1、Designation: C 740 97 (Reapproved 2004)Standard Practice forEvacuated Reflective Insulation In Cryogenic Service1This standard is issued under the fixed designation C 740; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of
2、 last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers the use of thermal insulationsformed by a number of thermal radiation shields positionedperpend
3、icular to the direction of heat flow. These radiationshields consist of alternate layers of a low-emittance metal andan insulating layer combined such that metal-to-metal contactin the heat flow direction is avoided and direct heat conductionis minimized. These are commonly referred to as multilayer
4、insulations (MLI) or super insulations (SI) by the industry.1.2 The practice covers the use of these insulation construc-tions where the warm boundary temperatures are below ap-proximately 450 K.1.3 Insulations of this construction are used when apparentthermal conductivity less than 0.007 W/mK (0.0
5、49 Btuin./hft2F) at 300k are required.1.4 Insulations of this construction are used in a vacuumenvironment.1.5 This practice covers the performance considerations,typical applications, manufacturing methods, material specifi-cation, and safety considerations in the use of these insulationsin cryogen
6、ic service.1.6 The values stated in SI units are to be regarded as thestandard. The values given in parentheses are for informationonly.1.7 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to es
7、tablish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. For specific safetyhazards, see Section 8.2. Terminology2.1 Symbols:a = accommodation coefficient, dimensionlessb = exponent, dimensionlessd = distance between confining surfaces
8、, mq = heat flow per unit time, WA = unit area, m2n = number of radiation shieldss = Stefan-Boltzmann constant, 5.67 3 108W/m2K4T = temperature, K; That hot boundary, Tcat cold bound-aryE = emittance factor, dimensionless; Eeff, system effectiveemittancee = total hemispherical emittance of a surface
9、, dimension-less; ehat hot boundary, ecat cold boundaryt = distance between the hot boundary and the cold bound-ary, mk = thermal conductivity, W/mKR = shielding factor, dimensionless; equivalent to 1/ED = degradation factor, dimensionlessP = mechanical loading pressure, Pa2.2 Definitions:2.2.1 evac
10、uated reflective insulationMultilayer compositethermal insulation consisting of radiation shield materialsseparated by low thermal conductivity insulating spacer mate-rial of cellular, powdered, or fibrous nature designed to operateat low ambient pressures.2.2.2 ohms per squareThe electrical resista
11、nce of avacuum metallized coating measured on a sample in which thedimensions of the coating width and length are equal. Theohm-per-square measurement is independent of sample dimen-sions.3. Insulation Performance3.1 Theoretical Performance:3.1.1 The lowest possible heat flow is obtained in an MLIwh
12、en the sole heat transfer mode is by radiation between freefloating shields of low emittance and of infinite extent. Theheat flow between any two such shields is given by the relation:q/A 5 EsTh42sTc4! (1)3.1.1.1 (Refer to Section 2 for symbols and definitions.) Theemittance factor, E, is a property
13、 of the shield surfaces facingone another. For parallel shields, the emittance factor isdetermined from the equation:E 5 1/1/eh1 1/ec2 1! 5 ehec/eh1 1 2 eh!ec(2)1This practice is under the jurisdiction of ASTM Committee C16 on ThermalInsulation and is the direct responsibility of Subcommittee C16.21
14、 on ReflectiveInsulation.Current edition approved April 1, 2004. Published April 2004. Originallyapproved in 1973. Last previous edition approved in 1997 as C 740 97.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.1.2 When these
15、opposing surfaces have the same totalhemispherical emittance, Eq 2 reduces to:E 5 e/2 2 e! (3)3.1.2 An MLI of n shields is normally isolated in a vacuumenvironment by inner and outer container walls. When thesurface emittance of the shields and of the container wallsfacing the shields have the same
16、value, then the emittancefactor is given by:E15 e/n 1 1!2 2 e! (4)where (n + 1) is the number of successive spaces formed byboth the container walls and the shields.3.1.3 When the surface emittance of the shields has a valuee 2.6929NANA3.090.986.7310313.43102notcalculated1014-milcrinkledpolyesterA1c
17、oatedonesidenoneyes 0.5%423080.0632.6929NANA1.890.604.1310317.23102notcalculated1114-milA1foilglassfiberpaperno299770.200.161.761.020.400.760.241.631034.6431020.02231031214-milA1foilrayonfabricno3611240.231.0911.71.320.520.570.181.231034.3431020.01931031314-milA1foilglassfiberwebno218300.170.283.022
18、.260.891.830.583.931038.231020.1093103AThicknessdeterminedbycircumferentialtapemeasurement.BBasedonmeasuredheatfluxcorrectedtowarmboundarytemperatureof+80Fandcoldboundary320F.Ce=q/AsTw4DBetweenboundarytemperaturegiveninfootnoteA.ENA,notavailable.C 740 97 (2004)65.2.4.1 MLIs can be formed to a wide v
