Abstract:
The invention relates to an added electrical heater ( 1 ), especially for a heating system or air conditioner of a motor vehicle. A plurality of heating elements ( 2 ) are provided which are formed by PTC elements while being fitted with at least one electrically conducting coating ( 3 ).

Description:
BACKGROUND 
     The invention relates to an additional electric heater, in particular for a heating or air-conditioning system of a motor vehicle. 
     In low-consumption vehicles the low level of heating which is available makes it necessary to provide additional heating power to heat up the passenger compartment and to rapidly remove precipitation (ice or water), in particular from the windshield. To do this it is known, in the case of heat exchangers which are constructed from flat tubes through which a heat transfer medium flows which outputs heat in the case of heating, to provide an additional heating means in the form of PTC heating elements at least on the outermost tubes, these elements being generally ceramic PTC assemblies which usually have a surface temperature of between 110 and 160° C. irrespective of the peripheral conditions such as applied voltage, nominal resistance, air flow. Owing to restrictions in the shaping and geometry, the mounting or the design of an additional electric heater is extremely costly. In addition, the ceramic PTC assemblies are relatively heavy. 
     WO 03/098124 A1 discloses a heat exchanger with an additional electric heater having a plurality of flat tubes which are arranged parallel to one another and through which a heat transfer medium flows, wherein at least some of the flat tubes are assigned a heating element as an additional heating means which is mounted after the heat exchanger has been soldered, is operated electrically and is mounted by means of a securing element on the end side of the heat exchanger with respect to the corresponding flat tube and so as to extend parallel to the latter by means of the securing element which also extends parallel to the flat tube. In this context, a polymer PTC material which is composed of a film and is placed around a central electrode in a U shape can be used, said material just filling, with a slight pressing tension, the space within the securing grill. In this context, the securing grill is at an intermediate potential and has to be electrically insulated from the heating element. However, such a configuration is not entirely satisfactory. 
     In addition, DE 10 2004 027 687 A1 discloses an electric heating device having a plurality of heating element panels which are arranged parallel to one another in order to define an air duct between two adjacent heating element panels, a positive electrode element which is connected to one end side of each heating element panel and a negative electrode element which is connected to the other end side of each heating element panel. The heating element panels are, for example, electrically conductive resin into which an electrically conductive filling material is mixed. In this context, the electrically conductive resin generally has a positive resistance/temperature characteristic curve in which the electrical resistance rises at a predetermined temperature or above said temperature. The current flows here from one electrode element through the heating element panels to the other electrode element. 
     BRIEF SUMMARY 
     Taking this prior art as a basis, the object of the invention is to make available an improved additional electric heater. This object is achieved by means of an additional electric heater, in particular for a heating or air-conditioning system of a motor vehicle. 
     According to the invention, an additional electric heater is provided, in particular for a heating or air-conditioning system of a motor vehicle, wherein a plurality of heating elements are provided which are formed by PTC elements, and the heating elements are provided with at least one electrically conductive surface coating, referred to below as coating. Providing an electrically conductive coating simplifies the current supply and/or current discharge so that contact over a large area can be dispensed with, which reduces the manufacturing costs and also the weight of the additional heater. The PTC elements are preferably provided with two electrically conductive coatings which are spatially separated from one another by the PTC element. Here, the coatings are arranged in such a way that there is a flow over the largest possible area through the PTC element and as a result it is correspondingly heated. In order to prevent non-uniform heating, the thickness and thus the distance between the two coatings of the PTC element are correspondingly made as constant as possible. 
     The PTC elements are preferably formed by plastic elements with PTC properties which can be manufactured more easily and in a more flexible way than the known ceramic PTC elements. 
     The PTC elements preferably have a hollow profile, wherein the hollow profile can also be formed from a plurality of assembled parts. In particular hollow-cylindrical or flat tube-like profiles are possible as hollow profile, but the shapes are not restricted. 
     The heating element can be embodied divided into a plurality of parts, in particular two parts, in the longitudinal direction. In this context, the parts can be embodied, for example, as half shells, but it is necessary to ensure that an electrically conductive coating of the outside does not come into contact with the electrically conductive coating of the inside, in order to avoid a short-circuit. The heating element can also be formed by two panels, wherein the panels can also have a structured configuration which serves in particular to space them apart. 
