©2014 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 56, no. 3, March 2014.
By Paul Lindahl, Member ASHRAE
About the Authors
Paul Lindahl is director, market development at SPX Cooling Technologies (Marley) in Overland Park, Kan.
The purpose of this article is to address primarily the cold weather operation of open circuit cooling towers associated with water-cooled chiller systems, including those with water side economizers. While similar in many ways with regard to freeze protection of the recirculating water in contact with cooling air outside the process coils, closed-circuit cooling towers and evaporative condensers have special requirements that are not covered in this article.
As mentioned in “Saving Energy With Cooling Towers” by Frank Morrison in the February issue, water-cooled systems offer the lowest energy option for most cooling duties. Many buildings require cooling year-round and use either airside or waterside economizers to further reduce energy. Indeed, ASHRAE Standard 90.1-2013 expanded the use of economization in more climate zones. For those buildings that use water economizers, the cooling towers must operate year-round as would more process-oriented buildings, such as data centers. In colder climates, many designers and operators are concerned with operating cooling towers in subfreezing temperatures.
By following some simple operating guidelines, cooling towers can, and have been, successfully operated in even in very cold climates, as shown in Figure 1, at –15°C (5°F) in Montreal. More than 24 hours without wet-bulb temperatures going above 32°F (0°C) can be considered “sustained freezing conditions,” as no daily freeze-thaw cycle will exist. Wind speeds and other factors should also be considered. In general, when the weather report has a wind chill factor forecasted below 32°F (0°C) for more than a day, operators should be thinking about freezing operation strategy. Preferably the strategy is built into the design, automated and in use all of the time.
In comparison to comfort cooling, data centers may operate year-round with a high load factor, resulting in the cooling tower sizing being driven by the economizer duty in cold weather. This can result in the cooling tower being oversized for the summer duty. Cooling towers operating in economizer mode must produce water temperatures that are at least equal to, or lower than, the chilled water temperatures that would otherwise be produced during conventional chiller operation. However, when such data centers are lightly loaded, which is typical in the early years of operation, a potential impact exists due to the larger cooling tower sizing under freezing conditions.
Cold Weather Operation of Cooling Towers
Cooling towers have been operated successfully in some of the most severe freezing conditions around the world. The colder the weather, the more that certain relatively simple protocols must be followed and precautions taken to avoid operational issues under such conditions. Fully loaded data centers are actually ideal candidates for water-side economization in freezing climates because of high year-round heat load.
The sidebar, “Basic Cold Weather Operation,” lists operating guidelines and the next section discusses each of these in more detail.
Maintaining Heat Load. Without a heat load, water flowing over a cooling tower will end up either at the air wet bulb temperature, or as ice, whichever occurs first, as shown in Figure 1. This will happen quickly with fans running—more slowly if they’re off. Note that wet-bulb temperature drives evaporative heat transfer, and is an equal or lower temperature than the dry bulb. For example, at 35° F (1.6°C) dry bulb, above commonly assumed freezing conditions, the wet-bulb temperature often can be less than 32°F (0°C) wet bulb—and the water flowing over a cooling tower can freeze without proper operation.
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