Modeling, Evaluation and Application Potential of Two Stage Indirect/Direct Evaporative Air Coolers

Abstract

Two -stage indirect/direct evaporative air cooler can provide summer comfort conditions as an environmentally clean, fresh supply air and energy efficient cooling system for some regions with wide variety climatic conditions. In this paper, a general mathematical he at and mass transfer model for evaporative devices, which take into account surface wetting factor and non -unity Lewis factor is developed and presente d. This model then has been adapted for indirect and direct evaporative air coolers. Cross flow configura tion in which dry and wet air streams travel perpendicularly to one another is the most straightforward way to prevent mixing air streams in an indirect evaporative air cooler. An iterative numerical solution used to predict cooling performance of the cros s flow indirect cooler; while for the direct cooler, solution reduces to a simple analytical one. Factors affecting on cooling performance of this system consists of operational and geometrical parameters and climatic conditions. Results show that the oper ational parameters which mainly control the cooling performance of the indirect stage are number of transfer units in both dry and wet airstreams in addition to air stream mass flow ratio and that of the direct stage is only number of transfer units. Where as poor wetting degrades cooling efficiency, water flow rate should be enough low to keep the surface wet. A climatic condition zone has been specified in which this cooling system provides comfort condition and so fills the gap between direct evaporative cooling system and conventional vapor compression cooling system extending the application of the evaporative cooling in multi climatic conditions such as Iran.