Engineering Manual o.. - HVAC.Amickracing

DAMPER SELECTION AND SIZINGtwist on the blades. Because the blade profile of conventionalsheet metal dampers is not streamlined, the stresses imposedon the damper blades due to air movement are dynamic in naturerather than static. To strengthen the damper blades, the gage ofmetal and the number and depth of longitudinal bends can beincreased or reinforcing plates can be spot welded to the bladeto increase the blade cross-section. Strengthened dampers alsorequire heavy duty linkage, bearings, and frame. The dynamicand static stresses and linkage and bearing loads all mean thatlarge actuators are needed.STATIC PRESSUREThe maximum static pressure that an air handling system candevelop across a damper occurs when the damper is fully closed.As the damper opens, system airflow increases and a portion ofthe total pressure is converted to velocity pressure and the forceson the damper become more dynamic than static. It is importantto determine the maximum possible static pressure in normaloperation and to consider this when selecting dampers.TEMPERATURESome dampers are capable of satisfactory operation in thetemperature range of –40 to 400F, primarily to meet hightemperature requirements. A maximum temperature rating of200F is usually satisfactory for HVAC use. The specifictemperature range for a given damper can be found in themanufacturer specifications.CORROSIONDampers used in conventional HVAC systems typicallyrequire galvanizing or zinc plating for corrosion protection.Damper applications that may encounter corrosive elementsand require additional protection include:— Buildings in immediate coastal areas where salt spraycan enter with the outdoor air.— Outdoor air applications where the outdoor air damper islocated very close to the outdoor air intake, when theoutdoor air intake is not protected from rain or snow byfixed louvers, or when the velocity of the outdoor airintake is in the range of 750 to 1000 fpm or more.— Face dampers near spray coils.— Dampers near electronic air cleaners with in-place washers.— Dampers near spray humidifiers.— Dampers used in cooling tower applications.— Dampers in exhaust ducts that carry corrosive fumes orvapors.All aluminum or all stainless steel construction is preferredin many cases. Optionally, protective finishes are available. Therequirement for corrosion resistant dampers usually necessitatesa custom built damper.TURBULENCEThe flow of air in an air handling system is turbulent.Excessive turbulence or pulsations can have the same effectson dampers as increasing air velocity. There is a directrelationship between air velocity and the turbulence caused byairflow through a damper. The effects of moderate turbulencecan be noticed on dampers located near abrupt duct transitionsor near elbows without turning vanes. Effects of severeturbulence, capable of destroying a damper, can be noticed ondampers located in close proximity to a fan. A damper locatednear the discharge of a fan should be inspected during actualoperation over a full range of positions (from full open to fullclosed) to be certain no severe vibration occurs (due to thedamper being in resonance with a frequency generated by thefan blades). If the damper encounters severe vibration, thevibration may be decreased by adding stiffening members tothe damper blades, extra damper linkage, or additional actuators.The preferred method for preventing these damper problems isinitial selection of a location with minimal turbulence. However,if high turbulence cannot be avoided, a custom heavy dutydamper may be required.NUCLEAR/SEISMIC APPLICATIONSDamper applications in nuclear power plants and other similarfacilities must be fully compatible with safety system designsand meet all applicable regulations. Some dampers in nuclearfacilities are required to operate during and after an earthquake.Seismic or earthquake susceptibility requirements vary and arespecific for each individual job or geographic location. Seismiccertification involves verification (usually through testing) thatthe control device can withstand specified levels of vibration.Test procedures include low-frequency, high-amplitude,multiaxis vibration. The tests vary in intensity, not only withdifferent geographic locations but also with the physicalelevation within the building. Therefore, test requirements fornuclear facilities must be carefully reviewed to accommodateall applications.ACTUATORS AND LINAKGESNORMALLY OPEN/NORMALLY CLOSEDActuators open and close dampers according to an electric,electronic, or pneumatic signal from a controller. Actuatorsprovide normally open or normally closed damper operation.In a normally closed application, the damper blades are closedENGINEERING MANUAL OF AUTOMATIC CONTROL429