Patent Publication Number: US-7914267-B2

Title: Multistage compressor for a CO2 cycle that includes a rotary compressing mechanism and a scroll compressing mechanism

Description:
TECHNICAL FIELD 
     The present invention relates to a multistage compressor suitable for applying to a supercritical refrigeration cycle (CO2 cycle) using CO2 refrigerant as an operating fluid. 
     BACKGROUND ART 
     In the related art, various types of multistage compressors applicable to an air conditioning apparatus are proposed. As a known example, there is a multistage compressor for two-stage compression in which a low-stage side rotary compressing mechanism is provided under an electric motor provided at a center portion in a closed housing, and compressed gas is discharged into the closed housing and intermediate pressure gas is taken into a high-stage side scroll compressing mechanism provided above the electric motor (for example, see Patent Document 1). 
     There is also proposed in Patent Document 2 a multistage compressor for two-stage compression in which an electric motor and low-stage side and high-stage side rotary compressing mechanisms are provided in the closed housing, intermediate pressure gas compressed by the low-stage side rotary compressing mechanism is discharged to a second sealed chamber provided in the closed housing, the intermediate pressure gas extracted from the side of a refrigerant circuit is injected into the second sealed chamber, and the intermediate pressure injection gas and the intermediate pressure gas compressed by the low-stage side rotary compressing mechanism are taken into the high-stage side rotary compressing mechanism. 
     There is proposed in Patent Document 3 a multistage compressor for two-stage compression in which R410A refrigerant is used and intermediate pressure gas compressed by a low-stage side rotary compression element is taken into a high-stage side rotary compression element via a gas pipe and the intermediate pressure gas extracted from the side of the refrigerant circuit is injected into the gas pipe, and in which the ratios of displacement volume of the low-stage side compression element and the high-stage side compression element are 1:0.65 to 1:0.85. 
     In Patent Document 4, there is proposed a multistage compressor for two-stage compression in which part of CO2 refrigerant gas compressed by the low-stage side rotary compression element is discharged into the closed housing, and the intermediate pressure CO2 refrigerant gas and the remaining intermediate pressure CO2 refrigerant gas is taken into the high-stage side rotary compression element via the gas pipe, and in which the volumetric ratios of the low-stage side compression element and the high-stage side compression element are 1:0.56 to 1:0.8. 
     [Patent Document 1] JP-A-5-87074 
     [Patent Document 2] JP-A-2000-54975 
     [Patent Document 3] JP-A-2006-152839 
     [Patent Document 4] JP-A-2001-73976 
     DISCLOSURE OF THE INVENTION 
     However, those disclosed in Patent Documents 1 to 3 are intended to provide a multistage compressor for a refrigeration cycle using chlorofluorocarbon refrigerant or HFC refrigerant, and hence desired compression performance cannot be obtained necessarily even when it is applied to a supercritical refrigeration cycle (CO2 cycle) using CO2 refrigerant as non-chlorofluorocarbon refrigerant as is. 
     In particular, in the CO2 cycle, when a system of injecting the intermediate pressure refrigerant gas extracted from the refrigerant circuit into the intermediate pressure gas discharged from the low-stage side compression element is employed, the effects depend on how the low-stage side compression element and the high-stage side compression element are combined while considering the refrigerant characteristics. Therefore, in the combination of technologies disclosed in Patent Document 1 to Patent Document 4, desired compression efficiency and compression performance are not achieved as the multistage compressor for the CO2 cycle employing a gas injection system, and hence there is still a problem to be solved. 
     In view of such circumstances, it is an object of the present invention to provide a multistage compressor employing a gas injection system for the CO2 cycle, which is able to improve the compression efficiency and compression performance thereof. 
     In order to solve the above-described problems, a multistage compressor according to the present invention includes the following solutions. 
     In other words, a multistage compressor according to an aspect of the present invention is a multistage compressor for a CO2 cycle that includes a low-stage side rotary compressing mechanism and a high-stage side scroll compressing mechanism driven by an electric motor in a closed housing, and carries out two-stage compression by discharging CO2 refrigerant gas compressed in the low-stage side rotary compressing mechanism into the closed housing and taking intermediate pressure refrigerant gas in the closed housing by the high-stage side scroll compressing mechanism, in which a gas injection circuit for injecting intermediate pressure CO2 refrigerant gas extracted from a refrigerant circuit into the closed housing is connected to the closed housing, and in which the pressure ratios of the low-stage side rotary compressing mechanism and the high-stage side scroll compressing mechanism are substantially equivalent, and the ratios of displacement volume are substantially equivalent. 
     According to the aspect described above, the refrigerant gas compressed by the low-stage side rotary compressing mechanism is discharged into the closed housing, and the intermediate pressure refrigerant gas from the refrigerant circuit is injected into the closed housing at the intermediate pressure via the gas injection circuit to allow the intermediate pressure refrigerant gas to be taken into the high-stage side scroll compressing mechanism. Therefore, an excessive pressure loss is not generated, and a high compression performance and a high COP (coefficient of performance) can be obtained owing to an economizer effect through gas injection. Since the pressure ratios of the low-stage side compressing mechanism and the high-stage side compressing mechanism are substantially equivalent, high efficiency is achieved. When pressure ratios are equivalent, the pressure difference of the high-stage side compressing mechanism is large. However, since the high-stage side compressing mechanism employed here is a scroll compressing mechanism in which the compression leakage at the time of high pressure difference is smaller than the rotary compressing mechanism, the compression efficiency of the high-stage side compressing mechanism is increased and hence the performance of a two-stage compressor is improved as much as possible. In addition, since the displacement volumes of the low-stage side rotary compressing mechanism and the high-stage side scroll compressing mechanism are substantially equivalent, a sufficient amount of refrigerant is taken into the high-stage side compressing mechanism even in the case of CO2 refrigerant which has high dryness function for the intermediate pressure refrigerant gas in the refrigerant characteristics. Therefore, the gas injection effects can be demonstrated satisfactorily and the compression efficiency and the compression performance of the two-stage compression can be sufficiently improved. 
