Abstract:
A detachable work extraction system includes an expansion engine including a self-sealing coupling adapted to detachably connect the expansion engine to a cold box, and a hydraulic work extractor operatively connected to the expansion engine. A gas can travel from the cold box to the expansion engine through the self-sealing coupling. The gas is cooled by expansion of the gas in the expansion engine and work produced by the expansion of the gas is dissipated by the hydraulic work extractor. The expansion engine includes a cylinder housing a piston. The cylinder has a first self-sealing coupling defining an inlet and a second self-sealing coupling defining an outlet. The first and second self-sealing couplings each have a spring loaded seal. In the absence of an external force applied to the seal, the seal prevents flow of gas through the couplings.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a detachable cryogenic refrigerator expander. 
     Systems operating at cryogenic temperatures generally have a cryogenic refrigeration unit attached to the system to minimize or eliminate boil-off of the cryogenic coolant. A cryogenic refrigeration unit including an expansion device, compressor, and heat exchanger is known. High pressure fluid from the compressor is passed through the heat exchanger and introduced into the expansion device. Expansion of the fluid in the expansion device reduces the temperature and pressure of the fluid. Heat energy is transferred from the expanding fluid by the performance of mechanical work. 
     When the expansion device must be removed, either for maintenance or replacement, the cryogenic system is typically exposed to warmer temperatures and potential contaminates. Maintenance of the unit generally requires that the cryogenic system be shut down for at least a day. 
     SUMMARY OF THE INVENTION 
     A light weight, easily maintained work extraction system for cooling a gas provides reliable, low cost refrigeration in about the 4 K to 40 K temperature range. The system includes an expansion engine assembly and a hydraulic work extractor assembly. When an inlet valve of the expansion engine assembly is opened, gas enters the cold end and expands, raising an expansion piston and raising a hydraulic piston coupled to the expansion piston. This forces hydraulic fluid through a needle valve creating a head due to flow friction. The inlet valve is closed when the piston is partially up the stroke to allow the gas to expand to a lower pressure. When the full stroke is reached the exhaust valve is opened and pneumatic spring pistons push the expansion piston back down, exhausting the gas in the cylinder for the next cycle. 
     According to the invention, a detachable work extraction system includes an expansion engine including a self-sealing coupling adapted to detachably connect the expansion engine to a cold box, and a hydraulic work extractor operatively connected to the expansion engine. A gas can travel from the cold box to the expansion engine through the self-sealing coupling. The gas is cooled by expansion of the gas in the expansion engine and work produced by the expansion of the gas is dissipated by the hydraulic work extractor. 
     Embodiments of this aspect of the invention may include one or more of the following features. 
     The self-sealing coupling includes a spring loaded seal. The expansion engine includes a cylinder housing a piston. The cylinder has a first self-sealing coupling defining an inlet and a second self-sealing coupling defining an outlet. The first and second self-sealing couplings each have a spring loaded seal. In the absence of an external force applied to the seal, the seal prevents flow of gas through the couplings. 
     An inlet valve assembly, for example, an electric actuated spring biased valve, controls the flow of gas through the inlet. An outlet valve assembly, for example, a pneumatic actuated spring biased valve, controls the flow of gas through the outlet. 
     In the illustrated embodiment, a return assembly, for example, a pneumatically controlled return assembly, lowers the piston. The work extraction system includes a displacement transducer for monitoring the position of the piston. 
     The hydraulic work extractor includes a cylinder, piston, and oil loop. A control valve of the hydraulic work extractor dissipates the work produced by the expansion of the gas. The control valve includes a throttle valve and a check valve. 
     According to another aspect of the invention, a refrigeration system includes a cold box and a detachable work extraction system. The cold box has a first self-sealing coupling, and an expansion engine of the work extraction system has a second self-sealing coupling for detachably connecting the expansion engine to the cold box self-sealing coupling. 
     Embodiments of this aspect of the invention may include a hydraulic work extractor operatively connected to the expansion engine. 
     According to another aspect of the invention, a method for connecting a cold box and a work extraction engine includes detachable connecting the work extraction engine to the cold box, and removably disconnecting the work extraction engine from the cold box without substantial loss of gas from the cold box. 
