Cylinder for drying a fibrous web

Abstract

The invention relates to a cylinder (1) for drying a fibrous web, comprising a cylinder shell (2), wherein an electric heating device (3) for heating the cylinder (1) is arranged within the cylinder (1). It is characterized in that the electric heating device (3) is arranged on the inner side of the cylinder shell (2) with the formation of a conductively thermally conducting connection, wherein the electric heating device (3) comprises at least one electric heating element (4) with carbon nanotubes. A cylinder with a heating device with improved efficiency is thus provided.

Description

[0001] CYLINDER FOR DRYING A FIBRE WEB

[0002] The invention relates to a cylinder for drying a fibrous web, comprising a cylinder jacket with an outer side for guiding the fibrous web to be dried and an inner side, wherein an electric heating device for heating the cylinder is arranged within the cylinder.

[0003] In pulp and paper production, fibrous webs are typically dried using steam-heated cylinders. Steam-heated cylinders are characterized by their high heat output. This results in even temperature distribution across the cylinder surface, although the edge area of ​​the cylinder jacket often has an excessively high temperature. Steam-heated cylinders must be designed as pressure vessels, which requires thicker cylinder walls, which is detrimental to heat transfer. The actual steam generation occurs during combustion processes and is therefore criticized for environmental reasons. Steam cylinders can be combined into groups, with each group being characterized by a specific steam pressure or temperature. This allows for profiling of the drying process in the machine direction by adjusting the heating output of the individual groups accordingly.However, profiling of the drying in the cross-machine direction is not possible using the steam-heated cylinders.

[0004] Also well-known are infrared dryers, which are characterized by radiant heat transfer rather than direct conduction. These infrared dryers include gas-fired dryers as well as electrically powered radiators.

[0005] The aim of the invention is to provide a cylinder for drying a fibrous web with an electric heating device with improved efficiency. A further aim of the invention is to improve the profile control of the heat transfer through the cylinder.

[0006] This is achieved according to the invention in that the electrical heating device is arranged on the inside of the cylinder jacket, forming a conductive heat-conducting connection, wherein the electrical heating device comprises at least one electrical heating element with carbon nanotubes. By arranging the electrical 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 is used for heat exchange between the cylinder jacket and the heating jacket. 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 recognized that, particularly with direct contact between the heating device and the inside of the cylinder jacket - ieby forming a conductive, thermally conductive connection, the heat flows required for drying can be achieved. It has also been recognized that the use of an electric heating element comprising carbon nanotubes allows the realization of larger heat flows, while providing excellent thermal conductivity and, in particular, improved efficiency in the use of heat from electricity. This allows the required temperatures of 90°C-130°C to be achieved on the outside of the cylinder jacket. Typically, the cylinder is made of steel or cast iron, as steel and cast iron have advantageous properties with regard to thermal conductivity and mechanical strength, respectively. This allows the creation of thin-walled cylinders that withstand mechanical stress and exhibit good heat transfer. Cylinder diameters in the range between 1500 mm and 1800 mm with wall thicknesses of less than 30 mm or less than 25 mm are also typical.

[0007] An advantageous embodiment of the cylinder is characterized in that the electric heating device, in particular the at least one electric heating element, is integrally connected to the inside of the cylinder casing, in particular by adhesive bonding, or is non-positively connected, in particular by being pressed against the inside of the cylinder casing. In both cases, advantageous heat transfer is ensured, with the heat transfer occurring reliably, precisely, and evenly.

[0008] In a further advantageous embodiment, the at least one electrical heating element is designed as a heating mat. This design as a heating mat allows for quick and easy coverage of the entire interior of the cylinder shell. In an equally advantageous cylinder design, the heating mat consists essentially of silicone with embedded carbon nanotubes. Such heating mats allow for a homogeneous temperature distribution, with a particularly homogeneous temperature distribution being achieved by arranging several heating mats in abutting relationship.

[0009] A further advantageous embodiment of the cylinder is characterized in that the electric heating device comprises insulation, with the at least one electric heating element being arranged between the insulation and the inside of the cylinder casing. This advantageous arrangement of the insulation allows heat loss to be avoided, with heat being transported primarily from the heating element toward the inside of the cylinder casing. The insulation of the heating device can be designed as a replacement for or as a supplement to the known cylinder cover insulation.

