U.S. patent application number 15/935826 was filed with the patent office on 2018-10-04 for cryostat arrangement comprising a neck tube having a supporting structure and an outer tube surrounding the supporting structure to reduce the cryogen consumption.
The applicant listed for this patent is Bruker BioSpin AG. Invention is credited to Steffen BONN, Patrick WIKUS.
Application Number | 20180283769 15/935826 |
Document ID | / |
Family ID | 61763802 |
Filed Date | 2018-10-04 |
United States Patent
Application |
20180283769 |
Kind Code |
A1 |
WIKUS; Patrick ; et
al. |
October 4, 2018 |
CRYOSTAT ARRANGEMENT COMPRISING A NECK TUBE HAVING A SUPPORTING
STRUCTURE AND AN OUTER TUBE SURROUNDING THE SUPPORTING STRUCTURE TO
REDUCE THE CRYOGEN CONSUMPTION
Abstract
A cryostat arrangement (1) with a vacuum tank (2) and a
cryogenic tank (3) are provided. The vacuum tank has at least one
neck tube, (4) leading to the cryogenic tank, with a supporting
structure (4a) and an outer tube (4b) surrounding the supporting
structure. The neck tube provides a connection from the cryogenic
tank to a region outside the vacuum tank to allow cryogenic fluid
to flow from the cryogenic tank into a region outside the vacuum
tank or vice versa. The neck tube mechanically suspends the
cryogenic tank inside the vacuum tank, and parts of the neck tube
form a diffusion barrier between the interior of the cryogenic tank
and the interior of the vacuum tank. The neck tube can connect to
other components of the cryostat arrangement in a fluid-tight
manner. Heat input from the neck tubes into the cryogenic tank can
be considerably reduced thereby.
Inventors: |
WIKUS; Patrick;
(Nuerensdorf, CH) ; BONN; Steffen; (Zurich,
CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bruker BioSpin AG |
Faellanden |
|
CH |
|
|
Family ID: |
61763802 |
Appl. No.: |
15/935826 |
Filed: |
March 26, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F25D 23/06 20130101;
F25B 2309/02 20130101; F25D 19/00 20130101; F25D 2201/14 20130101;
G01R 33/3815 20130101; F25B 19/005 20130101; G01R 33/3804
20130101 |
International
Class: |
F25D 23/06 20060101
F25D023/06; F25B 19/00 20060101 F25B019/00; F25D 19/00 20060101
F25D019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 29, 2017 |
DE |
10 2017 205 279.1 |
Claims
1. A cryostat arrangement, comprising a vacuum tank and a cryogenic
tank, which is arranged inside the vacuum tank, wherein the vacuum
tank comprises at least one neck tube having a supporting structure
surrounded by an outer tube, the neck tube leading to the cryogenic
tank, wherein the neck tube connects an internal volume of the
cryogenic tank to a region outside the vacuum tank so that
cryogenic fluid can flow out of the cryogenic tank into a region
outside the vacuum tank or from the region outside the vacuum tank
into the cryogenic tank, wherein parts of the neck tube used to
mechanically suspend the cryogenic tank inside the vacuum tank and
parts of the neck tube used to construct a diffusion barrier
between an interior of the cryogenic tank and an interior of the
vacuum tank are arranged to be spatially separated from one another
and are produced from materials which are optimized independently
of one another, such that the supporting structure supports a
weight of the cryogenic tank, and the outer tube is produced from a
material through which the cryogenic fluid cannot diffuse, or
through which essentially no cryogenic fluid can diffuse, and
wherein the outer tube is configured to connect to other components
of the cryostat arrangement in a fluid-tight manner.
2. The cryostat arrangement according to claim 1, wherein the
supporting structure is in a form of an inner tube, and wherein the
inner tube connects the internal volume of the cryogenic tank to a
region outside the vacuum tank so that the cryogenic fluid from the
cryogenic tank can flow into a region outside the vacuum tank or
from outside the vacuum tank to flow into a region inside the
cryogenic tank.
3. The cryostat arrangement according to claim 2, wherein the outer
tube is in direct contact with the inner tube.
4. The cryostat arrangement according to claim 2, wherein the outer
tube is at a distance from the inner tube, and a gap remains open
between the inner tube and the outer tube.
5. The cryostat arrangement according to claim 4, wherein the outer
tube and the inner tube are interconnected by a plurality of
axially arranged, radially extending thermal bridges.
6. The cryostat arrangement according to claim 1, wherein the
supporting structure is produced from plastics material that is
fiber reinforced.
7. The cryostat arrangement according to claim 1, wherein the
supporting structure made of plastics material comprises a metal
extension at each of its two ends.
