Patent Publication Number: US-6990822-B2

Title: Pressure adjusting device for air conditioning system and air conditioning system equipped with the same

Description:
TECHNICAL FIELD 
   The present invention relates to a pressure adjusting device for an air conditioning system and, more particularly, to a pressure adjusting device for adjusting the pressure in the indoor heat exchanger of an air conditioning system provided with an outdoor unit having a compressor and an outdoor heat exchanger, an indoor unit having an indoor heat exchanger, and a gaseous refrigerant pipe connecting the indoor heat exchanger to the compressor. The invention also relates to an air conditioning system equipped with such a pressure adjusting device. 
   BACKGROUND ART 
   An example of an air conditioning system that is divided into an outdoor unit and an indoor unit is shown in  FIG. 4 . The air conditioning system  101  has one air-cooled outdoor unit  102  and a plurality of (more specifically, three) indoor units  103 ,  104 ,  105  and is used to air-condition an office or the like. The outdoor unit  102  is equipped with a compressor  111  and an outdoor heat exchanger  112  and is installed outdoors. The indoor units  103 ,  104 ,  105  are each equipped with an expansion valve  113 ,  114 ,  115  and an indoor heat exchanger  123 ,  124 ,  125  and installed in an indoor room  133 ,  134 ,  135 . The outdoor heat exchanger  112  and the expansion valves  113 ,  114 ,  115  are connected together by a liquid refrigerant pipe  116 . The indoor heat exchangers  123 ,  124 ,  125  and the compressor  111  are connected together by a gaseous refrigerant pipe  117 . 
   In this air conditioning system  101 , as shown in  FIGS. 4 and 5 , the gaseous refrigerant is compressed by the compressor  111  from the state at point A 0  to a prescribed pressure Pd 0  (see point B 0  in  FIGS. 4 and 5 ) before being delivered to the outdoor heat exchanger  112 . In the outdoor heat exchanger  112 , the gaseous refrigerant exchanges heat with the outside air and condenses, changing to a liquid refrigerant state (see point C 0  in  FIGS. 4 and 5 ). This condensed liquid refrigerant is delivered from the outdoor heat exchanger  112  to the expansion valves  113 ,  114 ,  115  of the indoor units  103 ,  104 ,  105  through the liquid refrigerant pipe  116  and the pressure of the liquid refrigerant is reduced to Ps 0  (see point D 0  in  FIGS. 4 and 5 ) by the expansion valves  113 ,  114 ,  115 . In the indoor heat exchangers  123 ,  124 ,  125 , the pressure-reduced refrigerant exchanges heat with the air inside each respective room and evaporates, changing to a gaseous refrigerant state (see point A 0  in  FIGS. 4 and 5 ). The evaporation temperature of the refrigerant at the indoor heat exchangers  123 ,  124 ,  125  is the temperature T 0  corresponding to the pressure Ps 0 . The gaseous refrigerant is drawn into the compressor  111  through the gaseous refrigerant pipe  117 . In this way, the air inside the rooms is cooled. 
   Due to the increased use of computers in recent years, the floor space of offices and the like is often partitioned to provide server rooms for the computers. In this kind of server room, it is necessary to run the indoor unit in cooling mode constantly regardless of the season in order to process the heat discharged by the server equipment. 
   However, when the outside air temperature is low, such as in the winter, the refrigerant evaporated in the indoor heat exchangers  123 ,  124 ,  125  of the conventional air conditioning system  101  partially changes to a liquid (see point E 0  in  FIGS. 4 and 5 ) by the time it reaches the compressor  111  through the gaseous refrigerant pipe  117  after leaving the outlets of the indoor heat exchangers  123 ,  124 ,  125  (see point A 0  in  FIGS. 4 and 5 ). When this partially liquefied refrigerant is drawn into the compressor  111 , such problems as damage to the compressor  111  and insufficient intake of gaseous refrigerant occur. 
   Therefore, conventionally, the openings of the expansion valves  113 ,  114 ,  115  are adjusted such that the refrigerant pressure in the indoor heat exchangers  123 ,  124 ,  125  is lowered (see point D 1  and pressure Ps 1  in  FIG. 5 ) and the evaporation temperature of the refrigerant in the indoor heat exchangers  123 ,  124 ,  125  is brought to a temperature T 1  that is lower than the outside air temperature, thus preventing the gaseous refrigerant from liquefying inside the gaseous refrigerant pipe  117  (see point A 1  in  FIG. 5 ). 
   If the evaporation temperature of the refrigerant is lowered too much, however, the refrigeration cycle of the air conditioning system  101  will be along the lines joining points A 1 , B 1 , C 1 , and D 1  in  FIG. 5  and the indoor heat exchangers  123 ,  124 ,  125  will freeze. As a result, it will not be possible to continue running the indoor units  103 ,  104 ,  105 . When such a situation occurs, the indoor units  103 ,  104 ,  105  are generally run in fan-only mode to increase the temperature of the frozen indoor heat exchangers  123 ,  124 ,  125  and return them to an unfrozen state. In a room, such as server room (assume, for example, that room  133  in  FIG. 4  is a server room), where the amount of discharged heat is large, the temperature inside the room will rise rapidly when the cooling operation is stopped and the operation of the server equipment could possibly be impeded. 
   DISCLOSURE OF THE INVENTION 
   The present invention relates to an air conditioning system provided with an outdoor unit having a compressor and an outdoor heat exchanger, an indoor unit having an indoor heat exchanger, and a gaseous refrigerant pipe connecting the indoor heat exchanger to the compressor. The object of the present invention is to make it possible to run such an air conditioning system in cooling mode continuously even when the outside air temperature is low by preventing the indoor heat exchanger from freezing. 
   An air conditioning system pressure adjusting device for adjusting the pressure in the indoor heat exchanger of an air conditioning system that is provided with an outdoor unit having a compressor and an outdoor heat exchanger, an indoor unit having an indoor heat exchanger, and a gaseous refrigerant pipe connecting the indoor heat exchanger to the compressor. The pressure adjusting device is provided with a pressure detecting means, an electric powered expansion valve, and an opening adjusting means. The pressure detecting means detects the pressure value of the refrigerant in the indoor heat exchanger. The electric powered expansion valve is disposed in the gaseous refrigerant pipe. The opening adjusting means adjusts the opening of the electric powered expansion valve based on the pressure value of the refrigerant detected by the pressure detecting means such that the pressure value of the refrigerant is adjusted to a prescribed pressure setting value. 
