Cooling device

Abstract

To provide a cooling device that can improve energy saving performance while simplifying the configuration of a hot gas defrosting circuit.SOLUTION: A cooling device uses a multiple-stage compression freezing cycle 1 including: a circulation circuit in which at least compressors 20, 21 at multiple stages, a gas cooler 3, and an evaporator 5A, 5B are connected; and a hot gas defrosting circuit for defrosting the evaporator 5A, 5B. The hot gas defrost circuit is connected with a gas-liquid separation means 7 into which the thermal medium ejected from a higher-stage compressor 21 flows after being depressed to a medium pressure equal to or higher than the pressure of the thermal medium ejected from a lower-stage compressor 20 by a medium pressure adjustment means 8A. The liquid-phase thermal medium at the medium pressure separated by the gas-liquid separation means 7 is delivered to the evaporator 5B, and the gas-phase thermal medium at the medium pressure separated by the gas-liquid separation means 7 is suctioned into the high-stage compressor 21.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a cooling device using a multi-stage compression refrigeration cycle including a circulation circuit to which at least a multi-stage compressor, a gas cooler or a condenser, and an evaporator are connected, and a hot gas defrost circuit for defrosting the evaporator.

Background Art

[0002] Cooling devices used for cooling objects in various industrial fields are known. Such cooling devices are configured to cool an object using a refrigeration cycle with a heat medium. In recent years, natural refrigerants such as carbon dioxide and ammonia have attracted attention as heat media that do not impose a load on the global environment, replacing fluorocarbons. Since these natural refrigerants have a large pressure difference between high pressure and low pressure, for example, a multi-stage compression refrigeration cycle is adopted to improve compressor efficiency by compressing a low-pressure heat medium to a medium pressure and then further compressing the medium-pressure heat medium to a high pressure.

[0003] In a cooling device using a refrigeration cycle, since frost adheres to the surface of the evaporator during the cooling operation and the cooling capacity decreases, it is necessary to perform a defrost operation to remove the frost adhering to the evaporator. For example, in the cooling device of Patent Document 1 using a two-stage compression refrigeration cycle, a hot gas defrost circuit that enables the use of the high-pressure heat medium discharged from the high-stage compressor for the defrost operation is provided. Specifically, in the cooling device of Patent Document 1, in the defrost operation, after the defrosted gas-liquid two-phase heat medium that has passed through the evaporator is depressurized to a low pressure by an expansion valve provided on the downstream side of the evaporator in the hot gas defrost circuit, it is evaporated by heat exchange with the medium-pressure heat medium discharged from the low-stage compressor in a separately provided heat exchanger (heating device) to become a vapor-phase heat medium and then inhaled into the low-stage compressor, thereby preventing liquid backflow to the low-stage compressor by the heat medium after defrost.

Prior Art Documents

Patent Documents

[0004] [Patent Document 1] Microfilm of Japanese Utility Model Publication No. 55-10961 (pages 5-7, Figure 2) [Overview of the project] [Problems that the invention aims to solve]

[0005] However, while Patent Document 1 offers high reliability in preventing liquid backflow from the heat transfer medium to the lower-stage compressor after defrosting, it requires a separate heating device to evaporate the heat transfer medium after defrosting. This complicates the hot gas defrost circuit configuration, and furthermore, it necessitates compressing the heat transfer medium after defrosting in the lower-stage compressor. This increases the compression power for the heat transfer medium, which does not contribute to the cooling capacity, resulting in lower energy-saving performance.

[0006] This invention was made in view of these problems, and aims to provide a cooling device that can simplify the configuration of the hot gas defrost circuit while improving energy-saving performance. [Means for solving the problem]

