Machine for the production of paper

Abstract

Machine for the production of paper, comprising a drying zone (EZ) configured to supply heat to a sheet of fibrous material (S), wherein in the drying zone (EZ) the sheet of fibrous material (S) follows a predefined path along a direction (MD) crossing the drying zone (EZ), and wherein in the drying zone (Z) is provided a thermal regulation system configured to adjust the amount of heat supplied to the sheet of fibrous material (S) along a direction (CD) transverse to said crossing direction (MD); the thermal regulation system comprises a plurality of electrical heating devices (2) which can be selectively activated and deactivated and are arranged each at a respective position along said transversal direction (CD) for supplying heat to the sheet of fibrous material (S) realizing a thermal profile transverse to the sheet of fibrous material (S) depending on the activation or modulation or deactivation condition of the individual electrical heating devices (2).

Description

[0001] TITLE

[0002] Machine for the production of paper.

[0003] DESCRIPTION

[0004] The present invention relates to the production of paper, in particular tissue paper.

[0005] More specifically, the present invention concerns the realization of thermal regulation systems that can be used in paper production.

[0006] It is known that the machines for the production of tissue paper generally have a sheet forming section and several sections intended for removing water from the formed sheet. Generally, the actions that lead to the production of a sheet with a correct degree of dryness are the following:

[0007] - water drainage by centrifugation and vacuum effect;

[0008] - pressing of the sheet in contact with a felt;

[0009] - drying.

[0010] It is generally believed that a correct degree of dryness is, on average, around 95% (per 100g of product, 95 g of fibers and 5 g of residual moisture).

[0011] The present invention particularly concerns the last of the steps listed above, i.e. drying.

[0012] In the most performing machines, configured to ensure high production rates, high- temperature blowing hoods are used, consisting of two semi-hoods arranged around a Yankee dryer on the upper part of the latter. The semi-hoods are distinguished based on the zone of the Yankee around which they are arranged: the half-hood surrounding the inlet part where the adhesion of the paper to the Yankee occurs is referred to as "wet end hood " or “WE hood” because in this zone the paper has a greater quantity of water compared to the exit zone, while the semi-hood surrounding the detachment part of the paper exiting the Yankee is indicated as "dry end hood " or “DE hood ” since the paper reaching this zone has already undergone a reduction in water content by passing through the the WE semi-hood. The two half-hoods are arranged one after the other, with respect to the overall direction followed by the sheet being produced, so as to surround a portion of the cylindrical surface of the Yankee and are placed a few millimeters from the sheet of paper adhered to the surface of the same Yankee. Each of the two half-hoods has distributed inlet and outlet passages which allow hot air to flow directed against the paper and removal of the air that has transferred heat to the sheet and has become loaded with the humidity removed from the sheet.

[0013] The hoods used in high-performance tissue machines are part of a larger system, normally called "air system" in which are implanted ducts for supplying hot air to the WE and DE hoods, air heating systems and suitable waste heat recovery systems to increase the energy efficiency of the machine. The hoods can be divided into sections aligned along the so-called "cross direction" or "CD", i.e. along a direction parallel to the axis of rotation of the Yankee, and inside each section there is a damper which allows the flow of hot air directed towards the sheet of paper transported by the Yankee to be partitioned. The dampers are used to adjust the humidity profile of the sheet by reducing the quantity of hot air directed on those sections where the sheet is drier (i.e. in the presence of the so-called dry bands). However, this system is inadequate in the case of wet bands, i.e. when some areas of the sheet are wetter than others. In this case the only available possibility is to reduce the flow rate in all sections except the one corresponding to the most humid area, to increase the energy input in this area compared to the other parts of the sheet. However, this method has little effectiveness and is energetically unfavorable.

[0014] US2002 / 0152630A1 describes a system for heating fibrous webs in which the fibrous webs are compressed between two belts in a compression zone where they are also subjected to a temperature gradient whose purpose is to promote the removal of water.

[0015] CA211994A1 describes a hollow roller used in the production of paper material, internally provided with a heating device.

[0016] GB886589 describes a process for drying a material to produce corrugated cardboard, in which use is made of heating plates which are arranged along a path followed by said material and are mounted on supports allowing them to be moved away or closer to said path to regulate the quantity of heat transmitted to the material itself.

[0017] The main aim of the present invention is to obtain a system allowing the localized increase (i.e. only in the sections that require greater drying) of the drying capacity through the localized increase in the air temperature.

