A vacuum pump is a device that removes gas molecules from a sealed volume in order to
leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke,
and was preceded by the suction pump, which dates to antiquity.
Types Pumps can be broadly categorized according
to three techniques: Positive displacement pumps use a mechanism
to repeatedly expand a cavity, allow gases to flow in from the chamber, seal off the
cavity, and exhaust it to the atmosphere. Momentum transfer pumps, also called molecular
pumps, use high speed jets of dense fluid or high speed rotating blades to knock gas
molecules out of the chamber. Entrapment pumps capture gases in a solid or adsorbed state.
This includes cryopumps, getters, and ion pumps.
Positive displacement pumps are the most effective for low vacuums. Momentum transfer pumps in
conjunction with one or two positive displacement pumps are the most common configuration used
to achieve high vacuums. In this configuration the positive displacement pump serves two
purposes. First it obtains a rough vacuum in the vessel being evacuated before the momentum
transfer pump can be used to obtain the high vacuum, as momentum transfer pumps cannot
start pumping at atmospheric pressures. Second the positive displacement pump backs up the
momentum transfer pump by evacuating to low vacuum the accumulation of displaced molecules
in the high vacuum pump. Entrapment pumps can be added to reach ultrahigh vacuums, but
they require periodic regeneration of the surfaces that trap air molecules or ions.
Due to this requirement their available operational time can be unacceptably short in low and
high vacuums, thus limiting their use to ultrahigh vacuums. Pumps also differ in details like
manufacturing tolerances, sealing material, pressure, flow, admission or no admission
of oil vapor, service intervals, reliability, tolerance to dust, tolerance to chemicals,
tolerance to liquids and vibration. Performance measures
Pumping speed refers to the volume flow rate of a pump at its inlet, often measured in
volume per unit of time. Momentum transfer and entrapment pumps are more effective on
some gases than others, so the pumping rate can be different for each of the gases being
pumped, and the average volume flow rate of the pump will vary depending on the chemical
composition of the gases remaining in the chamber.
Throughput refers to the pumping speed multiplied by the gas pressure at the inlet, and is measured
in units of pressure·volume/unit time. At a constant temperature, throughput is proportional
to the number of molecules being pumped per unit time, and therefore to the mass flow
rate of the pump. When discussing a leak in the system or backstreaming through the pump,
throughput refers to the volume leak rate multiplied by the pressure at the vacuum side
of the leak, so the leak throughput can be compared to the pump throughput.
Positive displacement and momentum transfer pumps have a constant volume flow rate, but
as the chamber's pressure drops, this volume contains less and less mass. So although the
pumping speed remains constant, the throughput and mass flow rate drop exponentially. Meanwhile,
the leakage, evaporation, sublimation and backstreaming rates continue to produce a
constant throughput into the system. Positive displacement
A partial vacuum may be generated by increasing the volume of a container. To continue evacuating
a chamber indefinitely without requiring infinite growth, a compartment of the vacuum can be
repeatedly closed off, exhausted, and expanded again. This is the principle behind positive
displacement pumps, like the manual water pump for example. Inside the pump, a mechanism
expands a small sealed cavity to reduce its pressure below that of the atmosphere. Because
of the pressure differential, some fluid from the chamber is pushed into the pump's small
cavity. The pump's cavity is then sealed from the chamber, opened to the atmosphere, and
squeezed back to a minute size. More sophisticated systems are used for most
industrial applications, but the basic principle of cyclic volume removal is the same:
Rotary vane pump, the most common Diaphragm pump, zero oil contamination
Liquid ring high resistance to dust Piston pump, fluctuating vacuum
Scroll pump, highest speed dry pump Screw pump
Wankel pump External vane pump
Roots blower, also called a booster pump, has highest pumping speeds but low compression
ratio Multistage Roots pump that combine several
stages providing high pumping speed with better compression ratio
Toepler pump Lobe pump
The base pressure of a rubber- and plastic-sealed piston pump system is typically 1 to 50 kPa,
while a scroll pump might reach 10 Pa and a rotary vane oil pump with a clean and empty
metallic chamber can easily achieve 0.1 Pa. A positive displacement vacuum pump moves
the same volume of gas with each cycle, so its pumping speed is constant unless it is
overcome by backstreaming. Momentum transfer
In a momentum transfer pump, gas molecules are accelerated from the vacuum side to the
exhaust side. Momentum transfer pumping is only possible below pressures of about 0.1
kPa. Matter flows differently at different pressures based on the laws of fluid dynamics.
