An air-conditioning system includes a plurality of indoor units; a relay unit including an intermediate heat exchanger configured to exchange heat between refrigerant and a heat medium; and a heat source unit configured to supply cooling energy or heating energy to each of the plurality of indoor units via the relay unit. The heat source unit and the relay unit are connected by a heat-source connection pipe through which the refrigerant flows, and the relay unit and the plurality of indoor units are connected by a load connection pipe through which the heat medium flows. The load connection pipe comprises a main pipe connecting between the relay unit and one of the indoor units provided at an end of the load connection pipe opposite to the relay unit. The main pipe has branch parts associated with the indoor units.

CROSS REFERENCE TO RELATED APPLICATION

This application is a U.S. national stage application of International Application No. PCT/JP2018/003915, filed on Feb. 6, 2018, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an air-conditioning system including a relay unit between a heat source unit and an indoor unit.

BACKGROUND

Hitherto, there is known an air-conditioning system in which a plurality of indoor units are connected to a heat source unit. In this air-conditioning system, refrigerant may circulate from the heat source unit to each indoor unit to transfer cooling energy or heating energy (see, for example, Patent Literature 1).

Further, reduction of refrigerant amounts is made obligatory by various regulations in recent years and the goal of reduction of refrigerant amounts tends to be stricter every year. Therefore, there has been developed an air-conditioning system in which heat generated by a heat source unit on a refrigerant circuit is supplied to each indoor unit on a heat medium circuit (see, for example, Patent Literature 2). In the air-conditioning system of Patent Literature 2, an intermediate heat exchanger configured to exchange heat between refrigerant flowing through the refrigerant circuit and a heat medium flowing through the heat medium circuit is provided in an outdoor unit.

Here, specific heat of the heat medium such as water is larger than specific heat of the refrigerant. That is, the temperature of the heat medium such as water is less changeable compared with the temperature of the refrigerant. Further, when the heat medium such as water and the refrigerant flow through pipes having the same diameter, greater power is required to convey the heat medium than is required to convey the refrigerant.

PATENT LITERATURE

Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2009-144940Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2016-90178

In the air-conditioning system of Patent Literature 2, however, a heat medium pipe runs from the intermediate heat exchanger provided in the outdoor unit to points near the indoor units, and branches from the points toward the indoor units. Therefore, the pipe length increases. Thus, the heat medium that is relatively large in terms of specific heat and power required for flowing moves by a long distance. As a result, problems arise in that the operation efficiency of the entire system decreases and the amount of energy consumption increases.

SUMMARY

The air-conditioning system of the present disclosure has been made to overcome the problems described above and aims to provide an air-conditioning system in which the operation efficiency of the entire system is increased and energy saving is achieved.

An air-conditioning system according to an embodiment of the present disclosure includes a plurality of indoor units; a relay unit including an intermediate heat exchanger configured to exchange heat between refrigerant and a heat medium; and a heat source unit including a compressor and a heat source-side heat exchanger and configured to supply cooling energy or heating energy to each of the plurality of indoor units via the relay unit, wherein the heat source unit and the relay unit are connected by a heat-source connection pipe through which the refrigerant flows, wherein the relay unit and the plurality of indoor units are connected by a load connection pipe through which the heat medium flows, wherein the load connection pipe comprises a main pipe connecting between the relay unit and one of the indoor units provided at an end of the load connection pipe opposite to the relay unit, wherein the main pipe has branch parts associated with the indoor units other than the one of the indoor units provided at the end of the load connection pipe opposite to the relay unit among the plurality of indoor units, and wherein a length of the main pipe from a connection part connected to the relay unit to a first branch part, which is closest to the relay unit of the branch parts, is smaller than a length of the heat-source connection pipe.

According to the embodiment of the present disclosure, the length of the main pipe from the relay unit to the first branch part is smaller than the length of the heat-source connection pipe. Thus, the amount of the heat medium that is larger than the refrigerant in terms of specific heat and power required for flowing can be reduced. Accordingly, the operation efficiency of the entire system can be increased and energy saving can be achieved.

DETAILED DESCRIPTION

FIG. 1is a circuit diagram exemplifying the structure of an air-conditioning system according to Embodiment 1 of the present disclosure. As illustrated inFIG. 1, an air-conditioning system100includes a heat source unit10, a relay unit20, and a plurality of indoor units30ato30c.FIG. 1exemplifies a case where the air-conditioning system100includes three indoor units30ato30c.

The heat source unit10supplies cooling energy or heating energy to each of the indoor units30ato30cvia the relay unit20. The heat source unit10includes a compressor11, a four-way valve12, a heat source-side heat exchanger13, a heat source-side expansion device14, and an accumulator15. The heat source unit10further includes a heat source-side fan16and a heat source-side controller17. The relay unit20includes an intermediate heat exchanger21configured to exchange heat between refrigerant and a heat medium, a relay unit expansion device22, a pump23, and a relay unit controller24. Each of the indoor units30ato30cincludes a load-side heat exchanger31, a flow control valve32, a load-side fan33, and a load-side controller34.

That is, in the air-conditioning system100, the compressor11, the four-way valve12, the heat source-side heat exchanger13, the heat source-side expansion device14, the relay unit expansion device22, the intermediate heat exchanger21, and the accumulator15are connected via a refrigerant pipe41to form a refrigerant circuit40through which refrigerant circulates. Examples of the refrigerant that circulates through the refrigerant circuit40herein include single-component refrigerants such as R-22 and R-134a, near-azeotropic refrigerant mixtures such as R-410A and R-404A, and zeotropic refrigerant mixtures such as R-407C. Note that refrigerants such as CF3CF═CH2having a double bond in the chemical formula and having a relatively low global warming potential, mixtures of those refrigerants, and natural refrigerants such as CO2and propane may be used as the refrigerant that circulates through the refrigerant circuit40.

