TECHNICAL FIELD OF THE INVENTION  
       [0001]     This invention relates in general to fluid dynamic machining and, more particularly, to a system and method for delivering a cryogenic fluid.  
       BACKGROUND OF THE INVENTION  
       [0002]     In fluid dynamic machining the force resulting from the momentum change of the fluid stream is utilized to cut, abrade, or otherwise machine materials. For example, water is often used as a fluid to cut or abrade certain materials and various abrasive materials may be used to enhance material removal. However, water jet machining may suffer from problems relating to the collection of the water during the machining operation or problems relating to the potential contamination of the water or surrounding environment from the material removed from the workpiece.  
         [0003]     To address the foregoing problems, sublimable particles, such as dry ice, may be used as the cutting material. The primary advantage of using sublimable particles is that there is no secondary waste material to be collected: the dry ice particles change to gaseous carbon dioxide (CO 2 ) shortly after striking the workpiece. The gaseous carbon dioxide may then be discharged into the atmosphere. Liquid nitrogen may also be utilized as the fluid medium. Since both carbon dioxide and nitrogen are present in the atmosphere in substantial quantities, venting them into the atmosphere should not pose any problems.  
       SUMMARY OF THE INVENTION  
       [0004]     According to an embodiment of the present invention, a cryogenic fluid delivery system includes a vessel containing a cryogenic fluid at a first pressure and a first temperature, a first heat exchanger coupled to the vessel for receiving the cryogenic fluid and cooling the cryogenic fluid to a second temperature, a first pump coupled to the first heat exchanger for pressurizing the cryogenic fluid to a second pressure, a second pump for pressurizing the cryogenic fluid to a third pressure, a second heat exchanger coupled to the second pump for cooling the cryogenic fluid to a third temperature, and a nozzle coupled to the second heat exchanger for delivering a jet of the cryogenic fluid toward a target.  
         [0005]     Embodiments of the invention provide a number of technical advantages. Embodiments of the invention may include all, some, or none of these advantages. For example, in one embodiment, a cryogenic fluid delivery system provides a fluid stream capable of a high pressure and high velocity in order to cut or otherwise machine a wide variety of materials. Such a system may be used in medical applications, such as liver or other types of surgery. By utilizing a cryogenic fluid, such as nitrogen, no secondary waste material needs to be collected; the supercritical nitrogen evaporates shortly after cutting or striking a workpiece. Since nitrogen is present in the atmosphere in substantial quantities, venting into the atmosphere should not pose any problems.  
         [0006]     In another embodiment, a cryogenic fluid delivery system is utilized in cold spraying. Small metal particles or carbon dioxide may be entrained within the fluid stream before exiting a nozzle. Such a system may be used to perform functions such as sandblasting or to replace electroplating.  
         [0007]     Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a functional block diagram of a cryogenic fluid delivery system according to one embodiment of the present invention;  
         [0009]      FIG. 2  is a schematic of a subcooler and a pre-pump according to one embodiment of the present invention;  
         [0010]      FIG. 3  is a more detailed schematic of a pre-pump according to one embodiment of the present invention;  
         [0011]      FIG. 4  is a schematic of a swapper according to one embodiment of the present invention;  
         [0012]      FIG. 5  is a schematic of a pair of intensifiers according to one embodiment of the present invention;  
         [0013]      FIG. 6  is a schematic of a heat exchanger according to one embodiment of the present invention;  
         [0014]      FIG. 7  is a schematic of a hydraulic system according to one embodiment of the present invention;  
         [0015]      FIGS. 8A through 8C  are various schematics of a rotating nozzle assembly according to one embodiment of the present invention;  
         [0016]      FIG. 9A  is a schematic of a nozzle assembly according to one embodiment of the present invention; and  
         [0017]      FIG. 9B  is a schematic illustrating a different nozzle assembly according to one embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Embodiments of the present invention and some of their advantages are best understood by referring to  FIGS. 1 through 9 B of the drawings, like numerals being used for like and corresponding parts of the various drawings.  
         [0019]      FIG. 1  is a functional block diagram of a cryogenic fluid delivery system  100  according to one embodiment of the present invention. In the illustrated embodiment, delivery system  100  includes a liquid nitrogen supply  102 , a sub-cooler  104 , a pre-pump  106 , a swapper  108 , a pair of intensifier pumps  110 , a heat exchanger  112 , a nozzle assembly  114 , a power system  116 , a recirculation pump  118 , a dump valve assembly  120 , and a controller  122 . The present invention, however, contemplates delivery system  100  having more, less, or different components than those illustrated in  FIG. 1 . Generally, cryogenic fluid delivery system  100  provides a cryogenic fluid stream capable of high pressure and high velocity in order to cut, abrade, or otherwise suitably machine a wide variety of materials. The components of delivery system  100  may be incorporated into a single structure, such as a skid, or may be separate components arranged in any suitable manner. Details of the components of delivery system  100  are described below in conjunction with  FIGS. 2 through 9 B.  
         [0020]     Although not described in detail, each of the components may be coupled to one another via any suitable piping adapted to transport a suitable cryogen at various temperatures and pressures. This piping may include other suitable components, such as valves, pumps, and reducers, and may be any suitable size depending on the process criteria. As an example, piping from liquid nitrogen supply  102  to sub-cooler  104  may be a ¾ inch diameter pipe. Temperatures and pressures associated with system  100  may vary depending on the particular implementation of system  100 .  
         [0021]     Liquid nitrogen supply  102  functions to store nitrogen, typically in liquid form, although some gas nitrogen may be present. Although nitrogen is used throughout this detailed description as the cryogenic fluid, the present invention contemplates other suitable cryogens for use in delivery system  100 . In addition, the term “fluid” may mean liquid, gas, vapor, supercritical or any combination thereof. In one embodiment, liquid nitrogen supply  102  is a double wall tank storing liquid nitrogen at less than or equal to −270° F. and a pressure less than or equal to 80 psi. However, supply  102  may supply any suitable cryogen at any suitable temperature and any suitable pressure. In addition, supply  102  may function to provide system  100  with liquid nitrogen or other suitable cryogen at any suitable velocity, such as approximately three gallons per minute.  
         [0022]     Sub-cooler  104  functions to sub-cool the liquid nitrogen received from liquid nitrogen supply  102  before it enters pre-pump  106 . In one embodiment, sub-cooler  104  sub-cools the liquid nitrogen to approximately −310° F. In one embodiment, sub-cooler  104  is a shell-and-tube type heat exchanger; however, sub-cooler  104  may take the form of other suitable heat exchangers. In addition to receiving liquid nitrogen from liquid nitrogen supply  102 , sub-cooler  104  may also receive recycled nitrogen from pre-pump  106 , as described in greater detail below in conjunction with  FIG. 2 . This recycling of the nitrogen from pre-pump  106  to sub-cooler  104  may be accomplished by recirculation pump  118 .  
         [0023]     Pre-pump  106  boosts the pressure of the liquid nitrogen received from sub-cooler  104  to a higher pressure. In one embodiment, pre-pump  106  boosts the pressure of nitrogen to between approximately 15,000 and 20,000 psi for use by intensifier pumps  110 . Because of the boosting of the pressure of the nitrogen by pre-pump  106 , the temperature of the nitrogen drops from −310° F. to somewhere between approximately −170° F. and −190° F. Further details of pre-pump  106  are described below in conjunction with  FIG. 3 .  
         [0024]     Swapper  108  is a heat exchanger that receives the colder incoming supercritical nitrogen from pre-pump  106  and warmer supercritical nitrogen from intensifier pumps  110  in countercurrent flow directions. Heat is then swapped or exchanged between the two streams resulting in the heating of the incoming nitrogen prior to delivering it to intensifier pumps  110  and pre-cooling the discharge from the intensifier pumps  110  prior to feeding it to heat exchanger  112 . Details of swapper  108  are described in greater detail below in conjunction with  FIG. 4 .  
         [0025]     Intensifier pumps  110  raise the pressure of supercritical nitrogen, for example, from approximately 15,000 psi to 55,000 psi via compression. Details of intensifier pumps  110  are described below in conjunction with  FIG. 5 . Intensifier pumps  110  work in conjunction with swapper  108 , as described in greater detail below.  
         [0026]     Heat exchanger  112  cools the high pressure supercritical nitrogen from intensifier pumps  110  to approximately −235° F. In one embodiment, heat exchanger  112  is a suitable shell-and-tube type heat exchanger; however, heat exchanger  112  may be other suitable types of heat exchangers. Details of heat exchanger  112  are described below in conjunction with  FIG. 6 .  
         [0027]     Nozzle assembly  114  receives the cooled cryogenic fluid from heat exchanger  112  and produces a high velocity jet stream to be used for cutting, abrading, coating, or other suitable machining operations. Details of some embodiments of nozzle assembly  114  are described below in conjunction with  FIGS. 8 and 9 . In one embodiment, the velocity of the jet stream delivered by nozzle  114  may be approximately Mach  3 ; however, other suitable velocities are contemplated by the present invention. Dump valve assembly  120  functions to release supercritical nitrogen to the atmosphere in order to keep a smooth, responsive flow of nitrogen delivered to nozzle  114  if the stream to the nozzle should need to be interrupted for any reason (e.g., to reposition an item being cut or abraded). In one embodiment, dump valve assembly  120  comprises suitable three-way valves that are air operated; however, other suitable valves may be contemplated by the present invention for dump valve assembly  120 .  
         [0028]     Power system  116  provides power to both pre-pump  106  and intensifier pumps  110 . Power system  116  enables a smooth flow of supercritical nitrogen through delivery system  100  and may be any suitable power system, such as a hydraulic system, a pneumatic system, or an electrical system. Details of one embodiment of power system  116  are described below in conjunction with  FIG. 7 . Power system  116  may also provide power for re-circulation pump  118  and swapper  108  in some embodiments. In the case of a hydraulic system, power system  116  may include suitable reservoirs, piping, pumps, valves, and other components to operate pumps  106 ,  110 , and/or  118 .  
         [0029]     Controller  122  may be any suitable computing device having any suitable hardware, firmware, and/or software that controls cryogenic fluid delivery system  100 . For example, controller  122  controls the valves and valve sequencing of power system  116 , as described below in conjunction with  FIG. 7 , and generally monitors and controls temperatures and pressures throughout system  100  as well as other components, such as pressure relief valves to provide safe operation of system  100 . An embodiment where the components of delivery system  100  are all contained on one skid, controller  122  may or may not be separate from the skid. Controller  122  may also have the option of providing an operator of delivery system  100  with critical operating parameters. For example, via a touch-screen control panel, an operator may control the more relevant operating parameters, such as output temperature and output pressure. Both cool-down and ramp-up processes may also be controlled by controller  122 .  
         [0030]      FIG. 2  is a schematic of sub-cooler  104  and pre-pump  106  according to one embodiment of the present invention. In the illustrated embodiment, sub-cooler  104  includes a vessel  200  storing a coolant  201 , such as liquid nitrogen, and piping  202  disposed within vessel  200 . Piping  202  receives liquid nitrogen from liquid nitrogen supply  102  via a feedline  204 . Recirculation pump  118  is also coupled to piping  202  and is operable to deliver the cryogenic fluid running through piping  202  to pre-pump  106 .  
         [0031]     Recirculation pump  118  functions to raise the pressure of the liquid nitrogen from approximately 80 psi to approximately 130 psi in order to “prime” pre-pump  106 , which results in a good net positive suction head to prevent cavitation. Recirculation pump  118  also functions to recirculate liquid nitrogen running through a pair of jackets  205  associated with pre-pump  106  back to sub-cooler  104  via a feedback line  206 . In an embodiment where power system  116  ( FIG. 1 ) is pneumatic, recirculation pump  118  may not be needed.  
         [0032]     Feedback line  206  delivers the recirculated nitrogen back to feedline  204 . In addition, coupled to feedback line  206  is a line  210  having an associated valve  212 . Valve  212  works in conjunction with a automated level controller  208  associated with sub-cooler  104  in order to control the level of coolant  201  within vessel  200 . For example, if the level starts to drop, automated level controller  208  actuates valve  212  open so that nitrogen running through feedback line  206  may enter vessel  200  via line  210 .  
         [0033]     Automated level controller  208  may be any suitable differential pressure transducer, such as a bubbler, a float, a laser sensor, or other suitable level controller. Automated level controller  208  may couple to vessel  200  in any suitable manner and in any suitable location. Reasons for controlling the level of coolant  201  within vessel  200  are to maintain proper subcooling of the incoming process liquid nitrogen and to prevent coolant  201  overflowing from vessel  200 .  
         [0034]     Also illustrated in  FIG. 2 , is a gas phase separator  214  coupled between feedline  204  and line  210 . Gas phase separator  214  functions to direct any nitrogen gas within the nitrogen to line  210 . In one embodiment, gas phase separator  214  includes a hand valve and a solenoid valve in series; however, other suitable valve arrangements are contemplated for gas phase separator  214 .  
         [0035]      FIG. 3  is a schematic of pre-pump  106  according to one embodiment of the present invention. In the illustrated embodiment, pre-pump  106  is a double-acting linear intensifier driven in both directions by a double-ended linear hydraulic piston  309  located in double-acting hydraulic cylinder  300 . Power system  116  provides the power at a suitable pressure and flow rate to operate piston  309  in a linear reciprocating fashion. A pair of limit switches  306 , which may be incorporated into spacers  304 , signal the electronic controls to shift the directional control valve to reverse the direction of travel of piston  309 . Pre-pump  106  also includes a pair of cold ends  302  separated from hydraulic cylinder  300  with a pair of intermediate spacers  304 . Surrounding each cold end  302  is jacket  205  for accepting liquid nitrogen from sub-cooler  104  via recirculation pump  118  ( FIG. 2 ).  
         [0036]     As described above, pre-pump  106  functions as an amplifier that converts a low pressure liquid nitrogen to intermediate-pressure supercritical nitrogen. To accomplish this, pre-pump  106  is provided with a plunger  310  on each side of piston  309  to generate force in both directions of piston travel in such a way that while one side of pre-pump  106  is in the inlet stroke, the opposite side is generating intermediate-pressure discharge. Therefore, during the inlet stroke of plunger  310 , liquid nitrogen enters cold end  302  under suction through a suitable check valve assembly  311   a . After plunger  310  reverses motion of travel, nitrogen is compressed and exits at a predetermined elevated pressure through a suitable discharge check valve assembly  311   b . This intermediate-pressure supercritical nitrogen, which is between approximately 15,000 to 20,000 psi, is then delivered to swapper  108 .  
         [0037]     Intermediate spacers  304  may have any suitable length and function to provide heat isolation and facilitate proper mechanical coupling between hydraulic cylinder  300  and cold ends  302 . Intermediate spacers  304  may couple to hydraulic cylinder  300  in any suitable manner and cold ends  302  may couple to respective intermediate spacers  304  in any suitable manner, such as by a screwed connection. Also illustrated in  FIG. 3  is an accumulator  308  (also known as a surge chamber) to smooth out the flow of nitrogen by taking out any pressure ripple therein.  
         [0038]      FIG. 4  is a schematic of swapper  108  according to one embodiment of the present invention. In the illustrated embodiment, swapper  108  includes a solid body  400 , a resistance heater  402  running through body  400 , and a pair of conduits  404 ,  406  extending through body  400 . In one embodiment, body  400  is formed from solid aluminum; however, other suitable materials are contemplated by the present invention. Resistance heater  402  may be any suitable heating unit that provides heat to body  400 . Conduits  404 ,  406  may be any suitable size and shape and both function to transport nitrogen or other suitable cryogen therethrough.  
