Theory of pumping

In order to be able to design even the most elementary vacuum line, it is necessary to know something of the basic theory concerning the movement of molecules within the system. No attempt will be made here to instruct the reader in the details of vacuum physics and therefore the formulae given below have been kept simple. A more detailed discussion of the theory of gaseous flow can be found in Bushman s excellent review (Bushman, 1962). The object of the following discussion is simply to allow the reader to assess, without too much effort, the approximate efficiency of the system he is planning. 

A chemist s vacuum line will often be pumped out several times within any working day and therefore the time taken to reach a useful vacuum should be as short as possible. The time taken by a system to reach the described vacuum depends upon the volume of the system, its internal surface area, the materials used for its construction, its cleanness, the speed of the pumps and the aerodynamic conductance of the system. 

The conductance of any system depends on the nature of the gas flow at higher pressures a viscous flow regime prevails and at lower pressures (p 10 Torr) a molecular flow regime. Turbulent flow, seldom considered when assessing the efficiency of a vacuum system, is encountered only when the pressure in the system is close to atmospheric pressure. As a general rule, if the efficiency of the system is adequate for the viscous flow regime then it will also be suitable for turbulent flow. 

The change from a viscous to a molecular flow regime occurs when the mean free path L of the gas molecules in the system exceeds the minimum physical dimensions of the system. The mean free path is a measure of the average distance a molecule travels between collisions. The derivation of L involves a number of assumptions about the ideality of the gas and the nature of the collisions and by definition some 63.2% of the molecules in a particular gas collide with other molecules within the distance L. The mean free path for any gas can be calculated from Equation (1.1) 

Since the main pumping lines are usually constructed from tubing with an internal diameter (i.d.) of ca. 2.5 cm, viscous flow predominates until the pressure is reduced to ca. 10 Torr, when the high vacuum pump is usually switched into the system. 

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Fried, L.E. and Mukamel, S. (1990). A classical theory of pump-probe photodissociation for arbitrary pulse durations, J. Chem. Phys. 93, 3063-3071. 

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