ASTM C740/C740M-13(2019)
(Guide)Standard Guide for Evacuated Reflective Insulation In Cryogenic Service
Standard Guide 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 guide 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 technology of evacuated reflective insulation in cryogenic service, or MLI, first came about in the 1950s and 1960s primarily driven by the need to liquefy, store, and transport large quantities of liquid hydrogen and liquid helium. (1-6)2
1.2 The practice guide covers the use of these MLI systems where the warm boundary temperatures are below approximately 400 K. Cold boundary temperatures typically range from 4 K to 100 K, but any temperature below ambient is applicable.
1.3 Insulation systems of this construction are used when heat flux values well below 10 W/m2 are needed for an evacuated design. Heat flux values approaching 0.1 W/m2 are also achievable. For comparison among different systems, as well as for space and weight considerations, the effective thermal conductivity of the system can be calculated for a specific total thickness. Effective thermal conductivities of less than 1 mW/m-K [0.007 Btu·in/h·ft2·°F or R-value 143] are typical and values on the order of 0.01 mW/m-K have been achieved [0.00007 Btu·in/h·ft2·°F or R-value 14 300]. (7) Thermal performance can also be described in terms of the effective emittance of the system, or Εe.
1.4 These systems are typically used in a high vacuum environment (evacuated), but soft vacuum or no vacuum environments are also applicable.(8) A welded metal vacuum-jacketed (VJ) enclosure is often used to provide the vacuum environment.
1.5 The range of residual gas pressures is from -6 torr to 10+3 torr (from -4 Pa to 133 kPa) with or without different purge gases as required. Corresponding to the applications in cryogenic systems, three sub-ranges of vacuum are also defined: from -6 torr to 10-3 torr (from -4 Pa to 0.133 Pa) [high vacuum/free molecular regime], from 10-2 torr to 10 torr (from 1.33 Pa to 1333 Pa) [soft vacuum, transition regime], from 100 torr to 1000 torr (from 13.3 kPato 133 kPa) [no vacuum, continuum regime].(9)
1.6 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.
1.7 This 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use. For specific safety hazards, see Section 9.
1.8 This international standard was developed in accordance with ...
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Standards Content (Sample)
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation:C740/C740M −13 (Reapproved 2019)
Standard Guide for
Evacuated Reflective Insulation In Cryogenic Service
This standard is issued under the fixed designation C740/C740M; 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.
-6
1. Scope 1.5 The range of residual gas pressures is from <10 torr to
+3 -4
10 torr(from<1.33 Pato133kPa)withorwithoutdifferent
1.1 This guide covers the use of thermal insulations formed
purge gases as required. Corresponding to the applications in
by a number of thermal radiation shields positioned perpen-
cryogenic systems, three sub-ranges of vacuum are also de-
dicular to the direction of heat flow. These radiation shields
-6 -3 -4
fined: from <10 torr to 10 torr (from <1.333 Pa to 0.133
consist of alternate layers of a low-emittance metal and an
-2
Pa) [high vacuum/free molecular regime], from 10 torr to 10
insulating layer combined such that metal-to-metal contact in
torr(from1.33Pato1333Pa)[softvacuum,transitionregime],
the heat flow direction is avoided and direct heat conduction is
from 100 torr to 1000 torr (from 13.3 kPato 133 kPa) [no
minimized. These are commonly referred to as multilayer
vacuum, continuum regime].(9)
insulations(MLI)orsuperinsulations(SI)bytheindustry.The
technology of evacuated reflective insulation in cryogenic 1.6 The values stated in either SI units or inch-pound units
service, or MLI, first came about in the 1950s and 1960s are to be regarded separately as standard. The values stated in
primarily driven by the need to liquefy, store, and transport each system may not be exact equivalents; therefore, each
large quantities of liquid hydrogen and liquid helium. (1-6) system shall be used independently of the other. Combining
values from the two systems may result in non-conformance
1.2 The practice guide covers the use of these MLI systems
with the standard.
