DESCRIPTION

The present invention relates to a papermaking machine. In particular, although not exclusively, a machine according to the present invention can be used for the production of structured paper.

It is known that, in general terms, tissue paper is produced using a fibrous suspension containing cellulose fibers normally called pulp. In continuous machines, the pulp is introduced on draining web-like supports. In this phase the formation of the sheet takes place, that is, the fibers overlap and create bonds to form what, once the water has been removed, will be the sheet. The removal of water takes place progressively through actions of drainage, suction, pressing and evaporation until obtaining rolls of paper with a residual humidity of around 5%. According to a consolidated production scheme, the water present in the fibrous suspension is drained through a forming cloth, after which the formed sheet passes through a pressing section in which other water is eliminated and, subsequently, passes over a cylinder, commonly called "Yankee”, heated from the inside by introducing high pressure steam. The combined action of the heat transmitted to the sheet through the external surface of the Yankee and of the blowing of hot air and the suction operated by hoods arranged around the Yankee itself finally determines the achievement of the desired degree of dryness. The sheet of paper can be detached from the Yankee by means of a creping blade and wound up in the form of a reel. In alternative systems, the detachment of the sheet from the Yankee can take place in different known ways. The term "tissue" generally refers to low-weight paper products intended for hygienic and sanitary use, such as toilet paper, rolls of kitchen paper or towels, handkerchiefs, napkins, etc.

In a machine for the production of structured paper the sheet is supported by a conveyor belt whose shape determines a substantial three-dimensionality of the sheet.

The present invention relates to the production of a papermaking comprising a Yankee whose surface in contact with the sheet of paper is heated avoiding the use of steam.

This result has been achieved, in accordance with the present invention, by adopting the idea of realizing a machine having the features indicated in claim 1. Other features of the present invention are the subject of the dependent claims.

The present invention provides for the construction of a machine provided with a Yankee with an electromagnetic induction system inside it and a shell made of any electrically conductive material, such as for example carbon steel or cast iron, suitable to exploit the thermal effects connected to the electromagnetic induction. At the two ends of the mantle there are two respective end heads.

In accordance with the present invention, the Yankee mantle can also be stratified, with a primary layer, in correspondence with the radially inner surface of the cylindrical mantle, on which the electromagnetic field acts, made of electrically conductive material. In this case, the mantle layers adjacent to the primary layer heat up by conduction by being in contact with the material of the primary layer.

Preferably, the induction system placed inside the Yankee is static, i.e. it does not rotate together with the Yankee, and is supported by an axis passing through two pins formed on the end heads of the Yankee.

The internal surface of the Yankee, i.e. the internal surface of the mantle, is free from circumferential grooves. These grooves are normally present in Yankees heated by steam and perform the dual function of increasing the convective exchange surface with the internal steam (which according to the present invention is not used) and to collect the condensed steam (which according to the present invention is not formed as a result of the steam not being used).

In accordance with the present invention, the electromagnetic induction system comprises a single inductor or more inductors, at a radial distance preferably between 30 cm and 1 mm from the inner surface of the mantle without contact with the latter. More preferably, said radial distance is between 5cm and 2mm and, still more preferably, between 2cm and 5mm. In this case, the term radial distance means the radial distance between the internal electrically conductive cylindrical surface of the Yankee and the point of the induction system closest to it.

Inductors can be made in many ways.

The inductors are configured to heat the metallic inner surface of the mantle. The heating is obtained by generating alternating electromagnetic fields through the inductors, with variable frequency and amplitude according to the power to be transmitted. The electromagnetic fields, which vary over time, generate induced currents in the metallic material of the mantle which, due to the Joule effect, produce localized overheating. In this way, the heat necessary for drying the paper is generated.

The heat produced at the internal surface of the Yankee diffuses by conduction through the thickness of the mantle, until it reaches the external surface with which the sheet being dried is in contact. A part of the heat will diffuse inside the cylinder itself both by radiation and by convection. However, since the internal volume of the Yankee is substantially closed, the heat diffused inside will not be dispersed but will cause a rise in the temperature of the air contained inside it. In this way, the corresponding portion of thermal energy will not be dispersed into the external environment, with the exception of the part transmitted to the outside through the end heads. In any case, similarly to what occurs in the case of steam Yankee dryers, the heads can be suitably insulated with per se known systems so as to limit to a minimum the part of thermal energy transferred to the external environment, such that the maximum portion of thermal energy is transferred to the sheet being dried.

