Abstract:
The invention relates to an electric heating device especially for fluids such as liquid fuel ( 48 ), comprising a heat element with an electrical resistance ( 32 ). Said heat element can be connected to an electrical power source ( 44 ), and also comprises a carrier element ( 12 ) to which an electrically conductive heat layer ( 32 ) made out of a PTC material with positive temperature resistance co-efficients is applied. The heat layer ( 32 ) can, at least in two interspaced positions, be connected to the electrical power source ( 44 ).

Description:
BACKGROUND  
         [0001]    The present invention refers to an electrical heating device, in particular, for fluids such as liquid fuel, with a heating element which comprises an electrical resistance and which can be connected to an electrical power source.  
           [0002]    One such device is known in the market. In it, a so called “heating cartridge” is arranged in a fuel line, such as an oil supply line for an oil burner. The cartridge comprises a heating coil which has a thermo-element and extends into the fuel stream that is transported inside the fuel line. The thermo-element is used to record and regulate the temperature. If an electrical voltage is applied to the heating cartridge, it and the fuel running past will expand. The advantage of such a heating process is that the heated fuel has a low viscosity and therefore forms a very thin fuel film on the wall of the customary hollow-cone nozzle. This in turn reduces the flow even when the nozzle opening is relatively wide. A relatively wide nozzle opening is desirable to prevent plugging.  
           [0003]    In this manner, low performance atomizing burners can be used with a relatively wide nozzle diameter. Furthermore, due to the low viscosity, the nozzle atomizes the fuel more evenly and into a finer spray, thus improving ignition. The overall effect of heating the fuel is a reduction in fuel consumption.  
           [0004]    However, the device known in the market has the disadvantage that its design is complicated, and it is therefore difficult to integrate it into the oil supply line. Furthermore, regulating with a thermo-element and an electronic regulator and control unit is relatively complex, which increases the cost. Finally, in many cases, the heating coil for heating the fuel has to become quite hot, which causes the fuel that is in direct contact with the heating coil to evaporate and to form an insulating vapor lock between the remaining liquid fuel and the heating coil. This thermally insulating vapor lock reduces the heat transfer between the heating coil and the fuel, which means that more energy is required for heating the fuel, and the fuel is not evenly heated.  
           [0005]    Another electrical heating device of the above named type that is known in the market is used, for example, as a hot plate. In this device, too, the temperature of the hot plate is recorded by a thermo-element and adjusted via a separate regulator and control unit. This is also technically complex and, in addition, the reaction is sluggish.  
           [0006]    It is therefore the object of the present invention to further develop a heating device of the above named type such that it is easy to manufacture and easy to integrate, that its technical complexity is reduced, that the fuel can be heated evenly, and that a quick reaction time can be achieved for temperature regulation.  
         SUMMARY  
         [0007]    This object is achieved by means of the invention in that the heating element comprises a carrier element to which an electrically conducting heating layer made of PTC material with a positive resistance coefficient is applied, and that the layer can be connected to the electrical power supply in at least two separate places.  
           [0008]    When PTC material is used, it becomes completely unnecessary to provide an external regulator, since the specific resistance of this material changes according to the temperature; that is, as the temperature drops, the specific electrical resistance of the material decreases, which causes the current that passes through it to rise, provided that the voltage is constant. This in turn causes a higher heating output and a rise in temperature, with the opposite effect on the resistance. Therefore, such a material is self regulating. To set a certain temperature, it is only necessary to provide a constant voltage, while neither a regulator nor a thermo-element is necessary.  
           [0009]    Furthermore, due to the small mass of the heating layer, the heating device according to the invention can be very rapidly heated up, and its overall dynamics can be distinctive. This in turn leads to the high efficiency of the device. The maximum temperatures that can be achieved with the heating device are approximately between 500 and 1000° C.  
           [0010]    In a first aspect of the invention, an automatic limit can be established for the temperatures that can be achieved with the heating device in that a PTC material is used whose temperature resistance curve, starting at a certain temperature, changes its gradient in the form of a “kink”.  
           [0011]    In this, it is particularly preferred when the gradient change is formed such that a certain temperature can be held constant. In that case, the local temperature, which is at least partly determined by the characteristics of the material, is “self-adjusting”, and a temperature sensor and an associated control and/or regulator device is not absolutely necessary. This self-adjustment can also be a reliable means to prevent overheating. The fact that additional components can be eliminated considerably simplifies the manufacture of the heating device according to the invention.  
