Electric heater, method for operating the electric heater, and control unit

Abstract

The present disclosure relates to an electric heater for heating a fluid flowing along a direction of flow (10) in a fluid channel. The electric heater has a heating unit (1) which has a plurality of heating circuits (2, 4, 6) arranged in succession along the direction of flow (10). Furthermore, the electric heater has a control unit (20) which is configured such that it separately controls the heating circuits (2, 4, 6) in order to create respective heat outputs according to the arrangement of the heating circuits (2, 4, 6) along the direction of flow (10).

Description

[0001] Electric heater, method for operating the electric heater and control unit

[0002] Description

[0003] The present disclosure relates to an electric heater for heating a fluid flowing in a fluid channel along a flow direction, a method for operating the electric heater, and a control unit for the electric heater.

[0004] A fluid heater is known from the published patent applications DE 10 2019 133 039 A1 and DE 10 2019 133 043 A1, which originate from the applicant. The fluid heater disclosed therein comprises a circuit board on which two heating circuits formed by conductor tracks are mounted along a flow direction of a fluid to be heated. The two heating circuits are designed to provide different heating outputs, but are controlled in the same way, i.e., by applying the same voltage. In practice, the total heating output of the fluid heater is controlled by switching the two heating circuits on and off.

[0005] It is therefore an object of the present disclosure to provide an improved electric heater for heating a fluid flowing in a fluid channel along a flow direction, an improved method for operating the electric heater, and a control unit for the electric heater. This object is achieved by the electric heater, the method, and the control unit having the features according to the independent claims. Advantageous embodiments are the subject of the dependent claims.

[0006] An electric heater according to the disclosure is used to heat a fluid flowing in a fluid channel along a flow direction and comprises a heating unit with a plurality of heating circuits arranged successively along the flow direction. In this context, it should be noted that "plurality" is to be understood as two or more. It should also be noted that the fluid channel can be formed at least partially by the heating unit. For this purpose, the heating unit can have a fluid inlet and a fluid outlet and can be interposed in a fluid line.

[0007] The heating circuits can be designed as PTC elements, tubular heaters, heating plates, etc. Particularly preferably, the heating circuits can be designed as conductor tracks, which are made in particular from a metal such as copper, nickel, aluminum, gold, etc. The heating circuits designed as conductor tracks can be arranged in or on a common heating element or in or on separate heating elements. The heating element(s) can be formed by a circuit board(s). The conductor tracks can preferably be formed in a meandering shape. In addition, the meanders of the conductor tracks can spread along the flow direction, i.e. a current flowing in the conductor tracks flows in a meandering shape along or against the flow direction of the fluid. As a further advantage, a power density of the conductor tracks can be adapted along the flow direction, e.g. by changing, in particular enlarging, a cross-sectional area of ​​the conductor tracks.Consequently, the heat output of the heating circuit can be optimized for its width along the flow direction. It has also proven advantageous if the heating element(s) are designed as IMS circuit boards, which are in direct or indirect heat-conducting contact with the fluid channel via a metallic backing or metallic core. Heat exchangers can also be assigned to the heating circuits to transfer the heat generated by the heating circuits to the fluid. The heat exchangers can extend at least partially into the fluid channel and can be at least partially in contact with the fluid. The heat exchangers can be provided in the form of fins, louvers, grids, or turbulators. It is also possible to provide a common heat exchanger for all heating circuits.

[0008] In addition, the electric heater has an independently operable control unit or regulating unit configured to separately control or regulate the heating circuits to generate a respective heat output according to the arrangement of the heating circuits along the flow direction. The control unit preferably controls the heat output of the heating circuits via pulse width modulation (PWM). For this purpose, the control unit can have appropriately configured outputs, each assigned to the heating circuits. An amplifier circuit can be connected downstream of the outputs. The control unit can also be formed by several separate control units, each assigned to the heating circuits. Alternatively, the control unit can also output control commands to corresponding energy sources or an energy source with multiple outputs, which then apply or apply energy to the heating circuits according to the control commands from the control unit.The heat output of the heating circuits can thus be controlled using a PWM duty cycle. The electric heater according to the disclosure thus enables customized control of the respective heat outputs of the heating circuits. It should be noted that the aspects relating to the control unit can be claimed independently of the electric heater.

[0009] Preferably, the control unit can be configured such that it controls the heating circuits during simultaneous operation such that the respective heat output of the heating circuits progressively increases against the direction of flow. This means that all heating circuits are controlled simultaneously and the respective heat outputs increase against the direction of flow. While, with the same control, a last heating circuit along the flow direction reaches a maximum temperature, in particular its permissible limit temperature, for a specific operating point, heating circuits arranged upstream are only heated to a lower temperature due to a lower fluid temperature and thus better heat transfer. The upstream heating circuits can therefore be operated with higher heat output when controlled separately. Consequently, the total output of the heating circuits can be increased compared to with the same control of the heat output of the heating circuits.In this way, the size of the heating unit can also be reduced in order to achieve the same total output.

