This chapter shows that while the pressure drops for compressible isothermal flow and for compressible adiabatic flow are almost identical, they are both higher than that predicted from incompressible flow model by about 8%. From its derivation, it can be seen that the equation holds for any compressible flow (adiabatic, isothermal, flow with or without friction, etc.), as long as the pipe diameter remains constant throughout the length of the pipe. One can determine the pressure drop in the pipe by assuming three different conditions: incompressible flow, compressible isothermal flow, and compressible adiabatic flow. When the pipe is long and/or when the velocity is high, the three models (incompressible, isothermal compressible, and adiabatic compressible) will yield very different results. More about compressible flow of ideal gas through pipe for both subsonic and supersonic flows can be found in fluid mechanics texts.