THERMAL FLOW METER

Description

[0001] The invention relates to a device for determining and / or monitoring the mass flow rate and / or flow velocity of a fluid medium through a pipeline and to a method for manufacturing a device according to the invention. The device comprises at least one heating element, which heating element is at least partially and / or temporarily in thermal contact with the medium and which heating element can be actuated at least temporarily by means of a heating signal.

[0002] Thermal or calorimetric flowmeters are widely used in process and / or automation technology. The applicant manufactures and distributes corresponding field devices under names such as t-switch, t-trend, or t-mass. The measuring principles underlying thermal or calorimetric flowmeters are also known from the prior art. In principle, flow, and in particular mass flow, can be determined in two different ways.

[0003] According to a first measurement principle, a sensor element is exposed to a medium flowing through a pipeline and heated in such a way that its temperature remains essentially constant. Given known and at least temporarily constant medium properties, such as the medium temperature, density, or composition, the mass flow rate of the medium through the pipeline can be determined based on the heating power required to maintain the temperature at a constant value. The medium temperature is defined here as the temperature the medium would have without any additional heat input from a heating element.

[0004] In an alternative measuring principle, the heating element is operated at a constant heating power, and the temperature of the medium downstream of the heating element is measured. In this case, the measured temperature of the medium provides information about the mass flow rate.

[0005] In the case of both described measurement principles, especially with variable medium temperature, a second sensor element can also be used to measure the temperature of the medium.

[0006] The heating elements of thermal flow meters are often designed as resistance heaters. For example, so-called resistance elements, such as RTD (Resistance Temperature Detector) elements, particularly platinum elements, are used, which are also commercially available under the designations PT10, PT100, and PT1000. The resistance elements are heated by the conversion of electrical power supplied to them, e.g., as a result of an increased current supply.

[0007] To measure mass flow and / or flow velocity, the respective heating element is brought into thermal contact with the medium, at least temporarily and / or partially. For this purpose, the sensor element can, for example, be placed directly on or in the pipeline, or integrated into a sensor head, which in turn can be installed in an existing pipeline. These two possibilities, as well as others described in the prior art, are the subject of the present invention.

[0008] The determination of the flow rate is fundamentally based on the fact that the thermal resistance between the sensor element and the flowing medium changes with the prevailing flow velocity. If, for example, there is no flow in the pipeline, heat is dissipated from the heating element via conduction, radiation, and possibly also free convection within the medium. To maintain a specific temperature difference according to the first of the two described measurement principles, a constant amount of heat is required. However, if there is a flow, the sensor element is additionally cooled by the flowing, and especially colder, medium. This results in additional heat transfer due to forced convection.Therefore, as a result of the flow, an increased heating power must be supplied in order to maintain a fixed temperature difference.

[0009] The functional relationship between the heating power or measured temperature and the mass flow rate and / or flow velocity of the medium through the pipeline can also be expressed using the so-called heat transfer coefficient. In addition, various thermophysical properties of the medium, as well as the pressure prevailing in the pipeline, influence the measured flow rate. To account for the dependence of the flow rate on these factors, various thermophysical properties are stored within the electronic unit of the flow meter, for example, in the form of characteristic curves or as components of functional equations.

[0010] For high measurement accuracy, the thermal contact between the heating element and the flowing medium is crucial. For example, a change in the thermal resistance of the heating element, which leads to a change in heat transfer from the heating element to the medium under otherwise constant flow conditions, can cause significant measurement distortions. Such a change in thermal resistance is also known as sensor drift.

[0011] Regarding the thermal resistance in a flow meter, a distinction is made between internal and external thermal resistance. Internal thermal resistance depends, among other things, on individual components within the sensor element, such as those inside the housing. For example, sensor drift can occur due to defects in solder joints caused by tensile stress from material expansion or similar factors. External thermal resistance, on the other hand, is influenced by deposit formation, material erosion, or material transformation (e.g., corrosion) on the surfaces of the sensor element that come into contact with the medium. Changes in external thermal resistance are therefore particularly relevant during long-term operation and / or when in contact with aggressive media.In the case of gaseous or vaporous media, the measurement of mass flow or flow rate can also be impaired by condensation on at least one of the temperature sensors.

