Pressurisation Question

[VivaTheBeaver]

Can anyone help me with this scenario?

If I shipped a pressure vessel with a rubber bladder inside, (similar to a sealed heating system expansion vessel) and the bladder was pressurised to 7psi at sea level, what would the pressure be in the bladder when the aircraft has been pressurised and would this change at different altitudes and will the temperature also make a difference?

If the altitude makes a difference lets say 36000 feet on a modern jet airliner.

One additional question, would there be a difference in pressurisation between cargo aircraft or passenger aircraft with cargo in the hold.

VTB

[Donkey497]

Simple answer, if the bladder is sealed, the pressure within it will remain at 21.5 psia (pounds per square inch absolute), assuming that it's been filled at a nominal sea level atmospheric pressure of 14.5 psia or 0 pisg (pounds per square inch, guage).

Pressurisation of the aircraft is a bit of a misnomer as the (absolute)pressure within the aircraft will decay as the aircraft climbs to its cruising altitude. Where, if I recall correctly (it's on another thread here somewhere) the pressure will be around 8 to 12 psia. If you checked the pressue of the notional bladder with a standard, atmospheric referencing, pressure guage at cruising altitude, it would therefore read between 9.5 and 13.5 psig, although the "true" pressure would remain 21.5 psia.

Once aircraft reach an altitude where the external pressure matches the set pressure of the cabin discharge valves, the pressure within the cabin remains at that pressure, almost regardless of how low the external air pressure becomes. This is due to the cabin air being currently supplied by air bled from the main flow of air compressed by the engines for combustion and thrust, hence the concern in some quarters over engine lubricants being misted into the cabin. There is a physical ,limit to this though, as there are low pressure limits below which aircraft engines cannot work. However, the wings on a loaded commercial aircraft would probably stall before you got to these altitudes.

Hope this helps.

[BOAC]

The differential pressure across the rubber will increase from 7.5 psi at sea level to around 13 psi at height - ie almost double. If the 'item' were a balloon it would probably burst. If a football or pressure vessel as I think you describe, it will be 'supported' by the container and thus should not burst unless there is a weakness, say in a seam.

Holds are pressurised but I am never convinced they will be as airtight as the cabin, so diff pressure will be at least the same if not higher.

[BelArgUSA]

Hola VivaTheBeaver -
xxx
Some airliner pressurization numbers for you.
A regular airliner, call it 737, is generally pressurized to maintain 7,000 to 7,500 ft pressure altitude.
Typical pressude differential is 7.5 to 8.0 psi, in cruise.
Cargo airliners are pressurized to about same (or very similar) values...
Cargo compartments (belly) are pressurized at same values as main deck cabins.
xxx
At times, happens that some containers burst (i.e. in baggage) because of increased differential.
Remember also the old "fountain pens" with ink, that were leaking...
And yes, if I put a soccer ball in baggage, I deflate it prior to packing, valve to remain open.
Wife used to have some of her cosmetic creams/liquids leaking.
She thought it was baggage handlers (gorillas on duty) jumping on her bags...
xxx

Happy contrails

[BOAC]

Originally Posted by mig15 and for belarg

The only thing generally separating the pax cabin and the hold is a fibreglass honeycomb floorboard and a fireproof liner. Pressure should be the same.

- one would certainly hope so, hence

so diff pressure will be at least the same

but maybe you need to look a bit more closely on the walkround at the state of some of your cargo hold doors after they have been lovingly 'modified' by the odd baggage belt? I believe blow out panels are fitted to help relieve any XS diff pressure. Sometimes I'm surprised some of the doors actually shut.

Anyway, back to the topic - I reckon Viva should be ok, but best to check with the airline. Of course, "if the cabin air supply fails" all bets are off! Best not sit too near curry eaters too.

[WHBM]

Originally Posted by BOAC

Holds are pressurised but I am never convinced they will be as airtight as the cabin, so diff pressure will be at least the same if not higher.

Holds are absolutely the same pressure, there are quite significant vents between cabin (above) and hold (below) to ensure this.

The loss of the THY DC-10 near Paris in 1973 was caused by this. On climbout an underfloor cargo hatch blew out due to incorrect latching, and the hold depressurised immediately. Because there was an inadequate air path from the cabin above (poor design) the pressure in the cabin distorted and ruptured the floor itself, which is of course not designed to handle any pressurisation loads but was acting as a barrier. Control runs through the floor to the elevators were distorted and the aircraft became uncontrollable. It was a poor affair because it was not the first time this had happened on the DC-10, an American Airlines one had it happen just the same and was saved by skillful handling, and the lesson should have been immediately learned.

Originally Posted by BelArgUSA

Remember also the old "fountain pens" with ink, that were leaking...

Ah, you and me both I presume most FOs nowadays don't even know what a fountain pen is, let alone have had them leak all over the place at altitude !

Originally Posted by VivaTheBeaver

what would the pressure be in the bladder when the aircraft has been pressurised and would this change at different altitudes

You may not realise it but this is fundamentally how altimeters work. There is a sealed unit with a flexible section (usually thin metal) and this flexes in and out dependent on the outside air pressure force. A geared mechanism resting on this drives the altimeter display. It is surprising the accuracy you get. Aircraft altimeters are set to today's initial ground pressure, which varies with the weather, before you start, known (in the US) as Altimeter Setting (which is understandable) and everywhere else in the world as QNH (whose origin you will have to look up).

Barometers at home work in exactly the same way.

On a pressurised aircraft there are two altimeters, one for the outside air pressure to show wat altitude you are at, and one for the inside cabin pressure to show what the pressurisation is doing.

will the temperature also make a difference?

It does, to a very marginal extent. It is taken into account in some aircraft performance calculations but can be ignored for your situation here.

would there be a difference in pressurisation between cargo aircraft or passenger aircraft

Well no, because the crew still have to breathe ! However I have heard that crew of aircraft conveying livestock, which has a tendency to go wild from time to time, will go onto oxygen and depressurise just a bit more than normal, and the animals just calm down and go to sleep.