Engineering Manual o.. - HVAC.Amickracing

DAMPER SELECTION AND SIZING100CHARACTERISTIC RATIOS =SERIES RESISTANCEDAMPER RESISTANCECharacteristic ratio =or=total resistance – damper resistancedamper resistancetotal resistancedamper resistance – 1% FULL AIRFLOW20 10 5 3 2 1INHERENTCHARACTERISTICFor parallel blade dampers:2.5 =100damper resistance – 1Damper resistance = 29% of total resistance00 % DAMPER OPENING100C1498or29= 41% of series resistance100 – 29100Fig. 29. Damper System Characteristics ofParallel Blade Dampers.CHARACTERISTIC RATIOS =SERIES RESISTANCEDAMPER RESISTANCEFor opposed blade dampers:10010 =damper resistance – 1or damper resistance = 9% of total resistance9= 10% of series resistance100 – 9% FULL AIRFLOW20 105 3 1INHERENTCHARACTERISTICFor example, if a coil (Fig. 31) with a pressure drop of 0.140 kPais located in series with an opposed blade damper, the dampershould have a pressure drop of 0.014 kPa (10 percent of 0.140= 0.014).00% DAMPER OPENING1000.014 kPaDROP0.140 kPaDROPM15303C1499Fig. 30. Damper System Characteristics ofOpposed Blade Dampers.To achieve performance closest to the ideal linear flowcharacteristic, a characteristic ratio of 2.5 for parallel bladedampers (Fig. 29) and 10 for opposed blade dampers (Fig. 30)should be used. The percent of the total resistance needed bythe damper can be determined by:Total resistance (100%) = damper resistance + seriesresistanceCharacteristic ratioseries resistance(Fig. 29 and 30)=damper resistanceSubstituting (total resistance – damper resistance)for series resistance:Fig. 31. Typical Pressure Drop for Coil andDamper (Series Resistance).DETERMINING DAMPER SIZEThe desired relationship of damper resistance to seriesresistance developed in DAMPER CHARACTERISTICS isused to determine the desired damper pressure drop. Thispressure drop is then used in the damper sizing procedure inTable 4.For example, for a 915 mm by 1625 mm (1.487 m 2 ) ductwith an airflow of 9.45 m 3 /s and a pressure drop of 0.014 kPaacross a parallel blade damper, determined the damper size asshown in Table 5.ENGINEERING MANUAL OF AUTOMATIC CONTROL459