Abstract:
A hybrid two stage expander having a first stage Stirling expander coupled to a second stage pulse tube expander. Both stages are pneumatically driven by a common reciprocating compressor in a typical application. The first stage Stirling expander provides high thermodynamic efficiency which removes a majority of the heat load from gas within the cryocooler. The second stage pulse tube expander provides additional refrigeration capacity and improved power efficiency with little additional manufacturing complexity since it has no moving parts.

Description:
BACKGROUND 
     The present invention relates generally to cryocoolers, and more particularly, to a two stage cryocooler having a hybrid configuration employing a Stirling first stage expander and a pulse tube second stage expander. 
     Low temperature refrigeration needs can often be met more efficiently with multi-stage refrigerators than with traditional single stage devices. For applications requiring closed-cycle refrigeration where multiple stages have been deemed advantageous, previous designs have typically implemented two or more expander stages of the same type. Examples of these expanders include those of the Stirling, Gifford-McMahon, pulse tube, and Joule-Thomson designs. 
     It would therefore be an advantage to have an improved cryocooler that improves upon conventional single and multi-stage designs. Accordingly, it is an objective of the present invention to provide for a two stage cryocooler having a hybrid configuration that uses a Stirling first stage expander and a pulse tube second stage expander. 
     SUMMARY OF THE INVENTION 
     To meet the above and other objectives, the present invention provides for a two stage expander having a hybrid configuration that combines a first stage Stirling expander with a second stage pulse tube expander. Both stages are pneumatically driven by a common reciprocating compressor or motor. The two stage cryocooler is designed for long, highly reliable life and is sufficiently small and light weight to permit its use in spacecraft applications. 
     The use of the first stage Stirling expander provides high thermodynamic efficiency in that it removes a majority of the heat load from gas within the cryocooler. The use of the second stage pulse tube expander provides additional refrigeration capacity and improved power efficiency with little additional manufacturing complexity due to the simplicity of the pulse tube expander, which has no moving parts. One of the major refrigeration losses in a traditional single-stage pulse tube expander, regenerator pressure drop, is relatively small in the present hybrid two stage cryocooler since the pulse tube regenerator operates at a reduced temperature (higher density yields lower gas velocity, which results in a lower pressure drop). 
     The use of the second stage pulse tube expander enables the incorporation of a low-through heat exchanger at an interface between first and second stage expanders. This feature significantly improves first stage efficiency (relative to conventional single stage Stirling expanders) by virtue of the improved heat transfer coefficient at the thermal interface between the first and second stage expanders. Use of the first stage Stirling expander also reduces the total dead volume of the hybrid cryocooler compared to a pulse tube cooler (either one or two stage cooler having equivalent thermodynamic power). This reduces mass flow requirements, which in turn reduces the swept volume requirements of the compressor. This enables refrigeration to be accomplished with a smaller compressor. 
     The present invention may be adapted for use with cryogenic refrigerators used in military and commercial applications where the application demands high efficiency refrigeration at one or two temperatures, small size, low weight, long life, high reliability, and cost effective producibility. The primary intended use for the present invention is in space-based infrared sensors for civil and defense applications. 
     The present invention improves upon or displaces existing conventional cryocooler expanders including single and multi-stage Stirling expanders and single and multi-stage pulse tube expanders. The present hybrid expander achieves better performance at the same or lower manufacturing cost than either Stirling or pulse tube technology can deliver separately. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawing, wherein like reference numerals designate like structural elements, and in which: 
     FIGS.  1 - 4  illustrate several cross sectional views of an exemplary hybrid two stage expander in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing figures, FIGS.  1 - 4  illustrate cross sectional views of an exemplary hybrid two stage expander  10  in accordance with the principles of the present invention. The exemplary hybrid two stage expander  10  comprises first and second stages  20 ,  30 . The first stage  20  comprises a Stirling expander  20  and the second stage  30  comprises a pulse tube expander  30 . 
     The first stage Stirling expander  20  of the exemplary hybrid two stage cryocooler  10  comprises a flexure mounted Stirling expander  20 . The Stirling expander  20  has a plenum  22  and a cold head comprising a thin walled cold cylinder, an expander inlet  26  disposed at a fore end of the plenum  22 , a moveable displacer  23  or piston  23  disposed within the plenum  22 , and a first stage regenerator  21  and heat exchanger  24 . 
