Electric heating – Heating devices – With heating unit structure
Reexamination Certificate
2000-07-31
2002-05-21
Walberg, Teresa (Department: 3742)
Electric heating
Heating devices
With heating unit structure
C219S543000, C219S552000, C219S541000, C399S333000
Reexamination Certificate
active
06392209
ABSTRACT:
Electric heating elements are used in different sectors to generate heat; they require high voltages in order to generate a sufficiently high temperature. These high voltages, however, can constitute safety risks, particularly when used to heat media or when in contact with the human body. Moreover, because of the materials used in them, most traditional heating elements are suitable only for low temperatures, particularly in long-term operation. Other proposals of the prior art require a complex constitution of the heating element und hence limit possible applications of the heating element.
It is one object of the present invention to provide a heating element with which a high output per unit area and thus high temperatures can be generated even in long-term operation while low voltages prevail in the heating element. In addition, the heating element should be versatile in its applications and simple to provide with contact terminals. This is especially important for hollow structures to be heated such as pipes, transportation devices, e.g. heatable tanks or other containers, or heat roller shells.
Pipes are extensively employed, for instance, to conduct fluid media. When such pipes for instance are laid underground or as open-air piping in cold regions, the risk exists that the medium present in the pipe solidifies because of the low temperatures, and the pipe clogs.
Media such as gases or liquids often are transported in heatable tanks mounted on railway cars or on trucks. At low ambient temperatures, the medium in the tank can freeze and thus may even damage the tank. The installation of heating elements in such cars is highly demanding with respect to the heating element as well as to the heat transfer that can occur between the heating element and the car. Dangerous substances sometimes are transported in such tanks. It is important then that the heating element will not lead to any local temperature increase. But also a failure of the heating element, for instance as a result of its detachment from the tank, must be avoided in order to prevent freezing of the medium.
It is therefore a further object of the present invention to provide a heating element for a transportation device in which during transport a medium can be kept at a predetermined temperature, without creating safety risks such as freezing, an explosion or a fire.
Heat rollers which can be heated to a certain temperature, particularly for use in copying or foil-coating machines, are required in many areas of heating technology. Up to now such heat rollers have been produced with heating elements having resistance wires embedded in an insulating mass. Another operating mode of heat rollers, for instance in copiers, is the installation of a halogen emitter in the roller. Both of these versions have the disadvantage of being either very expensive in their manufacture or exhibiting a poor efficiency of heat transfer.
It is therefore a still further object of this invention to provide a heating element for a heat roller of simple design that can be operated with low voltage and at the same time has a high heat transfer efficiency. The heat roller should further be versatile in its applications.
The invention is based on the realization that all these objects can be reached by a heating element in which the heating current flows in an optimum way through a suitable resistive mass, the heating element being of flat shape and guaranteeing a heat transfer that is uniform across the area to be heated.
According to the invention, the objects are reached by a flat heating element comprising a thin resistance layer which contains an electro-conductive polymer and at least two flat electrodes arranged on one side of the resistance layer at a distance from each other, wherein the polymer has an intrinsic electric conductivity caused by a content of at least one metal or semimetal atom dopant.
These polymers which, according to the invention, are used in the resistance layer have a constitution such that the current flows along the polymer molecules. Owing to the polymer structure, the, heating current is conducted through the resistance layer along the polymers. Because of the electric resistance of the polymers, heat is generated which can be transferred to an object to be heated. Here the heating current cannot follow the shortest pathway between the two electrodes but follows the structure of the polymer arrangement. Thus, the length of the current path is predetermined by the polymers, so that even in the instance of small layer thicknesses, relatively high voltages can be applied without causing a voltage breakdown. Even in the instance of high currents such as making currents, one must not be afraid of a burn-out. Moreover, the distribution of the current in the first electrode and its subsequent conduction along the polymer structure in the resistance layer leads to a homogeneous temperature distribution within the resistance layer. This distribution arises immediately after applying voltage to the electrodes.
Because of the polymers employed according to the invention, the heating element, the pipe, the transportation device and the roller shell to be heated can be operated even at high voltages, for instance line voltage. As the attainable heating power increases with the square of operating voltage, the resistance-heating element, the pipe, the transportation device and the roller according to the invention can yield high heating power and hence high temperatures. According to the invention, the current density is minimized because a relatively long current path is provided along the electro-conductive polymers or because at least two zones are created which are electrically in series and contain the intrinsically electro-conductive polymer used according to the invention.
Moreover, the electro-conductive polymers used according to the invention exhibit long-term stability. This stability is explained above all by the fact that the polymers are ductile, so that a rupture of the polymer chains and thus interruption of the current path will not occur when the temperature is raised. The polymer chains are unharmed even after repeated temperature fluctuation. In conventional heating elements, to the contrary, where conductivity is created, for instance, by carbon black skeletons, such a thermal expansion would lead to interruption of the current path and hence to overheating. This would lead to a strong oxidation and to bum-out of the resistance layer.
Similar considerations apply to konwn intrinsically electro-conductive polymers containing chemical compounds and/or ions which sharply reduce the long-term stability of the resistance layer subject to electric currents. It was revealed that polymers which contain a higher percentage of ions have a low aging resistance when subject to electric currents, since electrolysis reactions lead to spontaneous destruction of the resistance layer.
The intrinsically electro-conductive polymer used according to the invention is not subject to such aging phenomena; they resist aging even in reactive environments such as air, let alone oxygen. Moreover, current conduction through the resistive mass is of the electronic conduction type. Hence even an autodestruction of the resistance layer by electrolysis reactions caused by electric currents will not occur in the heating element according to the invention in which time-dependent drops in heating power per unit area are very small and approximately zero, even at temperatures as high as e.g. 500° C. and at heating powers per unit area as high as e.g. 50 kW/m
2
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Due to the use of intrinsically electro-conductive polymers, the resistance layer as a whole which is used according to the invention presents a homogeneous structure that permits a heating that is uniform across the entire layer.
According to the invention, contact to the heating element is provided by two electrodes which preferably consist of a material of high electric conductivity and are arranged on one side of the resistance layer. This type of contact arrangement
Elasser Manfred
Fuqua Shawntina
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