The chilled ceiling (CC) displacement ventilation (DV) system is one of the air conditioning systems that have shown effectiveness in providing air quality through supplying 100% fresh air. The CC/DV system effectiveness and economic viability has been increased when return fraction was recirculated in the supply air stream where the maximum CO2 concentration level does not exceed 700 ppm in the occupied zone. However, it is important to investigate whether satisfying the allowable level of CO2 inside the space ensures that the World Health Organization (WHO) requirement for bacterial and fungal count of less than 500 CFU/m3 (14.2 CFU/ft3) is satisfied. If not, it will be interesting to study the maximum allowable return air ratio that provides good air quality in terms of acceptable CO2 level and acceptable bacterial and fungal count and at the same time improves the energy performance. Since the CC/DV system relies on buoyancy effects for driving the contaminated air upwards, infectious particles will recirculate inside the upper zone before entering the return duct. Such increase in the residence of infectious particles in the upper zone, will allow the effective utilization of upper-room ultraviolet germicidal irradiation (UVGI) to clean the air before recirculating it.

This paper uses an analytical model to investigate the effectiveness of upper-room UVGI mechanism for air disinfection when used with a mixed air CC/DV system. The simplified numerical model combines published multi-layer plume and three-zone mixing models to estimate the airborne bacteria concentration in the upper zone and occupied zone using the maximum allowable return air ratios. The results from this model are compared to a CFD simulation of the same room that incorporates the bacteria inactivation due to UV field.