JP2024022593A - refrigeration system - Google Patents

Abstract

[Problem] In a refrigeration system compatible with ultra-low temperature applications, a decrease in compressor efficiency is suppressed even when a refrigerant with a low environmental load is used. SOLUTION: A refrigeration system 1 includes a refrigerant circuit 2 having a low-stage multi-stage compressor 11, a high-stage multi-stage compressor 12, an oil separator 13, a condenser 14, and an evaporator 18. The refrigerant has a GWP of less than 750 and a saturation pressure at -80°C of 0.017 MPaA or more. The low-stage multi-stage compressor 11 is driven by a low-stage motor 11c, and the high-stage multi-stage compressor 12 is driven by another high-stage motor 12c. The refrigerant circuit 2 includes a first return line 41 that returns a part of the refrigerant between the condenser 14 and the evaporator 18 to an intermediate stage of the low-stage multi-stage compressor 11, and a first return line 41 that returns a part of the refrigerant between the condenser 14 and the evaporator 18 to an intermediate stage of the low-stage multi-stage compressor 11; A second return line 42 returns part of the refrigerant between the low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12, and a part of the refrigerant between the condenser 14 and the evaporator 18 is returned to the high stage. It has a third return line 43 that returns to the intermediate stage of the side multi-stage compressor 12. [Selection diagram] Figure 1

Description

TECHNICAL FIELD The present invention relates to refrigeration systems.

In this document, the symbol represented by "R" and the numbers and subscripts that follow it are refrigerant numbers defined in ISO817. The term "ultra-low temperature" is intended to refer to temperatures of, for example, -70°C to -90°C.

Patent Document 1 discloses a three-way refrigeration system that uses R23 as a refrigerant for medium-temperature side and low-temperature side refrigerators. When R23 is used as a refrigerant, an extremely low temperature of -80°C or lower can be obtained, for example.

International Publication No. 2020/095381

R23 has an extremely high GWP (Global Warming Potential) and will no longer be usable in the future. R32 (CH2F2) is attracting attention as an example of a refrigerant that can replace R23. While R32 has a low GWP, at extremely low temperatures such as -85°C, the suction pressure of the compressor becomes extremely low, making the compressor less efficient.

An object of the present invention is to suppress a decrease in the efficiency of a compressor in a refrigeration system for ultra-low temperature applications even when a refrigerant with a small environmental load is used.

One aspect of the present invention includes a refrigerant circuit that includes a low-stage multi-stage compressor, a high-stage multi-stage compressor, an oil separator, a condenser, and an evaporator, and circulates a refrigerant in this order, and the refrigerant is , the GWP is less than 750, and the saturation pressure at -80° C. is 0.017 MPaA or more, the low-stage multi-stage compressor and the high-stage multi-stage compressor are each driven by separate motors, and the refrigerant A circuit includes a first return line returning a portion of the refrigerant between the condenser and the evaporator to an intermediate stage of the lower stage multi-stage compressor, and a first return line between the condenser and the evaporator. a second return line that returns a portion of the refrigerant between the low-stage multi-stage compressor and the high-stage multi-stage compressor; and a second return line that returns a portion of the refrigerant between the condenser and the evaporator. A refrigeration system is provided, further comprising a third return line returning to an intermediate stage of the stage-side multi-stage compressor.

According to the above configuration, in the refrigerant circuit, the low-stage multi-stage compressor and the high-stage multi-stage compressor are connected in series. The refrigerant is circulated from the evaporator to the low-stage multi-stage compressor, passes through the low-stage multi-stage compressor and the high-stage multi-stage compressor in this order, and is sequentially compressed in the process. The combination of a low-stage multi-stage compressor and a high-stage multi-stage compressor includes at least four stages of compressors, and as a result, the compression ratio (ratio of suction pressure to discharge pressure, also known as "pressure ratio") of each compressor is ), it is possible to obtain high pressure and ultimately ultra-low temperature, and prevent a decrease in compressor efficiency.

When multiple compressors are connected in series, the temperature of the refrigerant increases with each compressor. According to the above configuration, a part of the refrigerant flowing from the condenser to the evaporator is returned to three different locations in the combination of the low-stage multi-stage compressor and the high-stage multi-stage compressor via the three return lines. The refrigerant is sequentially compressed while the discharge temperature of the compressor is lowered as needed. It is possible to prevent the refrigerant from becoming extremely hot on the downstream side of the high-stage multi-stage compressor. The high-stage multi-stage compressor can operate stably and compress the refrigerant to the pressure required to obtain ultra-low temperatures. In addition, deterioration of the refrigerant due to an increase in the temperature of the refrigerant can be prevented.

The refrigerant may be R32.

According to the above configuration, since the discharge temperature of the compressor can be lowered, an extremely low temperature can be obtained even if a refrigerant having a high specific heat ratio such as R32 is used. Since the GWP of R32 is less than 750, the environmental load of the refrigeration system can be reduced.

The refrigerant circuit includes a fourth return line that returns a portion of the refrigerant between the condenser and the evaporator between the low-stage multi-stage compressor and the high-stage multi-stage compressor; interposed between the refrigerant and the evaporator and on the fourth return line, between the refrigerant flowing from the condenser to the evaporator and the refrigerant flowing through the fourth return line. The refrigerant may further include a first economizer that performs heat exchange, and a first economizer expansion mechanism that reduces the pressure of the refrigerant supplied to the first economizer via the fourth return line.

According to the above configuration, a part of the refrigerant between the condenser and the evaporator is cooled by the economizer expansion mechanism, the economizer cools the refrigerant supplied from the condenser to the evaporator, and the low-stage multi-stage compression Return between the compressor and the high-stage multi-stage compressor. The refrigerant supplied from the condenser to the evaporator can be further cooled, improving the refrigeration capacity (coefficient of performance) of the refrigeration system. Moreover, the discharge temperature of the compressor can be further reduced.

The first return line returns the refrigerant between the first economizer and the evaporator, and the second return line and the third return line return the refrigerant between the condenser and the first economizer. Refrigerant may be returned.

According to the above configuration, the relatively low temperature refrigerant on the downstream side of the economizer is returned to the relatively upstream side in the combination of the low-stage multi-stage compressor and the high-stage multi-stage compressor. The relatively high temperature refrigerant upstream of the economizer is returned relatively downstream in the combination of the low stage multi-stage compressor and the high stage multi-stage compressor. Therefore, the discharge temperature of the compressor can be effectively cooled.

The refrigerant circuit includes a fifth return line that returns a portion of the refrigerant between the condenser and the evaporator to the intermediate stage of the low-stage multi-stage compressor, the first economizer and the evaporator. heat exchange between the refrigerant flowing from the first economizer to the evaporator and the refrigerant flowing through the fifth return line. The refrigerant may further include a second economizer and a second economizer expansion mechanism that reduces the pressure of the refrigerant supplied to the second economizer via the fifth return line.

According to the above configuration, since two stages of economizers are provided, the refrigeration capacity of the refrigeration system can be further improved, and the discharge temperature of the compressor can be further reduced.

The refrigeration system includes an oil return line that returns at least the oil recovered by the oil separator to the low-stage multi-stage compressor, and extracts the refrigerant dissolved in the oil from the oil flowing through the oil return line. A deaerator is used to remove the refrigerant, and the refrigerant separated by the deaerator is transferred to an intermediate stage of the low-stage multi-stage compressor or between the low-stage multi-stage compressor and the high-stage multi-stage compressor. It may further include a degassed refrigerant return line that returns the degassed refrigerant to the return destination, and a pressure regulating valve that adjusts the internal pressure of the deaerator to a higher pressure than the pressure of the return destination.

Here, if the refrigerant is supplied to the compressor together with the oil while being dissolved in the oil, there is a risk that the performance of the compressor will deteriorate due to evaporation (flash) of the refrigerant. According to the above configuration, since the refrigerant is removed from the oil in the deaerator, such performance deterioration can be prevented. Since the refrigerant is returned to the refrigerant circuit after being recovered by the deaerator, the amount of refrigerant in the refrigerant circuit can be prevented from decreasing.

The evaporator may be a heat exchanger that directly cools an object to be cooled with the refrigerant.

According to the above configuration, brine is not required, and the configuration of the refrigeration system can be simplified as a whole.

The refrigerant circuit further includes a liquid receiver interposed between the condenser and the heat exchanger serving as the evaporator, and the refrigerant stored in the liquid receiver is supplied to the heat exchanger. The gaseous refrigerant after heat exchange with the object to be cooled may be circulated to the lower stage multi-stage compressor.

