ASTM C740-97

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Designation: C 740 – 97
Standard Practice for
Evacuated Reflective Insulation In Cryogenic Service
This standard is issued under the fixed designation C 740; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) 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 describes 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 3 10 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
in the heat flow direction is avoided and direct heat conduction
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 = distance between the hot boundary and the cold bound-
tions where the warm boundary temperatures are below ap-
ary, m
proximately 450 K.
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—Multilayer composite
1.5 This practice describes 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
rial of cellular, powdered, or fibrous nature designed to operate
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-square measurement is independent of sample dimen-
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 a multi-
layer insulation when the sole heat transfer mode is by
2. Terminology
radiation between free floating shields of low emittance and of
2.1 Symbols:
infinite extent. The heat flow between any two such shields is
given by the relation:
4 4
a = accommodation coefficient, dimensionless q/A 5 E~sT 2sT ! (1)
h c
b = exponent, dimensionless
3.1.1.1 (Refer to Section 2 for symbols and definitions.) The
d = distance between confining surfaces, m
emittance factor, E, is a property of the shield surfaces facing
one another. For parallel shields, the emittance factor is
determined from the equation:
This practice is under the jurisdiction of ASTM Committee C-16 on Thermal
E 5 1/~1/e 1 1/e 2 1! 5 e e /e 1 ~1 2 e !e (2)
Insulation and is the direct responsibility of Subcommittee C 16.21on Reflective h c h c h h c
Insulation.
3.1.1.2 When these opposing surfaces have the same total
Current edition approved Nov. 10, 1997. Published June 1998. Originally
e1
hemispherical emittance, Eq 2 reduces to:
published as C 740 – 73. Last previous edition C 740 – 82 (1996) .
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
C740–97
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 The mechanical loading pressure imposed across the
insulation boundaries,
3.1.2 A multilayer insulation of n shields is normally
3.1.5.2 The composition and pressure level of the interstitial
isolated in a vacuum environment by inner and outer container
gas; and
walls. When the surface emittance of the shields and of the
3.1.5.3 Penetration such as mechanical supports, piping and
container walls facing the shields have the same value then the
wiring.
emittance factor is given by:
3.2 Mechanical Loading Pressure:
E 5 e/~n 1 1!~2 2 e! (4)
3.2.1 In practice, the shields of a multilayer insulation are
where (n + 1) is the number of successive spaces formed by
not free-floating. Compression between the layers due to the
both the container walls and the shields.
weight of the insulation or to pressures induced at the bound-
3.1.3 When the surface emittance of the shields has a value
aries, or both, can cause physical contact between the shields
e < 1.0 and the boundaries have an emittance of 1.0, then the
producing a direct conduction heat transfer path between the
emittance factor is given by:
shields, thereby increasing the total heat flux of the system.
E 5 e/~n ~2 2 e! 1 e! (5)
2 3.2.2 The effects of compression on the heat flux are usually
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
accuracy will be less than 10%.
b
proportional to P where b varies between 0.5 and 0.66.
3.1.4 Computed values of the theoretical MLI heat flow
Typical data for a number of MLI systems are presented in Fig.
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, several
important factors usually combine to reduce significantly the
Black, I. A., Glaser, P. E. and Perkins, P. “A Double-Guarded Cold-Plate
actual performance compared to the theoretical performance.
Thermal Conductivity Apparatus,” Thermal Conductivity Measurements of Insulat-
The principal sources of this degradation are: ing Materials at Cryogenic Temperatures, ASTM STP 411, 1967.