The electrical power to be transformed into thermal power for drying the sheet is of the same order of magnitude as the thermal power provided by the steam in traditional systems. In fact, the thermal power does not depend on the source, but on the use that must be made of it. The function of the Yankee is to provide heat to the paper in order to bring the dryness degree of the sheet adhered to it from a minimum value (normally in the range of 25%-50%) when the paper comes into contact with the external surface of the mantle, up to a maximum value (typically in the range of 55%-99%) when the paper is detached from the Yankee. The thermal energy required for the correct functioning of the Yankee will therefore depend on the programmed production parameters and, for example, on: paper weight, sheet width, input dryness degree and output dryness degree from the Yankee, Yankee rotation speed and input sheet temperature. These parameters make it possible to calculate the exact amount and enthalpy content of the water that must be removed from the sheet. Overall, the power required to evaporate the water contained in the sheet until the desired degree of dryness is reached varies approximately between 2MW and 15MW, depending on the production rate, the inlet temperature of the sheet on the Yankee and various other parameters (grammage, degree of creping, etc.).

In accordance with particular embodiments of the present invention, the Yankee may have an external cylindrical surface with an external layer that is harder than the base metal in order to increase its resistance to wear (due to the presence of paper and scrapers used for remove and/or crepe the dried sheet and/or clean the surface upstream of the sheet adhesion area). For example, said outer layer can be made by means of a metal deposition, or by a plating deposition (deposited by fusion) or by a surface treatment, for example by laser hardening.

Among the advantages offered by the present invention the following can be mentioned in particular. Decarbonisation: in traditional Yankee dryers, steam is normally produced in boilers that burn fossil fuels. Steam generation represents a large amount of the energy used to operate a paper mill. Increasingly, national and international regulations, in addition to the need to reduce the impact of the greenhouse effect on the climate, require that the production of carbon dioxide, one of the main greenhouse gases, be limited and possibly cancelled. The electrification of the heat production to be used in the Yankee for drying paper in accordance with the present invention can allow to reduce (up to canceling) the production of CO2 necessary for the operation of the Yankee since the required electrical energy can be produced using renewable sources and not the use of fossil fuels. Assuming that only electricity generated from renewable sources is used, it is possible to estimate a potential reduction of CO2 emissions between 2500tons/year and 25000tons/year, depending on the productivity of the machine.

Another advantage deriving from the invention concerns the simplification of the system and a greater controllability: in traditional Yankee dryers, the heat is introduced by using steam as afluid carrier. The steam, following the heat exchange with the sheet being dried, condenses, therefore, at the Yankee outlet there is a two-phase fluid composed of condensate and steam (introduced in excess precisely to maintain the driving force necessary for the extraction of the condensate, in order to avoid flooding inside the cylinder). The steam must have a sufficiently high temperature to allow the paper to be dried while maintaining high productivity (the heat exchanged will be a function of the gradient between the steam temperature and the sheet temperature). Therefore, the steam pressure will have to be kept above a minimum value depending on the process equilibrium. Modem, more performing machines, in order to produce paper with speeds even higher than 2000 mpm, require that the temperature of the internal steam exceeds 150-160°C, with peaks close to 190°C. These temperatures, being saturated steam, require that the pressure inside the Yankee can reach lObar. All of this requires the construction of a steam generation, treatment and transport plant completed with a series of components and pipes that have a heavy impact on the complexity of the paper production plant. In particular, the following components are present in most applications: steam generation boiler, transport pipes, thermo-compressors to raise, using higher pressure steam, the pressure of the exhausted steam in the cycle in order to regenerate it and keep it in the cycle itself, safety systems (safety and regulating valves), heat recovery systems, condensate separators and others. Furthermore, a system must be created inside the Yankee which allows the continuous removal of the condensate itself. These systems normally consist of a multiplicity of cannulas immersed in the grooves (in the case of grooved Yankees): the steam is introduced into the cylinder in excess with respect to what is necessary for the heat transfer to the paper; the excess steam under pressure finds the only way out through the openings of the cannulas, connected to manifolds distributed circumferentially inside the cylindrical surface of the Yankee and, from these, to a manifold coaxial to the Yankee itself. The action of the excess steam causes the condensate accumulated on the bottom of the grooves following the passage of heat to the paper to be forced to go back up through the cannulas themselves. In this way, following the steam flow, the condensate is continuously removed from inside the Yankee, avoiding flooding. In the case of non-grooved Yankees (typically used in less performing applications or for particular types of paper to be produced, such as MG paper), the condensate extraction system involves the use of large collection spoons rotating together with the Yankee: in this case, given the lower rotation speed of these systems, the condensate tends to accumulate on the lower part of the cylinder. The spoons (in number depending on the diameter of the cylinder) when they rotate, pass from the lower part, collect the condensate and guide it towards a central collector by exploiting the gravity and the particular path of the condensate removal ducts.

