HELIUM LIQUEFACTION AND RELIQUEFACTION

Definitions:

LIQUEFACTION

transforms 300 K gas into 4.2 K liquid. (This requires 1564 Joules of heat removed per gram liquefied).

RELIQUEFACTION

transforms 4.2 K gas into 4.2 K liquid. (this requires 21 Joules of heat removed per gram reliquefied).

The cooling capacity required to keep an apparatus cold usually can not be fully expressed as a single number. For example, a typical superconducting magnet system may require:

• 5 W at 77 K
• 1 W at 15 K
• 1 W at 4.2 K

Here, three numbers rather than a single number must be used to describe the system.

Note that the cooling capacity of the cold gas (as it warms) is 1564 J/g spread over the temperature range 4.2 K to 300 K. Compare this to only 21 J/g latent heat of evaporation at 4.2 K.

The cooling capacity of the evaporating helium gas as it warms from 4.2K to 300K is used to cool heat shields, dewar neck and magnet current leads. This is why the design of the dewar neck, neck plug, magnet leads and heat shields is critical. If neck flow is required, for example to cool magnet leads, then a liquefier is needed.

QUANTUMCOOLER COLD FINGER HELIUM RELIQUEFIER

A copper cold finger cooled to 4.0 K (0.2 K below the boiling point) is inserted into the top of the dewar. The helium stays in the dewar. There is no boil-off of liquid helium--a zero boil-off system.

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