AU2007223486B2

AU2007223486B2 - Refrigeration system

Info

Publication number: AU2007223486B2
Application number: AU2007223486A
Authority: AU
Inventors: Masahiro Yamada; Takahiro Yamaguchi
Original assignee: Daikin Industries Ltd
Current assignee: Daikin Industries Ltd
Priority date: 2006-03-06
Filing date: 2007-03-05
Publication date: 2010-03-04
Anticipated expiration: 2027-03-05
Also published as: EP1992887A4; CN101384862A; CN101384862B; AU2007223486A1; WO2007102463A1; KR20080087902A; JP4715561B2; KR100960196B1; US20100229582A1; JP2007240026A; EP1992887A1

Description

I REFRIGERATION SYSTEM TECHNICAL FIELD [0001] The present invention relates to a refrigeration system adapted to perform a gas 5 injection operation by supplying gas refrigerant of intermediate pressure to a compressor. BACKGROUND ART [0002] Refrigeration systems have been known in the past which perform a so-called gas injection operation (i.e., the operation of supplying gas refrigerant of intermediate pressure to a compressor) for the purpose of reducing the input to the compressor. For example, 10 there is disclosed in Figure 1 of a patent document (JP-A-2001-033117) a refrigeration system which performs a single-stage compression refrigerating cycle and, in this refrigeration system, gas refrigerant of intermediate pressure is supplied to the compression chamber of a compressor being in the process of compression. In addition, there is disclosed in Figure 13 of the patent document another refrigeration system which performs 15 a two-stage compression refrigerating cycle and, in this refrigeration system, gas refrigerant of intermediate pressure is supplied to between a lower stage compressor and a higher stage compressor. [0003] In order to perform such a gas injection operation, gas refrigerant of intermediate pressure must be generated. To this end, for example, the refrigeration system illustrated 20 in Figure 1 of the patent document is provided in its refrigerant circuit with a gas/liquid separator configured to separate intermediate pressure refrigerant into liquid refrigerant and gas refrigerant, and the gas refrigerant of intermediate pressure is supplied to the compressor from the gas/liquid separator. In addition, in a refrigeration system illustrated in Figure 9 of the patent document, intermediate pressure refrigerant is made to exchange 25 heat with high pressure liquid refrigerant in an intermediate pressure heat exchanger, whereby the intermediate pressure refrigerant evaporates and changes to gas refrigerant of intermediate pressure. This intermediate pressure gas refrigerant is supplied to the compressor from the intermediate pressure heat exchanger. [0004] For the case of such a refrigeration system, its components (e.g., compressors, 2 heat exchangers et cetera) disposed in the refrigerant circuit may actually in some cases be arranged at a distance from each other or at different height levels from each other. For example, an air-conditioning system which is a type of refrigeration system is often configured by establishing connection between an outdoor unit and an indoor unit by 5 interconnecting piping. And, in the case where an air-conditioning system is installed in a building or the like, the length of interconnecting piping may reach in some cases a length of about 100 meters. In addition, in some cases, the difference in height between the outdoor unit and the indoor unit may be somewhere between about 20 and about 30 meters. [0005] That is, the installation situation for the refrigeration system varies variously 10 depending on the application and so on. And, with respect to the aforesaid refrigeration system adapted to perform a gas injection operation, there is the possibility that smooth operation might become impossible depending on the installation situation. Hereinafter, description will be made in terms of this drawback. [0006] As described above, in the gas injection refrigeration system, gas refrigerant of 15 intermediate pressure may be in some cases supplied to the compressor from the gas/liquid separator. Liquid refrigerant and gas refrigerant coexist within the gas/liquid separator. That is, liquid refrigerant to be fed out of the gas/liquid separator is in the saturated state. When this type of refrigeration system performs an operation of cooling a target object, liquid refrigerant in the saturated state flowing out of the gas/liquid separator is delivered 20 to a utilization side heat exchanger. However, if the utilization side heat exchanger is installed at a far distance from the gas/liquid separator or if the utilization side heat exchanger is installed at a higher position relative to the gas/liquid separator, this causes a pressure drop in the refrigerant during its flow through piping from the gas/liquid separator towards the utilization side heat exchanger, and as a result, a part of the refrigerant may 25 evaporate. This produces the possibility that the amount of liquid refrigerant flowing into the utilization side heat exchanger may decrease, thereby to reduce the cooling capacity to be obtained in the utilization side heat exchanger. [0007] In addition, in the gas injection refrigeration system, intermediate pressure refrigerant may be in some cases made to exchange heat with high pressure liquid 30 refrigerant in the intermediate pressure heat exchanger for supplying of the evaporated intermediate pressure refrigerant from the intermediate pressure heat exchanger to the 3 compressor. When this type of refrigeration system performs an operation of heating a target object, a part of the refrigerant condensed in the utilization side heat exchanger is pressure reduced down to an intermediate pressure and is introduced into the intermediate pressure heat exchanger. However, if the intermediate pressure heat exchanger is installed 5 at a far distance from the utilization side heat exchanger or if the intermediate pressure heat exchanger is installed at a higher position relative to the utilization side heat exchanger, this causes a pressure drop in the refrigerant during its flow through piping from the utilization side heat exchanger towards the intermediate pressure heat exchanger, and as a result, a part of the refrigerant may evaporate thereby causing a temperature drop in the 10 refrigerant. This produces the possibility of reducing the difference in temperature between the high pressure refrigerant and the intermediate pressure refrigerant which are subjected to heat exchange with each other in the intermediate pressure compressor, thereby making it impossible to ensure that the intermediate pressure refrigerant is gasified in the intermediate pressure heat exchanger. 15 [0007a] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of 20 this application. [0007b] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 25 DISCLOSURE OF THE INVENTION [0008] The present invention was made in view of the above. There is disclosed a refrigeration system adapted to perform a so-called gas injection operation which is able to operate smoothly irrespective of the installation situation or the operation state of the refrigeration system. 