EP1082876B1 - Device for heating media - Google Patents

Abstract

The invention relates to a device for heating media which comprises at least one container for receiving the medium and at least one flat resistance heating element. The resistance heating element is positioned on at least part of the external side of the container and its resistance mass comprises an intrinsically electroconductive polymer.

Description

The present invention relates to a device for heating media, in particular liquids.

In conventional devices for heating media, e.g. in hot water boilers, the heating of the medium in a container is accomplished by placing heating elements in the container, e.g. in the form of heating rods, are introduced. The entry and exit points of the heating elements must be sealed in order to prevent the escape of the medium. This brings an increased design effort with it. In addition, in conventional devices, the heating element is in direct contact with the medium to be heated. If the heating element is damaged, it can thus be used with some media such as e.g. Water in addition to a security risk.

US-A-4571481 discloses a device for heating media comprising at least one container for receiving the medium and at least one sheet resistance element, wherein the resistance heating element is arranged in containers, and whose resistance mass comprises an intrinsically electrically conductive polymer.

US-A-5305419 discloses an apparatus in which the resistance heating element comprises an outer metal layer. A socket of intrinsically electrically conductive polymer is provided. This socket connects the layer to the conductive inner wall of the container.

Object of the present invention is to provide a device with which media can be heated quickly and reliably and in which there are no problems regarding the tightness of the container and an impact of the medium on the heating element.

The invention is based on the finding that this object can be achieved by a device in which a suitable heating element for heating the medium outside the container is arranged and allows a targeted penetration of the heat generated in the container.

The object is achieved by a device for heating _ media comprising at least one container for receiving the medium, and at least one planar resistance heating element, wherein the resistance heating element is arranged on at least part of the outside of the container and its resistance mass is an electrically conductive polymer includes.

In the arrangement according to the invention, the heating element is outside the container, whereby the container next to an opening for filling and emptying needs no further openings and the tightness of the container can thus be ensured without design effort. Furthermore, there are no security risks in the device according to the invention. The electrical resistance heating element does not come into contact with the medium to be heated. Penetration of the medium into the heating element and thereby caused short circuits or damage to the resistance heating element can thus be avoided.

In addition, when using an electrically conductive polymer, the resistance heating element serves as a black body, which can emit radiation of all wavelengths. With decreasing temperature, the wavelength of the radiated radiation shifts more and more to the infrared. This infrared radiation can thus penetrate into the container in container materials that allow the passage of infrared radiation, such as glass or radiation-permeable plastic. As a result, in addition to the heat conduction and radiant heat enter the medium and heat it. Due to the depth effect no particularly high temperatures are required in the heating element itself. In particular, in the heating of e.g. Water is thereby avoided precipitation of lime and other mineral salts.

By choosing a resistance mass comprising an electrically conductive polymer, the resistance heating element can withstand mechanical stresses. The polymers have such flexibility that they do not tend to break in the resistance mass even under load, whereby a local overheating would occur.

With this resistance mass thus a planar resistance heating element can be realized, which can reliably dissipate heat over large areas of the area. This uniform heat transfer over a large area is particular in the device according to the invention advantageous because the medium is heated evenly in the container and with large heating surfaces, the temperature of the heating element can be kept lower. As a result, local high temperatures are avoidable and decomposition of the medium to be heated is not to be feared.

The use of a resistance heating element, whose resistance mass comprises an electrically conductive polymer, has the further advantage that this generates sufficient heat even at low voltages. Mains voltage can be applied with the resistance heating element used according to the invention, thus producing power in the kW / m ² range of, for example, 20 to 60 kW / m ² . But it is also possible to generate lower power. In particular, when using plastic containers, the performance should be set to 30 to 60 W / m ² , preferably 30 to 40 W / m ² due to the lower heat output to the container.

