Vacuum pump

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The Roots blower is one example of a vacuum pump
The Roots blower is one example of a vacuum pump

A vacuum pump is a pump that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The vacuum pump was invented in 1650, see: History.

Contents

[edit] Types

Pumps can be broadly categorized according to three techniques:[1]

  • 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 gaseous molecules out of the chamber.
  • Entrapment pumps capture gases in a solid or absorbed state. This includes cryopumps, getters, and Ion pumps.

Positive displacement pumps are the most effective for low vacuums, but their high backstream flows through mechanical seals generally limits their usefulness in high vacuums. Momentum transfer pumps in series with positive displacement pumps are the most common configuration used to achieve high vacuums, but they stall at low vacuums. (Hence the need for a positive displacement pump in series.) Entrapment pumps can be added to reach ultrahigh vacuums, but they have a maximum operational time since they do not exhaust materials. They periodically saturate and require regeneration, which usually means bringing the system back up to higher pressures and temperatures. The available operational time is usually 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.

[edit] Performance measures

  • Pumping speed refers to the volume flow rate of a pump at its inlet, often measured in litres per second, cubic feet per minute, or cubic metre per hour. Because of compression, the volume flow rate at the outlet will always be much lower than at the inlet. Momentum transfer and entrapment pumps are more effective on some gases than others, so the pumping speed can be simultaneously different for each of the gases being pumped, and the average pumping speed 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 such as torr-litres/second. 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. (Think PV=nRT) When discussing a leak, backstreaming or outgassing, 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, (pumping speed,) 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 produce a constant throughput into the system. When the pump's mass flow drops to the same level as the mass flows into the chamber, the system asymptotically approaches a constant pressure called the base pressure.

Evaporation and sublimation into a vacuum is called outgassing, and the most common source is water absorbed by materials in the chamber. If the dominant mass flow into the vacuum system is chamber leakage or outgassing of materials under vacuum, then the vacuum can be improved simply by installing bigger pumps with a higher volume flow rate. However, there is a point where backstream leakage through the pump and outgassing of the pump oils become the dominant mass flows into the chamber. In this situation, the vacuum will approach the pump's ultimate pressure - the best vacuum that this type of pump can achieve under ideal conditions. Adding more pumps in parallel or bigger pumps of the same type can still improve the pump-down speed, but they will not reduce the base pressure below ultimate. Better pumping technologies must be used to go beyond this barrier.

[edit] Positive displacement

The manual water pump draws water up from a well by creating a vacuum that water rushes in to fill. In a sense, it acts to evacuate the well, although the high leakage rate of dirt prevents a high quality vacuum from being maintained for any length of time.
The manual water pump draws water up from a well by creating a vacuum that water rushes in to fill. In a sense, it acts to evacuate the well, although the high leakage rate of dirt prevents a high quality vacuum from being maintained for any length of time.
Mechanism of a scroll pump
Mechanism of a scroll pump

Fluids cannot be pulled, so it is technically impossible to create a vacuum by suction. Suction is the movement of fluids into a vacuum under the effect of a higher external pressure, but the vacuum has to be created first. The easiest way to create an artificial vacuum is to expand the volume of a container. For example, the diaphragm muscle expands the chest cavity, which causes the volume of the lungs to increase. This expansion reduces the pressure and creates a partial vacuum, which is soon filled by air pushed in by atmospheric pressure.

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 create a deep vacuum. Because of the pressure differential, some fluid from the chamber (or the well, in our example) 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:

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 until it is overcome by backstreaming.

[edit] Momentum transfer

A cutaway view of a turbomolecular high vacuum pump
A cutaway view of a turbomolecular high vacuum pump

In a momentum transfer pump, gas molecules are accelerated from the vacuum side to the exhaust side (which is usually maintained at a reduced pressure by a positive displacement pump). Momentum transfer pumping is only possible below pressures of about 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 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 force flow backstream 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. Diffusion pumps blow out molecules with jets of oil, while turbomolecular pumps use high speed fans. 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.

[edit] Entrapment

Entrapment pumps may be cryopumps, which use cold temperatures to condense gases to a solid or absorbed 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.

[edit] Other pump types

Venturi vacuum pump (aspirator) (10 to 30 kPa)

[edit] 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 series or in parallel) 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 (exhaust) port. Because of their mechanical limitations, such pumps can only achieve a fairly crude partial vacuum. To achieve a more-perfect vacuum, other techniques must then be used, typically in series (usually following an initial fast "pump down" using a positive displacement pump). 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 vacuum quality desired.

Achieving truly 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, greases, rubber, or plastic used to form gaskets and seals 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 O-ring seals such as Klein flanges or ISO flanges. 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 problematic source of outgassing when attempting to achieve high vacuums. With these standard precautions, vacuums of 1 mPa are easily achieved with off-the-shelf molecular pumps. With careful design and operation, 1μPa 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 (760 Torr, 101 kPa) to 25 Torr (3 kPa). Then a sorption pump would be used to bring the pressure down to 10-4 Torr (10 mPa). A cryopump or turbomolecular pump would be used to bring the pressure further down to 10-8 Torr (1 μPa). 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 conflat 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 and 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 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. Your system may be able to evacuate nitrogen, (the main component of air,) to the desired vacuum, but your chamber could still be full of residual atmospheric hydrogen and helium. Vessels lined with a highly gas-permeable material such as palladium (which is a high-capacity hydrogen sponge) create special outgassing problems.

[edit] Uses of vacuum pumps

Vacuum pumps are used in many industrial and scientific processes including:

Vacuum may be used to power mechanical devices. In diesel-engined automobiles, a pump fitted on the engine (usually on the camshaft) is used to produce vacuum. In gasoline-powered automobiles, instead, vacuum is obtained as a side-effect of the operation of the engine and the flow restriction created by the throttle plate. This vacuum may then be used to power:

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.

[edit] History of the vacuum pump

The vacuum pump was invented by Otto von Guericke.

[edit] See also

[edit] References

  1. ^ Van Atta, C. M..; and M. Hablanian (1991). "Vacuum and Vacuum Technology". Encyclopedia of Physics (Second Edition): pp. 1330-1333. Ed. Rita G. Lerner and George L. Trigg. VCH Publishers Inc.. ISBN 3-527-26954-1.
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