GB2025029A - Vacuum Insulated Vessels or Conduits - Google Patents

Abstract

The inner member (12) of a liquefied gas storage vessel or conduit is kept out of physical contact with an outer member (14) at ambient or other higher temperature by using the repulsive forces between the like polarised permanent magnets. The magnets (20, 26) are positioned in the evacuated space between the two members at locations spaced from where the members are connected together (16), so as to limit relative movement. <IMAGE>

Description

SPECIFICATION Vacuum-insulated Vessels The present invention relates to vacuuminsulated vessels or conduits and in particular, vacuum-insulated vessels or conduits for liquefied cryogenic gases. Such vessels or conduits have an outer member at or near ambient temperature, and an inner member at the temperature of the liquified gas, be it oxygen, nitrogen, helium etc.

Usually the inner member is wrapped with alternating layers of metal foil (to cut down the radiation of thermal energy between the two members) and glass tissue (to cut down conduction and convection losses). The space between the two members is evacuated of air to reduce conduction and convection losses still further, It is this latter provision which gives rise to the term 'vacuum-insulated'.

In some vacuum-insulated vessels or conduits there are one or more intermediate screens provided to give an even higher degree of thermal insulation between the outermost and innermost members. In the present description the term 'inner member' can be taken to mean a screen or the innermost member, and the term 'outer member' can be taken to mean a screen or the outermost member, wher the context permits.

In use, it is important to reduce to a minimum the paths by which heat can pass from the atmosphere to the liquified gas, because this results in undesired vaporisation of gas. With vacuum-insulated vessels and conduits it is necessary to provide some measure of mechanical support for the inner member to prevent it from moving excessively relative to the outer member. The support may be permanent, as in the case of spacers of plastics or other material positioned between the tubular inner and outer members or a vacuum-insulated conduit, or it may be transient, as in the case of snubbers fitted in storage vessels to limit deflection of the inner member relative to the outer member when subjected to sudden movement or shock.In the first case the spacers, whatever their shape or material, provide a direct thermal bridge between the two members, and are therefore inherently inefficient as thermal insulators. In the second case, the machining and assembly requirements are very high to ensure that the two members are kept spaced from each other except when subjected to transient deflection loads.

The present invention aims at providing means not relying on mechanical contact to keep two members at significantly different temperatures spaced from each other under all normal permanent or transient loads tending to force the two members into contact with each other.

According to the present invention, a vacuuminsulated vessel or conduit comprises an inner member spaced from an outer member, the space between the members being evacuated, and magnets in the evacuated space so arranged that when operational, they seek to maintain the members in their spaced relationship.

Preferably, the inner member is in the form of a tubular member for the passage therethrough of liquefied cryogenic gas which inner tubular member is arranged coaxially within the outer member which is also in tubular form, at intervals along their length, the outer surface of the inner tubular member and the inner surface of the outer tubular member are provided respectively with circular arrays of permanent magnets, an array on the inner tubular member being aligned with a complementary array on the outer tubular member such that both arrays carry the same number of permanent magnets with each pair of opposed magnets lying on the same radius and the magnets being polarised so that their opposing faces are of the same polarity.

Alternatively, the inner member is in the form of an inner vessel for the storage of liquid cryogenic gases which inner vessel is supported within the outer member which is in the form of an outer vessel, a post projecting from the inner vessel and carrying a circular array of angularly spaced permanent magnets, the post extending into but being spaced from a collar projecting from the outer vessel, the collar carrying a complementary array of angularly spaced permanent magnets, the arrays being aligned with each other and arranged such that each carries the same number of magnets with each pair of opposed magnets lying on the same radius, the magnets being polarised so that their opposing faces are of the same polarity.

Embodiments of the invention will now be described, by way of example, with reference to the Figures of the accompanying diagrammatic drawing, in which: Figure 1 is an axial cross-section of part of a vacuum-insulated conduit; Figure 2 is a view, partly in section and partly in elevation, of a vacuum-insulated storage vessel; Figure 3 is part of a cross-sectional view of a deep low temperature, vacuum-insulated storage vessel.

