Abstract:
In a refrigerating apparatus using a refrigerant so that the refrigerant discharged from a compressor becomes a supercritical state, a refrigerating ability runs short. Therefore, to rapidly perform cooling, the amount of the refrigerant to be filled has to be increased. On the other hand, another problem occurs that a large amount of excessive refrigerant is generated in a refrigerant circuit when where the refrigerating apparatus is sufficiently cooled. In the present invention, a refrigerating circuit in which a compressor, a gas cooler, a first pressure reducing unit and an evaporator are successively annularly connected to one another via pipes includes a second pressure reducing unit and a liquid receiver between the gas cooler and the first pressure reducing unit, and the liquid receiver is connected to the suction port of the compressor via a pipe. Then, the opening/closing degree of the second pressure reducing unit is controlled in accordance with a pressure difference between the discharge-side pressure of the compressor and the suction-side pressure thereof, whereby the amount of the refrigerant to be circulated is increased when the refrigerating ability runs short, and the excessive refrigerant is received in the liquid receiver when the refrigerating ability becomes excessive, so that the amount of the refrigerant to be circulated can be adjusted.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a refrigerating apparatus which includes a refrigerant circuit constituted by connecting a compressor, a gas cooler, a pressure reducing unit, an evaporator and the like via pipes and in which a natural refrigerant such as carbon dioxide (CO 2 ) is used with a supercritical pressure as the discharge-side pressure of the compressor. 
         [0003]    2. Description of the Related Art 
         [0004]    Heretofore, a chlorofluorocarbon-based refrigerant has been used in a refrigerating apparatus, but chlorofluorocarbon has a problem such as ozone layer destruction or global warming. Therefore, the use of chlorofluorocarbon has started to be strictly regulated, and the development of a refrigerating apparatus has been advanced in which a natural refrigerant such as CO 2  or hydrocarbon is used as a substitute refrigerant. 
         [0005]    In particular, CO 2  is the natural refrigerant having a small global warming coefficient, and is incombustible and nontoxic unlike hydrocarbon having inflammability or ammonia having toxicity. Therefore, CO 2  is expected as the next refrigerant that is eco-friendly and highly safe. 
         [0006]    However, CO 2  has a critical point of 31.1° C., 7.38 MPa, and hence a very high pressure is required for performing heat exchange accompanied by phase change such as evaporation or condensation in the refrigerating apparatus. Therefore, CO 2  compressed in the refrigerating apparatus is brought into a high-temperature high-pressure supercritical state and discharged from a compressor. 
         [0007]    It is known that a method of performing inner heat exchange by use of a cascade heat exchanger (an inner heat exchanger) as shown in  FIG. 1  is effective in a case where the refrigerant having the above-mentioned characteristics is used in the refrigerating apparatus (see Japanese Patent Application Laid-Open No. 2004-270517). In  FIG. 1 , CO 2  is used as the refrigerant, reference numeral  11  is a two-stage compressor,  12  is a gas cooler,  13  is a cascade heat exchanger,  23  is an expansion valve (a pressure reducing unit) and  15  is an evaporator. 
         [0008]    A low-pressure gas refrigerant sucked by the compressor  11  is compressed into a high-temperature high-pressure state by the two-stage compressor  11 , and discharged in a supercritical state. The refrigerant discharged in the supercritical state is cooled in the gas cooler  12 , and then flows into a high-pressure-side circuit  13 - a  of the cascade heat exchanger  13 . 
         [0009]    The refrigerant passed through the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13  has the pressure reduced by the expansion valve  23 , and the refrigerant in the evaporator  15  cools the evaporator  15  and the periphery of the evaporator. The refrigerant passed through the evaporator  15  has a low temperature and low pressure to flow into the low-pressure-side circuit  13 - b  of the cascade heat exchanger  13 . 
         [0010]    Here, the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13  usually has a temperature higher than that of the low-pressure-side circuit  13 - b , so that the heat exchange between both the circuits is performed. Therefore, the refrigerant cooled by the gas cooler  12  passes through the high-pressure-side circuit  13 - a , and is further cooled, whereby a refrigerating ability in the evaporator  15  improves. 