Further, in the air-conditioning system100, the pump23, the intermediate heat exchanger21, and the load-side heat exchangers31and the flow control valves32of the indoor units30ato30care connected via a heat medium pipe61to form a heat medium circuit60through which a heat medium circulates. Examples of the heat medium herein include water and brine.

The heat source unit10and the relay unit20are connected by a heat-source connection pipe50that constitutes the refrigerant pipe41. The heat-source connection pipe50is constituted by a liquid-side connection pipe51and a gas-side connection pipe52. The liquid-side connection pipe51constitutes a liquid pipe55of the refrigerant pipe41and connects between a connection part10xof the heat source unit10and a connection part25xof the relay unit20. Here, the liquid pipe55connects between the heat source-side expansion device14and the relay unit expansion device22. Liquid refrigerant or two-phase refrigerant flows through the liquid pipe55. In Embodiment 1, two-phase refrigerant mainly flows through the liquid pipe55. During a cooling operation, the liquid-side connection pipe51allows refrigerant flowing out of the heat source unit10to flow therethrough to the relay unit20. During a heating operation, the liquid-side connection pipe51allows refrigerant flowing out of the relay unit20to flow therethrough to the heat source unit10.

The gas-side connection pipe52constitutes a gas pipe56of the refrigerant pipe41and connects between a connection part10yof the heat source unit10and a connection part25yof the relay unit20. In Embodiment 1, the gas pipe56connects between the intermediate heat exchanger21and the four-way valve12. That is, the gas pipe56is constituted by a pipe connecting the four-way valve12to the connection part10y, the gas-side connection pipe52, and a pipe connecting the connection part25yto the intermediate heat exchanger21. During the cooling operation, the gas-side connection pipe52allows refrigerant flowing out of the relay unit20to flow therethrough to the heat source unit10. During the heating operation, the gas-side connection pipe52allows the refrigerant flowing out of the heat source unit10to flow therethrough to the relay unit20.

The relay unit20and each of the indoor units30ato30care connected by a load connection pipe70that constitutes the heat medium pipe61. The load connection pipe70includes a main pipe80connecting the relay unit20to an indoor unit provided at an end of the load connection pipe opposite to the relay unit20. The main pipe80has branch parts associated with indoor units other than the indoor unit provided at the end of the load connection pipe opposite to the relay unit among the plurality of indoor units.

In Embodiment 1, the indoor unit30cis provided at the end of the load connection pipe opposite to the relay unit20. That is, the main pipe80connects between a connection part26xof the relay unit20and a connection part30xof the indoor unit30cand a connection part26yof the relay unit20to a connection part30yof the indoor unit30c. Further, the main pipe80has a first branch part61aassociated with the indoor unit30aand a second branch part62aassociated with the indoor unit30b. The length of a first main pipe80aof the main pipe80from a connection part26aconnected to the relay unit20to the first branch part61aclosest to the relay unit20is smaller than the length of the heat-source connection pipe50.

Here, the specific heat of the heat medium and power required for the heat medium to flow are larger than the specific heat of the refrigerant and power required for the refrigerant to flow. Therefore, the amount of the heat medium can be reduced by reducing the total length of the heat medium pipe61. Thus, the operation efficiency of the entire system can be increased and energy saving can be achieved. Further, the reduction in the amount of the heat medium can lead to a reduction in the amount of heat to be supplied to the heat medium when the air-conditioning system100is activated. Thus, the activation time of the air-conditioning system100can be reduced. Note that the installation places of the indoor units are determined depending on, for example, the structure of a building and the layout of rooms and therefore the length of the heat medium pipe61in a range from the first branch part61acannot be adjusted prior to works on site. In this respect, in Embodiment 1, the length of the first main pipe80a, which can be adjusted prior to works on site, is set smaller than the length of the heat-source connection pipe50. Thus, the total length of the heat medium pipe61is reduced.

More specifically, the main pipe80includes an departure main pipe81that allows the heat medium flowing out of the relay unit20to flow therethrough toward each of the indoor units30ato30c, and a return main pipe82that allows the heat medium flowing out of each of the indoor units30ato30cto flow therethrough toward the relay unit20. The departure main pipe81connects between the connection part26xand each connection part30x. The departure main pipe81has a first departure branch part61xand a second departure branch part62x. The return main pipe82connects between the connection part26yand each connection part30y. The return main pipe82has a first return branch part61yand a second return branch part62y.

That is, the first branch part61aincludes the first departure branch part61xprovided on the departure main pipe81and the first return branch part61yprovided on the return main pipe82. The second branch part62aincludes the second departure branch part62xprovided on the departure main pipe81and the second return branch part62yprovided on the return main pipe82. Further, the first main pipe80aincludes a first departure main pipe81a, which is a part of the departure main pipe81and connects between the connection part26xand the first departure branch part61x, and a first return main pipe82a, which is a part of the return main pipe82and connects between the connection part26yand the first return branch part61y. Further, the total length of the first departure main pipe81aand the first return main pipe82ais smaller than the total length of the liquid-side connection pipe51and the gas-side connection pipe52. For example, both the first departure main pipe81aand the first return main pipe82amay be shorter than the liquid-side connection pipe51and the gas-side connection pipe52.

The load connection pipe70includes a branch pipe91connecting the main pipe80to the indoor unit30aand a branch pipe92connecting the main pipe80to the indoor unit30b. The branch pipe91is connected to the main pipe80at the first branch part61a. The branch pipe92is connected to the main pipe80at the second branch part62a.

The branch pipe91includes an departure branch pipe91xconnecting the first departure branch part61xto the connection part30xof the indoor unit30a, and a return branch pipe91yconnecting the connection part30yof the indoor unit30ato the first return branch part61y. The branch pipe92includes an departure branch pipe92xconnecting the second departure branch part62xto the connection part30xof the indoor unit30b, and a return branch pipe92yconnecting the connection part30yof the indoor unit30bto the second return branch part62y.

