Level measurement of a two-phase refrigerant

Abstract

Device and method for measuring the liquid level (10) of a liquid in an upright container (5), the container (5) having a vapor / gas inlet (6) at its top and a liquid outlet (7) at its bottom, a plurality of thermistors (2) mounted externally on the container wall (1) at different heights, with which temperature increase pulses can be sent and received, wherein the container wall (1) is highly thermally conductive and has a low heat capacity, the thermistors (2) are connected to voltage measuring devices (4) and to an evaluation unit, and the measurement is carried out using the 3-omega method. The devices and measuring methods are used in refrigeration circuits to determine the liquid levels in the heat exchangers (11, 17, 18) and to control the expansion valve (20).

Description

[0001] The invention relates to the measurement of a refrigerant level in a refrigeration circuit during operation. Such a measurement is difficult because the refrigerant typically exists in two or more phases during operation. Typical refrigerants, such as those used in refrigerators, air conditioners, and heat pumps, have two phases—vapor and liquid. They can be multiphase when refrigerant mixtures with miscibility gaps or supercritical fluids are used.

[0002] The problem of level measurement is also familiar in steam generators, where rising steam bubbles must share the boiler volume with boiling water. It's difficult from a control perspective to maintain a constant level when refilling with evaporated water, while the steam bubble volume decreases and refilling leads to an overall drop in the liquid level – control systems typically expect the opposite. Effects of this kind also occur in evaporators and condensers of refrigeration circuits.

[0003] In the past, this was addressed by generously dimensioning the collection volumes and filling the system with a larger refrigerant charge than was required for purely stationary operation. This made it possible to handle dynamic processes during load changes, although with the throttling devices typically used in the refrigeration circuit, one never knows exactly what flow rate is occurring at any given time, and there is therefore always a risk that the downstream evaporator will receive too little or too much refrigerant to perform its function satisfactorily.

[0004] If it receives too much refrigerant, the result is that the refrigerant can no longer be fully evaporated. Superheating is then no longer possible, and droplets could enter the downstream compressor. This would lead to cavitation, and the compressor would be damaged if it were not shut down immediately. However, if the condenser receives too much refrigerant vapor from the compressor, not all of the vapor can be condensed, and vapor bubbles will hit the expansion valve. Controlling the expansion valve then becomes difficult or even impossible. Both effects therefore lead to refrigeration failure.

[0005] Recently, the use of natural refrigerants that are flammable, toxic, or dangerous has been encouraged. These include hydrocarbons such as R290, R600a, and R1270, and inorganic substances such as R717 and R744. When these refrigerants are used in indoor units, either only small quantities may be used, or complex safety measures must be implemented to ensure that even in the event of a leak, there is no significant risk to occupants. Therefore, there is a strong interest in using as little refrigerant as possible. This exacerbates the problem of pumping the correct amount of refrigerant into the evaporator and condenser under every operating condition and maintaining a balance. This quickly leads to stability limits when ambient conditions and user requirements change rapidly.

[0006] Refrigerant level control has become known in many different forms in the prior art. DE 41 24 363 C2 describes a method in which the level is calculated after temperatures and pressures have been measured. Using the known vapor pressure curves of the refrigerant, the difference between the temperature corresponding to the value of the vapor pressure curve at the measured pressure and the measured temperature is then calculated, and the degree of subcooling is determined from this. Depending on the magnitude of this difference, underfilling or overfilling is determined. US 2005 / 0204756 A1 and US 2009 / 0260376 A1 also determine the liquid quantities in a similar way. DE 10 2011 052 775 B4 also uses a reference evaporator. However, these measurement methods all require stationary operation or even shutdown; they are not suitable for measurements during dynamic operation and therefore cannot be used for control purposes.Liquid levels in heterogeneous fluids or in multiphase states, such as boiling or condensing refrigerants, cannot be reliably measured this way. This also applies to refrigerants with temperature glide.

[0007] This principle measures the heat transfer of a modulated heat flow from a heater mounted externally on the collector, based on the Omega-3 method. The heat flow from the respective heater flows through the housing wall of the collector tank into the medium contained within the tank. Depending on the phase state of the medium, its thermal conductivity and specific heat capacity change. The system consisting of heater, temperature sensor, heat transfer through the tank, and heat transfer into the fluid can be assigned a temperature along the direction of heat flow for a given input power. The heater and sensor can be combined in a single element if the temperature coefficient of the heater, i.e., the resistance with R(T), is to be utilized.

