Abstract:
An electrical cartridge type heater ( 100 ) includes an outer metallic jacket ( 110 ) an electrical heating element ( 120, 121 ) arranged in an interior space ( 140 ) of the outer metallic jacket ( 110 ) and a device for monitoring the temperature ( 130 ), which is galvanically separated from the electrical heating element ( 120, 121 ) and is arranged in the interior space ( 140 ) of the outer metallic jacket ( 110 ). The device for monitoring the temperature ( 130 ) includes a wire or a tube, in addition to the electrical heating element ( 120, 121 ), made of a material that changes resistance with temperature change with a value of the temperature coefficient of the electrical resistance greater than 800 ppm/K, and especially preferably greater than 4,000 ppm/K between 20° C. and 105° C. The wire or the tube is directly embedded into an electrically non-conducting filler filling a remaining interior space ( 140 ) of the outer metallic jacket ( 110 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority under 35 U.S.C. §119 of German Application 20 2015 104 723.1 filed Sep. 4, 2015, the entire contents of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention pertains to an electrical cartridge type heater with an outer metallic jacket and with at least one electrical heating element arranged in an interior space of the outer metallic jacket, wherein at least one device for monitoring the temperature, which is galvanically separated from the electrical heating element, is arranged in the interior space of the outer metallic jacket of the electrical cartridge type heater. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electrical cartridge type heaters are a versatile kind of electrical heaters, in which an electrical heating element, which is typically configured as a hot wire or resistance wire, is arranged in the interior space of an outer jacket, the jacket often, but not necessarily, being formed by a tube, especially by a tube with a circular cross section. In this connection, this kind comprises, in addition to cartridge type heaters with only one metallic jacket, also hollow cartridges, which have a second, inner, often likewise tubular metallic jacket. 
         [0004]    It is important in many applications in which such electrical cartridge type heaters are employed to monitor the operation of the electrical cartridge type heater, and it often is a question of reaching or maintaining a temperature at one or more points of the electrical cartridge type heater within a predefined temperature window as well. In order to make this possible, it is known, e.g., from DE 20 2008 014 050 U1 and DE 20 2007 010 865 U1 to arrange a temperature sensor or a temperature probe or an integrated thermocouple in the interior of the electrical cartridge type heater, which is, however, usually sensitive to pressure, which is to be taken into consideration in case of a crimping or compressing of the cartridge type heater. In addition, only a local monitoring of the temperature at one point is achieved in this way, so that a temperature deviation occurring at another point can only be detected if it has an effect on the locally monitored point. 
         [0005]    Electrical heating elements with a temperature-dependent resistance behavior are known from DE 203 21 257 U1, for example. 
       SUMMARY OF THE INVENTION 
       [0006]    Therefore, an object of the present invention is to provide an electrical cartridge type heater and an electrical heater with such a cartridge type heater, in which the above-mentioned drawbacks are avoided. 
         [0007]    An electrical cartridge type heater according to the present invention has, as is usual for cartridge type heaters, an outer metallic jacket and at least one electrical heating element arranged in an interior space of the outer metallic jacket. Further, at least one device for monitoring the temperature, which is galvanically separated from the electrical heating element, is arranged in the interior space of the metallic jacket of the electrical cartridge type heater. 
         [0008]    It is essential to the present invention that the device for monitoring the temperature be at least one wire present in addition to the electrical heating element or one tube present in addition to the electrical heating element, the wire or the tube being made of a cold-conducting material (PTC material)—a material that has an electrical resistance that increases with increased temperature such as used with a Positive Temperature Coefficient (PTC) thermistor in which a resistance increases as temperature rises. A value (absolute value) of the temperature coefficient of the electrical resistance is greater than 800 ppm/K, especially preferably greater than 4,000 ppm/K between 20° C. and 105° for this cold-conducting material. Furthermore, according to the present invention, the wire or the tube is directly embedded into an electrically non-conducting filler, which may be configured especially as MgO powder or MgO granules, filling the remaining interior space of the outer metallic jacket. 