[0018] In this way the hood equipped with a thermal profiling system will have the possibility of correcting both positively and negatively the drying capacity of the individual sections, with the result of improving the quality of the finished product and increasing production without any reduction in energy efficiency.

[0019] This result was achieved, in accordance with the present invention, by adopting the idea of realizing a system having the features indicated in claim 1. Other features of the present invention are the subject of the dependent claims.

[0020] These and further advantages and characteristics of the present invention will be more and better understood by every person skilled in the art thanks to the following description and the attached drawings, provided by way of example but not to be considered in a limiting sense, in which: - Fig.1 represents a schematic top plan view of a paper production machine which can be provided with a thermal regulation system in accordance with the present invention;

[0021] - Fig.2 represents a schematic front view of the drying area of the machine shown in Fig.1;

[0022] - Fig.3 and Fig.4 schematically represent two possible operating conditions of the machine of Fig.l in proximity of the drying area;

[0023] - Fig.5 schematically represents the internal structure of a blowing hood in which can be arranged a thermal regulation system in accordance with the present invention;

[0024] - Fig.6 represents a detail relating to a sector of the structure of Fig.5;

[0025] - Fig.7 is a simplified block diagram relating to a possible configuration of an automatic control circuit for a thermal regulation system in accordance with the present invention;

[0026] - Fig.8 schematically represents a possible shape of the path of the sheet (S) in the drying station in correspondence with a hood to which is associated a thermal regulation system in accordance with the present invention;

[0027] - Fig.9 is a diagram that schematically represents a possible distribution of the thermal power provided by a thermal regulation system in accordance with the present invention in the "CD" direction.

[0028] The following description provides examples of implementation of the present invention.

[0029] For example, a thermal regulation system in accordance with the present invention can be associated with the blowing hoods used in paper production machines comprising, according to a per se known configuration represented in the diagram of Fig.l and Fig.2:

[0030] - a forming zone (FZ), in which a sheet (S) of fibrous material is formed;

[0031] - a drainage zone (DZ), in which part of the water contained in the sheet of fibrous material is drained while the sheet is moved away from the formation zone (FZ);

[0032] - a pressing zone (PZ), in which the sheet is subjected to compression to obtain further removal of water from the sheet; and

[0033] - a drying zone (EZ), arranged downstream of the other zones with respect to the direction (MD) followed by the sheet (S) inside the system, in which are arranged means adapted for supplying thermal energy to the sheet to cause the evaporation of residual water from the same sheet.

[0034] The means arranged in said zones (FZ, DZ, PZ, EZ) to obtain the formation of the sheet (S), the drainage, pressing and drying of the same are per se known to those skilled in the art and, therefore, not will be described in further detail. In the drying zone (EZ) a heating unit is arranged, such as a Yankee (Y). Typically, a Yankee is a heating body that has a cylindrical surface, on which the sheet (S) is guided, and from which it receives heat, while it is directed towards a collection station (CZ) arranged downstream of the drying area, where it is wound into a reel.

[0035] A hood (HO) is installed on the heating unit (Y) arranged in the drying zone (EZ), typically made up of two units, indicated as semi-hoods (HW, HD) in this description. A semi-hood (HW) is on the wet side, i.e. on the entry side of the sheet into the drying zone (EZ), and a semi-hood (HD) is on the dry side, i.e. on the exit side of the sheet from the same zone (EZ). The semi-hoods (HW, HD) have respective inlets and outlets which allow hot air to flow directed against the sheet (S) guided on the heating unit (Y), and removal of the air which has supplied heat to the sheet and has become loaded with humidity removed from the sheet itself due to the heating carried out by the heating unit (Y) and by the hot air directed against the sheet through the half-hoods. The half-hoods (HW, HD) are served by an air circuit which, similarly to what is provided in known paper production plants, is configured to supply hot air to each half-hood (HW, HD) and respectively to extract humid air from the drying zone as previously mentioned. Typically, the air circuit is divided into two sides, i.e. a wet side circuit serving the semi-hood (HW) on the wet side and a dry side circuit serving the semi-hood (HD) on the dry side. With reference to the exemplary diagram in Fig.2, an air circuit (AC) is arranged, including two sub-circuits (WC, DC) each of which feeds air to a respective halfhood (HW, HD) via respective supply ducts (1C, C2) and extracts humid air from the same semi-hood via respective humid air extraction ducts (1W, C8). Generally, the air extracted through the ducts (1W, C8) is conveyed to respective gas burners (or other heating systems, not visible in the drawings) for the production of hot air sent to the semi-hoods through the supply ducts (1C, C2). The air circuit (AC) has an exhaust duct (12) through which the exhausted air, i.e. the air that has exhausted its function as a thermal carrier within the same circuit (AC), is discharged to the outside.