8. The cryostat arrangement according to claim 7, wherein the metal
extensions each have a length of between 20 mm and 100 mm and a
cross-sectional area of stress of between 50 mm.sup.2 and 500
mm.sup.2.
9. The cryostat arrangement according to claim 1, wherein the outer
tube is produced from metal.
10. The cryostat arrangement according to claim 1, wherein inside
an inner tube of the neck tube and/or between the supporting
structure and the outer tube, baffles are installed, which absorb
thermal radiation and prevent convection.
11. The cryostat arrangement according to claim 11, wherein the
baffles are foldable.
12. The cryostat arrangement according to claim 2, wherein an upper
end of the inner tube is closed in a fluid-tight manner in normal
operation by a pressure relief valve or a rupture disk, allowing
the cryogenic fluid flowing away in normal operation to flow
through a gap between the inner tube and the outer tube.
13. The cryostat arrangement according to claim 1, wherein the
cryostat contains a Joule-Thomson (JT) cooler, in which the
cryogenic fluid is depressurized using a pump located outside the
vacuum tank, and the gap between the supporting structure and the
outer tube is part of a connecting line between the JT cooler and
the pump.
14. The cryostat arrangement according to claim 4, wherein the gap
between the inner tube and the outer tube comprises a flow
restrictor at an end of the neck tube which is near the cryogenic
tank.
15. The cryostat arrangement according to claim 1, wherein the
outer tube comprises at least one bellows portion so that the outer
tube does not absorb any axial forces.
16. The cryostat arrangement according to claim 1, wherein the neck
tube is produced from a material for which: .sigma. is a maximum
permissible mechanical stress, and .sigma.>100 MPa; .theta. is
an integral of the thermal conductivity .lamda. over the
temperature range .DELTA.T between 300 K and 4 K, and
.theta.<300 W/m; and wherein a ratio .sigma./.theta.>1/3
(MPam)/W.
17. The cryostat arrangement according to claim 1, wherein an
integral leakage rate out of the cryogenic tank into the vacuum
tank is less than 10.sup.-6 mbarl/s.
18. The cryostat arrangement according to claim 2, wherein the
outer tube is in thermal contact with the inner tube.
19. The cryostat arrangement according to claim 6, wherein the
fiber-reinforced plastics material is G10.
20. The cryostat arrangement according to claim 7, wherein the
metal extension is made of stainless steel.
21. The cryostat arrangement according to claim 8, wherein the
outer tube is made of stainless steel.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims foreign priority under 35 U.S.C.
.sctn. 119(a)-(d) to German Application No. 10 2017 205 279.1 filed
on Mar. 29, 2017, the entire contents of which are hereby
incorporated into the present application by reference.
FIELD OF THE INVENTION
[0002] An aspect of the invention relates to a cryostat
arrangement, comprising a vacuum tank and a cryogenic tank, which
is arranged inside the vacuum tank, the vacuum tank having at least
one neck tube having a supporting structure and an outer tube
surrounding the supporting structure, the neck tube leading to the
cryogenic tank and, wherein the neck tube produces a spatial
connection of an internal volume of the cryogenic tank to a region
outside the vacuum tank so that cryogenic fluid can flow out of the
cryogenic tank into a region outside the vacuum tank or vice
versa.
BACKGROUND
[0003] Aspects of the invention generally relate to the area of
cooling technical systems which should/must be kept at very low
(=cryogenic) temperatures during operation. Such systems can
include, for example, superconducting magnet arrangements of the
type used in the field of magnetic resonance, such as in MRI
topographies or NMR spectrometers. Superconducting magnet
arrangements of this type are conventionally cooled by liquid
helium as a cryogenic fluid.
[0004] An important feature of a superconducting NMR magnet system
is the helium consumption during operation. First, helium
consumption has effects on costs incurred for the operation of the
system; second, the refilling interval is crucially dependent on
the helium consumption. The shorter the refilling interval, the
longer the system can be operated without errors. In the case of a
constant refilling interval, a system having less helium
consumption can also have a more compact design, since the helium
tank can be smaller. The consequence of this is that the system is
cheaper to produce, and the requirements at the installation site
(e.g., room height) are reduced. One of the development aims for
superconducting magnet systems is thus to reduce the consumption of
liquid helium, which, in the case of bath-cooled systems, is
equivalent to a reduction in the thermal load on the helium
tank.
