Pumping of gases dry processes

For dry processes in which a non condensable gas mixture (e.g., air) is to be pumped, the pump to be used is clearly characterized by the required working pressure and the quantity of gas to be pumped away. The choice of the required working pressure is considered in this section. The choice of the required pump is dealt with in Section 2.3. 

For the production of pressures in the ultrahigh vacuum region, sputter-ion, and sublimation pumps, as well as turbomolecular pumps and cryopumps, are used in combination with suitable forepumps. The pump best suited to a particular UHV process depends on various conditions (for further details, see Section 2.5). 

Water vapor is frequently removed by pumps that operate with water or steam as a pump fluid, for example, water ring pumps or steam ejector pumps. This depends considerably on circumstances, however, because the economy of steam ejector pumps at tow pressures is generally far inferior to that of rotary pumps. For pumping a vapor - gas mixture in which the vapor portion is large but the air portion is small, the vapor can be pumped by condensers and the permanent gases, by relatively small gas ballast pumps (see Section 2.1.5). 

Comparatively, then, a pump set consisting of a Roots pump, condenser, and backing pump, which can transport 100 kg/h of vapor and 18 kg/h of air at an inlet pressure of 50 mbar, has a power requirement of 4 -10 kW (depending on the quantity of air involved). A steam ejector pump of the same performance requires about 60 kW without altering the quantity of air involved. 

For the pumping of water vapor, gas ballast pumps and combinations of gas ballast pumps, Roots pumps, and condensers are especially suitable. 

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