Abstract:
A refrigeration device includes a compression mechanism, a radiator, a first expansion mechanism ( 15 ), a liquid receiver ( 16 ), a second expansion mechanism, an evaporator, a temperature detector, a first pressure storing unit ( 23   a ), and a second pressure determining unit, a pressure detector, and a control unit ( 23   c ). The first pressure storing unit stores an upper limit and lower limit of an intermediate pressure. The second pressure determining unit determines an upper limit and lower limit of a high pressure based on the upper limit and lower limit of the intermediate pressure and on a temperature in a vicinity of an exit of the radiator. The control unit controls the first expansion mechanism and the second expansion mechanism so that a pressure detected by the pressure detector will be equal to or less than the upper limit and equal to or greater than the lower limit of the high pressure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. National stage application claims priority under 35 U.S.C. §119(a) to Japanese Patent Application No. 2006-246154, filed in Japan on Sep. 11, 2006, the entire contents of which are hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a refrigeration device, and particularly relates to a refrigeration device in which the refrigerant attains a supercritical state during the refrigeration cycle. 
     BACKGROUND ART 
     Conventional refrigeration devices are widely known that are provided with a refrigerant circuit in which a compressor, a radiator, a first expansion valve, a liquid receiver, a second expansion valve, and an evaporator are connected in sequence (see Japanese Laid-open Patent Application No. 10-115470, page 4, fifth column, line 12 through page 5, seventh column, line 39; FIG. 3). 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In cases where carbon dioxide or another supercritical refrigerant is employed as a refrigerant in a refrigerant circuit of such refrigeration devices, making the pressure (“intermediate pressure” hereunder) of the refrigerant that flows from the first expansion valve to the second expansion valve dramatically lower than the saturation pressure causes large quantities of refrigerant gas to be generated, and the refrigerant level in the liquid receiver harder to control. 
     It is an object of the present invention to enable stable control over the refrigerant level in the liquid receiver in a refrigeration device of the above description. 
     Solution to Problem 
     A refrigeration device according to a first aspect of the present invention comprises a compression mechanism, a radiator, a first expansion mechanism, a liquid receiver, a second expansion mechanism, an evaporator, a temperature detector, a first pressure storing unit, a second pressure determining unit, a pressure detector, and a control unit. The compression mechanism compresses a refrigerant. The radiator is connected to a refrigerant discharge side of the compression mechanism. The first expansion mechanism is connected to an exit side of the radiator. The liquid receiver is connected to a refrigerant outflow side of the first expansion mechanism. The second expansion mechanism is connected to an exit side of the liquid receiver. The evaporator is connected to a refrigerant outflow side of the second expansion mechanism and to a refrigerant intake side of the compression mechanism. The temperature detector is disposed between the exit side of the radiator and a refrigerant inflow side of the first expansion mechanism. The first pressure storing unit stores an upper limit and lower limit of the first pressure. The “first pressure” referred to herein is the pressure of the refrigerant that flows from the refrigerant outflow side of the first expansion mechanism to the refrigerant intake side of the second expansion mechanism. The second pressure determining unit determines the upper and lower limits of the second pressure from the upper and lower limits of the first pressure and the temperature detected by the temperature detector. The “second pressure” referred to herein is the pressure of the refrigerant that flows from the refrigerant discharge side of the compression mechanism to the refrigerant intake side of the first expansion mechanism. The pressure detector is provided between the refrigerant discharge side of the compression mechanism and the refrigerant inflow side of the first expansion mechanism. The control unit controls the first expansion mechanism and the second expansion mechanism in such a manner that the pressure detected by the pressure detector will be equal to or less than the upper limit and equal to or higher than the lower limit of the second pressure, and so that the first pressure will be equal to or less than the upper limit and equal to or higher than the lower limit of the first pressure. 
