Patent Publication Number: US-2005141888-A1

Title: Heating device and heating method for a fluid in a basin

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
      German Priority Application 103 04 398.5-34, filed Jan. 30, 2003, and German Priority Application 103 22 366.5, filed May 8, 2003, including the specifications, drawings, claims and abstracts, are incorporated herein by reference in their entirety. Further, U.S. patent application Ser. No. ______, Attorney Docket No. 027209-1101, filed concurrently herewith, is hereby incorporated by reference in its entirety.  
     BACKGROUND OF THE INVENTION  
      The invention relates to heating devices. In particular, the invention relates to heating devices for a fluid in a basin, such as a whirlpool, spa or bath tub, for example.  
     SUMMARY OF THE INVENTION  
      According to one embodiment of the invention, a heating device for a fluid in a basin includes a flow-through path for a fluid reservoir of the basin and a heater arranged in the flow-through path. The fluid is able to flow past the heater for the purpose of heating up. The device also includes at least one heating element arranged in the heater and a temperature sensor in thermal communication with the heater. The thermal communication is sufficient for the temperature sensor to determine a temperature of the heater. The temperature sensor is adapted to determine absolute temperatures and changes in temperatures based on the thermal communication.  
      The temperature sensor may be arranged in direct thermal contact with the heater. The temperature sensor may be arranged on an outer side of a tube forming at least a portion of the flow-through path.  
      A tube forming at least a portion of the flow-through path may have a contoured, flattened or recessed area. The temperature sensor may be arranged on an outer surface of the tube in that area. The contoured, flattened or recessed area may include a substantially flat surface. In a preferred embodiment, the heater is positioned on or near an inner wall of the tube substantially opposing the temperature sensor. A gap may be located between the heater and the inner wall of the tube in the flattened or recessed area, with the fluid being able to flow through the gap.  
      The at least one heating element is preferably an electrical resistor element having an extension in a longitudinal direction of the heater. The heating device may further include at least one thermal melting fuse adapted to interrupt an electrical circuit for the at least one heating element when a predetermined temperature is exceeded. The thermal melting fuse is preferably positioned in the vicinity of the temperature sensor.  
      The heating device may further include an evaluating device for evaluating signals of the temperature sensor. The evaluating device may be adapted to recognize whether a reduced volume throughput of fluid through the flow-through path is present based upon signals of the temperature sensor. Further, the evaluating device may be adapted to recognize whether a reversible case of malfunction or an irreversible case of malfunction is present when a reduced volume throughput is recognized. The evaluating device may be adapted to recognize whether any dry running of the heater is present based upon signals of the temperature sensor.  
      The heating device may include a circulation pump adapted to be switched off via the evaluating device. The circulation pump may be adapted to be switched off by the evaluating device when a minimum throughput is not reached. At least one of a signal line for controlling the switching off of the circulation pump and an electrical energy supply line may be guided through the heater.  
      In a preferred embodiment, the flow-through path is dimensioned to prevent any dry running of the heater in the case of a throughput of fluid above a minimum throughput.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a plan view of a basin, on which a heating device according to an embodiment of the invention is mounted;  
       FIG. 2  shows a side view of the arrangement of  FIG. 1  in the direction A;  
       FIG. 3  shows a plan view of a tube area of an embodiment of a heating device having a temperature sensor;  
       FIG. 4  shows a cross-sectional view of the tube according to  FIG. 3  along line  4 - 4 ; and  
       FIG. 5  shows an exemplary course of the temperature over the time during dry running (steep curve) and with a blocked inlet or outlet (flatter curve). 
    
    
     DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS  
      The following description of preferred embodiments serves to explain the invention in greater detail in conjunction with the drawings.  
      In accordance with an embodiment of the present invention, a heating device is provided with which an improved switch-off control of the heating is ensured.  
      In accordance with an embodiment of the invention, a temperature sensor is provided which is arranged so as to be in thermal communication with respect to the heater such that a heating temperature of the heater may be determined and absolute temperatures and changes in the temperature may be determined via the temperature sensor.  
