Abstract:
A cold head for cryogenic machines comprises a displacer mounted in a working chamber of a housing. The cold head also has a high-pressure connection for supplying highly compressed refrigerant and a low-pressure connection for discharging expanded refrigerant. Also provided is a control valve arrangement for controlling the supply and discharge of refrigerant. According to the invention there is a bypass channel connecting the high-pressure connection to the low-pressure connection.

Description:
BACKGROUND 
       [0001]    1. Field of the Disclosure 
         [0002]    The disclosure relates to a cold head for a cryogenic refrigerating machine. 
         [0003]    2. Discussion of the Background Art 
         [0004]    WO 94/29653 describes a cold head for a cryogenic refrigerating machine, which is operated with helium as the working gas and is connected to a high-pressure source and a low-pressure source. The cold head includes a multi-channel control valve that controls the respective connection of a high-pressure inlet and a low-pressure inlet to a piston-cylinder unit and to a working chamber of the cold finger on the warm side. At one of its ends, the displacer which may include a regenerator, delimits a warm-side working chamber and at the opposite end it delimits a cold-side working chamber. While the displacer is periodically reciprocated by the piston-cylinder unit, heat is constantly drawn from the housing of the cold head. With a cold head having a single-stage displacer, temperatures down to about 30 K can be generated. With two- or three-stage displacers, temperatures below 1 K can be generated. A thermodynamic cycle process (a Stirling process or a Gifford-McMahon process) is performed in the cold head using process gas, generally helium, the process gas being guided in a closed circuit. As a consequence, heat is drawn from one end region of the housing enclosing the displacer. 
         [0005]    The cold head is connected to a compressor. Since the circuit is a closed circuit, both the high-pressure connection and the low-pressure connection of the cold head are connected to the compressor. Such compressors typically include an overflow valve. The same is arranged in a return flow conduit arranged between the high-pressure side and the low-pressure side. Typically, overflow valves are spring-loaded check valves generally designed for a differential pressure between the high and low pressures of the compressor of 18 bar, for example. When a cold head, whose resistance is very high, is connected to the compressor the working pressure increases excessively on the high-pressure side at the compressor. For discharging this excess energy, the overflow valve opens so that the refrigerant, in particular helium, flows to the low-pressure side of the compressor via the return flow conduit. Due to the cyclic process of the cold head, a pulsed gas supply from the compressor to the cold head is realized. Here, gas oscillations may occur. In particular over a longer period, this may cause a frequent opening and closing of the overflow valve. Thereby, the overflow valve is subjected to significant overload. This may lead to damage to the valve seat of the overflow valve or even to the destruction of the same. Further, this causes the generation of significant noise and losses in performance. When an overflow valve is damaged, it may happen that oil gets into the refrigeration circuit. Another disadvantage is that performance losses occur due to the existing hysteresis of the overflow valve between the opening pressure and the closing pressure. 
         [0006]    It is an object of the disclosure to reduce the load on the overflow valve. 
       SUMMARY 
       [0007]    The cold head for a cryogenic refrigerating machine of the present disclosure has a working chamber in a housing, possibly a multi-part housing. A single- or multi-stage displacer is arranged in the working chamber. The cold head further comprises a high-pressure connection for supplying highly compressed refrigerant to the working chamber and a low-pressure connection for discharging expanded or low pressure refrigerant. Further, a control valve device is provided. The control valve device serves to control the supply and discharge of refrigerant into and from the working chamber. Here, the control device may comprise a plurality of valves, for example an inlet valve and an outlet valve. It is preferred that the control valve device has a multi-channel control valve which controls the connection between the high-pressure connection, the low-pressure connection and the working chamber. According to the disclosure he cold head has a bypass channel arranged between the high-pressure connection and the low-pressure connection and connecting the two connections. If needed, excess refrigerant can flow through said channel directly from the high-pressure connection to the low-pressure connection without flowing through the cold head. Such occurring surplus energy can thus be discharged via the bypass. As a result, the overflow valve integrated in the compressor is relieved. Possibly, the overflow valve may be omitted altogether from the compressor or it may be provided merely as a safety device. As a consequence, it is possible at least to use a significantly less costly overflow valve. 
