Refrigeration system and refrigeration method

Abstract

The present invention provides a refrigeration system and a refrigeration method which each make it possible to relax a structural constraint when a flammable refrigerant is used. A refrigeration system according to the present invention comprises a refrigerant line through which a flammable refrigerant flows, a refrigerant compression unit which is provided to the refrigerant line and which compresses the flammable refrigerant, a refrigerant expansion unit which is provided to the refrigerant line and which expands the flammable refrigerant that has been compressed by the refrigerant compression unit, a refrigerant heat exchange unit which is provided to the refrigerant line and which cools a cooling target by the flammable refrigerant having a temperature decreased due to expansion, and a working fluid line through which a working fluid having flammability lower than that of the flammable refrigerant passes, wherein the refrigerant compression unit receives, from the working fluid line, the working fluid that has been pressurized, and compresses the flammable refrigerant using the pressure of the working fluid.

Description

Refrigeration System and Refrigeration Method

[0001] The present disclosure relates to a refrigeration system and a refrigeration method.

[0002] Techniques for cooling a cooling target are known. For example, Patent Document 1 discloses a two-stage compression refrigeration device including a refrigerant circuit in which a two-stage compressor, a condenser, an intermediate cooler, an expansion valve, and an evaporator are connected by refrigerant pipes.

[0003] Japanese Patent Application Laid-Open No. 2007-255864

[0004] Although not disclosed in Patent Document 1, as a refrigerant used in a refrigeration system, a low-GWP refrigerant having a low global warming potential (GWP) is required. One example of a low-GWP refrigerant is a natural refrigerant. A natural refrigerant is a refrigerant using a substance that originally exists in the natural environment, and typical examples include ammonia, carbon dioxide, water, air, and hydrocarbons. Natural refrigerants include flammable refrigerants such as ammonia and hydrocarbons (hereinafter referred to as flammable refrigerants).

[0005] When a flammable refrigerant is used in a refrigeration system, in order to prevent ignition and explosion of the flammable refrigerant, it is required to provide an explosion-proof structure at the location where the flammable refrigerant flows. In particular, a compressor that compresses the refrigerant is generally provided with a prime mover such as an electric motor as a power source, and since the prime mover can become an ignition source for the flammable refrigerant, the need for an explosion-proof structure is high. When devices such as a compressor including an ignition source are designed with explosion-proof specifications, structural constraints become large. Therefore, it is desirable to relax the structural constraints when using a flammable refrigerant.

[0006] An object of the present disclosure is to provide a refrigeration system and a refrigeration method capable of relaxing structural constraints when using a flammable refrigerant.

[0007] To achieve the above objectives, the refrigeration system of the present disclosure comprises a refrigerant line through which a flammable refrigerant flows; a refrigerant compression unit provided in the refrigerant line for compressing the flammable refrigerant; a refrigerant expansion unit provided in the refrigerant line for expanding the flammable refrigerant compressed in the refrigerant compression unit; a refrigerant heat exchange unit provided in the refrigerant line for cooling an object with the flammable refrigerant whose temperature has been lowered by the expansion; and a working fluid line through which a working fluid less flammable than the flammable refrigerant flows, wherein the refrigerant compression unit receives the working fluid pressurized from the working fluid line and compresses the flammable refrigerant using the pressure of the working fluid.

[0008] Furthermore, the refrigeration method of this disclosure comprises the steps of: circulating a working fluid that is less flammable than a flammable refrigerant in a pressurized state through a working fluid line; compressing a flammable refrigerant flowing through a refrigerant line using the pressure of the working fluid supplied pressurized from the working fluid line; expanding the compressed flammable refrigerant; and cooling an object to be cooled with the flammable refrigerant whose temperature has decreased due to the expansion.

[0009] This disclosure provides a refrigeration system and a refrigeration method that can alleviate structural constraints when using flammable refrigerants.

[0010] Figure 1 is a schematic diagram showing a refrigeration system according to the first embodiment. Figure 2 is a p-h diagram illustrating the operation of the refrigeration system according to the first embodiment. Figure 3 is a schematic diagram showing a low-pressure compression system according to the second embodiment. Figure 4 is a schematic diagram showing a high-pressure compression system according to the second embodiment. Figure 5 is a schematic diagram showing a low-pressure compression system according to the third embodiment. Figure 6 is a schematic diagram showing a high-pressure compression system according to the third embodiment. Figure 7 is a schematic diagram showing a low-pressure compression system according to a modified example of the third embodiment. Figure 8 is a schematic diagram showing a high-pressure compression system according to a modified example of the third embodiment. Figure 9 is a schematic diagram showing a refrigeration system according to the fourth embodiment.

[0011] Preferred embodiments of the present disclosure will be described in detail below with reference to the drawings. However, these embodiments do not limit the present disclosure, and where there are multiple embodiments, they may be combinations of these embodiments. Furthermore, the components in the embodiments include those readily conceivable by those skilled in the art, those that are substantially identical, and those that are equivalent.

[0012] [First Embodiment] Figure 1 is a schematic diagram showing a refrigeration system according to the first embodiment. Figure 2 is a p-h diagram illustrating the operation of the refrigeration system according to the first embodiment. The symbols a to h in Figure 1 represent the refrigerant flow points in the refrigeration system 1, and Figure 2 shows the state of the refrigerant corresponding to the points a to h in Figure 1. In Figure 2, the vertical axis represents the refrigerant pressure and the horizontal axis represents enthalpy.

[0013] Refrigeration system 1 is a system that supplies cold energy to a cooling object 2 using a refrigerant. Refrigeration system 1 is installed in facilities such as factories, plants, logistics centers, and data centers. The cooling object 2 is not particularly limited. The cooling object 2 may be, for example, air or gas used in air conditioning equipment, freezers, or processing equipment, or it may be a secondary refrigerant supplied to such equipment. The cooling object 2 may also be something other than a fluid, such as a heat source provided in the equipment.

[0014] The refrigeration system 1 consists of a refrigeration cycle using a flammable refrigerant 3. The flammable refrigerant 3 is a refrigerant that is flammable and becomes a flammable gas in the gas phase. Flammability means the property of burning continuously. The flammable refrigerant 3 according to this embodiment is, for example, a refrigerant that corresponds to CLASS 3 (highly flammable) as defined in ISO 817:2014. When a flammable gas is mixed with a combustion-supporting gas such as oxygen at a concentration above a certain level, it may cause a combustion explosion depending on the temperature conditions, etc. Therefore, depending on the degree of flammability (ease of ignition or explosion), it is required that the equipment handling the flammable gas be equipped with an explosion-proof structure (such as a pressure-resistant explosion-proof structure, an oil-immersed explosion-proof structure, or an internal pressure explosion-proof structure).

[0015] The flammable refrigerant 3 according to the first embodiment is a flammable natural refrigerant. Natural refrigerants are preferred because they have a lower global warming potential compared to fluorocarbons and have a smaller impact on global warming. Examples of flammable natural refrigerants include hydrocarbon refrigerants and ammonia refrigerants. Examples of hydrocarbon refrigerants include propane, isobutane, and butane.

