Cryocooler based helium liquefier using an active thermosiphon process

Abstract

A helium liquefier with a production capacity of 17.4 L per day at standard temperature and pressure has been developed using a 1.5 W @4.2 K Gifford McMahon cryocooler to study the heat transfer dynamics between the cryocooler cold stages and flowing helium gas. Heat transfer studies are normally long drawn processes which require use of a large quantity of helium gas in the conventional mode of such liquefier. To minimise the usage of the precious helium gas, a thermosiphon loop connecting the liquid helium zone to the room temperature helium inlet line has been added to the system. A heater is used to evaporate liquid helium in the thermosiphon collection bottle and to re-circulate the evaporated helium gas from liquid to warm inlet gas on a continuous basis. A large number of temperature sensors were placed on the wall of the cryocooler and the liquefier to measure the thermal response of the system. In the 2nd stage regenerator region of the cryocooler, natural convection has been proposed to be the dominant mode of heat transfer between the cryocooler wall and the helium gas. Finite element analysis was carried out to understand the heat transfer process in this region. The result was validated against the observed thermal response. In this paper we present our studies on the liquefier, based on temperature response during the cool down time, system characterisation under various heat loads in the thermo-siphon region, and an FEA analysis of the heat transfer process in the 2nd stage regenerator zone.