A valve is a device that regulates, or controls the flow of a fluid (gases, liquids, or slurries) by opening, closing, or partially obstructing various passageways. In an open valve, fluid flows in a direction from higher pressure to lower pressure. Valves have many uses, including controlling water for irrigation, industrial uses for controlling processes, residential uses such as on/off and pressure control to dishwashers and washing machines for clothes as well as water taps in the home. Valves are also used in the military and transport sectors. In HVAC ductwork and other near-atmospheric air flows, the role of valves is more like a damper.
An actuator is a device that uses a form of power to convert a control signal into mechanical motion, for example, from electric door locks in automobiles, to ailerons on aircraft. A few types of actuators include electric motors, stepper motors, electroactive polymers, screw jacks, servomechanism, solenoids, and hydraulic cylinders. Therefore, it can be said that a valve is a physical part of a system that opens and closes to dictate the amount of fluid being allowed to pass through, while the actuator tells the valve when to operate.
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