   This air conditioning system pressure adjusting device makes it possible to adjust the pressure of the refrigerant in the indoor heat exchanger to a prescribed pressure setting by adjusting the opening of the electric powered expansion valve. Consequently, the pressure of the refrigerant in the indoor heat exchanger can be adjusted to a higher pressure than the pressure of the refrigerant in the gaseous refrigerant pipe between the electric powered expansion valve and the compressor. 
   Thus, even when the outside air temperature is low, the pressure of the refrigerant downstream of the electric powered expansion valve in the gaseous refrigerant pipe can be lowered so as to prevent the gaseous refrigerant from liquefying. At the same time, the pressure of the refrigerant in the indoor heat exchanger can be adjusted such that the evaporation temperature of the refrigerant is a temperature at which the indoor heat exchanger will not freeze, thus preventing the indoor heat exchanger from freezing. As a result, the air conditioning system can be run continuously in cooling mode. 
   In the air conditioning system pressure adjusting device the opening adjusting means is capable of providing the electric powered expansion valve with an opening value that is appropriate for oil recovery mode when the system is run in oil recovery mode in order to return lubricating oil that has accumulated in the refrigerant circuit to the compressor. 
   With this pressure adjusting device, the opening adjusting means not only provides an opening for adjusting the pressure of the refrigerant in the indoor heat exchanger but also makes it possible to provide an opening that is appropriate for oil recovery mode when the system is run in oil recovery mode. Thus, the air conditioning system can be run in an oil recovery mode similar to the oil recovery mode of conventional air conditioning systems. 
   Also, an air conditioning system pressure adjusting device is described, wherein the electric powered expansion valve is installed in the indoor portion of the gaseous refrigerant pipe. 
   When the electric powered expansion valve is disposed in the outdoor portion of the gaseous refrigerant pipe, the refrigerant in the portion of the gaseous refrigerant pipe upstream of the electric powered expansion valve is cooled by the outside air and becomes partially liquefied. Then, the partially liquefied refrigerant is reduced in pressure by the electric powered expansion valve and the liquid portion is evaporated again before being drawn into the compressor. Consequently, if there is a portion where liquid accumulation occurs readily due to the shape and routing of the gaseous refrigerant pipe, there is the possibility that liquid refrigerant and oil will accumulate in the portion of the gaseous refrigerant pipe upstream of the electric powered expansion valve subjecting the compressor to conditions of insufficient oil and insufficient gaseous refrigerant intake. 
   Conversely, with the air conditioning system pressure adjusting device claimed here, temporary liquefaction of the refrigerant in the gaseous refrigerant pipe can be prevented because the electric powered expansion valve is disposed indoors instead of outdoors. Thus, conditions of insufficient oil and insufficient gaseous refrigerant intake do not occur at the compressor and the compressor can be protected more reliably. 
   Moreover, an air conditioning system pressure adjusting device, is described, wherein the electric powered expansion valve, the pressure detecting means, and the opening adjusting means are constructed as a single integral unit. 
   Since this air conditioning system pressure adjusting device is a single unit, it can be installed easily in, for example, the gaseous refrigerant pipe of an existing air conditioning system in order to prevent freezing of the indoor heat exchanger. 
   Also, an air conditioning system is provided with an outdoor unit, a plurality of indoor units, a gaseous refrigerant pipe, and a pressure adjusting device. The outdoor unit has a compressor and an outdoor heat exchanger. The indoor unit has a compressor and an indoor heat exchanger. The gaseous refrigerant pipe has a plurality of gaseous refrigerant branch pipes connected to the indoor heat exchangers of the respective indoor units and a gaseous refrigerant convergence pipe into which the gaseous refrigerant branch pipes converge and which is connected to the compressor. The pressure adjusting device is connected to some of the gaseous refrigerant branch pipes. 
   In this air conditioning system, the pressure adjusting device is provided with respect to some of the indoor units, i.e., more than one indoor unit but less than all of the indoor units. Thus, the indoor units that are provided with a pressure adjusting device can be run in cooling mode continuously even when the outside air temperature is low. For example, when a server room or other room having a large thermal load is provided in an office or the like by partitioning, the indoor unit installed in the room having the large thermal load can be run in cooling mode continuously even when the outside temperature is low by providing a pressure adjusting device for that indoor unit only, thereby preventing the gaseous refrigerant in the portion of the gaseous refrigerant branch pipe downstream of the electric powered expansion valve and in the gaseous refrigerant convergence pipe from liquefying and preventing the indoor unit from freezing. 
   Moreover, an air conditioning system is described, wherein the indoor units corresponding to the gaseous refrigerant branch pipes that do not have a pressure adjusting device connected thereto are connected to the outdoor unit in such a manner that they can switch between a cooling mode and a heating mode. The operating capacity of the outdoor unit can be adjusted in accordance with the total operating load resulting from the cooling operation and heating operation of the plurality of indoor units. 
   This air conditioning system has indoor units connected to the outdoor unit in such a manner that they can switch between cooling mode and heating mode and the operating capacity of its outdoor unit can be adjusted in accordance with the total operating load resulting from the cooling operation and heating operation of the plurality of indoor units. In short, it is the type of air conditioning system that is capable of so-called simultaneous heating and cooling. In the winter when the outside temperature is low, this kind of air conditioning system (i.e., one capable of simultaneous heating and cooling) generally performs heating in all rooms except those having large thermal loads, such as server rooms. In short, only the indoor units installed in rooms having large thermal loads, e.g., server rooms, are run in cooling mode. Since the refrigerant leaving the indoor units that are running in cooling mode returns to the outdoor unit through the gaseous refrigerant pipe, there is the possibility that the indoor heat exchangers of the indoor units running in cooling mode will freeze. 
   However, since the indoor units installed in rooms having large thermal loads and used exclusively for cooling are provided with pressure adjusting devices, those indoor units can be run in cooling mode continuously even when the outside temperature is low because the pressure adjusting devices prevent the gaseous refrigerant in the portions of the gaseous refrigerant branch pipes downstream of the electric powered expansion valves and in the gaseous refrigerant convergence pipe from liquefying and also prevent the indoor unit from freezing. 

   
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
       FIG. 1  is a schematic view of the refrigerant circuit of an air conditioning system in accordance with a first embodiment of the present invention. 
