Cabin Pressurization

From:         tristar500@aol.com (TriStar500)
Organization: America Online, Inc. (1-800-827-6364)
Date:         18 Sep 95 12:06:58 
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Regarding the discussion concerning control of air from an engine and into
the cabin, here is the information that I got (from vendor manuals) on a
couple of pack valves.

MD-80 pack valve regulates flow to 65-89 lbs/min. With both packs
operating that's 130-178 lbs/min. I wasn't able to find the info for the
727, but since the 727 doesn't recirculate air like the MD-80 it may need
a higher flow rate.

B-757 has two flow rates depending on single or two pack operation. For
two-pack operation (low flow mode) each valve regulates to 72 lbs/min. For
single-pack (hi flow mode) operation the valve regulates to 105-117
lbs/min. This allows one pack to provide 80% of the air that is supplied
in two-pack operation.

Of course, I think the original question was, how is air bleed from the
engines controlled? It's controlled by the bleed valves (Pressure
Regulating and Shutoff Valve - PRSOV) on the engines that regulate
pressure in the pneumatic system to about 40psi and about 450 degrees F.
The way this is done depends, of course, on the airplane. The 757 bleed
system is quite different then that on a 727.

Hope this clears things up a little.

Dave