C740–97
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— Heat transfer by gas conduction other is to use the classical thermal conductivity term in spite
within a multilayer insulation may be considered of negligible of the fact that the thermal profile across these insulations is not
importance if the interstitial gas pressure is in the range from linear. Elaboration and a discussion of the limitations of these
−2 −3
10 to 10 Pa depending upon the type of spacer material approaches follow:
used. This pressure is achieved with ( a) a vacuum environment
3.4.2 Effective Emittance:
−3 −4
of approximately 10 to 10 Pa, and (b) with a well-vented
3.4.2.1 The effective emittance of an MLI has the same
shield-spacer system which provides communication between
meaning as the emittance factor, E or E , when it is applied to
the interstitial spaces and the vacuum environment. Failure to
the theoretical performance of the system. The effective
provide these minimal conditions results in a serious increase
emittance of an actual system is given by the ratio of the
in the thermal conductance of the insulation system. The effect
measured heat flux per unit area to the differences in the black
of excessive gas pressure on conductivity is illustrated for a
body emissions (per unit area) of the boundaries at their actual
number of insulation systems in Fig. 3.
temperatures as given by Eq 6.
3.4 Performance Factors:
4 4
E 5 ~q/A!/~sT 2sT ! (6)
eff h c
3.4.1 A number of factors have come into technical usage to
fill the need for expressing the thermal performance of a 3.4.2.2 The measured average total effective emittance of a
multilayer insulation by a single, simple, and meaningful given insulation will have different values depending upon the
value. Two schools of thought have predominated. One is to number of shields, the total hemispherical emittance of the
express the performance in terms of radiation transfer since shield materials, the degree of mechanical compression present
these insulations are predominantly radiation controlling. The between layers of the reflective shields, and the boundary
C740–97
NOTE 1—d = distance between confining surfaces
a = accommodation coefficient (dimensionless)
FIG. 3 Effect of Gas Pressure on Thermal Conductivity
temperatures of the system. This effective emittance factor can 3.4.4 Degradation Factor—The degradation factor, D,is
be used to compare the thermal performance of different the ratio of the actual system heat flux to the theoretical system
multilayer insulation systems under similar boundary tempera- heat flux, that is,
ture conditions.
D 5 ~q/A! actual / ~q/A! theoretical (7)
3.4.3 Shielding Factor:
this factor can only have values larger than 1.0. At a value of
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
and then subtracting 1.0 from the result for each space 3.4.5.1 The apparent thermal conductivity of an MLI system
traversed. can be defined by the ratio of the heat flow per unit area to the
3.4.3.2 The actual system shielding factor is the reciprocal average temperature gradient of the system in comparable units
of the effective emittance of the system, that is, R = 1/E. as follows:
C740–97
k 5 ~q/A!/~~T 2 T !/t! (8) 5.1.2 It is the objective of the multilayer insulation manu-
a h c
facturing techniques to:
3.4.5.2 Since radiative heat transfer present within a MLI
5.1.2.1 Reduce the solid conduction heat flow by minimiz-
system produces a nonlinear temperature gradient k will vary
ing the compression between the layers.
approximately as the third power of the mean temperature.
Thus, k can be used only for comparison of performance of
5.1.2.2 Reduce gas conduction heat flow by providing flow
different MLI systems when the boundary temperatures are the paths within the insulation so that the interstitial gas can be
same.
removed by the vacuum environment, and
3.4.5.3 A second 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 Multilayer
5.2.2 Spiral-Wrap Method:
Insulation—The thermal performance of multilayer insulations
5.2.2.1 The spiral-wrap technique is applicable mainly to
can vary over a wide range from system to system depending
the cylindrical segments of tanks. The shields and spacers are
largely upon the fabrication techniques, but also upon the
applied together from rolls onto the rotating cylinder in a
materials used for the shields and spacers. Typical performance
continuous manner until the desired thickness or number of
values of installed systems are shown in Table 1 as well as the
layers is achieved. This method is compatible with automatic
pertinent information concerning the system characteristics and
manufacturing techniques as well as with manual techniques.
installation data. Thermal performance for some systems was
The shield and spacer material may have the same width as the
shown in both the effective emittance and thermal conductivity
cylinder or segments of the cylinder. In some cases the shield
terms where this information 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 multilayer
either a storage or a distribution equipment category. Typical
insulation onto the cylindrical portion of the vessel so that it
storag
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