Method for controlling level of oil separator in refrigeration circuit and related system

Abstract

The invention relates to a method for controlling the liquid level of an oil separator (2) of a refrigeration circuit (K), downstream of the outlet of which a controllable valve (5) is connected, said method being intended to minimize the loss of pressure and energy as much as possible, while at the same time being low in maintenance cost and technically easy to implement. For this purpose, after opening the valve (5), a first temperature is measured in a region downstream of the valve (5), and a threshold value of at least one parameter characterizing the characteristic of the first temperature change is set, and the valve (5) is closed immediately if the parameter exceeds the corresponding threshold value.

Description

Technical Field

[0001] The invention relates to a method for regulating the liquid level of an oil separator in a refrigeration circuit, comprising a controllable valve connected downstream of the outlet of the oil separator, through which the oil is conveyed to an oil collecting container. The invention also relates to a system comprising an oil separator and an oil collecting container connected downstream of the outlet side via a valve, and a refrigeration circuit comprising the system. Background Art

[0002] A refrigeration circuit is a system used to cool equipment to a desired temperature, such as a food freezer. The refrigerant moving in a closed circuit undergoes various changes of state in sequence: The gaseous refrigerant is first compressed by a compressor and then condensed in a subsequent heat exchanger, releasing heat. The liquid refrigerant then expands due to the pressure change through a throttling element (e.g. an expansion valve or a capillary tube). In a second downstream heat exchanger (evaporator), the refrigerant evaporates at a low temperature (boiling cooling). The cycle can start again. The process must be kept running by external mechanical work (driving force) provided by the compressor.

[0003] Oil is used as a lubricant in compressors. Due to design reasons, some oil always finds its way into the discharged compressed refrigerant. Therefore, an oil separator is usually installed downstream of the compressor on the outlet side, which can be designed, for example, as an impact separator to separate the oil from the refrigerant. The separated oil is not discarded, but is conveyed via a separate oil line to an oil collection container and from there back to the compressor.

[0004] This separate oil circuit connects the outlet of the oil separator to the oil collection container, that is, a part of the refrigeration circuit downstream of the compressor (at high pressure) to the low-pressure inlet area of ​​the compressor. Therefore, it cannot be left open all the time, otherwise it will cause permanent loss of pressure and energy, so it needs to be equipped with a controllable valve.

[0005] Known methods include opening the valve as needed to detect the level of the oil separator, for example by optical detection or a float, or simple timing control, which regularly opens the valve for a specified time (period). However, measuring the level of the oil separator is susceptible to contamination and can be technically complex. In addition, they are usually not suitable for all temperatures in the oil collection container. On the other hand, timing control usually does not meet the requirements, resulting in the valve being open for too long.

[0006] Therefore, the object of the present invention is to provide a method of the above-mentioned type, which is capable of reducing the pressure and energy losses as low as possible and which is low in maintenance costs and easy to implement technically. Summary of the invention

[0007] According to the present invention, this object is achieved by measuring the first temperature in the pipeline between the valve and the oil collecting container after the valve is opened, setting a threshold value of at least one parameter characterizing the first temperature change characteristics, and closing the valve when the parameter exceeds the threshold value.

[0008] The invention is based on the idea that the oil separator can be emptied into the oil collecting container not only by opening the oil separator as required when an excessively high liquid level is detected, but also by precisely controlling the opening time of the valve. To this end, the valve should be closed again at a precise time after all the oil has been discharged from the oil separator. In order to accurately determine this time, it should be accurately determined whether oil or refrigerant is flowing through the valve, i.e. whether the oil in the oil separator has been exhausted. Since the refrigerant in the oil separator has usually been compressed before, it is at high pressure and in a gaseous state. An isenthalpic state change occurs in the valve between the oil separator and the oil collecting container. Since the refrigerant is gaseous and the oil is liquid, the two media behave differently under an isenthalpic state change: the expansion of the gaseous refrigerant leads to a temperature drop on both sides of the valve, while liquid oil does not experience this temperature change. Therefore, the flow of oil or refrigerant in the valve can be detected by means of this temperature characteristic. By specifying suitable temperature characteristic parameters and suitable threshold values, a precise closing of the valve after opening can be achieved to ensure that all the oil is transferred from the oil separator to the oil collecting container.

