Characterization of ACC Losses Due to Wind and Design of Optimal Wind-Screens

Abstract

Abstract Large mechanical draft Air-Cooled Condensers (ACC) are increasingly preferred, as a more environmentally friendly solution than wet cooling, in the cooling processes necessary to condense the working fluid in Rankine cycle-based power production systems. Such heat exchangers are conceived as an assembly of smaller modules each composed of an axial fan and inclined bundles where the fluid (usually steam) condensation actually takes place. The fan is usually installed suspended above the ground at a height between 10 and 25 m, depending on the size of the ACC, with upward flow. Such a layout makes ACCs prone to a decay of efficiency and the onset of fan blade fluctuations due the crossflow induced by neighbour fans. Such detrimental effects become fundamental (up to 50% reduction in ACC cooling potential at 10 m/s), eventually resulting in fan failures, in case of high-speed winds. Current work is devoted to the study of the wind effects on the cooling performance of the ACC and on blade load oscillations analyzing in detail the effect of wind speed and wind direction on a mid-sized ACC with 6 rows and 3 streets arrangement for 205.4 MW nominal thermal duty. The study is conducted by means of high-fidelity CFD consisting of Unsteady RANS computations of the entire ACC with all 18 axial fans under rotation and a direct meshing of all the 6 blades per fan. A common and relatively simple strategy used to mitigate such wind losses is the installation, directly on the ACC structural columns, of partially porous screens made of plastic fabric that act as wind breakers. The correct design of these wind screens is however quite challenging since they not only help reducing the wind speed below the ACC but also introduce additional aerodynamic losses that the fan needs to sustain to correctly feed the heat exchanger. This study verifies the gains obtainable with the installation of wind screens, proposing a detailed investigation about different windscreen layouts (e.g. perimeter, cruciform, one-bay-back and skirt), variable screen porosity and dimensions.