B737NG Pressurization
Hi, I'm studying the 737 air systems, in particular the pressurization
system, but I'm not quite sure that I've got it all right: I have a few doubts on how the outflow valve works, for example.
Basically, intake air is compressed by the engines so the air becomes hot and the pressure gets very high; bleed air is then passed to the packs and is conditioned, to cool it down; automatic systems then regulate the mixture of heat from the engines and the cold air from the packs.
Then this highly-pressurized air is sent to the cabin and the outflow is opened/closed to regulate the air going out of the a/c, determining the pressure inside the cabin. When the press.system is in AUTO or ALTRN mode the outflow valve is automatically opened/closed, while in MAN mode the crew controls it.
Is that correct?
Now, suppose the plane is flying at FL380, the cabin altitude is probably about 8000feet, right?
I suppose that if the outflow valve is completely opened, the plane depressurizes, as the pressure decreases, the differential pressure slowly goes to 0 and the cabin altitude reaches the a/c altitude, is that correct?
But what happens if the outflow valve is completely closed? I suppose the pressure in the cabin goes up, so the differential pressure goes up, does the cabin altitude stay at 8000ft or does the cab altitude descend?
Thank you for your help!
Gianluca (from Venice, Italy)