US3668880A

US3668880A - Capillary insulation

Info

Publication number: US3668880A
Application number: US81440A
Authority: US
Inventors: John P Gille
Original assignee: Martin Marietta Corp
Current assignee: Martin Marietta Corp
Priority date: 1970-10-16
Filing date: 1970-10-16
Publication date: 1972-06-13
Anticipated expiration: 1989-06-13
Also published as: JPS5511639B1; GB1370348A; FR2111456A5; DE2151467A1; DE2151467C2

Abstract

A capillary insulation assembly for insulating low temperature liquids consists of a cellular core defining discrete cells and a capillary cover having at least one or more openings per cell attached to the liquid side of the cellular core. A structurally stable liquid-gas interface forms at each of the openings preventing the entry of liquid into the cell and thereby positioning a layer of gas between the liquid and the container wall. A thermal barrier in the form of an insulative layer is interposed between the capillary cover and the contained liquid with the insulative layer being operative to insulate the gasliquid interface from subcooled liquid in the container thereby to maintain the temperature at the interface at the saturation temperature.

Description

[ June 13, 1972 [54] CAPILLARY INSULATION [72] Inventor: John P. Gille, Littleton, Colo.

Martin Marietta Corporation, Friendship lntl. Airport, Md.

[22] Filed: Oct. 16,1970

[2]] Appl.No.: 81,440

[73] Assignee:

[52] [1.8. CI ..62/45, 220/9 LG [51] Int. Cl. F 17c 13/00 [58] Field of Search ..220/9 A, 9 B, 9 D, 9 LG, 10, 220/15; 62/45 [56] References Cited UNITED STATES PATENTS 2,676,773 4/1954 Sanz et al. ..220/9 A X 2,937,780 5/1960 Beckwith ..220/9 LG 3,365,897 l/l968 Middleton et al ..62/45 2,859,895 11/1958 Beckwith ..220/65 2,947,438 8/1960 Clauson ..220/15 3,018,018 l/l962 Beckwith.... ..220/63 3,019,937 2/1962 Morrison ..220/9 LG X nix 3,150,794 9/ 1964 Schlumberger et al ..220/9 LG 3,208,621 9/ 1965 Dawson ...220/9 LG 3,261,087 7/1966 Schlumberger 220/9 A X 3,325,037 6/1967 Kohn et al 0220/9 A Primary Examiner-Meyer Perlin Assistant Examiner-Ronald C. Capossela Attorney-Phillip L. De Arment and Gay Chin [57] ABSTRACT tioning a layer of gas between the liquid and the container wall. A thermal barrier in the form of an insulative layer is interposed between the capillary cover and the contained liquid with the insulative layer being operative to insulate the gasliquid interface from subcooled liquid in the container thereby to maintain the temperature at the interface at the saturation temperature.

12 Claims, 3 Drawing Figures PATENTEDJUH 13 m2 3, 668,880

//VVEN7'0/? JOHN A G/LLE FIG. 2 V2 ATTORNEYS CAPILLARY INSULATION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 stat. 435; 42 U.S.D. 2457).

This invention relates to the field of insulation and, more particularly, to an improved insulation for use in the containment of low temperature or cryogenic liquids.

The need for lightweight, inexpensive, reliable non-vacuum insulation for the containment of cryogenic liquids has long been recognized. Such a cryogenic insulation is disclosed in copending application Ser. No. 44,678, filed June 9, 1970, and assigned to the assignee of this invention, and the disclosure thereof is incorporated herein by reference. This insulation provides a layer of gas between the liquid and the wall of the container with capillary forces being used to separate the liquid and gas phases. More specifically, the afore-mentioned application discloses a cellular or honeycomb core which forms discrete and separate cells attached to the container wall and a capillary cover attached to the liquid side of the cellular core. The capillary cover has one or more openings per cell. Through surface tension effects a stable gas interface, which acts as a stretched membrane, is formed at each of the openings thereby preventing the entry of liquid into the cell and positioning a layer of gas between the liquid and the container wall.

In addition to the importance of surface tension in the establishment of the liquid-gas interface, the thermodynamic state of the liquid at the interface is also of considerable importance in the proper functioning of the insulation. It may be apparent that a thermodynamic equilibrium must exist across this phase boundary. The pressure is slightly greater in the gas than in the adjacent liquid because of capillary pressure effects and the liquid immediately adjacent the interface must be saturated at this gas pressure. Assuming the gas in the cells is the vapor of the contained liquid, it is apparent that the temperature at the interface corresponds to the boiling temperature of the liquid at the gas pressure.