     The displacer  23  is suspended on fore and aft flexures  25 . The displacer  23  is controlled and moved by means of a motor  12  located at a fore end of the plenum  22 . A flexure suspended balancer  27  may be used to provide internal reaction against the inertia of the moving displacer  23 . 
     The second stage pulse tube expander  30  comprises a second stage regenerator  31  or regenerative heat exchanger  31 , a pulse tube  32 , and a surge volume  33 . The pulse tube  32  is coupled at one end to a second stage thermal interface  41 . The second stage thermal interface  41  has a first end cap  42  that seals the pulse tube gas column  32 , a second end cap  43  that seals the second stage regenerator  31  or regenerative heat exchanger  31 . A second stage heat exchanger  44  is provided in the second stage thermal interface  41  that is coupled between the pulse tube  32  and the scond stage regenerator  31 . 
     A flow-through heat exchanger  34  is disposed at a thermal interface  35  between first stage Stirling expander  20  and the second stage pulse tube expander  30 . The flow-through heat exchanger  34  includes a pulse tube inlet heat exchanger  51  and a pulse tube outlet heat exchanger  52 . A third end cap  53  seals the end of the pulse tube gas column  32  in the flow-through heat exchanger  34 . A port  54  is disposed in the flow-through heat exchanger  34  that is coupled to the surge volume  33  and provides a phase angle control orifice. 
     In the hybrid two stage expander  10 , a gas such as helium, for example, flows into the expander inlet  26  and into the first stage regenerator  21  and heat exchanger  24 . Gas flowing into the cold volume within the first stage Stirling expander  20  is regenerated by the first stage regenerator  21  and heat exchanger  24 . A portion of the gas remains in the first stage expansion volume of the first stage regenerator  21 . Progressively smaller portions of the gas continue to the second stage regenerator  31 , the pulse tube  32 , and the surge volume  33 . Gas return flow follows the same path in reverse. 
     A significant advantage of the hybrid two stage expander  10 , compared with other multistage expanders, is the ease of shifting refrigerating power between the two stages  20 ,  30 . This is accomplished by varying the stroke and/or phase angle of the displacer  23  in the Stirling first stage expander  20  and by means of the port  54 , which alters mass flow distribution into the surge volume  33 . This additional degree of control enables performance optimization at any operating point, including on orbit in the actual thermal environment of a spacecraft, for example. This feature provides for power savings when using the hybrid two stage expander  10 . 
     The first stage Stirling expander  20  has high thermodynamic efficiency when removing the majority of the heat load from gas within the expander  10 . The second stage pulse tube expander  30  provides additional refrigeration capacity and improved power efficiency. The second stage pulse tube expander  30  adds little additional manufacturing complexity because of its simplicity, in that it has no moving parts. 
     The flow-through heat exchanger  34  at the interface  35  between first and second stage expanders  20 ,  30  significantly improves first stage efficiency (relative to conventional single stage Stirling expanders) by virtue of the improved heat transfer coefficient at the thermal interface therebetween. The Stirling expander  20  reduces the total dead volume of the hybrid expander  10  compared to a conventional one or two stage pulse tube cooler having an equivalent thermodynamic power. The Stirling expander  20  thus reduces mass flow requirements, which reduces the swept volume of the compressor and enables refrigeration to be accomplished with a smaller compressor. 
     The regenerator pressure drop is relatively small in the hybrid two stage expander  10  because the pulse tube regenerator  31  operates at a reduced temperature. The gas thus has a higher density and produces a lower gas velocity, which results in a lower pressure drop. 
     The hybrid two stage expander  10  may be used in cryogenic refrigerators adapted for military and commercial applications where high efficiency refrigeration is required at one or two temperatures. The hybrid two stage expander  10  is also well suited for use in applications requiring small size, low weight, long life, high reliability, and cost effective producibility. The hybrid two stage expander  10  is particularly well suited for use in civil and defense space-based infrared sensors, such as those used in spacecraft infrared sensor systems, and the like. 
     Thus, an improved hybrid two stage expander has been disclosed. It is to be understood that the described embodiment is merely illustrative of some of the many specific embodiments which represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.