GB2533042A

GB2533042A - Air conditioner

Info

Publication number: GB2533042A
Application number: GB1521701.1A
Authority: GB
Inventors: Yamashita Koji
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 2013-08-30
Filing date: 2013-08-30
Publication date: 2016-06-08
Anticipated expiration: 2033-08-30
Also published as: GB201521701D0; JPWO2015029220A1; WO2015029220A1; GB2533042B; JP6017048B2

Abstract

When a first heat exchanger, namely a heat-source-side heat exchanger (12), is acting as a condenser, this air conditioner is controlled such that, as a refrigerant flows through a first bypass pipe (4a) to an inlet-side pipe for a compressor (10) via throttling devices (14a and 14d), said refrigerant also flows through a second bypass pipe (4b) via one of the throttling devices (14b) and is introduced into a compression chamber via an injection port. When the heat-source-side heat exchanger (12) is acting as an evaporator, the air conditioner is controlled such that the refrigerant flows through the second bypass pipe (4b) via the aforementioned throttling device (14b) and is introduced into the compression chamber via the injection port and the other throttling device (14d) is either closed completely or closed down such that almost no refrigerant flows therethrough.

Description

DESCRIPTION Title of Invention

AIR-CONDITIONING APPARATUS

Technical Field

[0001] The present invention relates to an air-conditioning apparatus to be applied to, for example, a multi-air-conditioning apparatus for a building.

Background Art

[0002] As air-conditioning apparatus such as a multi-air-conditioning apparatus for a building, there is given an air-conditioning apparatus including a refrigerant circuit for injecting (liquid-injecting) refrigerant in a liquid state from a high-pressure liquid pipe into a middle part in a compression stroke of a compressor in order to lower a discharge temperature of the compressor. In this air-conditioning apparatus, the discharge temperature can be controlled to a set temperature irrespective of an operation state (see, for example, Patent Literature 1).

[0003] Further, there is given an air-conditioning apparatus capable of injecting liquid refrigerant in a high-pressure state in a refrigeration cycle into a pipe arranged on a suction side of the compressor in both of a cooling operation and a heating operation (see, for example, Patent Literature 2).

[0004] Still further, there is given an air-conditioning apparatus including a subcooling heat exchanger on a refrigerant flow outlet side of a condenser and being configured to control a flow rate of the refrigerant to be caused to flow through the subcooling heat exchanger, thereby controlling the discharge temperature of the compressor (see, for example, Patent Literature 3).

Citation List Patent Literature [0005] Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2005-282972 (page 4, Fig. 1, etc.) Patent Literature 2: Japanese Unexamined Patent Application Publication No. Hei 2-110255 (page 3, Fig. 1, etc.) Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2001-227823 (page 4, Fig. 1, etc.)

Summary of Invention

Technical Problem [0006] For example, in the air-conditioning apparatus disclosed in Patent Literature 1 described above, there is only disclosed a method of injection into the middle of the compressor from the high-pressure liquid pipe, thus leading to a problem in that the air-conditioning apparatus is not capable of handling, for example, a case where a circulation path of the refrigeration cycle is reversed (the cooling and the heating are switched).

[0007] In the air-conditioning apparatus disclosed in Patent Literature 2, check valves are installed in parallel to expansion devices on both of the indoor side and the outdoor side, and hence suction injection of the liquid refrigerant can be carried out during both of the cooling and the heating. To achieve such an operation, however, a special indoor unit is required, and an ordinary indoor unit having no check valves connected in parallel to the expansion devices cannot be used, failing to achieve a configuration for general purpose.

[0008] In the air-conditioning apparatus disclosed in Patent Literature 3, an expansion device arranged together with the subcooling heat exchanger controls the flow rate of the refrigerant to be caused to flow through the subcooling heat exchanger, thereby controlling the discharge temperature. Therefore, both of the discharge temperature and a degree of subcooling at the outlet of the condenser cannot be controlled to target values individually, with the result that the discharge temperature cannot appropriately be controlled while maintaining an appropriate degree of subcooling.

Thus, when an extension pipe connecting the outdoor unit and the indoor unit is long, the control of the discharge temperature to the target value may hinder the control of the degree of subcooling at the outlet of the outdoor unit to the target value. Consequently, the refrigerant flowing into the indoor unit may be turned into two-phase refrigerant due to pressure loss occurring in the extension pipe. Therefore, when the expansion device is arranged in the indoor unit as in the case of, for example, the multi-air-conditioning apparatus, the two-phase refrigerant generated on an inlet side of the expansion device may result in a problem of noise or unstable control.

[0009] The present invention has been made to solve the problems described above, and provides an air-conditioning apparatus capable of stably controlling a discharge temperature of a compressor and a degree of subcooling of refrigerant.

Solution to Problem [0010] According to one embodiment of the present invention, there is provided an air-conditioning apparatus, including: a compression chamber and an injection port to introduce the refrigerant into the compression chamber; a first heat exchanger to exchange heat for the refrigerant; a refrigerant flow switching device to switch between causing the first heat exchanger to serve as a condenser and causing the first heat exchanger to serve as an evaporator; a subcooling heat exchanger including a first passage and a second passage, and being configured to subcool the refrigerant flowing through the first passage by exchanging heat between the refrigerant passing through the first passage and the refrigerant passing through the second passage; a second heat exchanger to exchange heat for the refrigerant; a first expansion device to adjust a pressure of the refrigerant passing through the second heat exchanger, the compressor, the first heat exchanger, the refrigerant flow switching device, the subcooling heat exchanger, the second heat exchanger, and the first expansion device being connected by pipes, to thereby form a refrigerant circuit for circulating the refrigerant; a first bypass pipe connecting between a pipe arranged between the first heat exchanger and the second heat exchanger and a refrigerant inflow side of the second passage of the subcooling heat exchanger, and between a refrigerant outflow side of the second passage of the subcooling heat exchanger and a pipe arranged on a refrigerant suction side of the compressor; a second expansion device to adjust a pressure of the refrigerant flowing in the first bypass pipe from the pipe arranged between the first heat exchanger and the second heat exchanger into the second passage of the subcooling heat exchanger; a third expansion device to adjust a pressure of the refrigerant flowing in the first bypass pipe from the second passage of the subcooling heat exchanger into the pipe arranged on the refrigerant suction side of the compressor; a second bypass pipe connecting between the refrigerant outflow side of the second passage of the subcooling heat exchanger and the injection port; and a fourth expansion device to control a flow rate of the refrigerant flowing through the second bypass pipe, wherein the air-conditioning apparatus is configured to, when the first heat exchanger serves as the condenser, cause the refrigerant to pass through the first bypass pipe via the second expansion device and the third expansion device so that the refrigerant flows into the pipe arranged on the refrigerant suction side of the compressor, and cause the refrigerant to pass through the second bypass pipe via the fourth expansion device so that the refrigerant is introduced into the compression chamber, and wherein, the air-conditioning apparatus is configured to, when the first heat exchanger serves as the evaporator, cause the refrigerant to pass through the second bypass pipe via the fourth expansion device so that the refrigerant is introduced into the compression chamber, and control an opening degree of the third expansion device to be fully closed or set at an opening degree at which substantially no refrigerant flows through the third expansion device. During the cooling operation, the refrigerant is injected into the compression chamber of the compressor while controlling the degree of subcooling of the refrigerant at an outlet of the subcooling heat exchanger to a higher extent, thereby being capable of lowering the discharge temperature of the compressor. The discharge temperature of the compressor can be lowered while preventing the refrigerant from being turned into two-phase refrigerant in an extension pipe. The compressor can be operated safely to extend the life of the device.

Advantageous Effects of Invention [0011] In the air-conditioning apparatus according to the one embodiment of the present invention, during the cooling operation, the refrigerant can be injected into the compression chamber of the compressor while maintaining the degree of subcooling at an outlet of the outdoor unit to an appropriate value, thereby being capable of preventing excessively high discharge temperature of the compressor. Therefore, damage to the compressor can be prevented to extend the life of the compressor.

Brief Description of Drawings [0012]

[Fig. 1] Fig. 1 is a schematic diagram illustrating an example of installation of an air-conditioning apparatus according to Embodiment 1 of the present invention.

[Fig. 2] Fig. 2 is a diagram schematically illustrating a configuration of the air-conditioning apparatus according to Embodiment 1 of the present invention.

[Fig. 3] Fig. 3 is a diagram illustrating a flow of refrigerant during a cooling operation mode of an air-conditioning apparatus 100 according to Embodiment 1 of the present invention when a discharge temperature is low.

[Fig. 4] Fig. 4 is a p-h diagram (pressure-enthalpy diagram) during a cooling operation of the air-conditioning apparatus 100 according to Embodiment 1 of the present invention when the discharge temperature is low.

[Fig. 5] Fig. 5 is a diagram illustrating a flow of the refrigerant during the cooling operation mode of the air-conditioning apparatus 100 according to Embodiment 1 of the present invention when the discharge temperature is high.

[Fig. 6] Fig. 6 is a p-h diagram (pressure-enthalpy diagram) during the cooling operation of the air-conditioning apparatus 100 according to Embodiment 1 of the present invention when the discharge temperature is high.

[Fig. 7] Fig. 7 is a diagram illustrating a flow of the refrigerant during a heating operation mode of the air-conditioning apparatus 100 according to Embodiment 1 of the present invention.

[Fig. 8] Fig. 8 is a p-h diagram (pressure-enthalpy diagram) during a heating operation of the air-conditioning apparatus according to Embodiment 1 of the present invention.

Description of Embodiments [0013]

An air-conditioning apparatus according to embodiments of the present invention is described referring to the drawings or the like. Note that, in the drawings referred to below including Fig. 1, components denoted by the same reference symbols correspond to the same or equivalent components. This is common throughout the embodiments described below. Further, the forms of the components described herein are merely examples, and the components are not limited to the forms described herein. In particular, the combinations of the components are not limited to only the combinations in each embodiment, and the components described in another embodiment may be applied to still another embodiment. Further, unless otherwise necessary to be distinguished or specified, a plurality of devices of the same type or other components, which are distinguished from one another by suffixes or in another way, may be described without the suffixes. Further, in the drawings, the size relationship between the components may be different from the actual size relationship. In addition, a high-and-low relationship or other relationships of temperatures, pressures, or other factors are not determined in relation to particular absolute values, but are determined in a relative manner based on a state, an operation, or other factors of systems, devices, or the like.

[0014] Embodiment 1 Fig. 1 is a schematic diagram illustrating an example of installation of an air-conditioning apparatus according to Embodiment 1 of the present invention. The example of installation of the air-conditioning apparatus according to this embodiment is described referring to Fig. 1. The air-conditioning apparatus according to this embodiment is capable of selecting any one of a cooling mode and a heating mode as an operation mode by utilizing a refrigeration cycle for causing refrigerant to change its phase.

[0015] In Fig. 1, the air-conditioning apparatus according to this embodiment includes one outdoor unit 1 serving as a heat source apparatus, and a plurality of indoor units 2. Further, the outdoor unit 1 and the indoor units 2 are connected by extension pipes (refrigerant pipes) 5 through which the refrigerant passes.

[0016] The outdoor unit 1 is usually arranged in an outdoor space 6, which is a space outside of a construction 9 such as a building (for example, on a rooftop). Further, the outdoor unit 1 generates cooling energy or heating energy, and delivers the energy to each of the indoor units 2 through the extension pipes 5. The indoor units 2 are arranged at positions at which air adjusted in temperature or other conditions can be supplied to an indoor space 7, which is a space inside the construction 9 (for example, residential room). The indoor units 2 supply cooling air or heating air to the indoor space 7.