       FIG. 2  is a schematic view of the pressure adjusting device of an air conditioning system in accordance with the first embodiment of the present invention. 
       FIG. 3  is a Mollier diagram showing the refrigeration cycle of an air conditioning system in accordance with the first embodiment of the present invention. 
       FIG. 4  is a schematic view of the refrigerant circuit of a conventional air conditioning system. 
       FIG. 5  is a Mollier diagram showing the refrigeration cycle of a conventional air conditioning system. 
       FIG. 6  is a schematic view of the refrigerant circuit of an air conditioning system in accordance with a second embodiment of the present invention. 
       FIG. 7  is a diagram illustrating the flow of the refrigerant during simultaneous heating and cooling operation in an air conditioning system in accordance with the second embodiment of the present invention. 
   

   PREFERRED EMBODIMENTS OF THE INVENTION 
   Embodiments of the present invention will now be described with reference to the drawings. 
   [First Embodiment] 
   (1) Constituent Features of the Air Conditioning System 
     FIG. 1  is a schematic view of the refrigerant circuit of an air conditioning system  1  in accordance with a first embodiment of the present invention. The air conditioning system  1  is equipped chiefly with one air-cooled outdoor unit  2  and a plurality of (three in this embodiment) indoor units  3 ,  4 ,  5  connected to the outdoor unit  2  in parallel. It is used, for example, to air-condition an office or the like. Among the indoor units  3 ,  4 ,  5 , the indoor unit  3  is installed in a room  33  that is a server room fitted with server equipment. Consequently, the room  33  has a larger amount of discharged heat than the rooms  34 ,  35  in which the other indoor units  4 ,  5  are installed. 
   The outdoor unit  2  is equipped chiefly with a compressor  11  and an outdoor heat exchanger  12  and is installed outdoors. The compressor  11  is a device for compressing gaseous refrigerant to a prescribed pressure. The outdoor heat exchanger  12  is a device that exchanges heat between the refrigerant and the outside air and is a so-called air-cooled heat exchanger. 
   The indoor units  3 ,  4 ,  5  are equipped chiefly with an expansion valve  13 ,  14 ,  15  and an indoor heat exchanger  23 ,  24 ,  25 . The expansion valves  13 ,  14 ,  15  serve to reduce the pressure of the liquid refrigerant that is condensed by the exchange of heat taking place in the outdoor heat exchanger  12 . The indoor heat exchangers  23 ,  24 ,  25  are devices for exchanging heat between the refrigerant that has been pressure-reduced by the expansion valves  13 ,  14 ,  15  and the air inside each room. 
   The outdoor heat exchanger  12  and the expansion valves  13 ,  14 ,  15  are connected together by a liquid refrigerant pipe  16 . The indoor heat exchangers  23 ,  24 ,  25  and the compressor  11  are connected together by a gaseous refrigerant pipe  17 . The liquid refrigerant pipe  16  has a liquid refrigerant convergence pipe  16   a  that is connected to the outlet of the outdoor heat exchanger  12  and liquid refrigerant branch pipes  16   b ,  16   c ,  16   d  that are connected between the liquid refrigerant convergence pipe  16   a  and each of the expansion valves  13 ,  14 ,  15 , respectively. The gaseous refrigerant pipe  17  has a gaseous refrigerant convergence pipe  17   a  that is connected to the inlet of the compressor  11  and gaseous refrigerant branch pipes  17   b ,  17   c ,  17   d  that are connected between the gaseous refrigerant convergence pipe  17   a  and each of the indoor heat exchangers  23 ,  24 ,  25 , respectively. A pressure adjusting device  6  is installed in the gaseous refrigerant branch pipe  17   b . Thus, the pressure adjusting device  6  is provided with respect to the indoor unit  3  installed in the room  33 . The pressure adjusting device  6  functions to adjust the pressure of the refrigerant in the indoor heat exchanger  23  which refrigerant has been pressure-reduced by the expansion valve  13  to a higher pressure than the refrigerant in the indoor heat exchangers  24 ,  25  of the other indoor units  4 ,  5 . 
   (2) Constituent Features of the Pressure Adjusting Device of the Air Conditioning System 
     FIG. 2  is a schematic view of the pressure adjusting device  6  of the air conditioning system  1 . The pressure adjusting device  6  is a single unit equipped with a pressure detecting means  61 , an electric powered expansion valve  62 , and an opening adjusting means  63  and is arranged externally to the indoor unit  3 . 
   The pressure detecting means  61  is a pressure gauge for detecting the pressure value of the refrigerant the indoor heat exchanger  23  of the indoor unit  3  and transmits the detected refrigerant pressure value to the opening adjusting means  63 . 
   The opening adjusting means  63  is a control device that executes feedback control to adjust the opening of the electric powered expansion valve  62  based on the pressure value of the refrigerant detected by the pressure detecting means  61  such that the pressure value of the refrigerant is adjusted to a prescribed pressure setting value. The pressure setting value of the opening adjusting means  63  can be changed. The opening adjusting means  63  is capable of forcefully providing the electric powered expansion valve  62  with an opening value that is appropriate for oil recovery mode when the system runs in oil recovery mode in order to return lubricating oil that has accumulated in the gaseous refrigerant pipe  17  to the compressor  11 ; it provides this opening value in response to an oil recovery mode signal issued from the main control unit  20  of the air conditioning system  1 . 
   The electric powered expansion valve  62  is disposed downstream of the pressure detecting means  61  and is an adjustable valve that can open an close automatically in response to a signal from the opening adjusting means  63 . 
   Due to the constituent features described heretofore, the pressure adjusting device  6  can adjust the pressure of the refrigerant in the indoor heat exchanger  23  of the indoor unit  3  to a higher pressure than the refrigerant in the indoor heat exchangers  24 ,  25  of the other indoor units  4 ,  5 . 
   (3) Operation of the Air Conditioning System and the Pressure Adjusting Device 
   The operation of the air conditioning system  1  and the pressure adjusting device  6  will now be described using  FIGS. 1 to 3 . 
