ASTM C740/C740M-13(2019)

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
sentativeofablackbody,thentheemittancefactorisgivenby:
free molecular range begins. However, some amount of gas
-6
E 5 e/~n ~2 2 e!1e! (5)
conduction still remains until the 10 torr level is reached.
For example, some mean free path values for air at room
For values of e ≤ 0.1, Eq 4 and Eq 5 can be simplified to E
-3
temperature are approximately 0.1 m for 10 torr and 100 m
= e/[2(n + 1)] and E = e/2n, respectively, and the loss in
-6
for 10 torr.
accuracy will be less than 10 %. Note also that e is a function
of temperature. For pure metals, e decreases with temperature. 4.2.2 The working definition of soft vacuum (SV) is a range
-2
Further considerations include the influence of the spacer on E of residual gas pressure from 10 torr to 10 torr (1.33 Pa to
(for example, the mutual emissivity of two adjacent reflector 1333 Pa) which represents a transition regime of the thermo-
layers increases when a spacer is present). physical behavior of the gas. The gaseous component of the
4.1.4 Computed values of the theoretical MLI heat flow heattransferthroughamaterialintheSVrangeisbetweenfree
obtained by using Eq 1 and Eq 5 are presented in Fig. 1.(10) molecular conduction and convection. This range is one of
Further information on the theory of heat transfer processes sharptransitionsandoftenassociatedwithstrongdependencies
associatedwithMLIsystemscanbefoundintheliterature.(11- on the morphology, composition, and construction of the
13) insulation materials. The molecular flow condition is for 1.0 >
4.1.5 Well-designed and carefully fabricated MLI systems Kn > 0.01. Thermal insulation systems operating in the soft
tested under ideal laboratory conditions can produce very low vacuumrangeoftenhaveallmodesofheattransferworkingin
heatfluxvalues.Inpractice,however,severalimportantfactors substantial proportions.
C740/C740M−13 (2019)
FIG. 1 MLI Theoretical Heat Flow for Various Shield Emittances and 1.0 Boundary Emittance
4.2.3 The working definition of no vacuum (NV) is a range 4.2.5 Also important are the type of spacer material used
ofresidualgaspressurefrom100torrto1000torr(13.3kPato and the layer density. A spacer material that is readily evacu-
133 kPa) which represents a continuum regime of the thermo- ated and very low outgassing is more conducive for obtaining
physical behavior of the gas. The continuum regime is associ- and maintaining the desired high vacuum condition. A lower
ated with viscous flow and convection heat transfer. The layer density typically promotes better evacuation and higher
molecular flow condition is for Kn < 0.01. While most MLI ultimate vacuum levels, but an exceptionally low layer density
systemsaredesignedtooperateunderhighvacuumconditions, can make maintenance of the high vacuum condition even
other MLI systems may be designed to operate under soft more critical.
vacuumornovacuumconditions.Inothercases,knowledgeof
4.2.6 An acceptable CVP is achieved with a well-vented
the performance sensitivity due to degraded vacuum or loss-
reflector-spacer system that provides communication between
of-vacuum conditions can be crucial for system operation and
the interstitial spaces and the vacuum environment. Failure to
reliability analysis. The three basic ranges of vacuum levels
provide proper venting can result in serious degradation of
(highvacuum,softvacuum,andnovacuum)aredepictedinthe
thermal performance.
MLI system performance curve given in Fig. 2.(17) In this
4.3 Mechanical Loading Pressure: .
example, the MLI system is 40 layers of aluminum foil and
4.3.1 In practice, the reflector layers are not free-floating.
micro-fiberglass paper under the following conditions: cold
Compression between the layers due to the weight of the
boundary temperature of 78 K, warm boundary temperature of
insulation or to pressures induced at the boundaries, or both,
293 K, and gaseous nitrogen as the residual gas.
can cause physical contact between the reflectors producing a
4.2.4 Cryopumping effects through the innermost layers
more direct conduction heat transfer path between the layers,
greatly aid in producing the desired high vacuum levels
therebyincreasingthetotalheatfluxofthesystem.Thegoalin
between the layers by freezing, condensing, and adsorbing the
designing any MLI system for high vacuum operation is to
some of the residual gases. The assumption here is that the
minimize the thermal contact as much as possible.
