Introduction

The word cryogenics is defined as the study of low-temperature phenomena. However, the temperature level at which refrigeration in the conventional sense ends and cryogenics begins is somewhat arbitrary. The temperature separating cryogenics from conventional refrigeration that is suggested by the workers at the National Institute for Standards and Technology in Boulder, Colorado, is −150°C (123 K) or −238°F (222°R) (Scott 1958, p. 1). This selection is logical because the normal boiling points (NBPs) of fluids that are important in the cryogenic industry, including helium, hydrogen, nitrogen, oxygen, and air, all lie below −150°C. The fluids commonly used in domestic refrigerators, air conditioners, and freezers all boil at temperature above −150°C at atmospheric pressure.