Abstract:
A pulse tube system for generating refrigeration for uses such as in magnetic resonance imaging systems wherein an oil-free compressor operating at a higher frequency generates pulsing gas which undergoes a frequency reduction and drives the pulse tube system at a more efficient lower frequency.

Description:
TECHNICAL FIELD 
   This invention relates generally to low temperature or cryogenic refrigeration and, more particularly, to pulse tube refrigeration. 
   BACKGROUND ART 
   A recent significant advancement in the field of generating low temperature refrigeration is the pulse tube system or cryocooler 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. One important application of the refrigeration generated by such cryocooler system is in magnetic resonance imaging systems. 
   One problem with conventional cryocooler systems is contamination of the pulsing gas by the pulse generating equipment. Moreover, a source of inefficiency is a mismatch between the most efficient operating frequency of the cryocooler system and the most efficient operating frequency of the pulse generating system. 
   Accordingly it is an object of this invention to provide an improved cryocooler or pulse tube system which has reduced contamination potential and more efficient operation. 
   SUMMARY OF THE INVENTION 
   The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is: 
   A method for operating a low frequency cryocooler system comprising:
     (A) generating pulsing gas at a frequency of at least 25 hertz by compressing a gas using a moving element moving proximate a surrounding wall wherein no oil is employed between the moving element and the surrounding wall;   (B) passing the pulsing gas through a frequency modulation valve and reducing the frequency of the pulsing gas to produce lower frequency pulsing gas; and   (C) passing the lower frequency pulsing gas to a regenerator which is in flow communication with a thermal buffer tube.   

   Another aspect of the invention is: 
   A low frequency cryocooler system comprising:
     (A) a compressor having a discharge and having a moving element proximate a surrounding wall wherein no oil is employed between the moving element and the surrounding wall;   (B) a regenerator, a frequency modulation valve, discharge conduit extending from the discharge to the frequency modulation valve, and regenerator input/output conduit extending from the frequency modulation valve to the regenerator; and   (C) a thermal buffer tube in flow communication with the regenerator.   

   As used herein the term “regenerator” means a thermal device in the form of porous distributed mass or media, such as spheres, stacked screens, perforated metal sheets and the like, with good thermal capacity to cool incoming warm gas and warm returning cold gas via direct heat transfer with the porous distributed mass. 
   As used herein the term “thermal buffer tube” means a cryocooler component separate from the regenerator and proximate the cold heat exchanger and spanning a temperature range from the coldest to the warmer heat rejection temperature for that stage. 
   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. 
   As used herein the term “direct heat exchange” means the transfer of refrigeration through contact of cooling and heating entities. 
   As used herein the term “frequency modulation valve” means a valve or system of valves generating oscillating pressure and mass flow at a desired frequency. 
   As used herein the term “discharge frequency modulating volume” means the total volume of the discharge conduit, and the reservoir if employed, extending from the compressor discharge to the frequency modulation valve. The discharge frequency modulating volume may be from 0.1 to 10 times the displacement volume of the compressor. 
   As used herein the term “suction frequency modulating volume” means the total volume of the suction conduit, and the reservoir if employed, extending from the frequency modulation valve to the compressor suction. The suction frequency modulation volume may be from 0.1 to 10 times the displacement volume of the compressor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of one preferred embodiment of the invention wherein the compressor is a linear compressor and the frequency modulation valve is a rotary valve. 
       FIG. 2  is a schematic representation of another preferred embodiment of the invention wherein the compressor is a linear compressor and the frequency modulation valve is a control valve system. 
   