[0010] According to a possibly independently claimable aspect, the electric heater can have a plurality of temperature sensors, each assigned to the plurality of heating circuits. The temperature sensors can be formed by a thermocouple, an NTC resistor, or a platinum measuring resistor. The control unit can then be configured to detect a respective temperature of the heating circuits, in particular an average temperature of the heating circuits, i.e., a temperature averaged across a width of the heating circuits along the flow direction, using the temperature sensors. For this purpose, the control unit can have corresponding inputs and additional converters for converting the input signals from the temperature sensors into digitally analyzable signals. Consequently, the temperature of the heating circuits can be detected.

[0011] Alternatively or additionally, according to a possibly independently claimable aspect, the control unit can be configured such that it detects a temperature-dependent operating parameter of the heating circuits. The temperature-dependent operating parameter can preferably be a resistance of the heating circuits. Even more preferably, the temperature-dependent operating parameter can be an ohmic resistance of the heating circuits. When using the heating circuits designed as conductor tracks, detecting the ohmic resistance has proven advantageous. From the detected temperature-dependent operating parameters of the heating circuits, the control unit can detect or derive a respective temperature of the heating circuits, in particular an average temperature of the heating circuits, i.e. a temperature averaged over a width of the heating circuits along the flow direction. Consequently, the temperature of the heating circuits can be detected in a simple manner.It should be noted that the temperature of a heating circuit can be recorded simultaneously using a temperature sensor and the temperature-dependent operating parameter can be recorded simultaneously.

[0012] The control unit can be configured to separately control the respective heat output of the heating circuits based on the recorded temperatures of the heating circuits. Consequently, the recorded temperatures of the heating circuits are taken into account when controlling or regulating the heat output, thereby achieving optimized control of the heating circuits according to their arrangement along the flow direction.

[0013] When recording the temperatures of the heating circuits, the control unit can be configured to use an average of the recorded temperatures of the heating circuits as the target value for the temperature-dependent control. Particularly preferably, a general manipulated variable, e.g. the PWM duty cycle, is determined beforehand based on a predetermined target output and a current heat output, i.e. an actual output. This general manipulated variable can then be adjusted for the individual heating circuits based on the average of the recorded temperatures. Consequently, the heat output generated by the heating circuits can be controlled such that the upstream heating circuits reach a higher temperature, thereby increasing the total output provided by all heating circuits.

[0014] As an alternative to the above procedure, the control unit can be configured to use one of the recorded temperatures of the heating circuits as the setpoint for temperature-dependent control. Again, a general control variable can be determined beforehand. The general control variable can be forwarded to the heating circuit whose temperature is used as the setpoint for temperature-dependent control, without temperature-dependent adjustment. In this case, too, the total output provided by all heating circuits can be increased.

[0015] The control unit can be configured to determine overheating of a heating circuit if the detected temperature of the heating circuit exceeds a predetermined maximum temperature. Alternatively or additionally, the control unit can be configured to determine overheating of a heating circuit if a change in at least two consecutively detected temperatures of the heating circuit exceeds a predetermined maximum change. The change, in particular a gradient of a temporal temperature profile, can be determined based on two or more consecutive temperature values. The change can also be determined as a moving average in order to smooth the temporal temperature profile. In this way, overheating of a heating circuit, which occurs, for example, due to a leak or dry running, can be reliably avoided.

[0016] Additionally, according to a possibly independently claimable aspect, the control unit can be configured to determine a flow direction based on the respective heat output of the heating circuits. This approach is possible because the respective heat output is controlled based on the temperature of the heating circuits and therefore a flow direction does not need to be known in advance. The increasing course of the heat output then occurs as a result of the set flow direction, since upstream heating circuits can more easily transfer heat to the colder fluid. The determined flow direction can, for example, also be used to determine an incorrect reversal of the flow direction. For example, when controlled in such a way that the heat output increases opposite to the flow direction, the heating circuit that generates the greatest heat output is arranged at the beginning of the fluid channel.The heating circuit that generates the lowest heat output is located at the end of the fluid channel. This allows the operation of the electric heater to be reliably monitored.

[0017] Furthermore, according to a possibly independently claimable aspect, the control unit can be configured to determine a lack of response of a heating circuit in response to the control or regulation if the detected temperature of a heating circuit falls below a predetermined minimum temperature. The minimum temperature can be determined, for example, based on a fluid temperature during normal operation of the electric heater. Falling below the minimum temperature indicates that the heating circuit is heated solely by the heat of the fluid and / or by a neighboring heating circuit and not by a current flow in the heating circuit. Consequently, an electrical open circuit of the heating circuit can be detected.

