Patent Application
20150128617
Early superconducting magnets for MRI systems were cooled by partial immersion in a bath of liquid cryogen. However, more recent designs have reduced the quantity of liquid cryogen required, typically by using a cooling loop arrangement. Such systems usually have a relatively small cryogen vessel positioned at the top of the cooling loop and below a recondensing refrigerator, to provide liquid cryogen which is fed into thermal contact with the magnet under the influence of gravity. Warmed, or boiled, cryogen returns through the cooling loop to the cryogen vessel for recooling. However, such an arrangement tends to have a height greater than the available height within a typical installation room. The positioning of the recondensing refrigerator at the top of the magnet is inconvenient for servicing and replacement of the recondensing refrigerator.
Some conventional arrangements provide a recondensing refrigerator at a reduced height, thermally linked to a cryogen vessel through a solid thermal bus-bar. However, at the temperature required to re-condense cryogens such as helium, a cooling power of only 1 W is common. The solid bus-bar may need to have a length of about 1 m and needs to be highly thermally conductive. Such bus-bars are typically made from high purity annealed copper or high purity annealed aluminum with a very large cross-sectional area in order to transfer cooling power from the refrigerator to the cryogen vessel without significant loss. They are very expensive to produce. For example, a conventional 50 cm bus bar may cause a temperature rise of 0.1 K or more. This makes the bus bar arrangement bulky and expensive to manufacture. It has been found difficult to produce perfectly flat contact surfaces for the thermal connection between the bus bar and the refrigerator. This tends to result in only point contacts which can be improved by filling the joint with indium or grease, or mechanically compressing the joints. However, such joints are difficult to make. Replacement of the recondensing refrigerator in the field may mean that the contacts need to be reproduced on site, which is found to be unreliable. Imperfect connections result in lost cooling power and may result in loss of cryogen.
The present disclosure relates to cooling arrangements for cryogen cooled superconducting magnets such as used in MRI systems. In particular it relates to arrangements for controlling the level of liquid cryogen in a cryogen vessel of compact magnet systems.
Background information on conventional arrangements is provided in US patents U.S. Pat. No. 4,464,904; U.S. Pat. No. 5,335,503; U.S. Pat. No. 5,549,142; U.S. Pat. No. 5,937,655; U.S. Pat. No. 6,996,994 and Japanese patent publication JP2004033260.
It is an object to provide an arrangement in which a cryogen vessel is arranged no higher than the top of a cryostat housing a superconducting magnet, yet which allows cryogen gas to be re-condensed and supplied to the cryogen vessel near the top of the superconducting magnet to be fed to a cooling loop.
Conventional MRI cooling systems have a recondensing refrigerator arranged to liquefy cryogen gas within a cryogen vessel. The liquefied cryogen gas is then fed to a cooling loop under the influence of gravity. In the present preferred embodiment, the recondensing refrigerator is not placed above the magnet, but is provided in association with a separate recondensing chamber, positioned rather lower than the cryogen vessel connected to the cooling loop.
Such arrangements are particularly useful where MRI systems are to be installed in locations where available height is limited.
In a cryogen cooling system for cooling a superconducting magnet, a cryogen vessel is linked to a cooling loop arrangement in thermal contact with the superconducting magnet. A recondensing chamber is arranged such that a lower extremity of the cryogen vessel is above a lower extremity of the recondensing chamber. A recondensing refrigerator is arranged to recondense cryogen gas within the recondensing chamber. A heater is positioned to heat gaseous cryogen within the recondensing chamber, and wherein the recondensing chamber is hydraulically connected to the cryogen vessel by a cryogen supply pipe. An upper end of the cryogen supply pipe is exposed to cryogen gas in the cryogen vessel and a lower end of cryogen supply pipe is exposed to an interior of the recondensing chamber towards or at its lower extremity.
The above and further objects characteristics and advantages of the present preferred embodiments will become more apparent from consideration of the following description of certain embodiments given by way of example only, in conjunction with the following accompanying drawings.
For the purposes of promoting an understanding of the principles of the invention, reference will now be made to preferred exemplary embodiments/best mode illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and such alterations and further modifications in the illustrated embodiments and such further applications of the principles of the invention as illustrated as would normally occur to one skilled in the art to which the invention relates are included herein.
