Patent Publication Number: US-11035589-B2

Title: Electric heating device

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electric heating device with a casing that forms inlet and outlet openings for the fluid to be heated and a circulation chamber as well as a connection chamber separated from the circulation chamber by way of a preferably fluidically tight partition wall. A heat-emitting surface of a PTC heating element is exposed in the circulation chamber. This heat-emitting surface is coupled to a PTC element in a heat conductive manner. The PTC element is contacted in an electrically conductive manner by electrical conductor tracks to different polarities, which are electrically connected in the connection chamber. 
     2. Description of the Related Art 
     Such an electric heating device can be configured according to EP 1 768 459 A1 and can be used as an electric heating device for heating air. However, it can in particular be designed as a liquid heating device, as is known, for example, from EP 1 872 986 A1. In this prior art, heating ribs protrude into the circulation chamber, through which the liquid fluid to be heated is flowed. The heating ribs form a U-shaped pocket in which the PTC element as well as the conductor tracks and insulation layers each on the outer side of the conductor tracks are accommodated. The PTC elements and the conductor tracks, which abut thereagainst on both sides for energizing, are accommodated in the heating rib in an electrically insulating manner. The heating rib and the partition wall are there made of metal. The heat generated by the PTC element is passed via the walls forming the heating rib outwardly to the outer surface of the heating rib and there delivered to the liquid medium to be heated. 
     SUMMARY OF THE INVENTION 
     The present invention relates, in particular, to an electric heating device for heating a motor vehicle, in particular a passenger car. Accordingly, the electric heating device is a component which must be produced economically. Furthermore, it is generally to be observed with built-in components for motor vehicles that the built-in components need to be designed to be weight-optimized. In view of this, EP 2 797 381 A1, which is likewise to be considered as being generic, proposes an electric heating device with a casing which comprises a casing cover forming the partition wall, said casing cover is produced as an aluminum diecasting component and is sealingly placed on a casing lower part circumferentially surrounding the circulation chamber and designed as an injection-molded plastic member and is thereby reduced in weight. 
     However, though the solution last discussed can be produced more economically and is also reduced in terms of weight as compared to earlier proposals with a pure metal casing, demands remain unmet, in particular, with regard to electromobility and the increasing use of electric heating devices as liquid heaters in motor vehicles. The present invention seeks to provide an electric heating device which is light in weight and inexpensive to manufacture. 
     To solve this problem, the present invention proposes an electric heating device having a casing which forms the circulation chamber and the connection chamber, where the fluid to be heated flows in the circulation chamber, and the electrical connection of the PTC heating element to the power current is effected in the connection chamber. It goes without saying that several PTC heating elements can be provided in the circulation chamber. They can be grouped to heating circuits in a known manner within the connection chamber by way of an electrically conductive plate or circuit board. This electrically conductive plate or circuit board can be distinguished by the absence of any electronic components and serves merely to group the individual PTC elements into heating circuits. It can, in particular, be configured according to EP 2 505 931 A1. In parallel thereto, a further electrically conductive plate or circuit board can be provided in the connection chamber which can be equipped with components in order to process or control the control current and/or the power current to the individual PTC heating elements. 
     The present invention is distinguished from prior art in that the casing is a plastic casing. Accordingly, the circulation chamber and the connection chamber as well as the partition wall separating the two chambers, which is preferably a fluidly tight partition wall, are largely completely surrounded by plastic walls. The walls which surround the connection chamber at least circumferentially, the walls circumferentially surrounding the circulation chamber, and the partition wall usually extending at a right angle thereto, are formed from plastic material. This part of the casing can be designed as an integrally formed casing base element, i.e. as an integrally formed component produced by way of injection molding. The cover covering the circulation chamber and/or the connection chamber on the upper side can also be made of plastic material, so that the connection chamber as a whole and the circulation chamber as a whole is/are formed and defined by plastic components. However, the casing cover can possibly also be made of a thin metal plate which contributes only insignificantly to increasing the dead weight of the casing. 
     According to the present invention, the plastic casing is further provided with a shielding. This shielding encloses the casing substantially entirely circumferentially to provide effective EMC protection for the electric heating device. The switching operations performed in the electric heating device then do not lead to any significant electromagnetic interference within the vehicle. The shielding is preferably provided with a ground connection. This ground connection is embodied in the form of an object in form of a ground interface, for example, in the form of a plug contact element or an attachment eyelet. The electric heating device can be connected via this ground connection to the minus pole of the vehicle after installation in the vehicle. The ground connection is commonly exposed on the outside of the casing. It can also be part of the attachment of the casing to the vehicle, so that the ground connection is established when a mounting bolt or screw is connected to the casing of the electric heating device to mount the electric heating device to the vehicle. 
     The plastic casing can be penetrated by a ground monitor connection, which, for example, projects into the circulation chamber in order to verify whether the electrical insulation of the PTC heating element within the circulation chamber is functional. Such a ground monitor can be exposed as a connector pin on the exterior of the casing of the electric heating device. The ground monitor can be provided, for example, in combination with an attachment eyelet with which the electric heating device is mounted and attached to a motor vehicle. The ground connection is commonly electrically coupled to the shielding of the casing. A ground monitor connection has no function of being used to electrically supply the PTC elements. It merely verifies the electrical insulation of the PTC element against the casing or the fluid to be heated. 