The resistance heating element may have a shape in which it comprises at least two electrodes extending longitudinally through the surface of the resistive mass, the current applied to the electrodes passing through the resistive mass perpendicular to the thickness of the resistive mass. In the following, the parts of the resistance heating element which serve to supply or discharge current to and from the resistance mass are referred to as electrodes. The area formed by the resistance mass is also referred to below as the resistance layer. The electrodes may also extend in the width direction through the surface of the resistive mass. As electrodes, for example, Lahn tapes can be used in this embodiment. The use of such a resistance heating element has the advantage that the resistance mass, which lies between the electrodes and heats up when a voltage is applied to the electrodes, is in direct contact with the outer surface of the container. Heat losses can be minimized.

In the device according to the invention, the surface of the resistive mass may comprise a grid, wherein the filaments of the grid are formed from a plastic of the electrically conductive polymer or the filaments of the grid consists of a different material and are coated with this plastic. This grid can be easily arranged on the outer wall of the container due to its flexibility. In this way, a good contact between the container and the resistance heating element can be produced and the heat transfer between the container and the resistance heating element can be improved. In addition, the grid has a certain flexibility. If the container bulges due to the filling with the medium or due to the heating of the medium, so the grid can absorb this mechanical stress, without it would come to an uneven heat transfer over the surface. On the one hand, the crossing points defined in a lattice do not allow a relative change in the distance between the individual threads of the lattice and, on the other hand, there is no danger of tearing off the threads and thus of the electrical line due to the selected material which comprises an electrically conductive polymer. Even with mechanical stress so a local temperature increase can not occur and there is still a uniform heat transfer over the surface of the grid.

According to a further embodiment, the resistance heating element may comprise two planar electrodes, between which the resistance mass is arranged in the form of a layer, wherein the electrodes at least partially cover these and at least one of the electrodes faces the outer wall of the container. In this embodiment, the current applied to the electrodes flows through the sheet resistance substantially in thickness. The electrodes are preferably made of a highly conductive material. Local overheating can be derived by the good thermal conductivity of the electrodes. Overheating can thus only in the direction of the layer thickness occur and affect but not negative due to the small thickness of the sheet-like resistance heating element. Another advantage of such a construction of the resistance heating element in the device according to the invention is that it can also be ideally compensated by a local temperature increase caused by the resistance heating element from the outside, eg from the container.

According to one embodiment, the electrically conductive polymer of the resistance mass of the resistance heating element has a positive temperature coefficient of electrical resistance. As a result, a self-regulating effect with respect to the maximum achievable temperature is achieved.

According to a preferred embodiment of the device according to the invention, this comprises two containers, between which a resistance heating element is arranged so that one surface of the resistance heating element on the outside of the one container and the opposite surface of the resistance heating element is arranged on the outside of the other container. Advantage of this embodiment is that the heat losses can be minimized. The resistance heating element releases heat in all directions. By arranging the two containers on each surface of the planar resistance heating element thus most of the heat generated is delivered to the container and thus to the medium to be heated. The arrangement of the resistance heating element between the two containers has the additional advantage that the resistance heating element is pressed against the container when they expand by the filling or by the heating. A detachment of the resistance heating element from the outside of the container is thus avoided and the contact between the resistance heating element and the containers is further improved, as a result of which the heat transfer is also optimized.

According to a further embodiment, the device according to the invention comprises at least two containers, which are connected to each other so that the medium can flow through the containers in succession. In this embodiment, the medium can be preheated in a first container and heated in a second or further container to the desired temperature.

The device according to the invention can also comprise at least two containers, which have devices which allow a separate filling and emptying of a respective container. Advantage of this embodiment is the rapid availability of the entire heated amount of the medium.

A preferred embodiment of the device according to the invention is a water heater. In this use of the device, the advantages of the invention can be particularly utilized. This way, the water can be heated very quickly without risk and calcification can be avoided. In addition, the device according to the invention also allows a flat construction, i. a construction in which the depth of the device to the width and height is low. This design brings both space savings and a uniform heating of the water with it. In a flat configuration, preferably one of the surfaces of the container is completely covered by a resistance heating element.

The invention will be described below with reference to the accompanying figures, which schematically illustrate the embodiments of the invention.