   [1] Operation When Outside Air Temperature is High (Non-Winter Season) 
   As shown in  FIGS. 1 and 3 , when the compressor  11  is started and the air conditioning system  1  is run, the gaseous refrigerant is compressed by the compressor  11  from the state at point A 0  in  FIGS. 1 and 3  to a prescribed pressure Pd 0  (see point B 0  in  FIGS. 1 and 3 ) before being delivered to the outdoor heat exchanger  12 . In the outdoor heat exchanger  12 , the gaseous refrigerant exchanges heat with the outside air and condenses to a liquid refrigerant state (see point C 0  in  FIGS. 1 and 3 ). The condensed refrigerant liquid is fed from the outdoor heat exchanger  12  to the expansion valves  13 ,  14 ,  15  of the indoor units  3 ,  4 ,  5  through the liquid refrigerant pipe  16 . 
   Next, the cycle from the expansion valves  13 ,  14 ,  15  to the gaseous refrigerant convergence pipe  17   a  will be explained. Since the construction of this portion of the refrigerant circuit is different for the indoor unit  3  in which the pressure adjusting device  6  is installed than for the other indoor units  4 ,  5 , the two different arrangements are described separately. 
   In the arrangement of the indoor units  4  and  5 , the liquid refrigerant is delivered from the outdoor heat exchanger  12  to the expansion valves  14 ,  15  of the indoor units  4 ,  5  through the liquid refrigerant convergence pipe  16   a  and the liquid refrigerant branch pipes  16   c ,  16   d  and the pressure of the liquid refrigerant is reduced to Ps 0  (see point D 0  in  FIGS. 1 and 3 ) by the expansion valves  14 ,  15 . In the indoor heat exchangers  24 ,  25 , the pressure-reduced refrigerant exchanges heat with the air inside each respective room  34 ,  35  and evaporates, changing to a gaseous refrigerant state (see point A 0  in  FIGS. 1 and 3 ). The evaporation temperature of the refrigerant in the indoor heat exchangers  24 ,  25  is the temperature T 0  corresponding to the pressure Ps 0 . This gaseous refrigerant passes through the gaseous refrigerant branch pipes  17   c ,  17   d  and converges into the gaseous refrigerant convergence pipe  17   a.    
   In the arrangement of the indoor unit  3 , the liquid refrigerant is delivered from the outdoor heat exchanger  12  to the expansion valve  13  of the indoor unit  3  through the liquid refrigerant convergence pipe  16   a  and the liquid refrigerant branch pipe  16   b  and the pressure of the liquid refrigerant is reduced to Ps 2  (see point D 2  in  FIGS. 1 and 3 ) by the expansion valve  13 . In the indoor heat exchanger  23 , the pressure-reduced refrigerant exchanges heat with the air inside the room  33  and evaporates, changing to a gaseous refrigerant state (see point A 2  in  FIGS. 1 and 3 ). The evaporation temperature of the refrigerant in the indoor heat exchanger  23  is the temperature T 2  corresponding to the pressure Ps 2 . Also, since the pressure adjusting device  6  is installed in the gaseous refrigerant branch pipe  17   b , the pressure of the refrigerant that evaporated in the indoor heat exchanger  23  is reduced by the electric powered expansion valve  62  of the pressure adjusting device  6  to the same pressure Ps 0  as the refrigerant in the other indoor heat exchangers  24 ,  25  before the refrigerant flows into the gaseous refrigerant convergence pipe  17   a . In short, the pressure adjusting device  6  detects the evaporation pressure of the indoor heat exchanger  23  of the indoor unit  3  with the pressure detecting means  61  and adjusts the opening of the electric powered expansion valve  62  using the opening adjusting means  63  such that prescribed pressure setting value Ps 2  is obtained. 
   Then, the gaseous refrigerant is drawn into the compressor  11  through the gaseous refrigerant convergence pipe  17   a . In this way, the air inside the rooms  33 ,  34 ,  35  is cooled. 
   [2] Operation When Outside Air Temperature is Low (Winter Season) 
   The operation when the outside air temperature is low is basically the same as when the outside air temperature is high. The differences between the operation when the outside air temperature is low and the operation when the outside air temperature is high will now be described. 
   When the outside air temperature is low, i.e., lower than the temperature of the gaseous refrigerant, it becomes easy for the gaseous refrigerant to be cooled and liquefied within the gaseous refrigerant pipe  17  as it travels from the outlets of the indoor heat exchangers  23 ,  24 ,  25  to the compressor  11  through the gaseous refrigerant pipe  17 . In order to prevent this from occurring, the intake pressure of the compressor  11  is set to a pressure Ps 3  that is lower than the pressure used when the outside temperature is high (pressure Ps 0 ). 
   Thus, the entire air conditioning system  1  operates at a lower refrigerant temperature. The indoor units  4  and  5  of the air conditioning unit  1  operate according to the refrigerant cycle indicated by the single-dot chain lines joining points A 1 , B 1 , C 1 , and D 1  in  FIG. 3  and the indoor unit  3  operates according to the refrigerant cycle indicated by the lines joining points A 1 , B 1 , C 1 , D 2 , A 2 , and A 1  in  FIG. 3 . 
   Since the intake pressure of the compressor  11  falls from Ps 0  to Ps 3 , the evaporation temperature of the refrigerant in the indoor heat exchangers  24 ,  25  of the indoor units  4 ,  5  falls to a temperature T 1  at which there is the possibility that the indoor heat exchangers  24 ,  25  will freeze. If the indoor heat exchangers  24 ,  25  for the rooms  34 ,  35  freeze, the expansion valves  14 ,  15  are closed and the indoor units  4 ,  5  are operated in fan-only mode so that the indoor heat exchangers  24 ,  25  can be returned from their frozen state to a normal state. Consequently, such temporary inconveniences as a rise in the temperature inside the rooms  34 ,  35  occur. However, this is not a serious problem because the thermal loads of the rooms  34  and  35  are smaller than the thermal load of the room  33 . 
   Meanwhile, the thermal load of the room  33  is large and the indoor heat exchanger  23  of the indoor unit  3  cannot be allowed to freeze if the server equipment is to be maintained at a normal operating state. Therefore, the pressure adjusting device  6  installed downstream of the indoor heat exchanger  23  adjusts the refrigerant pressure Ps 2  of the indoor heat exchanger  23  such that the evaporation temperature becomes a temperature T 2  (e.g., a temperature approximately equal to the evaporation temperature when the outside air temperature is high) at which freezing of the indoor heat exchanger  23  does not occur. 