30 [0009] According to the present invention, there is provided a refrigeration system 4 comprising a refrigerant circuit wherein the refrigerant circuit includes a compressor, a heat source side heat exchanger, and a utilization side heat exchanger which are connected to perform a refrigerating cycle, and wherein the refrigerant circuit is selectively operable either in a cooling mode in which the heat source side heat exchanger becomes a condenser 5 and the utilization side heat exchanger becomes an evaporator or a heating mode in which the utilization side heat exchanger becomes a condenser and the heat source side heat exchanger becomes an evaporator. In the refrigeration system, the refrigerant circuit further includes an injection passageway through which to supply to the compressor intermediate pressure refrigerant resulting from pressure reducing a part of high pressure 10 liquid refrigerant, an intermediate pressure heat exchanger in which intermediate pressure refrigerant flowing through the injection passageway towards the compressor exchanges heat with high pressure liquid refrigerant and evaporates, and a gas/liquid separator in which intermediate pressure refrigerant resulting from pressure reducing high pressure liquid refrigerant is separated into liquid refrigerant and gas refrigerant, and the refrigerant 15 circuit is configured such that its refrigerant circulation path is selectively changeable between the cooling mode during which gas refrigerant of intermediate pressure flowing through the injection passageway is supplied to the compressor and the heating mode during which gas refrigerant of intermediate pressure flowing out of the gas/liquid separator is supplied to the compressor. 20 [0010] In embodiments of the present invention, the source of intermediate pressure refrigerant to the compressor differs between the cooling mode and the heating mode. During the cooling mode, intermediate pressure refrigerant evaporated in the intermediate pressure heat exchanger is supplied to the compressor. At that time, in the intermediate pressure heat exchanger, high pressure liquid refrigerant is cooled by heat exchange with 25 the intermediate pressure refrigerant, thereby increasing the degree of supercooling of the high pressure liquid refrigerant. Consequently, even in the event that there is some pressure drop in the high pressure refrigerant during its flow from the intermediate pressure heat exchanger to the utilization side heat exchanger, either (i) the high pressure refrigerant to be supplied to the utilization side heat exchanger is maintained in the liquid 30 state or (ii) the amount, by which the high pressure refrigerant to be supplied to the utilization side heat exchanger evaporates along the way thereto, becomes reduced. On the other hand, during the heating mode, intermediate pressure refrigerant is introduced into 5 the gas/liquid separator and gas refrigerant within the gas/liquid separator is supplied to the compressor. Consequently, even in the event that there is some pressure drop in the refrigerant during its flow from the utilization side heat exchanger to the gas/liquid separator thereby causing a part of the refrigerant to evaporate, the supply of gas 5 refrigerant of intermediate pressure to the compressor is ensured because of the separation into gas refrigerant and liquid refrigerant by the gas/liquid separator. [0011] The refrigerant circuit may be formed by connecting, by interconnecting piping, a heat source side circuit in which the compressor and the heat source side heat exchanger are disposed and a utilization side circuit in which the utilization side heat exchanger is 10 disposed, and wherein the injection passageway, the intermediate pressure heat exchanger, and the gas/liquid separator are disposed in the heat source side circuit. [0012] In an embodiment of the present invention, the refrigerant circuit is composed of the heat source side circuit, the utilization side circuit, and the interconnecting piping. During the cooling mode, high pressure liquid refrigerant cooled during its passage 15 through the intermediate pressure heat exchanger flows into the utilization side heat exchanger by way of the interconnecting piping. Consequently, even in the case where the interconnecting piping is long or where the utilization side circuit is installed at a higher position relative to the heat source side circuit, either (i) the high pressure refrigerant to be supplied to the utilization side heat exchanger is maintained in the liquid state or (ii) the 20 amount, by which the high pressure refrigerant to be supplied to the utilization side heat exchanger evaporates along the way thereto, becomes reduced. On the other hand, during the heating mode, refrigerant condensed in the utilization side heat exchanger flows into the gas/liquid separator by way of the interconnecting piping, and gas refrigerant within the gas/liquid separator is supplied to the compressor. This ensures that even if the 25 interconnecting piping is long or if the utilization side circuit is installed at a higher position relative to the heat source side circuit, the gas refrigerant is supplied to the compressor. [0013] The gas/liquid separator may be formed by a container-shaped member which is arranged at a position in the refrigerant circuit, which position is located, during the 30 cooling mode, downstream of the heat source side heat exchanger and is located, during the heating mode, downstream of the utilization side heat exchanger, and wherein the 6 intermediate pressure heat exchanger may be formed by a heat exchange member which is housed in the inside of the container-shaped member and in which intermediate pressure refrigerant flowing through the injection passageway exchanges heat with liquid refrigerant within the container-shaped member. 5 [0014] In an embodiment of the present invention, the gas/liquid separator is formed by the container-shaped member and the intermediate pressure heat exchanger is formed by the heat exchange member. In the cooling mode, refrigerant (high pressure liquid refrigerant) condensed in the heat source side heat exchanger flows into the container shaped member. In addition, a part of the high pressure liquid refrigerant flows into the 10 injection passageway, is pressure reduced down to an intermediate pressure, and then flows into the heat exchange member. The intermediate pressure refrigerant admitted into the heat exchange member exchanges heat with the high pressure liquid refrigerant within the container-shaped member, evaporates, and is then supplied to the compressor. The high pressure liquid refrigerant within the container-shaped member cooled by heat exchange 15 with the intermediate pressure refrigerant is delivered towards the utilization side heat exchanger from the container-shaped member. On the other hand, in the heating mode, refrigerant, condensed in the utilization side heat exchanger and then pressure reduced down to an intermediate pressure, flows into the container-shaped member. Within the container-shaped member, the intermediate pressure refrigerant admitted thereinto is 20 separated into liquid refrigerant and gas refrigerant. From the container-shaped member, the liquid refrigerant is delivered towards the heat source side heat exchanger whereas the gas refrigerant is supplied through the injection passageway to the compressor. [0015] A supercooling heat exchanger, configured to cool high pressure liquid refrigerant by heat exchange with low pressure refrigerant resulting from pressure reducing 25 a part of the high pressure liquid refrigerant, may be arranged at a position in the refrigerant circuit, which position is located, during the cooling mode, downstream of the intermediate pressure heat exchanger. [0016] In an embodiment of the present invention, the refrigerant circuit is provided with the supercooling heat exchanger. During the cooling mode, in the supercooling heat 30 exchanger, high pressure liquid refrigerant passing through the intermediate pressure heat exchanger is cooled by heat exchange with low pressure refrigerant resulting from pressure 7 reduction of a part of the high pressure liquid refrigerant. That is, the degree of supercooling of the high pressure liquid refrigerant increases. The high pressure liquid refrigerant cooled in the supercooling heat exchanger is delivered to the utilization side heat exchanger. 