How it works
Outside air
Air is sucked in by the screw element (air-end) through the air inlet filter. The inlet filter makes sure that all the dust en dirt stays outside. It protects the screw element (which is very expensive and can be damaged.
It's also the first step in making sure that the compressed air is clean: all the dust that is sucked in will eventually end up in the compressed air system.
Load/unload control
Before the air enters the screw element, it passed the inlet valve or unloader valve. This valve opens and closes the air supply to the screw element.
When it's open, the compressor is in 'loaded' condition: it is actually compressing air and pumping it into the compressed air system. When the valve is closed, is shuts off the air supply to the compressor element: the motor and screw element are turning, but the compressor is not sucking any air in and is not pumping any air to the system.