[0017] As illustrated in Fig. 1, in the air-conditioning apparatus according to this embodiment, the outdoor unit 1 and the respective indoor units 2 are connected by the two extension pipes (refrigerant pipes) 5.

[0018] In this case, an example of a case where the indoor unit 2 is a ceiling cassette-type indoor unit is illustrated in Fig. 1, but the present invention is not limited thereto. Any type of the indoor unit such as a ceiling-concealed indoor unit or a ceiling-suspended indoor unit may be employed as long as the cooling air or the heating air can be supplied to the indoor space 7 directly, through a duct, or in other ways.

[0019] Further, an example of a case where the outdoor unit 1 is installed in the outdoor space 6 is illustrated in Fig. 1, but the present invention is not limited thereto. For example, the outdoor unit 1 may be installed in an enclosed space such as a machine room with a ventilation opening.

Moreover, the outdoor unit 1 may be installed inside the construction 9 as long as air can be exhausted to the outside of the construction 9 through an exhaust duct or in other ways. In addition, a water-cooled outdoor unit 1 or an outdoor unit of another type may be installed inside the construction 9. No particular problem is caused even if the outdoor unit 1 according to this embodiment is installed at any place.

[0020] Further, the numbers of the outdoor units 1 and the indoor units 2 to be connected are not limited to the numbers as illustrated in Fig. 1. For example, the numbers may be determined depending on the construction 9 in which the air-conditioning apparatus according to this embodiment is installed.

[0021] Fig. 2 is a diagram schematically illustrating a configuration of the air-conditioning apparatus according to Embodiment 1 of the present invention. In Fig. 2 and subsequent drawings, the air-conditioning apparatus according to this embodiment is referred to as an air-conditioning apparatus 100. Referring to Fig. 2, a detailed configuration of the air-conditioning apparatus 100 according to this embodiment is described. As described above, the outdoor unit 1 and the indoor units 2 are connected by the extension pipes 5 through which the refrigerant flows, to thereby construct a refrigerant circuit.

[0022] [Outdoor Unit 1] In the outdoor unit 1, a compressor 10, a refrigerant flow switching device 11 such as a four-way valve, a heat source-side heat exchanger 12, and an accumulator 15 are mounted in a serial connection by the refrigerant pipes, to thereby construct a main refrigerant circuit together with expansion devices 16 and use-side heat exchangers 17 of the indoor units 2. Further, the outdoor unit 1 includes a first bypass pipe 4a, a second bypass pipe 4b, a subcooling heat exchanger 13, an expansion device 14a, an expansion device 14b, an expansion device 14c, an expansion device 14d, and a liquid separator 18.

[0023] The compressor 10 sucks the refrigerant and compresses the refrigerant into a high-temperature and high-pressure state to discharge the refrigerant. It is preferred that the compressor 10 be a capacity-controllable inverter compressor, for example. In this case, the compressor 10 according to this embodiment has an injection port formed in a compression chamber for compressing the refrigerant inside the compressor 10. The injection port is capable of introducing the refrigerant from the outside of the compressor 10 to the inside of the compression chamber. Further, as the compressor 10, there is used, for example, a compressor having a low-pressure shell structure, in which a compression chamber is formed inside a hermetic container that is under a low-pressure refrigerant atmosphere, and low-pressure refrigerant inside the hermetic container is sucked into the compression chamber to compress the refrigerant. The refrigerant flow switching device 11 switches a flow of the refrigerant during a heating operation and a flow of the refrigerant during a cooling operation. The heat source-side heat exchanger 12 functions as an evaporator during the heating operation and functions as a condenser or a radiator during the cooling operation. The heat source-side heat exchanger 12 exchanges heat between air supplied from a blower (not shown) and the refrigerant, thereby causing the refrigerant to take away heat to be evaporated and gasified or to reject heat to be condensed and liquefied. The subcooling heat exchanger 13 is, for example, a double-tube heat exchanger. The subcooling heat exchanger 13 is a refrigerant-refrigerant heat exchanger including a first passage and a second passage and being configured to exchange heat between the streams of the refrigerant passing through the respective passages. The refrigerant flowing into and out of the heat source-side heat exchanger 12 passes through the first passage. The refrigerant passing through the expansion device 14a flows into the second passage, and flows out of the second passage to the first bypass pipe 4a. In this case, the subcooling heat exchanger 13 is not limited to the double-tube heat exchanger, but may have any structure as long as the subcooling heat exchanger 13 is capable of exchanging heat between the refrigerant passing through the first passage and the refrigerant passing through the second passage.

[0024] The expansion device 14a, which functions as a second expansion device of the present invention, regulates a pressure of the refrigerant and controls a flow rate of the refrigerant, which flows from the liquid separator 18 and passes through the first bypass pipe 4a to flow into the second passage of the subcooling heat exchanger 13. Further, the expansion device 14d, which functions as a third expansion device of the present invention, regulates a pressure of the refrigerant and controls a flow rate of the refrigerant, which flows from the second passage of the subcooling heat exchanger 13 and passes through the first bypass pipe 4a to flow into a pipe arranged on an upstream side of the accumulator 15 (suction side of the compressor 10). Moreover, the expansion device 14b, which functions as a fourth expansion device of the present invention, regulates a pressure of the refrigerant and controls a flow rate of the refrigerant, which passes through the second bypass pipe 4b. In addition, in this embodiment, the expansion device 14c regulates a pressure of the refrigerant flowing through a pipe arranged between the expansion device 14a and each of the expansion devices 16.

[0025] The accumulator 15 is connected to the compressor 10 through a pipe serving as a suction-side passage, and accumulates surplus refrigerant in the refrigerant circuit. In this case, the accumulator 15 may be provided as necessary. For example, when the surplus refrigerant is not generated or the amount of the surplus refrigerant is small in the refrigerant circuit, the accumulator 15 may not be provided. The liquid separator 18 is arranged at a position serving as a refrigerant outflow side of the heat source-side heat exchanger 12 in the refrigerant circuit during the cooling operation (on a pipe arranged between the heat source-side heat exchanger 12 and each of the expansion devices 16). The liquid separator 18 is arranged on a pipe arranged between the heat source-side heat exchanger 12 and each of the use-side heat exchangers 17 of the indoor units 2 (for example, between the heat source-side heat exchanger 12 and each of the extension pipes 5). The liquid separator 18 serves as a branching unit for branching the refrigerant. For example, when refrigerant in a two-phase gas-liquid state (two-phase refrigerant) passes through the liquid separator 18, the refrigerant is separated into a part of liquid refrigerant (which may be the entire liquid refrigerant) and the rest of the refrigerant (other part of the liquid refrigerant and gas refrigerant). Then, the part of the liquid refrigerant is caused to flow into the first bypass pipe 4a, and the rest of the refrigerant is caused to flow into the main refrigerant circuit.

[0026] The first bypass pipe 4a is a pipe for bypassing, for example, during the cooling operation, the refrigerant condensed and liquefied by the heat source-side heat exchanger 12 serving as the condenser to the suction-side passage of the compressor 10 (upstream side of the accumulator 15) through the expansion device 14a, the second passage of the subcooling heat exchanger 13, and the expansion device 14d as low-pressure superheated gas refrigerant. In this case, in Fig. 2, the first bypass pipe 4a is connected to the upstream side of the accumulator 15, but it is only necessary that the first bypass pipe 4a be connected to the suction-side passage of the compressor 10. Thus, the first bypass pipe 4a may be connected to a pipe arranged on an outlet side (downstream side) of the accumulator 15.

[0027] The second bypass pipe 4b is a pipe connecting between a refrigerant outflow side of the second passage of the subcooling heat exchanger 13 and the injection port of the compressor 10. During the cooling operation and the heating operation, high-pressure or first intermediate-pressure liquid refrigerant is depressurized by a function of the expansion device 14b in the second bypass pipe 4b so that two-phase refrigerant having a second intermediate pressure lower than the first intermediate pressure can be injected into the compression chamber through the second bypass pipe 4b. In this case, the high pressure is a pressure of the refrigerant on a discharge side of the compressor 10. The low pressure is a pressure of the refrigerant on the suction side of the compressor 10. Further, the intermediate pressure is a pressure that is lower than the high pressure and higher than the low pressure.

[0028] Further, the outdoor unit 1 includes a discharge refrigerant temperature detection device 21, a high-pressure detection device 22, a low-pressure detection device 23, a liquid refrigerant temperature detection device 24, a subcooling heat exchanger inlet refrigerant temperature detection device 25, a subcooling heat exchanger outlet refrigerant temperature detection device 26, and a controller 50. The discharge refrigerant temperature detection device 21 is a device for detecting a temperature of the refrigerant discharged from the compressor 10. The high-pressure detection device 22 is a device for detecting a pressure on the discharge side of the compressor 10, which corresponds to the high-pressure side in the refrigerant circuit. The low-pressure detection device 23 is a device for detecting a pressure on a refrigerant inflow side of the accumulator 15, which corresponds to the low-pressure side in the refrigerant circuit. The liquid refrigerant temperature detection device 24 is a device for detecting a temperature of the liquid refrigerant.

The subcooling heat exchanger inlet refrigerant temperature detection device 25 is a device for detecting a temperature of the refrigerant flowing into the second passage of the subcooling heat exchanger 13. The subcooling heat exchanger outlet refrigerant temperature detection device 26 is a device for detecting a temperature of the refrigerant flowing out of the second passage of the subcooling heat exchanger 13. Further, the controller 50 controls each of the devices of the outdoor unit 1 based on, for example, information pieces detected by the various detection devices and an instruction in a signal from a remote controller. For example, the controller 50 controls a frequency of the compressor 10, a rotation speed of the blower (not shown) (including turning ON/OFF), and switching of the refrigerant flow switching device 11, to thereby execute each of the operation modes described later. In this embodiment, the controller 50 controls, for example, the expansion device 14b, the expansion device 14c, and the expansion device 14d, thereby being capable of controlling the flow rate of the refrigerant, regulating the pressure of the refrigerant, and adjusting other conditions of the refrigerant, which is to be injected to the suction side of the compressor 10. Therefore, in a case of using a refrigerant such as an R32 refrigerant (hereinafter referred to as R32) that may easily cause high discharge temperature of the compressor 10, the discharge temperature of the compressor 10 can be lowered. Specific control operations performed by the controller 50 are described in the description of operations of each of the operation modes to be given later. In this case, the controller 50 is, for example, a microcomputer.

[0029] [Indoor Unit 2] Each indoor unit 2 includes the expansion device 16 and the use-side heat exchanger 17 mounted therein. The expansion device 16 and the use-side heat exchanger 17 are connected to the outdoor unit 1 by the extension pipes 5. The expansion device 16 such as an expansion valve or a flow control device, which functions as a first expansion device of the present invention, depressurizes the refrigerant passing through the expansion device 16. Further, the use-side heat exchanger 17 serving as a second heat exchanger of the present invention exchanges heat between air supplied from a blower such as a fan (not shown) and the refrigerant, to thereby generate the heating air or the cooling air to be supplied to the indoor space 7. In addition, although not illustrated in Fig. 2 or other drawings, each indoor unit 2 includes a controller for controlling the expansion device 16, the blower, and other devices.