5 [0017] A single-stage compression refrigerating cycle may be performed in the refrigerant circuit, and wherein the compressor may be configured such that gas refrigerant of intermediate pressure flows into a compression chamber being in the process of compression. [0018] In an embodiment of the present invention, gas refrigerant of intermediate 10 pressure is introduced into the compression chamber being in the process of compression in the compressor. The compressor draws in low pressure refrigerant evaporated in either the utilization side heat exchanger or the heat source side heat exchanger, whichever functions as an evaporator, and intermediate pressure refrigerant supplied from either the intermediate pressure heat exchanger or the gas/liquid separator, and then compresses them. 15 [0019] In the refrigerant circuit, a lower stage compressor and a higher stage compressor may be connected together in series to perform a two-stage compression refrigerating cycle, and wherein the refrigerant circuit may be configured such that gas refrigerant of intermediate pressure is supplied to a suction side of the higher stage compressor. [0020] In an embodiment of the present invention, gas refrigerant of intermediate 20 pressure is introduced to the suction side of the higher stage compressor. The higher stage compressor draws in refrigerant compressed in the lower stage compressor and gas refrigerant supplied from the intermediate pressure heat exchanger and the gas/liquid separator and compresses them. [0021] In embodiments of the present invention, it is arranged such that during the 25 cooling mode, intermediate pressure refrigerant evaporated in the intermediate pressure heat exchanger is supplied to the compressor and high pressure liquid refrigerant cooled in the intermediate pressure heat exchanger is supplied to the utilization side heat exchanger. As a result of such an arrangement, even in an installation situation that causes a considerable pressure drop in the high pressure refrigerant during its flow from the 30 intermediate pressure heat exchanger to the utilization side heat exchanger due to the fact 8 that the utilization side heat exchanger is arranged at a far distance from the intermediate pressure heat exchanger or due to the fact that the utilization side heat exchanger is arranged at a higher position relative to the intermediate pressure heat exchanger, it is still possible to maintain the high pressure refrigerant to be supplied to the utilization side heat 5 exchanger in the liquid state or to reduce the amount by which the high pressure refrigerant to be supplied to the utilization side heat exchanger evaporates along the way thereto. As a result, it becomes possible to ensure the amount of liquid refrigerant to be supplied to the utilization side heat exchanger during the cooling mode, thereby enabling the utilization side heat exchanger to satisfactorily demonstrate its cooling capacity. 10 [0022] In addition, in embodiments of the present invention, it is arranged such that during the heating mode, gas refrigerant of intermediate pressure is supplied to the compressor from the gas/liquid separator. As a result of such an arrangement, even in an installation situation that causes a considerable pressure drop in the refrigerant during its flow from the utilization side heat exchanger to the gas/liquid separator due to the fact that 15 the gas/liquid separator is arranged at a far distance from the utilization side heat exchanger or due to the fact that the gas/liquid separator is arranged at a higher position relative to the utilization side heat exchanger, it is still possible to ensure that gas refrigerant of intermediate pressure is supplied to the compressor. As a result, it becomes possible to avoid the situation where liquid refrigerant of intermediate pressure flows into 20 the compressor thereby causing damage to the compressor. [0023] As described above, in embodiments of the present invention, it is possible to enable the refrigeration system to smoothly operate during both the cooling mode and the heating mode, regardless of whatever state the refrigeration system is installed in. [0024] In an embodiment of the present invention, the refrigerant circuit is made up of 25 the heat source side circuit, the utilization side circuit, and the interconnecting piping. In many cases in such an arrangement, the heat source side circuit provided with the intermediate pressure heat exchanger and the gas/liquid separator is arranged at a far distance from the utilization side circuit provided with the utilization side heat exchanger or these two circuits are positioned at different levels of height. Accordingly, if in the 30 refrigeration system having the refrigerant circuit as configured according to this embodiment, the source of intermediate pressure refrigerant to the compressor is made to 9 differ between the cooling mode and the heating mode, this makes it possible to relax the constraints on the installation situation of the refrigeration system. [0025] In an embodiment of the present invention, the heat exchange member constituting the intermediate pressure heat exchanger is housed in the inside of the 5 container-shaped member constituting the gas/liquid separator. In other words, if the container-shaped member with the heat exchange member housed therein is connected to the refrigerant circuit, this is the same as arranging both the gas/liquid separator and the intermediate pressure heat exchanger in the refrigerant circuit. Therefore, in accordance with this embodiment of the present invention, the refrigerant circuit can be simplified in 10 configuration in comparison with the case where the gas/liquid separator and the intermediate pressure heat exchanger are formed separately from each other. [0026] In an embodiment of the present invention, it is arranged such that the supercooling heat exchanger is disposed in the refrigerant circuit thereby increasing the degree of supercooling of high pressure liquid refrigerant to be supplied to the utilization 15 side heat exchanger during the cooling mode. As a result of such an arrangement, even in the installation situation where there is some pressure drop in the high pressure refrigerant during its flow from the intermediate pressure heat exchanger to the utilization side heat exchanger, it is still possible to further ensure that the high pressure refrigerant to be supplied to the utilization side heat exchanger is maintained in the liquid state or to further 20 reduce the amount by which the high pressure refrigerant evaporates along the way to the utilization side heat exchanger. BRIEF DESCRIPTION OF THE DRAWINGS [0027] Figure 1 is a plumbing diagram illustrating the arrangement of a refrigerant circuit in an air 25 conditioning system according to a first embodiment of the present invention wherein Figure 1(A) shows a state in the cooling mode and Figure 1(B) shows a state in the heating mode. Figure 2 is a plumbing diagram illustrating the arrangement of a refrigerant circuit in an air conditioning system according to a second embodiment of the present invention wherein 10 Figure 2(A) shows a state in the cooling mode and Figure 2(B) shows a state in the heating mode. Figure 3 is a plumbing diagram illustrating the arrangement of a refrigerant circuit in an air conditioning system according to a first modification of another embodiment of the present 5 invention wherein Figure 3(A) shows a state in the cooling mode and Figure 3(B) shows a state in the heating mode. Figure 4 is a plumbing diagram illustrating the arrangement of a refrigerant circuit in an air conditioning system according to a second modification of the other embodiment of the present invention wherein Figure 4(A) shows a state during the cooling mode and Figure 10 4(B) shows a state in the heating mode. REFERENCE NUMERALS IN THE DRAWINGS [0028] 20 refrigerant circuit 21 liquid side interconnecting piping 15 22 gas side interconnecting piping The description continues on page 11.