         [0009]    In an embodiment of the present invention alternative thereto, provisions may also be made, with otherwise the same configuration, for the wire present in addition to the electrical heating element or the tube present in addition to the electrical heating element as a device (thermistor) for monitoring the temperature to be made of a heat-conducting material (NTC material)—a material that has an electrical resistance that decreases with increased temperature such as used with a Negative Temperature Coefficient (NTC) thermistor in which a resistance that decreases as temperature rises. The value (absolute value) of the temperature coefficient of the electrical resistance being greater than 250 ppm/K, especially preferably greater than 800 ppm/K (for example −900 ppm/K if the temperature coefficient is negative) and most preferably greater than 4,000 ppm/K between 20° C. and 105° C. for this heat-conducting material. In this embodiment of the present invention as well, according to the present invention the wire or the tube is directly embedded into an electrically non-conducting filler, which may be configured especially as MgO powder or MgO granules, filling the remaining interior space of the outer metallic jacket. 
         [0010]    The lower threshold of the necessary minimal value of the temperature coefficient of the electrical resistance between 20° C. and 105° C. for heat-conducting materials can be attributed to the fact that the resistance of the electrical heating element has, as a rule, a positive temperature coefficient of the electrical resistance between 20° C. and 105° C. 
         [0011]    In both alternative embodiments of the present invention, it is possible to measure, for example, the resistance of the wire or tube or variables, which can be correlated with the resistance, e.g., currents flowing at a predefined voltage, or the voltage that is necessary for reaching a predefined current, and to carry out a comparison with standard values to achieve the monitoring of the temperature. This comparison is preferably carried out in an automated manner in an electronic control and/or monitoring unit for cartridge type heaters, data of a resistance characteristic stored in a memory of this electronic control and/or monitoring unit then being preferably accessed. 
         [0012]    In addition to the greater robustness against pressure, which permits a compression of the electrical cartridge type heater, the configuration of the device for monitoring the temperature according to the present invention is characterized, besides by its extremely cost-effective feasibility, also in that the cartridge type heater can be annealed for bending, and especially also under protective gas, can be annealed under oxidizing or stress-relieved conditions after the compressing, which is not the case, for example, in the use of known PT-100 temperature sensors as a device for monitoring the temperature. 
         [0013]    It is especially preferred when the wire or tube is made of a material, the resistance of which has a temperature coefficient, the value of which is at least twice as high, and especially preferably at least five times as high in the range between 20° C. and 105° C. as the value of the temperature coefficient of the resistance of the present electrical heating elements in the range between 20° C. and 105° C. This leads to the possibility of a sufficient accuracy of the temperature monitoring being guaranteed even in the case of possible resistance tolerances as a result of the compression. 
         [0014]    If the device for monitoring the temperature is embodied in this way, a device for monitoring the temperature, in which the ambient temperature of each individual section of the wire or the tube made of cold-conducting or heat-conducting material contributes to the result of the temperature monitoring, is obtained in contrast to most devices for monitoring the temperature known at the priority date, which take a local temperature measurement. This may involve a faster response characteristic of the device for monitoring the temperature, since a change in temperature occurring locally because of a malfunction has a relatively direct effect on the nearest section of the wire or tube made of cold-conducting or heat-conducting material and thus one does not have to wait until the malfunction is manifested at the point monitored locally with a sensor or thermocouple. 
         [0015]    In addition, in contrast to the use of the temperature sensors, temperature probes or thermocouples known from the state of the art, a device for monitoring the temperature is obtained, which is mostly insensitive to pressure and the operating current or operating voltage of which is set such that the temperature dependence of this resistance influences the heat output nonessentially only, while optimization of the compression of the electrical cartridge type heater without additional effort for the mechanical protection of the device for monitoring the temperature is made possible. Accordingly, it is especially preferred when the electrical cartridge type heater is compressed or crimped in at least some sections, wherein at least one section of the wire or tube present as a device for monitoring the temperature runs in at least one compressed or crimped section. 
         [0016]    According to a preferred embodiment of the present invention, provisions are made for the device for monitoring the temperature to be configured as a tube made of a cold-conducting material or made of a heat-conducting material, and for the electrical heating element to be arranged in the interior space of this tube, so that the tube and the electrical heating element together form a coiled tube cartridge, the jacket of which represents the device for monitoring the temperature. For electrical heating elements configured as a hot wire or resistance wire, this implies that they are electrically insulated from the tube forming the device for monitoring the temperature, for example, by means of an insulating material filling, which can be accomplished, e.g., with magnesium oxide powder or granules. 
