Double-acting Pneumatic Actuator: The pneumatic actuator is closed while compressed air enters cavities at both ends through port B to push the piston to middle position; the pneumatic actuator is opened while compressed air enters the middle cavity through port A to push the piston to both ends. 

Spring-return Pneumatic Actuator: When the air is exhausted from port A, the spring pushes piston to middle position and the pneumatic actuator is closed thereby; when the compressed air enters into the middle cavity through port A, the piston is pushed by air to compress spring and the pneumatic actuator is opened thereby.