AU2009239038A1 - Refrigeration Apparatus - Google Patents

Refrigeration Apparatus Download PDF

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Publication number
AU2009239038A1
AU2009239038A1 AU2009239038A AU2009239038A AU2009239038A1 AU 2009239038 A1 AU2009239038 A1 AU 2009239038A1 AU 2009239038 A AU2009239038 A AU 2009239038A AU 2009239038 A AU2009239038 A AU 2009239038A AU 2009239038 A1 AU2009239038 A1 AU 2009239038A1
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Australia
Prior art keywords
refrigerant
heat exchanger
tube
air
pressure
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AU2009239038A
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AU2009239038B2 (en
Inventor
Shuji Fujimoto
Atsushi Yoshimi
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Daikin Industries Ltd
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Daikin Industries Ltd
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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B9/00—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point
    • F25B9/002—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point characterised by the refrigerant
    • F25B9/008—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point characterised by the refrigerant the refrigerant being carbon dioxide
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B1/00—Compression machines, plants or systems with non-reversible cycle
    • F25B1/10—Compression machines, plants or systems with non-reversible cycle with multi-stage compression
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B47/00—Arrangements for preventing or removing deposits or corrosion, not provided for in another subclass
    • F25B47/02—Defrosting cycles
    • F25B47/022—Defrosting cycles hot gas defrosting
    • F25B47/025—Defrosting cycles hot gas defrosting by reversing the cycle
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B13/00—Compression machines, plants or systems, with reversible cycle
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B2309/00—Gas cycle refrigeration machines
    • F25B2309/06—Compression machines, plants or systems characterised by the refrigerant being carbon dioxide
    • F25B2309/061—Compression machines, plants or systems characterised by the refrigerant being carbon dioxide with cycle highest pressure above the supercritical pressure
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B2313/00—Compression machines, plants or systems with reversible cycle not otherwise provided for
    • F25B2313/027—Compression machines, plants or systems with reversible cycle not otherwise provided for characterised by the reversing means
    • F25B2313/0272—Compression machines, plants or systems with reversible cycle not otherwise provided for characterised by the reversing means using bridge circuits of one-way valves
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B2313/00—Compression machines, plants or systems with reversible cycle not otherwise provided for
    • F25B2313/027—Compression machines, plants or systems with reversible cycle not otherwise provided for characterised by the reversing means
    • F25B2313/02741—Compression machines, plants or systems with reversible cycle not otherwise provided for characterised by the reversing means using one four-way valve
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B2313/00—Compression machines, plants or systems with reversible cycle not otherwise provided for
    • F25B2313/031—Sensor arrangements
    • F25B2313/0315—Temperature sensors near the outdoor heat exchanger
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B2400/00—General features or devices for refrigeration machines, plants or systems, combined heating and refrigeration systems or heat-pump systems, i.e. not limited to a particular subgroup of F25B
    • F25B2400/04—Refrigeration circuit bypassing means
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B2400/00—General features or devices for refrigeration machines, plants or systems, combined heating and refrigeration systems or heat-pump systems, i.e. not limited to a particular subgroup of F25B
    • F25B2400/07—Details of compressors or related parts
    • F25B2400/072—Intercoolers therefor
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B2400/00—General features or devices for refrigeration machines, plants or systems, combined heating and refrigeration systems or heat-pump systems, i.e. not limited to a particular subgroup of F25B
    • F25B2400/13—Economisers
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B2400/00—General features or devices for refrigeration machines, plants or systems, combined heating and refrigeration systems or heat-pump systems, i.e. not limited to a particular subgroup of F25B
    • F25B2400/23—Separators
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B2700/00—Sensing or detecting of parameters; Sensors therefor
    • F25B2700/21—Temperatures
    • F25B2700/2106—Temperatures of fresh outdoor air

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Mechanical Engineering (AREA)
  • Thermal Sciences (AREA)
  • General Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Compression-Type Refrigeration Machines With Reversible Cycles (AREA)
  • Air Conditioning Control Device (AREA)

Description

DESCRIPTION REFRIGERATION DEVICE TECHNICAL FIELD The present invention relates to a refrigeration apparatus, and particularly relates to a 5 refrigeration apparatus which has a refrigerant circuit configured to be capable of switching between a cooling operation and a heating operation and which performs a multistage compression refrigeration cycle by using a refrigerant that operates in a supercritical range. BACKGROUND ART As one conventional example of a refrigeration apparatus which has a refrigerant 0 circuit configured to be capable of switching between a cooling operation and a heating operation and which performs a multistage compression refrigeration cycle by using a refrigerant that operates in a supercritical range, Patent Document I discloses an air conditioning apparatus which has a refrigerant circuit configured to be capable of switching between an air-cooling operation and an air-warming operation and which performs a two 5 stage compression refrigeration cycle by using carbon dioxide as a refrigerant. This air conditioning apparatus has primarily a compressor having two compression elements connected in series, a four-way switching valve for switching between an air-cooling operation and an air-warming operation, an outdoor heat exchanger, and an indoor heat exchanger. 20 <Patent Document 1> Japanese Laid-open Patent Publication No. 2007-232263 SUMMARY OF INVENTION A refrigeration apparatus according to a first aspect of the present invention is a refrigeration apparatus which a refrigerant that operates in a supercritical range is used, 25 comprising a compression mechanism, a heat source-side heat exchanger which functions as a radiator or evaporator of refrigerant, an expansion mechanism for depressurizing the refrigerant, a usage-side heat exchanger which functions as an evaporator or radiator of refrigerant, a switching mechanism, an intermediate heat exchanger, and an intermediate heat exchanger bypass tube. The compression mechanism has a plurality of compression 30 elements and is configured so that the refrigerant discharged from the first-stage compression element, which is one of a plurality of compression elements, is sequentially compressed by the second-stage compression element. As used herein, the term "compression mechanism" refers to a compressor in which a plurality of compression elements are integrally incorporated, or a configuration that includes a compression mechanism in which a single compression element is incorporated and/or a plurality of compression mechanisms in which a plurality of compression elements have been incorporated are connected together. The phrase "the refrigerant discharged from a first-stage compression element, which is one of the plurality of compression elements, is sequentially compressed by a second-stage compression 5 element" does not mean merely that two compression elements connected in series are included, namely, the "first-stage compression element" and the "second-stage compression element;" but means that a plurality of compression elements are connected in series and the relationship between the compression elements is the same as the relationship between the aforementioned "first-stage compression element" and "second-stage compression element." 10 The switching mechanism is a mechanism for switching between a cooling operation state, in which the refrigerant is circulated through the compression mechanism, the heat source-side heat exchanger, and the usage-side heat exchanger in a stated order; and a heating operation state, in which the refrigerant is circulated through the compression mechanism, the usage side heat exchanger, and the heat source-side heat exchanger in a stated order. The heat 15 source-side heat exchanger is a heat exchanger having air as a heat source. The intermediate heat exchanger is a heat exchanger integrated with the heat source-side heat exchanger and having air as a heat source, is provided to an intermediate refrigerant tube for drawing into the second-stage compression element refrigerant discharged from the first-stage compression element, and functions as a cooler of the refrigerant discharged from the first 20 stage compression element and drawn into the second-stage compression element. The intermediate heat exchanger bypass tube is connected to the intermediate refrigerant tube so as to bypass the intermediate heat exchanger. In this refrigeration apparatus, the intermediate heat exchanger is disposed above the heat source-side heat exchanger, and when a reverse cycle defrosting operation is performed for defrosting the heat source-side heat 25 exchanger by switching the switching mechanism to the cooling operation state, the intermediate heat exchanger bypass tube is used to ensure that refrigerant does not flow to the intermediate heat exchanger. In a conventional air-conditioning apparatus, the critical temperature (about 31 *C) of carbon dioxide used as the refrigerant is about the same as the temperature of water or air as 30 the cooling source of an outdoor heat exchanger or indoor heat exchanger functioning as a cooler of the refrigerant, which is low compared to R22, R410A, and other refrigerants, and the apparatus therefore operates in a state in which the high pressure of the refrigeration cycle is higher than the critical pressure of the refrigerant so that the refrigerant can be cooled by the water or air in these heat exchangers. As a result, since the refrigerant discharged from the 2 second-stage compression element of the compressor has a high temperature, there is a large difference in temperature between the refrigerant and the water or air as a cooling source in the outdoor heat exchanger functioning as a refrigerant cooler, and the outdoor heat exchanger has much heat radiation loss, which poses a problem in making it difficult to 5 achieve a high operating efficiency. As a countermeasure to this problem, the intermediate heat exchanger which functions as a cooler of the refrigerant discharged from the first-stage compression element and drawn into the second-stage compression element is provided to the intermediate refrigerant tube for drawing refrigerant discharged from the first-stage compression element 0 into the second-stage compression element, the intermediate heat exchanger bypass tube is connected to the intermediate refrigerant tube so as to bypass the intermediate heat exchanger, the intermediate heat exchanger bypass tube is used to ensure that the intermediate heat exchanger functions as a cooler when the switching mechanism corresponding to the aforementioned four-way switching valve is set to a cooling operation 15 state corresponding to the air-cooling operation, and also that the intermediate heat exchanger does not function as a cooler when the switching mechanism is set to a heating operation state corresponding to the air-warming operation. This minimizes the temperature of the refrigerant discharged from the compression mechanism corresponding to the aforementioned compressor during the cooling operation, suppresses heat radiation from the intermediate heat 20 exchanger to the exterior during the heating operation, and prevents loss of operating efficiency. In cases in which a heat exchanger having air as a heat source is used as the heat source-side heat exchanger in this type of refrigeration apparatus, when the heating operation is performed while the air as the heat source is low in temperature, frost deposits form on the 25 heat source-side heat exchanger functioning as a heater of the refrigerant, and a defrosting operation for defrosting the heat source-side heat exchanger must therefore be performed by causing the heat source-side heat exchanger to function as a cooler of the refrigerant. Moreover, there is a danger that frost deposits will occur in the intermediate heat exchanger as well because a heat exchanger whose heat source is air is used as the intermediate heat 30 exchanger and the intermediate heat exchanger is integrated with the heat source-side heat exchanger; in this case, refrigerant must be passed through not only the heat source-side heat exchanger but also the intermediate heat exchanger and the intermediate heat exchanger must be defrosted. However, in this refrigeration apparatus, when the only measure taken during the 3 heating operation is to prevent the intermediate heat-exchanger from functioning as a cooler using an intermediate heat exchanger bypass tube, the amount of frost deposits in the intermediate heat exchanger is small and defrosting of the intermediate heat exchanger will conclude sooner than in the heat source-side heat exchanger. Therefore, if refrigerant 5 continues to flow to the intermediate heat exchanger even after defrosting of the intermediate heat exchanger is complete, heat is radiated from the intermediate heat exchanger to the exterior and the temperature of the refrigerant drawn into the second-stage compression element decreases, and as a result, the temperature of the refrigerant discharged from the compression mechanism decreases, creating a problem of the loss of defrosting capacity of 10 the heat source-side heat exchanger. In view of this, in the refrigeration apparatus according to a first aspect of the present invention, an intermediate heat exchanger is disposed above a heat source-side heat exchanger. Thereby, in this refrigeration apparatus, frost deposits are minimized in the border between the intermediate heat exchanger and the heat source-side heat exchanger 15 regardless of the intermediate heat exchanger being integrated with the heat source-side heat exchanger, and unlike cases in which the intermediate heat exchanger is disposed below the heat source-side heat exchanger, there is less risk that water that has melted and dripped down from the heat source-side heat exchanger due to the defrosting of the heat source-side heat exchanger will adhere, freeze, and spread on the intermediate heat exchanger, and the 20 intermediate heat exchanger therefore does not need to be defrosted when the reverse cycle defrosting operation is performed. In this refrigeration apparatus, taking advantage of the fact that the intermediate heat exchanger does not need to be defrosted during the reverse cycle defrosting operation, when the reverse cycle defrosting operation is performed, the intermediate heat exchanger bypass tube is used to ensure that the refrigerant does not flow to 25 the intermediate heat exchanger, thereby preventing heat from being radiated from the intermediate heat exchanger to the exterior and minimizing the decrease in the defrosting capacity of the heat source-side heat exchanger when the reverse cycle defrosting operation is performed. The reverse cycle defrosting operation can thereby be performed efficiently in this 30 refrigeration apparatus. The refrigeration apparatus according to a second aspect of the present invention is the refrigeration apparatus according to the first aspect, further comprising a second-stage injection tube for branching off the refrigerant whose heat has been radiated in the heat source-side heat exchanger or a usage-side heat exchanger and returning the refrigerant to a 4 second-stage compression element, wherein the second-stage injection tube is used during the reverse cycle defrosting operation to return the refrigerant fed to the usage-side heat exchanger from the heat source-side heat exchanger back to the second-stage compression element. 5 In this refrigeration apparatus, since a reverse cycle defrosting operation is used for defrosting the heat source-side heat exchanger by switching the switching mechanism to the cooling operation state, the usage-side heat exchanger is made to function as an evaporator of refrigerant regardless of the intention being to cause the usage-side heat exchanger to function as a radiator of refrigerant, and there is a problem encountered with a decrease in 10 temperature on the usage side. Since the reverse cycle defrosting operation is a cooling operation performed in a state in which the intermediate heat exchanger is made not to function as a cooler while the air as a heat source is low in temperature, the low pressure in the refrigeration cycle decreases, and the flow rate of the refrigerant drawn from a first-stage compression element is reduced. When this happens, another problem emerges that more 15 time is required for defrosting the heat source-side heat exchanger because the flow rate of refrigerant circulated through the refrigerant circuit is reduced and the flow rate of refrigerant flowing through the heat source-side heat exchanger can no longer be guaranteed. In view of this, in this refrigeration apparatus, when the reverse cycle defrosting operation is performed, the second-stage injection tube is used to ensure that the refrigerant 20 fed from the heat source-side heat exchanger to the usage-side heat exchanger is returned to the second-stage compression element, whereby the flow rate of the refrigerant flowing through the usage-side heat exchanger can be reduced, and the flow rate of the refrigerant flowing through the heat source-side heat exchanger can be guaranteed. It is thereby possible in this refrigeration apparatus to minimize the temperature 25 decrease on the usage side and to reduce the defrosting time of the heat source-side heat exchanger when the reverse cycle defrosting operation is performed. A refrigeration apparatus according to a third aspect of the present invention is the refrigeration apparatus according to the first or second aspect of the present invention, wherein the refrigerant that operates in the supercritical range is carbon dioxide. 30 BRIEF DESCRIPTION OF DRAWINGS FIG 1 is a schematic structural diagram of an air-conditioning apparatus as an embodiment of the refrigeration apparatus according to the present invention. FIG. 2 is an external perspective view of a heat source unit (in a state in which a fan grill has been removed). 5 FIG 3 is a side view of the heat source unit in a state in which a right panel of the heat source unit has been removed. FIG 4 is an enlarged view of section I in FIG 3. FIG 5 is a diagram showing the flow of refrigerant within the air-conditioning 5 apparatus during the air-cooling operation. FIG. 6 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation. FIG. 7 is a temperature-entropy graph representing the refrigeration cycle duri ng the air-cooling operation. .0 FIG. 8 is a graph showing the characteristics of the heat transfer coefficient in a case in which carbon dioxide of an intermediate pressure lower than the critical pressure flows through a heat transfer passage, and also of the heat transfer coefficient in a case in which carbon dioxide of a high pressure exceeding the critical pressure flows through the heat transfer passage. 5 FIG 9 is a diagram showing the flow of refrigerant within the air-conditioning apparatus during the air-warming operation. FIG 10 is a pressure-enthalpy graph representing the refrigeration cycle during the air-warming operation. FIG. 11 is a temperature-entropy graph representing the refrigeration cycle during 20 the air-warming operation. FIG 12 is a flowchart of the defrosting operation. FIG 13 is a diagram showing the flow of refrigerant within the air-conditioning apparatus during the defrosting operation. FIG 14 is a schematic structural diagram of an air-conditioning apparatus according 25 to Modification 1. FIG 15 is a diagram showing the flow of refrigerant within the air-conditioning apparatus during the air-cooling operation according to Modification 1. FIG 16 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation in the air-conditioning apparatus according to Modification 1. 30 FIG 17 is a temperature-entropy graph representing the refrigeration cycle during the air-cooling operation in the air-conditioning apparatus according to Modification 1. FIG 18 is a diagram showing the flow of refrigerant within the air-conditioning apparatus during the air-warming operation according to Modification 1. 6 FIG 19 is a pressure-enthalpy graph representing the refrigeration cycle during the air-warming operation in the air-conditioning apparatus according to Modification 1. FIG 20 is a temperature-entropy graph representing the refrigeration cycle during the air-warming operation in the air-conditioning apparatus according to Modification 1. 5 FIG 21 is a diagram showing the flow of refrigerant within the air-conditioning apparatus during the defrosting operation according to Modification 1. FIG 22 is a pressure-enthalpy graph representing the refrigeration cycle during the defrosting operation in the air-conditioning apparatus according to Modification 1. FIG 23 is a temperature-entropy graph representing the refrigeration cycle during 10 the defrosting operation in the air-conditioning apparatus according to Modification 1. FIG. 24 is a schematic structural diagram of an air-conditioning apparatus according to Modification 2. FIG 25 is a diagram showing the flow of refrigerant within the air-conditioning apparatus during the air-cooling operation according to Modification 2. 15 FIG 26 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation in the air-conditioning apparatus according to Modification 2. FIG 27 is a temperature-entropy graph representing the refrigeration cycle during the air-cooling operation in the air-conditioning apparatus according to Modification 2. FIG 28 is a diagram showing the flow of refrigerant within the air-conditioning 20 apparatus during the air-warming operation according to Modification 2. FIG 29 is a pressure-enthalpy graph representing the refrigeration cycle during the air-warming operation in the air-conditioning apparatus according to Modification 2. FIG 30 is a temperature-entropy graph representing the refrigeration cycle during the air-warming operation in the air-conditioning apparatus according to Modification 2. 25 FIG 31 is a diagram showing the flow of refrigerant within the air-conditioning apparatus during the defrosting operation according to Modification 2. FIG 32 is a pressure-enthalpy graph representing the refrigeration cycle during the defrosting operation in the air-conditioning apparatus according to Modification 2. FIG 33 is a temperature-entropy graph representing the refrigeration cycle during 30 the defrosting operation in the air-conditioning apparatus according to Modification 2. FIG. 34 is a schematic structural diagram of an air-conditioning apparatus according to Modification 3. FIG 35 is a diagram showing the flow of refrigerant within the air-conditioning apparatus during the air-cooling operation according to Modification 3. 