US3009600A - Thermal insulation - Google Patents

Description

NOV- 21, 1961 L.. c. MATscH 3,009,600

THERMAL INSULATION Filed Jan. 25, 1960 4 7?? Concentrlc 5 Composite j Laye.' Insulation |nsula'f'0n gia/@y giiiff' H mi'. 4*

Spiral Layer Insulation F ibrous Se ing Materia rarat INV EN TOR.

Path of Heat Transfer Arron Ei( LADISLAS C MATSCH United States Patent O 3,009,600 'I'HERMAL INSULATION Ladislas C. Matsch, Kenmore, N.Y., assignor to Union Carbide Corporation, a corporation of New York Filed Jan. 25, 1960, Ser. No. 4,298 2 Claims. (Cl. 220-9) This invention relates to an improved insulation having a high resistance to all modes of heat transfer and particularly concerns a low temperature, heat insulating material adapted to improve a Vacuum insulating system.

In the conservation and conveying of low temperature commercial products-for example, perishable commodities Which must I'be held at low temperatures for substantial periods of time and valuable volatile materials, such as liquied gases having boiling points at atmospheric pressure below 233 K., for example, liquid oxygen or nitrogen-a 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. In 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 liqueed gases are: for small containers, -the Dewar type high vacuum-polished metal surfaces system, and for large containers, the powder-in-vacuum 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-invacuum heat insulation is highly effective in reducing thermal heat loss in many systems, 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 temperature products, there, nevertheless, exists a great commercial need for more ecient insulating materials capable of meeting more rigid and exacting requirements, and which will provide even lower thermal conductivities than those afforded by either of the above-described insulations.

To give some insight into the problems that are presented in effecting further reductions in heat leak for small portable containers, assume 4for example that it is desired to insulate a double walled cylindrical container for low boiling liquefied gases such as oxygen, sothat 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 vessel diameter of 8 inches and an inner vessel total length 'of 48 inches. Using one of the best insulating materials of the prior art, for example, powder-in-vacuum 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 9.2)(10-4 B.t.u./(ihr.), (ft), ("F.) may be achieved. p In order to more fully appreciate the signiiicance of such a thermal conductivity, the insulating effects of the following insulation thicknesses are set forth. An insulation thickness of 1.66 inchesof a powder-in-vacuum 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 vessel. In other words, beyond the thickness of 1.66V inches, the bulk of the insulation which must be stored and/or transported becomes greater than ,the bulk of the contained stored material.

3,000,000 Patented Nov. 21, 1961 ICC Increasing the thickness of such an insulation to 4 inches reduces the loss rate to 3.6% per day, while simultaneously increasing the insulation volume to about 3 times that of the storage capacity of the inner vessel. It has been found that it is entirely impractical to consider insulating the vessel with a material having a conductivity as high as 9.2 l04 B.t.u./(hr.), (ft), (F.),

since calculations show the theoretical required insula tion thickness to be 105 inches.

Considering a straight vacuum insulating system in which the walls of the inner vessel and outer casing forming the insulation space are polished in order to reflect radiant heat energy, there is a problem of maintaining a suiiciently low vacuum to eliminate heat conduction by residual gas. For this purpose the absolute pressure within the insulating space must be maintained at a value l0 to 100 times lower than when a powder-invacuum insulating system is used. The vacuum should be less than 0.01 micron of mercury absolute pressure and preferably should be on the order of 0.001 micron mercury. This may be obtainable in special laboratory equipment, but it is an impractical specification for fabricated metal Vessels intended for industrial service. Assuming 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 reiiectivity for the vessel walls. To obtain a maximum loss rate of 1% per day, surface reflectivities of at least 99.6% must be obtained. Reflectivities of this order are only obtainable, if at all, under strictly controlled laboratory conditions, which may not be duplicated or maintained either during fabrication of the container, or after the container isV in service. f

A llower quality reiiective 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 limiting difficulties involved in such an arrangement is in assembling and supporting many reective 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 to contact in even a minute area, the insulating effect of one shield will be essentially eliminated. Moreover, the number of shields required depends on their surface reflectivity. If a very highly polished surface is provided for both vessel walls and the shields, the polished surface having a reflectivity of then at least 10 such shields must be used in order to achieve a 1% per day storage loss rate in the desecribed 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 ll spaces, and taking the thickness of the shields into consideration, would account for an overall thickness of at least 3 inches. Under these circumstances the fabrication of vessels having a storage loss rate of less than 1% per day would be costly and time-consuming.

