In order to reach very low temperatures (around 4°K) it is therefore necessary in practice to introduce a non-regenerative

cooling stage, and it is known in this context to make use of the Joule-Thomson (J-T) expansion effect, namely that a gas, under high pressure and at a temperature below its inversion temperature, becomes cooled when it is allowed to expand through a flow constrictor to a lower pressure. However, the inversion temperatures of many gases, including helium, are well below ordinary room temperature, and therefore they must first be pre-cooled before they can be further cooled by use of the J-T effect. The required pre-cooling may be effected by means of any suitable refrigerating apparatus, which may, for example, be a Stirling-cycle refrigerator such as one of those referred to above.

The present Invention relates, therefore, in one of its aspects, to a multi-stage cryogenic cooling apparatus having a closed-loop J-T expansion stage and at least one pre-cooler stage, the J-T stage comprising a gas compressor, a J-T expansion block, having an Inlet arranged to receive high pressure gas via a high-pressure line from the compressor and constituted as a flow-restricting expansion valve therefor and an outlet connected to the compressor via a low-pressure 'return line, and a J-T stage heat exchanger 1n which the high-pressure line and the low-pressure return line are in heat-exchanging relationship, and the pre-cooler stage being arranged to pre-cool gas in the high-pressure line before 1t enters the J-T stage heat exchanger. Such a cryogenic cooling apparatus is referred to hereinafter as apparatus of the defined kind.

In such apparatus of this defined kind, high pressure gas from the compressor is pre-cooled by the pre-cooler stage before passing, via the J-T stage heat exchanger, to the flow-restricting expansion valve through which it expands into the expansion block with the effect of cooling both itself and the expansion block. The resulting low-pressure gas, now at the lowest temperature in the whole system, returns from the expansion block to the compressor via the low-pressure return line, and in doing so it passes through the J-T stage heat

exchanger where it is in heat-exchanging relationship with the high-pressure gas, which is thereby cooled, before it reaches the expansion valve, to a temperature below that already achieved by means of the pre-cooler stage. The above-described further cooling of the high-pressure gas, below the temperature to which it ts pre-cooled by the pre-cooler stage, leads to a progressive cooldown of the expansion block until finally it (and the expanded low pressure gas whose temperature it follows) are at or just above the boiling point of the gas at its pressure on the low-pressure side of the expansion valve; but the rate at which this progressive cooldown occurs is related to the mass flow rate of the gas through the pre-coόler stage (since this governs the rate of heat removal from the J-T expansion stage). This leads to a problem, because the gas density at a given pressure decreases and its viscosity increases, with increasing temperature, with the result that a constricted expansion valve designed to provide a given mass flow rate at the very low designed operating temperature of the expansion block will limit the flow rate at higher temperatures to only a small fraction of the designed mass flow rate and will thus seriously limit the cooling effect and the rate of cooldown in the J-T stage. It has been proposed to overcome this problem by providing a variable orifice as the expansion valve and decreasing its size, as the temperature falls, until finally it allows the designed mass flow rate of gas at the low designed operating temperature of the expansion block; but this requires moving parts which are. accurately controllable at very low temperatures, a requirement which is very difficult to implement reliably - especially in, for example, a miniature helium refrigerator with a designed flow rate of only a few milligrams per second at a designed operating temperature of 4° . A variant which has also been proposed is to provide a fixed-orifice expansion valve dimensioned appropriately for the operating low-temperature conditions and to provide, in parallel with it and also within the expansion block, a bypass valve which, when

open, has a much larger orifice and allows a correspondingly increased flow of gas from the high-pressure line into the expansion block and thence into the low-pressure return line. In this case, the resulting increased flow rate of gas through the J-T stage heat exchanger (in both directions) and through the expansion block while the bypass valve is open leads to a more rapid cooldown of both components to the temperature of the pre-cooler stage, after which the bypass valve 1s closed so that subsequent flow is only through the constricted expansion valve; but this variant also suffers from the disadvantage that the bypass valve is required to be operable to close it in low-temperature conditions.

It is an object of the present invention to provide, in apparatus of the defined kind, means whereby the rate of cooldown of the J-T expansion stage may be increased without the use of components having moving parts which are required to operate under low-temperature conditions.

To that end, according to the invention, the high-pressure gas line of the J-T stage of apparatus of the defined kind is provided, upstream of its Interaction with the pre-cooler stage, with a branch leading through a bypass valve (when open) to a bypass line which opens Into the expansion block and offers a less constricted gas route than the flow-restricting expansion valve, the pre-cooler stage being arranged to cool gas flowing in the bypass line, downstream of the bypass valve, before it reaches the expansion block, and the bypass line then leading direct from the pre-cooler stage to the expansion block without passing through the J-T stage heat exchanger.