[0018] Alternatively or additionally, according to a possibly independently claimable aspect, the control unit can be configured to determine a lack of response of a heating circuit in response to the control if a change in at least two consecutively detected temperatures of a heating circuit falls below a minimum change. In this way, it can be determined that the temperature of the heating circuit is not changing due to a current flow in the heating circuit. The minimum change can be determined based on a temperature increase of the fluid upon start-up of the electric heater. Consequently, an electrical idle state of the heating circuit can also be detected.

[0019] Advantageously, the control unit can be configured to deactivate the electric heater and / or issue a warning via a dedicated signal line, for example, to an output unit, such as a visual or acoustic output unit, if overheating and / or a faulty reversal of the flow direction and / or a lack of response of a heating circuit is detected. Consequently, the electric heater is protected from faulty operation and potentially from major damage due to continued faulty operation.

[0020] In addition, the control unit can be configured to shut down the heating circuit for which overheating is detected. This allows the other functioning heating circuits to continue to operate, at least in emergency mode.

[0021] Advantageously, the control unit can be configured to separately control the respective heat output of the heating circuits based on a respective factor or a respective characteristic curve or a respective characteristic map. The factors can be defined based on a standard operating point. By using the characteristic curves or the characteristic maps, several operating points of the electric heater can be taken into account. The several operating points can be defined based on the temperature of the heating circuits and / or a fluid temperature and / or a supply voltage. The factors, the characteristic curves and the characteristic maps can be determined empirically in advance, e.g. by a manufacturer of the electric heater. In this way, a simple control system can be implemented. It should be noted that this type of control can also be combined with controlling the heat output based on the temperature of the heating circuits.By taking these factors into account during control, a structurally determined difference in heat output between heating circuits can be taken into account. The heating circuits can then be assumed to be essentially equal for control purposes based on their temperature.

[0022] Particularly preferably, the heating circuits can be designed essentially identically. Accordingly, a flow direction of the electric heater is reversible. The heat output of the heating circuits then only needs to be controlled in reverse order. Consequently, the electric heater can also be used for applications in which a fluid is to flow bidirectionally through the fluid channel. As already mentioned, the heating unit can have a heat exchanger or a plurality of heat exchangers, each of which is assigned to the plurality of heating circuits in order to transfer heat from the heating circuits to the fluid. The heat exchanger(s) can then preferably be designed essentially identically for all heating circuits. Consequently, a reversal of the flow direction is possible.

[0023] Furthermore, the present disclosure provides a method for operating an electric heater for heating a fluid flowing in a fluid channel along a flow direction. The electric heater comprises a heating unit with a plurality of heating circuits arranged one after the other along the flow direction. The heating circuits for generating a respective heat output are controlled separately according to their arrangement along the flow direction. Consequently, optimal control of the respective heat output of the heating circuits can be achieved.

[0024] Preferably, the heating circuits are controlled in such a way that the respective heat output of the heating circuits increases against the flow direction. While, with the same control, a last heating circuit along the flow direction reaches its limit temperature, upstream heating circuits are heated to a lower temperature due to a lower fluid temperature and thus better heat transfer. The upstream heating circuits can therefore be operated at higher power with separate control. Accordingly, the total output of the heating circuits can be increased compared to a similar control of the heat output of the heating circuits.

[0025] Furthermore, a respective temperature of the heating circuits, in particular an average temperature of the heating circuits, i.e., a temperature averaged across the width of the heating circuits along the flow direction, can be detected. The respective temperature can be detected, as described above, using temperature sensors and by detecting a temperature-dependent operating parameter of the heating circuits. The respective heat outputs of the heating circuits can be controlled separately based on the detected temperatures of the heating circuits. This allows for optimal control of the heat outputs of the heating circuits.

[0026] Preferably, an average of the recorded temperatures of the heating circuits is used as the target value for the temperature-dependent control. Particularly preferably, a general control variable, e.g., the PWM duty cycle, is determined beforehand based on a specified target output and a current heat output, i.e., an actual output. This general control variable can then be adjusted based on the average of the recorded temperatures for the individual heating circuits. Consequently, the heat output generated by the heating circuits can be controlled such that the upstream heating circuits reach a higher temperature, thereby increasing the total output provided by all heating circuits.

[0027] As an alternative to the above procedure, one of the recorded temperatures of the heating circuits can be used as the setpoint for temperature-dependent control. Again, a general control variable can be determined beforehand. The general control variable can then be forwarded to the heating circuit whose temperature is used as the setpoint for temperature-dependent control, without temperature-dependent adjustment. In this case, too, the heat output generated by each heating circuit can be optimally controlled.

[0028] In addition, overheating of a heating circuit can be determined if the recorded temperature of the heating circuit exceeds a predefined maximum temperature. The maximum temperature can be set to a limit temperature of the heating circuit. Alternatively or additionally, overheating of a heating circuit can be determined if a change in at least two consecutively recorded temperatures of the heating circuit exceeds a predefined maximum change. The change, in particular a gradient of a temporal temperature profile, can be determined based on two or more consecutive temperature values. The change can also be determined as a moving average in order to smooth the temporal temperature profile. In this way, overheating of a heating circuit, which occurs, for example, due to a leak or dry running, can be reliably avoided.