     In addition, the multistage compressor in the aspect described above, the ratio of displacement volume in the multistage compressor described above may be 1:0.8 to 1:1. 
     In this configuration, the range of the ratios of displacement volume of the low-stage side compressing mechanism and the high-stage side compressing mechanism, which are substantially equivalent, is 1:0.8 to 1:1. Therefore, the ratio of displacement volume is sufficiently larger than the displacement volume which is considered to be optimal in the case of the CO2 refrigerant multistage compressor without gas injection (approximately 1:0.6 to 1:0.8), and hence the refrigerant gas is allowed to be taken into the high-stage side compressing mechanism sufficiently even with the multistage compressor employing a system of injecting the intermediate pressure refrigerant gas into the closed housing. Therefore, the gas injection effect is sufficiently demonstrated and the compression performance and the COP may be improved as much as possible. 
     In any one of the multistage compressors described above, the multistage compressor in the aspect described above may be configured in such a manner that the low-stage side rotary compressing mechanism is provided on one side of the electric motor provided at the center portion of the closed housing so as to be connected to a crank portion provided at one end of a drive shaft driven by the electric motor, and the high-stage side scroll compressing mechanism is provided on the other side of the electric motor so as to be connected to a crank pin portion provided at the other end of the drive shaft. 
     In this configuration, the electric motor is provided at the center portion of the closed housing, the low-stage side rotary compressing mechanism is connected to the one end side of the drive shaft, and the high-stage side scroll compressing mechanism is connected to the other end side thereof. Therefore, the high-efficiency and high-performance multistage compressor may be manufactured with the combination of the rotary compressing mechanism and the scroll compressing mechanism having different configuration. 
     In any one of the multistage compressors described above, the multistage compressor in the aspect described above may be configured in such a manner that the low-stage side rotary compressing mechanism and the high-stage side scroll compressing mechanism are provided with an oil supply pump for supplying lubricating oil filled in the closed housing to required points of lubrication via oil supply holes provided in the drive shafts thereof, and the oil supply pump is a positive displacement oil supply pump. 
     In the multistage compressor in which the pressure in the closed housing is an intermediate pressure, it is difficult to supply lubricating oil filled in the closed housing to the high-stage side scroll compressing mechanism with the pressure difference. Therefore, in the present invention, the positive displacement oil supply pump having a high oil supply performance can be employed as an oil supply pump, and hence oil supply is reliably achieved for the required points of lubrication respective in the low-stage side compressing mechanism and the high-stage side compressing mechanism even with the multistage compressor in which the pressure in the closed housing is the intermediate pressure. Therefore, the stable lubrication can be achieved in both of the compressors. 
     In addition, in any one of the multistage compressors described above, the multistage compressor in the aspect described above may be configured in such a manner that the gas injection circuit is connected to the closed housing at a position on the opposite side of the axial line of the drive shaft of the compressing mechanism from the position of an oil discharge hole through which the lubricating oil after having lubricated the compressing mechanism is discharged in the range defined by a line orthogonal to the axial line of the drive shaft. 
     In this configuration, since the gas injection circuit is connected to the closed housing at the position on the opposite side of the axial line of the drive shaft of the compressing mechanism from the position of the oil discharge hole, a sufficient distance is secured between the oil discharge hole and the point of connection of the gas injection circuit, and hence the refrigerant gas injected into the closed housing can be prevented from coming into contact with the lubricating oil discharged from the oil discharge hole and whirling the lubricating oil upward. Accordingly, unnecessary discharge of the lubricating oil (oil discharge out of the compressor) can be prevented, and lowering of the volumetric efficiency of the high-stage side compressing mechanism by excessive mixing of the lubricating oil into the intermediate pressure refrigerant gas can be also prevented, so that the performance of the multistage compressor is improved. 
     In any one of the multistage compressors described above, the multistage compressor in the aspect described above may be configured in such a manner that a shielding panel is provided in the closed housing so as to oppose to an opening of the gas injection circuit toward the interior of the closed housing. 
     In this configuration, since the shielding panel is provided so as to oppose the opening of the gas injection circuit, the refrigerant gas injected into the closed housing and the lubricating oil dropped down into the closed housing after having lubricated the compressing mechanisms can be separated by the partitioning function of the shielding panel, so that the lubrication oil is prevented from whirling upward by the injected refrigerant gas. Accordingly, unnecessary discharge of the lubricating oil (oil discharge out of the compressor) can be prevented, and lowering of the volumetric efficiency of the high-stage side compressing mechanism by excessive mixing of the lubricating oil into the intermediate pressure refrigerant gas can be also prevented, so that the performance of the multistage compressor is improved. 
     In any one of the multistage compressors described above, the multistage compressor in the aspect described above may be configured in such a manner that the gas injection circuit is connected to and opening toward the interior of the closed housing at a position opposing a stator coil end of the electric motor. 
     In this configuration, since the gas injection circuit is connected to and opening toward the interior of the closed housing at the position opposing the stator coil end of the electric motor, the refrigerant gas injected into the closed housing the partitioning function of the stator coil end and the lubricating oil dropped down in the closed housing after having lubricated the compressing mechanism can be separated, and hence the lubricating oil can be prevented from whirling upward by the injected refrigerant gas. Accordingly, unnecessary discharge of the lubricating oil (oil discharge out of the compressor) can be prevented, and lowering of the volumetric efficiency of the high-stage side compressing mechanism by excessive mixing of the lubricating oil into the intermediate pressure refrigerant gas can be also prevented, so that the performance of the multistage compressor is improved. In addition, the motor stator can be cooled by the injected refrigerant gas, and hence the motor efficiency can be improved. 
