Fred wrote:

So that builders can determine the effect of using counter flow radiators
instead of normal cross flow radiators, I plotted the information in Kayes
and London, Compact Heat Exchangers in a format that assumes the heat
exchanger is used as a radiator (instead of an intercooler or other air to
air unit).  Result is attached.  The horizontal axis plots Number of Transfer
Units (NTU), essentially the thermal size of the radiator.  The vertical axis
plots the effectiveness of the radiator.  If it were 100% effectiveness, the
air temperature exiting the radiator would be the same as the hot coolant
temperature entering the radiator (maximum theoretical possible transfer to
air).

A key figure of merit is the capacity ratio which is the ratio of the heat
carrying capacity of the air stream compared to the coolant stream.  In
radiators the capacity of the coolant is generally much higher than the air
stream giving a capacity ratio of 0.1 to 0.3 for normal applications.  From
the chart, you can see that for a capacity ratio of zero (MUCH more coolant
capacity than air capacity) all configurations end up having the same
effectiveness.

The next curve down is for a pure counter flow heat exchanger with a capacity
ratio of 0.25.  In this configuration the flow of water and coolant would be
in exactly opposite directions, a bit tough to build in practice.  The third
curve down is from a cross flow configuration also at capacity ratio of 0.25
(normal radiator configuration).  The case of the two pass cross counter flow
configuration (a radiator with the coolant passes folded over like the
picture Paul sent out recently) would be in between the second and third
curves somewhere.

Note that even with a perfect counter flow radiator at an NTU value of 2, the
effectiveness is about 83% while for the cross flow it is about 80% - not
much of an improvement.  If the capaicty ratio were 1.0 (more like an
intercooler), it would be another story.  But as long as we use lots of
coolant flow, cross flow is just fine.

This is not to say counter flow is not worthwhile.  It should be used if the
coolant flow rate is low to get a bit more effectiveness due to configuration
effects, but primarily to increase the velocity in the radiator tubes and
thus improve the coolant heat transfer to the tube wall which might be too
low with low coolant flow rate.  Cross flow is also useful if you want the
coolant inlet and outlet lines to be on the same end of the radiator.

But keep in mind that if you fold the radiator to get some counter flow
effect, the pressure drop will be 8 (EIGHT) time greater than simple cross
flow.  The number of radiator tubes carrying the fluid is halved, so the
pressure drop per unit length is 2 squared, or four times as much.  And the
tube length is doubled (two passes).  So it is best to check and see if the
water pump is up to the task.  Normally for cross flow radiators, the
pressure drop is small, but 8 times a small number may be a problem.

Better (in my opinion) to put on a large flow water pump, and operate in
simple cross flow.  But if you cannot get a high enough flow rate for some
reason, then consider counter flow to compensate.

Fred Moreno

My argument is if you have more than one row of tubes and more than one
pass 
you might as well use counter flow because there are no downsides and 
there is an upside... however slight it may be.

Paul Lamar
 
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