Thermal insulation

Transcription

Thermal insulation

Thermal insulation
Institute for Technical Physics
Holger Neumann
Don‘t be afraid of low temperatures
KIT – die Kooperation von Forschungszentrum Karlsruhe GmbH und Universität Karlsruhe (TH)
www.kit.edu
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
2
Thermal insulation | H. Neumann | March 2009
Relevance of thermal insulation in cryogenics
Cryogenics → ∆T = TEnvironment – TFluid → great value
→ latent heat are very small
→ needed energy input for generating low temperatures is very high
(Carnot)
εC =
TFluid
TEnvironmen t − TFluid
Example 1:
The efficiency of a 4.4 K-refrigerator is about 10% of the CarnotCoefficient 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)
⇒
3
Increase of the operating costs of ~ 1000 €/year
or additional LHe-acquisition costs of ~ 2000 €/year
Overview of different insulation materials
air (1 bar) ~ 2,6 . 10
-2
powder
with small
pieces of
metal foils
MLI
microsphere
vacuum atmospheric pressure
powder
fibreglas
10-6.0
10-5.0
10-4.0
10-3.0
foams, powders
fibres
10-2.0
heat conductivity λ [W/(m . K)] between ~ 300 K - 77 K
4
10-1.0
Multi-layer insulation (MLI) – Superinsulation – Description
MLI is presently the most effective kind of thermal insulation
developed in the fifties by Peterson (Sweden)
first established in the sixties by space industry
MLI consists of:
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
Multi-layer insulation (MLI) – Superinsulation – 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
– Heat transfer calculations
&
&
Q
overall = Qi,i +1 =
+
+
σ
⋅ (Ti4 − Ti4+1) ⋅ (1 − f) ⋅ A i
1
1
+
−1
ε Ti ε Ti+1
radiation
κ +1 α
2 ⋅R
⋅
⋅ pi ⋅
⋅ (Ti − Ti +1) ⋅ (1 − f) ⋅ A i
κ −1 2 − α
8 ⋅ π ⋅ (Ti + Ti +1)
reine
pure Wärmestrahlung
radiation
λ Ti,i+1
s
Wärmestrahlung
radiation and conduction
und -leitung
pure
conduction
reine Wärmeleitung
⋅ (Ti − Ti+1) ⋅ f ⋅ C ⋅ A i
300
275
residual gas heat conduction
250
solid heat conduction
T [K]
225
200
175
150
125
100
5
10
15
N
7
20
25
8
9
– Special characteristics
influence of
contact pressure
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)
mW/(m .K)
effective heat conductivity λ
0.15
5
2
6
0.10
1
4
x
0.05
x
0
10
20
x
30
N/D
10
x
3
40
50 1/cm 60
45
empirical values for different transferlines and cryostats
with 20 - 50 layers MLI between RT and 80 K
(winding technique on tubes and cylinders)
40
3
2,5
35
q [W/m2]
2
25
1,5
20
15
1
10
.
q [W/m2] with 3 blankets (RT - 80 K)
0,5
5
.
0
0
0
200
400
diameter of tube [mm]
11
600
800
q [W/m]
.
.
q [W/m] = q [W/m2]. π.d
30
9
Twarm = 280 K
-6
p < 2 .10 mbar
8
only one aluminium
layer (LN2)
7
q [W/m2]
6
MLI winding technique
5
IR 300.12 MLI blanket technique
open / closed symbols
LHe / LN2 - experiments
4
3
2
2 b lan
ke ts
1 blanket
1
3 blank e
ts
0
0
50
100
150
200
d [mm]
12
250
300
350
400
14
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 mm
Jehier: FZK, THISTA mit d=219 mm
12
10
q [W/m2]
8
6
4
2
0
0
10
20
30
N
13
40
50
– 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
– Example: Thermal insulation development for a flexible
cryogenic line
Requirements on a economic applicable HTS-cable
1⋅
W
m2
≤
q& 300⋅K →80⋅K
≤
2⋅
W
m2
compact design → ∆insulation = 20 mm
1⋅ 10 − 4 ⋅
W
W
≤ λIsolation ≤ 2 ⋅ 10 − 4 ⋅
m ⋅K
m ⋅K
⇒ The use of MLI is mandatory
15
cryogenic line
protective outer PE-jacket
state of the technology
spacer
multilayer
insulation
vacuum
multilayer
insulation
superconducting
cabel
LHe
vacuum
welded tube
returned
(60/66 mm)
GHe
welded tube (100/110 mm)
& /m = 4,55 ⋅ W/m
Measurement results: Q
16
corresponding
q& = 8,52 ⋅ W/m 2
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
cryogenic line
New concept
supporting rings
bars
vacuum between the
welded tubes
protective outer PE-jacket
multilayer insulation
HTSC-cable
(cooled with LN2 )
18
welded tubes
cryogenic line
New concept
outer welded tube
multilayer insulation
bar
inner welded tube
with HTSC-cable
supporting ring
contact-points
part of the welded tube
floating-support
systems
19
cryogenic line
New concept
outer welded
tube
vertical connection
of the longitudinal bars
supporting
rings
longitudinal
bars
}
multilayer
insulation
evacuated
space
inner welded
tube
about 1.0 m
longitudinal cross section of the insulation of the HTSC-cable
20
about 0.1 m
symmetry line
cryogenic line
Experiments
21
cryogenic line Nexans
spiral support structure
GfK-support structure
Experiments
straight without weight
2
102
8
7
6
5
2
qk [W/m ]
4
3
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)
2
101
bended without weight
8
7
6
5
4
3
2
10-5
22
10-4
10-3
p [mbar]
10-2
10-1
100
cryogenic line
straight without weight
Experiments
q& m [ W / m2 ]
boundary condition:
Nexans
3,70
100%
3,17
∆ = 14,41%
85,59%
∆ = 32,70%
2,49
23
67,30%
cryogenic line
straight with weight
boundary condition: (lead rod)
Experiments
2
q& m [ W / m ]
~ 430 N/m
139,83%
6,60
∆ = 39,83%
Nexans
4,72
100%
∆ = 34,32%
3,10
24
67,30%
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 factors
factor ≥ 10
factors 30 – 100
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
25
Thank you
for your
attention
26

Thermal insulation

Transcription

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