A Goal Method and a Target Method for balancing exhaust ventilation duct systems with dampers

This work presents and discusses two approaches to adjusting dampers, here called the Goal and Target Methods. A form of the Goal Method is commonly employed but has not been clearly described or discussed elsewhere. The Target Method is a novel variation of so-called "proportional" methods. Detailed step-by-step procedures for each are presented. It is likely that the most common method of balancing the airflows in ventilation systems is simply to adjust the damper in each branch in turn until the observed airflow for each branch duct (Qbr) equals the goal airflow (Qbr-goal). This is a simple Goal Method. It is difficult and time-consuming due to interactions among branches and with the fan. As one adjusts a damper to reduce airflow in that duct, all other airflows increase. As each damper is inserted, the overall resistance of the system increases, reducing fan output. The Target Method uses target values of centerline velocity pressure (VPcl) or hood static pressure (SPh) computed from the measured values of VPcl and SPh multiplied by factors intended to account for (a) initial Qbrand Qbr-goal, (b) the interactive effects of dampers on branch airflows, and (c) a model that predicts the reduction in fan airflow due to the dampers. Simulations on a computer spreadsheet predict that the Target Method will produce better accuracy for fewer adjustments than the goal method, but it is more computationally difficult.