vacuum environment can be approximately the same as the
vacuum between the layers for a properly designed and 4.3.2 The effects of compression on the heat flux can be
executed MLI system. obtained experimentally using a flat plate calorimeter.(18)
C740/C740M−13 (2019)
FIG. 2 Variation of Heat Flux with Cold Vacuum Pressure: example MLI system of 40 layers foil and paper with boundary temperatures
of 78 K and 293 K and nitrogen as the residual gas. [Note: 1 millitorr = 0.133 Pa]
Experimental correlations have been obtained for a variety of 4.4.2.2 The measured average total effective emittance of a
reflector-spacer combinations that indicate that the heat flux is
given insulation will have different values depending upon the
b
proportionaltoP wherebvariesbetween0.5and0.66.Typical
number of reflectors, the total hemispherical emittance of the
data for a number of MLI systems are presented in Fig. 3 that
reflector materials, the degree of mechanical compression
illustrate this effect. The typical MLI systems listed here
present between layers of the reflectors, and the boundary
provide no significant mechanical strength as the compressive
temperatures of the system.This effective emittance factor can
forcesshouldbekeptnearzero,orlessthanabout10Pa(0.001
be used to compare the thermal performance of different MLI
psi)foroptimumperformance.Theoverallconfigurationofthe
systems under similar boundary temperature conditions.
installed system, whether horizontal or vertical, as well as the
4.4.2.3 Installation Factor—The installation factor, I,isthe
unit weight of the MLI must therefore be considered for an
ratio of the actual system heat flux to the theoretical system
accurateestimationofactualsystemthermalperformance.(19,
heat flux, that is,
20)
I 5 q ⁄q (8)
actual theoretical
4.4 Performance Factors:
4.4.1 Therearethreecomplementarywaysofexpressingthe
The installation factor can only have values larger than 1.0.
thermal performance of an MLI system. One way is to express
At a value of 1.0 the amount of degradation is zero and the
the performance in terms of radiation transfer since these
actualperformancecorrespondstothetheoreticalperformance.
insulations are predominantly radiation controlling. A second
Degradation factors can range from 1.5 to 10 for high vacuum
way is to calculate the steady-state heat flux.Athird way is to
conditions and can be much higher for even moderately
use the classical thermal conductivity term in spite of the fact
degraded vacuum conditions as indicated in Fig. 4. The
that the thermal profile across these insulations is not linear.
theoretical system heat flux is not necessarily known, but is
Elaboration and a discussion of these approaches follow:
generally taken to be the idealized blanket tested under
4.4.2 Effective Emittance:
laboratory conditions.
4.4.2.1 The effective emittance of an MLI has the same
meaningastheemittancefactor, E or E ,whenitisappliedto
4.4.3 Heat Flux:
1 2
the theoretical performance of the system. The effective
4.4.3.1 The heat flux, q, of a thermal insulation system can
emittance of an actual system is given by the ratio of the
be defined by the total heat flow rate divided by the effective
measured heat flux per unit area to the differences in the black
area of heat transfer in comparable units as follows:
body emissions (per unit area) of the boundaries at their actual
q 5 Q⁄A (9)
temperaturesasgivenbyEq7.Theeffectiveheattransferareas e
The effective heat transfer area, A , is the mean area through
e
for both warm and cold surfaces must be applied.
which heat moves from the hot boundary to the cold bound-
4 4
E 5 q/ σT 2 σT (7)
~ !
e h c ary and is further defined as follows:
C740/C740M−13 (2019)
Curve Numbers of
Reflector Spacer
No. Layers
1 10 1145—H19 Tempered Alu- Nylon Netting (11 layers)
minum
2 10 Aluminized (both sides) Glass Fabric (22 layers)
Polyester
3 10 Aluminized (both sides) Silk Netting (33 layers)
Polyester
4 10 Aluminized (both sides) 32 kg/m Polyurethane Foam (11 layers)
Polyester
5 10 Aluminized (both sides) Silk Netting with 0.1-mm by 12.7-mm Strips of Glass Mat
Polyester (11 layers)
6 10 Aluminized (both sides) Silk Netting with 0.2-mm by 6.4-mm Strips of Glass Mat
Polyester (11 layers)
FIG. 3Effect of Mechanical Compression on Heat Flux
π q 5 k ~∆ T ⁄ x! (13)
e
Forflatdiskgeometries: A 5 d (10)
e e
The Lockheed Equation gives an empirical form as follows:
where d is taken as the inner diameter of vessel or pipe
2.63
e
C *n¯ T 2 T * T 1 T
~ ! ~ !
s h c h c
plus one wall thickness of that same vessel or pipe.
q 5
2* n 1 1
~ !
d
o 4.67 4.67
C *e* T 2 T
~ !