         [0011]    Then, the refrigerant passed through the low-pressure-side circuit  13 - b  of the cascade heat exchanger  13  is again sucked by the two-stage compressor  11 , thereby forming a refrigerant circuit. 
         [0012]    However, the refrigerant discharged from the two-stage compressor  11  has very high temperature and pressure. Therefore, when the gas cooler  12 , the evaporator  15  and the like have a high temperature, the refrigerant passes through the gas cooler  12  and the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13 . Even after the cooling is performed, the refrigerant sometimes has a gas state. 
         [0013]    The amount of heat absorbed in the evaporator  15  by the refrigerant having the gas state and having the pressure reduced by the expansion valve  23  is smaller than that of heat absorbed in the evaporator  15  by a liquid refrigerant having the pressure reduced by the expansion valve  23 . Therefore, to effectively perform cooling in the evaporator  15 , the low-temperature liquid refrigerant is preferable. 
         [0014]    In a case where the refrigerant having the supercritical state when discharged from the compressor is used as a refrigerant, the amount of the refrigerant with which the refrigerating apparatus is to be filled has to be increased to rapidly perform the cooling. However, there occurs a problem that a large amount of liquefied excessive refrigerant is generated in the refrigerating apparatus in a case where the refrigerating apparatus is sufficiently cooled. 
       SUMMARY OF THE INVENTION 
       [0015]    A refrigerating apparatus according to a first aspect of the invention is characterized by a refrigerating apparatus in which a compressor, a gas cooler, a first pressure reducing unit and an evaporator are connected to one another via pipes and in which a natural refrigerant is used as a refrigerant, the apparatus comprising: a second pressure reducing unit and a liquid receiver between the gas cooler and the first pressure reducing unit, wherein the liquid receiver is connected to the suction port of the compressor via a pipe. 
         [0016]    A refrigerating apparatus according to a second aspect of the invention is characterized by a refrigerating apparatus in which a compressor, a gas cooler, a first pressure reducing unit and an evaporator are connected to one another via pipes and in which a natural refrigerant is used as a refrigerant, the apparatus comprising: a second pressure reducing unit and a liquid receiver between the gas cooler and the first pressure reducing unit, wherein the liquid receiver is connected to the intermediate pressure portion of the compressor via a pipe. 
         [0017]    A refrigerating apparatus according to a third aspect of the invention is characterized in that the refrigerating apparatus according to the first or second aspect of the invention further comprises: an inner heat exchanger between the gas cooler and the second pressure reducing unit, wherein the outlet of the evaporator is directly connected to the suction port of the compressor via a pipe in parallel with a separate pipe which connects the outlet of the evaporator to the suction port of the compressor via an opening/closing valve and the inner heat exchanger. 
         [0018]    A refrigerating apparatus according to a fourth aspect of the invention is characterized in that in the refrigerating apparatus according to any one of the first to third aspects of the invention, an intermediate portion between the heat exchanger and the second pressure reducing unit is connected to an intermediate portion between the liquid receiver and the first pressure reducing unit via the opening/closing valve and a pipe. 
         [0019]    A refrigerating apparatus according to a fifth aspect of the invention is characterized in that in the refrigerating apparatus according to any one of the first to fourth aspects of the invention, the opening/closing degree of the second pressure reducing unit is controlled in accordance with the suction-side pressure of the compressor. 
         [0020]    A refrigerating apparatus according to a sixth aspect of the invention is characterized in that in the refrigerating apparatus according to any one of the first to fourth aspects of the invention, the opening/closing degree of the second pressure reducing unit is controlled in accordance with a pressure difference between the discharge-side pressure of the compressor and the suction-side pressure thereof. 
         [0021]    According to the first aspect of the invention, the refrigerating apparatus in which the compressor, the gas cooler, the first pressure reducing unit and the evaporator are connected to one another via the pipes and in which the natural refrigerant is used as the refrigerant comprises the second pressure reducing unit and the liquid receiver between the gas cooler and the first pressure reducing unit. The liquid receiver is connected to the suction port of the compressor via the pipe. In consequence, the pressure of the refrigerant cooled in the gas cooler is reduced by the second pressure reducing unit to expand the refrigerant, whereby the refrigerant is further cooled, and the liquefied refrigerant can be received in the liquid receiver. Therefore, the liquid refrigerant can be supplied to the evaporator. Furthermore, the gas refrigerant in the liquid receiver can efficiently be sucked from the suction port of the compressor, so that a pressure reducing effect produced by the second pressure reducing unit can be improved. Therefore, in the refrigerating apparatus in which the liquid refrigerant is efficiently received in the liquid receiver and in which the natural refrigerant is used, a high refrigerating ability can be obtained. 
