Experimental investigation on the characteristics of pressure drop and air/vapor flow over horizontal tube bundle with water-spray falling film

The horizontal tube falling film evaporation technology is one of the most promising desalination methods owing to its various advantages. The pressure drop of vapor plays a pronounced role in evaporation efficiency in low-temperature multiple-effect desalination (LT-MED). In this study, an experimental test bench by using air in the horizontal direction across a tube bundle with a vertical downward falling film was designed, and the flow characteristics of vapor of a large-scale desalination plant were simulated. Falling film pressure drops (FFPDs) under different conditions of air-inlet velocity, water-spray density, heating flux, and tube column number were obtained and analyzed. The experimental results indicate that FFPD can quadratically increase with the rise in air velocity, linearly rise with increments of spray density, and rapidly grow with the cumulative increase in tube columns. However, heat flux presented a minimal effect on FFPD because of the predominant portion volume of airflow relative to that of evaporated vapor. On the basis of the dimensionless analysis and the least-square multiple regression, a brand-new fitting correlation was proposed to predict FFPD. The proposed method corresponded well with the experimental data within the deviation of ±15%. The correlation also estimated the inter-tube average vapor velocity to be within 5–20 m/s in the first-effect evaporator of field-scale LT-MED in engineering. The brand-new correlation can also provide a theoretical reference in engineering design that considers vapor velocity and flow resistance in LT-MED.