by Daniel Jones
As air-conditioning and refrigeration equipment ages, its ability to maintain temperatures and humidity levels declines. Most often, the culprit is reduced coil heat-transfer effectiveness, with air-handling unit (AHU) cooling coils less able to remove heat and moisture from the air. These inefficient heat-transfer rates derive primarily from the buildup of organic contaminants on and through the coil’s fin areas. Such buildup can be eliminated through the use of ultraviolet (UV) germicidal energy (UV-C light).
In the UV-C wavelength (253.7 nm), the light disassociates molecular bonds, disinfecting and disintegrating organic materials. Contrary to popular belief, UV-C lighting systems are not an exotic, new technology. They have been employed extensively since the mid-1990s to significantly improve HVAC/refrigeration (HVAC/R) airflow and heat-exchange efficiency, which can reduce energy use by up to 25 percent. Although UV-C by itself does not save energy, it restores cooling capacity and airflow to increase the potential for energy savings. In fact, researchers have found exposing a fouled cooling coil to UV-C results in a 10 percent decrease in pressure drop and a 14.55 percent increase in heat transfer coefficient levels at reference conditions. (For more on this research, see J.T. Firrantello, W.P. Bahnfleth, R. Montgomery, and P.K. Kremer’s “Field Study of Energy Use-related Effects of Ultraviolet Germicidal Irradiation of a Cooling Coil,” from 12th REHVA World Congress CLIMA 2016 [Aalborg, Denmark].)
This impacts energy use. As a cooling coil ages, microbial growth on the surface of its fins impedes its ability to remove heat from the air, thereby causing the temperature of the leaving air to increase. Additionally, the layer of growth produces pressure drop because it decreases the space between the coil fins, causing an increased demand on the fan to compensate for the additional pressure in the system. Many facilities speed the fan up and decrease the chilled water to meet demand for cool air. These measures typically add to energy use. By cleaning with UV-C, which reaches deep into the coil, the system can be ‘de-tuned’ to its original operating condition and save energy.
In new/original equipment manufacturer (OEM) equipment, UV-C keeps cooling coil surfaces, drain pans, air filters, and ducts free from organic buildup for the purpose of maintaining as-built cooling capacity, airflow conditions, and indoor air quality (IAQ). In retrofit applications, UV-C eradicates organic matter that has accumulated and grown over time, and prevents it from returning. UV-C accomplishes this for roughly $0.15 per CFM.
UV-C is a relatively simple technology; it merely involves shining lamps onto surfaces and adding simple on/off controls to facilitate maintenance. Nevertheless, many professionals are mystified about how the process works and how to apply it cost-effectively. It is important to address these aspects of UV-C technology and the applications that seem the most awkward, referencing American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) guidelines found in Chapter 60.8, “Ultraviolet Air and Surface Treatment,” in the 2015 ASHRAE Handbook–HVAC Applications.