Patent Publication Number: US-6990818-B2

Title: Device for the recondensation, by means of a cryogenerator, of low-boiling gases evaporating from a liquid gas container

Description:
This is a Continuation-In-Part application of international application PCT/EP02/07406 filed Jul. 4, 2002 and claiming the priority of German application 101 37 552.2 filed Aug. 1, 2001. 

   BACKGROUND OF THE INVENTION 
   The invention relates to a device for the re-condensation of low-boiling gases evaporating from a liquid gas container by means of a cryo-generator. With such a device for example a superconductive magnet which is cooled by immersion into liquid helium can be operated over an extended period by re-condensation of the helium evaporated. The device is a small refrigeration apparatus, a so-called cryo-cooler. In a similar way, such a device is used in connection with a superconductive magnet of high-temperature superconductive material which is cooled by immersion into liquid nitrogen. 
   Below the present state of the art is described shortly (see also  FIG. 4 ): 
   The cryo-container  1  consists of an inner container  2 , which is filled with the low-boiling liquid gas, for example, liquid helium, up to a level  7 . The superconductive apparatus, typically a magnetic coil  5  including the power supply lines  6   a ,  6   b  is immersed into the liquid gas. The helium evaporating as a result of the heat supplied to the container  2  is conducted, by way of a narrow tube  8 , to the ambient or rather to a collecting container. For reducing the heat influx, the helium container  2  is surrounded by an enclosure  3  and the space between the inner container  2  and the outer enclosure  3  is evacuated. For further reducing the heat influx, a radiation shield  4  is arranged in the vacuum space between the container  2  and the enclosure  3 . The radiation shield  4  is cooled by the helium gas by way of a contact ring  10  disposed on the tube  8 . On one hand, the tube  8  should be as narrow as possible in order to reduce the heat influx but, on the other hand, if, accidentally, the magnet becomes suddenly normally conductive, the tube  8  should have a sufficiently large cross-section to permit the discharge of the additional gas generated in order avoid in that case an excessive pressure increase in the container  2 . 
   When the helium level has dropped below a certain height the helium must be replenished from a transport container. This requires substantial efforts and expenditures. 
   There are small cooling devices (cryo-generators) by which the helium evaporating from the helium bath can again be liquefied and returned directly to the cryo-container. Some of these devices have two- or several stages and provide sufficient cooling energy for the cooling of radiation shields. The most important embodiments of such cryo-generators are presently the pulse tube cooler and the Gifford-McMahon cooler. 
   As far as this is possible with such low temperature cooling apparatus such a cryo-generator should be easy to handle, uncomplicated in its operation and easy to service. The low temperature-boiling gases used in these cooling apparatus are helium, He, Hydrogen H 2 ; Neon, Ne; nitrogen, N 2  which are also used in the superconductor technology as coolants. 
   A cryo-generator consists basically of cooling equipment with a so-called cold head. This cold head is mounted outside onto the apparatus and extends into the tube  8  down to the container  3  for the liquid gas. There, the cold area  26  is exposed to the liquid level  7  of the liquid gas. The single-stage cooling apparatus is so designed and installed that it can be removed and re-installed without heating the liquid gas. The cold head comprises a regenerator  21  and a pulse tube  23  with a heat exchanger  25  disposed therebetween. The heat exchanger  25  is embedded in the cold area  26 , which is exposed toward the liquid gas bath. 
   The components regenerator  21 , pulse tube  23  are surrounded each by a thermally isolating enclosure/heat shield ( 20 ,  30 ,  31 ,  32 ) in order to prevent thermal coupling to the outside or at least to maintain it within acceptable limits. 
   The cooling apparatus that is the cold head may be of different design, but it includes generally at least two stages. It also extends into the tubular neck  8  and its last stage cold area  28  is disposed above the liquid gas bath. Also, such a multistage cold head can be removed and re-installed without heating the liquid gas bath. Each stage of the cold head consists of a regenerator  21 , and, respectively,  22  and a pulse tube  23  and, respectively,  24 , with a heat exchanger  25  and, respectively,  27  disposed therebetween. Each heat exchanger is contained in a cold area  26  or, respectively,  28 . The cold area of the last stage extends with its exposed surface into the cold vapor space of the liquid gas container  2 . The components, the regenerator  21  and respectively,  22 , the pulse tube  23  and respectively,  24  of the respective stage are, like in the single stage embodiment, each surrounded by a thermally insulating tube  29 ,  30 ,  31 ,  32 . All the cold areas  26 , except for the last one, are disposed in the direction toward the next following stage co-axially opposite a heat transfer ring  10 , which is disposed at the respective location in the tubular neck  8  in good heat transfer relationship. The respective cold head area  26  extends in an axially movable manner, with a small equidistant gap around the circumference, into the associated heat transfer ring  10 , without coming into contact therewith at any point. In this way, there is always a gas passage open from the vapor space above the liquid gas bath to the flange of the cold head. The multistage cooling apparatus extending into the tubular neck  8 , which is mounted onto the flange cover  33  that is bolted onto a connector flange  9  of the corner wall  3 , can expand axially as a result of thermal effects without restrictions. 
