HARRISON, Stephen Mark (5 Weedon Close, Cholsey Wallingford, Oxfordshire OX10 9RD, GB)
KRUIP, Marcel Jan Marie (4 Kineton Road, Oxford, Oxfordshire OX1 4PG, GB)
SPACE CRYOMAGNETICS LTD (Building E1, Culham Science Centre Culha, Abingdon Oxfordshire OX14 3DB, GB)
ATKINS, Andrew Farquhar (5 Bennett Close, Chacombe Banbury, Oxfordshire OX17 2JZ, GB)
HARRISON, Stephen Mark (5 Weedon Close, Cholsey Wallingford, Oxfordshire OX10 9RD, GB)
KRUIP, Marcel Jan Marie (4 Kineton Road, Oxford, Oxfordshire OX1 4PG, GB)
CLAIMS
1. Acryogeniccooling arrangement comprising: a therm o siphon pipe of a therm ally conductive material filled with a 5 first liquid cryogen for placement in thermal contact with an article to be cooled; and a sump partially filled with a second liquid cryogen, at a pressure which allows it to boil at an operating temperature of interest, wherein a part of the thermo siphon pipe passes through the sump 10 such that the first liquid cryogen is cooled by the boiling second liquid cryogen to the temperature of interest before circulating through the thermosiphon to cool thearticleto be cooled; and wherein, the first liquid cryogen is held within the thermo siphon pipe at a pressure in excess of its critical pressure at the temperature of 15 interest, so as to eliminate any phase changes over a range of temperatures of operation.
2. A cryogenic cooling arrangement according to claim 1, further comprising a recondensing refrigerator arranged to recondense cryogen
20 vapour in thesump into cryogen liquid.
3. A cryogenic cooling arrangement according to claim 1, wherein cryogen vapour from thesump is allowed to vent to atmosphere.
254. A cryogenic cooling arrangement according to any preceding claim wherein the part of the thermo siphon pipe which passes through the sump includesaspiral or serpentine arrangement.
5. A cryogenic cooling arrangement according to any preceding claim, wherein the sump and the thermo siphon tube each contain a liquid cryogen chosen from the group comprising helium, nitrogen, hydrogen and neon.
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6. A cryogenic cooling arrangement according to any preceding claim, wherein the sump and thethermo siphon tube each contain liquid helium as a cryogen.
107. A cryogenic cooling arrangement according to any preceding claim, wherein the sump and the thermo siphon tube respectively contain different liquid cryogens.
8. A method for cryogeniccooling of an articlecomprising: 15 - providing a sump partially filled with a first liquid cryogen at a pressure which allows it to boil at a temperature of interest; providing a thermo siphon pipe of a thermally conductive material in thermal contact with the article, such that a part of the thermo siphon pipe passes through the sump; 0 - filling the thermo siphon pipe with a second liquid cryogen at a pressure in excess of its critical pressure at the temperature of interest, so as to remain liquid over a range of temperatures of operation, said range of temperatures of operation including temperatures in excess of the temperature of interest; 25 - cooling the second cryogen in the part of the thermo siphon pipe which passes through the sump to the temperature of interest, being the boiling point of thefirst cryogen in thesump; warming the second cryogen in the thermo siphon pipe above the temperature of interest by absorption of heat from the article, such that the second cryogen circulates back through the sump, where it is cooled to the temperature of interest by the first cryogen in the sump.
9. A method according to claim 8 further comprising removing the 5 heat drawn from the cryogen in the thermo siphon pipe by arecondensing refrigerator which condenses boiled cryogen in the sump back into its liquid phase.
10. A method according to claim 8 wherein cryogen vapour from the 10 sump is allowed to vent to atmosphere.
11. A method according to any one of claims 8-10 wherein the first and second cryogens are each chosen from the group comprising helium, nitrogen, hydrogen and neon.
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12. A method according to claim 11 wherein the first and second cryogens are each liquid helium.
13. A method according to claim 11 wherein the first and second 20 cryogens are each liquid nitrogen.
14. A method according to any one of claims 8-11 wherein the first cryogen is different from the second cryogen.
25 15. A cryogenic cooling arrangement substantially as described, and/or as illustrated in Rg.2 of the accompanying drawing.
