For dampers that throttle air flow, which blade arrangement provides the required control?

• A. Constant volume control
• B. Face and bypass damper action
• C. Parallel blade operation
• D. Opposed blade operation

Answer: (D)

Throttling air flow needs a damper that can narrow the passage smoothly and seal well at partial openings. Opposed blade operation uses two sets of blades that move toward each other from opposite directions, overlapping to form a tight, progressively smaller opening. This arrangement minimizes leakage near closed and provides a steady, linear change in flow as the damper position moves, giving precise control over the amount of air let through. In contrast, parallel blade operation can leave small gaps at partial openings, and face-and-bypass or constant-volume approaches involve different mechanisms or control strategies rather than the blade geometry that governs throttling precision. So, the opposed blade arrangement is best for providing the required throttling control.