GB2427672A

GB2427672A - A cryogenic cooling arrangement

Info

Publication number: GB2427672A
Application number: GB0513368A
Authority: GB
Inventors: Andrew Farquhar Atkins; Stephen Mark Harrison; Marcel Jan Marie Kruip
Original assignee: Siemens Magnet Technology Ltd; Space Cryomagnetics Ltd
Current assignee: Magnetica Ltd; Siemens Magnet Technology Ltd
Priority date: 2005-06-30
Filing date: 2005-06-30
Publication date: 2007-01-03
Also published as: GB0513368D0; WO2007003499A1

Abstract

The present invention provides a cryogenic cooling arrangement comprising a thermo siphon pipe (30) of a thermally conductive material for placement in thermal contact with an article to be cooled; and a sump (32) for containing boiling cryogen, wherein a part of the thermo siphon pipe passes through the sump. Preferably, 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, and the sump is partially filled with a liquid cryogen, at a pressure which allows it to boil at an operating temperature of interest A recondensing refrigerator (14) may be provided to cool the boiled off cryogen gas in a sump (32) back into a liquid.

Description

A CRYOGENIC COOLING ARRANGEMENT AND METHOD

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 has to be maintained at a cryogenic temperature. In a classically designed magnetic resonance imaging (MRI) system, for example, this means that the magnet coils and their supporting structure are held within a tank that is flood filled with a fluid 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 10K, the fluid 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 * : 20 temperature. S..

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 pipes laid into the heat exchangers or enclosed channels.

Fig. 1 illustrates an embodiment of such a system, known as a thermosiphon. The thermo siphon comprises pipe-work 10 arranged about the cooled article, such as superconducting magnet coil windings. Each end of the pipe-work 10 is connected into a sump 1 2 of relatively small volume.

In some embodiments of solenoidal superconducting magnets of 1.0 metre internal diameter, the sump 12 may have a volume of about 30 litres. A recondensing refrigerator 14 is preferably provided, arranged to condense any gaseous cryogen 16 in the sump into liquid cryogen 18 by cooling it below its boiling point. Alternatively, and particularly in embodiments where inexpensive, non-polluting cryogens such as nitrogen are employed in the sump, there may be no need to provide a recondensing refrigerator 14. The boiled off cryogen vapour may simply be allowed to vent to * atmosphere, and be replaced by more liquid cryogen when appropriate.

:.: The shape of pipe-work 10 required 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 * *: 20 "inversions" - inverted U' shapes in the pipe-work.

The pipe-work or channels 10 are then connected up such that the working cryogen 18 and thus the heat can be transported around the system. The motive source for this circulation is the natural pumping caused by warming 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 much smaller inventory of cryogen than used in immersion cooled systems discussed above. A thermo-siphon will pump most effectively when gravity transfers the fluid cryogen, in liquid phase, from the cold sink of the refrigerator to the heat source, being the cooled artic]e, where boiling of the cryogen takes place, and gaseous cryogen rises back to the refrigerator's heat sink. In embodiments requiring a meander in the cryogen carrying pipe-work 10 it is important that the liquid cryogen level is above the level of the highest inversion 20a. If, as illustrated in Fig. 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 maximum system height imposed on the design by the practicalities of incorporating such a system into a building such as a hospital or into a trailer in the case of mobile systems, where such equipment 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 possible to place the sump 12 on the top S...

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 * .:. 20 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 sump is to raise the operating pressure of the fluid cryogen above its critical pressure - in the case of helium, 2.2 bar (2.2xl0 Pa). The coolant will thus not undergo a phase change and the locking 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.

That benefit is that, with cooling sumps under atmospheric pressure, or thereabouts, such as shown in Fig. 1, the boiling temperature of the liquid cryogen 18 will define the temperature of the cryogen 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 the cryogen. * S.

The present invention accordingly provides methods and apparatus as defined in the appended claims. S. * S * *5*

*. 20 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: Fig. 1 represents a conventional thermo-siphon cryogenic cooling arrangement, illustrating the problems posed by an inversion extending above the level of the liquid cryogen in the sump; and Fig. 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 Fig. 2. The invention proposes a two-pressure thermo-siphon cryogenic cooling arrangement.

Pipe-work 30 is provided to cool the cooled article, typically superconducting magnet coils. While the embodiment illustrated in Fig. 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 Fig. 1.

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 (3x10 Pa).

