Abstract:
A refrigerator ( 1 ) has a housing ( 2, 14, 19 ), a cylindrical working chamber ( 15, 21 ), a cylindrical displacing member ( 11, 17 ), a gap ( 36, 38 ) which is located between the housing and the displacing member, a regenerator which is disposed inside the displacing member, and a device alternatingly supplying the working chamber with an effective high-pressure gas and an effective low-pressure gas. In order to overcome the disadvantages associated with gas streams occurring in the gap ( 36, 38 ), an additional regenerator (gap gas regenerator), ( 43 ) is assigned to the gap ( 36, 38 ).

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a refrigerator comprising a housing, a cylindrical working chamber, a cylindrical displacing member, a gap which is located between the housing and the displacing member, a regenerator which is disposed inside the displacing member, and a device alternatingly supplying the working chamber with a high-pressure gas and a low-pressure gas. 
   Refrigerators are low-temperature cooling machines in which thermodynamic cyclic processes take place (c.f. US-PS 29 06 01, for example). A single stage refrigerator comprises chiefly a working chamber with a displacing member. The working chamber is alternatingly connected to a high-pressure and a low-pressure gas source, so that during the forced reciprocating motion of the displacing member, the thermodynamic cyclic process (Stirling process, Gifford McMahon process, etc.) takes place. In doing so, the working gas is guided in a closed circuit. The result of this is that heat is removed from a certain area of the working chamber and the displacing member. Through two-stage refrigerators of this kind and with helium as the working gas, temperatures well under 10° K. can be produced. 
   A fundamental component of a refrigerator is the regenerator, through which the working gas flows before and after relaxation. The regenerator is commonly disposed within the displacing member substantially of cylindrical design. The regenerator material needs to exhibit, on the one hand, good heat storing properties so that a sufficiently high exchange of heat can take place between the working gas and the regenerator. On the other hand, both the displacing member, in particular the housing of the displacing member, as well as also the cylinder housing need to exhibit poor heat conducting properties, as otherwise the heat removed from the cold side of the working chamber and the displacing member would be replaced rapidly through heat conduction. 
   It is known to employ as the material for the cylindrical housing, stainless-steel. Stainless-steel exhibits, at the very low temperatures concerned here, a very low heat conductivity. However, stainless-steel as a material is not an option when the refrigerator is employed in the areas of magnetic fields (for example, within nuclear spin tomographs). In such instances the cylindrical housing is made of Novetex (plastic impregnated cotton wool fibre) or materials of similar properties. Novetex is well proven, in particular, as a material for the housing of the displacing member. As regenerator materials, mesh, balls or wool of bronze (preferably for the first stage) and lead balls (preferably for the second stage) are known. 
   In the instance of the refrigerators of the kind affected here, it is unavoidable that a gas flow is present also in the gap between the housing and displacing member. Said gas flow has also the disadvantageous effect, in that it contributes to the heat exchange between the cold and the warm end of the displacing member. The heat ingress into the expansion chamber (cold end of the working chamber) reduces the performance of the entire refrigerator. 
   In order to maintain the gas flow through the gap at a lower level compared to the gas flow through the regenerator, the designers of refrigerators of the type affected here have, in the past, employed the approach of designing this gap as narrow as possible, and/or have inserted seals. Measures of this kind are involved and thus costly. This applies in particular to seals which need to fulfil their task at extremely low temperatures. These seals consist commonly of plastic materials which shrink with increasing operating time. Maintaining of close tolerances is not possible. 
   A refrigerator of the aforementioned type is known from U.S. Pat. No. 5,481,879. For the purpose of reducing the disadvantages involved due to the flow through the gap it is proposed to equip either the outside surface of the displacing member or the inside surface of the housing with one or several helical grooves. Through this measure it shall be achieved that the gases dwell longer within the gap so that an improved temperature equalisation between the flowing gas and the adjacent components takes place. This solution is disadvantageous in that the gap still needs to be relatively narrow in order to achieve a helical gas flow. Moreover, a rapid heat exchange between the gas and the adjacent components does not take place, since these consist of materials which—as already detailed—not only exhibit a low heat conductivity, but also exhibit a very low heat storing capacity. 
   It is the task of the present invention to create a refrigerator of the aforementioned kind in which the disadvantages due to the gas flows in the gap have been removed. 
   SUMMARY OF THE INVENTION 
   Through the measures in accordance with the present invention, the gas flow through the gap is fully regenerated. The regeneration ability of the surfaces encompassing the gap, is created in the instance of a refrigerator in accordance with the present invention by embedding a material having a high thermal capacity within the surfaces encompassing the gap on the outside of the displacing member and/or on the inside of the cylinder housing, for example. The performance of the refrigerator is thus not only improved in that an unwanted heat ingress into the expansion chamber is minimized, but also in that the gas mass flow flowing through the regenerator of the displacing member being, in the instance of the state-of-the-art substantially effective alone, is increased by the regenerated gas mass flow through the gap. 
   It is expedient to rate the storage ability of the gap gas regenerator such that the gap gas mass flow may increase with increasing operating time of the cold head without impairing the performance of the cold head. The sealing effect between displacing member and cylinder wall is subject, in the instance of a gap gas regenerator, to entirely new operating conditions. In principle, it is unimportant how high the gap gas mass flow is. So much heat is given off to the gap gas, regenerator se that the gap gas mass flow enters into the expansion chamber at substantially the same temperature as that of the expansion chamber. A refrigerator in accordance with the present invention may be designed to be significantly less complex; above all, the seal can be significantly simplified or even omitted. Besides a production with easily to be implemented dimensional specifications, it is in addition possible to fall back on “standard sealing rings”. Thus the cooler becomes cheaper, more simple and offers a longer service life. 
