Patent Publication Number: US-8109116-B2

Title: Dual compressor air conditioning system with oil level regulation

Description:
TECHNICAL FIELD 
     This invention relates to a refrigerating air conditioner for use in the air conditioners or the refrigerating machines and including two or more hermetic vessels for containing a compression mechanism therein, and more particularly to an oil equalizing mechanism between the hermetic vessels. 
     BACKGROUND ART 
     Some of the refrigerating air conditioners for use in air conditioners or refrigerating machines comprise, in order to improve the COP (Coefficient of Performance), a main compressor for compressing a refrigerant and an expander including an expansion mechanism for expanding the refrigerant and a sub-compression mechanism for converting the expansion energy in the expansion mechanism into a mechanical energy. In such the refrigerating air conditioner, in order to prevent the decrease in the reliability of the main compressor and the expander due to the sticking or abnormal wear of the machine parts, the oil levels in the main compressor and the expander must be regulated so that shortage of the lubricating oil does not occur. 
     Therefore, in the conventional refrigerating air conditioner, the pressure within the hermetic vessel of the main compressor is arranged to be maintained at the suction pressure, the suction pipe to the main compression mechanism is disposed within the hermetic vessel, its opening portion is positioned above the oil level of the lubricating oil maintained in the hermetic vessel, and an oil recovery hole is provided below the opening portion and at the upper limit position of the adequate oil level within the hermetic vessel of the main compressor (see Patent Document 1, for example). 
     Also proposed is a refrigerating air conditioner having a first compressor and a second compressor and an equalizing oil pipe communicating the bottom portion of the first compressor with the bottom portion of the second compressor is provided (see Patent Document 2 and Patent Document 3, for example). 
     Patent Document 1: Japanese Patent Laid-Open No. 2004-325019 (page 8, FIGS. 8 and 9). 
     Patent Document 2: Japanese Patent Laid-Open No. 7-103594 (pages 3-4, FIG. 1). 
     Patent Document 3: Japanese Patent Laid-Open No. 6-109337, (page 3, FIG. 1) 
     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, in the refrigerating air conditioner disclosed in Japanese Patent Laid-Open No. 2004-325019, the suction pipe to the main compressor mechanism must be provided within the hermetic vessel for the main compressor, and the position of this suction pipe is also limited. 
     Also, in the refrigerating air conditioner disclosed in Japanese Patent Laid-Open No. 7-103594 and Japanese Patent Laid-Open No. 6-109337, it is a problem that the two compressors must be located at the same level in order to regulate the oil level of the lubricating oil. 
     The present invention has been made to solve the above-discussed problems and has as its object the provision of a refrigerating air conditioner that has no limitation on the structure of the main compressor mechanism, and that the lubricating oil levels within the first hermetic vessel and the second hermetic vessel can be regulated without the need for adjusting the installation levels of the first hermetic vessel containing the main compressor mechanism and the second hermetic vessel containing the sub-compressor mechanism. 
     Measure for Solving the Problems 
     With the above object in view, the present invention resides in a refrigeration air conditioner comprising a main compression mechanism for compressing a refrigerant; a gas cooler or a heat radiator for cooling the compressed refrigerant; an expansion mechanism for expanding the refrigerant flowing out from said gas cooler to recover power; a sub-compression mechanism disposed on discharge side or suction side of said main compression mechanism for compressing the refrigerant by the power recovered by said expansion mechanism; an evaporator for evaporating the refrigerant expanded at said expansion mechanism; a first hermetic vessel having contained therein said main compression mechanism and a lubricant oil and having an atmosphere at a suction pressure; a second hermetic vessel having contained therein said expansion mechanism, said sub-compression mechanism and said lubricant oil; a first equalizer pipe connecting a bottom portion of said first hermetic vessel and a bottom portion of said second hermetic vessel; a second equalizer pipe connecting a side of said second hermetic vessel at a position higher than the requisite minimum oil level and a suction side of said main compression mechanism; wherein a space within said second hermetic vessel is isolated from said expansion mechanism and said sub-compression mechanism; and a pressure within said second hermetic vessel is independent from the pressure within said expansion mechanism and the pressure within said sub-compression mechanism. 
     The refrigeration air conditioner of the present invention may comprises a main compression mechanism for compressing a refrigerant; a sub-compression mechanism disposed on discharge side or suction side of said main compression mechanism for compressing a refrigerant; a gas cooler for cooling the compressed refrigerant; an expansion valve for expanding the refrigerant flowing out from said gas cooler; an evaporator for evaporating the refrigerant expanded at said expansion valve; a first hermetic vessel having contained therein said main compression mechanism and a lubricant oil and having an atmosphere at a suction pressure; a second hermetic vessel having contained therein said sub-compression mechanism and said lubricant oil; a first equalizer pipe connecting a bottom portion of said first hermetic vessel and a bottom portion of said second hermetic vessel; a second equalizer pipe connecting a side of said second hermetic vessel at a position higher than the requisite minimum oil level and a suction side of said main compression mechanism; wherein a space within said second hermetic vessel is isolated from said sub-compression mechanism; and a pressure within said second hermetic vessel is independent from the pressure within said sub-compression mechanism. 
     According to the present invention, a refrigerating air conditioner is provided that has no limitation on the structure of the main compressor mechanism, and that the lubricating oil levels within the first hermetic vessel and the second hermetic vessel can be regulated without the need for adjusting the installation levels of the first hermetic vessel containing the main compressor mechanism and the second hermetic vessel containing the sub-compressor mechanism. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating the construction of the refrigerating air conditioner according to the embodiment 1 of the present invention. 
         FIG. 2  is a longitudinal sectional view illustrating the structure of the expander according to the embodiment 1 of the present invention. 
         FIG. 3  is a cross sectional view illustrating the expansion mechanism of the expander according to the embodiment 1 of the present invention. 
         FIG. 4(   a ) is a plan view of a second scroll and  FIG. 4(   b ) is a plan view of a second orbiting scroll of a sub-compression mechanism of the expander according to the embodiment 1 of the present invention. 
         FIG. 5  is a cross sectional view for explaining the contact seal function of the generally conventional contact seal. 
         FIG. 6  is a block diagram illustrating the construction of the refrigerating air conditioner according to the embodiment 2 of the present invention. 
         FIG. 7  is a block diagram illustrating the construction of the refrigerating air conditioner according to the embodiment 3 of the present invention. 
