Mass Balance Analysis of Air Change Rate (ACR) and Space Volume on Contaminant Dilution.

ASHRAE Winter Conference, Orlando, FL, 2020

Author:

Kishor Khankari

Abstract:

The ventilation airflow rates are 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. This study with the help of a mass balance approach evaluates the impact of space volume on the contaminant dilution. It indicates that the specification of the same ACH for all space volumes can make smaller spaces vulnerable to a higher dose of contaminant exposure. This study shows that by proportionately adjusting the ACH with the space volume can maintain the same level of dilution (steady state concentration) and occupant exposure for all space volumes, however, larger spaces may take longer time to reach the steady state. For a given space, with a constant volume, increasing ACH increases both the rate and level of dilution, though with diminishing returns – most of the initial reduction occurs at lower ACH. One log (90 percent) reduction in the contaminant concentration requires ten-fold increase in the supply airflow rate. The rate of concentration build-up and decay depends only on the ACH but not on the individual values of supply airflow rate and/or space volume. Whereas the level of dilution, that is, the steady state concentration for which occupants are exposed for a longer duration, depends only on the supply airflow rate and not on the ACH. The current practice of specifying the same ACH for all space volumes only maintains the same rates of concentration build-up and decay while making smaller spaces more vulnerable for the higher dose of contaminant exposure. This study shows that by proportionately adjusting the ACH with the space volume can maintain the same level of dilution (steady state concentration) and occupant exposure for all space volumes, however, larger space may take longer time to reach the steady state. The mass balance approach assumes well-mixed conditions within a space and cannot predict the non-uniformity in the breathing zone of occupants which requires a detailed three-dimensional transient Computational Fluid Dynamics (CFD) analysis.