     The ground monitor can be connected in an electrically conductive manner within the casing to the aforementioned ground terminal. The conductor path can be effected, for example, via the shielding of the casing. 
     In addition to the connection chamber, a control chamber, in which a control device is accommodated, can be formed by the plastic casing. This control device, which is provided as a structural unit with the actual heating part, is in terms of control operation coupled to the PTC heating element. An electrical connection, for example, in the form of a plug contact is commonly provided between a conductive plate forming the heating circuits and a circuit board of the control device. Such an electrical connection can be formed, for example, by a current bar which is sealingly received in the wall of the plastic casing by way of overmolding. The plastic casing is preferably formed from crosslinked plastic material, in particular, duroplastic material. The plastic casing is therefore preferably not formed from thermoplastic material. Crosslinked plastic material in a particular way offers the possibility of accommodating the current bar in a sealing manner in the wall of the plastic casing. The wall, which is penetrated by the current bar, can also be formed by the partition wall in order to electrically connect, for example, one or more PTC heating elements within the circulation chamber. The corresponding development can also by itself be essential to the invention. According to a preferred embodiment of the present invention, the shielding is formed by a metal mesh integrated in the plastic casing. In this preferred development, the shielding is regularly not located on the outer side of the casing. Instead, the shielding is provided within the casing, i.e., for example, lining the respective chambers or accommodated in the plastic casing by overmolding. For attaching it, the shield can be connected to the plastic casing in a simple manner by overmolding the plastic material forming the plastic casing. 
     According to a preferred development of the present invention, the casing accommodates a control device which is surrounded circumferentially by the shielding. This part of the shield, as well, can be formed by an integrated metal mesh. 
     Alternatively, the shielding can also be formed by a metal casing enclosing the plastic casing on the outside. This metal casing can be made of mesh material for weight reduction. Alternatively, a continuous metal sheet can also form the metal casing. 
     This metal casing is preferably formed from at least two shell elements which receive between them the inlet and outlet ports of the casing which form the inlet and outlet openings. 
     The metal casing can comprise recesses through which attachment projections protrude over the outer surface of the metal casing, where these attachment projections are formed by the plastic casing and serve to attach the electrical heating device to the vehicle, for example, to receive attachment bolts or screws therein. 
     The casing can comprise a casing base member formed from plastic material, which forms the partition wall and walls that circumferentially surround the circulation chamber and the connection chamber. This casing base member is designed as an integrally formed component. It forms the base of the casing, which is made of plastic material, preferably entirely of plastic material. The circulation chamber and the connection chamber are basically defined by the casing base member. Only covers on the end side need to be placed on the casing base member in order completely close off, firstly, the circulation chamber and, secondly, the connection chamber. The casing base member also forms the inlet and outlet openings to the circulation chamber, particularly preferably in the shape of connection ports for connecting a hose or tube for liquid. 
     In a cross-sectional view, the casing base member is preferably formed to be H-shaped, where the cross bar of the H is formed by the partition wall which defines the connection chamber on one side and the circulation chamber on the other side. Furthermore, a divider extending substantially perpendicularly from the partition wall and dividing the circulation chamber into an inlet circulation chamber and an outlet circulation chamber can be formed by the casing base member. Due to this divider, two heating chambers are formed within the circulation chamber in each of which at least one PTC element is disposed, where one of the heating chambers is in direct communication with the inlet opening and the other of the heating chambers with the outlet opening. Provided therebetween is commonly a connection channel. It can be formed by a channel wall that extends at a right angle to the partition wall and extends at a right angle to the divider, and is likewise formed as part of the casing base. The divider and the channel wall can be continued by corresponding wall segments which can be formed by a casing cover that covers the circulation chamber. Such a casing cover, however, usually rests with its outer circumferential surface directly on the walls surrounding the circulation chamber and formed by the casing base. 
     Due to the casing base member, the present invention therefore provides the possibility of forming the central element of the heating device as an integrally formed component. The casing base member can then form plug-in receptacles for the sealing arrangement of PTC heating elements, which are first attached to the partition wall by being plugged on. 
     The heating device according to the invention also comprises a control chamber for accommodating a control device which is designed as a structural unit together with the power component of the electric heating device. The power component is then, in particular, formed by the circulation chamber and the PTC heating elements accommodated therein. This control chamber can be formed together with the connection chamber. In this case, the electric heating device has only two chambers. Alternatively, the control chamber can also be configured as chamber separate from the connection chamber. In this case, the connection chamber accommodates, for example, only electrically conductive plates or circuit boards which serve to group individual PTC heating elements to a heating circuit. A printed circuit board fitted with electronic components is then not accommodated in the connection chamber. Such a printed circuit board is provided separately for this purpose in the control chamber. This control chamber is formed by the casing. According to a preferred development of the present invention, the casing base member also forms the walls circumferentially surrounding the control chamber. In this preferred development, at least three different chambers [are given], each of which is assigned a particular function, namely heating the preferably liquid fluid to be heated (circulation chamber), grouping the PTC heating elements to heating circuits (connection chamber), and finally, processing or controlling control signals and power current (control chamber). 
