CA1133382A - Cryostat with serviceable refrigerator - Google Patents
Cryostat with serviceable refrigeratorInfo
- Publication number
- CA1133382A CA1133382A CA377,132A CA377132A CA1133382A CA 1133382 A CA1133382 A CA 1133382A CA 377132 A CA377132 A CA 377132A CA 1133382 A CA1133382 A CA 1133382A
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- CA
- Canada
- Prior art keywords
- housing
- refrigeration
- refrigerator
- reservoir
- heat
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Abstract
213-P-US02472 CRYOSTAT WITH SERVICEABLE REFRIGERATOR ABSTRACT OF THE DISCLOSURE A cryostat including a source of refrigeration disposed within an evacuated chamber containing a reservoir of liquified cryogen wherein refrigeration is directed to heat stations disposed in the access means to the reservoir to intercept heat leak into said reservoir and to recondense cryogen boil-off from said reservoir. Thermal convective couplings are used to cool the heat stations and to minimize heat leakage in the event the refrigeration source is disabled. Cryogen boil-off is condensed in an apparatus utilizing low thermal conductive conduits to direct the vapors to a refrigeration zone and return the condensed cryogen to the reservoir.
Description
1~33~8Z
13 1. Field of the Invention 14 This invention relates to cryostats utilized to produce and maintain an inventory of a liquified cryogen such as helium., ' "
16' The inventory of liquified cryogen can be used as a low temperatur'e 17',, environment for such things as infra-red detectors and super- ' 18l conducting devices. Such devices re~uire extremely low temperature 19, environments for efficient operation. Temperatures on the order 20~, of 4.2 degrees Kelvin (K) (~268.9 degrees C) are necessary to 21,, maintain helium in the liquified state. Without supplementary 22 ~ refrigeration a bath of liquid helium would, due to heat infiltra-23 !, tion even in the best insulated Dewar, boil off and be lost.
241, Attempts have been made to combine a refrigerator with a reservoir ~5 1'~ ox Dewar to prevent excessive loss of liguid helium and thus 26 maintain the overall level of inventoried helium.
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`` 1133382 _. The Prior Art Cryostats for producing and malntaining an inventory of a llquified cryogen such as helium range from miniature systems such as disclosed in ~.S. patent 3,728,868 to large systems such as shown in U.S. patents 2,~58,894 and 3,360,955.
The miniature system of the '868 patent employs a source of high-pressure gas cooled through heat exchange and expanded through a Joule-Thompson valve to provide a small inventory of liquld helium at the bottom of a glass Dewar.
The ~94 patent discloses a system employing one or more expansiGn engines and multi-stage heat eY.changers combined with Joule-Thompson expansion valves to produce liquid helium for use in heat exchange with air to liquify the air for subsequent fractional distillation to produce, inter alia, oxygen.
The '955 patent also employs one or more expansion engines, multi-stage heat exchangers, and Joule-Thompson expansion valves to produce liquid helium to cool various types of electrical apparatus including super-conducting devices.
In relation to super-conducting electronic devices such as super-conducting magnets, super-conducting quantum inter-ference devices and Josephson Junction Devices, applicant in U.S. patent 4,223,540 discloses a cryostat wherein the refrigerator can be removed from the vacuum housing, for routine maintenance or in the event the refrigerator malfunctions, without disturbing the vacuum. As pointed out in the copending application, it is desirable to have a cryogenic refrigerator as part of the cryostat so that possible points of heat infiltration (e.g. neck tube accessing the liquid cryogen inventory) can be cooled to prevent heat infiltration and to recolldcnse normal cryogen boi]-off from the liquid reservoir.
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Il li3;:~3~32 1 ¦~ one cryogenic refrigerator ideally suited for this
13 1. Field of the Invention 14 This invention relates to cryostats utilized to produce and maintain an inventory of a liquified cryogen such as helium., ' "
16' The inventory of liquified cryogen can be used as a low temperatur'e 17',, environment for such things as infra-red detectors and super- ' 18l conducting devices. Such devices re~uire extremely low temperature 19, environments for efficient operation. Temperatures on the order 20~, of 4.2 degrees Kelvin (K) (~268.9 degrees C) are necessary to 21,, maintain helium in the liquified state. Without supplementary 22 ~ refrigeration a bath of liquid helium would, due to heat infiltra-23 !, tion even in the best insulated Dewar, boil off and be lost.
241, Attempts have been made to combine a refrigerator with a reservoir ~5 1'~ ox Dewar to prevent excessive loss of liguid helium and thus 26 maintain the overall level of inventoried helium.
.~
`` 1133382 _. The Prior Art Cryostats for producing and malntaining an inventory of a llquified cryogen such as helium range from miniature systems such as disclosed in ~.S. patent 3,728,868 to large systems such as shown in U.S. patents 2,~58,894 and 3,360,955.
The miniature system of the '868 patent employs a source of high-pressure gas cooled through heat exchange and expanded through a Joule-Thompson valve to provide a small inventory of liquld helium at the bottom of a glass Dewar.
The ~94 patent discloses a system employing one or more expansiGn engines and multi-stage heat eY.changers combined with Joule-Thompson expansion valves to produce liquid helium for use in heat exchange with air to liquify the air for subsequent fractional distillation to produce, inter alia, oxygen.