In view of these obstacles, it has heretofore been i-mpossible to approach, rnnch -less achieve, heat leaks of such small quantities for systems involving extended periods of storage of low boiling liquefied gases in portable storage containers.

It is, therefore, an important object of the present invention to provide a greatly improved insulation system for reducing heat transmission by all modes of heat trans' fer 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 3 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, a multiplicity of 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 reecting barriers so constructed and arranged as to irnpede the passage of radiant heat through said insulation without noticeably increasing the solid conductance 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, a multiplicity of radiation barriers comprising spaced and parallel foils of heat reective material for reducing the transfer of heat by radiation, and a spacing material between said radiation barriers, comprising a low-conductive, heat insulating material for reducing the transfer of heat by conduction between said barriers.

A still further object of the invention is to provide a vacuum, multi-layer composite insulation system which is superior to heretofore proposed vacuum insulating systems in impeding heat transfer without requiring the extremely high vacuums associated with straight vacuum systems.

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 insulation material for reducing the transfer of heat by conduction, and incorporates therein a multiplicity of (radiant heat) 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 4novel 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. Y

In the drawings:

FIG. 1 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 section-al view taken along line 4-4 of FIG. 1 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 restrict 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.

It has been discovered 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 within the insulating structure, and can also be employed as a novel means for elastically supporting the insulated inner container.

More specifically, it has been discovered that the transmission of heat across a solid-in-vacuurn 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 a multiplicity of radiation impervious shields to substantially eliminate heat leak by radiation.

Furthermore, it has been 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 30 microns of mercury, and preferably below 5 microns of mercury. For superior quality results, the pressure should preferably be below l micron of mercury.

According to the invention, a vacuum insulated space is provided with a low heat conductive material having incorporated therein a multiplicity of radiation barriers disposed substantially transversely to the direction of heat ow in spaced relation to each other. The radiation barriers or shields of the invention may comprise one or more sheets 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 reect 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. 1 is a double-walled heat insulating container having parallel inner vessel and outer casing 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. The insulation appears as a series of spaced reflectors 14 disposed substantially transversely to direction of heat ow and supportably carried by the 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 or lmetal coated material, such as aluminum coated plastic lm, or other radiation reective 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. Forv high-quality insulations, the foil should be as thin as practical consistent with strength requirements. Thinness is benecial because it facilitates folding and forming the insulation to t the contour of the insulation space. lt also minimizes the weight of the container. In cryogenic vessels, low density is additionally important becauseit reduces the time and the quantity of expensive refrigera- `tion needed to cool down the inner vessel and establish a stable temperature gradient through the insulation. Foil thicknesses between 0.2 mm. and 0.002 mm. are suitable, and when aluminum foil is employed, thickness between 0.02 mm. and 0.005 mm. are preferred.

A preferred reective shield is Mt 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 are tin, silver, gold, copper, cadmium or other metals. The omissivity of the reective 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 94.2% 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 .005 may be obtained. The above ranges represent an optimum balance between the high performance and high cost of low omissivity materials.

'I'he base or separating material of the invention is a Ilow heat conductive material such as ber insulation which is provided as very thin sheets in a precompacted state. Such sheets are known as papers or mats to those skilled in the art and are commonly prepared by uniformly depositing nely-spun bers at a desired rate on a moving belt and subsequently compressing the mat, as for example, between com-pression rolls or by vacuum. The bers may be deposited in either wet or dry state, but if deposited dry, they are usually wetted with water before compression. Without the wetting and compression steps, the resulting sheet would be a relatively thick fluffy web, but by precompaction the thickness is permanently reduced to less than one-half that of a web of otherwise equal composition. v

Since the alternative web-type materials disclosed in copending application Serial No. 824,690 filed July 2, 1959 in the name of L. C. Matsch, are relatively thick due to their fluffy state, they must be compressed rather severely in the insulation space in order to provide numbers of shields on the order of 40 to 250 per inch. Severe compression of webs withn the insulation space has been found detrimental to the overall effectiveness of the insulation, so that high performance cannot be achieved with such materials. While the addition of shields reduces heat transmission by radiation, the compression of the web materials increases heat transmission by solid conduction to a greater degree.

