copper, coils, bio-growth, antimicrobial and comparison
The growth of molds and other bio-organisms on heat exchangers in commercial air handling units (AHU’s) can reduce air flow thereby affecting the efficiency of the HVAC system and potentially affecting air quality, as well. This growth is especially prevalent on the moist surfaces of cooling coils. The use of ultraviolet germicidal irradiation (UVGI) to control bio-growth in commercial AHU’s has become a common practice with documented positive results. Although the use of UVGI is effective on those surfaces that receive sufficient doses of radiation, concerns about surfaces outside of the radiation field plus concerns about energy costs and maintenance issues related to UVGI equipment has led to a search for new methods and materials that are more effective and less costly over the life of an AHU. The antimicrobial properties of copper are well documented (Weaver et al, 2009). Over 500 copper alloys are now registered with the US EPA as antimicrobial agents for touch surfaces. For copper alloys used in HVAC applications, the EPA grants a “Treated Article Exemption” registration allowing manufacturers of copper HVAC components to make product protection claims of suppressed growth of bacteria and mold that can reduce system efficiency, cause product deterioration and cause foul odors. The study presented in this paper was designed to observe a side-by-side comparison of copper alloy heat exchangers to conventional copper tube-aluminum fin heat exchangers. At a middle school in New Jersey, one of 7 rooftop AHU’s was fitted with hydronic and chilled water heat exchangers made of copper tubes and copper fins. The copper heat exchangers included copper brackets and drip pans. This York AHU with copper coils and components was compared to 2 equally sized York AHU’s with copper tubes, aluminum fins, aluminum brackets and aluminum drip pans. One of the aluminum-fin AHU’s was equipped with UVGI and one was not. Bio-growth sampling involved swabs from the horizontal surfaces of fins on both the supply-side and discharge-side of the coils. Swabs were also taken on the horizontal surfaces of drip pans. Data points covered a full heating and cooling season. The data in this paper shows that all-copper coils had dramatically less bio-growth than coils with aluminum fins and aluminum bracketry, including the AHU equipped with UVGI. This data supports continued studies that quantify the economic impact of all-copper heat exchangers and total cost of ownership (TCO) related to AHU’s without UVGI.