[0016] The refrigeration system 1 according to the first embodiment includes a refrigerant line 10, a refrigerant compression unit 11, a refrigerant condensation unit 12, a high-pressure refrigerant tank 13, an intermediate heat exchanger 14, a refrigerant expansion unit 15, a refrigerant heat exchange unit 16, a working fluid line 17, a working fluid compression unit 18, a working fluid cooling unit 19, a first cooler 20, and a second cooler 21. In Figure 1, the refrigeration system 1 includes a multi-stage compression refrigeration cycle equipped with a plurality of refrigerant compression units 11. Figure 1 illustrates a two-stage compression refrigeration cycle including a low-pressure compression system CS1 having a refrigerant compression unit 11 and a high-pressure compression system CS2 having a refrigerant compression unit 11. The refrigeration cycle constituting the refrigeration system 1 may be a single-stage compression refrigeration cycle or a multi-stage compression refrigeration cycle with three or more stages.

[0017] The refrigerant line 10 is a flow path for the flammable refrigerant 3. The refrigerant line 10 has an annular structure for circulating the flammable refrigerant 3. The refrigerant line 10 is sequentially equipped with a low-pressure compression system CS1, a high-pressure compression system CS2, a refrigerant condenser 12, a high-pressure refrigerant tank 13, an intermediate heat exchanger 14, a refrigerant expansion unit 15, and a refrigerant heat exchange unit 16. The flammable refrigerant 3 flowing through the refrigerant line 10 is supplied sequentially from the low-pressure compression system CS1 to each device, passes through the refrigerant heat exchange unit 16, and is returned to the low-pressure compression system CS1.

[0018] The refrigerant line 10 includes a bypass line 10A. The bypass line 10A branches off from the refrigerant line 10 at the branching point between the high-pressure refrigerant tank 13 and the intermediate heat exchanger 14 and merges at the confluence point of the inlet of the high-pressure compression system CS2. The bypass line 10A is equipped with an expansion valve 22 and an intermediate heat exchanger 14.

[0019] The low-pressure compression system CS1 is connected to the refrigerant heat exchange unit 16 and the high-pressure compression system CS2 via the refrigerant line 10. The low-pressure compression system CS1 receives the gaseous flammable refrigerant 3 from the refrigerant heat exchange unit 16. The low-pressure compression system CS1 compresses the flammable refrigerant 3 in the first stage using the refrigerant compression unit 11. The low-pressure compression system CS1 supplies the first-stage compressed flammable refrigerant 3 to the high-pressure compression system CS2.

[0020] The high-pressure compression system CS2 is connected to the low-pressure compression system CS1 and the refrigerant condenser 12 via the refrigerant line 10. The first-stage compressed flammable refrigerant 3 from the low-pressure compression system CS1 and the flammable refrigerant 3 from the bypass line 10A merge at the confluence point. The high-pressure compression system CS2 receives the merged flammable refrigerant 3. The high-pressure compression system CS2 compresses the flammable refrigerant 3 in the second stage using the refrigerant compression section 11. The high-pressure compression system CS2 supplies the second-stage compressed flammable refrigerant 3 to the refrigerant condenser 12.

[0021] The refrigerant condenser 12 is connected to the high-pressure compression system CS2 and the high-pressure refrigerant tank 13 via the refrigerant line 10. The refrigerant condenser 12 receives the flammable refrigerant 3 from the high-pressure compression system CS2. The refrigerant condenser 12 cools and condenses the high-temperature, high-pressure flammable refrigerant 3 compressed in the refrigerant compression unit 11. The refrigerant condenser 12 may be air-cooled, cooling the flammable refrigerant 3 with air (atmosphere), or water-cooled, cooling the flammable refrigerant 3 with cooling water.

[0022] The high-pressure refrigerant tank 13 receives and stores the flammable refrigerant 3 condensed in the refrigerant condensation section 12. The high-pressure refrigerant tank 13 is connected to the branching point between the refrigerant line 10 and the bypass line 10A. A portion of the flammable refrigerant 3 that leaves the high-pressure refrigerant tank 13 is sent through the refrigerant line 10 to the high-temperature side flow path of the intermediate heat exchanger 14. The remaining flammable refrigerant 3 that leaves the high-pressure refrigerant tank 13 flows into the bypass line 10A and is sent through the expansion valve 22 to the low-temperature side flow path of the intermediate heat exchanger 14.

[0023] The intermediate heat exchanger 14 performs heat exchange between the flammable refrigerant 3 passing through the refrigerant line 10 and the flammable refrigerant 3 passing through the bypass line 10A. The intermediate heat exchanger 14 includes a high-temperature side flow path and a low-temperature side flow path, and the high-temperature side flow path and the low-temperature side flow path are separated by a heat transfer surface so that heat exchange can occur. The high-temperature side flow path is connected to the refrigerant line 10 and constitutes part of the refrigerant line 10. The low-temperature side flow path is connected to the bypass line 10A and constitutes part of the bypass line 10A. High-temperature, high-pressure flammable refrigerant 3 from the high-pressure refrigerant tank 13 flows through the high-temperature side flow path. Flammable refrigerant 3 that has expanded and cooled down in the expansion valve 22 flows through the low-temperature side flow path. The intermediate heat exchanger 14 cools the high-temperature flammable refrigerant 3 that has been sent to the intermediate heat exchanger 14 at high pressure by using the flammable refrigerant 3 that has expanded and cooled down from the flammable refrigerant 3 condensed in the refrigerant condensation section 12. The flammable refrigerant 3 that has passed through the intermediate heat exchanger 14 via the bypass line 10A is returned to the inlet of the high-pressure compression system CS2.

[0024] The refrigerant expansion section 15 is provided in the refrigerant line 10 and expands the flammable refrigerant 3 compressed in the refrigerant compression section 11. The refrigerant expansion section 15 is provided between the intermediate heat exchanger 14 and the refrigerant heat exchange section 16. The refrigerant expansion section 15 is composed of, for example, an expansion valve or an expansion turbine. In the example in Figure 1, the refrigerant expansion section 15 is composed of an expansion valve. The flammable refrigerant 3 expands in the refrigerant expansion section 15 and its temperature decreases.

[0025] The refrigerant heat exchange unit 16 is installed in the refrigerant line 10 and cools the object to be cooled by the flammable refrigerant 3 whose temperature has decreased due to expansion. The refrigerant heat exchange unit 16 is composed of a heat exchanger including a high-temperature side flow path and a low-temperature side flow path. The high-temperature side flow path and the low-temperature side flow path are separated by a heat transfer surface so that heat exchange can occur. The high-temperature side flow path constitutes part of the flow path through which the object to be cooled flows. The low-temperature side flow path is connected to the refrigerant line 10 and constitutes part of the refrigerant line 10. Low-temperature, low-pressure flammable refrigerant 3 that has passed through the refrigerant expansion unit 15 flows through the low-temperature side flow path. The refrigerant heat exchange unit 16 is an evaporator that evaporates the flammable refrigerant 3 by absorbing heat from the object to be cooled flowing through the high-temperature side flow path. The refrigerant heat exchange unit 16 cools the object to be cooled by utilizing the latent heat of vaporization of the flammable refrigerant 3.

[0026] The flammable refrigerant 3 vaporized in the refrigerant heat exchange section 16 is returned to the inlet side of the low-pressure compression system CS1 through the refrigerant line 10.

[0027] (Configuration of the low-pressure compression system and the high-temperature compression system) Next, the configurations of the low-pressure compression system CS1 and the high-pressure compression system CS2 will be described.

[0028] The low-pressure compression system CS1 and the high-pressure compression system CS2 are each provided with a refrigerant compression unit 11. The refrigerant compression unit 11 is located in the refrigerant line 10 and compresses the flammable refrigerant 3. The refrigerant compression unit 11 receives pressurized working fluid 4 from the working fluid line 17 and uses the pressure of the working fluid 4 to compress the flammable refrigerant 3. In the first embodiment, the refrigerant compression unit 11 receives compressed gas of the gaseous working fluid 4 from the working fluid line 17.