         FIG. 8  is a block diagram illustrating the construction of the refrigerating air conditioner according to the embodiment 4 of the present invention. 
     
    
    
     PREFERRED EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     Embodiment 1 
       FIG. 1  is a block diagram illustrating the construction of the refrigerating air conditioner according to the embodiment 1 of the present invention. The arrows in the figure show the direction of flow of the refrigerant. In the figure, the same reference numerals designate the identical or corresponding components and this applies to the entire specification. The embodiments disclosed in this specification are only illustrative and they are not limited thereto. It is assumed in the embodiment 1 of this invention that a refrigerant which reaches the super critical sate at the high pressure side, such as carbon dioxide, is used. 
     In  FIG. 1 , an expander  1  comprises an expansion mechanism  2  for expanding the refrigerant and recovering the power and a sub-compression mechanism  3  driven by a power recovered by the expansion mechanism  2  and compressing the refrigerant, the expansion mechanism  2  and the sub-compressor mechanism  3  being contained as an integral structure within the second hermetic vessel  4  in which a lubricating oil  9  for lubricating the sliding parts is maintained in the bottom portion. The main compressor  5  comprises a main compressor mechanism  7  driven by an electric motor mechanism  6  and compressing the refrigerant, and the electric motor mechanism  6  and the main-compressor mechanism  7  are housed as an integral structure within the first hermetic vessel  8  in which the lubricating oil  9  for lubricating the sliding parts is maintained in the bottom portion. As illustrated in  FIG. 1 , the height at which the second hermetic vessel  4  is installed is higher than the installation level of the first hermetic vessel  8 . Here, the installation height of the hermetic vessels  4 ,  8  refers to a height position at which the bottom plates of the hermetic vessels  4 ,  8  come into contact with the lubricant oil  9 . 
     The sub-compressor mechanism  3  is disposed on the discharge side of the main-compressor mechanism  7 , and the discharge side of the main compressor mechanism  7  and the suction side of the sub-compressor mechanism  3  are connected to each other by means of a main compressor discharge pipe  18  and a sub-compressor suction pipe  19 . Also, the discharge side of the sub-compressor mechanism  3  and inlet side of a gas cooler or a heat radiator  11  cooling the refrigerator are connected by a sub-compressor discharge pipe  20 . Further, the outlet side of the gas cooler  11  and the suction side of the expansion mechanism  2  are connected to each other by means of a gas cooler outlet pipe  25  and an expander suction pipe  15 , and a second expansion valve  14  is provided in the middle of the expander suction pipe  5 . 
     On the other hand, the outlet side of the gas cooler  11  and the inlet side of the evaporator  12  are connected to each other via a bypass pipe  26  and an evaporator flow pipe  27 , and a first expansion valve  13  is inserted in the bypass pipe  26 . Also, the outlet side of the expansion mechanism  2  and the inlet side of the evaporator  12  are connected to each other via an expander discharge pipe  16  and the evaporator flow pipe  27 . The expander suction pipe  15  and the bypass pipe  26  are connected to the gas cooler flow pipe  25  at a branch point  28 , and the bypass pipe  26  and the expander discharge pipe  16  are connected to the evaporator flow pipe  27  at the junction point  29 . The outlet side of the evaporator  12  and the suction side of the main compressor mechanism  7  are connected to each other via the main compressor suction pipe  17  and the first hermetic vessel  8 . 
     The inner space of the second hermetic vessel  4  is isolated from the expansion mechanism  2  and the sub compressor mechanism  3 , so that the pressure within the second hermetic vessel  4  does not depend upon the pressure within the expansion mechanism  2  or the pressure within the main compressor mechanism  3 . Also, the pressure within the first hermetic vessel  8  is at the suction pressure because the main compressor suction pipe  17  is connected to the first hermetic vessel  8 . 
     The bottom portion of the second hermetic vessel  4  and the bottom portion of the first hermetic vessel  8  are connected to each other by a first equalizer  21 , and the first equalizer pipe  21  is provided with a check valve  23  for preventing the flowing out of the lubricating oil  9  from the second hermetic vessel  4  into the first hermetic vessel  8 . An oil level A shown by a dash line In  FIG. 1  is a minimum and requisite for lubricating the bearings and the sliding portions. Hereinafter this level A or height will be referred to as “minimum requisite oil level”. The second hermetic vessel  4  is connected at the position higher than the position of the minimum requisite oil level A to the main compressor suction pipe  17  which is the suction side of the main compressor mechanism  7  by means of the second hermetic tube  22 . 
     The operation of the refrigerating air conditioner of this embodiment according to the present invention will now be described in conjunction with  FIG. 1 . 
     When the main compressor mechanism  7  is driven by the electric motor  6 , the refrigerant at low temperature and low pressure and in the gaseous state is suctioned from the main compressor suction pipe  17  into the first hermetic vessel  8 . The refrigerant suctioned from the first hermetic vessel  8  to the main compressor mechanism  7  is compressed to an intermediate pressure and discharged from the main compressor discharge pipe  18 . The refrigerant at the intermediate pressure introduced into the sub compressor suction pipe  19  from the main compressor discharge pipe  18  is further compressed by the sub compressor mechanism  3  to high temperature and high pressure and discharged into the sub compressor discharge pipe  20 . The refrigerant discharged into the sub compressor discharge pipe  20  dissipates its heat at the gas cooler  11  and flows into the gas cooler flow pipe  25 . The refrigerant flown out into the gas cooler flow pipe  25  is branched at the branch point  28  into one portion that flows into the expander suction pipe  15  and the other portion that flows into the bypass pipe  26 . 
     The refrigerant introduced into the expander suction pipe  15  is first depressurized at the second expansion valve  14  so that the operation is achieved at the adequate compression ratio at the expansion mechanism  2 , and then introduced into the expansion mechanism  2  via the expander suction pipe  15 . The refrigerant expanded at the expansion mechanism  2  is now in the vapor-liquid two phase state at low temperature and low pressure and discharged into the expander discharge pipe  16 . On the other hand, the refrigerant that flows into the bypass pipe  26  is expanded and depressurized by the first expansion valve  13  in order to regulate the flow rate when the operating conditions of the refrigerating air conditioner is changed. The refrigerant expanded and depressurize at the first expansion valve  13  is joined at the junction point  29  with the refrigerant supplied from the expander discharge pipe  16  and flows into the evaporator  12  via the evaporator flow pipe  27 . The refrigerant introduced into the evaporator  12  is heated and evaporated and then flows back into the first hermetic vessel  8  via the main compressor suction pipe  17 . 
