[ferdb]

Originally Posted by jaydee116

Being we are dealing with a system here you must take into account the other parts of it. If you double the rads surface area, then you also double the area the coolant will have to be pumped through. The coolants flow rate needs to be in here somewhere and adjusted accordingly.

Because of the high heat capacity of water the temperature drop of the coolant is less than 1C going through the radiator, this is far less than the temperature change in the incoming air to the outgoing air so water flow rates make little impact on the heat transfer out of the radiator and we can just ignore this aspect.

Problem with this is you are assuming identical fans. I don't know of any instanced this is true. Blade number and pitch and rpm will most likely be different on any fan that doubled the other fans CFM in the same size. (80mm, 90mm, 120mm etc..). This would be a usable fact if such a thing existed in this system. I don't think you can just double the RPM of the fans in it unless they are already turned down. And the faster the fan turns the more power it will take to keep it going because of drag (wind resistance?).

Let me preface this response with the fact that I build all my systems for quiet operation and am always concerned with fans running at reduced speed on controllers. I mentioned doubling and halving the RPM of the fans to make the illustration simple. The relationship still holds for other rpm ratios. You could go from a 120mm panaflo L1A at 1700rpm to an H1A at 2500rpm which is an RPM ratio of 1:1.47 and get an airflow increase of roughly the same ratio in the system, the two models have exactly the same impeller etc, only the motor is different. Or you could be using a speed controlled fan, increase the rpm by 2x and you increase airflow by 2x. The basic concepts still hold. And yes the faster the fan goes the more power it takes, roughly 4 times the power for double the rpm. But the fan power is relatively minor in the system and I've never seen a system where supplying fans with power was a problem.

Providing your coolants flow rate remained the same. Not likely though doubling the rad area and not changing the coolants flow rate.

The backpressure of the radiator is usually quite low compared to the water blocks and the rest of the system so it's not the controlling factor on flow rates. Doubling the radiator will probably change the flow rates very little. Even if the flow did change substantially, as I mentioned above the coolant temp changes very little going through the radiator compared to the temperature change of the air going through. This means changes in coolant flow rate have little effect on the heat transfer out of the radiator. If the coolant temp drops 0.5C on it's way through the radiator and you then halve the flow rate it will then drop by 1C going through the radiator. A system temperature change of only 0.5C. The effective thermal resistance of the water blocks vs flow rate is another matter.

Huh? This has to be system spacific to a certain heat load. Makes no sense to put a certain number on this for all systems.

I picked 2C as an example of a realistic system to illustrate the point that no matter how big a radiator and how many fans you stack on it you cannot reduce the coolant temperature below the temperature of the incoming air. Say your coolant was 5C above the temperature of the incoming air to the radiator, no matter what you do in the way of radiators and airflow you cannot reduce your coolant temp by more than 5C. Also you can see that if you double the airflow through the radiator you would reduce your coolant temp by only 2.5C or so. In other words there is a point of diminishing returns on radiators and fans.

Would love to see how you came up with this.

I used approximate numbers from my current system and airflow/pressure curves for the Panaflo fans and ran the numbers. Equations can be useful. You just need to know what simplifications you can make, which you can't, what amount of accuracy you can expect etc and then can be powerful guides to design. It's much better than buying every fan and radiator under the sun and just trying them out.

Let's not assume bigger rads are going to be better with the same pump. If the pump can't handle the larger rad then your results are not going to be much better.

As I mentioned before radiators don't typically have large pressure drops so they don't tend to determine the overall system flow rates much. Also a larger radiator may have a LOWER pressure drop and thus the flow rates would actually be higher than for a smaller radiator.

Thanks for reading over my posts and bringing up points where I may not have been very clear in what I was trying to get across.