COOLING DEVICE

Abstract

Cooling is provided for operation at an ultra-low temperature of -60 °C, which can both reduce mechanical losses and ensure the reliability of a compressor. A cooling system (100) according to the present disclosure comprises: a low-temperature-side cooling circuit (CL) in which a first refrigerant circulates; a high-temperature-side cooling circuit (CH) in which a second refrigerant circulates; and an intermediate heat exchanger (10) configured to connect the low-temperature-side cooling circuit and the high-temperature-side cooling circuit, so that heat exchange takes place between the first refrigerant and the second refrigerant, wherein the first refrigerant is a mixed refrigerant containing CO2 and R32, and the second refrigerant is CO2 and a refrigeration oil with a kinematic viscosity of 5.2 mm². 2 / s or more and 19.8 mm 2 / s or less at 40 °C is stored in a compressor (2) for the cooling circuit of the low temperature side.

Description

[Technical area]

[0001] The present disclosure relates to cooling, in particular cascade cooling. [Technical background]

[0002] In recent years, the use of refrigerants with a low GWP (Global Warming Potential) has been demanded for environmental protection reasons. Examples of refrigerants that can achieve a low GWP include HFC refrigerants, carbon dioxide (CO2), and the like. Well-known HFC refrigerants are R23, R32, and others. The GWP of R23 is 14,800. The GWP of R32 is 675. The GWP of CO2 refrigerant is 1.

[0003] Using refrigerants with a low GWP can impair the cooling performance of refrigeration equipment. This is especially true for refrigeration equipment with extremely low target temperatures of around -45°C to -70°C.

[0004] When using a refrigerant with a low GWP, one possible measure is to increase the compression ratio of the compressors used in the cooling system. However, simply increasing the compression ratio can lead to an excessively high compressor outlet temperature, preventing the desired cooling effect from being achieved.

[0005] Patent literature 1 describes the use of a cascade circuit configuration to implement a cooling device capable of achieving both low global warming potential and high cooling capacity. The cascade cooling system primarily consists of a low-temperature cooling circuit, a high-temperature cooling circuit, and an intermediate heat exchanger. In patent literature 1, the refrigerant circulating in the low-temperature cooling circuit is a mixture containing CO2 and R32, while the refrigerant circulating in the high-temperature cooling circuit is solely CO2.

[0006] Typically, HFC refrigerants are combined with refrigeration oils of ISO viscosity grades VG32, VG68, VG75, and the like. CO2 refrigerants are combined with refrigeration oils of ISO viscosity grade VG68.

[0007] For example, R23 is combined with ISO VG32 alkylbenzene oil (Barrel Freeze 32SAM, manufactured by MATSUMURA ​​OIL CO., LTD) and then used for conventional cooling applications compatible with ultra-low temperatures.

[0008] For example, a refrigeration oil of ISO viscosity class VG68 is used for both cooling cycles in a cascade refrigeration unit as described in patent literature 1. [Reference list][Patent literature]

[0009] [PTL 1] Japanese patent application Laid-Open No. 2022-55607 [Summary of the invention][Technical problem]

[0010] The cooling system described in patent literature 1 can operate at extremely low temperatures. However, operating at ultra-low temperatures reduces the viscosity of the refrigeration oil exiting the compressor, hindering oil recirculation. This increases mechanical losses and reduces the compressor's mechanical efficiency. While reducing the viscosity of the refrigeration oil can mitigate this reduction in viscosity, it can also decrease the oil film's load-carrying capacity and lead to lubrication problems such as bearing wear, shortened service life, seizing, or similar issues. Such problems can compromise the reliability of refrigeration equipment.

[0011] The present disclosure was made in view of these circumstances and aims to provide a cooling system for operation at an ultra-low temperature of -60 °C, which can both reduce mechanical losses and ensure the reliability of a compressor. [Solution to the problem]

[0012] To solve the above problem, the following solution is used in a cooling of the present disclosure.

[0013] A cooling system according to the present disclosure comprises: a low-temperature refrigeration circuit in which a first refrigerant circulates; a high-temperature refrigeration circuit in which a second refrigerant circulates; and an intermediate heat exchanger configured to connect the low-temperature refrigeration circuit and the high-temperature refrigeration circuit, such that heat exchange takes place between the first refrigerant and the second refrigerant, wherein the first refrigerant is a mixed refrigerant containing CO2 and R32, the second refrigerant is CO2, and a refrigeration oil with a kinematic viscosity of 5.2 mm 2 / s or more and 19.8 mm 2 / s or less is stored at 40 °C in a compressor for the cooling circuit of the low temperature side. [Advantageous effects of the invention]