Figur 1:

Perspektivische Ansicht einer Ausführungsform der erfindungsgemäßen Vorrichtung mit zwei Behältern und dazwischen angeordnetem Widerstandsheizelement;

Figur 2:

Perspektivische Ansicht einer erfindungsgemäßen Vorrichtung mit gitterartigem Widerstandsheizelement;

Figur 3:

Perspektivische Teilschnittansicht einer Ausführungsform der erfindungsgemäßen Vorrichtung mit einem Widerstandsheizelement mit flächigen Elektroden.

Figur 4:

Seitenansicht einer weiteren Ausführungsform der erfindungsgemäßen Vorrichtung mit flächigen Elektroden;

Figur 5 :

Seitenansicht eines erfindungsgemäß verwendeten Widerstandsheizelementes mit zwei Elektroden und mehreren leitenden Schichten;

Figur 6,7,8:

Draufsicht auf Vorrichtungen mit jeweils zwei Behältern.

Show it:

FIG. 1:

Perspective view of an embodiment of the device according to the invention with two containers and resistance heating element arranged between them;

FIG. 2:

Perspective view of a device according to the invention with grid-like resistance heating element;

FIG. 3:

Perspective partial sectional view of an embodiment of the device according to the invention with a resistance heating element with flat electrodes.

FIG. 4:

Side view of a further embodiment of the device according to the invention with planar electrodes;

FIG. 5:

Side view of a resistance heating element according to the invention with two electrodes and a plurality of conductive layers;

Figure 6,7,8:

Top view of devices with two containers.

In Figure 1, the device 1 of the invention consists of two containers 21, 22. Between these containers 21. 22, the resistance heating element 3 is arranged. The embodiment shown has a flat construction, in which the resistance heating element 3 rests against a flat side of the containers 21, 22 and covers them as far as possible.

FIG. 2 shows a device according to the invention, in which the resistance mass 33 of the resistance heating element 3 has a lattice-like structure. The resistance heating element 3 is arranged on one side of the container 2 and covers there the outer wall as far as possible. The electrodes 31, 32 extend longitudinally through the resistance mass 33, whereby the current applied to them flows through the resistance mass 33 perpendicular to the thickness of the resistance mass 33. This direction is indicated in Figure 2 by an arrow. The container 2 further comprises a Befulülleinrichtung 23 for supplying the medium to be heated and an emptying device 24 for discharging the heated medium. As shown in Figure 2, the electrodes 31. 32 extend beyond the resistance heating element 3 and the container 2, whereby the Connection of the electrodes 31, 32 to the power source (not shown) is made possible.

FIG. 3 shows a further embodiment of the device according to the invention, in which a planar resistance heating element 3 with planar electrodes 34 is used. In the embodiment shown, the electrode 34 bears against the outer wall of the container 2 and covers this side of the container as far as possible. On the electrode 34, the sheet resistor 35 is arranged. This comprises an electrically conductive polymer. On the sheet resistor 35, the second electrode 34 is arranged, which covers this.

In Figure 4, an electrically conductive layer 37 is disposed on the container 2, which is covered by a sheet resistor 33. On this sheet resistor 33, two outer flat electrodes 38, 39 are arranged spaced from each other.

If current is applied to the electrodes 31, 32 in the embodiment shown in FIG. 2, it flows through the resistance mass 33, which comprises electrically conductive polymer, and heats the latter. The heat thus generated penetrates via the outer wall of the container 2 into the interior and heats the medium therein. Due to the planar design of the resistance heating element 3, the heat generated over a large area to the container and thus the medium are discharged.

Due to the flexibility of the resistance heating element 3, in particular in the embodiment in which the resistance mass 33 is lattice-like, this can also create surfaces that are not flat. The container may thus also have other shapes than those shown.

FIGS. 6, 7 and 8 show further embodiments of the device according to the invention, which have two containers 21, 22. The resistance heating element 3 is disposed between the containers and preferably covers the complete side of the container against which the resistance heating element abuts.

The containers 21, 22 may be fixedly connected to each other by edge-mounted sleeves or clips (not shown).

