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.