Differential Pressure

[Eagle179]

Just a quick question about cabin pressurisation. I understand why they pressurise the cabin etc. I just wanted to know how the pressure outside affects the pressure inside i.e. differential pressure.

If anyone could explain I would be most Grateful

Regards.

[Eagle179]

pressure

So is the greater pressure caused by the rise in altitude, the cabin pressurisation or both? This is the bit that confuses me whether or not the outside pressure affects the inside and differential pressure. Because technically the air inside the cabin looses pressure in comparison to the ground doesn’t it.

Thanks alot.

[dixi188]

Hi.

As an A/C climbs, the air pressure outside reduces. The air pressure in the cabin also reduces but at a slower rate due to the controlled release through the outflow valve(s).

So in cruise at say 33,000 ft, the cabin altitude will be at about 6,000 ft with a cabin differential pressure of around 8 psi. (pounds per square inch).

As for max diff, most jet airliners have a max diff of between 8 and 9 psi. (approx)

This is required to maintain the cabin altitude at or below 8000 ft at the maximum cruising altitude of the A/C.

The highest I know of was the Concorde at 10.7 psi.



Hope this helps,

Dixi.

[Eagle179]

Thanks

So the pressure inside once pressurised would stay the same no mater what, but the differential pressure would change due to the change outisde. Is that right.

Thanks alot guys.

(Just got a bit confused about why you have a maximum dif pressure.)


"As for max diff, most jet airliners have a max diff of between 8 and 9 psi. (approx)

This is required to maintain the cabin altitude at or below 8000 ft at the maximum cruising altitude of the A/C."

[Conan The Barber]

Imagine you blow up a ballon, the more air you pump into it the greater the differential pressure becomes. At some point the ballon will explode, as the differential pressure exceeds the strenght of the material the ballon is made of. It's for the same reason that aircraft have a maximum differential pressure.

This also means that the higher you climb, the higher the cabin altitude will have to be in order not to exceed the max diff pressure. Or put another way, the differential pressure is a constant, while aircraft and cabin altitude is a variable.

[Eagle179]

Thanks

I think I’ve got it now it was just the bit about how as the difference between outside and cabin pressure (differential) increases the force on the airframe increased that I didn't get. So in theory it’s the actual pressure that’s limited as opposed to the dif pressure.


Thanks a lot for all of your help guys.

[Fargoo]

Nope, its the diff pressure that's limited. Overpressure valves open (usually sprung loaded) if the max diff pressure is exceeded.

It's possible to have 8 PSI diff pressure on the ground, obviously the pressure in the cabin at this point would be much higher than if the aircraft was at 40000 ft with 8 PSI diff pressure.

[Eagle179]

This is what is confusing me. I would have thought that it was the actual pressure that is limited due to airframe stress etc.

So lets say that you keep the aircraft at 8,000ft as mentioned as you climb then the pressure outside will decrease and therefore the diff pressure will increase right. I get that bit. So does that therefore mean that as the diff pressure increases the stress on the airframe increases? That’s what I don’t understand how the lower pressure outside is affecting the aircraft.

Sorry to be a pain.

Thanks again.

[Rainboe]

Look, aircraft pressurisation systems gradually increase the differential pressure as the aircraft climbs up to the structurally limiting differential pressure of up to about 9psi. As the aeroplane climbs, the cabin in effect 'climbs as well to a higher altitude, but not at the same rate or altitude that the aeroplane climbs to. The aeroplane will maintain a maximum diffferential pressure during further climbs, so the cabin will 'climb' as well, but not as much. Remember, we are talking about 'climbing' as being up a muntain at that altitude. So the aeroplane will be at an altitude of 37,000', the cabin will be at an altitude of 8,000', and the sea surface will be at an altitude of 0'.

If you can't follow that, go to post 2 and start reading it again!

[Eagle179]

Sorry if ive been a pain.

I think ive got the gist so thanks to everyone for contributing.

Regards

[Conan The Barber]

Eagle179,

It seems to me that you don't understand the basic concept of differential pressure.

Forget about actual pressure. What causes stress on any pressure vessel is differential pressure.

If I seal a bottle at sea level it will have 14.7 PSI of pressure inside, as the atmospheric pressure is also 14.7 PSI there is 0 PSI differential pressure. This means that there will be no force exerted on the walls of the bottle.

If I now take the sealed bottle into space, the pressure inside the bottle is still 14.7 PSI, but the pressure outside the bottle is now 0 PSI. This means that there will now be 14.7 pounds of pressure on each square inch of bottle wall, as there is nothing to counteract the pressure inside the bottle. The differential pressure is now 14.7 PSI.

I could also pressurize the bottle to 5000 PSI and if the pressure outside the bottle was also 5000 PSI, there would be no force or stress exerted on the bottle.

So forget about actual pressure. All that matters is differential pressure.

[Eagle179]

Eureka

That’s cleared it up it was that last bit post that helped so thank you for taking the time to explain it now makes sense. Sometimes all it needs is to go back to basics.