7 FIG 36 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation in the air-conditioning apparatus according to Modification 3. FIG 37 is a temperature-entropy graph representing the refrigeration cycle during the air-cooling operation in the air-conditioning apparatus according to Modification 3. 5 FIG 38 is a diagram showing the flow of refrigerant within the air-conditioning apparatus during the air-warming operation according to Modification 3. FIG 39 is a pressure-enthalpy graph representing the refrigeration cycle during the air-warming operation in the air-conditioning apparatus according to Modification 3. FIG 40 is a temperature-entropy graph representing the refrigeration cycle during 10 the air-warming operation in the air-conditioning apparatus according to Modification 3. FIG 41 is a diagram showing the flow of refrigerant within the air-conditioning apparatus during the defrosting operation according to Modification 3. FIG 42 is a pressure-enthalpy graph representing the refrigeration cycle during the defrosting operation in the air-conditioning apparatus according to Modification 3. 15 FIG 43 is a temperature-entropy graph representing the refrigeration cycle during the defrosting operation in the air-conditioning apparatus according to Modification 3. FIG 44 is a schematic structural diagram of an air-conditioning apparatus according to Modification 4. DESCRIPTION OF EMBODIMENTS 20 Embodiments of the refrigeration apparatus according to the present invention are described hereinbelow with reference to the drawings. (1) Configuration of air-conditioning apparatus FIG 1 is a schematic structural diagram of an air-conditioning apparatus 1 as an embodiment of the refrigeration apparatus according to the present invention. The air 25 conditioning apparatus I has a refrigerant circuit 10 configured to be capable of switching between an air-cooling operation and an air-warming operation, and the apparatus performs a two-stage compression refrigeration cycle by using a refrigerant (carbon dioxide in this case) for operating in a supercritical range. The refrigerant circuit 10 of the air-conditioning apparatus 1 has primarily a 30 compression mechanism 2, a switching mechanism 3, a heat source-side heat exchanger 4, a bridge circuit 17, a receiver 18, a first expansion mechanism 5a, a second expansion mechanism 5b, a usage-side heat exchanger 6, and an intermediate heat exchanger 7. In the present embodiment, the compression mechanism 2 is configured from a compressor 21 which uses two compression elements to subject a refrigerant to two-stage 8 compression. The compressor 21 has a hermetic structure in which a compressor drive motor 21 b, a drive shaft 21 c, and compression elements 2c, 2d are housed within a casing 21 a. The compressor drive motor 21b is linked to the drive shaft 21c. The drive shaft 21c is linked to the two compression elements 2c, 2d. Specifically, the compressor 21 has a so 5 called single-shaft two-stage compression structure in which the two compression elements 2c, 2d are linked to a single drive shaft 21 c and the two compression elements 2c, 2d are both rotatably driven by the compressor drive motor 21b. In the present embodiment, the compression elements 2c, 2d are rotary elements, scroll elements, or another type of positive displacement compression elements. The compressor 21 is configured so as to draw [0 refrigerant through an intake tube 2a, to discharge this refrigerant to an intermediate refrigerant tube 8 after the refrigerant has been compressed by the compression element 2c, to drawhe intermediate-pressure refrigerant discharged to the intermediate refrigerant tube 8 in the refrigeration cycle into the compression element 2d, and to discharge the refrigerant to a discharge tube 2b after the refrigerant has been further compressed. The intermediate 15 refrigerant tube 8 is a refrigerant tube for taking the intermediate-pressure refrigerant in the refrigeration cycle into the compression element 2d connected to the second-stage side of the compression element 2c after the refrigerant has been discharged from the compression element 2c connected to the first-stage side of the compression element 2c. The discharge tube 2b is a refrigerant tube for feeding refrigerant discharged from the compression 20 mechanism 2 to the switching mechanism 3, and the discharge tube 2b is provided with an oil separation mechanism 41 and a non-return mechanism 42. The oil separation mechanism 41 is a mechanism for separating refrigerator oil accompanying the refrigerant from the refrigerant discharged from the compression mechanism 2 and returning the oil to the intake side of the compression mechanism 2, and the oil separation mechanism 41 has primarily an 25 oil separator 41a for separating refrigerator oil accompanying the refrigerant from the refrigerant discharged from the compression mechanism 2, and an oil return tube 41b connected to the oil separator 41a for returning the refrigerator oil separated from the refrigerant to the intake tube 2a of the compression mechanism 2. The oil return tube 41b is provided with a depressurization mechanism 41c for depressurizing the refrigerator oil 30 flowing through the oil return tube 41b. A capillary tube is used for the depressurization mechanism 41c in the present embodiment. The non-return mechanism 42 is a mechanism for allowing the flow of refrigerant from the discharge side of the compression mechanism 2 to the switching mechanism 3 and for blocking the flow of refrigerant from the switching mechanism 3 to the discharge side of the compression mechanism 2, and a non-return valve is 9 used in the present embodiment. Thus, in the present embodiment, the compression mechanism 2 has two compression elements 2c, 2d and is configured so that among these compression elements 2c, 2d, refrigerant discharged from the first-stage compression element is compressed in 5 sequence by the second-stage compression element. The switching mechanism 3 is a mechanism for switching the direction of refrigerant flow in the refrigerant circuit 10. In order to allow the heat source-side heat exchanger 4 to function as a radiator of refrigerant compressed by the compression mechanism 2 and to allow the usage-side heat exchanger 6 to function as an evaporator of refrigerant cooled in the .0 heat source-side heat exchanger 4 during the air-cooling operation, the switching mechanism 3 is capable of connecting the discharge side of the compression mechanism 2 and one end of the heat source-side heat exchanger 4 and also connecting the intake side of the compressor 21 and the usage-side heat exchanger 6 (refer to the solid lines of the switching mechanism 3 in FIG 1, this state of the switching mechanism 3 being referred to below as the "cooling [5 operation state"). In order to allow the usage-side heat exchanger 6 to function as a radiator of refrigerant compressed by the compression mechanism 2 and to allow the heat source-side heat exchanger 4 to function as an evaporator of refrigerant cooled in the usage-side heat exchanger 6 during the air-warming operation, the switching mechanism 3 is capable of connecting the discharge side of the compression mechanism 2 and the usage-side heat 20 exchanger 6 and also of connecting the intake side of the compression mechanism 2 and one end of the heat source-side heat exchanger 4 (refer to the dashed lines of the switching mechanism 3 in FIG 1, this state of the switching mechanism 3 being referred to below as the "heating operation state"). In the present embodiment, the switching mechanism 3 is a four way switching valve connected to the intake side of the compression mechanism 2, the 25 discharge side of the compression mechanism 2, the heat source-side heat exchanger 4, and the usage-side heat exchanger 6. The switching mechanism 3 is not limited to a four-way switching valve, and may be configured so as to have a function for switching the direction of the flow of the refrigerant in the same manner as described above by using, e.g., a combination of a plurality of electromagnetic valves. 30 Thus, focusing solely on the compression mechanism 2, the heat source-side heat exchanger 4, and the usage-side heat exchanger 6 constituting the refrigerant circuit 10; the switching mechanism 3 is configured to be capable of switching between a cooling operation state in which the refrigerant is circulated sequentially through the compression mechanism 2, the heat source-side heat exchanger 4 functioning as a radiator of refrigerant, and the usage 10 side heat exchanger 6 functioning as an evaporator of refrigerant; and a heating operation state in which the refrigerant is circulated sequentially through the compression mechanism 2, the usage-side heat exchanger 6 functioning as a radiator of refrigerant, and the heat source side heat exchanger 4 functioning as an evaporator of refrigerant. 5 The heat source-side heat exchanger 4 is a heat exchanger that functions as a radiator or an evaporator of refrigerant. One end of the heat source-side heat exchanger 4 is connected to the switching mechanism 3, and the other end is connected to the first expansion mechanism 5a via the bridge circuit 17. The heat source-side heat exchanger 4 is a heat exchanger that uses air as a heat source (i.e., a cooling source or a heating source), and a fin 0 and-tube heat exchanger is used in the present embodiment. The air as the heat source is supplied to the heat source-side heat exchanger 4 by a heat source-side fan 40. The heat source-side fan 40 is driven by a fan drive motor 40a. The bridge circuit 17 is disposed between the heat source-side heat exchanger 4 and the usage-side heat exchanger 6, and is connected to a receiver inlet tube 18a connected to the .5 inlet of the receiver 18 and to a receiver outlet tube 18b connected to the outlet of the receiver 18. The bridge circuit 17 has four non-return valves 17a, 17b, 17c, and 17d in the present embodiment. The inlet non-return valve 17a is a non-return valve that allows only the flow of refrigerant from the heat source-side heat exchanger 4 to the receiver inlet tube 18a. The inlet non-return valve 17b is a non-return valve that allows only the flow of refrigerant from 20 the usage-side heat exchanger 6 to the receiver inlet tube 18a. In other words, the inlet non return valves 17a, 17b have a function for allowing refrigerant to flow from one among the heat source-side heat exchanger 4 or the usage-side heat exchanger 6 to the receiver inlet tube 18a. The outlet non-return valve 17c is a non-return valve that allows only the flow of refrigerant from the receiver outlet tube 18b to the usage-side heat exchanger 6. The outlet 25 non-return valve 17d is a non-return valve that allows only the flow of refrigerant from the receiver outlet tube 18b to the heat source-side heat exchanger 4. In other words, the outlet non-return valves 17c, 17d have a function for allowing refrigerant to flow from the receiver outlet tube 18b to the heat source-side heat exchanger 4 or the usage-side heat exchanger 6. The first expansion mechanism 5a is a mechanism for depressurizing the refrigerant, 30 is provided to the receiver inlet tube 18a, and is an electrically driven expansion valve in the present embodiment. In the present embodiment, during the air-cooling operation, the first expansion mechanism 5a depressurizes the high-pressure refrigerant in the refrigeration cycle that has been cooled in the heat source-side heat exchanger 4 nearly to the saturation pressure of the refrigerant before the refrigerant is fed to the usage-side heat exchanger 6 via the 11 receiver 18; and during the air-warming operation, the first expansion mechanism 5a depressurizes the high-pressure refrigerant in the refrigeration cycle that has been cooled in the usage-side heat exchanger 6 nearly to the saturation pressure of the refrigerant before the refrigerant is fed to the heat source-side heat exchanger 4 via the receiver 18. 5 The receiver 18 is a container provided in order to temporarily retain the refrigerant that has been depressurized by the first expansion mechanism 5a so as to allow storage of excess refrigerant produced according to the operation states, such as the quantity of refrigerant circulating in the refrigerant circuit 10 being different between the air-cooling operation and the air-warming operation, and the inlet of the receiver 18 is connected to the 10 receiver inlet tube 18a, while the outlet is connected to the receiver outlet tube 18b. Also connected to the receiver 18 is a first intake return tube 18f capable of withdrawing refrigerant from inside the receiver 18 and returning the refrigerant to the intake tube 2a of the compression mechanism 2 (i.e., to the intake side of the compression element 2c on the first-stage side of the compression mechanism 2). A first intake return on/off valve 18g is 15 provided to this first intake return tube 18f. The first intake return on/off valve 18g is an electromagnetic valve in the present embodiment. The second expansion mechanism 5b is a mechanism provided to the receiver outlet tube 18b and used for depressurizing the refrigerant, and is an electrically driven expansion valve in the present embodiment. In the present embodiment, during the air-cooling 20 operation, the second expansion mechanism 5b further depressurizes the refrigerant depressurized by the first expansion mechanism 5a to a low pressure in the refrigeration cycle before the refrigerant is fed to the usage-side heat exchanger 6 via the receiver 18; and during the air-warming operation, the second expansion mechanism 5b further depressurizes the refrigerant depressurized by the first expansion mechanism 5a to a low pressure in the 25 refrigeration cycle before the refrigerant is fed to the heat source-side heat exchanger 4 via the receiver 18. The usage-side heat exchanger 6 is a heat exchanger that functions as a radiator or an evaporator of refrigerant. One end of the usage-side heat exchanger 6 is connected to the first expansion mechanism 5a via the bridge circuit 17, and the other end is connected to the 30 switching mechanism 3. The usage-side heat exchanger 6 is a heat exchanger that uses water and/or air as a heat source (i.e., a cooling source or a heating source). The intermediate heat exchanger 7 is provided to the intermediate refrigerant tube 8, and in the present embodiment, the intermediate heat exchanger 7 is a heat exchanger capable of functioning as a cooler of refrigerant that is discharged from the first-stage compression 12 element 2c and drawn into the compression element 2d. The intermediate heat exchanger 7 is a heat exchanger that uses air as a heat source (a cooling source in this case), and a fin-and tube heat exchanger is used in the present embodiment. The intermediate heat exchanger 7 is integrated with the heat source-side heat exchanger 4. 5 Next, the configuration of the intermediate heat exchanger 7 integrated with the heat source-side heat exchanger 4, including their arrangement and other features, will be described in detail using FIGS. 2 through 4. FIG 2 is an external perspective view of a heat source unit 1 a (in a state in which a fan grill has been removed), FIG 3 is a side view of the heat source unit 1 a in a state in which a right panel 74 of the heat source unit l a has been 10 removed, and FIG 4 is an enlarged view of section I in FIG 3. The terms "left" and "right" in the following description are based on a case of viewing the heat source unit I a from the side of a front panel 75. First, in the present embodiment, the air-conditioning apparatus 1 is configured by a connection between the heat source unit 1 a which is provided primarily with a heat source 15 side fan 40, the heat source-side heat exchanger 4, and the intermediate heat exchanger 7; and a usage unit (not shown) which is provided primarily with the usage-side heat exchanger 6. The heat source unit la is a so-called upward blowing type in which air is suctioned from the side and air is blown out upward, and the heat source unit la has primarily a casing 71, and the heat source-side heat exchanger 4, the intermediate heat exchanger 7 and other refrigerant 20 circuit structural components, and the heat source-side fan 40 and other devices disposed inside the casing 71. In the present embodiment, the casing 71 is a substantially rectangular parallelepiped box, configured primarily from a top panel 72 constituting the top surface of the casing 71, and a left panel 73, the right panel 74, the front panel 75, and a rear panel 76 constituting 25 external peripheral surfaces of the casing 71. The top panel 72 is primarily a member constituting the top surface of the casing 71, and in the present embodiment, the top panel 72 is a plate-shaped member which in a plan view has a substantially rectangular shape with an air-blowing opening 71a formed substantially in the center. The top panel 72 is provided with a fan grill 78 so as to cover the air-blowing opening 71a from above. The left panel 73 30 is primarily a member constituting the left surface of the casing 71, and in the present embodiment, the left panel 73 is a plate-shaped member extending downward from the left edge of the top panel 72 and having a substantially rectangular shape in a side view. Intake openings 73a are formed throughout nearly the entire left panel 73, except for the top part. The right panel 74 is primarily a member constituting the right surface of the casing 71, and 13 in the present embodiment, the right panel 74 is a plate-shaped member extending downward from the right edge of the top panel 72 and having a substantially rectangular shape in a side view. Intake openings 74a are formed throughout nearly the entire right panel 74, except for the top part. The front panel 75 is primarily a member constituting the front surface of the 5 casing 71, and in the present embodiment, the front panel 75 is configured from a plate shaped member disposed in sequence downward from the front edge of the top panel 72 and having a substantially rectangular shape in a front view. The rear panel 76 is primarily a member constituting the rear surface of the casing 71, and in the present embodiment, the rear panel 76 is configured from a plate-shaped member disposed downward along from the 10 rear edge of the top panel 72 and having a substantially rectangular shape in a front view. Intake openings 76a are formed throughout nearly the entire rear panel 76, except for the top part. A bottom panel 77 is primarily a member constituting the bottom surface of the casing 71, and in the present embodiment, the bottom panel 77 is a plate-shaped member having a substantially rectangular shape in a plan view. 15 The intermediate heat exchanger 7 is integrated with the heat source-side heat exchanger 4 in a state of being disposed above the heat source-side heat exchanger 4, and is also disposed on top of the bottom panel 77. More specifically, the intermediate heat exchanger 7 is integrated by sharing heat transfer fins with the heat source-side heat exchanger 4 (see FIG 4). In the present embodiment, the integration of the heat source-side 20 heat exchanger 4 and the intermediate heat exchanger 7 forms a heat exchanger panel having a substantial U shape in a plan view, which is disposed so as to face the intake openings 73a, 74a, 76a. The heat source-side fan 40 is made to face an air-blowing opening 71 a of the top panel 72, and is disposed on the top side of the integrated heat source-side heat exchanger 4 and intermediate heat exchanger 7. In the present embodiment, the heat source-side fan 40 25 is an axial flow fan which is rotatably driven by the fan drive motor 40a, whereby air as a heat source is suctioned into the casing 71 from the intake openings 73a, 74a, 76a and passed through the heat source-side heat exchanger 4 and the intermediate heat exchanger 7, after which the air can be blown upward out through the air-blowing opening 71a (refer to the arrows indicating the flow of air in FIG 3). Specifically, the heat source-side fan 40 is 30 designed so as to supply air as a heat source to both the heat source-side heat exchanger 4 and the intermediate heat exchanger 7. Neither the outward shape of the heat source unit la or the shape of the integrated heat source-side heat exchanger 4 and intermediate heat exchanger 7 are limited to those described above. An intermediate heat exchanger bypass tube 9 is connected to the intermediate 14 refrigerant tube 8 so as to bypass the intermediate heat exchanger 7. This intermediate heat exchanger bypass tube 9 is a refrigerant tube for limiting the flow rate of refrigerant flowing through the intermediate heat exchanger 7. The intermediate heat exchanger bypass tube 9 is provided with an intermediate heat exchanger bypass on/off valve 11. The intermediate 5 heat exchanger bypass on/off valve 11 is an electromagnetic valve in the present embodiment. In the present embodiment, this intermediate heat exchanger bypass on/off valve 11 is controlled basically so as to close when the switching mechanism 3 is set to the cooling operation state and to open when the switching mechanism 3 is set to the heating operation state, except during the defrosting operation described hereinafter. In other words, the 0 intermediate heat exchanger bypass on/off valve 11 is closed when the air-cooling operation is performed and opened when the air-warming operation is performed. The intermediate refrigerant tube 8 is also provided with an intermediate heat exchanger on/off valve 12 in the portion extending from connection with the end of the intermediate heat exchanger bypass tube 9 on side near the first-stage compression element 5 2c to the end of the intermediate heat exchanger 7 on the side near first-stage compression element 2c. This intermediate heat exchanger on/off valve 12 is a mechanism for limiting the flow rate of refrigerant flowing through the intermediate heat exchanger 7. The intermediate heat exchanger on/off valve 12 is an electromagnetic valve in the present embodiment. In the present embodiment, the intermediate heat exchanger on/off valve 12 is 20 controlled basically so as to open when the switching mechanism 3 is set to the cooling operation state and to close when the switching mechanism 3 is set to the heating operation state, except during the defrosting operation described hereinafter. In other words, the intermediate heat exchanger on/off valve 12 is controlled so as to open when the air-cooling operation is performed and close when the air-warming operation is performed. 