      In this regard, the phrase “absolute temperature” is used to refer to a temperature level on any given scale such as Fahrenheit, Celsius and Kelvin, for example. The accuracy of the measurement or determination of “absolute temperature” may vary according to various components and their calibration, for example.  
      When fluid flows in the flow-through path, stationary temperature conditions ensue in the case where no malfunctions occur; the heater heats the fluid and heat is carried away from the heater by the fluid flowing past the heater. If the amount of fluid is reduced or the flow even ceases, heat is carried away from the heating element of the heater only partially or even not at all, causing the region of the heating elements to heat up beyond nominal temperature. This heating up is detected by the temperature sensor. As a result of the arrangement of the temperature sensor, deviations from the stationary temperature conditions may be measured exactly and with short reaction times.  
      Malfunctions may then be recognized early in order to be able to, accordingly, also switch off the heating early. On account of short reaction times and high accuracy which is brought about by the thermal communication between the heater and the temperature sensor, malfunctions can also be evaluated in order, for example, to determine whether the case of malfunction is a critical one, requiring a permanent switching off of the heating, or whether it is a less critical malfunction which does not require a permanent switching off. A switch-off control may then be realized which, on the one hand, does not make any unnecessary intervention of an operator on account of less critical malfunctions necessary but, on the other hand, causes a permanent switching off of the heating of the heater with a short reaction time in the case of critical malfunctions. In the case of critical malfunctions, an operator has, for example, to see to it that the malfunction is eliminated whereas in the case of uncritical malfunctions intervention by an operator is not necessary.  
      As a result of the fact that, in accordance with embodiments of the invention, changes in temperature can also be determined in addition to absolute temperatures, it may be recognized whether an increase in temperature is attributable to dry running or to a reduced throughput of fluid through the flow-through path. It may be recognized early, in particular, whether the transport of fluid through the flow-through path is interrupted. The heating can then be switched off immediately and dry running thereby prevented. In addition, it may also be recognized whether any dry running is present in order to achieve additional safety.  
      In accordance with embodiments of the invention, an “intelligent” monitoring of dry running may be realized, with which less critical cases of malfunction can be recognized.  
      The temperature sensor is advantageously in direct thermal contact with the heater. As will be explained below, a small gap may be formed between the heater and a heat-conducting element, to which the temperature sensor is thermally coupled, so that the heater and the heat-conducting element do not touch directly. In this respect, the gap may be sized such that the transfer of heat is still sufficient for the temperature sensor to recognize the temperature of the heater early enough to avoid damage in the case of any dry running.  
      In one advantageous embodiment, the temperature sensor is arranged on an outer side of a tube or pipe which forms the flow-through path. The temperature sensor may then be positioned on the tube in a simple manner. Lines which connect the temperature sensor to an evaluating device may be guided in a simple manner. The production resources required can be minimized as a result.  
      It is particularly advantageous when a tube which forms the flow-through path has a contoured, flattened or recessed (e.g., deepened) area, on which the temperature sensor is arranged. The temperature sensor may be arranged closer to the heater as a result of such a recessed area, thereby improving thermal contact. As a result, the temperature of the heater may be determined with short delay times. In addition, it is possible to be able to evaluate increases in temperature more precisely. Such a contoured, flattened or recessed area may be produced on the tube in a simple manner.  
      Furthermore, it is favorable when the contoured, flattened or recessed area has an essentially flat surface. A good thermal contacting between the temperature sensor and the tube may be achieved in the recessed area, and a good thermal contact can be provided.  
      With respect to a distance between a longitudinal axis of the heater and a limiting wall of the tube, the heater can be seated on the contoured, flattened or recessed area, or in direct vicinity of the contoured, flattened or recessed area. As a result, the distance between the limiting wall and the heater may be minimized in order to provide a good thermal contact.  
