DE20019890U1 - Electric heater - Google Patents

Description

F:\IJBDHF\DHFANM\ALL292O

Applicant:

Elias Russegger
Upper Allgäu 3 02
A-5440 Golling

3658005 22.11.2000

kna / fbo

Title: Electric heating device Description

The present invention relates to an electrical heating device, in particular for fluids such as liquid fuel, with a heating element comprising an electrical resistor which can be connected to an electrical power source.

Such a device is known from the market. In this device, a so-called "heating cartridge" is arranged in a fuel line, e.g. an oil feed pipe to an oil heating burner, which comprises a heating coil with a thermocouple, which is inserted into the

transported fuel stream. The thermocouple is used for temperature detection and temperature control. If an electrical voltage is applied to the heating cartridge, the heating coil and the fuel flowing past it heat up. The advantage of such heating is that the heated fuel has a low viscosity and therefore forms a very thin film of fuel on the wall of the usual hollow cone nozzle. This in turn reduces the flow even with a relatively large nozzle opening. A relatively large nozzle opening is desired because of the risk of clogging otherwise present.

In this way, atomizing burners can be operated with low power and a relatively large nozzle cross-section. In addition, the fuel is atomized more evenly through the nozzle and into a finer spray due to the low viscosity, and therefore ignites better. Overall, heating the fuel also reduces fuel consumption.

However, the device known from the market has the disadvantage that it is complicated in terms of its design and difficult to integrate into the oil supply pipe. In addition, the control with a thermocouple and an electronic control and regulation unit is relatively complex, which increases the costs. Finally, the

In many cases, the heating coil has to get quite hot to heat the fuel, so that the fuel that is in direct contact with the heating coil evaporates and forms an insulating vapor cushion between the remaining liquid fuel and the heating coil. This thermally insulating cushion reduces the heat transfer between the heating coil and the fuel, which means that more energy is required to heat the fuel and the fuel is not heated evenly.

Another electrical heating device of the type mentioned above that is known on the market is used as a hotplate, for example. Here too, the temperature of the hotplate is measured by a thermocouple and set via a separate regulation and control unit. This is also complex and also slow to react.

The present invention therefore has the object of developing a heating device of the type mentioned at the outset in such a way that it is simple in construction, easy to integrate, the effort required for temperature control is reduced, uniform heating of the material to be heated is possible and a fast reaction time is achieved when regulating the temperature.

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This object is achieved according to the invention in that the heating element comprises a carrier element onto which an electrically conductive heating layer made of a PTC material with a positive temperature resistance coefficient is applied, and the layer can be connected to the electrical power source at at least two spaced-apart locations.

When using PTC material, external control may be completely unnecessary, as this material changes its specific resistance with temperature: as the temperature drops, its specific electrical resistance decreases, which means that the current passing through it increases, assuming a constant voltage. This in turn leads to an increase in heating power and an increase in temperature with an inverse effect on the resistance. Such a material is therefore self-regulating. To set a certain temperature, only a constant voltage setting is required; control or even a thermocouple can be omitted.

Due to the low mass of the heating layer, very fast heating times and an overall pronounced dynamic of the heating device according to the invention can be achieved. This in turn leads to a high efficiency of the

Device. The maximum temperatures that can be achieved with the heating device are approximately between 500 and 1000 ⁰ C.

Advantageous further developments of the invention are specified in subclaims:

According to a first development of the invention, an automatic limitation of the maximum temperature that can be reached with the heating device can be achieved by using a PTC material whose temperature-resistance characteristic changes its slope in the manner of a "kink" from a certain temperature.

It is particularly preferred that the gradient change is designed in such a way that a certain temperature can be kept constant. In this case, the local temperature, which is at least partly defined by the properties of the material, regulates itself "by itself" without a temperature sensor and a corresponding control and/or regulating device being absolutely necessary. The automatic regulation also reliably prevents overheating. The possible elimination of additional components makes the manufacture of the heating device according to the invention considerably easier.

