Abstract:
Pulse tube coolers and Gifford-McMahon coolers are used to cool nuclear spin tomographs and cryopumps. To supply cooled working gas, gas compressors and in particular helium compressors are used with rotational or rotary valves. The rate at which compressed helium is introduced into the cooling device and let out again lies in the range of 1 Hz. A problem of conventional screw or piston processors is that oil from the compressor mixes with the working gas and thus contaminates the cooling device. By providing a second compressor stage, a common pump device can be used to pump in both directions, which results in a two-stage compressor device. The working gas is compressed in each flow direction of the working liquid, in one flow direction in the first compressor stage and in the opposite flow direction in the second compressor stage. Thus, the efficiency of the compressor device is improved.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is filed under 35 U.S.C. §111(a) and is based on and hereby claims priority under 35 U.S.C. §120 and §365(c) from International Application No. PCT/EP2015/070507, filed on Sep. 8, 2015, and published as WO 2016/038041 A1 on Mar. 17, 2016, which in turn claims priority from German Application No. 102014217897.5, filed in Germany on Sep. 8, 2014. This application is a continuation-in-part of International Application No. PCT/EP2015/070507, which is a continuation of German Application No. 102014217897.5. International Application No. PCT/EP2015/070507 is pending as of the filing date of this application, and the United States is an elected state in International Application No. PCT/EP2015/070507. This application claims the benefit under 35 U.S.C. §119 from German Application No. 102014217897.5. The disclosure of each of the foregoing documents is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates a cooling device equipped with a compressor device and to a method for operating the compressor device. 
       BACKGROUND 
       [0003]    Conventionally, pulse tube refrigerators and Gifford-McMahon coolers are used for cooling magnetic resonance scanners and cryo-pumps. These cooling devices use gas compressors, in particular helium compressors, in combination with rotary or turning valves. The rate at which compressed helium is introduced into and removed from the cooling device is in the range of 1 Hz. A problem of conventional screw or piston compressors is that oil from the compressor may get into the working gas and thus enter the cooling device, thereby contaminating the device. 
         [0004]    Moreover, acoustic compressors and high frequency compressors are known in which one or more pistons are put in linear resonance oscillation by a magnetic field. Those resonance frequencies are within a range of a few tens of Hertz and therefore are not suited for being used with pulse tube refrigerators and Gifford-McMahon coolers for generating very low temperatures, such as below ten degrees Kelvin. 
         [0005]    The Swiss patent document CH457147B discloses a membrane compressor or membrane pump that has a working chamber sub-divided into a gas volume and a liquid volume by an elastic, airtight and liquid-tight membrane. A liquid pump periodically forces liquid into the liquid volume of the working chamber causing the elastic membrane to expand in the direction of the gas volume and to compress the gas in a compression function or to retract away from the gas volume in a pumping function. The disadvantage is that the airtight, liquid-tight and pressure-resistant seal of the elastic membrane in the working chamber is comparatively expensive. The membrane is heavily loaded, particularly in the area of the seal, so that either very expensive materials must be used or a lower service life must be accepted. 
         [0006]    The German patent document DE10344698B4 discloses a heat pump and a refrigerating machine with a compressor device. The compressor device includes a compressor chamber in which a balloon is arranged. The balloon is periodically loaded with liquid so that the gas surrounding the balloon is periodically compressed and relaxed again. This has the disadvantage that the balloon casing can scrape or rub under certain operating states on the hard and possibly edged inner surface of the compressor chamber. As a result, perforations or fissures in the balloon casing can form due to the pressure conditions. Moreover, the permeability of the balloon casing is too high when helium is used as the working gas, causing substantial quantities of helium to be quickly lost. Thus, the service life of such systems that use balloons is unsatisfactory. 
         [0007]    The German patent document DE91837 discloses a membrane pump for liquids that may also be used as a “gas compression pump”. A liquid is introduced between a membrane and the piston valves such that the liquid is present in the gas chamber. The device thus is a compression device with a liquid seal. There is no physical separation of the gas to be compressed and the hydraulic liquid. 