For example, the compressor11includes a compressor motor (not illustrated) to be driven by an inverter to suck and compress refrigerant. The four-way valve12is connected to the compressor11and is controlled by the heat source-side controller17to switch refrigerant passages. The four-way valve12selects a solid-line passage inFIG. 1during the cooling operation in which cooling energy is supplied to each of the indoor units30ato30c. On the other hand, the four-way valve12selects a broken-line passage inFIG. 1during the heating operation in which heating energy is supplied to each of the indoor units30ato30c.

For example, the heat source-side heat exchanger13is a fin-and-tube heat exchanger and exchanges heat between outdoor air and refrigerant flowing through the refrigerant circuit40. The heat source-side heat exchanger13functions as a condenser during the cooling operation, that is, when the heat source unit10serves as a cooling energy source. On the other hand, the heat source-side heat exchanger13functions as an evaporator during the heating operation, that is, when the heat source unit10serves as a heating energy source. For example, the heat source-side expansion device14is an electronic expansion valve and expands refrigerant by reducing a pressure of the refrigerant. The heat source-side expansion device14is provided in downstream of the heat source-side heat exchanger13during the cooling operation. During the cooling operation, the heat source-side expansion device14is controlled by the heat source-side controller17to generate two-phase refrigerant by reducing a pressure of high-pressure refrigerant flowing into the heat source-side expansion device14from the heat source-side heat exchanger13. The accumulator15is provided in upstream of the compressor11and accumulates surplus refrigerant to suppress inflow of liquid refrigerant into the compressor11. The heat source-side fan16sends outdoor air to the heat source-side heat exchanger13.

For example, the intermediate heat exchanger21is a plate heat exchanger and is connected between the refrigerant circuit40and the heat medium circuit60. The intermediate heat exchanger21exchanges heat between the refrigerant circulating through the refrigerant circuit40and the heat medium circulating through the heat medium circuit60. The intermediate heat exchanger21functions as an evaporator during the cooling operation and as a condenser during the heating operation. For example, the relay unit expansion device22is an electronic expansion valve and expands refrigerant by reducing a pressure of the refrigerant. The relay unit expansion device22is provided in downstream of the intermediate heat exchanger21during the heating operation. During the heating operation, the relay unit expansion device22is controlled by the relay unit controller24to generate two-phase refrigerant by reducing a pressure of high-pressure refrigerant flowing into the relay unit expansion device22from the intermediate heat exchanger21.

For example, the pump23includes a motor (not illustrated) to be driven by an inverter. The pump23is driven by the motor serving as a power source to circulate the heat medium through the heat medium circuit60. That is, the pump23is controlled by the relay unit controller24to apply a pressure by which the heat medium circulates through the heat medium circuit60.

For example, the load-side heat exchanger31is a fin-and-tube heat exchanger and exchanges heat between indoor air and the heat medium flowing through the heat medium circuit60. For example, the flow control valve32is an electronic expansion valve and is controlled by the load-side controller34to control the amount of the heat medium to be caused to flow into the load-side heat exchanger31. It is appropriate that the flow control valve32be provided in downstream of the load-side heat exchanger31.

The load-side fan33sends indoor air to the load-side heat exchanger31. The load-side controller34controls the opening degree of the flow control valve32. The load-side controller34of each of the indoor units30ato30ccan perform data communication with the heat source-side controller17of the heat source unit10and with the relay unit controller24of the relay unit20.

The heat source unit10includes a suction pressure sensor11aand a discharge pressure sensor11b. The suction pressure sensor11ais provided on a suction side of the compressor11and measures a suction pressure Ps, which is a pressure of refrigerant to be sucked into the compressor11. The discharge pressure sensor11bis provided on a discharge side of the compressor11and measures a discharge pressure Pd, which is a pressure of refrigerant discharged from the compressor11. The suction pressure sensor11aand the discharge pressure sensor11boutput measurement data to the heat source-side controller17.

The relay unit20includes a first temperature sensor21aand a second temperature sensor21b. The first temperature sensor21ameasures a first temperature, which is a temperature of refrigerant flowing through a passage between the intermediate heat exchanger21and the compressor11. In Embodiment 1, the first temperature sensor21ais provided in the relay unit20between the intermediate heat exchanger21and the four-way valve12. The second temperature sensor21bis provided between the intermediate heat exchanger21and the relay unit expansion device22and measures a second temperature, which is a temperature of refrigerant flowing through a passage between the intermediate heat exchanger21and the relay unit expansion device22.

The first temperature sensor21ais provided in downstream of the intermediate heat exchanger21during the cooling operation. The second temperature sensor21bis provided in downstream of the intermediate heat exchanger21during the heating operation. The first temperature sensor21aand the second temperature sensor21boutput measurement data to the relay unit controller24.

The heat source-side controller17controls actions of the compressor11, the four-way valve12, and the heat source-side expansion device14. The heat source-side controller17includes a heat source-side storage17athat stores, for example, data for use in various arithmetic operations. The heat source-side controller17can perform data communication with the relay unit controller24of the relay unit20and with the load-side controller34of each of the indoor units30ato30c.

During the cooling operation, the heat source-side controller17determines a degree of superheat at an outlet of an evaporator by using the suction pressure Ps measured by the suction pressure sensor11aand the first temperature measured by the first temperature sensor21a. The degree of superheat at the outlet of the evaporator is a degree of superheat at an outlet of the intermediate heat exchanger21that functions as the evaporator during the cooling operation, and is hereinafter referred to as a degree of superheat. More specifically, the heat source-side controller17determines an evaporating temperature by converting the suction pressure Ps into a saturation temperature during the cooling operation. The heat source-side controller17acquires the first temperature via the relay unit controller24. The heat source-side controller17determines the degree of superheat by subtracting the evaporating temperature from the first temperature.