[0009] In an advantageous embodiment, the rate of change of the first temperature is used as a parameter. In other words, the mathematical derivative of the temperature curve measured downstream of the valve is determined and used itself or as a corresponding derived variable as a parameter for specifying the threshold value. As soon as a sharp drop in temperature is detected (i.e. exceeding the threshold value), this indicates that the medium in the valve has changed from oil to refrigerant. Thus, the change of the medium in the valve can be determined with only one temperature measurement point.

[0010] In a particularly easy-to-implement technical design, an initial temperature is determined and the difference between the current temperature and the initial temperature is used as a parameter. In other words, an initial temperature is determined when the valve is opened. This initial temperature can be fixed or dynamically determined when the valve is opened. Then, when the medium flows through the valve, the current temperature is continuously measured and the difference between it and the initial temperature is calculated. If the difference exceeds a certain value, the valve is closed again. Since the temperature decreases as the refrigerant expands instead of the oil, the difference will be negative, and "exceeding" here means exceeding the absolute value of the specified negative difference.

[0011] In a further advantageous embodiment, the oil in the oil collecting container is supplied to the compressor of the refrigeration circuit by regulating the oil level in the compressor. By combining the above-mentioned control with a separate oil collecting container, both the emptying of the oil separator can be controlled in a targeted and demand-oriented manner and the oil level of the compressor can be independently regulated in an optimized manner. Oil level regulation is based on the oil level in the compressor as a control variable and controls the valve between the oil collecting container and the oil inlet of the compressor. If the oil level is too low, the valve is opened; if the oil level is too high, the valve is closed. It is also possible to control the flow in a targeted manner so that the oil lost by the compressor through the refrigerant outlet is continuously replenished.

[0012] Considerable additional advantages arise when several compressors in a refrigeration circuit are supplied with oil from the same oil collecting container by regulating the oil level in each compressor individually. By decoupling the oil level control in the compressor from the oil level control in the oil separator, a central oil collecting container can be used, to which the oil separator or all oil separators in the refrigeration circuit are supplied by means of the above-mentioned valve control. At the same time, all compressors, whether they are located in the same refrigeration circuit or in separate sub-circuits, can obtain the best oil supply from the central oil collecting container by providing corresponding oil level control in each compressor.

[0013] In a further or alternative advantageous embodiment, a second temperature is further measured in the region upstream of the valve and the difference between the first temperature and the second temperature is used as a parameter. Thus, the absolute temperature difference upstream and downstream of the valve is used as a parameter. In this case, a change in the medium in the valve is detected by an increase in this temperature difference and a corresponding threshold value is assigned for this purpose.

[0014] In principle, it is also possible to use a combination of the above parameters, i.e. to determine both the absolute temperature difference and the rate of temperature change at the same time. This allows even more precise detection of changes in the medium in the valve.

[0015] Advantageously, the second temperature is measured in the line between the oil separator and the valve. A suitable additional temperature measuring device can be installed here to record the temperature of the flowing medium before the valve releases the pressure.

[0016] In another advantageous embodiment, a third temperature is further measured in the inlet region of the oil separator and used to determine the characteristic value and / or the threshold value. If the temperature at the inlet of the oil separator is evaluated as a reference temperature, the filling level can be controlled more precisely.

[0017] In the above method, the valve is preferably opened periodically. This means that the valve is opened according to a predetermined regularity, for example, once per a predetermined period or once every X seconds / minutes at a certain opening frequency. The valve may also be opened again after a period of time after being closed according to the above method.

[0018] The cycle length, i.e. the average interval between the opening processes, is preferably determined according to the capacity of the compressor. If the capacity of the compressor in the refrigeration circuit is increased due to the need for higher cooling capacity, the oil will also be pumped into the oil separator at a higher rate. The opening frequency is adjusted according to the capacity of the compressor, i.e. the larger the capacity, the more frequently the valve is opened, thus further improving the efficiency of this process.

[0019] Finally, in the above control system, it is preferred to set the minimum and / or maximum time for opening the valve. This increases the stability of the control system.

[0020] A system comprises an oil separator and an oil collecting container connected downstream of the outlet thereof through a valve. The system preferably comprises a plurality of temperature measuring devices and a control device suitable for executing the above method.