      It may be possible for the heater to touch the limiting wall of the tube on an inner side of the tube. It may also be provided for a gap, through which fluid can flow, to be located between the heater and an inner wall of the tube in the contoured, flattened or recessed area. This gap may be sized such that the transfer of heat does not significantly deteriorate. A typical magnitude for the size of the gap (e.g., transversely to a longitudinal axis of the tube) is in the range of about 0.1 mm. As a result of a gap being provided, an improved differentiation between dry running and reduced throughput can be achieved. The gap makes a space available between the heater and the inner wall of the tube. When the gap has fluid flowing through it, a good transfer of heat is present. Without any fluid located in the gap (e.g., “air filling” of the gap), it has the effect of a thermal insulator, and the transfer of heat deteriorates. The gap, therefore, serves a discriminator function which causes a differentiatable change in signal in the case of any dry running.  
      The at least one heating element is favorably an electrical resistor element which has an extension in a longitudinal direction of the heater. As a result of such a heating element, an effective heating up of the fluid flowing past the heater can be achieved. A corresponding heating element may be embedded in the heater in an effective manner.  
      It is favorable when at least one thermal melting fuse is provided, via which the electrical circuit for the at least one heating element can be interrupted when a critical temperature is exceeded. In this respect, an emergency switching off is achieved irrespective of any measurement of the temperature. If, for example, the temperature sensor fails, a “hardware switching off” of the heating can be carried out when the critical temperature is exceeded.  
      In this respect, the at least one thermal melting fuse is preferably arranged in the vicinity of the temperature sensor. For example, the thermal melting fuse can be arranged on a contoured, flattened or recessed area, on which the temperature sensor is likewise arranged. A small distance to the heater is predetermined by the flattened or recessed area, and any increase in temperature and, in particular, any exceeding of the critical temperature has an effect on the melting fuse with little delay.  
      It is particularly advantageous when an evaluating device is provided which evaluates the signals of the temperature sensor. A temperature surveillance device, which has a certain intelligence, may be formed via this evaluating device. Malfunctions may be recognized which are reversible and require only a temporary, limited switching off of the heating. However, critical cases of malfunction, such as, for example, any dry running, which make a permanent switching off of the heating necessary, may also be recognized.  
      In this respect, it may be recognizable, due to the evaluating device by means of the signals of the temperature sensor, whether a reduced volume throughput of fluid through the flow-through path is present and whether the volume throughput is interrupted. A reduced volume throughput can be attributable to an interruption in the transport by the pump or to a blockage of an exit point at a basin which is connected to an entry point of the flow-through path. When a reduced volume throughput and, in particular, a zero transport is recognized, the heating is switched off. It can be differentiated via the increase in temperature (e.g., ascending gradient) whether a sudden reduction in throughput is involved, which may be rectified when the heating is switched on again following a waiting period, or whether a gradual increase in temperature is present, which may require special intervention. A sudden reduction in throughput can occur when a person has positioned himself in the tub in front of a fluid outlet and is blocking it, for example. A gradual reduction in throughput is attributable, for example, to soiling of the filter.  
      Additionally, it may be recognizable, due to the evaluating device by means of the signals of the temperature sensor, whether any dry running of the heater is present. This may be recognized, for example, from the increase in temperature. A steep rise in the temperature may point to dry running, whereas a flatter rise in the temperature may be attributable to a reduced volume throughput.  
      It is particularly favorable when reversible and irreversible malfunctions can be recognized by means of the evaluating device, in particular, via the type of increase in the temperature.  
      Furthermore, in a preferred embodiment, a circulation pump can be switched off via the evaluating device. The circulation pump may be switched off by the evaluating device, for example, when a minimum throughput is not reached. As a result, the circulation pump is prevented from being operated when no fluid throughput takes place. Damage to the circulation pump, which can be caused by a zero throughput or by dry running, can be avoided in this way.  
      A compact heating device may be constructed when a signal line for controlling the switching off of the circulation pump or an electrical energy supply line to the circulation pump is guided through the heater. The circulation pump may then be switched off by means of the evaluating device, in that a switch-off signal may be supplied to the pump or the energy supply is interrupted, for example, via a relay.  
