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 multistage compression refrigeration cycle that includes a circulation circuit connected to at least a multistage compressor, a gas cooler or condenser, and an evaporator, and a hot gas defrosting circuit that defrosts the evaporator. [Background technology]

[0002] Chillers used to cool objects in various industrial fields are known. Such chillers are configured to cool objects using a refrigeration cycle with a heat transfer medium. In recent years, natural refrigerants such as carbon dioxide and ammonia have been attracting attention as environmentally friendly alternatives to chlorofluorocarbons. Because these natural refrigerants have a large pressure difference between high and low pressures, a multi-stage compression refrigeration cycle is employed, which improves compressor efficiency by, for example, compressing a low-pressure heat transfer medium to a medium pressure and then further compressing the medium-pressure heat transfer medium to a high pressure.

[0003] In a cooling system using a refrigeration cycle, frost forms on the surface of the evaporator during cooling operation, reducing the cooling capacity. Therefore, a defrosting operation is required to remove the frost that has formed on the evaporator. For example, the cooling system of Patent Document 1, which uses a two-stage compression refrigeration cycle, is equipped with a hot gas defrosting circuit that enables the use of a high-pressure heat transfer medium discharged from a high-stage compressor for defrosting operation. Specifically, in the cooling system of Patent Document 1, during defrosting operation, the defrosted gas-liquid two-phase heat transfer medium that has passed through the evaporator is reduced in pressure by an expansion valve provided downstream of the evaporator in the hot gas defrosting circuit. Then, in a separately provided heat exchanger (heating device), the defrosted gas-liquid two-phase heat transfer medium is evaporated by heat exchange with a medium-pressure heat transfer medium discharged from a low-stage compressor, resulting in a gas-phase heat transfer medium that is then drawn into the low-stage compressor. This prevents the defrosted heat transfer medium from returning to the low-stage compressor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Microfilm of Utility Model Publication No. 55-10961 (pages 5-7, Figure 2) Summary of the Invention [Problem to be solved by the invention]

[0005] However, although Patent Document 1 is highly reliable in terms of preventing the liquid backflow of the heat transfer medium after defrosting into the low-stage compressor, it requires a separate heating device to evaporate the heat transfer medium after defrosting, which not only complicates the configuration of the hot gas defrost circuit, but also requires that the heat transfer medium after defrosting be compressed in the low-stage compressor, which increases the compression power of the heat transfer medium that does not contribute to cooling capacity, resulting in problems of reduced energy-saving performance.

[0006] The present invention has been made in light 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] In order to solve the above problems, the cooling device of the present invention comprises: A cooling device using a multistage compression refrigeration cycle including a circulation circuit to which at least a plurality of compressors, a gas cooler or a condenser, and an evaporator are connected, and a hot gas defrosting circuit to defrost the evaporator, The hot gas defrost circuit is connected to a gas-liquid separation means into which a heat medium discharged from a high-stage compressor is decompressed by a medium-pressure adjustment means to a medium pressure equal to or higher than that of the heat medium discharged from a low-stage compressor, and into which the heat medium flows, 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 drawn into the high-stage compressor. According to this feature, the medium-pressure heat transfer medium discharged from the high-stage compressor and depressurized by the medium-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, and the medium-pressure liquid-phase heat transfer medium is sent to the evaporator to contribute to the cooling capacity, while the medium-pressure gas-phase heat transfer medium that does not contribute to the cooling capacity is drawn into only the high-stage compressor, thereby reducing the compression power of the low-stage compressor and thereby simplifying the configuration of the hot gas defrost circuit while improving energy-saving performance.

[0008] The intermediate pressure adjusting means is 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 defrosting circuit to a medium pressure using a pressure reducing means, a high-pressure heat transfer medium can be used to defrost the evaporator, thereby improving the defrosting efficiency of the evaporator.

[0009] The gas-liquid separation means is provided between the gas cooler or the condenser and the evaporator, and the heat medium is introduced into the gas-liquid separation means after being decompressed to a medium pressure equal to or higher than the pressure of the heat medium discharged from the low-stage compressor by a decompression means provided downstream of the gas cooler or the condenser. According to this feature, by combining the medium-pressure heat transfer medium obtained by decompressing the heat transfer medium after defrosting and the medium-pressure heat transfer medium obtained by decompressing the heat transfer medium that has passed through a gas cooler or condenser in the gas-liquid separation means, the medium-pressure liquid-phase heat transfer medium to be sent to the evaporator performing cooling operation can be collected in one place, thereby simplifying the overall configuration of a cooling device equipped with a hot gas defrost circuit.

