Abstract:
A method for manufacturing a multi-stage compression type rotary compressor which avoids the replacement of parts to be used as much as possible to reduce costs and also which enables easily setting an appropriate displacement volume ratio between first and second rotary compression elements without increasing the size of the compressor outer housing. This is done by altering the inner diameter of the cylinder of one of the rotary compression elements without altering the thickness (or height) of this cylinder to set a displacement volume ratio between the first and second rotary compression elements to an optimum value in accordance with the alteration.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a rotary compressor which compresses a refrigerant by a rotary compression element to discharge it, a method for manufacturing the same, and a defroster for a refrigerant circuit using the same. 
   2. Description of the Related Art 
   Conventionally, in a multi-stage compression type rotary compressor, a refrigerant gas-is sucked through a suction port of a first rotary compression element into a low-pressure chamber side of a cylinder, compressed by the operations of a roller-and a vane to have a medium pressure, and discharged into a, sealed vessel through a discharge port of the side of a high pressure chamber of the cylinder. Then, the refrigerant gas having the medium pressure in the sealed vessel is sucked through a suction port of a second rotary compression element into the low-pressure chamber side of the cylinder, undergoes second-stage compression through the operations of the roller and the vane to have a high temperature and a high pressure, and is discharged from the discharge port of the high-pressure chamber side. The refrigerant thus discharged from this compressor flows into a radiator to radiate its heat, is squeezed by an expansion valve to absorb heat at an evaporator, and sucked into the first rotary compression element, which cycle is repeated. 
   In such a multi-stage compression type rotary compressor, especially when, for example, carbon dioxide (CO 2 ) having a large difference between the high and low pressures is used as the refrigerant, as shown in  FIG. 5 , a pressure of the discharged refrigerant reaches 12 MPaG in the second rotary compression element where the refrigerant has the high pressure (HP) and becomes 8 MPaG (medium pressure: MP) in the first rotary compression element which is the lower-stage side (where a suction pressure LP of the first rotary compression element is 4 MPaG). As a result, a differential pressure at the second stage (difference between the suction pressure MP of the second rotary compression element and the discharge pressure HP of the second rotary compression element) becomes a large value of 4 MPaG. Especially when an outside air temperature is low, the discharge pressure MP of the first rotary compression element becomes lower and, therefore, the second-stage differential pressure (difference between the suction pressure MP of the second rotary compression element and the discharge pressure HP of the second rotary compression element) increases further, so that a compression load of the second rotary compression element increases to bring about a problem that durability and reliability deteriorate. 
   Therefore, conventionally, by altering a dimension of thickness (or height) of the cylinder of the first rotary compression element so that a displacement volume of the second rotary compression element may be smaller than that of the first rotary compression element, a displacement volume ratio has been set so as to reduce a differential pressure at a second stage. 
   By such a setting method, however, the thickness (or height) of the first cylinder becomes large, so that correspondingly all of a cylinder material and the roller of the first rotary compression element, an eccentric portion, etc. have had to be replaced. Furthermore, as the thickness (or height) of the cylinder increases, the thickness (or height) of a rotary compression mechanism also increases, so that overall size of the relevant multi-stage compression type rotary compressor becomes larger, thus bringing about a problem of a difficulty in miniaturization of the compressor. 
   It is to be noted that the vane attached to such a multi-stage compression type rotary compressor is inserted movably in a groove formed in a radial direction of the cylinder. Such a vane is pressed against the roller to divide an inside of the cylinder into a low-pressure chamber side and a high-pressure chamber side in such a configuration that on a rear side of the vane a spring is provided to urge this vane on a roller side and also in the groove a back pressure chamber is provided which communicates with the high-pressure chamber of the cylinder to urge this vane on the roller side. 
   It is to be noted that in an internal medium-pressure type rotary compressor a pressure is higher in the cylinder of the second rotary compression element than in the sealed vessel, so that a pressure on a refrigerant discharge side of the second rotary compression element is applied to the back pressure chamber which urges the vane of this second rotary compression element. 
   If, for example, carbon dioxide (CO 2 ) having a large difference between high and low pressures is used as the refrigerant, however, as shown in  FIG. 5 , a discharged refrigerant pressure reaches 12 MPaG in the second rotary compression element where it has the high pressure (HP). Accordingly, when a pressure on the refrigerant discharge side of the second rotary compression element is applied to the back pressure chamber, a pressure to press the vane against the roller becomes higher than necessary to thereby apply a large load on a portion where a tip of the vane slides along an outer periphery of the roller, thus bringing about a problem that the vane and the roller may be worn heavily or, in the worst case, be damaged. 
   Furthermore, a discharge-noise silencer chamber of each of the first and second rotary compression elements is provided with a discharge valve to prevent back-flow of the refrigerant when it is discharged into the discharge-noise silencer chamber, using which discharge valve the discharge port can be opened and closed when necessary. 
   It is to be noted that if, for example, carbon dioxide (CO 2 ) having a large difference between high and low pressures is used as the refrigerant, as shown in  FIG. 5 , the discharged refrigerant pressure reaches 12 MPaG at the second rotary compression element where it has the high pressure (HP) and, on the other hand, becomes 8 MPaG (medium pressure: MP) at the first rotary compression element which is a lower-stage side at an outside air temperature of 15° C. (where the suction pressure LP of the first rotary compression element is 4 MPaG). As a result, a differential pressure at the first stage (difference between the suction pressure LP of the first rotary compression element and the discharge pressure MP of the first rotary compression element) becomes a large value of 4 MPaG. Moreover, with an increasing temperature of an outside air, the discharge pressure MP of the first rotary compression element increases rapidly, so that the first-stage differential pressure (difference between the suction pressure LP of the first rotary compression element and the discharge pressure MP of the first rotary compression element) increases further. 
   When the first-stage differential pressure increases in such a manner, a pressure difference between an inside and an outside of the discharge valve which opens and closes the discharge port of the first rotary compression element becomes excess, thus bringing about a problem of deterioration in durability and reliability such as damages of the discharge valve. 
   If the outside air temperature drops to reduce an evaporation temperature of the refrigerant, the discharge pressure MP of the first rotary compression element decreases, so that the second-stage differential pressure (difference between the suction pressure MP of the second rotary compression element and the discharge pressure HP of the second rotary compression element) increases further. 
   When the second-stage differential pressure increases in such a manner, a pressure difference between an inside and an outside of the discharge valve of the second rotary compression element becomes excess, thus bringing about a problem that the discharge valve etc. of the second rotary compression element may be damaged by this pressure difference. 
   Furthermore, the vane used in the rotary compressor is inserted movably in a guide groove provided in a radial direction of the cylinder. This vane, however, needs to be pressed toward the roller side always, so that conventionally, in configuration, the vane has been urged on the roller side not only by a spring but also by a back pressure applied to a back pressure chamber formed in the cylinder beforehand, thus complicating a construction. 
   Especially at the second rotary compression element of such an internal medium-pressure, multi-stage compression type rotary compressor, a pressure in the cylinder is higher than the medium pressure in the sealed vessel, thus bringing about a problem that a communication path needs to be formed through which a high back pressure is applied to the back pressure chamber. 
   Furthermore, in a refrigerant circuit using such a multi-stage compression type rotary compressor, an evaporator is liable to be frosted and so needs to be defrosted; however, if, to defrost this evaporator, a high-temperature refrigerant discharged from the second rotary compression element is supplied to the evaporator without being decompressed at a decompression device (in both cases of being directly supplied to the evaporator and being supplied thereto only by being passed through the decompression device but not being decompressed therethrough), the suction pressure of the first rotary compression element rises to thereby increase the discharge pressure (medium pressure) of the first rotary compression element. Thus, when this refrigerant is discharged through the second rotary compression element, it is not decompressed, so that the discharge pressure of the second rotary compression element becomes almost the same as the suction pressure of the first rotary compression element, thus bringing about a problem that a pressure level relationship may be reversed when the refrigerant is discharged from or sucked into the second rotary compression element. 
   This reversion in pressure level relationship during discharge and suction at the second rotary compression element can be avoided by providing such a refrigerator circuit as to supply the evaporator with a refrigerant discharged from the first rotary compression element without decompressing it so that the evaporator can be defrosted by supplying, using this refrigerant circuit, it with also the refrigerant discharged from the rotary compression element. 
   In this case, however, a discharge side of the first rotary compression element and that of the second rotary compression element communicate to each other in construction, so that a same pressure appears on the suction side and the discharge side of the second rotary compression element, thus bringing about a problem of unstable operation of the second rotary compression element such as breakaway of the vane from the second rotary compression element. 
   SUMMARY OF THE INVENTION 
   To solve those problems of the conventional technologies, the present invention has been developed, and it is an object of the present invention to provide a method for manufacturing a multi-stage compression type rotary compressor which can avoid the replacement of parts to be used as much as possible to reduce costs and also which enables easily setting an appropriate displacement volume ratio while preventing the compressor from being increased in size. 
   That is, a multi-stage compression type rotary compressor manufacturing method according to the present invention is directed to, a method for manufacturing a multistage compression type rotary compressor which comprises an electrical-power element and first and second rotary compression elements driven by the electrical-power element in a sealed vessel and in which these first and second rotary compression elements are constituted of first and second cylinders and first and second rollers which are fitted to first and second eccentric portions formed on a rotary shaft of the electrical-power element so as to eccentrically revolves in these cylinders; and a refrigerant gas compressed in the first rotary compression element and discharged therefrom is sucked into the second rotary compression element, compressed and then discharged therefrom; wherein an inner diameter of the first cylinder is altered without altering its thickness (or height); and a displacement volume ratio between the first and second rotary compression elements is set in accordance with the alteration. 
   By the present invention, therefore, costs can be reduced without replacing all of the cylinder material and the roller of the first rotary compression element, the eccentric portion of the rotary shaft, etc. as much as possible, for example, by replacing only the roller or only the roller and the eccentric portion. Furthermore, it is possible to prevent an increase in overall size of the compressor, thus reducing dimensions thereof. 
   Furthermore, to satisfy the above-mentioned object, the multi-stage compression type rotary compressor manufacturing method according to the present invention sets a displacement volume of the second rotary compression element to not less than 40% and not more than 75% of that of the first rotary compression element. 
   By thus setting the displacement volume of the second rotary compression element at a value between 40% and 75%, both inclusive, of that of the first rotary compression element, a displacement volume ratio between the first and second rotary compression elements can be set optimally. 
   It is another object of the present invention to improve durability of a vane and a roller in an internal medium-pressure, multi-stage compression type rotary compressor, thus avoiding damages of the vane and the roller beforehand. 
   That is, in a multi-stage compression type rotary compressor according to the present invention comprising an electrical-power element and first and second rotary compression elements driven by this electrical-power element in a sealed vessel in such a configuration that a refrigerant gas compressed at the first rotary compression element is discharged into the sealed vessel and this discharged medium pressure refrigerant gas is compressed at the second rotary compression element, wherein there are provided a cylinder constituting the second rotary compression element, a roller which is fitted to an eccentric portion formed on a rotary shaft of the electrical-power element to eccentrically revolve in the cylinder, a vane which butts against this roller to divide an inside of the cylinder into a low-pressure chamber side and a high-pressure chamber side, a back pressure chamber for urging this vane on a roller side always, a communication path which communicates a refrigerant discharge side of the second rotary compression element and the back pressure chamber to each other, and a pressure adjustment valve for adjusting a pressure applied to the back pressure chamber through this communication path, so that by using this pressure adjustment valve, force for pressing the vane against the roller can be held appropriately. Furthermore, by holding a pressure of the back pressure chamber at a predetermined value which is lower than a pressure on a refrigerant discharge side of the second rotary compression element and higher than a pressure in the sealed vessel, it is possible to prevent a back pressure higher than necessary from being applied to the vane while preventing a so-called vane breakaway, thus optimizing force for urging the vane toward the roller. 
   Accordingly, it is possible to reduce a load applied to a portion where a tip of the vane slides along an outer periphery of the roller to thereby avoid damages of the vane and the roller beforehand, thus improving durability thereof. 
   Furthermore, by the present invention, in addition to this configuration, there are provided a support member which blocks an opening face of the cylinder and also which has a bearing for the rotary shaft of the electrical-power element and a discharge-noise silencer chamber arranged in this support member in such a configuration that the communication path is formed in the support member to communicate the discharge-noise silencer chamber and the back pressure chamber to each other and also the pressure adjustment valve is provided in the support member, so that it is possible to adjust a pressure in the back pressure chamber of the vane without complicating a construction while effectively utilizing an internal limited space of the sealed vessel. Furthermore, since the communication path and the pressure adjustment valve can be provided in the support member beforehand, a work efficiency in assembly can be improved. 
   It is a further object of the present invention to provide a multi-stage compression type rotary compressor which can avoid beforehand such deterioration in durability and reliability as to be caused by an excessive first-stage differential pressure. 
   That is, in a multi-stage compression type rotary compressor according to the present invention comprising an electrical-power element and first and second rotary compression elements driven by this electrical-power element in a sealed vessel in such a configuration that a refrigerant gas compressed in the first rotary compression element and discharged therefrom is sucked into the second rotary compression element to be compressed and discharged therefrom, there are provided a communication path which communicates a refrigerant suction side and a refrigerant discharge side of the first rotary compression element to each other and a valve device which opens and closes this communication path in such a manner as to open it if a pressure difference between the refrigerant suction side and the refrigerant discharge side of the first rotary compression element exceeds a predetermined upper limit value, so that it is possible to suppress the pressure difference between the refrigerant suction side and the refrigerant discharge side of the first rotary compression element, which is the first-stage differential pressure, down to the predetermined upper limit value or less. Accordingly, it is possible to avoid a trouble such as damaging of the discharge valve provided on the first rotary compression element caused by an excessive value of the first-stage differential pressure, thus improving durability and reliability of the rotary compressor. 
   Furthermore, by the present invention, there are also provided a cylinder constituting the first rotary compression element, a support member which blocks an opening face of this cylinder and which has a bearing for the rotary shaft of the electrical-power element, and a suction path and a discharge-noise silencer chamber which are arranged in this support member in such a configuration that the communication path is formed in the support member to communicate the suction path and the discharge-noise silencer chamber to each other and also the valve device is provided in the support member, so that the communication path and the valve device can be integrated into the cylinder of the first rotary compression element to realize miniaturization and also the valve device can be set into the cylinder beforehand, thus improving a work efficiency in assembly. 
   It is a still further object of the present invention to provide a multi-stage compression type rotary compressor which can avoid beforehand a damage and a trouble of the discharge valve etc. of the second rotary compression element caused by a second-stage differential pressure. 
   That is, a multi-stage compression type rotary compressor according to the present invention comprises an electrical-power element and first and second rotary compression elements driven by this electrical-power element in a sealed vessel so as to suck a medium pressure refrigerant gas compressed in the first rotary compression element into the second rotary compression element and then compress and discharge it therefrom, wherein there are provided a communication path which communicates a passage through which the medium pressure refrigerant gas passes as compressed at the first rotary compression element and a refrigerant discharge side of the second rotary compression element to each other and a valve device which opens and closes this communication path in such a manner as to open it if a pressure difference between the medium pressure refrigerant gas and the refrigerant gas on a refrigerant discharge side of the second rotary compression element exceeds a predetermined upper limit value, so that it is possible to suppress a pressure difference between a discharge pressure and a suction pressure of the second rotary compression element, that is, a second-sage differential pressure, down to the predetermined upper limit value or less. 
   Accordingly, it is possible to avoid an occurrence of a trouble such as damaging of the discharge valve of the second rotary compression element. 
   Furthermore, by the present invention, in addition to this configuration, there are provided a cylinder which constitutes the second rotary compression element and a discharge-noise silencer chamber which discharges a refrigerant gas compressed in this cylinder in such a configuration that a medium pressure refrigerant gas compressed at the first rotary compression element is discharged into the sealed vessel and then sucked into the second rotary compression element, the communication path is formed in a wall defining the discharge-noise silencer chamber to communicate an inside of the sealed vessel and the discharge-noise silencer chamber, and the valve device is provided in the wall, so that it is possible to integrate the communication path which communicates the passage for the medium pressure refrigerant compressed at the first rotary compression element and the refrigerant discharge side of the second rotary compression element to each other and the valve device which opens and closes the communication path into a wall of the second rotary compression element. 
