US3236406A

US3236406A - Spaced wall insulated container

Info

Publication number: US3236406A
Application number: US306410A
Authority: US
Inventors: Vincent E First; Eugene S Kordyban
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1963-08-29
Filing date: 1963-08-29
Publication date: 1966-02-22
Anticipated expiration: 1983-02-22

Description

Feb. 22, 1966 v. E. FIRST ETAL 3,236,406

SPACED WALL INSULATED CONTAINER Filed Aug. 29, 1963 '7 Sheets-Sheet 1 '5 Q5 m a: z z E o F g :1: u o O 0) D a z a ll 3 5 E 5 5 u a o I L '2 Q 5 s a m Lu U) o z z :2 a, a a g g o m I J Lu 8 [L2 :2 (D

u 3 u) N Q 3 8 8 o Q o d u u. 15 uu/mapxxuouvwnsm uam'l HLVNHHL'W 3H1 d0 All/\ILOHCINOD 'IVWHHHJ.

INVENTORS EUGENE SIKORDYBA/V BY V/NCE/VTE FIRST ATTORNEY Feb. 22, 1966 v. E. FIRST ETAL 3,236,405

SPACED WALL INSULATED CONTAINER Filed Aug. 29, 1963 7 Sheets-Sheet 2 on In I m N l 2 3 4 5 6 7 8 9 IO COMPESSION P, PS!

EUGENE S. KORDYBAN y VINCENT E FIRST ATTORNEY Feb. 22, 1966 v. E. FIRST ETAL 3,236,406

SPAGED WALL INSULATED CONTAINER Filed Aug. 29, 1963 7 Sheets-Sheet 5 HDNI lsaaxrl N a Q NE E -0: w u3 Q) I Q y a; O... 9 a, z Q m 3 a: n. a

\ ID IO o s s 8 a Q 2 2 m N N 9 Hum suzuv'l N INVENTORS EUGENE S KORDYBAN By V/NCENTE F/RST A TTORNE Y Feb. 22, 1966 v. E. FIRST ETAL 3,236,406

SPACED WALL INSULATED CONTAINER Filed Aug. 29, 1963 '7 Sheets-Sheet 4 30 ZZ I E L /7 I I INVENTORS EUGENE S. KO/PDYBA/V 66 /0 /0 BY V/NCENTE FIRST Feb. 22, 1966 v. E. FIRST ETAL 3,236,406

SPACED WALL INSULATED CONTAINER Filed Aug. 29, 1963 7 Sheets-Sheet 5 FIG. 50' FIG. 5 e

INVENTORS EUGENE S. KOROYBA/V BY V/IVCE/VTE FIRST A TTOR/VE) Feb. 22, 1966 v, FlRsT ETAL, 3,236,406

SPACED WALL INSULATED CONTAINER Filed Aug. 29, 1963 7 Sheets-Sheet 6 INVENTORS EUGENE .5. KORDYBA/V BY VINCE N 7' E FIRST .53 A 7'7'0RNE Y '7 Sheets-Sheet 7 V. E. FIRST ETAL SPACED WALL INSULATED CONTAINER FIG. /2

Feb. 22, 1966 Filed Aug. 29, 1963 n O T N E V m United States Patent 3,236,406 SPACED WALL INSULATED CONTAINER Vincent E. First, Tonawanda, and Eugene S. Kordyban, Buffalo, N.Y., assignors to Union Carbide Corporation, a corporation of New York Filed Aug. 29, 1963, Ser.'No. 306,410 6 Claims. (Cl. 220-9) This is a continuation-in-part application of application Serial No. 118,741, filed June 21, 1961, in the names of V. E. First et al.

This invention relates to thermal insulating structures enclosing an evacuated space as, for example, between the product holding inner vessel and the outer casing of double-walled containers. Still, more particularly, this invention relates to a multiple-layer type of thermal insulating system which is particularly suitable for minimizing the atmospheric heat inleak through the evacuated insulating space to product liquids as, for example, lowboiling cryogenic liquefied gases such as liquid oxygen and hydrogen.

As used herein, the expression thermal insulating structure includes any construction which is gas-tight and evacuable to sub-atmospheric pressure so as to reduce heat transfer through the structures cross-section. The term container includes any construction capable of holding valuable materials as, for example, tanks, pipelines and rectification equipment.

The art has recently discovered composite insulating systems which are remarkably effective in reducing heat transfer by conduction and radiation. These systems are a flexible composite of a low heat conductive material component and a radiant heat barrier material component assembled sufliciently closely to provide at least 4 flexible layers per inch of composite insulation. The low conductive material is disposed generally perpendicular to the direction of heat transfer across the evacuated'space, and serves to separate the several radiant heat barrier elements or components so that the latter are confined to substantially isothermal planes within the insulation. These systems are particularly suitable for insulating cryogenic liquids in double-walled containers from the effects of atmospheric heat inleak. The composite insulating material is installed in the evacuable spacebetween the cryogenic liquid storing inner vessel and the outer casing and has been found to be at least ten times as effective as the conventional powder-in-vacuum system. Such insulations are described and claimed in US. Patent Nos. 3,007,596 and 3,009,600, respectively. These disclosures are incorporated herein by reference, and describe a composite of low conductive permanently precompacted fibrous paper material with a radiation-impervious reflecting component such as aluminum foil. Another suitable type of low conductive-radiation impervious composite insulation is a reflective metal-coated, non-metallic low conductive substrate material as, for example, the aluminum-coated polyethylene terephthalate film described in US. Patent No. 3,018,016 to M. P. Hnilicka, Jr. Another satisfactory metal-coated substrate is thin metallized paper such as metallized glassine.

Another composite multi-layered insulation for use in a vacuum space between warm and cold boundaried consists of alternating layers of cellulose fiber paper of the kraft type anda radiation-imprevious reflecting component such as aluminum foil.

Still another composite multi-layered insulation for use in a vacuum space between warm and cold boundaries consist of the paper layers and finely-divided radiant heat reflecting bodies of less than about 500 microns in size, being incorporated in and uniformly dispersed through the layers, as well as a binder for cementing the heat reflecting bodies to the fibers.

Most multiple-layer insulations of the high quality, vacuum-type are extremely sensitive to compression within the range of densities corresponding to best performance. For example, FIG. 1 is illustrative of the general problem and shows the variation in conductivity for an alternate-layer fiber and foil insulation which results from compressing more and more layers into a given thickness of insulation. This particular insulation is composed of 0.00025 in. thick soft annealed aluminumfoil having an emissivity of 0.058, alternating with thin precompacted glass fiber paper weighing 1.6 gm. per sq. ft. and composed of fibers whose average diameter is between 0.5 and 0.75 micron. Curves A and B represent the contributions to overall heat conduction by radiation and solid conduction, respectively, while curve C is the total conduction obtained as the summation of curves A and B. Curve C is an accurate representation of the overall performance of the composite insulation, provided that gaseous conduction is eliminated by virtue of a low absolute pressure below 1.0 micron of mercury and preferably below 0.1 micron of mercury.

