Introduction to cryogenics

This paper aims at introducing cryogenics to non-specialists. It is not a cryogenics course, for which there exist several excellent textbooks mentioned in the bibliography. Rather, it tries to convey in a synthetic form the essential features of cryogenic engineering and to raise awareness on key design and construction issues of cryogenic devices and systems. The presentation of basic processes, implementation techniques, and typical values for physical and engineering parameters is illustrated by applications to helium cryogenics. 1 Low temperatures in science and technology Cryogenics is defined as that branch of physics which deals with the production of very low temperatures and their effect on matter [1], a formulation which addresses aspects both of attaining low temperatures which do not naturally occur on Earth, and of using them for the study of nature or in industry. In a more operational way [2], it is also defined as the science and technology of temperatures below 120 K. The reason for this latter definition can be understood by examining characteristic temperatures of cryogenic fluids (Table 1): the limit temperature of 120 K comprehensively includes the normal boiling points of the main atmospheric gases, as well as of methane which constitutes the principal component of natural gas. Today, liquid natural gas (LNG) constitutes one of the largest—and fastest-growing—industrial domains of application of cryogenics, together with the liquefaction and separation of air gases. The densification by condensation, and separation by distillation of gases was historically—and remains today—the main driving force for the cryogenic industry, exemplified not only by liquid oxygen and nitrogen used in chemical and metallurgical processes, but also by the cryogenic liquid propellants of rocket engines and the proposed use of hydrogen as a ‘clean’ energy vector in transportation. Table 1: Characteristic temperatures of cryogenic fluids [K] Cryogen Triple point Normal boiling point Critical point Methane 90.7 111.6 190.5 Oxygen 54.4 90.2 154.6 Argon 83.8 87.3 150.9 Nitrogen 63.1 77.3 126.2 Neon 24.6 27.1 44.4 Hydrogen 13.8 20.4 33.2 Helium 2.2 4.2 5.2 ∗ λ point. The quest for low temperatures, however, finds its origin in early thermodynamics, with Amontons’s gas pressure thermometer (1703) opening the way for the concept of absolute zero inferred a century later by Charles and Gay-Lussac, and eventually formulated by Kelvin. It is,