ESDU 91023 develops a prediction procedure for annular/dispersed flow based on that used for vertical downflow in ESDU 90024. It does not apply to bubble or slug flow but in those cases the pressure drop is small. The method uses existing correlations for two-phase adiabatic flow (Friedel with a roughness correction for the friction component, Premoli et al to establish the vapour density/liquid density ratio and hence gravitational component, and homogeneous theory for the momentum component). Correction factors to that prediction are developed by comparing results with experimental data extracted from the literature for the flow of steam or halogenated hydrocarbons over a wide range of pressure, quality, and heat and mass flux. Values are tabulated of the correction factor applying for given values of density ratio and condensation number (heat flux divided by the product of mass flux and latent heat of vaporisation). Also tabulated, in terms of those two parameters and the quality, are the number of test results available and the standard deviation. A step-by-step calculation process is set out and illustrated with a worked example. See ESDU 93014 for a Fortran program of this calculation method. In practice, the pressure drop at inlet to the tubes of a condenser dominates, and it is shown how it may be estimated using ESDU 89012.