For best results, the insulation characterized by FIG. 1 should be wound loosely with only about 60 layers per inch thickness, thereby achieving thermal conductivity near the minimum value of 0.021 l0 B.t.u./hr. sq. ft. F./ft. However, it has often been found impossible to obtain such loose density in commercial systems using prior art wrapping techniques. Densities considerably greater than the optimum can be produced in some instances simply by the weight of the materials themselves when stacked alternately on a horizontal surface to thicknesses of, for example, 3-4 inches. More severe overcompression results when the alternate-layer insulation is wound in thick layers on large cylindrical or spherical vessels. The portions of the insulation which rest on upward-facing surfaces of the vessel must support the remainder of the insulation which hangs about the vertical and under-surfaces of the vessel. Thus, the insulation on the top surfaces is severely over-compressed while that below the vessel tends to sag and produce large void spaces within the blanket of insulation. The shape of curve C in FIG. 1 shows that either departure from the optimum number of layers per inch is highly undesirable.

Consider, for example, a horizontal cylindrical tank 5 feet in diameter on which it is desired to install 4 inches of the insulation of FIG. 1. By wrapping the materials very loosely, an average density of 60 layers per inch thickness can be achieved corresponding to the optimum value of curve C. However, the density in the vicinity of the top center line of the tank will be about to layers per inch, while that at the bottom will average only perhaps 40 layers per inch throughout its thickness. Inspection of curve C of FIG. 1 shows that the large areas thus overand under-compressed will exhibit thermal conductivities much higher than the optimum value. A tank so insulated will also be diflicult to handle or move about without shifting the loose insulation. Standing the tank vertically, or imposing only slight axial accelerations, will cause the insulation to telescope and slide off the tank.

The magnitude of the problem may be better understood by considering the internal and compression exerted between layers of the insulation. Stability against sagging and slippage depends upon the development of restraining frictional forces between the various component layers. The coefficient of friction between fine glass fibers and aluminum foil is not high (about 0.6) so that a measurable pressure must be exerted normal to the layers in order to generate sufficient restraining force. However, if the insulation of FIG. 1 were assembled conventionally to the optimum 60 layers per inch, the interlayer pressure would be immeasurably small (below .001 psi), many-fold less than the pressure needed for stability under normal commercial service. In fact, the inter-layer compression at optimum density is so low that the insulation might be properly visualized as consisting of a series of essentially free-standing foil shields which make only slight contact with the fiber separator.

It follows that a critical problem often encountered in the use of high quality multi-layer insulations is the absence of sufficient inter-layer frictional force to maintain uniform density and avoid displacement of the insulation. Heretofore, a partial remedy for sagging and instability has been to wrap the insulation more tightly and thereby eliminate the voids and increase inter-layer friction. However, the consequences of obtaining stability by the simple expedient of tightening the layers as they are applied is very severe. It has been observed that in the range of low interlayer compression corresponding approximately to optimum conditions, only a slight increase in compression will produce an extremely large increase in thermal conductivity. For practical usage, the minimum interlayer pressure needed for stability, even in light service may be times greater than the ideal pressure. The thermal conductivity k corresponding to such high pressure can easily be two or three times the optimal low value which is sought.

Another damaging consequence of tightening the layers as they are applied is the drastic reduction in insulation thickness. This means that if a given number of total layers of insulation is tightened sufficiently to achieve a stable condition, the thermal conductivity k will not only increase greatly, but the thickness t will also be reduced, thereby producing two highly detrimental eifects on the insulation performance. The alternative of adding additional layers to overcome the reduction in thickness results in greatly increased cost and weight of the insulation.

These problems are partially overcome by a system for stabilizing multi-layer insulation described and claimed in copending application Serial No. 306,415 filed simultaneously with this application on August 29, 1963, and in the name of David L]. Wang. In this system compression members are provided between the inner walls of the outer casing and the outer surface of the composite insulating material. These compression members constitute means for concentrating the total frictional force between the layers of composite insulating material in a minor part of the total insulated area whereby such minor part is above its stable density and the remaining major part of such area is maintained below its stable density. As used herein, the phase stable density refers to the insulation density to which the entire composite insulation body would have to be compressed uniformly in order to achieve mechanical stability without use of the compression members, under the expected conditions of handling and service of the insulated apparatus.

There are, however, certain limitations in the compression member system when the inner wall of the outer casing is used as an anchorage for the compression members. These limitations are primarily due to thermal contraction and expansion of the insulated vessel in the axial direction. If the compression member assemblies are spaced along the length of a vessel, they cannot move with the vessel as it shortens or lengthens, and hence tend to tear or distort the insulation. As a consequence, casing-anchored compression members are most effectively used at only one location on the length of the vessel, and this location should be at or near a point which does not move with respect to the casing, i.e. at the point where the container is supported axially. It will be appreciated that such a requirement seriously limits the variety of vessel configurations in which the compression member system may be employed.

An object of this invention is to provide an improved multiple-layer composite insulation assembly which affords overall physical stability on one hand, and subsubstantially optimum heat-impeding performance of the insulation system.

Another object of this invention is to provide an improved thermal insulating structure containing multiplelayer composite insulation having very thin layers of low heat conductive material and radiant heat barrier material and possessing high overall stability and substantially optimum heat impeding quality.

A further object is to provide a double-walled cryogenic liquid container having an evacuable space between relatively warm and cold walls filled with a multiplelayer composite insulation assembly possessing high overall stability and substantially optimum heat-impeding quality.

A still further object is to provide an improved compression member support system for multiple-layer composite insulation in a double-walled container which may be positioned at any desired location along the length of the inner vessel, and does not tend to tear or distort the insulation during thermal contraction or expansion of such vessel.

Additional objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.

In the drawings:

FIG. 1 is a graph showing the eifect of increasing the number of layers per inch on the thermal conductivity of an illustrative multiple-layer type insulation;

FIG. 2 is a graph showing the effect of compression on the thermal conductivity of the multiple-layer insulation of FIG. 1;

FIG. 3 is a graph showing the effect of compression on the density of the multiple-layer insulation of FIG. 1;

FIG. 4 is an end view taken in cross-section of a novel support system construction for multiple-layer insulation, according to the invention;

FIGS. 5a and 5ce are a series of longitudinal views taken in cross-section of the various steps comprising one preferred method of assembling the insulation support construction of FIG. 4;

FIG. 5b is a plan view looking downwardly on the assembly of FIG. 5a;

FIGS. 6ac are a series of longitudinal views taken in cross-section of the various steps comprising a method for assembling another support construction similar to FIG. 4;

FIG. 7 is an end view taken in cross-section of an alternate support system construction for multiple-layer insulation, similar to that shown in FIG. 4;

FIG. 8 is a fragmentary end view taken in cross-section of another alternate support system construction;

FIG. 9 is a fragmentary end view taken in cross-section of still another alternate support system construction;

FIG. 10 is an isometric view of apparatus suitable for assembling the composite insulation of one embodiment of the invention;

FIG. 11 is an enlarged view taken in cross-section of a preferred composite insulation-support construction which can be partially assembled by the FIG. 10 apparatus, and

FIG. 12 is a vertical cross-sectional view of a container constructed according to the invention, for the storing of materials at low temperature.