At atmospheric pressure and mild vacuums, molecules interact with each other and push
on their neighboring molecules in what is known as viscous flow. When the distance between
the molecules increases, the molecules interact with the walls of the chamber more often than
with the other molecules, and molecular pumping becomes more effective than positive displacement
pumping. This regime is generally called high vacuum.
Molecular pumps sweep out a larger area than mechanical pumps, and do so more frequently,
making them capable of much higher pumping speeds. They do this at the expense of the
seal between the vacuum and their exhaust. Since there is no seal, a small pressure at
the exhaust can easily cause backstreaming through the pump; this is called stall. In
high vacuum, however, pressure gradients have little effect on fluid flows, and molecular
pumps can attain their full potential. The two main types of molecular pumps are
the diffusion pump and the turbomolecular pump. Both types of pumps blow out gas molecules
that diffuse into the pump by imparting momentum to the gas molecules. Diffusion pumps blow
out gas molecules with jets of oil or mercury, while turbomolecular pumps use high speed
fans to push the gas. Both of these pumps will stall and fail to pump if exhausted directly
to atmospheric pressure, so they must be exhausted to a lower grade vacuum created by a mechanical
pump. As with positive displacement pumps, the base
pressure will be reached when leakage, outgassing, and backstreaming equal the pump speed, but
now minimizing leakage and outgassing to a level comparable to backstreaming becomes
much more difficult. Diffusion pump
Turbomolecular pump Regenerative Pump
Regenerative pumps utilize vortex behavior of the fluid. The construction is based on
hybrid concept of centrifugal pump and turbopump. Usually it consists of several sets of perpendicular
teeth on the rotor circulating air molecules inside stationary hollow grooves like multistage
centrifugal pump. They can reach to 1×10−5mbar and directly exhaust to atmospheric pressure.
Examples of such pumps are Edwards EPX and Pfeiffer OnToolTM Booster 150. It is sometimes
referred as side channel pump. Due to high pumping rate from atmosphere to high vacuum
and less contamination since bearing can be installed at exhaust side, this type of pumps
are used in load lock in semiconductor manufacturing processes.
This type of pump suffers from high power consumption(~1 kW) compare to turbomolecular
pump at low pressure since most power is consumed to back atmospheric pressure. This can be
reduced by nearly 10 times by backing with a small diaphragm pump.
Entrapment Entrapment pumps may be cryopumps, which use
cold temperatures to condense gases to a solid or adsorbed state, chemical pumps, which react
with gases to produce a solid residue, or ionization pumps, which use strong electrical
fields to ionize gases and propel the ions into a solid substrate. A cryomodule uses
cryopumping. Ion pump
Cryopump Sorption pump
Non-evaporative getter Other pump types
Venturi vacuum pump Steam ejector
Techniques Vacuum pumps are combined with chambers and
operational procedures into a wide variety of vacuum systems. Sometimes more than one
pump will be used in a single application. A partial vacuum, or rough vacuum, can be
created using a positive displacement pump that transports a gas load from an inlet port
to an outlet port. Because of their mechanical limitations, such pumps can only achieve a
low vacuum. To achieve a higher vacuum, other techniques must then be used, typically in
series. Some examples might be use of an oil sealed rotary vane pump backing a diffusion
pump, or a dry scroll pump backing a turbomolecular pump. There are other combinations depending
on the level of vacuum being sought. Achieving high vacuum is difficult because
all of the materials exposed to the vacuum must be carefully evaluated for their outgassing
and vapor pressure properties. For example, oils, and greases, and rubber, or plastic
gaskets used as seals for the vacuum chamber must not boil off when exposed to the vacuum,
or the gases they produce would prevent the creation of the desired degree of vacuum.
