Abstract:
A two-stage pulse tube refrigerator comprises a pressure wave generator-compressor, first stage and second stage regenerators, first stage and second stage pulse tubes, heat exchangers and a hybrid phase shift mechanism for the first and second stage pulse tubes. The second stage phase shift mechanism includes double fixed orifices while the first stage shifter is an arrangement including one of a) 4 valves, b) 5 valves c) 2 active buffers or d) 3 active buffers. The double fixed orifice phase shifter is located either at room temperature or is thermally connected with the first stage cold end. Two-stage pulse tube refrigerators with a hybrid phase shifter have increased second stage regenerator performance at lower temperature. Pressure drop through the valves and compressor power consumption are decreased, and losses from phase interaction between each stage are eliminated.

Description:
BACKGROUND OF THE INVENTION 
     Pulse tube refrigeration without moving parts, operating at cryogenic temperature, is one attractive method for providing a reliable, vibration-free, long life, and simple cryocooler that can meet the requirements for cryogenic cooling in many applications. In order to produce cooling effect at a pulse tube cold end, it is necessary to cause a time-phasing [shifting] between gas pressure fluctuations and gas displacement inside the pulse tube. Such phase shift between the gas pressure fluctuation and the gas displacement inside the pulse tube is obtained by controlling the mass flow rate with a phase shifter located at the pulse tube warm end. 
     Several types of phase shifters have been developed for improvement in performance of the pulse tube refrigerator, such as double inlet, four valve, and active buffer type phase shifters. However, there are several disadvantages in present phase shifters for multiple stage pulse tube refrigerators. 
     In the double inlet type and four valve pulse tube refrigerator for producing large cooling capacity at relatively high temperature, a large amount of additional compressor work is expended due to mass flow in and out of a bypass line and valves. This added workload decreases overall efficiency of the machine. In multiple stage double inlet and four valve pulse tube refrigerators, phase interaction between each stage produces thermal losses and makes the refrigeration temperature unstable at each stage. 
     In the active buffer type pulse tube refrigerator producing small cooling capacity at very low temperature, regenerator inefficiency is very high due to larger mass flow rate through the regenerator cold end and poor phase shift effect at a higher ratio of regenerator void volume to pulse tube volume. 
     SUMMARY OF THE INVENTION 
     The present invention addresses these problems in the conventional pulse tube refrigerators. An objective of the present invention is to provide an improved two-stage pulse tube refrigerator which has higher overall efficiency at a higher temperature stage, and higher regenerator performance at a lower temperature stage, and less phase interaction losses. 
     In order to meet the above and other objectives, a two-stage pulse tube refrigerator in accordance with the invention comprises a pressure wave generator-compressor, first stage and second stage regenerators, first stage and second stage pulse tubes, heat exchangers, and a hybrid phase shift mechanism for the first and second stage pulse tubes. The second stage phase shift mechanism utilizes at least one fixed orifice. The fixed orifice phase shifter is either located at room temperature or thermally connected with the first stage cold end. The first stage phase shifter includes any one of a) 4 valves, b) 5 valves, c) 2 active buffers, or d) 3 active buffers. 
     In a pulse tube refrigerator with two active phase shifting valves, the valves are positioned at room temperature between the warm end of the first stage pulse tube and the compressor return and supply line. One orifice is positioned at room temperature between the warm end of the second stage pulse tube and one buffer where there is a moderate gas pressure. Another orifice is positioned at room temperature between the warm end of the first regenerator and the warm end of the second stage pulse tube. 
     In another pulse tube refrigerator with three active valves, the valves are positioned at room temperature between the warm end of the first stage pulse tube and the compressor return and supply line, and one active valve is positioned between the warm end of the first stage pulse tube and one buffer. One orifice is positioned at room temperature between the warm end of the second stage pulse tube and one buffer where there is a moderate gas pressure. Another orifice is positioned at room temperature between the warm end of the first regenerator and the warm end of the second stage pulse tube. 
     Still another pulse tube refrigerator has a hybrid phase shift mechanism with three buffers, three active valves and two orifices. The three active valves are positioned at room temperature between three buffers and the warm end of the first stage pulse tube. One orifice is positioned at room temperature between the warm end of the second stage pulse tube and one buffer where there is a moderate gas pressure. Another orifice is positioned at room temperature between the warm end of the first regenerator and the warm end of the second stage pulse tube. 
