Volume Control damper is widely used in heating, ventilation and air-conditioning applications in order to maintain or control the air volume inside the duct. These dampers facilitate the air flow to  each zone and each room to be controlled  during testing & commissioning. The blades may be parallel or opposed type depending on the requirement.
Volume control damper can be mounted either horizontally or vertically. However, care should be taken to mount the damper, by orienting the blades in the direction of the desired airflow.