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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 12:06:32 -0500
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Al, that is good input.  First of all, I am not doing this to solve a problem with the cooling system.  I agree that it is not going to improve anything in that regard.  I am considering for reliability reasons (no boil over), improved corrosion protection, and environmental friendlyness.

You exaggerate your numbers a little.  First, you are comparing the specific heat to that of pure water, but I cannot run pure water because I have a house in the mountains.  Even then, the diference is 35% not 40% (the numbers are right there in their table).  Also, you mention heat removed with a volume of water.  But specific heat is in units of mass.  Since NPG+ is more dense, you need to take that into account.

It works out that a volume of NPG+ is only 17% less efficient than the same volume of 50/50.  Once hot, the vescosity is not an issue, being only slightly more viscous than water (no system change usually necessary).

Add in that it is much more resistant to micro boiling, and it is reasonable that NPG may do a better job of keeping the very hottest parts cool.

Suppose your goal with cooling is to keep the o-rings from melting, and keep the hottest areas around the spark plugs under some critical temp.  We know that is possible with presurized water kept under 220, EG/water at say 223*F, but perhaps NPG could meet those goals at 240.

That higher temp, as long as it is safe for the engine, will decrease total heat rejection and improve BSFC and horse power.

But like I said, I am thinking about doing it only because I have adequate cooling now and would like the improved safety margin and corrosion protection.

Dave Leonard


> 
> Dave;
> 
> Adding my 2 cents worth; don’t overlook that it’s ability to cool is
> seriously less than water, or 50-50 w/g in the first place.  The specific
> heat is 40% less (I notice that is not in their table), so, as you know, for
> a given volume of flow it carries away 40% less heat.  Couple that with say
> a 10 times (depending on temperature) higher viscosity, which means lower
> flow rate, and lower thermal conductivity; you have seriously changed the
> temperature gradients and internal surface temps in the engine.  I don’t
> know that it would damage the engine, but there is a risk.  On the face of
> it, these things are serious disadvantages, and suggest to me that it is not
> likely the best path to a solution.  And I haven’t even mentioned the higher
> pumping power requirements, but that is a second order effect.
> 
> We have been all over this stuff in the past; and, personally, I wouldn’t do
> it; at least not with a stock coolant pump.  Doesn’t mean you shouldn’t try
> it; we will all get some more information.  Don’t they still tell you that
> you should change the coolant pump?  They used to sell the pumps as well.
> 
> And don’t overlook that Evans is promoting sales; which includes hype, and
> overlooks potential problems. Design the cooling system around the use of
> NPG, and it should be fine. But it is difficult, at least for me, to see how
> it is going to solve a problem cooling system.
> 
> Whether or not it “solves” problems in your cooling system depends on where
> the problem is, and if it solves one it may cause others.  Again, not that
> you shouldn’t try it; just that as a friend I wasn’t comfortable in not
> voicing my concerns.
> 
> Al
> 
>  
> 
> 
> 
> You mention coolant vapor around hot spots.  Evans touts its ability to
> resist that vapor film as one of its strongest points, and it makes sense.
> Even unpressurized 370 + degree boiling point is worlds better than the
> 265*F boiling point of pressurized 50/50.  So I do believe that NPG will be
> better at preventing that vapor film than 50/50 even pressurized up into the
> 25psi range.
> 
> I'm not saying that this stuff is magic.  And I don't believe it can
> compensate for an inadequate cooling system (like pure water might), but it
> does offer some advantages that I am starting to appreciate.
> 
> From the Evans Web site, here is some technical stuff:
> 
> 
> COMPARISON OF COOLANT PARAMETERS
> 
> 
>  
> 
>  
> 
> Water
> 
> 50/50 EGW
> 
> Evans NPG <http://www.evanscooling.com/html/npgben1.htm> 
> 
> Evans <http://www.evanscooling.com/html/npgPls.htm>  NPG+
> 
> 
>  
> 
>  
> 
>  
> 
>  
> 
>  
> 
>  
> 
> 
> Boiling Point
> 
>  
> 
> 121° C (250° F)
> (1 atm plus 15 psig)
> 
> 129° C (264° F)
> (1 atm plus 15 psig) 
> 
> 187° C (369° F)
> (1 atm plus 0 psig) 
> 
> 191° C (375° F)
> (1 atm plus 0 psig)
> 
> 
>   _____  
> 
> 
> Viscosity
> 
>  
> 
>  
> 
>  
> 
>  
> 
>  
> 
> 
>     10° C (50° F)
> 
> cp
> 
> 1.2
> 
> 5.0
> 
> 115
> 
> 58
> 
> 
>     80° C (176° F)
> 
> cp
> 
> .37
> 
> 1.0
> 
> 4.5
> 
> 3.7
> 
> 
>    100° C (212° F)
> 
> cp
> 
> .28
> 
> 0.7
> 
> 2.8
> 
> 2.3
> 
> 
>   
> 
>   _____  
> 
> 
> Density
> 
>  
> 
>  
> 
>  
> 
>  
> 
>  
> 
> 
>     20° C (68° F)
> 
> spec grav
> 
> 1.00
> 
> 1.066
> 
> 1.038
> 
> 1.091
> 
> 
>     20° C (68° F)
> 
> lbs/gal
> 
> 8.32
> 
> 8.87
> 
> 8.64
> 
> 9.08
> 
> 
>   _____  
> 
> 
> Specific Heat
> 
>  
> 
>  
> 
>  
> 
>  
> 
>  
> 
> 
>     80° C (176° F)
> 
> Btu/lb/°F
> 
> 1.00
> 
> 0.81
> 
> 0.68
> 
> 0.64
> 
> 
>    100° C (212° F)
> 
> Btu/lb/°F
> 
> 1.01
> 
> 0.82
> 
> 0.71
> 
> 0.66
> 
> 
>   _____  
> 
> 
> Heat of Vaporization
> 
> cal/mole
> 
> 9,700
> 
> 9,800
> 
> 12,500
> 
> 12,050
> 
> 
>   _____  
> 
> 
> Vapor Pressure 
> 
>  
> 
>  
> 
>  
> 
>  
> 
>  
> 
> 
>     80° C (176° F)
> 
> mm Hg 
> 
> 360
> 
> 270
> 
> 8
> 
> 6
> 
> 
>     80° C (176° F)
> 
> kPa
> 
> 475
> 
> 360
> 
> 11
> 
> 8
> 
> 
>   _____  
> 
> 
> Surface Tension 
> 
>  
> 
>  
> 
>  
> 
>  
> 
>  
> 
> 
>     25° C (77° F)
> 
> dyn/cm
> 
> 72
> 
> 56
> 
> 36
> 
> 44
> 
> 
>   _____  
> 
>  
> 
>  
> 
>  
> 
> 
> >
> > 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/
> > >>  Archive:   http://lancaironline.net/lists/flyrotary/List.html 
> 
> 
>