[0005] In typical bath cryostats, a majority of the overall thermal
load on the helium tank is caused by thermal conduction in the
so-called "neck tubes". These neck tubes connect the tank, in which
the liquid cryogen is stored, to the environment. The cryogenic
liquid can be refilled through the neck tubes and can then flow off
(also at a high flow rate, such as during a magnet quench or a
sudden loss of vacuum insulation). The neck tubes are also
necessary for accessing components located in the tank (e.g., the
electrical connections of a magnet coil). In many cryostats, the
neck tubes also support the weight of the tank. One of the greatest
contributing factors to the overall thermal load on the helium tank
originates from said neck tubes, the thermal conduction in the tube
being the dominant mechanism.
[0006] Conventionally, neck tubes have particularly thin walls;
wall thicknesses in the range of a few tenths of a millimeter are
not uncommon. The neck tubes are typically produced from a material
having low thermal conductivity. The wall of the neck tube must not
be permeable to gas so that the vacuum insulation of the cryostat
does not become contaminated and thus unusable. In addition, the
neck tube material must be suitable in terms of connection
technology (weldability, solderability). In many cases, stainless
steel is used.
[0007] U.S. Pat. No. 5,220,800 discloses a generic cryostat
arrangement for a NMR magnet system comprising a superconducting
magnet coil. The cryostat arrangement comprises a double-walled
neck tube, through the annular gap of which helium can flow (see,
for example, FIG. 4 in said document).
[0008] For the mechanical construction and for fastening outer and
inner tubes, U.S. Pat. No. 5,220,800 states that:
[0009] "The manner in which the chambers 1 and 2, radiation shields
21 and 22, and cooling tank 23 are suspended in the cryostat 4 on
suspension tubes 30 is depicted only schematically in FIG. 1. The
connecting elements used are thin-walled tubes or bundles of three
centering rods 26 each, a few millimeters in diameter, which have
extremely low thermal conductivity and high tensile strength."
[0010] The mechanical suspension of the neck tube and the provision
of a fluidic diffusion barrier must thus be physically together in
this case, in particular formed using the same material. Separate
optimizations of the purely mechanical function of a suspension and
the fluidic function of a diffusion barrier, for example, with
respect to the selection of material according to type and
strength, are therefore neither possible nor envisaged according to
the teaching of U.S. Pat. No. 5,220,800.
SUMMARY
[0011] An aspect of the present invention reduces the heat input
originating from the neck tubes into the cryogenic tank--generally
a helium tank--for a cryostat(s) as described herein.
[0012] For a cryostat arrangement of the type(s) as described
herein, the parts of the neck tube are used to mechanically suspend
the cryogenic tank inside the vacuum tank, and the parts of the
neck tube are used to construct a diffusion barrier between the
interior of the cryogenic tank and the interior of the vacuum tank
are arranged so as to be spatially separated from one another and
are produced from materials which are optimized independently of
one another. The supporting structure supports the weight of the
cryogenic tank and is produced from a material for which the ratio
.sigma./.theta. of a maximum permissible mechanical stress .sigma.,
where .sigma.>100 MPa, to .theta., which is the integral of the
thermal conductivity .lamda. over the temperature range .DELTA.T
between 300 K and 4 K, where .theta.<300 W/m, the following
applies: .sigma./.theta.>1/3(MPam)/W. The outer tube is produced
from a material through which cryogenic fluid cannot diffuse, or
through which only an unmeasurable amount of cryogenic fluid can
diffuse in practice, and which tube can be connected to other
components of the cryostat arrangement in a fluid-tight manner so
that the resulting integral leakage rate out of the cryogenic tank
into the vacuum tank is less than 10.sup.-6 mbar.about.l/s.
[0013] An aspect of the present invention comprises forming the
generally double-walled neck tube in such a way that the functions
of mechanically fastening the tank and producing the fluid-tight
connection (minimal permeation of the cryogen through the neck-tube
wall) are separate from one another. This approach allows each
function to be optimized independently of the other.
[0014] However, the reduction in the thermal load which is
achievable by an aspect of the present invention is of great
relevance not only for superconducting magnet systems. For this
reason, an aspect of the invention is also beneficially applicable
to other areas of cryogenics (e.g., storing cryogens such as helium
or hydrogen).
[0015] According to an aspect of the present invention, in
particular, the following advantages are achieved:
[0016] The thermal load on the cryogenic tank can be greatly
reduced by the design of the neck tube which is more efficient with
respect to its thermal properties.
[0017] In the case of actively cooled systems, this allows for the
use of a cooler having a lower cooling capacity, which has, for
example, an advantageous effect on power consumption and system
costs.
[0018] In the case of bath-cooled systems, this leads to a
reduction in the evaporation rate of the cryogenic fluid. First,
this results in a considerable reduction in the operating costs,
and second, the time interval in which the cryogenic fluid
(typically helium) has to be refilled also increases, which reduces
disruptions in long-lasting nuclear magnetic resonance measurements
and increases the availability of the system for NMR measurements
overall.
