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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>
Homepage: http://www.flyrotary.com/
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