           [0012]    However, it is also possible for the electrical heating device to comprise an electronic control and/or regulating device which can be used to set a certain temperature in at least one section of the heating layer. In this, the heating layer itself can be used as a temperature sensor, since its resistance is a measure of the temperature. The resistance of the heating layer can be evaluated in the control and regulating device, and the appropriate temperature can be determined.  
           [0013]    Advantageously, the heating layer is applied by means of a thermal process, in particular by means of plasma deposition, plasma spraying, high-speed flame spraying, etc. This is provided in another aspect of the invention. On the one hand, such a thermal process is cost-effective and, on the other hand, it allows optimal bonding between the layers and the carrier element and also between the layers themselves.  
           [0014]    Another aspect is characterized in that the heating layer comprises a ceramic powder. This makes manufacturing easy and provides the layer with special thermal stability. The heating layer may also comprise a metal powder, which improves its application to the carrier element.  
           [0015]    In the other aspect of the invention, the carrier element is made of an electrically conducting material, and an electrically insulating layer is provided between the heating layer and the carrier element. It is often better to use carrier elements made of metal because they are easier to manufacture, but metals are electrical conductors. Therefore, in such cases, the heating layer must be electrically insulated from the carrier element by a layer. Thanks to this aspect, the heating device according to the invention can therefore be used with the customary metal carrier elements.  
           [0016]    In this aspect, the electrical conductivity of the carrier element can be used to channel the electrical output to or from the heating layer. This is possible when, as provided in another aspect the electrical power source is a low voltage source and the heating layer is electrically connected in one place with the carrier element.  
           [0017]    A layer that electrically insulates the heating layer from the carrier element is unnecessary if the carrier element itself is made of an electrically insulating material. Such materials include, in particular, many temperature-resistant plastics, but also ceramic materials and glass. In that case, higher voltages can also be used as the electrical power supply for the heating layer without the necessity of an electrically insulating layer. As a general principle, the heating device according to the invention can be operated with voltages starting at about 1.5 V, although voltages up to 220 V and more are possible as well.  
           [0018]    User handling of the device according to the invention made easier by an aspect in which an electrically insulating layer is provided on the side of the heating layer that is opposite the carrier element. This layer protects the user to come in direct contact with the live heating layer.  
           [0019]    One example for a preferred material for the electrically conducting layers is aluminum oxide, Al 2 O 3 , and another is zircon oxide. As a general principle, at least the insulating layer between the heating layer and the carrier element should be a good electrical insulator, but a poor thermal insulator. Furthermore, the material should be temperature-resistant and able to follow the thermal expansion movements of the carrier element. This is the case with aluminum oxide and with zircon oxide.  
           [0020]    Good bonding of the insulation layers is achieved when they are applied by means of a thermal process in particular plasma deposition, vapor deposition or highspeed flame spraying.  
           [0021]    One aspect of the heating device is characterized in that the thickness of the heating layer varies over the length of the carrier element, such that the output distribution or output absorption varies also over the length of the carrier element. In this manner, a certain temperature profile can be achieved in the two directions of the carrier element&#39;s plane without the necessity of a complex regulator or control device. The variation in thickness can be continuous, which would also allow for a continuous output distribution.  
           [0022]    In this, it is particularly preferred if, during operation, the temperature difference between the carrier element and the material to be heated can be held constant over the length of the carrier element. This would take into account the fact that the temperature of the material to be heated may change over the length of the carrier element, e.g., colder temperature at the edge. By varying the temperature of the heating layer over the length of the carrier element, the heating up process can be optimized, and, if necessary, the length of the heating layer necessary for heating can be reduced.  
           [0023]    The thickness range of the layers can be selected to be optimal. Accordingly, the thickness of at least one of the layers is in the range of 0.002 to 0.2 mm, but preferably in the range of 0.005 and 0.1 mm. As far as the heating layer is concerned, layers of such thickness form a resistance that is necessary for achieving the required temperatures in the range of up to 400° C. On the other hand, with regard to the electrically insulating layers, a thickness in the above range ensures that the thermal insulation effect is as low as possible.  