[0014] According to the present disclosure, a compressor is provided with cooling for operation at an ultra-low temperature of -60 °C, which can both reduce mechanical losses and ensure reliability. [Brief description of the drawings] [ Fig. 1] Fig. Figure 1 is a refrigerant cycle diagram showing a cooling configuration according to an embodiment of the present disclosure. [ Fig. 2] Fig. Figure 2 is a cycle diagram of the cooling according to an embodiment of the present disclosure. [ Fig. 3] Fig. Figure 3 is a diagram illustrating the relationship between the viscosity of the refrigerant decomposition and the thickness of the oil film. [ Fig. 4] Fig. Figure 4 is a diagram showing a result of the investigation of the relationship between the viscosity of the coolant decomposition and the kinematic viscosity. [ Fig. 5] Fig. Figure 5 is a diagram showing a result of the investigation of the relationship between the viscosity of the refrigerant decomposition and the kinematic viscosity. [ Fig. 6] Fig. Figure 6 is a diagram illustrating the relationship between viscosity and mechanical efficiency. [Description of the embodiments]

[0015] An embodiment of a cooling system according to the present disclosure is described below with reference to the drawings. [Configuration of the cooling unit]

[0016] Fig. Figure 1 is a refrigerant circuit diagram showing a configuration of a cooling system 100 according to the present embodiment. The cooling system 100 is a cascade cooling system. The cooling device 100 comprises a low-temperature-side refrigeration circuit CL, a high-temperature-side refrigeration circuit CH, an intermediate heat exchanger 10 configured to connect the low-temperature-side refrigeration circuit CL and the high-temperature-side refrigeration circuit CH, and gas injection circuits 9.

[0017] In the low-temperature cooling circuit CL, heat exchange takes place between the indoor air and a refrigerant (a first refrigerant, described later). In the high-temperature cooling circuit CH, heat exchange takes place between the outdoor air and a refrigerant (a second refrigerant, described later). The refrigerant (the second refrigerant) undergoing heat exchange with the outdoor air is subjected to heat exchange with the refrigerant (the first refrigerant) on the low-temperature side of the cooling circuit in the intermediate heat exchanger 10. [Configuration of the side cooling circuit at low temperatures]

[0018] The low-temperature side cooling circuit CL has an evaporator 1, a first compressor 2, low-temperature side expansion valves 3 (a low-temperature side first expansion valve 31 and a low-temperature side second expansion valve 32) and a low-temperature side collector 81.

[0019] The first compressor 2, the low-temperature side first expansion valve 31, the low-temperature side collector 81, the low-temperature side second expansion valve 32 and the evaporator 1 are arranged in this order from upstream to downstream in the direction of flow of the first refrigerant and are connected to a circuit by a low-temperature side line P1.

[0020] The low-temperature side line P1 is filled with the first refrigerant. This first refrigerant is a blend containing carbon dioxide (CO2, GWP: 1) and R32 (difluoromethane, GWP: 675). The main component of the blend is CO2. (2) R32 may be present in a range of 16% by weight or more and 22% by weight or less, based on the total weight of the mixed refrigerant. The mixed refrigerant may contain CO2 and R32 in a ratio of CO2:R32 = 78 : 22 (by weight).

[0021] The first compressor 2 compresses a low-pressure gaseous refrigerant supplied by the evaporator 1 to produce a high-temperature, high-pressure gaseous refrigerant. The first compressor 2 has a larger capacity than a compressor (a second compressor 4, described later) for the high-temperature-side refrigeration circuit.

[0022] The first compressor 2, for example, is a two-stage compressor called Scrotary (a registered trademark). In the first compressor 2, a rotary compressor 21 is used on the low-pressure side (intake side) and a scroll compressor 22 is used on the high-pressure side (pressure side). The rotary compressor 21 and the scroll compressor 22 are coaxially connected.

[0023] The first compressor 2 comprises a radial upper bearing and a radial lower bearing (not shown). The upper radial bearing (drive bearing) is located at the end on the rear side of a rotating screw and rotatably supports a drive bushing. The radial lower bearing (main bearing) rotatably supports a drive shaft. In the first compressor 2, the rotary compressor 21 has a larger bearing sliding surface than the scroll compressor 22.

[0024] The first compressor 2 contains a reservoir (not shown) in which a first refrigeration oil is stored. The composition of the first refrigeration oil stored in the reservoir is one or more types of oils selected from a group consisting of ester oils, ether oils, glycol oils, and mineral oils. The first refrigeration oil acts as a lubricant on a sliding surface of a bearing in the first compressor 2. The first refrigeration oil is operational in an atmosphere of medium temperature and medium pressure.

[0025] In combination with the mixed refrigerant containing CO2 and R32, the viscosity of the decomposition of the refrigerant of the first refrigeration machine oil is 0.32 cP or more, preferably 0.43 cP or more and 0.51 cP or less, particularly preferably 0.47 cP, at a temperature of 4.4 °C and an absolute pressure of 2.5 MPa.

[0026] If the viscosity of the coolant decomposition at a temperature of 4.4 °C and an absolute pressure of 2.5 MPa is 0.32 cP or more, an oil film with a thickness that meets a criterion can form on the bearing's sliding surface.

