Oil diffusion vacuum pump
The oil diffusion pump is operated with an oil of low vapor pressure. A high speed jet of the Oil is generated by boiling the Oil and directing the vapor through a jet assembly. Note that the oil is gaseous when entering the nozzles. Within the nozzles, the flow changes from laminar, to supersonic and molecular. Often several jets are used in series to enhance the pumping action. The outside of the diffusion pump is cooled using either air flow or a water line. As the vapor jet hits the outer cooled shell of the diffusion pump, the working fluid condenses and is recovered and directed back to the boiler. The pumped gases continue flowing to the base of the pump at increased pressure, flowing out through the diffusion pump outlet, where they are compressed to ambient pressure by the secondary mechanical forepump and exhausted. Diffusion pumps cannot discharge directly into the atmosphere, so a mechanical forepump is typically used to maintain an outlet pressure around 0.1 mbar. Diffusion pumps have no moving parts and as a result are quite durable and reliable. They can function over pressure ranges of 10−10 to 10−2 mbar. They are driven only by convection and thus have a very low energy efficiency and low cost per unit pumping speed when compared with other types of pump used in the same vacuum range. One major disadvantage of diffusion pumps is the tendency to backstream oil into the chamber being evacuated. This oil can contaminate surfaces inside the chamber or upon contact with hot filaments or electrical discharges may result in carbonaceous or siliceous deposits. Generally cold traps and baffles are utilized between the chamber and the diffusion pump to minimize backstreaming, although this results in some loss of pumping ability. The oil of a diffusion pump cannot be exposed to the atmosphere when hot. If this occurs, the oil will burn and has to be replaced .
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