In the various figures, corresponding items have been identified by the same numbers in the interest of clarity.

The present invention uses at least two principles: first, that increasing the compression on the multiple-layer insulation does not produce proportionate increases in thermal conductivity. This will be apparent from a close inspection of FIG. 2 which is the thermal conductivity vs. compression plot of the insulation of FIG. 1. It can be seen from this graph that the rate of increase in thermal conductivity k in fact falls oif appreciably at higher values of compression P, whereas one might have expected this.

from a study ofFIG. 3.

relationship to be linear. For example, increasing the compression by 0.1 p.s.i. from essentially zero to 0.1 p.s;i.-

produces an increase in k in excess of 0.025 X l0 B.t.i1./

'hr. sq. ft. F./ft., which more than doubles the optimum conductivity. In contrast, increasing compression by 0.1 p.s.i. from 0.9 to 1.0 p.s.i., produces less than as much increase in thermal conductivity k.

The second principle is that increasing the compression on the insulation to high values does not produce proportionate increases in insulation density as can be seen Like FIG. 2, the rate of increase in layers per inch falls off appreciably at higher values of compression P.

The significance of these principles is as follows: Assumethat stability of a given insulation system requires that a certain minimum restraining frictional force defined as nPAc, be provided where 1 =the coelficient of friction P: the interlayer compression Ac=area over which compression P is applied The shape of the curves FIGS. 2 and 3 shows that for any required value of PAc, it is preferred to increase compression P to a high value over a small area than to increase P only slightly over a large area. Stability is best achieved with this insulation by maintaining almost all of the insulated area under essentially zero compression corresponding to minimum heat conductivity and optimal thickness and by compressing only a small portion of the area to a relatively high density. On the other hand, if thermal conductivity and density varied linearly with compression as might be expected, then there would be no advantage in reducing area Ac by increasing compression P. In this event, the same impractically high increase in thermal conductivity would be observed for any given value of frictional force '17PAC regardless of the relative values of P and Ac.

One embodiment of the invention contemplates a thermal insulating structure comprising gas-tight walls enclosing an evacuable space. A heat-insulative and radiation-impervious composite flexible material is provided within this space, and comprises a low heat conductive material component and a radiant heat barrier material component assembled sufiiciently closely to provide at least 4 layers of composite insulation per inch of evacuable space cross-section. Both components are disposed generally perpendicular to the direction of heat transfer across the evacuated space. Multiple compression member means are provided for concentrating the total frictional force between the layers of composite insulating material in a minor part of the total insulated area whereby the minor part is above its stable density. One end of each compression member is positioned against the outer surface of the composite insulating material. Girth means are arranged and positioned against the other end of the compression members under sufficient tension for the previously described concentration of total frictional force to maintain the minor part of the insulated area above its stable density. The girth means may, for example, be metal straps, elastically deformable members, coil springs or leaf springs.

A preferred upper limit of insulated area portion above its stable density is 5%, and best results are obtained when not more than 2.5% of such area is so compressed. The compressive stress should be at least 0.4 p.s.i. for satisfactory stability.

There are no known limits on the maximum compressive stress which may be applied to the minor part of the total insulated area, in accordance with this invention. When the composite insulation consists, for example, of alternating aluminum foil and glass fiber paper layers, the indicators are that an upper limit for compression of glass fibers without permanent crushing, if such limit exists, is far above expected practice and possibly on the order of several hundred p.s.i. Any low limit for compressive 6 stress will correspond to low g-loading and large area under compression. Stationary storage vessels which are normally free of acceleration loads due tomovement may be constructed with very low composite insulation localized compression sufiicient only to prevent sagging. Regardless of the service, it is not usually desirable to design insulation for substantially less than about l-g loading; otherwise, insulation displacement may occur due to handling the vessel during fabrication. A preferred lower limit of about 0.4 p.s.i. compressive stress corresponds approximately to l-g gravitational acceleration with 5% compression area for a typical multiple-layer insulation of 4 inches thickness. Using low compressive stresses causes substantial increases in heat transmission.

In a preferred embodiment, multiple band means are disposed in contiguous adjacency with the composite insulating material extending laterally around the vessel. Such band means are spirally wound with the composite insulating material only sufficiently tightly to impart about 0.1 to 0.5 p.s.i. compression to the insulating material area beneath the multiple band means. This embodiment af fords stability during the composite insulation wrapping operation.

In still another preferred embodiment, low conductive compressible filler material is provided with the multiple band means, and spirally wound therewith around the inner vessel. The compressible filler material is in contiguous adjacency and in coextensive relationship with the multiple band means, and positioned between such band means and the composite insulating material. The filler material serves to mitigate or eliminate the otherwise severe reduction in insulation thickness due to the high compressibility of the composite insulation.

The achievement of a floating barrier structure over the majority of the insulated area is a close approach to the idealized. concept of multiple radiation shields. Ideally, multiple shields are free-standing barriers of thin radiation impervious material, positioned as close together as surface and shape imperfections will permit, and separated only by a near-perfect vacuum.

The radiation barrier or shield material may comprise either a metal or metal-coated material, such as aluminumcoated plastic film, or other radiation reflective material. Radiation reflective materials comprising thin metallic foils are admirably suited in the practice of the present invention. The foils should have sufficient thickness to resist tearing or other damage during installation. For high-quality insulations, the foil should be as thin as practical, consistent with strength requirements. Thinness is beneficial because it facilitates folding and forming the insulation to fit the contour of the insulation space. It also minimizes the weight of the enclosing structure and reduces the weight of insulation which must be stabilized. In cryogenic vessels, low density is additionally important because it reduces the time and the quantity of expensive refrigeration needed to cool down the inner vessel and establish a stable temperature gradient through the insulation. Foil thickness between 0.2 mm. and 0.002 mm. are suitable, and when aluminum foil is employed, thickness between 0.05 mm. and 0.002 mm. are preferred.