The connection between the containers is preferably formed on the container bottom through a channel or through a pipe. Here, the channel or the tube to the individual containers 21, 22 screwed and sealed O-rings. However, it is also within the meaning of the invention to make the two containers in one piece with the channel connecting them, i. in one part e.g. to squirt.

The embodiment shown in Figure 6 is preferably used for the flat construction of the device according to the invention. The containers 21, 22 each have a triangular cross section, wherein the resistance heating element 3 rests against the respective hypotenuse of the triangular cross section. As a result, the largest outer side of the container 21, 22 is covered by the Widertandsheizelement. The ratio between the volume to be heated and the contact surface of the resistance heating element with the outside of the container is thus optimally adjusted. The filling device 23 and emptying device 24 are arranged in the embodiment shown in Figure 6 at the top of the container 21, 22.

In the embodiment illustrated in FIG. 7, the resistance heating element does not extend on a straight line between the two containers. Rather, the containers each have a wide 221 and a tapered portion 212, which are interconnected by an oblique transition 231. Due to this configuration of the container can on the one hand a small Gesamtdikke allows the device to be made possible and on the other hand, the provision of filling and emptying at the top. '

FIG. 8 shows a device in which the containers 21, 22 each have a semicircular cross-section. Between these semicircular containers, a flat resistance heating element 3 is arranged. Compared to conventional heating devices with heating rods, this embodiment has the advantage that due to the large area a low temperature must be present at the resistance heating, whereby a circulation of the medium can be avoided. Such circulation, in conventional heating elements, causes the medium discharged from the container to be removed from a portion of the apparatus in which cold and warm medium are mixed due to circulation. In the device according to the invention, however, the warm medium rises in the container upwards and can be removed from there.

In the embodiments with two containers shown in FIGS. 1, 6, 7 and 8, either heating elements with a resistance mass which extends in a planar manner between the electrodes and in which the current passes through the resistance mass perpendicular to the thickness or resistive heating elements with flat electrodes can be used where the current passes through the thickness of the resistive layer can be used. The latter type of resistance heating element is preferred here.

In the embodiment in which the resistive mass is latticed and the filaments of the grid are formed of the electrically conductive polymer, the diameter of the filaments of the grid running parallel to the electrodes may be smaller than the diameter of the filaments perpendicular to them be lost. Through this cross-sectional distribution, a uniform passage of current over the entire surface of the resistance mass can be achieved. This uniform passage of current can also be achieved by a suitable choice of the material of the threads. Here, the material for the threads, which extend perpendicular to the electrodes, chosen so that it has a higher conductance than that of the parallel to these extending threads. Here, a difference in conductivity between the parallel and the perpendicular threads of 15 to 25%, preferably 20%, sufficient to thus regulate the flow of current through the threads or their coating and to distribute ideally over the entire surface. As a resistance mass, a support fabric, for example made of plastic, which has been coated with the electrically conductive polymer on both sides and thus continuous layers were obtained on the supporting fabric serve. As a support fabric Vließ, knitted or fiber mats of, for example, polyamide, for example nylon, glass fibers, polyester or polypropylene may be used.

In the embodiment of the present invention, as schematically illustrated in Fig. 3, the current flows through the resistance mass in the thickness direction of the resistance heating element when the electrodes are connected to a power source (not shown). The generated heat is released to the container and thus the medium. It is also within the meaning of the invention to form the resistance layer of the heating element by glass fibers provided with electrically conductive polymer. For example, the resistive layer may comprise a glass fiber mat that has been soaked with electrically conductive polymer by immersion. Furthermore, resistive layers containing ceramic particles, e.g. Barium titanate include. Despite this ceramic, the temperature coefficient of the resistance mass of the resistor used in the present invention may be negative.

A negative temperature coefficient of electrical resistance requires a very low inrush current. In addition, the resistance mass used in the invention may be adjusted so that at elevated temperature the temperature coefficient of electrical resistance becomes positive. The inrush currents here can be 50% of the operating current.