The '955 patent also employs one or more expansion engines, multi-stage heat exchangers, and Joule-Thompson expansion valves to produce liquid helium to cool various types of electrical apparatus including super-conducting devices.
In relation to super-conducting electronic devices such as super-conducting magnets, super-conducting quantum inter-ference devices and Josephson Junction Devices, applicant in U.S. patent 4,223,540 discloses a cryostat wherein the refrigerator can be removed from the vacuum housing, for routine maintenance or in the event the refrigerator malfunctions, without disturbing the vacuum. As pointed out in the copending application, it is desirable to have a cryogenic refrigerator as part of the cryostat so that possible points of heat infiltration (e.g. neck tube accessing the liquid cryogen inventory) can be cooled to prevent heat infiltration and to recolldcnse normal cryogen boi]-off from the liquid reservoir.
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Il li3;:~3~32 1 ¦~ one cryogenic refrigerator ideally suited for this
2 application is sold as a DISPLEX Model CS-308 Closed Cycle
3 Helium Refrigeration System by Air Products and Chemicals, Inc.,
4 Allentown, Pennsylvania. U.S. Patent 3,620,029 discloses in its specification the displacer-expander refrigerator portion of the 6 model CS-308 refrigeration system. This type of refrigerator has 7 been used in many applications. In Mossbauer Spectroscopy it is 8 essential to isolate the sample holder from vibration. ThereLore, 9 non-contact transfer of reirigeration by circulating gas as disclosed in U.S. Patent 3,894,403 was developed for use in 11 precision instruments to cool sample holders.
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3 Applicant's Gopen-d~ng-ap~ ti~n describes a method 14 and apparatus for servicing a cryogenic refrigerator disposed inside a vacuum chamber by removing the entire refrigerator from 16 the chamber.
17 In order to provide a system wherein the refrigerator 18 can be serviced without removing it from the evacuated chamber, 19 in the event that only the moving parts need service, the cold end portion of the refrigerator including the associated heat 21 exchangers and Joule-Thompson expansion valve are permanently 22 fixed for projection into the evacuated chamber. Thus, if the 23 moving parts of the refrigerator need service, they can be service 24 without removing the entire refrigerator from the evacuated chamber.
26 Refrigeration produced inside the evacuated chamber is 27 used to cool heat stations disposed in the access way to the 28 liquid helium reservoir to minimize heat infiltration to the 29 liquid helium reservoir. The heat stations are cooled by con-- --- 1133;~8~
~ective couplings which incLu(le a source of fluid (e.g.
helium) under pressure which is caused to circulate around a closed path by cooling a portion of the convection coupling so that cool gas will fall to the bottom and force warmer gas up toward the refrigeration source. ~n the event the refrigerator is turned off, the cooler gas in the thermal coupling will sink to the bottom of the thermal coupling, gas will stratify in the coupling and the coupling will act as a thermal switch. 'I`l-le coupling has a characteristic of being a passive tllermaL disconllect.
Cryogen boil-off is controlled by providing mealls to condense the boil-off and return the same to the liquid reservoir. Such means includes elongated low thermally-conductive patlls to both direct the boi]-off to the condcnser and return the condensed cryogen to the reservoir.
In one specific aspect the present application, a division of Canadian Application No. 353,027, filed ~lay 29, 1980, is concerned with the provision of a thermal coupling interposed between a source of refrigeration and an object to be cooled comprising in combination:
an elongated fluid-tight housing having first and second ends, said first end adapted to mechanically contact a source of refrigeration and said second end adapted to mechanically contact said object to be cooled;
means within said housing having a flow path whereby a fluid in the form of a gas which remains gaseous at the lowest temperature provided by said refrigeration source and disposed within said housing, can when warmed by giving up refrigeration rise from said second end toward said first end where it is cooled and returned to said second end along a separate path without contacting warmed rising gas.
BRlEF DESCRIPTIO~ OF T~IE DRAWING
Figure 1 is a schematic representation of an apparatus ~3;~382 according to the invention.
Figure 2 is a cross-sectional view of a convective coupling suitable for use in the apparatus of Figure 1.
Figure 3 is a cross-sectional view oi a cryogen condenser suitable for use in the apparatus of Figure 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figure 1, the Cryostat 10 includes a vacuum housing 12 which may be a double-walled construction as is well known in the art. Housing 12 includes suitable (not shown) means for evacuating its interior.
Disposed within the housing ]2 is a reservoir 14 adaptecl to store an inventory of liquified cryogen (eOg. helium) 16.
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1 Reservoir 14 includes a neck or access way (tube) 18 which is 2 affixed to the top 20 of housing 12 and includes a removable 3 cover 22 so that super-conducting devices can be passed into the 4 liquid cryogen 16 from outside of the cryostat 10. The reservoir 14 and access way 18 are preferrably constructed of low thermal 6 conducting material. Reservoir 14 can be of a double-walled 7 construction as is well known in the art. Disposed within access way 18 are a heat stations 24, 26 adapted to prevent heat infil-9 trating from outside the cryostat 10 toward the liguid inventory10 16. The heat stations 24, 26 are made from a,perforated high 11 thermal conductivity material (e.g. copper) to permit gas to 12 expand and contract in the access tube 18. Thermally connected 13 to heat stations 24 and 26 are adapters 28 and 30 for receiving 14 thermal couplings 32 and 34 respectively, the details of which will be disclosed hereinafter.