It is also possible for the electric heating device to have an electronic control and/or regulating device with which a specific temperature of at least one area of the heating layer can be set. 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 corresponding temperature can be determined.

The heating layer is advantageously applied by a thermal process, in particular by plasma coating, vapor deposition, plasma spraying, high-velocity flame spraying, or the like. This is provided for in a further development of the invention. Such a thermal process is inexpensive on the one hand and enables optimal adhesion of the layers on the one hand to the carrier element and on the other hand to one another.

Another development is characterized by the fact that the heating layer contains a ceramic powder. This makes it particularly thermostable and easy to manufacture.

The heating layer may also comprise a metal powder, which improves its applicability to the carrier element.

In the other development of the invention, the carrier element is made of an electrically conductive material and an electrically insulating layer is present between the heating layer and the carrier element. Metal carrier elements are often suitable because they are easy to manufacture; however, metals are electrically conductive. In this case, the heating layer must therefore be electrically insulated from the carrier element by a layer. This development means that the heating device according to the invention can be used with conventional metal carrier elements.

The electrically conductive property of the carrier element provided in this development can be used to supply the electrical power to the heating layer or to remove it from the heating layer. This is possible if, as another development of the invention provides, the electrical power source is a low-voltage source and the heating layer is electrically connected to the carrier element at one point.

A layer that electrically insulates the heating layer from the carrier element can be dispensed with if the carrier element itself is made of an electrically insulating material. This includes in particular many

temperature-resistant plastics, but also ceramic materials and glass. In this case, higher voltages can also be used to supply electrical power to the heating layer without the need for an electrically insulating layer. In principle, the heating device according to the invention can be operated with voltages from approximately 1.5 volts. But voltages up to 220 volts and more are also possible.

The handling of the device according to the invention by persons is made easier by a further development in which an electrically insulating layer is present on the side of the heating layer opposite the carrier element. This layer protects the user from coming into direct contact with the current-carrying heating layer.

An example of a preferred material for the electrically insulating layers is aluminum oxide (Al ₂ O ₃ ), but also zirconium oxide. Basically, at least the insulating layer provided 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-stable and accommodate the thermal expansion of the carrier element. This is the case with both aluminum oxide and zirconium oxide.

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Good adhesion of the insulating layers is achieved when they are applied by a thermal process, in particular plasma coating, vapor deposition or high-velocity flame spraying.

A further development of the heating device is characterized in that the thickness of the heating layer varies over the extent of the carrier element, so that the power distribution or the power consumption varies over the extent of the carrier element. In this way, a certain temperature profile can be realized in the two surface directions of the carrier element without complex regulation or control being required. The variation of the thickness can be continuous, so that a continuous power distribution is also possible.

It is particularly preferred that a constant temperature difference is achieved between the carrier element and the material to be heated over the entire length during operation. This takes into account the fact that the temperature of the material to be heated can change over the length of the carrier element (e.g. colder temperature at the edge). By changing the temperature of the heating layer over the length, the heating can be

be optimized and, if necessary, the extent of the heating layer required for heating can be reduced.

The layer thicknesses specified in a subclaim have proven to be optimal. According to this, the layer thickness of at least one of the layers is in the range of 0.002 to 0.2 mm, preferably in the range of 0.005 to 0.1 mm. As far as the heating layer is concerned, the thicknesses mentioned form a resistance which is required to achieve the necessary temperatures in the range of up to 400° C. With regard to the electrically insulating layers, the layer thicknesses specified ensure the lowest possible thermal insulating effect.

In a further development of the invention, various particularly preferred applications are also mentioned. The heating device can preferably be used for heating heating oil to be fed to a burner, as a continuous flow heater for water, as a hotplate, for heating the cooling water in motor vehicles, for heating fuel filters for paraffin separation, for heating windows or mirrors, for de-icing the wings of aircraft, for heating walls in rooms or for heating floors (for example as antifreeze protection) or as a hotplate. The heating device according to the invention can also be used on

Any free-form surface with any geometry and also uneven and/or rough surfaces can be applied. The application can be carried out inexpensively using robots.

