Vacuum pump


A vacuum pump is a type of pump device that draws gas particles 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.

History

Early pumps

The predecessor to the vacuum pump was the suction pump. Dual-action suction pumps were found in the city of Pompeii. Arabic engineer Al-Jazari later described dual-action suction pumps as part of water-raising machines in the 13th century. He also said that a suction pump was used in siphons to discharge Greek fire. The suction pump later appeared in medieval Europe from the 15th century.
File:Molchanova by levitskiy.jpg|thumb|Student of Smolny Institute Catherine Molchanova with vacuum pump, by Dmitry Levitzky, 1776
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, or about. This limit was a concern in irrigation projects, mine drainage, and decorative water fountains planned by the Duke of Tuscany, so the duke commissioned Galileo Galilei to investigate the problem. Galileo suggested, incorrectly, in his Two New Sciences that the column of a water pump will break of its own weight when the water has been lifted to 34 feet. Other scientists took up the challenge, including Gasparo 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. A breakthrough was made by Galileo's student 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.
In 1650, Otto von Guericke invented the first vacuum pump. Four years later, he 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 that helped to produce the vacuum.
By 1709, Francis Hauksbee improved on the design further with his two-cylinder pump, where two pistons worked via a rack-and-pinion design that reportedly "gave a vacuum within about one inch of mercury of perfect." This design remained popular and only slightly changed until well into the nineteenth century.

19th century

invented the mercury displacement pump in 1855 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. The Sprengel pump was a widely used vacuum producer of this time.

20th century

The early 20th century saw the invention of many types of vacuum pump, including the molecular drag pump, the diffusion pump, and the turbomolecular pump.

Types

Pumps can be broadly categorized according to three techniques: positive displacement, momentum transfer, and entrapment. 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.

Positive displacement pump

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 a positive displacement pump, for example the manual water pump. 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:
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 pump

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 an oil or mercury vapor, 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, in this case called a backing 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.

Entrapment pump

An entrapment pump may be a cryopump, which uses cold temperatures to condense gases to a solid or adsorbed state, a chemical pump, which reacts with gases to produce a solid residue, or an ion pump, which uses strong electrical fields to ionize gases and propel the ions into a solid substrate. A cryomodule uses cryopumping. Other types are the sorption pump, non-evaporative getter pump, and titanium sublimation pump.

Other types

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 1×10−5 mbar and directly exhaust to atmospheric pressure. Examples of such pumps are Edwards EPX and Pfeiffer OnTool™ 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 compared 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 pump.