16 Helium condenser 36 is affixed to reservoir 14 so that 17 an aperture 38 in reservoir 14 will permit normal helium boil-off 18 va.pors to pass into condenser 36 so that the vapors can be recon-19 densed to liguid helium and liguid helium conducted back into the reservoir as will hereinafter be more fully explained.
21 Surrounding reservoir 14 and a major portion of access 22 tube 18 is one or more radiation shields, shown schematically as 23 40, to aid in preventing heat infiltration to the liguid cryogen 24 16 by radiation.
Generally parallel to access way 18 is the cold end 42 26 of a refrigerator 44. Cold end 42 of refrigerator 44 includes a 27 first stage 46 capable of producing refrigeration at approximately 28 60 degrees K (-213C) and a second stage 48 capab~e of producing 29 refrigeration at approximately 15 degrees K (-258C). The refrigerator 44 includes a high-pressure inlet line 50 for admit-I , 1 ting high-pressure fluid, e.g. helium, in gaseous form to the 2 refrigerator and an outlet line 52 for removing warm fluid at 3 lower pressure. Inlet line 50 is adapted to admit high-pressure 4 fluid (e.g. helium) through conduit 54 into a first heat exchanger 56 then through a first adsorber 58 in heat exchange with the 6 first stage 46 of refrigerator 44 through heat station 32 back 7 across the first stage 46 of refrigerator 44 into heat exchanger 8 60 through second adsorber 61 across second stage 48 of refrigera~
9 tor 44 through thermal coupling 34 back across second stage 48 of refrigerator 44 through a third heat exchange,r 62 through a third 11 adsorber 64 through a Joule-Thompson valve 66 through helium 12 condenser 36 then outwardly through the heat exchangers 62, 60 13 and 56 and conduit 68 for recovery and recycle as with the outlet 14 f:Luid of conduit 52.
The adsorbers 58, 61, and 64 are used to purify the in-16 coming helium gas to prevent residual contaminants from blocking 17 the various conduits. For example, adsorber 58 will cause water 18 and CO2 to freeze out of the helium gas, whereas adsorber 62 will 19 remove oxygen and nitrogen and adsorber 64 will remove neon and hydrogen which may be in the helium gas.
21 Joule-Thompson valve 66 includes a control stem 70 22 which can be made to extend outwardly of the housing 12 so that 23 the oxifice size of the valve can be varied. Inlet conduit 50 24 includes a third branch conduit 72 with suitable control valve 74 so that high-pressure fluid can be admitted to thermal coupling 26 32 and 34 respectively as needed. Conduit 72 includes a purge 27 valve 76 and a pressure relief valve 78.
28 Heat exchanger 62 includes a bypass conduit 80 on the 29 return (warm) side and bypass valve 82 to control flow through conduit 80. The bypass valve 82 is open only during cool-down of 113;~3~Z
the refrige~ Lor. ~elow 20 K (~253~) valve 82 must be closed in order for the ret~lrn fluid to pass through heat exchanger 62 to pre-cool the incoming fluid.
Purge valves 83, 84 are included in the heat exchanger circuit to permit purging of the system during startup or to remove contaminants if such condition should develop during operation of the system.
In operation, the refrigerator and all conduits and all covers for the cryostat 10 are made fluid-tight to hollsing 12, after an inventory of liquid helium is placed in re6ervoir 14, refrigerator 44 is activated and high-pressure helium is admitted to the refrigerator and the heat exchangers simultaneously. As refrigerator 44 cools down it produces two levels of refrigeration, at first stage 46 and at second stage 48. The high-pressure helium flowing in conduit 54 is cooled to the first level of refrigeration at first stage 46 of refrigerator 44 and cools the thermal coupling 32. As the helium exits thermal coupling 32, it is recooled by contact with first stage 46 of the refrigerator 44 conducted through the second heat exchanger 60, second adsorber 62 and cooled to a lower tempcrature by second stage 48 of refrigerator 44 after which it is used to cool thermal coupling 34. The helium warmed by cooling thermal coupling 34 is again cooled to the temperature of second stage 48 of refrigerator 44 conducted through heat exchanger 62 and expanded in Joule-Thompson valve 66 to produce a liquified helium. The liquified helium is then passed through condenser 36 to recondense helium boil-off and vaporized where the cold revaporized gas is returned through the heat exchangers 62, 60 and 56 to precool the incoming high-pressure gas by heat exchange, as is well known in the art. Refrigeration produced at thermal couplings 32 and 34 producei an . _ _ 333~2 1 equivalent amount of refrigeration at heat stations 24 and 26 to 2 prevent heat infiltration into the liquid cryogen 16 by providing 3 thermal stratification in the access tube 18. Normal helium 4 boil-off in reservoir 14 is recondensed by conde~ser 36.