   The numerals in the Drawings are the same for the common elements. 
   DETAILED DESCRIPTION 
   The invention will be described in detail with reference to the Drawings. Referring now to  FIG. 1 , an oil-free compressor generates a pulsing gas to drive the cryocooler or pulse tube system which comprises regenerator  20  and thermal buffer tube  40 . Oil-free compressors operate efficiently at high frequencies, typically at from 50 to 60 hertz. In the embodiment of the invention illustrated in  FIG. 1  the oil-free compressor is a linear compressor  1  driven by an electrically driven linear motor, i.e. axially reciprocating electromagnetic transducer  2 . Another example of an oil-free compressor which may be used in the practice of this invention is an oil-free guided rotary compressor driven by a rotary motor. 
   The oil-free compressor has a moving element proximate a surrounding wall. In the embodiment of the invention illustrated in  FIG. 1  the moving element is piston  3  which is driven back and forth by linear motor  2 . Piston  3  reciprocates within the volume defined by casing or surrounding wall  8  and is proximate surrounding wall  8  separated therefrom by clearance  7 . There is no oil in clearance  7  between piston  3  and surrounding wall  8 . Instead, the linear compressor employs gas bearings or flexure suspensions to ensure facile motion of piston  3 . 
   The reciprocating piston  3  generates gas having a pulsing or oscillating motion at the frequency of the alternating current power supplied of at least 25 hertz and typically about 50 to 60 hertz. Check valve system  4 , usually termed reed valves, converts the oscillating pressure wave to obtain a compression output at compressor discharge  5  which has small fluctuations at its operating frequency. Examples of gas which may be used as the pulsing gas generated by the oil-free compressor in the practice of this invention include helium, neon, hydrogen, nitrogen, argon, oxygen, and mixtures thereof, with helium being preferred. 
   The pulsing gas is cooled of the heat of compression in cooler  12  and passed in discharge conduit  18  to frequency modulation valve  17  which, in the embodiment illustrated in  FIG. 1 , is a rotary valve. Rotary valve  17  is driven by a motorized system which is not shown in  FIG. 1 . Preferably, as shown in  FIG. 1 , the high frequency pulsing gas in discharge conduit  18  passes through reservoir  13 . The discharge frequency modulating volume of discharge conduit  18  and reservoir  13  serves to decouple the pulse rate between the compressor and the crycooler by providing a steady gas supply at a relatively stable pressure to the valve. As the rotating part (not shown) of rotary valve  17  rotates, the bores alternatively connect the compressor discharge conduit  18  to the regenerator inlet/outlet conduit  62 , and the regenerator inlet/outlet conduit  62  to the compressor suction conduit  19 . These alternating connections generate oscillating pressure and mass flow thus a pressure-volume work at the rotation frequency of the valve  17 . 
   As the pulsing gas passes through the frequency modulation valve its frequency is reduced to the most efficient operating frequency of the cryocooler. The resulting lower frequency pulsing gas generally has a frequency less than 40 hertz, typically has a frequency less than 30 hertz, preferably less than 10 hertz, most preferably less than 5 hertz. The lower frequency pulsing gas is then passed to regenerator  20  of the cryocooler or pulse tube system. Regenerator  20  is in flow communication with thermal buffer tube  40  of the pulse tube system. 
   The lower frequency pulsing gas applies a pulse to the hot end of regenerator  20  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  20 . The hot working gas is cooled, preferably by indirect heat exchange with heat transfer fluid  22  in heat exchanger  21 , to produce warmed heat transfer fluid in stream  23  and to cool the compressed working gas of the heat of compression. Examples of fluids useful as the heat transfer fluid  22 ,  23  in the practice of this invention include water, air, ethylene glycol and the like. Heat exchanger  21  is the heat sink for the heat pumped from the refrigeration load against the temperature gradient by the regenerator  20  as a result of the pressure-volume work generated by the compressor and the frequency modulation valve. 
   Regenerator  20  contains regenerator or heat transfer media. Examples of suitable heat transfer media in the practice of this invention include steel balls, wire mesh, high density honeycomb structures, expanded metals, lead balls, copper and its alloys, complexes of rare earth element(s) and transition metals. The pulsing or oscillating working gas is cooled in regenerator  20  by direct heat exchange with cold regenerator media to produce cold pulse tube working gas. 
   Thermal buffer tube  40  and regenerator  20  are in flow communication. The flow communication includes cold heat exchanger  30 . The cold working gas passes in line  60  to cold heat exchanger  30  and in line  61  from cold heat exchanger  30  to the cold end of thermal buffer tube  40 . Within cold heat exchanger  30  the cold working gas is warmed by indirect heat exchange with a refrigeration load thereby providing refrigeration to the refrigeration load. This heat exchange with the refrigeration load is not illustrated. 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. 
   The working gas is passed from the regenerator  20  to thermal buffer tube  40  at the cold end. Preferably, as illustrated in  FIG. 1  thermal buffer tube  40  has a flow straightener  41  at its cold end and a flow straightener  42  at its hot end. As the working gas passes into pulse thermal buffer  40  it compresses gas in the thermal buffer tube and forces some of the gas through heat exchanger  43  and orifice  50  in line  51  into the reservoir  52 . Flow stops when pressures in both the thermal buffer tube and the reservoir are equalized. 
   Cooling fluid  44  is passed to 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. Resulting warmed or vaporized cooling fluid is withdrawn from heat exchanger  43  in stream  45 . Preferably cooling fluid  44  is water, air, ethylene glycol or the like. 
   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  52  into the thermal buffer tube  40 . The cold working gas is pushed into the cold heat exchanger  30  and back towards the warm end of the regenerator while providing refrigeration at heat exchanger  30  and cooling the regenerator heat transfer media for the next pulsing sequence. Orifice  50  and reservoir  52  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  40 . 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  40 . The expanded gas reverses its direction such that it flows from the thermal buffer tube toward regenerator  20 . The relatively higher pressure gas in the reservoir flows through valve  50  to the warm end of the thermal buffer tube  40 . In summary, thermal buffer tube  40  rejects the remainder of pressure-volume work generated by the compression and frequency modulation system (which comprises the oil-free compressor and the frequency modulation valve) as heat into warm heat exchanger  43 . 
   The expanded working gas emerging from heat exchanger  30  is passed in line  60  to regenerator  20  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 refrigerant sequence and putting the regenerator into condition for the first part of a subsequent pulse tube refrigeration sequence. Pulsing gas from regenerator  20  passes back to rotary valve  17  and in suction conduit  19  to suction  6  of compressor  1 . Preferably reservoir  16  is employed on suction conduit  19  and the suction frequency modulating volume of suction conduit  19  and reservoir  16  serves a purpose similar to that of the discharge frequency modulating volume. 
     FIG. 2  illustrates another embodiment of the invention. The elements common to the embodiments illustrated in  FIGS. 1 and 2  will not be described again in detail. In the embodiment illustrated in  FIG. 2  the rotary valve is replaced with dual control valves  14  and  15  on the output and input conduits respectively, with motor driven control valve  14  serving as the frequency modulation valve. 
   Now by the use of this invention a cryocooler, i.e. a pulse tube system, may operate at its most efficient frequency rather than being limited to operating at the frequency of the compressor while also avoiding complications caused by oil contamination of the pulsing gas. 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 within the spirit and the scope of the claims.