[0029] The refrigerant compression unit 11 according to the first embodiment has a turbo compressor that includes a centrifugal compressor 31 for compressing a flammable refrigerant 3 and an expansion turbine 32 which is connected to the centrifugal compressor 31 in a way that allows power to be transmitted and rotates by the expansion of the working fluid 4. The centrifugal compressor 31 and the expansion turbine 32 are connected by a shaft 33 in a way that allows power to be transmitted and rotate together. The refrigerant compression unit 11 is not provided with a prime mover such as a motor for rotating the turbine. The refrigerant compression unit 11 performs the compression work of the flammable refrigerant 3 by the expansion of the working fluid 4 and operates without a prime mover.

[0030] Specifically, the refrigerant compression unit 11 is connected to the refrigerant line 10 and the working fluid line 17, respectively. The working fluid line 17 circulates a working fluid 4 that is less flammable than the flammable refrigerant 3. The working fluid line 17 supplies compressed gas of the working fluid 4, compressed to a predetermined pressure, to the expansion turbine 32 of the refrigerant compression unit 11. The compressed gas of the working fluid 4 expands in the expansion turbine 32, thereby driving the expansion turbine 32 to rotate. As the expansion turbine 32 rotates, a centrifugal compressor 31, which is connected to the expansion turbine 32 by a shaft 33, rotates. The centrifugal compressor 31 is installed in the refrigerant line 10 and receives the flammable refrigerant 3. As the centrifugal compressor 31 rotates, it compresses the flammable refrigerant 3 to a predetermined pressure.

[0031] In the first embodiment, the working fluid line 17 is a circulation path. In the first embodiment, the working fluid line 17 is provided separately for the low-pressure compression system CS1 and the high-pressure compression system CS2. The working fluid 4 circulates separately in each working fluid line 17.

[0032] The low-pressure compression system CS1 includes, in addition to the refrigerant compression unit 11, a working fluid line 17, a working fluid compression unit 18, a working fluid cooling unit 19, a first cooler 20, and a second cooler 21. The working fluid line 17 of the low-pressure compression system CS1 is provided with an expansion turbine 32, a working fluid compression unit 18, a working fluid cooling unit 19, a first cooler 20, and a second cooler 21.

[0033] The working fluid compression unit 18 compresses the working fluid 4 that has expanded in the refrigerant compression unit 11 (expansion turbine 32). In the working fluid line 17 configured as a circulation path, the working fluid compression unit 18 compresses the working fluid 4, allowing the compressed gas of the working fluid 4 to be continuously supplied to the refrigerant compression unit 11. The structure of the working fluid compression unit 18 is not particularly limited. The working fluid compression unit 18 may be configured as either a positive displacement compressor or a turbo compressor, for example. The working fluid compression unit 18 is a compressor equipped with a prime mover such as an electric motor. The working fluid compression unit 18 performs the compression work on the working fluid 4 using the power of the prime mover.

[0034] The first cooler 20 is provided so as to span the working fluid line 17 and the refrigerant line 10. The first cooler 20 is located downstream of the expansion turbine 32 in the working fluid line 17. The first cooler 20 is located downstream of the centrifugal compressor 31 in the refrigerant line 10. The first cooler 20 is a heat exchanger having a high-temperature side passage and a low-temperature side passage. The high-temperature side passage and the low-temperature side passage are separated by a heat transfer surface so as to be able to exchange heat. The high-temperature side passage is connected to the refrigerant line 10 and constitutes a part of the refrigerant line 10. The low-temperature side passage is connected to the working fluid line 17 and constitutes a part of the working fluid line 17. As a result, the first cooler 20 exchanges heat between the flammable refrigerant 3 and the working fluid 4. The first cooler 20 cools the flammable refrigerant 3 compressed in the refrigerant compression section 11 by heat exchange with the working fluid 4 expanded in the refrigerant compression section 11.

[0035] The working fluid cooling unit 19 is located in the working fluid line 17, between the working fluid compression unit 18 and the expansion turbine 32. More specifically, the working fluid cooling unit 19 is located between the working fluid compression unit 18 and the second cooler 21. The working fluid cooling unit 19 cools the working fluid 4 between the working fluid compression unit 18 and the refrigerant compression unit 11 (i.e., the expansion turbine 32). The working fluid cooling unit 19 is a chiller that cools the working fluid 4, whose temperature rises due to the pressure increase in the working fluid compression unit 18. The working fluid cooling unit 19 has a flow path for circulating refrigerant supplied from an external refrigerator, and cools the working fluid 4 by heat exchange between the low-temperature refrigerant and the working fluid 4. The refrigerant supplied to the working fluid cooling unit 19 is less flammable than the flammable refrigerant 3. The refrigerant supplied to the working fluid cooling unit 19 is a refrigerant that does not require an explosion-proof structure, and is, for example, a liquid refrigerant containing water, such as brine.

[0036] The second cooler 21 is provided so as to span part and other parts of the working fluid line 17. The second cooler 21 is a heat exchanger having a high-temperature side passage and a low-temperature side passage. The high-temperature side passage and the low-temperature side passage are separated by a heat transfer surface so as to be heat exchangeable. The high-temperature side passage is located in the working fluid line 17 between the working fluid cooling section 19 and the expansion turbine 32 and constitutes part of the working fluid line 17. The low-temperature side passage is located between the first cooler 20 and the working fluid compression section 18 and constitutes part of the working fluid line 17. The second cooler 21 exchanges heat between the high-pressure working fluid 4 immediately before the expansion turbine 32 and the low-pressure working fluid 4 after passing through the first cooler 20. As a result, the second cooler 21 cools the working fluid 4 flowing between the working fluid compression section 18 and the working fluid cooling section 19 by heat exchange with the working fluid 4 that has passed through the first cooler 20. In other words, the second cooler 21 cools the working fluid 4 at the outlet of the working fluid cooling section 19 with the working fluid 4 that returns to the working fluid compression section 18 at a lower temperature, thereby further lowering the outlet temperature of the expansion turbine 32. As a result, it lowers the temperature of the flammable refrigerant 3 after heat exchange in the first cooler 20.

[0037] In the low-pressure compression system CS1, the working fluid 4, which has been compressed and heated in the working fluid compression section 18, is cooled in the working fluid cooling section 19, and then further cooled in the second cooler 21 before expanding in the expansion turbine 32. The expansion of the working fluid 4 causes the centrifugal compressor 31 to rotate, compressing the flammable refrigerant 3. The working fluid 4 decreases in temperature as it expands in the expansion turbine 32, while the flammable refrigerant 3 increases in temperature as it is compressed in the centrifugal compressor 31. The flammable refrigerant 3 is cooled by heat exchange between the low-temperature working fluid 4 and the high-temperature flammable refrigerant 3 in the first cooler 20. The working fluid 4 increases in temperature due to heat absorption from the flammable refrigerant 3, but remains at a lower temperature than the high-pressure working fluid 4 supplied to the second cooler 21. In the second cooler 21, the high-temperature, high-pressure working fluid 4 is cooled by the low-temperature, low-pressure working fluid 4. Therefore, after expansion, the working fluid 4 undergoes heat exchange (heat absorption) in two stages between the first cooler 20 and the second cooler 21 before flowing into the working fluid compression section 18. In the working fluid line 17 of the low-pressure compression system CS1, the working fluid 4 circulates in this cycle.

