Cylinder for drying a strip of fibrous material

Abstract

The invention relates to a cylinder (1) for drying a fibrous network, comprising a cylindrical housing (2) in which an electric heating device (3) is arranged for heating the cylinder (1). It is characterized in that the electric heating device (3) is arranged on the inner face of the cylindrical housing (2), forming a thermally conductive connection, and comprises at least one electric heating element (4) with carbon nanotubes. In this way, a cylinder with an improved heating device is obtained.

Description

[0001] The invention relates to a cylinder for drying a fibrous web, comprising a cylinder shell with an outer surface for guiding the fibrous web to be dried and an inner surface, wherein an electric heating device for heating the cylinder is arranged inside the cylinder.

[0002] In the pulp and paper industry, drying of fiber webs typically takes place using steam-heated cylinders. Steam-heated cylinders are characterized by high heat output. A uniform temperature distribution is achieved across the cylinder surface, although the outer edge of the cylinder wall often exhibits excessively high temperatures. Steam-heated cylinders must be designed as pressure vessels, which necessitates thicker cylinder walls, thus compromising heat transfer. The actual steam generation occurs through combustion processes and is therefore criticized for environmental reasons. Steam cylinders can be grouped together, with each group defined by a specific steam pressure or temperature. This allows for profiling of the drying process along the machine direction by adjusting the heating output of the individual groups.However, profiling the drying process in the transverse direction of the machine is not possible using the steam-heated cylinders.

[0003] Infrared dryers are also well-known; they are characterized by radiant heat transfer and do not rely on direct heat conduction. These infrared dryers include both gas-powered dryers and electrically operated radiators.

[0004] US 362 4353 A discloses a paper web drying cylinder comprising a cylindrical shell with several axial, annular regions arranged along the inner surface of the shell, wherein the annular regions include an electrical resistor and an adjustable electric current can be supplied to the resistor to provide an axial temperature gradient.

[0005] DE 1020 1320 4009 A1 further discloses a heating device comprising a support layer and a heating layer, wherein the thin heating layer is located on the support layer and the support layer is designed to be thermally insulating. This solution allows for a reduction in energy consumption by reducing the heated mass.

[0006] The object of the invention is to provide a cylinder for drying a fibrous web with an electric heating device offering improved efficiency. A further object of the invention is to improve the profile control of heat transfer through the cylinder.

[0007] According to the invention, this is achieved by arranging the electric heating device on the inside of the cylinder jacket, forming a conductive-heat-conducting connection, wherein the electric heating device comprises at least one electric heating element with carbon nanotubes. By arranging the electric heating device directly on the inside of the cylinder jacket, forming a conductive-heat-conducting connection, energy losses are avoided, and the maximum available surface area for heat exchange between the cylinder jacket and the heating jacket is utilized. In contrast, with heating elements arranged at a distance from the inner surface of the cylinder jacket, the heat flux density between the heating element and the cylinder jacket is reduced. Furthermore, it was found that, especially with direct contact of the heating device with the inside of the cylinder jacket—i.e.,By forming a conductive-thermal connection, the heat flows necessary for drying can be achieved. It has also been recognized that the use of an electric heating element comprising carbon nanotubes allows for the realization of larger heat flows, offering excellent thermal conductivity and, in particular, improved efficiency in the utilization of heat from electricity. This enables the required temperatures of 90°C–130°C to be reached on the outer surface of the cylinder. Typically, the cylinder is made of steel or cast iron, as these materials offer advantageous properties in terms of thermal conductivity and mechanical strength, respectively. This allows for the production of thin-walled cylinders that withstand mechanical stresses and exhibit good heat transfer. Cylinder diameters typically range from 1500 mm to 1800 mm, with wall thicknesses of less than 30 mm or less than 25 mm.

[0008] An advantageous embodiment of the cylinder is characterized in that the electric heating device, in particular the at least one electric heating element, is bonded to the inside of the cylinder shell, either by material contact, particularly by adhesive bonding, or by frictional contact, particularly by being pressed against the inside of the cylinder shell. In both cases, advantageous heat transfer is ensured, with the heat transfer being reliable, precise, and uniform.

[0009] In a further advantageous embodiment, at least one electric heating element is designed as a heating mat. This design as a heating mat allows for quick and easy coverage of the entire inside of the cylinder shell.

[0010] In an equally advantageous embodiment of the cylinder, the heating mat consists essentially of silicone with embedded carbon nanotubes. Such heating mats allow for a homogeneous temperature distribution, and a particularly homogeneous temperature distribution can be achieved by arranging several heating mats adjacent to each other.

[0011] Another advantageous embodiment of the cylinder is characterized in that the electric heating device comprises insulation, wherein the at least one electric heating element is arranged between the insulation and the inside of the cylinder shell. The advantageous arrangement of the insulation allows for the prevention of heat loss, with heat transfer occurring primarily from the heating element towards the inside of the cylinder shell. The insulation of the heating device can be implemented as a replacement for, or in addition to, the known cylinder lid insulation.

[0012] An equally advantageous embodiment of the cylinder is one in which the heating device comprises at least a first and a second zone, and each zone includes at least one electric heating element with carbon nanotubes, wherein the zones are arranged axially on the inside of the cylinder shell, and the first heating power of the first zone is adjustable independently of the second heating power of the second zone. The invention thus allows for profile control of the heat transfer through the cylinder in the axial direction. With the electric heating elements arranged axially, the axial profile can be adjusted by selectively setting the first or second heating power. According to the invention, the surface temperature profile of the cylinder can therefore be easily controlled.