      It is particularly advantageous when a free cross-section of the flow-through path is dimensioned such that any dry running of the heater in the case of a throughput of fluid above a minimum throughput is prevented. A flow channel of the flow-through path is correspondingly narrow in order to prevent any dry running. Thus, dry running can only result when the throughput is below the minimum throughput and, for example, when a zero throughput is present. It may, however, be monitored via the evaluation of the increase in temperature whether a minimum throughput is reached. Dry running may, therefore, be prevented by an early switching off. It is possible, as a result of the corresponding dimensioning of the flow-through path, to arrange the temperature sensor along the flow-through path at any practical position. It need not necessarily be arranged at a highest point in relation to the direction of gravity.  
      The embodiments of the invention relate, in addition, to a heating method for a fluid in a basin, with which the fluid from the basin runs through a heating path which may be located outside a reservoir of the basin and is heated by a heater, with at least one heating element arranged in the heater.  
      Also, in accordance with embodiments of the invention, a heating method is provided that can be carried out in a simple and reliable manner.  
      This can be accomplished in that the temperature of the heater is monitored via a temperature sensor which can determine absolute temperatures and temporal changes in the temperature at the heater.  
      The advantages of the inventive heating method have already been explained in conjunction with the inventive heating device.  
      Additional advantageous developments of embodiments of the inventive method have likewise already been explained in conjunction with the inventive heating device.  
      It can be, for example, determined via registration of a temperature course in time whether the fluid throughput through the heating path or loop is below a minimum throughput and, in particular, whether a zero throughput is present. The heating can then be switched off before the risk of any dry running occurs.  
      Furthermore, it may be provided for a circulation pump to be switched off when a minimum throughput is not reached and, in particular, at a zero throughput in order to avoid any damage to the pump.  
      Embodiments of the invention relate to a heating device for a fluid in a basin, comprising a flow-through path which can be positioned outside a fluid reservoir of the basin, a heater which is arranged in the flow-through path and has the fluid flowing past it for the purpose of heating up, and at least one heating element which is arranged in the heater. Such a heating device, which is used, in particular, for a whirlpool (spa) or a bath tub, is known, for example, under the name Laing Infinity Heater.  
      The heating is based on the principle of continuous flow heating, i.e., fluid is coupled out of the basin and heated up when passing through the flow-through path. Heated fluid is then coupled into the basin again. In this respect, the basic problem is that malfunctions can occur which lead to dry running of the flow-through path or loop. A reduced amount of fluid then flows in the flow-through path or no fluid at all flows through it. In such cases, it is necessary to switch the heating off.  
      Referring now to the Figures, an inventive heating device, of which one embodiment is shown in  FIG. 1  and designated as a whole as  10 , is used for heating or maintaining a temperature of a fluid in a basin  12 . The basin  12  has a reservoir  14  for the fluid which is contained within basin walls  16 .  
      The basin  12  with fluid accommodated in the reservoir  14  may be a whirlpool or a bath tub, for example, with water to be heated.  
      In the illustrated embodiment, the heating device  10  is arranged outside the reservoir  14  and positioned, for example, on an outer side of the basin wall  16  or positioned on a corresponding holding frame with respect to the basin  12 . The heating device  10  comprises a flow-through path  18  which is arranged outside the reservoir  14  and which comprises a heating path, in which the fluid can be heated up.  
      The flow-through path  18  has an entry end  20 , via which fluid from the reservoir  14  can enter the flow-through path  18 . For this purpose, the basin wall  16  may be provided with a continuous recess and may have an opening so that fluid from the reservoir  14  can be guided through the heating device  10 .  
      Furthermore, the flow-through path  18  has an exit end  22  which is in effective fluid communication with an entry point  24  for fluid heated up by the heating device  10  into the reservoir  14 . The exit end  22  may be coupled directly to the entry point  24  in a fluid-effective manner. It may also be provided, as shown in  FIG. 1 , for the exit end  22  of the flow-through path  18  to be coupled to an entry to a circulation pump  26 , wherein an exit  28  of the circulation pump  26  is then coupled to the entry point  24  into the reservoir  14  of the basin  12 .  