[0027] If the viscosity of the coolant decomposition at a temperature of 4.4 °C and an absolute pressure of 2.5 MPa is 0.43 cP or more, it is possible to ensure a difference in oil film thickness that exceeds the criterion by a margin of 20%. If the viscosity of the coolant decomposition at a temperature of 4.4 °C and an absolute pressure of 2.5 MPa is 0.51 cP or less, it is possible to ensure a mechanical efficiency of approximately 90% for existing machines.

[0028] In combination with the mixed refrigerant containing CO2 and R32, the viscosity of the decomposition of the refrigerant of the first refrigeration machine oil is 0.32 cP or more and 0.8 cP or less, preferably 0.70 cP or more and 0.78 cP or less, particularly preferably 0.74 cP, at a temperature of 12.3 °C and an absolute pressure of 2.5 MPa.

[0029] To ensure reliability, it is advantageous for the compressors to have a higher viscosity of the first refrigeration oil. However, an excessively high viscosity of the first refrigeration oil reduces mechanical efficiency. If the viscosity of the first refrigeration oil at a temperature of 12.3 °C and an absolute pressure of 2.5 MPa is 0.8 cP or less, a significant reduction in mechanical efficiency compared to existing machines can be avoided, while an oil film with a thickness approximately twice that of the criterion forms on the bearing's sliding surface. In this way, reliability and mechanical efficiency can be achieved simultaneously across a relatively wide range of operating points required for the low-temperature lateral refrigeration circuit.

[0030] The kinematic viscosity of the first refrigeration oil is 5.2 mm at 40 °C. 2 / s or more and 19.8 mm 2 / s or less, preferably 6.12 mm 2 / s or more and 16.5 mm 2 / s or less, preferably 6.12 mm 2 / s or more and 11.0 mm 2 / s or less, preferably 7.14 mm 2 / s or more and 8.14 mm 2 / s or less, most preferred 7.64 mm 2 / s. Kinematic viscosity is a value measured according to a method specified in JIS-K2283.

[0031] The density of the first refrigeration oil at 15 °C is 0.917 g / cm³. 3 or more and 0.928 g / cm² 3 or less, preferably 0.917 g / cm³ 3 or more and 0.921 g / cm² 3 or less, especially preferably 0.921 g / cm³ (3) Density is a value measured according to a method in accordance with JIS-K2249.

[0032] The flash point of the first cooling machine oil is 179 °C or more and 185 °C or less, preferably 179 °C or more and 181 °C or less, particularly preferably 181 °C. The flash point is a value measured according to a method in JIS-K2265.

[0033] A gaseous, high-temperature, high-pressure refrigerant generated by the first compressor 2 flows into the intermediate heat exchanger 10. In the intermediate heat exchanger 10, heat exchange takes place between the second refrigerant (a low-temperature, low-pressure liquid-phase refrigerant) in the high-temperature refrigeration circuit CH and the first refrigerant (a high-temperature, high-pressure gas-phase refrigerant) in the low-temperature refrigeration circuit CL. Accordingly, in the low-temperature refrigeration circuit CL, the gaseous refrigerant flowing through the intermediate heat exchanger 10 is condensed, producing a liquid, high-pressure refrigerant.

[0034] The high-pressure liquid-phase refrigerant passes through the first expansion valve 31 on the low-temperature side, the low-temperature receiver 81, and the second expansion valve 32 on the low-temperature side in this sequence. The high-pressure liquid-phase refrigerant flows through the first expansion valve 31 on the low-temperature side, reducing the pressure to a specific level and becoming a medium-pressure, medium-temperature liquid-phase refrigerant. This liquid-phase refrigerant is stored in the low-temperature receiver 81 and undergoes gas-liquid separation. The gas-phase components of the separated gas and liquid are fed to the first compressor 2 (specifically, a position upstream of the scroll compressor 22 on the high-pressure side) via a low-temperature circuit 91 as a gas injection circuit 9.The first refrigerant (gas phase component) at a low temperature is supplied to the first compressor 2 via the low-temperature side circuit 91 before it is compressed.

[0035] The medium temperature and pressure liquid refrigerant that has passed through the collector 81 on the low temperature side flows through the second expansion valve 32 on the low temperature side, resulting in a further pressure reduction and becoming a low temperature and low pressure liquid refrigerant.

[0036] The liquid refrigerant, which has been brought to low pressure via the second expansion valve 32 on the low-temperature side, flows into the evaporator 1. The evaporator 1 is located in a cold storage chamber (a space to be cooled). Heat exchange takes place in the evaporator 1 between the air in the cold storage chamber and the first refrigerant.

[0037] Note that it is advisable to use a fan to force air through the cooling unit towards evaporator 1. As the heat in the cooling chamber is absorbed by the low-temperature liquid refrigerant, the temperature in the cooling chamber decreases. This cools the interior of the cooling unit. The liquid refrigerant flowing through evaporator 1 consequently experiences a temperature increase and changes from the liquid to the gaseous phase.

