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Computational Fluid Dynamics (CFD) Analysis of Hospital Operating Room Ventilation Systems – Part II: Analyses of HVAC Configurations

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©2018 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 60, no. 6, June 2018.

By Kishor Khankari, Ph.D., fellow ASHRAE

About the Author: Kishor Khankari, Ph.D., is the president at AnSight LLC in Ann Arbor, Mich.

The primary objective of the hospital operating room (OR) ventilation system is to minimize surgical site infection due to airborne bacteria and to provide a comfortable environment for surgeons and other staff in the room. The key factor in reducing surgical site infection is to minimize the entrainment of airborne particulates from the non-sterile zone into the sterile zone.

Part I of this study analyzed the impact of air change rates on the airflow patterns, temperature distribution, and resulting flow path of airborne particulates. The study indicated that high air change rates (ACH) potentially provide better thermal environment and minimize the mixing and recirculation of airborne particulates in the non-sterile section of the OR; however, higher ACH cannot substantially alter the airflow patterns and the resulting flow path of airborne particulates that may entrain into the sterile zone from the non-sterile zone. This present study evaluates whether the HVAC layout, including the location and number of supply and exhaust grilles in the OR can influence the airflow patterns and alter the flow path of contaminants.
As mentioned in the previous article, air is the primary carrier of heat, moisture, contaminants, and airborne particulates in operating rooms. Therefore, the distribution of supply air determines the resulting air velocities, temperature, concentration of contaminants, and the flow path of airborne particulates in the room. These factors subsequently determine thermal comfort, air quality, and the potential for transmission of airborne particulates. In the operating room, ASHRAE/ASHE Standard170-2017 recommends a supply of unidirectional downward airflow passing over the operating table and surgical staff, which sufficiently covers the entire sterile zone. Furthermore, Standard170-2017 requires an exhaust through at least two low sidewall grilles placed on opposite walls of the OR.

Previous studies indicate that the HVAC configuration, including the location and type of supply diffusers, and exhaust grilles can affect the airflow patterns, which in turn can affect the flow path of contaminants. Analysis of several commonly used supply diffusers for a wide range of ACH showed that ventilation systems that provide laminar (unidirectional) flow conditions with adequate coverage of the sterile zone can yield better contamination control.

Another study indicated a single continuous array of supply diffusers without gaps can be more effective in promoting the airflow that sweeps particles away from the operating table. Yet another intuitive approach is to create a barrier by providing an air curtain surrounding the array of laminar diffusers.6 However, the performance of these systems depends on the proper design and operation of the air curtains, which includes the length and location of air curtain, airflow split between the core supply through laminar diffuser and through the air curtain; and the relative ratio of the discharge velocities.

A recent CFD study of a patient room ventilation systems showed that simply relocating the return over the patient’s bed can significantly alter the flow path of contaminants and can reduce recirculation and mixing of the contaminants in the room.

However, previous studies did not examine different HVAC configuration to potentially improve upon the legacy design to effectively reduce entrainment of air from the non-sterile zone back into the sterile zone.

The current CFD study analyzes the impact of three different HVAC configurations on the entrainment of the surrounding air into the sterile zone and on the resulting flow path of airborne particulates. A total of three HVAC configurations are evaluated, including the legacy configuration which was studied previously, and two additional conceptual modifications of the legacy design. The overall airflow patterns, temperature distribution, and the probable flow path of airborne particulates are compared. Also the impact of these configurations on the acceleration of centerline velocity of the supply air jet from the laminar diffusers is evaluated.

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