      In the case of whirlpools, an ozone device  30  is generally provided which is connected in front of the entry point  24  for fluid into the reservoir  14  (with respect to the direction of flow of the fluid) and is arranged, for example, between the entry point  24  and the outlet  28  of the circulation pump  26 . The fluid coupled into the reservoir  14  of the basin  12  can previously be disinfected by way of ozonization via the ozone device  30 . The ozone device  30  may be connected behind the heating device  10  and its flow-through path  18 .  
      In the case of bath tubs, no ozone device is generally provided, and the outlet  28  of the circulation pump  26  is coupled directly to the entry point  24  of the reservoir  14 .  
      In the case of a heating device  10  positioned on the basin  12 , the entry end  20  of the flow-through path  18  of the heating device  10  is generally, in the case of whirlpools, located above the exit end  22  of the flow-through path  18  with respect to the direction of gravity. As a result, fluid from the reservoir  14  is removed at a higher level than it is coupled into the reservoir  14  again.  
      In the case of bath tubs, the exit end of the flow-through path is generally at a higher level than the entry end (e.g., fluid to be heated is removed from the basin at the bottom and hot fluid flows into the basin above this).  
      The flow-through path  18  is formed in a tube  32  which can be bent in order to be able to position the heating device  10  on the basin  12  in an optimum manner.  
      The tube  32  has an extension in a longitudinal direction in order to form in this way the flow-through path  18  for the fluid coupled out of the reservoir  14 . This extension is not necessarily linear.  
      A heater  33  with at least one heating element  34  ( FIG. 4 ) is arranged in the tube  32  and, therefore, in the flow-through path and this heater extends along the tube  32 , adapted to its shape and, for example, to the curvatures in it. The heating element  34  follows this course. The heating element  34  may be an electrical resistance heating element. The fluid flowing past the heater  33  may be heated via such a heating element  34  over a relatively long distance, wherein the specific power density per area can be kept small.  
      The heater  33  arranged in the tube (pipe)  32  is preferably designed as a heating rod  36 . This heating rod  36  comprises a single or a plurality of recesses  38  which extend in its longitudinal direction and have, for example, a circular cross section. In the embodiment shown in  FIG. 4 , a single recess is provided. The heating element  34  is arranged in the recess  38 .  
      The heating rod  36  has a metallic sleeve  40  which serves as a protective casing for an area  42  of solid material ( FIG. 4 ). The recess  38  is formed in this area  42  of solid material. The material for the area  42  of solid material is preferably a solid-state material with a high heat conductivity which is electrically insulating. For example, magnesium oxide may be used as such a material.  
      The heating element  34  is surrounded by the solid material, wherein the heating element  34  touches the area of solid material in order to provide thermal contact. (In  FIG. 4 , the contact is not shown for reasons of illustration).  
      The heating rod  36  is securely arranged in the tube  32  and can be bent with the tube  32 . For example, the heating rod  36  with the heating element  34  is securely positioned in the tube  32 , and during the bending of the tube  32  into the desired position, the heating rod  36  is bent with it.  
      The tube  32  preferably has a generally circular outer cross section and a generally circular inner cross section. A contoured, flattened or recessed area  44  ( FIGS. 3 and 4 ) may be provided, at which the distance between a limiting wall  46  of the tube  32  and the heater  33  transversely to the longitudinal direction of the heater is decreased. In the illustrated embodiment, this area  44  is generally flattened.  
      The flattened area  44  extends in the longitudinal direction  48  of the tube  32  in a finite area and, for example, not over the entire length of the tube  32 .  
