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©2016 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 58, no. 12, December 2016

Diana Glawe, Ph.D., P.E., Associate Member ASHRAE; Marilyn Wooten, Ph.D.; Dennis Lye, Ph.D.

About the Authors
Diana Glawe, Ph.D., P.E., is an associate professor in the Engineering Science Department, and Marilyn Wooten, Ph.D. is an instructor in the Chemistry Department at Trinity University in San Antonio. Dennis Lye, Ph.D. is a research microbiologist at the U.S. EPA Office of Research and Development in Cincinnati.

Collecting condensate from large air-handling units (AHU) for on-site use is compelling, particularly in humid climates prone to drought. Identifying the optimal on-site use for the condensate requires knowledge of the quantity and quality of the condensate versus the quantity and quality required for potential on-site applications. This article provides evidence that condensate from properly maintained large AHUs is high-quality water, explains how system design and maintenance affect condensate quality, and highlights considerations for on-site applications of condensate.

The condensate addressed in this article refers strictly to condensate from the cooling coils of large AHUs such as those in commercial and institutional facilities, as opposed to condensate from steam systems, which is inherently different. Only large AHUs yield enough condensate to justify the expense of collecting and using condensate on site. This size threshold is reflected in the ASHRAE Standard 189.1 requirement to collect condensate for reuse from “air-conditioning units with capacity greater than 65,000 Btu/h (19 kW)... in regions where the ambient mean coincident wet-bulb temperature at 1% design cooling conditions is greater than or equal to 72°F (22°C).”

Figure 1 illustrates the fundamental components inside an AHU. As relatively moist and humid air flows over the cooling coils located inside an AHU, the moisture in the air condenses on the cooling coils and drips into a drain pan located beneath the cooling coils. This water, hereafter referred to simply as condensate, is removed from the AHU through an exit port. The condensate can then be either disposed of properly or used on site.

Rough estimates of the expected quantity of condensate produced by an AHU can be calculated using rules of thumb. More accurate estimates can be calculated using models based on climate data. Although condensate derived from the air in most locations is expected to be high-quality water, fear of contamination often deters its use as an alternative on-site water source. Contaminants in water can be defined as a physical, chemical, biological, or radiological substance or matter.

The contaminants in the condensate formed on the cooling coils originate from one of two sources. The first source is the air passing through the AHU. Air filters installed at the outside and return air entrances to the AHU act to capture contaminants suspended in the air (Figure 1). Contaminants that are not captured by the air filters pass through to the cooling coils. This is the reason condensate from AHUs in facilities like hospitals, where return air could contain pathogens, requires special consideration or is even disqualified from consideration for reuse.

The second source of contamination is the surface of the cooling coils and drain pan. Since the formation of condensate on the cooling coils occurs in a process similar to distillation, the resulting condensate is slightly acidic and lacks total dissolved solids. As such, condensate tends to react with the metal surface of the cooling coils and drain pan to form metal ions, a chemical contaminant.

In addition, if the AHU is poorly maintained, microbial growth may accumulate on the cooling coils or the drain pan and be picked up by the condensate. If antimicrobial tablets are placed in the drain pan as part of a preventative maintenance program, the ingredients in the tablets can become a source of chemical contamination as well.


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