         [0008]    The published international patent application WO2014/016415A2 discloses a compressor device that includes a metal bellows as the compressor element, which is impermeable to all working gases except hydrogen. The metal bellows allows the working gas to be kept oil-free. However, on account of an interaction with the working liquid, the efficiency of the compensation container is unsatisfactory. It is an object of the invention to provide a compressor device that uses a metal bellows as the compressor element, but yet that is more efficient than the compressor device of WO2014/016415A2. Furthermore, it is an object of the invention to provide a cooling device for the compressor device. 
       SUMMARY 
       [0009]    The invention relates to a compressor device, to a cooling device equipped therewith, and to a method for operating the compressor device. Pulse tube coolers and Gifford-McMahon coolers are used to cool nuclear spin tomographs and cryopumps. To supply cooled working gas to the cooling device, gas compressors and in particular helium compressors are used with rotational or rotary valves. The rate at which compressed helium is introduced into the cooling device and let out again lies in the range of 1 Hz. A problem of conventional screw or piston processors is that oil from the compressor can get into the working gas and thus the cooling device and can contaminate the cooling device. By providing a second compressor stage, a common pump device can be used twice, which results in a two-stage compressor device. The working gas is compressed in each flow direction of the working liquid, in one flow direction in the first compressor stage and in the opposite flow direction in the second compressor stage. Thus, the efficiency of the compressor device is improved. 
         [0010]    In a first embodiment, a compressor device includes a first compressor chamber, a second compressor chamber, a pump and a rotary valve. The first compressor chamber is divided by a first metal bellows into a first gas volume inside the first metal bellows and a first liquid volume outside the first metal bellows. A working gas is present in the first gas volume, and a working liquid is present in the first liquid volume. A first working liquid connection is connected to the first liquid volume, and a first high-pressure working gas connection is connected to the first gas volume. The second compressor chamber is divided by a second metal bellows into a second gas volume inside the second metal bellows and a second liquid volume outside the second metal bellows. The working gas is present in the second gas volume, and the working liquid is present in the second liquid volume. A second working liquid connection is connected to the second liquid volume, and a second high-pressure working gas connection is connected to the second gas volume. 
         [0011]    The pump compresses the working gas in the first gas volume by pumping the working liquid from the second liquid volume through the second working liquid connection, through the first working liquid connection and into the first liquid volume. The working gas in the second gas volume is compressed as the pump pumps the working liquid from the first liquid volume into the second liquid volume. The working gas flows through both the first high-pressure working gas connection and the second high-pressure working gas connection to the rotary valve. 
         [0012]    The compressor device also includes a cooling device, first and second heat exchangers, a low-pressure gas storage container and a high-pressure gas storage container. The rotary valve alternately allows working gas to flow from the high-pressure gas storage container into the cooling device and from the cooling device into the low-pressure gas storage container. The cooling device is a Gifford-McMahon cooler or a pulse tube refrigerator. The working gas flows from the first gas volume, through the first high-pressure working gas connection, through the rotary valve and into the cooling device and also flows from the second gas volume, through the second high-pressure working gas connection, through the rotary valve and into the cooling device. The working gas flows from the first gas volume, through the first high-pressure working gas connection, through the first heat exchanger and through the rotary valve. The working gas also flows from the second gas volume, through the second high-pressure working gas connection, through the second heat exchanger and through the rotary valve. The working gas flows from the rotary valve, into the low-pressure gas storage container, through the first low-pressure working gas connection and into the first gas volume. The working gas also flows from the rotary valve, into the low-pressure gas storage container, through the second low-pressure working gas connection and into the second gas volume. 
         [0013]    The compressor device also includes a first low-pressure check valve and a first high-pressure check valve. The first low-pressure check valve is connected to the first low-pressure working gas connection and permits working gas to flow only in a direction through the first low-pressure working gas connection and into the first gas volume. The first high-pressure check valve is connected to the first high-pressure working gas connection and permits working gas to flow only in a direction out of the first gas volume and through the first high-pressure working gas connection. 