19、ariety of surfacegeometries by the individual application of the shields andspacers. First, a spacer layer is placed onto the entire surface tobe insulated. This layer would be composed of surface seg-ments, which are stitched together at the joints to form a closedand conforming spacer. Next, the s
20、hield layer is placed over theentire surface. Again, like the spacer, the shield may becomposed of surface segments, and these segments are over-lapped at the joints whenever possible. The insulation system isbuilt up to the desired number of shields with the alternateapplication of spacers and shie
21、lds.5.2.4.2 It is important that there is no mechanical pressurebuildup between layers as each successive shield-spacer layeris applied. This is often accomplished, particularly on articleshaving the major dimension of a metre or less, by fabricatingeach layer (shield-spacer combination) on its own
22、dimension-ally accurate form. The layers are then removed from the formsand assembled together onto the insulated article in theappropriate sequence.5.2.5 Filament-Wound MethodThis method of insulationis done with automatic machinery. The insulation is applied inthe form of a strip up to several inc
23、hes wide consisting of boththe shield and spacer. The machinery rotates the item to beinsulated, positions the shield strip relative to the rotating tank,and adjusts the strip tension. Its action is very similar to afilament-winding machine for glass-fiber tank manufacture.Once initiated, the windin
24、g of the shield is continued until thedesired thickness is achieved.5.3 Insulation Attachment and Support:5.3.1 Because MLIs consist of separate layers of material, amethod of securing these layers in place must be used so thatthey will not slip or shift during fabrication or use.5.3.2 Shell Contain
25、mentThe insulation is frequently heldin position by containing it between two walls, one the surfacebeing insulated and the other an outer shell. Care must be takenhere to space the walls close enough to constrain the insulationmaterial in place and not too closely to overly compress theinsulation,
26、thereby degrading the insulation effectiveness.5.3.3 Cinch BandAnother approach to attachment to largeobjects is to apply narrow cinch bands around the object at aminimum number of positions after it is insulated, therebyapplying compression to only a small portion of the insulatedsurface area. Care
27、 must be taken to avoid internal metal-to-metal contact within the insulation system. Allowance must bemade to account for the local reduction in insulation perfor-mance caused by the application of the bands as well as anypossible effect they may have on the allowable evacuation rate.5.3.4 Penetrat
28、ion MembersLayers of MLI can be pinnedto the wall of the item being insulated or they can be stitchedor quilted together into blankets which can then be attached tothe item to be insulated.Again, allowance for the effect of thesepins or stitches must be made on the thermal performance ofthe insulati
29、on.5.3.5 ShinglesApplication of the material in the form ofshingles where one end of each piece of the material is attacheddirectly to the tank wall with adhesives and overlapping anadjacent shingle, is especially attractive where rapid venting ofgas between layers is desirable, such as on earth lau
30、nchedspace vehicles. In this method, the insulation effectiveness isgoverned by the length of the shingle since the lateral conduc-tion along the shields will now be added to that of the basicperformance of the multilayer configuration.5.4 Joints:5.4.1 The method of preparing joints between any twos
31、egments of MLI is critical to the thermal performance of thesystem. Continuity of layers shall be maintained to ensure thatmetal-to-metal contact is avoided and there shall be nosignificant permanent gaps or openings in the MLI at the jointlocations. Any relative motion between the two componentspro
32、duced either by the thermal or the mechanical environ-ments, or both, shall be taken into account during fabrication.Introduce features to prevent gaps and openings from devel-oping.5.4.2 Gaps can be avoided by generously overlapping theshields at the joint locations. If butt-joining of shield canno
33、t beavoided, then the shields of each component must be restrainedto prevent the gap from increasing. Alternatively, a strip ofshield material can be placed over the butt joint, overlappingthe shields at the joining locations.5.5 Penetrations:5.5.1 In any practical system, the penetration of the MLI
34、with pipes, supports, and wiring cannot be avoided. Thesepenetrations produce unacceptable thermal shorts unless theyare insulated and unless this insulation is properly integratedwith the main surface insulations and direct metal-to-metalcontact avoided.5.5.2 Because of the small thicknesses associ