[0038] The refrigerant, which has transitioned into the gas phase via the evaporator 1, is returned to the first compressor 2. This cycle is performed continuously, during which the temperature of the refrigerant chamber is adjusted to a desired value.

[0039] The specification of the cycle for lateral cooling at low temperatures is shown below as an example. Evaporation temperature−62[∘C](LP=0.2[MPaA]∗) HP / MP=2.4 / 0.6[MPaA] Nc=104[rps] Ts / Td=−55 / 95[∘C],O¨ltemperatur(inside the compressor)=0[∘C],oa¨.

[0040] * Since the pressure on the charge side (the LP side) is essentially constant and also essentially constant on the HP side (the LP side on the high-temperature side), the pressure state does not change significantly.

[0039] HP delivery pressure of a scroll compressor 42 MP Outlet pressure of a rotary compressor 41 LP Intake pressure of a rotary compressor 41 Nc real rotational speed Ts Inlet temperature Td discharge temperature [Configuration of the high-temperature side cooling circuit]

[0041] The high-temperature side cooling circuit CH has a second compressor 4, a heat radiant heater 5, a high-temperature side expansion valve 6 (a high-temperature side first expansion valve 61 and a high-temperature side second expansion valve 62) and a high-temperature side collector 82.

[0042] The second compressor 4, the heat radiant heater 5, the high-temperature-side first expansion valve 61, the high-temperature-side collector 82 and the high-temperature-side second expansion valve 62 are arranged in this order from upstream to downstream in the flow direction of the second refrigerant and are connected to form a loop by the high-temperature-side line P2.

[0043] The high-temperature side line P2 is filled with the second refrigerant. The second refrigerant contains only carbon dioxide (CO2).

[0044] The second compressor 4 compresses a low-pressure gaseous refrigerant supplied by the intermediate heat exchanger 10 to produce a gaseous refrigerant at a high temperature and high pressure. The second compressor 4 is a two-stage compressor, a so-called scrotary compressor. The rotary compressor 41 is used on the low-pressure side and the scroll compressor 42 on the high-pressure side. The rotary compressor 41 and the scroll compressor 42 are coaxially connected.

[0045] The second compressor 4 comprises a radial upper bearing and a radial lower bearing (not shown). The upper radial bearing (drive bearing) is located at the end on the rear side of a rotating screw and rotatably supports a drive bushing. The radial lower bearing (main bearing) rotatably supports a drive shaft. In the second compressor 4, the rotary compressor 41 has a larger bearing sliding surface than the scroll compressor 42.

[0046] The second compressor 4 contains a reservoir (not shown) in which a second refrigeration oil is stored. The second refrigeration oil stored in the reservoir contains, as a frame, one or more types of oils selected from a group consisting of ester oils, ether oils, glycol oils, and mineral oils. The ester oils may, for example, be polyol ester oils. The second refrigeration oil acts as a lubricant on a sliding surface of a bearing in the second compressor 4. The second refrigeration oil is operational in an atmosphere of medium temperature and medium pressure.

[0047] In combination with a CO2 refrigerant, the viscosity of the decomposition of the refrigerant of the second refrigeration oil is 9.96 cP or more and 10.04 cP or less, preferably 10 cP, at a temperature of 4.4 °C and an absolute pressure of 2.5 MPa.

[0048] In combination with the refrigerant CO2, the viscosity of the decomposition of the refrigerant of the second refrigeration machine oil is 12.16 cP or more and 12.24 cP or less, preferably 12.2 cP, at a temperature of 12.3 °C and an absolute pressure of 2.5 MPa.

[0049] The kinematic viscosity of the second refrigeration oil is 61.2 mm. 2 / s or more and 74.8 mm 2 / s or less, preferably 68 mm 2 / s at 40 °C.

[0050] The density of the second refrigeration oil at 15 °C is 0.96 g / cm³. (3) .

[0051] The flash point of the second cooling machine oil is 254 °C.

[0052] A gaseous refrigerant at high temperature and high pressure, generated by the second compressor 4, flows into the radiant heater 5. The radiant heater 5 is located outside the cooling unit (a compartment to be cooled). Heat exchange takes place in the radiant heater 5 between the second refrigerant and the outside air.

[0053] Note that it is advisable to use a fan (not shown) to force outside air into the radiant heater 5. This causes the gaseous refrigerant in the radiant heater 5 to condense, producing a high-pressure refrigerant in liquid form.

[0054] The high-pressure liquid-phase refrigerant passes through the first expansion valve 61 on the high-temperature side, the high-temperature receiver 82, and the second expansion valve 62 on the high-temperature side in this sequence. The high-pressure liquid-phase refrigerant flows through the first expansion valve 61 on the high-temperature side, reducing the pressure to a specific level and becoming a medium-pressure, medium-temperature liquid-phase refrigerant. This liquid-phase refrigerant is stored in the high-temperature receiver 82 and undergoes gas-liquid separation. The gas-phase components of the separated gas and liquid are fed to the second compressor 4 (specifically, a position upstream of the scroll compressor 42 on the high-pressure side) via a high-temperature circuit 92 as a gas injection circuit 9.In this way, the second refrigerant with low temperature (gas phase component) is fed to the second compressor 4 via the high-temperature circuit 92 before compression.

