Abstract:
A cryocooler assembly having a regenerator employing a plurality of high heat capacity screens positioned perpendicular to the regenerator longitudinal axis and in a sufficient linear density to serve as heat transfer media and also to serve as buffer structures to keep worm holes within particulate heat transfer media from forming and to reduce agglomeration of particles within the regenerator.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates generally to low temperature or cryogenic refrigeration and, more particularly, to cryocoolers for the generation of such cryogenic refrigeration.  
       BACKGROUND ART  
       [0002]     A recent significant advancement in the field of generating low temperature refrigeration is the pulse tube and other cryocooler systems wherein pulse energy is converted to refrigeration using an oscillating gas. Such systems can generate refrigeration to very low levels sufficient, for example, to liquefy helium.  
         [0003]     One problem with conventional cryocooler systems is the loss of effective load heat capacity and flow uniformity and the resulting heat transfer maldistribution in the regenerator portion of the cryocooler which leads to operational inefficiency. These problems are particularly troublesome when the cryocooler is operated to provide very low temperature refrigeration such as below 40K.  
       SUMMARY OF THE INVENTION  
       [0004]     A cryocooler assembly comprising a pressure wave generator, a regenerator and a thermal buffer volume wherein the regenerator contains heat transfer media comprising a plurality of screens oriented perpendicular to the longitudinal axis of the regenerator, said screens being comprised of or coated with high heat capacity material or alloy.  
         [0005]     As used herein the term “screen” means a thin plate with periodic openings such as mesh, perforated plate, wire mesh or corrugated dimpled plate used for transferring heat from and to gas while providing uniform flow through its openings.  
         [0006]     As used herein the term “high heat capacity material” means a chemical element or compound that has a higher volumetric or mass heat capacity than steel at temperatures below 100K. Some examples include lead, some copper alloys, and lanthanide series materials.  
         [0007]     As used herein the term “high heat capacity alloy” means a homogeneous mixture, solid solution or heterogeneous mixture comprising at least one high heat capacity material.  
         [0008]     As used herein the term “pressure wave generator” means an electromechanical, mechanical, or thermoacoustic device that produces pressure waves in the form of acoustic energy.  
         [0009]     As used herein the term “longitudinal axis” means an imaginary line running through a regenerator in the direction of the gas flow.  
         [0010]     As used herein the term “regenerator” means a thermal device containing heat transfer media which has good thermal capacity to cool incoming warm gas and warm returning cold gas via direct heat transfer with the heat transfer media.  
         [0011]     As used herein the term “thermal buffer volume” means a cryocooler component separate from the regenerator, proximate a cold heat exchanger and spanning a temperature range from the coldest to the warmer heat rejection temperature.  
         [0012]     As used herein the term “indirect heat exchange” means the bringing of fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.  
         [0013]     As used herein the term “direct heat exchange” means the transfer of refrigeration through contact of cooling and heating entities.  
         [0014]     As used herein the term “electroplated screen” means a base screen substrate which is covered by a high heat capacity material or alloy via electroplating and/or another thin film deposition means. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a representation of one preferred cryocooler assembly of this invention wherein the cryocooler is a pulse tube type cryocooler.  
         [0016]      FIG. 2  is a plan view of one preferred embodiment of a screen type for use in the practice of this invention.  
         [0017]      FIG. 3  is a plan view of another preferred embodiment of a screen type for use in the practice of this invention.  
         [0018]      FIG. 4  is a plan view of another preferred embodiment of a screen type for use in the practice of this invention.  
         [0019]      FIG. 5  is a cross sectional representational illustration of the screen interlayer embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0020]     The invention will be described in greater detail with reference to the Drawings. Referring now to  FIG. 1 , pressure wave generator  9 , which may be a compressor driven by a linear or rotary motor, generates a pulsing gas to drive a cryocooler such as the pulse tube cryocooler illustrated in  FIG. 1 . The pulsing working gas pulses within the pressure wave pathway which comprises the pressure wave generator, a regenerator and a thermal buffer volume. In the pulse tube type cryocooler illustrated in  FIG. 1 , the pressure wave pathway also includes a reservoir downstream of the thermal buffer volume. Typically the working gas comprises helium. Other gases which may be used as working gas in the practice of this invention include neon, argon, xenon, nitrogen, air, hydrogen and methane. Mixtures of two or more such gases may also be used as the working gas.  
         [0021]     The pulsing working gas applies a pulse to the hot end of the regenerator  10  thereby generating an oscillating working gas and initiating the first part of the pulse tube sequence. The pulse serves to compress the working gas producing hot compressed working gas at the hot end of the regenerator  10 . The hot working gas is cooled, preferably by indirect heat exchange with heat transfer fluid  41 ,  42  in hot heat exchanger  40  to cool the compressed working gas of the heat of compression. Heat exchanger  40  is also the heat sink for the heat pumped from the refrigeration load against the temperature gradient by the regenerator  10  as a result of the pressure-volume work generated by the pressure wave generator.  
         [0022]     Regenerator  10  contains heat transfer media as will be more fully described below. The pulsing or oscillating working gas is cooled in regenerator  10  by direct heat exchange with cold heat transfer media to produce cold pulse tube working gas.  
         [0023]     Thermal buffer volume or tube  16 , which in the arrangement illustrated in  FIG. 1  is a pulse tube, and regenerator  10  are in flow communication. The flow communication includes cold heat exchanger  14 . The cold working gas passes in line  17  to cold heat exchanger  14  and in line  18  from cold heat exchanger  14  to the cold end of thermal buffer tube  16 . Within cold heat exchanger  14  the cold working gas is warmed by indirect heat exchange with a refrigeration load thereby providing refrigeration to the refrigeration load. In  FIG. 1 , the refrigeration load is represented by stream  47  which is passed to cold heat exchanger  14  and which emerges therefrom as stream  46 . One example of a refrigeration load is for use in a magnetic resonance imaging system. Another example of a refrigeration load is for use in high temperature superconductivity.  