     In any one of the multistage compressors described above, the multistage compressor in the aspect described above may be configured in such a manner that the gas injection circuit is connected to and opened toward the interior of the closed housing obliquely toward the high-stage side scroll compressing mechanism. 
     In this configuration, since the gas injection circuit is connected to and opening toward the interior of the closed hosing obliquely toward the high-stage side scroll compressing mechanism, the refrigerant gas injected obliquely toward the high-stage side scroll compressing mechanism is taken into the high-stage side scroll compressing mechanism as is. Therefore, the lubricating oil dropped down into the closed housing after having lubricated the compressing mechanism can be prevented from whirling upward by the injection gas. Accordingly, unnecessary discharge of the lubricating oil (oil discharge out of the compressor) can be prevented, and lowering of the volumetric efficiency of the high-stage side compressing mechanism by excessive mixing of the lubricating oil into the intermediate pressure refrigerant gas can be also prevented, so that the performance of the multistage compressor is improved. 
     In any one of the multistage compressors described above, the multistage compressor in the aspect described above may be configured in such a manner that the low-stage side rotary compressing mechanism and/or the high-stage side scroll compressing mechanism includes the oil discharge hole for discharging the lubricating oil after having lubricated required points into the closed housing, and the oil discharge hole is provided with an oil discharge guide for guiding the discharged oil into an oil trap in the closed housing. 
     In this configuration, since the oil discharge guide for guiding the discharged oil into the oil trap in the closed housing is provided on the oil discharge hole provided on the low-stage side rotary compressing mechanism and/or the high-stage side scroll compressing mechanism, the refrigerant gas injected into the closed housing and the lubricating oil discharged from the oil discharge hole into the closed housing after having lubricated the compressing mechanism can be separated by the partitioning function of the oil discharge guide, so that the lubricating oil is prevented from whirling upward by the injected refrigerant gas. Accordingly, unnecessary discharge of the lubricating oil (oil discharge out of the compressor) can be prevented, and lowering of the volumetric efficiency of the high-stage side compressing mechanism by excessive mixing of the lubricating oil into the intermediate pressure refrigerant gas can be also prevented, so that the performance of the multistage compressor is improved. 
     In any one of the multistage compressors described above, the multistage compressor in the aspect described above may be configured in such a manner that the gas injection circuit is connected to and opening toward the interior of the closed housing at a position between the electric motor and the high-stage side scroll compressing mechanism. 
     In this configuration, since the gas injection circuit is connected to and opened toward the interior of the closed housing at a position between the electric motor and the high-stage side scroll compressing mechanism, the refrigerant gas injected into the closed housing can be prevented from being heated by the electric motor. Therefore, the intake efficiency of the high-stage side scroll compressing mechanism is improved and the performance of the multistage compressor can be improved. 
     In any one of the multistage compressors described above, the multistage compressor in the aspect described above may be configured in such a manner that the gas injection circuit is connected to and opening toward the interior of the closed housing at a position between the electric motor and the low-stage side rotary compressing mechanism. 
     In this configuration, since the gas injection circuit is connected to and opening toward the interior of the closed housing at the position between the electric motor and the low-stage side rotary compressing mechanism, the refrigerant gas injected into the closed housing flows around the electric motor. Accordingly, the electric motor can be cooled. Consequently, the motor efficiency is improved and the performance of the multistage compressor can be improved. 
     According to the present invention, an excessive pressure loss is not generated during gas injection, and hence a high compression performance and a high COP (coefficient of performance) can be obtained owing to an economizer effect through gas injection. Since the pressure ratios of the low-stage side compressing mechanism and the high-stage side compressing mechanism are equivalent and the high-stage side compressing mechanism in which the pressure difference is increased in this case is a scroll compressing mechanism in which the compression leakage at the time of high pressure difference is relatively small, the compression efficiency of the high-stage side compressing mechanism is increased, and the performance as the two-stage compressor can be improved as much as possible. In addition, the displacement volumes of the low-stage side rotary compressing mechanism and the high-stage side scroll compressing mechanism are substantially equivalent, so that a sufficient amount of refrigerant can be taken into the high-stage side compressing mechanism in the case of the CO2 refrigerant which has high dryness function for the intermediate pressure refrigerant gas. Therefore, the gas injection effects are demonstrated satisfactorily and the compression efficiency and the compression performance of the two-stage compression can be sufficiently improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a CO2 cycle to which a multistage compressor according to a first embodiment of the present invention is applied. 
         FIG. 2  is a vertical cross-sectional view of the multistage compressor according to the first embodiment of the present invention. 
         FIG. 3  is a lateral cross-sectional view of a positive displacement oil supply pump applied to the multistage compressor shown in  FIG. 2 . 
         FIG. 4  is a P-h diagram of the CO2 cycle shown in  FIG. 1 . 
         FIG. 5  is a lateral cross-sectional view of a principal portion of the multistage compressor according to a second embodiment of the present invention. 
         FIG. 6  is a vertical cross-sectional view of a principal portion of the multistage compressor according to a third embodiment of the present invention. 
         FIG. 7  is a vertical cross-sectional view of a principal portion of the multistage compressor according to a fourth embodiment of the present invention. 
         FIG. 8  is a vertical cross-sectional view of a principal portion of the multistage compressor according to a fifth embodiment of the present invention. 
         FIG. 9  is a vertical cross-sectional view of a principal portion of the multistage compressor according to a sixth embodiment of the present invention. 
         FIG. 10  is a vertical cross-sectional view of a principal portion of the multistage compressor according to a seventh embodiment of the present invention. 
