Explain the difference between a 'normally open' and a 'normally closed' solenoid valve, and describe a scenario where each would be preferred.

A solenoid valve is an electromechanical valve controlled by an electric current through a solenoid: a coil of wire, with a movable plunger in the center. The position of the plunger determines whether the valve is open or closed. The key difference between a 'normally open' (NO) and a 'normally closed' (NC) solenoid valve lies in their default state when the solenoid is not energized. A normally open solenoid valve is open when the solenoid is de-energized. This means that fluid can flow through the valve in its default state. When the solenoid is energized, the plunger moves, closing the valve and stopping the flow of fluid. A normally closed solenoid valve is closed when the solenoid is de-energized. In this state, fluid cannot flow through the valve. When the solenoid is energized, the plunger moves, opening the valve and allowing fluid to flow. The choice between a normally open and a normally closed solenoid valve depends on the application's specific requirements. A normally closed valve is preferred in applications where the valve is typically closed and only needs to be opened intermittently. This minimizes energy consumption because the solenoid only needs to be energized when flow is required. For example, a normally closed solenoid valve might be used to control the flow of coolant to a machine tool. The valve is normally closed to prevent coolant from flowing when the machine is not in use, and it is only opened when the machine is operating and requires cooling. A normally open valve is preferred in applications where the valve is typically open and only needs to be closed intermittently or in emergency situations. This ensures that flow continues in the event of a power failure or control system malfunction. An example is a safety shutdown system where a normally open solenoid valve controls the air supply to an actuator. If power is lost, the valve remains open, allowing the actuator to perform its safety function, such as closing a process valve.