   Accordingly, it is possible to simplify a construction and reduce overall size. 
   It is an additional object of the present invention to provide a rotary compressor which simplifies a construction related to a vane for dividing an inside of a cylinder into a low-pressure chamber and a high-pressure chamber. 
   That is, in a rotary compressor according to the present embodiment of the present invention comprising an electrical-power element and a rotary compression element driven by this electrical-power element in a sealed vessel to compress a CO 2  refrigerant, there are provided a cylinder constituting the rotary compression element, a swing piston having a roller portion which is engaged to an eccentric portion formed on a rotary shaft of the electrical-power element to eccentrically move in the cylinder, a vane portion which is formed on this swing piston in such a manner as to project from the roller portion in a radial direction to thereby divide an inside of the cylinder into a low-pressure chamber side and a high-pressure chamber side, and a holding portion which is provided on the cylinder to hold the vane portion of the swing piston in such a manner that the vane portion can slide and swing, so that as the eccentric portion of the rotary shaft revolves eccentrically, the swing piston correspondingly swings and slides round the holding portion as a center, and therefore the vane portion thereof always divides the inside of the cylinder into the low-pressure chamber side and the high-pressure chamber side. 
   Accordingly, it is possible to eliminate a necessity of conventionally providing a spring for urging the vane on a roller side, a back pressure chamber, or a structure for applying a back pressure to the back pressure chamber, thus simplifying a construction of the rotary compressor and reducing costs in manufacture. 
   Furthermore, in a rotary compressor according to the present invention comprising an electrical-power element and first and second rotary compression elements driven by this electrical-power element in a sealed vessel in such a configuration that a CO 2  gas compressed at the first rotary compression element is discharged into the sealed vessel and this discharged medium pressure gas is compressed at the second rotary compression element, there are provided a cylinder constituting the second rotary compression element, a swing piston having a roller portion which is engaged to an eccentric portion formed on a rotary shaft of the electrical-power element to eccentrically move in the cylinder, a vane portion which is formed on this swing piston in such a manner as to project from the roller portion in a radial direction in order to divide an inside of the cylinder into a low-pressure chamber side and a high-pressure chamber side, and a holding portion which is provided on the cylinder to hold the vane portion of the swing piston in such a manner that the vane can slide and swing, so that similarly, as the eccentric portion of the rotary shaft revolves eccentrically, the swing piston correspondingly swings and slides round the holding portion as a center, and therefore the vane portion thereof always divides the inside of the cylinder of the second rotary compression element into the low-pressure chamber side and the high-pressure chamber side. 
   Accordingly, it is possible to eliminate a necessity of conventionally providing a spring for urging the vane on the roller side, a back pressure chamber, or a structure for applying a back pressure to the back pressure chamber. Although as by the present invention the structure for applying a back pressure is complicated especially in a so-called multi-stage compression type rotary compressor which provides a medium pressure in a sealed vessel, by thus using a swing piston, it is possible to remarkably simplify a construction and reduce costs in manufacture. 
   Besides the above-mentioned configuration of the present invention, the holding portion is constituted of a guide groove which is formed in the cylinder and which the vane portion of the swing piston can enter movably and a bush which is provided rotatably at this guide groove to slidingly support the vane portion, so that it is possible to smooth swinging and sliding operations of the swing piston. Accordingly, it is possible to greatly improve performance and reliability of the rotary compressor. 
   It is another additional object of the present invention to provide a defroster which can prevent unstable operation from occurring during defrosting of an evaporator, in a refrigerant circuit using a multi-stage compression type rotary compressor. 
   In a refrigerant circuit comprising a multi-stage compression type rotary compressor including an electrical-power element and first and second rotary compression elements driven by this electrical-power element in a sealed vessel in such a configuration that a refrigerant compressed at the first rotary compression element is then compressed at the second rotary compression element, a gas cooler into which the refrigerant discharged from the second rotary compression element of this multi-stage compression type rotary compressor flows, a first decompression device connected to an outlet side of this gas cooler, and an evaporator connected to an outlet side of this first decompression device in such a configuration that the refrigerant discharged from this evaporator is compressed at the first rotary compression element., a defroster according to the present invention comprises a defrosting circuit for supplying the evaporator with the refrigerant, without decompressing it, discharged from the first and second rotary compression elements, a first flow-path control device which controls flow of the refrigerant through this defrosting circuit, a second decompression device provided along a refrigerant path for supplying the second rotary compression element with the refrigerant discharged from the first rotary compression element, and a second flow-path control device which controls whether the refrigerant is allowed to flow through this second decompression device or the refrigerant is allowed to bypass it, wherein this second flow-path control device allows the refrigerant to flow through the second decompression device, when the first flow-path control device allows the refrigerant to flow through the defrosting circuit, so that during defrosting operation of the evaporator, the refrigerant discharged from the first and second rotary compression elements is supplied to the evaporator without being decompressed, thus avoiding reversion in pressure level relationship at the second rotary compression element. 
   In particular, by the present invention, during such defrosting operation, a refrigerant is controlled to be supplied to the second rotary compression element through the decompression device provided along the refrigerant path, so that a predetermined pressure difference is established between suction and discharge sides of the second rotary compression element. 
   Accordingly, the second rotary compression element becomes stable in operation, thus improving reliability. Remarkable effects are obtained especially in the case of a refrigerant circuit using a CO 2  gas as a refrigerant. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a vertical cross-sectional view for showing a multi-stage compression type rotary compressor according to an embodiment of the present invention; 
       FIG. 2  is a front view for showing the rotary compressor of  FIG. 1 ; 
       FIG. 3  is a side view for showing the rotary compressor of  FIG. 1 ; 
       FIG. 4  is a diagram for showing a refrigerant circuit of a hot-water supply apparatus to which the rotary compressor of  FIG. 1  is applied; 
       FIG. 5  is a graph for showing a relationship between an outside air temperature and various pressures in the case of a multi-stage compression type rotary compressor; 
       FIG. 6  is a vertical cross-sectional view for showing a multi-stage compression type rotary compressor according to another embodiment of the present invention; 
       FIG. 7  is an expanded cross-sectional view for showing a pressure adjustment valve of a second rotary compression element of the multi-stage compression type rotary compressor of  FIG. 6 ; 
       FIG. 8  is a vertical cross-sectional view for showing a multi-stage compression type rotary compressor according to a further embodiment of the present invention; 
       FIG. 9  is an expanded cross-sectional view for showing a communication path portion of a first rotary compression element of the multi-stage compression type rotary compressor of  FIG. 8 ; 
       FIG. 10  is a bottom view for showing a lower-part support member of the multi-stage compression type rotary compressor of  FIG. 8 ; 
       FIG. 11  is a top view for showing an upper-part support member of the multi-stage compression type rotary compressor of  FIG. 8 ; 
       FIG. 12  is a bottom view for showing a lower cylinder of the multi-stage compression type rotary compressor of  FIG. 8 ; 
       FIG. 13  is a top view for showing an upper cylinder of the multi-stage compression type rotary compressor of  FIG. 8 ; 
       FIG. 14  is a vertical cross-sectional view for showing a multi-stage compression type rotary compressor according to a still further embodiment of the present invention; 
       FIG. 15  is an expanded cross-sectional view for showing a communication path of a second rotary compression element of the multi-stage compression type rotary compressor of  FIG. 14 ; 
       FIG. 16  is an expanded cross-sectional view for showing the communication path of the second rotary compression element of another multi-stage compression type rotary compressor which corresponds to  FIG. 15 ; 
       FIG. 17  is a bottom view for showing a lower-part support member of the multi-stage compression type rotary compressor of  FIG. 14 ; 
       FIG. 18  is a vertical cross-sectional view for showing a rotary compressor according to an additional embodiment of the present invention;. 
       FIG. 19  is an expanded cross-sectional view for showing a swing piston portion of a second rotary compression element of the rotary compressor of  FIG. 18 ; 
       FIG. 20  is a vertical cross-sectional view for showing a multi-stage compression type rotary compressor according to an additional embodiment of the present invention applied to a defroster for a refrigerant circuit; and 
       FIG. 21  is a diagram for showing a refrigerant circuit of a hot-water supply apparatus to which the rotary compressor of  FIG. 20  is applied. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following will detail embodiments of the present invention with reference to drawings. In figures, a reference numeral  10  indicates an internal medium-pressure, multi-stage compression type rotary compressor using carbon dioxide as a refrigerant which comprises a cylindrical sealed vessel  12  made of a steel plate and a rotary compression mechanism portion  18  which includes an electrical-power element  14  arranged and housed in an upper part of an internal space of the sealed vessel and a first rotary compression element  32  (first stage) and a second rotary compression element  34  (second stage) which are arranged below the electrical-power element  14  to be driven by a rotary shaft  16  of the electrical-power element  14 . The sealed vessel  12  has its bottom used as an oil reservoir and is composed of a vessel body  12 A which houses the rotary compression mechanism portion  18  and the electrical-power element  14  and a roughly cup-shaped end cap (lid)  12 B which blocks an upper part opening of the vessel body  12 A in such a configuration that the end cap  12 B has a circular attachment hole  12 D formed therein at a center of its top face, in which attachment hole  12 D a terminal  20  (wiring of which is omitted) is attached which supplies power to the electrical-power element  14 . 
   The electrical-power element  14  is composed of a stator  22  mounted annularly along an inner peripheral face of an upper-part space of the sealed vessel  12  and a rotor  24  disposed and inserted in the stator  22  with some gap set therebetween. This rotor  24  is fixed to the rotary shaft  16  which vertically extends centrally. 
   The stator  22  has a stack  26  formed by stacking donut-shaped electromagnetic steel plates and a stator coil.  28  wound round teeth of the stack  26  by direct winding (concentrated winding). Furthermore, similar to the stator  22 , the rotor  24  is also made of a stack  30  of electromagnetic steel plates and a permanent magnet MG inserted into the stack  30 . 
   An intermediate partition plate  36  is sandwiched between the first rotary compression element  32  and the second rotary compression element  34 . That is, a combination of the first rotary compression element  32  and the second rotary compression element  34  is composed of the intermediate partition plate  36 , an upper cylinder  38  and a lower cylinder  40  arranged above and below the intermediate partition plate  36  respectively, an upper roller  46  and a lower roller  48  which eccentrically revolve within the upper and lower cylinders  38  and  40  respectively at upper and lower eccentric portions  42  and  44  provided on the rotary shaft  16  with a phase difference of 180 degrees therebetween, vanes  50  and  52  which butt against the upper and lower rollers  46  and  48  to divide an inside of the respective upper and lower cylinders  38  and  40  into a low-pressure chamber side and a high-pressure chamber side, and an upper-part support member  54  and a lower-part support member  56  given as a support member for blocking an upper-side opening face of the upper cylinder  38  and a lower-side opening face of the lower cylinder  40  respectively to serve also as a bearing for the rotary shaft  16 . 
   The upper and lower cylinders  38  and  40  constituting the second and first rotary compression elements  34  and  32  respectively are made up of a material having the same thickness in the present embodiment. Furthermore, assuming an inner diameter of the cylinders  38  and  40  obtained by cutting them to be D 2  and D 1  respectively, when altering a displacement volume ratio between the first and second rotary compression elements  32  and  34 , this ratio is set by altering the inner diameter D 1  of the lower cylinder  40  of the first rotary compression element  32 . 
   It is to be noted that when the displacement volume ratio is set by altering thickness (or height) of the lower cylinder  40 , for example, it is necessary to alter all of a material of the lower cylinder  40  and thickness (or height) of the lower eccentric portion  44  and the lower roller;  48 . That is, in this case, it is necessary at least to alter the lower cylinder  40  and the lower roller  48  starting from their materials and also alter how to cut the rotary shaft  16  for the lower eccentric portion  44 . By the present invention, on the other hand, at least the lower cylinder  40  need not be altered in material but only needs to be altered in inner diameter when being cut. Furthermore, although the lower roller  48  needs to be altered at least in outer diameter, the lower eccentric portion  44  need not be altered if the inner diameter is the same. Thus, by the present invention, the displacement volume ratio can be altered without altering at least the material of the lower cylinder  40  but by altering only a cutting process of the lower cylinder  40  and an outer diameter of the lower roller  48  or outer and inner diameters of the lower roller  48  as well as the lower eccentric portion  44 . It is thus possible to set an optimal displacement volume ratio between the first and second rotary compression elements  32  and  34  while minimizing replacement of parts at the same time. It is to be noted that in the present embodiment a displacement volume of the second rotary compression element  34  is set in a range of not less than 40% through not more than 75% of that of the first rotary compression element  32 . 
   A combination of the upper-part support member  54  and the lower-part support member  56 , on the other hand, is provided therein with a suction path  60  (and an upper-side suction path not shown) which communicate with insides of the upper and lower cylinders  38  and  40  through suction ports not shown and discharge-noise silencer chambers  62  and  64  which are formed by concaving a surface partially and then blocking resultant concavities by an upper cover  66  and a lower cover  68  respectively. 
   It is to be noted that the discharge-noise silencer chamber  64  communicates with an inside of the sealed vessel  12  through a communication path which penetrates the upper and lower cylinders  38  and  40  and the intermediate partition plate  36  in such a configuration that at an upper end of the communication path, an intermediate discharge pipe  121  is provided as erected, through which a medium pressure refrigerant compressed at the first rotary compression element  32  is discharged into the sealed vessel  12 . 
   Furthermore, the upper cover  66  which blocks an upper-face opening of the discharge-noise silencer chamber  62  communicating with an inside of the upper cylinder  38  of the second rotary compression element  34  partitions the inside of the sealed vessel  12  into a side of the discharge-noise silencer chamber  62  and a side of the electrical-power element  14 . 
   In this configuration, by the present embodiment, as a refrigerant, carbon dioxide (CO 2 ) which is a natural refrigerant friendly to environments of the earth is used taking into account inflammability, toxicity, etc., while as a lubricant, such existing oil is used as mineral oil, alkyl-benzene oil, ether oil, ester oil, or poly-alkyl glycol (PAG). 
   Onto a side face of the vessel body  12 A of the sealed vessel  12 , sleeves  141 ,  142 ,  143 , and  144  are fixed by welding at positions that correspond to the suction path  60  (and an upper-side suction path not shown) of the respective upper-part support member  54  and the lower-part support member  56 , the discharge-noise silencer chamber  62 , and an upper side of the upper cover  66  (a lower end of the electrical-power element  14  roughly) respectively. The sleeves  141  and  142  are adjacent to each other vertically, while the sleeve  143  is roughly in a diagonal direction of the sleeve  141 . Furthermore, the sleeve  144  is positioned as shifted by about 90 degrees with respect to the sleeve  141 . 
   In the sleeve  141  is there inserted and connected one end of a refrigerant introduction pipe  92  for introducing a refrigerant gas to the upper cylinder  38 , which one end communicates with the suction path, not shown, of the upper cylinder  38 . This refrigerant introduction pipe  92  passes through an upper part of the sealed vessel  12  up to the sleeve  144 , while the other end is inserted and connected in the sleeve  144  to communicate with the inside of the sealed vessel  12 . 
   In the sleeve  142 , on the other hand, is there inserted and connected one end of a refrigerant introduction pipe  94  for introducing a refrigerant gas to the lower cylinder  40 , which one end communicates with the suction path  60  of the lower cylinder  40 . The other end of this refrigerant introduction pipe  94  is connected to a lower end of an accumulator  146 . Furthermore, in the sleeve  143  is there inserted and connected a refrigerant discharge pipe  96 , one end of which communicates with the discharge-noise silencer chamber  62 . 
   The accumulator  146  is a tank for separating an sucked refrigerant into vapor and liquid and attached via a bracket  148  thereof to the bracket  147  of a sealed vessel side welded and fixed to an upper-part side face of the vessel body  12 A of the sealed vessel  12  (FIG.  2 ). 