The heat source-side controller17controls the opening degree of the heat source-side expansion device14based on the determined degree of superheat. When the degree of superheat is higher than a reference degree of superheat, the heat source-side controller17performs control to increase the opening degree of the heat source-side expansion device14. When the degree of superheat is lower than the reference degree of superheat, the heat source-side controller17performs control to reduce the opening degree of the heat source-side expansion device14. For example, the reference degree of superheat is determined through tests conducted for an actual device. When the degree of superheat equals the reference degree of superheat, the refrigerant flowing into the liquid pipe55from the heat source-side expansion device14is in a two-phase state. For example, the reference degree of superheat is set to 1 degree Celsius to 2 degrees Celsius but may be changed as appropriate depending on characteristics of the refrigerant circuit40and an installation environment of the air-conditioning system100.

More specifically, the heat source-side storage17amay store, for example, a heat source-side opening degree deriving function for deriving the opening degree of the heat source-side expansion device14with the degree of superheat as a variable. In this case, the heat source-side controller17can determine the opening degree of the heat source-side expansion device14that is associated with the degree of superheat by substituting the degree of superheat into the heat source-side opening degree deriving function. Further, the heat source-side storage17amay store a heat source-side opening degree table in which the degree of superheat is associated with the opening degree of the heat source-side expansion device14. In this case, the heat source-side controller17can determine the opening degree of the heat source-side expansion device14that is associated with the degree of superheat by referring to the degree of superheat in the heat source-side opening degree table. Further, it is appropriate that the heat source-side controller17adjust the opening degree of the heat source-side expansion device14to the determined opening degree.

Further, if the reference degree of superheat is stored in the heat source-side storage17a, the heat source-side controller17may determine a difference between the degree of superheat and the reference degree of superheat and control the opening degree of the heat source-side expansion device14based on the determined difference. In this case, the heat source-side opening degree deriving function is a function in which the difference between the degree of superheat and the reference degree of superheat is used as a variable. Similarly, in the heat source-side opening degree table, the difference is associated with the opening degree of the heat source-side expansion device14. Here, the heat source-side opening degree deriving function and the heat source-side opening degree table may be provided so that an amount of adjustment for the opening degree of the heat source-side expansion device14is derived instead of the opening degree of the heat source-side expansion device14.

The relay unit controller24controls actions of the relay unit expansion device22and the pump23. The relay unit controller24includes a relay unit storage24athat stores, for example, data for use in various arithmetic operations. The relay unit controller24can perform data communication with the heat source-side controller17of the heat source unit10and with the load-side controller34of each of the indoor units30ato30c.

During the heating operation, the relay unit controller24determines a degree of subcooling at an outlet of a condenser by using the discharge pressure Pd measured by the discharge pressure sensor11band the second temperature measured by the second temperature sensor21b. The degree of subcooling at the outlet of the condenser is a degree of subcooling at an outlet of the intermediate heat exchanger21that functions as the condenser during the heating operation, and is hereinafter referred to as a degree of subcooling. More specifically, the relay unit controller24acquires the discharge pressure Pd via the heat source-side controller17during the heating operation and determines a condensing temperature by converting the acquired discharge pressure Pd into a saturation temperature. The relay unit controller24acquires the second temperature from the second temperature sensor21b. The relay unit controller24determines the degree of subcooling by subtracting the second temperature from the condensing temperature.

The relay unit controller24controls the opening degree of the relay unit expansion device22based on the determined degree of subcooling. When the degree of subcooling is higher than a reference degree of subcooling, the relay unit controller24performs control to increase the opening degree of the relay unit expansion device22. When the degree of subcooling is lower than the reference degree of subcooling, the relay unit controller24performs control to reduce the opening degree of the relay unit expansion device22. For example, the reference degree of subcooling is determined through tests conducted for an actual device. When the degree of subcooling equals the reference degree of subcooling, the refrigerant flowing into the liquid pipe55from the relay unit expansion device22is in a two-phase state. For example, the reference degree of subcooling is set to 5 degrees Celsius to 6 degrees Celsius but may be changed as appropriate depending on the characteristics of the refrigerant circuit40and the installation environment of the air-conditioning system100.

More specifically, the relay unit storage24amay store, for example, a relay unit opening degree deriving function for deriving the opening degree of the relay unit expansion device22with the degree of subcooling as a variable. In this case, the relay unit controller24can determine the opening degree of the relay unit expansion device22that is associated with the degree of subcooling by substituting the degree of subcooling into the relay unit opening degree deriving function. Further, the relay unit storage24amay store a relay unit opening degree table in which the degree of subcooling is associated with the opening degree of the relay unit expansion device22. In this case, the relay unit controller24can determine the opening degree of the relay unit expansion device22that is associated with the degree of subcooling by referring to the degree of subcooling in the relay unit opening degree table. Further, it is appropriate that the relay unit controller24adjust the opening degree of the relay unit expansion device22to the determined opening degree.

Further, if the reference degree of subcooling is stored in the relay unit storage24a, the relay unit controller24may determine a difference between the degree of subcooling and the reference degree of subcooling and control the opening degree of the relay unit expansion device22based on the determined difference. In this case, the relay unit opening degree deriving function is a function in which the difference between the degree of subcooling and the reference degree of subcooling is used as a variable. Similarly, in the relay unit opening degree table, the difference is associated with the opening degree of the relay unit expansion device22. Here, the relay unit opening degree deriving function and the relay unit opening degree table may be provided so that an amount of adjustment for the opening degree of the relay unit expansion device22is derived.

The heat source-side controller17, the relay unit controller24, and the load-side controller34of each of the indoor units30ato30ccan be constituted by a processor such as a microcomputer, and software that implements the functions described above in cooperation with the processor. Note that the heat source-side controller17, the relay unit controller24, and the load-side controller34of each of the indoor units30ato30cmay include hardware such as a circuit device that partially or entirely implements the functions described above.

