Development of porous media thermosyphon technology for vapor recovering in cross-current cooling towers

摘要

Cooling towers are known for their efficiency in cooling industrial water streams from moderate down to nearly ambient temperatures. However, 3-5% of the cooled water is lost to the environment in vapor phase, dragged by the air streams that exit the tower nozzles. This wasted water, usually replenished from local watershed, is being considered a major ecologic problem in some countries. The outcomes of a seven-year research, which resulted in a patent of a passive technology, designed to recover partially the vapor in wet cross-current cooling towers, is presented. The idea is to condensate vapor by removing heat from the hot air streams, using the environment as the heat sink. Two vapor condenser technologies are proposed: cooled flat plates (located after the nozzle exit) and cooled porous media, located in the cooling chamber plenum, within which humid hot air percolates before being pushed away by the fan. In both cases, thermosyphons are used to transfer heat to the ambient air, over the cooling tower. The resulting condensate is pulled to the tower bottom by means of gravity. Based on a typical year study, several numerical simulations were performed, showing that the cold plate technology is not viable. Parametric experimental studies in several porous media configurations were performed using a special setup and the best vapor recovering potential, of more than 30%, was found for stainless steel stripe commercial sponges. A prototype of the complete device, consisting of porous media plus tube arrays mimicking thermosyphons, was constructed in an especially designed reduced-scale cooling tower. The stainless steel commercial sponges attached to the evaporators of thermosyphon arrays, which condensers are located beside the nozzle exit, showed to be the best configuration for the vapor recovery device proposed, as it is passive, simple, cheap and adaptable to existing cooling towers. Although not yet optimized, the device is very promising, being able to recover, in average, 10% of the lost vapor, which is comparable with other available technologies. (C) 2016 Elsevier Ltd. All rights reserved.