     Here, the pressure at the suction side of the main compressor mechanism  7  and the pressure at the discharge side of the expansion mechanism  2  are referred to as a low pressure, and the pressure at the suction side of the expansion mechanism  2  and the pressure at the discharge side of the sub compressor mechanism  3  are referred to as a high pressure, and the pressure at the discharge side of the main compressor mechanism  7  which is at the suction side of the sub compressor mechanism  3  is referred to as an intermediate pressure. 
     Then the behavior of the lubricating oil  9  within the second hermetic vessel  4  and the first hermetic vessel  8  during the above-discussed operation will now be described in conjunction with  FIG. 1 . In  FIG. 1 , it is assumed that the difference in height between the oil level position of the lubrication oil level in the second equalizing pipe  22  and the second hermetic vessel  4  and the oil level position within the first hermetic vessel  8  discharge is H, then the pressure difference ΔP 1  produced by the level difference H is given by equation (1):
 
ΔP 1 =ρ o gH  (1)
 
     where, ρ o  is density of the lubricating oil  9  and g is the gravitational acceleration. 
     On the other hand, assuming that the flow speed of the vaporized refrigerant within the main compressor suction pipe  17  at the connecting point B between the second equalizer pipe  22  and the main compressor suction pipe  17  is V, then the dynamic pressure ΔP 2  is given by equation (2):
 
Δ P   2 =ρ r   V   2 /2  (2)
 
     where, ρ r  is the density of the vaporized refrigerant. 
     The pressure P b  within the second hermetic vessel  4  is the pressure that does not depend upon the pressure within the expansion mechanism  2  or the pressure within the sub compressor mechanism  3 , and since the second hermetic vessel  4  and the main compressor suction pipe  17  are connected to each other, the pressure P b  within the second hermetic vessel  4  is always lower than the pressure P a  within the first hermetic vessel  8  by ΔP 2 . Therefore, the dynamic pressure ΔP 2  generated by the flow speed V of the vaporized refrigerant is also given by equation (3):
 
Δ P   2   =P   a   −P   b   (3)
 
     When the flow speed V of the gaseous refrigerant in the main compressor suction pipe  17  is large and ΔP 2 &gt;ΔP 1 , the lubricating oil  9  flows from the first hermetic vessel  8  to the second hermetic vessel  4  through the first equalizer pipe  21  against the pressure difference ΔP 1  due to the height difference H between the oil level within the second hermetic vessel  4  and the first hermetic vessel  8 , whereby the oil level within the second hermetic vessel  4  is elevated. When the oil level within the second hermetic vessel  4  is elevated and reaches to the height of the second equalizer pipe  22 , the lubricating oil flows out through the second equalizer pipe  22  into the main compressor suction pipe  17 . The lubricating oil  9  introduced into the main compressor suction pipe  17  is lead into the first hermetic vessel  8 , increasing the oil amount within the first hermetic vessel  8 , whereby the oil level within the respective hermetic vessels  4  and  8  are regulated. 
     Contrary to the above, when the flow speed V of the gaseous refrigerant in the main compressor suction pipe  17  is small and ΔP 2 &lt;ΔP 1 , the lubricating oil  9  tends to flow from the side of the second hermetic vessel  4  into the first hermetic vessel  8 . However, the check valve  23  prevents the lubricating oil  9  from flowing into the first hermetic vessel  8  from the side of the second hermetic vessel  4 , but the oil level within the second hermetic vessel  4  is not lowered and is maintained. 
     Also, even when the second hermetic vessel  4  is installed at a high position and the height difference H between the oil level within the second hermetic vessel  4  and the oil level within the first hermetic vessel  8  is large, the oil levels within the respective hermetic vessels  4  and  8  are regulated by the above-discussed function. 
     As has been described, the refrigerating air conditioner according to the first embodiment of the present invention comprises the first equalizing pipe  21  connecting the first hermetic vessel  8  and the second hermetic vessel  4  at their bottom portions and the second equalizing pipe  22  connecting the second hermetic vessel  4  at a position on the side above the requisite minimum oil level A to the suction side of the main compressor mechanism  7 , the inside of the first hermetic vessel  8  being filled with an atmosphere at the suction pressure and the inside space of the second hermetic vessel  4  being isolated from the expansion mechanism  2  and the sub expansion mechanism  3 , and the pressure in the second hermetic vessel  4  does not depend upon the pressure within the expansion mechanism  2  or the sub compressor mechanism  3 . Therefore, the oil level in the respective hermetic vessels  4  and  8  can be automatically regulated irrespective of the flow speed V of the gaseous refrigerant in the main compressor mechanism  2  and the sub compressor mechanism  3 , or the amount of the height difference H between the oil level in the second hermetic vessel  4  and the first hermetic vessel  8 . Therefore, the decrease of the reliability due to the sticking or abnormal wear of the sliding parts of the main compressor  5  and the expander  1 . 
     Next, a scroll type expander will now be described in terms of its structure and the operation as an example of the expander  1  having the expansion mechanism  2  and the sub compressor mechanism  3  driven by the power recovered by the expansion mechanism  2  and compressing the refrigerant according to the first embodiment of the present invention. 
       FIG. 2  is a longitudinal sectional view illustrating the structure of the scroll type expander according to the embodiment 1 of the present invention. 
     In  FIG. 2 , in the lower portion of the second hermetic vessel  4 , the expansion mechanism  2  is disposed and in the upper portion of the expansion mechanism  2 , the sub compressor mechanism  3  is disposed. The expansion mechanism  2  comprises a first fixed scroll  51  having a scroll wrap  51   c  formed on a base plate  51   a  and a first orbiting scroll  52  having a scroll wrap  52   c  formed on a base plate  52   a , the scroll wrap  51   c  of the first fixed scroll  51  and the scroll wrap  52   c  of the first orbiting scroll  52  are being arranged to mesh with each other. The sub compressor mechanism  3  comprises a second fixed scroll  61  having a scroll wrap  61   c  formed on a base plate  61   a  and a second orbiting scroll  62  having a scroll wrap  62   c  formed on a base plate  62   a , the scroll wrap  61   c  of the second fixed scroll  61  and the scroll wrap  62   c  of the second orbiting scroll  62  are being arranged to mesh with each other. 
