Abstract:
The present provides a cryogenic thermal battery arrangement for maintaining a superconducting magnet coil or similar apparatus at cryogenic temperature for a required shipping period, such as thirty days, without consuming a significant amount of costly cryogen. According to another aspect, the invention allows extended shipping periods without incurring excessive costs.

Description:
[0001]     The present invention relates to a thermal battery, that is, a device which serves to hold a piece of equipment at a certain temperature without external refrigeration and despite a relatively elevated ambient temperature.  
       BACKGROUND OF THE INVENTION  
       [0002]     Superconducting magnets are well known and used for producing very high strength magnetic fields in applications such as magnetic resonance imaging. The superconducting magnet is typically made up of a number of coils of superconducting wire which, in use, must be held at cryogenic temperatures typically of the order of 4K, the boiling point of liquid helium. Alternatively, systems using so-called high temperature superconductors are known. These may operate at the temperature of boiling neon, hydrogen or nitrogen. Critical temperatures in the region of 18-80K are typical for such materials. While the present invention will be described with particular reference to systems operating at about 4K, which are cooled by boiling helium, the present invention may be applied to high temperature superconducting arrangements.  
         [0003]     A typical arrangement has superconducting coils immersed in a bath of liquid cryogen, for example helium. The liquid cryogen is allowed to boil, and maintains the coils at a steady temperature of its boiling point, about 4K in the case of helium. A recondensing refrigerator is provided, and provides cooling by recondensing the boiled-off cryogen vapour back into a liquid. Thus, the overall consumption of liquid cryogen is reduced to a low value, since most if not all of the boiled off cryogen is recondensed back into liquid form within the cryogen vessel. Cryogens other than helium may of course be used, depending on the cryogenic temperature required.  
         [0004]      FIG. 1  shows a cross-section of a former  10  and superconducting coils  12  mounted thereon, typical of a solenoidal superconducting magnet for MRI or NMR imaging systems. The former  10  and coils  12  are housed within a cryogen vessel  14 . The cryogen vessel  14  is typically partially filled with a liquid cryogen, not shown in the drawing. The cryogen vessel itself is housed within an outer vacuum chamber  16 , and the space between the cryogen vessel  14  and the outer vacuum chamber  16  is evacuated. A thermal shield  18  is typically also provided, in the evacuated space between the cryogen vessel and the outer vacuum container. This shield reduces the incidence of radiated heat from the outer vacuum chamber which may reach the cryogen vessel. A service neck  20  is typically included. In operation, this neck may house a recondensing refrigerator.  
         [0005]     A difficulty arises when the system is in transit, awaiting installation. Typically, the system is transported filled with liquid cryogen, but the recondensing refrigerator is unable to operate due to the absence of a suitable power source, or due to regulatory restrictions. During the transit time, the cryogen is allowed to boil, keeping the coils  12  at the required cryogenic temperature. The cryogen thus acts as a thermal battery. Service neck  20  provides an escape path for boiled off cryogen to leave the cryogen vessel  14 . The boiled off cryogen is allowed to vent to atmosphere. The system may be required to be capable of remaining in this boiling thermal battery state for a duration of up to about 30 days. When a cryogen such as helium or neon is used, the cost of the cryogen lost by boiling may become significant. When hydrogen is used a cryogen, an explosion risk may result.  
         [0006]     It is required to keep the coils at a cryogenic temperature, since otherwise the commissioning of the system on installation becomes expensive, difficult and time consuming. If the system has heated up to ambient temperature, which will happen if the liquid cryogen boils dry, the system must be cooled and refilled with the relatively costly liquid cryogen before being commissioned. In some regions of the world, it is very difficult to obtain the large supplies of the liquid cryogen required for such an operation if not planned for in advance. Such a re-cooling and refilling is also time consuming, and costly both in terms of the time a field engineer must spend on site installing the system, and the material cost of the cryogen used.  