FIG. 2is a p-h diagram illustrating states of refrigerant during the cooling operation in the refrigerant circuit ofFIG. 1.FIG. 3is a p-h diagram illustrating states of refrigerant during the heating operation in the refrigerant circuit ofFIG. 1. In the p-h diagrams ofFIG. 2andFIG. 3, the horizontal axis represents specific enthalpy and the vertical axis represents pressure. Further, in the p-h diagrams ofFIG. 2andFIG. 3, symbols representing the heat source-side expansion device14, the liquid pipe55, and the relay unit expansion device22are placed at points associated with changes of the state of refrigerant in the heat source-side expansion device14, the liquid pipe55, and the relay unit expansion device22, respectively. Actions of the refrigerant circuit40in the air-conditioning system100are described with reference toFIG. 2andFIG. 3.

First, an action of the refrigerant circuit40during the cooling operation is described with reference toFIG. 2. In Embodiment 1, the heat source-side controller17controls the opening degree of the heat source-side expansion device14during the cooling operation based on the degree of superheat of refrigerant flowing out of the heat source-side heat exchanger13so that the refrigerant flowing out of the heat source-side expansion device14is in a two-phase state. During the cooling operation, the relay unit controller24opens the relay unit expansion device22. The relay unit controller24may maximize the opening degree of the relay unit expansion device22during the cooling operation.

High-temperature and high-pressure gas refrigerant discharged from the compressor11(point A inFIG. 2) flows into the heat source-side heat exchanger13via the four-way valve12. During the cooling operation, the heat source-side heat exchanger13functions as the condenser. That is, the heat source-side heat exchanger13exchanges heat between the refrigerant flowing through the heat source-side heat exchanger13and outdoor air sent by the heat source-side fan16, and condensing heat of the refrigerant is released into the outdoor air. Thus, the refrigerant flowing into the heat source-side heat exchanger13is condensed into high-pressure liquid refrigerant (point B inFIG. 2).

The high-pressure liquid refrigerant flowing out of the heat source-side heat exchanger13flows into the heat source-side expansion device14and the pressure of the refrigerant is reduced. Thus, the refrigerant turns into medium-pressure two-phase refrigerant having a pressure lower than a high-pressure-side pressure of the refrigerant circuit40and higher than a low-pressure-side pressure of the refrigerant circuit40(point C inFIG. 2). The medium-pressure two-phase refrigerant flowing out of the heat source-side expansion device14flows through the liquid pipe55and then through the relay unit expansion device22. The pressure of the refrigerant flowing through the liquid pipe55and the relay unit expansion device22is reduced by pressure loss in the liquid pipe55and the relay unit expansion device22and the refrigerant turns into low-pressure two-phase refrigerant (point D and point E inFIG. 2).

The low-pressure two-phase refrigerant flowing through the relay unit expansion device22flows into the intermediate heat exchanger21. The intermediate heat exchanger21exchanges heat between the heat medium and the refrigerant flowing through the intermediate heat exchanger21. During the cooling operation, the intermediate heat exchanger21functions as the evaporator. That is, the refrigerant flowing into the intermediate heat exchanger21is evaporated into low-pressure gas refrigerant (point F inFIG. 2). On the other hand, the heat medium flowing into the intermediate heat exchanger21is cooled by a heat removing action of the refrigerant.

The low-pressure gas refrigerant evaporated by the intermediate heat exchanger21flows through the gas pipe56and the four-way valve12and the pressure of the refrigerant is reduced by pressure loss. Then, the refrigerant is sucked into the compressor11(point G inFIG. 2). The low-pressure gas refrigerant sucked into the compressor11is compressed into high-temperature and high-pressure gas refrigerant (point A inFIG. 2). During the cooling operation, the sequential cycle described above is repeated.

Next, an action of the refrigerant circuit40during the heating operation is described with reference toFIG. 3. During the heating operation, the heat source-side controller17operates the four-way valve12so that the passage is switched to the broken-line passage inFIG. 1. Thus, high-temperature and high-pressure refrigerant discharged from the compressor11flows into the intermediate heat exchanger21via the heat-source connection pipe50. During the heating operation, the heat source-side controller17opens the heat source-side expansion device14. The heat source-side controller17may maximize the opening degree of the heat source-side expansion device14during the heating operation. Further, the relay unit controller24controls the opening degree of the relay unit expansion device22during the heating operation based on the degree of subcooling of the refrigerant flowing out of the intermediate heat exchanger21so that the refrigerant flowing out of the relay unit expansion device22is in a two-phase state.

That is, the high-temperature and high-pressure gas refrigerant discharged from the compressor11(point A inFIG. 3) flows through the four-way valve12and the gas pipe56and the pressure of the refrigerant is reduced by pressure loss. Then, the refrigerant flows into the intermediate heat exchanger21(point B inFIG. 3). During the cooling operation, the intermediate heat exchanger21functions as the condenser. That is, the intermediate heat exchanger21exchanges heat between the heat medium and the refrigerant flowing through the intermediate heat exchanger21, and condensing heat of the refrigerant is transferred to the heat medium. Thus, the refrigerant flowing into the intermediate heat exchanger21is condensed into high-pressure liquid refrigerant (point C inFIG. 3). Note that the heat medium flowing into the intermediate heat exchanger21is heated by a heat rejecting action of the refrigerant.

The high-pressure liquid refrigerant condensed by the intermediate heat exchanger21flows into the relay unit expansion device22and the pressure of the refrigerant is reduced. Thus, the refrigerant turns into medium-pressure two-phase refrigerant (point D inFIG. 3). The medium-pressure two-phase refrigerant flowing out of the relay unit expansion device22flows through the liquid pipe55and through the fully open heat source-side expansion device14. The pressure of the refrigerant flowing through the liquid pipe55and the heat source-side expansion device14is reduced by pressure loss in the liquid pipe55and the heat source-side expansion device14and the refrigerant turns into low-pressure two-phase refrigerant (point E and point F inFIG. 3).

