ith the collider being divided into 8 underground sectors delimited by the access shafts, the same number of refrigerators is required to keep the LHC at its operating temperature.

For the LHC, two 12 kW capacity refrigerators at 4.5 K, formerly used for the LEP, have been upgraded in order to increase their capacity to 18 kW, and two non standard equipment for the largest new 18 kW machines have been designed and installed on the site. One of the refrigerators was used initially to test all the magnets to be installed at the LHC. Helium is cooled in the refrigerators’ heat exchangers to 80 K by using about 10 000 t of liquid nitrogen (Liquid nitrogen is never directly injected into the LHC to avoid any possible source of asphyxiation in the underground tunnel). But this represents only a first stage.

To guarantee the performance of the accelerator components, helium at 4,5 K must be further cooled to 1.8 K (-271,4°C). Helium at 4,5 K is injected into the magnets’ cold masses. Once the magnets are filled, the refrigeration units bring the temperature down to 1,9 K (-271.3ºC).To achieve this, the vapour pressure over the helium bath which has a boiling point of 4,5 K at atmospheric pressure has to be reduced.

If 15 mbars is to be attained volumetric compressors at ambient temperature need to be combined with hydrodynamic centrifugal compressors operating at low temperatures (Cold Compression System - CSS).

When helium is cooled further it undergoes a second phase change at about 2.17 K (–271.0 °C) to its "superfluid" state. Among many remarkable properties, superfluid helium has a very high thermal conductivity, which makes it the coolant of choice for the refrigeration and stabilization of large superconducting systems Routing the cold The cold power generated is routed to the superconducting magnets via a cryogenic line which has been named QRL. To keep up the supply, the two helium flows (at 4.5 K and 1.8 K) coming from the 18 kW refrigerators and CCS compressors have first of all to be collected. Today, five cryogenic distribution boxes (QUI), five "cryogenic islands", are installed in the access shafts in the LHC tunnel. With their vacuum casing, valves and connections to upstream and downstream devices, they provide the connection between the refrigerated equipment and the accelerator itself.  Since the cryo-magnets are spread over 27 km, the QRL cryogenic distribution line follows the same layout.

The LHC tunnel therefore contains two rings, the first consisting of the magnets and proton accelerating cavities, i.e. the LHC, and the second so-called “cryogenic ring” or QRL, which transmits the cold power needed to keep the magnets at 1.8 K. As with the LHC, the cryogenic line is divided into eight sectors, each of them connected to an 18 kW refrigerator and a cold compression system (CCS) via the distribution boxes (QUI ). To achieve what is in fact a first in the world, i.e. the QRL, which allows 100 tons of liquid helium to be run over a distance of 27 km.

The 27 km long cryogenic ring.