Vacuum Pumps

A vacuum pump is a pump that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. more...

Mechanisms of pump operation

Vacuum pumps operate according to various mechanisms; sometimes more than one 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. In a turbomolecular pump, the momentum transfer principle is used, accelerating the gas molecules from the vacuum side to the exhaust side (which is usually maintained at a reduced pressure by a positive displacement pump). Another alternative is cryogenic entrapment where individual gas molecules are liquified and contained in a cold trap.

Effectiveness of pumps

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.

Types of vacuum pumps

• Rubber and plastic sealed piston pump (1 to 50 kPa)
• Venturi vacuum pump (10 to 30 kPa)
• Scroll pump (10 Pa)
• Rotary vane oil pump (0.1 Pa)

• Transfer
  • Roots blower
  • Rotary compression
  • Diffusion pump
  • Turbomolecular pump
  • Liquid Ring

• Entrapment
  • Ion pump
  • Cryopump

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