Air conditioner

Abstract

An air conditioner includes a liquid pressure adjusting expansion valve that is located in an outdoor liquid refrigerant pipe at a part thereof closer to a liquid refrigerant communication pipe than to a refrigerant cooler and configured to reduce a pressure of a refrigerant so that a refrigerant flows through the liquid refrigerant communication pipe in a gas-liquid two-phase state and so that the refrigerant flows through an outlet of a refrigerant cooler in a liquid state.

Claims

1. An air conditioner comprising: a refrigerant circuit configured by connecting, via a liquid refrigerant communication pipe and a gas refrigerant communication pipe, an outdoor unit having a compressor and an outdoor heat exchanger, and a plurality of indoor units each having an indoor expansion valve and an indoor heat exchanger, with a refrigerant filled into the refrigerant circuit being circulated through in the sequence of the compressor, the outdoor heat exchanger, the liquid refrigerant communication pipe, the indoor expansion valve, the indoor heat exchanger, the gas refrigerant communication pipe, and the compressor, wherein an outdoor liquid refrigerant pipe connecting a liquid-side end of the outdoor heat exchanger and the liquid refrigerant communication pipe is connected to a refrigerant returning pipe that branches off a portion of the refrigerant flowing through the outdoor liquid refrigerant pipe and returns the portion of the refrigerant to the compressor, and the outdoor liquid refrigerant pipe is provided with a refrigerant cooler configured to cool the refrigerant flowing through the outdoor liquid refrigerant pipe using the refrigerant flowing through the refrigerant returning pipe, wherein a liquid pressure adjusting expansion valve configured to reduce a pressure of the refrigerant is located in the outdoor liquid refrigerant pipe at a part thereof between the liquid refrigerant communication pipe and the refrigerant cooler so that the refrigerant flows through the liquid refrigerant communication pipe in a gas-liquid two-phase state and so that the refrigerant flows through an outlet of the refrigerant cooler in a liquid state, wherein the refrigerant becomes a liquid state in the outdoor heat exchanger by releasing its heat through heat exchange with outside air during a cooling operation, wherein an outdoor expansion valve configured to reduce the pressure of the refrigerant is located in the outdoor liquid refrigerant pipe at a part thereof between the outdoor heat exchanger and the refrigerant cooler, wherein a liquid-side outdoor heat exchange sensor configured to detect a temperature of the refrigerant at the liquid-side end of the outdoor heat exchanger is located in the outdoor liquid refrigerant pipe at a part thereof between the outdoor heat exchanger and the outdoor expansion valve, wherein the outdoor unit and/or the plurality of indoor units have a controller that is configured to control the liquid pressure adjusting expansion valve, wherein the controller is configured to obtain a subcooling degree of the refrigerant at the liquid-side end of the outdoor heat exchanger from the temperature of the refrigerant detected by the liquid-side outdoor heat exchange sensor, and adjust the outdoor expansion valve to reduce the pressure of the refrigerant so that the subcooling degree of the refrigerant at the liquid-side end of the outdoor heat exchanger reaches a target subcooling degree during the cooling operation, and wherein the controller is further configured to adjust the liquid pressure adjusting expansion valve to reduce the pressure of the refrigerant so that the refrigerant flows through the liquid refrigerant communication pipe in the gas-liquid two-phase state and so that the refrigerant flows through the outlet of the refrigerant cooler in the liquid state during the cooling operation.

2. The air conditioner according to claim 1, further comprising: a refrigerant cooling-side sensor that is located in the outdoor liquid refrigerant pipe at a part thereof between the outdoor heat exchanger and the liquid pressure adjusting expansion valve.

3. The air conditioner according to claim 2, wherein the refrigerant cooling-side sensor is configured to detect a pressure of the refrigerant or a state quantity equivalent to the pressure, and the controller is further configured to obtain the pressure of the refrigerant in the outdoor liquid refrigerant pipe at the part thereof provided with the refrigerant cooler from the pressure of the refrigerant or the state quantity equivalent to the pressure detected by the refrigerant cooling-side sensor.

4. The air conditioner according to claim 1, wherein when the controller controls the opening degree of the liquid pressure adjusting expansion valve such that the pressure of the refrigerant in the outdoor liquid refrigerant pipe at the part thereof provided with the refrigerant cooler reaches a target liquid pressure, the controller controls the liquid pressure adjusting expansion valve in a range of a lower limit opening degree or higher, and revises the lower limit opening degree according to the opening degree of the outdoor expansion valve.

5. The air conditioner according to claim 1, wherein the refrigerant returning pipe is a refrigerant pipe that sends the refrigerant branched off from the outdoor liquid refrigerant pipe to a suction side of the compressor.

6. The air conditioner according to claim 1, wherein the refrigerant returning pipe is a refrigerant pipe that sends the refrigerant branched off from the outdoor liquid refrigerant pipe into a middle of a compression process in the compressor.

7. The air conditioner according to claim 2, wherein the outdoor expansion valve is located in the outdoor liquid refrigerant pipe at a part thereof closer to the outdoor heat exchanger than to the refrigerant cooler, the controller is further configured to adjust the opening degree of the liquid pressure adjusting expansion valve such that the pressure of the refrigerant in the outdoor liquid refrigerant pipe at the part thereof provided with the refrigerant cooler reaches a target liquid pressure.

8. The air conditioner according to claim 2, wherein the refrigerant returning pipe is a refrigerant pipe that sends the refrigerant branched off from the outdoor liquid refrigerant pipe to a suction side of the compressor.

9. The air conditioner according to claim 2, wherein the refrigerant returning pipe is a refrigerant pipe that sends the refrigerant branched off from the outdoor liquid refrigerant pipe into a middle of a compression process in the compressor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic configuration diagram of an air conditioner according to one embodiment of the present invention (with illustration of the flow of refrigerant during a cooling operation).

(2) FIG. 2 is a control block diagram of the air conditioner.

(3) FIG. 3 is a pressure-enthalpy graph illustrating a refrigeration cycle during the cooling operation.

(4) FIG. 4 is a pressure-enthalpy graph illustrating a refrigeration cycle in a case where only reduction in amount of the refrigerant to be filled is performed.

(5) FIG. 5 is a pressure-enthalpy graph illustrating a refrigeration cycle in a case where reduction in amount of the refrigerant to be filled is performed and decrease in pressure is performed using an outdoor expansion valve until the refrigerant flows in a gas-liquid two-phase state.

(6) FIG. 6 is a schematic configuration diagram of an air conditioner according to Modified Example B (with illustration of the flow of refrigerant during a cooling operation).

(7) FIG. 7 is a schematic configuration diagram of an air conditioner according to Modified Example D (with illustration of the flow of refrigerant during a cooling operation).

(8) FIG. 8 is a pressure-enthalpy graph illustrating a refrigeration cycle during the cooling operation according to Modified Example D.

DESCRIPTION OF EMBODIMENTS

(9) Below, an embodiment of an air conditioner according to the present invention will be described based on the drawings. Note that the specific configuration of an embodiment of the air conditioner according to the present invention is not limited by the following embodiment and modified examples, and modifications are possible without departing from the scope of the invention.

