Refrigeration cycle apparatus

Abstract

A refrigeration cycle apparatus is provided with a control unit (C) that controls a refrigerant circuit (R) so that the pressure of a refrigerant compressed by a compression element (30) becomes equal to or higher than the critical pressure and the pressure of the refrigerant decompressed by a first decompression valve (23) becomes lower than the critical pressure. The refrigerant circuit (R) includes: an inflow pipe (61) that sends, into a gas-liquid separator (50), the refrigerant decompressed by the first decompression valve (23); a liquid outflow pipe (62) that sends, toward second decompression valves (24, 83), a liquid refrigerant resulting from separation in the gas-liquid separator (50); and an oil return flow passage (OP) that returns, to the compression element (30), an oil resulting from separation in the gas-liquid separator (50).

Description

Refrigeration Cycle Equipment

[0001] The present disclosure relates to a refrigeration cycle device.

[0002] The refrigeration cycle apparatus described in Patent Document 1 includes a refrigerant circuit filled with carbon dioxide as a refrigerant. The refrigeration cycle apparatus performs a refrigeration cycle in which a compressor compresses the refrigerant to a critical pressure or higher. In such a refrigeration cycle apparatus, oil for lubricating the sliding parts of the compressor is discharged from the compressor along with the refrigerant. For this reason, an oil separator that separates the oil from the refrigerant is provided on the discharge side of the compressor in the refrigerant circuit. The oil separated by the oil separator returns to the suction side of the compressor through an oil return pipe.

[0003] Japanese Patent Application Laid-Open No. 2022-039365

[0004] Incidentally, in a refrigeration apparatus in which an oil separator is provided in a refrigerant circuit, there has been a demand for miniaturization thereof.

[0005] An object of the present disclosure is to reduce the size of a refrigeration cycle device.

[0006] The first aspect relates to a refrigeration cycle apparatus, the refrigeration cycle apparatus including a refrigerant circuit (R) having a compression element (30), a radiator (22, 83), a first pressure reducing valve (23), a gas-liquid separator (50), a second pressure reducing valve (24, 83), and an evaporator (83, 22), and a control unit (C) that controls the refrigerant circuit (R) so that the pressure of the refrigerant compressed by the compression element (30) is equal to or greater than a critical pressure and the pressure of the refrigerant reduced by the first pressure reducing valve (23) is lower than the critical pressure. The refrigerant circuit (R) includes an inlet pipe (61) for feeding the refrigerant reduced in pressure by the first pressure reducing valve (23) into the gas-liquid separator (50), a liquid outlet pipe (62) for feeding the liquid refrigerant separated in the gas-liquid separator (50) to the second pressure reducing valve (24, 83) side, and an oil return flow path (OP) for returning the oil separated in the gas-liquid separator (50) to the compression element (30).

[0007] In the first aspect, the first pressure reducing valve (23) reduces the pressure of the refrigerant to a pressure lower than the critical pressure. The reduced pressure refrigerant flows into the gas-liquid separator (50) through the inlet pipe (61). The gas-liquid separator (50) separates the refrigerant into gas refrigerant and liquid refrigerant. The gas-liquid separator (50) further separates oil from the refrigerant. Oil is more easily separated from refrigerant in a subcritical state than from refrigerant in a critical state. The oil separated in the gas-liquid separator (50) returns to the compression element (30) through the oil return channel (OP).

[0008] The gas-liquid separator (50) functions as an oil separator, thereby enabling the refrigeration system to be made smaller.

[0009] In the second embodiment, the refrigerant circuit (R) in the first embodiment has a gas introduction pipe (63) for feeding the gas refrigerant separated in the gas-liquid separator (50) to the compression element (30).

[0010] In the second embodiment, the gas refrigerant separated in the gas-liquid separator (50) can be returned to the compression element (30) through the gas inlet pipe (63).

[0011] In the third aspect, in the second aspect, the oil return channel (OP) has oil inlet ports (O4, O5) through which oil in the gas-liquid separator (50) flows in. The liquid outlet pipe (62) has a liquid inlet port (O2) through which liquid refrigerant in the gas-liquid separator (50) flows in. The oil inlet ports (O4, O5) are located lower than the liquid inlet port (O2).

[0012] In the third aspect, the oil inlets (O4, O5) are located lower than the liquid inlet (O2). Here, oil, liquid refrigerant, and gas refrigerant tend to accumulate in the gas-liquid separator (50) from the bottom to the top. This is because, during normal operation, the density of oil is greater than the density of liquid refrigerant, and the density of liquid refrigerant is greater than the density of gas refrigerant. This prevents oil from flowing out of the gas-liquid separator (50) to the liquid inlet (O2). As a result, oil from the gas-liquid separator (50) can be more easily returned to the compression element (30).

[0013] In a fourth aspect, in the third aspect, the oil return channel (OP) has a first oil return pipe (64) and a second oil return pipe (65). The first oil return pipe (64) has a first oil inlet (O4) as an oil inlet. The second oil return pipe (65) has a second oil inlet (O5) as an oil inlet. The second oil inlet (O5) is located at a higher position than the first oil inlet (O4).

[0014] In the fourth aspect, when the oil level in the gas-liquid separator (50) rises, the oil at a relatively high level can be sent through the second oil inlet (O5) to the second oil return pipe (65). As a result, the oil in the gas-liquid separator (50) can be returned to the compression element (30) not only through the first oil return pipe (64) but also through the second oil return pipe (65).

[0015] In a fifth aspect, in the fourth aspect, the second oil return pipe (65) is the gas introduction pipe (63). A gas inlet (O3) is formed at an end of the gas introduction pipe (63). The second oil inlet (O5) is located downstream of the gas inlet (O3) in the second oil return pipe (65), and connects the interior of the gas introduction pipe (63) to the interior of the gas-liquid separator (50).

[0016] In the fifth aspect, the gas refrigerant separated in the gas-liquid separator (50) flows into the gas inlet (O3) of the second oil return pipe (65), which is the gas introduction pipe (63). This gas refrigerant is sent to the compression element (30) through the second oil return pipe (65). In addition, the oil separated in the gas-liquid separator (50) flows into the second oil inlet (O5). This oil returns to the compression element (30) through the second oil return pipe (65). In this way, the gas introduction pipe (63) also serves as an oil return flow path.

[0017] In a sixth aspect, in any one of the first to fifth aspects, the refrigerant circuit (R) has an accumulator (26) arranged between the evaporator (83, 22) and the compression element (30). The oil return channel (OP) is connected to the refrigerant circuit (R) between the evaporator (83, 22) and the accumulator (26) or to the accumulator (26).

[0018] In the sixth aspect, when liquid refrigerant in the gas-liquid separator (50) flows into the oil return channel (OP), the liquid refrigerant can be stored in the accumulator (26), which prevents the liquid refrigerant from being sucked into the compression element (30), thereby avoiding so-called liquid compression.