A preferred reflective shield is A mil (0.00025 in. or 0.0062 mm. thick) plain, annealed aluminum foil without lacquer or other coating. Also, any film of oil resulting from the rolling operation should be removed as by washing. Other radiation reflective materials which are susceptible of use in the practice of the invention include: tin, silver, gold, copper, cadmium or other metals. The emissivity of the reflective shield material should be between about 0.005 and 0.2, and preferably between 0.015 and 0.06. Emissivities of 0.015 to 0.06 (98.5% to 94.0% reflectivity) are obtainable with aluminum and are preferred in the practice of this invention, while with more expensive materials such as polished silver, copper or gold, emissivities as low as 0.005 may be obtained. The above ranges represent an optimum balance between the high performance and high cost of low emissivity materials.

The base constituent of the insulation is preferably a suitably low conductive permanently precompacted fibrous paper or mat material which may be produced in sheet form. Since the individual fiber diameters are less than 20 microns, the sheets are thin enough to be flexibly bent. These materials are commonly prepared by uniformly depositing finely spun or alternated fibers at a desired rate on a moving belt and subsequently compressing the mat as, for example, between compression rolls or by vacuum. Among suitable fiber materials are glass, plastics, cellulose and ceramics.

A preferred composite insulation of the alternate layer type includes between about 4 and 200 layers per inch of both aluminum foil and glass fiber paper.

Alternatively, the low conductive material component and the radiant heat barrier material component may be bonded together as, for example, a reflective metal-coated plastic or paper. The low conductive substrate must be capable of remaining under vacuum for long periods of time without suffering damage and preferably should not contain volatiles which are slowly released to the evacuated space. The preferred substrates are organic plastic films which are free of volatile plasticizers. The plastic should contain no material having an equilibrium vapor pressure at 20 C. in excess of microns Hg abs. Polyester resins are satisfactory and a particularly suitable bonded composite is the previously mentioned aluminumcoated. polyethylene terephthalate film. The previously described paper sheets may also be employed as the substrate.

The substrate should have a thickness of between about 0.0002 inch and 0.002 inch, one suitable commercially available polyethylene terephthalate film being 0.00025 inch (0.000625 cm.) thick. Substrates of thickness less than 0.0002 inch become extremely difficult to handle and often contain an excessive number of pinholes resulting in discontinuities in the radiant barrier. On the other hand, substrate thicknesses greater than 0.002 inch merely add unrequired solid material which is heavy, expensive, and contributes needless solid conduction.

The reflective metal coating should be sufliciently thick to provide a low emissivity, preferably less than 0.06. Thus, a coating possessing high reflectivity may be thinner than a coating of lower reflectivity. Suitable metals include gold, silver, copper and aluminum, the latter being preferred from the standpoint of cost. An aluminum coating thickness of about 0.0025 micron on the polyethylene terephthalate film has been found satisfactory.

If desired, the metal coated-low conductive substrate may be crimped or crumpled prior to insertion in the thermal insulating structure, so that only point contact between the layers is achieve-d. For example, the composite may be permanently wrinkled or creased generally parallel to the length of the composite with the wrinkles spaced approximately %%1 inch.

As a further alternative, the radiant heat barrier material component may be in the form of finely divided refleeting bodies of sizes less than about 500 microns. These bodies are incorporated in and uniformly dispersed through the previously described precompacted paper layers in an amount between about 10% and 60% by Weight of the paper. A binder as, for example, colloidal silica is used for cementing the heat reflecting bodies to the individual fibers of the paper. Less than about 10% by weight reflecting bodies does not achieve a significant radiation barrier effect, whereas greater than 60% reflecting bodies produces bridging of such bodies through and along the paper surface. The latter results in a solid conductive path.

The fibers may, for example, be formed of glass, ceramic, quartz, or potassium titanate, depending on the temperatures to which the composite multi-layered insulation will be exposed. For example, at temperatures below about 900 F. glass fibers are preferred but at high temperatures glass tends to soften and the other enumerated materials are more suitable. When glass fibers are used, they are preferably of less than 5 microns diameter, while a fiber diameter range of 0.2 to 3.8 microns gives best results. The above range represents a preferred balance between increasing cost of relatively small diameter fibers, and increased conductance and gas pressure sensitivity of relatively large diameter fibers.

The finely divided radiant heat reflecting bodies may, for example, be formed of aluminum, copper, nickel and molybdenum. Again the selection of the reflecting body is influenced by the operating temperature of the insulating composite. Aluminum is stable at temperatures below about 900 F., and is preferred in this range. Best results are obtained when the radiant heat reflecting bodies are relatively small, with particle sizes of less than 50 microns as the major dimension. Aluminum and copper paint pigment flakes of less than 0.5 micron thickness are especially suitable for relatively low temperature systems.

The reflecting body-containing paper may, for example, be formed on standard paper-making machines using colloidal silica as a binder. This paper and its manufacture are described more completely in copending application Serial No. 211,176 filed July 29, 1962, in the names of W. J. Bodendorf and D. I-J. Wang, incorporated herein to the extent pertinent.

As a still further alternative, the low conductive material component may consist of cellulose fiber paper of the kraft type, although the insulation in such case will yield a performance of intermediate quality when compared to the performance of the very high quality insulation employing the low conductive component materials, aforementioned. For example, an insulation was composed of alternate layers of mil aluminum foil and a thin, hard surface cellulose fiber paper of the kraft type (6.05 gm./ sq. ft.) having fiber sizes in the range of 10-70 microns. This construction was found to be reasonably stable only at very high N-values of about 190-200 shields per inch, where its thermal conductivity under good vacuum below 1 micron Hg and between ambient and liquid nitrogen temperatures is about 0.070 l0 B.t.u./hr. sq. ft. F./ ft. Over of this heat conductivity is contributed by solid conduction which means that the construction lies well up on the right side of a performance curve of the type shown in FIG. 1. Furthermore, the bulk density of the composite is about 39 lb./ cu. ft. which is very high compared wibh the value of 6 lb./ cu. ft. readily achieved in other multiple layer insulations. Both lower density and lower thermal conductivity can be obtained by employing bulking strips between the layers using, for example, strips of cellulose paper of suitable thickness to reach the optimum point on the curve, in addition to the compression member means of this invention.

The bulking material, if it is employed as afore-described may also, for example, be formed of the same low conductive fibrous paper material employed as the insulation base constituent. Another suitable fiber material is used in an uncompacted, elastically compressible, resilient and fluffy state, and known as webs. Suitable fibers include clean glass filaments having diameters between 0.2 and 5 microns, such as those produced by the so-called flame attenuation process. A fiber diameter range of 0.5 to 3.8 microns is preferred. The web material is described more completely in U.S. Patent 3,009,601, incorporated herein to the extent pertinent. I

Still other suitable bulking materials include the re flecting-body containing paper described above and woven fibrous materials such as glass cloth.

