Air Change Rate (ACR) or HVAC Configuration Which Makes 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. Previous analysis indicated that high ACH does not necessarily create diluted indoor environment at all the time for all the occupants. 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 HVAC configuration including the location and type of supply diffusers, diffuser throws, size and locations of exhaust grilles, locations and strengths of heat sources, location and size of fume hoods, and arrangement of furniture and other airflow obstructions can influence the flow path of contaminants, which in turn, determines the strength and location of high concentration zones in labs.

This presentation with the help of Computational Fluid Dynamics (CFD) analysis will demonstrate the effect of HVAC configuration on the ventilation effectiveness of HVAC system. This study investigates the impact of number and location of exhaust grilles on the flow path of contaminants and the resulting transient and spatial distribution of contaminant concentrations in a typical lab. 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. 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 than 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 the distributed exhaust strategy with three exhaust grilles significantly reduced the (SI)TC values resulting in reduced concentration levels and chemical exposure (dose) of occupants. Interestingly the occupant closer to the exhaust grilles shows high level of exposure than the one closer to the contaminant source. 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 location and number of exhaust grilles on contaminant concentration levels and on exposure levels (dose) of occupants;
  • 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; and
  • 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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