         [0022]    In the second aspect of the invention, the refrigerating apparatus in which the compressor, the gas cooler, the first pressure reducing unit and the evaporator are connected to one another via the pipes and in which the natural refrigerant is used as the refrigerant comprises the second pressure reducing unit and the liquid receiver between the gas cooler and the first pressure reducing unit, wherein the liquid receiver is connected to the intermediate pressure portion of the compressor via the pipe. In consequence, the pressure of the refrigerant cooled in the gas cooler is reduced by the second pressure reducing unit to expand the refrigerant, whereby the refrigerant is further cooled, and the liquefied refrigerant can be received in the liquid receiver. Therefore, the liquid refrigerant can be supplied to the evaporator. Furthermore, the gas refrigerant in the liquid receiver can be sucked by the intermediate pressure portion of the compressor, so that the pressure reducing effect produced by the second pressure reducing unit can be improved. Therefore, in the refrigerating apparatus in which the liquid refrigerant is efficiently received in the liquid receiver and in which the natural refrigerant is used, the high refrigerating ability can be obtained. 
         [0023]    Moreover, in the third aspect of the invention, the refrigerating apparatus further comprises: the inner heat exchanger between the gas cooler and the second pressure reducing unit, and the outlet of the evaporator is directly connected to the suction port of the compressor via the pipe in parallel with the separate pipe which connects the outlet of the evaporator to the suction port of the compressor via the opening/closing valve and the inner heat exchanger. In consequence, when the refrigerating apparatus has a sufficient refrigerating ability, the refrigerant discharged from the gas cooler can be supercooled by the low-temperature low-pressure refrigerant discharged from the evaporator. Furthermore, the refrigerating ability in the evaporator is sufficiently secured, whereby a temperature difference between the high-temperature refrigerant and the low-temperature refrigerant can be increased in the inner heat exchanger. Therefore, a heat exchange efficiency can be improved. 
         [0024]    Furthermore, in the fourth aspect of the invention, the intermediate portion between the heat exchanger and the second pressure reducing unit is connected to the intermediate portion between the liquid receiver and the first pressure reducing unit via the opening/closing valve and the pipe, whereby the refrigerant can be supplied to the first pressure reducing unit without circulating the refrigerant through the second pressure reducing unit and the liquid receiver. In consequence, when the refrigerant is sufficiently condensed in the gas cooler and the inner heat exchanger, the refrigerant is not expanded in the second pressure reducing unit and the liquid receiver, and the condensed refrigerant is directly fed into the evaporator, whereby the refrigerating efficiency of the refrigerating apparatus can be improved. 
         [0025]    In addition, according to the fifth aspect of the invention, the opening/closing degree of the second pressure reducing unit is controlled in accordance with the suction-side pressure of the compressor, whereby the amount of the refrigerant to be received in the liquid receiver and the flow rate into the compressor can be controlled. Therefore, when the refrigerant gathers on the high pressure side of the compressor, the rise of the pressure can be prevented. 