16. A method for cryogenic cooling of an article substantially as described, and/or as illustrated in Rg.2 of the accompanying drawing. |
ACRYOGENICCOOLINGARRANGEMENTANDMETHOD
The present application relates to cooling apparatus for maintaining a cooled article at a predetermined cryogenic temperature. In particular, it relates to such an arrangement having a low inventory of liquid cryogen, which is reliable in use and maintains a precise cooling temperature.
In certain examples, the present invention may be employed to cool superconducting devices. In order for a superconducting device to operate, it hasto be maintained at a cryogenic temperature. In aclassically designed magnetic resonance imaging (MRI) system, for example, this meansthat the magnet coils and their supporting structure are held within atankthat isflood filled with afluid cryogen to provide immersion cooling of the magnet coils. As the magnet coils are classically made from low temperature superconductor, which has an operating temperature of below 1OK, thefluid cryogen used has typically been helium. Heat transfer away from the magnet coils is then provided by direct contact between the fluid cryogen and the magnet coils. Recently, the cost of helium has risen sharply. This cost rise has led to a desire to reduce the volume of liquid helium required for maintaining the superconducting device's operating temperature.
More recently cryogenic systems have been developed that integrate heat exchangers into the magnet coils. The heat exchangers are integral to the magnet coil structure and so are in good thermal contact with the magnet coils. This then assures the minimum thermal resistance and so temperature difference between the superconductive components of the magnet coils and the fluid cryogen. In this way the heat exchangers transfer heat generated in the magnet coils to fluid cryogen which is
contained in either pi pes laid into the heat exchangers or enclosed channels.
Rg. 1 illustrates an embodiment of such a system , known as a thermo- siphon. The thermo siphon com prises pipe-work 10 arranged about the cooled article, such as superconducti ng magnet coi l windings. Each end of the pipe-work 10 is connected into a sump 12 of relatively small vol ume. In some embodi ments of solenoidal superconducting magnets of 1.0 metre internal diameter, the sum p 12 may have a volume of about 30 litres. A recondensi ng refrigerator 14 is preferably provided, arranged to condense any gaseous cryogen 16 in the sum p i nto liquid cryogen 18 by cooli ng it below its boili ng point. Alternatively, and particularly in em bodi ments where i nexpensive, non-polluti ng cryogens such as nitrogen are em ployed in the sump, there may be no need to provide a recondensi ng refrigerator 14. The boiled off cryogen vapour may si mply be allowed to vent to atmosphere, and be replaced by more liquid cryogen when appropriate. The shape of pipe-work 10 requi red to provide effective cooling to the whole of the cooled article means that meanders 20 are typically required in the pipe-work. Some of these as illustrated at 20a, may provide "inversions" - inverted 'U' shapes i n the pipe-work.
The pipe-work or channels 10 are then connected up such that the worki ng cryogen 18 and thus the heat can be transported around the system . The motive source for this circulation is the natural pum ping caused by warm ing the fluid cryogen 18 in one section and cooling it in another, using a suitable refrigerator 14. Such passively driven thermo-siphon systems have been successfully deployed on cryogenic systems and require a m uch smaller inventory of cryogen than used in im mersion cooled systems discussed above. A thermo-si phon will pump most
effectively when gravity transfers the fl uid cryogen, in liquid phase, from the cold sink of the refrigerator to the heat sou rce, being the cooled article, where boili ng of the cryogen takes place, and gaseous cryogen rises back to the refrigerator's heat si nk. In embodi ments requi ring a meander in the cryogen carryi ng pipe-work 10 it is important that the liquid cryogen level is above the level of the highest inversion 20a. If, as illustrated in Rg. 1 , this requirement is not met then there is a risk that gaseous cryogen collects in the inversion 20a. Such trapped gaseous cryogen may stop circulation of the fluid cryogen. This would lead to localised heating and subsequent quench of the magnet.
In the case of an MRI magnet system there is a maxim um system height im posed on the design by the practicalities of incorporating such a system i nto a building such as a hospital or into a trailer in the case of mobile systems, where such equi pment is typically provided. This means that the requirement for the liquid level being above that of the highest inversion cannot be met, since it may not be possi ble to place the sump 12 on the top of the system unless the refrigerator is orientated on its side, which is not an ideal solution as it reduces the efficiency and increases the time required to service the refrigerator.
