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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.
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/
> >>
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