Flow Control Devices

Abstract

Dampers are used for the control of airflow to maintain temperatures and/or pressures. Some special types of dampers are used in HVAC equipment, such as mixing boxes and induction units, but the control dampers in air ducts and plenums are almost invariably the “multi-leaf” type. Two arrangements are available: parallel-blade and opposed-blade. As Figure 5-1 shows, in parallel-blade operation all the blades move in the same (or parallel) way. In opposed-blade operation adjacent blades move in opposite directions. Without going into the physics of the situation it would appear that the opposed-blade damper is superior to the parallel-blade damper for modulating control. This is, in fact, true. Parallel-blade dampers, like flat-seated valves, should be used only for two-position control except for special applications such as air mixing to avoid stratification. (See paragraph 6.3.)