         [0017]    In an especially preferred embodiment of the present invention, the wire present as a device for monitoring the temperature or the tube present as a device for monitoring the temperature is coiled. This makes possible a first variant of the present invention, in which the wire present as a device for monitoring the temperature or the tube present as a device for monitoring the temperature is wound together with at least one heating element, but galvanically separated from same, on a common coil body. Consequently, an especially simple manufacture of an electrical cartridge type heater with a device for monitoring the temperature is made possible. 
         [0018]    As an alternative or in addition to the variant of the present invention described above, the wire present as a device for monitoring the temperature or the tube present as a device for monitoring the temperature can be configured as coiled with different coil pitches. As was already mentioned further above, in the configuration of the device for monitoring the temperature according to the present invention, the ambient temperature of each individual section of the wire or tube made of cold-conducting or heat-conducting material contributes to the result of monitoring the temperature. In a low coil pitch in a given area of the electrical cartridge type heater, an extension of the section of the wire or tube affected by the change in the temperature in this area is achieved in this area, which leads to an increased sensitivity of the device for monitoring the temperature to changes in temperature in this area, while, conversely, areas with low sensitivity can be created by high coil pitches. Thus, due to different coil pitches in a device for monitoring the temperature according to the present invention, the sensitivity thereof can be configured variably in different sections of the electrical cartridge type heater and be optimally adapted to the requirements of the application. 
         [0019]    According to another advantageous embodiment of the present invention, the wire present as a device for monitoring the temperature or the tube present as a device for monitoring the temperature is arranged in the radial direction within the coils of at least one coiled electrical heating element. Possible local malfunctions can be especially readily detected at this position. This is especially the case when the electrical heating element is wound onto a coil body and when the wire present as a device for monitoring the temperature or the tube present as a device for monitoring the temperature runs in a hole or opening of the coil body in at least some sections. 
         [0020]    For all embodiments of the present invention, in which the wire present as a device for monitoring the temperature or the tube present as a device for monitoring the temperature is made of cold-conducting material, it has proven to be especially successful when the material of the electrical heating element is an alloy containing chromium and nickel or an alloy containing copper and nickel and when the material of the wire present as a device for monitoring the temperature or of the tube present as a device for monitoring the temperature is a pure metal, especially nickel, refined nickel or highly refined nickel. When there are a plurality of such devices for monitoring the temperature, a plurality of different pure metals may optionally also be used. 
         [0021]    The electrical heater according to the present invention comprises an electrical cartridge type heater with an outer metallic jacket and at least one electrical heating element arranged in an interior space of the outer metallic jacket, in which electrical heating element at least one device for monitoring the temperature, which is galvanically separated from the electrical heating element and which is configured as at least one wire present in addition to the electrical heating element or a tube present in addition to the electrical heating element, is arranged in the interior space of the metallic jacket of the electrical heating element. In this connection, the wire or the tube is each made either of a cold-conducting material or of a heat-conducting material, wherein the absolute value of the temperature coefficient of the electrical resistance is greater than 800 ppm/K, especially preferably greater than 4,000 ppm/K between 20° C. and 105° C. for this heat-conducting or cold-conducting material when it is a cold-conducting material and greater than 250 ppm/K, especially preferably greater than 800 ppm/K, especially most preferably greater than 4,000 ppm/K when it is a heat-conducting material. Furthermore, the wire or the tube is embedded directly into an electrically non-conducting filler filling the remaining interior space of the outer metallic jacket. 
         [0022]    In addition, the electrical heater according to the present invention has a power supply for energizing the at least one electrical heating element and a device for determining the resistance of the wire present as a device for monitoring the temperature or of the tube present as a device for monitoring the temperature and for assigning a temperature value to the determined resistance of the wire present as a device for monitoring the temperature or of the tube present as a device for monitoring the temperature. 
         [0023]    It is especially preferred in this connection when the device for determining the resistance of the wire present as a device for monitoring the device or of the tube present as a device for monitoring the temperature and for assigning a temperature value to the determined resistance of the wire present as a device for monitoring the device or of the tube present as a device for monitoring the temperature with the power supply for energizing the at least one electrical heating element is in signal communication, so that the energizing of the at least one electrical heating element can be changed as a function of the temperature value assigned to the resistance value by the device for determining the resistance. 
