LHC refrigeration unit @ 1.8K

The 1.8K refrigeration units of the LHC allow to pump helium until 14.5 mbar in order to decrease the temperature of saturated helium from 4.5K until 1.8K following the saturation line. They are coupled to the 4.5K refrigerators via the line C of the cryogenic distribution line (QRL) which provide liquid helium at 4.6K under 3bar and they are connected to the line B of QRL.

Along the LHC ring, there are 8 refrigeration units providing a refrigeration power of 2400W at 1.8K. Four of them were built by the french company Air-liquide and the others were built by German company Linde in cooperation with the Japanese company IHI. The Air-Liquide system uses three cold-compressors in series to pump cold helium until 14.5 mbar and two oil lubricated screw compressors in parallel to compress warm helium until 3.2 bar. Two heat exchangers are used to transfer heat from the high pressure side to the low pressure side and one turbine allows to expand helium at 1bar. The Linde system works on the same principles but four cold-compressors are used and the warm compressors are in series.