APPARATUSAND M EξTH OPS FOR TRANSPORTING CRYOGENICALLYCOOLED GOODSOR EQUIPMENT

The present invention relates to methods and apparatus for transporting cryogenically cooled goods or equipment, such as superconducting magnets for magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR) imaging systems. In particular, it relates to such methods and apparatus for ensuring that such equipment arrives at its destination still cryogenically cooled, with lim ited consum ption of cryogen material en route.

When a cryogenic system such as a superconducting magnet for magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR) imaging systems reaches its site of instal lation, it should arrive such that it can be deployed as rapidly as possible. The current approach is to place the magnet withi n a cryogen vessel fil led with a liquid cryogen before departure . In the case of low temperature superconductor (LTS), this liquid cryogen would typical ly be liquid hel ium . For arrangements using high temperature superconductors (HTS), liquid neon, liquid nitrogen or liquid hydrogen could be used. The liquid cryogen is al lowed to boil during transit, thereby ensuring a constant tem perature whilst the liquid cryogen is present. The boiled off gaseous cryogen i s vented to atmosphere, representi ng an economic loss and a waste of resources. The volume of cryogen i nitially provided within the system is defined such that there is sufficient fluid to ensure that some is left at the end of a certain period, such period being defined to encom pass expected transit time. The period is typically set at 30 days. Once the system arrives at its installation site, additional liquid cryogen may be added to top up the cryostat to its operational fill level . Such topping up, however, is not for the purpose of

cooling the system , since it wi ll still be at its operating tem perature, due to the continued presence of boiling cryogen.

The l iquid cryogens employed i n such methods are increasingly expensive, and in some cases are produced from non-renewable sources (e.g. helium , being derived from oil). For exam ple, liquid helium presently costs almost GB£2 (€3, US$3) per litre. During transit of known superconducti ng magnets for magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR) imaging systems, up to 100 litres of liquid cryogen is typically lost during transport. However, with present cryostats carrying in the region of 1750 litres of cryogen, the potential for cryogen loss is much greater. A far greater cost risk is associated with the possibility that the whole volume of the cryogen may boil off: that the cryostat will boil dry and the cooled equi pment will heat up to am bient tem perature, for example 300K. In order to cool the equipment back to its required operating temperature, for example 4K, large volumes of liquid cryogen would need to be added, much of which would boil off to atmosphere in cooling the apparatus.

The present invention aims to provide a method of shipping such cooled equipment which redu ces the volume of liquid cryogen consumed.

Alternative sol utions proposed include the followi ng. The system may be shipped empty, at ambient temperature and cooled when it arrives. However, this would result in a very significant consum ption of working cryogen at the installation site, as working cryogen is applied to cool the system to operating tem perature. The cost, com plexity and delay at the installation site render this option impractical . The cryogen vessel may be filled with an inexpensive, renewable, non-pol luting cryogen such as

liquid nitrogen. However, the system would need to be purged on site and then further cooled by application of working cryogen. This option is also costly and complex at the site of installation.

The present inventi on aims to prevent such loss of cryogen, while ensuring that the cooled equipment remains cooled throughout its journey, even though the journey itself may be prolonged beyond the normal maxim um shipping time, which is currently in the region of 30 days.

Accordingly, the present invention provides methods and apparatus as defined in the appended claims.

The above, and further, objects, advantages and characteristics of the present invention will become more apparent by reference to the following descripti on of certain em bodi ments thereof, given by way of examples only, with reference to the accom panying drawings, wherein : Rg. 1 sh ows a schematic drawi ng of a system according to an embodiment of the present invention ; and Rg. 2 shows a schematic perspective view of an embodiment of the present invention.

Current shipping methods i nvolveinitially fill ing the cryostat with cryogen to a level sufficient to maintain operating temperature for 30 days. The present invention aims to reduce this initial filling level to one sufficient to maintain operating temperature for a much shorter time period - for exam ple, three days. Such should be sufficient to maintain the system at its operating tem perature during air transit anywhere in the world. The volume of cryogen liquid placed in the system for transit m ay be reduced by up to 90%.

- A -

According to an aspect of the present invention, the cryostat containing the equipment to be cooled is provided with means for active refrigeration during transit. Such means may be active for most, or all , of the transit time. It is possible that operation of the active refrigeration means may not be perm itted when the cryostat is carried by certai n modes of transport, such as air, rail or sea. For this reason, liquid cryogen m ust be provided i n sufficient volume to mai ntai n the cooled equi pm ent at operating temperature for the maximum predicted duration of such carriage, al lowing time for customs clearance, until the cryostat may once again be accessed to restart active refrigeration.