         [0039]     As described above, swapper  108  is a heat exchanger that functions to receive incoming supercritical intermediate-pressure nitrogen from pre-pump  106  and supercritical nitrogen high-pressure discharge from intensifier pumps  110  in countercurrent flow directions. Both liquid streams are passed through body  400 , in which heat is exchanged between the two streams resulting in the heating of incoming supercritical nitrogen prior to feeding to intensifier pumps  110 , as indicated by reference numeral  409 , and pre-cooling the hot discharge from the high-pressure intensifier pumps  110  prior to feeding to heat exchanger  112 , as indicated by reference numeral  411 . Resistance heater  402  may be used to control or otherwise influence the exchange of heat between the two streams. In addition, the selection of material and dimensions of body  400  also influence this exchange.  
         [0040]     In one embodiment, the supercritical nitrogen from pre-pump  106  enters into conduit  404  at a temperature of approximately −170° F. to −190° F. and a pressure of between 15,000 and 20,000 psi. Swapper  108  warms this incoming nitrogen to between approximately −140° F. and −40° F. Intensifier pumps  110 , as described in greater detail below in conjunction with  FIG. 5 , raise the pressure of the nitrogen to approximately 55,000 psi and consequently, raise the temperature of the nitrogen to between approximately 50° F. and 150° F. before it re-enters body  400  via conduit  406 . After traveling through conduit  406 , the temperature of the nitrogen is then cooled to a temperature of between approximately +30° F. to −40° F. before being delivered to heat exchanger  112 . System  100  contemplates other suitable temperatures and pressures for the cryogenic fluid flowing through swapper  108 .  
         [0041]      FIG. 5  is a schematic of intensifier pumps  110  according to one embodiment of the present invention. For convenience,  FIG. 5  shows each of the intensifier pumps  110   a ,  110   b  with their respective components designated “a” or “b”. The following description refers generally to the components without the “a” or “b” designations. In the illustrated embodiment, each intensifier pump  110  includes a hydraulic cylinder  501  having a piston  502  disposed therein, a pair of intermediate spacers  503  coupled to hydraulic cylinder  501 , and a pair of high pressure cylinders  505  coupled to intermediate spacers  503 . Each intensifier pump  110  also includes a pair of plungers  506  at either end of piston  502  and a pair of limit switches  504 . The layout of intensifier pumps  110  are similar to pre-pump  106  except that intensifier pumps  110  do not include jackets around the high pressure cylinders  505  although these could be incorporated if desired. The operation of intensifier pumps  110  is similar to that of pre-pump  106 .  
         [0042]     Intensifier pumps  110  act as amplifiers converting the intermediate-pressure inlet nitrogen received from a feedline  500  into a high-pressure process discharge fluid before delivering it to heat exchanger  112 . To accomplish this, each of intensifier pumps  110  is provided with plungers  506  on each side of piston  502  to generate pressure in both directions of piston travel in such a way that while one side of intensifier pump is in the inlet stroke, the opposite side generates the high-pressure discharge fluid. Therefore, during the inlet stroke of plunger  506 , nitrogen enters high pressure cylinder  505  under suction through a suitable check valve assembly  511 . After plunger  506  reverses the motion of travel, the supercritical nitrogen is compressed and exits at an elevated pressure (which is determined by the nozzle orifice diameter and the pump pressure limits) through a suitable discharge check valve assembly  513 .  
         [0043]     Thus, in one embodiment, intensifier pumps  110  raise the pressure of supercritical nitrogen at between approximately 15,000-20,000 psi to supercritical nitrogen at approximately 55,000 psi by compression. Power system  116  ( FIG. 1 ) provides the power at a suitable pressure and suitable flow rate to operate piston  502  in a reciprocating fashion. Limit switches  504 , which may be incorporated into spacers  503 , signal electronic controls to shift the directional control valve to reverse the direction of the travel of piston  502 .  
         [0044]      FIG. 6  is a schematic of heat exchanger  112  in accordance with one embodiment of the present invention. As described above, heat exchanger  112  may be any suitable heat exchanger, such as a shell-and-tube type heat exchanger. In the illustrated embodiment, heat exchanger  112  includes a vessel  600  storing a liquid nitrogen bath  601 . Nitrogen may be received via a feedline  603 , which may come from liquid nitrogen supply  102  ( FIG. 1 ). Although liquid nitrogen is utilized for the cooling bath  601  in  FIG. 6 , other suitable coolants are also contemplated by system  100 .  
         [0045]     Heat exchanger  112  also includes one or more coils  602  that receive supercritical nitrogen from intensifier pumps  110  via a feedline  605 . Any suitable arrangement of coils  602  is contemplated by system  100 . Depending on the number of coils  602  associated with heat exchanger  112 , a distribution manifold  606  may be utilized to distribute the supercritical nitrogen through each of the three coils  602 . Liquid nitrogen bath  601  cools the supercritical nitrogen within coil  602  to a minimum temperature of approximately −235° F. for a given pressure of approximately 55,000 psi before delivering it to nozzle assembly  114 .  
         [0046]     Heat exchanger  112  also includes an automated level controller  608 . Similar to the automated level controller  208  of sub-cooler  104  ( FIG. 2 ), automated level controller  608  controls the level of nitrogen bath  601  within vessel  600  in order to control the temperature of the nitrogen exiting heat exchanger  112 . The controlling of the temperature of the nitrogen delivered to nozzle assembly  114  is important to the quality of the jet stream produced by nozzle assembly  114 .  
