Free Cooling with Water-Side Economizers

When it's cold outside, using environmental conditions to cool a building or process makes perfect sense. For facilities with chilled water plants, a water-side economizer, which uses the evaporative cooling capacity of a cooling tower, is an effective way to maintain temperature and humidity requirements while reducing or eliminating chiller use. According to a PG&E study, chilled water plant energy use is reduced by as much as seventy percent during water-side economizer operation.

Water-side economizers can be used throughout most of the United States, where wet bulb temperatures are lower than 55°F for at least 3,000 hours per year. They are commonly used in data centers, which produce a near-constant internal cooling load, and provide an extra level of cooling redundancy in the event of chiller failure. Water-side economizers are preferable to their air-side counterparts for applications in which specific minimum humidity levels are called for, such as laboratories and hospitals. They are best suited for new construction; while most chilled water systems can be retrofitted with a water-side economizer, retrofit can be somewhat costly in certain cases, depending on the setup and limitations of the existing system and the type of economizer used.

There are several types of water-side economizers. The strainer cycle, in which cooled water from the cooling tower is sent through a filter and circulated through the chilled water system, is the most efficient, but it is susceptible to fouling. In the plate-and-frame heat exchanger, cooling tower water does not enter the chilled water loop, which eliminates the chance of contamination. Other types include refrigerant migration, the indirect evaporative precooling coil, and the dry cooler, or closed-circuit cooling tower.

Use of a water-side economizer is an excellent way to lower energy costs, but it must be properly controlled and maintained to save energy. MSC frequently encounters economizers that are not functioning correctly or aren't working at all, and building owners and personnel don't even know it. Every good preventive maintenance plan, in addition to filter changes and chemical treatment, should include testing of economizer functionality. A properly functioning water-side economizer can be expected to yield between fifteen and forty percent energy savings. 

Mechanical Service Corp. 
41 South Jefferson Road 
Whippany, NJ 07981 
732-884-5000
http://www.mscnj.com