FIG. 4 illustrates a double-walled container constructed in accordance with the invention wherein the inner vessel 10 is wrapped with composite insulation 11, and compression members 12 are preferably spaced at uniform intervals around the periphery of such vessel. Also, the compression members 12 may be spaced at any desired intervals along the longitudinal axis of the vessel. Outer casing 13 surrounds the inner vessel 10 and is sufficiently large to form an annular evacuable, gas tight space 14a therebetween. The compression members 12 are preferably secured to the outer surface of the composite wrapped insulation 11 before assembly in the casing 13. The compression member 12 bear against girth means 14 so that all opposing forces are balanced by tension in the girth. This construction has the important advantages of easier assembly of inner vessel 10 within outer casing 13, of eliminating the need for having a rigid outer casing 13 of heavy construction, and of eliminating the need for cutting and sealing holes in the vacuum casing. The compression members 12 are also free to move axially within the container as it contracts and expands, so that any number of axial locations may be supported without physical damage to the insulation.

The girth means may, for example, be constructed of ordinary steel strap material as used in the packaging industry. Thisis true whether the container is used for cryogenic or high temperature service, since the insulation protects the strips from extreme thermal conditions.

FIGS. 5a-5e illustrate a preferred compression member mechanism for use in the FIG. 4 embodiment, and shows its 4-step installation. In Step 1 (FIGS. 5ab), the strap is prepared by cutting holes 15 along the length of a steel strap 14, and placing threaded nuts 16 under the holes. The hole spacing coincides with the desired circumferential spacing of the compression members, and the hole diameter is slightly larger than the root diameter of the nut thread. Prefabricated spring shoes consisting of a disk or square 17 of rigid material and a short tube 18 attached symmetrically and perpendicularly thereto are inserted loosely through the nuts 16 and strap holes 15. The free end of tube 18 has a diametric slot 19 for about one third of its length. Strap 14 with shoes in place is secured around the periphery of the preinsulated vessel with a predetermined girth length.

The girth length of the strap is carefully predetermined by calculation, as being the perimeter of the polygon which it forms when tensioned by the compression members (see FIG. 4). All dimensions needed for this calculation will be known in a properly planned insulation system, i.e., the inner vessel diameter, the composite insulation thickness under the compression member, the compressed length of this member, and the circumferential spacing of the compression members.

In Step 2 (FIG. 50), a parallel threaded nipple 20 having a small opening 21 drilled diametrically therethrough is threaded inside nut 16. Tube 18 is of such outside diameter that it slides closely into nipple 20. Strap 14 may be lifted off the surface of the composite insulation 11 by hand, permitting the shoe assembly to drop away from strap 14 so that nipple 20 may be screwed into nut 16 for a substantial portion of its length. Nut 16 and nipple 20 serve to secure the compression member mechanism to strap 14. Nipple 20 also guides and stabilizes the spring shoe when it is subsequently extended to further compress the composite insulation 11.

In Step 3 (FIG. 5d), coiled spring 22 is dropped inside tube 18 and is compressed, thus forcing the shoe assembly to extend part-way out of the nipple 20. Compressing the spring may require considerable force and tool 23 may be found convenient or necessary. Tool 23 is essentially a threaded clamp consisting of nut 24 which threads onto the end of nipple 20, and driver 25 which threads into a central hole in the nut 24. Screwing driver 25 downwardly in contact with the end of spring 22 compresses the spring to the height needed to produce the desired compression on the composite insulation.

Final adjustment is made in Step 4 (see FIG. 5e) by tightening or loosening the threaded nipple 20 so that the hole 21 drilled therethrough is just above the top of the compressed spring 22. Nipple 20 is rotated so that its hole 21 is aligned with the slot in tube 18, and cotter pin 26 is inserted through hole 21 to hold spring 22 in a permanently compressed stage. The end of driver 25 is also provided with a slot to permit insertion of pin 26. Tool 23 is now removed by loosening nut 24 from the end of nipple 20. Preferably a jam nut 27 is tightened in nipple 20 against the outer surface of strap 14 to prevent rotation or tipping of the compression member assembly in service. Any excess length of nipple 20 extending beyond jam nut 27 may be trimmed.

FIGS. 6a-c illustrate the assembly of another compression member mechanism suitable for use in the FIG. 4 container. Referring now to FIG. 6a, the spring shoe assembly consisting of retainer 17 and inner tube 18 is inserted loosely into an oversize hole 15 in tension strap 14. The strap 14 of the proper girth length is then buckled or tied snugly around the insulated inner vessel 10. Spring 22 is inserted inside tube 18, and outer tube 20 with attached metal strip 31 is placed over the exposed end of the spring. Next, the outer tube 30 is pressed downwardly through the strap hole 15 and around the inner tube 18. Simultaneously the strap 14 is pulled upwardly and sufiicient counterforces are applied so that strip 31 contacts the upper surface of strap 14 across the hole 15, as illustrated in FIG. 6b. With the components held in this position, the ends 32 of strip 31 extending over outer tube 30 are crimped around the edge of strip 14, thereby securing the assembly in the compressed condition (see FIG. 60). If desired, a tool (not shown) may be used to exert the counterforce between strip 14 and outer tube 30.

Comparing the FIGS. 5 and 6 assemblies, FIG. 5 has the advantage of adjustability by virtue of the threaded connection between the compression member assembly and tension strap 14. Therefore, the initial length of the strap for Step 1 (FIG. 5a) is not so critical since the desired degree of compression is readily obtained in the final adjustment. Although the FIG. 6 embodiment is not adjustable in the compression member mechanism, strap 14 may be loosened or tightened as necessary to obtain the required compressed height of the spring. The advantages of the FIG. 6 construction are its lower cost, simplicity, and faster assembly. Compression members fabricated according to FIG. 6 may also be shorter in length than those of FIG. 5 because spring 22 which controls the overall compression member height, extends completely through length of the member from end to end. Short compression members are desirable because they minimize the spacing between the inner and outer walls.

FIG. 7 illustrates another compression member support embodiment of the invention including rigid tubes 18 of fixed length. The inner end of tubes 18 is attached to retainers 17 which in turn press against the outer surface of the composite wrapped insulation in the assembled state. The girth means includes straps 35 attached to the outer end of tubes 18 and extending outwardly beyond opposite sides of such tubes for preferably equal distances in a direction generally normal to the vessel centerline axis. Tensioned elastic elements such as coil springs 36 are secured to the extremities of adjacent straps by suitable means as, for example, hooks 37. In this manner, the necessary compression may be transmitted through tubes 18 and retainers 17 to the composite insulation in a minor part of the total insulation area. The compression member-girth means assembly of FIG. 7 may be formed in a manner similar to the FIG. 6 embodiment, but modified to eliminate outer tube 30 and spring 22 and to attach strip 31 diametrically across the outer end of tube 18. The latter tube is inserted through a hole provided in strap 35, and is assembled with detachable shoe 17 after which tube 18 is secured as in FIG. 6.

FIGS. 8 and 9 show still further modifications of compression membergirth means assemblies in which leaftype springs are employed rather than the coil springs of FIG. 7.

