Proposal of an effort-benefit diagram to compare unit and room air-change rates applied to a literature review

The main task of every ventilation system is to dilute and extract pollutants from indoor air, most importantly in occupied space. This is usually achieved by exchanging polluted indoor air with lesser polluted outdoor air. In the case of a mechanical ventilation system, this process requires a fan power to be provided which is approximately proportional by the power of three to the resulting airflow. Because of this, reducing the necessary airflow to be provided by the ventilation unit by 10% would lead to a reduced power supply of about 27%. Vice versa, a necessary increase of 10% of the airflow provided by the unit, would result in an increased power supply of about 33%. To determine whether a reduced unit airflow is feasible or an increase in unit airflow is required, it is important to evaluate the ventilation efficiency for the occupied zone and any other zone with certain air change rate requirements. Unfortunately, this evaluation process is time-consuming as well as labor-intensive and can't be done for every indoor zone specifically yet. However, for those situations where it has been and will be performed, it would be helpful if there is a common procedure how to compare them in a comprehensible graphical way. This paper proposes a diagram to visualize how efficiently certain ventilation systems provide their unit air change rate as a room air change rate. As characteristic physical limits and isolines the values for ideal mixed ventilation and plug flow are included as well. Furthermore, the diagram has been applied to visualize air change rates from a literature review. The overview indicates, other than often assumed, real buildings do not necessarily reach ideal mixed ventilation. Pointing out that, it must be admitted as well, that currently the available data for such a comparison is often not based on a uniform measurement and evaluation procedure. Nevertheless, the proposed diagram can be a useful tool to communicate the air exchange performance of ventilation systems.