ABSTRACT
A heating, ventilating, and air conditioning (HVAC) automatic control system starts, stops, and regulates the flow of air and water. HVAC controls receive and transmit signals; they can be either analog or digital. The main components of an automatic control system are the sensor, the controller, the operator, and the controlled device. A direct acting controller increases its signal strength to the operator as the condition its sensing increases. A controlled device that moves toward the closed position as the controller signal strength decreases is normally closed. Pneumatic control systems are powered by compressed air. The source of the compressed air in a pneumatic system is an electrically driven, reciprocating, positive displacement air compressor. In electric control systems, as in pneumatic systems, the controller is a separate hardware component. Electric damper motors or valve operators provide either two-position or proportional control. Digital control provides greater reliability, greater flexibility, and more energy and cost savings than conventional pneumatic analog control.