where the warm boundary temperatures are below approxi-
1.7 This standard does not purport to address all of the
mately 400 K. Cold boundary temperatures typically range
safety concerns, if any, associated with its use. It is the
from 4 K to 100 K, but any temperature below ambient is
responsibility of the user of this standard to establish appro-
applicable.
priate safety, health, and environmental practices and deter-
1.3 Insulation systems of this construction are used when
mine the applicability of regulatory limitations prior to use.
heat flux values well below 10 W/m are needed for an
2 For specific safety hazards, see Section 9.
evacuated design. Heat flux values approaching 0.1 W/m are
1.8 This international standard was developed in accor-
also achievable. For comparison among different systems, as
dance with internationally recognized principles on standard-
well as for space and weight considerations, the effective
ization established in the Decision on Principles for the
thermal conductivity of the system can be calculated for a
Development of International Standards, Guides and Recom-
specifictotalthickness.Effectivethermalconductivitiesofless
mendations issued by the World Trade Organization Technical
than 1 mW/m-K [0.007 Btu·in/h·ft ·°F or R-value 143] are
Barriers to Trade (TBT) Committee.
typical and values on the order of 0.01 mW/m-K have been
achieved [0.00007 Btu·in/h·ft ·°F or R-value 14 300]. (7)
2. Referenced Documents
Thermal performance can also be described in terms of the
2.1 ASTM Standards:
effective emittance of the system, or Ε .
e
B571Practice for Qualitative Adhesion Testing of Metallic
1.4 These systems are typically used in a high vacuum
Coatings
environment (evacuated), but soft vacuum or no vacuum
C168Terminology Relating to Thermal Insulation
environments are also applicable.(8)Awelded metal vacuum-
E408Test Methods for Total Normal Emittance of Surfaces
jacketed (VJ) enclosure is often used to provide the vacuum
Using Inspection-Meter Techniques
environment.
3. Terminology
This guide is under the jurisdiction of ASTM Committee C16 on Thermal
3.1 Definitions of Terms Specific to This Standard:
Insulation and is the direct responsibility of Subcommittee C16.40 on Insulation
Systems.
Current edition approved Sept. 1, 2019. Published October 2019. Originally
approved in 1973. Last previous edition approved in 2013 as C740/C740M–13. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
DOI: 10.1520/C0740_C0740M-13R19. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
The boldface numbers in parentheses refer to a list of references at the end of Standards volume information, refer to the standard’s Document Summary page on
this standard. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C740/C740M−13 (2019)
3.1.1 cold boundary temperature (CBT)—The cold bound- may be smooth, crinkled, or dimpled. The reflector may be
ary temperature, or cold side, of the MLI system is the unperforated or perforated
temperature of the cold surface of the element being insulated.
3.1.13 residual gas—As a perfect vacuum is not possible to
The CBT is often assumed to be the liquid saturation tempera-
produce,anygaseousmaterialinsideoraroundtheMLIsystem
ture of the cryogen. The CBT can also be denoted as T .
c
istheresidualgas.Theconcentrationofresidualgasescanvary
significantly through the thickness of the system of closely
3.1.2 cold vacuum pressure (CVP)—The vacuum level un-
spaced layers. The residual gas between the layers is also
der cryogenic temperature conditions during normal operation,
referred to as interstitial gas.
but typically measured on the warm side of the insulation.The
-2
CVP can be from one to three orders of magnitude lower than
3.1.14 soft vacuum (SV)—residual gas pressure from 10
the WVP for a well-designed cryogenic-vacuum system.
torr to 10 torr (1.33 Pa to 1333 Pa) [transition regime].
3.1.3 effective thermal conductivity (k )—The k is the
e e 3.1.15 spacer material—A thin insulating layer composed
calculated thermal conductivity through the total thickness of
of any suitable low conductivity paper, cellular, powder,
the multilayer insulation system between the reported bound-
netting, or fabric material. A given spacer layer may be a
ary temperatures and in the specific environment.
single, double, or more thickness of the material.