     The casing base member is preferably formed from cross-linked plastic material, i.e. no thermosetting material, but, for example, duroplastic material. The selection of this material leads to a specific technical effect, provided that a current bar is provided extending between two different chambers of the electric heating device and accommodated into the casing base member by way of overmolding. In this case, the current bar is sealingly accommodated in the casing base member by overmolding, i.e. in a wall which separates one connection chamber from the other connection chamber. The current bar preferably forms contact strips on oppositely disposed ends for plugging on a conductive plate or a circuit board. As a result, electrical conductor tracks, for example, of a printed circuit board or of an electrically conductive plate, can be plugged onto the current bar in a simple manner. It has been found that duroplastic material establishes a close and sealing connection to the current bar after overmolding. 
     The respective wall can be formed by the partition wall between the circulation chamber and the connection chamber. However, it can also be formed by a wall which extends between the connection chamber and the control chamber. 
     Such a wall is preferably configured as a control chamber base by the casing base. Such a control chamber base preferably has a passage opening into which a cooling element is sealingly inserted. The cooling element is coupled with its one end to the circulation chamber in a thermally conductive manner and with its other end within the control chamber to a component of the control device generating thermal dissipation loss. The cooling element can be entirely made of metal, for example, aluminum or copper. In the case where the control chamber is identical to the connection chamber, the control chamber base presently mentioned corresponds to the partition wall, which is provided between the circulation chamber and the connection chamber, and is in this case provided with the passage opening. 
     According to a preferred development of the present invention, the cooling element is exposed in a cooling channel of the circulation chamber. This cooling channel can be the previously mentioned connection channel which is within the circulation chamber and separated from the heating chamber by a channel wall. Arranging the end of the cooling element to be cooled behind the channel wall results in radiation shielding between the cooling element and the PTC heating element provided in the circulation chamber. The cooling element is therefore not directly exposed to the radiant heat emitted by the PTC heating element. This formulation is intended to express the fact that the end of the cooling element to be cooled, though being exposed in the circulation chamber, does not “see” any PTC element. 
     The cooling element is preferably held in the passage opening by way of a bayonet lock. The cooling element can then be designed as a substantially cylindrical component, which is protruded on oppositely disposed circumferential segments by locking projections, which, after the bayonet lock has been joined, generally abut against one of the walls circumferentially surrounding the circulation chamber, whereby the cooling element is locked. It goes without saying that the passage opening must be provided with axially extending grooves which are designed to have the locking projections pass through, as is customary with a bayonet-type lock. For joining the cooling element, the latter is commonly inserted from the control chamber side in the direction toward the circulation chamber, commonly entraining and incorporating a sealing element, for example, in the form of a sealing ring, which seals the cooling element against the wall defining the passage opening. On its side facing away from the circulation chamber, the cooling element commonly has a collar which abuts against an edge defining the passage opening on the control side. 
     In the above-discussed development with a circulation chamber, a connection chamber, and a control chamber, where the connection chamber and the control chamber are provided separate from each other, the casing base member preferably forms a first plastic tub which forms the circumferential surfaces of the circulation chamber, a second plastic tub, which forms the circumferential surfaces of the connection chamber, and a third plastic tub, which forms the circumferential surfaces of the control chamber. As already mentioned, the first and the second plastic tubs usually share a base formed by the partition wall. The first and the second plastic tubs are therefore open to oppositely disposed sides and separated from each other by the partition wall. The third plastic tub extends at a right angle to the first and the second plastic tubs. Accordingly, the main direction of extension of a casing cover that covers the third plastic tub is perpendicular to the main direction of extension of the casing cover that covers the first and the second plastic tubs. The base of the third plastic tub is then commonly formed by a wall segment that is formed by the casing base member and that forms a circumferential wall of the connection chamber and a circumferential wall of the circulation chamber commonly in straight-line extension. 
     Each of the aforementioned casing covers to the connection chamber, the circulation chamber and the control chamber, respectively, is preferably made of plastic material, particularly preferably of duroplastic material. For the tight connection between the respective casing cover and the casing base member, the walls circumferentially surrounding the connection chamber, the circulation chamber and/or the control chamber are provided with a circumferential groove on the face side. Adhesive, in particular, epoxy adhesive, is introduced into this groove before the respective casing cover is joined. The casing cover has a circumferential rim which is introduced into the circumferential groove and, together with a further ridge running in parallel thereto, forms a mating groove into which commonly an inner ridge engages, which defines the circumferential groove, whereafter a kind of labyrinth seal is formed, which is substantially completely filled by adhesive after pressing the casing cover on due to the volumetric dosing of the adhesive into the groove. 
     According to a preferred development of the present invention, a current bar extending between two different chambers is provided and is sealingly accommodated in the casing base member by way of overmolding. The latter is, in particular for this preferred embodiment, formed from duroplastic material. The duroplastic material encloses and seals the current bar within the material such that a reliable seal is provided between the two chambers along the outer surface of the current bar. 