     According to this refrigeration device, the second pressure determining unit determines the upper and lower limits of the second pressure from the upper and lower limits of the first pressure and the temperature detected by the temperature detector. The control unit controls the first expansion mechanism and the second expansion mechanism in such a manner that the pressure detected by the pressure detector will be equal to or less than the upper limit and equal to or higher than the lower limit of the second pressure, and so that the first pressure will be equal to or less than the upper limit and equal to or higher than the lower limit of the first pressure. The first pressure and the second pressure can accordingly both be kept at optimal levels in the refrigeration device. Therefore, in the refrigeration device, the refrigerant level in the liquid receiver can be stably controlled as long as the upper limit and lower limit of the first pressure are set so that the refrigerant flowing out from the first expansion mechanism will attain a state close to the saturation line, but not a state close to the supercritical point. In a case where a supercooling heat exchanger (which may be an internal heat exchanger) is provided between the liquid receiver and the second expansion mechanism, it will be necessary to set the upper and lower limit of the first pressure while also ensuring there is a difference in temperature between the high and low pressures of the supercooling heat exchanger. If such an arrangement is followed, it will be possible to avoid increasing the scale of the supercooling heat exchanger. 
     A refrigeration device according to a second aspect of the present invention is the refrigeration device according to the first aspect of the present invention, further comprising a heat exchanger for cooling a refrigerant. The heat exchanger for cooling a refrigerant is disposed between the exit side of the radiator and a refrigerant inflow side of the first expansion mechanism. The temperature detector is disposed between the exit side of the heat exchanger for cooling a refrigerant and a refrigerant inflow side of the first expansion mechanism. 
     In the refrigeration device, the temperature detector is provided between the exit side of the heat exchanger for cooling a refrigerant and a refrigerant inflow side of the first expansion mechanism. It is thus possible for the control according to the present invention to be performed by the refrigeration device even in a case where a heat exchanger for cooling a refrigerant is provided. 
     Advantageous Effects of Invention 
     According to the refrigeration device of the first aspect of the invention, it is possible to keep the first pressure and second pressure at optimal levels. The refrigeration device thus enables the refrigerant level in the liquid receiver to be stably controlled as long as the upper limit and lower limit of the first pressure are set so that the refrigerant flowing out from the first expansion mechanism will attain a state close to the saturation line, but not a state close to the supercritical point. In a case where a supercooling heat exchanger (which may be an internal heat exchanger) is provided between the liquid receiver and the second expansion mechanism, it will be necessary to set the upper and lower limit of the first pressure while also ensuring there is a difference in temperature between the high and low pressures of the supercooling heat exchanger. If such an arrangement is followed, it will be possible to avoid increasing the scale of the supercooling heat exchanger. 
     According to the refrigeration device of the second aspect of the invention, it is possible to perform the control according to the present invention even in cases where a supercoooling heat exchanger is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the refrigerant circuit of an air conditioning device according to an embodiment of the present invention. 
         FIG. 2  is a block diagram showing functions of a control device provided to an air conditioning device according to an embodiment of the present invention. 
         FIG. 3  is a diagram for describing the control of the level in the liquid receiver by the control device of the air conditioning device according to an embodiment of the present invention. 
         FIG. 4  is a diagram showing the refrigerant circuit of the air conditioning device according to Modification (A). 
         FIG. 5  is a diagram for describing the control of the level in the liquid receiver by the control device of the air conditioning device according to Modification (B). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     &lt;Structure of Air Conditioning Device&gt; 
       FIG. 1  is a schematic view of a refrigerant circuit  2  of an air conditioning device  1  according to an embodiment of the present invention. 
     This air conditioning device  1  is an air conditioning device that is capable of cooling operation and heating operation using carbon dioxide as the refrigerant, and is primarily composed of a refrigerant circuit  2 , blower fans  26 ,  32 , a control device  23 , a high-pressure sensor  21 , a temperature sensor  22 , and other components. 
     The refrigerant circuit  2  is equipped primarily with a compressor  11 , a four-way switch valve  12 , an outdoor heat exchanger  13 , a first electric expansion valve  15 , a liquid receiver  16 , a second electric expansion valve  17 , and an indoor heat exchanger  31 , and the devices are connected via a refrigerant pipe, as shown in  FIG. 1 . 
