METHOD FOR EXTRACTING A LIQUID PHASE OF A CRYOGEN FROM A STORAGE DEWAR

Abstract

A method for extracting a liquid phase of a cryogen comprising the liquid phase and a vapour phase from an interior volume of a storage dewar through an extraction means, utilizing a push gas introduced into the vapour phase of the cryogen through an outlet of a supply line provided between a push gas supply and the interior volume of the storage dewar, the supply line partially extending through the liquid phase within the interior volume.

Claims

1. A method for extracting a liquid phase of a cryogen comprising the liquid phase and a vapour phase from an interior volume of a storage dewar through an extraction means, utilizing a push gas introduced into the vapour phase of the cryogen through an outlet of a supply line provided between a push gas supply and the interior volume of the storage dewar, the supply line partially extending through the liquid phase within the interior volume and wherein the push gas is led through a heat exchanger provided in a section of the supply line within the liquid phase.

2. The method according to claim 1, wherein the push gas is a helium gas.

3. A storage dewar defining an interior volume comprising a lower section and an upper section for storage of a cryogenic, comprising an extraction means for extracting the cryogenic from the interior volume, and a supply line for introducing a push gas into the interior volume, the supply line extending, in a first section, from the upper section of the interior volume to the lower section, and then, in a second section, back from the lower section to the upper section, an outlet, through which push gas exits the supply line being provided at or in the vicinity of a terminal end of the supply line in the upper section of interior volume, and wherein the supply line comprises an extension reversal section in the lower part of the interior volume and wherein the supply line is provided with a heat exchanger in the lower part of the interior volume.

4. The storage dewar according to claim 3, wherein the first section of the supply line within the interior volume is provided as a coaxial vacuum jacketed pipe, and/or the second portion of the supply line within the interior volume is provided as a single walled pipe.

5. The storage dewar according to claim 3, wherein the heat exchanger is arranged between the first section and the second section of the supply line.

6. The storage dewar according to claim 3, wherein the second section of the supply line is arranged concentrically around the first section.

7. The storage dewar according to claim 6, wherein the heat exchanger is provided concentrically around the first section and concentrically within the second section of the supply line.

Description

[0027] FIG. 1 shows a schematic side view of a system for performing a preferred embodiment of the method according to the invention including a preferred embodiment of a storage dewar used for filling a magnet of an MRI,

[0028] FIG. 2 shows a further preferred embodiment of a storage dewar, and

[0029] FIG. 2 a more detailed side sectional view of a section of a pipe or supply line by means of which push gas is introduced into the storage dewar of FIG. 2.

[0030] In FIG. 1, a cryogenic storage dewar for storing and transporting a cryogen 14 is generally designated 10. The cryogen comprises a liquid phase 14a and a vapour phase 14b above the liquid phase 14a. The dewar 10 serves to refill an MRI magnet 20 with liquid phase cryogen. Also, a cylinder 30 serving as a push gas supply is shown, which constitutes an external source for push gas. Cylinder 30 has a volume of preferably 10 to 20 litres. Be it noted that this volume is substantially smaller than that of cylinders used in conventional systems, which typically use cylinders with volumes of around 40 to 50 litres. This reduction in size, which also reduces transport costs, is possible due to the fact that a substantially smaller amount of push gas is required according to the invention, as will be further explained below. The push gas is pressurised at ambient or room temperature within the cylinder 30. Dewar 10 and cylinder 30 are made of non magnetic materials.

[0031] Dewar 10 is an insulating storage vessel and comprises an outer shell 11a and an inner shell 11b, the space 11c between inner and outer shell being partially evacuated. At its lower or bottom side, the dewar 10 can be provided with transportation means, such as wheels 11d. The space surrounded by inner shell 11b is defined as interior space 12 of the dewar 10.

[0032] At its upper side, the dewar 10 is provided with a sealable opening 11e, through which the dewar can be filled with cryogen. The sealable opening is provided with a top valve 16, through which liquid cryogen can be extracted from the dewar 10 and transported to the MRI magnet 20, as will be explained in the following.

[0033] Be it assumed in the following that the cryogen 14 included in the interior volume 12 of the dewar 10 is helium. This helium comprises liquid phase 14a, and above this liquid phase vapour phase 14b, as mentioned above. Under storage conditions, the liquid phase 14a and the vapour phase are in thermodynamic equilibrium. A typical temperature within the interior volume of the dewar 10 is 4.2 K. The push gas contained in cylinder 30 is also helium, which has ambient temperature, i.e. around 300 K.

[0034] The push gas from cylinder 30 can be introduced into the dewar via a supply line 18. As indicated in FIG. 1, the supply line 18 is provided with valves 18a, 18b, 18c, and extends from the cylinder 30 into the interior space 12 of dewar 10. The pressure of the push gas entering the dewar can be regulated by a conventional 2-stage high accuracy gas pressure regulator 32. On the side of the dewar 10, supply line 18 extends from valve 18c through the upper side of dewar 10, passing through outer shell 11a, space 11c, inner shell 11b into the upper section of interior volume 12, the so called head space or gas space, from where it extends vertically downwards into the lower section of interior volume 12, reverses its direction of extension in a reversal point 18e, from which it extends upwardly back into the upper section 14b.

[0035] In the vicinity of reversal point 18e there is provided a heat exchanger 17, which is advantageously provided as a finned heat exchanger, for heat exchange between the helium acting as push gas passing through supply line 18 and the liquid phase of the cryogen, i.e. liquid helium 14a, within the dewar 10. The supply line upstream of heat exchanger 17 (designated 18′) is provided as a coaxial vacuum jacketed pipe. Downstream from heat exchanger 17, the supply line 18 is provided as a single walled pipe (designated 18″).

