Simulation of Effective Plume-Chimney Above Natural Draft Air-Cooled Heat Exchangers

Effective Plume-Chimney Height (EPCH) was a factor engineers used to design and analyse the performance of natural convection in air-cooled heat exchangers particularly in the event of power outage. To date the number of papers in the open literature presenting data on natural convection performance of air-cooled heat exchangers is scarce. The aim of this study is to corroborate the experimental results and theoretical predictions of Effective Plume-Chimney Height (EPCH) using Computational Fluid Dynamics (CFD) in a laboratory-scale air cooled heat exchanger of 457mm × 457mm face area and an industrial-scale test rig of 2.4m × 6.0m face area forced draft air-cooled heat exchanger comprising of a bundle with 4 rows of annular finned tubes in staggered formation operating under natural convection. The CFD software Phoenics 2015 was employed to simulate the electrically-heated air-cooled heat exchanger fitted with a top screen which was built to study the aerodynamics of natural convection of air-cooled heat exchangers. The CFD geometry arrangement and dimensions were schematic in nature, where errors introduced were considered reasonably negligible. The laboratory-scale exchanger model experimental pressure drop data was found to have an insignificant effective plume-chimney height, as predicted by a theoretical equation. It was found that EPCH values calculated from CFD results agree closely to within −0.11m and +0.06m with both experiments and the theoretical prediction, confirming the same conclusion reached in an earlier report. However, for an industrial-scale test rig (ITR) in forced draft mode of large face dimensions the EPCH had been found to be non-negligible in an earlier work. Significant values of theoretical effective plume-chimney height were inserted in the heat transfer and pressure drop simulation that appeared to yield results that agreed with the experimental heat loads. The CFD simulations on the ITR have confirmed the existence of significant effective plume-chimney heights at more than 100 percent of the bundle depth, or the chimney height. The implication is that a solid-walled chimney can appear to have an efficiency of more than 100 per cent, if cold inflow can be prevented or the penetration to the central core hindered. Since the validation of the existence of EPCH by CFD here has used only a set of data from a single source, it is worthwhile to produce more experimental data and analysis to establish the concept for better predictions of air-cooled heat exchanger natural convection performance.