     In the present embodiment, the air conditioning device  1  is a separate-type air conditioning device, and can also be described as comprising an indoor unit  30  primarily having the indoor heat exchanger  31  and an indoor fan  32 ; an outdoor unit  10  primarily having the compressor  11 , the four-way switch valve  12 , the outdoor heat exchanger  13 , the first electric expansion valve  15 , the liquid receiver  16 , the second electric expansion valve  17 , the high-pressure sensor  21 , the temperature sensor  22 , and the control device  23 ; a first connecting pipe  41  for connecting the pipe for refrigerant fluid and the like of the indoor unit  30  and the pipe for refrigerant fluid and the like of the outdoor unit  10 ; and a second connecting pipe  42  for connecting the pipe for refrigerant gas and the like of the indoor unit  30  and the pipe for refrigerant gas and the like of the outdoor unit  10 . The first connecting pipe  41  and the pipe for refrigerant fluid and the like of the outdoor unit  10  are connected via a first close valve  18  of the outdoor unit  10 , and the second connecting pipe  42  and the pipe for refrigerant gas and the like of the outdoor unit  10  are connected via a second close valve  19  of the outdoor unit  10 . 
     (1) Indoor Unit 
     The indoor unit  30  primarily has the indoor heat exchanger  31 , the indoor fan  32 , and other components. 
     The indoor heat exchanger  31  is a heat exchanger for exchanging heat between the refrigerant and the indoor air, which is the air inside the room to be air-conditioned. 
     The indoor fan  32  is a fan for taking the air inside the air-conditioned room into the unit  30  and blowing conditioned air, which is the air after heat exchange with the refrigerant via the indoor heat exchanger  31 , back into the air-conditioned room. 
     Employing such a configuration makes it possible for the indoor unit  30  to cause heat to be exchanged between the indoor air taken in by the indoor fan  32  and the liquid refrigerant that flows through the indoor heat exchanger  31 , and generate conditioned air (cool air) during cooling operation, as well as to cause heat to be exchanged between the indoor air taken in by the indoor fan  32  and supercritical refrigerant that flows through the indoor heat exchanger  31 , and generate conditioned air (warm air) during heating operation. 
     (2) Outdoor Unit 
     The outdoor unit  10  primarily has the compressor  11 , the four-way switch valve  12 , the outdoor heat exchanger  13 , the first electric expansion valve  15 , the liquid receiver  16 , the second electric expansion valve  17 , an outdoor fan  26 , the control device  23 , the high-pressure sensor  21 , the temperature sensor  22 , and other components. 
     The compressor  11  is a device for sucking in low-pressure refrigerant gas flowing through an intake pipe and compressing the refrigerant gas to a supercritical state, and then discharging the refrigerant to a discharge pipe. 
     The four-way switch valve  12  is a valve for switching the flow direction of the refrigerant in accordance with each operation mode, and is capable of connecting the discharge side of the compressor  11  and the high-temperature side of the outdoor heat exchanger  13 , and connecting the intake side of the compressor  11  and the gas side of the indoor heat exchanger  31  during cooling operation; as well as connecting the discharge side of the compressor  11  and the second close valve  19 , and connecting the intake side of the compressor  11  and the gas side of the outdoor heat exchanger  13  during heating operation. 
     The outdoor heat exchanger  13  is capable of cooling the high-pressure supercritical refrigerant discharged from the compressor  11  using the air outside the air-conditioned room as a heat source during cooling operation, and evaporating the liquid refrigerant returning from the indoor heat exchanger  31  during heating operation. 
     The first electric expansion valve  15  reduces the pressure of the supercritical refrigerant (during cooling operation) that flows out from the low-temperature side of the outdoor heat exchanger  13 , or the liquid refrigerant (during heating operation) that flows in through the liquid receiver  16 . 
     The liquid receiver  16  stores refrigerant that occurs as excess depending on the operating mode or the air conditioning load. 
     The second electric expansion valve  17  reduces the pressure of the liquid refrigerant (during cooling operation) that flows in through the liquid receiver  16 , or the supercritical refrigerant (during heating operation) that flows out from the low-temperature side of the indoor heat exchanger  31 . 