         [0014]    In a second embodiment, the compressor device includes the first compressor chamber, the second compressor chamber, the pump, the rotary valve and a buffer storage container. The first compressor chamber includes the first metal bellows that divides the first compressor chamber into the first gas volume inside the first metal bellows and the first liquid volume outside the first metal bellows. The working gas is present in the first gas volume, and the working liquid is present in the first liquid volume. The high-pressure check valve permits the working gas to flow only in a direction out of the first gas volume. The second compressor chamber includes the second metal bellows that divides the second compressor chamber into the second gas volume inside the second metal bellows and the second liquid volume outside the second metal bellows. The working gas is present in the second gas volume, and the working liquid is present in the second liquid volume. The low-pressure check valve permits the working gas to flow only in a direction into the second gas volume. 
         [0015]    The pump pumps the working liquid between the first liquid volume and the second liquid volume. The working gas in the first gas volume is compressed as the pump pumps the working liquid from the second liquid volume into the first liquid volume. The working gas in the second gas volume is also compressed as the pump pumps the working liquid from the first liquid volume into the second liquid volume. The working gas flows from the first gas volume, through the high-pressure check valve, through the buffer storage container, through the low-pressure check valve and into the second gas volume. 
         [0016]    The compressor device of the second embodiment also includes a cooling device. The working gas flows from the second gas volume to the cooling device, and the working gas flows from the cooling device to the first gas volume. The cooling device is a Joule-Thomson cooler. 
         [0017]    Other embodiments and advantages are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention. 
           [0019]      FIG. 1  is a schematic diagram of a first embodiment of the invention as a non-transporting compressor device with two compressor stages. 
           [0020]      FIG. 2A  illustrates a first operating phase of the compressor device of  FIG. 1 . 
           [0021]      FIG. 2B  illustrates a second operating phase of the compressor device of  FIG. 1 . 
           [0022]      FIG. 2C  illustrates a third operating phase of the compressor device of  FIG. 1 . 
           [0023]      FIG. 2D  illustrates a fourth operating phase of the compressor device of  FIG. 1 . 
           [0024]      FIG. 2E  shows a repeat of the first operating phase in which compression takes place in the first compressor stage. 
           [0025]      FIG. 3  is a schematic diagram of a second embodiment of the invention as a transporting compressor device with two compressor stages. 
           [0026]      FIG. 4A  illustrates a first operating phase of the compressor device of  FIG. 3 . 
           [0027]      FIG. 4B  illustrates a second operating phase of the compressor device of  FIG. 3 . 
           [0028]      FIG. 4C  illustrates a third operating phase of the compressor device of  FIG. 3 . 
           [0029]      FIG. 4D  illustrates a fourth operating phase of the compressor device of  FIG. 3 . 
           [0030]      FIG. 5  shows an application of the compressor device of  FIG. 3  as a drive for a Joule-Thomson cooler. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Reference will now be made in detail to some embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
         [0032]    The compressor device according to the present invention can be designed either as a non-transporting compressor device  10  or as a transporting compressor device  11 . In the non-transporting configuration, a predetermined amount of working gas  12  is alternately compressed and relaxed in two transporting stages. No working gas is supplied from the outside or discharged to the outside in the non-transporting compressor device  10 . 
         [0033]      FIG. 1  shows a first embodiment of compressor device  10  that has a first compressor stage  13  and a second compressor stage  14 . Compressor device  10  is non-transporting. By expanding the effective compensation container that holds a working liquid  15  to the second compressor stage  14 , a common pump device  16  can be used twice. The working gas  12  is compressed in each direction of flow of the working liquid  15 , both in the direction of flow towards the first compressor stage  13  and in the opposite direction of flow towards the second compressor stage  14 . Thus, the efficiency of the compressor device  10  is enhanced. 
         [0034]    The gas flow is controlled in a simple manner during compression and relaxation of the gas using check valves  17  at high-pressure working gas connections  19 - 20  and by using check valves  18  at low-pressure working gas connections  21 - 22 . The compressed working gas  12  is cooled after each compression stroke in the two compressor stages  13 - 14  using heat exchangers  23 - 24  downstream of the high pressure working gas connections  19 - 20 . 
         [0035]    A high-pressure gas line  25  and a low-pressure gas line  26  are configured to store gas on account of their volume. Alternatively, a low-pressure gas storage container  27  and a high-pressure gas storage container  28  are provided in the high-pressure gas line  25  and in the low-pressure gas line  26 , respectively. 