[0029] Furthermore, a flow direction can be determined based on the respective heat output of the heating circuits. For example, if the control system is such that the heat output increases counter to the flow direction, the heating circuit generating the greatest heat output is arranged at the beginning of the fluid channel. The heating circuit generating the least heat output is arranged at the end of the fluid channel. Consequently, the operation of the electric heater can be reliably monitored.

[0030] Furthermore, a lack of response from a heating circuit in response to the control or regulation can be detected if the detected temperature of a heating circuit falls below a specified minimum temperature. The minimum temperature can be defined, for example, based on a fluid temperature during normal operation of the electric heater. If the temperature falls below the minimum temperature, this indicates that the heating circuit is being heated solely by the heat of the fluid and / or by a neighboring heating circuit and not by a current flowing in the heating circuit. Consequently, an electrical open circuit in the heating circuit can be detected.

[0031] Advantageously, the electric heater can be shut down and / or a warning can be issued via a dedicated signal line, for example, to an output unit, such as a visual or acoustic output unit, if overheating and / or a faulty reversal of the flow direction and / or a lack of response of a heating circuit is detected. Consequently, the electric heater is protected from faulty operation and potentially from major damage due to continued operation.

[0032] In addition, the heating circuit for which overheating is detected can be switched off. This means that the other functioning heating circuits can continue to be used, at least in emergency operation, if necessary. Advantageously, the heat output of the heating circuits can be controlled separately on the basis of a respective factor or a respective characteristic curve or a respective characteristic map. The factors can be defined using a standard operating point. By using the characteristic curves or the characteristic maps, several operating points of the electric heater can be taken into account. The several operating points can be defined based on the temperature of the heating circuits and / or a fluid temperature and / or a supply voltage. The factors, the characteristic curves and the characteristic maps can, in particular, be determined empirically in advance, e.g. by a manufacturer of the electric heater. In this way, a simple control system can be implemented.It should be noted that this control method can also be combined with heat output control based on the temperature of the heating circuits. These factors can be used to take into account, for example, a structurally determined difference in heat output between the heating circuits. The heating circuits can then be assumed to be essentially equal when controlling based on temperature.

[0033] Another disclosed electric heater for heating a fluid flowing in a fluid channel along a flow direction comprises a heating unit having a plurality of heating circuits arranged one after the other along the flow direction, and a control unit configured to control the heating circuits in a substantially identical manner to generate a respective heat output. The heating circuits are then configured to generate a heat output corresponding to the arrangement of the heating circuits along the flow direction. Alternatively or additionally, a plurality of heat exchangers, each associated with the plurality of heating circuits, is configured such that a respective heat transfer coefficient is determined according to the arrangement of the heat exchangers along the flow direction. Consequently, an optimal adjustment of the heat output along the flow direction can be achieved.

[0034] The heating circuits can advantageously be designed such that the heat output of the heating circuits increases against the direction of flow. With the same control, only the last heating circuit along the direction of flow reaches a maximum temperature, in particular its permissible limit temperature, and the upstream heating circuits are only heated to a lower temperature due to a lower fluid temperature and thus better heat transfer. By designing the heating circuits such that the respective heat output increases against the direction of flow, i.e. by designing the upstream heating circuits for a higher heat output, the upstream heating circuits also reach a higher temperature. Consequently, a higher total output of the heating circuits can be achieved. This also makes it possible to reduce the size of the heating unit in order to achieve the same total output.

[0035] Preferably, the heat exchangers can be designed such that the heat transfer coefficient of the heat exchangers decreases in the direction opposite to the flow direction. Consequently, the heat from the upstream heating elements is transferred less effectively, so that the temperature of the upstream heating circuits is increased. This not only increases the total power but also reduces the pressure loss at the beginning of the fluid channel, which occurs due to the heat transfer to the fluid.

[0036] The electric heaters according to the disclosure and the method for operating the electric heater according to the disclosure thus enable improved control or adjustment of the respective heat outputs of the plurality of heating circuits arranged along the flow direction.

[0037] Short description of the characters

[0038] Embodiments of the present disclosure are described below with reference to the figures. In the figures:

[0039] Fig. 1 shows a schematic structure of a heating unit according to a first embodiment, which has three heating circuits arranged along a flow direction of a fluid, as well as a heat flow diagram during conventional operation of the heating unit; Fig. 2 shows a schematic structure of a conventional control unit;

[0040] Fig. 3 shows the schematic structure of the heating unit according to the first embodiment and a heat flow diagram during operation of the heating unit according to the disclosure;

[0041] Fig. 4 shows a schematic structure of a control unit according to the disclosure according to a first and a second modification of the first embodiment;

[0042] Fig. 5 shows a schematic structure of a unit for temperature-dependent manipulated variable control according to a first modification;

[0043] Fig. 6 shows a schematic structure of a unit for temperature-dependent manipulated variable control according to a second modification; and

[0044] Fig. 7 shows a schematic structure of a heating unit according to a third embodiment, which has three heating circuits arranged along a flow direction of a fluid, which are designed for different heat outputs, as well as a heat curve diagram during operation of the heating unit.