According to the above configuration, the liquid phase refrigerant stored in the liquid receiver functions similarly to brine, and the brine itself becomes unnecessary.

The refrigeration system includes an intermediate pressure sensor that detects an intermediate pressure that is a pressure between the low-stage multi-stage compressor and the high-stage multi-stage compressor, and according to the intermediate pressure detected by the intermediate pressure sensor. The compressor may further include a control device that controls the rotation speed of the high-stage multi-stage compressor.

According to the above configuration, the compression ratios of the low-stage multi-stage compressor and the high-stage multi-stage compressor can be controlled so as to be balanced, and the power consumption of each multi-stage compressor can be optimized. In other words, the performance of the refrigeration system is improved.

The low stage multi-stage compressor includes a suction side bearing that rotatably supports a suction side end of a rotating shaft of the first stage compression section, and a suction side bearing chamber that accommodates the suction side bearing. The suction side bearing chamber is provided with a wall portion disposed on the opposite side of the first stage compression section with respect to the suction side bearing, and an oil is disposed between the suction side bearing and the wall portion. may be accumulated. Further, an oil supply line for guiding oil to the suction side bearing of the low stage side multi-stage compressor may not be provided.

According to the above configuration, the suction side bearing of the first stage compression part can be sufficiently lubricated with the oil stored between the suction side bearing of the first stage compression part and the wall part.

The low stage multi-stage compressor includes a suction side bearing that rotatably supports a suction side end of a rotating shaft of the first stage compression section, and a suction side bearing chamber that accommodates the suction side bearing. , an oil supply line is provided for guiding the oil of the oil separator to the suction side bearing; the oil supply line is provided with an opening/closing valve that is controlled to open and close by a control device; The opening and closing of the on-off valve may be controlled so that when the side multi-stage compressor is driven, the on-off valve is closed, and when the low-stage multi-stage compressor is stopped, the on-off valve is opened.

According to the above configuration, since oil is not directly supplied to the first suction side bearing chamber while the low stage side multistage compressor is being driven, it is possible to prevent a large amount of refrigerant gas from being generated in the first suction side bearing chamber. can. Furthermore, while the low-stage multi-stage compressor is stopped, oil is directly supplied to the first suction-side bearing, so the differential pressure is small, so refrigerant gas is less likely to be generated from the supplied oil. Further, since oil is directly supplied to the first suction side bearing, the first suction side bearing can be reliably lubricated, and a sufficient amount of oil can be stored near the first suction side bearing.

According to the present invention, in a refrigeration system compatible with ultra-low temperature applications, even if a refrigerant with a small environmental load is used, a decrease in compressor efficiency can be suppressed.

FIG. 1 is a circuit diagram of a refrigeration system according to a first embodiment. A schematic configuration diagram of a low-stage multi-stage compressor. A schematic diagram of a first suction side bearing and a wall portion. 5 is a flowchart of control executed by the refrigeration system according to the first embodiment. FIG. 2 is a Mollier diagram of the refrigeration system according to the first embodiment. FIG. 3 is a circuit diagram of a refrigeration system according to a second embodiment. FIG. 3 is a circuit diagram of a refrigeration system according to a third embodiment. FIG. 3 is a circuit diagram of a refrigeration system according to a fourth embodiment. FIG. 3 is a circuit diagram of a refrigeration system according to a fifth embodiment. FIG. 3 is a circuit diagram of a refrigeration system according to a sixth embodiment. FIG. 7 is a schematic configuration diagram of a low-stage multi-stage compressor of a refrigeration system according to a seventh embodiment. FIG. 3 is a circuit diagram of a refrigeration system according to a modified example.

Embodiments will be described below with reference to the drawings. Note that the same or corresponding elements are denoted by the same reference numerals throughout the drawings, and detailed explanation will not be repeated.

(First embodiment)
First, with reference to FIG. 1, the overall configuration of a refrigeration system 1 according to a first embodiment will be described.

The refrigeration system 1 is a so-called unitary type. The refrigeration system 1 includes a refrigerant circuit 2 that includes a low stage multi-stage compressor 11, a high stage multi-stage compressor 12, an oil separator 13, a condenser 14, a receiver 15, a dryer 16, an expansion valve 17, and an evaporator 18. The refrigerant circulates in this order. The refrigeration system 1 further includes an oil circulation circuit 3 that circulates oil required for stable operation of the multistage compressors 11 and 12. The oil separator 13 is a component of the refrigerant circuit 2 and is also a component of the oil circulation circuit 3. Oil is used to lubricate the multi-stage compressors 11 and 12. In this embodiment, the multistage compressors 11 and 12 are oil-cooled. Oil is also used to cool the multi-stage compressors 11 and 12.

The low-stage multi-stage compressor 11 compresses the gaseous refrigerant in stages. The high-stage multi-stage compressor 12 further compresses the gaseous refrigerant compressed by the low-stage multi-stage compressor 11 in stages. The oil separator 13 separates oil from the high-temperature, high-pressure gaseous refrigerant compressed by the high-stage multi-stage compressor 12 . The condenser 14 condenses the high-temperature, high-pressure gaseous refrigerant separated from the oil by the oil separator 13. The receiver 15 temporarily stores the medium temperature and high pressure liquid refrigerant condensed in the condenser 14, and separates the gas refrigerant and other gases from the liquid refrigerant. The dryer 16 removes moisture from the liquid refrigerant supplied from the receiver 15. The expansion valve 17 is configured to be able to adjust its opening degree, and expands the liquid refrigerant supplied from the dryer 16. The evaporator 18 evaporates the low-temperature, low-pressure liquid refrigerant expanded by the expansion valve 17 . The low-temperature, low-pressure gaseous refrigerant obtained in the evaporator 18 is returned to the low-stage multistage compressor 11.

The evaporator 18 serves as a cooler that cools the brine using the cold heat of the refrigerant. In this embodiment, the temperature of the brine can be set to an ultra-low temperature of -80°C. The brine is used to cool an object to be cooled (not shown).

The low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12 are, for example, two-stage compressors. The low-stage multi-stage compressor 11 includes a first low-stage compression section 11a as a first stage, and a second low-stage compression section 11b as a second stage (final stage). The high-stage multi-stage compressor 12 includes a first high-stage compression section 12a as a first stage, and a second high-stage compression section 12b as a second stage (final stage). In the process of passing through the first low-stage compression section 11a, the second low-stage compression section 11b, the first high-stage compression section 12a, and the second high-stage compression section 12b in this order, the refrigerant passes through each compression section 11a. , 11b, 12a, and 12b.

The low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12 are driven by separate motors 11c and 12c, respectively. More specifically, the low-stage multi-stage compressor 11 has a low-stage motor 11c that drives its compression sections 11a and 11b, and the high-stage multi-stage compressor 12 has a low-stage motor 11c that is separate from the low-stage motor 11c. It has a high stage side motor 12c that drives the compression parts 12a and 12b. The low-stage motor 11c and the high-stage motor 12c are AC motors. The low-stage motor 11c is supplied with alternating current from the low-stage inverter 11d, and rotates its motor drive shaft 11e. The high-stage motor 12c is supplied with alternating current from a high-stage inverter 12d separate from the low-stage inverter 11d, and rotates its motor drive shaft 12e. The switching operations of the low-stage inverter 11d and the high-stage inverter 12d are controlled by the control device 80. The low-stage motor 11c and the high-stage motor 12c can operate independently of each other. The rotation speeds of the low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12 are controlled independently from each other by the control device 80.

The refrigerant circuit 2 has a refrigerant circulation line 30 for circulating refrigerant. The refrigerant circulation line 30 includes a plurality of circulation lines 31, 32a, 32b, . It is composed of 32c, 33, 34, 35a, 35b, 36, and 37.

The outlet of the evaporator 18 is connected to the inlet of the first low-stage compression section 11a via a circulation line 31. The outlet of the first low-stage compression section 11a is connected to the inlet of the second low-stage compression section 11b via a circulation line 32a. The outlet of the second low stage compression section 11b is connected to the inlet of the first high stage compression section 12a via the circulation line 32b. The outlet of the first high-stage compression section 12a is connected to the inlet of the second high-stage compression section 12b via a circulation line 32c. The outlet of the second high-stage compression section 12b is connected to the inlet of the oil separator 13 via a circulation line 33.