It has been discovered unexpectedly that thin precompacted papers or mats are highly beneficial in the construction of a high quality thermal insulation employing large numbers of shields per unit thickness. Insulations ofhighest effectiveness containing 40 to 250 shields per inch thickness are readily provided with such materials. Oddly enough, permanent precompaction of the bers does not produce the detrimental increase in solid conduction observed when fluffy bers are elastically compressed during or after assembly of lthe composite webshield insulation to the same thickness as acomparable paper-shield insulation.

The physical properties of the paper materials must be closely controlled to obtain the highly efficient composite insulating material of the present invention. The bers must be extremely ne and must be so matted together as to provide a reasonably strong sheet without reliance upon bonding materials. Bonding agents cannot be tolerated in the insulation of the present invention because of the resulting excessive solid conduction. Resin bonding is frequently employed in the manufacture of fibrous materials, and while such procedure might be used as an intermediate step in producing effective heat insulating papers, the resins must subsequently be removed completely by such means as heat or a solvent.

Suitable fibers -for the practice of this invention include clean glass laments having diameters less than 5 microns, while a ber diameter range of 0.05 to 1.0 micron is preferred. The above range represents a preferred balance between increasing frailness and cost of relatively small diameter bers, and increased conductance and gas pressure sensitivity of relatively large diameter bers, as will be discussed later in detail. Furthermore, the low conductive separating material of this invention preferably comprises bers which are substantially randomly disposed within the plane in the installed condition, and the individual bers `are also preferably oriented in a direction substantially perpendicular to the flow of heat. It will be understood that as a practical matter, the bers will not be individually confined to a single plane,

. but rather, in a finite thickness of fibrous material, the

bers will be generally disposed in thin parallel strata with, of course, some indiscriminate cross weaving of fibers between adjacent strata. Fibers having diameters in the range of 0.2 to 0.5 microns such as those commercially designed as 104 or AAAA ber, and bers designated as 106 or AAA ber having diameters in the range of 0.5 to 0.75 microns are normally available as papers, and are suitable for practicing this invention.

A characteristic of the paper materials of this invention is that they are composed of relatively short bers. While the bers composing fluffy webs may be on the order of 11/2 inches long, those of the preferred paper materials are less than about 1A. inch in length. These bers are deposited in amount suicient to produce sheets weighing no more than 8 gms. per sq. ft. and preferably less than 3 gms. per sq. ft. |It is believed that the ability of the present glass papers to remain permanently compacted even a-fter the removal of the compressive load applied during manufacture is at least partly due to the relatively small diameter and short length of the bers. contrast to the larger and longer bers employed in the webs, which bers are more spring-like andv produce a resilient mass.

The reective shield separating layer must be glowV conductive in the sense that it presents a high resistance to Ithe flow of heat through the solid material of whichl it is composed. While we do not wish to be bound by any particular theory, it is believed the principal reasons for the far superior insul-ating effects achieved by the previously described ber orientation are the/relatively few bers traversing the thickness of the insulating layer and the very large number of point contacts established between crossing bers. These point contacts represent the points of bearing between adjacent bers in the direc.

tion of heat ow, and as such, constitute an extremely high resistance to the flow of heat by conduction. In a given thickness of low conductive material, it is clear that4 more point contact resistances will be present in very ne bers than in coarse bers. Alternatively, for a given number of point contact resistances, ne bers will permit a thinner separating layer than will coarse bers. This is one important reason why papers (mats) composed of extremely ne bers are preferred in :this invention.

The papers may be made very thin, for example, on the, order of 2 mils thick, and still provide ample point con-v tact resistances for low solid conductance This is in.

