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Computational Fluid Dynamics (CFD) Analysis of Hospital Operating Room Ventilation Systems Part I: Analysis of Air Change Rates

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©2018 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 60, no. 5, May 2018.

By Kishor Khankari, Ph.D., fellow ASHRAE

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

The primary objective of hospital operating room (OR) ventilation systems is to minimize surgical site infection due to airborne contaminants and 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 contamination of the sterile (clean) zone where the surgical procedures are performed. One source of infection in the OR is squames, which are skin scales shed from the exposed skin of occupants in the room. Once airborne, these bacteria-carrying particulates generally follow the path of airflow in the room. The OR ventilation system should effectively sweep these particulates out of the sterile zone and minimize their re-entrainment from non-sterile (contaminated) zones.

ASHRAE/ASHE Standard 170-2017 provides minimum requirements for the design and layout of the ventilation systems in operating rooms which presumably can maintain a sterile environment around the surgical site. According to this standard, diffuser array should provide airflow over the patient and surgical team. Furthermore, the coverage area of the primary supply diffuser array should extend a minimum of 12 in. (305 mm) beyond the footprint of the surgical table on each side. The room should be equipped with at least two low sidewall exhaust grilles placed at opposite corners, with the bottom of these exhaust grilles installed approximately 8 in. (203 mm) above the floor. In addition, the OR should maintain positive pressure with a total of 20 air changes per hour (ACH) supplied with 4 ACH outside air. The supply air should be unidirectional directed downward with an average discharge velocity of 25 to 35 fpm (0.13 to 0.18 m/s). These specifications for minimum discharge velocities are based on previous CFD studies, which concluded that such velocities and the coverage area of the diffuser array would overcome the rising buoyant plumes from the sensible heat sources (i.e., surgical lights in the sterile zone), as well as protect the surgical site by allowing a local thermal plume to develop from a relatively “warm” surgical site. The later assumption, however, could not be verified by the ASHRAE-funded research project on experimental evaluation of hospital OR ventilation systems. It should be noted that the role ASHRAE standards is to provide only “minimum requirements,” which may not be the optimal design guidelines.

Air is the primary carrier of heat, moisture, contaminants, and airborne particulates in operating rooms. The distribution of supply air and the associated flow path of the air determine the resulting air velocities, temperature, and concentration of contaminants, and flow path of airborne particulates at various locations in the room. Such distribution, in turn, determines thermal comfort, air quality, and potential for transmission of airborne particulates. Ideally, in an operating room the supply air should pass through the sterile zone and exit through exhaust grilles in a “single pass” manner without recirculation and mixing with the supply airstream. It is generally believed that high air change rates can yield a cleaner environment in the operating rooms. However, recent studies indicate increasing ACH does not necessarily provide a cleaner environment but substantially increases the operating costs.

The airflow patterns, temperature distribution, and resulting flow path of airborne contaminants can depend on several interrelated factors including the location, type, and number of supply diffusers; supply air change rates and supply air temperature; locations and strengths of various heat sources in a room, including the ambient and surgical lights; size and location of equipment in the room that can obstruct the flow path of the air and contaminants; size and locations of room returns; and perhaps also on the frequency of opening and closing OR doors. Physical testing and real time measurements of all the parameters that can affect the performance of the OR ventilation system including the airflow patterns and the resulting flow path of contaminants is not feasible, if not impossible.

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