[0055] The medium temperature and pressure liquid refrigerant that has passed through the collector 82 on the high temperature side is passed through the second expansion valve 62 on the high temperature side, resulting in a further pressure reduction and becoming a low temperature and low pressure liquid refrigerant.

[0056] The liquid refrigerant, which has cooled to a low temperature and low pressure via the second expansion valve 62 on the high-temperature side, flows into the intermediate heat exchanger 10. In the intermediate heat exchanger 10, heat exchange takes place between the second refrigerant (a liquid refrigerant with low temperature and low pressure) in the high-temperature cooling circuit CH and the first refrigerant (a gaseous refrigerant with high temperature and high pressure) in the low-temperature cooling circuit described later. In the high-temperature cooling circuit CH, the liquid-phase refrigerant flowing through the intermediate heat exchanger 10 experiences a temperature increase and changes from the liquid phase to the gas phase.

[0057] The refrigerant, which has transitioned into the gas phase via the intermediate heat exchanger 10, is fed back into the second compressor 4. This cycle is carried out continuously in the high-temperature cooling circuit CH. [Effects and Benefits]

[0058] In a cascade cooling system, the required compression ratios in the first compressor 2 and the second compressor 4 can each be reduced to a low value. This allows the temperatures (outlet temperatures) of the refrigerants exiting these compressors to be further lowered. In this way, the cooling capacity of the cooling unit 100 can be further increased.

[0059] Fig. Figure 2 shows a circuit diagram of the cooling process according to the embodiment described above. Fig. In Figure 2, the horizontal axis represents the specific enthalpy, the vertical axis the pressure, the solid line the high-temperature cooling circuit CH, and the dashed line the low-temperature cooling circuit CL. As shown in Figure 2. Fig. As shown in Figure 2, in the cascade cooling system the circuit diagrams of the high-temperature cooling circuit CH and the low-temperature cooling circuit CL overlap at an intermediate position (the intermediate heat exchanger 10).

[0060] Accordingly, the coolant temperature can be reduced to a lower temperature than in a case where only the high-temperature cooling circuit CH is used. For example, if the temperature at the outlet of the radiant heater of the high-temperature cooling circuit CH is 34 °C, the evaporation temperature in the low-temperature cooling circuit CL can be reduced to an extremely low temperature of approximately -68 °C.

[0061] In a cascade chiller, different types of refrigerants can be used for the low-temperature (CL) and high-temperature (CH) circuits. Using CO2 as the primary refrigerant allows for a low global warming potential (GWP). A blended refrigerant, such as R32 mixed with CO2, can further improve cooling performance compared to using only CO2.

[0062] If the combination ratio of R32 is 16 wt% or more and 22 wt% or less, the GWP can be controlled to 150 or less and may meet or fall below international limits. Accordingly, it is possible to provide a cooling unit that can achieve both a low GWP and high cooling capacity.

[0063] In the high-temperature side cooling circuit CH, only CO2 is used as the secondary refrigerant, so the density is not excessively low compared to a case where R32 is mixed. Accordingly, the required compression ratio in the second compressor 4 can also be controlled to a low value.

[0064] In the low-temperature side cooling circuit CL, a refrigeration oil with a kinematic viscosity of 5.2 mm is used. 2 / s or more and 19.8 mm 2 / s or less at 40 °C applied to the first compressor 2, ensuring both reliability and reducing mechanical losses.

[0065] In the low-temperature side cooling circuit CL, a refrigeration oil with a refrigerant decomposition viscosity of 0.43 cP or more and 0.51 cP or less at a temperature of 4.4 °C and an absolute pressure of 2.5 MPa is applied to the first compressor 2, so that both the guarantee of reliability and the reduction of mechanical losses can be achieved, even when operating at -60 °C.

[0066] Fig. Figure 3 illustrates the relationship between the viscosity of the decomposition of a refrigeration oil at a temperature during the operation of the first compressor 2 and the thickness of the oil film that forms on the bearing's sliding surface. The operating condition is that the temperature is 4.4 °C or 12.3 °C and the absolute pressure is 2.5 MPa. Fig. 3 The horizontal axis represents the viscosity of the refrigerant decomposition (cP), the vertical axis the oil film thickness (µm), the solid line the oil film thickness of the main bearing, and the dashed line the oil film thickness of the drive bearing.