(1) Configuration of Air Conditioner

(10) FIG. 1 is a schematic configuration diagram of an air conditioner 1 according to one embodiment of the present invention. The air conditioner 1 is a device for cooling interiors of buildings and the like using a vapor-compression type refrigeration cycle. The air conditioner 1 mainly includes an outdoor unit 2, a plurality of (two in this embodiment) indoor units 5a and 5b connected in parallel to each other, and a liquid refrigerant communication pipe 6 and a gas refrigerant communication pipe 7 both connecting the outdoor unit 2 and the indoor units 5a and 5b. A vapor-compression type refrigerant circuit 10 of the air conditioner 1 is configured by connecting the outdoor unit 2 and the indoor units 5a and 5b via the liquid refrigerant communication pipe 6 and the gas refrigerant communication pipe 7.

(11) <Indoor Units>

(12) The indoor units 5a and 5b are disposed inside of a building or the like. As described above, the indoor units 5a and 5b are connected to the outdoor unit 2 via the liquid refrigerant communication pipe 6 and the gas refrigerant communication pipe 7, and constitute a part of the refrigerant circuit 10.

(13) Next, the configuration of the indoor units 5a and 5b will be described. Note that the indoor unit 5a and the indoor unit 5b have the same configuration, and therefore, only the configuration of the indoor unit 5a will be described here, and the suffix “b” will be added with regard to the configuration of the indoor unit 5b instead of the suffix “a” indicating elements of the indoor unit 5a, and the description of each of the elements of the indoor unit 5b will be omitted.

(14) The indoor unit 5a mainly includes an indoor expansion valve 51a and an indoor heat exchanger 52a. The indoor unit 5a further includes an indoor liquid refrigerant pipe 53a connecting the liquid-side end of the indoor heat exchanger 52a and the liquid refrigerant communication pipe 6, and an indoor gas refrigerant pipe 54a connecting the gas-side end of the indoor heat exchanger 52a and the gas refrigerant communication pipe 7.

(15) The indoor expansion valve 51a is an electric expansion valve for adjusting the flow rate of a refrigerant flowing through the indoor heat exchanger 52a while reducing the pressure of the refrigerant to a low pressure of the refrigeration cycle, and is located in the indoor liquid refrigerant pipe 53a.

(16) The indoor heat exchanger 52a is a heat exchanger that functions as an evaporator of the refrigerant at the low pressure of the refrigeration cycle, to cool indoor air. Here, the indoor unit 5a has an indoor fan 55a that suctions the indoor air into the indoor unit 5a for heat exchange of the air with the refrigerant in the indoor heat exchanger 52a to supply it as supply air to the indoor. That is, the indoor unit 5a has the indoor fan 55a as a fan for supplying the indoor air that serves as a cooling source for the refrigerant flowing through the indoor heat exchanger 52a to the indoor heat exchanger 52a. Here, a centrifugal fan, a multi-blade fan, or the like driven by an indoor fan motor 56a may be used as the indoor fan 55a. In addition, here, the number of rotations of the indoor fan motor 56a is controllable by an inverter or the like, which makes the air volume of the indoor fan 55a controllable.

(17) The indoor unit 5a is provided with various types of sensors. More specifically, the indoor unit 5a is provided with a liquid-side indoor heat exchange sensor 57a for detecting a temperature Trl of the refrigerant at the liquid-side end of the indoor heat exchanger 52a, a gas-side indoor heat exchange sensor 58a for detecting a temperature Trg of the refrigerant at the gas-side end of the indoor heat exchanger 52a, and an indoor air sensor 59a for detecting a temperature Tra of the indoor air suctioned into the indoor unit 5a.

(18) The indoor unit 5a includes an indoor-side controller 50a for controlling the operations of each of components that constitute the indoor unit 5a. The indoor-side controller 50a includes a microcomputer, a memory, and the like that are provided to perform individual control of the indoor unit 5a, such that exchanging of control signals and the like is enabled with a remote control (not shown) for individually manipulating the indoor unit 5a, and exchanging of control signals and the like is enabled with the outdoor unit 2 via a communication line.

(19) <Outdoor Unit>

(20) The outdoor unit 2 is disposed externally on a building or the like. As described above, the outdoor unit 2 is connected to the indoor unit 5a and 5b via the liquid refrigerant communication pipe 6 and the gas refrigerant communication pipe 7 and constitutes a part of the refrigerant circuit 10.

(21) Next, the configuration of the outdoor unit 2 will be described.

(22) The outdoor unit 2 mainly includes a compressor 21 and an outdoor heat exchanger 24. The outdoor unit 2 further includes an outdoor liquid refrigerant pipe 25 connecting the liquid-side end of the outdoor heat exchanger 24 and the liquid refrigerant communication pipe 6, and an outdoor gas refrigerant pipe 26 connecting the suction side of the compressor 21 and the gas refrigerant communication pipe 7. A liquid-side shutoff valve 27 is located at the connecting part of the outdoor liquid refrigerant pipe 25 with the liquid refrigerant communication pipe 6, and a gas side shutoff valve 28 is located at the connecting part of the outdoor gas refrigerant pipe 26 with the gas refrigerant communication pipe 7. The liquid side shutoff valve 27 and the gas side shutoff valve 28 are valves that are manually opened and closed.

(23) The compressor 21 is a device for compressing the refrigerant in the refrigeration cycle to increase a low pressure thereof to a high pressure. Here, as the compressor 21, a compressor with a tightly sealed structure, where a positive-displacement compressor element of a rotary type, a scrolling type, or the like (not shown) is rotationally driven by a compressor motor 22 is used. In addition, here, the number of rotations of the compressor motor 22 is controllable by an inverter or the like, which makes the capacity of the compressor 21 controllable.

(24) The outdoor heat exchanger 24 is a heat exchanger that functions as a radiator for the refrigerant at a high pressure in the refrigeration cycle. Here, the outdoor unit 2 includes an outdoor fan 29 for suctioning outside air into the outdoor unit 2 and discharging the outside air to the outside after heat exchange of the outside air with the refrigerant has been carried out in the outdoor heat exchanger 24. That is, the outdoor unit 2 has the outdoor fan 29 as a fan for supplying the outside air to the outdoor heat exchanger 24, the air being to serve as the cooling source for the refrigerant flowing through the outdoor heat exchanger 24. Here, as the outdoor fan 29, a propeller fan or the like that is driven by an outdoor fan motor 30 is used. In addition, here, the number of rotations of the outdoor fan motor 30 is controllable by an inverter or the like, which makes the air volume of the outdoor fan 29 is controllable.

(25) The refrigerant filled into the refrigerant circuit 10 is to be circulated through in the sequence of the compressor 21, the outdoor heat exchanger 24, the liquid refrigerant communication pipe 6, the indoor expansion valves 51a and 51b, the indoor heat exchangers 52a and 52b, the gas refrigerant communication pipe 7, and the compressor 21.

