Don, Steve, et Listers,
I thought I would repeat a little bit of the basic stuff I put out on this
subject about a year ago, so my apologies to those of you for whom this is
repetitive. I also dug out some data on pressure variation with altitude
that might be helpful, and also some info on glycols. Here goes:
First, the old stuff. Heat transfer from the engine to the world (air) can
be considered a three-step process; transfer from the internal engine
surfaces to the coolant, transfer from the coolant to the radiator internal
surfaces, and finally, transfer by forced convection from the radiator
external surfaces to the air. The efficiency of this process is dominated
by the weakest link; normally the last step, transfer from the radiator
surfaces to the air. The effectiveness of the heat transfer step is a
function of coolant heat capacity and flow rate. If this is NOT a
limitation, then the coolant temperature will be the same throughout the
system; i.e., the same temperature at the outlet of the radiator as at the
inlet. I want to emphasize this last point, because it is often believed
that a temperature drop across the radiator is a good thing, when in fact
just the opposite is true. So what about water versus other coolant media
like glycols? Water is very interesting stuff, and among its interesting
properties are high heat capacity (the amount of energy needed to raise its
temperature), low viscosity, and a reasonably high boiling point (of course
non-toxic, cheap, ubiquitous, etc. too). Glycols have some advantages in
terms of low freezing and higher boiling points, low corrosion, etc., but
have lower heat capacities and higher viscosities. These latter
deficiencies only come into play if, at the flow rate through the system,
there is a significant temperature drop across the radiator. Experience
seems to show that somewhere up to around a 50% concentration, a mixture of
ethylene glycol with water has minimal affect on the cooling efficiency
while at the same time both increasing the boiling point and decreasing the
freezing point. One small point of possible confusion to clear up here is
that, up to a point, adding glycol will have no effect at all. It's not a
linear kind of effect where, for example, the heat capacity of water is
twice that of glycol, so the system only works half as well with glycol
coolant.
So what about these glycols; what are they? Glycol is a family of chemicals
of the form: CnH2n(OH)2 (the 2's and n's are subscripts) where n is an
integer which for ethylene glycol, is 1(unity). For propylene glycol, n=2
and it boils at 188C. Polypropylene glycol is one of a class of higher
molecular weight glycols used to make emulsifiers and detergents,
lubricants, hydraulic fluids, and solvents for vegetable oils and waxes,
etc. Polypropylene glycol is less soluble in water and more soluble in oils
than ethylene or propylene glycol. For this reason, I would worry a little
about its effects on rubber, seals, etc.
The affect of altitude on the boil-over point of the radiator has been
mentioned. The following is a table of pressure in pounds per square inch
versus altitude (above sea level)in feet.
Feet lbs/sq-in
0 14.7
1584 13.9
3168 ` 13.0
4752 12.3
6336 11.6
8448 10.9
10,032 10.2
13,200 9.0
16,368 7.9
19,536 7.0
This gives you a quantitative measure of the effect of altitude in reducing
the affective pressure of the radiator cap. For example, if you have a 14
#/sq-in cap, then in going from sea level to 6336 feet altitude, the
pressure drops 14.7 - 11.6 = 3.1 #/sq-in. The water in the radiator
responds to absolute pressure; i.e., the cap pressure plus atmospheric
pressure. So, at 6336 feet altitude, the total coolant pressure is 11.6 +
14 = 25.6 #/sq-in versus 14.7 + 14 = 28.7 #/sq-in at sea level. The net
result is that a 14 #/sq-in cap at 6336 feet is equivalent to a 14 - 3.1 =
10.9 #/sq-in cap a sea level. Or, the other way around, you would need a 14
= 3.1 = 17.1 #/sq-in cap at 6336 feet.
Well, I think that's probably enough cooling physics for now. Any questions?
Breathing easy at sea level in San Diego,
Bob
At 09:27 AM 9/23/98 -0700, Don Cordier wrote:
>Hi Steve,
>
>Long time - no talk...
>
>Great discussions, eh?
>
>RE: Doug Mallory's comments about boiling during racing...
>
>While I agree that the gauge or something probably needs to be checked,
>do you know the altitude of the track where Doug was racing and the
>pressure device on the radiator - if any.
>
>I do not know the boiling decrease (off the top of my head) of water
>with decreasing atmospheric pressure but this may have been a factor???
>
>Do you know what the "normal" decrease is?
>
>Keep in touch when possible.
>
>Best regards,
>
>Don Cordier
>(1968 Lotus Elan S4SE Coupe - S/N 36/7947 - Cal License "68 Lotus")
>Pictures (courtesy of Alan Perry) at:
>http://www.best.com/~esprit/elan.html
>(1963 Corvette Roadster - VIN: 30867S116306 - CA License "QAM 488")
>Aviation Consultant
>Glendale, California, USA
>doncordier@earthlink.net
>
>Office & Hangar: (818) 997-7640 (24 hours)
>FAX-Private: (805) 527-1034
>FAX-General: (818) 553-3667
>Member: NBAA, NATA, PHPA, AOPA
>
>
Bob Palmer
UCSD, AMES Dept.
rpalmer@ames.ucsd.edu
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