US3007596A

US3007596A - Thermal insulation

Info

Publication number: US3007596A
Application number: US597947A
Authority: US
Inventors: Ladislas C Matsch
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1956-07-16
Filing date: 1956-07-16
Publication date: 1961-11-07
Anticipated expiration: 1978-11-07
Also published as: DE1214711B; BE559232A; GB853585A; FR1178908A

Abstract

853,585. Liquefied gas storage vessels. UNION CARBIDE CORPORATION. July 1, 1957 [July 16, 1956], No. 20748/57. Class 8(2). The evacuated insulation space 11, Fig. 1 between double walls 10a, 10b of a liquefied gas storage vessel is occupied by alternative layers 13, Fig. 2 of low conductivity material e.g., fibre glass and layers 14 of radiant heat reflecting material, e.g. aluminium foil. The layers 13, 14 may be wound round the cylindrical body of the vessel or may be arranged concentric therewith ; the domed top being occupied by supplementary layers of insulation. The radiant heat reflecting layers 14 which may also comprise tin, silver, gold and also metal coated plastic vary in thickness between 0À002 and 0À2 millimetres and the low conductivity layers 13 may comprise fibres of diameter between one and 50 microns or powder insulation or a mixture of fibres and powder. Specification 853,584 is referred to.

Description

L. C. MATSCH THERMAL INSULATION LADISLAS C. MATSCH Nov. 7, 1961 posj're lnsula'rlon b V ,if

PaTh of Heat Transfer Com TTORNEY United rates 3,007,596 THERMAL INSULATION Ladislas C. Matsch, Kenmore, N.Y., assigner to Union This invention relates to an improved insulation having a high resistance to all modes of heat transfer, and particularly concerns la low temperature, heat insulating material adapted to improve a vacuum insulating system.

In the conservation and conveying of low temperature commercial productsfor example, perishable commodities which must be held at low temperatures for substantial periods of time, and valuable volatile materials, such as liquefied gases having boiling points at atmospheric pressure below 233 K., for example, liquid oxygen or nitrogenha major problem encountered is the control of heat leak to the material, which in the case of liquefied gases results in loss due to evaporation. ln the conventional double walled liquid-oxygen container, the space between the walls is suitably insulated to limit this evaporation loss. However, up to now it has not been possible to provide [an insulation system, particularly for small, portable containers having small volumes in comparison with the surface areas, which will limit the evaporation loss to satisfactorily low values.

The basic systems for insulating the conventional double walled container for the conveyance and storage of low boiling liquefied gases :are for small containers, the Dewar type high vacuum-polished metal surface system and for large containers, the powder-in-vacuurn insulation System, which uses an insulating powder in the vacuum space between the walls. This system is described in detail in U.S. Patent 2,396,459. Although powder-invacuuxn heat insulation is effective in reducing thermal heat loss, it is not as effective as straight vacuum-polished metal surface for containers up to two feet in diameter. While these systems have greatly affected the commercial considerations as applied to storage and conveyance of low ltemperature products, there, nevertheless, exists a great commercial need for more efiicient insulating materials capable of meeting more rigid and exacting requirements, and which will provide even lower thermal conductivitics than those `afforded by either of the above-described insulators. Provision of such materials would permit the Study, development and creation of important new and improved control techniques for many processes and products.

To give some insight into the problems that are presented in effecting further reductions in heat leak for small portable containers, suppose for example that it is desired to insulate `a double ywalled cylindrical container for low boiling liquefied gases such las oxygen, so that the evaporation loss due to heat leak will be less than 1% of the contained material per day. Assume further that the container will have hemispherical ends, an inner diameter of 8 inches and an inner total length of 48 inches. Using one of the best insulating materials ofthe prior art, for example, powder-invacuurn insulation, in accordance with U.S. Patent 2,396,459, the vacuum being on the 'order of 0.1 micron of mercury absolute, a thermal conductivity of 9.2X104 Btu/hr., sq. ft., F./iit. may be achieved. ln order to more fully appreciate the significance of such a thermal'conductivity, :the insulating effects of the following insulation thicknesses are set forth. An insulation thickness of 1.6.6 inches of a powder-in-vacuurn insulation will permit an evaporation loss. of 7.1% per day. Such an insulation thickness results in an insulation cross sectional area equal to the useful cross sectional area of the inner storage container. In other words, beyond the thickness of 1.66 inches, the bulk of the insulation which must be stored land/ or transported becomes greater than the bulk of the contained stored material.

