GB2525791A

GB2525791A - Refrigeration-cycle device and method for controlling refrigeration-cycle device

Info

Publication number: GB2525791A
Application number: GB1513809.2A
Authority: GB
Inventors: Kosuke Tanaka
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 2013-01-31
Filing date: 2013-01-31
Publication date: 2015-11-04
Anticipated expiration: 2033-01-31
Also published as: JP6021955B2; JPWO2014118953A1; WO2014118953A1; GB201513809D0; GB2525791B

Abstract

In this refrigeration-cycle device, a compressor, a heat-source-side heat exchanger, a pressure-reducing means, and a load-side heat exchanger are connected in that order, forming a refrigerant circulation circuit. The compressor and the heat-source-side heat exchanger are provided in a heat-source-side unit, the load-side heat exchanger is provided in a load-side unit, and the pressure-reducing means comprises a first pressure-reducing device provided in the heat-source-side unit and a second pressure-reducing device provided in the load-side unit. The first and second pressure-reducing devices are connected in series via first connecting tubing interposed between the heat-source-side unit and the load-side unit. Refrigerant flowing through the channel between the first and second pressure-reducing devices is in a gas-liquid two-phase state, and the amounts by which the first and second pressure-reducing devices reduce pressure are controlled so as to make the amount of refrigerant in some of the channels of the refrigerant circulation circuit, including at least the first connecting tubing, equal to a target refrigerant amount.

Description

DESCRIPTION

Title of Invention

REFRIGERATION CYCLE APPARATUS AND REFRIGERATION CYCLE

APPARATUS CONTROL METHOD

Technical Field

[0001] The present invention relates to a refrigeration cycle apparatus and a refrigeration cycle apparatus control method.

Background Art

[0002] In a conventional refrigeration cycle apparatus, a pressure reducing device (e.g., an expansion valve etc.) is installed in any one of a heat source-side unit and a load-side unit. In this refrigeration cycle apparatus, a refrigerant circuit is formed by connecting the pressure reducing device installed in the heat source-side unit and a load-side heat exchanger installed in the load-side unit, or, a heat source-side heat exchanger installed in the heat source-side unit and the pressure reducing device installed in the load-side unit, via a connecting pipe (hereinafter referred to as a first connecting pipe), and connecting a compressor installed in the heat source-side unit and the load-side heat exchanger installed in the load-side unit via a connecting pipe (hereinafter referred to as a second connecting pipe).

[0003] In such a refrigeration cycle apparatus, the high-pressure-side pressure and the low-pressure-side pressure change due to changes in the environmental conditions (e.g., the temperature of the medium that exchanges heat with the refrigerant at the heat source-side heat exchanger, the temperature of the medium that exchanges heat with the refrigerant at the load-side heat exchanger, etc.), the operating conditions (e.g., the operating capacity of the compressor etc.), and the like, with the result that a necessary refrigerant amount for the refrigerant circuit fluctuates due to changes in the refrigerant density of the first connecting pipe and the second connecting pipe.

[0004] Further, as a conventional refrigeration cycle apparatus, the refrigeration cycle apparatus may further include a flow switching device (e.g., a four-way valve etc.) so that by switching the circulation direction of the refrigerant in the refrigerant circuit with the flow switching device, the refrigeration cycle apparatus switches between a cooling operation in which the heat source-side heat exchanger serves as a condenser and the load-side heat exchanger serves as an evaporator, and a heating operation in which the heat source-side heat exchanger serves as an evaporator and the load-side heat exchanger serves as a condenser.

[0005] In such a refrigeration cycle apparatus, a difference between the necessary refrigerant amount during a cooling operation and the necessary refrigerant amount during a heating operation is produced due to changes in the necessary refrigerant amount of the first connecting pipe and the necessary refrigerant amount of the second connecting pipe caused by a change in the circulation direction of the refrigerant.

[0006] When a pressure reducing device is installed in the heat source-side unit, during a cooling operation, the refrigerant of the first connecting pipe is in a gas-liquid two-phase state, and, the refrigerant of the second connecting pipe is in a gaseous state, and during a heating operation, the refrigerant of the first connecting pipe is in a liquid state, and, the refrigerant of the second connecting pipe is in a gaseous state.

Because the necessary refrigerant amount is larger for refrigerant in a liquid state than for refrigerant in a gas-liquid two-phase state, the necessary refrigerant amount for a heating operation is larger than the necessary refrigerant amount for a cooling operation.

[0007] When a pressure reducing device is installed in the load-side unit, during a cooling operation, the refrigerant of the first connecting pipe is in a liquid state, and, the refrigerant of the second connecting pipe is in a gaseous state, and during a heating operation, the refrigerant of the first connecting pipe is in a gas-liquid two-phase state, and, the refrigerant of the second connecting pipe is in a gaseous state.

Because the necessary refrigerant amount is larger for refrigerant in a liquid state than for refrigerant in a gas-liquid two-phase state, the necessary refrigerant amount for a cooling operation is larger than the necessary refrigerant amount for a heating operation.

[0008] Further, because the refrigerant density at the condenser is larger than the refrigerant density at the evaporator, a difference between the necessary refrigerant amount during a cooling operation and the necessary refrigerant amount during a heating operation is also produced due to a difference between the internal volume of the heat source-side heat exchanger and the internal volume of the load-side heat exchanger.

[0009] When the internal volume of the heat source-side heat exchanger is larger than the internal volume of the load-side heat exchanger, the necessary refrigerant amount for a cooling operation, in which the heat source-side heat exchanger having the larger internal volume serves as a condenser in which the refrigerant density increases, and the load-side heat exchanger having the smaller internal volume serves as an evaporator in which the refrigerant density decreases, is larger than the necessary refrigerant amount for a heating operation, in which the heat source-side heat exchanger having a larger internal volume serves as an evaporator in which the refrigerant density decreases, and the load-side heat exchanger having a smaller internal volume serves as a condenser in which the refrigerant density increases.

[0010] When the internal volume of the heat source-side heat exchanger is smaller than the internal volume of the load-side heat exchanger, the necessary refrigerant amount for a heating operation, in which the heat source-side heat exchanger having a smaller internal volume serves as an evaporator in which the refrigerant density decreases, and the load-side heat exchanger having a larger internal volume serves as a condenser in which the refrigerant density increases, is larger than the necessary refrigerant amount for a cooling operation, in which the heat source-side heat exchanger having the smaller internal volume serves as a condenser in which the refrigerant density increases, and the load-side heat exchanger having the larger internal volume serves as an evaporator in which the refrigerant density decreases.

[0011] Consequently, in a conventional refrigeration cycle apparatus, a refrigerant reservoir, such as an accumulator (so-called ACC) or a receiver (so-called REC), is installed in the refrigerant circuit in order to store excess refrigerant produced due to the above-mentioned change in the necessary refrigerant amount caused by a change in environmental conditions, operating conditions, and the like, excess refrigerant produced due to the above-mentioned difference between the necessary refrigerant amount during a cooling operation and the necessary refrigerant amount during a heating operation, and the like (e.g., see Patent Literature 1).

