips are formed, and further comprising one or several inductive heating apparatuses (100;100') fitted in connection with said cylinder or roll (10) for said heating characterized in that the mantle of the cylinder or roll (10) consists primarily of an outer layer (3) of an electrically conductive ceramic material, said outer layer being heated by said apparatuses. Device as claimed in claim 9, characterized in that underneath said outer layer (3) there is an electrically non-conductive second ceramic layer (2) or a corresponding insulation layer, below which there is the frame mantle (1) of the cylinder or roll (10), made of structural metal. Device as claimed in claim 9, or 10, characterized in that the thickness s₃ of the outer layer (3) is within the range of s₃ = 0.1...3 mm, preferably s₃ = 0.1...0.5 mm, and/or that the thickness s₂ of the electrically non-conductive and preferably thermally insulating intermediate layer (2), which is placed inside said outer layer (3), is within the range of s₂ = 3...10 mm, preferably s₂ = 4...5 mm. Device as claimed in any of the claims 9 to 11, characterized in that the specific resistance of the outer layer (3) has been chosen within the range of < 10⁻² Ωm, preferably within the range of 2...8 Ωcm x 10⁻⁵. Device as claimed in any of the claims 9 to 12, characterized in that, as the electrically conductive ceramic material of the outer layer (3), boride ceramics are used, which are most appropriately based on diborides of titanium and zirconium.

Inventors: JAAKKOLA JYRKI, JAAKKOLA, JYRKI
Applicants: VALMET PAPER MACHINERY INC, VALMET PAPER MACHINERY INC.
Application year: 1989
Main classification: D21F3
IPC codes: D21G1, D21F5, D21F3