     A shaft  78  is rotatably supported at both ends by bearing portions  51   b  and  61   b  each disposed at the center of the first fixed scroll  51  and the second fixed scroll  61 . The first orbiting scroll  52  and the second orbiting scroll  62  are respectively passed through and supported at an eccentric bearing portions  52   b  and  62   b  formed in their centers by a crank portion  78   a  fitted on the shaft  79  so that they achieve orbiting motions. The requisite minimum oil level A is at the lower end of the shaft  78 , which is the minimum oil level of the lubricating oil  9  necessary for lubricating the bearing portion  51   b  and  61   b  as well as the eccentric bearing portions  52   b  and  62   b.    
     Provided on the side wall of the second hermetic vessel  4  and at the outer circumference of the expansion mechanism  2  are the expander suction pipe  15  for sucking the refrigerant and the expander discharge pipe  16  for discharging the expanded refrigerant. On the other hand, on the upper wall of the second hermetic vessel  4  and above the sub compressor mechanism  3 , the sub compressor mechanism suction pipe  19  for sucking the refrigerant is provided, and on the side wall of the second hermetic vessel  4  and at the outer circumference of the sub compressor mechanism  3 , the sub compressor discharge pipe  20  for discharging the compressed refrigerant is provided. 
     Also, on the bottom portion of the second hermetic vessel  4 , the first equalizer pipe  21  is connected for communicating the bottom portion of the first hermetic vessel  8 , and on the side wall of the second hermetic vessel  4  and at the position higher than the requisite minimum oil level A, the second equalizer pipe  22  for the connection to main compressor suction pipe  17 . 
     In the sub compressor mechanism  3 , the spiral teeth  61   c  and  62   c  of the first fixed scroll  61  and the second orbiting scroll  62  has mounted, on their respective tips, tip seals  71  for sealing a sub-compression chamber  3   a  defined between the scroll wrap  61   c  of the second fixed scroll  61  and the scroll wrap  62   c  of the second orbiting scroll  62 . Also, an inner circumferential seal  72   a  is disposed on the surface of the second orbiting scroll  62  facing to the second fixed scroll  61  and at the outer circumference of the eccentric bearing portion  62   b  to function as a seal member for sealing between the second orbiting scroll  62  and the fixed scroll  61 . Further, an outer circumferential seal  73   a  is disposed on the surface of the second fixed scroll  61  facing to the second orbiting scroll  61  and at the outer circumference of the scroll wrap  61   c  to function as a seal member for sealing between the second orbiting scroll  62  and the fixed scroll  61 . 
     On the other hand, in the expansion mechanism  2 , similarly to the sub-compressor mechanism  3 , an inner circumferential seal  72   b  is disposed on the surface of the first orbiting scroll  52  facing to the first orbiting scroll  52  and at the outer circumference of the eccentric bearing portion  52   b  to function as a seal member for sealing between the first orbiting scroll  52  and the first fixed scroll  51 . Further, an outer circumferential seal  73   b  is disposed on the surface of the first fixed scroll  51  facing to the first fixed scroll  51  and at the outer circumference of the scroll wrap  51   c  to function as a seal member for sealing between the first orbiting scroll  52  and the first fixed scroll  51 . Further, the outer circumference portion of the base plate  51   a  of the first fixed scroll  51  and the outer circumference portion of the base plate  52   a  of the first fixed scroll  52  are arranged to contact to each other. 
     The first orbiting scroll  52  and the second orbiting scroll  62  are joined together by a connecting element such as a pin and prevented from the rotation by an Oldham&#39;s ring  77  disposed in the sub-compressor mechanism  3 . Also, in order to cancel out the centrifugal forces generated by the rotation of the orbiting scrolls  52 ,  62 , balance weights  79   a ,  79   b  are attached to both ends of the shaft  78 . The first orbiting scroll  52  and the second orbiting scroll  62  may by made in one piece member with the base plates  52   a ,  62   a  arranged as a common member. 
     In the expansion mechanism  2 , the power is generated by the expansion of the high pressure refrigerant suctioned from the expander suction pipe  15  within the expansion chamber  2   a  defined by the scroll wrap  51   c  of the first fixed scroll  51  and the scroll wrap  52   c  of the first orbiting scroll  52 . The refrigerant expanded and decompressed within the expansion chamber  2   a  is discharged from the expander discharge pipe  16  to the exterior of the second hermetic vessel  4 . The power generated at the expansion mechanism  2  compresses and pressure-raises the refrigerant introduced from the sub-compressor suction pipe  19  within the sub-compression chamber  3   a  of the sub-compressor mechanism  3 . The refrigerant compressed and pressure-raised within the sub-compression chamber  3   a  is discharged from the sub-compressor machine discharge pipe  20  to the exterior of the second hermetic vessel  4 . 
     The expansion mechanism  2  achieves the expansion step from the high pressure to the low pressure, and the sub-compression mechanism  3  achieves the compression step from the intermediate pressure to the high pressure. Therefore, in the orbiting scrolls  52  and  62 , a high pressure acts on both of the expansion chamber  2   a  at the center and the sub-compression chamber  3   a  at the center, a low pressure acts on the expansion chamber  2   a , and an intermediate pressure acts on the outer circumferential  3   a . The sub-compression chamber  3   a  and the space within the second hermetic vessel  4  are isolated from by an inner circumferential seal  72   a  and an outer circumferential seal  73   a , and the expansion chamber  2   a  and the space defined in the second hermetic vessel  4 . 
       FIG. 3  is a cross sectional view taken along the line C-C of the expansion mechanism of the expander, shown in  FIG. 2 , according to the embodiment 1 of the present invention. 
     At the inner end portion of the scroll wrap  52   c  of the first orbiting scroll  52 , a thick portion  52   d  is provided, and the thick portion  52   d  has formed therein an eccentric bearing portion  52   b  extending therethrough for allowing a crank portion  78  to be inserted therein. Formed on the thick portion  52   d  of the first orbiting scroll  52  and in the outer circumference of the eccentric bearing portion  52   b  is an inner seal groove  52   g , and an inner seal  72   b  is inserted within the inner seal groove  52   g . Also, formed on the base plate  51   a  of the first fixed scroll  51  and in the outer circumference of the scroll wrap  51   c  is an outer seal groove  51   g , and an outer seal  73   b  is inserted therein. 
