Abstract:
An improvement is provided to a pressurized close-cycle machine that has a cold-end pressure vessel and is of the type having a piston undergoing reciprocating linear motion within a cylinder containing a working fluid heated by conduction through a heater head by heat from an external thermal source. The improvement includes a heat exchanger for cooling the working fluid, where the heat exchanger is disposed within the cold-end pressure vessel. The heater head may be directly coupled to the cold-end pressure vessel by welding or other methods. A coolant tube is used to convey coolant through the heat exchanger.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/959,571, filed Dec. 19, 2007 and entitled Coolant Penetrating Cold-End Pressure Vessel, now U.S. Publication No. US-2008-0092536-A1, published Apr. 24, 2008 which is a continuation of U.S. Pat. No. 7,325,399, issued Feb. 5, 2008, and entitled Coolant Penetrating Cold-End Pressure Vessel, both of which are hereby incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention pertains to the pressure containment structure and cooling of a pressurized close-cycle machine. 
     BACKGROUND OF THE INVENTION 
     Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker, Stirling Engines, Oxford University Press (1980), incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression. 
     In the prior art, the heat transfer structure between the working gas and the cooling fluid also contains the high pressure working gas of the Stirling cycle engine. The two functions of heat transfer and pressure containment produce competing demands on the design. Heat transfer is maximized by as thin a wall as possible made of the highest thermal conductivity material. However, thin walls of weak materials limit the maximum allowed working pressure and therefore the power of the engine. In addition, codes and product standards require designs that can be proof tested to several times the nominal working pressure. 
     SUMMARY OF THE INVENTION 
     In accordance with preferred embodiments of the present invention, an improvement is provided to a pressurized close-cycle machine that has a cold-end pressure vessel and is of the type having a piston undergoing reciprocating linear motion within a cylinder containing a working fluid heated by conduction through a heated head by heat from an external thermal source. The improvement includes a heat exchanger for cooling the working fluid, where the heat exchanger is disposed within the cold-end pressure vessel. The heater head may be directly coupled to the cold-end pressure vessel by welding or other methods. In one embodiment, the heater head includes a step or flange transfers a mechanical load from the heater head to the cold-end pressure vessel. 
     In accordance with a further embodiment of the invention, the pressurized close-cycle machine includes a coolant tube for conveying coolant to the heat exchanger from outside the cold-end pressure vessel and through the heat exchanger and for conveying coolant from the heat exchanger to outside the cold-end pressure vessel. The coolant tube may be a single continuous section of tubing. In one embodiment, a section of the coolant tube is contained within the heat exchanger. The section of the coolant tube contained within the heat exchanger may be a continuous section of tubing. An outside diameter of a section of the coolant tube that passes through the cold-end pressure vessel may be sealed to the cold-end pressure vessel. In one embodiment, a section of the coolant tube is wrapped around an interior of the heat exchanger. 
     In another embodiment, a section of the coolant tube is disposed within a working volume of the heat exchanger. The section of the coolant tube disposed within the working volume of the heat exchanger may include a plurality of extended heat transfer surfaces. At least one spacing element may be included to direct the flow of the working gas to a specified proximity of the section of coolant tube in the working volume of the heat exchanger. The heat exchanger may further include an annular heat sink surrounding the coolant tube wherein a flow of the working gas in the working volume of the heat exchanger is directed along at least one surface of the annular heat sink. The heat exchanger may further include a plurality of heat transfer surfaces on at least one surface of the heat exchanger. 
     In yet another embodiment, the cold-end pressure vessel contains a charge fluid and a section of coolant tube is disposed within the cold-end pressure vessel to cool the charge fluid. The pressurized close-cycle machine may also include a fan in the cold-end pressure vessel to circulate and cool the charge fluid. The section of coolant tube disposed within the cold-end pressure vessel may include extended heat transfer surfaces on the exterior of the coolant tube. In a further embodiment, the heat exchanger has a body formed by casting a metal over the coolant tube. The heat exchanger body may include a working fluid contact surface comprising a plurality of extended heat transfer surfaces. A flow constricting countersurface may be used to confine any flow of the working fluid to a specified proximity of the heat exchanger body. 
     In accordance with another aspect of the invention, a heat exchanger is provided for cooling a working fluid in an external combustion engine. The heat exchanger includes a length of metal tubing for conveying a coolant through the heat exchanger and a heat exchanger body that is formed by casting a material over the metal tubing. In one embodiment, the heat exchanger body includes a working fluid contact surface that comprises a plurality of extended heat transfer surfaces. The heat exchanger may further include a flow-constricting countersurface for confining any flow of the working fluid to a specified proximity to the heat exchanger body. 
