Ultraviolet germicidal coil cleaning: Impact on heat transfer effectiveness and static pressure drop

Cooling coil surfaces are ideal sites for biofilm formation due to the presence of adequate nutrients (deposited particles) and moisture (condensate), causing adverse impacts on heating, ventilation and air-conditioning (HVAC) energy usage and performance. In this study, an HVAC test apparatus was built in our laboratory to investigate the hypothesis that ultraviolet germicidal coil cleaning (UVG-CC) of heat exchanger surfaces improves heat transfer effectiveness and reduces the static pressure drop across the coil. The test apparatus consisted of two parallel ducts, each with its own cooling coil. One coil was treated with UVG-CC while the other was the control and left untreated. Thermodynamic properties of the air and water flowing through both heat exchangers were monitored over the course of two years with sensors and a data acquisition system. Differences in static pressure drop and coil effectiveness between the UV-treated and control coil were compared across multiple modes of coil operation (defined by presence of condensate). The effectiveness of UVG-CC was drastically affected by the presence of condensation on coil fins. We observed a statistically significant difference in the heat transfer effectiveness between the UV-treated and control coils in wetted conditions while no difference was observed in dry conditions. Sensor accuracy, however, contributed to large uncertainty in our result. The average heat transfer of the UV-treated coil was 3.0-6.4% higher compared to the control coil, with an uncertainty of ±2.7%. UVG-CC, however, did not significantly reduce static pressure drop.