   [3] Operation in Oil Recovery Mode 
   During partial load operation of the air conditioning system  1 , lubricating oil from the compressor  11  accumulates chiefly in the gaseous refrigerant pipe  17 . When this occurs, the system is operated in oil recovery mode, i.e., the expansion valves  13 ,  14 ,  15  disposed upstream of the indoor heat exchangers  23 ,  24 ,  25  are opened fully while running the compressor  11  in order to push the lubrication oil accumulated in the refrigerant circuit toward the inlet of the compressor  11 . Since the electric powered expansion valve  62  of the pressure adjusting device  6  can also be opened fully in response to the fuel recovery mode start command from the main control unit  20  of the air conditioning system  1 , the lubricating oil accumulated in the refrigerant piping of the indoor unit  3  is recovered in the same manner as the lubricating oil accumulated in the refrigerant piping of the indoor units  4  and  5 . 
   (4) Characteristic Features of the Air Conditioning System Pressure Adjusting Device and Characteristic Features of an Air Conditioning System Equipped with the Same 
   An air conditioning system pressure adjusting device and air conditioning system equipped with the same in accordance with this embodiment have the following characteristic features. 
   [1] Prevents Freezing of the Indoor Heat Exchanger 
   A pressure adjusting device  6  in accordance with this embodiment makes it possible to adjust the pressure of the refrigerant in the indoor heat exchanger  23  to a prescribed pressure setting by adjusting the opening of the electric powered expansion valve  62 . As a result, the pressure of the refrigerant in the indoor heat exchanger  23  can be adjusted to a higher pressure than the pressure of the refrigerant in the gaseous refrigerant pipe  17  between the electric powered expansion valve  62  and the compressor  11 . Thus, as shown in  FIG. 3 , even when the outside air temperature is low, the pressure of the refrigerant in the indoor heat exchanger  23  can be adjusted to a pressure Ps 2  that is higher than the pressure Ps 3  such that the gaseous refrigerant in the gaseous refrigerant pipe  17  downstream of the electric powered expansion valve  62  is prevented from liquefying and the evaporation temperature of the refrigerant becomes a temperature T 2  at which the indoor heat exchanger  23  will not freeze. As a result, freezing of the indoor heat exchanger  23  is prevented and the indoor unit  3  can be run in cooling mode continuously. 
   The refrigerant pressure Ps 2  of the indoor heat exchanger  23  can be adjusted easily by simply changing the pressure setting value of the opening adjusting means  63  of the pressure adjusting device. 
   Furthermore, in an air conditioning system  1  equipped with a plurality of indoor units  3 ,  4 ,  5 , the indoor unit  3  installed in the room  33  where the thermal load is high can be run in cooling mode continuously even when the outside temperature is low by installing this kind of pressure adjusting device  6  for that indoor unit  3  only. 
   [2] Oil Recovery Mode 
   A pressure adjusting device  6  in accordance with this embodiment is easy to interlock with a command from the main control unit  20  of the air conditioning system  1  because the electric powered expansion valve  62  is electrically driven. The opening adjusting means  63  not only provides the electric powered expansion valve  62  with an opening for adjusting the pressure of the refrigerant in the indoor heat exchanger  23  but can also provide an opening that is appropriate for oil recovery mode when the system is run in oil recovery mode. Thus, the air conditioning system can be run in an oil recovery mode similar to the oil recovery mode of conventional air conditioning systems. 
   [3] Improves Reliability of Compressor Protection 
   When, for example, the electric powered expansion valve  62  is arranged in the outdoor portion of the gaseous refrigerant pipe  17 , the refrigerant in the portion of the gaseous refrigerant pipe  17  upstream of the electric powered expansion valve  62  will be cooled by the outside air and partially liquefy. Then, the partially liquefied refrigerant is reduced in pressure by the electric powered expansion valve  62  and the liquid portion is evaporated again before being drawn into the compressor  11 . Consequently, if there is a portion where liquid accumulation occurs readily due to the shape and routing of the gaseous refrigerant pipe  17 , there is the possibility that liquid refrigerant and oil will accumulate in the portion of the gaseous refrigerant pipe  17  upstream of the electric powered expansion valve  62 , thus subjecting the compressor  11  to conditions of insufficient oil and insufficient gaseous refrigerant intake. 
   Conversely, with a pressure adjusting device  6  in accordance with this embodiment, temporary liquefaction of the refrigerant in the gaseous refrigerant pipe  17  can be prevented because the electric powered expansion valve  62  is disposed indoors instead of outdoors. Thus, conditions of insufficient oil and insufficient gaseous refrigerant intake do not occur at the compressor  11  and the reliability of the compressor protection can be improved. 
   [3] Integration 
   Since a pressure adjusting device  6  in accordance with this embodiment is a single unit integrating the electric powered expansion valve  62 , the pressure detecting means  61 , and the opening adjusting means  63 , it can be installed easily in, for example, the gaseous refrigerant pipe of an existing air conditioning system in order to prevent freezing of the indoor heat exchanger. 
   [Second Embodiment] 
   While the previous embodiment is an example of applying the present invention to an air conditioning system that is used exclusively for cooling, it is also acceptable to apply the invention to an air conditioning system designed for simultaneous heating and cooling. An air conditioning system  201  for simultaneous heating and cooling to which the present invention has been applied will now be described with reference to the drawings. 
   (1) Constituent Features of the Air Conditioning System 
     FIG. 6  is a schematic view of the refrigerant circuit of an air conditioning system  201  in accordance with a second embodiment of the present invention. The air conditioning system  201  is provided chiefly with one air-cooled outdoor unit  202  and a plurality of (three in this embodiment) indoor units  203 ,  204 ,  205  connected in parallel to the outdoor unit  202 . It is used, for example, to air-condition an office or the like. Among the indoor units  203 ,  204 ,  205 , the indoor unit  203  is installed in a room that is a server room fitted with server equipment, similarly to the first embodiment. The server room has a larger amount of discharged heat than the rooms in which the other indoor units  204 ,  205  are installed. The indoor units  204  and  205  are connected to the outdoor unit  202  in such a manner that they can be switched between cooling mode and heating mode while the indoor unit  203  runs in cooling mode. The outdoor unit  202  is constituted such that its operating capacity can be adjusted in accordance with the total operating load resulting from the cooling operation and heating operation of the indoor units  203 ,  204 ,  205 . 
