Home | Business News | Browse by Publication | H | HVAC & R Research

In-place testing of in-duct ultraviolet germicidal irradiation.

Article Excerpt
INTRODUCTION

Ultraviolet germicidal irradiation (UVGI) uses 254 nm ultraviolet radiation from low-pressure mercury discharge lamps to reduce the indoor concentration of airborne microorganisms in order to reduce the risk of disease transmission. Germicdal UV radiation damages the genetic material of microorganisms so that they are unable to replicate. UVGI can be applied utilizing a number of different approaches, but generally one of the following three methods is used for sanitizing air in public buildings: 1) upper-room UVGI, 2) cooling-coil UVGI, and 3) in-duct UVGI.

With upper-room UVGI, the air in the upper portion of a room is irradiated while UV radiation in the occupied portion of the room is kept below levels that would be harmful to occupants; sufficient vertical air circulation is essential for upper-room UVGI to be effective. The primary objective of upper-room UVGI is to disinfect air in the room where both the infected and exposed occupants are located so that airborne transmission of disease from the infected to the exposed does not take place. Secondarily, microorganisms present in recirculated air may also be inactivated. Beginning in the 1930s (Wells 1955; Riley and O'Grady 1961) and continuing to the present day (Miller et al. 2002; Xu et al. 2003; First et al. 2007), numerous experimental studies have demonstrated the efficacy of upper-room UVGI. For specific applications, such as hospital surgical rooms where occupants wear protective equipment and unoccupied rooms, whole-room UVGI is also used.

With cooling-coil UVGI, the cooling coils, drip pans, and nearby surfaces are continuously irradiated to control microbial growth. Cooling-coil UVGI is generally concerned with the control of environmental microorganisms. The three primary objectives of cooling-coil UVGI are to 1) minimize aerosolization of microorganisms and allergens, 2) maintain cooling-coil heat transfer and pressure drop, and 3) reduce objectionable odors. Levetin et al. (2001) showed that the use of cooling-coil UVGI resulted in a significant reduction in fungi within insulation near cooling coils. Menzies et al. (2003) measured a 99% reduction in viable microorganisms and endotoxins on irradiated metal surfaces located on cooling coils and drip pans, as well as a large decline in worker-reported symptoms. Other studies have also shown significant reduction in microbial growth on cooling coils (Menzies et al. 1999; RLW 2006). UVGI cooling-coil disinfection is widely used and its application is now a specification for all new U.S. General Services Administration buildings (GSA 2003).

For in-duct UVGI, the air in ductwork or plenums of a heating, ventilating, and air-conditioning (HVAC) system is irradiated with high levels of UV radiation. In-duct UVGI is generally concerned with the control of infectious microorganisms. The objective is to disinfect air that is supplied to an occupied space. Unlike cooling-coil UVGI, which occurs continuously, in-duct UVGI has only a brief period of time to irradiate the air as it flows through the system and, thus, much higher levels of UVGI are required. If in-duct UVGI can be installed in close proximity to the downstream side of the cooling coils and drip pans, they can be irradiated simultaneously. Although studies on in-duct UVGI have been done in the laboratory (VanOsdell and Foarde 2002; RTI 2005), we know of only two previous studies that were done in the field, and these were done long ago (Rentschler and Nagy 1940; Nagy et al. 1954). Portable room-size air-cleaning devices based on UVGI can also be categorized as in-duct UVGI because in these devices room air is passed through a chamber containing UVGI and then returned to the room.

The goals of this study were to develop a methodology for in-place testing of in-duct UVGI and then to apply this methodology to the HVAC system of an occupied building. Kowalski and Bahnfleth (2004) discussed testing and commissioning of in-duct UVGI systems. They suggested three possible methods to ensure that a system performs as designed: 1) air sampling upstream and downstream of the UV lamps while a challenge microorganism is injected upstream, 2) air sampling simultaneously upstream and downstream of the UV lamps for naturally occurring microorganisms, and 3) air sampling downstream of UV lamps for naturally occurring microorganisms before and about a week or so after UV lamps have been activated. A fourth option is air sampling for natural ambient microorganisms from two identical ventilation systems, one downstream of the UV lamps and the other in a comparable location, but without UVGI. Kowalski and Bahnfleth pointed out that the first option, injection of a challenge microorganism, would not likely be permitted. For the third option, downstream air sampling before and about a week or so after the UV lamps are turned on, the outcome cannot be attributed solely to in-duct UVGI because other factors can modify the mix of naturally occurring ambient microorganisms over time. For our work, we chose the second option, air sampling...

View this article FREE - Now for a Limited Time, try Goliath Business News
Free for 3 Days!