[0007] To solve the above problems, the cooling device of the present invention is A cooling system using a multi-stage compression refrigeration cycle comprising a circulation circuit to which at least multiple stages of compressors, gas coolers or condensers, and evaporators are connected, and a hot gas defrost circuit for defrosting the evaporators, The hot gas defrost circuit is connected to a gas-liquid separation means into which the heat transfer medium discharged from the high-stage compressor is reduced to an intermediate pressure higher than that of the heat transfer medium discharged from the low-stage compressor by an intermediate pressure adjustment means, and the heat transfer medium is then introduced. The medium-pressure liquid phase heat transfer medium separated by the gas-liquid separation means is sent to the evaporator. The medium-pressure gas phase heat transfer medium separated by the gas-liquid separation means is characterized by being drawn into the high-stage compressor. According to this feature, the intermediate-pressure heat transfer medium discharged from the high-stage compressor and reduced in pressure by the intermediate-pressure adjustment means in the hot gas defrost circuit is separated into a gas phase heat transfer medium and a liquid phase heat transfer medium in the gas-liquid separation means. The intermediate-pressure liquid phase heat transfer medium is sent to the evaporator to contribute to the cooling capacity, and the intermediate-pressure gas phase heat transfer medium that does not contribute to the cooling capacity is drawn only into the high-stage compressor. This reduces the compression power of the low-stage compressor, thus simplifying the configuration of the hot gas defrost circuit while improving energy-saving performance.

[0008] The aforementioned intermediate pressure adjustment means is characterized by being a pressure reducing means provided downstream of the evaporator. According to this feature, by reducing the pressure of the heat transfer medium after defrosting the evaporator in the hot gas defrost circuit to a medium pressure using a pressure reduction means, a high-pressure heat transfer medium can be used for defrosting the evaporator, thereby increasing the defrosting efficiency of the evaporator.

[0009] The gas-liquid separation means is provided between the gas cooler or condenser and the evaporator, and is characterized in that a heat medium, which has been reduced to an intermediate pressure equal to or greater than the pressure of the heat medium discharged from the low-stage compressor by a pressure reducing means provided downstream of the gas cooler or condenser, flows into it. This feature allows for the simplification of the overall configuration of a cooling system equipped with a hot gas defrost circuit. By combining the defrosted, medium-pressure heat transfer medium with the depressurized, medium-pressure heat transfer medium that has passed through a gas cooler or condenser, the medium-pressure liquid phase heat transfer medium supplied to the evaporator undergoing cooling can be collected in one place.

[0010] A check valve is provided in the pipeline connecting the gas-liquid separation means and the suction side of the high-stage compressor. This feature prevents the heat transfer fluid discharged from the lower-stage compressor and drawn into the higher-stage compressor from flowing back into the gas-liquid separation means.

[0011] The hot gas defrost circuit is characterized by having a pressure reduction mechanism for reducing the pressure of the high-pressure gas phase heat transfer medium discharged from the high-stage compressor and using it for defrosting the evaporator. This feature allows for proper control of the defrost pressure in the evaporator.

[0012] The aforementioned gas-liquid separation means is a receiver tank, The receiver tank is characterized by being equipped with a level sensor that detects the upper limit of the liquid level of the liquid phase heat transfer medium. This feature allows for proper control of the amount of heat transfer fluid flowing into the receiver tank. [Brief explanation of the drawing]

[0013] [Figure 1] This figure shows a refrigeration cycle used in a cooling device according to an embodiment of the present invention. Figure 1 shows a state in which cooling operation is being performed on both evaporators connected in parallel. [Figure 2] This figure shows the state in the refrigeration cycle of the embodiment where one of the evaporators connected in parallel has been switched to defrost operation. [Figure 3] This figure shows the state in the refrigeration cycle of the embodiment where the other evaporator connected in parallel has been switched to defrost operation. [Modes for carrying out the invention]

[0014] Embodiments for implementing the cooling device according to the present invention will be described below based on examples. [Examples]

[0015] The cooling device according to the embodiment will be described with reference to Figures 1 to 3. In this embodiment, the cooling device using a two-stage compression-two-stage expansion refrigeration cycle 1 (hereinafter referred to as "refrigeration cycle 1") in which carbon dioxide is circulated as the heat transfer medium will be described as an example.

[0016] As shown in FIGS. 1 to 3, the refrigeration cycle 1 in this embodiment is used in a cooling device such as an industrial refrigerator, and in addition to the low-stage compressor 20, the high-stage compressor 21, the gas cooler 3, the expansion valves 4A and 4B, and the evaporators 5A and 5B, a medium-pressure electric valve 6 (pressure-reducing means provided on the downstream side of the gas cooler or condenser) and a receiver tank 7 (gas-liquid separation means) are connected by pipelines to form a circulation circuit, and a hot gas defrost circuit to which pressure-reducing valves 8A and 8B (medium-pressure adjusting means, pressure-reducing means provided on the downstream side of the evaporator) for reducing the heat medium after defrosting of the evaporators 5A and 5B described later to a medium pressure are connected, mainly composed of these components.

