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Loop order and airflow with intake and exhaust rads

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Senior Member
Jun 21, 2002
My Nemesis GTX radiator came with this insert:

The radiator has a front and rear core, if you're looking at it head on towards the ports we'll call that the front. Instead of a left and right half with a U flow at the end, the coolant will flow across the entire width or the radiator but only half the thickness (say the back half), and once it reaches the end tank, it will flow back across the entire width of the radiator through the front half.

In the picture above, with front to back airflow, it is advised to flow coolant first through the back of the radiator (port A) and then through the front of the radiator to exit via port B. My understanding would be that the front chamber acts as an after-cool and the rear as a pre-cool. So air is that is case ambient is encountering coolant that has made one pass through the radiator. Air that has been warmed by passing through the front chamber then passes through the rear chamber, hopefully extracting more heat.

I'm extrapolating that the principle here is that warmer air should contact the coolant first (in terms of loop order), and cooler air should be last. The implication is that the coolant temperature will be different between passes in the radiator. If water fresh off of the components contacted case-ambient air and warmed it up, then in the second pass the delta T between the coolant and air as it flows through the rad would be decreased, compared to the advised configuration.

This contradicts the conventional wisdom that loop order doesn't matter because water temperature will be equalized. If water temperature was unchanged during passes, then the delta T would be decreased for the second pass by the same amount, regardless of which way the water flowed through the rad. If this were the case, I doubt that HWLabs would have placed this insert into their product packaging.

I can fully understand that the difference might not be significant when we're dealing with a component to coolant delta T of 30C - 60C (the coolant having a high enough specific heat that a minor difference in coolant temperature is not relevant to performance), but may make a difference when dealing with a coolant to air delta T of 10C - 20C. If your delta T is 10C, then a 1C difference is 10% and a 3C difference is 30%, meanwhile if your CPU is 70C and your coolant is 35C, a 1C increase is < 3%.

Do you guys agree with HWLabs? Or did they go to the trouble of including that for nothing?

Do you think this principle applies to using multiple rads in a case as well, as intake and exhaust (such that the intake rad is after the exhaust rad in loop order)?
The difference would be so small. Probably 1c or less. I would not be bothered either way.
Cool thanks for the heads up, I took a look url=https://www.xtremerigs.net/2015/02/14/hardwarelabs-nemesis-360-gtx-radiator-review/4/]here[/url].

I have read that to optimize this flow path that you should have the hot coolant tube side (inlet port) receiving the coolest intake air. That makes sense to remove the most heat in the first pass of the coolant, but leaves the second pass to be cooled by warm 2nd hand air from the 1st pass which depending on coolant flow rate and air flow may no longer do much. I began to wonder if possibly the second pass might be more efficient as a “final treatment” with the cool fresh air. Knowing that heat has already been removed from the first pass using possibly cooler air after the second pass treatment. I suspect each case will be better for a different setup, e.g. high air flow might favor hottest coolant meeting with coolest air. Both setups were tested. “OptiFlow” is what we call the better setup.


“OptiFlow” therefore makes a huge difference for cases when flow rate is lower. However when flow rate is high enough, “optiflow” or “reverse” doesn’t make a difference.

Although I'm not entirely sure which setup they're describing as "OptiFlow." Either way you're correct and my question has already been answered. Thank you!