   [1] Outdoor Unit 
   The outdoor unit  202  is installed outdoors and includes chiefly the following devices and valves, which are connected with refrigerant piping: a compressor  211 , an outdoor main heat exchanger  212   a , a four-way selector valve  213 , an outdoor expansion valve  214 , an outdoor auxiliary heat exchanger  212   b , an outdoor solenoid valve  216 , a liquid refrigerant shut-off valve  217 , a first gaseous refrigerant shut-off valve  218 , and a second gaseous refrigerant shut-off valve  219 . 
   The compressor  211  is a device for compressing gaseous refrigerant. The intake side of the compressor  211  is connected to the four-way selector valve  213  and the second gaseous refrigerant shut-off valve  219 . The discharge side of the compressor  211  is connected to the four-way selector valve  213  and the outdoor auxiliary heat exchanger  212   b.    
   The outdoor main heat exchanger  212   a  is a heat exchanger for evaporating and condensing the refrigerant using the outside air as a heat source and forms the outdoor heat exchanger  212  together with the outside auxiliary heat exchanger  212   b . The gas side of the outdoor main heat exchanger  212   a  is connected to the four-way selector valve  213 . The liquid side of the outdoor main heat exchanger  212   a  is connected to the liquid refrigerant shut-off valve  217 . The outdoor expansion valve  214  is provided between the liquid side of the outdoor main heat exchanger  212   a  and the liquid refrigerant shut-off valve  217 . The outdoor expansion valve  214  is an electric powered expansion valve configured such that it can adjust the amount of refrigerant flowing through the outdoor main heat exchanger  212   a.    
   The four-way selector valve  213  is a selector valve configured to make the outdoor main heat exchanger  212   a  function as either an evaporator or a condenser. The four-way selector valve  213  is connected to the gas side of the outdoor main heat exchanger  212   a , the intake side of the compressor  211 , the discharge side of the compressor  211 , and the first gaseous refrigerant shut-off valve  218 . When it makes the outdoor main heat exchanger  212   a  function as a condenser, the four-way selector valve  213  can connect the discharge side of the compressor  211  to the gas side of the outdoor main heat exchanger  212   a  and connect the intake side of the compressor  211  to the first gaseous refrigerant shut-off valve  218 . Conversely, when it makes the outdoor main heat exchanger  212   a  function as an evaporator, the four-way selector valve  213  can connect the gas side of the outdoor main heat exchanger  212   a  to the intake side of the compressor  211  and connect the discharge side of the compressor  211  to the first gaseous refrigerant shut-off valve  218 . 
   The outdoor auxiliary heat exchanger  212   b  is connected in parallel with the outdoor main heat exchanger  212   a  and serves to condense the refrigerant using the outside air as a heat source. The outdoor solenoid valve  216  that can be opened and closed when necessary is provided on the liquid side of the outdoor auxiliary heat exchanger  212   b . As a result, the overall refrigerant evaporation amount of the outdoor heat exchanger  212  can be adjusted. 
   [2] Indoor Units 
   The indoor units  203 ,  204 ,  205  are each equipped chiefly with an expansion valve  223 ,  224 ,  225  and an indoor heat exchanger  233 ,  234 ,  235  and these devices and valves are connected together with refrigerant piping. The indoor expansion valves  223 ,  224 ,  225  are electric powered expansion valves for reducing the pressure of the liquid refrigerant during operation in cooling mode. The indoor heat exchangers  233 ,  234 ,  235  function as refrigerant condensers during heating mode and as refrigerant evaporators during cooling mode. 
   [3] Refrigerant Piping 
   In this embodiment, the liquid refrigerant pipe  251 , the first gaseous refrigerant pipe  252 , and the second gaseous refrigerant pipe  253  are connected to the outdoor unit  202 . 
   The liquid refrigerant pipe  251  serves to connect the liquid refrigerant shut-off valve  217  of the outdoor unit  202  to the indoor units  203 ,  204 ,  205  and includes the following: liquid refrigerant branch pipes  251   b ,  251   c ,  251   d  corresponding to the respective indoor units  203 ,  204 ,  205 ; and a liquid refrigerant convergence pipe  251   a  into which the liquid refrigerant branch pipes  251   b ,  251   c ,  251   d  converge and which is connected to the liquid refrigerant shut-off valve  217 . The liquid refrigerant branch pipe  251   b  is connected to the indoor expansion valve  223  of the indoor unit  203 . The liquid refrigerant branch pipe  251   c  runs from its junction with the liquid refrigerant convergence pipe  251   a  and connects to the indoor expansion valve  224  of the indoor unit  204 , passing through the heating/cooling changeover device  207  (discussed later) in-between. The liquid refrigerant branch pipe  251   d  runs from its junction with the liquid refrigerant convergence pipe  251   a  and connects to the indoor expansion valve  225  of the indoor unit  205 , passing through the heating/cooling changeover device  208  (discussed later) in-between. 
   The first gaseous refrigerant pipe  252  serves to connect the first gaseous refrigerant shut-off valve  218  of the outdoor unit  202  to the indoor units  204 ,  205  (i.e., the indoor units other than the indoor unit  203 ) and includes the following: first gaseous refrigerant branch pipes  252   c ,  252   d  corresponding to the respective indoor units  204 ,  205 ; and a first gaseous refrigerant convergence pipe  252   a  into which the first gaseous refrigerant branch pipes  252   c ,  252   d  converge and which is connected to the first gaseous refrigerant shut-off valve  218 . The first gaseous refrigerant branch pipe  252   c  runs from its junction with the first gaseous refrigerant convergence pipe  252   a  and connects to the indoor heat exchanger  234  of the indoor unit  204 , passing through the heating/cooling changeover device  207  in-between. The first gaseous refrigerant branch pipe  252   d  runs from its junction with the first gaseous refrigerant convergence pipe  252   a  and connects to the indoor heat exchanger  235  of the indoor unit  205 , passing through the heating/cooling changeover device  208  in-between. 