[0036] It is understood that the sequence of the said zones (DZ), (PZ) and (EZ) could be different from that represented in the diagram of Fig.1, i.e. devices adapted to remove the water from the sheet through drainage, vacuum, pressing or heat supply could be arranged differently from what is represented in Fig.l. The sequence shown in Fig.l is therefore indicative of a possible configuration of a paper production machine in which can be installed a blowing hood with a thermal regulation or profiling system in accordance with the present invention. The diagram in Fig.2 is intended to represent a possible example of the use of a blowing hood in a paper production machine. It is understood, therefore, that the specific configuration of the air circuit and the individual elements that compose it may vary depending on specific plant requirements. More generally, the present invention applies to all paper production machines in which a temperature regulation system acting in a transversal direction with respect to the path followed by the sheet is provided.

[0037] For example, a temperature regulation system in accordance with the present invention can be associated with the half-hoods of a hot air blowing and suction unit placed around a drying cylinder.

[0038] The half-hoods (HW, HD) are divided into sectors (SE) aligned along the "cross direction" (CD) oriented transversely with respect to the direction (MD) followed by the sheet (S). With reference to the example previously described, said sectors (SE) are aligned along a direction parallel to the rotation axis (AY) of the Yankee (Y). Each sector (SE) is connected on its upper surface to a respective duct (1C; C2) via a corresponding duct fitting (TC). Moreover, each sector (SE) has a substantially box-like structure delimited by two lateral surfaces (SI), an upper surface (S2), a rear surface (S3), a front surface (S4) and a lower surface (S5). The side surfaces (SI), the rear surface (S3) and the bottom surface (S5) are closed surfaces, i.e. without openings. The duct fitting (TC) is formed on the upper surface (S2). The front surface (S4), facing the path followed by the sheet (S), is provided with openings (Al) allowing the hot air to be directed towards said path. The sectors (SE) are in a containment structure (SC) that forms a space within which the same sectors are located and in which the extraction ducts (1W, C8) operate. Said containment structure (SC), according to a per se known configuration, is closed at the top, rear, bottom and on both sides, while it is open on the side facing the path followed by the sheet (S). Since the sectors (SE) are spaced apart from each other, while hot air (HA) is blown towards the sheet (S) through the openings formed on the front side of the sectors (SE), simultaneously humid air (WA) is sucked by the ducts (1W, C8) through the space (HS) between the sectors (SE).

[0039] Advantageously, an electric heater (2), i.e. an electric heating device, is arranged within a pre- established number of sectors (SE) of one or both half-hoods (HW, HD). Preferably, an electric heater (2) is arranged inside a pre-established number of sectors (SE) of both halfhoods (HW, HD). Electric heaters (2) can also be provided in each sector (SE) of both halfhoods (HW, HD).

[0040] In the configuration exemplified above, in which a drying cylinder (Y) is used in the drying station (EZ), the electric heaters (2) are on the opposite side of the drying cylinder with respect to the sheet (S) that, therefore, can receive heat, from two opposite sides, emitted by both the drying cylinder and the electric heaters.

[0041] For example, said electric heaters (2) are electrical resistors that can be shaped as cylindrical bars, smooth plates, finned plates and, more generally, conveniently shaped to be positioned inside the sectors (2). Or, for example, said heaters (2) can be electric infrared heaters (for example, infrared ceramic heaters or quartz heaters).

[0042] By controlling the thermal power emitted by each electric heater (2) within the respective sector (SE) it is possible to effectively adjust the heat distribution along the cross direction (CD) on the path followed by the sheet (S). Depending on the desired heat distribution, all the electric heaters (2) or, selectively, a part of them can be activated.

[0043] The electric heaters (2) are preferably controlled by a control unit (20) which receives signals from a scanner (21), known per se, that is arranged between the drying zone (EZ) and the collection station (CZ) and is configured to detect residual moisture in the sheet (S) along the cross direction (CD) downstream of the drying zone (EZ). In this way, it is possible to realize a heating system that is activated automatically according to the humidity profile of the sheet (distribution or pattern of the residual humidity in the sheet along the cross direction) downstream of the drying zone.