[0021] A refrigeration circuit comprises a compressor and a heat exchanger, wherein the compressor is connected to the heat exchanger at an outlet side via a refrigerant pipeline, and the refrigeration circuit preferably comprises the aforementioned system.

[0022] The advantages achieved by the invention are particularly evident in that, by determining the oil separator level detection based on temperature changes, an energy-efficient and technically simplified on-demand discharge of oil from the oil separator into the oil collecting container in the refrigeration circuit is achieved. The level of the oil separator is controlled by the temperature of the discharged medium, without the need for adapters or sight glasses and without the susceptibility to contamination as in previous float systems. This solution can be particularly simple to adapt to different load conditions in the refrigeration circuit, is technically very easy to implement and is also less expensive. By using this system in conjunction with an oil collecting container, it is possible to achieve both an optimized discharge of the oil separator and a precise on-demand supply of oil to multiple compressors from the same oil collecting container. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The embodiments of the present invention are described in more detail with reference to the accompanying drawings. The accompanying drawings are shown as follows:

[0024] Figure 1 A refrigeration circuit with an oil separator is shown, and a temperature measuring device is provided upstream and downstream of the valve leading to the oil collecting container;

[0025] Figure 2 Shown with Figure 1 The same refrigeration circuit, but with only one temperature measuring device downstream of the valve;

[0026] Figure 3 Shown with Figure 1 For the same refrigeration circuit, a temperature measuring device is provided downstream of the valve and at the inlet of the oil separator respectively;

[0027] Figure 4A pressure-enthalpy diagram is shown showing two examples of isoenthalpic expansion in the valve as the oil separator empties and the gas expands.

[0028] In all the drawings, the same parts are marked with the same reference symbols. DETAILED DESCRIPTION

[0029] Figure 1 A refrigeration circuit K is schematically shown. The refrigeration circuit K is described below starting with the compressor 1. The refrigerant compressed in the compressor 1 (carbon dioxide in this embodiment) is first sent to the oil separator 2. In this example, the oil separator 2 is designed as an impact separator for separating oil mixed with the refrigerant during the operation of the compressor 1. On the refrigerant side, the oil separator 2 is connected to the heat exchanger 3 (designed as a gas cooler in this example), where the compressed refrigerant is cooled and liquefied. Subsequently, the refrigerant flows through the throttling element 4 into the heat exchanger 7 designed as an evaporator, where the refrigerant expands and absorbs heat, thereby achieving the desired cooling effect. The gaseous refrigerant flowing out of the heat exchanger 7 returns to the compressor 1, where it is compressed and the cycle starts again.

[0030] The above only describes the arrangement of the oil separator 2 in relation to the refrigerant channel in the refrigeration circuit K. With regard to the oil circuit layout, the oil separator 2 is arranged as follows: an outlet for separating oil is provided at the bottom of the oil separator 2. The outlet is connected to the oil collecting container 6 via a controllable valve 5 (designed as a solenoid valve in the embodiment example). The oil is returned from the oil collecting container 6 to the compressor 1 by adjusting the oil level of the compressor 1.

[0031] The design of the refrigeration circuit K described here is Figures 1 to 3 The only difference is the arrangement of the temperature sensors T1, T2, T3. However, this does not mean that the method for controlling the liquid level of the oil separator 2 described below is only applicable to such simple refrigeration circuits K - these are only for illustrative purposes. The method can be applied to any refrigeration circuit K having an oil separator 2.

[0032] In order to ensure that the process of draining the separated oil from the oil separator 2 does not cause unnecessary pressure losses in the refrigeration circuit K, the valve 5 should be opened only when necessary. The purpose of this is to ensure that only separated oil passes through the valve 5, and not gas (which would happen if the valve was open and the oil separator 2 was empty). To this end, the liquid level in the oil separator 2 is regulated by opening and closing the valve 5 as required, as follows:

[0033] The valve 5 is opened in a time-controlled manner, irrespective of the current filling level of the oil separator 2. The valve 5 should be opened periodically after a predetermined period of time after the last closing. In an embodiment, this period of time may be dynamically dependent on the capacity of the compressor 1, i.e. when the capacity of the compressor 1 is higher, the length of the period between two opening triggering processes is shortened. Of course, this period of time may also be fixed.

