Design your evaporative cooling tower to handle variable ambient conditions and find the optimal balance between energy and water use.
Water is commonly used to manage excess heat in many processing plants. Specifically, evaporative cooling towers dissipate heat from industrial processes 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, heat loads, and ambient weather conditions. Since cooling towers are designed to operate under the worst-case ambient summer conditions, engineers often overestimate the actual long-term water and/or energy consumption, especially in locations that experience seasonal weather extremes.
Cooling tower heat and mass transfer are based on evaporative cooling to the atmosphere. The maximum cooling tower efficiency is limited by the local air temperature and ambient humidity, collectively measured as the wet-bulb temperature. Although the amount of heat that needs to be extracted from the process is independent of the weather; the design and operation of wet cooling towers is dependent on ambient weather conditions. The cooling tower acts as the intermediate at the boundary between the plant process and its surrounding environment.
Wet cooling towers in the form of multicell arrangements are common at oil refining, petrochemical, and power-generation facilities. The cooled water leaves the bottom of the tower and serves as the cooling medium for process heat exchangers and the heated water returns to the tower to be cooled. In the chemical process industries (CPI), the process heat load design basis depends on the production rate. The design of an evaporative cooling tower that is open to the environment must balance water consumption via evaporation and energy consumption via fan rotation speed to attain adequate cooling.
This article presents a rule-of-thumb procedure to estimate water consumption for an evaporative cooling tower design subject to variable ambient conditions. The prediction of water use during different weather conditions demonstrates the water-energy tradeoff inherent in a design and the potential to optimize the tower design based on location and weather. The analysis focuses on wet towers that manage a relatively constant heat load (and consequently a relatively constant water recirculation rate) typically supplied by one or more process heat exchangers.
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