Often, all of the surfaces exposed to the vacuum must be baked at high temperature to
drive off adsorbed gases. Outgassing can also be reduced simply by desiccation
prior to vacuum pumping. High vacuum systems generally require metal chambers with metal
gasket seals such as Klein flanges or ISO flanges, rather than the rubber gaskets more
common in low vacuum chamber seals. The system must be clean and free of organic matter to
minimize outgassing. All materials, solid or liquid, have a small vapour pressure, and
their outgassing becomes important when the vacuum pressure falls below this vapour pressure.
As a result, many materials that work well in low vacuums, such as epoxy, will become
a source of outgassing at higher vacuums. With these standard precautions, vacuums of
1 mPa are easily achieved with an assortment of molecular pumps. With careful design and
operation, 1 µPa is possible. Several types of pumps may be used in sequence
or in parallel. In a typical pumpdown sequence, a positive displacement pump would be used
to remove most of the gas from a chamber, starting from atmosphere to 25 Torr. Then
a sorption pump would be used to bring the pressure down to 10−4 Torr. A cryopump or
turbomolecular pump would be used to bring the pressure further down to 10−8 Torr.
An additional ion pump can be started below 10−6 Torr to remove gases which are not
adequately handled by a cryopump or turbo pump, such as helium or hydrogen.
Ultra high vacuum generally requires custom-built equipment, strict operational procedures,
and a fair amount of trial-and-error. Ultra-high vacuum systems are usually made of stainless
steel with metal-gasketed vacuum flanges. The system is usually baked, preferably under
vacuum, to temporarily raise the vapour pressure of all outgassing materials in the system
and boil them off. If necessary, this outgassing of the system can also be performed at room
temperature, but this takes much more time. Once the bulk of the outgassing materials
are boiled off and evacuated, the system may be cooled to lower vapour pressures to minimize
residual outgassing during actual operation. Some systems are cooled well below room temperature
by liquid nitrogen to shut down residual outgassing and simultaneously cryopump the system.
In ultra-high vacuum systems, some very odd leakage paths and outgassing sources must
be considered. The water absorption of aluminium and palladium becomes an unacceptable source
of outgassing, and even the absorptivity of hard metals such as stainless steel or titanium
must be considered. Some oils and greases will boil off in extreme vacuums. The porosity
of the metallic vacuum chamber walls may have to be considered, and the grain direction
of the metallic flanges should be parallel to the flange face.
The impact of molecular size must be considered. Smaller molecules can leak in more easily
and are more easily absorbed by certain materials, and molecular pumps are less effective at
pumping gases with lower molecular weights. A system may be able to evacuate nitrogen
to the desired vacuum, but the chamber could still be full of residual atmospheric hydrogen
and helium. Vessels lined with a highly gas-permeable material such as palladium create special
outgassing problems. Uses of vacuum pumps
Vacuum pumps are used in many industrial and scientific processes including:
Composite plastic moulding processes Driving some of the flight instruments in
older and simpler aircraft without electrical systems.