Citation List Patent Literature [0012] Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2012-229893 (paragraphs [0095] to [0100], Fig. 1, Fig. 6, and Fig. 7)

Summary of Invention

Technical Problem [0013] In such a refrigeration cycle apparatus, there is a problem in that there is a large amount of excess refrigerant and hence the increased size of the refrigerant reservoir is required, which causes the cost and the size of the refrigeration cycle apparatus to increase. In particular, when a pipe length of the first connecting pipe and the second connecting pipe is long, there is a problem in that the amount of excess refrigerant is increased even further, which causes a further increase in the cost and size of the refrigeration cycle apparatus.

[0014] The present invention has been created in view of such problems in the related art. It is an object of the present invention to provide a refrigeration cycle apparatus in which increases in cost and size are suppressed. Further, it is an object of the present invention to provide a method of controlling a refrigeration cycle apparatus in which increases in cost and size are suppressed.

Solution to Problem [0015] According to one embodiment of the present invention, there is provided a refrigeration cycle apparatus, A refrigeration cycle apparatus, comprising: a compressor; a heat source-side heat exchanger; a pressure reducing device; and a load-side heat exchanger, the compressor, the heat source-side heat exchanger, the pressure reducing device, and the load-side heat exchanger being connected serially to form a refrigerant circuit, the compressor and the heat source-side heat exchanger being installed in a heat source-side unit, the load-side heat exchanger being installed in a load-side unit, the pressure reducing device including a first pressure reducing device installed in the heat source-side unit and a second pressure reducing device installed in the load-side unit, the first pressure reducing device and the second pressure reducing device being connected in series via a first connecting pipe arranged between the heat source-side unit and the load-side unit, the refrigeration cycle apparatus being configured to control a pressure reducing amount of the first pressure reducing device and a pressure reducing amount of the second pressure reducing device so that refrigerant flowing through a passage between the first pressure reducing device and the second pressure reducing device is in a gas-liquid two-phase state, and, so that a refrigerant amount in a passage of a part of the refrigerant circuit including at least the first connecting pipe is at a target refrigerant amount.

Advantageous Effects of Invention [0016] In the refrigeration cycle apparatus according to the one embodiment of the present invention, the pressure reducing device includes the first pressure reducing device installed in the heat source-side unit and the second pressure reducing device installed in the load-side unit. The first pressure reducing device and the second pressure reducing device are connected in series via the first connecting pipe arranged between the heat source-side unit and the load-side unit. The refrigeration cycle apparatus is configured to control the pressure reducing amount of the first pressure reducing device and the pressure reducing amount of the second pressure reducing device so that the refrigerant flowing through the passage between the first pressure reducing device and the second pressure reducing device is in a gas-liquid two-phase state, and, so that the refrigerant amount in the passage of a part of the refrigerant circuit including at least the first connecting pipe is at a target refrigerant amount. As a result, the amount of excess refrigerant is reduced, and increases in cost and size are suppressed.

Brief Description of Drawings

[0017] [Fig. 1] Fig. 1 is a diagram illustrating a configuration of an air-conditioning apparatus according to Embodiment 1 of the present invention.

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

[Fig. 3] Fig. 3 is a diagram illustrating a control flow according to Control Method 1 of the air-conditioning apparatus according to Embodiment 1 of the present invention.

[Fig. 4] Fig. 4 is a diagram illustrating a control flow according to Control Method 2 of the air-conditioning apparatus according to Embodiment 1 of the present invention.

[Fig. 5] Fig. 5 is a diagram showing a method of obtaining quality with the air-conditioning apparatus according to Embodiment 1 of the present invention.

[Fig. 6] Fig. 6 is a diagram illustrating a configuration of a modified example of the air-conditioning apparatus according to Embodiment 1 of the present invention.

[Fig. 7] Fig. 7 is a diagram illustrating a configuration of an air-conditioning apparatus according to Embodiment 2 of the present invention.

[Fig. 8] Fig. 8 is a diagram showing a relationship among an internal volume of an outdoor-side heat exchanger, an internal volume of an indoor-side heat exchanger, and a total of a necessary refrigerant amount of the outdoor-side heat exchanger and a necessary refrigerant amount of the indoor-side heat exchanger.

[Fig. 9] Fig. 9 is a diagram illustrating a control flow according to Control Method 1 of an air-conditioning apparatus according to Embodiment 3 of the present invention.

[Fig. 10] Fig. 10 is a diagram illustrating a control flow according to Control Method 2 of the air-conditioning apparatus according to Embodiment 3 of the present invention.

Description of Embodiments

[0018] A refrigeration cycle apparatus according to the present invention is now described with reference to the drawings. The refrigeration cycle apparatus according to the present invention performs a cooling operation for cooling a thermal conditioning target and a heating operation for heating a thermal conditioning target by circulating refrigerant of a refrigerant circuit to form a refrigeration cycle (heat pump cycle). Note that, in the following description, a case is described in which the refrigeration cycle apparatus according to the present invention is an air-conditioning apparatus, but the present invention is not limited to such a case. The refrigeration cycle apparatus according to the present invention may be some other refrigeration cycle apparatus forming a refrigeration cycle. Further, the parts, operations, and the like described below are just examples, and the present invention is not limited to these examples. Further, members or portions in the drawings that are the same or similar are denoted by the same reference symbols. In addition, some of the finer details of the parts have been simplified or omitted as appropriate. Further, some overlapping or analogous descriptions have been simplified or omitted as appropriate.

[0019] Embodiment 1 An air-conditioning apparatus according to Embodiment 1 of the present invention is described.

<Air-conditioning Apparatus Configuration> The configuration of the air-conditioning apparatus according to Embodiment 1 is now described below.

Fig. 1 is a diagram illustrating the configuration of the air-conditioning apparatus according to Embodiment 1 of the present invention. As illustrated in Fig. 1, an air-conditioning apparatus 1 includes an outdoor unit 11 and an indoor unit 21.

The outdoor unit 11 corresponds to a "heat source-side unit" in the present invention.

The indoor unit 21 corresponds to a "load-side unit" in the present invention.

[0020] The outdoor unit 11 includes a compressor 12, a four-way valve 13, an outdoor-side heat exchanger 14, an outdoor-side fan 15, a first expansion valve 16, and an accumulator 17. The indoor unit 21 includes an indoor-side heat exchanger 22, an indoor-side fan 23, and a second expansion valve 24. The first expansion valve 16 corresponds to a "first pressure reducing device" in the present invention. The second expansion valve 24 corresponds to a "second pressure reducing device" in the present invention.

[0021] The first expansion valve 16 of the outdoor unit 11 and the second expansion valve 24 of the indoor unit 21 are connected to one another via a first connecting pipe 31. The four-way valve 13 of the outdoor unit 11 and the indoor-side heat exchanger 22 of the indoor unit 21 are connected to one another via a second connecting pipe 32. A refrigerant circuit is formed by the compressor 12, the four-way valve 13, the outdoor-side heat exchanger 14, the first expansion valve 16, the first connecting pipe 31, the second expansion valve 24, the indoor-side heat exchanger 22, the second connecting pipe 32, and the accumulator 17.

[0022] The drive frequency of the compressor 12 is controlled by a controller 41.

Further, the air supply amount of the outdoor-side fan 15 and the air supply amount of the indoor-side fan 23 are controlled by the controller 41. In addition, the opening degree of the first expansion valve 16 and the opening degree of the second expansion valve 24 are controlled by the controller 41. Still further, the passages of the four-way valve 13 are controlled by the controller 41. Note that, the controller 41 may be installed in the outdoor unit 11, the indoor unit 21, or some other location.