     The outdoor fan  26  is a fan for taking the outdoor air into the unit  10  and discharging the air after heat exchange with the refrigerant via the outdoor heat exchanger  13 . 
     The high-pressure sensor  21  is provided to the discharge side of the compressor  11 . 
     The temperature sensor  22  is provided in proximity to an entry of the first electric expansion valve  15 . 
     The control device  23  has a communication connection with the high-pressure sensor  21 , the temperature sensor  22 , the first electric expansion valve  15 , the second electric expansion valve  17 , and other components, and controls the degree of opening of the first electric expansion valve  15  and the second electric expansion valve  17  on the basis of temperature information transmitted from the temperature sensor  22 , and high-pressure information transmitted from the high-pressure sensor  21 . The control device  23  is primarily composed of a storing unit  23   a , a computing unit  23   b , and a control unit  23   c , as shown in  FIG. 2 . The storing unit  23   a  stores information of upper limit UL 1  and lower limit LL 1  of the pressure of the refrigerant (“intermediate-pressure refrigerant” hereunder) that flows between the refrigerant outflow side of the first electrical expansion valve  15  and the refrigerant inflow side of the second electric expansion valve  17 . The upper limit UL 1  and lower limit LL 1  are set so that the refrigerant flowing out from the first electric expansion valve  15  will attain a state close to the saturation line, but not a state close to the supercritical point (see  FIG. 3 ). The computing unit  23   b  uses the information of the upper limit UL 1  and lower limit LL 1  of the pressure of the intermediate pressure refrigerant sent by the storing unit  23   a , as well as temperature information transmitted by the temperature sensor  22 , to compute an upper limit UL 2  and a lower limit LL 2  of the pressure of the refrigerant (“high-pressure-side refrigerant” hereunder) that flows between the refrigerant discharge side of the compressor  11  and the refrigerant inflow side of the first electric expansion valve  15 , as shown in  FIG. 3 . The upper limit UL 2  and lower limit LL 2  of pressure of the high-pressure-side refrigerant are set by determining the points at which each of the upper limit UL 1  and lower limit LL 1  of the pressure of the intermediate-pressure refrigerant intersect the saturation line on the low enthalpy side relative to the supercritical point K, extending hypothetical lines along the vertical axis from the points of intersection, and determining the points at which these hypothetical lines intersect the isothermal line Tm corresponding to the temperature information at that time, as is also shown in  FIG. 3 . Such computations can be readily performed by one skilled in the art using techniques for expressing functions and techniques for creating control tables. The control unit  23   c  controls the degree of opening of the first electric expansion valve  15  and the second electric expansion valve  17  so that the value shown by the high-pressure sensor  21  will fall between the upper limit UL 2  and lower limit LL 2  of the pressure of the high-pressure-side refrigerant as determined above, and so that the pressure of the intermediate pressure refrigerant will fall between the upper limit UL 1  and lower limit LL 1  of the pressure of the intermediate-pressure refrigerant. The pressure of the high-pressure-side refrigerant at this time is exclusively controlled by the first electric expansion valve  15 . The pressure of the intermediate-pressure refrigerant is controlled by the balance between the degree of opening of the first electric expansion valve  15  and the degree of opening of the second electric expansion valve  17 . The degree of opening of the second electric expansion valve  17  at this time can be readily determined provided that the degree of opening of the second electric expansion valve  17  is expressed in advance as a function using, e.g., the pressure of the intermediate-pressure refrigerant and the degree of opening of the first electric expansion valve  15  as variables. The average of the upper limit UL 1  and lower limit LL 1  or another value may be used as the pressure value of the intermediate-pressure refrigerant used at this time. 
     &lt;Operation of the Air Conditioning Device&gt; 
     The operation of the air conditioning device  1  will be described using  FIG. 1 . This air conditioning device  1  is capable of cooling operation and heating operation, as described above. 