         [0024]    The present invention is explained in detail below on the basis of figures, which show exemplary embodiments. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    In the drawings: 
           [0026]      FIG. 1 a    is a perspective view of a first electrical cartridge type heater; 
           [0027]      FIG. 1 b    is a section through the electrical cartridge type heater from  FIG. 1 a    in a direction parallel to its direction of extension; 
           [0028]      FIG. 2 a    is a perspective view of a second electrical cartridge type heater; 
           [0029]      FIG. 2 b    is a section through the electrical cartridge type heater from  FIG. 2 a    in a direction parallel to its direction of extension; 
           [0030]      FIG. 3  is a section through a third electrical cartridge type heater in a direction parallel to its direction of extension; 
           [0031]      FIG. 4 a    is a perspective view of a fourth electrical cartridge type heater; 
           [0032]      FIG. 4 b    is a section through the electrical cartridge type heater from  FIG. 4 a    in a direction parallel to its direction of extension; 
           [0033]      FIG. 5 a    is a perspective view of a fifth electrical cartridge type heater; 
           [0034]      FIG. 5 b    is a section through the electrical cartridge type heater from  FIG. 5 a    in a direction parallel to its direction of extension; 
           [0035]      FIG. 6 a    is a perspective view of a sixth electrical cartridge type heater; 
           [0036]      FIG. 6 b    is an exploded view of the electrical cartridge type heater from  FIG. 6   a;    
           [0037]      FIG. 6 c    is the electrical heating element from  FIG. 6 a    in the unwound state; 
           [0038]      FIG. 6 d    is a cross section through the electrical cartridge type heater from  FIG. 6 a    in a direction at right angles to its direction of extension; 
           [0039]      FIG. 7 a    is a section through a third electrical cartridge type heater in a direction parallel to its direction of extension; 
           [0040]      FIG. 7 b    is a first enlarged detail view from  FIG. 7   a;    
           [0041]      FIG. 7 c    is a second enlarged detail view from  FIG. 7 a   ; and 
           [0042]      FIG. 8  is a schematic perspective view an electrical heater. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    Referring to the drawings,  FIG. 1 a    shows a first electrical cartridge type heater  100  with a metallic jacket  110  that is tubular in this example, with an oval cross section, which is closed on each end face by means of end caps  111 ,  112  that are each traversed by three connecting bolts  114 - 119 . As the sectional view according to  FIG. 1 b    shows, two electrical heating elements  120 ,  121 , which are embodied as coiled hot wires, are arranged in the interior space  140  of the metallic jacket  110 , preferably under mechanical tension, between the connecting bolts  114  and  115  or  118  and  119 , while a device for monitoring the temperature  130  in the form of a coiled wire made of a cold-conducting material—(PTC material)—a material that has an electrical resistance that increases with increased temperature—or made of a heat-conducting material—(NTC material)—a material that has an electrical resistance that decreases with increased temperature—is arranged between the connecting bolts  116  and  117 , preferably under mechanical tension, which device, to clarify that it is not a third electrical heating element and a different material forms the wire, is shown to be thinner than the electrical heating elements  120 ,  121 , which, however, shall not be understood here as in all other figures to be an indication of a necessary difference with respect to the necessary dimensioning, especially with respect to the cross section. 
         [0044]    The preferably crimped insulating material, which may consist, e.g., of MgO powder or granules, actually filling the interior space  140  of the respective metallic jacket  110  is not shown in this figure as well as in all other figures for the sake of clarity. 
         [0045]      FIG. 2 a    shows a second electrical cartridge type heater  200  with a metallic jacket  210 , which is tubular in this example, with an essentially rectangular cross section with rounded corners, which is closed on one side on the end face by a bottom  211 . On the other end face, four connecting bolts  214 - 217  lead into the interior space  240  of the metallic jacket  210 . As the sectional view according to  FIG. 2 b    shows, an electrical heating element  220 , which is embodied as a coiled hot wire and which runs in an approximately U-shaped manner through the interior space  240  of the metallic jacket  210 , is arranged between the connecting bolts  214  and  215 , while a device for monitoring the temperature  230  in the form of a coiled wire, which is made of a cold-conducting material or made of a heat-conducting material, is arranged between the connecting bolts  216  and  217 , which device, to clarify that it is not a second electrical heating element and a different material forms the wire, is shown to be thinner than the electrical heating element  220  and likewise runs in an approximately U-shaped manner. 
