B737NG Pressurization

[tpgian]

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)

[Pub User]

All your assumptions are correct.

If the outflow valve were to stay closed, the differential pressure would increase to a maximum of 9.1 psi, then the Pressure Relief Valves would open to prevent overpressure of the fuselage structure.

[tpgian]

OK thanks :)

So if in cruise the outflow valve gets frozen/blocked in the closed position, the pressure increases, the differential pressure increases and what about the cabin altitude?

[john_tullamarine]

Two considerations here -

(a) the potential for overpressure is addressed by the relief valves and these can be backed up by the pilot's attempts to dump pressure manually if it came to that

(b) cabin altitude is a function of air pressure. Whatever pressure exists relates to the same pressure's being scheduled at a particular Hp in the standard atmosphere ie if the pressure in the cabin is the same as, say, that which occurs at 8000ft in the standard atmosphere, then the cabin altitude is 8000ft.

Inverse relationship - ie as the pressure goes down the cabin altitude goes up .. same as in the real world. So, for the case of a defective outflow valve, the pressure might increase a small amount which would see the cabin altitude decrease by the appropriate amount.