how do passenger airplanes regulate cabin oxygen levels?

Question

Oxygen masks will drop from ceiling during emergencies, how about during regular flights?

In: 9

Answer 1

Premise: Turbine engines work by compressing air with a series of “fans” one after the other, then the air is burned with fuel, the gas expands and turns the turbine section that spins the compressor that I mentioned above. The remaining energy propels the plane.

Pressurization: given the plane has already the big compressor that is part of each jet engine, there is a system that takes part of this clean compressed air, “steal” it from the engine. This compressed air is pretty hot due to the compression work, anything between 100 and 250 Celsius. This hot compressed air is sent to an -air cycle machine- which is, simply put, a massive heat exchanger with turbines and fans in it, capable of decompressing and cool that air. By regulating that thing, you can transform the compressed air stolen from the engine into -compressed air the temperature you like-.

The -compressed air at the temperature you like- is then routed to the cabin.

There is a valve (outflow valve) that regulates how much air can be expelled by the cabin. By adjusting this valve, you adjust the pressure in the cabin.

Recap: engine compresses air, air goes to “air cycle machine”, then goes to the cabin and pressurize the cabin, the outflow valve expel the excess of pressure from the cabin.

Result: the inside of the plane is always kept at a nice temperature and at a pressure that is ok for people to live in. Also, the massive flow of air in/air out guarantees you are breathing new air and not other people’s breath and farts.

Safety: system uses air from the two engines, plus eventually the APU engine (small backup engine, too long to explain here). There are generally at least two air cycle machine, and the outflow is controlled in 4 different and independent ways. As long as you have 1 of 3 sources, 1of 2 air cycle machines, and 1 of 4 controllers for outflow, you can breathe normally in the plane. Past that, pilot will have to descend to a lower altitude, meanwhile, you have your oxy mask if you need it. Pretty safe.