[0017] In this embodiment, the medium-pressure electric valve 6 is provided on the downstream side of the gas cooler 3, specifically, between the gas cooler 3 and the receiver tank 7. The medium-pressure electric valve 6 reduces the pressure of the high-pressure heat medium whose temperature has been lowered by passing through the gas cooler 3 to a pressure (PE3-2) not lower than the pressure (PE3-1) of the gas-phase heat medium discharged from the low-stage compressor 20, that is, the "medium pressure" in this embodiment. In this embodiment, the aspect where the pressure-reducing means provided on the downstream side of the gas cooler or condenser is the medium-pressure electric valve 6 is described, but it is not limited to this. The pressure-reducing means may have a function as a proportional automatic pressure regulating valve and may be constituted by, for example, an air-driven valve other than an electric valve.

[0018] In this embodiment, the "medium pressure" refers to a pressure within a range lower than the pressure of the high-pressure (PE1) gas-phase heat medium discharged from the high-stage compressor 21 and the defrost pressure (PE2, see FIGS. 2 and 3), and higher than the pressure of the low-pressure (PE4) liquid-phase heat medium decompressed by the expansion valves 4A and 4B.

[0019] In addition, the "medium-pressure" heat medium in the refrigeration cycle 1 of this embodiment is a vapor-phase heat medium of medium pressure (PE3-1) discharged from the low-stage compressor 20 described above, a gas-liquid two-phase heat medium of medium pressure (PE3-2) decompressed by the medium-pressure electric valve 6, and a gas-liquid two-phase heat medium of medium pressure (PE3-3) decompressed by the pressure-reducing valves 8A and 8B provided in the hot gas defrosting circuit. The pressure relationship of the "medium-pressure" heat medium in the refrigeration cycle 1 of this embodiment is PE3-3≒PE3-2≧PE3-1. That is, the pressure relationship of the heat medium in the refrigeration cycle 1 of this embodiment is PE1≧PE2>(PE3-3≒PE3-2≧PE3-1)>PE4.

[0020] In this embodiment, the receiver tank 7 is provided between the gas cooler 3 and the evaporators 5A and 5B, specifically, between the medium-pressure electric valve 6 provided on the downstream side of the gas cooler 3 and the expansion valves 4A and 4B. A gas-liquid two-phase heat medium of medium pressure (PE3-2) decompressed by the medium-pressure electric valve 6 flows into the receiver tank 7, and a gas-liquid two-phase heat medium of medium pressure (PE3-3) in which the heat medium after defrosting is decompressed by the pressure-reducing valves 8A and 8B provided in the hot gas defrosting circuit can also flow in. Further, the receiver tank 7 separates the incoming gas-liquid two-phase heat medium into a vapor-phase heat medium and a liquid-phase heat medium, sends the liquid-phase heat medium to the evaporators 5A and 5B, and can send the vapor-phase heat medium to the suction side of the high-stage compressor 21 through an injection pipe 101 (a pipe connecting the gas-liquid separation means and the suction side of the high-stage compressor) connected to the upper part of the receiver tank 7.

[0021] In this embodiment, the hot gas defrosting circuit branches from the downstream side of the high-stage compressor 21, specifically, between the high-stage compressor 21 and the gas cooler 3, and is provided with bypass pipelines 100A and 100B respectively connected to the upstream side of the evaporators 5A and 5B, specifically, between the expansion valves 4A and 4B and the evaporators 5A and 5B, so that the high-pressure (PE1) vapor-phase heat medium discharged from the high-stage compressor 21 can be used for defrosting the evaporators 5A and 5B.

