The common marketing statements by the Air-compressor manufacturers is that  to save Energy, consumed by the compressed air installation, is to install a VSD air-compressor !

What is not said that :

A centrifugal compressor is designed for fixed speed and not suited for VSD.  Exception for high speed turbos as they can be used to reduce the heavy losses at blow off.

An Oil-lubricated screw air-compressor has such a bad efficiency at part load that it needs to be reduced.  Therefor the VSD screw compressor does not save energy but reduces the losses !

An Oil-free screw compressor is not suited for lower speed than the design speed due to the low volumetric efficiency at lower speed

A VSD piston compressor has no benefits as the reciprocating compressor is already very efficient a partial load.

The Frequency converter continuously consumes 5% of the total consumed power.

On top, the Frequency converter itself has a short lifetime and costly to repair !

Wouldn't you think twice before installing a VSD ?