The oil separator 13 has a vertically long casing 13a and a demister 13b housed within the casing 13a (see FIG. 2). The inlet of the oil separator 13 is opened below the demister 13b within the casing 13a. The refrigerant containing oil is separated into liquid oil and gas refrigerant by the demister 13b. The refrigerant can flow out of the casing 13a via a refrigerant outlet provided above the demister 13b. A refrigerant outlet of the oil separator 13 is connected to a refrigerant inlet of the condenser 14 via a circulation line 34 .

The condenser 14 has a refrigerant passage 14a that connects a refrigerant inlet to a refrigerant outlet, and a coolant passage 14b through which a coolant flows. In the condenser 14, the refrigerant is cooled and condensed through heat exchange with the cooling liquid while flowing through the refrigerant passage 14a.

The refrigerant flows from the condenser 14 to the evaporator 18 via circulation lines 35a, 35b, 36, 37. A refrigerant outlet of the condenser 14 is connected to an inlet of the receiver 15 via a circulation line 35a. The outlet of the receiver 15 is connected to the inlet of the dryer 16 via a circulation line 35b. The outlet of the dryer 16 is connected to the inlet of the expansion valve 17 via a circulation line 36 . The outlet of the expansion valve 17 is connected to the refrigerant inlet of the evaporator 18 via a circulation line 37 .

The evaporator 18 according to this embodiment has a refrigerant passage 18a that connects a refrigerant inlet to a refrigerant outlet, and a brine passage 18b through which brine flows. In the evaporator 18, the refrigerant is heated and evaporated by heat exchange with brine while flowing through the refrigerant passage 18a.

The refrigerant has a GWP of less than 750 and a saturation pressure at -80°C of 0.017 MPaA or more. GWP is an index that quantitatively indicates how much global warming a target gas has, based on carbon dioxide. The higher the GWP value, the higher the global warming capacity, or the greenhouse effect.

A suitable example of a refrigerant that satisfies these conditions is R32 (difluoromethane). GWP of R32 is 675. The saturation pressure of R32 at -80°C is 0.018 MPaA. Another characteristic of R32 is that it has a high specific heat ratio. Therefore, when R32 is used as a refrigerant in the refrigeration system 1, particularly in a refrigeration system 1 expected to obtain cold heat at an extremely low temperature, measures must be taken to prevent an excessive temperature rise during compression.

Therefore, the refrigerant circuit 2 supplies a portion of the refrigerant between the condenser 14 and the evaporator 18 (more specifically, between the dryer 16 and the expansion valve 17) to the first A plurality of refrigerant return lines 40 are provided to return the refrigerant to a region between the stages and the oil separator 13. The refrigerant circuit 2 according to this embodiment is provided with a first return line 41 , a second return line 42 , a third return line 43 , and a fourth return line 44 as the refrigerant return line 40 .

The first return line 41 returns a portion of the refrigerant between the condenser 14 and the evaporator 18 to the intermediate stage of the low-stage multi-stage compressor 11 . The second return line 42 returns a portion of the refrigerant between the condenser 14 and the evaporator 18 to between the low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12. The third return line 43 returns a portion of the refrigerant between the condenser 14 and the evaporator 18 to the intermediate stage of the high-stage multi-stage compressor 12 . Similarly to the second return line 42, the fourth return line 44 also transfers a part of the refrigerant between the condenser 14 and the evaporator 18 to the lower stage multi-stage compressor 11 and the higher stage multi-stage compressor 12. Return it between. However, the fourth return line 44 is independent from the second return line 42. That is, the fourth return line 44 connects the circulation line 36 to the compressor side without fluidly communicating with the second return line 42.

In this document, the "intermediate stage" of the multistage compressors 11 and 12 means a portion between the first stage outlet and the final stage inlet of the multistage compressors 11 and 12. In this embodiment, since the multistage compressors 11 and 12 are two-stage, the "intermediate stage" connects the outlet of the first stage and the inlet of the second stage in each of the multistage compressors 11 and 12. (i.e., circulation lines 32a, 32c). When the multi-stage compressors 11 and 12 are of a type with three or more stages, the "intermediate stage" is a compressor located between the first stage and the final stage, in addition to the flow path connecting adjacent compression sections. It may be the part itself or the space housing the compressed part.

Furthermore, the refrigerant circuit 2 according to this embodiment includes an economizer 21 and an expansion mechanism 22 for the economizer 21. The economizer 21 is interposed between the condenser 14 and the evaporator 18 and is also interposed in the fourth return line 44 . More specifically, economizer 21 is interposed between dryer 16 and expansion valve 17. The expansion mechanism 22 reduces the pressure of the refrigerant supplied to the economizer 21 via the fourth return line 44 . The expansion mechanism 22 is realized by a valve whose opening degree can be adjusted.

More specifically, the economizer 21 has a supply channel 21 a that constitutes a part of the circulation line 36 and a return channel 21 b that constitutes a part of the fourth return line 44 . In other words, the circulation line 36 has an upstream section 36a that connects the outlet of the dryer 16 to the inlet of the supply channel 21a, the supply channel 21a, and the outlet of the supply channel 21a to the inlet of the expansion valve 17. and a downstream portion 36b. The fourth return line 44 has an upstream section 44a branched from the downstream section 36b of the circulation line 36 and connected to the inlet of the return channel 21b, a return channel 21b, and an outlet of the return channel 21b facing the compressor side. and a connecting downstream portion 44b. The expansion mechanism 22 is interposed in the upstream portion 44a of the fourth return line 44.

The economizer 21 causes heat exchange to occur between the refrigerant flowing through the supply flow path 21a and the refrigerant flowing through the return flow path 21b. The temperature of the refrigerant flowing through the return passage 21 b is lowered by the action of the expansion mechanism 22 before being supplied to the economizer 21 . The refrigerant flowing through the supply flow path 21a is cooled by the refrigerant flowing through the return flow path 21b.

The first to fourth return lines 41 to 44 branch from the circulation line 36. Here, the term "Nth branching point" is defined as the branching point of the Nth return channel from the circulation line 36 (N: natural number). The third branch point, the second branch point, the fourth branch point, and the first branch point are arranged in this order on the circulation line 36 in the flow direction of the refrigerant from the dryer 16 to the expansion valve 17. The third branch point and the second branch point are located in the upstream section 36a. The fourth branch point and the first branch point are located at the downstream portion 36b.

The first return line 41 returns the refrigerant between the economizer 21 and the evaporator 18 . The second return line 42 returns the refrigerant between the condenser 14 and the economizer 21 . The third return line 43 returns the refrigerant from a side closer to the condenser 14 than the second return line 42 . The fourth return line 44 returns the refrigerant through the economizer 21 from the side closer to the economizer 21 than the first return line 41 . The first return line 41 is connected to the circulation line 32a, the second return line 42 and the fourth return line 44 are connected to the circulation line 32b, and the third return line 43 is connected to the circulation line 32c. The second return line 42 and the third return line 43 only need to return the refrigerant between the condenser 14 and the economizer 21, and the third return line 43 returns the refrigerant from the side closer to the economizer 21 than the second return line 42. It's okay.

The refrigerant circuit 2 has a plurality of on-off valves and check valves. The on-off valves are electromagnetic valves, and the operation of each on-off valve is controlled by a control device 80. The on-off valve 27a is provided in the circulation line 31, the on-off valve 27b is provided in the circulation line 32b, the on-off valve 27c is provided in the circulation line 33, the on-off valve 27d is provided in the circulation line 35b, and the on-off valve 27e is provided in the circulation line 33. The downstream portion 36b of 36 is interposed therebetween. An on-off valve 27f is provided in the first return line 41, an on-off valve 27g is provided in the second return line 42, an on-off valve 27h is provided in the third return line 43, and an on-off valve 27i is provided upstream of the fourth return line 44. It is interposed in the portion 44a. The on-off valve 27e is provided between the first branch point and the expansion valve 17 on the downstream portion 36b. The on-off valve 27i is provided between the fourth branch point and the expansion mechanism 22 on the upstream portion 44a. Further, the check valve 28a is provided on the circulation line 31 closer to the first low-stage compression section 11a than the on-off valve 27a. The check valve 28b is provided on the circulation line 32b closer to the first high-stage compression section 12a than the on-off valve 27b. The check valve 28c is provided on the circulation line 33 closer to the oil separator 13 than the on-off valve 27c.

The oil circulation circuit 3 further includes an oil cooler 51 and an oil return line 60 in addition to the oil separator 13 described above. In the oil separator 13, oil is stored in the inner lower part of the casing 13a. Oil can flow out of the casing 13a through an oil outlet provided at the bottom of the casing 13a.

