Such ejectors are known for instance from German Patent DE 43 02 951 C1. Such ejectors operate on the Venturi principle. The filtered, lubricant-free compressed air flows into the ejector via a connection neck and reaches the propellant nozzle, where the flow velocity of the air acting as a propellant gas is exceeded to supersonic speed in the cross-sectional reduction. After emerging from the propellant nozzle, the air expands and flows via a receiver nozzle into the expansion portion and from there out into the open, optionally via a muffler. In the process, a negative pressure is created in the chamber surrounding the propellant nozzle, and this causes air to be aspirated via the suction connection. The aspirated air and the propellant gas introduced into the ejector emerge jointly into the open via the expansion portion.
Ejectors of this kind have the advantage over vacuum pumps of not having any rotating parts and therefore of requiring little maintenance. They also have low wear. They are also explosion-proof, since as a rule they are pneumatic in nature. Furthermore, they are simple in design and can be installed in an arbitrary position.
Also, they develop no heat and can be turned on and off at any time, which saves energy. In addition, the vacuum can be built up faster because of short line lengths between suction cups and the ejector. Finally, the compact design, low weight, and the possibility of combining multiple functions in one device, along with economies in the area of construction, work preparation, procurement, mechanical machining, installation, startup, and spare parts inventory, all play important roles.
For fast decay of the vacuum and ejection of the workpiece, in special ejectors the suction connection can be occupied by compressed air, so that an overpressure is applied to the workpiece instead of a negative pressure. However, it has been found that above all when there are high resistances in the suction connection, especially with long connecting lines between the ejector and a suction cup, for instance, engaging the workpiece, the compressed air does not escape, or only a negligible portion of it escapes, via the suction connection and instead it escapes through the receiver nozzle via the suction conduit. In other words, a pneumatic short circuit is created. The attempt has been made to overcome this disadvantage with check valves, but this has led to unsatisfactory results.