Do High Air Change Rates (ACR) Make Labs Safe?

Kishor Khankari, AnSight LLC

Often high airflow rates or air change rates per hour (ACH) for laboratory spaces are presumed to cover the risk of chemical exposure. Do they really cover safety of all occupants? This presentation with the help of Computational Fluid Dynamics (CFD) analysis attempts to answer this question. This study investigates the impact of ACH on the flow path of contaminants and the resulting transient and spatial distribution of contaminant concentrations in a typical lab. A total of four ACH levels varying from 4 to 10 are analyzed for the same generation rate of the contaminant and for the same layout of the ventilation system. Time varying concentration levels are predicted at the face level of three occupants located at three different locations in the lab as well as in the exhaust duct. Based on these concentrations the time varying chemical exposure (dose) for each occupant is calculated. Although high supply airflow rates can reduce the overall concentration of contaminants it may not ensure uniformity of concentrations below the acceptable level. Importantly, locations of high concentration, especially those in the breathing zone of occupants, can pose potentially higher exposure risk. The design of HVAC system and the flow path of contaminants play an important role in determining the strength and location of high concentration zones. This analysis shows depending on the position of the occupant relative to the source of contaminant and the location of exhaust grills the exposure levels can vary. High ACH does not necessarily create diluted indoor environment at all the time for all the occupants. These analyses indicate with increasing ACH the overall concentration levels in the space (detected in the exhaust duct) decrease, however, the flow path of the contaminants remains almost similar. The ventilation effectiveness of the HVAC system is analyzed with the help of two non-dimensional indices: Spread Index (SI)TC and Purge Time (PT)TC. (SI)TC quantifies the percent of the room volume presumed to be the high risk zone, where the contaminant concentrations are higher the desired target concentration (TC). Whereas the (PT)TC evaluates the time the ventilation system takes to bring the lab environment below the target concentration. This analysis shows high values of (SI)TC indicating larger spread of contaminants during the initial phase of the contaminant release. The analysis results will be presented with insightful animations showing the progression and movement of contaminant cloud in the space.

Learning Objectives

  • Understand the impact of supply airflow rate or air change rates per hour (ACH) on the distribution of contaminant concentration in a laboratory space.
  • Understand the impact of air change rates per hour (ACH) on the relative exposure levels (dose) of occupants and how do they vary with time and location in the lab space.
  • Understand the definitions of Spread Index (SI)TC and Purge Time (PT)TC and how they can be employed to evaluate the ventilation effectiveness of laboratory HVAC systems.
  • Understand how Computational Fluid Dynamics (CFD) can be employed in evaluating the ventilation effectiveness of laboratory HVAC systems and optimizing the ventilation system designs for the labs.


Dr. Kishor Khankari, Ph.D. is noted expert in Computational Fluid Dynamics (CFD) with several years of experience in providing engineering insights and optimized HVAC design solutions using analytical techniques. He has developed patented technology of exhaust fan assembly. He has published several technical papers and trade magazine articles. Dr. Khankari is ASHRAE Fellow member, Distinguished Lecturer, and recipient of the ASHRAE Exceptional and Distinguished Service Awards.


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