Method for controlling the fill level of an oil separator for a cooling circuit and associated system

Abstract

A method for controlling the fill level of an oil separator (2) for a cooling circuit (K), comprising a controllable valve (5) connected downstream of an outlet of the oil separator (2), is intended to enable the lowest possible loss of pressure and energy, and yet is low-maintenance and technically simple to implement. For this purpose, after the valve (5) is opened, a first temperature is measured in a region downstream of the valve (5), a limit value for at least one parameter that is characteristic of a change in the first temperature is specified, and a closing of the valve (5) results as soon as the parameter exceeds the specified limit value.

Description

[0001] Method for controlling the fill level of an oil separator for a refrigeration circuit and associated system

[0002] The invention relates to a method for regulating the fill level of an oil separator for a refrigeration circuit, comprising a controllable valve connected downstream of an outlet of the oil separator, through which valve oil is directed into an oil collection tank. It further relates to a system comprising an oil separator and an oil collection tank connected downstream of the outlet side via a valve, as well as to a refrigeration circuit with such a system.

[0003] A refrigeration cycle is a system used to cool a device to a desired level, for example a freezer for food. A refrigerant that is moved in the closed circuit undergoes various changes of state one after the other: The gaseous refrigerant is first compressed by a compressor. In the downstream heat exchanger it condenses, releasing heat. The liquid refrigerant is then expanded due to the change in pressure via a throttling device, for example an expansion valve or a capillary tube. In the downstream second heat exchanger (evaporator) the refrigerant evaporates at a low temperature, absorbing heat (evaporative cooling). The cycle can now start again. The process must be kept going from the outside by supplying mechanical work (drive power) via the compressor.

[0004] The compressor uses oil as a lubricant. Due to its design, a portion of the oil always ends up in the escaping compressed refrigerant. Therefore, an oil separator, which may be designed as an impact separator, is usually installed downstream of the compressor's outlet. This separator, for example, can be used as an impact separator, and in which the oil is removed from the refrigerant. The separated oil is not discarded but is channeled via a separate oil circuit into an oil collection tank and from there fed back into the compressor.

[0005] This separate oil circuit connects the outlet of the oil separator to the oil collection tank, i.e., a part of the refrigeration circuit downstream of the compressor and therefore under high pressure, with the low-pressure inlet area of ​​the compressor. It cannot therefore remain permanently open, as this would result in a permanent loss of pressure and thus energy, and is therefore equipped with a controllable valve.

[0006] Common options include either on-demand valve openings that detect the oil separator's fill level, e.g., via optical detection or a float, or simple time-controlled systems that open the valve for a specified time in a regular cycle. However, oil separator fill level measurements are susceptible to dirt and can be technically complex. Furthermore, they are often not suitable for all temperatures in the oil collection tank. Time-controlled systems, on the other hand, are often not tailored to requirements and result in the valve remaining open for too long.

[0007] It is therefore an object of the invention to provide a method of the type mentioned at the outset which enables the lowest possible pressure and energy loss and yet is low-maintenance and technically simple to implement.

[0008] This object is achieved according to the invention in that, after the valve has been opened, a first temperature is measured in a line between the valve and the oil collecting container, a limit value is specified for at least one parameter characteristic of a change in the first temperature, and the valve is closed as soon as the parameter exceeds the specified limit value.

[0009] The invention is based on the consideration that the oil separator can be emptied into the oil collection container as required, not only by opening the oil separator as needed when the fill level is detected to be too high, but also by precisely controlling the opening duration of the valve. To achieve this, the valve should be closed again after opening at the exact time when all of the oil has been removed from the oil separator. In order to determine this point in time precisely, it should be determined precisely whether oil is still flowing through the valve or whether refrigerant is already flowing through the valve, i.e. the oil supply in the oil separator is exhausted. Since the refrigerant in the oil separator has usually been compressed immediately beforehand, it is under high pressure and is in gaseous form. An isenthalpic change of state occurs in the valve between the oil separator and the oil collection container.The refrigerant is in gaseous form, while the oil is in the liquid phase, so that both media behave differently under the isenthalpic change of state: the expansion of the gaseous refrigerant leads to a reduction in temperature across the valve, whereas no such temperature change is to be expected with liquid oil. The flow of oil or refrigerant can therefore be detected in the valve by this temperature characteristic. By specifying suitable temperature-characteristic parameters and suitable limit values, the valve can be closed precisely after opening, which occurs exactly when the entire oil supply from the oil separator has been transferred to the oil collection container.