     In accordance with another aspect of the invention, a method is provided for fabricating a heat exchanger for transferring thermal energy from a working fluid to a coolant. The method includes forming a spiral shaped section of tubing and casting a material over the annular shaped section of tubing to form a heat exchanger body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional view of a Stirling cycle engine including working spaces in accordance with an embodiment of the present invention. 
         FIG. 2  is a cross-section taken perpendicular to the Stirling cycle engine in  FIG. 1  in accordance with an embodiment of the present invention; 
         FIG. 3   a  is a side views in cross section of a Stirling cycle engine including coolant tubing in accordance with an embodiment of the invention; 
         FIG. 3   b  is a side view in cross section of a Stirling cycle engine including coolant tubing in accordance with an alternative embodiment of the invention; 
         FIG. 3   c  is a side view in cross section of a Stirling cycle engine including coolant tubing in accordance with an alternative embodiment of the invention; 
         FIG. 3   d  is a side view in cross section of a Stirling cycle engine including coolant tubing in accordance with an alternative embodiment of the invention; 
         FIG. 4   a  is a perspective view of a cooling coil for heat exchange in accordance with an embodiment of the invention; 
         FIG. 4   b  is a perspective view of a cooling assembly cast over the cooling coil of  FIG. 4   a  in accordance with an embodiment of the invention; 
         FIG. 5   a  is a detailed cross sectional top view of the interior section of the over-cast cooling heat exchanger of  FIG. 4   b  showing vertical grooves in accordance with an embodiment of the invention; and 
         FIG. 5   a - 1  is a detailed view of a portion of  FIG. 5   a.    
         FIG. 5   b  is a detailed cross sectional top view of the interior section of the over-cast cooling heat exchanger of  FIG. 4   b  showing vertical and horizontal grooves creating heat exchange pins in accordance with another embodiment of the invention. 
         FIG. 5   b - 1  is a detailed view of a portion of  FIG. 5   b.    
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In accordance with embodiments of the present invention, the heat transfer and pressure vessel functions of the cooler of a pressurized close-cycle machine are separated, thereby advantageously maximizing both the cooling of the working gas and the allowed working pressure of the working gas. Increasing the maximum allowed working pressure and cooling both result in increased engine power. Embodiments of the invention achieve good heat transfer and meet code requirements for pressure containment by using small (relative to the heater head diameter) metal tubing to transfer heat and separate the cooling fluid from the high pressure working gas. 
     Referring now to  FIG. 1 , a hermetically sealed Stirling cycle engine, in accordance with preferred embodiments of the present invention, is shown in cross section and designated generally by numeral  50 . While the invention will be described generally with reference to a Stirling engine as shown in  FIG. 1  and  FIG. 2 , it is to be understood that many engines, coolers, and other machines may similarly benefit from various embodiments and improvements which are subjects of the present invention. A Stirling cycle engine, such as shown in  FIG. 1 , operates under pressurized conditions. Stirling engine  50  contains a high-pressure working fluid, preferably helium, nitrogen or a mixture of gases at 20 to 140 atmospheres pressure. Typically, a crankcase  70  encloses and shields the moving portions of the engine as well as maintains the pressurized conditions under which the Stirling engine operates (and as such acts as a cold-end pressure vessel). A free-piston Stirling engine also uses a cold-end pressure vessel to maintain the pressurized conditions of the engine. A heater head  52  serves as a hot-end pressure vessel. 
     Stirling engine  50  contains two separate volumes of gases, a working gas volume and a charge gas volume, separated by piston seal rings  68 . In the working gas volume, working gas is contained by heater head  52 , a regenerator  54 , a cooler  56 , a compression head  58 , an expansion piston  60 , an expansion cylinder  62 , a compression piston  64  and a compression cylinder  66  and is contained outboard of the piston seal rings  68 . The charge gas is a separate volume of gas enclosed by the cold-end pressure vessel  70 , the expansion piston  60 , the compression piston  64  and is contained inboard of the piston seal rings  68 . 
     The working gas is alternately compressed and expanded by the compression piston  64  and the expansion piston  60 . The pressure of the working gas oscillates significantly over the stroke of the pistons. During operation, there may be leakage across the piston seal rings  68  because the piston seal rings  68  are not hermetic. This leakage results in some exchange of gas between the working gas volume and the charge gas volume. However, because the charge gas in the cold-end pressure vessel  70  is charged to the mean pressure of the working gas, the net mass exchange between the two volumes is zero. 
