Mounting pumps

Positive displacement and kinetic pumps extract gases from the tank to be emptied and transfer them to a higher pressure, usually atmospheric. Attachment pumps trap gases by attaching them to their walls under conditions such that re-emission from the trapping surface is extremely low.

Standards NF X 10-501 and ISO 3529-2 define a fixation pump as "a vacuum pump on whose inner walls molecules are fixed by sorption or condensation".

All fastening pumps have favourable or unfavourable characteristics in common, which make them advisable or inadvisable depending on the application:

• they are not contaminating, in particular they do not emit oil vapour;

• they are closed cavities that do not communicate with the atmosphere; a failure of the power supply system therefore does not cause air to enter the vacuum chamber, nor back-diffusion of undesirable vapors;

• they have a sorption capacity limited to a certain quantity of gas; they should therefore not be used at high pressures (except for primary adsorption pumps) since the gas flow is proportional to the pressure; this pressure threshold can be different depending on the pump: atmospheric pressure for sorption pumps, or 10 ^-1 Pa to 10 ^-3 Pa for getter or ionic pumps, as well as for cryogenic pumps, but the phenomenon remains qualitatively the same;

• once saturated, the pump must be regenerated; the time required for this operation depends on the application; it can work for a very long time without regeneration, if it is only used at very low pressure and there is very little gas to pump (extremely low degassing and permeation, and virtually no leakage);

• the volume flow rate (pumping speed) of attachment pumps is highly selective, depending on the gases to be pumped. Some types can even have zero flow for certain gases, as we'll see later. It is therefore not uncommon to combine different types of pump with complementary performances. However, recent advances in vacuum pumping have limited the number of such combinations, which remain tricky to manage;

• Fastening pumps are static machines; they do not generate vibrations that could be transmitted to the frame (except in the case of certain cryogenic pumps).

Note: this article has been placed under the heading of vacuum pumps in the "Mechanical Engineering" treatise, as it concerns the means of obtaining a vacuum in an enclosure, a vacuum which is more generally obtained by purely mechanical means, but the principles used here fall...