Figure 2 shows details of thermal coupling 32 which 6 will illustrate the general structure and operation of both 7 thermal couplings. Thermal coupling 32 includes a housing 90 8 including a first fluid-tight cover 92 and a second fluid-tight 9 cover 94. Housing 90 also includes a flange 96 or other adapter so that the thermal coupling 32 can be affixed to adapter 28 for 11 thermal contact with heat station 24. Disposed within housing 90 12 is a chimney or draft tube 98 to provide a circulation path 13 within housing 90. Chimney 98 is of a shorter length than the 14 housing 90 and can be affixed to the housing by any suitable means such as webs, spiders, and the like or the extended surface 16 as will hereinafter be more fully described. Disposed around the 17 first or upper end 100 of chimney 98 is a heat exchanger or 18 extended surface apparatus 102 including an inlet conduit 104 and 19 an outlet conduit 106. For thermal coupling 32 conduit 104 is connected to the inlet line from adsorber 58 (Figure 1) after 21 being cooled by first stage 46 of refrigerator 44 and outlet 22 conduit 106 conducts the warmed fluid back to first stage 46 of 23 refrigerator 44. Disposed adjacent the bottom end 108 of chimney 24 98 is a second extended surface device 110. Both extended surface device 110 and ~eat exchanger 102 can be made from finned tubing 26 as is well known in the art. Extended surface 110 could be made 27 from perforated plates, screens, parallel plates and the like, it 28 being only reguired to have an extended surface for good heat 29 exchange of the circulating gas. In operation, high-pressure working fluid ~helium) is admitted through pressurization tube il 1133~2 1 ~1 . The housing 90 fille~ with gas and under refrigeration 2 causes the gas in the upper end 92 to become colder than the gas 3 at the bottom end 94, the cold gas falling to the bottom 14 of 4 coupling 32, thus causing warmer gas to rise up the chimney 98:
As the warmer gas rises up the chimney, it forces gas over the 6 top end 100 of chimney 98 past heat exchanger coils 102 and down 7 toward the bottom 94 of thermal coupling 32 between the chimney 8 9~ and the housing 32. The cold gas falling toward the bottom 94 9 causes the bottom plate (end cap) 94 to be cooled to the desired temperature. Housing 90 and chimney 98 are fabricated from 11 materials that are poor thermal conductors, (e.g. stainless 12 steel) whereas bottom plate 94 is fabricated from a good thermal 13 conductor such as copper. The process of warming and cooling and 14 circulation by convection is carried on as long as the refrigera-tion system is in operation. It is necessary that the waxm gas 16 rise up the center in the chimney and the cool gas return or flow 17 between the chimney and the housing 32 to provide effective 18 refrigeration. Therefore, thermal coupling device 32 can only be 19 operated in the vertical position where the refrigeration is produced at the top 92 and the warm fluid rises up through the 21 chimney 98 across extended surface 110. In the event the refrig-22 erator is turned off for service, the cold gas will drop to the 23 bottom of housing 90, the bottom 94 then becoming colder th~n the 24 top 92, the gas stratifies in the housing 90 and the device acts as a thermal switch. Thus, the device has a characteristic of 26 being a passive thermal disconnect when the refrigerator is shut 27 off.
28 When the refrigerator 44 is turned off helium boil-off 29 from reservoir 14 has a large heat capacity and further cools heat stations 26 and 24. The cooling of heat station 26 and 24 _ g _ .,,. ,~ . ,. .. _ 333~
1 in turn further cools the bottom ends of thermal coupling 34, 32 2 preventing heat lea~ through the couplings to the access way 18.
3 In order to promote gas circulation, the cold down- ¦
4 flowing gas is kept separate from the warm rising gas as explained ~bove. Driving potential is egual to the density difference of 6 the rising gas and the falling gas times the height of the chimney 7 98. The density difference is a function of gas temperature and 8 the gas pressure. Since mass circulation rate is proportional to 9 pressure, the device can be used as a variable conductance mecha-nism. Circulation rate is limited by flow friction in the heat 11 exchangers. Couplings, according to the present invention, have 12 been sized for 17 atmospheres internal pressure to operate under 13 the following conditions.
Heat Station First Second . - --~
16 Heat Transfer - Watts 15 2 17 Inlet Gas Temperature K 60.0 15 18 Outlet Gas Temperature K 72.1 16.3 19 Heat Station Temperature K 80.0 18.0 Figure 3 shows a helium condenser 36 including a mountin g 21 plate 120 adapted for fluid-type engagement with aperture 38 in 22 reservoir 14 shown schematically in figure 1. Extending through 23 mounting plate 120 are a plurality of heat conducting tubes 122, 24 124 of low thermal conductivity. The tubes 122, 124 extend through bottom closure 127 o~ housing 126 and terminate adjacent 26 a heat exchanger 128 disposed around an inner concentric tube 130 27 fixed to bottom closure 127 of housing 126. Housing 126 is 28 ~cl ed by a suitable fluid-tig t cover 132. ~eat exchanger 128 .11 J13;~3~3~
1 includes an inlet conduit 134 connected to the output line from 2 the JT valve 66 (Figure 1) and an outlet conduit 136 to return 3 warmed helium through the heat exchangers 62, 60 and 56. The 4 helium flowing in conduit 134 is at about 4.2 degrees K, thus helium boil-off rising through tubes 122, 124 and striking heat 6 exchanger 128 is recondensed and falls back through tubes l22, 7 124 into the reservoir 14. Suitable drainholes such as shown as 8 140 are included in the event liguid helium accumùlates inside 9 concentric tube 130 so that it can be returned to the reservoir 14 also. Condenser 36 also serves to isolate the reservoir 14 11 from thermal conduction in the event the refrigerator is turned 12 off, since the access conduits 122, 124 are of a low thermal 13 conductivity material~ The diameter of tubes 122, 124 are selecte 14 to avoid accoustical oscillation as is well known in the art.
