Positive placement pump
The positive placement pump uses a mechanism that repeatedly expands a cavity.
This is to allow gases to flow in from the chamber, seals off the cavity and exhausts the gas out to the atmosphere;
Positive displacement pumps are ideal for low vacuums but they tend to be ineffective in high vacuums.
Momentum transfer pump
The momentum transfer pump uses high speed jets of dense fluid or high speed rotating blades.
This is to knock gaseous molecules out of the chamber.
Momentum transfer pumps placed in series with positive displacement pumps are the most common configuration in the achievement of high vacuums but they often stall at low vacuums.
Entrapment pump
The entrapment pump that captures gases in a solid or absorbed state.
This type includes cryopumps, getters and ion pumps.
Entrapment pumps can be added in series to other types of pumps in order to reach ultra high vacuum states but they do not exhaust materials so their operational time is limited. Entrapment pumps periodically saturate and require regeneration, which generally means bringing the system up to higher temperatures and pressures.