30 outdoor circuit (heat source side circuit) 31 compressor 33 lower stage compressor 34 higher stage compressor 36 outdoor heat exchanger (heat source side heat exchanger) 40 intermediate pressure heat exchanger 43 injection piping (injection passageway) 51 gas/liquid separator 65 container-shaped member 66 heat exchange member 70 indoor circuit (utilization side circuit) 71 indoor heat exchanger (utilization side heat exchanger) BEST MODE FOR CARRYING OUT THE INVENTION [0029] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIRST EMBODIMENT OF THE INVENTION [0030] A first embodiment of the present invention will be described. The present embodiment is an air conditioning system (10) formed by a refrigeration system according to the present invention. [0031] As shown in Figure 1, the air conditioning system (10) of the present embodiment is provided with a single outdoor unit (11) and two indoor units (12). Note that the number of indoor units (12) shown here is a mere illustration. The outdoor unit (11) accommodates an outdoor circuit (30) which is a heat source side circuit (30). Each of the indoor units (12) accommodates a respective indoor circuit (70) which is a utilization side circuit.

[0032] In the air conditioning system (10), the outdoor circuit (30) and the indoor circuit (70) are connected together by liquid side interconnecting piping (21) and gas side interconnecting piping (22) to form a refrigerant circuit (20). In the refrigerant circuit (20), the two indoor circuits (70) are connected in parallel with each other to the single outdoor circuit (30). [0033] Each of the indoor circuits (70) is provided with a respective indoor heat exchanger (71) which is a utilization side heat exchanger and a respective indoor expansion valve (72). The indoor heat exchanger (71) is an air heat exchanger for heat exchange between indoor air and refrigerant. In each of the indoor circuits (70), the indoor heat exchanger (71) and the indoor expansion valve (72) are connected together in series. In each of the indoor circuits (70), one end thereof on the side of the indoor expansion valve (72) is connected to the liquid side interconnecting piping (21) and the other end thereof on the side of the indoor heat exchanger (71) is connected to the gas side interconnecting piping (22). [0034] The outdoor circuit (30) is provided with a compressor (31), a four-way selector valve (35), an outdoor heat exchanger (36) which is a heat source side heat exchanger, an outdoor expansion valve (37), and an accumulator (38). In addition, the outdoor circuit (30) is provided with an intermediate pressure heat exchanger (40), a gas/liquid separator (51), bypass piping (50), injection piping (43), and intermediate pressure gas piping (52). [0035] The compressor (31) is a positive compressor (31) and is configured such that it compresses refrigerant drawn into its compression chamber. The compressor (31) has an intermediate pressure port (32) through which to introduce refrigerant of intermediate pressure into the compression chamber being in the process of compression. The discharge side of the compressor (31) is connected to a first port of the four-way selector valve (35) and the suction side thereof is connected via the accumulator (38) to a second port of the four-way selector valve (35). Although only one compressor, i.e., the compressor (31), is disposed in the outdoor circuit (30) in the present embodiment, a plurality of compressors may be arranged in parallel. [0036] The outdoor heat exchanger (36) is an air heat exchanger for heat exchange between outdoor air and refrigerant. The intermediate pressure heat exchanger (40) is a heat exchanger intended for refrigerant to refrigerant heat exchange, such as a double-pipe heat exchanger, a plate type heat exchanger et cetera. Formed in the intermediate pressure heat exchanger (40) are a first flowpath (41) and a second flowpath (42). One end of the outdoor heat exchanger (36) is connected to a third port of the four-way selector valve (35) and the other end thereof is connected via the outdoor expansion valve (37) to one end of the first flowpath (41) of the intermediate pressure heat exchanger (40). The other end of the first flowpath (41) of the intermediate pressure heat exchanger (40) is connected via a first check valve (45) to the liquid side interconnecting piping (21). The first check valve (45) is arranged so that it permits only the flow and passage of refrigerant towards the liquid side interconnecting piping (21) from the intermediate pressure heat exchanger (40). [0037] The injection piping (43) forms an injection passageway. The start end of the injection piping (43) is connected to between the intermediate pressure heat exchanger (40) and the first check valve (45) and the termination end thereof is connected to the intermediate pressure port (32) of the compressor (31). The second flowpath (42) of the intermediate pressure heat exchanger (40) is arranged along the injection piping (43). The injection piping (43) is provided, between its start end and the second flowpath (42) of the intermediate pressure heat exchanger (40), with an injection expansion valve (44). [0038] The gas/liquid separator (51) is a hermetically closed container shaped like a vertically elongated cylinder. The gas/liquid separator (51) has a lower end part arranged along the bypass piping (50). The start end of the bypass piping (50) is connected to between the first check valve (45) and the liquid side interconnecting piping (21), and the terminal end thereof is connected to between the first flowpath (41) of the intermediate pressure heat exchanger (40) and the outdoor expansion valve (37). In addition, the bypass piping (50) is provided, between its termination end and the gas/liquid separator (51), with a second check valve (55). The second check valve (55) is arranged such that it permits only the flow and passage of refrigerant in the outflow direction from the gas/liquid separator (51). [0039] One end of the intermediate pressure gas piping (52) is connected to the top of the gas/liquid separator (51). The other end of the intermediate pressure gas piping (52) is connected to between the second flowpath (42) of the intermediate pressure heat exchanger 14 (40) and the compressor (31) in the injection piping (43). A solenoid valve (53) is disposed along the intermediate pressure gas piping (52). [0040] As described above, the first port of the four-way selector valve (35) is connected to the discharge side of the compressor (31), the second port is connected to the accumulator (38), and the third port is connected to the outdoor heat exchanger (36). In addition, the fourth port of the four-way selector valve (35) is connected to the gas side interconnecting piping (22). The four-way selector valve (35) is selectively switchable between a first state (shown in Figure 1(A)) and a second state (shown in Figure 1(B)). That is, when the four-way selector valve (35) is placed in the first state, this establishes fluid communication between the first port and the third port and fluid communication between the second port and the fourth port. On the other hand, when the four-way selector valve (35) is placed in the second state, this establishes fluid communication between the first port and the fourth port and fluid communication between the second port and the third port. RUNNING OPERATION [0041] The air conditioning system (10) is selectively operable either in a cooling or a heating mode. COOLING MODE [0042] Referring to Figure 1(A), running operation in the cooling mode will be described. In the cooling mode, refrigerant is circulated in the refrigerant circuit (20) in which the outdoor heat exchanger (36) becomes a condenser and the indoor heat exchanger (71) becomes an evaporator, in