Motivation:

In scientific research 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 1 K. Because Helium is very expensive on the open market, the II. Institute of Physics of the RWTH Aachen University runs a helium liquefaction system, that can recover used Helium.

Helium recovery system:

A hugh network of pipelines connects the helium users with the helium recovery system. Therefore the evaporated helium is collected at the experiments and is guided through the pipelines to a 20m³ gasballoon. After that the helium is compressed at a pressure of 200 bar and is stored in 8-10 bundles of gas bottles. The balloon is always filled to one third so that there is a constant overpressure of 40- 50 mbar on the return pipes. This avoids the contamination of the system by air or other gases. The liquefaction system is fed by the gas bundles.

Liquefaction:

The commercially available helium liquefation system of the company "Linde" has an output-flow of up to 22 liters helium per hour. First the helium gas is purificated from extrinsic gases. After that it is cooled down in two steps by adiabatic expansion to at least 4,2 K. The liquid helium is stored in a 2000 liter reservoir. From the reservoir the helium is filled into 100 liter dewers 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.

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.