[0030] In this case, in Fig. 2, there is illustrated an example of a case where four indoor units 2 are connected, and an indoor unit 2a, an indoor unit 2b, an indoor unit 2c, and an indoor unit 2d are illustrated in the stated order from the bottom of the drawing sheet. Similarly, correspondingly to the indoor unit 2a to the indoor unit 2d, the respective expansion devices 16 are illustrated as an expansion device 16a, an expansion device 16b, an expansion device 16c, and an expansion device 16d in the stated order from the bottom of the drawing sheet. Further, the respective use-side heat exchangers 17 are illustrated as a use-side heat exchanger 17a, a use-side heat exchanger 17b, a use-side heat exchanger 17c, and a use-side heat exchanger 17d in the stated order from the bottom of the drawing sheet. Although the four indoor units 2 are illustrated in Fig. 2, the number of the indoor units 2 to be connected according to this embodiment is not limited to four similarly to Fig. 1.

[0031] Next, each of the operation modes to be executed by the air-conditioning apparatus 100 is described. The air-conditioning apparatus 100 according to this embodiment determines the operation mode of the outdoor unit 1 as any one of a cooling operation mode and a heating operation mode based on, for example, an instruction from each indoor unit 2.

[0032] The air-conditioning apparatus 100 causes all the driven indoor units 2 to perform the same operation (cooling operation or heating operation) based on the determined operation mode, to thereby air-condition the indoor space 7. In this case, running and stopping of each indoor unit 2 may be switched freely in any of the cooling operation mode and the heating operation mode.

[0033] [Cooling Operation Mode (when Discharge Temperature is Low)] Fig. 3 is a diagram illustrating a flow of the refrigerant during the cooling operation mode of the air-conditioning apparatus 100 according to Embodiment 1 of the present invention when the discharge temperature is low. The description "the discharge temperature is low" means a case where the discharge temperature is less than 105 degrees C, for example. In Fig. 3, the cooling operation mode is described taking as an example a case where a cooling load is generated in all the use-side heat exchangers 17. In this case, in Fig. 3, the pipes indicated by the thick lines correspond to the pipes through which the refrigerant flows, and a direction of the flow of the refrigerant is indicated by the solid arrows.

[0034] In the case of the cooling operation mode illustrated in Fig. 3, in the outdoor unit 1, the controller 50 instructs the refrigerant flow switching device 11 to switch so that the refrigerant discharged from the compressor 10 flows into the heat source-side heat exchanger 12. Then, the compressor 10 compresses low-temperature, low-pressure refrigerant into high-temperature and high-pressure gas refrigerant to discharge the refrigerant. The high-temperature and high-pressure gas refrigerant discharged from the compressor 10 passes through the refrigerant flow switching device 11 to flow into the heat source-side heat exchanger 12. Then, the refrigerant is condensed and liquefied into high-pressure liquid refrigerant while rejecting heat to the outdoor air in the heat source-side heat exchanger 12. The high-pressure liquid refrigerant flowing out of the heat source-side heat exchanger 12 passes through the expansion device 14c in a fully-opened state and the first passage of the subcooling heat exchanger 13. The refrigerant passing through the first passage of the subcooling heat exchanger 13 is distributed to two branched passages. One branched flow of refrigerant passes through the liquid separator 18 to flow out of the outdoor unit 1. The other branched flow of refrigerant flows into the first bypass pipe 4a. The high-temperature, high-pressure liquid refrigerant flowing into the first bypass pipe 4a is depressurized into low-temperature, low-pressure two-phase refrigerant by the expansion device 14a, and passes through the second passage of the subcooling heat exchanger 13 and the expansion device 14d in a fully-opened state to flow into the passage located on the upstream side of the accumulator 15. At this time, the subcooling heat exchanger 13 exchanges heat between the high-temperature, high-pressure liquid refrigerant passing through the first passage and the low-temperature, low-pressure two-phase refrigerant passing through the second passage. Therefore, the refrigerant passing through the first passage is cooled by the refrigerant passing through the second passage, whereas the refrigerant passing through the second passage is heated by the refrigerant passing through the first passage.

[0035] The expansion device 14d is fully opened, and hence the flow rate of the refrigerant passing through the second passage of the subcooling heat exchanger 13 (first bypass pipe 4a) is controlled based on an opening degree (opening area) of the expansion device 14a. The controller 50 controls the opening degree (opening area) of the expansion device 14a so that a degree of subcooling of the refrigerant flowing on a downstream side of the first passage of the subcooling heat exchanger 13, which corresponds to a temperature difference between a saturation temperature of the pressure detected by the high-pressure detection device 22 and a temperature detected by the liquid refrigerant temperature detection device 24, approximates a target value. In this case, the opening degree of the expansion device 14a may be controlled so that a temperature difference (degree of superheat) of the refrigerant in the second passage of the subcooling heat exchanger 13, which corresponds to a temperature difference between a temperature detected by the subcooling heat exchanger outlet refrigerant temperature detection device 26 and a temperature detected by the subcooling heat exchanger inlet refrigerant temperature detection device 25, approximates a target value. Further, the liquid refrigerant temperature detection device 24 may be arranged on the other end side of the first passage of the subcooling heat exchanger 13.

[0036] The high-temperature, high-pressure liquid refrigerant flowing out of the outdoor unit 1 passes through the extension pipe 5 to flow into each of the indoor units 2 (2a to 2d). The high-temperature, high-pressure liquid refrigerant flowing into the indoor units 2 (2a to 2d) is expanded into low-temperature, low-pressure two-phase refrigerant by the expansion devices 16 (16a to 16d), and flows into the use-side heat exchangers 17 (17a to 17d), respectively, the use-side heat exchangers each serving as an evaporator. Then, the refrigerant takes away heat from air flowing around the use-side heat exchangers 17 to turn into low-temperature, low-pressure gas refrigerant. Further, the low-temperature, low-pressure gas refrigerant flows out of each of the indoor units 2 (2a to 2d), and passes through the extension pipe 5 to flow into the outdoor unit 1 again. Then, the refrigerant passes through the refrigerant flow switching device 11 to join the refrigerant that is be bypassed to the upstream side of the accumulator 15 through the first bypass pipe 4a. After that, the refrigerant flows into the accumulator 15, and is then sucked into the compressor 10 again.

[0037] At this time, an opening degree (opening area) of each of the expansion devices 16a to 16d is controlled so that a temperature difference (degree of superheat) between a temperature detected by a use-side heat exchanger gas refrigerant temperature detection device 28 and a temperature detected by a use-side heat exchanger liquid refrigerant temperature detection device 27 approximates a target value.

[0038] In this case, in this embodiment, the subcooling heat exchanger 13 is provided to keep the refrigerant subcooled reliably (keep the liquid refrigerant state) even when the extension pipe 5 is long (for example, 100 m). When the extension pipe 5 is long, pressure loss is increased in the extension pipe 5. Therefore, refrigerant with a small degree of subcooling may be turned into two-phase refrigerant before reaching the indoor unit 2. When the two-phase refrigerant flows into the indoor unit 2, the two-phase refrigerant flows into the expansion device 16. The expansion device such as the expansion valve or the flow control device has such a characteristic that noise is generated around the expansion device when the two-phase refrigerant flows into the expansion device. The expansion device 16 according to this embodiment is arranged inside the indoor unit 2 for delivering air adjusted in temperature to the indoor space 7, and hence when the generated noise is leaked to the indoor space 7, residents may feel a sense of discomfort. Further, when the two-phase refrigerant flows into the expansion device 16, the pressure is not stabilized, and the operation of the expansion device 16 becomes unstable. Therefore, it is necessary that refrigerant in a liquid state, which is subcooled reliably, be caused to flow into the expansion device 16. For this reason, the subcooling heat exchanger 13 is provided. The expansion device 14a is arranged in the first bypass pipe 4a. When the opening degree (opening area) of the expansion device 14a is increased to increase the flow rate of the low-temperature, low-pressure two-phase refrigerant flowing into the second passage of the subcooling heat exchanger 13, the degree of subcooling of the refrigerant flowing out of the first passage of the subcooling heat exchanger 13 increases. In contrast, when the opening degree (opening area) of the expansion device 14a is reduced to reduce the flow rate of the low-temperature, low-pressure two-phase refrigerant flowing into the second passage of the subcooling heat exchanger 13, the degree of subcooling of the refrigerant flowing out of the first passage of the subcooling heat exchanger 13 is reduced. As described above, through the adjustment of the opening degree (opening area) of the expansion device 14a, the degree of subcooling of the refrigerant at an outlet of the first passage of the subcooling heat exchanger 13 can be controlled to an appropriate value. However, it is not preferred from the viewpoint of reliability that the compressor 10 suck refrigerant having low quality (of dryness) with a large amount of liquid refrigerant mixed therein during a normal operation. Therefore, in this embodiment, the first bypass pipe 4a is connected to a pipe arranged on the refrigerant inflow side (upstream side) of the accumulator 15. The accumulator 15 accumulates surplus refrigerant therein. A majority of the refrigerant bypassed to the refrigerant inflow side of the accumulator 15 through the first bypass pipe 4a is accumulated inside the accumulator 15, thereby being capable of preventing a large amount of the liquid refrigerant from flowing back to the compressor 10.

[0039] The operation of each of the devices and the flow of the refrigerant in the refrigerant circuit in the cooling operation mode when the discharge temperature of the compressor 10 is low are described above. Description of a case where the discharge temperature of the compressor 10 is high is given later. The discharge temperature of the compressor 10 is low, and hence the expansion device 14b is fully closed or set at a small opening degree at which no refrigerant flows therethrough, thereby preventing the refrigerant from flowing through the second bypass pipe 4b.

In this case, in the cooling operation mode, the discharge temperature is not increased to be high unless a temperature around the heat source-side heat exchanger 12 is considerably high.

[0040] Fig. 4 is a p-h diagram (pressure-enthalpy diagram) during the cooling operation of the air-conditioning apparatus 100 according to Embodiment 1 of the present invention when the discharge temperature is low. In the cooling operation mode, the refrigerant compressed by the compressor 10 and discharged therefrom (point G in Fig. 4) is condensed and liquefied into high-pressure liquid refrigerant by the heat source-side heat exchanger 12 (point J in Fig. 4). Further, the refrigerant is cooled by the subcooling heat exchanger 13 with the refrigerant branched into the first bypass pipe 4a so that the degree of subcooling is increased (point L in Fig. 4). Then, the refrigerant flows into the liquid separator 18. A part of the liquid refrigerant branched into the second passage of the subcooling heat exchanger 13 by the liquid separator 18 is depressurized by the expansion device 14a and then heated by the subcooling heat exchanger 13 to be evaporated and gasified. Then, the gas thus obtained passes through the first bypass pipe 4a and the expansion device 14d in the fully-opened state to flow into the refrigerant inflow side of the accumulator 15. On the other hand, the high-pressure two-phase refrigerant passing through the liquid separator 18 flows out of the outdoor unit 1 to pass through the extension pipe 5, and flows into the indoor units 2 to be depressurized by the respective expansion devices 16 (16a to 16d) of the indoor units 2 (point K in Fig. 4). Further, the refrigerant is evaporated by each of the use-side heat exchangers 17 (17a to 17d). After that, the refrigerant flows out of each of the indoor units 2 and passes through the extension pipe 5 to flow into the outdoor unit 1. Then, the refrigerant passes through the refrigerant flow switching device 11 to join the refrigerant that is bypassed to the upstream side of the accumulator 15 through the first bypass pipe 4a. After that, the refrigerant flows into the accumulator 15 (point F in Fig. 4).