Referring back to figure 1, in the event that the 16 moving parts of the refrigerator 44 have to be serviced, the cold 17 end jacket 42 does not have to be removed from the housing 12 and 18 so the vacuum need not be broken. Those portions of the refriger-19 ator 44 requiring service can be readily removed and serviced.
If the refrigerator shuts down, then flow through the 21 Joule-Thompson loop (56, 58, 60, 61, ~2, 64, 66) ceases and the 22 refrigerator is thermally uncoupled from the liquid helium reser-23 voir 14. Thermal couplings 32, 34 will stay cold and in a typica 24 dewar liquid helium would boil off ovex a period of 10 to 20 days. If the Joule-Thompson loop becomes plugged with contaminant , 26 then it is necessary to warm up the thermal couplings and purge 27 the gas lines. The helium recondenser is designed so it can warm 28 up with only a small heat input into the liquid helium.
29 With an apparatus according to the invention it is possible to warm up the Joule-Thompson loop -(56, 58~ 32, 60, 61, ~3;~3l~Z
34, 62, 64, 66) to purge it of contaminants such as oil, water, gas, etc. with only a small increase in the boil-off rate of liquid helium, e.g. 0.1 to 1.0 Liquid liters per hour.
A device according to the foregoing description is an improvement over those of the prior art, since it isolates the refrigerator so it's moving parts can be removed for service without disturbing the vacuum. There are no moving parts contained within the vacuum envelope and it is possible to thermally isolate the inoperable refrigerator from the liquid helium.
It is also within the purview of the invention to include small heaters with each of the adsorbers 58, 61, 64 to warm the adsorbers if they become plugged.
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. . . _ _ 1 ~S~,,.Q~ 3~'~4~
3 Applicant's Gopen-d~ng-ap~ ti~n describes a method 14 and apparatus for servicing a cryogenic refrigerator disposed inside a vacuum chamber by removing the entire refrigerator from 16 the chamber.
17 In order to provide a system wherein the refrigerator 18 can be serviced without removing it from the evacuated chamber, 19 in the event that only the moving parts need service, the cold end portion of the refrigerator including the associated heat 21 exchangers and Joule-Thompson expansion valve are permanently 22 fixed for projection into the evacuated chamber. Thus, if the 23 moving parts of the refrigerator need service, they can be service 24 without removing the entire refrigerator from the evacuated chamber.
26 Refrigeration produced inside the evacuated chamber is 27 used to cool heat stations disposed in the access way to the 28 liquid helium reservoir to minimize heat infiltration to the 29 liquid helium reservoir. The heat stations are cooled by con-- --- 1133;~8~
~ective couplings which incLu(le a source of fluid (e.g.
helium) under pressure which is caused to circulate around a closed path by cooling a portion of the convection coupling so that cool gas will fall to the bottom and force warmer gas up toward the refrigeration source. ~n the event the refrigerator is turned off, the cooler gas in the thermal coupling will sink to the bottom of the thermal coupling, gas will stratify in the coupling and the coupling will act as a thermal switch. 'I`l-le coupling has a characteristic of being a passive tllermaL disconllect.
Cryogen boil-off is controlled by providing mealls to condense the boil-off and return the same to the liquid reservoir. Such means includes elongated low thermally-conductive patlls to both direct the boi]-off to the condcnser and return the condensed cryogen to the reservoir.
In one specific aspect the present application, a division of Canadian Application No. 353,027, filed ~lay 29, 1980, is concerned with the provision of a thermal coupling interposed between a source of refrigeration and an object to be cooled comprising in combination:
an elongated fluid-tight housing having first and second ends, said first end adapted to mechanically contact a source of refrigeration and said second end adapted to mechanically contact said object to be cooled;
means within said housing having a flow path whereby a fluid in the form of a gas which remains gaseous at the lowest temperature provided by said refrigeration source and disposed within said housing, can when warmed by giving up refrigeration rise from said second end toward said first end where it is cooled and returned to said second end along a separate path without contacting warmed rising gas.
BRlEF DESCRIPTIO~ OF T~IE DRAWING
Figure 1 is a schematic representation of an apparatus ~3;~382 according to the invention.
Figure 2 is a cross-sectional view of a convective coupling suitable for use in the apparatus of Figure 1.
Figure 3 is a cross-sectional view oi a cryogen condenser suitable for use in the apparatus of Figure 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Figure 1, the Cryostat 10 includes a vacuum housing 12 which may be a double-walled construction as is well known in the art. Housing 12 includes suitable (not shown) means for evacuating its interior.
Disposed within the housing ]2 is a reservoir 14 adaptecl to store an inventory of liquified cryogen (eOg. helium) 16.
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1 Reservoir 14 includes a neck or access way (tube) 18 which is 2 affixed to the top 20 of housing 12 and includes a removable 3 cover 22 so that super-conducting devices can be passed into the 4 liquid cryogen 16 from outside of the cryostat 10. The reservoir 14 and access way 18 are preferrably constructed of low thermal 6 conducting material. Reservoir 14 can be of a double-walled 7 construction as is well known in the art. Disposed within access way 18 are a heat stations 24, 26 adapted to prevent heat infil-9 trating from outside the cryostat 10 toward the liguid inventory10 16. The heat stations 24, 26 are made from a,perforated high 11 thermal conductivity material (e.g. copper) to permit gas to 12 expand and contract in the access tube 18. Thermally connected 13 to heat stations 24 and 26 are adapters 28 and 30 for receiving 14 thermal couplings 32 and 34 respectively, the details of which will be disclosed hereinafter.