         [0026]    Moreover, in the sixth aspect of the invention, the opening/closing degree of the second pressure reducing unit is controlled in accordance with the pressure difference between the discharge-side pressure of the compressor and the suction-side pressure thereof, whereby the amount of the refrigerant to be received in the liquid receiver and the flow rate into the compressor can be controlled. Therefore, when the refrigerant gathers on the high pressure side of the compressor, the rise of the pressure can be prevented. It is to be noted that the second pressure reducing unit is controlled so as to obtain a constant difference between the pressures before and after the compressor. Therefore, a substantially constant difference between the pressures before and after the first expansion valve is obtained, and the operation of the first pressure reducing unit can be stabilized. In consequence, the refrigerating ability of the refrigerating apparatus can be stabilized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  shows a refrigerant circuit in a conventional trans-critical refrigerating apparatus; 
           [0028]      FIG. 2  shows a refrigerant circuit according to one embodiment in a trans-critical refrigerating apparatus of the present invention; 
           [0029]      FIG. 3  shows the refrigerant circuit according to the embodiment of the present invention in a case where a refrigerating ability runs short; 
           [0030]      FIG. 4  shows the refrigerant circuit according to the embodiment of the present invention in a case where the refrigerating ability is sufficient; 
           [0031]      FIG. 5  shows the refrigerant circuit according to the embodiment of the present invention in a case where the refrigerating ability is excessive; 
           [0032]      FIG. 6  shows the refrigerant circuit according to the embodiment in the trans-critical refrigerating apparatus of the present invention in which a three-way valve is used; 
           [0033]      FIG. 7  shows a refrigerant circuit according to another embodiment in the trans-critical refrigerating apparatus of the present invention; 
           [0034]      FIG. 8  shows the refrigerant circuit according to the embodiment of the present invention in a case where a refrigerating ability runs short; 
           [0035]      FIG. 9  shows the refrigerant circuit according to the embodiment of the present invention in a case where the refrigerating ability is sufficient; 
           [0036]      FIG. 10  shows the refrigerant circuit according to the embodiment of the present invention in a case where the refrigerating ability is excessive; and 
           [0037]      FIG. 11  shows the refrigerant circuit according to the embodiment in the trans-critical refrigerating apparatus of the present invention in which a three-way valve is used. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    Next, embodiments of the present invention will be described in detail with reference to the drawings. 
       Embodiment 1 
     (1) Refrigerating Apparatus to which the Present Invention is Applied 
       [0039]      FIG. 2  shows a refrigerant circuit  1  of a refrigerating apparatus according to one embodiment to which the present invention is applied. In the drawing, reference numeral  11  is a compressor,  12  is a gas cooler,  13  is a cascade heat exchanger (an inner heat exchanger),  14  is a liquid receiver,  15  is an evaporator,  21  is a second expansion valve (a pressure reducing unit),  22 ,  24 ,  25  and  26  are electromagnetic valves (opening/closing valves), and  23  is a first expansion valve. 
         [0040]    It is to be noted that the compressor  11  is a multistage compressor of a single stage or two or more stages. A refrigerant has a sub-critical state on the low pressure side of this compressor  11 , and the discharged refrigerant has a supercritical state, so that the whole refrigerating apparatus has a trans-critical state. As one example of the refrigerant having such properties, carbon dioxide is used in the present embodiment. 
         [0041]    The supercritical refrigerant discharged from the compressor  11  flows into the gas cooler  12 , and is air-cooled by a blower fan  12 - a.    
         [0042]    The refrigerant discharged from the gas cooler  12  passes through a high-pressure-side circuit  13 - a  of the cascade heat exchanger  13 , and reaches the expansion valve  21  in a case where the electromagnetic valve  22  closes. The pressure of the refrigerant is reduced by the expansion valve  21  to expand and cool the refrigerant. The cooled and thus liquefied refrigerant is received in the liquid receiver  14 . When the electromagnetic valve  26  opens, the vaporized refrigerant is sucked into the suction port of the compressor  11  via a bypass circuit. 
         [0043]    The liquid refrigerant received in the liquid receiver  14  has the pressure reduced by the expansion valve  23 , flows into the evaporator  15 , and expands. In the present refrigerating apparatus, owing to two-stage expansion including the expansion performed by the expansion valve  21  and the expansion by the expansion valve  23 , a refrigerating ability is improved. 
         [0044]    On the other hand, when the electromagnetic valve  22  opens, the refrigerant discharged from the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13  reaches the expansion valve  23  via the electromagnetic valve  22 , and the refrigerant has the pressure reduced by the expansion valve  23  to flow into the evaporator  15 . 
         [0045]    The refrigerant which has flowed into the evaporator  15  evaporates to absorb heat, and outside air circulated by a blower fan  15 - a  is cooled. When the electromagnetic valve  24  closes and the electromagnetic valve  25  opens, the low-temperature low-pressure refrigerant discharged from the evaporator  15  is sucked from the low pressure side of the compressor  11 . 