[0010] In an advantageous embodiment, a rate of change of the first temperature is used as a parameter. In other words: The mathematical derivative of the temperature curve measured after the valve is determined and itself or a correspondingly derived value is used as a parameter for which a limit value is specified. As soon as a particularly rapid reduction in temperature (i.e., above the limit value) is detected, this indicates the change of the medium in the valve from oil to refrigerant. This makes it possible to detect the change in the medium in the valve with only one

[0011] To determine the temperature measuring point.

[0012] In a design that is particularly simple to implement from a technical perspective, a starting temperature is determined, and a difference between the current temperature and the starting temperature is used as a parameter. In other words, a starting temperature is determined when the valve opens. This can be fixed or determined dynamically at the time of opening. The current temperature is then continuously measured while the medium is flowing through the valve, and the difference to the starting temperature is calculated. If the difference exceeds a certain value, the valve is closed again. Since the temperature decreases as the refrigerant rather than the oil expands, the difference will be negative, and “exceeding” here means exceeding a predetermined absolute value of the negative difference.

[0013] In a further advantageous embodiment, a compressor in the refrigeration circuit is supplied with oil from the oil collection tank by means of a control of the oil fill level in the compressor. By combining a control system as described above with a separate oil collection tank, it is possible to both control the emptying of the oil separator in a targeted manner as needed and to independently adjust the oil fill level of the compressor in an optimized manner. Control of the oil fill level is understood to be a control system that is based on the oil fill level in the compressor as the controlled variable and controls a valve in an area between the oil collection tank and the compressor's oil inlet. If the oil fill level is too low, the valve is opened; if it is too high, it is closed. Targeted control of the flow rate is also possible so that the oil lost from the compressor via the refrigerant outlet is continuously replaced.

[0014] A considerable additional advantage arises if a plurality of compressors in the refrigeration circuit are advantageously supplied with oil from the same oil collection tank by means of separate control of the oil level in the respective compressor. By separating the oil level control in the compressor from the oil level control in the oil separator, it is possible to use a central oil collection tank to which the oil separator or all oil separators in the refrigeration circuit supply oil by means of the valve control described above. At the same time, all compressors, whether they are arranged in the same refrigeration circuit or in separate sub-circuits, can be optimally supplied with oil from the central oil collection tank by providing appropriate control of the oil level in each compressor as described above.

[0015] In an additional or alternative advantageous embodiment, a second temperature is measured in a region upstream of the valve, and the difference between the first and second temperatures is used as a characteristic. Thus, the absolute temperature difference before and after the valve is used as the characteristic. In this case, a change in the medium in the valve is detected by an increase in this temperature difference, and a corresponding limit value is specified.

[0016] In principle, combinations of the parameters mentioned can also be used, i.e., determining both the absolute temperature difference and the rate of temperature change. This may allow for even more precise detection of the change in the medium in the valve.

[0017] Advantageously, the second temperature is measured in a line between the oil separator and the valve. A suitable additional temperature measuring device can be arranged here to measure the temperature of the flowing medium before it is released in the valve.

[0018] In a further advantageous embodiment, a third temperature is measured in an inlet area of ​​the oil separator, and the third temperature is used to determine the parameter and / or the limit value. Knowing the temperature at the inlet of the oil separator as a reference temperature enables even more precise control of the fill level if it is included in the evaluation.

[0019] In the described methods, the valve is advantageously opened cyclically. This means that the valve is opened at a predetermined regularity, e.g., once per predetermined period of time or at a specific opening frequency every X seconds / minutes. It can also be opened after a certain time has elapsed following the valve's closing according to the described methods.

[0020] The cycle length, i.e., the average interval between opening cycles, is advantageously determined as a function of the compressor's performance. If the compressor's performance in a refrigeration circuit increases because greater cooling capacity is required, oil is also pumped into the oil separator at a higher rate. Adapting the opening frequency to the compressor's performance, meaning that higher performance also means a higher valve opening frequency, further improves the efficiency of the process.

