Pressure drop analysis of steam condensation in air-cooled circular tube bundles

Pressure losses on the condensing-side of an air-cooled condenser (ACC) have the potential to inhibit condenser performance and, ultimately, curtail plant efficiency. However, little information is available on the magnitude and effect of these losses in an ACC under typical Rankine cycle operating conditions. This article seeks to improve current understanding on steam-side pressure losses in ACCs by presenting an experimental study on the losses in a full-scale ACC circular tube bundle. Saturated steam at low pressure was condensed by a cross flow of cooling air, provided by a bank of axial fans. Full condensation occurred in all measurements, which were carried-out over a steam pressure and temperature range of approximately 0.05-0.14 bar absolute and 33-55 °C, respectively. These test parameters ensured that measurement program test conditions were representative of those expected in an operational thermoelectric power plant. Experimental mass fluxes, per individual tube, varied from 0.7 to 2 kg/m2 s during testing. The pressure drop characteristics were, therefore, analysed over a vapour Reynolds numbers range of 1890-5150 and liquid Reynolds number range of 25-95. Results indicate that the measured pressure drop through the tube bundle was relatively small, in the range of 130-250 Pa. As shown in this article, the reason for this was due to momentum recovery as the steam condenses to form liquid condensate. This phenomenon offsets the frictional losses, which are shown to be comparable in magnitude to momentum recovery in a condensing flow. However, this may not always be the case. Therefore, since the frictional component is traditionally the most problematic to predict, a range of liquid-gas two-phase frictional pressure drop predictive models were reviewed, and are presented herein. Comparisons between these models and the experimental data show that the most applicable model was found to be that of Lockhart & Martinelli. This demonstrated reasonable accuracy of ±18%.