[0067] The oil film thickness criterion for the main bearing is 0.68 µm, and for the drive bearing, it is 0.61 µm. These criteria represent the oil film thickness required for stable oil film formation during operation, preventing lubrication failure of any part of the bearing due to insufficient oil film. The required oil film thickness is determined by bearing specifications (clearance, bearing surface finish), rotational speed, temperature conditions, and other factors. Insufficient oil film thickness can lead to an unstable oil film, lubrication failure due to insufficient oil film, and ultimately, bearing failure.

[0068] Fig. Figure 3 shows that a refrigerant dissolution viscosity of 0.32 cP or higher can meet the criteria mentioned above. Using a refrigeration oil with a refrigerant dissolution viscosity of 0.32 cP or higher at a temperature of 4.4 °C and an absolute pressure of 2.5 MPa can ensure bearing reliability. Using a refrigeration oil with a refrigerant dissolution viscosity of 0.43 cP or higher and 0.51 cP or lower at a temperature of 4.4 °C and an absolute pressure of 2.5 MPa allows for a tolerance and ensures reliability with greater certainty.

[0069] As the temperature rises during operation, the viscosity of the refrigerant decomposition also increases. Excessively high viscosity of the refrigerant decomposition leads to a reduction in mechanical efficiency. According to Fig. 3. The refrigeration oil, with a refrigerant decomposition viscosity of 0.8 cP, can form an oil film on the bearing's sliding surface with a thickness approximately twice that of the criterion. This result indicates that an oil film of sufficient thickness can be obtained if the refrigerant decomposition viscosity is 0.8 cP or less at a relatively high temperature (12.3 °C) and an absolute pressure of 2.5 MPa.

[0070] Below are the results of a study on the relationship between the viscosity of the decomposition of refrigerants at a temperature of 4.4 °C and an absolute pressure of 2.5 MPa and the kinematic viscosity at 40 °C.

[0071] For arbitrary ester-based refrigeration oils A and B, the kinematic viscosities (at 40 °C and 100 °C) were measured according to a method specified in JIS K2283. Based on these measurements, the kinematic viscosities (at 4.4 °C and 12.3 °C) were calculated. Subsequently, for refrigeration oil A, the refrigerant solubility (%) and the refrigerant decomposition viscosity (cP) under operating conditions at a pressure of 2.5 MPa and temperatures of 4.4 °C and 12.3 °C, respectively, were determined from the temperature-pressure, decomposition viscosity, and refrigerant decomposition viscosity graphs. The decomposition viscosity of refrigeration oil B was calculated from the kinematic viscosity ratio of B / A and the decomposition viscosity of refrigeration oil A at each temperature.

[0072] The results are in Fig. 4 and Fig. Figure 5 shows the kinematic viscosity of the cutting machine oil A, which was 64.2 mm². 2 / s at 40 °C, 8.0 mm 2 / s at 100 °C, 363 mm 2 / s at 12.3 °C and 697 mm 2 / s at 4.4 °C. The kinematic viscosity of the cutting machine oil B was 7.6 mm². 2 / s at 40 °C, 2.1 mm 2 / s at 100 °C, 22 mm 2 / s at 12.3 °C and 33 mm 2 / s at 4.4 °C.

[0073] For refrigeration oil A at an absolute pressure of 2.5 MPa and a temperature of 12.3 °C, the refrigerant solubility was 19.1% and the refrigerant decomposition viscosity was 12.2 cP. For refrigeration oil A at an absolute pressure of 2.5 MPa and a temperature of 4.4 °C, the refrigerant solubility was 22.9% and the refrigerant decomposition viscosity was 10.0 cP.

[0074] For refrigeration oil B at an absolute pressure of 2.5 MPa and a temperature of 12.3 °C, the viscosity of the refrigerant decomposition was 0.74 cP. For refrigeration oil B at an absolute pressure of 2.5 MPa and a temperature of 4.4 °C, the viscosity of the refrigerant decomposition was 0.47 cP.

[0075] The viscosity of the refrigerant decomposition of 0.32 cP was converted into the kinematic viscosity mm 2 / s at 40 °C was converted by calculating a ratio of 0.32 cP to the viscosity of the refrigerant decomposition at an absolute pressure of 2.5 MPa and a temperature of 4.4 °C of refrigeration oil B with the kinematic viscosity at 40 °C of refrigeration oil B. The kinematic viscosity at 40 °C for the viscosity of the refrigerant decomposition of 0.32 cP was 5.202 mm². 2 / s.

[0076] The above result suggests that the use of a cooling machine oil with a kinematic viscosity of 5.2 mm² will result in 2 / s or more at 40 °C the reliability of a bearing can be guaranteed.

