US 3119238 A
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Jan. 28, 1964 w. H. CHAMBERLAIN ETI'AL 3,119,238
CRYOGENIC DEWAR Filed Feb. 18, 1963 M W M INVENTORS.
WILLIAM H. CHAMBERLA /N y HARVEY E. MASECK A TTORNE Y.
United States Patent Ofiice 3,119,238 Patented Jan. 28, 1964 3,119,238 CRYOGENIC DEWAR William H. Chamberlain, Walnut Creek, and Harvey E. Maseck, Oakland, tCalitl, assignors to the United States of America as represented by the United States Atomic Energy Commission "Filed Feb. 18, 1963, Ser. No. 259,471 3 Claims. (Cl. 62--54) The present invention relates to the dewars or multiple walled vessels used for the storage of cryogenic fluids and more particularly to a dewar having novel internal heat shielding whereby the evaporation loss of stored fluid is substantially reduced.
In recent years the use of cryogenic gases in the research laboratory and in various industrial processes has been increasing rapidly. Therefore the losses resulting from evaporation during the handling and storing of such cryogenic fluids has assumed much greater importance than formerly. Such losses are virtually always present as cryogenic gases must be maintained at temperatures far below normal atmospheric temperatures and therefore absorb heat by radiation and conduction from the surrounding materials. Absorbed heat causes the fluids to vaporize and this in turn requires that such fluids be stored in a container with an open vent to avoid pressure build-up. Thus under most storage conditions, some loss of the gas is inevitable.
Losses are aggravated where the specific heat of vaporization of the liquid gas is low as in the case of liquid helium for example as the amount of liquid which evaporates for a given heat input is proportionately high. The cost of some commonly used cryogenic gases is very high so that even a fairly small proportionate loss may result in considerable expense.
It is impossible to reduce these losses to zero under ordinary conditions as there will always be some vaporization which must be conducted away by way of a vent so that excessive pressures do not develop in the container. The loss can be minimized however by providing a container oflering maximum thermal insulation to the gas.
In the past, it has been the practice to store liquid helium and other cryogenic gases in a dewar comprised of an inner flask with a narrow neck through which the liquid is supplied or withdrawn. The flask must be vented through the neck at all times since the transfer of heat to the helium cannot be absolutely prevented and in the absence of a vent a dangerous pressure build-up would occur. For maximum thermal shielding of expensive gases a double walled outer envelope entirely surrounds the flask and its neck and is filled with a less expensive cryogenic gas such as liquid nitrogen to shield the inner flask. The dewar is so constructed that a space remains between the flask and the liquid nitrogen envelope, the space being evacuated to minimize the conduction of heat to the helium from the relatively warm nitrogen. An outer vacuum jacket surrounds the nitrogen envelope in spaced relationship therefrom to minimize heat transfer to the nitrogen itself.
This multi-walled construction has been found necessary to maintain cryogenic gases such as helium in liquid form for any reasonable period of time since in the absence of efficient heat shielding, evaporation occurs very rapidly. The liquid helium is expensive to procure, typically about $10.00 per liter, and in addition has a low latent heat of vaporization of the order of 29.8 milliwatt days per liter so that very good insulation is required to keep the loss by vaporization at a reasonable minimum.
In spite of this high loss rate, the low temperature of about 4.2 degrees K. at which liquid helium evaporates, at normal atmospheric pressure, makes its use essential for some applications.
The liquid nitrogen in the dewar is used to shield the helium inasmuch as it is relatively cheap, about 10 per liter, and has a specific heat of vaporization of about 1,870 milliwatt days per liter. This is about sixty times as high as the specific heat of vaporization for helium. The liquid nitrogen containing envelope is at 77.2 degrees K. and therefore offers considerable protection to the inner flask.
With all of these protections against the boiling away of the liquid helium the cost of the helium loss is still undesirably high. The liquid nitrogen envelope, for example, is at 772 K. and therefore still radiates considerable heat to the helium flask which is much colder at 42 K. Development work to minimize this problem led to the present invention.
In this invention a shield of sheet copper or other good heat conducting material is placed within the vacuum space between the inner flask containing the liquid helium and the liquid nitrogen envelope or the outer shell of the dewar where no nitrogen envelope is employed. The shield completely encloses the flask and part of its neck Without physical contact therewith or with the liquid nitrogen envelope. The shield contacts the neck of the flask however at an intermediate position thereon. Helium vapor rising in the neck of the flask keeps the neck at a very low temperature, near 4.2 degrees K. Since the shield is attached to the neck of the flask it is cooled to this low temperature and in turn operates to intercept heat radiation from the liquid nitrogen envelope which radiation otherwise would reach the liquid helium flask. Owing to the low temperature of the shield and the fact that it is formed of a good thermal conductor, such heat is not re-radiated, but is conducted to the evaporating gas in the neck of the flask. The introduction of the shield reduces the heat radiation to the helium by about 92 percent relative to a conventional dewar.