The oil return line 60 returns the oil recovered by the oil separator 13 to the multistage compressors 11 and 12. The oil return line 60 includes a common part 61 extending from the oil outlet of the oil separator 13, and a plurality of branch parts 62a branching from the common part 61 and extending to each of the plurality of compression parts 11a, 11b, 12a, and 12b. , 62b, 63a, and 63b.

The branch portion 62a supplies oil to the first low-stage compression portion 11a. The branch portion 62b supplies oil to the second low-stage compression portion 11b. The branch portion 63a supplies oil to the first high-stage compression portion 12a. The branch part 63b supplies oil to the second high-stage compression part 12b.

Oil cooler 51 is interposed in common portion 61 . The oil is cooled by an oil cooler 51 before being supplied to each compression section 11a, 11b, 12a, 12b. Note that the oil circulation circuit 3 may include an oil pump that pumps oil from the oil separator 13 to the multistage compressors 11 and 12. The oil pump may be provided within the oil separator 13 or on the common part 61.

Next, the structure of the low-stage multi-stage compressor 11 will be described with reference to FIGS. 2 and 3. Although the case where the compressor is a screw compressor will be explained as an example, the compressor may be other types of compressors such as a rotary compressor or a reciprocating compressor.

As shown in FIG. 2, the low-stage multi-stage compressor 11 includes a casing 70. The casing 70 is composed of a plurality of casing members sequentially connected in the axial direction of the low-stage multi-stage compressor 11. The casing 70 includes a motor chamber 70a, a first suction side bearing chamber 70b, a first compression chamber 70c, a first discharge side bearing chamber 70d, an intermediate pressure chamber 70e, a second suction side bearing chamber 70f, a second compression chamber 70g, and A second discharge side bearing chamber 70h is formed. These eight chambers are arranged in this order from one side to the other side in the axial direction of the low-stage multi-stage compressor 11.

The motor chamber 70a accommodates the low-stage motor 11c. The low-stage motor 11c is, for example, an inner rotor type. When the low-stage motor 11c is energized, the motor drive shaft 11e is rotationally driven.

The first low-stage compression section 11a is configured by a pair of male and female first screw rotors 71A, 71B, a first compression chamber 70c, and a portion of the casing 70 that defines the first compression chamber 70c. The first compression chamber 70c accommodates first screw rotors 71A and 71B that are meshed with each other. The first screw rotor 71A may be of a male type or a female type. The first screw rotor 71A is integrally provided with a first rotating shaft 73A, and the first screw rotor 71B is integrally provided with a first rotating shaft 73B.

The second low-stage compression section 11b is constituted by a pair of male and female second screw rotors 72A, 72B, a second compression chamber 70g, and a portion of the casing 70 that defines the chamber 70g. The second compression chamber 70g accommodates second screw rotors 72A and 72B that are meshed with each other. The second screw rotor 72A may be of a male type or a female type. The second screw rotor 72A is integrally provided with a second rotating shaft 74A, and the second screw rotor 72B is integrally provided with a second rotating shaft 74B.

The motor drive shaft 11e protrudes toward the other side in the axial direction with respect to the stator and rotor of the high-stage motor 11c. The first rotating shafts 73A, 73B protrude on both sides in the axial direction with respect to the corresponding first screw rotors 71A, 71B. The second rotating shafts 74A, 74B protrude on both sides in the axial direction with respect to the corresponding second screw rotors 72A, 72B. The motor drive shaft 11e, the first rotating shaft 73A, and the second rotating shaft 74A are coaxial with each other. The other end of the motor drive shaft 11e is connected to one end of the first rotating shaft 73A, and the other end of the first rotating shaft 73A is connected to one end of the second rotating shaft 74A. The rotation axes 73B and 74B are parallel to the rotation axes 73A and 74A, and are separated from each other in the axial direction.

The first suction side bearing chamber 70b accommodates first suction side bearings 75A and 75B that rotatably support one end of the first rotating shafts 73A and 73B, respectively. The first discharge side bearing chamber 70d accommodates first discharge side bearings 76A and 76B that rotatably support the other end portions of the first rotating shafts 73A and 73B, respectively. The second suction side bearing chamber 70f accommodates second suction side bearings 77A and 77B that rotatably support one end of the second rotating shafts 74A and 74B, respectively. The second discharge side bearing chamber 70h accommodates second discharge side bearings 78A and 78B that rotatably support the other ends of the second rotation shafts 74A and 74B, respectively.

The casing 70 has a suction port 70i that communicates with the first compression chamber 70c. The suction port 70i is connected to the evaporator 18 via the circulation line 31. When the motor drive shaft 11e rotates, the first rotation shaft 73A and the second rotation shaft 74A rotate together. As a result, the first screw rotors 71A, 71B rotate, and the second screw rotors 72A, 72B rotate. The refrigerant from the evaporator 18 is sucked into the space formed by the first screw rotors 71A and 71B in cooperation with the inner surface of the casing 70 through the suction port 70i, and is compressed by reducing the volume of the space. It is discharged from. The discharged refrigerant is sucked into the second compression chamber 70g via the intermediate pressure chamber 70e. The intermediate pressure chamber 70e corresponds to the circulation line 32a described above. The refrigerant from the intermediate pressure chamber 70e is sucked into the space formed by the second screw rotors 72A, 72B in cooperation with the inner surface of the casing 70, compressed by the reduction in the volume of the space, and discharged from the space. The discharged refrigerant is discharged to the outside of the casing 70 via a discharge port (not shown) communicating with the second compression chamber 70g. The discharge port is connected to the high-stage multi-stage compressor 12 via the circulation line 32b.

The first return line 41 is connected to the intermediate pressure chamber 70e. The refrigerant is returned to the intermediate pressure chamber 70e, ie, the circulation line 32a. Thereby, the refrigerant discharged from the first low stage compression section 11a is cooled within the intermediate pressure chamber 70e before being supplied to the second low stage compression section 11b.

The branch portion 62a described above supplies oil to the first compression chamber 70c and the first discharge side bearing chamber 70d. Thereby, the first screw rotors 71A, 71B and the first discharge side bearings 76A, 76B are lubricated and cooled. The aforementioned branch part 62b supplies oil to the second suction side bearing chamber 70f, the second compression chamber 70g, and the second discharge side bearing chamber 70h. Thereby, the second screw rotors 72A, 72B, the second suction side bearings 77A, 77B, and the second discharge side bearings 78A, 78B are lubricated and cooled. The supplied oil is discharged from the casing 70 through the discharge port together with the refrigerant, and is supplied to the high-stage multi-stage compressor 12.

As shown in FIGS. 2 and 3, the first suction side bearing chamber 70b is provided with wall portions 79A and 79B. The wall portions 79A, 79B are arranged on the opposite side (one side in the axial direction) from the first low stage compression portion 11a with respect to the first suction side bearings 75A, 75B. Further, the wall portions 79A, 79B have a predetermined distance from the first suction side bearings 75A, 75B. The wall portions 79A, 79B face the lower portions of the first suction side bearings 75A, 75B. For example, a virtual plane that includes the upper end of the wall portion 79A and is parallel to the axial direction of the first rotating shaft 73A is at the same height as or lower than the lowest end of the inner ring transfer surface of the first suction side bearing 75A. . A virtual plane parallel to the axial direction of the first rotating shaft 73B including the upper end of the wall portion 79B is at the same height as or lower than the lowest end of the inner ring transfer surface of the first suction side bearing 75B. Note that the shape of the wall portions 79A and 79B is not limited to the rectangular shape shown in the illustrated example, but may be an arcuate plate shape. The upper sides of the wall portions 79A, 79b are not limited to the right circles shown in the illustrated example, but may be arcuate.

By providing such wall portions 79A, 79B, oil can be stored between the first suction side bearings 75A, 75B and the wall portions 79A, 79B. The oil sufficiently lubricates the first suction side bearings 75A, 75B. If a line is provided to directly supply the oil from the oil separator 13 to the first suction side bearings 75A, 75B, since the refrigerant is dissolved in the oil, the oil will not be supplied to the first suction side bearing chamber 70b. A large amount of refrigerant is generated from the oil due to the pressure difference, and the lubrication efficiency of the first suction side bearing chamber 70b deteriorates. On the other hand, in this embodiment, since no line is provided for directly supplying oil to the first suction side bearings 75A, 75B, occurrence of such a problem can be prevented.

