Engineering Manual o.. - HVAC.Amickracing

DAMPER SELECTION AND SIZINGIn a low leakage damper, materials for the seals are selectedbased on the temperature of the air being controlled. Standardseals can be upgraded to withstand higher temperatures by usinga more heat resistant material. An example would be changinga blade edge seal from neoprene to silicone rubber.When duct static pressure is relatively low but leakage mustbe minimal, a low leakage damper with reduced static pressureratings may be used. Generally, as the strength of a given damperincreases, velocity and static pressure drop capabilities increase.SMOKE DAMPERSAny damper that controls airflow is capable of controllingsmoke. In order to apply dampers to smoke control systemsproperly, UL 555S, Standard for Leakage Rated Dampers forUse in Smoke Control Systems, provides classification basedon leakage, differential pressure across the damper, maximumvelocity when the damper is fully open, temperature, anddamper size. This classification includes the specific actuatorused. See Table 1 for leakage classifications.Table 1. UL 555S Maximum AllowableDamper Leakage Classifications.LeakageClassificationOIIIIIIIVApproximateSI EquivalentOIIIIIIIVCfm per sq ft at 1in. wc04104060m3/s per sq mat 0.249 kPa00.0200.0510.2030.305Cfm per sq ftat 4 in. wc082080120m3/s per sq mat 0.995 kPa00.0410.1020.4060.610Generally, Classes I, II, III, and IV are considered appropriatefor smoke control. The class specified should be based on theapplication requirements. For example, Classes I and II areappropriate for mixed air dampers on systems having return fans.Classes II and III are appropriate for zone dampers where moreleakage is acceptable. Classes III and IV are applicable to dampersthat always modulate, such as in stairwell pressurization systems.FIRE DAMPERSFire dampers are used in HVAC systems to prevent superheatedair, gases, or flames from crossing a fire barrier through an airduct. Fire dampers are usually not used in modulating airflowcontrol applications and are designed for extreme operatingenvironments. Fire dampers are rated in hours of exposure in aspecified test environment. Construction and performance of firedampers (Fig. 11 and 12) is governed by UL Standard 555.M10413Fig. 11. Wall/Partition Fire Damper.In designing a smoke damper, a manufacturer develops aproduct line with Maximum A and B and Minimum A and Bdimensions where:A = Overall damper size in the direction of the bladelength.B = Overall damper size perpendicular to the blade length.The three damper sizes tested by UL are Maximum A/MinimumB, Minimum A/Maximum B, and Maximum A/Maximum B.Damper testing includes meeting material constructionrequirements, cycling, temperature degradation, dust loading,salt-spray exposure, leakage, and operation at maximumpressure and velocity.In testing for temperature degradation, the damper is heatedin the closed position for 30 minutes and then cycled to see thatit operates as intended. Temperature classifications include121.1°C, 176.7°C, 232.2°C, etc., in increments of 55.6 kelvins.M10416Fig. 12. Ceiling Fire Dampers.450ENGINEERING MANUAL OF AUTOMATIC CONTROL