[0038] The working fluid line 17 in the high-pressure compression system CS2 is basically the same as that of the low-pressure compression system CS1. That is, in addition to the refrigerant compression unit 11, the high-pressure compression system CS2 includes the working fluid line 17, the working fluid compression unit 18, the working fluid cooling unit 19, and the first cooler 20. On the other hand, unlike the low-pressure compression system CS1, the high-pressure compression system CS2 does not have a second cooler 21. Therefore, in the working fluid line 17 of the high-pressure compression system CS2, the working fluid 4 that has passed through the first cooler 20 is returned directly to the working fluid compression unit 18.

[0039] In the high-pressure compression system CS2, the working fluid 4, which has been compressed and heated in the working fluid compression section 18, is cooled in the working fluid cooling section 19 and then expands in the expansion turbine 32. The expansion of the working fluid 4 causes the centrifugal compressor 31 to rotate, compressing the flammable refrigerant 3. The working fluid 4 decreases in temperature as it expands in the expansion turbine 32, while the flammable refrigerant 3 increases in temperature as it is compressed in the centrifugal compressor 31. The flammable refrigerant 3 is cooled by heat exchange between the low-temperature working fluid 4 and the high-temperature flammable refrigerant 3 in the first cooler 20. The working fluid 4, which has absorbed heat from the flammable refrigerant 3, flows into the working fluid compression section 18. In the working fluid line 17 of the high-pressure compression system CS2, the working fluid 4 circulates in this cycle.

[0040] Thus, the working fluid 4 according to the first embodiment has the function of a working fluid that serves as a driving source for the refrigerant compression unit 11, as well as the function of a refrigerant that intercools the flammable refrigerant 3 by exchanging heat with it.

[0041] In the first embodiment, the working fluid 4 is not particularly limited as long as it is a fluid that is less flammable than the flammable refrigerant 3. The working fluid 4 in the embodiment is, for example, a fluid (refrigerant) that corresponds to CLASS 1 (non-flammable) as defined in ISO 817:2014. For example, the working fluid 4 is selected from nitrogen gas, an inert gas, carbon dioxide gas, or air. In one example, the working fluid 4 is nitrogen gas. Each piece of equipment handling the working fluid 4 (working fluid line 17, working fluid cooling unit 19, first cooler 20, and second cooler 21) does not need to be provided with an explosion-proof structure. Thus, in the first embodiment, a compression cycle using nitrogen gas, which is also an inert gas, as the working fluid 4 is provided as a system for driving the refrigerant compression unit 11 of the flammable refrigerant 3.

[0042] Since the working fluid line 17 is a flow path independent of the refrigerant line 10, it is easy to sufficiently isolate the physical location of the working fluid compression unit 18 from areas where flammable refrigerant 3 is present (or where an explosive atmosphere may be formed), such as the refrigerant compression unit 11. Therefore, it is not necessary to make complex, prime mover-based equipment like the working fluid compression unit 18 explosion-proof.

[0043] Regarding the refrigerant compression section 11, since it handles the flammable refrigerant 3, at least the centrifugal compressor 31 adopts an explosion-proof structure. In the first embodiment, it is not necessary to provide a prime mover such as an electric motor in the refrigerant compression section 11, and since there is no ignition source, it is possible to easily realize an explosion-proof structure. That is, in the refrigerant compression section 11, an explosion-proof structure for a case where the degree of danger is lower than when equipped with an ignition source such as a prime mover can be adopted, so the structure for making the refrigerant compression section 11 explosion-proof can be simplified, and structural constraints are relaxed.

[0044] (Refrigeration method) Next, referring to FIGS. 1 and 2, the refrigeration method according to the first embodiment will be described. Note that the plots in the p-h diagram shown in FIG. 2 only conceptually illustrate the operation of the refrigeration system 1 and do not accurately represent the state of the refrigerant.

[0045] The refrigeration method according to the first embodiment includes a step of circulating a working fluid 4 having a lower flammability than the flammable refrigerant 3 in a pressurized state through the working fluid line 17. That is, in the first embodiment, in the low-pressure compression system CS1 and the high-pressure compression system CS2, the working fluid compression sections 18 operate respectively to pressurize and compress the working fluid 4 in the working fluid line 17. Thereby, the compressed gas of the working fluid 4 (that is, the gaseous working fluid 4 in a pressurized state) flows in the respective working fluid lines 17 of the low-pressure compression system CS1 and the high-pressure compression system CS2.

[0046] The refrigeration method according to the first embodiment includes a step of compressing the flammable refrigerant 3 flowing through the refrigerant line 10 by utilizing the pressure of the working fluid 4 pressurized and supplied from the working fluid line 17.

[0047] First, in the low-pressure compression system CS1, the working fluid 4 compressed by the working fluid compression unit 18 is cooled by the refrigerant in the working fluid cooling unit 19, and further cooled by the expanded working fluid 4 in the second cooler 21, and then expands in the expansion turbine 32 of the refrigerant compression unit 11. Due to the expansion of the working fluid 4, the centrifugal compressor 31 rotates and the combustible refrigerant 3 is compressed in the first stage (from point a to point b in FIG. 2). The combustible refrigerant 3 is cooled by the low-temperature working fluid 4 in the first cooler 20 (from point b to point c in FIG. 2), then merges with the combustible refrigerant 3 flowing through the bypass line 10A and is sent to the high-pressure compression system CS2.

[0048] In the high-pressure compression system CS2, the working fluid 4 compressed by the working fluid compression unit 18 is cooled by the refrigerant in the working fluid cooling unit 19 and then expands in the expansion turbine 32 of the refrigerant compression unit 11. Due to the expansion of the working fluid 4, the centrifugal compressor 31 rotates and the combustible refrigerant 3 is compressed in the second stage (from point c to point d in FIG. 2). The combustible refrigerant 3 compressed in two stages is cooled by the low-temperature working fluid 4 in the first cooler 20 and then sent to the refrigerant condensation unit 12.

[0049] The combustible refrigerant 3 sent out from the high-pressure compression system CS2 is condensed by being cooled through heat exchange with the atmosphere or cooling water in the refrigerant condensation unit 12 (from point d to point e in FIG. 2). The combustible refrigerant 3 passing through the refrigerant condensation unit 12 passes through the high-pressure refrigerant tank 13 (from point e to point f in FIG. 2) and is sent to the intermediate heat exchanger 14. A part of the combustible refrigerant 3 from the high-pressure refrigerant tank 13 is sent from the bypass line 10A to the intermediate heat exchanger 14, and the remaining part of the combustible refrigerant 3 flows through the refrigerant line 10 to the intermediate heat exchanger 14.

[0050] In the example of FIG. 1, in the intermediate heat exchanger 14, the combustible refrigerant 3 passing through the bypass line 10A cools the combustible refrigerant 3 passing through the refrigerant line 10. The combustible refrigerant 3 passing through the refrigerant line 10 decreases in temperature while remaining at high pressure due to cooling (from point f to point h in FIG. 2). The combustible refrigerant 3 passing through the bypass line 10A is depressurized and its temperature decreases by the expansion valve 22 (from point f to point g in FIG. 2), absorbs heat in the intermediate heat exchanger 14, and then returns to the high-pressure compression system CS2 (from point g to point c in FIG. 2).

[0051] The refrigeration method according to the first embodiment includes a step of expanding the compressed flammable refrigerant 3. That is, the refrigerant expansion unit 15 expands the flammable refrigerant 3 that has passed through the intermediate heat exchanger 14 in the refrigerant line 10, reducing the pressure and lowering the temperature (from point h to point i in Figure 2).