         [0047]      FIG. 7  is a schematic of power system  116  according to one embodiment of the present invention. Power system  116  functions to provide power to both pre-pump  106  and intensifier pumps  110  and, in the illustrated embodiment, is a hydraulic power system in which both pre-pump  106  and intensifier pumps  110  are fed by separate hydraulic oil pumps  700  and  702 , respectively. Pumps  700 ,  702  are pressure compensated, variable displacement (therefore, variable pressure) pumps that get their oil supply from a common reservoir  704 .  
         [0048]     Pump  700  provides pressurized oil to pre-pump  106  via hydraulic valves  706 . Additionally, oil from a pilot circuit in pump  700  flows through a series of external hydraulic valves  708  that control the displacement of pump  700  itself and thereby control the pressure that pump  700  delivers. External hydraulic valves  708  may be controlled by an operator via controller  122  ( FIG. 1 ) coupled to a programmable logic controller (“PLC”), thus providing flexibility in selecting an appropriate pressure for a particular application.  
         [0049]     Pump  700  is operable to provide pressurized oil in a range from approximately 300 psi up to approximately 3000 psi. This pressure is selectable by an operator via controller  122 . External hydraulic valves  708  perform the function of remotely varying the displacement and, hence, the pressure of pump  700 . Oil flow out of the pilot line enters normally closed proportional control valve (“PCV”)  710  and normally closed, manually adjustable pressure regulating valve (“HV”)  712 . In operation of one embodiment of the invention, HV  712  is set to a value less than 3000 psi as a redundant backup valve in case of a malfunction of PCV  710  during normal operation. PCV  710  is used to set hydraulic oil pump discharge pressures (all lower than that set by HV  712 ) via controller  122  and the PLC. Both of these valves allow flow of pilot circuit oil back to reservoir  704 .  
         [0050]     Pressure relief valve (“PRV”)  714  is included in external hydraulic valves  708  as a means of relieving any overpressure that may build up in the entire pre-pump hydraulic circuit as a result of hydraulic pump malfunction. It represents an added safety measure in the case of an hydraulic overpressure condition to pre-pump  106 .  
         [0051]     Hydraulic valves  706  include a 4-way solenoid operated directional flow control valve (“SV”)  716  that provides pressurized oil to pre-pump  106 . As described above in conjunction with  FIG. 3 , in one embodiment pre-pump  106  is a double-acting hydraulically driven pump including a double-acting actuator and two cold ends  302  capable of producing pressures of up to 20,000 psi or more. End of travel for piston  309  is determined via limit switches  306  that relay this information to the PLC, which in turn transmits signals to open and close the various control valve ports of SV  716 .  
         [0052]     In operation of one embodiment of the pre-pump portion of power system  116 , when end-of-travel (compression stroke) is sensed for one of the cold ends  302  by the respective limit switch  306 , the limit switch  306  relays this information to the PLC, which in turn signals solenoid control valve SV  716  to reverse the current hydraulic oil flow directions. In this embodiment, one port (A or B) on the solenoid control valve SV  716  sees a change from pressurized oil inflow to oil outflow back to reservoir  704  and, conversely, the other port of the solenoid control valve SV  716  sees a change from oil outflow to reservoir  704  to pressurized oil inflow. This has the effect of reversing the direction of movement of piston  309 , thereby toggling one cold end  302  from a compression stroke to a suction stroke, while simultaneously changing the opposite cold end  302  from a suction stroke to a compression stroke. This process is then repeated when the opposite cold end  302  reaches its end of travel. This valve sequencing repeats itself continuously, thus providing the pumping action required to pressurize the nitrogen to an intermediate pressure.  
         [0053]     Pump  702  provides pressurized oil to intensifier pumps  110  via a series of hydraulic valves  720 . Additionally, oil from a pilot circuit in pump  702  flows through a series of external hydraulic valves  722  that control the displacement of pump  702  itself and thereby control the pressure that pump  702  delivers. External hydraulic valves  722  may be controlled by an operator via controller  122  ( FIG. 1 ) coupled to the PLC, thus provide flexibility in selecting an appropriate pressure for a particular application.  
         [0054]     Pump  702  is capable of providing pressurized oil in a range from approximately 300 psi up to approximately 3000 psi. This pressure is selectable by an operator via controller  122 . External hydraulic valves  722  perform the function of remotely varying the displacement and, hence, the pressure of pump  702 . Oil flow out of the pilot line enters normally closed proportional control valve (“PCV”)  724  and normally closed, manually adjustable pressure regulating valve (“HV”)  726 . In operation of one embodiment of the invention, HV  726  is set to a value less than 3000 psi as a redundant backup valve in case of a malfunction of PCV  724  during normal operation. PCV  724  is used to set hydraulic oil pump discharge pressures (all lower than that set by HV  726 ) via controller  122  and the PLC. Both of these valves allow flow of pilot circuit oil back to reservoir  704 .  
         [0055]     Pressure relief valve (“PRV”)  728  is included in external hydraulic valves  722  as a means of relieving any overpressure that may build up in the entire intensifier hydraulic circuit as a result of pump  702  malfunction. It represents an added safety measure in the case of an hydraulic overpressure condition to intensifier pumps  110 .  