[0045] First embodiment

[0046] The following describes an electric heater according to a first embodiment of the present disclosure. The electric heater according to the disclosure comprises a heating unit 1 shown in Fig. 1, which has three heating circuits 2, 4, 6. The number of heating circuits 2, 4, 6 is not limited to three, but can also be two or more than three.

[0047] The heating circuits 2, 4, 6 are arranged in Fig. 1 on a common circuit board 8 as a heating element along a flow direction 10 of the fluid. However, the heating circuits 2, 4, 6 can also be arranged on separate circuit boards as separate heating elements. The circuit board 8 can preferably be designed as an IMS board, the rear side of which is in direct or indirect heat-conducting contact with the fluid channel formed below the circuit board 8. The fluid channel can be formed at least in sections by the heating unit 1. The heating unit 1 can form a fluid inlet and a fluid outlet and be interposed in a fluid line. A heat exchanger (not shown) can also be assigned to each of the heating circuits 2, 4, 6 in order to transfer the heat generated by the heating circuits 2, 4, 6 to the fluid flowing in the fluid channel along the flow direction. A common heat exchanger can also be provided for all heating circuits 2, 4, 6.

[0048] Heating circuit 2 will be described in detail as representative of the two heating circuits 4, 6. Heating circuit 2 is formed by a conductor track 12 that extends in a meandering manner along the flow direction. For this purpose, conductor track 12 has deflection points 14 that are parallel to the flow direction. Conductor track 12 is preferably made of copper. It should be noted that in the present embodiment, heating circuits 2, 4, 6 and the associated heat exchangers are essentially identical, so that heating unit 1 can also be operated with a reversed or opposite flow direction.

[0049] The conductor track 12 can be electrically contacted via conductor track terminals 16 and 18 in order to supply power to the conductor track 12. The power is preferably supplied by pulse width modulation (PWM) in order to control the supplied energy and thus the heat output of the heating circuit 2. The conductor track terminals 16 and 18 can be designed as plugs or contact tabs on the circuit board 8.

[0050] Fig. 2 shows a conventional control unit S which receives a predefined target output and a current heat output as input variables. From these input variables, a manipulated variable, which is preferably a PWM duty cycle, is determined by means of a manipulated variable control or a manipulated variable regulation. The heat outputs of the heating circuits 2, 4, 6 are then controlled according to the manipulated variable, e.g. by applying a voltage pulse-width modulated with the PWM duty cycle or a current pulse-width modulated with the PWM duty cycle to the conductor track connections 16 and 18 of the conductor track 12. As shown in the diagram in Fig. 1, with the control described above, a heating circuit temperature profile ■SH(X) and a fluid temperature profile ■SF(X) are established. It should be noted that the heating circuits 2, 4, 6 must not be heated above a predefined limit temperature ÖG, as otherwise they will be irreversibly damaged.The maximum heat output of heating circuits 2, 4, and 6 is determined by a minimum temperature difference A£(x). The temperature difference A£(x) corresponds to the difference between the limit temperature $G and the fluid temperature ■SF(X). As can be seen in the diagram in Fig. 1, the fluid temperature ■SF(X) increases over a length x of heating unit 1. As a result, heat can be transferred more effectively to the fluid near the inlet of the fluid channel, so that the first heating circuit 2 has a lower temperature for the same heat output. This difference AOu(x) between the limit temperature $G and the heating circuit temperature ■SH(X) thus corresponds to unused heat output potential. It should be noted that the limit temperature G is only an example of a maximum temperature at a specific operating point.By setting a different operating point of the electric heater, a different maximum temperature can be reached, which may be lower than the limit temperature. The temperature of the upstream heating elements 4, 6 is then lower than this maximum temperature.

[0051] Fig. 3 shows the heating unit 1 from Fig. 1. In this case, however, the heating unit 1 is operated using a control unit 20 shown in Fig. 4. The control unit 20 has a unit 22 for general manipulated variable control and a unit 24 for temperature-dependent manipulated variable control according to a first modification or a unit 26 for temperature-dependent manipulated variable control according to a second modification. The unit 22 for general manipulated variable control essentially corresponds to the conventional control unit S shown in Fig. 2. The unit 22 for general manipulated variable control receives a predetermined target power and a heat output as input parameters and determines a general manipulated variable from this. The general manipulated variable is then subsequently controlled by the unit 24 orThe unit 26 is adjusted according to the measured temperatures 1 to n (n corresponds to the number of heating circuits; in the present embodiment, n = 3) of the heating circuits 2, 4, 6. The temperature of the heating circuits 2, 4, 6 can be detected using temperature sensors (not shown) assigned to the heating circuits 2, 4, 6, respectively. The control unit 20 is then configured to detect a respective temperature of the heating circuits 2, 4, 6 using the temperature sensors. Alternatively or additionally, the control unit 20 can be configured to detect a respective temperature of the heating circuits 2, 4, 6 by detecting an ohmic resistance of the conductor track 12, which changes substantially proportionally to the temperature, as a temperature-dependent operating parameter. The control unit 20 is thus configured to perform a resistance measurement. The resistance measurement is performed in a predetermined cycle.

