Different Heat Exchanger Options for Natural Draft Cooling Towers

This paper examines the application of different heat transfer enhancement mechanisms to improve the performance of an air-cooled heat exchanger. Specific attention is paid to a natural draft cooling tower which is to be applied as the air-cooled condenser of a geothermal power plant. The conventional method of extending the heat transfer area by means of extruding fins is compared with a modern technique being the application of a metal foam heat exchanger applied as a layer to the outer surface of the tube. Both designs improve the heat transfer rate from the phase-change fluid flowing in the tube bundle albeit at the expense of a higher pressure drop compared to the bare tube as our reference case. Considering the heat transfer enhancement as the benefit and the excess pressure drop as the cost, the two cases are compared against each other. In order to achieve this goal, three different software have been implemented and the results were crossvalidated. These include two commercially available software being CFD-ACE and ASPEN B-JAC as well as a FORTRAN code developed in QGECE. A number of correlations are also proposed to predict the cost (excess pressure drop) and the benefit (heat transfer augmentation) of the extended surface.