      The flattened area  44  has an outer side  50  which is generally flat. The corresponding plane of the flattened area  44  has a normal direction which is oriented transversely and at right angles to the longitudinal direction of the heater  33 . A temperature sensor  52 , which may be, for example, a semiconductor temperature sensor, is seated on this outer side  50 . The temperature sensor  52  is arranged such that it is located at a minimal distance in relation to the heater  33 . It may, for example, be arranged centrally on the flattened area  44  of the tube  32 .  
      The temperature sensor  52  is in thermal contact with the limiting wall  46  of the flattened area  44 . For example, a heat paste  54  or the like may be arranged between the temperature sensor  52  and the limiting wall  46  in order to provide for a good thermal contact with the heater  33 .  
      In addition, the temperature sensor  52  may be pressed against the limiting wall  46  via a spring  56  in order to provide a good mechanical contact with the flatted area  44 , which may be a precondition for a good thermal contact with the heater  33 .  
      The spring  56  may be supported on a rear side of a plate  58  which bears electrical circuit elements for the temperature sensor  52 . An evaluating device  60 , for example, may be arranged on this plate  58 .  
      In the illustrated embodiment, the limiting wall  46  is in thermal contact with the heater  33 . In this respect, it may be provided for the heater  33  to touch an inner side of the limiting wall  46  and so a direct thermal contact may be provided.  
      In the embodiment shown in  FIG. 4 , the heater  33  is positioned a short distance d from the inner side of the limiting wall  46 . Thus, a gap  62  is formed, through which fluid can flow. The gap  62  has a sufficiently small extension with respect to the distance between the limiting wall  46  and the heater  33  such that a good transfer of heat from the heater  33  to the limiting wall  46  is brought about by the gap  62  with fluid located therebetween. A typical magnitude for the extension of this gap  62  in a transverse direction  64  at right angles to the longitudinal direction of the tube  32  is about 0.1 mm.  
      In addition, one or more thermal melting fuses  66  ( FIG. 3 ) can be arranged in the flattened area  44 . In a preferred embodiment, the melting fuse  66  is located in the vicinity of the temperature sensor  52 . The melting fuse  66  is connected to the thermal heating element  34  and may be, for example, pressed against it. When a critical temperature is exceeded, corresponding lines of the melting fuse  66  melt, and the circuit formed via the heating element  34  is interrupted. As a result, the heating of the heating element  34  is also interrupted since electrical current no longer flows through it.  
      The functioning of the illustrated heating device will now be described.  
      When fluid flows around the heater  33 , the fluid absorbs heat and conducts heat away from the heater  33 . If the fluid throughput is reduced or a partial or complete dry running of the heater  33  is present, the heater  33  can discharge heat only to a reduced extent or can no longer discharge any heat, thereby causing its temperature to rise.  
      As a result of the arrangement of the temperature sensor  52  with a reduced distance in relation to the heater  33  and the presence of thermal contact between the heater  33  and the temperature sensor  52 , an increase in the temperature may be detected exactly via the temperature sensor  52  with a minimal delay.  
      In this respect, absolute temperatures can be determined via the temperature sensor  52  through temporal changes in the temperature. The cause of the change in temperature may be concluded from the changes in temperature, for example, via the size of a temporal increase, as illustrated in  FIG. 5 .  
      A reduction in the throughput, for example, due to a blockage of the circulation pump  26  may lead to a temporary, slow increase in the temperature, as indicated by the temperature curve  68 . In the case of any dry running of the flow-through path  18 , steam can result, or air can enter. The temperature curve then may be very much steeper over time, as indicated by the temperature curve with the reference numeral  70  in  FIG. 5 . The evaluating device  60  can now recognize, for example, from the gradient of the temperature curve whether any dry running is present which makes an immediate, permanent switching off necessary because a malfunction which is considered not to be reversible has occurred, such as, for example, dry running with formation of steam.  
      If a reduction in throughput is ascertained, for example, via an increase in the temperature which exceeds a certain limit, indicating a reduction in throughput below a minimum throughput limit, the heating is likewise switched off. Signals may be transmitted to a primary control device or regulating device (not shown) for the basin  12  in order to initiate corresponding correction procedures. Alternatively, these correction procedures may be initiated directly. For example, the heating may be switched on again after a certain time without any external operator intervention being necessary in the case of malfunctions which are considered to be reversible.  
