Balancing Exhaust Ventilation Systems

Dampers are used to adjust the distribution of §airflows in duct systems. The common (i.e., most §commonly used) method is to adjust each damper in §turn so that the airflow through its branch equals §the desired level. Typically, the airflow through §each branch duct is estimated from the centerline §velocity pressure. To test the effectiveness of that §approach, dampers were adjusted on a seven branch, §full-sized experimental duct system. After adjusting §the dampers for a given condition, the percent §excess airflow (%Qexcess) for the system was §estimated as the amount above the ideal fan airflow §that would exist if the fan speed were adjusted so §that the lowest ratio of airflow to airflow goal for §any branch was unity. The lower the value of percent §excess airflow, the more perfectly balanced the §system was. The results varied with the level of the §target airflows. The excess airflow values were much §higher than the airflow values found by Dodrill §(2004) on the same system for the SPh Goal Ratio §method proposed by Guffey (2005). This analysis §should be useful to professionals in the fields of §HVAC and ventilation design.