Increasing the thickness of such an insulation to 4 inches reduces the loss rate to 3.6% per day, while increasing the insulation volume to about 3 times that of the storage capacity of the inner container. In fact, it is entirely impractical to consider insulating the vessel with ya material hav-ing aconductivity of 9.2. 10F4 B tu/ hr., sq. it., F./ft., since calculations show the theoretical required insulation thickness to be lGS inches.

Considering a straight vacuum insulating system in which the walls of the inner and outer vessels within the insulation space yare polished in order to reflect radiant heat energy, .there is a problem of maintaining a sufficiently low vacuum to eliminate heat conduction by residual gas. For this purpose the absolute pressure within the insulating space must be maintained et a value l0 to 100 times lower than when a powder-in vacuum insulating system is used. Consequently, the vacuum must be less than `0.()1 micron of mercury absolute pressure. This may be obtainable in special laboratory equipment, but it is quite lan impractical specification for fabricated vmetal vessels intended for industrial service. Considering for a moment the possibility that low vacuum conditions could be maintained so that heat transmission by conduction through the residual gas would be negligible, there still remains the problem of achieving the necessary reflectivity for the vessel walls. To obtain `a maximum loss nate of 1% per day, calcul-ations indicate that surface retlectivities of at least 99.6% must be obtained. Re iiectivities of this ord-er are only obtainable, if at all, under strictly controlled laboratory conditions, which may not be duplicated or maintained either during fabrication of the container, Ior after the container is in service.

A lower quality reflective surface may be tolerated by interposing several concentric reflective shields within the insulation space as described in U.S. Patent 2,643,022. However, one of the difficulties involved in such an arrangement is in assembling and supporting many reflective shields within a reasonable insulation thickness so that each shield is properly spaced from adjacent shields at all points. Proper spacing is an absolute necessity, for if two adjacent shields are permitted t0contact in even a minute area, the insulating effect of one shield will be essentially cancelled out. Moreover, the number of shields required depends on their surfacerellectivity. If

,a very highly polished surface is provided for both vessel walls and the shields, the polished surface having a reilectivity of then it may be shown by calculation that at least 10 such shields must be used in order to achieve a 1% per day heat loss rate in the described vessel. At the same time, to maintain a reasonable thickness of insulation, the shields must be spaced as close together as possible. Allowing for inaccuracies in forming and assembling the shields, the spacing of at least 1A inch would appear to be reasonable. Ten shields between the container walls would provide l1 spaces, and taking the thickness of the shields into consideration, would account for lan overall thickness of at least 3 inches. Under these circumstances the fabrication of vessels having a heat loss rate of less than 1% per day would be costly and time-consuming.

In view of these obstacles, up to the present time it has been impossible to approach, much less achieve, heat leaks of such restricted quantities for applications in- -volving extended periods of storage of low boiling liquefied gases in portable storage containers.

It is, therefore, animportant object of the present invention to provide a greatly improved insulation systcm for reducing heat transmission by all modes of heat transfer to values well below that of any previously known insulating system.

Another object of the present invention is to provide a novel insulating material in an insulation system where radiation would otherwise be an important mode of heat transfer.

Another object of the invention is to provide in a low heat conductive material wherein radiation is the predominant remaining mode of heat transfer, one or more parallel radiant heat barriers interposed in said low conductive material for substantially reducing the passage of radiant heat therethrough.

Yet another object of the invention is to provide in a low heat conductive insulation, a series of spaced, heat reflecting barriers so constructed and arranged as to impede the passage of radiant heat through said insulation without affecting the thermal conductivity thereof.