other words, cooling operation is carried out in the refrigerant circuit (20). [0043] More specifically, in the cooling mode, the four-way selector valve (35) is set in the first state. In addition, the outdoor expansion valve (37) is set in the fully opened state, the degree of opening of the injection expansion valve (44) and that of the indoor expansion valve (72) are properly regulated, and the solenoid valve (53) is closed. [0044] High pressure gas refrigerant discharged from the compressor (31) dissipates heat to the outdoor air in the outdoor heat exchanger (36) and condenses. This high 15 pressure liquid refrigerant exiting the outdoor heat exchanger (36) passes through the first flowpath (41) of the intermediate pressure heat exchanger (40) during which passage it dissipates heat to the refrigerant in the second flowpath (42). A part of the high pressure liquid refrigerant flowing out of the first flowpath (41) of the intermediate pressure heat exchanger (40) flows into the injection piping (43) whereas the rest of the high pressure liquid refrigerant is distributed to each indoor circuit (70) by way of the liquid side interconnecting piping (21). [0045] In each indoor circuit (70), the high pressure liquid refrigerant admitted thereinto passes through the indoor expansion valve (72) during which passage it is pressure reduced, absorbs heat from the indoor air in the indoor heat exchanger (71), and evaporates. The refrigerant evaporated in the indoor heat exchanger (71) returns to the outdoor circuit (30) by way of the gas side interconnecting piping (22) and is drawn into the compressor (31) by way of the accumulator (38). [0046] On the other hand, the high pressure liquid refrigerant admitted into the injection piping (43) passes through the injection expansion valve (44) during which passage it is pressure reduced down to an intermediate pressure and changes to intermediate pressure refrigerant in the gas/liquid two-phase state. This intermediate pressure refrigerant flows through the second flowpath (42) of the intermediate pressure heat exchanger (40) during which flow it absorbs heat from the refrigerant in the first flowpath (41) and evaporates. The intermediate pressure gas refrigerant exiting the second flowpath (42) of the intermediate pressure heat exchanger (40) is delivered to the intermediate pressure port (32) of the compressor (31). [0047] The compressor (31) draws low pressure refrigerant into its compression chamber through the accumulator (38) and compresses it. In addition, the intermediate pressure gas refrigerant admitted in through the intermediate pressure port (32) is introduced to the compression chamber being in the process of compression. And the compressor (31) compresses the refrigerant in the compression chamber up to a high pressure and discharges it. [0048] As described above, during the cooling mode, high pressure liquid refrigerant is cooled during its passage through the intermediate pressure heat exchanger (40) and, as a result, its degree of supercooling increases. Then, the high pressure refrigerant is delivered by way of the liquid side interconnecting piping (21) to the indoor circuit (70). Consequently, the high pressure refrigerant flowing into the indoor circuit (70) is maintained in the liquid single-phase state, even in the case where, due to the fact that the liquid side interconnecting piping (21) is more than a certain length or due to the fact that the indoor circuit (70) is positioned higher by a certain height than the outdoor circuit (30), the liquid refrigerant to be fed into the liquid side interconnecting piping (21) from the outdoor circuit (30) is placed in the saturated state or a part of the high pressure liquid refrigerant evaporates by the time it reaches the indoor circuit (70). In addition, even if a part of the high pressure liquid refrigerant evaporates by the time it reaches the indoor circuit (70), the amount of evaporation of the high pressure liquid refrigerant is reduced as compared to the case where the liquid refrigerant to be fed into the liquid side interconnecting piping (21) from the outdoor circuit (30) is in the saturated state. HEATING MODE [0049] Referring to Figure 1(B), running operation in the heating mode will be described. In the heating mode, refrigerant is circulated in the refrigerant circuit (20) in which the indoor heat exchanger (71) becomes a condenser and the outdoor heat exchanger (36) becomes an evaporator, in other words, heating operation is carried out in the refrigerant circuit (20). [0050] More specifically, in the heating mode, the four-way selector valve (35) is set in the second state. In addition, the degree of opening of the outdoor expansion valve (37) and that of the indoor expansion valve (72) are properly controlled. The injection expansion valve (44) is set in the fully closed state and the solenoid valve (53) is opened. [0051] High pressure gas refrigerant discharged from the compressor (31) is distributed by way of the gas side interconnecting piping (22) to each indoor circuit (70). In the indoor heat exchanger (71) of each indoor circuit (70), the high pressure gas refrigerant dissipates heat to the indoor air and condenses. In each indoor circuit (70), the high pressure liquid refrigerant flowing out of the indoor heat exchanger (71) passes through the indoor expansion valve (72) during which passage it is pressure reduced and changes to intermediate pressure refrigerant in the gas/liquid two-phase state. The intermediate pressure refrigerant flowing out of each indoor circuit (70) returns to the outdoor circuit (30) by way of the liquid side interconnecting piping (21) and flows into the gas/liquid separator (51) by way of the bypass piping (50). [0052] Of the intermediate pressure refrigerant admitted into the gas/liquid separator (51), liquid refrigerant is collected in a lower part of the gas/liquid separator (51) whereas gas refrigerant is collected in an upper part of the gas/liquid separator (51). The liquid refrigerant of intermediate pressure in the gas/liquid separator (51) again flows through the bypass piping (50), passes through the outdoor expansion valve (37) during which passage it is pressure reduced, and is introduced into the outdoor heat exchanger (36). In the outdoor heat exchanger (36), the refrigerant absorbs heat from the outdoor air and evaporates. The refrigerant evaporated in the outdoor heat exchanger (36) is drawn by way of the accumulator (38) into the compressor (31). On the other hand, the gas refrigerant of intermediate pressure in the gas/liquid separator (51) sequentially passes through the intermediate pressure gas piping (52) and the injection piping (43) and is introduced to the intermediate pressure port (32) of the compressor (31). [0053] The compressor (31) draws low pressure refrigerant into the compression chamber through the accumulator (38) and compresses it. In addition, the intermediate pressure gas refrigerant admitted in through the intermediate pressure port (32) is introduced into the compression chamber being in the process of compression. And the compressor (31) compresses the refrigerant in the compression chamber up to a high pressure and discharges it. [0054] As described above, during the heating mode, after returning to the outdoor circuit (30) by way of the liquid side interconnecting piping (21), the refrigerant is introduced into the gas/liquid separator (51) and separated into liquid refrigerant and gas refrigerant, and only the gas refrigerant within the gas/liquid separator (51) is supplied to the intermediate pressure port (32) of the compressor (31). Stated another way, even if refrigerant flowing into the outdoor circuit (30) is in the gas/liquid two-phase state, it is ensured that only gas refrigerant is supplied