   The second gaseous refrigerant pipe  253  serves to connect the second gaseous refrigerant shut-off valve  219  of the outdoor unit  202  to the indoor units  203 ,  204 ,  205  and includes the following: second gaseous refrigerant branch pipes  253   b ,  253   c ,  253   d  corresponding to the respective indoor units  203 ,  204 ,  205 ; and a second gaseous refrigerant convergence pipe  253   a  into which the second gaseous refrigerant branch pipes  253   b ,  253   c ,  253   d  converge and which is connected to the second gaseous refrigerant shut-off valve  219 . The second gaseous refrigerant branch pipe  253   b  runs from its junction with the second gaseous refrigerant convergence pipe  253   a  and connects to the indoor heat exchanger  233  of the indoor unit  203 , passing through the pressure adjusting device  206  (discussed later) in-between. The second gaseous refrigerant branch pipe  253   c  runs from its junction with the second gaseous refrigerant convergence pipe  253   a  and connects to the indoor heat exchanger  234  of the indoor unit  204 , passing through the heating/cooling changeover device  207  in-between. The second gaseous refrigerant branch pipe  253   d  runs from its junction with the second gaseous refrigerant convergence pipe  253   a  and connects to the indoor heat exchanger  235  of the indoor unit  205 , passing through the heating/cooling changeover device  208  in-between. 
   [4] Pressure Adjusting Device 
   Similarly to the pressure adjusting device  6  of the first embodiment, the pressure adjusting device  206  is a single unit equipped with a pressure detecting means  261 , an electric powered expansion valve  262 , and an opening adjusting means  263 . It is provided in the second gaseous refrigerant branch pipe  253   b , which connects the outdoor unit  202  and the indoor unit  203  together. The pressure adjusting device  206  can adjust the pressure of the refrigerant in the indoor heat exchanger  233  of the indoor unit  203  to a higher pressure than the refrigerant in the indoor heat exchangers  234 ,  235  of the other indoor units  204 ,  205 . Also, again similarly to the pressure adjusting device  6  of the first embodiment, the opening adjusting means  263  of the pressure adjusting device  206  is capable of forcefully providing the electric powered expansion valve  262  with an opening value that is appropriate for oil recovery mode in response to an oil recovery mode signal issued from the main control unit  20  of the air conditioning system  201  when oil recovery mode is executed. 
   [5] Heating/Cooling Changeover Device 
   The indoor units  207 ,  208  are each equipped chiefly with a subcooling heat exchanger  241 ,  242 , a low-pressure gaseous refrigerant return valve  243 ,  244 , and a high-pressure gaseous refrigerant supply valve  245 ,  246 . 
   The heating/cooling changeover devices  207 ,  208  are configured such that, when the indoor units  204 ,  205  run in cooling mode, liquid refrigerant can be supplied from the outdoor unit  202  to the indoor units  204 ,  205  through the liquid refrigerant branch pipes  251   c ,  251   d  of the liquid refrigerant pipe  251  and the subcooling heat exchangers  241 ,  242 . The heating/cooling changeover devices  207 ,  208  are further configured such that refrigerant evaporated in the indoor heat exchangers  234 ,  235  of the indoor units  204 ,  205  can be delivered to the second gaseous refrigerant branch pipes  253   c ,  253   d  of the second gaseous refrigerant pipe  253  through the low-pressure gaseous refrigerant return valves  243 ,  244 . 
   The heating/cooling changeover devices  207 ,  208  are configured such that, when the indoor units  204 ,  205  run in heating mode, gaseous refrigerant can be supplied from the outdoor unit  202  to the indoor units  204 ,  205  through the first gaseous refrigerant branch pipes  252   c ,  252   d  of the first gaseous refrigerant pipe  252  and the high-pressure gaseous refrigerant supply valves  245 ,  246 . The heating/cooling changeover devices  207 ,  208  are further configured such that refrigerant condensed in the indoor heat exchangers  234 ,  235  of the indoor units  204 ,  205  can be delivered to the liquid refrigerant branch pipes  251   c ,  251   d  of the liquid refrigerant pipe  251  through the subcooling heat exchangers  241 ,  242 . 
   The subcooling heat exchangers  241 ,  242  serve to subcool the liquid refrigerant supplied to the indoor units  204 ,  205  from the outdoor unit  202 . More specifically, the heating/cooling changeover devices  207 ,  208  each have a subcooling valve  247 ,  248  and a capillary  249 ,  250  for reducing the pressure of a portion of the liquid refrigerant that is supplied to the heating/cooling changeover devices  207 ,  208  from the liquid refrigerant branch pipes  251   c ,  251   d  during cooling mode. The subcooling heat exchangers  241 ,  242  cool the liquid refrigerant heading toward the indoor units  204 ,  205  to a subcooled state using this pressure-reduced refrigerant as a cooling source. Meanwhile, after the refrigerant used as a cooling source is evaporated in the subcooling heat exchangers  241 ,  242 , it is returned downstream of the low-pressure gaseous refrigerant return valves  243 ,  244  and converges with the refrigerant evaporated in the indoor units  204 ,  205 . 
   The indoor unit  203  differs from the indoor units  204 ,  205  in that it is a dedicated cooling unit connected to a pressure adjusting device  206  instead of a heating/cooling changeover device  207 ,  208 . In short, the air conditioning system  201  is configured such that it can perform simultaneous heating and cooling. Thus, for example, the indoor unit  203  installed in a server room can be run in cooling mode while the indoor units  204 ,  205  are run in heating mode or the indoor unit  203  and the indoor unit  204  can be run in cooling mode while the indoor unit  205  is run in heating mode. 
   (2) Operation of the Air Conditioning System 
   The operation of the air conditioning system  201  of this embodiment will now be described for a case in which the outside air temperature is low (winter season) using  FIG. 7 . In this description, it will be assumed that, when the outside air temperature is low (winter season), the indoor unit  203  of the air conditioning system  201  operates in cooling mode in order to cool the air inside the server room and the indoor units  204 ,  205  operate in heating mode. 
   During an operating mode in which heating and cooling are mixed in this manner, the refrigerant circuit of the air conditioning system  201  is configured as shown in  FIG. 7  (the flow of the refrigerant is indicated by arrows in the figure). 
   The outdoor unit  202  is configured such that, when the operating load for heating is larger than the operating load for cooling, the outdoor main heat exchanger  212   a  can be made to operate as an evaporator by switching the four-way selector valve  213  to the heating position (broken line in  FIG. 7 ) and the outdoor auxiliary heat exchanger  212   b  can be made to operate as a condenser by opening the outdoor solenoid valve  216  in accordance with the heating operating load. 
   First, except for a portion that is directed to the outdoor auxiliary heat exchanger  212   b , the gaseous refrigerant compressed by the compressor  211  is fed to the indoor units  204 ,  205  through the four-way selector valve  213 , the first gaseous refrigerant shut-off valve  218  and the first gaseous refrigerant pipe  252 . 