   In this configuration, a multi-stage compression type rotary compressor  10  of the present embodiment is used in a refrigerant circuit of a hot-water supply apparatus  153  such as shown in FIG.  4 . That is, the refrigerant discharge pipe  96  of the multi-stage compression type rotary compressor  10  is connected to an inlet of a gas cooler  154  for heating water. This gas cooler  154  is provided to a hot-water storage tank, not shown, of the hot-water supply apparatus  153 . The pipe exits the gas cooler  154  and passes through an expansion valve  156 , which serves as a decompression device, up to an inlet of an evaporator  157 , an outlet of which is connected to the refrigerant introduction pipe  94 . Furthermore, as shown in  FIG. 4 , a defrosting pipe  158  constituting the defrosting circuit branches from the refrigerant introduction pipe  92  at somewhere along it and is connected through an electromagnetic valve  159 , which serves as a flow-path control device, to the refrigerant discharge pipe  96  extending to the inlet of the gas cooler  154 . It is to be noted that the accumulator  146  is omitted in FIG.  4 . 
   The following will describe operations with reference to this configuration. It is to be noted that the electromagnetic valve  159  is supposed to stay closed during heating. When the stator coil  28  of the electrical-power element  14  is electrified through the terminal  20  and a wiring line not shown, the electrical-power element  14  is actuated, thus causing the rotor  24  to revolve. By this revolution, the upper and lower rollers  46  and  48  are fitted to the upper and lower eccentric portions  42  and  44  provided integrally with the rotary shaft  16 , to eccentrically revolve in the upper and lower cylinders  38  and  40  respectively. 
   Accordingly, a low-pressure refrigerant sucked into the low-pressure chamber side of the cylinder  40  from the suction port, not shown, through the refrigerant introduction pipe  94  and the suction path  60  formed in the lower-part support member  56  is compressed by operations of the roller  48  and the vane  52  to have a medium pressure, passed through the high-pressure chamber side of the lower cylinder  40 , a discharge port not shown, the discharge-noise silencer chamber  64  formed in the lower-part support member  56 , and the communication path not shown, and discharged into the sealed vessel  12  from the intermediate discharge pipe  121 . Thus, the medium pressure develops in the sealed vessel  12 . 
   Then, the medium pressure refrigerant gas in the sealed vessel  12  exits it through the sleeve  144 , passes through the refrigerant introduction pipe  92  and the suction path, not shown, formed in the upper-part support member  54 , and is sucked from the suction port, not shown, into the lower-pressure chamber side of the upper cylinder  38 . The medium pressure refrigerant gas thus sucked undergoes second-stage compression through operations of the roller  46  and the vane  50  to provide a high-temperature, high-pressure refrigerant gas, which in turn passes through the high-pressure chamber side, the discharge port not shown, the discharge-noise silencer chamber  62  formed in the upper-part support member  54 , and the refrigerant discharge pipe  96  to then flow into the gas cooler  154 . At this moment, the refrigerant has a raised temperature of about +100° C. and, therefore, such a high temperature, high pressure gas radiates heat to heat water in the hot-water storage tank, thus generating hot water having a temperature of about +90° C. 
   The refrigerant itself, on the other hand, is cooled at the gas cooler  154  and exits it. Then, the refrigerant is decompressed at the expansion valve  156 , flows into the evaporator  157  to evaporate there, passes through the accumulator  146  (not shown in FIG.  4 ), and is sucked into the first rotary compression element  32  through the refrigerant introduction pipe  94 , which cycle is repeated. 
   Thus, by altering the inner diameter D 1  of the lower cylinder  40  without altering its thickness (or height) to thus set the displacement volume of the second rotary compression element  34  at not less than 40% and not more than 75% of that of the first rotary compression element  32 , a displacement volume ratio between the first and second rotary compression elements  32  and  34  is set, so that it is possible to reduce a compression load of the second rotary compression element  34  while minimizing alterations of the cylinder material and parts such as the eccentric portions and rollers as much as possible, to thereby provide an optimal displacement volume ratio with a differential pressure suppressed as much as possible. Furthermore, the rotary compression mechanism portion  18  also stays as unexpanded in vertical size, thus enabling minimizing the multi-stage compression type rotary compressor  10 . 
   Although in the present embodiment the upper and lower cylinders  38  and  40  are supposed to have the same thickness (or height), the present invention is not limited thereto; for example, the displacement volume ratio may be set by altering the inner diameter of the cylinder of the first rotary compression element in a condition where the upper and lower cylinders  38  and  40  are different in thickness (or height) originally. 
   Furthermore, although the present embodiment has been described in all cases with reference to a multi-stage compression type rotary compressor in which the rotary shaft  16  is mounted vertically, of course the present invention can be applied also to a multi-stage compression type rotary compressor in which the rotary shaft is mounted horizontally. Furthermore, the multi-stage compression type rotary compressor has been described as a two-stage compression type rotary compressor equipped with first and second rotary compression elements, the present invention is not limited thereto; for example, the multi-stage compression type rotary compressor may be equipped with three, four, or even more stages of rotary compression elements. 
   Furthermore, although the present embodiment has used the multi-stage compression type rotary compressor  10  in a refrigerant circuit of the hot-water supply apparatus  153 , the present invention is not limited thereto; for example, the present invention may well be applied for warming of a room. 
   As detailed above, according to the present embodiment of the present invention, when manufacturing a multi-stage compression type rotary compressor which comprises an electrical-power element and first and second rotary compression elements driven by the electrical-power element in a sealed vessel in such a configuration that the first and second rotary compression elements are constituted of first and second cylinders and first and second rollers which are fitted to first and second eccentric portion formed on a rotary shaft of the electrical-power element so as to eccentrically revolve in the cylinders respectively and also that a refrigerant gas compressed in the first rotary compression element and discharged therefrom is sucked into the second rotary compression element to be compressed and discharged therefrom, an inner diameter of the first cylinder is altered without altering its thickness (or height) to thereby set a displacement volume ratio between the first and second rotary compression elements, so that costs can be reduced without replacing all of a cylinder material and the roller of the first rotary compression element, the eccentric portion of the rotary shaft, etc. as much as possible, for example, by replacing only the roller or only the roller and the eccentric portion. Furthermore, it is possible to prevent an increase in overall size of the compressor, thus reducing dimensions thereof. Also, for example, by setting the displacement volume of the second rotary compression element at not less than 40% and not more than 75% of that of the first rotary compression element, a displacement volume ratio between the first and second rotary compression elements can be optimized. 
   The following will describe a multi-stage compression type rotary compressor according to another embodiment of the present invention with reference to  FIGS. 6 and 7 .  FIG. 6  is a vertical cross-sectional view of the multi-stage compression type rotary compressor according to the present embodiment of the present invention and  FIG. 7 , an expanded cross-sectional view of a pressure adjustment valve  107  of the rotary compressor  10 . It is to be noted that the same reference numerals in  FIGS. 6 and 7  as those in  FIGS. 1-5  indicate the same or similar functions. 
   In the figures, a reference numeral  10  indicates the internal medium-pressure, multi-stage compression type rotary compressor using carbon dioxide (CO 2 ) as a refrigerant which comprises the cylindrical sealed vessel  12  made of a steel plate and the rotary compression mechanism portion  18  which includes the electrical-power element  14  arranged and housed in an upper part of an internal space of the sealed vessel  12  and the first rotary compression element  32  (first stage) and the second rotary compression element  34  (second stage) which are arranged below the electrical-power element  14  to be driven by the rotary shaft  16  of the electrical-power element  14 . 
   The sealed vessel  12  has its bottom used as an oil reservoir and is composed of the vessel body  12 A which houses the rotary compression mechanism portion  18  and the electrical-power element  14  and the roughly cup-shaped end cap (lid)  12 B which blocks an upper part opening of the vessel body  12 A in such a configuration that the end cap  12 B has the circular attachment hole  12 D formed therein at a center of its top face, in which attachment hole  12 D the terminal  20  (wiring of which is omitted) is attached which supplies power to the electrical-power element  14 . 
   The electrical-power element  14  is composed of the stator  22  mounted annularly along an inner peripheral face of an upper-part space of the sealed vessel  12  and the rotor  24  disposed and inserted in the stator  22  with some gap set therebetween. This rotor  24  is fixed to the rotary shaft  16  which vertically extends centrally. 
   The stator  22  has the stack  26  formed by stacking donut-shaped electromagnetic steel plates and the stator coil  28  wound round teeth of the stack  26  by direct winding (concentrated winding). Furthermore, similar to the stator  22 , the rotor  24  is also made of the stack  30  of electromagnetic steel plates and the permanent magnet MG inserted into the stack  30 . 
   The intermediate partition plate  36  is sandwiched between the first rotary compression element  32  and the second rotary compression element  34 . That is, a combination of the first rotary compression element  32  and the second rotary compression element  34  is composed of the intermediate partition plate  36 , the upper cylinder  38  and the lower cylinder  40  arranged above and below the intermediate partition plate  36  respectively, the upper roller  46  and the lower roller  48  which are fitted to the upper and lower eccentric portions  42  and  44  provided on the rotary shaft  16  with a phase difference of 180 degrees set therebetween so as to eccentrically revolve within the upper and lower cylinders  38  and  40  respectively, the upper and lower vanes  50  and  52  which butt against the upper and lower rollers  46  and  48  to divide respective upper and lower cylinders  38  and  40  into a low-pressure chamber side and a high-pressure chamber side, and the upper-part support member  54  and the lower-part support member  56  given as a support member for blocking an upper-side opening face of the upper cylinder  38  and a lower-side opening face of the lower cylinder  40  respectively to serve also as a bearing for the rotary shaft  16 . 
   It is to be noted that a displacement volume ratio between the first rotary compression element  32  and the second rotary compression element  34  is supposed to be (displacement volume of the second rotary compression element  34 )/(displacement volume of the first rotary compression element  32 )×100=30−75%. 
   As shown in  FIG. 7 , within the upper cylinder  38  constituting the second rotary compression element  34 , a guide groove  70  for housing the vane  50  is formed; and outside the guide groove  70 , that is, on a rear face side of the vane  50 , there is formed a housing portion  70 A for housing a spring  74  serving as a spring member. The spring  74  butts against a rear face side end of the vane  50  to thereby always urge the vane  50  on the roller  46 . The housing portion  70 A has an opening on a side of the guide groove  70  and a side of the sealed vessel  12  (vessel body  12 A) and is provided with a metal-made plug  137  on a side of the sealed vessel  12  with respect to the spring  74  housed in the housing portion  70 A for preventing fall-out of the spring  74 . Furthermore, on a peripheral face of the plug is there attached an O-ring, not shown, for sealing an inner face of this plug  137  and that of the housing portion  70 A off each other. 
   Furthermore, between the guide groove.  70  and the housing portion  70 A is there provided a back pressure chamber  99  which applies a refrigerant discharge pressure of the second rotary compression element  34  to the vane  50  to work with the spring  74  in order to always urge the vane  50  on the roller  46 . An upper face of this back pressure chamber  99  communicates with a later-described second path  106 . 
   Furthermore, a combination of the upper-part support member  54  and the lower-part support member  56  is provided therein the suction path  60  (and upper-side suction path not shown) communicating with insides of the upper and lower cylinders  38  and  40  respectively through a suction port not shown and the discharge-noise silencer chambers  62  and  64  formed by concaving a surface partially and blocking resultant concavities by the upper and lower covers  66  and  68  respectively. 
   It is to be noted that the discharge-noise silencer chamber  64  and an inside of the sealed vessel  12  communicate to each other through an communication path which penetrates the upper and lower cylinder  38  and  40  and the intermediate partition plate  36  in such a configuration that at an upper end of the communication path is there provided the intermediate discharge pipe  121  as erected, from which pipe  121  a medium pressure refrigerant gas-compressed at the first rotary compression element  32  is discharged into the sealed vessel  12 . 
   In this configuration, the upper cover  66  which blocks the upper-face opening of the discharge-noise silencer chamber  62  communicating with an inside of the upper cylinder  38  of the second rotary compression element  34  partitions an inside of the sealed vessel  12  into the discharge-noise silencer chamber  62  and a side of the electrical-power element  14 . 
   Furthermore, a communication path  100  is formed in the upper-part support member  54 . This communication path  100  is provided to communicate to each other the back pressure chamber  99  and the discharge-noise silencer chamber  62  which communicates with a discharge port, not shown, of the upper cylinder  38  of the second rotary compression element  34  and is constituted of a valve housing chamber  102  which penetrates the upper-part support member  54  vertically and has its upper side blocked by the upper cover  66 , a first path  101  which communicates an upper end of this valve housing chamber  102  and the discharge-noise silencer chamber  62  to each other, and a second path  106  which is positioned outside the valve housing chamber  102  to communicate this valve housing chamber  102  and the back pressure chamber  99  to each other as shown in FIG.  7 . 
   The valve housing chamber  102  is a cylindrical hole extending vertically and has its lower end blocked by a sealing agent  103 . On a upper side of the sealing agent  103  is there attached a lower end of a valve disc  104  (coil spring.), at an upper end of which is, in turn attached a valve disc  105 . This valve disc  105  is provided in the valve housing chamber  102  vertically movably and butts against a peripheral wall of this valve housing chamber  102  as sliding to divide the valve housing chamber  102  vertically. These valve disc  105  and spring member  104  constitute a pressure adjustment valve  107  of the present invention. 
   The second path  106  is formed from a position below a lower end of the valve housing chamber  102  by a predetermined distance down to the back pressure chamber  99  in such a configuration that if the valve disc  105  is above the path  106 , the communication path  100  is closed and, if an upper face of the valve disc  105  is below an upper end of the second path  106 , the communication path  100  is opened. The spring member  104  always urges this valve disc  105  in such a direction as to raise it. 
   Furthermore, the valve disc  105  receives downward force due to a high pressure refrigerant gas flowing through the first path  101  into the valve housing chamber  102  and upward force due to a pressure in the back pressure chamber  99  through the second path  106 . That is, the valve disc  105  moves downward and upward respectively owing to a pressure of the refrigerant gas compressed in the upper cylinder  38  of the second rotary compression element  34  and discharged into the discharge-noise silencer chamber  62  and a combination of urging force of the spring member  104  and a pressure in the back pressure chamber  99 . 
   The urging force of this spring member  104  is supposed to be set so that if, for example, a pressure difference between the discharge-noise silencer chamber  62  and the back pressure chamber  99  (pressure of the discharge-noise silencer chamber  62 —pressure of the back pressure chamber  99 ) becomes larger than, for example, 2 MPaG, an upper face of the valve is lowered below the upper end of the second path  106  to thereby open the communication path  100  and, if the pressure difference becomes 2 MPaG or less, the valve disc  105  is raised until its upper face exceeds in height the upper end of the second path  106  to thereby close the communication path  100 . 
   In this case, as a refrigerant, carbon dioxide (CO 2 ), which is a natural refrigerant friendly to environments of the earth, is used taking into account inflammability, toxicity, etc., while as a lubricant, such existing oil is used as mineral oil, alkyl-benzene oil, ether oil, ester oil, or poly-alkyl glycol (PAG). 
   On a side face of the vessel body  12 A of the sealed vessel  12 , the sleeves  141 ,  142 ,  143 , and  144  are fixed by welding at positions that correspond to the suction path  60  (and an upper-side suction path not shown) of the respective upper-part support member  54  and the lower-part support member  56 , the discharge-noise silencer chamber  62 , and an upper side of the upper cover  66  (a lower end of the electrical-power element  14  roughly) respectively. The sleeves  141  and  142  are adjacent to each other vertically, while the sleeve  143  is roughly in a diagonal direction of the sleeve  141 . Furthermore, the sleeve  144  is positioned as shifted by about 90 degrees with respect to the sleeve  141 . 
   In the sleeve  141  is there inserted and connected one end of the refrigerant introduction pipe  92  for introducing a refrigerant gas to the upper cylinder  38 , which one end communicates with a suction path, not shown, of the upper cylinder  38 . This refrigerant introduction pipe  92  passes through the upper part of the sealed vessel  12  up to the sleeve  144 , while the other end is inserted and connected in the sleeve  144  so as to communicate with an inside of the sealed vessel  12 . 