The conduit shown in Figure 1 includes an inner tubular member 2 through which a liquefied cryogenic gas is to be conveyed from a reservoir or other source to a load at which the gas is normally vaporised in order to extract significant amounts of heat from the load. The inner member 2 is spaced radially from an outer tubular member 4, the members 2 and 4 being coaxial. Normally, the inner member has its outer surface wrapped with alternating layers of glass fibre tissue and metal foil to reduce to a minimum the conveyance of thermal energy to the inner member 2 by radiation, conduction or convection from the outer member 4.

Vacuum-insulated conduit is normally made in sections of convenient length for handling on site, usually 7.5 metres. The ends of the factoryevacuated sections are normally mechanically connected together through thin sleeves of stainless steel or like material to form a long path for heat conduction but which is sufficiently strong to maintain the integrity of the vacuum between the inner and outer members.It is also usual to insert metal bellow or like members in the inner member 2 to accommodate differential thermal movement of the inner and outer members (unless the inner member is made of a material having a low coefficient of thermal expansion, such as that sold under the trademark INVAR) as the conduit is initially cooled down by the introduction of liquefied cryogenic gas, and as it later is allowed to warm up after all the gas has been vaporised, as can happen when the conduit is disused for a time. For clarity of illustration, the thermal insulation, lagging, thermal stand-offs and thermal movement compensation devices have been omitted from Figure 1, initially cooled down by the introduction of liquefied cryogenic gas, and as it later is allowed to warm up after all the gas has been vaporised, as can happen when the conduit is disused for a time.For clarity of illustration, the thermal insulation, lagging, thermal stand-offs and thermal movement compensation devices have been omitted from the drawing, and will not be described herein in any further detail.

Because of their lengths, it will be appreciated that there is a tendency for the inner tubular member 2 to sag relatively to the outer tubular member 4 under its own weight.as liquefied gas is introduced into, and passes through, the inner tubular member. At intervals along its length, the outer surface of the inner tubular member 2 and the inner surface of the outer tubular member 4 are provided with circular arrays of permanent magnets. Each array takes the form of a sleeve 6 of plastics material having embedded in it at regular angular intervals small but strong permanent magnets 8. A sleeve 6 is clamped to the inner tubular member 2, and another sleeve 6 is clamped to the outer tubular member 4. Both the sleeves 6 are aligned radially with each other, and both carry the same number of magnets 8, with each pair of opposed magnets lying on the same radius.The magnets 8 are polarised so that their opposing faces are of the same polarity, thus giving rise to very high magnetic repulsive forces between the magnets, and therefore between the inner and outer tubular members. By means which are not shown in the drawing, or futher described herein, both the sleeves are secured firmly to their respective members so that the relative orientation of the opposing magnets is not able to change significantly despite the very high repulsive forces exerted between each pair of magnets as they oppose any deflection.

Although the magnetic arrays are stated as being positioned directly on their associated members, it is within the purview of the present invention for them to be mounted on top of the usual 'superinsulation' wrapped around the inner member. This makes it possible for such superinsulation to be applied in the form of a continuous helix, giving rise to significant economies in manufacturing, but in this case it is important to ensure that the magnet array mounted on the insulation is not able to rotate about the axis of the inner tubular member 2 to a position in which magnets of each pair no longer face each other.

It is similarly practical to apply this support system as first described with 'superinsulation' as a multi-layered continuous helix between the opposing pole faces of the magnetic support system as the thermal insulator is itself nonmagnetic.

The gap between the opposing pole faces is dependant upon the maximum permissible deflection, which is governed by failure by buckling in the case of a vessel neck tubes. The gap would normally be in-the region of one to two millimetres. Normally, the-designed gap between the opposing pole faces would be greater than this value, with the intention that the gap would not become less than this figure under all foreseeable relative deflection forces exerted on the inner and outer members of the conduit.