         [0024]     The working gas is passed from the regenerator  10  to thermal buffer tube  16  at the cold end. As the working gas passes into thermal buffer volume  16 , it compresses gas in the thermal buffer volume or tube and forces some of the gas through warm heat exchanger  43  and orifice  20  in line  19  into the reservoir  22 . Flow stops when pressures in both the thermal buffer tube and the reservoir are equalized.  
         [0025]     Cooling fluid is passed in line  44  to warm heat exchanger  43  wherein it is warmed or vaporized by indirect heat exchange with the working gas, thus serving as a heat sink to cool the compressed working gas. The resulting warmed or vaporized cooling fluid is withdrawn from heat exchanger  43  in line  45 .  
         [0026]     In the low pressure point of the pulsing sequence, the working gas within the thermal buffer tube expands and thus cools, and the flow is reversed from the now relatively higher pressure reservoir  22  into the thermal buffer tube  16 . The cold working gas is pushed into the cold heat exchanger  14  and back towards the warm end of the regenerator while providing refrigeration at heat exchanger  14  and cooling the regenerator heat transfer media for the next pulsing sequence. Orifice  20  and reservoir  22  are employed to maintain the pressure and flow waves in phase so that the thermal buffer tube generates net refrigeration during the compression and the expansion cycles in the cold end of thermal buffer tube  16 . Other means for maintaining the pressure and flow waves in phase which may be used in the practice of this invention include inertance tube and orifice, expander, linear alternator, bellows arrangements, and a work recovery line connected back to the compressor with a mass flux suppressor. In the expansion sequence, the working gas expands to produce working gas at the cold end of the thermal buffer tube  16 . The expanded gas reverses its direction such that it flows from the thermal buffer tube toward regenerator  10 . The relatively higher pressure gas in the reservoir flows through valve  20  to the warm end of the thermal buffer tube  16 . In summary, thermal buffer tube  16  rejects the remainder of pressure-volume work generated by the compression as heat into warm heat exchanger  43 .  
         [0027]     The expanded working gas emerging from heat exchanger  14  is passed to regenerator  10  wherein it directly contacts the heat transfer media within the regenerator to produce the aforesaid cold heat transfer media, thereby completing the second part of the pulse tube refrigeration sequence and putting the regenerator into condition for the first part of a subsequent pulse tube refrigeration sequence.  
         [0028]     Conventional heat transfer media within the regenerator, such as particulate material, tend to promote flow maldistribution via worm hole and agglomerate formation in these materials. In the practice of this invention the heat transfer media is comprised of a plurality of screens oriented perpendicular to the longitudinal axis of the regenerator. The screens act as heat transfer media per se, and also serve as buffer structures, as in the preferred embodiments illustrated in  FIG. 3-5 , to combat worm hole formation and agglomeration when particles are also employed as heat transfer media within the regenerator. Preferably the screens which are employed in the practice of this invention are comprised of steel, copper, oxygen-free copper, copper bronze, phosphorous copper, etc.  
         [0029]     In one preferred embodiment of the invention illustrated in  FIG. 2 , the base support screen  61  is electroplated with high heat capacity material or alloy such as lead or rare earth  62  to form screens  60 . The screen bed within the regenerator produced by the use of such preferred screens exhibits optimum porosity and volumetric heat capacity, and also allows transverse equalization of temperatures. For a cryocooler operating at 80K the porosity of such screens may be within 50 to 90% for optimal performance, while a cryocooler operating at 20K could be optimized at a screen porosity within the range of from 10 to 50 percent.  
         [0030]     In another preferred embodiment of the invention illustrated in simplified form in  FIGS. 3 and 4 , the steel, copper bronze or oxygen-free copper screens  71  are made with openings large enough to accept particles  72  made of high heat capacity material or alloys such as lead or rare earth within their matrices. The addition of the particles within the screen matrix will provide higher heat transfer and high heat capacity at lower temperatures. The coarse screens will suppress particle agglomeration and minimize longitudinal heat conduction. The coarse screens filled with loose particles could be supported at both ends (top and bottom) by very fine screens  77 . Essentially, these fine screens will contain the particles in the regenerator bed assembly.  
         [0031]     In another preferred embodiment of the invention, which is illustrated in  FIG. 5 , screens are installed in the regenerator  10  in interlayers  82  perpendicular to the longitudinal axis or gas flow  84  to hold heat transfer particles  85  in layers  83  between the screens. This grading or layering of the screens and particulate material in alternating sequence allows better optimization of the regenerator bed thus resulting in better cryocooler performance. The screen-interlayers can be made of multiple diffusion-bonded or loose steel, copper bronze or oxygen-free copper screens. The diffusion-bonded screens can be made to various thicknesses for adequate support. Oscillating flow can be detrimental to screen integrity. Therefore screens must be adequately designed to support the particulate bed materials in operation. A reduced number of screens can be used with a reinforcing plate like structure to ensure larger ratios of rare earth to interlayer material, while maintaining structure integrity. Additionally, the screen interlayers will allow transverse equalization of temperatures and allows a graded regenerator bed. For example lead particles could be used at regenerator temperature zones of 70 to 30K and rare earth materials could be used at temperatures below 30K. There could be more than two different bed materials within the regenerator. The bed could also be graded with particle diameters where smaller diameter more expensive particles could be used in the colder zones of the regenerator.  
         [0032]     Preferably in the practice of this invention screens are positioned within the regenerator in a linear density within the range of from 150 to 600 screens per inch.  
         [0033]     Although the invention has been described in detail with reference to certain preferred embodiments, those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims.