Thermodynamics reminder
Helium liquefaction and medium and high energy consumption helium refrigeration.

Before tackling the subject, it is interesting to know the theoretical power required to carry out the refrigeration operation at any temperature: what is the ideal value (i.e. minimum calculated by describing a Carnot cycle) of the power to be expended at room temperature (generally, power taken from the electrical network) to be able to absorb one unit of thermal power dissipated at a temperature lower than room temperature? There are two cases to consider, depending on whether operation is isothermal or non-isothermal.