Compression and expansion of gases or vapors

Compression and expansion of compressible fluids, gases or vapors, are fundamental operations in the operation of thermal machines such as refrigeration machines or engines, whether these are reciprocating internal combustion engines (gasoline and diesel engines) or continuous flow (gas turbines) or external energy input engines such as steam turbines. In internal combustion engines, the two operations are performed in succession, whereas in steam turbines only expansion takes place, and in conventional refrigeration machines only compression.

These operations correspond to open transformations of a system, in the thermodynamic sense of the term, always involving mechanical energy and, depending on the case, thermal energy. Indeed, the search for maximum work production in the case of gas expansion, or minimum mechanical energy consumption in the case of compression, requires not only that we try to get as close as possible to a reversible process (second principle of thermodynamics), but also that we involve specific heat exchanges (first principle of thermodynamics). Since the simplest case of transformation is adiabatic, we need to know what penalty this type of evolution entails for machine performance.

The aim of this article on applied thermodynamics is to provide answers to all these questions, through a thorough understanding of the basic phenomena involved and a comparative study of the different types of transformations that can be envisaged and carried out. The development of these analyses involves the fundamental principles of thermodynamics and the various corresponding balances, including energy and exergy balances. It leads to the definition of various efficiencies with specific meanings, and the quantification of the advantages of one type of compression or expansion over another.