[0052] The structure of the temperature-dependent manipulated variable control unit 24 according to the first modification is shown in Fig. 5. The temperature-dependent manipulated variable control unit 24 receives the general manipulated variable determined by the general manipulated variable control unit 22, as well as the detected temperatures 1 to 3 of the heating circuits 2, 4, 6. An averaging unit 28 calculates an average from the temperatures 1 to 3 and outputs it to the subtractors 30, 32, 34, which each subtract the temperatures 1 to 3 from the average. The differences thus determined between the average and the temperatures 1 to 3 are then passed on to controllers 36, 38, 40, respectively. The controllers 36, 38, 40 can, in particular, be PI controllers or PID controllers.The variables output by controllers 36, 38, and 40 are then added to the previously determined general manipulated variable by adders 42, 44, and 46 to determine temperature-dependent manipulated variables 1 to 3, which are, in particular, a PWM duty cycle. Heating circuits 2, 4, and 6 are then controlled according to temperature-dependent manipulated variables 1 to 3.

[0053] The structure of the temperature-dependent manipulated variable control unit 26 according to the second modification is shown in Fig. 6. The temperature-dependent manipulated variable control unit 26 also receives the general manipulated variable and the detected temperatures 1 to 3. In the present example, the temperature-dependent manipulated variable control is based on temperature 1 of heating circuit 2. However, the temperature-dependent manipulated variable control can also be based on temperature 2 or temperature 3. Accordingly, the general manipulated variable is used as the temperature-dependent manipulated variable 1 for heating circuit 2. To calculate the two remaining temperature-dependent manipulated variables 2 and 3, temperatures 2 and 3 are first subtracted from temperature 1 by the subtractors 48, 50, respectively. These differences are then output to a controller 52 and a controller 54, respectively. The controllers 52, 54 can in turn be PI controllers or PID controllers.The variables determined by controllers 52, 54 are then added to the general manipulated variable by adders 56, 58 to calculate temperature-dependent manipulated variables 2 and 3 for heating circuit 4 and heating circuit 6. Heating circuits 2, 4, and 6 are then each controlled according to temperature-dependent manipulated variables 1 to 3.

[0054] Through the control and regulation described above, the heating circuit temperature curves H,-I (X), H,2(X), H,3(X) and a fluid temperature curve F(X) shown in the diagram of Fig. 3 are established. Again, the temperatures H,1, H,2, OH,3 of the heating circuits 2, 4, 6 should not exceed the limit temperature OG, which represents an example of a maximum temperature at a specific operating point. It can be seen that the respective temperatures of the heating circuits 2 and 4 are increased compared to the case shown in the diagram of Fig. 1, so that the respective average temperatures of the heating circuits are essentially the same across their width along the flow direction 10. It can also be seen that the heating circuit temperature curves OH,-I (X), OH,2(X), OH,3(X) each reach the limit temperature OG at the end of the respective heating circuits 2, 4, 6. As a result, the heat output potential of heating circuits 2, 4, and 6 can be utilized more efficiently.The total output of heating circuits 2, 4, and 6 can be increased by approximately 10% compared to a conventional control system. This allows the size of heating unit 1 to be reduced to achieve the same output.

[0055] In addition to the temperature-dependent control of heating circuits 2, 4, 6 described above, control unit 20 can be configured to determine overheating of a heating circuit 2, 4, 6 if the detected temperature 1 to 3 of heating circuit 2, 4, 6 exceeds a predetermined maximum temperature, in particular the limit temperature OG. Alternatively or additionally, control unit 20 can be configured to determine overheating of a heating circuit 2, 4, 6 if a change in at least two consecutively detected temperatures 1 to 3 of heating circuits 2, 4, 6 exceeds a predetermined maximum change.

[0056] Furthermore, the control unit 20 can be configured to determine a flow direction based on the respective heat outputs of the heating circuits 2, 4, 6. In the present embodiment, the heating circuits 2, 4, 6 are controlled such that their respective heat output increases counter to the flow direction 10. Accordingly, a heating circuit that generates the highest heat output, in this case heating circuit 2, is arranged at the beginning of the fluid channel. The heating circuit that generates the lowest heat output, in this case heating circuit 6, is arranged at the end of the fluid channel.

