Conventional modeling of condensers always assumes three zones: desuperheating, condensation and subcooling even condensation occurs in desuperheating zone at some conditions and that subcooling occurs during condensation. This paper discusses the actual situation and provides experimental validation of the hypothesis. The experimental results show heat transfer coefficients of CO_2 and R410A at mass fluxes 100 to 240 kg m~(-2)s~(-1) and reduced pressures from 68% to 100% in a horizontal smooth tube of 6.1 mm inner diameter. Data are compared to correlations proposed for other working fluids or other conditions. Results show much higher values of HTC than correlation proposed for single-phase turbulent flow in superheated zone. The occurrence of condensation in superheat zone is evident when tube wall temperature is below saturation temperature. The results suggest that simplified calculations of heat rejection in superheated zone significantly oversize the condenser. The semi-empirical correlation, which is here proposed as the combination of existing correlations for single-phase turbulent and saturated condensation, satisfactorily predicts the heat transfer coefficient of the superheat zone condensation.
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