# Centrifugal Compressor selection

I am currently trying to select a centrifugal compressor for a wind tunnel project.

The requirement is that the flow velocity in the test section is Ma 0.1. The geometry of the wind tunnel has already been defined, so all the dimensions of the contraction nozzle, the test section, and the diffuser are known. With this requirement, a volumetric flow rate of around 3900 m3/h is determined.

The way I'm currently trying to approach this is through static pressure calculations at every point in the wind tunnel (using Bernoulli's equation with no losses and continuity A1v1 = A2v2).

I came across this specific compressor curve:

The y-axis represents the static pressure and the x-axis represents the volume flow rate of the centrifugal compressor.

From the graph, a flow rate of 3900 m3/h gives about 1250 Pa of discharge static pressure. I'm assuming this means that the static pressure of air at the compressor inlet will increase by 1250 Pa at the outlet.

I have a couple of questions...

1. How does one select a compressor using this curve?
2. What does the static pressure at the diffuser outlet (also assumed the compressor inlet) and the compressor's outlet tell about the selection of the compressor? In other words, how does calculating them help in selecting the compressor?
3. Does the compressor's pressure ratio actually matter in maintaining the velocity required at the test section or is it just the rpm? Like would using another compressor which has the same rpm but a lower pressure ratio affect the flow velocity at the test section?
• you'll need to know the pressure needed to push the required air velocity through your wind tunnel, at the reference point in the tunnel cross section, at extremes of applicable temperature & pressure, and with installed equipment etc. Then simply read off the chart, the ensure the advertised volumetric flow is sufficient (pressure is the vertical axis apparently). good idea to leave oneself a comfortable safety margin Commented Jan 19 at 0:17
• it means that if the discharge static pressure is 1250 PA then it will discharge 3900 m3/h. In the US we would call this a fan, not a compressor Commented Jan 19 at 0:27
• This is a curve for a compressor. You use it to determine whether the compressor can meet your needs. If the pressure and flow rate point you need is below the curve, you may be able to use the compressor by bleeding off the excess. Generally one tries to have a lot of excess from a bare minimum condition to be able to simulate a range of velocities...
– Abel
Commented Jan 19 at 1:35