[0052] The refrigeration method according to the first embodiment includes a step of cooling the object to be cooled with a flammable refrigerant 3 whose temperature has decreased due to expansion. That is, the refrigerant heat exchange unit 16 cools the object to be cooled by heat exchange between the flammable refrigerant 3, which has passed through the refrigerant expansion unit 15 and has been depressurized and cooled, and the object to be cooled. The flammable refrigerant 3 vaporizes due to heat absorption from the object to be cooled (from point i to point a in Figure 2). In the refrigeration method according to the first embodiment, the flammable refrigerant 3 that has passed through the refrigerant heat exchange unit 16 is sent to the low-pressure compression system CS1 and returns to the compression step. The refrigeration method according to the first embodiment cools the object to be cooled by repeating each of the above steps.

[0053] [Second Embodiment] Figure 3 is a schematic diagram showing the low-pressure compression system according to the second embodiment. Figure 4 is a schematic diagram showing the high-pressure compression system according to the second embodiment. In the second embodiment, the configuration of the low-pressure compression system and the high-pressure compression system of the refrigeration system differs from that of the first embodiment. Note that in the second embodiment, the overall configuration of the refrigeration system is the same as in Figure 1, so a description is omitted.

[0054] In the refrigeration system 1 according to the second embodiment, the working fluid compression unit 18 has a centrifugal electric compressor. In the second embodiment, as shown in Figures 3 and 4, both the working fluid compression unit 18 of the low-pressure compression system CS1 and the working fluid compression unit 18 of the high-pressure compression system CS2 are composed of centrifugal electric compressors. Alternatively, only one of the working fluid compression units 18 may be composed of a centrifugal electric compressor. The configurations of the low-pressure compression system CS1 and the high-pressure compression system CS2 other than the working fluid compression unit 18 are the same as in the first embodiment, so their description is omitted.

[0055] In the examples shown in Figures 3 and 4, the centrifugal electric compressor employs an electric motor 51 as the prime mover and is a two-stage compression type compressor equipped with a low-pressure stage impeller 52 and a high-pressure stage impeller 53.

[0056] The electric motor 51 comprises, for example, a stator having a stator core with stator coils attached, and a rotor having a rotor core with permanent magnets attached. When current flows through the stator coils of the stator, the rotor rotates due to the attractive and repulsive forces of the resulting magnetic force. The rotor of the electric motor 51, the low-pressure stage impeller 52, and the high-pressure stage impeller 53 are connected by a rotating shaft so as to rotate as a single unit. The low-pressure stage impeller 52 and the high-pressure stage impeller 53 are impellers with blades formed thereon that push fluid radially outward when rotated. The low-pressure stage impeller 52 is positioned on one axial side relative to the electric motor 51, and the high-pressure stage impeller 53 is positioned on the other axial side relative to the electric motor 51.

[0057] When the low-pressure stage impeller 52 rotates, the working fluid 4 taken into the working fluid compression section 18 is accelerated and pressurized by the centrifugal force of the low-pressure stage impeller 52. The accelerated and pressurized working fluid 4 is then decelerated by the low-pressure side diffuser section and sent through the connecting passage to the high-pressure stage impeller 53. When the high-pressure stage impeller 53 rotates, the working fluid 4 compressed in the first stage by the low-pressure stage impeller 52 is accelerated and pressurized by the centrifugal force of the high-pressure stage impeller 53. The accelerated and pressurized air is then decelerated by the high-pressure side diffuser section and discharged from the outlet. As a result, the working fluid 4 compressed in the second stage by the high-pressure stage impeller 53 is sent to the working fluid line 17 toward the refrigerant compression section 11 (expansion turbine 32).

[0058] The working fluid compression unit 18, which is composed of a centrifugal electric compressor, can be made smaller in size, making it easy to secure installation space in the refrigeration system 1. Therefore, the working fluid compression unit 18 can be easily isolated from the area where the explosion-proof structure is employed without increasing the overall size of the refrigeration system 1.

[0059] [Third Embodiment] Figure 5 is a schematic diagram showing the low-pressure compression system according to the third embodiment. Figure 6 is a schematic diagram showing the high-pressure compression system according to the third embodiment. In the third embodiment, the configuration of the refrigerant compression section of the refrigeration system differs from that of the first embodiment. Note that in the third embodiment, the overall configuration of the refrigeration system is the same as in Figure 1, so a description is omitted.

[0060] As shown in Figures 5 and 6, in the refrigeration system 1 according to the third embodiment, the refrigerant compression unit 11 includes a plurality of centrifugal compressors 31 that compress the flammable refrigerant 3 in multiple stages, and a plurality of expansion turbines 32 that expand the working fluid 4 in multiple stages.

[0061] In the third embodiment, as shown in Figures 5 and 6, both the refrigerant compression section 11 of the low-pressure compression system CS1 and the refrigerant compression section 11 of the high-pressure compression system CS2 are composed of multi-stage compressors. Alternatively, only one of the refrigerant compression sections 11 may be configured as a multi-stage compressor. The working fluid compression section 18 may be the centrifugal electric compressor described in the second embodiment. The configurations of the low-pressure compression system CS1 and the high-pressure compression system CS2 other than the refrigerant compression section 11 are the same as in the first embodiment, so their description is omitted.

[0062] Figures 5 and 6 show an example of a multi-axis, multi-stage compression type refrigerant compression unit 11 in which a centrifugal compressor 31 and an expansion turbine 32 are provided on each of the multiple shafts 33. In the example of Figures 5 and 6, the refrigerant compression unit 11 has a two-axis, two-stage compression configuration comprising a low-pressure stage unit 61 and a high-pressure stage unit 62. The low-pressure stage unit 61 and the high-pressure stage unit 62 each include a centrifugal compressor 31 and an expansion turbine 32, which are connected by the shafts 33 in a manner that allows for power transmission.

[0063] The centrifugal compressor 31 of the low-pressure stage unit 61 receives the flammable refrigerant 3 from the refrigerant line 10. The expansion turbine 32 of the low-pressure stage unit 61 receives the compressed gas of the working fluid 4, which has been compressed by the working fluid compression unit 18 in the working fluid line 17. The low-pressure stage unit 61 extracts the expansion energy of the working fluid 4 as rotational power by expanding the compressed gas of the working fluid 4 in the expansion turbine 32 in the first stage. The low-pressure stage unit 61 rotates the centrifugal compressor 31 with the extracted rotational power, thereby compressing the flammable refrigerant 3 supplied from the refrigerant line 10 in the first stage.

[0064] The centrifugal compressor 31 of the high-pressure stage unit 62 receives the flammable refrigerant 3 that has been compressed in the first stage by the low-pressure stage unit 61. The expansion turbine 32 of the high-pressure stage unit 62 receives the compressed gas of the working fluid 4 after it has been expanded in the first stage by the low-pressure stage unit 61. The high-pressure stage unit 62 extracts the expansion energy of the working fluid 4 as rotational power by expanding the compressed gas of the working fluid 4 in the second stage using the expansion turbine 32. The high-pressure stage unit 62 uses the extracted rotational power to rotate the centrifugal compressor 31, thereby compressing the flammable refrigerant 3 that has been compressed in the first stage in the second stage.

[0065] This configuration makes it possible to achieve a high pressure ratio in the refrigerant compression unit 11. Figures 5 and 6 show an example in which the refrigerant compression unit 11 performs two-stage compression with a low-pressure stage unit 61 and a high-pressure stage unit 62, but the refrigerant compression unit 11 may be configured with three or more stages.

