ASTM C740-97(2004)
(Practice)Standard Practice for Evacuated Reflective Insulation In Cryogenic Service
Standard Practice for Evacuated Reflective Insulation In Cryogenic Service
ABSTRACT
This practice covers the use of thermal insulations formed by a number of thermal radiation shields positioned perpendicular to the direction of heat flow. These radiation shields consist of alternate layers of a low-emittance metal and an insulating layer combined such that metal-to-metal contact in the heat flow direction is avoided and direct heat conduction is minimized. These are commonly referred to as multilayer insulations (MLI) or super insulations (SI) by the industry. The performance considerations, typical applications, manufacturing methods, material specification, and safety considerations in the use of these insulations in cryogenic service are also discussed. MLI can be manufactured by any of the following: spiral-wrap method, blanket method, single layer method, and filament-wound method.
SCOPE
1.1 This practice covers the use of thermal insulations formed by a number of thermal radiation shields positioned perpendicular to the direction of heat flow. These radiation shields consist of alternate layers of a low-emittance metal and an insulating layer combined such that metal-to-metal contact in the heat flow direction is avoided and direct heat conduction is minimized. These are commonly referred to as multilayer insulations (MLI) or super insulations (SI) by the industry.
1.2 The practice covers the use of these insulation constructions where the warm boundary temperatures are below approximately 450 K.
1.3 Insulations of this construction are used when apparent thermal conductivity less than 0.007 W/mK (0.049 Btuin./hft 2F) at 300k are required.
1.4 Insulations of this construction are used in a vacuum environment.
1.5 This practice covers the performance considerations, typical applications, manufacturing methods, material specification, and safety considerations in the use of these insulations in cryogenic service.
1.6 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.7 his standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific safety hazards, see Section 8.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information.
Designation: C740 – 97 (Reapproved 2004)
Standard Practice for
Evacuated Reflective Insulation In Cryogenic Service
This standard is issued under the fixed designation C740; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
q = heat flow per unit time, W
A = unit area, m
1.1 This practice covers the use of thermal insulations
n = number of radiation shields
formed by a number of thermal radiation shields positioned
−8 2 4
s = Stefan-Boltzmann constant, 5.67 310 W/m ·K
perpendicular to the direction of heat flow. These radiation
T = temperature, K; T at hot boundary, T at cold bound-
h c
shields consist of alternate layers of a low-emittance metal and
ary
an insulating layer combined such that metal-to-metal contact
E = emittance factor, dimensionless; E , system effective
eff
intheheatflowdirectionisavoidedanddirectheatconduction
emittance
is minimized. These are commonly referred to as multilayer
e = total hemispherical emittance of a surface, dimension-
insulations (MLI) or super insulations (SI) by the industry.
less; e at hot boundary, e at cold boundary
h c
1.2 The practice covers the use of these insulation construc-
t = distancebetweenthehotboundaryandthecoldbound-
tions where the warm boundary temperatures are below ap-
ary, m
proximately 450K.
k = thermal conductivity, W/m·K
1.3 Insulations of this construction are used when apparent
R = shielding factor, dimensionless; equivalent to 1/E
thermal conductivity less than 0.007 W/m·K (0.049 Btu·in./
D = degradation factor, dimensionless
h·ft ·°F) at 300k are required.
P = mechanical loading pressure, Pa
1.4 Insulations of this construction are used in a vacuum
2.2 Definitions:
environment.
2.2.1 evacuated reflective insulation—Multilayercomposite
1.5 This practice covers the performance considerations,
thermal insulation consisting of radiation shield materials
typical applications, manufacturing methods, material specifi-
separated by low thermal conductivity insulating spacer mate-
cation, and safety considerations in the use of these insulations
rialofcellular,powdered,orfibrousnaturedesignedtooperate
in cryogenic service.
at low ambient pressures.
