Computational Fluid Dynamics (CFD) Analysis of Hospital Operating Room Ventilation System – Part II: Analyses of HVAC Configurations
ASHRAE Journal June 2018
Previous Computational Fluid Dynamics (CFD) studies of a legacy HVAC design for a hospital operating room indicate that when airborne particulates originate in the non-sterile zone (i.e., from the face of a scrubbing nurse), these particles can get entrained back into the sterile zone, irrespective of the supply airflow rate or ACH. This study evaluates whether modifications in the legacy HVAC configuration could alter the flow path of these contaminants and mitigate the issue of particulate entrainment. During this study CFD analyses of two conceptual modifications in the legacy HVAC design of a hospital operating room are performed to compare the airflow patterns, temperature distribution, and resulting flow path of airborne particulates. Additionally, the acceleration of centerline velocity of the supply air jet is also evaluated to understand the extent of entrainment for the three HVAC configurations analyzed.
These analyses indicate HVAC configuration, including the number and locations of supply diffusers and exhaust grilles, influences the HVAC performance of the ventilation systems of OR. Unlike the legacy design with low wall exhaust, when the exhaust grilles are placed in the ceiling with a small barrier around the sterile zone (“ceiling exhaust”), the entrainment of particulates from the non-sterile zone into the sterile zone are significantly reduced compared to the legacy design. Furthermore, in the “ceiling exhaust” configuration, mixing of the cold exiting air from the sterile zone with the warm air in the non-sterile zone reduced thermal gradients across the sterile zone. This may help in creating a more comfortable thermal environment for the occupants in the OR, reduce the possibility for contaminated air entrainment into the sterile zone, and reduce the acceleration of discharge air jet from the laminar diffusers.
In the second design modification, additional laminar diffusers are placed in the non-sterile zone surrounding the array of laminar diffusers in the sterile zone (“distributed supply”). The total supply airflow and the discharge area of the diffusers between the sterile and non-sterile zone were split in the proportion of 3:1, keeping the same discharge velocity from all the diffusers. This analysis showed the discharge air jets in the non-sterile zone get drifted towards the air jet in the sterile zone which can effectively increase the air entrainment. This was evident from the highest acceleration in the centerline velocity of the discharge air jet in the sterile zone. Thus, the particle movement from the non-sterile zone to the sterile zone did not show any significant improvement over the legacy design. However, unlike the legacy design, entrainment of the cold supply air from the non-sterile zone into the sterile zone results in lowering the temperatures surrounding the central core. In order for the discharge air jets in the non-sterile zone to sweep the occupants and prevent entrainment of the airborne particulates into the sterile zone, the flow split and the discharge velocities between the sterile and non-sterile zones may require careful balancing and adjustment. It should be noted the modifications in HVAC configuration presented in this study are only conceptual and not necessarily the “optimized” designs.
These studies indicate that HVAC configuration, rather than ACH, has a larger influence on the airflow patterns, temperature distribution, and hence, the resulting flow path of contaminants and thermal comfort of occupants. Therefore, in order to improve the effectiveness of OR ventilation systems high ACH should not be considered as the only potential solution. Rather, analysis and modification of the legacy HVAC configuration at a low ACH should be considered first before considering increasing ACH.