Neumann Thermalinsulation

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

Thermal insulation

Institute for Technical Physics

Holger Neumann

Dont be afraid of low temperatures


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Thermal insulation | H. Neumann | March 2009

KIT die Kooperation von Forschungszentrum Karlsruhe GmbH und Universitt Karlsruhe (TH)

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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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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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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

.




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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




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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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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 the

welded 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 the 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

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


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 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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Neumann Thermalinsulation