BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a refrigerator for producing refrigeration at two or more cryogenic temperatures by combining a first stage Brayton cycle engine with one or more Gifford-McMahon ("GM") expanders. Specifically, the invention relates to a refrigerator wherein the cold gas that circulates through the Brayton cycle engine is further cooled by one or more GM expanders and transport refrigeration to one or more remote heat exchangers. Therein, the can be used for example to cool a superconducting magnet at 30 K and a surrounding shield at 70 K.

2. Background Information

A refrigeration system that operates on the Brayton cycle consists of a compressor that supplies gas at a discharge pressure to a counterflow heat exchanger, which admits gas to an expansion space through a cold inlet valve, expands the gas adiabatically, exhausts the expanded gas (which is colder) through in outlet valve, circulates the cold gas through a load being cooled, then returns the gas through the counterflow heat exchanger to the compressor at a return pressure.

U.S. patent 3,045,436, by W. E. Gifford and H. O. McMahon describes the Gifford- McMahon ("GM") cycle. This refrigerator system also consists of a compressor that supplies gas at a discharge pressure to an expander which admits gas through an inlet valve to the warm end of a regenerator heat exchanges and then into an expansion space at the cold end of a piston from whence it returns back through the regenerator and a warm outlet valve to the compressor at a return pressure. The typical GM type expander being built today has the regenerator located inside the piston so the piston/regenerator becomes a displacer that moves from the cold end to the warm end with the gas at high pressure then from the warm end to the cold end with the gas at low pressure. An important difference between GM and Brayton type refrigerators is that Brayton cycle refrigerators can distribute cold gas to a remote load while the cold expanded gas in GM expanders is contained within the expansion space. U.S. Patent Application Publication 2011/0219810 dated 9/15/11 by R. C. Longsworth describes a reciprocating expansion engine operating on a Brayton cycle in which the piston has a drive stem at the warm end that is driven by a mechanical drive, or gas pressure that alternates between high and low pressures, and the pressure at the warm end of the piston in the area around the drive stem is essentially the same as the pressure at the cold end of the piston while the piston is moving. U.S. Patent application S/N 13/106,218 dated 5/12/11 by S. Dunn, et al, describes alternate means of actuating the expander piston. A compressor system that can be used to supply gas to these engines is described in published patent application U.S. 2007/0253854 titled "Compressor With Oil Bypass" by S. Dunn filed on 4/28/06. The engines described in these applications provide examples of Brayton engines that can be used in this present invention.

Adding a second piston to a Brayton engine requires a second pair of valves and their associated actuators whereas adding a GM displacer attached to the Brayton piston utilizes the first stage valves to cycle pressure to the second stage. It is thus an object of the present invention to combine the advantage of the Brayton engine to output cold gas that can be circulated to a remote heat station with the simplicity of construction of adding one or more additional stages of GM cooling to the Brayton engine and use the circulating gas to cool one or more remote heat stations at colder temperatures.

SUMMARY OF THE INVENTION The present invention combines a Brayton engine first stage with one or more GM colder stages that uses the flow from the Brayton engine to provide refrigeration at one or more remote heat stations. A hybrid expander for producing refrigeration at cryogenic temperatures operates with gas supplied from a source at a first pressure and returned to the source at a second pressure. The second pressure is lower than the first pressure. The hybrid expander includes

a Brayton expansion engine producing refrigeration at a first temperature, the Brayton expansion engine comprising a reciprocating piston having a piston warm end and a piston cold end, and

a Gifford-McMahon expander producing refrigeration at a second temperature, the second temperature being colder than the first temperature, the Gifford-McMahon expander comprising a displacer, the displacer being attached to the piston cold end and reciprocating contemporaneously with the piston.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic view of a hybrid expander 100 comprising a pneumatically actuated gas balanced Brayton engine first stage, a GM second stage, a remote first stage heat exchanger, and a remote second stage heat exchanger.

Fig. 2 is a schematic view of a hybrid expander 200 comprising a Brayton engine first stage, a GM second stage, a remote first stage heat exchanger, and an integral second stage heat exchanger.

Fig. 3 is a schematic view of a hybrid expander 300 comprising a Brayton engine first stage, a GM second stage, a GM third stage, a remote first stage heat exchanger, and a remote third stage heat exchanger. Fig. 4 is a schematic view of a hybrid expander 400 comprising a Brayton engine first stage, a GM second stage, and a remote second stage heat exchanger.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with one or more embodiments of the present invention, Figs. 1 to 4 use the same number and the same diagrammatic representation to identify equivalent parts.

