Chdmcmanus From United States of America, joined Mar 2001, 374 posts, RR: 2 Reply 1, posted (12 years 8 months 1 week 3 days 22 hours ago) and read 899 times:
Contrary to popular belief, acft aren't pressurized like a balloon, but more like a sink with constantly running water, the water is coming in at a pre-determined rate (the acft's pressurization system) and the level of water is controlled by regulating the stopper (the acft outflow valves). Using this method, fresh air is always being pumped in. There is a post prior to this one (Air filters /Heat Exchangers / ECS) in which NKP82 and Airplay provide an outstanding explanation of cooling/heating system operation. After this, the air is basically "pumped in" via the distribution system, and then regulated as it leaves the cabin.
To learn more about pressurization, first study the atmospheric pressure differences on increasing altitude, and the effect of the partial pressure of oxygen during this. Then move to the three methods of pressurization, Isobaric, Differential, and Iso-Differential.
I apologize, I don't have any good links for this subject, maybe someone else could help out? If you can't find any info, just post another question and I will reply in more detail.
NKP S2 From United States of America, joined Dec 1999, 1714 posts, RR: 6 Reply 2, posted (12 years 8 months 1 week 3 days 22 hours ago) and read 886 times:
Here's a "generic" explanation non-specific to any one A/C type: The air output from the air conditioning systems pressurizes the A/C as well. It is compressor bleed air, so it is at higher than ambient pressure..and very hot. The air conditioning system's job is to cool the air/mix the output with hot bleed air to provide the desired cabin temperature. ---- There is an outflow valve that regulates ( depending on the desired cabin differential ) how much of this air conditioning air exits the A/C...ie: it is the only exit point for air form the pressure vessel, except for emergency relief valves. This is how the cabin is pressurized. The air that exits through the outflow valve opening is replenished by the "new" air coming in from the air conditioning packs...at more or less a constant flow... ie: the valve doesn't close till pressure builds up, rather it finds a position slightly ajar to let air escape at a set rate comensurate with the pressure controllers commands. This valve is wide open the ground. --- BTW: For a more thorough explanation of the air conditioning itself, see the thread titled "Air filters/Heat Exchangers/ECS" further down on this forum. It was discussed in pretty decent detail.