[0044] The selective activation of the individual heaters (2) allows the heat directed towards the sheet (S) to be distributed according to a criterion that takes into account how the humidity is distributed in the sheet itself along the cross direction (CD), providing greater heat in the areas where residual humidity is greater. For example, with reference to the diagram in Fig.3, the scanner (21) detects excessive residual humidity compared to a pre-established value (for example, a residual humidity greater than 5%) along the longitudinal edges (LS) of the sheet (S) and the control unit (20) activates only the electric heaters of the outermost sectors of both half-hoods (HW, HD). Again, for example, with reference to the diagram in Fig.4, the scanner (21) detects excessive residual humidity compared to a pre-established value (for example, a residual humidity greater than 5%) along the central part (CS) of the sheet (S) and the control unit (20) activates only the electric heaters (2) of the central sectors of both half-hoods (HW, HD).

[0045] For example, commercially available microwave scanners can be used which exploit the ability of the water contained in the sheet to absorb radiation with a wavelength typical of the microwave field. By emitting microwaves and measuring the microwaves that pass through the sheet, it is possible to trace the energy absorbed by the sheet, which is greater the higher the degree of humidity. Indirectly, this measurement allows the moisture content to be measured with great speed and good precision.

[0046] Alternatively, or in addition to the automatic mode, the electric heaters (2) can be selectively activated in manual mode by the operators responsible for operating the machine on the basis of the data provided by the scanner (21). The thermal profiling system described above, comprising the electric heaters (2) acting on the path followed by the sheet (S) in the drying zone, can be associated with a control of the flow rate of the hot air introduced into the sectors (SE). For example, with methods known to those skilled in the art, this control can be achieved by flow adjustment carried out through dampers (3) positioned in the sectors (SE). The dampers (3) are surfaces which, depending on their orientation, allow the flow rate of hot air to be adjusted in the individual sectors (SE). The position of the dampers (3) is adjusted by respective actuators (30). In the maximum opening position, the dampers (3) allow the maximum flow of hot air towards the sheet (S). Conversely, in the closed position the dampers (3) reduce the flow of hot air towards the sheet (S) to a minimum value. Positions of the dampers (3) intermediate between the maximum opening and closing positions determine intermediate values of the flow of hot air towards the sheet (S). The electric heaters (2) can be positioned both upstream and downstream of the dampers (3) with respect to the direction of the hot air flow. It is understood that, whatever the structural configuration of the control system for the flow of hot air introduced into the sectors (SE), the electric heaters (2) are controlled independently: in practice, in any sector (SE) the respective electric heater (2) can be activated or deactivated regardless of the flow rate of hot air introduced into the same sector. For example, if the maximum heat input to the sheet (S) in correspondence with a given sector (SE) is desired, it will be possible to completely open the respective damper (3) and activate the respective electric heater (2) at the maximum power. Conversely, in order to minimize the heat input to the sheet (S) in correspondence with a given sector (SE), it will be possible to completely close the respective damper (3) and deactivate the respective electric heater (2). It will also be possible to finely adjust the heat input to the sheet (S) in correspondence with each sector (SE), for example by placing the respective damper in a given position and regulating the thermal power of the respective electric heater up to the desired value. In fact, the use of electric heaters allows for more precise and broader adjustments.

[0047] In a possible configuration of the present thermal profiling system, the actuators (30) are controlled by the same control unit (20) that controls the heaters (2) so as to realize an automatic control mechanism that manages both the heaters (2) and the dampers (3) in a coordinated way, based on the signals provided by the scanner (21) as schematically represented in Fig.7.

[0048] In accordance with the present invention, it is also possible to control the profile of the heat supply in the CD direction solely through electric heaters (2), i.e. without adjusting the flow rate of the hot air blown. In this case, for example, it is possible to set the power (P) emitted by each resistor (2) to a fixed value (for example a statistically predetermined value "M" based on experimental measurements) and modify that value, by increasing or decreasing it, based on the readings provided by the scanner (21) while the sheet (S) passes through the zone of the machine where the resistors (2) are installed. An exemplary graphic representation of said profile is provided in Fig.9 where the temperature values are shown on the ordinate axis (T) and the cross-direction (CD) is shown on the abscissa axis. In the example of Fig.9, the heaters arranged on the lateral parts of the path crossed by the sheet are at the "TM" temperature while the heaters arranged on the central part of the same path are at the "M" temperature.

