Engineering Manual o.. - HVAC.Amickracing

DAMPER SELECTION AND SIZINGDAMPER SELECTIONDAMPER TYPESPARALLEL AND OPPOSED BLADE DAMPERSParallel blade dampers are constructed so each blade rotatesin parallel with or in the same direction as the blade next to it(Fig. 1). The rotation changes the direction of airflow and canprovide mixing with only a small increase in airflow resistance.ROUND DAMPERSRound dampers (Fig. 3) are typically used to control flow inducts that usually have high static pressure and high velocitycharacteristics. Round dampers can be installed in air handlingsystems with spiral-wound ducts in sizes similar to rectangularducts. The smallest sizes of round dampers have a butterflytype blade while larger ones might be multiblade.PARALLELBLADEACTIONM10412Fig. 3. Round Damper.AIRFLOW THROUGHPARALLEL BLADE DAMPERAT ONE-THIRD CLOSED.M10414Fig. 1. Parallel Blade Damper.Opposed blade dampers are constructed so adjacent bladesrotate opposite to each other (Fig. 2). The rotation does not changethe direction of airflow, but it does increase resistance to andstratification of airflow since the air is funneled through a smalleropening. An opposed blade damper must be open further to obtainthe same resistance to airflow as a parallel blade damper.LOW LEAKAGE DAMPERSLow leakage dampers minimize the amount of leakage througha fully closed damper. This can increase the energy efficiency ofthe control system, the comfort level in occupied spaces, and thesafe operation of control elements such as coils. Low leakage isachieved through a variety of damper features and parameters,including blade edge seals, blade side seals, blade linkage, bladeand frame reinforcements, and seal material.Damper blade edge seals that work in compression betweenthe blades e.g., cellular foam strip, (Fig. 4) are effective whenblades are rigid enough to prevent misalignment and bending.Other types of blade edge seals include:BLADEEDGEOPPOSEDBLADEACTIONCELLULAR FOAM SEALWORKS WITHCOMPRESSIONC2385Fig. 4. Cellular Foam Blade Edge Seal.AIRFLOW THROUGHOPPOSED BLADE DAMPERAT ONE-THIRD CLOSED.M10415Fig. 2. Opposed Blade Damper.— Snap-on seals, extruded plastic or dual durometer elastomertypes (Fig. 5)— Over-center compression seals, extruded plastic or roll-formconstruction (Fig. 6)— Inflatable seals (rubber or silicone) that use the pressuredifferential across the damper for tight sealing when closed(Fig. 7)448ENGINEERING MANUAL OF AUTOMATIC CONTROL