[0036] The interior space 12 contains helium as a cryogenic, including a liquid phase 14a and a vapour phase 14b above the liquid phase, as already mentioned. Thus, within the interior space 12, the supply line 18 extends through the vapour phase 14b, then through the liquid phase 14a, and terminates in the vapour phase at an opening section 18f.

[0037] For transportation of liquid helium from the dewar 10 to the MRI magnet, a syphon 22a,22b is provided between the dewar 10 and the MRI magnet 20. In FIG. 1, two alternative syphon designs are shown: The syphon 22a is provided with top valve 16, mentioned above, as syphon valve. The syphon 22a can be inserted through the sealable opening 11e of the dewar and fixed therein. Syphon 22a is connected to a transportation line 24 for transporting liquid cryogen from the dewar 10 to the MRI magnet 20.

[0038] Alternatively, or additionally, the dewar 10 can be provided with a built-in syphon 22b and a built—in side outlet valve 23. Syphon 22b is connected to a transportation line 25 for transporting liquid cryogen from the dewar 10 to the MRI magnet 20. A further flow control valve 26 is provided in transfer line 24 and/or transfer line 25. Both syphon alternatives 22a,22b are shown in FIG. 1, although typically only one of the alternatives provided.

[0039] In order to transport liquid helium 14a from the interior space 12 of dewar 10 to the MRI magnet, pressurized gaseous helium from cylinder 30 is transported into the vapour phase within the interior volume through supply line 18 by means of the opening of valves 18a, 18b and 18c.

[0040] During its passage through the supply line 18, especially heat exchanger 17, within the dewar, this gaseous helium is cooled down to essentially the temperature of the cryogen within dewar 10, at the same time retaining its gaseous state. Advantageously, heat exchanger 17 is dimensioned such that a large part of the heat energy, preferably up to 99%, contained in the ambient temperature helium being uses as push gas, is transferred to the liquid helium in the dewar 10. This will cause part of the liquid helium to evaporate, thus increasing the pressure of the vapour phase in the head space of the dewar 10.

[0041] Thus, the pressure within dewar 10 increases not only by means of the push gas being introduced into the vapour phase, but also by means of the evaporated liquid phase. Utilising this effect, by opening valves 16 and/or 23, as well as valve 26, liquid helium will flow through syphon 22a and/or 22b and the transportation line 24 and/or 25 into the MRI magnet 20. As the pressure of the vapour phase in part increases due to evaporation of liquid helium, substantially less push gas is required to generate and maintain sufficient pressure in the vapour phase compared to prior art solutions.

[0042] In addition to cooling of the push gas in the heat exchanger 17, a further cooling is achieved in the single walled pipes in the section 18″ of the supply line downstream of the heat exchanger. By means of providing a push gas essentially cooled down to the temperature of the cryogen, the danger of introducing helium gas bubbles into the liquid phase 14a within the dewar 10 can be essentially eliminated, whereby quench effects within the MRI magnet can be avoided.

[0043] By providing the supply line 18′ upstream of the heat exchanger 17 as a vacuum jacketed pipe, it is possible to avoid or at least minimise heat transfer from the pipe into the gas phase in the header of the dewar. The heat transfer will increase as the liquid level in the dewar drops and more and more heat transferring area will be exposed to the gas phase. This means that the temperature of the gas phase can not be controlled. The advantage of focusing the heat transfer only to the heat exchanger submerged in the liquid in the lower part of the dewar is to ensure that the gas generated by evaporated liquid is vapor (gas with the same temperature as the liquid).

[0044] With the invention, a very low mass flow of push gas, typically smaller than 10 nl/minute (normal litre per minute, the “normal” reference condition being 0° C. and 1013 mbara) can be achieved. This means that the total usage of push gas for filling a MRI magnet will be 3-4 times lower compared to the conventional push gas methodology. Also, the heat exchanger 17 can work very efficiently, as it is submerged in the liquid phase 14a. As mentioned, the cooled push gas will have a temperature very close to that of the liquid phase 14a, and the liquid phase and the vapour phase will stay very close to thermodynamical equilibrium.

[0045] In FIGS. 2 and 3, a preferred embodiment of dewar 10 and heat exchanger 17 together with a preferred design of supply line 18 are shown. In this embodiment, a built in syphon 22b, as shown in FIG. 1, is typically utilised.

[0046] Supply line 18 from a push gas supply such as cylinder 30 enters dewar 10 via sealable opening 11e, as shown in FIG. 2. In downward direction, it passes through vapour phase 14b of the cryogenic within interior volume 12 into the liquid phase 14a. As especially visible in FIG. 3, this downwardly extending section 18′ of supply line 18 is provided as the centre of a concentric arrangement, section 18′ being concentrically surrounded by a finned heat exchanger 17 in its lower part, which itself is concentrically surrounded by upwardly extending section 18″ of supply line 18. Thus, pressurised push gas entering the dewar form the push gas supply in supply line 18 will flow downwardly through section 18′ in the centre of this concentric arrangement, further downwardly through heat exchanger 17, following which its direction of transportation will be reversed in reversal section 18e, and it will flow upwardly through section 18″, which concentrically surrounds section 18′ and heat exchanger 17, and exit supply line 18 into the vapour phase 14b at outlet 18f.

[0047] The concentric arrangement of supply line 18 with heat exchanger 17 provides an extremely compact and robust design.