       FIG. 2  shows a cross-section of the Stirling cycle engine in  FIG. 1  taken perpendicular to the view in  FIG. 1  in accordance with an embodiment of the invention. Stirling cycle engine  100  is hermetically sealed. A crankcase  102  serves as the cold-end pressure vessel and contains a charge gas in an interior volume  104  at the mean operating pressure of the engine. Crankcase  102  can be made arbitrarily strong without sacrificing thermal performance by using sufficiently thick steel or other structural material. A heater head  106  serves as the hot-end pressure vessel and is preferably fabricated from a high temperature super-alloy such as Inconel 625, GMR-235, etc. Heater head  106  is used to transfer thermal energy by conduction from an external thermal source (not shown) to the working fluid. Thermal energy may be provided from various heat sources such as solar radiation or combustion gases. For example, a burner may be used to produce hot combustion gases  107  that are used to heat the working fluid. An expansion cylinder (or work space)  122  is disposed inside the heater head  106  and defines part of a working gas volume as discussed above with respect to  FIG. 1 . An expansion piston  128  is used to displace the working fluid contained in the expansion cylinder  122 . 
     In accordance with an embodiment of the invention, crankcase  102  is welded directly to heater head  106  at joints  108  to create a pressure vessel that can be designed to hold any pressure without being limited, as are other designs, by the requirements of heat transfer in the cooler. In an alternative embodiment, the crankcase  102  and heater head  106  are either brazed or bolted together. The heater head  106  has a flange or step  110  that axially constrains the heater head and transfers the axial pressure force from the heater head  106  to the crankcase  102 , thereby relieving the pressure force from the welded or brazed joints  108 . Joints  108  serve to seal the crankcase  102  (or cold-end pressure vessel) and bear the bending and planar stresses. In an alternative embodiment, the joints  108  are mechanical joints with an elastomer seal. In yet another embodiment, step  110  is replaced with an internal weld in addition to the exterior weld at joints  108 . 
     Crankcase  102  is assembled in two pieces, an upper crankcase  112  and a lower crankcase  116 . The heater head  106  is first joined to the upper crankcase  112 . Second, a cooler  120  is installed with a coolant tubing  114  passing through holes in the upper crankcase  112 . Third, the expansion piston  128  and the compression piston  64  (shown in  FIG. 1 ) and drive components  140 ,  142  are installed. The lower crankcase  116  is then joined to the upper crankcase  112  at joints  118 . Preferably, the upper crankcase  112  and the lower crankcase  116  are joined by welding. Alternatively, a bolted flange may be employed as shown in  FIG. 2 . 
     In order to allow direct coupling of the heater head  106  to the upper crankcase  112 , the cooling function of the thermal cycle is performed by a cooler  120  that is disposed within the crankcase  102 , thereby advantageously reducing the pressure containment requirements placed upon the cooler. By placing the cooler  120  within crankcase  102 , the pressure across the cooler is limited to the pressure difference between the working gas in the working gas volume, including expansion cylinder  122 , and the charge gas in the interior volume  104  of the crankcase. The difference in pressure is created by the compression and expansion of the working gas, and is typically limited to a percentage of the operating pressure. In one embodiment, the pressure difference is limited to less than 30% of the operating pressure. 
     Coolant tubing  114  advantageously has a small diameter relative to the diameter of the cooler  120 . The small diameter of the coolant passages, such as provided by coolant tubing  114 , is key to achieving high heat transfer and supporting large pressure differences. The required wall thickness to withstand or support a given pressure is proportional to the tube or vessel diameter. The low stress on the tube walls allows various materials to be used for coolant tubing  114  including, but not limited to, thin-walled stainless steel tubing or thicker-walled copper tubing. 
     An additional advantage of locating the cooler  120  entirely within the crankcase  102  (or cold-end pressure vessel) volume is that any leaks of the working gas through the cooler  120  will only result in a reduction of engine performance. In contrast, if the cooler were to interface with the external ambient environment, a leak of the working gas through the cooler would render the engine useless due to loss of the working gas unless the mean pressure of working gas is maintained by an external source. The reduced requirement for a leak-tight cooler allows for the use of less expensive fabrication techniques including, but not limited to, powder metal and die casting. 