25 The intermediate refrigerant tube 8 is also provided with a non-return mechanism 15 for allowing refrigerant to flow from the discharge side of the first-stage compression element 2c to the intake side of the second-stage compression element 2d and for blocking the refrigerant from flowing from the intake side of the second-stage compression element 2d to the discharge side of the first-stage compression element 2c. The non-return mechanism 30 15 is a non-return valve in the present embodiment. In the present embodiment, the non return mechanism 15 is provided in the portion of the intermediate refrigerant tube 8 extending from the end of the intermediate heat exchanger 7 on the side near the second-stage compression element 2d to the end of the intermediate heat exchanger bypass tube 9 on the side near the second-stage compression element 2d. 15 Furthermore, the air-conditioning apparatus 1 is provided with various sensors. Specifically, the heat source-side heat exchanger 4 is provided with a heat source-side heat exchange temperature sensor 51 for detecting the temperature of the refrigerant flowing through the heat source-side heat exchanger 4. The air-conditioning apparatus 1 (the heat 5 source unit la in this case) is provided with an air temperature sensor 53 for detecting the temperature of the air as a heat source for the heat source-side heat exchanger 4 and intermediate heat exchanger 7. Though not shown in the drawings, the air-conditioning apparatus 1 also has a controller for controlling the actions of the compression mechanism 2, the switching mechanism 3, the expansion mechanism 5, the heat source-side fan 40, the 0 intermediate heat exchanger bypass on/off valve 11, the intermediate heat exchanger on/off valve 12, the first intake return on/off valve 18g, and the other components constituting the air-conditioning apparatus 1. (2) Action of the air-conditioning apparatus Next, the action of the air-conditioning apparatus I of the present embodiment will 15 be described using FIGS. I and FIGS. 5 through 13. FIG 5 is a diagram showing the flow of refrigerant within the air-conditioning apparatus 1 during the air-cooling operation, FIG 6 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation, FIG 7 is a temperature-entropy graph representing the refrigeration cycle during the air-cooling operation, FIG 8 is a graph showing the characteristics of the heat transfer 20 coefficient in a case in which carbon dioxide of an intermediate pressure lower than the critical pressure flows through a heat transfer passage, and also of the heat transfer coefficient in a case in which carbon dioxide of a high pressure exceeding the critical pressure flows through the heat transfer passage, FIG 9 is a diagram showing the flow of refrigerant within the air-conditioning apparatus I during the air-warming operation, FIG. 10 is a pressure 25 enthalpy graph representing the refrigeration cycle during the air-warming operation, FIG 11 is a temperature-entropy graph representing the refrigeration cycle during the air-warming operation, FIG 12 is a flowchart of the defrosting operation, and FIG 13 is a diagram showing the flow of refrigerant within the air-conditioning apparatus 1 during the defrosting operation. Operation control in the air-cooling operation, the air-warming operation, and the 30 defrosting operation described hereinbelow is performed by the aforementioned controller (not shown). In the following description, the term "high pressure" means a high pressure in the refrigeration cycle (specifically, the pressure at points D, D', and E in FIGS. 6 and 7, and the pressure at points D, D', and F in FIGS. 10 and 11), the term "low pressure" means a low pressure in the refrigeration cycle (specifically, the pressure at points A and F in FIGS. 6 and 16 7, and the pressure at points A and E in FIGS. 10 and 11), and the term "intermediate pressure" means an intermediate pressure in the refrigeration cycle (specifically, the pressure at points B, C in FIGS. 6 and 7, and the pressure at points B, C, and C' in FIGS. 10 and 11). <Air-cooling operation> 5 During the air-cooling operation, the switching mechanism 3 is brought to the cooling operation state shown by the solid lines in FIGS. 1 and 5. The opening degrees of the first expansion mechanism 5a and the second expansion mechanism 5b are adjusted. Since the switching mechanism 3 is in the cooling operation state, the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 is opened and the 10 intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9 is closed, thereby creating a state in which the intermediate heat exchanger 7 functions as a cooler. When the refrigerant circuit 10 is in this state, low-pressure refrigerant (refer to point A in FIGS. 1 and 5 through 7) is drawn into the compression mechanism 2 through the intake 15 tube 2a, and after the refrigerant is first compressed by the compression element 2c to an intermediate pressure, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point B in FIGS. 1 and 5 through 7). The intermediate-pressure refrigerant discharged from the first-stage compression element 2c is cooled in the intermediate heat exchanger 7 by heat exchange with the air as a cooling source supplied by the heat source 20 side fan 40 (refer to point C in FIGS. 1 and 5 through 7). The refrigerant cooled in the intermediate heat exchanger 7 is drawn into and further compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and the refrigerant is then discharged from the compression mechanism 2 to the discharge tube 2b (refer to point D in FIGS. I and 5 through 7). The high-pressure refrigerant discharged from 25 the compression mechanism 2 is compressed by the two-stage compression action of the compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pcp at the critical point CP shown in FIG. 6). The high-pressure refrigerant discharged from the compression mechanism 2 flows into the oil separator 41a constituting the oil separation mechanism 41, and the accompanying refrigeration oil is separated. The 30 refrigeration oil separated from the high-pressure refrigerant in the oil separator 41a flows into the oil return tube 41b constituting the oil separation mechanism 41 wherein it is depressurized by the depressurization mechanism 41c provided to the oil return tube 41b, and the oil is then returned to the intake tube 2a of the compression mechanism 2 and once more drawn into the compression mechanism 2. Next, having been separated from the 17 refrigeration oil in the oil separation mechanism 41, the high-pressure refrigerant is passed through the non-return mechanism 42 and the switching mechanism 3, and is fed to the heat source-side heat exchanger 4 functioning as a refrigerant radiator. The high-pressure refrigerant fed to the heat source-side heat exchanger 4 is cooled in the heat source-side heat 5 exchanger 4 by heat exchange with air as a cooling source supplied by the heat source-side fan 40 (refer to point E in FIGS. I and 5 through 7). The high-pressure refrigerant cooled in the heat source-side heat exchanger 4 then flows through the inlet non-return valve 17a of the bridge circuit 17 into the receiver inlet tube 18a, and the refrigerant is depressurized to a nearly saturated pressure by the first expansion mechanism 5a and is temporarily retained in 10 the receiver 18 (refer to point I in FIGS. 1 and 5). The refrigerant retained in the receiver 18 is fed to the receiver outlet tube 18b and is depressurized by the second expansion mechanism 5b to become a low-pressure gas-liquid two-phase refrigerant, and is then fed through the outlet non-return valve 17c of the bridge circuit 17 to the usage-side heat exchanger 6 functioning as a refrigerant evaporator (refer to point F in FIGS. 1 and 5 through 15 7). The low-pressure gas-liquid two-phase refrigerant fed to the usage-side heat exchanger 6 is heated by heat exchange with water or air as a heating source, and the refrigerant is evaporated as a result (refer to point A in FIGS. 1 and 5 through 7). The low-pressure refrigerant heated in the usage-side heat exchanger 6 is then drawn once more into the compression mechanism 2 via the switching mechanism 3. In this manner the air-cooling 20 operation is performed. Thus, in the air-conditioning apparatus 1 (refrigeration apparatus) of the present embodiment, the intermediate heat exchanger 7 is provided to the intermediate refrigerant tube 8 for drawing the refrigerant discharged from the compression element 2c into the compression element 2d, and during the air-cooling operation, since the intermediate heat 25 exchanger 7 is brought to a state of functioning as a cooler by opening the intermediate heat exchanger on/off valve 12 and closing the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9, both the temperature of the refrigerant drawn into the compression element 2d on the second-stage side of the compression element 2c (refer to points B and C in FIG 7) and the temperature of the refrigerant discharged from 30 the compression element 2d (refer to points D and D' in FIG. 7) decrease more than in a case in which the intermediate heat exchanger 7 is not provided (in this case, the refrigeration cycle is performed in the following sequence in FIGS. 6 and 7: point A -- point B - point D' -> point E -> point F). Therefore, in the heat source-side heat exchanger 4 functioning as a radiator of the refrigerant in this air-conditioning apparatus 1, operating efficiency can be 18 improved over cases in which no intermediate heat exchanger 7 is provided, because the temperature difference between the refrigerant and water or air as the cooling source can be reduced, and heat radiation loss can be reduced by an amount equivalent to the area enclosed by connecting points B, D', D, and C in FIG. 7. 5 Moreover, in the air-conditioning apparatus 1 of the present embodiment, since refrigerant that operates in the supercritical range (carbon dioxide in this case) is used, an air cooling operation is performed in which refrigerant of an intermediate pressure lower than the critical pressure Pcp (about 7.3 MPa with carbon dioxide) flows into the intermediate heat exchanger 7, and refrigerant of a high pressure exceeding the critical pressure Pcp flows into 10 the heat source-side heat exchanger 4 functioning as a radiator of refrigerant (see FIG. 6 and 7). In this case, as a result of the difference between the properties of the refrigerant at a pressure lower than the critical pressure Pcp and the properties of the refrigerant at a pressure exceeding the critical pressure Pcp (particularly the heat transfer coefficient and/or the specific heat at constant pressure) as shown in FIG 8, there is a tendency for the heat transfer 15 coefficient of the refrigerant in the intermediate heat exchanger 7 to be lower than the heat transfer coefficient of the refrigerant in the heat source-side heat exchanger 4 functioning as a radiator of refrigerant. FIG 8 shows the heat transfer coefficient values (corresponding to the heat transfer coefficient of the refrigerant in the intermediate heat exchanger 7) when carbon dioxide at 6 MPa flows at a predetermined quantity flow rate into a heat transfer 20 passage having a predetermined passage cross-sectional area, and also the heat transfer coefficient values (corresponding to the heat transfer coefficient of the refrigerant in the heat source-side heat exchanger 4) of carbon dioxide at 10 MPa in the same heat transfer passage and at the same quantity flow rate as the 6 MPa carbon dioxide, but it is clear from looking at this graph that within the temperature range (about 40 to 70*C) of the refrigerant flowing 25 through the intermediate heat exchanger 7 and/or the heat source-side heat exchanger 4 functioning as a radiator of refrigerant, the heat transfer coefficient values of the 6 MPa carbon dioxide are lower than the heat transfer coefficient values of the 10 MPa carbon dioxide. Therefore, in the heat source unit la of the air-conditioning apparatus I of the present embodiment (i.e., the heat source unit configured so as to suction air in from the side 30 and blow air out upward), if the intermediate heat exchanger 7 were to be integrated with the heat source-side heat exchanger 4 in a state of being disposed underneath the heat source-side heat exchanger 4, the intermediate heat exchanger 7 integrated with the heat source-side heat exchanger 4 would be disposed in the bottom part of the heat source unit I a where the air as a heat source flows at a low rate, the effect of the decrease in the heat transfer coefficient of the 19 air in the intermediate heat exchanger 7 caused by disposing the intermediate heat exchanger 7 in the bottom part of the heat source unit I a and the effect of the heat transfer coefficient of the refrigerant in the intermediate heat exchanger 7 being lower than the heat transfer coefficient of the refrigerant in the heat source-side heat exchanger 4 would combine to 5 reduce the overall heat transfer coefficient of the intermediate heat exchanger 7. Moreover, since there is a limit on the extent to which the heat transfer surface area of the intermediate heat exchanger 7 can be increased due to the intermediate heat exchanger 7 being integrated with the heat source-side heat exchanger 4, there would be a decrease in the heat transfer performance of the intermediate heat exchanger 7. However, in the present embodiment, 10 since the intermediate heat exchanger 7 is integrated with the heat source-side heat exchanger 4 in a state of being disposed above the heat source-side heat exchanger 4, the intermediate heat exchanger 7 is disposed in the top part of the heat source unit 1 a where the air as a heat source flows at a high rate (see FIGS. 2 through 4), and the heat transfer coefficient of the air in the intermediate heat exchanger 7 increases. As a result, the decrease in the overall heat 15 transfer coefficient of the intermediate heat exchanger 7 can be minimized, and the decrease in the heat transfer performance of the intermediate heat exchanger 7 can be minimized. <Air-warming operation> During the air-warming operation, the switching mechanism 3 is brought to the heating operation state shown by the dashed lines in FIGS. I and 9. The opening degrees of 20 the first expansion mechanism 5a and the second expansion mechanism 5b are also adjusted. Since the switching mechanism 3 is in the heating operation state, the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 is closed and the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9 is opened, whereby the intermediate heat exchanger 7 is brought to a state of not functioning as 25 a cooler. When the refrigerant circuit 10 is in this state, low-pressure refrigerant (refer to point A in FIGS. I and 9 through 11) is drawn into the compression mechanism 2 through the intake tube 2a, and after the refrigerant is first compressed by the compression element 2c to an intermediate pressure, the refrigerant is discharged to the intermediate refrigerant tube 8 30 (refer to point B in FIGS. 1 and 9 through 11). The intermediate-pressure refrigerant discharged from the first-stage compression element 2c passes through the intermediate heat exchanger bypass tube 9 (refer to point C in FIGS. I and 9 through 11) without passing through the intermediate heat exchanger 7 (i.e., without being cooled), unlike in the air cooling operation. The refrigerant is drawn into and further compressed in the compression 20 element 2d connected to the second-stage side of the compression element 2c, and is discharged from the compression mechanism 2 to the discharge tube 2b (refer to point D in FIGS. I and 9 through 11). The high-pressure refrigerant discharged from the compression mechanism 2 is compressed by the two-stage compression action of the compression 5 elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pcp at the critical point CP shown in FIG. 10), similar to the air-cooling operation. The high-pressure refrigerant discharged from the compression mechanism 2 flows into the oil separator 41a constituting the oil separation mechanism 41, and the accompanying refrigeration oil is separated. The refrigeration oil separated from the high-pressure refrigerant in the oil [0 separator 41 a flows into the oil return tube 41b constituting the oil separation mechanism 41 wherein it is depressurized by the depressurization mechanism 41c provided to the oil return tube 41b, and the oil is then returned to the intake tube 2a of the compression mechanism 2 and once more drawn into the compression mechanism 2. Next, having been separated from the refrigeration oil in the oil separation mechanism 41, the high-pressure refrigerant is 5 passed through the non-return mechanism 42 and the switching mechanism 3, fed to the usage-side heat exchanger 6 functioning as a radiator of refrigerant, and cooled by heat exchange with the water and/or air as a cooling source (refer to point F in FIGS. I and 9 through 11). The high-pressure refrigerant cooled in the usage-side heat exchanger 6 then flows through the inlet non-return valve 17b of the bridge circuit 17 into the receiver inlet 20 tube 18a, and the refrigerant is depressurized to a nearly saturated pressure by the first expansion mechanism 5a and temporarily retained in the receiver 18 (refer to point I in FIGS. I and 9). The refrigerant retained in the receiver 18 is fed to the receiver outlet tube 18b and is depressurized by the second expansion mechanism 5b to become a low-pressure gas-liquid two-phase refrigerant, and is then fed through the outlet non-return valve 17d of the bridge 25 circuit 17 to the heat source-side heat exchanger 4 functioning as a refrigerant evaporator (refer to point E in FIGS. 1 and 9 through 11). The low-pressure gas-liquid two-phase refrigerant fed to the heat source-side heat exchanger 4 is heated and evaporated in the heat source-side heat exchanger 4 by heat exchange with the air as a heating source supplied by the heat source-side fan 40 (refer to point A in FIGS. 1 and 9 through 11). The low-pressure 30 refrigerant heated and evaporated in the heat source-side heat exchanger 4 is then drawn once more into the compression mechanism 2 via the switching mechanism 3. In this manner the air-warming operation is performed. Thus, in the air-conditioning apparatus I (the refrigeration apparatus) of the present embodiment, the intermediate heat exchanger 7 is provided to the intermediate refrigerant 21 tube 8 for drawing the refrigerant discharged from the compression element 2c into the compression element 2d, and during the air-warming operation, the intermediate heat exchanger on/off valve 12 is closed and the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9 is opened, whereby the intermediate heat 5 exchanger 7 is brought to a state of not functioning as a cooler. Therefore, the decrease in the temperature of the refrigerant discharged from the compression mechanism 2 is minimized (refer to points D and D' in FIG. 10) more than in cases in which only the intermediate heat exchanger 7 is provided and/or cases in which the intermediate heat exchanger 7 is made to function as a cooler similar to the air-cooling operation described 10 above (in these cases, the refrigeration cycle is performed in the following sequence in FIGS. 9 and 10: point A -+ point B -> point C' -- point D' - point F -+ point E). Therefore, in the air-conditioning apparatus 1, heat radiation to the exterior can be minimized, temperature decreases can be minimized in the refrigerant supplied to the usage-side heat exchanger 6 functioning as a refrigerant cooler, loss of heating performance in the usage-side heat 5 exchanger 6 can be reduced, and loss of operating efficiency can be prevented, in contrast with cases in which only the intermediate heat exchanger 7 is provided or cases in which the intermediate heat exchanger 7 is made to function as a radiator similar to the air-cooling operation described above. Moreover, in the air-conditioning apparatus 1 of the present embodiment, the air 20 warming operation is performed while the air as a heat source of the heat source-side heat exchanger 4 is low in temperature, whereby, even when frost deposition occurs on the heat source-side heat exchanger 4 functioning as an evaporator of refrigerant, the frost deposition in the border between the intermediate heat exchanger 7 and the heat source-side heat exchanger 4 is minimized regardless of the intermediate heat exchanger 7 being integrated 25 with the heat source-side heat exchanger 4, because the intermediate heat exchanger 7 is disposed above the heat source-side heat exchanger. Unlike cases in which the intermediate heat exchanger 7 is disposed below the heat source-side heat exchanger 4, there is less risk that water that has melted and dripped down from the heat source-side heat exchanger 4 due to the defrosting of the heat source-side heat exchanger 4 will adhere, freeze, and spread on 30 the intermediate heat exchanger 7, and the intermediate heat exchanger 7 therefore does not need to be defrosted when the defrosting operation (described hereinafter) is performed. <Defrosting operation> First, in step S1, a decision is made as to whether or not frost deposits have formed in the heat source-side heat exchanger 4 during the air-warming operation. This is 22 determined based on the temperature of the refrigerant flowing through the heat source-side heat exchanger 4 as detected by the heat source-side heat exchange temperature sensor 51, and/or on the cumulative time of the air-warming operation. For example, in cases in which the temperature of refrigerant in the heat source-side heat exchanger 4 as detected by the heat 5 source-side heat exchange temperature sensor 51 is equal to or less than a predetermined temperature equivalent to conditions at which frost deposits occur, or in cases in which the cumulative time of the air-warming operation has elapsed past a predetermined time, it is determined that frost deposits have occurred in the heat source-side heat exchanger 4. In cases in which these temperature conditions or time conditions are not met, it is determined 10 that frost deposits have not occurred in the heat source-side heat exchanger 4. Since the predetermined temperature and predetermined time depend on the temperature of the air as a heat source, the predetermined temperature and predetermined time are preferably set as a function of the air temperature detected by the air temperature sensor 53. In cases in which a temperature sensor is provided to the inlet or outlet of the heat source-side heat exchanger 15 4, the refrigerant temperature detected by these temperature sensors may be used in the determination of the temperature conditions instead of the refrigerant temperature detected by the heat source-side heat exchange temperature sensor 51. In cases in which it is determined in step Si that frost deposits have occurred in the heat source-side heat exchanger 4, the process advances to step S2. 