     The base plate  51   a  of the first fixed scroll  51  is provided with a suction port  51   d  for sucking the refrigerant and a discharge port  51   e  for discharging the refrigerant. The suction port  51   d  has an elongated hole-shape for maintaining the opening area and is connected to the expander suction pipe  15 . Also, a notch portion  52   e  is provided in the thick portion  52   d  in order to decrease the area that closes the suction port  51   d  during the orbiting motion. The discharge port  51   e  is disposed at the position in which it does not interfere with the outer end portion of the scroll wrap  52   c  of the first orbiting scroll  52  and connected to the expander discharge pipe  16 . 
       FIG. 4(   a ) is a plan view of the second scroll and  FIG. 4(   b ) is a plan view of the second orbiting scroll of the sub-compressor mechanism of the expander according to the embodiment 1 of the present invention. 
     As shown in  FIGS. 4(   a ) and  4 ( b ), the scroll wraps  61   c ,  62   c  of the sub-compressor mechanism  3  are wrapped in the same scroll direction as the expansion mechanism  2 , so that, when the second orbiting scroll  62  and the first orbiting scroll  52  are combined, back-to-back, and make an orbiting motion together, they achieve compression on one side and expansion on the other side. 
     At the inner end portion of the scroll wrap  62   c  of the second orbiting scroll  62 , a thick portion  62   d  is provided and, similarly to the first orbiting scroll  52  of the expansion mechanism  2 , an eccentric bearing portion  62   b  into which a crank portion  78   a  is inserted is formed to extend therethrough. Also, the base plate  61   a  of the second fixed scroll  61  is provided with a suction port  61   d  for sucking the refrigerant and a discharge port  61   e  for discharging the refrigerant. The discharge port  61   e  has an elongated hole shape for maintaining the opening area and is connected to the sub-compressor suction pipe  20 . Also, a notch portion  62   e  is provided in the thick portion  62   d  in order to decrease the area that closes the discharge port  61   e  during the orbiting motion. The suction port  61   d  is disposed at the position in which it does not interfere with the outer end portion of the scroll wrap  62   c  of the second orbiting scroll  62  and connected to the sub-compressor suction pipe  19 . 
     In the tip end surfaces of the scroll wraps  61   c ,  62   c , tip seal grooves  61   f ,  62   f  are formed for receiving therein tip seals  71 . In the thick portion  62   d  of the second orbiting scroll  62  and in the outer circumference of the eccentric bearing portion  62   b , an inner circumference groove  62   g  for inserting an inner seal  72   a  is formed. Also, on the base plate  61   a  of the second fixed scroll  61  and in the outer circumference of the scroll wrap  61   c , an outer seal groove  61   g  for inserting the outer seal  73   a.    
       FIG. 5  is a cross sectional view for explaining the contact seal function of the tip seal. 
     In  FIG. 5 , the tip seal  71  is pressed from the left and the above as shown by the arrows, which are high pressure side, according to the pressure difference between the sub-compressor chamber  3   a  at both sides of the partition. Therefore, the tip seal  71  is urged within the tip seal groove  62  for the tip seal  71  against the right hand wall and the above base plate  61   a  to provide the contact seal between the second orbiting scroll  62  and the fixed scroll  61 . The contact seal functions of the inner seal  72   a  and  72   b  and the outer seal  73   a  and  73   b  are similar to contact seal function of the tip seal  71 . 
     In the expander of the scroll type as above described, the inner seals  72   a ,  72   b  are disposed on the inner circumference portion of the first orbiting scroll  52  and the inner circumference portion of the second orbiting scroll  62 , and the outer seals  73   a ,  73   b  are disposed on the outer circumference portion of the first fixed scroll  51  and the outer circumference portion of the second fixed scroll  61 . Therefore, the space within the second hermetic vessel  4  is isolated from the expansion mechanism  2  and the sub-compressor mechanism  3 , so that the pressure within the second hermetic vessel  4  does not depend upon the pressure within the expansion mechanism  2  and the pressure within the sub-compressor mechanism  3 , whereby the oil level can be stably regulated 
     In this embodiment, the inner seals  72   a ,  72   b  which are seal members are disposed on the inner circumference portion of the first orbiting scroll  52  and the inner circumference portion of the second orbiting scroll  62 , but the inner seals  72   a ,  72   b  which are seal members may be disposed on the inner circumference portion of the first fixed scroll  51  and the inner circumference portion of the second fixed scroll  52 . Also, in this embodiment, the outer seals  73   a ,  73   b  which are seal members are disposed on the outer circumference portion of the first fixed scroll  51  and the outer circumference portion of the second fixed scroll  61 , but the outer seals  73   a ,  73   b  which are seal members may be disposed on the outer circumference portion of the first orbiting scroll  52  and the outer circumference portion of the second orbiting scroll  62 . 
     Further, in this embodiment, the scroll type expander is described as the expander  1  used in the refrigerating air conditioner, but any type of expander such as multi-vane type or rotary type machine may equally be used as long as the pressure within the second hermetic vessel  4  does not depend upon the pressure within the expansion mechanism  2  and the pressure within the sub-compressor mechanism  3 . 
     Also, in this embodiment, the centrifugal pump  76  is described as the pump for feeding the lubricating oil  9  into the bearing and the sliding portion, but any type of pump such as a volume-type pump including the troquoid pump may equally be used. When a volume-type pump is used, the level of the suction port of the pump is the requisite minimum oil level. 
     Embodiment 2 
     In embodiment 1, the description has been made as to the refrigerating air conditioner in which the installation level of the second hermetic vessel  4  is higher than the installation level of the first hermetic vessel  8 . In embodiment 2, the description will be made as to a refrigerating air conditioner in which the installation level of the second hermetic vessel  4  is lower than the installation level of the first hermetic vessel B. 
       FIG. 6  is a block diagram illustrating the construction of the refrigerating air conditioner according to the embodiment 2 of the present invention 
     The refrigerating air conditioner of embodiment 2 of the present invention is different from the refrigerating air conditioner of the embodiment 1 in that, as shown in  FIG. 6 , the installation level of the second hermetic vessel  4  is lower than the installation level of the first hermetic vessel  8 , and an electromagnetic valve  24  instead of the check valve  23  is disposed in the first equalizer pipe  21 . In other respects, the structure is the same as that of the refrigerating air conditioner of embodiment 1. 