If the container is depressurized after equilibrium conditions have been established at the interface, the gas in the cells will bubble from the cells through the capillary openings until a new state of pressure equilibrium is established. In the same manner, if the pressure in the container is increased, additional liquid will enter the cell where it will vaporize to increase the gas pressure in the cell until a new equilibrium condition is established at the interface.

A particular problem arises in maintaining equilibrium conditions at the interface if the bulk liquid is subcooled. Thus, if the liquid at the interface becomes cooler than the saturation temperature, condensation of the gas in the cell will begin to occur allowing liquid to enter the cell. A percolating action will tend to develop with liquid alternately entering the cells and evaporating and then bubbling back through the capillary opening into the liquid. This percolating action will continue as along as the liquid immediately adjacent the capillary opening is subcooled. This percolating action substantially reduces the insulating effect of a cell and for practical purposes renders it inoperative.

It is the primary object of this invention to provide capillary insulation of the type heretofore described which is effective in storing subcooled liquids over long periods of time.

A subcooled liquid is one which is at a temperature below the saturation temperature or boiling temperature of the liquid. The boiling temperature of the liquid, of course, also varies with the pressure of the liquid, and the boiling temperature increases as the pressure of the liquid increases and, conversely, reduces as the pressure of the liquid reduces. Moreover, when considering the boiling of liquids in a closed container, it should be realized that the partial pressure of the vapor in contact with the liquid also has an effect on the boiling temperature of the liquid. As the partial pressure of the vapor increases the boiling temperature of the liquid also increases, and conversely. These well-known thermodynamic facts are important to an understanding of the operation of the present invention. The important point to realize is that boiling point of a liquid in a closed container can be controlled by controlling either, or both, the temperature of the liquid or the partial pressure of the vapor adjacent the liquid. The present invention contemplates utilizing either or both of these aspects in order to effectively solve the afore-mentioned percolating problem which occurs in the storing of subcooled liquids.

In accordance with the principles of this invention, there is provided a capillary insulation comprising a cellular core defining a plurality of discrete cells with the core being adapted to be secured to the walls of a container for storing low temperature liquid. A capillary cover extends across the liquid side of the cells with at least one opening per cell in the cover. The capillary openings in the cover are so designed that a structurally stable liquid-gas interface is formed at each of the openings, preventing the entry of liquid into the cells and thereby positioning a layer of gas between the liquid and the container wall.

In accordance with one aspect of the present invention, to maintain equilibrium conditions at the interface, and more specifically to maintain the temperature at the interface at the saturation temperature of the liquid, a thermal barrier of a porous material is placed in the liquid adjacent to the capillary cover to establish a temperature differential between the temperature of the liquid remote from the interface and the temperature at the interface. The temperature differential results from the thermal resistance of the insulating layer of the nor mal heat flux entering the bulk liquid through the insulation system. This thermal barrier, or interior insulation layer, permits liquid to pass therethrough but is effective to achieve a temperature Stratification in the bulk liquid so that stable equilibrium conditions are maintained at the interface, even though the bulk liquid may be subcooled. The thermal barrier or insulation layer is not sealed to the cellular core and is either inherently porous or perforated to permit the passage of liquid through the barrier to the capillary openings. The material from which the insulation layer is constructed and the thickness of that layer may vary depending on the particular conditions but, in general, the layer of insulation must be sufficient to establish a temperature differential across the layer which is equal to the difference between the boiling and bulk temperatures of the liquid at the minimum rates of thermal conduction or heat flux at which the system is to be operated.

Other aspects and features of the invention will be apparent upon a complete reading of the following description which, together with the attached drawing, discloses the invention. Referring to the drawings wherein like reference numerals indicate like parts in various views:

FIG. 1 is a fragmentary perspective view of an insulation assembly constructed in accordance with the principles of this invention;

FIG. 2 is a sectional view taken along line 22 of FIG. 1; and

FIG. 3 is an enlarged view of the capillary opening in FIG. 2.

Referring now more in detail to the drawings where the showings are for the purposes of illustrating a preferred embodiment only, there is illustrated in FIG. 1 an insulation assembly, indicated generally by the reference numeral 10. The insulation assembly 10 is adapted for long term storage of subcooled, low temperature boiling point liquids, such as liquified natural gas or liquid nitrogen, oxygen, hydrogen, etc. The insulation assembly 10 is adapted to be secured to a supporting surface 12 which normally will correspond to the walls of a suitable tank or container in which the liquid is to be stored.