   In the sleeve  142 , on the other hand, is there inserted and connected one end of the refrigerant introduction pipe  94  for introducing a refrigerant gas to the lower cylinder  40 , which one end communicates with the suction path  60  of the lower cylinder  40 . The other end of this refrigerant introduction pipe  94  is connected to a lower end of the accumulator  146 . Furthermore, in the sleeve  143  is there inserted and connected the refrigerant discharge pipe  96 , one end of which communicates with the discharge noise silencer chamber  62 . 
   The accumulator  146  is a tank for separating an sucked refrigerant into vapor and liquid and attached via the bracket  148  thereof to the bracket  147  of a sealed vessel side welded and fixed to an upper-part side face of the vessel body  12 A of the sealed vessel  12  (see FIG.  2 ). 
   Accordingly, the multi-stage compression type rotary compressor  10  of the present embodiment is used in a refrigerant circuit of a hot-water supply apparatus such as shown in FIG.  4 . That is, the refrigerant discharge pipe  96  of the multi-stage compression type rotary compressor  10  is connected to the inlet of the gas cooler  154  for heating water. This gas cooler  154  is provided to a hot-water storage tank, not shown, of the hot-water supply apparatus  153 . The pipe exits the gas cooler  154  and passes through the expansion valve  156  serving as a decompression device up to an inlet of the evaporator  157 , an outlet of which is connected to the refrigerant introduction pipe  94 . Furthermore, as shown in  FIG. 4 , the defrosting pipe  158  constituting the defrosting circuit branches from the refrigerant introduction pipe  92  at somewhere along it and is connected through the electromagnetic valve  159  serving as a flow-path control device to the refrigerant discharge pipe  96  extending to the inlet of the gas cooler  154 . 
   The following will describe operations with reference to this configuration. It is to be noted that the electromagnetic valve  159  is supposed to stay closed during ordinary heating. When the stator coil  28  of the electrical-power element  14  is electrified through the terminal  20  and a wiring line not shown, the electrical-power element  14  is actuated, thus causing the rotor  24  to revolve. By this revolution, the upper and lower rollers  46  and  48  are fitted to the upper and lower eccentric portions  42  and  44  provided integrally with the rotary shaft  16 , to eccentrically revolve in the upper and lower cylinders  38  and  40  respectively. 
   Accordingly, a low-pressure (first-stage suction pressure: 4 MPaG) refrigerant sucked into the low-pressure chamber side of the cylinder  40  from a suction port, not shown, through the refrigerant introduction pipe  94  and the suction path  60  formed in the lower-part support member  56  is compressed by operations of the lower roller  48  and the vane  52  to have a medium pressure (first-stage discharge pressure: 8 MPaG), passed through the high-pressure chamber side of the lower cylinder  40  and a discharge port not shown, and discharged into the discharge-noise silencer chamber  64  formed in the lower-part support member  56 . Then, the medium pressure refrigerant gas discharged into the discharge-noise silencer chamber  64  is discharged through the communication path into the sealed vessel  12  from the intermediate discharge pipe  121 , thus providing the medium pressure (8 MPaG) in the sealed vessel  12 . 
   Then, the medium pressure refrigerant gas in the sealed vessel  12  exits it through the sleeve  144 , passes through the refrigerant introduction pipe  92  and the suction path, not shown, formed in the upper-part support member  54 , and is sucked from a suction port, not shown, into the lower-pressure chamber side of the upper cylinder  38 . The medium pressure refrigerant gas thus sucked undergoes second-stage compression through operations of the roller  46  and the vane  50  to provide a high-temperature, high-pressure refrigerant gas (second-stage discharge pressure: 12 MPaG), which in turn passes from the high-pressure chamber side and a discharge port not shown to be discharged into the discharge-noise silencer chamber  62  formed in the upper-part support member  54 . 
   The refrigerant gas thus sucked into the discharge-noise silencer chamber  62  flows into the gas cooler  154  from the refrigerant discharge pipe  96 . At this moment, the refrigerant has a raised temperature of about +100° C. and, therefore, such a high temperature, high pressure gas radiates heat to heat water in the hot-water storage tank to thus generate hot water having a temperature of about +90° C. 
   The refrigerant itself, on the other hand, is cooled at the gas cooler  154  and exits it. Then, the refrigerant is decompressed at the expansion valve  156 , flows into the evaporator  157  to evaporate there, passes through the accumulator  146 , and is sucked into the first rotary compression element  32  through the refrigerant introduction pipe  94 , which cycle is repeated. 
   During such heating operation, a pressure in the discharge-noise silencer chamber  62  reaches an extremely high value of 12 MPaG as mentioned above, so that if a pressure of the back pressure chamber  99  is lower than the pressure in the discharge-noise silencer chamber  99  with a difference therebetween being larger than 2 MPaG, as mentioned above, the valve disc  105  of the pressure adjustment valve  107  opens the communication path  100 . Accordingly, the high-pressure refrigerant gas in the discharge-noise silencer chamber  62  flows into the back pressure chamber  99 . 
   If such introduction of the pressure increases a pressure in the back pressure chamber  99  until the difference between the pressure in the back pressure chamber  99  and the pressure in the discharge-noise silencer chamber  62  decreases to 2 MPaG, as mentioned above, the valve disc  105  of the pressure adjustment valve  107  closes the communication path  100 , thus stopping flow of the refrigerant gas into the back pressure chamber. 
   In such a manner, when the second-stage discharge pressure is 12 MPaG, a pressure in the back pressure chamber  99  is held at about 10 MPaG higher than the medium pressure 8 MPaG and lower than the second-stage discharge pressure 12 MPaG, so that it is possible to prevent the back pressure higher than necessary from being applied to the vane  50  while preventing a so-called vane breakaway, thus optimizing force for urging the vane  50  on the upper roller  46 . Accordingly, it is possible to reduce a load applied to a portion where a tip of the vane slides along an outer periphery of the roller to thereby improve durability of the vane  50  and the upper roller  46 , thus avoiding damages of the vane and the roller beforehand. 
   In this case, especially in a low outside-air temperature environment, heating operation causes the evaporator  157  to be frosted. In such a case, the electromagnetic valve  159  is opened and the expansion valve  156  is opened fully to defrost the evaporator  157 . Thus, a medium-pressure refrigerant in the sealed vessel  12  (including a small amount of high pressure refrigerant discharged from the second rotary compression element  34 ) passes through the defrosting pipe  158  to reach the gas cooler  154 . This refrigerant has a temperature of roughly +50° C. through +60° C. and so radiates no heat at the gas cooler  154  but, instead, absorbs heat at the beginning. Then, the refrigerant discharged from the gas cooler  154  passes through the expansion valve  156  to reach the evaporator  157 . That is, the roughly medium-pressure, comparatively high-temperature refrigerant is essentially supplied to the evaporator  157  directly without being decompressed, thus heating the evaporator  157  to defrost it. 
   Thus, the rotary compressor according to the present embodiment which comprises the electrical-power element  14  and the first and second rotary compression elements  32  and  34  driven by the electrical-power element  14  in the sealed vessel  12  in such a configuration that a refrigerant gas compressed at the first rotary compression element  32  is discharged into the sealed vessel  12  and this medium pressure refrigerant gas thus discharged is then compressed at the second rotary compression element  34 , wherein there are also provided the upper cylinder  38  constituting the second rotary compression element  34 , the upper roller  46  which is fitted to the upper eccentric portion  42  formed on the rotary shaft  16  of the electrical-power element  14  to thereby eccentrically revolves in the upper cylinder  38 , the vane  50  which butts against this upper roller  46  to divide an inside of the upper cylinder  38  into a low-pressure chamber side and a high-pressure chamber side, the back pressure chamber  99  which urges this vane  50  on a side of the upper roller.  46  always, the communication path  100  which communicates a refrigerant discharge side of the second rotary compression element  34  and the back pressure chamber  99  to each other, and the pressure adjustment valve  107  for adjusting a pressure applied to the back pressure chamber  99  through this communication path, so that by using this pressure adjustment valve  107  to set a pressure of the back pressure chamber  99  to a predetermined value lower than a high pressure on the refrigerant discharge side of the second rotary compression element  34  and higher than a medium pressure in the sealed vessel  12 , it is possible to prevent a back pressure higher than necessary from being applied to the vane  50  while preventing the so-called vane breakaway, thus optimizing force for urging the vane  50  on the upper roller  46 . 
   Accordingly, it is possible to reduce a load applied to a portion where a tip of the vane slides along an outer periphery of the upper roller  46  to thereby improve durability of the vane  50  and the upper roller  46 , thus avoiding damages of the vane and the roller beforehand. 
   In particular, the communication path  100  is formed in the upper-side support member  54  to communicate the discharge-noise silencer chamber  62  and the back pressure chamber  99  to each other and also the pressure adjustment valve  107  is provided in the upper-part support member  54 , so that it is possible to adjust a pressure in the back pressure chamber  99  of the vane  50  without complicating a construction while effectively utilizing an internal limited space of the sealed vessel  12 . Furthermore, since the communication path  100  and the pressure adjustment valve  107  can be provided in the upper-part support member  54  beforehand, a work efficiency in assembly can be improved. 
   It is to be noted that pressure values employed on the present embodiment are not restrictive and so may be set appropriately corresponding to a capacity and a function of various compressors. Furthermore, although the present embodiment has been described with reference to a multi-stage compression type rotary compressor  10  in which the rotary shaft  16  is mounted vertically, of course the present invention can be applied also to a multi-stage compression type rotary compressor in which the rotary shaft is mounted horizontally. 
   Furthermore, the multi-stage compression type rotary compressor has been described as a two-stage compression type rotary compressor equipped with first and second rotary compression elements, the present invention is not limited thereto; for example, the multi-stage compression type rotary compressor may be equipped with three, four, or even more stages of rotary compression elements. Furthermore, although the present embodiment has used the multi-stage compression type rotary compressor  10  in a refrigerant circuit of the hot-water supply apparatus  153 , the present invention is not limited thereto; for example, the present invention may well be applied for warming of a room. 
   As detailed above, by the present invention, in a multi-stage compression type rotary compressor according to the present embodiment which comprises an electrical-power element and first and second rotary compression elements driven by this electrical-power element in a sealed vessel in such a configuration that a refrigerant gas compressed at the first rotary compression element is discharged into the sealed vessel and this medium pressure refrigerant gas thus discharged is compressed at the second rotary compression element, there are also provided a cylinder constituting the second rotary compression element, a roller which is fitted to an eccentric portion formed on a rotary shaft of the electrical-power element to thereby eccentrically revolves in the cylinder, a vane which butts against this roller to divide an inside of the cylinder into a low-pressure chamber side and a high-pressure chamber side, a back pressure chamber which always urges this vane on a side of the roller, a communication path which communicates a refrigerant discharge side of the second rotary compression element and the back pressure chamber to each other, and a pressure adjustment valve for adjusting a pressure applied to the back pressure chamber through this communication path, so that by setting a pressure of the back pressure chamber at a predetermined value lower than a pressure on a refrigerant discharge side of the second rotary compression element and higher than a pressure in the sealed vessel  12 , it is possible to prevent a back pressure higher than necessary from being applied to the vane while preventing the so-called vane breakaway, thus optimizing force for urging the vane on the roller. 
   Accordingly, it is possible to reduce a load applied to a portion where a tip of the vane slides along an outer periphery of the roller to thereby improve durability of the vane and the roller, thus avoiding damages of the vane and the roller beforehand. 
   Furthermore, there are also provided a support member which blocks an opening face of the cylinder and also which has a bearing for the rotary shaft of the electrical-power element and a discharge-noise silencer chamber arranged in this support member in such a configuration that the communication path is formed in the support member to communicate the discharge-noise silencer chamber and the back pressure chamber to each other and also the pressure adjustment valve is provided in the support member, so that it is possible to adjust a pressure in the back pressure chamber of the vane without complicating a construction while effectively utilizing an internal limited space of the sealed vessel. Furthermore, since the communication path and the pressure adjustment valve can be provided in the support member beforehand, a work efficiency in assembly can be improved. 
   The following will describe a multi-stage compression type rotary compressor according to a further embodiment of the present invention with reference to  FIGS. 8-13 .  FIG. 8  is a vertical cross-sectional view of the multi-stage compression type rotary compressor according to the present embodiment. It is to be noted that the same reference numerals in these figures as those in  FIGS. 1-5  have the same or similar functions. 
   In  FIG. 8 , a reference numeral  10  indicates an internal medium-pressure, multi-stage compression type rotary compressor using carbon dioxide as a refrigerant which comprises the cylindrical sealed vessel  12  made of a steel plate and a rotary compression mechanism portion  18  which includes an electrical-power element  14  arranged and housed in an upper part of an internal space of the sealed vessel  12  and the first rotary compression element  32  (first stage) and the second rotary compression element  34  (second stage) which are arranged below the electrical-power element  14  to be driven by the rotary shaft  16  of the electrical-power element  14 . 
   The sealed vessel  12  has its bottom used as an oil reservoir and is composed of the vessel body  12 A which houses the rotary compression mechanism portion  18  and the electrical-power element  14  and the roughly cup-shaped end cap (lid)  12 B which blocks an upper part opening of the vessel body  12 A in such a configuration that the end cap  12 B has the circular attachment hole  12 D formed therein at a center of its top face, in which attachment hole  12 D the terminal  20  (wiring of which is omitted) is attached which supplies power to the electrical-power element  14 . 
   The electrical-power element  14  is composed of the stator  22  mounted annularly along an inner peripheral face of an upper-part space of the sealed vessel  12  and the rotor  24  disposed and inserted in the stator  22  with some gap set therebetween. This rotor  24  is fixed to the rotary shaft  16  which vertically extends centrally. 
   The stator  22  has the stack  26  formed by stacking donut-shaped electromagnetic steel plates and the stator coil  28  wound round teeth of the stack  26  by direct winding (concentrated winding). Furthermore, similar to the stator  22 , the rotor  24  is also made of the stack  30  of electromagnetic steel plates and the permanent magnet MG inserted into the stack  30 . 
   The intermediate partition plate  36  is sandwiched between the first rotary compression element  32  and the second rotary compression element  34 . That is, a combination of the first rotary compression element  32  and the second rotary compression element  34  is composed of the intermediate partition plate  36 , the upper and lower cylinders  38  and  40  arranged above and below this intermediate partition plate  36  respectively, the upper and lower rollers  46  and  48  which are fitted to the upper and lower eccentric portions  42  and  44  provided on the rotary shaft  16  with a phase difference of 180 degrees therebetween to thereby eccentrically revolve within these upper and lower cylinders  38  and  40  respectively, the upper and lower vanes  50  and  52  which butt against the upper and lower rollers  46  and  48  to divide an inside of the respective upper and lower cylinders  38  and  40  into a low-pressure chamber side and a high-pressure chamber side, and the upper-part support member  54  and the lower-part support member  56  given as a support member for blocking an upper-side opening face of the upper cylinder  38  and a lower-side opening face of the lower cylinder  40  respectively to serve also as a bearing for the rotary shaft  16 . 
   A combination of the upper-part support member  54  and the lower-part support member  56  is provided therein with the suction paths  58  and  60  which communicate with insides of the upper and lower cylinders  38  and  40  through suction ports  161  and  162  respectively and the concave discharge-noise silencer chambers  62  and  64  in such a configuration that openings of these two discharge-noise silencer chambers  62  and  64  are blocked by respective covers. That is, the discharge-noise silencer chamber  62  is blocked by the upper cover  66  serving as a cover and the discharge-noise silencer chamber  64 , by the lower cover  68  serving as a cover. 
   In this case, a bearing  54 A is formed as erected at a center of the upper-part support member  54 . At a center of the lower-part support member  56  is there formed a bearing  56 A as going through, so that the rotary shaft  16  is held by the bearing  54 A of the upper-part support member  54  and the bearing  56 A of the lower-part support member  56 . 