[0008] Based on the vertical arrangement, the phase state of the medium in the container can be determined at the respective position of the sensor element. To increase resolution, interpolation can be performed between two adjacent sensors. This effect works because the heat conduction path of an element is smeared around its position in both spatial directions. The sensor element essentially sees around its position.

[0009] The container's walls must not be too thick, otherwise the behavior will be determined solely by the container, not by its contents. The sensor becomes increasingly blind the thicker the walls. The sensor and actuator can be integrated, but can also be implemented separately.

[0010] The arrangement could also be used to detect mixed phases. For this purpose, radially oriented, thermally insulated feedthroughs are inserted into the vessel in a vertical direction (e.g., a thin copper pin inserted through a ceramic sleeve through the vessel directly into the fluid / gas. The resistance element is then seated on this pin). The possibility of interpolation is thus suppressed, as thermal insulation is provided in the lateral direction to improve the SNR.

[0011] The object of the invention is therefore to provide a device and a method with which varying fill levels of heterogeneous fluids in containers and apparatus can be measured reliably and quickly.

[0012] This involves using a collection reservoir, which can also be a refrigerant collector or a correspondingly designed reservoir in one or more of the refrigeration circuit's components. The fluid's thermal conductivity and heat capacity are used to determine the fluid level. These are measured by a large number of thermistors, preferably between 3 and 30, depending on the desired resolution.

[0013] The measurement method is thermal impedance measurement, in which the thermal conductivity of a liquid or solid is measured using applied thermal vibrations. To do this, the sample must be heated and then cooled back to its original value. Very small temperature changes in the range of hundredths of a degree Celsius are used during heating and cooling. Depending on the thermal conductivity of the material being traversed, these thermal vibrations are captured by a temperature sensor, and their phase shift is measured.

[0014] Thermal impedance is determined using the 3ω method, as the thermal conductivity of a liquid differs from that of a gas or vapor. The 3ω method is described in "Hainan Wang and Mihir Sen, Analysis of 3ω method for thermal conductivity measurement, Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556, October 7, 2008." It is also known from "M. Jaegle et al., Thermal-electrical impedance spectroscopy for fluid characterization, AMA Conferences 2017 - SENSOR 2017 and IRS 2017."

[0015] A special case is a boiling liquid, because small pulses of temperature increase lead to local evaporation, which shifts the vapor-liquid equilibrium in a two-phase system but does not change the temperature of the medium. This signal is also easily detectable using the 3ω method.

[0016] The invention solves the problem by a device for measuring a liquid level of a boiling liquid in an upright container, comprising - the container has a steam / gas inlet on its top and - has a liquid drain on its underside, - a large number of thermistors mounted on the outside of the container wall at different heights, with which temperature increase pulses can be sent and received, - the container wall is a good heat conductor and has a low heat capacity, - the thermistors are connected to voltage measuring devices and to an evaluation unit.

[0017] The vessel can also be designed as an annular gap similar to a double-walled tube, whereby the inner and outer tubes do not need to be concentric with each other. In this case, the inner surface also counts as the vessel wall, and a recess is also considered a vessel wall in this sense. The term "from the outside" should be interpreted to mean that the thermistors do not have direct contact with the fluid to be measured. The thermistors can therefore also be arranged alone in a recess.

[0018] In one embodiment of the device, measuring strips with a plurality of thermistors are provided both in a submerged area within the container and on the outer wall of the container. In a further embodiment, the thermistors in the measuring strip within the submerged area and the thermistors on the outer wall of the container are paired together.

[0019] In further embodiments, the containers are integrated into heat exchangers. Vertical plate heat exchangers are preferably used for this purpose. These can have a reservoir on the underside. Furthermore, the liquid level is preferably measured in a bypass of the heat exchanger.

[0020] The invention also relates to a measuring method for locating a liquid level in a container. Here, heat pulses are applied horizontally using thermistors according to the 3-omega method from at least one wall at different heights, and the system response is analyzed to determine whether the signal corresponds to that of a liquid or a vapor.

[0021] In some embodiments, the signal is additionally evaluated to determine whether demixing has occurred in the liquid phase and whether the signal has not led to a change in temperature, i.e. whether vapor-liquid equilibrium exists.

[0022] The invention also encompasses uses in refrigeration circuits of heat pumps and air conditioning systems. It is provided that devices and methods are carried out in containers that are installed in the refrigeration circuit upstream and / or downstream of the expansion valve in the connecting lines to the connected heat exchangers. Furthermore, it can be provided, additionally or alternatively, that devices and methods are used in an integrated form in the heat exchangers that serve for evaporation or condensation. Furthermore, it is provided that the refrigerant levels on the inlet and outlet sides of the expansion valve are continuously measured and used to adjust the dynamic control of the expansion valve. The refrigerant levels can be measured either in appropriately equipped heat exchangers or in separate collecting containers.

