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* Steels & Properties
* Heat Treatment


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Vacuum and Vacuum Pumps

What is a Vacuum

  • Vacuum is a space in which the pressure is below atmospheric pressure.
  • There is less air contained in a chamber under vacuum than when that chamber is open to the atmosphere.
  • The reference pressure for the best vacuum is zero pressure, and all other vacuum conditions will be positive pressures.

Reasons for using a vacuum

A process or physical measurement is generally performed in a vacuum for one or more of the following reasons:

  • To remove active atmospheric constituents that could cause a physical or chemical reaction (e.g. in electric lamps, during melting and sintering of metals, packaging of dairy products and in encapsulation of electronic components).
  • To achieve a pressure difference (e.g. for holding, lifting and transportation, and vacuum forming of plastics).
  • To decrease energy transfer (e.g. for thermal and electrical insulation).
  • To remove occluded or dissolved gas or volatile liquid from the bulk of material (e.g. degassing of oils and freeze drying).
  • To extend the distance that a particle travel before it collides with another, thereby helping the particles in a process to move without collision between source and target (e.g. vacuum coating; in cathode ray, X-ray and television tubes; ion implantation; particle accelerators; and in electron beam welding).
  • To produce clean surfaces (useful in the preparation of pure, thin films and in surface studies such as corrosion, catalysis and tribology).

Pressure Levels Vs Heat Treating

  • Positive pressures from 2 to 6 bar: Pressure used for high-speed gas quenching. Positive pressures from 1 atm to 2 bar may be used in standard vacuum furnaces chambers with special door clamping.
  • Pressures from 1 bar to 6 bar absolute (750 torr to 4500 torr): Pressure to which inert gas is backfilled into the chamber at the completion of the process cycle for accelerated work cooling. Work-cooling blowers and heat exchangers operate in this range.
  • Pressures from 100 to 10-1 torr (1 torr to 100 microns): Pressures to which inert gas is backfilled when it is used to suppress vapors, commonly used for brazing and some sintering operations.
  • Pressures from 10-1 to 10-4 torr (100 microns to 0.1 micron): Pressure range in which graphite hot-zone furnaces operate at temperature when used for brazing, heat treating, and most hard metal and steel sintering.
  • Pressures from 10-3 to 10-5 torr (1 micron to 0.01 micron): Pressure range used for refractory metal hot-zone furnaces at temperature when used for diffusion bonding, heat treating, and brazing.
  • Pressures from 10-5 to 10-6 torr (0.01 micron to 0.001 micron): Pressure range at temperature for aluminum brazing, reactive metal sintering, and heat treating.
  • Pressures from 10-7 to 10-9 torr: Pressure range used for semi-conductor applications.
Vacuum pump choice

Low and medium vacuum

Pumps such as the rotary-vane, rotary-piston and mechanical booster are all positive displacement types. They reduce the pressure in a system by repeatedly taking samples of gas into pump. The pump mechanism isolates the gas from the inlet, compresses it and then expels it through an outlet.

High and ultra-high vacuum

Diffusion pump

Here gas transport is achieved by a series of high-velocity vapor jets (normally oil vapor is used) emerging from an assembly within the pump body. In normal operation a portion of any gas arriving at the inlet jet is entrained, compressed and transferred to the next stage.

The advantage of a vapor jet pump is that it can pump a very wide variety of gases, including rare gases, such as helium, that are difficult to pump using cryo-pumps or ordinary turbo-pumps. Vapor jet pumps with very high pumping speeds can be obtained. The major disadvantages are that if improperly operated, oil vapor can back-stream into the system. Cold traps are used to prevent this migration and have an advantage of increasing the pumping speed for water. 

Turbomolecular pump

This contains a rotor with inclined blades moving at high speed between corresponding stationary blades in a stator. Gas molecules entering the inlet port acquire a velocity and preferred direction superimposed on their thermal velocity by repeated collisions with the fast-moving rotor. Rotational speeds for small pumps are typically 60 000rev min-1.

These pumps are generally clean and oil free, and provide high pumping speeds for most gases. Due to their precision construction, these pumps are particularly susceptible to particle contamination. A turbo-pump can be turned on or off faster then both the cryo and diffusion pumps. Hence its use may be preferred in application sensitive to pumping system failures.

Cryogenic pump

Operation is achieved by the condensation, freezing and/or sorption of gas at surfaces maintained at extremely low temperatures, thus removing them from the gas phase in the vacuum system.

Cyryo-pumps are oil free and have high pumping speeds, particularly for water. They are capture pumps and consequently require frequent regeneration to dispose of the captured gases. This can pose a problem when used with hazardous gases, because a significant accumulation of an explosive or toxic gas is difficult to deal with in an emergency shutdown.

Sputter-ion pump

This makes use of the gettering principle, in which a cathode material (usually titanium) is vaporized or sputtered by bombardment with high-velocity ions. The active gases by ionization and burial in the cathode, and the light gases by diffusion into the cathode.
 
They are most suited for systems that are not frequently opened to atmosphere. Because they are capture pumps, they cannot accept large gas loads. They are extremely useful for pumping of vacuum tubes and long-term experiments. They are clean and have no oil contamination.

 
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