Another reason for using extremely line bers is to reduce gaseous conduction through the insulation and to obtain an insulation which is relatively insensitive to moderate changes in residual gas pressure. The larger the particle size (eg, liber diameter) of the low conductive material, the larger will be the voids between the particles and the greater will be the heat transfer by solid conductance, Heat is transferred across the voids by molecules of the residual gas in the insulation space. However, the path of greatest resistance to heat flow is through the individual particles and across the point contacts between the particles. Gas conduction across the voids may, therefore, be viewed as a short circuit around the principal resistance. The rate of heat transfer by gaseous conduction is dependent upon the number of molecules present and upon the mean-free-path of molecular motion. Reducing the absolute pressure reduces the number of molecules present to transfer heat, and for this reason, a good vacuum is important. However, reducing the absolute pressure will increase the mean-free-path of the molecules and tend to increase gaseous conduction. If the voids are large so that their average dimension is comparable to or exceed the mean-free-molecular path, then the adverse effect of increasing the mean-free-path essentially cancels out the beneficial effect of fewer molecules. For this reason, reducing the absolute pressure will not reduce gaseous conduction until the mean-freepath has lengthened to the point that molecular motion is restricted by the dimensions of the void spaces. This is why extremely low absolute pressures (e.g., l-6 mm. Hg) are required in straight vacuum systems or in coarse particle fillers where the dimensions across the void spaces are relatively long. In such systems, a slight increase in absolute pressure not only increases the number of molecules present but also reduces the mean-free-path so that the voids no longer restrict molecular motion. The gas then attains its maximum heat carrying capacity, and the `full elfect of the short circuit by gaseous conduction develops rapidly. In commercial vessels constructed of metal and subject to rough treatment, it is usually impractical to maintain extremely low absolute pressures such as 106 mm. Hg in the insulation at all times, A paper material composed of very fine fibers between the shields relaxes the vacuum requirement for the insulation and results in a dependable high-quality insulation system. Accordingly, yfor the aforementioned reasons it has been found that ber diameters below microns provide far superior quality insulation than tibers with larger diameters and are required to practice this invention.

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

Referring to FIG. 4, radiant heat striking the rst sheet of aluminum foil will for the most part be reflected, and the remaining par-t absorbed. Part of this absorbed nadiation 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 fiber webs in what might be considered an irregular path, crossing relatively small areas of point contact between crossing bers until it reaches the second sheet of aluminum foil, where the heat reecting and absorbing process described above is repeated. Because of the particular orientation of the individual bers in the paper, the path of solid conduction from the first sheet of aluminum foil to the second is greatly lengthened, and encompasses an 'indefinitely large number of point con# tact resistances between contacting fibers. By analogy it will be seen that a multilayer insulation having a series 8 of lalternately radiation reliecting sheets and permanently precompacted paper layers of low conductive insulating material may be particularly efficient in preventing or diminishing yheat losses by radiation as well as by conf duction. i

A common characteristic of low conductive, particulate insulating materials including papers and webs is that they are compression-sensitive, which means that varying the compression on the insulation will change the thermal conductivity. Fine fibrous insulations with rehective shields are particularly sensitive to compression. Slight compression increases the number of reflective shields per unit thickness and thus tends to reduce the overall heat transmission by decreasing radiation. However, continued increase in compression compacts more and more solid heat conductive material in a xed volume and the ad: verse effect of increased solid conduction soon overrides the decrease in radiation to nroduoo a not rise in heat transmission.

Fiber materials in either paper or web form exhibit the above described compressionsensitivity, but when conn` bined with reflective shields it has been unexpectedly discovered that the overall thermal conductivity is strik.- ingly different for the two types of materials. A uffy web, if compressed suiciently'to provide a large number of shields per inch, will result in 'an insulation of mediocre quality, while a paper otherwise identical to the web and installed with the same number of shields per inch will require much less compresion and accordingly pro-` vide Aan excellent insulation, On the other hand, if the previously discussed web and paper insulations are both subjected to the same compressive load, the number of shields per inch in the paperftype insulation will be much larger than inthe web-'type insulation, and radiative heat transfer through the former will be relatively small. This difference in thermal conductivity exists even if the same quantity of identical fiber is provided between the shields for the` two types of insulation. While it seems that the controlling factor would be the total amount of solid heat conductive material present between the shields, it now appears equally or more important to provide this amount of material with minimum ofanniession on the composite insulation. Increasing the sheet density of fibrous mate` ria-l to achieve thinness is highly beneficial if aQGOIIlPlished by permanent precompaction,4 but high-ly detrimental if accomplished by compression ofthe installed insulation.-