Referring now to FIG. 8, S-shaped leaf springs 40 serve as the compression members, and one end presses against the center section is secured to tension strap 14.

H the outer surface of the assembled composite insulation 11. The other end of the S-shaped leaf springs 40 is attached to girth strap 14 as by welding or riveting. For installation, the leaf springs 40 may be retracted against .strap 14 and held temporarily by wiring or clamping (not shown). When the strap 14 is secured in position, the springs are released against the insulation.

If desired, the S-shaped leaf-spring may be doubleended, as shown in FIG. 9, to avoid bending moment in the strap. That is, each end of double-ended leaf-spring 41 bears against the assembled composite insulation, and This type of compression member is simple and inexpensive and requires very little headroom between the composite insulation 11 and the outer casing.

While the previously described embodiments of the invention permit thermal contraction and expansion of the insulated vessel without distortion or tearage of the composite insulation, no provision has been made for avoiding sagging while the insulation is being wrapped. Insulation displaced in this manner cannot be restored to the desired position unless it is removed and rewrapped.

This limitation has been overcome in another form of the invention in which multiple band means are disposed in contiguous adjacency with the composite insulating material extending laterally around the vessel. The bands are spirally wound with the composite insulation only sufficiently tightly to impart about 0.1 to 0.5 p.s.i. compression to the insulating material area beneath the multiple bands, the compressed area being a minor part of the total insulated area. Such bands and their assembly are more completely described in copending application Serial No. 306,408 filed simultaneously on August 29, 1963, in the name of E. S. Kordyban et al. However, in the last-mentioned application, the bands are spirally wound with the composite insulation sufiiciently tightly to concentrate the total frictional force between the composite sheet layers in a minor part of the total insulated area beneath the band means whereby the minor part is above its stable density and the remaining major part is maintained below its stable density. In the present invention, this last-mentioned function is performed by the previously described compression members and the bands are only spirally wound sufliciently tightly to lend uniform density and minimum stability required during the wrapping procedures. The cross-section area of the band should be minimized consistent with strength requirements so as to reduce the weight and heat capacity of the insulation. Materials suitable for banding most systems including aluminum stainless steel.

An important advantage of this invention is the compensation for relaxation or gradual permanent deformation of the materials used in the compressed area. This phenomenon is believed caused by slippage, yielding, and/or breakage of the minute fibers in the highly compressed low conductive layers and filler strips when fibers constitute the low conductive component.

When spiral bands are employed in this invention as previously described, further compensation for relaxation may be delivered by the use of elastic materials for spiral bands. Since the band spirals completely through the insulation thickness, the banding is preferably formed of materials which retain their requisite elasticity over wide ranges of temperatures. A strip of rubber or rubberoid material is suitable for systems in which the temperatures across the insulation are not widely different from room temperature. However, for cryogenic service, rubber in the vicinity of the cold wall becomes stiff and loses much of its elasticity. Other forms of elastic bands suitable for more extreme temperatures include a flat zig-zag spring formed of high yield-point wire, and discontinuous metal bands of material such as aluminum. The bands may, for example, be connected by spring wire clips or buckles. One advantage of the elastic spiralling band embodiment is its ability to stretch or contract with very little change in the hoop tension which produces and maintains stabilizing compression. Its length changes in concert with any dimensional changes which occur in the insulation or inner vessel.

It will be appreciated that a reduction in insulation thickness occurs in the banded areas and the areas under the compression members, due to the high compressibility of the fiber materials. The effect of this insulation thickness reduction is appreciable and can account for a substantial portion of the total heat conducted through the banded area. It will also be recalled from FIG. 2 that compression causes an increase in conductivity which, while not proportionate to compression, is nevertheless quite appreciable. Thus, the heat transport through the banded area, determined as Qc=kcAcAT/L, increases due to a higher kc and increases still further due to re duced thickness L.

In a still further preferred embodiment of the invention, any appreciable reduction in thickness L is avoided, and to a limited degree the thermal conductivity kc of the banded area is also reduced. These remarkable improvements are achieved by providing low conductive, compressible filler material in contiguous adjacency and coextensive relationship with the multiple band means. The filler material is positioned between such band means and the composite flexible material. For best results, the filler material is of such thickness in the installed compressed state that the number of layers of low conductive component per inch of composite insulation in the minor banded and compression member-loaded part of the total insulated area is substantially the same as the number of layers of low conductive component per inch in the remaining major part of such area.

A preferred filler material consists of strips of the same material used for the low conductive component layers. When the latter material is available in various thicknesses, combinations of thicknesses may be employed together to obtain the desired total amount of filler under the band. It is to be understood, however, that the invention is not limited to such common identity of the low conductive material component and filler materials.

Apparatus suitable for spirally wrapping the container 10 with an alternating layer composite insulation including a low conductive filler material is illustrated in FIG. 10. Container 50 is cylindrically shaped and mounted horizontally on shaft 51 driven by a power device 52. Stability band roll 53 is mounted on shaft 54 equipped with a variable-force friction brake 55, and low conductive filler material 56 is fed from roll 57 onto container 1i) beneath stability band 58. Simultaneously, low conductive sheet material and radiation barrier material are spirally wound on container 50 from rolls 59 and 60, respectively.

After wrapping has proceeded to the desired thickness, the free edges of the components may be secured by, for example, tapping or by a suitable adhesive compound. For insulating vessels longer than the width of the sheet material, several rolls of material may be aligned to end-to-end on a shaft, the combined length of the rolls being at least equal to that of the vessel. tension of all rolls assembled on a single shaft may be controlled by the same friction brake.

It would be understood that when the radiation barrier component and the low conductive material component are formed into a unitary sheet before they are wrapped onto a container, a single roll of the insulation would replace the separate rolls 59 and 60 in the apparatus shown in FIG. 10. This would be the case when the composite insulation being wrapped was of the reflective metal-coated non-metallic low conductive substrate material type, or of the finely divided radiant heat reflecting material-containing paper type.

The number of bands applied will depend upon the length of the vessel and on the stability desired. Only The than 25 microns of mercury.

one band may suflice in some instances, such as for small vessels subject to low accelerations. Normally at least two bands will be needed, one near each end of the vessel. Maximum band spacing along the vessel length will also be limited by the stiffness of the insulation. The blanket should be reasonably self-supporting between band locations so that it does not sag against the vessel and become over-compressed due to its own weight. With lightweight insulation consisting of thin glass fiber paper and /2 to 4 mil soft annealed aluminum foil, for example, experience has shown that 48-inch band spacing is suit able, while spacing beyond 72 inches may permit excessive sagging.

The total frictional force developed in the compressed areas should be based on the most severe stress expected in service. Normally, axi-al accelerations will produce the most severe slipping force on the insulation, and such force will be the controlling factor in planning for stability.

