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A vacuum ejector, or simply ejector is a type of
vacuum pump A vacuum pump is a device that draws gas molecules from a sealed volume in order to leave behind a partial vacuum. The job of a vacuum pump is to generate a relative vacuum within a capacity. The first vacuum pump was invented in 1650 by Otto ...
, which produces
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
by means of the
Venturi effect The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section (or choke) of a pipe. The Venturi effect is named after its discoverer, the 18th century Italian physicist, Giovanni Battista V ...
. In an ejector, a working fluid (liquid or gaseous) flows through a jet nozzle into a tube that first narrows and then expands in cross-sectional area. The fluid leaving the jet is flowing at a high velocity which due to
Bernoulli's principle In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy. The principle is named after the Swiss mathematic ...
results in it having low pressure, thus generating a vacuum. The outer tube then narrows into a mixing section where the high velocity working fluid mixes with the fluid that is drawn in by the vacuum, imparting enough velocity for it to be ejected, the tube then typically expands in order to decrease the velocity of the ejected stream, allowing the pressure to smoothly increase to the external pressure. The strength of the vacuum produced depends on the velocity and shape of the fluid jet and the shape of the constriction and mixing sections, but if a liquid is used as the working fluid the strength of the
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
produced is limited by the
vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases ...
of the liquid (for water, or 32
mbar The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000  Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea lev ...
at ). If a gas is used, however, this restriction does not exist. If not considering the source of the working fluid, vacuum ejectors can be significantly more compact than a self-powered vacuum pump of the same capacity.


Common types


Water aspirator

The cheap and simple water aspirator is commonly used in chemistry and
biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary ...
laboratories A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratory services are provided in a variety of settings: physicia ...
and consists of a
tee A tee is a stand used in sport to support and elevate a stationary ball prior to striking with a foot, club or bat. Tees are used extensively in golf, tee-ball, baseball, American football, and rugby. Etymology The word tee is derived from the ...
fitting attached to a tap and has a
hose barb Hose barbs are cylindrical pieces or parts for attaching and securing of hoses (tubing). The barb-like rings on the cylindrical piece allow for an easy push-connection of flexible-plastic or rubber tubing that is not so easily disconnected. Hose ...
at one side. The flow of water passes through the straight portion of the tee, which has a restriction at the intersection, where the hose barb is attached. The vacuum hose should be connected to this barb. In the past, water aspirators were common for low-strength vacuums in chemistry benchwork. However, they are water-intensive, and depending on what the vacuum is being used for (e.g. solvent removal), they can violate environmental protection laws such as the
RCRA The Resource Conservation and Recovery Act (RCRA), enacted in 1976, is the principal federal law in the United States governing the disposal of solid waste and hazardous waste.United States. Resource Conservation and Recovery Act. , , ''et seq., ...
by mixing potentially hazardous chemicals into the water stream, then flushing them down a drain that often leads directly to the municipal sewer. Their use has decreased somewhat as small electric vacuum pumps are far more effective, environmentally safe, and have become more affordable, but the unmatched simplicity and reliability of this device have caused it to remain popular for small labs or as a backup. Another, much larger version of this device is used in maritime operations as a device to dewater (drain) areas in a ship that have been flooded in emergency situations. Typically referred to as an eductor in these applications, this is preferred over electrical pumps due to their simplicity, compact size, and greatly mitigated risk of explosion in the event that flammable liquids and/or vapors are present. Additionally, unlike many mechanical pumps, they can also pass debris as the eductor has no moving parts that can be fouled. This makes an eductor especially useful in situations where fitting a debris strainer to the suction port will present more issues than it resolves. The size of the debris that can be passed depends on the physical size of the eductor. Sizes, flow ratings, and applications vary, including eductors that are permanently installed (typically used in very large spaces, such as a ship's main engine room), or portable models that can be lowered into spaces by a rope and supplied and drained through firefighting hoses. Most are supplied through a ship's firefighting main, and portable models can also be supplied by an emergency pump, provided it can supply sufficient flow to operate the eductor.