The development of the invention in which the carrier element comprises a tubular element is suitable for heating fluids.

In a particularly preferred development of the invention with a tubular support element, this comprises a fuel line with an inlet and an outlet, the heating layer being applied at least in part to a wall of the fuel line. The device according to the invention therefore completely dispenses with a heating coil and the associated small contact area with the fuel. Instead, at least one area of the wall of the fuel line is heated by the heating layer. The contact area created in this way between the heated wall of the fuel line and the fuel itself is considerably larger than the contact area between the fuel and a heating coil, so that the temperature of the wall itself can be lower, which reduces the risk of fuel vapor formation. Of course, this advantage is all the more noticeable the larger the wall area of the fuel line heated by the heating layer. It is therefore

12

It is best if the points at which the poles of the electrical power source are connected to the heating layer are as far apart as possible, e.g. at the inlet on the one hand and at the outlet on the other.

In a further development, 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 necessary, the injection nozzle can also be heated up by means of an appropriate coating, which further promotes the formation of an optimal fuel spray.

A particularly preferred development is specified in another subclaim: According to this, the fuel line comprises an annular space at least in the area of the heating layer through which the fuel is passed. In this way, the volume of fuel that actually has to be heated is reduced, which on the one hand reduces the heating power to be introduced and on the other hand improves the temperature distribution in the fuel flowing through the fuel line and shortens the control reaction time.

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Furthermore, a particularly easy-to-use electrical contact device for the heating layer is mentioned: According to this, the device has at least one contact ring that can be pushed onto the fuel line and that makes electrical contact with the heating layer. Such contact can be made, for example, via a cutting edge that, when pushed on, cuts open any electrical insulation layer that may be present on the heating layer and digs into the heating layer, thus ensuring a secure contact.

In the following, an embodiment of the invention is explained in detail with reference to the single figure.

In the figure, a device for heating fuel bears the reference number 10. It is shown partially broken away and comprises a pipe 12, which has an outlet 13 at its left end in the figure and is closed by a nozzle piece 14. The nozzle piece 14 is welded into the pipe 12. At its right end in the figure, the pipe 12 is open so that an inlet 11 is formed. It can be provided with a thread (not shown) at this open end, with which it can be connected to a fuel supply. In the nozzle piece 14 there is a central nozzle opening 16 with a cross-section that tapers outwards, through which

During operation, the fuel exits the pipe 12 and a fuel spray is generated.

Coaxially to the tube 12, there is a hollow displacement body 18 in its interior. It is supported at its left end in the figure by feet 20 in corresponding recesses 22 in the nozzle piece 16. At its right end in the figure, it is held by a spacer ring 24 against the inner wall of the tube 12. In the spacer ring, there are openings 26 distributed over the entire circumference through which the fuel can flow into an annular space 28 formed between the displacement body 18 and the tube 12 during operation.

The pipe 12 is a conventional steel pipe as used for fuel lines, e.g. supply lines for heating oil to heating oil burners.

An electrically insulating layer 30 made of aluminum oxide is applied to its radially outer surface by plasma coating. The thickness of the electrically insulating layer is approximately 0.1 mm. A heating layer 32 is applied to the electrically insulating layer 30 by powder plasma coating, which, however, in contrast to the insulating layer 30, does not extend over the entire length of the tube 12, but in

a distance a in front of the respective ends of the tube 12.

From the inlet end 11 to the outlet end 13, the thickness of the heating layer 32 decreases from initially approximately 0.1 mm to approximately 0.05 mm. The material of the heating layer 32 is a nickel-chromium-iron alloy, which has a positive resistance temperature coefficient (PTC). The nickel-chromium alloy is embedded in a base made of a powdered ceramic material. An outer electrically insulating layer 34 is applied to the heating layer 32, also by plasma coating, which also consists of aluminum oxide and is spaced from the respective ends of the tube 12 by a distance b. This distance b is slightly greater than the distance a, so that the respective ends of the heating layer 32 are exposed.