     
    
    
     EXPLANATION OF REFERENCE 
     
         
           1 : CO2 cycle 
           2 : multistage compressor 
           3 : closed housing 
           4 : low-stage side rotary compressing mechanism (low-stage side compressing mechanism) 
           5 : high-stage side rotary compressing mechanism (high-stage side compressing mechanism) 
           15 : gas injection circuit 
           20 : positive displacement oil supply pump 
           21 : oil supply hole 
           31 : electric motor 
           34 ,  35 ,  36 : crankshaft (drive shaft) 
           35 A: crank portion 
           36 A: crank pin 
           37 : lubricating oil 
           65 : oil discharge hole 
           66 : shielding panel 
           67 : stator coil end 
           68 : oil discharge guide 
         p: axial line of drive shaft 
         Q: line which is orthogonal to axial line 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring now to the drawings, embodiments of the present invention will be described. 
     First Embodiment 
     Referring to  FIG. 1  to  FIG. 4 , a first embodiment of the present invention will be described. 
       FIG. 1  shows a block diagram of a CO2 cycle (a supercritical refrigeration cycle using CO2 refrigerant)  1  using a multistage compressor  2  according to the first embodiment of the present invention. The CO2 cycle  1  includes the multistage compressor  2  in which two compressing mechanisms; a low-stage side compressing mechanism  4  and a high-stage side compressing mechanism  5  are provided in one single closed housing  3 . The configuration of the multistage compressor  2  will be described later in detail. 
     A discharge pipe  6  is connected to the high-stage side compressing mechanism  5  of the multistage compressor  2 , and the other end of the discharge pipe  6  is connected to a radiator  7 . High-temperature, high-pressure refrigerant gas discharged from the multistage compressor  2  is heat-exchanged with outside air sent by a radiator fan (not shown) and cooled in the radiator  7 . A vapor-liquid separator  10  is provided at downstream of the radiator  7  via a refrigerant pipe  8  and a first reducing valve  9 , and refrigerant depressurized by the first reducing valve  9  is separated into vapor and liquid. An evaporator  13  is connected to the downstream of the vapor-liquid separator  10  via a refrigerant pipe  11  and a second reducing valve  12 . 
     In the evaporator  13 , vapor-liquid two-phase refrigerant at a low temperature and a low pressure depressurized via the second reducing valve  12  is heat-exchanged with air sent by an evaporator fan (not shown) and absorbs heat from the air and evaporatively emitted. The refrigerant evaporated by the evaporator  13  is adapted to be taken into the low-stage side compressing mechanism  4  of the multistage compressor  2  via an intake pipe  14  connected between the evaporator  13  and the multistage compressor  2 . 
     A gas injection circuit  15  for injecting intermediate pressure refrigerant gas separated by the vapor-liquid separator  10  into the closed housing  3  is connected between the vapor-liquid separator  10  and the closed housing  3  of the multistage compressor  2 . 
     Referring now to  FIG. 2 , the configuration of the multistage compressor  2  will be described. 
     The multistage compressor  2  has a configuration in which the low-stage side compressing mechanism  4  is provided in a lower portion in the closed housing  3 , and the high-stage side compressing mechanism  5  is provided in an upper portion thereof. An accumulator  30  to which the intake pipe  14  is connected is provided integrally with the multistage compressor  2 . An electric motor  31  including a stator  32  and a rotor  33  is provided at a center portion of the closed housing  3 , and a crankshaft  34  is connected integrally with the rotor  33 . The lower end portion of the crankshaft  34  corresponds to a crankshaft  35  for the low-stage side compressing mechanism  4 , and an upper end portion corresponds to a crankshaft  36  for the high-stage side compressing mechanism  5 . Lubricating oil  37  is filled by a predetermined amount at the bottom of the closed housing  3 . The lubricating oil  37  is adapted to be supplied to required points of lubrication in the low-stage side compressing mechanism  4  and the high-stage side compressing mechanism  5  via an oil supply hole  21  formed in the axial direction of the crankshaft  34  by a positive displacement oil supply pump  20 , described later, provided at the lower end portion of the crankshaft  34 . 
     The low-stage side compressing mechanism  4  includes a rotary type compressing mechanism. The rotary type compressing mechanism  4  may be a general rotary type compressing mechanism having a cylinder chamber  41 , and including a cylinder body  40  to be fixedly provided on the closed housing  3 , an upper bearing  42  and a lower bearing  43  disposed respectively on the top and the bottom of the cylinder body  40 , a rotor  44  fitted to a crank portion  35 A of the crankshaft  35  so as to be capable of sliding rotation in the cylinder chamber  41 , a discharge cover  46  forming a discharge cavity  45 , and a blade and a blade holding spring (not shown). 
     In the low-stage side rotary type compressing mechanism  4 , refrigerant gas taken into the cylinder chamber  41  via an intake pipe  47  connected to the accumulator  30  is compressed to an intermediate pressure by the rotation of the rotor  44 , and then is discharged into the discharge cavity  45  and is discharged into the closed housing  3  via a discharge port provided on the discharge cover  46 . 
     The refrigerant gas at the intermediate pressure discharged into the closed housing  3  flows through an air gap or the like of the electric motor  31  into an upper space of the closed housing  3 , is joined with the intermediate pressure refrigerant gas injected from the gas injection circuit  15  connected to the closed housing  3  into the closed housing  3 , and then is taken into the high-stage side compressing mechanism  5 . 
     The gas injection circuit  15  is connected to the closed housing  3  at a point between the electric motor  31  and the high-stage side compressing mechanism  5 . 
     The high-stage side compressing mechanism  5  includes a scroll type compressing mechanism. 