[0012] Besides various efforts to ensure stable thermal contact through design measures, it is common practice to detect, monitor, and / or compensate for sensor drift. Documents DE102005057687A1, DE102007023823A1, US8590360B2, and WO / 2008 / 142075A1 disclose various flowmeters that can be used to diagnose changes in external thermal resistance by analyzing the sensor elements. This allows for an assessment of the condition of at least one of the sensor elements. Regarding internal thermal resistance, DE102014119223B3 discloses methods for localizing sensor drift. In particular, changes in internal thermal resistance can be inferred. US Patent 2008 / 184780 A1 discloses a thermal flow meter in which a sensor assembly is arranged in a protective sleeve.

[0013] The present invention is based on the objective of providing a flow meter with high measuring accuracy, as well as a method for its manufacture.

[0014] This problem is solved by a device according to claim 1 and by a method according to claim 12.

[0015] The device according to the invention serves to determine and / or monitor the mass flow rate and / or the flow velocity of a flowable medium through a pipeline and comprises at least one heating element, which heating element is at least partially and / or temporarily in thermal contact with the medium and which heating element can be acted upon at least temporarily by means of a heating signal.

[0016] According to the invention, at least in a partial area between the medium and the heating element, a unit comprising a material with anisotropic thermal conductivity is arranged. Various embodiments are conceivable in this respect, all of which fall within the scope of the present invention. The precise embodiment depends, among other things, on the specific measuring instrument and the specific geometric situation. For example, the heating element and the unit can be arranged one above the other or side by side. The heating element can also be at least partially surrounded by the unit in an area facing the medium. Such an embodiment, in connection with a thermometer, is known from the previously unpublished German patent application with file number 102017100267.7, to which full reference is made in the present application.It should be noted that, within the scope of the present invention, the unit can be arranged directly between the heating element and the medium. Likewise, further intermediate layers can be located between the heating element and the unit, or between the unit and the medium. For example, a pipe wall can be located between the unit and the medium.

[0017] To determine and / or monitor the mass flow rate and / or flow velocity of the respective medium, the heating element is brought into contact with the medium, particularly thermal contact. Heat exchange takes place between the device and the medium. The thermal contact resistance between the respective sensor element and the flowing medium changes with the prevailing flow velocity. Thus, heat propagates or flows from the heating element to the medium.

[0018] Many different situations are conceivable in which heat propagation from the heating element to the medium is not homogeneous or isotropic. For example, the heat flow from the heating element to the medium can lead to anisotropic heat propagation due to the specific arrangement, particularly the design, within a flow meter. Different distances between different areas of the heating element and the medium, as well as the quality of the various contacts within the meter, are examples of this. Consequently, in practice, the thermal resistance depends not only on the mass flow rate but also on the flow direction of the medium and on the installation conditions, such as the positioning relative to the flow direction.

[0019] This is particularly problematic if the measuring device is exposed to at least a temporary and / or partial dynamic and / or inhomogeneous thermal environment, for example for an application in a flowing medium with variable flow direction, for example in the case of an inhomogeneous flow profile.

[0020] It goes without saying that numerous other scenarios can lead to a distortion of the thermal resistance. As a consequence, significant measurement errors can occur in each case. To avoid this problem, according to the invention, the unit, which at least partially comprises the material with anisotropic thermal conductivity, is arranged at least in a partial area between the medium and the heating element. Since the thermal conductivity of the unit is direction-dependent, the heat flow from the heating element to the medium can be advantageously controlled for a measuring device, possibly for a specific intended application.

