Hey Mario, I'd rethink using an air compressor for vacuum hold down on a CNC. You have the sound of the compressor and also the noise of air escaping from a venturi nozzle. My guess is that the escaping air will be a lot louder and more annoying than the noise of the air compressor.
The compressor also needs to be continuous duty rated. As Peter mentioned, venturi nozzle applications are tough on a compressor because they're not designed to run continuously.
This little gem of knowledge is directly from CAT. The "New" SP Series of Ultra Quiet Air Compressor Motors have been designed to increase duty cycle and allow for longer continuous run times. So using this on a CNC that can run continuously for hours may cause a premature melt down.
Also the safety issue raises its ugly head. A venturi setup is an open loop system. If the compressor shuts off for some reason, you lose your vacuum now.
A vacuum pump is a closed loop system, if the vac pump shuts off for some reason, you still have a vacuum supply in the hoses.
I've tried this with my Vac Sys. I've attached a board to the Vac Sys and then shut down the Milwaukee vacuum pump. An hour later the board was still attached to the Vac Sys head. An important consideration when you're going to push the CNC start button and then walk away for some period of time. [smile]