Liquefaction and Refrigeration Systems

Although the emphasis of this book is on the development of physical understanding of helium as a cryogenic fluid, the discussion would be incomplete without reviewing the various methods of obtaining low-temperature helium. Such methods are based solidly in engineering thermodynamics and rely primarily on a combination of processes which make up a cycle. A cycle consists of a closed circuit where the working fluid, for example helium, is compressed, expanded, and heat exchanged in such a way as to achieve cooling. The simplest and most thermodynamically ideal cycle is the Carnot cycle which consists of a combination of isothermal and isentropic processes. The Carnot cycle is difficult to achieve in a practical system. However, real refrigeration and liquefaction systems are made up of similar processes which are usually compared in their performance to that of the ideal Carnot cycle. In all gas refrigeration systems, the cooling is achieved by an expansion process. There are essentially two types of expansion which are applied. Isentropic expansion is that where the fluid does work and expands keeping its entropy constant (ΔS = 0). This is the best method of expansion because it produces the largest temperature change over a given pressure change. The other expansion process is isenthalpic expansion,where the fluid undergoes a pressure change without heat transfer (ΔQ = 0). This method is in common application in practical refrigeration systems for its ease of use. However, it is of lower thermodynamic efficiency because it is an irreversible process leading to nonidealities.