All these systems are at the service of the correct flow of steam and condensate, in order to allow the transport of the required heat. Every single component requires checking and maintenance in order to keep the system efficient and to avoid corrosive/erosive phenomena. The steam circuit requires constant quality control of the flowing substances. For example, it is necessary to keep the PH level within pre-established limits and it is necessary to constantly guarantee the absence of free oxygen in the circuit in order to prevent corrosive phenomena which could cause damage not only to the components of the circuit, but also to the Yankee itself. For this reason, it is necessary to constantly monitor the operating condition and check the quality of the steam also through the use of specific chemicals.

All these problems and the resources required to keep these systems efficient are eliminated, in accordance with the present invention, by avoiding the use of steam as a heat transfer fluid.

The use of induction heaters makes it possible to eliminate the use of steam and all the components, systems and technical problems associated with the use of steam.

The need to use pressurized steam (up to 10 bar) inside traditional Yankee dryers also leads to a series of construction complications. Given the large volume of pressurized steam contained in the Yankee (even over 7m in diameter and over 5.6m wide), the resulting forces on the internal surfaces of the Yankee are significant (a circular head with an internal diameter of 6.6m, exposed to steam under pressure at lObar, bears a resulting thrust equal to about 14000tons). Furthermore, the use of pressurized steam means that the Yankee must be designed in compliance with the stringent rules imposed by national and international regulations for pressure vessels in order to avoid catastrophic accidents, such as an explosion of the Yankee. Therefore, the structure itself and the connection systems of the various structural components require care and very high levels of control in order to absolutely guarantee the structural reliability of the cylinder throughout its operating life, with a great increase in costs both for the materials used and for the necessary construction and control procedures, the latter to be repeated periodically during the life of the product. Normally, the surface of the Yankee, before coming into contact with the paper to be adhered, is sprinkled with substances whose function is to promote adhesion of the paper, to facilitate its detachment after drying, to give particular characteristics to the adhered paper and to protect the outer cylindrical surface of the Yankee. This is normally referred to as an organic coating and its chemical composition can vary greatly depending on the product, the condition of the Yankee surface and the recipe used by the paper mill. Some of the chemical substances contained in the coating (for example epichlorohydrin polyaminoamide resins which can be used in the adhesion components) require, in order to function correctly, to create cross-linking bonds which are facilitated by high temperatures. This aspect also has an impact on the sizing of the Yankee and of the machine configuration, in fact, having to guarantee the cross-linking of the coating before contact with the paper, this causes the coating spray bar (or more simply, the coating bar) to be placed at a certain distance from the point of contact with the paper (depending on the type of coating and the rotation speed of the Yankee). The surface between the coating bar and the point of contact with the paper develops along an arc which, in fact, is unused: the purpose of the Yankee is to transfer thermal power to the paper to dry and this can only take place in the zone where the paper adheres to the Yankee; if a part of the Yankee cannot accommodate the paper because the coating distributed on it is not "ready" to ensure correct adhesion, then the Yankee, in condition of same performance, must have a larger diameter to compensate for the unused area (the development of the wrapping of the paper around the Yankee and, therefore, the diameter of the Yankee, depend on the overall performance of the machine and of the Yankee, as well as on the grammage). On the other hand, the use of an induction system placed inside the Yankee and not rotating together with it, allows, by adopting a modular design, to adjust the intensity of thermal power also according to the involved sectors. In this way it is possible to control the temperature of the sector dedicated to the cross-linking of the coating. By optimizing the crosslinking process, it is possible to reduce the space dedicated to it. Reducing the space required for crosslinking allows a reduction of the Yankee diameter in conditions of same performance. Another potential advantage concerns the greater energy efficiency: the current system provides for the generation of thermal power outside the Yankee (the steam is produced in a boiler outside the Yankee and, from this, via ducts and regulation components, is brought inside the Yankee). The system object of the present invention instead provides that the thermal power is produced, thanks to the Joule effect caused by the induced currents, directly in the metal coat of the Yankee, i.e. in correspondence with the exchange zone with the drying paper. This allows to reduce all the losses due to the transport of fluid at high temperature from a remote area inside the Yankee. This fact, if associated with a source of electricity production from renewable sources (i.e. without the need to produce a heat transfer fluid for the production of electricity via combustion), allows the path of the thermal power to be significantly shortened, thus improving the overall efficiency.

