From: henry@spsystems.net   
      
   In article ,   
   Greg  wrote:   
   >While on the subject. Positive displacement pumps don't have the NPSH   
   >problems that dynamic pumps have.   
      
   The problem isn't entirely absent, although in a more subtle way.  The   
   delivery stroke of the pump is all push, no pull, so cavitation isn't an   
   issue there.  But the refill stroke is all pull, so cavitation *is*   
   possible then, depending on flow resistance etc., and it could cause   
   various kinds of trouble.   
      
   >...But what about other designs. Like a screw pump with a   
   >screw expander? That way you get the gas to do some work while   
   >expanding, rather than "workless" free expansion...   
      
   Please elaborate; I don't believe I've heard of screw pumps, and I can't   
   picture what you're envisioning.   
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