Thermal insulation. Don t be afraid of low temperatures. Institute for Technical Physics Holger Neumann

Transcription

1 Thermal insulation Institute for Technical Physics Holger Neumann Don t be afraid of low temperatures

2 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

3 Relevance of thermal insulation in cryogenics Cryogenics ε C = T T Environment T = T Environment T Fluid great value latent heat are very small needed energy input for generating low temperatures is very high Fluid (Carnot) T Fluid Example 1: The efficiency of a 4.4 K-refrigerator is about 10% of the Carnot- Coefficient of Performance (COP) ε = The heat load of 100 W at 4.4 K requires a power input of about 70 kw Example : 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 ~ 000 /year 3

4 Overview of different insulation materials air (1 bar) ~, MLI powder with small pieces of metal foils vacuum atmospheric pressure microsphere powder fibreglas foams, powders fibres heat conductivity λ [W/(m. K)] between ~ 300 K - 77 K 4

5 Description MLI is presently the most effective kind of thermal insulation developed in the fifties by Peterson (Sweden) MLI consists of: first established in the sixties by space industry reflecting layers reduction of heat transfer due to radiation spacer elements with low heat conductivity between the reflecting layers high vacuum prevention of convection minimisation of heat conduction of residual gas 5

6 Description 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) 6

7 Heat transfer calculations Q& overall = Q& i,i+ 1 = 1 ε T i σ 1 + ε T i+ 1 (T 1 4 i T 4 i+ 1 ) (1 f) A i radiation κ κ 1 + λ T i,i+ 1 s α α (T T i p i+ 1 solid heat conduction i R 8 π (T + T ) f C A i i i+ 1 (Ti T ) residual gas heat conduction i+ 1 ) (1 f) A T [K] i reine pure Wärmestrahlung radiation Wärmestrahlung radiation and conduction und -leitung reine pure Wärmeleitung conduction N 7

8 Heat transfer calculations 8

9 Heat transfer calculations 9

10 Special characteristics influence of contact pressure mw/(m. K) optimum number of layers / density of layers 1-3: Al layers with fibre glass paper of different thickness 4: Dracon Al-metallized with glass silk tissue 5-6: theoretical values (without solid heat conduction) 0.15 effective heat conductivity λ x x 1 3 x 4 x /cm 60 N/D 10

11 Special characteristics empirical values for different transferlines and cryostats with 0-50 layers MLI between RT and 80 K (winding technique on tubes and cylinders).. q [W/m] = q [W/m ]. π. d 3,5 q [W/m ] ,5 1 q [W/m] q [W/m ] with 3 blankets (RT - 80 K) 0, diameter of tube [mm] 0 11

12 Special characteristics T warm = 80 K p < mbar only one aluminium layer (LN ) q [W/m ] blankets MLI winding technique 3 blankets IR MLI blanket technique open / closed symbols LHe / LN - experiments d [mm] 1 blanket 1

13 Special characteristics q rad =f(e wall =0.1; e shield =0.03; T W =300 K; T C =77 K) IHI: Jacob IHI: FZK IHI: Ohmori [199] Jehier: FZK, TESSI mit d=30 mm Jehier: FZK, THISTA mit d=19 mm 8 q [W/m ] N 13

14 interim conclusion Superinsulation only meets this expression and expenditure if several possibilities of errors could be avoided 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 14

15 Example: Thermal insulation development for a flexible cryogenic line Requirements on a economic applicable HTS-cable W 1 q& 300 K 80 K m W m compact design insulation = 0 mm W m K λ Isolation 10 4 W m K The use of MLI is mandatory 15

16 Example: Thermal insulation development for a flexible cryogenic line state of the technology superconducting cabel multilayer insulation spacer vacuum multilayer insulation protective outer PE-jacket LHe vacuum welded tube returned (60/66 mm) GHe welded tube (100/110 mm) welded tube (130/143 mm) welded tube (198/0 mm) Measurement results: Q& /m = 4,55 W/m corresponding q& = 8,5 W/m 16

17 Example: Thermal insulation development for a flexible cryogenic line 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 17

18 Example: Thermal insulation development for a flexible cryogenic line New concept supporting rings bars vacuum between the welded tubes protective outer PE-jacket multilayer insulation HTSC-cable (cooled with LN ) welded tubes 18

19 Example: Thermal insulation development for a flexible cryogenic line New concept outer welded tube multilayer insulation bar supporting ring inner welded tube with HTSC-cable contact-points floating-support systems part of the welded tube 19

20 Example: Thermal insulation development for a flexible cryogenic line New concept outer welded tube supporting rings vertical connection of the longitudinal bars longitudinal bars multilayer insulation } evacuated space inner welded tube about 1.0 m about 0.1 m longitudinal cross section of the insulation of the HTSC-cable symmetry line 0

21 Example: Thermal insulation development for a flexible cryogenic line Experiments 1

22 Example: Thermal insulation development for a flexible cryogenic line Experiments Nexans GfK-support structure spiral support structure straight without weight q k [W/m ] Nexans: straight without weight Nexans: bended without weight Nexans: straight with weight GfK-support structure: straight with weight GfK- support structure: straight without weight GfK- support structure: bended without weight spiral support structure: straight with weight spiral support structure: straight without weight straight with weight (lead rod) bended without weight p [mbar]

23 Example: Thermal insulation development for a flexible cryogenic line q& Experiments m [W / m ] 3,70 Nexans boundary condition: 100% straight without weight spiral support structure = 14,41% 3,17 85,59% = 3,70% GfK-support structure,49 67,30% 3

24 Example: Thermal insulation development for a flexible cryogenic line Experiments & m [W / m ] q 6,60 spiral support structure boundary condition: 139,83% straight with weight (lead rod) ~ 430 N/m Nexans = 39,83% 4,7 100% GfK-support structure = 34,3% 3,10 67,30% 4

25 Conclusions For cryogenics application (T < 10 K), vacuum insulation technology is mandatory For LHe (4 K) and LH (0 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 factors factor 10 factors compared to other vacuum insulation materials 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 5

26 Thank you for your attention 6