[0021] In a preferred embodiment, the unit is designed to ensure that the thermal resistance of the device is independent of the flow direction of the medium or the prevailing flow profile of the medium. In particular, the unit ensures that homogeneous heat distribution from the sensor head to the medium can occur, especially regardless of the thermal environment of the device. Likewise, the unit preferably ensures that the thermal resistance is independent of certain manufacturing tolerances and design-related asymmetrical geometric conditions in the area of ​​the device.

[0022] According to the invention, the unit comprises a layer or a coating, which layer or coating consists at least partially of the material with anisotropic thermal conductivity, or a thin-walled film, which film consists at least partially of the material with anisotropic thermal conductivity. According to the invention, the layer exhibits a higher thermal conductivity in the longitudinal direction, i.e., along a plane parallel to the layer or coating, than perpendicular to it or to the plane. This allows the heat flow to be redistributed in a controlled manner along the unit.

[0023] According to the invention, the film also exhibits a higher thermal conductivity along its longitudinal direction than perpendicular to it.

[0024] According to a further particularly preferred embodiment, the material with anisotropic thermal conductivity is a material containing at least some carbon, in particular graphite, or hexagonal boron nitride.

[0025] In one embodiment of the device, the heating element and / or the unit are / are arranged at least partially within a sensor head. Preferably, the unit is designed and / or arranged such that it at least partially surrounds the temperature sensor and the measuring element. The unit thus ensures heat distribution in the circumferential direction of the sensor head.

[0026] It is advantageous if the device comprises a filler that is at least partially arranged within the sensor head and at least partially, preferably completely, fills the interior of the sensor head. The unit, which is also at least partially arranged within the sensor head, is preferably also at least partially surrounded by the filler. The filler is, in particular, a material with a substantially homogeneous thermal conductivity and / or thermal capacity. The filler thus ensures a substantially homogeneous, and in particular isotropic, heat distribution within the sensor head. For example, the filler can be a cement, in particular a mixture of magnesium oxide, zirconium silicate, and magnesium phosphate.

[0027] It is also advantageous if the unit is designed and / or arranged such that it has a higher, in particular at least 10 times higher, and especially at least 30 times higher, thermal conductivity parallel to the circumferential direction of the sensor head than perpendicular to the circumferential direction. This design is particularly advantageous for use in a thermal environment that is at least temporarily and / or partially dynamic and / or inhomogeneous, for example, for applications in flowing media with an inhomogeneous flow profile or in the case of variable flow velocities.

[0028] Finally, it is also advantageous for the design of the device with a sensor head if the unit is at least partially attached to a portion of the sensor head's wall facing the interior. The unit is thus arranged in the area of ​​the sensor head's surface facing the medium. The unit ensures a circumferential distribution of heat flow along the surface of the sensor head. Starting from the surface of the sensor head, and assuming a suitable sensor head design, this results in a substantially homogeneous heat distribution to the medium.

[0029] For example, the unit is attached to the wall of the sensor head by means of at least a portion of the filler material and / or by means of a fastening element, for example in the form of a tubular element. The tubular element is, in particular, a ceramic tube with an outer diameter that essentially corresponds to, or is slightly smaller than, the inner diameter of the sensor head, such that the tubular element can be arranged within the sensor head with a virtually perfect fit.

[0030] An alternative embodiment provides that the unit is attached to a wall of the pipeline or integrated into a pipeline. In this respect, all methods known to those skilled in the art for attaching the unit to the pipeline, for example by gluing, soldering or the like, as well as all generally known methods for integrating the unit into a pipeline, for example by potting or the like, are conceivable and fall within the scope of the present invention.

[0031] For this alternative design, it is advantageous if the heating element is attached to the unit, at least partially, such that the heating element is arranged in the area of ​​the unit facing away from the medium. In this case, the unit is therefore located, at least partially, preferably completely, between the medium and the heating element in the area facing the medium.

[0032] It is also advantageous if the unit is designed and / or arranged in such a way that it has a higher, in particular at least 10 times greater, in particular at least 30 times higher, thermal conductivity parallel to the circumferential direction of the pipeline than perpendicular to the circumferential direction.

