First, If you have enough air flow, the difference will be none or negligible. BUT if you don’t, there are different scenarios and cases where there could be a difference:
Situation 1: Two radiator take air from outside the case (corsair recommendation):
Water flow: CPU/GPU -> Rad1 -> Rad2
- Case 1: water reach room temperature on the First radiator. Then the second radiator is pointless because it cannot cool the water below room temp
- Case 2: water does not reach room temperature on the first radiator. Then a second radiator will be able to cool the water even more. BUT as the difference between the water and the air temperature is smaller than in the first radiator, the efficiency (the amount of heat extracted) will be smaller than in the first radiator.
Situation 2: Radiator 1 takes air from outside, the other from inside:
Water flow: CPU/GPU -> Rad1 (air from outside) -> Rad2 (air from inside)
- Case 1: water reach room temperature on the First radiator. Then the second radiator could be a problem because (in the worst case) it could be possible that the air going through the radiators is hotter than the water (because it absorbed heat from the first rad), and in that case, the radiator will increase the water temperature.
- Case 2: water does not reach room temperature on the first radiator. Then a second radiator will be able to cool the water even more. BUT as the difference between the water and the air temperature is even smaller than in the previous situation, so the efficiency (the amount of heat extracted) will be lower. Also, it won’t be able to lower the water temperature to room temperature because the air inside the case will be hotter than that.
Situation 3: Radiator 1 takes air from inside, the other from outside:
Water flow: CPU/GPU -> Rad1 (air from inside) -> Rad2 (air from outside)
- Case 1: water reach room temperature on the First radiator. Then the second radiator is pointless because it cannot cool the water below room temp.
- Case 2: water does not reach room temperature on the first radiator. Then a second radiator will be able to cool the water even more because the air passing through will be at room temp. Again, as the difference between the water and the air temperature is smaller, the amount of heat extracted will be lower. In this case it could be possible to cool the water to room temperature. An extra benefit is that the air pushed inside the case will be cooler than in situation 2 because this radiator will extract less heat into it.
Conclusion:
1- If you push enough air through the rads/case so it never gets hot, you can put the rads in any way you want. In that case all this analysis is pointless.
2- If you want the maximum efficiency with more than one rad, make them take always air from outside the case.
3- If you need to put the rads “in serial”, then you should connect the hottest water (the tube from CPU/GPU) to the rad that takes air from the case first.
4- If you need to stack rads, as in the Minecraft server, then you should connect the hottest water to the inner most rad and leave the outer most rad for last to maximize temperature difference.
5- The benefit of adding more rads to the loop, decreases with every rad as the difference in temperature is smaller.
