Analysis of gas separation, conductance and equivalent single gas approach for binary gas mixture flow expansion through tubes of various lengths into vacuum

• The flow of binary gas mixtures through tubes of various lengths into vacuum is considered.
• The gas separation intensity, the conductance and the equivalent single gas approach are analyzed.
• The effect of the molar mass ratio and fraction of the mixture components on the flow is investigated.
• Practical guidelines of the gas mixture flow behaviour are provided.

The steady-state binary gas mixture expansion through tubes of various lengths into vacuum is considered, focusing on the intensity of gas separation, the computation of conductance and the implementation of the equivalent single gas approach in terms of the mixture composition and its molar fraction in a wide range of the Knudsen number. The analysis is based on the dimensionless flow rates of He-Ne, He-Ar, He-Kr and He-Xe. Gas separation is characterized by the ratio of the dimensionless flow rates of the two species and it is increased as both the Knudsen number and the square root of the heavy over the light molar mass ratio of the components are increased. The gas mixture conductance is increased as the molar fraction of the light species is increased and it is bounded from below and above by the conductance of the heavy and light species respectively. The error introduced in the equivalent single gas approach is increased along with the difference between the molar masses of the species and the Knudsen number. These statements are valid for any tube length. Quantitative results are also provided. Practical guidelines, which may be useful in industrial applications and measurements under vacuum conditions, are deduced.