Correlations and analysis of heat transfer coefficient for seawater falling film evaporation on horizontal tube

Abstract

Falling film evaporators are crucial for producing freshwater through seawater desalination, where accurately predicting heat transfer coefficients (HTC) directly impacts equipment efficiency and energy use. While current average HTC correlations work well for estimating overall performance in horizontal tube evaporators, their inability to resolve circumferential HTC variations limits detailed thermal analysis and improvements. This study proposes local HTC correlations for seawater falling film evaporation through experiments on 25.4 mm tubes under conditions: saturation temperature T_sat = 45–70 °C, Reynolds number Re_Γ = 150–655, seawater salinities S = 30-60 g/kg. Results reveal that the circumferential HTC first drops quickly (0°-45°), then decreases gradually (45°-135°), and finally shows a small recovery (135°-180°). This trend leads to the identification of three distinct heat transfer zones, which are well predicted by proposed correlations. The HTC initially increases and subsequently decreases with spray density, while it consistently declines with higher saturation temperatures and seawater salinities. The new correlations enhance the understanding of seawater evaporation processes while providing practical guidelines for creating energy-efficient desalination systems.