     The scroll type compressing mechanism  5  may be a general scroll type compressing mechanism including a frame member  50  having a bearing  51  that supports the crankshaft  36  and being fixedly provided on the closed housing  3 , a fixed scroll  52  and an orbiting scroll  53  which are supported on the frame member  50  and define a pair of compression chambers  54  by being meshed with each other at a phase shifted from each other, a drive bush  55  connecting the orbiting scroll  53  and a crank pin  36 A provided at the axial end of the crankshaft  36  for driving the orbiting scroll  53  to orbit, an Oldham ring  56  provided between the orbiting scroll  53  and the supporting frame  50  for preventing the orbiting scroll  53  from rotating by itself and allowing the same to do an orbiting motion, a discharge valve  57  provided on the back face of the fixed scroll  52 , a discharge cover  59  fixedly provided on the back face of the fixed scroll  52  so as to define a discharge chamber  58  between the fixed scroll  52  and the discharge cover  59 , and the like. 
     In the high-stage side scroll type compressing mechanism  5 , the discharge pipe  6  is connected to the discharge chamber  58  to discharge refrigerant gas compressed to a high temperature and a high pressure out of the compressor. 
     In the high-stage side scroll type compressing mechanism  5 , intermediate pressure refrigerant gas compressed by the low-stage side rotary type compressing mechanism  4  to the intermediate pressure and discharged into the closed housing  3  and intermediate pressure refrigerant gas injected from the gas injection circuit  15  into the closed housing  3  are mixed in the closed housing  3 , and then taken into the pair of compression chambers  54  via an intake port  60 . The pair of compression chambers  54  are moved toward the center while being reduced in capacity by the orbiting motion of the orbiting scroll  53  and are joined into one single compression chamber  54 . During this period, the refrigerant gas is compressed from the intermediate pressure to a high pressure (discharge pressure), and is discharged from the center portion of the fixed scroll  52  via the discharge valve  57  into the discharge chamber  58 . The high-temperature, high-pressure refrigerant gas is discharged out of the multistage compressor  2  via the discharge pipe  6 . 
     As shown in  FIG. 2  and  FIG. 3 , the positive displacement oil supply pump  20  defines a cylinder chamber  22  hermetically closed at a lower opening portion by a thrust plate  23  and a cover plate  24  at the lower bearing  43  which constitutes the low-stage side rotary compressing mechanism  4 , and a rotor  26  fitted on an eccentric shaft  25  formed at the lower end of the crankshaft  34  to make the orbiting motion while being sliding contact with the inner peripheral surface of the cylinder chamber  22  is fitted into the cylinder chamber  22 . The rotor  26  is integrally provided with a blade  26 A which partitions the interior of the cylinder chamber  22  into an oil supply chamber  22 A and an oil discharge chamber  22 B. The lubricating oil  37  filled in the closed housing  3  is taken into the oil supply chamber  22 A via an intake port  27 , is discharged from the oil discharge chamber  22 B to the discharge port  28 , and is supplied through a communication channel  29  to an oil supply hole  21  by the positive displacement oil supply pump  20 . 
     The positive displacement oil supply pump  20  is described here for illustrative only, and any types of positive displacement oil supply pump may be employed in this case. 
     In this embodiment, the relation between the low-stage side rotary compressing mechanism  4  and the high-stage side scroll compressing mechanism  5  in the multistage compressor  2  described above are configured as shown below. 
     The low-stage side rotary compressing mechanism  4  and the high-stage side scroll compressing mechanism  5  are configured to have the substantially equivalent pressure ratio so as to achieve the highest efficiency in the case of the two-stage compression. 
     The low-stage side rotary compressing mechanism  4  and the high-stage side scroll compressing mechanism  5  are configured to have the substantially equivalent ratio of displacement volume on a premise of the equivalent pressure ratio described above. 
     To have the substantially equivalent ratio of displacement volume means that the ratios of the displacement volume V 1  of the low-stage side rotary compressing mechanism  4  and the displacement volume V 2  of the high-stage side scroll compressing mechanism  5  (V 1 :V 2 ) are 1:0.8 to 1:1. 
     Operations of the refrigeration cycle  1  and the multistage compressor  2  will be described below. In the low-stage side rotary compressing mechanism  4  of the multistage compressor  2 , low pressure refrigerant gas is taken from the accumulator  30  directly into the cylinder chamber  41  via the intake pipe  47 . The refrigerant gas is compressed to the intermediate pressure by the rotation of the rotor  44  via the electric motor  31  and the crankshaft  35 , and then discharged into the discharge cavity  45 , and then is discharged from the discharge cavity  45  through a discharge port provided on the discharge cover  46  into the closed housing  3 . Accordingly, the interior of the closed housing  3  is brought into an intermediate pressure atmosphere, so that the temperatures of the electric motor  31  and the lubricating oil  37  are substantially the same as the intermediate pressure refrigerant. 
     The refrigerant gas at the intermediate pressure separated by the vapor-liquid separator  10  is injected into the closed housing  3  at the intermediate pressure atmosphere via the gas injection circuit  15 . 
     The intermediate pressure refrigerant gas described above is mixed in the closed housing  3 , and is taken into the compression chambers  54  of the high-stage side scroll compressing mechanism  5  through the intake port  60  opening into the closed housing  3 . In the scroll compressing mechanism  5 , the compression operation is achieved when the electric motor  31  is driven and the orbiting motion of the orbiting scroll  53  with respect to the fixed scroll  52  via the crankshaft  36 , the crank pin  36 A and the drive bush  55 . Accordingly, the intermediate pressure refrigerant gas described above is compressed to a high-pressure state, and is discharged into the discharge chamber  58  via the discharge valve  57 . 
