"Air to open" and "air to close" refer to the action of pneumatic actuators used to control valves. These terms describe how the actuator responds to changes in air pressure to either open or close the valve. Here are examples of each:

1. Air to Open Valve:

   • In an air to open valve, the pneumatic actuator requires air pressure to open the valve. When air pressure is applied to the actuator, it moves in a direction that opens the valve, allowing fluid to flow through the system.
   • Example: A spring-return pneumatic actuator used in a control valve for a process system. When air pressure is applied to the actuator, it overcomes the force of the spring and moves the valve stem to an open position, allowing fluid (gas or liquid) to pass through the valve.

2. Air to Close Valve:

   • In an air to close valve, the pneumatic actuator requires air pressure to close the valve. When air pressure is applied to the actuator, it moves in a direction that closes the valve, stopping the flow of fluid through the system.
   • Example: A double-acting pneumatic actuator used in a shut-off valve for a pipeline. When air pressure is applied to one side of the actuator, it moves the valve stem in a direction that closes the valve. To open the valve, air pressure is applied to the other side of the actuator, causing it to move in the opposite direction.

In summary, the terms "air to open" and "air to close" describe the response of pneumatic actuators to changes in air pressure for controlling valve operation. In an air to open valve, air pressure opens the valve, while in an air to close valve, air pressure closes the valve. These concepts are fundamental in pneumatic control systems used in various industries, including manufacturing, petrochemical, water treatment, and HVAC (heating, ventilation, and air conditioning).