Hydraulically Operated Two-Phase Helium Expansion Engine

Abstract

Traditional helium refrigerators and liquefiers have generally contained two or three stages of refrigeration; namely, liquid-nitrogen precooling, one or two reciprocating expansion engines or turbine expanders, followed by a Joule-Thomson (JT) valve. In order to liquefy helium, the JT exchanger has to be precooled well below the inversion temperature, i.e. 30°K [¹]. For useful yields of liquid—10% or more of the JT stream—the gas has to be cooled to at least 15°K. Use of an expansion engine rather than a JT valve increases the maximum precooling temperature which can be used for the liquefaction of helium. For example, a helium gas stream entering a JT heat exchanger at 40° K and expanding through an engine of 90% efficiency can provide a liquid yield of 10%.