[0077] In the above description, the kinematic viscosity at 40 °C of refrigeration oil B is calculated from the viscosity of the refrigerant decomposition, and this method is only effective for the combination in the above embodiment of "the refrigerant of the low-temperature cascade refrigeration circuit," "the mixed refrigerant of CO2 and R32," and "the refrigeration oil containing an ester oil, an ether oil, a glycol oil, and a mineral oil as a frame." The inventors of the present invention have previously conducted studies, such as measuring the viscosity of the refrigerant decomposition of refrigeration oils A and B for a single-component CO2 refrigerant. (2)-refrigerant or an R32 refrigerant. The inventors then concluded that it is possible to convert the viscosity of the refrigerant decomposition into a kinematic viscosity when, in the above embodiment, "the refrigerant of the cascade cooling circuit at low temperature", "the mixed refrigerant of CO2 and R32", and "the refrigeration machine oil with an ester oil, an ether oil, a glycol oil, and a mineral oil as a frame" are combined. The above relationship between the viscosity of the refrigerant decomposition and the kinematic viscosity is not necessarily established under conditions that do not satisfy the combination of the above embodiment.

[0078] Fig. Figure 6 shows the relationship between the viscosity grade and the mechanical efficiency of compressors. Fig. In Figure 6, the horizontal axis represents the viscosity grade, the vertical axis the mechanical efficiency, the black circles the mechanical efficiency of rotary compressors (refrigerant: CO2 + R32, temperature: -45 °C), the white circles the mechanical efficiency of screw compressors (refrigerant: CO2 + R32, temperature: -45 °C), the black triangle the rotary mechanical efficiency (refrigerant: CO2, temperature: -10 °C), and the white triangle the mechanical efficiency of scroll compressors (refrigerant: CO2 + R32, temperature: -10 °C). Fig. 6 is the mechanical efficiency on the vertical axis normalized to the actual value of 96% as 1, which is achieved by a rotating mechanical efficiency of an existing machine with a CO2-based compressor (refrigerant: CO2, temperature: -10 °C). The viscosity grade is an ISO viscosity grade. The cooling machine oil is a rack-based ester oil.

[0079] The high-temperature cooling circuit uses a single-component refrigerant made of CO₂. (2) (Kältemittel The low-temperature refrigeration circuit (CO2, temperature: -10 °C) has a higher mechanical efficiency than the low-temperature circuit with a refrigerant mixture of CO2 and R32 (refrigerant: CO2 + R32, temperature: -45 °C). In the low-temperature circuit, the rotary compressor tends to have a higher mechanical efficiency than the scroll compressor, and this tendency becomes more pronounced with decreasing viscosity.

[0080] Preferably, a mechanical efficiency of 80% (an efficiency of 0.8 or more) of the mechanical efficiency of the existing CO2-based compressor machine should be ensured. According to Fig.For viscosity grade 6, the viscosity is 20 or less and the rotating mechanical efficiency of the low-temperature side of the refrigeration circuit is 0.8 or more. The kinematic viscosity of viscosity grade 20 is 19.8 mm. 2 / s or less at 40 °C. Therefore, by using a refrigeration oil with a kinematic viscosity of 19.8 mm², 2 / s or less at 40 °C an efficiency of 0.8 or more can be achieved compared to the existing machine.

[0081] In the cooling system according to the above embodiment, the first refrigerant or the second refrigerant is supplied at a low temperature to at least one of the first compressors 2 and the second compressor 4 before compression by the gas injection circuit 9. If the first compressor 2 and the second compressor 4 each consist of several compressor stages, it is possible to further reduce the final discharge temperature of the refrigerant by supplying a low-temperature refrigerant to the intermediate position of these multiple stages.

[0082] The gas injection circuit 9 is provided for both the low-temperature cooling circuit CL and the high-temperature cooling circuit CH. Accordingly, the outlet temperatures of the compressors (of the first compressor 2 and the second compressor 4) can be reduced in both the low-temperature cooling circuit CL and the high-temperature cooling circuit CH.

[0083] The gas injection circuits 9 are configured to supply refrigerant upstream of the high-pressure side scroll compressors 22, 42. The scroll compressors 22, 42 are configured so that the refrigerant flowing through the interior of the casing flows into the compression chamber without any restriction of flow direction or similar constraints. Therefore, it can be said that with the scroll compressors 22, 42, it is easier to add another refrigerant to the outside of the compression chamber than with the rotary compressors 21, 41. Consequently, refrigerant can be supplied more easily and smoothly to the gas injection circuits 9. [Additional note]

[0084] The cooling according to the embodiment described above is understood, for example, as follows.

[0085] The cooling system (100) according to the first aspect of the present disclosure comprises: the low-temperature-side cooling circuit (CL) in which a first refrigerant circulates; a high-temperature-side cooling circuit (CH) in which a second refrigerant circulates; and an intermediate heat exchanger (10) configured to connect the low-temperature-side cooling circuit and the high-temperature-side cooling circuit, such that heat exchange takes place between the first refrigerant and the second refrigerant, wherein the first refrigerant is a mixed refrigerant containing CO2 and R32, and the second refrigerant is CO2 and a refrigeration oil having a kinematic viscosity of 5.2 mm². 2 / s or more and 19.8 mm 2 / s or less is stored at 40 °C in a compressor (2) for the cooling circuit of the low temperature side.