     Such a configuration can be provided, for example, between the control chamber and the connection chamber, penetrating a control base. Such a control chamber base defines the control chamber on the underside and forms a wall circumferentially surrounding the connection chamber. In the development of the present invention, the current bar thus provided is embodied as a connection element between a circuit board received in the control chamber and the connection chamber. The current bar can be provided as a plug element and can be electrically contacted by, firstly, plug-on contacts to the printed circuit board and, secondly, a conductive plate provided within the connection chamber. This plate can be configured like in EP 2 505 931 A1. Within the connection chamber, the plug-on contact formed by the current bar preferably extends parallel to contact strips of the PTC heating element so that the plug-on contact to the PTC heating element and the plug-on contact to the current bar can be effected by plugging on an electrically conductive plate in a predetermined direction, commonly by lowering the conductive plate into the connection chamber. 
     According to a preferred embodiment of the present invention, the cooling element is sealingly accommodated in the casing by way of overmolding. In this embodiment as well, the casing is preferably formed from duroplastic material. On its outer circumferential surface, the cooling element can comprise one or more regularly circumferentially extending attachment collars, which together with the solidified plastic material of the casing form a labyrinth seal and accordingly improve both the reliable attachment of the cooling element in the casing as well as the tightness. During overmolding, a surface of the cooling element facing the connection chamber can be covered entirely or partially by plastic material forming the casing in order to provide an electrically insulating contact surface for a power transistor that is connected to the cooling element in a heat-conducting manner in order to dissipate the thermal dissipation loss produced by the power transistor to the medium to be heated within the circulation chamber. The insulation can be formed integrally with the casing as a thin film. This makes arranging a separate electrical insulation between the cooling element and the power transistor unnecessary. Production is simplified. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details and advantages of the present invention shall become apparent from the following description of an embodiment in combination with the drawing, in which: 
         FIG. 1  shows a perspective top view onto a first embodiment of a casing; 
         FIG. 2  shows a perspective side view of the embodiment shown in  FIG. 1 ; 
         FIG. 3  shows a cover plate  22 , provided in the first embodiment, in isolated representation together with PTC heating elements; 
         FIG. 4  shows the cover plate shown in  FIG. 3  when being fitted with a PTC heating element; 
         FIG. 5  shows a perspective sectional side view of a second embodiment of the present invention; 
         FIG. 6  shows detail VI according to the representation in  FIG. 5 ; 
         FIG. 7  shows a perspective view into the control tub of the second embodiment; 
         FIG. 8  shows a perspective longitudinal sectional view taken along line VIII-VIII according to the representation in  FIG. 7 ; 
         FIG. 9  shows a perspective and partially exposed side view of the second embodiment with a shielding provided on the plastic casing; 
         FIG. 10  shows a perspective side view of parts of the PTC heating element; 
         FIG. 11  shows a partially exposed perspective side view of parts of the PTC heating element; 
         FIG. 12  shows a perspective cross-sectional view of the PTC heating element; 
         FIG. 13  shows a longitudinal sectional view through the cover plate according to  FIGS. 3, 4  with a single PTC heating element accommodated therein; 
         FIG. 14  shows a perspective side of a further embodiment with the casing cover removed; 
         FIG. 15  shows a view similar to  FIG. 14  of the further embodiment without the casing cover, and 
         FIG. 16  shows a perspective sectional view through the partition wall of the further embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     1. Basic Configuration of the Heating Device 
       FIG. 1  shows a perspective top view of a casing, designated by reference numeral  2 , of an electric heating device configured as a water heater. The casing  2  has a casing tub element  4  made of plastic material. This plastic material is presently duroplastic material. The casing  2  forms an inlet port  6  and an outlet port  8  which are presently embodied formed integrally on the casing tub element  4 . The ports  6 ,  8  are designed as hose connection ports and form an inlet opening  10  and an outlet opening  12 , respectively, to a circulation chamber designated by reference numeral  14 . The circulation chamber  14  is divided into two heating chambers, of which only the heating chamber  16  shown in the front of the representation can be seen in  FIGS. 1 and 2 . The circulation chamber  14  is divided by a dividing wall  20 , which is formed integrally on the casing tub element  4  and protrudes inwardly from the base  18  thereof, and which is continued by a further dividing wall  24  identified by reference numeral  24  and formed on a cover plate designated by reference numeral  22 . On the side disposed opposite to the two nozzles  6 ,  8 , the two heating chambers  16  are connected to one another by way of a connection channel which can be seen in the second embodiment of a casing  2  according to  FIG. 5  and is designated there by reference numeral  26 . This connection channel  26  is separated from the heating chamber  16  by a channel wall segment  28 , which is integrally formed on the cover plate  22  and extends at a right angle to the partition wall  20 , and a channel wall segment  30  which projects from the base  18  of the casing tub element  4 . 
