Vertical tube length calculation based on available heat transfer coefficient expressions for the subcooled flow boiling region

Tube length calculation is usually an important step in the design process of water tube bank evaporators. In this work water enters a vertical tube as compressed liquid and leaves at low vapour quality values whereas an external hot gases stream perpendicular to the tube supplies the heat flux required to the evaporation process. Temperature and velocity of hot gases are considered uniform thus leading to non uniform wall temperature and heat flux along the tube. The first step was to apply the experimental information available on heat transfer coefficients to determine the tube length required in the case under study. A variety, uncertainty and, specially, a lack of continuity of the experimental heat transfer correlations in the subcooled flow boiling region is observed. As a consequence, and only for design purposes, new expressions for the heat transfer coefficient in this region as a function of the length are analysed and, finally, one of them is proposed. It presents the advantage of allowing continuity of the heat transfer coefficient from the single-phase liquid region to the two-phase saturated flow boiling one.