The low-pressure two-phase refrigerant flowing through the heat source-side expansion device14flows into the heat source-side heat exchanger13. During the heating operation, the heat source-side heat exchanger13functions as the evaporator. That is, the heat source-side heat exchanger13exchanges heat between the refrigerant flowing through the heat source-side heat exchanger13and outdoor air sent by the heat source-side fan16. Thus, the refrigerant flowing into the heat source-side heat exchanger13is evaporated into low-pressure gas refrigerant (point G inFIG. 3). The low-pressure gas refrigerant flowing out of the heat source-side heat exchanger13is sucked into the compressor11through the four-way valve12and is compressed into high-temperature and high-pressure gas refrigerant (point A inFIG. 3). During the heating operation, the sequential cycle described above is repeated.

Here, in the air-conditioning system100, the relay unit20is disposed as close to the indoor units as possible so that the total length of the heat medium pipe61decreases. Therefore, the amount of the heat medium is reduced. Thus, the total length of the refrigerant pipe41is larger than that in a case where the relay unit20is disposed closer to the heat source unit10. In this respect, in the air-conditioning system100, the refrigerant in the liquid pipe55is in the two-phase state during both the cooling operation and the heating operation as described above. That is, according to the air-conditioning system100, the density of the refrigerant in the refrigerant pipe41can be reduced and therefore the amount of refrigerant to be charged into the refrigerant pipe41can be reduced.

As described above, in the air-conditioning system100of Embodiment 1, the length of the first main pipe80aof the main pipe80from the relay unit20to the first branch part61ais smaller than the length of the heat-source connection pipe50. Thus, the amount of the heat medium that is larger than the refrigerant in terms of specific heat and power required for flowing can be reduced. Accordingly, the operation efficiency of the entire system can be increased and energy saving can be achieved.

That is, in the air-conditioning system100, the total length of the first departure main pipe81aand the first return main pipe82ais smaller than the total length of the liquid-side connection pipe51and the gas-side connection pipe52. Therefore, the amount of the heat medium can be reduced to the extent corresponding to the reduction in the length of the heat medium pipe61of the heat medium circuit60. Thus, the amount of heat to be supplied to the heat medium can be reduced and therefore the activation time can be reduced. Further, the power required for the heat medium to flow by the pump23can be reduced and the operation efficiency of the entire system can be increased.

Further, the diameter of the heat medium pipe through which the heat medium such as water circulates is larger than the diameter of the refrigerant pipe through which the refrigerant circulates. Therefore, the cost per unit length is higher in the heat medium pipe than in the refrigerant pipe and the installation cost is also higher in the heat medium circuit60than in the refrigerant circuit40. Further, the pipe length from each branch part of the main pipe of the heat medium pipe to each indoor unit cannot be set in advance because the pipe length is determined during works on site. In this respect, in the air-conditioning system100, the length of the main pipe80from the relay unit20to the first branch part61ais smaller than the length of the heat-source connection pipe50. Thus, the costs for materials and other costs can be reduced.

In addition, in the air-conditioning system100, the relay unit20including the intermediate heat exchanger21is interposed between the heat source unit10and each of the indoor units30ato30c. Therefore, the refrigerant amount and the activation time can be reduced compared with those in the structure of, for example, Patent Literature 1 in which the refrigerant circulates in a wide range from the heat source unit to each indoor unit.

Further, during the cooling operation in which the heat source-side heat exchanger13functions as the condenser, the refrigerant to be caused to flow into the relay unit20is brought into the two-phase state by the heat source-side expansion device14. More specifically, when the degree of superheat at the outlet of the intermediate heat exchanger21that functions as the evaporator is higher than the reference degree of superheat, the heat source-side controller17performs control to increase the opening degree of the heat source-side expansion device14. When the degree of superheat at the outlet of the intermediate heat exchanger21that functions as the evaporator is lower than the reference degree of superheat, on the other hand, the heat source-side controller17performs control to reduce the opening degree of the heat source-side expansion device14. With this structure, the refrigerant flowing from the heat source-side expansion device14into the liquid pipe55is in the two-phase state and therefore the density of the refrigerant can be reduced. Thus, the refrigerant charge amount can be reduced. That is, the volume of gas refrigerant is larger than the volume of liquid refrigerant and therefore the refrigerant amount can be reduced to the extent corresponding to the gas refrigerant in the two-phase gas-liquid refrigerant compared with the refrigerant amount in a case where liquid refrigerant flows through the liquid pipe55.

Further, during the heating operation in which the intermediate heat exchanger21functions as the condenser, the refrigerant to be caused to flow into the heat source unit10is brought into the two-phase state by the relay unit expansion device22. More specifically, when the degree of subcooling at the outlet of the intermediate heat exchanger21that functions as the condenser is higher than the reference degree of subcooling, the relay unit controller24performs control to increase the opening degree of the relay unit expansion device22. Further, when the degree of subcooling at the outlet of the intermediate heat exchanger21that functions as the condenser is lower than the reference degree of subcooling, the relay unit controller24performs control to reduce the opening degree of the relay unit expansion device22. With this structure, the refrigerant flowing from the relay unit expansion device22into the liquid pipe55is in the two-phase state and therefore the density of the refrigerant can be reduced. Thus, the refrigerant charge amount can be reduced.

That is, in the air-conditioning system100, the refrigerant in the liquid pipe55is in the two-phase state during both the cooling operation and the heating operation. Therefore, the refrigerant amount can be reduced compared with that in the case where liquid refrigerant flows through the liquid pipe55. Thus, according to the air-conditioning system100, the amount of flow of the heat medium can be reduced by reducing the amount of heat to be supplied to the heat medium, and the amount of flow of the refrigerant can be reduced by reducing the amount of heat to be supplied to the refrigerant. Thus, the operation efficiency of the entire system can be improved and energy saving can be achieved.

FIG. 4is a circuit diagram exemplifying the structure of an air-conditioning system according to Embodiment 2 of the present disclosure. An air-conditioning system200of Embodiment 2 differs from the air-conditioning system100of Embodiment 1 in terms of disposition of a part of the sensors. Components equivalent to those of Embodiment 1 are represented by the same reference signs and description thereof is omitted.