20 Next, in step S2, the defrosting operation is started. The defrosting operation is a reverse cycle defrosting operation in which the heat source-side heat exchanger 4 is made to function as a refrigerant radiator by switching the switching mechanism 3 from the heating operation state (i.e., the air-warming operation) to the cooling operation state. In the present embodiment, since the intermediate heat exchanger 7 is disposed above the heat source-side 25 heat exchanger as described above, frost deposition is minimized in the border between the intermediate heat exchanger 7 and the heat source-side heat exchanger 4 regardless of the intermediate heat exchanger 7 being integrated with the heat source-side heat exchanger 4, and unlike cases in which the intermediate heat exchanger 7 is disposed below the heat source-side heat exchanger 4, there is less risk that water that has melted and dripped down 30 from the heat source-side heat exchanger 4 due to the defrosting of the heat source-side heat exchanger 4 will adhere, freeze, and spread on the intermediate heat exchanger 7, and the intermediate heat exchanger 7 therefore does not need to be defrosted. In view of this, in this defrosting operation, when the reverse cycle defrosting operation described above is performed, the intermediate heat exchanger bypass tube 9 is used to ensure that refrigerant 23 does not flow to the intermediate heat exchanger 7 (by closing the intermediate heat exchanger on/off valve 12 and opening the intermediate heat exchanger bypass on/off valve 11). The air-cooling operation is thereby performed in a state in which the intermediate 5 heat exchanger 7 is not made to function as a cooler (the refrigeration cycle performed in the following sequence in FIGS. 6, 7, and 13: point A -* point B -> point D' -> point E -> point F), heat radiation from the intermediate heat exchanger 7 to the exterior can be prevented (i.e., it is possible to prevent heat radiation equivalent to the area enclosed by connecting points B, D', D, and C in FIG. 7), the decrease in the defrosting capacity of the heat source 10 side heat exchanger 4 can be minimized, and the reverse cycle defrosting operation can thereby be performed efficiently. Next, in step S3, a decision is made as to whether or not defrosting of the heat source-side heat exchanger 4 has concluded. This decision is made based on the temperature of refrigerant flowing through the heat source-side heat exchanger 4 as detected 15 by the heat source-side heat exchange temperature sensor 51, and/or on the operation time of the defrosting operation. For example, in the case that the temperature of refrigerant in the heat source-side heat exchanger 4 as detected by the heat source-side heat exchange temperature sensor 51 is equal to or greater than a temperature equivalent to conditions at which frost deposits do not occur, or in the case that the defrosting operation has continued 20 for a predetermined time or longer, it is determined that defrosting of the heat source-side heat exchanger 4 has concluded. In the case that the temperature conditions or time conditions are not met, it is determined that defrosting of the heat source-side heat exchanger 4 is not complete. In the case that a temperature sensor is provided to the inlet or outlet of the heat source-side heat exchanger 4, the temperature of the refrigerant as detected by either 25 of these temperature sensors may be used in the determination of the temperature conditions instead of the refrigerant temperature detected by the heat source-side heat exchange temperature sensor 51. In cases in which it is determined in step S3 that defrosting of the heat source-side heat exchanger 4 has completed, the process transitions to step S4, the defrosting operation ends, and the process for restarting the air-warming operation is again 30 performed. More specifically, a process is performed for switching the switching mechanism 3 from the cooling operation state to the heating operation state (i.e. the air warming operation). Thus, in the air-conditioning apparatus 1 (the refrigeration apparatus) of the present embodiment, the intermediate heat exchanger 7 is disposed above the heat source-side heat 24 exchanger 4, whereby frost deposition is minimized in the border between the intermediate heat exchanger 7 and the heat source-side heat exchanger 4 regardless of the intermediate heat exchanger 7 being integrated with the heat source-side heat exchanger 4, and unlike cases in which the intermediate heat exchanger 7 is disposed below the heat source-side heat 5 exchanger 4, there is less risk that water that has melted and dripped down from the heat source-side heat exchanger 4 due to the defrosting of the heat source-side heat exchanger 4 will adhere, freeze, and spread on the intermediate heat exchanger 7, and the intermediate heat exchanger 7 therefore does not need to be defrosted when the reverse cycle defrosting operation is performed. In this air-conditioning apparatus 1, taking advantage of the fact 10 that the intermediate heat exchanger 7 does not need to be defrosted during the reverse cycle defrosting operation, when the reverse cycle defrosting operation is performed, the intermediate heat exchanger bypass tube 9 is used to ensure that the refrigerant does not flow to the intermediate heat exchanger 7, whereby heat radiation from the intermediate heat exchanger 7 to the exterior is prevented and the decrease in the defrosting capacity of the heat 15 source-side heat exchanger 4 is minimized, and the reverse cycle defrosting operation can therefore be performed efficiently. (3) Modification 1 In the embodiment described above, in the air-conditioning apparatus 1 configured to be capable of switching between the air-cooling operation and the air-warming operation 20 via the switching mechanism 3, the intermediate heat exchanger 7 having air as a heat source is integrated in a state of being disposed above the heat source-side heat exchanger 4, and the intermediate heat exchanger bypass tube 9 is used to ensure that the refrigerant does not flow to the intermediate heat exchanger 7 when the reverse cycle defrosting operation is performed, whereby the decrease in the defrosting capacity of the heat source-side heat 25 exchanger 4 is minimized when the reverse cycle defrosting operation is performed, and the reverse cycle defrosting operation can be performed efficiently, but another possible consideration in addition to this configuration is to also provide a first second-stage injection tube 18c for branching off the refrigerant heated in the heat source-side heat exchanger 4 or the usage-side heat exchanger 6 and returning the refrigerant to the second-stage compression 30 element 2d. For example, in the embodiment described above in which the two-stage compression-type compression mechanism 2 is used, a refrigerant circuit 110 provided with the first second-stage injection tube 18c can be used as shown in FIG 14. The first second-stage injection tube 18c herein is a refrigerant tube capable of 25 performing intermediate pressure injection for extracting refrigerant from the receiver 18 and returning the refrigerant to the second-stage compression element 2d of the compression mechanism 2, and in the present modification, the first second-stage injection tube 18c is provided so as to connect the top part of the receiver 18 and the intermediate refrigerant tube 5 8 (i.e., the intake side of the second-stage compression element 2d of the compression mechanism 2). This first second-stage injection tube 18c is provided with a first second stage injection on/off valve 18d and a first second-stage injection non-return mechanism 18e. The first second-stage injection on/off valve 18d is a valve capable of opening and closing, and is an electromagnetic valve in the present modification. The first second-stage injection 10 non-return mechanism 18e is a mechanism for allowing the flow of refrigerant from the receiver 18 to the second-stage compression element 2d and blocking the flow of refrigerant from the second-stage compression element 2d to the receiver 18, and in the present modification, a non-return valve is used. The first second-stage injection tube 18c and the first intake return tube 18f are integrated in the portion near the receiver 18. The receiver 18 15 hereby functions as a gas-liquid separator for performing gas-liquid separation between the first expansion mechanism 5a and the second expansion mechanism 5b on the refrigerant flowing between the heat source-side heat exchanger 4 and the usage-side heat exchanger 6 in cases in which the first second-stage injection tube 18c and/or the first intake return tube 18f is used by opening the first second-stage injection on/off valve 18d and/or the first intake 20 return on/off valve 18g, and intermediate pressure injection by the receiver 18 can be performed, which is primarily for returning the gas refrigerant resulting from gas-liquid separation in the receiver 18 from the top part of the receiver 18 to the intake side of the second-stage compression element 2d of the compression mechanism 2 (the outlet side of the intermediate heat exchanger 7 of the intermediate refrigerant tube 8 in this case). 25 Next, the action of the air-conditioning apparatus 1 will be described using FIGS. 14 through 23. FIG. 15 is a diagram showing the flow of refrigerant within the air-conditioning apparatus 1 during the air-cooling operation, FIG 16 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation, FIG 17 is a temperature entropy graph representing the refrigeration cycle during the air-cooling operation, FIG. 18 is 30 a diagram showing the flow of refrigerant within the air-conditioning apparatus I during the air-warming operation, FIG. 19 is a pressure-enthalpy graph representing the refrigeration cycle during the air-warming operation, FIG. 20 is a temperature-entropy graph representing the refrigeration cycle during the air-warming operation, FIG. 21 is a diagram showing the flow of refrigerant within the air-conditioning apparatus 1 during the defrosting operation, 26 FIG 22 is a pressure-enthalpy graph representing the refrigeration cycle during the defrosting operation, and FIG 23 is a temperature-entropy graph representing the refrigeration cycle during the defrosting operation. Operation control in the air-cooling operation, the air warming operation, and the defrosting operation described hereinbelow is performed by the 5 aforementioned controller (not shown). In the following description, the term "high pressure" means a high pressure in the refrigeration cycle (specifically, the pressure at points D, D', and E in FIGS. 16, 17, 22, and 23, and the pressure at points D, D', and F in FIGS. 19 and 20), the term "low pressure" means a low pressure in the refrigeration cycle (specifically, the pressure at points A and F in FIGS. 16, 17, 22, and 23, and the pressure at points A and E 10 in FIGS. 19 and 20), and the term "intermediate pressure" means an intermediate pressure in the refrigeration cycle (specifically, the pressure at points B, C, Q, G', I, L, and M in FIGS. 16, 17, 19, 20, 22, and 23). <Air-cooling operation> During the air-cooling operation, the switching mechanism 3 is brought to the 15 cooling operation state shown by the solid lines in FIGS. 14 and 15. The opening degrees of the first expansion mechanism 5a and the second expansion mechanism 5b are adjusted. Since the switching mechanism 3 is in the cooling operation state, the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 is opened and the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass 20 tube 9 is closed, thereby creating a state in which the intermediate heat exchanger 7 functions as a cooler. Furthermore, the first second-stage injection on/off valve 18d is brought to an open state. When the refrigerant circuit 110 is in this state, low-pressure refrigerant (refer to point A in FIGS. 14 through 17) is drawn into the compression mechanism 2 through the 25 intake tube 2a, and after the refrigerant is first compressed by the compression element 2c to an intermediate pressure, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point A in FIGS. 14 through 17). The intermediate-pressure refrigerant discharged from the first-stage compression element 2c is cooled in the intermediate heat exchanger 7 by undergoing heat exchange with the air as a cooling source supplied by the heat source-side 30 fan 40 (refer to point C in FIGS. 14 through 17). The refrigerant cooled in the intermediate 7 is further cooled (refer to point G in FIGS. 14 through 17) by mixing with refrigerant being returned from the receiver 18 via the first second-stage injection tube 18c to the second-stage compression element 2d (refer to point M in FIGS. 14 through 17). Next, having been mixed with the refrigerant returning from the first second-stage injection tube 18c (i.e., 27 intermediate pressure injection is carried out by the receiver 18 which acts as a gas-liquid separator), the intermediate-pressure refrigerant is drawn into and further compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and the refrigerant is discharged from the compression mechanism 2 to the discharge tube 2b 5 (refer to point D in FIGS. 14 through 17). The high-pressure refrigerant discharged from the compression mechanism 2 is compressed by the two-stage compression action of the compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pep at the critical point CP shown in FIG. 16). The high-pressure refrigerant discharged from the compression mechanism 2 flows into the oil separator 41a constituting 10 the oil separation mechanism 41, and the accompanying refrigeration oil is separated. The refrigeration oil separated from the high-pressure refrigerant in the oil separator 41a flows into the oil return tube 41b constituting the oil separation mechanism 41 wherein it is depressurized by the depressurization mechanism 41c provided to the oil return tube 41b, and the oil is then returned to the intake tube 2a of the compression mechanism 2 and drawn once 15 more into the compression mechanism 2. Next, having been separated from the refrigeration oil in the oil separation mechanism 41, the high-pressure refrigerant is passed through the non-return mechanism 42 and the switching mechanism 3, and is fed to the heat source-side heat exchanger 4 functioning as a refrigerant radiator. The high-pressure refrigerant fed to the heat source-side heat exchanger 4 is cooled in the heat source-side heat 20 exchanger 4 by heat exchange with air as a cooling source supplied by the heat source-side fan 40 (refer to point E in FIGS. 14 through 17). The high-pressure refrigerant cooled in the heat source-side heat exchanger 4 then passes through the inlet non-return valve 17a of the bridge circuit 17 and flows into the receiver inlet tube 18a, and the refrigerant is depressurized to a nearly intermediate pressure by the first expansion mechanism 5a, 25 temporarily retained in the receiver 18, and subjected to gas-liquid separation (refer to points I, L, and M in FIGS. 14 through 17). The gas refrigerant having undergone gas-liquid separation in the receiver 18 is then extracted out of the top part of the receiver 18 by the first second-stage injection tube 18c, and this refrigerant mixes with the intermediate-pressure refrigerant discharged from the first-stage compression element 2c as described above. The 30 refrigerant retained in the receiver 18 is fed to the receiver outlet tube 18b and is depressurized by the second expansion mechanism 5b to become a low-pressure gas-liquid two-phase refrigerant, and is then fed through the outlet non-return valve 17c of the bridge circuit 17 to the usage-side heat exchanger 6 functioning as a refrigerant evaporator (refer to point F in FIGS. 14 through 17). The low-pressure gas-liquid two-phase refrigerant fed to 28 the usage-side heat exchanger 6 is heated by heat exchange with water or air as a heating source, and the refrigerant is evaporated as a result (refer to point A in FIGS. 14 through 17). The low-pressure refrigerant heated in the usage-side heat exchanger 6 is then drawn once more into the compression mechanism 2 via the switching mechanism 3. In this manner the 5 air-cooling operation is performed. Thus, in the air-conditioning apparatus I of the present modification, in addition to the intermediate heat exchanger 7 being made to function as a cooler similar to the air cooling operation in the embodiment described above, the first second-stage injection tube 18c is provided to branch off the refrigerant fed from the heat source-side heat exchanger 4 to 0 the expansion mechanisms 5a, 5b and return the refrigerant to the second-stage compression element 2d, and the temperature of the refrigerant drawn into the second-stage compression element 2d can therefore be kept even lower without heat being radiated to the exterior (refer to points C and G in FIG. 17). The temperature of the refrigerant discharged from the compression mechanism 2 is thereby kept lower (refer to points D and D' in FIG 17), and it is [5 possible to further reduce the heat radiation loss equivalent to the area enclosed by connecting points C, D', D, and G in FIG 17 to a greater extent than in cases in which the first second-stage injection tube 18c is not provided; therefore, the power consumption of the compression mechanism 2 can be further reduced, and operating efficiency can be further improved. 20 <Air-warming operation> During the air-warming operation, the switching mechanism 3 is brought to the heating operation state shown by the dashed lines in FIGS. 14 and 18. The opening degrees of the first expansion mechanism 5a and the second expansion mechanism 5b are adjusted. Since the switching mechanism 3 is in the heating operation state, the intermediate heat 25 exchanger on/off valve 12 of the intermediate refrigerant tube 8 is closed and the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9 is opened, thereby creating a state in which the intermediate heat exchanger 7 does not function as a cooler. Furthermore, the first second-stage injection on/off valve 18d is brought to an open state similar to during the air-cooling operation. 30 When the refrigerant circuit 110 is in this state, low-pressure refrigerant (refer to point A in FIGS. 14 and 18 through 20) is drawn into the compression mechanism 2 through the intake tube 2a, and after the refrigerant is first compressed by the compression element 2c to an intermediate pressure, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point B in FIGS. 14 and 18 through 20). This intermediate-pressure refrigerant 29 discharged from the first-stage compression element 2c passes through the intermediate heat exchanger bypass tube 9 (refer to point C in FIGS. 14 and 18 through 20) without passing through the intermediate heat exchanger 7 (i.e., without being cooled), similar to the air warming operation in the embodiment described above. This intermediate-pressure 5 refrigerant which has passed through the intermediate heat exchanger bypass tube 9 without being cooled by the intermediate heat exchanger 7 is cooled (refer to point G in FIGS. 14 and 18 through 20) by mixing with the refrigerant returned from the receiver 18 through the first second-stage injection tube 18c to the second-stage compression element 2d (refer to point M in FIGS. 14 and 18 through 20). Next, having been mixed with the refrigerant returning 10 from the first second-stage injection tube 18c (i.e., intermediate pressure injection is carried out by the receiver 18 which acts as a gas-liquid separator), the intermediate-pressure refrigerant is drawn into and further compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and the refrigerant is discharged from the compression mechanism 2 to the discharge tube 2b (refer to point D in FIGS. 14 and 18 15 through 20). The high-pressure refrigerant discharged from the compression mechanism 2 is compressed by the two-stage compression action of the compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pcp at the critical point CP shown in FIG. 19), similar to the air-cooling operation. The high-pressure refrigerant discharged from the compression mechanism 2 flows into the oil separator 41a constituting 20 the oil separation mechanism 41, and the accompanying refrigeration oil is separated. The refrigeration oil separated from the high-pressure refrigerant in the oil separator 41a flows into the oil return tube 41b constituting the oil separation mechanism 41 wherein it is depressurized by the depressurization mechanism 41 c provided to the oil return tube 41 b, and the oil is then returned to the intake tube 2a of the compression mechanism 2 and drawn once 25 more into the compression mechanism 2. Next, having been separated from the refrigeration oil in the oil separation mechanism 41, the high-pressure refrigerant is passed through the non-return mechanism 42 and the switching mechanism 3, fed to the usage-side heat exchanger 6 functioning as a radiator of refrigerant, and cooled by heat exchange with the water and/or air as a cooling source (refer to point F in FIGS. 14 and 18 through 20). 30 The high-pressure refrigerant cooled in the usage-side heat exchanger 6 then flows through the inlet non-return valve 17b of the bridge circuit 17 into the receiver inlet tube 18a, and the refrigerant is depressurized to a nearly intermediate pressure by the first expansion mechanism 5a, temporarily retained in the receiver 18, and subjected to gas-liquid separation (refer to points I, L, and M in FIGS. 14 and 18 through 20). Having undergone gas-liquid 30 separation in the receiver 18, the gas refrigerant is extracted out of the top part of the receiver 18 by the first second-stage injection tube 18c, and is mixed with the intermediate-pressure refrigerant discharged from the first-stage compression element 2c as described above. The liquid refrigerant retained in the receiver 18 is fed to the receiver outlet tube 18b and is 5 depressurized by the second expansion mechanism 5b to become a low-pressure gas-liquid two-phase refrigerant, and is then fed through the outlet non-return valve 17d of the bridge circuit 17 to the heat source-side heat exchanger 4 functioning as a refrigerant evaporator (refer to point E in FIGS. 14 and 18 through 20). The low-pressure gas-liquid two-phase refrigerant fed to the heat source-side heat exchanger 4 is then heated and evaporated by heat 10 exchange with the air as a heat source supplied by the heat source-side fan 40 (refer to point A in FIGS. 14 and 18 through 20). The low-pressure refrigerant heated and evaporated in the heat source-side heat exchanger 4 is then drawn once more into the compression mechanism 2 via the switching mechanism 3. In this manner the air-warming operation is performed. 15 Thus, in the air-conditioning apparatus 1 of the present modification, the intermediate heat exchanger 7 is brought to a state of not functioning as a cooler similar to the air-warming operation in the embodiment described above, and the first second-stage injection tube 18c is provided to branch off the refrigerant fed from the heat source-side heat exchanger 4 to the expansion mechanisms 5a, 5b and return the refrigerant to the second 20 stage compression element 2d; therefore, the temperature of the refrigerant drawn into the second-stage compression element 2d can be suppressed without heat being radiated to the exterior (refer to points C, G, and G' in FIG. 20). Thereby, although the temperature of the refrigerant discharged from the compression mechanism 2 decreases and the heating capacity per unit flow rate of the refrigerant in the usage-side heat exchanger 6 decreases (refer to 25 points D, D' and F in FIG 20), the flow rate of the refrigerant discharged from the second stage compression element 2d increases, the decrease in the heating capacity of the usage side heat exchanger 6 is therefore minimized, and as a result, the power consumption of the compression mechanism 2 can be reduced and operating efficiency can be improved. <Defrosting operation> 30 In the embodiment described above, since a reverse cycle defrosting operation is used for defrosting the heat source-side heat exchanger 4 by switching the switching mechanism 3 to the cooling operation state, the usage-side heat exchanger 6 is made to function as an evaporator of refrigerant regardless of the intention being to cause the usage side heat exchanger 6 to function as a radiator of refrigerant, and there is a problem in that the 31 temperature on the usage side decreases. Since the reverse cycle defrosting operation is an air-cooling operation performed in a state in which the intermediate heat exchanger 7 is not made to function as a cooler while the temperature of the air as a heat source is low, the low pressure in the refrigeration cycle decreases, and the flow rate of the refrigerant drawn from 5 the first-stage compression element 2c is reduced. When this happens, another problem emerges that more time is required for defrosting the heat source-side heat exchanger 4 because the flow rate of refrigerant circulated through the refrigerant circuit 10 is reduced and the flow rate of refrigerant flowing through the heat source-side heat exchanger 4 can no longer be guaranteed. Such problems are also encountered in the configuration of the [0 present modification. In view of this, in the present modification, in step S2 shown in FIG. 12, when the reverse cycle defrosting operation is performed, a state is created in which the intermediate heat exchanger 7 is not made to function as a cooler, and the first second-stage injection tube 18c is used (i.e., the first second-stage injection on/off valve 18d is opened and intermediate 15 pressure injection is performed by the receiver 18 as a gas-liquid separator) to perform the reverse cycle defrosting operation (see FIG. 21) while the refrigerant fed from the heat source-side heat exchanger 4 to the usage-side heat exchanger 6 is returned to the second stage compression element 2d. The air-cooling operation (the refrigeration cycle performed in the following 20 sequence shown in FIGS. 21 through 23: point A -+ point B, C -+ point G -+ point D -+ point E -> point I -> point L -> point F) accompanying intermediate pressure injection by the receiver 18 as a gas-liquid separator is thereby performed in a state in which the intermediate heat exchanger 7 is not made to function as a cooler, heat radiation from the intermediate heat exchanger 7 to the exterior is prevented (i.e., it is possible to prevent heat radiation equivalent 25 to the area enclosed by connecting points G D, D' and G' in FIG 23), the loss of defrosting capacity of the heat source-side heat exchanger 4 is minimized (this also applies to the defrosting operation in the embodiment described above), and the flow rate of the refrigerant flowing through the heat source-side heat exchanger 4 can be guaranteed while reducing the flow rate of the refrigerant flowing through the usage-side heat exchanger 6, whereby the 30 defrosting time of the heat source-side heat exchanger 4 can be reduced while minimizing the temperature decrease on the usage side when the reverse cycle defrosting operation is performed. The other steps Sl, S3, and S4 of the defrosting operation in the present modification are the same as those of the defrosting operation in the embodiment described above and are therefore not described herein. 