'Ilhe above discovery is illustrated by the following comparative tests conducted with sheets of unbonded glass fibers less than. 0.5 micron in diameter The sheets each weighed approximately 1. 6 grams per sq. ft., but one was I a permanently precompacted paper andthe other a tlutfy web. An insulation consisting of 60 shields per inch separated by the paper material required less than .01, lbpor Sq, in. compression and provided a thermal Conductivity of ,043.Xl0-3 B-Lu/(HLN-N" E). By comparison, insulation consisting of 60 identical shields por inch Separated. hy the web material required. 0.16 1b. por Sq. in. compression and exhibited, a. conductivity of 0.199 1O3 B.t,u. /(hr.i)v(fit.) 13.). Thus, the paper provides a 4.6- fold improvement in overall insulating effectiveness.

In the above controlled tests, gaseous conductance was eliminated by applying an extremely good vacuum. Radiation wlas found to be essentially the same for both forms of insulation. Therefore, the difference in performance is due to solid conductance which was determined as 0,02,8 l0*.3 B.t.u./(hr.)(ft.)(F.) and 0.18.4X10r3 B.t.u./ (hr.) (fh) F.) for the paper and web composites respectively. It is seen that a 6.6-fold improvement in solid conductanc@ iS only achieved with the paper material.

In compressible web-type materials, tiberdensity is a simple function of the compressive force, and the performance of insulations prepared with such material can be expressed generally in terms of the installed ibl density (pf), the emissivity of the shields (e), `and the number of shields per unit thickness (N). However, the ber density of paper materials is related to compression in an entirely different fashion within the range of compression loading normally employed in practice in that a much .smaller compressive load is needed to achieve a given number of layers per unit thickness, as previously illustrated.

Many tests have |been conducted on insulations composed of alternate layers of radiation shields and ber sheet materials. The results obtained with glass or glasslike materials in both paper and web form are found to conform reasonably well with the following empirical equation for solid conductance:

where ksc=solid conductance, B.t.\u./ (hr.) (ft.) F.)

k1=therrnal conductivity of the solid, base material of which the bers are composed, B.t.u./ (hr.) (ft.)( F.)

Df=average ber diameter, microns a=fraction of the ber sheet volume which is occupied by solid material when under compression p.

p=compression exerted on insulation, lb./sq. in.

The value of a may be determined yas the ratio pf/ p1 where pf is the density of the bulk fiber sheet under compression p, and p1 is Ithe density of the solid base material of which the bers arecomposed. The factor (ap) is a measure of the compressibility of the insulation and can be determined experimentally without difficulty for a given composite material. The constant -4 in Equation l is proper only for glass land glass-like materials and in general will be greater for materials having a lower modulus of elasticity or hardness.

Heat transmission by radiation may be expressed by the following equation:

im: 16j-VX 10-3 (2) where KR=radiative heat conductance B.t.u./ (hr.) (ft.) F.) e=eective emissivity ofthe shields N=number of shields per inch thickness If gaseous conduction is negligible by virtue of a good vaccum, the total apparent thermal conductivity of the insulation will be the sum of Equations 1 and 2 above. A graph of the summation equation exhibit a minimum total k-Vahie corresponding to a preferred compression (p) and number of shields (N). The factor Df in Equation 1 is limited by current manufacturing technology to a of about 0.1 micron ber diameter; similarly a is limited to a of about 0.0'1 (or a void fraction of 99%). Analysis of Equation 1 shows that an ultra-high quality insulation is achieved with brous glass materials only if the compressive load is less than` about 0.03 psi.