         [0007]     In magnet systems such as that illustrated in  FIG. 1 , the volume of the cryogen tank  14  is largely defined by the minimum allowable shipping time. To allow a shipping time with the recondenser inoperable of, say, thirty days, a large volume of liquid cryogen must be accommodated in the system to ensure that it does not run dry during transit. The volume of this required cryogen reservoir plays a large part in determining the final size of the whole system.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention addresses some of the problems of the prior art, to provide a cryogenic thermal battery arrangement for maintaining a superconducting magnet coil or similar apparatus at cryogenic temperature for a required shipping period, such as thirty days, without consuming a significant amount of costly cryogen. The present invention allows extended shipping periods without incurring excessive costs.  
         [0009]     The present invention provides methods and apparatus for maintaining a system at cryogenic temperatures over an extended period, such as thirty days, without active refrigeration, and with reduced consumption of expensive cryogens such as liquid helium.  
         [0010]     According to the present invention, a second cryogen different from the working cryogen is deployed in a tank, in thermal connection with the system to be cooled. When applied to the magnet system shown in  FIG. 1 , the tank may be placed in thermal connection with the former  10 , and thermal connection to coils  12  may be achieved through, the former, for example as shown at  22  in  FIG. 2 .  
         [0011]     The material of the second cryogen may be selected to be inexpensive, renewable and inexpensive to transport. It is therefore preferably a commonly used cryogen.  
         [0012]     The tank containing the second cryogen is preferably arranged to be separate from the main cryogen vessel  14  for holding the working cryogen. This will prevent contamination of the working cryogen by eh second cryogen. Such contamination is inconvenient, and possibly dangerous. The second cryogen is preferably solid at the temperature of operation, and has a boiling point higher than the boiling point of the working cryogen. Using a system normally cooled by helium as an example, the working cryogen vessel  14  may be held at about 4K by boiling helium. In the tank of the invention, a second cryogen such as solid nitrogen may be housed. During normal operation, the helium boils to cool the system to about 4K; the recondensing refrigerator recondenses boiled-off helium vapour back to liquid form, and the second cryogen remains solid and plays no part in cooling.  
         [0013]     During transport, the working cryogen—helium in this example—acts as a first thermal battery and boils to cool the system to its boiling point—about 4K for helium. After a certain period, this working cryogen will boil dry. The temperature of the system will begin to rise. This is the stage at which a prior art system would heat up to ambient temperature, leading to costly, time consuming and sometimes difficult re-cooling being required before installation. However, according to an aspect of the present invention, once the system has heated up to a certain, still cryogenic, temperature, the second cryogen in the tank will begin to change phase. For a second cryogen which is solid at the temperature of the working cryogen, the solid may begin to melt. This melting draws latent heat of fusion from its surroundings, cooling the system. Thus, the second cryogen acts as a second thermal battery. Once the second cryogen has melted, the temperature of the system will begin to rise again. Once the system has heated up to a certain, still cryogenic, temperature, the second cryogen in the tank will begin to change phase again. In this example, the liquid second cryogen may begin to boil. This boiling draws latent heat of evaporation from its surroundings, cooling the system. This represents a second thermal battery effect of the second cryogen. Once the second cryogen has boiled dry, the system will heat up to ambient temperature. However, by carefully selecting the material and quantity of the second cryogen provided in the system, this final stage of heating may be delayed such that a useful transit time may be allowed. Since the second cryogen does not need to maintain the system at its operating temperature, an inexpensive cryogen may be employed as the second cryogen.  
         [0014]     In order to reduce the cost of the working cryogen consumed, its volume in the system is minimised. This may be by carefully controlling the initial fill of the system, or at least part of the working cryogen may be reclaimed from the cryostat prior to transport.  
         [0015]     It may be advantageous to delay this final cooling stage until all, or almost all, of the second cryogen has boiled off. Alternatively, any remaining second cryogen may be recovered from the cryostat. This is to avoid the consumption of working cryogen in cooling the remaining volume of second cryogen. In the example of a nitrogen second cryogen with a helium working cryogen, cooling of a certain volume of nitrogen at 77K to 4K will require double that volume of helium, and is preferably avoided in the interest of reduced helium consumption.  