     The high-temperature, high-pressure refrigerant gas discharged into the discharge chamber  58  is discharged from the multistage compressor  2  via the discharge pipe  6  connected to the discharge chamber  58 , and is introduced into the radiator  7  as shown in an arrow in a solid line shown in  FIG. 1 . The refrigerant gas is heat-exchanged with air sent by the radiator fan in the radiator  7  and is discharged toward the air, so as to be brought into a supercritical state or a condensed state. The refrigerant is passed through the refrigerant pipe  8  and is depressurized by the first reducing valve  9 , thereby being brought into a vapor-liquid two-phase state, reaches the vapor-liquid separator  10 , where it is separated into intermediate pressure liquid refrigerant and intermediate pressure gas refrigerant. The separated intermediate pressure gas refrigerant passes through the gas injection circuit  15 , and is injected into the closed housing  3  as described above. On the other hand, the intermediate pressure liquid refrigerant passes through the refrigerant pipe  11 , is depressurized again by the second reducing valve  12 , and reaches the evaporator  13  in a state of low pressure vapor-liquid two-phase refrigerant. 
     The low-pressure, low-temperature vapor-liquid two-phase refrigerant is heat-exchanged with air sent from the evaporator fan while flowing in the evaporator  13  and is evaporatively emitted by absorbing heat from the air side. The low-pressure refrigerant gas passes through the intake pipe  14  and reaches the accumulator  30  provided integrally with the multistage compressor  2 , where liquid content (including oil) is separated, and only gas content is taken into the low-stage side rotary compressing mechanism  4  via the intake pipe  47 , and is compressed again. 
     While the cycle described above is repeated, space heating or heating can be achieved by using discharged heat from the radiator  7 , and space cooling or cooling can be achieved by using the heat-absorbing operation of the evaporator  13 . 
     During the above described cycles, in the multistage compressor  2 , the lubricating oil  37  filled in the closed housing  3  is supplied to a required points of oil supply in the low-stage side rotary type compressing mechanism  4  and the high-stage side scroll type compressing mechanism  5  via the oil supply hole  21  by the positive displacement oil supply pump  20 , so that the both compressing mechanisms  4 ,  5  are reliably lubricated. In other words, the lubricating oil  37  in the closed housing  3  is taken from the intake port  27  into the oil supply chamber  22 A, is discharged from the oil discharge chamber  22 B to the discharge port  28  by the rotating motion of the rotor  26 , and is sent to the oil supply hole  21  via the communication channel  29 . With this oil supply operation of the positive displacement oil supply pump  20 , oil supply is reliably achieved also for the high-stage side scroll type compressing mechanism  5  in which pressure-difference oil supply is difficult to achieve. 
       FIG. 4  shows a P-h diagram of the refrigeration cycle shown above. The change of the refrigerant characteristics will be described on the basis of this diagram. Low-pressure refrigerant taken into the multistage compressor  2  is compressed by the low-stage side rotary type compressing mechanism  4  from a point A to a point B, is then discharged into the closed housing  3 , and is joined with the intermediate pressure refrigerant gas injected from the gas injection circuit  15  and is brought into a state of a point C. In this state, it is taken into the high-stage side scroll type compressing mechanism  5  and is compressed again. The high-pressure refrigerant gas compressed to a point D by the high-stage side scroll type compressing mechanism  5  is cooled by discharging heat in the radiator  7 , and is brought into a supercritical state or a condensed state at a point E. 
     Accordingly, the refrigerant in the state of the supercritical state or the condensed state is depressurized to a point F by the first reducing valve  9  to be vapor-liquid two-phase intermediate pressure refrigerant, and is separated into the intermediate pressure gas refrigerant and the intermediate pressure liquid refrigerant in the vapor-liquid separator  10 . The intermediate pressure gas refrigerant is injected into the closed housing  3  via the gas injection circuit  15 , is joined with the refrigerant at the point B, and is brought into the state of the point C. The intermediate pressure liquid refrigerant cooled by the separation of the intermediate pressure gas refrigerant in the vapor-liquid separator  10  and brought into a state of a point G is further depressurized by the second reducing valve  12  so that the low-pressure refrigerant of vapor-liquid two-phase low-pressure refrigerant represented by a point H is obtained. This low-pressure two-phase refrigerant reaches the evaporator  13 , absorbs heat from air and hence is evaporated, and then is changed to the point A and is returned to the multistage compressor  2 . 
     Consequently, at the time of space heating or heating, the intermediate pressure refrigerant is added to the refrigerant flowing in the radiator  7  by gas injection. Therefore, the amount of circulation of the refrigerant is increased, and the space heating or heating performance is improved correspondingly. In the case of space cooling or cooling, the enthalpy of the refrigerant at the point H is increased from the point F to the point G, and hence the amount of heat of the refrigerant evaporated in the evaporator  13  is increased, so that the space cooling or cooling performance is improved correspondingly. Furthermore, in the multistage compressor  2 , a power required for compressing the refrigerant from the point A to the point D is reduced significantly by an economizer effect through gas injection. 
     Since the pressure ratios of the low-stage side rotary type compressing mechanism  4  and the high-stage side scroll type compressing mechanism  5  are substantially equivalent, the multistage compressor  2  is able to achieve the two-stage compression at the highest efficiency. In addition, since the ratios of displacement volume of the low-stage side rotary type compressing mechanism  4  and the high-stage side scroll type compressing mechanism  5  are 1:0.8 to 1, which are substantially equivalent, the gas injection function is sufficiently achieved by causing a sufficient amount of refrigerant to be taken into the high-stage side compressing mechanism even in the case of CO2 refrigerant which has high dryness function for the intermediate pressure refrigerant gas in the refrigerant characteristics. In other words, in the case of the CO2 refrigerant, the refrigerant gas in the intermediate stage which is expanded by one stage contains much gas content and is high in dryness function in comparison with R410A refrigerant or the like as is clear from  FIG. 4  in the refrigerant characteristics. Therefore, a sufficient amount of injection gas cannot be taken in the high-stage side compressing mechanism and the gas injection effect is lowered unless the ratio of displacement volume is increased in comparison with the multistage compressor for R410A refrigerant and the multistage compressor for CO2 refrigerant of a system without gas injection shown in Patent Documents 3 and 4 described above. 
