Quote:

I'm still not convinced that a longer tube has less efficiency over shorter, but numerically more, tubes.



I’ve read it in Corky Bell’s book, I understood it, I think I can explain it.

I suspect that you understand that hte greater the temperature difference, the greater (faster) that heat transfer will occur. It is therefore a non-linear event – with a constant ambient temperature of say 80°, hot coffee will cool faster from 150° to 145° than it will cool from 105° to 100°.

So, consider 1 tube 26" long. I’ll call heat removal ‘X’. Ambient temperature is constant for the whole radiator, so the 2nd section tries to exchange heat with a temperature differential that is lower/closer to ambient, so the 2nd section will not be able to exchange the same heat as the top half, so it will be less than ‘X’. Let’s just say perhaps 1.5 X for the top half and X for the bottom.

Now consider 2 tubes 13” long, and with all other conditions the same, we’ll have 2 times 1.5 X of heat removal. Without a bunch of coefficients, flowrates, and calculations we don’t know the difference between all the X’s, but mathematically/logically we see it.

The same principle explains why adding additional cores/thickness does not increase cooling by the same amount as the core before it. The first row of core receives ambient airflow, but the row of core tubes behind it receives warmer ambient air, and therefore does not exchange as much heat as core #1.

So now you say ’sure, we’ve got more heat rejection from teh 2 short tubes compared to the long one, but logically the overall temperature is lower at the end of the long tube’ (and I agree with that). But as heat rejection and temperature drop aren’t the same thing (there is more energy lost from 150-145 than from 105-100) I would say that as time goes on, the longer tube system would normalize to a higher temperature than it started, and higher than the 2 short tubes.

If anyone can provide a better/correct explanation of this, I'll be glad to edit/delete my wrong info.