   Especially advantageous is the utilization of the idea in accordance with the present invention in the second stage of a two-stage refrigerator. 
   Still further advantages of the present invention will be appreciated to those of ordinary skill in the art upon reading and understand the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. 
       FIG. 1  illustrates a two-stage refrigerator in accordance with the state-of-the-art, 
       FIG. 2  illustrates a partial sectional view of a gap gas regenerator in accordance with the present invention, 
       FIG. 3  illustrates a single stage refrigerator designed in accordance with the present inventions, and 
       FIG. 4  illustrates a further solution for the design of a gap gas regenerator. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , a two-stage Gifford McMahon refrigerator  1  according to the state-of-the-art is depicted. In the housing  2  a valve system, not depicted in greater detail, of a basically known design is accommodated. The valve system sequence connects a high-pressure and a low pressure gas source to the connection ports  3  and  4 , to the channels  5 ,  6  and  7 . The channel  6  opens out into a cylinder  8  in which there is located a drive piston  12  with the displacing member  9  of the first stage  11  of the refrigerator. Instead of the piston drive also a crank drive may be employed. A ring sealing the piston  12  with respect to the inside wall of the cylinder  8  is designated as  13 . With the aid of this drive, the displacing member  9  is reciprocated in the working chamber  15  formed by cylindrical housing  14 . Through the pin  16  in the displacing member  17  of the second stage  18  of the refrigerator it is linked to the displacing member  9  of the first stage, such that also the displacing member  17  of the second stage performs a reciprocating motion in the working chamber  21  formed by the cylindrical housing  19 . The axis of the entire system is designated as  10 . 
   The displacing members  9  and  17  are of a substantially cylindrical design. Their housings  22  and  23  form hollow chambers  20   a , respectively  20   b  serving the purpose of accommodating the regenerators. These consist, for example, of bronze mesh in the first stage and lead balls in the second stage. 
   The working gas is supplied, respectively discharged, through the channels  5  and  7 . It flows through the bores  24 , through the regenerator of the displacing member  9  and through the bores  37  into the expansion chamber  25  which is the bottom section of the working chamber  15 . There the gas expands and removes heat from this area of the first stage  11  of the refrigerator. The pre-cooled gas flows further through the bore  27  in the displacing member  17  of the second stage  18 , through the regenerator located in the inside chamber  20   b  of the displacing member  17  and through the bore  28  at the bottom end of the displacing member  17  into the expansion chamber  29  of the second stage  18 . There a further expansion is effected having in this area of the second stage a cooling effect. Through the same path the gas flows back and cools the regenerator materials so that the gases flowing in again in the next cycle are already pre-cooled in the regenerator. Sealing rings  31  and  32  which are accommodated in the outside grooves  33  and  34  of the walls of the displacing members serve the purpose of sealing the displacing members  9  and  17  with respect to their related chamber walls  14  and  19 . The gaps between the displacing members  11 ,  17  and the cylindrical housings  14 ,  19  of the working chambers  15 ,  21  are designated as  36  and  38  respectively. 
     FIG. 2  is a highly schematic partial sketch with a solution in accordance with the present invention which may be employed both in the first and also in the second stage of a refrigerator in accordance with  FIG. 1 . Through double arrows  41  in the regenerator (in hollow chamber  20   a ,  20   b  of the displacing member  9 , respectively  17 ), respectively  42  (in gap  36 ,  38 ) the main gas mass flow and the gap gas mass flow are indicated. To the gap gas mass flow  42  an additional regenerator  43  is assigned. This is a single layer coil extending in the axial direction, being embedded on the gap side in the housing wall  22 ,  23  of the displacing member  9 ,  17 . In the instance of employing the further generator  43  said coil is constructed in the first stage  9  of bronze wire, for example, and in the instance of being employed in the second stage it is constructed of lead, for example. It is true that also a seal  31 ,  32  is depicted; but it no longer needs to meet high sealing requirements. It may even be omitted provided it is ensured that the gap gas mass flow is regenerated substantially in its entirety. 
     FIG. 3  depicts a single-flow embodiment of a refrigerator  1 . In contrast to the solution in accordance with  FIG. 2  the gap gas regenerator  43  is a component of the housing wall  14  of the refrigerator housing. If need be, gap gas regenerators  43  of the kind detailed may also be arranged to both sides of the gaps  36 ,  38 . 
     FIG. 4  depicts an embodiment with a gap gas generator  43  which in the depicted embodiment is integrated in the displacing member  17  of the second stage  18 , specifically in the area of its warm end. For this purpose in housing  23  of the displacing member  17 , a hollow chamber  44  is provided in which the regenerator material is located. Through axially spaced radial bores  45 ,  46  the hollow chamber  44  is linked on the inlet and at the discharge side to gap  38 . Between the openings of the radial bores  45 ,  46  in the gap  38 , there is located a seal  47 . This seal too thus also does not need to meet high sealing requirements. It only needs to be ensured that the pressure difference which is created by the seal  47  is greater than the pressure difference created by the regenerator  43 . In this manner it is achieved that the gases flowing from the warm side of the displacing member  17  to its cold side through the gap  38  almost entirely flow through the regenerator  43  so that the desired regeneration effect occurs also with respect to the gap gases. 
   In order to restrict the quantity of the gases flowing through the gap  38 , a further seal  48  may be present in gap  38  at the end (warm end). However, in the instance of an optimised design of the flow resistances produced through the seal  47  and the regenerator  43 , said further seal can be omitted. 
   In connection with the solution in accordance with  FIG. 4 , a further variant is expedient. The chamber  44  may be linked through an approximately axially oriented bore directly to the channel  27 . This solution has the effect that the pressure difference across the seal  47  is lower, in particular when bore  45  is eliminated. 
   The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be constructed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.