16 Helium condenser 36 is affixed to reservoir 14 so that 17 an aperture 38 in reservoir 14 will permit normal helium boil-off 18 va.pors to pass into condenser 36 so that the vapors can be recon-19 densed to liguid helium and liguid helium conducted back into the reservoir as will hereinafter be more fully explained.
21 Surrounding reservoir 14 and a major portion of access 22 tube 18 is one or more radiation shields, shown schematically as 23 40, to aid in preventing heat infiltration to the liguid cryogen 24 16 by radiation.
Generally parallel to access way 18 is the cold end 42 26 of a refrigerator 44. Cold end 42 of refrigerator 44 includes a 27 first stage 46 capable of producing refrigeration at approximately 28 60 degrees K (-213C) and a second stage 48 capab~e of producing 29 refrigeration at approximately 15 degrees K (-258C). The refrigerator 44 includes a high-pressure inlet line 50 for admit-I , 1 ting high-pressure fluid, e.g. helium, in gaseous form to the 2 refrigerator and an outlet line 52 for removing warm fluid at 3 lower pressure. Inlet line 50 is adapted to admit high-pressure 4 fluid (e.g. helium) through conduit 54 into a first heat exchanger 56 then through a first adsorber 58 in heat exchange with the 6 first stage 46 of refrigerator 44 through heat station 32 back 7 across the first stage 46 of refrigerator 44 into heat exchanger 8 60 through second adsorber 61 across second stage 48 of refrigera~
9 tor 44 through thermal coupling 34 back across second stage 48 of refrigerator 44 through a third heat exchange,r 62 through a third 11 adsorber 64 through a Joule-Thompson valve 66 through helium 12 condenser 36 then outwardly through the heat exchangers 62, 60 13 and 56 and conduit 68 for recovery and recycle as with the outlet 14 f:Luid of conduit 52.
The adsorbers 58, 61, and 64 are used to purify the in-16 coming helium gas to prevent residual contaminants from blocking 17 the various conduits. For example, adsorber 58 will cause water 18 and CO2 to freeze out of the helium gas, whereas adsorber 62 will 19 remove oxygen and nitrogen and adsorber 64 will remove neon and hydrogen which may be in the helium gas.
21 Joule-Thompson valve 66 includes a control stem 70 22 which can be made to extend outwardly of the housing 12 so that 23 the oxifice size of the valve can be varied. Inlet conduit 50 24 includes a third branch conduit 72 with suitable control valve 74 so that high-pressure fluid can be admitted to thermal coupling 26 32 and 34 respectively as needed. Conduit 72 includes a purge 27 valve 76 and a pressure relief valve 78.
28 Heat exchanger 62 includes a bypass conduit 80 on the 29 return (warm) side and bypass valve 82 to control flow through conduit 80. The bypass valve 82 is open only during cool-down of 113;~3~Z
the refrige~ Lor. ~elow 20 K (~253~) valve 82 must be closed in order for the ret~lrn fluid to pass through heat exchanger 62 to pre-cool the incoming fluid.
Purge valves 83, 84 are included in the heat exchanger circuit to permit purging of the system during startup or to remove contaminants if such condition should develop during operation of the system.
In operation, the refrigerator and all conduits and all covers for the cryostat 10 are made fluid-tight to hollsing 12, after an inventory of liquid helium is placed in re6ervoir 14, refrigerator 44 is activated and high-pressure helium is admitted to the refrigerator and the heat exchangers simultaneously. As refrigerator 44 cools down it produces two levels of refrigeration, at first stage 46 and at second stage 48. The high-pressure helium flowing in conduit 54 is cooled to the first level of refrigeration at first stage 46 of refrigerator 44 and cools the thermal coupling 32. As the helium exits thermal coupling 32, it is recooled by contact with first stage 46 of the refrigerator 44 conducted through the second heat exchanger 60, second adsorber 62 and cooled to a lower tempcrature by second stage 48 of refrigerator 44 after which it is used to cool thermal coupling 34. The helium warmed by cooling thermal coupling 34 is again cooled to the temperature of second stage 48 of refrigerator 44 conducted through heat exchanger 62 and expanded in Joule-Thompson valve 66 to produce a liquified helium. The liquified helium is then passed through condenser 36 to recondense helium boil-off and vaporized where the cold revaporized gas is returned through the heat exchangers 62, 60 and 56 to precool the incoming high-pressure gas by heat exchange, as is well known in the art. Refrigeration produced at thermal couplings 32 and 34 producei an . _ _ 333~2 1 equivalent amount of refrigeration at heat stations 24 and 26 to 2 prevent heat infiltration into the liquid cryogen 16 by providing 3 thermal stratification in the access tube 18. Normal helium 4 boil-off in reservoir 14 is recondensed by conde~ser 36.