Another object of the present invention is to provide in a restricted gas-evacuated insulating space, a plurality of radiation barriers, said barriers being disposed in spaced relation to each other, and maintained in such spaced position by a low heat conductive spacing material.

Still another object of the present invention is to provide in a vacuum-solid insulating space for small portable containers, a multiplicity of radiation barriers comprising spaced and parallel foils of heat reflective material for reducing the transfer of heat by radiation, and a spacing material between said radiation barriers, cornprising a low-conductive, heat insulating material for reducing the transfer of heat by conduction between said barriers.

A further object of the present invention is to provide an improved method of fabricating and applying a heat insulation for cylindrical containers wherein the heat insulation comprises a low-conductive, heat insulating material for reducing the transfer of heat by conduction, and incorporates therein a multiplicity of radiation sheet barriers for reducing the transfer of heat by radiation.

A further object of the present invention is to provide in an enclosed volume defining a gas evacuated insulating space, a novel insulating structure adapted to fill the insulating space and effect contact with the wall surfaces defining the insulating space, said insulating space being characterized by the absence of gross voids, and having a low rate of heat transfer by conduction and radiation.

Other objects, features and advantages of the present invention will be apparent from the following detailed description.

In the drawings:

FIG. l is a front elevational view, partly in section, of a double walled liquid gas container embodying the principles of the invention;

FIG. 2 is an isometric view of the composite insulating material of the invention shown in a flattened position with parts broken away to expose underlying layers;

FIG. 3 is a greatly enlarged detail sectional view showing the irregular path of heat transfer through the composite insulating material of the invention;

FIG. 4 is a sectional view taken along line 4 4 of FIG. l, illustrating the spiral wrapping of insulating material of the invention;

FIG. 5 is a sectional view similar to FIG. 4, but showing a concentric layered modification thereof; and

FIG. 6 is a fragmentary elevational view, in section, of a modified double walled liquid gas container embodying the principles of the invention.

In the past, radiation shields used in vacuum spaces have been constructed for the most part to be supportingly suspended in spaced relation to each other. Numerous small diameter supports were employed in the vacuum space to support the insulated vessel and to maintain proper shield spacing. A minimum number of these supports were employed to constrict the passage of heat leak by conduction. The remaining space was left unfilled in order not to create additional pathways for thermal conduction. Furthermore, it was believed that the reflective characteristics of the shields would be seriously impaired by contact with an insulating filler.

lI have made the amazing discovery that the insulating qualities of an evacuated insulating space may be substantially enhanced to a degree never before attained with a novel insulating structure, which may occupy part of or the entire insulating space. Yet the insulating structure does not require numerous brace bars or other supports, does not provide gross voids in the insulating space, and can also be employed as a novel means for elastically supporting the insulated inner container.

More specifically, I have discovered that the transmission of heat across a solid-in-vacuum type insulation may be substantially reduced to a degree greater than has heretofore been possible by the use of a low heat conductive material which incorporates therein one or more radiation impervious shields to substantially eliminate heat leak by radiation.

Furthermore, I have discovered that the placing of reflective shields in direct contact with an insulating material does not substantially impair the radiation barrier qualities of the shields.

The term vacuum as used hereinafter is intended to apply to sub-atmospheric absolute pressure conditions not substantially greater than 5000 microns of mercury, and preferably below 1000 microns of mercury. For superior quality results, the pressure should preferably be below 25 microns of mercury.

According to the invention, a vacuum insulated space is provided with a low heat conductive material having incorporated therein one or more radiation barriers disposed substantially transversely to the direction of heat flow in spaced relation to each other. The radiation barriers or shields of the invention may comprise one or more sheets of heat absorbing material, or preferably thin sheets or layers of a material possessing high reflecting characteristics when exposed to infra-red radiation, such as aluminum or tin foil. The low conductive material also acts as a supporting and spacing material for retaining the radiation barrier sheets in uniformly spaced relation to each other independently of the thickness and stiffness of the barriers. In this manner it is possible for a large number of thin foils to be supportably mounted and maintained in position in an insulation space of limited thickness. A clearance of a few thousandths of an inch between foils is enough to effectively interrupt and reflect the radiant heat. In this way it is possible to provide a large number of shields in a very limited space, ranging up to several hundred shields per inch of composite insulation thickness.