         [0046]    On the other hand, when the electromagnetic valve  24  opens and the electromagnetic valve  25  closes, the low-temperature low-pressure refrigerant discharged from the evaporator  15  is sucked from the low pressure side of the compressor  11  via a low-pressure-side circuit  13 - b  of the cascade heat exchanger  13 . 
       (2) In a Case where the Refrigerating Ability of the Refrigerating Apparatus Runs Short 
       [0047]    In a case where the refrigerating ability of the refrigerating apparatus runs short, the refrigerant circuit  1  has a constitution shown in  FIG. 3  in which the electromagnetic valves  22  and  24  close and the electromagnetic valves  25  and  26  open. The refrigerant discharged from the compressor  11  and cooled by the gas cooler  12  reaches the expansion valve  21  via the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13 . 
         [0048]    When the refrigerating ability runs short, the refrigerant discharged from the compressor  11  has a very high temperature. Therefore, when the refrigerant is not sufficiently cooled by the gas cooler  12 , the refrigerant discharged from the gas cooler  12  is supposed to have a supercritical or trans-critical state. 
         [0049]    It is difficult to perform the sufficient cooling with the supercritical refrigerant in the evaporator  15 . Therefore, this refrigerant has the pressure reduced by the expansion valve  21 , and is thus cooled, and a mixed state of a liquid and a gas is brought in the liquid receiver. In consequence, the liquid refrigerant is received in the lower part of the liquid receiver  14 , and the gas refrigerant is received in the upper part of the liquid receiver. 
         [0050]    However, when the liquid receiver  14  is filled with the gas refrigerant and the inner pressure of the liquid receiver  14  rises, the evaporation of the refrigerant is limited, so that the cooling effect due to the pressure reduction of the expansion valve  21  lowers. 
         [0051]    In the present invention, the upper part of the liquid receiver  14  is connected to the suction port of the compressor  11  via the electromagnetic valve  26 , whereby the gas refrigerant with which the liquid receiver  14  has been filled is sucked by the compressor  11 , and the inner pressure of the liquid receiver  14  is reduced. Therefore, the refrigerant can sufficiently be expanded in the liquid receiver  14 , so that the refrigerant can efficiently be cooled and liquefied. 
         [0052]    Moreover, the refrigerant directly flows into the low pressure portion of the compressor  11  from the evaporator  15 , and is directly sucked by the compressor  11  from the liquid receiver  14 , so that the amount of the refrigerant to be circulated increases and the refrigerating ability further improves. 
       (3) In a Case where the Refrigerating Ability of the Refrigerating Apparatus is Sufficient 
       [0053]    In a case where the refrigerating ability of the refrigerating apparatus is sufficient, the refrigerant circuit  1  has a constitution shown in  FIG. 4  in which the electromagnetic valves  22  and  24  open, and the expansion valve  21  and the electromagnetic valves  25  and  26  close. The refrigerant discharged from the compressor  11  and cooled by the gas cooler  12  reaches the expansion valve  23  via the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13 . 
         [0054]    When the refrigerating ability is sufficient, the refrigerant cooled and liquefied in the gas cooler  12  flows into the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13 . Moreover, the refrigerant discharged from the evaporator  15  in a state in which the refrigerating ability is sufficient has a low temperature and low pressure, so that the refrigerant of the high-pressure-side circuit  13 - a  is supercooled by the refrigerant of the low-pressure-side circuit  13 - b  in the cascade heat exchanger  13 . 
         [0055]    The supercooled refrigerant has the pressure reduced by the expansion valve  23  via the electromagnetic valve  22 , and flows into the evaporator  15 . In the evaporator  15 , the liquid refrigerant absorbs heat while evaporating, whereby the outside air circulated by the blower fan  15 - a  is cooled. 
         [0056]    The gas refrigerant brought to the low temperature and low pressure flows into the low-pressure-side circuit  13 - b  of the cascade heat exchanger  13  via the electromagnetic valve  24  to cool the refrigerant flowing through the high-pressure-side circuit  13 - a . The refrigerant discharged from the low-pressure-side circuit  13 - b  is sucked on the low pressure side of the compressor  11 , thereby constituting the refrigerating apparatus. 