[0033] A particularly preferred embodiment of the device according to the invention includes the fact that the unit is dimensioned and / or arranged depending on the heating signal in such a way that a predefinable amount of heat is transferred from the heating element to the medium.

[0034] In this respect, preferably at least one area of ​​at least one partial area of ​​the unit is dimensioned as a function of the heating signal such that a predefinable amount of heat is transferred from the heating element to the medium.

[0035] The object of the invention is further achieved by a method for manufacturing a device according to the invention, wherein a heating element is arranged such that the heating element is at least partially and / or temporarily in thermal contact with the medium, and wherein at least in a partial area between the medium and the heating element a unit comprising a material with anisotropic thermal conductivity is arranged.

[0036] For example, the heating element and / or the unit will be at least partially located in the sensor head.

[0037] For example, the unit is attached to a wall of the pipeline, with the heating element being attached to the unit at least section by section in such a way that the heating element is located in the area of ​​the unit facing away from the medium.

[0038] It should be noted that the embodiments explained in connection with the device are also applicable, mutatis mutandis, to the proposed method and vice versa.

[0039] The invention is explained in more detail with reference to the following drawings. They show: Fig. 1 : a schematic representation of a flow meter in which a heating element and a unit according to the invention are arranged in a sensor head, Fig. 2 : a schematic representation of a flow meter in which the unit and the heating element are attached to a wall of the pipeline, and Fig. 3 : a schematic diagram to illustrate the influence of the geometric design of the unit on the determination of the flow rate.

[0040] In the figures, identical features are marked with the same reference symbols.

[0041] In Fig. 1 A thermal flow meter 1 is shown, in which a heating element 2 is arranged in a sensor head 3. The sensor head 3, in turn, projects into a pipe 5 carrying a medium 4. For attaching the measuring device 1 to the pipe 5, all possibilities known to those skilled in the art are conceivable within the scope of the present invention and fall under its scope.

[0042] The heating element 2 is in at least partial and at least temporary thermal contact with the medium 4 by means of the sensor head 3. In the present example, the sensor head 3 is cylindrical; however, the present invention is by no means limited to cylindrical symmetries of the sensor head 3. In other embodiments of the present invention, the device can also comprise two heating elements 2 or one heating element 2 and another temperature sensor [not shown]. The heating element 2 is connected to an electronic unit 7 by means of the connecting wires 6, which serves for signal acquisition, evaluation, and power supply. It should be noted that the number of connecting wires used can vary from embodiment to embodiment.

[0043] The heating element 2 is in thermal contact with the medium 4 at least partially and / or temporarily by means of the sensor head 3.

[0044] During continuous operation, the heating element 2 can be heated to a predetermined temperature T by supplying a variable heating power P, or operated at a variable temperature T by supplying a constant heating power P. In principle, a heat exchange takes place between the measuring device 1 and the medium 4, or a heat propagation or heat flow occurs from the heating element 2 to the medium 4. The thermal contact resistance between the heating element 2 and the flowing medium 4 changes with the prevailing flow velocity v.

[0045] To ensure high measurement accuracy, it is essential to guarantee that the heat distribution from the heating element 2 to the medium 4 is as homogeneous as possible. For example, the heat flow from the heating element 2 to the medium 4 can lead to anisotropic heat distribution due to the specific arrangement, particularly the design, within the flow meter 1. Examples of such anisotropic heat distribution include varying distances between different areas of the heating element 2 and the medium, as well as the quality of the various contacts within the flow meter 1. Consequently, in practice, the thermal resistance depends not only on the mass flow rate but also on the flow direction of the medium 4 and on the installation conditions, such as the positioning relative to the flow direction of the medium 4.This is particularly problematic if the measuring instrument 1 is exposed to at least a temporary and / or partial dynamic and / or inhomogeneous thermal environment, for example for an application in a flowing medium 4 with variable flow direction, for example in the case of an inhomogeneous flow profile.