     The cover plate  22  is made of plastic material. In the present case, the cover plate  22  is made of duroplastic material. In principle, the use of duroplastic material for producing the casing  2  or parts thereof has the advantage that duroplastic material exhibits relatively high temperature stability of about 150° C. and also shows no brittleness at temperatures of −40° C. This results in better temperature resistance for a casing formed entirely or partially from duroplastic material, for requirements which arise during heating operation and when operating vehicles in arctic regions. However, duroplastic material is also preferable over thermosetting materials because duroplastic material has better resistance against leakage current than thermosetting material. 
       FIG. 4  shows the cover plate  22  without the casing tub element  4 . The cover plate  22  has a circumferential edge  32  which interacts with a circumferential groove  34  formed on the casing tub element  4 . This groove  34  is filled with adhesive, in the present case epoxy glue, before the edge  32  penetrates into the groove  34  in order to adhesively bond the cover element  22  to the casing tub element. The segments  24  and  28  are joined to the corresponding wall elements  20  and  30 , respectively, of the casing tub element  4  in the same way. The circulation chamber  14  is then sealed in a fluidly tight manner between the base  18  of the casing tub element  4  and the cover element  22  and circumferentially surrounded by walls formed by the casing tub element  4  and extending at a right angle to the base  18  thereof. 
     Prior to assembly, the cover element  22  is fitted PTC heating elements  40  in the manner shown in  FIGS. 3 and 4 . For this purpose, the PTC heating elements  40  are inserted on the connection side, i.e. where the PTC heating elements  40  each have two contact strips  42 , into female plug element holding fixtures  44  which are integrally formed in the cover plate  22 . These plug element holding fixtures  44  are each provided with a longitudinal slot  46  which is penetrated by the contact strips  42  (cf.  FIGS. 1, 3 ). A longitudinal slot  46  is an example of a through-hole opened in the cover plate  22  as an example of an inner wall. 
       FIG. 4  illustrates the insertion of the PTC heating element  40  into the plug element holding fixture  44 . For the plug connection established after insertion, the PTC heating element  40  has a sealing collar  48  which is configured to be adapted to the dimension of the plug element holding fixture  44  and is formed from elastomeric material which is sealingly injected into the plug element holding fixture  44 , so that the slots  46  are sealed fluidly tight against the circulation chamber  14 . Furthermore, by injecting the material forming the sealing collar  48 , the PTC heating element  40  is held frictionally engaged within the plug element holding fixture  44 , namely, also in the direction of insertion, which corresponds to the direction of longitudinal extension of the contact strips  42 . Joined in this manner, the PTC heating elements  40  adhere to the cover plate  22  even if the latter with the plug element holding fixtures  44  is oriented downwardly, as shown in  FIG. 3 . 
     2. Casing Base Member 
       FIG. 5  in a sectional view illustrates the nature of an embodiment of a casing base member which is designated by reference numeral  50 . The casing base member  50  forms walls that are designated by reference numerals  52  and surround the circulation chamber  16  circumferentially, i.e. the heating chambers  16  and the connection channel  26 . The cover plate  22 , which is provided as a separate component in the previously described embodiment, is presently embodied integrally with the casing base member  50  and presently also forms a partition wall  54 , like in the first embodiment, which separates the circulation chamber  16  from a connection chamber  56  in which presently all contact strips  42  of all the PTC heating elements  40  are exposed. Walls designated by reference numeral  58  and surrounding the connection chamber circumferentially are also formed by the casing base member  50 . Finally, in the extension of the ports  6 ,  8  and in this direction behind the circulation chamber  16  or the connection chamber  56 , respectively, walls  60  are formed by the casing base member  50  and surround a control chamber  62  circumferentially. A control chamber base, designated by reference numeral  64 , from which the walls  60  rise, is in part formed by one of the walls  52  and one of the walls  58 . In a cross-sectional view extending parallel to the direction of extension of the nozzles  6 ,  8 , the casing base member therefore accordingly appears to have an H-shaped cross-section with a U adjoining at one side of the H. The casing base member  50  is made of plastic material and configured as an integrally formed component. The casing base member  50  is presently formed from duroplastic material. 
     The walls  52 ,  58 ,  60  are each provided with a circumferential groove  66 ,  68 ,  70  on the face side. The function of these grooves  66 ,  68 ,  70  is illustrated in particular by  FIG. 6 . A cover element  72  to the control chamber  62  is there illustrated, which is configured as a substantially flat plate with a circumferential rim  74 . This rim  74  engages in the associated groove  70  and displaces adhesive applied there (in the present case, epoxy adhesive). The rim  74  forms an outer edge of a mating groove  76  which is formed on the cover element  72  and is defined on the inside by an inner ridge  78 . A kind of labyrinth seal is formed thereby, into which displaced adhesive can ingress, which has been introduced either into the groove  70  or the mating groove  76  prior to the installation of the cover element  72 . Correspondingly, the casing cover  80  to the circulation chamber  14  and  82  to the connection chamber  56 , respectively, is formed on the edge side and attached to the casing base member  50  and sealed against it. 
     As also illustrated in  FIG. 6 , the control chamber base  64  is provided with a passage opening  84  into which a cooling element  86  is introduced. A sealing ring  88  is provided between the cooling element  86  and the inner circumferential wall of the passage opening  84  for sealing the circulation chamber  14  against the control chamber  62 . On the control casing side, the cooling element  86  forms an annular stop  90  which abuts against the face side end surface of the passage opening  84 , which is extended in the direction towards the control chamber  62  as a projecting collar  92  and formed by the material of the casing base member  50 . 