The overall reduction in helium loss brought about by the present invention is partly a function of dewar size and proportions but is of the order of one half for a dewar of typical configuration. Similar savings occur where the invention is used for the storage of gases other than liquid helium.
Therefore it is an object of the present invention to reduce the loss of cryogenic gases resulting from heat transfer thereto.
It is another object of the invention to provide means for extending the period that liquified gases may be stored at atmospheric pressure.
It is a further object of this invention to provide improved thermal insulation for cryogenic gases.
Another object of the present invention is to provide an improved dewar for storing cryogenic gases with minimum evaporation losses.
Still another object of this invention is to provide means for utilizing the vapors of evaparation arising from a cryogenic gas to inhibit such evaporation. V
A still further object of the present invention is to provide a container for storing cryogenic gases with provision for reducing the radiation of heat to the gas to a minimal value.
Another object of this invention is the provision of a refrigerated shielding for a flask holding cryogenic liquids said refrigeration being provided by vapor from the evaporating cryogenic fluid.
Other objects and advantages of this invention will become obvious to those skilled in the art upon considera tion of the following description and accompanying drawing which is an elevational section view of a cryogenic gas dewar constructed according to the invention.
Referring now to the drawing the dewar 11 includes a cylindrical outer casing 12 having a conical upper section 12', the casing being mounted on a wheeled dolly platform 13 for convenience in moving the dewar. The dewar 11 has an inner flask 14 which may be globular in shape and which is made of a thin sheet of stainless steel. Inner flask 14 has a long thin upwardly projecting neck 16 through which the flask may be filled and drained and which provides for the escape of vapor from the de- War. A downwardly opening cup shaped element 17, made of thermal insulating material, is secured to the bottom of flask 14 for support purposes as will be hereinafter described in greater detail.
Surrounding the flask 14 is a spherical thermal shield 18 which is constructed of copper or other good heat conductor and which has a greater diameter than the flask so that it is spaced a small distance therefrom. The shield 18 also has a neck 18' which surrounds the lower portion of neck 16 of the flask 14 for about one third of the length thereof, and at that point the neck of the shield is pressed to a tight contact fit around the neck of the flask to provide good thermal conduction therebetween. The shield 18 has an aperture at the bottom through which the cup element 17 extends, the aperture providing a gas through the shield in order that the region on both sides thereof may be evacuated.
To surround the inner flask 14 with a volume of liquid nitrogen, a second and larger spherical stainless steel flask 19 encloses the shield 18 and is spaced therefrom to provide a vacuum region therebetween. Flask 19 is formed with a neck 21 which terminates about halfway up the neck 16 of the inner flask 14. At this level an annular element 22 of thermal insulating material seals the end of the neck 21 of flask 19 to the neck 16 of flask 14. To eliminate heat conduction towards the inner flask 14, the space between flasks 14 and 19 is evacuated.
To form the outer wall of the liquid nitrogen volume, a third and still larger stainless steel flask 23 surrounds the second flask 19 in concentric relationship therewith. To provide a thermal insulating vacuum region around the liquid nitrogen, still a fourth stainless steel flask 24 surrounds the third flask 23 and is of larger diameter in order to be spaced therefrom. The neck 26 of the third flask 23 and the neck 27 of the fourth flask 24 both enclose the neck 16 of inner flask 14 around the lower fourfifths of the latter, the necks being radially spaced. The space between the upper ends of the necks 2 6 and 27 is sealed by a downwardly extending rim on an annular thermally insulating cap 28 thus creating an enclosed vacuum region between flasks 23 and 24. Insulating cap 28 encircles the neck 16 of the flask 14 but is not secured directly thereto. A distendable cylindrical bellows 29 is disposed coaxially around neck 16 above cap 28 and seals the space between flasks 19 and 23 which constitutes the reservoir for liquid nitrogen.
Although all but the outermost flask 24 is suspended from cap 28, a support element 31 of insulating material extends between the inner surface of flask 23 and the outer surface of flask 19 at the bottom thereof to add rigidity to the assembly of flasks and to reduce swaying during transportation. For similar purposes, a smaller cylindrical support 32 projects upwardly from the base of flask 19 into the cup shaped element 17.
A small diameter tube 33 enters through the insulating cap 28 and extends to a location near the bottom of the liquid nitrogen space between flasks 19 and 23. The tube 33 thus serves as fill line for the liquid nitrogen used to insulate the inner flask 14. A second small tube 34 entering through the insulating cap 28 terminates just below the lower surface of the cap and serves as a vent for evaporating liquid nitrogen. A third tube 36 passes through the insulating element 22 into the space between the flasks 14 and 19. This serves as a pump-out line to provide a vacuum in the space between flask 14 and 19 and is sealed after the pump-out. Since the tube 36 enters this space at a point outside the shield 18 it will be 'noted that air inside the shield is evacuated through the opening around element 17. A fourth small tube 37 enters through the neck 27 into the space between flasks 23 and 24 and serves as a pump-out line for this space. Tube 37 is also sealed off after evacuation of flask 24.