Forcylindricalgeometries: A 52π~L !x⁄1n (11)
S D R h c
e e
d 1
i
n
where L is the effective heat transfer length of the cylinder
e
0.52 0.52
C *P* T 2 T
and do and di are the outer and inner diameters, respectively, ~ !
G h c
1 (14)
of the insulation system.
n
All three modes of heat transfer are accounted for by the
Forsphericalgeometries: A 5 πd d (12)
e o i
leading coefficients: solid conduction (C ), radiation (C ),
where d and d are the outer and inner diameters, S R
o i
and gaseous conduction (C ). Even at high vacuum levels,
respectively, of the insulation system. The heat flux can be G
some gas molecules do exist between the layers of radiation
computed based on the MLI or the total system. For
shields and spacers necessitating a term for gaseous conduc-
example, the outer diameter of the MLI is chosen for the
MLI heat flux while the inner diameter of the vacuum jacket
tion. The Lockheed Equation (21) is based primarily on data
is chosen to compute the total system heat flux. Accordingly,
from MLI systems comprised of double-aluminized mylar
the heat flux should be stated as for the MLI only or for the
radiation shields with silk net spacers and tested using a flat
total system. The basic form using the Fourier rate equation
for heat conduction is given as:
C740/C740M−13 (2019)
plate boiloff calorimeter.) Alternatively, the general form for 4.5 Typical Thermal Performance of MLI—The thermal
the physics-based equation developed by McIntosh (22) is performance of MLI systems can vary over a wide range
depending largely upon the fabrication techniques, but also
given as follows:
upon the materials used for the reflectors and spacers. (24)
4 4
σ T 2 T
~ !
h c
q 5 Performance will vary in accordance with different boundary
1 1
1 2 1 temperatures.Performancecanalsovarywidelyfortanks,rigid
S D
ε ε
h c
piping, and flexible piping applications. In all cases, under-
1C Pα~T 2 T !
G h c
standing the total system performance, including MLI,
~T 2 T !
supports, attachments, penetrations, getters, etc., is the main
h c
1C fk (15)
s
x
point. Testing methods and equipment include a wide range of
The McIntosh Equation, as well as the Lockheed Equation,
both boiloff calorimetric and electrical-based techniques.(25-
has three terms: one for radiation between shields, one for
31)
solid conduction through the spacers, and one for gaseous
conduction due to any residual gas molecules among the
5. Practical Performance and Applications
layers. The term f is the relative density of the spacer com-
5.1 Insulations of the type described above are generally
pared to the solid form of the material. The use of these or
other equations available in the literature requires adequate used when lower conductivities are required than can be
understanding of all three heat transfer modes as well as the
obtained with other evacuated insulations or with gas-filled
testing methodologies used and the influences of installation
insulations.This may be dictated by the value of the cryogenic
for a given application.
fluid being isolated or by weight or thickness limitations
imposed by the particular application. Generally these fall into
4.4.4 Effective Thermal Conductivity:
either a storage or a distribution equipment category. Typical
4.4.4.1 The effective thermal conductivity (k)ofanMLI
e
storage applications include the preservation of biologicals,
systemcanbedefinedbytheratiooftheheatflowperunitarea
onboard aviation breathing gas, piped-in hospital oxygen
to the average temperature gradient of the system in compa-
systems, welding and heat-treating requirements, distribution
rable units as follows:
storage reservoirs, and industrial users whose requirement
k 5 ~Q ⁄ A ! ⁄ ~∆ T ⁄ x! (16)
cannot be economically met with gas storage. Distribution
e e
For highly-evacuated MLI systems, the effective thermal
applications include railroad tank cars, highway trucks and
conductivity can be expressed as follows (23):
trailers,pipelines,portabletankageofvarioussizes,allserving
21 2 2
the metal industry, medicine, and space exploration programs.