While-glass and especially borosilicate glass is the preferred ber material, other silicaceous materials including ceramics and quartz are also suitable. The material should preferably be selected for low values of k1 and for high values of hardness and modulus of elasticity E.

From the foregoing discussion, it is seen that the ability to install many layers per unit thick-ness without appreciable compression is an important requirement for achieving very rhighquality insulations. Permanently precompacted [papers are ideally suited for insulations requiring many shields per inch while lower-cost webs are best suited for less exacting insulation requirements where a relatively'small number of shields are adequate. In this respect, the striking distinction between papers and webs may be clearly shown by assuming an illustra-tive insulation consisting of 40 shields per inch. Papers readily provide this number of shields with less than 0.01

10 p.s.i. compression, while webs require more than 0.1 p.s.i. compression. Since it has previously been shown that ultra-high quali-ty insulation lcannot be achieved under a compressive load above 0.03 p.s.i., it follows that such insulation cannot be obtained with elastically compresv sible webs.

As may be concluded from the previous discussion an important advantage of the insulation of this invention is' the very 4low coeicients of heat transmission which may be obtained. For example, using an insulation consisting for 54 alternate layers per inch or aluminum foil having an effective emissivity of 0.05 8 Iand a 1.5 gm/sq. ft. paper of oriented, unbonded type AAAA glass ber a thermal conductivity coeicient of 0.025 X103 B.t.u./(hr.)(ft.) F.) has been obtained. In order to further demonstrate the effectiveness of this insulation Table I compares its thermal conductivity with that of the prior art insulations.

Table I Absolute Thermal Pressure Conductiv- Type of Insulation in Vacuum ity B.t.u

Space (hr.) (it Microns F.) Mercury Powder-in-vacuum insulating systems in accordance with U.S. Patent 2,396,459.-. 0.1 9. 2X10-4 High vacuum-polished metal surface system with radiation shields in accordance With U.S. Patent 2,643,022 0.01 1.9X10l Alternate layer insulation consisting of 2.5 to 3.8 micron diameter ber web having a density of 4.7 gms/sq. ft. alternating with 94.2% reflective aluminum foil compressed to a ber density of 1.6 lbs/cu. ft. in accordance with copending applicacation Ser. No. 824,690 0. 1 1.18)(10-4 Insulation illustration of this invention: AAAA ber paper 1.5lgm./ft.2; 94.2% reflective aluminum foil; 54 layers of each per inch 0. 1 0. 25)(10-4 It is thus seen that the quality of the present insulation is about forty times that of the powder-in-vacuum type. Compared with the high vacuum-polished surface type this invention reduces the conductivity by a factor of at least 7 land simultaneously permits use of a practical vacnum. By using foils of higher reectivity and papers of lighter weight, still lower coefficients of thermal conductivity are obtainable.

Another advantage of the paper-type alternate 'layer insulation is its low weight per unit heat flow resistance. This is Ianimportant characteristic for two reasons; first, it achieves tare weight in portable and llyable containers (eg. aboard missiles), and thus facilitates handling `and reduces transportation costs; second, by minimizing the insulation weight one also reduces the amount of expensive refrigeration needed for cooling the inner vessel to operating temperature and for establishing a stable temperature gradient through the insulation thickness.

Another of the many important advantages in the thermal insulation of the present invention is that the ilexibility of the layers of thin reflective shields and permanently precompacted paper 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 composite material of the invention is adapted to be applied to contoured surfaces, in addition to being particularly well suited for insulating either flat or cylindrical surfaces.

Obviously the multiple shield insulation of the invention maybe mounted in the insulation space in any one of a variety of ways. For example, in FIG. 5, the insulation 12 may be mounted concentrically with respect to the inner container 10a, or it may be, as in FIG. 4, spirally wrapped around the inner vessel with one end of the insulation wrapping in contact with the inner vessel 10a, and the other end nearest the outer casing 10b or 11 in actual contact therewith, the latter form of mounting being preferred and illustrated herein. Referring to FIG. 4, the metal foil may be loosely spirally wrapped around the inner vessel a, the tightness and number of turns being selected preferably to obtain best performance as discussed above.

