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article-morrison.jpg

©2015 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 57, no. 8, August 2015

Frank Morrison, Member ASHRAE

About the Author
Frank Morrison is manager, global strategy at Baltimore Aircoil Company in Jessup, Md. He is past chair of ASHRAE TC 3.6, Water Treatment.

Saving water with cooling towers. At first glance, this statement seems counterintuitive. Cooling towers save energy but aren’t they major users of water? This article will help readers understand the critical role evaporative heat transfer systems play in a sustainable environment, explore how water is consumed in such systems, and review the strategies that help minimize the use of both water and energy.

The first water-cooled systems used potable water to provide heat rejection with the cooling water wasted to a drain. Cooling towers were developed to recycle more than 98% of this water, resulting in tremendous reductions in both water and energy use as these systems grew in size and popularity. Evaporative heat rejection also enables higher system efficiencies, which conserves water at the power plant and reduces emissions of greenhouse gases and other pollutants. This is because thermoelectric power generation accounts for 38% of freshwater withdrawals in the United States—essentially equal to that withdrawn for irrigation.

By reducing the electrical energy consumed at the site, less power is required to be generated and less water is used at the power plant and in the extraction and processing of the plant’s fuel source. For example, in some climates, the total water use (source and site) between air- and water-cooled chillers is almost equal.2 The lower energy use also enables a higher percentage of renewable, clean power from solar and wind at a given facility. Energy is also required to treat and distribute water. The balance between the use of these two natural resources is often referred to as the “energy/water nexus.”

Today, water supplies are challenged in many areas of the world, including Atlanta (recent drought) and California (current drought). Therefore, it is critical that all water consuming systems, no matter where they are located, optimize their use of this resource. Methods to conserve water include using low-flow bathroom fixtures, repairing leaks in water-distribution systems, and taking advantage of relatively simple techniques to help ensure that evaporative heat rejection systems use only the amount of water required to save energy, maintain optimized system performance, minimize system maintenance, and ensure a long system life.

Evaporative heat rejection systems encompass open circuit cooling towers, closed circuit cooling towers, and evaporative condensers. For the purposes of this article, the term “cooling tower” will be used synonymously for all of these devices, except as noted.

 

Evaporation

The primary consumption of water in a cooling tower is through evaporation—a process that is also used by the human body to help regulate its internal temperature. In a cooling tower, the warm water from the system comes into contact with the entering air, usually over a heat transfer surface such as fill, where a small portion of the recirculating water evaporates, cooling the remaining flow. This process is very energy efficient as approximately 1,000 Btu (1055 kJ) are required to evaporate 1 lb (0.454 kg) of water at standard design conditions (1,000 Btu/lb [2,326 kJ/kg]).

In contrast, air-cooled heat exchangers must move far more air to reject the same heat, consuming additional fan energy in the process, usually at a much higher system temperature since the dry-bulb temperature is higher than the wet-bulb temperature of the air. These higher temperatures result in greater energy use by the cooling system, often 30% or more as in the case of an air-cooled versus a water-cooled chiller.

 

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Comments

  • 29 Sep 2015 | Joel Sampson
  • Thanks Frank! The prospect of cooling towers working a lot like sweat is interesting to me. My brother in law works at a company where they use cooling towers. I wonder if he's ever thought of it like that? Thanks for the great article.
  • Reply

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