[0023] A first pressure sensor 51 and a second pressure sensor 52 are connected to the controller 41. The first pressure sensor 51 is configured to detect the pressure of the refrigerant discharged from the compressor 12. The second pressure sensor 52 is configured to detect the pressure of the refrigerant to be sucked into the compressor 12.

[0024] Further, a first temperature sensor 61, a second temperature sensor 62, a third temperature sensor 63, a fourth temperature sensor 64, a fifth temperature sensor 65, and a sixth temperature sensor 66 are connected to the controller 41. The first temperature sensor 61 is configured to detect the temperature of the refrigerant discharged from the compressor 12. The second temperature sensor 62 is configured to detect the temperature of the refrigerant flowing between the outdoor-side heat exchanger 14 and the first expansion valve 16. The third temperature sensor 63 is configured to detect the temperature of the refrigerant flowing between the first expansion valve 16 and the second expansion valve 24. The fourth temperature sensor 64 is configured to detect the temperature of the refrigerant flowing between the second expansion valve 24 and the indoor-side heat exchanger 22. The fifth temperature sensor 65 is configured to detect the temperature of the refrigerant flowing between the indoor-side heat exchanger 22 and the four-way valve 13. The sixth temperature sensor 66 is configured to detect the temperature of the refrigerant to be sucked into the compressor 12. Note that, in Fig. 1, a case is illustrated in which the third temperature sensor 63 is installed in the outdoor unit 11, but the third temperature sensor 63 may be installed in the indoor unit 21 or in the first connecting pipe 31.

[0025] <Air-conditioning Apparatus Operations> The operations performed by the air-conditioning apparatus according to Embodiment 1 are described below.

(Operations during a Cooling Operation) The operations performed by the air-conditioning apparatus 1 during a cooling operation are now described.

The controller 41 switches the passages of the four-way valve 13 so that the refrigerant discharged from the compressor 12 is guided to the outdoor-side heat exchanger 14 and the refrigerant from the indoor-side heat exchanger 22 is guided to the suction side of the compressor 12. Note that, the passages of the four-way valve 13 during a cooling operation are indicated by the solid lines in Fig. 1.

[0026] High-temperature, high-pressure gaseous refrigerant discharged from the compressor 12 passes through the four-way valve 13 and flows into the outdoor-side heat exchanger 14. The high-temperature, high-pressure gaseous refrigerant exchanges heat with a medium, such as outside air, supplied to the outdoor-side heat exchanger 14 by the outdoor-side fan 15, thereby condensing into high-pressure liquid refrigerant. The high-pressure liquid refrigerant passes through the first expansion valve 16, the first connecting pipe 31, and the second expansion valve 24, thereby turning into low-pressure, gas-liquid two-phase state refrigerant, which flows into the indoor-side heat exchanger 22. The low-pressure, gas-liquid two-phase state refrigerant exchanges heat with a medium, such as indoor air, supplied to the indoor-side heat exchanger 22 by the indoor-side fan 23, thereby evaporating into low-pressure gaseous refrigerant. The low-pressure gaseous refrigerant passes through the second connecting pipe 32 and the four-way valve 13, flows into the accumulator 17, and is then sucked back into the compressor 12. In other words, during a cooling operation, the outdoor-side heat exchanger 14 serves as a condenser, and the indoor-side heat exchanger 22 serves as an evaporator. Note that, the circulation direction of the refrigerant during a cooling operation is indicated by the solid arrows in Fig. 1.

[0027] (Operations during a Heating Operation) The operations performed by the air-conditioning apparatus 1 during a heating operation are now described.

The controller 41 switches the passages of the four-way valve 13 so that the refrigerant discharged from the compressor 12 is guided to the indoor-side heat exchanger 22 and the refrigerant from the outdoor-side heat exchanger 14 is guided to the suction side of the compressor 12. Note that, the passages of the four-way valve 13 during a heating operation are indicated by the dotted lines in Fig. 1.

[0028] High-temperature, high-pressure gaseous refrigerant discharged from the compressor 12 passes through the four-way valve 13 and the second connecting pipe 32 and flows into the indoor-side heat exchanger 22. The high-temperature, high-pressure gaseous refrigerant exchanges heat with a medium, such as indoor air, supplied to the indoor-side heat exchanger 22 by the indoor-side fan 23, thereby condensing into high-pressure liquid refrigerant. The high-pressure liquid refrigerant passes through the second expansion valve 24, the first connecting pipe 31, and the first expansion valve 16, thereby turning into low-pressure, gas-liquid two-phase state refrigerant, which flows into the outdoor-side heat exchanger 14. The low-pressure, gas-liquid two-phase state refrigerant exchanges heat with a medium, such as outdoor air, supplied to the outdoor-side heat exchanger 14 by the outdoor-side fan 15, thereby evaporating into low-pressure gaseous refrigerant. The low-pressure gaseous refrigerant passes through the four-way valve 13, flows into the accumulator 17, and is then sucked back into the compressor 12. In other words, during a heating operation, the indoor-side heat exchanger 22 serves as an evaporator, and the indoor-side heat exchanger 22 serves as a condenser. Note that, the circulation direction of the refrigerant during a heating operation is indicated by the dotted arrows in Fig. 1.

[0029] <Controller Operations> Fig. 2 is a P-H diagram of the air-conditioning apparatus according to Embodiment 1 of the present invention. In Fig. 2, point A corresponds to a compressor suction side, point B corresponds to a compressor discharge side, point C corresponds to an inlet side of the expansion valve positioned upstream of other (hereinafter referred to as an upstream-side expansion valve a) from among the first expansion valve 16 and the second expansion valve 24, and point D corresponds to an outlet side of the expansion valve positioned downstream of other (hereinafter referred to a downstream-side expansion valve b) from among the first expansion valve 16 and the second expansion valve 24. The first connecting pipe 31 corresponds to point E during a cooling operation and during a heating operation.

Further, the second connecting pipe 32 corresponds to point A during a cooling operation and to point B during a heating operation.

[0030] As shown in Fig. 2, the air-conditioning apparatus 1 is configured to control so that, during a cooling operation and during a heating operation, the refrigerant between the first expansion valve 16 and the second expansion valve 24 is in a gas-liquid two-phase state. Further, the air-conditioning apparatus 1 is configured to control so that, during a cooling operation and during a heating operation, the total of the refrigerant amount of the first connecting pipe 31 and the refrigerant amount of the second connecting pipe 32 is at a target refrigerant amount. The control performed so that the refrigerant between the first expansion valve 16 and the second expansion valve 24 is in a gas-liquid two-phase state, and the control performed so that the total of the refrigerant amount of the first connecting pipe 31 and the refrigerant amount of the second connecting pipe 32 is at a target refrigerant amount are both carried out by controlling the opening degrees of the first expansion valve 16 and the second expansion valve 24 with the controller 41.

[0031] The following two control methods are described as specific examples. Note that, in the following description, a case is described in which the first connecting pipe 31 and the second connecting pipe 32 have the same pipe length, namely, a case in which setting the total of the refrigerant amount of the first connecting pipe 31 and the refrigerant amount of the second connecting pipe 32 at a target refrigerant amount has the same meaning as setting the total of the refrigerant amount per pipe unit length of the first connecting pipe 31 and the refrigerant amount per pipe unit length of the second connecting pipe 32 at a target refrigerant amount per pipe unit length.