k 5 N ⁄ x ⁄ h 1 σ e T 2 T T 2 T ⁄ 2 2 e
~ ! @ ~ !~ ! ~ !#
e c h c h c
Specialized applications such as surgical operating tools and
(17)
spacevehicleoxidizerandfueltankshavealsoseensignificant
The effective thermal conductivity is determined from Fouri-
er’s law for heat conduction through a flat plate as given by development.
equation (Eq 18), between concentric cylinders as given by
5.2 Thermal Performance Data—ExampleThe typical ther-
equation (Eq 19), and between concentric spheres as given
mal performance data for a number of MLI systems are given
by equation (Eq 20):
in Fig. 4and Fig. 5.(9, 14, 17)These figures show the variation
4Qx
of heat flux with cold vacuum pressure (the residual gas is
FlatPlate: k 5 (18)
e 2
πd ∆T
e
nitrogen in all cases). The data were obtained using a cylin-
d
drical boiloff calorimeter. The thermal performance including
o
Q1n
S D
d
the effect of residual gas pressure on effective thermal conduc-
i
Cylindrical: k 5 (19)
e
2πL ∆T
tivityisshowninFig.6fornitrogengas (9, 14, 17)andinFig.
e
7 for helium gas. (10) Table 1 includes the pertinent informa-
Qx
Spherical: k 5 (20)
tion concerning the materials, system characteristics, and
e
πd d ∆T
o i
installation methods. (9, 10, 14, 17) Thermal performance is
4.4.4.2 Because radiation heat transfer within an MLI sys- shown for effective emittance, heat flux, and thermal conduc-
tem produces a nonlinear temperature gradient, k will vary
tivity terms where this information was available. These data
e
approximately as the third power of the mean temperature. are for the boundary temperatures of approximately liquid
Thus, k can be properly used for comparison of performance nitrogen (77 K) and ambient (295 K). Design specifications
e
ofdifferentMLIsystemsonlywhentheboundarytemperatures and thermal performance test results for an example cryogenic
are the similar. MLI system are given in Table 2. (17) Additional thermal
performancedatafordifferentMLIsystemscanbefoundinthe
4.4.4.3 The total insulation thickness must be carefully
literature. (32, 33)
defined. Whenever k is used to describe the thermal perfor-
e
mance of an MLI system, a statement indicating the method
5.3 Detailed Performance Considerations:
usedinmakingthethicknessmeasurementandtheaccuracyof
5.3.1 Residual Gas Effects—The type and amount (vacuum
such measurement is needed. In some cases, an estimate of a
pressure)ofresidualgashasastronginfluenceontheresulting
range of thicknesses for a given installation may be appropri-
thermal performance of MLI systems. The vacuum level, if
ate.Alternatively, the appropriate diameter of the vacuum can known, is usually measured at the warm boundary or vacuum
orjacketcanbeusedtoestablishathicknessfordeterminingan
enclosure. The vacuum levels between layers are generally
overall system thermal conductivity (k ). unknown and can have significant effects on the thermal
s
C740/C740M−13 (2019)
FIG. 4Variation of Heat Flux with Cold Vacuum Pressure for Various MLI Systems for the Full Vacuum Range [Note: 1 millitorr = 0.133
Pa]
FIG. 5Variation of Heat Flux with Cold Vacuum Pressure for Various MLI Systems for the High Vacuum Range [Note: 1 millitorr = 0.133
Pa]
performance. Understanding and applying all the available result in additional compression between the layers and give
informationfromtheheating,purging,evacuation,andvacuum diminishing returns or even reduced thermal performance.(35)
monitoring steps can help to account for residual gas effects 5.3.3 Layer Density—The layer density (z) is crucial for
and explain the overall thermal performance results. estimating the thermal performance of MLI systems. An
5.3.2 Number of Layers—The number of layers (n) for MLI optimum layer density must be considered in light of the
systems can be from 1 to 100 or more. If size and weight are performance targets as well as the practicality of installation
not an issue, then more layers are generally better for reducing techniques. The optimum layer density often varies with
the heat flux.(34) However, sagging in thicker blankets can different combinations of reflector and spacer materials.(36)
C740/C740M−13 (2019)
FIG. 6Variation of Effective Thermal Conductivity with Cold Vacuum Pressure. [Note: 1 millitorr = 0.133 Pa]
FIG. 7Variation of Effective Thermal Conductivity with Cold Vacuum Pressure for Helium Residual Gas. [Note: 1 millitorr = 0.133 Pa]
Variable density MLI systems, when carefully executed, can of 300 K), the influence of lower temperature can have
provide increased thermal performance.(36-40) profoundeffectsonthecoldvacuumpressureandheattransfer
5.3.4 Cold Boundary Temperature Effects—Thecoldbound- mechanisms.Testingandanalysisareneededtounderstandthe
ary temperatures typically range from 111 K for liquefied influenceofdifferentCBT.Extrapolationsandinterpolationsof
natural gas to 4 K for liquid helium. Liquid nitrogen at a test data are often unavoidable, but such performance predic-
normalboilingpointof77Kisofcourserightinthemiddleof tions should be taken with precaution.