         [0056]     Hydraulic valves  720  provide pressurized hydraulic oil to hydraulic cylinders  501  of intensifier pumps  110 , which compress nitrogen as a supercritical fluid up to 60,000 psi or more. In addition to providing directional flow control of the hydraulic oil to and from each of hydraulic cylinders  501  using two separate directional flow control valves,  730  and  732  (4-way solenoid-operated directional flow control valves), hydraulic valves  720  also sequence the supply of oil to each hydraulic cylinders  501  via “sequencing” valves, PRV  734  and PRV  736 , which in one embodiment are ventable, adjustable, pilot-operated pressure relief valves. One PRV is dedicated to each hydraulic cylinder  501 , with vent ports of both PRV  734  and PRV  736  controlled by a “phasing” valve SV  738  (a 3-way, solenoid-operated directional flow control valve), which enables and disables the pilot function of each sequencing valve in a phased manner. Opening the vent ports of PRV  734  and PRV  736  (vents pilot flow oil to reservoir  704 ) disables the pilot function of these same valves and thus bypasses any pressure relief capability the valves possess thereby transmitting the full hydraulic pump pressure once any minimal main stage spring pressure has been overcome. Conversely, when the pilot function is re-enabled (pilot flow is not vented to reservoir), the pressure relief capability of the valves is also re-enabled.  
         [0057]     In operation of one embodiment of the intensifier pump portion of power system  116 , and with reference to  FIG. 5 , one intensifier hydraulic piston  502   b  is coming to the end of its stroke and its corresponding plunger  506   b  is in the almost fully extended position. Correspondingly, high pressure cylinder  505   b  is delivering maximum supercritical fluid pressure to a single common high-pressure discharge line that has a pressure-developing orifice installed at its exit. At this same time the limit switch  504   b  is about to signal the end of travel for piston  502   b . Sequencing valve PRV  736  is fully open (phasing valve SV  738  has opened a route for the vented pilot flow to flow to reservoir  704 ) thus disabling the pilot function of the sequencing valve PRV  736  and disabling the pressure relief capability of the valve. This configuration transmits hydraulic oil through directional flow control valve SV  732  to hydraulic piston  502   b  at the full pressure being generated at the discharge port of pump  702  (excluding line and valve losses).  
         [0058]     Simultaneously, the vent port of sequencing valve PRV  734  does not have a flow route to reservoir  704  because phasing valve SV  738  has blocked this flow path, which enables the pilot function of the valve and thus the pressure relief capability of PRV  734 . The impact of enabling the pressure relief capability of PRV  734  is that there is created a differential pressure, ΔP (which may be set manually) across PRV  734  (oil pressure downstream is lower) and consequently SV  730  and hydraulic cylinder  501   a , equal in magnitude to the pressure created by the adjustable spring setting of PRV  734 . This differential pressure, ΔP, translates into a reduction in the discharge pressure exiting high pressure cylinder  505   a  and into the common high pressure discharge line, which is equal to the product of ΔP times the high-pressure cylinder intensification factor.  
         [0059]     The pressure in the common single high-pressure discharge line at this point is at the pressure generated previously by high pressure cylinder  505   b , which was un-impacted by any ΔP-derived pressure reduction, since conditions for the development of a ΔP did not exist for high pressure cylinder  505   b  (the pressure relief capability of PRV  736  was disabled). This combination of conditions causes hydraulic piston  502   a  to stall at an intermediate travel position because the product of the reduced hydraulic oil pressure times the intensification factor of the high pressure cylinder creates an intensifier discharge pressure, less than the back-pressure in the single common high pressure discharge line it must act against. This prevents hydraulic piston  502   a  from progressing any further.  
         [0060]     Given this current starting point state, the PLC receives a signal from limit switch  504   b  of high pressure cylinder  505   b  that plunger  506   b  has now reached its end of travel. The PLC then sends a signal to directional flow control valve SV  732  to toggle the hydraulic oil flow directions so that piston  502   b  can begin reversing direction, i.e., oil starts to flow into the opposite side of hydraulic cylinder  501   b  while flowing out of the previously pressurized side. Simultaneously, the PLC sends a signal to phasing valve SV  738  that then shifts and blocks the pilot oil vent flow path of sequencing valve PRV  736  (thus enabling the pressure relief capability of this valve, which in turn creates the previously described differential pressure ΔP) and unblocks the pilot oil vent flow path of PRV  734  to reservoir  704 , thus disabling the pressure relief capability and eliminating the pressure differential ΔP.  
         [0061]     Elimination of the pressure differential ΔP now enables the full oil pressure developed at the discharge port of hydraulic pump  702  to be effective in driving hydraulic cylinder  501   a , thereby allowing piston  502   a  to complete its previously stalled compression stroke. This may now occur because the back-pressure in the common high-pressure discharge line is no longer greater than the pressure being discharged from high pressure cylinder  505   a . Pressurized hydraulic oil from pump  702  continues to flow into the opposite side of hydraulic cylinder  501   b  until piston  502   b  now reaches a stalled intermediate travel position (because of the generation of the differential pressure ΔP on the downstream side of sequencing valve PRV  736 . Correspondingly, high pressure plunger  506   a  driven by piston  502   a  has reached its end of travel and corresponding limit switch  504   a  sends a signal to the PLC, which then sends a signal to directional flow control valve SV  730  to toggle the direction of the hydraulic oil flow so that piston  502   a  can begin reversing direction, i.e., oil starts to flow into the opposite side of hydraulic cylinder  501   a  while flowing out of the previously pressurized side.  
