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Cryosystems

The thermal insulation of all systems is critical for the storing and transfer of liquified gases (e.g. liqufied nitrogen at -196 °C). The most effective insulation is vacuum with multilayer insulation foils.

This method is superior to all other techniques as it is demonstrated by the comparison below.

F o a m i n s u l a t i o n 0,035 W/mK V a c u u m p a n e l 0,004 W/mK Perlite with vacuum 0,001 W/mK High vacuum with refl. foils 0,00025 W/mK
Heat loss (Watt)---> 1 m pipe length, 30 mm diameter 20 16 4 1
Insulation thickness (cm) 10 1 1 1

  Precondition for a first class transfer line:

- vacuum <1x10E-5 mbar in cold condition
- vacuum stable for min. 2 years
- 30 layers super insulation
- vacuum checkable
- no cold spots on outer surface

Vacuum insulated transfer lines are the industry standard for the transfer of liquified gases.
If these systems are flexible in addition, they give excellend property for use.

 

Above you can see a typical transfer line.

 

VACUUM
A very low vacuum of less than 1x10E-5 mbar is important for a good insulation behaviour. At a vacuum of 1x10E-4 mbar the insulation is reduced by more than 50%. At 1x10E-3 mbar the insulation is reduced by several factors.
For long term vacuum it is not sufficient to pump it just down to a value of about 1x10E-5 mbar or lower. The use of getter, molecular sieve or charcoal helps to have a good vacuum behaviour for 10 years or longer.

Pressure Range Pressure (mbar) Molecules per cm Mean Free Path
Atmospheric pressure 1013,25 2,7 x 10E19 68 nm
Low vacuum 300 -> 1 10E19 -> 10E16 0,01 -> 100 m
Medium vacuum 1 -> 10E-3 10E16 -> 10E13 0,1 -> 100 mm
High vacuum (HV) 10E-3 -> 10E-7 10E13 -> 10E9 100 mm -> 1 km
Ultra high vacuum (UHV) 10E-7 -> 10E-12 10E9 -> 10E4 1 -> 10E5 km
Extremely high vacuum <10E-12 <10E4 >10E5 km

Examples
Vacuum cleaner 800 10E19 70 nm
Rotary pump 1 -> 10E-3 10E16 -> 10E13 100 m -> 10 cm
High vacuum insulated pipe 10E-4 -> 10E-5 10E12 -> 10E11 1 m -> 10 m
Pressure on the moon ~10E-11 4 x 10E5 10000 km
Interplanetary space 10
Interstellar space 1

Vacuum Gauges

Pirani
Penning
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Spinning Rotor Gauge
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Flexible transfer lines are the best choice for the new High Temperature Superconducting Cables (HTSC).

For some years HTS Cables can be manufactured to very long length with a working temperature at about -200°C.

 

What means LNG?
If you cool down natural gas to about -163°C, it will become liquid. It is known as LNG (liquid natural gas). The volume is 1/600 of gas, the weight is 45% of water. Natural gas is 90% methan, rest is ethan, propan and others.

Natural gas is the fuel of the future, it is environmental friendly. Ferries will be driven by methan instead of diesel.

Using of LNG
At a temperature of -163°C tanks and tube systems should have a good insulation. Vacuum insulation plays an important roll. Flexible double tube systems can be used for insulation and for safety requirements.

  Data on Liquefied Gases

Heat Conductivity

LN2 / N2 phase

Material Heat Conductivity W/(m*K)
Edelstahl 15
Glas 0,76
Glaswool 0,035-0,05
Polyurethan 0,024-0,035
Cork 0,035-0,046
Air 0,0261
Vacuum panel 0,004-0,006
Multilayer-Vacuum Insulation 0,00025

1 Watt -> 18,2 g/h Evaporation of liquid nitrogen

1 Watt -> 173,5 g/h Evaporation of liquid helium

Liquid for cooling N2 H2 He
Metal (Stainless Steel) after cooling 77 K 20,4 4,2 K
Metal (Stainless Steel) before cooling 300 K 300 K 300 K
Use of kg liquid / kg metal kg liquid / kg metal kg liquid / kg metal
Evaporation 0,43 0,2 4,2
Evaporation + enthalpie of cold gas 0,27 0,036 0,1
Data: J.B. Jacobs (1962) Adv. Cryog. 8.529

Attention: Data sheet does not correspond to calculated data!!!