   It is to be noted that a communication path  200  is formed in the lower-part support member  56  between the suction path  60  of the first rotary compression element  32  and the discharge-noise silencer chamber  64 . This communication path  200  communicates, to each other, the suction path  60  which is on a refrigerant suction side of the first rotary compression element  32  and the discharge-noise silencer chamber  64  which is on a refrigerant discharge side where a medium refrigerant compressed at the first rotary compression element  32  is discharged, details of which path  200  are shown in FIG.  9 . That is, one end of a first path  201  opens into the discharge-noise silencer chamber  64 , while the other end thereof opens into a valve-device housing chamber  202 , thus communicating the discharge-noise silencer chamber  64  and the valve-device housing chamber  202  to each other. 
   This valve-device housing chamber  202  is formed vertically in such a configuration that an upper-part opening thereof toward the suction path  60  and a lower-part opening thereof toward the lower cover  68  are blocked by sealing, agents  204  and  205  respectively. 
   Above a position where the first path  201  opens into the valve-device housing chamber  202 , one end of a second path  203  opens into it and the other end thereof opens into the suction path  60 , thus communicating the valve device housing chamber  202  and the suction-path  60  to each other. These first and second paths  201  and  203  and valve-device housing chamber  202  are formed in the lower-part support member  56 , thus constituting the communication path  200 . In this valve-device housing chamber  202  is there vertically movably housed a valve device  206  which functions as a release valve. On an upper face of this valve device is there provided a telescoping spring  207  in a condition where one end thereof butts against it and the other end thereof is fixed to the sealing agent  204 , so that the valve device  206  is downward urged by the spring  207  always. 
   Furthermore, if the valve device  206  is placed between an opening position of the first path  201  and that of the second path  203  as shown in  FIG. 9 , a combination of a pressure in the suction path  60  (low pressure LP) and force of the spring  207  downward urges the valve device  206 , whereas the medium pressure upward urges the valve device  206  through the first path  201 . That is, the valve device  206  moves up and down in the valve-device housing chamber  202  owing to a pressure difference between a pressure of a low-pressure refrigerant gas on a refrigerant suction side plus urging force of the spring  207  and that of a medium-pressure refrigerant gas on a refrigerant discharge side. 
   Furthermore, by the present embodiment, if the pressure difference between a pressure of the low-pressure refrigerant gas and that of the medium-pressure refrigerant gas is 5 MPaG or less, the valve device  206  housed in the valve-device housing chamber  202  is put in a state shown in  FIG. 9  in being positioned between the other end of the first path  201  and the second path  203  in the valve-device housing chamber  202 , so that the refrigerant suction side and the refrigerant discharge side are not communicated to each other but blocked from each other by the valve device  206 . 
   The urging force of the spring  207  is set so that if the medium pressure rises until the pressure difference between a pressure of the low-pressure refrigerant gas and that of the medium-pressure refrigerant gas increases up to 5 MPaG (upper limit value), the valve device  206  is raised above the second path  203  by the mediate-pressure refrigerant gas flowing through the first path  201  to communicate the first path  201  and the second path  203  to each other (open the communication path  200 ) in order to flow the medium-pressure refrigerant gas on the refrigerant discharge side into the suction path  60  on the refrigerant suction side. If the pressure difference between the two becomes less than 5 MPaG, on the other hand, the valve device  206  is lowered to a position between a communication position of the first path  201  below the second path  203  and a communication position of the second path  203  to block the first path  201  and the second path  203  from each other, thus closing the communication path  200 . In such a manner, it is possible to regulate below the upper limit value a first-stage differential pressure, that is, a pressure difference between the refrigerant discharge side and the refrigerant suction side of the first rotary compression element  32 . 
   The lower cover  68 , on the other hand, is made of a donut-shaped circular steel plate and fixed upward to the lower-part support member  56  by main bolts  129  disposed peripherally, to block a lower-part opening of the discharge-noise silencer chamber  64  communicating with an inside of the lower cylinder  40  of the first rotary compression element  32  through the discharge port  41 . Tips of these main bolts  129  are screwed to the upper-part support member  54 .  FIG. 10  shows a bottom of the lower-part support member, in which a reference numeral  128  indicates a discharge valve of the first rotary compression element  32  for opening and closing the discharge port  41  in the discharge-noise silencer chamber  64 . 
   Further, the discharge-noise silencer chamber  64  and a face of the upper cover  66  on a side of the electrical-power element  14  in the sealed vessel  12  are communicated to each other through a communication path, not shown, which penetrates the upper and lower cylinders  38  and  40  and the intermediate partition plate  36 . In this case, at an upper end of the communication path is there provided the intermediate discharge pipe  121  as erected, through which a medium-pressure refrigerant is discharged into the sealed vessel  12 . 
   Furthermore, the upper cover  66  blocks an upper-face opening of the discharge-noise silencer chamber  62  communicating with an inside of the upper cylinder  38  of the second rotary compression element  34  through a discharge port  39 , thus partitioning an inside of the sealed vessel  12  into the discharge-noise silencer chamber  62  and a side of the electrical-power element  14 . As shown in  FIG. 11 , this upper cover  66  is made of a roughly donut-shaped circular steel plate in which a hole is formed through which the bearing  54 A for the upper-part support member  54  extends through and fixed downward to the upper-part support member  54  by main bolts  78  peripherally. Tips of these main bolts  78  are screwed to the lower-part support member  56 . It is to be noted that a reference numeral  127  in  FIG. 11  indicates a discharge valve of the second rotary compression element  34  for opening and closing the discharge port  39  in the discharge-noise silencer chamber  62 . 
   It is to be noted that discharge valves  127  and  128  are made of an elastic member such as a vertically long metal plate, one sides of which valves  127  and  128  butt against the discharge ports  39  and  41  respectively in close contact therewith and the other sides of which are fixed by screws, not shown, in screw holes, not shown, formed somewhere distant from the discharge ports  39  and  41  by a predetermined spacing. The discharge valves  127  and  128  butt against the discharge ports  39  and  41  with constant urging force to open and close the discharge ports  39  and  41  by elasticity respectively. 
   In  FIG. 8 , a reference numeral  94  indicates a suction pipe of the first rotary compression element  32 , which suction pipe is attached and communicated to the suction path  60  of the lower-part support member  56 . Reference numerals  92  and  96  indicate a suction pipe and a discharge pipe of the second rotary compression element  34 , one end of which suction pipe  92  communicates to an inside of the sealed vessel  12  above the upper cover  66  and the other end of which communicates with the suction path  58  of the second rotary compression element  34 . The discharge pipe  96  is attached and communicated to the discharge-noise silencer chamber  62  of the second rotary compression element  34 . 
   In this case, as a refrigerant, carbon dioxide (CO 2 ) which is a natural refrigerant friendly to environments of the earth is used taking into account inflammability, toxicity, etc., while as a lubricant, such existing oil is used as mineral oil, alkyl-benzene oil, ether oil, or ester oil. 
   The following will describe operations with reference to this configuration. When the stator coil  28  of the electrical-power element  14  is electrified through the terminal  20  and a wiring line not shown, the electrical-power element  14  is actuated, thus causing the rotor  24  to revolve. By this revolution, the upper and lower rollers  46  and  48  are fitted to the upper and lower eccentric portions  42  and  44  provided integrally with the rotary shaft  16 , to eccentrically revolve in the upper and lower cylinders  38  and  40  respectively. 
   Accordingly, a low-pressure (LP) refrigerant sucked into the low-pressure chamber side of the lower cylinder  40  from the suction port  162  shown in  FIG. 12  illustrating a bottom of the lower cylinder  40  through the suction pipe  94  and the suction path  60  formed in the lower-part support member  56  is compressed by operations of the lower roller  48  and the lower vane  52  to have a medium pressure (MP), passed through the high-pressure chamber side of the lower cylinder  40  and the discharge port  41 , and discharged into the discharge-noise silencer chamber  64  formed in the lower-part support member  56 . 
   At this moment, if a pressure difference of the refrigerant gas between a pressure of a refrigerant gas in the suction path  60  on a refrigerant suction side and that in the discharge-noise silencer chamber  64  on a refrigerant discharge side is less than 5 MPaG, the valve device  206  is positioned between the communication position of the first path  201  and that of the second path  203  in the valve device housing chamber  202 , so that the communication path  200  is blocked. Then, a medium-pressure refrigerant gas discharged into the discharge-noise silencer chamber  64  passes through a communication path not shown and is discharged into the sealed vessel  12  from the intermediate discharge pipe  121 . Accordingly, the sealed vessel  12  has the medium pressure therein. 
   In this case, for example, if an outside air temperature rises to increase an evaporation temperature of a later-described evaporator and thereby increase the medium pressure until the pressure difference of the refrigerant gas between a pressure of the refrigerant gas in suction path  60  on a low pressure side and that in the discharge-noise silencer chamber  64  on a medium pressure side reaches the upper limit value of 5 MPaG, this increased medium pressure causes the valve device  206  to be pressed upward above the communication position of the second path  203  in the valve device housing chamber  202 , so that the first path  201  and the second path  203  communicate with each other, thus flowing the medium-pressure refrigerant gas into the suction path  60  on the lower pressure side. When the medium-pressure refrigerant is thus discharged to the suction side to thereby reduce the pressure difference between the two below 5 MPaG, the valve device  206  returns downward to a position below the communication position of the second path  203 , so that the communication path  200  (first path  201 , valve device housing chamber  202 , and second path  203 ) is closed by the valve device  206 . 
   Then, the medium-pressure refrigerant gas in the sealed vessel  12  exits it and passes through the suction pipe  92 , enters the suction path  58  formed in the upper-part support member  54 , and is sucked therethrough into a low-pressure chamber side of the upper cylinder  38  from the suction port  161  shown in  FIG. 13  illustrating a top of the upper cylinder  38 . The medium-pressure refrigerant gas thus sucked undergoes second-stage compression through operations of the upper roller  46  and the upper vane  50  to provide a high-temperature, high-pressure refrigerant gas (HP), which passes from a high-pressure chamber side through the discharge port  39  and is sucked from the discharge-noise silencer chamber  62  formed in the upper-part support member  54  and through the discharge pipe  96  into the gas cooler  154  shown in  FIG. 4  provided outside the multi-stage compression type rotary compressor  10 . Then, it flows from the gas cooler  154  into the expansion valve  156  and the evaporator  157  sequentially. 
   Thus, in the multi-stage compression type rotary compressor  10  comprising the electrical-power element  14  and the first and second rotary compression elements  32  and  34  driven by the electrical-power element  14  in the sealed vessel  12  in such a configuration that a refrigerant gas compressed at the first rotary compression element  32  and discharged therefrom is sucked into the second rotary compression element  34  to be compressed and discharged therefrom, there are provided the communication path  200  which communicates a refrigerant suction side and a refrigerant discharge side of the first rotary compression element  32  to each other and the valve device  206  which opens and closes the communication path  200  in such a manner as to open it if a pressure difference between the refrigerant suction side and the refrigerant discharge side of the first rotary compression element  32  exceeds a predetermined upper limit value (5 MPaG), so that it is possible to suppress a first-stage differential pressure down to the upper limit value or less. Accordingly, it is possible to suppress a pressure difference between an inside and an outside of the discharge valve  127  of the first rotary compression type element  32  down to the upper limit value or less, thus avoiding a trouble that the discharge valve  127  may be damaged by the pressure difference. 
   Furthermore, by the present embodiment, the suction path  60  and the discharge-noise silencer chamber  64  arranged in the lower-part support member  56  which blocks an opening face of the lower cylinder  40  constituting the first rotary compression element  32  and also which has a bearing for the rotary shaft  16  of the electrical-power element  14  are communicated to each other through the communication path  200  formed in the lower-part support member  56  and the valve device  206  is also provided in the lower-part support member  56 , so that the communication path  200  and the valve device  206  can be integrated into the lower-part support member  56  to realize miniaturization. Furthermore, it is possible to form the communication path  200  in the lower-part support member  56  beforehand to attach and set the valve device  206  thereto, thus improving a work efficiency in assembly of the multi-stage compression type rotary compressor  10 . 
   It is to be noted that although the present embodiment has been described in all cases with reference to the multi-stage compression type rotary compressor  10  in which the rotary shaft  16  is mounted vertically, of course the present invention can be applied also to a multi-stage compression type rotary compressor in which the rotary shaft is mounted horizontally. Furthermore, the upper limit of the first-stage differential pressure given in the present embodiment is not restricted to the above-mentioned value and so may be set appropriately corresponding to a capacity and an employed pressure of the rotary compressor. 
   Furthermore, the multi-stage compression type rotary compressor has been described as a two-stage compression type rotary compressor equipped with first and second rotary compression elements, the present invention is not limited thereto; for example, the multi-stage compression type rotary compressor may be equipped with three, four, or even more stages of rotary compression elements. 
   As detailed above, according to the present embodiment of the present invention, in a multi-stage compression type rotary compressor comprising an electrical-power element and first and second rotary compression elements driven by this electrical-power element in a sealed vessel in such a configuration that a refrigerant gas compressed in the first rotary compression element and discharged therefrom is sucked into the second rotary compression element to be compressed and discharged therefrom, there are provided a communication path which communicates a refrigerant suction side and a refrigerant discharge side of the first rotary compression element to each other and a valve device which opens and closes this communication path in such a manner as to open it if a pressure difference between the refrigerant suction side and the refrigerant discharge side of the first rotary compression element exceeds a predetermined upper limit value, so that it is possible to suppress the pressure difference between the refrigerant suction side and the refrigerant discharge side of the first rotary compression element which is the first-stage differential pressure down to the predetermined upper limit value or less. Accordingly, it is possible to avoid a trouble such as damaging of the discharge valve provided on the first rotary compression element caused by an excessive value of the first-stage differential pressure, thus improving durability and reliability of the rotary compressor. 
   Furthermore, by the present invention, there are provided a cylinder constituting the first rotary compression element, a support member which blocks an opening face of this cylinder and also which has a bearing for the rotary shaft of the electrical-power element, and a suction path and a discharge-noise silencer chamber which are arranged in this support member in such a configuration that the communication path is formed in the support member to communicate the suction path and the discharge-noise silencer chamber to each other and also the valve device is provided in the support member, so that the communication path and the valve device can be integrated into the cylinder of the first rotary compression element to realize miniaturization and also the valve device can be set into the cylinder beforehand, thus improving a work efficiency in assembly. 
   The following will describe a multi-stage compression type rotary compressor according to a still further embodiment of the present invention with reference to  FIGS. 14-17 .  FIG. 14  shows a vertical cross-sectional view of the multi-stage compression type rotary compressor according to the present embodiment. It is to be noted that the same reference numerals in these figures as those in  FIGS. 1-3  have the same or similar functions. 
   In  FIG. 14 , a reference numeral  10  indicates an internal medium-pressure, multi-stage compression type rotary compressor using carbon dioxide as a refrigerant which comprises the sealed vessel  12  composed of the cylindrical vessel body  12 A made of a steel plate and the roughly cup-shaped end cap (lid body)  12 B which blocks an upper-part opening of this vessel body  12 A and the rotary compression mechanism portion  18  which includes the electrical-power element  14  arranged and housed in an upper part of an internal space of the vessel body  12 A of the sealed vessel  12  and the first rotary compression element  32  (first stage) and the second rotary compression element  34  (second stage) which are arranged below this electrical-power element  14  to be driven by the rotary shaft  16  of the electrical-power element  14 . It is to be noted that the sealed vessel  12  has its bottom used as an oil reservoir. Furthermore, the end cap  12 B has the circular attachment hole  12 D formed therein at a center of its top face, in which attachment hole  12 D the terminal  20  (wiring of which is omitted) is attached for supplying power to the electrical-power element  14 . 
   The electrical-power element  14  is composed of the stator  22  mounted annularly along an inner peripheral face of an upper space of the sealed vessel  12  and the rotor  24  disposed and inserted in the stator  22  with some gap set therebetween. To this rotor  24 , the rotary shaft  16  which vertically extends is fixed. 
   The stator  22  has the stack  26  formed by stacking donut-shaped electromagnetic steel plates and the stator coil  28  wound round teeth of the stack  26  by direct winding (concentrated winding). Furthermore, similar to the stator  22 , the rotor  24  is also made of the stack  30  of electromagnetic steel plates and the permanent magnet MG inserted into the stack  30 . 