1.6 The values stated in SI units are to be regarded as the
2.2.2 ohms per square—The electrical resistance of a
standard. The values given in parentheses are for information
vacuum metallized coating measured on a sample in which the
only.
dimensions of the coating width and length are equal. The
1.7 This standard does not purport to address all of the
ohm-per-squaremeasurementisindependentofsampledimen-
safety concerns, if any, associated with its use. It is the
sions.
responsibility of the user of this standard to establish appro-
3. Insulation Performance
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use. For specific safety
3.1 Theoretical Performance:
hazards, see Section 8.
3.1.1 The lowest possible heat flow is obtained in an MLI
when the sole heat transfer mode is by radiation between free
2. Terminology
floating shields of low emittance and of infinite extent. The
2.1 Symbols:
heatflowbetweenanytwosuchshieldsisgivenbytherelation:
4 4
q/A 5 E~sT 2sT ! (1)
h c
a = accommodation coefficient, dimensionless
3.1.1.1 (RefertoSection2forsymbolsanddefinitions.)The
b = exponent, dimensionless
emittance factor, E, is a property of the shield surfaces facing
d = distance between confining surfaces, m
one another. For parallel shields, the emittance factor is
determined from the equation:
E 51/~1/e 11/e 21! 5 e e /e 1 ~1 2 e !e (2)
This practice is under the jurisdiction of ASTM Committee C16 on Thermal h c h c h h c
Insulation and is the direct responsibility of Subcommittee C16.21 on Reflective
3.1.1.2 When these opposing surfaces have the same total
Insulation.
hemispherical emittance, Eq 2 reduces to:
Current edition approved April 1, 2004. Published April 2004. Originally
approved in 1973. Last previous edition approved in 1997 as C740–97. DOI:
10.1520/C0740-97R04.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
C740 – 97 (2004)
FIG. 1 MLI Theoretical Heat Flow for Various Shield Emittances and 1.0 Boundary Emittance
E 5 e/ 2 2 e! (3)
~ 3.1.5.1 Themechanicalloadingpressureimposedacrossthe
insulation boundaries,
3.1.2 An MLI of n shields is normally isolated in a vacuum
3.1.5.2 Thecompositionandpressureleveloftheinterstitial
environment by inner and outer container walls. When the
gas; and
surface emittance of the shields and of the container walls
3.1.5.3 Penetration such as mechanical supports, piping and
facing the shields have the same value, then the emittance
wiring.
factor is given by:
3.2 Mechanical Loading Pressure:
E 5 e/~n 11!~2 2 e! (4)
3.2.1 Inpractice,theshieldsofanMLIarenotfree-floating.
where (n+1) is the number of successive spaces formed by
Compression between the layers due to the weight of the
both the container walls and the shields. insulation or to pressures induced at the boundaries, or both,
3.1.3 When the surface emittance of the shields has a value can cause physical contact between the shields producing a
direct conduction heat transfer path between the shields,
e < 1.0 and the boundaries have an emittance of 1.0, then the
emittance factor is given by: thereby increasing the total heat flux of the system.
3.2.2 Theeffectsofcompressionontheheatfluxareusually
E 5 e/~n ~2 2 e! 1 e! (5)
obtained experimentally using a flat plate calorimeter. Experi-
For values of e# 0.1, Eq 4 and Eq 5 can be simplified to
mental correlations have been obtained for a variety of
E= e/(2 (n+1)) and E= e/2 n, respectively, and the loss in
shield-spacer combinations which indicate that the heat flux is
b
accuracy will be less than 10%.
proportional to P where b varies between 0.5 and 0.66.
3.1.4 Computed values of the theoretical MLI heat flow
TypicaldataforanumberofMLIsystemsarepresentedinFig.
obtained by using Eq 1 and Eq 5 are presented in Fig. 1.
2 that illustrate this effect.
