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AHU Condensate Collection Economics: A Study of 47 U.S. Cities


©2012 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 54, no. 5, May 2012.


By Thomas Lawrence, Ph.D., P.E., Member ASHRAE; Jason Perry, Associate Member ASHRAE; Tyler Alsen, Student Member ASHRAE

About the Authors
Thomas Lawrence, Ph.D., P.E., is senior public service associate in the Faculty of Engineering, Jason Perry is research engineer at the Engineering Outreach Service, and Tyler Alsen is a graduate student, at the University of Georgia in Athens, Ga.

Using recycled or reclaimed water systems is a regular part of the urban infrastructure in some regions of the world. In areas that are normally thought of as "humid" or at least not as water-stressed, recent concerns about water availability due to population growth or shifting climate are opening up a much larger market for water reclaim and reuse in the built environment.

To avoid confusion, the term "condensate collection" in this article refers strictly to the capture and reuse of cooling coil condensate from air-conditioning systems, and does not include the very different process of collecting condensate in steam systems for reuse. This water source is being increasingly recognized as a valuable resource, particularly in warm-humid or hot-humid climate zones.

The overall intent here is to apply recent techniques in evaluating condensate collection potential and reuse to see if easy to apply generalizations can be made regarding where condensate collection would be recommended or perhaps even mandated. The use of reclaimed water sources such as condensate collection is one strategy for reducing overall potable water consumption.

ASHRAE and the International Code Council (ICC) have programs developed for green building construction. ASHRAE released its Standard 189.1-2009 in January 2010, while the ICC has just published its International Green Construction Code, or IgCC.

Both Standard 189.1 and the IgCC require the use of condensate collection and reuse for new construction and major renovation projects. However, limited guidance is found in the literature concerning where and when mandating condensate collection would be recommended. In some climates, the amount of condensate expected is practically zero, so requiring a condensate collection system would not be as practical or recommended. The primary motivation for this study was to develop and present a method for predicting the amount of condensate and making recommendations on when condensate collection should be considered mandatory.

Prior Studies

Several attempts to estimate condensate have been previously published. A typical hourly condensate production rate was reported by Guz for buildings in San Antonio, Texas, of between 0.1 and 0.3 gallons of water per ton of cooling (0.11 and 0.32 L per kW of cooling), or approximately 0.5 gallons/hour per 1,000 ft2 of conditioned floor space (2.0 L/hour per 100 m² of conditioned floor space). At these rates of collection, condensate recovery systems were determined to be financially viable for buildings in excess of 100,000 ft2 (9300 m²). While this provides a useful guideline for areas with a climate similar to San Antonio, it is not readily applicable to other areas and climates.

Bryant and Ahmed reported in 2008 a simplified model based on empirical data from a case study in Qatar, predicting condensate generation for a "normal" commercial air-conditioning system to be 8 gallons of condensate per ton of cooling (8.6 L per kW of cooling) for each day with a dew-point temperature in excess of 60°F (15.5°C). Painter reported a methodology to predict condensate production from dedicated outdoor air-handling units with energy recovery systems for buildings in Dallas, Houston and San Antonio, Texas, comparing the difference in humidity ratio across the system cooling coils.

Lawrence, et al., developed a method for evaluating the amount of condensate collected from a typical air-handling unit based on the amount of incoming outdoor air and its temperature and relative humidity. This method predicts the collected condensate using hourly weather data for when mechanical cooling would be expected, and accounts for the potential of using economizer cooling when that would make sense. The method was validated using data collected from a field study during the cooling season of 2009. The method predicts the volume of condensate annually collected from a unit volume flow of incoming outdoor air. Application of this method to the varied climatic conditions of the U.S. was described in Lawrence and Perry.

Methodology

This study approached the topic of condensate collection with three specific purposes. The first was to develop a method to characterize the total annual amount of condensate collected using correlations with local weather data parameters. Next, was to evaluate the economics associated with a typical condensate collection system. Finally, these results would be analyzed for generalizations regarding regions where condensate collection would be recommended from an economic and/or environmental impact perspective.

Selection of Cities and Weather Data Parameters

A set of 47 cities in the United States was selected for this study, with these shown on the map in Figure 1. Although this study only looked at cities in the U.S., developing correlations between the amount of condensate and weather data parameters allows the results and conclusions to be applicable anywhere. The first step toward this goal is to check if correlations can be derived for the amount of condensate collected with respect to readily available weather data parameters.

 

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Figure 1