[0041] In this case, it is desired that the expansion device 14a be a device capable of changing its opening area, such as an electronic expansion valve. When the electronic expansion valve is used, the flow rate of the refrigerant passing through the second passage of the subcooling heat exchanger 13 can be controlled to any desired rates, thereby being capable of finely controlling the degree of subcooling of the refrigerant flowing out of the outdoor unit 1. However, the expansion device 14a is not limited to the electronic expansion valve. For example, there may be employed a configuration in which on-off valves such as small-size solenoid valves are combined so that the opening degree can selectively be controlled in a plurality of levels. Further, there may be employed a configuration in which a capillary tube is provided to enable subcooling depending on the pressure loss of the refrigerant.

Although the controllability is slightly degraded as compared to the case of the electronic expansion valve or other devices, the degree of subcooling can approximate a target value. On the other hand, any device may be employed as the expansion device 14b as long as the device is capable of closing the passage. Further, in the cooling operation mode when the discharge temperature of the compressor 10 is low, the expansion device 14d is in the fully-opened state, and any device may be employed as long as the device does not cause a large pressure loss. For example, there may be employed a solenoid valve or other devices, which open and close the passage, or an electronic expansion valve or other devices, which are capable of changing the opening area. In this case, the description "fully-opened state" does not mean a state in which no pressure loss is caused, but means a state in which the pressure loss is not excessively large.

[0042] When the cooling operation mode is executed, the refrigerant is not required to be caused to flow into the use-side heat exchanger 17 without a heat load (including a thermostat-off state), and hence the operation is stopped. At this time, the expansion device 16 corresponding to the non-operating (stopped) indoor unit 2 is fully closed or set at a small opening degree at which no refrigerant flows therethrough.

[0043] [Cooling Operation Mode (when Discharge Temperature is High)] Fig. 5 is a diagram illustrating a flow of the refrigerant during the cooling operation mode of the air-conditioning apparatus 100 according to Embodiment 1 of the present invention when the discharge temperature is high. The description "the discharge temperature is high" means a case where the discharge temperature in a non-injection state is 105 degrees C or more, for example. In Fig. 5, the cooling operation mode is described taking as an example a case where a cooling load is generated in all the use-side heat exchangers 17. In this case, in Fig. 5, the pipes indicated by the thick lines correspond to the pipes through which the refrigerant flows, and a direction of the flow of the refrigerant is indicated by the solid arrows.

[0044] In the case of the cooling operation mode illustrated in Fig. 5, in the outdoor unit 1, the controller 50 instructs the refrigerant flow switching device 11 to switch so that the refrigerant discharged from the compressor 10 flows into the heat source-side heat exchanger 12. Then, the compressor 10 compresses low-temperature, low-pressure refrigerant into high-temperature and high-pressure gas refrigerant to discharge the refrigerant. The high-temperature and high-pressure gas refrigerant discharged from the compressor 10 passes through the refrigerant flow switching device 11 to flow into the heat source-side heat exchanger 12. Then, the refrigerant is condensed and liquefied into high-pressure liquid refrigerant while rejecting heat to the outdoor air in the heat source-side heat exchanger 12. The high-pressure liquid refrigerant flowing out of the heat source-side heat exchanger 12 passes through the expansion device 14c in a fully-opened state and the first passage of the subcooling heat exchanger 13. The refrigerant passing through the first passage of the subcooling heat exchanger 13 is distributed to two branched passages. One branched flow of refrigerant passes through the liquid separator 18 to flow out of the outdoor unit 1. The other branched flow of refrigerant flows into the first bypass pipe 4a. The compressor 10 compresses low-temperature, low-pressure refrigerant into high-temperature and high-pressure gas refrigerant to discharge the refrigerant. The high-temperature and high-pressure gas refrigerant discharged from the compressor 10 passes through the refrigerant flow switching device 11 to flow into the heat source-side heat exchanger 12. Then, the refrigerant is condensed and liquefied into high-pressure liquid refrigerant while rejecting heat to the outdoor air in the heat source-side heat exchanger 12. The high-pressure liquid refrigerant flowing out of the heat source-side heat exchanger 12 passes through the expansion device 14c in a fully-opened state and the first passage of the subcooling heat exchanger 13. The refrigerant passing through the first passage of the subcooling heat exchanger 13 is distributed to two branched passages. One branched flow of refrigerant passes through the liquid separator 18 to flow out of the outdoor unit 1. The other branched flow of refrigerant flows into the passage to reach the expansion device 14a. The high-temperature, high-pressure liquid refrigerant flowing into the first bypass pipe 4a is depressurized into first intermediate-pressure two-phase refrigerant by the expansion device 14a, and passes through the second passage of the subcooling heat exchanger 13 and the expansion device 14d to flow into the passage located on the upstream side of the accumulator 15. At this time, the subcooling heat exchanger 13 exchanges heat between the high-temperature, high-pressure liquid refrigerant passing through the first passage and the first intermediate-pressure two-phase refrigerant passing through the second passage. Therefore, the refrigerant passing through the first passage is cooled by the refrigerant passing through the second passage, whereas the refrigerant passing through the second passage is heated by the refrigerant passing through the first passage.

[0045] The flow rate of the refrigerant passing through the second passage of the subcooling heat exchanger 13 is controlled based on an opening degree (opening area) of the expansion device 14a. The controller 50 controls the opening degree (opening area) of the expansion device 14a so that a degree of subcooling of the refrigerant flowing on a downstream side of the first passage of the subcooling heat exchanger 13 approximates a target value. The degree of subcooling here corresponds to a temperature difference between a saturation temperature of the pressure detected by the high-pressure detection device 22 and a temperature detected by the liquid refrigerant temperature detection device 24,.

[0046] The high-temperature, high-pressure liquid refrigerant flowing out of the outdoor unit 1 passes through the extension pipe 5 to flow into the indoor units 2 (2a to 2d). The high-temperature, high-pressure liquid refrigerant flowing into the indoor units 2 (2a to 2d) is expanded into low-temperature, low-pressure two-phase refrigerant by the respective expansion devices 16 (16a to 16d), and flows into the use-side heat exchangers 17 (17a to 17d) each serving as an evaporator. Then, the refrigerant takes away heat from air flowing around the use-side heat exchangers 17 to turn into low-temperature, low-pressure gas refrigerant. Further, the low-temperature, low-pressure gas refrigerant flows out of each of the indoor units 2 (2a to 2d), and passes through the extension pipe 5 to flow into the outdoor unit 1 again. Then, the refrigerant passes through the refrigerant flow switching device 11 to join the refrigerant that is bypassed to the upstream side of the accumulator 15 through the first bypass pipe 4a. After that, the refrigerant flows into the accumulator 15, and is then sucked into the compressor 10 again.

[0047] At this time, an opening degree (opening area) of each of the expansion devices 16a to 16d is controlled so that a temperature difference (degree of superheat) between a temperature detected by the use-side heat exchanger gas refrigerant temperature detection device 28 and a temperature detected by the use-side heat exchanger liquid refrigerant temperature detection device 27 approximates a target value.

[0048] As described above, in this embodiment, the subcooling heat exchanger 13 is provided to keep the refrigerant subcooled reliably even when the extension pipe 5 is long. Further, through the adjustment of the opening degree (opening area) of the expansion device 14a, the degree of subcooling of the refrigerant at the outlet of the first passage of the subcooling heat exchanger 13 can be controlled to an appropriate value.

[0049] In this case, when a refrigerant such as R32 that may easily cause higher discharge temperature of the compressor 10 than the case where R41OA is used as the refrigerant, the discharge temperature is required to be lowered in order to prevent deterioration of refrigerating machine oil, burnout of the compressor 10, and other undesirable situations. Therefore, a part of the refrigerant passing through the second passage of the subcooling heat exchanger 13 is branched and caused to pass through the second bypass pipe 4b and the expansion device 14b and flow into the compression chamber of the compressor 10 through the injection port formed in the compressor 10. The refrigerant is caused to flow in through the injection port so that a temperature of the refrigerant to be discharged from the compressor 10 can be lowered. Thus, the refrigerant can be used safely.

[0050] In this case, the second bypass pipe 4b is connected to the injection port formed in the compression chamber of the compressor 10. To protect the compressor 10, it is necessary to prevent excessively high discharge temperature of the compressor10. For example, even when the refrigerant is bypassed to the refrigerant flow inflow side (upstream side) of the accumulator 15, a majority of the refrigerant is accumulated in the accumulator 15, and only a part of the refrigerant flows into the compressor 10. Therefore, the refrigerant in the two-phase state having high quality can directly be injected into the compression chamber through the second bypass pipe 4b and the injection port of the compressor 10.

[0051] The flow rate of the refrigerant passing through the second bypass pipe 4b is controlled based on an opening degree (opening area) of the expansion device 14b.

When the opening degree (opening area) of the expansion device 14b is increased to increase the flow rate of the refrigerant flowing through the second bypass pipe 4b, the discharge temperature of the compressor 10 is lowered. In contrast, when the opening degree (opening area) of the expansion device 14b is reduced to reduce the flow rate of the refrigerant flowing through the second bypass pipe 4b, the discharge temperature of the compressor 10 is increased. Therefore, through the adjustment of the opening degree (opening area) of the expansion device 14b, the discharge temperature of the compressor 10, which is a detection value of the discharge refrigerant temperature detection device 21, can be controlled to approximate a target value.

[0052] Further, in the cooling operation mode, in a case of, for example, a high-temperature outside air cooling in which the temperature around the heat source-side heat exchanger 12 is high, the discharge temperature of the compressor 10 is increased. Accordingly, the injection through the second bypass pipe 4b is necessary.

[0053] Fig. 6 is a p-h diagram (pressure-enthalpy diagram) during the cooling operation of the air-conditioning apparatus 100 according to Embodiment 1 of the present invention when the discharge temperature is high. The details of the action of the injection are described referring to Fig. 6. In the cooling operation mode, the refrigerant compressed by the compressor 10 and discharged therefrom (point I in Fig. 6) is condensed and liquefied into high-pressure liquid refrigerant by the heat source-side heat exchanger 12 (point J in Fig. 6). Further, the refrigerant is cooled by the subcooling heat exchanger 13 with the refrigerant branched into the first bypass pipe 4a so that the degree of subcooling is increased (point L in Fig. 6).

Then, the refrigerant flows into the liquid separator 18.

[0054] A pad of the liquid refrigerant, which is branched by the liquid separator 18 and flows toward the first bypass pipe 4a, is depressurized into a first intermediate pressure by the expansion device 14a (point M in Fig. 6). Then, the refrigerant is separated into refrigerant kept flowing through the first bypass pipe 4a and refrigerant branched into the second bypass pipe 4b. Of the streams of the refrigerant, the stream of the refrigerant flowing through the second bypass pipe 4b is depressurized into a second intermediate pressure through the expansion device 14b, and is injected into the compression chamber through the injection port formed in the compression chamber of the compressor 10. Then, the refrigerant joins the refrigerant sucked into the compressor 10 to be compressed into a second intermediate pressure. Further, the refrigerant flowing through the first bypass pipe 4a is depressurized into a low pressure through the expansion device 14d, and flows into the passage located on the upstream side of the accumulator 15.

[0055] On the other hand, the high-pressure two-phase refrigerant passing through the liquid separator 18 flows out of the outdoor unit 1 to pass through the extension pipe 5, and flows into the indoor units 2 to be depressurized by the respective expansion devices 16 (16a to 16d) of the indoor units 2 (point K in Fig. 6). Further, the refrigerant is evaporated by each of the use-side heat exchangers 17 (17a to 17d). After that, the refrigerant flows out of each of the indoor units 2 and passes through the extension pipe 5 to flow into the outdoor unit 1. Then, the refrigerant passes through the refrigerant flow switching device 11 to join the refrigerant that is caused to flow through the first bypass pipe 4a so as to be bypassed to the upstream side of the accumulator 15. After that, the refrigerant flows into the accumulator 15. Further, the refrigerant flowing out of the accumulator 15 passes through the suction-side passage to be sucked into the compressor 10.