     (1) Cooling Operation 
     During cooling operation, the four-way switch valve  12  is in the state indicated by the solid line in  FIG. 1 , i.e., a state in which the discharge side of the compressor  11  is connected to the high-temperature side of the outdoor heat exchanger  13 , and the intake side of the compressor  11  is connected to the second close valve  19 . The first close valve  18  and the second close valve  19  are also open at this time. 
     When the compressor  11  is activated in this state of the refrigerant circuit  2 , the refrigerant gas is sucked into the compressor  11  and compressed to a supercritical state, and then sent through the four-way switch valve  12  to the outdoor heat exchanger  13  and cooled in the outdoor heat exchanger  13 . 
     This cooled supercritical refrigerant is sent to the first electric expansion valve  15 . The supercritical refrigerant sent to the first electric expansion valve  15  is depressurized to a saturated state, and then sent to the second electric expansion valve  17  via the liquid receiver  16 . The refrigerant in a saturated state sent to the second electric expansion valve  17  is depressurized to liquid refrigerant, and then fed to the indoor heat exchanger  31  via the first close valve  18 , where the refrigerant cools the indoor air and evaporates into refrigerant gas. 
     The refrigerant gas is again sucked into the compressor  11  via the second close valve  19 , and the four-way switch valve  12 . Cooling operation is performed in this manner. 
     (2) Heating Operation 
     During heating operation, the four-way switch valve  12  is in the state indicated by the dashed line in  FIG. 1 , i.e., a state in which the discharge side of the compressor  11  is connected to the second close valve  19 , and the intake side of the compressor  11  is connected to the gas side of the outdoor heat exchanger  13 . The first close valve  18  and the second close valve  19  are also open at this time. 
     When the compressor  11  is activated in this state of the refrigerant circuit  2 , the refrigerant gas is sucked into the compressor  11  and compressed to a supercritical state, and then is fed to the indoor heat exchanger  31  via the four-way switch valve  12  and the second close valve  19 . 
     The supercritical refrigerant heats the indoor air, and is cooled in the indoor heat exchanger  31 . The cooled supercritical refrigerant is sent through the first close valve to the second electric expansion valve  17 . The supercritical refrigerant sent to the second electric expansion valve  17  is depressurized to a saturated state, and then sent to the first electric expansion valve  15  via the liquid receiver  16 . The refrigerant in a saturated state sent to the first electric expansion valve  15  is depressurized to liquid refrigerant, and then sent to the outdoor heat exchanger  13  and evaporated to refrigerant gas in the outdoor heat exchanger  13 . This refrigerant gas is again sucked into the compressor  11  via the four-way switch valve  12 . Heating operation is performed in this manner. 
     &lt;Characteristics of the Air Conditioning Device&gt; 
     According to the air conditioning device  1  of the present embodiment, the information of upper limit UL 1  and lower limit LL 1  to the effect that the intermediate-pressure refrigerant will attain a state close to the saturation line, but not a state close to the supercritical point, are stored in the storing unit  23   a ; and the computing unit  23   b  uses the information of the upper limit UL 1  and the lower limit LL 1 , as well as temperature information transmitted by the temperature sensor  22 , to compute upper limit UL 2  and lower limit LL 2  of the pressure of the high-pressure-side refrigerant. The control unit  23   c  controls the degree of opening of the first electric expansion valve  15  and the second electric expansion valve  17  so that the value shown by the high-pressure sensor  21  will fall between the upper limit UL 2  and lower limit LL 2  of the pressure of the high-pressure-side refrigerant as determined above, and so that the pressure of the intermediate-pressure refrigerant will fall between the upper limit UL 1  and lower limit LL 1  of the pressure of the intermediate-pressure refrigerant. In the air conditioning device  1 , the pressure of the intermediate-pressure refrigerant and the pressure of the high-pressure-side refrigerant can accordingly be held at optimal levels. Therefore, in the air conditioning device  1 , the refrigerant level in the liquid receiver  16  can be stably controlled. 