[0010] A check valve is provided in a pipe connecting the gas-liquid separating means and the suction side of the high-stage compressor. According to this feature, it is possible to prevent the heat medium discharged from the low-stage compressor and sucked into the high-stage compressor from flowing back toward the gas-liquid separating means.

[0011] The hot gas defrost circuit is characterized by including a pressure reducing mechanism for reducing the pressure of the high-pressure gas phase heat transfer medium discharged from the high-stage compressor and using the reduced pressure to defrost the evaporator. According to this feature, the defrost pressure in the evaporator can be appropriately controlled.

[0012] the gas-liquid separation means is a receiver tank; The receiver tank is characterized by being provided with a level sensor for detecting the upper limit of the liquid level of the liquid-phase heat transfer medium in the receiver tank. According to this feature, the amount of heat transfer medium flowing into the receiver tank can be appropriately controlled. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a diagram showing a refrigeration cycle used in a cooling device of an embodiment according to the present invention, and Fig. 1 shows a state in which cooling operation is performed by both evaporators connected in parallel. [Figure 2] FIG. 10 is a diagram showing a state in which one of the evaporators connected in parallel is switched to a defrosting operation in the refrigeration cycle of the embodiment. [Figure 3] FIG. 10 is a diagram showing a state in which the other evaporator connected in parallel is switched to a defrosting operation in the refrigeration cycle of the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A cooling device according to an embodiment of the present invention will be described below with reference to the accompanying drawings. [Example]

[0015] A cooling device according to an embodiment will be described with reference to Fig. 1 to Fig. 3. In this embodiment, a 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 a heat medium will be described as an example.

[0016] As shown in Figures 1 to 3, the refrigeration cycle 1 in this embodiment is used in cooling devices such as industrial freezers, and is mainly composed of a low-stage compressor 20 and a high-stage compressor 21, a gas cooler 3, expansion valves 4A, 4B, evaporators 5A, 5B, as well as a circulation circuit in which a medium-pressure electric valve 6 (pressure reduction means provided downstream of the gas cooler or condenser) and a receiver tank 7 (gas-liquid separation means) are connected by pipes, and a hot gas defrost circuit to which pressure reduction valves 8A, 8B (medium-pressure adjustment means, pressure reduction means provided downstream of the evaporators) are connected for reducing the pressure of the heat medium to medium after defrosting of the evaporators 5A, 5B, which will be described later.

[0017] In this embodiment, the medium pressure motor operated valve 6 is provided downstream of the gas cooler 3, more specifically, between the gas cooler 3 and the receiver tank 7. The medium pressure motor operated valve 6 reduces the pressure of the high-pressure heat medium, the temperature of which has been reduced by passing through the gas cooler 3, to a pressure (PE3-2) that is equal to or higher than the pressure (PE3-1) of the gas-phase heat medium discharged from the low-stage compressor 20, i.e., the "medium pressure" in this embodiment. Note that, in this embodiment, an embodiment has been described in which the pressure reducing means provided downstream of the gas cooler or condenser is the medium pressure motor operated valve 6, but the present invention is not limited to this. The pressure reducing means may be any means that has the function of a proportional automatic pressure control valve, and may be configured as an air-driven valve or the like other than a motor operated valve.

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

[0019] Furthermore, the "medium-pressure" heat transfer medium in the refrigeration cycle 1 of this embodiment is the medium-pressure (PE3-1) gas-phase heat transfer medium discharged from the above-mentioned low-stage compressor 20, the medium-pressure (PE3-2) gas-liquid two-phase heat transfer medium depressurized by the medium-pressure motor-operated valve 6, and the medium-pressure (PE3-3) gas-liquid two-phase heat transfer medium depressurized by pressure reducing valves 8A and 8B provided in the hot gas defrost circuit. The pressure relationship of the "medium-pressure" heat transfer medium in the refrigeration cycle 1 of this embodiment is PE3-3 ≒ PE3-2 ≧ PE3-1. That is, the pressure relationship of the heat transfer 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 disposed between the gas cooler 3 and the evaporators 5A, 5B, more specifically, between the medium-pressure motor-operated valve 6 and the expansion valves 4A, 4B, which are disposed downstream of the gas cooler 3. The receiver tank 7 receives medium-pressure (PE3-2) two-phase gas-liquid heat transfer medium reduced in pressure by the medium-pressure motor-operated valve 6, and also receives medium-pressure (PE3-3) two-phase gas-liquid heat transfer medium reduced in pressure by pressure reducing valves 8A, 8B disposed in the hot gas defrost circuit after defrosting. The receiver tank 7 separates the incoming two-phase gas-liquid heat transfer medium into gas-phase and liquid-phase heat transfer medium, and delivers the liquid-phase heat transfer medium to the evaporators 5A, 5B. The receiver tank 7 also delivers the gas-phase heat transfer 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 top of the receiver tank 7.