     The behavior of the lubricating oil  9  within the second hermetic vessel  4  and the first hermetic vessel  8  of embodiment 2 will now be described in terms of FIG.  6 . In  FIG. 6 , since the installation level of the second hermetic vessel  4  is lower than the installation level of the first hermetic vessel  8 , the pressure difference ΔP 1  generated due to the height difference H between the oil level in the second hermetic vessel  4  and the oil level in the first hermetic vessel  8  causes the oil level within the first hermetic vessel  8  to be lowered. Also, the pressure difference ΔP 2  given by equation (2) generates a force that lowers the oil level within the first hermetic vessel  8 , so that the lubricating oil  9  flows out through the second equalizer pipe  23  into the main compressor suction pipe  17  irrespective of the flow speed V of the gas refrigerant in the main compressor suction pipe  17 . 
     The lubricating oil  9  flows into the main compressor suction pipe  17  is introduced into the first hermetic vessel  8  to increase the oil amount within the first hermetic vessel  8 , so that the oil level in each of the hermetic vessels  4  and  8  is regulated. Therefore, the check valve  23  is not necessary in the first equalizer pipe  21 . When the refrigeration air conditioner is not operated, it is necessary to prevent the lubricating oil  9  within the first hermetic vessel  8  from moving into the second hermetic vessel  4  via the first equalizer pipe  21  due to the level difference H. Thus, the arrangement is such that the electromagnetic valve  24  disposed in the first equalizer pipe  21  is closed when the refrigerating air conditioner is not operated. The electromagnetic valve  24  is open when the refrigerating air conditioner is operated. 
     As has been described, the refrigerating air conditioner according to embodiment 2 of the present invention comprises the first equalizer pipe  21  connecting the bottom portion of the first hermetic vessel  8  and the bottom portion of the second hermetic vessel  4 , and the second equalizer pipe  22  connecting the side of the second hermetic vessel  4  at a position higher than the requisite minimum oil level A and the suction side of the main compression mechanism  7 , the inside of the first hermetic vessel  8  being filled with an atmosphere at the suction pressure, the space within the second hermetic vessel  4  being isolated from the sub-compression mechanism  3 , and the pressure within the second hermetic vessel  4  being independent from the pressure within the sub-compression mechanism  3 . Therefore, the oil level in the respective hermetic vessels  4  and  8  can be automatically regulated irrespective of the flow speed V of the gaseous refrigerant in the main compressor mechanism suction pipe  17 , or the amount of the level difference H between the oil level in the second hermetic vessel  4  and the oil level in the first hermetic vessel  8 . Therefore, the decrease of the reliability due to the sticking or abnormal wear of the sliding parts of the main compressor  5  and the expander  1 . 
     In embodiment 2 of the present invention, the description has been made as to the refrigerating air conditioner in which the installation level of the second hermetic vessel  4  is lower than the installation level of the first hermetic vessel  8 , but the same applies to the refrigerating air conditioner in which the installation level of the second hermetic vessel  4  is the same as the installation level of the first hermetic vessel  8 . When the installation level of the second hermetic vessel  4  is the same as the installation level of the first hermetic vessel  8 , the electromagnetic valve  24  is not necessary. 
     As apparent from embodiment 1 and embodiment 2, the refrigeration air conditioner of the present invention comprises the first equalizer pipe  21  connecting the bottom portion of the first hermetic vessel  8  and the bottom portion of the second hermetic vessel  4 , and the second equalizer pipe  22  connecting the side of the second hermetic vessel  4  at the position higher than the requisite minimum oil level A and the suction side of the main compression mechanism  7 , and the space within the second hermetic vessel  4  is isolated from the expansion mechanism  2  and the sub-compression mechanism, and the pressure within the second hermetic vessel  4  is independent from the pressure within the expansion mechanism  2  and the pressure within the sub-compression mechanism  3 . Therefore, the oil level in the respective hermetic vessels  4  and  8  can be automatically regulated irrespective of the installation level of each of the first hermetic vessel  8  and the second hermetic vessel  4 . Therefore, the decrease of the reliability due to the sticking or abnormal wear of the sliding parts of the main compressor  5  and the expander  1 . 
     Embodiment 3 
     In the embodiment 1 and embodiment 2, the refrigerant air conditioner having the sub-compression mechanism  3  disposed on the discharge side of the main compression mechanism  7  is described. In embodiment 3, a refrigerating air conditioner in which the sub-compression mechanism  3  is disposed on the suction side of the main compression mechanism  7 . 
       FIG. 7  is a block diagram illustrating the construction of the refrigerating air conditioner according to the embodiment 3 of the present invention. 
     In  FIG. 7 , the sub-compression mechanism  3  is disposed on the suction side of the main compression mechanism  7 , and the discharge side of the sub-compression mechanism  3  and the suction side of the main compression mechanism  7  are connected to each other via the sub-compression discharge pipe  20 , the main compressor suction pipe  17  and the first hermetic vessel  8 . Also, the discharge side of the main compression mechanism  7  and the inlet side of the gas cooler  11  are connected to each other via the main compressor discharge pipe  18 . On the other hand, the outlet side of the evaporator  12  and the suction side of the sub-compression mechanism  3  are connected via the sub-compressor suction pipe  19 . As seen from  FIG. 7 , the installation level of the second hermetic vessel  4  is lower than the installation level of the first hermetic vessel  8 . In other respects, the arrangement is the same as that of the refrigerating air conditioner of embodiment 2. 
     The operation of the refrigerating air conditioner according to embodiment 3 of the present invention will now be described in conjunction with  FIG. 7 . 
     When the main compression mechanism  7  is driven by the electric motor mechanism  6 , the refrigerant in the gas state pressurized to the intermediate pressure in the sub-compression mechanism flows from the main compressor suction pipe  17  into the first hermetic vessel  8 , and it is suctioned by the main compression mechanism  7  when the first hermetic vessel  8  reaches to the intermediate pressure atmosphere. The refrigerant in the gas state that is further compressed in the main compression mechanism  7  into the high temperature, high pressure refrigerant, is discharged into the main compressor discharge pipe  18 . The refrigerant in the gas state discharged into the main compression discharge pipe  18  flows out to the gas cooler flow pipe  25  after it dissipates heat in the gas cooler  11 . One portion of the refrigerant flowed into the gas cooler flow pipe  25  is lead to the expander suction pipe  15  at the junction  28 , the remaining portion being lead to the bypass pipe  26 . 