FIG. 11 is a fragmentary view through the evacuable space of a double-walled container constructed in accordance with this invention, the layer thickness being exaggerated for clarity. The composite spirally wrapped insulation 11 comprises alternate layers of low conductive component, e.g. fibrous sheet 65, and radiation barrier component, e.g. metal foil 66, with compression members 12 spaced at preferably uniform intervals along the length of such insulation. The inner end of compression member assemblies 12 bears against the outer surface of the assembled composite insulation 11, and

the outer end of such assemblies bears against girth means such as metal strap 14 under tension.

Compression member assemblies 12 are preferably placed over each spiral band 58 and low conductive filler 56 location in axial alignment therewith so that the compression member retainers 17 only bear on the banded area. While noticeable reduction in composite insulation thickness still occurs from the high compression imparted by assemblies 12, the degree of insulation thinning is reduced significantly by the band 58 and filler strip 56. Some thinning is inevitable whenever supporting pressure is applied after wrapping is completed.

In spite of the reduction in thickness under the compression members, this construction for stabilizing composite insulation produces thermal performance comparable to the previously described spirally banded insulation. The reason is that very small fractions of the total area are under appreciable compression. Whereas high band tension limits supporting pressure obtained by spiral banding to about p.s.i., the force exerted by compression members can easily be as high as 30 psi. Thus, the area under compression needed for a given value of PA can be very small. It will be recalled that FIGS. 2 and 3 show that high compression and small percent-area under compression tend to provide low heat conductance. This tendency more than compensates for the reduction in thickness under the compression members.

FIG. 12 illustrates a container constructed according to the present invention for the storing of materials at low temperature, including an inner vessel 10 having rigid, self-supporting walls for holding such material and a larger outer gas-tight shell 13 extending about said inner vessel. An intervening evacuable insulation space 67 is provided between the inner vessel 10 and outer shell 13 at an absolute pressure not substantially greater Inner vessel 10 is supported by suitable means as, for example, bottom tube 68 and top tube 69, and various conduits and valving for introducing and discharge of fluid may be employed, as will be understood by those skilled in the art. These elements have not been shown in the interest of simplicity and do not constitute parts of the invention. Suitable piping and valving is, for example, illustrated and described in US. Patent No. 2,986,163, issued January 17, 1961, to John H. Beckman.

The insulation space contains a composite insulation 11 in a series of spaced layers. Each composite insulation layer comprises a low conductive material component and a radiation barrier material component. In a preferred embodiment the low conductive material component comprises a fibrous low heat conductive oriented material wherein the fiber diameters are less than about 20 microns, and the radiation barrier material component comprises a series of spaced metal foils of thickness between 0.002 mm. and 0.2 mm. coextensive with and being separated and supported by the fibrous oriented material. The components of this insulation 11 are not shown in detail in FIG. 12, but may be assembled in the manner of FIG. 10.

The radiant heat barrier component-low conductive component composite insulating material is spirally wound around the inner vessel sufficiently closely to provide at least 4 composite layers per inch of composite insulation, and disposed generally perpendicular to the direction of heat transfer across the space. The previously described compression members and girth means for concentrating the total frictional force between the layers of the composite insulation are also provided as may be the spirally Wound band means and filler strips. Thus, FIG. 11 illustrates a suitable composite insulation-support assembly which may be employed in the evacuable annular space 67 of FIG. 12.

While it is desirable to minimize the quantity of low coductvie component used in each layer, it is not possible to eliminate this component altogether. In FIG. 11, for example, low conductive fiber paper sheet 65 is preferably provided as thin and light in weight as possible, but it cannot be omitted completely from the assembly. When reflective foil shields are used, an essential function of the low conductive component is to serve as a guard membrane preventing direct contact between adjacent radiation shields. Thermal short circuit between two shields even over a small area cancels the effectiveness of one shield, and it is practically impossible to avoid all sagging and irregular alignment of foils, particularly in curved insulated surfaces.

In one construction employing this invention, a 1%; in. thickness of the thin precompacted glass fiber paper aluminum foil composite insulation of FIGS. 1 and 2 was installed on a mobile liquid hydrogen tank having an inner vessel 78 inches in diameter and over 36 ft. long. The compression member assemblies of FIGS. 4 and 6 was employed with the spiral banding and filler of FIG. 11. The filler was one layer of 1.6 gm./ sq. ft. glass fiber paper. Nine aluminum banding strips 1% inches wide and 0.002 inch thick were spirally wound with the composite insulation, and 24 coil spring-type compression members were positioned at uniform intervals around the vessel circumference and over each band. Each spring base was 1 /24 in. x 1 /2 in., so that the springs pressed against 0.4% of the insulated cylindrical area of the inner vessel. Each spring exerted a force of 7 lbs., resulting in a compression of 3.8 psi. The stationary evaporation rate of hydrogen from the tank was 0.42% capacity per day, and the apparent thermal conductivity of the insulation was less than O.025 1O" B.t.u./sq. ft. F./ft. The latter figure approaches the ideal unsupported performance of the multiple-layer insulation.

Although preferred embodiments of this invention have been described in detail, it will be appreciated that modifications may be made and that some parts may be used without others, all within the contemplation and spirit of the invention. For example, multiple layer insulations other than the alternating foil and fiber type will benefit from the stabilization method of this invention.

The mechanics of stabilization appears to be about the same for both glass paper-aluminum foil composite insulation and aluminum-coated polyethylene terephthalate film composite insulation. The coefficients of friction between the components of both insulation, for example, are

about equal. An enlarged cross-section of the metalcoated low conductvie substrate-support construction assembly would appear very similar to the FIG. 11 construction except that the reflective radiation barrier component 66 is bonded to the low conductvie component substrate 65. The same filler strips 56 could be employed. Also, the assembly method and apparatus of FIG. would be appropriate for this embodiment except that component rolls 59, 60 would be replaced with a single composite roll.

This invention may similarly be advantageously employed for stabilization of the previously described radiant heat reflecting body-containing paper by employing the principles of the FIGS. 4-11 construction thereto. In such cases, the radiation barrier reflective sheet 66 in FIG. 11 would be optional since the paper layer in such case would contain uniformly dispersed small reflecting bodies.

Similarly vacuum type composite insulations of intermediate quality will also benefit from the stabilization methods and apparatus of this invention. An example of such an insulation is a composite comprising alternate layers of metal foil, e.g. aluminum, as the radiation barrier component and cellulose fiber paper of the kraft type as the low conductvie component material.

As another variation, the invention has been specifically described in terms of insulating containers having curved outer surfaces, but it is equally suitable for flat surfaced, e.g. rectangular, panels for insulating refrigerators. Also, the present insulation construction need not completely enclose the source of heat or cold as long as it is contiguously associated and in heat transfer relation with at least part of the source. For example, gas evacuated insulating structure spaces may be provided on the sides of a rectangular container and not on the bottom thereof.