this range and offers a popular test condition for many MLI 5.3.5 Very Low Temperature Effects—The radiative trans-
systems.Whiletheoverallchangein ∆Tisnotextremelylarge missivity of metalized coatings should be considered for
going from 77 K to 4 K (only 223 K versus 296 K for a WBT cryogenic environments colder than approximately 40 K. At
C740/C740M−13 (2019)
TABLE 1 Performance and Weight Summary for Typical Installed MLI Systems
System Characteristics Installation Data Thermal Performance
E D
No. nx t Ae ρ CVP CBT WBT qk
e
A,B C
Reflector Spacer
2 3 2
n/mm mm m kg/m µK K W/m mW/m-K
N01 Foil Paper 60 2.5 24.5 0.349 . 0.010 86 295 0.734 0.086
N02 Foil paper 40 3.6 11.2 0.324 . 0.001 78 293 0.586 0.030
N03 Foil paper 80 3.8 21.1 0.341 0.003 78 293 0.516 0.051
N04 DAM paper 30 2.1 19.0 0.338 . 0.001 78 293 0.373 0.033
N05 DAM Polyester fabric 40 3.0 13.6 0.328 87 0.008 78 300 0.639 0.039
N06 DAM Polyester fabric 10 1.6 6.4 0.316 . 0.008 78 293 0.557 0.016
N07 DAM Polyester net 40 2.6 15.5 0.342 . 0.010 78 293 0.398 0.028
N08 DAM Polyester net 60 1.4 42.6 0.377 55 0.002 78 293 0.366 0.073
N09 DAM Polyester net 30 4.3 7.0 0.317 . 0.011 78 298 0.883 0.029
P01 Poly film goldized both sides 3 layers silk netting 5 . . 3.67 . HV 78 300 1.04 .
P02 DAM 2 layers silk netting 5 . . 3.67 . HV 78 300 1.36 .
P03 DAM 2 layers glass fabric 5 . . 3.67 . HV 78 300 1.67 .
P04 DAM, 1.9% perforated 2 layers glass fabric 5 . . 3.67 . HV 78 300 3.28 .
P05 DAM 0.5-mm polyurethane foam 10 . . 2.19 . HV 78 300 1.23 .
P06 DAM 0.9-mm polyurethane foam 37 . . 2.92 . HV 78 300 0.54 .
P07 DAM 0.07-mm paper 30 . . 5.48 . HV 78 300 1.42 .
P08 DAM 0.013-mm polyester Dimplar 36 . . . . HV 78 300 1.92 .
Aluminized both sides
P09 Crinkled poly film aluminized none 42 . . 2.69 . HV 78 300 1.89 .
one side, 0.5% perforated
P10 Foil, 0.006-mm glass fiber paper 29 . . 0.16 . HV 78 300 0.76 0.038
P11 Foil, 0.006-mm rayon fabric 36 . . 1.09 . HV 78 300 0.57 0.033
P12 Foil, 0.006-mm glass fiber web 21 . . 0.28 . HV 78 300 1.83 0.189
A
Double-Aluminized Mylar (DAM) is polyester film, 0.006-mm (0.25 mil) thick, and metalized on both sides unless otherwise noted.
B
Foil film is aluminum foil unless otherwise noted.
C
Paper spacer is micro-fiberglass paper unless otherwise noted.
D
All CVP for high vacuum condition; where the measurement is unknown, the designation HV is given (assumed to be 0.005 µ on charts). Note: 1µ=1millitorr = 0.133 Pa.
