Subject: [FlyRotary]
Re: Switching to Evans NPG+
Marv, I agree completely with everything you
said. I fully plan to keep my pressure system in place and continue to
monitor the pressure as a way to detect leaks. The beauty of NPG is that
if the system does loose pressure, it does not change the ability to cool.
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
|
Evans
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