Shown in FIG. l is a double walled heat insulating vessel having parallel inner and

outer container walls

10a and 10b and an evacuated insulating space 11 therebetween. Disposed within the insulation space 11 is a composite insulation material 12 embodying the principles of the invention, and comprising essentially a low heat conductive material 13 having incorporated therein multiple reflective shields or radiation barriers 14 in contiguous relation for diminishing the transfer of heat by radiation across the insulating space 11. In assembly, the insulation occupies the entire insulating space 11, and appears as a series of spaced reflectors 14 disposed substantially transversely to direction of heat flow and supportably carried by the solid, low conductive insulating material. The insulating material. uniformly contacts and supports the entire surface of each radiation shield in superposed relation and, in addition to its primary purpose of serving as an insulating material, constitutes a carrier and spacing material for maintaining a separation space between adjacent shields. No other supports are required to maintain the insulation in operative assembled relation.

The radiation shield material 14 to be used in the insulation material 12 of the invention may comprise either a metal, metal oxide, or metal coated material, such as aluminum coated plastic lm, or other-radiation refiective or radiation absorptive material or a suitable combination thereof. Radiation reflective materials comprising thin metallic foils are admirably suited in the practice of the present invention, while reective sheets of aluminum foil having a thickness between 0.2 mm. and @.002 mm. are preferred. Other radiation reiiective materials which are susceptible of use in the practice of the invention are tin, silver, gold, copper, cadmium or other metals.

The base material lf3 of the invention may be a suitably low conductive material such as fiber insulation, which may be produced in sheet form. It should preferably be thin enough to -be liexibly bent. Among the spacing materials which give excellent results are the porous unbonded fiber-type insulating materials which do not give ofi gas, for example, a filamentary glass material such as glass wool and ber glass, the latter being preferred because of its low conductivity and foraminous structure, and the ease with which the air spaces within the brous structure may be evacuated. The base material may also comprise a combination of the fibers with low conductive powder insulation, vas specified hereinafter, or any other combination of suitably low conductive materials.

ri`he invention will be described in connection with a multi-layer insulation in a vacuum space having a pressure less than 0.1 micron of mercury, using aluminum foils as the radiant heat barriers, and commercial grades of fiber glass sheetings as low conductive insulation, and as the supporting and separating media between sheets of aluminum foil. This composite insulation has been found to aord a particularly efficient means for minimizing the transfer of heat by all modes of heat transfer across a vacuum insulated space, but it is to be understood that the same principles apply regardless of the nature of the speciiic material used.

As a preferred feature of the invention, the low conductive sheet of ber glass separating material i3 to be used in the present vacuum-solid insulating system should be fabricated in such manner that its fibers are, for all intents and purposes, randomly disposed within the plane of the separating sheet, and oriented in a direction substantially perpendicular to the flow of heat. it will be understood that as a practical matter, the fibers will not be individually confined to a single plane, but rather, in a finite thickness of fibrous material, the fibers will be generally disposed in thin parallel strata with, of course, some indiscriminate cross weaving of iibers across the various strata.

While we do not wish to be bound by any particular theory, we believe the principal reasons for the far superior insulating effects achieved by such a fiber orientation are the relatively few iibers traversing the thickness o-f the insulating sheet and the point contacts established between crossing fibers. These point contacts represent thepoints of ioinder between adjacent fibers in the direction of heat flow, and as such, constitute an extremely high resistance to the flow of heat by conduction. ln this fashion it is possible to achieve in a finite thickness of insulating material, an extremely high degree of conductive insulation between proximate sheets of aluminum foil. Best results have been obtained when the fiber diameter is less than 1.0 micron, although larger diameter ibers up to about 10.0 microns, and in some instances as high as 50 microns, depending upon the insulation thickness employed, still produce results equal to or better than the best known practical insulation of the prior art.

