Cooling water is used for heat rejection in most process plants. 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 ambient weather conditions. 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 realize over 40% heat transfer by non-evaporative air-cooling during reduced ambient air temperatures.
This article presents a rule-of-thumb procedure to estimate water consumption for a select evaporative cooling tower system design when subject to varying ambient conditions. The prediction of the water usage at different weather conditions is used to demonstrate the water -- energy tradeoff inherent in a design selection. The discussion will show how much flexibility is available in a typical cooling system, such as for water usage.
This analysis focuses on chemical process industry cooling systems consisting of wet towers that manage a relatively constant heat load, and thus water recirculating rate, typically supplied from a common set of process loop heat exchangers.