     As shown in  FIG. 7 , the outer circumferential surface of the cooling element  88 , which is formed substantially cylindrical at least in the direction toward the circulation chamber  14 , is protruded circumferentially on opposite sides by locking projections  94 , which interact with locking catches  96 , which at the inner circumference project over the substantially cylindrical passage opening  84  to form a bayonet lock with which the cooling element  86  is fixed with respect to the casing base member  50 . The sealing ring  88  can be configured such that it holds this lock subject to axial and/or radial pretension. In addition, the control chamber base  64  can be provided with depressions adjacent to the mouth of the passage opening  84  on the sides of the circulation chamber  14  into which the locking projections  94  engage and which hold the cooling element  86  in a positive-fit anti-rotational manner in order to prevent the bayonet lock  94 ,  96  from being released unintentionally. 
       FIG. 7  provides a view into a control casing tub which is formed by the control chamber base  64  and the walls  60  and which is closed by the cover element  72  in the final assembled state. 
       FIG. 5  shows, above the partition wall  54 , a connection tub  100  formed by the partition wall  54  and the walls  58 . On the side which is in the representation according to  FIG. 5  located therebeneath, a circulation pan  102  is formed by the partition wall  54  and the walls  52  through the casing base member  50 . The casing base member  50  forms substantially all the chambers  14 ,  56 ,  52 , which only need to be covered by the cover elements  72 ,  80 ,  82 , in order to create an electric heating device which is operational and formed as a lightweight plastic part. 
     3. Sealing Overmolded Current Bar 
       FIG. 8  illustrates a current bar  104  which is provided as a conductor track between the connection chamber  56  and the control chamber  62 . The current bar  104  is bent twice by 90° in opposite directions at the height of the control chamber base  64  and within the latter. One end of the current bar  104  located within the control chamber  62  is configured as a plug connection for a printed circuit board assembly  106  provided within the control chamber (cf.  FIG. 6 ). The other end is provided within the connection chamber as a connection strip  108  for a distributor plate provided inside the connection chamber and omitted in the previous figures. The distributor plate can be configured like in EP 2 505 931 A1. Several connection strips  108  are exposed within the connection chamber  56  by several current bars  104  provided in an identical manner. The connection strips  108  extend parallel to the contact strips  42  to the PTC heating elements. The distributor plate can there be lowered into the connection chamber from above in order to contact all contact and connection strips  42 ,  108 . 
     4. Shielding 
     The sheet metal strips  128  are formed from punched sheet metal and represent embodiments of contact plates. 
       FIG. 9  shows a partial sectional view of the casing  2  described above. This plastic casing  2  is surrounded by two metal shells  110 ,  112  which form a shielding for the electric heating device so that electromagnetic radiation cannot be emitted from the latter. The dividing plane of the metal shells  110 ,  112  is located approximately at the center height of the two nozzles  6 ,  8 . The metal shells  110 ,  112  substantially completely surround the casing  2 . Only attachment projections  114  formed on the plastic casing  2  project outwardly beyond the metal shells  110 ,  112  and through recesses  115  formed on the metal shells  110 ,  112  and through the shielding formed thereby. Furthermore, connector casings protrude which are designated by reference numerals  116  and  118 , where the connector casing  116  is presently provided for the high-voltage power current and the connector casing  118  for the low-voltage control lines. The embodiment shown in  FIG. 9  allows for the attachment of the casing  2  by way of the attachment elements  110  made of plastic material. In using them, the requirement to fasten the casing  2  at the desired location within a motor vehicle can be complied with in a simple manner. In  FIG. 9 , the metal shells  110 ,  112  are formed to be substantially end-to-end. They can just as well be formed from a perforated plate without the effect of the metal shells  110 ,  112  as an EMC protection being lost. 
     5. PTC Heating Element as a Frame Element 
       FIG. 10  shows a PTC element  120  which is provided with an insulation layer  124  on its oppositely disposed main side surfaces  122 . The insulation layer  124  is presently a ceramic plate made from aluminum oxide. However, it can also be applied as a coating onto the PTC element  120  or as a combination of a coating with a single or multiple layer insulation coat. The PTC element  120  is designed as a platelet having a width B or a length L, respectively, that is greater by the factor of at least 10 than the thickness D which corresponds to the distance between the two main side surfaces  122 . Sheet metal strips  128  substantially extending in the direction of the length L are provided on mutually oppositely disposed face side surfaces  126  and are glued to the PTC element  120  and are connected in an electrically conductive manner to a surface metallization of the PTC element  128 , which can be applied as a protection by way of PVD or CVD. These sheet metal strips each form conductor tracks to different polarities with which the PTC element  120  is energized. The sheet metal strips  12  consist of a contact ridge  130  which is relatively narrow and the contact strip  42  which is widened in relation to the contact ridge  130  in the direction of the width B. 