[0019] In a seventh aspect, in any one of the first to sixth aspects, the compression element (30) includes a low-stage compression section (31) that compresses a refrigerant, and a high-stage compression section (32) that compresses the refrigerant compressed in the low-stage compression section (31). The oil return flow path (OP) includes a low-stage oil return pipe (66) that returns oil separated in the gas-liquid separator (50) to the low-stage compression section (31), and a high-stage oil return pipe (67) that returns oil separated in the gas-liquid separator (50) to the high-stage compression section (32).

[0020] In the seventh aspect, the oil separated in the gas-liquid separator (50) can be returned to both the low-stage compression section (31) and the high-stage compression section (32).

[0021] In an eighth aspect, in any one of the first to seventh aspects, the refrigerant is carbon dioxide, and the oil is polyalkylene glycol.

[0022] In the gas-liquid separator (50) of the eighth aspect, carbon dioxide as a refrigerant is brought to a subcritical state, which makes it particularly easy to separate polyalkylene glycol from the refrigerant.

[0023] Fig. 1 is a piping system diagram of an air conditioning apparatus according to an embodiment. Fig. 2 is a schematic configuration diagram showing a gas-liquid separator and piping connected thereto. Fig. 3 is a block diagram of the main equipment of the air conditioning apparatus. Fig. 4 is a diagram corresponding to Fig. 1 of Modified Example 1. Fig. 5 is a diagram corresponding to Fig. 2 of Modified Example 1. Fig. 6 is a diagram corresponding to Fig. 2 of Modified Example 2. Fig. 7 is a diagram corresponding to Fig. 1 of Modified Example 3. Fig. 8 is a diagram corresponding to Fig. 1 of Modified Example 4. Fig. 9 is a diagram corresponding to Fig. 1 of another embodiment.

[0024] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below, and various modifications are possible within the scope of the technical concept of the present disclosure. Since the drawings are intended to conceptually explain the present disclosure, dimensions, ratios, or numbers may be exaggerated or simplified as necessary to facilitate understanding.

[0025] The refrigeration cycle device of the present disclosure constitutes an air conditioner (10). The air conditioner (10) adjusts the temperature of air in an indoor space, which is a target space. The air conditioner (10) of this embodiment performs cooling operation and heating operation.

[0026] (1-1) Overall Configuration of Air Conditioning Apparatus As shown in FIG. 1, an air conditioner (10) has an outdoor unit (20), an indoor unit (80), a liquid-side connecting pipe (2), and a gas-side connecting pipe (3). The outdoor unit (20) is installed outdoors, and the indoor unit (80) is installed indoors. In the air conditioner (10), the outdoor unit (20) and the indoor unit (80) are connected to each other via the liquid-side connecting pipe (2) and the gas-side connecting pipe (3), thereby forming a refrigerant circuit (R). In the refrigerant circuit (R), a refrigeration cycle is performed by circulating a refrigerant.

[0027] The outdoor unit (20) has an outdoor fan (21) and outdoor equipment provided in a refrigerant circuit (R). The outdoor equipment mainly includes a compression element (30), an outdoor heat exchanger (22), a first pressure reducing valve (23), a gas-liquid separator (50), and an outdoor expansion valve (24). The indoor unit (80) has an indoor fan (81) and indoor equipment provided in the refrigerant circuit (R). The indoor equipment mainly includes an indoor heat exchanger (82) and an indoor expansion valve (83).

[0028] (1-2) Details of the Outdoor Unit The outdoor equipment of the refrigerant circuit (R) includes a first pressure reducing valve (23) and a gas-liquid separator (50). The outdoor equipment further includes a four-way switching valve (25), a bridge circuit (B), and an accumulator (26).

[0029] The compression element (30) of this embodiment includes a low-stage compressor (31) serving as a low-stage compression section and a high-stage compressor (32) serving as a high-stage compression section. The low-stage compressor (31) compresses low-pressure refrigerant in the refrigerant circuit (R) to intermediate-pressure refrigerant and discharges the intermediate-pressure refrigerant. The high-stage compressor (32) compresses intermediate-pressure refrigerant discharged from the low-stage compressor (31) to high-pressure refrigerant and discharges the high-pressure refrigerant. A suction pipe (40) is connected to the suction side of the compression element (30) (strictly, the low-stage compressor (31)). A discharge pipe (41) is connected to the discharge side of the compression element (strictly, the high-stage compressor (32)). An intermediate pipe (42) is connected between the discharge side of the low-stage compressor (31) and the suction side of the high-stage compressor (32).

[0030] The outdoor heat exchanger (22) is a heat source side heat exchanger, and exchanges heat between the refrigerant flowing therethrough and the outdoor air transported by the outdoor fan (21).

[0031] The gas-liquid separator (50) is a sealed container. The gas-liquid separator (50) separates the refrigerant into a gas refrigerant and a liquid refrigerant. The gas-liquid separator (50) has a function of separating oil from the refrigerant. The gas-liquid separator (50) will be described in detail later.

[0032] The bridge circuit (B) has first to fourth pipes (P1, P2, P3, P4) and a check valve (CV) provided in each of these pipes (P1, P2, P3, P4). The check valves (CV) allow refrigerant to flow in the direction indicated by the arrows in Figure 1 and prohibit refrigerant from flowing in the opposite direction.

[0033] One end of an inlet pipe (61) is connected to the junction between the outlet end of the first pipe (P1) and the outlet end of the second pipe (P2). The other end of the inlet pipe (61) is connected to the gas-liquid separator (50). One end of a liquid outlet pipe (62) is connected to the junction between the inlet end of the third pipe (P3) and the inlet end of the fourth pipe (P4). The other end of the liquid outlet pipe (62) is connected to the gas-liquid separator (50). The junction between the inlet end of the second pipe (P2) and the outlet end of the fourth pipe (P4) is connected to the liquid side end (low temperature side end) of the outdoor heat exchanger (22) via a relay pipe (43). The junction between the inlet end of the first pipe (P1) and the outlet end of the third pipe (P3) is connected to the liquid side end of the outdoor heat exchanger (22) via a liquid side connection pipe (2).

[0034] The first pressure reducing valve (23) is provided in the inlet pipe (61). The first pressure reducing valve (23) is an electronic expansion valve whose opening is variable. The first pressure reducing valve (23) reduces the pressure of high-pressure refrigerant equal to or higher than the critical pressure to intermediate-pressure refrigerant lower than the critical pressure.

[0035] The outdoor expansion valve (24) is provided in the relay pipe (43). The outdoor expansion valve (24) is an electronic expansion valve whose opening is variable. The outdoor expansion valve (24) serves as a second pressure reducing valve that reduces the pressure of intermediate-pressure refrigerant to low pressure during heating operation.

