Performance enhancement of heat exchanger coolers with evaporative cooling

Abstract

Air or water cooled heat exchangers (HX) are typically utilized as condensers or coolers for air-conditioning, refrigeration or process cooling applications in both commercial and industrial sector. However, air cooled heat exchanger performance degrades considerably with rise in ambient air temperature and water cooled coolers require considerable pumping power, a cooling tower and may consume a significant amount of water which may come from fresh water sources. Evaporative cooling offers a unique solution to this problem, where a small amount of wetting water evaporates on HX surface to boost performance in high ambient air temperature conditions. In this study, several evaporative cooling technologies were applied to three wavy-fin HXs to quantify capacity enhancement ratio (CER) and air-side pressure drop penalty ratio (PRΔPa) compared to respective dry case baseline values. Effect of varying wetting water flow rate, air velocity, fin spacing, hydrophilic coatings, spray orientation and inlet air temperature and relative humidity was investigated on hybrid heat exchanger performance. Several new performance comparison parameters were defined to compare different evaporative cooling approaches. Deluge cooling achieved overall highest CER but at a PRΔPa that was similar in magnitude to the CER. This limitation was found to be inherent to the nature of wetting water distribution method itself. Although front spray cooling tests indicated PRΔPa~1, front spray evaporative cooling technology was found to have up to 23-75 % lower CER at 60-100% lower PRΔPa compared to deluge cooling. In order to understand the wetting behavior a novel visualization method was proposed and implemented, which consisted of borescope assisted flow mapping of water distribution within the HX core as a function of air velocities and wetting water flow rates. It was found that up to 85% of HX volume remained dry during front spray cooling which accounted for lower capacity enhancement and deluge cooling forms non-uniform and thick water film which causes bridging and increased PRΔPa, A larger component level testing with HX size similar to commercial units allowed to identify constraints of different evaporative cooling methods, which would not be possible if tests were performed at a smaller segment or fin level. A novel spray cooling technology utilizing internal jet spray cooling within HX volume was both proposed and implemented and a provisional patent # 61/782,825 was obtained. Compared to front spray cooling at a given spray rate, internal spray cooling could either achieve up to 35% higher HX cooler capacity, or obtain same HX cooler capacity at approximately three times lower air-side pressure drop. Alternatively, at same air-side pressure drop wetting water savings of up to 68-97% are achieved. Internal spraying combines advantages of conventional technologies and overcomes the drawbacks, by getting CER of approx. 3.8, without film carryover and at PRΔPa=1, while getting maximum wetting uniformity. Intermittent cooling combined with internal spraying could reduce water consumption as evaporative cooling sustains though the brief period when spray is turned off. Thus, potential for significant energy and water savings, targeted cooling, and retrofit design offers significant commercialization opportunity for future hybrid evaporative coolers. Discussions are underway for the inclusion of this technology into product line up of a leading HX manufacturing company.