[0057] Furthermore, the control unit 20 can be configured to determine a lack of response of a heating circuit 2, 4, 6 in response to the control if the detected temperature 1 to 3 of a heating circuit 2, 4, 6 falls below a predetermined minimum temperature. Alternatively or additionally, the control unit 20 can be configured to determine a lack of response of a heating circuit 2, 4, 6 in response to the control if a change in at least two consecutively detected temperatures 1 to 3 of a heating circuit 2, 4, 6 falls below a minimum change. Consequently, the heating circuit 2, 4, 6 is heated only by the fluid and / or by a neighboring heating circuit 2, 4, 6 and not by a current flow through the conductor track 12 of the heating circuit 2, 4, 6. Consequently, a line to the conductor track 12 or the conductor track 12 itself is interrupted, resulting in an electrical open circuit.

[0058] Second embodiment

[0059] An electric heater according to a second embodiment comprises the heating unit 1 of the electric heater according to the first embodiment and a control unit configured to separately control the respective heat output of the heating circuits 2, 4, 6 based on a respective factor or a respective characteristic curve or a respective characteristic map. For example, for heating circuits 2 and 4 that are not fully exploiting their heat output potential, a fixed factor by which the maximum temperature of heating circuit 2 or 4 is lower than the limit temperature G can be determined for a standard operating point. The general manipulated variable can then be multiplied by this factor, so that the total output of heating circuits 2, 4, 6 is increased. Alternatively, a characteristic curve or a characteristic map that takes multiple operating points into account can be determined for each heating circuit 4, 6.

[0060] Consequently, the total output of heating circuits 2, 4, 6 can also be optimized at other operating points.

[0061] Third embodiment

[0062] An electric heater for heating a fluid according to a third embodiment is shown in Fig. 7. The electric heater comprises a heating unit 60 having a plurality of heating circuits 62, 64, 66. The heating circuits 62, 64, 66 are arranged on a common circuit board 68 along a flow direction 70.

[0063] The electric heater has a control unit (not shown) configured to control the heating circuits 62, 64, 66 in a substantially identical manner to generate a respective heat output. The control unit, in particular, drives the heating circuits 62, 64, 66 with the same energy or power. The control unit thus corresponds to the conventional control unit S shown in Fig. 2.

[0064] To increase the total output, the heating circuits 62, 64, 66 are configured such that they generate a heat output corresponding to the arrangement of the heating circuits 62, 64, 66 along the flow direction. The heating circuits are configured such that their respective heat output increases counter to the flow direction 70. In the present embodiment, this is achieved by different cross-sectional areas of the conductor tracks 72, 74, 76 that form the heating circuits 62, 64, 66. In particular, a cross-sectional area of ​​the conductor tracks 72, 74, 76 decreases counter to the flow direction 70, so that with the same control, a higher heat output is achieved by the upstream heating circuits. It should be noted that the cross-sectional areas are set according to a standard operating point.

[0065] Alternatively or additionally, a heat exchanger or a plurality of heat exchangers assigned to the plurality of heating circuits can be configured to transfer heat from the heating circuits to the fluid, such that a respective heat transfer coefficient is determined according to an arrangement of the heating circuits 72, 74, 76 along the flow direction 70. In particular, the heat exchangers can be configured such that a heat transfer coefficient of the heat exchanger or heat exchangers decreases counter to the flow direction 70. Accordingly, heat transfer for the upstream heating circuits can be adjusted such that they reach a higher temperature.

[0066] As can be seen in the diagram in Fig. 7, a similar course of the heating circuit temperatures ÖH,I (X), 0H,2(X), £H,3(X) can be achieved by the electric heater according to the third embodiment.

[0067] The present disclosure is not limited to the embodiments described above. For example, the heating circuits and the heat exchanger(s) in the first embodiment need not be substantially identical and may be differently configured, as in the third embodiment. Furthermore, the control based on the factors or the characteristic curves or the characteristic maps according to the second embodiment can also be combined with the control according to the first embodiment. The control based on the factors or the characteristic curves or the characteristic maps according to the second embodiment can also be used for the electric heater according to the third embodiment. List of Reference Symbols

[0068] 1 heating unit

[0069] 2 heating circuits

[0070] 4 heating circuits

[0071] 6 heating circuits

[0072] 8 circuit boards

[0073] 10 Flow direction

[0074] 12 conductor tracks

[0075] 14 Deflection point

[0076] 16 conductor track connection

[0077] 18 Conductor track connection

[0078] 20 Control unit

[0079] 22 Unit for general manipulated variable control

[0080] 24 Unit for temperature-dependent manipulated variable control

[0081] 26 Unit for temperature-dependent manipulated variable control

[0082] 28 averaging unit

[0083] 30 subtractors

[0084] 32 subtractors

[0085] 34 subtractors

[0086] 36 controllers

[0087] 38 controllers

[0088] 40 controllers

[0089] 42 adders

[0090] 44 adders

[0091] 46 adders

[0092] 48 subtractors

[0093] 50 subtractors

[0094] 52 controllers

[0095] 54 controllers

[0096] 56 adders

[0097] 58 Adder heating unit

[0098] Heating circuit

[0099] Heating circuit

[0100] Heating circuit

[0101] circuit board

[0102] Flow direction

[0103] conductor track

[0104] conductor track

[0105] conductor track

[0106] Control unit

Claims

Claims 1. An electric heater for heating a fluid flowing in a fluid channel along a flow direction (10), comprising: a heating unit (1) having a plurality of heating circuits (2, 4, 6) arranged one after the other along the flow direction (10), characterized by a control unit (20) configured to separately control the heating circuits (2, 4, 6) to generate a respective heat output in accordance with the arrangement of the heating circuits (2, 4, 6) along the flow direction (10).

