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Thermal insulation
Institute for Technical Physics
Holger Neumann
Dont be afraid of low temperatures
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Content
Relevance of thermal insulation in cryogenics
Overview of different insulation materials
Multi-layer insulation (MLI) Superinsulation
Description
Heat transfer calculations
Special characteristics
Example: Thermal insulation development for a flexible cryogenic line
Conclusions
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Relevance of thermal insulation in cryogenics
Example 1:
The efficiency of a 4.4 K-refrigerator is about 10% of the Carnot-Coefficient of Performance (COP)
= 0.0015
The heat load of 100 W at 4.4 K requires a power input ofabout 70 kW
Example 2:1000 litres-vessel LHe with an evaporation rate of 1%/day
decrease of the insulation quality of 10% (~ 30 mW)
Increase of the operating costs of ~ 1000 /year
or additional LHe-acquisition costs of ~ 2000 /year
FluidtEnvironmen
FluidC
TT
T
=
Cryogenics T = TEnvironment TFluid great value
latent heat are very small
needed energy input for generating low temperatures is very high
(Carnot)
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Overview of different insulation materials
10-6.0 10-5.0 10-4.0 10-3.0 10-2.0 10-1.0
MLI
micro-sphere
powderwith smallpieces ofmetal foils
fibreglas
powder
atmospheric pressurevacuum
air (1 bar) ~ 2,6 10-2.
heat conductivity [W/(m K)] between ~ 300 K - 77 K .
foams, powdersfibres
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Multi-layer insulation (MLI) Superinsulation Description
MLI consists of:
reflecting layers reduction of heat transfer due to radiationspacer elements with low heat conductivity between the reflecting layers
high vacuum
prevention of convection
minimisation of heat conduction of residual gas
MLI is presently the most effective kind of thermal insulationdeveloped in the fifties by Peterson (Sweden)
first established in the sixties by space industry
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SI-materials:
reflecting layers: mostly aluminium metallized mylar films / pure aluminium foils
spacer elements: mostly net of glas fibre or foils / paper or polyester / tulle or silk
or
unit of reflector and spacer:
metallized mylar films, crinkled or embossed to reduce the contact surface between the
reflecting layers without spacer elements
attention: SI-anisotropy
delicate regarding installation (many bugs are possible)
Multi-layer insulation (MLI) Superinsulation Description
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Multi-layer insulation (MLI) Superinsulation
Heat transfer calculations
i1ii
T
i1ii
1ii
i
i4
1i4i
TT
1ii,overall
ACf)T(Ts
Af)(1)T(T)T(T8
R2p21
1
Af)(1)T(T
1
1
1QQ
1ii,
1ii
+
+
++
+
==
+
+
+
++
+
+
&&
100
125
150
175
200
225
250275
300
[K]
5 10 15 20 25
N
reine Wrmestrahlung
Wrmestrahlung und -leitung
reine Wrmeleitung
pure radiationradiation and conductionpure conduction
radiation
residual gas heat conduction
solid heat conduction
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Multi-layer insulation (MLI) Superinsulation
Heat transfer calculations
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Multi-layer insulation (MLI) Superinsulation
Heat transfer calculations
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Multi-layer insulation (MLI) Superinsulation
Special characteristicsinfluence ofcontact pressure
1-3: Al layers with fibre glass paper of different thickness4: Dracon Al-metallized with glass silk tissue
5-6: theoretical values (without solid heat conduction)
optimum number of layers / density of layers
x
x
xx
0 10 20 30 40 50 1/cm
N/D
0.05
0.10
0.15
mW/(m K).
1
2 3
4
56
effectiveheatconductivity
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.0
5
10
15
20
25
30
35
40
45
0 200 400 600 800
diameter of tube [mm]
q
[W/m2]
0
0,5
1
1,5
2
2,5
3
q[W/m]
empirical values for different transferlines and cryostatswith 20 - 50 layers MLI between RT and 80 K(winding technique on tubes and cylinders)
q [W/m] = q [W/m ] d2
. .. .
q [W/m ] with 3 blankets (RT - 80 K)2
.
