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Date: Fri, 30 Mar 2001 11:21:01 EST
Subject: Re:  Air Conditioning?
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In a message dated 3/29/01 11:07:28 PM, you wrote:

<<Has anyone considered a liquid nitrogen based system?>>

This one scares me.  I assume the liquid nitrogen is to be used as a heat 
sink, boiling off nitrogen as heat is transferred to it somehow.  The danger 
comes from the nitrogen gas.  This stuff will kill you quickly and silently.  
You should have an oxygen monitor if you use this, but a better solution is 
to use liquid air rather than liquid nitrogen.  I don't know how well the 
liquid air stays mixed (does the oxygen settle to the bottom?) but it's gotta 
be safer than nitrogen.

However, there are still risks.  One can envision a system whereby the 
boiloff gases are ducted overboard.  One could even claim that there's enough 
airflow though the cabin to purge it of excess nitrogen.  If a fitting 
breaks, though, what happens?  Does a puddle of liquid nitrogen form on the 
bottom of the airplane, and the structure become fragile, and then break?  
Imagine the two seats falling out the bottom of the airplane and you're 
falling thru the sky watching the Lancair fly on without you, saying to your 
buddy "What just happened?"  (Okay, so I have a vivid imagination.)  A more 
likely scenario is the cabin filling with nitrogen on the ground between 
flights, you jumping in and being asphxiated by nitrogen gas, and being found 
dead in the cockpit in your hangar, broken fitting or not.

Don't even think of using liquid oxygen to avoid the asphxiation hazard.  The 
fire hazard is worse.  While the risks can be mitigated, liquid oxygen 
provides a significant fire hazard which is readily avoided by choosing 
liquid air or nitrogen.

Don't take these comments as a wet blanket saying the idea has no merit.  But 
please be mindful of the risks and do a careful "what if?" analysis, more 
formally known as a "failure modes and effects analysis".  And understand 
that the first indication you might get of overabundant nitrogen is suddenly 
losing consciousness.

- Rob Wolf

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