Monatomic working gases vs air for the closed Brayton cycle

Monatomic gas mixtures have low Prandtl numbers, as low as 0.25 or even lower, compared to about 0.7 for air. In a closed Brayton cycle using a low Prandtl number working gas, the three heat exchangers in the cycle will in general be smaller than in a comparable cycle using air. A system study has been made to compare several promising monatomic gases with air, and to explore the economic incentive to replace air in the closed Brayton cycle with a monatomic gas.