[0032] Note that, when the first connecting pipe 31 and the second connecting pipe 32 have different pipe lengths, the total of the refrigerant amounts each obtained by multiplying the refrigerant amount per pipe unit length by the pipe length may be set at the target refrigerant amount. Further, when the first connecting pipe 31 and the second connecting pipe 32 have the same pipe length, and, have the same cross-sectional area, the total of the refrigerant density of the first connecting pipe 31 and the refrigerant density of the second connecting pipe 32 may be set at a target refrigerant density In addition, when the first connecting pipe 31 and the second connecting pipe 32 have different pipe lengths, but have the same cross-sectional area, the total of the refrigerant amounts per pipe unit cross-sectional area may be set at a target refrigerant amount per pipe unit cross-sectional area.

[0033] (Control Method 1) The controller 41 sets and changes the drive frequency of the compressor 12 based on the air-conditioning load, namely, so that the indoor unit 21 is capable of exhibiting a target performance. Further, the controller 41 sets and changes the air supply amount of the outdoor-side fan 15 so that during a cooling operation the condensing temperature is at a target condensing temperature, and, during a heating operation the evaporating temperature is at a target evaporating temperature. Note that, the condensing temperature is obtained by, for example, converting a detected pressure Pd of the first pressure sensor 51 into a saturated temperature, and the evaporating temperature is obtained by, for example, converting a detected pressure P5 of the second pressure sensor 52 into a saturated temperature. Further the controller 41 sets and changes the air supply amount of the indoor-side fan 23 based on a setting set by the user.

[0034] The controller 41 sets and changes the opening degree of the upstream-side expansion valve a so that a degree of subcooling SC is at a target degree of subcooling SCM. Note that, the degree of subcooling SC is obtained by, during a cooling operation, for example, obtaining a difference between a temperature obtained by converting the detected pressure Pd of the first pressure sensor 51 into a saturated temperature and a detected temperature TH2 being a temperature detected by the second temperature sensor 62, or, during a heating operation, for example, obtaining a difference between a temperature obtained by converting the detected pressure Pd of the first pressure sensor 51 into a saturated temperature and a detected temperature TH4 being a temperature detected by the fourth temperature sensor 64. The target degree of subcooling SCm is one fixed value set in advance.

As the target degree of subcooling SCm, two fixed values may be set, and the controller 41 may perform control so that the degree of subcooling SC is between these two fixed values.

[0035] The controller 41 controls the opening degree of the downstream-side expansion valve b so that a total of the refrigerant amount per pipe unit length of the first connecting pipe 31 and the refrigerant amount per pipe unit length of the second connecting pipe 32 (hereinafter referred to as a total refrigerant amount Mp per unit length) is at a target total refrigerant amount Mpm per unit length. The target total refrigerant amount Mpm per unit length is one fixed value set in advance. As the target total refrigerant amount Mp per unit length, two fixed values may be set, and the controller 41 may perform control so that the total refrigerant amount Mp per unit length is between these two fixed values.

[0036] The total refrigerant amount Mp per unit length is obtained based on Expression (1) using a cross-sectional area Si [m2] of the first connecting pipe 31, a cross-sectional area S2 [m2] of the second connecting pipe 32, refrigerant density p pi [kg/rn3] at the first connecting pipe 31, and refrigerant density p p2 [kg/rn3] at the second connecting pipe 32.

[0037] [Math. 1] (1) [0035] The refrigerant density p p1 at the first connecting pipe 31 is obtained based on, during a cooling operation, for exarnple, a detected temperature THa being a temperature detected by the third temperature sensor 63 and the enthalpy converted frorn the detected temperature TH2 of the second temperature sensor 62, and during a heating operation, the detected temperature THa of the third temperature sensor 63 and the enthalpy converted from the detected temperature TH4 of the fourth temperature sensor 64. Note that, the method of obtaining the refrigerant density p pi at the first connecting pipe 31 is described in detail later.

[0039] The refrigerant density p p2 at the second connecting pipe 32 may be converted from, during a cooling operation, for example, the detected pressure P5 of the second pressure sensor 52, or converted from the detected pressure P of the second pressure sensor 52 and a detected tern perature TH6 being a temperature detected by the sixth temperature sensor 66. When converting the refrigerant density p p at the second connecting pipe 32 from the detected pressure P of the second pressure sensor 52 and the detected temperature TH6 of the sixth temperature sensor 66, a degree of superheat SH is reflected in the result, which improves the computation accuracy of the refrigerant density p p2 at the second connecting pipe 32. During a heating operation, the refrigerant density p p2 at the second connecting pipe 32 is converted from the detected pressure Pd of the first pressure sensor 51 and the detected temperature THi of the first temperature sensor 61.

[0040] Fig. 3 is a diagram illustrating a control flow according to Control Method 1 of the air-conditioning apparatus according to Embodiment 1 of the present invention.

As illustrated in Fig. 3, in Step SlOl, the controller 41 determines whether or nota cooling operation or a heating operation is being carried out. When it is determined that a cooling operation is being carried out, the processing proceeds to Step S 102, and when it is determined that a heating operation is being carried out, the processing proceeds to Step S106.

[0041] In Step S102, the controller 41 obtains the degree of subcooling SC, and the processing then proceeds to Step S103. In Step Si 03, the controller 41 obtains the total refrigerant amount Mp per unit length, and the processing proceeds to Step S 104. In Step S104, the controller 41 compares the degree of subcooling SC with the target degree of subcooling SCm. When it is determined that the degree of subcooling SC is larger than the target degree of subcooling SCm, the controller 41 increases the opening degree of the first expansion valve 16. When it is determined that the degree of subcooling SC is smaller than the target degree of subcooling SCm, the controller 41 decreases the opening degree of the first expansion valve 16.

Then, the processing proceeds to Step S105. The change amount of the opening degree may be determined based on how large the difference is between the degree of subcooling SC and the target degree of subcooling SCm. In Step SlOb, the controller 41 compares the total refrigerant amount Mp per unit length with the target total refrigerant amount Mpm per unit length. When the total refrigerant amount Mp per unit length is larger than the target total refrigerant amount Mp per unit length, the controller 41 increases the opening degree of the second expansion valve 24.

When the total refrigerant amount Mp per unit length is smaller than the target total refrigerant amount Mp per unit length, the controller 41 decreases the opening degree of the second expansion valve 24. The change amount of the opening degree may be determined based on how large the difference is between the total refrigerant amount Mp per unit length and the target total refrigerant amount Mpm per unit length.

[0042] In Step S106, the controller 41 obtains the degree of subcooling SC, and the processing then proceeds to Step S107. In Step Si 07, the controller 41 obtains the total refrigerant amount Mp per unit length, and the processing proceeds to Step S 108. In Step 5108, the controller 41 compares the degree of subcooling SC with the target degree of subcooling SCm. When it is determined that the degree of subcooling SC is larger than the target degree of subcooling SC, the controller 41 increases the opening degree of the second expansion valve 24. When it is determined that the degree of subcooling SC is smaller than the target degree of subcooling SCm, the controller 41 decreases the opening degree of the second expansion valve 24. Then, the processing proceeds to Step S109. The change amount of the opening degree may be determined based on how large the difference is between the degree of subcooling SC and the target degree of subcooling SC.

