A Goal Method and a Target Method for Balancing Exhaust Ventilation Duct Systems with Dampers

Abstract

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.

Experimental Validation of a Target Method for Balancing Exhaust Ventilation Duct Systems with Dampers

“…(3) When a damper on one branch is pushed in, the airflow in that branch decreases, the fan airflow increases by somewhat less, while the airflows in each of the other branches increase by differing amounts depending on their location in the system. As each succeeding damper is adjusted, the airflows through those branches already adjusted become increasingly greater than their value of Q br-goal .…”

“…The Target Method (3) tested in this study is a proportional method designed to address the problems of inappropriate fan speed and shifting airflow among branches. Because the relative distribution of airflows among branches will change very little even if the fan airflow changes a great deal, (6,7) the Target Method is designed to allow adjustment of dampers to achieve branch airflow ratios equal to Q fan-final /Q fan-goal (Eq.…”

“…Instead, it uses target values that account for the problems of both inappropriate fan speed and shifting airflows among branches. (3) The desired levels of airflow are then achieved by adjusting the fan output after all dampers are adjusted. That is, like other proportionality methods, the Target Method does not aim to attain a specific airflow level in each branch but instead seeks to obtain a fraction of total airflow.…”

“…The system had seven branches designed to have diverse resistances to flow due to their different duct lengths, number of elbows, and types of hood transition (Figure 1, Table I). The 20-gauge, galvanized 3.00, 4.00, 5.00, and 6.00 inches) immediately before measurements were taken. The time average was set at 1 sec to measure velocity pressure and at 5 sec to measure static pressure.…”

“…Increasing the resistance of a branch reduces its flow and increases the flows to other branches. (1) By accounting for this shift in airflow, the Target Method (3) aims to be less tedious and difficult.…”