   The intermediate partition plate  36  is sandwiched between the first rotary compression element  32  and the second rotary compression element  34 . That is, a combination of the first rotary compression element  32  and the second rotary compression element  34  is composed of the intermediate partition plate  36 , the cylinders  38  and  40  arranged above and below the intermediate partition plate  36  respectively, the upper and lower rollers  46  and  48  which are fitted to the upper and lower eccentric portions  42  and  44  provided on the rotary shaft  16  with a phase difference of 180 degrees therebetween to thereby eccentrically revolve within the upper and lower cylinders  38  and  40  respectively, the upper and lower vanes  50  and  52  which butt against these upper and lower rollers  46  and  48  to divide an inside of the respective upper and lower cylinders  38  and  40  into a low-pressure chamber side and a high-pressure chamber side, and the upper-part support member  54  and the lower-part support member  56  given as a support member for blocking an upper-side opening face of the upper cylinder  38  and a lower-side opening face of the lower cylinder  40  respectively to serve also as a bearing for the rotary shaft  16 . 
   Furthermore, as shown in  FIGS. 11-13  and  FIG. 17 , a combination of the upper-part support member  54  and the lower-part support member  56  is provided therein with the suction paths  58  and  60  which communicate with insides of the upper and lower cylinders  38  and  40  through the suction ports  161  and  162  respectively and the discharge muffler chambers  62  and  64  formed by blocking concavities in the upper-part support member  54  and the lower-part support member  56  by covers serving as a wall respectively. That is, the discharge muffler chamber  62  is blocked by the upper cover  66  serving as a wall defining the discharge muffler chamber  62  and the discharge muffler chamber  64 , by the lower cover  68  serving as a wall defining the discharge muffler chamber  64 . 
   In this case, the bearing  54 A is formed as erected at a center of the upper-part support member  54 . At a center of the lower-part support member  56  is there formed the bearing  56 A as going through, so that the rotary shaft  16  is held by the bearing  54 A of the upper-part support member  54  and the bearing  56 A of the lower-part support member  56 . 
   Furthermore, the lower cover  68  is made of a donut-shaped circular steel plate to define the discharge-noise silencer chamber  64  communicating with an inside of the lower cylinder  40  of the first rotary compression element  32 , and it is fixed upward to the lower-part support member  56  by the main bolts  129  disposed peripherally, tips of which are screwed to the upper-part support member  54 .  FIG. 17  shows a bottom of the lower-part support member  56 , in which a reference numeral  128  indicates the discharge valve of the first rotary compression element  32  for opening and closing the discharge port  41  in the discharge-noise silencer chamber  64 . 
   Further, the discharge-noise silencer chamber  64  of the first rotary compression element  32  and the inside of the sealed vessel  12  communicate with each other through an communication path, which is a hole, not shown, penetrating the upper cover  66 , the upper and lower cylinders  38  and  40 , and the intermediate partition plate  36 . In this case, at an upper end of the communication path is there provided the intermediate discharge pipe  121  as erected, through which a medium-pressure refrigerant is discharged into the sealed vessel  12 . 
   Furthermore, the upper cover  66  defines the discharge-noise silencer chamber  62  communicating through the discharge port  39  with an inside of the upper cylinder  38  of the second rotary compression element  34 , above which upper cover  66  is there provided the electrical-power element  14  with a predetermined spacing present therebetween. Similarly, as described with reference to  FIG. 11 , this upper cover  66  is made of a roughly donut-shaped circular steel plate in which a hole is formed through which the bearing  54 A for the upper-part support member  54  extends through and fixed by the main bolts  78  peripherally. Therefore, tips of these main bolts  78  are screwed to the lower-part support member  56 . 
   It is to be noted that the discharge valves  127  and  128  are constituted of an elastic member made of a vertically long rectangular metal plate, one sides of which valves  127  and  128  butt against the discharge ports  39  and  41  respectively to seal them and the other sides of which are fixed by screws, not shown, provided somewhere distant from the discharge ports  39  and  41  by a predetermined spacing therebetween. The discharge valves  127  and  128  butt against the discharge ports  39  and  41  with constant urging force to open and close the discharge ports  39  and  41  by elasticity respectively. 
   Furthermore, in the upper cover  66  of the second rotary compression element  34  is there provided a communication path  300  according to the present embodiment of the present invention. This communication path  300  communicates, to each other, the inside of the sealed vessel  12  which provides a path through which a medium-pressure refrigerant gas compressed at the first rotary compression element  32  and the discharge-noise silencer chamber  62  on a refrigerant discharge side of the second rotary compression element, in such a configuration that, as shown in  FIG. 15 , one end of a horizontally extending first path  301  communicates with the inside of the sealed vessel  12  and the other end of the first path  301  communicates with a valve device housing chamber  302 . This valve device housing chamber  302  is a hole penetrating the upper cover  66  vertically in such a configuration that an upper face thereof opens into the sealed vessel  12  and a lower face thereof opens into the discharge-noise silencer chamber  62 . Furthermore, upper and lower openings of this valve device housing chamber  302  are blocked by sealing agents  303  and  304  respectively. 
   In the sealing agent  304  provided at a bottom of the valve device housing chamber  302  is there formed a second path  305  which communicates the valve device housing chamber  302  and the discharge-noise silencer chamber  62  to each other. These first path  301 , valve device housing chamber  302 , and second path  305  are combined to constitute the communication path  300 . Furthermore, in the valve device housing chamber  302  of this communication path  300  is there housed a spherical valve device  307 , a top face of which is abutted by one end of a telescoping spring  306  (urging member). The other end of this spring  306  is fixed at the upper side sealing agent  303 , so that the valve device  307  is always downward urged by this spring  306  to thereby block the second path  305  always. 
   Furthermore, in construction, a medium pressure refrigerant in the sealed vessel  12  flows through the first path  301  into the valve device housing chamber  302  to downward urge the valve device  307 , while a high pressure refrigerant in the discharge-noise silencer chamber  62  flows through the second path  305  formed in the lower side sealing agent  304  into the valve device housing chamber  302  to upward urge the valve device  307  at its bottom. 
   Thus, the valve device  307  is downward urged by the medium pressure refrigerant gas and the spring  306  from a side where the spring  306  butts against, that is, from the above and, from an opposite side, upward urged by the high pressure refrigerant gas. Therefore, the bottom of the valve device  307  always butts against the second path  305  to be sealed, so that the communication path  300  is blocked by the valve device  307  always. 
   It is to be noted that the urging force of the spring  306  is supposed to be set so that when a pressure difference between a pressure of a medium pressure refrigerant gas in the sealed vessel  12  and that of a high pressure refrigerant gas in the discharge-noise silencer chamber  62  has reached an upper limit value of, for example, 8 MPaG, the valve device  307  abutted against the first path  305  to close it may be pressed upward by the high pressure refrigerant gas flowing in through the second path  305 . Therefore, if this pressure difference exceeds 8 MPaG (upper limit value), the first path  301  and the second path  305  communicate with each other through the valve device housing chamber  302 , so that the high pressure refrigerant gas in the discharge-noise silencer chamber  62  flows into the sealed vessel  12 . If this pressure difference is reduced below 8 MPaG, on the other hand, the spring  306  abuts the valve device  307  against the second path  305  to close it, so that the valve device  307  blocks the first path  301  and the second path  305  from each other. Thus, a second-stage differential pressure can be prevented beforehand from becoming excess. 
   As described above, as a refrigerant, carbon dioxide (CO 2 ) which is a natural refrigerant friendly to environments of the earth is used taking into account inflammability, toxicity, etc., while as a lubricant, such existing oil is used as mineral oil, alkyl-benzene oil, ether oil, or ester oil. 
   The following will describe operations with reference to this configuration. When the stator coil  28  of the electrical-power element  14  is electrified through the terminal  20  and a wiring line not shown, the electrical-power element  14  is actuated, thus causing the rotor  24  to revolve. By this revolution, the upper and lower rollers  46  and  48  are fitted to the upper and lower eccentric portions  42  and  44  provided integrally with the rotary shaft  16 , to eccentrically revolve in the upper and lower cylinders  38  and  40  respectively. 
   Accordingly, a low-pressure refrigerant sucked into the low-pressure chamber side of the lower cylinder  40  from the suction port  162  through the suction path  60  formed in the lower-part support member  56  as shown in  FIG. 11  is compressed by operations of the lower roller  48  and the lower vane  52  to have a medium pressure, passed through the high-pressure chamber side of the lower cylinder, and the discharge port  41 , the discharge-noise silencer chamber  64  formed in the lower-part support member  56 , and a communication path not shown, and is discharged into the sealed vessel  12  from the intermediate discharge pipe  121 . 
   Then, the medium-pressure refrigerant gas in the sealed vessel  12  passes through a refrigerant path not shown and the suction path  58  formed in the upper-part support member  54 , and is sucked into the low-pressure chamber side of the upper cylinder  38  from the suction port  161  shown in FIG.  13 . The medium-pressure refrigerant gas thus sucked undergoes second-stage compression through operations of the upper roller  46  and the upper vane  50  to provide a high-temperature, high-pressure refrigerant gas, which passes from the high-pressure chamber side through the discharge port  39  and is sucked into the discharge-noise silencer chamber  62  formed in the upper-part support member  54 . 
   If, a this moment, a pressure difference between a pressure of the medium pressure refrigerant gas in the sealed vessel  12  and that of the high pressure refrigerant gas in the discharge-noise silencer chamber  62  is less than 8 MPaG, as mentioned above, the valve device  307  is abutted against the second path  305  to close it in the valve-device housing chamber  302 , so that the communication path  300  is not opened and, therefore, the high pressure refrigerant gas discharged into the discharge-noise silencer chamber  62  all flows through a refrigerant path not shown into the gas cooler  154  ( FIG. 4 ) provided outside the multi-stage compression type rotary compressor  10 . 
   After flowing into the gas cooler  154 , the refrigerant radiates heat to exert a heating action. After exiting the gas cooler  154 , the refrigerant is decompressed at the expansion valve  156  and enters the evaporator  157  to evaporate there. Finally, the refrigerant is sucked to the suction path  60  of the first rotary compression element  32 , which cycle is repeated. 
   It is to be noted that if an outside air temperature drops to reduce an evaporation temperature of the refrigerant in the evaporator, as described above, it is difficult also for a pressure (medium pressure) of a refrigerant discharged from the first rotary compression element  32  into the sealed vessel  12  to rise. Thus, when a pressure difference between a pressure of a medium pressure refrigerant gas in the sealed vessel  12  and that of a high pressure refrigerant gas in the discharge-noise silencer chamber  62  has reached 8 MPaG, the valve device  307  abutted against the second path  305  by a pressure in the discharge-noise silencer chamber  62  is pressed upward against the spring  306  to be released from the second path  305 , so that the first path  301  and the second path  305  communicate with each other to flow the high pressure refrigerant gas into the sealed vessel  12  on a medium pressure side. If the pressure difference between the two drops below 8 MPaG, on the other hand, the valve device  307  butts against the second path  305  to close it, thus blocking the second path  305 . 
   As described above, in the present embodiment comprising the electrical-power element  14  and the first and second rotary compression elements  32  and  34  driven by this electrical-power element  14  in the sealed vessel  12  in such a configuration that a medium pressure refrigerant gas compressed at the first rotary compression element  32  is sucked into the second rotary compression element  34  to be compressed and discharged therefrom, there are provided the communication path  300  which communicates a passage for the medium pressure refrigerant compressed at the first rotary compression element  32  and a refrigerant discharge side of the second rotary compression element  34  to each other and the valve device which opens and closes this communication path  300 , wherein a pressure difference between a pressure of the medium pressure refrigerant gas and that of a refrigerant gas on a refrigerant discharge side of the second rotary compression element  34  exceeds a predetermined upper limit value of 8 MPaG, the valve device  307  opens the communication path  307 , so that it is possible to suppress a second-stage differential pressure below the upper limit value, thus avoiding damaging of the discharge valve  128  of the second rotary compression element  34  beforehand. 
   Furthermore, there are also provided the upper cylinder  38  constituting the second rotary compression element  34 , the discharge-noise silencer chamber  62  into which a refrigerant gas compressed in this upper cylinder  38  is discharged, and the upper cover  66  serving as a wall defining this discharge-noise silencer chamber  62  in such a configuration that the communication path  300  is formed in the upper cover  66  to communicate an inside of the sealed vessel  12  and the discharge-noise silencer chamber  62  to each other and also the valve device  307  is provided in the upper cover  66 , so that it is possible to suppress the second-stage differential pressure without complicating a construction of the communication path  300 . 
   Although the present embodiment has been described in all cases with reference to the multi-stage compression type rotary compressor  10  in which the rotary shaft  16  is mounted vertically, of course the present invention can be applied also to a multi-stage compression type rotary compressor in which the rotary shaft is mounted horizontally. 
   Furthermore, the multi-stage compression type rotary compressor has been described as a two-stage compression type rotary compressor equipped with first and second rotary compression elements, the present invention is not limited thereto; for example, the multi-stage compression type rotary compressor may be equipped with three, four, or even more stages of rotary compression elements. 
   It is to be noted that although the present embodiment has employed a spherical valve device  307 , the present invention is not limited thereto; for example, a cylindrical valve device  317  such as shown in  FIG. 16  may be employed. In this case, the valve device  317  is arranged to butts against a wall face of the valve-device housing chamber  302  to seal it in such a configuration that it is ordinarily placed in the valve-device housing chamber  302  between the first path  301  and the second path  305  to thereby block the communication path  300 . In this configuration, if the pressure difference exceeds 8 MPaG, the valve device  317  is pressed upward above the first path  301  to thereby communicate the first path  301  and the second path  305  to each other, thus flowing a high pressure refrigerant gas into the sealed vessel  12  having a medium pressure. If the pressure difference between the two drops below 8 MPaG, the valve device  317  returns back below the first path  301 , thus blocking the first path  301  and the second path  305  from each other. 
   As detailed above, according to the present embodiment of the present invention, in a multi-stage compression type rotary compressor comprising an electrical-power element and first and second rotary compression elements driven by this electrical-power element in a sealed vessel in such a configuration that a medium pressure refrigerant gas compressed at the first rotary compression element is sucked into the second rotary compression element to be compressed and discharged therefrom, there are provided a communication path which communicates a passage for the medium pressure refrigerant compressed at the first rotary compression element and a refrigerant discharge side of the second rotary compression element to each other and a valve device which opens and closes this communication path in such a manner as to open it if a pressure difference between a pressure of the medium pressure refrigerant gas and that of a refrigerant gas on the refrigerant discharge side of the second rotary compression element exceeds a predetermined upper limit value, so that it is possible to suppress a pressure difference between a discharge pressure and a suction pressure of the second rotary compression element, that is, a second-stage differential pressure, below the predetermined upper limit value. 
   Accordingly, it is possible to avoid an occurrence of a trouble such as damaging of the discharge valve of the second rotary compression element. 
   Furthermore, there are provided also a cylinder which constitutes the second rotary compression element and a discharge-noise silencer chamber which discharges a refrigerant gas compressed in this cylinder in such a configuration that a medium pressure refrigerant gas compressed at the first rotary compression element is discharged into the sealed vessel and then sucked into the second rotary compression element, the communication path is formed in a wall defining the discharge-noise silencer chamber to communicate an inside of the sealed vessel and the discharge-noise silencer chamber to each other, and the valve device is provided in the wall, so that it is possible to integrate the communication path which communicates the passage for the medium pressure refrigerant compressed at the first rotary compression element and the refrigerant discharge side of the second rotary compression element to each other and the valve device which opens and closes the communication path into a wall of the second rotary compression element. 
   Accordingly, it is possible to simplify a construction and reduce overall size. 
   The following will describe a multi-stage compression type rotary compressor according to an additional embodiment of the present invention with reference to  FIGS. 18 and 19 .  FIG. 18  shows a vertical cross-sectional of a multi-stage compression type rotary compressor according to the present embodiment. It is to be noted that the same reference numerals in these figures as those in  FIGS. 1-17  have the same or similar functions. 