32 (4) Modification 2 In Modification 1 described above, in the air-conditioning apparatus 1 configured to be capable of switching between the air-cooling operation and the air-warming operation via the switching mechanism 3, the first second-stage injection tube 18c is provided for 5 performing intermediate pressure injection through the receiver 18 as a gas-liquid separator, and intermediate pressure injection is performed by the receiver 18 as a gas-liquid separator, but instead of intermediate pressure injection by the receiver 18, another possible option is to provide a second second-stage injection tube 19 and an economizer heat exchanger 20 and to perform intermediate pressure injection through the economizer heat exchanger 20. 10 For example, as shown in FIG 24, a refrigerant circuit 210 can be used which is provided with a second second-stage injection tube 19 and an economizer heat exchanger 20 instead of the first second-stage injection tube 18c in Modification 1 described above. The second second-stage injection tube 19 has a function for branching off and returning the refrigerant cooled in the heat source-side heat exchanger 4 or the usage-side 15 heat exchanger 6 to the second-stage compression element 2d of the compression mechanism 2. In the present modification, the second second-stage injection tube 19 is provided so as to branch off refrigerant flowing through the receiver inlet tube 18a and return the refrigerant to the intake side of the second-stage compression element 2d. More specifically, the second second-stage injection tube 19 is provided so as to branch off and return the 20 refrigerant from a position on the upstream side of the first expansion mechanism 5a of the receiver inlet tube 18a (i.e., between the heat source-side heat exchanger 4 and the first expansion mechanism 5a when the switching mechanism 3 is in the cooling operation state, or between the usage-side heat exchanger 6 and the first expansion mechanism 5a when the switching mechanism 3 is in the heating operation state) to a position on the downstream side 25 of the intermediate heat exchanger 7 of the intermediate refrigerant tube 8. The second second-stage injection tube 19 is provided with a second second-stage injection valve 19a whose opening degree can be controlled. The second second-stage injection valve 19a is an electrically driven expansion valve in the present modification. The economizer heat exchanger 20 is a heat exchanger for carrying out heat 30 exchange between the refrigerant from which heat has been released in the heat source-side heat exchanger 4 or the usage-side heat exchanger 6 and the refrigerant that flows through the second second-stage injection tube 19 (more specifically, the refrigerant that has been depressurized to near intermediate pressure in the second second-stage injection valve 19a). In the present modification, the economizer heat exchanger 20 is provided so as to perform 33 heat exchange between the refrigerant flowing through a position in the receiver inlet tube 18a upstream of the first expansion mechanism 5a (i.e., between the heat source-side heat exchanger 4 and the first expansion mechanism 5a when the switching mechanism 3 is in the cooling operation state, or between the usage-side heat exchanger 6 and the first expansion 5 mechanism 5a when the switching mechanism 3 is in the heating operation state) and the refrigerant flowing through the second second-stage injection tube 19, and the economizer heat exchanger 20 has a passage through which both refrigerants flow against each other. In the present modification, the economizer heat exchanger 20 is provided upstream of the second second-stage injection tube 19 of the receiver inlet tube 18a. Therefore, the 0 refrigerant from which heat has been released in the heat source-side heat exchanger 4 or usage-side heat exchanger 6 is branched off in the receiver inlet tube 18a into the second second-stage injection tube 19 before undergoing heat exchange in the economizer heat exchanger 20, and heat exchange is then conducted in the economizer heat exchanger 20 with the refrigerant flowing through the second second-stage injection tube 19. 5 Furthermore, the air-conditioning apparatus 1 of the present modification is provided with various sensors. Specifically, the intermediate refrigerant tube 8 or the compression mechanism 2 is provided with an intermediate pressure sensor 54 for detecting the pressure of the refrigerant that flows through the intermediate refrigerant tube 8. The outlet of the second second-stage injection tube 19 side of the economizer heat exchanger 20 is provided 20 with an economizer outlet temperature sensor 55 for detecting the temperature of the refrigerant at the outlet of the second second-stage injection tube 19 side of the economizer heat exchanger 20. Next, the action of the air-conditioning apparatus I will be described using FIGS. 24 through 33. FIG. 25 is a diagram showing the flow of refrigerant within the air-conditioning 25 apparatus I during the air-cooling operation, FIG 26 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation, FIG. 27 is a temperature entropy graph representing the refrigeration cycle during the air-cooling operation, FIG. 28 is a diagram showing the flow of refrigerant within the air-conditioning apparatus I during the air-warming operation, FIG. 29 is a pressure-enthalpy graph representing the refrigeration 30 cycle during the air-warming operation, FIG. 30 is a temperature-entropy graph representing the refrigeration cycle during the air-warming operation, FIG. 31 is a diagram showing the flow of refrigerant within the air-conditioning apparatus I during the defrosting operation, FIG. 32 is a pressure-enthalpy graph representing the refrigeration cycle during the defrosting operation, and FIG. 33 is a temperature-entropy graph representing the refrigeration cycle 34 during the defrosting operation. Operation control in the air-cooling operation, the air warming operation, and the defrosting operation described hereinbelow is performed by the aforementioned controller (not shown). In the following description, the term "high pressure" means a high pressure in the refrigeration cycle (specifically, the pressure at points 5 D, D', E, H in FIGS. 26, 27, 32, and 33, and the pressure at points D, D', F, and H in FIGS. 29 and 30), the term "low pressure" means a low pressure in the refrigeration cycle (specifically, the pressure at points A and F in FIGS. 26, 27, 32, and 33, and the pressure at points A and E in FIGS. 29 and 30), and the term "intermediate pressure" means an intermediate pressure in the refrigeration cycle (specifically, the pressure at points B, C, G, 10 G', J, and K in FIGS. 26, 27, 29, 30, 32, 33). <Air-cooling operation> During the air-cooling operation, the switching mechanism 3 is brought to the cooling operation state shown by the solid lines in FIGS. 24 and 25. The opening degrees of the first expansion mechanism 5a and the second expansion mechanism 5b are adjusted. 15 Since the switching mechanism 3 is in the cooling operation state, the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 is opened and the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9 is closed, thereby creating a state in which the intermediate heat exchanger 7 functions as a cooler. Furthermore, the opening degree of the second second-stage injection valve 19a 20 is also adjusted. More specifically, in the present modification, so-called superheat degree control is performed wherein the opening degree of the second second-stage injection valve 19a is adjusted so that a target value is achieved in the degree of superheat of the refrigerant at the outlet in the second second-stage injection tube 19 side of the economizer heat exchanger 20. In the present modification, the degree of superheat of the refrigerant at the 25 outlet in the second second-stage injection tube 19 side of the economizer heat exchanger 20 is obtained by converting the intermediate pressure detected by the intermediate pressure sensor 54 to a saturation temperature and subtracting this refrigerant saturation temperature value from the refrigerant temperature detected by the economizer outlet temperature sensor 55. Though not used in the present embodiment, another possible option is to provide a 30 temperature sensor to the inlet in the second second-stage injection tube 19 side of the economizer heat exchanger 20, and to obtain the degree of superheat of the refrigerant at the outlet in the second second-stage injection tube 19 side of the economizer heat exchanger 20 by subtracting the refrigerant temperature detected by this temperature sensor from the refrigerant temperature detected by the economizer outlet temperature sensor 55. The 35 opening degree adjustment of the second second-stage injection valve 19a is not limited to superheat degree control, and the opening degree may be opened to a predetermined opening degree in accordance with the flow rate of refrigerant circulating in the refrigerant circuit 210 or other factors, for example. 5 When the refrigerant circuit 210 is in this state, low-pressure refrigerant (refer to point A in FIGS. 24 through 27) is drawn into the compression mechanism 2 through the intake tube 2a, and after the refrigerant is first compressed by the compression element 2c to an intermediate pressure, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point A in FIGS. 24 through 27). The intermediate-pressure refrigerant discharged 10 from the first-stage compression element 2c is cooled in the intermediate heat exchanger 7 by undergoing heat exchange with the air as a cooling source supplied by the heat source-side fan 40 (refer to point C in FIGS. 24 through 27). The refrigerant cooled in the intermediate heat exchanger 7 is further cooled (refer to point G in FIGS. 24 through 27) by being mixed with refrigerant being returned from the second second-stage injection tube 19 to the second 15 stage compression element 2d (refer to point K in FIGS. 24 through 27). Next, having been mixed with the refrigerant returning from the second second-stage injection tube 19 (i.e., intermediate pressure injection is carried out by the economizer heat exchanger 20), the intermediate-pressure refrigerant is drawn into and further compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and the 20 refrigerant is discharged from the compression mechanism 2 to the discharge tube 2b (refer to point D in FIGS. 24 through 27). The high-pressure refrigerant discharged from the compression mechanism 2 is compressed by the two-stage compression action of the compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pcp at the critical point CP shown in FIG. 26). The high-pressure refrigerant 25 discharged from the compression mechanism 2 flows into the oil separator 41a constituting the oil separation mechanism 41, and the accompanying refrigeration oil is separated. The refrigeration oil separated from the high-pressure refrigerant in the oil separator 41a flows into the oil return tube 41b constituting the oil separation mechanism 41 wherein it is depressurized by the depressurization mechanism 41c provided to the oil return tube 41b, and 30 the oil is then returned to the intake tube 2a of the compression mechanism 2 and drawn once more into the compression mechanism 2. Next, having been separated from the refrigeration oil in the oil separation mechanism 41, the high-pressure refrigerant is passed through the non-return mechanism 42 and the switching mechanism 3, and is fed to the heat source-side heat exchanger 4 functioning as a refrigerant radiator. The high-pressure 36 refrigerant fed to the heat source-side heat exchanger 4 is cooled in the heat source-side heat exchanger 4 by heat exchange with air as a cooling source supplied by the heat source-side fan 40 (refer to point E in FIGS. 24 through 27). The high-pressure refrigerant cooled in the heat source-side heat exchanger 4 flows through the inlet non-return valve 17a of the bridge 5 circuit 17 into the receiver inlet tube 18a, and some of the refrigerant is branched off into the second second-stage injection tube 19. The refrigerant flowing through the second second stage injection tube 19 is depressurized to a nearly intermediate pressure in the second second-stage injection valve 19a and is then fed to the economizer heat exchanger 20 (refer to point J in FIGS. 24 through 27). The refrigerant after being branched off into the second 10 second-stage injection tube 19 then flows into the economizer heat exchanger 20, where it is cooled by heat exchange with the refrigerant flowing through the second second-stage injection tube 19 (refer to point H in FIGS. 24 through 27). The refrigerant flowing through the second second-stage injection tube 19 is heated by heat exchange with the high-pressure refrigerant cooled in the heat source-side heat exchanger 4 as a radiator (refer to point K in 5 FIGS. 24 through 27), and is mixed with the intermediate-pressure refrigerant discharged from the first-stage compression element 2c as described above. The high-pressure refrigerant cooled in the economizer heat exchanger 20 is depressurized to a nearly saturated pressure by the first expansion mechanism 5a and is temporarily retained in the receiver 18 (refer to point I in FIGS. 24 and 25). The refrigerant retained in the receiver 18 is fed to the 20 receiver outlet tube 18b and is depressurized by the second expansion mechanism 5b to become a low-pressure gas-liquid two-phase refrigerant, and is then fed through the outlet non-return valve 17c of the bridge circuit 17 to the usage-side heat exchanger 6 functioning as a refrigerant evaporator (refer to point F in FIGS. 24 through 27). The low-pressure gas liquid two-phase refrigerant fed to the usage-side heat exchanger 6 is heated by heat 25 exchange with water or air as a heating source, and the refrigerant is evaporated as a result (refer to point A in FIGS. 24 through 27). The low-pressure refrigerant heated in the usage side heat exchanger 6 is then drawn once more into the compression mechanism 2 via the switching mechanism 3. In this manner the air-cooling operation is performed. Thus, in the air-conditioning apparatus 1 of the present modification, in addition to 30 the intermediate heat exchanger 7 being made to function as a cooler similar to the air cooling operation in the embodiment described above, the second second-stage injection tube 19 and the economizer heat exchanger 20 are provided to branch off the refrigerant fed from the heat source-side heat exchanger 4 to the expansion mechanisms 5a, 5b and return the refrigerant to the second-stage compression element 2d, and the temperature of the refrigerant 37 drawn into the second-stage compression element 2d can therefore be kept even lower without heat being radiated to the exterior (refer to points C and G in FIG. 27), similar to Modification 1 described above. The temperature of the refrigerant discharged from the compression mechanism 2 is thereby suppressed (refer to points D and D' in FIG. 27), and it 5 is possible to further reduce the heat radiation loss equivalent to the area enclosed by connecting points C, D', D, and G in FIG. 27 more than in cases in which the second second stage injection tube 19 and the economizer heat exchanger 20 are not provided; therefore, the power consumption of the compression mechanism 2 can be further reduced, and operating efficiency can be further improved. 10 Moreover, the intermediate pressure injection by the economizer heat exchanger 20 used in the present modification is more beneficial than the intermediate pressure injection by the receiver 18 as a gas-liquid separator used in Modification 1 described above, because in a refrigerant circuit configuration in which no significant depressurizing operations are performed except for the first expansion mechanism 5a as a heat source-side expansion 15 mechanism after the refrigerant is cooled in the heat source-side heat exchanger 4 as a radiator and the pressure difference from the high pressure in the refrigeration cycle to the nearly intermediate pressure of the refrigeration cycle can be used, the quantity of heat exchanged in the economizer heat exchanger 20 can be increased, and the flow rate of the refrigerant passing through the second second-stage injection tube 19 and returning to the 20 second-stage compression element 2d can thereby be increased. Particularly in cases in which refrigerant that operates in the supercritical range is used as in the present modification, the intermediate pressure injection by the economizer heat exchanger 20 is extremely beneficial because there is an extremely large pressure difference from the high pressure in the refrigeration cycle to the nearly intermediate pressure of the refrigeration 25 cycle. <Air-warming operation> During the air-warming operation, the switching mechanism 3 is brought to the heating operation state shown by the dashed lines in FIGS. 24 and 28. The opening degrees of the first expansion mechanism 5a and the second expansion mechanism 5b are adjusted. 30 Since the switching mechanism 3 is in the heating operation state, the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 is closed and the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9 is opened, thereby creating a state in which the intermediate heat exchanger 7 does not function as a cooler. Furthermore, the opening degree of the second second-stage injection valve 19a 38 is adjusted in the same manner as in the air-cooling operation. When the refrigerant circuit 210 is in this state, low-pressure refrigerant (refer to point A in FIGS. 24 and 28 through 30) is drawn into the compression mechanism 2 through the intake tube 2a, and after the refrigerant is first compressed by the compression element 2c 5 to an intermediate pressure, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point B in FIGS. 24 and 28 through 30). This intermediate-pressure refrigerant discharged from the first-stage compression element 2c passes through the intermediate heat exchanger bypass tube 9 (refer to point C in FIGS. 24 and 28 through 30) without passing through the intermediate heat exchanger 7 (i.e., without being cooled), similar to the air [0 warming operation in the embodiment and the modification described above. This intermediate-pressure refrigerant that has passed through the intermediate heat exchanger bypass tube 9 without being cooled by the intermediate heat exchanger 7 is further cooled (refer to point G in FIGS. 24 and 28 through 30) by mixing with the refrigerant returned from the second second-stage injection tube 19 to the second-stage compression element 2d (refer 15 to point K in FIGS. 24 and 28 through 30). Next, having been mixed with the refrigerant returning from the second second-stage injection tube 19 (i.e., intermediate pressure injection is carried out by the economizer heat exchanger 20), the intermediate-pressure refrigerant is drawn into and further compressed in the compression element 2d connected to the second stage side of the compression element 2c, and the refrigerant is discharged from the 20 compression mechanism 2 to the discharge tube 2b (refer to point D in FIGS. 24 and 28 through 30). The high-pressure refrigerant discharged from the compression mechanism 2 is compressed by the two-stage compression action of the compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pcp at the critical point CP shown in FIG 29), similar to the air-cooling operation. The high-pressure refrigerant 25 discharged from the compression mechanism 2 flows into the oil separator 41a constituting the oil separation mechanism 41, and the accompanying refrigeration oil is separated. The refrigeration oil separated from the high-pressure refrigerant in the oil separator 41a flows into the oil return tube 41b constituting the oil separation mechanism 41 wherein it is depressurized by the depressurization mechanism 41c provided to the oil return tube 41b, and 30 the oil is then returned to the intake tube 2a of the compression mechanism 2 and drawn once more into the compression mechanism 2. Next, having been separated from the refrigeration oil in the oil separation mechanism 41, the high-pressure refrigerant is passed through the non-return mechanism 42 and the switching mechanism 3, fed to the usage-side heat exchanger 6 functioning as a radiator of refrigerant, and cooled by heat exchange with 39 the water and/or air as a cooling source (refer to point, F in FIGS. 24 and 28 through 30). The high-pressure refrigerant cooled in the usage-side heat exchanger 6 flows through the. inlet non-return valve 17b of the bridge circuit 17 into the receiver inlet tube 18a, and some of the refrigerant is branched off into the second second-stage injection tube 19. The 5 refrigerant flowing through the second second-stage injection tube 19 is depressurized to a nearly intermediate pressure in the second second-stage injection valve 19a and is then fed to the economizer heat exchanger 20 (refer to point J in FIGS. 24 and 28 through 30). The refrigerant after being branched off to the second second-stage injection tube 19 then flows into the economizer heat exchanger 20, where it is cooled by heat exchange with the 10 refrigerant flowing through the second second-stage injection tube 19 (refer to point H in FIGS. 24 and 28 through 30). The refrigerant flowing through the second second-stage injection tube 19 is heated by heat exchange with the high-pressure refrigerant cooled in the usage-side heat exchanger 6 functioning as a radiator (refer to point K in FIGS. 24 and 28 through 30), and is mixed with the intermediate-pressure refrigerant discharged from the 15 first-stage compression element 2c as described above. The high-pressure refrigerant cooled in the economizer heat exchanger 20 is depressurized to a nearly saturated pressure by the first expansion mechanism 5a and is temporarily retained in the receiver 18 (refer to point I in FIGS. 24 and 28). The refrigerant retained in the receiver 18 is fed to the receiver outlet tube 18b and is depressurized by the second expansion mechanism 5b to become a low 20 pressure gas-liquid two-phase refrigerant, and is then fed through the outlet non-return valve 17d of the bridge circuit 17 to the heat source-side heat exchanger 4 functioning as a refrigerant evaporator (refer to point E in FIGS. 24 and 28 through 30). The low-pressure gas-liquid two-phase refrigerant fed to the heat source-side heat exchanger 4 is heated and evaporated in the heat source-side heat exchanger 4 by heat exchange with the air as a heat 25 source supplied by the heat source-side fan 40 (refer to point A in FIGS. 24 and 28 through 30). The low-pressure refrigerant heated and evaporated in the heat source-side heat exchanger 4 is then drawn once more into the compression mechanism 2 via the switching mechanism 3. In this manner the air-warming operation is performed. Thus, in the air-conditioning apparatus 1 of the present modification, the 30 intermediate heat exchanger 7 is brought to a state of not functioning as a cooler similar to the air-warming operation in the embodiment described above, and the second second-stage injection tube 19 and economizer heat exchanger 20 are provided to branch off the refrigerant fed from the usage-side heat exchanger 6 to the expansion mechanisms 5a, 5b and return the refrigerant to the second-stage compression element 2d; therefore, the temperature of the 40 refrigerant drawn into the second-stage compression element 2d can be suppressed without heat being radiated to the exterior (refer to points C, G and G' in FIG. 30). Thereby, although the temperature of the refrigerant discharged from the compression mechanism 2 decreases and the heating capacity per unit flow rate of the refrigerant in the usage-side heat 5 exchanger 6 decreases (refer to points D, D' and F in FIG 30), the flow rate of the refrigerant discharged from the second-stage compression element 2d increases, the decrease in the heating capacity of the usage-side heat exchanger 6 is therefore minimized, and as a result, the power consumption of the compression mechanism 2 can be reduced and operating efficiency can be improved. 