     A fourth embodiment of a pulse tube refrigerator in accordance with the invention has a double fixed orifice phase shifter for a second stage thermally connected with the first stage cold end. The warm end of the second stage pulse tube is thermally connected with the first stage cold end. One orifice is positioned between the first stage cold end and the second stage pulse tube warm end, and another orifice is positioned between the warm end of the second stage pulse tube and one buffer at the first stage cold end. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a full understanding of the invention reference is had to the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram of a two-stage pulse tube refrigerator in accordance with the invention; 
     FIG. 2 is a timing graph for active valves in the refrigerator of FIG. 1; 
     FIG. 3 is a schematic diagram of an alternative embodiment of a two-stage pulse tube refrigerator in accordance with the invention; 
     FIG. 4 is a valve timing chart associated with the embodiment of FIG. 3; 
     FIG. 5 is a schematic of another alternative embodiment of a two-stage pulse tube refrigerator in accordance with a the invention; 
     FIG. 6 is a valve timing chart associated with the embodiment of FIG. 5; 
     FIG. 7 is a schematic diagram of yet another alternative embodiment of a two-stage pulse tube refrigerator in accordance with the invention; 
     FIG. 8 is a schematic diagram of an fifth alternative embodiment of a two-stage pulse tube refrigerator in accordance with the invention; and 
     FIG. 9 is a schematic diagram of a sixth alternative embodiment of a two-stage pulse tube refrigerator in accordance with the invention. 
     FIG.  10 ( a ) and FIG.  10 ( b ) are pressure-volume diagrams of gas volumes at respective cold ends of the two pulse tubes of the embodiment of FIG. 9; and 
     FIG. 11 is a valve timing chart associated with the embodiment of FIG.  9 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to the Figures, a two-stage pulse tube refrigerator in accordance with the invention includes a first pulse tube  12  and a second pulse tube  14 , a first regenerator  16  connected to a second regenerator  18 . The first pulse tube  12  has a warm end heat exchanger  20  and a cold end and heat exchanger  22 , and the second pulse tube  14  has respective warm and cold end heat exchangers  24 ,  26 . 
     A line  28  connects between the cold end heat exchanger  22  of the first pulse tube  12  and the colder end of the first regenerator and warmer end of the second regenerator  18 . A line  30  connects between the cold end heat exchanger  26  of the second pulse tube  14  and the cold end of the second regenerator  18 . The warm end of the first regenerator  16  connects to the low pressure side of a compressor  32  by way of the on/off valve  36 , and, the warm end heat exchanger  20  of the first pulse tube  12  also connects to the low pressure inlet of the compressor  32  by way of the on/off valve  37 . The high pressure discharge of the compressor  32  connects with the warm end of the first regenerator  16  by way of the valve  34  and to the warm end heat exchanger  20  in the first pulse tube  12  by way of the valve  35 . 
     A buffer  38  connects to the warm end heat exchanger  24  of the second pulse tube  14  by way of the fixed orifice  40 , and the warm end of the first regenerator  16  connects to the warm end heat exchanger  24  of the second pulse tube  14  by way of the fixed orifice  42 . 
     The term “fixed orifice” does not mean that this device is not adjustable but rather that the device if adjustable is not adjusted or varying physically during steady-state operation of the refrigerator. 
     These refrigerators are improved in general by reducing system losses and by increasing the work effected by gas expansion at the cold end of the pulse tube. Refrigerant gas flowing in and out of the pulse tubes at each end is controlled to affect the gas expansion work by sequenced operation of the valves  34 - 37 . Operation of each valve in a cycle shifts the phase between the gas pressure fluctuation and the gas displacement inside the pulse tubes. 
     FIG. 2 indicates the timing for each valve  34 - 37 . That is, the crossed hatched rectangles indicate periods within a single operating cycle when the particular valve is open, permitting flow of gas therethrough. The cycle begins with each of the valves  34 - 37  closed and the cycle finishes in the same state. 
     In another embodiment of a two-stage pulse tube refrigerator in accordance with the invention (FIG.  3 ), the physical configuration is substantially similar to that in FIG. 1, except that a fifth on/off valve  44  has been added connecting the buffer  38  to the warm end heat exchanger of the first pulse tube  12 . Similar reference numerals are used in FIG. 3 (and in all drawings), to designate the same elements that appear in several embodiments in the application. 
     FIG. 4 illustrates the timing for opening and closing each of the valves in one cycle of the refrigerator of FIG.  3 . 
     In the embodiment in accordance with the invention of FIG. 5 the connection between the compressor  32  and the warm end heat exchanger  20  of the first pulse tube  12  is replaced by additional buffers  46 ,  48 . FIG. 6 illustrates the valve timing cycle associated with the embodiment of FIG.  5 . In FIG. 5, three active valves  35 ,  37 ,  44  are positioned at room temperature between three buffers  38 ,  46 ,  48  and the warm end of the first stage pulse tube  12 . FIG. 6 illustrates valve timing for a single cycle of operation. 
     The two-stage pulse tube refrigerator of FIG. 7 is an embodiment in accordance with the invention wherein the double fixed orifice phase shifter for the second stage is thermally connected with the first stage cold end. Further, the second stage pulse tube  14  warm end is thermally connected with the first stage pulse tube  12  cold end. One orifice  42  is positioned between the first stage pulse tube  12  cold end and the second stage pulse tube  14  warm end. Another orifice  40  is positioned between the warm end of the second stage pulse tube  14  and one buffer  38  at the first stage pulse tube  12  cold end. 
