Effect of room air recirculation delay on the decay rate of tracer gas concentration

Tracer gas measurements are used to estimate the flow rate of fresh air into a room or building. These methods commonly account for the decay of tracer gas concentration as the result of ventilation air supply and infiltration, using a well-mixed model of the space. Some researchers also have considered the effect of leakage in the ventilation ductwork.

This paper considers the effect of recirculation through ventilation ducts on the calculated fresh air supply rate. Transport delay in the ducts can significantly alter the time evolution of tracer concentration, and hence alter the estimated air change rate.

This result could be important when interpreting experimental measurements of tracer gas decay in a space with recirculating ventilation. For instance, transport delays longer than ten minutes have been observed due to low airspeeds in a ceiling return plenum. This paper shows that such long delays can have a significant impact on the estimated building air change rate.

Research presented at 9th International Conference on Air Distribution in Rooms - RoomVent 2004, in Coimbra, Portugal. Page 6. 

Abstract for conference proceedings: Tracer gas measurements are commonly used to estimate the fresh air exchange rate in a room or building. Published tracer decay methods account for fresh air supply, infiltration, and leaks in ductwork. However, the time delay associated with a ventilation system recirculating tracer back to the room also affects the decay rate. We present an analytical study of tracer gas decay in a well-mixed, mechanically-ventilated room with recirculation. The analysis shows that failing to account for delays can lead to under- or over-estimates of the fresh air supply, depending on whether the decay rate calculation includes the duct volume.