2. Electric heater according to claim 1, wherein the control unit (20) is configured to control the heating circuits (2, 4, 6) during simultaneous operation of the heating circuits (2, 4, 6) such that the respective heat output of the heating circuits (2, 4, 6) progressively increases against the flow direction (10).

3. An electric heater according to claim 1 or 2, comprising a plurality of temperature sensors respectively associated with the plurality of heating circuits (2, 4, 6), wherein the control unit (20) is configured to detect a respective temperature of the heating circuits (2, 4, 6) using the temperature sensors, and / or wherein the control unit (20) is configured to detect a respective temperature of the heating circuits (2, 4, 6) by detecting a temperature-dependent operating parameter of the heating circuits (2, 4, 6), preferably a resistance and more preferably an ohmic resistance, and wherein the control unit (20) is configured to separately control the respective heat output of the heating circuits (2, 4, 6) based on the detected temperatures of the heating circuits (2, 4, 6).

4. Electric heater according to claim 3, wherein the control unit (20) is configured to use an average value of the detected temperatures of the heating circuits (2, 4, 6) or one of the detected temperatures of the heating circuits (2, 4, 6) as a target value in the temperature-dependent control.

5. Electric heater according to one of claims 3 or 4, wherein the control unit (20) is configured to determine overheating of a heating circuit (2, 4, 6) when the detected temperature of the heating circuit (2, 4, 6) exceeds a predetermined maximum temperature and / or a change in at least two consecutively detected temperatures of the heating circuit (2, 4, 6) exceeds a predetermined maximum change.

6. Electric heater according to one of claims 3 to 5, wherein the control unit (20) is configured to determine a flow direction (10) based on the respective heat output of the heating circuits (2, 4, 6).

7. Electric heater according to one of claims 3 to 6, wherein the control unit (20) is configured to determine a lack of response of a heating circuit (2, 4, 6) in response to the control when the detected temperature of a heating circuit (2, 4, 6) falls below a predetermined minimum temperature and / or a change in at least two consecutively detected temperatures of a heating circuit (2, 4, 6) falls below a minimum change.

8. Electric heater according to one of claims 1 to 7, wherein the control unit (20) is configured to separately control the respective heat output of the heating circuits (2, 4, 6) on the basis of a respective factor or a respective characteristic curve or a respective characteristic map.

9. Method for operating an electric heater for heating a fluid flowing in a fluid channel along a flow direction (10), wherein a heating unit (1) of the electric heater has a plurality of heating circuits (2, 4, 6) arranged one after the other along the flow direction (10), characterized in that the heating circuits (2, 4, 6) for generating a respective heat output according to their arrangement along the flow direction (10) are controlled separately.

10. Control unit (20) for an electric heater for heating a fluid flowing in a fluid channel along a flow direction (10), wherein a heating unit (1) of the electric heater has a plurality of heating circuits (2, 4, 6) arranged one after the other along the flow direction (10), characterized in that the control unit (20) is configured to separately control the heating circuits (2, 4, 6) to generate a respective heat output in accordance with the arrangement of the heating circuits (2, 4, 6) along the flow direction (10). 11.An electric heater for heating a fluid flowing in a fluid channel along a flow direction (70), comprising: a heating unit (60) having a plurality of heating circuits (62, 64, 66) arranged one after the other along the flow direction (70), a control unit configured to control the heating circuits (62, 64, 66) in a substantially identical manner to generate a respective heat output, characterized in that the heating circuits (62, 64, 66) are designed to generate a heat output corresponding to the arrangement of the heating circuits (62, 64, 66) along the flow direction (70), and / or a heat exchanger or a plurality of heat exchangers respectively assigned to the plurality of heating circuits to transfer heat from the heating circuits (62, 64, 66) to the fluid are designed such that a respective heat transfer coefficient corresponds to an arrangement of the Heating circuits (62, 64, 66) are fixed along the flow direction (70).

12. Electric heater according to claim 11, wherein the heating circuits (62, 64, 66) are designed such that their respective heat output increases counter to the flow direction (70), and / or the heat exchanger or heat exchangers are designed such that a heat transfer coefficient of the heat exchanger or heat exchangers decreases counter to the flow direction.

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