[0049] From the previous description it is evident that a machine in accordance with the present invention is a paper production machine, comprising a drying zone (EZ) configured to supply heat to a sheet of fibrous material (S) coming from a forming zone (FZ) of the same machine and possibly subjected to drainage and pressing before reaching the drying zone (EZ), wherein in the drying zone (EZ) the sheet of fibrous material (S) follows a predefined path along a direction (MD) crossing the drying zone (EZ), and wherein in the drying zone (Z) is provided a thermal regulation system configured to adjust the amount of heat supplied to the sheet of fibrous material (S) along a direction (CD) transverse to said crossing direction (MD), wherein: said thermal regulation system comprises a plurality of electrical heating devices (2) which can be selectively activated and deactivated and are arranged each at a respective position along said transversal direction (CD) for supplying heat to the sheet of fibrous material (S) so as to realize a thermal profile transverse to the sheet of fibrous material (S) depending on the activation or deactivation condition of the individual electrical heating devices (2); in the drying zone (Z) there is a drying cylinder (Y) oriented along said transversal direction (CD) and placed in a position opposite to that of the electric heating devices (2) with respect to said path; on the drying cylinder (Y) there is a hood (HO) configured to blow hot air towards the path followed by the sheet of fibrous material (S) in correspondence with the drying cylinder (Y) and the electric heating devices (2) are positioned inside the hood (HO).

[0050] From the previous description it is also evident that a machine in accordance with the present invention can have one or more of the following further features, even combined with each other: the hood (HO) has a plurality of internal sectors (SE) each of which blows hot air towards a corresponding portion of the said path and the electrical heating devices (2) are arranged in said sectors (SE). inside each sector (SE) there is a device (3) for adjusting the flow rate of the hot air that passes through the sector itself. the dryer cylinder (Y) is a Yankee. the machine comprises a programmable control unit (20) programmed to selectively activate and deactivate the electrical heating devices (2) on the basis of data detected by a scanner (21) adapted to detect residual humidity in the sheet of fibrous material (S) downstream of the drying zone (Z).

[0051] In practice, the execution details can however vary in an equivalent way as regards the individual elements described and illustrated without departing from the idea of the solution adopted and therefore remaining within the limits of the protection granted to this patent in accordance with the following claims.

Claims

CLAIMS1) Machine for the production of paper, comprising a drying zone (EZ) configured to supply heat to a sheet of fibrous material (S) coming from a forming zone (FZ) of the same machine and possibly subjected to drainage and pressing before reaching the drying zone (EZ), wherein in the drying zone (EZ) the sheet of fibrous material (S) follows a predefined path along a direction (MD) crossing the drying zone (EZ), and wherein in the drying zone (Z) is provided a thermal regulation system configured to adjust the amount of heat supplied to the sheet of fibrous material (S) along a direction (CD) transverse to said crossing direction (MD), characterized in that: said thermal regulation system comprises a plurality of electrical heating devices (2) which can be selectively activated and deactivated and are arranged each at a respective position along said transversal direction (CD) for supplying heat to the sheet of fibrous material (S) so as to realize a thermal profile transverse to the sheet of fibrous material (S) depending on the activation or deactivation condition of the individual electrical heating devices (2); in the drying zone (Z) there is a drying cylinder (Y) oriented along said transversal direction (CD) and placed in a position opposite to that of the electric heating devices (2) with respect to said path; and on the drying cylinder (Y) there is a hood (HO) configured to blow hot air towards the path followed by the sheet of fibrous material (S) in correspondence with the drying cylinder (Y) and the electric heating devices (2) are positioned inside the hood (HO).2) Machine according to claim 1 characterized in that the hood (HO) has a plurality of internal sectors (SE) each of which blows hot air towards a corresponding portion of the said path and the electrical heating devices (2) are arranged in said sectors (SE).3) Machine according to claim 2 characterized in that inside each sector (SE) there is a device (3) for adjusting the flow rate of the hot air that passes through the sector itself.4) Machine according to claim 1 characterized in that the dryer cylinder (Y) is a Yankee.5) Machine according to any of the preceding claims characterized in that it comprises a programmable control unit (20) programmed to selectively activate and deactivate the electrical heating devices (2) on the basis of data detected by a scanner (21) adapted to detect residual humidity in the sheet of fibrous material (S) downstream of the drying zone (Z).

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