         [0046]    In contrast to the electrical cartridge type heater  100  to be connected on both sides shown in  FIGS. 1 a  and 1 b   , the electrical cartridge type heater  200  according to  FIGS. 2 a  and 2 b    is thus a cartridge type heater  200  to be connected on one side; further, the electrical cartridge type heaters  100 ,  200  differ with respect to their cross section. 
         [0047]    The electrical cartridge type heater  300  shown in a sectional view in  FIG. 3  includes a tubular metallic jacket  310 , end caps  311 ,  312 , which is embodied as a coiled, preferably self-supporting hot wire, which is arranged in the interior space  340  of the metallic jacket  310 , an electrical heating element  320 , which is arranged between the connecting bolts  314  and  315  and a device for monitoring the temperature  330  which is arranged in the interior space  340  of the metallic jacket  310  in the form of a coiled wire made of a cold-conducting material or made of a heat-conducting material, the ends of which lead directly out of the tubular metallic jacket  310  and are thus not provided with connecting bolts. The special feature of the embodiment according to  FIG. 3  is that the device for monitoring the temperature  330  is arranged coaxially within the coils of the electrical heating element  320 , which makes possible a very high sensitivity and an especially fast response of the device for monitoring the temperature  330  to a local failure of the electrical heating element  320 . 
         [0048]    In the fourth electrical cartridge type heater  400  shown in  FIG. 4 a    and  FIG. 4 b   , the tubular metallic jacket  410  is cylindrical and closed with a bottom  411  formed in one piece with it as well as with an end cap  412 . The electrical heating element  420 , which is arranged in the interior space  440  of the tubular metallic jacket  410  and is embodied as a coiled hot wire wound onto a coil body  450 , is passed through the end cap  412  with its ends  420   a ,  420   b . The ends  430   a ,  430   b  of the device for monitoring the temperature  430 , which is embodied here in the form of a U-shaped coiled wire made of a cold-conducting material or made of a heat-conducting material lying in a central plane of the coil body  450 , are also passed through the end cap  412 . 
         [0049]    In the fifth electrical cartridge type heater  500  shown in  FIG. 5 a    and  FIG. 5 b   , the tubular metallic jacket  510  is likewise cylindrical and closed with a bottom  511  as well as an end cap  512 , the bottom  511  having a recess  511   a  as a positioning aid for a coil body  550 . As in the electrical cartridge type heater  400  according to  FIGS. 4 a,b   , the electrical heating element  550 , which is arranged in the interior space  540  of the tubular metallic jacket  510  and which is embodied as a coiled hot wire wound onto a coil body  550 , is passed through the end cap  512  with its ends  520   a ,  520   b . Also passed through the end cap  512  are the ends  530   a ,  530   b  of the device for monitoring the temperature  530 , which is embodied here in the form of a U-shaped, coiled wire made of a cold-conducting material or made of a heat-conducting material, lying in a central plane of the coil body  550 , wherein the sections  530   c ,  530   d  of the device for monitoring the temperature  530  forming the two legs of the U are passed through holes in the coil body and thus are arranged radially within the turns or coils of the electrical heating element  520  wound onto the coil body  550 . 
         [0050]      FIGS. 6 a  through 6 d    show a sixth exemplary embodiment of an electrical cartridge type heater  100 . As is especially readily seen in the exploded view of  FIG. 6 b   , the electrical cartridge type heater  600  has an electrical heating element  620 , which is arranged between an outer metallic jacket  610  and an inner metallic jacket  660 , which are connected to one another by means of a bottom surface  661  facing away from the viewer and hence not visible in  FIG. 6 a   . As can be readily seen in  FIG. 6 a   , the electrical heating element  620  is arranged in the interior space  640  of the outer metallic jacket  610  between an outer shaped ceramic part  670 , which at the same time guarantees an electrical insulation to the outer metallic jacket  610 , and a central shaped ceramic part  671 , and is optionally additionally embedded in an electrically insulating material, not shown, e.g., MgO granules, which, however, is not shown in  FIGS. 6 a  through 6 c    for the sake of clarity. 