E
Designations starting with an N indicate a cylindrical test configuration; the P designations indicate a flat plate test configuration.

C740/C740M−13 (2019)
TABLE 2 Design Specifications and Thermal Performance Test Results for an Example Cryogenic MLI System
A245 MLI Baseline (10, 1.6, 6.4) Test CVP Flow WBT CBT Q q k
e
Mylar & Poly Fabric - Original (millitorr) (sccm) (K) (K) (W) (W/m ) (mW/m-K)
System
Layers = 10 pairs 1 0.004 76 293.1 78 0.316 1.00 0.030
Layer density = 1.6 layers/mm 2 0.050 130 293.0 78 0.536 1.70 0.050
Total thickness = 6.4 mm 3 0.132 146 292.9 78 0.603 1.91 0.057
Ae = 0.316 m 4 0.326 171 293.0 78 0.706 2.24 0.066
Total mass = 126 g 5 1.02 277 292.9 78 1.148 3.64 0.108
System density = 53 kg/m 6 9.96 1456 292.6 78 6.030 19.10 0.567
Initial Pumping & Heating = 24 7 99 7684 292.8 78 31.80 100.7 2.99
hours
even colder temperatures, for example at liquid helium (4 K), requirements for bending as well as unique challenges in
near field thermal radiation effects including the phenomenon
fabrication and assembly.
of radiation tunneling may become significant in some cases.
5.4.3 Effect of Supports—Support considerations may in-
For very low temperatures the longer radiation wavelengths
clude the manner of physically supporting the MLI materials
can become comparable with the spacing between layers and
themselves, the inner pipe or vessel, or any number of
lead to unexpected consequences in heat transfer.(41-43)
mechanical combinations.(44, 45) These support structures or
5.3.6 Other Considerations—System requirements can vary
otherstructuralelementsusedbetweenthewarmboundaryand
greatly with regard to boundary temperatures, temperature
thecoldboundarycancauseincreasedheatleakinatleastthree
matchingamongconductivelayers,vaporshields,andsoforth.
ways. First is the solid conduction of heat through the support
Fullunderstandinganddelineationofthetotalsystemtempera-
itself. This heat leakage rate may be easy to calculate, but the
tureprofile,fromwarmesttocoldesttemperature,isneededfor
thermalcontactresistanceatthecoldsurfaceisoftenunknown
design the proper MLI system to meet the overall system
anddifficulttoestimate.Thesecondincreaseinheatleakisany
requirements.
gap or crack that is created underneath or adjacent to the
5.4 System Performance Considerations:
support structure. This gap allows a direct view for radiation
5.4.1 Temperatures—As previously discussed, the warm
heattransfer.Thethirdincreaseinheatleakisthedisruptionin
and cold boundary temperatures are the first and foremost
continuity of the MLI layers that leads to compressed edges or
factorsinthethermalperformanceofthesystem.Thereareany
damage.
number of combinations of boundary temperatures that may
5.4.4 Vacuum Life and Getters—Both the vacuum level and
come into play. Some of the more common combinations are
the operational vacuum life are major factors in the design of
listed, for example, as follows in terms of WBT/CBT (in units
MLI systems for vacuum jacketed applications. (16) Vacuum
of K): 293/77, 350/90, 300/20, 300/4, 77/4, 50/2, etc. The two
retention, maintenance schedule, and getters should be consid-
keyfactorsherearethetemperaturedifferenceandthedepthof
ered for the planned system life cycle. Vacuum retention
the cold (that is, the cold boundary temperature). The cold
depends on the leak tightness of the jacket and the outgassing
boundary temperature is often determined by the cryogen or
cold product to be insulated. The normal boiling points of of all materials inside the annular space. Getters may include
common cryogens at one atmosphere pressure are summarized chemical getters such as palladium oxide or silver zeolite for
as follows: liquid methane (112 K), liquid oxygen (90 K), hydrogengas,oradsorbentssuchamolecularsieveorcharcoal
liquidargon(87K),liquidnitrogen(77K),liquidhydrogen(20
for water vapor and other contaminants. The amounts,
K), and liquid helium (4 K). For reference, the boiling point of
packaging, and placement for the selected getters should be
liquid carbon dioxide is 217 K (at 519 kPa).
carefully chosen in accordance with life cycle requirements.