More generally, it is an object of the invention to provide a controlled connection, or heat switch, between a source of cryogenic cooling and an item which is to be cooled thereby, and to do so by the use of control valve means which is not itself subjected to the cryogenic temperatures involved. According to this more general aspect of the Invention, the invention provides cooling means comprisng a source of flow of a fluid, a supply

line for supplying fluid from said source to a first heat exchanger, where it is cooled by a source of cryogenic cooling, and thereafter to a second heat exchanger where it is in heat-exchanging relationship with an item to be cooled by the cryogenic cooling source, and a return line for return flow of the fluid from the second heat exchanger to the fluid flow source, the return line and the supply line between the fluid flow source and the first heat exchanger being In heat exchange relationship with one another in a third heat exchanger, wherein there is included in the supply line or the return line between the fluid flow source and the third heat exchanger a control valve whereby the flow of fluid through the supply line and from the first to the second heat exchanger can be controlled.

In terms of this more general aspect of the invention, cooling means according to the invention as just outlined, and having the said control valve included 1n the said supply line, may have, in parallel with the latter through the third heat exchanger (in heat exchanging relationship with the return line) and through the first heat exchanger (to be cooled by the cryogenic cooling source) a further supply line connected to supply fluid from the fluid source to the second heat exchanger, with the second heat exchanger constituting a Joule-Thomson expansion block and the further supply line opening thereinto through an inlet constituted as a flow-restricting expansion valve therefor. The return line then constitutes a fluid outlet from the Joule-Thomson expansion block, and the fluid supply line having the said control valve connected in it opens into the expansion block through a less restricting inlet than that provided for the said further supply line, whereby fluid flow into the expansion block is preferentially through the supply line havi.ng the control valve connected in it or through the further supply line, respectively, according as the control valve is open or closed.

These and other aspects and advantageous and preferred features of the invention will be disclosed below 1n the

following description with reference to the accompanying drawings, in which:-

Figure 1 is a schematic diagram of apparatus of the defined kind which incorporates the invention, Figure 2 is a view, partly 1n elevation and partly in axial longitudinal section, of a preferred practical embodiment of that part of the apparatus represented in

Figure 1 which is shown in the right-hand half of that

Figure, and Figure 3 is a schematic diagram of apparatus in which the invention provides a heat switch between a source of cryogenic cooling and an item which is to be cooled thereby.

The cryogenic cooling apparatus represented 1n Figure 1 comprises a closed-loop J-T expansion stage, using helium as its working fluid, and a two-stage Stirling-cycle refrigerator which provides two successive pre-cooler stages for the helium of the

J-T stage. The Stirling-cycle refrigerator, which is of the known kind described 1n the above-cited paper by the inventors of the present invention, comprises a pair of electrically driven compressors 11 and 12 which are mounted rigidly with respect to one another, in alignment but in mechanical opposition so that cyclical momentum changes in the one are balanced and cancelled out by the equal and opposite changes in the other, and which act in phase with one another, though a common output line 13, on a displacer unit 14 in which accordingly they effect alternate compression and decompression, suitably at a cycle frequency of about 35Hz, of the Stirling-cycle working fluid which conveniently may also be helium. The displacer unit 14 comprises a stepped cylinder having larger-diameter and small-diameter sections 15 and 16 respectively within which a stepped displacer piston (not shown) is reciprocated, by electrical drive means

(not shown, but housed within a housing 17), at the same frequency as that of the compressors 11 and 12 but with a phase displacement of approximately one quarter of ^* a cycle relative thereto. Preferably the displacer unit drive means 1s a. oving

coil motor comprising, in known manner, a coil mounted on the stepped displacer piston for axial movement therewith and disposed in a coaxial annular gap of a permanent magnet system so as to be excited into axial oscillation when supplied with an alternating current from an a.c. current source (not shown); and the compressors 11 and 12, preferably, similarly comprise moving coil motors supplied, with the required phase displacement relative to the displacer unit, with driving current from the same source. Larger-diameter and smaller-diameter sections of the stepped piston of the displacer unit 14 are both hollow to accommodate respective axially-extending regenerator units communicating at their ends with respective working chambers defined between the stepped cylinder 15, 16 and the stepped piston disposed within it; two of these chambers are located, within the stepped cylinder, at the upper ends of its sections 15 and 16 respectively, and operation of the Stirling-cycle refrigerator results in cooling of the adjacent parts of the cylinder wall, and of respective thermally-conductive collars 18 and 19 mounted thereon in good thermal contact therewith, to temperatures which may be as low as about 100°K and 20° respectively. The collar 18 has two apertures in which are mounted two pre-cooler units 20 and 21 which are in good thermal contact with the collar 18; and the collar 19 is similarly provided with two further gas pre-cooler units 22 and 23.

