METHOD FOR DETERMINING AN OUTPUT TEMPERATURE OF A FLUID

Abstract

The invention relates to a method for determining an output temperature of a fluid after flowing through a PTC heater with a PTC heating element. In the method a current of the PTC heating element, a supply voltage of the PTC heating element and a duty cycle of the supply voltage are determined, and the output temperature of the fluid is calculated based on the current (I), the supply voltage and the duty cycle (PWM). The invention also relates to a PTC heater for carrying out the method.

Claims

1. A Method for determining an output temperature of a fluid after flowing through a PTC heater heating the fluid with a PTC heating element, wherein in the method: a current of the PTC heating element, a supply voltage of the PTC heating element and a duty cycle of the supply voltage are determined, and the output temperature of the fluid is calculated based on the current, the supply voltage and the duty cycle.

2. The method according to claim 1, wherein: the output temperature of the fluid is calculated independently of an input temperature of the fluid before flowing through a PTC heater, and/or the output temperature of the fluid is calculated without a measurement of temperatures prevailing in the PTC heater and/or in the fluid, and/or the output temperature of the fluid is calculated exclusively based on the supply voltage, the duty cycle of the supply voltage and the current with the addition of a predetermined characterization constant of the PTC heater.

3. The method according to claim 1, wherein the output temperature of the fluid is calculated exclusively from an electrical heating power of the PTC heating element, a temperature (T.sub.CERAMIC) prevailing at the PTC heating element and a characterization constant (K.Math.S) of the PTC heater.

4. The method according to claim 3, wherein the electrical heating power of the PTC heating element is calculated as a product of the supply voltage, the duty cycle of the supply voltage and the current.

5. The method according to claim 3, wherein when calculating the output temperature of the fluid, an electrical resistance (R.sub.CERAMIC) of the PTC heating element is calculated from the supply voltage and the current.

6. The method according to claim 3, wherein when calculating the output temperature of the fluid, the temperature (T.sub.CERAMIC) prevailing at the PTC heating element is read out from a predetermined matrix ({f, U, R.sub.CERAMIC, T.sub.CERAMIC}) depending on the supply voltage, the frequency of the duty cycle of the supply voltage and the calculated electrical resistance (R.sub.CERAMIC) of the PTC heating element.

7. The method according to claim 3, wherein the characterization constant (K.Math.S) of the PTC heater is a product of a heat-transferring area of the PTC heater which is flowed around by the fluid and a factor which indicate an electrical heating power per surface per Kelvin transmitted by the PTC heating element to the fluid.

8. The method according to claim 3, wherein the output temperature of the fluid is calculated as a difference of the temperature (T.sub.CERAMIC) prevailing at the PTC heating element and a double quotient of the electrical heating power (P.sub.EL) of the PTC heater by the characterization constant (K.Math.S) of the PTC heater.

9. The Method according to claim 1, wherein: the output temperature of the fluid is compared with a predetermined limit temperature (T.sub.THRESHOLD), wherein the electrical heating power (P.sub.EL) is reduced and the method is continued when the limit temperature (T.sub.THRESHOLD) is exceeded, and wherein the method is continued without the reducing of the electrical heating power (P.sub.EL) when the limit temperature (T.sub.THRESHOLD) is not exceeded, and/or the output temperature of the fluid is output to a user.

10. A PTC heater with a PTC heating element, wherein the PTC heater can be flowed through by a fluid and is provided for heating the fluid to an output temperature (T.sub.OUT), characterized in that the PTC heater is provided for carrying out the method according to claim 1.

Description

[0029] Preferred embodiments of the invention are shown in the drawings and will be explained in more detail in the following description, wherein identical reference signs refer to identical or similar or functionally identical components.

[0030] It shows, each schematically

[0031] FIG. 1 a sectional view of a PTC heater with a PTC heating element according to the invention;

[0032] FIG. 2 a schematic diagram of a method according to the invention.

[0033] FIG. 1 is a sectional view of a PTC heater 1 according to the invention. The PTC heater 1 comprises a PTC heating element 2 made of a PTC ceramic, two electrical conductive contact plates 3a and 3b, two dielectrical insulating plates 4a and 4b, a housing 5 made of a thermally conductive material such as metal, and two ribs 6a and 6b made of a thermally conductive material such as metal. The PTC heating element 2 is arranged between the contact plates 3a and 3b and is in electrical conductive contact therewith. The insulating plates 4a and 4b are arranged on the contact plates 3a and 3b facing away from the PTC heating element 2. The housing encloses the PTC heating element 2, the contact plates 3a and 3b and the insulating plates 4a and 4b. The insulating plates 4a and 4b are arranged between the housing 5 and the contact plates 3a and 3b and insulate the contact plates 3a and 3b from the housing 5. The ribs 6a and 6b are arranged on the outside of the housing 5. The ribs 6a and 6b are connected to the housing 5 in a heat-transferring manner, and the housing 5 is connected to the PTC heating element 2 in a heat-transferring manner via the insulating plates 4a and 4b and the contact plates 3a and 3b. The PTC heating element 2, the contact plates 3a and 3b, the insulating plates 4a and 4b, the housing 5, and the ribs 6a and 6b are suitably fixed to each other. The PTC heater 1 also contains a control unit for controlling the PTC heating element 2, but the control unit is not shown here.

