Oil diffusion pump is a momentum transfer jet vacuum pump with low pressure and high speed oil vapor as working medium. Pump works in high vacuum area. The oil diffusion pump has the advantages of simple structure, convenient operation and low cost per unit pumping speed. It has no mechanical moving parts, long service life, high reliability and convenient maintenance. Oil diffusion pump is especially suitable for the process of requiring rapid evacuation of large working containers and rapid elimination of sudden emission of large amounts of gas from treated materials during treatment. For example, vacuum metallurgy and vacuum heat treatment equipment.
When working, the oil diffusion pump and system are first pumped into a preparatory vacuum by the front pump. The lower boiler of the pump is equipped with pump oil, which is heated by an electric heater to boil and evaporate to produce oil vapor. Because of the vacuum pre-pumping in the pump and the low pressure, the pump oil can be evaporated at a lower temperature (related to the type of working medium, generally 370-420K). Diffusion pump oil volatilizes rapidly into vapor in vacuum, and oil vapor rises along the guide pipe and is ejected from umbrella nozzles at all levels. Umbrella nozzles can be set up in the first, second, third and fourth levels. The high-speed oil vapor sprays on the water wall and condenses into liquid, which flows back to the bottom of the pump and is re-heated and gasified, thus continuously circulating. The gas molecules in the pumped system diffuse into the vapor jet through the intake port, and are carried to the next stage jet under the collision of a large number of high-speed directional moving oil molecules. The gas molecules are compressed by the step-by-step diffusion, and are pumped out by the front stage pump through the outlet nozzle and the exhaust port.
There are two harmful reverse diffusions in this diffusion pumping process:
(1) The extracted gas molecule is compressed to the exit El of the pump. There is a density difference between the exit and the population, and the extracted gas molecule at the exit will diffuse back to the entrance side.
(2) The directional movement of oil molecules collides with the pumped gas molecules, and some of them will change the direction of movement into the pumped system. In addition, the molecular density of oil vapor at the nozzle is greater than that of the pumped system, and the oil molecules will diffuse back into the pumped system. These two harmful back-diffusion are one of the reasons that affect the increase of the limit vacuum degree of oil diffusion pump, and also cause oil vapor pollution to the vacuum system.
Improving the design of nozzle and the quality of diffusive pump oil, setting liquid nitrogen cold trap at the intake end of pump, etc., can improve the ultimate vacuum degree of pump to a certain extent and reduce the oil pollution of the system. The limit vacuum of ordinary oil diffusion pump can reach 10-5 Pa, and that of ultra-high vacuum oil diffusion pump can reach 5 X 10-8 Pa.
The oil diffusion pump can work normally only after the system pressure is pre-pumped below the initial working pressure of the diffusion pump by the front pump, and the oil diffusion pump must heat the pump oil before the normal pumping operation. The oil diffusion pump must be cleaned and filled with sufficient working fluid before assembly. The cooling water jacket of the pump body must be supplied with sufficient pressure cooling water. Do not heat diffusion pump oil without cooling water. When shutdown, stop heating first, and then stop cooling water after oil cooling.