         [0051]    In addition, a device for monitoring the temperature  630 , which is embodied in the form of a wire made of a cold-conducting material or made of a heat-conducting material, is arranged between the central shaped ceramic part  671  and an inner shaped ceramic part  672 , the wire likewise depicting a space curve, which can be obtained by winding a basic shape in a meandering pattern. It is preferred in this case when the space curve, which depicts the device for monitoring the temperature  630 , can be converted, by scaling in the radial direction, into the space curve, which depicts the electrical heating element  620 . Further, it is preferred when an alignment is present, in which points corresponding to one another in the same direction of curved arcs  628 ,  629  or  638 ,  639  of the meandering structures of the device for monitoring the temperature  630  or of the electrical heating element  620  lie each on a common radius r, as is shown by example in  FIG. 6 d   . In this way, a section of the device for monitoring the temperature  630  is directly assigned to each section of the electrical heating element  620 , which leads to especially reliable detection of temperature deviations. 
         [0052]    The end face of the electrical cartridge type heater  600 , which can be seen by the viewer in  FIG. 6 a   , is closed with a circular-ring-shaped cap  662  in the assembled state. The outer metallic jacket  610  and the inner metallic jacket  660  are each configured as a cylindrical tube and are arranged concentrically to one another. The direction of extension of the outer metallic jacket  610  and of the inner metallic jacket  660  is thus predefined by the cylinder axis. 
         [0053]    As can be especially readily seen in  FIG. 6 c   , which shows the electrical heating element  620  in the wound or unwound state, i.e., not in the state, in which it is installed, the electrical heating element has a meandering shape with meandering loops. A connecting bolt  614 ,  615  each with a hole, which cannot be seen in  FIG. 6 c   , into which an end section of the electrical heating element  620  is received and electrically contacted, is at both ends of the electrical heating element  620 . 
         [0054]    In the electrical cartridge type heater  700  shown in  FIGS. 7 a  through 7 c   , as can be especially readily seen in the detailed view of  FIG. 7 b   , the electrical heating element  720  is arranged in the form of a hot wire centrally in the interior of a device for monitoring the temperature, which device is embodied as a tube made of a cold-conducting material or made of a heat-conducting material and is electrically insulated from same with magnesium oxide powder  735 . Thus, the electrical heating element  720  and the device for monitoring the temperature together form a coiled tube cartridge, which is arranged in some sections coiled in the interior space  740  of a cup-shaped outer metallic jacket  710 . The filling, consisting of a readily heat-conducting, preferably electrically non-conducting material, especially MgO powder or granulates, filling this interior space and ensuring the heat conduction to the cup-shaped metallic jacket  710 , has been omitted for the sake of clarity. Contact plates  713 ,  714  and connecting bolts  721 ,  722  are provided for contacting the electrical heating element  720 . The contacting of the device for monitoring the temperature  730  is carried out via contact plates  715 ,  716  and connecting wires  717 ,  718 , as can be especially readily seen in  FIG. 7   c.    
         [0055]    The electrical heater  1000  shown in  FIG. 8  has, besides an electrical cartridge type heater  200 , as it was already described above on the basis of  FIGS. 2 a,b    and which was hence identified in  FIG. 8  with the same reference numbers as in  FIGS. 2 a,b   , a power supply  1010  for energizing the at least one electrical heating element  220  and a device for determining the resistance  1020  of the wire present as a device for monitoring the temperature  230  and for assigning a temperature value to the determined resistance of the device for monitoring the temperature, which are both combined in a control device  1030  in this example. Accordingly, the power supply  1010  and the electrical heating element  220  are connected to one another by electrical lines  1011 ,  1012  via the connecting bolts  214 ,  215 , and the device for determining the resistance  1020  and the device for monitoring the temperature  230  are connected to one another by electrical lines  1021 ,  1022  via the connecting bolts  216 ,  217 . 
         [0056]    In addition, the device for determining the resistance  1020  of the device for monitoring the temperature  230  is in signal communication with the power supply  1010  via a signal line  1023 , so that the energizing of the at least one electrical heating element  220  can be changed as a function of the temperature value assigned to the resistance value by the device for determining the resistance  1020 . 
         [0057]    While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.