What's the difference between parallel-blade and opposed-blade dampers?

Parallel-blade dampers have all blades rotating in the same direction, producing a non-linear flow characteristic best suited for two-position (open/close) applications and where pressure drops are low. Opposed-blade dampers have adjacent blades rotating in opposite directions, providing a more linear flow characteristic ideal for modulating control applications. Per ASHRAE guidance, opposed-blade configurations generate lower torque requirements because aerodynamic forces on adjacent blades partially cancel each other. For VAV system economizer sections, opposed-blade dampers are strongly preferred to maintain stable mixed air temperature control.