[0036] The accumulator (26) is provided in the suction pipe (40). The accumulator (26) is a sealed container that stores liquid refrigerant. The accumulator (26) prevents the liquid refrigerant from being sucked into the compression element (30). The suction pipe (40) includes an upstream suction pipe (40a) located upstream of the accumulator (26) and a downstream suction pipe (40b) located downstream of the accumulator (26).

[0037] The four-way selector valve (25) is provided on the discharge side of the compression element (30). The four-way selector valve (25) is switchable between a first state (a state indicated by a solid line in FIG. 1 ) and a second state (a state indicated by a dashed line in FIG. 1 ). In the first state, the four-way selector valve (25) connects the discharge side of the compression element (30) to the gas side end of the outdoor heat exchanger (22) and also connects the suction side of the compression element (30) to the gas side end of the indoor heat exchanger (82). In the second state, the four-way selector valve (25) connects the discharge side of the compression element (30) to the gas side end of the indoor heat exchanger (82) and also connects the suction side of the compression element (30) to the gas side end of the outdoor heat exchanger (22).

[0038] In the refrigerant circuit (R) during cooling operation, the four-way selector valve (25) is in the first state, the outdoor heat exchanger (22) functions as a radiator, and the indoor heat exchanger (82) functions as an evaporator. In the refrigerant circuit (R) during heating operation, the four-way selector valve (25) is in the second state, the indoor heat exchanger (82) functions as a radiator, and the outdoor heat exchanger (22) functions as an evaporator.

[0039] (1-3) Details of the Indoor Unit The indoor heat exchanger (82) is a utilization-side heat exchanger, and exchanges heat between the refrigerant flowing therethrough and the outdoor air transported by the indoor fan (81).

[0040] The indoor expansion valve (83) is provided in the refrigerant circuit (R) between the liquid side end of the indoor heat exchanger (82) and the liquid side connecting pipe (2). The indoor expansion valve (83) is an electronic expansion valve whose opening is variable. The indoor expansion valve (83) serves as a second pressure reducing valve that reduces the pressure of intermediate-pressure refrigerant to low pressure during cooling operation.

[0041] (2) Refrigerant and Oil The refrigerant circuit (R) is filled with carbon dioxide as a refrigerant. In the refrigerant circuit (R), a so-called supercritical cycle is performed in which the refrigerant compressed by the compression element (30) reaches or exceeds its critical pressure. In the refrigerant circuit (R) of this embodiment, a two-stage compression / two-stage expansion refrigeration cycle is performed.

[0042] The refrigerant contains oil as refrigeration oil, which constitutes a lubricant for lubricating the sliding parts of the compression element (30). In this embodiment, the oil is PAG (polyalkylene glycol).

[0043] The solubility of PAG in carbon dioxide varies depending on the pressure and temperature of carbon dioxide. When the pressure of carbon dioxide is equal to or higher than the critical pressure, in other words, when carbon dioxide is in a critical state, PAG becomes more soluble in carbon dioxide. Therefore, carbon dioxide and PAG become less likely to separate from each other. On the other hand, when the pressure of carbon dioxide is lower than the critical pressure, in other words, when carbon dioxide is in a subcritical state, PAG becomes less soluble in carbon dioxide. Therefore, carbon dioxide and PAG become more likely to separate from each other.

[0044] (3) Gas-Liquid Separator and Its Surrounding Structure (3-1) Details of the Gas-Liquid Separator As shown in FIG. 2, the gas-liquid separator (50) is a hollow, sealed container forming an internal space (S). The gas-liquid separator (50) is formed continuously with a cylindrical body (50a), a bottom (50b) formed below the body (50a), and a top (50c) formed above the body (50a). The gas-liquid separator (50) is installed outdoors. The gas-liquid separator (50) separates a refrigerant in a two-phase gas-liquid state into a gas refrigerant and a liquid refrigerant.

[0045] Basically, carbon dioxide in a subcritical state flows into the gas-liquid separator (50). As described above, when carbon dioxide is in a subcritical state, PAG is less soluble in carbon dioxide. Therefore, oil is more likely to separate from the refrigerant in the gas-liquid separator (50). The gas-liquid separator (50) functions as an oil separator that separates the refrigerant from the oil. Therefore, in the refrigerant circuit (R) of this embodiment, no oil separator is provided on the discharge side (high-pressure line) of the compression element (30).

[0046] During normal operation of the air conditioner (10), oil, liquid refrigerant, and gas refrigerant accumulate in the internal space (S) from bottom to top in this order. In other words, an oil layer (51) which is an oil layer, a liquid layer (52) which is a liquid refrigerant layer, and a gas layer (53) which is a gas refrigerant layer are formed in this order from bottom to top in the internal space (S).

[0047] (3-2) Details of Piping Connected to Gas-Liquid Separator As shown in FIGS. 1 and 2 , the refrigerant circuit (R) of this embodiment has an inlet pipe (61), a liquid outlet pipe (62), a gas inlet pipe (63), and an oil return channel (OP). The oil return channel (OP) is the entire channel for returning the oil separated in the gas-liquid separator (50) to the compression element (30). The oil return channel (OP) of this embodiment has a first oil return pipe (64). These pipes are provided in the outdoor unit (20).

[0048] The inlet pipe (61) is a pipe that sends the refrigerant, whose pressure has been reduced by the first pressure reducing valve (23), into the gas-liquid separator (50). In this embodiment, the inlet pipe (61) forms a flow path from the bridge circuit (B) to the gas-liquid separator (50). The inlet pipe (61) passes through the top (50c) of the gas-liquid separator (50). The inlet pipe (61) has an outlet (O1) through which the refrigerant flows into the gas-liquid separator (50). The outlet (O1) opens toward the internal space (S). The outlet (O1) faces downward.

[0049] The liquid outlet pipe (62) is a pipe for sending the liquid refrigerant separated in the gas-liquid separator (50) to the second pressure reducing valve (24, 83). In this embodiment, the liquid outlet pipe (62) forms a flow path from the gas-liquid separator (50) to the bridge circuit (B). The liquid outlet pipe (62) passes through the body portion (50b) of the gas-liquid separator (50). The liquid outlet pipe (62) has a liquid inlet (O2) through which the refrigerant in the gas-liquid separator (50) flows. The liquid inlet (O2) opens toward the internal space (S). The liquid inlet (O2) faces downward.

[0050] The gas introduction pipe (63) is a pipe for sending the gas refrigerant separated in the gas-liquid separator (50) to the compression element (30). The gas introduction pipe (63) of this embodiment sends the gas refrigerant to the suction side of the high-stage compressor (32). The outlet end of the gas introduction pipe (63) is connected to the intermediate pipe (42). The gas introduction pipe (63) passes through the top (50c) of the gas-liquid separator (50). The gas introduction pipe (63) has a gas inlet (O3) through which the gas refrigerant in the gas-liquid separator (50) flows. The gas inlet (O3) is formed at an end of the gas introduction pipe (63). The gas inlet (O3) opens toward the internal space (S). The gas inlet (O3) faces downward.