[0046] According to the invention, the thermal flowmeter 1 further comprises a unit 8 made of a material with anisotropic thermal conductivity to avoid measurement inaccuracies caused by such problems. For example, the unit consists at least partially of graphite. The unit 8 is arranged at least in a partial area between the medium and the heating element 2. In the present embodiment, it is arranged together with the heating element 2 in the sensor head 3 and surrounds the heating element 2. A heat flow emanating from the heating element 2 can be selectively influenced by means of the unit 8, in particular with regard to the direction of propagation of the respective amount of heat. In the present embodiment, the unit 8 is a thin-walled film in the form of a cylindrical sleeve with a base surface B and a lateral surface M, which completely surrounds the heating element 2 in the area facing the medium 4.Other possible embodiments of a unit 8 according to the invention can be found, for example, in the previously unpublished German patent application with file number 102017100267.7. Furthermore, in a further manufacturing step, any remaining cavities within the sensor head 3 can be filled with a suitable, in particular electrically insulating, filler material [not shown], especially cement.

[0047] Since the thermal conductivity of unit 8 is direction-dependent, the heat flow from the heating element 2 to the medium 4 for the measuring device 1 can be advantageously controlled, possibly for a specific intended application. In the present example, unit 8 ensures that the heat flow to the medium 4 is isotropic.

[0048] Preferably, the unit 8 is designed to ensure that the thermal resistance is independent of the flow direction of the medium 4 through the sensor head 3, and independent of the prevailing flow profile of the medium 4. Likewise, the unit 8 preferably ensures that the thermal resistance is independent of certain manufacturing tolerances and design-related asymmetrical geometric conditions in the area of ​​the measuring device 1, particularly in the area of ​​the sensor head 3. In the present example, the thermal conductivity of the unit 8 is preferably greater in the circumferential direction, i.e., parallel to the lateral surface M of the cylindrically shaped film, than perpendicular to it. Thus, heat propagation to the medium 4 occurs homogeneously in all directions from the wall of the sensor head 3, regardless of the arrangement of the heating unit 2 within the sensor head 3.

[0049] A second possible embodiment of a thermal flow meter 1 according to the invention is described in Fig. 2 As shown, in this example, unit 8 and heating element 2 are mounted on the outer wall of pipe 5 carrying the medium 4. Alternatively, unit 8 and / or heating element 2 can also be at least partially integrated into pipe 5. In any case, unit 8 is located between heating element 2 and the medium 4.

[0050] Unit 8 itself can, for example, be used in the same way as in the case of the Fig. 1 The unit 8 can be designed as a thin-walled film or as a coating applied to the pipe 5. In this configuration, the thermal conductivity of the unit 8 is preferably greater parallel to the wall of the pipe 5, i.e., along surface A, than perpendicular to it. A quantity of heat emanating from the heating element 2 will therefore first propagate along surface A and then from the unit 8 via the wall of the pipe 5 to the medium 4.

[0051] Electronic unit 7 is used for the example of the Fig. 2 separately from the sensor head 3 and connected to the heating element 2 via the connecting cables 6. Alternatively, the following configuration is also possible: Fig. 1 a separately arranged electronic unit 7 can be used, or the flow meter 1 according to Fig. 2 in compact design as in Fig. 1 depicted, executed.

[0052] The geometric design and dimensioning of unit 8 determines the size of a so-called heating surface and can be selected according to the invention such that, depending on the heating signal used, a predefinable amount of heat is transferred from the heating element 2 to the medium 4 by unit 8. In the case of Fig. 1 The heating surface can be appropriately selected, for example, by varying the surface area M or the area of ​​the surface M of unit 8; in the case of the design according to Fig. 2 In contrast, the area of ​​surface A parallel to the wall of the pipe 5 can be varied.