Note that the high-stage multi-stage compressor 12 has almost the same structure as the low-stage multi-stage compressor 11 shown in FIGS. 2 and 3 except that it does not include the wall portions 79A and 79B. Duplicate explanations will be omitted.

In the high-stage multi-stage compressor 12, the motor chamber accommodates the high-stage motor 12c. The first compression chamber constitutes the first high-low stage compression section 12a. The second compression chamber constitutes the second high-stage compression section 12b. The suction port is connected to the low-stage multi-stage compressor 11 via the circulation line 32b, and corresponds to the inlet of the first high-stage compression section 12a. The intermediate pressure chamber corresponds to the circulation line 32c and is connected to the third return line 43. A discharge port is connected to the oil separator 13 via a circulation line 33. The branch portion 63a supplies oil to the first compression chamber and the first discharge side bearing chamber. The branch portion 63b supplies oil to the second suction side bearing chamber, the second compression chamber, and the second discharge side bearing chamber.

Returning to FIG. 1, the refrigeration system 1 includes a control device 80 for arithmetic processing and control of the entire device. The control device 80 includes, for example, a CPU (Central Processing Unit) or an MPU (Micro Processing Unit) that cooperates with software to realize predetermined functions. The control device 80 may be configured with a hardware circuit such as a dedicated electronic circuit or a reconfigurable electronic circuit designed to realize a predetermined function, or may be configured with various semiconductor integrated circuits. good. Examples of various semiconductor integrated circuits include, in addition to CPUs and MPUs, microcomputers, DSPs (Digital Signal Processors), FPGAs (Field Programmable Gate Arrays), and ASICs (Application Specific Integrated Circuits). Further, the control device 80 may include a storage device such as a RAM (Random Access Memory) and a ROM (Read Only Memory). Specifically, the control device 80 may be configured with, for example, an information processing device such as a desktop computer, a notebook computer, a workstation, or a tablet terminal, or a printed circuit board having equivalent functions.

The refrigeration system 1 is connected to the control device 80 and includes a plurality of sensors that detect the state of the refrigeration system 1. The control device 80 controls the operations of the low-stage inverter 11d, the high-stage inverter 12d, the expansion valve 17, the expansion mechanism 22, and the on-off valves 27a to 27i based on the detection results by the sensors. The on-off valves 27a to 27i are typically open while the refrigeration system 1 is operating and the evaporator 18 is functioning as a cooler.

Although detailed illustrations are omitted, the sensors connected to the control device 80 include a temperature sensor that detects the temperature of the circulation line 31 (that is, the temperature of the refrigerant discharged from the evaporator 18) and a temperature sensor that detects the temperature of the refrigerant discharged from the evaporator 18, and a temperature sensor that detects the temperature of the refrigerant discharged from the evaporator 18. A temperature sensor may be included that detects the temperature of the downstream portion 44b (that is, the temperature of the refrigerant returned from the economizer 21 to the compressor side).

The control device 80 controls the operation of the expansion valve 17 and the expansion mechanism 22 based on the temperature detected by the temperature sensor described above. For example, the control device 80 adjusts the opening degree of the expansion valve 17 based on the temperature detected by a temperature sensor provided in the circulation line 31 so that the degree of superheat of the refrigerant in the circulation line 31 is constant. Further, the control device 80 controls the expansion mechanism 22 as an expansion valve so that the degree of superheat of the refrigerant in the fourth return line 44 is constant based on the temperature detected by the temperature sensor provided in the fourth return line 44. Adjust the opening.

The sensor connected to the control device 80 has a sensor connected between the low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12 (that is, between the discharge port of the low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12). An intermediate pressure sensor 81 is included that detects an intermediate pressure Pm that is the pressure between the pump and the suction port. The control device 80 controls the rotation speed of the high-stage multi-stage compressor 12 according to the detected intermediate pressure Pm. The main purpose of this control is to balance the compression ratio of the low-stage multi-stage compressor 11 and the compression ratio of the high-stage multi-stage compressor 12, and furthermore, to balance the compression ratio of the low-stage multi-stage compressor 11 and the compression ratio of the high-stage multi-stage compressor 12, and to The purpose is to balance the compression ratios of 12a and 12b.

The flow shown in FIG. 4 is repeatedly executed at predetermined control intervals (for example, every 5 milliseconds). The control device 80 sequentially acquires the intermediate pressure Pm detected by the intermediate pressure sensor 81 at every control cycle (step S1).

The control device 80 compares the obtained intermediate pressure Pm with two different threshold values, and determines to which of the three numerical ranges defined by the two threshold values the obtained intermediate pressure Pm belongs. (Step S2). That is, the control device 80 determines whether the intermediate pressure Pm is less than the first threshold, greater than or equal to the first threshold and less than the second threshold, or greater than or equal to the second threshold.

Note that the second threshold is a larger value than the first threshold. In the flow shown in FIG. 4, the first threshold is 0.1 MPaA and the second threshold is 0.2 MPaA, just as an example. However, the specific numerical values of the first threshold value and the second threshold value may be changed as appropriate depending on the specifications of the low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12, the magnitude of cold heat required by the evaporator 18, etc. be done.

When the intermediate pressure Pm is less than the first threshold value (for example, Pm<0.1), the control device 80 performs a switching operation of the high-stage inverter 12d to reduce the rotation speed of the high-stage multistage compressor 12. (step S3a). As a result, the frequency of the alternating current supplied to the high-stage motor 12c is adjusted, and the rotational speed of the high-stage multi-stage compressor 12 is reduced. The compression ratio of the high-stage multi-stage compressor 12 decreases, and the compression ratio of the low-stage multi-stage compressor 11 can be increased accordingly. The intermediate pressure Pm increases, exits the numerical range below the first threshold value, and enters the numerical range above the first threshold value and below the second threshold value.

When the intermediate pressure Pm is equal to or higher than the second threshold (for example, 0.2≦Pm), the control device 80 performs a switching operation of the high-stage inverter 12d to increase the rotation speed of the high-stage multi-stage compressor 12. (step S3b). As a result, the frequency of the alternating current supplied to the high-stage motor 12c is adjusted, and the rotation speed of the high-stage multi-stage compressor 12 increases. The compression ratio of the high-stage multi-stage compressor 12 increases, and accordingly, the compression ratio of the low-stage multi-stage compressor 11 can be increased. The intermediate pressure Pm decreases, leaves the numerical range of the second threshold value or more, and enters the numerical value range of the first threshold value or more and less than the second threshold value.

When the intermediate pressure Pm is greater than or equal to the first threshold and less than the second threshold (for example, 0.1≦Pm<0.2), the control device 80 maintains the rotation speed of the high-stage multi-stage compressor 12, The switching operation of the high-stage inverter 12d is controlled (step S3c). Control is performed so that the intermediate pressure Pm is within a numerical range that is neither too high nor too low, and the compression ratio of the low-stage multi-stage compressor 11 and the compression ratio of the high-stage multi-stage compressor 12 are sufficiently balanced. ing.

FIG. 5 is a Mollier diagram of the refrigerant circulating in the refrigerant circuit 2 in the refrigeration system 1 according to the present embodiment.

In this embodiment, after the enthalpy increases from E1 to E2 in the evaporation process, the pressure increases from P1 to P2 in the compression process. This compression step is performed separately in a total of four stages of compression sections 11a, 11b, 12a, and 12b. Therefore, the compression ratio per stage can be reduced. Even if the compression ratio per stage is small, high pressure and ultimately extremely low temperatures can be obtained, and a decrease in compressor efficiency can be prevented.

When a large number of compression sections 11a, 11b, 12a, 12b are connected in series, the temperature of the refrigerant increases in each compression section 11a, 11b, 12a, 12b. A part of the refrigerant flowing from the condenser 14 to the evaporator 18 passes through the first to third return lines 41 to 43 to three different locations in the combination of the low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12. will be returned to. The refrigerant is sequentially compressed while reducing the discharge temperature of the compression sections 11a, 11b, and 12a as needed. It is possible to prevent the refrigerant from becoming extremely hot on the downstream side of the high-stage multi-stage compressor 12. The high-stage multi-stage compressor 12 can operate stably and compress the refrigerant to a pressure P2 required to obtain an ultra-low temperature. In addition, deterioration of the refrigerant due to an increase in the temperature of the refrigerant can be prevented.