     Cooler  120  is used to transfer thermal energy by conduction from the working gas and thereby cool the working gas. A coolant, either water or another fluid, is carried through the crankcase  102  and the cooler  120  by coolant tubing  114 . The feedthrough of the coolant tubing  114  through upper crankcase  112  may be sealed by a soldered or brazed joint for copper tubes, welding, in the case of stainless steel and steel tubing, or as otherwise known in the art. 
     The charge gas in the interior volume  104  may also require cooling due to heating resulting from heat dissipated in the motor/generator windings, mechanical friction in the drive, the non-reversible compression/expansion of the charge gas and the blow-by of hot gases from the working gas volume. Cooling the charge gas in the crankcase  102  increases the power and efficiency of the engine as well as the longevity of bearings used in the engine. 
     In one embodiment, an additional length of coolant tubing  130  is disposed inside the crankcase  102  to absorb heat from the charge gas in the interior volume  104 . The additional length of coolant tubing  130  may include a set of extended heat transfer surfaces  148 , such as fins, to provide additional heat transfer. As shown in  FIG. 2 , the additional length of coolant tubing  130  may be attached to the coolant tubing  114  between the crankcase  102  and the cooler  120 . In an alternative embodiment, the length of coolant tubing  130  may be a separate tube with its own feedthrough of the crankcase  102  that is connected to the cooling loop by hoses outside of the crankcase  102 . 
     In an another embodiment, the extended coolant tubing  130  may be replaced with extended surfaces on the exterior surface of the cooler  120  or the drive housing  72 . Alternatively, a fan  134  may be attached to the engine crankshaft to circulate the charge gas in interior volume  104 . The fan  134  may be used separately or in conjunction with the additional coolant tubing  130  or the extended surfaces on the cooler  120  or drive housing  72  to directly cool the charge gas in the interior volume  104 . 
     Preferably, coolant tubing  114  is a continuous tube throughout the interior volume  104  of the crankcase and the cooler  120 . Alternatively, two pieces of tubing could be used between the crankcase and the feedthrough ports of the cooler. One tube carries coolant from outside the crankcase  102  to the cooler  120 . A second tube returns the coolant from the cooler  120  to the exterior of the crankcase  102 . In another embodiment, multiple pieces of tubing may be used between the crankcase  102  and the cooler in order to add tubing with extended heat transfer surfaces inside the crankcase volume  104  or to facilitate fabrication. The tubing joints and joints between the tubing and the cooler may be brazed, soldered, welded or mechanical joints. 
     Various methods may be used to join coolant tubing  114  to cooler  120 . Any known method for joining the coolant tubing  114  to the cooler  120  is within the scope of the invention. In one embodiment, the coolant tubing  114  may be attached to the wall of the cooler  120  by brazing, soldering or gluing. Cooler  120  is in the form of a cylinder placed around the expansion cylinder  122  and the annular flow path of the working gas outside of the expansion cylinder  122 . Accordingly, the coolant tubing  114  may be wrapped around the interior of the cooler cylinder wall and attached as mentioned above. 
     Alternative cooler configurations are presented in  FIGS. 3   a - 3   d  that reduce the complexity of the cooler body fabrication.  FIG. 3   a  shows a side view of a Stirling cycle engine including coolant tubing in accordance with an embodiment of the invention. In  FIG. 3   a , cooler  152  includes a cooler working space  150 . Coolant tubing  148  is placed within the cooler working space  150 , so that the working gas can flow over an outside surface of coolant tubing  148 . The working gas is confined to flow past the coolant tubing  148  by the cooler body  152  and a cooler liner  126 . The coolant tube passes into and out-of the working space  150  through ports in either the cooler  152  or the drive housing  72  (shown in  FIG. 2 ). The cooler casting process is simplified by having a seal around coolant lines  148 . In addition, placing the coolant line  148  in the working space improves the heat transfer between the working fluid and the coolant fluid. The coolant tubing  148  may be smooth or may have extended heat transfer surfaces or fins on the outside of the tubing to increase heat transfer between the working gas and the coolant tubing  148 . In another embodiment, as shown in  FIG. 3   b , spacing elements  154  may be added to the cooler working space  150  to force the working gas to flow closer to the coolant tubes  148 . The spacing elements are separate from the cooler liner  126  and the cooler body  152  to allow insertion of the coolant tube and spacing elements into the working space. 