[0051] The first oil return pipe (64) is a pipe for returning the oil separated in the gas-liquid separator (50) to the compression element (30). The outflow end of the first oil return pipe (64) in the present embodiment is connected to the suction pipe (40). Specifically, the outflow end of the first oil return pipe (64) is connected between the evaporator (83, 22) and the accumulator (26) in the refrigerant circuit (R). The evaporator (83, 22) mentioned here is the indoor heat exchanger (82) during the cooling operation or the outdoor heat exchanger (22) during the heating operation. The first oil return pipe (64) penetrates the bottom (50b) of the gas-liquid separator (50). The first oil return pipe (64) has a first oil inlet (O4) as an oil inlet through which the oil in the gas-liquid separator (50) flows in. The first oil inlet (O4) opens toward the internal space (S). The first oil inlet (O4) faces upward. The first oil inlet (O4) and the bottom surface of the gas-liquid separator (that is, the upper surface of the bottom (50b)) are substantially flush. In other words, the first oil inlet (O4) is formed in the bottom (50b) of the gas-liquid separator (50).

[0052] As shown in FIG. 1, a first flow rate regulating valve (71) is provided in the gas introduction pipe (63). The first flow rate regulating valve (71) is, for example, an electronic expansion valve with a variable opening degree. A second flow rate regulating valve (72) is provided in the first oil return pipe (64). The second flow rate regulating valve (72) is, for example, an electronic expansion valve with a variable opening degree.

[0053] (3-3) Relationship of the height positions of the pipe openings The relationship of the height positions of the openings of the inflow pipe (61), the liquid outflow pipe (62), the gas introduction pipe (63), and the first oil return pipe (64) in the internal space (S) of the gas-liquid separator (50) will be described in detail while referring to FIG. 2.

[0054] In FIG. 2, the height position of the outlet (O1) of the inflow pipe (61) is h1, the height position of the liquid inlet (O2) of the liquid outflow pipe (62) is h2, the height position of the gas inlet (O3) of the gas introduction pipe (63) is h3, and the height position of the opening of the first oil return pipe (64) is h4. In the present embodiment, the relationship h4 < h2 < h1 < h3 is satisfied. Note that the height position mentioned here means altitude.

[0055] Specifically, in this embodiment, the first oil inlet (O4) is located lower than the liquid inlet (O2). The first oil inlet (O4) is located near the bottom (50b) of the gas-liquid separator (50). The gas inlet (O3) is located higher than the outlet (O1). The gas inlet (O3) is located near the top (50c) of the gas-liquid separator (50). The liquid inlet (O2) is located lower than the outlet (O1). The liquid inlet (O2) is located closer to the bottom of the internal space (S), and the outlet (O1) is located closer to the top of the internal space (S).

[0056] (4) Controller and Sensor As shown in FIG. 3 , the air conditioner (10) has a control unit (C). The control unit (C) controls the refrigerant circuit (R). The control unit (C) has an outdoor control unit (C1) and an indoor control unit (C2). The outdoor control unit (C1) and the indoor control unit (C2) are configured to be able to communicate with each other wirelessly or via a wire. The outdoor control unit (C1) and the indoor control unit (C2) include an MCU (Micro Controller Unit), an electric circuit, and an electronic circuit. The MCU includes a CPU (Central Processing Unit), a memory, and a communication interface. The memory stores various programs to be executed by the CPU.

[0057] The outdoor control unit (C1) is provided in the outdoor unit (20). The outdoor control unit (C1) controls the rotation speed of the low-pressure side compressor (31), the rotation speed of the high-pressure side compressor (32), the rotation speed of the outdoor fan (21), the opening degree of the first pressure reducing valve (23), the opening degree of the outdoor expansion valve (24), the opening degree of the first flow control valve (71), the opening degree of the second flow control valve (72), the state of the four-way selector valve (25), etc. The indoor control unit (C2) is provided in the indoor unit (80). The indoor control unit (C2) controls the rotation speed of the indoor fan (81), the opening degree of the indoor expansion valve (83), etc.

[0058] The air conditioner (10) has a plurality of sensors. The plurality of sensors includes a refrigerant-side sensor that detects the temperature and pressure of the refrigerant. The refrigerant-side sensors include a high-pressure pressure sensor (44) and an intermediate pressure sensor (45). The high-pressure pressure sensor (44) is provided in the discharge pipe (41). The high-pressure pressure sensor (44) detects the pressure of the refrigerant discharged from the compression element (30) (strictly speaking, the high-stage compressor (32)). The intermediate pressure sensor (45) is provided, for example, in the liquid outlet pipe (62). The intermediate pressure sensor (45) detects the pressure of the intermediate-pressure refrigerant after being reduced in pressure by the first pressure reducing valve (23) and before being reduced in pressure by the second pressure reducing valve (24, 83).

[0059] (5) Operation The air conditioner (10) performs cooling operation and heating operation. In Fig. 1, the flow of refrigerant during cooling operation is indicated by solid arrows, and the flow of refrigerant during heating operation is indicated by dashed arrows.

[0060] In the cooling operation and the heating operation, the control unit (C) controls the refrigerant circuit (R) so that, in principle, the pressure of the refrigerant compressed by the compression element (30) is equal to or greater than the critical pressure and the pressure of the refrigerant reduced by the first pressure reducing valve (23) is lower than the critical pressure. Specifically, the control unit (C) controls the rotation speeds of the low-stage compressor (31) and the high-stage compressor (32) so that the pressure of the refrigerant compressed by the compression element (30) is equal to or greater than the critical pressure. The pressure of the refrigerant compressed by the compression element (30) corresponds to the pressure detected by the high-pressure pressure sensor (44). The control unit (C) controls the aperture of the first pressure reducing valve (23) so that the pressure of the refrigerant reduced by the first pressure reducing valve (23) is equal to or greater than the critical pressure. The pressure of the refrigerant reduced by the first pressure reducing valve (23) corresponds to the pressure detected by the intermediate pressure sensor (45).

[0061] (5-1) Cooling Operation In cooling operation, the control unit (C) operates the outdoor fan (21), the low-stage side compressor (31), and the high-stage side compressor (32), sets the four-way switching valve (25) to the first state, adjusts the openings of the first pressure reducing valve (23) and the indoor expansion valve (83), and fully opens the outdoor expansion valve (24).

