CFD Analysis for Design Optimization of a Pharmaceutical Cleanroom
ASHRAE Journal, Vol. 65, no. 8, August 2023
Pharmaceutical cleanrooms are employed for the manufacturing of drugs and medical products and require a high level of cleanliness to maintain the aseptic environment. Often high air changes per hour (ACH) are specified to ensure a higher level of cleanliness. Such specifications are often based on engineering judgment, assume well-mixed indoor environmental conditions, and target a certain average value for the particle concentration in a cleanroom. However, in reality, the cleanroom indoor environment is seldom well-mixed with non-uniform distribution of particles. Airflow patterns play a crucial role in determining the level of non-uniformity and cleanliness levels in the critical zones of a cleanroom. Several inter-related factors can affect the airflow patterns and resulting flow path of airborne contaminants.
This CFD study systematically evaluates the impact of ACH and the locations of supply diffusers on the airflow patterns and the resulting distribution of particles in the critical zone of a pharmaceutical cleanroom. Spread Index – a CFD-based metrics is used to compare the contamination control performance for three levels of ACH and two different layouts of laminar diffusers.
These analyses indicate that the airflow patterns are mostly non-unidirectional and independent of the ACH levels. The resulting particle concentration at a critical plane shows non-uniform distribution with local zones of high concentration levels. Though increasing the ACH levels enhances dilution and reduces the overall particle concentration, it neither ensures the required level of cleanliness in the critical zone of a cleanroom nor creates well-mixed uniform concentration in the space. Rather than the ACH, the layout of supply diffusers plays a prominent role in determining the effectiveness of contaminant removal.. Short-circuiting of the supply air without sweeping the sources of contaminant and recirculating airflow patterns promote formation of high concentration zones. Placement of supply diffusers over the sources of contaminants (operators) improve the contaminant removal and reduce the particle concentration without increasing the air change rates. These analyses indicate that the cleanroom contamination control criterion based on a single value of ISO class or a single value of an average particle concentration based on well-mixed assumption cannot address the non-uniform particle concentration and the required level of cleanliness in the critical zone of a cleanroom. CFD analyses can help optimize the design and operating conditions of a cleanroom to achieve a higher level of cleanliness with lower air change rates.
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