The present preferred embodiment provides a cryogen cooling system comprising two hydraulically connected reservoirs. The system operates a closed cryogenic cycle, including liquefaction and evaporation.
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According to an aspect of the exemplary embodiment, liquid helium from recondensing chamber 16 is driven up through pipe 18 to the cryogen vessel 15 by controlled variation in the pressure of cryogen gas within recondensing chamber 16.
An example method of operation of the present exemplary embodiment may proceed as follows. In a steady state, recondensing refrigerator 20 operates at a working temperature of 4.2 K in contact with helium gas at a pressure of 14.5 psi (100 kPa) absolute within the recondensing chamber 16. The recondensing refrigerator may have an effective cooling power of 1 W at 4.2 K. Helium gas within the cryogen vessel 15 is a standard pressure of 15.3 psi (105.5 kPa) absolute. This pressure, being higher than the gas pressure within the recondensing chamber 16, drives helium gas from cryogen vessel 15 into recondensing chamber 16. In the recondensing chamber 16, the helium gas is liquefied and maintains a gas pressure lower than that in the cryogen vessel 15. Accordingly, cryogen gas tends to flow from cryogen vessel 15 to recondensing chamber 16. Once a predetermined quantity of liquid helium has accumulated within the recondensing chamber 16, operation of the recondensing refrigerator 20 may cease, heater 24 is then energised and warms cryogen gas within the recondensing chamber 16. The recondensation stops and the pressure of cryogen gas within the recondensing chamber 16 rises. Once the pressure of cryogen gas within the recondensing chamber 16 rises above the pressure of cryogen gas in cryogen vessel 15 by a sufficient amount, liquid cryogen 22 is forced from the recondensing chamber 16 through pipe 18 into the cryogen vessel 15. Once the level of liquid cryogen within the recondensing chamber 16 reaches a predetermined minimum level, which may be “empty”, the heater 24 is de-energised and re-condensation of helium gas will recommence. This operation will continue cyclically to ensure that a supply of liquid cryogen is always provided within cryogen vessel 15, by periodically replenishing the cryogen vessel 15 with liquid cryogen from the re-condensing chamber 16.
In this example, the present exemplary embodiment makes use of the relatively low density of liquid helium and the relatively large coefficient of thermal expansion of gaseous helium. Atmospheric pressure corresponds to approximately 10 m height of water or 80 m height of liquid helium. This means that a relatively low pressure difference is required. Only about 10 mbar or 1% of atmospheric pressure should be sufficient to raise liquid helium over 800 mm.
There is only a very small temperature difference between liquid cryogen in the recondensing chamber 16 and liquid cryogen in the cryogen vessel 15, and the thermal conductivity of liquid helium is relatively low, so any heat load on the recondensing vessel due to the pipe 18 is likely to be insignificant.
The upper end of pipe 18 is exposed to cryogen gas in the cryogen vessel 15. The lower end of pipe 18 is exposed to the interior of recondensing chamber 16 towards or at its lower extremity. Once a small quantity of liquid cryogen is recondensed in the recondensing chamber 16, the lower end of pipe 18 will be immersed in liquid cryogen forming a hydraulic lock. The pressure difference in gaseous cryogen between the cryogen vessel 15 and the recondensing chamber 16 will be sufficient to draw gaseous cryogen from cryogen vessel 15 through pipe 18 to recondensing chamber 16 when recondensing refrigerator 20 is in operation. Recondensation of gaseous cryogen in the recondensing chamber 16 will keep the gas pressure within the recondensing chamber 16 lower than gas pressure in cryogen vessel 15.
