Re: TB spread through aircraft air

From: (Jim Hogan)
Organization: Eskimo North (206) For-Ever
Date:         14 Mar 95 02:34:35 
References:   1 2 3
Followups:    1
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In article <airliners.1995.242@ohare.Chicago.COM>, says...
>I'm cross posting this to sci.aeronautics.airliners, because you've
>made it a technical question (followups to s.a.a).  I have a tough
>time believing that getting sufficient fresh air into the cabin
>to have 100% fresh air would be a problem.  I can understand it
>might cost a little more, but other than that, what is the technical

The main problem is the limited supply of high pressure engine bleed air. 
I'm not a propulsion type, so bear with me...
In a high bypass engine, about 75 % of the air bypasses the engine core. 
The remaining 25% that travels through the combustor and high pressure 
stages provides the power to drive the low pressure compressor that 
imparts it's energy on the bypass air (and consequently provides the 
majority of the engine's thrust).  This high pressure air is also bled 
from the engine to power the air conditioning packs and provides the 
thermal anti-icing functions on the wing leading edge and engine cowls.  
Due to the pressures and temperatures required to run the AC packs, only 
air bled from the core of the engine is suitable.

This is the crux of the problem - the more bleed we take, the less thrust 
the engines produce, and the less fuel efficient they become.  The 
penalty of bleed air can not be taken lightly - when looking at aircraft 
stretches, we find that the increased bleed necessary to bump cabin 
inflows enough to meet pressurization requirements can often be the make 
or break factor in whether or not the existing engine is "big" enough - 
not the increased weight of the fuselage.

As far as the recirc vs. no recirc argument, there is really no 
detriment to recirculated air.  I have spent a great deal of time working 
on this issue and can elaborate if there is interest.