Rg. 1 sh ows a schematic drawi ng of a system according to an embodiment of the present invention. A cryostat 10 containing a superconductive magnet 12 for MRI or NMR imaging and partially fil led with liquid cryogen 1 1 is provided with a refrigerator 14 for active cooling. Such refrigerator may be of any of the known types of cryogenic refrigerators, such as a pulse tube refrigerator, a Gifford-McMahon refrigerator, or a Stirling cycle refrigerator. To enable the refrigerator to operate, a supply 16 of high pressure gas, and a gas return path 18, m ust be provided. This gas is typically helium , although other gases could be used, dependent upon the operating tem perature of the cryostat. Depending of the type of refrigerator em ployed, an electrical supply 20 may also be required to the refrigerator. Accordingly, auxiliary equipment is provided to furnish the refrigerator with its required supplies. An electrical generator 22 is provided. This may conveniently be a diesel powered three-phase electrical generator providing 400V AC at 50HZ at up to 2OkW. The electrical generator is connected to supply electrical power to a chi ller 24 and a cryogen compressor 26. The chi H er 24 cools and provides a supply

of cooling fl uid for cooling the cryogen com pressor 26. The chil ler 24 may be a model ICS TAE-020 of 9kW cooling power, which consumes approximately 3kW of electrical power. A forward cool ing fluid flow path 30 and return feed cooling fluid path 31 are provided. In a certain em bodiment, a valve 27 and flow meter 28 were provided to restrict the flow of water cooling fluid flow through the chi ller to 8 litres per m inute. The chi l l er may typical ly operate to cool a water cool ing fl uid flowing through it to a temperature of 10-20 O. While water may conveniently be employed as the cooling fluid, other cool ing fluids may be used as appropriate.

The cryogen com pressor 26 receives cooled cooling fluid 30 from the chil ler 24, and electrical power 20 from the generator 22. It provides compressed cryogen gas supply 16 and a gas return path 18 to/from the cryogenic refrigerator 14. The cooled cooling fluid circuit serves to keep the com pressor 26 cool . Further or alternative cooli ng could be provided using the material of the container as a heat sink. Cooling fluid and gas tanks, not shown in Rg. 1 , may also be provided to mai ntai n a supply of gas and cool ing fluid for the chi l l er and the com pressor. A supply of fuel 31 , for exam ple diesel fuel , is provided for the generator 22 in a tank 32.

The whole system illustrated in Rg. 1 , with its cooli ng fl uid and gas tanks, is mounted on a transportable carrier. In a particularly preferred embodiment, the transportable carrier is in the form of a standard freight container, modified to provide an exhaust port for gases boiled off from the cryostat, and exhaust gases generated by the generator. It may also be requi red to provide an externally accessible switch (shown at 38 i n Rg.2), which may be lockable, in order to allow the generator to be tu rned off when necessary for transport.

Rg. 2 illustrates a perspective view of a standard shi ppi ng container 40 modified according to an embodiment of the present invention. Cool i ng fluid 34, gas 36 and fuel 32 tanks will also be provided within the container, to supply the chi l ler 24, gas compressor 26 and electrical generator 22 respectively. Rg. 2 provides only a very schematic representation , but one skil led in the art would easily design and incorporate suitable apparatus to fit in the container. Supply and retu rn paths such as electrical supply 20, cooling fl uid paths 30, 31 com pressed gas 16, 18 are preferably routed along the walls and ceili ng of the container 40 so as not to i mpede access for operators.

As illustrated, the cryostat 10 housing the cooled equipment, such as a superconducting magnet, is placed on a vibration reducing mounting 40, provided to lim it horizontal acceleration applied to the cryostat. Such vibration reducing m ou ntings are provided to restrain side-to-side oscil lations. Vi bration reducing mountings may also be provided for other components of the system . Such vibration reducing mounts serve to reduce the likelihood of damage to the system in transit, and ensure efficient operation of the various com ponents during transit. They also serve to reduce the level of mechanical vibrations and acoustic noise transmitted to the body of the shipping container 40.

An exhaust vent 42 is provided in the wall or roof of the shipping container, to provide an exit path for exhaust gases from generator 22. A further vent 44 is also provided to enable boiled -off cryogen gas to escape from the container. Suitable screening or shielding should be provided to prevent ingress of foreign bodies through these vents 42, 44.

Alternatively, the two gas exhaust paths may be combined within the container, and a single exhaust vent provided.

