Re: Cabin pressure

From: (George Hull)
Organization: DirecTell L.C. - Park City, UT. - 1.801.647.0214
Date:         01 Jun 95 05:00:58 
References:   1
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> If the cabin in a commercial passenger aircraft is
> pressurized, how is it possible to exhange fresh
> air from the outside since I would assume the aircraft
> is somewhat airtight?  Also, is the pressure managed
> by a air-compressor or is it simply "scooped" from the
> outside via some sort of duct-work into the cabin thereby
> generating the pressure?

On the airliners on which I have worked the air for pressurization is
"bled" from the compressor section of the engines, fed through an air
conditioning "pack" where it is filtered and temperature controlled and
then introduced into the ducting in the cabin.  The actual pressure
differential in the cabin is a result of how much of that air is allowed
to escape, rather than how much is actually introduced.  An airliner cabin
has lots of small "holes" in the form of vents and other leaks, but the
outflow valve which controls the pressurization is a big hole with a big
valve which is modulated to keep the pressure at the desired
differential.  So there is a constant exchange of new air from the engines
. . sometimes augmented by the infamous and much-maligned recirc fans.