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Venturi-type vacuum pumps
Many machines that require vacuum also use compressed air. And if vacuum is required only intermittently, the compressed air that already is available can be used to generate vacuum through a device called a vacuum generator, also known as a vacuum ejector or vacuum pump. Furthermore, the compressed air also can be used in combination with a vacuum cup by producing a puff of air to hasten release of the workpiece.
Vacuum generators operate based the venturi principle, Figure 4. Filtered, non-lubricated compressed air enters through inlet A. A diffuser orifice (nozzle), B, causes the air stream to increase in velocity, thereby lowering its pressure, which creates a vacuum in channel C. The air stream exhausts to atmosphere through muffler D.
Figure 4. Venturi-type vacuum generator produces vacuum from stream of compressed air. Most recent designs pull vacuum to 27 in.-Hg from a source of compressed air of less than 50 psig.
Vacuum generators offer several advantages. They are compact and lightweight, so they often can be mounted at or near the point of use. They are inexpensive, and because they have no moving parts, do not require the maintenance associated with mechanical vacuum pumps. They do not need an electrical power source because they generate vacuum by tapping into an existing compressed air system. However, if retrofitted into a machine, capacity of the existing pneumatic system may have to be increased. Heat generation, which often is the limiting factor with mechanical vacuum pumps, is of little concern with vacuum generators.
Mechanical pumps most often are specified to provide a machine with vacuum on a continuous basis. But many of these machines actually use vacuum only intermittently at many different locations. In cases like this, vacuum generators can provide a practical alternative by supplying vacuum intermittently at each source rather than continuously for the entire machine.
Vacuum generators are controlled simply by initiating or terminating compressed air flow to the nozzle. Vacuum generators have been used for decades, but relatively recent improvements have led to nozzle designs that provide higher operating efficiencies.
Another development using venturis is the multi-stage vacuum generators. In this configuration, two or more vacuum generators are piped in series to produce greater vacuum flow without using more compressed air. Essentially, the exhaust from the first nozzle (which determines the maximum attainable vacuum level) serves as input for a second stage. Exhaust from the second stage then serves as input for a third stage. This means that a multi-stage generator evacuates a given volume faster than a single-stage generator does, but they both will eventually pull the same vacuum level.
Selecting a vacuum generator depends on the lifting force required and the volume of air that must be evacuated. Lifting force depends on the vacuum level the generator can pull — which, in turn, depends on the air pressure supplied — and the effective area of the vacuum cup. In most applications it is important that a generator be able to pull the required vacuum in as short a time as possible to minimize air consumption.
Vacuum pump selection
The first major step in selecting the right vacuum pump is to compare application vacuum requirements with the maximum vacuum ratings of commercial pumps. At low levels, there is a wide choice of pumps. But as vacuum level increases, the choice narrows, sometimes to the point where only one type of pump may be available.
To calculate a system's vacuum needs, consider all work devices to be driven. The working vacuum of the devices can be determined by calculations based on handbook formulas, theoretical data, catalog information, performance curves, or tests made with prototype systems. Once you know the vacuum required, you can begin looking for pumps that can accommodate application requirements.
The maximum vacuum rating for a pump is commonly expressed for either continuous or intermittent duty cycles, and can be obtained from pump manufacturers. Because the maximum theoretical vacuum at sea level is 29.92 in.-Hg, actual pump capabilities are based on and compared to this theoretical value. Depending on pump design, the vacuum limit ranges from 28 to 29.5 in.-Hg or about 93% or 98% of the maximum theoretical value. For some pump types, the maximum vacuum rating will be based on this practical upper limit. For others, where heat dissipation is a problem, the maximum vacuum rating might also take into account allowable temperature rise.