   The gaseous refrigerant fed to the indoor units  204 ,  205  is directed through the high-pressure gaseous refrigerant supply valves  245 ,  246  of the heating/cooling changeover devices  207 ,  208  and into the indoor heat exchangers  234 ,  235  of the indoor units  204 ,  205 , where it condenses and heats the air in the respective rooms. Then, the condensed refrigerant passes through the indoor expansion valves  224 ,  225  and the subcooling heat exchangers  241 ,  242  of the heating/cooling changeover devices  207 ,  208  and into the liquid refrigerant pipe  251 . Except for a portion of the refrigerant that is fed into the liquid refrigerant branch pipe  251   b  to facilitate the cooling mode operation of the indoor unit  203 , the condensed refrigerant passes through the liquid refrigerant convergence pipe  251   a  and returns to the outdoor unit  202 . 
   Meanwhile, the portion of the gaseous refrigerant compressed by the compressor  211  that is directed to the outdoor auxiliary heat exchanger  212   b  is condensed. This condensed refrigerant is mixed with the refrigerant returning from the indoor units  204 ,  205  through the liquid refrigerant pipe  251 , reduced in pressure by the outdoor expansion valve  214 , and directed into the outdoor main heat exchanger  212   a , where it is evaporated. Then, the evaporated refrigerant is drawn into the compressor  211  again through the four-way selector valve  213 . In short, the flow rate of the gaseous refrigerant supplied from the outdoor unit  202  to the indoor units  204 ,  205  through the first gaseous refrigerant pipe  252  is adjusted by the condensation of refrigerant performed by the outdoor auxiliary heat exchanger  212   b  and the flow rate adjustment executed by the outdoor expansion valve  214 . 
   The portion of refrigerant condensed in the indoor units  204 ,  205  is directed to the indoor unit  203  through the liquid refrigerant branch pipe  251   b . Then, after the refrigerant is reduced in pressure by the indoor expansion valves  223 , it is evaporated in the indoor heat exchanger  233  and cools the air inside the server room before being fed to the pressure adjusting device  206 . Similarly to the first embodiment, the pressure adjusting device  206  adjusts the refrigerant pressure in the indoor heat exchanger  233  (corresponds to Ps 2  in  FIG. 3 ) so as to achieve an evaporation temperature (corresponds to T 2  in  FIG. 3 ) at which the indoor heat exchanger  233  does not freeze. After having its pressure reduced by the pressure adjusting device  206 , the refrigerant is returned to the intake side of the compressor  211  of the outdoor  202  unit through the second gaseous refrigerant pipe  253 . 
   There are times when the heating load of the indoor units  204 ,  205  is small. In particular, in recent office buildings the amount of heat emitted from computers and OA equipment in rooms other than the server room is large and, consequently, there are times when the heating load is small even in the winter when the outside air temperature is low. In such a situation, the flow rate of gaseous refrigerant returning to the outdoor unit  202  through the liquid refrigerant pipe  251  from the indoor units  204 ,  205  becomes small and the flow rate of gaseous refrigerant returning to the outdoor unit  202  through the second gaseous refrigerant pipe  253  from the indoor unit  203  becomes relatively large. 
   Under such conditions, without the pressure adjusting device  206 , the refrigerant pressure inside the indoor heat exchanger  233  would become too low and the possibility of the indoor heat exchanger  233  freezing would be high. Furthermore, if the system were operated at a refrigerant pressure at which the indoor heat exchanger  233  does not freeze, the influence of the gaseous refrigerant returned to the outdoor unit  202  through the second gaseous refrigerant pipe  253  from the indoor unit  203  would become large and it would be possible for the gaseous refrigerant to liquefy on the intake side of the compressor  211 . Conversely, since the system is provided with a pressure adjusting device  206 , even when the outside air temperature is low, the indoor unit  203  can be run continuously in cooling mode because the gaseous refrigerant in the second the gaseous refrigerant pipe  253  is prevented from liquefying and the indoor heat exchanger  233  is prevented from freezing. 
   As described heretofore, when the present invention is applied to an air conditioning system  201  that is capable of simultaneous heating and cooling, the same effects as the first embodiment can be obtained. Even when the outside air temperature is low, the room (e.g., a server room) having a large thermal load can be cooled continuously while performing simultaneous heating and cooling. 
   [Other Embodiments] 
   Although embodiments of the present invention have been described herein with reference to the drawings, the specific constituent features are not limited to those of these embodiments and variations can be made within a scope that does not deviate from the gist of the invention. 
   (1) Although the previously described embodiments applied the invention to air conditioning systems used for cooling only or for simultaneous heating and cooling, the invention can also be applied to an air conditioning system that switches between cooling and heating modes. 
   (2) The numbers of rooms are not limited to the numbers mentioned in the embodiments. 
   (3) In the first embodiment, the pressure adjusting device is operated even during non-winter seasons such that the refrigerant pressure in the corresponding indoor heat exchanger is higher than the refrigerant pressure in the other indoor heat exchangers. However, it is also acceptable to open the electric powered expansion valve fully during non-winter seasons such that the corresponding indoor heat exchanger is used at the same refrigerant pressure as the other indoor heat exchangers and to operate the pressure adjusting device only during the winter season. 
   (4) In the second embodiment, one of the indoor units making up the simultaneous heating and cooling type air conditioning system is a dedicated cooling unit that is not connected to a heating/cooling changeover device, but the invention is not limited to such an arrangement. For example, the simultaneous heating and cooling type air conditioning system could be configured such that all of the indoor units are connected to a heating/cooling changeover device and the indoor unit used to cool the server room or other room with a high thermal load could have a pressure adjusting device connected in series with the heating/cooling changeover device. 
   APPLICABILITY TO INDUSTRY 
   By using the present invention, the refrigerant pressure in the indoor heat exchanger can be adjusted to a higher pressure than the refrigerant pressure in the gaseous refrigerant pipe between the electric powered expansion valve and the compressor. Therefore, even when the outside air temperature is low, the refrigerant pressure in the gaseous refrigerant pipe downstream of the electric powered expansion valve can be lowered so as to prevent the gaseous refrigerant from liquefying and the refrigerant pressure in the indoor heat exchanger can be adjusted such that the evaporation temperature of the refrigerant is a temperature at which the indoor heat exchanger will not freeze, thus preventing the indoor heat exchanger from freezing. As a result, continuous operation in cooling mode can be accomplished even when the outside air temperature is low.