to the intermediate pressure port (32) of the compressor (31). This maintains the refrigerant flowing into the intermediate pressure port (32) of the compressor (31) in the gas single-phase state, even in the case where, due to the fact that the liquid side interconnecting piping (21) is more than a certain length or due to I d the fact that the outdoor circuit (30) is positioned higher by a certain height than the indoor circuit (70), a part of the high pressure liquid refrigerant evaporates by the time it reaches the indoor circuit (70). ADVANTAGEOUS EFFECTS OF THE FIRST EMBODIMENT [0055] It is arranged such that during the cooling mode of the air conditioning system (10), intermediate pressure refrigerant evaporated in the intermediate pressure heat exchanger (40) is supplied to the intermediate pressure port (32) of the compressor (31) and high pressure liquid refrigerant cooled in the intermediate pressure heat exchanger (40) is supplied to the indoor circuit (70). As a result of such an arrangement, even in an installation situation that causes a considerable pressure drop in the refrigerant during its flow through the liquid side interconnecting piping (21) due to the fact that the liquid side interconnecting piping (21) for connection between the outdoor circuit (30) and the indoor circuit (70) is extremely long or due to the fact that the indoor circuit (70) is arranged at a higher position relative to the outdoor circuit (30), it is still possible to maintain the high pressure refrigerant to be supplied to the indoor circuit (70) in the liquid state or to reduce the amount by which the high pressure refrigerant to be supplied to the indoor circuit (70) evaporates along the way thereto. As a result, it becomes possible to ensure the amount of liquid refrigerant to be supplied to the indoor circuit (70) during the cooling mode, thereby enabling the indoor unit (12) to satisfactorily demonstrate its cooling capacity. [00561 Here, in the case where a plurality of indoor circuits (70) are connected in parallel with one another (as in the air conditioning system (10)), the degree of opening of the indoor expansion valve (72) of each indoor circuit (70) is controlled individually to regulate the rate of distribution of the refrigerant to each indoor circuit (70), in order to properly control the cooling capacity of each indoor unit (12). However, if the refrigerant passing through the indoor expansion valve (72) enters the gas/liquid two-phase state, then the indoor expansion valve (72) lacks stability in the characteristic of flow rate, thereby producing the possibility that it becomes impossible to properly regulate the rate of distribution of the refrigerant to each indoor circuit (70). On the other hand, in the air conditioning system (10) according to the present embodiment, it becomes easy to hold the refrigerant flowing into the indoor circuit (70) during the cooling mode in the liquid state. Therefore, in accordance with the present embodiment, it becomes possible to accurately control, in the air conditioning system (10) provided with a plurality of indoor units (12), the cooling capacity of each indoor unit (12). [0057] In addition, it is arranged such that during the heating mode of the air conditioning system (10), refrigerant returned to the outdoor circuit (30) from the indoor circuit (70) is separated by the gas/liquid separator (51) into liquid refrigerant and gas refrigerant and only gas refrigerant of intermediate pressure is supplied to the compressor (31) from the gas/liquid separator (51). As a result of such an arrangement, even in an installation situation that causes a considerable pressure drop in the refrigerant during its flow through the liquid side interconnecting piping (21) due to the fact that the liquid side interconnecting piping (21) for connection between the outdoor circuit (30) and the indoor circuit (70) is extremely long or due to the fact that the outdoor circuit (30) is arranged at a higher position relative to the indoor circuit (70), it is still possible to ensure that only gas refrigerant is supplied to the intermediate pressure port (32) of the compressor (31). This therefore makes it possible to avoid the situation where liquid refrigerant of intermediate pressure flows into the compressor (31) thereby causing damage to the compressor (31). [0058] As described above, in accordance with the present embodiment, it is possible to enable the air conditioning system (10) to smoothly operate during both the cooling mode and the heating mode, regardless of whatever state the air conditioning system (10) is installed in. SECOND EMBODIMENT OF THE INVENTION [0059] A second embodiment of the present invention will be described. The second embodiment is an embodiment characterized in that the air conditioning system (10) of the first embodiment additionally includes a supercooling heat exchanger (60) and supercooling piping (63). Here, the difference from the first embodiment in terms of the air conditioning system (10) of the present embodiment is described below. [0060] As shown in Figure 2, the supercooling heat exchanger (60) is disposed in the outdoor circuit (30). The supercooling heat exchanger (60) is a heat exchanger intended for refrigerant to refrigerant heat exchange, such as a double-pipe heat exchanger, a plate type heat exchanger et cetera. A first flowpath (61) and a second flowpath (62) are formed in the supercooling heat exchanger (60). The first flowpath (61) of the supercooling heat zU exchanger (60) lies between the intermediate pressure heat exchanger (40) and the first check valve (45) in the outdoor circuit (30). [0061] The supercooling piping (63) has a start end and a termination end the former of which is connected to between the supercooling heat exchanger (60) and the first check valve (45) and the latter of which is connected to between the accumulator (38) and the four-way selector valve (35). The second flowpath (62) of the supercooling heat exchanger (60) lies along the supercooling piping (63). The supercooling piping (63) is provided, between its start end and the second flowpath (62), with a supercooling expansion valve (64). RUNNING OPERATION COOLING MODE [0062] As shown in Figure 2(A), in the refrigerant circuit (20) in the cooling mode, refrigerant is circulated in substantially the same way as in the first embodiment. More specifically, the second embodiment has a refrigerant circulation path which differs from that of the first embodiment only in the following points. That is, high pressure liquid refrigerant leaving the intermediate pressure heat exchanger (40) flows, after passing through the supercooling heat exchanger (60), into the liquid side interconnecting piping (21) and a part of the high pressure liquid refrigerant flows into the supercooling piping (63). [0063] When the air conditioning system (10) of the present embodiment is in the cooling mode, the degree of opening of the supercooling expansion valve (54) is properly controlled. High pressure liquid refrigerant flowing out of the first flowpath (41) of the intermediate pressure heat exchanger (40) passes through the first flowpath (61) of the supercooling heat exchanger (60) during which passage it dissipates heat to the refrigerant in the second flowpath (62). A part of the high pressure liquid refrigerant flowing out of the first flowpath (61) of the supercooling heat exchanger (60) flows into the supercooling piping (63) and the rest of the high pressure liquid refrigerant is distributed by way of the liquid side interconnecting piping (21) to each indoor circuit (70). To sum up, the high pressure liquid refrigerant cooled in both the intermediate pressure heat exchanger (40) and the supercooling heat exchanger (60) is supplied to each indoor circuit (70).