[0021] Finally, in the described control system, a minimum and / or maximum time for opening the valve is advantageously specified at a higher level. This increases the stability of the control system. A system comprising an oil separator and an oil collection container connected downstream of a valve on the outlet side advantageously comprises a number of temperature measuring devices and a control device designed to carry out the method described above.

[0022] A refrigeration circuit comprising a compressor and a heat exchanger, wherein a refrigerant line connects the compressor to the heat exchanger on the output side, advantageously comprises such a system.

[0023] The advantages achieved by the invention are in particular that by determining the fill level of the oil separator based on temperature changes, an energy-efficient and technically simple demand-based emptying of the oil separator into the oil collection tank of a refrigeration circuit is achieved. The control of the fill level in the oil separator via the temperature of the outflowing medium does not require adapters or sight glasses and is not susceptible to contamination like previous systems with floats. It enables particularly simple adaptation to different load conditions of the refrigeration circuit and is technically particularly simple to implement and therefore cheaper. By using such a system in combination with an oil collection tank, it is possible to combine optimized emptying of the oil separator with the demand-based oil supply, even to several compressors, from a common oil collection tank.

[0024] Exemplary embodiments of the invention are explained in more detail with reference to the drawings. In the drawings: FIG. 1 shows a refrigeration circuit with an oil separator and a temperature measuring device before and after the valve to the oil collecting tank,

[0025] FIG 2 the refrigeration circuit as in FIG 1 with only one temperature measuring device after the valve, and

[0026] FIG 3 the refrigeration circuit as in FIG 1 with a temperature measuring device after the valve and at the inlet to the oil separator, and

[0027] FIG 4 A pressure-enthalpy diagram with two examples of isenthalpic expansion in the valve when the oil separator is empty and gas is expanded.

[0028] Identical parts are provided with the same reference symbols in all figures.

[0029] FIG 1 shows a schematic representation of a refrigeration circuit K. The refrigeration circuit K is described below starting with a compressor 1. The refrigerant compressed in the compressor 1 - in the exemplary embodiment carbon dioxide - is first fed into an oil separator 2. The oil separator 2 is designed as an impact separator in the exemplary embodiment and separates oil which has been mixed with the refrigerant in the compressor 1 for operational reasons. On the refrigerant side, a heat exchanger 3, designed as a gas cooler in the exemplary embodiment, in which the compressed refrigerant is cooled and liquefied, is connected to the oil separator 2. From there, it flows via a throttle element 4 into a heat exchanger 7 designed as an evaporator. Here, the refrigerant is expanded and absorbs heat, so that the desired cooling effect is achieved. From the latter heat exchanger 7, the now gaseous

[0030] Refrigerant flows back into the compressor 1, is compressed there and the cycle begins again. So far, only the arrangement of the oil separator 2 with regard to the refrigerant channel in the refrigeration circuit K has been described. With regard to the oil flow, the oil separator 2 is arranged as follows: At the bottom of the oil separator 2 there is an outlet for the separated oil. This outlet is connected to an oil collecting tank 6 via a controllable valve 5 which, in the exemplary embodiment, is designed as a solenoid valve. The oil is fed back to the compressor 1 from the oil collecting tank 6, this being done by regulating the oil fill level in the compressor 1.

[0031] The design of the refrigeration circuit K described so far is the same for all embodiments of FIGS. 1 to 3. Differences arise only in the arrangement of the temperature sensors T1, T2, T3. However, this does not mean that the method described below for controlling the fill level of the oil separator 2 is only applicable to such simple refrigeration circuits K - these serve only as an explanation. The method is applicable to any refrigeration circuits K that have an oil separator 2.

[0032] To ensure that the process of draining the separated oil from the oil separator 2 does not cause an unnecessary pressure loss in the cooling circuit K, the valve 5 should only be opened when required. The aim here is that only separated oil flows through the valve 5 and not gas, which would be the case if the valve were open and the oil separator 2 were emptied. For this purpose, the fill level of the oil separator 2 is regulated by opening and closing the valve 5 as required as follows: The valve 5 is opened in a time-controlled manner regardless of the current fill level of the oil separator 2. The provision is for the valve 5 to be opened regularly and cyclically after a predetermined period of time after it was last closed. In exemplary embodiments, this period can be dynamically dependent on the output of the compressor 1, i.e. with higher output of the compressor 1, the cycle length between two opening trigger processes is shortened. However, it can also be fixed.

