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.) .
 >
 > 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

Paul,

  I disagree. What Fred said is "the heat transfer coefficient in a good
radiator or oil to air cooler is about 20-30 btu/square ft". I assume the
definition of " a good radiator" to be one that is equipped with fins, not
a core with no fins. What in Fred's statement above makes you think that he
did not include the fins? I see nothing in his email that says "assume
there are no fins and the water and air area are the same". I in fact
assume the opposite.

Mike Wills

"the heat transfer coefficient in a good radiator or oil to air cooler is 
about 20-30 btu/square ft"

That leaves out a lot of variables such as air flow reynolds number, air density, 
water flow rate through the core and last but not least water temperature itself. 
I don't  really know... perhaps he meant square foot of radiator frontal area 
regardless of the thickness. I just thought it was a bit vague and he was talking
about the heat transfer between metal and air or metal and water.

Perhaps he meant:

"the heat transfer coefficient in a good radiator or oil to air cooler 
is about 20-30 btu/square ft of core frontal area"

or;

"the heat transfer coefficient in a good radiator or oil to air cooler 
is about 20-30 btu/square ft of core frontal area in a typical aircraft scenario"

The following  statement made me think he was talking about the basic HTC
between  differing fluids and metal walls.

"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.)"

I probably interpreted it wrongly.

Paul Lamar
 
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