[0056] In this case, the compressor 10 according to this embodiment is a low-pressure shell-type compressor. The sucked refrigerant and oil flow into a lower part of the compressor 10. Further, a motor is arranged at a middle part of the compressor 10. In addition, in an upper part of the compressor 10, the high-temperature and high-pressure refrigerant compressed in the compression chamber is discharged into a discharge chamber defined inside the hermetic container, and is then discharged from the compressor 10. Therefore, the metal hermetic container of the compressor 10 has a part exposed to the high-temperature and high-pressure refrigerant and a part exposed to the low-temperature, low-pressure refrigerant. Thus, a temperature of the hermetic container is an intermediate temperature between a temperature of the high-temperature and high-pressure refrigerant and a temperature of the low-temperature, low-pressure refrigerant. Further, a current flows through the motor, and hence the motor generates heat. Therefore, the low-temperature, low-pressure gas refrigerant sucked into the compressor 10 is heated to be raised in temperature by the hermetic container and the motor of the compressor 10 (point F in Fig. 6).

Then, the refrigerant is sucked into the compression chamber and is compressed into a second intermediate pressure in the compression chamber (point N in Fig. 6). Then, when the refrigerant is injected into the compression chamber of the compressor 10 through the second bypass pipe 4b, a temperature of the refrigerant in the compression chamber at the joined part between the refrigerant sucked therein from the suction-side passage and the refrigerant thus injected is lowered (point H in Fig. 6). Then, the refrigerant in the compression chamber continues to be further compressed into high-pressure gas refrigerant, and is then discharged from the compressor 10. Therefore, when the refrigerant is caused to flow into the compressor 10 through the second bypass pipe 4b, the temperature of the refrigerant is lowered at the joined part. Thus, the discharge temperature is lowered (point I in Fig. 6) as compared to a 10discharge temperature of the compressor, the discharge temperature in the case where the refrigerant is not caused to flow into the compressor 10 through the second bypass pipe 4b (point G in Fig. 6). The discharge temperature of the compressor 10 can be lowered also in the case of using, for example, a refrigerant such as R32 that may cause higher discharge temperature of the compressor 10 than R410A. Thus, the refrigerant can be used safely. Further, the reliability is enhanced.

[0057] In the case of the cooling operation mode when the discharge temperature is high, it is required to control both of the degree of subcooling in the first passage of the subcooling heat exchanger 13 and the discharge temperature of the compressor 10 to approximate the target values. For example, the controller 50 performs the following control. First, in order that the degree of subcooling of the refrigerant passing through the first passage of the subcooling heat exchanger 13 approximates the target value, the opening degree (opening area) of the expansion device 14a is changed to control the flow rate of the refrigerant flowing through the second passage of the subcooling heat exchanger 13. Further, in order that the discharge temperature of the compressor 10 approximates the target value, the opening degree (opening area) of the expansion device 14b is changed to control the flow rate of the refrigerant flowing into the injection port of the compressor 10 through the second bypass pipe 4b.

[0058] At this time, the refrigerant flowing through the second passage of the subcooling heat exchanger 13 is branched into the first bypass pipe 4a to which the expansion device 14d is provided and the second bypass pipe 4b to which the expansion device 14b is provided. When the discharge temperature of the compressor 10 is low, the expansion device 14b is set to be fully closed or at a small opening degree at which no refrigerant flows therethrough, and the expansion device 14d is set to be fully opened. In a case where the discharge temperature is high, if the opening degree of the expansion device 14b is changed while keeping the expansion device 14d set to be fully opened, the flow resistance inside the expansion device 14d provided to the first bypass pipe 4a is low, and hence the substantially entire refrigerant passes and flows through the first bypass pipe 4a. Therefore, the discharge temperature of the compressor 10 cannot be controlled through the control of the flow rate of the refrigerant flowing into the injection port of the compressor 10 through the second bypass pipe 4b.

[0059] Therefore, to change the flow rate of the refrigerant flowing into the second bypass pipe 4b through the control of the opening degree of the expansion device 14b, it is necessary to change an opening degree (opening area) of the expansion device 14d in accordance with the change in opening degree (opening area) of the expansion device 14b. For example, when the discharge temperature of the compressor 10 is low, the expansion device 14d is fully opened and the expansion device 14b is fully closed or set at a small opening degree at which no refrigerant flows therethrough. Even when the discharge temperature of the compressor 10 is increased from this state, the discharge temperature of the compressor 10 is controlled not to be excessively increased while preventing significant change in flow rate of the refrigerant flowing through the second passage of the subcooling heat exchanger 13. Thus, the expansion device 14b and the expansion device 14d are controlled in association with each other On a substantially simultaneous manner) while preventing significant change in total of the opening area of the expansion device 14b and the opening area of the expansion device 14d. For example, it is only necessary that a value obtained by subtracting the opening area of the expansion device 14b, which is controlled so that the discharge temperature approximates the target value, from the opening area of the expansion device 14d in the fully-opened state be determined as a renewed opening area of the expansion device 14d.

[0060] In general, the expansion device is designed so that the opening degree and the opening area correspond to each other in a substantially linear form in many cases. In such a case, it is only necessary that a value obtained by subtracting the opening degree of the expansion device 14b, which is controlled so that the discharge temperature approximates the target value, from the opening degree of the expansion device 14d in the fully-opened state be determined as a renewed opening degree of the expansion device 14d. For example, such a case is assumed that the expansion device 14b and the expansion device 14d have the same capacity (substantially the same opening area at the full-open opening degree) and are expansion devices of the same type. More specifically, the full-open opening degree of the expansion device 14b and the expansion device 14d is 1,500 pulses, and the expansion device 14b and the expansion device 14d have such a characteristic that no refrigerant flows therethrough at the opening degree of 100 pulses or less. Further, such a case is assumed that the expansion device 14b and the expansion device 14d are each designed so that the opening degree and the opening area correspond to each other in a substantially linear form.

[0061] In such assumptions, when the discharge temperature is low, the expansion device 14d is set to 1,500 pulses, and the expansion device 14b is set to 100 pulses. At this time, a total of the opening degree of the expansion device 14d and the opening degree of the expansion device 14b is 1,600 pulses. A case is assumed in which the discharge temperature of the compressor 10 is increased, and the opening degree of the expansion device 14b is controlled so that the discharge temperature approximates the target value, thereby causing the opening degree of the expansion device 14b to reach 600 pulses. At this time, the opening degree of the expansion device 14d is also controlled substantially simultaneously with the control of the expansion device 14b. The opening degree of the expansion device 14d is set to 1,000 pulses obtained by subtracting 600 pulses corresponding to the opening degree of the expansion device 14b from 1,600 pulses corresponding to the total opening degree of the two expansion devices. With this, the flow resistance inside the expansion device 14d installed on the first bypass pipe 4a is increased, and the flow resistance inside the expansion device 14b installed on the second bypass pipe 4b decreases, thereby being capable of causing the refrigerant to flow through the second bypass pipe 4b. Every time the opening degree of the expansion device 14b is controlled, the opening degree of the expansion device 14d is also controlled along therewith, thereby being capable of appropriately controlling the discharge temperature of the compressor 10.

[0062] In this case, the case where the expansion device 14b and the expansion device 14d are controlled substantially at the same time as each other is described. However, in actuality, the individual expansion devices are not controlled simultaneously, but are sequentially controlled at respective control timings.

Therefore, the description "substantially at the same time" in this embodiment means that the individual expansion devices are controlled at the same control timing (for example, every 30 seconds). Even when the expansion devices are controlled at the same control timing, in actuality, the individual expansion devices are not controlled simultaneously, but are sequentially controlled in many cases. For example, the opening degree of the expansion device 14b is controlled, and the opening degree of the expansion device 14d is next controlled. The expansion devices may be sequentially controlled as described above or simultaneously controlled. Further, when the expansion devices are sequentially controlled, the opening degree of the expansion device 14d may be first controlled, and after that, the opening degree of the expansion device 14b may be controlled. Further, even when the control timings of the expansion device 14b and the expansion device 14d are slightly deviated (for example, from about 30 seconds to about 1 minute), no serious problem is caused, and both of the degree of subcooling of the subcooling heat exchanger 13 and the discharge temperature of the compressor 10 can be controlled to the target values. Note that, when the individual expansion devices are controlled at the same control timing, better controllability is achieved, and the discharge temperature reaches the target value earlier. Further, when the expansion device 14b and the expansion device 14d are not sequentially controlled, but are simultaneously controlled, better controllability of the discharge temperature is achieved.

[0063] As described above, when the expansion device 14b and the expansion device 14d have the same capacity (substantially the same opening area at the full-open opening degree), it is desired to control the expansion devices so that the total of the opening areas of the two expansion devices is substantially the same. In this case, the description "the total of the opening areas of the two expansion devices is substantially the same" refers to, for example, a case where the total opening area of both of the two expansion devices falls within a range of about ±1%. For example, as in the example described above, when the total opening degree is 1,600 pulses, it is only necessary that the total opening degree fall within a range of 1,600±16 pulses. Further, for example, when the opening degree of the expansion device 14b is 600 pulses, it is only necessary that the opening degree of the expansion device 14d be set to pulses falling within a range of from 984 pulses to 1,016 pulses.

[0064] Further, although the controllability is slightly degraded as compared to the case where the total opening area is set to fall within a range of about ±1%, when the total of the opening areas of the two expansion devices is set to fall within a range of about ±10%, the discharge temperature of the compressor 10 can normally be controlled to the target value. For example, as in the example described above, when the total opening degree is 1,600 pulses, setting the total opening degree to fall within a range of 1,600±160 pulses suffices the above purpose. For example, when the opening degree of the expansion device 14b is 600 pulses, it is only necessary that the opening degree of the expansion device 14d be set to pulses falling within a range of from 840 pulses to 1,160 pulses. Further, when the total of opening areas of both the two expansion devices falls within a range of about ±20%, although the controllability is further degraded and the responsiveness of the control is slightly degraded, a problem in control is not caused, and the discharge temperature can be controlled to the target value. For example, as in the example described above, when the total opening degree is 1,600, it is only necessary to set the total opening degree to fall within 1,600±320 pulses. For example, when the opening degree of the expansion device 14b is 600 pulses, it is only necessary that the opening degree of the expansion device 14d be set to pulses falling within a range of from 680 pulses to 1,320 pulses.

[0065] In this case, the case where the expansion device 14b and the expansion device 14d have the same capacity (substantially the same opening area at the full-open opening degree) is described. However, the present invention is not limited thereto, and expansion devices of different types or expansion devices having different capacities may be employed. When the types or other features are different from each other, it is only necessary to control both of the expansion devices so that the total opening area of the opening area of an expansion portion of the expansion device 14b and the opening area of an expansion portion of the expansion device 14d is substantially the same.