[0022] Furthermore, the bypass pipelines 100A and 100B are equipped with pressure reducing mechanisms 90A and 90B for reducing the pressure (PE1) of the gas phase heat transfer medium discharged from the high-stage compressor 21 and using it for defrosting the evaporators 5A and 5B, thereby enabling proper control of the defrost pressure (PE2). In this embodiment, the pressure reducing mechanisms 90A and 90B are configured so that the opening degree of the electric valves 9A and 9B can be electrically controlled by PICs 91A and 91B based on the defrost pressure (PE2). In this embodiment, the pressure reducing mechanisms 90A and 90B are described as being composed of PICs 91A and 91B and electric valves 9A and 9B, but the mechanism is not limited to this configuration. The valves constituting the pressure reducing mechanism may be any valves that function as proportional automatic pressure regulating valves, and may be composed of, for example, air-driven valves in addition to electric valves.

[0023] Furthermore, the hot gas defrost circuit branches downstream of evaporators 5A and 5B, specifically between evaporators 5A and 5B and solenoid valves 50A and 50B, and merges downstream of pressure reducing valves 8A and 8B. It is equipped with a bypass pipeline 100C that connects to the receiver tank 7. This allows the heat transfer medium after defrosting to be depressurized by pressure reducing valves 8A and 8B and flow into the receiver tank 7 as a medium-pressure (PE3-3) gas-liquid two-phase heat transfer medium. Note that the medium-pressure (PE3-3) heat transfer medium flowing from the hot gas defrost circuit into the receiver tank 7 may be a liquid-phase heat transfer medium, for example, immediately after the start of defrost operation.

[0024] In this embodiment, the pressure reducing valves 8A and 8B reduce the pressure of the heat transfer medium after defrosting to approximately the same pressure (PE3-3) as the pressure (PE3-2) of the liquid phase heat transfer medium reduced by the intermediate pressure electric valve 6, i.e., the "intermediate pressure" in this embodiment. Note that the pressure reducing valves 8A and 8B may be either automatic valves (feedback opening control) or manual valves (fixed opening).

[0025] Here, we will explain the state in which cooling operation is being performed on both evaporators 5A and 5B, which are connected in parallel, using Figure 1. As shown in Figure 1, in the state in which cooling operation is being performed on both evaporators 5A and 5B, the electric valves 9A and 9B provided in the bypass pipes 100A and 100B that constitute the hot gas defrost circuit, and the solenoid valves 80A and 80B provided in the bypass pipe 100C are closed.

[0026] As shown by the solid arrows in Figure 1, when cooling operation is being performed in both evaporators 5A and 5B, the intermediate-pressure (PE3-1) gas-phase heat transfer medium discharged from the low-stage compressor 20 is drawn into the high-stage compressor 21, and the high-pressure (PE1) gas-phase heat transfer medium discharged from the high-stage compressor 21 passes through the gas cooler 3 and then through the intermediate-pressure electric valve 6, where it is depressurized to become an intermediate-pressure (PE3-2) gas-liquid two-phase heat transfer medium, which is stored in the receiver tank 7.

[0027] The medium-pressure (PE3-2) liquid phase heat transfer medium supplied from the receiver tank 7 to the evaporators 5A and 5B passes through solenoid valves 40A and 40B, and check valves 41A and 41B in that order. After passing through expansion valves 4A and 4B, it is depressurized to become a low-pressure (PE4) liquid phase heat transfer medium, which then flows into the evaporators 5A and 5B to cool the target (external fluid). The low-pressure (PE4) gas phase heat transfer medium that has passed through the evaporators 5A and 5B passes through solenoid valves 50A and 50B, respectively, and is then drawn into the low-stage compressor 20. Note that depending on the cooling capacity of the evaporators 5A and 5B, the heat transfer medium that has passed through may not evaporate sufficiently and may remain as a gas-liquid two-phase heat transfer medium. Therefore, for example, an accumulator or the like may be provided between the solenoid valves 50A and 50B and the low-stage compressor 20 to reliably prevent liquid backflow into the low-stage compressor 20.

