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Creating oil free vacuum with rotary pump accessories

During a vacuum process, substances harmful to rotary pumps can be present in a vacuum chamber. Various accessories to prevent contamination are available, as described below. In particular, reducing water vapor, oil vapor and dust particles through the use specialized filters or adsorption traps is discussed.

Elimination of water vapor

Water vapor arises in wet vacuum processes. This can cause water to be deposited in the inlet line. If this condensate reaches the inlet port of the pump, contamination of the pump oil can result. The pumping performance of oil-sealed pumps can be significantly impaired in this way. Moreover, water vapor discharged through the outlet valve of the pump can condense in the discharge outlet line. The condensate can, if the outlet line is not correctly arranged, run down and reach the interior of the pump through the discharge outlet valve. Therefore, in the presence of water vapor and other vapors, the use of condensate traps is strongly recommended. If no discharge outlet line is connected to the gas ballast pump (e.g., with smaller rotary vane pumps), the use of discharge filters is recommended. These catch the oil mist discharged from the pump.

Some pumps have easily exchangeable filter cartridges that not only hold back oil mist, but also clean the circulating pump oil. Whenever the amount of water vapor present is greater than the water vapor tolerance of the pump, a condenser should always be installed between the vessel and the pump. (For further details, see Condensers)

Elimination of dust

Solid impurities, such as dust and grit, significantly increase the wear on the pistons and the surfaces in the interior of the pump housing. If there is a danger that such impurities can enter the pump, a dust separator or a dust filter should be installed in the inlet line of the pump. Today, not only conventional filters having fairly large casings and matching filter inserts are available, but also fine mesh filters which are mounted in the centering ring of the small flange. If required, it is recommended to widen the cross section with KF adaptors.

Elimination of oil vapor

The attainable ultimate pressure with oil-sealed rotary pumps is strongly influenced by water vapor and hydrocarbons from the pump oil. Even with two-stage rotary vane pumps, a small amount of back-streaming of these molecules from the pump interior into the vacuum chamber cannot be avoided. For the production of hydrocarbon-free high and ultrahigh vacuum, for example, with sputter-ion or turbomolecular pumps, a vacuum as free as possible of oil is also necessary on the forevacuum side of these pumps. To achieve this, medium vacuum adsorption traps (see Fig. 2.40) filled with a suitable adsorption material (e.g., LINDE molecular sieve 13X) are installed in the inlet line of such oil-sealed forepumps. The mode of action of a sorption trap is similar to that of an adsorption pump. For further details, see Sorption pumps.

Fig 2.40 Cross section of a medium vacuum adsorption trap.

Housing
Basket holding the sieve
Molecular sieve (filling)
Sealing flanges
Intake port with small flange
Upper section
Vessel for the heater or refrigerant
Connection on the side of the pump with small flange

If foreline adsorption traps are installed in the inlet line of oil-sealed rotary vane pumps in continuous operation, two adsorption traps in parallel are recommended, each separated by valves. Experience shows that the zeolite used as the adsorption material loses much of its adsorption capacity after about 10 – 14 days of running time, after which the other, now regenerated, adsorption trap can be utilized; hence the process can continue uninterrupted. By heating the adsorption trap, which is now not connected in the pumping line, the vapors escaping from the surface of the zeolite can be most conveniently pumped away with an auxiliary pump. In operation, pumping by the gas ballast pump generally leads to a covering of the zeolite in the other, unheated adsorption trap and thus to a premature reduction of the adsorption capacity of this trap.

Reduction of the effective pumping speed

All filters, separators, condensers, and valves in the inlet line reduce the effective pumping speed of the pump. On the basis of the values of the conductances or resistances normally supplied by manufacturers, the actual pumping speed of the pump can be calculated. For further details, see Calculating conductance.

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