RE: OIL/WATER COOLER

[Robert Myers <rmyers@inetone.net>]

Contents - probably more than you will ever want to know about cooling
liquids.  Be warned...  <Delete> at any time.

---snip---
>
>>I believe Avi meant that oil coolant never drops below _its_ freezing
>>point. However maybe in some parts of Canada....?
>
>>Phil Rose
>		
>So if the coolants' freezing point is -50, and the outside temperature is
>-100, are you saying the coolant will remain at -50? 
>
>Neato! 
>
>What's keeping it from getting as cold as its surrounding? 
>Does this happen to all materials? 
>Does *everything* stay at the temperature that it becomes a solid at?
>
>*COOL*.........

>Steve Bigelow ICQ 22399818

---snip again---

Neato! Indeed.  

The rate of cooling of a liquid depends upon several factors.  Among them
are deltaT (difference in temperature between the heat source and the heat
sink), surface area through which the transfer is being made, and the
"thermal resistance" (insulating capability of the interface).  Probably
several others also.

A plot of temperature vs. time for a pure liquid starting at a warm
temperature is always a smooth curve going downward with time.  It is a
curve, not a straight line since as cooling happens deltaT decreases.  The
temperature is approaching ambient temperature asymptotically.  The
temperature eventually falls slightly below the normal freezing point of
the liquid and something happens to cause crystallization (freezing) to
commence.  Heat (the term is latent heat of fusion) is produced by this
action [ liquid -->  solid + heat ].  The heat rewarms the sample back up
to the normal freezing point (probably less than 1 degree C above the
temperature at which crystallization commenced) and the temperature then
remains relatively constant until all the "extra heat" has been lost and
the sample is now entirely solid.  At this time the sample, now completely
solid continues to lose heat to the surroundings.  It approaches the
ambient temperature asymptotically.  "In theory" it will never quite reach
that temperature but in practice the difference between the sample's
temperature and the surrounding's temperature become vanishingly small -
small enough to be ignored - small enough to be essentially unmeasurable.

Thus, pure water at, say, 90 degrees C surrounded by -30 degree C will lose
  heat and cool at a decreasing rate until it reaches (slightly - called
super cooling) below 0 degrees C.  The water starts to freeze.  The
temperature rises (again, slightly) back up to 0 degrees C.  Once the water
has frozen solid, the sample (now ice) continues to cool right on down to
-30 degrees C.

The coolant under question, a mixture of water and antifreeze, will behave
similarly, but not exactly the same.  The freezing point of the mixture
depends upon the concentration of the mixture.  A 50-50 mixture, I vaguely
remember, starts to freeze at about -30 C (-50 C?- machts nicht).  As water
freezes from the mixture, the resulting remaining solution will have an
increasing concentration of ethylene glycol and will have, therefore a
lower freezing  point than the solution had before some water froze from
the mixture.  This complicates the picture somewhat (the plateau tends to
slow downward rather than remain relatively constats as it did with pure
water) but the basic description of the pure water system remains a
suitable comparison.  The coolant at operating temperature, when placed in
a -100 degrees C environment, will cool to its freezing point (perhaps -30
C?).  Freezing will commence.  The temperature will tend to stabilize,
slowly falling as the composition of the mixture and its resulting freezing
point change during the freezing process.  The contents of the cooling
system will, about this time, have the consistency of slush.  

The final result will depend upon the temperature at which the entire
coolant mixture will eventually freeze.  I have no data handy so I can only
guess.  If the freeing point of the final mixture is above the surrounding
-100 C then the coolant will freeze solid.  If the freezing point is below
the surrounding -100 C then the solution will remain slushy.

Whatever the case, Steve, given enough time, the temperature of the engine
and its coolant will reach (very closely anyway) the ambient surrounding
temperatures.


___
   Bob
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