[0028] Next, the state in which defrosting operation is performed in either evaporator 5A or 5B will be explained using Figures 2 and 3. As shown in Figure 2, when defrosting operation is performed in evaporator 5A, compared to the state in Figure 1, the electric valve 9A provided in the bypass pipeline 100A that constitutes the hot gas defrost circuit and the solenoid valve 80A provided in the bypass pipeline 100C are open, and the solenoid valve 40A provided upstream of the expansion valve 4A and the solenoid valve 50A provided downstream of evaporator 5A are closed. Also, as shown in Figure 3, when defrosting operation is performed in evaporator 5B, compared to the state in Figure 1, the electric valve 9B provided in the bypass pipeline 100B that constitutes the hot gas defrost circuit and the solenoid valve 80B provided in the bypass pipeline 100C are open, and the solenoid valve 40B provided upstream of the expansion valve 4B and the solenoid valve 50B provided downstream of evaporator 5B are closed.

[0029] As shown by the solid arrows in Figure 2, when defrosting is performed in evaporator 5A, the intermediate pressure (PE3-1) mainly discharged from the low-stage compressor 20 is drawn into the high-stage compressor 21, and a portion of the high-pressure (PE1) gas phase heat medium discharged from the high-stage compressor 21 is reduced in pressure by passing through the electric valve 9A provided in the bypass pipeline 100A, becoming the gas phase heat medium at defrost pressure (PE2), which flows into evaporator 5A to perform defrosting. Note that the high-pressure (PE1) gas phase heat medium does not necessarily need to be reduced in pressure by passing through the electric valve 9A, and may be used for defrosting of evaporator 5A while maintaining the high pressure (PE1).

[0030] The defrosted heat transfer fluid that has passed through the evaporator 5A passes through the solenoid valve 80A located in the bypass pipe 100C, and then through the pressure reducing valve 8A, where it is reduced in pressure to become a medium-pressure (PE3-3) gas-liquid two-phase heat transfer fluid, which then passes through the check valve 81A and flows into the receiver tank 7.

[0031] The medium-pressure (PE3-3) gas-liquid two-phase heat transfer medium flowing into the receiver tank 7 is separated into a gas-phase heat transfer medium and a liquid-phase heat transfer medium. The liquid-phase heat transfer medium separated in the receiver tank 7 passes through the gas cooler 3 and the medium-pressure electric valve 6, is mixed with the liquid-phase heat transfer medium separated and stored in the receiver tank 7, and is sent to the evaporator 5B to contribute to the cooling capacity of the evaporator 5B. The cooling operation of the evaporator 5B is the same as when the cooling operation is performed in both evaporators 5A and 5B as described above, so a detailed explanation is omitted.

[0032] Furthermore, the gaseous heat transfer medium separated in the receiver tank 7 is sent to the suction side of the high-stage compressor 21 through the injection pipe 101 connected to the top of the receiver tank 7 and is drawn into the high-stage compressor 21.

[0033] In this embodiment, the injection pipe 101 is provided with a check valve 110 to prevent the heat transfer fluid discharged from the low-stage compressor 20 and drawn into the high-stage compressor 21 from flowing back into the receiver tank 7.

[0034] Furthermore, in this embodiment, the receiver tank 7 is equipped with a level sensor 70 that detects whether the liquid level of the liquid phase heat transfer medium has reached its upper limit, thereby enabling proper control of the amount of heat transfer medium flowing into the receiver tank 7.

[0035] Furthermore, as shown in Figure 3, the state in which defrost operation is being performed in evaporator 5B is substantially the same as the state in which defrost operation is being performed in evaporator 5A described above, except that the operating modes of evaporators 5A and 5B are swapped, so a detailed explanation will be omitted.

[0036] As described above, in this embodiment, the cooling device reduces the pressure of the defrosted heat transfer medium to a pressure (intermediate pressure) equal to or greater than that of the gas phase heat transfer medium discharged from the low-stage compressor 20 by a pressure reducing means (either pressure reducing valve 8A or 8B) provided downstream of the evaporator where defrosting is performed. The receiver tank 7 into which this intermediate-pressure heat transfer medium flows is connected to the hot gas defrost circuit (bypass pipeline 100C). The intermediate-pressure liquid phase heat transfer medium separated by the receiver tank 7 is supplied to the evaporator where cooling is performed to contribute to the cooling capacity, while the intermediate-pressure gas phase heat transfer medium that does not contribute to the cooling capacity is drawn only into the high-stage compressor 21. This reduces the compression power of the low-stage compressor 20, eliminating the need for a heating device for the defrosted heat transfer medium that was required in conventional hot gas defrost circuits. This simplifies the configuration of the hot gas defrost circuit while improving energy-saving performance. Furthermore, the simplification of the hot gas defrost circuit reduces the initial cost of the cooling device and reduces the installation space required.

