Ventilation Effectiveness Parameters Resulting from Mechanical Ventilation with Recirculation

The effectiveness of a ventilation system in terms of the age of air and its ability to remove contaminants will be significantly affected if part of the exhaust air is recirculated. In this paper the consequences of recirculation of air in mechanical ventilation systems on ventilation effectiveness parameters is examined. Two alternative methods of establishing ventilation effectiveness parameters relating to air change efficiency and contaminant removal effectiveness when recirculation of air is present are presented. The procedures are compared by way of a four-zone example, which demonstrates equivalence between the two methods. The effect of recirculation on quality of air in terms of local mean age and local air quality index are investigated as is the sensitivity of internal air quality to the fraction of the supply air which is recirculated.