3.1.4 evacuated reflective insulation—Multilayer insulation
3.1.16 system thermal conductivity (k )—The k is the ther-
s s
(MLI)systemconsistingofreflectorlayersseparatedbyspacer
mal conductivity through the thickness of the total system
layers. An MLI system is typically designed to operate in a
including insulation materials and all ancillary elements such
high vacuum environment but may also be designed for partial
aspackaging,supports,getterpackages,andvacuumjacket.As
vacuum or gas-purged environments up to ambient pressures.
with k , the k must always be linked with the reported
e s
Additional components of an MLI system may include tapes
boundary temperatures and in the specific environment.
and fasteners, and mechanical supports; closeout insulation
3.1.17 warm boundary temperature (WBT)—The warm
materialsandgapfillersforpenetrationsandfeedthroughs;and
boundary temperature, or hot side, of the MLI system is the
getters, adsorbents, and related packaging for maintaining
temperature of the outermost layer of the MLI system.
vacuum conditions.
Alternatively, the WBT can be specified as the temperature of
3.1.5 getters—The materials included to help maintain a thevacuumcanorjacket.TheWBTcanalsobedenotedas T .
h
high vacuum condition are called getters. Getters may include
3.1.18 warm vacuum pressure (WVP)—The vacuum level
chemical getters such as palladium oxide or silver zeolite for
under ambient temperature conditions
hydrogengas,oradsorbentssuchamolecularsieveorcharcoal
3.2 Symbols:
for water vapor and other contaminants.
3.1.6 heat flux—The heat flux is defined as the time rate of l = mean free path for gas molecular conduction, m
Kn = Knudsennumber,ratioofthemolecularmeanfreepath
heat flow, under steady-state conditions, through unit area, in a
lengthtoarepresentativephysicallengthscale,dimen-
direction perpendicular to the plane of the MLI system. For all
sionless
geometries, the mean area for heat transfer must be applied.
-10
ξ = diameter of gas molecule, m (nitrogen, 3.14 ×10 m)
-6
3.1.7 high vacuum (HV)—residual gas pressure from <10
Q = heat flow per unit time, W
-3 -4
torr to 10 torr (<1.33 Pa to 0.133 Pa) [free molecular 2
q = heat flux, W/m
regime].
A = unit area, m
k =m thermal conductivity, mW/m·K
3.1.8 hot vacuum pressure (HVP)—Thevacuumlevelofthe
k = effective thermal conductivity through the total thick-
system under the elevated temperatures during a bake-out e
ness of the insulation system, mW/m-K
operation. SI units: Pa; in conventional units: millitorr (µ); 1 µ
k = system thermal conductivity through the total thick-
s
= 0.133 Pa.
nessoftheinsulationsystemandallancillaryelements
3.1.9 layer density (x)—The layer density is the number of
such as packaging, supports, getter packages,
reflector layers divided by the total thickness of the system.
enclosures, etc., mW/m-K
The number of reflector layers is generally referred to as the 2
A = effective area of heat transfer, m
e
number of layers (n) for an MLI system.
d = effective diameter of heat transfer, m
e
d = inner diameter of vessel or piping, m
3.1.10 novacuum(NV)—residualgaspressurefrom100torr
i
d = outer diameter of vessel or piping, m
to 1000 torr (13.3 kPa to 133 kPa) [continuum regime]. o
L = effective length of heat transfer area, m
e
3.1.11 ohms per square—Theelectricalsheetresistanceofa 3
ρ = bulk density of installed insulation system, kg/m
vacuum metalized coating measured on a sample in which the
n = number of reflector layers or number of layer pairs
dimensions of the coating width and length are equal. The
(one layer pair = one reflector and one spacer)
ohm-per-squaremeasurementisindependentofsampledimen-
z = layer density, n/mm
sions.