     The embodiment in accordance with the invention of FIG. 8, is similar to the embodiment of FIG. 3 except that the fixed orifice  50  in FIG. 8 replaces the valve  44  in the embodiment of FIG.  3 . Valve timing is similar to FIG.  2 . 
     The embodiment of a two-stage pulse tube refrigerator in accordance with the invention of FIG. 9 differs from FIG. 8 in that the orifice  42  of FIG. 8 is replaced by on/off valves  52 ,  54  that are between the warm end heat exchanger  24  of the second pulse tube  14  and the compressor  32  inlet and discharge respectively. FIG. 11 indicates the timing sequence for the six valves in the embodiment of FIG. 9 for a single refrigeration cycle. 
     Operation of the two-stage pulse tube refrigerator in accordance with the invention of FIG. 9, a preferred embodiment, is now explained. For purposes of this discussion, the internal volume of the first pulse tube is divided into three parts, namely a hot volume Vh 1  at the warm end of the first stage pulse tube  12 , a cold volume Vc 1  at the cold end of the pulse tube  12 , and the intermediate volume Vp 1  that is the gas piston, as will be understood by those skilled in the pulse tube arts. 
     The second stage pulse tube  14  is similarly divided showing Vh 2 , Vc 2  and the intermediate Vp 2 . FIG. 10 a  is a PV diagram showing changes of pressure and volume of the gas represented by Vc 1  in the first stage pulse tube  12 , and FIG. 10 b  is a similar PV cycle diagram for the cold gas volume Vc 2  in the second stage pulse tube  14 . It will be appreciated that the purpose of phase shifting is to increase the area enclosed in the PV cycle diagram. This enclosed area represents cooling capacity made available by the refrigerator. 
     Basic Principle of Operation for Hybrid Two-Stage Pulse Tube Refrigerator (FIG. 9) 
     In comparison to a G-M refrigerator, the gas within the pulse tube works as a compressible displacer (as a piston). This gas piston has to move with correct relative timing for a desired refrigeration cycle by using a phasing control mechanism located at the pulse tubes warm ends. The thermodynamic process of the hybrid two-stage pulse tube refrigerator of the present invention is described as follows: 
     Process 1-2: Starting at point  1  with all valves closed and the pulse tubes at low pressure, gases from the buffer flow into the pulse tubes through the orifices  50  (O 1 ) and  40  (O 2 ). The pressure in the pulse tubes is thereby increased and the gas pistons Vp 1  and Vp 2  move toward the cold ends of the pulse tubes and the volumes Vc 1  and Vc 2  are decreased. 
     Process 2-3: With gas pistons near the respective bottoms of the pulse tube cold ends, the inlet valve  52  (V 5 ) is opened first and the valve  35  (V 3 ) is opened later, the pressures in the pulse tubes are further increased by connection to the compressor discharge. The gas pistons move to the bottoms of the pulse tubes so that Vc 1  and Vc 2  are zero. 
     Process 3-4: With the inlet valves V 5  and V 3  still opened, the inlet valve V 1  is opened, and the pressures in the pulse tubes are increased to high pressure. The gas pistons in the pulse tubes start to move from the cold ends toward the hot ends of the pulse tubes, and Vc 1  and Vc 2  increase. 
     Process 4-5: With the inlet valve V 1  still opened, V 3  is closed first and V 5  is closed later. Thus, the gas piston in each pulse tube continues to move from the cold ends to the hot ends of the pulse tubes, and Vc 1  and Vc 2  increase at relatively constant pressure. 
     Process 5-6: All valves are closed and the pulse tubes have high pressure. Gases from the pulse tubes flow into the buffer through the orifices O 1  and O 2 . The pressure in the pulse tubes is thereby decreased and the gas pistons Vp 1  and Vp 2  move toward the hot ends of the pulse tubes. Vc 1  and Vc 2  increase. 
     Process 6-7: With the gas pistons near the tops of the pulse tube hot ends, the outlet valve V 6  is opened first and V 4  is opened later, the pressures in the pulse tubes are further decreased by connection to the compressor suction. The gas pistons move to the warm tops of the pulse tubes. 
     Process 7-8: With the outlet valves V 6  and V 4  still opened, the outlet valve V 2  is opened, and the pressures in the pulse tubes are decreased to low pressure. The gas pistons in the pulse tubes start to move from the hot ends toward the cold ends of the pulse tubes. 
     Process 8-1: With the inlet valve V 2  still opened, V 4  is closed first and V 6  is closed later. Thus the gas piston in the pulse tube continue to move from hot ends to cold ends of the pulse tubes to complete the cycle. 
     Operation of the pulse tube refrigerators of FIGS. 1,  3 ,  5 ,  7 ,  8 , are similar to the process described above when considered with their associated timing charts for valve operation, and will be readily understood by those skilled in the art. 
     It will thus be seen that the objects set forth above, those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above constructions without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limited sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention, which as a matter of language might be said to fall between there.