0 Moreover, the intermediate pressure injection by the economizer heat exchanger 20 used in the present modification is more beneficial than the intermediate pressure injection by the receiver 18 as a gas-liquid separator used in Modification I described above, similar to the air-cooling operation, because in a refrigerant circuit configuration in which no significant depressurizing operations are performed except for the first expansion mechanism 15 5a as a heat source-side expansion mechanism after the refrigerant is cooled in the usage-side heat exchanger 6 as a radiator and the pressure difference from the high pressure in the refrigeration cycle to the nearly intermediate pressure of the refrigeration cycle can be used, the quantity of heat exchanged in the economizer heat exchanger 20 can be increased, and the flow rate of the refrigerant passing through the second second-stage injection tube 19 and 20 returning to the second-stage compression element 2d can thereby be increased. Particularly in cases in which refrigerant that operates in the supercritical range is used as in the present modification, the intermediate pressure injection by the economizer heat exchanger 20 is extremely beneficial because there is an extremely large pressure difference from the high pressure in the refrigeration cycle to the nearly intermediate pressure of the refrigeration 25 cycle. <Defrosting operation> In the embodiment described above, since a reverse cycle defrosting operation is used for defrosting the heat source-side heat exchanger 4 by switching the switching mechanism 3 to the cooling operation state, the usage-side heat exchanger 6 is made to 30 function as an evaporator of refrigerant regardless of the intention being to cause the usage side heat exchanger 6 to function as a radiator of refrigerant, and there is a problem in that the temperature on the usage side decreases. Since the reverse cycle defrosting operation is an air-cooling operation performed in a state in which the intermediate heat exchanger 7 is not made to function as a cooler while the temperature of the air as a heat source is low, the low 41 pressure in the refrigeration cycle decreases, and the flow rate of the refrigerant drawn from the first-stage compression element 2c is reduced. When this happens, another problem emerges that more time is required for defrosting the heat source-side heat exchanger 4 because the flow rate of refrigerant circulated through the refrigerant circuit 10 is reduced and 5 the flow rate of refrigerant flowing through the heat source-side heat exchanger 4 can no longer be guaranteed. Such problems are also encountered in the configuration of the present modification. In view of this, in the present modification, in step S2 shown in FIG. 12, when the reverse cycle defrosting operation is performed, a state is created in which the intermediate 10 heat exchanger 7 is not made to function as a cooler, and the second second-stage injection tube 19 is used (i.e., the second second-stage injection valve 19a is opened and intermediate pressure injection is performed by the economizer heat exchanger 20) to perform the reverse cycle defrosting operation (see FIG 31) while the refrigerant fed from the heat source-side heat exchanger 4 to the usage-side heat exchanger 6 is returned to the second-stage 15 compression element 2d. Opening degree control is herein performed so that the opening degree of the second second-stage injection valve 19a is greater than the opening degree of the second second-stage injection valve 19a during the air-cooling operation and/or during the air-warming operation. In a case in which the opening degree of the second second stage injection valve 19a when fully close is 0%, the opening degree when fully open is 20 100%, and the second second-stage injection valve 19a is controlled during the air-cooling operation and air-warming operation within the opening-degree range of 50% or less, for example; the second second-stage injection valve 19a in step S2 is controlled so that the opening degree increases up to about 70%, and this opening degree is kept constant until it is determined in step S3 that defrosting of the heat source-side heat exchanger 4 is complete. 25 The air-cooling operation (the refrigeration cycle performed in the following sequence shown in FIGS. 31 through 33: point A - point B, C - point G -+ point D -+ point E -+ point H -+ point F) accompanying intermediate pressure injection by the economizer heat exchanger 20 is thereby performed in a state in which the intermediate heat exchanger 7 is not made to function as a cooler, heat radiation from the intermediate heat 30 exchanger 7 to the exterior is prevented (i.e., it is possible to prevent heat radiation equivalent to the area enclosed by connecting points G D, D' and G' in FIG. 33), the loss of defrosting capacity of the heat source-side heat exchanger 4 is minimized (this also applies to the defrosting operation in the embodiment described above), and the flow rate of the refrigerant flowing through the heat source-side heat exchanger 4 can be guaranteed while reducing the 42 flow rate of the refrigerant flowing through the usage-side heat exchanger 6, whereby the defrosting time of the heat source-side heat exchanger 4 can be reduced while minimizing the temperature decrease on the usage side when the reverse cycle defrosting operation is performed. The other steps Si, S3, and S4 of the defrosting operation in the present 5 modification are the same as those of the defrosting operation in the embodiment described above and are therefore not described herein. Moreover, in the present modification, since it is possible to control the flow rate of the refrigerant passing through the second second-stage injection tube 19 and returning to the second-stage compression element 2d by controlling the opening degree of the second 10 second-stage injection valve 19a, the flow rate of the refrigerant returning to the second-stage compression element 2d can be greatly increased by performing opening degree control so that the opening degree of the second second-stage injection valve 19a is greater than during the air-cooling operation and/or the air-warming operation as described above, for example, and the flow rate of the refrigerant flowing through the heat source-side heat exchanger 4 can 15 thereby be further increased while the flow rate of the refrigerant flowing through the usage side heat exchanger 6 is further reduced. Thus, in the present modification, since intermediate pressure injection by the economizer heat exchanger 20 is used, the effect of reducing the defrosting time of the heat source-side heat exchanger 4 while suppressing the temperature decrease on the usage side can be further improved in comparison with using 20 intermediate pressure injection by the receiver 18 in Modification I described above. (5) Modification 3 In the refrigerant circuit 210 (see FIG 24) in Modification 2 described above, in both the air-cooling operation in which the switching mechanism 3 is brought to the cooling operation state and the air-warming operation in which the switching mechanism 3 is brought 25 to the heating operation state, the temperature of the refrigerant discharged from the second stage compression element 2d is reduced, the power consumption of the compression mechanism 2 is reduced, and operating efficiency can be improved by performing intermediate pressure injection by the economizer heat exchanger 20 as described above. The intermediate pressure injection by the economizer heat exchanger 20 is believed to be 30 beneficial in a refrigerant circuit configuration having a single usage-side heat exchanger 6, and wherein the pressure difference from the high pressure in the refrigeration cycle to the nearly intermediate pressure of the refrigeration cycle can be used. However, there are cases in which the configuration has a plurality of usage-side heat exchangers 6 connected to each other in parallel with the objective of performing air 43 cooling and/or air warming corresponding to air-conditioning loads for a plurality of air conditioned spaces, and usage-side expansion mechanisms 5c are provided between the receiver 18 as a gas-liquid separator and the usage-side heat exchangers 6 so as to correspond to each of the usage-side heat exchangers 6, in order to make it possible to control the flow 5 rates of refrigerant flowing through each of the usage-side heat exchangers 6 and obtain the refrigeration loads required in each of the usage-side heat exchangers 6. For example, although the details are not shown, in the refrigerant circuit 210 (see FIG 24) having a bridge circuit 17 in Modification 2 described above, another possibility is to provide a plurality (two in this case) of usage-side heat exchangers 6 connected to each 0 other in parallel, to provide usage-side expansion mechanisms 5c (see FIG 34) between the receiver 18 as a gas-liquid separator (more specifically, the bridge circuit 17) and the usage side heat exchangers 6 so as to correspond to each of the usage-side heat exchangers 6, to omit the second expansion mechanism 5b that had been provided to the receiver outlet tube 18b, and to provide a third expansion mechanism (not shown) for depressurizing the 15 refrigerant to a low pressure in the refrigeration cycle during the air-warming operation instead of the outlet non-return valve 17d of the bridge circuit 17. In such a configuration, intermediate pressure injection by the economizer heat exchanger 20 is beneficial, similar to Modification 2 described above, in conditions in which the pressure difference from the high pressure in the refrigeration cycle to the nearly 2O intermediate pressure of the refrigeration cycle can be used without any significant depressurizing operations being performed except for the first expansion mechanism 5a as a heat source-side expansion mechanism after the refrigerant is cooled in the heat source-side heat exchanger 4 as a radiator, as in the case in the air-cooling operation in which the switching mechanism 3 is brought to the cooling operation state. 25 However, in conditions in which each of the usage-side expansion mechanisms 5c control the flow rate of the refrigerant flowing through each of the usage-side heat exchangers 6 as radiators so as to obtain the refrigeration loads required in each of the usage side heat exchangers 6 as radiators, and the flow rate of the refrigerant passing through each of the usage-side heat exchangers 6 as radiators is mostly determined by depressurizing the 30 refrigerant by controlling the opening degrees of the usage-side expansion mechanisms 5c provided downstream of each of the usage-side heat exchangers 6 as radiators and upstream of the economizer heat exchanger 20, as in the case in the air-warming operation in which the switching mechanism 3 is brought to the heating operation state; the extent to which the refrigerant is depressurized by controlling the opening degrees of the usage-side expansion 44 mechanisms 5c fluctuates not only according to the flow rate of the refrigerant flowing through each of the usage-side heat exchangers 6 as radiators but also according to the state of the flow rate distribution among the plurality of usage-side heat exchangers 6 as radiators, and there are cases in which a state arises in which the extent of depressurization differs 5 greatly among the plurality of usage-side expansion mechanisms 5c, or the extent of depressurization in the usage-side expansion mechanisms 5c is comparatively large. Therefore, there is a risk that the pressure of the refrigerant in the inlet of the economizer heat exchanger 20 will decrease, in which case there is a risk that the rate of heat exchange in the economizer heat exchanger 20 (i.e., the flow rate of the refrigerant flowing through the 10 second second-stage injection tube 19) will decrease and use will be difficult. Particularly in cases in which this type of air-conditioning apparatus 1 is configured as a separate-type air conditioning apparatus in which a heat source unit including primarily the compression mechanism 2, the heat source-side heat exchanger 4, and the receiver 18 is connected by a communication tube with a usage unit including primarily the usage-side heat exchanger 6, [5 the communication tube could be extremely long depending on the arrangement of the usage unit and the heat source unit; therefore, the pressure drop has an effect, and the pressure of the refrigerant in the inlet of the economizer heat exchanger 20 decreases further. In cases in which there is a risk that the pressure of the refrigerant in the inlet of the economizer heat exchanger 20 will decrease, it is beneficial to use intermediate pressure injection by the 20 receiver 18 as a gas-liquid separator in Modification 1, which can be used even in conditions in which there is a small pressure difference between the pressure in the receiver 18 and the intermediate pressure in the refrigeration cycle (the pressure of the refrigerant flowing through the intermediate refrigerant tube 8 in this case). In cases in which the configuration has a plurality of usage-side heat exchangers 6 25 connected to each other in parallel with the objective of performing air cooling and/or air warming corresponding to air-conditioning loads for a plurality of air-conditioned spaces, and a configuration is used which is provided with usage-side expansion mechanisms 5c between the receiver 18 and the usage-side heat exchangers 6 so as to correspond to each of the usage side heat exchangers 6 in order to make it possible to control the flow rates of refrigerant 30 flowing through each of the usage-side heat exchangers 6 and obtain the refrigeration loads required in each of the usage-side heat exchangers 6 as described above; during the air cooling operation, the refrigerant depressurized by the first expansion mechanism 5a to a nearly saturated pressure and temporarily retained in the receiver 18 (refer to point L in FIG 34) is distributed to each of the usage-side expansion mechanisms 5c, but if the refrigerant 45 fed from the receiver 18 to each of the usage-side expansion mechanisms 5c is in a gas-liquid two-phase state, there is a risk of the flow being uneven when the refrigerant is distributed among the usage-side expansion mechanisms 5c, and it is therefore preferable that the refrigerant fed from the receiver 18 to each of the usage-side expansion mechanisms 5c is 5 brought as much as possible to a subcooled state. In view of this, in the present modification, the configuration of Modification 2 described above (see FIG 24) is replaced by a refrigerant circuit 310 in which the first second-stage injection tube 18c is connected to the receiver 18 in order to allow the receiver 18 to function as a gas-liquid separator and enable intermediate pressure injection to be 0 performed, intermediate pressure injection by the economizer heat exchanger 20 can be performed during the air-cooling operation, intermediate pressure injection by the receiver 18 as a gas-liquid separator can be performed during the air-warming operation, and a subcooling heat exchanger 96 as a cooler and a second intake return tube 95 are between the receiver 18 and the usage-side expansion mechanisms 5c, as shown in FIG 34. 5 The second intake return tube 95 herein is a refrigerant tube for branching off the refrigerant fed from the heat source-side heat exchanger 4 as a radiator to the usage-side heat exchangers 6 and returning the refrigerant to the intake side of the compression mechanism 2 (i.e., the intake tube 2a). In the present modification, the second intake return tube 95 is provided so as to branch off the refrigerant fed from the receiver 18 to the usage-side 0 expansion mechanisms 5c. More specifically, the second intake return tube 95 is provided so as to branch off the refrigerant from a position upstream of the subcooling heat exchanger 96 (i.e., between the receiver 18 and the subcooling heat exchanger 96) and return the refrigerant to the intake tube 2a. This second intake return tube 95 is provided with a second intake return valve 95a whose opening degree can be controlled. The second intake 5 return valve 95a is an electrically driven expansion valve in the present modification. The subcooling heat exchanger 96 is a heat exchanger for performing heat exchange between the refrigerant fed from the heat source-side heat exchanger 4 as a radiator to the usage-side heat exchangers 6 as evaporators and the refrigerant flowing through the second intake return tube 95 (more specifically, the refrigerant that has been depressurized in the 0 second intake return valve 95a to a nearly low pressure). In the present modification, the subcooling heat exchanger 96 is provided so as to perform heat exchange between the refrigerant flowing through a position upstream of the usage-side expansion mechanisms 5c (i.e., between the position where the second intake return tube 95 branches off and the usage side expansion mechanisms 5c) and the refrigerant flowing through the second intake return 46 tube 95. In the present modification, the subcooling heat exchanger 96 is provided farther downstream than the position where the second intake return tube 95 branches off. Therefore, the refrigerant cooled in the heat source-side heat exchanger 4 as a radiator is branched off to the second intake return tube 95 after passing through the economizer heat 5 exchanger 20 as a cooler, and in the subcooling heat exchanger 96, heat exchange is performed with the refrigerant flowing through the second intake return tube 95. The first second-stage injection tube 18c and the first intake return tube 18f are integrated in the portion near the receiver 18, similar to Modification 1. The first second stage injection tube 18c and the second second-stage injection tube 19 are integrated in the 10 portion near the intermediate refrigerant tube 8. The first intake return tube 18f and the second intake return tube 95 are integrated in the portion on the intake side of the compression mechanism 2. In the present modification, the usage-side expansion mechanisms 5c are electrically driven expansion valves. In the present modification, since the second second-stage injection tube 19 and the economizer heat exchanger 20 are used 5 during the air-cooling operation, and on the other hand the first second-stage injection tube 18c is used during the air-warming operation as described above, there is no need for the direction of refrigerant flow to the economizer heat exchanger 20 to be constant during both the air-cooling operation and the air-warming operation, and the bridge circuit 17 can therefore be omitted to simplify the configuration of the refrigerant circuit 310. 20 An intake pressure sensor 60 for detecting the pressure of the refrigerant flowing through the intake side of the compression mechanism 2 is provided to either the intake tube 2a or the compression mechanism 2. The outlet of the subcooling heat exchanger 96 on the side near the second intake return tube 95 is provided with a subcooling heat exchange outlet temperature sensor 59 for detecting the temperature of the refrigerant in the outlet of the Z5 subcooling heat exchanger 96 on the side near the second intake return tube 95. Next, the action of the air-conditioning apparatus I will be described using FIGS. 34 through 43. FIG. 35 is a diagram showing the flow of refrigerant within the air-conditioning apparatus I during the air-cooling operation, FIG. 36 is a pressure-enthalpy graph representing the refrigeration cycle during the air-cooling operation, FIG. 37 is a temperature 0 entropy graph representing the refrigeration cycle during the air-cooling operation, FIG. 38 is a diagram showing the flow of refrigerant within the air-conditioning apparatus I during the air-warming operation, FIG. 39 is a pressure-enthalpy graph representing the refrigeration cycle during the air-warming operation, FIG. 40 is a temperature-entropy graph representing the refrigeration cycle during the air-warming operation, FIG. 41 is a diagram showing the 47 flow of refrigerant within the air-conditioning apparatus I during the defrosting operation, FIG 42 is a pressure-enthalpy graph representing the refrigeration cycle during the defrosting operation, and FIG 43 is a temperature-entropy graph representing the refrigeration cycle during the defrosting operation. Operation control in the air-cooling operation, the air 5 warming operation, and the defrosting operation described hereinbelow is performed by the aforementioned controller (not shown). In the following description, the term "high pressure" means a high pressure in the refrigeration cycle (specifically, the pressure at points D, D', E, H, I, R in FIGS. 36, 37, 42, and 43, and the pressure at points D, D', and F in FIGS. 39 and 40), the term "low pressure" means a low pressure in the refrigeration cycle 10 (specifically, the pressure at points A, F, S, and U in FIGS. 36, 37, 42, and 43, and the pressure at points A and E in FIGS. 39 and 40), and the term "intermediate pressure" means an intermediate pressure in the refrigeration cycle (specifically, the pressure at points B, C, G, G', J, and K in FIGS. 36, 37, 42, 43, and the pressure at points B, C, G, G', I, and L in FIGS. 39 and 40). 