How Ultraviolet Germicidal Irradiation Mitigates Coil Biofouling in HVAC Systems
From eSociety, July 2018
A two-part series in July’s Science and Technology for the Built Environment reflects on the same ASHRAE research project.
Focusing on field measurement and modeling of UVC cooling coil irradiation for HVAC energy use reduction, the article's first part addresses field measurements, and the second part addresses energy, indoor air quality and economic modeling.
The articles stem from RP 1738, which concluded in March 2017. That research project focused on ultraviolet germicidal irradiation (UVGI) as one way to mitigate coil biofouling in HVAC systems.
The ASHRAE study performed field measurements of change in coil performance (pressure drop and heat transfer) after treatment with UVGI. The final report details modeled energy use impact of coil irradiation and monetization of UVGI benefits including first cost, energy cost, maintenance cost and collateral health benefits.
More than a year after the completion of RP 1738, the principal investigator William Bahnfleth, Ph.D., P.E., Presidential Member/Fellow ASHRAE; Joseph Firrantello, Ph.D., Member ASHRAE; and Paul Kremer, Member ASHRAE, discuss the significance of both the research project and the STBE articles.
1. What is the significance of this research?
Irradiation with germicidal UVC light has been used for many years to control biological growth on cooling coils that increases resistance to air flow and decreases heat transfer coefficient, both of which can increase cooling system energy use.
Previously, no high-quality measurements of the type obtained in our study had been published.
2. Why is it important to explore this topic now?
The use of cooling coil irradiation is common and increasing, so it is important to have reliable, objective data on performance and economics to support decisions on the application of this technology.
3. What does Part 1 focus on and what does Part 2 focus on?
The first part of this project obtained and analyzed extensive interval data from field installations to determine the size of these effects.
The second part used modeling to predict the energy savings and economic performance of coil irradiation for a range of building types and climates, as well as the impact on indoor air quality of what we called collateral disinfection of air passing through an irradiated cooling coil.
4. Why was the research split into two parts?
The most important aspect of the research, from our perspective, was to obtain credible field measurements, but modeling was needed to generalize our findings to make them more useful in practice.
5. What lessons, facts, and/or guidance can an engineer working in the field take away from this research?
Our measurements confirmed improvements in heat transfer and flow resistance, both about 15%, for a system in a hot and humid climate (Tampa, Fla.). However, irradiation did not fully remove pre-existing fouling, so irradiated coils should be new or receive an initial cleaning.
The economic performance of coil irradiation depends on operating conditions, in particular, how much of the time coil surfaces are dry.
We also found that energy cost savings may be exceeded by operational savings versus periodic conventional coil cleaning, and by the benefits of coincidental air disinfection by the lamps irradiating coil surfaces.
6. How can this research further the industry's knowledge on this topic?
Those who wish to consider using coil irradiation now have a set of detailed field measurements documenting the performance of coil irradiation. Additionally, the extensive parametric modeling study we conducted helps to clarify the economic impact of these systems in a variety of facility types and climates.
7. Were there any surprises or unforeseen challenges for you when preparing this research?
A pleasant surprise was that collaborators doing a similar measurement study in Singapore obtained results quite similar to those from our hot and humid site.
Comparing before and after irradiation values of flow resistance and heat transfer presented some interesting statistical and analytical challenges that are described in the papers.
8. What is the significance of this being ASHRAE research?
While there are some very interesting fundamental aspects to this work, it was conceived primarily to help answer significant questions related to application, and the results of the work will go directly into practitioner resources like the ASHRAE Handbook. This very much fits what we understand to be the objectives of ASHRAE’s research program, i.e., to support research the industry needs that is not being funded by others.
ASHRAE members have free online access to Science and Technology for the Built Environment using their existing ASHRAE member login information.