[0010] Equally advantageous is an embodiment of the cylinder wherein the heating device comprises at least a first and a second zone, and the zones each comprise at least one electrical heating element with carbon nanotubes, wherein the zones are arranged in the axial direction of the cylinder on the inside of the cylinder jacket, and a first heating power of the first zone can be adjusted independently of a second heating power of the second zone. Advantageously, the invention allows profile control of the heat transfer through the cylinder in the axial direction. When the electrical heating elements are arranged in the axial direction, the axial profile can be adjusted by specifically adjusting the first or second heating power. According to the invention, the course of the surface temperature of the cylinder can thus be easily controlled.

[0011] An advantageous embodiment of the cylinder is characterized in that it comprises covers connected to the cylinder jacket at the end face, wherein a connection for supplying the electric heating device with electric current is led through at least one cover. Since the cylinder is typically rotatable and the power supply takes place via a non-rotatable environment, the connection for supplying the electric heating device with electric current is advantageously led through at least one cover. For example, the cover can also have pins, wherein the connection for the supply is led through the pin. Advantageously, the connection further comprises a slip ring body that moves with the cylinder, wherein the slip ring body is 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 in order to establish an electrical connection of the heating device to an electrical power source.

[0012] The invention further relates to a cylinder group comprising at least a first and a second cylinder according to the invention, wherein the electric heating device of the first cylinder can be controlled independently of the electric heating device of the second cylinder. Advantageously, the first and second cylinders are thus independent of each other.

[0013] The invention will now be described with reference to Figure 1. Figure 1 shows a cylinder 1 according to the invention for drying a fibrous web, having a cylinder jacket 2. A fibrous web to be dried can be guided on the outer side of the cylinder jacket 2. An electrical heating device 3 for heating the cylinder 1 is arranged within the cylinder 1, wherein the electrical heating device 3 is arranged on the inner side of the cylinder jacket 2 to form a conductive, heat-conducting connection, and the electrical heating device 3 comprises at least one electrical heating element 4 with carbon nanotubes. Advantageously, the at least one electrical heating element 4 is designed as a heating mat, wherein the heating mats can be installed very easily on the inner side of the jacket 2.It is also advantageous if the electrical 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 jacket, wherein a first heating output of the first zone 5 can be adjusted independently of a second heating output of the second zone 6. This allows zonal regulation and thus cross-profiling of the heating output of the cylinder. The cylinder 1 can be designed with a cover 7, wherein the covers 7 are connected to the cylinder jacket 2 at the end face. Advantageously, the electrical heating device 3 is supplied with electrical current via a connection which is guided through at least one cover.

[0014] Typically, the connection further comprises a slip ring body that moves with the cylinder 1 and is designed to receive electrical current. The present invention offers numerous advantages; in particular, the cylinder has an electric heating device with improved efficiency, whereby the heat flows required for drying a fibrous web can be realized. According to the invention, a homogeneous temperature distribution on the outside of the cylinder jacket is adjustable, but axial profiling of the heat transfer through the cylinder is also possible. In particular, the electric heating elements designed as heating mats are easy to install.

[0015] Reference symbol

[0016] 1 cylinder

[0017] 2 cylinder jacket

[0018] 3 Electric heater

[0019] 4 Electric heating element

[0020] 5 First Zone

[0021] 6 Second Zone

[0022] 7 lids

Claims

Claims: Cylinder (1) for drying a fibrous web, comprising a cylinder shell (2) with an outer surface for guiding the fibrous web to be dried and an inner surface, wherein an electric heating device (3) for heating the cylinder (1) is arranged inside the cylinder (1), characterized in that the electric heating device (3) is arranged on the inner surface of the cylinder shell (2) forming a conductive-heat-conducting connection, wherein the electric heating device (3) comprises at least one electric heating element (4) with carbon nanotubes. Cylinder (1) according to claim 1, wherein the electric heating device (3), in particular the at least one electric heating element (4), is materially bonded and in particular glued to the inner surface 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 force-fitted to the inside of the cylinder shell (2) and is in particular pressed against the inside of the cylinder shell (2). Cylinder (1) according to any one of the preceding claims, wherein the at least one electric heating element (4) is designed as a heating mat. Cylinder (1) according to claim 4, wherein the heating mat consists essentially of silicone with embedded carbon nanotubes. Cylinder (1) according to any one of the preceding claims, wherein the electric heating device (3) comprises insulation and the at least one electric heating element (4) is arranged between the insulation and the inside of the cylinder shell (2).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). Cylinder (1) according to one of the preceding claims comprising covers (7) connected to the end face of the cylinder shell (2), wherein at least one The lid (7) has a connection for supplying the electric heating device (3) with electric current. A cylinder (1) according to claim 8, wherein the connection comprises a slip ring body moving with the cylinder (1) and the slip ring body is designed to receive electric current. A 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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