[0013] An advantageous embodiment of the cylinder is characterized by the inclusion of covers connected to the cylinder shell at its end face, wherein at least one cover provides a connection for supplying the electric heating device with electric current. Since the cylinder is typically rotatable, and the power supply is provided via a non-rotating environment, the connection for supplying the electric heating device with electric current is advantageously routed through at least one cover. For example, the cover can also have pins, with the connection for the supply routed through the pin. Advantageously, the connection further includes a slip ring body that moves with the cylinder, the slip ring body being designed to receive electric current.For example, stationary contact pins can be arranged in the area of ​​the non-rotating environment, which are in contact with the slip ring body to establish an electrical connection between the heating device and an electrical power source.

[0014] The invention further relates to a cylinder group comprising at least a first and a second cylinder according to the invention, wherein the electrical heating device of the first cylinder is controllable independently of the electrical heating device of the second cylinder. Advantageously, the first and second cylinders are thus independent of each other.

[0015] The invention will now be demonstrated using the Figure 1 described. Figure 1Figure 1 shows a cylinder 1 according to the invention for drying a fibrous web, comprising a cylindrical shell 2. A fibrous web to be dried can be guided on the outside of the cylindrical shell 2. An electric heating device 3 for heating the cylinder 1 is arranged inside the cylinder 1, wherein the electric heating device 3 is arranged on the inside of the cylindrical shell 2 forming a conductive-heat-conducting connection and the electric heating device 3 comprises at least one electric heating element 4 with carbon nanotubes. Advantageously, the at least one electric heating element 4 is designed as a heating mat, whereby the installation of the heating mats on the inside of the shell 2 can be carried out very easily.It is also advantageous if the electric heating device 3 comprises at least a first and a second zone 5, 6, wherein the zones 5, 6 are arranged offset in the axial direction of the cylinder on the inside of the cylinder shell, and wherein a first heating power of the first zone 5 is adjustable independently of a second heating power of the second zone 6. This allows zonal regulation and thus cross-profiling of the cylinder's heating power. The cylinder 1 can be equipped with a cover 7, wherein the cover 7 is connected to the cylinder shell 2 at its end face. Advantageously, the electric heating device 3 is supplied with electric current via a connection that passes through at least one cover. Typically, the connection further comprises a slip ring body that moves with the cylinder 1 and is designed to receive electric current.

[0016] The present invention offers numerous advantages; in particular, the cylinder features an electric heating device with improved efficiency, enabling the heat flows necessary for drying a fibrous web to be achieved. According to the invention, a homogeneous temperature distribution on the outside of the cylinder shell can be set, and axial profiling of the heat transfer through the cylinder is also possible. In particular, the electric heating elements, designed as a heating mat, can be easily installed. Reference sign

[0017] 1 Cylinder 2 Cylinder casing 3 Electric heating device 4 Electric heating element 5 First zone 6 Second zone 7 Cover

Claims

1. Cylinder (1) for drying a fibrous web, comprising a cylinder shell (2) with an outer side for guiding the fibrous web to be dried and an inner side, wherein an electric heating device (3) for heating the cylinder (1) is arranged inside the cylinder (1), whereby the electric heating device (3) is arranged on the inner side of the cylinder shell (2), forming a conductive thermal connection, characterised in that the electric heating device (3) comprises at least one electric heating element (4) with carbon nanotubes and is designed to generate a temperature of 90°C-130°C on the outside of the cylinder shell.

2. Cylinder (1) according to claim 1, wherein the electric heating device (3), in particular the at least one electric heating element (4), is bonded, in particular glued, to the inside of the cylinder shell (2).

3. Cylinder (1) according to claim 1, wherein the electric heating device (3), in particular the at least one electric heating element (4), is non-positively connected to the inside of the cylinder shell (2) and, in particular, is pressed against the inside of the cylinder shell (2).

4. Cylinder (1) according to one of the preceding claims, wherein the at least one electric heating element (4) is designed as a heating mat.

5. Cylinder (1) according to claim 4, wherein the heating mat consists essentially of silicone with embedded carbon nanotubes.

6. Cylinder (1) according to one of the preceding claims, wherein the electric heating device (3) comprises an insulation and that at least one electric heating element (4) is arranged between the insulation and the inside of the cylinder shell (2).

7. Cylinder (1) according to one of the preceding claims, wherein the electric heating device (3) comprises at least a first and a second zone (5,6), and the zones (5,6) each comprise at least one electric heating element (4) with carbon nanotubes, wherein the zones (5,6) are arranged offset in the axial direction of the cylinder on the inside of the cylinder shell, and a first heating power of the first zone (5) is adjustable independently of a second heating power of the second zone (6).

8. Cylinder (1) according to one of the preceding claims, comprising end covers (7) connected to the cylinder shell (2) on the end face, wherein a connection for supplying the electric heating device (3) with electrical current is routed through at least one end cover (7).

9. Cylinder (1) according to claim 8, wherein the connection comprises a slip ring body that moves with the cylinder (1) and the slip ring body is designed to carry electric current.

10. Cylinder group comprising at least a first and a second cylinder (1) according to one of the preceding claims, wherein the electric heating device (3) of the first cylinder (1) can be operated independently of the electric heating device (3) of the second cylinder (1).

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