I was seeing the specifications of several ASU(Air start unit)s and found out that makers of such machines provide the flow rate of the ASU in PPM. Why? why not Volumetric flow rate?
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PPM stands for Pounds Per Minute (not to be confused with Parts Per Million, which also uses PPM), which is a unit of mass flow rate.
The specification gives the mass flow rate rather than the volumetric flow rate because the mass of air going in and out of the ASU is the same, where the volume going in and out is different because of the compression.