Effect of recirculation on the local mean age of air

When it is required to predict the local mean age of air, the calculations are often carried out assuming that there is no recirculation, that is that all of the supply air is fresh. However, local mean age values will be greater than in the fresh air case when some of the exhaust air is recirculated. This paper examines and discusses the effect of recirculation and shows that, for many systems, the increase in local mean age due to recirculation can be calculated by means of an elementary analysis that leads to a simple algebraic expression. The paper also shows how the zonal model may be adapted for the general case of multiple recirculation paths. Compatibility between the elementary approach and the general zonal method is demonstrated.