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Lack of precision and inconsistencies in existing correlations, an attempt was successfully made to develop a new and more mechanistically based correlation (Kattan, 1996 [1]) for boiling inside plain tubes. This new general correlation is original in that it takes into account the effect of two phase flow patterns on evaporative heat transfer. As a first step, the Steiner (1993, [4]) flow map was rewieved and a proposal was made to better account for the transition boundaries between stratified-wavy and annular flow in particular. The importance of the heat flux on the transition between annular flow and annular with partial dryout was identified and original propositions were developed for predicting its occurence, which allows a significantly more accurate prediction of the heat transfer coefficients at high vapor qualities in the boiling process. An extension of the new plain tube correlation is proposed for microfin tubes using enhancement factors which are calculated empirically for the effects of the fins on convective and nucleate boiling contributions to heat transfer.