An extensive database has been collected for the evaporation of two hydrofluorocarbon refrigerants (HFC- 134a and the zeotropic mixture HFC- 407C) and the natural refrigerant ammonia (R-717). A model to predict the onset of nucleate boiling has been successfully developed to differentiate pure convective evaporation from mixed convective and nucleate boiling heat transfer during horizontal evaporation inside a horizontal tube of 14mm I.D. The analysis of the Stratified, Stratified-Wavy and mainly Annular flow patterns during evaporation with differente heat flux ranges showed very accurate results in terms of mean local heat transfer coefficient using this new onset of nucleate boiling criterion.