      If a reduced throughput and, in particular, a throughput under a minimum limit, is detected which is, for example, to a zero conveyance of the circulation pump  26 , to blocked filters or to a blocked entry point  24 , the heating may be switched off. Thus, dry running of the heater  33  can be avoided. In addition, it can also provided for the evaluating device  60  to deliver a switch-off signal to the circulation pump  26  in order to prevent any running of the pump with a lack of conveyance and, therefore, minimize the risk of damage to the circulation pump  26 .  
      In this respect, a signal line may be guided from the evaluating device  60  through the heater  33  to the circulation pump  26 . The evaluating device  60  generates a corresponding switch-off signal which can be transmitted through this line to the circulation pump  26 . Alternatively, it may be provided for an energy supply line for the circulation pump  26  to be guided through the heater  33 , wherein the supply of energy can be interrupted for switching off the circulation pump  26  by means of the evaluating device  60 . For example, a relay may be arranged on the plate  58  for this purpose, and the energy supply line may be coupled to the relay.  
      Such a reduced throughput may be recognized due to the disclosed arrangement of the temperature sensor  52  and due to the ability to recognize increases in temperature—qualitative and quantitative.  
      The gap  62  also contributes to this. When fluid is flowing in the gap  62 , a considerable transfer of heat to the limiting wall  46  is present, in contrast to the case when air is located in the gap  62 . As a result, a greater differentiation between the cases of reduced throughput and dry running is achieved, thereby increasing the accuracy of evaluation.  
      In one embodiment, the flow-through path  18  is designed with respect to a free flow cross section to have a sufficiently narrow flow channel that dry running results only when the transport of fluid through the flow-through path  18  is interrupted. Thus, dry running results from interruption of the transport of fluid in the flow-through path  18 . In accordance with embodiments of the invention, a reduced throughput and an interruption of the throughput may be recognized, and the heating may be switched off prior to any dry running occurring.  
      As a result, it is possible to arrange the temperature sensor  52  at any optional location along the flow-through path  18  in relation to the heater  33 . Thus, the flattened area  44  may be formed at any optional location.  
      In a preferred embodiment, the temperature sensor  52  is arranged at or near the spatially highest point of the flow-through path  18  with respect to the direction of gravity. The highest point of the flow-through path  18  with respect to the direction of gravity is a particularly critical point since steam or air can accumulate at this point. Such an accumulation of steam or air occurs, for example, when a reduced amount of fluid is flowing through the flow-through path. The transfer of heat is worse in an area in which a bubble of steam or a cushion of air is seated, or in which a two-phase flow is present, than an area with a single-phase fluid flow. The heater may no longer be cooled effectively in such a two-phase flow area, which makes a safety switch-off necessary.  
      As a result of the detection of reduced throughput or an interruption of the transport of fluid through the flow-through path  18  and the heating then being switched off, the formation of steam or the accumulation of air can be excluded from the beginning, and the positioning of the temperature sensor  52  can thus be planned as required.  
      In accordance with an embodiment of the invention, a temperature surveillance device  72  is made available which comprises the temperature sensor  52  and the evaluating device  60 .  
      Embodiments of the temperature surveillance device  72  may have a certain intelligence. It may be recognized via this device whether, for example, a reversible stagnation (e.g., an uncritical, reversible case of malfunction) is present. The occurrence of any dry running may be prevented.  
      It may also be recognized, for example, whether a permanent dry running is present. Thus, additional safety is provided.  
      This safety is increased further via the thermal melting fuse  66 .  
      While particular embodiments of the present invention have been disclosed, it is to be understood that various different modifications and combinations are possible and are contemplated within the true spirit and scope of the appended claims. There is no intention, therefore, of limitations to the exact abstract and disclosure herein presented.