   In  FIG. 18 , a reference numeral  10  indicates an internal medium-pressure, multi-stage compression type rotary compressor using carbon dioxide (CO 2 ) as a refrigerant which comprises the cylindrical sealed vessel  12  made of a steel plate and the rotary compression mechanism portion  18  which includes the electrical-power element  14  arranged and housed in an upper part of an internal space of the sealed vessel  12  and the first rotary compression element  32  (first stage) and the second rotary compression element  34  (second stage) which are arranged below this electrical-power element  14  to be driven by the rotary shaft  16  of the electrical-power element  14 . 
   It is to be noted that in the rotary compressor  10  of the present embodiment, a displacement volume of the second rotary compression element  34  is set smaller than that of the first rotary compression element  32 . 
   The sealed vessel  12  has its bottom used as an oil reservoir and is composed of the vessel body  12 A which houses the electrical-power element  14  and the rotary compression mechanism portion  18  and the roughly cup-shaped end cap (lid)  12 B which blocks an upper part opening of this vessel body  12 A in such a configuration that at a top face of the end cap  12 B is there attached the terminal  20  (wiring of which is omitted) which supplies power to the electrical-power element  14 . 
   The electrical-power element  14  is composed of the stator  22  mounted annularly along an inner peripheral face of an upper-part space of the sealed vessel  12  and the rotor  24  disposed and inserted in the stator  22  with some gap set therebetween. This rotor  24  is fixed to the rotary shaft  16  which vertically extends centrally. 
   The stator  22  has the stack  26  formed by stacking donut-shaped electromagnetic steel plates and the stator coil  28  wound round teeth of the stack  26  by direct winding (concentrated winding). Furthermore, similar to the stator  22 , the rotor  24  is also made of the stack  30  of electromagnetic steel plates and the permanent magnet MG inserted into the stack  30 . 
   The intermediate partition plate  36  is sandwiched between the first rotary compression element  32  and the second rotary compression element  34 . A combination of the first rotary compression element  32  and the second rotary compression element  34  is composed of the intermediate partition plate  36 , the upper and lower cylinders  38  and  40  arranged above and below the intermediate partition plate  36  respectively, the upper and lower eccentric portions  42  and  44  which are positioned in the upper and lower cylinders  38  and  40  respectively and provided on the rotary shaft  16  with a phase difference of 180 degrees therebetween, and the upper-part support member  54  and the lower-part support member  56  given as a support member for blocking an upper-side opening face of the upper cylinder  38  and a lower-side opening face of the lower cylinder  40  respectively to serve also as a bearing for the rotary shaft  16 . 
   The first rotary compression element  32  is provided with the lower roller  48  which eccentrically revolves as engaged to the lower eccentric portion  44  and the vane  52  which butts against this lower roller  48  to thereby divide an inside of the lower cylinder  40  into a low-pressure chamber side and a high-pressure chamber side. The cylinder  40  is provided with a guide groove for housing the vane  52  in such a manner that the vane  52  can slide therein and a spring  76  arranged outside this guide groove, so that this spring  76  butts against an outer end portion of the vane  52  to always urge the vane  52  on the roller  48 . Furthermore, on a side of the sealed vessel  12  in a housing of this spring  76  is there provided a metallic plug  437  which serves to prevent fall-out of the spring  76 . 
   The guide groove in the cylinder  40  communicates with an inside of the sealed vessel  12  on a side of the outer end of the vane  52 , so that a later-described medium pressure in the sealed vessel  12  is applied as a back pressure for the vane  52  in configuration. 
   Furthermore, the upper cylinder  38  of the second rotary compression element  34  is provided therein with a swing piston  410 , which is constituted of a roller portion  412  and a vane portion  414  (FIG.  19 ). The roller portion  412  is engaged to the upper eccentric portion  42  of the rotary shaft  16 , so that as the upper eccentric portion  42  revolves in this roller portion  412  eccentrically, correspondingly the roller portion  412  itself moves eccentrically as butting against an inner face of the upper cylinder  38 . 
   The vane portion  414 , which projects from this roller portion  412  in a radial direction, enters a holding groove  416 A in a later-described bush  416  and is held therein to thereby divide an inside of the upper cylinder  38  into a low-pressure chamber-side and high-pressure chamber side in configuration (FIG.  19 ). 
   Furthermore, in the upper cylinder  38  is there formed the guide groove  70  extending from an inner circumference in a radial direction, at an inner end of which guide groove  70  is there formed as expanded a roughly cylindrical holding hole  88  vertically. Into this holding hole  88  the bush  416  described above is inserted-to be held therein as rotating round a vertical axis as a center. 
   The holding groove  416 A described above is formed through in this bush  416  along its center in a direction of a diameter of this bush  416  (radial direction of the upper cylinder  38 ), in such a configuration that the vane portion  414  of the swing piston  410  enters the guide groove  70  and passes through this holding groove  416 A to be held in this holding groove  416 A in such a manner that it can slide. In this condition, the vane portion  414  can move in the guide groove  70  and also, when the bush  416  itself rotates, the swing piston  410  itself is held in such a manner that it can slide and swing. 
   That is, the swing piston  410  has the roller portion  412  which eccentrically moves in the upper cylinder  38  in a condition where it is engaged to the upper eccentric portion  42  formed on the rotary shaft  16  of the electrical-power element  14  and is provided with the vane portion  414  which projects from this roller portion  412  in a radial direction to divide an inside of the upper cylinder  38  into a low-pressure chamber side and a high-pressure chamber side. In this configuration, as the upper eccentric portion  42  revolves eccentrically, the swing piston  410  swings in the upper cylinder  38 . In the present embodiment, the guide groove  70  and the bush  416  constitute the holding portion of the present invention. 
   In this case, a spacing between the holding hole  88  and the bush  416  and that between the holding groove  416 A and the vane portion  414  are dimensioned so that they may be sealed off from each other with oil therebetween respectively, to prevent a discharge pressure of the second rotary compression element  34  from being released. Such a construction eliminates a necessity of a spring on the second rotary compression element  34  for urging the vane  52  provided on the first rotary compression element  32  on the roller  48 . If the second rotary compression element  34  is configured like the first rotary compression element  32 , on the other hand, a back pressure is to be applied to the vane to urge it on the roller; a necessity of applying the back pressure to the vane, however, is rendered unnecessary because the second rotary compression element  34  is provided with the swing piston  410 . This swing piston  410  is held by the bush  416  in such a manner that it can swing and slide, so that it is possible to smooth operations of the vane portion  414  owing to the swing piston  410 , thus greatly improving performance of the rotary compressor  10 . 
   The upper-part support member  54  and the lower-part support member  56 , on the other hand, have the concave discharge-noise silencer chambers  62  and  64  formed therein, openings of which are blocked by respective covers. That is, the discharge-noise silencer chamber  62  is blocked by the upper cover  66  serving as a cover, while the discharge-noise silencer chamber  64  is blocked by the lower cover  68  serving as a cover. 
   It is to be noted that a portion of the upper cover  66  on a side of the electrical-power element  14  in the discharge-noise silencer chamber  64  and the sealed vessel  12  penetrates the upper and lower cylinders  38  and  40  and the intermediate partition  36  to communicate with an inside of the sealed vessel  12  through a communication path, not shown, which opens into the sealed vessel  12 . 
   In this case also, as a refrigerant, carbon dioxide (CO 2 ) which is a natural refrigerant friendly to environments of the earth is used taking into account inflammability, toxicity, etc., while as a lubricant, such existing oil is used as mineral oil, alkyl-benzene oil, ether oil, or ester oil. 
   On a side face of the vessel body  12 A of the sealed vessel  12 , the sleeves  141 ,  142 ,  143 , and  144  are fixed by welding at positions that correspond to the upper-side support member  54 , the lower-part support member  56 , the discharge-noise silencer chamber  62 , and an upper side of the upper cover  66  (a lower end of the electrical-power element  14  roughly) respectively. The sleeves  141  and  142  are adjacent to each other vertically, while the sleeve  143  is roughly in a diagonal direction of the sleeve  141 . Furthermore, the sleeve  144  is positioned as shifted by about 90 degrees with respect to the sleeve  141 . 
   In the sleeve  141  is there inserted and connected one end of the refrigerant introduction pipe  92  for introducing a refrigerant gas to the upper cylinder  38 , which one end communicates with a suction path of the upper cylinder  38 . This refrigerant introduction pipe  92  passes through an upper part of the sealed vessel  12  up to the sleeve  144 , while the other end is inserted and connected in the sleeve  144  so as to communicate with an inside of the sealed vessel  12 . 
   In the sleeve  142 , on the other hand, is there inserted and connected one end of the refrigerant introduction pipe  94  for introducing a refrigerant gas to the lower cylinder  40 , which one end communicates with a suction path of the lower cylinder  40 . The other end of this refrigerant introduction pipe  94  is connected to a lower end of an accumulator. Furthermore, in the sleeve  143  is there inserted and connected the refrigerant discharge pipe  96 , one end of which communicates with the discharge-noise silencer chamber  62 . It is to be noted that a reference numeral  147  indicates the bracket for holding the accumulator. 
   The following will describe operations with reference to this configuration. When the stator coil  28  of the electrical-power element  14  is electrified through the terminal  20  and a wiring line not shown, the electrical-power element  14  is actuated, thus causing the rotor  24  to revolve. By this revolution, a roller portion  112  of the swing piston  410  engaged to the upper eccentric portion  42  integrally provided with the rotary shaft  16  revolves in the upper cylinder  38  as described above, so that the roller  48  engaged to the lower eccentric portion  44  revolves eccentrically in the lower cylinder. 
   Accordingly, a low-pressure (first-stage suction pressure LP: 4 MPaG) refrigerant gas sucked into the low-pressure chamber side of the cylinder  40  from a suction port, not shown, through the refrigerant introduction pipe  94  and a suction path formed in the lower-part support member  56  is compressed by operations of the lower roller  48  and the vane  52  to have a medium pressure (MP 1 : 8 MPaG), passed through the high-pressure chamber side of the lower cylinder  40 , a discharge port not shown, and the discharge-noise silencer chamber  64  formed in the lower-part support member  56 , and is discharged into the sealed vessel  12  from the communication path described above. Thus, the sealed vessel  12  has the medium pressure (MP 1 ) therein. 
   Then, the medium pressure refrigerant gas in the sealed vessel  12  exits it through the sleeve  144 , passes through the refrigerant introduction pipe  92  and a suction path formed in the upper-part support member  54 , and is sucked from a suction port, not shown, into the lower-pressure chamber side of the upper cylinder  38 . The medium pressure refrigerant gas thus sucked undergoes second-stage compression through swinging of the swing piston  410  (the vane portion  414  and the roller portion  412 ) held slidingly in the holding groove  416 A provided in the bush  416  held rotatably in the holding groove  88  in the upper cylinder  38  to thereby provide a high-temperature, high-pressure refrigerant gas (second-stage discharge pressure HP: 12 MPaG), which in turn passes from the high-pressure chamber side through a discharge port not shown, the discharge-noise silencer chamber  62  formed in the upper-part support member  54 , and the refrigerant discharge pipe  96 , and is discharged to an outside. This discharged refrigerant flows into the gas cooler  154 . At this moment, the refrigerant has a raised temperature of about +100° C. and, therefore, such a high temperature, high pressure gas radiates heat to heat water in, for example, the hot-water storage tank to thus generate hot water having a temperature of about +90° C. 
   The refrigerant itself, on the other hand, is cooled at the gas cooler  154  and exits it. Then, the refrigerant is decompressed at the expansion valve  156 , flows into the evaporator  157  to evaporate there, passes through the accumulator described above, and is sucked into the first rotary compression element  32  through the refrigerant introduction pipe  94 , which cycle is repeated. 
   Thus, the present embodiment according to the present embodiment comprises the upper cylinder  38  which constitutes the second rotary compression element  34  and the swing piston  410  which has the roller portion  412  which is engaged to the upper eccentric portion  42  formed on the rotary shaft  16  of the electrical-power element  14  to thereby move in the upper cylinder  38  eccentrically, in which on the swing piston  410  is there formed the vane portion  414  which projects from the roller portion  412  in a radial direction to divide an inside of the upper cylinder  38  into a low-pressure chamber side and a high-pressure chamber side in such a configuration that the vane portion  414  of the swing piston  410  is held at the upper cylinder  38  in such a manner that the vane portion  414  can slide and swing, so that a conventional construction to apply a back pressure to the vane and a spring to urge the vane on the roller are rendered unnecessary. Especially in an internal medium-pressure, multi-stage compression type rotary compressor according to the present embodiment, it is unnecessary to provide a construction to apply a discharge pressure of the second rotary compression element  34  to the vane as a back pressure, thus simplifying a construction of the rotary compressor  10  and greatly reducing productions costs. 
   Although the present embodiment has provided the swing piston  410  on the second rotary compression element  34 , the present invention is not limited thereto; for example, the swing piston  410  may be provided on the first rotary compression element  32  instead. By providing the swing piston  410  only to the second rotary compression element  34  as in the case of the present embodiment, costs of parts can be reduced. Furthermore, although the present embodiment has applied the present invention to an internal medium-pressure, multi-stage compression type rotary compressor, the present invention is not limited thereto; for example, the present invention may be applied to an ordinary single-cylinder type roller. 
   As detailed above, by the present invention, in a rotary compressor for compressing a CO 2  refrigerant according to the present embodiment which comprises an electrical-power element and a rotary compression element driven by this electrical-power element in a sealed vessel, there are provided a cylinder constituting the rotary compression element, a swing piston having a roller portion which is engaged to an eccentric portion formed on a rotary shaft of the electrical-power element to eccentrically moves in the cylinder, a vane portion formed on this swing piston in such a manner as to project from the roller portion in a radial direction to thereby divide an inside of the cylinder into a low-pressure chamber side and a high-pressure chamber side, and a holding portion provided on the cylinder to hold the vane portion of the swing piston in such a manner that the vane portion can slide and swing, so that as the eccentric portion of the rotary shaft revolves eccentrically, the swing piston correspondingly swings and slides round the holding portion as a center and, therefore, the vane portion thereof always divides the inside of the cylinder into the low-pressure chamber side and the high-pressure chamber side. 
   Accordingly, it is possible to eliminate a necessity of conventionally providing a spring for urging the vane on a roller side, a back pressure chamber, or a structure for applying a back pressure to the back pressure chamber, thus simplifying a construction of the rotary compressor and reducing costs in production. 
   Furthermore, in a rotary compressor comprising an electrical-power element and first and second rotary compression elements driven by this electrical-power element in a sealed vessel in such a configuration that a CO 2  refrigerant gas compressed at the first rotary compression element is discharged into the sealed vessel and this discharged medium pressure gas is compressed at the second rotary compression element, there are provided a cylinder constituting the second rotary compression element, a swing piston having a roller portion which is engaged to an eccentric portion formed on a rotary shaft of the electrical-power element to eccentrically move in the cylinder, a vane portion which is formed on this swing piston in such a manner as to project from the roller portion in a radial direction to thereby divide an inside of the cylinder into a low-pressure chamber side and a high-pressure chamber side, and a holding portion which is provided on the cylinder to hold the vane portion of the swing piston in such a manner that the vane can slide and swing, so that similarly, as the eccentric portion of the rotary shaft revolves eccentrically, the swing piston correspondingly swings and slides round the holding portion as a center and, therefore, the vane portion thereof always divides the inside of the cylinder of the second rotary compression element into the low-pressure chamber side and the high-pressure chamber side. 
   Accordingly, it is possible to eliminate a necessity of conventionally providing a spring for urging the vane on the roller side, a back pressure chamber, or a structure for applying a back pressure to the back pressure chamber. Especially in a so-called multi-stage compression type rotary compressor in which a medium pressure develops in a sealed vessel as in the case of the present invention, a structure for applying a back pressure is complicated; by using a swing piston, however, it is possible to simplify the structure remarkably and reduce production costs. 
   Furthermore, the holding portion is constituted of a guide groove which is formed in the cylinder and which the vane portion of the swing piston can enter movably and a bush which is provided rotatably at this guide groove to slidingly support the vane portion, so that it is possible to smooth swinging and sliding operations of the swing piston. Accordingly, it is possible to greatly improve performance and reliability of the rotary compressor. 