(26) In addition, here, the outdoor liquid refrigerant pipe 25 is connected to a refrigerant returning pipe 31, and is provided with a refrigerant cooler 35 and an outdoor expansion valve 36. The refrigerant returning pipe 31 is a refrigerant pipe for branching off a portion of the refrigerant flowing through the outdoor liquid refrigerant pipe 25 and returning this portion of the refrigerant to the compressor 21. The refrigerant cooler 35 is a heat exchanger for cooling the refrigerant flowing through the outdoor liquid refrigerant pipe 25 using the refrigerant flowing through the refrigerant returning pipe 31. The outdoor expansion valve 36 is an electric expansion valve located in the outdoor liquid refrigerant pipe 25 at a part thereof between the outdoor heat exchanger 24 and the refrigerant cooler 35. Moreover, here, a liquid pressure adjusting expansion valve 37 for reducing the pressure of the refrigerant is located in the outdoor liquid refrigerant pipe 25 at a part thereof between the liquid refrigerant communication pipe 6 and the refrigerant cooler 35 (here, at a part thereof between the refrigerant cooler 35 and the liquid side shutoff valve 27) so that the refrigerant flows through the liquid refrigerant communication pipe 6 in a gas-liquid two-phase state and so that the refrigerant flows through the outlet of the refrigerant cooler 35 in a liquid state. Here, the liquid pressure adjusting expansion valve 37 is comprised of an electric expansion valve.

(27) The refrigerant returning pipe 31 is a refrigerant pipe for sending the refrigerant branched off from the outdoor liquid refrigerant pipe 25 to the suction side of the compressor 21. The refrigerant returning pipe 31 mainly includes a refrigerant returning inlet pipe 32 and a refrigerant returning outlet pipe 33. The refrigerant returning inlet pipe 32 is a refrigerant pipe for branching a portion of the refrigerant flowing through the outdoor liquid refrigerant pipe 25 off from a part between the liquid-side end of the outdoor heat exchanger 24 and the liquid pressure adjusting expansion valve 37 (here, a part between the outdoor expansion valve 36 and the refrigerant cooler 35) and sending to the inlet of the refrigerant cooler 35 on the refrigerant returning pipe 31 side. The refrigerant returning inlet pipe 32 is provided with a refrigerant returning expansion valve 34 for adjusting the flow rate of the refrigerant flowing through the refrigerant cooler 35 while reducing the pressure of the refrigerant that flows through the refrigerant returning pipe 31 to the low pressure of the refrigeration cycle. Here, the refrigerant returning expansion valve 34 is comprised of an electric expansion valve. The refrigerant returning outlet pipe 33 is a refrigerant pipe for sending refrigerant from the outlet of the refrigerant cooler 35 on the refrigerant returning pipe 31 side to the outdoor gas refrigerant pipe 26 connected to the suction side of the compressor 21. The refrigerant cooler 35 is configured to cool the refrigerant flowing through the outdoor liquid refrigerant pipe 25 using the refrigerant flowing through the refrigerant returning pipe 31 at the low pressure in the refrigeration cycle.

(28) The outdoor unit 2 is provided with various types of sensors. More specifically, the outdoor unit 2 is provided with, in the vicinity of the compressor 21, a suction pressure sensor 38 for detecting a suction pressure Ps of the compressor 21, a suction temperature sensor 39 for detecting a suction temperature Ts of the compressor 21, a discharge pressure sensor 40 for detecting a discharge pressure Pd of the compressor 21, and a discharge temperature sensor 41 for detecting a discharge temperature Td of the compressor 21. In addition, a liquid-side outdoor heat exchange sensor 42 for detecting a temperature Tol of the refrigerant at the liquid-side end of the outdoor heat exchanger 24 is located in the outdoor liquid refrigerant pipe 25 at a part thereof between the outdoor heat exchanger 24 and the refrigerant cooler 35 (here, at a part thereof between the outdoor heat exchanger 24 and the outdoor expansion valve 36). Furthermore, an outside air sensor 43 for detecting a temperature Toa of the outside air suctioned into the outdoor unit 2 is located in the vicinity of the outdoor heat exchanger 24 or the outdoor fan 29. A refrigerant cooling-side sensor 44 for detecting a pressure Pol of the refrigerant in the outdoor liquid refrigerant pipe 25 at a part thereof provided with the refrigerant cooler 35 is located in the outdoor liquid refrigerant pipe 25 at a part thereof between the outdoor heat exchanger 24 and the liquid pressure adjusting expansion valve 37 (here, at a part thereof between the outdoor expansion valve 36 and the liquid pressure adjusting expansion valve 37). Furthermore, the refrigerant returning outlet pipe 33 is provided with a refrigerant returning-side sensor 45 for detecting a temperature Tor of the refrigerant flowing through the outlet of the refrigerant cooler 35 on the refrigerant returning pipe 31 side.

(29) The outdoor unit 2 includes an outdoor-side controller 20 for controlling the operations of each of components that constitute the outdoor unit 2. The outdoor-side controller 20 has a microcomputer, a memory, and the like provided in order to perform control of the outdoor unit 2, so that control signals and the like can be exchanged between the outdoor-side controller 20 and the indoor-side controllers 50a and 50b of the indoor units 5a and 5b via a communication line. That is, a controller 8 for performing control of the operations of the entire air conditioner 1 is configured by connecting the indoor-side controllers 50a and 50b and the outdoor-side controller 20 via the communication line. As shown in FIG. 2, the controller 8 is connected so as to be able to receive detection signals from the respective sensors 38 to 45, 57a to 59a, and 57b to 59b and is also connected so as to be able to control the respective devices 21, 29, 34, 36, 37, 51a, 55a, 51b, 55b, and the like based on these detection signals and the like. Here, FIG. 2 is a control block diagram of the air conditioner 1.

(2) Operations and Features of Air Conditioner

(30) Next, the operations and features of the air conditioner 1 will be described with reference to FIGS. 1 to 5. Here, FIG. 3 is a pressure-enthalpy graph illustrating a refrigeration cycle during a cooling operation, FIG. 4 is a pressure-enthalpy graph illustrating a refrigeration cycle in a case where only reduction in amount of the refrigerant to be filled is performed, and FIG. 5 is a pressure-enthalpy graph illustrating the refrigeration cycle in a case where reduction in amount of the refrigerant to be filled is performed and decrease in pressure is performed using the outdoor expansion valve 36 until the refrigerant flows in a gas-liquid two-phase state.

(31) <Operations>

(32) The air conditioner 1 mainly performs a cooling operation in which the refrigerant filled into the refrigerant circuit 10 is circulated through in the sequence of the compressor 21, the outdoor heat exchanger 24, the liquid refrigerant communication pipe 6, the indoor expansion valves 51a and 51b, the indoor heat exchangers 52a and 52b, the gas refrigerant communication pipe 7, and the compressor 21. In addition, in the cooling operation, an operation of cooling the refrigerant flowing through the outdoor liquid refrigerant pipe 25 is also performed using the refrigerant cooler 35 located in the outdoor liquid refrigerant pipe 25 as well as the refrigerant returning pipe 31 connected to the outdoor liquid refrigerant pipe 25 that connects the liquid-side end of the outdoor heat exchanger 24 and the liquid refrigerant communication pipe 6. Furthermore, in the cooling operation, an operation of reducing the pressure of the refrigerant is also performed using the liquid pressure adjusting expansion valve 37 located in the outdoor liquid refrigerant pipe 25 at a part thereof between the liquid refrigerant communication pipe 6 and the refrigerant cooler 35 so that the refrigerant flows through the liquid refrigerant communication pipe 6 in a gas-liquid two-phase state and so that the refrigerant flows through the outlet of the refrigerant cooler 35 in a liquid state. Note that the operations of the air conditioner 1 described below are performed by the controller 8 that controls the constituent components of the air conditioner 1.