The production of most types of electric lamps, vacuum tubes, and CRTs where the device is
either left evacuated or re-filled with a specific gas or gas mixture
Semiconductor processing, notably ion implantation, dry etch and PVD, ALD, PECVD and CVD deposition
and soon in photolithography Electron microscopy
Medical processes that require suction Uranium enrichment
Medical applications such as radiotherapy, radiosurgery and radiopharmacy
Analytical instrumentation to analyse gas, liquid, solid, surface and bio materials
Mass spectrometers to create an ultra high vacuum between the ion source and the detector
Vacuum coating on glass, metal and plastics for decoration, for durability and for energy
saving, such as low-emissivity glass Hard coating for engine components
Ophthalmic coating Milking machines and other equipment in dairy
sheds Vacuum Impregnation of porous products such
as wood or electric motor widings. Air conditioning service - removing all contaminants
from the system before charging with refrigerant Trash compactor
Vacuum engineering Sewage systems
Freeze Drying Fusion Research
Vacuum may be used to power, or provide assistance to mechanical devices. In hybrid and diesel
engined motor vehicles, a pump fitted on the engine is used to produce vacuum. In petrol
engines, instead, vacuum is typically obtained as a side-effect of the operation of the engine
and the flow restriction created by the throttle plate, but may be also supplemented by an
electrically operated vacuum pump to boost braking assistance or improve fuel consumption.
This vacuum may then be used to power the following motor vehicle components:
The vacuum servo booster for the hydraulic brakes
Motors that move dampers in the ventilation system
The throttle driver in the cruise control servomechanism
Door locks or trunk releases In an aircraft, the vacuum source is often
used to power gyroscopes in the various flight instruments. To prevent the complete loss
of instrumentation in the event of an electrical failure, the instrument panel is deliberately
designed with certain instruments powered by electricity and other instruments powered
by the vacuum source. History of the vacuum pump
The predecessor to the vacuum pump was the suction pump, which was known to the Romans.
Dual-action suction pumps were found in the city of Pompeii. Arabic engineer Al-Jazari
also described suction pumps in the 13th century. He said that his model was a larger version
of the siphons the Bizantines used to discharge the Greek fire. The suction pump later reappeared
in Europe from the 15th century. By the 17th century, water pump designs had
improved to the point that they produced measurable vacuums, but this was not immediately understood.
What was known was that suction pumps could not pull water beyond a certain height: 18
Florentine yards according to a measurement taken around 1635. This limit was a concern
to irrigation projects, mine drainage, and decorative water fountains planned by the
Duke of Tuscany, so the Duke commissioned Galileo to investigate the problem. Galileo
advertised the puzzle to other scientists, including Gaspar Berti who replicated it by
building the first water barometer in Rome in 1639. Berti's barometer produced a vacuum
above the water column, but he could not explain it. The breakthrough was made by Evangelista
Torricelli in 1643. Building upon Galileo's notes, he built the first mercury barometer
and wrote a convincing argument that the space at the top was a vacuum. The height of the
column was then limited to the maximum weight that atmospheric pressure could support; this
is the limiting height of a suction pump, and the same as the maximum height of a siphon,
which operates by the same principle. Some people believe that although Torricelli's
experiment was crucial, it was Blaise Pascal's experiments that proved the top space really
contained vacuum. In 1654, Otto von Guericke invented the first
vacuum pump and conducted his famous Magdeburg hemispheres experiment, showing that teams
of horses could not separate two hemispheres from which the air had been evacuated. Robert
Boyle improved Guericke's design and conducted experiments on the properties of vacuum. Robert
Hooke also helped Boyle produce an air pump which helped to produce the vacuum. The study
of vacuum then lapsed until 1855, when Heinrich Geissler invented the mercury displacement
pump and achieved a record vacuum of about 10 Pa. A number of electrical properties become
observable at this vacuum level, and this renewed interest in vacuum. This, in turn,
led to the development of the vacuum tube. In the 19th century, Nikola Tesla designed
an apparatus that contains a Sprengel Pump to create a high degree of exhaustion.
Hazards
Old vacuum-pump oils that were produced before circa 1980 often contain a mixture of several
different dangerous polychlorinated biphenyls, which are highly toxic, carcinogenic, persistent
organic pollutants. See also
An Experiment on a Bird in the Air Pump Vacuum sewerage
References