Function of a vacuum generator

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How does a vacuum generator work?

Venturidüse de.png

Festo vacuum generators operate according to the venturi principle. The compressed air flows from the pressure supply port into the ejector. The constriction in the venturi nozzle increases the flow velocity of the air to supersonic speed. After exiting the venturi nozzle, the air expands and flows through the receiver nozzle into the outlet port (silencer). In the process, a vacuum is created in the chamber between the venturi and receiver nozzles, which causes air to be drawn in from the vacuum port. The vacuumed air and exhaust air both leave through the outlet port (silencer). 

Why must vacuum generators always be operated with unlubricated compressed air?

If lubricated compressed air is used, dust and dirt particles from the air which is sucked in can settle in the silencer or even in the vacuum generator, thus impairing the suction performance.