Contemporary vapor-jet vacuum pumps work on the principle of diffusion (entrainment) of the pumped gas into a supersonic vapor jet flowing into the vacuum space of the pump from a Laval nozzle. There exist different types of vacuum pumps, which differ mainly with respect to working fluid, preheating power, designs of nozzles and number of stages. However, for the majority of types of high-vacuum pumps, the use as a working fluid of high-molecular oil with molecular weight u 200 to 500, low adiabatic index n - 1.15 to 1.05 and vapor pressure in the evaporator of the order of several mm of Hg is characteristic.
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