Guy Gistau-Baguer

Air liquide 1.8 K refrigeration units for CERN LHC project

The Large Hadron Collider (LHC) will be CERN’s next research instrument for high energy physics. This 27 km long circular accelerator will make intensive use of superconducting magnets, operated below 2.0 K. It will thus require high capacity refrigeration below 2.0 K [1, 2]. Coupled to a refrigerator providing 18 kW equivalent at 4.5 K [3], these systems will be able to absorb a cryogenic power of 2.4 kW at 1.8 K in nominal conditions. Air Liquide has designed one Cold Compressor System (CCS) pre-series for CERN-preceding 3 more of them (among 8 in total located around the machine). These systems, making use of cryogenic centrifugal compressors in a series arrangement coupled to room temperature screw compressors, are presented. Key components characteristics will be given.

High Power Refrigeration at Temperatures Around 2.0 K

Publisher Summary This chapter reviews the possible technologies for pumping on the liquid helium bath. It emphasizes on cryogenic dynamic compressors, their bearings and their wheels. It analyzes arrangement of the compressors and their behavior during transient situations. In order to achieve the required high compression ratios, pumping on the liquid helium bath can be made at room temperature, at cryogenic temperatures or partially at cryogenic then at room temperatures. The equipment to be cooled at temperatures around 2.0 K generate heat loads which are lower during periods when the system is not operating at nominal. During turn-down situations it is generally required that the pressure on the helium bath is kept constant, so the compression ratio is to be kept constant when the mass flow rate is lower. For volumetric machines, there is no problem. However, the situation is delicate with dynamic compressors. H. Quack proposes to allow the discharge pressure of the dynamic compressors to decrease with the mass flow, in combination with temperature stabilization at the inlet of the third stage compressor. The disadvantage is to have part of the room temperature system operating at sub-atmospheric pressure. CERN proposes to operate at a lower first stage suction pressure. There is still a significant amount of work to be done in this area.

Two Large 18 KW (Equivalent Power at 4.5 K) Helium Refrigerators for Cern’s LHC Project, Supplied by Air Liquide

CERN in Switzerland has decided to build a new accelerator project called LHC (Large Hadron Collider). This 27 km long accelerator will, for the first time at a such large scale, operate superconducting magnets and radiofrequency cavities. For that purpose Air Liquide is now building two custom designed refrigerators of cryogenic power 18 kW equivalent at 4.5 K. The thermodynamical cycle, chosen to fit the LHC cryogenic loads with a very high efficiency, is discussed. A special emphasis is put on the cold end that makes use of a cryogenic expansion turbine discharging into the double phase domain. As these refrigerators are designed to be able to be operated at reduced cryogenic power with reduced electrical power consumption, chosen solutions to adapt the refrigerator operation to reduced load are described.

A High Reliability Gas-Driven Helium Cryogenic Centrifugal Compressor

A helium cryogenic compressor was developed and tested in real conditions in 1996. The achieved objective was to compress 0.018 kg/s Helium at 4 K @ 1000 Pa (10 mbar) up to 3000 Pa (30 mbar). This project was an opportunity to develop and test an interesting new concept in view of future needs. The main features of this new specific technology are described. Particular attention is paid to the gas bearing supported rotor and to the pneumatic driver. Trade off between existing technologies and the present work are presented with special stress on the bearing system and the driver. The advantages are discussed, essentially focused on life time and high reliability without maintenance as well as non pollution characteristic. Practical operational modes are also described together with the experimental performances of the compressor. The article concludes with a brief outlook of future work.

Operating a Train of Cryogenic Centrifugal Compressors at Lower Flow Rates and Constant Compression Ratio

Cryogenic centrifugal compressors are presently the only «industrial» machines used to reach temperatures lower than 4.4 K (Tore Supra, Fermilab, TJNAL)1,2,3. When the full cryogenic power is not needed, the processed flow is reduced. But the flexibility of a centrifugal compressor train is limited: reducing the processed flow rate leads to troubles (stall) and subsequently, surge. One of the easiest way to avoid such a situation is to inject electrical power into the liquid helium bath in order to provide the compressor train with a constant flow rate. This is not a very satisfactory procedure on a thermodynamical point of view... Other solutions have been proposed which generally lead to the operation of some parts of the room temperature helium circuits at sub atmospheric pressure, which is not consistant with a very long term reliable operation. Operating two parallel trains, each one processing half the nominal flow is another solution not very much flexible but which investment is roughly doubled.