As shown in Figure 2, the storage vessel is such as is normally used for storing liquid air, oxygen or nitrogen. It includes an inner member in the form of an inner vessel 12 and an outer member also in the form of a vessel 14, the two vessels being connected together at their necks 1 6. The space between the two vessels 12, 14 normally contains superinsulation and is evacuated of air for the reasons discussed above.

As will be appreciated from a consideration of the construction of the vessel, particularly when the inner vessel 12 is full of a liquefied cryogenic gas, sudden accelerations or decelerations applied to the vessel can lead to significant flexural forces applied to the interconnected necks 1 6. It is important to ensure that the relative movement of the two vessels is limited to such an extent that the distortion does not become excessive. To this end, it is usual to provide a 'snubber' between the bottoms of the two vessels.

As shown, the snubber includes a post 18 projecting from the bottom end of the inner vessel 12, the post carrying a circular array of angularly- spaced permanent magnets 20. Projecting upwardly from the base 22 of the outer vessel 14 is a collar 24 which is welded to, or integral with, the base. The collar 24 carries a complementary array of permanent magnets 26 which are aligned relatively to the magnets 20 so that each pair of magnets lie on a common radius, with their opposing pole faces being spaced from each other by a predetermined distance and being of the same magnetic polarity.

The radial spacing between the opposing pole faces is such that, even if the opposing faces did come into contact with each other, the resultant mechanical contact would limit the maximum deflection permitted to the inner vessel 12.

However, in practice, the magnets 20 and 26 are so strong that the repulsive forces between the respective pairs of magnets would increase so rapidly as they neared each other that physical contact between the magnets would be virtually impossible, under the operating conditions for which the vessel was designed.

It will be appreciated that, even if such contact did take place, it would last for such a relatively short time that there would be an insignificant leakage of heat from collar 24 to post 1 8 before the magnetic repulsive forces were able to bias the contacting magnets out of contact with each other.

Similarly to what was described above in connection with that embodiment of the invention illustrated in Figure 1, one or both arrays of biasing magnets may be mounted on thermal insulation positioned in the evacuated space between the vessels, rather than directly on members in direct thermal contact with their respective vessels. Again, the method of mounting the magnets must be such as not to allow them to distort out of the their radially opposing positions when subjected to the very high repulsive forces which can be applied to both magnets of a pair as they are forced towards each other by externally applied forces.

The storage vessel shown in Figure 3 is intended for use for liquid helium, which has to be stored at temperatures as low as 4K. It is even more important for such usage to keep down to an absolute minimum the leakage of heat from the outer vessel 14 to the inner vessel 12 (parts common to the vessels shown in Figure 2 being given the same references). To this end the space between the two vessels is shown as containing two intermediate screens 28 and 30, each carrying its own body 38, 40 of superinsulation material. When screens are used, it is important to ensure that the screens are thermally insulated both from each other and from the body of the inner vessel. The present invention provides a way in which the screens are kept so spaced by virtue of magnetic repulsion forces.

The embodiment shown in Figure 3 is for use with vessels for storing liquefied helium. Because of the very high thermal differences between the temperature of the liquid helium and the atmosphere, it is necessary for the thermal insulation between the inner container and the outer one to be as efficient as possible. In known fashion, this includes positioning a pair of gascooled shields between the inner vessel 12 for the liquid helium and the outer vessel 14 at ambient temperature. The inner shield 28 and the outer shield 30 both have flanged ends 32 and 34 which are soldered to the neck 36 of vessel 12.

Because it is impossible to prevent the liquid helium from absorbing some heat, and using this heat to be vaporised, there is a constant flow of very cold vaporised helium up neck 36. This cold gas is effective to remove heat, through the soldered connections, from shields 28 and 30, together with the bodies of superinsulation 38 and 40 carried by the shields.

Because the neck 36 and the soldered connections between this and flanges 32 and 34 are not of infinite mechanical strength, it is important to limit deflection of the inner vessel and the shields from the illustrated position. This limitation of deflection is achieved by sets of repulsive forces enanating from magnets.