[0019] Evaporating cryogen is in thermal contact with parts of the
neck tube. This makes it possible to use the enthalpy of the cold
gas to "absorb" heat which flows in the neck tube from the warm end
to the cold end using thermal conduction. In the prior art,
corresponding non-fluid-tight supporting structures are always in
vacuum, and as a result, these structures lack the feature of
thermodynamically advantageous exhaust-gas cooling.
[0020] Most preferred is an embodiment of the cryostat arrangement,
according to an aspect of the invention, in which the supporting
structure is in the form of an inner tube, and wherein the inner
tube produces a spatial connection of the internal volume of the
cryogenic tank to a region outside the vacuum tank so that
cryogenic fluid can flow out of the cryogenic tank into a region
outside the vacuum tank or vice versa. Using the inner tube, access
to the cryogenic tank can be achieved, e.g., to provide electrical
connections to a superconducting magnet coil. In addition, a
sufficiently large cross section is available to allow cryogen
having a high flow rate out of the helium tank to leak out of the
cryogenic tank without an excessive increase in pressure (e.g., in
the case of a quench of a superconducting magnet coil or in the
case of a loss of vacuum).
[0021] In an embodiment, the outer tube is in direct, preferably
thermally well-conducting, contact with the inner tube. The
diffusion barrier can be applied directly to the supporting tube.
The inner tube and outer tube are thus in good thermal contact with
one another. Cryogen, which flows off through the inner tube, cools
the inner tube first and foremost but, since the tubes are in good
thermal contact, is also able to absorb heat from the outer tube.
The greatest advantages of this embodiment lie in the simplicity of
the construction and the thermodynamic efficiency.
[0022] In an alternative embodiment, the outer pipe can be at a
distance from the supporting structure, and a gap can remain open
between the inner tube and the outer tube. This gap can be used in
various ways, as described herein. It is thus possible, for
example, to let cryogen flow through the gap between the supporting
structure and the outer tube, which allows particularly efficient
cooling of the two tube walls. In addition, it is possible to use
the gap as a pump line for operating a Joule-Thomson (JT)
cooler.
[0023] If, in this embodiment, the supporting structure is in the
form of an inner tube which is closed during normal operation, the
advantages already mentioned above (e.g., a large cross section for
electrical connections and quenches/loss of vacuum) can be
achieved, while the "exhaust-gas cooling" of the neck tube can
simultaneously be optimized by a corresponding selection of the gap
geometry (e.g., small gap dimensions for high heat-transfer
coefficients).
[0024] More preferred are variants of this development in which the
outer tube and the supporting structure are interconnected by a
plurality of axially arranged, radially extending thermal bridges.
As a result, even when the supporting structure is at a distance
from the outer tube, good thermal contact between the two tubes and
fluid flowing in the inner tube or gap can be ensured. Thermal
bridges of this type can be, for example, beryllium copper springs
which are fixed to the supporting structure in a thermally
conductive manner. If the supporting structure is then pushed into
the outer tube during system assembly, the beryllium copper springs
press against the inner face of the outer tube, by which good
thermal transfer can be achieved.
[0025] In further advantageous embodiments, the gap between an
inner tube and the outer tube comprises a flow restrictor at the
end of the neck tube which is closer to the cryogenic tank. If the
inlet of the gap is protected by a restrictor, and the
room-temperature-side outlet of the gap has sufficiently large
dimensions, the quench pressure does not have to be taken into
consideration when dimensioning the outer tube, since high pressure
cannot build up in the gap. This makes it possible to form the
outer tube, which tends to be produced from a material having a
relatively great thermal conductivity (e.g., stainless steel) in
order to achieve a sufficiently fluid-tight connection, with
particularly thin walls, which in turn minimizes the axial thermal
conduction in the outer tube.
[0026] The inner tube will typically have a wall thickness of
between 0.5 mm and 3 mm. In order to minimize the heat input into
the cryogenic tank, the wall thickness should be as thin as
possible--in particular, as thin as the mechanical strength
requirements allow. Magnet coils of a size similar to those
referenced herein result in a range of wall thicknesses as
indicated above, for typical neck tube diameters and using the
materials mentioned further below.