The use of a standard shipping container as the transportable carrier sim plifies l ifti ng and loading and unloadi ng operations for transferri ng the system onto and off of lorries, trains, shi ps and ai rcraft. Such shipping containers are famil iar sights all over the world, and provide a convenient storage and transport container for all manner of goods. The containers may be carried singly on lorries or railway carriages, or in large quantities on cargo ships. They may be loaded into aircraft cargo holds for ai r freight carriage. The containers are typical ly provided in standard length of 20 feet (6.1 m) or 40 feet (12.2m). The containers are typically formed from corrugated sheet steel mounted on a frame of steel mem bers. The shipping container employed in the present invention may be one such typical container, or may be a specially designed and constructed container which meets the standards of external dimensions and other required characteristics of such standard shippi ng containers. The chil ler, generator and cryogen com pressor wi ll generate significant levels of heat within the container. Care must be taken to provide adequate ventilation for cooling the interior of the container. This may take the form of several openi ngs in the container, preferably accompanied by a fan to assist exhaust of hot air.

In an alternative em bodiment, an open 'flat-rack' container may be employed, preferably with a surrounding frame. A flat rack container is essentially an open frame the dimensions of a shipping contai ner, and having all the advantages of ease of l ifting and transport, but having no enclosed side or end panels. The use of a surrounding frame may be particularly appropriate for air transport. The use of this type of shipping

container may enable the system to be transported in the cargo hold of an ai rcraft.

In one method according to the present invention, the cryostat 10 is loaded into a transportable container as described. The generator 22 is turned on, and active cooling operates during the length of the transit until the cryostat reaches its installation si te. The cryostat is removed from the transportable container and connected to corresponding suppl ies at its installation site. The level of liquid cryogen 1 1 may be topped up if necessary. The cooled equipment 12 and the cryostat 10 may be very rapidly i nstal led and brought into service.

In another method according to the present invention, the cryostat 10 is loaded into a transportable container as described. The generator 22 is turned on, and active cooling operates until the cryostat needs to be loaded onto an aircraft, ship or other transport which does not allow operation of the active refrigeration in transit. The generator is turned off, for example using switch 38, and the transportable container is placed on the transport. As the generator 22 is not operating, the liquid cryogen 1 1 wi l l begin to boil off, so maintai ning the cooled equipment 12 at the boiling point of the l iquid. The cryostat 10 will remain i n this condition unti l it is removed from the transport. The transportable carrier may then be placed on a lorry, for exam ple, where active refrigeration is perm itted. The generator 22 may be restarted and active refrigeration will be provided until the cooled equipment 12 reaches its installation site. The cryostat 10 is removed from the transportable container and connected to correspondi ng suppl ies at its installation site. The level of liquid cryogen 1 1 may be topped up if necessary. The cooled equipment 12 and the cryostat 10 m ay be very rapidly installed and brought into service. Since

the duration of cooling by boiling cryogen is likely to last 3 days at the most, the vol ume and cost of boi led off cryogen is m uch less than the conventional method of cooling by boiling cryogen for the duration of the transit.

In a further method according to the present invention, two transportable containers as described are required. The cryostat 10 is loaded into a first transportable container. The generator is turned on, and active cooling operates until the cryostat needs to be loaded onto an aircraft, ship or other transport which does not allow operation of the active refrigeration in transit. The generator is turned off and the cryostat 10 housing the cooled equipment 12 is removed from the transportable contai ner and is placed on the transport. The l iquid cryogen will begin to boil off, so cooling the cooled equipment at the boiling point of the liquid. The cryostat 10 will remain in this condition until it is removed from the transport. The cryostat is then placed in the second transportable container and placed on a lorry, for example, where active refrigeration is permitted. A generator 22 of the second transportable carrier is then started and active refrigeration wil l be provided unti l the cooled equipment reaches its installation site. The cryostat is removed from the second transportable container and connected to corresponding supplies at its installation site. The level of liquid cryogen 1 1 may be topped up if necessary. The cooled equipment 12 and the cryostat 10 may be very rapidly i nstal led and brought i nto service. Since the duration of cooling by boili ng cryogen is likely to last 3 days at the most, the volume and cost of boiled off cryogen is m uch less than the conventional method of cooling by boiling cryogen for the duration of the transit.

