increasing compressor efficiency

Use a multi step compression with inter-coolers.

Reduce frictions as well in the gas path as in the mechanical part of the machine. This is depending on the original design and not easy to do.

There are some good information here: http://www.ifps.org/Education/WhitePapers/index.htm

that may give you some pointers. Pay particular attention to LEAKS or other Blow Off applications.

Even simple things like, use a Pressure Regulator to reduce pressure to actuators can reduce air consumption. I often use lower reduced pressure to return cylinders when no work is needed. Cut pressure in half and the compressor only uses half the volume. Does'nt sound like much but check out some cylinder manufacturers charts to see the SCFM savings.

The cost of the added regulator is quickly paid back in compressor running cost.

Or, even simpler, reduce the pressure on any running circuit that has had pressure increased since someone thinks it makes more, better, faster production.

I would like to add a comment. What I mentioned before concerns only the compressor (I respected the question as it is) if you want to optimise the whole system it would be better to use a variable speed drive ( frequency variation of the supply for the asynchronous motor) and adapt the air copression to the real need of your circuit. The overal economy is impressive.

thanks a lot for the information and the help!

Take a look at this article on Air Compressor Efficiency.

http://www.p2pays.org/ref/32/31312.pdf