[0062] The refrigerant compressed by the low-pressure stage compressor (31) is further compressed to above critical pressure by the high-pressure stage compressor (32). The refrigerant discharged from the compression element (30) dissipates heat to outdoor air in the outdoor heat exchanger (22). The heat-dissipating refrigerant is decompressed by the first pressure reducing valve (23). After decompression, the refrigerant becomes a two-phase gas-liquid refrigerant in a subcritical state, which is lower than the critical pressure. This refrigerant flows into the gas-liquid separator (50). In the gas-liquid separator (50), the refrigerant is separated into liquid refrigerant, gas refrigerant, and oil. The liquid refrigerant separated in the gas-liquid separator (50) is decompressed by the indoor expansion valve (83) and then absorbs heat from indoor air in the indoor heat exchanger (82) to evaporate. The evaporated refrigerant is drawn into the low-pressure stage compressor (31).

[0063] (5-2) Heating Operation In heating operation, the control unit (C) operates the outdoor fan (21), the low-stage side compressor (31), and the high-stage side compressor (32), sets the four-way switching valve (25) to the second state, adjusts the apertures of the first pressure reducing valve (23) and the outdoor expansion valve (24), and fully opens the indoor expansion valve (83).

[0064] The refrigerant compressed by the low-pressure stage compressor (31) is further compressed to above critical pressure by the high-pressure stage compressor (32). The refrigerant discharged from the compression element (30) dissipates heat to the indoor air in the indoor heat exchanger (82). The refrigerant that has dissipated heat is reduced in pressure by the first pressure reducing valve (23). After being reduced in pressure, the refrigerant becomes a two-phase gas-liquid refrigerant in a subcritical state, which is lower than the critical pressure. This refrigerant flows into the gas-liquid separator (50). In the gas-liquid separator (50), the refrigerant is separated into liquid refrigerant, gas refrigerant, and oil. The liquid refrigerant separated in the gas-liquid separator (50) is reduced in pressure by the outdoor expansion valve (24) and then absorbs heat from the indoor air in the outdoor heat exchanger (22) to evaporate. The evaporated refrigerant is drawn into the low-pressure stage compressor (31).

[0065] (5-3) Gas Introduction Operation In cooling operation or heating operation, a gas introduction operation is performed to send the gas refrigerant in the gas-liquid separator (50) to the compression element (30). In the gas introduction operation, the control unit (C) opens the first flow control valve (71) to a predetermined opening. The gas refrigerant in the gas layer (53) in the gas-liquid separator (50) flows into the gas introduction pipe (63) and passes through the first flow control valve (71). This refrigerant passes through the intermediate pipe (42) and is sucked into the high-stage compressor (32).

[0066] (5-4) Oil Return Operation During cooling and heating operations, an oil return operation is performed to return the oil in the gas-liquid separator (50) to the compression element (30). In the oil return operation, the control unit (C) opens the second flow control valve (72) to a predetermined opening. The oil in the oil layer (51) in the gas-liquid separator (50) flows into the first oil return pipe (64) and passes through the second flow control valve (72). This oil flows into the upstream suction pipe (40a), passes through the accumulator (26), and is then sucked into the low-stage compressor (31). As a result, the sliding parts of the low-stage compressor (31) can be lubricated. Since oil flows out of the low-stage compressor (31) together with the refrigerant, this oil can also be returned to the high-stage compressor (32). As a result, the sliding parts of the high-stage compressor (32) can be lubricated.

[0067] When the liquid refrigerant in the liquid layer (52) flows out into the first oil return pipe (64), the liquid refrigerant can be captured in the accumulator (26). This prevents the liquid refrigerant from being directly sucked into the low-stage compressor (31), thereby preventing so-called liquid compression in the compression element (30).

[0068] The first oil return pipe (64) is connected to the suction side of the low-stage compressor (31), and therefore a sufficient pressure difference can be ensured between the inlet and outlet sides of the first oil return pipe (64), thereby ensuring that the oil in the gas-liquid separator (50) is returned to the compression element (30).

[0069] The temperature of the oil in the gas-liquid separator (50) is lower than the temperature of the high-pressure refrigerant, for example. Therefore, by sending the low-temperature oil in the gas-liquid separator (50) to the compression element (30), the compression efficiency of the compression element (30) can be improved compared to, for example, returning the oil in the high-pressure refrigerant to the compression element (30).

[0070] (6) Features (6-1) Issues Related to Oil Separators Conventional air conditioners may have an oil separator on the high-pressure side of the refrigerant circuit (e.g., in the discharge pipe) to return oil contained in the refrigerant discharged from the compression element to the compression element. On the other hand, when a refrigeration cycle is performed in which the refrigerant (e.g., carbon dioxide) is compressed to a critical pressure or higher in the refrigerant circuit, it is necessary to increase the thickness of the oil separator to ensure its pressure resistance. As a result, the oil separator may become, for example, large in size, heavy in weight, or expensive.

[0071] Therefore, in this embodiment, the oil separator is omitted from the refrigerant circuit (R) and the gas-liquid separator (50) is given the function of oil separation, thereby making it possible to reduce the size of the refrigeration cycle device (air conditioner (10)).

[0072] (6-2) Effects of the Embodiment (6-2-1) The control unit (C) controls the refrigerant circuit (R) so that the pressure of the refrigerant compressed by the compression element (30) is equal to or greater than the critical pressure and the pressure of the refrigerant reduced by the first pressure reducing valve (23) is lower than the critical pressure. The refrigerant circuit (R) includes an inlet pipe (61) for feeding the refrigerant reduced in pressure by the first pressure reducing valve (23) into the gas-liquid separator (50), a liquid outlet pipe (62) for feeding the liquid refrigerant separated in the gas-liquid separator (50) to the second pressure reducing valve (24, 83), and an oil return path (OP) for returning oil separated in the gas-liquid separator (50) to the compression element (30).

[0073] In this configuration, subcritical refrigerant can be stored in the gas-liquid separator (50), which facilitates separation of oil from the refrigerant. The oil separated in the gas-liquid separator (50) returns to the compression element (30) through the oil return path (OP). Therefore, the sliding parts of the compression element (30) can be lubricated with oil without providing an oil separator in the refrigerant circuit (R).

[0074] In addition, the oil in the gas-liquid separator (50) has a lower temperature than, for example, the oil in the high-pressure refrigerant. Therefore, compared to returning high-pressure oil to the compression element (30), lower-temperature oil can be returned to the compression element (30). This improves the compression efficiency of the compression element (particularly, the low-stage compressor (31) in this embodiment).

[0075] (6-2-2) The first oil inlet (O4) of the oil return passage (OP) (strictly speaking, the first oil return pipe (64)) is located lower than the liquid inlet (O2) of the liquid outlet pipe (62). This prevents oil near the bottom (50b) of the gas-liquid separator (50) from flowing out to the liquid outlet pipe (62). As a result, insufficient oil lubrication in the compression element (30) can be prevented.