15 <Air-cooling operation> During the air-cooling operation, the switching mechanism 3 is brought to the cooling operation state shown by the solid lines in FIGS. 34 and 35. The opening degrees of the first expansion mechanism 5a and the usage-side expansion mechanisms 5c as heat source-side expansion mechanisms are adjusted. Since the switching mechanism 3 is in the 20 cooling operation state, the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 is opened and the intermediate heat exchanger bypass on/off valve 11 of the intermediate heat exchanger bypass tube 9 is closed, thereby creating a state in which the intermediate heat exchanger 7 functions as a cooler. When the switching mechanism 3 is brought to the cooling operation state, intermediate pressure injection by the receiver 18 as a 25 gas-liquid separator is not performed, but intermediate pressure injection is performed by the economizer heat exchanger 20 which returns to the second-stage compression element 2d the refrigerant that has been passed through the second second-stage injection tube 19 and heated in the economizer heat exchanger 20. More specifically, the first second-stage injection on/off valve 18d is closed, and the opening degree of the second second-stage injection valve 30 19a is adjusted in the same manner as in Modification 2 described above. Furthermore, when the switching mechanism 3 is in the cooling operation state, the opening degree of the second intake return valve 95a is adjusted as well because the subcooling heat exchanger 96 is used. More specifically, in the present modification, so-called superheat degree control is performed wherein the opening degree of the second intake return valve 95a is adjusted so 48 that a target value is achieved in the degree of superheat of the refrigerant at the outlet in the second intake return tube 95 side of the subcooling heat exchanger 96. In the present modification, the degree of superheat of the refrigerant at the outlet in the second intake return tube 95 side of the subcooling heat exchanger 96 is obtained by converting the low 5 pressure detected by the intake pressure sensor 60 to a saturation temperature and subtracting this refrigerant saturation temperature value from the refrigerant temperature detected by the subcooling heat exchanger outlet temperature sensor 59. Though not used in the present modification, another possible option is to provide a temperature sensor to the inlet in the second intake return tube 95 side of the subcooling heat exchanger 96, and to obtain the 10 degree of superheat of the refrigerant at the outlet in the second intake return tube 95 side of the subcooling heat exchanger 96 by subtracting the refrigerant temperature detected by this temperature sensor from the refrigerant temperature detected by the subcooling heat exchanger outlet temperature sensor 59. Adjusting the opening degree of the second intake return valve 95a is not limited to the superheat degree control, and the second intake return 15 valve 95a may be opened to a predetermined opening degree in accordance with the flow rate of refrigerant circulating within the refrigerant circuit 310, for example. When the refrigerant circuit 310 is in this state, low-pressure refrigerant (refer to point A in FIGS. 34 through 37) is drawn into the compression mechanism 2 through the intake tube 2a, and after the refrigerant is first compressed by the compression element 2c to 20 an intermediate pressure, the refrigerant is discharged to the intermediate refrigerant tube 8 (refer to point A in FIGS. 34 through 37). The intermediate-pressure refrigerant discharged from the first-stage compression element 2c is cooled in the intermediate heat exchanger 7 by undergoing heat exchange with the air as a cooling source supplied by the heat source-side fan 40 (refer to point C in FIGS. 34 through 37). The refrigerant cooled in the intermediate 25 heat exchanger 7 is further cooled (refer to point G in FIGS. 34 through 37) by being mixed with refrigerant being returned from the second second-stage injection tube 19 to the second stage compression element 2d (refer to point K in FIGS. 34 through 37). Next, having been mixed with the refrigerant returning from the second second-stage injection tube 19 (i.e., intermediate pressure injection is carried out by the economizer heat exchanger 20), the 30 intermediate-pressure refrigerant is drawn into and further compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and the refrigerant is discharged from the compression mechanism 2 to the discharge tube 2b (refer to point D in FIGS. 34 through 37). The high-pressure refrigerant discharged from the compression mechanism 2 is compressed by the two-stage compression action of the 49 compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pcp at the critical point CP shown in FIG 36). The high-pressure refrigerant discharged from the compression mechanism 2 flows into the oil separator 41a constituting the oil separation mechanism 41, and the accompanying refrigeration oil is separated. The 5 refrigeration oil separated from the high-pressure refrigerant in the oil separator 41a flows into the oil return tube 41b constituting the oil separation mechanism 41 wherein it is depressurized by the depressurization mechanism 41c provided to the oil return tube 41b, and the oil is then returned to the intake tube 2a of the compression mechanism 2 and drawn once more into the compression mechanism 2. Next, having been separated from the [0 refrigeration oil in the oil separation mechanism 41, the high-pressure refrigerant is passed through the non-return mechanism 42 and the switching mechanism 3, and is fed to the heat source-side heat exchanger 4 functioning as a refrigerant radiator. The high-pressure refrigerant fed to the heat source-side heat exchanger 4 is cooled in the heat source-side heat exchanger 4 by heat exchange with air as a cooling source supplied by the heat source-side [5 fan 40 (refer to point E in FIGS. 34 through 37). Some of the high-pressure refrigerant cooled in the heat source-side heat exchanger 4 is then branched off to the second second stage injection tube 19. The refrigerant flowing through the second second-stage injection tube 19 is depressurized to a nearly intermediate pressure in the second second-stage injection valve 19a and is then fed to the economizer heat exchanger 20 (refer to point J in 20 FIGS. 34 through 37). The refrigerant after being branched off to the second second-stage injection tube 19 then flows into the economizer heat exchanger 20, where it is cooled by heat exchange with the refrigerant flowing through the second second-stage injection tube 19 (refer to point H in FIGS. 34 through 37). The refrigerant flowing through the second second-stage injection tube 19 is heated by heat exchange with the high-pressure refrigerant 25 cooled in the heat source-side heat exchanger 4 as a radiator (refer to point K in FIGS. 34 through 37), and is mixed with the intermediate-pressure refrigerant discharged from the first-stage compression element 2c as described above. The high-pressure refrigerant cooled in the economizer heat exchanger 20 is depressurized to a nearly saturated pressure by the first expansion mechanism 5a and is temporarily retained in the receiver 18 (refer to point I in 30 FIGS. 34 through 37). Some of the refrigerant retained in the receiver 18 is then branched off to the second intake return tube 95. The refrigerant flowing through the second intake return tube 95 is depressurized to a nearly low pressure in the second intake return valve 95a and is then fed to the subcooling heat exchanger 96 (refer to point S in FIGS. 34 through 37). The refrigerant branched off into the second intake return tube 95 then flows into the 50 subcooling heat exchanger 96, where it is further cooled by heat exchange with the refrigerant flowing through the second intake return tube 95 (refer to point R in FIGS. 34 through 37). The refrigerant flowing through the second intake return tube 95 is heated by heat exchange with the high-pressure refrigerant cooled in the economizer heat exchanger 20 5 (refer to point U in FIGS. 34 through 37), and is mixed with the refrigerant flowing through the intake side of the compression mechanism 2 (here, the intake tube 2a). The refrigerant cooled in the subcooling heat exchanger 96 is then fed to the usage-side expansion mechanisms 5c and depressurized by the usage-side expansion mechanisms 5c to become a low-pressure gas-liquid two-phase refrigerant, and is then fed to the usage-side heat 10 exchangers 6 functioning as evaporators of refrigerant (refer to point F in FIGS. 34 through 37). The low-pressure gas-liquid two-phase refrigerant fed to the usage-side heat exchanger 6 is heated by heat exchange with water or air as a heating source, and the refrigerant is evaporated as a result (refer to point A in FIGS. 34 through 37). The low-pressure refrigerant heated in the usage-side heat exchanger 6 is then drawn once more into the [5 compression mechanism 2 via the switching mechanism 3. In this manner the air-cooling operation is performed. Thus, in the air-conditioning apparatus 1 of the present modification, in addition to the intermediate heat exchanger 7 being made to function as a cooler similar to the air cooling operation in Modification 2 described above, the second second-stage injection tube 20 19 and the economizer heat exchanger 20 are provided to ensure that the refrigerant fed from the heat source-side heat exchanger 4 to the expansion mechanisms 5a, 5c is branched off and returned to the second-stage compression element 2d, and the temperature of the refrigerant drawn into the second-stage compression element 2d can therefore be suppressed even lower (refer to points C and G in FIG 37) without radiating heat to the exterior, similar to Z5 Modification 2 described above. Thereby, the temperature of the refrigerant discharged from the compression mechanism 2 is kept low (refer to points D and D' in FIG. 37), and the power consumption of the compression mechanism 2 can be further reduced and operating efficiency further improved in comparison with cases in which the second second-stage injection tube 19 and the economizer heat exchanger 20 are not provided, because heat 0 radiation loss can be further reduced in equivalent to the area enclosed by connecting points C, D', D, and G in FIG. 37. Moreover, in the present modification, since the refrigerant fed from the receiver 18 to the usage-side expansion mechanisms 5c (refer to point I in FIGS. 34 through 37) can be cooled by the subcooling heat exchanger 96 to a subcooled state (refer to point R in FIGS. 36 51 and 37), it is possible to reduce the risk of the flows being uneven when the refrigerant is distributed to each of the usage-side expansion mechanisms 5c. <Air-warming operation> During the air-warming operation, the switching mechanism 3 is brought to the 5 heating operation state shown by the dashed lines in FIGS. 34 and 38. The opening degrees are adjusted in the first expansion mechanism 5a and the usage-side expansion mechanisms 5c as heat source-side expansion mechanisms. Since the switching mechanism 3 is in the heating operation state, the intermediate heat exchanger on/off valve 12 of the intermediate refrigerant tube 8 is closed and the intermediate heat exchanger bypass on/off valve 11 of the 10 intermediate heat exchanger bypass tube 9 is opened, thereby creating a state in which the intermediate heat exchanger 7 does not function as a cooler. When the switching mechanism 3 is brought to the heating operation state, intermediate pressure injection by the economizer heat exchanger 20 is not performed, but intermediate pressure injection is performed by the receiver 18 whereby the refrigerant is passed through the first second-stage 15 injection tube 18c and returned from the receiver 18 as a gas-liquid separator to the second stage compression element 2d. More specifically, the first second-stage injection on/off valve 18d is brought to an opened state and the second second-stage injection valve 19a is brought to a fully closed state. Furthermore, when the switching mechanism 3 is brought to the heating operation state, the second intake return valve 95a is also brought to the fully 20 closed state because the subcooling heat exchanger 96 is not used. When the refrigerant circuit 310 is in this state, low-pressure refrigerant (refer to point A in FIGS. 34 and 38 through 40) is drawn into the compression mechanism 2 through the intake tube 2a, and after the refrigerant is first compressed by the compression element 2c to an intermediate pressure, the refrigerant is discharged to the intermediate refrigerant tube 8 25 (refer to point B in FIGS. 34 and 38 through 40). This intermediate-pressure refrigerant discharged from the first-stage compression element 2c passes through the intermediate heat exchanger bypass tube 9 (refer to point C in FIGS. 34 and 38 through 40) without passing through the intermediate heat exchanger 7 (i.e., without being cooled), similar to the air warming operation in the embodiment and modifications described above. This 30 intermediate-pressure refrigerant that has passed through the intermediate heat exchanger bypass tube 9 without being cooled by the intermediate heat exchanger 7 is cooled (refer to point G in FIGS. 34 and 38 through 40) by mixing with the refrigerant returned from the receiver 18 through the first second-stage injection tube 18c to the second-stage compression element 2d (refer to point M in FIGS. 34 and 38 through 40). Next, having been mixed with 52 the refrigerant returning from the first second-stage injection tube 18c (i.e., intermediate pressure injection is carried out by the receiver 18 which acts as a gas-liquid separator), the intermediate-pressure refrigerant is drawn into and further compressed in the compression element 2d connected to the second-stage side of the compression element 2c, and the 5 refrigerant is discharged from the compression mechanism 2 to the discharge tube 2b (refer to point D in FIGS. 34 and 38 through 40). The high-pressure refrigerant discharged from the compression mechanism 2 is compressed by the two-stage compression action of the compression elements 2c, 2d to a pressure exceeding a critical pressure (i.e., the critical pressure Pcp at the critical point CP shown in FIG 39), similar to the air-cooling operation. 10 The high-pressure refrigerant discharged from the compression mechanism 2 flows into the oil separator 41a constituting the oil separation mechanism 41, and the accompanying refrigeration oil is separated. The refrigeration oil separated from the high-pressure refrigerant in the oil separator 41a flows into the oil return tube 41b constituting the oil separation mechanism 41 wherein it is depressurized by the depressurization mechanism 41 c 15 provided to the oil return tube 41b, and the oil is then returned to the intake tube 2a of the compression mechanism 2 and drawn once more into the compression mechanism 2. Next, after the refrigeration oil has been separated in the oil separation mechanism 41, the high pressure refrigerant is fed through the non-return mechanism 42 and the switching mechanism 3 to the usage-side heat exchangers 6 functioning as radiators of refrigerant, and 20 the refrigerant is cooled by heat exchange with the water and/or air as a cooling source (refer to point F in FIGS. 34 and 38 through 40). After the high-pressure refrigerant cooled in the usage-side heat exchangers 6 is then depressurized to a nearly intermediate pressure by the usage-side expansion mechanisms 5c, the refrigerant is temporarily retained in the receiver 18 and subjected to gas-liquid separation (refer to points I, L, and M in FIGS. 34 and 38 25 through 40). The gas refrigerant that has undergone gas-liquid separation in the receiver 18 is then extracted out from the top part of the receiver 18 by the first second-stage injection tube 18c and mixed with the intermediate-pressure refrigerant discharged from the first-stage compression element 2c as described above. The liquid refrigerant retained in the receiver 18 is then depressurized by the first expansion mechanism 5a to become a low-pressure gas 30 liquid two-phase refrigerant, which is fed to the heat source-side heat exchanger 4 functioning as an evaporator of refrigerant (refer to point E in FIGS. 34 and 38 through 40). The low-pressure gas-liquid two-phase refrigerant fed to the heat source-side heat exchanger 4 is then heated and evaporated in the heat source-side heat exchanger 4 by heat exchange with the air as a heat source supplied by the heat source-side fan 40 (refer to point A in FIGS. 53 34 and 38 through 40). The low-pressure refrigerant heated and evaporated in the heat source-side heat exchanger 4 is then drawn once more into the compression mechanism 2 via the switching mechanism 3. In this manner the air-warming operation is performed. Thus, in the air-conditioning apparatus 1 of the present modification, the 5 intermediate heat exchanger 7 is brought to a state of not functioning as a cooler similar to the air-warming operation in Modification 1 described above, and the first second-stage injection tube 18c is provided to branch off the refrigerant fed from the heat source-side heat exchanger 4 to the expansion mechanisms 5a, 5c and return the refrigerant to the second stage compression element 2d; therefore, the temperature of the refrigerant drawn into the 10 second-stage compression element 2d can be minimized without heat being radiated to the exterior (refer to points C, G, and G' in FIG. 40). Thereby, although the temperature of the refrigerant discharged from the compression mechanism 2 decreases and the heating capacity per unit flow rate of the refrigerant in the usage-side heat exchangers 6 decreases (refer to points D, D' and F in FIG 40), the flow rate of the refrigerant discharged from the second 15 stage compression element 2d increases, the decrease in the heating capacity of the usage side heat exchangers 6 is therefore minimized, and as a result, the power consumption of the compression mechanism 2 can be reduced and operating efficiency can be improved. <Defrosting operation> In the embodiment described above, since a reverse cycle defrosting operation is 20 used for defrosting the heat source-side heat exchanger 4 by switching the switching mechanism 3 to the cooling operation state, the usage-side heat exchangers 6 are made to function as evaporators of refrigerant regardless of the intention being to cause the usage-side heat exchangers 6 to function as radiators of refrigerant, and there is a problem in that the temperature on the usage side decreases. Since the reverse cycle defrosting operation is an 25 air-cooling operation performed in a state in which the intermediate heat exchanger 7 is not made to function as a cooler while the temperature of the air as a heat source is low, the low pressure in the refrigeration cycle decreases, and the flow rate of the refrigerant drawn from the first-stage compression element 2c is reduced. When this happens, another problem emerges that more time is required for defrosting the heat source-side heat exchanger 4 30 because the flow rate of refrigerant circulated through the refrigerant circuit 10 is reduced and the flow rate of refrigerant flowing through the heat source-side heat exchanger 4 can no longer be guaranteed. Such problems are also encountered in the configuration of the present modification. In view of this, in the present modification, in step S2 shown in FIG. 12, when the 54 reverse cycle defrosting operation is performed, a state is created in which the intermediate heat exchanger 7 is not made to function as a cooler, and the second second-stage injection tube 19 is used (i.e., the second second-stage injection valve 19a is opened and intermediate pressure injection is performed by the economizer heat exchanger 20) to perform the reverse 5 cycle defrosting operation (see FIG. 41) while the refrigerant fed from the heat source-side heat exchanger 4 to the usage-side heat exchangers 6 is returned to the second-stage compression element 2d, similar to Modification 2 described above. The second second stage injection valve 19a herein is subjected to the same opening degree control as in Modification 2 described above. Moreover, in step S2, the second intake return tube 95 is 0 used (i.e., the second intake return valve 95a is opened) to perform the reverse cycle defrosting operation (see FIG 41) while the refrigerant fed from the heat source-side heat exchanger 4 to the usage-side heat exchangers 6 is returned to the intake side of the compression mechanism 2. The opening degree of the second intake return valve 95a is herein controlled so that the opening degree is greater than the opening degree of the second 15 intake return valve 95a during the air-cooling operation. For example, in cases in which the opening degree of the second intake return valve 95a in the fully closed state is 0%, the opening degree in the fully open state is 100%, and the second intake return valve 95a is controlled within an opening degree range of 50% or less during the air-cooling operation; the second intake return valve 95a in step S2 is controlled so that its opening degree increases 20 up to about 70%, and this opening degree is kept constant until it is determined in step S3 that defrosting of the heat source-side heat exchanger 4 is complete. The air-cooling operation (the refrigeration cycle performed in the following sequence shown in FIGS. 41 through 43: point A -- point B, C -> point G -> point D -> point E -> point H -> point I -+ point R -> point F) accompanying intermediate pressure 25 injection by the economizer heat exchanger 20 is thereby performed in a state in which the intermediate heat exchanger 7 is not made to function as a cooler, heat radiation from the intermediate heat exchanger 7 to the exterior is prevented (i.e., it is possible to prevent heat radiation equivalent to the area enclosed by connecting points G, D, D' and G' in FIG. 43), the loss of defrosting capacity of the heat source-side heat exchanger 4 is minimized (this also 30 applies to the defrosting operation in the embodiment described above), and the flow rate of the refrigerant flowing through the heat source-side heat exchanger can be guaranteed while reducing the flow rate of the refrigerant flowing through the usage-side heat exchangers 6, whereby the defrosting time of the heat source-side heat exchanger 4 can be reduced while minimizing the temperature decrease on the usage side when the reverse cycle defrosting 55 operation is performed, similar to Modification 2 described above. The other steps Si, S3, and S4 of the defrosting operation in the present modification are the same as those of the defrosting operation in the embodiment described above and are therefore not described herein. 