     The electrically conductive coating is preferably formed by a metallic coating, in particular by a silver, aluminum, copper or gold layer which can be applied by any desired method, for example by means of dipping, PVD or CVD methods. Alternatively, it is also possible to provide metal foils as electrically conductive coatings, metal foils made of aluminum, an aluminum alloy, copper or a copper alloy being particularly suitable. 
     For reasons of protection against short-circuits in a motor vehicle, the outer electrically conductive coating is preferably connected to the negative pole and/or the inner electrically conductive coating is preferably connected to the positive pole. 
     The heating element is preferably composed of plastic with PTC properties, in particular of a polymer, in particular preferably a polyolefin, with electrically conductive filling materials, in particular with carbon, in particular in the form of soot particles. Providing a plastic PTC heating element increases the possible variety of shapes and permits the flow paths for the medium which is to be heated through the additional heater to be optimized. In addition to air, it is also possible to heat other media, in particular gases or fluids which are not electrically conductive or are electrically conductive only to a minimum degree, for example an oil, in a corresponding additional heater. 
     The surface around which the medium which is to be heated flows is preferably provided with a surface-enlarging structured configuration which can be directly integrally molded on. 
     The surface-enlarging structured configuration can alternatively also be formed by panels or corrugated ribs which are good conductors of heat and which are fitted onto or between the heating elements. These are preferably sheet metal panels, in particular made of aluminum, copper or silver, which are good conductors of heat. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in detail below by means of a plurality of exemplary embodiments and with reference to the drawing, in which: 
         FIGS. 1   a - 1   e  show schematically illustrated details of an additional electric heater according to the first exemplary embodiment in various views, 
         FIGS. 2   a - 2   d  show schematically illustrated details of a variant of the first exemplary embodiment in various views, 
         FIGS. 3   a - d  show schematically illustrated details of an additional electric heater according to the second exemplary embodiment in various views, 
         FIGS. 4   a - d  show schematically illustrated details of a variant of the second exemplary embodiment in various views, 
         FIGS. 5   a - d  show schematically illustrated details of an additional electric heater according to the third exemplary embodiment in various views, 
         FIGS. 6   a - d  show schematically illustrated details of a variant of the third exemplary embodiment in various views, 
         FIGS. 7   a - d  show schematically illustrated details of an additional electric heater according to the fourth exemplary embodiment in various views, 
         FIGS. 8   a - d  show schematically illustrated details of a first variant of the fourth exemplary embodiment in various views, 
         FIGS. 9   a - d  show schematically illustrated details of a second variant of the fourth exemplary embodiment in various views, 
         FIGS. 10   a - d  show schematically illustrated details of an additional electric heater according to the fifth exemplary embodiment in various views, 
         FIGS. 11   a - d  show schematically illustrated details of a first variant of the fifth exemplary embodiment in various views, 
         FIGS. 12   a - d  show schematically illustrated details of a second variant of the fifth exemplary embodiment in various views, 
         FIGS. 13   a - d  show schematically illustrated details of a third variant of the fifth exemplary embodiment in various views, 
         FIGS. 14   a - d  show schematically illustrated details of an additional electric heater according to the sixth exemplary embodiment in various views, with the last part of a heating element being illustrated once more individually on the right, 
         FIGS. 15   a - d  show schematically illustrated details of a first variant of the sixth exemplary embodiment in various views, with the last part of a heating element being illustrated once more individually on the right, 
         FIGS. 16   a - d  show schematically illustrated details of a second variant of the sixth exemplary embodiment in various views, with the last part of a heating element being illustrated once more individually on the right, 
         FIGS. 17   a - d  show schematically illustrated details of a third variant of the sixth exemplary embodiment in various views, with the last part of a heating element being illustrated once more individually on the left, 
         FIGS. 18   a - d  show sectional and schematically illustrated details of the sixth exemplary embodiment from  FIGS. 14   a  to  14   d  in various views, with the last part of a heating element being illustrated once more individually on the right, 
         FIGS. 19   a - d  show schematically illustrated details of a fourth variant of the sixth exemplary embodiment in various views, with the last part of a heating element being illustrated once more individually on the right, 
         FIGS. 20   a - d  show schematically illustrated details of an additional electric heater according to the seventh exemplary embodiment in various views, with the last part of a heating element being illustrated once more individually on the right, 
         FIGS. 21   a - d  show schematically illustrated details of a first variant of the seventh exemplary embodiment in various views, 
         FIGS. 22   a - c  show detailed views of a refinement of the fourth variant of the sixth exemplary embodiment ( FIGS. 19   a - d ), 
         FIGS. 23   a - e  show schematically illustrated details of an additional electric heater according to the eighth exemplary embodiment in various views, 
         FIG. 24   a  shows a detail of a schematically illustrated additional electric heater according to the ninth exemplary embodiment, with just one heating element being illustrated, and 
         FIG. 24   b  is a schematic illustration of the additional heater from  FIG. 24   a , with a plurality of heating elements being illustrated. 
