How do big planes like 737 manage fresh air for passengers during flight?

Question

How do the big planes (e.g. A380 or 737) flying on high altitudes manage to get fresh air for passengers and crew during the flight?

I know the inner space of the plane used for passengers and crew is sealed and the pressure is maintained within some range.

However in standard flight altitude of about 10-14 km there is not enough oxygen to breathe. I am not sure whether the standard oxygen ratio is still about 21% and you only have less air (lower pressure).

We talked two possible solutions and maybe I am still wrong:

1. using some pumps and small pressure tanks to suck in air from outside till we get higher pressure and then use some valves to "replace" the breathed up air with the fresh one

2. use some pressure tanks loaded with oxygen + some cleaning devices to clean the air and add oxygen to keep the same oxygen level - but this would need to get the oxygen before take-off and such tanks would be really dangerous

So how is it in reality?

Answer 1

They bleed hot compressed air from inside the jet engines, cool it down and pass it into the cabin. There are outlet valves in the fuselage that allow stale air out and which control cabin pressure and air-refresh rates.

A few aircraft types, such as the 787, now use a compressor to take in air from under the aircraft. They do this because it can be more fuel-efficient (not for any other reason). It is arguable whether it creates an additional fire hazard.

See also

• Why is the compressed air (from Engine Compressors) taken from High Pressure (HP) stage instead of Low Pressure (LP) stage
• Can an aircraft re-pressurize at high altitude?
• Is constant air pressure maintained inside the fuselage?

Answer 2

Airplanes basically draw in outside air, compress it, and send it to the cabin. RedGrittyBrick lists some related questions about this process. Most airplanes use bleed air from the engines, using the compressors in the engines to do the compressing. Although in normal operation this will only bring in outside air, engine issues can cause fumes to enter the cabin. The 787 uses an electric compressor that draws from separate intakes.

The problem with breathing at high altitudes is not necessarily the lack of oxygen, but the low pressure. Humans need a certain partial pressure of oxygen for the lungs to work properly, and as pressures drop, the oxygen is less effective at getting into the bloodstream, which results in hypoxia. So keeping the cabin pressurized is what allows the occupants to breathe normally.

For more information about the cabin environment, see this paper. Ventilation can be expressed in "outside air changes/hour", or how often the total volume of air is replaced by new air. Airplanes have 10-15 changes/hour (every 4-6 minutes), which is 2-3 times more changes/hour than hospital delivery and operating rooms, and 4-15 times more than a typical building. New air is constantly being circulated into the cabin, while old air escapes through relief valves.

Answer 3

Bleed air from the engine is used to power a Pneumatic Air Cycle Kit (PACK) which takes in outside air heats it and compresses it.

The mechanism is entirely mechanical using turbines and heat exchangers and engine bleed air is isolated from the air destined to the cabin to avoid oils and fumes entering the cabin.

The answer to How hot would pressurized air get if the air conditioning failed? ex^plained in detail how a PACK works.