Abstract
A study tested whether measures of equivalent resistance (X values) and ratios of static pressure (SPratio) for given ducts of contaminant control exhaust ventilation systems were independent of substantial changes to airflow level and to changes to resistance of other ducts within the same full-scale five-branch system. In a factorial study design, four airflow levels were achieved by changing fan rotation rate while resistances to flow for specific branch ducts were changed independently by adjusting slidegate dampers to various settings. For each damper insertion depth (including fully open), the results demonstrated substantial invariance for branch X values (few greater than 5%), SPratio (few greater than 3%), and fraction of airflow to each duct (few greater than 2%). X-values for submains were much less stable, changing by 20% or more with changes to other parts of the system. For the same conditions, hood static pressures changed by as much as 96% (with standard deviation of 40%). The results suggest that before and after values of X and SPratios should be more reliable bases for indicating alterations than comparison of observed static pressures. The stability of airflow distributions with substantial changes in airflow suggests that one could adjust airflow distribution (e.g., with dampers) without considering whether the fan speed was set correctly, leaving fan adjustments for a final step.
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