It will be recognized that because of the difliculty 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 over these irregular contours 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 flat 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 a thermal insulation such as disclosed in the co-pending application to LC. Matsch et al., Serial No. 580,897, filed April 26, 1956, or any other suitably low conductive material.

Coupled with the composite insulation 12, the supplemental insulation 16 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.

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 vessel is maintained in substantially centered position, and the need for lateral braces or other centering devices is obviated, thus further reducing the heat leak into the container. It is to be understood, however, that the present invention does not provide support for the weight of the inner vessel, nor support for the walls of' the vacuum space against external loads and, hence, is external load-free and that specific means for such support must be provided.

From the above description it will, therefore, be seen that the present invention provides in a solid-in-vacuum type insulation, a permanently precompacted low heat conductive material having incorporated therein multiple radiation shields for impeding radiative heat transmission through the insulation, while minimizing the flow of heat by conductionV 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 fibrous structure oriented in a direction perpendicular to the direction of heat flow. Possessed of a relatively small percentage of solid material per unit volume, 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 low conductive, permanently precompacted paper-type insulating material,

12 have been found to be superior to any known insulating system.

It will be understood that variations and modifications may be effected without departing from the novel concepts of the present invention.

This is a continuation-in-part application of my application Serial No. 597,947, filed July 16, 1956.

What is claimed is:

. l. In an apparatus provided with a vacuum insulating space, a composite multi-layered, external load-free insulation in said space comprising low conductive fibrous sheet material layers composed of fibers for reducing heat transfer by gaseous conduction and thin, flexible radiant heat reflecting shields, said radiant heat reflecting shields being supportably carried in superposed relation by said fibrous sheet layers to provide a large number of radiant heat reflecting shields 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 radiant heat reflecting shield being disposed in contiguous relation on opposite sides with a layer of the fibrous sheet material, the fibers of said fibrous sheet material being oriented substantially parallel to the heat reflecting shields and substantially perpendicular to the direction of heat inleak across the insulating space, said fibrous sheet material being a permanently precompacted paper composed of unbonded fibers having diameters less than 5 microns and a length of less than about 0.5 inch, said radiant heat reflecting shields having a thickness less than about 0.2 mm., and said multi-layered composite insulation being generally spirally wound in the insulation space to provide more than 40 radiant heat reflecting shields per inch of said composite insulation.

2. In an apparatus provided with a vacuum insulating space, a composite multi-layered external load-free insulation in said space comprising low conductive fibrous sheet material layers composed of fibers for reducing heat transfer by gaseous conduction and thin, flexible radiant heat reflecting shields, said radiant heat reflecting shields being supportably carried in superposed relation by said fibrous sheet layers to provide a large number of radiant heat reflecting shields 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 radiant heat reflecting shield disposed in contiguous relation on opposite sides with a layer of the fibrous sheet material, the fibers of said fibrous sheet material being oriented substantially parallel to the heat reflecting shields and substantially perpendicular to the direction of heat inleak across the insulating space, said fibrous sheet material being a permanently precompacted paper composed of unbonded fibers having diameters less than about 5 microns and a length of less than about 0.5 inch so as to provide low gaseous conductance, the absolute pressure in said insulating space being below 30 microns of mercury, said radiant heat reflecting shields having a thickness less than about 0.2 mm., and said multi-layered composite insulation being generally spirally wound in the insulation space to provide more than 4() radiant heat reflecting shields per inch of said composite insulation.

References Cited in the file of this patent UNITED STATES PATENTS 2,776,776 Strong et al Ian'. 8, 1957 FOREIGN PATENTS 143,219 Great Britain Dec. 9, 1920 683,855 Great Britain Dec. 3, 1952 715,174 Great Britain Sept. 8. 1'954 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N6, 3,009,600 November 21, 1961 Ladislas C, Matseh E It is herebycertfed that error appears in the above numbered patent requiring correction and that the said Letters Patent shouldread as corrected below.

Column 5, line 57,- for "Withn" read within =5 column 6, line 34, for "designed" read designated signed and s661661 this 1st dafy of May 1962a SEAL) Attest:

DAVID L. LADD ERNEST W, SWIDER Commissioner 0f Patents Attesting Officer

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