The invention has also been specifically described in terms of a structure for minimizing the heat inleak to a container storing low boiling liquefied gases. Other cold materials which may be thermally separated from atmospheric heat by employment of this invention include quick frozen biological specimens, living tissues and other perishable commodities.

The invention may also be used to great advantage in the insulation of a heat transferable container at high temperature as, for example, a rocket motor. In this event, heat is transferred to the atmosphere and the present structure serves to minimize the transfer.

It should be recognized that materials selected for construction of the present structure should be stable at the temperatures to be encountered in usage. For example, if the thermal insulating structure is to be exposed to moderately high temperatures and a low conductvie substrate is to be employed, the fluorinated polymers (e.g. polymers of tetrafluoroethylene) are especially suitable as the substrate.

As another variation, elastically deformable compression members other than coil springs (FIG. 4) and leaf springs (FIGS. 8 and 9) may be employed as, for example, rubber or rubberlike columns.

Referring again to FIG. 11, when only slight compression is needed to avoid sagging and slippage during the wrapping procedure, the previously described spiral bands 58 may be omitted but the filler material 56 retained as a bulking component between at least some of the spaced layers and beneath the compression members 12. Such bulking component has a total surface area contiguously associated with a minor part of the spaced layer surface area. In this embodiment, one of the components of the composite flexible material, e.g. foil sheet 66, is tensioned against the bulking material 56 to provide that portion of the total frictional force necessary for uniform wrapping. Such use of compression bulking material is described more completely in copending application Serial No, 306,371 filed simultaneously on August 29, 1963, in

16 the names of L. C. Matsch, D. L]. Wang and J. A. Paivanas. This application is incorporated herein by reference. The bulking component may, for example, be the previously described glass fiber paper or Web materials.

The use of this invention has been explained in a simplified manner based on the assumption that the low conductive component material or radiation barrier component density which is optimal for ideal, unsupported insulation is also the optimal density for insulation having the instant compression members associated therewith. For example, it is assumed that 60 layers/inch is always the best shield density for the 0.00025-inch thick aluminum foil-1.6 gm./ sq. ft. glass fiber paper combination. This is not necessarily true in all embodiments. The overall (average) conductivity of compression membered insulation is the sum of contributions of the uncompressed and the compressed fractions. If the fibrous sheet density of this specific embodiment is reduced below the optimal 60 layers/inch, the contribution of the uncompressed factor will tend to increase; but because the total weight of the insulation to be stabilized is reduced, the contribution of the compressed fraction will tend to decrease.

Thus, the desired minimum value of overall thermal conductivity Will usually correspond to a low conductive component material density somewhat different from that corresponding to minimum thermal conductivity of uncompressed composite insulation. A few trial-and-error computations will disclose the true optimal low conductive component material density for compression mem bered insulation. t is apparent that when the contribution of the compressed area is small, then the optimal density will correspond closely to minimum thermal conductivity of uncompressed composite insulation and little added benefit will result from such trial-and-error computation. However, if the contribution of the compressed fraction is large, then the best shield density may be significantly different from that corresponding to minimum uncompressed thermal conductivity.

In one instance which clearly illustrates the above effect, it was found that a thick layer of the FIG. 1 insulation subjected to high g-loading would exhibit lowest overall thermal conductivity if wrapped with a shield density of only about 45 layers/inch. Two thicknesses of 1.6 gms./ sq. ft. bulking component were used to obtain this low density.

What is claimed is:

1. A thermal insulating structure comprising gas-tight walls enclosing an evacuable space; multiple layers of heat insulative and radiation-impervious composite flexible insulating material within said space comprising a low heat conductive material component and a radiant heat barrier material component assembly sufficiently close to provide at least 4 layers per inch of composite insulation, and disposed generally perpendicular to the direction of heat transfer across the insulation space; multiple compression member means within said space for concentrating the total frictional force between the layers of composite insulating material in a minor part of the total insulated area whereby said minor part is above its stable density, one end of each compression member being positioned against the outer surface of said composite insulating material, and girth means in said space positioned against the other end of each of said compression members under sufficient tension for said concentrating the total frictional force to maintain said minor part above its stable density.

2. A container for storing materials at low temperature comprising an inner vessel having rigid, self-supporting walls for holding such material; a larger outer gas-tight shell extending about said inner vessel; an intervening evacuable insulation space at an absolute pressure not substantially greater than 25 microns of mercury; multiple layers of heat insulative and radiation-impervious composite flexible insulating material Within said space comprising a low heat conductive material component and a radiant heat barrier material component assembled sufiiciently closely to provide at least 4 layers per inch of composite insulation, and disposed generally perpendicular to the direction of heat transfer across the insulation space; multiple compression member means within said space for concentrating the total frictional force between the layers of composite insulating material in a minor part of the total insulated area whereby said minor part is above its stable density, one end of each compression member being positioned against the outer surface of said composite insulating material, said compression members being arranged in at least one row around the circumference of said inner vessel and perpendicular to the longitudinal axis of such vessel; and girth means extending laterally around said inner vessel, being arranged and positioned against the other end of said compression members under sufiicient tension for said concentrating the total frictional force to maintain said minor part above its stable density.

3. A container according to claim in which multiple band means are disposed in contiguous adjacency with said composite insulating material extending laterally around said inner vessel and being spirally Wound with said composite insulating material only sufiiciently tightly to impart about 0.1 to 0.5 psi. compression to the composite insulating material area beneath said multiple band means, the compressed area being a minor part of said total insulated area.

4. A container according to claim 2 having multiple band, means and low conductive, compressible filler material disposed in contiguous adjacency with said composite insulating material extending laterally around said vessel and being spirally Wound with said composite insulating material only sufficiently tightly to impart about 0.1 to 0.5

p.s.i. compression to the composite insulating material area beneath said multiple band means, the compressed area being a minor part of said total insulated area, the compressible filler material being in contiguous adjacency and in coextensive relationship with said multiple band means, and between such band means and said composite flexible material.

5. A container according to claim 2 in which said filler is of such thickness in the installed compressed state that the number of low conductive material component layers per inch of composite insulation in the minor part of the total insulated area beneath the band means is substantially the same as the number of layers per inch in the remaining uncompressed major part of such area.

6. A container according to claim 2 in which a low heat conductive filler material is disposed beneath said multiple compression member means and between at least some of said spaced layers of low heat conductive material component, and having a total surface area contiguously associated with a minor part of the spaced layer surface area.

References Cited by the Examiner UNITED STATES PATENTS 1,730,153 10/1929 Lindsay 220-9 2,239,109 4/1941 Lundvall 20-4 2,928,565 3/1960 Glasoe 220-9 3,009,601 11/1961 Matsch 220-9 FOREIGN PATENTS 257,225 12/ 1926 Great Britain. 840,952 7/ 1960 Great Britain.

THERON E. CONDON, Primary Examiner.

GEORGE E. LOWRANCE, Examiner.