Convection, air mixing, and ultraviolet air disinfection in rooms

The efficiency with which airborne test organisms were removed from the lower part of a room when the upper air was irradiated with ultraviolet light (UV) was used to evaluate convective air mixing between the upper and lower parts of the room. The temperature of air entering the room through four diffusers in the ceiling was 10 to 15 F hotter or colder than lower room air during the studies. Rates of disappearance of test organisms atomized into the air were more than twice as fast when cold air entered at the ceiling as when hot air entered. Mean vertical mixing rates were estimated to be 20 air changes per hour (AC/hr) with hot air entering at the ceiling and 150 to 300 AC/hr with cold air entering at the ceiling. These large differences resulted from the large temperature gradients which favored or inhibited vertical mixing of air.