In the embodiment shown in Figure 4, two flat electrodes are arranged on the side facing away from the container and separated from each other by an insulation. This isolation can be done by providing an insulating piece of insulating material or by air. On the opposite side of the resistance mass, which faces the container, a layer of electrically conductive material is applied, for example in the form of a metal foil or metal sheet. This electrically conductive layer is not connected to the power source. In this embodiment, when the two planar electrodes are connected to a power source, the current flows from the one contacted electrode through the thickness of the resistive mass to the electrically conductive layer, is forwarded therethrough, and flows through the resistive mass back to the second contacted electrode. Such a structure of the resistance heating element has the advantage that it can be operated by a very low voltage. The voltage can be reduced by up to half the voltage required in the construction with two flat electrodes sandwiching the resistance mass between them. The reduction of the supply voltage is due, inter alia, to the resistance formed by the insulation between the adjacent electrodes. If air is selected as insulation, the resistance is determined by the distance of the electrodes from each other and thus by the surface resistance. In particular, when using the device according to the invention for heating media as a hot water boiler operation with very low voltage is of particular advantage, as this can minimize the security risk. The arrangement of the two contacted with the power source electrodes on one side of the sheet resistor has the further advantage that the Contact between the container and the electrically conductive foil or sheet is not disturbed by means of power supply.

FIG. 5 shows a resistance heating element in which a thin resistance layer 33 is present. On both sides of the resistive layer 33 are each a planar electrode 38, 39 and a plurality of conductive layers 37 are arranged. The electrodes 38, 39 are respectively provided at the opposite end of the resistance layer 33. The electrode 38 and the conductive layers 37 are spaced from each other and offset from the electrode 39 and the conductive layers 37 disposed on the opposite side of the resistor layer 33. In this structure, the current applied to the electrodes 38, 39 flows through the resistive layer 33 and the conductive layers 37 in the direction indicated by arrows in the drawing. In this current flow, the resistance layer 33 serves as a series circuit of a plurality of electrical resistances, whereby a high performance can be achieved. Here, both the resistance in the thickness of the resistance layer 33 and the surface resistance in the spaces between the electrically conductive layers 37 and the electrically conductive layer 37 and the electrodes 38 and 39 are utilized.

In addition, the large spatial distance between the electrodes offers the advantage that direct contact between them can be avoided. As electrodes and electrically conductive layers can serve metal foils or sheets. In particular, in the embodiment of the resistance heating element shown in Figure 5, the resistance layer may be very thin and have a thickness of eg 1 mm. It is also within the scope of the invention to modify the resistance heating element shown in Figure 5 so that both electrodes are arranged on one side of the resistive layer. In this embodiment, the conductive layers are spaced apart between the electrodes disposed at the ends of the resistor layer. On the opposite side of the resistive layer are the conductive layers offset and arranged spaced from each other. Even with this arrangement, even with a small layer thickness of the resistive layer, high powers can be achieved by using the resistive layer as a series connection of a plurality of resistors and the simultaneous use of the surface resistance.

Even with resistance heating elements, as shown in Figures 3 and 4, thin resistive layers are possible. However, these must have a high-resistance electrical resistance.

To improve the electrical contact between the resistance mass and the electrodes, the resistance mass may be provided in the region of the electrodes with a sprayed-on layer of metal. In the embodiment in which the resistance mass is in the form of a layer, the surface of the layer is sprayed with metal and then the planar electrode is applied. If the mass is constructed in the manner of a grid, then the area of the grid through which the electrodes should extend can be sprayed with metal prior to the introduction of the electrodes.

The contacting of the electrodes with the current source, which preferably applies the resistance heating element with a mains voltage of 220 V, can be effected by conventional contacting methods. Since, even in embodiments with two containers, the heating element preferably covers an entire side of the respective container outer wall and thus extends to the edge, in both embodiments of the resistance heating element, i. in the case of sheet-like electrodes and in the case of electrodes extending longitudinally through the resistance mass, they can be accessed from the outside and can be connected there.

In the embodiment shown in FIG. 1, both a planar resistance heating element, in which the flow of current takes place over the surface of the resistance mass, and a planar resistance heating element, that in the thickness direction The current flows through it. The resistance heating element is arranged between the containers such that the largest possible area of the outer walls of the containers is covered by the resistance heating element. When a voltage is applied, the resistance heating element releases heat in all directions and thus simultaneously heats the medium of the two containers. By this arrangement, the heating of a large amount of the medium is ensured with low power consumption.

