Few experimental test data are available for evaporation of ammonia inside tubes and numerous new data have been measured and presented here. An improved approach to the prediction of flow boiling heat transfer in horizontal tubes has been proposed through the study of each flow pattern separately, incorporating a new criterion defining the onset of nucleate boiling as a function of the critical convective heat transfer coefficient representative of the location where nucleate boiling might occur. A new function, based on a pseudo-Biot number delineates two different mean heat fluxes on the perimeter of the tube in stratified types of flow, one in contact with the liquid and one in contact with the vapor. Considering pure convective heat transfer, or mixed convective and nucleate heat transfer, this division allows the use of a common criterion to be applied to each flow pattern. Even if the database showed that the flow conditions in the annular liquid film were close to, or in the turbulent to laminar flow transition, and even if the major part of the experimental points where purposely obtained close to the various flow pattern transitions, the new model showed very good agreement with the experimental database of refrigerants HFC-134a and ammonia. Due to the precision of the new flow pattern map and the effectiveness of the onset on nucleate boiling criterion, this new heat transfer model accurately predicts the heat transfer conditions during evaporation.