If the gas pressure within the recondensing chamber 16 is sufficient, and the cooling power of the recondensing refrigerator 20 is known, one can calculate the rate at which cryogen will condense, and cycles of replenishing the cryogen vessel 15 may simply be timed. For instance, cooling helium at 4.2K at a power of 1 W will cause about 1 litre of liquid helium to recondense in one hour. At predetermined time intervals, corresponding to expected quantities of recondensed cryogen, operation of the recondensing refrigerator 20 may cease, and heater 24 may be energized. The resulting heating of gaseous cryogen will cause expansion of the gas and an increase of gas pressure within recondensing chamber 16. Once that gas pressure exceeds the gas pressure within the cryogen vessel 15 sufficiently, liquid cryogen 22 will be driven from recondensation chamber 16 into cryogen vessel 15. The heating may be continued for a fixed period of time, deemed sufficient to drive all, or a predicted quantity, of the liquid cryogen from recondensation chamber 16 to cryogen vessel 15. Alternatively, sensors may be provided to detect a minimum level of liquid cryogen 22 in the recondensing chamber 16, or a maximum level of liquid cryogen in the cryogen vessel 15, and the heater 24 may be de-energized as soon as one of these sensors indicates that one of these conditions has been reached. Similarly, a sensor may be provided to detect a maximum level of liquid cryogen within the recondensing chamber 16, or a minimum level of liquid cryogen within the cryogen vessel 15 and to energise heater 24 once this maximum level is reached.
In the case of helium cryogen, the expansion coefficient at about 4K is so large that only a very small temperature rise, in the order of 10 mK, is expected to be sufficient to create a pressure increase of about 10 mbar and drive liquid helium up a height H of about 800 mm.
If heater 24 remains energized for long enough, all of the liquid cryogen within recondensing chamber 16 will be driven into the cryogen vessel 15. At this point the gas pressures within recondensing chamber 16 and cryogen vessel 15 will equalize. Heater 24 is then de-energized and the sequence repeats.
In alternative arrangements, the refrigerator 20 may remain operational while heater 24 is energized, the cooling power of the refrigerator 20 being easily overcome by the heating effect of a simple electrical heater 24.
The heater may be a simple coil of resistive wire provided with an electrical current by electrical connections integrated into the refrigerator 20. The heater itself may be integrated into the recondensing refrigerator, for example being attached to an outer surface. Alternatively, the heater may be placed on the outer surface of recondensing chamber 16, in thermal contact with it.
The depth of liquid helium within recondensing chamber 16 may be monitored by temperature sensors placed at appropriate locations either within or on the external surface of the recondensing chamber 16. The temperature sensors may be electrical resistors, or any other known level gauges such as those relying upon capacitance, superconduction etc., suitably connected to a control system. The control system may use measurements provided by the sensors to control the operation of the heater 24 and a drive the hydraulic lift of liquid cryogen from recondensing chamber 16 into cryogen vessel 15.
The recondensing chamber 16 need not be located immediately adjacent to the cryogen vessel 15 but may be positioned elsewhere in a room with the MRI system. However, suitable thermal insulation must be provided around the pipe 18 connecting the recondensing chamber 16 with the cryogen vessel 15. Such thermal insulation will typically include vacuum insulation.
Also shown is an optional further valve 34, which may be also or alternatively be used to prevent ingress of air into the cryogen vessel 15 during servicing operations within the recondensing chamber, such as replacement of recondensing refrigerator 20.
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It is preferred that the open end of pipe 18 should be quite near the upper surface of the cryogen vessel, such that the warmer cryogen gas nearer the top of the cryogen vessel is taken for recondensation. This is believed to result in improved thermal stability of the cryogen vessel.
The total quantity of cryogen within the recondensing chamber 16 and cryogen vessel 15 is preferably determined such that the upper end of pipe 18 always remains above the liquid level in cryogen vessel 15.
The present exemplary embodiments accordingly provide apparatus and methods for replenishing a cryogen vessel arranged for cooling a superconducting magnet in which a recondensing refrigerator is provided at a height significantly below that of conventional arrangements the inventive arrangement is simple and compact. The cost of manufacture of the arrangement of the present exemplary embodiments are significantly less than that of conventional arrangements. The relative positioning of the recondensing chamber and the cryogen vessel is very flexible and allows easy replacement of the refrigerator when required. The use of the optional valve allows for the warming of the recondensing chamber to room temperature while maintaining cryogenic temperatures within the cryogen vessel.
Although preferred exemplary embodiments are shown and described in detail in the drawings and in the preceding specification, they should be viewed as purely exemplary and not as limiting the invention. It is noted that only preferred exemplary embodiments are shown and described, and all variations and modifications that presently or in the future lie within the protective scope of the invention should be protected.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1209853.9 | Jun 2012 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/058556 | 4/24/2013 | WO | 00 |