     The contact ridges  130  presently form the electrodes to the PTC element are electrically connected to the metallization of the PTC element  120 . The thickness of the contact strip  42  is smaller than the thickness of the contact ridge  30 . Its thickness is in turn smaller than the thickness D of the PTC element. The sheet metal strip  128  is provided such that it does not project over the main side surfaces  122  of the PTC element  120  at any point. As can be seen in particular in  FIGS. 11 and 12 , the insulation layers  124  project laterally beyond the PTC element  120 . The insulation layers  124  accordingly have a base area which is larger than the base area of the main side surfaces  122  of the PTC element  120 . Accordingly, the outer edges of the insulation layers  124  receive the contact ridge  130  between themselves on both sides (see  FIG. 12 ). Each contact ridge  130  is glued to one of the oppositely disposed face side surfaces  126 . The insulation layer  124  is likewise glued to the PTC element  120 . The insulation layer  124  abuts directly against the PTC element. One of the insulation layers  124  therefore directly contacts the associated main side surface  122  of the PTC element. 
     The outer side of the respective sheet metal strips  128  is completely flat and extends strictly in the direction of the length L. The contact strips  42  are in the width direction formed larger than the contact ridge  130  and project over the PTC element  120  towards each other (cf.  FIG. 10 ). The contact strip  42  is also thinner than the contact ridge  130 . 
     The region of the sheet metal strip  128 , which is broadened as compared to the contact ridge  130 , is received in a heating element casing  134  surrounding the PTC element  120  in a frame-like manner. This plastic frame  134  encloses the circumferential edges of the insulation layers  124 . The edge surfaces  132  are also sealed by the material forming the plastic frame  134 . The plastic frame  134  is formed as an elastomeric plastic frame  134  by overmolding elastomeric material. With the finished PTC heating element  40 , only the contact strips  42  project over the plastic frame  134  on a face side. All other functional parts of the PTC heating element  40  used for heat generation and current conduction are accommodated within the plastic frame  134 . As illustrated in particular by  FIG. 12 , the PTC element  120  with its oppositely disposed main side surfaces  122  is located behind frame openings which are designated by reference numeral  136  and are formed by the plastic frame  134 . These frame openings  136  are covered by the insulation layer  124 . Almost no covering of the plastic frame  134  with the main side surfaces  122  of the PTC element presently arises so that this exposes approximately 100% of its main side surfaces  122  behind the frame openings  136 . 
     The frame ties of the plastic frame  134  protruding at a right angle from the sealing collar  48  are each provided with abutment ridges  137  on the edge side. The abutment ridges  137  are designed in the manner of tapered slats which have a thickness that tapers as compared to the thickness of the plastic frame  134 . As can be seen, for example, in  FIG. 6 , the PTC heating elements  40  are each provided slightly staggered one behind the other such that each first PTC heating element  40  abuts against the wall of the casing  2  on the left side and is sealed there by the abutment ridge  137  which sealingly abuts against a border wall of the circulation chamber  14 . Therefore, no fluid can flow past the PTC element  40  on this side, but it can flow past the PTC heating element that is in the flow direction located upstream or downstream. The latter, in turn, with its abutment ridge  137  abuts against the wall defining the circulation chamber  14  the opposite side and redirects the flow channel there. A meandering flow channel is thus formed by the alternating arrangement of the PTC heating elements  40 . 
     6. Plug Connection 
     The plastic frame  134  further forms the previously described sealing collar  48 , which is provided with segments  138  tapering toward the free end of the contact strips  42  (cf.  FIG. 13 ). Three of these segments  138  are presently provided one behind the other in the direction of longitudinal extension of the contact strip  42 . An annular stop  140  is formed by the heating element casing  134  between the frame opening  136  and these segments  138 . As illustrated by  FIG. 13 , this stop is in sealing abutment against a cylindrical mating surface  142  formed by the female plug element holding fixture  44 . 
     As illustrated in  FIGS. 2 and 5 , the oppositely disposed side of the PTC elements  40  abuts the base  18  of the casing  2  after installation. Provided in the base  18  for exact positioning are depressions  144  in which the face side surfaces of the heating element casing  134  are received. This ensures exactly right-angled positioning of the PTC heating elements relative to the partition wall  54 . Furthermore, a holding force is generated which prevents the PTC heating elements  40  plugged into the partition wall  54  from dropping out of the female plug element holding fixtures  44  during operation and in particular due to the vibration of the vehicle. This base-side abutment of the PTC heating elements  40  also ensures the contact pressure within the plug contact of the PTC heating elements, so that they securely and also permanently seal the longitudinal slots  46  to the connection chamber  56 . 
     It is understood that the depressions  144  can also be dispensed with. Similar to the abutment ridges  137 , an abutment ridge  137  can also be provided on the underside of the plastic frame  134  disposed opposite to the sealing collar  48  and seal there and be supported and in abutment against the base  18  of the casing  2  subject to elastic compression. This not only provides a fluidly tight seal between the underside of the PTC heating element  40  and the casing  2 . Rather, a compressive force is also generated due to the compression of the abutment ridge and holds the PTC heating element  40  captively in the female plug element holding fixture  44  even when strong vibrations act upon the electric heating device. 
