Mike Wills wrote:

Russell,

  I found the email from Fred. The appropriate term I was looking for in my
previous email was "heat transfer coefficient". I'd agree that it doesnt
make sense that you can get something for nothing by going from
oil-to-water-to-air. My feeling is that whatever efficiency you may give up
(if any) by transfering the oil heat thru the coolant to the air is offset
by the greater delta T between the coolant and the air at the radiator.

  Seems to me that the advantage in the oil/water cooler is the flexibility
it allows in oil cooler placement and the simplification of ducting. Trade
off is weight and cost.

   I'm currently going with a conventional Setrab air/oil cooler but at the
moment its location and associated ducting look somewhat questionable. This
current location is the only place it fits within the confines of my
cheekless RV4 cowl. I have space available to increase radiator core size
enough to accomodate the additional heat load an oil/water exchanger would
create. I havent found an air/oil cooler with the appropriate form factor
to fit this space so cant go that route but the larger radiator core would
be relatively easy. Also going this route in my installation would greatly
simplify my plumbing and ducting requirements. There may still be an
oil/water exchanger in my future.

I'd like to know how the oil/water exchanger that Powersport is using is
working out, how big it is, what it weighs, what it costs, how its plumbed,
etc...?

Mike Wills

 From Fred Moreno:

Fred wrote:
 >
 > To further the confusion:
 >
 > In transferring heat from fluid A to fluid B across a wall, the rate of
heat
 > transfer is proportional to the temperature difference between A and B, the
 > area between them, and the heat transfer coefficient for the respective
 > fluids. (Temperature drop in the wall is generally negligible.)
 > Transferring heat from oil to coolant, it is true that the temperature drop
 > is much less than transferring to air, but the heat transfer coefficient
for
 > air in a good radiator or oil to air cooler is about 20-30 btu/square foot
 > per hour per degree F. On the coolant side it is about 1000. So the very
 > high heat transfer coefficient more than makes up for the low delta T and
 > permits also a reduction in the area if there is liquid on both sides. (Oil
 > is not quite so good as coolant, but still much better than air.) .
 >
 > Based on the few oil to coolant heat exchangers I have seen, the problem is
 > that they are not large enough for the rotary which dumps so much of its
heat
 > to oil. However, with a big enough liquid to liquid heat exchanger, it
 > should work just fine. Aeroquip makes one for race cars, but as I recall it
 > would take 2 or 3 to do the job. See their performance charts and check it
 > out.
 >
 > Fred

It is not quite this simple. The area Fred refers to is the metal area
of the heat exchanger. The ratio of water to air btu/square foot stated 
assumes the water area and the air area are the same. The water area is 
much much less than the air area due to the fins. Then the temp drop in 
the fins as you get away from the metal part in contact with the water 
becomes a factor.

Paul Lamar
 
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