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ASHRAE Research Project: UV-C Cleans Coils & Reduces Energy Use
For years, UV-C users and manufactures have shared anecdotal experiences and case studies of successful installations of UV-C systems where the result was a clean cooling coil and reduced energy use.
ASHRAE TC 2.9—the technical committee responsible for studying Ultraviolet Air and Surface Treatment—sought to prove, through an independent ASHRAE sponsored research project, that UV-C can clean coils, improve coil performance and save energy.
Researchers from Pennsylvania State University were awarded the project entitled “Field Measurement and Modeling of UVC Cooling Coil Irradiation for HVAC Energy Use Reduction”.*
Based on field tests, investigators found that the germicidal wavelength yields an average pressure drop reduction of 21% and an increase in heat transfer coefficient of 14%. In this study, AHUs with UV-C installed downstream of the cooling coil were capable of extracting more heat, thus using less energy to perform the same task.
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Specifically, the researchers found that the majority of HVAC energy savings derived from applying UV-C occurred in fan energy (80%), followed by cooling (17%) and pump energy (3%).
Why heat-transfer efficiency is important
Evaporator coils are vital for removing heat from occupied spaces. As part of this heat-transfer process, the condensation produced by the coil acts as a magnet to attract dust and other particulates contained in the airstream. Once present, these particulates act as a food source for bacteria and microbes which quickly multiply due to the moist and dark environment inside an air handler that serves as an ideal growth medium for bio-film.
As the coil fins become packed with additional contaminants, pressure drop across the coil increases and less heat is removed, the entire system must work harder to overcome this loss in cooling capacity.
As heat transfer efficiency decreases, operators are forced to compensate with more energy-intense adjustments to try and meet demand, such as increasing fan speeds and/or pumping additional chilled water.
Because light in the UV-C wavelength destroys microbial infestations 24/7/365, these adverse impacts are reduced. The pressure drop across the coil, and air velocity between the coil fins (interstitially) are optimized, while fin and tube surfaces are kept clean to maximize system heat-exchange efficiency.
Study methodology & conclusions
The 14-month study tested the improvement in coil performance (specifically how AHUs adapted to cooling coils fouled with microbial growth) using two test sites, located in Tampa, FL and State College, PA. Field measurements were taken four months before UV-C fixtures were installed and 10 months after.
Researchers drew the following conclusions:
- The UV-C wavelength can reduce HVAC pressure drop and improve heat transfer efficiency due to biofouling. The study’s average pressure drop (21.7%) and increase heat transfer coefficient (14.7%) are similar to other research findings
- Use of UV-C to treat coils helped save roughly 4.5% in HVAC energy, when modeled across multiple building types and climates. Most of these savings were derived from reduced fan use.
- The potential improvements in indoor air quality from the use of UV-C exceeds the energy savings.
- Improvements in heat transfer and pressure drop occurred quickly, one month or less at the two test sites.
- Modeling of energy use impacts showed that energy savings from UV-C coil treatment in hot, humid climates with year-round wet coil operation will benefit more from UV-C
* Bahnfleth, W., and J. Firrantello. 2017. Field Measurement and Modeling of UVC Cooling Coil Irradiation for HVAC Energy Use Reduction (RP-1738). Atlanta, GA: ASHRAE.