Sure,Originally Posted by Fragger
It's due to one specific thing. The air's ability to store heat is very poor, it's referred to as the "specific heat value". Better yet if you want to compare volume, the "Volumetric Heat Capacity" is an even better comparison.
Basically the air can only store a certain amount of heat until it's pretty well used up.
Water has a fantastic specific heat value, but air is very poor. That's why air is actually considered a good insulator rather than a conductor.
http://en.wikipedia.org/wiki/Specific_heat_capacity
Volumetric Specific Heat Value
Comparison of the ability to store heat per unit of volume:
Air = 0.001297 J cm−3 K−1
Water = 4.186 J cm−3 K−1
So, water per unit of volume, can store over 3000X more heat than air of the same volume
This is also part of why flow rate is "Everything" when it comes to heat exchangers/radiators, but flow rate is relatively insignificant when it comes to our loops and blocks. Specific heat isn't the only important thing though, thermal conductivity is equally important, but in terms of looking at using up a substance to exchange heat, specific heat value is the criteria to look at. When water passes through a block the temperature rise is many times only .1C, but in a radiator it can be 10 or even 20 degrees or more and the volume of air moving through the radiator is many many times larger than the volume of water moving through the loop. If you have fans moving 20CFM of air through a triple radiator, that's 60CFM which equals 450 Gallons per minute. The water loop is moving maybe 1.5GPM, so the volume of air going through the radiator is 450/1.5 or about 25X more.
It's too bad you don't live next to a spring fed river. You could put an 80mm radiator in the river and cool many systems strung together. Water is awesome, air sucks or does it "Blow"..?..
You can see that in the Air out temperature of the radiator testing. If the air out temperature was equal to the water out temperature, you would have used up 100% of the air's ability to store heat. You can never get 100%, but the difference between slim and double thickness when using slow speed fans is pretty small.
Take a close look at the percentages of the air out vs. water out of the MCR320:
At 600 and 1000RPM, the % is around 67-68%. That means the air's ability to cool is already used up about 67-68% at that flow rate. But notice as the air speeds increase, the % reduces down into the 54% range at the ultra high speed fans.
Now take a look at my TFC480 chart.
At 1000RPM, the % is up to about 86% at the low end and 70% at the high end.
So the double thickness does help, but you can only do so much with that remaining capacity left in the air. The first thickness is doing a much higher percentage of cooling and the air is about 60-70% used up by the time it reaches the second layer, so the second layer is only cooling maybe 70% of 30% that's left.
There are gains to be had, but not nearly as much heat exchange that occurs at that first layer that comes into contact with cool fresh air. This is why the frontal area is so key to performance. Add 30% more frontal area and you'll likely gain nearly that 30% in full. That's not the case with thickness because air has a very poor specific heat value, and that become more and more of an issue the slower the air moves.
In addition, you have fans that have a very poor ability in overcoming restriction and you're left with a system that's very sensitive to fan power and density of the exchanger. A simple thing like changing the FPI count by 2FPI is enough to make a measurable change in the radiators optimization.
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