[0033] Essential for the control method for the fill level of the oil separator 2 presented here is the immediate closing of valve 5 after the oil separated in the oil separator 2 has been completely removed. For this purpose, in all exemplary embodiments, the medium flowing through the open valve 5 is detected – as long as oil is still flowing, valve 5 remains open; as soon as refrigerant is flowing, valve 5 is immediately closed.

[0034] To detect the flowing medium in valve 5, it is utilized that the refrigerant in oil separator 2 is gaseous, while the oil is liquid. In valve 5, an isenthalpic expansion of the medium takes place, so that based on the temperature change across valve 5, it can be determined whether the flowing medium is liquid or gaseous: If oil separator 2 is empty and gas is expanded, the temperature drops. When oil is passed through valve 5, its temperature downstream of valve 5 is approximately the same as before valve 5. This can be detected using suitable temperature measuring devices TI, T2, T3. In all embodiments, temperature measuring devices TI, T2, T3 are connected to a control device (not shown), which controls the opening and closing of valve 5 using the data from

[0035] Temperature measuring devices TI, T2, T3 with corresponding

[0036] Controls hardware and software.

[0037] In the embodiment shown in FIG. 1, two temperature measuring devices T1, T2 are provided for this purpose. These are arranged in the supply lines upstream and downstream of valve 5, i.e., between oil separator 2 and valve 5 (T1) and between valve 5 and oil collection tank 6 (T2). The temperature measuring devices T1, T2 measure the temperature of the medium in the supply lines.

[0038] As explained above, a gaseous medium flowing through valve 5 results in a temperature drop across valve 5. Therefore, in the method of the exemplary embodiment shown in FIG. 2, the temperature difference between the measured temperatures at both temperature measuring devices T1, T2 is used as a parameter. A temperature difference is specified as a limit value. As soon as this limit value is exceeded, i.e., the temperatures deviate by more than the limit value, valve 5 is closed again.

[0039] In the embodiment of FIG. 2, however, in contrast to FIG. 1, only one temperature measuring device T2 is provided. This is arranged between valve 5 and oil collection tank 6 and measures the temperature of the flowing medium immediately after valve 5. Therefore, only a single temperature value is available in the control device; no difference can be calculated.

[0040] Instead, the embodiment shown in FIG. 2 provides for detecting the sharp drop in temperature at the temperature measuring device T2 when the medium in valve 5 changes from oil to refrigerant after the oil separator 2 has been emptied. For this purpose, the control device continuously calculates the derivative of the measured temperature value after the valve 5 has been opened and uses this derivative as a parameter. A limit value for the negative derivative is specified. As soon as this limit value is exceeded, i.e., the temperature decreases even faster than the limit value specifies, valve 5 is closed again.

[0041] A further alternative embodiment, which requires structurally identical devices as in FIG. 2, is explained below. Only the type of control differs from the previous example explained with reference to FIG. 2. In this embodiment, a starting temperature is first determined when the valve 5 is opened. For this purpose, a fixed value, e.g. -20°C, is initially specified. The current temperature is then recorded at the temperature measuring device T2. If the temperature is higher, this recorded value becomes the starting temperature; otherwise, the starting temperature remains at the aforementioned example value of -20°C. The difference between the starting temperature and the current temperature at the temperature measuring device T2 is then continuously determined. If this value exceeds a predetermined limit, e.g. 4°C (i.e.the current temperature is lower than the start temperature minus the specified limit), the valve is closed again.

[0042] Optionally, the described embodiment also provides for a higher-level time control, which specifies a minimum and maximum opening duration that is maintained regardless of the temperature behavior. For example, the control could be set so that valve 5 remains open for a minimum of 10 seconds and a maximum of 60 seconds. Only within these limits will it be closed again prematurely due to the described temperature control.

[0043] FIG. 3 shows another embodiment with two temperature measuring devices T2, T3. The temperature measuring device T2 is arranged at the same location as the temperature measuring device T2 in FIG. 2. The temperature measuring device T3, however, is arranged at the inlet to the oil separator 2 and ultimately measures the temperature of the medium flowing out of the compressor 1 (compressed refrigerant with an admixture of oil).