         [0016]     If any second cryogen does remain within the cryostat, once it has been cooled to operating temperature then it does not drain any parasitic thermal load, but simply remains at operating temperature typically solidified and taking no part in the cryogenic cooling process. It is however preferable to minimise the mass of second cryogen remaining within the cryostat, since this will represent an additional thermal mass which will need to be cooled to operating temperature, resulting in unnecessary consumption of working cryogen.  
         [0017]     Examples of possible second cryogens include, but are not limited to, nitrogen, neon, hydrogen, oxygen. Nitrogen is an attractive material to use as a second cryogen. It is very inexpensive and very abundant. It is neither explosive nor combustible. Nitrogen has useful physical properties as will be briefly discussed.  
         [0018]     Nitrogen finds stable temperatures at 35K, 70K and 77K as latent heat is absorbed. As nitrogen undergoes one more phase change than most cryogens, it is an effective cryogen in this application. The volume of a certain mass of solid nitrogen at under 35K is about 0.8% less than its volume as a liquid.  
         [0019]     As the temperature rises to 35K, solid nitrogen undergoes a solid-solid phase change, absorbing latent heat. At 70K, nitrogen melts to become a liquid, absorbing latent heat again. Nitrogen boils at 77K under atmospheric pressure, again absorbing latent heat. At temperatures above 77K and under atmospheric pressure, nitrogen is a gas.  
         [0020]     When solid nitrogen is used as a second cryogen, therefore, much thermal heat must be absorbed from the surroundings before it boils. The specific heat capacity of nitrogen is relatively large compared to other candidates such as hydrogen, neon or oxygen.  
         [0021]     If the system of the present invention is cooled by a nitrogen second cryogen, after the working cryogen has boiled off, the system may heat to one of these transition temperatures—35K, 70K or 77K, but it will be much less difficult to cool the system from one of these temperatures back to 4K than to cool from ambient (in the region of 300K). As the system may be held at a cryogenic temperature by the second cryogen, there is less need for costly cryogen such as liquid helium to be provided for transport. The volume of the working cryogen vessel  14  may accordingly be reduced, with reduced cost for filling, and a smaller overall system. The system will become lighter, and the requirement for liquid cryogen in the tank will be reduced. In a known system, a required cryogen fill of 1500 litres of liquid helium would weigh 400 kg, and would cost about £3300 at current prices. According to the present invention, the equivalent system could be shipped with 100 litres of nitrogen, weighing 80 kg, to provide equivalent cooling at minimal material cost. For example, a system may be cooled to operating temperature by boil off of 100 litres of liquid nitrogen, followed by boil off of a minimum quantity of working cryogen required to cool the system from the boiling point of nitrogen to the boiling point of the working cryogen.  
         [0022]     Since nitrogen requires a large amount of energy to heat up from 4K to above 77K, it is an effective cryogen. In terms of simple latent heat capacity of evaporation, 10 litres of liquid nitrogen absorb latent heat of evaporation equal to that absorbed by 55 litres of boiling helium. Thus, a given mass of nitrogen may be seen to be over five times as effective at cooling at its boiling point as an equivalent mass of helium. Of course, the boiling points of the cryogens are different, so that boiling helium will hold a temperature of 4K, while boiling nitrogen will hold a temperature of 77K. As described above, nitrogen has a solid phase transition in addition to melting and boiling transitions. Heating nitrogen from below 35K to above 77K requires ten times as much energy as required for heating the same mass of helium over the same temperature range. Thus, for the same cooling effect, one-tenth of the volume of nitrogen may be used as compared to an equivalent thermal battery using helium as the cryogen. For example, if a cryogenic system currently requires an inventory of 1500 litres of liquid helium to ensure a permissible transit time of thirty days, the same transit time may be provided with an inventory of 150 litres of nitrogen. The overall system may then be reduced in size, since the cryogen vessel  14  need not be so large, and the expense of transporting and installing the system is reduced, since large quantities of liquid helium are no longer required. In another example, the total volume of cryogen may be kept the same but a proportion of the helium may be replaced by nitrogen. For a cryogen volume designed to provide boiling helium cooling for up to 31 days, it has been calculated that replacing one-third of the volume of helium with nitrogen would provide in transit cooling at boiling helium temperature for 20 days, followed by cooling at liquid or boiling nitrogen temperature for 120 days—a transit time far in excess of those provided by present arrangements. A disadvantage of such an arrangement is that the system may be at a temperature of up to 77K on arrival, instead of 4K as would be the case with a system cooled only by boiling helium.  