         [0036]    In the transporting configuration of a second compressor device  11 , the working gas  12  is first compressed or pre-compressed in the first compressor stage  13  and stored intermediately in a buffer storage container  29 . The second compressor stage  14  operates in an idle mode and serves as a compensation container  31  for the working liquid  15 . When an amount of working gas  12  at a middle pressure Pmid is reached in the buffer storage  29 , which corresponds to the second gas volume  33  in the second compressor stage  14 , during the next compressor stroke the pre-compressed working gas  12  from the buffer storage  29  is compressed in the second compressor stage  14  to an end pressure Pend. The working gas  12  compressed to an end pressure Pend is then released to the outside or stored in the high-pressure gas storage container  28 . 
         [0037]    In the transporting configuration of the first compressor device  10 , the working gas  12  is first compressed or pre-compressed in the first compressor stage  13  and at the same time is transferred into the second gas volume  33  of the second compressor stage  14 . In the second compressor stage  14 , the working gas  12  which has been pre-compressed to a middle pressure Pmid is then compressed to the end pressure Pend. The working gas  12  which has been compressed to the end pressure Pend is then released to the outside or stored in the high-pressure gas storage container  28 . 
         [0038]    Hydraulic oil as defined by the German Industry Standard DIN 51524 is preferably used as the working liquid  15 , which is additionally water-free or desiccated. In the first and second compressor devices  10  and  11 , the hydraulic oil is present in a closed system comprising the pump  16 , the working liquid compensation container  31  and a liquid volume  30  in the compressor chamber  13  such that during operation no water from the environment can be absorbed by the hydraulic oil. Alternatively, water can also be used as the working liquid  15 . Water is also advantageous as the working liquid because in the case of a defect, water that has penetrated into a downstream cryo-cooler can be removed more easily than can hydraulic oil that has penetrated into a downstream cooler. Water also is more advantageous as a working liquid in explosion-protected applications because water is noncombustible and non-explosive. Moreover, water is non-toxic and therefore environmentally friendly. 
         [0039]    For cryo-applications, helium, neon or nitrogen are preferably used as the working gas  12 , depending on the temperature range. 
         [0040]      FIG. 1  shows the compressor device  10  as the first embodiment, which has a first compressor stage  13  and a second compressor stage  14 . Device  10  is a non-transporting compressor device because the working gas is not transported out of the device. Each of the two compressor devices  13 - 14  has a compressor chamber  34 - 35  that is closed in an airtight manner. A metal bellows  36 - 37  is arranged in each of the two compressor chambers  34 - 35 . The metal bellows  36  subdivides the compressor chamber  34  into a first gas volume  32  and a first liquid volume  30 . The metal bellows  37  subdivides the compressor chamber  35  into a second gas volume  33  and a second liquid volume  31 . The first gas volume  32  and the second gas volume  33  contain the working gas  12 . The first liquid volume  30  and the second liquid volume  31  contain the working liquid  15 . 
         [0041]    The two compressor stages  13 - 14  are constructed in the same way such that both of the gas volumes  32 - 33  are equal and both of the liquid volumes  30 - 31  are equal. The gas volumes  32 - 33  are inside the metal bellows  36 - 37 , and the liquid volumes are outside the metal bellows  36 - 37 . A connection  38 - 39  for the working fluid leads out of each of the liquid volumes  30 - 31 . The gas volumes  32 - 33  are each connected both to a high-pressure working gas connection  19 - 20  and to a low-pressure working gas connection  21 - 22 . The low-pressure working gas connections  21 - 22  are provided with check valves  18  that are permeable in the direction of compressor stages  13 - 14 . The high-pressure working gas connections  19 - 20  are provided with check valves  17  that, in contrast to check valves  18  at the low-pressure working gas connections  21 - 22 , have opposite forward directions. Thus, the check valves  17  permit working gas to flow only in the direction out of the gas volumes  32 - 33 , and the check valves  18  permit working gas to flow only in the direction into the gas volume  32 - 33 . The high-pressure working gas connections  19 - 20  are connected to the common high-pressure gas line  25  via the check valves  17 . The low-pressure working gas connections  21 - 22  are connected to the low-pressure gas line  26  via the check valves  18 . 
