Helium Liquefaction System

Motivation

Because Helium is very expensive on the open market, the 2nd Institute of Physics of the RWTH Aachen University runs a helium liquefaction system, that can recover used Helium.The liquefaction systems are situated in a separate building next to the Physikzentrum in order to keep the mechanical vibrations of the laboratories as low as possible.
In scientific research liquid helium is necessary as a cooling medium to reach very low temperatures. 4He condensates at a temperature of 4,2 K. By evaporation cooling it is even possible to reach temperatures down to 800 mK.

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Building of the gas liquefaction systems

Liquefaction system with gas purification stages
Liquid helium reservoir (4200 liters) with filling station
High performance compressors of the liquefaction system

Liquefaction

The commercially available helium liquefation system of the company “Linde” has an output-flow of up to 22 liters helium per hour.In combination with liquid nitrogen pre-cooling the production rate reaches 30 liter per hour. First the helium gas is purified from extrinsic gases. After that it is cooled down in three temperature steps by adiabatic expansion to at least 4,2 K. The liquid helium is stored in a 4200 liter reservoir. From the reservoir the helium is filled into 100 liter dewars which are carried to the helium consumers at the institutes of the RWTH. Beneath the physical institutes, also several chemical and semiconductor institutes are supplied with our liquid helium. The annual liquefaction rate is more than 85000 liters.

Helium recovery system

A huge network of pipelines is connecting the helium users (laboratories) of the whole Physikzentrum with the new helium recoverysystem. It consists of a 30000-liter gas balloon serving as a helium buffer and two high-pressure compressors. The helium is taken from the balloon and is compressed at 200 bar fo the storage in 29 bundles of gas bottles. This storage unit contains almost 3.5 million liters of helium gas. The balloon is always filled more than one third in order to keep a constant overpressure of 5 mbar in the pipelines. This avoids the contamination of the system by air or other gases. The helium liquefaction system is fed by the gas bundles.
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Helium balloon filled with 30 m³ helium gas

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High pressure compressors with gas cleaning system and gas bundles in the background

Losses

There are different reasons for helium losses in the circuit (precooling of the cryostats, gas-purification, diffusion while storing in gas-bundles, leakages, etc.). This part is replaced by purchasing helium from the open market. The leakage rate of the whole system is about 5 % helium gas per year.