         [0023]     Once the system has arrived on site, it is relatively simple to cool from 77K to 4K, for instance by using an on-site closed loop refrigerator. The closed loop refrigerator may require a helium charge of only about three litres. Further explanation of a closed loop refrigerator will be described below. Alternatively, the system may be cooled by direct contact refrigeration of the former  10 , or by addition and boil off of working cryogen.  
         [0024]     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     Certain specific examples of a cryostat according to the present invention will now be described in connection with the figures of the drawing, in which:  
         [0026]      FIG. 1  shows a cross-section of a magnet system suitable for use in an embodiment of the invention;  
         [0027]      FIG. 2  shows a cross-section of a magnet system according to another embodiment of the invention;  
         [0028]      FIG. 3  shows a detail of a portion of  FIG. 2 ; and  
         [0029]      FIG. 4  shows a detail of another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     In the simplest embodiment of the present invention, the structure of  FIG. 1  is unchanged. The cryogen vessel  14  is partially filled with a second cryogen, such as nitrogen. This may be achieved by filling with liquid nitrogen, or by operating an associated recondensing refrigerator to liquid nitrogen temperature and leaving the cryogen vessel open to an appropriate source of nitrogen. Once a required volume of second cryogen has been introduced, a required volume of working cryogen such as helium is introduced. The system may now be held at operating temperature by an associated recondensing refrigerator. The second cryogen will remain solid within the cryogen vessel, and will take no active part in maintaining the system at operating temperature.  
         [0031]     If the recondensing refrigerator stops for some reason, for example because the system is being transported, the working cryogen will boil off, cooling the system. If the working cryogen boils dry, the second cryogen will begin to melt, then boil, providing cryogenic cooling over an extended time period, at the boiling point of the second cryogen.  
         [0032]      FIG. 2  shows another embodiment of the invention. A second cryogen tank  22  is provided, around the former  10  and in thermal contact with it. The tank may be filled with second cryogen through access neck  24  by a method as mentioned above. The use of the tank for cooling the system is as for the embodiment described above, except that the second cryogen is held in thermal contact with the former, by second cryogen tank  22 , giving efficient cooling of the coils  12 . The provision of second cryogen tank  22  retains the second and working cryogens separately, and avoids any contamination of the working cryogen by the second cryogen. When emptied, the second cryogen tank  22  may serve as a displacement volume. This will allow a given depth of working cryogen to be provided in the cryogen tank with a correspondingly reduced volume of cryogen, or, alternatively, a conventional volume of cryogen may be introduced into the tank, to immerse the superconducting magnet assembly to a greater depth. This would ensure that a greater proportion of the superconducting magnet is cooled by contact with the liquid cryogen, providing more constant cooling.  
         [0033]     The tank  22  may be filled by the following process. Access to the tank  22  is connected to a suitable source of nitrogen. The former  10  is cooled by a refrigerator according to any known arrangement. This cooling cools the walls of the tank  22  to below the boiling point of nitrogen. Nitrogen begins to condense on the inside of the tank. The vacuum caused by the condensation of the nitrogen draws more nitrogen into the tank. A nitrogen thermal battery may thus be self filling: as the tank is cooled, nitrogen liquefies then solidifies, drawing more nitrogen gas in.  