3.1.5 Well-designed and carefully fabricated MLI systems
have produced measured heat flows within approximately
50% of their theoretical performance. In practice, however,
Black, I. A., Glaser, P. E., and Perkins, P. “A Double-Guarded Cold-Plate
several important factors usually combine to reduce signifi-
Thermal ConductivityApparatus,” Thermal Conductivity Measurements of Insulat-
cantly the actual performance compared to the theoretical
ing Materials at Cryogenic Temperatures, ASTM STP 411, ASTM International,
performance. The principal sources of this degradation are: 1967.
C740 – 97 (2004)
Curve Numbers of
Material
No. Layers
110 1145—H19 Tempered Aluminum
11 Nylon Netting
210 Aluminized (both sides) Polyester
22 Glass Fabric
310 Aluminized (both sides) Polyester
33 Silk Netting
410 Aluminized (both sides) Polyester
11 2 lb/ft Polyurethane Foam
510 Aluminized (both sides) Polyester
11 Silk Netting with 0.004-in. by 0.5-in. Strips of Glass Mat
610 Aluminized (both sides) Polyester
11 Silk Netting with 0.008-in. by 0.25-in. Strips of Glass Mat
FIG. 2 Effect of External Compression on the Heat Flux Through Multilayer Insulations
3.3 Interstitial Gas—Heattransferbygasconductionwithin thermal profile across these insulations is not linear. Elabora-
an MLI may be considered of negligible importance if the tion and a discussion of the limitations of these approaches
−2 −3
follow:
interstitial gas pressure is in the range from 10 to 10 Pa
3.4.2 Effective Emittance:
dependinguponthetypeofspacermaterialused.Thispressure
is achieved with (a) a vacuum environment of approximately 3.4.2.1 The effective emittance of an MLI has the same
−3 −4
meaning as the emittance factor, E or E , when it is applied to
10 to 10 Pa, and (b) with a well-vented shield-spacer
the theoretical performance of the system. The effective
system which provides communication between the interstitial
emittance of an actual system is given by the ratio of the
spaces and the vacuum environment. Failure to provide these
measured heat flux per unit area to the differences in the black
minimal conditions results in a serious increase in the thermal
body emissions (per unit area) of the boundaries at their actual
conductance of the insulation system. The effect of excessive
temperatures as given by Eq 6.
gas pressure on conductivity is illustrated for a number of
4 4
insulation systems in Fig. 3.
E 5 ~q/A!/~sT 2sT ! (6)
eff h c
3.4 Performance Factors:
3.4.2.2 The measured average total effective emittance of a
3.4.1 Anumberoffactorshavecomeintotechnicalusageto
given insulation will have different values depending upon the
fill the need for expressing the thermal performance of an MLI
number of shields, the total hemispherical emittance of the
by a single, simple, and meaningful value. Two schools of
shieldmaterials,thedegreeofmechanicalcompressionpresent
thought have predominated. One is to express the performance
between layers of the reflective shields, and the boundary
in terms of radiation transfer since these insulations are
temperatures of the system.This effective emittance factor can
predominantly radiation controlling. The other is to use the
be used to compare the thermal performance of different MLI
classical thermal conductivity term in spite of the fact that the systems under similar boundary temperature conditions.
C740 – 97 (2004)
NOTE 1—d=distance between confining surfaces
a=accommodation coefficient (dimensionless)
FIG. 3 Effect of Gas Pressure on Thermal Conductivity
3.4.3 Shielding Factor: thisfactorcanonlyhavevalueslargerthan1.0.Atavalueof
3.4.3.1 The theoretical shielding factor, R, is the reciprocal 1.0 the amount of degradation is zero and the actual perfor-
of the emittance factor. This factor can also be obtained by
mance corresponds to the theoretical performance.
summing the reciprocal emittances of each shield surface as
3.4.5 Thermal Conductivity:
one proceeds from one of the system boundaries to the other
3.4.5.1 TheapparentthermalconductivityofanMLIsystem
and then subtracting 1.0 from the result for each space
can be defined by the ratio of the heat flow per unit area to the
traversed.
