Performance impact of mismatched radiators

[targ]

So, this is about placement of radiators on intake and exhaust fans in a water cooling loop. I understand why someone would choose one or the other, but I have also noticed that some people use both placements simultaneously in a single loop. I assume this is because they want additional radiators but they don't have room to configure them all as exhaust or all as intake. What I don't understand is how this can be beneficial over just using less radiators.

Are there any tests that show the actual impact of using two radiators as both exhaust and intake compared to the same radiators on their own? I don't see how it could help at all unless they were segregated somehow.

Suppose you have two identical radiators, one on a top front intake and one on the top exhaust. The intake one removes a certain amount of heat from the water and exhausts it, but this is the same flow that provides intake for the top radiator. Basically, it will receive cooler water, but also air that is warmer to the same degree that the water was cooled, which balances out to zero. Wouldn't you see exactly the same performance if you removed the radiator from the intake fan?

 

[BrightCandle]

No because you never maintain water temperatures anywhere near ambient temperature. In practice what will happen is the air will gain 1-2C in going through the radiator but the water will be +10C of the ambient temperature after leaving the radiator. So yes cooling will be reduced somewhat if the only source of air is the intake from the other radiator, but in practice the air is also coming from intake fans in the case as well.

The reason this happens is that air and water require different amounts of energy to raise their temperature by 1C. This mismatch and the fact that more energy is transferred when the difference in temperature is higher results in this situation where energy is conserved but also that even in this constrained situation that both intake and outlet are worth having.

 

[targ]

So basically a 5C gain in air temperature doesn't mean a 5C gain in water temperature because of the energy requirements. That makes sense.

However, aren't there still cases where this wouldn't work? Let's say the air gains 1C for every 2C lost from the water. With 20C ambient and 50C water, if a radiator can drop the water to 30C (-20C), its exhaust will also be 30C (+10C), so a second radiator in its path can't do anything. What are the actual numbers / how do you determine whether this would be worth while?