In particular, in this embodiment, R32, which has a high specific heat ratio, is used as the refrigerant. Even if the specific heat ratio of the refrigerant is large, an excessive temperature rise can be suppressed, and the refrigeration system 1 can be constructed to obtain an extremely low temperature. The GWP of R32 is less than 750, and the environmental load of the refrigeration system 1 can be reduced.

Further, a part of the refrigerant between the condenser 14 and the evaporator 18 is cooled by the economizer expansion mechanism 22, and the economizer 21 cools the refrigerant supplied from the condenser 14 to the evaporator 18, and the refrigerant is cooled in the lower stage. It returns between the side multi-stage compressor 11 and the high-stage multi-stage compressor 12. The refrigerant supplied from the condenser 14 to the evaporator 18 can be further cooled, and the refrigeration capacity (coefficient of performance) of the refrigeration system 1 is improved. Moreover, the discharge temperature of the compression section can be further reduced.

The first return line 41 returns the refrigerant between the economizer 21 and the evaporator 18 . The second return line 42 returns the refrigerant between the condenser 14 and the economizer 21, and the third return line 43 returns the refrigerant between the condenser 14 and the economizer 21. The relatively low-temperature refrigerant on the downstream side of the economizer 21 is returned to the relatively upstream side in the combination of the low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12. The relatively high temperature refrigerant on the upstream side of the economizer 21 is returned to the relatively downstream side in the combination of the low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12. Therefore, the discharge temperature can be effectively cooled.

(Second embodiment)
Next, with reference to FIG. 6, a refrigeration system 1 according to a second embodiment will be described, focusing on differences from the first embodiment.

In this embodiment, the oil circulation circuit 3 includes a deaerator 52 and a pressure regulating valve 53. The refrigerant circuit 2 has a degassed refrigerant return line 49 .

The deaerator 52 is tank-shaped. The deaerator 52 removes refrigerant dissolved in the oil from the oil flowing through the oil return line 60. The oil return line 60 is roughly divided into two systems: branch parts 62a and 62b that go from the common part 61 to the low-stage multi-stage compressor 11, and branch parts 63a and 63b that go to the high-stage multi-stage compressor 12. It is divided into The deaerator 52 is provided in a portion of the oil return line 60 that goes toward the low-stage multi-stage compressor 11. The branch portions 63a and 63b extend from the common portion 61 toward the high-stage multi-stage compressor 12 without passing through the deaerator 52.

In other words, the common portion 61 connects the oil outlet of the oil separator to the oil inlet of the deaerator 52. The branch parts 63a and 63b branch from the common part 61 on the downstream side of the oil cooler 51 and are connected to the high-stage multi-stage compressor 12, while the branch parts 62a and 62b connect the oil outlet of the deaerator 52. It is connected to the lower stage side multi-stage compressor 11.

The degassed refrigerant return line 49 returns the refrigerant separated by the deaerator 52 between the low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12. The refrigerant separated from the oil in the deaerator 52 accumulates in the upper part of the deaerator 52. A degassed refrigerant return line 49 connects a refrigerant outlet provided at the top of the deaerator 52 to the circulation line 32b.

The degassed refrigerant return line 49 returns the refrigerant separated by the deaerator 52 between the low-stage multi-stage compressor 11 and the high-stage multi-stage compressor 12. The refrigerant separated from the oil in the deaerator 52 accumulates in the upper part of the deaerator 52. A degassed refrigerant return line 49 connects a refrigerant outlet provided at the top of the deaerator 52 to the circulation line 32b.

The pressure regulating valve 53 is provided between the oil cooler 51 and the deaerator 52 in the common part 61 of the oil return line 60, and controls the internal pressure of the deaerator 52 between the lower stage multistage compressor 11 and the higher stage side. The pressure is adjusted to be higher than the pressure between the multistage compressor 12 (intermediate pressure Pm). Therefore, the degassed refrigerant return line 49 can be used to return the refrigerant recovered by the deaerator 52 to the circulation line 32b. Further, the destination of the refrigerant is not limited to the circulation line 32b. The refrigerant separated by the deaerator 52 may be returned to an intermediate stage of the low-stage multi-stage compressor 11, for example, to the circulation line 32a. In this case, the pressure regulating valve 53 adjusts the internal pressure of the deaerator 52 to a pressure higher than that of the intermediate stage of the low-stage multi-stage compressor 11.

This embodiment also has the same effects as the first embodiment. Here, if the refrigerant is supplied to the compressor together with the oil while being dissolved in the oil, there is a risk that the performance of the compressor will deteriorate due to evaporation (flash) of the refrigerant. According to this embodiment, since the refrigerant is removed from the oil in the deaerator 52, such performance deterioration can be prevented. Since the refrigerant is returned to the refrigerant circuit 2 after being recovered by the deaerator 52, the amount of refrigerant in the refrigerant circuit 2 can be prevented from decreasing.

(Third embodiment)
Next, with reference to FIG. 7, a refrigeration system 1 according to a third embodiment will be described, focusing on differences from the first embodiment.

The evaporator 18 according to this embodiment has a refrigerant passage 18a through which a refrigerant flows, but does not include a brine passage 18b (see FIG. 1). The object to be cooled 99 is in contact with the outer surface of the evaporator 18 . The evaporator 18 is configured as a heat exchanger that directly cools the object 99 to be cooled with the refrigerant flowing through the refrigerant passage 18a.

This embodiment also has the same effects as the first embodiment. Furthermore, since brine is not required, the overall configuration of the refrigeration system 1 can be simplified.

(Fourth embodiment)
Next, with reference to FIG. 8, a refrigeration system 1 according to a fourth embodiment will be described, focusing on differences from the third embodiment.

The refrigerant circuit 2 according to this embodiment further includes a liquid receiver 19 interposed between the condenser 14 and a heat exchanger serving as an evaporator 18, and a pump 29. The outlet of the expansion valve 17 is connected to the liquid inlet of the liquid receiver 19 via a circulation line 37a. The liquid receiver 19 is tank-shaped and stores liquid refrigerant supplied from the expansion valve 17 at the bottom. A liquid outlet is provided at the bottom of the liquid receiver 19, and the liquid outlet of the liquid receiver 19 is connected to the inlet of the evaporator 18 via a circulation line 37b. Pump 29 is interposed on circulation line 37b. The outlet of the evaporator 18 is connected to the gas inlet of the receiver 19 via a circulation line 31a. A gas outlet is provided at the upper part of the liquid receiver 19, and the gas outlet of the liquid receiver 19 is connected to the lower stage side multistage compressor 11 via a circulation line 31b.

When the pump 29 is driven, the refrigerant in the liquid receiver 19 passes through the evaporator 18, becomes a gas in the evaporator 18, and returns to the liquid receiver 19. The refrigerant that has become a gas in the evaporator 18 is supplied to the low-stage multi-stage compressor 11 via the liquid receiver 19 .

Note that the refrigeration system 1 may include a level sensor that detects the height of the liquid level of the refrigerant in the liquid receiver 19 as a sensor connected to the control device 80. The control device 80 may adjust the opening degree of the expansion valve 17 based on the height of the liquid level detected by the level sensor.

This embodiment also has the same effects as the third embodiment. The liquid-phase refrigerant stored in the liquid receiver 19 functions similarly to brine, and the brine itself becomes unnecessary when cooling the object 99 to be cooled.

(Fifth embodiment)
Next, with reference to FIG. 9, a refrigeration system according to a fifth embodiment will be described, focusing on differences from the first embodiment.

The refrigerant circuit 2 according to this embodiment includes two economizers, a first economizer 21A and a second economizer 21B. Further, the refrigerant return line 40 further includes a fifth return line 45 that returns a portion of the refrigerant between the condenser 14 and the evaporator 18 to the intermediate stage of the low-stage multi-stage compressor 11. The fifth return line 45 is independent from the first return line 41. That is, the fifth return line 45 connects the circulation line 36 to the compressor side without fluidly communicating with the first return line 41.

The first economizer 21A is similar to the economizer 21 (see FIG. 1, etc.) according to the first to fourth embodiments. The refrigerant circuit 2 has an expansion mechanism 22A for the first economizer 21A. The first economizer 21A has a supply channel 21Aa and a return channel 21Ab. The supply channel 21Aa constitutes a part of the circulation line 36, like the supply channel 21a according to the first embodiment, and the return channel 21Ab, like the return channel 21b according to the first embodiment, It constitutes a part of the fourth return line 44.