[0037] Furthermore, by combining the medium-pressure heat transfer medium, which has been defrosted by the pressure reducing valves 8A and 8B provided in the hot gas defrost circuit, with the medium-pressure heat transfer medium, which has been depressurized after passing through the gas cooler 3, in the receiver tank 7, the medium-pressure liquid phase heat transfer medium that is supplied to the evaporator under cooling operation can be collected in one place, thus simplifying the overall configuration of the cooling system equipped with a hot gas defrost circuit.

[0038] Furthermore, the intermediate pressure adjustment means in this embodiment is a pressure reducing valve 8A, 8B provided downstream of the evaporators 5A, 5B. By reducing the pressure of the heat transfer medium after defrosting of the evaporators 5A, 5B in the hot gas defrost circuit to an intermediate pressure using the pressure reducing valves 8A, 8B, a high-pressure (defrost pressure (PE2)) heat transfer medium can be used for defrosting the evaporators 5A, 5B, thereby increasing the defrosting efficiency of the evaporators 5A, 5B.

[0039] Furthermore, the bypass pipes 100A and 100B that constitute the hot gas defrost circuit are equipped with pressure reducing mechanisms 90A and 90B to reduce the pressure of the high-pressure (PE1) gas phase heat transfer medium discharged from the high-stage compressor 21 and use it for defrosting the evaporators 5A and 5B, thereby allowing the design pressure of the evaporators 5A and 5B to be lowered.

[0040] Although embodiments of the present invention have been described above with reference to the drawings, the specific configurations are not limited to these embodiments, and any changes or additions that do not depart from the spirit of the present invention are also included.

[0041] For example, in the above embodiment, the receiver tank 7 is configured to receive a medium-pressure gas-liquid two-phase heat transfer medium that has been depressurized by the medium-pressure electric valve 6, and to allow the flow of a medium-pressure gas-liquid two-phase heat transfer medium that has been depressurized by the pressure reducing valves 8A and 8B provided in the hot gas defrost circuit. However, the configuration is not limited to this, and it is sufficient that the receiver tank 7 is configured to allow the flow of a medium-pressure gas-liquid two-phase heat transfer medium that has been depressurized by the pressure reducing valves 8A and 8B provided in the hot gas defrost circuit. In other words, the receiver tank 7 does not need to be located between the gas cooler 3 and the evaporators 5A and 5B, and a separate receiver tank into which the medium-pressure gas-liquid two-phase heat transfer medium that has been depressurized by the medium-pressure electric valve 6 flows may be provided.

[0042] Furthermore, although the above embodiment described the intermediate pressure adjustment means as pressure reducing valves 8A and 8B provided downstream of the evaporators 5A and 5B, it is not limited to this. For example, without providing pressure reducing valves 8A and 8B, the pressure of the heat medium may be adjusted by electric valves 9A and 9B that constitute the pressure reducing mechanism 90A and 90B in the hot gas defrost circuit, and the heat medium after defrosting of the evaporators 5A and 5B may be controlled to reach an intermediate pressure (PE3-3). In this case, the intermediate pressure adjustment means would be the electric valves 9A and 9B that constitute the pressure reducing mechanism 90A and 90B and the evaporators 5A and 5B that are defrosted.

[0043] Furthermore, although the above embodiment was described assuming that the gas-liquid separation means is a receiver tank, the gas-liquid separation means is not limited to this. Any gas-liquid separator or gas-liquid separation device can be used as long as it can separate the gas-liquid two-phase heat transfer medium into a gas phase heat transfer medium and a liquid phase heat transfer medium, send the separated liquid phase heat transfer medium to an evaporator undergoing cooling operation, and send the gas phase heat transfer medium to the suction side of a high-stage compressor.

[0044] Furthermore, although the above embodiment described a configuration in which two evaporators 5A and 5B are connected in parallel in the refrigeration cycle 1, the configuration is not limited to this, and for example, a configuration in which three or more evaporators are connected in parallel may also be used.

