From: henry@spsystems.net   
      
   In article ,   
   Jake McGuire  wrote:   
   >The air in the airlock (at STP, or some approximation thereof) is   
   >expanded through a turbine or a reciprocating "expander" down to   
   >something like Martian surface pressure in an external holding tank,   
   >and the shaft work thereby obtained is used to drive the compressor of   
   >a refrigeration system used to progressively liquefy the (cooled   
   >during expansion) air from the holding tank...   
      
   Venting an airlock into a low-pressure holding tank, which is in turn   
   cleared by a scavenging system, has certainly been proposed before.   
   (Although for rover applications, remember that the holding tank's volume   
   has to be many times the airlock's air volume, i.e. the holding tank will   
   be big.)  But I don't recall anyone suggesting liquefying it.   
      
   Bear in mind that you *cannot* liquefy a gas below its triple-point   
   pressure.  Below that pressure, there is no liquid phase, only gas and   
   solid.  (This is why dry ice doesn't melt, but goes directly to gas --   
   there is no liquid CO2 at 1atm.)  Unfortunately, nitrogen's triple-point   
   pressure is about 125mbar, many times Martian surface pressure.  Oxygen's   
   is much lower, and you could liquefy oxygen at Mars pressure, but not   
   nitrogen.   
      
   Whether this matters will depend on the cabin atmosphere, but a long-stay   
   expedition is going to have at least some nitrogen in the air for health   
   reasons, and preferably quite a lot to make air-cooled electronics   
   possible.  Cabin pressure might still be somewhat lower than Earth's to   
   reduce prebreathing requirements, especially if the suit pressure is   
   relatively low.   
      
   >...It looks like the work available from the depressurization is   
   >approximately equal to the work needed to liquefy the air...   
      
   Note that even aside from the triple-point issue, at a low pressure you'll   
   need to cool it down somewhat farther to liquefy it.   
      
   >Is this insane?   
      
   Aside from the triple-point problem, it's not ridiculous... but it's not   
   obvious what advantage it would have.  It is more complex and probably   
   needs more power than just compressing the holding-tank air for injection   
   back into the cabin as a gas.   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
   --- SoupGate-Win32 v1.05   
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