         [0062]     Piston  502   a  reverses direction until it stalls at which point piston  502   b  (waiting in the stalled position) will no longer be stalled and will complete its full stroke. Piston  502   b  then reaches its end of travel and reverses, at which point piston  502   b  stalls and piston  502   a  (now waiting in the stalled position) resumes and completes its full stroke. In this manner all the high pressure cylinders on each of the intensifier pumps  110   a ,  110   b , get to play their equal parts. The entire intensifier pumping cycle presented repeats itself continuously, thus providing high-pressure supercritical nitrogen at pressures up to and exceeding 60,000 psi if so desired.  
         [0063]     The dual intensifier operation without the use of a surge chamber, wherein one high pressure cylinder compresses nitrogen to a certain pressure and then stalls while another high pressure cylinder now completes its previously-stalled compression stroke, therefore achieves a steady, relatively “pressure-spike free” flow of high pressure supercritical nitrogen to the nozzle by allowing some overlap of the suction and compression phases (“phasing”) of the different high pressure cylinders. Without this approach the variations in pressure at the nozzle caused by the time lag between the suction phase and the compression phase of each cylinder, may be quite marked, were the cylinders operated in a fully sequential manner.  
         [0064]      FIGS. 8A, 8B  and  8 C are various schematics of a rotating nozzle assembly  800  according to one embodiment of the present invention. The present invention contemplates nozzle assembly  800  being adaptable for different platforms, such as being coupled to a robotic arm, a hand held wand, or other suitable active or passive platform depending on the application.  
         [0065]     In the illustrated embodiment, nozzle assembly  800  includes a housing  802 , a rotatable shaft  804  having a bore  805  running therethrough, a feed chamber  808 , a rotating seal  810 , a seal backup disc  812 , a bearing housing  827  housing a radial bearing  824  and a pair of angular contact bearings  826 , a grease nipple  828 , and a universal head  830 . The present invention contemplates more, less, or different components for nozzle assembly  800  than those shown in  FIGS. 8A-8C .  
         [0066]     Housing  802  may be any suitable size and shape, and may be formed from any suitable material. Rotatable shaft  804  is partially disposed within housing  802  and has an upstream portion  806  associated with feed chamber  808  in order to receive high pressure cryogenic fluid. Rotatable shaft  804  may have any suitable length and be formed from any suitable material. Bore  805  may also have any suitable diameter. Rotatable shaft  804  may be rotated in any suitable manner, such as a suitable drive assembly (not illustrated).  
         [0067]     In the illustrated embodiment, shaft  804  is rotatable with respect to housing  802  by radial bearing  824  and angular contact bearings  826 . Any suitable number and any suitable type of bearings may be used in lieu of radial bearing  824  and angular contact bearings  826 . In one embodiment, bearings  824 ,  826  are lubricated with a suitable lubricant. In a particular embodiment of the invention, bearings  824 ,  826  are lubricated with a cryogenically-rated aerospace grease. In one embodiment, the cryogenically-rated aerospace grease is a perfluoropolyether grease. For example, the grease may be Christo-Lube® MCG-106 manufactured by Lubrication Technology, Inc. In another particular embodiment of the invention, bearings  824 ,  826  are bearings that require no lubrication. In the embodiment where bearings are used that require no lubrication, bearings may be sputter coated bearings, ceramic bearings, or other suitable bearings that require no lubrication. For example, bearings  824 ,  826  may be sputter coated with a permanent low friction coating, such as tungsten disulphide.  
         [0068]     In order to prevent high pressure nitrogen from leaking from feed chamber  808  into bearing housing  828 , seal  810  is disposed within feed chamber  808  and surrounds an upstream portion of rotatable shaft  804 . Seal backup disc  812  is disposed proximate the downstream end of seal  810  to keep seal  810  in place as shaft  804  rotates. Seal  810 , in one embodiment, is a rotating seal and is described in greater detail below in conjunction with  FIG. 8C .  
         [0069]     Referring now to  FIG. 8B , seal backup disc  812  includes an orifice  814  that surrounds an outside diameter  818  of rotatable shaft  804 . In one embodiment, diameter  818  is between 0.187 and 0.1875 inches. According to the teachings of one embodiment of the invention, orifice  814  has a diameter  816  such that, when a cryogenic fluid such as supercritical nitrogen is flowing through bore  805  of rotatable shaft  804 , rotatable shaft  804  can freely rotate while seal  810  prevents cryogenic fluid from seeping past seal  810 . In one embodiment, this is accomplished by having an orifice diameter  816  of at least 0.191 inches and no greater than 0.193 inches.  
         [0070]     Referring to  FIG. 8C , seal  810  comprises a body  820  and a spring member  822  disposed within a groove  823  on an upstream end of seal  810 . In one embodiment, body  820  is formed from an ultra-high molecular weight polyethylene (“UHMW PE”), which may be oil-filled; however, other suitable materials may be utilized for body  820 . Spring member  822 , in one embodiment, is a cantilever spring member having a V-shaped cross section; however, spring member  822  may have other suitable cross sections, such as circular. In a particular embodiment of the invention, an inside diameter of seal  810  is between 0.188 and 0.191 inches.  