The second economizer 21B is interposed between the first economizer 21A and the evaporator 18, and is also interposed in the fifth return line 45. The second economizer 21B performs heat exchange between the refrigerant flowing from the first economizer 21A to the evaporator 18 and the refrigerant flowing through the fifth return line 45. The expansion mechanism 22B for the second economizer 21B reduces the pressure of the refrigerant supplied to the second economizer 21B via the fifth return line 45. The expansion mechanism 22B is realized by a valve whose opening degree can be adjusted.

The second economizer 21B has a supply flow path 21Ba that forms part of the circulation line 36 and a return flow path 21Bb that forms part of the fifth return line 45. The upstream section 36a of the circulation line 36 connects the outlet of the dryer 16 to the inlet of the supply channel 21Aa of the first economizer 21A, and the downstream section 36b of the circulation line 36 connects the outlet of the supply channel 21Ba of the second economizer 21B. is connected to the inlet of the expansion valve 17. The outlet of the supply channel 21Aa of the first economizer 21A is connected to the outlet of the supply channel 21Ba of the second economizer 21B via the intermediate portion 36c of the circulation line 36.

Therefore, the upstream portion 44a of the fourth return line 44 branches from the intermediate portion 36c of the circulation line 36, and is connected to the inlet of the return flow path 21Ab of the first economizer 21A. The fifth return line 45 includes an upstream section 45a that branches from the downstream section 36b of the circulation line 36 and is connected to the inlet of the return channel 21Bb of the second economizer 21B, a return channel 21Bb, and an outlet of the return channel 21Bb. and a downstream portion 45b that connects the compressor to the compressor side. The expansion mechanism 22B is interposed in the upstream portion 45a of the fifth return line 45. Note that the on-off valve 27j is also interposed in the upstream portion 45a.

The third branch point, the second branch point, the fourth branch point, the first branch point, and the fifth branch point are arranged in this order in the flow direction of the refrigerant from the dryer 16 to the expansion valve 17 on the circulation line 36. They are lined up. The third branch point and the second branch point are located at the upstream portion 36a, similar to the first to fourth embodiments. The fourth branch point and the first branch point are located at the intermediate portion 36c. The fifth branch point, which is the most downstream, is located at the downstream portion 36b.

Although not shown in detail, the refrigeration system 1 uses a sensor connected to the control device 80 to detect the temperature of the downstream portion 45b of the fifth return line 45 (i.e., the temperature of the refrigerant returned from the second economizer 21B to the compressor side). ) is included. Based on the temperature detected by the temperature sensor, the control device 80 adjusts the opening degree of the expansion mechanism 22B as an expansion valve so that the degree of superheating of the refrigerant in the fifth return line 45 is constant.

This embodiment also has the same effects as the first embodiment. Furthermore, since two stages of economizers are provided, the refrigerating capacity of the refrigeration system is further improved, and the discharge temperature of the compressor can be further reduced.

(Sixth embodiment)
Next, with reference to FIG. 10, a refrigeration system according to a sixth embodiment will be described, focusing on differences from the first embodiment.

In this embodiment, the economizer is omitted from the refrigerant circuit 2. The refrigerant return line 40 has first to third return lines 41 to 43. This embodiment also has the same effects as the first embodiment.

(Seventh embodiment)
FIG. 11 is a schematic diagram of the main parts of a refrigeration system according to a seventh embodiment of the present invention. This embodiment differs from the first embodiment in that the oil return line 60 is provided with a branch portion 62c extending to the first suction side bearings 75A, 75B, and the branch portion 62c is provided with an on-off valve 64.

Opening and closing of the on-off valve 64 is controlled by a control device 80. More specifically, the control device 80 controls the opening and closing of the on-off valve 64 so that when the low-stage multi-stage compressor 11 is driven, the on-off valve 64 is closed, and when the low-stage multi-stage compressor 11 is stopped, the on-off valve 64 is opened. It is meant to be controlled. Therefore, when the low-stage multi-stage compressor 11 is driven, oil is not directly supplied to the first suction-side bearing chamber 70b, so it is possible to prevent a large amount of refrigerant gas from being generated in the first suction-side bearing chamber 70b.

Furthermore, when the low-stage multi-stage compressor 11 stops, the on-off valve 91 opens, so oil from the branch portion 62c is directly supplied to the first suction-side bearings 75A, 75B. In this case, since the differential pressure is small, even if oil is supplied to the first suction side bearings 75A, 75B, refrigerant gas is unlikely to be generated from the supplied oil. Further, since oil is directly supplied to the first suction side bearings 75A, 75B, the first suction side bearings 75A, 75B can be reliably lubricated. Further, while the low-stage multi-stage compressor 11 is stopped, oil can be sufficiently stored near the first suction-side bearings 75A and 75B.

(Modified example)
Although the embodiments have been described so far, the above configuration can be modified as appropriate within the scope of the spirit of the present invention.

As shown in FIG. 12, the deaerator 52 may be interposed at another location on the oil return line 60, and how to connect the piping from the oil cooler 51 to the deaerator 52 or the pressure regulating valve 53. can be changed as appropriate. The deaerator 52 may be provided in the middle of the branch portions 62a and 62b of the oil return line 60.

The present disclosure may include the following aspects.
(Aspect 1)
Equipped with a refrigerant circuit that includes a low-stage multi-stage compressor, a high-stage multi-stage compressor, an oil separator, a condenser, and an evaporator, and circulates refrigerant in this order,
The refrigerant has a GWP of less than 750 and a saturation pressure at -80°C of 0.017 MPaA or more,
The low-stage multi-stage compressor and the high-stage multi-stage compressor are each driven by separate motors,
The refrigerant circuit is
a first return line that returns a portion of the refrigerant between the condenser and the evaporator to an intermediate stage of the low-stage multi-stage compressor;
a second return line that returns a portion of the refrigerant between the condenser and the evaporator between the low-stage multi-stage compressor and the high-stage multi-stage compressor;
a third return line that returns a portion of the refrigerant between the condenser and the evaporator to an intermediate stage of the high-stage multi-stage compressor;
A refrigeration system further comprising:
(Aspect 2)
the refrigerant is R32;
Refrigeration system according to aspect 1.
(Aspect 3)
The refrigerant circuit is
a fourth return line that returns a portion of the refrigerant between the condenser and the evaporator between the low-stage multi-stage compressor and the high-stage multi-stage compressor;
The refrigerant is interposed between the condenser and the evaporator and is interposed on the fourth return line, and flows from the condenser to the evaporator, and the refrigerant flows through the fourth return line. a first economizer that performs heat exchange between the
a first economizer expansion mechanism that reduces the pressure of the refrigerant supplied to the first economizer via the fourth return line;
The refrigeration system according to aspect 1 or 2, further comprising:
(Aspect 4)
the first return line returns the refrigerant between the first economizer and the evaporator;
The second return line and the third return line return the refrigerant between the condenser and the first economizer.
Refrigeration system according to aspect 3.
(Aspect 5)
The refrigerant circuit is
a fifth return line that returns a portion of the refrigerant between the condenser and the evaporator to the intermediate stage of the low-stage multi-stage compressor;
The refrigerant interposed between the first economizer and the evaporator and on the fifth return line, the refrigerant flowing from the first economizer to the evaporator and the refrigerant flowing through the fifth return line. a second economizer that performs heat exchange between the
a second economizer expansion mechanism that reduces the pressure of the refrigerant supplied to the second economizer via the fifth return line;
The refrigeration system according to aspect 3 or 4, further comprising:
(Aspect 6)
an oil return line that returns the oil recovered by the oil separator to at least the low-stage multi-stage compressor;
a deaerator that removes the refrigerant dissolved in the oil from the oil flowing through the oil return line;
A degassed refrigerant that returns the refrigerant separated by the deaerator to an intermediate stage of the low-stage multi-stage compressor or a return destination between the low-stage multi-stage compressor and the high-stage multi-stage compressor. return line and
a pressure regulating valve that adjusts the internal pressure of the deaerator to a higher pressure than the pressure of the return destination;
The refrigeration system according to any one of aspects 1 to 5, further comprising:
(Aspect 7)
The evaporator is a heat exchanger that directly cools the object to be cooled with the refrigerant.
The refrigeration system according to any one of aspects 1 to 6.
(Aspect 8)
The refrigerant circuit further includes a liquid receiver interposed between the condenser and the heat exchanger as the evaporator,
The refrigerant stored in the liquid receiver is supplied to the heat exchanger, and the gaseous refrigerant after heat exchange with the object to be cooled is circulated to the lower stage multi-stage compressor.
Refrigeration system according to aspect 7.
(Aspect 9)
an intermediate pressure sensor that detects an intermediate pressure that is a pressure between the low-stage multi-stage compressor and the high-stage multi-stage compressor;
a control device that controls the rotation speed of the high-stage multi-stage compressor according to the intermediate pressure detected by the intermediate pressure sensor;
The refrigeration system according to any one of aspects 1 to 8, further comprising:
(Aspect 10)
At least one of the low-stage multi-stage compressor and the high-stage multi-stage compressor,
a suction side bearing rotatably supporting the suction side end of the rotating shaft of the first stage compression section;
a suction side bearing chamber that accommodates the suction side bearing;
including;
The suction side bearing chamber is provided with a wall portion disposed on the opposite side of the first stage compression section with respect to the suction side bearing, and oil is stored between the suction side bearing and the wall portion. be able to,
A refrigeration system according to any one of aspects 1 to 9.
(Aspect 11)
An oil supply line for guiding oil to the suction side bearing of the low stage side multi-stage compressor is not provided.
Refrigeration system according to aspect 10.
(Aspect 12)
The low-stage side multi-stage compressor is
a suction side bearing rotatably supporting the suction side end of the rotating shaft of the first stage compression section;
a suction side bearing chamber that accommodates the suction side bearing;
including;
An oil supply line is provided that guides oil from the oil separator to the suction side bearing,
The oil supply line is provided with an on-off valve that is controlled to open and close by a control device,
The control device controls opening and closing of the on-off valve so that when the low-stage multi-stage compressor is driven, the on-off valve is closed, and when the low-stage multi-stage compressor is stopped, the on-off valve is opened.
The refrigeration system according to aspect 1 or 2.