[0066] Further, even when the total of the opening area of the expansion device 14b and the opening area of the expansion device 14d is different to some extent, the opening degree (opening area) of the expansion device 14d may be changed in an opposite direction in accordance with the change in opening degree (opening area) of the expansion device 14b. With this, the opening degree (opening area) of the expansion device 14a can be adjusted to control the degree of subcooling in the first passage of the subcooling heat exchanger 13, and the opening degree (opening area) of the expansion device 14b installed on the second bypass pipe 4b can be adjusted to control the discharge temperature of the compressor 10. In this case, the description "be changed in an opposite direction" refers to, for example, control of changing the opening degree (opening area) of the expansion device 14d so as to be reduced when the opening degree (opening area) of the expansion device 14b is increased, or control of changing the opening degree (opening area) of the expansion device 14d so as to be increased when the opening degree (opening area) of the expansion device 14b is reduced. For example, it is assumed that the opening area at the maximum opening degree of the expansion device 14b is 1.5 times as large as the opening area at the maximum opening degree of the expansion device 14d, and the opening degree and the opening area have a linear relationship. Further, it is assumed that the expansion device 14b has the full-open opening degree of 2,000 pulses and the minimum opening degree of 200 pulses, at which no refrigerant flows therethrough. In addition, it is assumed that the expansion device 14d has the full-open opening degree of 1,000 pulses and the minimum opening degree of 100 pulses, at which no refrigerant flows therethrough. In such a condition, when the discharge temperature is low, the expansion device 14d is set to 1,000 pulses corresponding to the maximum opening degree, and the expansion device 14b is set to 200 pulses corresponding to the minimum opening degree. It is assumed that, when the discharge temperature of the compressor 10 is increased and the opening degree of the expansion device 14b is controlled so that the discharge temperature of the compressor 10 reaches the target value, the controller 50 determines an appropriate opening degree of the expansion device 14b as 800 pulses. At this time, the opening degree of the expansion device 14d is controlled to be closed from 1,000 pulses corresponding to the full-open opening degree in accordance with control of opening the opening degree of the expansion device 14b from 200 pulses corresponding to the minimum opening degree to 800 pulses corresponding to a renewed opening degree. Further, the opening degree of the expansion device 14d is determined based on, for example, Expression (1). According to this expression, the expansion device 14d is closed at the same rate as a change rate at which the opening degree of the expansion device 14b is opened.

[0067] [Math. 1] Renewed opening degree of expansion device 14d=minimum opening degree of expansion device 14d+(maximum opening degree of expansion device 14d-minimum opening degree of expansion device 14d)x(renewed opening degree of expansion device 14b-minimum opening degree of expansion device 14b)/(maximum opening degree of expansion device 14b-minimum opening degree of expansion device 14b) (1) [0068] Specific values based on the conditions are substituted into Expression (1) to obtain Expression (2). As a result of computation of Expression (2), the renewed opening degree of the expansion device 14d is 400 pulses.

[0069] [Math. 2] Renewed opening degree of expansion device 14d=100+(1,000-100)x(800- 200)1(2,000-200) *** (2) [0070] Therefore, the controller 50 controls the opening degree of the expansion device 14b to 400 pulses at the control timing at which the opening degree of the expansion device 14d is controlled to 800 pulses. At this time, regarding change amounts of the opening degrees of the individual expansion devices, the expansion device 14d is (800-200)=600 pulses, and the expansion device 14b is (400-100)=300 pulses. Each of both of the expansion devices is changed by about 33% of a difference between the maximum opening degree and the minimum opening degree of each of the expansion devices.

[0071] As described above, even when the capacities (opening areas at the maximum opening degrees) of the expansion device 14b and the expansion device 14d are different from each other, the change amount of the opening degree of the expansion device 14d is determined in accordance with the change in opening degree of the expansion device 14b, and a direction of changing the opening degree of the expansion device 14b and a direction of changing the opening degree of the expansion device 14d are controlled to directions opposite to each other, thereby being capable of controlling the discharge temperature of the compressor 10 to the target value. Note that, in this case, the total area of the opening area of the expansion device 14b and the opening area of the expansion device 14d varies before and after the control. Therefore, it is necessary to control the opening degree of the expansion device 14a to maintain the degree of subcooling in the first passage of the subcooling heat exchanger 13 to the target value.

[0072] For example, when the capacities of the expansion device 14b and the expansion device 14d are different from each other, better controllability is achieved in the case where the opening degree and the opening area have the linear relationship. However, the present invention is not limited thereto. The same control can be performed as long as the expansion device has the opening area to be increased along with increase in opening degree irrespective of a form in a correspondence relationship between the opening degree and the opening area.

[0073] As described above, in the cooling operation mode when the discharge temperature of the compressor 10 is high, the expansion device 14a is controlled so that the degree of subcooling in the first passage of the subcooling heat exchanger 13 approximates the target value, to thereby control the flow rate of the refrigerant flowing through the second passage of the subcooling heat exchanger 13. Further, the opening degree of the expansion device 14b is controlled so that the discharge temperature of the compressor 10 approximates the target value, to thereby control the flow rate of the refrigerant flowing into the second bypass pipe 4b connected to the injection port of the compressor 10. At this time, the expansion device 14d is controlled in association with the expansion device 14b, and the opening degree (opening area) of the expansion device 14d is computed and set based on the opening degree (opening area) of the expansion device 14b after the control, thereby being capable of appropriately controlling both of the degree of subcooling of the subcooling heat exchanger 13 and the discharge temperature of the compressor 10.

[0074] In this case, the direction of changing the opening degree of the expansion device 14b and the direction of changing the opening degree of the expansion device 14d are controlled to the directions opposite to each other. For example, when the controller 50 controls the opening area of the expansion device 14b to be increased, the controller 50 controls the opening area of the expansion device 14d to be reduced. Further, when the controller 50 controls the opening area of the expansion device 14b to be reduced, the controller 50 controls the opening area of the expansion device 14d to be increased.

[0075] In many cases, the injection port of the compressor 10 is generally opened to the compression chamber at a position having a small angle with respect to a position at which a suction port of the compression chamber is closed. In this case, the pressure inside the compression chamber in which the injection port is positioned corresponds to a value close to the low pressure (pressure on the suction side). At this time, the expansion device 14b and the expansion device 14d having the same capacity (substantially the same opening area at the full-open opening degree) are used. However, the flow rate of the refrigerant flowing through the expansion device is proportional to the square root of a pressure difference between the front and back of the expansion device. For example, when the injection port of the compressor 10 is opened to the compression chamber after a considerably large rotation angle is formed with respect to the position at which the suction port of the compression chamber is closed, the pressure inside the compression chamber is increased sufficiently from the low pressure. Therefore, the pressure difference between the front and back of the expansion device 14b is smaller than the pressure difference between the front and back of the expansion device 14d. Therefore, better controllability is achieved in a case where the capacity (opening area at the full-open opening degree) of the expansion device 14b is set to be higher than the capacity (opening area at the full-open opening degree) of the expansion device 14d. Also in this case, the expansion device 14b and the expansion device 14d are controlled in association with each other. It is only necessary that the controller 50 compute the opening degree of the expansion device 14d or the change amount of the opening degree, or the opening area of the expansion device 14d or the change amount of the opening area based on the opening degree of the expansion device 14b or the change amount of the opening degree, or based on the opening area of the expansion device 14b or the change amount of the opening area.

[0076] In this case, it is desired that the expansion device 14a be a device capable of changing its opening area, such as the electronic expansion valve, but the expansion device 14a is not limited thereto For example, the expansion device 14a may be the combination of on-off valves such as the small-size solenoid valves, or the capillary tube.

[0077] Further, the expansion device 14d only needs to change the flow resistance in accordance with the change in opening degree of the expansion device 14b.

Therefore, it is desired that the expansion device 14d be an electronic expansion valve or other devices, which are capable of changing the opening area, but the expansion device 14d is not limited thereto. For example, a plurality of solenoid valves may be combined with each other in parallel to enable change in opening area in a plurality of levels in accordance with the change in opening degree of the expansion device 14b. In this case, the opening area cannot be changed continuously, and hence the accuracy of control of the discharge temperature is slightly degraded. However, the discharge temperature can be controlled not to exceed a limit value, thereby causing no problem.

[0078] Further, as described above, the refrigerant is not required to be caused to flow into the use-side heat exchanger 17 without the heat load (including the thermostat-off state), and hence the operation is stopped. At this time, the expansion device 16 corresponding to the non-operating indoor unit 2 is fully closed or set at a small opening degree at which no refrigerant flows therethrough.

[0079] As described above, the two bypass pipes are provided on a downstream side of the second passage of the subcooling heat exchanger 13. The first bypass pipe 4a is constructed such that the refrigerant is caused to flow into the passage located on the upstream side of the accumulator 15 (suction side of the compressor 10) through the expansion device 14d, and the second bypass pipe 4b is constructed such that the refrigerant can be injected into the compression chamber of the compressor 10 through the expansion device 14b. At this time, the expansion device 14b and the expansion device 14d are controlled in association with each other. Thus, even when the extension pipe 5 is long, the refrigerant flowing into the indoor unit 2 can be brought into a state in which the degree of subcooling is secured reliably. In addition, under the condition that the discharge temperature of the compressor 10 is high, the discharge temperature of the compressor 10 can reliably be controlled not to exceed the upper limit.

[0080] In this case, the refrigerant passing through the second passage of the subcooling heat exchanger 13 is low-temperature two-phase refrigerant or gas refrigerant. For example, when the discharge temperature of the compressor 10 is slightly higher than a limit temperature, it is only necessary to cause the low-temperature gas refrigerant to flow into the compression chamber of the compressor through the second bypass pipe 4b. On the other hand, when the discharge temperature is considerably higher than the limit temperature, it is necessary to cause the low-temperature two-phase refrigerant to flow into the compression chamber of the compressor 10. When the refrigerant passing through the second passage of the subcooling heat exchanger 13 is brought into the two-phase state, the refrigerant in the two-phase state is branched into the first bypass pipe 4a and the second bypass pipe 4b. In this case, the refrigerant flowing into the branch portion for the first bypass pipe 4a and the second bypass pipe 4b is caused to flow, for example, upward from below in the gravity direction (height direction). Then, passages for the branched streams of the refrigerant are constructed such that the branched streams of the refrigerant flow at substantially the same height in the height direction. With such a configuration, the refrigerant in the two-phase state can accurately (equally) be branched without biasing the liquid refrigerant toward one passage. A T-shaped joint or a Y-shaped joint is used for the branch portion in this case. Further, even when the branch portion is slightly inclined, no problem is caused as long as the inclination is only slight inclination, and the two-phase refrigerant is accurately distributed. The allowable inclination angle is about 15 degrees or less, and the two-phase refrigerant can be distributed without any problem as long as the inclination angle is about 15 degrees or less. Note that, the branch portion for the first bypass pipe 4a and the second bypass pipe 4b is not limited thereto, and any structure may be employed as long as the structure is capable of accurately branching the refrigerant in the two-phase state without biasing the refrigerant liquid toward one side.

[0081] [Heating Operation Mode] Fig. 7 is a diagram illustrating a flow of the refrigerant during the heating operation mode of the air-conditioning apparatus 100 according to Embodiment 1 of the present invention. In Fig. 7, the heating operation mode is described taking as an example a case where a heating load is generated in all of the use-side heat exchangers 17. In this case, in Fig. 7, the pipes indicated by the thick lines are the pipes through which the refrigerant flows, and directions in which the refrigerant flows are indicated by the solid arrows.