(33) The refrigerant filled into the refrigerant circuit 10 is first suctioned into the compressor 21 and compressed to increase the pressure from low to high in the refrigerant cycle, to be discharged therefrom (see the points A and B in FIGS. 1 and 3). The refrigerant discharged in a gas state from the compressor 21 flows into the gas-side end of the outdoor heat exchanger 24.

(34) The refrigerant flowing into the gas-side end of the outdoor heat exchanger 24 becomes a refrigerant in a liquid state in the outdoor heat exchanger 24 by releasing its heat through heat exchange with the outside air supplied through the outdoor fan 29, and flows out from the liquid-side end of the outdoor heat exchanger 24 (see the point C in FIGS. 1 and 3).

(35) The refrigerant flowing out from the liquid-side end of the outdoor heat exchanger 24 flows through the outdoor liquid refrigerant pipe 25 and is reduced in pressure by the outdoor expansion valve 36 (see the point D in FIGS. 1 and 3). The refrigerant reduced in pressure by the outdoor expansion valve 36 flows into the inlet of the refrigerant cooler 35 on the outdoor liquid refrigerant pipe 25 side. Here, the controller 8 controls an opening degree MVoo of the outdoor expansion valve 36 such that a subcooling degree SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger 24 reaches a target subcooling degree SCot. The controller 8 obtains the subcooling degree SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger 24 from the temperature Tol of the refrigerant detected by the liquid-side outdoor heat exchange sensor 42. More specifically, the controller 8 obtains the subcooling degree SCo of the refrigerant by subtracting the temperature Tol of the refrigerant from a temperature Toc of the refrigerant that is obtained by converting a discharge pressure Pd detected by the discharge pressure sensor 40 to a saturation temperature. The target subcooling degree SCot is set to be as small as possible (for example, 1 to 3° C.) so that it is easy to maintain the refrigerant, which flows through the outdoor liquid refrigerant pipe 25 after being reduced in pressure by the outdoor expansion valve 36, in a state of having a high level of wetness (see the point D in FIGS. 1 and 3). Then, the controller 8 performs control to increase the opening degree MVoo of the outdoor expansion valve 36 when the subcooling degree SCo is larger than the target subcooling degree Scot, and performs control to decrease the opening degree MVoo of the outdoor expansion valve 36 when the subcooling degree SCo is smaller than the target subcooling degree SCot.

(36) The refrigerant flowing into the inlet of the refrigerant cooler 35 on the outdoor liquid refrigerant pipe 25 side becomes a refrigerant in a subcooled state (that is, a liquid state) by further being cooled through heat exchange in the refrigerant cooler 35 with the refrigerant flowing through the refrigerant returning pipe 31 (see the point E in FIGS. 1 and 3). At this point of time, a portion of the refrigerant reduced in pressure by the outdoor expansion valve 36 is branched off to the refrigerant returning pipe 31 and is reduced in pressure until it reaches a pressure close to the low pressure in the refrigerant cycle by the refrigerant returning expansion valve 34. The refrigerant flowing through the refrigerant returning pipe 31 after being reduced in pressure by the refrigerant returning expansion valve 34 flows into the inlet of the refrigerant cooler 35 on the refrigerant returning pipe 31 side. The refrigerant flowing into the inlet of the refrigerant cooler 35 on the refrigerant returning pipe 31 side becomes a refrigerant in a gas state by being heated through heat exchange in the refrigerant cooler 35 with the refrigerant flowing through the outdoor liquid refrigerant pipe 25. Then, the refrigerant cooled in the refrigerant cooler 35 flows out from the outlet of the refrigerant cooler 35 on the outdoor liquid refrigerant pipe 25 side and is sent to the liquid pressure adjusting expansion valve 37. The refrigerant heated in the refrigerant cooler 35 flows out from the outlet of the refrigerant cooler 35 on the refrigerant returning pipe 31 side and is returned to the suction side of the compressor 21 (here, the outdoor gas refrigerant pipe 26). Here, the controller 8 controls an opening degree MVor of the refrigerant returning expansion valve 34 so that a superheating degree SHo of the refrigerant at the outlet of the refrigerant cooler 35 on the refrigerant returning pipe 31 side reaches a target superheating degree SHot. The controller 8 obtains the superheating degree SHo of the refrigerant at the outlet of the refrigerant cooler 35 on the refrigerant returning pipe 31 side by subtracting a temperature Tos of the refrigerant obtained by converting the suction pressure Ps detected by the suction pressure sensor 38 to the saturation temperature from the temperature Tor of the refrigerant detected by the refrigerant returning-side sensor 45. The target superheating degree SHot is set to a value of about 3 to 10° C. so that the refrigerant suctioned into the compressor 21 (see the point A in FIGS. 1 and 3) does not enter a state that has a high level of wetness. Then, the controller 8 performs control to increase the opening degree MVor of the refrigerant returning expansion valve 34 when the superheating degree SHo is larger than the target superheating degree SHot, and performs control to decrease the opening degree MVor of the refrigerant returning expansion valve 34 when the superheating degree SHo is smaller than the target superheating degree SHot.

(37) The refrigerant sent to the liquid pressure adjusting expansion valve 37 is reduced in pressure by the liquid pressure adjusting expansion valve 37 so that the refrigerant flows through the liquid refrigerant communication pipe 6 in a gas-liquid two-phase state and so that the refrigerant flows through the outlet of the refrigerant cooler 35 in a liquid state (see the points E and F in FIGS. 1 and 3). Here, the controller 8 controls an opening degree MVop of the liquid pressure adjusting expansion valve 37 such that the pressure Pol of the refrigerant in the outdoor liquid refrigerant pipe 25 at a part thereof provided with the refrigerant cooler 35 reaches a target liquid pressure Polt. The controller 8 obtains the pressure Pol of the refrigerant in the outdoor liquid refrigerant pipe 25 at a part thereof provided with the refrigerant cooler 35 from the pressure of the refrigerant detected by the refrigerant cooling-side sensor 44. The target liquid pressure Polt is set to be as high as possible so that the refrigerant flows through the outlet of the refrigerant cooler 35 in a liquid state. Then, the controller 8 performs control to increase the opening degree MVop of the liquid pressure adjusting expansion valve 37 when the pressure Pol of the refrigerant is higher than the target liquid pressure Polt, and performs control to decrease the opening degree MVop of the liquid pressure adjusting expansion valve 37 when the pressure Pol of the refrigerant is lower than the target liquid pressure Polt.

(38) The refrigerant reduced in pressure by the liquid pressure adjusting expansion valve 37 is sent to the liquid refrigerant communication pipe 6 via the liquid side shutoff valve 27. At this point of time, because the refrigerant flowing through the liquid refrigerant communication pipe 6 is in a gas-liquid two-phase state, as compared to a case where the refrigerant flowing through the liquid refrigerant communication pipe 6 is in a liquid state (that is, a case where the configuration of PTL 3 is employed), the liquid refrigerant communication pipe 6 is filled with a reduced amount of a refrigerant in a liquid state, and thereby the amount of the refrigerant in the liquid refrigerant communication pipe 6 can be reduced by the reduced amount. The refrigerant sent to the liquid refrigerant communication pipe 6 is sent to the indoor units 5a and 5b after being reduced in pressure due to a loss in pressure that is caused corresponding to the length and diameter of the pipe (see the point G in FIGS. 1 and 3).