     
    
    
     DETAILED DESCRIPTION 
     An additional electric heater  1  for a motor vehicle air-conditioning system (not illustrated) is composed, according to the first exemplary embodiment illustrated in  FIGS. 1   a  to  1   e , of a plurality of heating elements  2  which are arranged in a packet and constructed in the form of round tubes from a plastic with PTC properties, in the present case of a polyolefin with soot particles. The heating elements  2  are arranged here downstream of the heater in the air stream flowing through the motor vehicle air-conditioning system, indicated by arrows. 
     The individual heating elements  2  which are manufactured by means of injection molding are electrically conductively coated on the inside and outside (coatings  3 ), wherein, for reasons of protection against short-circuits in the motor vehicle, the inner coating  3   i  is connected here to the positive pole, and the outer coating  3   a  to the negative pole of the voltage source. The electrically conductive (surface) coating has been applied by means of a PVD method, and according to the first exemplary embodiment it is formed by a thin layer of silver. Here, the inner coating  3   i  and the outer coating  3   a  are spaced apart from one another in such a way that an electrical short-circuit is ruled out. 
     According to the first exemplary embodiment, a panel  4 , which serves as an injection molding panel for the manufacture of the individual heating elements  2 , is provided in the center, but it does not necessarily need to be arranged in the center. 
     If a voltage, for example a voltage of up to 42 V from a 42 V on-board power system, is present, power can be supplied and distributed in a relatively loss-free way through the individual heating elements  2  via the electrically conductive coatings, and there is subsequently a transverse flow through the PTC material, in which case heat is generated owing to the electrical resistance. In this context, the electrical resistance is dependent on the heat owing to the PTC properties so that the heating elements  2  have self-regulating properties. Power is then discharged in a relatively loss-free fashion via the other coating  3  so that heat is essentially generated only in the desired regions. 
     In the variant of the first exemplary embodiment which is illustrated in  FIGS. 2   a  to  2   d , the individual heating elements  2  are spaced apart from one another further so that the maximum pressure drop on the air side is reduced, but with a reduced, heat-transmitting surface. Otherwise, the variant corresponds to the first exemplary embodiment. 
     According to the second exemplary embodiment, flat tubes are provided as the heating elements  2  instead of round tubes, said tubes being correspondingly arranged in order to provide the smallest possible air resistance. In this case also, the tubes are provided on the inside and outside with an electric coating  3 , and in this case by means of a copper layer which has been applied by dipping. The electric wiring corresponds to that in the first exemplary embodiment. 
     The flat tubes can also be aligned in different ways, for example in the manner of fish bones according to the variant of the second exemplary embodiment illustrated in  FIGS. 4   a  to  4   d  so that, for example, selective eddying of the through-flowing air is generated. 
     In addition to the embodiments of the tubes illustrated in the drawing, any other desired geometries are also possible, for example drop-shaped or triangle-shaped geometries. A combination of different geometries and/or different distances between the heating elements and/or different alignments of the heating elements in an additional electric heater is also possible, for example also as a function of individual climatic zones in multi-zone motor vehicle air-conditioning systems. 
     According to the third exemplary embodiment illustrated in  FIGS. 5   a  to  5   d , the heating elements  2  which are embodied in the manner of flat tubes are designed with a structured configuration  5  in order to permit improved transfer of heat to the through-flowing air. According to the third exemplary embodiment, a panel  4  is again provided for the injection molding of the heating elements  2 , but this time, in contrast to the first exemplary embodiment, it is provided at one end of the heating elements  2 . According to the third exemplary embodiment, an aluminum layer, which has been applied by means of a CVD method, is provided on the inner face and outer face of the heating elements  2 . 
     In the variant of the third exemplary embodiment which is illustrated in  FIGS. 6   a  to  6   d , the panel  4  is arranged centrally in accordance with the first exemplary embodiment. 
     In  FIGS. 7   a  to  7   d , the fourth exemplary embodiment is illustrated, the heat-transferring surface of the individual heating elements  2  which are embodied as round tubes here being enlarged by flat disks  6 , with a circular shape, which are also injection molded within the scope of the manufacture of the heating elements  2 . The electrically conductive outer coating  3   a  is also provided here continuously on the surfaces and on the outer circumference of the disks  6 . 