   The following will describe a defroster for a refrigerant circuit according to another additional embodiment of the present invention with reference to  FIGS. 21 and 21 .  FIG. 20  shows a vertical cross-sectional of a multi-stage compression type rotary compressor used in this case. It is to be noted that the same reference numerals in these figures as those in  FIGS. 1-19  indicate the same or similar functions. 
   In  FIG. 20 , a reference numeral  10  indicates an internal medium-pressure, multi-stage compression type rotary compressor using carbon dioxide (CO 2 ) as a refrigerant which comprises the cylindrical sealed vessel  12  made of a steel plate and the rotary compression mechanism portion  18  which includes the electrical-power element  14  arranged and housed in an upper part of an internal space of the sealed vessel  12  and the first rotary compression element  32  (first stage) and the second rotary compression element  34  (second stage) which are arranged below the electrical-power element  14  to be driven by the rotary shaft  16  of the electrical-power element  14 . 
   The sealed vessel  12  has its bottom used as an oil reservoir and is composed of the vessel body  12 A which houses the electrical-power element  14  and the rotary compression mechanism portion  18  and the roughly cup-shaped end cap (lid)  12 B which blocks an upper part opening of the vessel body  12 A. Furthermore, the end cap  12 B has the circular attachment hole  12 D formed therein at a center of its top face, in which attachment hole  12 D the terminal  20  (wiring of which is omitted) is fixed by welding which supplies power to the electrical-power element  14 . 
   The electrical-power element  14  is composed of the stator  22  mounted annularly along an inner peripheral face of an upper-part space of the sealed vessel  12  and the rotor  24  disposed and inserted in the stator  22  with some gap therebetween in such a configuration that to this rotor  24  is there fixed the rotary shaft  16  which vertically extends centrally. 
   The stator  22  has the stack  26  formed by stacking donut-shaped electromagnetic steel plates and the stator coil  28  wound round teeth of the stack  26  by direct winding (concentrated winding). Furthermore, the rotor  24  is constituted of the stack  30  of electromagnetic steel plates and the permanent magnet MG inserted into the stack  30 . 
   The intermediate partition plate  36  is sandwiched between the first rotary compression element  32  and the second rotary compression element  34 . That is, a combination of the first rotary compression element  32  and the second rotary compression element  34  is composed of the intermediate partition plate  36 , the upper and lower cylinders  38  and  40  arranged above and below the intermediate partition plate  36  respectively, the upper and lower rollers  46  and  48  which are fitted to the upper and lower eccentric portions  42  and  44  provided on the rotary shaft  16  with a phase difference of 180 degrees therebetween so as to eccentrically revolve within the upper and lower cylinders  38  and  40  respectively, upper and lower vanes  50  and  52 , not shown, which butt against the upper and lower rollers to divide an inside of the respective upper and lower cylinders  38  and  40  into a low-pressure chamber side and a high-pressure chamber side, and the upper-part support member  54  and the lower-part support member  56  given as a support member for blocking an upper-side opening face of the upper cylinder  38  and a lower-side opening face of the lower cylinder  40  respectively to serve also as a bearing for the rotary shaft  16 . 
   Furthermore, a combination of the upper-part support member  54  and the lower-part support member  56  is provided therein with the suction paths  58  and  60  communicating with insides of the upper and lower cylinders  38  and  40  through the suction ports  161  and  162  respectively and the discharge-noise silencer chambers  62  and  64  which are formed by concaving a surface partially and then blocking resultant concavities by the upper cover  66  and the lower cover  68  respectively. 
   It is to be noted that the discharge-noise silencer chamber  64  communicates with an inside of the sealed vessel  12  through a communication path, not shown, which penetrates the upper and lower cylinders  38  and  40  and the intermediate partition plate  36  in such a configuration that at an upper end of the communication path, an intermediate discharge pipe  121  is provided as erected, through which a medium pressure refrigerant compressed at the first rotary compression element  32  is discharged into the sealed vessel  12 . 
   Furthermore, the upper cover  66  defines the discharge-noise silencer chamber  62  communicating with an inside of the upper cylinder  38  of the second rotary compression element  34 , above which upper cover  66  is there provided the electrical-power element  14  with a predetermined spacing therebetween. 
   In this case also, as a refrigerant, carbon dioxide (CO 2 ) which is a natural refrigerant friendly to environments of the earth is used taking into account inflammability, toxicity, etc., while as a lubricant, such existing oil is used as mineral oil, alkyl-benzene oil, ether oil, ester oil, or poly-alkyl glycol (PAG). 
   Onto a side face of the vessel body  12 A of the sealed vessel  12 , sleeves  141 ,  142 ,  143 , and  144  are fixed by welding at positions that correspond to the suction paths  58  and  60  of the respective upper-part support member  54  and the lower-part support member  56 , the discharge-noise silencer chamber  62 , and an upper side of the upper cover  66  (a lower part of the electrical-power element  14  roughly) respectively. The sleeves  141  and  142  are adjacent to each other vertically, while the sleeve  143  is roughly in a diagonal direction of the sleeve  141 . Furthermore, the sleeve  144  is positioned as shifted by about 90 degrees with respect to the sleeve  141 . 
   In the sleeve  141  is there inserted and connected one end of a refrigerant introduction pipe  92  serving as a refrigerant path for introducing a refrigerant gas to the upper cylinder  38 , which one end communicates with the suction path  58  of the upper cylinder  38 . This refrigerant introduction pipe  92  passes through an upper part of the sealed vessel  12  up to the sleeve  144 , while the other end is inserted and connected in the sleeve  144  to communicate with the inside of the sealed vessel  12 . 
   In the sleeve  142 , on the other hand, is there inserted and connected one end of a refrigerant introduction pipe  94  for introducing a refrigerant gas to the lower cylinder  40 , which one end communicates with the suction path  60  of the lower cylinder  40 . The other end of this refrigerant introduction pipe  94  is connected to a lower end of an accumulator not shown. Furthermore, in the sleeve  143  is there inserted and connected the refrigerant discharge pipe  96 , one end of which communicates with the discharge-noise silencer chamber  62 . 
   This accumulator is a tank for separating an sucked refrigerant into vapor and liquid and attached via a bracket thereof, not shown, to the bracket  147  of a sealed vessel side welded and fixed to an upper-part side face of the vessel body  12 A of the sealed vessel  12 . 
   Next,  FIG. 21  shows a refrigerant circuit of a hot-water supply apparatus  553  to which the present embodiment of the present invention is applied, in which the multi-stage compression type rotary compressor  10  constitutes part of a refrigerant circuit of the hot-water supply apparatus  553  shown in FIG.  21 . That is, the refrigerant discharge pipe  96  of the multi-stage compression type rotary compressor  10  is connected to an inlet of a gas cooler  154 , which is provided to a hot-water storage tank, not shown, of the hot-water supply apparatus  553  in order to heat water and generate hot water. The pipe exits the gas cooler  554  and passes through an expansion valve  556  serving as a decompression device up to an inlet of an evaporator  557 , an outlet of which is connected via the accumulator described above (not shown) to the refrigerant introduction pipe  94 . 
   Furthermore, a defrosting pipe  558  constituting a defrosting circuit branches from somewhere along the refrigerant introduction pipe (refrigerant path)  92  for introducing a refrigerant in the sealed vessel  12  into the second rotary compression element  34  and is connected through an electromagnetic valve  559  constituting a first flow-path control device to the refrigerant discharge pipe  96  extending to the inlet of the gas cooler  554 . 
   Another defrosting pipe  568  is provided to communicate, to each other the refrigerant discharge pipe  96  and a pipe interconnecting the expansion valve  556  and the evaporator  557 , to which defrosting pipe  568  is there equipped another electromagnetic valve  569  constituting the first flow-path control device. Furthermore, to the refrigerant introduction pipe  92  on a downstream side of a branching point  570  of the defrosting pipe  558  are there provided a capillary tube  560  serving as a second decompression device and an electromagnetic valve  563  connected in parallel with this capillary tube  560  to serve as a second flow-path control device. 
   In this configuration, the electromagnetic valves  559 ,  569 , and  563  are controlled in opening and closing by the control device  564 . The electromagnetic valve  563  is opened by the control device  563  in ordinary defrosting operation. Accordingly, during defrosting operation, a refrigerant gas supplied to the second rotary compression element  34  is decompressed through the capillary tube  560  (decompression device) provided to the refrigerant introduction pipe  92  (refrigerant path) and then supplied to the second rotary compression element  34 . In such a way, as described later, a pressure difference develops between an suction side and a discharge side of the second rotary compression element  34  to thereby prevent breakaway of the vane, thus avoiding unstable operation during defrosting for improvements in reliability. 
   The following will describe operations with reference to this configuration. It is to be noted that the control device  564  closes the electromagnetic valves  559  and  569  and opens the electromagnetic valve  563  in heating operation as described above. When the stator coil  28  of the electrical-power element  14  is electrified through the terminal  20  and a wiring line not shown, the electrical-power element  14  is actuated, thus causing the rotor  24  to revolve. By this revolution, the rollers  46  and  48  fitted to the upper and lower eccentric portions  42  and  44  provided integrally with the rotary shaft  16  revolve eccentrically in the upper and lower cylinders  38  and  40  respectively. 
   Accordingly, a low-pressure (first-stage suction pressure LP: 4 MPaG) refrigerant sucked into the low-pressure chamber side of the cylinder  40  from a suction port  562  through the refrigerant introduction pipe  94  and the suction path  60  formed in the lower-part support member  56  is compressed by operations of the lower roller  48  and the vane to have a medium pressure (MP 1 : 8 MPaG), passed through the high-pressure chamber side of the lower cylinder  40 , a discharge port not shown, and the discharge-noise silencer chamber  64  formed in the lower-part support member  56  and is discharged into the sealed vessel  12  from a communication path not shown. Thus, the sealed vessel  12  has the medium pressure (MP 1 ) therein. 
   Then, the medium pressure refrigerant gas in the sealed vessel  12  exits it through the refrigerant introduction pipe  92  of the sleeve  144  (where an intermediate discharge pressure is MP 1  described above), passes through the electromagnetic valve  563  connected in parallel with the capillary tube  560  of this refrigerant introduction pipe  92  and the suction path  58  formed in the upper-part support member  54 , and is sucked into the low-pressure chamber side of the upper cylinder  38  from the suction port  161  (second-stage suction). The medium pressure refrigerant gas thus sucked undergoes second-stage compression through operations of the roller  46  and a vane not shown to thereby provide a high-temperature, high-pressure refrigerant gas (second-stage discharge pressure HP: 12 MPaG), which in turn passes from the high-pressure chamber side through a discharge port not shown, the discharge-noise silencer chamber  62  formed in the upper-part support member  54 , and the refrigerant discharge pipe  96 , and flows into the gas cooler  554 . At this moment, the refrigerant has a raised temperature of about +100° C. and, therefore, such a high temperature, high pressure gas radiates heat through the gas cooler  554  to heat water in the hot-water storage tank to thus generate hot water having a temperature of about +90° C. 
   The refrigerant itself, on the other hand, is cooled at the gas cooler  554  and exits it. Then., the refrigerant is decompressed at the expansion valve  556 , flows into the evaporator  557  to evaporate there (while absorbing heat from surroundings), passes through the accumulator, and is sucked into the first rotary compression element  32  through the refrigerant introduction pipe  94 , which cycle is repeated. 
   Especially in a low outside-air temperature environment, such heating operation causes the evaporator  557  to be frosted. Therefore, periodically or according to an arbitrary instruction for operation, the control device  564  opens the electromagnetic valves  559  and  569  and closes the electromagnetic valve  563  and, furthermore, opens the expansion valve  556  fully to thereby defrost the evaporator  557 . When the electromagnetic valves  559  and  569  are opened, a refrigerant gas discharged from the first rotary compression element  32  into the sealed vessel  12  flows either through the refrigerant introduction pipe  92 , the defrosting pipe  558 , the refrigerant discharge pipe  96 , and the defrosting pipe  568  toward a downstream side of the expansion valve  556  or through the gas cooler  554  and the expansion valve  556  (opened fully), in both cases of which the refrigerant directly flows into the evaporator  557  without being decompressed. 
   Furthermore, a refrigerant gas discharged from-the second rotary compression element  34  passes through the refrigerant discharge pipe  96  and the defrosting pipe  568  to flow toward the downstream side of the expansion valve  556  into the evaporator  557  directly without being decompressed. When such a high-temperature, high-pressure refrigerant gas flows into the evaporator  557 , it is heated and defrosted as melting. 
   In this case, when the electromagnetic valves  559  and  569  are opened, a discharge side and a suction side of the second rotary compression element  34  communicate with each other through the refrigerant discharge pipe  96 , the defrosting pipe  558 , and the refrigerant introduction pipe  92  and so have the same pressure naturally; by the present invention, however, the electromagnetic valve  563  is closed in defrosting operation, so that the capillary tube  560  is interposed between the suction side (side of the refrigerant introduction pipe  92 ) and the discharge side (side of the refrigerant discharge pipe  96 ) of the second rotary compression element  34  in configuration. 
   Accordingly, a refrigerant gas to be compressed at the first rotary compression element  32 , discharge into the sealed vessel  12 , and supplied to the second rotary compression element  34  through the refrigerant introduction pipe  92  is actually supplied through this capillary tube  560  to the second rotary compression element  34 . That is, since the refrigerant gas is decompressed at the capillary tube  560 , a pressure difference occurs between a suction side and a discharge side of the second rotary compression element  34  to thereby prevent breakaway of the vane in order to avoid unstable defrosting operation, thus improving reliability. 
   Such defrosting operation ends, for example, when the evaporator  557  reaches a predetermined defrosting temperature or time. When defrosting ends, the control device  564  closes the electromagnetic valves  559  and  569  and opens the electromagnetic valve  563  to return to ordinary heating operation. 
   Although the present embodiment has used the multi-stage compression type rotary compressor  10  in a refrigerant circuit of the hot-water supply apparatus  553 , the present invention is not limited thereto; for example, it may well be applied for warming of a room. Furthermore, although the present embodiment has employed an internal medium-pressure multi-stage compression type rotary compressor, the present invention is not limited thereto; for example, it is applicable also to such a configuration that a refrigerant discharged from the first rotary compression element  32  is supplied through the refrigerant introduction pipe  92  to the second rotary compression element  34  without passing it through the sealed vessel  12 . 
   As detailed above, according to the present embodiment of the present invention, in a refrigerant circuit comprising a multi-stage compression type rotary compressor including an electrical-power element and first and second rotary compression elements driven by this electrical-power element in a sealed vessel in such a configuration that a refrigerant compressed at the first rotary compression element is then compressed at the second rotary compression element, a gas cooler into which the refrigerant discharged from the second rotary compression element of this multi-stage compression type rotary compressor flows, a first decompression device connected to an outlet side of this gas cooler, and an evaporator connected to an outlet side of this first decompression device in such a configuration that the refrigerant discharged from this evaporator is compressed at the first rotary compression element, there are provided a defrosting circuit for supplying the refrigerant discharged from the first and second rotary compression elements to the evaporator without decompressing it, a first flow-path control device which controls flow of the refrigerant through this defrosting circuit, a second decompression device provided along a refrigerant path for supplying the second rotary compression element with the refrigerant discharged from the first rotary compression element, and a second flow-path control device which controls whether the refrigerant is allowed to flow through this second decompression device or the refrigerant is allowed to bypass it, wherein when the refrigerant is controlled by the first flow-path control device to flow to the defrosting circuit, this second flow-path control device controls the refrigerant to flow to the second decompression device, so that during defrosting operation of the evaporator, the refrigerant discharged from the first and second rotary compression elements is supplied to the evaporator without being decompressed, thus avoiding reversion in pressure level relationship at the second rotary compression element. 
   In particular, by the present invention, during such defrosting operation, a refrigerant is controlled to be supplied to the second rotary compression element through the decompression device provided along the refrigerant path, so that a predetermined pressure difference is established between suction and discharge sides of the second rotary compression element. 
   Accordingly, the second rotary compression element becomes stable in operation, thus improving reliability. In particular, remarkable effects are obtained in the case of a refrigerant circuit using a CO 2  gas as a refrigerant.