   It is the object of the present invention to provide an improved device for the re-condensation of low boiling gases evaporating in a liquid gas container. 
   SUMMARY OF THE INVENTION 
   In a device for the re-condensation of low-boiling gases evaporating from a liquid gas container having a tubular neck in which a cold head of a cryo-generator is supported, the cold head includes a pulse tube with a heat exchanger and a cold area having an annular projection extending into an annular recess formed in a heat transfer ring mounted in the tubular neck in closely spaced relationship with the walls of the annular recess so as to provide a gas passage therethrough and permitting relative axial movement between the cold head and the liquid gas container. 
   Preferably, the thermally isolating shield  20 ,  30 ,  31 ,  32  consists of a layer which is disposed on the respective component and consists of a material which has a low heat conductivity and which prevents or severely limits axial and radial heat transfer. 
   Thermal insulation is provided by an evacuated space extending from end to end of an envelope. To this end, the respective component is surrounded by a thin-walled cylindrical tube with low heat conductivity which, because of its shape or a support structure, remains so stiff that the exterior pressure—that is generally the ambient pressure, in fault situations such as sudden transition of the immersed coil from a superconductive to a normally conductive state generating excess pressure—cannot move the cylindrical tube into contact with the envelope wall over an extended area. Preferably, also the support structures which stiffen the outer wall of the vacuum space consist of a material with low heat conductivity. The support structure may include a rope wound helically around the component from the top to the bottom or vice versa. In place of such a continuous rope, rope sections may be provided on the circumference of the component which are not in contact with one another. Other measures known from the state of the art of insulation engineering may also be used if applicable. 
   In another effective way of providing a vacuum chamber, the outside wall of the vacuum chamber is a thin-walled corrugated tube whose inner open diameter is slightly larger than the component disposed within so that, if contacts are formed, they are established only as short line contacts with the outer wall of the component. Such a chamber may also be formed by a thin-walled tube which has projections or line-like reinforcements so that contacts can be provided only in spots or over short lines. 
   The outer wall of the vacuum chamber may furthermore consist of a thin-walled corrugated tube which has an inner open width which is also slightly larger than that of the one which is surrounded thereby and is held in spaced relationship by rod elements which helically surround the component or by axial rods disposed in circumferentially spaced relationship on the component. 
   For a low-resistance gas flow particularly during a fault each of the cold areas  26  is provided with at least one bore  37   a  or more bores  37   a  uniformly distributed over the circumference. 
   The advantages of the device according to the invention obtained as a result of the design features disclosed will be described below with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a device for the re-condensation of low boiling gases with a cryo-generator including two pulse tube coolers, 
       FIG. 2   a  shows a rope wound helically around a pulse tube cooler tube for ensuring a certain spacing, 
       FIG. 2   b  shows the pulse tube disposed in a corrugated hose for ensuring a certain spacing, 
       FIG. 3  shows the arrangement with two McMahon coolers, and 
       FIG. 4  shows the diagram of a cryostat. 
   

   DESCRIPTION OF THE ARRANGEMENTS OF THE INVENTION AND THE ADVANTAGES THEREOF 
     FIG. 1  shows schematically the construction of the cold head of the two-stage pulse tube cooler and its installation in a cryostat. The pulse tube cooler and its components are only shown to the extent they are needed for an understanding of the invention. 
   The two-stage cooler consists of the regenerator  21  with a connecting line  37  to a compressor which is not shown and which supplies the pulsating gas flow. The pressure varies typically between about 10 bar and 25 bar. At the other end of the regenerator, the gas flow is divided so that a first partial flow is admitted through the first heat exchanger  25  to the first pulse tube  23 . At the opposite end thereof, a second gas flow is admitted by way of the connection  34 . With suitably adjusted values and a time shift of these gas flows a cooling effect is achieved in the area of the heat exchanger  25  providing for a refrigeration output. With this refrigeration output, the radiation shield  4  is cooled down to a first temperature level, which is already substantially below the ambient temperature. For the thermal coupling of the radiation shield  4  to the location of the refrigeration output the heat transfer device  26  comprises a structure with good heat conductivity, the so-called first cold area  26 . At the side adjacent the heat transfer ring  10  which is connected to the tubular neck  8 , the first cold area  26  has a circumferentially toothed structure and the heat transfer ring  10  has a complementary structure. This toothed structure is so designed that at the interface areas which extend in the figure vertically between the cold area  26  and the transfer ring  10  a very narrow gap remains which is filled with the gas evaporating in the container. On the other hand, the tooth engagement is such that a displacement in the vertical direction is possible. In this way, on one hand, a good thermal coupling is achieved and, on the other hand, relative displacement as it occurs for example with different thermal expansions and contractions, is possible. 
