How does the pressurization system on a commercial aircraft work?
Keeping it simple: High pressure air is tapped from the compressor section of the engines. The compressor is before any fuel is introduced, so this is pure clean air. It is still very hot, though, just due to the heating as a result of compression. This air is run through an air cycle machine, which is basically a mechanical air conditioner, to cool it and regulate the temperature of the cabin. The conditioned air is then ducted into the cabin. The pressurization is controlled by a regulator which opens and closes a valve called the outflow valve. If the outflow valve is closed, the pressure in the cabin will increase. If the valve is open, the pressure will decrease. In all reality, the regulator continuously and automatically repositions the outflow valve to keep the cabin pressure at the selected value.