     The refrigerant lead into the expander suction pipe  15  is decompressed by the second expansion valve  14  so that it is worked in the expansion mechanism  2  at a proper compression ratio and then lead from the expander suction pipe  15  into the expansion mechanism  2 , where it is expanded. The refrigerant expanded in the expansion mechanism  2  becomes into the low temperature and low pressure liquid-gas phase state and discharged into the expander discharge pipe  16 . On the other hand, the refrigerant lead into the bypass pipe  26  is expanded and decompressed by the first expansion valve  13  so the flow rate may be regulated when the operating conditions of the refrigerating air conditioner are changed. The refrigerant expanded and decompressed at the first expansion valve  13  joins with the refrigerant discharged into the expander discharge pipe  16  at the junction point  29  and introduced into the evaporator  12  via the evaporator inlet pipe  27 . The refrigerant introduced into the evaporator  12  is suctioned into the sub-compression mechanism  3  via the sub-compressor suction pipe  19  after it is heated and evaporated. The refrigerant suctioned into the sub-compression mechanism  3  is compressed to the intermediate pressure and discharged into the sub-compressor discharge pipe  20 . The refrigerant discharged into the sub-compressor discharge pipe  20  flows through the main compressor suction pipe  17 , flows into the first hermetic vessel  8  and again suctioned into the main compression mechanism  7 . 
     Here, the pressure on the suction side of the sub-compression mechanism  3  and the pressure on the discharge side of the expansion mechanism  2  are referred to as low pressures, the pressure on the suction side of the expansion mechanism  2  and the pressure on the discharge side of the main compression mechanism  7  are referred to as high pressures, and the pressure on the discharge side of the sub-compression mechanism  3  which is the pressure on the suction side of the main compression mechanism  7  are referred to as intermediate pressures. 
     Next, the behavior of the lubricating oil  9  within the second hermetic vessel  4  and within the first hermetic vessel  8  during the above operation will now be described in conjunction with  FIG. 7 . In  FIG. 7 , the pressure P a  in the first hermetic vessel  8  is an intermediate pressure and, since the pressure P b  in the second hermetic vessel  4  is independent of the pressure in the expansion mechanism  2  and the pressure in the sub-compression mechanism  3 , the pressure difference ΔP 2   2  is given by the equation (2) in the similar manner as to embodiments 1 and 2. 
     Therefore, as in the refrigerating air conditioner according to embodiment 2, the lubricating oil  9  flows through the second equalizer pipe  22  to flows out from the second hermetic vessel  4  into the main compressor suction pipe  17 . The lubricating oil  9  flows into the main compressor suction pipe  17  is lead into the first hermetic vessel  8  to increase the oil amount within the first hermetic vessel  8 , whereby the oil levels in the respective hermetic vessels are regulated. 
     As has been described, the refrigerating air conditioner according to embodiment 3 of the present invention comprises the first equalizer pipe  21  connecting the bottom portion of the first hermetic vessel  8  and the bottom portion of the second hermetic vessel  4 , and the second equalizer pipe  22  connecting the side of the second hermetic vessel  4  at a position higher than the requisite minimum oil level A and the suction side of the main compression mechanism  7 , the inside of the first hermetic vessel  8  being filled with an atmosphere at the suction pressure, the space within the second hermetic vessel  4  being isolated from the sub-compression mechanism  3 , and the pressure within the second hermetic vessel  4  being independent from the pressure within the sub-compression mechanism  3 . Therefore, the oil level in the respective hermetic vessels  4  and  8  can be automatically regulated irrespective of the flow speed V of the gaseous refrigerant in the main compressor mechanism suction pipe  17 , or the amount of the level difference H between the oil level in the second hermetic vessel  4  and the oil level in the first hermetic vessel  8 . Therefore, the decrease of the reliability due to the sticking or abnormal wear of the sliding parts of the main compressor  5  and the expander  1 . 
     The description has been made as to the refrigerating air conditioner in which the installation level of the second hermetic vessel  4  is lower than the installation level of the first hermetic vessel  8 , but even when the installation level of the second hermetic vessel  4  is the same as the installation level of the first hermetic vessel  8 , the behavior of the lubricating oil is the same and similar advantageous results can be obtained. When the installation level of the second hermetic vessel  4  is higher than the installation level of the first hermetic vessel  8 , the lubricating oil  9  operates in a manner similar to that of embodiment 1 and similar advantageous results discussed in conjunction with the refrigerating air conditioner according to embodiment 1 can be obtained. 
     Accordingly, as apparent from embodiment 1 to embodiment 3, the refrigeration air conditioner of the present invention comprises the first equalizer pipe  21  connecting the bottom portion of the first hermetic vessel  8  and the bottom portion of the second hermetic vessel  4 , and the second equalizer pipe  22  connecting the side of the second hermetic vessel  4  at the position higher than the requisite minimum oil level A and the suction side of the main compression mechanism  7 , the inside of the first hermetic vessel  8  being filled with an atmosphere at the suction pressure, the space within the second hermetic vessel  4  is isolated from the expansion mechanism  2  and the sub-compression mechanism, and the pressure within the second hermetic vessel  4  is independent from the pressure within the expansion mechanism  2  and the pressure within the sub-compression mechanism  3 . Therefore, the oil level in the respective hermetic vessels  4  and  8  can be automatically regulated irrespective of the installation level of each of the first hermetic vessel  8  and the second hermetic vessel  4 . Therefore, the decrease of the reliability due to the sticking or abnormal wear of the sliding parts of the main compressor  5  and the expander  1 . 
     Embodiment 4 
     In the embodiment 1 to embodiment 3, the refrigerant air conditioner having the expansion mechanism  2  and the sub-compression mechanism  3  disposed within hermetic vessel  4 . In embodiment 4, a refrigerating air conditioner in which the sub-compression mechanism  3  driven by the electric motor mechanism  6  is disposed within the second hermetic vessel  4 . 
       FIG. 8  is a block diagram illustrating the construction of the refrigerating air conditioner according to the embodiment 4 of the present invention. 
     In  FIG. 8 , a sub-compressor  81  comprises the sub-compression mechanism  3  driven by an electric motor mechanism  82  to compress the refrigerant, and the electric motor mechanism  82  and the sub-compression mechanism  3  are housed as one unit within the second hermetic vessel  4  in which the lubricating oil  9  is maintained at the bottom portion thereof. The main compressor  5  comprises the main compression mechanism  7  driven by the electric motor mechanism  6  to compress the refrigerant, and the electric motor mechanism  6  and the main compression mechanism  7  are housed as one unit within the first hermetic vessel  8  in which the lubricating oil  9  is maintained at the bottom portion thereof. As shown in  FIG. 8 , the installation level of the second hermetic vessel  4  is higher than the installation level of the first hermetic vessel  8 . 