Since expansion engines are usually oriented with the cold end down, in order to minimize convective losses in the heat exchanger, the movement of the piston from the cold end toward the warm end is frequently referred to as moving up, thus the piston moves up and down or to the top and bottom. The figures do not show the warm mounting plate on which warm flange 7 is mounted and the vacuum housing below the warm flange that separates the cold components from the air outside.

Fig. 1 illustrates the Brayton engine drive mechanism described in U.S. Patent Application Publication 2012/0285181 published 11/14/12 and titled "Gas Balanced Cryogenic Expansion Engine," while Figs. 2, 3, and 4 illustrate generic drive mechanisms to which the present invention is modified.

In Fig. 1, a hybrid expander 100 comprises a Brayton piston/GM displacer assembly, a drive assembly at the warm end, a cylinder assembly, and a piping assembly that includes multiple heat exchangers. Brayton piston 1 is attached to drive stem 2 at the warm end and connected by coupling 60 at the cold end to GM displacer 20 which contains regenerator 21. Seal 51 prevents gas from by-passing from displaced volume DVs 5, which is disposed above drive stem 2, to displaced volume DVw 4, which is disposed above Brayton piston 1. Seal 50 prevents gas from by-passing from DVw 4 to displaced volume DVc 3, which is below Brayton piston 1. Seal 52 prevents gas from by-passing from displaced volume DVc 3 to displaced volume 23 below displacer 20. This piston/displacer assembly reciprocates in a cylinder assembly. The cylinder assembly includes warm flange 7, first stage cylinder 6, first stage end cap 9, second stage cylinder 22, and second stage end cap 24. The pneumatic drive assembly includes components that are not shown which open inlet valve Vi 10, when piston 1 is near the cold end and closes it when piston 1 is near the top, and which open outlet valve Vo, 11, when piston 1 is at the top and closes it when piston 1 is near the bottom. Gas pressure in displaced volumes 3 and 23, as well as regenerator 21, is nearly the same, differing only due to pressure drop through regenerator 21 when gas is moving.

When piston 1 reaches the top, displaced volumes DVc 3 and DVw 4 have gas at a pressure near high pressure Ph in them. With inlet valve Vi 10 closed, outlet valve Vo 11 opens and allows cold gas to flow out to low pressure PL The pressure difference between displaced volumes DVw 4 and DVc 3 causes piston 1 to move down and draw gas into displaced volume DVw 4 from high pressure supply line 30 through a warm inlet valve Vwi 15, inlet check valve CVi 13, and connecting lines 33. The speed at which piston 1 moves down is controlled by the setting of warm inlet valve Vwi 15.

When piston 1 reaches the bottom, outlet valve Vo 11 is closed and time is allowed for gas to continue to flow into DVw 4 until the pressure is near pressure Ph. Inlet valve Vi 10 is then opened to admit gas at pressure Ph. The force imbalance due to pressure Ph acting at the cold end of piston 1 and pressure PI acting on drive stem 2 compresses gas in displaced volume DVw 4 above pressure Ph, i.e., a third pressure, and pushes it is out through outlet check valve CVo 12, warm outlet valve Vwo, 14, aftercooler 48, and connecting lines 34 to high pressure line 30. The speed at which piston 1 moves up is controlled by the setting of warm outlet valve Vwo 14. Displaced volume DVs 5 is connected through line 32 to low pressure return line 31; thus, it is always at pressure PL The piping assembly comprises

a counterflow heat exchanger 40 between room temperature and a first stage temperature,

a counterflow heat exchanger 41 between a first stage temperature and a second stage temperature, a remote heat exchanger 43 which receives heat from a load at temperature Tl, a remote heat exchanger 44 which receives heat from a load at temperature T2, a heat exchanger 46 which transfers heat from the circulating gas to the gas in GM expansion space 23 through second stage cold end 24, and

connecting piping.

The piping is identified as connecting the previously listed components in the system. Therein,

line 30 carries gas at pressure Ph from the compressor to inlet valve Vi 10 through heat exchanger 40,

line 35 carries gas at pressure PI from outlet valve Vo 11 to flow splitter 16, line 36 which carries a first fraction of the gas from splitter 16 through heat exchanger 43 to tee 17,

line 37 carries the balance of the gas through heat exchangers 41, 46, 44, and 41 to tee 17, and

line 31 returns gas to the compressor from tee 17 through heat exchanger 40.