[0023] The invention is based on the Fig. 1 to Fig. 7 is explained in more detail. It shows: Fig. 1 a schematic representation of a thermistor, Fig. 2 the measuring principle of the 3-Omega method, Fig. 3 a container with a plurality of thermistors on one side, Fig. 4 a container with a plurality of thermistors in a submerged Fig. 5 a container with a plurality of thermistors on two opposite sides, Fig. 6 a plate heat exchanger with a plurality of thermistors, Fig. 7 a refrigeration circuit with two separate collecting tanks, Fig. 8 a refrigeration circuit with two plate heat exchangers with integrated collecting tanks.

[0024] Fig. Figure 1 shows a schematic representation of a wall 1 to which a thermistor 2 is applied. In this case, the wall 1 is that of a container in a refrigeration circuit. To obtain a good measurement result, the wall should be particularly thin and have good thermal conductivity. Depending on the pressure conditions in the container, the container must be encapsulated for this purpose, with the encapsulation ensuring pressure stability. An electric current 3 flows through the thermistor 2, with the resistance being measured by the voltmeter 4. The thermistor heats up slightly according to its resistance and emits a short heat pulse onto or through the wall 1. Shortly thereafter, it receives a system response when the container contents heat or cool the thermistor.The temperature increases used and the associated heat flows are very small, but their phase shift and magnitude are characteristic and one can clearly distinguish whether the system on the other side of the wall is a liquid or vapor or a two-phase system that does not heat up but in which only the phase equilibrium changes.

[0025] Fig. Figure 2 shows in general terms, analogous to the Wikipedia entry “3-Omega Method”, the basic relationship between the individual measured quantities current I, power P, temperature T, resistance R, voltage U and the third harmonic U 3rd , which form an oscillating system. For the known mathematical analysis, please refer to the literature.

[0026] Fig. 3 shows a container 5 with a plurality of thermistors 2 on one side, each of the thermistors having the same structure as in Fig. 1. This results in signals that differ significantly from one another depending on whether vapor, liquid, or a two-phase system is located behind the vessel wall 1. Multiphase systems can also be differentiated in this way, for example, refrigerant as vapor, as liquid, as a two-phase system, and an oil phase released from the compressor. The vessel 5, which serves as a collecting tank here, has a submerged inlet 6 and an outlet 7. The upper part contains the vapor space 8, and the lower part contains the liquid 9; the two phases are separated by level 10, which is determined by the thermistors 2.

[0027] Fig. Figure 4 shows a vessel 5 with a plurality of thermistors in a recess 23. The thermistors 2 are arranged in a protected manner within this recess. Compared to mounting on the outer wall of the vessel 5, this has the advantage that the recess can be made of a particularly thin material and that its interior can be pressurized. This reduces the differential pressure to the refrigerant and increases stability without posing a safety risk.

[0028] Fig. Figure 5 shows a container 5 with a plurality of thermistors 2 on two opposite sides. For this purpose, a submerged 23 with thermistors as in Fig. 4 is used. The distances between the opposing thermistors can be chosen very small, but care must be taken to avoid distortions caused by capillary effects. Each thermistor receives a signal from the opposite side, which increases precision and speed.

[0029] Fig. Figure 6 shows a plate heat exchanger 11 with a plurality of thermistors 2 arranged in a bypass 24, wherein the bypass 24 is connected as a communicating tube to the upright plate heat exchanger 11. The plate heat exchanger 11 can be a condenser or an evaporator, both of which can have an identical structure. This is particularly advantageous if the refrigeration circuit is to be operated as a heat pump at one time and as an air conditioning system at another.

[0030] The refrigerant enters the vapor space 8 of the plate heat exchanger 11 via inlet 6, which is operated here as a condenser. A heat sink medium, for example, a brine from a heating circuit medium, flows in at inlet 12 and is heated, while the incoming vapor condenses. Thermistors can be used to measure whether condensation is proceeding as intended and whether the condenser is filling up or running dry. The heat sink medium exits the condenser at return 13. A refrigerant reservoir 14 is also located in the lower area.

[0031] The structure of an evaporator is identical, with inlet 12 and return 13 being heat source media and the refrigerant being supplied from below.

