What is the difference in sliding valve and VSD Compressors chillers and what the advantages and disadvantages of both?

I work in the field of compressed air, but I 've never seen a device operating with a valve propotionel , but I know lthough VOD , to come to your question , both are designed to give you a range of reduced pressure ( almost constant) , but the advantage for a VSD compressor , this is the economy of energy that can go up to 40 %