[0066] Figures 5 and 6 show examples of multi-axis multi-stage compression type refrigerant compression units 11, but a single-axis multi-stage compression type refrigerant compression unit 11 may also be used. Figure 7 is a schematic configuration diagram showing a low-pressure compression system according to a modified example of the third embodiment. Figure 8 is a schematic configuration diagram showing a high-pressure compression system according to a modified example of the third embodiment.

[0067] Figures 7 and 8 show an example of a multi-axis, multi-stage compression type refrigerant compression unit 11 in which multiple centrifugal compressors 31 and multiple expansion turbines 32 are provided on a single shaft 33. In the example shown in Figures 7 and 8, the refrigerant compression unit 11 has a single-axis, two-stage compression configuration with two centrifugal compressors 31 and two expansion turbines 32 on a single shaft 33.

[0068] The two centrifugal compressors 31 and the two expansion turbines 32 are mounted on a single shaft 33 so as to transmit power and rotate as a single unit. One of the two centrifugal compressors 31 is a low-pressure stage centrifugal compressor 31A, and the other is a high-pressure stage centrifugal compressor 31B. One of the two expansion turbines 32 is a low-pressure stage expansion turbine 32A, and the other is a high-pressure stage expansion turbine 32B.

[0069] The low-pressure stage centrifugal compressor 31A receives the flammable refrigerant 3 from the refrigerant line 10. The low-pressure stage centrifugal compressor 31A rotates in conjunction with the rotation of the shaft 33, compressing the flammable refrigerant 3 supplied from the refrigerant line 10 in the first stage. The high-pressure stage centrifugal compressor 31B receives the flammable refrigerant 3 that has been compressed in the first stage by the low-pressure stage centrifugal compressor 31A. The high-pressure stage centrifugal compressor 31B rotates in conjunction with the rotation of the shaft 33, compressing the flammable refrigerant 3 that has been compressed in the first stage in the second stage.

[0070] The high-pressure stepped expansion turbine 32B receives the compressed gas of the working fluid 4, which has been compressed by the working fluid compression section 18 in the working fluid line 17. The high-pressure stepped expansion turbine 32B rotates by expanding the compressed gas of the working fluid 4 in the first stage, and extracts the expansion energy of the working fluid 4 as rotational power. The low-pressure stepped expansion turbine 32A receives the compressed gas of the working fluid 4 that has been expanded in the first stage by the high-pressure stepped expansion turbine 32B. The low-pressure stepped expansion turbine 32A rotates by expanding the working fluid 4 that has been expanded in the first stage in the second stage, and extracts the expansion energy of the working fluid 4 as rotational power. As a result, the rotational power extracted by the high-pressure stepped expansion turbine 32B and the low-pressure stepped expansion turbine 32A performs the compression work on the low-pressure centrifugal compressor 31A and the high-pressure stepped expansion turbine 32B.

[0071] Figures 7 and 8 show an example in which the refrigerant compression unit 11 performs two-stage compression, but the refrigerant compression unit 11 may be configured with three or more stages. In other words, the number of centrifugal compressors 31 and expansion turbines 32 installed on the shaft 33 may be three or more. Alternatively, for example, two or more centrifugal compressors 31 may be installed to create a multi-stage system, while only one expansion turbine 32 may be installed to create a single-stage system.

[0072] The multi-axis multi-stage type shown in Figures 5 and 6 may be combined with the single-axis multi-stage type shown in Figures 7 and 8. In other words, the high-pressure stage unit 62 and low-pressure stage unit 61 shown in Figures 5 and 6 may be replaced with the single-axis multi-stage type units shown in Figures 7 and 8.

[0073] Figures 5 to 8 show the low-pressure compression system CS1 and the high-pressure compression system CS2, respectively. However, since a high pressure ratio can be obtained by using multiple stages, the refrigeration system 1 may be configured with only a single-stage compression system. For example, in Figure 1, only one of the compression systems, the low-pressure compression system CS1 or the high-pressure compression system CS2, may be used, and that compression system may be configured to include a multi-stage compression type refrigerant compression unit 11 as shown in Figures 5 to 8.

[0074] [Fourth Embodiment] Figure 9 is a schematic diagram showing a refrigeration system according to the fourth embodiment. In the fourth embodiment, a working fluid compression unit 18 is not provided in the working fluid line 17, and the compressed gas of the working fluid 4 is supplied from outside the refrigeration system 1. In the fourth embodiment, the configuration of the part through which the flammable refrigerant 3 flows is the same as in the first embodiment, so a description is omitted.

[0075] As shown in Figure 9, in the refrigeration system 1A according to the fourth embodiment, the working fluid line 80 has a fluid inlet 81 connected to a compressed fluid supply source 71 that pressurizes and supplies the working fluid 4, and a fluid outlet 82 that discharges the working fluid 4 that has passed through the refrigerant compression section 11. The working fluid line 80 according to the fourth embodiment is a non-circulating line that is open to the compressed fluid supply source 71 outside the refrigeration system 1A.

[0076] Furthermore, unlike the first embodiment in which the working fluid lines 17 were independent for the low-pressure compression system CS1 and the high-pressure compression system CS2, the working fluid line 80 according to the fourth embodiment is provided in common to both the low-pressure compression system CS1 and the high-pressure compression system CS2, and distributes and supplies the working fluid 4 to the low-pressure compression system CS1 and the high-pressure compression system CS2, respectively. In the fourth embodiment, the working fluid line 80 is not provided with a working fluid compression unit 18.

[0077] In the example shown in Figure 9, the working fluid line 80 has one fluid inlet 81 and one fluid outlet 82. The working fluid line 80 receives the working fluid 4 from the fluid inlet 81 and flows the working fluid 4 toward the fluid outlet 82. The working fluid line 80 receives the compressed gas of the working fluid 4 from the fluid inlet 81. Between the fluid inlet 81 and the fluid outlet 82, the refrigerant compression section 11 (expansion turbine 32) of the low-pressure compression system CS1 and the refrigerant compression section 11 (expansion turbine 32) of the high-pressure compression system CS2 are arranged. As the working fluid 4 flows from the fluid inlet 81 toward the fluid outlet 82, it expands in the expansion turbine 32 of the refrigerant compression section 11, providing rotational power to the centrifugal compressor 31. The working fluid line 80 discharges the working fluid 4, which has expanded in the refrigerant compression section 11, to the outside through the fluid outlet 82.

[0078] The compressed fluid supply source 71 is not particularly limited. For example, the compressed fluid supply source 71 is a compressor installed in the facility where the refrigeration system 1A is installed. Such a compressor is provided to supply compressed gas of the working fluid 4 to the refrigeration system 1A and other demand equipment within the facility, and is installed, for example, in a machine room within the facility. Alternatively, for example, the compressed fluid supply source 71 may be a high-pressure storage facility for the working fluid 4, such as an accumulator or a gas tank. The fluid inlet 81 of the working fluid line 80 is connected to a supply port 72 that is connected to the compressed fluid supply source 71. This supplies the working fluid 4 from the compressed fluid supply source 71 to the fluid inlet 81.

[0079] The fluid outlet 82 of the working fluid line 80 is connected, for example, to a connection port 74 of an exhaust line 73 installed in the facility where the refrigeration system 1A is installed. The working fluid 4 is discharged from the fluid outlet 82 to the exhaust line 73. Depending on the type of gas of the working fluid 4, the exhaust line 73 may discharge the working fluid 4 into the atmosphere or to a gas treatment device. Alternatively, the working fluid 4 sent to the exhaust line 73 may be sent to a compressor, which is a compressed fluid supply source 71, and recompressed.