[0076] In addition, the liquid refrigerant in the gas-liquid separator (50) can be prevented from flowing out into the first oil return pipe (64), thereby preventing a decrease in the amount of liquid refrigerant sent to the evaporator (83, 22).

[0077] (6-2-3) The oil return channel (OP) (strictly speaking, the first oil return pipe (64)) is connected between the evaporator (83, 22) and the accumulator (26) in the refrigerant circuit (R). Therefore, when liquid refrigerant in the gas-liquid separator (50) flows into the oil return channel (OP), the liquid refrigerant can be stored in the accumulator (26). As a result, the liquid refrigerant can be prevented from being sucked into the compression element (30), and so-called liquid compression can be avoided.

[0078] (6-2-4) The refrigerant is carbon dioxide and polyalkylene glycol. In these combinations of refrigerant and oil, the oil is easily separated from the refrigerant, particularly when the refrigerant is brought to a subcritical state in the gas-liquid separator (50). This improves the oil separation rate in the gas-liquid separator (50).

[0079] (7) Modifications The above-described embodiment may have the following modified configurations. The following describes the differences from the above-described embodiment.

[0080] (7-1) Variant Example 1 As shown in FIGS. 4 and 5, the oil return passage (OP) of Variant Example 1 has the same first oil return pipe (64) as in the embodiment and a second oil return pipe (65). The first oil return pipe (64) and the second oil return pipe (65) are pipes for returning the oil separated in the gas-liquid separator (50) to the compression element (30). The second oil return pipe (65) penetrates, for example, the body portion (50a) of the gas-liquid separator (50). The second oil return pipe (65) has a second oil inlet (O5) as an oil return port into which the oil in the gas-liquid separator (50) flows. The second oil inlet (O5) opens toward the internal space (S). The second oil inlet (O5) faces downward. The outflow end of the second oil return pipe (65) is connected to the suction pipe (40). Specifically, the outflow end of the second oil return pipe (65) is connected between the evaporators (83, 22) and the accumulator (26) in the refrigerant circuit (R).

[0081] Let the height position of the second oil inlet (O5) of the second oil return pipe (65) be h5. In Variant Example 1, the relationship h4 < h5 < h2 < h1 < h3 is satisfied. The second oil inlet (O5) is at a higher position than the first oil inlet (O4). The second oil inlet (O5) is at a lower position than the liquid inlet (O2). A third flow rate control valve (73) is provided in the second oil return pipe (65). The second flow rate control valve (72) is, for example, an electronic expansion valve whose opening degree is variable.

[0082] In the gas-liquid separator (50), the densities of the oil and the refrigerant change according to the temperature of the refrigerant or oil inside. For example, when the temperature of the refrigerant or oil is 20°C, in the internal space (S), the oil, liquid refrigerant, and gas refrigerant accumulate in this order from bottom to top. On the other hand, when the temperature of the refrigerant or oil is extremely low (e.g., -20°C), the density of the liquid refrigerant may become greater than the density of the oil. In this case, in the internal space (S), the liquid refrigerant, oil, and gas refrigerant accumulate in this order from bottom to top. Therefore, in the internal space (S), the oil layer (51) and the liquid layer (52) may reverse vertically.

[0083] In the first modification, the second oil return pipe (65) is connected to the gas-liquid separator (50), and the second oil inlet (O5) is located higher than the first oil inlet (O4). Therefore, when the oil layer (51) and the liquid layer (52) are upside down, the oil in the oil layer (51) can be returned to the compression element (30) through the second oil return pipe (65).

[0084] Specifically, when the first condition is satisfied, the control unit (C) of the first modification closes the second flow rate control valve (72), which is an on-off valve, and opens the third flow rate control valve (73), which is an on-off valve. As a result, oil in the oil layer (51) above the liquid layer (52) can be returned to the compression element (30) through the second oil return pipe (65). On the other hand, closing the second flow rate control valve (72) can prevent the liquid refrigerant in the liquid layer (52) below the oil layer (51) from flowing through the second oil return pipe (65).

[0085] The first condition is a predetermined condition under which there is a possibility of inversion of the oil layer (51) and the liquid layer (52). The first condition is a condition indicating that the temperature of the refrigerant in the gas-liquid separator (50) is lower than a predetermined temperature. Specifically, the first condition includes a condition in which the outside air temperature is lower than a predetermined temperature at the timing when the air conditioner (10) starts operating.

[0086] The outlet end of the second oil return pipe (65) of the first modification may be connected to a portion of the first oil return pipe (64) downstream of the second flow rate control valve (72). In this configuration, part of the piping of both pipes can be shared.

[0087] In the first modification, the second oil inlet (O5) may be located at a higher position than the liquid inlet (O2).

[0088] (7-2) Modification 2 In Modification 2 shown in Fig. 6, the gas introduction pipe (63) also serves as the second oil return pipe (65) of Modification 1. The gas introduction pipe (63) has a first piping section (63a), a second piping section (63b), and a third piping section (63c).

[0089] The first pipe section (63a) penetrates through the top (50c) of the gas-liquid separator (50). The first pipe section (63a) is a vertical pipe extending in the vertical direction. The first pipe section (63a) extends to near the bottom (50b) of the gas-liquid separator (50). One end of the second pipe section (63b) is continuous with the lower end of the first pipe section (63a). The second pipe section (63b) is formed in a U shape. The third pipe section (63c) is continuous with the other end of the second pipe section (63b). The third pipe section (63c) is a vertical pipe extending in the vertical direction. The third pipe section (63c) extends from near the bottom (50b) of the gas-liquid separator (50) to near the top (50c) of the gas-liquid separator (50). A gas inlet (O3) is formed at the end of the third pipe section (63c), specifically, at the upper end. The gas inlet (O3) opens toward the internal space (S). The gas inlet (O3) faces upward.

[0090] A second oil inlet (O5) is formed in the peripheral wall (63d) of the gas introduction pipe (63). The second oil inlet (O5) has the same function as the second oil inlet (O5) in the first modification. The second oil inlet (O5) is located on the downstream side of the gas inlet (O3) in the gas introduction pipe (63). In other words, the second oil inlet (O5) is formed in the middle of the gas introduction pipe (63) inside the gas-liquid separator (50). The second oil inlet (O5) communicates the internal space (S) of the gas-liquid separator (50) with the inside of the gas introduction pipe (63).

[0091] When the height position of the second oil inlet (O5) is h5, in the second modification as well, the relationship h4 < h5 < h2 < h1 < h3 is satisfied. Here, strictly speaking, the height position h5 of the second oil inlet (O5) is the height position of the lower end of the second oil inlet (O5).