         [0034]      FIG. 3  shows one end of the former in detail, modified according to another embodiment of the invention. In this embodiment, in order to achieve good thermal contact with the coils  12 , a tank  32  is provided as an integral part of the former. As illustrated, the tank is of toroidal form, coaxial with the former  10 , and is braised or welded to the former at appropriate points. Tank  32  may be filled by the method described with reference to tank  22  of  FIG. 2 .  
         [0035]      FIG. 4  shows one end of the former in detail, modified according to another embodiment of the invention. In this embodiment, in order to achieve good thermal efficiency of the system, a tank  42  is provided in the form of a thermal shield, but also in thermal contact with the former. As illustrated, the tank is of cylindrical form, co-axial with the former  10 , and is braised or welded to the former at appropriate points. By making the tank  42  a full cylinder about the magnet&#39;s axis, attached to the former  10  at each end, the heat load onto the cooled former  10  is reduced as the tank  42  acts to shield at least some of the coils  12  and former  10  from incoming thermal radiation. If nitrogen is used as the second cryogen, the tank  42  should preferably be made of aluminium, since aluminium has emissivity characteristics at low temperatures, which are beneficial here. The tank  42  may be filled by a process similar to that described with reference to  FIG. 2 .  
         [0036]     The coils  12  and former  10  may be cooled by inclusion in a cryogen vessel such as illustrated in  FIGS. 1 and 2 . However, an alternative arrangement for cooling the former and coils  12  down to operating temperature is illustrated in  FIG. 5 , which illustrates closed-loop refrigeration but does not itself illustrate an embodiment of the present invention.  
         [0037]     In its simplest form, a closed loop refrigerator may comprise a loop of tube of a thermally conductive material, whose ends are connected to a recondensing refrigerator. The recondensing refrigerator should be arranged at the top of the loop. The tube is placed in thermal contact with the article to be cooled, and is at least partially filled with a liquid cryogen. Heat is absorbed through the material of the tube and results in the boiling of a proportion of the cryogen. The boiled off cryogen gas travels upwards to the recondensing refrigerator. The refrigerator cools the cryogen gas and recondenses it back into a liquid, which runs back through the tube. By suitable arrangement of the refrigerator and the loop of tube, a circulation current may be established around the loop. Being a closed loop system, this arrangement can cool apparatus to the boiling point of the cryogen used, but does not consume cryogen in operation, and requires only a small charge of cryogen.  
         [0038]     The second cryogen provided according to the present invention should be cooled to operating temperature by active cooling, for example using a closed loop refrigeration system. The alternative, to cool the second cryogen to the operating temperature by addition of working cryogen, may be economically unsound. For example, to cool nitrogen to 4K by addition of liquid helium would require a quantity of helium double the volume of the nitrogen cooled. No cost saving would be made.  
         [0039]     As can be seen from  FIG. 5 , no cryogen vessel  14  or outer vacuum container  16  are required. A coolant tube  54  containing a liquid coolant such as liquid helium is provided in thermal contact with former  10 . The coolant tube may comprise one or more toroidal hoops braised or welded onto the former  10  at appropriate positions. Alternative or additional positions for cooling tubes are shown at  54 ′ and  54 ″. A refrigerator is connected to cool and pass a cryogenic coolant through these cooling tubes, forming a closed loop refrigeration system. Because no cryogen tank is required, the overall system may be made significantly smaller. The only working cryogen required in such embodiments is the cryogen required to fill cooling tube  54 . This may have a volume of the order of three litres. Of course, such a small volume of working cryogen cannot be expected to maintain the system at a cryogenic temperature for any significant amount of time when operating as a thermal battery in the absence of external refrigeration. By combining this arrangement with a tank of second cryogen according to an embodiment of the present invention, the system may be maintained at a cryogenic temperature for many days. The tank arrangement of  FIG. 4  may be particularly advantageously applied to such embodiments, as it would provide thermal shielding which may otherwise be lacking.  
         [0040]     Once cooled to operating temperature, in operation of the magnet in persistent mode, coils may be cooled by thermal conduction or by thermosiphon cooling. Both of such methods may be realised with a closed loop system requiring a small cryogen inventory.  