In the drawings attached by way of example, not to be considered in a limiting sense:

Fig.l shows a papermaking machine provided with a Yankee in accordance with the present invention in a possible operating configuration;

Fig.2 shows a papermaking machine equipped with a Yankee in accordance with the present invention in a further possible operating configuration;

Fig.3 represents a schematic vertical sectional view illustrating some parts of a Yankee dryer according to the present invention;

Fig.4 represents a detail relating to a possible operating configuration of a Yankee dryer in accordance with the present invention;

Fig.5 A represents a schematic longitudinal section view of a Yankee in accordance with the present invention;

Fig.5B represents an enlargement of Fig.5A and highlights the magnetic field lines which intercept the Yankee mantle;

Figs.6-8 show three different views of a possible embodiment of an inductor for a Yankee in accordance with the present invention;

Figs.9-12 show four different views of a further possible embodiment of an inductor for a Yankee in accordance with the present invention; Figs.13-15 show three different views of a possible embodiment of an inductor group for a Yankee dryer according to the present invention;

Figs.16-20 show five different views of a further possible embodiment of a group of inductors for a Yankee dryer according to the present invention.

Reduced to its essential structure and with reference to the figures of the attached drawings, a Yankee (1) for a machine according to the present invention is a body comprising a metal mantle (2) with a circular cross section and two end heads (3) on which two respective coaxial pins (4) are formed or mounted, arranged along an axis of rotation (x-x) of the Yankee. A transmission (5) acts on the pins (4) by which the rotation speed of the Yankee around the axis (x-x) is controlled. The outer surface of the mantle (2) forms a heat exchange surface with a sheet of paper produced upstream of the Yankee to reduce its water content, i.e. increase its dryness. The sheet of paper can be produced, with methods per se known to those skilled in the art, in a machine for the production of structured paper which, for example, comprises a forming zone (A) in which the sheet is formed starting from a fibrous suspension of predefined composition, a pre-drying zone (B) of the sheet downstream of the formation zone (A), along which the water content of the sheet is progressively but partially reduced, and a drying zone (C) downstream of the pre-drying area (B), where the Yankee (1) is installed. In the area (A) for forming the sheet there is arranged and acting a pick-up belt (NA) which transports the sheet towards the pre-drying area (B). In the pre-drying station (B) a structured conveyor web (NB) is arranged and acting which, at the entrance to the pre-drying station (B) receives the sheet from the pickup belt (NA).

The structured paper is produced by adhering the sheet to the Yankee (1) when the sheet has a relatively high degree of dryness (typically greater than 30% and preferably greater than 60%). The sheet has a three-dimensional structure due to the conformation of the structured belt (NB) which transports it from the previous stations (A, B) of formation and partial reduction of the water content, up to the contact with the Yankee. The sheet is made to adhere to the surface of the Yankee by effect of a slight pressure exerted by a transfer roller or by effect of a slight winding around the cylindrical surface of the Yankee by means of the web itself. In this case, the pressure exerted by the roller or by the web is preferably the minimum required to guarantee the transfer of the sheet onto the surface of the Yankee, thus avoiding damage to the structure of the same paper. The transition of the paper from the structured web to the Yankee can also be carried out using a higher level of pressure or using pressure rollers of different types (for example suction presses) in order to obtain also a partial removal of the water contained in the sheet and to facilitate the adhesion of the sheet to the mantle of the Yankee, at the cost of greater compression exerted on the sheet with consequent partial flattening of the three-dimensional structure. In Fig. l and Fig.2, in the entry section of the drying area (C) there is a transfer roller (6) which pushes the sheet of paper onto the Yankee (1) with a predetermined pressure.