A contact ring 36 or 38 is pushed onto the tube 12 with the layers 30, 32 and 34 at both ends, i.e. at points spaced apart from one another. The two contact rings 36 and 38 are made of an electrically insulating material, e.g. a plastic. Their ring surface facing the tube 12, i.e. radially inward, is slightly stepped in order to take into account the fact that the outer electrically insulating

Layer 34 by a distance b and the heating layer 32 by a distance a in front of the respective

end of the tube 12. A contact pin 40 or 42 is inserted into the contact rings 36 and 38, which is made of an electrically conductive material and on the one hand contacts the heating layer 32 and on the other hand can be connected to a voltage source 44 via an electrical line 45 and a control unit 46.

The device 10 functions as follows:

Fuel, e.g. heating oil, is fed through the pipe 12 from the inlet 11 to the outlet 13 according to the arrows 48 in the figure via a pump (not shown in the figure) and a supply line (also not shown). In the pipe 12, in the area of the longitudinal extension of the heating layer 32, only the annular space 28 between the displacement body 18 and the radially inner surface of the pipe 12 is available. It should be noted at this point that neither the displacement body 28 nor the heating layer 32 or the insulating layers 30, 34 necessarily have to extend over the entire length of the pipe 12. However, the larger the contact area between the heated wall and the fuel, the better the heating of the fuel.

When the control unit 46 causes the circuit formed by the voltage source 44, the line 45, the contact pins 40 and 42 and the heating layer 32 to be closed, this leads to heating of the heating layer 32 between the contact rings 36 and 38, i.e. essentially over the entire length of the pipe 12. Due to the greater layer thickness of the heating layer 32, this heating is less in the area of the inlet end 11 than at the outlet end 13, where the heating layer 32 has a smaller layer thickness. The heating of the heating layer 32 is transferred through the electrically insulating layer 30 to the pipe 12, so that the pipe 12 is heated in the desired manner essentially over its entire length and its entire circumferential extent. By selecting a suitable material for the nozzle piece 14, it is also ensured that the nozzle piece 14 and the nozzle opening 16 are heated accordingly by heat conduction.

On the way from the inlet end 11 to the outlet end 13, the fuel (arrows 48) flows past the heated inner wall of the pipe 12 and is thereby also heated along the flow path. Due to the change in the thickness of the heating layer 32, the temperature of the pipe 12 also increases from the inlet end 11 to the outlet end 13, so that the

Heat transfer between pipe 12 and fuel 4 8
significant temperature difference across the flow path of the fuel 48 is kept essentially constant
In this way, a large amount of thermal energy can be transferred into the
Fuel 48 can be coupled in.

The insulating layers 30 and 34 protect the
Device 10 reliably protects the people handling it from contact with live elements.
In addition, the material and thickness of the layer for the outer insulating layer 34 can be selected so that it also provides thermal insulation, whereby the
It is also understood that instead of a direct current source, a
AC source with higher voltage can be used
can.

In an embodiment not shown, the radially outer surface of the
Displacement body 18 has a structure consisting of insulating layers and a heating layer, so that the two boundary walls of the annular space formed are heated and an even better heating of the fuel is achieved
can be.

In another embodiment (not shown), the contact ring is omitted at one end of the tube. Instead, the inner electrically insulating layer, i.e. the one that electrically insulates the heating layer from the tube, is slightly recessed at this end so that the heating layer makes electrical contact with the tube at this point. In this case, which is of course only possible when using a low-voltage source for safety reasons, the tube can be used as one of the two feeds for the electrical power.

