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Thermodynamics of loop order?

post #1 of 26
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So, long ago, I ran a WC setup. Like LONG ago, when we were using submerged aquarium pumps with heater cores and the blocks were either hand made in your garage, or bought from someone who hand made them in his. smile.gif Is there a thread somewhere to post pics of ancient tech? I've still got a few pieces laying around here that might generate a chuckle or two.

Anyways, I moved on to other things (including being able to afford bleeding edge equipment, so I didn't have as much need to overclock and as a result could make do with cheapish air cooling.) and haven't had my hand in watercooling since it's blossomed in popularity and availability.

I'm planning on returning to it for my next build, and as such I've been reading as much as possible.

My strengths don't lay in Science, (The limit of my understanding of the thermodynamics involved in water cooling is: Water takes the heat from the heat source and passes it into the air via the radiator.) so one thing I'm having trouble wrapping my head around is why exactly the placement of the components in your loop doesn't matter. Or, alternately, if it DOES matter.

The voices who seem to be the most knowledgeable in these threads (I judge by whether or not they use "then" when they mean "than." One's a reference to time, the other's a comparison. Pay attention in English class kids, they're not interchangeable.) claim that it makes no difference if a loop is:

pump > rad > CPU > rad > GPU > res

or

pump > rad > CPU > GPU > rad > res


To the layman, it would certainly seem that the former of those two loops would be better, as it would be introducing a lower temperature coolant to the GPU.

Can anyone explain in sciencey (but not too sciencey) terms, why this is or isn't the case?

IS there a difference? And if there actually is a difference but it's negligible, I'd appreciate knowing that, as opposed to just being told, "There's no difference."


I've done some searching here & elsewhere but haven't been able to find anything really answers my question in a way that satisfies me. Hopefully someone here will be able to clear it up for me or point me to an article where it's already done.

Using my rudimentary intelligence I've got an idea as to the workings of this and why neither loop order is superior, but I'd like to hear/read an explanation before I share my crackpot voodoo science theories.


Thanks for any enlightenment. smile.gif
     
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post #2 of 26
I don't have much WCing experience, but if your pump can push water around fast enough, it shouldn't matter, at least in theory. Otherwise, like you said, the 1st loop would be better at keeping both components cool, as the warm water would be cooled by the rad before going to the GPU.
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post #3 of 26
Only reason it really matters is to make sure that your pump is directly following the source of coolant to alleviate the possibility of running dry. as far as i understand it anyway. With my own wc setup i go Res>>Pump>>Rad>>cpu>>Rad>>Gpu>>res
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post #4 of 26
The quick and short version of why the order isn't important is due to the high flow rate and high heat capacity of water. When the flow rate is around 1 gallon per minute, the heat load required to raise the temperature of the water 1C is several hundred watts (I will do the calculation in a second and give a definite number).
Therefore as long as you have a decent flow rate, even a load of 1000 W will only raise the temperature of the water a few degrees as it flow past the components.

Of course it is best to have the coolest water flow over a block, however the temperature difference between having 25C or 26C water flowing over a block is very small. Keep in mind while the water maybe 30C, the CPU cores will much hotter, at around 60C or so. The mount of a block is of much more importance than a 1-2C change in water temperature.
 
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post #5 of 26
The order does not matter as the pumps move the water really quickly and the difference in order is about less than 1 degrees C. However, the order should be the reservoir feeding the pump first, if its a combo then you're fine! Not sure if you're familiar with pumps used and how they're fed is that the only thing to keep in mind now is have the reservoir above the pump so that the pump has plenty water and no bubbles in the impeller.
post #6 of 26
Loop order does matter, but only trivially, and it's only really worth considering in the most extreme situations. I believe the actual numbers as to why it doesn't really matter are this: it takes roughly 250w of heat to raise water that's flowing at 1 gpm by 1C. This means going from Rad > CPU > GPU, your gpu is going to be getting water that's less than 1C warmer than the CPU was getting. Once you get up to extreme setups, like 4x gtx580's, you start to see 3-4C temp increase after the last GPU, which is why you probably want to run your CPU before the GPU's, but even a 4C water increase is only going to result in 4C higher temperatures, you're not looking at a major difference. This is why for all intents and purposes loop order doesn't matter, 1-2C isn't going to limit your overclocking capability or cause a component failure.
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post #7 of 26
i have wasted alot of tubing to find that in a short amount of time the entire loop is going to equalize (within a degree or 2)

while i agree it is best to put your pump just downstream of the reservoir and to put radiators right in front of the hottest components, case layout and rad placement often doesnt allow for it.
    
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post #8 of 26
I say just do your loop in as clean a way as possible, using as little tubing as possible.
    
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post #9 of 26
The loop order might affect the temps at the first couple of runs that the water goes around your loop, but everything will equalize at certain point, its a loop after all. Your rad might not be able to immediately get rid of The heat that the CPU emits if you put your rad before your CPU, but that heat will gonna go through the rad no matter what, again, its a loop. I can't understand why some people believes tjat the loop order matters, it just does not makes sense, care to elaborate?
post #10 of 26
Thread Starter 
Thank you to the couple of guys who attempted to answer the question I had. The rest of you ... Well, thanks as well, I guess. You tried. Though you seemingly didn't actually read and/or comprehend my query here.


mdatmo, you said that was the short version. Do you have time for the longer one? smile.gif


A question: Say a component needs to shed 150w worth of heat to remain at a safe temp. Water in the system will always be able to absorb that amount of heat, no matter what temp the water is? So water that hits the GPU at 50C will pull off the same amount of heat as would 45C water?

If I don't have enough radiator and run the system under stress 24/7 would the water temps keep rising theoretically until it boiled? (I assume that the chip would burn out long before then, of course.)


bakageta, you get extra special rep for using "intents and purpose" correctly. Oh how I cringe when I hear "all intensive purposes." smile.gif
     
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