         [0041]     A significant advantage of the present invention is that one may be confident of the state of a cryogenic system upon delivery. Provided that delivery takes place within the determined maximum delivery time, which may be extended beyond the normal range by the present invention without incurring penalties in terms of cost or size of the equipment, one may be sure that the system will be cooled to 77K or below. It will be necessary to then expend a certain amount of working cryogen, such as liquid helium on cooling the system from 77K to the operating temperature, 4K in this example, but this amount of helium may be accurately estimated and provided for in advance. In the prior art systems, one may have expected the system to arrive still cooled by boiling helium, and requiring a helium top-up of about 400 litres, say. If there is a delay in shipping, the system may in fact arrive at ambient temperature, and require initial nitrogen cooling plus a large quantity of liquid helium to put the system in an operable condition. Such large volumes of cryogen may not be readily available at the desired installation site.  
         [0042]     Certain cryogens, for example nitrogen, have a low thermal conductivity in their solid state. When such a cryogen is operating as a thermal battery, outer regions of the cryogen may melt, and even boil, before sufficient heat has reached other parts of the cryogen to melt it. This may result in ineffective cooling. On the other hand, such effect may be advantageous if a lower rate of cooling is required over a more extended time period. According to certain embodiments of the invention, thermally conductive paths may be provided within the solid cryogen, to improve contact surface area of the second cryogen. For example, a second cryogen tank such as shown at  32  in  FIG. 3  or at  42  in  FIG. 4 , may be partly filled with a thermally conductive open-celled material, such as copper wool. Such material would disperse heat more evenly through the second cryogen, and allow a more even heating of the second cryogen, and a more constant rate of cooling to the cooled system. Finned structures, akin to heat sinks found on electrical equipment, may alternatively or additionally be provided inside the second cryogen tanks for the same purpose. This is particularly effective if the second tanks are manufactures by an extrusion process, for example using aluminium. By controlling the material and density of such thermally conductive structures, the rate and uniformity of cooling provided by the second cryogen may be adapted to suit the required application.  
         [0043]      FIG. 6  illustrates temperature curves which may be obtained with a nitrogen second cryogen, according to certain embodiments of the present invention. A first curve  61  illustrates the temperature variation over time of an example superconductive magnet cooled by a nitrogen second cryogen in a tank according to an embodiment of the present invention. As can be seen, the temperature of the magnet may rise above the boiling point of the second cryogen, due to the poor thermal conductivity of the cryogen in its solid state. A second curve  62  illustrates the temperature variation over time of the example superconductive magnet cooled by an otherwise identical tank of nitrogen second cryogen, which tank is partially filled with an open celled thermally conductive material such as copper wool. As seen in  FIG. 6 , the cooling is more effective, maintaining the temperature of the magnet below 77K at all times. This arrangement is critically damped; it just avoids heating to temperatures above 77K and ensures that the cryostat will be stable at boiling nitrogen temperature for a significant period of time, beginning at around 22 days, a typical shipping time. Certain applications may require a certain time constant. By adapting the provision of thermally conductive material within the cryogen tank, effectively varying the surface area of the solid cryogen, an optimal time constant for a particular application may be obtained.  
         [0044]     While the present invention has been described with reference to a limited number of specific embodiments, it will be clear to one skilled in the art that other modifications and variations of the present invention may be made within the scope of the invention as defined in the appended claims.  
         [0045]     For example, while the present invention has been described with particular reference to nitrogen as the second cryogen, other cryogens may be used. The second cryogen chosen should have a high heat capacity when frozen, and preferably should also have a high latent heat capacity in its phase changes. The chosen second cryogen should not expand as it solidifies. If a second cryogen is chosen which does expand on solidifying, then the tank containing it must either relax to accommodate the expanding contents, or must be strong enough to withstand the pressure put on it by the expanding cryogen.  
         [0046]     Furthermore working cryogens other than helium may be used.  
         [0047]     While the invention has been particularly described with reference to superconductive magnets for MRI systems, the present invention may be applied to the cooling of other equipment.  
         [0048]     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.