Fluid Network Studio

Compressor and line, a worked set-ratio example

This worked example is a compressor feeding a discharge line, the building block of any system that needs to raise pressure rather than just move air. It is a built-in network in Fluid Network Studio, so you can open it, solve it, and read the discharge temperature and shaft power without building anything first.

The scenario

A compressor takes air at a suction pressure and raises it to a set pressure ratio. The compressed air then travels down a line to a delivery point held at a fixed pressure. Two questions matter here that do not arise with a fan. First, how hot does the air get when you compress it, because discharge temperature drives material limits and any aftercooling you might need. Second, how much shaft power does the duty take. This example answers both.

The real setup

The example uses these parameters exactly as built:

  • A suction boundary at 200 kPa absolute (about 1 bar gauge against the default atmosphere).
  • A compressor between the suction and a junction, set to a pressure ratio of 2.0, so it lifts the air to 400 kPa absolute. Isentropic efficiency 0.75.
  • A line, 150 m long, 60 mm internal diameter, roughness 4.6e-5 m, from the junction to the delivery point.
  • A delivery boundary at 300 kPa absolute.
  • Working fluid set to the gas phase, air at 15 degrees C, gas constant 287.05 J/kg.K, viscosity 1.81e-5 Pa.s.

The physics and method

The compressor is a set-point machine, not a curve-driven one. You fix the ratio and the solver makes it hold. Internally it splits the network at the machine and runs a bracketed root-find on the suction-side flow so the delivered pressure lands where the downstream line and boundary demand. The discharge temperature comes from the isentropic relation corrected by the efficiency you set, so a less efficient machine reports a hotter discharge for the same ratio. The downstream line is solved by the compressible gas kernel in absolute pressure, in pressure-squared, with the Churchill friction factor. The network solves isothermally, and the compressor reports its outlet temperature alongside the hydraulic result.

What the solver computes and what you learn

Solve it and you get the flow the system settles at, the pressure along the line, and on the compressor itself the discharge temperature and the shaft power. The lesson is that the compressor and the line are coupled. The 2:1 ratio sets the discharge pressure, but the line and the 300 kPa delivery boundary decide the flow, and the flow in turn sets the power. Reading the discharge temperature is the quickest way to check whether you need aftercooling before the air enters the line.

Compressible gas is part of the Advanced plan, so solving this example needs that plan.

Take it further

This example sits behind the compressed air system design page, which covers fans versus compressors and sizing the downstream pipe to match. The glossary explains the terms the solver uses. Fluid Network Studio supports your engineering work and does not replace a qualified engineer, and it makes no claim of compliance with any standard.

Open this example in FNS and change the pressure ratio to watch the discharge temperature respond.