Figure 2 shows details of thermal coupling 32 which 6 will illustrate the general structure and operation of both 7 thermal couplings. Thermal coupling 32 includes a housing 90 8 including a first fluid-tight cover 92 and a second fluid-tight 9 cover 94. Housing 90 also includes a flange 96 or other adapter so that the thermal coupling 32 can be affixed to adapter 28 for 11 thermal contact with heat station 24. Disposed within housing 90 12 is a chimney or draft tube 98 to provide a circulation path 13 within housing 90. Chimney 98 is of a shorter length than the 14 housing 90 and can be affixed to the housing by any suitable means such as webs, spiders, and the like or the extended surface 16 as will hereinafter be more fully described. Disposed around the 17 first or upper end 100 of chimney 98 is a heat exchanger or 18 extended surface apparatus 102 including an inlet conduit 104 and 19 an outlet conduit 106. For thermal coupling 32 conduit 104 is connected to the inlet line from adsorber 58 (Figure 1) after 21 being cooled by first stage 46 of refrigerator 44 and outlet 22 conduit 106 conducts the warmed fluid back to first stage 46 of 23 refrigerator 44. Disposed adjacent the bottom end 108 of chimney 24 98 is a second extended surface device 110. Both extended surface device 110 and ~eat exchanger 102 can be made from finned tubing 26 as is well known in the art. Extended surface 110 could be made 27 from perforated plates, screens, parallel plates and the like, it 28 being only reguired to have an extended surface for good heat 29 exchange of the circulating gas. In operation, high-pressure working fluid ~helium) is admitted through pressurization tube il 1133~2 1 ~1 . The housing 90 fille~ with gas and under refrigeration 2 causes the gas in the upper end 92 to become colder than the gas 3 at the bottom end 94, the cold gas falling to the bottom 14 of 4 coupling 32, thus causing warmer gas to rise up the chimney 98:
As the warmer gas rises up the chimney, it forces gas over the 6 top end 100 of chimney 98 past heat exchanger coils 102 and down 7 toward the bottom 94 of thermal coupling 32 between the chimney 8 9~ and the housing 32. The cold gas falling toward the bottom 94 9 causes the bottom plate (end cap) 94 to be cooled to the desired temperature. Housing 90 and chimney 98 are fabricated from 11 materials that are poor thermal conductors, (e.g. stainless 12 steel) whereas bottom plate 94 is fabricated from a good thermal 13 conductor such as copper. The process of warming and cooling and 14 circulation by convection is carried on as long as the refrigera-tion system is in operation. It is necessary that the waxm gas 16 rise up the center in the chimney and the cool gas return or flow 17 between the chimney and the housing 32 to provide effective 18 refrigeration. Therefore, thermal coupling device 32 can only be 19 operated in the vertical position where the refrigeration is produced at the top 92 and the warm fluid rises up through the 21 chimney 98 across extended surface 110. In the event the refrig-22 erator is turned off for service, the cold gas will drop to the 23 bottom of housing 90, the bottom 94 then becoming colder th~n the 24 top 92, the gas stratifies in the housing 90 and the device acts as a thermal switch. Thus, the device has a characteristic of 26 being a passive thermal disconnect when the refrigerator is shut 27 off.
28 When the refrigerator 44 is turned off helium boil-off 29 from reservoir 14 has a large heat capacity and further cools heat stations 26 and 24. The cooling of heat station 26 and 24 _ g _ .,,. ,~ . ,. .. _ 333~
1 in turn further cools the bottom ends of thermal coupling 34, 32 2 preventing heat lea~ through the couplings to the access way 18.
3 In order to promote gas circulation, the cold down- ¦
4 flowing gas is kept separate from the warm rising gas as explained ~bove. Driving potential is egual to the density difference of 6 the rising gas and the falling gas times the height of the chimney 7 98. The density difference is a function of gas temperature and 8 the gas pressure. Since mass circulation rate is proportional to 9 pressure, the device can be used as a variable conductance mecha-nism. Circulation rate is limited by flow friction in the heat 11 exchangers. Couplings, according to the present invention, have 12 been sized for 17 atmospheres internal pressure to operate under 13 the following conditions.
Heat Station First Second . - --~
16 Heat Transfer - Watts 15 2 17 Inlet Gas Temperature K 60.0 15 18 Outlet Gas Temperature K 72.1 16.3 19 Heat Station Temperature K 80.0 18.0 Figure 3 shows a helium condenser 36 including a mountin g 21 plate 120 adapted for fluid-type engagement with aperture 38 in 22 reservoir 14 shown schematically in figure 1. Extending through 23 mounting plate 120 are a plurality of heat conducting tubes 122, 24 124 of low thermal conductivity. The tubes 122, 124 extend through bottom closure 127 o~ housing 126 and terminate adjacent 26 a heat exchanger 128 disposed around an inner concentric tube 130 27 fixed to bottom closure 127 of housing 126. Housing 126 is 28 ~cl ed by a suitable fluid-tig t cover 132. ~eat exchanger 128 .11 J13;~3~3~
1 includes an inlet conduit 134 connected to the output line from 2 the JT valve 66 (Figure 1) and an outlet conduit 136 to return 3 warmed helium through the heat exchangers 62, 60 and 56. The 4 helium flowing in conduit 134 is at about 4.2 degrees K, thus helium boil-off rising through tubes 122, 124 and striking heat 6 exchanger 128 is recondensed and falls back through tubes l22, 7 124 into the reservoir 14. Suitable drainholes such as shown as 8 140 are included in the event liguid helium accumùlates inside 9 concentric tube 130 so that it can be returned to the reservoir 14 also. Condenser 36 also serves to isolate the reservoir 14 11 from thermal conduction in the event the refrigerator is turned 12 off, since the access conduits 122, 124 are of a low thermal 13 conductivity material~ The diameter of tubes 122, 124 are selecte 14 to avoid accoustical oscillation as is well known in the art.
