What kind of equipment is used to turn the airflow on or off in a system?

The correct answer is Parallel Blade Dampers. These devices are specifically designed to control airflow within a duct system by opening and closing their blades to regulate the volume of air that passes through. When the blades are positioned parallel to the airflow, they allow air to pass freely; when they rotate to a closed position, they effectively block the airflow. This capability makes parallel blade dampers essential for maintaining desired temperature and ventilation conditions in a facility.

In contrast, a flow valve typically regulates the flow rate of liquids rather than air and is not used primarily for turning airflow on or off. An air compressor serves a different purpose, as it increases the pressure of air for various applications rather than controlling airflow direction or volume in a ventilation or heating system. Lastly, a pressure regulator is intended to maintain a specific pressure within a system rather than controlling the flow of air, thereby making it unsuitable for the task of simply turning airflow on or off.