5.4.2 Configuration—MLI systems are generally designed
Any material placed in the annular space should be evaluated
foreithertankorpipingconfigurations.Eachconfigurationwill
for oxygen compatibility requirements.
have its own unique requirements and limitations. The design
5.4.5 Loss of Vacuum / Degraded Vacuum—The effects of
must take into account the basic geometry, the available
loss of vacuum (46-49) or degraded vacuum (9, 50) can be a
annular space, support structures, the fabrication and assembly
particular concern with most vacuum-jacketed (evacuated)
sequence, and the thermal performance requirements. The
systems.Issuesmayincludesafety,increasedmaintenance,and
thermal performance requirement for a long term storage tank
dramatically reduced thermal insulating effectiveness.
will likely be much less stringent than a pipeline providing a
5.4.6 Valves and Instrumentation—Vacuum-jacketed appli-
high velocity flow.Tanks are generally cylindrical or spherical
cations usually require the use of at least one pressure relief
but also have annular space piping to be insulated. The
valve (port) that can also serve as the connection for evacua-
complex geometries such as ellipsoidal ends of tanks make for
tion. Vacuum pressure instrumentation can be incorporated
additionalchallengesinthedesign.Pipingsystemscanberigid
with this port, be made separate, or not used depending on the
or flexible and each type has its own special requirements and
overall system needs for performance validation. Consider-
terminations (end assemblies) to consider. Rigid piping assem-
ation to the getter replacement or regeneration can also be
blies may have complex geometries and angles to address.
Flexible piping assemblies can be long lengths with specific made in concert with the port design. System life cycle and
C740/C740M−13 (2019)
maintenance requirements, such as vacuum regeneration capa- 6.1.2.1 Reduce the solid conduction heat flow by minimiz-
bility and vacuum monitoring, may define further needs for ing the compression or thermal contact between the layers.
additional valves, ports, and instrumentation.
6.1.2.2 Reduce gas conduction heat flow by providing flow
paths within the insulation so that the interstitial gas can be
5.4.7 Other Considerations—The actual system perfor-
removed by the vacuum environment (or if not high vacuum
mance may largely depend on the method of manufacture and
requirement then a purge gas can be selected to meet the
installation of the MLI materials and ancillary parts. These
thermal performance criteria), and
details are discussed in Section 6. Table 2 gives the design
6.1.2.3 Reduce the radiation heat flow by utilizing low-
specifications and thermal performance test results for an
emittance reflector materials and by the elimination of gaps,
example cryogenic MLI system.
spaces, or openings in each reflector layer. Most importantly,
5.5 Applications—MLI systems are generally used when
through-thickness gaps or cracks must be eliminated.
lower heat leakage rates than those obtained with other
6.1.2.4 Select the materials, number of layers, and layer
evacuatedinsulationsarerequired.Otherevacuatedinsulations
density for the desired thermal performance with additional
include, for example, perlite powder, glass bubbles, or aerogel
considerations for space and weight.
bulk-fill, which can provide heat flux values in the range of
6.1.2.5 Provide a design to allow for adequate venting
approximately 5 to 20 W/m .(51) The choice for an MLI
during evacuation and/or purging cycles such that the system
system may be dictated by the value of the cryogenic fluid
remains physically intact with all layers and joints in their
beingisolatedorbyweightorthicknesslimitationsimposedby
proper locations.
the particular application. Applications generally fall into the
6.1.2.6 Minimize particulate and molecular contamination,
following categories: storage, transfer, thermal protection, and
asrequired,andprovideanynecessarygetteringandadsorbent
low-temperature processes. Typical storage applications liquid
materials to maintain the proper high vacuum condition as
helium dewars, liquid hydrogen storage tanks, liquefied indus-
required for the design life of the overall system.
trialgasvesselsincludingargonandnitrogen,onboardaviation
6.2 Installation:
breathing gas, high energy cryostats, and industrial power
6.2.1 A wide variety of insulation techniques are available.
applications. Transfer applications include hospital oxygen
They include, but are not limited to, the spiral-wrap, blanket,
piping systems, welding supply, computer equipment
layer-by-layer, and filament-wound techniques. As not all
manufacturing,beveragebottling,cryogenicpropellantloading
geometri
...