Air-change effectiveness: theory and calculation methods

This paper reports the development of methods for calculating a ventilation performance metric that is a measure of the airflow pattern in a room or zone of a multi-zone ventilation system. Temporal mixing theory is used as the basis for these methods. The methods are applicable to all ventilated systems that can be modeled as a set of interconnected chambers. Relations between the ventilation performance metric defined in this paper and those defined previously are derived. The theoretical results of this paper are consistent with published experimental findings. They also illustrate that the conclusions in some experimental studies about the airflow patterns in working buildings may be incorrect. Re-analysis of previously published data illustrates how common features of mechanically ventilated buildings, such as recirculation of return air and multiple chambers, confound information about airflow patterns in tracer gas data. The calculation methods developed in this paper can be used to undo this confounding.