(39) The refrigerant sent to the indoor units 5a and 5b is reduced in pressure until it reaches a pressure close to the low pressure of the refrigerant cycle by the indoor expansion valves 51a and 51b (see the point H in FIGS. 1 and 3). The refrigerant after being reduced in pressure by the indoor expansion valves 51a and 51b flows into the liquid-side ends of the indoor heat exchangers 52a and 52b. The refrigerant flowing into the liquid-side ends of the indoor heat exchangers 52a and 52b becomes a refrigerant in a gas state by being evaporated through heat exchange in the indoor heat exchangers 52a and 52b with the indoor air supplied by the indoor fans 55a and 55b, and flows out from the gas-side ends of the indoor heat exchangers 52a and 52b (see the point I in FIGS. 1 and 3). Further, the indoor air cooled through heat exchange with the refrigerant in the indoor heat exchangers 52a and 52b is supplied to indoors thereby perform cooling of the indoors. Here, the controller 8 controls an opening degree MVrr of the indoor expansion valves 51a and 51b such that superheating degree SHr of the refrigerant at the gas-side ends of the indoor heat exchangers 52a and 52b reaches a target superheating degree SHrt. The controller 8 obtains the superheating degrees SHr of the refrigerant at the gas-side ends of the indoor heat exchangers 52a and 52b by subtracting the temperatures Trl of the refrigerant detected by the liquid-side indoor heat exchange sensors 57a and 57b from the temperatures Trg of the refrigerant detected by the gas-side indoor heat exchange sensors 58a and 58b respectively. The target superheating degree SHrt is set to a value of about 3 to 10° C. so that the refrigerant suctioned into the compressor 21 (see the point A in FIGS. 1 and 3) does not enter a state that has a high level of wetness. Then, the controller 8 performs control to increase the opening degrees MVrr of the indoor expansion valves 51a and 51b when the superheating degree SHr is larger than the target superheating degree SHrt, and performs control to decrease the opening degrees MVrr of the indoor expansion valves 51a and 51b when the superheating degree SHr is smaller than the target superheating degree SHrt.

(40) The refrigerant flowing out from the gas-side ends of the indoor heat exchangers 52a and 52b is sent to the gas refrigerant communication pipe 7. The refrigerant sent to the gas refrigerant communication pipe 7 is then sent to the outdoor unit 2 after being reduced in pressure due to the loss in pressure that is caused corresponding to the length and diameter of the pipe, and is suctioned again into the compressor 21 along with the refrigerant from the refrigerant returning pipe 31 via the gas side shutoff valve 28 and the outdoor gas refrigerant pipe 26 (see the point A in FIGS. 1 and 3).

(41) In this manner, a cooling operation is performed in the air conditioner 1.

(42) <Features>

(43) Here, as described above, in a configuration including the refrigerant circuit 10 configured by connecting, via the liquid refrigerant communication pipe 6 and the gas refrigerant communication pipe 7, the outdoor unit 2 having the compressor 21 and the outdoor heat exchanger 24, and the plurality of indoor units 5a and 5b having the indoor expansion valves 51a and 51b and the indoor heat exchangers 52a and 52b, first, the refrigerant returning pipe 31 and the refrigerant cooler 35 are located in the outdoor liquid refrigerant pipe 25 that connects the liquid-side end of the outdoor heat exchanger 24 and the liquid refrigerant communication pipe 6. Here, the refrigerant returning pipe 31 is a refrigerant pipe for sending the refrigerant branched off from the outdoor liquid refrigerant pipe 25 to the suction side of the compressor 21, which thereby provides the refrigerant cooler 35 with a cooling function that is obtained by utilizing a difference in pressure between the pressure of the refrigerant flowing through the outdoor liquid refrigerant pipe 25 and the low pressure of the refrigeration cycle. Furthermore, by providing the liquid pressure adjusting expansion valve 37 in the outdoor liquid refrigerant pipe 25 at a part thereof between the liquid refrigerant communication pipe 6 and the refrigerant cooler 35 as described above, the refrigerant flowing through the outdoor liquid refrigerant pipe 25 is reduced in pressure (see ΔPef in FIG. 3) so that the refrigerant flows through the liquid refrigerant communication pipe 6 in a gas-liquid two-phase state (see the points F and G in FIG. 3) and so that the refrigerant flows through the outlet of the refrigerant cooler 35 in a liquid state (see the point E in FIG. 3).

(44) For these reasons, here, the pressure of the refrigerant flowing through the refrigerant cooler 35 is unlikely to fall, and the refrigerant can flow through the refrigerant cooler 35 with a high level of wetness, and also a difference in pressure (see ΔPad in FIG. 3) can be easily secured between the refrigerant flowing through the outdoor liquid refrigerant pipe 25 and the refrigerant flowing through the refrigerant returning pipe 31, and therefore, a cooling function (see ΔQde in FIG. 3) can be adequately fulfilled in the refrigerant cooler 35. As a result, the flow rate of the refrigerant sent to the plurality of indoor units 5a and 5b can be reduced, and the loss in pressure (refer to ΔPai in FIG. 3) in the gas refrigerant communication pipe 7 and the like can be decreased, and therefore, improving the refrigeration capacity (see ΔQhi in FIG. 3) and operating efficiency (the value obtained by dividing ΔQhi by Wab in FIG. 3).

(45) In this manner, here, in the air conditioner 1 including the refrigerant circuit 10 configured by connecting, via the liquid refrigerant communication pipe 6 and the gas refrigerant communication pipe 7, the outdoor unit 2 having the compressor 21 and the outdoor heat exchanger 24, and the plurality of indoor units 5a and 5b having the indoor expansion valves 51a and 51b and the indoor heat exchangers 52a and 52b, the amount of the refrigerant to be filled into the refrigerant circuit 10 can be reduced while the refrigeration capacity and operating efficiency using the refrigerant returning pipe 31 and the refrigerant cooler 35 are improved.

(46) Moreover, here, the controller 8 controls the opening degree MVop of the liquid pressure adjusting expansion valve 37 such that the pressure Pol of the refrigerant in the outdoor liquid refrigerant pipe 25 at a part thereof provided with the refrigerant cooler 35 reaches the target liquid pressure Polt, in order to achieve the operation of reducing pressure in the outdoor liquid refrigerant pipe 25 as described above.

(47) For this reason, here, the pressure Pol of the refrigerant flowing through the refrigerant cooler 35 can be maintained to be high, which enables the refrigerant with a high level of wetness to reliably flow through the refrigerant cooler 35. Note that it is possible to accurately obtain the pressure Pol of the refrigerant in the outdoor liquid refrigerant pipe 25 at a part thereof provided with the refrigerant cooler 35 using the refrigerant cooling-side sensor 44 provided in the outdoor liquid refrigerant pipe 25 at a part thereof between the outdoor heat exchanger 24 and the refrigerant cooler 35 (here, a part between the outdoor expansion valve 36 and the liquid pressure adjusting expansion valve 37), and therefore, the liquid pressure adjusting expansion valve 37 can be controlled in a precise manner.

