Pump fluid backstreaming

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

The attainabie uitimate pressure depends on the construction of the pump, the vapor pressure of the pump fluid used, the maximum possibie condensation of the pump fluid, and the cieaniiness of the vessei. Moreover, backstreaming of the pump fluid into the vessei shouid be reduced as far as possibie by suitabie baffles or coid traps (see Section 2.1.6.4). 

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... 

In practice, however, complete suppression of oil-backstreaming is never attained. There are always a few pump-fluid molecules that, as a result of collisions with one another, reach the vessel without having hit one of the cooled surfaces of the baffle or the cold trap. Moreover, there are alvrays a... 

Backstreaming vapor pump fluids, vapors of oils, rotary pump lubricants, and their cracking products can significantly disturb various working processes In vacuum. Therefore, It Is recommended that certain applications use pumps and devices that reliably exclude the presence of hydrocarbon vapors. 

A possible problem with diffusion pumps is that of backstreaming whereby pump fluid molecules emerge, particularly from the top jet, in the direction of the chamber, where they can condense. Oil contamination can be significantly reduced by the use of baffles (cold-cap, shell) and traps between the pump and the system. 

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