The device according to the invention may also comprise more than two containers, wherein preferably resistance heating elements are arranged so that they can give off heat to two containers. The flow of the medium to be heated can be performed so that it flows through only one or more containers.

The resistance mass also acts as a "black spotlight". The radiation in the infrared range results in containers made of a material that transmits this radiation. e.g. Glass, to heat the medium inside the container. In this case, no high temperatures occur at the contact surface of the resistance heating element on the outer wall of the container. Unwanted decomposition processes in the medium are thus avoided. The depth effect of the resistance heating element also has the advantage that the medium is heated evenly.

The means for filling and emptying can be arranged as required at different locations of the container. Preferably, they are arranged so that they have the greatest possible distance from the terminals of the electrodes. As a result, it is possible to rule out contact of the medium with the electrodes and resulting disadvantages even in the case of possible leaks in the device. As a further safety measure, between the resistance heating element and the outer wall of the container, in particular be provided in metal containers, an electrically insulating layer. Polyester, polyimide or polytetrafluoroethylene is preferably used for this layer.

Preferably, the container has the shape of a cuboid, in which the height and length of the container is large in relation to its width. This shape of the container allows a large contact surface with the container and a favorable ratio between this contact surface and the volume of the container in a flat resistance heating element. The heating surface is large in relation to the volume of the medium to be heated, whereby a rapid and uniform heating can take place. The container may also have, for example, a round cross-section. To this cylinder-shaped container, another container, which has an annular cross-section, be arranged, and the resistance heating element may be provided in a gap formed thereby between the containers.

It is also within the meaning of the invention that the side walls of the container are completely covered with the resistance heating element. Here, the heat is released from all sides into the container and the medium is heated quickly.

If a plurality of containers are provided, which are to be successively flowed through by the medium, they can be arranged one above the other, wherein the means for emptying the upper container is connected at its bottom with the means for filling a container disposed thereunder at the top thereof. If the containers are arranged next to one another, the media flow can be effected by means of pumps which can be arranged outside the containers.

The containers can be made of metal. Glass or plastic. Preferably, polycarbonate is used. Due to the low temperature at the resistance heating element, which is sufficient due to the depth effect of the resistance heating elements to heat the medium, there is no risk of melting the container even with the use of plastic containers.

The walls of the containers are preferably thin. As a result, on the one hand, the heat from the heating element can be released well to the medium in the container. On the other hand, the container can expand, in particular bulge during filling and heating. By this bulging the contact with the heating element located on the container is reinforced, especially in embodiments with two containers, between which a heating element is arranged. In this embodiment, ribs may additionally be provided in the container on the wall of the container which is not provided with the resistance heating element. Such ribs create a reinforcement on these walls. Through these wall reinforcements, the test pressure of 8 bar, which is normally prescribed for example for water regulators, can also be maintained with plastic containers.

The thickness of the walls of the containers is between 1 and 8 mm, preferably 3 to 5 mm. The thickness of the walls should be chosen according to the size of the container and the capacity.

With the device according to the invention, e.g. Temperatures of water which have been heated in the device can be adjusted from 20 to 90 ° C.

The device according to the invention can have very low overall thicknesses. It can be produced total thicknesses of 6 to 10 cm, preferably 8 cm, wherein the surface heating element has a thickness of 0.1 to 5 mm, preferably 1 to 2 mm. For example, a hot water boiler with a thickness of only 8 cm can be produced with the device according to the invention. This small thickness allows attachment of the device eg behind veneers in kitchens or bathroom equipment.

The electrically conductive polymer used according to the invention is preferably produced by doping a polymer. The doping may be a metal or semimetal doping. In these polymers, the interfering conductor is chemically bound to the polymer chain and generates an impurity. The doping atoms and the matrix molecule form a so-called charge-transfer complex. In doping, electrons are transferred from filled bands of the polymer to the dopant. The resulting electron holes give the polymer semiconductor-like electrical properties. By chemical reaction, in this embodiment, a metal or semimetal atom is incorporated into or attached to the polymer structure so as to generate free charges which allow the flow of current along the polymer structure. The free charges are in the form of free electrons or holes. It thus creates an electron conductor.