The sequence of modes of heat transfer which might occur in a typical multi-layer insulation of aluminum foils which are proximately spaced from each other by sheetings of glass fiber having a fiber orientation substantially parallel to the aluminum foils and transverse to the direction of heat flow, might be as follows.

Referring to FIG. 3, radiant heat striking the rst sheet of aluminum foil will for the most part be reected, and the remaining part absorbed. Part of this absorbed radiation will tend to travel toward the next barrier by reradiation, where again it will be mostly reflected, part will travel by solid conduction, and a minor part by conduction through the residual gas. According to the solid conduction method of heat transfer, the heat leak proceeds along the fibers in what might be considered an irregular path, crossing relatively small areas of point contact between crossing iibers until it reaches the second sheet of aluminum foil, where the hea-t reiiecting and absorbing process described above is repeated. Because of the particular orientation of the glass fibers, the path of solid conduction from the first sheet of aluminum foil to the second is greatly lengthened, and encompasses an indeinitely large number of point contact resistances between contacting iibers. Bn analogy it will be seen that a multi-layer insulation having a series of heat reflecting sheets and a fiber oriented sheet of low conductive insulating material .therebetween may be particularly ehicient in preventing or diminishing heat losses by radiation, as well as by conduction.

in the practice of the present invention, the radiation shield spacing may vary from one-half to two hundred la yers per inch. Where the quality of the insulation is of prime importance, the preferred number of shields may vary between 4 and l0() shields per inch. For the purposes of this invention, the insulation may be also related tothe size of the container to which it is applied. Thus, for small containers having a diameter less than two feet, an insulation thickness up to three inches is desirable, using at least l5 layers of shields per inch in a glass fiber insulation having a fiber diameter less than one micron. For vessels above two feet in diameter a coarser fiber diameter up to l0 microns may be used, and for very large containers of tank car size or larger, iiber diameters up to 50 microns may be employed; the insulation thickness may vary up to 24 inches, and the shield spacing may be as great as 1/2 shield per inch or one shield for every two inches of insulation thickness. In any case, the insulation thickness should contain at least one radiation shield, and the shield spacing should not exceed 5% of the container diameter. As a practical matter, the usual space should preferably contain a minimum number of at least three shields. It is to be understood, however, that the invention is not necessarily limited to the above ranges, and that a larger or smaller number of shields may be satisfactorily employed in the practice of the invention, depending upon the particular conditions involved in the application of the invention.

@ne of the important advantages in fthe thermal insulation of the present invention is that the flexibility of the layers of aluminum foil and fiber glass allows the insulation thickness as a whole -to be pliably bent so as to conform to irregularities and changes in the surface conditions of the container to be insulated. The cornposite material of the invention is adapted to be applied to contoured surfaces, and is particularly well suited for insulating either iiat or cylindrical surfaces.

Gbviously the multiple foil insulation of the invention may be mounted in the insulation space in any one of a variety oi' ways. For example, in FdG. 5, the insulation 12 may be mounted concentricaily with respect to the inner container lita, or it may be, as in FlG. 4,' spirally wrapped around the inner container with one end of the insulation wrapping in contact with the inner container dita, and `the other end nearest the outer container 10b or in actual contact therewith, the latter form `of Vmounting being preferred and illustrated herein. In

either case, the composite insulation of the present invention does not support the walls of the vacuum space against external loads and, hence, is external load-free. Referring to FiG. 4, the metal foil may be loosely spirally wrapped around ythe inner container "a, the tightness and number of turns being varied to `suit the particular conditions, or the requirements desired. Tightening of the insulation wrapping causes the low conductive and resilient fibrous material to be compressed into a smaller space. This action decreases the percentage voids in the fibrous material, and increases the cross sectional area of the effective path of solid conduction. However, the voids are reduced in size, which results in the insulation being less sensitive to changes in casing pressure. On the other hand, wrapping the insulation too loosely decreases the number of turns of radiation shielding in the insulation space, and increases heat leak by radiation. Optimum results obtain somewhere between these extremes when the sum of the heat leaks due to radiation and conduction is a minimum. By providing a large number of turns of insulation wrappings, the passage of radiative heat is substantially eliminated, while the conductive heat flow along the spiral path is effectively reduced owing to the lengthened heat path.