The structured web (NB) can consist, for example, of a fabric suitable for structuring paper (for example a TAD type fabric). In this case the structured paper, i.e. characterized by a three-dimensional structure along its thickness, which takes up the shape of the fabric itself. Normally, in these cases, the paper is brought into contact with the cylindrical surface of the Yankee and made to adhere to it only in correspondence with the upper protuberances ("knuckles"), which represent, compared to the surface, a limited percentage (typically between 10% to 60%, preferably between 20% and 40%).

The structured web (NB) can also be made from a layer of absorbent or draining material with a relief reported on the paper side, preferably made of polymeric elastic material such as polyurethane, having a distributed structure, preferably to form a lattice, whose function is to create compression surfaces of the sheet against the Yankee surface distinct from other areas in which the sheet undergoes very little, if any, compression. This solution makes it possible to create on the same sheet a weave of compressed fibers which give high resistance to the product, while at the same time maintaining areas with a high absorption capacity in correspondence with the areas which have undergone limited compression. In jargon, in this case, the paper is defined as textured or marked paper.

In Fig.l and Fig.2 the arrow “MD” shows the direction of the sheet along the machine.

Downstream of the Yankee (1) a section (D) can be arranged for collecting the sheet in the form of reels (Rl, R2).

A hood (7) can be arranged on the Yankee (1).

In Fig.l the pre-drying area (B) comprises a compression unit (Z2) with a pressure roller and an underlying counter-roller to exert a pressure of a predetermined value on the sheet supported by the structured web (NB). Downstream of the compression unit (Z2), with respect to the direction followed by the sheet directed towards the Yankee (1), there is a TAD section (TD). In the variant of Fig.2, the compression unit (Z2) comprises two pressure rollers and an underlying counter-roller to exert a pre-set pressure on the sheet supported by the structured web (NB). Also in this case, downstream of the compression unit (Z2), with respect to the direction followed by the sheet directed towards the Yankee (1), there is a TAD section (TD). In a further variant embodiment, not shown in the drawings, the pre-drying zone (B) can optionally comprise only a TAD section upstream of the Yankee (1).

In the drying section (C) the sheet is in contact with the outer surface of the mantle of the Yankee (1) from the entry point (Pl) to the exit point (P2) for a section with an angular extension (a) greater than 180°. In the scheme of Fig.2, the sheet is indicated by the reference "S". At said exit point (P2) a creping scraper (8) can be arranged for detaching the sheet from the Yankee (1) and, downstream of the creping scraper (8) with respect to the direction of rotation (1C) of the Yankee, there is a cleaner scraper (80). The sheet, in alternative configurations, could be detached from the Yankee even without the use of a scraper, but through the use of support belts which collect the sheet at the end of its passage from the Yankee surface.

For example, the Yankee (1) has an external diameter comprised between 2.00 m and 7.500 m, and an axial length comprised between 3.00 m and 7.400 m.

Inside the Yankee (1) there is a fixed electromagnetic induction heating system made up of one or more inductors (H; HN) positioned and configured to produce alternating electromagnetic fields with pre-established frequency and amplitude according to the thermal power to be generated. In case of installation of a plurality of inductors (HN) distributed in the axial direction and fed independently or in groups, it is possible to carry out a partial control of the thermal power generated, in the axial direction, in the mantle. In this way it is possible to compensate for any unevenness in the transversal humidity profile of the dried paper, by acting on the solenoids acting on the areas corresponding to the bands where the paper has greater humidity, by increasing the electric power supply. The electromagnetic fields produced by the inductors, which vary over time, induce the generation of electric currents in the metallic material of the Yankee mantle and, due to the Joule effect, the mantle itself is subject to heating. The heat thus generated is used to dry the sheet (F). The induction heating system makes it possible to generate a uniform thermal imprint on the surface of the mantle (2) of the Yankee, i.e. a uniform heat distribution along the generatrix lines of the cylindrical surface which defines the mantle.

The electromagnetic induction heating system extends axially, i.e. parallel to the axis (x-x) of the Yankee, along the inner surface of the mantle (2) but is preferably shorter than the latter, leaving a free space (10) in front of each of the end heads (3). In this way, the electromagnetic interaction with the mantle (2) extends solely or almost exclusively to a central area (20) of the latter intended for contact with the sheet (F). This reduces the energy transmission to the lateral parts of the mantle (2) which do not come into contact with the sheet (F).

Preferably, the radially outer side of the electromagnetic induction system is at a radial distance from the inner surface of the mantle (2) comprised between 20cm and 1mm, more preferably said radial distance is comprised between 5cm and 2mm and, even more preferably, between 2cm and 5mm.