[0080] In the example shown in Figure 9, the working fluid line 80 branches into a first line 83 and a second line 84 between the fluid inlet 81 and the fluid outlet 82. A low-pressure compression system CS1 is provided in the first line 83, and a high-pressure compression system CS2 is provided in the second line 84.

[0081] In other words, the first line 83 is provided with the working fluid cooling unit 19, the second cooler 21, the expansion turbine 32, and the first cooler 20 of the low-pressure compression system CS1 in that order. The working fluid 4 flowing through the first line 83 is cooled by the refrigerant in the working fluid cooling unit 19, cooled by heat exchange with the expanded working fluid 4 in the second cooler 21, and then expanded in the expansion turbine 32. The working fluid 4, which has expanded and cooled in the expansion turbine 32, absorbs heat from the flammable refrigerant 3 in the first cooler 20, and further absorbs heat from the high-pressure, high-temperature working fluid 4 in the second cooler 21, before exiting the low-pressure compression system CS1 and flowing to the fluid outlet 82.

[0082] The second line 84 is sequentially equipped with a working fluid cooling unit 19, an expansion turbine 32, and a first cooler 20 for the high-pressure compression system CS2. The working fluid 4 flowing through the second line 84 is cooled by a refrigerant in the working fluid cooling unit 19, and then expands in the expansion turbine 32. The working fluid 4, which has expanded and cooled in the expansion turbine 32, absorbs heat from the flammable refrigerant 3 in the first cooler 20, and then flows out of the high-pressure compression system CS2 to the fluid outlet 82.

[0083] The first line 83 and the second line 84 merge at a confluence point just before the fluid outlet 82. The working fluid 4 from the low-pressure compression system CS1 and the working fluid 4 from the high-pressure compression system CS2 merge at the confluence point and are discharged to the fluid outlet 82.

[0084] Thus, in the refrigeration system 1A according to the fourth embodiment, since there is no working fluid compression unit 18 for compressing the working fluid 4, the overall size of the refrigeration system 1A can be reduced. Furthermore, since there is no working fluid compression unit 18 which could potentially be an ignition source for the flammable refrigerant 3, an explosion-proof structure can be realized even more easily. Even if a compressor is provided in the facility as the compressed fluid supply source 71, it is easy to isolate the refrigerant line 10 of the flammable refrigerant 3 and the area where an explosive atmosphere may be formed in the refrigeration system 1A from the compressed fluid supply source 71.

[0085] [Effects of the Embodiment] The refrigeration system 1 according to the first embodiment includes a refrigerant line 10 through which a flammable refrigerant 3 flows, a refrigerant compression unit 11 provided in the refrigerant line 10 for compressing the flammable refrigerant 3, a refrigerant expansion unit 15 provided in the refrigerant line 10 for expanding the flammable refrigerant 3 compressed in the refrigerant compression unit 11, a refrigerant heat exchange unit 16 provided in the refrigerant line 10 for cooling the object to be cooled with the flammable refrigerant 3 whose temperature has decreased due to expansion, and a working fluid line 17 through which a working fluid 4 that is less flammable than the flammable refrigerant 3 flows. The refrigerant compression unit 11 receives compressed gas of the working fluid 4 from the working fluid line 17 and compresses the flammable refrigerant 3 using the pressure of the working fluid 4.

[0086] According to the first embodiment of the refrigeration system 1, the refrigerant compression unit 11 is not directly driven by a prime mover such as an electric motor, but rather the flammable refrigerant 3 flowing through the refrigerant line 10 is compressed using the pressure of the working fluid 4 supplied from the working fluid line 17. As a result, in a normal compressor, the refrigerant compression part and the prime mover are structurally close together, but with this structure, the prime mover, which may generate an ignition source (such as an electric spark), can be excluded or isolated from the area in the refrigerant compression unit 11 where the flammable refrigerant 3 (or explosive atmosphere) is present. Consequently, the requirements for achieving an explosion-proof structure in the refrigerant compression unit 11 are relaxed, thus easing the structural constraints when using flammable refrigerant 3.

[0087] The refrigeration system 1 according to the second embodiment is the refrigeration system 1 according to the first embodiment, wherein the refrigerant compression unit 11 includes a centrifugal compressor 31 for compressing a flammable refrigerant 3, and an expansion turbine 32 connected to the centrifugal compressor 31 so as to be able to transmit power, and which rotates due to the expansion of the working fluid 4, and has a turbo compressor that operates without a prime mover. As a result, the refrigerant compression unit 11 that compresses the flammable refrigerant 3 by the expansion of the working fluid 4 can be realized with a simple structure that does not include an ignition source (prime mover). As a result, an explosion-proof structure for the refrigerant compression unit 11 can be realized with an easy and simple configuration.

[0088] The third embodiment of the refrigeration system 1 is the refrigeration system 1 according to the first or second embodiment, wherein the working fluid line 17 is a circulation path and further comprises a working fluid compression section 18 that compresses the working fluid 4 expanded in the refrigerant compression section 11. This allows the circulating working fluid 4 to be reused by compressing it in the working fluid compression section 18. Since the working fluid line 17 is a separate circulation path from the refrigerant line 10, the working fluid compression section 18 can be easily isolated from areas requiring explosion protection, such as the refrigerant compression section 11. Therefore, it is possible to use a compressor with a prime mover in the working fluid compression section 18 without an explosion-proof structure, and there are no structural constraints.

[0089] The refrigeration system 1 according to the fourth embodiment is the refrigeration system 1 according to the third embodiment, further comprising: a working fluid cooling unit 19 that cools the working fluid 4 between a working fluid compression unit 18 and a refrigerant compression unit 11; and a first cooler 20 that cools the flammable refrigerant 3 compressed in the refrigerant compression unit 11 by heat exchange with the working fluid 4 expanded in the refrigerant compression unit 11. This allows the working fluid 4 to be used not only as a drive source to drive the refrigerant compression unit 11, but also as a refrigerant to cool the flammable refrigerant 3. This improves the cooling performance (energy efficiency) of the refrigeration system 1.

[0090] The fifth embodiment of the refrigeration system 1 is the same as the fourth embodiment of the refrigeration system 1, further comprising a second cooler 21 that cools the working fluid 4 flowing between the working fluid compression unit 18 and the working fluid cooling unit 19 by heat exchange with the working fluid 4 that has passed through the first cooler 20. As a result, the working fluid 4 before cooling the flammable refrigerant 3 can be pre-cooled in the second cooler 21 with the working fluid 4 that has been cooled in the first cooler 20. As a result, the amount of heat required to cool the working fluid cooling unit 19 can be reduced, further improving the cooling performance (energy efficiency) of the refrigeration system 1.

[0091] The refrigeration system 1 according to the sixth embodiment is the refrigeration system 1 according to the third embodiment, wherein the working fluid compression unit 18 has a centrifugal electric compressor. This makes it possible to provide a working fluid compression unit 18 that is small and has high compression performance. As a result, the refrigeration system 1 can be made smaller.

[0092] The seventh embodiment of the refrigeration system 1 is the same as the second embodiment of the refrigeration system 1, wherein the refrigerant compression unit 11 includes a plurality of centrifugal compressors 31 that compress the flammable refrigerant 3 in multiple stages, and a plurality of expansion turbines 32 that expand the working fluid 4 in multiple stages. This makes it possible to achieve a high compression ratio even without using prime mover power.