     Therefore, in this embodiment, since the ratio of displacement volume is sufficiently higher in comparison with the ratio of displacement volume shown in Patent Documents 3 and 4, the desired gas injection effect can be demonstrated. 
     Therefore, according to this embodiment, the following effects are achieved. 
     Since the pressure ratios of the low-stage side rotary compressing mechanism  4  and the high-stage side scroll compressing mechanism  5  are substantially equivalent, the two-stage compression at the highest efficiency can be achieved. 
     When the pressure ratios of the low-stage side and the high-stage side are set to be equivalent, the difference in pressure of the high-stage side compressing mechanism is large. However, since the scroll compressing mechanism having a smaller compression leakage at the high pressure difference than the rotary compressing mechanism is employed as the high-stage side compressing mechanism, the compression efficiency is increased and the performance of a two-stage compressor can be improved as much as possible. 
     Since a sufficient amount of refrigerant gas is taken into the high-stage side scroll compressing mechanism  5  even when the ratios of displacement volume of the low-stage side rotary compressing mechanism  4  and the high-stage side scroll compressing mechanism  5  are set to (1:0.8 to 1:1), which are substantially equivalent, and the CO2 refrigerant having the high dryness function for the intermediate pressure refrigerant gas is employed, the gas injection effects are demonstrated satisfactorily and the compression efficiency and the compression performance of the two-stage compressor can be sufficiently improved. 
     Since the positive displacement oil supply pump  20  which has a high oil supply performance is employed in the oil supply pump for supplying lubricating oil to the respective compressing mechanisms  4  and  5 , the interior of the closed housing is maintained at the intermediate pressure. Therefore, even with the multistage compressor  2  in which pressure-difference oil supply to the high-stage side compressing mechanism is difficult to achieve, oil supply is reliably achieved for each of the required points of lubrication of the low-stage side compressing mechanism  4  and the high-stage side compressing mechanism  5 . Therefore, stable lubrication can be achieved for the both compressing mechanisms. 
     Since the electric motor  31  is provided at the center of the closed housing  3 , the low-stage side rotary compressing mechanism  4  is connected to one end side  35  of the drive shaft (crankshaft)  34 , and the high-stage side scroll compressing mechanism  5  is connected to the other end side  36 , manufacture of the high-performance multistage compressor  2  having the rotary compressing mechanism  4  and the scroll compressing mechanism  5  combined to each other is enabled. 
     Furthermore, in this embodiment, the gas injection circuit  15  is connected to a point between the electric motor  31  and the high-stage side scroll compressing mechanism  5 . Therefore, the injected refrigerant gas can be prevented from being heated by the electric motor  31 . Therefore, the intake efficiency of the high-stage side scroll compressing mechanism  5  can be increased, so that the performance of the multistage compressor  2  is improved. 
     Second Embodiment 
     Referring now to  FIG. 2  and  FIG. 5 , a second embodiment of the present invention will be described. 
     This embodiment is different from the first embodiment in that the point of connection of the gas injection circuit  15  is specified. Other points are the same as the first embodiment, and hence description thereof will be omitted. 
     In the high-stage side scroll compressing mechanism  5 , lubricating oil which has lubricated the required points of lubrication is collected in a recess of the frame member  50 , and from this recess, is dropped down to the bottom of the closed housing  3  via an oil discharge hole  65  (see  FIG. 2 ). The gas injection circuit  15  is connected to the closed housing  3  at a position 180° opposite from the oil discharge hole  65  with respect to an axial line P of the crankshaft  34  as shown in  FIG. 5 . 
     In this manner, a sufficient distance is secured between the oil discharge hole  65  and the connecting position of the gas injection circuit  15  by connecting the gas injection circuit  15  to the closed housing  3  at a position 180° opposite from the oil discharge hole  65  with respect to the axial line P of the crankshaft  34 . Therefore, the refrigerant gas injected into the closed housing  3  can be restrained from coming into contact with the lubricating oil  37  discharged from the oil discharge hole  65  and whirling the lubricating oil  37  upward. Accordingly, unnecessary discharge of the lubricating oil  37  (oil discharge out of the compressor) can be prevented, and lowering of the volumetric efficiency of the high-stage side compressing mechanism  5  by excessive mixing of the lubricating oil  37  into the intermediate pressure refrigerant gas can be also prevented. Therefore, the performance of the CO2 cycle  1  and the multistage compressor  2  can be improved. 
     The point of connection of the gas injection circuit  15  does not necessarily have to be the position 180° opposite side of the axial line P of the crankshaft  34  from the oil discharge hole  65 , but must simply be apart therefrom by a distance which prevents upward whirling of the lubricating oil  37  discharged from the oil discharge hole  65 , and must simply be connected to the closed housing  3  at a position opposite side of the axial line P of the crankshaft  34  from the position of the oil discharge hole  65  in a range R defined by a line Q which is orthogonal to the axial line P. 
     An oil discharge hole may be provided on the low-stage side rotary compressing mechanism  4  as needed, and in this case, the relation with respect to the gas injection circuit  15  is the same as described above. 
     Third Embodiment 
     Referring now to  FIG. 6 , a third embodiment of the present invention will be described. 
     This embodiment is different from the first embodiment in the configuration of a connecting portion of the gas injection circuit  15  connected to the closed housing  3 . Other points are the same as the first embodiment, and hence description thereof will be omitted. 
     In this embodiment, as shown in  FIG. 6 , a shielding panel  66  for covering an opening of the gas injection circuit  15  at a predetermined distance is provided inside the closed housing  3  so as to oppose the point of connection of the gas injection circuit  15  to the closed housing  3 . 
     With the provision of the shielding panel  66  as described above, refrigerant gas injected from the gas injection circuit  15  into the closed housing  3  and the lubricating oil  37  dropped down into the closed housing  3  after having lubricated the compressing mechanism  5  are separated from each other, and hence the lubricating oil  37  is prevented from whirling upward by the injected refrigerant gas. 