averagetemperaturegradientofthesystemincomparableunits
3.4.3.2 The actual system shielding factor is the reciprocal
as follows:
of the effective emittance of the system, that is, R = 1/E.
k 5 ~q/A!/~~T 2 T !/t! (8)
3.4.4 Degradation Factor—The degradation factor, D,is a h c
theratiooftheactualsystemheatfluxtothetheoreticalsystem
3.4.5.2 Since radiative heat transfer present within an MLI
heat flux, that is,
system produces a nonlinear temperature gradient, k will vary
D 5 ~q/A!actual/ ~q/A!theoretical (7) approximately as the third power of the mean temperature.
C740 – 97 (2004)
Thus, k can be used only for comparison of performance of 5.1.2.2 Reduce gas conduction heat flow by providing flow
differentMLIsystemswhentheboundarytemperaturesarethe paths within the insulation so that the interstitial gas can be
same. removed by the vacuum environment, and
3.4.5.3 Asecond difficulty associated with the use of a k for
5.1.2.3 Reduce the radiation heat flow by utilizing low-
MLI systems is the necessity of defining the insulation thick- emittance shield materials and by the elimination of gaps,
ness. This is possible only in certain types of measurement
spaces, or openings in each shield layer.
apparatus and in mechanized MLI systems. Thus, whenever k
5.2 Application:
is used to describe the thermal performance of an MLI, it
5.2.1 The user has a wide variety of application techniques
should be accompanied by a statement indicating the method
available to him. They include, but are not limited to, the
used in making the thickness measurement or the accuracy
spiral-wrap, blanket, single-layer, and filament-wound tech-
with which such a measurement was made.
niques.
3.5 Typical Thermal Performance of MLI—The thermal
5.2.2 Spiral-Wrap Method:
performance of MLIs can vary over a wide range from system
5.2.2.1 The spiral-wrap technique is applicable mainly to
to system depending largely upon the fabrication techniques,
the cylindrical segments of tanks. The shields and spacers are
but also upon the materials used for the shields and spacers.
applied together from rolls onto the rotating cylinder in a
Typical performance values of installed systems are shown in
continuous manner until the desired thickness or number of
Table 1 as well as the pertinent information concerning the
layers is achieved. This method is compatible with automatic
system characteristics and installation data. Thermal perfor-
manufacturing techniques as well as with manual techniques.
mance for some systems was shown in both the effective
Theshieldandspacermaterialmayhavethesamewidthasthe
emittance and thermal conductivity terms where this informa-
cylinder or segments of the cylinder. In some cases the shield
tion was available.
segments are butt-joined. It is the recommended and general
practice, however, to eliminate the possibility of gaps devel-
4. Typical Applications
oping between segments by providing a generous overlap of
4.1 Insulations of the type described above are generally the shield segments. Overlaps of 51 mm (2 in.) are typically
used when lower conductivities are required than can be used.
obtained with other evacuated insulations or with gas-filled 5.2.2.2 To obtain the best thermal performance, the shields
insulations.This may be dictated by the value of the cryogenic
and spacers of the tank ends are applied individually in a
fluid being isolated or by weight or thickness limitations manner such that the end shields are interleaved with the side
imposed by the particular application. Generally these fall into
shields.An alternative procedure is to apply the MLI onto the
either a storage or a distribution equipment category. Typical cylindricalportionofthevesselsothatitextendsoverthetank
storage applications include the preservation of biologicals,
endadistancecomparabletotheradius.Thetankendscanthen
onboard aviation breathing gas, piped-in hospital oxygen be insulated by folding the extended portion of insulation over
systems, welding and heat-treating requirements, distribution
the tank end.Alternatively, the extended portion of the insula-
storage reservoirs, and industrial users whose requirement tions can be appropriately
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.