     In another embodiment, as shown in  FIG. 3   c , the coolant tubing  148  is overcast to form an annular heat sink  156  where the working gas can flow on both sides of the cooler body  152 . The annular heat sink  156  may also include extended heat transfer surfaces on its inner and outer surfaces  160 . The body of the cooler  152  constrains the working gas to flow past the extended heat exchange surfaces on heat sink  156 . The heat sink  156  is typically a simpler part to fabricate than the cooler  120  in  FIG. 2 . The annular heat sink  156  provides roughly double the heat transfer area of cooler  120  shown in  FIG. 2 . In another embodiment, as shown in  FIG. 3   d , the cooler liner  126  can be cast over the coolant lines  148 . The cooler body  152  constrains the working gas to flow past the cooler liner  162 . Cooler liner  126  may also include extended heat exchange surfaces on a surface  160  to increase heat transfer. 
     Returning to  FIG. 2 , a preferred method for joining coolant tubing  114  to cooler  120  is to overcast the cooler around the coolant tubing. This method is described, with reference to  FIGS. 4   a  and  4   b , and may be applied to a pressurized close-cycle machine as well as in other applications where it is advantageous to locate a cooler inside the crankcase. 
     Referring to  FIG. 4   a , a heat exchanger, for example, a cooler  120  (shown in  FIG. 2 ) may be fabricated by forming a high-temperature metal tubing  302  into a desired shape. In a preferred embodiment, the metal tubing  302  is formed into a coil using copper. A lower temperature (relative to the melting temperature of the tubing) casting process is then used to overcast the tubing  302  with a high thermal conductivity material to form a gas interface  304  (and  132  in  FIG. 2 ), seals  306  (and  124  in  FIG. 2 ) to the rest of the engine and a structure to mechanically connect the drive housing  72  (shown in  FIG. 2 ) to the heater head  106  (shown in  FIG. 2 ). In a preferred embodiment, the high thermal conductivity material used to overcast the tubing is aluminum. Overcasting the tubing  302  with a high thermal conductivity metal assures a good thermal connection between the tubing and the heat transfer surfaces in contact with the working gas. A seal is created around the tubing  302  where the tubing exits the open mold at  310 . This method of fabricating a heat exchanger advantageously provides cooling passages in cast metal parts inexpensively. 
       FIG. 4   b  is a perspective view of a cooling assembly cast over the cooling coil of  FIG. 4   a . The casting process can include any of the following: die casting, investment casting, or sand casting. The tubing material is chosen from materials that will not melt or collapse during the casting process. Tubing materials include, but are not limited to, copper, stainless steel, nickel, and super-alloys such as Inconel. The casting material is chosen among those that melt at a relatively low temperature compared to the tubing. Typical casting materials include aluminum and its various alloys, and zinc and its various alloys. 
     The heat exchanger may also include extended heat transfer surfaces to increase the interfacial area  304  (and  132  shown in  FIG. 2 ) between the hot working gas and the heat exchanger so as to improve heat transfer between the working gas and the coolant. Extended heat transfer surfaces may be created on the working gas side of the heat exchanger  120  by machining extended surfaces on the inside surface (or gas interface)  304 . Referring to  FIG. 2 , a cooler liner  126  (shown in  FIG. 2 ) may be pressed into the heat exchanger to form a gas barrier on the inner diameter of the heat exchanger. The cooler liner  126  directs the flow of the working gas past the inner surface of the cooler. 
     The extended heat transfer surfaces can be created by any of the methods known in the art. In accordance with a preferred embodiment of the invention, longitudinal grooves  504  are broached into the surface, as shown in detail in  FIG. 5   a . Alternatively, lateral grooves  508  may be machined in addition to the longitudinal grooves  504  thereby creating aligned pins  510  as shown in  FIG. 5   b . In accordance with yet another embodiment of the invention, grooves are cut at a helical angle to increase the heat exchange area. 
     In an alternative embodiment, the extended heat transfer surfaces on the gas interface  304  (as shown in  FIG. 4   b ) of the cooler are formed from metal foam, expanded metal or other materials with high specific surface area. For example, a cylinder of metal foam may be soldered to the inside surface of the cooler  304 . As discussed above, a cooler liner  126  (shown in  FIG. 2 ) may be pressed in to form a gas barrier on the inner diameter of the metal foam. Other methods of forming and attaching heat transfer surfaces to the body of the cooler are described in co-pending U.S. patent application Ser. No. 09/884,436, filed Jun. 19, 2001, entitled Stirling Engine Thermal System Improvements, which is herein incorporated by reference. 
     All of the systems and methods described herein may be applied in other applications besides the Stirling or other pressurized close-cycle machines in terms of which the invention has been described. The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.