         [0008]    In a particularly preferred development of the disclosure a throughflow regulation device is arranged in the bypass channel. This may for example be a nozzle and/or a valve. The throughflow regulation device may be adjustable. In this context it is possible that a fixed setting is made prior to operation, so that the valve opens for example when a pressure difference is exceeded. Further, it is possible to allow for an adjustment of the throughflow regulation device from outside, i.e. from outside the cold head. As such, it may possibly be allowed to make corresponding adjustments also during operation. 
         [0009]    Since the present disclosure provides a bypass in the cold head, which preferably includes a pressure regulation device, it is possible to achieve a significant reduction of costs for the compressors used. Further, the operating safety of the compressors can be enhanced and the compressor performance can be increased. The risk of an oil breakthrough caused by a damaged overflow valve in the compressor is also reduced. Further, service life is extended and a constant noise behavior can be achieved. 
         [0010]    In a preferred development of the disclosure the cold head has a movement device for moving the displacer. The movement device may be a motor. By means of the motor, which may be an electric motor, for example, the displacer can be moved using a slotted guide. This may be effected e.g. through an eccentric so that the rotational movement of the motor is converted into a longitudinal movement of the displacer in a simple manner. As an alternative, a piston/cylinder unit may be provided for moving the displacer. The piston-cylinder unit may for example be driven via a separate hydraulic system. However, for the purpose of movement, it is preferred to connect the piston-cylinder unit to the high-pressure connection and the low-pressure connection. In a preferred embodiment, the actuation of the piston-cylinder unit and thus the movement of the displacer are realized by means of the refrigerant. 
         [0011]    It is further preferred that the cold head has a distributor body in which at least a first connecting channel is provided. The first connecting channel serves to connect the high-pressure connection to the working chamber. Preferably, this connection is made via the control valve device so that the first connecting channel is arranged between the control valve device and the working chamber. Preferably, the distributor body additionally has a second connecting channel arranged between the control valve device and the low-pressure connection. 
         [0012]    In a particularly preferred embodiment the valve body is designed such that is also comprises a control channel. The control channel serves to supply and discharge control medium to the movement device, i.e. in particular to the piston-cylinder unit. The control medium preferably is the refrigerant. 
         [0013]    The disclosure will be explained in detail hereinafter with reference to a preferred embodiment and to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In the Figures: 
           [0015]      FIG. 1  is a schematic illustration of a cryogenic refrigeration machine of the prior art, 
           [0016]      FIG. 2  is a schematic illustration of a cryogenic refrigeration machine of the present disclosure and 
           [0017]      FIG. 3  is a schematic sectional view of an embodiment of a cold head according to the present disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    A cryogenic refrigeration machine of the prior art ( FIG. 1 ) comprises a compressor  10  which compresses a refrigerant such as helium. On the high-pressure side, the compressor  10  is connected to a high-pressure connection  14  of a cold head  16  via a conduit  12 . A low-pressure connection  18  of the cold head  16  is connected to the low-pressure side of the compressor  10  via a conduit  20 . To avoid an overload on the compressor  10  a check valve  24  is arranged in a return flow conduit  22  that connects the high-pressure side of the compressor  10  to the low-pressure side of the compressor  10 . 
         [0019]    Inside the cold head  16  a working chamber  26  is provided in which a displacer piston is arranged that is not illustrated in  FIG. 1 . An inlet valve  28  is connected to the high-pressure connection  14  so that compressed refrigerant flows into the working chamber  26  when the inlet valve  28  is open. Expanded refrigerant may be guided to the low-pressure connection  18  via an outlet valve  30 . 
         [0020]    In the basic configuration of a system of the present disclosure illustrated in  FIG. 2 , similar and identical components are identified by the same reference numerals. 
         [0021]    According to the disclosure schematically illustrated a bypass channel  32  is provided between the inlet valve  28  of the cold head  16  and the outlet valve  30  of the cold head  16 , in which bypass channel a throughflow regulation device may possibly be arranged. As shown in broken lines in  FIG. 2 , providing the bypass channel  32  of the present disclosure makes it possible to omit the return flow conduit  22  and the overflow valve  24 . 
         [0022]    A preferred embodiment of the cold head  16  is illustrated in a schematic sectional view in  FIG. 3 . 
         [0023]    The cold head  16  has a housing formed by the two housing parts  34  and  36 . In the housing part  34  two cylindrical cold-side working chambers  38  and  40  are provided for the two displacer stages  42  and  44 . 
