INTRODUCTION TO CRYOGENIC SYSTEMS

Introduction

The word cryogenics means, literally, the production of icy cold; how­ever, the term is used today as a synonym for low temperatures.
The point on the temperature scale at which refrigeration in the ordinary sense of the term ends and cryogenics begins is not sharply defined. The workers at the National Bureau of Standards at Boulder, Colorado, have chosen to consider the field of cryogenics as that involving temperatures below -150°C (123 K) or - 240°F (220⁰R). This is a logical dividing line, because the normal boiling points of the so-called permanent gases, such as helium, hydrogen, neon, nitrogen, oxygen, and air, lie below - 150°C, while the Freon refrigerants, hydrogen sulfide, ammonia, and other con­ventional refrigerants all boil at temperatures above -150°C.

In the field of cryogenic engineering, one is concerned with developing and improving low-temperature techniques, processes, and equipment. As contrasted to low-temperature physics, cryogenic engineering primarily involves the practical utilization of low-temperature phenomena, rather than basic research, although the dividing line between the two fields is not always clear-cut. The engineer should be familiar with physical phenomena in order to know How to utilize them effectively; the physicist should be familiar with engineering principles in order to design experiments and apparatus.

Present areas involving cryogenic engineering

Present-day applications of cryogenic technology are widely varied, both in scope and in magnitude. Some of the areas involving cryogenic engineering include:

1.Rocket propulsion systems. All the large United States launch vehicles use liquid oxygen as the oxidizer. The Space Shuttle propulsion system uses both cryogenic fluids, liquid oxygen, and liquid hydrogen.

2.Studies in high-energy physics. The hydrogen bubble chamber uses liquid hydrogen in the detection and study of high-energy particles produced in large particle accelerators.

3. Electronics. Sensitive microwave amplifiers, called masers, are cooled to liquid-nitrogen or liquid-helium· temperatures so that thermal vibrations of the atoms of the amplifier element do not seriously interfere with absorption and emission of microwave energy. Cryogenically cooled masers have been used in missile detectors, in radio astronomy to listen to faraway galaxies, and in space communication systems.