Oil diffusion pumps generate a high vacuum and work with a fast vapor jet of the operating fluid (oil vapor). All propellant pumps use a similar pump mechanism in which the operating medium (oil) is heated by an electric heater and the oil vapor expands after passing through the jet nozzles. The pumped gas molecules diffuse into the vapor stream and leave the pump chamber.
The molecules of the pump liquid flow are transferred to the gas molecules by impact pulses in the direction of flow, which moves the gas to be pumped into an area with higher pressure. This technology is wear-free as it requires no moving parts. With oil diffusion pumps, the corresponding vapor pressure increases during operation depending on the type of operating fluid (silicone or mineral oil), temperature and nozzle configuration.
After exiting through the nozzles, the vapor jet condenses on the cooled outer walls of the pump and does not enter the process chamber. A modern baffle plate design reduces oil return flow and consumption through an improved condensation surface. This simple physical mechanism makes the diffusion pump a robust technology.
The upstream pressure in the pump must be low enough for the vapor to flow out. Appropriate vacuum pumps are used to ensure this. The pre-vacuum also influences the vapor jet and becomes problematic if its value exceeds a certain critical value.