     An advantage of this system is its ease of maintenance. Self sealing couplings allow the expander module to be removed and replaced without warming the system or contaminating the inner components. With a dual expander arrangement, the system may not even be required to be shut down. The hydraulic work extraction device allows the expander module to be light enough to be removed by one person and reattached relatively easily. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the invention will be apparent from the following description taken together with the drawings in which: 
     FIG. 1 is a schematic of a work extraction system according to the invention, shown during a gas intake portion of its cycle; 
     FIG. 2 is a schematic of the work extraction system of FIG. 1, shown during a gas exhaust portion of its cycle; 
     FIGS. 3A and 3B are side and front views, respectively, of an expansion engine assembly of the work extraction system of FIG. 1; 
     FIG. 4 shows inlet and outlet couplers connecting the expansion engine assembly to a cold box; 
     FIG. 5A is a cross-sectional side view showing a male coupler assembly attached to the expansion engine assembly; 
     FIG. 5B is an exploded view of a poppet valve of the male coupler assembly of FIG. 5A; and 
     FIG. 6 is a cross-sectional side view showing a female coupler assembly attached to the cold box. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an illustrative schematic diagram, a work extraction system  10  for cooling a gas includes an expansion engine assembly  12  and a hydraulic work extractor assembly  14 . When the cooled gas is to be used, for example, as a refrigerant for a superconducting magnet, pre-cooled, high pressure gas, for example, helium gas at a temperature of 80 K, and a pressure of 250 psi, enters expansion engine assembly  12  at an inlet  20  and expands within a piston chamber  22 . Due to the expansion of the gas, the helium gas exiting expansion engine assembly  12  at an outlet  24  is at a lower pressure, for example,  30  psi, and lower temperature, for example, 40 K, than the inlet gas. The work produced by expansion of the gas is dissipated by hydraulic work extractor assembly  14 . 
     Referring to FIG. 3A, which illustrates assembly  12  in more detail, inlet  20  and outlet  24  of expansion engine assembly  12  are connected to a cold box  30 . Except for couplings for connecting the inlet and outlet to the cold box, described below, cold box  30  is a conventional pre-cooling unit housing the heat exchanger and valves of the refrigeration system. Cold box  30  is connected to a compressor  32  by high and low pressures lines  34 ,  36 , respectively. The cooled gas entering cold box  30  from outlet  24  is fed to the superconducting magnet. 
     Referring again to FIG. 1, expansion engine assembly  12  includes a piston cylinder  40  housing a piston  42 . Piston chamber  22  defined between cylinder  40  and piston  42  is sealed, for example, by o-rings  44 . A connecting rod  46  is attached to piston  42  for movement with piston  42 . Connecting rod  46  is attached to a hydraulic piston rod  48  of hydraulic work extractor assembly  14  by a coupler  50 , for example, a U-joint. The use of a U-joint for coupler  50  accounts for any misalignment between connecting rod  46  and hydraulic piston rod  48 . Hydraulic piston rod  48  extends into a hydraulic cylinder  52  and terminates in a hydraulic piston head  54 . 
     A control bar  60  is attached to hydraulic piston rod  48  for movement with piston rod  48 . Two pneumatic springs  62 ,  64  are attached to control bar  60 . Each pneumatic spring  62 ,  64  includes a pneumatic cylinder  66 , a piston  68 , and a piston rod  70  attached to control bar  60  for movement with control bar  60 . Air can be introduced and bled from a top portion  71  of cylinder  66  to push piston  68  downward and allow piston  68  to move upward. A linear variable differential transducer (LVDT)  72  is attached to control bar  60  by an arm  73  to monitor the position of hydraulic piston head  54 . 
     Oil is contained within hydraulic cylinder  52 . An oil line  80  is connected to hydraulic cylinder  52  at an upper section  52   a  of cylinder  52 . Oil line  80  splits at  82  into a main flow path  84  and a secondary flow path  86 . The two flow paths  84 ,  86  join at  88  to form an oil line  90  which is connected to hydraulic cylinder  52  at a lower section  52   b  of cylinder  52 . A valve  92  is located in main flow path  84 . Valve  92 , for example, a Model Number PF600 BV40 Flow Control Valve from Parker Motion &amp; Control, Elyria, Ohio, includes a throttle control valve  94  for metering flow through valve  92  and a check valve  96 . A fan  102  is used to cool the hydraulic fluid as it flows through control valve  94 . Located in secondary flow path  86  is a solenoid valve  98 , and connected to oil line  90  is an oil buffer  100 , for purposes described below. 