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 Fig. 1 filling with gas and stalling the thermo-siphon effect. :. 20

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-siphons and immersion cooled cryostats, the liquid cryogen boils and remains at a steady 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 (1x105 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-polluting cryogens such as nitrogen are employed in the sump, there may be no need to provide a recondensing refrigerator 14. The boiled off cryogen vapour may simply 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 sump 32. This certain length 34 should be arranged so as to provide good thermal contact between the liquid cryogen in the pipes 30 and the boiling liquid cryogen in the sump 32. For example, the certain length 34 may include a spiral or serpentine arrangement 36 to increase the surface area of contact between the pipes 30 and the cryogen in the sump 32. At least part of the certain length 34 should be constructed of a material of high thermal conductivity.

For example, copper and aluminium 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 pipes 30 carrying a liquid cryogen at a pressure in excess of its critical pressure are arranged in thermal contact with an article to be *:, 20 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 aluminium are possible examples. The liquid cryogen in the pipes 30 may be heated above its normal boiling point since it is held at a pressure in excess of its critical pressure.

Heating 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 Pa). The cryogen in the pipes 30 is accordingly cooled to the steady temperature of the boiling point of the cryogen in the sump 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 the sump back into the pipes 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 Fig. 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, this temperature would be 4.2K.

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.

A sump is provided, to contain a boiling cryogen. A thermo siphon pipe is provided for thermal contact with the article(s) to be cooled, and part of the thermo siphon pipe passes through the sump. The thermo siphon pipe is of a thermally 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 boll at an operating temperature of interest. Cryogen in the thermo siphon pipe 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 passes through the pipe 30 in thermal contact with the article(s) to be cooled. By the thermo 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 :. 20 recondensing refrigerator 14 which condenses boiled cryogen in the sump back into liquid phase.

* : While the 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. Tn embodiments where inexpensive, non-polluting cryogens such as nitrogen are employed in the sump 32, there may be no need to provide a recondensing refrigerator 14. The boiled off cryogen vapour may simply be allowed to vent to atmosphere, and be replaced by more liquid cryogen when appropriate. In certain embodiments, different cryogens may be provided in the sump and the thermo siphon pipe. Care should be taken, however, that the cryogen in the thermo siphon pipe remains liquid at the temperatures and pressures applied to the thermo siphon pipe. * ** * S * S.. * a... * S a... *. * S * SSS **

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Claims

1. A cryogenic cooling arrangement comprising: - a thermo siphon pipe of a thermally conductive material for placement in thermal contact with an article to be cooled; and - a sump for containing boiling cryogen, wherein a part of the therrno siphon pipe passes through the sump.

2. A cryogenic cooling arrangement according to claim 1 wherein, 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 eliminate any phase changes over a range of temperatures of operation, and the sump is partially filled with a liquid cryogen, at a pressure which allows it to boil at an operating temperature of interest.

* **

3. A cryogenic cooling arrangement according to claim 1 or claim 2, further comprising a recondensing refrigerator arranged to recondense cryogen vapour in the sump into cryogen liquid. a. * 4 * *4*

4. A cryogenic cooling arrangement according to claim 1 or claim 2, wherein cryogen vapour from the sump is allowed to vent to atmosphere. *s**a a a

5. A cryogenic cooling arrangement according to any preceding claim wherein the part of the thermo siphon pipe which passes through the sump includes a spiral or serpentine arrangement.

6. A cryogenic cooling arrangement according to any preceding claim, wherein the sump and the thermo siphon tube each contain liquid helium as a cryogen. -11 -

7. 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 cryogenic cooling of an article comprising: - 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; - 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; :.: - cooling the second cryogen in the part of the thermo siphon pipe S.. .

which passes through the sump to the temperature of interest, being the boiling point of the first cryogen in the sump; * .*.

:. 20 - 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 heat drawn from the cryogen in the thermo siphon pipe by a recondensing refrigerator which condenses boiled cryogen in the sump back into its liquid phase. - 12-

10. A method according to claim 8 wherein cryogen vapour from the 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 liquid helium.

12. A method according to any one of claims 8-10 wherein the first and second cryogens are each liquid nitrogen.

13. A method according to any one of claims 8-10 wherein the first cryogen is different from the second cryogen.

14. A cryogenic cooling arrangement substantially as described, and/or as illustrated in Fig. 2 of the accompanying drawing.

* ,*

15. A method for cryogenic cooling of an article substantially as :: e described, andlor as illustrated in Fig. 2 of the accompanying drawing. I. * * * *e.

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