The pre-cooler units 20 and 21, and 22 and 23, provide pre-cooler stages for the Joule-Thomson section of the apparatus. This comprises a compressor unit composed of two compressors 25 and 26 arranged in series with a buffer volume or receiver 27 between them. The compressors 25 and 26 are preferably similar to the compressors 11 and 12 and, like them, mounted in alignment but in mechanical opposition so that oscillating momentum forces tend to cancel one another; but the compressors 25 and 26 differ in that they, unlike the compressors 11 and 12, are fitted with one-way inlet and outlet valves so

that low-pressure helium drawn into the compressor 25 is fed under pressure into the receiver 27 and is then further compressed by the compressor 26 and fed to a high pressure gas line 28 fitted, preferably, with a liquid nitrogen trap 29 and a getter 30 for other Impurities in the helium. The trap 29 and getter 30 are shown in Figure 1 as being introducible into and removable from the line 28 at will, by appropriate operation of associated valves; but in practice the trap 29, which is used as the means by which the Joule-Thomson section of the apparatus is filled with its working fluid, would usually thereafter be permanently removed whereas the getter 30 would usually be left permanently in the circuit.

The high-pressure line 28 has a branch 29 leading to a normally closed valve 30 which, when open, allows helium into a bypass line 31; and the high-pressure line 28 and the bypass line 31 pass together via a manifold 32 into a first countercurrent heat exchanger 33 in which they are in heat- exchanging relationship with low-pressure helium which has undergone the Joule-Thomson expansion and which emerges from the manifold 32 to connect via a return, line 34 with a low-pressure helium receiver 35 which supplies the inlet side of the compressor 25. At its end remote from the manifold 32, the heat exchanger 33 has a manifold 36 from which the high-pressure line 28 and bypass line 31 emerge to open into the pre-cooler units 20 and 21 respectively. Extensions 28a and 31a of the lines 28 and 31 respectively then lead from the pre-cooler units 20 and 21 respectively through a manifold 37 into a second countercurrent heat exchanger 38, to emerge therefrom via a manifold 39 and open into the pre-cooler units 22 and 23 respectively. A further extension 28b of the high pressure line 28 leads from the pre-cooler unit 22 via a manifold 40 into a third countercurrent heat exchanger 41 from which it emerges via a manifold 42 to pass finally via a filter 43a and an inlet line 43b into the expansion chamber of a Joule-Thomson expansion block 43 in which the Inlet line 43b terminates in a restricted-orifice expansion valve 44.

The pre-cooler unit 23, on the other hand, is connected by a further extension 31b of the bypass line, which bypasses the third heat exchanger 41, directly into the expansion chamber of the expansion block 43, into which it opens without any constriction comparable to the expansion valve 44.

The low-pressure return line 34 opens, through the manifold 32, to the space surrounding the high-pressure and bypass lines 28 and 31 within the outer, tube of the heat exchanger 33, and that space communicates through the manifold 36 and a duct 34a with the manifold 37 and, therethrough, with the similar space within the outer tube of the heat-exchanger 38. That space, similarly, communicates through the manifold 39 and a duct 34b with the manifold 40 and, therethrough, with the space surrounding the high-pressure line section 28b within the outer tube of the heat exchanger 41; and the space within the heat exchanger 41 communicates, through the manifold 42, with the expansion chamber of the expansion block 43 by means of a low-pressure outlet line section 34c which includes a load 45 whose cryogenic cooling 1t is the purpose of the above-described apparatus to provide. Thus low-pressure helium, leaving the expansion block 44 through the outlet section 34c, flows in turn through the load to be cryogenically cooled and then through the heat exchangers 41, 38 and 33 and, via the line 34, back into the receiver 35. With the valve 30 open, compressed helium flowing through the bypass line 31 Is cooled in the heat exchangers 33 and 38 by countercurrent heat exchange with the expanded helium returning to the receiver 35, and also by its passage through the pre-cooler units 21 and 23 which are chilled to about 100°K and 20°K respectively. The relatively large rate of flow of helium through this route, via the valve 30, enables the temperature of the expansion block 43 to be reduced relatively quickly to a level at which the J-T effect is efficient and flow rate through the valve 44 aproaches its designed value. Closure of the valve 30 then prevents further flow through the bypass route, and