35、ated with MLI,it is necessary to increase the effective length of the penetrationbetween the cold and warm boundary temperatures. MLI isplaced around the penetration and extends from the mainsurface outward along the penetration several diameters (theexact length to be established by the user).5.5.3
36、 Because MLIs are anisotropic, the best possible ther-mal isolation of the penetration at the joint is obtained byinterleaving the shields of the penetration MLI with the mainsurface MLI. This is accomplished by cutting gores in theshields of one of the components at the joint and overlappingthe gor
37、e segments with the shields of the second component.Alternatively, a preformed corner shield can be placed at thecorner locations in a manner that they overlap the shields ineach component.5.5.4 Alternatively, the corner formed by the two compo-nents can be filled with a preformed isotropic insulati
38、ngmaterial such as plastic foam, glass wool, and encapsulatedpowders.5.6 Evacuation RatesEvacuation of multilayer reflectiveinsulations, whether by vacuum pump or by ascent through theatmosphere (for example, on space vehicles), must occurwithout damage to the insulation. During evacuation, a gaspre
39、ssure gradient will exist within the insulation. The user musteither control the evacuation rate such that the pressuregradient does not damage or blow off the insulation, or if thiscannot be accomplished, then the rate at which the enclosedgas (air or a purge gas) can escape from between the shield
40、smust be enhanced. This is usually done by perforating theshields to provide broadside flow in addition to that via theC 740 97 (2004)7edges. However, the effect of these perforations on the overallthermal efficiency must be taken into account.6. Cleanliness6.1 It is essential that the materials use
41、d be clean, and thatthe wrapping area be clean. Dust, organic materials, etc., cancause significant outgassing, and certain foreign materials cancorrode reflective surfaces and thereby increase the emittance,that is, reduce the reflectance. Particularly, fingerprints shouldbe avoided, because body a
42、cid can cause corrosion of foil, andcan even cause the reflective coating of plastics to disappear intime.6.2 If sorbers or chemical getters are used, it may benecessary to protect these from contamination prior to pump-down. In some cases, insulation rooms should not only beclean, but also dry.6.3
43、It is recommended that wrapping always be done in aclean room, and that materials be protected by clean paperwrapping when not actually being applied.6.4 It is recommended that clean clothing and gloves beworn by any person actually handling the insulation materials.7. Materials Specifications7.1 Mu
44、ltilayer reflective insulation systems always havemultiple sheets of reflector material, each separated by lowconductance separator material. Considering reflector sheetsfirst, these depend on the low emissivity characteristic of cleansmooth metal surfaces. The metal can be a sheet of foil, or itcan
45、 be a coating of some appropriate nonmetal. The two mostcommonly used materials are (1) thin aluminum foil, and (2)vapor deposited aluminum on polyester film.7.2 Foils:7.2.1 Since the materials used must be thin and highlyreflective, the foils are usually high-purity metals having highthermal conduc
46、tivity. Such metals as gold, silver, and alumi-num could be used, but the choice is obviously aluminumbecause of cost. The most commonly used aluminum foil is1145-0. This material has a 99.45 % purity, is soft, and can beobtained in thin highly reflective sheets. Other alloys ofaluminum, or even oth
47、er metals, are acceptable if highlyreflective throughout the range of temperatures expected. Oneside, at least, should reflect 95 % or more of the thermalradiation incident on it, that is, the emittance at all temperaturesof interest should be 0.05 or less. The other side should notdiffer greatly. A
48、 foil with sufficient reflectance will appearbright and shiny on both sides, although perhaps only one ofthe sides will be mirror-like (normal bright finish), the otherhaving a semi-matte finish. Noticeably dull or tarnished sur-faces are cause for rejection. Contamination, such as oil orpigment coa
49、tings, or even numerous fingerprints, is also causefor rejection.7.2.2 Mechanically, there are several requirements. The foilused should be thin enough to restrict lateral heat conduction,and to give flexibility for easy folding without stiffness. Thelatter property is usually assured for thin enough foils becauseonly soft metal can be cheaply rolled into very thin sheets. Asa general rule, aluminum foil should not be more than 12.5 m(0.0005 in.) thick, although heavier weights have been used. Athickness of 7.5 m (0.0003 in.) or slightly