[0027] In further preferred embodiments of the invention, the
supporting structure is produced from plastics material, preferably
from fiber-reinforced plastics material, in particular from
glass-fiber reinforced plastic (GRP), more preferably from the
fiber-reinforced composite G10. In the case of GRP, the minimum
achievable wall thickness is restricted by manufacturing
limitations. GRP cannot be made as thin as desired. G10 is a
popular material in cryogenics which is characterized by a
particularly low ratio of thermal conductivity to strength. G10 is
therefore ideally suited to fastening structures having low thermal
conductivity. In addition, G10 is relatively cheap and can be
molded into numerous shapes. Metal connection pieces can be
connected in a simple manner to G10 components by adhesive bonding
or, even better, can be laminated directly during the production of
the G10 component. When using fiber-reinforced composites, it is
possible to orientate the fibers within the matrix in such a way
that the anisotropic properties of the fibers are optimally
utilized, and e.g., the tensile strength of a support tube in the
axial direction is maximized.
[0028] In further advantageous embodiments of the invention, the
supporting structure made of plastics material comprises a metal
extension, preferably made of stainless steel, at each of its two
ends. Typically, the inner tubes are connected by metal parts in
the cryostat (e.g., to the vacuum tank or the cryogenic tank). The
assembly of the cryostat is particularly simple when the plastics
pipe is already equipped with metal sleeves at its ends. In that
case, no metal-plastics composite has to be produced during the
assembly of the cryostat. This is advantageous, since reliable
metal-plastics composites are technologically difficult to produce
during the assembly process but can easily be integrated directly
into the production of the supporting structure made of plastics
material (e.g., by directly embedding the metal sleeve in plastics
material) or can readily be produced (e.g., by adhesion) in a
separate work process before the assembly of the cryostat. It is
easy to carry out welding using metal sleeves at the tube ends.
Welding is a process which can easily be integrated in the assembly
process of a cryostat.
[0029] More preferred are developments of these embodiments in
which the metal extensions each have a length of between 20 mm and
100 mm, preferably approximately 50 mm, and a cross-sectional area
of stress of between 50 mm.sup.2 and 500 mm.sup.2. As already
mentioned above, the metal sleeves are typically connected by other
metal parts in the cryostat (e.g., to the vacuum tank or the
cryogenic tank) using welding. The heat input required during
welding could damage the plastics tube if there is insufficient
distance between the plastics tube and the weld. The dimensions
indicated above ensure that the plastics tube is not heated
excessively when welding in the metal sleeve.
[0030] Another advantageous embodiment of the cryostat arrangement
according to an aspect of the invention is that the outer tube is
produced from metal, preferably from stainless steel. Metals can be
used as a particularly effective diffusion barrier for cryogens. In
addition, it is simple to produce reliable and fluid-tight
connections between metals (e.g. by welding, brazing or soldering).
Stainless steel is one of the most popular materials in cryostat
construction. Its low thermal conductivity and good weldability
make it ideal for the embodiments described here. In addition, very
thin-walled stainless steel tubes having wall thicknesses of a few
tenths of a millimeter are readily commercially available and can
also be produced cheaply by rolling and longitudinal welding. For
cryostats of NMR magnet systems, the beneficial magnetic properties
and the low electrical conductivity are also advantageous. The
thermal conduction within the diffusion barrier can thus be further
reduced (in addition to the favorable material selection).
[0031] In the case of a preferred category of embodiments of the
invention, inside the neck tube, in particular inside an inner tube
and/or between the supporting structure and the outer tube,
so-called baffles are installed, which absorb thermal radiation and
prevent convection. For the operation of the magnet system, in
several respects, it is advantageous for the inner tube to have a
large diameter. This makes it easier for example to introduce
components, such as power supply lines, signal lines, valve rods,
etc., into the cryogenic tank. As the diameter of the inner tube
increases, however, the thermal load on the cryogenic tank also
increases--first, due to the greater cross-sectional area which is
available for transporting thermal radiation and for thermal
conduction in the gas column, and second, due to a more favorable
geometry for forming convection eddies and thermoacoustic
oscillations (Taconis oscillations). If baffles are installed in
the neck tube, the thermal radiation is substantially isolated (the
baffles act as gas-cooled radiation shields), and the formation of
large convection eddies and thermoacoustic oscillations inside the
inner tube is prevented. In so doing, the baffles prevent the mass
flow. The thermal conduction in the gas column is reduced, since
heat transfer resistances occur at each baffle.
[0032] More preferred variants of this category of embodiments
include foldable baffles. A significant advantage of a large inner
tube diameter consists in the fact that, in the case of a large
thermal load on the cryogen (e.g., due to a break in the insulation
vacuum, in a superconducting magnet system, e.g., by a quench),
high pressure cannot build up in the cryogenic tank, since
sufficiently large flow-off cross sections are available. If the
free cross section of the inner tube is minimized by baffles,
however, this advantage is lost. Therefore, it is particularly
beneficial for the baffles to be foldable so that, as soon as a
large pressure increase, and thus a large mass flow from the
cryogenic tank into the region outside the vacuum tank, occurs, the
baffles are folded upwards by the out-flowing gas and release the
pressure via the cross section of the inner tube.