Fig. 2 shows hybrid expander 200 comprising a Brayton piston/GM displacer assembly, a cylinder assembly, and a piping assembly that includes multiple heat exchangers. The means for driving the piston/displacer assembly up and down is not shown but can be either a mechanical or pneumatic mechanism. The Brayton piston/GM displacer assembly and the cylinder assembly are the same as expander 100. Expander 200 differs from expander 100 in that the piping assembly has only one remote heat station. Cold gas flows from outlet valve Vo 11 through remote heat exchanger 43 to heat exchanger 40 through line 38. Heat flows into heat exchanger 43 from a load at temperature Tl and flows directly into cold end 24 from a colder load at temperature T2. Fig. 3 shows hybrid expander 300 comprising a Brayton piston/GM displacer assembly, a cylinder assembly, and a piping assembly that includes multiple heat exchangers. The means for driving the piston/displacer assembly up and down is not shown but can be either a mechanical or pneumatic mechanism. The first two stages of the Brayton piston/GM displacer assembly are the same as shown in Figs. 1 and 2. This embodiment of the invention includes a third stage GM displacer 25 containing regenerator 26 and seal 53 and which is coupled to second stage displacer 20 by coupling 61 and reciprocates within the extension of the cylinder assembly consisting of cylinder 27 and cold end 29. Refrigeration is produced by gas expanding in displaced volume 28.

The piping in expander 300 differs from expander 100 in that the piping to the colder remote heat exchanger is different. The first fraction of cold gas that flows through line 36 from splitter 16 through remote heat exchanger 43 to tee 17 is the same. The balance of the flow is cooled to lower temperatures as it flows in line 39 from splitter 16 through heat exchangers 41, 46, 42, and 47, and then is warmed as it flows through heat exchangers 45, 42, and 41 before joining the first fraction of flow at tee 17. Heat is transferred from a load at temperature Tl to heat exchanger 43 and from a load at temperature T3 to heat exchanger 45. Fig. 4 shows hybrid expander 400 comprising the same Brayton piston/GM displacer assembly and cylinder assembly as expander 100. The means for driving the piston/displacer assembly up and down is not shown but can be either a mechanical or pneumatic mechanism. The piping assembly shows the options of circulating gas at pressure Ph through a single remote heat exchanger which is at temperature T2. Gas at pressure Ph has a greater density than gas at pressure PI thus the piping can be smaller. Fig 4 shows the option of using all of the first stage refrigeration to remove thermal losses in heat exchanger 40 and having all of the flow that is circulated by the engine go through heat exchanger 44 at temperature T2. The piping in expander 400 includes line 30 which extends from the compressor through heat exchangers 40, 41, 44, 46, and 41 to inlet valve Vi 10. Gas at pressure PI flows through line 38 from outlet valve Vo 11 to heat exchanger 40.

These embodiments provide examples of many ways that the basic concepts can be applied, in accordance with one or more embodiments of the present invention, a GM displacer to the cold end of a Brayton engine piston, thus using the inlet and outlet valves of the Brayton engine to cycle gas to the volumes displaced by the piston and displacer. In accordance with one or more embodiments of the present invention, a gas is circulated by the Brayton engine to remove heat from one or more remote locations, the gas being at either high or low pressure. Counter-flow heat exchangers can be used between the GM expander stages to make the gas available to transfer heat from remote loads to the cold stage(s) of the GM expander(s) with small thermal losses imposed on the GM stage(s) by the counter- flow heat exchanger(s). Heat can alternately be transferred directly to a GM heat station. The drive mechanism for the Brayton engine and the means for opening and closing the inlet and outlet valves is a matter of choice. Helium is the preferred gas for most cryogenic refrigerators but other gases such as hydrogen and neon might be used. Calculations have been made for the cooling that would be expected for hybrid expanders 100 and 400 from a compressor that compresses 11 g/s of helium at room temperature from 0.8 MPa to 2.2 MPa and draws about 14 KW of power. A hybrid expander 100 having a Brayton engine piston with a diameter of 100 mm and a GM displacer with a diameter of 50 mm would provide about 200 W of cooling at a remote heat exchanger at 80 K and 100 W of cooling at a remote heat exchanger at 30 K. A hybrid expander 400 having a Brayton engine piston with a diameter of 100 mm and a GM displacer with a diameter of 75 mm would provide about 175 W of cooling at a remote heat exchanger at 30 K and no cooling at 100 K. In this design the cooling from the Brayton engine is only used to remove the thermal losses in heat exchanger 40.