In Step S109, the controller 41 compares the total refrigerant amount Mp per unit length with the target total refrigerant amount Mpm per unit length. When the total refrigerant amount Mp per unit length is larger than the target total refrigerant amount Mp per unit length, the controller 41 increases the opening degree of the first expansion valve 16. When the total refrigerant amount Mp per unit length is smaller than the target total refrigerant amount Mpm per unit length, the controller 41 decreases the opening degree of the first expansion valve 16. The change amount of the opening degree may be determined based on how large the difference is between the total refrigerant amount Mp per unit length and the target total refrigerant amount Mpm per unit length.

[0043] (Control Method 2) The controller 41 sets and changes the drive frequency of the compressor 12 based on the air-conditioning load, namely, so that the indoor unit 21 is capable of exhibiting a target performance. Further, the controller 41 sets and changes the air supply amount of the outdoor-side fan 15 so that during a cooling operation the condensing temperature is at a target condensing temperature, and, during a heating operation the evaporating temperature is at a target evaporating temperature.

Further, the controller 41 sets and changes the air supply amount of the indoor-side fan 23 based on a setting set by the user.

[0044] The controller 41 sets and changes the opening degree of the downstream-side expansion valve b so that a degree of superheat SH is at a target degree of superheat SHm. Note that, the degree of superheat SH is obtained by, during a cooling operation, for example, obtaining a difference between a detected temperature TH5 being a temperature detected by the fifth temperature sensor 65 and a temperature obtained by converting the detected pressure P of the second pressure sensor 52 into a saturated temperature, or, during a heating operation, for example, obtaining a difference between a detected temperature TH6 being a temperature detected by the sixth temperature sensor 66 and a temperature obtained by converting the detected pressure P of the second pressure sensor 52 into a saturated temperature. The target degree of superheat SHm is one fixed value set in advance. As the target degree of superheat SHm, two fixed values may be set, and the controller 41 may perform control so that the degree of superheat SH is between these two fixed values.

[0045] The controller 41 controls the opening degree of the upstream-side expansion valve a so that the total refrigerant amount Mp per unit length is at the target total refrigerant amount Mp per unit length. The total refrigerant amount Mp per unit length is obtained in the same manner as Control Method 1.

[0046] Fig. 4 is a diagram illustrating a control flow according to Control Method 2 of the air-conditioning apparatus according to Embodiment 1 of the present invention.

As illustrated in Fig. 4, in Step S201, the controller 41 determines whether a cooling operation or a heating operation is being carried out. When it is determined that a cooling operation is being carried out, the processing proceeds to Step 5202, and when it is determined that a heating operation is being carried out, the processing proceeds to Step S206.

[0047] In Step S202, the controller 41 obtains the degree of superheat SH, and the processing then proceeds to Step S203. In Step S203, the controller 41 obtains the total refrigerant amount Mp per unit length, and the processing proceeds to Step S204. In Step S204, the controller 41 compares the degree of superheat SH with the target degree of superheat SHm. When it is determined that the degree of superheat SH is larger than the target degree of superheat SH,11, the controller 41 increases the opening degree of the second expansion valve 24. When it is determined that the degree of superheat SH is smaller than the target degree of superheat SHm, the controller 41 decreases the opening degree of the second expansion valve 24. Then, the processing proceeds to Step 5205. The change amount of the opening degree may be determined based on how large the difference is between the degree of superheat SH and the target degree of superheat SHm. In Step S205, the controller 41 compares the total refrigerant amount Mp per unit length with the target total refrigerant amount Mpm per unit length. When the total refrigerant amount Mp per unit length is larger than the target total refrigerant amount Mpm per unit length, the controller 41 decreases the opening degree of the first expansion valve 16. When the total refrigerant amount Mp per unit length is smaller than the target total refrigerant amount Mp per unit length, the controller 41 increases the opening degree of the first expansion valve 16. The change amount of the opening degree may be determined based on how large the difference is between the total refrigerant amount Mp per unit length and the target total refrigerant amount Mp per unit length.

[0048] In Step S206, the controller 41 obtains the degree of superheat SH, and the processing then proceeds to Step S207. In Step S207, the controller 41 obtains the total refrigerant amount Mp per unit length, and the processing proceeds to Step S208. In Step S208, the controller 41 compares the degree of superheat SH with the target degree of superheat SHm. When it is determined that the degree of superheat SH is larger than the target degree of superheat SH, the controller 41 increases the opening degree of the first expansion valve 16. When it is determined that the degree of superheat SH is smaller than the target degree of superheat SHm, the controller 41 decreases the opening degree of the first expansion valve 16.

Then, the processing proceeds to Step S209. The change amount of the opening degree may be determined based on how large the difference is between the degree of superheat SH and the target degree of superheat SHm. In Step 5209, the controller 41 compares the total refrigerant amount Mp per unit length with the target total refrigerant amount Mpm per unit length. When the total refrigerant amount Mp per unit length is larger than the target total refrigerant amount Mp per unit length, the controller 41 decreases the opening degree of the second expansion valve 24.

When the total refrigerant amount Mp per unit length is smaller than the target total refrigerant amount Mpm per unit length, the controller 41 increases the opening degree of the second expansion valve 24. The change amount of the opening degree may be determined based on how large the difference is between the total refrigerant amount Mp per unit length and the target total refrigerant amount Mpm per unit length.

[0049] (Method of Calculating Refrigerant Density at the First Connecting Pipe) The refrigerant density p pi at the first connecting pipe 31 is obtained using Expression (2) based on a saturated gas density pgi [kg/m3], a saturated liquid density pH [kg/rn3], and a void fraction fi (surface area ratio of included air bubbles per unit cross-sectional area of the fluid) of the refrigerant of the first connecting pipe 31.

[0050] [Math. 2] pp1=f1xpg1+(l-f3xpl1 (2) [0051] The saturated gas density pgi and the saturated liquid density ph are converted from, for example, the detected tern perature THa of the third tern perature sensor 63. The void fraction fi is obtained using Expression (3) based on a quality xi, the saturated gas density pgi and the saturated liquid density ph, of the refrigerant of the first connecting pipe 31. Note that, in Expression (3), the computation accuracy of the void fraction fi improves if e is set at 0.4.

[0052] [Math. 3] ( / 1/2 l'2 li( i_i = [iiJe+[' [x__ x, ) pi, p/I X1 Ph) X1 1+11 -1k (0 0/ [0053] Fig. 5 is a diagram showing a method of obtaining quality with the air-conditioning apparatus according to Embodiment 1 of the present invention. The quality xi is, for example, obtained using Expression (4) based on an enthalpy Hi [kJ/kg], a saturated liquid enthalpy Hsli [kJ/kg], and a saturated gas enthalpy Hsgi [kJ/kg] shown in Fig. 5. Note that, the enthalpy Hi is converted from, for example, during a cooling operation, the detected temperature TH2 of the second temperature sensor 62, and during a heating operation, the detected temperature TH4 of the fourth temperature sensor 64. The saturated liquid enthalpy Hsli and the saturated gas enthalpy Hsgi are converted from, for example, the detected temperature TH3 of the third temperature sensor 63. Note that, instead of the detected temperature TH3 of the third temperature sensor 63, a pressure sensor may be installed, and the saturated liquid enthalpy Hsli and the saturated gas enthalpy Hsgi may also be converted from the pressure detected by that pressure sensor.

