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

ASHRAE Journal May 2018

Author:

Kishor Khankari

Abstract:

Computational Fluid Dynamics (CFD) analyses of a hospital operating room are performed to analyze the impact of supply airflow rates or air change rate per hour (ACH) on the airflow patterns, temperature distribution, and resulting flow path of airborne particulates. Additionally, the impact of supply airflow rates on the acceleration of centerline velocity of the supply air jet was also evaluated.

These analyses indicate the supply airflow rate, and hence, the discharge velocity of the unidirectional air jet, has a little impact on the overall airflow patterns, resulting thermal stratification, and on the flow path of airborne contaminants in the hospital operating room (OR). When the particulates are originated within a sterile zone or at the edge of the sterile zone they are generally swept away into the non-sterile zone where they can recirculate within the non-sterile zone without significant re-entrainment. However, when such particulates originate in the non-sterile zone, for example from the face of a scrubbing nurse, irrespective of the supply airflow rate, they get entrained into the sterile zone. For all the cases of supply airflow rates, and irrespective of the origin of the release, the particulates tend to stay and circulate in the non-sterile zone before exiting the OR which potentially can increase the probability of deposition of these particles on the back table which is generally located in the non-sterile zone.

Increasing the supply airflow rates and the associated heat capacity of the supply air helps in reducing the thermal gradients between the sterile and non-sterile zone which in turn reduces the contraction of the supply air jet, indicating the potential reduction in the entrainment of air from the non-sterile zone into the sterile zone. This study also indicates the velocity of the discharge air jet increases at it travels towards the operating table. However such acceleration in the discharge velocity reduces with increasing the supply airflow rate.

High ACH can potentially reduce the overall temperature levels in the OR; reduce the thermal gradients across the sterile and non-sterile zones; reduce the acceleration of the discharge air jet; and potentially minimize recirculation of airborne particulates in the non-sterile zone. However, high ACH add to the initial and operating costs of OR ventilation systems and it cannot alter the overall airflow patterns and the resulting flow path of the airborne contaminates (including possible entrainment of airborne particulates from the non-sterile zone into the sterile zone). HVAC configuration including the size, number, and locations of supply and return of the air may play a role in determining the flow path of airborne contaminants, especially in the non-sterile zone. By altering the airflow patterns in the non-sterile zone, the flow of path of these particulates may be altered to avoid entrainment. The legacy HVAC design for hospital operating rooms involving a ceiling array of laminar supply diffusers and low wall exhaust grilles on the opposite walls needs further evaluation in order to minimize the transfer of airborne particulates from non-sterile to sterile zones.