[0034] The control unit of the PTC heater 1 can apply a supply voltage U to the PTC heating element 2 via the contact plates 3a and 3b. The supply voltage U is a DC voltage and can be pulse width modulated with a variable duty cycle PWM. Current I flows then in the PTC heating element 2 and the PTC heating element 2 generates an electrical heating power P.sub.EL. The PTC heating element 2 heats up to a temperature T.sub.CERAMIC depending on the applied supply voltage U i.e. the duty cycle PWM of the supply voltage U. The PTC heating element 2 has an electrical resistance R.sub.CERAMIC depending on the temperature T.sub.CERAMIC due to its PTC properties. A fluid can flow around the ribs 6a and 6b and can be heated by the PTC heater 1 i.e. the PTC heating element 2 from an input temperature to an output temperature T.sub.OUT. The PTC heater 1 is characterized by an area S and a factor K. The area S is the heat-transferring surface of the PTC heater 1 flowed around by the fluid, and the factor K is the electrical heating power transferred by the PTC heater 2 to the fluid per surface per Kelvin. The area S and the factor K are constant for the given PTC heater 1.

[0035] FIG. 2 shows a schematic diagram of a method 7 according to the invention for determining the output temperature T.sub.OUT of the fluid after flowing through the PTC heater 1. The method 7 is carried out by the PTC heater 1 for example via the control unit of the PTC heater 1.

[0036] In the method 7, the supply voltage U, the current I and the duty cycle PWM of the supply voltage U are determined in a step 8. The supply voltage U and the current I can be measured at the PTC heating element 2. The duty cycle PWM is a controlled variable of the PTC heater 1 and can be read out from the control unit.

[0037] Then, in a step 9, the electrical heating power P.sub.EL of the PTC heater 1 is calculated using the equation:

P.sub.EL=U.Math.I.Math.PWM.

[0038] Then, the temperature T.sub.CERAMIC prevailing at the PTC heating element 2 is determined. Thereby, in the step 10, the electrical resistance R.sub.CERAMIC of the PTC heating element 2 is calculated using the equation:

[00008]$R_{\mathrm{CERAMIC}}=\frac{U}{I}.$

[0039] Subsequently, in a step 11, the temperature T.sub.CERAMIC of the PTC heating element 2 is read out from a predetermined matrix {U, R.sub.CERAMIC, T.sub.CERAMIC} as a function of the previously measured supply voltage U. The matrix {U, R.sub.CERAMIC, T.sub.CERAMIC} is originally of the form {f, U, R.sub.CERAMIC, T.sub.CERAMIC}, wherein the frequency f of the duty cycle PWM of the supply voltage U is constant. The matrix {U, R.sub.CERAMIC, T.sub.CERAMIC} i.e. {f, U, R.sub.CERAMIC, T.sub.CERAMIC} can be determined i.e. calculated in preliminary tests and/or read out from known reference works and can be stored in the control unit of the PTC heater 1.

[0040] After determining the temperature T.sub.CERAMIC of the PTC heating element 2, in a step 13 the output temperature T.sub.OUT of the fluid is calculated using the equation:

[00009]$T_{OUT}=T_{CERAMIC}-2\frac{P_{EL}}{K.Math.S}.$

[0041] There, a constant characterization constant K.Math.S of the PTC heater 1 is used. The characterization constant K.Math.S is given by a constant area S and a constant factor K. The area S is a heat-transferring surface of the PTC heater 1 flowed around by the fluid. The factor K is an electrical heating power transmitted by the PTC heating element 2 to the fluid per surface per Kelvin. The area S and the factor K can be determined by preliminary tests on the respective PTC heater 1 and stored in the control unit of the PTC heater 1.

[0042] Then, the calculated output temperature T.sub.OUT of the fluid is output to a user in a step 14 and compared to a limit temperature T.sub.THRESHOLD in a step 15. If the output temperature T.sub.OUT is greater than the limit temperature T.sub.THRESHOLD, the heating power P.sub.EL is reduced. For this purpose, the duty cycle PWM of the supply voltage U can be reduced up to zero. Then the method 7 is continued with the step 8. If the output temperature T.sub.OUT is lower than the limit temperature T.sub.THRESHOLD, the method 7 is continued with the step 8 without the step 16.

