Performance investigation of plain circular and oval tube evaporatively cooled heat exchangers

The performance of two evaporatively cooled heat exchangers is analysed, one has plain circular tubes while the other one has plain oval tubes. Both are investigated under similar operating conditions in relation to airflow rates and inlet hot water temperatures. The circular tube is 10 mm o.d., and the oval tube (axes ratio 3.085) is formed from an 18 mm o.d. circular tube whose perimeter is preserved after forming. It is concluded that the average mass transfer Colburn factor (jm) for the oval tube is 89% of that for the circular tube, while the average friction factor (f) for the oval tube is 46% of that for the circular tube. The ratio (jm/f) for the oval tube is 1.93–1.96 times that for the circular tube. This means that the oval tube has a better combined thermal–hydraulic performance. The heat-mass transfer analogy showed lower values for the mass transfer coefficient estimated from dry heat transfer correlations when compared with wet measurements.