Don‘t be afraid of low temperatures - IRFM Thermal insulation development for a flexible cryogenic line Conclusions. ... – Example: Thermal insulation development for a flexi ble

KIT – die Kooperation von Forschungszentrum Karlsruhe GmbH und Universität Karlsruhe (TH) www.kit.edu

Thermal insulation

Institute for Technical PhysicsHolger Neumann

Don‘t be afraid of low temperatures

Thermal insulation | H. Neumann | March 2009

KIT – die Kooperation von Forschungszentrum Karlsruhe GmbH und Universität Karlsruhe (TH)

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Content

Relevance of thermal insulation in cryogenics

Overview of different insulation materials

Multi-layer insulation (MLI) – SuperinsulationDescriptionHeat 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)⇒ ε = 0.0015

⇒ The heat load of 100 W at 4.4 K requires a power input of about 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 €/yearor additional LHe-acquisition costs of ~ 2000 €/year

FluidtEnvironmen

FluidC TT

T−

=ε

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 Kλ .

foams, powdersfibres



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Multi-layer insulation (MLI) – Superinsulation – Descript ion

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 foilsspacer 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 poss ible)

Multi-layer insulation (MLI) – Superinsulation – Descript ion



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Multi-layer insulation (MLI) – Superinsulation– Heat transfer calculations

i1iiT

i1ii1ii

i

i4

1i4i

TT

1ii,overall

ACf)T(Ts

Af)(1)T(T)T(T8

R2p

211

Af)(1)T(T1

ε

1ε

1QQ

1ii,

1ii

⋅⋅⋅−⋅λ

+

⋅−⋅−⋅+⋅π⋅

⋅⋅⋅α−

α⋅−κ+κ+

⋅−⋅−⋅−+

σ==

+

++

++

+

+

&&

100

125

150

175

200

225

250

275

300

T[K

]

5 10 15 20 25

N

reine WärmestrahlungWärmestrahlung und -leitungreine Wärmeleitung

pure radiationradiation and conductionpure conduction

radiation

residual gas heat conduction

solid heat conduction



8




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Multi-layer insulation (MLI) – Superinsulation– Special characteristics

influence of contact pressure

1-3: Al layers with fibre glass paper of different thickness4: Dracon Al-metallized with glass silk tissue5-6: theoretical values (without solid heat conduction)

optimum number of layers / density of layers

x

x

xx

0 10 20 30 40 50 1/cm 60

N/D

0.05

0.10

0.15

mW/(m K).

1

2 3

4

56

effe

ctiv

e h

eat c

ond

uctiv

ity λ



11

.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 π. . . .

q [W/m ] with 3 blankets (RT - 80 K)2.




12

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 Kp < 2 10 mbar

warm-6.

only one aluminiumlayer (LN )2

1 blanket

2 blankets

3 blankets

IR 300.12 MLI blanket techniqueopen / closed symbols LHe / LN - experiments 2

MLI winding technique




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0 10 20 30 40 50

N

0

2

4

6

8

10

12

14q

[W/m

2 ]

qrad=f(ewall=0.1; eshield=0.03; TW=300 K; TC=77 K)IHI: JacobIHI: FZKIHI: 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 conditionsperforated layers

MLI with integrated getter materials

Superinsulation only meets this expression and expenditure if several possibilities of errors could be avoided



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Multi-layer insulation (MLI) – Superinsulation– Example: Thermal insulation development for a flexi ble

cryogenic line

Requirements on a economic applicable HTS-cable

compact design → ∆insulation = 20 mm

⇒⇒⇒⇒ The use of MLI is mandatory

2K80K3002 mW

2qmW

1 ⋅≤≤⋅ ⋅→⋅&

KmW

102Km

W101 4

Isolation4

⋅⋅⋅≤λ≤

⋅⋅⋅ −−



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cryogenic line

superconductingcabel

welded tube(60/66 mm)

welded tube (100/110 mm)



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


cryogenic line



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cryogenic line

protective outer PE-jacket

welded tubesHTSC-cable(cooled with LN )2

multilayer insulation

bars

supporting rings

vacuum between thewelded tubes

New concept



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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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cryogenic line

New concept

outer weldedtube

inner weldedtube

supportingrings

longitudinalbars

vertical connectionof the longitudinal bars


about 1.0 m about 0.1 m

longitudinal cross section of the insulation of th e HTSC-cable symmetry line

evacuatedspace}



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cryogenic line

Experiments



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10-5 10-4 10-3 10-2 10-1 100

101

102

2

3

4

5

678

2

3

4

5678

2

Nexans: straight without weightNexans: bended without weightNexans: straight with weightGfK-support structure: straight with weightGfK- : support structure straight without weightGfK- : support structure without weightbended spiral : support structure straight with weightspiral support structure straight without weight:

p [mbar]

q k[W

/m2 ]

Nexans GfK-support structure spiral support structure

straight without weight

straight with weight(lead rod)

bended without weight


cryogenic line

Experiments



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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


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 is mandatory

For LHe (4 K) – and LH2 (20 K) – applications, the use of the best kind of insulation, so MLI, is warrantable or just enough respectively

MLI is the best kind of thermal insulation if it is used professional

improvement factorsfactor ≥ 10 compared to other vacuum insulation materialsfactors 30 – 100 compared to evacuated powder insulation

further improvement factors of ~ 30 are possible by the use of evaporation enthalpy – multishield-technique

MLI can be flexible adapted very compact if the accessibility is ensured



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Thank youfor yourattention

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Don‘t be afraid of low temperatures - IRFM Thermal insulation development for a flexible cryogenic line Conclusions. ... – Example: Thermal insulation development for a flexi ble