(48) In addition, here, by providing the outdoor expansion valve 36 in the outdoor liquid refrigerant pipe 25 at a part thereof between the outdoor heat exchanger 24 and the refrigerant cooler 35, the opening degree MVoo of the outdoor expansion valve 36 is controlled such that the subcooling degree SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger 24 (see the point C in FIG. 3) reaches the target subcooling degree SCot. For this reason, the pressure Pol of the refrigerant in the outdoor liquid refrigerant pipe 25 is likely to fall at the part provided with the refrigerant cooler 35 (see ΔPcd in FIG. 3). In contrast to this, here, the opening degree MVop of the liquid pressure adjusting expansion valve 37 is controlled such that the pressure Pol of the refrigerant in the outdoor liquid refrigerant pipe 25 at a part thereof provided with the refrigerant cooler 35 reaches the target liquid pressure Polt as described above.

(49) For this reason, here, although the outdoor expansion valve 36 reduces the pressure of the refrigerant flowing through the outdoor liquid refrigerant pipe 25 at a part thereof between the outdoor heat exchanger 24 and the refrigerant cooler 35, the pressure Pol of the refrigerant flowing through the refrigerant cooler 35 can be maintained to be high, which enables the refrigerant with a high level of wetness to reliably flow through the refrigerant cooler 35. Note that it is possible to correctly obtain the subcooling degree SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger 24 using the liquid-side outdoor heat exchange sensor 42 located in the outdoor liquid refrigerant pipe 25 at a part thereof between the outdoor heat exchanger 24 and the outdoor expansion valve 36, and therefore, the outdoor expansion valve 36 can be controlled in a precise manner.

(50) In contrast to this, a case is assumed where, in a configuration having the refrigerant returning pipe 31 and the refrigerant cooler 35, an amount of the refrigerant to be filled is reduced without the liquid pressure adjusting expansion valve 37 in the outdoor liquid refrigerant pipe 25 at a part thereof between the liquid refrigerant communication pipe 6 and the refrigerant cooler 35. That is, a case is assumed where only reduction in amount of the refrigerant to be filled is performed in the same configuration as PTL 3. In this case, unlike the refrigeration cycle illustrated by the two-dot chain line (that is, the refrigeration cycle of FIG. 3), as shown in FIG. 4, due to the less amount of the refrigerant to be filled, a refrigerant in a gas-liquid two-phase state is apt to flow out from the liquid-side end of the outdoor heat exchanger 24 (see the point C in FIG. 4). As a result, the refrigerant flows through the liquid refrigerant communication pipe 6 in a gas-liquid two-phase state, but the refrigeration capacity (ΔQhi 1 in FIG. 4) is decreased (ΔQhi 1<ΔQhi), which creates a necessity to increase an amount of the refrigerant circulation flow to compensate for the decreased capacity. However, when the amount of the refrigerant circulation flow is increased, the loss in pressure (see ΔPai1 in FIG. 4) in the gas refrigerant communication pipe 7 and the like is increased (ΔPai1>ΔPai). Consequently, the power consumption of the compressor 21 (Wab 1 in FIG. 4) increases (Wab 1>Wab) and also the operating efficiency (the value obtained by dividing ΔQhi1 by Wab1) falls.

(51) To address such a changing of the refrigerant into a gas-liquid two-phase state at the liquid-side end of the outdoor heat exchanger 24 that is caused by reduction in amount of the refrigerant to be filled, the pressure in the refrigerant can be significantly reduced by the outdoor expansion valve 36 connected to the liquid-side end of the outdoor heat exchanger 24. That is, in a configuration similar to that of PTL 3, the pressure in the refrigerant can be reduced by the outdoor expansion valve 36 connected to the liquid-side end of the outdoor heat exchanger 24 such that the refrigerant flows through the liquid refrigerant communication pipe 6 in a gas-liquid two-phase state, as in PTL 1 and 2. However, in this case, unlike the refrigeration cycle illustrated by the two-dot chain line (that is, the refrigeration cycle of FIG. 3), as shown in FIG. 5, a significant reduction in pressure of the refrigerant (see ΔPcd2 in FIG. 5) is caused by the outdoor expansion valve 36 connected to the liquid-side end of the outdoor heat exchanger 24, and then a pressure Pol2 of the refrigerant flowing through the refrigerant cooler 35 falls (Pol2<Pol), which disables a flow of the refrigerant with a high level of wetness through the refrigerant cooler 35 (see the points D, E, and F in FIG. 5). In addition, it is difficult to secure a difference in pressure (see ΔPad2 in FIG. 5) between the refrigerant flowing through the outdoor liquid refrigerant pipe 25 and the refrigerant flowing through the refrigerant returning pipe 31 (ΔPad2<ΔPad), and the cooling function (ΔQde2 in FIG. 5) of the refrigerant cooler is no longer able to be adequately fulfilled (ΔQde2<ΔQde). As a result, similarly to the case where only reduction in amount of the refrigerant to be filled is performed (refer to FIG. 4), the refrigerant flows through the liquid refrigerant communication pipe 6 in a gas-liquid two-phase state, but the refrigeration capacity (ΔQhi1 in FIG. 4) is decreased (ΔQhi1<ΔQhi), which creates a necessity to increase an amount of the refrigerant circulation flow to compensate for the decreased capacity. Then, when the amount of the refrigerant circulation flow is increased, the loss in pressure (refer to ΔPai2 in FIG. 5) in the gas refrigerant communication pipe 7 and the like is increased (ΔPai2>ΔPai). For this reason, the power consumption of the compressor 21 (Wab2 in FIG. 5) increases (Wab2>Wab) and also the operating efficiency (the value obtained by dividing ΔQhi2 by Wab2) falls.

(52) As described above, in the case (see FIG. 4) where only reduction in amount of the refrigerant to be filled is performed and the case (see FIG. 5) where reduction in pressure of the refrigerant is performed by the outdoor expansion valve 36 connected to the liquid-side end of the outdoor heat exchanger 24 so that the refrigerant flows through the liquid refrigerant communication pipe 6 in a gas-liquid two-phase state, unlike the case (see FIG. 3) where the liquid pressure adjusting expansion valve 37 is located in the outdoor liquid refrigerant pipe 25 at a part thereof between the liquid refrigerant communication pipe 6 and the refrigerant cooler 35, the amount of refrigerant to be filled into the refrigerant circuit 10 cannot be reduced while the refrigeration capacity and operating efficiency are improved using the refrigerant returning pipe 31 and the refrigerant cooler 35.

(3) Modified Examples

(53) <A>

(54) In the embodiment described above, in order to flow the refrigerant through the liquid refrigerant communication pipe 6 in a gas-liquid two-phase state and to flow the refrigerant through the outlet of the refrigerant cooler 35 in a liquid state, the opening degree MVoo of the outdoor expansion valve 36 is controlled such that the subcooling degree SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger 24 reaches the target subcooling degree SCot, and the opening degree MVop of the liquid pressure adjusting, expansion valve 37 is controlled such that the pressure Pol of the refrigerant in the outdoor liquid refrigerant pipe 25 at a part thereof provided with the refrigerant cooler 35 reaches the target liquid pressure Polt.