[0054] [Math. 4] H1 -Hs11 (j) Ticg1 -Hg!1 [0055] Note that, in the air-conditioning apparatus 1, the refrigerant density p pi at the first connecting pipe 31 and the refrigerant density p p at the second connecting pipe 32 are obtained based on the detected temperature, detected pressure, and the like of the respective sensors arranged in the outdoor unit 11 or in the indoor unit 21.

However, when a difference occurs between the actual refrigerant density of the first connecting pipe 31 or the second connecting pipe 32 and the refrigerant density obtained based on the detected temperature, detected pressure, and the like of the respective sensors arranged in the outdoor unit 11 or in the indoor unit 21 due to the first connecting pipe 31 and the second connecting pipe 32 having a large pressure loss, the refrigerant density obtained based on the detected temperature, detected pressure, and the like of the respective sensors arranged in the outdoor unit 11 or in the indoor unit 21 may preferably be corrected using, for example, information on the pipe length of the first connecting pipe 31 and the second connecting pipe 32 to reflect the pressure loss. Further, for example, the refrigerant density may be obtained at a plurality of locations by adding a temperature sensor between the first connecting pipe 31 and the second expansion valve 24, and an average value of the obtained refrigerant densities may be used for control.

[0056] <Air-conditioning Apparatus Effects> The effects of the air-conditioning apparatus according to Embodiment 1 are now described.

The air-conditioning apparatus 1 performs control so that the refrigerant between the first expansion valve 16 and the second expansion valve 24 is in a gas-liquid two-phase state, and the total of the refrigerant amount of the first connecting pipe 31 and the refrigerant amount of the second connecting pipe 32 is at a target refrigerant amount (e.g., is constant). As a result, fluctuations in the necessary refrigerant amount for the refrigerant circuit caused by changes in the high-pressure-side pressure and the low-pressure-side pressure due to changes in the environmental conditions, the operating conditions, and the like are suppressed, with the result that the size of the refrigerant reservoir may be reduced.

[0057] Further, in the air-conditioning apparatus 1, the control performed so that the refrigerant between the first expansion valve 16 and the second expansion valve 24 is in a gas-liquid two-phase state, and so that the total of the refrigerant amount of the first connecting pipe 31 and the refrigerant amount of the second connecting pipe 32 is at a target refrigerant amount is carried out using a common (e.g., the same) target refrigerant amount during a cooling operation and during a heating operation. As a result, the occurrence of a difference in the necessary refrigerant amounts caused by a change in the circulation direction of the refrigerant is suppressed, with the result that the size of the refrigerant reservoir may be reduced even more.

[0058] Note that, the air-conditioning apparatus 1 is capable of switching between a cooling operation and a heating operation, but the air-conditioning apparatus 1 may perform only a cooling operation or only a heating operation. Further, when the air-conditioning apparatus 1 is capable of switching between a cooling operation and a heating operation, the air-conditioning apparatus 1 may perform control so that the refrigerant between the first expansion valve 16 and the second expansion valve 24 is in a gas-liquid two-phase state, and so that the total of the refrigerant amount of the first connecting pipe 31 and the refrigerant amount of the second connecting pipe 32 is at a target refrigerant amount by using a target refrigerant amount for only a cooling operation or only a heating operation, or, a target refrigerant amount that is different for a cooling operation and for a heating operation. Even in these cases, fluctuations in the necessary refrigerant amount for the refrigerant circuit caused by changes in the high-pressure-side pressure and the low-pressure-side pressure due to changes in the environmental conditions, the operating conditions, and the like are suppressed, with the result that the size of the refrigerant reservoir may be reduced.

[0059] Further, although the air-conditioning apparatus 1 performs control so that the total of the refrigerant amount of the first connecting pipe 31 and the refrigerant amount of the second connecting pipe 32 is at a target refrigerant amount, the air-conditioning apparatus 1 may also control so that only the refrigerant amount of the first connecting pipe 31 is at a target refrigerant amount. Even in such a case, fluctuations in the necessary refrigerant amount for the refrigerant circuit caused by changes in the high-pressure-side pressure or the low-pressure-side pressure due to changes in the environmental conditions, the operating conditions, and the like are suppressed, with the result that the size of the refrigerant reservoir may be reduced.

Further, when the air-conditioning apparatus 1 is configured to switch between a cooling operation and a heating operation, the refrigerant of the first connecting pipe 31 no longer changes into a gas-liquid two-phase state and a liquid phase state due to changes of the refrigerant in the circulation direction, and the occurrence of a difference in the necessary refrigerant amounts is efficiently suppressed, with the result that the size of the refrigerant reservoir may be reduced.

[0060] All of the cases described above have the additional effects of reducing cost and improving environmental performance. In other words, because the refrigerant of the first connecting pipe 31 does not turn into a liquid phase state, the necessary refrigerant amount per se of the refrigerant circuit is reduced, with the results that, for example, the costs of the refrigerant per se are reduced. Further, by reducing the necessary refrigerant amount per se of the refrigerant circuit, harm to the environment and living creatures if refrigerant leaks is reduced. This enables, for example, R32, HFOl234yf, HFOl234ze, propane, and the like to be used as the refrigerant, thereby improving environmental performance evaluation indices such as actual global-warming potential (GWP) (=GWPxrefrigerant amount) and annual energy consumption efficiency (APF).

[0061] Further, there is also the effect that, when a difference in the necessary refrigerant amount, namely, the occurrence of excess refrigerant, is suppressed, the coefficient of performance (COP) of the refrigerant circuit is improved. In other words, when there is a large amount of excess refrigerant, for example, in order to suppress an excessive increase in the high-pressure-side pressure due to excess refrigerant accumulating in the condenser, a limit may be placed on the refrigeration capacity of the refrigerant circuit by performing control so that the refrigerant at the evaporator outlet is in a gas-liquid two-phase state. However, when the occurrence of excess refrigerant is suppressed, the limit on the refrigeration capacity does not need to be set strictly, or may not even be required, and hence the coefficient of performance (COP) of the refrigerant circuit is improved.

[0062]

Fig. 6 is a diagram illustrating a configuration of a modified example of the air-conditioning apparatus according to Embodiment 1 of the present invention. As illustrated in Fig. 6, the air-conditioning apparatus 1 includes the outdoor unit 11 and a plurality of indoor units 21-1 and 21-2. The first expansion valve 16 of the outdoor unit 11 may be connected via the first connecting pipe 31 to a second expansion valve 24-1 of the indoor unit 21-1 and a second expansion valve 24-2 of the indoor unit 21-2. The four-way valve 13 of the outdoor unit 11 may be connected via the second connecting pipe 32 to an indoor-side heat exchanger 22-1 of the indoor unit 21-1 and an indoor-side heat exchanger 22-2 of the indoor unit 21-2. Note that, in Fig. 6, a case is illustrated in which there are two indoor units 21-1 and 21-2, but the number of indoor units 21 may be three or more.

[0063] Embodiment 2 An air-conditioning apparatus according to Embodiment 2 of the present invention is described.

Note that, descriptions that overlap with or are similar to those in Embodiment 1 have been simplified or omitted as appropriate. Also note that, because the operations of the air-conditioning apparatus 1 and the operations of the controller 41 are the same as in Embodiment 1, a description thereof is omitted here.