           [0024]    Furthermore, the aspects of the invention name a number of especially preferred applications. Thus, the heating device can preferably be used for the heating of oil supplied to a burner, as an instantaneous water heater, as a hot plate, for heating the coolant in automobiles, for heating fuel filters for paraffin precipitation, for heating windshields or mirrors, for the de-icing of aircraft wings, for heating the walls of rooms or for heating floors, e.g., to prevent freezing, or as a warming plate. The heating device according to the invention can be applied to any regular or irregular surfaces with any geometry, and also to uneven and/or rough surfaces. It is possible to apply it cost effectively with a robot.  
           [0025]    An aspect of the invention in which the carrier element comprises a tubular element is suitable for the heating of fluids.  
           [0026]    In a particularly preferred aspect of the invention with a tubular carrier element, the element comprises a fuel line with an inlet and an outlet, whereby at least some parts of the heating layer are applied to a wall of the fuel line. Thus, a heating coil or the small contact surface between the same and the fuel are completely unnecessary in the device according to the invention. Instead, at least one section of the fuel line wall is heated by the heating layer. The contact surface thus created between the heated fuel line wall and the fuel itself is considerably larger than the contact surface between the fuel and a heating coil, which means that the temperature of the wall itself can be lower, thus reducing the danger of fuel vapor formation. Of course, the more the surface of the fuel line wall increases, the more apparent this advantage becomes. It is therefore best if the places where the poles of the electrical power source are connected with the heating layer are separated from each other as far as possible, for example, one at the inlet and one at the outlet.  
           [0027]    In one aspect, the fuel line has an injection nozzle at one end. This has the advantage that the path from the heated area to the injection nozzle is relatively short. If need be, the injection nozzle can also be heated as well by means of an appropriate coating, which in addition promotes the formation of an optimal fuel spray.  
           [0028]    In a particularly preferred aspect, the fuel line, at least in the area of the heating layer, comprises an annular chamber through which the fuel is led. In this manner, the actual fuel volume to be heated is reduced, thus, on the one hand, reducing the required heat output and, on the other hand, improving temperature distribution in the fuel that flows through the fuel line, as well as shortening the regulating reaction time.  
           [0029]    Furthermore, a contact means for the heating layer should be provided that is particularly easy to handle. According to it, the device is provided with at least one contact ring that can be pulled onto the fuel line and establishes an electrical contact with the heating layer. For example, such a contact can be made via a blade which, when pulled on, scores an electrical insulation layer that may exist on the heating layer and digs into the heating layer, thus ensuring a safe contact.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0030]    Below, one aspect of the invention is described in detail with reference to the sole drawing. 
     
    
     DETAILED DESCRIPTION  
       [0031]    In the drawing, a device for the heating of fuel is identified by the overall reference number  10 . The device  10  is shown with partial sections cut out and comprises a tube which at the end which is at the left end is provided with an outlet  13  and which is closed by a nozzle piece  14 . Nozzle piece  14  is welded into tube  12 . Tube  12  is open at its right end, thus forming an inlet  11 . At this open end, it can be provided with a thread not shown, to connect it to a fuel supply. Nozzle piece  14  is provided with a central nozzle opening  16 , whose cross section tapers toward the outside, through which the fuel is discharged from the tube  12  during operation, and a fuel spray is generated.  
         [0032]    A hollow displacer  18  is provided coaxially to the tube  12  and in the interior of said tube. The left end of the displacer  18  is supported by feet  20  in associated recesses  22  in the nozzle piece  16 . At its right end, it is fastened to the interior wall of tube  12  by a washer  24 . Along its entire circumference, this washer  24  is provided with holes  26  through which during operation the fuel can flow into an annular chamber  28  formed between the displacer  18  and tube  12 .  
         [0033]    Tube  12  is a conventional steel tube, such as is used for fuel lines, e.g., for oil lines leading to heating-oil burners, etc.  
         [0034]    On its radially outside casing surface, an electrically insulating layer  30  of aluminum oxide is applied by means of plasma deposition. The thickness of the electrically insulating layer is about 0.1 mm. To the electrically insulating layer  30 , a heating layer  32  is applied by means of powder plasma deposition, but this layer, in contrast to the insulating layer  30 , does not extend over the entire length of tube  12 , but ends at a distance a before the two ends of tube  12 .  