         [0042]    The check valves  17  in the high-pressure working gas connections  19 - 20  are permeable in the direction of the common high-pressure gas line  25 , and the check valves  18  in the low-pressure working gas connections  21 - 22  are permeable in the direction of the compressor stages  13 - 14 . The common high-pressure gas line  25  and the common low-pressure gas line  26  end in a motor rotary valve  40  and alternately connect the high-pressure gas line  25  and the low-pressure gas line  26  to a cooling device  41 . The cooling device  41  may be a Giffon-McMahon cooler or a pulse tube refrigerator. In some aspects, the cooling device is considered part of the compressor device. In other aspects, the compressor device and the cooling device are separate components. Due to their volume, the high-pressure gas line  25  and the low pressure gas line  26  act as gas storage. In addition, the low-pressure gas storage container  27  and the high-pressure gas storage container  28  are provided in the high-pressure and low-pressure gas lines  25 - 26 . The heat exchangers  23 - 24  for cooling the compressed working gas are connected downstream of check valves  17  on the two high-pressure working gas connections  19 - 20 . The two working liquid connections  38 - 39  are connected to a common electromotive pump device  16  that alternatingly pumps working liquid  15  into the first and second liquid volumes  30 - 31  of the first and second compressor stages  13 - 14 . Either the working liquid  15  is pumped from the second liquid volume  31  into the first liquid volume  30  or vice versa. 
         [0043]      FIGS. 2A-2E  illustrate the different operating phases of the compressor device  10  of  FIG. 1 . In a first phase shown in  FIG. 2A , working liquid  15  is pumped by the common pump device  16  from the second liquid volume  31  of the second compressor stage  14  into the first liquid volume  30  of the first compressor stage  13 . The first metal bellows  36  is compressed, and the working gas  12  therein is forced into the high-pressure storage  28  via the first high-pressure working gas connection  19 , the first heat exchanger  23  and the common high-pressure gas line  25 . The second metal bellows  37  expands through working gas  12  that flows back from the low-pressure working gas storage  27  via the low-pressure gas line  26  and the second low-pressure working gas connection  22 . The rotary valve  40  connects the cooling device  41  via low-pressure gas line  26  to low-pressure gas storage  27 . 
         [0044]    In the second operating phase shown in  FIG. 2B , compression in the first compressor stage  13  is completed, and the rotary valve  40  connects the high-pressure gas storage  28  to the cooling device  41  so that compressed working gas  12  cooled in the first heat exchanger  23  enters the cooling device  41 . 
         [0045]    In the third phase shown in  FIG. 2C , the flow of working liquid is reversed, and the pumping device  16  now pumps working liquid  15  from the first liquid volume  30  of the first compressor stage  13  into the second liquid volume  31  in the second compressor stage  14 . In so doing, the second metal bellows  37  is compressed, and the working gas  12  therein is compressed and forced into the high-pressure gas storage  28  via the second high-pressure working gas connection  20 , the second heat exchanger  24  and the common high-pressure gas line  25 . The first metal bellows  36  expands through working gas  12  flowing back from the low-pressure gas storage  27  via the low-pressure gas line  26  and the first low-pressure working gas connection  21 . 
         [0046]    In the fourth phase shown in  FIG. 2D , compression in the second compressor stage  14  is completed, and the rotary valve  40  again connects the high-pressure gas storage  28  to the cooling device  41  via the common high-pressure gas line  25  so that compressed working gas  12  cooled in the second heat exchanger  24  enters the cooling device  41 . 
         [0047]    The operating phase illustrated in  FIG. 2E  is again the first phase in which compression takes place in the first compressor stage  13 .  FIG. 2A  is distinguishable from  FIG. 2E  only in that the first metal bellows  36  in  FIG. 2E  is still relaxed and the second metal bellows  37  is still compressed. In  FIG. 2A , compression in the first compressor stage  13  is completed and the first metal bellows  36  is compressed while the second metal bellows  37  is relaxed. 
         [0048]    By providing a high-pressure storage  28  and a low-pressure storage  27 , the rotational frequency of rotary valve  40  is decoupled from the frequency of compression in the two compressor stages. Alternatively, the rotational frequency of rotary valve  40  is synchronized with the frequency of the compressor strokes. In that case, the high-pressure and low-pressure storage volumes  28 ,  27  may be dispensed with. 