     Therefore, unnecessary discharge of oil discharged out of the compressor  2  due to the lubricating oil  37  mixed into the refrigerant gas can be prevented, and lowering of volumetric efficiency of the high-stage side compressing mechanism  5  by excessive mixing of the lubricating oil into the intermediate pressure refrigerant gas can be prevented, so that the performance of the multistage compressor  2  is improved. 
     Fourth Embodiment 
     Referring now to  FIG. 7 , a fourth embodiment of the present invention will be described. 
     This embodiment is different from the first embodiment in the configuration of the connecting portion of the gas injection circuit  15  connected to the closed housing  3 . Other points are the same as the first embodiment, and hence description thereof will be omitted. 
     In this embodiment, as shown in  FIG. 7 , the gas injection circuit  15  is connected to the closed housing  3  at a position opposing a stator coil end  67  of the electric motor  31 . 
     As described above, by connecting the gas injection circuit  15  so as to oppose the stator coil end  67  of the electric motor  31 , refrigerant gas injected into the closed housing  3  and the lubricating oil dropped down into the closed housing  3  after having lubricated the compressing mechanism  5  are separated from each other, and hence the lubricating oil can be prevented from whirling upward by the injected refrigerant gas using the stator coil end  67 . 
     Accordingly, unnecessary discharge of oil discharged out of the compressor  2  due to the lubricating oil  37  mixed into the refrigerant gas can be prevented, and lowering of volumetric efficiency of the high-stage side compressing mechanism  5  by excessive mixing of the lubricating oil into the intermediate pressure refrigerant gas can be prevented, so that the performance of the multistage compressor  2  is improved. In addition, since the motor stator  32  is cooled by the injected refrigerant gas, the motor efficiency can be improved. 
     Fifth Embodiment 
     Referring now to  FIG. 8 , a fifth embodiment of the present invention will be described. 
     This embodiment is different from the first embodiment in the configuration of the connecting portion of the gas injection circuit  15  connected to the closed housing  3 . Other points are the same as the first embodiment, and hence description thereof will be omitted. 
     In this embodiment, as shown in  FIG. 8 , the gas injection circuit  15  is connected to the closed housing  3  toward obliquely upward from below, so that the injected refrigerant gas is directed toward the high-stage side scroll compressing mechanism  5 . 
     As described above, by connecting the gas injection circuit  15  to the closed housing  3 , refrigerant gas injected into the closed housing  3  is taken into the high-stage side scroll compressing mechanism  5  without bringing into much contact with the lubricating oil  37  which drops down into the closed housing  3  after having lubricated the high-stage side scroll compressing mechanism  5 . Therefore, the lubricating oil can be prevented from whirling upward by the injection gas. 
     Therefore, unnecessary discharge of oil discharged out of the compressor  2  due to the lubricating oil  37  mixed into the refrigerant gas can be prevented, and lowering of volumetric efficiency of the high-stage side compressing mechanism  5  by excessive mixing of the lubricating oil into the intermediate pressure refrigerant gas can be prevented, so that the performance of the multistage compressor  2  is improved. 
     Sixth Embodiment 
     Referring now to  FIG. 9 , a sixth embodiment of the present invention will be descried. 
     This embodiment is different from the first embodiment in the point of connection of the gas injection circuit  15  connected to the closed housing  3 . Other points are the same as the first embodiment, and hence description thereof will be omitted. 
     In this embodiment, as shown in  FIG. 9 , the gas injection circuit  15  which is connected to the position shown by a chain line in the first embodiment is connected to the closed housing  3  at a position below the electric motor  31 , that is, a position between the electric motor  31  and the low-stage side rotary compressing mechanism  4 . 
     As described above, by connecting the gas injection circuit  15  to the closed housing  3  at a position between the electric motor  31  and the low-stage side rotary compressing mechanism  4 , the refrigerant gas injected into the closed housing  3  circulates upward around the electric motor  31 , so that the electric motor  31  is cooled by the refrigerant gas. 
     Accordingly, the motor efficiency is increased, and the performance of the multistage compressor  2  can be improved. 
     Seventh Embodiment 
     Referring now to  FIG. 10 , a seventh embodiment of the present invention will be described. 
     This embodiment is different from the first and second embodiments in the configuration of the oil discharge hole  65 . Other points are the same as the first and second embodiments, and hence description thereof will be omitted. 
     In this embodiment, as shown in  FIG. 10 , an oil discharge guide  68  for guiding discharged oil into an oil trap in the closed housing  3  is provided at the oil discharge hole  65  for discharging lubricating oil after having lubricated the high-stage side scroll compressing mechanism  5  into the closed housing  3 . 
     As described above, by providing the oil discharge guide  68 , refrigerant gas injected into the closed housing  3  and the lubricating oil discharged from the oil discharge hole  65  into the closed housing  3  after having lubricated the compressing mechanism  5  are separated from each other, and the lubricating oil can be prevented from whirling upward by the injected refrigerant gas. 
     Accordingly, unnecessary discharge of oil discharged out of the compressor  2  due to the lubricating oil  37  mixed into the refrigerant gas can be prevented, and lowering of volumetric efficiency of the high-stage side compressing mechanism  5  by excessive mixing of the lubricating oil into the intermediate pressure refrigerant gas can be prevented, so that the performance of the multistage compressor  2  is improved. 
     The oil discharge guide may be provided also in the case in which the oil discharge hole is provided in the low-stage side rotary compressing mechanism  4 . 
     The present invention is not limited to the embodiments shown above, and may be modified without departing the scope of the present invention. 
     For example, the system of the CO2 cycle may be of a system in which an internal heat exchanger is provided and the intermediate pressure refrigerant gas extracted from the internal heat exchanger is injected instead of the injection system using the vapor-liquid separator.