[0082] In the case of the heating operation mode illustrated in Fig. 7, in the outdoor unit 1, the controller 50 instructs the refrigerant flow switching device 11 to switch the flow of the refrigerant discharged from the compressor 10 so that the refrigerant flows out of the outdoor unit 1 and flows into the indoor unit 2 without passing through the heat source-side heat exchanger 12. Then, the compressor 10 compresses low-temperature, low-pressure refrigerant into high-temperature and high-pressure gas refrigerant to discharge the refrigerant. The high-temperature and high-pressure gas refrigerant discharged from the compressor 10 passes through the refrigerant flow switching device 11 to flow out of the outdoor unit 1. The high-temperature and high-pressure gas refrigerant flowing out of the outdoor unit 1 passes through the extension pipe 5 to flow into the indoor units 2 (2a to 2d). The high-temperature and high-pressure gas refrigerant flowing into the indoor units 2 (2a to 2d) flows into the use-side heat exchangers 17 (17a to 17d), and is condensed and liquefied into high-temperature, high-pressure liquid refrigerant while rejecting heat to air flowing around the use-side heat exchangers 17 (17a to 17d). The liquid refrigerant flowing out of the use-side heat exchangers 17 (17a to 17d) is expanded into first intermediate-pressure two-phase refrigerant by the respective expansion devices 16 (16a to 16d), and flows out of the indoor units 2 (2a to 2d). The first intermediate-pressure two-phase refrigerant flowing out of the indoor unit 2 passes through the extension pipe 5 to flow into the outdoor unit 1 again.

[0083] At this time, the opening degree (opening area) of each of the expansion devices 16a to 16d is controlled so that a temperature difference (degree of subcooling) between a condensing temperature in each of the use-side heat exchangers 17a to 17d and the temperature detected by the use-side heat exchanger liquid refrigerant temperature detection device 27 approximates a target value. A value of the condensing temperature in each of the use-side heat exchangers 17a to 17d is, for example, determined by the controller 50 of the outdoor unit 1, and a signal of the value is transmitted to a controller (not shown) of each of the indoor units 2 through communication.

[0084] The first intermediate-pressure two-phase refrigerant flowing into the outdoor unit 1 flows into the liquid separator 18. A part of the liquid refrigerant flowing into the liquid separator 18 flows out of the liquid separator 18 through an oufflow port, which is formed on a lower side of the liquid separator 18 in the gravity direction, and flows through the first bypass pipe 4a to pass through the expansion device 14a.

The liquid refrigerant having first intermediate-pressure is depressurized into two-phase refrigerant having second intermediate-pressure by the expansion device 14a, and passes through the second passage of the subcooling heat exchanger 13. The refrigerant passes through the second passage of the subcooling heat exchanger 13, and hence is heated into two-phase refrigerant having high quality or gas refrigerant by the refrigerant passing through the first passage of the subcooling heat exchanger 13. Then, the refrigerant passes through the second bypass pipe 4b and the expansion device 14b in the fully-opened state to flow into the compression chamber through the injection port formed in the compressor 10.

[0085] Further, the rest of the two-phase refrigerant flowing into the liquid separator 18 flows out of the liquid separator 18 through an outflow port, which is formed in the vicinity of a center or on an upper side of the liquid separator 18 in the gravity direction, and flows into the first passage of the subcooling heat exchanger 13. The first intermediate-pressure two-phase refrigerant flowing into the first passage of the subcooling heat exchanger 13 is slightly cooled by the second intermediate-pressure refrigerant flowing through the second passage of the subcooling heat exchanger 13, and flows out with its quality being slightly reduced. Then, the refrigerant passes through the expansion device 14c to be depressurized into low-temperature, low-pressure two-phase refrigerant, and flows into the heat source-side heat exchanger 12. The low-temperature, low-pressure two-phase refrigerant in the heat source-side heat exchanger 12 takes away heat from air flowing around the heat source-side heat exchanger 12 to be evaporated into low-temperature, low-pressure gas refrigerant, and flows out of the heat source-side heat exchanger 12. Then, the refrigerant passes through the refrigerant flow switching device 11 and the accumulator 15 to be sucked into the compressor 10 again.

[0086] As described above, in the cooling operation mode, the refrigerant is caused to flow into the second passage of the subcooling heat exchanger 13, and the refrigerant flowing through the first passage of the subcooling heat exchanger 13 is subcooled. Then, the refrigerant is sent out to the extension pipe 5. In this case, in the heating operation mode, it is unnecessary to subcool the refrigerant flowing through the extension pipe 5. A first purpose of causing the refrigerant to flow into the second passage of the subcooling heat exchanger 13 is to lower the discharge temperature when the discharge temperature of the compressor 10 is excessively high. Further, a second purpose thereof is to enhance the heating capacity. Both of the operations are required to be performed in a case of, for example, a low outside air temperature. Therefore, when the discharge temperature of the compressor 10 is not excessively high, and further, the heating capacity is not required to be increased such as when the outside air temperature is relatively high (when, for example, the discharge temperature in a non-injection state is 105 less than degrees C), the controller 50 controls the expansion device 14b to be fully closed or set at a small opening degree at which no refrigerant flows therethrough, thereby preventing the flow of the refrigerant into the second bypass pipe 4b. Further, in the heating operation mode, the expansion device 14d is generally set to be fully closed or at a small opening degree at which no refrigerant flows therethrough, thereby preventing the refrigerant from flowing into the first bypass pipe 4a.

[0087] In this case, when a refrigerant such as R32 that may cause higher discharge temperature of the compressor 10 than the case where R410A is used as the refrigerant, the discharge temperature becomes high as compared to the case where R410A is used, at a slightly higher outside air temperature. The discharge temperature is required to be lowered in order to prevent, for example, deterioration of refrigerating machine oil and burnout of the compressor 10.

[0088] As described above, the refrigerant in the two-phase state having high quality can directly be injected into the compression chamber of the compressor 10 through the second bypass pipe 4b connected to the injection port of the compressor 10. [0089] The flow rate of the refrigerant passing through the second bypass pipe 4b is controlled based on the opening degree (opening area) of the expansion device 14b. When the opening degree (opening area) of the expansion device 14b is increased to increase the flow rate of the refrigerant flowing through the second bypass pipe 4b, the discharge temperature of the compressor 10 is lowered. In contrast, when the opening degree (opening area) of the expansion device 14b is reduced to reduce the flow rate of the refrigerant flowing through the second bypass pipe 4b, the discharge temperature of the compressor 10 is increased. Therefore, through the adjustment of the opening degree (opening area) of the expansion device 14b, the discharge temperature of the compressor 10 can be controlled. For example, the heating operation mode is set such that the refrigerant in the two-phase state having high quality is injected into the compression chamber of the compressor 10. At this time, a degree of discharge superheat is calculated based on the discharge temperature detected by the discharge refrigerant temperature detection device 21 and the saturation temperature of the pressure detected by the high-pressure detection device 22, and the opening degree (opening area) of the expansion device 14b is controlled so that the degree of discharge superheat falls within a target range. With this, the amount of the refrigerant that can be injected is increased, thereby being capable of increasing the heating capacity.

[0090] For example, as described above, when R32 is used as the refrigerant, the discharge temperature is easily increased, and hence the refrigerant is injected into the compressor 10. In such a case, basically, the heating capacity may not be increased, and hence the controller 50 may control the amount of the refrigerant to be injected so that the discharge temperature approximates the target value. As a matter of course, even when R32 is used, the injection amount may be controlled so that the degree of discharge superheat approximates the target value in order to increase the heating capacity.

[0091] Further, the expansion device 14c is controlled to regulate the pressure of the refrigerant flowing between the expansion device 16 and the expansion device 14a (liquid separator 18) to a first intermediate pressure. As described above, the first intermediate pressure is the pressure lower than the high pressure on the discharge side of the compressor 10, and is the pressure higher than the second intermediate pressure that is the pressure on a downstream side of the second bypass pipe 4b or the pressure in the injection port of the compression chamber of the compressor 10.

When the pressure of the refrigerant flowing between the expansion device 16 and the expansion device 14c is maintained to the first intermediate pressure, a pressure difference between the front and back of the expansion device 14a can be secured, thereby being capable of reliably injecting the refrigerant into the compressor 10. In this case, the opening degree (opening area) of the expansion device 14c is controlled so that the intermediate pressure, which is determined by converting the temperature detected by the liquid refrigerant temperature detection device 24 into a saturation pressure, approximates a target value.

[0092] Fig. 8 is a p-h diagram (pressure-enthalpy diagram) during the heating operation of the air-conditioning apparatus according to Embodiment 1 of the present invention. In the heating operation mode, the refrigerant, which is compressed by the compressor 10 to be discharged therefrom (point I in Fig. 8), passes through the refrigerant flow switching device 11 to flow out of the outdoor unit 1, and passes through the extension pipe 5 to flow into the indoor unit 2. Then, the refrigerant is condensed by the use-side heat exchanger 17 of the indoor unit 2. After that, the refrigerant passes through the expansion device 16 and the extension pipe 5 to flow back to the outdoor unit 1. Further, the refrigerant passes through the liquid separator 18 and the first passage of the subcooling heat exchanger 13 to flow into the expansion device 14c. Through the adjustment of the opening degree of the expansion device 14c, the pressure of the refrigerant flowing between the expansion device 16 and the expansion device 14c (refrigerant flowing on an upstream side of the expansion device 14c) is adjusted to the first intermediate pressure (point J in Fig. 8). A part of the liquid refrigerant is branched by the liquid separator 18 from the first intermediate-pressure refrigerant flowing between the expansion device 16 and the expansion device 14c (point Ji in Fig. 8). The part of the liquid refrigerant thus branched passes through the first bypass pipe 4a, and is depressurized into second intermediate-pressure refrigerant by the expansion device 14a (point M in Fig. 8). Further, the refrigerant passes through the second passage of the subcooling heat exchanger 13 while exchanging heat with the refrigerant flowing through the first passage of the subcooling heat exchanger 13 so that the refrigerant is heated into second intermediate-pressure two-phase refrigerant having high quality (point Mt in Fig. 8). Then, the refrigerant passes through the second bypass pipe 4b and the expansion device 14b in the fully-opened state to be injected into the compression chamber through the injection port of the compressor 10.

[0093] On the other hand, the part of the liquid refrigerant is branched by the liquid separator 18, and hence the quality of the rest of the refrigerant is slightly increased (point J2 in Fig. 8). Further, the rest of the refrigerant passes through the first passage of the subcooling heat exchanger 13 while exchanging heat with the refrigerant flowing through the second passage of the subcooling heat exchanger 13 so that the refrigerant is cooled (point J3 in Fig. 8). Further, the refrigerant is depressurized into low-pressure two-phase refrigerant by the expansion device 14c (point K in Fig. 8). The low-pressure two-phase refrigerant is evaporated by the heat source-side heat exchanger 12, and then passes through the refrigerant flow switching device 11 to flow into the accumulator 15. Further, the refrigerant flowing out of the accumulator 15 passes through the suction-side passage to be sucked into the compressor 10.