subsequent flow of high-pressure helium from the line 28 is through all three heat exchangers 33, 38 and 41, as well as through the two pre-cooler units 20 and 22., whereafter the expansion of the helium through the expansion valve or nozzle 44 provides the final cooling down to about 4°K. In this final, operating, condition of the apparatus there will be a substantial temperature difference between the expansion block 43 and the pre-cooler unit 23, between which the final section 31b of the now-inoperative bypass line extends; but 1t should be noted that undesired thermal leakage along the section 31b can be made satisfactorily small because section 31b will usually be a fine tube of small cross-section and can be of substantial length".

A practical embodiment of an assembly constituting the major part of the right-hand side of Figure 1 is shown in Figure 2, in which the same reference numerals are used as for the corresponding elements in Figure 1. As shown in Figure 2, the larger- and smaller-diameter sections 15 and 16 of the stepped cylinder of the displacer unit 14 of the Stirling-cycle refrigerator constitute a central spine around which the assembly Is built. The collar 18, mounted on the shoulder between the sections 15 and 16, has two apertures in which the pre-cooler units 20 and 21 respectively are received as interference fits and thereby located; and the pre-cooler units 22 and 23 are similarly located as interference fits in apertures in the collar 19 which is secured on the free upper end of the section 16. Also mounted on the upper end of the section 16 are two pillars 46 of a good thermal insulating material, on the upper ends of which is mounted a thermally conductive support 47 on which the filter 43a and the Joule-Thomson expansion block 43 are secured in thermal contact with the support and thus with one another.

The three heat exchangers 33, 38 and 41 in this embodiment are all, as shown in Figure 2, of the coiled tube-in-tube type.

An annular mandrel 48 is secured in place round the displacer unit cylinder section 15, coaxial therewith, and the heat exchanger 33 is coiled round the mandrel, seated in a spiral

groove 49 thereof. At its upper end, the outer tube of the heat exchanger 33 is brazed into a lateral opening of the manifold 36 and thereby opens into an axial bore of the manifold. The high-pressure line 28 and bypass line 31 emerging from the end of 5 the heat exchanger outer tube extend across the axial bore of the manifold 36 and out of the manifold through two small lateral openings, in which they are sealed by brazing, opposite the larger bore in which the end of the outer tube of the heat exchanger 33 1s brazed (and thereby sealed). The emerging

10 high-pressure line 28 and bypass line 31 are led to apertures in the upper ends of the pre-cooler units 20 and 21 respectively, in which they are brazed so as to seal those apertures whilst being communication with the interiors of the units.

The manifold 37 for the heat exchanger 38 is brazed in place

15 on the manifold 36 and has an axial internal bore communicating with that of the manifold 36 and constituting therewith the duct 34a identified in Figure 1. The manifold 37 has a lateral opening in which the lower end of the outer tube of the heat exchanger 38 is brazed, and thereby sealed, in communication with

20 the duct 34a. The inner tubes 28a and 31a of the heat exchanger 38, where they emerge from the lower end of its outer tube, extend across the duct 34a and emerge from the manifold 37 through two lateral openings (in which they are sealed by brazing) to be led to apertures in the lower ends of the

25 _. pre-cooler units 20 and 21 respectively into which they are sealed by brazing so as to be in communication through the units 20 and 21 with the gh-pressure line 28 and the bypass line 31 respectively.

The manifolds 39 and 40 are formed and connected in similar

30 manner as the manifolds 36 and 37, so that they provide an internal duct 34b through which the outer tubes of the heat exchangers 38 and 41 are in communication with one another. The upper ends of the inner tubes 28a and 31a of the heat exchanger 38 emerge from the manifold 39 and are sealed into the lower ends

35 of the pre-cooler units 22 and 23 respectively, and the single

inner tube 28b of the heat exchanger 41 emerges at its lower end from the manifold 40 and is sealed into the upper end of the pre-cooler unit 22. The upper end of the tube 28b emerges from the manifold 42 and is sealed into the lower end of the filter 43a, the upper end of which is connected to the Joule-Thomson expansion block 43 by the Inlet line 43b which terminates, within the block 43, in the restricted orifice or valve 44 through which the Joule-Thomson expansion takes place. The bypass line extension 31b, which bypasses the heat exchanger 41, extends from the upper end of the pre-cooler unit 23, is led past the filter

43a (in good thermal contact with it so as to cool it) and opens

Into the upper end of the Joule-Thomson block 43 adjacent the valve 44 but without itself having any comparable constriction.

