Engineering Manual o.. - HVAC.Amickracing

DAMPER SELECTION AND SIZINGTORQUE REQUIREMENTSOperating and close-off torque requirements of dampers andtheir actuator sizing guidelines are typically shown in manufacturerspecifications. Occasionally a brief explanation of the theory orbasis for the actuator torque ratings accompanies this data.Two conditions must be considered when establishingminimum torque requirements of a damper. One is closingtorque which is the torque required to force the blades togethersufficiently to achieve minimum possible leakage. The other isthe dynamic torque required to overcome the effect of highvelocity airflow over the blades. The maximum dynamic torquewill occur somewhere in the middle two-thirds of the bladerotation depending on the damper design.VELOCITY RATINGSApproach velocity of the air is an important physicallimitation that applies to all dampers and should be consideredwhen sizing dampers. Generally, the maximum velocity ratingincreases as the overall performance rating of a damperincreases. In practical terms, a higher velocity rating of onedamper compared to another indicates the former damper hasstiffer blade and linkage design and that the bearings may alsobe capable of higher loads. The velocity rating of controldampers is usually a statement of the maximum value allowedfor the particular design under conditions of normal (notexcessive) turbulence. Velocity ratings must be severely reducedunder excessively turbulent conditions.Uneven velocity profiles due to locations near fan discharges,duct transitions, and elbows require derating of the velocity value.Table 2. Maximum Static Pressure DifferentialPressureDifferentialDamper Typein. wcStandard Damper 3Standard and High Temperature, Low6Leakage DamperLow Static, Low Leakage Damper 2The ratings are probably conservative for smaller sizes sincedamper blades tend to deflect more as their length increases.An alternative method is a listing of differential static capabilityas a function of damper blade lengths, such as Table 3.Table 3. Maximum Static Pressure Differential CapabilityMax Close-OffDamper Length (in.)Static (in. wc)12 824 836 648 4UL CLASSIFICATION (FIRE/SMOKE)Performance criteria for fire or smoke dampers bearing theUL rating are contained in specifications UL 555 (Standard forSafety/Fire Dampers and Ceiling Dampers) and UL 555S(Standard for Leakage Rated Dampers for Use in Smoke ControlSystems). These two specifications govern the design,performance, installation, and marking (labeling) of the devicesbearing the UL classification.TEMPERATURE RATINGSThe maximum operating temperature of control dampers isthe maximum temperature at which they will function normally.Increased temperature ratings on dampers indicate that bearingsand seals are constructed of heat resistant materials. Statedtemperature limits apply to the operating life that would beexpected under normal ambient conditions.PRESSURE RATINGSThe pressure rating of a control damper is the maximum staticpressure differential which may be applied across the assemblywhen the blades are closed. Excessive leakage (caused bydeflection of blades) and abnormally high operating torque (dueto forces on blades, and loads on bearings and linkages) canresult from high differential pressure. In extreme cases, physicaldamage to the dampers could occur. Typical ratings are statedin Table 2.APPLICATION ENVIRONMENTMost HVAC system requirements can be met using standarddampers from major manufacturers. Many manufacturers alsobuild custom dampers with special features to satisfy particularapplication requirements. Custom features or modificationsinclude blade reinforcement, heavy duty linkage, bearings, axlesand frames, special seals, and construction or finish of corrosionresistant materials. The application must be carefully examinedto ensure job requirements are met. Some of these specialfeatures add significantly to the cost of the damper so theyshould be furnished only when needed.VELOCITYAs the velocity in a system increases, dampers in the systemencounter higher forces. The impact pressure of the airmovement increases the bending force on the damper bladesand the airflow over the damper blades may cause a torque orENGINEERING MANUAL OF AUTOMATIC CONTROL428