The insulation assembly 10 comprises a cellular or honeycomb core indicated at 14 which may be constructed from any lightweight material which is compatible with the liquid being stored and which has a low thermal conductivity. For example, the core may be constructed from polyimide, Nomex, nylon or plastic impregnated Kraft paper.

The core 14 comprises a plurality of strips or ribbons 16 which are assembled on edge and secured together at spaced points. Each strip has an undulating cross-sectional configuration with adjacent strips cooperating to define discrete cells 18 therebetween. The edges of the assembled strips cooperate to define opposed, generally planar surfaces with one of the surfaces being secured to the tank wall 12.

The cells 18 are substantially closed on the liquid side of the core 14 by a capillary cover 20. The cover 20 may be made from a material such as 1 mil Mylar film or a 1 mil Kapton film. Cover 20 extends across each of the cells 18 and is secured to the cellular core by suitable means, such as adhesive bonding. The portion of the cover over each cell may be dimpled or concave in configuration, as is disclosed in copending application, Ser. No. 81,400, filed Oct. 16, 1970.

The combination of the cellular core 14, the cover 20 and the surface 12 cooperates to define a plurality of separate confined areas or voids corresponding to the cells 18. The cover 20 includes a plurality of small capillary openings 22 with at least one opening per cell. These openings provide communication between the interior of the cells 18 and the liquid stored in the container. In each of these confined areas of cells a gas column is established to insulate the stored liquid from the tank wall 12.

The insulating gas column in each of the cells 18 is established in the following manner. A container having the insulation 20 applied to the walls 12 thereof will be filled with the liquid to be stored. As the liquid enters the container and contacts the capillary cover 20, a portion of the liquid will pass through the capillary openings 22 into the cells 18. The liquid in the cells 18 will vaporize and build up a gas pressure in each cell until the pressure of the gas columns in the cells equalizes with the pressure of the liquid. When this condition is reached, a liquid gas interface 24 forms at each of the openings 22 with liquid on one side of the interface and gas on the other side of the interface. Because of the small size of the openings and the characteristic of surface tension, this liquid-gas interface, under equilibrium conditions, is structurally stable and acts as a stretched membrane. While the membrane or interface 24 has been illustrated in a particular location in the opening 22, it is to be appreciated that the membrane may form at any of various locations within an opening.

As long as the conditions remain relatively constant, the gas columns in the cells 18 act as an insulative layer which will insulate the contained liquid from the container walls. If the tanks are depressurized after the stable membranes 24 have been established, gas will bubble from the cells 18 to establish a new state of pressure equilibrium. On the other hand, if there is a pressure increase in the tank, additional liquid will flow into the cells 18 and vaporize thereby increasing the pressure in the gas columns until a new state of pressure equilibrium has been established.

In addition to pressure equilibrium, it is apparent that a thermodynamic equilibrium also must exist across either side of the membrane; although the pressures on either side of the membrane have been described as being equalized, the pressure in the gas may be slightly greater than the pressure in the adjacent liquid because of capillary pressure forces and it is important that the liquid immediately adjacent the membrane be at the saturation temperature at this pressure. In other words, it is apparent that the boiling temperature of the liquid, under the pressure conditions present, must exist at the interface.

When a subcooled liquid, that is, one that is at a temperature substantially below its boiling temperature at its local pressure, is in contact with the cover 20, maintaining the liquid at the interface at the boiling temperature is difficult to achieve. If the liquid at the interface is below the boiling temperature thereof, condensation of the gas or vapor in the cells will begin to occur. As the vapor condenses, the pressure in the cells will drop and the stable interface is destroyed. As a result, liquid enters the cells 18 with a resultant flooding thereof. As the liquid enters the cells, some of the liquid may vaporize thereby increasing the gas pressure in the cells and causing some of the gas to bubble back into the stored liquid. A percolating action will occur and continue as long as the liquid in the vicinity of the cover 20 is subcooled.

