Continuous flow of gases and vapors
Fluid flow in piping systems

Compressible fluids (gases and vapors) behave differently from liquids, particularly in their flow. Liquids are generally considered to have a constant density ρ, independent in particular of pressure; and, more often than not, their temperature hardly varies, in the absence of heat transfer.

Gases, on the other hand, are subject to significant variations in density, due to pressure variations caused, for example, by pressure drop or velocity variations in the flow, and associated with temperature variations. With the exception of short piping lengths (see § 2.1.4