Diffusion pump

The oil diffusion pump is a high vacuum pump which operates on the principle of a jet pump. It has no moving parts and is therefore very reliable and easy to maintain. It is a further development of Wolfgang Gaede 1916 presented diffusion pump with mercury as a propellant.

Operation

The pump consists of a cylindrical water - cooled or air- pump body. The bottom of the pump body is heated ( usually electric). In the boiling space is with the propellant. The boiling chamber is the nozzle system through which the heated propellant flowing at high pressure. Occur three to four conical steam jets (one pump stage ) in which condense on the cold walls.

Gas molecules, which enter the steam jet can be entrained and thus conveyed to the bottom of a higher pressure region. There is to be output from the diffusion pump of the side arm. By heating the propellant molecules come out again and be pumped out of the backing pump (eg rotary vane pump).

Although the gas molecules can spontaneously emerge from the propellant, however, it is not possible for them to reach a deeper level pressure (that is, back to the recipient ).

Oil diffusion pumps require a backing pressure of about 0.1 mbar; they reach discharge pressures up to about 10-9 mbar, depending on the vapor pressure of the propellant.

Propellant

The type of blowing agent is crucial. Normally mineral oils, which have been removed under high vacuum from the components with a high vapor pressure are used. Such oils have vapor pressures between 5 x 10-7 and 2 x 10-8 mbar at room temperature and a good oxidation resistance. Greater demand for vapor pressure ( Ultimate ) or oxidation resistance silicone oils, esters or certain Pentaphenylether (vapor pressure: 10-10 mbar ) can be used.

The blowing agent was formerly exclusively used mercury. Besides the fact that mercury is toxic, mercury has in cooling water temperature has a relatively high vapor pressure (about 10-3 mbar). One must therefore turn a cold trap between the pump and vacuum vessel to reduce the vapor pressure at sufficiently low values. Mercury diffusion pumps are only used in special cases, such as when in the vessel to be evacuated anyway mercury is present and therefore no cold trap must be interposed.

Oil return flow

Oils are very unpopular in vacuum technology basically. To prevent oil backflow into the recipient, Düsenhutdampfsperren (cold cap) and / or shell baffles ( baffle ) are connected between the vacuum chamber and pump. Vapor barriers are made of cooled baffles that prevent the direct flying through oil molecules. To increase the temperature difference, and liquid nitrogen (< 77 K ) is used for cooling.

Disadvantages

Significant disadvantages of this type of pump are long heating and cooling times. This in batch mode (if container is repeatedly opened and closed) to get around, is pre-evacuated with a bypass (bypass) the recipient before the oil diffusion pump is switched on.

Applications

• High vacuum pump for high vacuum range ( 10-3 to 10-9 mbar, with capacities of up at 10-6 mbar)

• Jet pump
• Vacuum pump