Media vacuum

Horizontal Rotating Pan Filters. These filters (Fig. 10) represent a further development of the tipping pan filter for continuous operation. They consist of a circular pan rotating around the central filter valve. The pan is divided into wedge-shaped sections covered with the filter medium. Vacuum is appHed from below. Each section is provided with a drainage pipe which connects to a rotary filter valve of the same type as in dmm filters. 

Rough vacuum Medium vacuum High vacuum Ultra-high vacuum... 

Rough vacuum Medium vacuum High vacuum Ultra high vacuum 760 torr to 1 torr 1 to 10-3 torr 10-3 to 10 torr 10 7 torr and below... 

Pumping time From normal pressure To medium vacuum approx. 1 x 10-3 torr To high vacuum approx. 1 x 10-6 torr... 

Rough vacuum (RV) Medium vacuum (MV) Fligh vacuum (FIV) Ultrahigh vacuum (UFIV)... 

The transitional range between viscous flow and molecular flow is known as Knudsen flow. It is prevalent in the medium vacuum range = d. 

Fig. 2.40 Cross section of a medium vacuum adsorption trap...

For the pumping of large quantities of gas in this pressure region, vapor ejector pumps are by far the most suitable. With mercury vapor ejector pumps, completely oil-free vacua can be produced. As a precaution, the insertion of a cold trap chilled with liquid nitrogen is recommended so that the harmful mercury vapor does not enter the vessel. With the medium vacuum sorption traps described under a), it is possible with two-stage rotary vane pumps to produce almost oil-free vacua down to below 10" mbar. 

Absolutely oil-free vacua may be produced in the medium vacuum region with adsorption pumps. Since the pumping action of these pumps for the light noble gases is only small, vessels initially filled with air can only be evacuated by them to about 10 2 mbar. Pressures of lO mbar or lower can then be produced with adsorption pumps only if neither neon nor helium is present in the gas mixture to be pumped. In such cases it can be useful to expel the air in the vessel by first flooding with nitrogen and then pumping it away. 

In the medium vacuum region, the gas evolution or the leak rate becomes significantly evident. From the nomogram 9.10 in Section 9, the corresponding calculations of the pump-down time in this vacuum region can be approximated. 

Classical physics teaches and provides experimental confirmation that the thermal conductivity of a static gas is independent of the pressure at higher pressures (particle number density), p > 1 mbar. At lower pressures, p < 1 mbar, however, the thermal conductivity is pressure-dependent (approximately proportional 1 / iU). It decreases in the medium vacuum range starting from approx. 1 mbar proportionally to the pressure and reaches a value of zero in the high vacuum range. This pressure dependence is utilized in the thermal conductivity vacuum gauge and enables precise measurement (dependent on the type of gas) of pressures in the medium vacuum range. 

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