Pump fluid backstreaming and its suppression Vapor barriers, baffles

4 Pump fluid backstreaming and its suppression (vapor barriers, baffles) 

In the vapor stream from the topmost nozzle of a diffusion pump, pump fluid molecules not only travel in the direction of streaming to the cooled pump wall, but also have backward components of velocity because of intermolecular collisions. They can thus stream in the direction of the vessel. In the case of LEYBODIFF and Dl pumps, the oil-backstreaming amounts to a few micrograms per minute for each square centimeter of inlet cross-sectional area. To reduce this backstreaming as much as 

A cold cap baffle is constructed so that it can be mounted immediately above the high vacuum nozzle. The cold cap baffle is made of metal of high thermal conductivity in good thermal contact with the cooled pump wall, so that in practice it is maintained at the cooling-water temperature or, vi/ith aircooled diffusion pumps, at ambient temperature. In larger types of pumps, the cold cap baffle is water cooled and permanently attached to the pump body. The effective pumping speed of a diffusion pump is reduced by about 10 % on installation of the cold cap baffle, buf the oil backsfreaming is reduced by about 90 to 95 %. 

For the smaller air-cooled, oil diffusion pumps, plate baffles are used. The air-cooled arrangement consists of a copper plate with copper webs to the housing wall. The temperature of the plate baffle remains nearly ambient during the operation of the diffusion pump. 

If extreme demands are made on freedom from oil vapor with vacuum produced by oil diffusion pumps, cold traps should be used that are cooled with liquid nitrogen so that they are maintained at a temperature of-196 °C. 

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