     Entry and exhaust of gas from expansion engine assembly  12  through inlet  20  and outlet  24  are controlled by an inlet valve assembly  109  and an outlet valve assembly  111 , respectively. An inlet valve  110  of assembly  109 , for example, an electrically actuated control valve, has a first, closed position in which gas is prevented from flowing through inlet  20  into chamber  22 , and a second, open position permitting the flow of gas into chamber  22 . Inlet valve  110  is biased closed by a spring  114 , and is opened against the force of spring  114  by a solenoid  116 . Inlet valve assembly  109  is rated, for example, at a 20% duty cycle and can lift a fifty pound load. 
     An outlet valve  112  of outlet valve assembly  111 , for example, a pneumatically actuated control valve, has a first, closed position in which gas is prevented from flowing through outlet  24 , and a second, open position permitting the flow of gas through outlet  24 . Outlet valve  112  is biased closed by a spring  118 , and is opened against the force of spring  118  by a pneumatic piston  120 . Pneumatic piston  120  includes a piston cylinder  126  housing a piston head  128 . A three-way valve  122  controls the flow of air to and from a lower chamber  124  of piston cylinder  126  to raise and lower outlet valve  112  between its opened and closed positions. Outlet valve assembly  111  is rated, for example, at a 50% duty cycle and can lift an eighty pound load. 
     In operation, with outlet valve  112  in its closed position, inlet valve  110  is opened to permit the high pressure gas to enter chamber  22 . The gas pushes against piston  42  lifting piston  42 . The motion of piston  42  causes hydraulic piston rod  48 , control bar  60 , piston rods  70 , and arm  73  of LVDT  72  to rise. As the hydraulic piston head  54  rises, oil is forced into oil line  80 , through control valve  94 , and through oil line  90  to the lower end  52   b  of hydraulic cylinder  52  (check valve  96  and solenoid  98  are closed at this point in the operation). 
     When control bar  60  has risen, for example, about one inch, as measured by LVDT  72 , a control signal is sent to solenoid  116  by a controller (not shown). Solenoid  116  is actuated to close inlet valve  110 . With inlet valve  110  and outlet valve  112  both closed, the gas trapped within chamber  22  expands isentropically lowering the pressure and temperature of the gas within chamber  22 . The work produced by the expansion of the gas within chamber  22  is dissipated in the form of heat by the resulting flow of oil through throttle valve  94 . 
     As piston head  54  reaches the top of its stroke, solenoid valve  98  is opened. This allows the oil to bypass throttle valve  94  and flow through secondary flow path  86 , thus reducing the friction against which the oil flows. This permits the stroke of piston head  54  to be maximized, allowing the pressure of the gas in chamber  22  to drop lower, thus increasing the efficiency of the system. 
     At the end of the intake stroke (see FIG.  2 ), as measured by LVDT  72 , three way valve  122  is positioned to allow air to flow into lower chamber  124  of piston cylinder  126  raising piston head  128 . This opens valve  112  permitting the low temperature, low pressure gas to exit chamber  22 . Concurrently with the opening of valve  112 , check valve  96  is opened, solenoid  98  is closed, and pressurized air is delivered to top portion  71  of pneumatic springs  62 ,  64 . Pneumatic springs  62 ,  64  act to lower control bar  60  and thus lower piston head  54 , piston rod  48 , connecting rod  46 , and piston  42 . The lowering of piston  42  forces the low temperature, low pressure gas to exit chamber  22  through outlet  24 . 
     As piston head  54  lowers, oil is forced from the lower section  52   b  of cylinder  52  through flow line  90 , up flow path  84  through check valve  96  and up flow path  86 , and out oil line  80  into the upper section  52   a  of cylinder  52 . As piston head  54  reaches the bottom of the stroke, the intake and exhaust cycle in repeated. 
     Oil buffer  100  provides space for accommodating the change in oil volume in cylinder  52  which results from the movement of piston rod  48  into and out of cylinder  52 . Oil buffer  100  also acts as an oil reservoir in case of oil leakage from hydraulic work extractor assembly  14 . 
