(171c) Predicting Long Term Evaporative Cooling Tower Water Usage:  the Energy-Water Balance

Evaporative Cooling Towers are commonly used to dissipate heat from industrial process operations by cooling water through a combination of heat and mass transfer. An evaporative cooling tower can operate over a wide range of recirculating water rates, air velocity rates, and heat loads with variation in the approach of cold water to the ambient wet-bulb air temperature. Designed to manage summer worst case ambient conditions, the actual long term water and/or energy usage is overestimated, increasingly so for locations which experience extremes in seasonal weather. For instance, some evaporative cooling systems can cool a heat load entirely by non-evaporative air-cooled heat transfer; during reduced ambient air temperatures.

This paper presents a procedure for predicting long term water usage of a cooling system based on regional climactic data. The study focuses on industrial cooling systems consisting of wet towers that supply a common set of process loop heat exchangers, typically arranged in parallel; and will provide guidance on design options for water or energy minimization. The estimated water usage on a typical monthly, seasonal or annual basis is presented in case studies to demonstrate the energy –water tradeoff for sites across the continental United States. The results estimate how much operational flexibility is available for a cooling system and offers an understanding of the interaction between water and energy requirements under varying climate conditions. Also, the tradeoffs of different water capacity systems will be reviewed with respect to securing a water supply and estimating Potential-to-Emit emissions for air permitting.


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