Field measurements of air change effectiveness using tracer gas techniques.

The present paper reports on tracer gas measurements performed in five large buildings during normal operating conditions. In all buildings air was supplied through ceiling diffusers and returned through a ceiling plenum. The measurements were taken during summer with the systems in cooling mode, i.e. the supply temperature was lower than the room temperature. The global air change effectiveness and the occupied zone average air change effectiveness were calculated based on the age-of-air concept. The local air change effectiveness i.e., for one point in the space, was calculated in two ways: (1) Age-of-air in the return duct divided by local age-of-air at breathing level, and (2) Age-of-air at return grille in ceiling divided by local age-of-air at breathing level. To measure age-of-air the tracer gas stepup method was used. The global air change effectiveness as well as the average air change effectiveness in the occupied zone for all systems indicated complete mixing. The local air change effectiveness showed, however, larger differences, indicating that the air in the occupied zone was not uniform mixed in all buildings.