To eliminate this percolating action and to enable the use of the capillary insulation concepts in the storage of subcooled liquids, it is contemplated by the principles of this invention that a thermal barrier may be interposed between the capillary cover 20 and the bulk liquid. This thermal barrier may take the form of a layer of conventional insulating material which is placed over the capillary cover 20. Thus, as illustrated in each of FIGS. 1 through 3, a thermal barrier layer, indicated generally by the reference numeral 30, extends across the cellular core 14 and between the capillary cover 20 and the liquid being stored. This thermal barrier or insulation layer may be of any suitable conventional insulation material, such as cork or a foam material. The function of this insulation layer is to insulate, but not isolate, the gas-liquid interface 24 from the bulk liquid. Hence, the layer 30 need only be secured to the insulation assembly 10 in such a manner as will maintain it in position over the cover 20. The layer should not be sealed to the cover and, in fact, it is preferred that a relatively porous material be employed as the insulation layer so that the liquid will pass through the insulation layer to establish the desired stable capillary gas-liquid interface in the openings in the cover. If the material selected for the insulation layer is not inherently porous, apertures or openings through the material should be provided to permit the liquid to pass therethrough. By permitting free passage of the liquid through the insulation layer, pressure differences across this layer are prevented and, therefore, no structural loading is developed within the insulation system.

The additional insulation layer 30 functions to insulate the liquid adjacent the membrane from the bulk liquid. If a porous insulation material is used for the additional insulation layer, the porous insulation material provides a stagnant layer of liquid adjacent the cover 20. By stagnant" is meant that the insulation prevents free convection currents to be established in the liquid. The stagnant liquid provides thermal resistance adequate to establish the necessary temperature difference.

As shown in FIG. 2, the insulation layer 30 is illustrated as being in direct engagement with the capillary cover 20 with a small space 32 between the concave portions of the cover and layer 30. However, the layer 30 may also be spaced a small distance from the capillary cover 20 if so desired.

The composition of the insulation layer, as well as its thickness, may vary depending on many factors including the minimum heat flux at which the system is to be operated and the degree of subcooling of the stored liquid. In general, the layer of insulation must be sufficient to create a temperature stratification with the minimum operating heat flux through the insulation system which, as measured across the insulation layer, would correspond to a temperature differential equal to the difference between the boiling and bulk temperatures of the liquid. Thus, referring specifically to FIGS. 2 and 3, the temperature of the bulk liquid L remote from the interface is at the subcooled temperature, while the temperature of the liquid L, adjacent the interface 24 is at the boiling or saturation temperature. This temperature differential is maintained primarily by the insulation layer 30 and by the inherent transfer of some heat inward from the walls of the container. However, in designing a system for subcooled liquids, the insulation layer 30 must be so designed that at the minimum permissible heat flux for the system, the temperature differential across the insulation layer is such as to maintain the liquid L, at the boiling temperature.

If the heat transfer to the liquid L, increases above the minimum design level, some boiling of the liquid in the space 32 between the insulation layer 30 and the capillary cover 20 may occur. However, this boiling does not adversely affect the insulative function of the insulation assembly, since it will merely cause the liquid-gas interface 24 to intermittently move away from the capillary openings 22. However, there will be no liquid entry through the openings 22 into the cells 18. When the temperature in this region subsequently returns to the boiling temperature level, the liquid will return to the region between the insulation layer 30 and the capillary cover and the stable capillary interface will immediately form again in the capillary openings 22.

A primary use of the insulation assembly with the layer is in a relatively large container, where the pressure due to the hydrostatic head at the bottom of the container will be substantially greater than the pressure at, for example, a point in the upper region of the container. This differential in pressure will result in a differential in the boiling temperature of the liquid at the bottom of the container as compared to the boiling temperature of the liquid at the top of the container. Since the hydrostatic pressure increases with depth in the tank and, consequently, the boiling temperature increases with depth, the degree of subcooling of the bulk liquid increases with depth in the tank. Accordingly, it may be desirable to utilize the additional layer at the bottom of the container but dispense with the insulation layer near the upper portion of the container. Alternatively, the insulation layer may be used throughout the container but it may be varied in thickness, being thicker at the bottom portion of the container than at the top. Still further, different insulative materials may be used to form the thermal barrier along different portions of the capillary insulation assembly. Thus, one insulative material might be used at the bottom of the container, while a different material might be used at the top of the container.

While the embodiment described above solves the problem of storing subcooled liquids by providing a thermal barrier between the vapor in the cell and the subcooled liquid, it is also possible to solve the problem created by the subcooled liquid by controlling the vapor pressure within the cell. As noted above, by controlling the pressure of the vapor in the cell, it is also possible to control the boiling temperature of the liquid. If the cell is partially filled with a gas that is not the vapor of the stored liquid, such as helium, then the saturation pressure of the stored liquid at the interface corresponds to the partial pressure of the vapor phase of the contained liquid existing within the cell. Accordingly, this, in effect, reduces the pressure of the vapor within the cell and thereby results in a reduction in the boiling temperature of the liquid. By properly controlling the amount of helium, the partial pressure of the vapor can be controlled within the cell and, ac cordingly, the problem of maintaining a stable interface with a subcooled liquid can be controlled. However, over a long period of time, it might be expected that the helium (or other gas) may be absorbed into the liquid or displaced by transient bubbling and refilling of the cell due to pressure or level fluctuation. Accordingly, this is a solution to the problem created by the subcooled liquid in those environments where storage is for a short period of time.