[0034] After the oil separated in the oil separator 2 is completely discharged, the valve 5 is immediately closed, which is the key to the method for controlling the filling level of the oil separator 2 proposed in the present application. In all embodiments, the medium flowing through the open valve 5 is detected for this purpose - as long as the oil is still flowing, the valve 5 remains open; once the refrigerant flows, the valve 5 is immediately closed.

[0035] The flow medium in the valve 5 is detected by utilizing the property that the refrigerant in the oil separator 2 is gaseous and the oil is liquid. The medium undergoes isenthalpic expansion in the valve 5, so whether the flow medium is liquid or gaseous can be determined by the temperature change on both sides of the valve 5: if the oil separator 2 is empty and the gas expands, the temperature drops. When the oil passes through the valve 5, the temperature downstream of the valve 5 is approximately the same as the temperature upstream. This can be detected by suitable temperature measuring devices T1, T2, T3. In all embodiments, the temperature measuring devices T1, T2, T3 are connected to a control device (not shown), which uses the data from the temperature measuring devices T1, T2, T3 and the corresponding hardware and software to control the opening and closing of the valve 5.

[0036] exist Figure 1 In the embodiment of the present invention, two temperature measuring devices T1, T2 are provided for this purpose. They are arranged in the supply pipeline upstream and downstream of the valve 5, i.e. between the oil separator 2 and the valve 5 (T1) and between the valve 5 and the oil collecting container 6 (T2). The temperature measuring devices T1, T2 measure the temperature of the medium in the supply pipeline.

[0037] As mentioned above, the gaseous flow medium in the valve 5 causes a temperature drop on both sides of the valve 5. Figure 1 In the embodiment of the invention, the temperature difference between the temperatures measured by the two temperature measuring devices T1 and T2 is used as a parameter. A temperature difference is designated as a threshold value. Once the threshold value is exceeded (i.e. the temperature deviation exceeds the threshold value), the valve 5 will be closed again.

[0038] and Figure 1 different, Figure 2 In the embodiment of FIG. 5 , only one temperature measuring device T2 is provided. The temperature measuring device is arranged between the valve 5 and the oil collecting container 6 and is used to measure the temperature of the flowing medium immediately after the valve 5. Therefore, the control device has only one temperature value and cannot form a temperature difference.

[0039] On the contrary, Figure 2 In the example of embodiment of , the aim is to detect a sharp drop in temperature at the temperature measuring device T2 when the medium in the valve 5 changes from oil to refrigerant after the oil separator 2 is emptied. To this end, after the valve 5 is opened, the derivative of the measured temperature value is continuously calculated in the control device and used as a parameter. A threshold value is specified for the negative derivative. Once this threshold value is exceeded (i.e. the temperature drop rate exceeds the threshold value), the valve T5 will be closed again.

[0040] Another alternative embodiment example is explained below, and its device structure is similar to Figure 2 Same as above, only the control method is the same as above Figure 2 The examples are different. In this embodiment example, when the valve 5 is opened, an initial temperature is first determined. For this purpose, a fixed value is initially specified, for example -20°C. The current temperature is then recorded at the temperature measuring device T2. If the temperature is higher, this recorded value becomes the initial temperature, otherwise the initial temperature remains at the specified example value -20°C. The difference between the initial temperature and the current temperature at the temperature measuring device T2 is then continuously calculated. If the difference exceeds a predetermined threshold, a predetermined limit of, for example, 4°C, (i.e., the current temperature is lower than the initial temperature minus the predetermined threshold), the valve is closed again.

[0041] Optionally, in the exemplary embodiment described, a higher level of time control is also provided, in which the minimum and maximum time for the valve to be open are specified, and these time thresholds will be maintained regardless of temperature changes. For example, "control" can be set to keep valve 5 open for at least 10 seconds and at most 60 seconds. Only within the above time range will valve 5 be closed early due to temperature control.

[0042] Figure 3 A further exemplary embodiment with two temperature measuring devices T2, T3 is shown. The temperature measuring device T2 is arranged at Figure 2 The temperature measuring device T3 is arranged at the inlet of the oil separator 2, and finally measures the temperature of the medium (compressed refrigerant mixed with oil) flowing out of the compressor 1.

