A multi-stage down-draft evaporative cool tower for semi-enclosed spaces. Part II: Water spraying system

In a conventional down-draft evaporative cool tower (DECT), dry ambient air is drawn in at the top and cooler moist air is delivered at the bottom. Most of the cooling occurs near the inlet, where water is introduced and air temperature approaches the wet bulb. A novel DECT was developed which incorporates a secondary air inlet and complex longitudinal section that comprises two partly overlapping cones. The complex section and the addition of a secondary air intake near the middle required a sophisticated water spraying system. In addition to maximizing the cooling potential, the spraying system was designed to limit spray drift beyond the base of the tower, to reduce maintenance costs (especially due to clogging of the sprayers) and to minimize pumping energy. Analysis shows that maximum cooling may be obtained either by employing a very fine spray, requiring the introduction of a relatively small volume of water, or by spraying a larger volume of coarser drops. However, spraying fine drops requires more pumping power, finer nozzles are more likely to clog and small drops of water aggravate the problem of drift near the tower base. If full evaporation of the water spray is not required and excess water is collected for reuse, the second option is thus preferable. In addition to the theoretical analysis, the paper presents experimental findings on temperature reduction, water consumption and cooling output of an 8-meter high prototype tower constructed at Sde Boqer, Israel.