[0033] Also advantageous are embodiments of the cryostat
arrangement according to an aspect of the invention comprising a
tubular supporting structure, in which the upper end of the inner
tube is closed in a fluid-tight manner in normal operation, in
particular by a pressure relief valve or a rupture disk, so that
cryogenic fluid flowing away in normal operation has to flow
through the gap between the inner tube and the outer tube. Cold gas
which is produced by the evaporation of the liquid cryogen in the
cryogenic tank can still provide considerable cooling performance
in the temperature range between the boiling point of the cryogen
and the temperature of the vacuum tank. During flow off, the cold
gas sweeps along the tube walls and absorbs heat which flows using
thermal conduction inside the tube walls of the vacuum tank into
the cryogenic tank ("counter-current cooling"). If the cold gas is
conducted through the gap between the inner tube and the outer
tube, it comes into thermal contact with both the inner tube and
the outer tube, by which particularly efficient counter-current
cooling can be provided. By selecting a suitable gap geometry, a
good balance between a good thermal transfer of the fluid with the
walls of the gap and the loss of pressure in the flowing fluid can
be achieved.
[0034] Further advantageous embodiments of the invention are
characterized in that the cryostat contains a JT cooler, in which
cryogen is depressurized using a pump located outside the vacuum
tank, and in that the gap between the supporting structure and the
outer tube is part of the connecting line between the JT cooler and
the pump. In turn, the above-described advantage can be utilized in
that the cold gas flow off out of the cryostat is used efficiently
to reduce the thermal load using thermal conduction. A JT cooler
can thus be integrated in a cryostat in a particularly simple
manner, since it is not necessary to provide a separate pump line.
Absolute fluid-tightness between the pump line (annular gap) and
the cryogenic tank (or the volume in the inner neck tube) is not
necessary. A low leakage flow is acceptable if it is low by
comparison with the flow which is pumped out by the
refrigerator.
[0035] Furthermore, in embodiments of the cryostat arrangement
according to an aspect of the invention, at least one bellows
portion can be present in the outer tube so that the outer tube
does not absorb any axial forces. If the inner tube and the outer
tube are produced from different materials, it is very likely that
these two materials have different coefficients of thermal
expansion. If the cryostat is cooled down, large mechanical
stresses would therefore be produced in the neck tube arrangement.
These stresses can be counteracted if a bellows is installed in the
outer tube. Said bellows ensures that the tube remains
substantially free from stress.
[0036] Most preferred are variants of the invention in which the
cryostat arrangement is part of an apparatus for nuclear magnetic
resonance, in particular for magnetic resonance imaging (=MRI) or
for magnetic resonance spectroscopy (=NMR), which preferably
comprises a superconducting magnet arrangement. Superconducting
magnets for MRI or NMR are conventionally cooled by liquid helium.
However, the availability of helium and its price are an essential
factor for minimizing helium losses.
[0037] Further advantages of aspects of the invention can be found
in the description and the drawings. Likewise, the features
mentioned above and set out in the following, according to aspects
of the invention, can each be used individually per se or together
in any combinations. The embodiments shown and described are not to
be understood as a definitive list, but rather are in fact examples
for describing aspects of the invention.
DESCRIPTION OF THE DRAWINGS
[0038] Aspects of the invention are shown in the drawings and
described with reference to exemplary embodiments. In the
drawings:
[0039] FIG. 1 is a schematic vertical sectional view of a first
embodiment of the cryostat arrangement according to an aspect of
the invention.
[0040] FIG. 2 is a schematic vertical sectional view of a second
embodiment of the cryostat arrangement comprising baffles in the
neck tube according to an aspect of the invention.
[0041] FIG. 3 is a schematic vertical sectional view of a third
embodiment of the cryostat arrangement comprising a JT cooler and a
thermal barrier in the cryogenic tank according to an aspect of the
invention.
[0042] FIG. 4 is a schematic vertical sectional view of a fourth
embodiment of the cryostat arrangement comprising a bellows portion
in the neck tube according to an aspect of the invention.
[0043] FIG. 5A is a graph of the thermal conductivity integral of
stainless steel versus temperature.
[0044] FIG. 5B is a graph of the thermal conductivity integral of
G10 versus temperature.
DETAILED DESCRIPTION
[0045] FIGS. 1 to 4 of the drawings each show, in a schematic view,
embodiments of the cryostat arrangement according to an aspect of
the invention for storing a cryogen fluid, in particular, for
cooling a superconducting magnet arrangement.
