How to Select Venturi Vacuum Generators
Image Credit: Clippard Instrument Laboratory, Inc. | Techno, Inc. | Gast Group
Venturi vacuum generators create vacuums using a venturi chamber designed to move gases or fluids out of a region of space. Venturi or fluid jet vacuum generators rely on the flow of compressed air, gas, or liquid as the "motive" fluid to pull or create a vacuum at a desired port. They contain no moving parts and require no other power than the compressed fluid.
Venturi vacuum generators have a number of advantages over other vacuum types:
No moving components
Image Credit: Fox Venturi Educators
The venturi principle (as shown in the diagrams above and below) involves sending a motive stream horizontally through a constricting nozzle. This movement creates an area of low pressure at the expanding side of the nozzle which pulls gas molecules into the flow from an attached inlet. The intake provides the vacuum or suction force needed for the application.
Venturi principle. Image Credit: Prospekti GmbH
Types of Venturi Vacuum Generators
Venturi jet vacuum generator types vary primarily based on the type of motive fluid used. Different types include air, steam, and liquid.
Venturi air jets create vacuum by a flow of compressed air used as the motive fluid. This relatively inexpensive technology is used in a wide range of vacuum applications, from very small sampling uses to large industrial vacuum systems. They are usually used to create vacuum for air or gas systems, and are often unsuitable for liquid vacuum uses. These pumps can be noisy since the noise level is the same as that of a venting compressed air line.
Steam ejectors use steam as the primary motive fluid to generate vacuum. These are very reliable systems that are used widely in industrial vacuum systems. Ejectors will handle both condensable and non-condensable gas loads as well as small amounts of solids or liquids, however accidental entrainment of liquids can cause a momentary interruption in vacuum. An interruption of this nature will not cause damage to the ejector.
Liquid eductors and liquid ejectors use liquid, such as oil or water, as the motive fluid to provide vacuum. These are typically used for creating vacuum in liquid systems. They are commonly used for pump priming evacuating closed vessels, and pumping mixtures of liquids and gases.
Venturi jet ejectors and eductors can be sourced based on a number of different specifications that define performance, sizing and connections, vacuum pump stages, and materials of construction.
The most important specifications to consider when selecting venturi vacuum generators are those that describe product performance.
Ultimate (maximum) operating vacuum or ultimate pressure is the lowest pressure which the venturi vacuum pump can generate. Buyers should note the conditions or assumptions used to obtain this value, since manufacturers may provide this rating using assumptions that are not realistic under normal operating conditions (e.g. ignoring the pressure of condensable gases like water vapor). Vacuum ratings which follow the ISO 21360-1 standard adhere to standard methods for measuring vacuum-pump performance.
Evacuation time may be provided by some manufacturers to specify the time/flow (typically s/ft3 or s/L) required to reach different vacuum pressure levels. Evacuation must be low enough in order for the device to reach the required vacuum pressure for the application in sufficient time.
- Air/fluid consumption is the volume rate of air, steam, or other motive fluid used by a venturi vacuum generator. This indicates how much fluid is needed and must be supplied to the device for operation at a given pressure.
Pumping speed or vacuum flow is the volumetric rate at which the vacuum intake (gas, air, fumes, moisture, etc.) is transported across a plane, typically given in ft3/min (cfm), m3/s, L/min, or gal/min (gpm). Vacuum flow ratings listed for venturi vacuum generators are typically referenced to STP (standard temperature and pressure), in which it is given in scfm (standard ft3/min) or similar units. Keep in mind that the vacuum flow is dependent on the supplied pressure and flow of the motive fluid.
Operating temperature is the temperature at which the venturi vacuum generator is designed or able to operate. The motive liquid temperature is the primary limiting factor of operation in a venturi, since pressure affects the fluid's boiling point temperature. For very low suction pressures, consideration must be given to the motive liquid vapor pressure to prevent it from flashing to a gas.
Vacuum Pump Stages
The stage of the venturi vacuum generator defines how many sequential sections, chambers, or pumps are packaged together in the unit. Multi-stage systems may incorporate all of the same technology type or two or more different type pumps packaged together. For many applications, using a multi-stage pump may be more efficient for achieving a vacuum than using multiple separate pumps in series.
Single stage vacuum ejectors move gas molecules directly from the evacuated chamber into the atmosphere. They typically cover vacuum ranges from 30mm HgA to atmospheric pressure. Size typically ranges from 1 inch to 6 inch, however larger sizes are available if required.
Two stage vacuum ejectors evacuate gas molecules in two stages for lower absolute pressure. In operation, they typically consist of a low vacuum ejector working in conjunction with a high vacuum ejector. Two stage steam ejector systems may also incorporate condensers in the first stage to help reduce the gas load being passed to the next stage. This reduces motive fluid consumption and allows the ejectors to be smaller in size.
Three stage pumps use three separate chambers in succession. They cover vacuum ranges between 0.8mm HgA and 25mm HgA, and usually use steam as the motive fluid. In operation, a three stage system is similar to a two stage system, but incorporates a booster pump stage in addition to the low vacuum and high vacuum ejectors. Three stage systems are also usually condensing.
Pumps with four or more stages are employed in applications requiring extremely high vacuums. They incorporate one or more additional boosters which are equipped with steam jackets to prevent ice forming within the ejectors.
Materials of Construction
Various metals & plastics can be employed for eductor/ejector body and nozzle construction depending on service conditions and the properties of the fluids handled. Standard materials of construction are carbon steel or stainless steel, but they can come in a variety of metals (including cast-iron, brass, titanium, Hastalloy, and Monel) or plastics (including PVC, CPVC, Teflon, and polypropylene).
Sizing and Connections
A venturi vacuum generator's size is given based on the nominal outer diameter (OD) of the inlet and/or outlet connection. Often these connections are threaded and are sized based on NPT (National Pipe Thread) standards. Sizes are specified in either inches (") or millimeters (mm).
Venturi vacuum generators may incorporate a number of additional features to enhance operation and application. Integrated components such as automatic blow-off controls, vacuum and blow-off solenoids, pressure sensors, check valves and filters are options that can reduce overall mounting space, reduce cycle time and can offer air conservation functions as well as emergency stop modes. More advanced venturi vacuum systems may incorporate OEM controllers or valve sequencing control to replace manual operation of regulator knobs and valves for pressure and flow control.
Venturi vacuum generators are used in a range of different applications and industries. Common uses include evacuating vapors or fumes from process vessels, priming pumps, vacuum drying of air conditioners and other systems, initializing a siphon, or machine shop workholding operations. Machine shop operations include pick and place, clamping, chucking, alignment, and surface mounting. For all types of workholding, the hold down force required for a part can be calculated by the following equation:
F = Apart x Pvac
where F is the holding force, Apart is the area of the part being held in sq. inches (in2), and Pvac is the vacuum pressure in psi. This video demonstrates the use of a venturi part gripper:
Venturi gripper. Video Credit: Youtube - jkortberg
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