     7. Electric Connection of the PTC Heating Elements 
       FIGS. 14-16  illustrate the electric connection of the PTC heating elements  40  using the example of a further embodiment.  FIG. 15  shows the embodiment without the casing  2  and parts thereof, similar to the illustration in  FIG. 3 . For the electric connection, pieces of punched sheet metal are provided in the connection chamber  56  as current bars  146 ,  148 ,  150 , comprising contact projections  152  formed by punching and bending which abut against the contact strips  42  subject to elastic pretensioning and contact them. The contact projections  152  project into receptacle openings  154  which are recessed in the sheet metal strips of the current bars  146 ,  148 ,  150 . The current bar, designated by reference numeral  148 , has a corresponding receptacle opening  154  with contact projections  152  for the current bar  104 . 
     The current bars, designated by reference numbers  146  and  148 , are each equipped with a power transistor  156  which is connected in an electrically conductive manner to the respective current bar  146 ,  150  and mounted thereon. The power transistors  156  are connected on the input side to conductors  158  which are likewise formed as current bars, and which are again provided with receptacle openings  154  with contact projections  152  for respectively contacting via a current bar  104 . Electric conduction within the containment chamber  56  [sic] is effected solely by way of sheet metal elements. These sheet metal elements are equipped exclusively with the power transistors  156  as electric or electronic components. No further electric or electronic components are provided. 
     As shown in particular in  FIG. 16 , these metal plates  146 ,  148 ,  150 ,  158  are inserted in a manner electrically insulated from one another into the connection chamber  56  and positioned there. The control signals for actuating the power transistors  146  processed on the circuit board  106  are supplied to the respective power transistor  156  via at least one of the conductors  158 . 
       FIG. 16  illustrates a sectional view at the height of a cooling element, denoted by reference numeral  86  also in  FIG. 15 . An electrical insulation coat  160  made, for example, of Kapton or a ceramic plate is implemented between the cooling element  86  and the power transistor  156 . The current bar  146 ,  150  provided between this insulation coat  160  and the power transistor  156  results in a certain spread of the heat emitted by the power transistor  156 . For this purpose, the current bar  146  or  150 , respectively, can be configured to have a greater thickness at least in the region of the power transistor  156 . It is equally possible to form the current bar  146  or  150 , respectively, from copper and thus from a material that exhibits good thermal conductivity. The sheet metal material can have an increased thickness in the region of the power transistor  156  due to contour rollers. 
     The current bars  146 ,  148 ,  150 ,  158  can be provided with a crimping tab or the like by way of punching which interacts with the casing  2  to attach the respective current bar  146 ,  148 ,  150 ,  158  to the casing  2 , and/or to abut or press the power transistor  146  against the cooling element  86  subject to pretension. 
       FIG. 16  also shows a variant of the PTC heating element  40 . According thereto, the contact strips  42  are received within the plastic frame  134  in an attachment plug  162  formed from a hard plastic component. The contact strips  42  are received in the attachment plug  162  by way of overmolding. The attachment plug  162  is made of hard plastic material with good resistance to leakage current and sparkover. The attachment plug  162  forms a hard core for the male plug element formed by the PTC heating element  40 . This attachment plug  162  is merely covered by a thin coating of the soft-elastic material of the sealing collar  48 , whereby the compression force formed by the sealing collar  48  within the female plug element holding fixture  44  is increased. The attachment plug  162  can be effected, for example, by overmolding two sheet metal strips  128 , each of which individually forms the contact ridges  130  and the contact strips  42  as an integrally formed component. Prepared in this manner and positioned spatially and physically relative to one another, the attachment plug  162  together with the sheet metal strip  128  can be overmolded with the soft-elastic plastic component for forming the plastic frame  134 , as described above. 
     The sealing collar is formed to be elongate also in the variant according to  FIGS. 14-16 . However, it goes without saying that each individual contact strip  42  can be arranged in a cylindrical sealing collar with a basically circular or oval cross-sectional area. The female plug element holding fixture  44  is then likewise provided with a cross-section that is circular or oval, respectively. Such an embodiment can increase the tightness as compared to the specifically described embodiment. It is understood that polygonal cross-sectional areas are also possible. 
       FIG. 16  further shows the attachment of the cooling element  86  within the partition wall  54 . At the height of the partition wall  54 , the cooling element  86  has an attachment collar  164  on its outer circumference which projects beyond the cooling element  86  on the edge side and is sealed in a positive-fit manner by overmolding the plastic material forming the partition  54 . The design of the attachment collar  164  incidentally also improves the tightness between the circulation chamber  14  and the connection chamber  56 . 
     According to a variant (not shown), the surface of the cooling element  86  projecting to the connection chamber  56  can also be covered with the plastic material forming the partition wall  54 . When the cooling element  46  is overmolded, a thin film can be formed by injection molding between the surface of the cooling element  86  and the interior of the connection chamber  56 , via which the power transistor  156  abuts against the cooling element  68  in an electrically insulated manner. This further simplifies production. A separate electrical insulation layer does not need to be provided between the power transistor  156  and the cooling element  86 .