An experimental investigation was conducted to evaluate the steam heat removal capacity in the presence of non-condensable gases (e.g. air

helium) over a vertical tube external surface under moderate wall subcooling. Under steam/air condition

condensation heat transfer coefficients were obtained under wall subcooling ranging from 13℃ to 25℃

total pressure ranging from 0.4 MPa to 0.6 MPa and air mass fraction ranging from 0.07 to 0.52. The experiments for the influence of wall subcooling on steam condensation heat transfer with a fixed pressure and air mass fraction were made. Under the same pressure with the same non-condensable gases mass fraction

the effect of wall subcooling on condensation heat transfer coefficient with non-condensable gases was negative. An empirical correlation for heat transfer coefficient was developed

covering all data points within 15%. Under steam/air/helium (simulating hydrogen) condition

the effect of helium mole fraction in non-condensable gases on heat transfer coefficient was investigated under wall subcooling ranging from 18℃ to 27℃

total pressure ranging from 0.53 MPa to 0.6 MPa

steam mass fraction ranging from 0.6 to 0.92 and helium mole fraction in non-condensable gases 0.3. The condensation heat transfer coefficients obtained from steam/air/helium condition were lower than those obtained from steam/air case. Helium stratification was not found under the experimental conditions. With the same non-condensable gases mass fraction

presence of helium lowered condensation heat transfer coefficient by around 20%.