It will be recognized that because of the difficulty involved in conformably applying the composite insulation material 12 of the invention to surfaces other than flat or cylindrical surfaces without sacrificing insulating qualities, for maximum benefit it may be advantageous in some instances to employ a supplementary low heat conductive `material in combination with the insulation 12.

In the modification shown in FIG. 6 the composite insulation material 12 of the invention may be employed in the cylindrical portion 11a of the insulation space 11, and the end portions 11b of the insulation space, including the fiat bottom portion and the upper spherical portion, provided with a supplemental low heat conductive material 16. The supplemental low heat conductive materials which may be used in the terminal sections 11b may comprise a finely divided powder of the type disclosed in U.S. Patent No. 2,396,459, or any other suitably low conductive material.

Coupled with the composite insulation 12, the supplemental insulation 16 maintains the extremities of the individual foil barriers 14 in spaced apart relation, and provides the means for producing low thermal heat transfers in containers of a wide variety of shapes. The cooperative relationship between the supplemental insulation 16 and the composite insulation 12 meets the requirements of the most critical present day insulation standards, and has extended the usefulness and capabilities of the present invention.

I have found that a very significant advantage of the present invention arises from the elastic properties of the insulation, particularly when a fibrous insulation is employed in the annular insulating space of a double Walled container. The ability of the insulation to give and resist movement of the inner container, and to restore or expand itself when the forces exerted upon it are relaxed, enables it to operate along the lines of a shock mount. Obvious advantages to using the insulation as an elastic support are that the inner container is maintained in substantially centered position, and the need for braces or other supports is obviated, thus further reducing the heat leak into the container.

On the surface it might seem that a more effective insulation may be obtained by providing additional turns of insulation wrapping. In a number of actual tests to determine thermal conductivity, the insulating system of the invention was found to be affected by a number of variables. These tests were conducted under carefully controlled conditions, using a double walled container having an evacuated insulation space containing the insulation of the invention, one Wall of the container being maintained at a lower temperature of either 183.2 C. or -195.9 C., and the other wall having an upper temperature ranging from approximately 20 C. to approximately 50 C. Representative results of these tests are tabulated below in Tables I and II. In Table I, the

pressure in the insulation space is less than 0.1 micron of mercury, while in Table II the pressure is varied as indicated therein.

TABLE I Thermal conductivilies of nsulatons of the present invention Temp. of Double Walled Container, K 103 C. Btu/hr., Material sq. ft., F./ft. Higher Lower Temp. Temp.

78 sheets of glass ber per inch separated individually by aluminum foil (fiber diameter 0.2 to 0.5 micron). 97 sheets ofhglatss ber peri inci wth warmer s ee s separate in ivi ually and colder sheets separated iu 5 pairs by aluminum foil (ber diameter 0.5 to 0.75 micron). 9% sheets of glass ber per inch separated individually by aluminum foil (ber diameter 2.5 to 3.8 microns) 22 195. 9 0.075 24 layers per inch of finely divided 47.0 `183.2 0.274 silica separated by aluminum foil. 21.5 183. 2 0. 242

TABLE II Eect of pressure in insulation space on insulations of the present invention [Higher temperature of container wall 20 C. Lower temperature of container wall l95.9 0.]

Itis evident from the above that Ian important variable influencing the performance of the present insulating system is the ber diameter of the low conductive spacing material. Depending upon the thickness of insulation and the number of radiation shields, ber diameters up to about 50 microns may be satisfactorily employed in the practice of the invention. A small ber diameter of less than 10 microns is desirable, while a fiber diameter less than 1 micron is preferred for superior quality insulations.

Another variable affecting insulating performance is the gas pressure in the insulating space. Obviously, from the data given in Table II, the insulating system of the present invention is affected by changes in pressure, the thermal conductivity increasing with increasing air pressure. As an additional feature of the invention, the adverse effect of increased pressure may be minimized by filling the voids between the fibers for example with a very fine, low conductive powder.