         [0024]    The upper displacer stage  42  delimits a warm-side working chamber  46  and is provided with a drive piston  48  arranged in a cylinder  50  of a distributor body  52 . Thus, the displacer  42 ,  44  is arranged in a working chamber  38 ,  40 ,  46  formed by a plurality of partial chambers. 
         [0025]    The distributor body  52  delimits the warm-side working space  46 . It is provided with bores that form a control channel  54 , a first connecting channel  56 , as well as a second connecting channel  57 . The first connecting channel  56  opens into the working chamber  46  and serves to supply working gas to this chamber. All three channels are controlled by the control valve  58 . The first connecting channel  56  connects the control valve  58  to the warm-side working chamber  46 , the control channel  54  connects the valve  58  to the cylinder  50  and the second  57  connects the vale  58  to a low-pressure connection  60 . The control valve  58  is further connected to a chamber  62  that is in communication with a high-pressure connection  64 . The high-pressure connection  64  supplies helium gas at a pressure of ca. 20 bar, while helium at a pressure of about 5 bar prevails at the low-pressure connection  18 . Both pressures are supplied to corresponding connections (not illustrated) of the control valve  58  via the chamber  62  and the second connecting channel  57 , respectively. All conduits lead into the upper side of the distributor body  52  and from there to the valve  58 . 
         [0026]    The housing part  36  accommodates a motor  66  that drives the control valve  58  via a shaft  68 . The valve is acted upon by a compression spring  70 . 
         [0027]    In the embodiment illustrated the process gas subjected to the thermodynamic cycle process and the drive gas for the piston-cylinder unit  48 ,  50  are identical. Suitably, helium is used. It is possible to use a different gas than the process gas as the drive gas. 
         [0028]    Instead of the piston-cylinder unit  48 ,  50  provided in the embodiment illustrated for moving the displacers  72 ,  76 , the displacers  72 ,  76  may also be moved by a motor, e.g. using an electric motor. For this purpose, the electric motor may be provided with an eccentric and a slotted guide so that the rotation of the eccentric is converted into a linear movement. 
         [0029]    In the cylindrical working chamber  46 , the displacer stage  42  has a tubular displacer  72  filled with a thermal regenerator  74  that is permeable to gas. The regenerator  74  serves to store cold and to give off stored cold to the inflowing warm gas. 
         [0030]    Similarly, the displacer stage  44  that has a smaller diameter than the displacer stage  42 , includes a tubular displacer  76  shiftable in the axial direction in the cylindrical working chamber  40 , said displacer being connected to the displacer  72  and also being filled with a gas-permeable regenerator  78 . 
         [0031]    In operation of the cold finger, the working chamber  46  on the warm side is first connected to the high-pressure connection  64  via the first connecting channel  56  and the control valve  58 . At the same time, high pressure is introduced into the cylinder  50  through the control channel  54 . The displacers  72  and  76  are shifted towards the cold side (downward). The gas under high pressure flows through the regenerators  74  and  78  to the cold side as well. In doing so it expands while cooling, with further expansion being effected by heat exchange with the regenerators. 
         [0032]    In the second phase the control channel  54  is connected to the low-pressure connection. Under the effect of the high pressure, the displacers  72 ,  76  are shifted towards the warm side so that the working chamber  46  on the warm side becomes smaller and gas flows into the working chamber  40  on the cold side through the regenerators  74  and  78 . 
         [0033]    In the third phase the control valve  58  causes the connection of the working chamber  40  to the low-pressure connection  60  via the conduit  56 . Thereby, the gas in all working chambers of the cold head expands while cooling. 
         [0034]    Thereafter the displacers  72  and  76  are moved to the cold side, whereby the volume of the cold-side working chamber  40  shrinks so as to be prepared for the next cycle. In this phase the cold gas flows from the working chamber  40  into the regenerators  74  and  78  which are thereby cooled further. 
         [0035]    The frequency of the working cycle described is about 2 Hz. 
         [0036]    Further, in the embodiment illustrated, a bypass channel  80  according to the disclosure is provided in the distributor body  52 . The bypass channel  80  connects the second control channel  57  to the chamber  62 . The bypass channel  80  thus connects the high-pressure connection  64  to the low-pressure connection  60 . As schematically illustrated, a throughflow regulation device, such as a valve  82 , is arranged in the bypass channel  80 . In case of an undesired high pressure increase in the chamber  62 , a part of the refrigerant thus flows directly through the bypass channel  80  back into the channel  57  connected to the low-pressure connection  60 .