One possible option to avoid the collection of gaseous cryogens in inversions which are higher than the level of the cryogen in the sum p is to raise the operating pressure of the fluid cryogen above its critical pressure - in the case of helium , 2.2 bar (2.2x10 5 Pa). The coolant will thus not undergo a phase change and the locki ng behaviour is thus avoided. However, by moving to such a high pressure single phase system , a major benefit of the thermo siphon is also lost.
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That benefit is that, with cooling sumps under atmospheric pressure, or thereabouts, such as shown in Rg.1, the boiling temperature of the liquid cryogen 18 will define the temperature of thecryogen in the sump, so long as the rate of heat energy transfer to the sump does not exceed the cooling power of the sump and any refrigerator 14 provided. Thus at atmospheric temperature with helium used as the transport medium an isothermal wet surface of 4.2K is defined.
By losing this isothermal behaviour by operating at a higher pressure in the system, insufficient, or inconsistent, heating may result.
The present invention addresses the problems caused by the prior art, and provides a system which avoids the presence of gaseous cryogen in inversions, while still providing isothermal behaviour at the boiling point of thecryogen.
The present invention accordingly provides methods and apparatus as defined in the appended claims.
The above, and further, objects, characteristics and advantages of the present invention will become more apparent from a consideration of the following description of certain embodiments thereof, given by way of examples only, in conjunction with the accompanying drawings wherein:
Rg. 1 represents a conventional thermo-siphon cryogenic cooling arrangement, illustrating the problems posed by an inversion extending abovethelevel of theliquid cryogen in thesump; and
Rg. 2 illustrates an embodiment of a cryogenic cooling arrangement according to an embodiment of the present invention.
An embodiment of the invention is illustrated in Rg.2. The invention
5 proposes a two-pressure thermo-siphon cryogenic cooling arrangement.
Pipe-work 30 is provided to cool the cooled article, typically superconducting magnet coils. Whilethe embodiment illustrated in Rg.2 shows multiple pipes arranged in parallel, the pipe work 30 may be arranged as a single coil or serpentine arrangement such as shown in Rg.
101.
According to an aspect of the present invention, the pipes 30 are filled with a fluid cryogen at a pressure above its critical pressure. For example, the pipes 30 may be filled with liquid helium at a pressure of 3 bar (3χ10 5 Pa).
15 At this pressure, the thermo siphon effect continues, but the liquid cryogen does not boil, even when raised to a temperature in excess of its normal boiling point. For this reason, no gaseous cryogen is generated, and there is no risk of an inversion such as 20a in Rg.1 filling with gas and stalling the thermo-siphon effect. 0
According to another aspect of the invention, there is provided a cryogen sump 32 which contains a quantity of liquid cryogen at a pressure below its critical pressure. As with conventional thermo-siphonsand immersion cooled cryostats, the liquid cryogen boils and remains at a steady
25 temperature of its boiling point whilst it remains wet. For example, the sump 32 may be partially filled with liquid helium at a pressure of 1 bar (1x10 5 Pa), that is approximately atmospheric pressure. A recondensing refrigerator 14 is preferably also provided, to cool the boiled-off cryogen gas in sump 32 back into a liquid. Alternatively, and particularly in
embodiments where inexpensive, non-polluti ng cryogens such as nitrogen are em ployed in the sum p, there may be no need to provide a recondensing refrigerator 14. The boi led off cryogen vapour may sim ply be allowed to vent to atmosphere, and be replaced by more liquid cryogen when appropriate.
A certain length 34 of the tube 30 is routed through the sum p 32. This certain length 34 should be arranged so as to provide good thermal contact between the liquid cryogen i n the pipes 30 and the boiling liquid cryogen in the sum p 32. For example, the certain length 34 may include a spiral or serpenti ne arrangement 36 to increase the surface area of contact between the pi pes 30 and the cryogen in the sum p 32. At least part of the certain length 34 should be constructed of a material of high thermal conductivity. For exam ple, copper and al umi nium are inexpensive but suitable materials. Other materials such as silver or gold have superior thermal conduction, but their cost is usually prohibitive.