   Furthermore, the cold head can be removed and re-installed when necessary without heating the cryostat. 
   The second partial flow of gas out of the first regenerator  21 , which has an intermediate temperature, is conducted, by way of the second heat transfer structure  27 , into the second pulse tube  24  to which, by way of the gas conduit  36  at the upper end thereof, also a pulsating gas flow is supplied. In this way, in the area of the second heat transfer structure  27 , the temperature is further reduced. Such coolers are in accordance with the state of the art so constructed that at the first stage a first temperature reduction in the range of 30° K and 100° and in the second stage a cooling energy with a much smaller temperature reduction in a temperature range of 5° K which is available for the condensation of helium is available. If the second heat exchanger  27  is embedded into the second cold area  28 , which is a second heat conductive structure also with good heat conductivity and a large surface area on the side of the evaporating helium, the helium evaporating in the container  2  can be condensed and it can return to the bath disposed below. 
   Because of the method of operation of the cooler with a pulsating gas stream, the temperature varies slightly in each operating cycle at the surfaces subjected to the internal pressure. In the pulse tubes  23  and  24 , this effect is particularly pronounced. With the temperature change at the side adjacent the evaporating helium a locally limited expansion of this gas occurs. This however, results in a movement of the gas in the whole container neck formed by the tubes  8   a  and  8   b . As a result, there is a heat flow from the warm upper support flange  33  to the cold gas space  7 , which is undesirable. There is furthermore an additional effect, which results from the different temperature distributions in the regenerators and the pulse tubes. As a result, these components may have different temperatures at the same level. This unavoidably results in a natural convection, which may also cause a detrimental heat transport. 
   Both effects are avoided if both regenerators  21 ,  22  and both pulse tubes  23 ,  24  have thermally insulated walls  29  to  32 . The pulse tubes  23 ,  24  can be insulated by enclosing them in a layer of plastic which has a low heat conductivity or by providing an evacuated intermediate space that is a vacuum chamber. The numeral  30  designates the thermally insulating tube  29  surrounding the first regenerator,  29  designates the tube surrounding the second regenerator and  32  the tube surrounding the second pulse tube. It is however a disadvantage that through the wall of such a thermally insulating tube an additional heat flow to the respective cold end is established. In order to reduce this effect, the insulating tube must be as thin-walled as possible. However, if the wall is too thin, the tube may be bulged inwardly because of the external pressure effective thereon. The measures schematically shown in  FIGS. 2   a  and  2   b  help to avoid such bulging.  FIG. 2   a  shows an example of such a component with the larger diameter, that is for the regenerator  21 , wherein the tube  30  is provided with a support structure disposed on the inner tube  21   a  for stabilizing the tube. A second solution is shown in  FIG. 2   b . In this case, the thin-walled tube is in the form of a corrugated tube. If the open width of this corrugated tube is slightly greater than the outer diameter of the inner tube, only point-like contacts with negligible heat transfer bridges can form. These tubes may be permanently sealed or they may be connected to communication lines leading to a vacuum pump. 
   Under normal operating conditions, the helium gas assumes within the tubular neck  8   a  and  8   b  a stationary temperature distribution without internal connection and the connecting line  37  is closed. Only when the pressure in the gas space exceeds a predetermined value because of a fault, the exhaust gas line  37  is opened for example by way of a pressure relief valve. If it is necessary to release a large amount of gas, the body  26  at the first cold area may be provided with bores  37   a  which facilitate the discharge of gases from the lower neck part with the surrounding wall  8   b  into the part with the surrounding wall  8   a.    
     FIG. 3  shows schematically the important components of the Gifford McMahon cooler for helium re-condensation, specifically the analog solution for a two stage Gifford McMahon cooler. The first stage is formed by a circular structure  41 . Its lower front end surface forms the first cold area  26 . The following second cylinder  43  with smaller diameter forms the second stage. The pressure pulsations in the interior of these cylinders  41 ,  43  and the movement of the regenerators result in temperature changes at the outer walls. To avoid the undesirable heat flow caused thereby the wall surfaces of both cylinders should be thermally insulated. In the representation of  FIG. 3 , a corrugated tube structure  42 ,  44  is shown for that purpose. The other solutions described above can also be used in connection with the Gifford McMahon cooler.