It seems bizarre, but it is possible to make vacuum with compressed air. Vacuum eductors, sometimes called ejectors, or venturi use a flow of compress through a specialized nozzle to create vacuum without a mechanical vacuum pump. The process in its simplest form uses no moving parts, so it is very reliable. It is also very energy intensive; like most compressed air powered devices, a vacuum venturi consumes about 10 times the power of a mechanical vacuum pump for the same flow rating. This is not the fault of the generator, but reflects on the inefficiency of compressed air.
Due to the high cost of compressed air, it is wise to always assess your vacuum generators for cost. Is it a continuous requirement or intermittant. Often, continuous duty requirements can best be supplied using a mechanical vacuum pump.
Consider the vacuum educator pictured in the photograph in Figure 1. This unit consumes a continuous 50 cfm and runs on a 24-hour x 7 day per week duty cycle, feeding sand to a silo in a metal foundry. It consumes about $10,000 of electricity per year. Use of a mechanical vacuum source, or even better, a mechanical sand elevator could save substantial annual cost.
On the other hand, low duty cycle applications like the vacuum pick-up devices in Figure 2 can often be more economically supplied by compressed air powered generators. With proper control, vacuum pick up can be set to use minimal compressed air during pick up operation and turn off the vacuum when not required. More sophisticated vacuum venturi can also be purchased with a built-in sensor that shuts off the flow of compressed air when the vacuum level within the pick-up cups is adequate. If this application was suppled by a dedicated mechanical vacuum pump, which would likely run continuously, the electrical cost would be higher.
When generating vacuum from compressed air, it is best to consider the duty cycle of the application. With good control, the cost can be reduced substantially — even with compressed air powered generators.