Multi-layer insulation (MLI) Superinsulation
Special characteristics
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0
1
2
3
4
5
6
7
8
9
0 50 100 150 200 250 300 350 400
d [mm]
q[W/m2]
T = 280 K
p < 2 10 mbarwarm
-6.only one aluminium
layer (LN )2
1 blanket
2blankets
3blankets
IR 300.12 MLI blanket techniqueopen / closed symbolsLHe / LN - experiments2
MLI winding technique
Multi-layer insulation (MLI) Superinsulation
Special characteristics
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Multi-layer insulation (MLI) Superinsulation Special characteristics
0 10 20 30 40 50
N
0
2
4
6
8
10
12
14
q
[W/m2]
qrad=f(ewall=0.1; eshield=0.03; TW=300 K; TC=77 K)
IHI: Jacob
IHI: FZK
IHI: Ohmori [1992]
Jehier: FZK, TESSI mit d=320 mmJehier: FZK, THISTA mit d=219 mm
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Multi-layer insulation (MLI) Superinsulation interim conclusion
Important
quasi-isothermal parting points
Avoiding of gaps causes disproportionately high heat transfer
Avoiding of mechanical stress
causes exponentially increase of degradation with p
Relation between heat conduction and radiation = f(T)
MLI is especially effective at high temperatures
MLI is less effective or disadvantageous at T < 100 K
optimal layer density
vacuum conditions
perforated layers
MLI with integrated getter materials
Superinsulation only meets this expression and expenditure if severalpossibilities of errors could be avoided
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Multi-layer insulation (MLI) Superinsulation Example: Thermal insulation development for a flexible
cryogenic line
Requirements on a economic applicable HTS-cable
compact design insulation = 20 mm
The use of MLI is mandatory
2K80K3002 mW2q
mW1 &
Km
W102
Km
W101 4Isolation
4
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Multi-layer insulation (MLI) Superinsulation Example: Thermal insulation development for a flexible
cryogenic line
superconductingcabel
welded tube(60/66 mm)
welded tube (100/110 mm)
welded tube (130/143 mm)
welded tube (198/220 mm)
multilayerinsulation
multilayerinsulation
vacuum
vacuum
spacer
protective outer PE-jacket
LHe
returnedGHe
state of the technology
W/m4,55/mQ =&Measurement results: corresponding2W/m8,52q =&
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Improvement actions
Separation of MLI and supporting structures
Solid heat conduction of the supporting structures
as low as possible small contact areas and cross sections
low heat load at the disconnecting points
Multi-layer insulation (MLI) Superinsulation Example: Thermal insulation development for a flexible
cryogenic line
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Multi-layer insulation (MLI) Superinsulation Example: Thermal insulation development for a flexible
cryogenic line
protective outer PE-jacket
welded tubesHTSC-cable(cooled with LN )2
multilayer insulation
bars
supporting rings
vacuum between the
welded tubes
New concept
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Multi-layer insulation (MLI) Superinsulation Example: Thermal insulation development for a flexible
cryogenic line
New concept
bar
part of the welded tube
contact-points
outer welded tube
inner welded tubewith HTSC-cable
multilayer insulation
floating-supportsystems
supporting ring
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Multi-layer insulation (MLI) Superinsulation Example: Thermal insulation development for a flexible
cryogenic line
New concept
outer weldedtube
inner weldedtube
supportingrings
longitudinalbars
vertical connectionof the longitudinal bars
multilayerinsulation
about 1.0 m about 0.1 m
longitudinal cross section of the insulation of the HTSC-cable symmetry line
evacuatedspace
}
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Multi-layer insulation (MLI) Superinsulation Example: Thermal insulation development for a flexible
cryogenic line
Experiments
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10-5 10-4 10-3 10-2 10-1 100
101
102
2
3
4
5
6
78
2
3
4
5
6
78
2
Nexans: straight without weight
Nexans: bendedwithout weightNexans: straight with weightGfK-support structure:straight with weight
GfK- :support structure straight without weightGfK- :support structure without weightbended
spiral :support structure straight with weightspiral support structure straight without weight:
p [mbar]
qk[W/m2]
Nexans GfK-support structure spiral support structure
straight without weight
straight with weight
(lead rod)
bended without weight
Multi-layer insulation (MLI) Superinsulation Example: Thermal insulation development for a flexible
cryogenic line
Experiments
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Multi-layer insulation (MLI) Superinsulation Example: Thermal insulation development for a flexible
cryogenic line
Experiments boundary condition:
]m/W[q 2m& Nexans
3,70
3,17
2,49
100%
85,59%
67,30%
= 14,41%
= 32,70%
GfK-support structure
spiral support structure
straight without weight
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]m/W[q 2m&
6,60
4,72
3,10
100%
139,83%
67,30%
= 39,83%
= 34,32%
~ 430 N/m
Multi-layer insulation (MLI) Superinsulation Example: Thermal insulation development for a flexible
cryogenic line
Experiments
spiral support structure
Nexans
GfK-support structure
boundary condition:straight with weight(lead rod)
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Conclusions
For cryogenics application (T < 120 K), vacuum insulation technology ismandatory
For LHe (4 K) and LH2 (20 K) applications, the use of the best kind ofinsulation, so MLI, is warrantable orjust enoughrespectively
MLI is the best kind of thermal insulation if it is used professional
improvement factors
factor 10 compared to other vacuum insulation materials
factors 30 100 compared to evacuated powder insulation
further improvement factors of ~ 30 are possible by the use of evaporationenthalpy multishield-technique
MLI can be flexible adapted very compact if the accessibility is ensured
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Thank youfor yourattention