[0045] Furthermore, in the above embodiment, the refrigeration cycle 1 was described as a two-stage compression and two-stage expansion system in which the heat medium, compressed in two stages by the low-stage compressor 20 and the high-stage compressor 21, is expanded in two stages by the intermediate-pressure electric valve 6 and the expansion valves 4A and 4B before flowing into the evaporators 5A and 5B. However, the present invention is not limited to this, and the refrigeration cycle may, for example, be a compression system with three or more stages, or a single-stage expansion system in which the heat medium discharged from the high-stage compressor is expanded in only one stage.

[0046] Furthermore, the multi-stage compressor is not limited to one in which a low-stage compressor and a high-stage compressor are provided separately, as in the above embodiment, but may also be a single-shaft multi-stage compressor.

[0047] Furthermore, in the above embodiment, a gas cooler is used because carbon dioxide is circulated as the heat transfer medium. However, if, for example, a fluorocarbon or ammonia is circulated as the heat transfer medium, it is preferable to use a condenser instead of a gas cooler. [Explanation of symbols]

[0048] 1 Refrigeration cycle 3. Gas cooler 4A, 4B Expansion valve 5A, 5B Evaporator 6. Intermediate-pressure electric valve (a pressure reducing means provided downstream of the gas cooler or condenser) 7. Receiver tank (gas-liquid separation means) 8A, 8B Pressure reducing valve (intermediate pressure adjustment means, pressure reducing means provided downstream of the evaporator) 9A, 9B Electric Valve 20 Low-stage compressor 21 High-stage compressor 40A, 40B Solenoid Valves 41A, 41B Check valve 50A, 50B Solenoid Valves 70-level sensor 80A, 80B Solenoid Valves 81A, 81B Check valve 90A, 90B Pressure Reduction Mechanism 100A~100C Bypass Pipeline 101 Injection pipe (pipe connecting the gas-liquid separation means and the suction side of the high-stage compressor) 110 Check valve

Claims

1. A cooling system using a multi-stage compression refrigeration cycle comprising a circulation circuit connected to at least multiple stages of compressors, gas coolers or condensers, and two or more evaporators connected in parallel, and a hot gas defrost circuit for defrosting the evaporators, The hot gas defrost circuit is connected to a pressure reducing mechanism provided upstream of the evaporator for controlling the pressure of the heat transfer medium discharged from the high-stage compressor to a reduced pressure, a pressure reducing means provided downstream of the evaporator for reducing the pressure of the heat transfer medium discharged from the evaporator to an intermediate pressure higher than that of the heat transfer medium discharged from the low-stage compressor, and a gas-liquid separation means into which the heat transfer medium reduced in pressure by the pressure reducing means flows. When defrosting is being performed in one or more of the evaporators, the medium-pressure liquid phase heat transfer medium separated by the gas-liquid separation means is sent to the evaporator where defrosting is not being performed. The medium-pressure gas phase heat transfer medium separated by the gas-liquid separation means is drawn into the high-stage compressor. The high-pressure heat transfer fluid discharged from the aforementioned high-stage compressor is controlled to be depressurized by the aforementioned depressurization mechanism and is drawn into the evaporator where the defrosting is being performed. The heat transfer fluid discharged from the evaporator where the defrosting is performed is depressurized by the depressurization means and flows into the gas-liquid separation means. A cooling device characterized by the following features.

2. The cooling apparatus according to claim 1, wherein the gas-liquid separation means is provided between the gas cooler or the condenser and the evaporator, and a heat medium that has been reduced to an intermediate pressure equal to or greater than the pressure of the heat medium discharged from the low-stage compressor by a pressure reducing means provided downstream of the gas cooler or the condenser flows into it.

3. The cooling device according to claim 1 or 2, characterized in that a check valve is provided in the piping connecting the gas-liquid separation means and the suction side of the high-stage compressor.

4. The aforementioned gas-liquid separation means is a receiver tank, The cooling device according to claim 1 or 2, characterized in that a level sensor is provided to detect the upper limit of the liquid level of the liquid phase heat transfer medium in the receiver tank.

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