<Air-conditioning Apparatus Configuration> The configuration of the air-conditioning apparatus according to Embodiment 2 is now described below. Fig. 7 is a diagram illustrating the configuration of the air-conditioning apparatus according to Embodiment 2 of the present invention. As illustrated in Fig 7, an air-conditioning apparatus 1 does not include the accumulator 17. The internal volume of the outdoor-side heat exchanger 14 is 0.7 to 1.3 times the internal volume of the indoor-side heat exchanger 22. The remaining configurations of the air-conditioning apparatus 1 are the same as those in Embodiment 1, and hence a description thereof is omitted here.

[0064] <Air-conditioning Apparatus Effects> Fig. 8 is a diagram showing a relationship among an internal volume of an outdoor-side heat exchanger, an internal volume of an indoor-side heat exchanger, and a total of a necessary refrigerant amount of the outdoor-side heat exchanger and a necessary refrigerant amount of the indoor-side heat exchanger. As shown in Fig. 8, the total of the necessary refrigerant amount of the outdoor-side heat exchanger 14 and the necessary refrigerant amount of the indoor-side heat exchanger 22 (hereinafter referred to as the heat exchanger total necessary refrigerant amount) increases from, for example, 1 kg to 3 kg as an internal volume (VOC) of the outdoor-side heat exchanger 14 and an internal volume VIC of the indoor-side heat exchanger 22 increase. In Fig. 8, an area F shown by the hatching is an area in which the internal volume VOC of the outdoor-side heat exchanger 14 is 0.7 to 1.3 times the internal volume VIC of the indoor-side heat exchanger 22.

[0065] In the heat exchanger total necessary refrigerant amount, the necessary refrigerant amount of the heat exchanger acting as the condenser is dominant.

Accordingly, when the internal volume VOC of the outdoor-side heat exchanger 14 is larger than the internal volume VIC of the indoor-side heat exchanger 22, the heat exchanger total necessary refrigerant amount increases during a cooling operation in which the outdoor-side heat exchanger 14 serves as the condenser, with the result that excess refrigerant is produced during a heating operation. In other words, the heat exchanger total necessary refrigerant amount in the area beneath the area F is a heat exchanger total necessary refrigerant amount M0 during a cooling operation.

[0066] Further, when the internal volume VOC of the outdoor-side heat exchanger 14 is smaller than the internal volume VIC of the indoor-side heat exchanger 22, the heat exchanger total necessary refrigerant amount increases during a heating operation in which the indoor-side heat exchanger 22 serves as the condenser, with the result that excess refrigerant is produced during a cooling operation. In other words, the heat exchanger total necessary refrigerant amount in the area above the area F is a heat exchanger total necessary refrigerant amount Mh during a heating operation.

[0067] In addition, when the internal volume VOC of the outdoor-side heat exchanger 14 is 0.7 to 1.3 times the internal volume VIC of the indoor-side heat exchanger 22, the difference between the heat exchanger total necessary refrigerant amount M during a cooling operation and the heat exchanger total necessary refrigerant amount Mb during a heating operation is almost eliminated, and hence almost no excess refrigerant is produced. In other words, the heat exchanger total necessary refrigerant amount in the area F is the heat exchanger total necessary refrigerant amount M0 during a cooling operation or the heat exchanger total necessary refrigerant amount Mh during a heating operation.

[0068] Note that, the heat exchanger total necessary refrigerant amount M during a cooling operation and the heat exchanger total necessary refrigerant amount Mh during a heating operation are obtained using Expression (5) and Expression (6) based on the internal volume VOC [m3] of the outdoor-side heat exchanger 14, the internal volume VIC [m3] of the indoor-side heat exchanger 22, refrigerant density c [kg/m3] of the condenser, and refrigerant density Pa [kg/rn3] of the evaporator.

[0069] [Math. 5] M0zVOCxp+VICxp (5) [0070] [Math. 6] M1,=VOCxo+VtCxp (6) [0071] Then, in the air-conditioning apparatus 1, the internal volume VOC of the outdoor-side heat exchanger 14 is 0.7 to 1.3 times the internal volume VIC of the indoor-side heat exchanger 22, and the opening degrees of the first expansion valve 16 and the second expansion valve 24 are controlled so that during a cooling operation and during a heating operation, the total of the refrigerant amount at the first connecting pipe 31 and the refrigerant amount at the second connecting pipe 32 is at a common (e.g., the same) target refrigerant amount. Therefore, the necessary refrigerant amount of the refrigerant circuit, which is approximated by the total of the necessary refrigerant amount at the first connecting pipe 31, the necessary refrigerant amount at the second connecting pipe 32, and the heat exchanger total necessary refrigerant amount, is almost the same during a cooling operation and during a heating operation.

[0072] Note that, a necessary refrigerant amount Mr of the refrigerant circuit during a cooling operation is obtained using Expression (7) based on the heat exchanger total necessary refrigerant amount M0 during a cooling operation, a total refrigerant amount Mp per unit length, and pipe lengths L Em] of the first connecting pipe 31 and the second connecting pipe 32. When the pipe length of the first connecting pipe 31 and the pipe length of the second connecting pipe 32 are different, the MpxL of Expression (7) is substituted with the total of multiplying the pipe length and the pipe cross-sectional area of each connecting pipe by the refrigerant density of each connecting pipe.

[0073] [Math. 7] Mr.1'1.--Mp xL. . (7) [0074] Further, a necessary refrigerant amount Mrh of the refrigerant circuit during a heating operation is obtained using Expression (8) based on the heat exchanger total necessary refrigerant amount Mh during a heating operation, a total refrigerant amount Mp per unit length, and pipe lengths L [ml of the first connecting pipe 31 and the second connecting pipe 32. When the pipe length of the first connecting pipe 31 and the pipe length of the second connecting pipe 32 are different, the MpxL of Expression (8) is substituted with the total of multiplying the pipe length and the pipe cross-sectional area of each connecting pipe by the refrigerant density of each connecting pipe.

[0075] [Math. 8] A14=k1+1Vfp x[ (8) [0076] In other words, in the air-conditioning apparatus 1, because there is almost no difference between the necessary refrigerant amount Mr of the refrigerant circuit during a cooling operation and the necessary refrigerant amount Mrh of the refrigerant circuit during a heating operation, almost no excess refrigerant is produced, with the result that the refrigerant reservoir may be unnecessary, or may be reduced further in size.

[0077] Embodiment 3 An air-conditioning apparatus according to Embodiment 3 of the present invention is described.

Note that, descriptions that overlap with or are similar to those in Embodiment 1 and Embodiment 2 have been simplified or omitted as appropriate. Also note that, because the operations of the air-conditioning apparatus 1 are the same as in Embodiment 1, a description thereof is omitted here.

<Air-conditioning Apparatus Configuration> The configuration of the air-conditioning apparatus according to Embodiment 3 is now described below. An air-conditioning apparatus 1 does not include the accumulator 17. The internal volume of the outdoor-side heat exchanger 14 may not be 0.7 to 1.3 times the internal volume of the indoor-side heat exchanger 22. The remaining configurations of the air-conditioning apparatus 1 are the same as those in Embodiment 1, and hence a description thereof is omitted here.

[0078] <Controller Operations> The air-conditioning apparatus 1 performs control so that, during a cooling operation and during a heating operation, the refrigerant between the first expansion valve 16 and the second expansion valve 24 is in a gas-liquid two-phase state.

