Mechanical pumps backstreaming

The use of a quartz crystal microbalance for measuring vapour backstreaming from mechanical pumps Vacuum, Volume 16 (11), 633-637,1966... 

All of the aforementioned pumps require oil for operation. One of the drawbacks to standard mechanical pumps is their propensity toward backstreaming of their oils, causing the potential contamination of the system. Once the pump has achieved its greatest vacuum, its efficiency drops to zero. At that point there is lit-... 

Among the duties of oils in standard mechanical pumps are those of protection. Since scroll pumps do not have oil, they are dependent upon traps and baffles for protection against gaseous and particulate contamination. Although backstreaming of pump oils is not a concern from a scroll pump, the need for traps still remains. Ironically, the demands of maintenance on traps for scroll pumps is greater than for regular pumps due to the total lack of protection the pump has for itself. 

Oils used in mechanical pumps have significantly higher vapor pressures than the oils used within diffusion pumps. Therefore, it is important to prevent backstreaming of mechanical pump oils into the diffusion pump. This barrier can be done either with liquid nitrogen cold traps, molecular sieves, water-cooled thimbles, or chevron baffles. 

Backstreaming can be one of the main limitations for mechanical pumps to achieve a better vacuum. Because molecular sieve and Micromaze traps are so efficient at capturing (and not releasing) vapors, Strattman experimented with a Micromaze foreline trap to trap the hydrocarbon oils from a mechanical pump. After proper baking and cooling, he was able to achieve pressures of 10 5 torr consistently with only a mechanical vacuum pump. 

Backstreaming. There are two types of backstreaming both are not desirable, but one can be quite deleterious. Once the vacuum system achieves its ultimate vacuum and is in molecular flow, gases from the system can drift back into the system. This is simply the results of equilibrium. On the other hand, if oils or vapors from the diffusion pump drift into the system, or oils from the mechanical pump drift into the diffusion pump, the effects are more profound. 

If there are any pressure blips in the system and the mechanical pump cannot remove them fast enough, a diffusion pump will be swamped. That is, there is not enough backing pressure to maintain the gradient pressures within the diffusion pump and the gases contained within backstream into the system. The only pre-... 

Uncontrollable backstreaming can occur if there is a total system power failure that controls your vacuum system. If power in your building should go out and all mechanical pumping ceases, the efficiency of the traps and baffles to maintain a barrier in a static system will be tested. If someone is around at the time, close all stopcocks between each pump from any other and the stopcocks between the pumping system and the vacuum system. 

The freeze-dryer is equipped with a mechanical pumping system that removes noncondensable gases. With oil sealed mechanical pumps, one should be careful to operate the dryer so that no hydrocarbon vapours from the pump can backstream into the drying chamber. 

The hydrocarbon lubricating and sealing oils used in mechanical pumps must not be allowed to backstream or creep to the DP and contaminate the DP oil Power failure, cooling failure, or mistakes in operating a diffusion-pumped system can result in pump oil contaminating the processing chamber. In some applications, cryopumps or turbopumps are used instead of DPs to avoid the possibility of oil contamination. 

Elimination of backstreaming from mechanical vacuum pumps J. Vac. Sci. Technol. A5 (4), 1987, 2612-2615... 

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