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

[0022] The bypass lines 100A, 100B are equipped with pressure reducing mechanisms 90A, 90B that reduce the pressure of the high-pressure (PE1) gas-phase heat transfer medium discharged from the high-stage compressor 21 and use it to defrost the evaporators 5A, 5B, thereby enabling appropriate control of the defrost pressure (PE2). In this embodiment, the pressure reducing mechanisms 90A, 90B are configured so that the openings of the motor-operated valves 9A, 9B are electrically controlled by the PICs 91A, 91B based on the defrost pressure (PE2). While this embodiment describes a configuration in which the pressure reducing mechanisms 90A, 90B are each composed of the PICs 91A, 91B and the motor-operated valves 9A, 9B, the present invention is not limited thereto. The valves constituting the pressure reducing mechanisms may be any valves capable of functioning as proportional automatic pressure regulating valves, and may be configured, for example, as air-operated valves other than the motor-operated valves.

[0023] The hot gas defrost circuit is provided with a bypass line 100C that branches off downstream of the evaporators 5A, 5B, more specifically between the evaporators 5A, 5B and the solenoid valves 50A, 50B, joins downstream of the pressure reducing valves 8A, 8B, and is then connected to the receiver tank 7, so that the heat transfer medium after defrosting is reduced in pressure by the pressure reducing valves 8A, 8B and flows into the receiver tank 7 as a medium-pressure (PE3-3) two-phase gas-liquid 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 in the liquid phase, 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 defrost to a pressure (PE3-3) that is substantially the same as the pressure (PE3-2) of the liquid phase heat transfer medium reduced by the medium pressure motor-operated valve 6, i.e., to the "medium pressure" in this embodiment. The pressure reducing valves 8A and 8B may be either automatic valves (feedback opening control) or manual valves (fixed opening).

[0025] Here, a state in which both evaporators 5A and 5B connected in parallel are performing cooling operations will be described with reference to Fig. 1. As shown in Fig. 1, when both evaporators 5A and 5B are performing cooling operations, motor-operated valves 9A and 9B provided in bypass pipes 100A and 100B constituting the hot gas defrost circuit and solenoid valves 80A and 80B provided in 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 medium-pressure (PE3-1) gas-phase heat transfer medium discharged from the low-stage compressor 20 is sucked 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 further passes through the medium-pressure electric valve 6, whereby it is reduced in pressure to become a medium-pressure (PE3-2) gas-liquid two-phase heat transfer medium and is stored in the receiver tank 7.

[0027] The medium-pressure (PE3-2) liquid-phase heat transfer medium sent 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, and then passes through expansion valves 4A and 4B, respectively, where it is reduced in pressure to become a low-pressure (PE4) liquid-phase heat transfer medium. This flows into the evaporators 5A and 5B, respectively, and cools 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 the evaporators 5A and 5B may not evaporate sufficiently and become a two-phase gas-liquid heat transfer medium. Therefore, for example, a separate 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 to the low-stage compressor 20.

[0028] Next, a state in which a defrosting operation is being performed in one of the evaporators 5A and 5B will be described with reference to Figures 2 and 3. As shown in Figure 2, when a defrosting operation is being performed in the evaporator 5A, compared to the state in Figure 1, motor-operated valve 9A provided in bypass pipe 100A and solenoid valve 80A provided in bypass pipe 100C, which constitute the hot gas defrosting circuit, are opened, and solenoid valve 40A provided upstream of expansion valve 4A and solenoid valve 50A provided downstream of evaporator 5A are closed. Also, as shown in Figure 3, when a defrosting operation is being performed in the evaporator 5B, compared to the state in Figure 1, motor-operated valve 9B provided in bypass pipe 100B and solenoid valve 80B provided in bypass pipe 100C, which constitute the hot gas defrosting circuit, are opened, and solenoid valve 40B provided upstream of expansion valve 4B and solenoid valve 50B provided downstream of evaporator 5B are closed.