5 Moreover, in the present modification, since refrigerant is returned to the intake side of the compression mechanism 2 through the second intake return tube 95 and it is possible to control the flow rate of the refrigerant returning through the second intake return tube 95 by controlling the opening degree of the second intake return valve 95a, the flow rate of the refrigerant returning to the second-stage compression element 2d can be greatly increased by 10 performing opening degree control so that the opening degree is greater than the opening degree of the second intake return valve 95a during the air-cooling operation as described above, for example, and the flow rate of the refrigerant flowing through the heat source-side heat exchanger 4 can thereby be further increased while the flow rate of the refrigerant flowing through the usage-side heat exchangers 6 is further reduced. Thus, in the present 15 modification, since refrigerant is returned to the intake side of the compression mechanism 2 through the second intake return tube 95 during the reverse cycle defrosting operation as well, the effect of suppressing the temperature decrease on the usage side can be further improved in addition to the operational effects in Modification 2 described above. (6) Modification 4 20 In the above-described embodiment and the modifications thereof, a two-stage compression-type compression mechanism 2 is configured such that the refrigerant discharged from the first-stage compression element of two compression elements 2c, 2d is sequentially compressed in the second-stage compression element by one compressor 21 having a single-axis two-stage compression structure, but other options include using a 25 compression mechanism having more stages than a two-stage compression system, such as a three-stage compression system or the like; or configuring a multistage compression mechanism by connecting in series a plurality of compressors incorporated with a single compression element and/or compressors incorporated with a plurality of compression elements. In cases in which the capacity of the compression mechanism must be increased, 30 such as cases in which numerous usage-side heat exchangers 6 are connected, for example, a parallel multistage compression-type compression mechanism may be used in which two or more multistage compression-type compression mechanisms are connected in parallel. For example, the refrigerant circuit 310 in Modification 3 described above (see FIG 34) may be replaced by a refrigerant circuit 410 that uses a compression mechanism 102 in 56 which two-stage compression-type compression mechanisms 103, 104 are connected in parallel instead of the two-stage compression-type compression mechanism 2, as shown in FIG. 44. In the present modification, the first compression mechanism 103 is configured 5 using a compressor 29 for subjecting the refrigerant to two-stage compression through two compression elements 103c, 103d, and is connected to a first intake branch tube 103a which branches off from an intake header tube 102a of the compression mechanism 102, and also to a first discharge branch tube 103b whose flow merges with a discharge header tube 102b of the compression mechanism 102. In the present modification, the second compression 10 mechanism 104 is configured using a compressor 30 for subjecting the refrigerant to two stage compression through two compression elements 104c, 104d, and is connected to a second intake branch tube 104a which branches off from the intake header tube 102a of the compression mechanism 102, and also to a second discharge branch tube 104b whose flow merges with the discharge header tube 102b of the compression mechanism 102. Since the [5 compressors 29, 30 have the same configuration as the compressor 21 in the embodiment and modifications thereof described above, symbols indicating components other than the compression elements 103c, 103d, 104c, 104d are replaced with symbols beginning with 29 or 30, and these components are not described. The compressor 29 is configured so that refrigerant is drawn from the first intake branch tube 103a, the drawn refrigerant is 20 compressed by the compression element 103c and then discharged to a first inlet-side intermediate branch tube 81 that constitutes the intermediate refrigerant tube 8, the refrigerant discharged to the first inlet-side intermediate branch tube 81 is caused to be drawn into the compression element 103d by way of an intermediate header tube 82 and a first outlet-side intermediate branch tube 83 constituting the intermediate refrigerant tube 8, and the 25 refrigerant is further compressed and then discharged to the first discharge branch tube 103b. The compressor 30 is configured so that refrigerant is drawn through the second intake branch tube 104a, the drawn refrigerant is compressed by the compression element 104c and then discharged to a second inlet-side intermediate branch tube 84 constituting the intermediate refrigerant tube 8, the refrigerant discharged to the second inlet-side 30 intermediate branch tube 84 is drawn into the compression element 104d via the intermediate header tube 82 and a second outlet-side intermediate branch tube 85 constituting the intermediate refrigerant tube 8, and the refrigerant is further compressed and then discharged to the second discharge branch tube 104b. In the present modification, the intermediate refrigerant tube 8 is a refrigerant tube for drawing refrigerant discharged from the 57 compression elements 103c, 104c connected to the first-stage sides of the compression elements 103d, 104d into the compression elements 103d, 104d connected to the second stage sides of the compression elements 103c, 104c, and the intermediate refrigerant tube 8 primarily comprises the first inlet-side intermediate branch tube 81 connected to the 5 discharge side of the first-stage compression element 103c of the first compression mechanism 103, the second inlet-side intermediate branch tube 84 connected to the discharge side of the first-stage compression element 104c of the second compression mechanism 104, the intermediate header tube 82 whose flow merges with both inlet-side intermediate branch tubes 81, 84, the first discharge-side intermediate branch tube 83 branching off from the 10 intermediate header tube 82 and connected to the intake side of the second-stage compression element 103d of the first compression mechanism 103, and the second outlet-side intermediate branch tube 85 branching off from the intermediate header tube 82 and connected to the intake side of the second-stage compression element 104d of the second compression mechanism 104. The discharge header tube 102b is a refrigerant tube for feeding 15 refrigerant discharged from the compression mechanism 102 to the switching mechanism 3. A first oil separation mechanism 141 and a first non-return mechanism 142 are provided to the first discharge branch tube 103b connected to the discharge header tube 102b. A second oil separation mechanism 143 and a second non-return mechanism 144 are provided to the second discharge branch tube 104b connected to the discharge header tube 102b. The first 20 oil separation mechanism 141 is a mechanism whereby refrigeration oil that accompanies the refrigerant discharged from the first compression mechanism 103 is separated from the refrigerant and returned to the intake side of the compression mechanism 102. The first oil separation mechanism 141 mainly has a first oil separator 141a for separating from the refrigerant the refrigeration oil that accompanies the refrigerant discharged from the first 25 compression mechanism 103, and a first oil return tube 141b that is connected to the first oil separator 141a and that is used for returning the refrigeration oil separated from the refrigerant to the intake side of the compression mechanism 102. The second oil separation mechanism 143 is a mechanism whereby refrigeration oil that accompanies the refrigerant discharged from the second compression mechanism 104 is separated from the refrigerant 30 and returned to the intake side of the compression mechanism 102. The second oil separation mechanism 143 mainly has a second oil separator 143a for separating from the refrigerant the refrigeration oil that accompanies the refrigerant discharged from the second compression mechanism 104, and a second oil return tube 143b that is connected to the second oil separator 143a and that is used for returning the refrigeration oil separated from 58 the refrigerant to the intake side of the compression mechanism 102. In the present modification, the first oil return tube 141b is connected to the second intake branch tube 104a, and the second oil return tube 143c is connected to the first intake branch tube 103a. Accordingly, a greater amount of refrigeration oil returns to the compression mechanism 103, 5 104 that has the lesser amount of refrigeration oil even when there is an imbalance between the amount of refrigeration oil that accompanies the refrigerant discharged from the first compression mechanism 103 and the amount of refrigeration oil that accompanies the refrigerant discharged from the second compression mechanism 104, which is due to the imbalance in the amount of refrigeration oil retained in the first compression mechanism 103 [0 and the amount of refrigeration oil retained in the second compression mechanism 104. The imbalance between the amount of refrigeration oil retained in the first compression mechanism 103 and the amount of refrigeration oil retained in the second compression mechanism 104 is therefore resolved. In the present modification, the first intake branch tube 103a is configured so that the portion leading from the flow juncture with the second oil [5 return tube 143b to the flow juncture with the intake header tube 102a slopes downward toward the flow juncture with the intake header tube 102a, while the second intake branch tube 104a is configured so that the portion leading from the flow juncture with the first oil return tube 141b to the flow juncture with the intake header tube 102a slopes downward toward the flow juncture with the intake header tube 102a. Therefore, even if either one of the 20 two-stage compression-type compression mechanisms 103, 104 is stopped, refrigeration oil being returned from the oil return tube corresponding to the operating compression mechanism to the intake branch tube corresponding to the stopped compression mechanism is returned to the intake header tube 102a, and there will be little likelihood of a shortage of oil supplied to the operating compression mechanism. The oil return tubes 141b, 143b are 25 provided with depressurization mechanisms 141c, 143c for depressurizing the refrigeration oil that flows through the oil return tubes 141b, 143b. The non-return mechanism 142, 144 are mechanisms for allowing refrigerant to flow from the discharge side of the compression mechanisms 103, 104 to the switching mechanism 3, and for cutting off the flow of refrigerant from the switching mechanism 3 to the discharge side of the compression 30 mechanisms 103, 104. Thus, in the present modification, the compression mechanism 102 is configured by connecting two compression mechanisms in parallel; namely, the first compression mechanism .103 having two compression elements 103c, 103d and configured so that refrigerant discharged from the first-stage compression element of these compression 59 elements 103c, 103d is sequentially compressed by the second-stage compression element, and the second compression mechanism 104 having two compression elements 104c, 104d and configured so that refrigerant discharged from the first-stage compression element of these compression elements 104c, 104d is sequentially compressed by the second-stage 5 compression element. In the present modification, the intermediate heat exchanger 7 is provided to the intermediate header tube 82 constituting the intermediate refrigerant tube 8, and the intermediate heat exchanger 7 is a heat exchanger for cooling the conjoined flow of the refrigerant discharged from the first-stage compression element 103c of the first compression 0 mechanism 103 and the refrigerant discharged from the first-stage compression element 104c of the second compression mechanism 104 during the air-cooling operation. Specifically, the intermediate heat exchanger 7 functions as a shared cooler for two compression mechanisms 103, 104 during air-cooling operation. Accordingly, the circuit configuration is simplified around the compression mechanism 102 when the intermediate heat exchanger 7 is 5 provided to the parallel-multistage-compression-type compression mechanism 102 in which a plurality of multistage-compression-type compression mechanisms 103, 104 are connected in parallel. The first inlet-side intermediate branch tube 81 constituting the intermediate refrigerant tube 8 is provided with a non-return mechanism 81a for allowing the flow of 20 refrigerant from the discharge side of the first-stage compression element 103c of the first compression mechanism 103 toward the intermediate header tube 82 and for blocking the flow of refrigerant from the intermediate header tube 82 toward the discharge side of the first-stage compression element 103c, while the second inlet-side intermediate branch tube 84 constituting the intermediate refrigerant tube 8 is provided with a non-return mechanism 84a 25 for allowing the flow of refrigerant from the discharge side of the first-stage compression element 104c of the second compression mechanism 103 toward the intermediate header tube 82 and for blocking the flow of refrigerant from the intermediate header tube 82 toward the discharge side of the first-stage compression element 104c. In the present modification, non return valves are used as the non-return mechanisms 81a, 84a. Therefore, even if either one 30 of the compression mechanisms 103, 104 is stopped, there are no instances in which refrigerant discharged from the first-stage compression element of the operating compression mechanism passes through the intermediate refrigerant tube 8 and travels to the discharge side of the first-stage compression element of the stopped compression mechanism. Therefore, there are no instances in which refrigerant discharged from the first-stage 60 compression element of the operating compression mechanism passes through the interior of the first-stage compression element of the stopped compression mechanism and exits out through the intake side of the compression mechanism 102, which would cause the refrigeration oil of the stopped compression mechanism to flow out, and it is thus unlikely 5 that there will be insufficient refrigeration oil for starting up the stopped compression mechanism. In the case that the compression mechanisms 103, 104 are operated in order of priority (for example, in the case of a compression mechanism in which priority is given to operating the first compression mechanism 103), the stopped compression mechanism described above will always be the second compression mechanism 104, and therefore in this 0 case only the non-return mechanism 84a corresponding to the second compression mechanism 104 need be provided. In cases of a compression mechanism which prioritizes operating the first compression mechanism 103 as described above, since a shared intermediate refrigerant tube 8 is provided for both compression mechanisms 103, 104, the refrigerant discharged from the 5 first-stage compression element 103c corresponding to the operating first compression mechanism 103 passes through the second outlet-side intermediate branch tube 85 of the intermediate refrigerant tube 8 and travels to the intake side of the second-stage compression element 104d of the stopped second compression mechanism 104, whereby there is a danger that refrigerant discharged from the first-stage compression element 103c of the operating 20 first compression mechanism 103 will pass through the interior of the second-stage compression element 104d of the stopped second compression mechanism 104 and exit out through the discharge side of the compression mechanism 102, causing the refrigeration oil of the stopped second compression mechanism 104 to flow out, resulting in insufficient refrigeration oil for starting up the stopped second compression mechanism 104. In view of Z5 this, an on/off valve 85a is provided to the second outlet-side intermediate branch tube 85 in the present modification, and when the second compression mechanism 104 is stopped, the flow of refrigerant through the second outlet-side intermediate branch tube 85 is blocked by the on/off valve 85a. The refrigerant discharged from the first-stage compression element 103c of the operating first compression mechanism 103 thereby no longer passes through the 30 second outlet-side intermediate branch tube 85 of the intermediate refrigerant tube 8 and travels to the intake side of the second-stage compression element 104d of the stopped second compression mechanism 104; therefore, there are no longer any instances in which the refrigerant discharged from the first-stage compression element 103c of the operating first compression mechanism 103 passes through the interior of the second-stage compression 61 element 104d of the stopped second compression mechanism 104 and exits out through the discharge side of the compression mechanism 102 which causes the refrigeration oil of the stopped second compression mechanism 104 to flow out, and it is thereby made even more unlikely that there will be insufficient refrigeration oil for starting up the stopped second 5 compression mechanism 104. An electromagnetic valve is used as the on/off valve 85a in the present modification. In the case of a compression mechanism which prioritizes operating the first compression mechanism 103, the second compression mechanism 104 is started up in continuation from the starting up of the first compression mechanism 103, but at this time, 10 since a shared intermediate refrigerant tube 8 is provided for both compression mechanisms 103, 104, the starting up takes place from a state in which the pressure in the discharge side of the first-stage compression element 103c of the second compression mechanism 104 and the pressure in the intake side of the second-stage compression element 103d are greater than the pressure in the intake side of the first-stage compression element 103c and the pressure in 15 the discharge side of the second-stage compression element 103d, and it is difficult to start up the second compression mechanism 104 in a stable manner. In view of this, in the present modification, there is provided a startup bypass tube 86 for connecting the discharge side of the first-stage compression element 104c of the second compression mechanism 104 and the intake side of the second-stage compression element 104d, and an on/off valve 86a is 20 provided to this startup bypass tube 86. In cases in which the second compression mechanism 104 is stopped, the flow of refrigerant through the startup bypass tube 86 is blocked by the on/off valve 86a and the flow of refrigerant through the second outlet-side intermediate branch tube 85 is blocked by the on/off valve 85a. When the second compression mechanism 104 is started up, a state in which refrigerant is allowed to flow 25 through the startup bypass tube 86 can be restored via the on/off valve 86a, whereby the refrigerant discharged from the first-stage compression element 104c of the second compression mechanism 104 is drawn into the second-stage compression element 104d via the startup bypass tube 86 without being mixed with the refrigerant discharged from the first stage compression element 103c of the first compression mechanism 103, a state of allowing 30 refrigerant to flow through the second outlet-side intermediate branch tube 85 can be restored via the on/off valve 85a at a point in time when the operating state of the compression mechanism 102 has been stabilized (e.g., a point in time when the intake pressure, discharge pressure, and intermediate pressure of the compression mechanism 102 have been stabilized), the flow of refrigerant through the startup bypass tube 86 can be blocked by the on/off valve 62 86a, and operation can transition to the normal air-cooling operation or air-warming operation. In the present modification, one end of the startup bypass tube 86 is connected between the on/off valve 85a of the second outlet-side intermediate branch tube 85 and the intake side of the second-stage compression element 104d of the second compression 5 mechanism 104, while the other end is connected between the discharge side of the first-stage compression element 104c of the second compression mechanism 104 and the non-return mechanism 84a of the second inlet-side intermediate branch tube 84, and when the second compression mechanism 104 is started up, the startup bypass tube 86 can be kept in a state of being substantially unaffected by the intermediate pressure portion of the first compression 10 mechanism 103. An electromagnetic valve is used as the on/off valve 86a in the present modification. The actions of the air-cooling operation, air-warming operation, and/or defrosting operation of the air-conditioning apparatus 1 of the present modification are not described herein because they are essentially the same as the actions in Modification 3 described above 15 (FIGS. 34 through 43 and their relevant descriptions), except for the points of modification owing to the somewhat higher level of complexity of the circuit configuration surrounding the compression mechanism 102 due to the compression mechanism 102 being provided instead of the compression mechanism 2. The same operational effects as those of Modification 3 described above can also be 20 achieved with the configuration of the present modification. (7) Other embodiments Embodiments of the present invention and modifications thereof are described above with reference to the drawings; however, the specific configuration is not limited to these embodiments or their modifications, and can be changed within a range that does not deviate 25 from the scope of the invention. For example, in the above-described embodiment and modifications thereof, the present invention may be applied to a so-called chiller-type air-conditioning apparatus in which water or brine is used as a heating source or cooling source for conducting heat exchange with the refrigerant flowing through the usage-side heat exchanger 6, and a 30 secondary heat exchanger is provided for conducting heat exchange between indoor air and the water or brine that has undergone heat exchange in the usage-side heat exchanger 6. The present invention can also be applied to other types of refrigeration apparatuses besides the above-described chiller-type air-conditioning apparatus, as long as the apparatus has a refrigerant circuit configured to be capable of switching between a cooling operation 63 and a heating operation, and the apparatus performs a multistage compression refrigeration cycle by using a refrigerant that operates in a supercritical range as its refrigerant. The refrigerant that operates in a supercritical range is not limited to carbon dioxide; ethylene, ethane, nitric oxide, and other gases may also be used. 5 INDUSTRIAL APPLICABILITY If the present invention is used, the reverse cycle defrosting operation can be performed efficiently in a refrigeration apparatus which has a refrigerant circuit configured to be capable of switching between a cooling operation and a heating operation and which uses a refrigerant that operates in the supercritical range to perform a multistage compression-type .0 refrigeration cycle. REFERENCE SIGNS LIST I Air-conditioning apparatus (refrigeration apparatus) 2, 102 Compression mechanisms 3 Switching mechanism 5 4 Heat source-side heat exchanger 6 Usage-side heat exchanger 7 Intermediate heat exchanger 8 Intermediate refrigerant tube 9 Intermediate heat exchanger bypass tube 20 18c First second-stage injection tube 19 Second second-stage injection tube 64

Claims (3)

1. A refrigeration apparatus (1) that uses a refrigerant that operates in a supercritical range, the refrigeration apparatus comprising: a compression mechanism (2, 102) having a plurality of compression elements and 5 configured so that refrigerant discharged from a first-stage compression element of the plurality of compression elements is sequentially compressed by a second-stage compression element; a heat source-side heat exchanger (4) which is a heat exchanger in which air is used as a heat source and which functions as a radiator or evaporator of refrigerant; 10 a usage-side heat exchanger (6) which functions as an evaporator or radiator of refrigerant; a switching mechanism (3) for switching between a cooling operation state wherein the refrigerant is circulated through the compression mechanism, the heat source-side heat exchanger, and the usage-side heat exchanger in a stated order; and a heating operation state 15 wherein the refrigerant is circulated through the compression mechanism, the usage-side heat exchanger, and the heat source-side heat exchanger in a stated order; an intermediate heat exchanger (7) which is a heat exchanger integrated with the heat source-side heat exchanger and having air as a heat source, the intermediate heat exchanger provided to an intermediate refrigerant tube (8) for drawing into the second-stage 20 compression element refrigerant discharged from the first-stage compression element, and functioning as a cooler of the refrigerant discharged from the first-stage compression element and drawn into the second-stage compression element; and an intermediate heat exchanger bypass tube (9) connected to the intermediate refrigerant tube so as to bypass the intermediate heat exchanger; wherein 2S the intermediate heat exchanger is disposed above the heat source-side heat exchanger; and when a reverse cycle defrosting operation is performed for defrosting the heat source-side heat exchanger by switching the switching mechanism to the cooling operation state, the intermediate heat exchanger bypass tube is used to ensure that refrigerant does not 30 flow to the intermediate heat exchanger.
2. The refrigeration apparatus (1) according to claim 1, wherein a second-stage injection tube (18c, 19) is also provided for branching off the refrigerant whose heat has been radiated in the heat source-side heat exchanger (4) or the usage-side heat exchanger (6) and for returning the refrigerant to the second-stage 65 compression element; and when the reverse cycle defrosting operation is performed, the second-stage injection tube is used to return to the second-stage compression element the refrigerant fed from the heat source-side heat exchanger to the usage-side heat exchanger. 5
3. The refrigeration apparatus (1) according to claim I or 2, wherein the refrigerant that operates in the supercritical range is carbon dioxide. 66
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