(55) However, the controls of the two expansion valves 36 and 37 are likely to affect each other, and this tends to make the opening degrees MVoo and MVop of both of the expansion valves 36 and 37 unstable. For example, in a state where the outdoor expansion valve 36 and the liquid pressure adjusting expansion valve 37 are stabilized at certain opening degrees (that is, a state of being stabilized at the target subcooling degree SCot and at the target liquid pressure Polt), when the opening degree MVoo of the outdoor expansion valve 36 is controlled to be increased, the pressure Pol of the refrigerant on the downstream side of the outdoor expansion valve 36 (that is, in the outdoor liquid refrigerant pipe 25 at a part thereof between the outdoor expansion valve 36 and the liquid pressure adjusting expansion valve 37) is changed for increasing. The change in pressure of the refrigerant caused by the change in the opening degree MVoo of the outdoor expansion valve 36 occurs considerably suddenly, and therefore, swift control of the opening degree MVop of the liquid pressure adjusting expansion valve 37 is required, but if the control sensitivity is excessively raised, the stability is impaired. As a result, the opening degree MVop of the liquid pressure adjusting expansion valve 37 and furthermore the opening degrees MVoo and MVop of both of the expansion valves 36 and 37 are likely to be unstable.

(56) Therefore, here, in controlling the liquid pressure adjusting expansion valve 37, the changeable range of opening degrees is restricted to the lower limit opening degree MVopm or higher, and the lower limit opening degree MVopm is revised according to the opening degree MVoo of the outdoor expansion valve 36, so as not to excessively raise the control sensitivity, but to swiftly follow the change in pressure of the refrigerant on the downstream side of the outdoor expansion valve 36 (that is, in the outdoor liquid refrigerant pipe 25 at a part thereof between the outdoor expansion valve 36 and the liquid pressure adjusting expansion valve 37) caused by controlling the opening degree of the outdoor expansion valve 36. Here, to revise the lower limit opening degree MVopm of the liquid pressure adjusting expansion valve 37, a function is set such that the lower limit opening degree MVopm of the liquid pressure adjusting expansion valve 37 increases as the opening degree MVoo of the outdoor expansion valve 36 increases, and thereby it is possible to revise the lower limit opening degree MVopm according to the function.

(57) As a result, here, although the controlling of the opening degree of the outdoor expansion valve 36 and the controlling of the opening degree of the liquid pressure adjusting expansion valve 37 are likely to affect each other, both of the expansion valves 36 and 37 can be controlled with good stability and follow-up performance.

(58) <B>

(59) In the embodiment and Modified Example A described above, controlling of the opening degree of the liquid pressure adjusting expansion valve 37 is performed by obtaining the pressure Pol of the refrigerant in the outdoor liquid refrigerant pipe 25 at a part thereof provided with the refrigerant cooler 35 from the pressure value of the refrigerant detected by the refrigerant cooling-side sensor 44 located in the outdoor liquid refrigerant pipe 25 at a part thereof between the outdoor expansion valve 36 and the liquid pressure adjusting expansion valve 37, as shown in FIG. 1.

(60) However, the pressure Pol of the refrigerant need not be obtained from the pressure of the refrigerant detected by the refrigerant cooling-side sensor 44 that is comprised of a pressure sensor, but may be obtained from a state quantity equivalent to the pressure of the refrigerant. For example, the refrigerant at the liquid-side end of the outdoor heat exchanger 24 including the downstream side of the outdoor expansion valve 36 is almost in the state of a saturation liquid (see the points C and D in FIG. 3), and therefore, as shown in FIG. 6, the pressure Pol of the refrigerant in the outdoor liquid refrigerant pipe 25 at a part thereof provided with the refrigerant cooler 35 may be obtained by providing the refrigerant cooling-side sensor 44 comprised of a temperature sensor in the outdoor liquid refrigerant pipe 25 at a part thereof between the outdoor heat exchanger 24 side and the liquid pressure adjusting expansion valve 37 and by converting a temperature value for the refrigerant detected by the refrigerant cooling-side sensor 44 into a saturation pressure.

(61) <C>

(62) In the embodiment and Modified Examples A and B described above, in order to flow the refrigerant through the liquid refrigerant communication pipe 6 in a gas-liquid two-phase state and in order to flow the refrigerant through the outlet of the refrigerant cooler 35 in a liquid state, the opening degree MVoo of the outdoor expansion valve 36 is controlled such that the subcooling degree SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger 24 reaches the target subcooling degree SCot, and the opening degree MVop of the liquid pressure adjusting expansion valve 37 is controlled such that the pressure Pol of the refrigerant in the outdoor liquid refrigerant pipe 25 at a part thereof provided with the refrigerant cooler 35 reaches the target liquid pressure Polt.

(63) However, the control to achieve the refrigerant that flows through the liquid refrigerant communication pipe 6 in a gas-liquid two-phase state and the refrigerant that flows through the outlet of the refrigerant cooler 35 in a liquid state is not limited to the one described above, and other control may be used. For example, although in the embodiment and Modified Examples A and B described above, the opening degree of the outdoor expansion valve 36 is controlled such that the subcooling degree SCo of the refrigerant at the liquid-side end of the outdoor heat exchanger 24 reaches the target subcooling degree SCot, the outdoor expansion valve 36 may be fully opened and then the controller 8 may control the opening degree MVop of the liquid pressure adjusting expansion valve 37 such that the subcooling degree SCo of the refrigerant reaches the target subcooling degree SCot. Note that the outdoor expansion valve 36 is fully open in the above control, but the control is not limited thereto, and the outdoor expansion valve 36 may be omitted.

(64) In this case, the opening degree of the liquid pressure adjusting expansion valve 37 is controlled to make the subcooling degree SCo of the refrigerant reach the target subcooling degree SCot, which facilitates the maintenance of the refrigerant in a liquid state that flows in the outdoor liquid refrigerant pipe 25 at a part thereof between the outdoor heat exchanger 24 and the liquid pressure adjusting expansion valve 37. As a result, as in the embodiment and Modified Examples A and B described above, the pressure of the refrigerant flowing through the refrigerant cooler 35 is unlikely to drop, and this enables to flow the refrigerant with a high level of wetness through the refrigerant cooler 35, and makes it easy to secure a difference in pressure (see ΔPad in FIG. 3) between the refrigerant flowing through the outdoor liquid refrigerant pipe 25 and the refrigerant flowing through the refrigerant returning pipe 31, and therefore, the cooling function (see ΔQde in FIG. 3) can be adequately fulfilled in the refrigerant cooler 35. Consequently, the flow rate of refrigerant sent to the plurality of indoor units 5a and 5b can be reduced, and the loss in pressure in the gas refrigerant communication pipe 7 and the like (see ΔPai in FIG. 3) can be decreased, which improves the refrigeration capacity (see ΔQhi in FIG. 3) and operating efficiency (the value obtained by dividing ΔQhi by Wab in FIG. 3).

(65) In this manner, even with the control configuration of this Modified Example, in the air conditioner 1 including the refrigerant circuit 10 configured by connecting, via the liquid refrigerant communication pipe 6 and the gas refrigerant communication pipe 7, the outdoor unit 2 having the compressor 21 and the outdoor heat exchanger 24, and the plurality of indoor units 5a and 5b having the indoor expansion valves 51a and 51b and the indoor heat exchangers 52a and 52b, reduction in the amount of the refrigerant to be filled into the refrigerant circuit 10 can be achieved while the refrigeration capacity and operating efficiency are improved using the refrigerant returning pipe 31 and the refrigerant cooler 35.

