How Everything Works
How Everything Works How Everything Works

Question 1416

Since a typical commercial jetliner cruises at around 30,000 feet (higher than Mt. Everest), where the air is very rarified, is there a mechanism to concentrate the air around the engine intake? — P
There certainly is such a mechanism. The air at a jetliner's cruising altitude is much too thin to support life so it must be compressed before introducing it into the airplane's passenger cabin. The compressed air is actually extracted from an intermediate segment of the airplane's jet engines. In the course of their normal operations, these engines collect air entering their intake ducts, compress that air with rotary fans, inject fuel into the compressed air, burn the mixture, and allow the hot, burned gases to stream out the exhaust duct through a series of rotary turbines. The turbines provide the power to operate the compressor fans. Producing the stream of exhaust gas is what pushes the airplane forward.

But before fuel is injected into the engine's compressed air, there is a side duct that allows some of that compressed air to flow toward the passenger cabin. So the engine is providing the air you breathe during a flight.

There is one last interesting point about this compressed air: It is initially too hot to breathe. Even though air at 30,000 feet is extremely cold, the act of compressing it causes its temperature to rise substantially. This happens because compressing air takes energy and that energy must go somewhere in the end. It goes into the thermal energy of the air and raises the air's temperature. Thus the compressed air from the engines must be cooled by air conditioners before it goes into the passenger cabin.


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