[0043] The specification is understood with reference to the following Numbered Paragraphs: [0044] Numbered Paragraph 1. A Method (7) for determining an output temperature (T.sub.OUT) of a fluid after flowing through a PTC heater (1) heating the fluid with a PTC heating element (2), wherein in the method: [0045] a current (I) of the PTC heating element (2), a supply voltage (U) of the PTC heating element (2) and a duty cycle (PWM) of the supply voltage (U) are determined, and [0046] the output temperature (T.sub.OUT) of the fluid is calculated based on the current (I), the supply voltage (U) and the duty cycle (PWM). [0047] Numbered Paragraph 2. Method according to Numbered Paragraph 1, characterized [0048] in that the output temperature (T.sub.OUT) of the fluid is calculated independently of an input temperature of the fluid before flowing through a PTC heater (1), and/or [0049] in that the output temperature (T.sub.OUT) of the fluid is calculated without a measurement of temperatures prevailing in the PTC heater (1) and/or in the fluid, and/or [0050] in that the output temperature (T.sub.OUT) of the fluid is calculated exclusively based on the supply voltage (U), the duty cycle (PWM) of the supply voltage (U) and the current (I) with the addition of a predetermined characterization constant (K.Math.S) of the PTC heater (1). [0051] Numbered Paragraph 3. Method according to Numbered Paragraph 1 or 2, [0052] characterized in that the output temperature (T.sub.OUT) of the fluid is calculated exclusively from an electrical heating power (P.sub.EL) of the PTC heating element (2), a temperature (T.sub.CERAMIC) prevailing at the PTC heating element (2) and a characterization constant (K.Math.S) of the PTC heater (1). [0053] Numbered Paragraph 4. Method according to Numbered Paragraph 3, [0054] characterized in that the electrical heating power (P.sub.EL) of the PTC heating element (2) is calculated as a product of the supply voltage (U), the duty cycle (PWM) of the supply voltage (U) and the current (I). [0055] Numbered Paragraph 5. Method according to Numbered Paragraph 3 or 4, [0056] characterized in that, when calculating the output temperature (T.sub.OUT) of the fluid, an electrical resistance (R.sub.CERAMIC) of the PTC heating element (2) is calculated from the supply voltage (U) and the current (I). [0057] Numbered Paragraph 6. Method according to any one of Numbered Paragraphs 3 to 5, [0058] characterized in that, when calculating the output temperature (T.sub.OUT) of the fluid, the temperature (T.sub.CERAMIC) prevailing at the PTC heating element (2) is read out from a predetermined matrix ({f, U, R.sub.CERAMIC, T.sub.CERAMIC}) depending on the supply voltage (U), the frequency (f) of the duty cycle (PWM) of the supply voltage (U) and the calculated electrical resistance (R.sub.CERAMIC) of the PTC heating element (2). [0059] Numbered Paragraph 7. Method according to any one of Numbered Paragraphs 3 to 6, [0060] characterized in that the characterization constant (K.Math.S) of the PTC heater (1) is a product of a heat-transferring area (S) of the PTC heater (1) which is flowed around by the fluid and a factor (K) which indicate an electrical heating power per surface per Kelvin transmitted by the PTC heating element (2) to the fluid. [0061] Numbered Paragraph 8. Method according to any one of Numbered Paragraphs 3 to 7, [0062] characterized in that the output temperature (T.sub.OUT) of the fluid is calculated as a difference of the temperature (T.sub.CERAMIC) prevailing at the PTC heating element (2) and a double quotient of the electrical heating power (P.sub.EL) of the PTC heater (1) by the characterization constant (K.Math.S) of the PTC heater (1). [0063] Numbered Paragraph 9. Method according to any one of the preceding Numbered Paragraphs, [0064] characterized [0065] in that the output temperature (T.sub.OUT) of the fluid is compared with a predetermined limit temperature (T.sub.THRESHOLD), wherein the electrical heating power (P.sub.EL) is reduced and the method (7) is continued when the limit temperature (T.sub.THRESHOLD) is exceeded, and wherein the method (7) is continued without the reducing of the electrical heating power (P.sub.EL) when the limit temperature (T.sub.THRESHOLD) is not exceeded, and/or [0066] in that the output temperature (T.sub.OUT) of the fluid is output to a user. [0067] Numbered Paragraph 10. PTC heater (1) with a PTC heating element (2), wherein the PTC heater (1) can be flowed through by a fluid and is provided for heating the fluid to an output temperature (T.sub.OUT), characterized in that the PTC heater (1) is provided for carrying out the method (7) according to one of the preceding Numbered Paragraphs.