In a particular quality control application, a product may be moving down a conveyor system.
When the product moves through a quality checkpoint, a sensor (perhaps a weigh scale or some sort of color scanner) determines if that product meets the quality set points. If it does not, a signal is sent to a PLC which in turn sends a signal to an actuator on a 2/2 air valve. The 2-2-air valve shifts, and compressed air flows through the valve to a nozzle, to blow the defective part from the conveyor. All this takes place in a split second.
In another application for a 2/2 valve, parts may be exiting a manufacturing operation, and if debris is noted by the quality checking sensors - then, a signal is generated that actuates the 2/2 valve allowing it to blow air through the air knife onto the part to remove debris from the part.
Typically, the 2-2 valve that could be used in the quality control or cleaning operations as noted above is a solenoid actuated valve. In the graphic just above is a small 2/2 air valve with a solenoid operator and a DIN electrical connection.
These venturi compressed-air-driven appliances are used to provide simplistic cooling to control cabinets where the application cannot use other typical forms of cabinet cooling such as fans or refrigeration.
Typically, when the temperature inside the cabinet reaches a high level set point, a signal is generated which, in turn, opens the 2/2 air valve. Compressed air flows to and through the cabinet cooler, and lowers the temperature in the cabinet to an acceptable level. When the low temperature set point is reached the air valve shifts to a closed position until the high set point is again reached and the cycle repeats.
As an typical example, an air motor may be powering a fixture to turn tops onto bottles. When a bottle reaches a set spot in the assembly line, a signal is generated. The PLC fires the 2/2 valve which powers the air motor, and possibly also triggers an air cylinder to position the motor, and the air motor then turns the cap onto the bottle.
A compressed air regulator could be used to modulate air motor force, allowing the air motor to stall out before it over torques the cap.
Or a simple timer may be used, which allows the air to drive the air motor for just the time it takes to install the cap, and then shuts off the signal to the valve, and stops the air motor.
For example, picture a submersible air pump in a tank. A float switch signals when the tank level reaches a set point, a signal is sent to the PLC and then to the 2/2 valve. The pumps starts and runs until the level in the tank reaches a low set point, at which point, the PLC is signalled again, and the valve is de-energized, allowing the pump to stop. The cycle repeats as the level in the tank reaches the start set point again.
No 2/2 air valve can be used to supply air to any device that is a closed vessel as once the 2/2 air valve is shut off, compressed air in the air line from the valve to the application, and any compressed air in the application itself - both now at full line pressure - is trapped! Compressed air cannot get back out if it is a 2-2 air valve that is used to supply air to that application.
As a result, 2/2 air valves are normally only used when the application ultimately leads to atmosphere. Then, when the valve is shut off, air in the downstream line can bleed off, through the application, to atmosphere.
Of course, as the need for more air flow for the application is determined, larger ported 2/2 valves are selected.
It seems more economical to make larger bodied valves multi-ported and then to have the unused ports plugged for different applications than it is to purchase tooling to manufacture many different body styles. And, there are fewer applications for really large 2/2 air valves too.
Or, 2/2 valves can be plumbed individually in line as needed.
Here's where you learn how to draw the schematic for a 2/2 air valve.