What are the single acting and double acting of Rack & Pinion pneumatic actuators?

1、 Single acting  pneumatic actuator :
Single acting Pneumatic Actuators open valves when ventilated and automatically close when the gas source is not available. Single acting Pneumatic Actuators rely on springs to automatically reset and are commonly used in hazardous conditions, such as conveying flammable gases or liquids. In the event of an emergency situation due to a loss of gas source, single acting pneumatic actuators can automatically reset to minimize danger, while double acting pneumatic actuators are generally not easy to reset.
Single acting Pneumatic Actuators are generally divided into normally open and normally closed types. Normally open type: ventilation off, gas cut-off on; Normally closed type: ventilation on, shut-off off.

2、 Double acting pneumatic actuator:
A double acting pneumatic actuator starts rotating and opening the valve when it is ventilated. When the valve needs to be closed, the other side can only be closed by ventilation. It is reset by the cylinder and can only remain in place when the air source is lost; Simply put, when you give air, the pneumatic actuator starts to rotate and open the valve. When you want to close the valve, the other side needs to be raised to close it! And the single effect is that it opens when you give air, and automatically closes when you don't give air.
Double acting pneumatic actuators are commonly used in general operating conditions, as they do not have springs and therefore have lower costs compared to single acting  pneumatic actuators .