Vacuum Science and Technology (VST), e-chapter 3 Flow of gases through tubes and orifices
Evacuation of a vacuum vessel by pumping requires the vessel to be somehow connected to the pump. This connection, usually consisting of valves and pipelines, will always result in some resistance for the gas on its way from vessel to pump. Obviously, for an unobstructed gas flow it will be important to keep this resistance as low as possible. A good understanding of the various gas flow types is essential to be able to determine the correct dimensions of the different necessary system components. In some cases, the proper operation of a vacuum system is undermined by a mismatch between successive vacuum components. Formulas for the laminar and molecular conductances of orifices and tubes of different dimensions are derived by application of the relevant thermodynamic and flow laws. In relation to the operation of vapour stream pumps (see chapter 4), special attention is paid to the supersonic flow through a Laval nozzle and the occurence of a choked gas flow and shock wave. The chapter is concluded with the definition of the concept ‘pumping speed’ and some calculation examples in a simple vacuum system.