Steam ejector

The industrial steam ejector (also called the "steam jet ejector", "steam aspirator", or "evactor") uses steam as a working fluid and multistage systems can produce very high vacuums. Due to the lack of delicate moving parts and the flow of steam providing somewhat of cleaning action, steam ejectors can handle gas flows containing liquids, dust, or even solid particles that would damage or clog many other vacuum pumps. Ejectors made entirely from specialised materials such as
PTFE Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemour ...
or
graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on lar ...
have allowed usage of extremely corrosive gasses, since steam ejectors have no moving parts they can be constructed in their entirety from almost any material that has sufficient durability. In order to avoid using too much steam or impractical operating pressures, a single steam-ejector stage is generally not used to generate vacuum below approximately 10
kPa KPA may refer to: * Keele Postgraduate Association, Keele University, UK, formerly Keele Research Association (KRA) * Kensington (Olympia) station, London, England, National Rail station code * Kenya Ports Authority * ''Kiln phosphoric acid'', a ...
(75
mmHg A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly pascals. It is denoted mmHg or mm Hg. Although not an ...
).''High Vacuum Pumping Equipment'', B. D. Power, New York: Reinhold Publishing Corporation, 1966, chapter 4. To generate higher vacuum, multiple stages are used; in a two-stage
steam ejector An injector is a system of ducting and nozzles used to direct the flow of a high-pressure fluid in such a way that a lower pressure fluid is entrained in the jet and carried through a duct to a region of higher pressure. It is a fluid-dynamic ...
, for example, the second stage provides vacuum for the waste steam output by the first stage. Condensers are typically used between stages to significantly reduce the load on the later stages. Steam ejectors with two, three, four, five and six stages may be used to produce vacuums down to 2.5
kPa KPA may refer to: * Keele Postgraduate Association, Keele University, UK, formerly Keele Research Association (KRA) * Kensington (Olympia) station, London, England, National Rail station code * Kenya Ports Authority * ''Kiln phosphoric acid'', a ...
, 300 Pa, 40 Pa, 4 Pa, and 0.4 Pa, respectively. Steam ejectors are also suitable for pumping many liquids since if the steam can be easily condensed into the liquid then there is no need to separate the working fluid or manage a mist of liquid droplets. This is the manner in which a steam ''injector'' operates. An additional use for the injector technology is in vacuum ejectors in continuous train braking systems, which were made compulsory in the UK by the
Regulation of Railways Act 1889 The Regulation of Railways Act 1889 (52 & 53 Vict c 57) is an Act of the Parliament of the United Kingdom. It is one of the Railway Regulation Acts 1840 to 1893.The Short Titles Act 1896 The Short Titles Act 1896 (59 & 60 Vict c 14) is an ...
. A vacuum ejector uses steam pressure to draw air out of the vacuum pipe and reservoirs of continuous train brake. Steam locomotives, with a ready source of steam, found ejector technology ideal with its rugged simplicity and lack of moving parts. A steam locomotive usually has two ejectors: a large ejector for releasing the brakes when stationary and a small ejector for maintaining the vacuum against leaks. The exhaust from the ejectors is invariably directed to the smokebox, by which means it assists the blower in draughting the fire. The small ejector is sometimes replaced by a reciprocating pump driven from the
crosshead In mechanical engineering, a crosshead is a mechanical joint used as part of the slider-crank linkages of long reciprocating engines (either internal combustion or steam) and reciprocating compressors to eliminate sideways force on the piston. ...
because this is more economical of steam and is only required to operate when the train is moving.


Air ejector

Commonly called an air ejector, Venturi pump, or vacuum ejector. This ejector is similar in operation to the steam ejector but uses high-pressure air as the working fluid. Multistage air ejectors can be used, but since air cannot easily be condensed at room temperature, an air ejector is usually limited to two stages as each subsequent stage would have to be significantly larger than the last. These are commonly used in pneumatic handling equipment when a small vacuum is required to pick up objects since compressed air is often already present to power other parts of the equipment. Air ejectors used to suction liquids directly will produce a fine mist of droplets, this is how airbrushes and many other spraying systems operate, but when a spray is not required it is typically an undesirable effect that limits the applications to gas suction.


See also

*
Diffusion pump Diffusion pumps use a high speed jet of vapor to direct gas molecules in the pump throat down into the bottom of the pump and out the exhaust. They were the first type of high vacuum pumps operating in the regime of free molecular flow, where th ...
*
Injector An injector is a system of ducting and nozzles used to direct the flow of a high-pressure fluid in such a way that a lower pressure fluid is entrained in the jet and carried through a duct to a region of higher pressure. It is a fluid-dynamic ...
*
Vacuum pump A vacuum pump is a device that draws gas molecules from a sealed volume in order to leave behind a partial vacuum. The job of a vacuum pump is to generate a relative vacuum within a capacity. The first vacuum pump was invented in 1650 by Otto ...
*
Water eductor A water eductor or water dredge is an eductor-jet pump-based tool used by underwater archaeologists to remove sediments from an underwater archaeological site. Airlifts may be used for the same purpose. It consists of a large bore straight tube ...


References

{{Laboratory equipment Laboratory equipment Vacuum pumps