     According to the first variant of the fourth exemplary embodiment which is illustrated in  FIGS. 8   a  to  8   d , panels  7  which are fitted subsequently over the round tubes serving as heating elements  2  are provided instead of the disks  6  which are also injection molded, said panels  7  being designed to hold the tubes with a plurality of circular openings. According to the first variant, the panels  7  here are composed of the same material as the heating elements  2 . The individual tubes are, as in the first exemplary embodiment, integrally injection molded onto both sides of an injection molding panel  5 , with the manufacture being as a composite here. 
     The round tubes according to the second variant illustrated in  FIGS. 9   a  to  9   d  which serve as heating elements  2  are manufactured individually and introduced into circular openings which are bored or punched into aluminum panels  7  which serve as ribs. In this context, the ribs are arranged spaced apart from one another. It is possible to dispense with an injection molding panel  4 . 
     According to a further variant (not illustrated in the drawing), gills can be additionally provided on the aluminum panels in order to improve the transfer of heat further. 
       FIGS. 10   a  to  10   d  show various views of the fifth exemplary embodiment according to which a single row, running perpendicularly to the direction of flow, of PTC heating elements  2  which are embodied in the manner of flat tubes are provided, the longitudinal sides of which PTC heating elements  2  each point to the adjacent heating element  2 . In turn, an injection molding panel  4  is provided in the center. In accordance with the previous exemplary embodiments, the heating elements  2  are provided on the outside and inside with an electrically conductive coating, with the outer coating being connected to the negative pole and the inner coating being connected to the positive pole. 
     According to a first variant of the fifth exemplary embodiment which is illustrated in  FIGS. 11   a  to  11   d , the heating elements  2  have, essentially in accordance with the fourth exemplary embodiment, enlarged, heat-transferring surfaces which are enlarged by flat plates  6  with an approximately rectangular shape, which are also injection molded onto the individual heating elements  2  within the scope of the manufacture of said heating elements  2 . The individual plates  6  are each arranged spaced apart from one another. The electrically conductive outer coating  3   a  is in turn provided so as to also extend over all of the surfaces and the outer sides of the plates  6 . 
     In a second variant of the fifth exemplary embodiment which is illustrated in  FIGS. 12   a  to  12   d , panels  7  which extend over all the surfaces instead of the plates  6  are provided with openings which correspond to the shape of the heating elements  2  in order to enlarge the heat-transferring surface, which openings are fitted onto the heating elements  2  which are manufactured as a composite here (injection molding panel  5 ). 
     According to a third variant of the fifth exemplary embodiment which is illustrated in  FIGS. 13   a  to  13   d , the injection molding panel  5  is dispensed with since the individual heating elements  2  are manufactured separately and are positioned and connected to one another by means of the panels  7 . 
     According to a fourth variant of the fifth exemplary embodiment, instead of approximately rectangular plates  6  which are also injection molded on (cf. first variant of the fifth exemplary embodiment), correspondingly embodied aluminum plates which have openings for the heating elements  2  are fitted onto the individual heating elements after they have been electrically conductively coated. 
       FIGS. 14   a  to  14   d  and  18   a  to  18   d  show the sixth exemplary embodiment which corresponds, in the assembled form, approximately to the first variant of the fifth exemplary embodiment. Here, the individual heating elements  2  are not formed by continuous hollow profiles but rather by assembled half shells  8  on which surface-enlarging structured configurations  5  are formed. The half shells  8  are closed on one side (cf.  FIG. 14   a ), with the underside, which is provided with the outer coating  3   a  like the entire lateral outer surface, being connected on the outside to the negative pole. The upper side is of open design, with the inner coating  3   i  extending as far as the upper side and being connected to the positive pole. Here, the coatings  3   a  and  3   i  are provided spaced apart from one another on the upper side so that no electrical short-circuit takes place. In addition, the end faces of the half shells  8  which are in contact with one another are not electrically conductively coated. 
     According to the first variant of the sixth exemplary embodiment which is illustrated in  FIGS. 15   a  to  15   d , electrical contact is formed with the inner coating  3   i  by means of contacting rails  9  which are inserted inwardly and are connected to the positive pole. Otherwise, the variant corresponds to the sixth exemplary embodiment. 