Referring back to figure 1, in the event that the 16 moving parts of the refrigerator 44 have to be serviced, the cold 17 end jacket 42 does not have to be removed from the housing 12 and 18 so the vacuum need not be broken. Those portions of the refriger-19 ator 44 requiring service can be readily removed and serviced.
If the refrigerator shuts down, then flow through the 21 Joule-Thompson loop (56, 58, 60, 61, ~2, 64, 66) ceases and the 22 refrigerator is thermally uncoupled from the liquid helium reser-23 voir 14. Thermal couplings 32, 34 will stay cold and in a typica 24 dewar liquid helium would boil off ovex a period of 10 to 20 days. If the Joule-Thompson loop becomes plugged with contaminant , 26 then it is necessary to warm up the thermal couplings and purge 27 the gas lines. The helium recondenser is designed so it can warm 28 up with only a small heat input into the liquid helium.
29 With an apparatus according to the invention it is possible to warm up the Joule-Thompson loop -(56, 58~ 32, 60, 61, ~3;~3l~Z
34, 62, 64, 66) to purge it of contaminants such as oil, water, gas, etc. with only a small increase in the boil-off rate of liquid helium, e.g. 0.1 to 1.0 Liquid liters per hour.
A device according to the foregoing description is an improvement over those of the prior art, since it isolates the refrigerator so it's moving parts can be removed for service without disturbing the vacuum. There are no moving parts contained within the vacuum envelope and it is possible to thermally isolate the inoperable refrigerator from the liquid helium.
It is also within the purview of the invention to include small heaters with each of the adsorbers 58, 61, 64 to warm the adsorbers if they become plugged.
~ll. -12-
Claims (5)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A thermal coupling interposed between a source of refrigeration and an object to be cooled comprising in combination:
an elongated fluid-tight housing having first and second ends, said first end adapted to mechanically contact a source of refrigeration and said second end adapted to mechanically contact said object to be cooled;
means within said housing having u flow path whereby fluid in the form of a gas which remains gaseous at the lowest temperature provided by said refrigeration source and disposed within said housing, can when warmed by giving up refrigeration rise from said second end toward said first end where it is cooled and returned to said second end along a separate path without contacting warmed rising gas.
an elongated fluid-tight housing having first and second ends, said first end adapted to mechanically contact a source of refrigeration and said second end adapted to mechanically contact said object to be cooled;
means within said housing having u flow path whereby fluid in the form of a gas which remains gaseous at the lowest temperature provided by said refrigeration source and disposed within said housing, can when warmed by giving up refrigeration rise from said second end toward said first end where it is cooled and returned to said second end along a separate path without contacting warmed rising gas.
2. A thermal coupling according to Claim 1 wherein said first end of said housing includes an extended surface cooled by said refrigeration source.
3. A thermal coupling according to Claim 1 wherein said flow path is defined by a chimney within said housing extending from a location adjacent said first end toward a location adjacent said second end whereby warm fluid can rise up said chimney and cooled fluid can flow downwardly between said chimney and said housing.
4. A thermal coupling according to Claim 1 wherein an extended surface is disposed adjacent said second end of said housing.
5. A thermal coupling according to Claim 1 including means to regular fluid pressure inside said housing.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA377,132A CA1133382A (en) | 1979-06-22 | 1981-05-07 | Cryostat with serviceable refrigerator |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/051,462 US4277949A (en) | 1979-06-22 | 1979-06-22 | Cryostat with serviceable refrigerator |
| US051,462 | 1979-06-22 | ||
| CA000353027A CA1118680A (en) | 1979-06-22 | 1980-05-29 | Cryostat with serviceable refrigerator |
| CA377,132A CA1133382A (en) | 1979-06-22 | 1981-05-07 | Cryostat with serviceable refrigerator |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1133382A true CA1133382A (en) | 1982-10-12 |
Family
ID=27166696
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA377,132A Expired CA1133382A (en) | 1979-06-22 | 1981-05-07 | Cryostat with serviceable refrigerator |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1133382A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2431462A (en) * | 2005-02-05 | 2007-04-25 | Siemens Magnet Technology Ltd | A service neck and refrigerator assembly for a cryostat |
-
1981
- 1981-05-07 CA CA377,132A patent/CA1133382A/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2431462A (en) * | 2005-02-05 | 2007-04-25 | Siemens Magnet Technology Ltd | A service neck and refrigerator assembly for a cryostat |
| GB2431462B (en) * | 2005-02-05 | 2008-01-09 | Siemens Magnet Technology Ltd | Recondensing service neck for cryostat |
| US7475552B2 (en) | 2005-02-05 | 2009-01-13 | Siemens Magnet Technology Ltd. | Recondensing service neck for cryostat |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry | ||
| MKEX | Expiry |
Effective date: 19991012 |