     The sub compression mechanism  3  is disposed on the discharge side of the main compression mechanism  7 , and the discharge side of the main compression mechanism  7  and the suction side of the sub-compression mechanism  3  are connected to each other via the main compressor discharge pipe  18  and the sub-compressor suction pipe  19 . Also, the discharge side of the sub-compressor  3  and the inlet side of the gas cooler  11  for cooling the refrigerant are connected to each other via the sub-compressor discharge pipe  20 . Further, the outlet side of the gas cooler  11  and the inlet side of the evaporator  12  are connected to each other via the gas cooler flow pipe  25 . The first expansion valve  13  for expanding the refrigerant is disposed in the gas cooler flow pipe  25 . The outlet side of the evaporator  12  and the suction side of the main compression mechanism  7  are connected to each other via the main compressor suction pipe  17  and the first hermetic vessel  8 . 
     Here, since the space within the second hermetic vessel  4  is isolated from the sub-compression mechanism  3 , the pressure within the second hermetic vessel  4  is not dependent upon the pressure within the sub-compression mechanism  3 . Also, the pressure within the first hermetic vessel  8  is the suction pressure because the main compressor suction pipe  17  is connected to the first hermetic vessel  8 . 
     The bottom portion of the second hermetic vessel  4  and the bottom portion of the first hermetic vessel  8  are connected to each other via the first equalizer pipe  21 , and the first equalizer pipe  21  is provided therein with the check valve  23  for preventing the flow of the lubricating oil  9  from the second hermetic vessel  4  to the first hermetic vessel  8 . Also, the side of the second hermetic vessel  4  at the position higher than the requisite minimum oil level A and the main compressor suction pipe  17  which is the suction side of the main compression mechanism  7  are connected to each other via the second equalizer pipe  22 . 
     The operation of the refrigerating air conditioner according to embodiment 4 of the present invention will now be described in conjunction with  FIG. 8 . 
     When the main compressor mechanism  7  is driven by the electric motor mechanism  6 , the refrigerant at low temperature and low pressure and in the gaseous state is suctioned from the main compressor suction pipe  17  into the first hermetic vessel  8 . The refrigerant suctioned from the first hermetic vessel  8  to the main compressor mechanism  7  is compressed to an intermediate pressure and discharged through the main compressor discharge pipe  18 . The refrigerant at the intermediate pressure introduced into the sub compressor suction pipe  19  from the main compressor discharge pipe  18  is further compressed by the sub compressor mechanism  3  to be high temperature and high pressure and discharged into the sub compressor discharge pipe  20 . The refrigerant discharged into the sub compressor discharge pipe  20  dissipates its heat at the gas cooler  11  and flows into the gas cooler flow pipe  25 . The refrigerant flown out into the gas cooler flow pipe  25  is expanded at the first expansion valve  13  to become into the vapor-liquid two phase state at low temperature and low pressure state and flows into the evaporator  12 . The refrigerant introduced into the evaporator  12  is heated and evaporated and then flows back into the first hermetic vessel  8  via the main compressor suction pipe  17 . 
     Here, the pressure at the suction side of the main compressor mechanism  7  is referred to as a low pressure, the pressure at the discharge side of the sub compression mechanism  3  is referred to as a high pressure, and the pressure at the discharge side of the main compression mechanism  7  which is at the suction side of the sub compression mechanism  3  is referred to as an intermediate pressure. 
     The behavior of the lubricating oil  9  within the second hermetic vessel  4  and the first hermetic vessel  8  in the above-described operation is similar to that described in relation to the refrigerating air conditioner of embodiment 1 and the oil level in each of the hermetic vessels  4  and  8  is automatically regulated. 
     As has been described, the refrigeration air conditioner according to embodiment 4 of the present invention comprises the first equalizer pipe  21  connecting the bottom portion of the first hermetic vessel  8  and the bottom portion of the second hermetic vessel  4 , and the second equalizer pipe  22  connecting the side of the second hermetic vessel  4  at the position higher than the requisite minimum oil level A and the suction side of the main compression mechanism  7 , the inside of the first hermetic vessel  8  being filled with an atmosphere at the suction pressure, the space within the second hermetic vessel  4  is isolated from the sub-compression mechanism, and the pressure within the second hermetic vessel  4  is not dependent upon the pressure within the sub-compression mechanism  3 . Therefore, the oil level in the respective hermetic vessels  4  and  8  can be automatically regulated irrespective of the flow speed V of the gaseous refrigerant within the main compressor suction pipe  17 , the level difference H between the oil level within the second hermetic vessel  4  and the oil level within the first hermetic vessel  8 . Therefore, the decrease of the reliability due to the sticking or abnormal wear of the sliding parts of the main compressor  5  and the expander  1 . 
     In embodiment 4, the description has been made as to the refrigerating air conditioner in which the installation level of the second hermetic vessel  4  is higher than the installation level of the first hermetic vessel  8 , but even when the installation level of the second hermetic vessel  4  is lower than the installation level of the first hermetic vessel  8 , or even when the installation level of the second hermetic vessel  4  is the same as the installation level of the first hermetic vessel  8 , advantageous results similar to those discussed above can be obtained. When the installation level of the second hermetic vessel  4  is lower than the installation level of the first hermetic vessel  8 , or the installation level of the second hermetic vessel  4  is the same as the installation level of the first hermetic vessel  8 , the check valve  23  is not necessary. When the installation level of the second hermetic vessel  4  is lower than the installation level of the first hermetic vessel  8 , the electromagnetic valve  24 , which closes when the refrigerating air conditioner is not operated, may be provided in the first equalizer pipe  21 , as in the case of embodiment 2. When the refrigerating air conditioner is not operated, the electromagnetic valve  24  prevents the lubricating oil  9  from moving from the first hermetic vessel  8  to the second hermetic vessel  4  through the first equalizer pipe  21 . 
     While embodiment 4 is described in terms of the sub-compression mechanism  3  disposed on the discharge side of the main compression mechanism  7 , the sub-compression mechanism  3  may be disposed on the suction side of the main compression mechanism  7  and advantageous results as above discussed can be obtained. Also, the main compression mechanism  7  and the sub-compression mechanism  3  are directly connected in series in embodiment 4, but similar advantageous results can also be obtained when the main compression mechanism  7  and the sub-compression mechanism  3  are connected in parallel.