û û_{o}1 S(P_{f}P_{b})Appendix ADOE-HDBK-1015/1-93BASIC SEPARATION THEORYRev. 0CH-01Page A-5BarrierMeasurementsThe ability of a barrier to transmit gas is usually expressed as û (gamma), thepermeability. This is a measure of the total flow through a barrier and may be definedas the ratio of the number of molecules which pass through the barrier to the numberwhich would pass through the space if the barrier were not there. This total flowthrough a barrier is known to be a combination of several different types; namely,diffusive flow, film flow, and viscous flow. As was previously explained, the type offlow that is of greatest importance for a gaseous diffusion plant is diffusive flow.However, as film flow and viscous flow affect the separation through a barrier, theycannot be ignored.Film flowrefers to the transport of molecules under the influence of the force fieldbetween the molecules and the barrier surface. When such force fields are significant,a large number of molecules will not have sufficient velocity normal to the surface toescape from this field so that their motion will be confined to the barrier surface andbarrier pore surface. Such molecules would pass through the barrier as a film on thesurface of the pores.Viscous flowoccurs when molecules flow as a group in the manner of ordinary flowthrough a tube. Referring back to permeability, it is known that the flow through abarrier increases as the pressure increases. Also, as the pressure increases, so does theviscous component of flow. The relation between permeability and viscous flow can bestbe shown by the following equation.In this equation where P equals the fore or high side pressure and P equals the back orf blow side pressure. û equals the permeability, û, whenoP + P = 0.f bS is called the slope factor and is inversely proportional to the viscosity of the gas.In the above equation, the term S(P + P ) is a measure of the viscous component off bflow. Figure A-2 illustrates the variation of permeability with the slope factor andchanges in pressure.