nycstev said:
Some good analysis, but I always thought higher velocity doesn't neccesarily mean greater cooling. I would think there is an optimum velocity to allow for the correct rate of heat transfer. In other words if the air is moving too fast there isn't enough time for the air too cool down. AC is always a little cooler
when running on slower speeds:dontknow: ?
There are indeed optimums as well for the cooling air ( outside ) as for the SC air ( inside). For a air-air cooler of course.... The factor that gives the amount of heat that will be transferred to the tubewall ( we call them alfa-values ) raise with airspeed almost linear to a certain value were they become almost constant. So indeed , no need to go higher as you will only get more and more pressure drop and almost no more heat transfer.
Be carefull in thinking that "time" is here relevant. It is not :
the cooling capacity = the surface off the exchanger x K-value x delta T.
K-value : sum of the 2 alfa-values and the tubewallthickness/heat transfer of the material.
Delta T : average difference in temperature between SC air and ambient air.
So you see, there's no "time" involved.
Now air has a very bad alfa-value ( water has a much better one ) , that's why radiators have fins outside. The water can give great heat transfer to the tubes but they cannot give it to the air.So for compensating the low alfa value that side there are fins to increase the surface . ( again those fins have optimum height and distance )
When now on a air-air cooler the air velocitys would be the same , there would be no fins needed . But because the inside velocity is much larger , so is the inside alfa value , there wil be fins needed outside .
( we even use sometimes tubes with fins inside were the problem is otherwise.. )
can everybody follow .. ?
