Deterministic simulation and assessment of air-recirculation performance of unidirectional-flow cleanrooms that incorporate age of air concept

The airflow characteristics of a unidirectional cleanroom are largely determined by the airflow in the supply air plenum, the return air plenum and the return air shaft. A deterministic computational fluid dynamic model that incorporates fan-performance characteristics was applied to investigate and compare the air-recirculation performance of the two general designs of unidirectional-flow cleanrooms. Typical flow-resistance models of the various components in the air path were used in the simulation. Non-uniformity (NU) of supply air velocity (at the filter face), airflow parallelism (deflection angle, α), and the distribution of the mean age of air were computed for the two types of unidirectional-flow cleanrooms. The results show that the performance of the fan filter unit system is generally superior to that of an axial fan system in terms of NU, deflection angle of airflow in the working zone, and the air-change efficiency derived from the age of air distribution. It was also found that the three performance indices provide a good assessment of the air-recirculation performance of unidirectional-flow cleanrooms.