     Referring to FIGS. 3A and 3B, expansion engine assembly  12  includes a vacuum insulated housing  200  enclosing piston cylinder  40  and a charcoal filter  202 . High pressure gas entering through inlet  20  flows through an inlet gas line  204  to charcoal filter  202 . The gas exits charcoal filter  202  through gas line  206  which is connected to inlet valve assembly  109 . Gas exiting piston cylinder  40  travels through an outlet gas line  208  connected between outlet valve assembly  111  and outlet  24 . Inlet gas line  204  and outlet gas line  208  each include relief valves, not shown, for relieving over pressure, for example, pressures over  300  pounds in inlet gas line  204  and pressures over 50 pounds in outlet gas line  208  where the high pressure gas entering through inlet  20  is at 250 psi. 
     Referring to FIG. 4, a self-sealing bayonet coupling  300  allows the work extraction system  10  to be easily replaced at regular maintenance intervals without contaminating the refrigeration system (cold box  30  and compressor  32 ). Coupling  300  includes a high pressure inlet bayonet  302  defining inlet  20 , and a low pressure exhaust bayonet  304  defining outlet  24 . Each bayonet has a male coupler assembly  306  and a female coupler assembly  308 . The inlet and outlet male couplers  306  are mounted to expansion engine assembly  12 , and the inlet and outlet female couplers  308  are mounted to cold box  30 . 
     Referring to FIG. 5A, each male coupler assembly  306  has a body  310  defining a flow passage  312 . A poppet valve  314  is mounted to the end  315  of body  310 . Referring also to FIG. 5B, poppet valve  314  includes a seal  316  defining a flow path  317  in fluid communication with flow passage  312 . Seal  316  is biased to a closed position by a spring  320 . In the closed position, seal  316  is held against a seat  318  defined by a cover  319  of poppet valve  314 . When a force is applied to seal  316  (along the direction of arrow  322 ), seal  316  is moved away from seat  318  against the force of spring  320 . The movement of seal  316  places flow path  317  in fluid communication with an aperture  324  defined by cover  319  permitting flow of gas through male coupler assemblies  306 . Cover  319  is attached to body  310  by, for example, welding. Each male coupler assembly  306  includes a connector  326  for detachable coupling the male coupler assembly to a respective female coupler assembly  308 . 
     Referring to FIG. 6, each female coupler  308  has a body  330  defining a flow passage  332 . A poppet valve  334  (identical to poppet valve  314 ) is mounted within an end  336  of body  330 . Each female coupler assembly  308  includes a connector  346  for detachable coupling the female coupler assembly to a respective male coupler assembly  306 . 
     Referring again to FIG. 4, when expansion engine assembly  12  is connected to cold box  30 , male coupler assemblies  308  are slid into respective female coupler assemblies  306 . This causes poppets  314 ,  334  to press against each other forcing both seals  316  against their springs, opening the gas passages between the cold box  30  and the expansion engine assembly  12  (as shown in FIG.  4 ). An o-ring  333  (see FIGS. 5A and 5B) provides a seal between the two poppets. When the bayonets are fully engaged, a set of external clamps  341  hold the female and male coupler assemblies together. When maintenance is required, work extraction system  10  can be easily removed by unclamping the assembly and pulling the male coupler assemblies  308  out of the female coupler assemblies  306 . The poppets automatically seal to maintain the pressure of both sides of the system and prevent substantial loss of gas from the cold box. 
     Depending upon the application, gas entering work extraction system  10  is generally at a pressure of about 250 psi, and the pressure of the exhaust gas is in the range of about 0 to 50 psi. Work extraction system  10  produces a temperature drop to about half the intake gas temperature. Depending upon the application, the intake gas will generally be selected to be between about 8 K and room temperature. 
     Work extraction system  10  is a dry expander, that is, the system is designed for use where the intake and exhaust are a gas. Pistons  42  and  54  of work extraction system  10  can be run up to about seventy strokes/minute. The percentage of time in a single intake stroke that the inlet valve is open, that is, the cut-off time, is about 30%. The overall size of work extraction system  10  is, for example, about 4 feet long and 6 inches in diameter. System  10  weighs, for example, about 40 pounds. 
     Other embodiments are within the scope of the following claims. For example, a pneumatic rather than an electric actuator can be used to control input valve  110  if input valve  110  is being run at a duty cycle greater than about 30%. An electric actuator can control output valve  112  if the system is running at low pressure.