Further, the air-conditioning apparatus 1 performs control so that the necessary refrigerant amount of the refrigerant circuit is at a target necessary refrigerant amount. The control performed so that the refrigerant between the first expansion valve 16 and the second expansion valve 24 is in a gas-liquid two-phase state, and the control performed so that the necessary refrigerant amount of the refrigerant circuit is at the necessary refrigerant amount are both carried out by controlling the opening degrees of the first expansion valve 16 and the second expansion valve 24 with the controller 41.

[0079] The following two control methods are described as specific examples. Note that, in the following description, a case is described in which the first connecting pipe 31 and the second connecting pipe 32 have the same pipe length. However, even if the first connecting pipe 31 and the second connecting pipe 32 have different pipe lengths, the total of multiplying the pipe length and the pipe cross-sectional area of each connecting pipe by the refrigerant density of each connecting pipe may be obtained. Further, in the following description, only the parts that are different from Control Method 1 and Control Method 2 of Embodiment 1 are described.

[0080] (Control Method 1) The controller 41 controls the opening degree of the downstream-side expansion valve b so that the necessary refrigerant amount of the refrigerant circuit is at a target necessary refrigerant amount Mrm. The target necessary refrigerant amount Mrm is one fixed value set in advance. As the target necessary refrigerant amount Mr, two fixed values may be set, and the controller 41 may perform control so that the necessary refrigerant amount of the refrigerant circuit is between these two fixed values.

[0081] The necessary refrigerant amount Mr of the refrigerant circuit during a cooling operation is obtained using Expression (7) into which Expression (5) has been substituted. Further, the necessary refrigerant amount Mrh of the refrigerant circuit during a heating operation is obtained using Expression (8) into which Expression (6) has been substituted. During these computations, information on the internal volume VOC of the outdoor-side heat exchanger 14, the internal volume VIC of the indoor-side heat exchanger 22, and the pipe lengths L of the first connecting pipe 31 and the second connecting pipe 32 is input in advance into the controller 41. The controller 41 may acquire these pieces of information automatically.

[0082] The refrigerant density c at the condenser is converted from, for example, the detected pressure Pd of the first pressure sensor 51. Further, the refrigerant density e at the evaporator is converted from, for example, the detected pressure P5 of the second pressure sensor 52.

[0083] Fig. 9 is a diagram illustrating a control flow according to Control Method 1 of the air-conditioning apparatus according to Embodiment 3 of the present invention.

As illustrated in Fig. 9, in Step S303, the controller 41 obtains the necessary refrigerant amount Mr of the refrigerant circuit, and the processing then proceeds to Step S304. In Step S305, the controller 41 compares the necessary refrigerant amount Mr of the refrigerant circuit with the target necessary refrigerant amount Mrm.

When it is determined that the necessary refrigerant amount Mr6 is larger than the target necessary refrigerant amount Mr, the controller 41 increases the opening degree of the second expansion valve 24. When it is determined that the necessary refrigerant amount Mr0 is smaller than the target necessary refrigerant amount Mrm, the controller 41 decreases the opening degree of the second expansion valve 24.

The change amount of the opening degree may be determined based on how large the difference is between the necessary refrigerant amount Mr of the refrigerant circuit and the target necessary refrigerant amount Mrm.

[0084] In Step S307, the controller 41 obtains the necessary refrigerant amount Mrh of the refrigerant circuit, and the processing then proceeds to Step S308. In Step S309, the controller 41 compares the necessary refrigerant amount Mrh of the refrigerant circuit with the target necessary refrigerant amount Mrm. When it is determined that the necessary refrigerant amount Mrh is larger than the target necessary refrigerant amount Mrm, the controller 41 increases the opening degree of the first expansion valve 16. When it is determined that the necessary refrigerant amount Mrh is smaller than the target necessary refrigerant amount Mr, the controller 41 decreases the opening degree of the first expansion valve 16. The change amount of the opening degree may be determined based on how large the difference is between the necessary refrigerant amount Mrh of the refrigerant circuit and the target necessary refrigerant amount Mr.

[0085] (Control Method 2) Fig. 10 is a diagram illustrating a control flow according to Control Method 2 of the air-conditioning apparatus according to Embodiment 3 of the present invention.

As illustrated in Fig. 10, in Step S403, the controller 41 obtains the necessary refrigerant amount Mr of the refrigerant circuit, and the processing then proceeds to Step S404. In Step S405, the controller 41 compares the necessary refrigerant amount Mr of the refrigerant circuit with the target necessary refrigerant amount Mrm.

When it is determined that the necessary refrigerant amount Mr0 is larger than the target necessary refrigerant amount Mr, the controller 41 decreases the opening degree of the first expansion valve 16. When it is determined that the necessary refrigerant amount Mr is smaller than the target necessary refrigerant amount Mrm, the controller 41 increases the opening degree of the first expansion valve 16. The change amount of the opening degree may be determined based on how large the difference is between the necessary refrigerant amount Mr0 of the refrigerant circuit and the target necessary refrigerant amount Mr.

[0086] In Step S407, the controller 41 obtains the necessary refrigerant amount Mrh of the refrigerant circuit, and the processing then proceeds to Step S408. In Step S409, the controller 41 compares the necessary refrigerant amount Mrh of the refrigerant circuit with the target necessary refrigerant amount Mrm. When it is determined that the necessary refrigerant amount Mrh is larger than the target necessary refrigerant amount Mrm, the controller 41 decreases the opening degree of the second expansion valve 24. When it is determined that the necessary refrigerant amount Mrh is smaller than the target necessary refrigerant amount Mrm, the controller 41 increases the opening degree of the second expansion valve 24. The change amount of the opening degree may be determined based on how large the difference is between the necessary refrigerant amount Mrh of the refrigerant circuit and the target necessary refrigerant amount Mr.

[0087] <Air-conditioning Apparatus Effects> In the air-conditioning apparatus 1, because the necessary refrigerant amount Mr of the refrigerant circuit during a cooling operation and the necessary refrigerant amount Mrh of the refrigerant circuit during a heating operation are each controlled so as to be at the target necessary refrigerant amount Mrm, even when the internal volume of the outdoor-side heat exchanger 14 is not 0.7 to 1.3 times the internal volume of the indoor-side heat exchanger 22, almost no excess refrigerant is produced, with the result that the refrigerant reservoir may be unnecessary, or may be reduced further in size. Note that, the internal volume of the outdoor-side heat exchanger 14 may be 0.7 to 1.3 times the internal volume of the indoor-side heat exchanger 22.

[0088] The present invention has been described with reference to Embodiment 1, Embodiment 2, and Embodiment 3, but the present invention is not limited to these embodiments. For example, each of the embodiments and each of the modified examples may be combined.

Reference Signs List [0089] 1 air-conditioning apparatus 11 outdoor unit 12 compressor 13 four-way valve 14 outdoor-side heat exchanger 15 outdoor-side fan 16 first expansion valve 17 accumulator 21 indoor unit 22 indoor-side heat exchanger 23 indoor-side fan 24 second expansion valve 31 first connecting pipe 32 second connecting pipe 41 controller 51 first pressure sensor 52 second pressure sensor 61 first temperature sensor 62 second temperature sensor 63 third temperature sensor 64 fourth temperature sensor 65 fifth temperature sensor 66 sixth temperature sensor a upstream-side expansion valve b downstream-side expansion valve