As illustrated inFIG. 4, the air-conditioning system200includes, in place of the first temperature sensor21a, a first temperature sensor15aprovided in upstream of the accumulator15in a heat source unit210. The first temperature sensor15aoutputs the measured first temperature to a heat source-side controller217.

That is, the heat source-side controller217determines an evaporating temperature by converting the suction pressure Ps measured by the suction pressure sensor11ainto a saturation temperature during the cooling operation. Further, the heat source-side controller217directly acquires the first temperature from the first temperature sensor15a. Then, the heat source-side controller217determines the degree of superheat at the outlet of the evaporator by subtracting the evaporating temperature from the first temperature. The other structure of the heat source-side controller217is similar to that of the heat source-side controller17of Embodiment 1.

Here, in the air-conditioning system100of Embodiment 1, the heat source-side controller17acquires the first temperature from the first temperature sensor21adisposed in the relay unit20. However, a part of the refrigerant pipe41between the intermediate heat exchanger21and the accumulator15is exposed to, for example, outdoor air and therefore the refrigerant temperature changes while the refrigerant is flowing through this part. Thus, when the degree of superheat is determined by using the first temperature measured by the first temperature sensor21a, a deviation may occur in the degree of superheat.

In this respect, in Embodiment 2, the heat source-side controller217determines the degree of superheat by using the first temperature measured by the first temperature sensor15aprovided in the upstream of the accumulator15. Thus, according to the air-conditioning system200of Embodiment 2, the degree of superheat can be determined with higher accuracy and therefore a low-energy operation can be achieved in consideration of loss of heat removal or rejection in the pipe between the intermediate heat exchanger21and the accumulator15.

Further, in the air-conditioning system200of Embodiment 2, the amount of the heat medium that is larger than the refrigerant in terms of specific heat and power required for flowing can be reduced similarly to the air-conditioning system100of Embodiment 1. Thus, the operation efficiency of the entire system can be increased and energy saving can be achieved. Other advantages are similar to those of Embodiment 1.

FIG. 5is a circuit diagram exemplifying the structure of an air-conditioning system according to Embodiment 3 of the present disclosure. An air-conditioning system300of Embodiment 3 differs from the air-conditioning systems of Embodiments 1 and 2 in terms of the structures of the sensors configured to measure various types of data. Components equivalent to those of Embodiments 1 and 2 are represented by the same reference signs and description thereof is omitted.

As illustrated inFIG. 5, the air-conditioning system300includes a first pressure sensor55aand a second pressure sensor55b. The first pressure sensor55ais provided on the liquid pipe55in a heat source unit310and measures a first pressure Pm1of refrigerant flowing through the liquid pipe55. The first pressure sensor55aoutputs the measured first pressure Pm1to a heat source-side controller317. The second pressure sensor55bis provided on the liquid pipe55in a relay unit320and measures a second pressure Pmt of the refrigerant flowing through the liquid pipe55. The second pressure sensor55boutputs the measured second temperature to a relay unit controller324.

During the cooling operation, the heat source-side controller317controls the opening degree of the heat source-side expansion device14based on the first pressure Pm1measured by the first pressure sensor55a. When the first pressure Pm1is lower than a first reference pressure, the heat source-side controller317performs control to increase the opening degree of the heat source-side expansion device14. When the first pressure Pm1is higher than the first reference pressure, the heat source-side controller317performs control to reduce the opening degree of the heat source-side expansion device14. For example, the first reference pressure is determined through tests conducted for an actual device. When the first pressure Pm1equals the first reference pressure, the refrigerant flowing into the liquid pipe55from the heat source-side expansion device14is in a two-phase state. The first reference pressure may be changed as appropriate depending on, for example, the characteristics of the refrigerant circuit40and an installation environment of the air-conditioning system300.

More specifically, the heat source-side storage17amay store, for example, a heat source-side opening degree deriving function for deriving the opening degree of the heat source-side expansion device14with the first pressure Pm1as a variable. In this case, the heat source-side controller317can determine the opening degree of the heat source-side expansion device14that is associated with the first pressure Pm1by substituting the first pressure Pm1into the heat source-side opening degree deriving function. Further, the heat source-side storage17amay store a heat source-side opening degree table in which the first pressure Pm1is associated with the opening degree of the heat source-side expansion device14. In this case, the heat source-side controller317can determine the opening degree of the heat source-side expansion device14that is associated with the first pressure Pm1by referring to the first pressure Pm1in the heat source-side opening degree table. Further, it is appropriate that the heat source-side controller317adjust the opening degree of the heat source-side expansion device14to the determined opening degree.

Further, if the first reference pressure is stored in the heat source-side storage17a, the heat source-side controller317may determine a difference between the first pressure Pm1and the first reference pressure and control the opening degree of the heat source-side expansion device14based on the determined difference. In this case, the heat source-side opening degree deriving function is a function in which the difference between the first pressure Pm1and the first reference pressure is used as a variable. Similarly, in the heat source-side opening degree table, the difference is associated with the opening degree of the heat source-side expansion device14. The heat source-side opening degree deriving function and the heat source-side opening degree table may be provided so that an amount of adjustment for the opening degree of the heat source-side expansion device14is derived instead of the opening degree of the heat source-side expansion device14. The other structure of the heat source-side controller317is similar to that of the heat source-side controller17of Embodiment 1.

During the heating operation, the relay unit controller324controls the opening degree of the relay unit expansion device22based on the second pressure Pm2measured by the second pressure sensor55b. When the second pressure Pm2is lower than a second reference pressure, the relay unit controller324performs control to increase the opening degree of the relay unit expansion device22. When the second pressure Pm2is higher than the second reference pressure, the relay unit controller324performs control to reduce the opening degree of the relay unit expansion device22. For example, the second reference pressure is determined through tests conducted for an actual device. When the second pressure Pm2equals the second reference pressure, the refrigerant flowing into the liquid pipe55from the relay unit expansion device22is in a two-phase state. The second reference pressure may be changed as appropriate depending on, for example, the characteristics of the refrigerant circuit40and the installation environment of the air-conditioning system300.