[0053] Fig. 3Figure 1 shows, as an example, two measuring devices 1, each with an identical heating element 2 arranged in an identically designed sensor head 3, and two units 8 made of the same material, each with different surface areas M. Each unit 8 shows a so-called sensor characteristic curve, which specifies a heating voltage U as a function of the mass flow rate m. Characteristic curve S1 corresponds to a small surface area M, and characteristic curve S2 to a large surface area M. The exact dimensions of the respective surface areas are irrelevant for the observed trends, which are highlighted here. The respective characteristic curve can be specifically influenced by appropriately selecting the surface area of ​​unit 8. For example, increasing the heating surface, which can be achieved, for instance, by distributing the same heating power over a larger area using unit 8, leads to a flatter characteristic curve.In this way, a larger dynamic range is covered. Conversely, smaller heating surfaces are advantageous when higher sensitivities of the respective measuring device are required, and especially in the case of low flow velocities. Unit 8 thus enables not only a homogenization of the heat distribution, but also the possibility of targeted adaptation of the sensor characteristic curve to a specific application, for example, to the flow conditions of the medium 4.

Claims

1. A device (1) for determining and / or monitoring the mass flow and / or the flow velocity of a flowable medium (4) through a pipeline (5), comprising at least one heating element (2), said heating element (2) being at least partly and / or temporarily in thermal contact with the medium (4), and said heating element (2) being at least temporarily exposed to a heating signal, characterized in that a unit (8) comprising a material with anisotropic thermal conductivity is arranged at least in a section between the medium (4) and the heating element (2), the unit (8) comprises a layer or a coating, said layer or coating consisting at least partly of the material with anisotropic thermal conductivity, or a thin-walled film, said film consisting at least partly of the material with anisotropic thermal conductivity, and in that the layer, coating or film has higher thermal conductivity in its longitudinal direction than perpendicular to its longitudinal direction.

2. The device (1) as claimed in at least one of the preceding claims, wherein the material with anisotropic thermal conductivity is a material containing at least some proportion of carbon, or is hexagonal boron nitride.

3. The device (1) as claimed in at least one of the preceding claims, wherein the material containing at least some proportion of carbon is graphite.

4. The device (1) as claimed in at least one of the preceding claims, wherein the heating element (2) and / or the unit (8) is / are arranged at least partly in a sensor head (3).

5. The device (1) as claimed in claim 4, comprising a filler, which is arranged at least partly inside the sensor head (3) and at least partly fills an interior space of the sensor head (3).

6. The device (1) as claimed in at least one of claims 4 or 5, wherein the unit (8) is configured and / or arranged in such a way that it has higher thermal conductivity parallel to the outer surface of the sensor head (3) than perpendicular to the outer surface.

7. The device (1) as claimed in at least one of claims 1 to 3, wherein the unit (8) is mounted on a wall of the pipeline (5), or is integrated into a pipeline (5).

8. The device (1) as claimed in claim 7, wherein the heating element (2) is attached to the unit (8) at least in one section in such a way that the heating element (2) is arranged in the area of the unit (8) facing away from the medium (4).

9. The device (1) as claimed in one of claims 7 or 8, wherein the unit (8) is configured and / or arranged in such a way that it has higher thermal conductivity parallel to the outer surface of the pipeline (5) than perpendicular to the outer surface.

10. The device (1) as claimed in at least one of the preceding claims, wherein the unit (8) is dimensioned and / or arranged based on the heating signal in such a way that a specifiable quantity of heat is transferred from the heating element (2) to the medium (4).

11. The device (1) as claimed in claim 10, wherein at least a surface area of a sub-area (M,A) of the unit (8) is dimensioned based on the heating signal in such a way that a specifiable quantity of heat is transferred from the heating element (2) to the medium (4).

12. A method for manufacturing a device (1) as claimed in at least one of the preceding claims, wherein a heating element (2) is arranged in such a way that the heating element (2) is at least partly and / or temporarily in thermal contact with the medium (4), and wherein a unit (8) comprising a material with anisotropic thermal conductivity is arranged at least in a section between the medium (4) and the heating element (2).