OK, I clearly don't pick up a thread and run with it (not quickly,
anyways). A week ago, a couple of questions appeared about my writing on
engine coolants...
I. Greg Meboe asks:
"One question I have, I was considering lightly sandblasting the
core of my rad to get off the layers of black paint. This because I
assume whatever thermoplastic is in the paint is a poorer conductor of
heat than the metal it covers. I see race cars with bare metal
raiators, and I assume that they're doing this for cooling
efficiency."
Response:
Although I understand heat transfer better than electrochemistry,
I can't answer this question with as much precision as you might like.
Here is what I know:
1.According to the symposium papers, the primary loss of cooling
efficiency in a brass and copper radiator occurs when the
soldered joint between the tubes and the fins. Typically, brass
and copper radiators lose about 5% efficiency in their first
couple of years. Presumably they stabilize after a this initial
period of time.
2. Adding a thin layer of paint will certainly add a little bit of
resistance to the heat transfer path. I do not know how
significant this is. Obviously a thick layer will be worse than a
thin layer.
3. I found out whether a matte black finish has a higher convective
heat transfer rate than bare metal. I suspect that it does have a
better convective heat transfer rate than polished metal, but is
probably little different from brushed metal.
4. A radiator exposed to salt air or road salts will definitely
experience external corrosion. This will irreversibly affect the
solder joints.
Here is what I opine:
Although I don't know anyone who races to consult, what I believe
is that a racecar requires greater cooling efficiency than a street
car because it is running continuously at high rpms. A race car
radiator probably gets replaced every couple of seasons and generally
does not have sufficient exposure to salt water or salt air to corrode
significantly.
Most road cars have excess cooling capacity and can easily
accomodate a corroded and normally painted radiator. I know that this
is certainly true of my TR7. This is not universally true. The
Spitfire, for instance, can have cooling problems. If your car
experiences chronic overheating problems, an oil cooler is probably
the best solution. Of course, it is possible that some dreaded
previous owner has actually added more paint to the radiator, in which
case paint removal could make a significant difference.
There is the matter of metal hardness. I have *no* practical
knowledge of sandblasting or its effect on copper. I would suspect
that having the radiator steamed or dipped would be a better way to
clean it. If you do clean the radiator, it is possible to have it
anodized. This will put a thin protective metal oxide coating on the
radiator, which will *probably* enhance its durability with no
noticeable effect on cooling. I think that some radiator cores may
initially be anodized instead of painted.
Finally, whatever you do, I doubt that you can cause permanent
and irreversible damage to your car as a whole. As long as the tanks
are good, a brass and copper radiator can be easily re-cored. If you
can safely blast the radiator for free and have the resources to re-
core if things go awry, then go for it!
II. Joe Gorin writes:
"My experience of having to rod-out the radiator after 12 kmiles
implies that time matters, not just mileage, in coolant lifetime. ...
But, let me look at it from a theoretical angle. ... [1]The rate at
which the silicates are "used up" probably doubles every 10 degrees C.
Therefore, the rate of decay depends on the period of time during
which the coolant is near its maximum temperature. A short trip to
the store and back, say 2 miles each way, might result in 6 minutes of
operation, but 30 minutes of near-maximum coolant temperature. Thus,
the coolant lifetime might be 5 times shorter in miles on a low usage
car than on a car driven for 30 minutes at a time on freeways. ...
[2]Consider a car that is kept in the garage only for 5 years, or
44,000 hours. The coolant would have to be used up at a rate that is
lower by a factor of 73 (44,000 hours vs. 600 hours) to be equally
used up by sitting in the garage. For this relative rate of "using
up," the weighted average temperature in the garage need be 61.9
degrees C (111 degrees F) lower than the stabilized operating
temperature. ... I don't know if these models have any accuracy, but I
doubt that the common recommendation to replace coolant based on time-
or-mileage-whichever-comes-first is totally bogus, though it is
obviously not an accurate model."
You have certainly caught a mistake, or as I prefer, "an
oversight" in my original article. The proper recommendation for a
low-use car would be to keep the coolant no longer than its shelf-
life. I have located no real information on coolant shelf-life but I
would bet on 3 or 4 years, tops. There were no manufacturer's numbers
on the two containers I have at home. As soon as I get hard numbers,
I will update the archived copy of the article.
Addressing your arguments about temperature and coolant
consumption more directly, both are correct in principle. I think that
the first [1] is definitely a perceptive observation. I suppose that
one could adjust the estimate of average speed, and hence hours per
mile, to reflect the type of driving done. The second [2] argument
moves into chemistry. In simple approximation, the rate of loss of an
inhibitor would depend exponentially upon the temperature. However,
this rate incorporates a significant constant that we don't, a priori,
know. We could calculate this constant from the shelf-life (assuming
some temperature) and the bench test results (and operating
temperature) by fitting with an Arrhenius type expression.
As a practical matter, rate increases of 100 to 10,000 times are
not unreasonable for reactions at 90C (194F) vs 28C (82F). This
temperature difference is an absolute temperature increase of 20%
(362K vs 297K).
Not knowing the shelf-life of coolants but applying your
empirical evidence, I would suggest that 3 or 4 years is the outer
limit of coolant life in an aluminum engine.
I do appreciate your reading and comments on the article. It is
nice to know that people are willing to think critically about it.
Gregory Fieldson fieldson@jhunix.hcf.jhu.edu
Department of Chemical Engineering -- The Johns Hopkins University
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