Air Change Effectiveness and Pollutant Removal Efficiency During Adverse Mixing Conditions

The air change effectiveness (ACE), an indicator of the indoor airflow pattern, was measured in twenty‐six laboratory experiments. Ventilation air was supplied through induction‐type diffusers located in the ceiling and removed through a ceiling mounted return grille. The tracer‐gas step‐up measurement procedure was employed. In five of the experiments, pollutant removal efficiencies were also measured for simulated pollutant emissions from the floor covering and for simulated emissions from occupants. In experiments with heated supply air, supply airflow rates typical of the minimum supply flow rates of VAV ventilation systems, and 100% outside air, the ACE ranged from 0.69 to 0.89. These results indicate that significant short‐circuiting of ventilation air between the supply air diffuser and return air grille does occur under these adverse conditions. Mechanical recirculation of air, so that the supply air contained approximately 50% outside air, increased the ACE by about 0.05. When the supply air was cooled, the ACE ranged from 0.99 to 1.15, adding to existing evidence that short‐circuiting is rarely a problem when the building is being cooled. The pollutant removal efficiency for simulated pollutant emissions from the floor covering (PRE_floor) was strongly correlated with ACE (R²= 0.98) and the values of PRE_floor were within approximately 0.1 of the values of ACE. The pollutant removal efficiency for simulated pollutant emissions by occupants varied between workstations and was not as well correlated with the ACE.