In embodiments described above, the boiled-off cryogen 11 is vented to atmosphere, while the electrical generator is powered by a fuel source such as a tank of diesel 32. A preferred embodiment of the invention may be employed in instances where the cryogen is flam mable. For exam ple, if the cryogen used is liquid hydrogen, the boiled off hydrogen may be used to power the generator. Such an arrangement may usefully provide a negative feedback arrangement: when the cryogen is boiling relatively rapidly, a plentiful supply of fuel is avai lable for the generator, ensuring effective active cooling. If the cryogen is boiling relatively slowly, there wi ll be a reduced supply of fuel avai lable for the generator, in which case less active cooling may be required, saving on fuel consu m ption . Such embodiments have the added advantage of reducing, or elimi nating, the em ission of flam mable gases to atmosphere. Burning of hydrogen to fuel the generator has the further advantage of producing no pollutants. The water vapour generated could be safely vented to atmosphere, or recondensed by a recondenser fed by the return cooling fluid supply to the cooler. Hydrogen or another fuel may be burned i n a gas turbi ne to provide rotary power for the generator. As an alternative to burning fuel , the required electrical power may be provided by supplyi ng hydrogen to power a fuel cell to generate electricity directly, without com bustion. Transport of systems containing liquid hydrogen, or other flammable gases, may be forbidden on certain modes of transport. The transport of systems em itting flammable gases wi ll be even more strictly controlled.

During an experiment, transport of a cryostat containing a superconducting magnet was arranged as described above. The cryostat was transported from Oxford, England to Erlangen, Germany and back in four days. On the outward journey, the generator was stopped for the duration of transport on a train through the channel tunnel . On the return

journey, the cryostat was transported by sea ferry, and the generator was kept running. On departure, the magnet was filled to 74% capacity with liquid helium cryogen . On arrival i n Erlangen, Germany, cryogen boil off had raised the pressure i n the cryostat to about 2.5psi (17kPa) above atmospheric pressure. The cryostat was vented to release this pressure to atmospheric pressure. Some cryogen gas was lost at this point, reducing the cryogen fil l level to 73%. At the end of the return journey, on arrival at Oxford, England, boil off of cryogen had again raised the pressure in the cryostat to about 2.5psi (17kPa) above atmospheric pressure. The cryostat was again vented to release this pressure to atmospheric pressure. Some cryogen gas was lost at this point, reducing the cryogen fill level to 71 %.

This experiment showed that the increase in pressure within the cryostat was no more that would be expected when keeping the refrigerator running with the cryostat stationary . The helium loss on each leg of the journey was about 1 %. This compares very favourably to the normal average loss of 6-7% over the same journey. The net reduction in cryogen consum ption, 5%, represents a saving of 87.5 litres of cryogen for a 1750 litre capacity cryostat. A corresponding cost saving is real ised on installation of the system , where a red uced volume of cryogen would be required for toppi ng up the cryostat. Alternatively, a m inim um cryogen level on arrival may be specified. This may, for exam ple, be 60%. If a consum ption of cryogen in transit can be reliably l im ited to 1 %, the cryostat m ay be initially filled to only 61 % capacity. To allow for unexpected delays and com plications, it may be more prudent to fi l l the cryostat to, say, 64%. This would represent a reduction in cryogen fill at the point of departure equivalent to 10% cryogen fill , or 175 litres of cryogen for a 1750 litre capacity cryostat. The added costs of running the generator and depreciation of the transportable carrier and its equipment

are estimated to be i nsignificant com pared to such savings, provided that such shipments are frequently made.

If it is acceptable for only a very small quantity of cryogen to be present in the cryostat on arrival , only a small supply - say sufficient for three days' boil ing - may be provided in the cryostat. This provides much reduced costs at the point of despatch, with correspondingly increased top-up costs at the point of arrival .

The financial savings in terms of cryogen consumption are complemented by an increased predictabi lity of the state of the cryostat on arrival at its desti nati on . With the conventional arrangement of fil ling with cryogen and allowing it to boil off during transit, there is a risk that delays in transit may cause the cryostat to boil dry, allowing the cooled equipment to heat u p to ambient temperature. This then requires an expensive, inconvenient and time consuming re-cooling on arrival . Sufficient volumes of cryogen may not be readily available at the installation site. With the present invention, delays in transit will not cause significant loss of cryogen, provided that the generator is kept running. Use of a diesel powered generator is particularly convenient in this respect, since diesel fuel is readily available in most regions of the world and the cryostat m ay be maintained with an acceptable fill level virtually i ndefinitely, provided that the generator is kept running .

While the present invention has been described with particular reference to the transport of superconducting magnets for magnetic resonance imaging (MRI) or nuclear magnetic resonance (NMR) imaging, the present invention may usefully be applied to the transport of any cryogenically cooled goods or equipment. In some embodiments, it may be preferred to

transport more than one cryostat per container, resulting in a further reduction in shipping costs.