The outlet duct 34c from the base of the block ^' 43 leads to the load (45 in Figure 1, but not shown 1n Figure 2) which is to be cooled cryogenically, and the return duct 34c ¹ from the load communicates through the manifold 42 with the interior of the outer tube of the heat exchanger 41. The ducts 34c and 34c' are preferably not in direct thermal contact, but are mechanically located relative to one another by a spacer member 50, which supports the weight of the heat exchanger 41.

It will be seen that the assembly of the heat exchangers 33, 38 and 41 together with the manifolds 36, 37, 39, 40 and 42 forms an integrated structure which is supported at its upper end by the spacer member 50 and at its lower end by the mandrel 48 but which 1s otherwise out of physical and thermal contact with the remainder of the apparatus apart from the connections of the ends of the heat-exchanger inner tubes to the pre-cooler units 20, 21, 22 and 23. This arrangement is effective to minimise unwanted heat leakage between the heat exchangers and other parts of the apparatus. The desired heat transfers within the pre-cooler units are maximised by providing them with a gas-permeable filling, such as the illustrated filling 20a of the unit 20, which has high thermal conductivity and 1s ^* in good thermal contact with the walls of the pre-cooler unit and therethrough

with the cold collar 18 or 19 respectively. The filling 20a may be in the form, for example, of a stack of circular discs cut from a sheet of metal gauze, or may be a strip of such gauze wound into a roll. The filter 43a may be provided with a similar filling to act as a filter element, and a similar filling may also be provided in the expansion block 43 to maximise thermal contact with the cold expanded gas issuing from the expansion nozzle 44.

As suggested earlier, the provision of the valve 30, remote from cryogenic conditions, to control the flow of helium through the pre-cooler units 21 and 23 and thence to the expansion block 43 to effect cryogenic cooling thereof in the apparatus illustrated in Figures 1 and 2 may be seen as one aspect or instance of the invention. Another embodiment of the invention, in its broader aspect as referred to above, is illustrated in Figure 3 and will now be described with reference thereto.

As shown 1n Figure 3, a source 55 of cryogenic cooling is represented by a Stirling-cycle refrigerator, and an item 56 is to be cooled by it, under control of a valve which is not, itself, to be subjected to the cryogenic conditions. There is therefore provided a circulating pump 57 with one-way inlet and outlet valves, for providing a flow of fluid through a supply line 58 to a first heat exchanger 59 in which it is cooled by the cryogenic cooling source 55 and thereafter to a second heat exchanger 60 in which it is in heat exchanging relationship with the item 56 which is to be cooled. A return line 61 for flow of the fluid from the heat exchanger 60 back to the pump 57 is also provided, as is a third heat exchanger 62 in which the return line 61 is in heat-exchanging relationship with the supply line 58 between the pump 57 and the first heat exchanger 59. Between the pump 57 and the third heat exchanger 62 there is provided (in the supply line 58 as illustrated, though it might equally well be in the return line 61) a valve 63 by which fluid flow through the supply line to the heat exchanger 59, and from it to the heat exchanger 60, can be controlled. The circuit just described may

be one of a plurality of such circuits, all supplied by the pump 57 : thus a second such circuit, controlled by a valve 63' and including a heat exchanger 62', may be provided for cooling the item 56 by means of a heat exchanger 60' receiving cooled fluid from a heat exchanger 59' which is cooled by a second source 55' of cryogenic cooling.

With the pump 57 operating, opening the valve 63 causes fluid to flow through the heat exchanger 59 and be cooled by the cooling source 55, and thereafter to cool the Item 56 through the heat exchanger 60. The heat exchanger 62, which may be of tube-in-tube type, operates to ^" minimise the unwanted heat load on the cooling source 55. If the source 55 should fail, closing the valve 63 effectively isolates it from the Item 56; and opening of another valve, such as the valve 63', enables cooling of the item 56 to be continued by an alternative cooling source, such as the source 55', in one of the alternative circuits. Alternatively, in normal operation the Item 56 may be cooled simultaneously by a plurality of cooling sources such as the source 55, with a plurality of the valves such as the valve 63 being normally open. In that case if one of the cooling sources fails it may be isolated from the item 56 by closing the corresponding valve, with the result that the failed cooling source Imposes minimum heat loading on the item 56.