Preferably, the polymer has been doped with a doping material in an amount such that the ratio of atoms of the dopant to the number of polymer molecules is at least 1: 1, preferably between 2: 1 and 10: 1. By this ratio it is achieved that substantially all polymer molecules are doped with at least one atom of the doping material. By selecting the ratio, the conductance of the polymers and thereby the resistance layer, as well as the temperature coefficient of the resistance of the resistive layer can be adjusted.

Although the electrically conductive polymer used according to the invention can also be used without the addition of graphite in the device according to the invention as a material for the resistance layer, according to a further embodiment, the resistance layer may additionally comprise graphite particles. These particles can contribute to the conductivity of the The entire resistive layer preferably does not contribute and preferably does not touch each other and in particular does not form lattice or skeletal structures. The graphite particles are not firmly bound in the polymer structure, but are freely movable before. If a graphite particle is in contact with two polymer molecules, the current can jump from one chain through the graphite to the next chain. The conductivity of the resistance layer can thus be increased even further. At the same time, due to their free mobility in the resistance layer, the graphite particles can reach the electrodes and cause an improvement in the contact there.

The graphite particles are preferably present in an amount of at most 20% by volume, more preferably at most 5% by volume, based on the total volume of the resistance layer, and have a mean diameter of not more than 0.1 μm. Due to this small amount of graphite and the small diameter, the formation of a graphite lattice that would lead to a conduction of the current through these lattices can be avoided. It is thus ensured that the flow of current continues to take place essentially via the polymer molecules by electron lines and thus the advantages mentioned above can be achieved. In particular, the conduit does not have to pass through a graphite lattice where the graphite particles must touch and which is easily destroyed by mechanical and thermal stress, but along the stretchable and age-resistant polymer.

As electrically conductive polymers it is possible to use both electrically conductive polymers such as polystyrene, polyvinyl resins, polyacrylic acid derivatives and copolymers thereof, as well as electrically conductive polyamides and their derivatives, polyfluorohydrocarbons, epoxy resins and polyurethanes. Preference is given to polyamides, polymethyl methacrylates. Epoxies, polyurethanes and polystyrene or mixtures thereof used. In this case, polyamides additionally have good adhesive properties, which are advantageous for the production of the device according to the invention for heating media, since this facilitates the attachment to the device. Some polymers, such as polyacetylenes excreted due to their low resistance to aging by reactivity with oxygen for the inventive use.

The length of the polymer molecules used varies widely depending on the type and structure of the polymer, but is preferably at least 500, more preferably at least 4000 Å.

According to the invention, the electrically conductive polymer used in the resistance surface of the device can be, in particular, those polymers which are conductive by metal or semimetal atoms which are attached to the polymers. These polymers preferably have a volume resistivity in the range of values achieved by semiconductors. It can be up to 10 ² Ω · cm, preferably it is higher, but at most 10 ⁵ Ω · cm. Such polymers can be obtained by a process in which polymer dispersions. Polymer solutions or polymers with metal or metalloid compounds or their solution are added in an amount such that a polymer molecule comes close to a metal or semimetal atom. This mixture is added a reducing agent in slight excess or formed by known thermal decomposition of metal or semimetal atoms. Subsequently, the formed or remaining ions are washed out and the dispersion solution or the granules can optionally be treated with graphite or carbon black.

