1、Designation: C740/C740M 97 (Reapproved 2009)Standard Practice forEvacuated Reflective Insulation In Cryogenic Service1This standard is issued under the fixed designation C740/C740M; the number immediately following the designation indicates the yearof original adoption or, in the case of revision, t
2、he year of last revision. A number in parentheses indicates the year of last reapproval.A superscript epsilon () 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 position
3、edperpendicular 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 m
4、ultilayerinsulations (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
5、W/mK 0.049 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
6、cryogenic service.1.6 The values stated in either SI units or inch-pound unitsare to be regarded separately as standard. The values stated ineach system may not be exact equivalents; therefore, eachsystem shall be used independently of the other. Combiningvalues from the two systems may result in no
7、n-conformancewith the standard.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 establish appro-priate safety and health practices and determine the applica-bility of regulatory limitatio
8、ns prior to use. For specific safetyhazards, see Section 8.2. Terminology2.1 Definitions of Terms Specific to This Standard:2.1.1 evacuated reflective insulationMultilayer compositethermal insulation consisting of radiation shield materialsseparated by low thermal conductivity insulating spacer mate
9、-rial of cellular, powdered, or fibrous nature designed to operateat low ambient pressures.2.1.2 ohms per squareThe electrical resistance 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
10、of sample dimen-sions.2.2 Symbols:a = accommodation coefficient, dimensionlessb = exponent, dimensionlessd = distance between confining surfaces, 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
11、 boundary, Tcat cold bound-aryE = emittance factor, dimensionless; Eeff, system effectiveemittancee = total hemispherical emittance of a surface, dimension-less; ehat hot boundary, ecat cold boundaryt = distance between the hot boundary and the cold bound-ary, mk = thermal conductivity, W/mKR = shie
12、lding factor, dimensionless; equivalent to 1/ED = degradation factor, dimensionlessP = mechanical loading pressure, Pa3. Insulation Performance3.1 Theoretical Performance:3.1.1 The lowest possible heat flow is obtained in an MLIwhen the sole heat transfer mode is by radiation between freefloating sh
13、ields 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 of the shield surfaces facingone another. For parallel shields, the e
14、mittance factor isdetermined from the equation:1This practice is under the jurisdiction of ASTM Committee C16 on ThermalInsulation and is the direct responsibility of Subcommittee C16.21 on ReflectiveInsulation.Current edition approved Nov. 1, 2009. Published December 2009. Originallyapproved in 197
15、3. Last previous edition approved in 2004 as C740 97(2004). DOI:10.1520/C0740-97R09.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.E 5 1/1/eh1 1/ec2 1! 5 ehec/eh1 1 2 eh!ec(2)3.1.1.2 When these opposing surfaces have the same totalh
16、emispherical 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 value, then the emittancefactor is giv
17、en 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 2.69 29 NA NA 3.09 0.98 6.7 3 10313.4 3 102not calculated1014-mil crinkled polyesterA1 coated one sidenone yes0.
18、5%42 308 0.063 2.69 29 NA NA 1.89 0.60 4.1 3 10317.2 3 102not calculated1114-mil A1 foil glass fiber paper no 29 977 0.20 0.16 1.76 1.02 0.40 0.76 0.24 1.6 3 1034.64 3 1020.022 3 1031214-mil A1 foil rayon fabric no 36 1124 0.23 1.09 11.7 1.32 0.52 0.57 0.18 1.2 3 1034.34 3 1020.019 3 1031314-mil A1
19、foil glass fiber web no 21 830 0.17 0.28 3.02 2.26 0.89 1.83 0.58 3.9 3 1038.2 3 1020.109 3 103AThickness determined by circumferential tape measurement.BBased on measured heat flux corrected to warm boundary temperature of + 80F and cold boundary 320F.C =q/AsTw4DBetween boundary temperature given i
20、n footnote A.ENA, not available.C740/C740M97(2009)65.2.4.1 MLIs can be formed to a wide variety 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, whi
21、ch are stitched together at the joints to form a closedand conforming spacer. Next, the shield 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 isbu
22、ilt up to the desired number of shields with the alternateapplication of spacers and shields.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 dimen
23、sion of a metre or less, by fabricatingeach layer (shield-spacer combination) on its own 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 wi
24、th automatic machinery. The insulation is applied inthe form of a strip up to several inches 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
25、 to afilament-winding machine for glass-fiber tank manufacture.Once initiated, the winding 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
26、 be used so thatthey will not slip or shift during fabrication or use.5.3.2 Shell ContainmentThe 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 constra
27、in the insulationmaterial in place and not too closely to overly compress theinsulation, 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 insulate
28、d, therebyapplying compression to only a small portion of the insulatedsurface area. Care 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
29、 well as anypossible effect they may have on the allowable evacuation rate.5.3.4 Penetration 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 t
30、he effect of thesepins or stitches must be made on the thermal performance ofthe insulation.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 especial
31、ly attractive where rapid venting ofgas between layers is desirable, such as on earth launchedspace 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
32、multilayer configuration.5.4 Joints:5.4.1 The method of preparing joints between any twosegments 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 ope
33、nings in the MLI at the jointlocations. Any relative motion between the two componentsproduced 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
34、generously overlapping theshields at the joint locations. If butt-joining of shield cannot 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 joi
35、ning locations.5.5 Penetrations:5.5.1 In any practical system, the penetration of the MLIwith 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 insulatio
36、ns and direct metal-to-metalcontact avoided.5.5.2 Because of the small thicknesses associated 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 alon
37、g the penetration several diameters (theexact length to be established by the user).5.5.3 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
38、 cutting gores in theshields of one of the components at the joint and overlappingthe gore 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, t
39、he corner formed by the two compo-nents can be filled with a preformed isotropic insulatingmaterial 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
40、 space vehicles), must occurwithout damage to the insulation. During evacuation, a gaspressure 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
41、the rate at which the enclosedgas (air or a purge gas) can escape from between the shieldsmust be enhanced. This is usually done by perforating theshields to provide broadside flow in addition to that via theC740/C740M 97 (2009)7edges. However, the effect of these perforations on the overallthermal
42、efficiency must be taken into account.6. Cleanliness6.1 It is essential that the materials used 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 emittanc
43、e,that is, reduce the reflectance. Particularly, fingerprints shouldbe avoided, because body acid 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
44、prior to pump-down. In some cases, insulation rooms should not only beclean, but also dry.6.3 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 bew
45、orn by any person actually handling the insulation materials.7. Materials Specifications7.1 Multilayer reflective insulation systems always havemultiple sheets of reflector material, each separated by lowconductance separator material. Considering reflector sheetsfirst, these depend on the low emiss
46、ivity characteristic of cleansmooth metal surfaces. The metal can be a sheet of foil, or itcan 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
47、 thin and highlyreflective, the foils are usually high-purity metals having highthermal conductivity. 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 s
48、oft, and can beobtained in thin highly reflective sheets. Other alloys ofaluminum, or even other 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
49、all temperaturesof interest should be 0.05 or less. The other side should notdiffer greatly. A 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 coatings, or even numerous fingerprints, is also causefor rejection.7.2.2 Mechanically, there are several requirements. The foilused should be thin enou
