A circular CAV damper has been developed, based on the investigation of the pressure loss, and the flow-resisting moment by the damper blade. When a torsional spring is attached to the damper axle, and the setting angle is adjusted to around $82^{circ}$, the volume rate across the damper is almost independent of the static pressure. Such a CAV characteristic appears at an opening angle between $40^{circ}$ and $60^{circ}$, where the normalized moment decreases linearly with the angle. In addition, by adjusting the setting angle, the volume rate can be controlled to within 10% error, regardless of the pressure loss.