[0046] A cryostat arrangement 1 according to an aspect of the
invention comprises a vacuum tank 2 and a cryogenic tank 3, which
is arranged inside the vacuum tank 2, the vacuum tank 2 comprising
at least one neck tube 4 having a supporting structure 4a and an
outer tube 4b surrounding the supporting structure 4a, the neck
tube 4 leading to the cryogenic tank 3 and, wherein the neck tube 4
produces a spatial connection of an internal volume of the
cryogenic tank 3 to a region outside the vacuum tank 2 so that
cryogenic fluid can flow out of the cryogenic tank 3 into a region
outside the vacuum tank 2 or vice versa (from a region outside the
vacuum tank 2 into the cryogenic tank 3).
[0047] The cryostat arrangement 1 according to an aspect of the
invention is characterized, in that, firstly the parts of the neck
tube 4 used to mechanically suspend the cryogenic tank 3 within the
vacuum tank 2, and secondly the parts of the neck tube 4 used to
construct a diffusion barrier between the interior of the cryogenic
tank 3 and the interior of the vacuum tank 2 are arranged so as to
be spatially separated from one another and are produced from
materials which are optimized differently in each case, in that the
supporting structure 4a supports the weight of the cryogenic tank 3
and is produced from a material in the case of which, for the ratio
.sigma./.theta. of a maximum permissible mechanical stress .sigma.,
where .sigma.>100 MPa, to .theta., where .theta. is the integral
of the thermal conductivity .lamda. over the temperature range
.DELTA.T between 300 K and 4 K, where .theta.<300 W/m, the
following applies: .sigma./.theta.>1/3 (MPam)/W, and in that the
outer tube 4b is produced from a material through which cryogenic
fluid cannot diffuse, or through which only an unmeasurable amount
of cryogenic fluid can diffuse in operation, and in which the neck
tube can be connected to other components of the cryostat
arrangement 1 in a fluid-tight manner so that the resulting
integral leakage rate out of the cryogenic tank 3 into the vacuum
tank 2 is less than 10.sup.-6 mbarl/s.
[0048] In the embodiments of the invention shown in FIGS. 1 to 4 of
the drawings, the supporting structure 4a is in the form of an
inner tube, and wherein the inner tube produces a spatial
connection of the internal volume of the cryogenic tank 3 to a
region outside the vacuum tank 2 so that cryogenic fluid can flow
out of the cryogenic tank 3 into a region outside the vacuum tank 2
or vice versa. In this case, the outer tube 4b can be in direct,
preferably thermally well-conducting, contact with the inner
tube--but this is not shown in the drawings. Alternatively, as
shown in FIG. 1-4, the outer tube 4b can be at a distance from the
inner tube, and a gap 4c can remain open between the inner tube and
the outer tube 4b. In this case, the outer tube 4b and the inner
tube are preferably interconnected by a plurality of axially
arranged, radially extending thermal bridges.
[0049] As can be seen in FIGS. 1, 2 and 4, flow off cryogen may
pass through the annular gap 4c between the inner tube 4a and the
outer tube 4b in order to optimally use the enthalpy of the cold
gas for absorbing the heat which flows from the outer tank (e.g.,
vacuum tank 2) along the neck tube 4 into the cryogenic tank 3.
[0050] The outer tube 4b produces a fluid-tight connection to the
insulation vacuum. The wall thickness is designed in such a way
that the maximum differential pressure between the annular gap 4c
and the insulation vacuum can be absorbed, and no significant
diffusion of the cryogen into the insulation vacuum takes place.
The material is selected in such a way that a fluid-tight
connection to other parts of the cryostat (e.g., the cover plate of
the cryogenic tank 3) can be produced reliably and cheaply. The
outer tube 4b can be produced e.g., from stainless steel which has
excellent welding properties. The wall thickness of said tube can
be selected so as to be very thin, since the tube does not have to
accommodate the entire weight of the cryogenic tank 3 (and the
components located in it).
[0051] The inner tube 4a supports the weight of the cryogenic tank
3. However, it does not have to be hermetically sealed, as a result
of which a material can be selected which is primarily
characterized by the high ratio of mechanical strength and thermal
conductivity. In this case, for example, fiber-reinforced plastics
materials are considered. Thus, for example, the supporting
structure 4a can be produced in particular from GRP, more
preferably from the fiber-reinforced composite G10.
[0052] The GRP tube is connected to a stainless-steel sleeve at its
two ends. Connection options between GRP and a stainless-steel
sleeve are known to a person skilled in the art. The
stainless-steel sleeve must have a certain minimum length
(typically 50 mm).