While the insulation assembly has been disclosed in connection with preferred embodiments, neither the disclosed embodiments nor the terminology employed in the description thereof is to be limiting; rather, it is intended that the invention be limited only by the scope of the appended claims.

Having thus described my invention, 1 claim:

1. An insulation assembly for reducing heat transfer between a surface and a liquid comprising,

means for providing a layer of gas between the liquid and the surface with a stable liquid-gas interface between said gas layer and the liquid, said means including a plurality of cells having a cover closing the liquid side thereof and having an opening in the cover in which the stable interface is formed, and

means associated with said cover for controlling the thermodynamic relationship between the liquid at each stable liquid-gas interface and the vapor of the contained liquid within the respective cells to ensure that the liquid at each stable liquid-gas interface is not below its boiling temperature to thereby control the formation of said interface. 2. The insulation assembly as defined in claim 1 wherein said means associated with said cover for controlling the thermodynamic relationship comprises means for thermally insulating the interface from the liquid remote to the interface.

3. The insulation assembly of claim 2 wherein said means for insulating said interface comprises a thermal barrier operative to establish a differential between the temperature of the liquid remote from said interface and the temperature at the interface.

4. The insulation assembly of claim 3 wherein said temperature differential corresponds to the difference between the boiling temperature of the liquid at the interface and the temperature of the liquid remote from said interface.

5. The insulation assembly of claim 4 wherein said thermal barrier comprises a layer of material having low thermal conductivity interposed between said interface and the bulk of said liquid,

said layer of material being so constructed that the liquid may pass from one side of said material remote from said interface through the material to the other side thereof adjacent to said interface.

6. The insulation assembly of claim 3 wherein said thermal barrier comprises a layer of porous material adapted to receive and store liquid in a relatively stagnant layer to reduce free convection within said layer of material.

"I. The insulation assembly of claim 5 wherein said layer of material is positioned over said cover.

8. The insulation assembly of claim 1 wherein said means for controlling the thermodynamic relationship includes a gas in said cells other than the vapor of the liquid.

9. In an insulation assembly for use in insulating the surface of a container for storing a low boiling point liquid therein, and which assembly includes means defining a plurality of cells for containing respective columns of gas between the liquid to be stored and the surface of said container, and a capillary means for said cells for establishing a capillary interface between each gas column and the liquid, the improvement comprising means for establishing a thermodynamic relationship between the liquid at each capillary interface and the vapor of the contained liquid within the cell to ensure that the liquid at each capillary interface is not below its boiling temperature.

10. The assembly of claim 9 wherein said means for maintaining the temperature comprises a thermal barrier positioned between said interface and the bulk of said liquid.

11. The assembly of claim 9 wherein said means for maintaining the temperature is operative to maintain said boiling temperature when the temperature of the liquid remote from said interface is lower than said boiling temperature.

12. The assembly of claim 9 wherein said means for establishing the temperature of the interface comprises a gas in the cells other than the vapor of the liquid so that the pressure of the vapor of the liquid in the cells comprises a partial pressure of the total pressure in the cells.

Claims

1. An insulation assembly for reducing heat transfer between a surface and a liquid comprising, means for providing a layer of gas between the liquid and the surface with a stable liquid-gas interface between said gas layer and the liquid, said means including a plurality of cells having a cover closing the liquid side thereof and having an opening in the cover in which the stable interface is formed, and means associated with said cover for controlling the thermodynamic relationship between the liquid at each stable liquid-gas interface and the vapor of the contained liquid within the respective cells to ensure that the liquid at each stable liquid-gas interface is not below its boiling temperature to thereby control the formation of said interface.

2. The insulation assembly as defined in claim 1 wherein said means associated with said cover for controlling the thermodynamic relationship comprises means for thermally insulating the interface from the liquid remote to the interface.

5. The insulation assembly of claim 4 wherein said thermal barrier comprises a layer of material having low thermal conductivity interposed between said interface and the bulk of said liquid, said layer of material being so constructed that the liquid may pass from one side of said material remote from said interface through the material to the other side thereof adjacent to said interface.

7. The insulation assembly of claim 5 wherein said layer of material is positioned over said cover.