     In accordance with the second variant of the sixth exemplary embodiment which is illustrated in  FIGS. 16   a  to  16   d , the heating elements  2  which are formed by two half shells  8  are of open design on both sides, with the heating elements  2  ending just at the bottom. According to the third variant of the sixth exemplary embodiment, the heating elements  2  end at the bottom and at the top with a laterally extended edge whose lateral dimensions correspond to those of the surface-enlarging structured configurations  5 . 
     The fourth variant (illustrated in  FIGS. 19   a  to  19   d ) of the sixth exemplary embodiment corresponds essentially to the sixth exemplary embodiment, but no connections of the flat-tube shaped heating elements  2  are provided on the upper side. The composite is held together at the bottom by a connecting panel  10 . Alternatively or additionally, improved cohesion is possible by means of surface-enlarging structured configurations  5  which are embodied as panels. 
     Detailed illustrations of a modification of the fourth variant of the sixth exemplary embodiment are given in  FIGS. 22   a  to  22   c , wherein no planar panels are provided for enlarging the surface but rather a corrugated rib, which can also be embodied with gills, is provided as the structured configuration  5  of the surface. 
     In accordance with the additional electric heater  1  which is illustrated in  FIGS. 20   a  to  20   d  according to the seventh exemplary embodiment, instead of half shells individual heating panels  11  made of PTC material are provided, said heating panels  11  being provided with an electrically conductive coating  3  on the sides which lie opposite one another, wherein no electrical connection is provided on the sides of the panels. An electric contact  12  in the form of a piece of sheet metal is inserted between two such heating panels  11  in a way which corresponds to the contacting rails  9  of the first variant of the sixth exemplary embodiment. On the side of the heating panels  11  facing away from the electric contact  12 , a corrugated rib made of a bent piece of sheet aluminum is provided as a surface-enlarging structured configuration  5 . In this context, the corrugated rib can also be embodied with gills. 
     In the first variant of the seventh exemplary embodiment which is illustrated in  FIGS. 21   a  to  21   d , the individual heating panels  11  are bent outward at the bottom and away from the electric contact. In this context, the coatings are embodied in such a way that no short-circuit arises between the inserted contact plate (electric contact  12 , positive pole) and the negative pole which is provided at the base in the form of a second contact plate. 
       FIGS. 23   a  to  23   e  show an additional electric heater according to the eighth exemplary embodiment. The individual heating elements  2  are composed of individual PTC panels of approximately constant thickness which have, however, structures  13 , in the present case elongate, knob-shaped structures. They have in turn an electrically conductive coating  3  on each of their sides, said coatings  3  not being electrically conductively connected to one another. 
     The structures  13  are embodied in an approximately mirror-inverted fashion with respect to an electric contact  12  arranged between two PTC panels in each case, said structures  13  being formed by one thin sheet metal panel which is arranged in the manner of a comb and is connected to the positive pole. The outer side is connected to the negative pole so that the PTC material of the heating elements  2  has a relatively uniform flow over the panel thickness, as a result of which the heating elements  2  are heated uniformly until the resistance becomes so large, due to the temperature, that no further heating is possible anymore and a stable state is established, with constant emission of heat. The structures  13  determine the distance between two heating elements  2  and thus determine the free intermediate space through which the air to be heated flows. On the other side, the electric contact  12  determines the distance between two PTC panels. 
     According to the ninth exemplary embodiment which is illustrated in  FIGS. 24   a  and  24   b , the heating elements  2  for an additional electric heater  1  are embodied in a zig-zag shape and are provided on both sides with a coating  3  composed of electrically conductive metal foils. The metal foils here are metal foils made of an aluminum alloy, but other metals/metal alloys, such as in particular ones based on copper, are also possible. On the outer sides of the bending points, the coating  3  makes contact with panel-shaped contact elements  14  which are composed of sheet aluminum plates and are each arranged between two heating elements  2  and on the outer sides of the outermost heating elements  2 . The connection is made here by means of bonding, for which an electrically conductive bonding agent is used according to the exemplary embodiment. The contact elements  14  are arranged with alternate polarity. 
     Due to the metal foils, there is a relatively uniform flow over the entire surface of the heating elements  2  so that uniform heating takes place. 
     In addition to zig-zag-shaped heating elements  2  it is, of course, also possible to use other structures such as, for example, corrugated, U-shaped or meandering ones. However, in order to ensure that there is a uniform flow, the thickness of the structures, i.e. the minimum distance between the two coatings, should be relatively constant.