Claims (22)

1. Electric heating device, in particular for fluids such as liquid fuel ( 48 ), with a heating element comprising an electrical resistor ( 32 ) which can be connected to an electrical power source ( 44 ), characterized in that the heating element comprises a carrier element ( 12 ) onto which an electrically conductive heating layer ( 32 ) made of a PTC material with a positive temperature resistance coefficient is applied, and the heating layer ( 32 ) can be connected to the electrical power source ( 44 ) at at least two spaced-apart locations. 2. Electric heating device according to claim 1, characterized in that a PTC material is used whose temperature-resistance characteristic curve has a change in slope in the manner of a "kink" at a certain temperature. 3. Electric heating device according to claim 2, characterized in that the gradient change is designed such that a certain temperature can be kept constant. 4. Electric heating device according to one of the preceding claims, characterized in that it has an electronic control and/or regulating device with which a specific temperature of at least one region of the heating layer can be set. 5. Electric heating device according to one of the preceding claims, characterized in that the heating layer ( 32 ) is applied to the carrier element ( 12 ) by a thermal process, in particular plasma coating, vapor deposition, high-velocity flame spraying or plasma spraying. 6. Electric heating device according to one of the preceding claims, characterized in that the heating layer comprises a ceramic powder. 7. Electric heating device according to one of the preceding claims, characterized in that the heating layer comprises a metal powder. 8. Electric heating device according to claim one of the preceding claims, characterized in that the carrier element ( 12 ) is made of an electrically conductive material and an electrically insulating layer ( 30 ) is present between the heating layer ( 32 ) and the carrier element ( 12 ). 9. Electric heating device according to claim 8, characterized in that the electrical power source is a low-voltage source and the heating layer is electrically connected to the carrier element at one point. 10. Electric heating device according to claim 9, characterized in that the carrier element is made of an electrically insulating material, in particular of a temperature-resistant plastic. 11. Electric heating device according to one of the preceding claims, characterized in that an electrically insulating layer ( 34 ) is present on the side of the heating layer ( 32 ) opposite the carrier element ( 12 ). 12. Electric heating device according to one of the preceding claims, characterized in that the insulating layers ( 30 , 34 ) comprise aluminum oxide Al ₂ O ₃ . 13. Electric heating device according to one of the preceding claims, characterized in that the insulating layers ( 30 , 34 ) are applied by a thermal process, in particular plasma welding. 14. Electric heating device according to one of the preceding claims, characterized in that the thickness of the heating layer ( 32 ) varies over the extension of the carrier element ( 12 ), so that the power distribution varies over the extension of the carrier element. 15. Electric heating device according to claim 14, characterized in that during operation a constant temperature difference over the extension is achieved between the carrier element ( 12 ) and the material to be heated ( 48 ). 16. Electric heating device according to one of the preceding claims, characterized in that the layer thickness of at least one of the layers ( 30 , 32 , 34 ) is in the range from 0.002 to 0.2 mm, preferably in the range from 0.005 to 0.1 mm. 17. Electric heating device according to one of the preceding claims, characterized in that the heating element is used for heating heating oil to be supplied to a burner, as a hotplate, as a continuous flow heater for water, as a cooking or warming plate, for heating the cooling water in motor vehicles, for fuel filter heating, for window heating, for de-icing aircraft wings, for wall heating of rooms or for heating floors. 18. Electric heating device according to claim 17, characterized in that the support element comprises a tubular element ( 12 ). 19. Electric heating device according to claim 18, characterized in that the tubular element comprises a fuel line ( 12 ) with an inlet ( 11 ) and an outlet ( 13 ) and the heating layer ( 32 ) is applied at least in regions to a wall of the fuel line ( 12 ). 20. Electric heating device according to claim 19, characterized in that the fuel line ( 12 ) has an injection nozzle ( 16 ) at one end. 21. Electric heating device according to one of claims 19 or 20, characterized in that the fuel line ( 12 ) comprises, at least in the region of the heating layer ( 32 ), an annular space ( 28 ) through which the fuel ( 48 ) is guided. 22. Electric heating device according to one of claims 19 to 21, characterized in that it has at least one contact ring ( 36 , 38 ) which can be pushed onto the fuel line ( 12 ) and which electrically contacts the heating layer ( 32 ).