[0029] 2, when the defrosting operation is being performed in the evaporator 5A, the medium-pressure (PE3-1) gaseous heat transfer medium discharged mainly from the low-stage compressor 20 is sucked into the high-stage compressor 21, and a portion of the high-pressure (PE1) gaseous heat transfer medium discharged from the high-stage compressor 21 is reduced in pressure by passing through the motor-operated valve 9A provided in the bypass pipe 100A, becoming a gaseous heat transfer medium at a defrosting pressure (PE2), which flows into the evaporator 5A and performs defrosting. Note that the high-pressure (PE1) gaseous heat transfer medium does not necessarily have to be reduced in pressure by passing through the motor-operated valve 9A, and may be used for defrosting the evaporator 5A while maintaining the high pressure (PE1).

[0030] The defrosted heat transfer medium that has passed through the evaporator 5A passes through the solenoid valve 80A provided in the bypass line 100C, and then passes through the pressure reducing valve 8A to reduce the pressure and become a medium pressure (PE3-3) gas-liquid two-phase heat transfer medium, 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 that flows 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 motor-operated valve 6, and is mixed with the liquid phase heat transfer medium that is 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. Note that the cooling operation of the evaporator 5B is the same as the state in which the cooling operation is performed by both the evaporators 5A and 5B described above, and therefore a detailed description thereof will be omitted.

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

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

[0034] In addition, in this embodiment, the receiver tank 7 is provided with a level sensor 70 that detects whether the liquid level of the liquid-phase heat transfer medium has reached its upper limit, allowing the amount of heat transfer medium flowing into the receiver tank 7 to be appropriately controlled.

[0035] Also, 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 detailed explanation will be omitted.

[0036] As described above, in this embodiment, the cooling system uses a pressure reducing means (either pressure reducing valve 8A or 8B) downstream of the defrosting evaporator to reduce the pressure of the defrosted heat transfer medium to a pressure (medium pressure) equal to or higher than the gaseous heat transfer medium discharged from low-stage compressor 20. The medium-pressure heat transfer medium flows into receiver tank 7, which is connected to hot gas defrost circuit (bypass pipe 100C). The medium-pressure liquid heat transfer medium separated by receiver tank 7 is sent to the evaporator in cooling operation, contributing to cooling capacity. The medium-pressure gaseous heat transfer medium, which does not contribute to cooling capacity, is drawn only into high-stage compressor 21, thereby reducing the compression power of low-stage compressor 20. This eliminates the need for a heating device for the defrosted heat transfer medium, which is required in conventional hot gas defrost circuits. This simplifies the hot gas defrost circuit configuration and improves energy efficiency. Furthermore, the simplified hot gas defrost circuit configuration reduces the initial cost of the cooling system and reduces installation space.

[0037] In addition, by combining the medium-pressure heat transfer medium obtained by reducing the pressure of the heat transfer medium after defrosting using pressure reducing valves 8A and 8B provided in the hot gas defrost circuit and the medium-pressure heat transfer medium obtained by reducing the pressure of the heat transfer medium after passing through the gas cooler 3 in the receiver tank 7, the medium-pressure liquid-phase heat transfer medium to be sent to the evaporator performing cooling operation can be collected in one place, thereby simplifying the overall configuration of the cooling device equipped with a hot gas defrost circuit.

[0038] In addition, the medium pressure adjustment means in this embodiment are pressure reducing valves 8A, 8B provided downstream of the evaporators 5A, 5B, and by reducing the pressure of the heat transfer medium after defrosting of the evaporators 5A, 5B in the hot gas defrost circuit to medium pressure by the pressure reducing valves 8A, 8B, it is possible to use a high pressure (defrost pressure (PE2)) heat transfer medium for defrosting the evaporators 5A, 5B, thereby improving the defrosting efficiency of the evaporators 5A, 5B.

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

[0040] Although the embodiments of the present invention have been described above with reference to the drawings, the specific configuration is not limited to these embodiments, and the present invention also includes modifications and additions that do not deviate from the gist of the present invention.