h = solid conductance of spacer material,W/K
c
-23
k = Boltzmann constant, 1.381 × 10 J/K
B
3.1.12 reflector material—A radiation shield layer com-
-8 2
σ = Stefan-Boltzmann constant, 5.67 × 10 W/m ·K4
posed of a thin metal foil such as aluminum, an aluminized
T = temperature, K; Th at hot boundary, T at cold bound-
c
polymeric film, or any other suitable low-emittance film. The
ary
reflector may be reflective on one or both sides. The reflector
C740/C740M−13 (2019)
usually combine to significantly degrade the actual perfor-
�T = temperature difference, T –T or WBT – CBT
h c
mance compared to the theoretical performance. The principal
Ε = emittance factor, dimensionless
sources of this degradation are listed as follows: (1) Compo-
Ε = effective emittance of system, dimensionless
e
e = total hemispherical emittance of a surface, dimension- sition and pressure level of the interstitial gas between the
less;e atthehotboundaryande atthecoldboundary layers; (2) Penetrations such as mechanical supports, piping
h c
x = total thickness of the insulation system, mm
and wiring; (3) Mechanical loading pressure imposed across
I = installation factor, dimensionless
the insulation boundaries; and (4) Localized compression and
,
P = mechanical loading pressure, Pa
structuralirregularitiesduetofabricationandinstallation.14 15
p = absolute gas pressure, Pa
4.2 Residual Gas:Heattransferbygasconductionwithinan
µ = vacuum level, millitorr (1 µ = 0.1333 Pa)
MLI may be considered negligible if the residual gas pressure
-6 -3
under cold conditions (CVP) is below 7.5 torr (10 Pa).
4. Theroretical Performance and Definition
However, the CVPis typically measured on the warm side and
4.1 Theoretical Performance:
the residual gas pressure between the layers is usually impos-
4.1.1 The lowest possible heat flow through an MLI system
sible to measure. The vacuum level inside the layers will
is obtained when the sole heat transfer mode is radiation
therefore vary greatly from the vacuum level measured in the
between free floating reflectors of very low emittance and of
surrounding annular space or warm-side vacuum environment.
infinite extent. The heat flow between any two such reflectors
The outer vacuum environment is at a vacuum level corre-
is given by the relation:
sponding to the WBT while the cold inner surface is at a
4 4
q 5 E~σT 2 σT ! (1) vacuum level corresponding to the CBT. The CVP, or amount
h c
of residual gas, can be imposed by design or can vary in
4.1.1.1 Theemittancefactor, E,isapropertyofthereflector
responsetothechangeinboundarytemperaturesaswellasthe
surfaces facing one another. For parallel reflectors, the emit-
surface effects of the insulation materials.
tance factor is determined from the equation:
4.2.1 For the purposes of this guide, the working definition
E 51/ 1/e 11/e 21 5 e e /e 1 1 2 e e (2)
~ ! ~ !
h c h c h h c of high vacuum (HV) is a range of residual gas pressure from
-6 -3 -4
<10 torrto10 torr(<1.33 Pato0.133Pa)whichrepresents
4.1.1.2 When these opposing surfaces have the same total
a free molecular regime of the thermophysical behavior of the
hemispherical emittance, Eq 2 reduces to:
gas. In order for free molecular gas conduction to occur, the
E 5 e/~2 2 e! (3)
mean free path of the gas molecules must be larger than the
spacingbetweenthetwoheattransfersurfaces.Theratioofthe
4.1.2 An MLI of n reflectors is normally isolated in a
meanfreepathtothedistancebetweensurfacesistheKnudsen
vacuumenvironmentbyinnerandoutercontainerwalls.When
number (Kn). The molecular flow condition is for Kn > 1.0.
the surface emittances of the reflectors and of the container
Themeanfreepath(l)forthegasmoleculemaybedetermined
walls facing the reflectors have the same value, then the
from the following equation:
emittance factor is given by:
k T
E 5 e/~n11!~2 2 e! (4)
B
l 5 (6)
=
2πξ P
where (n+1) is the number of successive spaces formed by
If the mean free path is significantly larger than the separa-
both the container walls and the reflectors.
tion between the hot side and cold side, then gaseous con-
4.1.3 When the surface emittance of the shields has a value
duction will be reduced.16 For many systems, a vacuum
E < 1.0 and the boundaries have an emittance of 1.0, repre-
pressure of roughly 50 millitorr is the point below which the
sentativeof
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