Date: Sat, 12 Dec 2009 18:28:31 -0700
Reply-To: Keith Hughes <keithahughes@Q.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Keith Hughes <keithahughes@Q.COM>
Subject: Re: About coolant
In-Reply-To: <BAY0-PAMC2-F5kK1Lap001f35fa@BAY0-PAMC2-F5.Bay0.hotmail.com>
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> Date: Sat, 12 Dec 2009 12:20:09 -0800
> From: Jim Arnott <jrasite@EONI.COM>
> Subject: Re: About coolant
>
> Short answer: yes. Water transfer heat better than glycol.
>
> Longer answer: When I worked in aerospace about a century ago, one of
> my first assignments was process control for the heat treat area. We
> solution heat treated a bunch of aluminum. (Solution heat treat: <http://www.azom.com/Details.asp?ArticleID=2540
> > This was done to soften the material for forming. To reduce
> residual stress, we used a quenchant of 20% polyalkalene glycol. This
> provided a slower quench than water. Quenching in water would have
> resulted in much more residual stress and distortion. Slower quench =
> slower heat transfer.
>
> Numbers here:
> http://www.engineeringtoolbox.com/ethylene-glycol-d_146.html
> http://www.engineeringtoolbox.com/overall-heat-transfer-coefficients-d_284.html
>
> Water is about 14% more efficient at 220 deg F.
>
> Jim
Uhm, well, you forgot the last half of the "longer answer"; Yes pure
*liquid* water has a higher specific heat than does glycol or
glycol/water mixtures. However, the glycol raises the boiling point
significantly, and *that* is the key parameter. When the cooling water
reaches the boiling point (i.e. when the system temperature results in a
system pressure greater than the reservoir cap bypass valve setpoint -
when water begins to dump into the overflow) then the dissolved gases
will begin to come out of solution. This causes two major problems,
both of which increase with additional temperature rise; cavitation in
the water pump, which reduces fluid flow, and a water/vapor mixture
(foam if you will) at the heat transfer interfaces (water/metal contact
point in the water jackets and radiator). Both of these result in a
dramatic reduction in cooling efficiency. So yes, as long as the
cooling water remains below boiling, pure water is better (for cooling,
not, certainly for corrosion) than water/glycol. In real life
operation, in hot climes, pure water is almost always the wrong choice
for a vehicle, like the stock Vanagon, with a pretty marginal cooling
system.
Keith Hughes
'86 Westy Tiico (Marvin) in Phoenix
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