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UVGI in air handlers.

Ultraviolet germicidal irradiation (UVGI) is the use of ultraviolet radiation to inactivate microorganisms. UVGI systems predominantly use Ultraviolet C (UVC) radiation at a wavelength of ~254 nm that is produced by low pressure mercury vapor or amalgam lamps. UVGI is applied in a variety of ways. Upper air systems disinfect air in rooms as it circulates into a region irradiated by fixtures located safely above the occupied zone. In-duct systems irradiate ventilation airstreams to inactivate microorganisms "on the fly" before air is delivered to occupied spaces. The primary purpose of upper air and in-duct systems is to reduce the risk of infectious disease transmission.

A third application is the use of UVC to irradiate surfaces inside air-handling units (AHUs) that may become fouled by microbial growth, especially cooling coils and condensate pans. A typical coil irradiation system is shown in Photo 1. While the control of microbial growth on AHU surfaces may have indoor air quality (IAQ) benefits, the benefits used to justify coil and condensate pan irradiation are often operational: reduced cleaning costs and energy savings resulting from improved performance.

Current information on all three applications can be found in the 2008 ASHRAE Handbook--HVAC Systems and Equipment, Chapter 16, prepared by ASHRAE Technical Committee 2.9, Ultraviolet Air and Surface Treatment. This article focuses on cooling coil irradiation: how it is done, how the benefits associated with it are achieved, and what evidence actually exists to document those benefits.

Cooling Coil Irradiation Systems

The primary purpose of cooling coil irradiation is to suppress microbial growth on wet cooling coil surfaces and, in most cases, also in condensate pans. To do so, lamps must be positioned so they have a good view of the surfaces to be treated. To irradiate coil surfaces, lamps can be placed on either the upstream or downstream side of the coil. However, the view of condensate pans through the closely spaced fins of a typical cooling coil is poor, so the downstream configuration may be preferred.

A consideration favoring upstream location is the impact of the wind chill effect (decrease in output due to the convective cooling effect of air velocity and temperature) on lamp output. The nominal lamp capacity required for a system upstream of a cooling coil may be half what is required downstream where air temperature is generally much lower. (1)

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The inactivation of microorganisms by UVC is a function of the dose delivered, which is proportional to the intensity of radiation and the duration of exposure. Because exposure time for microorganisms growing on a surface is very long, radiation intensity can be much lower for surface treatment applications than for fly-by air disinfection applications. A system sized for air disinfection and located in such a way as to irradiate the coil should be more than sufficient to treat the coil surfaces and condensate pans as a collateral benefit. Systems for coil irradiation only may be sized to provide as little as 0.16 Btu/h-[ft.sup.2] (0.5 W/[m.sup.2]) on the coil surface (although much larger values are specified in some cases) compared to in-duct system average irradiances, which are typically two orders of magnitude greater. (2) Manufacturers can assist with the prediction of the intensity distribution to determine an efficient arrangement of lamps.

Coil irradiation systems may be installed in new systems, where the coils they are intended to maintain are initially clean. They are also frequently installed in existing AHUs that may have widely varying degrees of cleanliness. In either case, attention must be given to safety (such as disconnects, interlocks, sight glasses, signage, and ozone generation associated with some types of lamps) and material degradation issues (use of UVC compatible materials or shielding of those that are not).

Benefits

As noted previously, primary benefits expected from cooling coil irradiation are energy savings, maintenance cost savings and increased capacity, with IAQ improvements a potential additional benefit. Surface irradiation also treats a potential source of odors and allergens in air distributed by AHUs, but this is typically viewed as a collateral benefit.

The expectation of energy savings is based on a presumption that the suppression of biofilms on coil surfaces will reduce fan energy use by reducing air-side resistance, and will increase the overall heat transfer coefficient with a resulting decrease in refrigeration equipment energy use. Maintenance cost savings are expected, relative to the labor-intensive cost of periodic mechanical or chemical cleaning. It is also claimed that coil irradiation will result in lower energy use than is associated with conventional cleaning because a periodically cleaned coil will be fouled more heavily, on average, than one continuously cleaned by coil irradiation. Capacity increase is expected because of the combined effect of increased airflow and heat transfer coefficient.

All of these claims are plausible, although unanticipated effects may result in counterintuitive behavior. For example, while cleaning of a coil may reduce its airside resistance, fan energy use in a constant volume system may not decrease significantly because of the corresponding increase in flow rate that will occur. It has also been noted in some studies that small amounts of coil fouling under certain flow conditions may actually increase the heat transfer coefficient, although this would certainly not be true for high levels of microbial growth.