More specifically, the relay unit storage24amay store, for example, a relay unit opening degree deriving function for deriving the opening degree of the relay unit expansion device22with the second pressure Pm2as a variable. In this case, the relay unit controller324can determine the opening degree of the relay unit expansion device22that is associated with the second pressure Pm2by substituting the second pressure Pm2into the relay unit opening degree deriving function. Further, the relay unit storage24amay store a relay unit opening degree table in which the second pressure Pm2is associated with the opening degree of the relay unit expansion device22. In this case, the relay unit controller324can determine the opening degree of the relay unit expansion device22that is associated with the second pressure Pm2by referring to the second pressure Pm2in the relay unit opening degree table. Further, it is appropriate that the relay unit controller324adjust the opening degree of the relay unit expansion device22to the determined opening degree.

Further, if the second reference pressure is stored in the relay unit storage24a, the relay unit controller324may determine a difference between the second pressure Pm2and the second reference pressure and control the opening degree of the relay unit expansion device22based on the determined difference. In this case, the relay unit opening degree deriving function is a function in which the difference between the second pressure Pm2and the second reference pressure is used as a variable. Similarly, in the relay unit opening degree table, the difference is associated with the opening degree of the relay unit expansion device22. The relay unit opening degree deriving function and the relay unit opening degree table may be provided so that an amount of adjustment for the opening degree of the relay unit expansion device22is derived. The other structure of the relay unit controller324is similar to that of relay unit controller24of Embodiment 1.

As described above, in the air-conditioning system300of Embodiment 3, the amount of the heat medium that is larger than the refrigerant in terms of specific heat and power required for flowing can be reduced similarly to Embodiments 1 and 2. Thus, the operation efficiency of the entire system can be increased and energy saving can be achieved.

Further, in the air-conditioning system300, the refrigerant to be caused to flow into the relay unit20is brought into the two-phase state by the heat source-side expansion device14during the cooling operation, and the refrigerant to be caused to flow into the heat source unit10is brought into the two-phase state by the relay unit expansion device22during the heating operation. Thus, the refrigerant amount can also be reduced in the air-conditioning system300compared with that in the case where liquid refrigerant flows through the liquid pipe55. Other advantages are similar to those of Embodiments 1 and 2.

Incidentally, in Embodiment 3, description is made of the exemplary case where the opening degree of the heat source-side expansion device14is adjusted based on the first pressure Pm1and the opening degree of the relay unit expansion device22is adjusted based on the second pressure Pm2. However, the adjustment is not limited to that in this case. For example, during the cooling operation, the heat source-side controller317may control the opening degree of the heat source-side expansion device14based on a pressure difference obtained by subtracting the first pressure Pm1from the discharge pressure Pd. At this time, it is appropriate that the relay unit controller324control the opening degree of the relay unit expansion device22based on the degree of superheat at the outlet of the evaporator. Further, during the heating operation, the opening degree of the heat source-side expansion device14may be controlled based on a pressure difference obtained by subtracting the suction pressure Ps from the second pressure Pm2. At this time, it is appropriate that the relay unit controller324control the opening degree of the relay unit expansion device22based on the degree of subcooling at the outlet of the condenser. With this structure, the density of the refrigerant in the liquid pipe55can be made constant irrespective of the operating condition of the air-conditioning system. Thus, it is possible to suppress a decrease in performance due to a change in the refrigerant amount in the liquid pipe55.

Embodiments 1 to 3 are preferred specific examples of the air-conditioning system and the technical scope of the present disclosure is not limited to Embodiments 1 to 3. For example, in each of the air-conditioning systems of Embodiments 1 to 3, the relay unit controller of the relay unit may perform centralized control over the entire system. More specifically, in the case of Embodiment 1, during the cooling operation, the relay unit controller24may determine the degree of superheat by using the suction pressure Ps and the first temperature and control the opening degree of the heat source-side expansion device14based on the determined degree of superheat. That is, the relay unit controller24may determine the opening degree of the heat source-side expansion device14that is associated with the determined degree of superheat and transmit a control signal showing the determined opening degree to the heat source-side controller17, thereby controlling the opening degree of the heat source-side expansion device14via the heat source-side controller17. In the cases of Embodiments 2 and 3, it is appropriate to employ a structure similar to the structure described above. When this structure is employed in the case of Embodiment 2, the relay unit controller24needs to acquire the first temperature measured by the first temperature sensor15afrom the heat source unit210via the heat source-side controller217. In the case of Embodiment 1, on the other hand, the relay unit controller24can directly acquire the first temperature measured by the first temperature sensor21a. Thus, the control can be simplified in the case where the structure described above is applied to Embodiment 1 than in the case where the structure described above is applied to Embodiment 2.

In Embodiments 1 to 3, description is made of the exemplary heat source units10,210, and310capable of supplying both cooling energy and heating energy by switching the refrigerant passages with the four-way valve12. However, the heat source unit is not limited thereto. Each of the heat source units10,210, and310may supply cooling energy or heating energy without the four-way valve12. That is, each of the air-conditioning systems100,200, and300may perform the cooling operation or the heating operation. Further, each of the air-conditioning systems100,200, and300may perform a cooling and heating simultaneous operation by selecting individual operating conditions of the indoor units.

Further, in Embodiments 1 to 3, description is made of the exemplary case where each of the air-conditioning systems100,200, and300includes three indoor units. However, the air-conditioning system is not limited thereto. Each of the air-conditioning systems100,200, and300may include two indoor units or may also include four or more indoor units. Note that, if each of the air-conditioning systems100,200, and300includes two indoor units, only the first branch part is associated with the indoor unit other than the indoor unit provided at the end of the load connection pipe opposite to the relay unit.