A collar 38 is mounted at the top of outer casing section 12' and is secured around the neck 27 of the outer flask 24 to serve as a mounting for supporting the neck assembly of the multiple flasks. A. broad cylindrical support 39 mounted on the dolly base 13 extends upwardly to the lower part of the outer flask 24 and provides additional support therefor.
In operation, the region between flasks 14 and 19 and the region between flasks 23 and 24, are both pumped out creating vacuums therein and the pump-out tubes 36 and 37 are sealed off as previously discussed. The region between flasks 19 and 23 is then filled with liquid nitrogen after which the inner flask 14 is ready to be filled with with liquid helium or any other cryogenic gas which is to be stored. Any loss of vacuum in the evacuated regions becomes evident immediately in the form of excess boil-ofl of cryogenic liquids and may be corrected by repeating the pump-out and seal-off operations.
Since no container can provide perfect thermal insulation, the helium boils and vapor forms in the top 0 flask 14, the vapor being vented through the neck 16 of the flask. This keeps the lower portion of neck 16 at about the boiling temperature of liquid helium or about 4.2 degrees K. Through the contact of shield 18 with the lower part of neck 16, heat is removed from the shield thus maintaining the shield at a temperature only slightly above 4.2 degrees K. Accordingly, and in contrast to prior cryogenic dewars, very little heat is radiated to the helium flask 14. The thermal radiation from flask 19, at the much higher temperature of boiling liquid nitrogen which is about 77.2 degrees K, goes to the shield 18 wherein it is conducted to neck 16 and transferred to the escaping helium vapor. In prior forms of dewar such radiatnt heat was transferred to the helium flask and promoted evaporation losses.
The overall improvement obtained in dewars of the above described construction is a reduction of the losses resulting from thermal radiation to the cryogenic gas by about 92 percent. As heat is transferred to the gas by processes other than radiation, this amounts to an overall reduction of helium loss typically of about one half.
As conventional dewars age, and surfaces tarnish, the thermal radiation induced losses increase. However in the present invention such losses do not increase very severely as surfaces tarnish but remain substantially at the original low figures.
It will be apparent that modifications in the general configuration of the invention may be made for specialized applications. Where the contained cryogenic gas serves to enclose and refrigerate such apparatus as particle accelerator targets or superconducting magnet coils, the several flasks and shields may be suitably conformed to the outline of such structures. Similarly, it is not essential that the liquid nitrogen envelope be employed if somewhat greater evaporation losses are acceptable.
Thus while the present invention has been disclosed with respect to a preferred embodiment, it will be evident to those skilled in the art that many variations are possible within the spirit and scope of the invention. Therefore it is not intended to limit the invention except as defined by the following claims.
What is claimed is:
1. In a container for a first cryogenic gas, the combination comprising an inner flask for holding said first gas and having a long narrow upwardly projecting vent tubulation, a double-walled envelope providing a vented container for a second more expendable cryogenic gas and formed to surround said inner flask and a lower portion of said vent tubulation, said double-walled envelope being spaced apart from said inner flask providing vacuum space between said inner flask and said envelope, a heat conductive shield surrounding said inner flask within said vacuum space between said inner flask and said double-Walled envelope and having an inwardly turned upper portion contacting an annular zone of the outer surface of said vent tubulation whereby evaporating vapors within said tubulation cool said shield to a very low temperature providing means for intercepting heat radiated from said envelope toward said flask, and an outer shell surrounding said doublewalled envelope in spaced apart relationship thereto and providing vacuum space therebetween.
2. A container for cryogenic gas as described in claim 1 wherein said inner flask, said shield, said double walled envelope and said outer shell are of substantially globular shape and are disposed substantially concentrically, said elements being secured together solely at the upper ends thereof in the region of said vent tubulation.
3. In a storage vessel for cryogenic gases, the combination comprising an inner flask for receiving said cryogenic gas, said flask having a long narrow upwardly projecting neck with a vent opening at the top thereof whereby evaporating gas may escape, an outer envelope substantially surrounding said inner flask in spaced relation therefrom to form a vacuum region therebetween, a double-walled container for a second more expendable cryogenic gas disposed between said inner flask and said outer envelope,
said double-walled container substantially enclosing said flask, and an intermediate shield disposed in said vacuum region between said inner flask and said double-walled container in spaced apart relationship to each thereof, said intermediate shield being formed of a material having high thermal conductivity and having an upper portion which is turned inwardly and secured against a portion of said neck of said inner flask whereby heat radiated from said double-walled container towards said inner flask is intercepted and conducted to escaping cold vapor within said neck.
References Cited in the file of this patent UNITED STATES PATENTS Dana et al Oct. 9, 1934 OTHER REFERENCES