[0041] For example, in the above embodiment, a configuration has been described in which the medium-pressure gas-liquid two-phase heat medium depressurized by the medium-pressure motor-operated valve 6 flows into the receiver tank 7, and the medium-pressure gas-liquid two-phase heat medium depressurized after defrosting by the pressure reducing valves 8A, 8B provided in the hot gas defrost circuit can also flow into the receiver tank 7. However, the present invention is not limited to this, and it is sufficient that at least the medium-pressure gas-liquid two-phase heat medium depressurized after defrosting by the pressure reducing valves 8A, 8B provided in the hot gas defrost circuit can flow into the receiver tank 7. In other words, the receiver tank 7 does not need to be provided between the gas cooler 3 and the evaporators 5A, 5B, and a separate receiver tank may be provided into which the medium-pressure gas-liquid two-phase heat medium depressurized by the medium-pressure motor-operated valve 6 flows.

[0042] In the above embodiment, the intermediate pressure adjusting means is described as being the pressure reducing valves 8A, 8B provided downstream of the evaporators 5A, 5B, but this is not limiting. For example, instead of providing the pressure reducing valves 8A, 8B, the pressure of the heat transfer medium may be adjusted by the motor-operated valves 9A, 9B constituting the pressure reducing mechanisms 90A, 90B in the hot gas defrost circuit, and the heat transfer medium may be controlled to be at an intermediate pressure (PE3-3) after defrosting in the evaporators 5A, 5B. In this case, the intermediate pressure adjusting means is the motor-operated valves 9A, 9B constituting the pressure reducing mechanisms 90A, 90B and the evaporators 5A, 5B where defrosting is performed.

[0043] Furthermore, in the above embodiment, the gas-liquid separation means is described as being a receiver tank, but this is not limited to this. The gas-liquid separation means may be any of various gas-liquid separators or gas-liquid separation devices, 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, can send the separated liquid-phase heat transfer medium to an evaporator performing cooling operation, and can send the gas-phase heat transfer medium to the suction side of the high-stage compressor.

[0044] In addition, in the above embodiment, the refrigeration cycle 1 is described as having two evaporators 5A, 5B connected in parallel, but this is not limited to this, and the refrigeration cycle 1 may also be configured, for example, with three or more evaporators connected in parallel.

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

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

[0047] In addition, in the above embodiment, a gas cooler is used because carbon dioxide is circulated as a heat transfer medium. However, if, for example, chlorofluorocarbon, ammonia, or the like is circulated as a 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. Medium pressure motor-operated valve (pressure reducing means installed downstream of gas cooler or condenser) 7 Receiver tank (gas-liquid separation means) 8A, 8B Pressure reducing valve (medium pressure adjusting means, pressure reducing means provided downstream of the evaporator) 9A, 9B Electric valve 20 Low-stage compressor 21 High-stage compressor 40A, 40B solenoid valve 41A, 41B check valve 50A, 50B solenoid valve 70 Level Sensor 80A, 80B solenoid valve 81A, 81B check valve 90A, 90B pressure reduction mechanism 100A~100C Bypass pipe 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 device using a multistage compression refrigeration cycle including a circulation circuit to which at least a plurality of compressors, a gas cooler or a condenser, and an evaporator are connected, and a hot gas defrosting circuit to defrost the evaporator, The hot gas defrost circuit is connected to a gas-liquid separation means into which a heat medium discharged from a high-stage compressor is decompressed by a medium-pressure adjustment means to a medium pressure equal to or higher than that of the heat medium discharged from a low-stage compressor, and into which the heat medium flows, The medium-pressure liquid-phase heat transfer medium separated by the gas-liquid separation means is sent to the evaporator, The cooling device is characterized in that the medium-pressure gas-phase heat transfer medium separated by the gas-liquid separation means is sucked into the high-stage compressor.

2. 2. The cooling device according to claim 1, wherein the intermediate pressure adjusting means is a pressure reducing means provided downstream of the evaporator.

3. 3. The cooling device according to claim 2, 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 in pressure to an intermediate pressure equal to or higher 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 the gas-liquid separation means.

4. 4. The cooling system according to claim 1, wherein a check valve is provided in a pipe connecting said gas-liquid separating means and the suction side of said high-stage compressor.

5. 4. The cooling device according to claim 2, wherein the hot gas defrosting circuit includes a pressure reducing mechanism for reducing the pressure of the high-pressure gas phase heat transfer medium discharged from the high-stage compressor and using the reduced pressure to defrost the evaporator.

6. the gas-liquid separation means is a receiver tank; 4. The cooling device according to claim 1, further comprising a level sensor for detecting an upper limit of the liquid level of the liquid heat transfer medium in the receiver tank.

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