[0086] In the refrigeration system described above, a cascade circuit type is used, with the refrigeration circuit on the low-temperature side and the refrigeration circuit on the high-temperature side. This allows the compression ratios required in the compressors for the respective cooling cycles to be controlled to a low value. As a result, the temperatures of the refrigerants exiting these compressors (outlet temperatures) can be further reduced, and the cooling capacity of the refrigeration unit can be further increased.

[0087] In the cooling device described above, the first refrigerant of the refrigeration circuit, on the low-temperature side, and the second refrigerant of the refrigeration circuit, on the high-temperature side, undergo heat exchange in an intermediate heat exchanger. Consequently, the refrigerant temperature can be reduced to a lower temperature than in a case where only the high-temperature side of the refrigeration circuit is used. This makes it possible to provide a cooling device compatible with ultra-low temperatures, achieving a temperature of -60 °C in the cooling chamber.

[0088] In the cooling device of the above aspect, different types of refrigerants can be used for the low-temperature side cooling circuit CL and the high-temperature side cooling circuit CH.

[0089] The mixed refrigerant, containing CO2 and R32, is used as the first refrigerant circulating in the low-temperature side's cooling circuit. Using CO2 as the refrigerant allows for a low global warming potential (GWP). The R32 blend further improves cooling performance compared to using only CO2. By using CO2 as the primary refrigerant component and adjusting the R32 blend ratio, it's possible to achieve a GWP of 150 or less and a coefficient of performance (COP) of 0.5. Therefore, it's possible to provide a cooling unit capable of both low GWP and high cooling capacity.

[0090] In the high-temperature cooling circuit, only CO2 is used as the secondary refrigerant, so the density is not excessively low compared to a case where R32 is mixed. Consequently, the required compression ratio in the compressor for the high-temperature cooling circuit can be kept to a low value.

[0091] The refrigeration oil with the kinematic viscosity described above is applied to the compressor for low-temperature cooling, allowing an oil film of sufficient thickness to form on the sliding parts and ensuring reliability. Using refrigeration oil with the kinematic viscosity described above can reduce mechanical losses.

[0092] In the cooling device according to the second aspect of the present disclosure, the refrigeration machine oil can have a viscosity of the refrigerant decomposition of 0.43 cP or more and 0.51 cP or less at a temperature of 4.4 °C and an absolute pressure of 2.5 MPa.

[0093] If the viscosity of the decomposition of the refrigeration oil is within the range described above, it is possible to further control the reduction of mechanical losses while ensuring the tolerance of the oil film thickness.

[0094] In the cooling device according to the third aspect of the present disclosure, the refrigeration machine oil can have a viscosity of refrigerant decomposition of 0.70 cP or more and 0.78 cP or less at a temperature of 12.3 °C and an absolute pressure of 2.5 MPa.

[0095] If the viscosity of the decomposition of the refrigeration oil is within the range described above, it is possible to further control the reduction of mechanical losses while ensuring the tolerance of the oil film thickness. [List of reference signs] 1 evaporator 2 first compressor (compressor) 3 Low-temperature side expansion valve 4-second compressor 5 radiant heaters 6 High-temperature side expansion valve 9 Gas injection circuit 10 intermediate heat exchangers 21 Rotary compressor 22 scroll compressors 31 Low temperature side first expansion valve 32 Low-temperature side second expansion valve 41 Rotary compressor 42 scroll compressors 61 High-temperature side first expansion valve 62 High-temperature side second expansion valve 81 Low-temperature side receivers 82 High-temperature side receivers 91 Cold-side circuit 92 High-temperature side switch 100 cooling devices CH High-temperature side cooling CL Low-temperature side cooling P1 Low-temperature side pipe P2 High-temperature side tube QUOTES INCLUDED IN THE DESCRIPTION

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

[0000] JP 2022-55607

[0009]

Claims

[1] A cooling system, including: a low-temperature cooling circuit in which a first refrigerant circulates; a high-temperature cooling circuit in which a second refrigerant circulates; and an intermediate heat exchanger configured to connect the refrigeration circuit on the low-temperature side and the refrigeration circuit on the high-temperature side, so that heat exchange takes place between the first refrigerant and the second refrigerant, the first refrigerant is a mixed refrigerant containing CO2 and R32, where the second refrigerant is CO2, and wherein a refrigeration oil with a kinematic viscosity of 5.2 mm 2 / s or more and 19.8 mm 2 / s or less is stored at 40 °C in a compressor for the cooling circuit of the low temperature side. [2] Cooling device according to claim 1, wherein the refrigeration oil has a refrigerant decomposition viscosity of 0.43 cP or more and 0.51 cP or less at a temperature of 4.4 °C and an absolute pressure of 2.5 MPa. [3] Cooling device according to claim 1, wherein the refrigeration oil has a refrigerant decomposition viscosity of 0.70 cP or more and 0.78 cP or less at a temperature of 12.3 °C and an absolute pressure of 2.5 MPa.

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