(66) <D>

(67) In the embodiment and Modified Examples A to C described above, the refrigerant returning pipe 31 is used as a refrigerant pipe for sending the refrigerant branched off from the outdoor liquid refrigerant pipe 25 to the suction side of the compressor 21, and a difference in pressure between the pressure of the refrigerant flowing through the outdoor liquid refrigerant pipe 25 and the low pressure of the refrigeration cycle is utilized to obtain the cooling function in the refrigerant cooler 35.

(68) However, the refrigerant returning pipe 31 is not limited thereto, and for example, as shown in FIG. 7, the refrigerant returning pipe 31 may be a refrigerant pipe for sending the refrigerant branched off from the outdoor liquid refrigerant pipe 25 into the middle of the compression process in the compressor 21, and a difference in pressure between the pressure of the refrigerant flowing through the outdoor liquid refrigerant pipe 25 and an intermediate pressure of the refrigeration cycle may be utilized, to obtain the cooling function in the refrigerant cooler 35. Note that in order to switch the refrigerant returning pipe 31 so as to make it function also as the refrigerant pipe for sending the refrigerant branched off from the outdoor liquid refrigerant pipe 25 to the suction side of the compressor 21, the refrigerant returning outlet pipe 33 of the refrigerant returning pipe 31 is configured to branch into two, whereby one branched pipe is connected into the middle of the compression process in the compressor 21 via a check valve 46, and the other branched pipe is connected to the suction side of the compressor 21 via a solenoid valve 47.

(69) In this case, unlike the embodiment and Modified Examples A to C described above, a portion of the refrigerant reduced in pressure by the outdoor expansion valve 36 and branched off by the refrigerant returning pipe 31 is reduced in pressure until it reaches a pressure close to the intermediate pressure of the refrigeration cycle by the refrigerant returning expansion valve 34. The refrigerant flowing through the refrigerant returning pipe 31 after being reduced in pressure by the refrigerant returning expansion valve 34 flows into the inlet of the refrigerant cooler 35 on the refrigerant returning pipe 31 side. The refrigerant flowing into the inlet of the refrigerant cooler 35 on the refrigerant returning pipe 31 side becomes a refrigerant in a gas state by being heated through heat exchange in the refrigerant cooler 35 with the refrigerant flowing through the outdoor liquid refrigerant pipe 25, flows out from the outlet of the refrigerant cooler 35 on the refrigerant returning pipe 31 side, and is returned into the middle of the compression process in the compressor 21. However, even in this case, as shown in FIG. 8, by providing the liquid pressure adjusting expansion valve 37 in the outdoor liquid refrigerant pipe 25 at a part thereof between the liquid refrigerant communication pipe 6 and the refrigerant cooler 35, the refrigerant flowing through the outdoor liquid refrigerant pipe 25 is reduced in pressure (see ΔPef in FIG. 8) so that the refrigerant flows through the liquid refrigerant communication pipe 6 in a gas-liquid two-phase state (see the points F and G in FIG. 8) and so that the refrigerant flows through the outlet of the refrigerant cooler 35 in a liquid state (see the point E in FIG. 8).

(70) For this reason, here, the pressure of the refrigerant flowing through the refrigerant cooler 35 is unlikely to fall, and the refrigerant can flow with a high level of wetness through the refrigerant cooler 35, and also a difference in pressure (see ΔPdj in FIG. 8) can be easily secured between the refrigerant flowing through the outdoor liquid refrigerant pipe 25 and the refrigerant flowing through the refrigerant returning pipe 31, and therefore, a cooling function (see ΔQde in FIG. 8) can be adequately fulfilled in the refrigerant cooler 35. Moreover, here, it is possible to increase the flow rate of the refrigerant to be returned into the middle of the compression process in the compressor 21 (see the point J in FIG. 8) via the refrigerant returning pipe 31, and therefore, the power consumption of the compressor 21 (see Wab in FIG. 8) also can be lowered. Consequently, it is possible to lower the flow rate of the refrigerant to be sent to the plurality of indoor units 5a and 5b and to decrease the loss in pressure in the gas refrigerant communication pipe 7 and the like (see ΔPai in FIG. 8), and therefore, improving the refrigeration capacity (see ΔQhi in FIG. 8) and operating efficiency (the value obtained by dividing ΔQhi by Wab).

(71) In this manner, even with the configuration of this modified example, in the air conditioner 1 including the refrigerant circuit 10 configured by connecting, via the liquid refrigerant communication pipe 6 and the gas refrigerant communication pipe 7, the outdoor unit 2 having the compressor 21 and the outdoor heat exchanger 24, and the plurality of indoor units 5a and 5b having the indoor expansion valves 51a and 51b and the indoor heat exchangers 52a and 52b, reduction in the amount of the refrigerant to be filled into the refrigerant circuit 10 can be achieved while the refrigeration capacity and operating efficiency are improved by the refrigerant returning pipe 31 and the refrigerant cooler 35.

(72) <E>

(73) In the embodiment and Modified examples A to D described above, the present invention is applied to the configuration having the refrigerant circuit 10 for performing a cooling operation as an example, but the present invention is not limited thereto, and it is possible to apply the present invention to any configuration for performing at least a cooling operation, including a configuration that includes a four-path switching valve in the outdoor unit 2 and has a refrigerant circuit so as to enable switching between a cooling operation and a heating operation. In addition, here, an air heat source type outdoor unit that has the outdoor fan 29 for supplying outside air as a heat source to be used in heat exchange with the refrigerant to the outdoor heat exchanger 24 is adopted as the outdoor unit 2, but the outdoor unit 2 is not limited thereto, and a water heat source type outdoor unit may be used as the outdoor unit 2 which does not have the outdoor fan 29 and uses water as a heat source to be used in heat exchange with the refrigerant in the outdoor heat exchanger 24.

INDUSTRIAL APPLICABILITY

(74) The present invention is widely applicable to an air conditioner that includes a refrigerant circuit configured by connecting, via a liquid refrigerant communication pipe and a gas refrigerant communication pipe, an outdoor unit having a compressor and an outdoor heat exchanger and a plurality of indoor units each having an indoor expansion valve and an indoor heat exchanger, and a refrigerant filled into the refrigerant circuit is circulated through in the sequence of the compressor, the outdoor heat exchanger, the liquid refrigerant communication pipe, the indoor expansion valve, the indoor heat exchanger, the gas refrigerant communication pipe, and the compressor.

REFERENCE SIGNS LIST

(75) 1 Air conditioner 2 Outdoor unit 5a, 5b Indoor unit 6 Liquid refrigerant communication pipe 7 Gas refrigerant communication pipe 8 Controller 10 Refrigerant circuit 21 Compressor 24 Outdoor heat exchanger 25 Outdoor liquid refrigerant pipe 31 Refrigerant returning pipe 35 Refrigerant cooler 36 Outdoor expansion valve 37 Liquid pressure adjusting expansion valve 42 Liquid-side outdoor heat exchange sensor 44 Refrigerant cooling-side sensor 51a, 51b Indoor expansion valve 52a, 52b Indoor heat exchanger

CITATION LIST

Patent Literature

(76) [Patent Literature 1 (PLT 1)]

(77) Japanese Unexamined Patent Application Publication No. S63-197853
[Patent Literature 2 (PLT 2)] Japanese Unexamined Patent Application Publication No. H5-332630
[Patent Literature 3 (PLT 3)] Japanese Unexamined Patent Application Publication No. 2010-236834