[0064] On the other hand, the high pressure liquid refrigerant admitted into the supercooling piping (63) passes through the supercooling expansion valve (64) during which passage it is pressure reduced down to a low pressure and changes to low pressure refrigerant in the gas/liquid two-phase state. This low pressure refrigerant passes through the second flowpath (62) of the supercooling heat exchanger (60) during which passage it absorbs heat from the refrigerant in the first flowpath (61) and evaporates. This low pressure gas refrigerant existing the second flowpath (62) of the supercooling heat exchanger (60) is drawn, together with the low pressure refrigerant returned to the outdoor circuit (30) from the indoor circuit (70) by way of the gas side interconnecting piping (22), into the compressor (31). HEATING MODE [0065] As shown in Figure 2(B), in the refrigerant circuit (20) in the heating mode, refrigerant is circulated in completely the same way as in the first embodiment. More specifically, in the heating mode, the supercooling expansion valve (64) is fully closed. And, intermediate pressure refrigerant admitted into the outdoor circuit (30) from the liquid side interconnecting piping (21) passes through the bypass piping (50), flows into the gas/liquid separator (51), and is separated there into liquid refrigerant and gas refrigerant. ADVANTAGEOUS EFFECTS OF THE SECOND EMBODIMENT [0066] In the present embodiment, the supercooling heat exchanger (60) is disposed in the outdoor circuit (30) thereby increasing the degree of supercooling of the high pressure liquid refrigerant to be supplied to the indoor circuit (70) during the cooling mode. Consequently, even in an installation situation that causes a pressure drop in the high pressure refrigerant during its flow from the outdoor circuit (30) to the indoor circuit (70), it is still possible to further ensure that the high pressure refrigerant to be supplied to the indoor circuit (70) is maintained in the liquid state or to further reduce the amount by which the high pressure refrigerant to be supplied to the indoor circuit (70) evaporates along the way thereto. ANOTHER EMBODIMENT [0067] The foregoing embodiments may be configured as follows. FIRST MODIFICATION [0068] In each of the foregoing embodiments, the gas/liquid separator (51) and the intermediate pressure heat exchanger (40) may be made integral with each other. Here, referring to Figure 3, description will be made in terms of an example in which the present modification is applied to the air conditioning system (10) of the second embodiment. [0069] The gas/liquid separator (51) of the present modification is formed by a container-shaped member (65) shaped like a somewhat vertically elongated cylinder. The container-shaped member (65) constituting the gas/liquid separator (51) is connected, at its bottom, to a portion of the outdoor circuit (30) between the outdoor expansion valve (37) and the supercooling heat exchanger (60). Note that in the outdoor circuit (30) of the present modification, the bypass piping (50), the first check valve (45), and the second check valve (55) are omitted. [0070] The container-shaped member (65) contains a heat exchange member (66) which is a heat transfer tube shaped like a coil spring. The heat exchange member (66) is arranged in the inner bottom of the container-shaped member (65) so that it is immersed in the liquid refrigerant collected in the container-shaped member (65). The heat exchange member (66) is arranged upstream of the injection expansion valve (44) in the injection piping (43). In the present modification, the heat exchange member (66) constitutes the intermediate pressure heat exchanger (40). [0071] Description will be made in terms of operation in the cooling mode. In the cooling mode, the degree of opening of the injection expansion valve (44) and that of the supercooling expansion valve (64) are properly controlled and the solenoid valve (53) is closed. [0072] In the cooling mode, refrigerant condensed in the outdoor heat exchanger (36) passes through the outdoor expansion valve (37) in the fully closed state and then flows into the container-shaped member (65). The high pressure liquid refrigerant within the container-shaped member (65) dissipates heat to the intermediate pressure refrigerant flowing in the heat exchange member (66). In other words, in the container-shaped member (65), the high pressure liquid refrigerant is cooled by heat exchange with the intermediate pressure refrigerant within the heat exchange member (66), and the degree of supercooling of the high pressure liquid refrigerant increases. A part of the high pressure liquid refrigerant cooled within the container-shaped member (65) flows into the injection piping (43) and the rest of the high pressure liquid refrigerant is cooled to a further extent during its passage through the first flowpath (61) of the supercooling heat exchanger (60). [0073] The high pressure liquid refrigerant cooled in the supercooling heat exchanger (60) is supplied through the liquid side interconnecting piping (21) to the indoor circuit (70). On the other hand, the high pressure liquid refrigerant admitted into the injection piping (43) passes through the injection expansion valve (44) during which passage it is pressure reduced down to an intermediate pressure refrigerant and changes to intermediate pressure refrigerant. Then, the intermediate pressure refrigerant is fed to the heat exchange member (66). The intermediate pressure refrigerant admitted into the heat exchange member (66) absorbs heat from the high pressure liquid refrigerant within the container shaped member (65), evaporates, and is supplied to the intermediate pressure port (32) of the compressor (31). [0074] Description will be made in terms of operation in the heating mode. In the heating mode, the injection expansion valve (44) and the supercooling expansion valve (64) are fully closed and the solenoid valve (53) is opened, as in the second embodiment. [0075] In the heating mode, refrigerant, condensed in the indoor heat exchanger (71), passes sequentially through the indoor expansion valve (72) during which passage it is pressure reduced down to an intermediate pressure, through the liquid side interconnecting piping (21), and through the first flowpath (61) of the supercooling heat exchanger (60) and flows into the container-shaped member (65). Within the container-shaped member (65), the intermediate pressure refrigerant in the gas/liquid two-phase state is separated into liquid refrigerant and gas refrigerant. And the gas refrigerant of intermediate pressure, collected in an inner upper portion of the container-shaped member (65), is supplied through the injection piping (43) to the intermediate pressure port (32) of the compressor (31). On the other hand, the liquid refrigerant of intermediate pressure, collected in an inner lower portion of the container-shaped member (65), passes through the outdoor expansion valve (37) during which passage it is pressure reduced down to a low pressure and is then introduced into the outdoor heat exchanger (36). [0076] As described above, in the present modification, the heat exchange member (66) constituting the intermediate pressure heat exchanger (40) is housed in the inside of the container-shaped member (65) constituting the gas/liquid two-phase separator (51). In other words, if the container-shaped member (65) with the heat exchange member (66) housed therein is connected to the outdoor circuit (30), this is the same as arranging both the gas/liquid separator (51) and the intermediate pressure heat exchanger (40) in the outdoor circuit (30). Therefore, in accordance with the present modification, the outdoor circuit (30) can be simplified in configuration in comparison with the case where the gas/liquid separator (51) and the intermediate pressure heat exchanger (40) are formed separately from each other. SECOND MODIFICATION [0077] In each of the foregoing embodiments, it may be arranged such that a lower stage compressor (33) and a higher stage compressor (34) are disposed in the outdoor circuit (30) for the refrigerant circuit (20) to perform a two-stage compression refrigerating cycle. Here, referring to Figure 4, description will be made in terms of an example in which the present modification is applied to the air conditioning system (10) of the second embodiment. [0078] In the outdoor circuit (30) of the present modification, the lower stage compressor (33) and the higher stage compressor (34) are connected together in series. More specifically, the suction side of the lower stage compressor (33) is connected through the accumulator (38) to the second port of the four-way selector valve (35). The discharge side of the lower stage compressor (33) is connected to the suction side of the higher stage compressor (34). The discharge side of the higher stage compressor (34) is connected to the first port of the four-way selector valve (35). In addition, in the present modification, the termination end of the injection piping (43) is connected to piping by which the discharge side of the lower stage compressor (33) and the suction side of the higher stage compressor (34) are connected together. And, gas refrigerant of intermediate pressure flowing through the injection piping (43) is drawn, together with intermediate pressure refrigerant discharged from the lower stage compressor (33), into the higher stage compressor (34). [0079] It should be noted that the above-described embodiments are merely preferable exemplifications in nature and are no way intended to limit the scope of the present invention, its application, or its application range. INDUSTRIAL APPLICABILITY [0080] As has been described above, the present invention finds useful application in a refrigeration system that performs a gas injection operation by supplying gas refrigerant of intermediate pressure to a compressor.