         [0071]     Universal head  830  can be any suitable universal head depending on the application for nozzle assembly  800 . For example, if nozzle assembly  800  is a rotating nozzle assembly, then universal head  830  may have a plurality of bores in fluid communication with bore  805  in order to perform a sand blasting operation, for example.  
         [0072]      FIG. 9A  is a schematic of a nozzle assembly  900  according to one embodiment of the present invention. Nozzle assembly  900  may be used for abrading, sandblasting, cold spraying, or other suitable machining or manufacturing process. It may also have the potential of replacing common electroplating. In the illustrated embodiment, nozzle assembly  900  includes a housing  902 , a high pressure nitrogen feed  904 , an abrasive material feed  906 , a mixing chamber  908 , and a nozzle  910 . The present invention contemplates more, less, or different components for nozzle assembly  900  than those shown in  FIG. 9A . In addition, the present invention contemplates combining features of rotating nozzle assembly  800  in  FIG. 8A  to facilitate rotating with abrasive materials.  
         [0073]     Housing  902  may be any suitable size and shape and may be formed from any suitable material, such as stainless steel. Housing  902  may couple to high-pressure supercritical nitrogen feed  904  in any suitable manner, such as a screwed connection. High-pressure supercritical nitrogen feed  904  delivers high-pressure supercritical nitrogen or other suitable cryogen into mixing chamber  908 . Before entering mixing chamber  908 , the supercritical nitrogen flows through an orifice  913 . Orifice  913  may have any suitable diameter, for example approximately 0.012 inches, to control the flow of nitrogen into mixing chamber  908 . Mixing chamber  908  may be formed from any suitable material; however, in one embodiment, mixing chamber  908  is formed from a hard material, such as tungsten carbide.  
         [0074]     Abrasive material feed  906  may couple to housing  902  in any suitable manner, such as a screwed connection. Abrasive material feed  906  delivers an abrasive material  907  into mixing chamber  908 . Abrasive material  907  may be any suitable abrasive material, such as grit, crystalline compounds, glass, metal particles, and carbon dioxide. Abrasive material  907  mixes with supercritical nitrogen in mixing chamber  908 , and exits chamber  908  towards a target (not illustrated) via nozzle  910 .  
         [0075]     Nozzle  910  couples to housing  902  in any suitable manner, such as a collet  915  that is screwed onto housing  902 . In one embodiment, nozzle  910  is sized such that the high pressure supercritical nitrogen jet does not lose coherence (i.e., become unstable and lose significant energy) before striking the target. In one embodiment, this is accomplished by having a length  912  of exposed nozzle  910  of no more than two inches. Nozzle  910  may be formed from any suitable material. For example, nozzle  910  may be formed from boron nitride, tungsten carbide, or other suitable hard abrasion resistant material. In one embodiment, the high-pressure supercritical nitrogen exits nozzle  910  at a temperature no colder than −235° F. at a given pressure of no more than 55,000 psi.  
         [0076]     Although not illustrated in  FIG. 9A , a vacuum shroud or other suitable vacuum system may be associated with nozzle assembly  900  in order to remove any abrasive material  907  exiting nozzle  910  after striking the target. This reduces or eliminates any potential for contamination of the environment.  
         [0077]      FIG. 9B  is a schematic illustrating a different nozzle assembly  920  according to one embodiment of the present invention. As illustrated, nozzle assembly  920  includes a venturi nozzle  922 , which may also be a straight nozzle in some embodiments. Venturi nozzle  922  facilitates entrainment of abrasives and a lateral dispersion  924  of the nitrogen/abrasive particle mixture exiting nozzle  922  for the purposes of providing a large area of contact suitable for cleaning and abrading. A length  923  of nozzle  922  may be any suitable length. In addition, nozzle  922  may have any suitable diameters associated therewith. Venturi nozzle  922  may be formed from any suitable material, such as a metal. In one embodiment, venturi nozzle  922  is lined with a ceramic material.  
         [0078]     Nozzle assembly  920  also includes a housing  925 , to which a high pressure nitrogen line  926  and an abrasive particle feed  938  is coupled thereto in any suitable manner. A seal  930  surrounds an outside perimeter of nitrogen line  926  and may be any suitable seal formed from any suitable material. Nitrogen line  926  includes an orifice  932  formed in an end thereof that may have any suitable diameter, such as between approximately 10 and 12 mils.  
         [0079]     Abrasive particle feed  938  may be either a positive feed or a venturi-suction feed that directs abrasive particles into housing  925  for mixing with nitrogen. Any suitable abrasive particles may be utilized.  
         [0080]     Although embodiments of the invention and some advantages are described in detail, a person skilled in the art could make various alterations, additions, and omissions without departing from the spirit and scope of the present invention as defined by the appended claims.
According to an embodiment of the present invention, a cryogenic fluid delivery system includes a vessel containing a cryogenic fluid at a first pressure and a first temperature, a first heat exchanger coupled to the vessel for receiving the cryogenic fluid and cooling the cryogenic fluid to a second temperature, a first pump coupled to the first heat exchanger for pressurizing the cryogenic fluid to a second pressure, a second pump for pressurizing the cryogenic fluid to a third pressure, a second heat exchanger coupled to the second pump for cooling the cryogenic fluid to a third temperature, and a nozzle coupled to the second heat exchanger for delivering a jet of the cryogenic fluid toward a target.