         [0049]      FIG. 3  shows the compressor device  11  as the second embodiment that transports working gas  12  and includes the two compressor stages  13 - 14 . Device  11  is a transporting compressor device because the working gas  12  is transported out of the device. Similar components in both devices  10  and  11  are labeled with the same reference numeral. The structure of the two compressor stages  13 - 14  and the connection of the two compressor stages  13 - 14  with the common pump device  16  corresponds to the structure shown in  FIGS. 1 and 2 . Likewise, the structure of the two heat exchangers  23 - 24  corresponds to the configuration of the first embodiment. In the embodiment of  FIG. 3 , the working gas  12  is first compressed in the first compressor stage  13  from an outlet pressure P 0  to a first middle pressure Pmid 1  and subsequently in the second compressor stage  14  from a second middle pressure Pmid 2  to an end pressure Pend. Throughout the operation, the first middle pressure Pmid 1  is greater than the second middle pressure Pmid 2 . 
         [0050]    Compressor device  11  includes the buffer storage container  29  that is connected via a first gas line  42  and a first lock valve  43  to the second low-pressure working gas connection  22  of the second compressor stage  14 . The first high-pressure working gas connection  19  is connected to the buffer storage  29  via the first heat exchanger  23  and a second gas line  44 . The low-pressure gas storage  27  is connected via a third gas line  45  and check valve  18  to the first low-pressure working gas connection  21  of the first compressor stage  13 . Working gas  12  from the low-pressure gas storage  27  that is to be compressed is supplied to the first compressor stage  13  via the first low-pressure working gas connection  21 . The second high-pressure working gas connection  20  of the second compressor stage  14  is connected to the high-pressure gas storage  28  via check valve  17 , the second heat exchanger  24  and a fourth gas line  46 . 
         [0051]      FIGS. 4A-4D  illustrate the operation of compressor device  11  of  FIG. 3 .  FIG. 4A  shows a first operating phase in which working liquid  15  is pumped through the common pump device  16  from the first liquid volume  30  of the first compressor stage  13  into the second liquid volume  31  of the second compressor stage  13 . As the first metal bellows  36  expands, uncompressed working gas  12  flows via the third gas line  45 , the check valve  18  and the first low-pressure working gas connection  21  into the first gas volume  32 . The first lock valve  43  in the first gas line  42  is closed. The second compressor stage  14  merely serves as a compensation container for working liquid  15 . In the relaxed state, the pressure in the second gas volume  33  is at the second middle pressure Pmid 2 . In the compressed state, the pressure in the second gas volume  33  is approximately at the end pressure Pend. 
         [0052]      FIG. 4B  illustrates the second operating phase in which the flow direction of the working liquid  15  reverses. The working gas  12  in the first compressor stage  13  is compressed and forced into buffer storage  29  through the first high-pressure working gas connection  19 , the check valve  17 , the first heat exchanger  23  and the second gas line  44 . The check valve  17  on the first high-pressure working gas connection  19  prevents working gas  12  that has been compressed to the middle pressure Pmid from flowing back into the first gas volume  32 . The first lock valve  43  continues to be closed, and the second compressor stage  14  acts only as a compensation container  31  for working liquid  15 . 
         [0053]    The operating phases illustrated in  FIGS. 4A and 4B  are performed repeatedly and for so long as the amount of working gas  12  in the buffer storage  29  that was compressed to the first middle pressure Pmid 1  is sufficient to generate the middle pressure Pmid 2  in the second gas volume  33  after the buffer storage  29  is connected to the second gas volume  33  through the first gas line  42  and the opened lock valve  43 . 
         [0054]      FIG. 4C  illustrates the flow of working gas  29  and the first middle pressure Pmid 1  has been reached in the buffer storage  29 . When the sufficient amount of gas is reached to achieve the first middle pressure Pmid 1  in the buffer storage  29 , the first lock valve  43  is opened during the next compression stroke in the first compressor stage  13  so that the working gas  12  that was pre-compressed to the first middle pressure Pmid 1  may flow from buffer storage  29  via the open first lock valve  43  and the first gas line  42  into the second gas volume  33  of the second compressor stage  14 , resulting in the second middle pressure Pmid 2  in the storage  29  and volume  33 . 