The electrically conductive polymers used according to the invention are preferably free of ions. The maximum content of free ions is 1% by weight, based on the total weight of the resistance layer. The ions are either washed out as described above or a suitable reducing agent is added. The reducing agent is added in such a ratio that the ions can be completely reduced. The low proportion of ions, preferably the freedom from ions, of the electrically conductive polymers used according to the invention results in a long resistance of the resistance layer under the action of electrical currents. It has been found that polymers containing ions at a higher percentage have little resistance to aging upon exposure to electrical currents, as electro-lyse reactions cause self-destruction of the resistive layer. The electrically conductive polymer used according to the invention, however, is resistant to aging due to the low ion concentration even with prolonged application of electricity. As the reducing agent for the above-described process for producing an electroconductive polymer used in the present invention, there are used those reducing agents which either do not form ions because they are thermally decomposed during processing, such as hydrazine, or chemically react with the polymer itself, such as formaldehyde or those whose excess or reaction products are easily washed out, such as hypophosphites. As metal or semimetals are preferably silver, arsenic, nickel. Graphite or molybdenum used. Particular preference is given to those metal or semimetal compounds which, by pure thermal decomposition, form the metal or semimetal without interfering reaction products. In particular arsenic or nickel carbonyl have proven to be particularly advantageous. The electrically conductive polymers used according to the invention can be prepared, for example, by reacting the polymer with 1-10% by weight (based on the polymer) a masterbatch prepared according to one of the following recipes. Example 1: 1470 parts by wt. Dispersion of fluorocarbon polymer (55% solids in water), 1 part by weight of wetting agent, 28 parts by weight of silver nitrate solution 10%, 6 parts by weight of chalk, 8 parts by weight of ammonia, 20 parts by weight Carbon black, 214 parts by weight of graphite, 11 parts by weight of hydrazine hydrate. Example 2: 1380 parts by weight of acrylic resin dispersion 60 wt .-% in water, 1 part by weight of wetting agent. 32 parts by weight of silver nitrate solution 10%, 10 parts by weight of chalk, 12 parts by weight of ammonia. 6 parts by weight of carbon black, 310 parts by weight of graphite, 14 parts by weight of hydrazine hydrate. Example 3: 2200 parts by weight of dist. Water. 1000 parts by weight of styrene (monomer), 600 parts by weight of ampholyte soap (15%), 2 parts by weight of sodium pyrophosphate, 2 parts by weight of potassium persulfate, 60 parts by weight of nickel sulfate, 60 parts by weight of sodium hypophosphite, 30 parts by weight of adipic acid. 240 parts by weight graphite.

Claims (9)

1. An apparatus (1) for heating media, comprising at least one container (2) for receiving the medium, and at least one planar resistance heating element (3), the resistance heating element (3) being disposed on at least part of the outer side of the container (2) and its resistance mass (33) comprising an electroconductive polymer.
2. The apparatus according to claim 1, characterized in that the resistance heating element (3) comprises at least two electrodes (31, 32) which extend in the longitudinal direction through the surface of the resistance mass (33), the current applied to the electrodes (31, 32) flowing through the resistance mass (33) perpendicular to the thickness of the resistance mass (33).
3. The apparatus according to claim 2, characterized in that the resistance mass (33) comprises a grid, the filaments of the grid being formed from a synthetic material comprising the electroconductive polymer, or the filaments of the grid being made of another material and coated with said synthetic material.
4. The apparatus according to claim 1, characterized in that the resistance heating element (3) comprises at least two planar electrodes (34) between which the resistance mass (35) is disposed in the form of a layer, the electrodes (34) covering it at least partly, and at least one of the electrodes (34) facing the outer wall of the container (2).
5. The apparatus according to any of the above claims, characterized in that the electroconductive polymer has a positive temperature coefficient of electric resistance.
6. The apparatus according to any of the above claims, characterized in that it comprises two containers (21, 22), and a resistance heating element (3) is so disposed between the containers (21, 22) that one surface of the resistance heating element (3) is disposed on the outer side of one container (21) and the opposite surface of the resistance heating element (3) on the outer side of the other container (22).
7. The apparatus according to any of the above claims, characterized in that it comprises at least two containers (21, 22) that are so interconnected that the medium flows through the containers (21, 22) one after the other.
8. The apparatus according to any of claims 1 to 6, characterized in that it comprises at least two containers (21, 22) having devices (23, 24) that permit separate filling and emptying of one container (21, 22) in each case.
9. The apparatus according to any of the above claims, characterized in that it is a water heater.

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