1 Refrigeration system 2 Refrigerant circuit 3 Oil circulation circuit 11 Low-stage multi-stage compressor 11a First low-stage compression section 11b Second low-stage compression section 11c Low-stage motor 11d Low-stage inverter 11e Motor drive shaft 12 High-stage multi-stage Compressor 12a First high-stage compression section 12b Second high-stage compression section 12c High-stage motor 12d High-stage inverter 12e Motor drive shaft 13 Oil separator 13a Casing 13b Demister 14 Condenser 14a Refrigerant passage 14b Coolant passage 15 Receiver 16 Dryer 17 Expansion valve 18 Evaporator 18a Refrigerant passage 18b Brine passage 19 Liquid receiver 21 Economizer 21A First economizer 21B Second economizer 21a, 21Aa, 21Ba Supply channel 21b, 21Ab, 21Bb Return channel 22, 22A, 22B Expansion mechanisms 27a to 27j On-off valves 28a to 28c Check valve 29 Pump 30 Refrigerant circulation lines 31 to 37 Circulation line 36a Upstream section 36b Downstream section 36c Intermediate section 40 Refrigerant return lines 41 to 45 First to fifth return lines 44a, 45a Upstream section 44b, 45b Downstream section 49 Degassed refrigerant return line 51 Oil cooler 52 Deaerator 53 Pressure adjustment valve 60 Oil return line 61 Common section 62a, 62b, 62c, 63a, 63b Branch section 64 Opening/closing valve 70 Casing 70a Motor chamber 70b First suction side bearing chamber 70c First compression chamber 70d First discharge side bearing chamber 70e Intermediate pressure chamber 70f Second suction side bearing chamber 70g Second compression chamber 70h Second discharge side bearing chamber 70i Suction ports 71A, 71B 1st Screw rotors 72A, 72B Second screw rotors 73A, 73B First rotating shafts 74A, 74B Second rotating shafts 75A, 75B First suction side bearings 76A, 76B First discharge side bearings 77A, 77B Second suction side bearings 78A, 78B Second discharge side bearings 79A, 79B Wall portion 80 Control device 81 Intermediate pressure sensor 99 Cooling object Pm Intermediate pressure

Claims (12)

Equipped with a refrigerant circuit that includes a low-stage multi-stage compressor, a high-stage multi-stage compressor, an oil separator, a condenser, and an evaporator, and circulates refrigerant in this order,
The refrigerant has a GWP of less than 750 and a saturation pressure at -80°C of 0.017 MPaA or more,
The low-stage multi-stage compressor and the high-stage multi-stage compressor are each driven by separate motors,
The refrigerant circuit is
a first return line that returns a portion of the refrigerant between the condenser and the evaporator to an intermediate stage of the low-stage multi-stage compressor;
a second return line that returns a portion of the refrigerant between the condenser and the evaporator between the low-stage multi-stage compressor and the high-stage multi-stage compressor;
a third return line that returns a portion of the refrigerant between the condenser and the evaporator to an intermediate stage of the high-stage multi-stage compressor;
A refrigeration system further comprising: the refrigerant is R32;
A refrigeration system according to claim 1. The refrigerant circuit is
a fourth return line that returns a portion of the refrigerant between the condenser and the evaporator between the low-stage multi-stage compressor and the high-stage multi-stage compressor;
The refrigerant is interposed between the condenser and the evaporator and is interposed on the fourth return line, and flows from the condenser to the evaporator, and the refrigerant flows through the fourth return line. a first economizer that performs heat exchange between the
a first economizer expansion mechanism that reduces the pressure of the refrigerant supplied to the first economizer via the fourth return line;
The refrigeration system according to claim 1 or 2, further comprising: the first return line returns the refrigerant between the first economizer and the evaporator;
The second return line and the third return line return the refrigerant between the condenser and the first economizer.
Refrigeration system according to claim 3. The refrigerant circuit is
a fifth return line that returns a portion of the refrigerant between the condenser and the evaporator to the intermediate stage of the low-stage multi-stage compressor;
The refrigerant interposed between the first economizer and the evaporator and on the fifth return line, the refrigerant flowing from the first economizer to the evaporator and the refrigerant flowing through the fifth return line. a second economizer that performs heat exchange between the
a second economizer expansion mechanism that reduces the pressure of the refrigerant supplied to the second economizer via the fifth return line;
4. The refrigeration system of claim 3, further comprising: an oil return line that returns the oil recovered by the oil separator to at least the low-stage multi-stage compressor;
a deaerator that removes the refrigerant dissolved in the oil from the oil flowing through the oil return line;
A degassed refrigerant that returns the refrigerant separated by the deaerator to an intermediate stage of the low-stage multi-stage compressor or a return destination between the low-stage multi-stage compressor and the high-stage multi-stage compressor. return line and
a pressure regulating valve that adjusts the internal pressure of the deaerator to a higher pressure than the pressure of the return destination;
The refrigeration system according to claim 1 or 2, further comprising: The evaporator is a heat exchanger that directly cools the object to be cooled with the refrigerant.
The refrigeration system according to claim 1 or 2. The refrigerant circuit further includes a liquid receiver interposed between the condenser and the heat exchanger as the evaporator,
The refrigerant stored in the liquid receiver is supplied to the heat exchanger, and the gaseous refrigerant after heat exchange with the object to be cooled is circulated to the lower stage multi-stage compressor.
Refrigeration system according to claim 7. an intermediate pressure sensor that detects an intermediate pressure that is a pressure between the low-stage multi-stage compressor and the high-stage multi-stage compressor;
a control device that controls the rotation speed of the high-stage multi-stage compressor according to the intermediate pressure detected by the intermediate pressure sensor;
The refrigeration system according to claim 1 or 2, further comprising: The low-stage multi-stage compressor is
a suction side bearing rotatably supporting the suction side end of the rotating shaft of the first stage compression section;
a suction side bearing chamber that accommodates the suction side bearing;
including;
The suction side bearing chamber is provided with a wall portion disposed on the opposite side of the first stage compression section with respect to the suction side bearing, and oil is stored between the suction side bearing and the wall portion. be able to,
The refrigeration system according to claim 1 or 2. An oil supply line for guiding oil to the suction side bearing of the low stage side multi-stage compressor is not provided.
Refrigeration system according to claim 10. The low-stage multi-stage compressor is
a suction side bearing rotatably supporting the suction side end of the rotating shaft of the first stage compression section;
a suction side bearing chamber that accommodates the suction side bearing;
including;
An oil supply line is provided that guides oil from the oil separator to the suction side bearing,
The oil supply line is provided with an on-off valve that is controlled to open and close by a control device,
The control device controls opening and closing of the on-off valve so that when the low-stage multi-stage compressor is driven, the on-off valve is closed, and when the low-stage multi-stage compressor is stopped, the on-off valve is opened.
The refrigeration system according to claim 1 or 2.

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