[0032] Fig. 7 shows a refrigeration circuit 15 with a compressor 16, a condenser 17 and an evaporator 18. Before and after the expansion valve 20, two separate refrigerant collectors 19 and 21 are provided, the structure of which is similar to that shown in Fig. 3 to Fig. 5, and with its help, it can be determined whether the flow rate of the expansion valve corresponds to the respective capacity of condenser 17 and evaporator 18. This is the case when the levels of the two receivers remain constant. For safety reasons, a droplet separator 22 is provided upstream of compressor 16, as usual.

[0033] Fig. 8 shows the refrigeration circuit 15, in which the condenser 17 and evaporator 18 with refrigerant collectors 19 and 21 are arranged accordingly. Fig. 6 are equipped.

[0034] Deviating from Fig. 7 and Fig.8, only a single collecting tank 19, 21 can be provided or integrated into one of the heat exchangers 17, 18 or a single collecting tank is combined with an integrated collecting tank. List of reference symbols 1 wall 2 thermistors 3 Current flow 4 Voltage measurement 5 containers 6 Inlet 7 Procedure 8 Steam room 9 Liquid 10 levels 11 plate heat exchangers 12 Heat sink medium inlet 13 Heat sink medium return 14 Refrigerant reservoir 15 Refrigeration circuit 16 compressors 17 Capacitor 18 evaporators 19 Refrigerant collector 1 20 Relief valve 21 Refrigerant receiver 2 22 droplet separators 23 Dive 24 Bypass QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 41 24 363 C2

[0006] US 2005 / 0204756 A1

[0006] US 2009 / 0260376 A1

[0006] DE 10 2011 052 775 B4

[0006] Cited non-patent literature

[0000] Hainan Wang and Mihir Sen, Analysis of 3ω method for thermal conductivity measurement, Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, IN 46556, October 7, 2008

[0014] M. Jaegle ea, Thermal-electrical impedance spectroscopy for fluid characterization, AMA Conferences 2017 - SENSOR 2017 and IRS 2017

[0014]

Claims

[1] Device for measuring a liquid level (10) of a liquid in an upright container (5), comprising - the container (5) has a steam / gas inlet (6) on its top and - has a liquid drain (7) on its underside, characterized by , that - a plurality of thermistors (2) are provided at different heights on a wall (1) on the container (5), the opposite side of which is exposed to the fluid to be measured, with which temperature increase pulses can be transmitted and received, - wherein the wall (1) is a good heat conductor and has a low heat capacity, - the thermistors (2) are connected to voltage measuring devices (4) and to an evaluation unit. [2] Device according to claim 1, characterized by that a plurality of thermistors (2) are additionally provided in an encapsulated immersion (23) in the container (5). [3] Device according to claim 2, characterized by that the thermistors (2) within the immersion (23) and the thermistors (2) on the outer wall (1) of the container (5) are assigned to each other in pairs. [4] Device according to one of claims 1 to 3, characterized by that the containers (5, 19, 21) are integrated into heat exchangers (11, 17, 19). [5] Device according to claim 4, characterized by that the heat exchangers (11) are vertical plate heat exchangers with a reservoir (14) at the bottom and an internal bypass. [6] Measuring method for locating a liquid level (10) in a container of a device according to one of claims 1 to 5, characterized bythat heat pulses are applied by thermistors (2) according to the 3-omega method from at least one wall (1) at different heights in a horizontal direction and the system response is analyzed to determine whether the signal corresponds to that of a liquid (9) or that of a vapor (8). [7] Measuring method according to claim 6, characterized by that the signal of the system response to demixing in the liquid phase is analyzed. [8] Measuring method according to claim 6, characterized by that the system response signal is analyzed for a two-phase region of vapor-liquid equilibrium. [9] Use of devices and measuring methods according to the preceding claims 1 to 8 in a refrigeration circuit (15) which has at least one compressor (16), one evaporator (18), one condenser (17) and one expansion valve (20), characterized bythat appropriately equipped containers (19, 21) are installed in the refrigeration circuit before and / or after the expansion valve (20) in the connecting lines to the connected heat exchangers (17, 18). [10] Use of devices and measuring methods according to the preceding claims 1 to 8 in a refrigeration circuit (15) which has at least one compressor (16), one evaporator (18), one condenser (17) and one expansion valve (20), characterized by that appropriately equipped heat exchangers (17, 18) which serve for evaporation or condensation have containers (19, 21) in integrated form. [11] Use according to one of claims 9 or 10, characterized by that the refrigerant levels (10) on the inlet side and the outlet side of the expansion valve (20) are continuously measured and used to adjust the dynamic control of the expansion valve (20).

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