Can ACH Specifications Safeguard Smaller Labs? A CFD Approach

Kishor Khankari, AnSight LLC

The ventilation airflow requirement is generally specified in terms of air changes per hour (ACH). Such specification requires higher airflow rates for larger spaces irrespective of the strength of contaminants in a space. This study, with the help of computational fluid dynamics (CFD), evaluates the impact of space volume on the distribution and spread of contaminants in a space. Comparative analyses are performed between the traditional constant ACH and newly proposed volume adjusted ACH approaches under identical design and operating conditions. A spread index is developed to analyze the ventilation effectiveness for each case.

The space averaged concentrations at the exhaust grille as predicted by these CFD analyses confirm the conclusions of the previous mass balance study that the current practice of specifying the same ACH for all space volumes can make smaller spaces more vulnerable for the higher dose of contaminant exposure, while proportionately adjusting the ACH with the space volume can maintain the similar level of occupant exposure for all space volumes. Although the mass balance approach can correctly predict the overall trends for the space averaged concentrations, these CFD analyses show significant spatial and temporal variations in the contaminant concentration and resulting exposure levels. These analyses indicate that the exposure levels in the breathing zone of occupants can be significantly higher than those predicted by the space averaged "well-mixed" conditions. The spread index analyses indicate that with the constant ACH approach and due to reduced supply airflow rate, the contaminants would reach higher concentrations and spread extensively in smaller spaces than larger spaces. It further indicates that with the volume adjusted ACH approach such spread can be significantly reduced for smaller spaces and equalized across the space volumes.

These analyses were performed for identical configurations of supply and return diffusers without adjusting their number, location, and throws. As a result in the case of volume adjusted ACH, slightly higher exposure levels and contaminant spread are predicted in the breathing zone of larger spaces. It should be noted that each space is unique, and therefore, HVAC configuration should be optimized by preforming such CFD analyses to maximize the ventilation effectiveness before increasing the supply airflow rates.

Learning Objectives

  • Understand the difference between simple mass balance and CFD approaches in predicting the dilution levels of contaminant in a space;
  • Understand the impact of space volume on spatial and temporal variations of contaminant concentration with the same ACH;
  • Understand the approach of volume adjusted ACH and its impact on dilution for various space volumes; and
  • Understand how Computational Fluid Dynamics (CFD) can be employed in evaluating the ventilation effectiveness of laboratory HVAC systems.


Dr. Kishor Khankari, Ph.D, is a 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 an ASHRAE Fellow member, Distinguished Lecturer, and recipient of the ASHRAE Exceptional and Distinguished Service Awards.


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