In operation, the pi pes 30 carrying a liquid cryogen at a pressure in excess of its critical pressure are arranged i n thermal contact with an article to be cooled. In an example, superconductive magnet coils are cooled. Heat is absorbed from the articles to be cooled by thermal conduction through the material of the pipes 30 into the liquid cryogen in the pipes. The pipes 30 are therefore preferably constructed of a material of high thermal conductivity. Again, copper and alum inium are possible exam ples. The liquid cryogen in the pipes 30 may be heated above its normal boil ing point si nce it is held at a pressure in excess of its critical pressure.
Heati ng of the cryogen in the pipes 30 sets up a thermo siphon effect in a known manner by thermal convection flow of the liquid cryogen in pipes
30. The cryogen begins to circulate in the direction shown by arrows 38. The liquid cryogen in the sump 32 is heated by heat transferred from the cryogen in pipes 30 through the walls of the certain length 34 of the pipe 30. This heating causes the liquid cryogen in sump 32 to boil, maintaining a steady temperature of its boiling point. For helium, this is approximately 4.2K at a pressure of 1 bar (1x10 5 Pa). The cryogen in the pipes 30 is accordingly cooled to the steady temperature of the boiling point of the cryogen in thesump 32.
Recirculation of the cryogen in pipes 30 by the thermo siphon effect takes cryogen at the steady temperature of the boiling point of the cryogen in thesump back into the pi pes 30, to provide cooling at that temperature.
The present invention accordingly provides a thermo siphon arrangement which avoids the problems of the prior art. The advantageous effect of avoiding gaseous cryogen locks in inversions 20a of pipes 10 of Rg.1 is provided by filling pipes 30 with a liquid cryogen at a pressure in excess of its critical temperature. This also allows the cryogen in the pipes 30 to be heated above the temperature of its normal boiling point, while remaining in liquid phase. The advantageous effect of providing cooling at a stable temperature is provided by passing the cryogen in tubes 30 through a sump 32 containing a cryogen at its boiling point. This ensures that the cryogen provided in tubes 30 to the articles to be cooled is always at a fixed temperature. In the case of a helium cryogen in sump 32, thistemperature would be4.2K.
While the present invention may be embodied with various modifications and variations from the particularly disclosed embodiments, the following features are believed to be essential to the present invention.
Asump is provided, to contain a boiling cryogen. Athermo siphon pipe is provided for thermal contact with the article(s) to be cooled, and part of thethermo siphon pipe passesthrough thesump. Thethermo siphon pipe is of athermally conductive material.
In use, the thermo siphon pipe is filled with a liquid cryogen at a pressure in excess of its critical pressure at the temperature of interest, so as to remain liquid over a range of temperatures of operation. The sump is partially filled with a liquid cryogen, at a pressure which allows it to boil at an operating temperature of interest. Cryogen in thethermo siphon pipe 30 is cooled in the sump to the operating temperature of interest, being the boiling point of the cryogen in the sump. The cryogen in the thermo siphon pipe is then warmed above the boiling point of the cryogen in the sump as it passesthrough the pipe 30 in thermal contact with thearticle(s) to be cooled. By thethermo siphon effect, the warmed cryogen in tubes 30 is circulated back through the sump, where it is cooled by boiling of the cryogen in the sump, back to the operating temperature of interest. The heat drawn from the cryogen in pipe 30 may be removed by a recondensing refrigerator 14 which condenses boiled cryogen in thesump back into liquid phase.
Whilethe present invention has been particularly described with reference to a helium cryogen, suitable to cool conventional superconducting materials, the present invention may be applied to cool to boiling point temperatures of other cryogens. For example, the sump and the thermo siphon may each be filled with liquid nitrogen, at appropriate pressures, to provide cooling at a temperature suitable for so-called high temperature superconductors. In embodiments where inexpensive, non-polluting
cryogens such as nitrogen are em ployed i n the sump 32, there may be no need to provide a recondensing refrigerator 14. The boiled off cryogen vapour may sim ply be allowed to vent to atmosphere, and be replaced by more liquid cryogen when appropriate. In certain em bodiments, different cryogens may be provided in the sum p and the thermo si phon pipe. In preferred embodi ments of the present i nvention, the or each cryogen may be selected from the group comprising helium , nitrogen, hydrogen and neon, although other cryogens may be em ployed to suit a particular application. Care should be taken, however, that the cryogen in the thermo siphon pipe remains l iquid at the tem peratures and pressures appl ied to the therm o si phon pipe.
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