       (4) In a Case where the Refrigerating Ability of the Refrigerating Apparatus is Excessive 
       [0057]    In a case where the refrigerating ability of the refrigerating apparatus becomes sufficient and the refrigerant becomes excessive on the high pressure side of the compressor, the refrigerant circuit  1  has a constitution shown in  FIG. 5  in which the electromagnetic valves  22 ,  24  and  26  open, and the electromagnetic valve  25  closes. The refrigerant discharged from the compressor  11  and cooled by the gas cooler  12  reaches the expansion valve  23  via the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13 . 
         [0058]    When the refrigerating ability becomes sufficient, the expansion valve  23  is substantially closed, so that the low-pressure-side pressure of the compressor  11  decreases. When this state continues for a long time, the refrigerant gathers on the high pressure side of the compressor  11 , and hence the high-pressure-side pressure of the compressor  11  rises. 
         [0059]    Carbon dioxide for use as the refrigerant in the present embodiment has a very high pressure in a trans-critical state. Therefore, when the pressure rises on the high pressure side of the compressor  11 , the safety of the refrigerating apparatus is impaired, and weight increase is caused owing to the rise of the durable pressure of the elements constituting the refrigerating apparatus. 
         [0060]    Moreover, when a difference between the high-pressure-side pressure of the compressor  11  and the low-pressure-side pressure thereof increases, a difference between the pressures before and after the expansion valve  23  also increases, so that the malfunction of the expansion valve  23  might occur. In consequence, the operation of the whole refrigerating apparatus becomes unstable. 
         [0061]    Here, the expansion valve  21  is opened to receive the liquid refrigerant liquefied in the liquid receiver  14 , and the gas/liquid bypasses the compressor  11 . In consequence, the refrigerant which gathers on the high pressure side of the compressor  11  is received in the liquid receiver  14  and discharged to the compressor  11 , whereby the high-pressure-side pressure of the compressor  11  can be lowered. 
         [0062]    At this time, the valve opening degree of the expansion valve  21  is controlled so that the high-pressure-side pressure of the compressor  11  becomes a predetermined value or less, whereby the safety of the refrigerating apparatus can be improved. 
         [0063]    It is to be noted that the valve opening degree of the expansion valve  23  is controlled based on the high-pressure-side pressure and low-pressure-side pressure of the compressor  11 , but may be controlled based on a high-pressure-side temperature and a low-pressure-side temperature to stabilize the refrigerating apparatus. 
         [0064]    Moreover, in the present embodiment, the refrigerant circuit is controlled with the electromagnetic valves, but this is not restrictive. For example, the refrigerant circuit may be constituted using a three-way valve  30  as shown in  FIG. 6 . 
       Embodiment 2 
       [0065]    Next, another embodiment of the present invention will be described in detail with reference to  FIGS. 7 to 11 . 
       (5) Refrigerating Apparatus to which the Present Invention is Applied 
       [0066]      FIG. 7  shows a refrigerant circuit  1  of a refrigerating apparatus according to another embodiment to which the present invention is applied. In the drawing, reference numeral  11  is a compressor,  12  is a gas cooler,  13  is a cascade heat exchanger (an inner heat exchanger),  14  is a liquid receiver,  15  is an evaporator,  21  is a second expansion valve (a pressure reducing unit),  22 ,  24 , and  26  are electromagnetic valves (opening/closing valves), and  23  is a first expansion valve. 
         [0067]    It is to be noted that the compressor  11  is a multistage compressor of two or more stages in which a refrigerant can be sucked not only from a low pressure portion but also from an intermediate pressure portion. The refrigerant has a sub-critical state on the low pressure side of this compressor  11 , and the discharged refrigerant has a supercritical state, so that the whole refrigerating apparatus has a trans-critical state. As one example of the refrigerant having such properties, carbon dioxide is used in the present embodiment. 
         [0068]    The supercritical refrigerant discharged from the compressor  11  flows into the gas cooler  12 , and is air-cooled by a blower fan  12 - a.    