[0093] The refrigeration system 1 according to the eighth embodiment is a refrigeration system 1 according to the first or second embodiment, wherein the working fluid line 17 has a fluid inlet 81 connected to a compressed fluid supply source 71 that pressurizes and supplies working fluid 4, and a fluid outlet 82 that discharges the working fluid 4 that has passed through the refrigerant compression section 11. This makes it possible to supply pressurized working fluid 4 to the refrigerant compression section 11 without providing a compressor for the working fluid 4 in the refrigeration system 1. Therefore, the structural constraints for making the refrigeration system 1 explosion-proof can be further relaxed.

[0094] The refrigeration system 1 according to the ninth embodiment is a refrigeration system 1 according to any one of the first to eighth embodiments, wherein the flammable refrigerant 3 is a flammable natural refrigerant. This effectively alleviates the structural constraints required to realize a refrigeration system 1 using a low GWP flammable refrigerant 3.

[0095] The refrigeration system 1 according to the tenth embodiment is a refrigeration system 1 according to any one of the first to ninth embodiments, wherein the working fluid 4 is selected from nitrogen gas, an inert gas, carbon dioxide gas, or air. This allows the use of a gas with low flammability (or non-flammability) as the working fluid 4.

[0096] The 11th embodiment of the refrigeration method comprises the steps of: circulating a working fluid 4, which is less flammable than a flammable refrigerant 3, in a pressurized state through a working fluid line 17; compressing the flammable refrigerant 3 flowing through a refrigerant line 10 using the pressure of the working fluid 4 supplied pressurized from the working fluid line 17; expanding the compressed flammable refrigerant 3; and cooling the object to be cooled with the flammable refrigerant 3 whose temperature has decreased due to the expansion.

[0097] According to the refrigeration method of the eleventh embodiment, instead of compressing the flammable refrigerant 3 with a prime mover-driven compressor, the pressure of the working fluid 4 supplied from the working fluid line 17 can be used to compress the flammable refrigerant 3 flowing through the refrigerant line 10. This makes it possible to eliminate or isolate the prime mover, which may generate an ignition source (such as an electrical spark), from the area where the flammable refrigerant 3 (or explosive atmosphere) is present. As a result, the requirements for realizing an explosion-proof structure for the compressor are relaxed, and structural constraints when using the flammable refrigerant 3 can be eased.

[0098] Although embodiments of the present disclosure have been described above, the embodiments are not limited to those described herein. Furthermore, the aforementioned components include those that can be easily conceived by those skilled in the art, those that are substantially the same, and those that fall within the so-called equivalent range. Moreover, the aforementioned components can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the components can be made without departing from the gist of the embodiments described above.

[0099] For example, in the above embodiment, a bypass line 10A, an expansion valve 22, and an intermediate heat exchanger 14 are provided, but these components do not need to be provided.

[0100] Furthermore, in the above embodiment, an example was shown in which the working fluid 4 functions as a working fluid that drives the refrigerant compression unit 11, and also functions as a refrigerant that intercools the compressed flammable refrigerant 3 by exchanging heat with it, but the invention is not limited to this. The working fluid 4 does not have to be used as a refrigerant, and heat exchange between the flammable refrigerant 3 and the working fluid 4 is not required. In that case, the first cooler 20 and the second cooler 21 do not have to be provided.

[0101] Furthermore, although the above embodiment shows an example in which the working fluid 4 is supplied to the refrigerant compression unit 11 under pressure in the state of compressed gas (gas phase), the working fluid 4 may also be supplied to the refrigerant compression unit 11 under pressure in the state of liquid phase. The refrigerant compression unit 11 is not particularly limited as long as it has a structure that performs the compression work of the flammable refrigerant 3 using the pressure of the working fluid 4, and may include components other than an expansion turbine.

[0102] 1, 1A Refrigeration system 2 Cooling target 3 Flammable refrigerant 4 Working fluid 10 Refrigerant line 10A Bypass line 11 Refrigerant compression section 12 Refrigerant condensation section 13 High-pressure refrigerant tank 14 Intermediate heat exchanger 15 Refrigerant expansion section 16 Refrigerant heat exchange section 17, 80 Working fluid line 18 Working fluid compression section 19 Working fluid cooling section 20 First cooler 21 Second cooler 22 Expansion valve 31 Centrifugal compressor 31A Low-pressure stage centrifugal compressor 31B High-pressure stage centrifugal compressor 32 Expansion turbine 32A Low-pressure stage expansion turbine 32B High-pressure stage expansion turbine 33 Shaft 51 Electric motor 52 Low-pressure stage impeller 53 High-pressure stage impeller 61 Low-pressure stage unit 62 High-pressure stage unit 71 Compressed fluid supply source 72 Supply port 73 Exhaust line 74 Connection port 81 Fluid inlet 82 Fluid outlet 83 First line 84 Second line CS1 Low-pressure compression system CS2 High-pressure compression system

Claims

1. A refrigeration system comprising: a refrigerant line through which a flammable refrigerant flows; a refrigerant compression unit provided in the refrigerant line for compressing the flammable refrigerant; a refrigerant expansion unit provided in the refrigerant line for expanding the flammable refrigerant compressed in the refrigerant compression unit; a refrigerant heat exchange unit provided in the refrigerant line for cooling an object with the flammable refrigerant whose temperature has decreased due to expansion; and a working fluid line through which a working fluid less flammable than the flammable refrigerant flows, wherein the refrigerant compression unit receives the working fluid pressurized from the working fluid line and compresses the flammable refrigerant using the pressure of the working fluid.

2. The refrigeration system according to claim 1, wherein the refrigerant compression unit includes a centrifugal compressor for compressing the flammable refrigerant and an expansion turbine connected to the centrifugal compressor so as to be able to transmit power, and which rotates due to the expansion of the working fluid, and has a turbo compressor that operates without a prime mover.

3. The refrigeration system according to claim 1, wherein the working fluid line is a circulation path and further comprises a working fluid compression section for compressing the working fluid that has expanded in the refrigerant compression section.

4. The refrigeration system according to claim 3, further comprising: a working fluid cooling unit that cools the working fluid between the working fluid compression unit and the refrigerant compression unit; and a first cooler that cools the flammable refrigerant compressed in the refrigerant compression unit by heat exchange with the working fluid expanded in the refrigerant compression unit.

5. The refrigeration system according to claim 4, further comprising a second cooler that cools the working fluid flowing between the working fluid compression unit and the working fluid cooling unit by heat exchange with the working fluid that has passed through the first cooler.

6. The refrigeration system according to claim 3, wherein the working fluid compression unit has a centrifugal electric compressor.

7. The refrigeration system according to claim 2, wherein the refrigerant compression section includes a plurality of centrifugal compressors for compressing the flammable refrigerant in multiple stages, and a plurality of expansion turbines for expanding the working fluid in multiple stages.

8. The refrigeration system according to claim 1, wherein the working fluid line has a fluid inlet connected to a compressed fluid supply source that pressurizes and supplies the working fluid, and a fluid outlet for discharging the working fluid that has passed through the refrigerant compression section.

9. The refrigeration system according to claim 1, wherein the flammable refrigerant is a flammable natural refrigerant.

10. The refrigeration system according to claim 1, wherein the working fluid is selected from nitrogen gas, an inert gas, carbon dioxide gas, and air.

11. A refrigeration method comprising: circulating a working fluid that is less flammable than a flammable refrigerant under pressurized conditions through a working fluid line; compressing a flammable refrigerant flowing through a refrigerant line using the pressure of the working fluid supplied under pressurized conditions from the working fluid line; expanding the compressed flammable refrigerant; and cooling an object to be cooled with the flammable refrigerant whose temperature has decreased due to the expansion.

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