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From: <daveleonard@cox.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Switching to Evans NPG+
Date: Tue, 18 Jan 2005 11:28:00 -0500
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Alex, you are of course right.  I didn't feel like debating Marv on that point.  

One typo - the vapor pressure needs to LOWER than ambient to avoid boiling, not higher.

The vapor pressure of NPG at 170*F is less than 10 mBar, you would have to climb well above 100,000 feet to get it to boil. Therefore I don't recommend they use it in space ship one  :-)

Dave Leonard
> 
> I have to disagree with you comments about pressure being needed to keep
> coolant in contact with an engine's surfaces. All that matters is that the
> ABSOLUTE pressure of the coolant does not exceed the vapor pressure of the
> coolant. 
> Yes, it is VERY important to consider all operating environments of the
> fluid (0 to 15,000 feet+) it would be a foolish mistake to do otherwise.
> However, it is important to remember that any system in our atmosphere has
> pressure on it (atmospheric pressure). For any coolant system we have the
> pressure of the atmosphere + the (optional) pressure of the coolant system.
> This gives us the absolute pressure keeping the coolant in contact with the
> engine. The only way the coolant will lose contact with the engine is if the
> vapor pressure of the coolant exceeds the absolute pressure of the cooling
> system (vapor pressure is temperature dependant variable). When that happens
> coolant vaporizes (boils) and allows the liquid coolant to separate form the
> surface being cooled.   
> 
> So yes a coolant system must have some absolute pressure. However it
> irrelevant if this pressure is above, at, or possibly below atmospheric
> pressure as long as the vapor pressure of the cooling liquid is exceeded. 
> 
> What someone needs to do is determine the max coolant temp in an engine
> (remember the local coolant temp inside the engine may exceed the exit
> coolant temp by several degrees). Now call up Evans and see what the vapor
> pressure of NPG+ is at that temp. If the vapor pressure exceeds the
> atmosphere pressure of you max altitude (+ a safety factor) then you do not
> have to worry about vaporizing coolant. The reverse is also possible find
> out what you min atmospheric pressure is and see what coolant temperature it
> corresponds to.
> 
> Alex Madsen
> Mechanical Engineering SR
> Rose-Human Institute of Technology
>  
> PS I think it would be easy to figure out what changes need to be made to a
> cooling system in terms of fluid flow and pipe diameter to convert from H20
> to NPG+. If someone will provide some data I can dig out my Heat Transfer
> Text and crunch the numbers.
> 
> 
> 
> -----Original Message-----
> From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
> Behalf Of Marvin Kaye
> Sent: Monday, January 17, 2005 10:56 PM
> To: Rotary motors in aircraft
> Subject: [FlyRotary] Re: Switching to Evans NPG+
> 
> Try and remember that the rules for what happens inside an engine at sea
> level 
> are different than what goes on at 15,000 feet.  While the NPG+ may not boil
> 
> until 396*F at sea level, I'm certain that it's a different beast at
> altitude. 
>  The guy who engineered the Eagle was livid when we told him that Evans 
> recommended a pressureless system... part of the reason for the pressure is
> to 
> keep the coolant pressed firmly against the metal surfaces it's trying to 
> cool.  Even if you have a high boiling point, when the metal temperatures 
> exceed it the boiling will happen and without the pressure to insure coolant
> 
> contact, pretty soon everything is surrounded by a cloud of PG steam (well, 
> maybe not a "cloud", but all the hotspots will be working overtime keeping
> the 
> coolant boiling next to them).  Those metal temps quickly build, the areas 
> where the coolant has turned to vapor grow, and the problem feeds on itself 
> until the system goes completely out of control.  At this point your 
> pressureless system vents itself, throwing out what's left of the coolant
> and 
> the engine is toast.  The point here is that there are more reasons for
> having 
> a pressurized system in an airplane than meets the eye.
> 
> One more thing... with a pressurized system you can alarm it for a low 
> pressure situation.  If something goes wrong with the coolant system (like
> you 
> spring a leak) the pressure will likely go down before you see a rise in 
> temps.  If the system is setup to run at 20psi and you alarm it at 15, when 
> you see that master warning you know that pretty soon you're probably going
> to 
> overheat.  Just another chance to get a jump on things that you pass up 
> without pressure.
> 
>   <marv>
>   
> 
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