In order to indicate still more fully the nature of the present invention, the following comparison of heat transfer rates between 1) A solid-in-vacuum insulating system using insulating powder in the vacuum space in accordance with U.S. Patent No. 2,396,459,

(2) A high vacuum-polished metal surface system using radiation shields in the vacuum space in accordance with U.S. Patent No. 2,643,022, and

(3) An insulating system constructed in accordance with the principles of the present invention, are set forth in Table IH, it being understood that the data presented therein is illustrative only, and not intended to limit the scope of the invention.

TABLE III Insulation characteristics between +20 C. and -1832 C.

Pressure in Thermal Con- Type of Insulation Vacuum Space, ductivity, Mierons Btn/hr., Mercury sq. ft., F./it.

0.1 9. 2)(10-4 0. 01 1. 9 l04 0.1 u. axis-4 It is seen from this table that the heat insulating efficiency of the present invention is approximately l times better than insulation (2), and more than 45 times higher than insulation (l).

From the above description it will, therefore, be seen that the present invention provides in a solid-in-vacuum type insulation, a low heat conductive material having incorporated therein multiple radiation shields for impeding radiative heat transmission through the insulation, while minimizing the ow of heat by conduction therethrough. The low conductive material uniformly supports and maintains the radiation shields in spaced relation. A low conductive material which is admirably suited for use in the practice of the invention is one having a iibrous structure oriented in a direction perpendicular to the direction Vof heat iiow. Possessed or" a high percentage of voids, the low conductive insulating material provides a very small, solid conduction heat path between radiation foils, and is remarkably efficient in minimizing the transmission of heat leak by conduction.

insulating systems of the invention, using a ne diameter, low conductive, fiber-type insulating material, have been found to be superior to any known insulating systern. Employing coarser, low-conductive insulating iibers, the present insulation achieves low thermal conductivities, which are comparable or superior to those obtained with either high quality straight vacuums or the best powderin-vacuum systems known, yet is considerably less eX- pensive than either of these forms of insulation, and does not require as low absolute pressures as straight vacuumpolished metal insulating systems.

It will be understood that variations and modifications may be eiected without departing from the novel concepts of the present invention. For example, vvhile a single layer thickness of low conductive material is shown between adjacent heat retlecting shields, it should be understood that more than one layer may be employed in the practice of the invention.

What is claimed is:

1. In a Vacuum insulating space, a low heat conductive fibrous material, a multiplicity of radiation-impervious sheets supportably carried by said fibrous material, said radiation-impervious sheets being disposed in parallel spaced relation to each other, and said fibrous material having a iber orientation substantially parallel to said sheets and substantially perpendicular to the direction of heat ow across said space, and a line low-conductive powder in the voids between the bers of said iibrous material, whereby said sheets and iibrous material are effective in reducing the transmission of radiant heat across said space without perceptibly increasing the heat transmission by conduction thereacross, and whereby said powder reduces the variation in thermal conductivity of said fibrous material and sheets due to changes in pressure conditions in said space.

2. An apparatus provided with a gas evacuated insulating space surrounding a storage container and being enclosed by rigid, self-supporting walls, a heat insulative and radiation-impervious composite flexible material comprising a multiplicity of thin-flexible, radiant heat barrier layers of thickness between 0.002 mm. and 0.2 mm. in said insulating space disposed in spaced relation to each 10 other for reducing the transmission of radiant heat across said space, and a multiplicity of low heat conductive porous, librous sheet layers composed of fibers having diameters less than about li) microns, being ycoextensive with such radiation barrier layers and disposed in contiguous adjacency between and in spirally wound alternating sequence with said barriers around said storage container for separating and supporting said barriers spaced from each other, the radiant heat barrier lowconductive yfibrous sheet composite insulating material being spirally wound suliiciently tightly to provide at least 4 radiant heat barriers per inch of composite insulation and disposed generally perpendicular to the direction of heat inleak across said space.

3. An apparatus provided with a vacuum insulating space according to claim 2, the radiant heat barrier layers consisting of metal foil.

4. An apparatus provided with a vacuum insulating space according to claim 2, the radiant heat barrier layers consisting of aluminum foil.