         [0069]    The refrigerant discharged from the gas cooler  12  passes through a high-pressure-side circuit  13 - a  of the cascade heat exchanger  13 , and reaches the expansion valve  21  in a case where the electromagnetic valve  22  closes. The pressure of the refrigerant is reduced by the expansion valve  21  to expand and cool the refrigerant. The cooled and thus liquefied refrigerant is received in the liquid receiver  14 . When the electromagnetic valve  26  opens, the vaporized refrigerant is sucked into the intermediate pressure portion of the compressor  11  via a bypass circuit. 
         [0070]    The liquid refrigerant received in the liquid receiver  14  has the pressure reduced by the expansion valve  23 , flows into the evaporator  15 , and expands. In the present refrigerating apparatus, owing to two-stage expansion including the expansion performed by the expansion valve  21  and the expansion by the expansion valve  23 , a refrigerating ability is improved. 
         [0071]    On the other hand, when the electromagnetic valve  22  opens, the refrigerant discharged from the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13  reaches the expansion valve  23  via the electromagnetic valve  22 , and the refrigerant has the pressure reduced by the expansion valve  23  to flow into the evaporator  15 . 
         [0072]    The refrigerant which has flowed into the evaporator  15  evaporates to absorb heat, and outside air circulated by a blower fan  15 - a  is cooled. When the electromagnetic valve  24  closes and the electromagnetic valve  25  opens, the low-temperature low-pressure refrigerant discharged from the evaporator  15  is sucked from the low pressure side of the compressor  11 . 
         [0073]    On the other hand, when the electromagnetic valve  24  opens and the electromagnetic valve  25  closes, the low-temperature low-pressure refrigerant discharged from the evaporator  15  is sucked from the low pressure side of the compressor  11  via a low-pressure-side circuit  13 - b  of the cascade heat exchanger  13 . 
       (6) In a Case where the Refrigerating Ability of the Refrigerating Apparatus runs Short 
       [0074]    In a case where the refrigerating ability of the refrigerating apparatus runs short, the refrigerant circuit  1  has a constitution shown in  FIG. 8  in which the electromagnetic valves  22  and  24  close and the electromagnetic valves  25  and  26  open. The refrigerant discharged from the compressor  11  and cooled by the gas cooler  12  reaches the expansion valve  21  via the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13 . 
         [0075]    When the refrigerating ability runs short, the refrigerant discharged from the compressor  11  has a very high temperature. Therefore, when the refrigerant is not sufficiently cooled by the gas cooler  12 , the refrigerant discharged from the gas cooler  12  is supposed to have a supercritical or trans-critical state. 
         [0076]    It is difficult to perform the sufficient cooling with the supercritical refrigerant in the evaporator  15 . Therefore, this refrigerant has the pressure reduced by the expansion valve  21 , and is thus cooled, and a mixed state of a liquid and a gas is brought in the liquid receiver. In consequence, the liquid refrigerant is received in the lower part of the liquid receiver  14 , and the gas refrigerant is received in the upper part of the liquid receiver. 
         [0077]    However, when the liquid receiver  14  is filled with the gas refrigerant and the inner pressure of the liquid receiver  14  rises, the evaporation of the refrigerant is limited, so that the cooling effect due to the pressure reduction of the expansion valve  21  lowers. 
         [0078]    In the present invention, the upper part of the liquid receiver  14  is connected to the intermediate pressure portion of the compressor  11  via the electromagnetic valve  26 , whereby the gas refrigerant with which the liquid receiver  14  has been filled is sucked by the intermediate pressure portion of the compressor  11 , and the inner pressure of the liquid receiver  14  is reduced. Therefore, the refrigerant can sufficiently be expanded in the liquid receiver  14 , so that the refrigerant can efficiently be cooled and liquefied. 
         [0079]    Moreover, the refrigerant directly flows into the low pressure portion of the compressor  11  from the evaporator  15 , and is directly sucked by the intermediate pressure portion of the compressor  11  from the liquid receiver  14 , so that the amount of the refrigerant to be circulated increases and the refrigerating ability further improves. 
       (7) In a Case where the Refrigerating Ability of the Refrigerating Apparatus is Sufficient 
       [0080]    In a case where the refrigerating ability of the refrigerating apparatus is sufficient, the refrigerant circuit  1  has a constitution shown in  FIG. 9  in which the electromagnetic valves  22  and  24  open, and the expansion valve  21  and the electromagnetic valves  25  and  26  close. The refrigerant discharged from the compressor  11  and cooled by the gas cooler  12  reaches the expansion valve  23  via the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13 . 