[0043] exist Figure 3 In the embodiment example, the control method is basically the same as Figure 2 The same as described in , that is, the derivative of the temperature value at the temperature measuring device T2 is used as the characteristic value. However, the temperature value at the temperature measuring device T3 is used as a reference value here, that is, when necessary, the characteristic value and / or threshold value is dynamically modified according to the reference value.

[0044] Often, mixed forms are possible, i.e. Figures 1 to 3The key is to detect as quickly as possible the change in the flow medium through the valve from liquid oil to gaseous refrigerant. This is achieved by isoenthalpic expansion cooling in valve 5, as Figure 4 shown. Figure 4 A pressure-enthalpy diagram for a typical refrigerant is shown. Two examples of isoenthalpic expansions in valve 5 are shown, from point 1' to 2' and from point 1 to 2. In both examples, significant temperature drops of 20 K and 30 K can be seen, respectively, which are detected by the above method.

[0045] The control system described above has additional advantages for refrigeration circuits not shown in the drawings, which are equipped with multiple compressors 1. This applies both to refrigeration circuits with independent sub-circuits and to simple refrigeration circuits using multiple compressors in parallel (for example due to required capacity considerations). The control system can have multiple or single oil separators 2. Whatever the configuration, the control system can use only one oil collection container 6, because the oil discharge control of the oil separator 2 and the oil filling control of the compressor 1 are operated independently of each other.

[0046] Reference Symbols List

[0047] 1 Compressor

[0048] 2 Oil separator

[0049] 3 Heat exchanger

[0050] 4 Throttle element

[0051] 5. Valve

[0052] 6 Oil collection container

[0053] 7 Heat exchanger

[0054] K Refrigeration circuit

[0055] T1 Temperature measuring device

[0056] T2 Temperature measurement device

[0057] T3 Temperature measuring device

Claims

1. A method for controlling the liquid level of an oil separator (2) in a refrigeration circuit (K), wherein a controllable valve (5) is connected downstream of the outlet of the oil separator, and the controllable valve delivers oil to an oil collecting container (6) through its outlet, characterized in that: After the valve (5) is opened, measuring a first temperature in the pipeline between the valve (5) and the oil collecting container (6); Setting a threshold value of at least one parameter characterizing the first temperature change characteristic; When the parameter exceeds the threshold value, the valve (5) is immediately closed.

2. The method of claim 1, wherein: The rate of change of the first temperature is used as the parameter.

3. The method of claim 1, wherein: An initial temperature is determined, and a difference between the current temperature and the initial temperature is used as a characteristic variable.

4. The method according to any one of claims 1 to 3, wherein: The oil in the oil collecting container (6) is supplied to the compressor (1) of the refrigeration circuit (K) by adjusting the oil level in the compressor (1).

5. The method according to claim 4, wherein: By adjusting the oil level of each compressor (1) respectively, the oil in the same oil collecting container (6) is supplied to the plurality of compressors (1) of the refrigeration circuit (K).

6. The method according to any one of claims 1 to 5, wherein: A second temperature is further measured in the upstream region of the valve (5), and the difference between the first temperature and the second temperature is used as a characteristic parameter.

7. The method according to claim 6, wherein: The second temperature is measured in the pipeline between the oil separator (2) and the valve (5).

8. The method according to any one of claims 1 to 7, wherein: A third temperature is further measured in the inlet region of the oil separator (2), and the third temperature is used to determine the characteristic parameter and / or the threshold value.

9. The method according to any one of claims 1 to 8, wherein: The opening of the valve (5) is a periodic operation.

10. The method according to claim 9, wherein: The cycle length is determined according to the capacity of the compressor (1).

11. The method according to any one of claims 1 to 10, wherein: The shortest and / or longest time for the valve (5) to be opened is preset.

12. A system comprising an oil separator (2) and an oil collecting container (6) connected to its outlet downstream through a valve (5), as well as a plurality of temperature measuring devices (T1, T2, T3) and a control device, wherein the control device is configured to perform the method according to any one of claims 1 to 11.

13. A refrigeration circuit (K), comprising a compressor (1) and a heat exchanger (3), wherein: The outlet of the compressor (1) is connected to the heat exchanger (3) via a refrigerant pipeline, and further comprises the system of claim 12.

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