[0044] In the embodiment of FIG. 3, the control is essentially applied as described for FIG. 2, i.e., by deriving the temperature value at the temperature measuring device T2 as a parameter. However, the temperature value at the temperature measuring device T3 is included as a reference value, i.e., the parameter and / or the limit value are dynamically modified, if necessary, based on this reference value.

[0045] In general, mixed forms are also possible, ie combined parameters and limit values ​​from the variables and values ​​described for FIGS. 1 to 3. It is essential to detect the change of the flowing medium through the valve from liquid oil to gaseous refrigerant as quickly as possible. This is achieved via cooling during isenthalpic expansion in valve 5, as shown in FIG. 4. FIG. 4 shows a pressure-enthalpy diagram of a typical refrigerant. Two examples of isenthalpic expansion in valve 5 are shown as examples, from point 1' to 2' and from point 1 to 2. In both cases, significant temperature drops of 20 or 30 K are recognizable, which are detected using the methods described above. In refrigeration circuits not shown in the diagram, which have several

[0046] Compressor 1, the described system offers additional advantages. This applies both to refrigeration circuits with separate sub-circuits and to simple refrigeration circuits that use several compressors in parallel, e.g. due to the required power. These can have several or a single oil separator 2. In any case, the described control system makes it possible to use only a single oil collection tank 6, since the control of the emptying of the oil separator 2 and that of the refilling of the compressor 1 with oil are independent of one another.

[0047] List of reference symbols

[0048] 1 compressor 2 oil separator

[0049] 3 heat exchangers

[0050] 4 Throttle organ

[0051] 5 Valve

[0052] 6 Oil collection tank 7 Heat exchanger

[0053] K Refrigeration circuit

[0054] TI temperature measuring device

[0055] T2 temperature measuring device

[0056] T3 temperature measuring device

Claims

Patent claims Method for regulating the fill level of an oil separator (2) for a refrigeration circuit (K), comprising a controllable valve (5) downstream of an outlet of the oil separator (2), through which valve oil is conducted into an oil collecting container (6), wherein after the valve (5) is opened, a first temperature is measured in a line between the valve (5) and the oil collecting container (6), a limit value is specified for at least one parameter characteristic of a change in the first temperature, and the valve (5) is closed as soon as the parameter exceeds the specified limit value. Method according to one of the preceding claims, in which a rate of change of the first temperature is used as the parameter. Method according to claim 1, in which a starting temperature is determined, and a difference between the current temperature and the starting temperature is used as the parameter.Method according to one of the preceding claims, in which a compressor (1) of the refrigeration circuit (K) is supplied with oil from the oil collection tank (6) by regulating the oil level in the compressor (1). Method according to the preceding claim, in which a plurality of compressors (1) of the refrigeration circuit are supplied with oil. (K) is supplied with oil from the same oil collecting tank (6) by means of a separate control of the oil level in the respective compressor (1).

6. Method according to one of the preceding claims, in which a measurement of a second temperature is also carried out in a region upstream of the valve (5) and the difference between the first and the second temperature is used as a characteristic variable.

7. Method according to the preceding claim, in which the measurement of the second temperature takes place in a line between the oil separator (2) and the valve (5).

8. Method according to one of the preceding claims, further comprising measuring a third temperature in an inlet region of the oil separator (2), and using the third temperature in determining the characteristic variable and / or the limit value.

9. Method according to one of the preceding claims, in which the opening of the valve (5) is carried out cyclically.

10. Method according to the preceding claim, wherein the cycle length is determined as a function of a power of a compressor (1).

11. Method according to one of the preceding claims, in which a minimum and / or a maximum time for opening the valve is specified at a higher level. System comprising an oil separator (2) and an oil collecting container (6) connected downstream on the outlet side via a valve (5), further comprising a number of temperature measuring devices (TI, T2, T3) and a control device designed to carry out the Method according to one of the preceding claims. Refrigeration circuit (K) comprising a compressor (1) and a heat exchanger (3), wherein a refrigerant line connects the compressor (1) to the heat exchanger (3) on the output side, further comprising a system according to the preceding claim.

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