In the diagrams of Fig.5 A, Fig.6, Fig.7 and Fig.8 there is a single inductor (H) in the form of a solenoid developed around the (x-x) axis of the Yankee. The solenoid is formed by coil turns concentric with said axis (x-x).

With reference to Fig.5A, the inductor (H) extends axially inside the Yankee (1) for a length (LH) shorter than the length (L2) of the mantle (2).

The terminals (HT) of the inductor (H) can be passed through one of the pins (4).

In the diagrams of Figs. 9-12 there is a single toroidal inductor (H) developed parallel to the axis (x-x) of the Yankee. The toroidal inductor is formed by coils parallel to the inner surface of the mantle (2). Also in this case, preferably the radially outer side of the inductor (H) is at a radial distance from the inner surface of the shell (2) between 20cm and 1mm, more preferably said radial distance is between 5cm and 2mm and, even more preferably, between 2cm and 5mm. Also in this case, the terminals (HT) of the inductor (H) can be passed through one of the pins (4).

In the diagrams of Figs.13-15 there are a plurality of inductors (HN) each of which consists of a solenoid developed around the axis of the Yankee and formed by coil turns concentric to said axis. In this example, the inductors (HN) are arranged axially side by side. Also in this case, the terminals (HT) of the inductors (HN) can be passed through one of the pins (4).

In the diagrams of Figs.16-20 more inductors (HN) are provided, each of which is formed by a solenoid consisting of coil turns wound around respective radial axes (RHN). The inductors (HN) in this case are arranged circumferentially around the axis of the Yankee. Also in this case, the terminals (HT) of the inductors (HN) can be passed through one of the pins (4). In the drawings, the reference "HP" indicates cores of ferromagnetic material which can optionally be used to make the inductors (HN).

By using a plurality of inductors (HN) arranged as in the diagrams of Figs. 13-15 it is also possible to control the thermal power transmitted to the mantle (2) in a differentiated manner along the axial direction of the Yankee to produce axially variable thermal profiles. By using a plurality of inductors (HN) arranged as in the diagrams of Figs. 16-20 it is also possible to control the thermal power transmitted to the mantle (2) in a circumferentially differentiated way to vary the heat transmission in the circumferential direction.

Preferably, the inductor consists of one or more solenoids geometrically configured such that the electromagnetic field produced is intercepted exclusively or almost exclusively by the cylindrical mantle (2) of the Yankee, minimizing the projection of the electromagnetic field lines towards the end heads (3), with which the sheet does not come into contact.

In practice, the Yankee mantle constitutes an armature in "Near Field" geometric conditions.

In this way, in addition to directing the electromagnetic energy towards the mantle with greater spatial precision, reducing the start-up transients, the projections of the electromagnetic fields towards the end heads are reduced, which also contributes to increasing safety for operators on the site where the Yankee is installed.

The substantial absence of electromagnetic fields outside the volumetric areas affected by the presence of the mantle therefore makes it possible to limit the amount of energy transmitted by the electromagnetic fields to the end heads, reducing the amount of energy dispersed in the environment and not effectively directed towards the sheet being dried.

The use of the Near Field for the induction of the mantle allows, in other words, to intercept the greatest possible number of electromagnetic field lines by the mantle, reducing the leakage of electromagnetic fields from the volume delimited circumferentially by the mantle and simplifying the realization of the end heads.

From the construction point of view, preferably in accordance with the present invention, the following condition is satisfied:

S<P-d, where “S” is the radial distance of the inductor from the internal surface of the Yankee mantle, “d” is the diameter of the conductors forming the coil turns (HS) and “P” is the pitch between the coil turns of the inductor (or individual inductors).

It is noted that, in general, all other conditions being equal, turns more spaced apart from each other will imply a lower spatial density of the local field, therefore a lower temperature generated on the mantle, compared to turns wound with a smaller winding pitch, i.e. closer turns.

More preferably, in accordance with the present invention, S and the difference P-d are of the same order of magnitude and have substantially the same value, i.e. the following condition is satisfied: S=P-d, where S, d and P have the meaning indicated above. Accordingly, according to the present invention, a geometric configuration of the inductor system (understood as a single solenoid or as a plurality of solenoids) is achieved in which the distance between the spiral windings of the inductor system and the internal surfaces of the armature consisting of the Yankee mantle is approximately equal or less than the pitch between the coils (HS) reduced by said diameter (d).