Claims

1. A refrigeration system comprising a refrigerant circuit wherein the refrigerant circuit includes a compressor, a heat source side heat exchanger, and a utilization side heat 5 exchanger which are connected to perform a refrigerating cycle, and wherein the refrigerant circuit is selectively operable either in a cooling mode in which the heat source side heat exchanger becomes a condenser and the utilization side heat exchanger becomes an evaporator or a heating mode in which the utilization side heat exchanger becomes a condenser and the heat source side heat exchanger becomes an evaporator, 10 wherein the refrigerant circuit further includes: an injection passageway through which to supply to the compressor intermediate pressure refrigerant resulting from pressure reducing a part of high pressure liquid refrigerant, an intermediate pressure heat exchanger in which intermediate pressure 15 refrigerant flowing through the injection passageway towards the compressor exchanges heat with high pressure liquid refrigerant and evaporates, and a gas/liquid separator in which intermediate pressure refrigerant resulting from pressure reducing high pressure liquid refrigerant is separated into liquid refrigerant and gas refrigerant, and 20 wherein the refrigerant circuit is configured such that its refrigerant circulation path is selectively changeable between the cooling mode during which gas refrigerant of intermediate pressure flowing through the injection passageway is supplied to the compressor and the heating mode during which gas refrigerant of intermediate pressure flowing out of the gas/liquid separator is supplied to the compressor. 25

2. The refrigeration system of clairn 1, wherein the refrigerant circuit is formed by connecting, by interconnecting piping, a heat source side circuit in which the compressor and the heat source side heat exchanger are disposed and a utilization side circuit in which the utilization side 30 heat exchanger is disposed, and wherein the injection passageway, the intermediate pressure heat exchanger, and the gas/liquid separator are disposed in the heat source side circuit. 27

3. The refrigeration system of claim 1, wherein the gas/liquid separator is formed by a container-shaped member which is arranged at a position in the refrigerant circuit, which position is located, during the cooling mode, downstream of the heat source side heat exchanger and is located, 5 during the heating mode, downstream of the utilization side heat exchanger, and wherein the intermediate pressure heat exchanger is formed by a heat exchange member which is housed in the inside of the container-shaped member and in which intermediate pressure refrigerant flowing through the injection passageway exchanges heat with liquid refrigerant within the container-shaped member. 10

4. The refrigeration system of claim 1, wherein a supercooling heat exchanger, configured to cool high pressure liquid refrigerant by heat exchange with low pressure refrigerant resulting from pressure reducing a part of the high pressure liquid refrigerant, is arranged at a position in the refrigerant circuit, which position is located, during the cooling mode, 15 downstream of the intermediate pressure heat exchanger.

5. The refrigeration system of claim 1, wherein a single-stage compression refrigerating cycle is performed in the refrigerant circuit, and 20 wherein the compressor is configured such that gas refrigerant of intermediate pressure flows into a compression chamber being in the process of compression.

6. The refrigeration system of claim 1, wherein in the refrigerant circuit, a lower stage compressor and a higher stage 25 compressor are connected together in series to perform a two-stage compression refrigerating cycle, and wherein the refrigerant circuit is configured such that gas refrigerant of intermediate pressure is supplied to a suction side of the higher stage compressor. 30

7. A refrigeration system substantially as hereinbefore described with reference to the accompanying drawings.