     &lt;Modifications&gt; 
     (A) 
     In the embodiment described above, the invention of the present application is applied to a separate-type air conditioning device  1  in which one indoor unit  30  is provided for one outdoor unit  10 , but the invention of the present application may also be applied to a multi-type air conditioning device  101  in which a plurality of indoor units is provided for one outdoor unit, such as shown in  FIG. 4 . In  FIG. 4 , the same reference numerals are used to refer to components that are the same as those of the air conditioning device  1  according to the embodiment described above. In  FIG. 4 , the reference numeral  102  refers to a refrigerant circuit,  110  refers to an outdoor unit,  130   a  and  130   b  refer to indoor units,  31   a  and  31   b  refer to indoor heat exchangers,  32   a  and  32   b  refer to indoor fans,  33   a  and  33   b  refer to second electric expansion valves,  34   a  and  34   b  refer to indoor control devices, and  141  and  142  refer to connecting pipes. In this case, the control device  23  controls the second electric expansion valves  33   a ,  33   b  via the indoor control devices  34   a ,  34   b . The second electric expansion valves  33   a ,  33   b  are housed in the indoor units  130   a ,  130   b  in the present modification, but the second electric expansion valves  33   a ,  33   b  may alternatively be housed in the outdoor unit  110 . 
     (B) 
     In the air conditioning device  1  according to the embodiment described above, although not particularly mentioned in the above description, a supercooling heat exchanger (which may be an internal heat exchanger) may be provided between the liquid receiver  16  and the second electric expansion valve  17 . In such cases, it will be necessary to set the upper limit UL 1  and lower limit LL 1  of the pressure of the intermediate-pressure refrigerant while also ensuring there is a difference in temperature between the high and low pressures of the supercooling heat exchanger. If such an arrangement is followed, it will be possible to avoid increasing the scale of the supercooling heat exchanger. The refrigeration cycle in such circumstances will be as shown in  FIG. 5 . 
     (C) 
     In the air conditioning device  1  according to the embodiment described above, the first electric expansion valve  15 , the liquid receiver  16 , the second electric expansion valve  17 , and other components are disposed in the outdoor unit  10 , but the positioning of these components is not particularly limited. For example, the second electric expansion valve  17  may be disposed in the indoor unit  30 . 
     (D) 
     An electric expansion valve is used as the means for reducing the pressure of the refrigerant in the air conditioning device  1  according to the embodiment described above, but an expansion device or the like may instead be used. 
     (E) 
     Although not particularly mentioned in the air conditioning device  1  according to the embodiment described above, the liquid receiver  16  and the intake pipe of the compressor  11  may be connected to form a gas release circuit. In this case, an electric expansion valve, an electromagnetic valve, or the like is preferably provided to the gas release circuit. 
     (F) 
     Although not particularly mentioned in the air conditioning device  1  of the embodiment described above, it is possible to provide an intermediate-pressure sensor at any position between the refrigerant outflow side of the first electric expansion valve  15  and the refrigerant inflow side of the second electric expansion valve  17 . In such cases, the control unit  23   c  controls the degree of openings of the first electric expansion valve  15  and the second electric expansion valve  17  so that the value shown by the high-pressure sensor  21  will fall between the upper limit UL 2  and lower limit LL 2  of the pressure of the high-pressure-side refrigerant as determined above, and so that the value shown by the intermediate-pressure sensor will fall between the upper limit UL 1  and lower limit LL 1  of the pressure of the intermediate-pressure refrigerant. 
     (G) 
     Although not specifically mentioned in the air conditioning device  1  of the previous embodiment, it is possible to provide a heat exchanger for cooling a refrigerant (which may be an internal heat exchanger) between the low-temperature side (or liquid side) of the outdoor heat exchanger  13  and the temperature sensor  22 . In such cases, it will be possible to prevent the refrigerant flowing out of the first electric expansion valve  15  from assuming a state close to the supercritical point. According to the air conditioning device  1 , therefore, the level in the liquid receiver can be stably controlled. 
     INDUSTRIAL APPLICABILITY 
     The refrigeration device of the present invention has the characteristic of enabling the refrigerant level in the liquid receiver to be stably controlled, and the present invention is particularly useful in a refrigeration device in which carbon dioxide or the like is used as the refrigerant.