[0053] The drawings in FIGS. 5A and 5B show the thermal
conductivity integrals for stainless steel (FIG. 5A) and for the
fiber-reinforced composite G10 (FIG. 5B). As can be seen, the
thermal conductivity integral of stainless steel is approximately
30 times greater than that of G10. However, stainless steel is also
much stronger than G10, as a result of which the ratio of strength
to thermal conductivity must be used as a key indicator. The 0.2%
yield strength of stainless steel, which would be used for the
design, is typically 360 MPa (for 1.4301); the tensile strength of
G10 is approximately 270 Mpa.
[0054] Even under the conservative assumption that, in the case of
G10, a safety factor of 3 is applied with respect to the tensile
strength and that stainless steel can be loaded up to the yield
strength, the thermal conduction of a stainless steel tube [(270/3
Mpa)/(1 W/cm)]/[(360 Mpa)/(30 W/cm)]=7.5 times as great as a GRP
tube having the same load capacity.
[0055] Preferably, the supporting structure 4a made of plastics
material will support a metal extension 5a', 5a'', preferably made
of stainless steel, at each of its two ends. The metal extensions
5a', 5a'' each have a length of between 20 mm and 100 mm,
preferably approximately 50 mm, and a cross-sectional area of
stress of between 50 mm.sup.2 and 500 mm.sup.2.
[0056] As shown in FIG. 1, in embodiments of the invention, the
upper end of the inner tube 4a can be closed in a fluid-tight
manner in normal operation, in particular by a pressure relief
valve or a rupture disk 9 so that cryogenic fluid flowing away in
normal operation has to flow through the gap 4c between the inner
tube 4a and the outer tube 4b. The gap 4c between the inner tube 4a
and the outer tube 4b comprises a flow restrictor 7 at the end of
the neck tube 4 which is closer to the cryogenic tank 3.
[0057] FIG. 2 shows an embodiment of the invention which is an
alternative thereto, in which, inside the neck tube 4, in
particular inside the inner tube 4a and/or between the supporting
structure 4a and the outer tube 4b, so-called baffles 6 are
installed, which absorb thermal radiation and prevent convection.
The baffles 6 are preferably foldable so that, in the case of a
rapid flow-off of the cryogen (e.g., in the case of a quench), the
baffles release the cross section of the inner tube 4a and, in this
way, restrict the pressure increase in the cryogenic tank 3.
[0058] In other embodiments of the invention, the annular gap 4c
can also be used as a pump line for supercooled systems. If the
inlet of the pump line is protected by a restrictor, the quench
pressure does not have to be taken into consideration when
dimensioning the outer tube 4b. Absolute fluid-tightness between
the pump line (annular gap 4c) and the cryogenic tank 3 (or the
volume in the inner tube 4a) is not necessary. A low leakage flow
is acceptable if it is minor by comparison with the flow which is
pumped out by the refrigerator.
[0059] FIG. 3 shows an embodiment designed in this manner in which
the cryostat contains a JT cooler, in which cryogenic fluid is
depressurized using a pump located outside the vacuum tank 2 (not
shown in the drawings), wherein the gap 4c between the supporting
structure 4a and the outer tube 4b is part of the connecting line
between the JT cooler and the pump. FIG. 3 shows a cryogenic tank
which is divided into two regions using a thermal barrier 20. The
thermal barrier 20 is thermally insulating but allows pressure
equalization between the two regions (e.g., a flexible membrane
made of thermally insulating material). Above the thermal barrier,
cryogen is, for example, at atmospheric pressure in the saturation
state. Below the barrier, the cryogen is in the supercooled state
(e.g., atmospheric pressure, but a temperature which is below the
equilibrium temperature). Therefore, heat must be conducted away
from the JT cooler. Such an arrangement is ideally suited to the
operation of superconducting magnet coils at temperatures below 4.2
K.
[0060] In order to avoid mechanical redundancy of the system, a
bellows portion 8 can be provided in the outer tube 4b. The outer
tube 4b thus cannot absorb any axial forces and therefore also
cannot be unduly strained by axial forces. An embodiment of the
invention which is configured in this way is shown in FIG. 4.
[0061] Also conceivable are embodiments of the invention in which,
by omitting the exhaust-gas cooling in the annular gap 4c or the
availability as a pump line, a variant without an annular gap is
implemented, or in which the supporting structure--unlike as shown
in the drawings--is not in the form of a tube, but rather of an
individual rod or a plurality of rods.
[0062] The features of all the above-described embodiments of the
invention can--largely--also be combined with one another.
* * * * *