Why and How Are Aircraft Pressurized?
Pressurization of the aircraft cabin is an integral part of any commercial flight. Without it, the simple act of breathing would not be possible. At sea level, humans are exposed to just under 15 pounds of pressure from the air (14.7 psi). However, as your altitude increases, the amount of air pressure decreases quickly. The pressure decreases because the air molecules are spreading father apart. As a result of this, when you breathe, your lungs take in less air and oxygen.
At 18,000 feet elevation, atmospheric pressure is at 7.3, less than half of the pressure at sea level. In these conditions, the average adult has only 20-30 minutes of breathing before unconsciousness. Commercial airlines fly at double this elevation, between 30,000 and 43,000 feet, where the air pressure is a meager 4 psi. This pressure provides less than a minute of consciousness before detrimental health risks. For a variety of reasons, aircraft cannot fly at lower altitudes, so the air pressure problem must be solved through another means.
This is where cabin pressurization systems come in. Cabin pressurization systems keep the interior pressure between a comfortable 11 and 12 psi at cruise altitude while simultaneously pumping fresh air in and allowing old air to exit. This is done through the use of a part called an outflow valve, a valve that opens and closes to allow air to enter or keep already-present air inside. Once the air enters, it is pressurized by, usually, one of two ways. The first way is through engine bleed air.
Most modern airliners take excess air from the compressor section of the engine and use it to pressurize the cabin. While totally clean, this is extremely hot air that must be cooled before entering the cabin. The other way air is pressurized is through electric compressors. This is a newer method, brought by the Boeing 787 Dreamliner. Electric compressors were used in older aircraft, but advances in technology have made the new system far more efficient. Control of the pressurization is also incredibly simple.
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