         [0055]      FIG. 4D  illustrates the next operating phase in which the working liquid  15  is pumped through the common pump device  16  into the second compressor stage  14 . The working gas  12  present in the second gas volume  33  and pre-compressed to a second middle pressure Pmid 2  is continued to be compressed to an end pressure Pend and is forced into the high-pressure storage  28  via the second heat exchanger  24  and the fourth gas line  46 . Thus, a compression cycle from an outlet pressure P 0  to an end pressure Pend is terminated and the cycle starts again. 
         [0056]    In an alternative embodiment to that of  FIG. 3 , the first high-pressure working gas connection  19  is connected to the low-pressure working gas connection  22  of the second compressor stage  14  through the gas lines  42  and  44 . The buffer storage  29  and the first lock valve  43  are not used. In that case, the working gas  12  in the first compressor stage  13  is pre-compressed to a single middle pressure Pmid. In the countermovement of the common electromotive pump device  16 , the working gas  12  is then compressed to the end pressure Pend in the second compressor stage  14 . The working gas  12  compressed to the end pressure Pend is then released to the outside or stored in a high-pressure storage  28 . 
         [0057]      FIG. 5  shows an application of the second embodiment  11  as a drive of a Joule-Thomson cooler  47  with a closed working gas loop. 
         [0058]    Hydraulic oils as defined by German Industry Standard DIN 51524 are suited as the working liquid  15 . The H, HL, HLP and HVLP oils are oils that are readily compatible with customary sealing plastics, such as NBR (acrylonitrile butadiene rubber). However, NBR is not sufficiently helium-impermeable. HF oils are frequently incompatible with customary sealing materials, as described at http://de.wikipedia.org/wiki/Liste_der_Kunststoffe. 
         [0059]    Alternatively, water can also be used as the working liquid  15 . Water as the working liquid is also advantageous because in the case of defects in a downstream cryo-cooler, penetrated water can more easily be removed than can hydraulic oil that has penetrated into a cooler connected downstream. In addition, water is appropriate as the working liquid in applications protected against explosions because water is non-combustible and non-explosive. Moreover, water is non-toxic and therefore environmentally friendly. 
       REFERENCE NUMERALS 
       [0060]    P 0  outlet pressure 
         [0061]    Pmid 1  middle pressure  1   
         [0062]    Pmid 2  middle pressure  2   
         [0063]    Pend end pressure 
         [0064]      10  transporting compressor device 
         [0065]      11  non-transporting compressor device 
         [0066]      12  working gas 
         [0067]      13  first compressor stage 
         [0068]      14  second compressor stage 
         [0069]      15  working liquid 
         [0070]      16  common electromotive pump device 
         [0071]      17  check valves 
         [0072]      18  check valves 
         [0073]      19  first high-pressure working gas connection 
         [0074]      20  second high-pressure working gas connection 
         [0075]      21  first low-pressure working gas connection 
         [0076]      22  second low-pressure working gas connection 
         [0077]      23  first heat exchanger 
         [0078]      24  second heat exchanger 
         [0079]      25  high-pressure gas line 
         [0080]      26  low-pressure gas line 
         [0081]      27  low pressure gas storage 
         [0082]      28  high-pressure gas storage 
         [0083]      29  buffer storage 
         [0084]      30  first liquid volume 
         [0085]      31  second liquid volume 
         [0086]      32  first gas volume 
         [0087]      33  second gas volume 
         [0088]      34  first compressor chamber 
         [0089]      35  second compressor chamber 
         [0090]      36  first metal bellows 
         [0091]      37  second metal bellows 
         [0092]      38  first working liquid connection 
         [0093]      39  second working liquid connection 
         [0094]      40  electromotive rotary valve 
         [0095]      41  cooling device 
         [0096]      42  first gas line 
         [0097]      43  first lock valve 
         [0098]      44  second gas line 
         [0099]      45  third gas line 
         [0100]      46  fourth gas line 
         [0101]      47  Joule-Thomson cooler 
         [0102]    Although the present invention has been described in connection with certain specific embodiments for instructional purposes, the present invention is not limited thereto. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.