[0092] In the second modification, too, when the oil layer (51) and the liquid layer (52) are upside down, the oil in the oil layer (51) can be made to flow into the second oil inlet (O5). In this case, the oil that has flowed into the second oil inlet (O5) returns to the compression element (30) through the gas inlet pipe (63). As described above, in the second modification, the gas inlet pipe (63) constitutes the second oil return pipe (65), thereby reducing the number of parts. When the first condition is satisfied, the control unit (C) in the second modification closes the second flow rate control valve (72), which is an on-off valve, and opens the first flow rate control valve (71), which is an on-off valve.

[0093] (7-3) Modification 3 The oil return flow path (OP) of Modification 3 shown in Fig. 7 has a low-stage oil return pipe (66) and a high-stage oil return pipe (67). The low-stage oil return pipe (66) returns oil separated in the gas-liquid separator (50) to the low-stage compressor (31). The high-stage oil return pipe (67) returns oil separated in the gas-liquid separator (50) to the high-stage compressor (32).

[0094] An inlet end of the low-stage oil return pipe (66) is connected to the gas-liquid separator (50). An outlet end of the low-stage oil return pipe (66) is connected to the suction pipe (40) on the suction side of the low-stage compressor (31). An inlet end of the high-stage oil return pipe (67) is connected to the gas-liquid separator (50). An outlet end of the high-stage oil return pipe (67) is connected to the intermediate pipe (42) on the suction side of the high-stage compressor (32). The oil inlet of the low-stage oil return pipe (66) and the oil inlet of the high-stage oil return pipe (67) may be at the same height position, or may be at different height positions, similar to, for example, the first oil return pipe (64) and the second oil return pipe (65) of Modification 1.

[0095] A fourth flow control valve (74) is provided in the low-stage oil return pipe (66), and a fifth flow control valve (75) is provided in the high-stage oil return pipe (67). The fourth flow control valve (74) and the fifth flow control valve (75) are, for example, electronic expansion valves whose opening degrees are variable.

[0096] When the control unit (C) opens the fourth flow control valve (74) to a predetermined opening, the oil in the gas-liquid separator (50) is returned to the low-stage compressor (31) through the low-stage oil return pipe (66). When the control unit (C) opens the fifth flow control valve (75) to a predetermined opening, the oil in the gas-liquid separator (50) is returned to the high-stage compressor (32) through the high-stage oil return pipe (67). When the control unit (C) opens both the fourth flow control valve (74) and the fifth flow control valve (75) to a predetermined opening, the oil in the gas-liquid separator (50) is returned to both the low-stage compressor (31) and the high-stage compressor (32).

[0097] In variant 3, the oil return flow path (OP) may have one main return pipe connected to the gas-liquid separator (50), and a low-stage oil return pipe (66) and a high-stage oil return pipe (67) that branch off from the outlet end of the main return pipe.

[0098] (7-4) Modification 4 As shown in FIG. 8 , the refrigerant circuit (R) of Modification 4 has an oil separator (90). The oil separator (90) is provided in the discharge pipe (41). The oil separator (90) is formed of a sealed container. The oil separator (90) separates oil from the refrigerant compressed by the compression element (30). In this example, the compression element (30) has a low-stage compressor (31) and a high-stage compressor (32) connected in series. Strictly speaking, the oil separator (90) is provided on the discharge side of the high-stage compressor (32) in the refrigerant circuit (R).

[0099] The refrigerant circuit (R) of the fourth modification has a discharge oil return channel (91). One end (inlet end) of the discharge oil return channel (91) is connected to the oil separator (90), and the other end (outlet end) of the discharge oil return channel (91) is connected to the suction pipe (40). In this example, the other end of the discharge oil return channel (91) is connected to the downstream suction pipe (40b) downstream of the accumulator (26). The other end of the discharge oil return channel (91) may be connected to the upstream suction pipe (40a) upstream of the accumulator (26). In the refrigerant circuit (R) of the fourth modification, the accumulator (26) may be omitted. A discharge oil return valve (92) is provided in the discharge oil return channel (91). In this example, the discharge oil return valve (92) is a flow rate control valve, but it may also be an on-off valve.

[0100] In the fourth modification, the discharge-side oil return valve (92) is opened, so that the oil separated in the oil separator (90) can be returned to the compression element (30) through the suction pipe (40). Specifically, the oil separated in the oil separator (90) is returned to the low-stage compressor (31) through the suction pipe (40).

[0101] In the fourth modification, too, the refrigerant in a subcritical state can be stored in the gas-liquid separator (50), which facilitates separation of oil from the refrigerant. The oil separated in the gas-liquid separator (50) returns to the compression element (30) through the oil return path (OP). Therefore, in the refrigerant circuit (R) of the fourth modification, the gas-liquid separator (50) performs the oil return function of the oil separator (90), which allows the oil separator (90) to be made smaller, and further allows the refrigeration cycle apparatus (air conditioner (10)) to be made smaller.

[0102] In the fourth modification, the other end of the discharge oil return channel (91) may be directly connected to the compression chamber of the low-stage compressor (31). The other end of the discharge oil return channel (91) may be connected to the intermediate pipe (42) between the low-stage compressor (31) and the high-stage compressor (32), or may be directly connected to the compression chamber of the high-stage compressor (32). When returning the oil in the oil separator (90) to the intermediate pipe (42) or the compression chamber of the high-stage compressor (32), it is preferable that the control section (C) fully closes the first flow control valve (71).

[0103] In the fourth modification, two branch paths may be provided at the other end of the discharge oil return path (91), one of which is connected to the suction side or compression chamber of the low-pressure compressor (31) and the other to the suction side or compression chamber of the high-pressure compressor (32). In this case, it is preferable to provide a discharge oil return valve in each branch path. In this configuration, the oil separated in the oil separator (90) can be selectively returned to the low-pressure compressor (31) or the high-pressure compressor (32).

[0104] (8) Other Embodiments As shown in Fig. 9, the compression element (30) may be configured with one single-stage compressor (33). The air conditioner (10) of this example operates in a single-stage compression / two-stage expansion refrigeration cycle. Oil in an oil return pipe (e.g., the first oil return pipe (64)) is sent to the suction side of the single-stage compressor (33).

[0105] The compression element (30) may be configured as a single two-stage compression compressor having a low-stage compression mechanism serving as a low-stage compression section and a high-stage compression mechanism serving as a high-stage compression section.

[0106] The oil return passage (OP) may return the oil in the gas-liquid separator (50) directly to the compression chamber of the compression mechanism of the low-stage compressor (31), directly to the compression chamber of the compression mechanism of the high-stage compressor (32), or directly to the compression chamber of the compression mechanism of the single-stage compressor (33).

[0107] The oil return channel (OP) may return the oil in the gas-liquid separator (50) to the downstream side of the accumulator (26) in the suction pipe (40). The oil return channel (OP) may return the oil in the gas-liquid separator (50) directly to the accumulator (26). In other words, the outlet end of the oil return channel (OP) may be directly connected to the accumulator (26).