         [0081]    When the refrigerating ability is sufficient, the refrigerant cooled and liquefied in the gas cooler  12  flows into the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13 . Moreover, the refrigerant discharged from the evaporator  15  in a state in which the refrigerating ability is sufficient has a low temperature and low pressure, so that the refrigerant of the high-pressure-side circuit  13 - a  is supercooled by the refrigerant of the low-pressure-side circuit  13 - b  in the cascade heat exchanger  13 . 
         [0082]    The supercooled refrigerant has the pressure reduced by the expansion valve  23  via the electromagnetic valve  22 , and flows into the evaporator  15 . In the evaporator  15 , the liquid refrigerant absorbs heat while evaporating, whereby the outside air circulated by the blower fan  15 - a  is cooled. 
         [0083]    The gas refrigerant brought to the low temperature and low pressure flows into the low-pressure-side circuit  13 - b  of the cascade heat exchanger  13  via the electromagnetic valve  24  to cool the refrigerant flowing through the high-pressure-side circuit  13 - a . The refrigerant discharged from the low-pressure-side circuit  13 - b  is sucked on the low pressure side of the compressor  11 , thereby constituting the refrigerating apparatus. 
       (8) In a Case where the Refrigerating Ability of the Refrigerating Apparatus is Excessive 
       [0084]    In a case where the refrigerating ability of the refrigerating apparatus becomes sufficient and the refrigerant becomes excessive on the high pressure side of the compressor, the refrigerant circuit  1  has a constitution shown in  FIG. 10  in which the electromagnetic valves  22 ,  24  and  26  open, and the electromagnetic valve  25  closes. The refrigerant discharged from the compressor  11  and cooled by the gas cooler  12  reaches the expansion valve  23  via the high-pressure-side circuit  13 - a  of the cascade heat exchanger  13 . 
         [0085]    When the refrigerating ability becomes sufficient, the expansion valve  23  is substantially closed, so that the low-pressure-side pressure of the compressor  11  decreases. When this state continues for a long time, the refrigerant gathers on the high pressure side of the compressor  11 , and hence the high-pressure-side pressure of the compressor  11  rises. 
         [0086]    Carbon dioxide for use as the refrigerant in the present embodiment has a very high pressure in a trans-critical state. Therefore, when the pressure rises on the high pressure side of the compressor  11 , the safety of the refrigerating apparatus is impaired, and weight increase is caused owing to the rise of the durable pressure of the elements constituting the refrigerating apparatus. 
         [0087]    Moreover, when a difference between the high-pressure-side pressure of the compressor  11  and the low-pressure-side pressure thereof increases, a difference between the pressures before and after the expansion valve  23  also increases, so that the malfunction of the expansion valve  23  might occur. In consequence, the operation of the whole refrigerating apparatus becomes unstable. 
         [0088]    Here, the expansion valve  21  is opened to receive the liquid refrigerant liquefied in the liquid receiver  14 , and the gas/liquid bypasses the intermediate pressure portion of the compressor  11 . In consequence, the refrigerant which gathers on the high pressure side of the compressor  11  is received in the liquid receiver  14  and discharged to the compressor  11 , whereby the high-pressure-side pressure of the compressor  11  can be lowered. 
         [0089]    At this time, the valve opening degree of the expansion valve  21  is controlled so that the high-pressure-side pressure of the compressor  11  becomes a predetermined value or less, whereby the safety of the refrigerating apparatus can be improved. 
         [0090]    It is to be noted that the valve opening degree of the expansion valve  23  is controlled based on the high-pressure-side pressure and low-pressure-side pressure of the compressor  11 , but may be controlled based on a high-pressure-side temperature and a low-pressure-side temperature to stabilize the refrigerating apparatus. 
         [0091]    Moreover, in the present embodiment, the refrigerant circuit is controlled with the electromagnetic valves, but this is not restrictive. For example, the refrigerant circuit may be constituted using a three-way valve  30  as shown in  FIG. 11 .