         [0035]    From end  11  at the outlet to end  13  at the inlet, the thickness of the heating layer  32  decreases from about 0.1 mm to about 0.05 mm. The material of heating layer  32  is a nickel, chromium and iron alloy with a positive temperature coefficient PTC. The nickel, chromium and iron alloy is embedded in a base of powdery ceramic material. An outer electrically insulating layer  34  is applied in turn to heating layer  32 , also by means of plasma deposition. This layer  34  also consists of aluminum oxide and is separated by a distance b from the two ends of tube  12 . This distance b is slightly greater that distance a, which means that the two ends of heating layer  32  are free.  
         [0036]    At the two ends of tube  12 , i.e., in separate places, contact rings  36 ,  38  are pulled onto the tube  12  and its layers  30 ,  32  and  34 . The two contact rings  36  and  38  are made of an electrically insulating material, such as plastic. The radially inside annular surface of the contact rings  36  and  38 , which faces tube  12 , is slightly graduated to take into consideration that the outer electrically insulating layer  34  ends by a distance b before the end of pipe  12  and the heating layer  32  by a distance a. Contact pins  40 ,  42  are inserted into contact rings  36 ,  38 . These contact rings  36 ,  38  are made of an electrically conducting material; on the one side the contact rings  36 ,  38  contact heating layer  32  and, on the other side, the contact rings  36 ,  38  can be connected to a voltage source  44  via an electrical line  45  and a control unit  46 .  
         [0037]    Device  10  functions as follows: Via a pump, not shown, and via a supply line, also not shown, a fuel, such as heating oil, is conducted through tube  12  from inlet  11  to outlet  13  as shown by arrows  48 . In tube  12 , only annular space  28  between displacer  18  and the radially inside casing surface of tube  12  is available in the area of the lengthwise extension of heating layer  32 . At this point it should be noted that neither displacer  28  nor heating layer  32  or insulating layers  30 ,  34  necessarily have to extend over the entire length of tube  12 . However, the larger the contact surface between the heated wall and the fuel, the more heat is generated for the fuel.  
         [0038]    When control unit  46  causes the closing of the circuit formed by voltage source  44 , line  45 , control pins  40 ,  42  and heating layer  32 , heating layer  32  occurs between contact rings  36 ,  38 , i.e., substantially over the entire length of tube  12 . Due to the fact that heating layer  32  is thicker at inlet end  11  than at outlet end  13 , more heat is generated at the former than at the latter. The electrically insulating layer  30  transfers the heating of heating layer  32  through tube  12 , such that tube  12  is substantially heated as desired over its entire length and its entire circumferential area. If the right kind of material is chosen for nozzle piece  14 , this ensures that as the result of heat transfer, nozzle piece  14  and nozzle opening  16  are heated as well.  
         [0039]    The fuel, on the way from the inlet end  11  to the outlet end  13 , passes the heated inner wall of tube  12  in the direction of arrow  48  and is thus also heated along its flow path. Due to the fact that the thickness of heating layer  32  differs from one end to the other, the temperature of tube  12  also rises from inlet end  11  toward outlet end  13 , such that the temperature difference, which determines the heat transfer between tube  12  and fuel  48 , can be held substantially constant throughout the flow path of fuel  48 . In this manner, a large amount of thermal energy can be injected into fuel  48  over a relatively short flow distance.  
         [0040]    Insulation layers  30 ,  34  reliably protect the operators who are handling device  10  from contact with live elements. Furthermore, the material and the thickness of the outer insulating layer  34  can be selectively chosen, which means that this layer provides thermal insulation as well, thus reducing the energy requirement even more. It will also be appreciated that instead of a D.C. current source, an A.C. current source of higher voltage can be used as well.  
         [0041]    Furthermore, in an aspect not shown, the radially outside casing surface of displacer  18  is also provided with insulating layers and a heating layer, which means that the two boundary walls of the annular space thus formed can be heated, causing the fuel to be heated even more efficiently.  
         [0042]    In another aspect not shown, there is no contact ring at one end of the tube. Instead, the inner electrically insulating layer, i.e., the layer which electrically insulates the heating layer from the tube, is slightly recessed at that end, which means that there is electrical contact in that place between the heating layer and the tube. In that case, which for safety reasons is naturally possible only with a low voltage source, the tube can be used as one of the two supply lines for the electrical output.