[0094] In this case, the compressor 10 according to this embodiment is the low-pressure shell-type compressor. The sucked refrigerant and the oil flow into the lower part of the compressor 10. Further, the motor is arranged at the middle part of the compressor 10. In addition, in the upper part of the compressor 10, the high-temperature and high-pressure refrigerant compressed in the compression chamber is discharged into the discharge chamber defined inside the hermetic container, and is then discharged from the compressor 10. Therefore, the metal hermetic container of the compressor 10 has the part exposed to the high-temperature and high-pressure refrigerant and the part exposed to the low-temperature, low-pressure refrigerant. Therefore, the temperature of the hermetic container is the intermediate temperature between the temperature of the high-temperature and high-pressure refrigerant and the temperature of the low-temperature, low-pressure refrigerant. Further, the current flows through the motor, and hence the motor generates heat. Therefore, the low-temperature, low-pressure gas refrigerant sucked into the compressor 10 is heated to be raised in temperature by the hermetic container and the motor of the compressor 10 (point F in Fig. 8). Then, the refrigerant is sucked into the compression chamber and is compressed into the second intermediate pressure in the compression chamber (point N in Fig. 8). Then, when the refrigerant is injected into the compression chamber of the compressor 10 through the second bypass pipe 4b, the temperature of the refrigerant in the compression chamber at the joined part between the refrigerant sucked therein from the suction-side passage and the refrigerant thus injected is lowered (point H in Fig. 8). Then, the refrigerant in the compression chamber continues to be further compressed into the high-pressure gas refrigerant, and is then discharged from the compressor 10. Therefore, when the refrigerant is caused to flow into the compressor 10 through the second bypass pipe 4b, the temperature of the refrigerant is lowered at the joined pad. Thus, the discharge temperature is lowered (point I in Fig. 8) as compared to the 10discharge temperature of the compressor, the discharge temperature in the case where the refrigerant is not caused to flow into the compressor 10 through the second bypass pipe 4b (point G in Fig. 8). The discharge temperature of the compressor 10 can be lowered also in the case of using, for example, the refrigerant such as R32 that may cause higher discharge temperature of the compressor 10 than the case where R410A is used Thus, the refrigerant can be used safely. Further, the reliability is enhanced.

[0095] In this case, it is desired that the expansion device 14c be a device capable of changing its opening area, such as the electronic expansion valve. When the electronic expansion valve is used, the first intermediate pressure that is the pressure of the refrigerant flowing on the upstream side of the expansion device 14c can be adjusted to an arbitrary pressure, thereby being capable of finely controlling the discharge temperature. However, the expansion device 14c is not limited to the electronic expansion valve. For example, there may be employed the configuration in which on-off valves such as the small-size solenoid valves are combined so that the opening degree can selectively be controlled in a plurality of levels. Further, there may be employed the configuration in which the capillary tube is provided to enable the subcooling depending on the pressure loss of the refrigerant. Although the controllability is slightly degraded as compared to the case of the electronic expansion valve, the discharge temperature can approximate the target value. [0096] Further, the description is given of the method of determining the first intermediate pressure by converting the temperature detected by the liquid refrigerant temperature detection device 24 into a saturation pressure. With this method, the system can be constructed at low cost. Note that, the present invention is not limited thereto. For example, a pressure sensor may be provided so that a directly detected pressure is used to determine the first intermediate pressure. The expansion device 14a is the electronic expansion valve or other devices, which are capable of changing the opening area. Further, the controller 50 controls the opening area of the expansion device 14a so that the degree of discharge superheat calculated based on a temperature difference between the discharge temperature of the compressor 10, which is detected by the discharge refrigerant temperature detection device 21, and the saturation temperature of the high pressure, which is detected by the high-pressure detection device 22, falls within the target range. Further, when it is determined that the discharge temperature exceeds a certain value (for example, 110 degrees C), the expansion device 14a may be controlled to be opened by a certain opening degree, for example, by 10 pulses. Further, the target temperature may be set in a range instead of being set to the certain value, and the expansion device 14a may be controlled so that the discharge temperature falls within the range of the target temperature (for example, from 100 degrees C to 110 degrees C). Note that, in the heating operation mode, normally, the expansion device 14b is held in the fully-opened state, and the expansion device 14d is held in the fully-closed state. At the time of activation or other times, the expansion device 14b may be fully closed.

[0097] In this case, when the heating operation mode is executed, the refrigerant is not required to be caused to flow into the use-side heat exchanger 17 without a heat load (heating load) (including the thermostat-off state). However, when the expansion device 16 corresponding to the use-side heat exchanger 17 without a heating load is fully closed or set at a small opening degree at which no refrigerant flows therethrough in the heating operation mode, the refrigerant is cooled and condensed inside the use-side heat exchanger 17 of the non-operating indoor unit 2 by ambient air so that the refrigerant may stagnate therein, which may cause shortage of the refrigerant in the entire refrigerant circuit. Therefore, in this embodiment, during the heating operation, the opening degree (opening area) of the expansion device 16 corresponding to the use-side heat exchanger 17 without a heat load is set to a large opening degree such as the fully-opened state so that the refrigerant can pass through the expansion device 16. Therefore, the stagnation of the refrigerant can be prevented.

[0098] The opening degree (opening area) of the expansion device 14a is controlled so that the degree of discharge superheat approximates the target value (for example, 40 degrees C) or falls within the target range (for example, from 30 degrees C to 40 degrees C). The target value of the degree of discharge superheat is set at different values depending on the outside air temperature, and is set at such a value that the degree of increase in heating capacity, which is exerted by the indoor unit 2, is increased to the extent possible in each of the outside air temperatures, and that the discharge temperature does not exceed the limit temperature.

[0099] Further, the opening degree (opening area) of the expansion device 14a may be controlled so that the discharge temperature of the compressor 10 approximates the target value. The target value of the discharge temperature is set smaller than the limit value of the discharge temperature. Note that, the heating capacity exerted by the indoor unit 2 can be large with higher discharge temperatures. Thus, it is desired to set the temperature as high as possible. For example, when the limit of the discharge temperature of the compressor 10 is 120 degrees C, to prevent the discharge temperature from exceeding 120 degrees C, the frequency of the compressor 10 is set to be decreased when the discharge temperature exceeds 110 degrees C. Therefore, when the discharge temperature of the compressor 10 is lowered through the injection, it is preferred that the target value of the discharge temperature be set at a temperature in a range of from 100 degrees C to 110 degrees C (for example, 105 degrees C), which is a temperature slightly lower than 110 degrees C that is the temperature at which the frequency of the compressor 10 is decelerated. In this case, when the frequency is not decelerated at 110 degrees C, it is preferred that the target value of the discharge temperature be set at a temperature in a range of from 100 degrees C to 120 degrees C (for example, 115 degrees C).

[0100] Embodiment 2 Although not particularly illustrated in Embodiment 1 described above, a four-way valve is generally used as the second refrigerant flow switching device 19. Note that, the second refrigerant flow switching device 19 is not limited thereto. A plurality of two-way passage switching valves, a plurality of three-way passage switching valves, or a plurality of other valves may be used so that flow switching similar to that in the four-way valve can be performed.

[0101] Further, the case where four indoor units 2 are connected is described as an example. However, the same effect as that in Embodiment 1 is achieved irrespective of the number of the connected indoor units 2.

[0102] In this case, detailed description of the configuration of the liquid separator 18 is not particularly given in Embodiment 1 described above. For example, it is only necessary that the liquid separator 18 have one inlet-side passage and two outlet-side passages and be capable of separating the liquid refrigerant from the refrigerant flowing therein through the inlet-side passage to cause the liquid refrigerant to flow out of the liquid separator 18 through one outlet-side passage to the first bypass pipe 4a. Further, some degree of gas refrigerant may be mixed into the refrigerant flowing out to the first bypass pipe 4a, and separation efficiency for the liquid refrigerant by the liquid separator 18 may not be 100% as long as the degree of gas refrigerant mixture is kept low enough to avoid significant influence on the control of the expansion device.

[0103] Further, the case of one outdoor unit 1 is described in the embodiments described above. However, a plurality of outdoor units 1 may be connected. At this time, a branch part or a joined part for the streams of the refrigerant to flow into or flow out of the plurality of the outdoor units 1 may be positioned on the outside of the respective outdoor units 1.

[0104] Further, the case where the low-pressure shell-type compressor is used as the compressor 10 is described as an example. However, the same effect is achieved even when, for example, a high-pressure shell-type compressor is used.

[0105] Refrigerants other than R32 are not mentioned in Embodiment 1 described above. The effect of the present invention is significantly exerted when a refrigerant such as R32 that may easily cause increase in discharge temperature is used. However, the refrigerant is not limited to R32. For example, in addition to R32, there may be used a refrigerant mixture of R32 and HF01234yf, HF01234ze, or other refrigerants, which are tetrafluoropropene-based refrigerants having a low global warming potential and being represented by the chemical formula of CF3CF=CH2 (zeotropic refrigerant mixture). In the case where, for example, R32 is used as the refrigerant, the discharge temperature is raised by about 20 degrees C under the same operation state as compared to the case where R410A is used. Therefore, R32 needs to be used while lowering the discharge temperature, and hence the effect of the injection of the present invention is significantly exerted. In the case where the refrigerant mixture of R32 and HF01234yf is used and a mass fraction of R32 is 62% (62 wt%) or more, the discharge temperature is increased by 3 degrees C or more as compared to the case where the R410A refrigerant is used. Therefore, the effect of lowering the discharge temperature through the injection of the present invention is significantly exerted. Further, in the case where the refrigerant mixture of R32 and HF01234ze is used and the mass fraction of R32 is 43% (43 wt%) or more, the discharge temperature is increased by 3 degrees C or more as compared to the case where the R410A refrigerant is used. Therefore, the effect of lowering the discharge temperature through the injection of the present invention is significantly exerted. In addition, the kinds of refrigerant in the refrigerant mixture are not limited to those described above. Also in a case of a refrigerant mixture containing a small amount of other refrigerant components, the discharge temperature is not affected significantly so that the same effect is achieved. Further, for example, a refrigerant mixture containing R32, HF01234yf, and a small amount of other refrigerants may also be used. The discharge temperature needs to be lowered in any refrigerant that may cause the discharge temperature to be increased as compared to the case of R410A. Thus, the same effect is achieved.

[0106] Further, blowers for delivering air to promote the condensation or the evaporation of the refrigerant are generally mounted on the heat source-side heat exchanger 12 and the use-side heat exchangers 17a to 17d in many cases, but the present invention is not limited thereto. For example, panel heaters utilizing radiation may be used as the use-side heat exchangers 17a to 17d. Further, a water-cooled heat exchanger for exchanging heat using liquid such as water or an antifreeze solution may be used as the heat source-side heat exchanger 12. Any device may be used as long as the device allows the refrigerant to reject or take away heat. [0107] Further, in this case, description is given taking as an example the direct-expansion air-conditioning apparatus in which the outdoor unit 1 and the indoor units 2 are connected by pipes so that the refrigerant is circulated, but the present invention is not limited thereto. For example, a relay unit is arranged between the outdoor unit 1 and the indoor units 2. Further, the present invention is also applicable to an air-conditioning apparatus for conditioning air by circulating the refrigerant between the outdoor unit and the relay unit, and circulating a heat medium such as water or brine between the relay unit and the indoor units so that heat is exchanged between the refrigerant and the heat medium in the relay unit, which attains the same effect. The installation of the liquid separator 18 is unnecessary in such an air-conditioning apparatus.

Reference Signs List [0108] 1 outdoor unit 2, 2a, 2b, 2c, 2d indoor unit 4a first bypass pipe 4b second bypass pipe 5 extension pipe 6 outdoor space 7 indoor space 9 construction 10 compressor 11 refrigerant flow switching device 12 heat source-side heat exchanger 13 subcooling heat exchanger14a, 14b, 14c, 14d expansion device 15 accumulator 16, 16a, 16b, 16c, 16d expansion device 17, 17a, 17b, 17c, 17d use-side heat exchanger 18 liquid separator 19 second refrigerant flow switching device 21 discharge refrigerant temperature detection device 22 high-pressure detection device 23 low-pressure detection device 24 liquid refrigerant temperature detection device 25 subcooling heat exchanger inlet refrigerant temperature detection device 26 exchanger outlet refrigerant temperature detection device27 exchanger liquid refrigerant temperature detection device 28 exchanger gas refrigerant temperature detection device 50 air-conditioning apparatus subcooling heat use-side heat use-side heat controller 100