Similarly to the arrangement shown in Figure 2, the outer shield has its deflection limited by means of a sleeve 42 projecting upwardly from the base of vessel 14, and carrying a circular array of magnets 44. Projecting downwardly from the base of the outer shield 30 is a nub 46 which overlaps axially with sleeve 42. The nub 46 carries a similar series of magnets 48, which are radially aligned with magnets 44, having like poles facing each other. The forces between the pairs of aligned magnets are such that, no matter in which direction the outer shield tends to be displaced relatively to the outer vessel 14, the magnet forces tend to restore it to the illustrated coaxial position in which all the magnetic forces balance each other.

A similar arrangement is used to hold the inner shield 28 in position relatively to the outer shield, and the inner vessel 12 relatively to the inner shield 28. Both arrangements use arrays of magnets mounted on collars opposing arrays of magnets mounted on nubs. For clarity of illustration, the references to the two inner sets of sleeves, nubs and magnets have been omitted from Figure 3.

It will thus be seen that the present invention provides means relying on the repulsive forces between two (ike-poled' magnets to keep two members at significantly different temperatures from each other out of mechanical contact with each other despite the various forces to which the two members are subjected during use, transport and storage.

It is conceivable but not likely that the permanent magnets could be replaced by electrically operated coil magnets.

Claims (6)

Claims

1. A vacuum insulated vessel or conduit comprising an inner member spaced from an outer member, the space between the members being evacuated, and magnets in the evacuated space so arranged that when operational, they seek to maintain the members in their spaced relationship.

2. A vacuum insulated conduit as claimed in claim 1, in which the inner member is in the form of a tubular member for the passage therethrough of liquefied cryogenic gas which inner tubular member is arranged coaxially within the outer member which is also in tubular form, at intervals along their length, the outer surface of the inner tubular member and the inner surface of the outer tubular member are provided respectively with circular arrays of permanent magnets, an array on the inner tubular member being aligned with a complementary array on the outer tubular member such that both arrays carry the same number of permanent magnets with each pair of opposed magnets lying on the same radius and the magnets being polarised so that their opposing faces are of the same polarity.

3. Avacuum insulated conduit as claimed in claim 2, in which each array comprises a sleeve of plastics material clamped to its respective surface and having embedded in it, at regular intervals, permanent magnets.

4. A vacuum insulated vessel as claimed in claim 1, in which the inner member is in the form of an inner vessel for the storage of liquid cryogenic gases which inner vessel is supported within the outer member which is in the form of an outer vessel, a post projecting from the inner vessel and carrying a circular array of angularly spaced permanent magnets, the post extending into but being spaced from a collar projecting from the outer vessel, the collar carrying a complementary array of angularly spaced permanent magnets, the arrays being aligned with each other and arranged such that each carries the same number of magnets with each pair of opposed magnets lying on the same radius, the magnets being polarised so that their opposing faces are of the same polarity.

5. A vacuum insulated vessel as claimed in claim 1, in which the inner member is in the form of an inner vessel for the storage of liquid cryogenic gases, which inner vessel is supported within the outer member which is in the form of an outer vessel, and a shield positioned in the space between the inner vessel and the outer vessel, the outer vessel having a collar projecting therefrom which carries a circular array of angularly spaced magnets which are in alignment with a circular array of angularly spaced magnets carried by a nub extending from the screen, the cooperating arrays being such that they both carry the same number of magnets with each pair of opposed magnets lying on the same radius, the magnets being polarised so that their opposing faces are of the same polarity, the screen carrying a second circular array of angularly spaced permanent magnets which array is in alignment with a circular array of angularly spaced magnets carried by a post extending from the inner vessel, both arrays carry the same number of magnets with each pair of opposed magnets lying in the same radius, the magnets being polarised so that their opposing faces are of the same polarity.

6. A vacuum insulated vessel or conduit constructed and arranged substantially as hereinbefore described with reference to and as illustrated in Figure 1 or Figure 2 or Figure 3 of the accompanying drawings.