Abstract
Condensation conditions of the motive fluid in an oil diffusion pump are considered with particular attention to the backstreaming problem. The backstreaming rate is correlated with the temperature of the “cold caps” surrounding the pump nozzle and it is demonstrated that an upper temperature limit exists near 200 °F where such devices cease to function effectively. The effect of “oleophobic” surfaces on cold caps and baffles is discussed. Conditions existing at the inlet of diffusion pumps and in baffles do not warrant attempts to introduce dropwise condensation. The condensation coefficient of pumping vapor at the temperature of the water cooled wall and with the flow rates used at the top nozzle appears to be very close to unity. The usefulness of creep barriers with modern pumping fluids and trap designs is judged to be questionable.
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