In this way, the magnetic fields of the individual turns intercept the mantle material being influenced in a limited way by the magnetic fields of the adjacent turns. Therefore, it is possible to concentrate the magnetic field effect of the single turns more effectively. Furthermore, given that once the induced magnetic field has been created in the metallic material of the mantle, the resulting magnetic field is reduced (Faraday cage effect), it is easier to prevent electromagnetic induction from extending to the end heads, reducing the amount of energy which would be dispersed in the external environment.

With reference to the example shown in Figs.9- 12, the inductor (H) has a side (HM) radially closer to the mantle (2) and a side (HW) radially more distant from the mantle (2), and said side (HM) radially closest to the mantle (2) is at a given radial distance from the inner surface of the mantle (2) and also in this case the inductors are formed by turns (HS) made up of conductors of predefined diameter spaced apart of a pitch of predefined value (P). This case is characterized by the fact that the value of the radial distance of the side (HM) from the inner surface of the mantle (2) is less than or equal to the difference between said pitch (P) and said diameter (d). The same applies if instead of the single inductor (H) there are several inductors thus constructed and aligned along the axis of the Yankee.

From the description provided above, it is evident that a papermaking machine according to the present invention comprises a Yankee (1) consisting of a body comprising a metal mantle (2) with circular cross-section and two end heads (3) on which are formed or mounted two respective coaxial pins (4) arranged along a rotation axis (x-x) of the Yankee, said body being configured to rotate with a predetermined angular speed around said rotation axis (x-x), characterized in that inside said body is arranged a fixed electromagnetic induction heating system comprising one or more inductors (H; HN) interacting electromagnetically with the mantle (2) to produce induced electric currents in the same mantle, said one or more inductors (H; HN) being arranged near the radially innermost surface of the mantle (2), and in which is arranged a structured web (NB) configured for transporting a sheet (F) of fibrous material up to the contact of the latter with the mantle (2) of the Yankee (1). From the description provided above, it is also evident that a machine according to the present invention may also have one or more of the following features possibly combined between them: the Yankee (1) has an external diameter comprised between 2.00 m and 7.500 m, and the mantle (2) has an axial length comprised between 3.00 m and 7.400 m. the electromagnetic induction heating system extends axially, i.e. parallel to the axis (x-x) of the Yankee, along the inner surface of the mantle (2) but is shorter than the latter, leaving a free space (10) in front of each of the end heads (3). the radially outermost side of the electromagnetic induction system is at a radial distance from the internal surface of the mantle (2) comprised between 20 cm and 1 mm, more preferably at a radial distance comprised between 5 cm and 2 mm and, even more preferably, comprised between 2cm and 5mm. the electromagnetic induction system comprises a single inductor (H) in the form of a solenoid developed around the axis (x-x) of the Yankee, the solenoid being formed by coils concentric to said axis (x-x). the electromagnetic induction system comprises a single toroidal inductor (H) developed parallel to the axis (x-x) of the Yankee. the electromagnetic induction system includes a plurality of inductors (HN) each of which consists of a solenoid developed around the axis of the Yankee and formed by coil turns concentric to said axis. the electromagnetic induction system comprises a plurality of inductors (HN) axially placed side by side. the electromagnetic induction system comprises a plurality of inductors (HN), each of which consists of a solenoid formed by coil turns parallel to the inner surface of the mantle (2). the electromagnetic induction system comprises a plurality of inductors (HN) arranged circumferentially around the axis of the Yankee. the inductor (H) or the inductors (HN) are at a radial distance (S) from the inner surface of the mantle (2), the inductors are formed by coils (HS) consisting of conductors with a predefined diameter (d) spaced apart by a pitch of predefined value (P), and the value of the said radial distance (S) is less than or equal to the difference between said pitch (P) and said diameter (d). the inductor (H) or the inductors (HN) are at a radial distance (S) from the inner surface of the mantle (2), the inductors are formed by coils (HS) consisting of conductors with a predefined diameter (d) spaced apart by a pitch of predefined value (P), and the value of the said radial distance (S) is about equal to the difference between said pitch (P) and said diameter (d). the pitch between the turn coils (HS) is adjustable. In practice, the execution details can in any case vary in an equivalent way as regards the individual elements described and illustrated, without thereby departing from the scope of the solution adopted and therefore remaining within the limits of the protection granted by the present patent in accordance with the attached claims.