Gas-flow Calculations: Don't Choke
Trey Walters, P.E. Applied Flow Technology - Published in Chemical Engineering, January 2000
Flow of gases in pipe systems is commonplace in chemical-process plants. Unfortunately, the design and analysis of gas-flow systems are considerably more complicated than for liquid (incompressible) flow, due mainly to pressure-induced variations in the gas-stream density and velocity. Here, we review practical principles and present some key equations governing gas flow, and assess several assumptions and rules of thumb that engineers sometimes apply in order to simplify gas-flow analysis and calculations.
In a broad sense, the appropriate term for gas flow is compressible flow. In a stricter sense, however, such flow can be categorized as either incompressible or compressible, depending on the amount of pressure change the gas undergoes, as well as on other conditions.
Accurately calculating truly compressible flow in pipe systems, especially in branching networks, is a formidable task. Accordingly, engineers often apply rules of thumb to a given design situation involving gas flow, to decide whether the use of (simpler) incompressible-flow calculations can be justified. Such rules of thumb are helpful, but they can lead one astray when used without a full understanding ofthe underlying assumptions.