[0108] The inlet pipe (61), the liquid outlet pipe (62), the gas introduction pipe (63), the first oil return pipe (64), the second oil return pipe (65), the low-stage oil return pipe (66), and the high-stage oil return pipe (67) do not necessarily have to pass through the gas-liquid separator (50). In this case, the openings (O1 to O5) at the ends of these pipes are formed in the gas-liquid separator (50).

[0109] The refrigerant circuit (R) may be configured so as to omit the gas introduction pipe (63).

[0110] In the above-described embodiment, the outlet end of the gas introduction pipe (63) is connected to the intermediate pipe (42). However, the outlet end of the gas introduction pipe (63) may be connected to the oil return channel (OP). Specifically, the outlet end of the gas introduction pipe (63) may be connected to the first oil return pipe (64) downstream of the second flow rate control valve (72) or the second oil return pipe (65) downstream of the third flow rate control valve (73). Furthermore, the outlet end of the gas introduction pipe (63) may be connected to the suction pipe (40). In this case, the outlet end of the gas introduction pipe (63) may be connected to the upstream suction pipe (40a) or the downstream suction pipe (40b).

[0111] The refrigerant circuit (R) may have an oil separator on the discharge side of the compression element (30). In this case, the gas-liquid separator (50) also functions as an oil separator, so that the oil separator on the discharge side of the compression element (30) can be made smaller, and the refrigeration cycle apparatus (air conditioner (10)) can be made smaller.

[0112] The refrigerant may be a refrigerant other than carbon dioxide, for example, a natural refrigerant.

[0113] The oil contained in the refrigerant may be POG (polyol ester) or PVE (polyvinyl ether).

[0114] The outdoor equipment of the refrigerant circuit (R) may include, for example, four on-off valves, two three-way valves, or one four-way valve instead of the bridge circuit (B). The air conditioner (10) may not have a four-way selector valve (25) and may perform only cooling operation or only heating operation. In this case, the bridge circuit (B) is omitted.

[0115] The refrigerant circuit (R) may have an electromagnetic on-off valve or a capillary tube instead of each flow rate control valve provided in the oil return pipe.

[0116] The refrigeration cycle device may be a cooling device that cools the interior of a container or trailer, a heat pump unit that generates cold water or hot water, or a water heater that stores the generated hot water in a tank.

[0117] Although the embodiments and modifications have been described above, it will be understood that various modifications in form and details are possible without departing from the spirit and scope of the claims. Furthermore, the above embodiments, modifications, and other embodiments may be combined or substituted as appropriate as long as the functionality of the subject matter of the present disclosure is not impaired.

[0118] The above-mentioned descriptions such as "first," "second," "third," etc. are used to distinguish the words to which these descriptions are attached, and do not limit the number or order of the words.

[0119] INDUSTRIAL APPLICABILITY As described above, the present disclosure is useful for refrigeration cycle devices.

[0120] 10 Refrigeration cycle device (air conditioner) 22, 83 Radiator 23 First pressure reducing valve 24, 83 Second pressure reducing valve 26 Accumulator 30 Compression element 31 Low stage compressor (low stage compression section) 32 High stage compressor (high stage compression section) 50 Gas-liquid separator 61 Inlet pipe 62 Liquid outlet pipe 63 Gas introduction pipe 83, 22 Evaporator C Control section O2 Liquid inlet O3 Gas inlet O4 First oil inlet O4, O5 Oil inlet O5 Second oil inlet R Refrigerant circuit

Claims

1. A refrigeration cycle device comprising: a refrigerant circuit (R) having a compression element (30), a radiator (22, 83), a first pressure reducing valve (23), a gas-liquid separator (50), a second pressure reducing valve (24, 83), and an evaporator (83, 22); and a control unit (C) that controls the refrigerant circuit (R) so that the pressure of the refrigerant compressed by the compression element (30) is equal to or greater than a critical pressure and the pressure of the refrigerant reduced by the first pressure reducing valve (23) is lower than the critical pressure, wherein the refrigerant circuit (R) includes: an inlet pipe (61) for feeding the refrigerant reduced in pressure by the first pressure reducing valve (23) into the gas-liquid separator (50); a liquid outlet pipe (62) for feeding liquid refrigerant separated in the gas-liquid separator (50) to the second pressure reducing valve (24, 83); and an oil return path (OP) for returning oil separated in the gas-liquid separator (50) to the compression element (30).

2. The refrigeration cycle device according to claim 1, wherein the refrigerant circuit (R) has a gas introduction pipe (63) for feeding gas refrigerant separated in the gas-liquid separator (50) to the compression element (30).

3. The refrigeration cycle apparatus according to claim 2, wherein the oil return passage (OP) has oil inlet ports (O4, O5) through which oil in the gas-liquid separator (50) flows, the liquid outlet pipe (62) has a liquid inlet port (O2) through which liquid refrigerant in the gas-liquid separator (50) flows, and the oil inlet ports (O4, O5) are located lower than the liquid inlet port (O2).

4. The refrigeration cycle device according to claim 3, wherein the oil return flow path (OP) has a first oil return pipe (64) and a second oil return pipe (65), the first oil return pipe (64) has a first oil inlet (O4) as the oil inlet, the second oil return pipe (65) has a second oil inlet (O5) as the oil inlet, and the second oil inlet (O5) is located at a higher position than the first oil inlet (O4).

5. The refrigeration cycle apparatus according to claim 4, wherein the second oil return pipe (65) is the gas introduction pipe (63), a gas inlet (O3) is formed at an end of the gas introduction pipe (63), and the second oil inlet (O5) is located downstream of the gas inlet (O3) in the second oil return pipe (65), and connects the inside of the gas introduction pipe (63) with the inside of the gas-liquid separator (50).

6. The refrigeration cycle device according to any one of claims 1 to 5, wherein the refrigerant circuit (R) has an accumulator (26) arranged between the evaporator (83, 22) and the compression element (30), and the oil return passage (OP) is connected to the refrigerant circuit (R) between the evaporator (83, 22) and the accumulator (26) or to the accumulator (26).

7. A refrigeration cycle device according to any one of claims 1 to 6, wherein the compression element (30) includes a low-stage compression section (31) that compresses a refrigerant, and a high-stage compression section (32) that compresses the refrigerant compressed in the low-stage compression section (31), and the oil return flow path (OP) includes a low-stage oil return pipe (66) that returns oil separated in the gas-liquid separator (50) to the low-stage compression section (31), and a high-stage oil return pipe (67) that returns oil separated in the gas-liquid separator (50) to the high-stage compression section (32).

8. The refrigeration cycle device according to any one of claims 1 to 7, wherein the refrigerant is carbon dioxide and the oil is polyalkylene glycol.

Images