5. An apparatus provided with a vacuum insulating space according to claim 2, the fibrous sheet layers consisting of filamentary glass material.

6. An apparatus provided `with a vacuum insulating space according to claim 2, the iibrous sheet layers consisting of glass liber.

7. A container for the holding of materials'at low temperatures, comprising an inner vessel having rigid, self-supporting walls for holding such material, a larger outer gas-tight shell also having rigid, self-supporting Walls extending about said inner vessel, providing therewith an intervening evacuable external load-tree insulation space at an absolute pressure not substantially greater than 25 microns of mercury, said insulation space containing a series of spaced layers of porous, fibrous, low heat conductive, oriented material wherein the liber diam'- eters are less than about l0 microns, and a series of spaced radiation barriers of thickness between 0.002 mm. 0.2 mm. coextensive with and being separated and supported by said iibrous, oriented material for reducing the transmission of radiant heat across said space without perceptively incr-easing the heat transmission by conduction thereacross, the radiant heat barrier-low conductive fibrous sheet composite insulating material being spirally around said inner vessel sufficiently tightly to provide at least 4 radiant heat barriers per inch of composite insulation and disposed generally perpendicular to the direction of heat in eak across said space.

8. in a double-walled low-temperature boiling liqueed-gas container having a cylindrical vacuum space between uniformly spaced rigid inner and outer walls, an external load-free solid-in-vacuum thermal insulation disposed Within such space comprising the combination with a multi-convoluted wrapping on such inner wall consisting of a continuous thin sheet of reective metal foil of thickness between 0.002 mm. and 0.2 mm.; of relatively thin, continuous, porous felt batting of glass iiber sandwiched between the convolutions of such foil, which eliectively supports and separates adjacent convolutions of such foil sheet from each other and completely -fills the space therebetween with clean, iieecy, compressible, unbonded, tine fibers of glass having ber diameters less than about l0 microns and being oriented by transverse compression thereof between such convolutions of metal foil sheet, in directions substantially perpendicular to the ow of heat in-leak; such glass fiber felt batting and foil sheet composite insulation substantially lling such cylindrical space sutciently tightly to provide at least 4 layers oi' foil sheet per inch of composite insulation and being highly effective to substantially decrease heat flow therein between such outer and inner walls, thereby substantially reducing heat iii-leak to the interior of the container by all modes of heat transfer, by virtue of the many convolutions of such insulation that can be made in such space that is available therefor, while miniinizing heat in-leak by conduction across the space occupied by such felt batting compressed between such foil convolutions; said insulation having a thermal conductivity valve of not more than 0.74 103, when such heat-leak is measured at a temperature difference between such Walls of +20 C. to 195 C., under a vacuum in such space of 25 microns pressure (maximum) of mercury, when the total heat flow is measured in B.t.u.s, the time in hours, the area in square feet, and the temperature difference in F., and total thickness of insulation in feet.

9. In an apparatus provided with a vacuum insulating space, a composite multi-layered, external load-free insulation in said space comprising low conductive l'ibrous sheet material layers composed of ibers for reducing heat transfer by gaseous conduction and thin, ilexible sheet radiation barrier layers, said radiation barrier layers being supportably carried in superposed relation by said brous sheet layers to provide a large number of radiation barrier layers in a limited space for reducing the transmission of radiant heat across said space Without perceptively increasing the heat transmission by solid conduction thereacross, each radiation barrier layer being disposed in contiguous relation on opposite sides with a layer of the fibrous sheet material, the bers of said brous sheet material being oriented substantially parallel to the radiation barrier layers and substantially perpendicular to the direction of heat inleak across the insulating space, said brous sheet material being composed of bers having diameters less than about 10 microns, said radiation barrier sheet having a thickness less than about 0.2 mm., and said multi-layered composite insulation being disposed in the insulation space to provide more than 12 4 radiation barrier layers per inch of said composite insulation.

10. An apparatus provided with a vacuum insulating space according to claim 9, the radiant heat barrier layers consisting of metal plated plastic sheet.

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