Evidence of Effectiveness

Research into the ability of UVC to inactivate microorganisms in the air and on surfaces has been going on for roughly a century. However, there is little directly relevant peer-reviewed literature on coil cleaning applications, especially careful measurements of flow resistance, heat transfer and energy consumption changes resulting from coil irradiation. Evidence of coil irradiation effectiveness rests on published sources that include 1) trade magazine articles that give essentially anecdotal reports of energy savings, (3) 2) articles that document the generic benefits of coil cleaning but do not deal specifically with UVGI, (4) 3) studies that document the reduction of microbial growth on coil and other AHU surfaces as a result of UVC irradiation, (5) and 4) a small number of studies that address airflow and heat transfer characteristics of coils fouled by particles or biofilms. (6)

In addition, there is a large body of anecdotal evidence provided by equipment manufacturers and satisfied owners. A further indication of the level of acceptance of coil irradiation is given by the current U.S. General Services Administration requirement for coil and condensate pan irradiation in its mechanical standards for new and renovated buildings. (7) However, the open literature, including the ASHRAE Handbook, does not provide performance-based guidance on the sizing of systems or reliable methods of estimating the cost-benefit of coil irradiation. It is straightforward to define how to conduct such an analysis, but key elements of required data are missing. Some sources in the trade literature do provide estimates of energy and operating cost savings, but in a way that is difficult to generalize and, in some cases, exaggerated by comparison to a baseline that represents poor system performance. For example, Keikavousi3 estimates an annual benefit of $5,000 in increased cooling capacity for a 6,000 cfm AHU that was initially 50% obstructed following installation of a $2,000 coil irradiation system.

What's Needed

From the author's perspective, there is no doubt that irradiation of coils with UVC reduces or eliminates biofilm growth and that there is clear evidence that, under most practical conditions, a cleaner coil will perform better than a dirty coil. In combination, these two observations strongly suggest that coil irradiation is a valuable technology. However, there are many important questions that still need to be answered so that systems can be designed properly and their benefits quantified in a defensible way.

One need is a much better understanding of the baseline against which savings should be calculated. Anecdotal accounts of energy savings for many technologies are based on remediation of systems that are poorly maintained and are not representative. Estimates of energy savings frequently incorporate assumptions about fouling rates that have little or no support in research literature. How dirty is a typical coil that is properly maintained? What is a typical progression of coil fouling--rate of increase of flow resistance and decrease of heat transfer coefficient? Does fouling vary in a predictable way with outdoor air conditions, level of particulate filtration, and other factors? How is the performance of coil irradiation systems affected by other organic and inorganic material that may collect on coils in addition to biofilms? It is of secondary concern to know precisely what is in the biofilms on typical cooling coils before and after irradiation, but that is also an area in which more research would be welcome.

Another important need is a better understanding of the relationship between the effectiveness of coil irradiation systems and cooling coil characteristics, system operating conditions and UVGI system characteristics. For example, there are no quantitative studies, to the author's knowledge, of the effect on performance of upstream vs. downstream lamp placement, coil depth and fin density, or surface UVC intensity. Availability of such information would contribute to more reliable and optimized designs.

A third, critically important need is detailed, generalizable analysis of energy impacts of coil UVGI. There are no published studies that support the kind of predictive capability needed to accurately estimate the savings that will result in a particular application. Potential impacts on energy use need to be stated in fundamental forms such as changes in airflow resistance and heat transfer coefficients, as well as in the form of case studies reporting submetered energy savings.

Finally, safety concerns frequently arise in discussions of UVGI and will only be resolved by publication of reliable measurements of low or undetectable levels of ozone and secondary air contaminants produced by coil irradiation.

Fortunately, there is growing interest in all of these issues and results may begin to appear over the next few years. As it does, the case for coil irradiation will become clearer--and very likely stronger in appropriate applications--than it is at present.

References

(1.) Lee, B., W. Bahnfleth, K. Auer. 2009. "Life-cycle cost simulation of in-duct ultraviolet germicidal irradiation systems." Proceedings of Building Simulation 2009.

(2.) Kowalski, W. 2009. Ultraviolet Germicidal Irradiation Handbook. Chapter 10.1 "Cooling Coil Irradiation." Heidelberg: Springer.

(3.) Keikavousi, F. 2004. "UVC: Florida hospital Puts HVAC maintenance under a new light." Engineered Systems (3).

(4.) Montgomery, R., R. Baker. 2006. "Study verifies coil cleaning saves energy." ASHRAE Journal 48(11):34 - 36.

(5.) Levetin, E., R. Shaughnessy, C. Rogers, and R. Scheir. 2001. "Effectiveness of germicidal UV radiation for reducing fungal contamination within air-handling units." Applied and Environmental Microbiology 67(8):3712 - 3715.

(6.) Pu, H., et al. 2010. "Air-side heat transfer and friction characteristics of biofouled evaporator under wet conditions." Front. Energy Power Eng. China. 4(3):306 - 312.

(7.) GSA. 2010. Facilities Standards for the Public Buildings Service, Section 5.8, HVAC Systems and Components. Washington D.C.: U.S. General Services Administration.

By William P. Bahnfleth, Ph.D., P.E., Fellow ASHRAE

William P. Bahnfleth, Ph.D., P.E., is professor of architectural engineering and director of the Indoor Environment Center at Pennsylvania State University in University Park, Pa.
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Title Annotation:IAQ APPLICATIONS; ultraviolet germicidal irradiation
Author:Bahnfleth, William P.
Publication:ASHRAE Journal
Article Type:Reprint
Geographic Code:1USA
Date:Apr 1, 2011
Words:1856
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